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Basalts and picrites from a plume-type ophiolite in the South Qilian Accretionary Belt, Qilian Orogen : accretion of a Cambrian Oceanic Plateau?

Zhang, Y.Q. and Song, S.G. and Yang, L.M. and Su, L. and Niu, Y.L. and Allen, M.B. and Xu, X. (2017) 'Basalts and picrites from a plume-type ophiolite in the South Qilian Accretionary Belt, Qilian Orogen : accretion of a Cambrian Oceanic Plateau?', Lithos., 278-281 . pp. 97-110.

Abstract

Oceanic plateaus with high–Mg rocks in the present-day oceanic crust have attracted much attention for their proposed mantle-plume origins and abnormally high mantle potential temperatures (Tp). However, equivalent rocks in ancient oceanic environments are usually poorly preserved because of deformation and metamorphism. Here we present petrological, geochronological and geochemical data for pillow lavas from Cambrian ophiolites in the Lajishan and Yongjing regions of the South Qilian Accretionary Belt (SQAB), from the southern part of the Qilian Orogen, northern China. Three rock groups can be identified geochemically: (1) sub-alkaline basalts with enriched mid- ocean ridge basalt (E-MORB) affinity; (2) alkaline basalts with oceanic island basalt (OIB) features, probably derived from partial melting of an enriched mantle source; and (3) picrites with MgO (18–22 wt.%). Cr-numbers [Cr# = Cr/(Cr + Al)] of spinels from the picrites suggest 18–21% degree of partial melting at the estimated mantle potential temperature (Tp) of 1489–1600 °C, equivalent to values of Cenozoic Hawaiian picrites (1500–1600 °C). Zircons from one gabbro sample yielded a U–Pb Concordia age of 525 ± 3 Ma, suggesting the oceanic crust formed in the Cambrian. Available evidence suggests that Cambrian mantle plume activity is preserved in the South Qilian Accretionary Belt, and influenced the regional tectonics: “jamming” of the trench by thick oceanic crust explains the emplacement and preservation of the oceanic plateau, and gave rise to the generation of concomitant Ordovician inner-oceanic island arc basalts via re-organisation of the subduction zones in the region.

Item Type:Article
Full text:(AM) Accepted Manuscript
Available under License - Creative Commons Attribution Non-commercial No Derivatives.
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Status:Peer-reviewed
Publisher Web site:https://doi.org/10.1016/j.lithos.2017.01.027
Publisher statement:© 2017. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/
Date accepted:28 January 2017
Date deposited:10 February 2017
Date of first online publication:08 February 2017
Date first made open access:08 February 2018

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