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

As an indicator of extensional tectonic events, MORB-like magmas could provide great insights into tectonic evo lution of subduction zones. In this study, we carry out an integrated study of geochronology , geochemistry and Sr-Nd isotopic compositions for the Early Paleozoic MORB-like Lashuixia gabbro in Hualong, southern Central Qilian block. U-Pb 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 epsilon(Nd)(t) values of +5.8 - +7.0, but their initial( 87)Sr/Sr-85 ratios ( (Sr-87/Sr-86)(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. (C) 2018 Elsevier By. All rights reserved.