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Crustal assimilation during turbulent magma ascent (ATA); new isotopic evidence from the Mull Tertiary lava succession, N. W. Scotland

Kerr, Andrew Craig ORCID:, Kempton, P. D. and Thompson, R. N. 1995. Crustal assimilation during turbulent magma ascent (ATA); new isotopic evidence from the Mull Tertiary lava succession, N. W. Scotland. Contributions to Mineralogy and Petrology 119 (2-3) , pp. 142-154. 10.1007/BF00307277

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Assimilation of crustal rocks with concomitant fractional crystallisation (AFC) is a well documented phenomenon in many igneous suites, but geochemical evidence from the Tertiary Mull lava succession suggests that in these magmas crustal contamination occurred by a distinctly different mechanism. Lavas from the lower half of the Mull Plateau group (MPG) can be divided into two broad sub-types; high (>8%) MgO basalts with elevated Ba and K; and lower MgO (<8%) basaltic-hawaiites with lower ba and k. the lower crust and most of the upper crust beneath mull is probably of lewisian age. the sr-, nd-and pb-isotope compositions of local lewisian crustal samples yield the following ranges>87Sr/86Sr=0.71002–0.72348, 143Nd/144Nd=0.51045–0.51058 and 206Pb/204Pb=14.0–14.6. Ten lavas have also been analysed and yield the following ranges: 87Sr/86Sr=0.7028–0.7042, 143Nd/144Nd=0.51214–0.51230 and 206Pb/204Pb=15.1–17.9. However, within this range, it is predominantly the more primitive mafic compositions, with elevated Mg, Ba and K, that show the lowest Nd- and Pb-, and the highest Sr-isotope values. Modelling of these isotopic results, in conjunction with major and trace element data, show that: (1) contamination by Lewisian lower crustal material does occur; (2) that the process involved was not one of assimilation with concomitant fractional crystallisation (AFC). The proposed contamination process is one whereby the hottest (most MgO rich) magmas have assimilated acidic partial melts of Lewisian lower crust during turbulent ascent (ATA) through thin, poorly connected dyke- and sill-like magma chambers. The chemical composition of the contaminated lavas can be modelled successfully through addition of 5% acidic Lewisian crust to an uncontaminated lava. In contrast, the more evolved magmas — which probably fractionated at sub-crustal levels — were either not hot enough to molt significant amounts of crust, or did not ascend turbulently because of their higher viscosity, and so are less contaminated with crust.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Environmental Sciences
Subjects: G Geography. Anthropology. Recreation > GC Oceanography
Q Science > QE Geology
Publisher: Springer Verlag
ISSN: 0010-7999
Last Modified: 18 Oct 2022 12:22

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