Spectacular shallow-level migmatization of ferrogabbroic rocks occurs in
a metamorphic contact aureole of a gabbroic pluton of the Tierra Mala
massif (TM) on Fuerteventura (Canary Islands). In order to improve our
knowledge of the low pressure melting behavior of gabbroic rocks and to
constrain the conditions of migmatization of the TM gabbros, we
performed partial melting experiments on a natural ferrogabbro, which is
assumed as protolith of the migmatites. The experiments were performed
in an internally heated pressure vessel (IHPV) at 200 MPa, 930-1150
degreesC at relatively oxidizing conditions. Distinct amounts of water
were added to the charge.
From 930 to 1000 degreesC, the observed experimental phases are
plagioclase (An(60-70)), clinopyroxene, amphibole (titanian
magnesiohastingsites), two Fe-Ti oxides, and a basaltic, K-poor melt.
Above 1000 degreesC, amphibole is no longer stable. The first melts are
very rich in non-native plagioclase (>70 wt.%). This indicates that at
the beginning of partial melting plagioclase is the major phase which is
consumed to produce melt. In the experiments, plagioclase is stable up
to high temperatures (1060 degreesC) showing increasing An content with
temperature. This is not compatible with the natural migmatites, in
which An-rich plagioclase is absent in the melanosomes, while amphibole
is stable. Our results show that the partial melting of the natural
rocks cannot be regarded as an ``in-situ'' process that occurred in a
closed system. Considerable amounts of alkalis probably transported by
water-rich fluids, derived from the mafic pluton underplating the TM
gabbro, were necessary to drive the melting reaction out of the
stability range of plagioclase. A partial melting experiment with a
migmatite gabbro showing typical ``in-situ'' textures as starting
material supports this assumption.
Crystallization experiments performed at 1000 degreesC on a glass of the
fitised ferrogabbro with different water contents added to the charge
show that generally high water activities could be achieved
(crystallization of amphibole), independently of the bulk water content,
even in a system with very low initial bulk water content (0.3 wt.%).
Increasing water contents produce plagioclase richer in An, reduces the
modal proportion of plagioclase in the crystallizing assemblage and
extends the melt fraction. High melt fractions of >30 wt.% could only
be observed in systems with high bulk water contents (> - 2 wt.%). This
indicates that the migmatites were generated under water-rich conditions
(probably water-saturated), since those migmatites, which are
characterized as ``in-situ'' formations, show generally high amounts
of leucosomes (>30 wt.%). (C) 2003 Elsevier B.V. All rights reserved.