DEPTH-PROFILING PETROCHRONOLOGY OF TITANITE FROM THE SOLUND BASIN, WESTERN NORWAY

Laser-Ablation Split-Stream Inductively Coupled Plasma Mass Spectrometry (LASS-ICPMS) was performed on 12 titanite samples from the eclogite-bearing gneisses within the Solund region in Western Norway, using both depth-profiling on grain separates, and in situ conventional LASS petrochronology. All titanites yield some Caledonian dates, indicating at least some modification of the titanite at, or around the time of eclogite-facies metamorphism. Mafic lenses—specifically those in proximity to eclogite—are wholly Caledonian, along with a garnet-bearing gneiss in close proximity to a large eclogite body. The textural and spatial relations of these particular samples indicate transformation of these rocks during subduction and/or exhumation; i.e., the mafic lenses were likely eclogite bodies, and the garnet gneiss is likely a retrogressed high-pressure gneiss.

Limited fluid availability may be the cause for lack of complete age resetting in the majority of the gneiss samples. This lack of resetting suggests that much of the region that was subducted to c. 2.3 GPa and 700°C remained metastable; i.e., much of the region did not convert to eclogite-facies assemblages. Nevertheless, the depth-profile age and trace-element data indicate that partial age resetting was likely caused by neocrystallization of new titanite or recrystallization of existing grains, rather than thermally mediated volume diffusion. Rims of some titanite from the mixed-age samples yield only Caledonian ages, whereby other grains from the same sample have much smaller or non-existent Caledonian rims. Furthermore, the profiles of trace-element concentrations match the age data, rather than displaying diffusion-coefficient dependent profiles.

To summarize, we have two main findings from this study. First, titanite can survive rather high metamorphic conditions without being reset by thermally mediated volume diffusion at temperatures in excess of its previously determined closure temperature. Second, given that titanite is the major Ti-bearing phase in the rock, its lack of participation in metamorphic reactions during Caledonian orogenesis suggests that much of the subducted crust also remains metastable during subduction and exhumation.