Crystallization sequence and tectonic significance of andalusite, kyanite, and sillimanite ‘triple point’ localities, including a new locality: Lesjaverk, Norway.

Abstract

Rocks containing three Al2SiO5 polymorphs (andalusite, kyanite, sillimanite) are uncommon; only ten localities have previously been reported. By determining the crystallization sequence of the polymorphs, tectonic/metamorphic histories can be unlocked. Two crystallization sequences have been proposed: (1) kyanite -> sillimanite -> andalusite (Idaho, New Mexico, Spain, Italy), and (2) andalusite -> kyanite -> sillimanite (Colombia, Turkey, Iran, Russia, South Korea, and Japan). The newest locality is Lesjaverk, Norway. Sequence (1) suggests continental collision in which exhumation of moderate-P/T (Barrovian) rocks was followed by high-P / low-T conditions, whereas Sequence (2) suggests contact metamorphism preceded more typical Barrovian metamorphism. In the Lesjaverk, Norway, rock that contains andalusite, kyanite, and sillimanite, observation of the Al2SiO5 polymorphs in thin section indicate crystallization Sequence (2). Temperatures were calculated using Zr-in-rutile thermometry; results of 580-650°C indicate that rutile likely equilibrated in the kyanite or sillimanite stability field. Analysis of trace elements in rutile further indicate that Zr, Nb, Cr, and Fe participate in elemental substitution in rutile.