Geochronology and petrogenesis of granitic rocks in ...

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Geochronology and petrogenesis of granitic rocks in Gangdese batholith, southern Tibet JI WeiQiang1,2, WU FuYuan1†, LIU ChuanZhou1 & CHUNG SunLin3 1

State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China; 2 Graduate University of Chinese Academy of Sciences, Beijing 100049, China; 3 Department of Geoscience, Taiwan University, Taipei 106, China

Based on petrological and geochemical characteristics such as rock assemblage, petrogeochemistry, Sr-Nd isotope, zircon U-Pb age, and Hf isotope, we studied geochronological framework, magma types, source characters, and petrogenesis of different stages of magmatism of the granitic rocks from the Gangdese batholith in southern Tibet. The magmatic activities of the Gangdese batholith can be divided into three stages. The Mesozoic magmatism, induced by northern subduction of Neotethyan slab, was continuously developed, with two peak periods of Late Jurassic and Early Cretaceous. The Paleocene-Eocene magmatism was the most intensive, and resulted from a complex progress of Neotethyan oceanic slab, including subduction, rollback, and subsequent breakoff. And the Oligocene-Miocene magmatism was attributed to the convective removal of thickened lithosphere in an east-west extension setting after India-Asia collision. Isotopically, zircons from these granitic rocks are characterized by positive εHf(t) values, suggesting that the magmatic source of the Gangdese batholith might be an arc terrane, which was accreted to the southern margin of Asia during Late Paleozoic. Therefore, the chronological framework and Hf isotopic characteristics of the Gangdese batholith are distinct from the granitic rocks in adjacent areas, which can be served as a powerful tracer in studying source-to-sink relation of sediments during the uplift and erosion of Tibetan Plateau. Tibet, Gangdese batholith, granitic rock, geochronology, petrogenesis

Granite, a major constituent of the continental crust, possesses valuable geological information about the tectonic-magmatic process through which the rocks were formed, and thus is vital to the study of the growth and evolution of our continental crust. In Tibet, granitic rocks are extensively developed, especially along the north of Indus-Tsangpo suture with a well-known name of Transhimalayan batholith[1] (Figure 1). This huge batholith extends from Kohistan and Ladakh in the west through southern Tibet and western Yunnan (Dianxi) to Burma, and can be divided into three parts, the Kohistan-Ladakh batholith in the west, the Gangdese batholith in the middle, and the Zayu-Dianxi-Burma batholith in the east, with a total length of >3000 km, comparable to the Andes/Cordillera batholiths in western America.

Therefore, the Gangdese batholith is important for us to study the growth and evolution of the southern Tibet orogen, and decipher the relationship between granitic magmatism and crustal growth/ evolution. In this paper, a summarization is made for the granitic rocks occurred in Gangdese batholith, including rock assemblages, high-quality data of zircon U-Pb ages, and Sr-Nd-Hf isotopic data. Based on this summarization, we redefined the geochronological framework of the area, discussed the magmatic nature and source characteristics, Received May 4, 2009; accepted August 4, 2009 doi: 10.1007/s11430-009-0131-y † Corresponding author (email: [email protected]) Supported by Knowledge Innovation Project of the Chinese Academy of Sciences (Grant No. KZCX2-YW-Q09-06) and National Natural Science Foundation of China (Grant No. 40721062)

Citation: Ji W Q, Wu F Y, Liu C Z, et al. Geochronology and petrogenesis of granitic rocks in Gangdese batholith, southern Tibet. Sci China Ser D-Earth Sci, 2009, 52(9): 1240-1261, doi: 10.1007/s11430-009-0131-y

Figure 1

Distribution of granitoids in the Gangdese batholith.

and finally illuminated the tectonic-magmatic history of the northern Lhasa since the Mesozoic.

1 Geological background Tibetan Plateau consisted of several blocks, such as the India continent, Lhasa, Qiangtang, and Songpan-Garzê terranes. These blocks are separated, from south to north, by Yarlung-Tsangpo, Bangong-Nujiang, and Jinsha sutures[2]. The Lhasa terrane, also called Gangdese oro－ genic belt[3 5], is an east-westward tectonic-magmatic belt with a length of ~2500 km and a maximum width of ~150－300 km. This terrane is also characterized by numerous east-westward faults, and it can be divided － into several sub-units and related magmatic belts[3 6]. Generally, the granitic rocks within the Lhasa terrane can be divided into the northern plutonic belt dominated by S-type granites, and the southern Gangdese belt dominated by I-type granites[7,8]. Recently, identification

of the Songduo eclogite with oceanic crust affinities[9,10] led to the suggestion that the Lhasa terrane should be divided into the northern and southern parts[11,12]. However, Zhu et al.[5] argued that the Lhasa terrane should be divided into the southern Gangdese, Gangdese back-arc fault uplift, middle Gangdese, and north Gangdese, among which the southern and northern Gangdeses consisted of Neoproterozoic crust whereas the middle Gangdese and Gandese back-arc fault uplift possessed Paleo- and Mesoproterozoic basements. The Gangdese batholith is here referred to the narrow and east-west trending granite belt located in southern margin of the Lhasa terrane, immediately to north of the Yarlung-Tsangpo suture. This definition is identical to that by Chu et al.[7] and Wen et al.[8], and is equivalent to the southern Gangdese granite of Zhu et al.[5,6] . The batholith stretches from Kailas in the west, where it is separated from Kohistan-Ladakh batholith by Karakorum fault, to Nyingchi area in the east, composed mainly of diorite

Ji W Q et al. Sci China Ser D-Earth Sci | Sep. 2009 | vol. 52 | no. 9 | 1240-1261

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and I-type granite[13,14]. To north of the batholith, Meso-Cenozoic volcanic rocks and sedimentary strata were also developed, including Lower-Middle Jurassic － Yeba Formation[15 18], Upper Jurassic-Lower Cretaceous － Sangri Group[19 21], and Paleogene Linzizong volcanic－ sedimentary strata[22 27]. To south of the batholith, the Xigaze fore-arc flysch suite, i.e., the Xigaze Group, is locally distributed around the Xigaze area[28,29]1).

