Journal of Petrology, Volume 40, Issue 5: May 1999.

Diversity of Mafic Rocks in the Ronda Peridotite: Evidence for Pervasive Melt-Rock Reaction during Heating of Subcontinental Lithosphere by Upwelling Asthenosphere

CARLOS J. GARRIDO^1,2 AND JEAN-LOUIS BODINIER²

¹DP. MINERALOGIA Y PETROLOGIA, UNIVERSIDAD DE GRANADA, 18002 GRANADA, SPAIN
²ISTEEM, Lab. Tectonophysique, CNRS & UNIVERSITÉ DE MONTPELLIER II, CC 57, PLACE EUGÈNE BATAILLON, 34095 MONTPELLIER CEDEX 05, FRANCE

On the basis of their structural and compositional characteristics, mafic rocks in the Ronda peridotite are classified into four groups: group A (garnet-bearing rocks) occurring in the spinel tectonite domain; group B (spinel websterites) along the recrystallization front; group C (Ti-rich spinel pyroxenites) in the granular tectonite and plagioclase tectonite domains; and group D (Cr-rich pyroxenites) as composite layers along the recrystallization front, dykes in the spinel tectonites, and single layers in the other domains. The spinel tectonite domain and group A layers represent the vestiges of an old, veined subcontinental lithosphere. The other mafic groups were formed during a later magmatic event that originated the present petrological zoning of the massif, and, in most cases, have replaced older mafic layers (groups B and D) or peridotites (group C). The magmatic event that led to the diversity and zoning of mafic layers was caused by melting of the base of thinned subcontinental lithosphere by upwelling asthenosphere, followed by infiltration of asthenospheric melts. The different groups of mafic layers record several stages of this event: (1) in the thermal climax, group B was formed at the expense of group A by melt-rock reactions at increasing melt mass. This stage was coeval with partial melting of the host lithospheric peridotites and the development of the recrystallization front; (2) upon cooling of the lithosphere, group C was formed by near-solidus, melt-consuming reactions between melt and peridotite at high melt/rock ratios. Group C parental melts were mixtures of partial melts of lithospheric peridotites and garnet pyroxenites, and alkaline melts infiltrated from the underlying asthenosphere; (3) in the waning stages of this magmatic event, group D was formed by magmatic replacement of group A, B and C layers by small-volume melt fractions that percolated pervasively throughout the massif and had a refractory, calc-alkaline character. Locally, these melts were tapped into cracks forming intrusive dykes.

Keywords: Ronda peridotite;mafic layers;pyroxenites; subcontinental lithospheric mantle

Pages 729-754