Petrogenesis of the Ultrapotassic Fanshan

Intrusion in the North China Craton:

Implications for Lithospheric Mantle

Metasomatism and the Origin of Apatite Ores

Tong Hou1,2,3, Zhaochong Zhang1*, Jakob K. Keiding4 and

Ilya V. Veksler4,5

1State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Beijing

100083, China, 2Institut fur Mineralogie, Leibniz Universitat Hannover, Callinstrasse 3, Hannover, D-30167, Germany,3Department of Petrology and Economic Geology, Geological Survey of Denmark and Greenland (GEUS), Øster

Voldgade 10, DK-1350 Copenhagen K, Denmark, 4Helmholtz Centre Potsdam GFZ German Research Centre for

Geosciences, Section 4.2, Telegrafenberg, Potsdam, D-14473, Germany and 5Department of Mineralogy, Technical

University Berlin, Ackerstrasse 71–76, Berlin 13555, Germany

*Corresponding author. Telephone: þ86 10 82322195. Fax: þ86 10 82323419.

E-mail: zczhang@cugb.edu.cn.

Received February 23, 2014; Accepted April 8, 2015

ABSTRACT

The Fanshan intrusion in the North China Craton (NCC) is concentrically zoned with syenite in the

core (Unit 1), surrounded by ultramafic rocks (clinopyroxenite and biotite clinopyroxenite; Unit 2),

and an outer rim of garnet-rich clinopyroxenite and orthoclase clinopyroxenite and syenite (Unit 3).

The intrusive rocks are composed of variable amounts of Ca-rich augite, biotite, orthoclase, melanite,

garnet, magnetite and apatite, with minor primary calcite. Monomineralic apatite rocks, nelsonite

and glimmerite exclusively occur in Unit 2. Geochemically, the Fanshan rocks are highly enriched in

light rare earth elements (LREE) and large ion lithophile elements (LILE), moderately depleted in highfield strength elements (HFSE), and have a limited range of Sr–Nd–O isotopic compositions. The

similar mineralogy, mineral compositions, and trace element characteristics of the three units sug-

gest that all the rocks are co-magmatic. The parental magma is ultrapotassic and is akin to kamafu-

gite. Very low-degree partial melting of metasomatized lithospheric mantle best explains the geo-

chemistry and petrogenesis of the parental magmas of the Fanshan intrusion. We propose that the

mantle source may have been metasomatized by a hydrous carbonate-bearing melt, which has im-printed the enriched Sr–Nd isotopic signature and incompatible element enrichment with conspicu-

ous negative Nb–Ta–Zr–Hf–Ti anomalies and LREE enrichments. The mantle source enrichment may

be correlated with oceanic sediment recycling during southward subduction of the Paleo-Asian oce-

anic plate during the Carboniferous and Permian. We propose that crystal settling and mechanical

sorting combined with repeated primitive magma replenishment and mixing with previously fractio-

nated magma is the predominant process responsible for the formation of the apatite ores.

Key words: apatite; ultrapotassic; Fanshan Intrusion; North China Craton; mantle metasomatism

INTRODUCTION

The study of ultrapotassic rocks, which are rare and

volumetrically minor, has been justified by their geneticlink with terrestrial mantle evolution and specific

tectonic settings (e.g. Foley et al., 1987; Miller et al.,

1999; Conticelli et al., 2007, 2009, 2013), as well as their

role in the generation of economic mineral deposits

(e.g. Dill, 2010). Although they can provide valuable

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J O U R N A L O F

P E T R O L O G Y

Journal of Petrology, 2015, 1–26

doi: 10.1093/petrology/egv021

Original Article

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information about their mantle source owing to their

extreme trace element characteristics, the origin of

ultrapotassic rocks has been a matter of substantial de-

bate (see Conticelli et al., 2013; Prelevic et al., 2014, and

references therein). Apatite deposits associated withultrapotassic intrusions are even rarer, with an equally

controversial origin; for example, Fanshan in China

(Jiang et al., 2004; Niu et al., 2012). Compared with

other occurrences of apatite-rich rocks (e.g. phoscorite)

much less information is available to constrain the ori-

gin and formation of monomineralic apatite ores.

The Fanshan alkaline intrusion in the North ChinaCraton (NCC; Fig. 1) hosts orebodies composed of alter-

nating layers of nelsonite and nearly monomineralic

apatite rocks (�38�7 wt % P2O5; Cheng & Sun, 2003).

One of its notable characteristics is the ultrapotassic na-

ture of bulk-rock compositions, with whole-rock K2O

concentrations up to 14% (Jiang et al., 2004). Its potas-sic nature contrasts with the Na-rich nepheline syenites

that host economic apatite deposits elsewhere in the

world, such as the Lovozero and Khibina intrusions,

Kola Peninsula, Russia (Downes et al., 2005; Zaitsev

et al., 2014). Thus, the Fanshan apatite deposits repre-

sent a unique type of magmatic apatite accumulationformed from an unusual type of parental magma. To

understand the origin of the deposits it is necessary to

constrain the composition of the parental magma and

the geochemical characteristics of its source. Although

some work has been done on the Fanshan intrusion

(e.g. Jiang et al., 2004; Niu et al., 2012), the origin of the

intrusion and its apatite deposits is still open to debate.For example, on the basis of Os isotopes Niu et al.

(2012) suggested that the Fanshan parental magma had

been contaminated by crustal rocks during magma em-

placement at crustal levels. However, the low Os con-

centrations (<300 ppt) in all the Fanshan rocks cast

doubts on such a conclusion.

In this contribution, we present comprehensive min-eral, whole-rock major and trace element and Sr–Nd–O

isotopic data for the Fanshan intrusion. The oxygen iso-

tope data are of special interest because they are prob-

ably a more robust indicator of crustal contamination

than Os isotopes (James, 1981). These new data are

used to constrain the composition of the parental

magma and to shed new light on the origin of phos-phorus mineralization in ultrapotassic intrusions.

GEOLOGICAL BACKGROUND

The Fanshan intrusion is located �100 km NW of

Beijing (Fig. 1) at the northern margin of the NCC. The

up to c. 3�85 Ga NCC (Liu et al., 1992) is bounded by

the Central Asian Orogenic Belt to the north and theQinling–Dabie and Sulu Orogens to the south (Fig. 1)

and consists of Paleoarchean to Paleoproterozoic

crystalline basement rocks that are mainly composed

of mafic granulites and amphibolites and tonalite–

trondhjemite–granodiorite (TTG) gneisses, overlain un-

conformably by Mesoproterozoic to Cenozoic sediment-

ary strata (Zhao et al., 2001).The northern margin of the NCC was strongly influ-

enced by the southward subduction of the Paleo-Asian

oceanic plate during Carboniferous to Permian times

(Xiao et al., 2003) with development of an Andean-style

continental margin during the Late Carboniferous–Early

Permian (Zhang et al., 2009). The final closure of the

North China Craton

Qilianshan Orogen

Qinling-Dabie OrogenYangtze Craton

Su-Lu Orogen240-210Ma

LA-ICP-MS

224±4MaSHRIMP

234±2MaLA-ICP-MS

Fanshan218±2MaSHRIMP

220±2MaSHRIMP

225-209MaLA-ICP-MS

220±5MaSHRIMP

Beijing

223±2Ma;222±4MaLA-ICP-MS300km

N40°

N30°

E100° E110° E120° E130°

N40°

N30°

Late Triassicalkaline intrusion

231±1MaSHRIMP

221±5MaSHRIMP

224±2MaSHRIMP

Solonker sutureCentral Asian Orogenic Belt

EAST CHINA SEA

Fault

Datong

Yaojiazhuang

Fig. 1. Simplified tectonic map of North China showing the locations of Late Triassic alkaline intrusions [modified from Ren et al.(2009)]. Age data and analytical methods are compiled from Zhang et al. (2012) and references therein.

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Paleo-Asian Ocean and amalgamation of Mongolian arc

terranes with the NCC along the Solonker suture

occurred between the Late Permian and earliest Triassic

(Zhang et al., 2012). Following this closure, post-colli-

sional Triassic (250–200 Ma) alkaline intrusions wereemplaced along an east–west-trending belt parallel to

the northern margin of the NCC (Fig. 1). The Early

Triassic alkaline rocks consist mainly of monzogranite,

K-feldspar granite and minor monzonite, whereas the

Middle–Late Triassic alkaline rocks include syenite and

peralkaline granite, as well as associated mafic–ultra-

mafic intrusions such as the Fanshan and Yaojiazhuangintrusions, which host magmatic apatite ores (Fig. 1).

Coeval lamprophyre and carbonatite dyke swarms have

been recognized in the Datong region of Shanxi prov-

ince, close to the northern margin of the NCC (Shao

et al., 2003; Fig. 1). In contrast to the Fanshan intrusion,

which contains nearly monomineralic apatite layers, apa-

tite is present throughout the Yaojiazhuang intrusion, lo-

cally in modal proportions up to 15% (Chen et al., 2013).

FANSHAN INTRUSION

GeologyThe Fanshan intrusion was emplaced at c. 218 Ma (Ren

et al., 2009; Niu et al., 2012), at the intersection between

pre-existing NNW–SSE- and east–west-trending faults,

into Meso- to Neoproterozoic limestones and clastic

rocks of the Wumishan Formation. It is an �6� 5 km

oval-shaped body in plan (Fig. 2). However, much of the

pluton is covered by over 100 m of Quaternary sedi-ments, and thus the present geometry of the intrusion

Syenite dykeClinopyroxenesyenite

Fe-P orebodies

Clinopyroxenitedominated rocks

Orthoclaseclinopyroxenite

Garnet-richclinopyroxenite

Late Cretaceous granodiorite

Z2w

Z2w

N

Z2wMesoproterozoic limestones

Z2w

BA

-200

200

600(m)

Height

(a)

Quaternary sediments

Drill hole

of samples from units 2-3

Samples from unit 1(b)

LENDEND

B

A

0 1km

Unit 1

Unit 2

Unit 3

Unit 1

Unit 2

Unit 3

Fig. 2. Geological map (a) and cross-section (b) of the Fanshan intrusion. Modified from Jiang et al. (2004). Most of the intrusion iscovered by Quaternary sediments, and the geological map is largely inferred from borehole and geophysical data. The dashed linein (a) indicates the approximate near-surface locations of samples collected from Units 2 and 3. A–B indicates the line of section in(b). The position of the borehole from which the clinopyroxene syenite samples of Unit 1 were obtained is indicated.