2 Research history of the Gangdese batholith Extensive researches on Gangdese batholith had not started until the 1960s when the scientific field investigation and joint cooperation project were organized by Chinese Academy of Sciences. These studies covered the distribution, rock assemblage, and K-Ar age of the batholith, and suggested that there existed Mesozoic granites because some granites were covered uncomformably by Late Cretaceous Xigaze Group[30,31]. During the 1970s and 1980s, more studies were done and most of them were focused on the middle segment of the batholith, especially the Quxu segment. The works include spatial- temporal distribution, rock association, petro-geochemistry, trace element geochemistry, Sr-Pb-O isotope geochemistry, source characteristics, － magmatic evolution, genetic mechanism and so on[32 39]. The above researches indicated that (1) Gangdese batholith was composite in nature, consisting of gabbro, gabbro-diorite, diorite, tonalite, granodiorite, ganite, and some gneissic ganites; (2) Rb-Sr ages suggested that the batholith was formed between the Late Paleozoic and Cenozoic; (3) the granitic rocks have low 87Sr/86Sr ratios, and the variations implied an assimilation and fractional crystallization during their magmatic evolution. In addition, systematically obtained K-Ar age data from the batholith and adjacent regions were comparable to those obtained recently[40,41]. A three-year French-Chinese cooperation project on Tibet (1980―1982) improved the geological study and － drew international attentions[42 46]. It was the first time to achieve the high quality of zircon U-Pb data, which indicated that the Gangdese magmatism ranges from Late Cretaceous to Eocene (94－41 Ma)[45]. Combined

with other related data, Schärer et al.[45] suggested that these granites resulted from the Neotethyan subduction, and the subsequent India-Asia collision was later than 41 Ma. These U-Pb data was fundamental for understanding the tectonic evolution of Gangdese batholith, although nowadays we found the magmatism was more complex than previously thought. Another study with regard to Gangdese batholith involving the granites of northern Tibet and Himalaya was accomplished by Debon, including petrology, major and trace element geochemistry, Sr-O isotopic geochemistry, and Rb-Sr dating. It is indicated that the Gangdese batholith was composed of Cretaceous (113 － 82 Ma) and Paleocene-Eocene (60－40 Ma) granites, attributed to Neotethyan subduction and India-Asia collision, respectively. Sr-O isotopic data suggested that these granites were probably derived from depleted-mantle sources and then assimilated or mixed with enriched materials. Furthermore, the French-Chinese cooperation project obtained some 40Ar/39Ar age data of Middle Cretaceous[43], consistent with present results[8,47], and achieved a geochr－ onological framework proved by later studies[48 54]. Subsequently, a joint Sino-British Tibet geotraverse in 1985 enriched the research achievements. Harris et al.[14] proposed that the Gangdese was a calc-alkaline composite batholith according to the major and trace element geochemistry, and the batholith was formed above a north-dipping subduction zone at the southern margin of Lhasa terrane before India-Asia collision. The mafic rocks of the batholith were derived probably from the garnet-bearing mantle source, with subsequent fractional crystallization and crustal assimilation to form the felsic granites. In addition, it was suggested that the Eocene granites were formed within a subduction setting before the India-Asian collision. It was also thought that the Nyainqentanglha batholith, belonging to northern Gangdese belt, is the exposed deep level of the Gangdese batholith[14]. Based on the Sr-Nd isotopes, Harris et al.[55] suggested the formation of the Gangdese batholith was related to anatexis of the Mesoproterozoic basement and crustal assimilation of mantle-derived melt. In terms of Nd isotopes, it was found that granitic rocks from the Gangdese batholith have positive εNd(t) values (+3.1－ +3.5) and young Nd model ages (481－305 Ma), distinct

1) Wu F Y, Ji W Q, Liu C Z, et al. Detrital zircon U-Pb and Hf isotopic constraints from the Xigaze fore-arc basin, southern Tibet on the source provenance and Transhimalaya magmatic evolution. J Geophys Res, 2009, in revision.

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from those of the Nyainqentanglha granites, which possess negative εNd(t) values (−6.9―−6.2) and old TDM ages (1382－1040 Ma). In the 1990s, along with more researches on the Gangdese granitoids[56], some related monographs were － published[57 59]. Then in the new century, another round of geological survey is initiated, which makes some important progresses, including better established temporal-spatial distribution of granitoids[3,5,8,47,60], identification of magma underplating and magma mixing event in the peak stage of ~50 Ma[50,51,61], ascertainment of the relationship between the Miocene adakitic magmatism － and mineralization[62 65].

3 Geochronological framework of the Gangdese batholith Geochronological method, like K-Ar, Ar-Ar, Rb-Sr and U-Pb, plays a fundamental role in the study of granites. The dating can be either based on a single crystal or using different minerals and whole-rock to construct an isochron. Zircon has the high closure temperature of U-Pb system. Thus, zircon U-Pb age is most approximate to the emplacement and crystallization age of the pluton. In recent years, in situ U-Pb dating techniques on zircon have been widely developed. The techniques are convenient and fast, and thus become the optimal choice. In the present paper, we summarized the published zircon U-Pb ages of Gangdese batholith measured by different methods, including TIMS, SHRIMP, CAMECA IMS, and LA ICP-MS. The magmatism of Gangdese batholith can be divided into four stages, i.e., 205―152, 109－80, 65－41, and 33－13 Ma (Figure 2).

Figure 2 Geochronological framework of Gangdese batholith. Age data for granitoids in the Gangdese batholith are from refs. [7, 8, 45, 47－54, 62－64, 66－73]. All the data are analyzed by zircon U-Pb method, summing to 129, including TIMS of 4, CAMECA IMS 1270 of 18, LA ICP-MS of 53, and SHRIMP of 54.

ported from Gangdese batholith up to date. Zhang et al.[71] obtained an age of 178 Ma from a deformed granite near Nyêmo bridge and Qu et al.[72] identified two Jurassic ages of 179 and 175 Ma from granodiorites of Xiongcun Cu-Au deposit by using SHRIMP. Yang et al.[73] found Early Jurassic granite yielding zircon U-Pb age of 182.3 Ma in Qulong deposit. Furthermore, early Mesozoic volcanic rocks (Yeba Formation: 190－174 Ma) are also outcropped in southern Lhasa terrane[17,18]. Therefore, Late Triassic-Early Jurassic magmatism has been found in an east-west narrow belt of 400 km in the middle segment of Gangdese batholith, and we believe more magmatic rocks of this stage will be identified in the future studies.

