Portal de Periódicos da CAPES

Metamorphic evolution of Archean ultrahigh-temperature mafic granulites from the western margin of Qian’an gneiss dome, eastern Hebei Province, North China Craton: Insights into the Archean tectonic regime

2018; Elsevier BV; Volume: 318; Linguagem: Inglês

10.1016/j.precamres.2018.10.007

1872-7433

Ting Liu, Chunjing Wei,

earthquake and tectonic studies

The supracrustal rocks of granulite facies occurs as enclaves in TTG gneisses in eastern Hebei terrane, North China Craton. Their peak P–T conditions, paths and tectonic regime remain controversial. Three samples of mafic two-pyroxene granulites from the western margin of Qian’an dome were chosen for detailed studies of petrography and P–T evolution from REE- and major element-based thermobarometers and pseudosection modelling. Their peak temperatures were constrained to be 1000–1100 °C using REE–in–clinopyroxene–orthopyroxene and REE–in–plagioclase–clinopyroxene thermometers. The major element-based clinopyroxene–orthopyroxene and amphibole–plagioclase thermobarometers yield cooling conditions of 740–830 °C/7–8 kbar. Pseudosection modelling suggests that the mafic granulites experienced a cooling evolution with decompression from UHT peak conditions at >1000 °C and 10–11 kbar in garnet-stability fields to normal granulite conditions at 820–900 °C and 7–8 kbar constrained from the stabilities of final solidus assemblages, which are well consistent with the results of REE- and major element-based thermobarometers. Both the P–T conditions and the cooling paths in P–T pseudosections are constrained by (i) the measured plagioclase compositions, especially the spectacular zoning patterns with increasing XAn [=Ca/(Ca + Na + K)] (0.46 → 0.52) from core to mantle followed by decreasing XAn (0.52 → 0.46) outwards in the rims; (ii) the measured zoning with core–rim increasing CaM2 [Ca in the M2 site] in clinopyroxene; and (iii) measured Ti in amphibole or biotite that yields temperature constraints for the final assemblages. The P–T evolution for the pre-peak stages is inferred to be a compression process based on the presence of a rounded magnetite–ilmenite grain included in clinopyroxene. Thus, an anticlockwise P–T path is inferred. Zircon U–Pb age dating yields a metamorphic age of ∼2.55 Ga, defining the protoliths of the mafic granulites were formed between 2.57 and 2.55 Ga. A summary of available age data reveals that the TTG magma activity, the formation of supracrustal rocks and their metamorphism took place simultaneously in a time span between 2.48 and 2.56 Ga. Therefore, a sagduction regime is favored to interpret the metamorphic evolution of supracrustal granulites in the east Hebei terrane.