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is largely deduced from geophysical studies and

boreholes. The intrusion consists of three main litholo-

gical units (Fig. 2): an inner, irregular, central syenitic

core (Unit 1), followed by rings of ultramafic layered

rocks (Unit 2) and garnet–orthoclase-clinopyroxenite(syenite) (Unit 3). Minor syenite dykes and veins occur

within the intrusion and cut through the entire succes-

sion; some of these may have been displaced by later

faulting.

The contact relationships between the three units are

sharp. At the contact between Units 2 and 3, a breccia

of Unit 2 clinopyroxenite can be seen in drill core, indi-cating that Unit 2 was the earlier intrusive phase (Mu

et al., 1988). Unit 1 cuts both Units 3 and 2 (Fig. 2) and

contains breccias of Units 2 and 3, and is thus the latest

intrusive phase. No country-rock xenoliths have been

found, but the country-rocks are locally changed into

marble by contact metamorphism.Unit 1 consists predominantly of syenitic rocks, which

are relatively homogeneous and show no layering, al-

though the mafic mineral content varies locally. Unit 2 is

the most complex and is the only unit containing Fe–P

ore, including monomineralic apatite rocks and nelson-

ite. It is composed predominantly of coarse-grained cli-nopyroxenite and locally shows well-developed

rhythmic layering. The clinopyroxenite locally grades

into biotite-clinopyroxenite, orthoclase clinopyroxenite

and glimmerite, and is intercalated with nine layers of

Fe–P ore (Fig. 3). Each rhythmic layer contains a clinopyr-

oxenite layer in the lower part and a biotite clinopyroxen-

ite layer in the upper part (Fig. 3). Although the rhythmiclayers have variable thickness, from a few centimetres to

several tens of meters, the clinopyroxenite and biotite cli-

nopyroxenite layers are typically several meters to 90 m

thick. The monomineralic apatite rocks and nelsonites lo-

cally show prominent rhythmic layering (Fig. 4a) and are

present as paired lithologies. They are recognized only inthe southeastern part of the intrusion; their absence in

the northwestern part could be attributed to removal by

the intrusion of the younger syenitic core (Unit 1). The

intercalated monomineralic apatite rocks and nelsonite

layers generally have a constant thickness and steep in-

ward concentric dips (Fig. 2b). Six phosphorus orebodies

with >4�5 wt % P2O5 have been identified in Unit 2 (Muet al., 1988); however, the cut-off grade for economic

mining activity is 7 wt % P2O5, and therefore only two

orebodies have been exploited. The glimmerite layers

are always intercalated with the Fe–P ore (Fig. 4b) and

are typically less than 1 m thick. The modal mineralogy

varies considerably in the glimmerites; apatite or Ca-richaugite locally reaches more than 25 modal %.

Unit 3 predominantly comprises orthoclase clinopyr-

oxenite and garnet-rich clinopyroxenite and syenite.

The garnet-rich clinopyroxenites dominate the rim

of the intrusion (Unit 3; Figs 2 and 3), and are character-

ized by the occurrence of melanite garnet, which locally

can be up to 35 modal % (Mu et al., 1988) in the outerpart of this unit. The contacts of these rock types are

gradual.

PetrographyCa-rich augite, biotite, magnetite, apatite and K-feldspar

(orthoclase) are the five main minerals in each unit.

Accessory minerals may include melanite, garnet, titan-

ite, pyrite, chalcopyrite, calcite, and rutile.

Unit 1The syenite is grayish white, and consists of euhedral–

subhedral K-feldspar (>70 modal %), Ca-rich augite(�20 modal %) and minor anhedral garnet, biotite, apa-

tite, titanite and magnetite. Biotite and magnetite typic-

ally account for less than 5 modal %, and apatite

accounts for less than 2 modal %. The syenite is rela-

tively homogeneous and has a cumulate texture evi-

denced by the orientation of euhedral to subhedral,

medium- to coarse-grained K-feldspar and Ca-rich aug-ite (Fig. 5a). The syenitic dykes are porphyritic and con-

sist of K-feldspar phenocrysts (up to 30 modal %) set in

a matrix of fine-grained K-feldspar.

Unit 2The modal mineralogy of Unit 2 is summarized in

Table 1 and illustrated in Fig. 6.

The clinopyroxenite in this unit exhibits a cumulatetexture, except locally within the ore horizons (Fig. 6a);

in most cases the crystals of Ca-rich augite have a pre-

ferred orientation, which is probably caused by magma

flow. These rocks are coarse-grained, composed essen-

tially of euhedral Ca-rich augite (up to 5 mm in length;

>80 modal %) and variable amounts of euhedral apatite(up to 10 modal %), with minor interstitial magnetite,

biotite and calcite (Fig. 5b and c). Euhedral apatite can

occur as inclusions in the Ca-rich augite. The calcite is

believed to be primary and magmatic in origin, as evi-

denced by textures (Fig. 5c) similar to those described

from alkaline intrusions associated with carbonatites

(e.g. Le Roex & Lanyon, 1998; Veksler et al., 1998;Zaitsev et al., 2014).

Biotite clinopyroxenite contains less Ca-rich augite

and magnetite but more apatite and biotite than clino-

pyroxenite. In addition, it displays a somewhat different

texture from the clinopyroxenite in which intercumulus

biotite occurs as subhedral grains but locally biotitecrystallized earlier than clinopyroxene (Fig. 6). Both bio-

tite and Ca-rich augite exhibit the same preferred orien-

tation. In some samples, biotite commonly forms

coarse poikilitic plates enclosing Ca-rich augite (Fig. 5d).

Glimmerites are medium- to coarse-grained cumu-

late rocks composed mainly of oriented, euhedral

brown biotite (>70 modal %; Fig. 6c) and small amountsof Ca-rich augite, apatite, and interstitial magnetite

(Fig. 5e). However, the modal proportions in this rock

type vary considerably, and either apatite or Ca-rich

augite locally reaches more than 25 modal % (Fig. 6).

Ca-rich augite is subhedral and occurs as granular ag-

gregates. Biotite grains are also subhedral and can con-tain small euhedral apatite grains, but no Ca-rich augite

inclusions were found. Biotite grains also exhibit a

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FS01FS02

FS03FS04FS05FS06

FS07FS08FS09

FS10FS11

FS12

FS13

FS15

FS16FS17

FS19

FS22

FS14

FS18

FS20

FS23FS24

FS-1-01

FS-1-02

FS-1-03FS-1-04FS-1-05

FS-1-06FS-1-07FS-1-08FS-1-09FS-1-10

FS-1-11

FS-1-12FS-1-13

SFS01

SFS05

SFS06

SFS07SFS08

Layer

9

8

7

6

5

4

3

2

1

Thickness (m)

Sampling

42

24

45

49

63

72.7

36

94

827

48

59

34

11

33

14

63

25

90 Clinopyroxenite

Clinopyroxenite

Clinopyroxenite

Clinopyroxenite

clinopyroxenite

Clinopyroxenite

clinopyroxenite

Clinopyroxenite

clinopyroxenite andglimmerite)

clinopyroxenite

Nelsonite

Clinopyroxenite(Interlayered orthoclase clinopyroxenite)

Orthoclaseclinopyroxenite

Clinopyroxenite

clinopyroxenite

Clinopyroxenite

clinopyroxenite

Clinopyroxenite

LithofaciesColumnarCentre of

the complex

Mesoproterozoiclimestones

0m

~900m

~2200m

~3100m

Syenite vein

ClinopyroxenesyeniteFe-P orebodies

Clinopyroxenitedominated rocks

Orthoclaseclinopyroxenite

Garnet-richclinopyroxenite

FS-2-27FS-2-25FS-2-19FS-2-09FS-2-08

Unit 2

Unit 3

Unit 1

Fig. 3. Schematic stratigraphy of the Fanshan intrusion showing the stratigraphic position of the samples of Unit 2 and garnet clino-pyroxenite in Unit 3 [modified from Cheng & Sun (2003)]. The orthoclase clinopyroxenite samples of Unit 3 were collected from atunnel in which a complete lithological succession is exposed through mining, whereas for the syenite in Unit 1, four core samples15–25 cm long were collected randomly from a recently drilled borehole. The 0 m reference level corresponds to the contact zonebetween garnet clinopyroxenite and Mesoproterozoic limestones of the Wumishan Formation.

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preferred orientation, but are usually bent and show

undulose extinction. Magnetite is rare.

Monomineralic apatite rocks and nelsonite dominate

the ore horizons (Fig. 6) and are composed mainly ofapatite and magnetite, with minor Ca-rich augite and

biotite. The monomineralic apatite rocks are light yel-

low to green in colour and very friable. They are me-

dium grained, granular, with a subhedral texture;

apatite contents can be up to 95 modal % (Fig. 6b). In

some of these rocks apatite displays 120� triple junc-tions (Fig. 5f). In those rocks containing biotite, apatite

typically occurs between euhedral or subhedral biotite

grains, and both show a preferred orientation. Most bio-

tite grains are bent and have undulose extinction. Ca-

rich augite is a minor interstitial phase and occurs as

subhedral or anhedral grains that crystallized later than

the biotite and apatite. Some inclusions composed ofbiotite and Ca-rich augite (Fig. 5g) with minor carbonate

(dolomite and calcite) have been observed within

apatite.

Nelsonite is also an important component of the ore

horizons; it is gray–black in colour (Fig. 4a), and con-

tains less apatite and biotite but more magnetite (up to40 modal %; Fig. 6d) and Ca-rich augite (up to 10 modal

%) than the monomineralic apatite rocks. Apatite is eu-

hedral and ranges from 0�5 to 5 mm in cross-section,

and up to 10 mm in length. Ca-rich augite occurs as eu-

hedral to subhedral aggregates and locally contains eu-

hedral apatite grains. In some samples biotite forms

coarse poikilitic plates enclosing small Ca-rich augitegrains. K-feldspar is absent in the nelsonite.

In most samples, Ca-rich augite occurs as a primoc-

ryst phase suggesting that it crystallized first, followed

by apatite, biotite and K-feldspar. Magnetite and calcite

are interstitial to the main rock-forming minerals in theultramafic rocks, and crystallized at a late stage. Apatite

accumulates together with magnetite in Units 2 and 3

(Fig. 5h), forming the layered Fe–P-rich lithologies

(Figs 3, 4a and 6).

