Geochronology Of The Chuancigou A-type Granite In The North Qilian Belt And Its Significances

Some geologists believe that the South Ophiolite Belt in North Qilian is a fragment of oceanic lithosphere formed in ocean ridge in Early Paleozoic.Characteristic of an A-type granite and peridotite in the South Ophiolite Belt will help to understand the time sequence and structural framework of this belt.This granite is mainly composed of alkali feldspar?⁴5 vol.%?,quartz?³0 vol.%?,plagioclase?¹5 vol.%?and aegirine augite?⁵ vol.%?with perthite,myrmekite and micrographic texture.This granite is high in SiO₂?72.58⁷3.96 wt%?,Na₂O+K₂O?10.12¹0.39 wt%?and Ga/Al?10000Ga/Al=2.59³.49?.Its A/NK ratios are below 1.0,which means the granite is peralkaline.Normalized rare earth elements and trace elements patterns show that this pluton has seagull-shaped REE patterns with obvious negative Eu anomaly??Eu=0.56⁰.60?,enriched in Rb,Ba,Th,K and depleted in Sr,P,Ti,Nb,Ta.Petrological and geochemical characteristics suggest that this granite is an A-type one,which is subdivided into A₂-type,i.e.,formed in post-orogenic environment or active continental margin.U-shaped REE patterns and the enrichment of LILE and Th,Zr,Hf of the Chuancigou peridotite illustrate a source of fossil mantle wedge modified by melts or fluids released from subducting slab.SHRIMP U-Pb analysis on zircon of the granite yield a weighted mean age of 501±5 Ma,interpreted as the emplacement age,which is earlier than the event of continent-continent collision and later than continent rifting of the North Qilian orogenic belt.In the Middle-Late Cambrian forearc of active continental margin of North Qilian,it is inferred that asthenosphere melted due to the extension caused by subduction and the magma underplated to form the Chuancigou A-type granite.