18 O/ 16 O, D/H, and 13 C/ 12 C studies of the Tertiary igneous complex of Skye, Scotland

Abstract

In southern Skye, δ18O analyses of 350 samples show that almost all
\nthe rocks within 4 km of the central intrusive complexes are depicted in 18O due to
\nextensive interaction with heated meteoric ground waters. Because of higher permeabilities,
\nthe plateau lavas are most widely affected (outward to 8 km). Whole-rock δ18O
\nvalues (SMOW) of basalts (-7.1 to +8.4), Mesozoic shales (-0.6 to +12.4), and Precambrian
\nsandstones (-6.2 to + 10.8) systematically decrease inward toward the center
\nof the plutonic complex. In the Cuillin gabbro, δ18O of plagioclase (-7.1 to +2.5) and
\npyroxene (-0.5 to +3.2) decrease outward toward the margins of the pluton. Most of
\nthese 18O depletions were produced by subsolidus exchange, but it also appears that
\nthe original Cuillin gabbro magma was depleted in 18O, with a δ18O ≅ +3.5. The
\nRed Hills epigranite plutons have δ18O quartz (-2.7 to +7.6) and feldspar (-6.7 to
\n+6.0) that suggest about 80 percent of the exchange took place at subsolidns temperatures;
\nprofound disequilibrium quartz-feldspar fractionations (up to 12.0) are characteristic.
\nThe early epigranites were, however, apparently intruded as low-18O magmas
\n(18O depletions of up to 3 per mil), with δ18O of quartz decreasing progressively with
\ntime. Successively later intrusions exhibit increasing δ18O quartz, but a major δ18O discontinuity
\nis observed at the geologic break between intrusion of the granites of the
\nWestern and Eastern Red Hills. The Southern Porphyritic epigranite was intruded as
\na low-18O magma with an extremely non-homogeneous δ18O of -3.0 to +3.0, suggesting
\nthat it was formed by partial melting of hydrothermally altered country rocks at
\ndepth; a similar origin may apply to the Coire Uaigneich granophyre. A good correlation
\nexists between grain size and δ18O for the unique, relatively norma1-18O Beinn
\nan Dubhaich granite, which intrudes limestone having a δ18O range of +0.5 to +20.8
\nand δ18O (PDB) of -4.9 to -1.0. The δD values (SMOW) of sericites (-104 to -107),
\nand amphiboles, chlorites, and biotites (-105 to -128) from the igneous rocks are
\nall low compared to "normal" igneous rocks, indicating that Eocene surface waters at
\nSkye had δD ≅ -85 and δ18O ≅ -12. The average integrated water/rock ratio for the
\nSkye hydrothermal system is approximately unity; at least 2000 km3 of heated meteoric
\nwaters were cycled through these rocks.