A Review of Tectonics and Metallogeny of the Tethyan Orogen

The Paleotethys ocean opened between Laurasia and Gondwana in the mid-Paleozoic, and was then closed in the Late Triassic by subduction to the north and opening of the Neotethys ocean to the south. Intervening between these two ocean basins were several small continental fragments, the Cimmerian continents (including the cores of present day Turkey, Iran, Tibet, and Indochina), which rifted from the northern Gondwana margin in the Permian–Early Triassic. These fragments swept northward as the Neotethys opened to the south, and were loosely accreted to the Laurasian margin in the Late Triassic–Early Jurassic. At this time, the southern margin of Laurasia likely resembled the archipelagos of the SW Pacific, with numerous small remnant Paleotethyan and new back-arc basins forming between the accreting blocks and the main continental margin. The Neotethyan ocean basin closed progressively throughout the late Mesozoic and Cenozoic by subduction to the north beneath the accretionary Laurasian margin (Fig. 1). In the Balkans region, collision between fragments of proto-Italy and Turkey resulted in complex oblique collisional tectonics, and closure of the Vardar ocean basin in the early Cenozoic (Fig. 1A–C). This was followed by convergence of Africa-Arabia with Eurasia, with diachronous collision along the Turkish–Iranian section of the orogen in the early–middle Miocene (Fig. 1D). To the east, collision between the rapidly northward drifting Indian plate and Eurasia began in the early Eocene (~55–50 Ma) and progressed in several stages until “hard collision” in the latest Eocene–early Oligocene (~34 Ma; Fig. 1C–D). Early “soft collision” between the Greater India extended continental margin in the Eocene-Oligocene resulted in accretion of crustal materials that form the Himalayas, and uplift of the Tibetan plateau. Collision of the main continental mass of India is ongoing at ~5 cm/yr, and involves extensive underthrusting of the Indian plate beneath Tibet, the full effects of which are yet to be observed but appear to include extensive crustal melting. Rocks generated during the formation and destruction of the Paleotethys ocean are not well preserved, and less detail is therefore known about the history of this ocean. In contrast, the diachronous and ongoing closure of the Neotethys ocean preserves a wide range of lithologies that can be related with varying degrees of certainty to specific tectonic processes, including normal and oblique subduction, back-arc extension, terrane collision, continent –continent collision, and continental subduction. The timing of various basin-forming events is relatively well constrained by sedimentological studies, while attempts have been made to constrain convergence processes from lithogeochemical and isotopic characteristics and age of associated igneous rocks. However, such petrogenetic studies have proven less diagnostic of tectonic processes than might have been expected, largely because the compositions of derivative magmas do not appear to change significantly from subduction to collisional processes. Essentially, the same subduction-modified mantle and deep crustal sources are involved in magmagenesis in both settings, and tend to yield similar calc-alkaline to mildly alkaline intermediate-composition magmas, with mixed, non-unique isotopic compositions. Trace element compositions have been used to argue for detailed petrogenetic models, but many of the distinctive characteristics used, such as high Sr/Y and La/Yb ratios, can be generated by multiple processes (e.g., oceanic slab melting, deep crustal melting, hydrous magmatic fractionation) and thus also do not provide unique Jeremy P. RICHARDS, 2014. A Review of Tectonics and Metallogeny of the Tethyan Orogen. Acta Geologica Sinica (English Edition), 88(supp. 2): 923-925.