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Mesozoic Volcanism And Constraint On Deep Geological Process In Yanshan Area

Posted on:2004-02-18Degree:DoctorType:Dissertation
Country:ChinaCandidate:X Y LiFull Text:PDF
GTID:1100360092496260Subject:Structural geology
Abstract/Summary:PDF Full Text Request
On the basis of detailed and systematic studies on the petrography, major and trace elements and Sr-Nd isotopic data of Mesozoic volcanic rocks, especially the mafic volcanic rocks in Yanshan area, we successfully studied the deep processes related to magma generation and their relationship with shallow-level tectonic evolution. The major conclusions are summarized as below.1. The Mesozoic volcanic rocks in Yanshan area comprise a wide spectrum of rock types including calc-alkaline, high-K calc-alkaline and shoshonitic lava. All mafic rocks show significant enrichment in LREE, LILE and depletion in HFSE with variable Sr anomalies. Among the felsic rocks, samples from the Zhangjiakou Formation (Fm.) are characterized by moderately to strongly Eu anomalies, and variable negative anomaly of Sr, Ti and P, whereas samples from the Donglingtai Fm. exhibit enriched LREE and flat HREE patterns with Nb/Ta fractionation and high Zr/Sm ratios. All of the mafic samples have age-corrected moderate enriched Sr and enriched Nd isotopic ratios and the felsic rocks span larger range in the initial Sr and Nd isotopic ratios than mafic volcanics, similar to isotopic compositions of lower crust of North China Block.2. Early Jurassic mafic volcanic rocks in Yanshan area were derived from decompression melting of an ancient continental lithospheric mantle, which had been previously metasomatized by subducted slabs during the closure of Paleo-Asian Ocean; the mantle source of late Jurassic mafic volcanic rocks were contributed by dominant EMI and insignificant subducted Metasomatic components; Early Cretaceous mafic volcanic rocks were derived from decompression melting of an ancient continental lithospheric mantle (EMI). With regard to the basaltic lavas, the felsic rocks resulted from partial melting of continental crust, which closely related to basaltic underplating.3. The mantle source for the Mesozoic mafic volcanic rocks in Yanshan area exhibit transition character in space and time: the mantle source components vary from subduction-related metasomatic components and EMI components throughdominant EMI components and slight subduction-related metasomatic components to EMI components; spatially, mantle source of the volcanic rocks show a regular variation trend from an enriched lithospheric mantle with EMI-like signatures in the western Shandong province through EMI and/or subduction-related metasomatic components in Yanshan area to subduction-related metasomatic lithospheric mantle in the Hinggan-Mongolian Orogen.4. The relative fertile lithospheric characters of Yanshan area, unique plate tectonic situation and developed deep faults can explain reasons of the volcanism and tectonic events in Yanshan area throughout the Mesozoic.5. The Mesozoic volcanism in Yanshan area temporarily and spatially corresponds with the regional tectonic events, which might be a consequence of deep lithospheric processes and multi-plate interaction.
Keywords/Search Tags:Deep processes, Petrogenesis, Volcanism, Mesozoic, Yanshan area
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