Comparison of hydrothermal systems in layered grabbros and granites, and the origin of low-¹⁸O magmas

Abstract

The style of hydrothermal alteration in layered gabbros is very different from that in granitic plutons. Non-equilibrium ¹⁸O/¹⁶O effects are observed in both types of bodies, but whereas the granitic rocks with ¹⁸O-exchanged feldspars commonly contain abundant chlorite, sericite, epidote, etc., most such gabbros are mineralogically virtually unaltered (e.g., they contain fresh olivine). These difference simply that the bulk of the externally-derived hydrothermal fluid passes through the gabbros at temperatures of 450° -900°C, much higher than the range of 250° -450°C generally found in the granites. This contrasting behavior is in part explained by the higher solidus temperature, higher latent heat of crystallization, and generally higher melt/phenocryst ratio of the gabbro magmas, However, even more important is: (1) The geometry of crystallization. Cumulate gabbros crystallize from the floor upward, and the last part to crystallize is a sub-horizontal sheet of liquid near the roof. This magma sheet provides a thermally insulating lid that is impermeable to the (hydrostatic) hydrothermal convection system in the country rocks, typically producing two decoupled hydrothermal systems, a lower-temperature system at high water/rock ratios above the intrusion, and a much higher-temperature system at low water/rock ratios within the cumulate gabbro below the late-stage magma sheet. (2) Presence of a magmatic H₂O envelope. Granitic magmas typically contain much more H₂O than tholeiitic gabbro melts; hence, evolution of a magmatic gas phase will usually occur at the late stages of crystallization of granites. This envelope of magmatic water fill savailable fractures and is under lithostatic pressure. It is thus impermeable to the outer, convecting meteoric-hydrothermal fluids, which gain access to the pluton only after the magmatic H₂O has dissipated and the body has cooled. The two major Cenozoic occurrences of low-¹⁸O magmas are Iceland and the Yellowstone Plateau, Wyoming. These types of magmas are much less common than heretofore believed, and they typically seem to be developed only in extensional tectonic environments where brittle fracture of the crust allows continued replenishment of the magma reservoir from below, as well as penetration of meteoric ground waters down to great depths. Such magmas are formed by melting or assimilation, or both, of hydrothermally altered volcanic roof rocks by the underlying magma reservoir, not by influx of low-¹⁸O ground waters directly into the melt.