Unit 3Unit 3 consists predominantly of orthoclase- and gar-

net-clinopyroxenite and syenite. Compared with the cli-

nopyroxenites in Unit 2, the orthoclase-clinopyroxenite

in this unit contains more interstitial K-feldspar, al-

though the contents of K-feldspar and Ca-rich augite

vary considerably. Locally, K-feldspar is the most abun-

dant mineral in lithologies such as Ca-rich augite syen-ite. The garnet-rich clinopyroxenites typically contain

�5 modal % melanite garnet, which locally reaches as

much as 35 modal %. They are composed of subhedral

garnet, Ca-rich augite, biotite, orthoclase and magnet-

ite. Euhedral apatite is common and titanite is also

present.

SAMPLE PREPARATION AND ANALYTICALMETHODS

Owing to the different conditions in the mine workings,we sampled the Fanshan intrusion using different meth-

ods. A total of 41 samples of Unit 2, 24 samples of Unit

Apatite rock

Nelsonite

(a) (b)

Glimmerite

Apatite rock

Syenite

Clinopyroxenite

(c) (d)

Garnet-richclinopyroxenite

Syenite

Fig. 4. Field photographs of Fanshan rocks from underground excavations. (a) Rhythmic layered rocks comprising alternatinglayers of monomineralic apatite rock and nelsonite, Unit 2. (b) Monomineralic apatite rocks occurring as enclaves in glimmerite,Unit 2. (c) Clinopyroxenite intruded by a syenite vein, Unit 2. (d) Garnet-rich clinopyroxenite intruded by a syenite vein, Unit 3.

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Cpx

Mt

Ap

0.2mm

Cpx

Bt

0.2mm

Ap

Bt

Cpx

0.2mm

Ap

ApAp

0.1mm

0.2mm

Mt

Ap

0.1mmBt

Cc

Cpx

Ap

Bt

50µm

0.2mm

Kfs

KfsCpx

(b)

(d)

(e) (f)

(g)

(c)

(h)

(a)

Cc

Kfs Cc

Fig. 5. Representative photomicrographs of Fanshan rocks. (a) Clinopyroxene syenite, Unit 1; plane-polarized light. (b)Clinopyroxenite, Unit 2; plane-polarized light. (c) Occurrence of calcite, Unit 2; plane-polarized light. (d) Biotite-clinopyroxenite, Unit2; cross-polarized light. (e) Glimmerite, Unit 2; cross-polarized light. (f) Monomineralic apatite rock, Unit 2; cross-polarized light. (g)Back-scattered electron image of clinopyroxene and biotite forming a crystallized melt inclusion in apatite, Unit 2. (h) Nelsonite,Unit 2; plane-polarized light. Ap, apatite; Bt, biotite; Cpx, clinopyroxene; Kfs, K-feldspar; Mt, magnetite; Cc, calcite.

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Tab

le1

:M

od

alm

ine

ralo

gy

of

the

Fa

nsh

an

intr

usi

on

.

Sa

mp

le:

FS

01

FS

02

FS

-1-0

1F

S0

3F

S-1

-02

FS

04

FS

05

FS

06

FS

-1-0

3F

S-1

-04

FS

-1-0

5F

S0

7F

S0

8F

S0

9F

S-1

-06

FS

-1-0

7F

S-1

-08

FS

-1-0

9U

nit

:2

22

22

22

22

22

22

22

22

2R

ock

typ

e:

CC

CC

NC

NN

AA

CG

CA

CA

CC

Dis

tan

ce(m

):1

49

01

47

01

46

01

42

01

40

51

40

41

40

21

40

11

39

91

39

81

39

71

39

51

39

01

38

71

38

51

38

21

38

01

37

7

Cli

no

py

rox

en

e6

67

27

15

88

60

85

0�5

3�5

59

14

76

17

61

67

69�5

Ap

ati

te7

98

5�4

41�5

12

40

36

94�5

90

18

25

10

90

58

81

76

Bio

tite

11�5

5�5

61

51

05

51

53�5

45

54

15

89�9

6�5

10

Ma

gn

eti

te7

98

5�6

38

16

44�5

40

0�5

0�5

16

11

21

90�1

61

1�5

K-f

eld

spa

r2

35

15

2�5

72

30�5

2tr

ace

51

31

0�9

32

Ga

rne

t0�1

0�2

Ca

lcit

e0�5

10�5

11

0�5

0�5

Tit

an

ite

10�5

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8 Journal of Petrology, 2015, Vol. 0, No. 0

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nloaded from

3 and four samples of the syenitic dykes were collected

from a tunnel in which a complete lithological succes-

sion is exposed (Figs 2 and 3). For the syenite in Unit 1,four core samples 15–25 cm long were collected from a

recently drilled borehole.

Weathered or altered surfaces were removed from

the samples before jaw-crushing. Fresh chips were then

selected for analysis using a binocular microscope and

pulverized into powders using agate mortars. Augite

grains for O isotope analysis were crushed to �0�05 mmto avoid oxide inclusions, washed and separated by

standard gravimetric and magnetic methods, and final

hand-picking under a binocular microscope.

Mineral chemistryElectron microprobe analyses of Ca-rich augite, mag-

netite, biotite, K-feldspar, garnet and apatite were ob-

tained using the JEOL Superprobe JXA-8200 electron

microprobe at Washington University in St. Louis, USA,

and the JXA 8230 electron microprobe at the German

Research Centre for Geosciences, GFZ, Potsdam,

Germany. A focused beam of 2mm was employed, ex-cept for K-feldspar where a 5 mm beam was used.

Accelerating voltage was 15 kV and beam current

was 15 nA. Elements were analysed with wavelength-

dispersive spectrometers and were calibrated by

reference to oxide and mineral standards using the PAP

correction routine. The precision for oxide concentra-tions is better than 1%. The counting times were 20 s on

the peak and 10 s on the background.

Trace element concentrations in clinopyroxene (Ca-

rich augite) were determined on thin sections by laser

ablation sector-field inductively coupled plasma massspectrometry (LA-SF-ICP-MS) at the Geological Survey

of Denmark and Greenland. A UP213 frequency-quin-

tupled Nd:YAG solid-state laser system from New Wave

Research employing two-volume cell technology was

coupled to an Element2 double-focusing single-collec-

tor magnetic sector-field ICP-MS system from Thermo-

Fisher Scientific. The mass spectrometer was equippedwith a Fassel-type quartz torch shielded with a

grounded Pt electrode and a quartz bonnet. Operating

conditions and data acquisition parameters are listed

in the Supplementary Data (supplementary data are

available for downloading at http://www.petrology.

oxfordjournals.org). Standards used were the BCR-2and NIST-614 glass reference standards, and for the

60mm square-spot laser analyses also the BB-2 cpx

(Norman et al., 1996), during the analytical sequence,

yielding internal 2SE precision and accuracy of <10%

for all elements measured. Data were acquired from

single round-spot analyses 40 mm in size or by 60 mm

square ablation patterns, using nominal laser fluence of10–12 J cm–2 and a pulse rate of 10 Hz. Total acquisition

times for single analyses were c. 120 s, including 50 s

gas blank followed by laser ablation for 30–50 s and

washout for 30 s. Factory-supplied software from

Thermo-Fisher Scientific was used for the acquisition of

the transient data, obtained through pre-set spot loca-tions. Data reduction was performed off-line using the

1500m

1400m

1300m

1200m

1100m

1000m

0 20 40 60 80 100

Ca-rich augite(wt.%) (wt.%) (wt.%) (wt.%)

K-feldspar(wt.%)

20 40 60 80 100 20 40 60 80 10 20 30 40 50 5 10 15 20

DIST

ANCE

(m)

Cpx Ap Bt Mt Kfs

cumulus/intercumulus

(a) (b) (c) (d) (e)

Fig. 6. Mineral modes (see Supplementary Data) of (a) Ca-rich augite, (b) apatite, (c) biotite, (d) magnetite and (e) K-feldspar in Unit2 of the Fanshan intrusion as a function of stratigraphic height and stratigraphy of cumulus (grey) and intercumulus (white) phasesin Unit 2. The grey bands in (a)–(e) indicate the ore horizons.

Journal of Petrology, 2015, Vol. 0, No. 0 9

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software Iolite version 2.5 (Hellstrom et al., 2008; Paton

et al., 2011), the Trace_Elements_IS routine and 29Si as

the internal standard.

Bulk-rock major and trace elementsSelected chips were crushed in an agate mortar and

ground in an agate mill to powders of �200 mesh.

Major elements were analysed on fused glass discsusing a scanning wavelength dispersion X-ray fluores-

cence spectrometer at the ICP-MS Laboratory of the

National Research Centre for Geoanalysis, Beijing and

at the Department of Earth Sciences, Nanjing

University, Nanjing, China. The analytical uncertainties

are less than 1%, estimated from repeat analyses of twostandards (andesite GSR-2 and basalt GSR-3). Loss on

ignition (LOI) was determined gravimetrically after heat-

ing the samples at 980�C for 30 min. The ferrous oxide

content was determined by wet chemical methods.

Bulk-rock trace elements were analysed by solution

ICP-MS at the National Research Centre for

Geoanalysis, Beijing and the Department of EarthSciences, Nanjing University, Nanjing, China. For the

analyses, rock powders (�40 mg) were dissolved in dis-

tilled HFþHClO4 in 15 ml Savillex Teflon screw-cap

breakers. The precision for most elements was typically

better than 5% RSD (relative standard deviation), and

the measured values for Zr, Hf, Nb and Ta were within10% of the certified values of the two employed stand-

ards (granite GSR-1 and basalt GSR-3).

