Ridge subduction and slab windows in the Central Asian Orogenic Belt: Tectonic implications for the evolution of an accretionary orogen

Abstract

Cenozoic ridge subduction and the resultant slab windows have been well documented worldwide, especially along the western margins of North and South America. The principal characteristics of ridge subduction, which can be used to recognize the process in ancient orogens, include intrusion of ridge-generated magmas into a forearc in a near-trench position; this can be regarded as the hallmark of ridge subduction. The key magmatic products are adakites that include plutonic and volcanic rocks with intermediate- to high-SiO2, high Sr/Y and La/Yb ratios, elevated MgO, Na2O, K2O, Ni and Cr contents, and high LILE and LREE elements. They range from hornblende/biotite-bearing high-Mg diorites, and granodiorites to hornblende/pyroxene high-Mg andesites. They are typically associated with a wide variety of coeval and compositionally diverse volcanic and plutonic rocks such as high-Ca boninites, tholeiitic, high-T basalts, alkaline basalts, charnockites, peraluminous TTGs, I-type granites, and mid-ocean ridge (MOR) (not supra-subduction) ophiolites. The trace element chemistry of these rocks demonstrates that they were generated by partial melting of dehydrated subducted oceanic crust and of lower continental crust, contaminated and metasomatized by juvenile melts that originated by upwelling of asthenospheric mantle. It is this combination of mixed sources that gives rise to the diagnostic and distinctive features of coeval and associated ridge subduction rocks in a near-trench environment. Other important features of ridge subduction are high-temperature metamorphism closely associated with near-trench plutons created by heat released through a slab window, and porphyry gold-copper-zinc mineralization associated with MOR ophiolites and generated in hydrothermal vents at a MOR. When a ridge subducts under a continental margin, the diverging plates associated with the ridge continue to separate creating a slab window that forms between the separating plates. Adakitic melts tend to be generated close to the hot plate margins, whereas A-type granites and porphyry Cu-Au deposits form in the centre of the opening slab window.Most of the above features have been described and analyzed in many parts of the Central Asian Orogenic Belt (CAOB), particularly in West Junggar, Chinese Altai, Russian Altai, East Junggar, Beishan, Chinese Tianshan, Inner Mongolia, Kyrgyzstan, and at Alxa. Prominent in one of two slab windows defined in West Junggar are two swarms of late Carboniferous high-Mg hornblende-biotite diorite dykes with sanukitoid chemistry that intruded through the slab window. One generation was orthogonal to an ophiolite-strewn paleotrench (suture), and the second was parallel to a transform fault that was parallel to the paleo-trench. The dikes provide the best evidence so far for the orientation and opening of a slab window in the CAOB, and for the relative movement directions of the downgoing and overriding plates during the subduction and closure of the Paleo-Asian Ocean. However, these are early days, because most parts of the CAOB have yet to be investigated for evidence of ridge subduction and slab windows, so the prospects are high.