Metamorphic Evolution of Neoarchean Amphibolites in the Hongshanzi Area, Yinshan Block, North China Craton: Insights Into a Drip/Sagduction Regime

ABSTRACT The metamorphism of amphibolite facies Archean supracrustal rocks has variably been interpreted to relate to subduction–collision or drip/sagduction tectonics. This paper presents a systematic metamorphic study of supracrustal amphibolites from the Hongshanzi area in the Yinshan Block, North China Craton, to decipher their metamorphic evolution and tectonic implications. The amphibolites occur as rafts within host TTGs (trondhjemite–tonalite–granodiorite) and are metamorphosed under lower to upper amphibolite facies conditions. Two samples of lower‐amphibolite facies rocks record one episode of metamorphism that exhibits clockwise P–T paths involving pre‐ P max burial heating and post‐ P max decompressional heating and subsequent cooling. The P max conditions are constrained to ~595°C, 8.5–9 kbar and ~650°C, 8 kbar, using isopleths of the maximum Ti content in amphibole and minimum X An (anorthite) in plagioclase in P–T pseudosections. This approach is quite effective for the P–T estimation especially of subsolidus amphibolites. The mAl (Al in M2 site) and tAl (total Al) in amphibole have high potentials to be geobarometers, but they are commonly modified during decompression and tend to underestimate pressures for amphibolites. One sample of upper‐amphibolite facies records two episodes of metamorphism. The early episode is characterized by a decompression and cooling evolution from the peak at 790°C–830°C, 7–8 kbar, defined using Ti and tAl in amphibole in P–T and T–M (TiO 2 ) pseudosections because the rock is unsaturated with respect to TiO 2 . The later metamorphic event shows a clockwise P–T path and peak condition of 560°C–600°C, 6–7.5 kbar. Zircon dating yields a crystallization age of 2.53–2.52 Ga and three metamorphic ages at 2.52–2.51 Ga, ~2.46 Ga and 1.86–1.82 Ga that are interpreted to date amphibolite facies metamorphism, modification and the overprinting event, respectively. The Neoarchean metamorphism is characterized by variable apparent geothermal gradients from 18°C/km to 29°C/km that occurred coeval with the regional TTG magmatism and that was more likely correlated with a drip or sagduction regime. In addition, the Yinshan Block was involved in the Paleoproterozoic orogeny at 1.86–1.82 Ga.