Geochemistry of picrites and associated lavas of a Devonian island arc in the northern Junggar terrane, Xinjiang (NW China): Implications for petrogenesis, arc mantle sources and tectonic setting

Abstract

The Junggar terrane in China's Xinjiang Province sits adjacent to the Siberian craton and Kazakhstan block. A nearly 100-m thick series of high-Mg lava flows is found in a Devonian arc in the Junggar terrane. The highly porphyritic high-Mg lavas are stratigraphically in the lower part of the Middle Devonian Beitashan Formation, and are overlain by basalts and andesites. Based on the chemistry and phenocryst contents, the high-Mg lavas are classified into picrites and ankaramites. The former are characterized by a large amount of olivine phenocrysts (Fo 79–82) with minor clinopyroxene (En 44–50Fs 1–10Wo 44–48) and rare chrome spinel with Cr# (Cr/Cr + Al) values between 0.63 and 0.86, whereas clinopyroxene phenocrysts are dominant in ankaramite. The groundmass in the picrites and ankaramites consists of plagioclase, clinopyroxene, and Ti-magnetite. Glass is not preserved, but minor metal sulfides are present. Based on olivine-melt equilibrium, the primitive magmas had approximately 9 wt.% MgO, and the high-Mg lavas contain accumulated olivine (picrites) and clinopyroxene + olivine (ankaramites). All lavas have similar primitive mantle normalized trace element patterns, characterized by negative Nb, Ta and Ti anomalies, as typical for island arc volcanic rocks. These relationships suggest that the rock suites are co-magmatic. The Zr/Nb ratios (23–66) of the picrites and basalts resemble MORBs (10–66), suggesting MORB-like sources for the picrites. The picritic rocks have slightly LREE-enriched patterns with (La/Yb) n ranging from 1.6 to 3.5 and (La/Sm) n from 1.2 to 2.1. In contrast, basaltic rocks have flat to slightly LREE-enriched patterns with (La/Yb) n ranging from 1.2 to 5.7 and (La/Sm) n from 0.8 to 2.5, whereas andesitic rocks have modestly LREE-enriched patterns with (La/Yb) n ranging from 2.8 to 9.1 and (La/Sm) n from 1.7 to 2.9. The REE patterns for all rocks do not exhibit Eu anomalies. The rocks have similar ( 87Sr/ 86Sr) t (0.7033–0.7043, t = 385 Ma) and ε Nd( t) values (6.4–7.3), overlapping with the present day island arc field. Consequently, primary magmas were most likely generated in N-MORB-type mantle, which was modified by the addition of a fluid component derived from altered subducted oceanic crust as indicated by elevated Sr/Nd and low Th/Yb ratios. REE modelling suggests that the primary magmas were derived from garnet–spinel transition zone mantle.