Middle Paleozoic mafic magmatism and ocean plate stratigraphy of the South Tianshan, Kyrgyzstan

作者：Safonova I; Biske G; Romer R L; Seltmann R; Simonov V; Maruyama S

来源：Gondwana Research, 2016, 30: 236-256.

DOI：10.1016/j.gr.2015.03.006

• Central Asian Orogenic Belt
• TurkeStan Ocean
• Ferghana-Atbashi-Kokshaal
• Geochemistry
• Sr-Nd-Pb isotopes
• Petrogenesis

The tectonic history of the Kyrgyz South Tianshan in the western Central Asian Orogenic Belt (CAOB) remains controversial, first of all, due to the limited amount of geochemical and isotope data. Our paper presents the first results of a detailed geochemical study (major and trace elements, Sr, Nd and Pb isotopes) of Middle Paleozoic mafic volcanic and subvolcanic rocks of the Ferghana and Atbashi-Kokshaal accretionary belts of the South Tianshan orogen in Kyrgyzstan, which formed during the evolution of the Turkestan Ocean. A special focus is given to the relation between magmatic rocks and sedimentary units of marine origin, chert, siliceous shale/mudstone, volcanogenic-carbonate clastics, seamount carbonates, and turbidites, which we consider as elements of Ocean Plate Stratigraphy (OPS). The age range of marine sediments is Late Silurian to Early Carboniferous, but the age of the most volcanic rocks associated with fossil-bearing OPS sediments is Devonian. The magmatic rocks have geochemical affinity to oceanic island basalts (OIB-type) and, to a lesser extent, mid oceanic ridge (MORE-type) basalts associated with hemipelagic siliceous mudstone and pelagic chert. The rocks with OIB-type affinity are associated with chert, siliceous shale and carbonaceous clastics and carbonates. They are enriched in TiO2, LREE (La/Sm-n = 1.9), and Nb (Zr/Nb-av, = 10), have differentiated HREE (Gd/Yb-n = 2.0), medium to low epsilon Nd (similar to 5.7) and are characterized by clear Nb positive anomalies in normalized multi element plots (Nb/Th-pm = 13, Nb/La-pm = 1.1). The OIBs formed by relatively low degrees of melting (<5%) of mantle sources in the garnet stability field and erupted in an oceanic island setting. The MORE-type samples associated with siliceous mudstone and chert are less enriched in incompatible elements, possess flat REE and multi-element pattern, and show higher eNd values (similar to 9.1); they were probably produced by high-degree melting of spinel lherzolite and/or harzburgite and erupted in a mid-oceanic ridge setting. The geological, lithological and geochronological data suggest that the OPS units with dominantly OIB-type basalts formed at one or several sea mount chains of the Turkestan Ocean, which were accreted to the Kazakhstan continent, and thus contribute to our understanding of the Paleozoic tectonic evolution of the western CAOB during the Serpukhovian-Bashldrian.

• 出版日期2016-2