Petrogenesis of the Paleoarchean Chentaigou supracrustal rocks in Anshan, North China Craton: Evidence for plume−continental nucleus interaction

Paleoarchean to Eoarchean supracrustal rocks are rare worldwide. This rarity is the major hindrance to understanding the surface environment and processes and mantle evolution in the early history of Earth. This contribution presents geological data, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating, Nd-Hf-O isotopic analysis, and whole-rock geochemical study of the Paleoarchean Chentaigou supracrustal rocks in the Anshan area, North China Craton. The supracrustal rocks are composed of meta-ultramafic, amphibolite, and meta-siliceous rock units. Zircon dating on eight samples (including four samples of trondhjemitic dikes intruding the different rock units) constrains the formation age of the supracrustal rocks between 3.36 Ga and 3.27 Ga. The zircons have εHf(t) values of −11.62 to +6.98 and two-stage Hf model [tDM2(Hf)] ages of 4.31−3.34 Ga. The meta-ultramafic rocks preserve no igneous textures or mineralogy but compositionally resemble Al-undepleted (Munro-type) komatiite, commonly having flat rare earth element (REE) patterns and low total REE (ΣREE) contents, and are proposed to have formed as a result of a mantle plume. The amphibolites show flat REE patterns and are enriched in large ion lithophile elements (LILEs), with slight Nb depletion and positive εNd(t) values of +3.11 to +5.13. The meta-siliceous rocks are mainly quartzite (including ferruginous quartzite and fuchsite quartzite), (hornblende-)biotite-plagioclase gneiss, and banded gneiss, all of which are characterized by being low in ΣREE content and showing flat REE patterns. They have large δ18O(V-SMOW) (relative to Vienna standard mean ocean water) variations from 4.4‰ to 11.6‰. The ferruginous quartzite is of chemical sedimentary origin, but the fuchsite quartzite is a highly silicified ultramafic rock. The (hornblende-)biotite-plagioclase gneiss and banded gneiss are interpreted to have formed by seafloor silicification of basaltic protoliths; the difference in appearance between the two may be because of variation in alteration intensity. Additionally, most quartzites are final products of strong silicification of volcanic or volcaniclastic rocks. It is speculated that the Paleoarchean supracrustal rocks are composed of tectonic slices of different ages, types, and origins brought together by late tectonic processes. Interaction of a mantle plume with a continental nucleus within oceanic crust is the preferred geodynamic for the formation of the Paleoarchean Chentaigou supracrustal rocks.