Multi-stage Magmatism In Laojunshan Of SE Yunnan,China:Geochemistry,Geodynamic Implication And Related Mineralization

As a major component of continental crust,the origin of granites is closely associated with the evolution of the continental crust.The early Paleozoic granite and Mesozoic granite are both the most important plutons in the Laojunshan district,west of South China Block.The petrogenesis of these rocks remains unclear,particularly with regard to the nature of source regin and geodynamic setting.The Early Paleozoic Orogen in South China has been a topic of hot debate;in particular with regard to the relative contributions of crustal and mantle sources to the generation of granites.To address this issue,we report detailed geochronological and geochemical data for three related suites of granites?Tuantian,Suitel;Nanlao,Suite 2;and Laochengpo,Suite 3?from the Laojunshan district in the southeast Yunnan Province,western South China Block?SCB?.LA-ICP-MS U-Pb dating of zircons indicates magmatic crystallization ages of?436Ma,?430Ma and?427 Ma for Suites 1,2,and 3 granites,respectively.The ?8Hf?t?values of magmatic zircons in all three suites are heterogeneous,ranging between +3 and-14.All three suites also contain inherited zircon cores with Proterozoic U-Pb and Hf model ages.Bulk compositions form well-defined mixing arrays,with Suites 1 and 2 being strongly peraluminous?A/CNK>1.1?,whereas Suite 3 is weakly peraluminous?A/CNK = 1.0-1.1?.Suites 1 and 2 have lower TFe₂O₃,Al₂O₃,MnO,MgO,CaO,Ti02,Na₂O,Mg#and Nb/Ta but higher K₂O,Rb/Sr,Rb/Ba and ?Nd?t?than those of Suite 3.These results suggest that Suite 1 and Suite 2 were formed by partial melting of Proterozoic metasedimentary rocks with little or no input of mantle-derived materials,but Suite 3 was derived from a mixture of a crustal melt with mantle derived mafic magma.The apparent paradox that the mantle-derived component has lower ?Nd?t?values than the crustal components is consistent with the very low sNd?t?values found in mantle-derived basalts in the SCB.The Wuyi-Yunkai orogenic collapse may have caused lower crustal melting of the Cathaysia Block.The early Paleozoic granitoid magmatism with mafic magma input indicates an extensional environment in the western SCB,in response to post-collisional orogenic collapse.Asthenospheric upwelling and basaltic underplating probably contributed heat and melts triggering felsic magmatism in the western SCB.In this study,three types of mineralization-related granites were identified in the Laojunshan district,including the Dulong coarse-grained granite?DCG?,the Dulong fine-grained granite?DFG?,and the Dulong porphyritic granite?DPG?.Detailed studies are presented on zircon U-Pb ages,major and trace elements,mineral chemical and Nd-Hf isotopic compositions of the tin-bearing granites from the Laojunshan district.LA-ICP-MS U-Pb dating of zircon grains from these three granite bodies yields ages of 90.1 ± 0.7 Ma,89.7 ± 0.8 Ma and 86.0 ± 0.5 Ma,respectively,indicating the same emplacement age of the three bodies.Geochemically,the granites are strongly peraluminous,with high contents of alkalis,enrichment in P,Li,Rb,Cs,Ta,Sn,W and U,depletion in Ti,Mg,Co,Ni,Sr,Ba,Zr,Hf,Th and rare earth elements.Fractional crystallization of plagioclase and K-feldspar was the principal process of magmatic differentiation that controlled Rb,Sr,Ba and Eu concentrations,whereas rare earth elements were fractionated by accessory minerals,such as apatite and monazite.The geochemical data suggest that the rocks are highly fractionated S-type granites.The granites show bulk rock ?Nd?t?values in the range of-12.2 to-10.8 and zircon ?Hf?t?values from-15.5 to-2.5,with Meso-Paleoproterozoic TDM2 ages for both Nd and Hf isotopes.Geochemical and isotopic data suggest that these highly fractionated S-type granites DCG,DFG and DPG were originated from the same episode of partial melting of the protolith,which have analogous components of metamorphosed pelitic rocks from the