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Accretion of the lower oceanic crust in the Troodos ophiolite: textural and geochemical constraints from drill core Cy-4, Cyprus

Banks, Graham John 2004. Accretion of the lower oceanic crust in the Troodos ophiolite: textural and geochemical constraints from drill core Cy-4, Cyprus. PhD Thesis, Cardiff University.

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Many processes involved in the accretion of lower oceanic crust remain enigmatic. To investigate some of these processes, the principal focus of this study is drill core 'CY-4', an unbroken 2.2km long section through sheeted dykes and mafic and ultramafic cumulates of the Troodos ophiolite lower crust, Cyprus. The generally accepted hypothesis states CY-4 intersected an axial magma chamber that was intruded by an off-axis pluton, based on a 'stepped' increase in clinopyroxene titanium content at 1331 m depth. This comprehensive structural, textual and trace element examination throughout CY-4 indicates a single, open system, plutonic sequence. It is comprised of high-level, varitextured gabbros, underlain by foliated gabbronorites and low-level, banded, gabbronorites and ultra mafic cumulates. Steeply inclined planar and linear fabrics were generated by upward migration of magma. Flow was predominantly channelised through the upper cumulates and diffuse through the lower cumulates, with corresponding differences in preserved structures and textures. Channelised flow and magma remobilisation in the rapidly cooled and poorly equilibrated upper cumulates is responsible for the decametre-scale textural and chemical variations. Syn-tectonic porous flow through the slower-cooled banded cumulates led to grain-scale textural and chemical interactions. Any initial mineral co-variations were erased during supra-solidus equilibration and efficient expulsion of incompatible element-rich melts. Any early-formed layering was transposed. A new model of CY-4 accretion is proposed in which a broad range of depleted and boninitic magma compositions were efficiently homogenised in the lower crust to yield the narrower ranges of extruded compositions. Correlations with Troodos Lower an Upper Pillow Lava series may indicate sources within the foliated gabbronorites, and the deeper ultramafic region respectively. The structural resemblance of the CY-4 cumulate series to fast-spreading lower crust can be reconciled with the slow spreading rate calculated for Troodos if CY-4 intersected a ridge axial node of magma upwelling

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Date of First Compliant Deposit: 30 March 2016
Last Modified: 09 Jan 2018 20:30

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