The petrology, geochemistry and petrogenesis of the upper pillow lavas, Troodos Ophiolite Complex, Cyprus

Langdon, George S. (1982) The petrology, geochemistry and petrogenesis of the upper pillow lavas, Troodos Ophiolite Complex, Cyprus. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Upper Pillow Lavas of the Troodos Ophiolite Complex, Cyprus, were formed in a back-arc basin environment by a second-stage partial melting event, which produced magmas depleted in large ion lithophile elements and enriched in magnesium. Renewed, second-stage melting to produce these magmas was facilitated by dehydration of subducted oceanic lithosphere as the spreading axis along which they were extruded moved away from a subduction zone. These parental magmas migrated upward to accumulate in shallow magma chambers and ultimately were extruded off-axis at the Upper Pillow Lava Series. -- The Upper Pillow Lavas exhibit genetic ties with the underlying, Lower Pillow Lavas of the Troodos Ophiolite Complex insofar as the latter represent- the 'normal' first-stage partial melting (21-23%) of 'fertile' upper mantle, erupted at a back-arc basin spreading centre above a subduction zone. The Upper Pillow Lavas, however, are considered distinct in character from the Lower Pillow Lavas, based on compositional and metamorphic criteria. The-petrographic and field classification of Smewing (1975) has been slightly modified, and the Upper Pillow Lavas are divided into four types: ultrabasic rocks, basaltic komatiites, olivine basalts and aphyric basalts. Major element extraction calculations and trace element modelling have shown that the magma parental to the Upper Pillow Lavas series could have been produced ~5% partial melting of a depleted source peridotite, and that the less primitive rocks of the Upper Pillow lavas (olivine and aphyric basalts) could have been derived by the fractional crystallization of 24-32% olivine from this parental magma; the ultramafic lavas represent accumulation of this olivine. This concept of olivine extraction was tested by comparing the calculated composition of olivines which would crystalize from the parental liquid, and the actual composition of the cumulus olivines. -- Projections into the basalt tetrahedron and C-M-A-S systems confirm that early-crystallizing olivine was followed down-temperature by orthopyroxene and clinopyroxene in the komatiites, and that quenching of the groundmass glass occurred before plagioclase became a liquidus phase.

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