Distinguishing Phenocrysts From Xenocrysts; Dating the Onset of Volcanic Activity on the Isle of Rum, Scotland.

Details Distinguishing Phenocrysts From Xenocrysts; Dating the Onset of Volcanic Activity on the Isle of Rum, Scotland. Distinguishing Phenocrysts From Xenocrysts; Dating the Onset of Volcanic Activity on the Isle of Rum, Scotland.

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.v13b0537t&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #V13B-0537

Mathematics

Probability

1036 Magma Chamber Processes (3618), 8400 Volcanology, 8410 Geochemical Modeling (1009, 3610)

Scientific paper

Major volcanic activity on the Isle of Rum started with eruption of rhyodacite, now preserved as intrusive and extrusive parts of the intra-caldera succession. A thick sequence of intra-caldera ignimbrites and sedimentary breccias are preserved in the north and south of the island. Twenty feldspar crystals of the rhyodacite were dated using Ar/Ar and yielded a mean apparent age of 60.83 ± 0.27Ma (MSWD = 3.4), consistent with previously published dates for the crosscutting (i.e. younger) ultrabasic-layered intrusion of 60.53 ± 0.08Ma, Hamilton et al., (1998, Nature). On an age versus probability plot the feldspars do not, however, show a simple Gaussian distribution, but a major peak at 60.33Ma and two smaller shoulders at approx 61.4Ma and 63Ma. Our preliminary interpretation of the older ages is that they include a) xenocrysts derived from earlier Tertiary plutonics (61.4Ma peak) and b) older feldspars that have largely re-equilibrated within the rhyodacite magma chamber, possibly derived from Lewisian gneiss (63Ma peak). This may imply a residence time of these xenocrysts in the magma chamber of up to several years; cf. Gansecki et al., (1996, Earth Planet Sci. Lett.). The youngest and strongest age peak at 60.33Ma is suggested to represent the rhyodacite event. The oxygen isotope composition of the rhyodacite feldspars (6.88 ‰) is in the range of magmatic phenocrysts (6-7.5 ‰) and Lewisian gneisses (5-8 ‰) and well above the very low oxygen isotope values usually associated with high-T alteration. Hydrothermal overprint due to the layered ultrabasic intrusion was therefore probably minimal. We suggest the 60.33 ± 0.21Ma crystal age represents the rhyodacite eruption/intrusion event, implying that the ultrabasic-layered suite was already forming at depth and emplaced at shallow structural levels quickly thereafter. These new age dates tie in very well with recent work by Chambers et al., (2005, Lithos), highlighting a very quick succession of events: doming, violent ignimbrite eruption, intrusion of granite and ultrabasic rocks, followed by the rapid erosion of the Rum super-volcano, all in probably less than 1 million years.

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