Study On Mineralization-Related Mesozoic Granitoids In Chongyi-Shangyou Counties, South Jiangxi, And Comparison To Corresponding Granitoids In The Nanling Region, South China

Granitoids are the important ingredients of the continental crust, and the effective explorer to understand the deep information of the Earth. Although granitoids are composed of more simple minerals than other types of rocks, a series of problems about origin and evolution of granitoids have not been settled. With the development of high-resolution technology and advanced theories, geologists mastered new analytical measures to make more and more progress of study on granites.There are all kinds of granites with different epochs in the Nanling region and rare metal-related Mesozoic granitoids distribute abroad, which is the cradle of study on the relationship of granite and metallogenesis. However, Mesozoic granites also have multistage pulses. In order to understand the subduction of ancient Pacific plate, interaction of crust and mantle, geodynamic evolution of the Earth, Mesozoic granites should be studied in detail.This paper detailedly studies of geology, mineralogy, litho-geochemistry, and isotope geochemistry of three granitic intrusions including Triassic Keshuling-Xian'etang granite, Mid-Jurassic Yingqian granodiorite, Late-Jurassic Jiulongnao granite in southwestern Jiangxi province. Through comparisons with four periods (Triassic, Mid-Jurassic, Late-Jurassic, and Cretaceous) similar granitoids, significance of different phases granites to tectonic background and lithosphere evolution in Mesozoic in Nanling region has been deeply discussed.1. The Keshuling-Xian'etang intrusion has been verified as a Triassic magmatic rock (251.5±6.6Ma and 202±15Ma), which are the same as the muscovite 40Ar/39Ar plateau age (231.4±2.4Ma) from ore-bearing quartz veins in Xian'etang section. From the characteristics of petrology and isotopic geochemistry, it was mainly divided from ancient crustal materials. But it was not come from single felsic magma chamber, or its source region was impacted by mantle materials according to zircon Hf isotopic values. Triassic granites should be high-K peraluminous granitoids, which mainly come from crustal materials with a few of mantle-derived magmas.2. The Yingqian intrusion consists of granitoids of two periods, mega-porphyritic granodiorites and fine-grained granites. There are a lot of melanic monzonitic enclaves in the former. Zircon SHRIMP U-Pb dating gives the ages of 172.2±3.1Ma for the earlier granodiorites,167.6±3.3Ma for enclaves, 168±3Ma for later granites, indicating that the Yingqian pluton is a Mid-Jurassic magmatic intrusion. According to petrology, geochemistry, Sr-Nd and Hf isotopic values, it was result of partial melting of ancient crustal materials, interacted with mantle-derived magmas. Mid-Jurassic granitoids along the "Shi-Hang" fault and paralleling NE-trending faults should be high-K calc-alkaline series and metaluminous-peraluminous granitoids those were formed in a continental crustal thinning setting, which is a favorable event to interaction with crustal and mantle.3. The Taoxikeng wolframite-quartz vein type tungsten deposit is located in North to Jiulongnao intrusion. SHRIMP zircon U-Pb analysis of the granite which hosts the Taoxikeng deposit has yielded ages of 158.7±3.9Ma and 157.6±3.5Ma, which are interpreted as the emplacement age of the granite. Muscovites separated from greisen between the host granite and country rocks yield 40Ar/39Ar plateau ages of 155.0±1.4Ma,153.4±1.3Ma and 152.7±1.5Ma. Fluid inclusions separated from ore-bearing quartz veins yields a Rb-Sr isochron age of 157.1±2.9Ma. These ages obtained from three independent geochronological techniques constrain the ore-forming age of the Taoxikeng tungsten deposit and link ore genesis to that of the hosting granite intrusion. Through petrology, geochemistry, Sr-Nd and Hf isotopic studies of Jiulongnao granitic intrusion, buried granite of Taoxikeng tungsten deposit, buried granite of Tianjingwo tungsten deposit and buried granite of Baoshan skarn-type Pb-Zn deposit, the three formers were derived from Pale-Proterozoic crust with few mantle. Although SHRIMP zircon U-Pb age of Baoshan buried granite is 157.7±2.7Ma that is the same as the age of Jiulongnao granite, Sr-Nd and Hf isotopic data are near to enriched mantle, so it can be inferred that more mantle-derived materials were involved in diagenetic process. Combined with regional research information, Late-Jurassic granites are mostly complex and it is difficult to divide them to various genetic classifications because of rock-melt interaction, but characteristics of geology, mineral and geochemistry indicate that I-type, S-type and A-type coexist in the Late-Jurassic granitoids. According to Chondrite-normalized REE pattern diagrams, the Late-Jurassic granitoids can be divided to two types, "sea-gull type" and "inclining type", and the latter has been included more mantle-derived materials. According to Sr-Nd-Hf-O-Pb isotopic data,Ⅰ-type and A-type granitoids concentrated in South Hunan and North Guangxi are mainly related to tin-mineralization, but S-type granitoids concentrate in South Jiangxi are mainly related to tungsten-mineralization. Maybe spatial distribution of "East tungsten and West tin" is controlled by "Shi-Hang" fault.4. Through comparing on geological, mineralogical, geochronological, geochemical, and Sr-Nd isotopic characteristics, there are obvious distinctions of four-stage granitoids including Triassic, Mid-Jurassic, Late-Jurassic and Cretaceous. Triassic and Cretaceous granitoids belong to high-K calc-alkaline series, but Mid-Jurassic and Late-Jurassic granitoids belong to shoshonitic and high-K calc-alkaline series. The total REE contents of Triassic granitoids are lower than others. The Chondrite-normalized REE pattern diagrams of Mid-Jurassic and Late-Jurassic granitoids are extraordinarily different with other two. The REE of some Triassic granitoids, some Late-Jurassic granitoids and Cretaceous granitoids have "tetrad effect". These four-stage granitoids fall into different positions in tectonic illustrated diagrams from Pearce J A et al (1984). Above characteristics show that different kinds of granitoids can be generated with the development of geological history. On the base of previous achievements, we suggest a new model. The formation mechanism of Mesozoic granitoids is as follows:Triassic granitoids were produced in post-collision setting owing to collision between the South China Block and the Indo-Chinese Block; In Mid-Jurassic the NW-trending subducing ancient Pacific plate was tore along the "Shi-Hang" fault and paralleling NE-trending deep faults; In Late-Jurassic because the ancient Pacific plate subduced wholly below the South China Block leading to delamination of lower crust and crust-mantle interaction, there was an overall "extension and thinning" dynamical setting; In Cretaceous there were two stages including NW-trending subduction and NE-trending sliding.