An Oxygen and Hydrogen Isotope Study of the Skaergaard Intrusion and its Country Rocks: a Description of a 55 M.Y. Old Fossil Hydrothermal System

Abstract

Oxygen isotope analyses have been obtained on rocks and coexisting minerals, principally plagioclase and clinopyroxene, from about 400 samples of the Skaergaard layered gabbro intrusion and its country rocks. The δ^(18)O values of plagioclase decrease upward in the intrusion, from ‘normal’ values of about +6.0 to +6.4 in the Lower Zone and parts of the Middle Zone, to values as low as −2.4 in the Upper Border Group. The ^(18)O depletions of the plagioclase all took place under subsolidus conditions, and were produced by the Eocene meteoric-hydrothermal system established by this pluton. Clinopyroxene, which is more resistant to ^(18)O exchange than is plagioclase, also underwent depletion in ^(18)O, but to a lesser degree (δ^(18)O = +5.2 to +3.5). The ^(18)O-depleted rocks typically show reversed Δ^(18)Oplag−px fractionations, except at the top of the Upper Zone, where the pyroxenes are very fine-grained aggregates pseudomorphous after ferrowollastonite; these inverted pyroxenes were much more susceptible to subsolidus ^(18)O exchange (δ^(18)O = +3–9 to +0.7). D/H analyses of the chloritized basalt country rocks and of the minor quantities of alteration minerals in the pluton (δD = −116 to −149) confirm these interpretations, indicating that the rocks interacted with meteoric groundwaters having an original δD ≈ −100. and δ^(18)O ≈ −14. Low δD values (≈ −125) were also found throughout the biotites of the Precambrian basement gneiss, requiring that small amounts of water penetrated downward to depths of at least 6 to 10 km. These values, together with the lack of ^(18)O depletion of the gneiss, imply that the overall water/rock ratios were very small in that unit (<0.01), and thus that convective circulation of these waters was much more vigorous in the overlying highly jointed plateau basalts (δ^(18)O ≈ −4.0 to +4–0) than in the relatively impermeable gneiss (δ^(18)O ≈ +7–3 to +7–7). This contrast in permeabilities of the country rocks is also reflected in the distribution of δ^(18)O values in the pluton; the plagioclases with ‘normal’ δ^(18)O values all lie stratigraphically beneath the projection of the basalt-gneiss unconformity through the pluton. Elsewhere, the ^(18)O depletions are correlated with abundance of fractures and faults, particularly in the NE portion of the intrusion, where the Layered Series is very shallow-dipping and permeable basalts underlie the gabbro. The transgressive granophyres in the lower part of the intrusive have δ^(18)O values identical to those of the basement gneiss, indicating they were probably formed by partial melting of stoped blocks of gneiss. In the upper part of the intrusion these granophyre dikes have δ^(18)O values similar to the adjacent host gabbro; this suggests that much of the hydrothermal alteration occurred after their emplacement. However, because of the rarity of low-temperature hydrous alteration minerals, it is also clear that most of the influx of H_2O into the layered gabbro occurred at very high temperatures (>400–500 °C). Prior to flowing into the gabbro, these fluids had exchanged with similar mineral assemblages in the basaltic country rocks, explaining the lack of chemical alteration of the gabbro. Xenoliths of roof rock basalt and of Upper Border Group leucogabbro were strongly depleted in ^(18)O by the hydrothermal system prior to their falling to the bottom of the magma chamber and being incorporated in the layered series. This proves that the hydrothermal system was established very early, at the time of emplacement of the Skaergaard intrusion. However, no measurable ^(18)O depletion of the gabbro magma could be detected, indicating that very little H_2O penetrated directly into the liquid magma, in spite of the fact that a hydrothermal circulation system totally enveloped the magma chamber for at least 100,000 years during its entire period of crystallization. Only as crystallization proceeded was the hydrothermal system able to collapse inward and interact with the solidified and fractured portions of the gabbro. Nevertheless some H_2O was clearly added directly to the magma by dehydration of the stoped blocks of altered roof rock. It is also plausible that small amounts of meteoric water diffused directly into the magma, most logically in the vicinity of major fracture zones that penetrated close to, or were underneath, the late-stage sheet of differentiated ferrodiorite magma. It is suggested that such influx of meteoric waters was responsible for many of the gabbro pegmatite bodies that are common in the Marginal Border Group; also, such H_2O might have produced local increases in Fe^(+3)/Fe^(+2) in the magma that in turn could explain some of the asymmetric crystallization effects in the magma chamber. Local lowering of the liquidus temperature would also occur, perhaps leading to topographic irregularities on the floor of the magma chamber (e.g. the trough bands?).