Early Devonian mafic igneous rocks in the East Kunlun Orogen, NW China: Implications for the transition from the Proto- to Paleo-Tethys oceans

Abstract

The tectonic evolution of the Proto- and Paleo-Tethys oceans had a significant influence on ocean-continent distributions in East Asia in the Phanerozoic, and major implications for continental growth in the region. However, it remains ambiguous when and how the Proto-Tethys Ocean transformed into the Paleo-Tethys Ocean. Here we present petrologic, mineralogical, chronological and geochemical data for Early Devonian mafic igneous rocks, located in the East Kunlun Orogen. The mafic igneous rocks include basaltic lavas and diabase dykes, and are tholeiitic in composition. Geochemical and Sr–Nd isotopic data indicate that the basaltic lavas were derived from melting of a spinel-bearing asthenospheric mantle (E-MORB) at normal mantle potential temperatures (1384–1400 °C) with negligible (1–4%) crustal contamination. The diabase dykes probably originated from melting of a spinel-bearing lithospheric mantle metasomatized by subduction-related fluids, with 5–20% crustal contamination, and crystallized at 1100–1135 °C. Both basaltic lavas and diabase dykes have the geochemical characteristics of within-plate basalts. Magmatic zircons from the mafic rocks yield Early Devonian ages (407–403 Ma), postdating the East Kunlun ultrahigh-pressure metamorphism by 19–25 Myr. Comparing our results with the location and timing of the high- and ultrahigh-pressure metamorphic belt, we conclude that the mafic igneous rocks formed in a post-collisional extensional setting. Their generation was associated with both the terminal stages of the Proto-Tethys orogenic belt, (with orogenic collapse promoted by repeated and localized delamination of lithospheric mantle), and early continental rifting related to the evolution of the Paleo-Tethys Ocean to the south. The period of ~426–390 Ma is important for the transition from Proto- to Paleo-Tethys oceans in the East Kunlun Orogen.