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

Abstract

The paper presents data on the transformations in the mineralogy and mineral assemblages of quartz and feldspar free magnesian orthopyroxene-clinopyroxene-biotite metaultramafic granulites con verted 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 = 0.6, X (Na,K)F = 0.3, and = 0.1. 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 petro logical 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 charnocki toids, 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 sim ple closed system partial melting induced by an increase in temperature.

Experimental study of amphibole interaction with H2O–NaCl Fluid at 900°C, 500 MPa: toward granulite facies melting and mass transfer

“…Such fluid phases should obviously be less dense and less viscous and hence more mobile than a granite melt. It is quite possible that interaction between host rocks and such fluids saturated with granitic components brought about granitization (charnockitization) of mafic (Khodor evskaya, 2005(Khodor evskaya, , 2009Korikovsky and Aranovich, 2010) and, particularly, ultramafic (Korikovsky and Aranovich, 2015) rocks.…”

“…Melting of mag nesian pargasite in the presence of pure H 2 O and fluids with various H 2 O/CO 2 ratios, and anhydrous melting of Prg, was studied in (Holloway, 1973;Holloway and Ford, 1975), whereas amphibole interaction with NaCl solutions has never been examined experimen tally, although field observations indicate that varia tions in the composition of this mineral may be induced by its interaction with aqueous salt solutions. For example, variations in the fluid composition can account for a systematic increase in the Fe mole frac tion and concentrations of alkalis in this mineral and, in certain instances, also its enrichment in halogens in zones of metasomatic granitization produced by infil trating fluids (Korikovsky and Khodorevskaya, 2006;Korikovsky and Aranovich, 2015).…”