Petrography Characteristics Of Huluhe Group Hongtubao In Tianshui Area, Gansu Province, And Their Tectonic Significance

The working area is located mostly in Tianshui area of Gansu province, between Qilian Orogenic belt and Western Qinling Orogenic. The dissertation mainly studies Hongtubao early palaeozoic metabasic volcanic rocks in geochemistry combining with structure geology, regional geologyand petrography etc. The main conclusions are:1.Qinshui-zhangjiachuan meta-volcanic rocks are mainly composed of south Hongtubao metabasic volcanic rocks and north Chenjiahe Group meta-intermediate-acidic volcanic rocks. They are structural contact. 2.The Hongtubao metabasic volcanic rocks are mainly composed of grayish green metabasalt, intercalated by a few siilcalite. The metabasic volcanic rocks are of tholeiitic series. The metabasic volcanic rocks are of tholeiitic series. Their chondrite-normalized REE patterns are of weakly LREE-enriched type, with relatively high E REE. There is no pronounced Eu anomaly or there are only weak positive anomalies; There is a pronounced negative Ce anomaly. The geological and geochemical characteristics show that the metabasic rocks formed in a back-arc rift-small ocean basin tectonic environment at convergent plate margins. 3.The Chenjiahe Group meta-intermediate-acidic volcanic rocks are mainly composed of metarhyolite and metadacite. The meta-intermediate-acidic rocks are of low-K calcalkali series. Their chondrite-normalized REE pattern are of LREE-enriched type, with relatively high∑REE. There is a pronounced negative Eu anomaly. Their MORB-normalized trace element spidergram shows that the rocks are enriched in large-ion lithophile elements (K, Rb, Ba), observably enriched in Th and depleted in high field strength elements (P, Ti, Cr). Nb and Ta show negative anomaly. The geological and geochemical characteristics show that the meta-intermediate-acidic rocks formed in a volcanic are tectonic environment at continental margin.4.Consulting information of regional structure, the eastern end of Qilian orogenic belts extended in early Ordovician period and formed Hongtubao basic volcanic rocks belt of back-arcrift-small ocean basin tectonic environment at early palaeozoic plate tectonics evolvement stage. This ocean crust subducted northward at middle-late Ordovician period and formed island are type intermediate-acidic volcanic are type intermediate-acidic volcanic rocks represented by Chenjiahe Group at continental margin.