The tectonically controlled emplacement of a vertically sheeted gabbro-pyroxenite intrusion: Feeder-zone of an ocean-island volcano (Fuerteventura, Canary Islands)


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The tectonically controlled emplacement of a vertically sheeted gabbro-pyroxenite intrusion: Feeder-zone of an ocean-island volcano (Fuerteventura, Canary Islands)
Allibon J., Bussy F., Lewin E., Darbellay B.
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The Miocene PX1 gabbro-pyroxenite pluton, Fuerteventura, Canary Islands,
is a 3.5 x 5.5 km shallow-level intrusion (0.15-0.2 GPa and 1100-1120
degrees C), interpreted as the feeder-zone to an ocean-island volcano.
It displays a vertical magmatic banding expressed in five 50 to 100
metre-wide NNE-SSW trending alkaline gabbro sequences alternating with
pyroxenites. This emplacement geometry was controlled by brittle to
ductile shear zones, generated by a regional E-W extensional tectonic
setting that affected Fuerteventura during the Miocene.
At a smaller scale, the PX1 gabbro and pyroxenite bands consist of
metre-thick differentiation units, which suggest emplacement by periodic
injection of magma pulses as vertical dykes that amalgamated, similarly
to a sub-volcanic sheeted dyke complex. Individual dykes underwent
internal differentiation following a solidification front parallel to
the dyke edges. This solidification front may have been favoured by a
significant lateral/horizontal thermal gradient, expressed by the
vertical banding in the gabbros, the fractionation asymmetry within
individual dykes and the migmatisation of the wall rocks. Pyroxenitic
layers result from the fractionation and accumulation of clinopyroxene
+/- olivine +/- plagioclase crystals from a mildly alkaline basaltic
liquid. They are interpreted as truncated differentiation sequences,
from which residual melts were extracted at various stages of their
chemical evolution by subsequent dyke intrusions, either next to or
within the crystallising unit. Compaction and squeezing of the crystal
mush is ascribed to the incoming and inflating magma pulses. The
expelled interstitial liquid was likely collected and erupted along with
the magma flowing through the newly injected dykes. Clinopyroxene
mineral orientation - as evidenced by EBSD and micro X-ray tomography
investigations - displays a marked pure-shear component, supporting the
interpretation of the role of compaction in the generation of the
pyroxenites. Conversely, gabbro sequences underwent minor melt
extraction and are believed to represent crystallised coalesced magma
batches emplaced at lower rates at the end of eruptive cycles.
Clinopyroxene orientations in gabbros record a simple shear component
suggesting syn-magmatic deformation parallel to observed NNE-SSW
trending shear zones induced by the regional tensional stress field.
This emplacement model implies a crystallisation time of 1 to 5 years
for individual dykes, consistent with PX1 emplacement over less than 0.5
My. A minimum amount of approximately 150 km(3) of magma is needed to
generate the pluton, part of it having been erupted through the Central
Volcanic Centre of Fuerteventura.
If the regional extensional tectonic regime controls the PX1 feeder-zone
initiation and overall geometry, rates and volumes of magma depend on
other, source-related factors. High injection rates are likely to induce
intrusion growth rates larger than could be accommodated by the regional
extension. In this case, dyke intrusion by propagation of a weak tip,
combined with the inability of magma to circulate through previously
emplaced and crystallised dykes could result in an increase of
non-lithostatic pressure on previously emplaced mushy dyke walls; thus
generating strong pure-shear compaction within the pluton feeder-zone
and interstitial melt expulsion. These compaction-dominated processes
are recorded by the cumulitic pyroxenite bands. (C) 2010 Elsevier B.V.
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01/10/2012 20:07
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