Petrogenesis And Crustal Evolution Of The Mesozoic Granites From Nanling, South China

The petrogenesis of granites and related crustal evolution history are always a hot topic among the geologist. Questions revolve around:How are granites formed? Where are their sources? What are roles of mantle input in the genesis of most granites? We started our work based on these questions. This thesis is divided into two parts:the first part talks about whole rock geochemistry, and isotopes, zircon U-Pb dating, Lu-Hf isotopic of Mesozoic granites in Nanling area, South China which aims to unlock the origin of those granites and the crustal evolution history in this area. The second part focuses on the petrogenesis of a gabbro-diorite-granite complex in Xialan area, Guangdong Province. In addition, we compared the source of Phanerozoic granites in Nanling area with the Precambrian magmatic rocks in east Jiangnan Orogen, which provided insights into the Phanerozoic tectonic-magmatic evolution history in this area. Then, we discussed the boundary between the Yangtze and Cathasia blocks.Thirty-two granitoid samples were investigated by zircon LA-ICP-MS dating and LA-MC-ICPMS Lu-Hf isotopic analysis. Combined with the published Lu-Hf isotopic data for the granitic rocks in the study and adjacent areas, three domains with different crustal formation histories have been identified in the southern part of the South China Block (SCB):eastern side, middle part and western side. The southern part of the SCB could be subdivided into three parts with two boundaries:Changning-Daoxian-Gongcheng and Yongxing-Linwu-Lianshan. The west part shows zircon Hf TDM2 ages ranging from 1.8 to 2.2 Ga. The eastern side extends to the coastal area of the SCB with zircon Hf TDM2 ages within the range of 2.2-1.6 Ga. The middle part displays zircon Hf TDM2 ages ranging from 1.6 to 1.0 Ga. The granites in the middle part are characteristic as A2 type geochemical features.The old zircon Hf model ages observed in the granites from the western side and eastern side suggest that these granites might be the products of partial melting of deep buried old felsic crustal materials. LA-ICP-MS U-Pb analyses of zircons indicate that the A2 granites in the middle part intruded mainly from ca 160-150 Ma. Although similar in their U-Pb ages, two types of zircons (magmatic and late-magmatic hydrothermal) were distinguished based on zircon trace element geochemistry. The late-magmatic hydro-thermal zircons have high contents of metallogenic elements (e.g., W and Sn) and may be closely related with the ore deposits in the study area. Close U-Pb ages between these two types of zircons suggest that the mineralization happened in a relatively short period of time after the emplacement of the granite.The Hf model ages of granites, as well as zircon xenocrysts ages within the A2 granites in the middle part, indicate that the middle part has similar crustal features as those of the eastern Jiangnan orogen. We thus proposed that this low TDM2 age granite belt (Chenzhou-linwu fault area) is part of the early Neoproterozoic arc-continent collision belt between different crustal blocks (possibly Yangtze and Cathaysia) and the A2 type granites are the reworking products of Proterozoic juvenile crustal materials.Xialan complex records bimodal magmatism in Guangdong province and it is composed of gabbros, diorites and granites. Zircon U-Pb dating results suggest that these rocks were all formed in a short time span (192±2 Ma). Whole rock geochemistry, zircon Hf isotopes and whole rock Sr-Nd-Pb isotopic studies suggest that the gabbros belong to sub-alkaline series, and are probably derived from isotopically depleted asthenospheric mantle in intraplate setting. The diorite is unlikely formed neither by fractional crystallization of the gabbroic magma nor by magma mixing between gabbros and granites. The granites belong to A2 type according to their geochemical features. The similar geochemistry and isotopic features between the diorites and granites suggest that they are probably derived from the same, isotopically homogeneous juvenile crustal source. The granite may be generated by factional crystallization of dioritic magma。The intraplate Xialan complex might be formed during the post-orogenic stage of Indosinian movement which resulted in tectonic extension at ca 192 Ma. At 150～160 Ma, magmatism in the Nanling area might be controlled by the subduction of the paleo-Pacific plate which led to regional mantle convection beneath the back-arc region. The high geothermal gradient in the back-arc area due to the convection induced melting of the early Neoproterozoic arc-volcanic materials and sediments in the area. Since this middle part area is the early southwestward extension of the Jiangnan orogen between the Yangtze and Cathaysia blocks, the magma source may contain a great amount of Neoproterozoic juvenile crustal materials, which may explain the A2 type signatures and relatively young Hf model ages compared with the granites in the western and eastern sides.Collectively, our work suggests that the Mesozoic (probably including Proterozoic and Phanerozoic) granites in South China might originate from deep buried crustal materials. Mantle-derived magma might serve as the heat source during the genesis of these granites, and the mass input might be negligible. Even the associated diorite may not be derived directly from the mantle magma but the product of remelting of the juvenile intermediate to mafic crustal. The source of the granites related with the W, Sn ore deposits in Nanling could be the deep buried Neoproterozoic juvenile crustal materials.