Origin, age, and significance of deep‐seated granulite‐facies migmatites in the Barrow zones of Scotland, Cairn Leuchan, Glen Muick area

Abstract

Petrological modelling of granulite‐facies mafic and semipelitic migmatites from Cairn Leuchan, northeast Scotland, has provided new constraints on the pressure (P) and temperature (T) conditions of high‐grade metamorphism in the type‐locality Barrow zones. Phase diagrams constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 system have constrained the P−T conditions of peak metamorphism in the Glen Muick region of the upper sillimanite zone (Sill+Kfs) to have been at least ∼840°C at ∼9 kbar (high‐P granulite facies). These conditions are ∼120°C and ∼3 kbar higher than those recorded by lower sillimanite zone (Sill+Ms) units located only a few kilometres away to the southeast at Glen Girnock, indicating the presence of a significant thermal and barometric high exposed within the Scottish Dalradian, and supporting previous suppositions of a potential tectonic break between the two regions. U–Pb zircon geochronology performed on these mafic migmatites produced ages of c. 540–470 Ma from grains with both igneous and metamorphic morphological characteristics. Their basaltic protoliths likely formed during a period of volcanism dated at c. 570 Ma, associated with passive‐margin extension prior to the onset of Iapetus Ocean closure, and high‐grade metamorphism and partial melting is interpreted to have taken place at c. 470 Ma, synchronous with sillimanite‐grade metamorphism recorded elsewhere in the Dalradian. These high‐grade Cairn Leuchan lithologies are interpreted as representing a fragment of Grampian Terrane lower crust that was exhumed via displacement along a steeply dipping tectonic discontinuity related to the Portsoy–Duchray Hill Lineament, and are not pre‐Caledonian Mesoproterozoic basement, as suggested by some previous studies. Veins within some mafic migmatites in the Cairn Leuchan area, composed almost entirely (>80%) of garnet, with minor quartz, plagioclase, amphibole, and clinopyroxene, are interconnected with leucosomes and are interpreted to represent former garnet‐bearing melt segregations that have been locally drained of almost all melt. Thus, mafic components of the lower crust, currently underlying relatively lower grade metasediments exposed to the southeast, may represent a potential source rock for widely documented, post‐orogenic felsic plutons, sills, and dykes that occur throughout the Grampian Terrane.

Thermal and Physical Properties of Barrovian Metamorphic Sequence Rocks in the Ailao Shan‐Red River Shear Zone, and Implications for Crustal Channel Flow

“…Rocks in this high-P granulite belt experienced extensive melt production and loss during exhumation, and are likely source rocks for the widespread felsic plutons, sills, and dykes that occur throughout the BMS. Such a phenomenon has also been reported in the type-locality Barrovian metasediments of northeast Scotland (Palin et al, 2018) and in the Greater Himalayan Sequence in the Himalaya (Khanal et al, 2021). Thus, the discovery of high-P granulite belt is an important extension of the traditional BMS conditions reported in metapelites of the ARSZ into the suprasolidus field.…”

“…High‐strain continental‐scale shear zones are typically characterized by low‐strength lithosphere, and show notable vertical and horizontal changes in rock types and mineral assemblages according to variations in deformation‐related temperature ( T ) and pressure ( P ) conditions (Bürgmann & Dresen, 2008; Cao & Neubauer, 2016; Ceccato et al., 2020; Jamieson & Beaumont, 2011). Exhumed Barrovian metamorphic sequence (BMS) rocks in tectonically active orogenic‐scale shear zones can record physico‐chemical variations that occur between brittle rocks in the shallow crust, through the brittle–ductile transition, to the highly deformable and partially melted lower crust (Masters & Ague, 2005; Palin et al., 2018). BMS units in the Himalaya have been used to constrain the thermal evolution and convergence processes during collision between Greater India with Eurasia since c. 50 Ma (Anczkiewicz et al., 2014; Burg & Moulas, 2022; Carosi et al., 2010; Jessup et al., 2016; Khanal et al., 2021; Mottram et al., 2014; Smit et al., 2014; St‐Onge et al., 2013; Tewari et al., 2021).…”

Neoproterozoic and Early Paleozoic magmatism in the eastern Lhasa terrane: Implications for Andean-type orogeny along the northern margin of Rodinia and Gondwana

“…Powell & Holland, 2008), thus allowing discrete changes in crustal depth and thermal state during the burial and exhumation cycle to be defined (e.g. Weller et al, 2015;Palin et al, 2018). Typical uncertainties associated with cation exchange thermometers and net transfer barometers are at least ± 50 °C and ± 1 kbar at 1.…”

Petrology and U–Pb geochronology of high-grade metavolcano-sedimentary rocks from central Xolapa Complex, southern Mexico

“…Reaction textures among these phases are however absent in metapelite A57. Conditions of peak metamorphism can be further constrained by correlating observed and predicted proportions of temperature-dependent phases (Powell and Holland, 2008;Palin et al, 2018) like biotite and hercynite (Fig. 7d).…”