The Remula region, located within the Cona Rift of the Himalayan orogenic belt, extensively exposes Miocene leucogranites. Geochemical characteristics indicate that the granites in this area show high potential for rare-metal mineralization. LA-MC-ICP-MS zircon U-Pb dating indicate that the granites crystallized at 16.86 +/- 0. 19Ma. Based on whole-rock major, trace and rare earth element compositions, the leucogranites can be classified into two types: Type A leucogranites and Type B Li-rich aplites. Type A leucogranites are characterized by: (1) high SiO2, Al2O,, K2O and Na2O, but low FeO, MgO, MnO, CaO and TiO2 contents; (2) elevated Rb but low Sr and Ba, together with high Rh/Sr ratios; (3) enrichment in LREEs and depletion in HREEs with a pronounced negative Eu anomaly; and (4) high initial (8)7Sr/Sr-86(1) ratios but low epsilon(ND) (1) values. The highly correlated variations of trace elements in Type A leucogranites suggest their formation was controlled by fractional crystallization of minerals such as plagioclase and mica from granitic melts. Compared to Type A leucogranites (Li = 64 * 10 (- 6) - 151 x 10 (-) 6) Type B Li-rich aplites (Li=141 x 10-(6) 970 * 10(-6) ) exhibit the following characteristics: (1) Relatively higher Al2O, and K2O, but lower SiO2, Na2O, FeO, MgO, CaO and TiO2 contents; (2) elevated Li, Ba, Rb, Sr contents and K/Rb ratios, but lower Rb/Sr ratios; (3) relative depletion in LREE with slight enrichment in HREE, and a weak negative Eu anomaly; and (4) markedly lower initial Sr/Sr(1) ratios but higher epsilon(ND) ((t)) values. The elemental and isotopic characteristics of Type B Li-rich aplites, combined with foamy textures in zireon cathodoluminescence images, indicate substantial metasomatic modification by K-rich hydrothermal fluids. Based on the geochemical features of Type B Li-rich aplites and published data from the Cuona area, it can be inferred that potassium-rich hydrothermal fluids derived from deep sources metasomatized the Miocene granites within the rift tectonic setting, leading to the formation of Li-rich aplites. This genetic model provides a new perspective for understanding rare-metal mineralization processes in the Himalayan orogenic belt.

Cenozoic leucogranites in the Himalayan orogenic belt represent the product of crustal analexis followed by various degrees of magma differentiation during the continental collisional orogeny. They provide a key litho-probe to unravel the tectonic-metamorphic-magmatic processes responsible for the construction of continental orogens. This paper focuses on the rutile- and andalusite-bearing leucogranites occurred in the Nyalam area of southern Tibet. Through detailed petrographie observations, zircon U-Pb geochronology, mineral trace element geochemistry, and whole-rock geochemical analysis, we constrain the genetic types of andalusite and rutile in these rocks and further discuss their physio-chemical formation conditions and regional tectonic dynamic setting. The results indicate that the Nyalam granite formed in the Early Miocene (ca. 17.5Ma and show high temperature (similar to 745 degrees C ) and. strongly peraluminous characteristics. Petrographic analysis reveals that the granular rutile in the leucogranite is of magmatic origin, crystallized directly from the magma, whereas the acicular rutile in the granite is a product of hydrothermal alteration of biotite. Euhedral andalusites in these leucogranites show prominent compositional zoning and well-developed reaction texture and suggest that they are of magmatic origin and crystallized at relatively high temperature conditions, These features observed in these Miocene leucogranites suggest that the Nyalam region was under an E-W extensional tectonic setting in the Early Miocene, possibly resulted from asthenospheric upwelling and heating. Such a process provided the crucial heat source for deep crustal partial melting and the crystallization of magmatic andalusite and rutile.

