Petrogenesis and source rocks of the high-K calc-alkaline and shoshonitic I-type granitoids in the northwestern part of East Junggar, NW China

Abstract

The metaluminous I-type granitoids in the northwestern part of East Junggar include the Kala'an high-K calc-alkaline granodiorite and the Zhalate shoshonitic quartz monzonite, which were emplaced at 331 Ma and 323 Ma, respectively. They represent formations from two independent magmatic events. The I-type granitoids have arc geochemical features verified by the following characteristics: (1) enrichments in large ion lithophile element (LILE) and light rare earth element (LREE) relative to high field strength element (HFSE), with strong positive Pb and negative Ti anomalies; (2) εHf(t) values (+13.8 to +12.7) that are similar to juvenile crust generated in modern island arcs; (3) (87Sr/86Sr)i (0.7035–0.7043) and εNd(t) (5.8–6.7) values and REE pattern that are consistent with arc volcanic rocks in East Junggar. Granodiorite and quartz monzonite show mantle-like weighted mean δ18O values of 5.3‰ and 5.9‰, respectively, which precludes stagnant oceanic crust as the source rock. The metamorphosed underplating mafic intrusion buried at depth most probably served as the source rock for the granodiorite and the quartz monzonite.The quartz monzonite exhibits remarkably higher contents of K2O, Na2O, Al2O3, REE and other incompatible elements, and lower contents of FeO, MgO and CaO than the granodiorite. These distinct chemical compositions and geochemical features of the quartz monzonite and the granodiorite were most likely inherited from differences between their source rocks, which were jadeite- and phengite-bearing garnet amphibolite, respectively. The coeval occurrence of high-K calc-alkaline granodiorite and shoshonitic quartz monzonite in the northwestern part of East Junggar suggests a post-collisional setting, and the subduction must has ceased before 331–323 Ma.