PARTIAL MELTING OF BASIC DIKES IN THE CONTACT METAMORPHIC AUREOLE OF A PYROXENITIC INTRUSION (BASAL COMPLEX, FUERTEVENTURA) AIDED BY THE PRESENCE OF METEORIC WATER: EVIDENCE FROM OXYGEN ISOTOPE COMPOSITIONS

Migmatites produced by the low pressure partial melting of basic dikes are found in a contact metamorphic aureole around a pyroxenitic intrusion, PX2, in the Basal Complex of Fuerteventura (Canary Islands). Anatexis at such shallow levels was attained by successive intrusion of numerous magma batches, producing a continuously high heat flow, along with the inferred presence of fluids. In order to gain insight into the nature and origin of these fluids, oxygen isotope compositions (δ¹⁸O) of rocks and mineral separates of clinopyroxene, feldspar and biotite from in and around the PX2 aureole were measured.

Dikes from beyond the PX2 aureole, unaffected by any hydrothermal events, have fairly normal mantle magmatic δ¹⁸O values (approx. +5 ‰). However, the dikes migmatized and hornfelsed by the intrusion of PX2, and greenschist dikes just outside the aureole, show unequivocal evidence for interaction with meteoric water, with low or negative δ¹⁸O (+0.2 to -3.4 ‰) whole-rock values. The δ¹⁸O values decrease towards the intrusion, which may be attributed to an increase in hydrothermal advection and dehydration of hydrous minerals with increasing proximity to the heat source. Diopside, plagioclase (An_32-38) and biotite separated from the migmatite leucosomes, yield consistent δ¹⁸O values of -4.2, -3.1 and -4.4 ‰, respectively. Hence, the melt (leucosome) must have been in equilibrium with a meteoric fluid. Nevertheless, the larger relict clinopyroxenes (in PX1 gabbros and as phenocrysts in dikes), that did not recrystallize during partial melting but still lie within the aureole, appear to almost completely preserve their magmatic compositions i.e. they did not reequilibrate with the surrounding melt. Fractionation factors calculated between plagioclase and diopside from the leucosome (assuming they are in equilibrium), yield an equilibrium temperature of 890°C, interpreted as a cooling rather than crystallization temperature.

Hypotheses involving the intrusion itself as a source for fluids can therefore be discarded. This is also supported by trace element geochemical studies. Dikes probably acted as conduits for meteoric fluids, with increasing advection in the vicinity of the heat source, causing greenschist metamorphism outside the aureole and aiding partial melting close to the intrusion.