Lithos 264 (2016) 125–140

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Mantle inputs to Himalayan anatexis: Insights from petrogenesis of theMiocene Langkazi leucogranite and its dioritic enclaves

Yuan-chuan Zheng a,⁎, Zeng-qian Hou b, Qiang Fu b, Di-Cheng Zhu a, Wei Liang a, Peiyan Xu a

a School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, PR Chinab Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, PR China

⁎ Corresponding author at: School of Earth Sciences andGeosciences, 29# Xueyuan Road, Haidian District, Beijing

E-mail address: zhengyuanchuan@gmail.com (Y. Zhen

http://dx.doi.org/10.1016/j.lithos.2016.08.0190024-4937/© 2016 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 10 March 2016Accepted 18 August 2016Available online 28 August 2016

Oligocene andMiocene Himalayan anatexis is generally thought to have been induced by intracrustal heating orprocesses without the involvement of mantle-derived heat and materials, suggesting that the Himalayanleucogranites are typical examples of purely crustal melts. This study focuses on a Miocene leucogranite atLangkazi within the Himalayan orogen, an intrusion that contains a large number of dioritic enclaves. Theseenclaves have typical igneous textures, contain acicular apatites, and have back-veining structures, quenchedmargins, and crystallization ages identical to the hosting two-mica granites, indicating that these enclaves aremagmatic. Although the enclaves are evolved, the most primitive samples contain high concentrations of MgO(up to 4.3 wt.%), Cr (up to 159 ppm), and Ni (up to 102 ppm), are strongly enriched in large-ion lithophile ele-ments, are depleted in high-field-strength elements, have negative εNd(t) values (–8.6 to –6.1), and have relative-ly high 87Sr/86Sr(i) values (0.7085–0.7137), suggesting that theywere derived from a relatively enriched region ofthe lithospheric mantle source. Whole-rock geochemical data indicate that the hosting Langkazi leucograniteformed frommagmas generated by the partial melting of metapelite material within the High Himalaya crystal-line sequence, and these magmas subsequently mixed with mantle-derived melts that are now represented bythe Langkazi dioritic enclaves. This indicates thatmantle-derivedmaterial played an important role in the gener-ation of the Langkazi intrusions. The whole-rock geochemical compositions of samples from the study area alsoindicate that the primary melts that formed the Langkazi enclaves were significantly contaminated by the rela-tively juvenile Himalayan lower crustal material, suggesting in turn that these mantle-derived magmasunderwent MASH (crustal melting, melt assimilation, magma storage, and homogenization) processes at thebase of crust, introducing heat to the lower crust. This mantle-derived heat may have induced partial meltingof the Himalayan lower crust, forming adakite-like magmas. Although the small volumes of these dioritic en-claves suggest that this heat may have assisted the genesis of the north Himalayan granites and the HighHimalayan leucogranites, whereas in situ crustal radiogenic heating, shear heating, and decompression meltingdominated the melting processes in this region.

© 2016 Elsevier B.V. All rights reserved.

Keywords:LeucogranitesDioritic enclavesMantle inputsMioceneHimalayan orogen

1. Introduction

Two vast Oligocene–Miocene leucogranite belts within theHimalayas define an E–W-striking igneous intrusive province that isca. 1500 km long (Fig. 1A). These leucogranites are generally thoughtto be representative of purely crustal melts that are uncontaminatedby mantle material. This conclusion is based on: (1) the fact thatthese leucogranites have elevated silica contents and are stronglyperaluminous; (2) their anomalously radiogenic Sr and Pb andunradiogenic Nd isotopic compositions, along with their elevated δ18Ocompositions; (3) a lack of spatial and temporal relationships between

Resources, China University of100083, PR China.g).

these leucogranites andmaficmagmatism (i.e., mafic intrusions or lavasand enclaves); and (4) experimental research indicates that melting ofmetasedimentary rocks generates peraluminous, silica-rich melts (Gaoand Zeng, 2014; Harris and Massey, 1994; Harrison et al., 1998; Kinget al., 2011; Le Fort et al., 1987; Z.C. Liu et al., 2014; Liu et al., 2016;Parrish and Hodges, 1996; Patiño Douce and Harris, 1998; Zhanget al., 2004a, 2004b). This means that comparatively little research hasconsidered whether Himalayan anatexis could have been influencedby mantle inputs.

A mantle-free model for anatexis within the Himalayan belt meansthat this process must therefore be closely linked to: (1) shear heatingassociated with rapid and prolonged movement along the main centralthrusts (MCT) and south Tibetan detachments (STD) within theHimalaya (Harrison et al., 1997, 1998; Whittington et al., 2009); (2) de-compressionmelting associated with rapid regional or local exhumation

81 E E78° °

93 E°90 E°

28 N°

Indian Plate

Qiangtang Terrane

LhasaXigaze

ShiquanheBNG

IYS

IYS

STD

MBT

THS

HHCLHSMCT

THS

LHS

20

20-1316

1117-10

19-17

13-8

2314-13

16

16-17

18

17

25-16

24

23

24-16

18-13

1521

32-24

16

84 E°

30 N°

A

Paro 24-23Makalu 23.4

Maja 9.5

Lhagoi Kangri15

17

14.3

Manaslu22.4

Malashan 17.6

Shisha Pangma20-17

Gongotri22.4

Shivling 22

Annapurna31-22

Kuday 28

14.5

Dinggye 15.8

Yadong 26-23

100 km

Fig. 1B

Oligocene-Miocene adakite-like rock(32-10 Ma)

N-S normal fault

Oligocene-Miocene leucogranite(31-8 Ma)

North Himalayan gneiss dome

Eocene adakite-like rock (46-42 Ma)

Age of the magma18

Ultrapotassic rock (25-8 Ma)

Luozha

Wagye La

12 Ma12.5 Ma

Ramba 8 Ma

Langkazi 11.6 Ma

Yad

on

g r

ift

Langkazi

Ramba domeB

STD

Kangmardome

Yelaxiangbo46-42 Ma

Fig. 1. (A) Simplified geological map of southern Tibet showing the locations of themain tectonic units, gneiss domes, and outcrops of the Oligocene–Miocene leucogranites, adakite-likegranites, and ultra-potassic rocks,modified fromSearle et al. (1997), Yang et al. (2009), Zhao et al. (2009), Hou et al. (2009), and Zheng et al. (2012a). (B) Geologicalmap showing outcropsof the Langkazi two-mica granites in the THS. Abbreviations: BNS, Banggong–Nujiang suture; IYS, Indus–Yarlung suture; LHS, lesser Himalayan sequences; MBT, Main Boundary Thrust;MCT, Main Central Thrust; STD, South Tibetan Detachment; THS, Tethyan Himalayan sedimentary sequence.

126 Y. Zheng et al. / Lithos 264 (2016) 125–140

of deeper rocks (Harris and Massey, 1994; Patiño Douce and Harris,1998; Royden, 1993); (3) radiogenic heating as a result of the decay ofradioactive elements (e.g., U, Th, and K), all of which are highly enrichedwithin the metasediments of the High Himalaya crystalline sequence(HHC) (Molnar et al., 1983); and/or (4) partial melting in the presenceof free aqueous fluids (Debon et al., 1986; Le Fort et al., 1987). Thishas led to the development of various petrogeneticmodels for the Hima-layan leucogranites. Early research suggested that these Oligocene–Miocene leucogranites were dominantly derived from muscovitedehydration melting of the HHC metapelites (Harris and Massey, 1994;King et al., 2011; Patiño Douce and Harris, 1998; Zhang et al., 2004a),with more recent research suggesting that the fluid-fluxing melting ofmetapelites was also an important step in the generation of graniticmelts containing relatively high concentrations of CaO (N1 wt.%) (Gaoand Zeng, 2014; Guo and Wilson, 2012; King et al., 2011; Prince et al.,2001; Zeng et al., 2012; Zhang et al., 2004b). Irrespective of whichmodel is correct, it is generally considered that these magmas are de-rived from melting of a single metapelites source region within theHHC (Guo and Wilson, 2012; King et al., 2011; Le Fort et al., 1987;Zhang et al., 2004a). However, a large number of dioritic enclaves havebeen found in the Miocene Langkazi two-mica granite and may repre-sent another end-member involved in the genesis of the Himalayanleucogranites. These enclaves may provide more constraints on the pet-rogenesis of the host rocks than already provided by the silicicleucogranites themselves, as well as insights into the deep-seatedgeodynamic processes that occurred during the Oligocene–Mioceneevolution of the Himalayan orogen.

