Geochemistry,Geochronology And Significance Of Intermediate-acid Intrusive Rocks In Quanji Area, Gangcha, Qinghai

Quanji Area, Gangcha, Qinghai province is located in southen Qilian magmatic belt, There are many intermediate-acid intrusive rocks in Quanji area, wide distribution but poor performance. We mainly study intermediate-acid intrusive rocks by means of 1:50000 regional geological and mineral survey. Geological and geochemical means are used through field geological investigation, testing of geological samples. Petrology, geochemistry, geochronology have been studied, the author find out the lithologic features, occurrent setting and intrusion age of intermediate-acid intrusive rocks. Also its significance is investigated, achieved main cognitive as follows:(1)In study area, intermediate-acid intrusive rocks mainly includes adamellite, granodiorite, tonalite, then orthoclase granite, quartz diorite, diorite and so on.(2)granitoid is rich in K and excessives Al, enrichment in LILE(Rb,K and Ba), depletion in HFSE(Nb,Ti and P) and weak europium negative anomaly. Diorites are rich in K, and quasi-aluminous and europium negative anomaly is not obvious. Granite and granodiorite have weak europium negative anomaly. Quartz diorite is not obvious. Intermediate-acid intrusive rocks have characteristics of continental collision environment from formation Environment. Based on regional tectonic setting and previous research results, intermediate-acid intrusive rocks were Caledonian products formed from Late Ordovician to Early Silurian under subduction of Lajishan and plate collision.(3)LA-ICP-MS zircon U-Pb date show that, the intrusion age of granitoid rocks is 428 Ma~455 Ma, indicating that magma was Late Ordovician, Early Silurian, Middle Silurian, representing 3 phases of a magmatic intrusion in study area.(4)The characteristics of petrology, geochemistry, geochronology show that intermediate-acid intrusive rocks are zonally distributed spatially. Al content is increased in number,but the hornblende is declined and muscovite is appeared in succession. Those exhibit a conversion from subduction into collision.(5)U-Pb age determination results show that, the granodiorite was formed at 444±11 Ma, indicating that granodiorite belongs to Late Ordovician. Thus, the Balonggonggaer Formation should be earlier than the Late Ordovician.