A Study On Geochronology, Geochemistry And Genesis Of Mashan Complex, Southeast Guangxi

Liuwan Moutains in Southeast Guangxi is located in the intersection of Tethysand Pan-Pacific tectonic zones, as well as the conjoint of Yantze and Cathaysianblocks, in side of Qinzhou-Hangzhou assemblage belt. Resulting from integratedactivities of Yantze, Cathaysia and Indochina blocks, this area has complicatedtectonic evolution and magmatic features. This study investigates the geochronologyand geochemistry characteristic of the complicated Mashan Complex, offering insightto the controversial Mesozoic tectonic evolution model in South China.Mashan Complex located in Liuwan Moutains, Southeast Guangxi, as complexlithology due to the multistage invasions with age gap reaches ca.10Ma. During theprolonged formation, the source and tectonic environment may have undergonedrastic change, but there is no precise isotopic age spectrum for it. The geochemicalcharacteristics of Mashan Complex show shoshonite features, whose typicalenvironment is island arc and active continental margin, only very minor shoshoniteare produced in intraplate setting, which raise special attention in recent decade.Therefore, the shoshonite in Mashan Complex in this study has important scientificvalue for the discussion of mantle nature, interaction of mantle and crust as well astectonic setting, etc.This thesis focuses on Mashan Complex, utilizing methods including petrology,mineralogy, elemental and isotopic geochemistry and zircon dating, systematicallystudy its petrology, geochemistry and age, discuss its genesis and tectonic setting,furthermore discuss the Mesozoic extension stages of Southeast Guangxi. Thefollowing points are made by this work:1. Through systematically field survey and sampling, gathering the existedinformation, it is discovered that the massif of Mashan Complex consists of graniticporphyry, syenite, quartz-monzonite, diorite, pyroxenite, and basalt.2. By analyzing the data of LA-ICPMS and U-Pb dating on zircon, it shows atleast three stages of magmatism, the diagenesis ages are247Ma,185Ma and164~166Ma respectively. The age of basalt is247Ma, the age of granitic porphyry is185Ma, and the age of syenite, quartz-monzonite, diorite and pyroxenite is between164~166Ma. 3. The geochemistry of major elements in Mashan Complex shows enrichmentin alkali, belonging to shoshonite-type rock. The contents of rare earth elements variesin a wide range, characterized by enrichment in LREE and a deficit of in HREE. In thetrace element spectrum, the massif is rich in large ion lithophile elements (Rb, Ba, Th,U, K) and depleted in HREE, P and T, the depletion of Nb and Ta is not obvious.Sr-Nd-Hf isotopes generally show low ISr and High εNd(t), εHf(t), and young T2DMmodel age. The source shows mantel affinity and all zircons’ trace elements isascribed as magmatic zircons originating from continental crust.4. Mashan basalt is mainly a product of fractional crystallization, sufferingcomparatively little crust conatamination during the uplifting invasion. The source isEMII mantl and it’s petrology composition is phlogopite-garnet-bearing peridotite;syenite, quartz-monzonite, diorite and pyroxenite are the product of the same stage,which spatially intergrow and compose a ultramafic–mafic-intermediate evolvementsequence, mainly controlled by fractional crystallization and crustal contamination.The source is probably the mix of asthenosphere and enriched lithosphere, made upby amphibole-bearing garnet peridotite, suffering different degrees of crustalcontamination during emplacement; granitic porphyry shows the features of A-typegranite, suggesting it was the product of partial melting of mafic lower crust in lateNeoproterozoic and mafic magma was derived from the enriched mantle, whichrecorded enrich messages of it.5. The tectonic setting of Mashan Complex is intraplate extension-rifting.6. At least three crustal extension have taken place in southeast Guangxi duringMesozoic, which are well-corresponded in South China. Mashan basalt might beconsequence of crustal extension after thrust-nappe structures, which provided theadvantageous space for basaltic magma intrusions; Magma underplating could havebeen the geodynamic setting responsible for the early Yanshanian magmatism ofMashan complex, which provided heat for extensive partial melting and crustalcontamination of lower crust. The extension of crust causes the lithosphere thinningand asthenosphere uplift, leads to the partial melting in asthenosphere or the are on theboundary of asthenosphere and lithosphere, forming basaltic magma and theconsequential underplating of basaltic magma.