Petrogenesis And Tectonic Implications Of A-Type Granites

A-type granites have received considerable interest globally in recent times.In Qinling orogenic belt,a series of A-type magmatism were emplaced during Paleozoic and Mesozoic periods.However,their origin and tectonic setting remains in debate.This study focuses on the spatially-related Paleozoic and Mesozoic A-type granites from Suizhou-Zaoyang-Tongbai regions of South Qinling in order to determine their origin and tectonic significance within the geodynamic context of Qinling orogenic belt.To achieve this objective,systematic petrological,mineral chemistry,geochemical,LAICP-MS zircon U-Pb ages,zircon Lu-Hf,and Nd isotopic analysis were conducted for the A-type granites and syenite from these three regions of South Qinling.The results show that the syenites and peralkaline A-type granites were episodically emplaced in Suizhou-Zaoyang region between 449±2 Ma and 441±7 Ma,which corresponds to Late Ordovician and Early Silurian periods,respectively.Petrologically,the syeniteperalkaline granite association,comprises nepheline-normative syenite and alkali granite in Guanzishan and quartz-normative syenite and alkali granite in Huangyangshan.The syenite –granite association are ferroan to alkali in composition and are characterized by intermediate to high Si O2(59.35 to 72.20 wt.%),high Al2O3(10.98-18.76 wt.%),K2O(3.65-6.78 wt.%),Na2O(4.62-6.28 wt.%),Fe O(3.32-9.08 wt.%)and low Ca O(0.12-1.08),P2O5(0.01-0.11 wt.%).They depict characteristics typical of OIB originated A-type granites in normalized multi-elements spider diagram and Yb/Ta vs.Y/Nb as well as Ce/Nb versus Y/Nb binary plots.Significant depletion in Ca O,Mg O,Ti O2,Cr,Ni,Ba,Sr,P,Ti,and Eu indicates fractionation of pyroxene,feldspars,biotite,calcic amphiboles and titanium-rich augite.The epsilon neodymium isotope ? Nd(t)values of Guanzishan nepheline syenite and alkali granite are +1.81 and +2.26 respectively,and the calculated two-stage model ages for these rocks are1040 and 1003 Ma,respectively.On the opposite hand,the Huangyangshan alkaline granite has ? Nd(t)values ranging from +2.61-+3.46 and a relatively younger twostage Nd model age values ranging from 906-975 Ma,respectively.In addition,Lu-Hf isotopic data for Guanzishan yielded ? Hf(t)of +0.87-+9.48 and a two-stage model age ranging from 774-1250 Ma.Based on these data,it is inferred that the Guanzhishan nepheline syenite and granites were formed from fractional crystallization of OIB-comparable mafic magmas originated from upwelling of the metasomatized asthenospheric mantle.The Huangyangshan quartz syenite and granite,on the other hand,were formed from similar magmas by means of fractional crystallization in addition to a little contribution from juvenile crustal rocks.Typically,the rocks exhibit A1-type granite affinity and classified as within plate granites associated with the Ordovician crustal extension and Silurian rifting.During Early Paleozoic,the PaleoQinling Ocean subducted southward beneath the Qinling-Dabie unit,which resulted into slab break-off.Such slab break-off triggered the upwelling of the asthenosphere,giving rise to back-arc extension around 449 Ma.The nepheline syenite in Guanzishan probably emplaced in the back-arc extension setting.More extension and consequent rifting around 441 Ma led to the emplacement of Huangyangshan granitoids in a riftrelated setting.The Paleozoic extension and rifting episodes in Qinling and adjoining Kunlun-Qilian orogens have significant geodynamic implications to the continental crustal evolution in Central China and probably marked the opening of Paleo-Tethys Ocean in the region.Similarly,a combined petrological,geochemical,U-Pb ages,Zircon Lu-Hf and Nd isotopic study has also been carried out for the A-type granites from Songpa intrusive dikes in Tongbai region.The results show that two distinct peraluminous A-type granites occur in Tongbai-region between 129±1 Ma and 130±2 Ma,corresponding to the Early Cretaceous period.In terms of mineralogy,the granitoids from Songpa comprise of alkali granite and quartz-normative syenite.The granitoids are ferroan to alkali in composition.They portray characteristics representative of crustally-derived A-type granites in normalized multi-elements spider diagram and Y/Nb as well as Th/Ta ratios.Depletion in Ba,Sr,Nb,Ta and Eu and very low Sr/Y ratios(<1)indicates the dominant role of fractional crystallization during magma evolution.Also,the depletion of the Ba,Sr,Nb,Ta together with high LREE and clear Hf and Zr anomalies is typical of crustally-derived magmas.Furthermore,Nd isotopic analysis yielded ? Nd(t)values of-13 to-17.65 and-18.75 for the Songpa quartz syenite and alkali granites,respectively.Both intrusions have two stages Nd model ages ranging between 2.02 and 2.44 Ga and yield epsilon hafnium ? Hf(t)values ranging from-2.34 to-16.69 and from-9.22 to-12.88,respectively.Calculated two stage Hf model for both intrusions are 1186-1974 Ma.Based on these data,it can be inferred that the Songpa alkali granite and quartz syenite,were formed from partial melting of Paleoproterozoic continental crustal materials which was accompanied by fractional crystallization during magma evolution.Notably,the rocks exhibit A2-type granite affinity and are categorized as post-collisional granites linked to the destruction of NCC and the creation of Qinling orogenic belt during the late Triassic to Early Cretaceous period.