comparison of the compositions of olivines and clinopyroxenes from mantle and “crustal”...
TRANSCRIPT
ISSN 1028�334X, Doklady Earth Sciences, 2011, Vol. 438, Part 1, pp. 705–710. © Pleiades Publishing, Ltd., 2011.Original Russian Text © A.Yu. Selyatitskii, V.V. Reverdatto, 2011, published in Doklady Akademii Nauk, 2011, Vol. 438, No. 3, pp. 384–389.
705
Bodies of garnet and spinel peridotites are charac�teristic components of lithospheric blocks that under�went collisional metamorphism at high/ultrahighpressures. These rocks represent one of the key petro�graphic types of collisional zones and carry importantgenetic information, because of their close connectionwith the mantle with respect to the composition, aswell as P–T parameters of their formation. They areconsidered as a result of crust–mantle interaction andallow us to obtain the data on the character of subduc�tion and exhumation, structure, properties, and mate�rial composition of the upper mantle and lower crust.
As is known, in addition to the low SiO2 concentra�tion, ultrabasic rocks are characterized by high con�
centrations of MgO, low FeO, and usually Al2O3, aswell as relatively enriched in such rare elements as Crand Ni and depleted in REE, as well as Zr, Y, and Nb.This is typical practically for all peridotites worldwide.They are named Alpine�type, orogenic, or mantle�derived. However, there are several collisional zonescontaining peridotites, which differ from all others bythe chemical composition. They are relativelyenriched in FeO, TiO2, Al2O3, Zr, Y, Nb, REE anddepleted in MgO, Cr, and Ni [1]. Such rocks wereoriginally discovered by D. Carswell et al., who studiedgarnet peridotites of the Western Gneiss Region inNorway [2]. Based on peculiarities of the peculiaritiesof the petrochemical composition, he distinguished
Comparison of the Compositions of Olivines and Clinopyroxenes from Mantle and “Crustal” Peridotites
of Collisional High�Pressure/Ultrahigh�Pressure ZonesA. Yu. Selyatitskii and Academician V. V. Reverdatto
Received January 14, 2011
DOI: 10.1134/S1028334X11050370
Sobolev Institute of Geology and Mineralogy, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russiae�mail: [email protected]
GEOCHEMISTRY
Table 1. Applied analytical database on the compositions of olivines and orthopyroxenes from mantle and crustal peridotites
Crustal peridotites (ol 63, opx 37) Mantle peridotites (ol 120; opx 140)
Kazakhstan, Kokchetav massif Norway, Western Gneiss Region [2, 5]
(ol 51, opx 19) (ol 11, opx 16)
(our data)
China, Dabie–Sulu terrain [4–9]
Norway, Western Gneiss Region [2] (ol 43, opx 31)
(ol 7, opx 13)
Indonesia, Sulavesi Island [10, 11]
(ol 11, opx 17)
China, Dabie–Sulu terrain [4]
(ol 5, opx 5) Europe
(ol 52, opx 73):
Blan massif, Czech Republic [12]
Western Alps [13]
Central Alps [14]
Ligurian peridotites of Italy [15]
Note: ol, olivine; opx, orthopyroxene; numerals denote the number of applied analyses.
706
DOKLADY EARTH SCIENCES Vol. 438 Part 1 2011
SELYATITSKII, REVERDATTO
Tabl
e2.
Rep
rese
nta
tive
ch
emic
al a
nal
yses
of
oli
vin
es a
nd
ort
ho
pyr
oxe
nes
fro
m “
cru
stal
” p
erid
oti
tes
of
the
Ko
kch
etav
mas
sif
Sam
ple
SiO
2T
iO2
Al 2
O3
Cr 2
O3
FeO
Mn
ON
iOM
gOC
aON
a 2O
K2O
To
tal
Si
Ti
Al
Cr
Fe
Mn
Ni
Mg
Ca
Na
Kf
Oli
vin
e, K
um
dy
Ko
l
SK
Ch
�19
39.9
70.
050.
000.
0016
.21
0.08
0.04
44.0
70.
010.
010.
0010
0.43
1.00
0.00
0.00
0.00
0.34
0.00
0.00
1.65
0.00
0.00
0.00
0.17
SK
Ch
�19/
340
.14
0.01
0.00
0.00
14.5
10.
040.
0645
.10
0.03
0.01
0.00
99.8
91.
000.
000.
000.
000.
300.
000.
001.
680.
000.
000.
000.
15
Ku
md
�139
.99
0.02
0.00
0.00
13.6
30.
040.
0745
.65
0.00
0.01
0.00
99.4
11.
000.
000.
000.
000.
290.
000.
001.
710.
000.
