David R. M. Pattison

Age of the Ballachulish and Glencoe Igneous Complexes (Scottish Highlands), and paragenesis of zircon, monazite and baddeleyite in the Ballachulish Aureole

U–Pb zircon ages are presented for the Ballachulish Igneous Complex (207Pb–206Pb age 427 ± 1 Ma; 206Pb–238U age 423 ± 0.3 Ma) and Glencoe Volcanic Complex (207Pb–206Pb age 406 ± 6 Ma) of the Scottish Highlands. These ages are significantly more precise than pre-existing age constraints, and discriminate a previously unresolved age difference of c. 20 Ma between the two complexes. This difference in age provides an explanation for the c. 10 km difference in crustal level between the two magmatic events, and constrains exhumation rates for the Argyll region to be on average c. 0.4 km Ma−1 over the period c. 425–405 Ma. With improved age constraints on the Ballachulish Igneous Complex in place, the associated metamorphic aureole is used as a type locality to investigate the metamorphic behaviour of the U-bearing accessory minerals baddeleyite, zircon and monazite. Baddeleyite formed in impure dolomites by the reaction of detrital zircon with dolomite and gives the same U–Pb age as the intrusive complex, consistent with its contact metamorphic origin. Detrital zircon appears to be inert throughout much of the metamorphic aureole, with contact metamorphic zircon growth being restricted to migmatite-grade rocks. A possible exception is the development of enigmatic, very narrow overgrowths at temperatures between c. 500 and 600 °C. Monazite exhibits a variety of textures in lower-grade parts of the aureole (<550 °C), but occurs as distinctive clusters and trails of tiny ovoid grains at temperatures above about 560–600 °C. Monazite thus appears sensitive to metamorphism at lower temperatures than zircon and may therefore be a better target for metamorphic age measurements in rocks that reach mid-amphibolite facies but do not experience partial melting.

Instability of Al2SiO5 “triple-point” assemblages in muscovite+biotite+quartz-bearing metapelites, with implications

Abstract This paper uses constraints from experiments, thermodynamic modeling, and natural mineral assemblages to argue that Al2SiO5 “triple-point” assemblages, in which all three Al2SiO5 minerals are in stable equilibrium, are not possible in common muscovite(Ms)+biotite(Bt)+quartz(Qtz)-bearing metapelitic rocks because the reactions that first introduce an Al2SiO5 mineral to these bulk compositions occur at higher temperature than the triple point. Less-common, highly aluminous bulk compositions may develop Al2SiO5 minerals at temperatures below the triple point such that stable triple-point assemblages are theoretically possible. The “invisibility” of the triple-point to common Ms+Bt+Qtz-bearing metapelites calls into question most metapelitic triple-point localities reported in the literature, and carries implications for the topology of the metapelitic petrogenetic grid, the bathozone/bathograd scheme of Carmichael (1978), and the possibility of prograde kyanite → andalusite → sillimanite sequences. Re-examination of reported triple-point localities suggests that in most if not all cases, the Al2SiO5 minerals grew at different times in the metamorphic history of the rock

