Late Orogenic Stage Magma Intrusion And Lithospheric Delamination,

The vast-scaled granitic magmatism has been considered to be connected with underplating of mantle-derived magmas during middle-late Triassic in the Kunlun orogenic belt. The underplating of mantle-derived magmas caused delamination during late Triassic in the East Kunlun Mountains followed by accumulation of a large number of sediments in the Chaidamu basin since Jurassic. It is still unknown whether the geologic model can work or not. An analysis on the progressive metamorphism of mafic rocks combined with a numerical simulation for understanding connection between delamination and mantle-derived magma underplating gives a clue to better demonstrating process of lithospheric delamination occurred in the east Kunlun orogenic belt. A comprehensive study have been carried out on a great number of the Triassic granitoid plutons of magma mixing type, especially the Shengliqiao pluton in the west Kunlun Mountains. Following are the main achievements of this study. (1) The numerical simulation supports occurrence of delamination after underplating of mantle-derived magmas in the East Kunlun orogenic belt. The amount of lithospheric delamination is different in the north Kunlun, middle Kunlun, and south Kunlun. (2) Special physical characteristics of mafic rocks play an important role in connection of mantle-derived magma underplating with lithospheric delamination. The differential density caused by lithofacies variation of mafic rocks is responsible for lithospheric delamination. (3) Occurrence or no occurrence of intense magmatic activities after lithospheric delamination depends on the amount and range of lithospheric delamination. (4) Based on the difference in geological, petrological and geochemical information of the northern Kuda,Shengliqiao and Saitula plutons in the western Kunlun Mountains, a classification of magma mixing has been proposed, and its geological significance also discussed. (5) The highly precise isotopic geochronological measurement combined with the magma mixing and material source tracing study on the Shengliqiao pluton in the western Kunlun Mountains provides some lines of evidence for existence of underplating of mantle-derived magmas in the Kunlun orogenic belt, and is consistent with its playing a very important role in the middle-late Triassic magmatism. (6) The Shengliqiao pluton in the western Kunlun Mountains is a composite batholith. The gradual increasing in density of magmatic rocks with increasing of depth resulted from cooling and lithofacies variation of the solidification products of underplated magmas derived from mantle leads to lithospheric delamination. This study is significant to better understanding orogenic process of the east Kunlun and even other orogenic belts.