Is the Songshugou Complex,Qinling Belt,China,an Eclogite Facies Neoproterozoic Ophiolite?  

作　　者：Thomas Bader Lifei Zhang Xiaowei Li 

机构地区：[1]Key Laboratory of Orogenic Belts and Crustal Evolution,Ministry of Education, School of Earth and Space Sciences, Peking University,Beijing 100871, China [2]State Key Laboratoiy of Geological Processes and Mineral Resources,School of Earth Sciences and Resources, China University of Geosciences, Beijing 100083, China

出　　处：《Journal of Earth Science》2019年第3期460-475,共16页地球科学学刊（英文版）

基　　金：supported by the National Natural Science Foundation of China (Nos. 41350110224, 41750110483, 41730426, 41872066);the Open Research Project of the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (No. GPMR201820)

摘　　要：Ophiolites play a key role in understanding subduction-accretion-collision processes. Herein, we discuss origin and metamorphic evolution of an enigmatic, Neoproterozoic ophiolite candidate—the mafic-ultramafic Songshugou Complex, Qinling belt, China—summarizing published thermobarometr);U/Pb geochronology, and geochemistry and presenting new phase equilibrium modeling. Garnet, rarely preserved in amphibolites of the Songshugou Complex, has prograde zoning and low-pvrope cores |Alm54-71(Grs+And)25-30PrP1-6Sps5-12. It formed at quartz eclogite facies conditions of 1.93-2.54 GPa, 462-542℃. During exhumation, garnet first was mantled by plagioclase-rich coronas at about 0.7-1.2 GPa, 660-710℃. During an isothermal uplift to 0.5-0.8 GPa, these coronas evolved widely into n-shaped aggregates and eventually into whitish ribbons oriented with a steeply southwest dipping mineral stretching lineation. The exhumation into middle-upper crustal levels proceeded till the Late Devonian. The oceanic protoliths of the eclogites were emplaced into continental crust in the Neoproterozoic and dragged into a subduction zone in North Qinling in the Cambrian. The ultramafic rocks of the Songshugou Complex were not subducted with the mafic rocks in a coherent block given the absence of garnet but ubiquitous occurrence of spinel implies a P maximum of ?1.7 GPa. Rather, mafic and ultramafic rocks belonged to downgoing and overriding plate, respectively. They were juxtaposed at 0.8-1.7 GPa at Early Ordovician time.Ophiolites play a key role in understanding subduction-accretion-collision processes. Herein, we discuss origin and metamorphic evolution of an enigmatic, Neoproterozoic ophiolite candidate—the mafic-ultramafic Songshugou Complex, Qinling belt, China—summarizing published thermobarometry, U/Pb geochronology, and geochemistry and presenting new phase equilibrium modeling. Garnet, rarely preserved in amphibolites of the Songshugou Complex, has prograde zoning and low-pyrope cores [Alm54–71(Grs+And)25–30 Prp1–6 Sps5–12]. It formed at quartz eclogite facies conditions of 1.93–2.54 GPa, 462–542 oC. During exhumation, garnet first was mantled by plagioclase-rich coronas at about 0.7–1.2 GPa, 660–710 oC. During an isothermal uplift to 0.5–0.8 GPa, these coronas evolved widely into σ-shaped aggregates and eventually into whitish ribbons oriented with a steeply southwest dipping mineral stretching lineation. The exhumation into middle-upper crustal levels proceeded till the Late Devonian. The oceanic protoliths of the eclogites were emplaced into continental crust in the Neoproterozoic and dragged into a subduction zone in North Qinling in the Cambrian. The ultramafic rocks of the Songshugou Complex were not subducted with the mafic rocks in a coherent block given the absence of garnet but ubiquitous occurrence of spinel implies a P maximum of ～1.7 GPa. Rather, mafic and ultramafic rocks belonged to downgoing and overriding plate, respectively. They were juxtaposed at 0.8–1.7 GPa at Early Ordovician time.

关 键 词：QINLING BELT Songshugou COMPLEX garnet isopleth THERMOBAROMETRY ECLOGITE Early Paleozoic 

分 类 号：P[天文地球]