The Himalayan Cenozoic leucogranites record important information on the geochemical and tectonic physical effects of crustal partial melting and granite emplacement. Compared with the Eocene granites formed under thickened crustal conditions and Miocene granites formed during extensional process, the Oligocene granites are relatively distributed in a limited area, and there is still controversy about their sources and formation mechanisms. The Nyalam region is located in the central part of the Himalayan orogenic belt, where the 32. 1Ma tourmaline-biotite-bearing granite, the 29. 8Ma biotite-bearing granite, and the 26. 6Ma tourmaline-biotitebearing pegmatitic granite intruded into the upper part of the High Himalayan Crystalline Sequence. The three groups of Oligocene granites have the following characteristics: ( 1) high SiO2, Al2O3, K2O and Na2O contents, with A/CNK > 1. 0; ( 2) negative anomalies of Ba, Nb, Ta, Sr and Ti; (3) slightly enriched in light rare earths, depleted in heavy rare earths, highly variable Eu anomalies and weak negative Nd anomalies; (4) uniform initial Sr and Nd isotope ratio, with (87) Sr/ (86) Sr( t) =0. 7463 similar to 0. 7471 and eNd (t) = 15.0 similar to -14. 6. Furthermore, the tourmaline-biotite-bearing granites and pegmatitic granites have higher Sr and Ba and lower Rb/Sr ratios than the other one, and their Rb/Sr ratios are not correlated with Ba and Sr contents, which suggest that they are products of B -rich fluid -present partial melting of metamorphic sedimentary rocks. While the biotite-bearing granites have lower Sr and Ba contents and higher Rb/Sr ratios, and Rb/Sr ratio negatively correlated with Ba and Sr contents, which suggest that they are products of fluid -absent melting of metamorphic sedimentary rocks. Combined with the published research results in the Himalayan orogenic belt, it is concluded that the source of the Oligocene granites is transformed from lower crustal basic material to middle crustal metamorphic sedimentary rocks, demonstrating the process of upward migration of the source, in addition, the Oligocene granites undergo an important period of magmatic hydrothermal metasomatism. The Oligocene partial melting recorded the response of the deep crust during the transition stage from thickening to extensional collapse in the Himalayan orogenic belt and promoted the exhumation of the High Himalayan Crystalline Sequence.

High silica granites ( HSGs) are common to large granitic batholiths worldwide. It is important for understanding crustal anatexis during arc magmatism and the evolution of continental crust. Within the Gangdese batholith, field observations and zircon U-Pb dating reveal a suite of HSGs occurred in the Yangbajing area, which formed at 64.9 +/- 0. 3Ma similar to 64. 1 0. 3Ma. Here, the petrographic, mineralogical, whole -rock geochemical and isotopic composition were carried out on the Yangbajing granite, and the results show that: (1) it belongs to the high -K calc-alkaline series, with high content of SiO2 and K2O, but low MgO, MnO, CaO and P2O5, and a gull -like REE pattern, and strong negative Eu anomalies, and enrichment in K, Rb, Pb, Th and U, and depletion of Nb, Ti and P; (2) magmatic garnets with almandine and spessartite significant enrichment in HREE, Sc, Y, and Zn, attributed to strong fractional crystallization of plagioclase; (3) fractional crystallization of plagioclase with high An components has also resulted in the decrease of Al/NK ratio in the evolved magma, which increased the solubility of zircon; and (4) zircon Hf and whole rock Nd isotopic compositions show enrichment characteristics (epsilon(Hf)(t) = -17. 1 similar to -11. 7; epsilon(Nd) ( = - 9. 0 similar to - 7. 6), suggesting that the melting of ancient crust magmatic materials and then underwent strong fractional crystallization, leading to the formation of I -type garnet -bearing granite.

The Gangdese batholith in southern Tibet preserves various magmatic rocks that can be used to infer tectono-magmatic processes associated with the northward subduction of the Neo-Tethyan oceanic lithosphere and with the Indian -Eurasian continental collision. The granitic pluton in Taizhao area in this batholith consists of two groups of granites, i. e., Group I with high Na/K ratios and Group II with low Na/K ratios, respectively. Furthermore, a set of gabbro and diorite developed in Basum Co area, several kilometers to the east of Taizhao area. The Group II granites are treated as the main object in this paper to the studies for comparing with the other two ones. Zircon U-Pb dating results indicate that Group II granites formed at 190 similar to 194Ma, which is coeval with Group I granites. The Group II granites are characterized by slightly more enriched Hf (epsilon(Hf)( t) = 7. 2 similar to -2. 5) and Nd(epsilon(Nd)(t) = 4. 1 3. 7) and lower Sr (Sr-87/Sr-86( t) = 0. 7061 - 0. 7070) isotope compositions than those in Group I granites, which is similar to the isotope compositions of intermediate-mafic rocks. Such features indicate that Group II granites represent the product derived from fractional crystallization of intermediate-mafic magmas. The intermediate-mafic magmas originated from melting of melt -modified sub -arc mantle. The contents of SiO2 of Group II granites display negative correlations with Al2O3 CaO, TiO2, FeOT, MgO, Sr and Sc, but positive correlations with Na2O, Rb, Nb and Ta. These geochemical features suggest the important role of hornblende and plagioclase and potassium feldspar as fractional crystallization phases during the differentiation of more -primitive mafic magma and the formation of high -Si granites, respectively. Both of them possibly represent the contemporaneous magmatism during the northward subduction of the Neo-Tethyan Oceanic lithosphere.

