Eocene Arc-related Magmatism within the Neotethys Subduction System in NW Iran; Geochemistry and SHRIMP zircon U-Pb Dating on Magmatic Phases in the Shahjahan Batholith

Granitoids of the composite Shahjahan batholith in the northernmost part of the Urmia-Dokhtar magmatic arc of Iran, and southernmost of the Lesser Caucasus (South Armenia) show SHRIMP zircon ages of 37.1±1.2 to 47.1±4.5 Ma. Dioritic rocks of the pluton with an age of 46.6 ± 4.6 to 47.1 ± 4.5 Ma are calk-alkaline to high-K calc-alkaline, metaluminous and I-type. They show arc-related affinities, characterized by LREE and LILE enrichment and HREE and HFSE depletion, especially negative Ti, Nb and Ta anomalies (TNT effect) in the normalized spider diagrams. low Ce/Pb, Nb/La and high Ba/Nb, U/Th and Hf/Zr ratios along with positive Pb, K, Th and Sr anomalies in the normalized spider diagrams for the studied samples are compatible with magma contamination with crustal materials during ascend to the lower crustal levels. Felsic dikes with granodiorite and syenite compositions and 37.1 ± 1.2 to 38.57 ± 0.41 Ma old, are characterized by high-K calc-alkaline to shoshonitic, metaluminous, and A2- type affinities which show post-collision tectonic setting geochemical features. The REE patterns for all studied samples and the composition of the trace element ratios indicate a geochemically enriched spinel-lherzolite lithospheric mantle source for the magmas, which underwent a low degree of partial melting. Dating arc-related dioritic samples and post collision felsic dikes put constrain on timing of Neotethys Ocean closure in NW Iran. Based on the present study, Middle to Upper Eocene is suggested as closure time of the Neotethys Ocean, Arabia and Central Iran plates’ collision and crustal thickening in Northwest Iran.

Closing the Neotethys Ocean in western Anatolia: Insights from forearc and foreland sedimentary basin records

<p>Across the Tethyan realm, subduction zones are characterized by phases of forearc and backarc extension, and subsequent collisions are protracted and polygenetic, often resulting in significant discrepancies among proxies of collision age. The closure of the northern branch of the Neotethys Ocean along the İzmir-Ankara-Erzincan suture in Anatolia has been variously estimated from the Late Cretaceous to Eocene. It remains unclear whether this age range results from a protracted, multi-phase collision or disparities between proxies and geographic location. Near-continuous Jurassic through Eocene deposition in the forearc-to-foreland Central Sakarya Basin system in western Anatolia makes it an ideal location to integrate pre-collisional extension and multi-stage collision into a holistic reconstruction of subduction through collision. The Central Sakarya Basin system is located north of the Izmir-Ankara-Erzincan suture, where the Gondwanan-derived Anatolide and Tauride terranes to the south collided with the Laurasian-derived Pontide terrane in the north. By integrating new sandstone petrography and detrital zircon U-Pb and Hf isotopes with other geologic proxies, we demonstrate four phases of evolution of subduction and collision. (1) Magmatism began on the Pontides at 110 Ma, potentially the signal of subduction (re-)initiation, and is coincident with extension in the forearc. (2) Forearc obduction began around 94 Ma with initial subduction of lower plate continental lithosphere. Extension migrated to the backarc and opened the Black Sea. (3) The onset of intercontinental collision at 76 Ma is marked by gradual arc shutdown, basement exhumation, and uplift of the suture zone. (4) This first contractional phase is followed by thick-skinned deformation and basin partitioning starting around 54 Ma, coeval with regional syn-collisional magmatism. The 20-Myr protracted collision in western Anatolia could be explained by three non-exclusive mechanisms that produced a change in plate coupling: relict basin closure, progressive underthrusting of thicker lithosphere, and slab breakoff.</p>

Integration of bedrock, seismic tomographic and plate kinematic constraints to test models of the India-Asia collision

