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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.; Song, S.G.; Yang, L.M.; Su, L.; Niu, Yaoling; Allen, M.B.; Xu, X.

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


Authors

Y.Q. Zhang

S.G. Song

L.M. Yang

L. Su

Yaoling Niu

X. Xu



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.

Citation

Zhang, Y., Song, S., Yang, L., Su, L., Niu, Y., Allen, M., & 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, 97-110. https://doi.org/10.1016/j.lithos.2017.01.027

Journal Article Type Article
Acceptance Date Jan 28, 2017
Online Publication Date Feb 8, 2017
Publication Date May 1, 2017
Deposit Date Feb 10, 2017
Publicly Available Date Mar 28, 2024
Journal Lithos
Print ISSN 0024-4937
Publisher Elsevier
Peer Reviewed Peer Reviewed
Volume 278-281
Pages 97-110
DOI https://doi.org/10.1016/j.lithos.2017.01.027

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