Mineral-chemical constraints on the miocene calc-alkaline and shoshonitic volcanic rocks of western Turkey: Disequilibrium phenocryst assemblages as indicators of magma storage and mixing conditions


Aldanmaz E.

TURKISH JOURNAL OF EARTH SCIENCES, cilt.15, ss.47-73, 2006 (SCI İndekslerine Giren Dergi)

  • Cilt numarası: 15 Konu: 1
  • Basım Tarihi: 2006
  • Dergi Adı: TURKISH JOURNAL OF EARTH SCIENCES
  • Sayfa Sayısı: ss.47-73

Özet

Early to Middle Miocene (22 to 15 Ma) volcanic activity across western Turkey produced a series of lavas and pyroclastic deposits with calc-alkaline and shoshonitic affinities. The erupted magmas show a broad range of compositions from basaltic to rhyolitic (48-75 SiO2 wt%) and are composed of variable phenocryst assemblages. Petrographic and mineral-chemical characteristics suggest that the magmas underwent hydrous crystallization in deep crustal magma chambers that was dominated by plagioclase + pyroxene + pargasitic amphibole fractionation. Subsequent crystallization in shallower magma chambers followed two different trends: (1) anhydrous (pyroxene + plagioclase-dominated); and (2) hydrous (plagioclase + edenitic amphibole + pyroxene-dominated). Application of magnetite-ilmenite, hornblende-plagioclase and two-pyroxene geothermometry to the western Anatolian volcanic rocks has yielded temperature estimates in the range of 585-1086 degrees C for the Early Miocene and 768-1095 degrees C for the Middle Miocene rocks. Pressure estimates from pyroxene and Al-in-hornblende geobarometers are in the range of 2.1-8.6 kbar for the Early Miocene and 6.5-9.1 kbar for the Middle Miocene rocks. The presence of amphibole with clear signs of disequilibrium. and plagioclase as inclusions in pyroxenes and other phenocryst cores, suggest that magma mixing was operational during the formation of the volcanic suites. Strong compositional variations and reverse zoning patterns in single phenocrysts, as well as considerably variable pressures of crystallization, further indicate that the magmas forming the volcanic suite had a polybaric origin and are the composite products of more than one petrogenetic stage. The observed range of phenocryst assemblages and different compositional trends are likely to have originated from fractionation of magmas with different initial water contents, under variable pressures of crystallization. The repeated occurrence of magmas from different suites during a single period of activity suggests that the magmatic system comprised several conduit systems and magma reservoirs dispersed at different levels of crustal magma chambers. The results suggest that the episodic intrusion of mafic magmas provided the necessary heat and perhaps contributed to the ascent of the magma to shallow crustal depths where it reequilibrated before the onset of eruption.