The Kongbugang granite pluton is located within the Sangri-Cuona rift system in the eastern part of the Tethys Himalayan belt. It consists of various types of leucogranites, including two -mica granite, muscovite leucogranite, garnet -bearing leucogranite and leucosome. LA-MC-ICP-MS zircon U-Pb dating results show that the Kongbugang muscovite leucogranite was formed at 16. +/- 0. 1Ma. Geochemical analysis data indicate that the two -mica granites are Na-rich and peraluminous granites with high Sr (107 x 10(-6) -141 x 10(-6)) and Ba (230 x10(-6) -311 x10, but low Rb (108 x10(-6) -221 x10(-6)) and Rb/Sr ratio (0.78 -2.07). They are defined by nearly constant Rb/Sr ratio despite large variations in Ba concentrations, which is a distinctive feature of fluid -fluxed melting of muscovite. The muscovite leucogranites are K -rich and peraluminous granites with high Rb ( >270 x 10(-6)) and high Rb/Sr ratio (5. 2 -9. 5), but low Sr ( <58. 5 x10(-6)) and Ba ( <167 x10(-6)). The Ba contents are negatively correlated with Rb/Sr ratios, which is considered to be a typical characteristic of fluid -absent melting of muscovite. The garnet -bearing leucogranites have significant Ba, Th, Nb, Ta, P, Sr, Eu, Zr and Ti negative anomalies and highly correlated relationships between different trace element contents, which indicates that they are the product from granitic melt by fractional crystallization of zircon, plagioclase and mica. The leucosomes have high SiO2, K2O, TiO2, Ba, Nb, Ta, Zr, Hf contents, and enriched in light rare earth elements or heavy rare earth elements with significant positive Eu anomalies. During the formation of the leucosomes, inherited accessory minerals ( such as zircon, monazite, garnet and Ti -rich minerals) and peritectic feldspar were captured from the source rocks. Combined with the published data in the Cuona area, we can infer that the Miocene leucogranite in Kongbugang region is related to the initiation of the Sangri-Cuona rift. The fluid released from the Lesser Himalayan Sequence rose through the rift system and promoted fluid -fluxed melting of muscovite within the High Himalayan Crystalline Series. The initiated time of the Sangri-Cuona rift should be no later than 22. 5Ma.

A number of Cenozoic high slica leucogranites in the Himalayan orogenic belt are enriched in rare metals ( e. g., Nb, Ta, W, Sn, Be, Li) and have high potential to produce economic rare metal deposits. In addition to the Cenozoic granites, the Early Paleozoic granites are widely distributed and have similar geochemical characteristics to the Cenozoic granites. Whether they also have mineralization potential is an important question to be further explored. The Xiaru Gneiss Dome hosts dominantly Early Paleozoic granitic gneisses intruded by Cenozoic leucogranites and pegmatites. These granitic gneisses show similar geochemical characteristics to those in the Cenozoic leucogranites and can be subdivided into two groups in terms of their enrichment in Sn-W and Nb-Ta. Group -1 granitic gneisses are enriched in W and Sn ( W =5 x 10(-6) similar to 42 x 10(-6), Sn = 12 x 10(-6) similar to 35 x 10(-6)), whereas Group -2 enriched in Nb and Ta ( Nb =23 x10(-6) 108 x10(-6), up to 217 x10(-6) Ta =8 x10(-6) similar to 38 x 10(-6), up to 143 x10(-6)). Compared with Group -1 granitic gneisses, Group -2 ones are characterized by (1) higher Na20 but lower K-2 O, FeOT, TiO2, P2O(5) Sr, Zr; ( 2) slightly enrichment in MREE, but depletion in LREE and HREE with pronounced Eu negative anomalies; and (3) higher Nb, Ta, but lower W, Sn. The systematic relationship of elements suggests that (1) both groups of granite gneisses are the products from more primary magmas by different degrees of fractional crystallization of plagioclase, zircon, mica, and ( 2) Group -2 granites represent the most evolved melts. Both groups of granitic gneisses in the Xiaru Dome have Nb/Ta lower than 5, but Zr/Hf ratios in Group -1 are greater than 20 in contrast to less than 20 in Group -2. Therefore, we suggest that with fractional crystallization of granitic magma, when Zr/Hf ratios drop to 20, the melt structure changes substantially in conjunction with changes in the melt' s composition from potassium -rich to sodium -rich. Such changes further induce changes in the dissolution behaviors and subsequent enrichment of Sn, W, Nb, and Ta in the melts of granitic compositions as the magma differentiation proceeds. Results yielded in this study demonstrate that (1) both the Cenozoic granites and the Early Paleozoic granites within the Himalayan orogenic belt can enrich in rare metal elements, and ( 2) change of melt structure in the evolving granitic magma plays a key control on the enrichment of rare metals in granites.

