Abstract Sumatra is located at the western end of the Sunda Arc, which resulted from the subduction of the Indo-Australian Plate beneath the Eurasian Plate. In this study, we report detailed zircon U–Pb and Hf isotope data for Cenozoic igneous rocks from the entire island of Sumatra to better constrain the temporal and spatial distribution of arc magmatism. The new dataset, combined with literature information, identifies the following two magmatic stages: (1) Paleocene to Early Eocene (66–48 Ma) and (2) Early Miocene to Recent (23–0 Ma), with a 25 myr-long period of magmatic quiescence in between. The magmatic zircons show predominantly positive and high ε Hf ( t ) values, ranging from +19.4 to +7.1 in western Sumatra, +17.1 to +1.6 in central Sumatra and +18.0 to +7.0 in eastern Sumatra, indicating an isotopically juvenile magma source in the mantle wedge along the western Sunda Arc. We explain the negative and low ε Hf ( t ) values (+0.5 to −13.1) of young samples around the supervolcano Toba as evidence for the subduction of sediment. We argue for a change in the subduction processes, where the first magmatic stage ceased owing to the termination of the Neo-Tethyan subduction and the following stage corresponded to the modern Sunda subduction.
Abstract The Meso-Tethys was a late Paleozoic to Mesozoic ocean basin between the Cimmerian continent and Gondwana. Part of its relicts is exposed in the Bangong–Nujiang suture zone, in the north-central Tibetan Plateau, that played a key role in the evolution of the Tibetan plateau before the India-Asia collision. A Penrose-type ophiolitic sequence was newly discovered in the Ren Co area in the middle of the Bangong–Nujiang suture zone, which comprises serpentinized peridotites, layered and isotropic gabbros, sheeted dikes, pillow and massive basalts, and red cherts. Zircon U-Pb dating of gabbros and plagiogranites yielded 206Pb/238U ages of 169–147 Ma, constraining the timing of formation of the Ren Co ophiolite. The mafic rocks (i.e., basalt, diabase, and gabbro) in the ophiolite have uniform geochemical compositions, coupled with normal mid-ocean ridge basalt-type trace element patterns. Moreover, the samples have positive whole-rock εNd(t) [+9.2 to +8.3], zircon εHf(t) [+17 to +13], and mantle-like δ18O (5.8–4.3‰) values. These features suggest that the Ren Co ophiolite is typical of mid-ocean ridge-type ophiolite that is identified for the first time in the Bangong–Nujiang suture zone. We argue that the Ren Co ophiolite is the relic of a fast-spreading ridge that occurred in the main oceanic basin of the Bangong–Nujiang segment of Meso-Tethys. Here the Meso-Tethyan orogeny involves a continuous history of oceanic subduction, accretion, and continental assembly from the Early Jurassic to Early Cretaceous.
