This paper reports the systematic study on petrology, geochemistry, LA ICPMS zircons U-Pb dating, and in situ Hf isotope geology of the four plutons in the central-southern Jiangxi Province, an important part of the South China Block. In the outcrops, rocks are gradually changed from wall rock (slate or schist) to pluton (gneissic granite); some residual blocks of sandy rock occur in the margin of pluton, and the foliations of residual blocks are parallel to those of both wall rock and gneissic granite. The thin-section observations show that the four plutons contain peraluminous minerals such as muscovite and sillimanite. The flattened and elongated feldspar and quartz grains are often visible in the gneissic granite, parallel to direction of lineation, suggesting that the granitic rock were subjected to a strong ductile shearing. Geochemically, the A/CNK values from 13 granitic samples are between 1.03 and 1.37 with an average of 1.16, indicating that the granites are of strongly peraluminous plutons. The REE compositions of the 13 samples are similar, showing higher REE contents, with enrichment in LREEs, depletion in Eu and REE patterns with relative LREE-enrichment and negligible Eu anomalies. They show enrichment in Rb, Th, U and depletion in Ba, Sr, Nb, Ti, belonging to a low Ba-Sr type of granite. Thus, the four bodies should be derived from the same magmatic source. Zircons used as U-Pb dating mostly exhibit euhedral shape and high Th/U values from 0.52 to 1.54 with an average of 1.08, suggesting that most zircons are of magmatic genesis. The zircons from four plutons yielded rather similar 206 Pb/ 238 U vs. 207 Pb/ 235 U concordia ages: 436.1±5.7 Ma for the Tangwan granite, 440.6±4 Ma for the Jiekou gneissic granite, 435.9±6.2 Ma for the Dongbao gneissic granite, and 441.9±3.1 Ma for the Jinxi K-granite, respectively, corresponding to Silurian Llandovery. Several xenocrysts yielded U-Pb ages around 700 Ma, implying that a breakup event took place during Neoproterozoic in the South China Block. In si
Two types of the Jurassic basins are distinguished in SE China based on their geodynamic features: the Late Triassic-Early Jurassic post-orogenic basins and the Middle Jurassic intra-continental extensional basins. The Lower Jurassic sequence shows a change from coarseto fine-grained accumulation, suggesting a gradually deepening depositional environment from river to shore-lake and to deep-water lake. In contrast, the Middle Jurassic accumulation was changed from claystone to conglomerate along the coastal provinces in SE China, indicative of an initial crustal uplift. The Wuyi Mountains have been a paleogeograghic separating zone since the Middle Jurassic. The Late Jurassic strata are absent in most areas of SE China. A large-scale bimodal intra-continental rift-type volcanism occurred during the Middle Jurassic along a 40-60 km wide and 200 km long area in western Fujian and southern Jiangxi provinces, which is most likely the strongest volcanism in SE China since the Cambrian. The SHRIMP zircon U-Pb analyses on the rhyolite from the Dongkeng basin in the southern Jiangxi area yield a concord U-Pb age of 160±0.5 Ma, providing an upper age limit for the bimodal volcanic eruption. The analyses of the basin features indicate a change of the depositional environment during the interval from Middle Triassic to Late Triassic from a shallow-sea to an intra-continent in SE China in response to the strong collision between the Yangtze and North China Blocks. Sedimentary structures record a southward direction of Early Jurassic paleo-currents, reflecting that their source areas were to the north side. We propose that the Wuyi region was uplifted as early as Middle Jurassic, followed by a wide E-W-trending extended depression and bimodal volcanism in the western foot of the Wuyi Mountains. Presumably the uplift of the Wuyi domain changed the Middle Jurassic paleo geographic outline and formed the transformational tectonic regime from compression to extension as a tectonic response to the Pacific plate subduction.
SHU LiangShuWANG YanSHA JinGengJIANG ShaoYongYU JinHaiWANG YanBin
Using the U-Pb LA-ICP-MS analysis technique we analyzed geochronological features of detrital zircons from Devonian and Ordovician coarse sandstone in southern Jiangxi Province,northern Cathaysia Block.Abundant ancient crustal information was obtained.The 350 groups of U-Pb age center on five ranges:2600-2300 Ma(peak at 2470 Ma),1100-900 Ma(peak at 980 Ma),900-700 Ma(peak at 800 Ma),650-520 Ma(peak at 600 Ma) and 450-400 Ma(peak at 440 Ma).We also found a detrital zircon of ~3.5 Ga.This is the oldest age in northern Cathaysia Block obtained so far.From the analysis we concluded that:(1) the 2600-2300 Ma period,characterized by a global continent-building,records late Neoarchean magmatism that did not occur in the neighboring area of Cathaysia;(2) the marked peak at 1100-900 Ma corresponds with the assembly time of the Neoproterozoic supercontinent,Rodinia,suggesting that the Cathaysia Block was once a part of Rodinia,and numerous euhedral zircons with similar ages likely resulted from the Grenville event;(3) the peak at 900-700 Ma corresponds to the breakup of Rodinia,as evidenced by wide occurrence of Neoproterozoic granite,basic dyke swarms and continental rift-type deposition;(4) the 650-520 Ma period is the typical time of the Pan-African event,but as yet no associated magmatic rock has been reported in this area;and(5) the peak at 450-400 Ma,representing the early Paleozoic orogeny,was recorded in various igneous rocks.Abundant Silurian-Lower Devonian granitic plutons,orthogneisses and their zircon U-Pb dating ages(450-400 Ma) are important evidence of an early Paleozoic orogenic event.Geological data support the interpretation of an Early Paleozoic tectonic heat event in Cathaysia,which was likely to be caused by intracontinental collision.
