Geochronology And Geochemistry Of The Maiga Batholith In Coqen, Tibet

The magma source and petrogenesis of Early Cretaceous granitoids whichwidely exposed in the Central Lhasa Terrane, Tibet, remain unconstrainted so far. Abetter understanding of these issues will help to reveal magmatic processes andmineralization background of the Lhasa Terrane during the Cretaceous. This thesisreports zircon U-Pb age, bulk-rock geochemical, Sr-Nd isotope data, and zircon Hfisotope data from the Maiga batholith, one of the representative Early Cretaceousbatholiths in the west areas of the Central Lhasa Terrane. These new data, incombination with the data of the Nixiong pluton in the west, Xainza pluton in themiddle, and Sangba pluton in the east of the Central Lhasa Terrane, the origin andpetrogenesis of these plutons and their constraints on the mineralization backgroundof the Lhasa Terrane have been explored.The Maiga batholith is mainly composed of monzogranite and granodiorite, withabundant dioritic enclaves. Zircon U-Pb dating for the Maiga batholith yieldsemplacement ages of122±1Ma and113±2Ma for granitic rocks. The dioriticenclaves have been dated to be113±2Ma. Granitoids of122±1Ma arecharacterized by high-Si, high-K, and high aluminum saturation index （A/CNK）, andare slightly peraluminous and high-K calc-alkaline I-type granites. These rocksexhibit high （87Sr/86Sr）i（ca.0.7147） values, low negative bulk-rock εNd（t）（ca.-12.0）and zircon εHf（t）（-15.7^-11.1）. Host granitoids of113±2Ma are metaluminous andhigh-K calc-alkaline I-type granite. Compared to granitoids of122±1Ma, they showrelatively low （87Sr/86Sr）i（0.7094⁰.7156） and enhanced bulk-rock εNd（t）（-12.1^-7.3） and zircon εHf（t）（-11.1⁰.1）. Dioritic enclaves （113±2Ma） are metaluminousand medium-to high-K calc-alkaline, and are characterized by varying （87Sr/86Sr）i（0.7058⁰.7105）, negative bulk-rock εNd（t）（-10.7^-9.8）, and zircon εHf（t）（-14.0^-5.6）.A synthetical compiliation for existing data indicate that the122±3Ma and113±3Ma magmatic rocks are widely exposed in the Central Lhasa Terrane from thewest, via middle to the east of this terrane. We found that SiO₂, K₂O₃, A/CNK, and the degrees of REE fractionation and Eu anomaly increase from west to east, while Al₂O₃,TFe₂O₃, MgO, CaO, and Mg decrease from west to east. Existing data reveal that thegranitoids from Coqen in the west and Xainza in the middle of the Central LhasaTerrane are mainly composed of I-type granites, which can be accounted for byanatexis of an ancient middle-lower crust with varying contributions of basaltic melts.However, the coeval granitoids from Sangba in the east of the Central Lhasa Terraneare mainly composed of S-type granites derived from partial melting of a thickenedcrust, with insignificant input of basaltic melts. Thesse differences are likelycontrolled by the different nature of basement and lithospheric architecture beneaththe Central Lhasa Terrane, possibly resulting in the differences of geological,geophysical, and metallogenic features observed in the present-day Central LhasaTerrane from west to east. The increased contributions of basaltic melts at about113Ma observed in the Maiga batholith and the other coeval batholiths in the CentralLhasa Terrane further verify the model of slab break-off of the southward subductionof the Bangong Nujiang Ocean seafloor at that time.