The geology and evolution of the Ballachulish Igneous Complex and Aureole

Synopsis The Ballachulish Igneous Complex and Aureole is one of the world’s most intensively studied plutonic-metamorphic complexes. The 412 ± 28 Ma igneous complex was emplaced at a depth of about 10 km in regionally deformed and metamorphosed mica schists, quartzites and siliceous carbonates belonging to the Dalradian Supergroup. The intrusion is exposed over an area of c. 7.5 × 4.5 km2 and is roughly cylinder-shaped to about 4 km beneath the surface, with the exception of two areas where the intrusion lies shallowly beneath the country rocks. The intrusion consists of an outer orthopyroxene-bearing diorite shell (emplacement temperature c. 1100°C) surrounding a central body of granite (emplacement temperature c. 850°C), the latter emplaced when the central portion of the diorite was still partially molten. A small, late leucocratic body in the centre of the granite is associated with weak Cu–Mo mineralization. The different intrusive phases represent separate magma batches which underwent little physical or chemical interaction during emplacement. A well-developed contact aureole surrounds the intrusive complex. Isograds in pelitic rocks, the most abundant rock type in the aureole, can be mapped around the intrusion and range from development of cordierite ‘spots’ (c. 550°C) up to anatectic migmatization (700–800°C). Isograds in siliceous carbonates rocks range from development of talc (< 480°C) up to periclase formation (c. 750°C). In the quartzites, recrystallization and coarsening of clastic quartz occurs, and clastic feldspar grains develop a high-temperature structural state, as the contact is approached. The agreement of the sequence and spacing of isograds in pelites, siliceous carbonates and quartzites with equilibrium phase diagrams indicates no significant kinetic control on the positioning of isograds, although some metamorphic processes appear to have been kinetically controlled. The contact metamorphism was mainly caused by intrusion of the diorite phase, with the later granite having little effect. Variations in width of the aureole are mainly due to variations in shape of the intrusion, temperature of the different intrusive phases, and the relative proportions of quartzite and pelite in the country rocks. The duration of the contact metamorphic event, for temperatures above conditions of the cordierite isograd (c. 550°C), was about 500 ka, whereas rocks were hot enough to be partially molten (temperatures above ca. 660°C) for about 270 ka. With the exception of some extensively fluid-fluxed partial melting on the west flank of the complex (Chaotic Zone), fluid communication between the intrusion and aureole was generally limited. Fluid fluxes in siliceous carbonates from the inner aureole on the east flank ranged from 100–1000 moles fluid cm−2. There is no evidence for the development of a large-scale hydrothermal circulation system.

Petrography and Mineral Chemistry of Pelites

This chapter describes the petrography and mineral chemistry of an exceptionally well-developed sequence of prograde mineral zones in pelitic and semipelitic rocks in the Ballachulish aureole. Two schematic petrogenetic grids are derived: the first is for mineral assemblages below the onset of partial melting, which define the mapped isograds in Maps 1 and 2 and Figure. 8.1; and the second is for high-grade mineral assemblages which occur sporadically within the zone of partial melting (Harte et al., Chap. 9, this Vol.) and within pelitic screens within the igneous complex. The two grids, when linked, provide a continuous petrogenetic grid from the lowest to highest grade in the aureole. In Pattison (Chap. 16, this Vol.) the continuous grid is calibrated in P-T space.

Field Relations and Petrography of Partially Melted Pelitic and Semi-Pelitic Rocks

Evidence of partial melting is seen in pelitic and semi-pelitic metasediments within the Ballachulish aureole in many locations immediately adjacent to the intrusive complex (within Zone V of Pattison and Harte 1985, and Chap. 8, this Vol.). In this chapter we summarize that evidence as seen both in the field and under the optical microscope. We endeavour in our presentation to largely keep description and interpretation in separate sub-sections, in an attempt to lay out the evidence for melting as objectively as possible. The chapter is concerned with textures and structures, their interpretation, and their implications for the rheological and crystallization behaviour of the melts. The reader is referred to Pattison and Harte (1985, 1988; Sect. 8.5, this Vol.) for notes concerning the melting reactions.

Beyond the equilibrium paradigm: How consideration of kinetics enhances metamorphic interpretation

Abstract The equilibrium model of prograde metamorphism, in which rocks are regarded as departing only negligibly from equilibrium states as they recrystallize, has generated a wealth of petrologic insights. But mounting evidence from diverse approaches and observations over a range of scales has revealed that kinetic impediments to reaction may prevent metamorphic rocks from attaining rock-wide chemical equilibrium along their prograde crystallization paths. To illustrate the resulting potential for inaccurate interpretation if kinetic factors are disregarded, we briefly review several case studies, including: out-of-sequence, metastable, and displaced isograds in contact aureoles; paragenetic sequences documenting overstepped, disequilibrium reaction paths; patterns of compositional zoning in garnet demonstrating partial chemical equilibrium; petrologic incongruities between observation and thermodynamic prediction; and inhibited reaction progress revealed by petrologically constrained numerical simulations of garnet crystallization. While the equilibrium model provides an indispensable framework for the study of metamorphic systems, these examples emphasize that all reactions require departures from rock-wide equilibrium, so all rocks must traverse kinetically sensitive reaction paths during recrystallization. Mindfulness of the potential significance of kinetic influences opens new avenues for petrologic investigation, thereby enhancing both analysis and interpretation.

