NJG96-3-04
2016
Orogenic degassing, scapolitization and K-metasomatism during Caledonian exhumation, Helgeland, Norway
96
3
223-244

Metasomatism associated with barren quartz veins cross-cutting hornblende schist in the upper part of the metasedimentary sequence of the W– Mo-mineralized Bjellatinden area was studied using P–T modeling of the mafic schist host and alteration assemblages, fluid inclusions, and isocon diagrams. Two main alteration assemblages are associated with the quartz veins: A biotite-scapolite-dominated zone (BSZ), and a zone dominated by chlorite, muscovite and calcite (CMZ), replacing the former BSZ and extending further into the wall rock. Three types of fluid inclusions are observed: CO2-rich (type I), low-salinity H2O-rich (type II), and high-salinity H2O-rich (type III). Type III fluids are strongly associated with the CMZ, and are not observed in the BSZ, whereas type I and II fluids are observed in quartz and scapolite in the BSZ. Type I and II fluid inclusions found in quartz in the CMZ are interpreted to be relics from the earlier BSZ alteration. Plagioclase-amphibole thermobarometry of grains in the hornblende schist indicates a near-isothermal pressure drop from ca. 660 to ca. 560 MPa at ca. 690°C. Type I and II fluids are interpreted to have infiltrated extension fractures at ca. 240 MPa and 600°C giving rise to the BSZ, and only minor mass exchange with the hornblende schist. Type III fluids are responsible for K- metasomatism of the existing alteration assemblage, and infiltrated the pre-existing fracture system over a large temperature range, from ca. 600°C to ca. 350°C during near-isobaric cooling at ca. 200 MPa. The CMZ was stabilized towards the end of this fluid infiltration. The high-salinity fluids are interpreted to have exsolved from an underlying, unexposed, granitic intrusion. This intrusion may also be the source of the W–Mo mineralizations in the area.

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