The Proterozoic Egersund Anorthosite Complex is composed of anorthosite and related rock types, which intrude intercalated charnockitic and garnetiferous migmatites. The highest metamorphic temperatures are spatially associated with the intrusion, and previous workers recognized high-temperature mineral isograds for pigeonite-in, osumilite (OSM)-in and orthopyroxene (OPX)-in. The P-T conditions at a distance of 16 km from the contact are about 700 ^oC and 5 kbar and increase to more than 1000 ^oC and 5 kbar at 2.5 km.

The high-temperature metamorphism in the country rocks found near the contacts of the northeastern part Rogaland intrusive complex (surface area of about 1000 square km) cannot be explained by assuming the heat source was a simple, single-phase of intrusion. In order to obtain the observed metamorphic temperatures and isograd distribution, thermal modelling indicates that the heat source must have had at least two main phases that were separated by a hiatus of about 3 - 4.5 m.y. In this model, emplacement and crystallization of the anorthosite (areal extent: 30 x 40 km) produces a thermal gradient (750-600 °C) in the country rocks. While this thermal gradient is developing, a second, smaller intrusion (Bjerkreim-Sokndal lopolith, areal extent: 9 x 12 km) is emplaced at the anorthosite-country rock contact. Because the country rocks nearest the anorthosite have undergone appreciable heating, the second intrusive can provide enough thermal input to obtain the observed high-temperatures. Recent work indicates that the entire magmatic emplacement occurred over a time interval of about 10 m.y. (930-920 Ma) which is consistent with the thermal model of the metamorphism.

The P-T estimates from 5 and 10 km from the intrusive contact and data from thermal model were used to estimate cooling rates and time intervals. Maximum temperatures are 860°C and 800°C and temperatures greater than 750°C (apparent end of new garnet growth) were maintained for 4.5 to 8.5 m.y. About 5 km from the intrusion calculated post-peak cooling rates indicate a decrease from 30° to 10°/m.y. over about 7 m.y., while at about 10 km calculated post-peak cooling rates decrease from 12° to 7°/m.y. over about 7 m.y.

During this time interval, retrograde and fine-grained Gar-Qz rims formed around orthopyroxene and primary garnet (outermost 150-200µ) present in assemblages of garnet-quartz-plagioclase-orthopyroxene +/- spinel. Based on the widths of preserved zoning profiles in garnet and orthopyroxene and diffusion data, estimates of cooling rates that can be made and these agree well with the calculated temperatures and cooling rates from the model. Where garnet and orthopyroxene are in direct contact, the retrograde exchange may have continued as long as 15-25 m.y., which resulted in more highly developed Fe/Mg-zoning at the rims (outermost 40-240µ) of both orthopyroxene and garnet.

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