Geochemistry and origin of ultramafic enclaves and their basanitic host rock from Kula Volcano, Turkey

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• 作者：Tobias Gr眉tzner ; Dejan Prelevi膰 ; C眉neyt Akal
• 关键词：Mediterranean ; Continental volcanism ; cumulates ; Rhö ; nite ; Green-core clinopyroxene ; Olivine
• 刊名：Lithos
• 出版年：November, 2013
• 年：2013
• 卷：180-181
• 期：Complete
• 页码：58-73
• 全文大小：3115 K

文摘

The Quaternary Kula Volcanic Province is located in western Anatolia, Turkey. This Na-alkaline anorogenic volcanism includes exposures of around 80 cinder cones, lava flows, and tuffs, representing one of the youngest volcanic activities in this region (1.9-0.026 Ma). The magmatism is related to an extensional regime and is interpreted as being derived predominantly from the asthenospheric mantle.

The lava flows are mostly of a basanitic composition and host rare comagmatic enclaves. The enclaves are composed of two dominant lithologies: amphibolites and clinopyroxenites with and without olivine. Amphibole is usually resorbed and replaced by a rhoenite-rich breakdown corona. The mineral composition of the breakdown corona suggests an eruption temperature slightly below 1100 掳C. Pressure and temperature calculations show that clinopyroxene crystallization began in the magma at 12-15 kbar and 1150-1200 掳C. Together with amphibole and later crystallized olivine, clinopyroxene was probably stored as cumulates at the walls of feeder dykes or small chambers.

The compositional variation of clinopyroxene from cumulates and lavas indicates two compositionally different clinopyroxene types. Mg-rich clinopyroxene is present in both lavas and cumulates, while the green-core Mg-poor clinopyroxene is observed exclusively in host lavas. Trace element analyses of clinopyroxene indicate that the clinopyroxenes from cumulates and lavas are comagmatic and crystallized in equilibrium with liquids whose compositions were the same as those of the lavas. On the other hand, the green-core clinopyroxenes were derived from a different melt source from the lavas. They crystallized in the lithospheric mantle and were incorporated into the basanitic melt.

In contrast to the primitive composition of lava olivines, the more evolved composition of enclave olivines is a result of fractional crystallization processes, whose cumulate products (clinopyroxenites and amphibolites) are directly observable. For example, a lower Fo component and Sc and V depletion in olivine from enclaves are mirrored by Sc and V enrichment in olivine from lavas.

In the initial phases of Kula volcanism, mantle-derived primary melts underwent deep-pressure fractionation of pyroxenites and amphibolites at the base of the crust. Calculations of the ascent rate suggest that the ascent of the magma from the Moho to the surface took only 4-11 days. After a period of time, a second batch of melt rose and incorporated the cumulates as enclaves. This new melt most probably originated from a slightly different mantle source, indicated by the presence of the green-core clinopyroxenes.