Herein, we present new zircon U–Pb and biotite 40Ar/39Ar ages andwhole-rock geochemical and Sr–Nd isotopic data for both the hostrocks and the dioritic enclaves within the Langkazi two-mica granite,an intrusion located within the Tethyan Himalayan sedimentary se-quences (THS). Our study indicates that the Langkazi two-mica granite

dominantly formed from crustal melts, whereas the dioritic enclavesmay have formed from mantle melts, providing direct evidence for theinvolvement ofmantle-derivedmagmas in the genesis of theHimalayanleucogranites. Combining our new data with the results of previous re-search provides new constraints on the petrogenesis of the Oligocene–Miocene granites within the Himalaya and the nature of their sources.Ultimately, these data provide new insights into the genetic relationshipbetween the host rocks and their dioritic enclaves, the origin of and heatsources involved in Himalayan anatexis, and the geodynamic setting ofthe Himalayan orogen.

2. Geological setting and petrography

2.1. Geological setting

The Himalayan–Tibetan Plateau is dominated by four continentalblocks, which from north to south are the Songpan–Ganzi, Qiangtang,Lhasa, and Himalayan terranes. The Himalayan orogen is bounded tothe north by the Indus–Yarlung suture (IYS; Fig. 1A), which is the resultof the northwards drift of the Indian Plate and its collision with theAsian Plate at ca. 55–50 Ma (Besse et al., 1984; Klootwijk et al., 1992;Leech et al., 2005; Patriat and Achache, 1984). The Himalayan orogen,from north to south, is loosely divided into three broadly parallel, later-ally continuous litho-tectonic units (Fig. 1; Le Fort, 1975, 1996; Yin andHarrison, 2000): (1) The THS, a deformed package of predominantlyNeoproterozoic to Eocene metasedimentary rocks and thick Permianto Upper Cretaceous passive Indian continental margin sequencesbounded by the IYS and STD (Burchfiel et al., 1992; Dipietro andPogue, 2004; Garzanti, 1999). From north to south (Fig. 1A), the THSconsists of a deformed, broadly southward-younging, Triassic toCretaceous sequence of predominantly low-grade metasediments(Aikman et al., 2008, 2012), which varies southward from thick Triassic

127Y. Zheng et al. / Lithos 264 (2016) 125–140

clastic-dominated sediments to Cretaceous marine clastics and carbon-ate platform deposits. This unit containswidespread pre-Cenozoic igne-ous rocks that are dominated by Permian volcanic rocks and Cretaceousvolcanic and plutonic rocks. The Permian volcanic rocks within this unitare thought to have formed in an extensional tectonic setting and aredominantly basalts interlayered with sedimentary units (Noble et al.,2001). The Cretaceous igneous rocks (ca. 132Ma) in the THS are charac-terized by abundant volcanic rocks (mostly basalts with some silicicrocks), voluminous mafic intrusions, and minor ultramafic rocks(Zhu et al., 2007, 2008, 2009a, 2009b). (2) The HHC, a Neoproterozoicto Ordovician high-grade metasedimentary sequence that forms analmost continuous belt along the Himalayan range and was exhumedbetween the STD and MCT (DeCelles et al., 2000; Dipietro and Pogue,2004; Parrish and Hodges, 1996). The HHC has a broadly similar struc-tural, metamorphic, and intrusive history to that the gneiss domesthat outcrop within the THS (Fig. 1B), suggesting that the THS is under-lain by the HHC in southern Tibet (Hou et al., 2012; Zhang et al., 2004a).(3) The lesser Himalayan sequences (LHS), a sequence of dominantlyPaleoproterozoic–Cambrian low-grade metamorphosed siliciclasticrocks bounded by the MCT and the main boundary thrust (MBT;Fig. 1A; Le Fort, 1975, 1996; Yin, 2006).

The STD system is a north-dipping, low-angle normal-fault systemthat was active between the late Oligocene and Miocene (25–12 Ma),juxtaposing the THS in the hangingwall against the HHC in the footwall(Sachan et al., 2010; Searle et al., 2003; Yin, 2006). Well-developedshear-sense indicators within mylonites beneath the STD provideevidence of northeastward or northwestward displacement of thehangingwall (e.g., Burchfiel et al., 1992) with a minimum displacementof 35–40 km (Hodges, 2000). The MCT has been interpreted as a shearzone along which the HHC was emplaced southward over the LHS,with theMBT acting as a thrust that placed the LHS over the Indian con-tinent (Fig. 1A). Both the MCT and MBT appear to sole into the low-angle main Himalayan thrust (MHT) beneath the THS, indicating thatthemain part of the Indian continent was thrust beneath the Himalayasalong this structure.

2.2. Magmatism in the Himalaya

Two distinct Cenozoic belts of granite in the Himalaya form an E–W-striking igneous intrusive province that is ca. 1500 km long (Fig. 1A).The northern belt, termed the north Himalayan granites (NHGs), wasemplaced into a series of gneiss domes that collectively form thenorth Himalayan antiform (NHA), an E–W, crustal scale anticlinesome 50 km south of the IYS in southern Tibet. At least eight gneissdomes (e.g., the Sakya–Kuday, Ramba, and Yelaxiangbo domes) havebeen recognized (Fig. 1A), and they are in fault contact with the overly-ing THS. They contain exposed Indian continental rocks and high-grademetamorphic rocks that record the ductile extension that occurredduring normal slip along the STD (Aoya et al., 2005; Chen et al., 1990;Lee and Whitehouse, 2007; Lee et al., 2006). The Cenozoic magmaswithin the north Himalayan domes (NHD) were dominantly intrudedbetween 28 and 8 Ma (Edwards and Harrison, 1997; Hodges et al.,1996; King et al., 2011; Z.C. Liu et al., 2014; Searle et al., 1999; Zhanget al., 2004a), similar in timing to some of the leucocratic dikes to thenorth of NHA in the Kuday and Ramba areas that are geochemicallysimilar to adakites and were emplaced at ca. 28–9 Ma (King et al.,2007; Z.C. Liu et al., 2014).

The southern belt contains numerous high Himalayan leucogranite(HHL) intrusions that were emplaced at ca. 31–12 Ma and vary in size(Z.C. Liu et al., 2014; Searle and Szluc, 2005; Vidal et al., 1982), althoughthe majority are stocks or sheets that were emplaced either within theHHC or near the interface of the HHC and the overlying THS (Hodgeset al., 1996; Searle et al., 1999; Vidal et al., 1982). The intrusives withinboth belts of granite in this region are similar in terms of structure, petrol-ogy, and geochemistry. These granites are dominated by biotite-bearingleucogranites (also known as two-mica granites), tourmaline-bearing

leucogranites, and garnet-bearing leucogranites (King et al., 2011; Z.C.Liu et al., 2014; Zhang et al., 2004b). They vary in grain size from fine-to coarse- grained, and the majority are peraluminous, with elevatedinitial 87Sr/86Sr ratios [87Sr/86Sr(i)], high values of δ18O values, and neg-ative Nd isotopic values [εNd(t)], whereas the leucocratic dikes withadakitic affinities have higher εNd(t) and lower87Sr/86Sr(i) values thanthe granitic intrusions in this area (King et al., 2007; Z.C. Liu et al., 2014).

Recent research has identified granites within the Himalaya that areolder than the Oligocene–Miocene NHGs and HHLs (i.e., N42 Ma), indi-cating that crustal anatexis in this area occurred before this time(Aikman et al., 2008; Hou et al., 2012; Larson et al., 2010; Zeng et al.,2009, 2011). Unlike the NHGs and HHLs, the Eocene granitoids withinthe Himalaya are restricted to the Yelaxiangbo and Ramba domes andsurrounding areas of the THS (Fig. 1A). Zircon U–Pb dating of thesegranitoids yields ages between ca. 46 and 42 Ma, and these intrusionsare dominated by two-mica granites and have clearly adakitic affinities(Aikman et al., 2008; Hou et al., 2012; Zeng et al., 2009, 2011).

The Ramba gneiss dome is located in the vicinity of the north–southtrending Yadong rift within the THS, is characterized by a core ofleucogranite that is mantled by amphibolite-facies paragneisses, and isoverlain by low-grade Triassic metasedimentary rocks (Fig. 1B). TheRamba and Langkazi leucograniteswere emplaced into the paragneisseswithin the dome, with some intensely deformed dikes that varyfrom several centimeters to meters wide intruding into the Triassicmetasedimentary rocks in this region. Zircon U–Pb dating indicatedthat these intensely deformed dikes were emplaced at ca. 44 Ma and28 Ma (Z.C. Liu et al., 2014), whereas, systematic zircon, monazite, andxenotime U–(Th)–Pb dating indicates that the Ramba leucograniteswere intruded at ca. 8 Ma (Z.C. Liu et al., 2014; Ratschbacher et al.,2011). Here, we focus on the petrogenesis of the Langkazi granitoidswithin the Ramba dome.

