Multi-stage metallogeny in the southwestern part of South China,and paleotectonic and climatic implications:A high precision geochronologic study

The South China Block(SCB)is among the large-scale W-Sn mineralized regions of the globe.The Laojunshan W-Sn-dominant ore area(LOA)in the western part of the SCB preserves the records of the tectonic history of the Tethys realm extending through North Vietnam,and Yangtze to Cathaysia blocks,with coeval formation of giant metallic deposits.The prolonged tectonic activities and their control on the genesis and spatio-temporal distribution of giant metallic deposits in the LOA provide a window for a holistic understanding of the tectono-metallogenesis of the SCB.In this study,we present results from a multi-chronologic study to determine the timing of formation of the cassiterite-wolframite-schee lite mineralization.The results suggest three distinct tectono-metallogenic periods in the LOA during the geodynamic evolution of the surrounding tectonic units.The opening of the Proto-Tethys Ocean between the Yangtze-Indochina blocks and the westward Paleo-Pacific subduction beneath the Cathaysia block(420–380 Ma)jointly contributed to the Silurian to early Devonian intracontinental orogeny in the middle of the SCB that involved top-to-the-north thrusting along NE-striking shear zones.This event generated the Dulong-Song Chay granitoids,together with the formation of Xinzhai Sn deposit related to sheared mylonitic granites(ca.419 Ma)and pegmatites(ca.389 Ma),which include the early-stage Sn-sulfide skarn(ca.418 Ma)and the late-stage Sn-bearing schist(ca.389 Ma).During the Late Permian to Late Triassic(260–220 Ma),with the closure of the Proto-Tethys oceans in the west and ongoing Paleo-Pacific westward subduction in the east,the SCB and Indochina Block(IB)were amalgamated which also marks the time of formation of the Nanwenhe scheelite skarn deposit.The subducted PaleoTethys oceanic crust was likely entrained by the nearby rising Emeishan mantle plume(270–259 Ma),which formed the Maguan diabase(ca.260 Ma)that shows significantly older Re-Os model age of ca.268 Ma,suggesting that the Nanwenhe mineralization is potentially derived from ca.260 Ma source.Furthermore,the intraplate shortening induced thin skinned crustal deformation and low grade metamorphism(ca.230 Ma),with the main stage of scheelite-Sn-Mo mineralization(229.9,229.8 and 219 Ma)and contemporary formation of the pegmatite(230.7 Ma).The Late Cretaceous involved two episodes of alternate extension and shortening,driven by the subduction polarity change from northwestward subduction of the Okhotomorsk block to northward subduction of the NeoTethys seafloor.The evolution of the LOA consists of the NW–SE transpression ending ca.100 Ma,the WNW–ESE extension in the earlier episode lasting from 100 Ma to 86 Ma,the WNW–ESE transpression beginning at ca.85 Ma and the NAS extension in the later episode during the latest Cretaceous,which produced the extension-related three periods of Laojunshan granitic magmatism and coeval Sn-W mineralization,with ages in the range of 90–89 Ma,87–85 Ma and 83–79 Ma.We also evaluate the implications of magmaticmetamorphic-metallogenic degassing on the regional paleoclimatic history.