Metamorphic evolution and Petrogenesis of garnet–corundum silica–undersaturated metapelitic granulites: A new case study from the Mianlüe Tectonic Zone of South Qinling, Central China

Abstract

Newly identified garnet–corundum silica-undersaturated pelitic granulites in the Mianlüe Tectonic Zone of the South Qinling belt, central China, preserve a distinct mineral assemblage of garnet + corundum + biotite + plagioclase ± kyanite + sillimanite + staurolite and are therefore useful for deciphering the metamorphic evolution of the zone. Most of the garnet porphyroblasts preserve prominent major- and trace-element zoning, which provides evidence for two stages of garnet growth, under subsolidus conditions and then fluid-absent conditions, during prograde metamorphism. The distinctive yttrium annuli in garnet grains are most likely the result of the breakdown of xenotime. Four stages in the pressure–temperature (P–T) evolution of the granulites are inferred from the results of P–T pseudosection analysis together with petrographic observations. The first stage records the early prograde evolution, comprising isobaric heating at ~7 kbar from 580 to ~700 °C, and the later-stage prograde evolution with increasing pressure and temperature to the peak-pressure stage. The second stage is marked by the maximum XGrs in the garnet mantle, indicating the peak conditions in the high-P–high-T granulite facies of 830 °C at ~11.7 kbar. The third stage is characterized by a continued increase in pyrope in the garnet rim, suggesting decompression with slight heating from the peak pressure to reach the peak temperature at ~9.4 kbar and ~850 °C. The late retrograde cooling stage is represented by further decompression after the peak temperature, accompanied by the growth of staurolite. 7–13 mol% melt was produced during prograde metamorphism via the reaction sill + bi + pl → g + crn + melt. Although later melt loss reduced the bulk-rock Si and Al, the low-Si and high-Al–Fe bulk compositions of restite were inherited mainly from the protolith, which comprised high-Fe sediments (bauxites) derived from weathering of adjacent mafic rock. U–Pb dating of metamorphic zircon rims yielded two age clusters: ~227 Ma represents the timing close to peak-pressure conditions, and 222–217 Ma represents the timing of post-Tmax cooling to the solidus. Monazite U–Pb ages of 212–206 Ma are interpreted as the timing of further retrograde cooling under the solidus. The geochronological data and clockwise P–T paths indicate the garnet–corundum pelitic granulites record a complete metamorphic P–T–t history, including crustal thickening during continental collision/subduction and subsequent exhumation and cooling during the early Mesozoic. Combined with recent high-precision early Neoproterozoic data, we proposed that the MTZ was formed as a result of Grenvillian oceanic plate subduction and the assembly of Rodinia during the early Neoproterozoic and that it also records processes related to continental collision/subduction during the early Mesozoic.