Neoarchean polymetamorphism in the West Shandong massif, North China Craton: Implications for a multiphase sagduction model

Archean cratons commonly experienced a protracted evolutionary history with multiphase tonalite-trondhjemite-granodiorite (TTG) and mafic-ultramafic magmatism and corresponding polymetamorphism. Reliably reconstructing the pressure-temperature-time (P-T-t) history of Archean polymetamorphic cratons is crucial for understanding the tectonic regimes that operated on early Earth. In this paper, we conduct a comprehensive study of petrography, mineral chemistry, phase equilibrium modeling, and zircon and titanite dating for three representative samples from the Mengjiatun area in the West Shandong massif, North China Craton. Three phases of metamorphism are identified. The first phase is upper amphibolite to granulite facies with an anticlockwise P-T path, and the peak condition is 790−830 °C and ∼8 kbar, registering a geothermal gradient of 27−28 °C/km of the low-P/T type. The second phase is lower amphibolite facies marked by clockwise P-T paths and peak conditions of 610−650 °C and 7−9 kbar, corresponding to a geothermal gradient of 20−21 °C/km of the medium-P/T type. The third phase is lower amphibolite facies with a possible clockwise P-T path and a peak P-T condition at ∼500 °C and ∼4 kbar, revealing a geothermal gradient of ∼34 °C/km of the low-P/T type. Zircon and titanite dating results and age compilation indicate the metamorphic ages of the three-phase metamorphism are 2.70−2.66 Ga, 2.64−2.58 Ga, and 2.54−2.48 Ga, which are coeval with the TTG and mafic-ultramafic magmatic activity at 2.74−2.66 Ga, 2.64−2.58 Ga, and 2.56−2.50 Ga, respectively. The polyphase metamorphism, coupled with coeval magmatism, could be attributed to the multiphase sagduction of supracrustal rocks into underlying granitoid intrusions, which was in turn dynamically driven by the episodic upwelling of asthenospheric mantle with accretion of mantle-derived magmas. This multiphase sagduction model is of significance for other Archean polymetamorphic cratons.