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- MakaleYalnız künye2021Nature of the Early Cretaceous lamprophyre and high-Nb basaltic dykes, NE Turkey: Constraints on their linkage to subduction initiation of Neotethyan oceanic lithosphere
Karsli, Orhan; İlhan, Mehdi; Kandemir, Raif; Dokuz, Abdurrahman; Aydin, Faruk; Uysal, İbrahim; Duygu, Levent
Late Mesozoic evolution of the Neotethys Ocean is critical and still remains unclear along the entire Sakarya Zone (SZ), Turkey, because previous interpretations of the Middle to Late Jurassic rifting of the SZ have not been consensually accepted. We identified, for the first time, the Early Cretaceous dykes intruded into the Late Jurassic and Late Carboniferous bodies in the Çamlıkaya (Erzurum) area in the SZ. Here, we present a data set of new field observations, 40Ar/39Ar geochronology, whole-rock geochemical and Sr–Nd isotopes to find out the tectonic setting of previously undated basic to intermediate igneous rocks. 40Ar/39Ar technique on the groundmass separations yielded the plateau ages of 128.78 ± 1.02 to 122.89 ± 0.82 Ma. The investigated dykes are compositionally divided into two sub-groups: calc-alkaline lamprophyres (CAL) and high-Nb alkaline basalts (HNB). They show enrichment in large ion lithophile elements (LILE: e.g., Rb, Ba, K) and depletion in high field strength elements (HFSE: Nb and Ti) as well as moderate fractionation of light rare earth elements relative to heavy rare earth elements. The HNB have high Mg# (47–66), and relatively unradiogenic Sr [87Sr/86Sr(t) = 0.70385 to 0.70510] and Nd [εNd(t) = +0.5 to +5.3] isotopic composition, whereas the CAL possess moderate to high Mg# (32–72), and relatively high Sr [87Sr/86Sr(t) = 0.70586 to 0.71379] and Nd [εNd(t) = +1.5 to −9.9] isotopic composition. Early Cretaceous high-angle subduction triggered upwelling of the asthenosphere at 126 Ma, which likely generated a heat anomaly. This anomaly has probably induced melting of the oceanic slab and the metasomatized mantle to form the CAL and HNB. The HNB were generated by low degrees of melting of mantle domain that was previously reacted by oceanic slab-related adakitic melts at the depths of the garnet stability field (≥70 km). However, the CAL were probably formed at shallower mantle levels within the garnet-amphibole field by a relatively higher degree of melting of the mantle, also modified by subducted slab sediments. A new field, age, and petrochemical data from the Early Cretaceous dykes, combined with the presence of Aptian deep-water sediments to the north and the Late Jurassic-Early Cretaceous platform carbonates deposited in the passive margin setting along the entire SZ, imply the initiation of active-margin magmatism in the SZ. We suggest that the coexistence of the Early Cretaceous CAL and HNB points to the formation of an Andean-type active margin above a north-dipping Neotethyan oceanic slab. Their emplacement ages are associated with the inception of the magmatic arc structure and refer to a significant geodynamic turnover from passive-margin to active-margin tectonics along the SZ.
- MakaleYalnız künye2020Silurian to Early Devonian arc magmatism in the western Sakarya Zone (NW Turkey), with inference to the closure of the Rheic Ocean
Karsli, Orhan; Sengün, 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.