Structural features of ophiolitic chromitites in the Zambales Range,Luzon, Philippines |
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| Authors: | M Hock G Friedrich |
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| Institution: | (1) Institut für Mineralogie und Lagerstättenlehre, Rheinisch Westfälische Technische Hochschule Aachen, Wuellnerstrasse 2, D-5100 Aachen, Germany;(2) Present address: Hahn-Meitner-Institut für Kernforschung Berlin GmbH, Bereich Kernchemie und Reaktor, AG Geochemie, Glienicker Strasse 100, D-1000 Berlin 39, Germany |
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| Abstract: | The chromitite-bearing peridotites of the Zambales mafic-ultramafic complex form the lowermost level of the Zambales ophiolite, which exposes a complete ophiolitic sequence. The chromitites occur close to the peridotite/gabbro transition zone.The chromite orebodies are structurally classified into three major types: (1) concordant tabular deposits, (2) strings of pods and (3) pocketlike deposits.Concordant tabular deposits show a gradational transition from chromitite to host rock (modal grading) and are characterized by the parallelism of ore and host-rock structures. Primary magmatic features like inch-scale layering, size grading, glomeroporphyric chromite aggregates, skeletal chromite growth and adcumulus growth (cumulus textures) are common.The concordant chromite bodies are often tectonically disrupted and boudined forming strings of pods or fault-controlled pocketlike deposits. With increasing tectonization chromite shows pull-apart textures and lineations (plastic deformation), shearing, prismatic jointing, brecciation and mylonitization (brittle deformation). Recrystallization of cataclastic chromite occurs on a microscopic scale.Plastic deformation is caused by mantle flow and/or the volume increase of the peridotites during serpentinization. The influence of mantle flow is indicated by the orientation of the pod strings and lineations in chromitite perpendicular to the ridge axis. Brittle deformation of chromite (cataclasis) and disruption by faults is related to the emplacement of the ophiolite. |
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