000.
000.
14
Ku
md
�239
.84
0.00
0.00
0.00
13.5
30.
060.
0046
.33
0.00
0.02
0.00
99.7
90.
990.
000.
000.
000.
280.
000.
001.
720.
000.
000.
000.
14
Ku
md
�339
.93
0.04
0.00
0.00
14.8
80.
060.
0045
.10
0.00
0.02
0.00
100.
031.
000.
000.
000.
000.
310.
000.
001.
680.
000.
000.
000.
16
Oli
vin
e, E
nbe
k�B
erly
k
SK
Ch
�1/1
t35
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0.05
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37.3
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0226
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0.00
0.03
0.00
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21.
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000.
880.
010.
001.
110.
000.
000.
000.
44
SK
Ch
�11/
1p38
.96
0.00
0.00
0.00
19.2
50.
310.
0041
.14
0.03
0.01
0.00
99.7
01.
000.
000.
000.
000.
410.
010.
001.
580.
000.
000.
000.
21
SK
Ch
�11/
2t39
.15
0.03
0.00
0.00
18.6
60.
240.
0041
.91
0.00
0.04
0.00
100.
061.
000.
000.
000.
000.
400.
010.
001.
600.
000.
000.
000.
20
SK
Ch
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3p39
.31
0.04
0.00
0.00
17.5
60.
280.
0042
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0.02
0.01
0.00
99.3
81.
000.
010.
000.
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380.
010.
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600.
000.
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19
SK
Ch
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4p38
.13
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0.05
0.01
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360.
980.
000.
000.
000.
410.
010.
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630.
000.
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000.
20
SK
Ch
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5p38
.54
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60.
990.
000.
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610.
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20
SK
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0010
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640.
990.
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t39
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081.
000.
000.
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23
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991.
000.
000.
000.
000.
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000.
000.
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23
Ort
ho
pyr
oxe
ne,
Ku
md
y K
ol
Ku
md
�1�t
55.9
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730.
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0.15
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51.
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000.
090.
000.
290.
000.
001.
670.
010.
000.
000.
15
Ku
md
�1�t
57.3
50.
091.
420.
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840.
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0532
.61
0.18
0.03
0.01
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641.
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000.
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000.
260.
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000.
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13
Ort
ho
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ne,
En
bek�
Ber
lyk
SK
Ch
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56.2
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440.
260.
0099
.31
2.04
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0.09
0.00
0.55
0.01
0.00
1.20
0.02
0.02
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0.31
SK
Ch
�11/
1р55
.00
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0.22
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560.
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19
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Ch
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2t56
.27
0.11
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18
SK
Ch
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0.18
Not
e:A
naly
ses
wer
e pe
rfor
med
on
a m
icro
prob
e C
ameb
ax�M
icro
at
the
Inst
itut
e of
Geo
logy
and
Min
eral
ogy,
Sib
eria
n B
ranc
h, R
ussi
an A
cade
my
of S
cien
ces,
ana
lyst
E.N
. Nig
mat
ulin
a.F
orm
ula
unit
s fo
r ol
ivin
es a
nd o
rtho
pyro
xene
s w
ere
calc
ulat
ed fo
r 4
and
6 ox
ygen
ato
ms,
res
pect
ivel
y. f
= F
e/(F
e +
Mg)
, iro
n m
ole
frac
tion
.
DOKLADY EARTH SCIENCES Vol. 438 Part 1 2011
COMPARISON OF THE COMPOSITIONS OF OLIVINES AND CLINOPYROXENES 707
two types among them: Mg–Cr and Fe–Ti. Later Fe–Ti type peridotites were named “crustal,” whereasMg–Cr type peridotites are related to typical mantleformations. Following Carswell, rocks with suchunusual composition were found in the Dabie–Suluterrain (Eastern China) and Kokchetav massif(Northern Kazakhstan) [1 and references therein].Chinese peridotites were divided into group A peridot�ites (mantle) and group B peridotites [3] (“crustal”).
Mantle peridotites represent the restitic mantle andpreserved all geochemical features of rocks of mantleorigin, whereas “crustal” peridotites are formed from
precollisional ultrabasic and basic predecessors of lowpressures, which primarily occurred in the Earth’scrust and then were subducted to the mantle togetherwith the crust. Protoliths of “crustal” peridotitesunderwent significant chemical alterations beforemetamorphism of high/ultrahigh pressures [1, 3].