Meso-Paleoproterozoic continental crust.The Sn content of the three granitic rocks is much higher than average upper crustal and lower crustal,we thus attribute the three granites to tin-bearing granites.In-situ U-Pb data on cassiterite was also obtained by LA-MC-ICP-MS from the Dulong tin deposit in Laojunshan district.In this study,we obtain the 206Pb/207Pb-238U/207Pb isochron age of cassiterite samples was 86.6?90.6 Ma.These ages indicate that the Sn-mineralization take place during Late Cretaceous,which are similar to the Late Mesozoic magmatic event in Laojunshan area.The close temporal and spatial link between the pluton's emplacement and tin mineralization suggest that the late Cretaceous magmatic hydrothermal events were the key factor of the Sn polymetallic deposit mineralization in the Dulong area.The electron-microprobe analyses of rock-forming minerals indicate that the tin-bearing granitic magma,rich in volatile component?F and Cl?,was derived from partial melting of the crust with high maturity.The oxygen fugacity calculated by biotite compositions is below Ni-NiO?NNO?,which shows a low oxygen fugacity of magma.At low oxygen fugacity,tin may be present in the bivalent state with big ionic radius,thereby precluding its widespread entry into rock-forming minerals and favoring accumulation in the residual liquid.Extreme fractional crystallization resulted in enrichment of tin in the evolved granitic magma,and the reduced magmas?fO₂ below NNO?would efficiently remove Sn into a hydrothermal fluid,and lead to deposition of Sn-rich mineral phases.The Dulong superlarge Sn-Zn deposit,located in the Southwest of the Laojunshan district,is the third largest cassiterite sulfide deposit?-30 Mt of Sn?in China.Besides,the Gejiu superlarge Sn deposit,adjacent area from the Dulong orefield?Laojunshan district?,is the largest Sn deposit in China.As world-class Sn-polymetallic ore deposits in west of South China Block,the widespread late Cretaceous magmatism and mineralization occurred in Dulong?Laojunshan district?and Gejiu district.Therefore,the close spatial and temporal association of the two units prompted us to model them as contrasting tin-bearing granitic suites to provide better understanding of the nature of source region and geodynamic setting in the west of South China Block.Compared with the coeval granites in Gejiu ore district close to our study area,the Laojunshan granites have lower TFe₂O₃,Al₂O₃,MgO,TiO₂,P₂O₅ and REE as well as ?Nd?t?values but higher Rb,Rb/Sr,initial 87Sr/86Sr ratios and zircon ?Hf?t?values.Geochemical data indicate that the Laojunshan granites,interpreted as the felsic end-member with strongly peraluminous components in Laojunshan-Gejiu area,were formed by partial melting of Meso-to Paleoproterozoic basement dominated by pelitic composition without input of mantle-derived materials,and the Gejiu granites was from a mixing source of the pure crustal magma?Laojunshan granites?and the mantle mafic magma?Gejiu Gabbro-MME rocks?.The association of Late Cretaceous Laojunshan and Gejiu granitoids indicates that the magmatism in the western South China was produced in the intraplate setting with extensional environment.The upwelling of asthenospheric triggered partial melting of metasedimentary rocks in the overlying crust formed the S-type granitic magmas such as Laojunshan granites,whereas the injection and pounding of these mantle-derived melts into the more thinned crust close to the Laojunshan district,forming the Gejiu granites as well as the Gabbro-MME rocks.The different magmatism between Laojunshan and Gejiu distict also responds the distinct metallogenic specificity for these two regions.Considering a higher content of Cu and lower content of Sn in the mantle than in the crust,it is likely that extensive Sn-Zn mineralization is associated with crustal-derived magmas in the Pengshan ore district,whereas widespread Sn-Cu mineralization is associated with magmatism related to crustal-mantle magmas in the Gejiu ore district.