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Pseudoleucite syenites at Loch Borralan, Scotland; petrology and a genetic model

Rebekka Reich, Michael A. W. Marks, Thomas Wenzel and Gregor Markl
Pseudoleucite syenites at Loch Borralan, Scotland; petrology and a genetic model
The Canadian Mineralogist (November 2020) 58 (5): 597-623


The alkaline Loch Borralan intrusion (Assynt Region, NW Highlands of Scotland) consists of a composite arrangement of several ultramafic to felsic plutonic rock bodies which were emplaced around 430 Ma into the Moine Thrust Zone during the Caledonian Orogeny. Some of the Loch Borralan rocks are ultrapotassic and contain pseudoleucite, i.e., a pseudomorph of alkali feldspar and nepheline after leucite. In total, 25 samples have been investigated, representing garnet-bearing pseudoleucite syenites and accompanying rock types such as nepheline-garnet-bearing syenites, alkali feldspar syenites, an amphibole syenite, a biotite-clinopyroxene syenite, and calcite-bearing glimmerites. Pseudoleucite is always associated with garnet, biotite, orthoclase, and minor clinopyroxene and titanite. Mineral chemical data indicate rather primitive magma compositions with no major differences between the various investigated main rock units. The abundant occurrence of up to 2 cm large, mostly euhedral pseudoleucite crystals and petrological phase considerations suggest that magmatic leucite physically separated from its host magma as a flotation cumulate. Based on our data and a comparison with previous field-based and experimental work, K-rich basanitic to tephriphonolitic melts that originated from a K-enriched mantle source may be parental to these rocks. The high liquidus temperatures at low pressures (e.g., approximately 1100 degrees C at 1 bar PH2O) required to crystallize leucite could have resulted from the ascent of successive melt batches in a composite intrusion. Later melt batches would increase the temperature in earlier, already partially cooled batches, causing an increase in temperature and a decrease in pressure during ascent. The subsequent decomposition of leucite to pseudoleucite is interpreted to result from either dry breakdown or autometasomatism, i.e., involvement of late-magmatic fluids.

ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 58
Serial Issue: 5
Title: Pseudoleucite syenites at Loch Borralan, Scotland; petrology and a genetic model
Affiliation: Eberhard Karls University, Department of Geosciences, Tuebingen, Germany
Pages: 597-623
Published: 20201124
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 70
Accession Number: 2021-003297
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 11 tables, sketch map
N57°49'60" - N58°30'00", W05°25'00" - W03°34'60"
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202103
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