Rosatelli, Gianluigi; (2001) The petrogenesis of carbonatitic rocks and their relation to mantle amphibole and carbonate as exemplified in contrasting volcanoes from Vulture, Italy and Rangwa, Kenya. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis is a study of the relationships between carbonate and Ca-amphibole in experimental petrology and natural systems. Amphibole and carbonate are important phases in upper mantle mineralogy, being their introduction related to metasomatic processes and products which influence magma petrogenesis. In the experimental system studied, at high pressure (10-22 kbar) and temperature (980-1400 °C), pargasitic amphibole is not stable in presence of Ca-carbonate but dehydrates to produce mainly Ca-rich clinopyroxene (fassaite) plus oxides. At Mt. Vulture (Southern Italy) detailed study of amphibole megacrysts reveals that interaction between Ca-amphibole and carbonate melt produced similar textures and compositions to those obtained experimentally. Petrological features and geochemical signatures of ultramafic xenoliths and xenocrysts led to the definition of the metasomatic and magmatogenetic phases which affected the mantle beneath Mt. Vulture. A carbonated Ca-rich silicate melt produced vein metasomatism with introduction in the anhydrous peridotites of phlogopite and amphibole and transformed the harzburgitic and Iherzolitic wall rocks into wehrlite, by orthopyroxene reaction to form clinopyroxene. Along with the modal metasomatism the peridotite wall-rocks were enriched in LILE. A small batch of Ca-carbonate melt infiltrated the mantle at least one time during a short-lived heating episode. It produced epitaxial overgrowth of fassaitic clinopyroxene on amphibole. The presence of amphibole xenocrysts with clear signs of reaction with carbonatite melt, along with coarse-grained layered carbonatite (sovite) ejecta, in the lowermost pyroclastic deposits of Mt. Vulture volcano, predate the carbonatite activity with respect to the other only reported carbonatitic occurrence at the volcano summit. For comparison has been studied the extrusive carbonatite occurrences of Rangwa Caldera Complex (Kenya). Althought these carbonatite rocks preserve rarely original textures and compositions due to the intense post-emplacement alkali metasomatism (fenitization), highly explosive Ca-carbonatite and K-rich silicate activity is recorded in the intracalderic pyroclastic deposits. The study of the Rangwa silicate- carbonatite rocks did not reveal any petrogenetic similarity with the analogue Vulture rocks.

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