Geochemical signature and rock associations of ocean ridge-subduction: Evidence from the Karamaili Paleo-Asian ophiolite in east Junggar, NW China

Liu, XJ, Xiao WJ, Xu JF, Castillo PR, Shi Y.  2017.  Geochemical signature and rock associations of ocean ridge-subduction: Evidence from the Karamaili Paleo-Asian ophiolite in east Junggar, NW China. Gondwana Research. 48:34-49.

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a-type granites, age, CAOB, continental growth, East Junggar, high-field strength, ophiolite, orogenic belt, Paleo-Asian Ocean, phanerozoic crustal growth, ridge subduction, slab-window formation, southern alaska, sr isotopic geochemistry, trace-element evidence, u-pb


Subduction of active spreading ridges most likely occurs throughout Earth's history. Interaction or collision between spreading center and trench, with the active spreading ridge downgoing and shallowly being buried in subduction zone, results in low-pressure but high-temperature near-trench magmatism in the forearc and accretionary prism setting. The Central Asian region, a complex orogenic belt created during the evolution and closure of the Paleo-Asian Ocean (PAO) at similar to 1000-300 Ma, provides an ideal place to study the subduction of PAO spreading ridges beneath ancient continental margins. It had been suggested that the low-pressure and high-temperature mafic and intermediate to felsic magmas from the Karamaili ophiolite (KO) in the NE corner of the Junggar basin (NW China) in Central Asia were likely produced by ridge subduction (Liu et al., 2007). In this paper, we combine our new geochemical data with previous results to show that the geochemical characteristics of the bulk of KO mafic rocks range from arc basalt-like to mid-ocean ridge basalt-like and ocean island basalt-like. Their trace element patterns range from depleted to enriched in highly incompatible elements, but depleted in Nb and Ta, indicating a subduction-influenced origin. The KO intermediate to felsic rocks are talc-alkaline and boninitic in composition and have trace element signatures similar to the associated mafic rocks. The low Nb/Ta ratios of some of the mafic rocks and boninitic character of some of the intermediate to felsic rocks reflect a highly depleted source, perhaps due to prior backarc magmatism. Major and trace element models indicate complex fractional crystallization histories of parental KO magmas to generate both the mafic and intermediate to felsic rocks, but in general, crystal fractionation occurred at 1000 to 1200 degrees C and moderate to low (0.5 kbar to 10 kbar) pressure or <23 km depth. We conclude that the KO was formed in a forearc region of a subduction system that experienced ridge subduction. (C) 2017 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.