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Calcic amphibole growth and compositions in calc-alkaline magmas: Evidence from the Motru Dike Swarm (Southern Carpathians, Romania)

¹ Laboratoire de Géochimie Isotopique et Géodynamique Chimique, DSTE, Université Libre de Bruxelles (CP 160/02) 50, av. Roosevelt 1050 Bruxelles, Belgium
² Université de La Rochelle, av. M. Crépeau, 17042 La Rochelle cedex 1, France
³ UMR CNRS 6112, UFR des Sciences et Techniques, BP 92208, 44322 Nantes Cedex 3 France
⁴ Institutul Geologic al Romaniei, Bucuresti 78344, Romania
⁵ "Sabba S. Stefanescu" Institute of Geodynamics of the Romanian Academy 19–21, Jean Louis Calderon str., Bucuresti 37, RO-70201, Romania

Correspondence: ^* E-mail: ofemenia{at}ulb.ac.be

A Late Pan-African calc-alkaline dike swarm (basalt-andesite-daciterhyolite) has been investigated in a region of over 2000 km² in the Alpine Danubian window, South Carpathians (Romania). Amphibole phenocrysts and microphenocrysts have been investigated by wavelength-dispersive microprobe analysis and BSE imaging. The Ca-amphibole population, represented in all the lithologies, displays a large compositional range, interpreted as the result of two processes: (1) magmatic evolution (kaersutite Ti-pargasite pargasite Ti-magnesiohastingsite magnesiohastingsite edenite tschermakite magnesiohornblende) linked to magmatic differentiation from andesitic basalt to rhyolite; and (2) deuteritic alteration of the primary amphibole related to late-emplacement hydrothermal activity (yielding numerous varieties comprising those cited above). In all rock types, amphibole phenocrysts equilibrated at a nearly constant pressure of about 0.6 ± 0.1 GPa, but their temperatures of crystallization ranged from 1000–900 °C for basaltic andesites to 700–600 °C for dacites. In rhyolites, edenite to magnesiohornblende crystals reflect a continuous range of P-T conditions from 700 °C/0.6 GPa to 600 °C/0.1 GPa, in agreement with their change of habit from euhedral to subhedral. Complex zonations in pargasite-magnesiohastingsite (including resorption) are interpreted in term of self-organization of oscillatory zoning without significant heating and/or magma mixing.

Key Words: Crystal structure • calcic amphibole • crystal growth • magmatic zonation • igneous petrology • calcalkaline magma • thermobarometry

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