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- MakaleAçık Erişim2011Scopus SJRQ1Transition from shoshonitic to adakitic magmatism in the eastern Pontides, NE Turkey: Implications for slab window melting
Eyuboglu, Yener; Chung, Sun-Lin; Santosh, M.; Dudas, Francis O.; Akaryali, Enver
The formation of the eastern Pontides orogenic belt has been widely assigned to a northward subduction of the Neotethyan oceanic slab during the late Mesozoic-Cenozoic. Here we provide an alternate model based on new geological, geochemical and isotopic data. The magmatic activity in the far south of the belt started in the early Campanian with shoshonitic trachyandesites and associated pyroclastics. This sequence is covered by the late Campanian-early Maastrichtian reefal limestones and another stage of high-K volcanism represented by analcimized leucite-rich ultrapotassic rocks of the Maastrichtian-early Paleocene (?) ages. The shoshonitic and ultrapotassic rocks, with K2O contents ranging from 0.26 to 6.95 wt.%, display broadly similar rare earth and multi-element distribution patterns. Both rock types are enriched in LILE and LREE and depleted in HFSE (Nb,Ta and Ti), suggesting a subduction-enriched mantle source for the magma generation. Subsequently, during the late Paleocene, a stage of acidic magmatism (SiO2 of 53.25-73.61 wt.%) that shows adakitic geochemical characteristics including high Sr/Y (46-416) and La/Yb (11-51) and low Y (2.6-12.2 ppm), is documented characterized by melting of a mafic source such as the MORB crust with garnet in the residue. The adakitic magmatism began at similar to 56 Ma and migrated toward the north through time, culminating with porphyritic andesites (similar to 47 Ma) that were emplaced in the Gumushane-Bayburt line and its vicinity. North of this line, coeval magmas show typical calc-alkaline nature and continued to develop toward further north until the middle to late Eocene. Based on the spatial and temporal variations in the magmas generated in the eastern Pontides orogenic belt, we propose a new geodynamic model to explain the tectonomagmatic evolution of these rocks and correlate the adakitic magmatism to ridge subduction and slab window process within a south-dipping subduction zone. Our model is in contrast to the previous proposals which envisage partial melting or delamination of thickened lower continental crust due to the collision in the south during the Paleocene-Eocene. (C) 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
AtıfScopus172WoS166OpenAlex176
İncele - MakaleYalnız künye2010Scopus SJRQ1Late Cretaceous high-K volcanism in the eastern Pontide orogenic belt: Implications for the geodynamic evolution of NE Turkey
Eyüboǧlu, Yener
High-K volcanics were produced in two different Late Cretaceous cycles during the infilling of back-arc basins of the eastern Pontides. The first cycle, represented by shoshonitic trachyandesites and associated pyroclastics, containing high K2O (2.74-4.81 wt-%) and NaK2O (3.60-5.51 wt-%), overlies a mid-Cretaceous ophiolitic-olistostromal melange formed during the rifting stage of a back-arc basin (Neotethys). 40Ar39Ar dating of biotite from trachyandesite at the base of the first cyle indicates that shoshonitic volcanism in the far south of the eastern Pontides started in the early Campanian. Volcanic rocks of this cycle are overlain by upper Campanian-Maastrichtian rudist-bearing reefal limestones. The second cycle of high-K volcanism is represented by analcime-bearing volcanic rocks erupted in a Neotethyan lagoonal environment. These volcanic rocks, intercalated with continental detritus, are characterized by high a2O (3.22-7.16 wt-%), now concentrated in secondary analcime crystals. Their K2O contents also range between 0.83 and 6.05 (wt-%). All of these units are disconformably overlain by Eocene turbidites with a basal conglomerate. Volcanic rocks belonging to both cycles show various enrichment degrees in LILE, HFSE, LREE, and HREE with respect to primitive mantle and chondrite [(La/Lu) cn=11.10-25.89]. Negative Nb, Ta, Zr, Hf, and Ti anomalies are typical of these subduction-related arc magmas. Geochemical similarities between the two volcanic cycles suggest that trachyandesites and analcime-bearing volcanics were derived from similar enriched mantle sources, and that they formed in the same geotectonic setting of a back-arc basin environment of the eastern Pontide magmatic arc during the Late Cretaceous. In addition, Nd-Sr and Pb isotope ratios of the investigated volcanic units indicate that their mantle melt sources were similar. The new geological, geochemical and isotopic data suggest that southward subduction of the Palaeotethyan oceanic crust can be separated into two main stages for the late Mesozoic Pontide orogenic belt. Early southward subduction ended by slab break-off during the mid-Cretaceous, resulting in upwelling and injection of asthenospheric mantle that caused the opening of the eastern Pontide back-arc basin (Neotethys). Resumption of southward subduction began with intense TH-CA bimodal volcanism in the northern front of the arc during the Turonian-Coniacian. The intensity of this Late Cretaceous magmatism in the north decreased towards the south, and a transition to monogenic calcalkaline-alkaline (CA-A) Campanian andesitic volcanism developed in the southern zone. Yet farther south, andesitic igneous activity graded into the Campanian-Maastrichtian shoshonitic and ultrapotassic volcanism in the Bayburt-Maden, Tokat, Amasya, and Gümüshaciköy areas. This zoning of the Late Cretaceous arc magmatism (TH-CA, CA-A and shoshonitic-ultrapotassic, from north to south) also supports the concept of southward subduction during late Mesozoic evolution of the eastern Mediterranean region.
AtıfScopus120WoS110OpenAlex117İncele