ABSTRACT
Figure 9.1 Photomicrographs of mafic rocks in thin section, showing mineralogy and textures with increasing metamorphic grade. All are in plane polarized light and have field widths of 4 mm. A) Greenschist facies, with the assemblage chlorite – calcite – white mica – actinolite. The large, apparently euhedral actinolite crystal is a pseudomorph, having replaced an igneous augite phenocryst. Littleton, New Hampshire, USA. B) Epidote amphibolite facies, with the assemblage hornblende – epidote – chlorite – plagioclase – calcite – quartz. Kvithylla, outer Trondheimsfjord, Sør Trøndelag, Norway. C) Amphibolite facies. This relatively iron-rich rock has the assemblage hornblende – garnet – plagioclase – quartz. Quabbin Reservoir, Massachusetts, USA. D) Amphibolite
Figure 9.1 (Continued) facies, to contrast with C. This relatively magnesium-rich rock has the somewhat different assemblage hornblende – gedrite – plagioclase (no quartz). Mafic rocks can have a relatively broad range of Mg/Fe ratios, so it is important to remember that composition, in addition to the metamorphic conditions, can affect mineralogy. North Orange, Massachusetts, USA. E) Transition between the amphibolite and granulite facies, with the assemblage hornblende – augite – enstatite – plagioclase – quartz – biotite. Warren, Massachusetts, USA. F) Granulite facies, with the assemblage augite – enstatite – plagioclase – quartz – biotite. Union, Connecticut, USA. There is a general progression of increasing grain size, and less prominent foliation, with increasing grade. Larger grain size is promoted by higher temperatures, which hastens recrystallization and grain growth. Foliation becomes less pronounced because of the loss of sheet silicates such as chlorite and white micas, and eventually hornblende and some biotite. Abbreviations: A, actinolite; B, biotite; C, chlorite; E, epidote; G, gedrite; Gt, garnet; H, hornblende; P, plagioclase; PQ, plagioclase and quartz; Px, pyroxene.
