Geochimie et mineralogie des granites de la region de Hetai province de Guangdong Chine meridionale =

Abstract

The Hetai Area, located in the western Guangdong Province of South China, has been affected by four orogenies, from the Caledonian (Early Paleozoic) to the Yanshanian (Late Mesozoic). Two major northeast trending faults, the Guangning-Loudin and the Wuchuan-Sihui, characterized by wide deformation zones, traverse the region. Granitic rocks, which cover about half the surface area, are related to each of the four orogenies. Previous regional studies have been premised on the importance of in situ granitization of Precambrian strata for the formation of the older granitic rocks as well as the formation of the Hetai gold deposits.This study discusses the genesis and evolution of ten granitic intrusions from the Hetai area based on their petrography, mineralogy and geochemistry. The granites of the Hetai Area are defined as "older granites" (Caledonian and Hercynian granites) and "younger granites" (Indosinian and Yanshanian granites) for comparison. Detailed field and microscopic observations show that all granitic rocks have a magmatic flow foliation. This foliation is overprinted by various degrees of solid-state deformation in the older (Caledonian and Hercynian) granite bodies.The geochemical analyses of granites also distinguish them into different types. The older granites are characterized mainly by peraluminous chemistry (A/CNK>1.05), high LILE/HFSE, HREE/LREE and a negative Eu anomaly, whereas the younger granites are characterized by slightly peraluminous and metaluminous chemistry (A/CNK<1.05), a significant change of LILE/HFSE, HREE/LREE and a negative Eu anomaly ranging from low to high with the calc-alkaline -> alkaline evolution of the magma. Comparison of the granites of the Hetai Area to those of well studied world areas by selected geochemical diagrams such as (Nb+Ta) vs (V+Sc) and Rb/Sr shows that the older granites are similar to S-type granite, but the younger granites are coincident with I-type and A-type (?) granites.Au contents are higher in younger granites than those in older granites. Au distributions in different type granites of the Hetai Area suggest that the granites derived Chemical analyses of biotite show that biotites from the older granites contain high FeO (>22%) and A1 2 C>3(>15%) but low MgO (<8%), TiO 2 (<3%) and SiO 2 (<36%), whereas those from the younger granites have high MgO (>8%), TiO 2 (>3%) and SiO 2 (>36%) but low FeO (<22%) and A1 2 O3 (<15%). In general, the biotites from older evolution of the different granites shows a geotectonic evolution from the peraluminous collision-type granites to mafic and calc-alkaline subduction-type granites finally to alkaline extension-type granites in the Hetai Area. IV RESUMECette thèse porte sur la genèse et l'évolution des roches granitiques de la région de Hetai et sur leurs relations possibles avec la minéralisation aurifère. Cette région de la Chine Méridionale est située à l'ouest de la Province de Guangdong. La Chine Méridionale a une histoire géotectonique complexe et l'étude des granites de la région de Hetai, qui...