|Abstract:The Gangdese magmatic belt, located in southern margin of the Lhasa terrane, was related to the Neo-Tethys oceanic evolution and Asian-Indian collision. Therefore, the Gangdese magmatic belt is a still research focus of the Tibetan geosciences. However, some basic and essential issues have been under discussion and debated, such as the Triassic-Jurassic evolutionary characteristics or nature of the Neo-Tethys oceanic crust, formation ages and dynamic mechanisms of ductile shear zones and denudation and uplift models of the Gangdese batholith in southern Tibet. These issues cloud the understanding and tectonic framework of the Tibetan plateau and the Neo-Tethys oceanic evolution, especially in assessing accurately ore deposits and met-allogenic belt for the whole Gangdese magmatic belt.
A series of detailed field surveys were conducted in the southern region of the middle Gangdese batholith, leading to discoveries of the Triassic plutons, which were analyzed by means of petrology, mineralogy, geochemistry and zircon Lu-Hf isotopes as well as geochronology. Besides, the early-middle Jurassic granitoids were studied using petrology, geochronology and Lu-Hf isotopes. We also have selectively taken Eocene granitic samples from the middle Gangdese for geochemistry and geochronology analysis. About geological structures, research group conducted much work, including geological observations and experimental researches (thin section observation, 40Ar-39Ar dating, U-Pb dating and EBSD measurements) for the Xaitongmoin-Quxu and Sefu-Jigong ductile shear zones. Combining with published data of the fission tracks in the Gangdese belt, we supplemented new samples and obtained new data about apatite fission track. Based on those above-mentioned data and regional geological features, we try to establish tectonic framework and discuss evolutionary model in southern Tibet.
1. Mylonitic granite was generated in an active continental margin during 206~212 Ma, varying from 8.95 to 12.91 for zircon εHf(t) values in Kazi township of Namling county. Zircons from the hornblende gabbros yield crystallization age of ca.210Ma, and εHf(t) values show highly positive values from 9.56 to 14.75 as well as model ages exhibit tDM1 from 256~459 Ma. Zircons from Daga show that granites crystallized during 225~230Ma, showing a highly positive εHf(t) values (from 13.91 to 15.54) and relatively young model ages. The geochronology, petrology, minerology and geochemistry as well as Lu-Hf isotopes suggest that late Triassic granites and hornblende gabbros have a close relationship with the northward subduction of the Neo-Tethys oceanic crust, and further indicate that the hornblende gabbros might be generated by the hydrous partial melting of depleted mantle wedge matasomatized by fluids released from down-going oceanic slab. The granites were formed due to partial melting of juvenile crustal materials, and the mantle materials play a significant role in generating granites. These late Triassic magmatic events indicate that the northward subduction of the Neo-Tethys oceanic crust no later than the Norian stage of Triassic and did not terminate until at least the early stage of Cenozoic.
2. The diorite, granodiorite and granite from Numa and Nyemo areas in the middle Gangdese belt provide an excellent opportunity for further studying rock suites and interactions, by means of geochronology, geochemistry, minerology and Lu-Hf isotopes. The LA-ICP-MS U-Pb dating results demonstrate that the granitoid suites crystallized at 169~191Ma, belonging to the early-middle Jurassic magmatism. Chemically, granitoid suites are enriched in LREEs and LILEs, depleted in HFSEs, such as Nb and Ta et al, revealing a volcanic arc granitic feature. Additionally, granitoids belong to typical sub-alkaline and show dominantly medium K calc-alkaline series. The granitoid suites have highly depleted zircon Hf isotopic compositions, with εHf(t) values of 10.10~15.44. Those data reach a consensus that the early-middle Jurassic magmatisms have the same tectonic setting with the late Triassic, and strengthen the notion that the southern Tibet was typical of an active continental margin. And those granitoid suites were originated from middle-lower crustal partial melting.
