Contact and fracture ultramafic assemblages from Norway: Rb-Sr evidence for crustal contamination

Abstract

Strontium isotopes are used as tracers of crustal contamination of alpine-type ultramafic rocks from the Basal Gneiss Complex of the Caledonides of southern Norway. Minerals from anhydrous assemblages that occur in the cores of these ultramafic lenses give Sr⁸⁷/Sr⁸⁶ ratios (0.7011 to 0.7047) that reflect the expected ambient Sr⁸⁷/Sr⁸⁶ conditions of the ancient upper mantle. Rb-Sr evidence for crustal contamination is found in hydrous assemblages that occur within fractures and around the margins of the ultramafio bodies. Olivine, enstatite, amphibole, and magnesite from these assemblages have present-day Sr⁸⁷/Sr⁸⁶ ratios (0.7049 to 0.7085) that are significantly higher than those of compositionally equivalent minerals from the interiors of the ultramafic bodies. The high Sr⁸⁷/Sr⁸⁶ values were acquired as a result of the reaction between the ultramafic rock and ion-charged hydrous solutions carrying strontium with the ambient Sr⁸⁷/Sr⁸⁶ ratio (around 0.713) of the enclosing country rook during the waning phases of the Caledonian Orogeny. Mineral separates from the interiors of these ultramafic bodies can yield useful information on the ancient upper mantle. Wholerock samples, however, will show some evidence of contamination from the crust as a result of the formation of at least trace amounts of secondary hydrous minerals. Most whole-rook Sr⁸⁷/Sr⁸⁶ ratios from alpine-type ultramafic rocks from other orogenic belts show evidence of this contamination.