Geochemistry And Tectonic Setting Of The Yongan Granitoid Pluton In Southeastern Guangxi

The Yongan granitoid pluton, which located in the southern Qinhang suture zone and belongs to Indosinian granites in South China, is a part of the Darongshan – Shiwadashan composite pluton in the southeasern Guangxi province. The shape of the pluton which intruded into the Paleozoic Bobai basin is controlled by both Cenxi-Bobai fault zone and Tengxian-Lingshan fault zone. The lithology of the pluton is porphyritic cordieritebiotite granodiorite.Combined with the observation of the each stage pluton in the field and the previous studies on geochronology for each stage of the composite pluton, the porphyritic cordierite-biotite granodiorite of the Yongan pluton is an early intrusive product of the composite pluton. The invasion revealed the prelude of Indosinian magmatic evolution in southeastern Guangxi. The results of Zircon U-Pb dating indicated that the ages of the Yongan pluton is 252.8±3.3 Ma, belonging to the Late Permian granites.The geochemical characteristics of the granites of Yongan pluton show that the rocks are rich in Al, K, relatively poor in Fe, Mg, supersaturation of Al. A/CNK are higher than 1.1. The standard mineral corundum molecular are more than 1%. All of that show the rocks belong to strongly peraluminous, high-K calc-alkaline series. In addition, the rocks are enriched in LREE, and depleted in HREE, and obvious fractionated between the LREE and HREE. The Chondrite-normalized REE patterns show right declining, which enriched in LREE and slightly negative Eu anomalies. The granites are enriched in large ion lithophile elements(Rb, Ba, K, Sr), and depletedin high-field strength elements( Nb, P, Ti,Ta) and Ba, Sr.With the combination of previous data and analysis on mineralogy, geochemistry, the zircon Hf isotopes and trace elements, it could be concluded that the Yongan pluton is strongly peraluminous S-type granites derived from the crustal source. The source rocks, which are formed from pelite by regional metamorphism, are cordierite- contained metamorphic rocks of senior amphibolite facies. The zircon saturation temperature of granite is 800-851 ℃. Though the mantle does not provide the rock material, the heat for the formation of magma may be provided by the mantle underplating.The research on the tectonic environment for the Yongan pluton shows that the special tectonic position of Qinfang region in Indosinian lead to the difference of the emplacement scale, characteristic mineral, tectonic environment between the Darongshan-Shiwandashan pluton and the contemporaneous granites of South China. With the effect of the Indosinian movement, the orogenic event caused by the collision of Yangtze block and Cathaysia block was happened in Qinfang area. The granite formed in extensional environment of the post-collision stage, which the emplacement mechanism is rockrigid displacement and tectonic-extension mechanism.