The Closure Of Tethys And Cretaceous Mineralization

South China is famous for its extensive magmatism and polymetallic mineralization in the Mesozoic,especially for W,Sn,Sb,Nb,Ta,Cu and U mineralization.These igneous rocks and deposits have temporal and spatial rules.They mainly can be classified into four episodes: Triassic?230?210 Ma?,Jurassic?180?150 Ma?,Early Cretaceous?140?125 Ma?and Middle-Late Cretaceous?110?80 Ma?.Compared to other episodes,Middle-Late Cretaceous?110?80 Ma?have great ore prospecting potential but attracted few attentions.More importantly,this episode is mainly distributed in southern South China,which belongs to the joint of the Pacific tectonic domain and Tethys tectonic domain.Therefore,there is a dispute in tectonic setting.The Yangchun basin is chosen as our study area.Cretaceous deposits of the Shilu Cu?Mo deposit,the Xishan Sn?W deposit and the Yingwuling W?Sn deposit in the Yangchun basin are studied through zircon U?Pb dating,molybdenite Re?Os dating,cassiterite U?Pb dating,whole-rock major and trace elements and Sr?Nd isotopes and zircon Hf isotopes,aiming at constraining on the genesis of Cretaceous deposits in the Yangchun basin.Combined with the distribution of Middle-Late Cretaceous igneous rocks and deposits and the tectonic evolution of Southeastern Asia,we discuss the tectonic setting of Cretaceous igneous rocks and deposits in South China.Shilu is a large porphyry-skarn Cu–Mo deposit in the Yangchun basin,South China.The lithology of the Shilu pluton is granodiorite and quartz diorite,both of which are high-K calc-alkaline series,with high Sr?>400 ppm?contents,along with low Y and Yb contents.Most of the samples have characteristics of adakite except for a few samples that have slightly higher Y and Yb contents,which may be plausibly explained by crustal contamination.LA-ICP-MS zircon U–Pb dating shows that the Shilu pluton formed between 106.6 ± 1.3 Ma and 103.9 ± 0.5 Ma,with multiple magmatic pulses.Molybdenite Re–Os isochron age is 102.2 ± 2.9 Ma?MSWD = 9.4?Ma,which is identical to the youngest zircon U–Pb age?103.9 ± 0.5 Ma?within error.The Shilu pluton has high oxygen fugacity as indicated by high zircon Ce⁴⁺/Ce³⁺ and ?Eu ratios.Considering the geochemical characteristics?high Sr,and low Y and Yb contents?,high oxygen fugacity and copper mineralization of the Shilu pluton,it was most likely formed by partial melting of a subducted young oceanic slab.Whole-rock Sr–Nd isotope,zircon Hf isotope,and whole-rock trace element show that Shilu adakitic magmas may have interacted with enriched mantle?EMII?and/or crustal materials during ascent.South China had been affected by the Pacific tectonic regime to the east and the Neo-Tethys tectonic regime to the south in the Cretaceous.Based on the Pacific plate drifting and rotation history,it is hard for the Pacific plate to be subducted and melted,forming adakitic rocks in the Shilu region.Considering the evolution history of Southeast Asia and the South China Sea,the Neo-Tethys trench should be much closer to the South China block in the Cretaceous,and thus a greater impact on the South China block.Based on the subduction direction,time of subduction and distance between the Neo-Tethys subduction zone and the Shilu deposit,subduction of the NeoTethys ridge is the best mechanism for Shilu adakitic rocks and Cu–Mo mineralization.The Xishan Sn–W deposit is spatially related to K-feldspar granite in the Yangchun basin,western Guangdong Province,South China.Laser Ablation Inductively Coupled Plasma Mass Spectrometry?LA-ICP-MS?zircon U–Pb dating defines an emplacement age of the K-feldspar granite as ⁷9 Ma?78.1 ± 0.9 Ma;79.0 ± 1.2 Ma;79.3 ± 0.8 Ma?.LA-ICP-MS cassiterite U–Pb ages of 78.1 ± 0.9 Ma and 79.0 ± 1.2 Ma for the cassiterite-quartz vein and agrees well with molybdenite Re–Os isochron age is 79.4 ± 4.5 Ma,both of which represent the mineralization age of the Xishan Sn–W deposit.The mineralization age is consistent with the emplacement age of the Xishan K-feldspar granite,indicating a close genetic relationship.Xishan samples are characterized by high K₂O?4.43?7.12 wt.%?and total alkali?Na₂O + K2 O = 7.88?10.07 wt.