Petrology, chemistry and phase relations of borosilicate phases in phlogopite diopsidites and granitic pegmatites from the Tranomaro belt, SE Madagascar; boron-fluid evolution
Published:January 01, 2010
Théodore Razakamanana, Brian F. Windley, Dietrich Ackermand, 2010. "Petrology, chemistry and phase relations of borosilicate phases in phlogopite diopsidites and granitic pegmatites from the Tranomaro belt, SE Madagascar; boron-fluid evolution", The Evolving Continents: Understanding Processes of Continental Growth, T. M. Kusky, M.-G. Zhai, W. Xiao
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The boron-bearing minerals grandidierite, werdingite, serendibite and sinhalite are common in high-grade rocks of the Tranomaro belt in southeastern Madagascar. The mutual occurrence of these phases allows a new understanding of the role of boron-rich fluids in the crustal evolution of Gondwana, and we provide critical borosilicate data to constrain that development. We distinguish two types of grandidierite depending on their B2O3 and Al2O3 contents and on their relations with associated borosilicate phases. (1) At Vohibola the presence of sinhalite and serendibite associated with phlogopite lenses in metasedimentary diopsidites indicates an evaporitic origin from calc-silicate sediments. (2) At Cape Andrahomana borosilicates are associated with pegmatites and granites that were emplaced along shear zones on the boundary of the Tranomaro belt. The shear zones acted as conduits for boron-bearing fluids and for granitic partial melts, which had derived their boron from calc-silicate sedimentary protoliths. Using geothermometry and geobarometry of minerals from associated rocks, we calculate that ambient pressures and temperatures changed in time from 7.5 to 4.0 kbar and from c. 800 °C to 700 °C. Our results confirm the important role of shear zones in channelling the fluid flow of boron-bearing fluids that were derived from crustal melt granites in the same shear zones, but that ultimately derived their boron from early metasediments. We provide new information on the mineralogy, phase assemblages and paragenetic history of multiple borosilicates.
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The Evolving Continents: Understanding Processes of Continental Growth
This volume honours the career of Brian F. Windley, who has been hugely influential in helping to achieve our current understanding of the evolution of the continental crust, and who has inspired many students and scientists to pursue studies on the evolution of the continents. Brian has studied processes of continental formation and evolution on most continents and of all ages, and has educated and inspired two generations of geologists to undertake careers in studies of continental evolution. The volume is organized into six sections, including: oceanic and island arc systems and continental growth; tectonics of accretionary orogens and continental growth; growth and stabilization of continental crust; collisions and intraplate processes; Precambrian tectonics and the birth of continents; and active tectonics and geomorphology of continental collision and growth zones.