Tectonic Setting of Ordovician Volcanic Rocks in Northwestern Xiaoxing'anling, Heilongjiang Province
小兴安岭西北部奥陶系火山岩形成环境

The Ordovician volcanic rocks,occurring in northwes tern Xiaoxing'anling,Heilongjiang Province,have the following features:1. They generally belong to calc-alkaline seriers on Na2O+K2O+K2O-SiO2 plot and AFM diagram despite strong potassic,silicic and spilite aftemation. They mainly belong to CAB on TiO2-MnO×10- P2O5×10 diagram.2. NMORB-normalized incompatible element distributions of volcanic rocks in this area are similar to that of continental-margin arc Cafe-alkaline series basalt. Their differences are that volcanic rocks in this area have high TI,Ta,Nb, Ce and low Cr. High Th, Ta,Nb contents indicate that the magma was composed of rutile, sphene and zircon that enriched in Th, Ta, Nb, Ce. Low Cr reflects that magma came from pert-melting upper mantle.3. Rare earth element distribution patterns of volcanic rocks are similar to that of island arc cafe-alkaline series basalt. Alternation resulted in ion of Ca2+ and depletion of Uranium.So the authors think that middle Ordovician volcanic rocks were formed in continental-margin arc serring relating to plate subduction.