3.1 Late Trassic-Jurassic granitoids (205－152 Ma)

3.2 Cretaceous granitoids (109－80 Ma)

Early Mesozoic granitoids, including tonalite, granodiorite, granite, and minor gabbro, have been locally discovered in the middle segment of Gangdese batholith from Qulong copper of Maizhokunggar county in the east to Xiongcun Cu-Au deposit of Xietongmen county in the west. Chu et al. [ 7] first reported a Jurassic Bi-granite from Wuyu basin with a zircon SHRIMP U-Pb age of 188.1±1.4 Ma. More recently[47], multiple Jurassic ages from 194.0 to 151.8 Ma have been reported in granites from north of Dagzhuka. Furthermore, a Triassic age of 205.3 Ma has also been obtained in granite with foliated texture, which is the oldest age re-

Currently, only five granites with Early Cretaceous ages have been reported, which are distributed in Nang County (108－101 Ma)[66], Quxu-Nyimo area (102.2 Ma)[8], and North of Dagzhuka (108.6 Ma)[47]. However, Late Cretaceous granitoids are widely distributed and they are one of the first identified magmatism. Granitoids of this stage, including gabbro, gabbro-diorite, diorite, tonalite, granodiorite, and granite, have been widely reported in the middle segment of Gangdese batholith (from Milin to Xigaze)[8,47,66,67]. More recently, we discovered granites with middle Early Cretaceous (~120 Ma) and Late Cretaceous (79－72 Ma) ages in the east-


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ern segment of the Gangdese batholith, which correspond to the Early Cretaceous volcanic rocks (Sangri Group) (136.5 Ma)[21]. 3.3 Paleocene-Eocene granitoids (65－41 Ma) Schärer et al.[45] first demonstrated that Paleocene- Eocene is the period with the strongest magmatic activity of Gangdese batholith. For example, more than half of the collected age data (i.e., 65 out of 129) falls into this stage. These data display a peak of ~50 Ma[8,47] (Figure 2), during which basic magmas were intensively developed[50,51]. The rock assemblage of peak magmatism was highly variable, including gabbro, gabbro-diorite, monzodiorite, monzonite, quartz monzonite, diorite, tonalite, granodiorite, and granite. Volcanic rocks were also extensively erupted associated with this peak magmatism. The well-known Linzizong volcanic rock in the southern Lhasa terrane during 65―41 Ma was contemporaneous with the Paleocene-Eocene granitic magma[23,24,26,27] tism . 3.4 Oligocene-Miocene granitoids Oligocene-Miocene developed another important stage of magmatism in the Gangdese batholith, involving monzonite, quartz monzonite, granodiorite, and granite. They are outcropped as dike swarms or small-volume plugs that crosscut or intrude the Gangdese batholith. Due to their association with porphyry Cu (Au) deposit, such as Dongga, Chongjiang, Tinggong, Qulong and Jiama[64,65], granites of this stage have been well stud－－ ied[47,62 64,68 70]. Harrison et al.[48] first identified Yaja granite with an age of 30.4 Ma in northeast of Zêtang[48]. Chung et al.[62] found granites of this stage have adakitic affinities and suggested they belong to collision-type adakite. They also divided it as a post-collisional magmatic stage, i.e., Oligocene-Miocene (26―10 Ma). Based on the summary of Miocene magmatism from some deposits in Gangdese batholith, Hou et al.[63] argued the peak stage was during 18－10 Ma. More recently, Ji et al.[47] reported a monzogranite of 32.5 Ma from Qulin, southeast of Nyêmo, and suggested the magmatism of this stage was commenced from early Oligocene. The geochronological results of Gangdese batholith were restricted to the samples collected from surface granitoids. To better reveal the magmatic history of

Gangdese batholith, we have combined the ages of detrital zircon from Xigaze Group (Figure 2)1). The results showed that two obvious peaks, Late Jurassic (170－150 Ma) and Early Cretaceous (130－100 Ma), existed in the Mesozoic detrital zircons. Most zircons display characteristics similar to zircons from Gangdese batholith[47], i.e., high 176Hf/177Hf ratios, positive εNd(t) values and young model ages, which suggested part of Mesozoic igneous rocks were eroded and transported to Xigaze fore-arc basin and became provenance of Xigaze Group[28,29]1). According to the geochronological framework of Gangdese batholith and the ages of detrital zircons from Xigaze Group, the magmatism of Gangdese batholith can be divided to the following stages: (1) Mesozoic magmatism, from Late Triassic to Late Cretaceous, with two peaks in Late Jurassic and Early Cretaceous; (2) Paleocene-Eocene magmatism (65－41 Ma), which is the most prominent stage; And (3) Oligocene-Miocene magmatism (33－13 Ma).

4 Geochemistry of granitoids from the Gangdese batholith Gangdese batholith is a composite batholith and comprises magmatic rocks ranging from mafic to felsic composition. To disclose the assemblage, petrology, geochemistry, and the nature of magmastism in different times, here we summarize the published geochemical data of granites with zircon U-Pb ages. 4.1 Major elements Most granitoids fall into subalkaline area in the TAS diagram (Figure 3(a))[74], and belong to medium-K and high-K calc-alkaline series in the SiO2-K2O diagram (Figure 3(b))[75]. Some Miocene samples from Jiama deposit have very high potassium contents and belong to shoshonitic series[63]. Except for several highly evolved samples from Qulong deposit[73], most Mesozoic granites contain low potassium and belong to medium-K calc-alkaline series. Especially, the Late Cretaceous samples are obviously enriched in sodium, having Na2O/K2O ratios ranging from 1.24 to 3.13 with most of them above 2. Potassium contents of Paleocene-Eocene samples are higher than those of the Mesozoic samples but lower than those of the Oligocene-Miocene samples.

1) See 1) on page 1242. 1244


Figure 3 (a) TAS diagram (from [74]) and (b) Si2O-K2O diagram (from ref. [75]) of granitoids from Gangdese batholith. Major elements data are from refs. [7, 48, 61－63, 67, 71, 73, 76－78] and unpublished data.

