The SHRIMP zircon U-Pb geochronology ofthree typically Indosinian granitic plutons with peralumi-nous and potassium-rich affinities (Tangshi ultraunit forWeishan and Baimashan, and Longtan ultraunit forGuandimiao) is presented in Hunan Province, South China.The analyses of zircons from biotite monozonite granites forWeishan, Baimashan and Guandimiao plutons show the sin-gle and tight clusters on the concordia, and yield theweighted mean 206Pb/238U ages of 244±4, 243±3 and 239±3Ma, respectively, representing the crystallized ages of theseIndosinian granites. These data suggest that the Indosiniangranitic plutons as previously thought formed at a narrowage span. In combination with other data, it is inferred thatthe Indosinian granites within the South China Block proba-bly distributed in Hunan, Jiangxi, Guangxi and Guangdongprovinces as planar shape, and were the derivation of thecrustal materials in the intracontinental thickening setting.These precisely geochronological data provide importantconstraints for better understanding the spatiotemporal pat-tern of the Indosinian peraluminous granites and earlyMesozoic tectonic evolution of the South China Block.
Early Yanshanian (Jurassic) granitoids are widespread in the Nanling Range of South China, and are associated with numerous non-ferrous and rare metal mineral deposits. These granitoids consist mainly of slightly peraluminous biotite monzogranites and K-feldspar granites that are closely associated in time and space with subordinate amphibole-bearing granites and muscoviteand garnet-bearing granites. In most previous studies, the biotite-bearing granites were classified as crustal transformation-type (corresponding to the S-type) granites which were interpreted to be derived from the regional Paleoproterozoic meta-sedimentary rocks. In this paper, we re-analyze the geochemical characteristics of a number of representative Early Yanshanian Nanling granitoids. There exists a clear negative correlation between SiO2 and P2O5 for the studied granitoids. The Early Yanshanian Nanling granitoid suites (including amphibole-bearing granodiorites→biotite monzogranites→K-feldspar granites→two-mica (muscovite) granites) are metaluminous to slightly peraluminous I-type or fractionated I-type granites. They were derived predominantly from Proterozoic igneous protoliths. The juvenile crust and/or newly mantle-derived materials might also have been involved in some of these granites. In addition, the Iand fractionated I-type granites are closely associated in time and space with minor amount of A-type felsic and mafic volcanic and intrusive rocks as well as alkaline rocks. All these rocks constitute a typical assemblage of anorogenic, intraplate magmatism, suggesting a dominant lithospheric extensional regime for the Nanling Range and neighboring region during the Early Yanshanian period.
Yunkaidashan, located at the southern South China block (SCB) and closely adjacent to the Indochina block, is an ideal region for better understanding the temporal and spatial framework of tectonothermal overprinting at the southern SCB since Mesozoic. Apatite and zircon fission track thermochronology of various-type rocks from Yunkaidashan is presented in this paper. The results show, no matter what rocks are, the apparent ages of zircon fission track range from 97.4 to 133.0 Ma, and those of apatite fission track from 43.2 to 68.4 Ma. The length of apatite fission track yields an average confined track length of ～13 μm and shows normal distribution of unimodal frequency. It is inferred that the uplifting amplitude has been more than 5 km in Yunkaidashan since late Mesozoic. The difference of fission track apparent ages at different locations in Yunkaidashan suggests a paleophysiognomic scenario of the heterogeneous uplift/denudation. These data of the fission track thermochronology provide new constraints for better understanding the tectonophysiognomic pattern of the SCB since late Mesozoic.
