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

摘要

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. 299-290 Ma), subduction initiation (ca. 294-280 Ma), ridge-trench collision (ca. 281-273 Ma) and slab break-off (ca. 255-248 Ma) in the Permian. A fraction of proto-arc crust (ca. 45 km long, up to 8 km wide) is preserved as a volcanic-plutonic sequence and is juxtaposed against a wide (ca. 30-80 km) forearc melange. 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 ~(206)pb/~(238)U ages of 284.0 ± 4.0 and 288.0 ± 6.0 Ma. The forearc melange 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 ± 2.3 Ma) that contains granite xenoliths (312.6 ± 1.8 and 313.6 ± 3.1 Ma). Other dated blocks include a trondhjemite (323.9 ± 2.7 Ma), a gabbro (296.6 ±1.7 Ma) and a tonalite (294.9 ± 2.4 Ma). Small bodies of diabase, andesite and diorite in the forearc melange exhibit a wide variety of geochemical signatures. We dated zircons from an N-MORB-like diabase (274.4 ±2.5 Ma), an E-MORB-like diabase (252.5 ±2.3 Ma), a transitional sanukitoid/adakite (andesite, 250.2 ± 2.4 Ma), a sanukitoid (high-Mg diorite; 251.8 ±1.1 Ma) and an anorthosite (252.2 ±1.7 Ma). The N-MORB-like diabase contains ca. 301-394 Ma 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. 269-295 Ma; ca. 301-495 Ma; and ca. 923-2501 Ma). 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. 269-273 Ma) may define the maximum depositional age of trench sediments.