THERMODYNAMIC MODELING OF METASEDIMENTARY ROCKS AND DIATEXITE, HELGELAND NAPPE COMPLEX, NORDLAND, NORWAY

Bulk rock P-T diagrams and porphyroblast composition data may be used to determine P-T paths of metamorphic and migmatitic rocks, and identify regional metamorphic gradients. The island of Ylvingen is underlain by 476 Ma granite that intrudes metasedimentary rocks, including diatexite. The pluton–host rock contact is not exposed so it is unclear if it is intrusive, therefore P-T diagrams were calculated with Theriak-Domino to determine if there is a metamorphic gradient within the host rocks. Fluid compositions were determined using binary diagrams. Samples modeled include two metapelites near the pluton–host rock contact, a metagreywacke 500 m from the contact, and a diatexite within the pluton.

The presence of kyanite, staurolite, biotite and rutile and the absence of magnesite constrain peak P-T conditions of one metapelite to 6.8-8.2 kbar and 560-620 °C and the presence of ilmenite and kyanite to sillimanite reaction textures suggest a clockwise P-T path. The presence of biotite and garnet and the absence of sillimanite, staurolite, calcite, and rutile constrain peak P-T conditions of the second metapelite to 4.5-7.5 kbar and 500-675 °C. Plagioclase composition, the presence of muscovite, dolomite, and garnet, and the absence of amphibole constrain peak temperature of metagreywacke to 550 °C at 6.5 kbar, which is lower than that of the pelites. The diatexite contains staurolite, Al-silicate, biotite, garnet, and muscovite but textures suggest that this does not represent an equilibrium assemblage. The presence of garnet, staurolite and biotite, absence of cordierite, and garnet composition also suggest a clockwise P-T path with a minimum pressure of 5.5 kbar at 400 °C, max pressure of 8.5 kbar at 610 °C, minimum solidus temperature of 660 °C at 3.9 kbar, and max temperature of 840 °C at 8.0 kbar.

A temperature gradient was modeled in the host rocks that indicates some heat for metamorphism probably came from the pluton, and suggests that the contact is intrusive. Similar assemblages between the first metapelite and the diatexite suggest that diatexite intruded from a structurally lower level or partially melted after being entrained within the pluton. These models constrain temperature to < 550 °C and pressure to 6.8-8.2 kbar prior to plutonism and suggest that regional metamorphism resulted from crustal thickening.