Tracking The Genesis Of Paleozoic Granites From Qinling Orogen, Central China By Zircon U-Pb Geochronological,Geochemical And Isotopic Data

The genesis of granites received extensive attention because recording important information of continental evolution.The Qinling orogenic belt contains a series of Paleozoic granites,and their origin is one of the critical issues during the past decades.This study chooses the high-Mg granodiorite pluton from the North Qinling orogen and peralkaline A-type granites in the Tongbai orogen.The petrogenesis of the Paleozoic granites and the genetic links with metasomatized mantle has been discussed.To achieve this target,we conducted an integrated investigation of in situ zircon U-Pb dating,whole-rock major and trace elements geochemical,as well as whole-rock Sr-Nd and zircon Hf-O isotopic compositions for the Kanfenggou pluton from the North Qinling orogen and the Huangyangshan pluton in the Tongbai orogen.A systematic LA-ICP-MS zircon U–Pb geochronology suggests that the Erlangping high-Mg granodiorite pluton was formed at ca.440 Ma.The high Mg-granodiorite from the Erlangping unit have intermediate SiO₂?60.48-64.67 wt.%?and K₂O?1.21 to 2.10 wt.%?,but high Al₂O₃?15.44 to 16.51 wt.%?and Na₂O?4.01 to 4.81 wt.%?contents.The samples are elevated in MgO ranging from 2.30 to 3.44 wt.%and Mg^#values of 53.35 to 56.66,implying they are high-Mg granodiorites.They also display crustal-like trace element:very high Ba?524-1132 ppm?and Sr?684-980 ppm?contents,but depleted in HREE,and high?La/Yb?_N ratios of 6.34 to 16.5 and slightly negative to weak positive Eu anomalies?Eu/Eu*=0.68-1.09?.The mantle-like geochemical signatures?high Mg#values,Cr and Ni contents?but enriched in LILE from the high-Mg granodiorite samples suggest an enriched mantle source.These geochemical features suggest that the pluton belongs to the sanukitoid series.The high-Mg granodiorite samples show relatively depleted radiogenic isotopic compositions with initial ⁸⁷Sr/⁸⁶Sr ratios varying from 0.7044 to 0.7047,?_Nd?t?values from0.31 to 4.21,and zircon?_Hf?t?values from 7.3 to 8.3.The zircons have a mean?¹⁸O value of5.20±0.17‰.Based on the trace element geochemical features,the metasomatic agent was suggested to be the fluids generated from dehydration of subducted slab.Therefore,suggest two-stage processes for the formation of the Erlangping high-Mg granodiorites are proposed:?1?interaction between slab fluids and mantle peridotite;?2?partial melting of mantle wedge peridotite metasomatized generated the high-Mg granodiorite magmas.LA-ICP-MS zircon U-Pb dating from the Huangyangshan pluton yields a weighted mean ²⁰⁶Pb/²³⁸U age of 444.6±6.4 Ma for a representative sample,representing the timing of the pluton.The Huangyangshan pluton shows peralkaline signature?A/CNK=0.78-0.86?and belongs to shoshonite and high-K calc-alkaline series.The granitoid samples exhibit the geochemical signatures of A-type granite:high SiO₂,total alkali contents?Na₂O+K₂O?,Fe-number,TiO₂/MgO,Ga/Al ratios,and LREE and HFSEs?i.e.,Zr,Nb,Ce and Y?,but depletion in MgO,CaO,and P₂O₅.These samples show enriched LREE and flat HREE patterns with strongly negative Eu anomalies and are enriched in most LILE and high-field-strength elements with negative Eu,Ba,Sr and Ti anomalies in primitive mantle-normalized trace element diagrams.The samples have very high Zr contents?175?776 ppm?,which yield the high magma temperatures of 967?1180?.The Huangyangshan pluton displays initial zircon?_Hf?t??+6.6 to+10.0?and whole-rock?_Nd?t?values?2.86 to 3.62?.The zircon grains show a mean?¹⁸O value of 5.14±0.26‰,suggesting no incorporation of hydrothermally altered crustal materials.The geochemical signatures of the Huangyangshan pluton indicate that the magmas were derived via low degrees partial melting from a relatively depleted mantle.The tectonic evolution for the Early Paleozoic granites in the Qinling orogenic belt can be summarized as the followings:in the early Paleozoic,the Paleotethyan Ocean and the North Qinling unit were subducted northward beneath the Erlangping unit,which generated fluids that metasomatized the mantle wedge.Low degree partial melting of the metasomatized mantle wedge generated the Erlangping high Mg granodiorite pluton?ca.440Ma?in a back-arc setting during the asthenosphere mantle upwelling triggered by the slab rollback.In the Tongbai orogen,the lithospheric mantle was metasomatized by asthenospheric mantle generated OIB-like melts.At ca.444 Ma,the Huangyangshan peralkaline A-type granites was formed asthenospheric upwelling and lithospheric extension in a passive continental margin in the northern Yangtze Block,followed by the opening of the Paleotethys Ocean.