藏南喜马拉雅造山带新生代高硅淡色花岗岩富集稀有金属元素(Nb、Ta、W、Sn、Be、Li等),成矿潜力大,是未来矿产勘探的重要靶区.除了新生代花岗岩,早古生代花岗岩分布广泛,具有和新生代花岗岩相似的地球化学特征,是否也具有成矿潜力,是有待深入研究的一个重要课题.夏如穹窿主体由早古生代花岗片麻岩以及侵入其中的新生代淡色花岗岩和伟晶岩组成,这些花岗岩具有与新生代高硅淡色花岗岩相似的地球化学特征,在Sn-W和Nb-Ta系统关系上,可以分成两组:一组富集W和Sn(W=5×10-6～42×10-6,Sn=12×10-6～35×10-6),另一组富集Nb和Ta(Nb=23×10-6～108×10-6,个别高达217×10-6,Ta=8×10-6～38×10-6,个别高达143×10-6).与富集W-Sn的花岗片麻岩相比,富集Nb-Ta的花岗片麻岩具有:(1)较高的Na2 O,为富Na花岗岩,(2)较低的K2 O、FeOT、TiO2、P2 O5、Sr、Zr;(3)略微富集MREE,亏损LREE和HREE,显著的负Eu异常;(4)较高的Nb、Ta,但较低的W、Sn.元素的系统关系表明,这两类花岗片麻岩都是较原始岩浆经历了不同程度斜长石、锆石、云母等矿物分离结晶作用的产物,富集Nb-Ta的花岗片麻岩分异程度最高.夏如早古生代两类花岗岩的Nb/Ta比值都小于5,但富集W-Sn花岗岩中Zr/Hf>20,富集Nb-Ta花岗岩中Zr/Hf<20.随着花岗质岩浆的分异逐步增强,当Zr/Hf=20时,熔体结构发生实质性变化,花岗质熔体从富钾质变成富钠质,从富集W-Sn变成富集Nb-Ta.本研究表明,在喜马拉雅造山带,不仅新生代花岗岩,而且古生代花岗岩都富集稀有金属元素,熔体结构的改变是控制花岗岩富集稀有金属的主要因素.

Documenting the occurrence of A-type granites is crucial to evaluating the nature and the mechanism for magmatism in extensional tectonic background. We present zircon U-Pb age, whole-rock geochemical and isotopic (Sr and Nd) data for Zhegu Co granodiorites in Tethyan Himalaya, South Tibet. The granodiorites were emplaced at 260. 3 +/- 2. 8Ma and intruded by a diorite with an age of 146Ma. The granodiorites are high-K, calc-alkaline, and metaluminous, with A/CNK and A/NK values in range 0. 74 similar to 0.76 and 1. 87 similar to 2. 72, respectively. They are enriched in light rare-earth-elements ( LREE) and depleted in heavy rare-earth-elements ( HREE), display negative Nb, Ta and Ti anomalies, and positive Zr and Hf anomalies, and ENd ( t) values of + 1. 1 to + 2. 3, characteristics of alkaline granite are also shown in petrography, all of which are similar to those in A2-type granites. However, the Zhegu Co granodiorites are distinct from typical arc magmatic rocks, and they are similar to magmatic rocks formed in extensional tectonic setting. The granodiorites are derived from partial melting of sediments or metamorphosed sediments modified by depleted mantle material. Based on these observations and combined with literature data, our new results suggest that the Late Permian felsic magmatism is related to the rifting of northern Gondwana and the opening of Neo-Tethyan Ocean.

