Caledonian plutonism and major lineaments in the SW Scottish Highlands

Abstract

Within the SW Grampian Highlands several NE–SW-trending shear zones and faults related to the Caledonian transpressional collision are recognized as being distinct from an intersect­ing set of NW–SE-trending pre-Caledonian crustal lineaments which were reactivated during Caledonian orogenesis. The geochemical and isotopic characteristics of the 425–400 Ma granitoids of this region show that they were derived by anatexis of the lower crust associated with a mantle component which provided an influx of mantle heat and/or metasomatic fluids; their spatial distribution suggests a close genetic relationship with Caledonian shear zones. Having made the fundamental distinction between ascent (transport) and emplacement (arrival) configurations possible for granitoid magmas, new data are presented for these plutons which show that: (i) a common modal sequence of intrusive phases can be recognized; (ii) these phases are all sited at shear zone or lineament intersections where transtensional zones allowed and facilitated ascent; (iii) emplacement was often by a process of localized in situ ballooning. The existence of such lineaments in these orientations in the SW Highlands may indicate that the structure of the lower crust can be regarded as a series of blocks bounded by intersecting ductile zones of high strain. The establishment of such a pattern of structural interactions within the lower crust could provide a mechanistic framework within which the location of anatectic zones, siting and ascent pathways and any subsequent emplacement phenomena can be explained in orogenic belts.