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

Research areas:
Year:
2006
Authors:
  • O Femenias
  • JCC Mercier
  • C Nkono
  • Hervé Diot
  • T Berza
  • M Tatu
  • D Demaiffe
Journal:
AMERICAN MINERALOGIST
Volume:
91
Number:
1
Pages:
73-81
Month:
January
ISSN:
0003-004X
BibTex:
Abstract:
A Late Pan-African calc-alkaline dike swarm
(basalt-andesite-dacite-rhyolite) has been investigated in a region of
over 2000 km(2) 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 -> magnesiohasting
site -> 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 degrees C for
basaltic andesites to 700-600 degrees C for dacites. In rhyolites,
edenite to magnesiohornblende crystals reflect a continuous range of
P-Tconditions from 700 degrees C/0.6 GPa to 600 degrees 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.