Unlike the previously reported leucogranites in both the NHGs andHHLs belts, the Langkazi two-mica granite contains a large number ofdioritic enclaves. These enclaves are elliptical and vary in size from 5to 20 cm in size (Fig. 2A–B). The hosting leucogranite is undeformedand is dominated by quartz, plagioclase, muscovite, biotite, and K-feldspar (Fig. 2C–D), whereas the dioritic enclaves have medium-grained igneous textures, contain plagioclase, biotite, K-feldspar, andquartz, and are free of muscovite and tourmaline (Fig. 2E–F). The en-claves also contain abundant acicular biotite and apatite (Fig. 2E–F),have back-veining structures (Fig. 2A–B), and have quenched margins.The enclaves are also gray in color, but some parts of the enclaves arelighter in color as a result of their lower biotite abundances.

3. Results

3.1. Leucogranite and enclave ages

The timing of the magmatic event that formed the Langkazileucogranite was constrained using zircon and biotite separates from asample of the hosting leucogranite (sample LKZ-10) and a sample ofthe dioritic enclaves within the intrusion (sample LKZ-17) and U–Pbdating and 40Ar/39Ar dating, respectively.

3.1.1. Zircon U–Pb agesZircon grains separated from the hosting leucogranite and the

dioritic enclave are similar, with both being transparent, colorless tolight brown, and generally euhedral. These zircons are long to shortprismatic, have average crystal lengths of 100 to 300 μm, and length-to-width ratios of ca. 2:1. The majority of these zircons have core–mantle–rim textures, although some lack mantles. The zircons withcomplete core–mantle–rim textures have cores characterized by irregu-lar or oscillatory zoning that are surrounded by gray and homogeneousmantles, with rims that are euhedral but have relatively dark oscillatoryzoning (Fig. 3A–B). The majority of these zircons have narrow rimsthat were still wide enough to be dated (Fig. 3) and, as such, the

A

Ms

Pl

Qz

Kf

Bi

PlKf

Pl

Qz

BiBi

leucogranite

enclave

enclave

back vein

4 cm

C 400 um

E 400 um F

Bi

Bi

Ap

Ap

100 um

4 cm

quenchedmargin

enclaveenclave

back vein

B

400 um

Qz

Bi

Bi

Bi

Pl

MsKf

Ms

Ms

Pl

KfKf

Pl MsMs

Ms

D

Bi

Fig. 2. Petrographical and microphotograph photos of the Langkazi two-mica granites. (A–B) Field photographs of Langkazi two-mica granites. Microphotographs of (C–D) the host rock,(E–F) dioritic enclave in the Langkazi two-mica granites. Abbreviations: Ap, apatite; Bi, biotite; Kf, K-feldspar; Ms., muscovite; Pl, plagioclase, Qz, quartz.

128 Y. Zheng et al. / Lithos 264 (2016) 125–140

SHRIMP U–Pb analysis undertaken to constrain the timing of thismagmatic event focused on oscillatory zoned zircon rims (Fig. 3A–B).

A total of 16 zircons from the hosting leucogranite (sample LKZ-10)and 20 zircons from the enclave (sample LKZ-17) were analyzed duringthis study. Zircons within sample LKZ-10 (hosting leucogranite) have Uconcentrations of 270–4539 ppm, Th concentrations of 40–678 ppm,and Th/U ratios of 0.03–0.39, whereas the enclave sample LKZ-17 con-tains zircons with U concentrations of 173–5544 ppm, Th concentra-tions of 50–2740 ppm, and Th/U ratios of 0.02–0.76 (Suppl. Table 1).The spot analysis of four magmatic domains within the zircons fromthe leucogranite sample LKZ-10 yielded a mean 206Pb/238U age of11.6 ± 0.5 Ma [MSWD (mean square of weighted deviates) = 1.7,Fig. 3C], with 12 additional concordant spot analyses yielding older206Pb/238U zircon ages from 15.7 to 613.7 Ma. The zircons within

enclave sample LKZ-17 yield similar ages, with seven spot analysesyielding a mean 206Pb/238U age of 12.2 ± 0.4 Ma (MSWD = 2.4,Fig. 3D), with 12 additional concordant spot analyses yielding older206Pb/238U zircon ages from 13.7 to 843.2 Ma.

3.1.2. 40Ar/39Ar cooling agesThe Langkazi hosting leucogranite and associated enclaves have gra-

nitic textures and contain euhedral to subhedral plagioclase, K-feldspar,biotite, and muscovite, with anhedral quartz. The quartz within theintrusion has uniform extinction patterns, indicating that this granitoidhas not undergone any intense metamorphism or deformation.

The biotite within sample LKZ-10 of the Langkazi two-mica graniteyielded a plateau age of 10.98 ± 0.17 Ma, representing 99% cumulative39Ar released (Fig. 4A, Suppl. Table 2). An isochron for the sample yields

206 P

b/23

8 U

206 P

b/23

8 U

207Pb/235U 207Pb/235U

100

200

300

400

500

600

700

10

14

18

22

26

30

Mean=11.6±0.5 Ma n=4, MSWD = 1.7

LKZ-10C

1.20 0.2 0.4 0.6 0.8 1.00.00

0.02

0.04

0.06

0.08

0.10

0.12

11

13

15

17

Mean=12.2±0.4 Ma n=7, MSWD = 3.3

200

400

600

800

1000LKZ-17D

0.4 0.8 1.2 1.60

0.04

0.08

0.12

0.16Langkazi two-mica granite Langkazi enclave

100um

15.111.1±.3

12.111.8±.2

4.15.1

19.1±.4

520±9

6.1

1.125.8±.6

9.1

11.9±.215.7±.4

13.111.7±.3

2.1 6.1495±10

10.1487± 8

9.1516±8

8.111.112.1±.2

12.7±.212.1

12.6±.2483±13

15.1

18.112.1±.2

16.1 19.113.7±.3

12 .2±

519.9±7.91.1

11.4±.32.2

484±10

3.17.1

497±9 11.7±.2100umLKZ-17Langkazi two-mica granite LKZ-10 Langkazi dioritic enclaveA B

Fig. 3. Cathodoluminescence images for the (A) Langkazi two-mica granite (sample LKZ-10) and (B) enclave (sample LKZ-17) analyzed in situ for U–Pb isotopes. Circles indicate theSHRIMPII analysis spots for U–Pb isotopes. Numbers near the analysis spots are the U–Pb ages. U–Pb concordia diagrams for zircons from (C) host rock (sample LKZ-10) and(D) dioritic enclave (sample LKZ-17) from the Langkazi two-mica granites; “Mean” in each plot indicates the 206Pb/238U age.

Age

(Ma)

0

20

40

60

80

100

120

140

0 20 40 60 80 100

070oC−1130oCPlateau age = 10.98 ± 0.17 Ma (2σ)

Includes 99.09% of the 39 Ar

A LKZ-10 biotiteLangkazi two-mica granite

Cumulative 39 Ar Percent Cumulative 39 Ar Percent

8

12

16

20

24

0 20 40 60 80 100

070oC−820oCMean age = 11.08 ± 0.16 Ma (2σ)

Includes 55.29% of the 39Ar

LKZ-17 biotiteLangkazi enclave

B

0

400

800

1200

1600

0 200 400 600 800 1000 1200

700oC − 1010oC36

Initial 40Ar/36Ar = 298.8 ± 1.5MSWD = 1.8

Age = 10.81 ± 0.16 Ma

0

400

800

1200

1600

2000

0 400 800 1200 1600

700oC − 820oC

Initial 40 Ar/36Ar = 299 ± 20MSWD=14

Age = 10.88 ± 0.86 Ma

LKZ-10 biotite LKZ-17 biotiteC D

39Ar/36Ar

40A

r/36

Ar

39Ar/36Ar

Fig. 4. 40Ar/39Ar Age spectra for (A)–(C) biotite grains from the host rock (sample LKZ-10) and (B)–(D) dioritic enclave (sample LKZ-17) of the Langkazi two-mica granites.

129Y. Zheng et al. / Lithos 264 (2016) 125–140

130 Y. Zheng et al. / Lithos 264 (2016) 125–140

an age of 10.81 ± 0.16 Ma, which is within the 2σ uncertainty of theplateau age, with an intercept on the 36Ar/40Ar axis corresponding toatmospheric value (i.e. 40Ar/36Ar = 298.8 ± 1.5; Fig. 4C). Biotite withinenclave sample LKZ-17 yielded a plateau age of 11.08 ± 0.16 Ma,representing 55% cumulative 39Ar released (Fig. 4B, Suppl. Table 2).An isochron for the same sample yields an age of 10.88 ± 0.86 Ma,which is within the 2σ uncertainty of the plateau age, with an intercepton the 36Ar/40Ar axis corresponding to atmospheric value (i.e.40Ar/36Ar = 299 ± 20; Fig. 4D).

