A geotraverse across two paleo-subduction zones in Tien Shan, Tajikistan

Abstract

We present first LA-ICP-MS U–Pb zircon ages as well as geochemical and Sr–Nd–Pb isotope data for 14 magmatic rocks collected along ca. 400km profile across the Chatkal-Kurama terrane in the Mogol-Tau and Kurama ranges and the Gissar Segment of the Tien Shan orogen in Tajikistan. These new data from supra-subduction and post-collisional magmatic rocks of two Late Paleozoic active margins constrain a tectonic model for terrane motions across two paleo-subduction zones: (1) The 425Ma old Muzbulak granite of the Mogol-Tau range formed in a supra-subduction setting at the northern margin of the Turkestan Ocean. The north-dipping plate was subducted from the Early Silurian to the earliest Middle Devonian. Thereafter the northern side of the Turkestan Ocean remained a passive margin until the Early Carboniferous. (2) In the Early Carboniferous, subduction under the northern margin of the Turkestan Ocean resumed and the 315 to 305Ma old Kara-Kiya, Muzbek, and Karamazar intrusions formed in a supra-subduction setting in the Mogol-Tau and Kurama ranges. (3) At the same time, in the Early Carboniferous, rifting of the southern passive margin of the Turkestan Ocean formed the short-lived Gissar Basin, separated from the Turkestan Ocean by the Gissar micro-continent. North-dipping subduction in the Gissar Basin is documented by the 315Ma Kharangon plagiogranite and the voluminous ca. 321–312Ma Andean-type supra-subduction Gissar batholith. The Kharangon and Khanaka gabbro-plagiogranite intrusions of the southern Gissar range have geochemical and Sr–Nd isotopic compositions (87Sr/86Sr(t) 0.7047–0.7056, εNd of +1.5 to +2.3) compatible with mantle-derived origin typical for plagiogranites associated with ophiolites. The supra-subduction rocks from the Gissar batholith and from the Mogol-Tau Kurama ranges have variably mixed Sr–Nd–Pb isotopic signatures (87Sr/86Sr(t) 0.7057–0.7064, εNd of −2.1 to −5.0) typical for continental arcs where mantle-derived magmas interact with continental crust. (4) In the latest Carboniferous, the Turkestan Ocean and the Gissar Basin were closed. The Early Permian Chinorsay (288Ma) and Dara-i-pioz (267Ma) post-collisional intrusions, emplaced in the northern part of the Gissar micro-continent after a long period of amagmatic evolution, have intraplate geochemical affinities and isotopic Sr–Nd–Pb isotopic compositions (87Sr/86Sr(t) 0.7074–0.7086, εNd of −5.5 to −7.4) indicating derivation from Precambrian continental crust which is supported by old Nd model ages (1.5 and 1.7Ga), and by the presence of inherited zircon grains with ages 850–500Ma in the Chinorsay granodiorite. The post-collisional intrusions in the southern Gissar and in the Mogol-Tau and Kurama ranges (297–286Ma), emplaced directly after supra-subduction magmatic series, have geochemical and isotopic signatures of arc-related magmas. The distinct shoshonitic affinities of post-collisional intrusions in the Mogol-Tau and Kurama ranges are explained by the interaction of hot asthenospheric material with subduction-enriched wedge of lithospheric mantle due to slab break-off at post-collisional stage. Despite origination from different tectonic environments, all magmatic rocks have relatively old Nd model ages (1.7–1.0Ga) indicating a significant proportion of Paleoproterozoic or older crustal material in their sources and their model ages are similar to those of post-collisional intrusions from the Alai and Kokshaal Segments of the South Tien Shan.