Geochemical diversity in oceanic basalts hosted by the Zasur’ya accretionary complex, NW Russian Altai, Central Asia: Implications from trace elements and Nd isotopes

Abstract

Diverse types of Late Cambrian–Early Ordovician basalts are present as tectonic sheets and fragments in the Early–Middle Paleozoic Charysh-Terekta suture-shear zone of NE Russian Altai. La and Nb depleted, transitional and enriched basalts coexist within this geological structure. Mg# varies from 56 to 20 over a relatively narrow range of SiO 2 (46–52 wt.%). Iron, Zr, Nb, Y, LREE Ti, Cr, Ni and Th variably decrease with Mg#. Depleted tholeiitic basalts have weakly to undepleted LREE (La/Sm n = 0.6–1.0), weakly fractionated HREE patterns (Gd/Yb n = 1.1–1.3), relatively high ε N d (7.7–10.3) and negative Nb and Th anomalies relative to La (Nb/La pm = 0.37–0.8, Th/La pm = 0.4–0.85). Transitional basalts have moderately fractionated REE (La/Sm n = 1.6–2.0, Gd/Yb n = 1.3–2.4), medium ε N d (5.6–5.7) and positive Nb and negative Th anomalies relative to La (Nb/La pm = 1.2–1.6, Th/La pm = 0.7–0.9). Enriched basalts are the prevalent volcanic rock type characterized by LREE-enriched patterns with fractionated HREE (La/Sm n = 1.9–4.7, Gd/Yb n = 1.5–2.5), low ε N d (2.6–3.3), positive Nb and zero to positive Th anomalies (Nb/La pm = 1.0–1.8, Th/La pm = 0.6–1.15). Mantle melting processes, crustal contamination, or fractional crystallization can be ruled out as the main cause of the anomalies except for the moderate effect of alteration and metamorphism on the major-element composition of basalts. The enriched basalts show stronger REE fractionation (La/Yb n = 6–9), higher positive Nb anomalies, like Phanerozoic HIMU ocean island basalts, than the transitional counterparts (La/Yb n = 3–4), and also higher Al 2O 3 contents with convex-up trace-element patterns through Th–Nb–La. Both varieties have higher Zr/Hf ratios than depleted basalts and ε N d values close to those in HIMU. The LREE-depleted basalts are interpreted to have been a mid-oceanic ridge, whereas the transitional and LREE-enriched basalts – a chain of oceanic islands derived from a heterogeneous multi-component plume tapping the moving oceanic lithosphere. The oceanic islands were fragmented in the accretionary complex and tectonically mixed with underlying MOR tholeiites.