The distribution of metamorphic temperatures around the Strontian Granodiorite

Abstract

SummaryThe Strontian Granodiorite, a ‘Newer Granite’ emplaced late in the Caledonian orogenic cycle, has an aureole which is strongly asymmetrical: the metamorphic zones are broader to the east of the intrusion than to the west. The asymmetry is attributed to a gradient in the background, regional temperatures. The muscovite-out isograd, marking completion of dehydration of muscovite + quartz to sillimanite + K feldspar (estimated temperature 645 °C), lies within 2 km of the contact on the west, whereas the cordierite isograd, due to the onset of dehydration of biotite+sillimanite+quartz (estimated temperature 690 °C) is at least 2.5 km from the contact on the east. This temperature interval of 45 °C, which is rather insensitive to small errors in the estimated pressure (preferred value 4.1 kbar), is used in a simple geometrical treatment to estimate the regional gradient in the plane of present exposure: approximately 5 °C/km in a direction 30° S of E. Because the intrusion does not appear to have been tilted, this result is regarded as an estimate of the horizontal component of the temperature gradient on the west flank of a regional thermal dome, inherited from the regional metamorphic climax and decaying on a timescale which is suggested, by radiometric data in the literature, to be ˜ 10 Ma. A tectonic origin for the perturbed regional temperature distribution (uplift of hot rocks in the east relative to colder rocks further west) is suggested by the metamorphic petrology of the region, and supported by literature data on structural movements, notably of the Sgurr Beag Slide.