CALEDONIAN PLUTONISM AND MAJOR LINEAMENTS IN THE SW SCOTTISH HIGHLANDS

Within the SW Grampian Highlands several NE-SW-trending shear zones an d faults related to the Caledonian transpressional collision are recog nized as being distinct from an intersecting set of NW-SE-trending pre -Caledonian crustal lineaments which were reactivated during Caledonia n orogenesis. The geochemical and isotopic characteristics of the 425- 400 Ma granitoids of this region show that they were derived by anatex is of the lower crust associated with a mantle component which provide d an influx of mantle heat and/or metasomatic fluids; their spatial di stribution suggests a close genetic relationship with Caledonian shear zones. Having made the fundamental distinction between ascent (transp ort) and emplacement (arrival) configurations possible for granitoid m agmas, new data are presented for these plutons which show that: (i) a common modal sequence of intrusive phases can be recognized; (ii) the se phases are all sited at shear zone or lineament intersections where transtensional zones allowed and facilitated ascent; (iii) emplacemen t was often by a process of localized in situ ballooning. The existenc e of such lineaments in these orientations in the SW Highlands may ind icate that the structure of the lower crust can be regarded as a serie s 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 loc ation of anatectic zones, siting and ascent pathways and any subsequen t emplacement phenomena can be explained in orogenic belts.