Microstructural features of albite porphyroblasts as indicators of sequential Barrovian metamorphic mineral growth in the Caledonides of the SW Scottish Highlands

Abstract

Inclusion -porphyroblast and porphyroblast -porphyroblast relationships show that abundant albite in mica schists in the Caledonides of the SW Scottish Highlands are part of the Barrovian metamorphic assemblage. Growth early in the D2 deformational phase of porphyroblast cores followed the growth of Mn-rich garnet but preceded the growth of porphyroblasts of the index mineral almandine. Two sets of inclusion trails in the albite correspond to the regionally expressed S1 and S2. Straight trails of muscovite, chlorite, quartz, epidote and the earliest growth of biotite make up S1. Crenulated trails express deformation of S1 early in D2 with muscovite, chlorite, biotite, quartz, epidote and the Mn-rich garnet associated with the development of S2 crenulation cleavage. The geometries of these trails uniquely record early stages of D2 deformational history. An 0)3 growth is related to the temporal coincidence of the formation of S1-S2 crenulation cleavage hinges as favourable sites for nucleation and the release of large amounts of water from prograde reactions during tectonothermal reconstitution of first cycle immature sediments with a volcanic component. The main characteristics of the regionally expressed D2 schistosity were developed during the major grain coarsening that followed both albite and almandine porphyroblast growth. Essentially inclusion-free An 4)19 rims grew on the inclusioncontaining cores in the almandine zone in the later stages of schistosity growth and unoriented porphyroblasts of muscovite, biotite and chlorite indicate that mineral growth extended from the later stages of D2 to post-D2. Previous interpretations of the albite porphyroblast growth having been during D4 to post-D4 contemporaneous with retrogression are inconsistent with the microstructural evidence.

Major nappe-like D2 folds in the Dalradian rocks of the Beinn Udlaidh area, Central Highlands, Scotland

“…Dempster & Harte 1986) with respect to the timing of regional metamorphism in the overlying Tay Nappe. However, Roberts and Treagus (1975) reported pre-to syn-D2 growth of garnet from rocks above the Iltay Boundary Slide, and a recent detailed study of the timing of regional metamorphism in the area north of Loch Lomond (10-20 km S of Beinn Udlaidh) shows a very similar, albeit slightly later, sequence of mineral growths (Mathavan & Bowes 2004). In the latter area, garnet growth occurred from pre-to syn-D2 and, as at Beinn Udlaidh, biotite is post-D2 in age.…”