Geochronology and petrogenesis of Neoproterozoic mafic dykes in the Aktash Tagh, SE Tarim Craton: New evidence for its tectonic setting and location in the Rodinia supercontinent

Abstract

Although Neoproterozoic magmatic rocks extensively occur both in the southern and northern margins of the Tarim Craton (TC), their distinct characteristics indicate not only different petrogenesis but also different tectonic settings. What caused these differences remains unknown. Here, we present new geochronological and geochemical data for mafic dykes collected from the Aktash Tagh area in the southeastern TC, in order to constrain the petrogenesis and reconstruct the Neoproterozoic tectonic evolution of the TC. Zircon U–Pb dating of the mafic dykes yielded a weighted mean 206Pb/238U age of 885 ± 4 Ma, which represents the crystallization age. The εHf(t) values of the zircons from the dykes vary from + 4.8 to + 14.2 with depleted mantle model ages (TDM1) of 1.25–0.87 Ga, among which the highest εHf(t) value nearly approaches to that of the coeval depleted mantle. The mafic dykes are characterized by relative uniform geochemical compositions with low SiO2 (46.89–48.49 wt%), K2O (0.36–0.70 wt%), and high Na2O (1.90–2.31 wt%), Fe2O3T (9.35–11.98 wt%) and TiO2 (0.96–1.50 wt%) components, which show tholeiitic affinities. Their slightly fractionated chondrite-normalized rare earth element patterns are similar to those of E-MORB, together with the positive εNd(t) (+4.9 to +6.3) of the dykes, indicating a slightly modified asthenospheric mantle source. In addition, the mafic dykes exhibit moderate enrichment in large ion lithophile elements, but show unconspicuous anomalies of the high field strength elements. On the basis of these geochemical and isotopic compositions, we suggest that these dykes were formed in an extensional setting likely related to intra-continent rifting and generated by ∼ 20% partial melting of a garnet- and spinel-bearing asthenospheric mantle source that had been modified by ancient slab-derived fluids. In combination with the previous studies on the Neoproterozoic magmatic rocks both of the southern and northern TC, we propose that the southern TC was located in the periphery of Rodinia and it most likely faced to the interior of this supercontinent during the time at 885–740 Ma.