3.2. Geochemistry

The major and trace element data and whole-rock Sr–Nd isotopiccompositions of the samples analyzed during this study are given inSuppl. Tables 3 and 4, respectively. The Langkazi two-mica granitescontain lower concentrations of SiO2 (68.82–71.25 wt.%) and higherconcentrations of CaO (1.97–2.33 wt.%) and MgO (0.58–0.84 wt.%)than other NHGs and HHLs, and are peraluminous to stronglyperaluminous (A/CNK (Al2O3/(CaO + Na2O + K2O)) = 1.00–1.15)(Figs. 5–6, Suppl. Table 3). They also have moderately fractionatedrare earth element (REE) patterns characterized by (La/Yb)N ratios of13.4–21.4, and moderately negative Eu anomalies with Eu/Eu* values

B

Na 2O

+K

2O(%

)

40 50 60

16

14

12

10

8

6

4

2

0

Sub-Alkalic

syenite

alkali granite

granite

quratz-diorite(granodiorite)

diorite

syeno-diorite

syenite

gabbro

nepheline-syenite

ijolite

gabbro

0.5

1.0

1.5

2.0

2.5

0.8

A/N

K

metaluminous peraluminous

B

NHG

Langkazi enclave

HHL

Langkazi two-mica granite

experimental productadakite-like rock

Alkalic

70 80

SiO2(%)

A/CNK

1.50.9 1.0 1.1 1.2 1.3 1.4

A

Fig. 5. (A) Rock suite classification in the plot of (Na2O + K2O) versus SiO2, according toWilson (2001); (B) A/CNK (Al2O3/(CaO + Na2O + K2O)) versus A/NK (Al2O3/(Na2O + K2O)) diagram. Data for NHGs and HHLs are from Visoná and Lombardo(2002), Tong et al. (2003), Zhang et al. (2004a), King et al. (2011), Yu et al. (2011),Huang et al. (2013), Zeng et al. (2012, 2014), Gao and Zeng (2014), and Z.C. Liu et al.(2014). Data for adakite-like rocks from King et al. (2007). Data for experimentalproducts from Patiño Douce and Harris (1998).

of 0.56–0.83 (Fig. 7A). These rocks are also relatively depleted in thehigh field strength elements (HFSEs; TiO2 of 0.22–0.32 wt.%, Nb of5.73–7.80 ppm, Ta of 0.60–0.94 ppm) and are enriched in the large ionlithophile elements (LILEs; K2O of 3.57–3.96 wt.%, Rb of 170–185 ppm,Ba of 575–750 ppm; Fig. 7B).

In general, the dioritic enclaves contain higher concentrations ofMgO (1.56–4.26 wt.%), CaO (2.55–4.11 wt.%), and compatible elements(Cr of 16.0–159 ppm, Ni of 16.4–102 ppm) than their host rocks, adifference that is especially pronounced in samples LKZ-8 and LKZ-16(Figs. 5–6, Suppl. Table 3). These enclaves are also enriched in theLILE relative to the HFSE, and have primitive-mantle-normalizedmulti-element patterns that are characterized by pronounced negativeNb–Ta–Ti and positive Pb anomalies (Fig. 7B). These rocks also havemoderately fractionated REE patterns that have (La/Yb)N ratios of8.62–37.4 and distinct negative Eu anomalies with Eu/Eu* values of0.35–0.65 (Fig. 7A).

Compared with previously published data for leucogranites of theNHG and HHL, the Langkazi two-mica granites and the dioritic enclaveshave relatively unradiogenic Sr and radiogenic Nd isotopic composi-tions, with the Langkazi host rocks having εNd(t) values of –12.6 to –10.3 and 87Sr/86Sr(i) values of 0.7162–0.7307 (n = 6), whereas theenclaves have 87Sr/86Sr(i) = 0.7085–0.7137 and have εNd(t) = –8.6 to–6.1 (n = 8).

4. Discussion

4.1. Interpretation of U–Pb and 40Ar/39Ar ages

Well-developed oscillatory growth zoning in zircons is indicative ofamagmatic origin, although themajority of the zircons analyzed duringthis study have relatively low Th/U ratios (b0.4). The 206Pb/238U age ofthe Langkazi host granite is within 2σ uncertainty of the age of the en-claves, indicating that these ages represent the timing of crystallizationof both the hosting leucogranites and associated enclaves. The presenceof ancient inherited zircon cores indicates that formed these units wereeither generated by the partial melting of ancient crustal material orassimilated crustal material during their ascent.

The presence of a typical granitic texture with free of deformationindicates that the biotite40Ar/39Ar ages obtained during this studymost likely record the timing of cooling of the Langkazi two-mica gran-ite to b335 °C, which is significantly lower than the closure temperatureof the zircon U–Pb isotopic system. The 40Ar/39Ar age for biotites ofthe Langkazi host granite is within the 2σ uncertainty of the zircon206Pb/238U ages obtained during this study, as is expected for granitesthat are free of deformation.

4.2. Origin of the Langkazi leucogranite and its enclaves

Previous research suggested that the NHGs and HHLs within theHimalaya were derived from magmas generated by the partial meltingof metapelites within the HHC (Debon et al., 1986; Deniel et al., 1987;Searle et al., 1997; Vidal et al., 1982; Zhang et al., 2004a). These granitesare thought to be derived frommagmas generated frommetapelites bymuscovite dehydration melting (Ayres and Harris, 1997; Breton andThompson, 1998; Guillot and Le Fort, 1995; Harris and Massey, 1994;Harris et al., 1995; Harrison et al., 1997; King et al., 2011; Knesel andDavidson, 2002; Patiño Douce and Harris, 1998; Zhang et al., 2004a),by fluid-fluxed muscovite melting (Gao and Zeng, 2014; Zeng et al.,2012), by fluid-fluxed biotite melting (King et al., 2011; Zhang et al.,2004a). These granites are geochemically similar and are characterizedby high SiO2 and low Sr contents, high Rb/Sr and Rb/Ba, low Sr/Y ratios,highly negative εNd(t) values, and elevated [87Sr/86Sr(i)] values. In addi-tion, recent research has identified that amphibolite within the lowercrust may be an important fertile component that was susceptible tomelting during the India–Eurasia continental collision. The melting ofthis material generates granites that are significantly enriched in CaO,

0.6

60 65 70 80

0.5

60 65 70 75 80

P2O

5 (%

)

10

12

14

16

18

20

60 65 70 75 80

Al 2O

3 (%

)

0

1

2

3

4

5

MgO

(%)

0

1

2

3

Fe 2O

3 (%

)

A

D

B

E

C

F

0

1

2

3

4

5

4

5

NHG

Langkazi enclave

HHL

Langkazi two-mica granite

experimental productadakite-like rock

interaction with

Indian lower-crust

magmamixing trend

magmamixing trend

magmamixing trend

magmamixing trend

interaction with Indian lower-crust

magmamixing trend

magmamixing trend

interaction with

Indian lower-crust

SiO2 (%)SiO2 (%)SiO2 (%)

TiO

2 (%

)C

aO (

%)

0.4

0.3

0.2

0.1

0.0

0.9

0.8

0.7

0.5

0.4

0.3

0.2

0.1

0.075

Fig. 6. (A–F) Harker variation diagrams showing the major element variations in the Oligocene-Miocene leucogranites in the Himalayan orogen. Data sources the same as Fig. 5.

131Y.Zheng

etal./Lithos264

(2016)125–140

Rb Ba Th U Nb Ta K La Ce Pb Nd Sr Sm Hf Zr Ti Eu Gd Tb Y Yb

La Sm Tm

Sam

ple/

Prim

ativ

e m

antle

1

1

Sam

ple/

Cho

ndrit

e

NHGsHHLs

HHLs

NHGs

Langkazi enclave

Langkazi two-mica granite

1000

100

10

0.1

1000

100

10

0.1

Ce Pr Nd Eu Gd Tb Dy Ho Er Yb Lu

Fig. 7. (A) Chondrite-normalized REE and (B) primitive mantle-normalized patterns forthe Langkazi two-mica granites in the Himalayan orogen. Chondrite and primitivemantle normalization factors are from Sun and McDonough (1989). Data sources are thesame as Fig. 5.

0.70 0.72 0.73

ε Nd(

t)

99 98

95

90

80

70

50

30 10

HHC

NHGs

Langkazi enclave

HHLs

Langkazi two-mica granite

Himalayanlower crust

MgO 3.7 %MgO 4.3 %

-3

-5

-7

-9

-11

-13

-15

-170.71 0.74

87Sr/86Sr(i)

0.75 0.76 0.77 0.78 0.79

Fig. 8. Whole-rock εNd(t) versus 87Sr/86Sr(i) compositions in Oligocene–Mioceneleucogranites in the Himalayan orogen. Data for the lower crust of the Himalyan orogenare from King et al. (2007), and are represented by Miocene lower-crust-derivedadakite-like felsic dikes in the THS. Data for HHC, NHGs, and HHLs are from Vidal et al.(1982), Deniel et al. (1987), Ahmad et al. (2000), Gao and Zeng (2014), and Z.C. Liuet al. (2014).

