Early Neoproterozoic continuous oceanic subduction along the northern margin of the Tarim Block: Insights from ca. 910–870 Ma arc-related magmatism in the Aksu area, NW China

Abstract

The Tarim Block in NW China records the tectonic transition from compression to extension during the Neoproterozoic, and hence is regarded as a component of the Rodinia supercontinent. To unravel the role of the Tarim Block in the Rodinia assembly during the early Neoproterozoic, new geochronological and geochemical analyses were conducted on the gabbroic diorite, plagiogranite and andesite exposed in the Aksu area, northwestern Tarim. Zircon U-Pb dating indicates that these magmatic rocks were formed at ca. 900–870 Ma. Geochemically, these magmatic assemblages have typical features of arc-type rocks, showing enrichment of LILEs and depletion of HFSEs (such as Nb, Ta and Ti). They all have positive zircon εHf(t) values (+6.8 to +12.7), positive εNd(t) values (+3.6 to +6.0), and low and constant (87Sr/86Sr)i ratios (0.70252 to 0.70485). The gabbroic diorites display geochemical features of boninitic rocks such as high SiO2 (53.4–56.3 wt%), MgO (5.79–9.15 wt%), low TiO2 (0.29–0.41 wt%) contents, indicating that they were derived by partial melting of residual mantle. The plagiogranites are characterized by high SiO2 (74.7–79.6 wt%), Na2O (3.87–4.74 wt%), moderate Al2O3 (11.5–13.2 wt%), low K2O (0.15–0.22 wt%) contents, with very low TiO2 (0.08–0.17 wt%) and total REEs (6.3–19.0 ppm) contents similar to oceanic plagiogranite, indicating a hydrous low-pressure melting of mafic protolith. The positive εNd(t) and zircon εHf(t) values and low (87Sr/86Sr)i ratios of the andesitic rocks in the Aksu area suggest a depleted mantle origin. The analysis of trace elements (e.g. Th, Zr, Nb) of these intrusive and volcanic rocks in the Aksu area indicates that their magma sources had been metasomatized by slab-derived fluids. Combined with trace element features, we infer that these early Neoproterozoic magmatic rocks in the Aksu area developed in an oceanic subduction setting with arc-back-arc system probably analogous to the Izu-Bonin (IB) subduction system in the western Pacific. These findings lead us to propose that continuous oceanic subduction beneath the northern margin of the Tarim Block occurred during ca. 910–870 Ma, corresponding to the Neoproterozoic circum-Rodinia subduction-accretion system, which provides new evidence for the accretionary process of the Tarim Block to Rodinia during the early Neoproterozoic.