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.

Refined model of incremental emplacement based on structural evidence from the granodioritic Newry igneous complex, Northern Ireland

“…Several deepseated crustal lineaments are thought to have influenced ascent and emplace ment of magma in the UK (Jacques and Reavy, 1994;Hutton and Alsop, 1996;Stevenson et al, 2006;Cooper et al, 2013). These lineaments are known primarily from geophysics (gravity and magnetic data) and are thought to trend approxi mately northsouth over much of the UK to the north of the Iapetus suture.…”

“…These lineaments are known primarily from geophysics (gravity and magnetic data) and are thought to trend approxi mately northsouth over much of the UK to the north of the Iapetus suture. The origin of the lin eaments is unclear, although they are proposed to represent preCaledonian faults (Jacques and Reavy, 1994;Hutton and Alsop, 1996;Cooper et al, 2013).…”

“…This is intimately connected to ascent, which is the process through which magma rises within the crust or uppermost mantle to reach its em placement site (Jacques and Reavy, 1994). Since the nineteenth century, it has been recognized that emplacement requires inflow of magma into relocated crustal space (commonly referred to as "generated" space; Westropp, 1867), although the processes through which this occurs remain strongly debated (Hutton, 1988;Molyneux and Hutton, 2000;Petford et al, 2000;Stevenson et al, 2007a;Miles et al, 2016).…”

The Dalradian rocks of Scotland: an introduction

“…Various lines of evidence suggest a late Silurian to Early Devonian age for the main strike-slip movements, which had a close temporal relationship with, and partly controlled the emplacement of, certain late-Caledonian plutons and dyke-swarms (Morris and Hutton, 1993;Jacques and Reavy, 1994). The intrusion of large volumes of silicic magma, starting at c. 425 Ma, suggests that by mid-Silurian time sinistral transtensional stresses had begun to replace oblique convergence and transpression.…”

Laurentian margin evolution and the Caledonian orogeny—A template for Scotland and East Greenland

“…This change in plate kinematics may ultimately have resulted in development of the crustal scale sinistral wrench faults which are believed to have acted as fundamental controls on the locus of magma emplacement (Hutton and Reavy, 1992;Jacques and Reavy, 1994). …”