A synthesis of geochemistry and Sm–Nd isotopes of Archean granitoid gneisses in the Jiaodong Terrane: Constraints on petrogenesis and tectonic evolution of the Eastern Block, North China Craton

Abstract

The Jiaodong Terrane is situated in the northeastern part of the Shandong Province in eastern China, experiencing multi-stage magmatism during the Mesoarchean (∼2.9Ga) and Neoarchean (∼2.7Ga and ∼2.5Ga). It consists dominantly of granitoid gneisses with minor amphibolite enclaves or lenses. This study presents new whole-rock geochemical and Sm–Nd isotopic data for the Archean granitoid gneisses from the Jiaodong Terrane in order to constrain their petrogenesis and tectonic setting. Geochemical studies and petrological features suggest that the protoliths of most granitoid gneisses are typical Archean tonalitic–trondhjemitic–granodioritic (TTG) suites, which are high in SiO2 (60.9–76.8wt.%), Al2O3 (12.6–16.8wt.%), Na2O (3.3–5.3wt.%), Sr (135–604ppm) and Sr/Y ratios (13.9–84), but low in MgO (0.4–4.5wt.%), K2O (0.7–3.4wt.%), TiO2 (0.2–0.9wt.%), Cr (<0.5–37.9ppm), Ni (<0.5–24ppm), Y (2.8–19ppm) and Mg number (Mg#=30–45). They are generally enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE), with slight Eu anomalies. These geochemical features suggest that they were derived from partial melting of metabasaltic rocks leaving eclogite or garnet–amphibolite dominant in the residue. Whole-rock Nd isotopes reveal that the protoliths of Mesoarchean (∼2.9Ga) granitoid gneisses were derived mainly from juvenile sources, whereas the early Neoarchean (∼2.7Ga) granitoid gneisses were derived from juvenile sources with significant additions of crustal material and the late Neoarchean (∼2.5Ga) granitoid gneisses were mainly derived from continental crustal sources. Combined with other geological considerations, a mantle plume model is favored to account for the generation of both early and late Neoarchean granitoid gneisses, whereas the tectonic setting for the Mesoarchean granitoid gneisses remains uncertain due to lack of geological data.