The Glas Bheinn Appinitic Complex, Glen Roy: a model for foliation development during emplacement

Abstract

SynopsisThe Glas Bheinn Appinitic Complex forms part of a suite of Silurian calc-alkaline igneous intrusions exposed in the Glen Roy area. The complex comprises three main components: (1) hornfelsed schist and schist breccia; (2) quartzite and quartzite breccia; and (3) a suite of mafic and ultramafic, variably foliated and/or lineated appinitic intrusive igneous rocks. Emplacement of the complex post-dated the main tectonothermal history of the country rock, which is entirely of Leven Schist (Appin Group), and predated the intrusion of the Ben Nevis Dyke Swarm. A four stage model for the emplacement of the Glas Bheinn Complex has been erected: (1) development of a breccia pipe with minor amounts of ultramafic material; (2) compaction and consolidation of the breccia;(3) intrusion of the dioritic (appinitic) magma; and (4) crystallization and foliation development within the appinitic diorites. A detailed petrological study indicates that fabric development within the appinitic igneous rocks occurred in response to the heterogeneous collapse of a primocryst framework prior to full crystallization, probably in response to deformation associated with continued forceful intrusion. The residual melt expelled during this process migrated into regions of low strain and crystallized as leucocratic domains within the appinitic diorite. Initial results suggest that final equilibration of the plagioclase-K-feldspar-amphibole assemblage occurred at temperatures of c.

A new ‘microstructural mapping’ methodology for the identification, analysis and interpretation of polyphase deformation within subglacial sediments

“…Small-scale pressure variations within the deforming bed may result in the remobilisation of the liquefied till, which would be "squeezed out" of relatively higher pressure areas where the tillmatrix frame work is collapsing and clast microfabric development is well underway, and injected into lower strain areas to form cross-cutting veinlets and/or patches of massive till. This process is analogous to the collapse of the primary crystal framework and foliation development in the Glas Bheinn appinite during the forceful emplacement of this dioritic intrusion and described by Phillips & May (1996) (compare Figure 22 with Figures 8 and 9). The stratification/layering present in samples N7126 (Figure 2) and N7128 ( Figure 19) may have developed in response to the injection of remobilised till during the earlier stages of subglacial deformation, potentially explaining the marked variation in the intensity of S1 within different areas of these samples (compare S1 fabric strength in areas 1 and 4, with that in areas 5, 2 and 3 on Figure 22).…”

“…The potential similarities between fabric development within water saturated tills and partially crystallised igneous rocks has previously been highlighted by Evans et al (2006). Pre-full crystallisation fabrics or foliations, as the name suggests, form during the emplacement of the igneous intrusions due to displacement (flow) of the partially crystallised magma prior to final solidification (Borradaile 1986;Hibbard 1987;Hutton 1988;Paterson et al, 1989;Cruden 1990;Phillips and May 1996). Paterson et al (1989) published a number of criteria for recognising foliations in granitic rocks formed by magmatic flow, these are: (i) a preferred orientation of primary igneous minerals that show no evidence of plastic deformation or recrystallisation of either of the crystals or interstitial minerals;…”

“…An example of a pre-full crystallisation fabric developed within an appinitic diorite intrusion from Glas Bheinn, Glen Roy in the Scottish Highlands (Phillips and May 1996) is shown in Figure 8. The spaced foliation in this diorite is defined by variably shape-aligned amphibole phenocrysts and elongate, lenticular domains composed of biotite; the overall morphology of this igneous fabric is comparable to the clast microfabrics present in the tills illustrated in Figures 2 and 5.…”

“…The spaced foliation in this diorite is defined by variably shape-aligned amphibole phenocrysts and elongate, lenticular domains composed of biotite; the overall morphology of this igneous fabric is comparable to the clast microfabrics present in the tills illustrated in Figures 2 and 5. Phillips and May (1996) argued that foliation development within these appinitic rocks occurred in response to the heterogeneous collapse of a crystal framework which formed prior to full crystallisation ( Figure 9), probably in response to deformation associated with the continued forceful emplacement of the intrusion. The collapse of this framework led to the passive rotation of amphibole and biotite crystals to form a well-developed domainal foliation with the remaining interstitial melt locally being expelled to form the more felsic domains richer in late crystallising, randomly orientated plagioclase and quartz.…”

“…Well-developed pre-full crystallisation fabrics developed within an appinitic quartz-diorite to granodiorite from the Glas Bheinn Appinitic Complex, Glen Roy (Phillips and May 1996). (a) Moderately developed domainal fabric defined by parallel to weakly anastomosing, discontinuous biotite-rich domains (brown) and variably shape-aligned amphibole (pale green) porphyroblasts.…”