Formation Mechanism of the Kuday Leucogranite from the Sakya Dome and Constraints on the Evolution of the Himalayan Orogen
The Sakya Dome is located in the central part of the Northern Himalayan Gneiss Dome(NHGD)Belt.Field observations show that the northern section of the Sakya Dome can be divided into three structural levels:the upper level of shallowly metamorphosed Tethyan Himalayan Sequence,the middle level of mylonitic garnet gneisses,and the lower level of mylonitic orthogneiss and paragneiss of the dome core.There is a continuous increase in metamorphism and deformation from the upper to the middle level without an obvious discontinuity.But a detachment fault developed between the middle and the lower tectonic level.Uniform north-dipping foliation and north-plunging lineation were formed in the three levels,and all the kinematic features indicate a uniform top-down-to-north shearing,suggesting that the shear zone in the northern section of the Sakya Dome should be an outcrop of the Southern Tibetan Detachment System(STDS).The Kuday leucogranites(muscovite and garnet leucogranites)intruded into the mylonitic gneiss of the lower level,in the forms of layer-parallel(foliated)and layer-cutting dykes,showing obvious syn-tectonic characteristics with the STDS.Zircon U-Pb dating results of muscovite leucogranites were about 27Ma,and the garnet leucogranites were formed at 27.5～28.1Ma.For whole-rock geochemistry,both muscovite leucogranites and garnet leucogranites are characterized by high SiO2(73.26％～74.87％),A12O3(14.37％～15.03％)and CaO(1.51％～1.81％)with A/CNK indices between 1.08 and 1.16.The muscovite leucogranites are characterized by high Ba(398 × 10-6～739 × 10-6),Sr(135 × 10-6～165 × 10-6),low Rb(180 ×10-6～201 × 10-6),Rb/Sr ratio(1.22～1.34),various Eu anomalies(0.76～1.47)and obvious differentiation of HREE and LREE.The chondrite-normalized REE distribution pattern shows an obvious right-leaning trend.The garnet leucogranites are characterized by low Ba(204 × 10-6～282 ×10-6),Sr(81 × 10-6～103 × 10-6),high Rb(243 × 10-6～281 ×10-6),Rb/Sr ratio(2.37～3.22)and obvious negative Eu anomalies(Eu/Eu*=0.46～0.66).The differentiation of HREE and LREE is not obvious.The chondrite-normalized REE distribution pattern is"seagull-type".The initial 87Sr/86Sr ratio and εNd(t)range from 0.762732 to 0.766668 and-13.1 to-12.5,respectively,which is consistent with the Greater Himalayan Crystalline(GHC).Based on the analysis of petrography,chronology,kinematics and geochemistry,it is believed that the Kuday muscovite leucogranites originated from the water-present melting of muscovite in the GHC,the source rocks are the meta-graywacke in a deep position of the GHC,and the garnet leucogranites are the results of further crystallization differentiation of muscovite leucogranites.The syn-tectonic Kuday leucogranites indicate that the STDS has been active in the Early Oligocene,which induced the water-present melting of muscovite in the deep crust to form leucogranites and emplacement along the STDS.During the Oligocene(28～26Ma),the Himalayan orogen has changed from a convergent thickening setting to an extensional tectonic background caused by the relaxation and exhumation of the orogen.
NETL
NSTL
万方数据
维普
