Geochemistry of the Early Paleozoic Baiyin Volcanic Rocks (NW China):Implications for the Tectonic Evolution of the North Qilian Orogenic Belt

摘要

The Qilian Mountains in NW China comprise the North Qilian Orogenic Belt,Central Qilian Block,and South Qilian Orogenic Belt.The North Qilian Orogenic Belt consists of the Northern and Southern terranes separated by a volcanic rock belt.This belt is composed mainly of felsic and mafic volcanic rocks.Volcanic rocks in the Baiyin area of the eastern part of the belt include rhyolites,rhyodacite,andesitic basalts,and basalts.New zircon U-Pb isotopic data yield a crystallization age of ca.445 Ma for the rhyolite,30 m.yr.younger than the associated basalts.The mafic volcanic rocks are relatively enriched in Th,Sr,and light rare earth element with(La/Yb)_N ratios ranging from 4.2 to 5.6 and La_N ranging from 40 to 49,and depleted in high field strength elements,with high Th/Nb ratios(0.9-1.3).These features together with their eNd(T)values(-1.4 to +3.1)are consistent with a subduction-related origin,most likely in a mature island arc or an arc built on thin continental crust in an active continental margin.The felsic volcanic rocks show a calc-alkaline affinity and a strong suprasubduction zone signature with negative Nb,Sr,and Ti anomalies and relatively high Th/Nb ratios(0.8-1.6).They have significantly high eNd(T)values(+4.4 to +7.7)relative to the mafic volcanic rocks.Such radiogenic Nd isotopic compositions rule out a crustal origin and indicate the derivation from a depleted mantle source in a volcanic arc environment.Therefore,the geochemistry of the mafic and felsic volcanic rocks demonstrates an Ordovician volcanic arc above a northward subduction zone.The northward drifting of the Central Qilian Block eventually resulted in the amalgamation of the Northern and Southern terranes to form the North Qilian Orogenic Belt in the Early Paleozoic.