Abstract

The Ronda massif is an outcrop of generally unaltered peridotite covering approximately 300 km2 on the southern coast of Spain west of Malaga. An emplacement age between Triassic and Miocene is indicated from geological evidence. Steeply dipping contacts, vertical gneissocity in contact rocks, a steep gravity-anomaly gradient, and a wide metamorphic aureole suggest that the mass forms a steep-walled ridge through the crust.

Associated metamorphic rocks include gneiss, hornfels, marble, and skarn. An undisturbed, prograde metamorphic aureole is present adjacent to one steep margin of the massif. The assemblage sequence ranges from quartz-sericite-chlorite over 6 km from the contact to quartz–K–feldspar–biotite–cordierite–garnet–plagioclase at the contact. Comparison of the observed reactions with published experimental work suggests that the maximum-temperature thermal gradient had a uniform value of approximately 80°C/km; the uniformity of this gradient suggests a dynamic model of origin for this aureole. A contact temperature of 800° C ± 100° C is estimated by extrapolation of the thermal gradient, and a final overburden pressure of 4.3 ± 1 kb is deduced from the coincidence of the andalusite-sillimanite and muscovite plus quartz reactions in the assemblage sequence. The petrogenesis of kyanite suggests that the inner aureole rock was dragged up by the intrusion from depths near the base of the crust.

The reaction biotite + sillimanite + quartz = cordierite + garnet + K-feldspar + H2O is estimated to have occurred in this aureole sequence at approximately 730° C ± approximately 30° C, 4.3 ± 1 kb (PH2O nearly equal to Ptotal), and the slope was calculated from analytical data to be approximately 12 bars/°C. The reaction that actually took place was strongly influenced by kinetic factors. The placement of this reaction in pressure-temperature space, in conjunction with published experimental work, delimits the range of the pressure sensitive assemblages: quartz–K-feldspar–sillimanite–cordierite–garnet–plagioclase and quartz–K–feldspar–biotite–cordierite–garnet–plagioclase; these assemblages can be used as geobarometers.

Rock of the same bulk composition as the marginal aureole rock contains a different assemblage sequence on the roof of the intrusion and shows no mineralogical indication of having experienced higher pressures (for example, no kyanite). The differential metamorphic development of these two sequences is explained by a model incorporating a different pressure history for each sequence. The marginal rock was dragged up from depth during metamorphism and vertical intrusion of the hot solid plug, finally equilibrating at approximately 4.3 kb. The penetration of the intrusion through the crust required that the roof rock be continually displaced outward from the area above the intrusion, and therefore the rock presently forming the roof cooled from maximum temperature at the end of intrusion at pressures of approximately 3.8 kb without experiencing higher pressures.

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