Petrogenesis and tectonic regime of two types of Neoarchaean amphibolites in the northern margin of the North China Craton

Abstract

ABSTRACT The Neoarchaean crustal evolutionary processes of the North China Craton (NCC) provide a window to understanding the crust–mantle interaction in the Early Earth. The Jiefangyingzi amphibolites are located in the Bainaimiao arc belt, along the northern margin of the NCC. LA–ICP–MS U–Pb isotopic data reveal that the Type 1 amphibolites were formed at ~2562 Ma, and the Type 2 amphibolites were formed at ~2504 Ma, but were subsequently subjected to metamorphism at ~1889 Ma. Lu–Hf isotopic data of the Type 1 amphibolite show positive εHf(t) values (+1.2–+6.0) and TDM from 2612 Ma to 2791 Ma, suggesting a significant episode of 2.6–2.8 Ga crustal growth within the study area. Geochemically, the Type 1 amphibolites show high SiO2 contents (53.37%–55.19%), moderate LREE–enriched patterns (La/Sm)N = 2.42–3.15 and HREE–depleted patterns (Gd/Yb)N = 1.86–2.64, with negative Nb anomalies, which are similar to IAB–like rocks, suggesting that they were derived from partial melting of arc–related depleted mantle with intense subduction–related fluids metasomatism. The Type 2 amphibolites have lower SiO2 contents from 44.28% to 50.77%, and show LREE–depleted REE patterns ((La/Sm)N = 0.67–1.02), nearly unfractionated HREE patterns ((Gd/Yb)N = 0.89–1.77), with negative Nb, Th and positive Sr, Zr anomalies, which are close to N–MORB–like rocks, indicating that they were generated from partial melting of depleted mantle with slight fluid metasomatism. The geochronological, Lu–Hf isotopic and geochemical features show that the IAB–like rocks were formed in an intra–oceanic arc environment, coincide with the significant ~2.5–2.6 Ga episode of crustal growth in the NCC; The slightly younger MORB–like rocks reflect the spreading of the oceanic crust might last until ~2.5 Ga in the central zone.