Regresar

Evolution of a Permian intraoceanic arc-trench system in the Solonker suture zone, Central Asian Orogenic Belt, China and Mongolia

Abstract:

The identification of a fossil arc-trench system from the ophiolite-decorated Solonker suture zone in the southernmost Central Asian Orogenic Belt (CAOB) enables us to constrain the timing of pre-subduction extension (ca. 299-290Ma), subduction initiation (ca. 294-280Ma), ridge-trench collision (ca. 281-273Ma) and slab break-off (ca. 255-248Ma) in the Permian. A fraction of proto-arc crust (ca. 45km long, up to 8km wide) is preserved as a volcanic-plutonic sequence and is juxtaposed against a wide (ca. 30-80km) forearc mélange. This proto-arc crust comprises two distinct magma series, island arc tholeiite (IAT) and mid-ocean ridge basalt (MORB), both of which have strong supra-subduction zone (SSZ) geochemical signatures. Zircons from a gabbro and a plagiogranite yielded weighted mean 206Pb/238U ages of 284.0±4.0 and 288.0±6.0Ma. The forearc mélange consists of numerous ophiolite fragments and continental margin-derived olistoliths/blocks that predate the ophiolite. The olistoliths are best represented by a gabbroic block (291.8±2.3Ma) that contains granite xenoliths (312.6±1.8 and 313.6±3.1Ma). Other dated blocks include a trondhjemite (323.9±2.7Ma), a gabbro (296.6±1.7Ma) and a tonalite (294.9±2.4Ma). Small bodies of diabase, andesite and diorite in the forearc mélange exhibit a wide variety of geochemical signatures. We dated zircons from an N-MORB-like diabase (274.4±2.5Ma), an E-MORB-like diabase (252.5±2.3Ma), a transitional sanukitoid/adakite (andesite, 250.2±2.4Ma), a sanukitoid (high-Mg diorite; 251.8±1.1Ma) and an anorthosite (252.2±1.7Ma). The N-MORB-like diabase contains ca. 301-394Ma zircon xenocrysts suggesting assimilation of trench sediments when a spreading ridge intersected a trench. The other dated rocks simultaneously formed near the Permian/Triassic boundary and captured abundant zircon xenocrysts (ca. 269-295Ma; ca. 301-495Ma; and ca. 923-2501Ma). Our new formation ages constrain a magmatic episode in response to slab break-off beneath a fossil forearc in a young post-collisional setting, and the youngest xenocryst ages (ca. 269-273Ma) may define the maximum depositional age of trench sediments. © 2010 Elsevier B.V.