Most Miocene porphyrites with high potassium contents belong to high-K series, and some of them belong to shoshonitic series. However, they also contain high sodium contents and have Na2O/K2O ratios of 0.5－2 with most larger than 1. The potassium contents of granitoids increase from Paleocene-Eocene to Oligocene-Miocene. As a whole, they are relatively enriched in sodium. According to the Aluminum Saturation Index, Gangdese granitoids mainly belong to metaluminous and weakly peraluminous group, with some being strongly peraluminous (A/CNK>1.1) (Figure 4). However, all of the samples are hornblende-bearing granitoids. There-

Figure 4 A/CNK-A/NK diagram of granitoids from Gangdese batholith. Data sources are the same as Figure 3. A/CNK: Al2O3/(CaO+Na2O+K2O) (molar ratio); A/NK: Al2O3/(Na2O+K2O) (molar ratio).

fore, they are I-type granites rather than S-type[13,79]. Very high A/CNK values in a few samples might be due to analytical reasons. As shown in Figure 4, granitoids in different periods fall into different areas. Mesozoic samples all belong to peraluminous group and Paleocene-Eocene samples are mainly metaluminous, whereas Oligocene-Miocene samples fall into the both areas. This may be attributed to the evolution of the magma, i.e., the aluminum contents of magma increasing along with the crystallization and fractionation of hornblendes. 4.2 Trace elements As shown in Figure 5, both rare earth element (REE) and trace element patterns of granitoids with different ages from Gangdese batholith indicate that all the samples bear an arc magma affinity. Furthermore, previous studies have suggested that there were many stages of granites possessing adakitic characteristics. Chung et al.[62] first recognized that the Miocene porphyrite bear adakitic characteristics, which was confirmed by later studies[63,76,77]. Recently, Wen et al.[67] found some Late Cretaceous granodiorites (83－80 Ma) also have adakitic characteristics. The trace element geochemistry of granitoids with different ages from Gangdese batholith can be summarized as follows: (1) Late Triassic-Jurassic granitoids contain moderate contents of REE and other trace elements. Except for the different Eu anomalies, their REE patterns are consistent (Figure 5(a-1)). Porphyrites from Qulong deposit are the


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Figure 5 (a) Chondrite normalized REE patterns; (b) primitive mantle normalized trace element spidergrams of granitoids from Gangdese batholith. Chondrite and Primitive mantle normalization data are from refs. [80, 81], respectively. Data sources are the same as Figure 3.

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only samples with obvious negative Eu anomaly, whereas samples from Nyêmo bridge and North of Dagzhuka show weak negative Eu anomaly or positive anomaly[7] (unpublished data). Samples of this period display characteristics similar to arc calc-alkaline magmatic rocks (Figure 5(b-1)), i.e., enriched in large ion lithophile elements (LILEs: Rb, Ba, Th, U, K) and light rare earth elements (LREEs: La, Ce) but relatively depleted in high field strength elements (HFSEs: Nb, Ta, Ti). Furthermore, some highly evolved samples show obvious negative anomalies of Sr, P, and Eu. Depletion of both Sr and Eu can be explained either by the fractional crystallization of plagioclase or the residue of plagioclases in the source, while the depletion of P might reflect the crystallization of apatites. (2) Up to now, geochemical data of Cretaceous granites have been rarely reported. Wen et al.[67] suggested that granodiortes of ca. 80 Ma from Nangxian-Lilong bear adakitic affinities. Recently, Ji et al.[47] reported a Hb-diorite of Early Cretaceous (06FW170: 108.6 Ma) and a granodiorite of Late Cretaceous (06FW114: 86.4 Ma). The Hb-diorite sample is slightly differentiated in REE and has high contents of heavy rare earth elements (HREEs), whereas the granodiorite sample is highly differentiated in REE and contains low contents of HREEs. The latter bears the characteristics similar to the adakitic granodiorites reported by Wen et al.[67]. As shown in the spider diagram (Figure 5(b-2)), all samples are enriched in LILEs and LREEs but depleted in HREEs and HFSEs, such as Nb, Tb and Ti. Furthermore, the adakitic samples have lower contents of HREEs and Y. However, the Cretaceous samples do not have negative anomalies of both Sr and Eu but with weak negative P anomaly. (3) Both rock associations and trace elements of Paleocene-Miocene intrusive rocks are highly variable. However, they display consistent REE patterns and weak negative Eu anomalies (Figure 5(a-3)). They are also enriched in both LILEs and LREEs, but depleted in HFSEs, with clear negative anomalies in Nb, Ta and Ti, and positive Pb anomaly in the spidergram (Figure 5(b-3)). (4) Geochemical compositions of Oligocene-Miocene samples have been widely studied. They all possess adakitic characteristics, such as high differentiated in REEs, low content of HREEs (Yb, Lu), and Y, high Sr/Y and La/Yb ratios (Figure 5(a-4), (b-4)). This stage of magmatism started at 32.5 Ma (06FW157)[47], and this sample also exhibited adakitic affinity (unpublished data).

All samples in this stage show similar Eu anomalies, i.e., weak negative Eu anomalies or positive Eu anomalies, but their trace element compositions are distinctly different, such as Sr and REE[64,77]. 4.3 Nature of the magmas in Gangdese batholith As inferred from the previous studies, the Gangdese batholith has the following characteristics: (1) The Mesozoic intrusive rocks consist of various lithologies from mafic to felsic rocks rather than only highly evolved rock like granites as previous thought[3]. Both rock assemblage and trace element compositions of the Mesozoic rocks are similar to arc calc-alkaline rocks. Furthermore, some Late Cretaceous rocks bear the adakitic affinity; (2) The Paleocene-Eocene intrusive rocks have compositions ranging from mafic to felsic. They belong to medium-K and high-K series, with rock assemblage and geochemical composition of arc affinity; (3) Most of the Oligocene-Miocene intrusive rocks are intermediate-felsic and belong to medium-K and high-K series. They also have adakitic affinity. However, some samples have obviously high potassium contents and are consistent with high-K calc-alkaline series. Kay and Kay[82] and Mantle and Collins[83] suggested that the differentiation degrees of the igneous rocks reflect their source depths, i.e., the crustal thickness at the formation of the granites. Experimental study has suggested that depletion of both Nb and Ta in the arc magmas requires the rutile residual in the source, i.e., the formation depth should be larger than 50 km[84]. Comparisons of trace element compositions and the depletion degree of both Nb and Ta in granitoids of different ages from the Gangdese batholith suggest that the crustal thickness increased from Late Triassic to Cretaceous (205－80Ma) but decreased before Paleocene-Eocene. However, the crust was significantly thickened during Miocene.

