Geochemical And Tectonic Evolution Of Mesozoic Volcanic Rocks From Yindaosi Area In West Segment Of North Qinling Mts. And Late Proterozoic Volcanic Rocks From Xinglongshan Formation In Eastern Qilian Mts.

Based on the former data, field investigation and major samples analysis and study, this paper emphasizes particularly on geochemical and tectonic evolution of Mesozoic volcanic rocks from Yindaosi area in west segment of North Qinling Mts., and Late Proterozoic volcanic rocks from Xinglongshan Formation in Eastern Qilian Mts., by means of geochemistry, regional geology, tectonic geology, petromorphology, isotope chronology and microphysiography. The purpose is to determine their age, identify their petrogenesis and geotectonic setting, and clarify the relationship between the petrogenesis and tectonic environment. To study on the two groups volcanic rocks provided us important information for discussing the basic characteristics of Chinese continental orogenic zone and continental dynamics mechanism.Through the study, new findings are as following:1. The results of U-Pb zircon La-ICPMS dating for volcanic rocks from Yindaosi area in west segment of North Qinling Mts. and the petrography study are as follow:The results of U-Pb zircon La-ICPMS dating for volcanic rocks from Yindaosi area in west segment of North Qinling Mts. and the elements geochemistry study show that Yindaosi rhyolite is 211±2.6Ma, at the later Indo-Chinese epoch.As a whole, the element geochemistry characteristics of this rhyolite are following.(1) High SiO₂, Na₂O + K₂O and low TiO₂, and it belongs to sub-alkali series.(2) LREE enriched, distinct negative abnormity on Eu, and the primitive-mantle normalized trace element patterns is of the intraplate, with the markedly depletion on Ba, Sr, P and Ti.(3) The traits of Pb isotope show that the source region of Yindaosi rhyolite is the basement rocks in the North Qinling Mts., which are produced by the partial melting earth's crust. The comparison of Chondrite-normalized patterns of REE and the primitive-mantle normalized trace element patterns suggests that the matter in the source region may be biotite- hornblende-anorthose gneiss of Paleoagnotozoic Qinling Group. The residue may be mainly plagioclase, containing hornblende, and the limited separate crystal of alkalic feldspar and biotite existed in the magma process.(4) Combining the specific conversion tectonic setting in Indo-Chinese epoch with certain faults in the Yindaosi area in west of North Qinling Mts., the rhyolite magma erupted particularly in the region.2. The results of U-Pb zircon La-ICPMS dating for volcanic rocks from Xinglongshan Formation in Eastern Qilian Mts. and the petrography study are as follow:(1) Xinglongshan Group consisting mainly of detrital rocks and basic volcanic rocks, which can be divided into lower, middle and upper formations. The result of U-Pb zircon LA-ICPMS dating for upper formation basic lava is 713-824Ma, together with the regional geological characteristics, we suggest that Neoproterozoic is the formation age of Xinglongshan Group.(2) Based on petrographic, major and trace element data, the lower formation of basalt or basaltic and middle and upper formations of basic volcanic rocks of Xinglongshan Group are different distinctly. The former exhibits LREE enriched, distinct Nb-Ta ant Ti depletion in trace element patterns. The latter with LREE depleted and HREE enriched, their primitive-mantle normalized diagram show a"flat-type"patterns. The Xinglongshan Group volcanic rocks are dominantly tholeiitic in composition.(3) The parent magma of Xinglongshan Group basic volcanic rocks subjected to shallow level gabbroic (palg+cpx[±ol]) fractionation. The basic lavas in the lower formation occurs within plate rift setting, and crustal contamination has also contributed significantly to the formation of the basalt. In contrast, the middle and upper formation form in initial oceanic basin setting(4) The basalt or basaltic in the lower formation with En(composition enriched), exhibits approximate mantle plume in asthenosphere subjects crustal contamination in the process of uplifting , mainly forming in the primitive garnet stability field. The basic lavas in the middle, upper formation derived from depleted mantle source similar to N-MORB in inspinel-iherzolite stability field.(5) Xinglongshan Group is the response of the evolvement history in eastern Qilian orogenic belt of continental extension-continental rift development-formation of initial ocean basin, after the formation of Rodinia Supercontinent.