Sr, Nd and O isotopesSr and Nd isotopes were analysed at the Department of

Earth Sciences, Nanjing University, China, using a

Finnigan Triton TI mass spectrometer. The samples

were analysed for Rb, Sr, Sm and Nd concentrations,

and 87Sr/86Sr and 143Nd/144Nd isotope ratios. Rb, Sr, Nd

and Sm concentrations were determined by isotope di-lution using 85Rb–84Sr and 150Nd–149Sm spikes. The ac-

curacy is 60�5% for Rb and Sr, 61�1% for 87Rb/86Sr and

60�01% for 87Sr/86Sr (2r), 60�5% for Sm and Nd, 60�5%

for 147Sm/144Nd and 60�005% for 143Nd/144Nd (2r). Sr

and Nd isotopic ratios were normalized against86Sr/88Sr¼ 0�1194 and 146Nd/144Nd¼ 0�7219, respect-ively. 87Sr/86Sr for the NIST987 Sr standard was

0�710268 6 0�000007 (2r, n¼ 8), and 143Nd/144Nd for the

La Jolla Nd standard was 0�511842 6 0�000006 (2r,

n¼ 6). Total blanks were 100 pg for Sr and 80 pg for Nd,

and negligible for the determination of isotopic com-

positions. eNd(t) values were calculated using the pre-

sent-day values for a chondritic uniform reservoir(CHUR) 143Nd/144Nd¼0�512638 and 147Sm/144Nd¼0�1967 (Jacobsen & Wasserburg, 1980).

The oxygen isotopic compositions of Ca-rich augite

were measured at the Institute of Mineral Resources,

Chinese Academy of Sciences. Ca-rich augite was

chosen for study because the mineral is present in allthe rocks of the intrusion. Oxygen isotope measure-

ments were performed using the bromine pentafluoride

method of Vennemann & Smith (1990). The analyses

were compared with those of an internal standard, cali-

brated relative to NBS-28 (d18OSMOW¼þ9�6%), and no

data correction was needed. Almost all samples have

been duplicated with analytical precision 6 0�2% (1r) orbetter.

RESULTS

Mineral chemistryRepresentative chemical compositions and structural

formulae of Ca-rich augite, K-feldspar, biotite, garnet,

magnetite and apatite from Fanshan are listed in the

Supplementary Data. Those of Ca-rich augite in Unit 2

are reported in Table 2.

Ca-rich augiteCa-rich augites in the three units of the Fanshan intru-

sion show similar compositions with a limited range of

Wo46�42–50�09En29�72–42�02Fs8�26–19�49 (Table 2; Figs 7a, b

and 8a). These compositions are comparable with those

reported for clinopyroxene from ultrapotassic volcanic

rocks of Central Italy that have been considered to becontaminated by carbonate sediments (Cellai et al.,

1994; Gaeta et al., 2006; Mollo & Vona, 2014; and refer-

ences therein). The Mg-number [Mg-number (Mg#) is

defined as Mg/(MgþFetot), in atoms per formula unit]

varies from 0�70 to 0�92, and exhibits several gradual re-

versals from the base of Unit 2 upwards (Fig. 7a). TiO2

concentrations (0�07–1�76 wt %) generally correlate withthe change of Mg-number (Fig. 7b). The relatively low

SiO2 and high Al2O3 contents of the Ca-rich augites

from the Fanshan intrusion probably reflect the SiO2-

undersaturated nature of the parental magma (see

below).

Ca-rich augite has total rare earth element (REE) con-tents ranging from 108 to 124 ppm (Table 3) and shows

‘hump-shaped’ light REE (LREE)-enriched chrondrite-

normalized REE patterns (Fig. 9a), coupled with Hf en-

richment and negative Nb–Ta anomalies in primitive

mantle-normalized trace element patterns (Fig. 9b).

K-feldsparK-feldspar in the Fanshan intrusion shows only a

small range of compositional variation through the in-

trusion (Fig. 8b). It is almost pure orthoclase Or90�6–100

Ab0�1–9�6An0–0�3.

BiotiteBiotite exhibits much larger compositional variations inFe and Mg compared with Ca-rich augite, especially in

Unit 2; our data compare well with those previously re-

ported for the Fanshan intrusion (Niu et al., 2012). Total

Fe2O3 ranges from 8�5 to 24�4 wt %, and MgO from 8�8to 19�5 wt %. The TiO2 contents of biotite in Unit 2 range

from 1�2 to 5�3 wt %. The MgO, K2O and TiO2 contentsexhibit several gradual reversals from the base of Unit 2

upwards (Fig. 7c–e); MgO contents are notably higher in

10 Journal of Petrology, 2015, Vol. 0, No. 0

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ay 26, 2015http://petrology.oxfordjournals.org/

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the ore horizons (Fig. 7c). On an Fe3þ–Fe2þ–Mg diagram

most of the Fanshan biotites suggest crystallization

under relatively normal redox conditions for plutonic

rocks close to the fayalite-magnetite–quartz–magnetite

oxygen buffer (FMQ; Fig. 8c).

Melanite, magnetite and apatiteAll the analysed garnets are Ti-rich (1�8–12�3 wt %) and

classified as melanite (i.e. Fe3þ>Ti in the octahedral

position; Deer et al., 1992) with Ti¼ 0�142–0�778, Ca¼2�898–3�018 and Fe3þ¼ 1�193–1�620 (a.p.f.u.). Magnetite

shows a wide compositional range with TiO2 contents

ranging from 0�1 to 10�9 wt %, 0�0–3�6 wt % MgO and0�2–1�6 wt % MnO. However, no systematic trend was

found between the units and within the layered rocks

(Fig. 7f). Apatites in Units 2 and 3 are predominantly flu-

orapatite characterized by high F (1�8–2�1 wt %) and low

Cl contents (0�04–0�06 wt %); the F/Cl ratios are constant

(35–53) throughout Unit 2 (Fig. 7g).

Bulk-rock major and trace element compositionsAll analysed samples have low or negligible LOI values

except those containing considerable amounts of apatite

and biotite (Table 4). As one would expect from stronglymodally layered cumulates, the bulk-rocks exhibit large

compositional variations (Table 4). Specifically, the

(biotite-)clinopyroxenites in Unit 2 have low SiO2 and

high CaO, consistent with the high content of apatite in

these rocks. They also have variable Mg-numbers

(Table 4). The Al2O3, K2O and Na2O contents are lowerthan those of other rock types in the Fanshan intrusion.

The two glimmerite samples (FS-07 and FS-1-10) are

characterized by high MgO contents and relatively low

contents of total Fe2O3 compared with the clinopyroxen-

ites. Generally, the compositional variations can be

attributed to the varying proportions of Ca-rich augite

and biotite and to the variable amounts of intercumulus

phases (mainly orthoclase, apatite, and magnetite).Compared with the clinopyroxenite and glimmerite in

Unit 2, the syenites in Unit 1 have much higher SiO2,

Al2O3, K2O and Na2O contents, but lower CaO, MgO and

total Fe2O3, consistent with the dominance of orthoclase

in these rocks. The garnet-rich clinopyroxenites and

orthoclase-clinopyroxenites in Unit 3 show transitional

chemical compositions between the ultramafic rocks [i.e.(biotite-)clinopyroxenite and glimmerite] and the syenites

(Table 4), but the former exhibit dispersion of the data for

Na2O and TiO2, possibly owing to the presence of melan-

ite. In particular, the garnet-rich clinopyroxenites have

low Na2O contents and are characterized by higher TiO2

contents, with limited variation in SiO2, Al2O3, K2O andMgO, but variable CaO and total Fe2O3. The monominer-

alic apatite rocks have the highest P2O5 contents, corres-

ponding to almost pure apatite; the nelsonites also have

high Fe2O3 and P2O5 contents (Table 4).

Representative trace element compositions of the

Fanshan intrusive rocks are given in Table 4 and illus-

trated in chondrite-normalized and mantle-normalizeddiagrams in Figs 10 and 11. All the rock types are char-

acterized by significant enrichment in large ion litho-

phile elements (LILE), such as Sr, Ba and Rb, and LREE,

and display prominent troughs in Nb, Ta, Zr, Hf and Ti.

The garnet-clinopyroxenite and syenite samples

(Fig. 11e and f) show convex-upward REE patterns with

1500m

1400m

1300m

1200m

1100m

1000m

0.6 0.7 0.8 0.9 1.0Mg# in Cpx

0 0.5 1.0 1.5 2.0TiO2(wt.%) in Cpx

10 20 300MgO(wt.%)in Bt

K2O(wt.%)in Bt

9 9.5 10 10.5TiO2(wt.%)in Bt

0 2 4 6 0 5 10 15TiO2(wt.%)in Mt

0 20 40 60F/Cl in Ap

(a) (b) (c) (d) (e) (f) (g)

DIST

ANCE

Fig. 7. Major element compositional variations of (a) Ca-rich augite (Mg#), (b) Ca-rich augite (TiO2), (c) biotite (MgO), (d) biotite(K2O), (e) biotite (TiO2), (f) magnetite (TiO2), and (g) apatite (F/Cl ratio) with stratigraphic position in Unit 2 of the Fanshan intrusion.The grey bands indicate the ore horizons.

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relatively flat heavy REE (HREE), which is probably due

to the presence of melanite garnet.

Sr–Nd–O isotope dataThe analysed Fanshan rocks show limited variations in

Sr, Nd and O isotopic composition (Table 4). The age-

corrected 87Sr/86Sr ratios (t¼ 218 Ma, Ren et al., 2009;

Niu et al., 2012), range from 0�70513 to 0�70601, andeNd(t) values vary from –6�8 to –5�5; d18O values range

from þ7�5 to þ8�1%. Our data (Table 3) are consistent

with those of previous isotopic studies on the rocks

from the three units (Mu et al., 1988; Niu et al., 2012).

The data for our samples overlap the field of ultrapo-

tassic rocks from the Roman Magmatic Province

(Fig. 12), and plot above the fields of the Late Paleozoicto Early Mesozoic carbonatite intrusions in the NCC.

The Yaojiazhuang intrusion and the Precambrian base-

ment rocks in the NCC have much lower eNd(t) and

higher initial (87Sr/86Sr) ratios. Notably, the Fanshan

samples have a slightly more enriched Nd–Sr isotopic

signature compared with the Palaeozoic lithosphericmantle of the NCC represented by kimberlite-hosted

xenoliths (Zheng & Lu, 1997; Xu et al., 2004).

DISCUSSION

Parental magma compositionThe similar mineralogy, mineral compositions, chon-

drite-normalized REE patterns (Figs 10 and 11) and

Sr–Nd isotopic compositions (Fig. 12) observed in the

three units suggest that all the rocks were derived from

a common parent magma by similar processes of mag-matic differentiation. Unfortunately, neither chilled mar-

gins nor melt inclusions in primitive cumulates have

been found that could help to constrain the parent

magma composition. Previous attempts to estimate the

parental magma composition have been mainly based

on the area of surface outcrops and data from several

boreholes in the part of the intrusion buried underQuaternary sediments (Mu et al., 1988). Those recon-

structions and associated calculations suggest low SiO2

(35�26 wt %) and Al2O3 (5�68 wt %), but high CaO

(19�46 wt %) and K2O (2�01 wt %) contents and Fe2O3/

FeO ratios (1�2) in the parental magma (Mu et al., 1988).