5 Source characteristics of granitoids from the Gangdese batholith 5.1 Sr-Nd isotopes Both Rb-Sr and Sm-Nd systematics are commonly applied to constrain the source characteristics of granites, as old crustal materials always show higher 87Sr/86Sr ratios, lower 143Nd/144Nd ratios and older Nd model ages than the juvenile crust[85]. This inverse Sr-Nd correlation makes it an effective method to trace the magma source and crustal evolution.


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The early studies of Sr-Nd isotopes of the Gangdese batholith commenced in the 1980s, when the Rb-Sr isotopes were utilized to study the various rocks from Quxu － batholith and Lhasa pluton[13,38,39, 86 88]. It has been showed that the granitoids possess low 87Sr/86Sr ratios, which show no relationship with their SiO2 contents. This suggests that they were mainly derived from juvenile crustal materials. Although some advancement on Sr-Nd isotopes of the Gangdese batholith has been achieved in recent years, newly reported data are still rare. The summary of Sr-Nd isotopes of granitoids from Lhasa terrane[3] shows that the Gangdese batholith displays positive εNd(t) values (+1.64―+5.21) and has young model ages (50 Ma[124].


Based on the above analysis of multidisciplinary evidence, the time of India-Asia collision was focused on 55±5 Ma. However, Aitchison and his group insisted that the collision happen at the end of Eocene (ca. 35 － Ma)[125 129], although they made many hypotheses to sustain their model, it was irrational and lack of credible evidence[12,130]. 6.2 Petrogenesis of the Mesozoic magmatic rocks The facts that no magmatism in Gangdese batholith developed in the early stage of Cretaceous (>109 Ma) but magmas of this time are extensively distributed in the － north Lhasa terrane[7,14,46,131 133] lead some researchers to explain it as a result of the low angle subduction of － Neotethyan slab[134 136]. Recently, it was indicated that the dating results of detrital zircons from Xigaze fore-arc basin were distributed in the whole Mesozoic with two obvious peaks of 170－150 Ma and 130－100 Ma1). These zircons possessed high 176Hf/177Hf ratios, positive εNd(t) values, and young model ages, resembling the characters of zircons from granitoids in Gangdese batholith, which implied that Gangdese batholith had served as important source of Xigaze Group in Cretaceous with many Mesozoic magmatic rocks having been eroded and deposited in Xigaze fore-arc basin[28,29]1). The extensive lack of Cretaceous strata at southern margin of Lhasa terrane, except for Lhasa-Linzhou area, indicated these places had been strongly uplifted and eroded in Cretaceous[133]. The scarcity of Pre-Cretaceous granitoids may be attributed to this as well. Combined with the newly reported dating result about Sangri Group volcanic rocks in the early stage of Early Cretaceous (136.5 Ma)[21], and our unpublished data about Early and Late Cretaceous granites (120 Ma and 79－72 Ma), it was proved that Mesozoic magmatism of southern Lhasa terrane was successive from Late Triassic to the latest Cretaceous. As shown in Figure 6(b-1) and (b-2), the Mesozoic granitoids of Gangdese batholith bear arc affinity, and are enriched in LILEs (Rb, Ba, Th, U and K) and LREEs (La and Ce), but relatively depleted in HFSEs (Nb, Ta and Ti). It becomes a consensus that Cretaceous granitoids of Gangdese batholith was produced by the northward subduction of Neotethyan slab[8,45,134]. However, it is still controversial on whether the earlier Mesozoic

granitoids were produced either by earlier Neotethyan subduction[7,47,71] or by the southern subduction of Bangong-Nujiang oceanic slab[4,16]. The temporal and spatial distribution of Mesozoic magmatic rocks in Lhasa terrane[5] is more likely related to the northward subduction of Neotethyan slab. The earliest Mesozoic magmatic rocks have been identified in Lhasa terrane are: (1) Luozha granitoids from Namling area, including megaporphyritic granodiorite (zircon TIMS U-Pb age: 217 Ma)[137], two-mica granite (205 Ma) and granodiorite (202 Ma) by zircon LA ICP-MS U-Pb method[138]; (2) Menba biotite monzogranite (zircon SHRIMP U-Pb age: 207.5 Ma) and Zhongda Bt-Hb-granodiorite (Hb 40Ar/39Ar plateau age: 215.2 Ma)[139]; (3) granites from North of Namling (zircon LA ICP-MS U-Pb age: 220-200 Ma) (unpublished data). These samples are distributed in south margin of Gangdese back-arc fault uplift, with a distance of 100－ 150 km from Yarlung Tsangpo suture and 200－250 km from Bangong-Nujiang suture. Given the crustal shortening, the distance from north suture should be even larger, so it is more likely related to the northward subduction of Neotethyan slab. Furthermore, many Late Triassic-Early Jurassic granitoids[7,47,73,138] and coeval volcanic rocks of Yeba Formation (190－174 Ma)[17,18] are developed along the southern margin of Lhasa terrane. The Early Jurassic magmatic rocks were mainly distributed in south and middle Lhasa terrane, but rare in the northern Lhasa terrane. Most of the earliest magmatic rocks in northern Lhasa terrane are of Middle Jurassic and Early Cretaceous[5]. The dating results of detrital zircon from Xigaze Group indicate early Mesozoic magmatism was also developed in the Gangdese batholith1). In conclusion, the temporal and spatial distribution of magmatic rocks in Lhasa terrane reflects that the magmatism became younger from south to north, which was consistent with the characters of active continental margin, i.e., Mesozoic magmatism of Lhasa terrane was related to northward subduction of the Neotethyan slab. 6.3 Petrogenesis of the Paleocene-Eocene magmatic rocks The Paleocene-Eocene granitoids comprise the majority of Gangdese batholith[8,47]. Their similarities to Mesozoic

1) See 1) on page 1242.


