COMPOSITION, GEOLOGICAL STRUCTURE AND FORMATION MODEL OF THE SUBDUCTION MELANGE OF THE ALAMBAY MELANGE ZONE OF SALAIR (NORTHWEST 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, Altai-Sayan orogen, melanges, olistostromes, subduction, accretionary prism, flysch, oceanic islands, subduction.
Abstract
The Alambay melange zone (AMZ) is located in the interior of the Early Paleozoic Salair orogeny (Northern part CAOB) and represent the northern extension of the Alambay-Kaim zone, Salair and the Altai Mountains. In paleotectonic terms, Salair is a part of the Kuznetsk-Altai paleo-island arc system. AMZ ophiolites are characterized by the absence of igneous rocks with the geochemical characteristics of MORB, as well as large allochthonous nappes composed of the undisturbed oceanic plate stratigraphy units. Melange complexes play an important role in the geological structure of the AMZ. Two types of melanges, serpentine and clastic, are distinguished in the AMZ. The both types of melanges contain blocks of microquartzites, basalts, limestones; also in clastic mélange a block of garnet amphibolites is found, and in serpentine melange there are blocks of gabbro. Gabbro and garnet amphibolites from AMZ melanges can be classified as island arc rocks based on their geochemical features. Chromite from serpentinized hyperbasites of the zone has an increased content of chromium and magnesium, which is also typical for chromite from suprasubduction hyperbasites. Melange basalts are characterized by high titanium and niobium contents and belong to the OIB type. There are two morphological types of the clastic melanges. The first type is characterized by the predominance of inclusions over the matrix, and the dominance of OIB basalts and limestones in the composition of the inclusions. The second type is characterized by a predominance of matrix over inclusions, and the main type of inclusions are microquartzites (metacherts). To explain the origin of AMZ melanges, the OPS melange model can be used [Wakita, 2015]. This model describes the relations between subduction melanges composition and the position of the decolement, separating subducting plate from the accretionary prism. Within this model, the first type of terrigenous subduction melange is the limestone-basalt type (ls-bs), which is the result of seamount subduction, and the second type corresponds to the sandstone-siliceous type (ss-ch). Formation of sandstone-siliceous melange corresponds to the position of the decolement along the base sedimentary layer of the subducting plate. Subduction melange formed later than 500 million years and earlier than the accumulation of the flysch Late Cambrian-Early Ordovician Green-Violet Group, which allows to estimate the age of the AMZ melange to the Late Cambrian. Wide distribution island-arc rocks as inclusion in the composition of serpentinite and clastic subduction mélanges suggests that the Cambrian subduction zone was characterized by a tectonic erosion regime, in which material from the subduction hanging wall was carried into the subduction zone. The separation faults, along which material was exhumed from subduction channel into the accretionary wedge, were located along the base of the oceanic sediment layer, at the base of the seamounts, and inside the hanging wall of the subduction zone as well.
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