Petrology, phase equilibria and monazite geochronology of granulite-facies metapelites from deep drill cores in the Ordos Block of the North China Craton

Abstract

Among the various Precambrian crustal blocks in the North China Craton (NCC), the geology and evolution of the Ordos Block remain largely enigmatic due to paucity of outcrop. Here we investigate granulite-facies metapelites obtained from deep-penetrating drill holes in the Ordos Block and report petrology, calculated phase equilibria and in-situ monazite LA-ICP-MS geochronology. The rocks we studied are two samples of cordierite-bearing garnet–sillimanite–biotite metapelitic gneisses and one graphite-bearing, two-mica granitic gneiss. The peak metamorphic age from LA-ICP-MS dating of monazite in all three samples is in the range of 1930–1940Ma. The (U+Pb)–Th chemical ages through EPMA dating reveals that monazite occurring as inclusions in garnet are older than those in the matrix. Calculated metamorphic phase diagrams for the cordierite-bearing metapelite suggest peak P–T conditions ca. 7–9kbar and 775–825°C, followed by decompression and evolution along a clockwise P–T path. Our petrologic and age data are consistent with those reported from the Khondalite Belt in the Inner Mongolia Suture Zone in the northern part of the Ordos Block, suggesting that these granulite-facies metasediments represent the largest Paleoproterozoic accretionary belt in the NCC.