<p>The India-Asia collision is one of the most well-studied orogenic events on Earth; it recorded the terminal stages of the central Tethys ocean basins and offers invaluable insight into the geological processes associated with continental collision. In this study, we integrate bedrock datasets, observations of subducted slabs in the mantle, and plate kinematic constraints, to constrain models for the India-Asia collision and the central Tethys oceans.</p><p>Previously proposed models for the India-Asia collision differ in terms of subduction zone configurations and paleogeographic reconstructions of Greater India, which represents to northern passive margin of India prior to collision. Five distinct subduction zone configurations have been proposed previously, which differ in the number of active trenches (one or two trenches) in the central Neotethys Ocean and differ in the respective timing, duration, location and migration of those trenches. Three distinct paleogeographic reconstructions of Greater India have been proposed previously, which differ in size and structure. Here, we consider the validity of these subduction zone configurations and Greater India reconstructions with respect to the bedrock record, plate kinematics and the deep mantle structure of subducted slabs beneath the Indian hemisphere.</p><p>Following the assumption that slabs sink vertically through the mantle, the positions and geometries of subducted slabs determined from seismic tomography constrain the locations and kinematics of paleo-subduction zones. Integrating this with bedrock constraints allows us to constrain post-Triassic subduction zone configurations for the central Tethys oceans. Our analysis demonstrates that the Neotethys Ocean was consumed by at least two subduction zones since the Jurassic. At the onset of the India-Asia collision at 59±1 Ma, one subduction zone was active along the southern Asian continental margin at ~20°N. At that time, a second may have been active at subequatorial latitudes, but support for this from a bedrock perspective is lacking. This subduction zone configuration allows for three reconstructions for Greater India: The (1) minimum-area; (2) enlarged-area; and (3) Greater India Basin reconstructions. We integrate these reconstructions and subduction zone configurations in a plate kinematic framework to test their validity for the India-Asia collision.</p><p>Our findings show that no single model is entirely satisfactory and each invokes assumptions that challenge accepted concepts. These include our understanding of suture zones, subduction-erosion processes, and the limits of continental subduction. We explore these challenges and their implications for our understanding of the India-Asia collision and continental collisions in general.</p>

A new Changhsingian brachiopod fauna from the Xiala Formation at Tsochen in the central Lhasa Block and its paleogeographical implications

Permian faunal affinity in the Lhasa Block plays a critical role in reconstructing its paleogeographic evolution. Cisuralian and Guadalupian faunas have been described from the Lhasa Block, but very few Lopingian (late Permian) brachiopods have been reported so far. In this paper, a new diverse brachiopod fauna consisting of 17 species of 17 genera and an unidentifiable Orthotetoidea is described from the uppermost part of the Xiala Formation at the Aduogabu section in the central part of the Lhasa Block. The age of this fauna can be assigned to the Changhsingian (late Lopingian) as indicated by the associated foraminifersColaniella parva(Colani, 1924) andReichelina pulchraMiklukho-Maklay, 1954. Characteristic brachiopods includeSpinomarginifera chengyaoyenensisHuang, 1932,Haydenella wenganensis(Huang, 1932), andAraxathyriscf.dilatatusShen, He, and Zhu, 1992. They also generally suggest a Changhsingian age. Paleobiogeographically, this fauna is uniformly composed of typical Tethyan elements represented bySpinomarginiferaHuang, 1932 andHaydenellaReed, 1944, and some cosmopolitan elements, but no typical cold-water taxa of Gondwanan affinity. This is in contrast to the contemporaneous brachiopod faunas from the Tethys Himalayan region that are characterized by typical cold-water taxa of Gondwanan affinity, e.g.,Costiferina indica(Waagen, 1884),Retimarginifera xizangensisShen et al., 2000,Neospirifer(Quadrospina)tibetensisDing, 1962. Thus, it is strongly indicative that the Lhasa Block had drifted into a relatively warm-water regime during the Changhsingian. An analysis of the paleobiogeographic change of brachiopods in the Lhasa Block throughout the entire Permian further suggests that the Lhasa Block probably had rifted away from the northern peri-Gondwanan margin between the latest Cisuralian and middle Guadalupian, that is, the Neotethys Ocean had opened before middle Guadalupian.