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  • MakaleYalnız künye2004Scopus SJRQ2
    Magma interaction recorded in plagioclase zoning in granitoid systems, Zigana Granitoid, eastern Pontides, Turkey

    Karsli, Orhan; Aydin, Faruk; Sadiklar, M. Burhan

    Plagioclase crystals characterized by compositional zoning are typical of the Zigana Granitoid (ZG), NE Turkey. The zoned crystals, which show textural equilibrium with the assemblage quartz + plagioclase + K-feldspar + hornblende + biotite + pyroxene, + magnetite + ilmenite, exhibit oscillatory zoning. The zoned plagioclase crystals, ranging in size from 3 to 6 mm, are oval and larger than normal lath-shaped crystals. The zoned crystals are divided petrographically into two main types: (i) crystals zones only at their rims, with spongy cores; and (ii) completely zoned ones. Composition of the zoned crystals ranges from An23 (oligoclase) to An70 (labradorite). Plagioclase crystals with oscillatory zoning are governed by strong substitution mechanisms of Ca2+ ⌊ Na+, Al 3+ ⌊ Sl4+ and weak substitution of Fe3+ for Sl4+. The Fe2O3 (t) (wt %) contents of the subject plagioclases increase with increasing An content. The petrographic and compositional changes from rim to rim (oscillatory zoning), and positive correlation between Fe2O3(t) (wt %) and An (mol %) contents of the zoned plagioclases, suggest recurrent mafic magma injection into a felsic magma chamber, and thus magma interaction as a dominant magmatic process during the evolution of the Zigana Granitoid (ZG).

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  • MakaleYalnız künye1998
    Growth rates of early Campanian rudists in a siliciclastic-calcareous setting (Pontid MTS., North-Central Turkey)

    Steuber, Thomas; Yilmaz, Cemil; Löser, Hannes

    Rudist communities that developed on the southern slope of the Pontid magmatic arc during the late Cretaceous thrived in a tectonically unstable setting, and in environments that were strongly influenced by siliciclastic sedimentation. Strontium isotope stratigraphy indicates an early Campanian age of the investigated association at Höbek Tepe, near Amasya. Stable isotope analyses of sclerochronological profiles of several shells of the three dominating hippuritid species yielded cyclic variations of δ18O that delineate seasonal variations in palaeotemperature. This allowed for the determination of individual life spans, skeletal growth rates and the reconstruction of the living growth fabric. Annual vertical shell growth of 35-40 mm was maintained by cylindrical morphotypes of Vaccinites ultimus during several successive years, while adult annual shell accretion of conical morphotypes was only 27 to 30 mm. Vertical growth of the co-occurring Yvaniella alpani was considerably slower and rarely exceeded 10mm/year. Differential growth rates of the two most abundant species, and morphological characters of Yvaniella which indicate that skeletal growth capacity was similar to sediment accumulation, allowed for a consistent reconstruction of the original growth fabric. CaCO3 production of the rudist communities amounted to 20 kg x year-1 x m-2 and contributed significantly to the sedimentary budget. Large-scale formation of fine-grained calcareous sediments by bioeroders occurred only in environments of less rapid burial, indicated by associations of slowly-growing geniculate rudist morphotypes and patellate corals. The implications of these results for the distribution of rudists and corals in Cretaceous shallow-marine environments are discussed.

    AtıfScopus37OpenAlex40
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