Such chemical characteristics correspond to those ofGroup II ultrapotassic rocks as defined by Foley et al.

(1987), including kamafugites, which have consistently

low SiO2 (<46%) and Al2O3, but high CaO contents.

On the basis of the partition coefficients of trace

elements between clinopyroxene and silicate melt

(Bedard, 2014), we calculated the trace element com-

position of the melt that would be in equilibrium withthe early stage Ca-rich augite of Unit 2 (see

Supplementary Data). The calculated trace element pat-

terns (Fig. 9c) are similar to those of a representative

Group II kamafugite from the Western Qinling province,

NCC (Guo et al., 2014). Considering that previous esti-

mates of the parental magma also belong to Group II

Table 3: Trace element content of clinopyroxene and calculated equilibrated melts in Unit 2 of the Fanshanintrusion

Sample: SFS-7 Calculated SFS-6 Calculated SFS-5 Calculated Kamafugite(n¼5) melt-sfs-7 (n¼5) melt-sfs-6 (n¼5) melt-sfs-5

V 114 35 134 41 196 60 161Cr 6�18 1�48 21�42 5�13 1�79 0�43 661Zn 25�44 58�54 28�26 65�03 68�30 157�16 117Ga 7�43 24�30 9�22 30�15 15�93 52�09 15�9Rb 0�01 0�63 0�02 1�91 0�19 17�51 30�5Sr 528 6290 554 6608 1142 13617 1367Y 12�40 25�05 13�53 27�34 13�48 27�23 32�8Nb 0�169 21�030 0�255 31�757 0�292 36�361 136Ba 0�302 157 0�302 157 0�462 240 729La 9�328 157 9�880 166 14�948 251 122Ce 38�72 392 45�32 459 42�63 432 237Pr 6�8 44�9 7�8 51�2 7�72 50�7 26�8Nd 32�16 147 35�54 162 34�36 157 102Sm 7�11 20�72 9�34 27�23 8�06 23�49 18Eu 2�53 5�92 2�62 6�14 2�28 5�34 5�17Gd 4�66 10�48 5�48 12�33 6�18 13�91 13�4Tb 0�77 1�62 0�92 1�94 0�75 1�57 1�74Dy 3�37 6�88 3�60 7�35 3�64 7�44 8�05Ho 0�44 0�90 0�54 1�10 0�55 1�12 1�22Er 1�05 2�15 1�35 2�78 1�22 2�51 2�82Tm 0�16 0�34 0�13 0�28 0�20 0�41 0�32Yb 0�65 1�34 0�98 2�02 1�15 2�38 1�74Lu 0�17 0�36 0�17 0�34 0�22 0�46 0�22Hf 6�48 38�47 9�20 54�59 16�71 99�15 8�09Ta 0�09 4�02 0�08 3�85 0�06 2�58 6�19Th 0�05 3�50 0�07 5�64 0�11 8�60 16U 0�01 0�64 0�01 0�76 0�01 0�65 3�41Total REE 108 124 124

n, number of analysed points that are adjacent to the core of the crystals. Partition coefficient values are fromBedard (2014); the composition of a kamafugite from West Qinling, North China Craton is from Guo et al. (2014).

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ultrapotassic rocks, we propose that the parental

magma of the Fanshan intrusion is compositionally

similar to that of Group II kamafugites. However, the

notable differences in Nb–Ta–Hf concentration between

kamafugites and our calculated parent magma may in-

dicate a different magma source or degree of crustalcontamination as discussed below.

Crustal contaminationThe Fanshan rocks are characterized by crustal-like

trace element and isotopic compositions such as the

relative depletion in high field strength elements (HFSE)(Figs 10 and 11) and enriched Sr–Nd isotopic compos-

itions [87Sr/86Srt¼ 0�70513–0�70601, eNd(t)¼ –6�8 to

–5�5]. Niu et al. (2012) proposed that the Fanshan parent

magma experienced contamination by mafic lower

crust, on the basis of Os isotope data. The relatively

high d18O values (þ7 to þ9%; Table 4) also indicate thepossibility of crustal contamination. Because the intru-

sion is emplaced into limestones of the Wumishan

Formation, contamination by limestone seems inev-

itable. Assimilation of limestone will drive the

crystallization of Ca-rich clinopyroxene, resulting in

desilication of the melt and an increase of Si-under-

saturation (Gaeta et al., 2006; Mollo & Vona, 2014), as

supported by several experimental studies (e.g. Iacono-Marziano et al., 2007, 2008, 2009; Freda et al., 2008;

Conte et al., 2009). However, the assimilation of carbon-

ate will have no significant effect on the 87Sr/86Sr and

LREE/HREE ratios (Conticelli et al., 2002; Perini et al.,

2004), and may be visible only in terms of oxygen

isotope ratios (Gaeta et al., 2006). Assimilation andfractional crystallization (AFC) modelling of the

Wo

En Fs

Wo(a)

(c)

Or60

80

30Ab

An(b)

Syenite, unit 1Clinopyroxenite, unit 2(Garnet-)clinopyroxenite, unit 3

Alm+Sp

from potassic and ultrapotassicrocks in central Italy

MH

NNOFMQ

Fe2+

Mg

Fe3+ Fe3+

Fig. 8. Compositional variations of minerals from the Fanshanintrusion. (a) Clinopyroxene (Ca-rich augite); field of clinopyr-oxene from potassic and ultrapotassic rocks in central Italy isfrom Cellai et al. (1994), Gaeta et al. (2006), Melluso et al.(2008) and Mollo & Vona (2014). (b) K-feldspar. (c) Biotite(Wones & Eugster 1965). Data for rocks from Unit 3 from Jianget al. (2004) and Niu et al. (2012) are also plotted.

100

1000

Th Nb Ta La Ce Pr Nd Hf Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu

10

Calculated parent

Kamafugitefrom Western Qinling

1

Th Nb Ta La Ce Pr Nd Hf Sm Eu Gd Tb Dy Y Ho Er Tm Yb Lu

(a)

(c)

Ca-rich augite from unit 2

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Sam

ple/

Chro

ndrit

e

Ca-rich augite from unit 2

(b)1000

100

10

1

0.1

100

10

1

Fig. 9. Chondrite-normalized REE patterns (a) and primitivemantle-normalized trace element patterns (b) for the Ca-richaugite in Unit 2. (c) Calculated parent magma compositionscompared with that of kamafugite from Western Qinling in theNCC (Guo et al., 2014).

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Table 4: Representative bulk-rock major (wt %), trace element (ppm) and Sr–Nd–O isotopic compositions for the Fanshan intrusion

Sample: FS01 FS03 FS07 FS08 FS19 FS22 FS23 SFS-08 FS-2-01 FS-2-07 FS-2-10Unit: 2 2 2 2 2 2 2 2 3 3 3Rock type: C C G C C C C C OC OC OC

SiO2 33�91 41�59 32�03 28�5 34�58 35�76 37�28 38�68 37�31 38�71 33�96TiO2 2�27 1�49 2�84 2�19 2�72 2�27 2�41 2�09 2�71 2�1 3�06Al2O3 5�06 10�78 7�68 4�98 8�18 6�86 8�04 8�37 7�22 7�14 7�5Fe2O3-t 17�6 15�85 10�91 15�33 12�21 16�94 12�27 20�2 20�06 14�51 21�14CaO 18�7 12�61 18�3 24�73 15�05 16�45 15�75 14�78 17�25 18�97 15�49MgO 10�65 6�13 14�87 10�89 15�04 13�19 14�68 6�9 7�48 7�59 8�68MnO 0�17 0�12 0�1 0�14 0�12 0�21 0�12 0�23 0�25 0�21 0�25K2O 1�98 5�54 4�72 2�22 5�06 3�22 4�75 4�42 1�81 2�88 2�94Na2O 0�25 1�03 0�17 0�23 0�11 0�39 0�18 0�29 1�33 1�05 0�86P2O5 3�16 1�68 6�74 9�11 4�12 2�25 2�93 1�52 1�55 2�66 2�91CO2 (%) 0�94 1�29 0�77H2Oþ (%) 1�38 1�02 0�94LOI 0�8 2�03 1�06 0�78 1�88 1�71 1�04 1�48 1�45 2�28 0�78Total 94�55 98�85 99�44 99�1 99�07 99�27 99�44 98�96 98�42 98�1 97�57Mg# 56�16 45�01 74�27 60�07 72�3 62�25 71�69 41�97 44�12 52�56 46�51

