Detailed oxygen, hydrogen and carbon isotope studies have been carried out on igneous and metamorphic rocks of the StonyMountain complex, Colorado, and the Isle of Skye, Scotland, in order to better understand the problems of hydrothermal meteoricwater-rock interaction.
The Tertiary Stony Mountain stock (~1.3 km in diameter), is composed of an outer diorite, a main mass of biotite gabbro,and an inner diorite. The entire complex and most of the surrounding country rocks have experienced various degrees of18O depletion (up to 10 per mil) due to interaction with heated meteoric waters. The inner diorite apparently formed from alow-18Omagma with δ18O ≃ +2.5, but most of the isotopic effects are a result of exchange between H2O and solidified igneous rocks. The low-18O inner diorite magma was probably produced by massive assimilation and/or melting of hydrothermally alteredcountry rocks. The δ18Ovalues of the rocks generally increase with increasing grain size, except that quartz typicallyhas δ18O = +6 to +8, and is more resistant to hydrothermal exchange than any other mineral studied. Based on atom % oxygen,the outer diorites, gabbros, and volcanic rocks exhibit integrated water/rock ratios of 0.3 ± 0.2, 0.15 ± 0.1, and 0.2 ± 0.1,respectively. Locally, water/rock ratios attain values greater than 1.0. Hydrogen isotopic analyses of sericites, chlorites,biotites, and amphiboles range from -117 to -150. δD in biotites varies inversely with Fe/Fe+Mg, as predicted by Suzuokiand Epstein (1974), and positively with elevation, over a range of 600 m. The calculated δD of the mid-to-late-Tertiary meteoricwaters is about -100. Carbonate δ13C values average -5.5 (PDB), within the generally accepted range for deep-seated carbon.
Almost all the rocks within 4 km of the central Tertiary intrusive complex of Skye are depleted in 18O. Whole-rockδ18O values of basalts (-7. 1 to +8.4), Mesozoic shales (-0.6 to + 12.4), and Precambrian sandstones (-6.2 to + 10.8) systematicallydecrease inward towards the center of the complex. The Cuillin gabbro may have formed from a 18O-depleted magma(depleted by about 2 per mil); δ18O of plagioclase (-7.1 to + 2.5) and pyroxene (-0.5 to + 3.2) decrease outward toward themargins of the pluton. The Red Hills epigranite plutons have δ18O quartz (-2.7 to + 7.6) and feldspar (-6.7 to + 6.0) thatsuggest about 3/4 of the exchange took place at subsolidus temperatures; profound disequilibrium quartz-feldspar fractionations(up to 12) are characteristic. The early epigranites were intruded as low-18O melts (depletions of up to 3 per mil)with δ18O of the primary, igneous quartz decreasing progressively with time. The Southern Porphyritic Epigranite was apparentlyintruded as a low-18O magma with δ18O ≃ -2.6. A good correlation exists between grain size and δ18O for the unique, high-18O Beinn an Dubhaich granite which intrudes limestone having a δ18O range of +0.5 to +20.8, and δ13C of -4.9 to -1.0. The δD values of sericites (-104 to -107), and amphiboles, chlorites, and biotites (-105 to -128) from the igneous rocks , indicatethat Eocene surface waters at Skye had δD ≃ -90. The average water/rock ratio for the Skye hydrothermal system is approximatelyone; at least 2000 km3 of heated meteoric waters were cycled through these rocks.
Thus these detailed isotopic studies of two widely separated areas indicate that (1) 18O-depleted magmas are commonlyproduced in volcanic terranes invaded by epizonal intrusions; (2) most of the 18O-depletion in such areas are a result ofsubsolidus exchange (particularly of feldspars); however correlation of δ18O with grain size is generally preserved onlyfor systems that have undergone relatively minor meteoric hydrothermal exchange; (3) feldspar and calcite are the mineralsmos t susceptible to oxygen isotopic exchange, whereas quartz is very resistant to oxygen isotope exchange; biotite, magnetite,and pyroxene have intermediate susceptibilities; and (4) basaltic country rocks are much more permeable to the hydrothermalconvective system than shale, sandstone, or the crystalline basement complex.
