MetamorphicP‒Tpaths and Zircon U–Pb ages of Archean ultra‐high temperature paragneisses from the Qian’an gneiss dome, East Hebei terrane, North China Craton

Abstract

AbstractThe East Hebei terrane of North China Craton is characterized by the dome‐and‐keel structure, a common feature in most Archean cratons, where supracrustal rocks of granulite facies commonly occur as enclaves or rafts in tonalite–trondhjemite–granodiorite (TTG) gneisses. The metamorphicP–Tpaths of the granulites are significant for addressing the Archean tectonic regimes. Two types of granulite facies paragneiss with pelitic and greywacke compositions from the western margin of Qian'an gneiss dome are documented for their petrography, mineral chemistry, phase equilibria modelling usingthermocalc,and zircon dating. AnticlockwiseP–Tpaths involving the pre‐peak pressure increase to the ultra‐high temperature peak conditions and post‐peak cooling and decompression processes were recognized. The pre‐peak pressure increase process was constrained for a pelitic granulite mainly based on the spinel and cordierite inclusions in garnet and rutile corona around ilmenite, where the transition from spinel to garnet is modelled at 6–7 kbar at a fixedT = 1,000°C. For greywacke granulite, the pre‐peak pressure increase evolution can be ascertained from the textural relation that orthopyroxene is surrounded by garnet, and the outwards increasing grossular (from 0.03 to 0.05) in the core of the atoll‐like garnet (Grt‐A), to occur from ~7 kbar at ~1,000°C. The peakP–Tconditions for pelitic granulite are roughly limited to 7–11 kbar/890–1,050°C on the basis of the stability of the inferred peak assemblage involving garnet, perthite, sillimanite, rutile/ilmenite, and quartz. The peakP–Tcondition for greywacke granulite can be well constrained as 9–10 kbar/>1,000°C on the basis of the maximum grossular content (XGrs = 0.045–0.050) in the core of subhedral garnet (Grt‐B) and the mantle of Grt‐A together with an average re‐integrated anorthite content (XAn = 0.07) in K‐feldspar. The peak temperature condition is consistent with the ternary feldspar thermometer results mostly of 950–1,020°C for antiperthite and perthite in greywacke granulite, and in accordance with the development of oriented needle‐like exsolution of Ti±Fe oxides in garnet from pelitic granulite. The post‐peak cooling and decompression process was consistent with the decreasingXGrsin the mantle of Grt‐A and core of Grt‐B in greywacke sample, and the final‐stage cooling conditions can be well constrained from the stability of final assemblages marked by the later growth of biotite, as 8–9 kbar/820–880°C for pelitic granulite and 6–9 kbar/840–890°C for greywacke granulite. Zircon dating yields provenance ages from 3.34 to 2.57 Ga and metamorphic ages ofc.2.50 Ga for the two types of granulite. The metamorphic ages overlap the final pulse of the Neoarchean magmatic activity of TTGs that ranges fromc.2.56 toc.2.48 Ga with a peak atc.2.52 Ga. Combining the development of dome‐and‐keel structures, the penecontemporaneity between the metamorphism of supracrustal rocks and TTG magmatic activity, and also the unique anticlockwiseP–Tpaths, we prefer a vertical sagduction regime to interpret the tectonic evolution of the East Hebei terrane, which may be also significant for other Archean cratons.