Metamorphic Facies: Reminder of principle metamorphic changes: 1/ Recrystallization changes in grain size responding to T & P changes. Coarsening of grains is common e.g. Quartzite. 2/ Neomineralization: Growth of new minerals is common. 3/ Development of oriented fabric A pervasive planar fabric defined by parallel structural planes & lineation of minerals. 4/ Metasomatism affected by hot fluids if the bulk chemical composition is metasomatized. NOTE: Hot inter-granular fluids (commonly H2O & CO2 ) speed up metamorphic reactions and fluids are heated by geothermal gradient or igneous intrusion. Porphyroblasts - Commonly one metamorphic mineral grows much larger than other constituent minerals porphyroblasts which may grow over an extended period or at late-stage of a metamorphic event. In a foliated rock, foliation may wrap around a porphyroblast and slightly coarser grains may develop in ‘pressure shadows’ on either side of porphyroblast. porphyroblast pressure shadow The evidence of shear stress and resulting rotation of porphyroblasts is often preserved as subparallel S-shaped inclusions within them. The line of inclusions appear continuous with the surrounding groundmass foliation. When porphyroblast growth foliation occurs at a late stage of metamorphism then the porphyroblasts simply overprints the general fabric. Porphyroblasts may grow over long period of time and record a history of temp & pressure change therefore the study of porphyroblasts is of importance to metamorphic petrologists. Shear stress & accompanying brittle to ductile deformation & cataclastic textures. mylonites In a fault zone environment, a layered rock consisting of bands of hard, brittle rocks in a matrix of softer, clay-rich layers will develop ‘lenses’ of brittle mineral boudins (from French word meaning ‘sausage’) Augen structure Boudins Cataclasite lava block with stretched phenocrysts from shear zone in Chaos Crags dome Normal lava block from Chaos Crags, N. California Metamorphic Facies: A metamorphic petrologist can decipher the times at which a metamorphic rocks of a region were subjected to different P-T conditions. In other words, the evolutionary history of such a region in terms of pressure-temp-time (P-T-t). 1893 George Barrow carried out field-based study in Scotland using mineralogical changes as a function of metamorphic intensity in mudrock protolith (or pelite in metamorphic petrology). Barrow showed that distinct zones or boundaries are marked by appearance/disappearance of a specific mineral (or index mineral) and this can be mapped at outcrop scale, not just in Highlands but globally. These zones are: 1. Chlorite zone: Chlorite + Muscovite + Quartz + Albite 2. Biotite zone: Biotite + Chlorite + Muscovite + Albite + Quartz 3. Garnet zone: Garnet + Quartz + Biotite + Muscovite + Albite 4. Staurolite zone: Staurolite + Garnet Quartz + Muscovite + Biotite + Plag 5. Kyanite zone: Kyanite + Garnet + Muscovite + Biotite + Quartz + Plag + K-feldspar Barrow’s interpretation that the metamorphic grade/intensity increased from Chlorite to the Sillimanite zones was based on observation that grain size also increased. These zones Barrovian Zones and are recognized as representative of intermediate P-T metamorphism. Tilley extended this study and introduced the concept of an isograd which is a contour on a geological map that marks the first appearance & disappearance of an index mineral. Buchan zones Barrovian zones Subduction zones Garnet & Omphacite pyroxene Glaucophane amphibole Orogenic belts Mid-ocean ridges Note: White lines are isograds Metamorphic facies + tectonic associations. Later studies found different types of metamorphic zonation in rocks of pelitic composition worldwide. Close to Barrows study area, in the Buchan area of eastern Dalradians, a very different sequence of metamorphic zones occurs in a pelitic protolith. Here, the index mineral sequence is: Buchan sequence: Staurolite - Cordierite - Andalusite - Sillimanite Bulk composition (inc. fluid composition) has an important control on type of mineral reactions in a protolith thus affecting the mineral-based isograds. Bulk composition also dictates what minerals may form at specific P-T & fluid composition. The list below highlights the different rocks forming under similar P-T conditions: Sandstone Limestone Basalt Granite Shale Peridotite - quartzite marble amphibolite garnet-gneiss sillimanite gneiss olivine-tremolite schist Metamorphic Facies: Despite a wide variation in bulk composition of protoliths, these rocks develop: 1/ Metamorphic assemblages with simple mineralogies where each rock has 4 or 5 of following minerals: quartz, K-feldspar, plagioclase, cordierite, wollastonite, diopside, hypersthene and garnet. 2/ For a particular bulk composition, the mineral assemblage is the same. IMPORTANT: A metamorphic facies is not a single rock-type but a wide range of minerals that form under similar P-T and fluid composition conditions. A general facies diagram was developed and names of each facies are based on those mineral assemblages that develop when a mafic bulk composition undergoes various P-T conditions. These facies are: Zeolite facies - zeolites Prehnite-Pumpellyite - Prehnite + pumpellyite Blueschist facies - glaucophane +lawsonite or epidote (+ albite + chlorite) Greenschist Facies - hlorite + albite + epidote + actinolite Epidote-Amphibolite facies - plagioclase + hornblende + +/- garnet Amphibolite facies - plagioclase + hornblende + garnet Granulite facies -orthopyroxene + clinopyoxene + plag + hornblende + garnet Eclogite facies - omphacitic pyroxene + garnet Boundaries between 2 facies is gradational. IMPORTANT: Although rocks undergo metamorphism over increasing (or prograde) as well as decreasing (retrograde during exhumation process) set of P-T conditions, the assignment of a rock to a facies is always based on the peak metamorphic conditions it reached. The retrograde conditions are generally incapable of obliterating the peak P-T conditions reached. Metamorphic Facies Series & Plate Tectonics Miyashiro noted the consistent differences between Barrovian & Buchan-type sequences in his study of Japan metamorphic belts. He noted 3 sequences, mainly formed due to a variation in pressure. 1/ Zeolite - prehnite-pumpellyite-blueschist-eclogite (HIGH P-T) 2/ Greenschist-epidote-amphibolite-amphibolite-granulite-(INTERMED. P-T) 3/ Greenschist-amphibolite-granulite (LOW P-T) Even prior to the concept of plate tectonics, Miyashiro recognized sub-parallel belts of high P-T adjacent to low P-T metamorphic rocks parallel to the Trench & called them paired metamorphic belts. Paired metamorphic belts of Japan The low P-T belt is composed of andalusite-sillimanite facies assemblages, occuring to the NW of a major tectonic discontinuity and the high P-T belt occuring to SE of it. High P-T belt consists of zeolite facies to blueschist / greenschist facies and some amphibolite rocks. Miyashiro also noted paired metamorphic belts around the entire Pacific Rim. In Japan, the high P-T belt mirrors the location of the subduction zone where the subducting plate moves to the NW. The low P-T belt is an ancient island arc that has been thrust against the high P-T belt. Thrusting is common in subduction zones.