KFMASH System Research Articles

A petrogenetic grid for pelitic schists in the system SiO2-Al2O3-FeO-MgO-K2O-H2O

A quantitative petrogenetic grid for pelitic schists in the system KFMASH that includes the phases garnet, chlorite, biotite, chloritoid, cordierite, staurolite, talc, kyanite, andalusite, sillimanite, and pyrophyllite (with quartz, H2O and muscovite or K-feldspar in excess) is presented. The grid is based on thermodynamic data of Berman et al. (1985) and Berman (1988) for endmember KFASH and KMASH equilibria and natural Fe-Mg partitioning for the KFMASH system. Calculation of P-T slopes and the change in Fe/(Fe+Mg) along reactions in the KFMASH system were made using the Gibbs method. In addition, the effect on the grid of MnO and CaO is evaluated quantitatively. The resulting grid is consistent with typical Buchan and Barrovian parageneses at medium to high grades. At low grades, the grid predicts an extensive stability field for the paragenesis chloritoid+biotite which arises because of the unusual facing of the reaction chloritoid+biotite + quartz+H2O = garnet+chlorite+muscovite, which proceeds to the right with increasing T in the KFMASH system. However, the reaction proceeds to the left with increasing T in the MnKFASH system so the assemblage chloritoid + biotite is restricted to bulk compositions with high Fe/(Fe+Mg+Mn). Typical metapelites will therefore contain garnet+chlorite at low grades rather than chloritoid + biotite.

Chemographic relationships of biotite and cordierite in the McGerrigle thermal aureole, Gaspé, Quebec

Abstract Biotite and cordierite occur in a 1‐km wide zone of pelitic hornfelses around the McGerrigle pluton. These phases display systematic changes in XFe that can be attributed to continuous reactions involving chlorite or andalusite in the system KFMASH. Through much of the zone biotite and cordierite were products of the ‘breakdown’of chlorite. Close to the pluton this continuous reaction was terminated by a discontinuous reaction that introduced andalusite. Pelites which interdigitate with apophyses of the intrusive at the pluton margin contain assemblages that record a continuous reaction between biotite, cordierite, andalusite, muscovite, and quartz or, alternatively, the discontinuous breakdown of muscovite and quartz to K‐feldspar and andalusite.The mole fraction of Fe in biotite and cordierite increased significantly with the progress of the first continuous reaction and apparently decreased during the second continuous reaction. The KD of Fe‐Mg between the minerals decreased and apparently increased, respectively, during the two reactions.Biotite‐cordierite‐chlorite assemblages are interpreted to have been stable at temperatures between 525° C and 615° C and biotite‐cordierite‐andalusite assemblages stable at temperatures between 615° C and 635° C. The confining pressure was estimated to have been < 2 kbar.The results of this study suggest that the KD of Fe‐Mg between biotite and cordierite is a function of temperature, the Fe‐Mg exchange characteristics of the controlling continuous reaction and non‐ideal mixing of Fe and Mg.

Alpine metamorphism of pelitic schists in the Nufenen Pass area, Lepontine Alps

Abstract The effects of Tertiary Alpine metamorphism on pelitic Mesozoic cover rocks have been studied along a cross‐section in the central Lepontine Alps in the Nufenen Pass area, Switzerland.Greenschist facies to amphibolite facies conditions are indicated by the formation of the index minerals chloritoid, garnet, staurolite and kyanite in pelitic rocks. Regional metamorphism reached maximum conditions during the interkinematic period between a main Alpine penetrative (D2) and a late Alpine (D3) crenulation type deformation phase or synchronous with the late Alpine deformation. Based on AFM phase relationships four different metamorphic zones can be distinguished: (1) chloritoid zone; (2) staurolite + chlorite zone; (3) staurolite + biotite zone; and, (4) kyanite zone.The isograds that separate these zones can be modelled by univariant reactions in the KFMASH system. The conditions of metamorphism calculated from geological ther‐mobarometers for the maximum post‐D2 por‐phyroblast stage are from North to South: 500° C at 5‐6 kbar and 600° C at 7‐8 kbar.Detailed thermobarometry of garnet por‐phyroblasts with complex textures suggests that maximum temperature was reached later than maximum pressure. Early garnet growth occurred along a prograde P‐T‐path, post‐D2 rims grew with increasing temperature but decreasing pressure, and finally post‐D3 garnet formed along a retrograde P‐T‐path.It may be concluded from the calculated pressure and temperature difference over a short distance (3 km) across the mapped area that the isogradic surfaces of the post‐D2 metamorphism are steeply oriented. The data also suggest that isobaric and isothermal surfaces are parallel.Much of the observed metamorphic pattern can be explained as the result of a significant post‐D2 differential uplift of the hot Pennine area relative to the Helvetic area along a tectonic contact zone. The closely spaced isograds (isotherms) in the North may then be interpreted as a thermal effect owing to the emplacement of the hot Pennine rocks against the Got‐thard massif with its cover. Whereas, in the Pennine metasediments, post‐D2 porphyroblast formation can be related to the decompression path which was steep enough for dehydration reactions to proceed. It is also remarkable that late kyanite porphyroblasts probably formed with decreasing pressure.The interpretation given here for the Nufenen Pass area may also apply to the Luk‐manier Pass area where similar metamorphic patterns have been reported by Fox (1975). The formation of the ‘Northern Steep Belt’;, as denned by Milnes (1974b), and the associated late Alpine fold zones may, therefore, have significantly modified the metamorphic pattern of the Helvetic‐Penninic contact zone.

Pelite facies series and the temperatures and pressures of Dalradian metamorphism in E Scotland

Summary An attempt is made to synthesise data on the regional metamorphism of pelites in the eastern Scottish Dalradian. Zonal sequences (facies series) of mineral assemblages are presented and variants of the traditionally recognised Barrovian and Buchan metamorphism are separated. A Schreinemakers net and petrogenetic grid for the system KFMASH is presented and fixed in P-T space using experimental data. The P-T data for the various facies series are used in conjunction with other constraints to develop a model of isotherm and isobar distribution in the region, and the effects of post metamorphic folding of the isopleths are noted. P-T gradients normal to isobars suggest overall ‘geothermal’ gradients for most of the area to be convex to the T-axis, and it is suggested that they result from magma intrusion at depth. The region adjacent to the Highland Boundary fault shows strong horizontal and vertical temperature gradients which are tentatively interpreted to indicate the presence of a synmetamorphic tectonic boundary to the Dalradian metamorphic belt.