Arc magmatism as a window to plate kinematics and subduction polarity:Example from the eastern Pontides belt,NE Turkey

The Eastern Pontides orogenic belt in the Black Sea region of Turkey offers a critical window to plate kinematics and subduction polarity during the closure of the Paleotethys. Here we provide a brief synthesis on recent information from this belt. We infer a southward subduction for the origin of the Eastern Pontides orogenic belt and its associated late Mesozoic--Cenozoic magmatism based on clear spatial and temporal variations in Late Cretaceous and Cenozoic arc magmatism, together with the exis- tence of a prominent south-dipping reverse fault system along the entire southern coast of the Black Sea. Our model is at variance with some recent proposals favoring a northward subduction polarity, and illus- trates the importance of arc magmatism in evaluating the geodynamic milieu associated with convergent margin orocesses.

Origin of Qushenla Formation Volcanic Rocks in the Nawucuo Area,Northern Tibet,and Constraints on the Subduction Polarity of the Bangong-Nujiang Tethys Ocean

The Qushenla Formation volcanic rocks are widely exposed in the northern margin of the Bangong-Nujiang suture zone(BNSZ).Research on these rocks is of great significance for understanding the tectonic evolution of the Bangong-Nujiang Tethys Ocean(BNTO).In this study,a systematic geological survey was conducted on the Qushenla Formation volcanic rocks that are widely exposed in the Nawucuo area,in the northern margin of the western segment along the BNSZ.The whole-rock geochemistry,zircon U-Pb dating,and in situ zircon Lu-Hf isotopes were carried out in this study,aiming to constrain the formation age,rock genesis,magma source and tectonic setting of the volcanic rocks.The zircon U-Pb dating shows that the Qushenla Formation volcanic rocks in the western BNSZ erupted during the period of 120–108 Ma,i.e.,Early Cretaceous.The Qushenla Formation volcanic rocks are a suite of intermediate-basic volcanic and pyroclastic rocks belonging to the medium-K calc-alkaline series.They are relatively enriched in light rare earth elements(LREEs)and incompatible elements such as Rb,K,La,Th,Sm,and Hf,whereas depleted in heavy REEs(HREEs)and high field strength elements(HFSEs)such as Nb,P,Zr,and Ti.The in situ zirconεHf(t)values of the volcanic rocks range from 8.95 to 12.57,with an average of 10.40.The Mg#,Th/La and Th/Ce values are between those of the mantle-derived magma and the continental crust.The formation of the Qushenla Formation volcanic rocks can be explained through the following process:(1)As the nospheric materials that upwelled during the rollback of the subducting Tethys Ocean slab induced the large-scale partial melting of the mantle wedge and the formation of the initial basaltic magma.(2)These mantle-derived magmas ascended and induced the partial melting of the lower crust to generate peraluminous melts.(3)The mixing of the peraluminous melts and mantle-derived melts generated the initial magma with homogeneous Sr-Nd isotope compositions.(4)Last,the eruption of the magma produced the widespread Qushenla Formation volcanic rocks at the surface.When combining this information with the regional geological background,it is believed that the Qushenla Formation volcanic rocks,the Meiriqicuo Formation volcanic rocks and the Late Jurassic–Early Cretaceous intrusive rocks together constitute the tectonic magmatic arc of the active continental margin on the southern margin of Qiangtang,which was formed in the tectonic setting of the northward subduction of the Bangong-Nujiang oceanic crust beneath the Qiangtang Block.

Northward subduction-related orogenesis of the southern Altaids:Constraints from structural and metamorphic analysis of the HP/UHP accretionary complex in Chinese southwestern Tianshan,NW China

The Chinese Tianshan belt of the southern Altaids has undergone a complicated geological evolution. Different theories have been proposed to explain its evolution and these are still hotly debated. The major subduction polarity and the way of accretion are the main problems. Southward, northward subduction and multiple subduction models have been proposed. This study focuses on the structural geology of two of the main faults in the region, the South Tianshan Fault and the Nikolaev Line. The dip direction in the Muzhaerte valley is southward and lineations all point towards the NW. Two shear sense motions have been observed within both of these fault zones, a sinistral one, and a dextral one, the latter with an age of 236-251 Ma. Structural analyses on the fault zones show that subduction has been northward rather than southward. The two shear sense directions indicate that the Yili block was first dragged along towards the east due to the cloclkwise rotation of the Tarim block. After the Tarim block stopped rotating, the Yili block still kept going eastward, inducing the dextral shear senses within the fault zones.

Formation of South Tianshan suture and its geological significance

South Tianshan–Solonker suture,is the largest and southernmost suture within the Central Asian orogenic belt(CAOB).It records the ultimate collision between Tarim–North China cratons and Siberia craton,and is commonly interpreted as marking the eventual closure of Paleo-Asian Ocean.South Tianshan suture belongs to the western segment of the suture zone,and its evolutionary features are important for defining the formation age of the South Tianshan–Solonker suture.In this paper,the authors review the geochronological,geochemical,petrographic,and paleontological evidence within South Tianshan suture to delineate its formation era and closure characteristics,and thus further revealing the ultimate evolutionary pattern of the western segment of Paleo-Asian Ocean.This suture records strong plate collision before Late Carboniferous,forming a series of high-pressure metamorphic rocks,characterized by the presence of blue schist,eclogite and mica schist.In Permian,the whole area was under a relatively stable post-orogenic setting,with the formation of bimodal volcanic rocks,post-collisional granites,and terrestrial molasses.Sedimentary facies gradually changed from marine to either lacustrine or fluvial during this period.An Early Permian granite dike crosscuts the HP metamorphic belt,and the HP metamorphic rocks also underwent retrograde metamorphism at this time,indicating the formation of the South Tianshan suture was earlier than Permian.Hence,the western section of Paleo-Asian Ocean closed during Late Carboniferous,and Tarim Craton moved northward to collide with Kazakhstan–Yili Block,leading to the formation of the South Tianshan suture.