Relics of a Paleoproterozoic orogen: New petrological, phase equilibria and geochronological studies on high-pressure pelitic granulites from the Pingdu-Laiyang areas, southwest of the Jiaobei terrane, North China Craton

Abstract

High-pressure (HP) pelitic granulites have been used as important targets in evaluating ancient geodynamic processes similar to subduction, collision and exhumation operating in modern plate tectonics. In this contribution, relic HP pelitic granulites were identified from the Pingdu-Laiyang areas of the Jiaobei terrane that are dominated by low- to medium-pressure (LMP) pelitic granulites, suggesting their linear distribution and that at least some of LMP types should be retrograde products of the HP pelitic granulites. Their metamorphic P-T-t paths were traced by integrated methods of petrographic observation, mineral chemistry, P-T-X pseudosection modeling, Zr-in-rutile thermometry, and zircon and monazite geochronology. These HP pelitic granulites record consistent clockwise P-T-t paths that are manifested by similar peak metamorphism of high-pressure granulite-facies, with P-T conditions of P > 13 kbar, T > 850 °C possibly during 1950–1900 Ma, followed by near isothermal decompression (ITD) to mid-crustal level at 1900–1850 Ma and subsequent near isobaric cooling (IBC) to low temperature during 1850–1800 Ma. The consistent clockwise P-T-t paths and linear distribution of HP pelitic granulites explicitly indicate a sequence of geodynamic processes similar to modern-style plate tectonics must have operated during formation of the Paleoproterozoic Jiao-Liao-Ji belt (JLJB). These include a process similar to subduction that brought sedimentary precursors of HP pelitic granulites down, which was followed by continental orogeny during 1950–1900 Ma, leading to crustal thickening (40–50 km) and high-pressure granulite-facies metamorphism, and a post-orogenic collapse during 1900–1800 Ma that resulted in exhumation and cooling of HP granulites along ITD and IBC paths. Therefore, it is argued that ancient HP pelitic granulites can be regarded as relics of orogen and ascertain tectonic interpretation of the dominated LMP pelitic granulites in the same Precambrian metamorphic terranes. Notably, the apparent thermal gradients estimated from peak metamorphism are 15–20 °C/km, much higher than those of typical products in Phanerozoic plate tectonics, and the HP pelitic granulites that contain well-preserved kyanite probably require relatively high exhumation rates during ITD to middle crustal level. These seem to mark the differences in thermal regimes and tectonic mechanisms between Paleoproterozoic and Phanerozoic collisional orogenic systems.