Because of the chemical composition not typicalfor ultrabasic rocks, rare abundance, and complexgeological history, “crustal” peridotites are unique for�mations. Since minerals of typical mantle peridotitesreflecting the bulk composition of rocks are character�ized by a high magnesium mole fraction, the composi�
1.0
0 0.35 2.10Mg
Fe
0.8
0.6
0.4
0.2
1.751.401.050.70
0.6
0 0.1 0.5Fe/(Fe + Mg)
NiO
0.5
0.4
0.3
0.1
0.40.30.2
0.2
0.7
0 0.1 0.5Fe/(Fe + Mg)
MnO
0.6
0.4
0.3
0.1
0.40.30.2
0.5
0.2
1
2
3
4
5
6
7
Fig. 1. Results of comparison of the compositions of olivines from mantle and “crustal” peridotites (see Table 1). (1–3) “Crustal”peridotites: (1) Western Gneiss Region, (2) Dabie–Sulu terrain, (3) Kokchetav massif; (4–7) mantle peridotites: (4) WesternGneiss Region, (5) peridotites of Europe (see Table 1), (6) Dabi–Sulu terrain, (7) Sulavesi Island. Dotted lines indicate the fieldsof “crustal” compositions; filled areas denote the fields of “mantle” compositions. Fe and Mg concentrations are given in formulaunits; MnO, TiO2, NiO, and Cr2O3, in wt %.
708
DOKLADY EARTH SCIENCES Vol. 438 Part 1 2011
SELYATITSKII, REVERDATTO
tions of the two types of peridotites differ significantly,whereas the phases composing them are the same (oliv�ine, pyroxenes, garnet, and spinel), it becomes impor�tant to compare the peculiarities of the compositions ofminerals from “crustal” and mantle peridotites.
The results of such comparison are presented inthis paper. The analytical database applied in the studyis given in Table 1. We mainly compare lherzolites and,to a lesser extent, harzburgites. Almost all the rockscontain garnet. Our own analytical data obtained fortitanclinohumite garnet ultrabasites of the Kumdy KolLake area in the western part of the massif and spinelharzburgites in the area of the Enbek�Berlyk in the east�ern part were used for the Kokchetav massif (Table 2).
We studied the concentrations of FeO, MgO,MnO, TiO2, Cr2O3, and NiO. The representativeresults of the comparison are shown in Figs. 1–3. Thestrongest and typical differences are reflected in theconcentrations of FeO, MgO, and NiO for olivinesfrom various types of peridotites. Olivines from“crustal” peridotites are distinguished by the high ironmole fraction (0.21 on average), whereas the averageiron mole fraction of mantle olivines is 0.09. The com�positions of mantle olivines on the diagrams Mg–Fe(Fig. 1) and f(ol)–f(opx) (Fig. 3) form compact fields,whereas “crustal” olivines have significant composi�tional diversity. In olivines from mantle peridotites,the iron mole fraction ranges from 0.04 to 0.11 (one
0.6
0 2.0Mg
Fe
0.5
0.3
0.2
0.1
1.51.00.5
1.0
0 0.08 0.32Fe/(Fe + Mg)
Cr2O3
0.8
0.6
0.4
0.2
0.240.16
0.4
0.36
0 0.24Fe/(Fe + Mg)
TiO2
0.30
0.18
0.12
0.06
0.180.120.06
0.36
0 0.06 0.24Fe/(Fe + Mg)
MnO
0.30
0.24
0.12
0.06
0.180.12
0.24
0.18
Fig. 2. Results of comparison of the compositions of orthopyroxenes from mantle and “crustal” peridotites. See Fig. 1 forsymbols.
DOKLADY EARTH SCIENCES Vol. 438 Part 1 2011
COMPARISON OF THE COMPOSITIONS OF OLIVINES AND CLINOPYROXENES 709
value of 0.14 is close to the field of “crustal” olivines),whereas in olivines from “crustal” peridotites themaximal values of the iron mole fraction reach 0.44and do not decrease below 0.14. Such a high iron molefraction is quite unusual for olivines from ultrabasicrocks. Among the “crustal” compositions, olivinesfrom Chinese peridotites have the most stable compo�sition and the lowest iron mole fraction (0.15–0.16).
Most olivines from “crustal” peridotites are char�acterized by very low concentrations of NiO, usually<0.01 wt % (0.02 wt % on average), whereas the NiOconcentration in olivines from mantle peridotites isalmost usually >0.3 wt % (0.38 wt % on average). Sin�gle olivine analyses from mantle peridotites of China,Norway, and Indonesia are characterized by low NiOconcentrations (0.17, 0.04, and 0.00 wt %, respec�tively) (Fig. 1).
