Nd-Sr Isotopic Study of Magmatic Rocks and 40Ar/39Ar Dating of the Mafic Dike of the Proterozoic Ulan-Sar’dag Ophiolite Mélange (Southern Siberia, East Sayan, Middle Belt, Russia)

We report the first radiogenic Nd-Sr isotope data in the magmatic rocks island-arc ophiolite assemblage from the middle branch of the East Sayan ophiolite complexes to better constrain geodynamic processes in this segment of the CAOB in southern central Siberia. The magmatic rocks belong to the following geochemical types: (1) Ensimatic island-arc boninites; (2) island-arc assemblage; (3) enriched basalts of mid-ocean ridges; and (4) oceanic island-like basalts. The boninites have a positive value εNd (T), which is generated from a depleted mantle source (N-MORB). The island-arc assemblage has negative or slightly positive εNd (T) and was formed from an enriched mantle source due to the subduction of terrigenous rocks. The source of the terrigenous material was most likely the rocks of the Archean TTG (Trondhjemite Tonalite Granodiorite) complex of the Gargan block. Isotopic ratios for E-MOR and OIB-like basalts are characterized by positive or slightly negative values of εNd (T). The mafic dike, which crosscut ophiolite rocks, corresponds to OIB-like basalts. The values of εNd (T), measured 87Sr/86Sr and I (Sr), in the mafic dike correspond to the EM I mantle source. The E-MOR and OIB-like basalts appear to be formed in late-stage asthenospheric mantle melting via the decompression melting processes. The obtained isotope geochemical data for the E-MOR and OIB-like basalts probably indicate the mixing of island-arc melts with asthenospheric melts. We undertook 40Ar/39Ar dating of the mafic dike, which crosscut the ophiolite unit. The mafic dike has a whole-rock 40Ar/39Ar weighted mean plateau age of 799 ± 11 Ma. The dating constrains the minimum age of the ophiolite and island-arc magmatism in the region.

Supplemental Material: Eocene thickening without extra heat in a collisional orogenic belt: A record from Eocene metamorphism in mafic dike swarms within the Tethyan Himalaya, southern Tibet

Table S1: EPMA data of minerals compositions for garnet amphibolite; Table S2: Zircon U-Pb SHRIMP data for garnet amphibolite T0526; Table S3: Major and trace element compositions of garnet amphibolites.

Supplemental Material: Eocene thickening without extra heat in a collisional orogenic belt: A record from Eocene metamorphism in mafic dike swarms within the Tethyan Himalaya, southern Tibet

Table S1: EPMA data of minerals compositions for garnet amphibolite; Table S2: Zircon U-Pb SHRIMP data for garnet amphibolite T0526; Table S3: Major and trace element compositions of garnet amphibolites.

Structural Evolution of Extended Continental Crust Deciphered From the Cretaceous Batholith in SE China, a Kinmen Island Perspective

<p>The continental crust of southeast Asia underwent from thickening, thinning to almost rifting during the Mesozoic era as the active continental margin transformed into a passive one. Such crustal thinning history is well-preserved in the Kinmen Island, as the lower crustal granitoids retrograded and rapidly exhumed to surface that were crosscutted by mafic dike swarm. Kinmen Island is situated on the SE coast of Asia, featured by the widespread Cretaceous magmatism as the Paleo-Pacific plate subducted and rollbacked underneath the South China block. Although these complex magmatism are well reported and studied, their associated structural evolution and plate kinematics have not been clearly deciphered. Detailed field mapping, structural measurement, and petrographic analysis of the Kinmen Island were conducted. Up to five deformation events accompanied with five relevant magmatic episodes as well as their corresponding kinematic setting are reconstructed. The ∼129 Ma Chenggong Tonalite (G<sub>1</sub>) preserved all deformation events identified in this study, which marks the lower bound timing of all reported events. D<sub>1</sub> formed a gneiss dome with the Taiwushan Granite (∼139 Ma) at the core bounded by moderately dipping gneissic foliation (S<sub>1</sub>) as crust extended. D<sub>2</sub> formed subhorizontal S-tectonite (S<sub>2</sub>) with further exhumation of D<sub>1</sub> gneiss dome due to middle-to-lower crustal flow associated with further crustal thinning. D<sub>3</sub> formed a sinistral ENE-WSW striking steeply S dipping shear belts with well-developed S/C/C’ fabrics. The moderately E-plunging lineation on C surface indicates its transtensional nature. Widespread garnet-bearing leucogranite (G<sub>2</sub>) associated with decompressional melting showed long lasting intrusion prior to D<sub>2</sub> until post D<sub>3</sub>. D<sub>4</sub> was the intrusion of biotite-bearing Tienpu Granite (∼100 Ma; G<sub>3</sub>) that truncated G<sub>1</sub>, G<sub>2</sub>, and all fabrics, which was followed by the intrusion of E-W striking, steeply dipping biotite-bearing pegmatite (G<sub>4</sub>) as the crust further extended. The youngest deformation event (D<sub>5</sub>) was NE-SW striking subvertical mafic dike swarm (G<sub>5</sub>; 90–76 Ma) due to mantle upwelling through significantly thinned crust. By integrating the structural evolution and the previously reported strain pattern, we delineate the slab rollback direction of the Paleo-Pacific plate, which changed from northeastward (129∼114 Ma) to southeastward (107∼76 Ma). This plate kinematic movement switched during 114–107 Ma.</p>