Geochronology, Geochemistry And Implications Of The San-dstones In Shexing Formation In Maxiang Area, Tibet

The Maxiang unconformable contact between the wild deformed CretaceousShexing Formation sedimentary rocks, and the overlying Linzizong volcanic rocks, iswell outcropping in the Maxiang area, along the Qinghai-Tibet express way, in thesouthern Lhasa terrane. This cross-section near Maxiang is well-known by manygeological researchers in Tibet, and is widely regards as one of the most importantrocks which containing the information of the tectonic evolution before the initialcollision of India and Asia in southern Tibet. However, knowledge of this Shexingformation is still few. In this thesis, based on the sampling on the Maxiangcross-section, major and trace elements, zircon U-Pb dating, trace elements and Hfisotopic geochemistry were carried out on the sedimentary rocks and over-lyingvolcanic rocks from the base of the Linzizong session. Further discussion arepresented concerning the sedimentary environment, source regions, tectonicbackground, and their implications for the early uplift in the southern Lhasa terrane.The rocks collected from Shexing formation are sandstones, including finegrained sandstone, fine-grained feldspar sandstone, with lithic feldspar and quartz.The quartz is subangular or angular, with higher euhedral degree of feldspar, debrisare mainly volcanic affinity, reflecting the in-situ rapid accumulation features,perhaps related to the uplift of the Tibetan plateau. This is also the indicator of thenature of the source region, and the distance from the source to the deposit place.According to the analysis of major and trace elements, it is concluded that the tectonicsetting of Shexing formation was in active continental arc and island arc. Thesediments provenance mainly comes from the felsic magmatic rocks. The rocks, withnegative Eu anomaly, relative enrichment of light rare earth elements (REE), the REEdistribution, show very similar compositions to the continental-and island-arc relatedmagmatic rocks.In the zircon cathodoluminescence (CL) images, zonal vibration bands are welldeveloped, showing magmatic origin of the detrital zircons separated from theShexing formation sandstones. The most prominent detrital zircon U-Pb data peaks in the age histogram are the groups of88Ma and260Ma, which represent the two mostimportant source regions of the Shexing basin. The youngest detrital zircon age (81±1Ma, with the concordant degree is72%) can be used to constrain the depositionalupper limit of Shexing formation.Zircons with the age less than100Ma have positiveHf(t) values (1.5~14.0) andrelative younger crust model ages (0.3~1.2Ga). More than80%zircons have agesbetween100Ma and300Ma, which show much enrichedHf(t)(-16.1~-1.2), andrelative older crust model ages (1.0~2.2Ga). Comparing the Hf isotope data of thiswork with that published data from different areas in the Lhasa block, it suggest thatthe200Ma cluster zircons provenance were from the central Lhasa terrane. Whereas,the provenance of the88Ma zircon groups should have come from the Gangdese arcmagmatic rocks of the southern Gangdese arc.The sedimentation intervals (81~70Ma) of the rocks on Shexing formationimply that the materials came from very close region after a very quick uplift anderosion processes. This further suggest that the southern part of the Gangdese arc havealready uplifted to a rather high altitude, mostly like that occurred in the Andes insouthern America which cause by the oceanic subduction, instead of a continentalsubduction.