Thermal evolution of two textural types of mafic granulites in the North China craton: Evidence for both mantle plume and collisional tectonics

Abstract

Mafic granulites from the North China craton can be divided into two textural types, referred to as A- and B-types. A-type mafic granulites display garnet + quartz symplectic coronas, and outcrop in the eastern and western zones of the craton, whereas B-type mafic granulites exhibit orthopyroxene + plagioclase ± clinopyroxene symplectites or coronas, and are mainly exposed in the central zone of the craton. Most A-type mafic granulites preserve the prograde (M 1), peak (M 2) and post-peak near-isobaric cooling (M 3) assemblages, which are represented respectively by inclusions of hornblende + plagioclase + quartz, a peak mineralogy of orthopyroxene + clinopyroxene + plagioclase + quartz + garnet, and overprinted by garnet + quartz sympletic coronas. These mineral assemblages and their P-T (pressure-temperature) estimates define anticlockwise P-T evolutionary paths. The B-type mafic granulites preserve the peak (M 1), post-peak near-isothermal decompression (M 2) and cooling (M 3) assemblages, which are represented by the peak assemblage of orthopyroxene + clinopyroxene + plagioclase + quartz + garnet ± hornblende, post-peak orthopyroxene + plagioclase ± clinopyroxene symplectites or coronas, and later hornblende + plagioclase + magnetite symplectites, respectively. These mineral assemblages and their P-T estimates define clockwise P-T paths. The anticlockwise P-T paths of the A-type mafic granulites in the eastern and western zones of the North China craton are consistent with a model of underplating and intrusion of mantle-derived magmas. In combination with lithological, structural and geochronological data, the eastern and western zones of the North China craton are considered to represent two continental blocks that developed through the interaction of mantle plumes with the lithosphere from the Palaeoarchaean to the Neoarchaean era. The B-type mafic granulites and associated rocks in the central zone represent a magmatic arc that was metamorphosed and deformed during amalgamation of the eastern and western continental blocks in the late Palaeoproterozoic era. The mineral reaction relations and clockwise P-T paths of the B-type mafic granulites from the central zone record the tectonothermal history of the collision that resulted in the final assembly of the North China craton at c. 1800 Ma.