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Magmatism Of Phanerozoic Granitoids In Southeastern Hunan Province, China And Its Evolution Regularity

Posted on:2007-02-11Degree:DoctorType:Dissertation
Country:ChinaCandidate:P C LiFull Text:PDF
GTID:1100360185953203Subject:Structural geology
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The northeastern Hunan Province, which connects the Yangtze and Cathaysian blocks, located in the central section of the Jiangnan Orogen, not only has idiographic tectonic characteristics, but also is famous for intensive granitic magmatism and extensive gold ploymetallic ore formation. In this paper, the auther has detailedly and systemically researched the characteristics such as tectonics, geology, petrochemistry and Sr-Nd-Pb isotopic geochemistry of Caledonian, Indosinian and Yanshanian granitoids, and contrasted it to Meso- and Neo- Proterozoic meta-sedimentary rocks, then discussed the scientific problems such as sources, mechanisms of genesis, deep dynamic processes and tectono-magmatism evolutions.The Proterozoic basement is composed of meta-sedimentary rocks of the Mesoproterozoic Lengjiaxi and Neoproterozoic Banxi Groups, which have similar petrochemical characteristics. We suggest that the felsic component rocks were dominant in the source area which has undergone middling chemical weathering. These sedimentary rocks were deposited in active continental margins.The Caledonian granitoids are mostly composed by Banshanpu, Hongxiaqiao and Zhangfang granitic bodies. And their main minerals are orthoclase, plagioclase, biotite, quartz and hornblende,etc. They have high Sr content and low Y and Yb contents, therefore possessing high Sr/Y and La/Yb ratios. They are enriched in large ion lithopile elements(LIFE) such as Cs, Ba and Th, and depleted in high field strength elements(HFSE) such as Nb, Ta and Ti. Those compositional characteristics, similar to those of adakite suggest that the granitoids, related to subduction, were most likely derived from lower crustal partial melting, which caused by basaltic magma underplating caused by subduction and interacted to some extent with basaltic magma material.The Indosinian granitoids in southeastern Hunan Province are geochemically similar to adakite, with characteristics of high Al2O3 and Sr contents and La/Y and Sr/Y ratios, low Y and Yb concentrations, very weak negative Eu anomalies, enrichment in large ion lithopile elements(LIFE) such as Cs, Ba and Th and depletion in high field strength elements(HFSE) such as Nb and Ta. We believe that they are likely resulted from partial melting of thickened lower crust, hot mantle liquids probably provide the heat energy.There are intensive magmatism in Yanshanian epoch in northeastern Hunan Province. Based on geochemical characteristics, the Yanshanian granitoids can be divided into two types: a high-Sr and low-Y (adakite-like) and a low-Sr and low-Y.The former has high SiO2, Al2O3, Na2O, Sr concentrations, but low K2O,Y and Yb concentrations, therefore possessing high La/Yb and Sr/Y ratios. They enriched in LILE and depleted in HFSE, with very weak negative Eu anomalies. These geochemical features are similar to those of adakites. But the Sr-Nd compositions are different from those of adakite generated by slab-melting, and its Pb compositions are similar to those of adakites from assimilation and fractional crystallization (AFC) processes involving a basaltic magma. The high-Sr and low- Y type rock may be derived from partial melting of thickened lower crust, followed contamination of crustal intermediate-rock.The latter, on the one hand, is different from the former by its low Sr content and obviously negative Eu anomaly;on the other hand, is similar to adakite by its low Y and HREE contents. Its Sr-Nd-Pb and other major and trace elements compositions are similar to those of high-Sr and low-Sr type. Therefore, it is suggested that the low-Sr and low-Y type, is also generated by partial melting of relatively thick lower crust, which closely related to the basaltic magma underplating.The tectono-magmatism in northeastern Hunan Province might be a consequence of interaction between the Yangtze and Cathaysian plates and deep lithospheric processes. Phanerozoic tectono-magmatism evolution experienced subduction of the south China sea, continental-continental collision which thickened crust, extension-cracking and introcontinental rifted orogen, and finally, resulted in "basin-range" type tectonic landforms, large-scale granitic generation and extensive gold ploymetallic ore formation.
Keywords/Search Tags:Granitoids, Petrogenesis, Tectono-magmatism evolution, Phanerozoic, Southeastern Hunan Province, China
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