Trace elements (ppm)Sc 13�71 7�17 12�54 43�57 19�81 8�99 17�95 29�2 30�9 32�8 31�3Ti 12239 8552 15481 11454 14008 12212 12420 12360 16260 12600 18360V 419�07 331�39 265�02 395�6 227�35 374�33 241�08 520 506 377 539Cr 33�12 33�46 151�05 32�2 139�63 77�42 21�63 4�26 4�98 34�7 4�85Co 69�23 123�73 56�09 76�13 75�99 71�85 58�54 60�2 69�1 56�6 67Ni 51�61 31�56 81�2 51�63 139�46 52�63 76�69 17�5 20�9 30�3 39�2Cu 436�46 490�5 36�8 421�76 1246�38 170�24 693�48 149 230 331 723Ga 16�27 16 15�39 16�56 13�34 17�08 14�41 17�4 16�9 16�7 18Rb 68�12 547�87 218�46 87�89 134�06 106�81 190�53 257 101 102 115Sr 1380 745 1765�09 2184 1152 1754 1103�07 1700 1362 2212 1933Y 36�39 19�73 48�48 63�09 26�88 19�95 25 28�9 33�9 43�1 34�6Zr 254�69 163�17 112�02 171�78 131�08 206�14 170�49 258 396 451 244Nb 4�05 1�77 5�31 3�63 5�99 11�34 6�41 5�3 17�1 10�8 16�5Ba 594 396 3728 1211 3216 3517 2443 911 497 1175 3486La 103�86 58�47 180�38 220�78 96�4 89�02 80�98 87�7 102 145 122Ce 276�29 156�13 447�56 557�78 249�42 218�75 189�73 197 237 326 282Pr 36�24 20�06 58�01 65�98 31�89 26�31 25�14 26�4 31�7 41�8 36�3Nd 159�68 89�01 249�82 309�9 137�28 107�31 112 112 148 194 169Sm 30�1 17�18 44�59 55�02 26�31 18�42 21�58 20�9 24�8 31 26�6Eu 7�87 4�43 11�55 14�15 6�84 5�09 5�71 5�68 6�69 8�15 6�88Gd 20�96 11�82 31�64 38�65 18�01 13�14 15�47 15�8 17�9 21�6 18�2Tb 2�01 1�16 2�91 3�52 1�69 1�22 1�45 1�89 2�15 2�64 2�15Dy 9�97 5�69 13�82 17�12 7�88 6�01 7�23 7�75 9�4 11�4 9�18Ho 1�58 0�9 2�11 2�63 1�26 0�95 1�13 1�21 1�27 1�52 1�21Er 3�6 1�93 4�56 5�65 2�78 2�1 2�49 2�89 3�23 3�96 3�11Tm 0�37 0�2 0�43 0�55 0�27 0�22 0�26 0�3 0�32 0�4 0�3Yb 1�91 1�04 2�1 2�65 1�4 1�15 1�35 1�73 2�05 2�38 1�74Lu 0�29 0�15 0�29 0�38 0�2 0�17 0�2 0�24 0�28 0�34 0�25Hf 10�27 6�19 4�1 6�32 4�97 7�26 6�25 10�9 14�2 14�6 8�83Ta 0�29 0�14 0�32 0�22 0�35 0�46 0�34 0�21 0�84 0�78 0�81Pb 9�83 0�74 0�92 8�21 54�33 4�71 19�46 12�4 6�3 4�49 45�4Th 6�47 3�79 8�74 13�36 5�37 5�71 5�46 5�53 6�78 9�53 7�72U 1�12 0�58 1�22 1�59 0�93 1�71 1�02 0�69 0�99 1�93 1�0187Sr/86Sr 0�70559 0�71267 0�70651 0�70636 0�70693 0�70573 0�70703 0�70576 0�70583 0�70577 0�705842r 3 8 3 4 5 5 4 3 3 3 4143Nd/144Nd 0�51222 0�51223 0�51222 0�51221 0�51222 0�51221 0�51224 0�51222 0�51222 0�5122 0�51222r 3 4 2 2 3 4 3 3 2 3 387Rb/86Sr 0�1428 2�1279 0�3581 0�1165 0�3368 0�1762 0�4998 0�1762 0�2146 0�1334 0�1721147Sm/144Nd 0�1139 0�1166 0�1078 0�1073 0�1158 0�1037 0�1164 0�1127 0�1012 0�0965 0�0951(87Sr/86Sr)t 0�70514 0�70601 0�70539 0�706 0�70587 0�70517 0�70546 0�70522 0�70517 0�70536 0�70531eNd(t) –5�8 –5�8 –5�7 –5�8 –5�8 –5�7 –5�5 –5�7 –5�6 –5�8 –5�7(143Nd/144Nd)t 0�51206 0�51206 0�51206 0�51206 0�51206 0�51206 0�51207 0�51206 0�51207 0�51206 0�5120718O (%) 7�5 7�9 7�6 7�9 7�7 7�6 7�5 7�8 7�6 7�8 7�6

Sample: FS-2-13 FS-2-26 FS-2-09 FS-2-27 ST01 ST02 ST03 FS05 FS09 FS11Unit: 3 3 3 3 1 1 1 2 2 2Rock type: OC OC GC-ST GC-ST ST ST ST N A A

SiO2 38�45 40�12 43�48 42�5 50�81 50�87 50�52 7�7 5�46 10�75TiO2 2�4 1�47 2�21 4�17 1�35 1�33 1�42 4�31 0�34 1�35Al2O3 7�4 13�38 15�02 12�7 14�66 14�76 14�64 1�68 0�77 3�4Fe2O3-t 17�98 11�77 9�69 11�79 7�89 7�89 8�14 41�34 4�16 4�96CaO 17�57 10�38 7�62 13�97 7�41 7�46 7�75 24�15 55�4 44�31MgO 7�8 4�56 2�31 2�85 5�25 4�96 5 4�46 1�76 5�81MnO 0�23 0�22 0�13 0�22 0�14 0�14 0�14 0�22 0�05 0�05K2O 3�34 8�92 10�54 6�21 5�58 5�59 5�74 0�17 0�38 2�26Na2O 0�49 0�37 0�01 1�13 3�73 3�61 3�37 0�12 0�13 0�15P2O5 1�41 0�81 0�27 0�28 1 1 1�02 14�81 29 24�03CO2 (%) 1�46 4�63 5�57 1�2H2Oþ (%) 0�7 1�52 2�22 1�64LOI 1�9 5�89 7�36 2�81 1�6 1�73 1�61 0 0�82 1�24Total 98�97 97�89 98�64 98�63 99�43 99�34 99�35 98�96 98�27 98�31Mg# 47�88 45�07 33�55 33�86 58�49 57�08 56�55 18�58 47�26 71�24

Trace elements (ppm)Sc 28�9 9�58 8�35 7�96 11�21 11�47 11�72 12�73 6�78 5Ti 14400 8820 13260 25020 7475 8206 8104 24103 1830 6474V 427 324 284 480 210�61 219�26 218�21 1078�19 237�71 237�29Cr 3�47 3�86 1�52 2�1 82�43 89�15 85�78 23�08 15�67 29�14Co 57�3 44�9 27�1 31�7 21�83 24�75 26�47 122�49 24�3 23�36

(continued)

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Alban Hills (Roman Province) clinopyroxene d18O and87Sr/86Sr isotopic compositions is able to reproduce the

observed variation in terms of fractionation of high-Caclinopyroxeneþ leucite 6 apatite 6 magnetite coupled

with assimilation of 5–10% of limestone (Gaeta et al.,

2006). This may also be the case for Fanshan, as evi-

denced by the similarities in crystallizing phases and

the clinopyroxene composition between Fanshan and

Central Italy (Fig. 8a).Nevertheless, a limited degree of crustal contamin-

ation (5–10%) at source is suggested by the combined

Sr–O isotopic characteristics of the Fanshan samples

(Fig. 13a), which show relatively higher initial Sr iso-

topic values compared with O isotopic values. This vari-

ability of Sr–O isotopic compositions could be

indicative of differing amounts of crustal components inthe mantle source in addition to shallow-level assimila-

tion. This is similar to the model proposed for some of

the ultrapotassic rocks in Western Qinling that also

show a crustal signature in their mantle source (Guo

et al., 2014). The addition of crustal materials to the

mantle source could also be used to explain the differ-ence in HFSE between inferred and calculated parental

magmas (Fig. 9c) because crustal materials usually

have lower Nb–Ta–Hf relative to LILE and REE (Rudnick

& Gao, 2003).

P–T conditions and MELTS modellingThe parental magmas of the Fanshan intrusion may

have experienced variable degrees of fractional crystal-

lization, en route from source to surface. The morph-

ology of the Ca-rich augite crystals, which are up to5 mm in length, strongly indicates that they crystallized

at depth under higher pressure rather than crystallized

in situ. Using the thermobarometer proposed by Putirka

(2008), most of the Ca-rich augite crystallized at pres-

sures of 7�4–16�4 kbar, corresponding to depths of 24–

54 km, at moderately high temperatures (1200–1300�C;

Table 2). On the basis of the total thickness ofMesoproterozoic to Early Triassic (�218 Ma) sequences

in the area, the final emplacement depth of the Fanshan

intrusion is estimated to be �5 km (�2 kbar; Mu et al.,

1988). Thus, the Fanshan magmas probably experi-

enced variable amounts of crystallization at different

crustal levels.We use the geochemical modeling program MELTS

(Ghiorso & Sack, 1995) to test if a parent magma com-

position similar to a kamafugite from Western Qinling

(sample LN10-001-8 of Guo et al., 2014; Supplementary

Data) could produce the cumulus phases present in the

Fanshan intrusion by crystal fractionation. The result of

the MELTS modelling is shown in Fig. 14a. In the model,we use fO2¼FMQ, a starting temperature of 1250�C, a

Table 4. Continued

Sample: FS-2-13 FS-2-26 FS-2-09 FS-2-27 ST01 ST02 ST03 FS05 FS09 FS11Unit: 3 3 3 3 1 1 1 2 2 2Rock type: OC OC GC-ST GC-ST ST ST ST N A A