132 Y. Zheng et al. / Lithos 264 (2016) 125–140

Ba, and Sr (typically N400 ppm), are depleted in Rb and the heavy REE(Fig. 6), have high Sr/Y and low Rb/Sr and Rb/Ba ratios, and have rela-tively low 87Sr/86Sr(i) and high εNd(t) values, all ofwhich are significantlydistinct from the compositions of the granites derived frommetapelitesdescribed above. The classic examples of the NHGs and HHLs aregenerally devoid of any microgranular igneous enclaves, meaning thatprevious research exclusively focused on the leucogranite host rocks,with little to no research undertaken on the igneous enclaves. Asdiscussed above, the Langkazi two-mica granite contains a large num-ber of dioritic enclaves. As such, we initially focus on the petrogenesisof these enclaves as they provide insights into the petrogenesis of thehosting Langkazi two-mica granite.

4.2.1. Langkazi dioritic enclavesMafic to dioritic enclaves within silicic igneous rocks are commonly

interpreted to be either residual material from the site of melting(i.e., restite) (Chappell and White, 1992), the preserved primitivemagmas that eventually formed the hosting granite (Li et al., 2010;Zheng et al., 2014), or relics of a mafic igneous component that wasadded to an intermediate to silicic magma chamber by magma mixing(Holden et al., 1987; Yang et al., 2004; Zheng et al., 2012b). Enclavesin the restitemodel are generally thought to represent residualmaterialthat progressively unmixed from themelt during the ascent of a crystalmush from its source region or from the surrounding wall rocks(Chappell et al., 1987). The majority of the enclaves within the NHGsand the HHLs are this type of enclave, all of which consist of various

types of gneiss (Z.C. Liu et al., 2014). However, the dioritic enclaves inthe Langkazi two-mica granite have typical igneous textures, containacicular apatite and biotite, and have quenched margins (Fig. 2), all ofwhich are distinct from the metamorphic textures expected for restiteenclaves (Eichelberger, 1980; Vernon, 1984). In addition, the U–Pb zir-con ages of restite enclaves are generally much older than the age ofcrystallization of their hosting granites (Chappell et al., 1987), whereasthe Langkazi enclaves and host granite in this study have nearly identi-cal (i.e., overlap within uncertainty) ages, indicating that the Langkazidioritic enclaves most likely have magmatic origins.

Mafic or dioritic enclaves can also represent relict blebs of the prima-ry melts present during the earliest stages of themagmatic evolution oftheir host rocks. Thismodel generates host rocks and enclaves that havesimilar whole-rock Sr–Nd isotopic compositions (Li et al., 2010; Zhenget al., 2014), something that is not the case for the Langkazi enclavesand their hosting leucogranites (Fig. 8). In addition, crustal contamina-tion during magma emplacement could result in the evolved magmashaving differing Sr–Nd isotopic compositions to the primitive magmaswithin the same system. However, this would also generate either neg-ative or positive correlations between the 87Sr/86Sr(i) values and majorelement concentrations (SiO2 and MgO) and the εNd(t) values andmajor element concentrations (SiO2, MgO) of the evolved and primitivemagmas within the system, something that is again absent from thesamples from the study area (Fig. 9). As such, the relict bleb modelcannot explain the presence and the evolution of enclaves within theLangkazi leucogranites (Guan et al., 2012; Yang et al., 2004).

The final possibility is that the Langkazi enclaves represent anotherigneous component that was added to the leucogranite magmachamber. This model is supported by the data presented in this study,primarily as this magma mixing model can explain the large isotopicdifferences but nearly identical crystallization ages of the hosting gran-ites and the entrained enclaves (Figs. 8–9) (Guan et al., 2012; Yang et al.,2004; Zheng et al., 2012b). The presence of quenched textures (acicularapatite and biotite and quenched margins) within the dioritic enclavesalso provides evidence of an igneous component at a relatively hightemperature that was injected into a cooler magma, consistent withthe results of the whole-rock Zr saturation thermometry undertakenduring this study (Suppl. Table 3) (Watson and Harrison, 1983). This,combined with the whole-rock Sr–Nd isotopic compositions described

mag

ma

mix

ing

tren

d

Indian lower-crust

Lower crust-derived melts

HHL

interaction with

Indian lower-crust

.705

.710

.715

.720

.725

.730

.735

0 1 2 3 4 5

Indian lower-crust

interaction with

Indian lower-crust

mag

ma

mix

ing

tren

d

Lower crust-derivedmelts

HHL

Langkazi host rockLangkazi enclave

-13

-11

-9

-7

-5

-3

SiO2 (%)

ε Nd(

t)

60 62 64 66 68 70 72

Indian lower-crust

magm

a

mixing trend

interaction with

Indian lower-crust

HHL

Lower crust-derived melts

0.6 1.0 1.4 1.8 2.2

-13

-11

-9

-7

-5

-3

ε Nd(

t)

Yb (ppm)m

agm

am

ixin

g tr

end

Indian lower-crust

Lower crust-derived melts

HHL

interaction with

Indian lower-crust

-13

-11

-9

-7

-5

-3

0 1 2 3 4 5MgO (%)

ε Nd(

t)interaction with

Indian lower-crust

Indian lower-crust

mag

ma

mix

ing

tren

d

HHL

Lower crust-derived melts

1 10 100Ni (ppm)

-13

-11

-9

-7

-5

-3

ε Nd(

t)

1 10 100Ni (ppm)

.705

.710

.715

.720

.725

.730

.735

87S

r/86

Sr (i)

87S

r/86

Sr (i)

Indian lower-crust

Lower crust-derived melts

mag

ma

mix

ing

tren

d

HHL

interaction with

Indian lower-crust

A

MgO (%)

C

E

B

D

F

Fig. 9.Whole-rock εNd(t) and 87Sr/86Sr(i) compositions varying with major and trace elements. Whole-rock εNd(t) versus (A) MgO, (B) SiO2, (C) Ni, and (D) Yb; 87Sr/86Sr(i) versus (E) MgOand (F) Ni for the Langkazi two-mica granites in the Himalayan orogen. Data sources the same as Fig. 8.

133Y. Zheng et al. / Lithos 264 (2016) 125–140

above, indicates that the dioritic enclavesweremost likely derived froma different source than the hosting leucogranites.

Some parts of the enclaves are lighter in color than the majority ofthe enclaves, with these lighter regions containing variable amountsof plagioclase megacrysts, indicating that these enclaves may havebeen partially replaced by the host melt during magma mixing. Assuch, the influence of magmamixing was restricted during the analysisundertaken in this study by only analyzing enclaves with clearlyquenched margins, an approach that was successful as evidenced by alack of negative or positive correlations in plots of 87Sr/86Sr(i) andεNd(t) against SiO2 and MgO (Fig. 9).

Dioritic (andesitic) melts are generated by either the partial meltingof metabasaltic material (i.e., amphibolites) within the lower or middlecrust or by the evolution of mantle-derived basaltic or basaltic andesitemelts (Annen et al., 2006; Jackson et al., 2003; Petford and Atherton,1996; Prouteau and Scaillet, 2003; Sisson and Grove, 1993; Q. Wang

et al., 2014). However, the presence of a negative correlation betweenεNd(t) and MgO concentrations (Fig. 9A) indicates that the enclavescannot be derived from a single source rock, suggesting that theirprimitive melts may have mixed with other melts or interacted withcrustal material during their ascent. This means that either the highestor lowest Mg sample within the enclave suite could represent themost primitive melt.

The first possibility is that the sample with lowest MgO content rep-resents the most primitive melt within the Langkazi dioritic enclavesamples analyzed during this study. If this is correct, then the enclavescould be derived frommelts generated by the partialmelting of a crustalsource, with interactions during ascent increasing the MgO content ofthe melts. Such a scenario could only occur in a foundering part of thelower crust during the Oligocene–Miocene. Melts derived from thefoundering lower crust would likely significantly increase in MgO andcompatible element (e.g., Cr and Ni) contents, and undergo changes in

134 Y. Zheng et al. / Lithos 264 (2016) 125–140

their Sr–Nd isotopic compositions as they interacted with peridotiteduring their ascent through the overlying mantle (Stern and Kilian,1996;Wang et al., 2007; Zhu et al., 2009b). This suggests that upwellingasthenospheric material in this area should have a relatively enrichedSr–Nd isotopic composition,which is inconsistentwith the compositionof the asthenosphere beneath the Indian Plate (Zhu et al., 2008, 2009b),indicating that this hypothesis can be ruled out.

