Archean magmatism and crustal evolution in the northern Tarim Craton: Insights from zircon U–Pb–Hf–O isotopes and geochemistry of ~2.7 Ga orthogneiss and amphibolite in the Korla Complex

Abstract

manycratons of the world, but the geodynamic setting during this period is complicated because both arc- andplume-related metavolcanic rocks are closely associated in several greenstone belts. Here, we presentin situ zircon U–Pb–Hf–O isotopic and whole-rock geochemical data for meta-igneous rocks from theKorla Complex, northern Tarim Craton, NW China. SHRIMP and/or LA-ICP-MS zircon U–Pb dating indi-cates that two orthogneisses and an amphibolite crystallized at ~2.71–2.74 Ga and were affected by atleast two metamorphic events at ~2.0–1.8 Ga and 0.8–0.6 Ga, respectively. These are the oldest rockswith reliable crystallization ages so far identified in the northern Tarim Craton. These rocks exhibit het-erogeneous zircon Hf isotopic compositions, with the most radiogenic analyses (average eHf(t)= +7.4,n = 8) plotting on the depleted mantle evolution array and the most unradiogenic eHf(t)extending downto -5.6. This implies that both ~2.7 Ga depleted mantle and ancient continental crust at least as old as3.4–3.5 Ga contributed to the magma source. This conclusion is supported by zircon O isotopic data andHf–O isotopic modeling of the orthogneisses. Available zircon U–Pb ages and Hf isotopic data show thattwo important Neoarchean magmatic events occurred at ~2.71 and ~2.55 Ga in northern Tarim, andthat both events involved synchronous crustal growth and reworking. This observation suggests thatthe apparent peaks of zircon Hf crustal model ages of these rocks do not represent the time of crustalgrowth but are artifacts of magma mixing. Geochemical data show that the parent mafic magmas of theamphibolites follow two distinct evolution trends: a Fenner trend of extreme Fe–Ti enrichment (GroupI) and a typical tholeiitic trend with Fe–Ti enrichment followed by Fe–Ti depletion (Group II). This wasprobably controlled by the fractionation of Fe–Ti oxides, which was in turn controlled by magma oxygenfugacity. The orthogneisses follow the second trend and can be interpreted as the products of assimilationand fractional crystallization of the Group II magmas. Trace element systematics indicate that the parentmagma of Group I amphibolites resembled Nb-enriched or high-Nb arc basalts, whereas the Group IIamphibolites were probably derived from tholeiitic arc basalts. Such a rock assemblage is similar to theisland arc volcanic association found in ~2.7 Ga greenstone belts in other cratons and indicates subduc-tion of young and hot oceanic crust under an ancient continental block, which was probably an importantprocess