Pressure-temperature-time evolution of a blueschist and an eclogite from the Vestgötabreen Complex, Svalbard

The pressure-temperature-time (P-T-t) history of a blueschist and an eclogite from the high pressure-low temperature Vestgötabreen Complex, Svalbard, has been constrained using the conventional geothermobarometry, trace elements thermometry, and elastobarometry coupled with Lu-Hf garnet, U-Pb monazite, and U-Pb zircon dating. Three evolutionary stages for the eclogite have been distinguished thanks to the different textural positions and zoning of major minerals. The prograde growth (M1) happened at 15.9 kbar and 460°C, then the peak-P conditions (M2) 23.5 kbar at 507°C, followed by peak-T conditions (M3) of 21.4 kbar at 553°C. Only peak conditions of ca. 18 kbar at 520-550°C have been estimated for the blueschist. These P-T results indicate a low geothermal gradient of 7-8°C, as suggested by Agard et al. (2005). Secondary ion mass spectrometry (SIMS) analyses of zircon rims from the eclogite yielded the lower intercept of concordia at 478±17 Ma (n=11, MSWD=1.1), which is interpreted as a prograde growth. Monazite from the matrix and inclusions in garnet rim give a ²⁰⁶Pb/²³⁸U weighted mean age of 471±6 Ma (n=7, MSWD=1.4). Monazite could have formed due to florencite and/or lawsonite breakdown somewhere between M2 and M3 stages. Garnet in the eclogite is strongly zoned and Lu is concentrated mostly in the rims. Lu-Hf dating yields the age of ca. 471 Ma for the biggest fraction and ca. 466 Ma for smaller garnet separates. Monazite from the blueschist gives a ²⁰⁶Pb/²³⁸U weighted mean age of 486±6 Ma (n=4, MSWD=0.32) interpreted as a prograde growth. Lu-Hf dating of garnet from the blueschist provides an age of a peak metamorphism of 471.1±3.8 Ma (n=10, MSWD=2.8). in our opinion, the Vestgötabreen Complex represents the earliest Paleozoic subduction system, which could have developed proximally to the Baltican margin.

This work is supported by the National Science Centre of Poland project no. 2021/43/D/ST10/02305.

References:

Agard P, Labrousse L, Elvevold S, Lepvrier C (2005). Discovery of Palaeozoic Fe–Mg carpholite (Motalafjella, Svalbard Caledonides): a milestone for subduction zone gradients. Geology 33: 761–764.