Ultrahigh-temperature metamorphism in Daqingshan, Inner Mongolia Suture Zone, North China Craton

Abstract

The Inner Mongolia Suture Zone marks the Paleoproterozoic collisional boundary between the Yinshan Block and Ordos Block in the North China Craton. The ultrahigh-temperature (UHT) orogen developed within this collisional suture reflects the extreme thermal conditions associated with the tectonic processes during the incorporation of the North China Craton within the Columbia supercontinent. Here we report the results from a systematic petrological and phase equilibria modeling study of sapphirine-bearing granulites from Daqingshan, a region within the central domain of the Inner Mongolia Suture Zone. The sapphirine–spinel–garnet–sillimanite–biotite-bearing granulites in Daqingshan occur typically as dark lenses or layers within spinel–garnet–sillimanite–biotite-bearing gneisses, and are associated with mafic granulites carrying orthopyroxene + clinopyroxene + plagioclase + calcic amphibole. The sapphirine in these rocks is magnesian ( X Mg = 0.73–0.79) and occurs in various microstructural associations such as: (1) matrix coarse-grain mineral in plagioclase aggregates, (2) subidioblastic mineral mantling spinel or in direct contact with spinel, (3) fine-grained phase in sillimanite, and (4) vermicular symplectitic aggregates with plagioclase. Garnet + plagioclase + sapphirine + sillimanite + ilmenite + inferred melt is considered to represent the peak assemblage in these rocks. We apply systematic phase equilibria analysis in the system Na 2O–CaO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O–TiO 2–Fe 2O 3 (NCKFMASHTO) to constrain the peak P–T conditions of the Daqingshan Mg–Al granulites. Our pseudosection analysis defines the stability of the peak assemblage at T > 970 °C at P > 11 kbar. The peak temperature derived from pseudosection analysis is broadly consistent with the temperature estimate from conventional thermometry of sapphirine–spinel pairs (ca. 940 °C). The extreme metamorphism recorded from the Mg–Al granulites from Daqingshan is also consistent with the occurrence of dry orthopyroxene + clinopyroxene + plagioclase, and orthopyroxene + pargasite + plagioclase in the associated mafic granulites, although conventional thermometry yields lower temperatures from these rocks (830–840 °C) marking the post-peak retrograde stage. Our data confirm UHT conditions in Daqingshan and indicate that the extreme metamorphism is of regional extent within the Inner Mongolia Suture Zone, in agreement with the model of ultra-hot metamorphic orogen developed within a Paleoproterozoic subduction–collision system.