Petrogenesis and tectonic setting of Middle Silurian A-type granite in Kaerqueka area, East Kunlun

A lot of porphyritic monzogranites are outcropped in the Kaerqueka area, west of the East Kunlun orogenic belt.Zircon U-Pb age and geochemistry of the porphyritic monzonites were studied.Zircon U-Pb dating yields an age of 425±2.3 Ma, indicating Middle Silurian.The rocks are characterized by high contents of SiO₂(69.0%~71.2%) and K₂O(4.3%~5.54%), enrichment of large ion lithophile element(Rb, K, Th and U), depletion of high field strength element(Nb, Ta and Ti), and strong Eu negative anomaly(Eu/Eu^*=0.40~0.51).Their lower content of Cr, Ni(Cr=13.81×10^-6~32.9×10^-6, Ni=5.48×10^-6~12.05×10^-6) and the lower Ce/Pb (2.01~4.19) and Nb/U(4.1~7.57) values indicate that they were originated from partial melting of the upper crust.The high ratio of 10000Ga/Al (3.22~3.42), high contents of Zr+Nb+Ce+Y(459×10^-6~656×10^-6) and zircon saturation temperature(827~881℃) imply they belong to A-type granite.Tectonic diagram and high Y/Nb ratio(2.93~3.15) indicate that they are A₂ type granites, and were formed in the post-collision extension setting.Combined with the spatial and temporal distribution of magmatic rocks in the region, it is suggested that the East Kunlun Orogen entered the post-collision extension stage from the Middle Silurian period.