Ni 17�9 5�53 1�65 3�49 35�19 42�34 38�98 64�26 17�5 29�45Cu 400 39�8 65�5 102 28�25 110�02 74�33 382�32 380�88 69�22Ga 17�2 21 17�2 19�1 23�95 24�63 23�94 25�65 16�49 16�56Rb 190 285 472 326 86�27 99�87 91�36 8�08 16�53 84�87Sr 1270 1993 1015 1212 3004�69 3757�17 3515�16 2646 6038 4945Y 28�5 27�6 90�6 166 24�13 25�22 26�1 81�79 120�6 129�99Zr 417 368 836 2145 523�53 407�84 421�72 74�98 35�12 43�59Nb 10�2 15�4 13�1 28�1 45�32 31�24 34�18 2�27 0�78 2�56Ba 858 2720 1484 1241 2227 2993 2876 98 152 1579La 83�9 120 49�8 74 111�59 113�92 121�96 320�5 729�3 601�33Ce 194 229 147 258 235�27 249�83 268�49 806�26 1739�35 1472�92Pr 26�6 25�9 23�5 46�3 24�4 28�09 29�08 97�02 222�93 189�05Nd 130 108 137 285 95�03 101�96 105�82 416�8 956�54 769�81Sm 21�4 15�1 34�3 65�4 14�71 15�68 16�3 78�83 169�91 148�41Eu 5�8 3�64 10�6 19�7 4�05 4�59 4�83 19�82 42�61 37�25Gd 15�5 9�94 29�9 52 10�23 11�05 11�77 54�91 114�32 101�69Tb 1�85 1�18 4�39 7�48 1�08 1�15 1�24 4�91 10�16 8�99Dy 7�93 5�49 21�9 36�4 6�09 6�57 6�37 23�1 47�21 42�31Ho 1�11 0�79 3�59 5�83 1�04 1�14 1�16 3�38 7�01 6�55Er 2�7 2�13 8�95 15�2 2�67 2�78 2�95 7�41 14�66 13�5Tm 0�27 0�22 1�14 1�93 0�34 0�35 0�37 0�7 1�3 1�31Yb 1�85 1�37 7�24 12�8 2 2�02 2�08 3�32 5�92 6�05Lu 0�28 0�21 1�06 1�84 0�29 0�31 0�32 0�43 0�77 0�78Hf 15�8 11�5 27�4 60�7 12�15 10�75 11�53 2�71 1�01 1�09Ta 0�44 0�58 1�21 2�87 2�41 1�74 1�9 0�21 0�06 0�16Pb 23�5 6�21 2�71 3�62 30�24 26�9 27�24 9�37 16�81 5�15Th 7�75 11�8 8�59 15 21�35 13�27 15�3 17�82 12�48 21�6U 1�78 3�18 1�89 5�32 8�25 3�94 4�19 1�82 4�39 3�5487Sr/86Sr 0�70661 0�70663 0�70949 0�70773 0�7054 0�70544 0�70545 0�70518 0�70515 0�705332r 6 4 4 3 3 4 4 3 4 3143Nd/144Nd 0�51221 0�51218 0�51228 0�51226 0�51214 0�51214 0�51214 0�51222 0�51221 0�512212r 4 4 3 3 3 7 4 2 2 287Rb/86Sr 0�4329 0�4138 1�3456 0�7783 0�0831 0�0769 0�0752 0�0088 0�0079 0�0497147Sm/144Nd 0�0995 0�0845 0�1513 0�1386 0�0935 0�0929 0�093 0�1143 0�1073 0�1165(87Sr/86Sr)t 0�70529 0�70537 0�70538 0�70535 0�70515 0�70521 0�70522 0�70515 0�70513 0�70518eNd(t) –5�6 –5�8 –5�7 –5�8 –6�8 –6�8 –6�8 –5�9 –5�8 –6�1(143Nd/144Nd)t 0�51207 0�51206 0�51206 0�51206 0�512 0�51201 0�51201 0�51205 0�51206 0�5120418O (%) 7�5 7�6 7�7 7�5 8 8�1 7�9 7�8 7�7 8

LOI, weight loss on ignition at 1000�C. Total iron oxide expressed as Fe2O3 (Fe2O3-t); Mg#¼ [molar Mg/(MgþFe2þ)]�100, assum-ing 15% of total iron is ferric. Chondritic uniform reservoir (CHUR) values [(143Sm/144Nd)CHUR

0¼0�512638, (143Nd/144Nd)CHUR0¼0

�1967] are used for the calculation. kRb¼1�42�10�11 a–1 (Steiger & Jager, 1977), kSm¼6�5�10�12 a–1 (Lugmair & Harti, 1978).(87Sr/86Sr)t and eNd(t) were calculated at 218 Ma. Rock type abbreviations as in Table 1.

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final temperature of 760�C, H2O¼ 4 wt % and a pressure

of 10 kbar. The MELTS modelling shows that the

sequence of mineral crystallization is augite–biotite–

apatite–magnetite–garnet. This is consistent with petro-

graphic observations from Unit 2 and some of the rocks

in Unit 3. However, K-feldspar, which is an interstitialphases in these two units, is not present in the model

assemblage. It is probable that owing to the high water

content in the model parent magma most of the potas-

sium entered biotite. To test this, we conducted another

model run using the same starting composition with an

elevated K2O content (3 wt %) but low water content

(0�5 wt %) under lower pressure (2 kbar). The resultsshow that sanidine (K-feldspar) crystallizes after augite

and biotite, consistent with the mineral assemblage in

Unit 1. Therefore, our modelling suggests that the par-

ental magma generating the Fanshan intrusion may be

composionally similar to the kamafugite from Western

Qinling but with higher K2O and lower H2O contents.

Magma generation and nature of the mantlesourceLithospheric mantlePartial melting of metasomatized subcontinental litho-

spheric mantle is widely regarded as the most likelyprocess to explain the origin of Group II potassic and

ultrapotassic rocks (Peccerillo, 2005). Indeed, the Sr and

Nd isotope compositions of the investigated samples

(Fig. 12) fall far outside the ranges for oceanic basalts

[mid-ocean ridge basalt (MORB) and ocean-island bas-

alt (OIB)]. This argues against exclusively astheno-

spheric or mantle plume sources. The enrichment of

LREE and LILE and depletion of HFSE (Figs 10 and 11)support an origin from the lithospheric mantle. The

Fanshan rocks have La/Yb and Nb/La ratios consistent

with an origin from the lithospheric mantle (Fig. 13b;

Condie, 1997).

Subduction-related metasomatismThe enrichment of LILE (Rb, K, Th, U, Sr, and Pb) and

depletion of the high-field strength elements (HFSE; Nb

and Ti) and the HREE (Yb) are characteristic features of

magmas generated in suprasubduction-zone settings

(e.g. Wilson, 1989; Castillo & Newhall, 2004). The high

Th/Yb ratios above the MORB–OIB array (Fig. 15a) are

presumed to reflect the influence of subduction-zonefluids or melts enriched in Th in their petrogenesis. The

Fanshan intrusive rocks plot in the fields of arc volcanic

rocks in Fig. 15a and b. Thus, a likely scenario for the

petrogenesis of the ultrapotassic magmas is that a fluid

or melt derived from subducted pelagic or terrigeneous

sediments was channelled in the overlying lithosphericmantle, forming a zone of hybrid veined mantle

(Conticelli et al., 2013). As stated above, the northern

Fig. 10. Primitive mantle-normalized trace element and chondrite-normalized rare earth element (REE) patterns for the clinopyrox-ene syenite of Unit 1, and the monomineralic apatite rocks and nelsonites of Unit 2 of the Fanshan intrusion. The primitive mantleand chondrite normalizing values are from Sun & McDonough (1989).

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glimmeriteglimmerite

Fig. 11. Primitive mantle-normalized trace element and chondrite-normalized rare earth element (REE) patterns of the (biotite-)clino-pyroxenite and glimmerite of Unit 2, the orthoclase clinopyroxenite, garnet-rich clinopyroxenite and syenite of Unit 3 and syeniticdykes from the Fanshan intrusion. The primitive mantle and chondrite normalizing values are from Sun & McDonough (1989).

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margin of the NCC was strongly influenced by the

southward subduction of the Paleo-Asian (Mongolian)

oceanic plate during Carboniferous to Permian times

(Zhang et al., 2009). This is evidenced by the occurrence

of Late Paleozoic Andean-type continental arc magma-

tism on the northern margin of the craton (390–280 Ma;Zhang et al., 2007, 2009). The termination of the sub-

duction of the Paleo-Asian Ocean beneath the north-

ern margin of the NCC was at c. 280 Ma. The

218 Ma Fanshan intrusion and many other contem-

poraneous Late Triassic alkaline intrusions are re-

stricted to the northern margin of the NCC, forming an

east–west-trending alkaline–ultramafic magmatic belt,

suggesting a genetic link to the Palaeozoic subduction

of the Paleo-Asian oceanic slab. Furthermore, an exten-

sional tectonic regime probably developed in the north-

ern margin of the NCC during the late Triassic following

the final collision of the Mongolian oceanic arc terraneswith the NCC. We thus propose that the parental mag-

mas of the Fanshan intrusion were generated by

decompression melting of enriched mantle peridotite in

a post-collision, extensional tectonic setting. The en-

riched lithospheric mantle source was probably meta-

somatized by infiltration of subduction zone fluids

Fig. 12. Variation of eNd(t) vs (87Sr/86Sr)t for the Fanshan intrusion. Plotted for comparison are Sr–Nd isotopic compositions ofPrecambrian basement rocks from the northern North China Craton (calculated at 218 Ma) and those of Late Paleozoic–earlyMesozoic intrusions (calculated at 218 Ma) in the northern margin of the NCC from Jiang (2005) and Zhang et al. (2009, 2012). Thecomposition of the lower continental crust is after Jahn et al. (1999). The composition of Paleozoic lithospheric mantle in the NCC(represented by kimberlite-hosted xenoliths; Zheng & Lu, 1997; Xu et al., 2004), the Roman Magmatic Province (Prelevic et al., 2008,2013; Boari et al., 2009) and Yaojiazhuang intrusion (Chen et al., 2013) are shown for comparison. The compositions of igneousrocks of the Devonian Kola Alkaline Carbonatite Province in NW Russia and eastern Finland are compiled from Downes et al. (2005)and Lee et al. (2006).

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and/or melts during the late Palaeozoic when the Paleo-

Asian oceanic slab was subducted beneath the northern

margin of the NCC.

Mantle mineralogy and melting processesConsidering the ultrapotassic composition of the

Fanshan rocks, we believe that a K-bearing phase, such

as amphibole (K-richterite) or phlogopite, was present

in the mantle source. These minerals have long beenrecognized as important reservoirs in the mantle for K,

Rb and Ba as well as volatiles (e.g. Wilson, 1989;

Rudnick et al., 1993). As stated above, both melts and

aqueous fluid phases were probably responsible for the

enrichment of the mantle source in LILE and REE (Baker

& Wyllie, 1992).The elevated [Tb/Yb]N ratios (Fig. 15d) indicate that

the magmas parental to the Fanshan intrusion were

probably derived from a garnet-facies peridotite source

region rather than a spinel-bearing source (Xu, 2001).

Thus, the Fanshan magmas could have been derived

from an amphibole- or phlogopite-bearing garnet-facies

5101520

30354045

SiO2

25H2O

Al2O3

MgO

CaO

FeO

Na2O

P2O5

Fe2O3

K2O

TiO2

7008009001000110012001300024681012141618

wt.%

Mineral phase in the systemP=10kbar, fO2=FMQ, H2O(wt.%)=3%

Augite

Garnet

7008009001000110012001300

(a)

(b)

5101520

30354045

25

wt.%

SiO2

Al2O3

MgOCaO FeO

0

2

4

6

8

10

12

Fe2O3

TiO2

K2OP2O5

Na2O

70080090010001100 0060021Mineral phase in the system

P=2kbar, fO2=FMQ, H2O(wt.%)=0.5%,elevated K2O content(3wt.%)Augite

Sanidine

70080090010001100 0060021

Fig. 14. Result of MELTS modelling (Ghiorso & Sack, 1995)using a starting magma composition based on a kamafugitefrom Western Qinling, NCC (Guo et al., 2014) assumingfO2¼FMQ, a starting temperature of 1250�C and final tempera-ture of 730�C: (a) H2O¼3 wt %, pressure of 10 kbar (�30 km);(b) H2O¼0�5 wt %, pressure of 2 kbar. The sequence of appear-ance of mineral phases in each of the MELTS runs is displayedby the grey bars. In (b) K2O content of the starting magma com-position has been increased to 3 wt %.

