Pyroxenes of Basic Rocks and Rodingites from an Ophiolite Mélange, South-Eastern Turkey

Robert Hall
Department of Geology, Queen Mary College, London University, London E1 4NS

None of the blocks examined is completely unaffected by alteration or recrystallization, yet despite the different metamorphic and metasomatic events affecting the different blocks there is a surprisingly small range in the composition of the clinopyroxenes. There is a trend of iron enrichment and sympathetic decrease in Cr2O3 from picrites, through gabbros and rodingites, to meta-basalts, and enrichment in TiO2 in the meta-basalt groundmass pyroxenes. This pattern of chemical variation, with the textural evidence, is taken to indicate that the clinopyroxenes are igneous in origin, that they have survived metamorphism and metasomatism without re-equilibration, and that most of the basic rocks originated in a single igneous complex.

Two exceptions occur. The pyroxenes of meta-basalt RH95 are separated on the pyroxene quadrilateral (fig. 1) from all the other clinopyroxenes, and their more calcic character is thought to be an original feature. It is possible that the pyroxenes of RH95 crystallized from a melt of different composition from that of the other rocks. Rodingite RM17 contains two chemically and texturally distinct clinopyroxenes. The augite resembles pyroxenes from other basic rocks and is considered to be of igneous origin. The second clinopyroxene is a super-calcic diopside characterized by low Cr, Ti, Na, Mn, and Al, and high Ca and Si. It is intimately intergrown with idocrase and grossular and is interpreted as metasomatic in origin. Its unusual chemistry is probably a reflection of the fluids associated with serpentinization. These are likely to have been similar to calcium hydroxide waters thought to be associated with present-day serpentinization in the western United States (Barnes and O'Neil, 1969).

The chemistry of the original igneous clinopyroxenes should indicate the nature of the original basic magma. Coombs (1963) has suggested the use of pyroxene norms as aids in the discrimination of basaltic types. The norms indicate that the pyroxenes of meta-basalt RH95 are alkalic in character. If, as suggested above, the remaining rocks originated in a single complex their norms indicate that they are more likely to be tholeiitic than alkalic. However, comparison of the compositions of the clinopyroxenes with those of the Bushveld complex, and of the North Atlantic and Pacific ocean floors, indicate that the mélange pyroxenes tend to be more calcic than those from typical tholeiites. It seems likely that the mélange rocks are transitional between tholeiitic and alkalic basaltic rocks.

Data from thirteen meta-volcanics from the mélange supports suggestions that the elements Ti, Zr, and Y are immobile during low-grade metamorphism, and indicates that the meta-volcanics belong to the group of ‘within-plate oceanic basalts’ (Pearce and Cann, 1971, 1973), which includes tholeiites and intermediate- and ultra-alkaline rocks from ocean islands.

This work indicates that igneous clinopyroxenes can survive low-grade metamorphism at moderate to high pressures, and may also be resistant to extensive calcium metasomatism under certain conditions. Metasomatic clinopyroxenes are clearly different from those of igneous origin. The range and type of chemical variation in the igneous clinopyroxenes, and the whole-rock trace element data, indicate that most of the basic rocks of the Mutki ophiolitic mélange once formed part of a single igneous complex, but are different from typical ocean-ridge tholeiites.

Mineralogical Magazine; December 1978 v. 42; no. 324; p. 511-512; DOI: 10.1180/minmag.1978.042.324.16
© 1978, The Mineralogical Society
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