COMPOSITION, GEOLOGICAL STRUCTURE, AND FORMATION MODEL OF THE SUBDUCTION MÉLANGE OF THE ALAMBAY MÉLANGE ZONE OF SALAIR (northwestern Central Asian orogenic belt)
F.I. Zhimulev, A.V. Kotlyarov, M.A. Fidler, N.A. Bekhterev
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: Ophiolites, mélange, olistostromes, subduction, accretionary prism, flysch, oceanic island basalt, Altai-Sayan orogenic area
Abstract
The Alambay mélange zone is located within the interior of the early Paleozoic Salair Orogen (fold-thrust structure) and is the northern link of the Alambay-Kaim tectonic zone of the Salair Ridge and the Altai Mountains. In paleotectonic terms, the Salair Orogen is part of the Kuznetsk-Altai paleoisland-arc system. The Alambay mélange zone is comprised of two types of mélange: serpentinite and terrigenous. The mélange contains microquartzite, basalt, and limestone blocks. The terrigenous mélange contains a block of garnet amphibolite, and the serpentinite mélange has gabbroid blocks. In terms of geochemical features, gabbroid and garnet amphibolite from the Alambay mélange zone can be attributed to island-arc rock. Chromite from serpentinized hyperbasite in this zone has an increased content of chromium and magnesium, which is also typical for chromite from suprasubduction hyperbasite. The mélange basalt is categorized as an oceanic island basalt and distinguished by a high content of titanium and niobium. There are two morphological varieties of terrigenous mélange. One of them is characterized by the prevalence of inclusions over the matrix and the dominance of limestone and oceanic island basalt within inclusions. The other variety is notable for the prevalence of the matrix over inclusions, and its main type of inclusions is aposilicic microquartzite. The genesis of the Alambay mélanges can be explained using the oceanic plate stratigraphy model [Wakita, 2015]. This model describes the dependence of the composition of subduction mélange on the position of a sole thrust between the subducting plate and the accretionary prism. Within the framework of this model, two varieties of a terrigenous subduction mélange are identified. One of them is a limestone-basalt mélange formed during seamount subduction, and the other one is a sandstone-cherty mélange whose formation corresponds to décollement position along the base of the sedimentary bed of the subducting plate. The formation of the mélange assemblage dates back to 500 Ma and precedes the accumulation of the Late Cambrian-Early Ordovician Green-Violet Group flysch and its analogs, suggesting that the Alambay mélange zone is of Late Cambrian age. The important role of fragments of the island-arc base in the serpentinite and terrigenous subduction mélanges indicates that the Cambrian subduction zone was characterized by tectonic erosion conditions under which the material of the advancing plate was entrained into the subduction zone. The sole thrusts in the subduction zone, along which the material was exhumed into the accretionary wedge, were located along the base of the oceanic sediments, at the base of seamounts, and inside the hanging wall of the subduction zone.
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