The second possibility is that the sample with highest MgO contentrepresents most primitive melt. This hypothesis predicts that subse-quent interaction after the generation of these melts causes a decreasein MgO contents. Experimental research into the partial melting ofmetabasalts indicates that andesitic melts with MgO contentsN3% areonly generated under high temperature conditions (N950 °C) (Wolfand Wyllie, 1994). If the enclaves are derived from partial melting ofthe lower crust then the low-Mg end-member should represent eitherthe Himalayan upper crust or melts derived from this material. Theseinteractionswould increase the 87Sr/86Sr(i) and decrease the εNd(t) com-position of the enclaves, decrease the MgO and Ni content of the en-claves, and increase the SiO2 content of the enclaves, all of which areopposite to the trends defined by the enclaves analyzed during thisstudy (Fig. 9). In addition, the lower crustally derived melts coevalwith the Langkazi granites have adakitic affinities, whereas the highestMgO sample of the Langkazi enclaves is characterized by high Y and lowSr/Y ratios, plotting outside of the adakitefield (Fig. 10). As such, it is un-likely that the Langkazi enclaves were formed frommagmas generatedby the partial melting of lower crustal material beneath the THS.

Thismeans that the only remaining potential source for the high-Mgenclaves is the mantle beneath the Himalayan orogen, which is consis-tent with the enclaves containing relatively high concentrations of MgO(up to 4.3wt.%), Cr (up to 159 ppm), andNi (102ppm), although typicalmantle-derived basaltic melts have not yet been identified within theIndian–Asian collision-related Himalaya (Yin et al., 2010). The mantle-derived melts could have crossed the Moho before stalling in thelower crust, where these melts underwent crustal melting, melt assim-ilation, magma storage, and homogenization (i.e., MASH processes;Hildreth andMoorbath, 1988), that caused thesemelts to evolve and as-similate significant amounts of lower crustal material. The assimilationof lower crustal material could decrease the 87Sr/86Sr(i) values and in-crease the εNd(t) values of mantle-derived melts (Fig. 9), although thisrequires the lower crust of the Himalayan orogen to be relativelydepleted. The presence of negative correlations between εNd(t) valuesand Y and Yb concentrations (Fig. 9C–D) and positive correlationsbetween 87Sr/86Sr(i) and Y and Yb concentrations provides more evi-dence that these mantle-derived melts intensively interacted withgarnet-bearing juvenile lower crustal material (D. Liu et al., 2014).This, combined with the presence of Oligocene–Miocene adakite-like

A

Sr/

Y

250

200

150

100

50

00 5 15 20 25 30 35 4010 45 50

Adakite

Arc magmatic rocks

NHG

Langkazi enclave

HHL

Langkazi two-mica granite

experimental productadakite-like rock

Y (ppm)

Fig. 10. (A) Sr/Y vs. Y and (B) (La/Yb)N vs. YbN discrimination diagrams showing data for adakiteFig. 5.

rocks, strongly suggests that the Himalaya contains relatively juvenilelower crustal material (King et al., 2007; Z.C. Liu et al., 2014).

Oligocene (28 Ma) and Miocene (11–13 Ma) granitic dikes withadakite-like affinities have been identified within the Ramba domeand around the Kuday dome (King et al., 2007; Z.C. Liu et al., 2014).These rocks contain high concentrations of SiO2, Al2O3, and Sr (typicallyN400 ppm), and low concentrations of Y (b14 ppm) and Yb (b1 ppm),yielding high Sr/Y and La/Yb ratios and ensuring that these samples plotin the adakite field on Sr/Y versus Y and (La/Yb)N versus YbN diagrams(Fig. 10). The Oligocene–Miocene granites in the study area also havelow Mg# values [Mg# = MgO / (FeO + MgO)] and low MgO and con-tain low concentrations of compatible elements (e.g., Cr and Ni), similarto the abundances within lower crustally derived adakite-like melts(Chung et al., 2003; Hou et al., 2004). These features indicate that themelts that formed these units were derived from a source regionwithinthe garnet stability field under thickened lower crustal conditions (Zenget al., 2011). In addition, the relatively unradiogenic Sr (0.7072–0.7106)and radiogenic Nd (–7.9 to –4.0) isotopic compositions of these rockssuggest that the lower crust beneath the THS is dominated by juvenilematerial rather than by Archean Indian-derived material, although theorigin of this juvenile lower crustal material remains controversial(King et al., 2007; Z.C. Liu et al., 2014).

In summary, we suggest that the highest MgO enclave samples(samples LKZ–8 and LKZ–16) represent most primitive melt of the en-clave samples analyzed during this study, with these melts generatedby the partial melting of an enriched region of the mantle beneath theTHS. These melts interacted with juvenile lower crustal material priorto emplacement. However, we cannot constrain the nature of theresidual minerals within the enriched mantle source for these melts asa result of the evolved nature of these enclaves.

4.2.2. Langkazi leucograniteThe Rb–Sr–Ba systematics of silicate melts are controlled by a num-

ber of variables that include temperature, pressure, and H2O activity, al-though the most dominant control is the composition of the protolith(Gao and Zeng, 2014; Harris and Inger, 1992; Sylvester, 1998). Thefact that Rb is concentrated within mica, whereas Ca is the majorstructural component of plagioclase, and Sr and Ba can substitute forCa in plagioclase means that the Rb–Sr–Ba systematics of metapelitesare evidently distinct from the systematics of metapsammites andmetagreywackes, primarily as a result of the variations inmica and feld-spar contents (Harris and Inger, 1992). Plagioclase is usually a minorphase within pelites, indicating that the melting of metapelitic materialcould generate melts containing low concentrations of Sr, Ba, and Caconcentrations and high Rb/Ba and Rb/Sr ratios, whereas melts derivedfrom metapsammite and metagreywacke material have the opposite

MORBAdakite or high-Al TTDLower-Al TTDPartial melting lines

10

10

10

25

505050 B

100

50

00 5 10 15 20

Eclogite

25% Garnet-amphibolite

10% Garnet-amphibolite

Amphibolite

MORB

(La/

Yb)

N

YbN

B

s and normal calc–alkaline rocks (Defant and Drummond, 1990). Data sources the same as

135Y. Zheng et al. / Lithos 264 (2016) 125–140

(i.e., high Sr, Ba, and Ca, and low Rb/Ba and Rb/Sr) (Atherton andPetford, 1993; Wolf andWyllie, 1991). As shown in Fig. 11, the distinc-tively high Rb, low Sr–Ba and high Rb/Sr and Rb/Ba of themajority of theNHGs and HHLs indicate that these granites generally formed frommagmas derived from a metapelite-dominated source region.

The Langkazi leucogranites are similar to the previously reportedNHGs and HHLs in that they contain high concentrations of SiO2, lowconcentrations of MgO, CaO, and Fe2O3 (Figs. 5–6), have low CaO/Na2O and Al2O3/TiO2 ratios (Fig. 11), have elevated 87Sr/86Sr(i) values,and have negative εNd(t) values (Figs. 8–9), indicating that they formedfrom magmas derived from a metasedimentary-dominated source re-gion (Debon et al., 1986; Deniel et al., 1987; Searle et al., 1997; Vidalet al., 1982; Zhang et al., 2004a). However, the leucogranites withinthe Himalaya also have highly variable major element (e.g., Si, Mg, Al,Ca, Na, and K), trace element (e.g., Rb, Sr, Ba, and REE), and Sr–Nd isoto-pic compositions. Previous research suggested that the geochemicalheterogeneities present in typical NHGs and HHLs may be resulted ofthe progressive melting of a single metasedimentary source (Gao andZeng, 2014; Knesel and Davidson, 2002; Zeng et al., 2005a). However,there is also significant evidence that the geochemistry of the Langkazitwo-mica granites are suggestive of a magma mixing origin involvingthe melts that formed the dioritic enclaves in the study area, ratherthan being derived from melts entirely generated by the metapelites;this evidence is summarized below:

(1) As discussed above, the petrography of the enclaves is indicativeof mixing between the enclave melts and the host magma,indicating that the geochemical composition of the host meltsmust have been modified by the enclave magmas duringmagma mixing.

(2) The Langkazi leucogranites contain lower concentrations of SiO2

(Fig. 5A), have lower Rb/Sr and Rb/Ba ratios (Fig. 11B), and con-tain higher concentrations ofMgO, CaO, and TiO2 (Fig. 6) than thepreviously reported NHGs and HHLs. These differences are alsoclear in Harker diagrams, where the Langkazi rocks and otherleucogranites define different differentiation trends (e.g., Fe2O3

and P2O5 versus SiO2; Fig. 6C, F) indicating that the Langkazirocks could not represent more primitive compositions of themagmas that formed the Oligocene–Miocene Himalayanleucogranites. However, plotting major element concentrationsagainst SiO2 does generally define linear arrays containing boththe dioritic enclaves and the Langkazi granitoids (with the latterhaving the highest SiO2; Fig. 6) indicative of binary mixing(Clynne, 1999).

90%CaO

/Na 2O

10 100 10000.1

0.3

1.0

5.0Basalt

50%

20%

10%

Pelite-derivedmelt

Field ofSP Granites

A2lO3/TiO2

Rb/

Ba

0.1 10.01

0.1

1

10

Clay-poor

source

s

Clay-ri

ch

sour

ces

Shale

GreywackeBasalt

60%

30%

A BB

NHG

Langkazi enclave

HHL

Langkazi two-mica granite

experimental productadakite-like rock

Rb

A

Fig. 11. (A) CaO/Na2O versus Al2O3/TiO2 ratios and (B) Rb/Ba versus Rb/Sr ratios (after Sylvestsources the same as Fig. 5.

