Caledonian magmatism record within Hebridean Terrane? Loch Roag dyke (Lewis Island, northern Scotland) non-peridotitic xenoliths and megacrysts as messengers from deep lithosphere.
International audience The northernmost part of Scotland - the Hebridean Terrane - is formed of Archean rocks originally being part of the Laurentian North Atlantic Craton. The geological history of the terrane is well recognised, however details of its internal structure remain unknown. The Eocene...
Main Authors: | , , , , , , |
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Other Authors: | , , , |
Format: | Conference Object |
Language: | English |
Published: |
HAL CCSD
2020
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Subjects: | |
Online Access: | https://insu.hal.science/insu-03669367 https://doi.org/10.5194/egusphere-egu2020-14255 |
Summary: | International audience The northernmost part of Scotland - the Hebridean Terrane - is formed of Archean rocks originally being part of the Laurentian North Atlantic Craton. The geological history of the terrane is well recognised, however details of its internal structure remain unknown. The Eocene (Faithfull et al. 2012, JGS) Loch Roag monchiquite (Lewis Island) sampled deep-seated lithologies, providing insight on evolution and geological structure of the deeper lithosphere of the Hebridean terrane. The monchiquite comprises abundant xenoliths of ultramafic, mafic and felsic rocks. The peridotitic xenoliths represent pieces of Archean mantle underlying marginal parts of the North Atlantic Craton, whereas the origin of non-peridotitic lithologies is uncertain.The studied suite of samples comprises two groups: 1) "xenoliths" of diorites (plagioclase, clinopyroxene, orthopyroxene, apatite, opaques) and biotite clinopyroxenites (+apatite), 2) "megacrysts" of clinopyroxene and K-feldspar, both with inclusions of clinopyroxene, biotite and apatite. Megacrysts of alkali-rich feldspar associated with corundum and HFSE-bearing minerals, and composite xenoliths formed of pyroxenite and K-feldspar-rich lithology have also been described from this locality (Menzies et al., 1986, Geol. Soc. Australia Spec. Pub.; Upton et al., 2009, Mineral. Mag.).We interpret the "xenoliths" as products of crystallization of fractionated mafic melt(s). The primary character of Sr isotopic ratios in plagioclase (87Sr/86Sr <0.702) suggests that parental melt of those lithologies originated from melting of depleted lithospheric mantle sources. The "megacrysts" represent fragments of disintegrated alkaline pegmatite(s) formed from melt of plausible mantle origin, possibly enriched (87Sr/86Sr in feldspar >0.704).Trace element composition, similar Sr isotopic ratios of minerals and textural features of "xenoliths" and "megacrysts" groups suggest their close genetic relationship. This geochemical resemblance may reflect crystallisation ... |
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