Paleoproterozoic P–T–t evolution in the Hengshan–Wutai–Fuping area, North China Craton: Evidence from petrological and geochronological data

Abstract

This contribution carries out zircon dating and a review of the previous metamorphic and geochronological work on the Hengshan–Wutai–Fuping area in the central trans-North China Orogen, for elucidating its Paleoproterozoic orogenic evolution. The Hengshan Complex can be subdivided into North Hengshan and South Hengshan Complexes, separated by the strike-slip Zhujiafang shear zone. High-pressure (HP) mafic granulite and garnet-absent amphibolite collected from the North Hengshan Complex yield metamorphic ages of 1963–1769Ma and 1882–1793Ma, respectively, both with weighted mean ages of ∼1.85Ga. In the HP mafic granulite, zircons of >1.90Ga and those of <1.90Ga have consistent REE patterns and low crystallization temperatures of 650±50°C calculated by Ti-in-zircon thermometer, where the temperatures are compatible with the cooling temperatures of the rock. Thus the zircon ages are interpreted to mainly represent the cooling stage. Garnet amphibolite collected from the Zhujiafang shear zone yields a metamorphic age of 1864±31Ma (crystallization T=650±50°C), consistent with the deformation age (amphibole/mica 40Ar/39Ar method) of the shear zone. Metamorphic zircons in this rock show steep REE patterns and high concentration of HREE, possibly indicating that they grew later than the relict garnet. Therefore, these zircons could have grown or re-crystallized during mylonitization of the shear zone. A summary of metamorphic ages and metamorphic P–T paths in the Hengshan–Wutai–Fuping area indicates that age peaks increase regularly from granulite facies North Hengshan and Fuping Complexes (∼1.85Ga), to upper-amphibolite facies South Hengshan Complex (∼1.92Ga) and to lower-amphibolite facies Wutai Complex (∼1.95Ga). The Hengshan–Wutai–Fuping area may have undergone a protracted P–T–t evolution, including a crustal thickening at ∼1.95Ga, followed by exhumation and (isobaric) cooling of high-grade terranes from 1.92 to 1.85Ga. The final uplift of the Hengshan Complex could be attributed to the strike-slip movement of the Zhujiafang shear zone.