(3) Experimental studies and field observations indicate that themelting of a pelitic source could generate melts with differingεNd(t) and 87Sr/86Sr(i) values than the source undergoing partialmelting, following one of two paths in εNd–87Sr/86Sr ratiospace (Knesel and Davidson, 2002; Zeng et al., 2005b). Thefluid-fluxed melting of muscovite and/or biotite will generatemelts with lower 87Sr/86Sr and εNd values than their source, pri-marily as apatite (but not monazite) will remain as a residualmineral, whereas melts derived from the dehydration meltingof muscovite and/or biotite will have higher 87Sr/86Sr and εNdvalues than their sources as apatite (but not monazite) meltingwould have an important role in the generation of these melts(Zeng et al., 2005b). These processes were used to interpret thesignificant Sr–Nd isotopic variations within the Himalayanleucogranites (Gao and Zeng, 2014; Zeng et al., 2012), althoughneither of the trends within these leucogranites are present inthe samples from the study area (Fig. 8). In contrast, the Langkazitwo-mica granites clearly have a negative correlation between87Sr/86Sr(i) and εNd(t) values, which is consistent with binarymixing between enriched and depleted end-members (Fig. 8).

(4) Recent research has identified Miocene two-mica granites withhigh CaO contents (N1.5 wt.%) within the North Himalayangneiss dome. These granites are thought to have formed frommelts generated by the fluid-present melting of muscovite inmetapelites under relatively high pressure conditions (Gao andZeng, 2014; Zeng et al., 2012). This is based on the fact thatthese high CaO two-mica granites have relatively low and con-stant Rb/Sr ratios with large variations in Ba concentrations(Gao and Zeng, 2014; Harris and Inger, 1992). However, asshown in Fig. 11C, both the Langkazi enclaves and their hostingleucogranites also have low and constant Rb/Sr ratios withlarge variations in Ba concentrations (trend A in Fig. 11C), sug-gesting that these features may not be reliable evidence of thefluid-present melting of biotite within metapelites, but insteadmay be evidence of the mixing of leucogranite magmas withmantle-derived melts.

In conclusion, the melts that formed the Langkazi leucograniteswere predominantly derived from crustal metasedimentary rocks, butmantle-derived melts (represented by the Langkazi dioritic enclaves)may also have had an important role in the genesis of theseleucogranites. In addition, the NHGs and HHLs that have similar miner-alogical and geochemical characteristics to the Langkazi leucogranites(Fig. 6) suggest that mantle-derived melts may also have had a more

10

Calculatedpelite-derivedmelt

Calculatedpsammite-derivedmelt

Rb/

Sr

C0.01

0.1

1

10

100

0 200 400 600 800 1000 1200 1400 1600

Mus (VP)

Bi (VA)Mus (VA)

Trend A

Trend B

/Sr100

Ba (ppm)

er, 1998); (C) Rb/Sr versus Ba in Oligocene–Miocene leucogranites of southern Tibet. Data

136 Y. Zheng et al. / Lithos 264 (2016) 125–140

important influence on the petrogenesis of the NHGs and HHLs than iscurrently thought. However, further detailed field, petrological, andgeochemical research into the genesis of the NHGs and the HHLs isrequired.

5. Implications

5.1. Implications for Himalayan anatexis

The anatexis ofmetapelites in the upper crust can be induced by var-ious mechanisms, including the heating by intrusion of mantle-derivedmafic magmas (Shellnutt et al., 2011), shear heating during deforma-tion (Harrison et al., 1997, 1998; Whittington et al., 2009), radiogenicheating by the decay of radioactive elements (e.g., U, Th, andK) (Molnar et al., 1983), decompression melting associated with therapid exhumation of deep-seated rocks (Harris and Massey, 1994;Patiño Douce and Harris, 1998; Royden, 1993), and partial melting inthe presence of free aqueous fluids (Debon et al., 1986; Le Fort et al.,1987). The lack of spatial and temporal associations with maficmagmatism (i.e., mafic intrusions or lavas and enclaves) has led previ-ous studies to suggest that the anatexis in the Himalaya was inducedby intracrustal heating or geological processes without the involvementof mantle-derived heat (Debon et al., 1986; Harris and Massey, 1994;Harrison et al., 1997, 1998; Le Fort et al., 1987; Molnar et al., 1983;Royden, 1993; Whittington et al., 2009).

Some studies have suggested that frictional heating along the STDandMCT raised temperatures sufficiently to generate melting in the Hi-malayan orogenic belts (Harrison et al., 1997, 1998; Le Fort, 1975;Whittington et al., 2009). However, Molnar et al. (1983) indicated thatthis shear heating hypothesis requires implausibly high velocity, stress,and time constants to provide sufficient heating for the anatexis knownto have occurred in this region. More recently, voluminous mid-uppercrustal melts with ages from 28 to 35Ma have been identified in this re-gion (King et al., 2011; Liu et al., 2016; Zeng et al., 2012), indicating thatmovement on the STDs and MCTs must have been initiated severalmillion years after the melting events in this area (Le Fort, 1975; Yin,2006). This indicates that although anyMiocene (b24Ma) crustal melt-ing may have resulted from frictional heating along fault zones, someother source of heat is almost certainly required to generate the crustalmelting in this area prior to the Miocene (Royden, 1993).

Crustal radioactivity may be another possible source of heat to drivethe anatexis within the Himalaya. Molnar et al. (1983) suggested thatradioactive decay in the crust could generate temperature conditionsat and above the solidus for granitemelts. However, other research sug-gests that reasonable amounts of radioactive heating will not inducemelting unless the thickened crustal layerwheremelting takes place re-mains motionless for ca. 100 Ma (Bird, 1978; Bird et al., 1975; Toksözand Bird, 1977), a period of time that is significantly longer than thetime available within the Himalayan belt (Harrison et al., 1997, 1998;Le Fort et al., 1987).With the exception of high heat flows, an unusuallysteep transient geotherm is also required for the development ofvoluminous and widespread leucogranites within the Himalayanorogen (Harris and Massey, 1994). This suggests that the formation oftheHimalayan leucogranites is intrinsically linked to decompression as-sociatedwith the rapid exhumation of deeper rocks (Harris andMassey,1994; Patiño Douce and Harris, 1998; Royden, 1993), but does not pre-clude any possible contributions from frictional or radiogenic heating.As such, it is clear that none of the present models can clearly explainthe origin of the granites within the Himalayan orogen. Thus, thepresence of mantle-derived dioritic enclaves within leucogranites inthe study area suggests that the anatexis of the Himalayan crust mayhave been facilitated by the addition of mantle-derived heat.

As discussed above, aside from the typical NHGs andHHLs, the studyarea contains coeval adakite-like dikes derived frommelts generated bythe partialmelting ofmafic lower crustalmaterial beneath theHimalaya(King et al., 2007; Z.C. Liu et al., 2014). Experimental research indicates

that the solidus of hornblendewithin amphibolite is higher than the sol-idus of micas within metapelites (Martin, 1987; Peacock et al., 1994;Rapp et al., 1991; Wyllie, 1983). For example, dehydration melting ata depth of 25 km would begin at 680 °C for muscovite, at ca. 760 °Cfor biotite, and at ca. 910 °C for hornblende. This indicates that moreheat is required to melt thickened mafic lower crustal material thanthe upper crust (Martin, 1987; Rapp et al., 1991;Wyllie, 1983). Potentialheat sources include strain heating during deformation, radiogenicheating, and heat flux from themantle, although the 40Ma of thrustingalong a shear zone at a depth of 50 km in the study area indicates thatstrain heating could not reach the solidus temperature of schists withina thickened region of the crust (Whittington et al., 2009), meaning thatshear heating was insufficient to trigger the partial melting of thethickened mafic lower crust beneath the Himalaya. In addition, radio-genic heating of the crust over a period of 30 Ma at a depth of 50 kmalso cannot reach temperatures of 800 °C (Molnar et al., 1983). This, to-gether with the relatively low abundances of radioactive elements inthe lower crust, suggest that radiogenic heat was also insufficient totrigger the melting of the mafic lower crust within 30 Ma of theinitiation of the Indian–Asian collision. As such, we suggest that hotmantle-derived melts (as evidenced by the Langkazi enclaves) crossedtheMoho before stalling in the lower crust. Thesemelts would undergointensiveMASH processes at the base of the crust as well as introducingheat into the lower crust. This mantle-derived heat may have inducedthe partial melting of the thickened Himalayan lower crust, formingthe adakite-like dikes that are present in the study area.

