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

Abstract

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)25−30Prp1−6Sps5−12]. It formed at quartz eclogite facies conditions of 1.93–2.54 GPa, 462–542 °C. During exhumation, garnet first was mantled by plagioclase-rich coronas at about 0.7–1.2 GPa, 660–710 °C. 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.