Geochemical Characteristics Of The Early Palaeozoic Meta-volcanic Rocks From Qingshui-Zhangjiachuan In The Tianshui Area, Gansu, And Their Tectonic Significance

This dissertation mainly studies Qingshui-Zhangjiachuan early Palaeozoic metabasic and meta-intermediate-acidic volcanic rocks in Tianshui, Gansu in geochemistry combining with regional geology, structure, tectonics, isotopic geochronology and petrography etc., to discover their ages, cause of formation and tectonic settings, and to discuss their regional adscription. Further it illustrates the evolving history of paleo-plate in the study area, solves the questions which remain in argument in former studies, and puts forward new ideas. Further to this, it also provides important basic scientific data for discussing the joining relationship, forming and evolvement of the tectonic joint of Qilian and Western Qinling orogenic belts and the basic characteristics of continental orogenic belts, and exploring the continental dynamics in China.The following new progresses and knowledge have been procured by the studies of Qingshui-Zhangjiachuan early Palaeozoic meta-volcanic rocks:(1) Qingshui-Zhangjiachuan meta-volcanic rocks are mainly composed of south early Ordovician Epoch Hongtubao metabasic volcanic rocks and north Ordovician Period Chenjiahe Group meta-intermediate-acidic volcanic rocks. They are structural contact relationship.(2) The Hongtubao metabasic volcanic rocks are mainly composed of grayish green metabasalt, intercalated by a few offwhite-light-grey sillcalite, and intruded along the bedding by some contemporaneous diabase dikes of the same composition. The metabasic volcanic rocks are of tholeiitic series. Their chondrite-normalized REE patterns are of weakly LREE-enriched type, with relatively high ∑REE. There is no pronounced Eu anomaly or there are only weak positive anomalies; there is a pronounced negative Ce anomaly. Their MORB-normalized trace element spidergram shows that the rocks are slightly enriched in large-ion lithophile elements (K, Rb, Sr, Ba), enriched in Th and depleted in Cr and Ni. Nb and Ta show negative anomaly, La/Nb>1, and Ti anomaly is not pronounced. Trace element and Sr-Nd-Pb isotope characteristics show that the basic magma roots in enriched mantle wedge metasomatized by fluid in subduction belt, hybridized by continental crust material. 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 volcanic rocks are of low-K — K calcalkali series. Their chondrite-normalized REE pattern are of LREE-enriched type, with relatively high ∑ REE, and the HREE pattern are relatively flat. 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. Sr-Nd-Pb isotope characteristics show that the intermediate-acidic magma roots in an intermixed source of enriched mantle metasomatized by fluid in subduction belt and continental crust. The geological and geochemical characteristics show that the meta-intermediate-acidic rocks formed in a volcanic arc 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-arc rift-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 arc type intermediate-acidic volcanic rocks represented by Chenjiahe Group at continental margin. The analyses of geological and geochemical characteristics prove basically the tectonic settings of Qingshui-Zhangjiachuan metabasic and meta-intermediate-acidic volcanic rocks and the tectonic evolving process of Yangjiasi-Hongtubao-Fenggeling metabasic rocks belt and Humao-Xinchen meta-intermediate-acidic volcanic-sedimentary belt in early Palaeozoic.(5) The comparative studies between Qingshui-Zhangjiachuan meta-volcanic rocks and Eastern Qinling early Palaeozoic Caotangou Group meta-volcanic rocks and Yinggezui Ophiolitic Melange,and Ordovician Baiyinchang orefield meta-volcanic rocks in Northern Qilian orogenic belts show that Qingshui-Zhangjiachuan meta-volcanic rock series do not extend to Northern Qinling, do not belong to Erlangping meta-volcanic rocks belt, and are not the west extension of Shangdan tectonic belt. They also do not belong to the south meta-volcanic rock series of Northern Qilian Caledonides tectonic belts. These comparative results provide necessary proof for discussing the joining relationship of the tectonic joint of Qilian and Western Qinling orogenic belts at early Palaeozoic.(6) The joining relation of Qilian and Northern Qinling tectonic belts in early Palaeozoic is that Northern Qinling extended westward to become many limited oceanic basin and small oceanic basin systems divided by microlandmasses. These oceanic basin systems went through opening and developing various ophiolite or rift volcanic rocks to subducting northward along many subduction zones, which led to many volcanic magma island arc tectonic belts formation, untill subduction, collision and orogenesis in early Palaeozoic. Early Palaeozoic Yangjiasi-Hongtubao-Fenggeling back-arc rift-small ocean basin and Humao-Xincheng intermediate-acidic volcanic island arc belts composed of Hongtubao basic volcanic rock series and Chenjiahe Group volcanic-sedimentary rock series are one of the volcanic magma island arc tectonic belts. These elementary knowledge provide important basic scientific data for discussing the joining relationship, formation and evolvement of the tectonic joint of Qilian and Western Qinling orogenic belts and the basic characteristics of continental orogenic belts, and exploring the continental dynamics in China.