This study clearly indicates that mantle-derived heat could haveplayed a role in the anatexis of the Himalayan upper and lower crust.However, the fact that only small volumes of mantle-derived melt arelikely to be involved suggests that although any mantle-derived meltsinjected into the crust assisted the genesis of the NHGs andHHLswithinthe Himalayan orogen, it is more likely that mid-upper crustal anatexisin this region was dominantly caused by in situ crustal radiogenicheating, shearing heating, and decompression melting during theIndian–Asian collision. However, melting of lower crustal materialwas predominantly induced by the underplating of mantle-derivedmagmas.

5.2. Implications for the geodynamics of the Himalaya

Partial melting of the mantle lithosphere in post-collisional settingsrequires specific heat source. This heat is generally thought to bederived from the replacement of the lower part of the lithosphere byhotter asthenospheric material (Chung et al., 2003; Turner et al., 1996;Q. Wang et al., 2014). The upwelling of the asthenospheric material be-neath theHimalayanorogen has not previously been discussed in detail,primarily as a result of a lack of documentation of the involvement ofmantle-derivedmelts in the Asian–Indian continental collision. Howev-er, the mantle-derived Langkazi enclaves may provide evidence of theupwelling of asthenospheric material beneath the Himalaya, with thisupwelling acting as a heat source that caused the continuous meltingof the Indian continental lithosphere during the Indian–Asian collision.

The upwelling of asthenospheric material has also been used to ex-plain the genesis of both the post-collisional Oligocene–Miocenelithospheric-mantle-derived ultrapotassic–potassic and lower crustallyderived adakite-like magmas within the Lhasa terrane to the north ofthe IYS (Chung et al., 2003, 2005; England and Houseman, 1989; Guoet al., 2007; Hou et al., 2004; Turner et al., 1996; Williams et al., 2001).Zircon U–Pb and Ar–Ar age data dating indicate that the post-collisional leucogranites within the Himalaya formed between 31 and8 Ma in a magmatic event (King et al., 2011; Z.C. Liu et al., 2014;Searle et al., 2003; Zhang et al., 2004b) that was contemporaneouswith both the ultrapotassic–potassic magmatism (ca. 25–8 Ma: Dinget al., 2003; Gao et al., 2007; Zhao et al., 2009) and the emplacementof adakite-like magmas within the Lhasa terrane (ca. 30–9 Ma: Houet al., 2004; Zhao et al., 2009; Zheng et al., 2012a, 2012b; R. Wang

137Y. Zheng et al. / Lithos 264 (2016) 125–140

et al., 2014a; R. Wang et al., 2014b; Yang et al., 2015). The similar ageranges for these igneous rocks suggest that the upwelling of theasthenospheric material beneath the two plates in this region occurredsimultaneously, indicating that this upwelling was controlled by thesame deep-seated lithospheric processes. This in turn suggests thatthe widespread and voluminous Oligocene–Miocene magmatism onboth sides of the Indian–Asian collision zone was induced by theupwelling of asthenospheric material caused by either the convectiveremoval of the lower lithosphere of southern Tibet (Chung et al.,2003; Hou et al., 2004; Williams et al., 2001), or by rollback (Dinget al., 2003) or breakoff (Ji et al., 2016; Mahéo et al., 2002) of thesubducted Indian continental lithosphere, a process that would haveelevated the geothermal gradient in this region.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.lithos.2016.08.019.

Acknowledgments

This study is supported by grants from the National Key Researchand Development Program of China (2016YFC0600310), the Ministryof Science and Technology of China (973 Project 2015CB452600,2011CB4031006), NSFC (41472076), IGCP/SIDA-600, the Programof the China Geological Survey (1212011121255, 12120113037900),and the Fundamental Research Funds for the Central Universities(53200859424), 111 project (B07011). This is CUGB petro-geochemicalcontribution NO. PGC-201508 (E7110214). We are most grateful to thetwo anonymous referees for their critical and constructive reviews ofthis manuscript.

Appendix A

A.1. Descriptions of analytical methods

A.1.1. Zircon U–Pb datingIn this study, zircons were separated from granitic samples using

conventional heavy-liquid and magnetic separation techniques. Zircongrains, together with a zircon U–Pb standard (TEMORA; cf. Black et al.,2004),were cast in an epoxymount, whichwas then polished to sectionthe grains in half for analysis. Zircon were documented with transmit-ted and reflected light micrographs as well as cathodoluminescenceimages to reveal their internal structures, and the mount wasvacuum-coated with an ~500 nm layer of high-purity gold. Zircon U–Pb isotopic analyses were performed using the sensitive high-resolution ion microprobe (SHRIMP) in Beijing.

Ages of the Langkazi two-mica granites (sample LKZ-10) and dioriticenclave (sample LKZ-17) were obtained using the SHRIMP equipped atthe Beijing SHRIMP center, Institute of Geology, Chinese Academy ofGeological Sciences. The uncertainties of individual analyses are report-ed at the 1σ level. The mean dates for 206Pb/238U analyses are used toindicate crystallization age of granitoids, with 95% confidenceinterval (2σ). Operating and data processing procedures follow thoseestablished in RSES, Australian National University. Standard materialfor measurements of U–Th abundance and U–Pb–Th isotopic ratios ofanalyzed samples is TEM standard zircon with 206Pb/238U = 0.0668 at417 Ma. The mass resolution used for determining Pb/U and Pb/Pbisotopic ratios is about 5000. Common 206Pb was corrected using non-radiogenic 204Pb.

A.1.2. 40Ar/39Ar datingFresh biotite grains were separated for 40Ar/39Ar dating using con-

ventional magnetic and gravimetric methods and hand-picking undera binocular microscope. The biotite grains were washed with methanoland rinsed many times with deionized water in an ultrasonic bath, andthen irradiated for 48h at the BeijingNuclear Research Institute Reactor.Also irradiated was the Fangshan biotite (ZBH-25) whose age is132.7 ± 1.2Ma and potassium content is 7.6 wt.% was used to calculate

the irradiation factor J. K2SO4 and CaF2 were used to determine correc-tion factors for the interfering neutron reactions. The sample wasstep-heated using a radiofrequency furnace. Its Ar isotopes weremeasured with a MM-1200B Mass Spectrometer at the Laboratory ofIsotope Geochronology at the Institute of Geology, Chinese Academyof Geosciences. The procedure for of the analyses and age calculationsis the same as that described by Chen et al. (2002). Measured isotopicratios were corrected for mass discrimination, atmospheric Ar compo-nent, blanks and irradiation induced mass interference. The correctionfactors for the interfering isotopes produced during the irradiationwere determined by analysis of the irradiated pure K2SO4 and CaF2salts. The final results are: (36Ar/37Ar)Ca = 0.000240; (40Ar/39Ar)K =0.004782; (39Ar/37Ar)Ca = 0.000806. The blanks of m/e = 40, 39, 37,and 36 are less than 6 × 10−15, 4 × 10−16, 8 × 10−17 and2 × 10−17 mol, respectively. The decay constant is taken as λ =5.543 × 10−10 a−1 (Steiger and Jäger, 1977). 37Arwere corrected for ra-diogenic decay (half-life 35.1 days).

A.1.3. Major and trace elementsSamples for geochemical analyses were ground to pass through a

200 mesh and further ground and homogenized in an agate mortarunder alcohol. Major element oxides, trace elements and rare earthelements (REEs) of those samples were analyzed by X–ray fluorescence(XRF) and by inductively coupled plasmamass spectrometry (ICP–MS),respectively, at the National Research Centre for Geoanalysis, ChineseAcademy of Geological Science (Beijing). The analytical uncertainty ofXRF analyses for major elements was within 5%, and the uncertaintyof the elements examined here was also less than 5% for the ICP-MSanalyses.

A.1.4. Whole-rock Sr–Nd isotopesSr–Nd isotopic analysis was done by a Triton mass spectrometer

(TIMS) at the Isotope Geology Lab, Chinese Academy of Geological Sci-ence (Beijing). For the NBS987 standard, the ratio of 87Sr/86Sr =0.71025 ± 2 (2σ). The measurement accuracy of the Rb/Sr ratio wasbetter than 0.1% and themass fractionation of Sr isotopes was correctedby using 88Sr/86Sr = 8.37521; the J&M ratio of 143Nd/144Nd =0.511125 ± 8 (2σ), while the measurement accuracy of the Sm/Ndratio was better than 0.1% and the mass fractionation of Nd isotopeswas corrected by using 146Nd/144Nd = .7219. The 87Rb/86Sr and147Sm/144Nd ratios were calculated using the Rb, Sr, Sm and Nd concen-trations obtained by ICP–MS.

Appendix B. Reference

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Chen W., Zhang Y., Ji Q., Wang S. S. and Zhang J. X. (2002) Themagmatism and deformation times of the Xidatan rock series, EastKunlun Mountain. Sci. China D 45, 20–27.

Steiger R. H. and Jäger E. (1977) Subcommission on geochronology:convention on the use of decay constants in geo- and cosmochronology.Earth Planet. Sci. Lett. 36, 359–362.

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