Early Silurian to Early Carboniferous ridge subduction in NW Junggar: Evidence from geochronological, geochemical, and Sr-Nd-Hf isotopic data on alkali granites and adakites

Abstract

The Central Asian Orogenic Belt (CAOB) evolved through a long-lived orogeny involving multiple episodes of subduction and accretion marking a major phase of continental growth during the Paleozoic. The northern part of the Western Junggar region (NW Junggar) offers a window into these processes, particularly to constrain the timing of closure of the Paleo-Asian Ocean. Here we report geochemical, geochronological, and isotopic data from K-feldspar granites and adakitic rocks from the NW Junggar region. Zircon U-Pb ages suggest that the granites were emplaced during Early Silurian to the Early Carboniferous (434–328Ma). The granites show geochemical characteristics similar to those of A-type granites, with high SiO2 (71.13–76.72wt%), Na2O+K2O (8.00–9.59wt%), and Al2O3 (12.28–14.08wt%), but depleted Sr, Nb, Ta and Eu. They display moderate to high positive εNd(t) and εHf(t) values (4.26–8.21 and 7.69–14.60, respectively) and young Nd and Hf model ages (T2DM-Nd=489–740 Ma and T2DM-Hf=471–845 Ma), suggesting magma derivation through partial melting of lower crust in the Boshchekul-Chingiz and Zharma-Saur arcs. The adakites are characterized by high Sr content (406.5–751.6ppm), and low Y (13.8–16.4ppm) and Yb (1.5–1.8ppm) content, yielding relatively high Sr/Y ratios (25.38–49.41) similar to those of modern adakites. They have high positive εNd(t) and εHf(t) values (7.85–8.25 and 13.23–15.97, respectively) and young Nd and Hf model ages (T2DM-Nd=429–535Ma and T2DM-Hf=355–550Ma), indicating that their source magmas were likely derived from partial melting of the oceanic crust beneath the Boshchekul-Chingiz arc. The petrogenesis and distribution of the A-type granites and adakites, as well as the tectonic architecture of the region, suggest that a ridge subduction event might have occurred during the Early Silurian to Early Carboniferous. In combination with previous studies in the Chinese Altai, we suggest a two-sided ridge subduction model for the Junggar-Altai region.