Structural Geology and Chronology of Shear Zones along the Shangdan Suture in Qinling Orogenic Belt,China:Implications for Late Mesozoic Intra-Continental Deformation of East Asia

The Shangdan suture zone(SDZ)in the Qinling orogenic belt(QOB)is a key to understanding the East Asia tectonic evolution.The SDZ gives information about convergent processes between the North China Block(NCB)and South China Block(SCB).In the Late Mesozoic,several shear zones evolved along the SDZ boundary that helps us comprehend the collisional deformation between the NCB and SCB,which was neglected in previous studies.These shear zones play an essential role in the tectonic evolution of the East Asia continents.This study focuses on the deformation and geochronology of two shear zones distributed along the SDZ,identified in the Shaliangzi and Maanqiao areas.The shear sense indicators and kinematic vorticity numbers(0.54–0.90)suggest these shear zones have sinistral shear and sub-simple shear deformation kinematics.The quartz’s dynamic recrystallization and c-axis fabric analysis in the Maanqiao shear zone(MSZ)revealed that the MSZ experienced deformation under green-schist facies conditions at∼400–500℃.The Shaliangzi shear zone deformed under amphibolite facies at∼500–700℃.The^(40)Ar/^(39)Ar(muscovite-biotite)dating of samples provided a plateau age of 121–123 Ma.Together with previously published data,our results concluded that QOB was dominated by compressional tectonics during the Late Early Cretaceous.Moreover,we suggested that the Siberian Block moved back to the south and Lhasa-Qiantang-Indochina Block to the north,which promoted intra-continental compressional tectonics.

Geochemistry of the E-MORB type ophiolite and related volcanic rocks from the Wushan area,West Qinling

The mafic-ultramafic assemblages,which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen,Lijiahe,Lubangou and Gaojiahe area,consist mainly of meta-peridotites,gabbros and basalts.The meta-peridotites are characterized by high SiO_(2) and MgO contents,lowΣREE,as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite.The basalts from the Yuanyangzhen,Lijiahe and Lubangou area are characterized by relatively high TiO_(2) content,low Al_(2)O_(3) content and Na_(2)O>>K_(2)O.Above all,it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB.In comparison,the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting.From studies of the regional geology,petrology,geochemistry,geo-chronology and all above evidence,it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite,which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean.It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading.This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault,and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.