Geochronology and geochemistry of the Neoarchean Lulong Complex in the eastern Hebei Province, North China Craton: Implications on regional crustal evolution

摘要

The Lulong Complex, exposed in the southeastern part of the eastern Hebei Province in the North China Craton (NCC), is mainly composed of meta-basalt, biotite plagioclase gneiss, migmatite, amphibolite and granitoids. Mineral assemblages indicate the rocks in this Complex underwent upper amphibolite facies metamorphism. Zircon U-Pb and Lu-Hf data constrain that both protolith of the biotite plagioclase gneiss and the homogeneous domain of the migmatite formed at 2.53 Ga and were mainly derived from the reworking of 2.8-2.7 Ga crustal source. The similar mineral association, zircon U-Th-Pb systematics and geochemical features suggest that their protolith should be dominated by felsic volcanic sequences with limited contributions from older crust. The protolith of the amphibolite (gabbro or diabase) was metamorphosed at 2477 +/- 20 Ma. Geochemistry and zircon Lu-Hf systems indicate that these meta-mafic rocks originated from an enriched subcontinental lithospheric mantle source. The granitoids exposed in the Lulong area yield zircon U-Pb ages of 2507 +/- 21 Ma to 2473 +/- 38 Ma and record similar to 2.5 Ga regional metamorphic event. The initial zircon Hf-176/Hf-177 ratios of the granite are in the range of 0.280640-0.281678 and suggest a complicated source where as those from the granodiorite indicate melting of juvenile crustal materials.