Multi-stage metamorphic evolution of retrograde eclogite with a granulite-facies overprint in the Zhaigen area of the North Qinling Belt, China

Abstract

Retrograde eclogite from the central part of the Qinling Complex, Zhaigen area of the North Qinling Belt, was studied using integrated petrology, mineral chemistry, pseudosection modeling, and geochronology. Microstructures and mineral relationships reveal five metamorphic stages and associated mineral assemblages as follows: (1) pre-peak stage M1, which is recorded by the inner cores of garnets together with mineral inclusions of clinopyroxene (Cpx1)+amphibole (Am1)+plagioclase (Pl1)±quartz±rutile, occurred under conditions of 760–770°C and 11.4–14.0kbar; (2) eclogite-facies stage M2, recorded by garnet cores+relic omphacite (with a high jadeite content up to 31%)+rutile+quartz under conditions of >16.7kbar and 679–765°C; (3) high-pressure granulite-facies stage M3, characterized by clinopyroxene (Cpx2)+plagioclase (Pl2) symplectites under conditions of 14.5–15.6kbar and 800–850°C; (4) medium-pressure granulite-facies stage M4, characterized by the growth of plagioclase+orthopyroxene coronas around garnet under conditions of 8.3–10kbar and 795–855°C; and (5) retrogressive amphibolite-facies stage M5, which is represented by amphibole (Am3)+plagioclase (Pl3) kelyphitic rims around garnet at conditions of <4kbar and <620°C. Based on Laser Raman analysis of mineral inclusions, cathodoluminescence images, in situ trace element concentrations from different domains within zircon grains, and LA-ICP-MS and SHRIMP U–Pb dating, the protolith age of the Zhaigen retrograde eclogite is suggested at 786±10Ma and the eclogite-facies metamorphic age recorded by metamorphic zircon cores is limited within 501–497Ma. The retrograde zircon rims display ages of 476–447Ma and 425Ma that probably reflect the timing of two stages of retrograde metamorphism, respectively. The mineral assemblages, P–T conditions, and zircon U–Pb data define a clockwise P–T–t path for the retrograde eclogite, suggesting that the Neoproterozoic protolith of the retrograde eclogite might evolved into continental subduction and eclogite-facies metamorphism during 501–497Ma before undergoing retrograde metamorphism during an initial stage of exhumation to middle–upper crust level at 474–447Ma and subsequent exhumation to shallow upper crust at ~420Ma.