Phase equilibria of margarite-bearing schists and chloritoid + hornblende rocks from western new Hampshire, USA

A variety of uncommon garnet-grade assemblages have been found in rocks from three outcrops in the western part of central New Hampshire, and include the associations Grt+Mrg±Cld, Grt+Bt+Cld±Mrg, and Mrg+Cld+Hbl±Grt (all rocks contain Ms, Chl, Ilm, and Qtz). These unusual rocks coexist with more typical Grt+Bt+Chl+Pl metapelites and amphibolites. Rim P-T conditions are ̃490±35°C and 5{dot operator}75±1{dot operator}25 kbar.Projection of the assemblages from Qtz, H₂O, and Ilm into the Ca-Al'-Na-(Fe+Mg) tetrahedron, and from Qtz, Ilm, H₂O, and Chl into the Ca-Al'-Fe'-Mn tetrahedron indicates that Ca/(Ca+Na) and Mn differ among the assemblages in a systematic fashion. Common Grt+Bt+Chl+Pl assemblages are restricted to relatively high Mn and low Ca/(Ca+Na) values, whereas Cld+Bt+Mrg and Cld+Hbl+Mrg assemblages are stable at low Mn and high Ca/(Ca+Na). These data suggest that at this grade Cld+Bt is more stable than Grt+Chl in the KFMASH system, whereas in the Ca-KFMASH system, Hbl+Cld assemblages are stable.Composition space analysis using the singular value decomposition method indicates that compositions of minerals from individual samples are consistent with local equilibrium, but that different outcrops may not have all equilibrated at the same P-T-a_H2O conditions. Thermodynamic analysis suggests that a garnet-zone prograde sequence of ferromagnesian associations for these bulk compositions would be Hbl+Cld+Grt+Chl←Bt+Cld+Grt+Chl←Bt+Grt+Chl.Staurolite-grade rocks from the same stratigraphic units are exposed across strike, and contain the assemblage Grt+St±Bt±Pl (all rocks contain Ms, Qtz, Chl, and Ilm). Margarite is commonly present as inclusions in the cores of garnets, but is absent as inclusions near garnet rims and from the matrix; conversely, staurolite inclusions are present towards the rims of the garnets, but are absent from the cores. These inclusion relations suggest that margarite may react to form staurolite and garnet with increasing grade via a reaction such as. chlorite+margarite=staurolite+garnet+H₂O.Biotite is common in the matrix but is not typically abundant, and appears to have been the last phase to join the assemblage. Biotite is inferred to have joined the Grt+St+Chl assemblages after margarite breakdown through the reaction. Grt+Chl+Ms=St+Bt+H₂O.Thus, uncommon margarite assemblages may evolve into common Grt+Bt+St+Chl assemblages.