The early Late Cretaceous magmatic rocks in the Gangdese batholith provide an important record on the nature of the NeoTethys Ocean subduction and the crustal growth in southern Tibet. Zircon U-Pb dating yields that diorite and granodiorite formed at 92. 4 - 86. 9Ma and granite emplaced in 91. 9 88. 6Ma, respectively. The diorite and granodiorite are high -K calc-alkaline aluminous rocks with relatively high zircon Hf ( ed,f ( t) = + 8. 3 - + 13.2, average + 10. 9) and whole rock Nd ( eNd ( t) = + 3. 2 + 2. 7) isotope compositions. They were originated from partial melting of mantle wedge metasomatized by fluids and melts derived from subducted materials. The diorites were derived from a shallower source metasomatised by subducted-slab-derived fluids, whereas the granodiorites from much deeper source metasomatised by sediment -derived melts. Granites can be divided into two groups according to their geochemical compositions. Group -1 is of low -potassium calc-alkaline series with high Na20/K20 > 3. 0, high REE contents, strong negative Eu anomalies ( Eu/Eu * = 0. 35 - 0. 40), but low Sr/Y ratios ( < 7. 2). Linear relationship in major elements of the granites with those in the diorite as well as in the granodiorite suggests that these granites might represent the derivative product from the intermediate -acid magma through fractional crystallization. Group -2 is of high potassium calc-alkaline series characterized by 1) low Na2 0/K2 0 ( < 1. 0) and relatively elevated aluminum saturation index ( A/CNK = 1. 01 - 1. 02); 2) low REE contents and weakly negative to no Eu anomalies ( Eu/Eu * = 0. 88 -1. 12); 3) high Sr/Y ratios ( 33. 8 55. 4) and the presence of hornblende; and 4) relatively low zircon Hf (eHf(t) = + 4. 1 + 10. 8) and whole rock Nd ( eNd (t) = + 0. 8) isotope compositions. Group -2 granites represents the melts derived from the newly formed lower crust. Combined with those literature data, our new data suggest that a major change in the geometry of subducted Neo-Tethys Ocean slab. High angle subduction in the Early Cretaceous was overtaken by slab retreat in the early Late Cretaceous, which resulted in intensified upwelling asthenosphere and melting of the mantle wedge metasomatized by melts/fluids. This process could produce an elevated amount of mafic magma which further induced partial melting newly formed lower crust and account for the formation of a spectrum of magmatic rocks of similar ages in the eastern Gangdese batholith.

In the Borong Gneiss Dome and Malashan Gneiss Dome within the Tethyan Himalaya, zircon U-Pb ages and whole-rock geochemical data demonstrate that at least two episodes of leucogranite formed at 19.8-19.4 Ma and 18.6-18.5 Ma, respectively. Each of them contains at least two suites of granite: two-mica granite and garnet-bearing leucogranite. Although the two types of granites are characterized by distinct element geochemistry and mineral compositions, they show similar Sr-Nd-Hf isotope ratios and regular variations in the compositions of whole-rock elements. From two-mica granite to garnet-bearing leucogranite, SiO2, Na2O, Rb, Nb, and Ta contents and Rb/Sr ratios increase; in contrast, Al2O3, CaO, MgO, FeO, TiO2, Sr, Ba, Eu, Zr, U, Th, light rare earth elements (LREEs) and Sc contents, as well as Zr/Hf and Nb/Ta ratios, decrease. Such systematic variations imply that two-mica granite and garnet-bearing leucogranite are cogenetic and that two-mica granite represents the more primary melt, whereas garnet-bearing leucogranite is a more evolved melt. During granitic magma evolution, fractional crystallization induces substantial changes in the melt structure and in turn major changes in the dissolution behavior of accessory phases (e.g., zircon, monazite, and apatite) and the geochemistry of key trace elements. Such changes might be the key factors that resulted in the subordinate W-Sn-Nb-Ta-Be anomalies in the more evolved granites, which implies that the Himalayan Cenozoic leucogranites have high potential to produce economic rare metal deposits.

Knowledge of the nature of the earliest metamorphism experienced by collisional orogenic belts is essential for reconstruction of tectonic processes that build high mountain chains and their environmental consequences. Understanding the metamorphic nature of Eohimalayan-phase orogeny of the Himalayan orogen, one of the typical examples of orogenic belts worldwide, could provide some important constraints to test different tectonic models (shallow continental subduction vs. slab breakoff) for the early phases of the development of large-scale orogenic belts. As exhumed middle-to lowercrustal rocks in the Kangmar gneiss dome, the garnet amphibolites with a protolith age of 176.4 +/- 3.6 Ma experienced a phase of metamorphism at 47.2 +/- 1.8 Ma with an increase in pressure as well as temperature from 3-5 kbar and 550-600 degrees C to over similar to 11 kbar and 650 degrees C. This suggests that the middle- to lower-crustal rocks experienced heating at least by similar to 50 degrees C while they underwent compression and thickening. Heat-flow estimation further demonstrates that the self-produced heat was high enough to achieve the observed pressure-temperature conditions recorded by the garnet amphibolite. Therefore, an additional heat supply is not required during early Eocene metamorphism. A breakoff of the leading part of the subducting Indian continental slab, if it occurred, should be younger than ca. 47 Ma.

