Petrology of metapelites in the Bugaboo aureole, British Columbia, Canada

Abstract

Contact metamorphism of greenschist facies Neoproterozoic turbidites by the Cretaceous Bugaboo Batholith in southeastern British Columbia has resulted in a well‐developed contact aureole. The aureole is about 1 km wide and can be divided into three main zones: (i) spotted phyllite zone, extending from the first appearance of spots of cordierite or andalusite to the last occurrence of primary chlorite; (ii) cordierite + andalusite + biotite zone, comprising hornfelses or schists with abundant porphyroblasts of cordierite and andalusite and, at higher grades, fibrolitic sillimanite; and (iii) K‐feldspar zone, characterized by hornfelses and schists that, in the inner part of this zone, are variably migmatitic. Four parts of the aureole were examined, three of which are characterized by schists, and one of which (Cobalt Lake area) is characterized by hornfelses and has exceptional exposure and comparatively unaltered rocks. Petrographic, modal, mineral‐compositional and whole rock‐compositional data were collected from the Cobalt Lake transect, allowing the prograde reaction sequence to be inferred. Notable features of the aureole at Cobalt Lake include: initial development of andalusite and plagioclase at the expense of paragonite‐rich white mica; a narrow interval across which cordierite, andalusite and biotite increase markedly at the expense of chlorite; gradual development of andalusite and biotite at the expense of cordierite and muscovite upgrade of chlorite consumption; and near‐simultaneous development of andalusite + K‐feldspar and sillimanite, the latter indicating a pressure of contact metamorphism of ~3 kbar. In other parts of the aureole, the development of sillimanite downgrade of the initial development of K‐feldspar suggests slightly higher pressures of contact metamorphism. Lack of correspondence between the observed sequence of reactions in the aureole and those predicted thermodynamically suggests that modifications to some of the thermodynamic data or activity–composition models may be required. Textural features in the aureole suggest the influence of kinetic factors on metamorphic recrystallization, including: (i) deformation‐catalysed reaction in the schists compared to the hornfelses, as indicated by different mineral‐growth sequences inferred from microstructures, and (ii) heating rate‐controlled recrystallization, as indicated by the decrease in grain size of hornfelses with increasing metamorphic grade.

Kinetic control of staurolite–Al2SiO5 mineral assemblages: Implications for Barrovian and Buchan metamorphism

“…The predicted subsolidus phase equilibria using “tc331” show poorer agreement with natural constraints than the earlier a – x models (e.g. compare figure 13b,c of Pattison & DeBuhr, ). Neglecting the “tc331” position of reaction , the intersection of reaction and the kyanite–sillimanite curve is constrained to the interval 6.5 ± 0.5 kbar and 630 ± 20°C.…”

“…Figure e shows the positions of reactions , and for the average Nelson pelite composition calculated using different thermodynamic data sets and a – x relations: the same Holland and Powell () data set as used in Figure c (ds5.5), but with the 2007 “Thermocalc331” a – x relations, labelled as “tc331” in Figure e: ( http://www.metamorph.geo.uni-mainz.de/thermocalc/software/index.html); thermodynamic data set ds6.2 of Holland and Powell () and the a – x relations of White, Powell, Holland, Johnson, and Green () and White, Powell, and Johnson (), labelled as “ds62” in Figure e; and data set SPac14, a modification of the Spear and Cheney () data set, described in Pattison and DeBuhr (, p. 457), which is labelled in Figure e as “SPaC14”. The maximum difference is between the first two.…”

2. Phase Relations, Reaction Sequences and Petrochronology

“…Because we use phase equilibria modelling, it is assumed that 367 there are no kinetic controls (e.g., nucleation barriers or sluggish diffusion) that impact the 368 growth or dissolution of major and accessory minerals. Although this assumption may not be 369 strictly valid in some circumstances (Watt and Harley 1993;Pattison and Tinkham 2009; Gaides 370 et al 2011;Pattison and Debuhr 2015), the modelling here provides a general framework for 371 investigating reaction sequences. 372 373…”

Melt Reintegration Modelling: Testing against a Subsolidus Reference Assemblage

“…The absence of andalusite in a fluid-undersaturated system suggests that H 2 O plays a large role in its stability, at least using the dataset and activity models used in this study [8][9][10]43]. It should be noted that the use of different activity-composition models can result in differences in mineral stability [57]. Whereas modelling with subsolidus H 2 O saturation arguably represents a closed system from which H 2 O produced from dehydration reactions is unable to escape [56], a fluid-rich composition appears to be necessary to reproduce the subsolidus andalusite that we interpret was present.…”

“…Therefore, at this stage, we cannot incorporate orthoamphibole into the modelling with ds62 with any confidence. It should also be noted that low-P subsolidus equilibria involving porphyroblastic cordierite like in this study have been known to be problematic to model in ds62 e.g., [9,57,71]. Instead, the final composition produced after melt reintegration modelling (Figure 6b) was used to model the subsolidus assemblages in the previous THERMOCALC dataset ds55 [72], to investigate the effects of incorporating orthoamphibole into the modelling e.g., [69,73].…”