Compositional evolution of REE- and Ti-bearing accessory minerals in metamorphic schists of Atomfjella series, Western Ny Friesland, Spitsbergen and its petrogenetic significance

Akbarpuran Haiyati S. A., Gulbin Yu. L., Sirotkin A. N., Gembitskaya I. M.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 5. P. 1-28

https://doi.org/10.31857/S0869605520050020

Language: Russian

Abstract

Accessory REE- and Ti-bearing minerals have been investigated in rocks of the northern part of the anticlinorium of West Ny Friesland, Spitsbergen Archipelago. The anticlinorium is formed by Paleoproterozoic and Early Riphean metamorphic complexes which are overlain with angular unconformity by Late Riphean – Early Paleozoic metasedimentary strata of the ancient platform cover. Representative samples of metapelites with the assemblage Ms–Bt–Grt–Qz–Pl and calcic pelitic schists with additional calcite and clinozoisite were studied. The special attention was paid to the microstructure of aggregates of accessory minerals and its petrogenetic interpretation. There were outlined several accessory assemblages corresponded to distinct “time sections” of the metamorphic history. The earlier assemblage consists of REE-bearing minerals: monazite-(Ce) as well as REE-rich clinozoisite and epidote overgrown on detrital (?) allanite-(Ce). These minerals were appeared before the garnet porphyroblast nucleation (<530–540 °C, <5.5–7.5 kbar) and in initial stages of its growth. Clinozoisite rims on allanite-(Ce) grains were formed during the same time period as evidenced by zoned crystals of clinozoisite with allanite cores which is preserved as inclusions within garnet. In the late stages of porphyroblast grow, rutile was stabilized and the assemblage of Fe–Ti oxides included, besides rutile itself, metastable ilmenite were originated due to attainment of peak temperature and pressure (670–690 °С, 10–11 kbar). During the retrograde metamorphic stage at 450–470 °C and 3–5 kbar rutile and ilmenite were replaced by titanite associated with late chlorite. To account for such facts, conditions and mechanisms of phase reactions controlling accessory mineral formation are discussed.

Keywords: allanite, clinozoisite, rutile, ilmenite, titanite, monazite, metapelites, calcic pelitic schists, Theriak/Domino, isochemical diagrams, Spitsbergen