CONTRASTING MINERAL ASSEMBLAGE SEQUENCES IN THE AUREOLE OF THE TILTED NELSON BATHOLITH, BRITISH COLUMBIA: IMPLICATIONS FOR THE METAPELITIC PETROGENETIC GRID AND FOR PRESSURE DETERMINATION IN LOW PRESSURE METAPELITES

The Nelson Batholith is a ca. 40 x 70 sq. km Jurassic intrusion in SE British Columbia surrounded by a 0.7-2.0 km wide contact aureole developed in graphitic argillaceous rocks that show only minor variations in bulk composition. The batholith and aureole have undergone at least two episodes of post-metamorphic flexure and tilting. The result is a smooth progression of contrasting prograde mineral assemblage sequences as one goes around the aureole. These variations are ascribed to subtly different pressures of contact metamorphism, all at low pressure within the stability field of andalusite. The assemblages show the following progression going from lowest to highest pressure (all assemblages contain muscovite+biotite+quartz): (1) cordierite(Crd)-only assemblages, locally with Crd+K-feldspar(Kfs) and andalusite(And)+ Kfs assemblages at higher grade; (2) mix of Crd and Crd+And assemblages, locally with Crd+Kfs and And+Kfs assemblages at higher grade; (3) And-only assemblages, with sillimanite(Sil)+And assemblages and locally Sil+Kfs assemblages at higher grade; (4) staurolite(St)+And assemblages, with garnet(Grt)+Sil-bearing and locally Sil+Kfs assemblages at higher grade. Early-formed, Mn-rich garnet, Grt(e), was developed locally at low grade and persisted to higher grade. The above progression in mineral assemblage sequences represents a series of approximately isobaric slices through the metapelitic petrogenetic grid below the Al2SiO5 triple point, corresponding to facies series 1b, 1c, 2a and 2b of Pattison & Tracy (1991, RiM v. 26). The total pressure range represented by the contrasting mineral assemblage sequences is about 1.5 kbar, and shows that the mineral assemblages themselves represent a more sensitive method of pressure estimation in andalusite-bearing rocks than conventional or multi-equilibrium geothermobarometry. P-T and T-X(Fe-Mg) pseudosections using the thermodynamic data set of Spear, Pattison & Cheney (2000, unpubl.), reported in Pattison et al. (2002, JMG), model the mineral assemblages and their compositions fairly well and are compared with those using the Holland & Powell (1998) data set. In both cases there remain problems with staurolite (not stable to low enough pressure).