PALEOZOIC RIDGE SUBDUCTION AND SLAB WINDOWS IN THE CENTRAL ASIAN OROGENIC BELT: TECTONIC IMPLICATIONS FOR THE EVOLUTION OF AN ACCRETIONARY OROGEN

Cenozoic ridge subduction and slab windows are well documented worldwide, especially along the margins of the Pacific. The principal characteristics of ridge subduction include intrusion of ridge-generated magmas into a near-trench forearc, the hallmark of ridge subduction. The key products are plutonic-volcanic adakites 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, which range from hornblende/biotite-bearing high-Mg diorites, and granodiorites to hornblende/pyroxene high-Mg andesites. They are typically associated with coeval, compositionally-diverse rocks such as high-Ca boninites, tholeiitic, high-T basalts, alkaline basalts, charnockites, peraluminous TTGs, and I-type granites. Their trace element chemistry demonstrates generation by partial melting of dehydrated subducted oceanic crust and lower continental crust, metasomatized by juvenile melts from upwelling asthenospheric mantle. This combination of mixed sources gives the diagnostic features of coeval ridge subduction rocks in a near-trench environment. Other important features are high-temperature metamorphism created by ridge-heat released through a slab window, and porphyry AuCu mineralization. When a ridge subducts under a continental margin, the diverging plates continue to separate creating a slab window. Adakitic melts are generated close to the hot plate margins, whereas A-type granites and porphyry Cu-Au deposits form in the center of the opening slab window.

Most of these features have been described in the Central Asian Orogenic Belt (CAOB), e.g. West-East Junggar, the Chinese-Russian Altai, Beishan, Chinese Tianshan, Inner Mongolia, Kyrgyzstan, and Alxa. Prominent in a West Junggar slab window are two swarms of late-Carboniferous high-Mg hornblende-biotite sanukitoid diorite dykes that intruded the slab window, orthogonal and parallel to a paleotrench. 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 subduction 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.