Sapphirine-bearing granulites from the Tongbai orogen, China: Petrology, phase equilibria, zircon U-Pb geochronology and implications for Paleozoic ultrahigh temperature metamorphism

Abstract

We report here for the first time the occurrence of sapphirine-bearing granulites within the Qinling Group of the Qinling-Tongbai orogen and provide robust evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions. We document the UHT indicator of sapphirine and spinel in a mafic granulite consisting of orthopyroxene, biotite, plagioclase, amphibole and rutile/ilmenite. The ferromagnesian minerals in the sapphirine- bearing granulitehavehigh X Mg [Mg/(Mg+ Fe)](orthopyroxeneX Mg = 0.84–0.95;biotiteX Mg = 0.81; amphibole X Mg = 0.87–0.96).Thephaseequilibriamodelingdemonstratesthattheearlyspinel-bearingassemblageisstableat 923–950 °Cand6.7–8.9 kbar,andthepeakassemblageofOpx+Pl+Spr/Spl+Amp+Bt+Ilm(+melt)defines a field at 922–947 °C and 8.4–10.2 kbar. Rutiles have variable Zr concentrations but mostly cluster at ca. 1,500 and 3400 ppm. Zr-in-rutile geothermometry yielded high temperatures of up to 890–940 °C. Zircon U–Pb dating of the granulite constrains the timing of the immediate post-peak and retrograde metamorphic stages as 429 ± 7 Ma and 412 ± 4 Ma, respectively. The UHT metamorphism, together with extensive occurrence of coeval magmatic suites suggests that the Tongbai orogen experienced a Paleozoic Andean-type orogeny probably derived from mid-oceanic ridge subduction of the Qinling Ocean.