Mesozoic Tectonic Evolution In The Xiangdong Tungsten Deposit, Central South China And Its Geotectonic Implications

As an essential part of the middle segment of the Nanling tectonic-magmatic-metallogenic belt, Dengfuxian-Xitian granitic pluton is situated at the junction of Yangtze and Cathaysian sub-Block. This composite granite is composed of Indosinian coarse porphyritic biotite granite, as the majority, Yanshanian medium-grained porphyritic muscovite and biotite granite and fine-grained alkali feldspar granite. The complex is separated by a Cretaceous basin into two individual plutons: Dengfuxian pluton to the north and Xitian pluton to the south.Located in the southeastern corner of the Mesozoic Dengfuxian pluton, the Xiangdong tungsten deposit is a typical quartz vein type tungsten-tin polymetallic deposit. Tungsten-bearing quartz veins have an inhomogeneous concentration throughout the deposit. They are grouped into four sets, namely North, Middle, South, and Southernmost, from north to south respectively. The east-northeast trending Laoshanao fault, as the largest fault in the deposit area, separated the nourh vein set from the rests.In this paper, we undertake detailed structural observations on both the pluton and the deposit. Systemic research on pre-metallogenic, metallogenic and post-metallogenic has been taken. The major conclusions are summarized below:(1) There exist two types of foliations in Indosinian porphyritic granite, namely flow foliations and tectonic gneissosity.In flow foliations, feldspar(plagioclase or K-feldspar) phenocrysts keep euhedral, without any flattened. They tend to parallel the contact between the pluton and the wall rocks, and are non-developed, underdeveloped, and developed towards the center, corresponding to three pulse of emplacement. On this point, the ballooning model can best explain the emplacement of the pluton. The strain intensity, however, tends to increase towards the center, as is different from the strain distribution observed in classic ballooning plutons, maybe owing to the dramatic decrease in magma supply at the final pulse.(2) Feldspar phenocrysts in the tectonic gneissosity, discriminating from the former, are extremely flattened, which illustrates strongly ductile deformation. Gradural or abrupt contact with the flow foliations, the tectonic one distributed irregularly. This kind of gneissosity is much more locally distributed, however, as a whole, strikes towards northeast, indicating a NW–SE regional compression in the subsolidus or solidus state.(3) Two types of veins can be recognized in deposit, say single-segment veins and multiple-segment ones. Both of them have wide range of aspect ratio. Still, all of them have an aspect ratio of no less than 1.0×10-2, which is much larger than the measurements made at the outcrop scale. This may be caused by crack-sealing, fluid overpressure, ductility of host rocks or even vein size.(4) In the 11 th level of Xiangdong tungsten deposit, a lamprophyre vein injected into the granite, then again, cut by ore veins. according to the zircon U-Pb isotopic age of the lamprophyre vein, and the cross-correlation between it and the tungsten-bearing quartz vein, the age of the mineralization of the deposit can be constrainted between 152.1±1.0 Ma and 149.2±2.1 Ma(Cai et al., 2012).(5) Structural observations within the Laoshanao fault zone and nearby, comparison in mineral composition, structure and horst deformation between ore sets in the walls, and the type and distribution of alteration in the area reveal a post-metallogenic, normal nature of the fault. Consider both the North and South-South Ore Sets have a contact-parallel, banded structure, an alternation of black and white bands, we believe that they should have come from a single ore set before the fault formation. Accordingly, the Laoshanao Fault would have a horizontal throw of 2.4-3.1km.(6) Fault-slip data measured in the deposit were inverted for stress using the method proposed in this paper. Based upon this result and field observations, three major tectonic phases are established such as NNW–SEE stretch during early hypothermal mineralization, NNE–SSW contraction during late, and approximately radial extension after mineralization. They presumably resulted from both the westward subduction of the Izanagi plate in the West Pacific and, particularly in the second phase, the collisions between the Siberia and Mongolia–Sino-Korean plates to the north and between the Lhasa and Qiangtang blocks to the west. The second phase has an estimated age of 148~145 Ma, in coincidence with the turning point at magnetic anomaly M21(~147 Ma) of the Izanagi plate.