Geochronology,Petrogenesis And Tectonic Implications Of Silurian Gabbro In The Erlangping Tectonic Belt From Xinyang,Tongbai Orogen

The Qinling-Tongbai orogen,as a typical collision orogenic belt,has carried out a mass of research on the orogenic process of the Early Paleozoic period before the Triassic collision.The Early Paleozoic tectonic model of“trench-arc-back arc basin”system on eastern Qinling is accepted by most researchers.However,there are still disputes over the orogenic process,arc-basin framework,and tectonic setting of the late Early Paleozoic on the eastern extended areas of the Qinling Orogen.Compared to Hong’an and Dabie orogen,the Early Paleozoic tectonic units are most exposed in Tongbai orogen.Thus,the Tongbai orogen is a key region to understanding the above question.In contrast to those on western part of the Tongbai Orogen,the studies on the Xinyang area,eastern part of Tongbai orogen,which is located in the junction of Tongbai and Dabie-Sulu area,is still comparatively limited.There are some problems in the Xinyang area that need to be further determined,mainly about whether the Paleozoic period developed rock assemblages with similar characteristics to those in the Tongbai area,and whether they could be unified with the Qinling orogen in the eastern segment in the same tectonic setting and orogenic process.Erlangping unit,as one of the Paleozoic metamorphic units,is significant for understanding the tectonic evolution of the Paleozoic orogenic events in Tongbai orogen.The Erlangping tectonic belt include volcanic-sedimentary assemblages similar to those in the East Qinling area and ultramafic-mafic intrusive rocks.These intrusive rocks provide a key for reconstructing the tectonic framework of the Tongbai orogen in Paleozoic and comparing the tectonic evolution of the Paleozoic orogenic events in Qinling-Tongbai orogen.In this paper,we present zircon U-Pb data,and whole-rock elemental and Sr-Nd isotopic results for the gabbros in Erlangping tectonic belt in Shanshuwan and Zhangjiachong areas,and aim to constrain the formation age and tectonic background of gabbro,further highlight the tectonic framework and evolution of Paleozoic orogenic event in Tongbai orogen.The main conclusions are as follows:According to the regional geological data,the Erlangping unit in the eastern of Tongbai orogen including Dalishu formation,Zhangjiadazhuang formation and Liushanyan formation from bottom to top,which generally presents NNE trending monoclinal structure.Our field geological surveys show that Dalishu formation mainly consist of metaspilite.Zhangjiadazhuang Formation mainly consist of mica-quartz schist,plagioclase-leptynite and metaclastic rock.Liushanyan Formation mainly composed of spilite,rhyolite,volcanic breccia and mylonite.The ultramafic-mafic intrusive rocks in the Erlangping tectonic belt are gabbro and serpentinite.The Erlangping unit was intruded by early Paleozoic granite（such as Huangjiawan,Duhudian and Bingmasichong granitic plutons）.LA–ICP–MS zircon U–Pb dating from two samples in Shanshuwan area yield the two weighted mean ages of 432.2±4.3 Ma and 441.3±9.5 Ma.Geochemical analysis shows that the gabbro in Shanshuwan belongs to tholeiitic basalts series.The Si O₂content ranged from 41.65 to 48.24 wt.%,Na₂O+K₂O ratios range from 0.20 to 2.45 wt.%,and Na₂O/K₂O range from 6.51 to 34.42.The value of Mg^#range from 77.0 to 84.1.The chondrite-normalized REE patterns diagram exhibit relative LREE depletion,with positive Eu anomalies（δEu=1.46～4.13）.In the primitive mantle-normalized spider diagrams,they are enrichment in LILEs（Ba,Rb,and Sr）and depleted of HFSE（Nb,Ta,Zr and Hf）.These rocks have low initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7041 to 0.7042 and positiveε_Nd（t）values ranging from+2.3 to+5.3.Zircon U–Pb dating of gabbro in Zhangjiachong area yields ages of 430.9±3.3 Ma and432.1±1.6 Ma for magma crystallization.These samples exhibit variable content of Si O₂from 46.99 to 50.58 wt.%,Na₂O and K₂O content in range of 1.50 to 2.86 wt.%with Na₂O/K₂O ratios range 2.49 to 22.14.The Mg-number of gabbro in Zhangjiachong area from 80.1 to 85.3.Study samples display left-sloping pattern and no obvious Eu positive anomalies（δEu=1.05～1.63）in the chondrite-normalized REE spider diagrams,and exhibit enrichment in large ion lithophile elements（Ba,Rb,and Sr）but depletion in high field strength elements（Nb,Ta,Zr and Hf）relative to LILE in the primitive mantle-normalized spider diagrams.As mentioned above,our data shows that the gabbro in the Erlangping tectonic belt in the Xinyang has the hybrid natures of N-MORB and IAB.These mafic rocks might be derived from the spinel peridotite mantle source area.The magma experienced the metasomasis of melt produced by partial melting of subducted sediments.Based on previous studies,we consider that the Erlangping back-arc basin has been closed during～432Ma,and the gabbro developed in the Erlangping structural belt may be related to slab rollback during the northward subduction of the Shangdan ocean or the mafic-magma underplating event caused by the subduction of the mid ocean ridge.These gabbro rocks and Erlangping ophiolites in eastern Qinling belong to different periods and tectonic environments.