Spatial Variation Of Zircon U-Pb Ages And Hf Isotopic Compositions Of The Gangdese Granitoids And Its Geologic Implications

The Himalayan-Tibetan orogenic system is the most distinctive landform on our planet. However, the geological evolution of the Tibetan plateau still hotly debated. The Tibetan Plateau has experienced the subduction of Tethyan oceanic lithosphere (involving the Proto-Tethys, Paleo-Tethys, Neo-Tethys) and the India-Asia continental collision, in the processes of which widespread magmatic activities occurred. In the Tibetan Plateau, the Lhasa terrane locating in the most south of the Asian continent is characterized by the widespread granitoids, which occupy 80% area of granitoid within Tibet. Especially, the granitoid magmatism is the most widesrpread in the Gangdese belt, the southern portion of the Lhasa terrane. The Gangdese belt formed as the result of subduction of the Neo-Tethyan oceanic slab and the India-Asia continental collision as well as post-collisional convergence, and thus can provide constraints on the evolution of the Neo-Tethys and the Tibetan uplift. In addition, the Gangdese belt is an important porphyry copper (molybdenum, gold) ore belt, which is either the product of magmatism, or closely related to tectonic-hydrothermal activity after magmatism. Therefore, the investigation of Gangdese magmatism could provide important constraint for the geological background of a large-scale mineralization, looking for mineral resources and the development of the national economy.In the paper, we present field geology, petrography, LA-ICPMS zircon U-Pb geochronology, geochemistry and Sr-Nd-Hf isotopic compositons for the Gangdese granitoids, with the aim of constraining the petrogenesis and origin of these rocks and the tectonic evolution of the Gangdese belt. Combining with published data, we compared the Gangdese granitoids with the the western Kohistan-Ladakh-Karakorum granitoids and the eastern Bomi-Basu-rawu-Chayu Granitoids, and discussed their affinity. The main research results are as follows:1. Forty-two granitoid samples from the Gangdese belt and the Middle Lhasa terrane were selected for in situ zircon U-Pb analyses. These ages range from～205 Ma to～12 Ma, with a peak age at～50 Ma. According to the zircon U-Pb and Hf isotopic data as well as previous study, the magmatic activities are divided into five intrusive stages:(1)205-202 Ma, (2)-178 Ma, (3)94-87 Ma, (4)68-40 Ma and (5)25-13 Ma. The most granitoids were collected from the Gangdese belt, except for two Late-Triassic granitoids, which were obtained from the Middle Lhasa terrane.2. The paper reports geochemistry and zircon Hf isotopic compositions for two Late-Triassic plutons (a two-mica granite and a granodiorite) from the Middle Lhasa terrane. The two-mica granite is strongly peraluminous, with A/CNK=1.16-1.20. The two-mica granite is characterized by enrichments of Rb, Th and U etc. They have Eu/Eu*=0.29-0.41, Rb/Sr=2.6～5.5 and Rb/Ba=1.1～1.3. Dated zircon Hf isotopic compositions exhibitεHf(t) values ranging from-12.4 to-1.8. The geochemistry of the two-mica granite is comparable to the Himalayan Tertiary leucogranites, suggesting that the magma of the two-mica granite was dominantly derived from partial melting of argillaceous rocks in crust. The granodiorite is metaluminous, with A/CNK=0.96-0.98. They display K2O/Na2O=1.42-1.77, Eu/Eu*=0.54-0.65 and (La/Yb)N=6.76-13.35.