DISCOVERY OF SAPPHIRINE-BEARING MG-AL GRANULITES IN THE NORTH CHINA CRATON: IMPLICATIONS FOR PALEOPROTEROZOIC ULTRAHIGH TEMPERATURE METAMORPHISM

Abstract

We report here for the first time the occurrence of Mg–Al granulites within the khondalite belt of the North China Craton and provide robust evidence for extreme crustal metamorphism at ultrahigh-temperature (UHT) conditions in this region. The UHT indicator assemblages in these rocks include equilibrium sapphirine+quartz, high alumina orthopyroxene+sillimanite+quartz and high temperature perthites. The surrounding khondalites also carry spinel+quartz equilibrium assemblage. The sapphirines occurring in various microstructural associations are markedly magnesian (XMg=0.69–0.77) with a composition varying from 7:9:3 to 2:2:1 end members. Spinel is a solid solution of Mg-spinel and hercynite (XMg=0.41–0.54) with ZnO content varying from 0.48 to 1.79 wt.%. Garnet shows a compositional range between pyrope and almandine (XMg=0.39–0.48) with minor grossular (2.6–3.8 mol%) and spessartine (0.5–1.8 mol%). The orthopyroxenes display marked compositional variations in terms of Al2O3 content, with the core of the medium-grained orthopyroxene adjacent to sillimanite in the matrix showing the highest Al2O3 content (8.4 wt.%). The history of prograde to peak metamorphism of these rocks is indicated by inclusions of sapphirine, spinel, quartz and sillimanite within garnet. Sapphirine formed during the peak stage through the reaction: Spl+Qtz+Sil→Spr.