The Late Cretaceous Polymetallic Mineralization Related To Granitoid Systems In The Yidun Terrane, East Tibet

The Yidun Terrane is located between the western Songpan-Garze Fold Belt and the eastern Qiangtang-Changdu Block,separated by the Garze-Litang Suture Zone to the east and the Jinshajiang Suture Zone to the west.It starts from the Zengke county in the north and ends to the Zhongdian county in the south,extending over 500 km.It is suggested that the Yidun Terrane can be divided into the Northern Yidun Terrane?NYT?and Southern Yidun Terrane?SYT?by 28°30'N.Distinct Late Cretaceous granitic magmatism and mineralization types occur in the NYT and SYT.The NYT develops Sn-Pb-Zn-Ag polymetallic mineralization,while the SYT develops Cu-Mo-W polymetallic mineralization.Which factors have caused the mineralized diversity?This thesis has systematically studied the Late Cretaceous polymetallic mineralization occurred in the Yidun Terrane,and focused on the Xiasai giant magmatic-hydrothermal Sn-Pb-Zn-Ag deposit and Tongchanggou porphyry-skarn Mo-Cu deposits,to identify the mineralized intrusions,compare the geochemical and isotopic characteristics of causative intrusions and evaluate the P-T-fO₂ conditions for their formation.These unraveled that the mineralization diversity ultimately depends on the distinctive magmatic source affinities of their parental magmas.The NYT Sn-Pb-Zn-Ag-bearing granites belong to highly fractionated granites,and they are derived from ancient crustal materials,with minor input of metasedimentary materials;their parental magmas experience intensive fractionated crystallization of plagioclase at the shallow crust scale or plagioclase presents as residue in the source;the amphibole phenocrysts are absent in these granites,indicating low H₂O contents in these magmas.The SYT Cu-Mo-bearing granitic porphyries show typical affinities of I-type granites,and their parental magmas derived from the partial melting of the garnet-amphibole middle-lower crust or lower crust;These magmas have relatively high oxygen fugacities and high H₂O contents?4.5⁴.9wt.%?.The SYT Cu-Mo-W-bearing granitic porphyries still remain the mineralogical and geochemical signatures of I-type granites,with lower degree of fractionation than the NYT granites,but higher degree of fractionation than the SYT Cu-Mo-bearing granitic porphyries;their parental magmas originated from the partial melting of amphibolites with minor garnet;they are characterized by low oxygen fugacities and low H2O contents.The increasing Sr/Y and La/Yb ratios of the Late Cretaceous granitoids also indicate their changing magmatic sources from north to south of the Yidun Terrane.The terrane-scale Hf-Nd isotopic mapping of Triassic magmatic rocks indicate that the NYT is characterized by enriched Hf-Nd isotopic signatures??Hf?t?=-20.6⁺0.4,?Nd?t?=-11.3^-2.0?,while the SYT is characterized by relatively depleted Hf-Nd isotopic signatures??Hf?t?=-6.0⁺7.0,?Nd?t?=-4.9⁺0.7?.The different isotopic affinities between the NYT and SYT were attributed to changing subduction dip in space and time,with earlier steeper subduction at²35-230 Ma,and later shallow-dip subduction from²28-220 Ma.During the steeper dip subduction phase,it is likely that the NYT experienced a slightly greater degree of extension than the SYT,causing the extensive interaction between the normal arc mantle wedge and the old igneous or metasedimentary lower crust in the NYT,and more mantle-derived or juvenile crustal materials were added into the SYT magmatic source.This suggestion can effectively explain the respective occurrences of Late Triassic VMS-type Pb-Zn polymetallic deposit and porphyry-type Cu deposits in the NYT and SYT.The different melting depths of NYT and SYT in the Late Cretaceous might be related to the continuous more extension in the NYT than SYT.In addition,the SYT Late Cretaceous granitic porphyries have inherited the arc magma affinities,with the enrichments of LILEs and the depletions of HFSEs.Their Sr-Nd isotopic compositions suggest the input of ca.215 Ma juvenile arc crustal materials during the generation of SYT Late Cretaceous porphyries.This indicates that the remelting of the residual sulfide phase within the Triassic juvenile crust could have provided Cu and S for the formation of ca.80 Ma porphyry-skarn deposits.