Evidence of mantle xenoliths included in triassic basalts of Sierra de Valle Fértil, province of San Juan

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Maria Gimena Lopez
Brígida Castro de Machuca


Ultramafic xenoliths hosted in alkaline basaltic rocks of the Sierra de Valle Fértil are described and analyzed for the first time. These are dark gray subrounded fragments, up to 8 cm in diameter, with granular texture and microfractured but without evidence of intracrystalline deformation. They are composed of clinopyroxene, spinel group minerals, and eventual olivine. In the different mineral phases, zoning and disequilibrium evidences, such as sieve or spongy texture, exsolution lamellae, Schiller texture, and kelyphitic rims, were identified. The contents of Al2O3, Cr2O3, Mg# and the ratio AlIV/AlVI in the clinopyroxene crystals and spinel group minerals, together with geothermobarometric calculations in the clinopyroxene, allowed establishing slight differences between the analyzed xenoliths. Moreover, it was possible to verify that the values found are significantly different from those of the magmatic clinopyroxene which composes the basalts. Pressure-temperature calculations in clinopyroxene of the ultramafic xenoliths vary between 18.1 kbar to 12 kbar in average, with temperatures between 1250 °C to 1228 °C. In the magmatic clinopyroxene, the obtained P-T average values are ≈ 8.9 kbar and 1058 °C. It is proposed that the origin of the ultramafic xenoliths in the alkaline basalts would be related to non-reabsorbed mantle-derived fragments from the mantle.

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Lopez, M. G., & Castro de Machuca, B. (2023). Evidence of mantle xenoliths included in triassic basalts of Sierra de Valle Fértil, province of San Juan. Revista De La Asociación Geológica Argentina, 80(3), 559-574. Retrieved from https://revista.geologica.org.ar/raga/article/view/1632


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