The geometry and high-temperature brittle deformation of the Skaergaard Intrusion

Abstract

The Skaergaard magma chamber formed approximately 55 m.y. ago along the embryonic rift between North America and Europe as tholeiitic basalt magma flowed upward along fractures in basement gneiss and then infiltrated the stratigraphic unconformity at the base of a 7- to 9-km-thick section of continental basalts. The magma deflected and faulted the overburden as it formed a 4.5-km-thick, 189 km3, laccolithlike chamber with elliptical form (a = 6 km, b = 4 km) in map view. As the chamber grew, its feeder pipes were eroded into conical depressions; blocks of gneiss were rafted to the chamber top, and some blocks were fused and entrained in the main magma as "immiscible" fluids. Crystallization and cooling produced at least four distinct fracture events: (1) At 1050°–1000°C, residual magma accumulated in fractures in the Layered Series, forming gabbro pegmatites, (2) at 1050°–700°C, near-vertical fractures were formed, providing channels for the main pulse of meteoric-hydrothermal activity; these fractures developed near the margin of the magma chamber, then expanded outward into the permeable basalts and inward, following the gabbro-magma interface. Ground waters derived from joints in the surrounding basalts flowed into the gabbro, were heated, lowered the 18O/16O ratio of the intrusion, and filled its fractures with hornblende, clinopyroxene, biotite, and magnetite-ilmenite, (3) at 800°–750°C, volatile-rich granophyric melts derived from sloped blocks of basement gneiss expanded and crystallized as both sill-like and dike-like bodies in the gabbro, and (4) below 700°C, fractures continued to form and hydrothermal activity continued to cool the intrusion. The relative age, abundance, continuity, and mineralogy of the veins are consistent with parameters used in previous studies of this intrusion that predict the occurrence of a high-temperature hydrothermal system and a time- and volume-averaged permeability of 10−13 cm2. Our new data indicate that the permeability of the layered gabbro decreased with time because the flow channels were sealed by high-temperature mineral deposition. We thus conclude the following: (1) layered gabbros fracture in response to local stress at conditions just below their solidus temperature if the confining pressures are typical of the upper crust. This observation contravenes the conceptual viewpoint that the style of deformation at such elevated temperatures is only by plastic flow, and (2) because an extensive fracture network develops at these near-solidus temperatures in layered gabbros, the bulk of the hydrothermal alteration of such bodies takes place at extremely high temperatures. This helps clarify the apparent paradox that extreme 18O depletions are found in "fresh" layered gabbros.

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