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

Abstract

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

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