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Petrogenesis Of K-rich Volcanic Rocks Of Niangniangshan Formation In Ningwu Basin

Posted on:2014-02-08Degree:MasterType:Thesis
Country:ChinaCandidate:L ShiFull Text:PDF
GTID:2250330401989166Subject:Institute of Geochemistry
Abstract/Summary:PDF Full Text Request
The volcanic rocks of Longwangshan formation, Dawangshan formation, Gushanformation and Niangniangshan formation, distributed from up to bottom, locate in Ningwubasin of the Middle and Lower Yangtze Valley. Researches on Longwangshan formation,Dawangshan formation and Gushan formation reveal that they are originated from partialmelting of the LILE and water enriched mantle which slightly contaminated by crustalmaterial. The dominent volcanic rock type of Niangniangshan formation isnosean-phonolitic ignimbrite which are ultrapotassic signature and belong to shoshoniteseries. Sr-Nd isotopic characters of Niangniangshan formation are remarkably differentfrom the underlied volcanic rocks in the aspect of higher εNd(t) value. Its higher potassicvalues,87Sr/86Sr(t) ratios, εNd(t) values and enrichment of radioactive caused Pb isotopesuggest the source of the magama is consist of depleted mantle, together with GLOSS-likematerial which derived from subducted slab. Their Sr-Nd-Pb isotopic charactors canaccommodated by93%DMM+7%GLOSS. It is originated from depleted mantle sourcealternated by fluid derived from subducted slab. The zircon U-Pb age of theNiangniangshan formation volcanic rocks is130.6±1.1Ma, which belongs to the secondstage of late Mesozoic magma activity in the Middle and Lower Yangtze Valley.Petrogenesis of the volcanic rocks of Niangniangshan formation indicates that there isnot exact evidence of the contribution of the fluid separated from subducted plate until theNiangniangshan vocanic rocks occur, suggesting that the magmas in first stage and earlysecond stage not directly contributed by the material of subduction, but are produced byupwelling of the asthenosphere, partial melting of the lithosphere, the bottom invasion ofthe basaltic magma and crust-mantle interaction and a series of deep geological processesunder a tectonic setting of tension. Signature of fluid separated from subducted ancientpacific plate appeared in the volcanic rocks of Niangniangshan formation indicates that thetectonic setting of Ningwu basin turned from back-arc to continental arc during the periodof about130Ma.
Keywords/Search Tags:Niangniangshan formation, late Mesozoic, K-rich volcanic rocks, Ningwubasin, the Middle and Lower Yangtze Valley
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