Age and composition of Neoproterozoic diabase dykes in North Altyn Tagh, northwest China: implications for Rodinia break-up

Abstract

ABSTRACT Whether or not the Altyn Tagh terranes in northwest China were part of the Rodinia supercontinent is unclear due to the apparent absence of rift-related igneous rocks. We present U-Pb geochronology, mineral chemistry, whole-rock major and trace element and Sr-Nd isotope data for the Yapuqiasayi mafic dykes in the North Altyn Tagh. Zircon and baddeleyite from a mafic dyke sample yield indistinguishable U-Pb results and a weighted mean 207Pb/206Pb date of 740 ± 11 Ma, interpreted as the igneous crystallization age. The dykes are tholeiitic in composition, with high TiO2, Zr/Sm and Zr/Y, resembling intraplate basalts. Whole-rock major and trace elements, together with Sr and Nd isotope data, indicate that the dyke magmas originated by 3–5% partial melting of a garnet-poor (<1%) asthenospheric mantle source and were variably modified by crustal contamination. These features, together with mineral chemistry and field observations, indicate that these mafic dykes formed in a continental rift setting, possibly induced above a mantle plume. We show that the Yapuqiasayi dykes are similar in age and petrogenetic characteristics to mafic igneous rocks in Australia, South China, Seychelles, Laurentia, and Tarim, and speculate that their emplacement in North Altyn Tagh was associated with rifting and break-up of the Rodinia supercontinent above a Neoproterozoic superplume.