The Caledonian And Indosinian Magmatism In The Tangzang Area Of The Western Qinling Orogenic Belt And Their Tectonic Implications

The cognition of slab subduction and continental collision is crucial to develop plate tectonics.Composite collisional orogens commonly experienced very complicated formation and evolution,such as ocean subduction,arc-continent collision,continent-continent collision,and intraplate evolution,along with geodynamic transition at mantle depth and wholesale magmatic rocks,playing a major role in shaping the evolution of continental crust.The Qinling orogenic belt formed in the Mesozoic is a well-known giant collisional orogenic zone in Asia and experienced muti-stage continental collision between the North China Craton and South China Block.The early Paleozoic and Triassic continental collisions formed strong mafic-felsic magmatism and resulted in significant crustal growth and reworking.Therefore,the study of Caledonian and Indosinian magmatism in the Qinling orogenic belt is crucial to decipher geodynamic setting and associated crustal evolution during ocean subduction and continental collision.However,the petrogenesis of the igneous rocks,tectonic evolution and crustal evolution in the Caledonian and Indosinian are still open to debate.The main questions are as follows:1.The petrogenesis and geodynamic setting of the Caledonian and Indosinian magmatic rocks,especially the Indosinian adakitic rocks;2.The timing of continental collision and Caledonian-Indosinian tectonic evolution in the Qinling orogenic belt;3.The Caledonian-Indosinian crustal evolution of the Qinling orogenic belt.To elucidate the above questions,the present study involved integrated analyses of in-situ U–Pb dating,trace element,and Hf isotopic compositions of zircon,and whole-rock major and trace element and Sr–Nd-Hf isotopic compositions of the mafic-felsic rocks in the Tangzang area of the western Qinling orogenic belt,and obtained the following results:?1?The western Qinling orogenic belt developed the strong Caledonian magmatism and volcanic activites,with the exposure of high Ba-Sr intrusions and Caotangou Group volcanic rocks in the research area.The high Ba-Sr intrusions can be divided into K-rich high Ba-Sr intrusion represented by the Yangjiazhuang quartz diorites and Honghuapu diorites and Na-rich high Ba-Sr intrusion represented by the Tangzang quartz diorites.LA–ICP–MS zircon U–Pb dating indicated that the K-rich dioritic intrusions were formed at 439–438 Ma,and the Na-rich intrusion at 429–425Ma.Rocks of the three intrusions are characterized by metaluminous to weakly peraluminous,high Ba and Sr concentrations,and relatively homogeneous Sr and Nd isotopic compositions with(⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7032–0.7059,?_Nd?t?values of-0.65to-2.78,indicative of an enriched mantle source.The K-rich intrusions exhibit minor Nd–Hf isotopic decoupling(??_Hf?t?=2.93–5.01)and have low Nb/U ratios and Mg numbers,which suggest that they were formed through fractional crystallization of partial melts of enriched mantle metasomatized by terrigenous-sediment-derived fluids.The Na-rich high Ba-Sr intrusion exhibits distinct??_Hf?t?values?5.72–9.35?,high Na₂O and Nb contents,and highly variable Rb/Y,Nb/Y,Th/Zr,and Nb/Zr ratios,indicating that they were formed through crustal assimilation and fractional crystallization of partial melts of enriched mantle previously fertilized by terrigenous sediment-derived fluids and slab melts.The Caotangou Group sodic rhyolites have LA–ICP–MS zircon U–Pb age of 435Ma,high Na₂O contents,low Mg numbers and K₂O contents,and obvious negative Eu anomalies.They have enriched Sr and Nd isotopic compositions((⁸⁷Sr/⁸⁶Sr)_i=0.70612–0.70647;?_Nd?t?=1.03–1.62)and elder two-stage Nd model ages,which indicate that the sodic rhyolites were formed by fractional crystallization and crustal assimilation of back-arc basin basalts.?2?The western Qinling orogenic belt developed the strong Indosinian mafic-felsic magmatism,with the exposure of pyroxenite-syenite complex,adakitic intrusive rocks?granodiorite,granite porphyry,and dacite porphyry?,and biotite granite porphyry in the research area.The pyroxenite-syenite complex is mainly composed of biotite pyroxenite,gabbro,quartz monzonite and syenite with ages of233–231 Ma.They show fractionated and parallel REE patterns,negligible Eu anomalies,parallel PM-normalized multi-element plots,extremely high Ba and Sr contents,and enriched Sr-Nd isotopes((⁸⁷Sr/⁸⁶Sr)_i=0.70535–0.70652,?