Chromite composition as evidence of metamorphism in komatiites, greenstone belt, Mozambique
نویسندگان
چکیده
Archaean komatiites of the greenstone belt in Manica area, western Mozambique contains accessory amount of disseminated chromite grains. The komatiite rocks comprise peridotitic and basaltic komtiite. These komatiites are affected by regional metamorphism as all of the studied area. It is observed that only chromian spinel is preserved as the primary mineral and that all the other primary minerals in the komatiite samples are completely altered. The present work gives first time data on the effect of metamorphism on texture and composition of chromite enclosed in komatiite, Manica area, Mozambique. The primary chromian spinel has high Cr # (Cr/(Cr+Al) atomic ratio; approximate value = 0.9, which is comparable with Cr # value of chromian spinel in the worldwide komatiites. The chromian spinel have low Al and Mg value (up to 0.26 and 0.21 respectively, low Mg # and high Cr # and Fe # values. Most of the studied spinel grains rimmed by ferritchromite which is transformed outwards to magnetite. Textural features and mineral chemistry of chromian spinel enclosed in these komatiites indicate postmagmatic alteration and metamorphism in greenchist to amphibolite transition facies. The metamorphism also involves reequilibration of the chromian spinel cores with the surrounding silicate assemblage.
منابع مشابه
Fresh Melt Inclusions in 3.3 Ga Komatiitic Olivines from the Barberton
Introduction: Studies of komatiitic lavas continue to play a significant role in refining our understanding of early planetary tectonism, and thermal and compositional structure of the crust-mantle. Komatiites further represent a major lithologic component of the primitive landscapes which fostered early life on Earth, and may have played a significant part in modification of early atmospheric ...
متن کاملPetrology and Geochemistry of 3.3 Ga Komatiites– Weltevreden Formation, Barberton Greenstone Belt
Introduction: Komatiites provide several ways of assessing the chemical and thermal evolution of the mantle. Komatiitic liquids are produced from very high degrees of partial melting of the deep mantle, and erupted as extremely fluid lavas. Komatiites are important in the study of the early Earth because they are a significant constituent of the Archean crust. It has been proposed that komatiit...
متن کاملConstraints on the pre-metamorphic trace element composition of Barberton komatiites from ion probe analyses of preserved clinopyroxene
The trace element compositions of igneous augite preserved in metamorphosed komatiite samples from the lower Komati Formation, Barberton Mountainland, South Africa (3.49 Ga), were analyzed in order to estimate the pre-metamorphic bulk composition of the samples. The chemical effects of metamorphism are evaluated by calculating the trace element (TE) composition of a hypothetical augite in equil...
متن کاملA Petrologic, Geochemical and Osmium Isotopic Study of Selected Precambrian Komatiites
Title of Dissertation: A PETROLOGIC, GEOCHEMICAL AND OSMIUM ISOTOPIC STUDY OF SELECTED PRECAMBRIAN KOMATIITES Amitava Gangopadhyay, Ph. D., 2004 Dissertation Directed By: Professor Richard J. Walker, Department of Geology The major and trace elements, and Re-Os isotope systematics of the ca. 2.7Ga komatiites from the Alexo and Dundonald Beach areas in the Abitibi greenstone belt, Canada, and ca...
متن کامل