Geochronology of fluid-induced eclogite and amphibolite facies metamorphic reactions in a subduction–collision system, Bergen Arcs, Norway

Abstract

Rb-Sr multimineral isochron data for metamorphic veins allow to date separate increments of the mineral reaction history of polymetamorphic terranes. Granulite facies rocks of the Lindås nappe, Bergen Arcs, Norway, were subducted and exhumed during the Caledonian orogeny. The rocks show petrographic evidence for two distinct events of local fluid infiltration and vein formation, along fractures and shear zones. The first occurred at eclogite facies (15-21 kbar, 650-750 °C) and a later one at amphibolite facies conditions (8-10 kbar, 600°C). The presence of fluids enabled local metamorphic equilibration only near fluid pathways. In fluid-absent domains, preexisting assemblages were metastably preserved. This resulted in a heterogeneity of metamorphic signatures on m-to µm-scales. Well-preserved granulite facies rocks preserve their Proterozoic Rb-Sr mineral ages, as does the U-Pb system of zircon in most lithologies. Six Rb/Sr multimineral isochron ages for eclogite facies veins and their immediate wallrocks date the fluid-induced eclogitization at 429.9 ± 3.5 Ma (2σ, weighted average, MSWD = 0.39). An eclogite facies vein has yield metamorphic zircon with concordant U-Pb ages of 429 ± 3 Ma, identical to the U-Pb age of 427.4 ± 0.9 Ma for zircon xenocrysts in an amphibolite facies vein. Seven Rb/Sr mineral isochron ages date amphibolite-facies fluid infiltration at 414.2 ± 2.8 Ma (MSWD = 1.5), an age value testifying to residence of the rocks in the deep orogenic crust at temperatures >600°C for nearly 15 Ma. The new data show that Rb-Sr mineral isochron ages effectively date fluid-induced (re)crystallization events rather than stages of cooling. The direct link between isotopic ages and distinct petrographic equilibrium assemblages aids to constrain the evolution of rocks in the P-T-reaction-time space, which is essential for understanding exhumation histories and the internal dynamics of orogens in general.

Evidence from Rb–Sr mineral ages for multiple orogenic events in the Caledonides of Shetland, Scotland

“…(1) Growth or partial or complete recrystallization of mica (Willigers et al 2004;Glodny et al 2008;Villa 2010). Of the samples presented in this study, all micas and feldspars show evidence of dynamic recrystallization, and are therefore interpreted as recording metamorphic or deformational ages unless otherwise stated.…”

“…The Rb-Sr system in micas has been shown to be robust to temperatures greater than 600°C over orogenic timescales (>15 Ma), provided the rock is free of fluids (e.g. Glodny et al 2008). We therefore consider it unlikely that white mica ages relate to cooling through a specific closure temperature, as isotopic equilibrium is more likely to be achieved by fluid-induced recrystallization than by temperature-driven diffusional processes.…”

U–Pb geochronology of two discrete Ordovician high-pressure metamorphic events in the Seve Nappe Complex, Scandinavian Caledonides

“…1), dated at 405 Ma (Young et al 2007), and 430 Ma eclogites in the Lindås Nappe in the Middle Allochthon (Glodny et al 2008) share a broadly similar tectonic history with the WGC eclogites: all were derived from Baltica and were formed from subduction of the Baltica margin during its final collision with Laurentia. In contrast, 452 Ma eclogite formation in the Tromsø Nappe in the Uppermost Allochthon ( Fig.…”

Tracing the effects of high-pressure metasomatic fluids and seawater alteration in blueschist-facies overprinted eclogites: Implications for subduction channel processes

“…It has also been demonstrated that fluids in HP metamorphic rocks play a key role in promoting equilibration of isotopic systems used as geochronometers (Glodny et al, 2003(Glodny et al, , 2008. In turn, some isotopic systems may still record the effects of previous seafloor alteration or the isotopic signature of the precursor rock (Halama et al, 2010(Halama et al, , 2011John et al, 2011;Miller et al, 1988;Putlitz et al, 2000).…”

“…7B) also suggest Sr isotope homogenization. Homogenization of Sr isotopes by HP fluid-rock interaction was previously recognized for eclogite-and amphibolite facies fluid infiltration events and used to date HP fluid-rock interaction (Glodny et al, 2003(Glodny et al, , 2008. The existence of the FTS 4 isochron strongly points to a metasomatic fluid infiltration event that caused resetting of the isotopic system in the Tian Shan pillow metabasalts because of two reasons: First, from seawater alteration alone a wide range of initial Sr isotope compositions would be expected, as observed for seawater-altered eclogites (e.g., Miller et al, 1988;Halama et al, 2011).…”