Critical elements are the indispensable strategic resource for global high-tech industries, and the mechanisms for the enrichment and mineralization for such elements is currently one of the key areas in the international mineral deposit studies. In the Gyirong and Yadong areas along the Himalayan orogen, a systematic geochemical study of leucogranites indicate that the leucogranites which invaded the Southern Tibet Detachment System (STDS) contain a relatively elevated content of Sn, Cs, Tl, Be, W, B, Li and Bi. Whole -rock element geochemical data indicate that (1) these leucogranites are enriched in critical elements; (2) they represent the residual melts, which resulted from fractional crystallization of plagioclase, zircon, monazite, apatite, and mica of more primitive magmas; and (3) enrichment and mineralization of critical elements are closely related to the evolution of such highly fractionated granites. As magmas evolved into high Si02 and volatile magmas by intensive fractional differentiation, critical elements with a high fluid compatibility are enriched in the residual melt and might eventually form economic ore deposits. Due to geochemical similarity, Cs and Tl are incorporated into mica by replacement of K or Rb. These granites enriched with critical elements are syntectonic granites spatially and temporally associated with the STDS whose activities promote extensive fractional crystallization of the more primitive magmas and later enrichment of critical elements such as Rb, Cs, and Tl.

Syn-exhumation mafic magmatism during continental collision provides insights into the crust-mantle reaction during deep subduction and the nature of omgenic lithospheric mantle in collisional omgens. In this study, we present a comprehensive data set of zircon U-Pb ages and whole-rock major-trace elements as well as Sr-Nd-Pb isotopes of alkaline mafic rocks from the southern Liaodong Peninsula, eastern China. Zircon U-Pb analyses yield Late Triassic age of 213 +/- 3 to 217 +/- 3 Ma, younger than the Middle Triassic ultrahigh-pressure metamorphic rocks of the Dabie-Sulu orogen. Thus, the alkaline mafic rocks are products of syn-exhumation magmatism during continental collision of the South and North China blocks. The rocks show shoshonitic affinities with high K2O (3.78-5.23 wt%) and K2O/Na2O (0.71-1.22). They are characterized by arc-like trace-element patterns with enriched LILE, Pb, and LREE, and depleted HFSE. They exhibit enriched Sr-Nd isotopic compositions with high initial Sr-87/Sr-86 isotopic ratios of 0.7058-0.7061 and negative epsilon(Nd)(t) values of -13.0 to -15.1. These results suggest involvement of recycled continental crust in their mantle source. The mantle source likely formed by the metasomatic reaction of subducted continental crust-derived melts with the overlying subcontinental lithospheric mantle during the Triassic continental collision. Decompressional melting of this metasomatized mantle formed syn-exhumation mafic magmas during the transition from convergent to extensional tectonics in the Late Triassic. Accordingly, mafic rocks from the southern Liaodong Peninsula provide a geochemical record of the subduction and recycling of continental crust into the mantle and melt-mantle reaction induced metasomatism within the orogen.

Early Jurassic plutonic rocks from the Central Lhasa block that range from gabbro to granite yield similar Pb-206/U-238 ages ranging from 187.6 Ma to 196.0 Ma. Gabbros and diorites are characterized by similar radiogenic Sr-87/Sr-86(t) ratios that range from 0.7053 to 0.7060, unradiogenic epsilon(Nd)(t) values ranging from -4.2 to -2.5 and epsilon Hf(t) values ranging from -8.8 to -2.8. Compared with the intermediate to mafic rocks, the granites have much higher Sr-87/Sr-86(t) ratios, from 0.7102 to 0.7127, and lower epsilon(Nd)(t) values, from -8.7 to -7.5, and epsilon(Hf)(t) values from -11.9 to -7.5. Elemental and isotopic data suggest that the gabbros were derived from a melt-modified subarc mantle, diorites represent derivatives produced by hornblende fractional crystallization of gabbros, and granites were generated by the melting of ancient crustal basement rocks. Combined with literature data, these results suggest that the Central Lhasa block, as well as parts of the Southern and Northern Lhasa blocks, also contains ancient crystalline basement rocks. (C) 2021 Elsevier B.V. All rights reserved.

