STABLE ISOTOPE CHEMISTRY AND GEOCHRONOLOGY OF SCAPOLITE-BEARING PEGMATITES IN NORDØYANE, WESTERN GNEISS REGION, NORWAY: A MONITOR FOR THE ROLE OF FLUIDS IN (U)HP PARTIAL MELTING

Abstract

Scapolite ((Ca, Na)4Al3-6Si6-9O24(SO4, CO3, Cl)), a volatile-bearing framework silicate, is found in pegmatites and leucosomes in close field association with (ultra)-high pressure ((U)HP) eclogites in the Nordøyane domain of the Western Gneiss Region (WGR) of Norway. Study of the field relationships, stable isotope geochemistry, and geochronology of these scapolite-bearing rocks provides insights into the role of fluids in partial melting of (U)HP rocks during the Scandian Orogeny (425-380 Ma). Scapolite-bearing pegmatites in Nordøyane were divided into three types based on mineralogy, texture, and scapolite chemistry. The most abundant Type 1 pegmatites are associated with granodioritic rocks which cut amphibolite-facies fabrics in adjacent basement gneisses and host numerous eclogite bodies. The pegmatites, which are concentrated in low-strain areas, are interpreted as a late-stage differentiate of the granodiorites. Scapolites from Type 1, 2 and 3 pegmatites, and basement leucosomes, all share similar δ13C (-7.97 to -2.24 ‰) and δ34S (-1.1 to 5.7‰) compositions, interpreted to indicate a common source of volatiles, derived from the mantle. Type 1 pegmatites crystallized at ~ 395 Ma under amphibolite-facies conditions (ca. 750 °C, assuming P = 1.0 GPa), at the same time as Type 3 pegmatites (396±2 Ma; Gordon et al., 2013). An older ~ 420 Ma subpopulation of titanites is interpreted as inherited, likely from adjacent basement gneisses. Scapolite δ18O (16.21 to 20.05‰) and titanite εNd (-6.8 to -10.8) values indicate that Type 1 pegmatites crystallized from a crustal melt, suggesting that the introduction of mantle fluids promoted melting of crustal rocks. Mantle fluids are interpreted to have infiltrated subducted Baltican continental crust during subduction (ca.415-400 Ma), or in the early stages of extension and exhumation (ca. 399-395 Ma). These fluids may have been present in Laurentian lithospheric mantle prior to the Caledonian orogeny, or introduced from the down-going slab during early Caledonian subduction.