There are no radical differences for such minorcomponents as MnO and TiO2 for olivines from differ�ent genetic types of ultrabasic rocks (Fig. 1). The con�centration of TiO2 in olivines from both peridotitetypes ranges from 0.00 to 0.n wt %. The concentrationof MnO is 0.02–0.65 wt % (0.22 wt % on average) in“crustal” olivines and 0.01–0.24 wt % (0.12 wt % onaverage) in mantle olivines. The maximal concentra�tions of manganese are typical for olivines from“crustal” peridotites of the Kokchetav massif (0.04–0.65 wt %).
The differences in the compositions of orthopy�roxenes from mantle and “crustal” ultrabasites areshown in Figs. 2 and 3. Orthopyroxenes from“crustal” peridotites, as well as olivines, are more Fe�rich in comparison with mantle ones: the average ironmole fraction is 0.18 for the first and 0.09, for the sec�ond. The range of the iron mole fraction for mantleorthopyroxenes is 0.04–0.11, rarely up to 0.14, and forcrustal orthopyroxenes, 0.13–0.21, rarely up to 0.26and 0.32. The analysis of Norwegian orthopyroxenewith the iron mole fraction of 0.26 is not shown inFig. 2 (Fe/(Fe + Mg)–Cr2O3 diagram), because thechromium concentration in it is not known.
Orthopyroxenes of different genetic types are sig�nificantly distinguished by the concentrations ofCr2O3 as well (Fig. 2). “Crustal” orthopyroxenes arecharacterized by very low Cr2O3 concentrations: from0.00 to 0.06 wt % with an average of 0.01 wt %. Mostmantle orthopyroxenes are enriched in Cr2O3: theaverage concentration of this component in them is0.31 wt %, and the maximal values reach 0.91 wt %.However, the Cr2O3 concentrations in some mantleorthopyroxenes are not high and comparable with the“crustal” set of orthopyroxenes; this is the most typicalfor orthopyroxenes from mantle peridotites of China.The highest Cr2O3 concentrations were registered inorthopyroxenes from mantle peridotites of Europe.
Similarly to olivines, the concentrations of MnOand TiO2 in orthopyroxenes from various types ofultrabasic rocks do not demonstrate significant differ�ences (Fig. 2). The concentration of MnO is 0.03–
0.43 wt % (0.20 wt % on average) in “crustal” ortho�pyroxenes and 0.01–0.28 wt % (0.14 wt % on average)in mantle orthopyroxenes. The concentration of TiO2is 0.00–0.17 wt % (0.07 wt % on average) in “crustal”orthopyroxenes and 0.00–0.36 wt % (0.09 wt % onaverage) in mantle minerals. Orthopyroxenes frommantle peridotites of Europe are characterized by themaximal titanium concentrations often exceeding theTiO2 concentration in “crustal” peridotites.
We can make the following conclusions from thediscussed data. So�called “crustal” peridotites of theKokchetav massif in Northern Kazakhstan, Dabie–Sulu terrain in Eastern China, and Western GneissRegion in Norway are characterized by an unusuallyhigh iron mole fraction of olivines and orthopy�roxenes, as well as quite low concentrations of NiO inolivines and Cr2O3 in orthopyroxenes, which is nottypical for the minerals from ultrabasic rocks of typicalmantle origin. Olivines and orthopyroxenes from“crustal” peridotites of the Kokchetav massif andWestern Gneiss Region are characterized by strongdiversity of the FeO and MgO concentrations. Miner�als from ultrabasic rocks of China are characterized bythe most stable composition among “crustal” peridot�ites.
According to the concentrations of FeO, MgO,Cr2O3, and NiO, olivines and orthopyroxenes reflectthe differences in the bulk composition of peridotitesof various genetic types. Because of this, in addition tothe compositions of rocks on their own, the revealedpeculiarities and differences in the compositions of
0.35
0 0.5f(ol)
f(opx)
0.30
0.15
0.10
0.05
0.40.30.1
0.20
0.25
0.2
Fig. 3. f(ol)–f(opx) diagram for pairs of coexisting olivineand orthopyroxene: 91 pairs from the mantle and 31 pairsfrom “crustal” peridotites. See Fig. 1 for symbols.
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DOKLADY EARTH SCIENCES Vol. 438 Part 1 2011
SELYATITSKII, REVERDATTO
olivines and orthopyroxenes from mantle peridotitesof the Mg–Cr type and “crustal” peridotites of theFe–Ti type may be applied for diagnostics of ultrabasicrocks in Precambrian cratons and collisional zones ofhigh/ultrahigh pressures concerning the origin ofrocks and revealing of the nature of their protoliths.
ACKNOWLEDGMENTS
This study was supported by the Russian Founda�tion for Basic Research (project nos. 10�05�00217 and08�05�00166) and Integration Project of the SiberianBranch, Russian Academy of Sciences (no. 2).
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