Scapolite-group minerals are widespread volatile -bearing aluminosilicates in granulite facies metamorphic rocks, but the formation mechanism and the behavior of scapolite is poorly understood. EPMA and TEM analyses of the scapolite and its exsolution lamellae from a mafic granulite in Dinggye area show that: (1) the scapolite is rich in S02,-; (2) the lamellae are troilite with (001) parallel to the host crystal (100); and (3) its unit cell of the lamellae is: a = b =0. 5968nm, c =1. 174nm, a = p =90, -y =120. The scapolite before exsolution formed in eclogite facies conditions and its upper limit of pressure is strongly controlled by Fe content. During later granulite facies metamorphism, S042- with these scapolites experienced a self-redox reaction to produce the lamellae plus 02 in a H20 -free condition, the reaction equation is described as follows: Fee{+SO4- =FeS +202. Similar reactions could lead to an elevated oxygen fugacity in the lower crust than in the middle and upper crust. The formation and breakdown of scapolite provide a valuable record on the deep processes during the tectonic evolution of the Himalayan orogen. In addition, these processes might play an important role controlling the geochemical behaviors of volatile components during the subduction and exhumation of continental orogenic belts. This exploits a new tool for the investigation of deep -Earth oxygen fugacity adjustment.

The subduction of the Neo-Tethyan oceanic lithosphere might experienced a major shift in the subdcution geometry in the Early Cretaceous time during its long-lasting subducion. Knowing the geochemical nature and the mode of the Early Cretaceous magmatism in the Gangdese batholith is critical to deccriminate the subdcution processes in this time. Data from zircon U-Pb geochronology and geochemical ( whole -rock element and isotope, zircon Hf) analyses on various rock types preserved in the Langxian complex show that: 1) the mafic rocks formed at 103. 6 - 100. 8Ma are of high -K calc-alkaline aluminous in composition. They are characterized by relatively low zircon e,(t) ( +0. 3 - +5. 7) and bulk eNd ( t) ( -0. 8 and -0. 3), indicating that they were drived from partial melting of depeled mantle wedge metasomatized by fluids from subducted sediment; 2) the intermediate rocks formed at 99. 8 - 97. 6Ma and have slightly higher Nd isotope compositions with eNd ( t) = +1. 1. They display a good linear relationship in major oxides with those in the mafic ones, suggesting that they are derivative products from the mafic suite; 3) the felsic rock ( pluton and dike) crystallized at 124. 1 -95. 3Ma. They could be subdivided into two types based on their isotopic compositions. The first type has a lower eNd ( t) ( - 8. 3 - 6. 0) but higher /Dm, (1385 -1586Ma). They formed by remelting of ancient crustal materials. In contrast, eidf(t) and eNd(t) in the second type granitic rocks vary widely with eHf ( t) ranging from -2. 8 to +3. 2 for the plutonic granite and from + 0. 4 to +8. 1 for the dike, respectively. Together with relatively low Nd isotope ( ENd ( t) = + 0. 1 and + 0. 8) and /Dm (428 906Ma), these characteristics suggest a relative young source region due to modification of ancient crustal materials by melts derived from fluid -enriched mantle; 4) enclaves hosted by the granitic rocks show good linear relationship in major oxides with the host rocks. They display a rather large shift of - 13 epsilon units in zircon Hf isotope compositions ( EHf ( t) = -9. 3 - +4. 1) and might represent the melts formed by various degrees of mixing an acidic end member of the hosted granite wih a mafic end member of hornblende gabbro. Combined with literature data of the Gangdese batholith, magmatic rocks from the Langxian complex preserved a key record of a major magmatic process during the Early Cretaceous ( 122 97Ma) Melting and mixing of various source components ( ancient crustal material, fluid -enriched mantle, and depleted mantle) is one of the important features of the Early Cretaceous magmatism due to subduction of the Neo-Tethyan ocean and resulted in large shifts ( up to - 13 epsilon units) in zircon Hf as well as in bulk Sr-Nd isotope compositions. To introduce various components into and achieve a large isotopic heterogeneity in the source region, it requires a major change in the subduction geometry in the long subduction of the Neo-Tethyan subdction system. Since the initation of the subduction at -240Ma, subdcition of the Neo-Tethyan Ocean might experinecd a critical reorganization enabling the introduction of an increased amount of sediments and fluids into the subduting system and the modification of the mantle wedge in the Early Cretaceous ( -120Ma). The mafic magmas caused remelting of the overlying ancient crustal material and finally formed the complex magmatic activity in the Early Cretaceous, which is likely to be a sign of the beginning of the second subduction of the Neo-Tethyan Ocean.