_Nd?t?=-1.40to-3.42).The Nd isotopic data has a tendency toward lower values as SiO₂ contentsincrease,which suggests that the complex was formed by fractional crystallization and crustal assimilation of enriched-mantle-derived-melts.The adakitic granodiorite,granite porphyry,and dacite porphyry have zircon U–Pb ages of 237–216 Ma and present the following feature:metaluminous to weakly peraluminous,enriched large-ion lithophile elements,depleted high-field-strength elements,high Sr/Y?31.8–77.5?and?La/Yb?_N?13.6–72.6?ratios,low Y?5.3–16.3 ppm?and Yb?0.35–1.53 ppm?contents,and negligible Eu anomalies.Their(⁸⁷Sr/⁸⁶Sr)_i ratios?0.70715–0.70884?,?_Nd?t??-5.93 to-3.07?and zircon?_Hf?t??-2.34 to+0.85?values indicate that they are likely partial melts of the Neoproterozoic lower crust,with input of enriched-mantle-derived mafic magma.There is no evidence supporting high-pressure magmatic processes,while partial melting modeling indicates that the high-Sr/Y signature of these intrusive rocks may have been inherited from source rocks?the lower crustal rocks of the South Qinling terrane?.Therefore,we suggest that the simple association of high Sr/Y rocks with a specific geodynamic setting,such as thickened and delaminated crust,needs to be more cautious.The biotite granite porphyries were formed at 215 Ma and have low P₂O₅ contents and Mg numbers.They have peaks at large-ion lithophile elements,troughs at high-field-strength elements,and present mildly fractionated REE patterns and negative Eu anomalies.Their Sr and Nd isotopes((⁸⁷Sr/⁸⁶Sr)_i=0.70565 to 0.70648,?_Nd?t?=-4.43 to-5.33)indicate that the biotite granite porphyries were derived from the Neoproterozoic mafic lower crust of the South Qinling terrane.Besides,the low Mg numbers and the obvious negative Eu anomalies demonstrate that the biotite granite porphyries might result from fractional crystallization of crustal melts.?3?The Caledonian tectonic evolution of the Qinling orogenic belt:The belt is controlled by the northward subduction of the Shangdan Ocean during the Cambrian and early Silurian;The closure of the Shangdan Ocean and initial collision of the North China Craton and South China Block in the Qinling orogenic belt occurred at ca 433Ma;The Ordovician–Early Silurian mafic-felsic intrusions such as K-rich HiBaSr intrusions were formed in a slab subduction setting via the asthenosphere convection in mantle wedge induced by slab rollback?slab sinking?;The Middle-Late Silurian Na-rich HiBaSr intrusions and I-,S-,A-type granites were controlled by slab break-off after continental collision.?4?The Hercynian-Indosinian tectonic evolution of the Qinling orogenic belt:The opening of the Mianlue Ocean and the separation of the South Qinling terrane from the South China Block occurred in the Late Paleozoic?ca 420-400Ma?;The initial northward subduction of the Mianlue Ocean occurred in the Middle Hercynian?ca 320Ma?;The closure of Mianlue Ocean and Indosinian collision of the North China Craton and South China Block experienced a scissor-like,northwestward propagating continental collision from ca.230 Ma to ca.200Ma;A southeastward slab tearing triggered asthenosphere upwelling and subsequent mafic-felsic magmatism during the Indosinian scissor-like continental collision in the Qinling orogenic belt.?5?The voluminous mantle-derived mafic rocks and their fractionates,such as high Ba-Sr rocks,intruded into the Western Qinling orogenic belt in the Ordovician-Early Silurian,which was manifested as net crustal growth;This belt evolved into the syn-/post-collision setting in the Middle-Late Silurian and was intruded by crustal I-,S-,A-type granitoids,which was exhibited as crustal reworking;This belt was successively intruded by the mantle-derived mafic rocks,intrusive rocks formed by magma mixing,and crustal rocks from old to young in the Triassic,which indicated that the Qinling collisional orogen experienced a gradual transition from net crustal growth to crustal reworking.