εHf(t) values from the dated zircons range from -8.2 to -5.5. The geochemical signatures and zircon Hf isotopic compositions suggest that the magma of granodiorite formed by partial melting of basaltic rocks in crust. The occurring of the strongly peralumineous granite reveals Lhasa crustal thickening prior to Late Indosinian, and gives an impelling evidence that the Lhasa terrane took place an Early Indosinian orogenic event.3. LA-ICPMS zircon U-Pb dating results show that the deformed granite in the southern edge of the Gangdese belt yield a magma crystallization age of～178 Ma. The deformed granites are high silicon calc-alkline series, with SiO2=73.38-76.06% and A/CNK=1.03～1.07. The deformed granite is characterized by low large ion lithophile element (LILE) contents and low high field strength element (HFSE) contents, indicating that the granite has an island-arc-type geochemical affinity. Zircon Hf isotopic compositions from the granite displayεHf(t) values ranging from +14.1 to +17.7, suggesting that the magma was derived from partial melting of juvenile crust. Based on the study of the Late-Triassic and Early-Jurassic igneous rocks and their petrogenesis in the Gangdese belt, the beginning time of the Neo-Tethyan oceanic slab subduction is not later than Early Jurassic. The Neo-Tethyan Ocean has long lasting tectonic evolution.4. The Gangdese batholith are composed mainly of the Paleocene to Eocene (68-40 Ma) granitoids, which include granite, granodiorite, quartz-monzonite, diorite, monzonite and gabbric diorite etc. These granitoids are dominantly high-potassic calc-alkaline series, and are metaluminous or weakly peraluminous with A/CNK=0.80-1.06. These granitoids are relatively enriched in Rb, Th, U and K, and show negative Nb, Ta, P and Ti anomalies, indicating that the granitoids have an island-arc-type geochemical affinity. They have ISr values ranging from 0.70369 to 0.70585 andεHf(t) values ranging from +5.6 to -4.1. Zircon Hf isotopic compositions from these granitoids displayεHf(t) values ranging from +14.7 to -6.4, suggesting that the magmas were mainly derived from partial melting of juvenile crust with incorporation of the ancient crustal material deriving from the Middle Lhasa terrane. While the Neo-Tethys subduction-related granitoids in the Mesozoic distributed limitedly in the Gangdese belt, the granitoid magmatism during the Paleocene-Eocene has extended into the Middle Lhasa terrane, suggesting that a light and hot residual Neo-Tethyan ocean slab was underthrust beneath the Lhasa terrane with a low angle. Its dehydration and thermal effect caused the extensive magmatism in both the Gangdese belt and the Middle Lhasa terrane.5. The large-volume Pagu and Nanmuqie granitoids are firstly identified to be adakitic rocks, which are intruded by three adakitic porphyries in the Lhasa. LA-ICPMS zircon U-Pb dating for the Pagu granodiorite and Nanmuqie granite yielded identical magma crystallization ages of 14.0～14.4 Ma, which is indistinguishable from their associated adakitic porphyries (14.2～14.6 Ma). The granitoid was intruded at middle-crust depth, whereas the porphyry was intruded at upper-crust depth, indicating that the Lhasa terrane has experienced a rapid crustal uplift during the magma emplacement. Zircon Hf isotopic and whole-rock Sr-Nd isotopic compositions for these granitoids and porphyries suggest that their magmas were dominantly derived from partial melting of crustal materials. The granitoids and porphyries haveεHf(t) values overlapping with the mafic granulites in the Himalayan terrane (Indian plate). Their Sr-Nd isotopic compositions show two-endmember mixing between the Himalayan mafic lower crust and the ultrapotassic lavas/the Lhasa lower crust. We suggest that the adakitic magmas in the Lhasa terrane could be derived from partial melting of subducted Indian mafic lower crust with incorporation of the ultrapotassic lava and/or the Lhasa lower crust components. Our study suggests a new model for the adakitic magma generation in the Lhasa terrane and provides a line of geochemical evidence that the Indian continental crust was subducted beneath the southern Lhasa terrane in the Early-Middle Miocene.6. The evolutional history of the Gangdese belt is inferred from the granitoid magmatism, based on this study and published data. The Gangdese belt did not exist before opening of the Neo-Tethyan oceanic basin. The Late-Triassic to Early-Jurassic