Synopsis

Growth histories have been determined from microstructural relationships and mineral chemistries of porphyroblasts of muscovite, Mn-rich garnet, almandine, biotite and chlorite. With data for albite, these chart the progressive development of biotite- and almandine-zone mineral assemblages during Barrovian metamorphism of Dalradian pelitic rocks near Loch Lomond in the SW Highlands of Scotland. Dimensionally aligned, fine-grained products of low greenschist facies dynamothermal metamorphism formed during D1 are included in early D2 Mn-rich garnets and in the cores of porphyroblasts of muscovite. Abundant albite porphyroblasts (An0–3) also grew early in D2 enclosing trails of inclusions that define open folds, with some showing two mineral growth fabrics. Almandine grew later during D2 when temperature was at its peak. Matrix grain coarsening, the development of the regionally expressed schistosity and intense dissection of F2 folds took place during late D2. Inclusion-free overgrowths of almandine and plagioclase (An4–17) grew during the end part of D2 to post-D2 along with porphyroblasts of muscovite, biotite and chlorite that are randomly disposed to the schistosity. Growth of the micaceous porphyroblasts is related to a change from the regionally expressed dynamothermal conditions of D2 to regional thermal growth conditions. Their development prior to D3 is shown by S3 minerals cutting across them and across the S2 schistosity. Both S3 and S4 developed during retrogressive low greenschist facies dynamothermal metamorphism. The timing of peak temperatures in the Loch Lomond district differs from that in the type area for Barrovian zones in the SE Scottish Highlands.

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