Yazar "Andersen, Tom" seçeneğine göre listele

Listeleniyor 1 - 2 / 2
  • [ X ]
    Öğe
    Melting of the juvenile lower crust in a far-field response to roll-back of the southern Neotethyan oceanic lithosphere: the Oligocene adakitic dacites, NE Turkey
    (Elsevier, 2020) Karsli, Orhan; Caran, Semsettin; Coban, Hakan; Sengun, Firat; Tekkanat, Osman; Andersen, Tom
    Late Cenozoic tectono-thermal events and associated magmatism in the Sakarya Zone (SZ) are still contested. Although documented in the western part of the SZ, thus far, no magmatic activity has been identified in the eastern part of the Oligocene SZ. Here, we report a newly identified Oligocene magmatism to interpret the genesis with tectonic setting and gain new insight into the geological evolution of the eastern SZ. We present extensive geochemical, bulk-rock Sr-Nd and zircon Hf isotope, and zircon U-Pb chronological analyses for the Tepebasi dacites in the Artvin area, NE Turkey. Zircon UPb dating analyses revealed a dacite formation age of similar to 29.8 +/- 0.3 Ma. Geochemically, with a K2O/Na2O ratio of 0.5 to 0.6, they are composed of rocks of a medium-K calc-alkaline adakitic affinity. The samples are further characterized by low Y (6-7 ppm), and high Sr (362-588 ppm) and Sr/Y ratios (58-98), with low Mg# (41-45) values, demonstrating a close affinity with the crustal source of adakitic rocks. They have slightly radiogenic isotope concentrations (Sr-87/Sr-86(t) = 0.70460-0.70544, epsilon(Nd)(t) = +1.7 to +2.0), and single-stage Nd model ages of T-DM1 = 0.61-0.63 Ga, as well as uniform and positive epsilon(Hf) (t) of 8.2-10.5, with young Hf depleted mantle ages (T-DM1 = 0.31-0.41 Ga). These isotopic features, in combination with the geochemical signature, preclude a mantle origin. Instead, they most likely originated from a juvenile mafic lower crustal material by low degree partial melting (<%5) rather than through partial fusion of a subducting slab or thickened lower crust. Trace element modeling reveals that the mafic juvenile lower crust is composed of <10% garnet-bearing amphibolite. Furthermore, trace element compositions imply that adakitic melts formed in an extensional setting without delamination of a thick mafic lower continental crust. We conclude that the Oligocene adakitic magmatism originated in an intracontinental setting, which was subjected to far-field extensional forces induced by roll-back of south Neotethyan oceanic lithosphere just before its detachment in the collision zone. We believe that hot asthenospheric upwelling due to the far-field extension induced by the roll-back of the southern branch of the Neotethyan oceanic lithosphere triggered adakitic magmatism. The heat induced by the upwelling of the asthenosphere likely led to the heat-fluxed melting of juvenile mafic crustal material in such an extensional tectonic setting during the Oligocene epoch in the eastern SZ. (C) 2020 Elsevier B.V. All rights reserved.
  • [ X ]
    Öğe
    Silurian to Early Devonian arc magmatism in the western Sakarya Zone (NW Turkey), with inference to the closure of the Rheic Ocean
    (Elsevier, 2020) Karsli, Orhan; Sengun, Firat; Dokuz, Abdurrahman; Kandemir, Raif; Aydin, Faruk; Andersen, Tom
    The Rheic Ocean is the most significant Paleozoic ocean that detached peri-Gondwana terranes from the northern Gondwana margin throughout the closure of the Iapetus Ocean. The suture of the Rheic Ocean spreads from Mexico to the Middle East, and the timing of its final closure is well-documented by the rocks formed in the Variscan-Alleghanian-Ouachita orogeny which led to the formation of the supercontinent Pangaea. However, as robust paleomagnetic and quantitative data are mostly lacking, the onset and evolution of the subduction of the Rheic Ocean are highly speculative, and they require further confirmation. Recently, the well-preserved metagranitoids along the western Sakarya Zone (SZ) in Anatolia have been identified, and they provide new data that improve our knowledge on the evolution of the Early Paleozoic Rheic Ocean along the northern Gondwana. Here, we present new geochronological, in situ zircon Hf isotope, and whole-rock geochemical analyses of these metagranitoids from the western SZ to enhance our understanding of the subduction processes of the Rheic Ocean. IA-ICP-MS zircon U-Pb dating demonstrated that the Boziiyuk and Borcak metagranitoids from the western SZ were emplaced during the Silurian to Early Devonian (431 +/- 2.7 to 403 +/- 3.5 Ma). The both granitoids have medium- to high-K. calc-alkaline magmatic character, and exhibit peraluminous to slightly metaluminous geochemical signature. They show a typical arc pattern in terms of trace elements and have a uniform, moderate negative epsilon(Hf) (t) of -3.2 to -9.7, with Mesoproterozoic Hf depleted mantle model ages (T-DM1 = 12 to 1.4 Ga). The geochemical and isotopic characteristics are not consistent with those of depleted mantle melts and melts derived from the crustal rocks in an intracontinental environment. Instead, the parental magma is likely generated from the partial melting of a homogeneous and enriched mantle wedge source. We propose that the Silurian to Early Devonian arc-related magmatism is associated with a northward subduction episode of the Rheic Oceanic lithosphere beneath the peri-Gondwana terranes. Hence, we consider that the opening of Paleotethys Ocean formed in a back-arc basin of subduction in Andean style to the north rather than a continental rift to the south in response to south-directed and short-lived supra-subduction zone (SSZ)-type subduction during the Silurian to Early Devonian. (C) 2020 Elsevier B.V. All rights reserved.