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| Titel |
Mineral chemical compositions of late Cretaceous volcanic rocks in the Giresun area, NE Turkey: Implications for the crystallization conditions |
| VerfasserIn |
Simge Oğuz, Faruk Aydin, İbrahim Uysal, Cüneyt Şen |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250132256
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| Publikation (Nr.) |
 EGU/EGU2016-12746.pdf |
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| Zusammenfassung |
| This contribution contains phenocryst assemblages and mineral chemical data of late
Cretaceous volcanic (LCV) rocks from the south of Görele and Tirebolu areas (Giresun, NE
Turkey) in order to investigate their crystallization conditions. The LCV rocks in the study
area occur in two different periods (Coniasiyen-Early Santonian and Early-Middle
Campanian), which generally consist of alternation of mafic-intermediate (basaltic to
andesitic) and felsic rock series (dacitic and rhyolitic) within each period. The basaltic and
andesitic rocks in both periods generally exhibit porphyritic to hyalo-microlitic porphyritic
texture, and contain phenocrysts of plagioclase and pyroxene, whereas the dacitic and
rhyolitic rocks of the volcanic sequence usually show a vitrophyric texture with predominant
plagioclase, K-feldspar, quartz and lesser amphibole-biotite phenocrysts. Zoned plagioclase
crystals of the mafic and felsic rocks in different volcanic periods are basically different in
composition. The compositions of plagioclase in the first-stage mafic rocks range from
An52 to An78 whereas those of plagioclase from the first-stage felsic rocks have
lower An content varying from An38 to An50. Rim to core profile for the zoned
plagioclase of the first-stage mafic rocks show quite abrupt and notable compositional
variations whereas that of the first-stage felsic rocks show slight compositional
variation, although some of the grains may display reverse zoning. On the other
hand, although no zoned plagioclase phenocryst observed in the second-stage mafic
rocks, the compositions of microlitic plagioclase show wide range of compositional
variation (An45−80). The compositions of zoned plagioclase in the second-stage
felsic rocks are more calcic (An65−81) than those of the first-stage felsic rocks, and
their rim to core profile display considerable oscillatory zoning. The compositions
of pyroxenes in the first- and second-stage mafic-intermediate rocks vary over a
wide range from diopside and augite to pigeonite (Wo27−49En35−48Fs9−30 with
Mg#=58-83 for the first-stage and Wo3−43En43−69Fs13−39 with Mg#=56-78 for the
second-stage). Although most of the pyroxene phenocrysts in both stages show no
zoning, some are chemically heterogeneous and exhibit zoning as variations in
parameters, such as Mg# [Mg/(Mg+Fe2+)], which are not systematic from core to
rim. This may be related to changing physicochemical conditions through magma
ascent and/or to the local effects of disequilibrium crystallization. The investigated
amphiboles and biotites of the volcanic sequence are particularly present in the
second-stage felsic rocks. The amphibole crystals are considered to be members of the
calcic amphibole with magnesio-hornblende to actinolite with their Mg# ranging
between 45 and 66. Biotites have relatively high Mg# (50-76). All compositional
variations for the zoned plagioclase and clinopyroxene phenocrysts imply that complete
equilibrium has not been reached during crystallization of the mafic and felsic magmas.
Moreover, abrupt compositional variations in some plagioclases require rapid changes
in the conditions of crystallization (P, T, fO2, etc.). However, slight variations in
oscillatory zoning are best interpreted as a result of local effects of disequilibrium
crystallization.
Acknowledgments
This study was supported by TUBITAK 112Y365 project. |
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