Delineation of polymetamorphism from the middle-low-grade metamorphic complex in the Dengfeng area, southern North China Craton, resolved by multiple mineral petrochronology and phase equilibrium modelling

Abstract

Polymetamorphic garnet–bearing amphibolites from the Dengfeng Complex (Central Henan, China) exhibit intricate microstructural and mineral-chemical relationships. Two generations of garnet porphyroblastand various mineral age populations have been identified in this middle-low-grade metamorphic complex. Garnet1, which co-exists with titanite in the ‘dark domain’, exhibits a trend of decreasing Mn, constant Mg and notably increasing Ca from its cores to rims, indicating growth zoning associated with prograde metamorphism from ∼590 °C–650 °C and ∼3.8–4.5 kbar to ∼650 °C–680 °C and ∼5.0–5.6 kbar by phase equilibrium modelling. By contrast, garnet2 from the ‘light domain’ displays a decrease in Ca alongside an increase in Mg and low Mn, reflecting clockwise pressure–temperature (P–T) paths with peak P–T conditions reached at ∼630 °C–645 °C and ∼10.9–11.6 kbar. Laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) zircon U–Pb and amphibole ⁴⁰Ar/³⁹Ar dating yielded polymetamorphic events with extensive Neoarchean records of ∼2.49–2.53 Ga and sporadic Palaeoproterozoic records of ∼1.94–1.86 Ga, respectively. By contrast, LA–(HR)–ICP–MS titanite U–Pb data from the ‘dark domain’ only recorded late Neoarchean metamorphic ages ranging from ∼2.44 to ∼2.54 Ga. The early metamorphism from the ‘dark domain’ is characterised by low pressure and high temperature, indicating a high T/P gradient towards ∼1200 °C–1300 °C/Gpa at the Neoarchean. By contrast, the late metamorphism from the ‘light domain’ underwent clockwise metamorphic cycles at an intermediate T/P gradient of ∼540 °C–590 °C/Gpa during the Palaeoproterozoic. The diachronous evolution of these amphibolites indicates that the Dengfeng Complex in the southern North China Craton has undergone low-pressure facies series metamorphism during the late Neoarchean, which was subsequently incorporated into supercontinent Columbia during the late Palaeoproterozoic cold subduction and continent–continent collision along the Trans-North China Orogen.