Research Article| October 01, 1998 Thermochronological evidence for the movement of the Ailao Shan–Red River shear zone: A perspective from Vietnam Pei-Ling Wang; Pei-Ling Wang 1Department of Geology, National Taiwan University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Ching-Hua Lo; Ching-Hua Lo 1Department of Geology, National Taiwan University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Tung-Yi Lee; Tung-Yi Lee 2Department of Earth Sciences, National Taiwan Normal University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Sun-Lin Chung; Sun-Lin Chung 1Department of Geology, National Taiwan University, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Ching-Ying Lan; Ching-Ying Lan 3Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan Search for other works by this author on: GSW Google Scholar Nguyen Trong Yem Nguyen Trong Yem 4Institute of Geological Sciences, National Center for Natural Sciences and Technology, Hanoi, Vietnam Search for other works by this author on: GSW Google Scholar Geology (1998) 26 (10): 887–890. https://doi.org/10.1130/0091-7613(1998)026<0887:TEFTMO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Pei-Ling Wang, Ching-Hua Lo, Tung-Yi Lee, Sun-Lin Chung, Ching-Ying Lan, Nguyen Trong Yem; Thermochronological evidence for the movement of the Ailao Shan–Red River shear zone: A perspective from Vietnam. Geology 1998;; 26 (10): 887–890. doi: https://doi.org/10.1130/0091-7613(1998)026<0887:TEFTMO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract In order to explore the timing of strike-slip movement along the Ailao Shan–Red River shear zone, an 40Ar/39Ar thermochronological study of the Day Nui Con Voi metamorphic massif in northern Vietnam was undertaken. The massif, exposed in the southeastern segment of the Ailao Shan–Red River shear zone, reveals a rapid cooling in the early Miocene (25–21 Ma) after a very slow cooling ca. 34–25 Ma. The slow cooling period most likely resulted from a geothermal perturbation in the lithosphere owing to the late Paleogene (40–30 Ma) extension in eastern Tibet and western Yunnan, China. The rapid cooling, consistent with evidence of a wider rapid cooling span from 27 to 17 Ma compiled from the entire Ailao Shan–Red River shear zone, constrains the duration of left-lateral shearing, which eventually offset Indochina from South China by about 600 km. The time constraints we have established reinforce the argument that the onset of the Ailao Shan–Red River shear zone postdates the opening of the South China Sea that began ca. 30 Ma. This result highlights the necessity for reevaluating the Cenozoic tectonic models of Southeast Asia. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract Major and trace elements and Sm–Nd isotopic data are presented for metabasites that are present as lenses within Paleozoic metasediments in the Chenxing and Bangxi regions, central Hainan Island, Southeast (SE) China. Most metabasites are metamorphosed cumulated gabbroic rocks tholeiitic in nature, and characterized by varying degrees of depletion in Th, Nb, Ta and light rare earth elements (LREE). Moreover, they show high positive ∈Nd(T) values of approximately +7, similar to those of mid‐ocean ridge basalts (MORB). A Sm–Nd isochron age of 333 ± 12 Ma obtained for the metabasites is interpreted as their crystallization age. The combined geochemical and Sm–Nd isotopic data suggest that the metabasites were generated by dynamic partial melting from a MORB‐like mantle source in an oceanic regime. These rocks probably represent remnants of fragmented oceanic crust of the eastern part of Paleo‐Tethys. They were obducted onto the continental crust as part of the ‘Shilu Mélange’ in earliest Mesozoic time when southern Hainan (part of the Indochina block) collided with northern Hainan (part of South China). Alternatively, they could be formed in a volcanic rifted passive margin at the sea‐floor spreading stage as part of MORB‐like seaward‐dipping reflector break‐up packages.
The petrogenesis of adakites holds important clues to the formation of the continental crust and copper ± gold porphyry mineralization. However, it remains highly debated as to whether adakites form by slab melting, by partial melting of the lower continental crust, or by fractional crystallization of normal arc magmas. Here, we show that to form adakitic signature, partial melting of a subducting oceanic slab would require high pressure at depths of >50 km, whereas partial melting of the lower continental crust would require the presence of plagioclase and thus shallower depths and additional water. These two types of adakites can be discriminated using geochemical indexes. Compiled data show that adakites from circum-Pacific regions, which have close affinity to subduction of young hot oceanic plate, can be clearly discriminated from adakites from the Dabie Mountains and the Tibetan Plateau, which have been attributed to partial melting of continental crust, in Sr/Y-versus-La/Yb diagram. Given that oceanic crust has copper concentrations about two times higher than those in the continental crust, whereas the high oxygen fugacity in the subduction environment promotes the release of copper during partial melting, slab melting provides the most efficient mechanism to concentrate copper and gold; slab melts would be more than two times greater in copper (and also gold) concentrations than lower continental crust melts and normal arc magmas. Thus, identification of slab melt adakites is important for predicting exploration targets for copper- and gold-porphyry ore deposits. This explains the close association of ridge subduction with large porphyry copper deposits because ridge subduction is the most favorable place for slab melting.
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