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granitoids lead some authors to suggest that they resulted from the continuous subduction of Neotethyan slab[1,45]. Analysis of relevant data about India-Asia collision suggests that the initial collision between the Indian and the Asian plates more likely happened at ca. 55±5 Ma. Therefore, Miocene granites with arc affinity cannot be attributed to Neotethyan subduction, and the magmatism with arc signatures also does not suggest the ongoing of subduction. As to the coeval of the Paleocene-Eocene magmatism with the India-Asia collision, two opinions existed for its origin. Based on the study of Lizizong volcanic rocks, Mo et al.[23] argued this magmatism was formed in syn-collision setting and the coeval granitoids belonged to syn-collision granitoids[3,26]. The other argument is that the magmatism resulted from a composite geological process, including Neotethyan slab subduction, rollback, and subsequent breakoff[8,25]. As noted above, the temporal and spatial distribution of Mesozoic magmatic rocks should be attributed to north subduction of Neotethyan slab. The Mesozoic magmatism of north Lhasa terrane was focused on 130－110 Ma with a peak of ~110 Ma, and ceased at the latest Early Cretaceous[5,140,141]. Recently, Kang et al.[142,143] obtained some ages of middle Early Cretaceous from Zenong Group (113.6 Ma) and Duoni Formation (116－115 Ma), which were extensively distributed in north-middle Lhasa terrane. Currently, ages of Late Cretaceous magmatism in northern Lhasa terrane are still rare in the literature. The magmatic activities migrated southward and were concentrated in southern Lhasa terrane in Late Cretaceous[8,47]. Two processes occurred during the subduction would impede the magmatism above the front of slab. Firstly, continuous dehydration decreases the water content in the front slab. Secondly, the changes of lithologies and mineral phases during pressure increase will increase the density of subducted slab, which decreases the buoyancy force and accelerates the sinking of the slab. This is more obvious in the leading end and steepens the subducted slab. These processes are consistent with the transfer of magmatic activities. Based on the distribution of volcanic rocks in Lhasa terrane, Lee et al.[27] argued that the southward migration of magmatism reflects the slab rollback, which has been proposed by Chung et al.[60] and Wen et al.[8] to emphasize the magma gap of 80－65 Ma. Rollback of the Neotethyan slab would change the 1252

asthenospheric corner flow of the mantle wedge, which results in the migration and intensification of the magmatism towards the trench. This is supported by the facts that Paleocene-Eocene granitoids in Gangdese batholith and coeval Linzizong volcanism were very intense and were only distributed in a narrow belt along the southern margin of Lhasa terrane. After the India-Asia collision, the subducted oceanic slab would be detached from the underthrusting continental slab due to gravitational drag, which leads to the upwelling of the asthenosphere. The anomalous heat would result in large scale partial melting of different sources involving lithospheric mantle and crust, and produce magma with heterogenous chemical and isotopic compositions[144,145]. Both magmatism of Gangdese batholith and Linzizong volcanism were coevally culminated at 50 Ma[8,25,27,47], and the volcanic rocks of Pana Formation, Linzizong Group, have various rock types and variable Sr-Nd isotopic compositions[25]. Furthermore, granitoids of 50－40 Ma in Gangdese batholith also possessed a large range of Hf isotopes, and in some granites zircons exhibited negative εHf(t) values[7,47,91] (Figure 7), which reflected the extensive melting of magma source with some old crustal materials. Because the crust and lithospheric mantle of Gangdese batholith have identical depleted Hf isotopic composition, no obvious change in the magmatic Hf isotopes is expected, let alone the negative εHf(t) values. Thus, the old crustal materials should come from the subducted Indian continent, which suggests the India-Asia collision was initiated before 50 Ma[47]. 6.4 Petrogenesis of the Oligocene-Miocene magmatic rocks The Oligocene-Miocene magmatism was the latest magmatic activity of Gangdese batholith, which resulted in the adakitic porphyrite in a post-collision set－ ting[62,63,76,77,146 150]. These magmatic rocks are highly evolved felsic rocks, including tonalite, granodiorite, quartz monzonite, and granite, and have variable K2O contents ranging from medium-K calc-alkaline series to shoshonitic series (Figure 4). They have a large scale of Sr-Nd isotopes, especially the samples from Jiama deposit and west of Xigaze, which display mixing trends with enriched component. Their small variation in Hf isotopes may be due to the lack of data from Jiama deposit and west of Xigaze. These adakitic rocks might assimilate some enriched materials possessing high potassium contents, which is more obvious in the sho-


nitic rocks from west Gangdese batholith[151]. Therefore, Oligocene-Miocene magmatism should be derived mainly from partial melting of a depleted source, i.e., the source of Gangdese batholith, with various assimilations of enriched lithospheric mantle, i.e., the Indian continent. Source material and heat are the two major factors controlling the generation of Oligocene-Miocene magmatism. Based on the geochemistry of magmatism, some researchers emphasized that the magma source had been metasomatized by subducted materials[76,77]. Several possible mechanisms have been proposed to provide the heat, such as slab breakoff[144], lithospheric delamination[152] or convective removal[153]. Some authors have discussed the probability that the Oligocene-Miocene magmatism was controlled by the breakoff of Neotethyan subduction[154,155]. In Oligocene-Miocene, however, the subduction was terminated and the Tibetan Plateau was under a post-collision setting. Therefore, the Oligocene-Miocene magmatism is unlikely related to the Neotethyan subduction. Furthermore, slab breakoff, being a catastrophic event, would result in intense magmatism in a short time, and the magmatic activities should be concentrated in a narrow belt along the trench[142], as the Paleocene-Eocene magmatism in the Gangdese batholith. Both delamination and convective removal of thickened lithosphere have been commonly invoked to explain the magmatism － in Oligocene-Miocene[60,62,144 147,156]. The delamination is also a catastrophic event, which would lead to strong magmatic activities immediately. Considering the small volumes and long duration of the Paleocene-Eocene magmatism, we favor the model of convective removal － of lithosphere[144 147,153]. This model also can interpret the following characteristics, (1) the adakitic geochemical characteristics due to the thickened lower crust, (2) the depleted isotopic composition of the Gangdese batholith. The potassic-ultrapotassic magmas, derived from the enriched Indian lithosphere, provided heat, some potassium and enriched components, which could match the geochemical characteristics of the porphyrites. However, Maheo et al.[157] argued that the narrow belt of post-collisional porphyrite in the southern Lhasa terrane cannot be explained by either the delamination or convective removal. We suggest that both the potassic-ultrapotassic magmas and the coeval porphyrite belt were formed in a post-collisional setting, and their dif-

ferences are due to the diversity of source components, which is supported by the transitional characteristics in some adakitic porphyrites, and potassic-ultrapotassic rocks[76]. The distribution of the Paleocene-Eocene adakitic porphyrites represents the range of depleted source in Gangdese batholith, i.e., a young arc terrane that was accreted to the southern Lhasa terrane.

