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Published November 10, 1984 | public
Journal Article

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


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.

Additional Information

This study has enjoyed the contributions of many colleagues and friends whom we wish to thank. The expeditions to the outcrop would not have been possible without the support, advice, and physical effort of the Greenlanders, C. K. Brooks, T. F. D. Nielsen, A. R. McBirney, numerous mariners and their ships, G. Sigurdsson, and the Ragnarsson family. We are also grateful to D. Montgomery, J. Coleman, J. Woodward, A. Troutner, E. Creigh, R. Rogers, J. Wellington-Miller, and C. Manning for their patience and cooperation in the preparation of illustrations, maps, and manuscript. For the opportunity to study this system with modem graphics techniques, we thank Steve Sorenson for his robust, user-friendly software and patience with geologic thinking. Various aspects of this study were supported by the National Science Foundation (NSF grants EAR-78-16874, EAR-7919765, EAR-8007828, and EAR-8215120), the Department of Energy's Office of Basic Science, and the California Institute of Technology. The scientific legacy established by L. R. Wager and then expanded by G. M. Brown, C. K. Brooks, A. R. McBirney, T. N. Irvine, H. R. Naslund, H. R. Blank, and M. E. Gettings provided an unusually high-quality vantage point from which to conduct our observations. We also wish to acknowledge the discussions and reviews of this work by T. N. Irvine, A. R. McBimey, J. Johnson, C. Manning, J. Ganguly, H. C. Helgeson, R. 8. Knapp, R. Capuano, S. R. Titley, J. R. Delaney, and R. Gregory.

Additional details

August 22, 2023
October 24, 2023