Megacrysts and mafic–ultramafic xenolith-bearing ignimbrites from Sirwa Volcano, Morocco: phase petrology and thermobarometry |
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Authors: | M Bondi L Morten P Nimis P L Rossi C A Tranne |
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Institution: | (1) Dipartimento di Scienze della Terra e Geologico-Ambientali, Università di Bologna, Italy, IT;(2) Dipartimento di Mineralogia e Petrologia, Università di Padova, Italy, IT |
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Abstract: | Summary
A suite of clinopyroxene and amphibole megacrysts and mafic–ultramafic xenoliths are present in ignimbritic rocks of trachybasaltic–andesitic
composition from the Sirwa volcanic district, Morocco. The stumpy prismatic and sometimes euhedral clinopyroxene megacrysts
are Ti–Al-rich diopsides with mg values in the range 0.82–0.87 and Ca/(Ca + Mg) ratios in the range 0.53–0.54. The prismatic, elongated amphibole megacrysts
are calcic kaersutites–kaersutites with a narrow mg range (0.66–0.68). The xenoliths are represented by gabbroic and pyroxenitic types. In the gabbroic xenoliths two distinct
textural types can be distinguished: medium-sized granular and banded. The granular type is characterized by the mineral assemblage
Pl + Amph + Spl + Ilm + Ap. The banded type is distinct for the absence of Ilm and the presence of Cpx and Opx and shows alternating
bands enriched in Pl and Amph, respectively. The megacrysts and, probably, the xenoliths are considered not cognate with the
present host rocks since the calculated liquids in equilibrium with clinopyroxene and amphibole megacrysts over a wide range
of physical conditions have different trace and rare earth element contents. The observed phase relations and thermobarometric
calculations indicate that the megacrysts and xenoliths crystallized from their parent melts at P ≥ 10 kbar and T ≤ 1160 °C, i.e., in the upper mantle or near the crust-mantle boundary. A deep ( ≥ 30 km) magmatic chamber, where the megacrysts
and xenoliths originated, and a shallow volcanic chamber, energetically activated up to explosive conditions by injection
of deep-originated melts, is suggested to explain the occurrence of high-pressure megacrysts and xenoliths in the Sirwa volcanic
explosive products.
Received October 8, 2000; revised version accepted September 9, 2001 |
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