The Petrogenesis Of Early Paleozoic Intracontinental Magmatism In The Zhuguang-Wanyang Mts District,Cathaysia Block

The formation and evolution of continental crust and felsic magmas are always major concerns in igneous petrology.Recently,studies on silicic large igneous province indicate that the under-and intra-plating of "hidden" large-scale basaltic magmas are the major mechasims inducing the extensive partial melting of the lower to middle crust.The early Paleozoic Cathaysia Block experienced extensive intracontinental felsic magmatism,therefore,the studies on these early Paleozoic magmatic rocks will enhance our understanding of the Paleozoic geological evolution of the Cathaysia Block,and provide insights into the petrogenesis of intracontinental felsic magmatism.In this contribution,we present a detailed investigation of the early Paleozoic mafic-felsic magmatic rocks from the Zhugang-Wanyang district,interior the Cathaysia Block.Through an integrated study of petrological,mineralogical,and geochemical data,and results from previous studies,we discussed the contribution of crust-mantle interaction in the formation of the early Paleozoic magmatic rocks in the Cathaysia Block.Based on mineral assemblages,the early Paleozoic granitoids from the Zhugang-Wanyang district are divided into three groups:Group A-amphibole-bearing granitoids characterized by relatively low initial 87Sr/86Sr ratios(0.7052 to 0.7116),high ?Nd(t)values(-7.0 to-3.0),and high zircon ?Hf(t)values(-6.9 to-2.8);Group C-two-mica granites that have high initial 87Sr/86Sr ratios(0.7153 to 0.7219),low ?Nd(t)values(-9.4 to-7.3),and low zircon ?Hf(t)values(-9.0 to-7.2);and Group B-biotite granites that have geochemical and isotopic compositionsthat are roughly intermediate between those of Group A and Group C.Zircon U-Pb ages of these early Paleozoic granitoids generally decrease from Group A(amphibole-bearing granitoids)to Group B(biotite granites)and then to Group C(two-mica granites),although they overlap within the range of analytical error(from 443 Ma to 431 Ma).A mafic microgranular enclave(MME)from the Guantian quartz dioritic pluton(Group A)shows Sr-Nd isotopic signatures sim i lar to its host,but higher zircon ?Ht(t)values(-1.0±0.5).The Group A granitoids show weak negative Eu anomalies,whereas Group B and Group C granitoids show moderate to strong negative Eu anomalies.Recently,a number of coeval ?-type granite and mafic rocks were identified,along with the considerable amount of MMEs in most of the early Paleozoic granitoid plutons,we suggest that the under-and intra-plating of extensive mantle-derived mafic magmas contributed sufficient heat and minor juvenile components in the formation of the widespread early Paleozoic granitoids in the Cathaysia Block,as opposed to being derived solely from crustal anatexis without any mantle contribution.The study of crust-mantle interaction in the formation and evolution of felsic rocks could not be well constrained without efforts on mafic rocks.It is shown that the Fuxi monzonorite samples have signatures of both mantle-derived mafic magmas(substantial contents of MgO,Cr.and Ni;presence of olivine and orthopyroxene)and crust-derived magmas(substantial contents of SiO2.K2O,Rb,Ba,and light rare earth elements;presence of K-feldspar and low-calcium plagioclase).Interestingly,the monzonorite,granodiorite and the MME are remarkably uniform in Sr-Nd-Hf isotopic signatures,with high initial 87Sr/86Sr ratios(0.7081-0.7098),low?Nd(t)values(-6.8 to-6.3)and low zircon ?Hf(t)values(-8.0 to-7.4).Combined with previous studies,it is shown that the early Paleozoic mafic magmatic rocks in the Cathaysia Block demonstrate extremely heterogeneous isotopic signatures from enriched to depleted in a large array(initial 87Sr/86Sr ratios of 0.7041-0.7102,?Nd(t)values of-8.4 to +1.8,and weighted mean zircon ?Hf(t)values of-7.4 to +5.2).It is also shown that these early Paleozoic mafic rocks have significant low Ce/Pb and Nb/U ratios(0.59-13.1 and 3.5-20.9,respectively),in contrast to typical mantle-derived magmas(e.g.,MORB and OIB).Particularly,the high-MgO(>10 wt.%)mafic rocks tend to have low ?Nd(t)values(<-5),while the relatively low-MgO(<10 wt.%)mafic rocks could have relatively higher ?Nd(t)values(>-4),suggesting crustal contamination of the high-MgO mafic rocks.We proposed that the high-MgO mafic rocks with fertile compositions may not derived from an enriched mantle sources and the early Paleozoic lithospheric mantle beneath the Cathaysia Block is more likely moderately depleted.The geodynamic mechanism for the early Paleozoic intracontinental magmatism in the Cathaysia Block remains debated.According to geological,geochemical,geochronological,paleomagnetic and paleontological data,the South China Block maintained a position adjacent to the western Australia and northeast India in the east Gondwanaland before early Devonian,and subsequently drifted across the Tethys Ocean before accreting to present day Asia.Considering the style of deformation in the early Paleozoic South China Block,we speculate that the separation of South China Block from east Gondwanaland began in the late Ordovician and finished prior to early Devonian.During the breakup process,stresses originated from plate boundary triggered asthenospheric upwelling and associated decompression melting beneath rift-related local weak zones(e.g.,failed rift basin)in the Cathaysia Block,which induced the widespread partial melting of the lower to middle crust and the formation of widely distributed contemporaneous intracontinental magmatic rocks.