Initial back-arc extension: Evidence from petrogenesis of early Paleozoic MORB-like gabbro at the southern Central Qilian block, NW China

Abstract

As an indicator of extensional tectonic events, MORB-like magmas could provide great insights into tectonic evolution of subduction zones. In this study, we carry out an integrated study of geochronology, geochemistry and SrNd isotopic compositions for the Early Paleozoic MORB–like Lashuixia gabbro in Hualong, southern Central Qilian block. UPb zircon dating yields magma crystallization age of 506 ± 6 Ma for the gabbro. Geochemically, the gabbros display weakly fractionated REE patterns with (La/Yb)N = 1.17–1.61 and flat trace element patterns except for enrichment of large ion lithophile elements (LILEs) and depletion of Nb. Isotopically, the gabbros display pronouncedly positve εNd(t) values of +5.8 − +7.0, but their initial 87Sr/86Sr ratios ((87Sr/86Sr)i = 0.7046–0.7070) are slightly high because of post-magmatic alteration and metamorphism. Their geochemical and isotopic features indicate that several endmembers (i.e. MORB-sourced asthenospheric mantle, subcontinental lithospheric mantle and oceanic slab-derived fluid) were involved into their source region. Our modal batch partial melting calculation indicates that 7–17% partial melting of 90% asthenospheric mantle plus 10% subcontinental lithospheric mantle at spinel-stable depth accords well with most incompatible trace element compositions of the gabbros. In combination with regional data, we propose that rock association of arc-type pillow lava (ca. 494 Ma), Nb-enriched basalt (NEB)-like diabase (ca. 491 Ma), boninite (ca. 483 Ma) and later OIB-like basalt (ca. 468 Ma) in the Lajishan belt witnessed evolution of this belt from continental arc splitting to back-arc basin spreading due to rollback of the subducted North Qaidam-West Qinling Oceanic slab. We suggest that the Lashuixia gabbro, ca. 10 Ma earlier than the arc-type pillow lava, recorded the initial back-arc lithospheric extension. As with those in many modern arc-basin systems, MORB-like gabbro with overprinting of other endmembers in paleo-subduction zones could not only be an important indicator for back-arc extension, but also make a constraint on the time when the back-arc extension initiated.