TY - JOUR

T1 - An experimental study on the shallow-level migmatization of ferrogabbros from the Fuerteventura Basal Complex, Canary Islands

AU - Koepke, Jürgen

AU - Berndt, Jasper

AU - Bussy, Francois

N1 - Funding Information: Otto Diedrichs's careful sample preparation is gratefully acknowledged. Stefan Grassmann is thanked for his analytical work and for the characterization of the grain size of the starting powder. Lesley Perg improved the English considerably. The manuscript has been substantially improved after thorough reviews by A.E. Patino Douce and M.B. Wolf. The work was funded by grants from the Deutsche Forschungsgemeinschaft.

PY - 2003/8

Y1 - 2003/8

N2 - 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 °C at relatively oxidizing conditions. Distinct amounts of water were added to the charge.From 930 to 1000 °C, the observed experimental phases are plagioclase (An60-70), clinopyroxene, amphibole (titanian magnesiohastingsites), two Fe-Ti oxides, and a basaltic, K-poor melt. Above 1000 °C, amphibole is no longer stable. The first melts are very rich in normative 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 °C) 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 °C on a glass of the fused 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.%).

AB - 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 °C at relatively oxidizing conditions. Distinct amounts of water were added to the charge.From 930 to 1000 °C, the observed experimental phases are plagioclase (An60-70), clinopyroxene, amphibole (titanian magnesiohastingsites), two Fe-Ti oxides, and a basaltic, K-poor melt. Above 1000 °C, amphibole is no longer stable. The first melts are very rich in normative 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 °C) 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 °C on a glass of the fused 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.%).

KW - Canary Islands

KW - Experimental petrology

KW - Gabbro

KW - Migmatites

KW - Partial melting

UR - http://www.scopus.com/inward/record.url?scp=0041629295&partnerID=8YFLogxK

U2 - 10.1016/S0024-4937(03)00049-5

DO - 10.1016/S0024-4937(03)00049-5

M3 - Article

AN - SCOPUS:0041629295

VL - 69

SP - 105

EP - 125

JO - LITHOS

JF - LITHOS

SN - 0024-4937

IS - 3-4

ER -