%?contents,and belong to high-K calc-alkaline to shoshonitic series.Rare earth elements?REE = 167?539 ppm?and high field-strength elements?Nb = 48.1–78.5 ppm,Ta = 8.5–25.3 ppm?show positive anomalies,while Sr,Eu,Ba and Ti are negative anomalies.Mineralogical and geochemical characteristics?10000*Ga/Al >2.6,Zr + Nb + Ce + Y > 350 ppm?indicate that the K-feldspar granite belongs to A-type granite,and it is further classified as A2-type granite.High Rb/Sr and K/Rb ratios and low Nb/Ta and Zr/Hf of the K-feldspar granite indicate that it has been experienced a strong magmatic differentiation.The K-feldspar granite has initial 87Sr/86 Sr ratios of 0.705256 to 0.706181 and ?Nd?t?values of-5.4 to-4.8 and zircons from the K-feldspar granite give ?Hf?t?values ranging from-7.8 to 1.6,suggesting a mixed magma source.The Kfeldspar granite was generated at a low oxygen fugacity condition as indicated by low zircon Ce⁴⁺/Ce³⁺ ratios of 12?88,which is key for high Sn contents in initial magmas.High volatile?e.g.fluorine?and highly evolved magmas play important roles in oreforming element transportation and concentration.A-type granite characteristics show that the Xishan pluton formed in an extensional environment.High F and low Cl characteristics of the K-feldspar granite are most probably attributed to phengite decomposition induced by slab rollback.Asthenosphere upwelling and geothermal gradient rising induced by slab rollback,leading to the decomposition of phengite and releasing F-rich and Cl-poor fluids that result in the partial melting of the overlying metasomatized mantle and the lower crust.In the Late Cretaceous,the Xishan deposit located near the interaction of the circum-Pacific and the Tethys tectonic realms.The Shilu Cu?Mo deposit in the Yangchun basin has been proved to be the product of the Tethys subduction.Therefore,the Xishan deposit is possibly attributed to the rollback of the Tethys slab.The Yingwuling deposit is a W?Sn polymetallic deposit in the Yangchun basin,west of the Xishan Sn?W deposit.There are a variety of minerals and magmatic dykes,which is more complicated than the Xishan deposit.The Yingwuling pluton can be classified into K-feldspar granite and biotite granite.Both of which have low Nb/Ta and Zr/Hf ratios,indicating that they experience highly fractionated.Compared to biotite granite,K-feldspar granite is more enriched in LREE but depleted in HREE,which means biotite granite experience more strongly fractionated.LA-ICP-MS zircon U?Pb dating show that they have identical age of ⁷9 Ma within age error,consistent with the Xishan pluton in the Yangchun basin,belonging to the product of Late Cretaceous magmatic activity in the west of Guangdong.Cassiterite U–Pb dating yields a age of 79.6 ± 0.9 Ma,which agrees well with the age of the Yingwuling pluton.Therefore,there is a genetic relationship between magmatism and mineralization.Both K-feldspar granite and biotite granite show A-type granite characteristics in mineral and geochemical compositions.The high temperature?>800 °C?of initial magmas are also consistent with A-type granite characteristic.The Yingwuling pluton has low Ce⁴⁺/Ce³⁺ and ?Eu values,indicating low oxygen fugacity.This is important for Sn concentration in the initial magmas.The Yingwuling pluton has identical age and similar geochemical compositions with the Xishan pluton.They should be formed in the same tectonic setting.In the Late Cretaceous,the slab rollback of the Neo-Tethys caused the formation of the Yingwuling pluton.Compilation data reveal that cretaceous igneous rocks and W?Sn deposits can be divided into two belts: the NE-trending igneous rocks belt in southeastern coast and the EW-trending igneous rocks and W?Sn deposits belt in southern South China.They are different in distribution direction,rock association and mineralization.The NE-trending igneous rocks belt is compatible with the subduction direction of the Pacific plate in the Late Cretaceous.The EW-trending igneous rocks and W?Sn deposits belt is in accord with the direction of the Neo-Tethys slab rollback,but disagrees with the SE-trending rollback of the Pacific plate.Therefore,they are influenced by the Pacific plate and Neo-Tethys plate,respectively.Taken together,Neo-Tethys still influenced the South China Block in the Cretaceous.The ridge of the Neo-Tethys subducted beneath the Yangchun district and formed the Shilu Cu?Mo deposit in ¹04 Ma.As the subduction going,Neo-Tethys slab began to rollback.The EW-trending igneous rocks and W?Sn deposits belt is attributed to the slab rollback of the Neo-Tethys.