Compositional Variations, Zoning Types and Petrogenetic Implications of Low-pressure Clinopyroxenes in the Neogene Alkaline Volcanic Rocks of Northeastern Turkey


Aydin F. , Karsli O., Sadiklar M. B.

TURKISH JOURNAL OF EARTH SCIENCES, cilt.18, ss.163-186, 2009 (SCI İndekslerine Giren Dergi) identifier identifier

  • Cilt numarası: 18 Konu: 2
  • Basım Tarihi: 2009
  • Doi Numarası: 10.3906/yer-0802-2
  • Dergi Adı: TURKISH JOURNAL OF EARTH SCIENCES
  • Sayfa Sayıları: ss.163-186

Özet

Clinopyroxene phenocrysts and microphenocrysts in different series of the Neogene alkaline volcanic rocks from the eastern Pontides (NE Turkey) record various stages in the crystallization conditions and evolution history of the alkaline melt as well as its origin. Crystal chemical studies reveal that the clinopyroxenes in each rock series show strong textural and compositional similarities, which all reflect a common petrogenetic affinity. They have relatively high Mg-numbers (0.68-0.95), variable Al(2)O(3) (1.3-9.6 wt%), low TiO(2) (<2.7 wt%) and Na(2)O (<0.9 wt%) contents and low Al([6])/Al([4]) ratios (mostly <0.25), suggesting relatively low-pressure crystallization conditions of the magma in the storage region. The pressures calculated for the clinopyroxenes in each series are nearly similar and vary in the range of 2.4-4.6 +/- 0.9 kbars, which approximately corresponds to a crystallization depth of 714 +/- 3 km. The analyses of the compositional trends of the clinopyroxenes indicate the following types of zoning: (i) oscillatory and sectorial zoning related to melt crystallization (i.e. rapid cooling and crystallization), (ii) oscillatory, reverse zoning related to the different crystallization paths under a variable fluid regime, (iii) normal zoning related to the differentiation and fractional crystallization of the magma. Based on the primitive mantle- and chondrite-normalized trace and rare earth element patterns, all clinopyroxenes have high abundances of incompatible elements (i.e. La, Ce) with negative high field strength element anomalies (i.e. Zr, Ti) and low Nb/Y (0.1-0.2), Th/Y (<0.1) and Rb/Y (<0.03) ratios, suggesting derivation from a similar source. Obtained textural and mineral chemical data, as well as whole-rock compositions, thus suggest that the clinopyroxenes may have started to crystallize from alkaline basaltic magma derived from a homogeneous lithospheric mantle enriched by an earlier subduction event. After this process, the alkaline magma, from which early clinopyroxenes crystallized, underwent a relatively low-pressure fractional crystallization process. This was in closed magma chambers at different levels of the crust (or within a volcanic conduit system devoid of interaction processes), shown by variations in the different crystallization paths and in the fluid regime of the melt during differentiation and ascent of the magma, in a post-collisional extensional tectonic regime which affected the eastern Pontides during the Neogene.