Deep‐crustal mineral assemblages and potential for crustal rocks below the Moho in the Scottish Caledonides

Abstract

The Scottish Caledonian lower crust is strongly seismically reflective. Beneath the Moho, at about 30 km, very few reflections are seen on seismic records, with the exception of a strong reflector at about 50 km depth. The rocks between the Moho and this strong reflector have been interpreted as crustal, but with mantle velocities and densities. We report tests of this, in which the densities and seismic velocities of likely crustal precursors are assessed through determination of the stable mineral assemblages along geotherms appropriate to the region. Likely crustal precursors were selected from suites of exhumed deep‐crustal rocks and deep‐crustal xenoliths, and from a variety of basaltic rocks. Mineral assemblages were estimated using a variant of the lowest‐free‐energy search procedure of Wood (1987), with account taken of non‐ideal mixing in solid solutions. Geotherms were calculated from the heatdow‐heat‐production relationship for the Scottish Caledonides, using the methods of Pollack & Chapman (1977). Seismic velocities were estimated from empirical density‐velocity relationships and from the velocities of individual mineral species weighted for modal proportions. Mafic rocks, including a variety of modern basaltic types and mafic gneisses from deep‐crustal sections, have densities calculated to be greater than 3200kg m−3 along Caledonian geotherms. Calculated seismic velocities for mafic suites are high, and variations in seismic velocity are low; mafic rock suites in metamorphic equilibrium would probably be indistinguishable from peridotitic upper mantle at sub‐Moho depths. Intermediate and felsic rocks have velocities too low to be mistaken for peridotites and would give rise to much greater reflectivity than is observed beneath the Caledonide Moho if present in significant quantities. Restites from partial melting of aluminous sediments, on the other hand, have densities and seismic velocities calculated to be similar to those of mafic rocks. The region between the Moho and the mantle reflector in the Caledonides could therefore be crustal if it were largely of mafic composition to begin with, if it had been invaded by voluminous gabbroic melts at some stage in its history, or if it had undergone substantial partial melting with extraction of granitoid melts, for which there is considerable evidence in the geological record. The development of dense mineral assemblages in the lower parts of thickened crust in the Caledonides obviates the need for crustal thinning following crustal thickening during orogeny. This scenario may apply to other regions of thickened continental crust which, like the Caledonides, have probably experienced widespread lower crustal anatexis, resulting in the removal of low‐density material and the development of cool temperatures at depth. Copyright © 1995, Wiley Blackwell. All rights reserved