Convergent margin magmatism and crustal evolution during Archean-Proterozoic transition in the Jiaobei terrane: Zircon U–Pb ages, geochemistry, and Nd isotopes of amphibolites and associated grey gneisses in the Jiaodong complex, North China Craton

Abstract

Mafic metavolcanic rocks and grey gneisses are the dominant components of many Archean terranes and preserve significant imprints and information of the crustal evolution. Here we present in situ zircon U–Pb geochronology, whole-rock geochemistry and Nd isotopes of the amphibolites and associated grey gneisses in the drill holes from the Jiaobei terrane, North China Craton. The LA-ICP-MS zircon U–Pb data show that the magmatic protoliths of two amphibolites and two grey gneisses crystallized at 2457–2503Ma followed by the metamorphism at ∼1.8–1.9Ga. These rocks exhibit heterogeneous whole-rock Nd isotopic compositions, with the most radiogenic analyses (ɛNd(t)=+3.8 to +6.9) plotting on or close to the depleted mantle evolution array and the most unradiogenic ɛNd(t) extending down to −2.4. The data suggest that both ∼2.5Ga depleted mantle and ancient continental crust at least as old as 3.05 Ga contributed to the magma source. In combination with previous zircon Hf isotopic data, it is suggested that the ∼2.5–2.6Ga magmatic events in the entire Jiaobei terrane were mostly derived from the reworking of the pre-existing ∼2.7–2.8Ga crustal material. However, large variations in Hf–Nd isotopic compositions of abundant rocks at ∼2.5Ga and ∼2.7–2.8Ga in the Jiaobei terrane were likely to have resulted from interaction of mantle-derived juvenile magma with ancient crustal materials in the source region. Thus, the peaks of zircon Hf crustal model ages from these rocks do not represent the actual time of crustal growth, but could be artifacts of interaction of magmas from different sources. Geochemical features show that the protoliths of the amphibolites are tholeiitic basalts and can be subdivided into two groups: high-Mg tholeiite basalts (Group I) and high-Fe tholeiite basalts (Group II). Trace element systematics indicate that the Group I amphibolites show good affinity to subduction-related tholeiitic arc basalts, whereas the Group II amphibolites represent back-arc lavas modified by subduction components. The two groups of amphibolites and associated grey gneisses collectively define an arc-back-arc tectonic setting in the study area at ∼2.5Ga. Integrated with the previous data from other areas in Jiaobei, it can be inferred that Archean-Proterozoic transition in the whole Jiaobei terrane witnessed subduction-related magmatism in a convergent margin.