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岩石学报 2009
Zircon U-Pb dating and geochemical study of volcanic rocks from Sunjiahe Formation of Xixiang Group in northern margin of Yangtze Plate
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Abstract:
The Sunjiahe Formation is a suit of low-metamorphosed volcano-sedimentary association, and is composed of basic, intermediate-basic and acid volcanic rocks with interbedding tuff, sediment-tuff, mudstone and siliceous rocks, the major rock types of volcanic rock are basalt, andesite, dactie and rhyolite. LA-ICPMS zircon U-Pb dating of rhyolite and basalt show us the ages of 832.9 ±4.9Ma and 845.0±17Ma respectively, implying rhyolites and basalts occurred at the same time in Neoproterozoic. Based on major, trace element, and Sr-Nd isotope data, the basalts can be classified into tholeiitic magma series and have the geochemistical characteristics of crust-assimilated continental within-plate basaltic. In addition, basalts, andesites and dacites exhibit the same REE chondrite-normalized and trace element primitive mantle normalized patterns with overlapping one another, and samples plot along crystallized curve in Th-Cr diagram can be explained by crystallization from a common basic parental magma. On the other hand, the remarkable difference in REE and trace element normalized patterns of basalt, andesite, dacite and rhyolite suggests the different source. All the ε_(Nd)(t) values of basalts, andesites and dacites greater than 0 and ploting in OIB region in ε_(Nd) (t) -(~(87)Sr/~(86)Sr)_t diagram indicate their source is similar to mantle source of OIB. Whereas, the rhyolites have the same ε_(Nd) (t) values as basalts, suggesting rhyolite is generated by partial melting of juvenile basaltic curst. The age, tectonic setting of Sunjiahe volcanic series in this article and the development of continental rift intrusions in Yangtze Plate and in other areas in the world at the same time, showing the existence of Neoproterozoic rocks in the Sunjiahe Formation, and the Sunjiahe Formation is the product of continental riftism in Neoproterozoic, which is the precursor of the Neoproterozoic break-up of Redinia supercontinent.
