Mesozoic Tectonic-Magma-Mineralization In Southeastern Hunan Province

As a northern part of the middle segment of the Nanling EW-trending tectonic-magmatic-metallogenic belt, Southeast Hunan Province is situated at the junction of Yangtze Block and South China Block. There occur a great number of the Mesozoic granites associated with abundant metallic mineral resources. A chronological, geological, petrological, petrochemical and isotopic geochemical study on the Mesozoic granites combined with a comprehensive research on the other geological records such as structures, basin sediments and volcanic rocks has been carried out, which provides some new insight into the Mesozoic dynamic process of tectonism-magmatism-metallogeny in Southeast Hunan Province. The following are the major progresses and inclusions. 1. The formation time of the Mesozoic granites in Southeast Hunan Province is defined for the first time to be in the range of 233～210 Ma in the Indosinian, 174～135 Ma in the Early Yanshanian, and 130～85 Ma in the Late Yanshanian. 2. The Mesozoic granites occurred in Southeast Hunan Province can be divided into two genetic types. The first type is a crust-derived type represented by the Indosinian granites, and the other is a crust-and mantle-derived type predominated by the Yanshanian granites. 3. The tectonic settings for formation of the Mesozoic granites of different stages are different. The Indosinain granites were formed in an intracontinental syn-orogenic environment, the Early Yanshanian granites in a post-orogenic extensional environment, and the Late Yanshanian granites in an intraplate rifting environment. 4. The regional structure framework and the lithospheric structure play an important role in controlling the Mesozoic granitic magmatic activities. In the late Indosinian, intensive thickening caused by superposition of subducting slabs with transverse shortening in the upwelling region to the east of the Chaling-Chenzhou fault resulted in the partial melting of crystalline schist and gneiss in the middle crust to form granitic magma with high acidity. In contrast, slight thickening in the down-warping region to the west of the fault led to the partial melting of basic rocks in the lower crust to form sub-basic magma. In the Early Yanshanian, large scale delamination of the lithosphere possibly owing to occurrence of the thicker crust and mantle in the Yanlin-Ruchen upwelling region and the local upwelling places of the Hengyang-Guiyang down-warping region was responsible for the partial melting of crystalline schist in the middle crust to form bigger plutons with high acidity. In contrast, the magma produced by partial melting of crystalline schist in the middle crust on a smaller scale caused by weak interaction of crust with mantle and its thermal disturb in the down-warping region with thinner lithosphere mixed with mantle-derived magma and moved along faults up to shallow level of crust to form small bodies of granite porphyry, quartz porphyry, and granodiorite. 5. Different tectonic regimes are responsible for the existence of difference in possibility of mineralization between the Indosinian and Early Yanshanian granites. The transformation of the regional tectonic regime from Indosinian compression to Early Yanshanian post-orogenic extension led to the less development of open fault and fracture systems in the Indosinian granites associated with weak mineralization, but development of open fault and fracture systems in the Early Yanshanian granites associated with strong mineralization. 6. A new consideration is brought forward that the special tectonic setting is the major cause for occurrence of metallogenic difference and specialization in Early Yanshanian in Southeast Hunan Province. In the upwelling region to the east of the Chaling-Chenzhou fault and the local upwelling places of the down-warping region to the west of the Chaling-Chenzhou fault, partial melting of plenty of schist in the middle crust followed by crystallization and fractionation of magmas in a large-scale magma chamber resulted in the formation of larger monzogranite and alkali granite plutons and enrichment of incompatible elements such as W and Sn, etc. to form W-Sn-polymetallic deposits. In the down-warping region to the west of the Chaling-Chenzhou fault, magma produced by partial melting of crystalline schist in the middle crust on a smaller scale mixed with mantle-derived magma rich in compatibe elements such as Pb, Zn, Cu and Au to form small bodies of granite porphyry, quartz porphyry, and granodiorite associated with responding ore deposits.