Neo-Tethyan oceanic subduction beneath the Lhasa micro-continental block (i.e., the Middle Lhasa terrane) yielded Yeba volcanic rocks and the contemporaneous granitoids in oceanic crust along south of the Lhasa micro-continental block. From Late-Jurassic to Cretaceous, the lasting oceanic subduction produced the Sangri volcanics and Late-Cretaceous granitoids in the Gangdese belt. Along with closing of the Neo-Tethyan oceanic basin during the Paleocene-Eocene, the India-Asia continental collision initiated. During the continental collision, light and hot residual Neo-Tethyan ocean slab was underthrust beneath the Lhasa terrane with a low angle, which induced the widespread magmatism in both the Gangdese belt and the Middle Lhasa terrane. Then the Indian continental margin began to be subducted beneath the Lhasa terrane with a slow subduction velocity and low subduction angle. By～25 Ma, the subducted felsic part was detached from the subjacent mafic part and lithosphere mantle of the Indian plate. The above felsic part was exhumed due to the buoyancy. This felsic part is equivalent to the Great Himalayan Sequence in the Himalayan terrane. The residual mafic part and lithosphere mantle became steeper from low-angle underthrusting to high-angle underthrusting, even forming a subvertical lithosphere slab. A consequence of this downwelling would be a deficit of asthenosphere, which should be balanced by an upwelling counterflow, and thus warm both the Asian mantle and the Indian mafic lower crust. It would induce partial melting of the Indian mafic lower crust to produce the pristine adakitic melt. During the magma migration and emplacement, the magma may incorporate the ultrapotassic magmas and/or the Lhasa lower crust materials.7. Zircons from granitoids in the Gangdese belt have dominantly high 176Hf/177Hf ratios, corresponding to positiveεHf(t) values and young Hf model age of 0.1 to 1.1 Ga, with a peak at～0.3 Ga. Among them, the Mesozoic zircons have homogeneous Hf isotopic compositions, withεHf(t)=+17.7～+9.5. However, the Cenozoic zircons have heterogeneous Hf isotopic compositions (εHf(t)=+14.7～-6.4). By comparison, zircons from granitoids in the Middle Lhasa terrane display dominantly negativeεHf(t) values (-14.2～+6.0). Their Hf model ages are mainly Paleoproterozoic and early Mesoproterozoic (0.8～2.1 Ga), with a peak at～1.6 Ga. Therefore the Hf isotopic compositions of zircons for granitoids in the Gangdese belt are significantly different from those in the Middle Lhasa terrane, indicating distinct sources. In addition, the Middle Lhasa terrane has widespread magmatism during Early Cretaceous, which is also different from the Gangdese belt. Either zircon U-Pb ages or Hf isotopic compositions of granitoids in the Kohistan-Ladakh island arc terrane and the Karakorum terrane are comparable to the Gangdese belt and the Middle Lhasa terrane, respectively. It suggests that the western Kohistan-Ladakh island arc terrane could be correlated with the Gangdese belt, and the Karakorum terrane could be correlated with the Middle Lhasa terrane. In the east, the framework of zircon U-Pb ages for granitoids in the Bomi-Basu-ranwu-Chayu area and Gaoligong-Tengchong-Yingjiang area is indistinguishable from that of the Middle Lhasa terrane. The Mesozoic zircons display dominantly negativeεHf(t) values (+5.0～-15.0), corresponding to old Hf model ages (1.0～2.2 Ga) with a peak at～1.7 Ga, which are also consistent with that of the Middle Lhasa terrane. Both the U-Pb ages and the Hf isotopic compositions suggest that the eatern area could be correlated with the Middle Lhasa terrane. However, there are also a small amount of Cenozoic granitoids along the Indus-Yarlung suture. They have zirconεHf(t) values overlapping largely with the granitoids in the Gangdese belt. It may imply that the eastern area either appear the Gangdese belt, or only present the southern margin of the Middle Lhasa terrane.