7 Implications 7.1 Evolutional history of the Neotethys and its subduction Since identification of the Early Jurassic arc granite (188.1 Ma) by Chu et al.[7], the initial subduction has been advanced to Early Jurassic from Cretaceous[45]. Recently, Ji et al.[47] found a successive magmatism from Late Triassic (205.3 Ma) to Jurassic, their arc signatures indicated that the Neotethyan subduction may have initiated at least in Late Triassic (205.3 Ma), which is supported by other lines of geological evidence, including Middle-Late Triassic radiolarian chert[158,159] and Late Triassic ophiolite[160]. In addition, a Late Triassic granitic belt developed to north of Namling. Although some granites are peraluminous with appearance of muscovite, most of them are I-type with rock types of granodiorite and monzogranite[137,138]. Combined with their distribution in an east-west belt to north of the Yarlung Tsangpo suture, it is suggested that they probably resulted from the northward Neotethyan subduction, which had begun at least in Early Triassic. During Late Triassic-Cretaceous (205－80 Ma), the southern Lhasa terrane developed a continuous magmatism, including the granitic intrusions in Gangdese batholith and coeval volcanism. Adakitic rocks have also been identified in this terrane, such as the Mamen andesites of Early Cretaceous (136.5 Ma)[21] and NangxianLilong granodiorites of Late Cretaceous (83－80 Ma)[67]. Zhu et al.[21] thought that the former was derived from partial melting of obliquely subducted Neotethyan slab, along with involvements of sediment and fluid, and subsequent hybridization by mantle wedge peridotite, whereas Wen et al.[67] argued that the latter Cretaceous adakitic granodiorites were attributed to Neotethyan flat-slab subduction, which also resulted in a magma gap of 80－68 Ma in the Gandese batholith[8,67]. It was previously suggested, however, that flat-slab subduction would squeeze out the mantle wedge, and thus generate


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adakitic magma in front edge above the subducted slab with a distance of >200km far from trench, such as demonstrated in Ecuador and Chile[161]. As Nang County-Lilong area is close to the trench, the adakitic rocks cannot be attributed to flat-slab subduction, even if it was proceeding at that time. Wen et al.[67] also proposed a complex history of the subducted Neotethyan slab during Late Cretaceous, including stable/oblique subduction (100－85 Ma), flat-slab subduction (83－80 Ma), rollback (~60 Ma), and subsequent breakoff (~50 Ma). Although this model could interpret the phenomena they observed, it was unreasonable that the subducting angle underwent repeated changes. Generally, magmatism would cease after flat-slab subduction[162]. Furthermore, the recently identified granitoids with age of 79－72 Ma challenged the magmatic gap of 80－68 Ma, hence the flat-slab subduction. In short, our observations suggest that the stable/oblique subduction of Neotethyan slab resulted in a successive magmatism in southern Lhasa terrane during Late Triassic-Cretaceous, and due to the increase of subducting angle, the magmatism ceased in northern Lhasa terrane from middle-late stage of Early Cretaceous (~110 Ma), then it moved southward and intensified. In Cenozoic, magmatism was concentrated in southern Lhasa terrane, and resulted from steepening of the subduction angle. After India-Asia collision, the Neotethyan slab broke off at ~50 Ma. This is the whole history of the Neotethyan slab, as presented in Figure 8. 7.2 Discrimination of arc magmatism Arc/Andes-type magmatic rocks, first recognized along the circum-Pacific subduction belt, generally possess enriched LILEs (Rb, Ba, Th, U and K) and LREEs (La and Ce), and relatively depleted HFSEs (Nb, Ta and Ti). These rocks are generally derived from partial melting of mantle wedge metasomatized by fluids from the dehydration of oceanic slab and sediments. Their arc-type geochemistry is inherited from the fluids that are enriched in LILEs and LREEs and depleted in HFSEs. Traditionally, the magmatism of Gangdese batholith, which exhibits arc geochemical affinity, was considered to represent the subduction of Neotethyan slab[44,163,164] and its termination was used to constrain the continental collision of India-Asia[1,114,115]. As Miocene granites also show arc geochemical characters, it is usually implied that the subduction should continue to Miocene. This conclusion, however, is evidently irrational. 1254

Figure 8 Schematic diagram of tectonic evolution of Gangdese batholith. (a) Late Triassic-Cretaceous: Stable and oblique subduction of Neotethyan slab and generation of Mesozoic granitoids. From Middle Cretaceous, due to tectonic uplift of southern Lhasa terrane, many materials were eroded from Gangdese batholith and deposited in Xigaze fore-arc basin. (b) 65－ 41 Ma: Late subduction, rollback and subsequent breakoff of Neotethyan slab and generation of Paleocene-Eocene granitoids. (c) 33－13 Ma: Extensive convective removal of thickened lithospheric mantle in an east-west extensional setting after India-Asia collision, and generation of adakitic rocks and coeval potassic-ultrapotassic magmatism.

Traditionally, Rb-(Y+Nb) (Figure 9(a)) and Ta-Yb (Figure 9(b)) diagrams were usually used to discriminate ocean ridge (OR), within plate (WP), volcanic arc (VA) and syn-collision (syn-COL) granites[165], especially for discrimination of VAG and syn-COLG granites[166]. In Figure 9(a) and (b), however, all spots fall into VAG fields, indicating that the above diagrams cannot discriminate the Mesozoic arc granites, Cenozoic syn-collision granites and post-collision granites. Therefore, the diagrams presented by Pearce et al.[165] may not be taken as an effective method to discriminate granites formed in different settings.


Figure 9 Discrimination diagrams[165] for tectonic setting of granitoids from Gangdese batholith. Data sources are the same as Figure 3. ORG, ocean ridge granites; syn-COLG, syncollision granites; VAG, volcanic arc granites; WPG, within plate granites.