Petrology and geochemistry of the Neoproterozoic Guaxupé granulite facies terrain, southeastern Brazil

Abstract Geochemical and oxygen isotopic data on the Neoproterozoic granulite facies rocks (enderbites, charnockite and charnokitic augen gneisses) of Guaxupe, Brazil are assembled. Geochemical discrimination function calculations show an igneous origin for majority of the rock types in the area. Major and trace element data, especially REE, are similar to tonalite and granodiorite. Oxygen isotope composition for charnockitic gneisses and mafic granulites yield δ 18O values in the range of +8.8 to +11‰ and fall in the range of quartzofeldspathic rocks of medium- to high-grade terrains elsewhere. A small-scale variation of δ 18O values, compatible with the chemical data, imply the preservation of pre-metamorphic oxygen isotope compositions. Minimum peak pressure-temperature conditions are 8.5 kbar and 850°C. The high-grade metamorphism did not appear to have altered significantly the geochemical and oxygen isotope compositions of the protoliths. Approximately 2.5 kbar of pressure decrease accompanied by 200°C cooling has been recorded for a portion of the cooling path. The P–T path is consistent with a single event, namely the formation and uplift of the Guaxupenappe in the Neoproterozoic Brasiliano cycle.

Regional Geology of the Ballachulish Area

The purpose of this chapter is to describe the stratigraphy, structure and regional metamorphism of the host rocks to the Ballachulish Igneous Complex. The chapter opens with a description of the geography of the Ballachulish area, and concludes with a discussion of pre-intrusion and post-intrusion uplift in the area.

Stable Isotope Geochemistry on the Intrusive Complex and Its Metamorphic Aureole

Variations in stable isotope ratios can be particularly helpful in studying the origins of igneous rocks, as well as the fluid interactions that may accompany magma generation or which may occur during and after magma emplacement and solidification (see e.g., the recent review in Valley et al. 1986). In certain instances, and to certain extents, granitic magmas may inherit and preserve the isotopic composition of their source region (O’Neil and Chappel 1977; O’Neil et al. 1977; Chivas et al. 1982; Vidal et al. 1984; Hill et al. 1986). More commonly, mafic partial melts may interact with isotopically evolved crust during magma ascent through AFC-type processes which produce correlated stable-radiogenic isotope covariations within genetically related plutonic suites (Michard-Vitrac et al. 1980; Halliday et al. 1980; Clayburn et al. 1983; Fleck and Criss 1985). Finally, interaction with externally derived fluids at either magmatic or subsolidus stages can be an important process (Friedman et al. 1964; Forester and Taylor 1976,1977; Hildreth et al. 1984; Fourcade and Javoy 1985; Wickham and Taylor 1987; Bickle et al. 1988).

Evidence of Fluid Phase Behaviour and Controls in the Intrusive Complex and Its Aureole

This chapter brings together evidence pertaining to the presence, composition, influence and mobility of fluids rich in volatile species (principally H2O and CO2) in the development of the Ballachulish intrusive complex and its aureole. We examine fluid-mineral-rock interactions over the complete range of scales from that of the small-scale systems represented by individual rocks, to that of the large-scale system represented by the combined intrusive complex and its aureole. This will allow comparison with models of fluid behaviour identified in other low- to high-grade metamorphic situations (see reviews in Walther and Wood 1986; and Valley 1986), such as the hydrothermal circulation system of the Hebridean intrusive complexes (Taylor and Forester 1971; Forester and Taylor 1977), and the high fluid-pressure model of regional metamorphism in orogenic belts (Etheridge et al. 1983, 1984).