Petrological and Geochemical Characteristics of Indosinian Granites in Hongshishan Area, Gansu Province and Discussion on Its Genesis

The Indosinian granites are located in the ophiolite magmatitic complex of Hongshishan belt,Gansu.It consists of three complexes:North Maanshan complex,Xiaocaohu Lake complex,and Hongshishan complex,including granodiorite,tonalite,and syenogranite;they also contain many xenoliths composed by amphibolitie inclusions and dark microgramular mafic inclusions.Its isochronal age is 237.8 to 240 Ma,indicating that the emplacement occurred in Middle Triassic.According to the geological characteristic,petro-chemistry,and geochemistry,these intrusions can be proximately divided into two types: a high-Sr and low-Y(Maanshanbei complex) and a low-Sr and Y type(Xiaocaohu and Hongshishan complex),The first type of intrusions has the geochemical features of adakite,and therefore it is inferred to be a possible product of partial melting of the over-thickened continental lower crust with a residual phase of eclogite or amphibole eclogite.The second type,on one hand,is different from the first one by its low Sr and Al content and obviously negative Eu anomaly;on the other hand,it is similar to adakite by its low Y and HREE contents.Therefore,it is suggested that the low-Sr,low-Y type intrusions were generated by partial melting of relatively thick lower crust under high pressure granulite facies conditions with plagioclase to be the dominant residual phase.