Evidence for regional metamorphism in a continental rift during the Rodinia breakup

Abstract

Continental rifts are tectonic zones where substantial material and heat can be transferred from the asthenospheric mantle into the crust. Because of their high thermal gradients, they were hypothesized as a setting for regional high-temperature (HT)/low-pressure (LP) metamorphism. However, the cited example is located in an intracontinental setting, where not only the compressional regime was prominent during collisional orogeny but also the envisaged continental rift failed to run into rupture. This led to ambiguous identification of the continental rift in intraplate regions and thus rejection of this hypothesis. In the present study, we identify for the first time typical HT/LP metamorphic rocks that show the occurrence of both andalusite and sillimanite in a Neoproterozoic continental rift zone. Andalusite O isotopes confirm the continental rift setting, and comparison with zircon O isotopes supports the metamorphic origin of aluminosilicates. Pseudosection calculations indicate that the HT/LP metamorphism occurred at 1.0–3.5 kbar and 560–660 °C. U-Pb dating of metamorphic titanite yields ages of ca. 750 Ma for the HT/LP metamorphism, consistent with the peak age of Rodinia breakup. Premetamorphic protoliths show bimodal lithochemistry and arc-like geochemical signatures, suggesting that they are rift magmatic rocks. These rocks would be produced during the development of continental rifting on a Grenvillian accretionary orogen that formed due to the Rodinia assembly. The continental rifting is coupled with stretching of the thinned orogenic lithosphere, accounting for the high heat flow from the asthenospheric mantle into the upper crust for the HT/LP metamorphism at shallow depths. Heat flow estimates from crustal Th, U and K contents are significantly lower than that from peak mineral assemblages, indicating that the anomalously high heat flow from the asthenospheric mantle did occur in the continental rift. Therefore, continental rifts are an important site for high thermal gradients and thus for regional HT/LP metamorphism.