Fig. 13. (a) Theoretical two-component mixing curves for d18Ovs initial 87Sr/86Sr. Ratios shown on each curve denote the pro-portion of Sr in the mantle or mantle-derived end-member tothe proportion of Sr in the crustal contaminant or slab-derivedfluid (after James, 1981). (b) Nb/La vs La/Yb variation inthe Fanshan samples. The black lines separating fields of as-thenospheric, lithospheric and mixed lithospheric–astheno-spheric mantle are from Abdel-Rahman (2002). Symbols are asin Fig. 12.

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lithospheric mantle source that was metasomatized by

subduction-related melts or fluids prior to magmageneration.

Experimental studies suggest that small degrees of

melting (<10%) of mantle peridotite can yield alkali-rich

primary magmas (Hirschmann et al., 1998) with LREE-

enriched REE patterns (Henderson, 1984). During partial

melting, apatite and hydrous phases are rapidly con-sumed over a small temperature interval close to the

solidus (Wilson, 1989; Hammouda et al., 2010).

Experimental melting studies of phlogopite-bearing

harzburgite or lherzolite indicate that under F-rich con-

ditions and elevated pressure (>12 kbar for harzburgite

or >18 kbar for lherzolite) melt compositions change

from silica-saturated to silica-undersaturated (Melzer &

Foley, 2000, and references therein). A decrease in the

degree of silica-saturation of potassic melts has alsobeen observed experimentally under F-poor, H2O-rich

conditions at elevated pressure (Foley, 1992, 1993). The

degree of silica-saturation of primary potassic melts,

however, is also controlled by the fluid composition

during partial melting. A predominance of CO2 over

H2O during magma generation will suppress the stabil-ity field of olivine, favouring the formation of silica-

undersaturated melts (Wendlandt & Eggler, 1980).

Therefore, low-degree melting of such a mantle source

under high-pressure conditions could well explain the

geochemical characteristics of the Fanshan parental

magmas, analagous to models proposed for the origin

of Group II ultrapotassic rocks in Central Italy and

glimmerite,

Fig. 15. Variation of (a) Th/Yb vs Nb/Yb, (b) Ba/Nb vs La/Nb, and (c) [Ta/La]N vs [Hf/Sm]N for the Fanshan intrusion. The trends ofsubduction- and carbonatite-related metasomatism are from LaFleche et al. (1998) and references therein. (d) Variation of Tb/Yband La/Sm normalized to primitive mantle values (Sun & McDonough, 1989). The boundary between products of spinel- and gar-net-dominated melting is from Wang et al. (2002) and references therein; OIB from Sun & McDonough (1989).

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Western Qinling in the NCC (e.g. Conticelli et al., 2013;

Guo et al., 2014).

Origin of the apatite oresThe Fanshan intrusion includes a distinctive type of

magmatic apatite-rich rocks comprising layers of mono-

mineralic apatite. Cumulates with very high apatitemodes approaching monomineralic facies have been

reported from other alkaline intrusions such as the

Kihibina Complex in the Kola Peninsula (e.g. Notholt,

1979; Veksler et al., 1998; Zaitsev et al., 2014) but the

Fanshan apatite-rich rocks are to our knowledge unpar-

alleled in terms of size and the unique cumulus assem-blage characterized by the coexistence of glimmerite

with apatite 6 magnetite rocks. Like silica-poor phoscor-

ites (e.g. Zaitsev et al., 2014), the almost silica-free char-

acteristics of the Fanshan ores (monomineralic apatite

rocks and nelsonite) probably preclude the possibility

that these rocks crystallized from immiscible P-rich con-jugates (e.g. Veksler et al., 1998, 2006, 2008).

Fractional crystallization and magmareplenishment?It is notable that the monomineralic apatite rocks andnelsonites contain biotite and are often associated with

glimmerite. According to our studies, the Fanshan rocks

exhibit a large range of major element compositional

variation (Table 4), yet all have similar Sr–Nd isotopic

and trace element characteristics (Figs 10–12), suggest-

ing that the parental magmas have experienced varyingdegrees of fractional crystallization and crystal accumu-

lation after emplacement. The order of crystallization

can be deduced from the field relations, petrographic

observations and MELTS modelling. Except for melan-

ite, the following crystallization sequence is proposed:

Ca-rich augite is the earliest phase on the liquidus,

followed by biotite, apatite, magnetite and finallyK-feldspar. The absence of apatite in the early formed

clinopyroxenite suggests undersaturation of apatite

during the early stage. Further fractionation of silicate

minerals after magma emplacement may have driven

the elevated concentrations of phosphorus until the

monomineralic apatite rocks and nelsonites started toform (Tollari et al., 2006). However, MELTS modelling

suggests that apatite crystallized simultaneously with

several other silicate phases including Ca-rich augite

and biotite. Therefore, simple fractional crystallization is

incapable of explaining the formation of monomineralic

rocks.

The combination of large variations in modal min-eralogy and evidence of fractionation suggest that even

Unit 2 does not exhibit closed-system behaviour. The

first and most important line of evidence is the presence

of several Mg-number reversals in Ca-rich augite com-

position from the base of Unit 2 upwards. At these

stratigraphic levels, the concentration of Ti in Ca-richaugite shifts to higher values (Fig. 7b), consistent with

abrupt increases of these elements in the crystallizing

magma. Thus, these intervals probably record the re-

charge and mixing of more primitive magmas with the

fractionating magma in the chamber. Such open-

system behaviour has previously been proposed for

other phoscorite intrusions (e.g. alkaline plutons of theKola Peninsula, Russia; Verhulst et al., 2000).

In the high-level Fanshan magma chamber the dens-

ity of the evolved magma crystallizing Ca-rich augite,

biotite, apatite and magnetite is likely to be lower than

that of the primitive magma replenished from below.

Thus, when a new pulse of primitive magma arrived it

would have formed a layer at the base of the magmachamber (Campbell & Turner, 1989; Snyder & Tait,

1995). The widespread planar foliation and lineation

shown by the main minerals in the rocks of Unit 2 may

indicate that magmatic currents and a laminar flow re-

gime may have resulted from the replenishment of

magma (e.g. Wager & Brown, 1967; Irvine, 1987;Conrad & Naslund, 1989). Thus the thick monomineralic

apatite rocks could be produced by apatite crystalliza-

tion from fractionated P-rich magmas frequently

recharged with more primitive magma from a deeper

crustal magma chamber. If this is the case, the Fanshan

magma is required to be low in viscosity to facilitatemagma flow. Addition of H2O is known to reduce melt

viscosities (e.g. Baker & Vaillancourt, 1995; Giordano

et al., 2008), and fluorine also acts to significantly de-

crease melt viscosity (e.g. Dingwell & Hess, 1998;

Zimova & Webb, 2006). Compaction and subsolidus

growth of minerals probably also contributed to the de-

velopment of mineral foliation (McBirney & Hunter,1995) as reflected by the presence of 120� triple junc-

tions of apatite in the monomineralic apatite rocks and

deformed biotite in the glimmerite.

In conclusion, we consider that crystal settling and

mechanical sorting, combined with repeated magma re-

plenishment and mixing with the fractionated chambermagma, is the predominant process responsible for the

formation of the apatite ores. However, we admit that

the exact mechanism of apatite and biotite concentra-

tion in monomineralic layers is still unclear.

SUMMARY AND CONCLUSIONS

New data presented in this study show that the

Fanshan intrusion reflects open-system magma cham-

ber processes. The parental magmas are deduced to be

kamafugitic in composition (Group II ultrapotassic)

but with relatively high K2O and low water contents;

these evolved in a deep-seated magma chamber via

fractional crystallization and assimilation of wall-rocklimestone. Low degrees of melting of an enriched litho-

spheric mantle source (apatite–carbonate–amphibole–

phlogopite-bearing garnet lherzolite) could explain the

petrogenesis of the Fanshan primary magma. Mantle

enrichment probably resulted from metasomatism

associated with oceanic sediment recycling duringsouthward subduction of the Paleo-Asian oceanic plate

in Carboniferous to Permian times. The P-rich Fanshan

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rocks are clearly cumulates and we consider that crystal

settling and mechanical sorting is the predominant pro-

cess responsible for their formation. The presence of

several Mg# and TiO2 reversals in Ca-rich augite com-

position from the base of Unit 2 upwards indicates anopen system. We favour a model in which the Fanshan

intrusion marks the periodic injection of primitive mag-

mas derived from a subduction-modified lithospheric

mantle source into an upper crustal magma chamber

where they mixed with previously fractionated magma,

leading to the production of exotic and enigmatic

monomineralic lihologies (monomineralic apatite rocksand glimmerite).

ACKNOWLEDGEMENTS

We are grateful to B. Ronald Frost, Dejan Prelevic, and

Editor Marjorie Wilson for their thoughtful and con-structive comments. Tonny Bernt Thomsen at the

Geological Survey of Denmark and Greenland, Paul

Carpenter of Washington University in St. Louis and

Oona Appelt at the Helmholtz Centre GFZ Potsdam are

thanked for their assistance with laser ablation and elec-

tron microprobe analysis; Ziliang Jin and Liu Han arethanked for their help in the field. Zhenhui Bian is

acknowledged for provision of logistical support in the

Fanshan Phosphorus Mine.

FUNDING

Parts of this work were supported by 973 Program(2012CB416806), the National Natural Science Founda-

tion of China (Nos 40925006 and 40821061), the ‘Funda-

mental Research Funds for the Central Universities’, the

111 Project (B07011), and PCSIRT, and DFG grant VE 619/

2-1. I.V.V. also acknowledges support by the Russian Sci-

ence Foundation grant No. 14-17-00200.

SUPPLEMENTARY DATA

Supplementary data for this paper are available at

Journal of Petrology online.

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