To sum up, the arc characteristics of the Gangdese batholith cannot represent the subduction process. These characters can be generated in different settings. For example, if HFSE-rich minerals, such as rutile and ilmenite, remain in magma source during the partial melting, the magma would deplete these HFSEs (Nb, Ta, Ti and so on) and hence exhibit arc affinity. This understanding is important for the study of granites in other places. 7.3 Magmatic source of the Gangdese batholith and comparison to the ancient basement of the Lhasa terrane Based on the Sr-Nd-Hf isotopic data, the magmatic source of the Gangdese batholith is depleted and juvenile, whereas that of the northern granite is enriched and ancient. This isotopic boundary between two plutonic belts divides the Lhasa terrane into two parts along the Maila-Luobadui-Milashan fault[5,6]. Chu et al.[7] found that Wuyu granite had high εHf(t) (+10.2－+17.6) and εNd(t) (+5.5) values, and young Phanerozoic model ages, whereas adjacent northern granite belt possessed obviously low and negative εHf(t) (−13.7－−3.9) and εNd(t) (−9.4－−9.3) values, and Paleoprterozoic and Mesoproterozoic model ages. In the Namling area, from Gangdese batholith in the south to north granite belt in the north, the εHf(t) values changed abruptly from +9.7－ +13.7 to −10.6－−3.0 (unpulished data). Furthermore, other previously published data[3,55,57,167] also suggested that north granite belt shows negative εHf(t) values (−17.3－−3.4) and old model ages (2.5－1.0 Ga), such as Bange and Gazha plutons in the east, Jiangba and Bangba plutons in the west, and Nyainqentanglha granite and gneiss in the middle.

Zhu et al.[141] found that some magmatic rocks from the north Gangdese belt exhibited positive εHf(t) values and Neoproterozoic model ages, indicating that north Gangdese possessed obviously young basement than middle Gangdese and Gangdese back-arc fault uplift belt. We found in this study an abrupt change in the source characters between Gangdese batholith and north granite belt while the change was gradual between north and middle Gangdese belt. Therefore, the ancient Lhasa terrane, including middle Gangdese and Gangdese back-arc fault uplift belt, has Paleo- and Mesoproterozoic basement. Recently, eclogite (~262 Ma) with oceanic affinity has been found in Songduo. Based on the regional geology survey of 1:200000, several other kinds of rocks have been also identified, including the ultramafic rocks in the south, representing ancient ophiolite, and Permian arc volcanic rocks in the north[168,169]. These lines of evidence suggested an ancient suture near Songduo with polarity of northward subduction, along the MailaLuobadui-Milashan fault[5,6], which, also being a geochemical boundary, divided the Lhasa terrane into two parts. Therefore, the middle-north Lhasa terrane, including middle Gangdese and Gangdese back-arc fault uplift belt, was an ancient block possessing Paleoproterozoic and Mesoproterozoic basement[3,141], whereas the area occupied by the Gangdese batholith may be a young arc terrane of late Paleozoic which accreted to the ancient Lhasa terrane later[47]. There existed many lines of geological evidence supporting the above proposed model: (1) only Mesozoic strata developed in the southern Lhasa terrane, whereas the Paleozoic stata widely developed to the north of


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Maila-Luobadui-Milashan fault; (2) inherited zircons older than Paleozoic are rarely found in Gangdese bathoith, but the Pan-African and Proterozoic inherited zircons were abundant in the northern granite belt; (3) In the Gangdese batholith, crust-derived granites and mantle-derived mafic rocks have identical and depleted Hf isotopic composition, suggesting a young arc terrane formed in the late Paleozoic, much different from that of the middle Lhasa terrane; (4) Potassic-ultrapotassic volcanic rocks developed extensively in Lhasa and Qiangtang terranes, whereas the adakitic rocks only developed in Gangdese batholith, which was attributed to the diversity of magmatic source. Therefore, the distribution of adakitic rocks reflected the spatial extent of the accreted arc terrane. 7.4 Zircon U-Pb ages and Hf isotopes of the Gangdese batholith and their applications in sediment tracing Zircon U-Pb age and Hf isotopic data of granites from Gangdese batholith and adjacent areas are summarized in Figure 10. Gangdese batholith is characterized by positive εHf(t) values, except for some zircons of 50－40 Ma showing εHf(t) values around 0. This Hf isotopic feature is similar to that of the Kohistan-Ladakh batho-

Figure 10 Comparisons of zircon Hf isotopic compositions from granitoids in the Gangdese batholith and adjacent areas. Data of Gangdese batholith are from refs. [6, 47, 71] and 1). Data of Kohistan-Ladakh batholith are from refs. [170－172]. Data of Zayu-Dianxi-Burma batholith are from refs. [173, 174]. Data of north Lhasa granite belt are from ref. [7] and unpublished data.

lith, but obviously different from that of the north Lhasa granite belt and Zayu-Dianxi-Burma batholith, which possesses negative Hf isotopic composition. Furthermore, the geochronological framework of Gangdese batholith differs from that of the Kohistan-Ladakh batholith. These unique characteristics in both age and Hf isotopes could be used as an effective tracer to discriminate the provenance of sediments. For example, Wu et al.[95] used this method to study the sedimentary composition of Indus molasses, and revealed the India-Asia collision time and tectonic evolution in west syntaxis. Liang et al.[173] carried out studies on detrital zircons from Upper Miocene sandstone in Inner-Burma Tertiary basin, and disclosed the history of river captures in eastern Himalaya. Therefore, the unique zircon age and Hf isotopic compositions of granitoids in the Gangdese batholith can effectively fingerprint the tectonic uplift and erosion history of Tibet[95,173]2).

8 Conclusions Through the systematic summary of granitic rocks in Gangdese batholith, we have come to the following conclusions: (1) The complicated magmatic history of the Gangdese batholith can be divided into three stages, i.e., Mesozoic (Late Triassic-Cretaceous), Paleocene-Eocene (65－41 Ma), and Oligocene-Miocene (33－13 Ma). (2) Mesozoic magmatism was caused by the northward subduction of the Neotethyan slab, whereas the Paleocene-Eocene magmatism resulted from a composite process of Neotethyan slab, including its subduction, roll-back, and subsequent break-off. In contrast, the Oligocene-Miocene magmatism was related to convective removal of the thickened lithosphere in an east-west extensional setting. (3) The magmatic source of Gangdese batholith in southern Lhasa terrane has affinity to an arc terrane accerated in late Paleozoic, which possesses depleted isotopic composition, markedly different from the ancient basement of the Lhasa terrane. We thank staffs of the MC ICP-MS laboratory at institute of Geology and Geophysics, Chinese Academy of Sciences for their help during zircon analyses. Xianhua Li is appreciated for his kind invitation to write this paper, which is improved by constructive comments from three anonymous reviewers.

1) See 1) on page 1249. 2) See 1) on page 1242. 1256


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