Reactions involving S-bearing apatite, monazite and scapolite during amphibolitization of granulites, Bergen Arcs, W-Norway

Abstract

This master project focuses on the mineralogical changes and re-distribution of S between apatite, monazite and scapolite in the Lindås Nappe, Bergen Arcs, W-Norway. The assemblage developed during the Sveconorwegian granulite facies events and was reworked during Caledonian amphibolitization. Granulite facies anorthosite from the Lindås Nappe records a concordant U-Pb zircon age of 911 ± 2 Ma, reflecting growth of metamorphic zircon associated with the granulite facies event. Granulite facies mangerite yields U-Pb zircon ages at 948 ± 6 Ma and ≤ 917 ± 7 Ma, interpreted as an older, magmatic zircon component, and partial resetting and new-/re-crystallization during Sveconorwegian granulite facies metamorphic conditions, respectively. Apatite in mangerite comprises up to 6.4 modal% of the rock, contains up to 0.36 wt% SO3 and includes needles of monazite, presumably developed by dissolutionreprecipitation at the end of the granulite facies event. SO3 values in monazite reach up to 2.73 wt%. Scapolite makes up around 4 modal% of the granulites and contains up to 6.34 wt% SO3. During the Caledonian event, apatite and scapolite were fractured, deformed and hydrated, precipitating pyrite. Scapolite, apatite and monazite also produce sulphates and allanitic epidote upon breakdown. Ce2O3 (up to 0.62 wt%), Nd2O3 (up to 0.41 wt%) and other REE are leached from apatite during hydration and fluid infiltration associated with amphibolitization. The biggest change is the decrease of SO3 from 0.29 wt% to 0.15 wt% and the decrease in the amount of oriented monazite inclusions in apatite with increasing degree of amphibolitization. ID-TIMS dating of apatite with monazite inclusions resulted in Caledonian ages, with an 238U-206Pb isochron age of 433 ± 44 Ma, indicating isotopic resetting during amphibolitization of the mangerite.