The Qulin pluton is located at the southern edge of the middle part of Gangdese belt. As an Oligocene-Miocene pluton, its main body is coarse-grained granite porphyry, which was later staged by lamproitic, granodioritic, and granitic dikes which are the products of multi-stage magmatism, with an exposed area of about 8km(2). The zircon U-Pb dating results of two samples of granite porphyry are similar to 30Ma. Whole rock geochemical studies show that granite porphyry is high potassium and quasi-aluminum, characterized by low MgO together with enriched light rare earth elements ( LREE) and large ion lithophile elements ( LILE) , depleted heavy rare earth elements ( HREE) and high field strength elements ( HESE). In addition, it has the characteristics of high Sr, Sr/Y, ( La/ Yb) (N) ; low Y and Yb, and weak Eu negative anomaly. A series of nearly N-S trending granodiorite porphyrite dikes occur in the pluton, among which the zircon U-Pb dating results of two samples are 15. 5 +/- 0. 1Ma and 14. 4 +/- 0. 1Ma, respectively. They have similar distribution patterns of rare earth and trace elements as the main body of pluton. And they are also rich in LREE and LILE, along with depleting HREE and HFSE. The Sr-87/Sr-86 ( (i )) values of granite porphyry range from 0. 706102 to 0. 706202, epsilon(Nd) ( t ) of which from -0. 6 to +0. 6, and zircon epsilon(Hf) ( t ) of which from +4. 9 to +7. 9; while for granodiorite porphyrite dikes, the Sr-87/Sr-86(( i )) values range from 0. 705429 to 0. 705474, epsilon(Nd) ( t ) from - 1. 4 to - 0. 2, and zircon epsilon(Hf) ( t ) from + 2. 6 to + 7. 6. The new data and literature data results show that : both the granite porphyry and granodiorite porphyrite dikes of the Qulin pluton origined from the partial melting of the thickened lower crust beneath Lhasa terrane, possibly related to the upwelling of asthenosphere induced by delamination of thickened lithosphere or tearing of the Indian lithospheric slab.

The Gangdese batholith consists dominantly of magmatic rocks with ages from Mesozoic to Cenozoic (ca. 230 similar to 10Ma), however, increasing number of studies have documented that magmatic suites of Late Paleozoic age could provide critical constraints on the tectonic processes in the Lhasa terrane prior to the subduction of Neo-Tethyan oceanic lithosphere. Zircon U-Pb dating yields that the protoliths of the granitic gneisses from Jiacha and Lang formed at similar to 344. 8Ma and ca. 362. 0 similar to 344. 0Ma, respectively, which bracket the range of timing reported in the literature. Combined those literature data, new geochemical data show that these Carboniferous granites are characterized by high SiO2 (67. 46% similar to 75. 33%), but low Al2O3 (12. 66% similar to 15. 82%), CaO (0. 79% similar to 4. 32%), FeO (0.48% similar to 3. 00%) and MgO (0. 28% similar to 1. 64%). Based on their K2O/Na2O ratios, they can be divided into two series : potassium-rich and sodium-rich, respectively. These rocks are characterized by (1) enrichment in light rare earth elements (LREE); (2) substantial negative Eu anomalies; and (3) depletion in Nb, Ta, and Ti, but no Zr and Hf negative anomalies. Geochemical and geochronological data suggest that: (1) the Carboniferous granites in the eastern Gangdese were formed in a back-arc extension setting, possibly related to the subduction of the Paleo-Tethys Ocean to the northern margin of Gondwana continent; (2) sodium-rich granites represent the differentiation products of the mafic parental magmas, whereas the potassium-rich ones from mixing of mantle-derived magma with middle-lower crustal; (3) this phase of magmatism lasted at least similar to 30Myr.

Layered migmatite formed by partial melting of amphibolites from mid to lower crust levels is widespread in the Namche Barwa. Titanite, as a key metamorphic residual and magmatic phase, occurs in the migmatite as well as in the leucosome, which provides a good opportunity to investigate the role of titanite in regulating the trace element compositions in the amphibolite-derived melts. Compared with migmatites, the leucosomes are depleted in elements such as Ti, V, REE, Y, Nb, Ta and U, which are enriched in titanite. Such characteristics indicate that the trace element compositions in the anatectic melts are mainly controlled by the geochemical behavior of titanite. During partial melting of amphibolite, due to low solubility of TiO2 in felsic melts, titanite grains commonly retain in the residue and in turn result in the depletion of those key elements compatible in titanite structure. The trace element partition coefficients between titanite and silicic melts are significantly affected by the melt's compositions. Residual metamorphic titanite with D-Nb/Ta <1 will result in higher Nb/Ta in melt than that in the protolith. However, when equilibrating with high Si-Al melts, magmatic titanite preferred Nb over Ta with D-Nb/Ta > 1. Separation of such titanite grains will decrease the Nb/Ta ratio in the residual melts. Data and results presented in this study demonstrate that titanite, similar to other accessory phases, could strongly affect the geochemical characteristics of melts produced by melting mafic rocks.