Details

Autor(en) / Beteiligte

• Korikovsky, S. P.
• Aranovich, L. Ya

Titel

Charnockitization of feldspar-free orthopyroxene-clinopyroxene-phlogopite metaultramafite in the lapland granulite belt, southern Kola Peninsula: Compositional trends of rocks and minerals, P-T parameters, and fluid regime

Ist Teil von

• Petrology, 2015-05, Vol.23 (3), p.189-226

Ort / Verlag

Moscow: Pleiades Publishing

Erscheinungsjahr

2015

Quelle

Alma/SFX Local Collection

Beschreibungen/Notizen

• The paper presents data on the transformations in the mineralogy and mineral assemblages of quartz- and feldspar-free magnesian orthopyroxene-clinopyroxene-biotite metaultramafic granulites converted into orthopyroxene-clinopyroxene-potassic feldspar-plagioclase-quartz charnockite. Within a 4 to 6-m aureole around the fluid conduit, the granulite has been affected by Na-K-Si-H 2 O-CO 2 -Cl-F brine. Thereby the magnesian metaultramafite has undergone progressive debasification and leucocratization: potassic feldspar, oligoclase, and quartz have been metasomatically formed in the interstitial space between the mafic minerals and replaced ortho- and clinopyroxene and biotite. The Fe mole fractions X Fe of all mafic minerals increase toward the fluid conduit as follows: from 0.25 to 0.54 for Opx , from 0.15 to 0.32 for Cpx , and from 0.16 to 0.56 for Bt . The whole-rock compositions suggest that Na, K, and Si have been introduced and Mg, Fe, and Ca removed from the protolith, so that the metaultramafic rock has gradually been transformed first into a mesocratic rock and then, when partial melting started in its most significantly debasified domains, into leucocratic nebulitic migmatite with skialiths of the modified granulite, then into charnockite migma, and eventually into magma. The composition of the charnockite-forming fluid was estimated as , X (Na,K)F = 0.3, and . The unusual F-rich composition of the fluid is reflected in that both Bt and Hbl are enriched in F and contain almost no Cl. The P - T parameters of the process, which took place at the metamorphic peak, were T ∼ 780°C, P = 8.5 kbar. Material balance plots of the rocks revealed three petrological trends of the charnockite-forming process controlled solely by the composition of the brine: (a) a trend that did not produce either Bt or Hbl during the metasomatic and anatectic stages, (b) that associated with intense amphibolization and biotitization during the metasomatic stage, and (c) a trend associated with the origin of melanocratic metasomatic Opx - Cpx - Bt - Hbl ± Pl selvages around newly formed charnockitoids, with the composition of the selvages close to the melanocratic veins produced in the peripheries of the charnockitization zone by the rapid redeposition of Mg, Fe, and Ca mobilized in the course of debasification. It follows that charnockitization proceeded according to the model of nonisochemical migmatization in an open system, a process driven first of all by the inflow of deep brines. This process differed strongly from simple closed system partial melting induced by an increase in temperature.