Clinopyroxene Phenocryst Formation in an Alkaline Magma: Interpretations from Oscillatory Zoning

Abstract

Oscillatory zoning is observed in a large variety of igneous mineral species and reflects changes in the crystal-melt system. Theoretical (numerical modelling) and experimental investigations of this common phenomenon lag behind the ability to accurately measure oscillating chemical compositions on a micro-scale. For this reason only general models for the production of oscillatory zoning exist (Shore and Fowler, 1996). Although the formation of oscillatory zoning patterns (OZP) commonly observed in accessory minerals such as zircon probably require a complex feed-back mechanism between the crystal and melt, major element OZPs in most rock forming minerals can be adequately explained without a feed-back mechanism and, therefore, probably reflect true oscillations in the broad chemical environment. These oscillations may occur as changes in the system's pressure, temperature, or composition (e.g. Simonetti et al., 1996). The sample for the present study is from the eastern Pontides, near the city of Trabzon, Turkey. Volcanic rocks from the region formed during three cycles of Phanerozoic eruptive activity in the Liassic, Upper Cretaceous and ?Eocene. The third stage coincides with maximum regional crustal thickening, during which time alkaline volcanism was dominant between Tonya and Trabzon, along the modern coastline. Exposed ?Eocene alkaline rocks at Trabzon and include a range of lava and pyroclastic compositions. The rocks are generally microlitic porphyritic to hyalo-prophyritic, and contain clinopyroxene, plagioclase, analcite, olivine and phlogopite as major phases, and sanidine, nepheline, cancrinite, apatite and Fe-Ti oxides as minor phases. In addition, the rocks from Trabzon area often contain cognate or cumulate xenoliths. The magmas derived from an E-type MORB source which experienced low degrees of melt segregation, rising to shallow depths (-1 kbar) and evolving during ascent by fractionation of garnet (residual), olivine, clinopyroxene, and plagioclase (Arslan et al., 1997). Diopside chemistry