Metamorphic evolution of a garnet-bearing schist from the Bogegga Formation, Svalbard

The Bogegga Formation crops out on Oscar II Land in the western part of Svalbard archipelago. It is part of the Kongsvegen Group which belongs to the Southwestern Basement Province. This unit contains garnet-bearing mica schists and gneisses, pegmatites, and calc-schists which experienced up to a medium grade metamorphism (Hjelle et al., 1999). However, the petrological studies including estimation of the pressure-temperature (P-T) conditions have not been performed so far. Here we present the petrological characteristics of the highest grade garnet-bearing mica schist and the P-T estimates using a combined approach.

The studied schist consists of garnet porphyroblasts, white mica, biotite, quartz, and plagioclase. Tourmaline, epidote, allanite, zircon, and zoisite are accessory minerals. Garnet shows two distinctive compositions. Garnet-I forms cores and its composition is Alm_76-81Grs_6-9Prp_8-14Sps_2-4. It contains voluminous quartz inclusions. Garnet-II is generally calcium richer and forms rims or fills cracks within garnet-II. Its chemical composition can be characterized as Alm_71-72Grs_18-23Prp_4-7Sps_2-3. White mica is muscovite with Si content varying from 3.075 to 3.162 a.p.f.u. Biotite shows chemical zonation between the inclusions within garnet-I (X_Fe = 0.36 to 0.50) and matrix (X_Fe = 0.64 to 0.68). Plagioclase is dominated by albite endmember and its composition is Ab_77-97An_2-22Or_1-2. Rims of bigger porphyroclasts are albite rich, whereas cores are enriched in anorthitic component. Two metamorphic phases M1 and M2 were distinguished based on the petrological studies and P-T estimates. Preliminary P-T estimates suggest garnet-I growth at  4.3 – 8.5 kbar and 415 – 560 °C (M1), followed by garnet-II and matrix minerals formation at higher pressures and temperatures of 7.5 – 10.8 kbar and 590 – 675 °C (M2).

Amphibolite facies rocks that experienced similar P-T conditions are known from SW Svalbard (f.E. Müllerneset Formation, Berzeliuseggene unit, Isbjørnhamna Group, Pinkie unit). The correlations of the Boggega Formation with other amphibolite facies units cropping out along southwestern Svalbard require further studies including detailed geochronological analyses. This work was partly funded by the National Science Centre of Poland project no. 2021/43/D/ST10/02305.

References:

Hjelle A., Piepjohn K., Saalmann K., Ohta Y,. Salvigsen O., Thiedig W., Dallmann W.K. (1999). Geological Map, Svalbard 1:100 000, A7G Kongsfjorden, Norsk Polarinstitutt, Tromsø.