3. The granitic complexes developed in the southern region of the middle Gangdese batholith provide a very valuable opportunity to study and assess Eocene magmatisms directly, with aim of supplying new geochronology and geochemical constraints on Asian-Indian collision. In this paper, we take typical samples and conducted experiments, including geochronology, petrology and Lu-Hf isotopes. The LA-ICP-MS U-Pb data demonstrate that granitoids crystallization ages range from 40 to 55 Ma. In geochemical aspects, the samples, enrichment of HREEs and LILEs and depletion of Nb, Ta and P, show a volcanic arc affinities. Moreover, analyzed samples, located in calc-alkaline and high K calc-alkaline fields, are dominantly by metaluminous nature and marked by I-type granitic characteristics. Dominantly positive εHf(t) values and geochemical compositional discrimination diagrams suggest that Eocene granitoids might be generated from partial melting of juvenile crustal materails, which include meta-tonalite partial melting and small amounts of meta-sandstone, with a little mantle materials injection. The Sr-Yb discrimination diagram show that the Eocene granitoid samples plot into collision fields, indicating the collision of Asian-Indian plates at the very early Eocene. According to numerical and geochemical combinations, the Eocene magmatism was related to breakoff of the Neo-Tethys slab.
4. Detailed geologic survey, structural analysis, thin section observations and EBSD fabric measurements together determined structural nature and evolutionary dynamic process for the Xaitongmoin-Quxu shear zone. The Xaitongmoin-Quxu ductile shear zone, consisted of strong deformational zones and weak deformational zone, includes foliated granite, mylonitic granite, protomylonite, mylonite and small amounts of phyllonite. Structural geometry and dynamics reveal that the shear zone has the same kinematic features, showing a shearing sense top to the north dominately. In addition, kinematic vorticities, over 0.71, demonstrate that the shear zone is marked by simple shear, namely pure shear play a little role in shearing process. According to quantitative formula, we obtained mean thickness reduction value about 20% for the Xaitongmoin-Quxu shear zone. Quartz EBSD measurements of mylonites reveal that two patters of deformation in the shear zone are observed which are middle-temperature and low-temperature, respectively. Obviously, owing to lacking of high-temperature prismatic fabrics, the slip systems are dominantly by prismatic, rhombohedral and basal glides.
5. Zircon U-Pb ages of mylonites and un-deformed veins and Ar-Ar ages play significant roles in constraining activity limits of the Xaitongmoin-Quxu shear zone. Integrated results show that the Xaitongmoin-Quxu shear zone was motivated in the late stage of Eocene, namely 35~38 Ma.
6. Field geological survey, thin sections observed and EBSD measurements arrive at a consensus that shearing sense of Sefu-Jigong ductile shear zone is dominantly dextral strike slip. Mineral deformation thermometer and EBSD measurements suggest that the Sefu-Jigong shear zone mainly experienced middle-temperature structural deformations, indicating about 500~550℃. Zircons from syn-tectonic granitic dykes exposed in the shear zone were conducted by LA-ICP-MS, suggesting activity time during 35~38 Ma.
7. Eleven samples collected from the Gangdese batholith proposed that denudation and uplift are complex and multiple stages. Early uplift was due to collision of Asian-Indian plates, but quick denudation was caused by major thrusts in the Gangdese since 23Ma. However, very short non-active phase also existed during active periods. From 10Ma to present, the significant tectonic and climatic changes occurred in the Gangdese belt, for instance, quick fluvial erosion of the Yarlung Tsangpo drainage systems and formation of the Asian monsoon, which resulted in enhanced erosion in the Gangdese. Consequently, tectonic and climatic interactions have continued to shape southern Tibet to the present.
8. Integrated studies suggest that the Gangdese magmatic belt have experienced a long evolution from late Triassic to 55Ma. Due to Asian-Indian collision, many thrusts and strike-slip fauts occurred and southern Tibet began uplift during Eocene and Oligocene. After 23Ma, the E-W extensional collapse owing to lateral variation of crust thickened gradient in the Lhasa terrane might have contributed to generation of normal faults and formation of granite porphyry (with metallogenic explosion), and quick uplift and denudation occurred in Gangdese magmatic belt.