lacking the muscovite and some biotite that it would have at lower grade. The first
two field photos (Figs. 8.3, 8.4) show the effects of different amounts of deformation
on what were once probably almost identical granites. The next set (Figs. 8.5–8.7)
shows gneisses that have experienced relatively low-grade metamorphism and so
have not experienced any partial melting. The next two are transitional, having
melted to only a small degree (about 1–2%, Figs. 8.8, 8.9). The third set have expe-
rienced large amounts of partial melting (about 5–20%, Figs. 8.10–8.15). Partial
melting produces silicate liquid, essentially magma, which can flow into fractures
or other opening spaces. This separation of melt from residual solid rock is called
melt segregation. The result is a mixed rock with a metamorphic part (restite, the
residual solid) and the igneous part (the liquid, now crystallized to rock). Such rocks
are called migmatites and are important enough in high-grade terranes that they
have their own chapter (21). Many migmatites are also gneisses, so here is your first
introduction to them.
Figure 8.1 Illustration of some ways gneissic rocks can get their layering. The top row shows initial
rocks, and the bottom row shows the resulting appearance after deformation. A and C are entirely igneous, with a homogeneous rock (say, diorite, gray) hosting pegmatite dikes (A) or granite xenoliths (C). E is already a gneissic metamorphic rock (gray, say, a diorite gneiss) that has partially melted, with the granitic liquid having segregated into discontinuous layers where it crystallized. After strong deformation, it is difficult to tell what the granitic layers in the three different gneisses, B, D, and F, originally were prior to deformation.
Figure 8.2 Photomicrographs of gneissic rocks in thin section. Images on the left are in
80 A pictorial guide to metamorphic rocks in the field
Figure 8.2 (Continued) widths are 4 mm. A, B) Poorly foliated granitic gneiss, with the epidote
amphibolite facies assemblage quartz – plagioclase – microcline – biotite – epidote – titanite, plus a little retrograde muscovite. Pike, New Hampshire, USA. C, D) Poorly foliated tonalitic gneiss, with the amphibolite facies assemblage quartz – plagioclase – biotite – hornblende. Quabbin Reservoir, Massachusetts, USA. E, F) Essentially unfoliated charnockitic gneiss (orthopyroxene- bearing granitic gneiss) with the transitional granulite facies assemblage quartz – microperthite – plagioclase – hornblende – orthopyroxene. In outcrop this rock has strongly deformed and folded layers, but recrystallization and grain growth after deformation has erased most foliation at thin section scale. Notice the exsolved albite (F, light-gray, irregular spots) in darker-gray K-feldspar. Schroon Lake, Adirondacks, New York, USA. G, H) Moderately foliated aluminous gneiss, prob- ably metamorphosed shale. It has the assemblage quartz – microcline – plagioclase – garnet – kyanite – biotite. Despite its origins as a shale, the scarcity of micas makes this rock gneissic rather than schistose. Fjørtoft, Nordøyane, Møre og Romsdal, Norway. Abbreviations: B, biotite; E, epidote; G, garnet; H, hornblende; K, kyanite; Kf, K-feldspar; M, muscovite; O, orthopyroxene; P, plagioclase; Q, quartz; QF, quartz and feldspar; T, titanite.
Figure 8.3 Coarse-grained, relatively undeformed granitic gneiss, metamorphosed to epidote
amphibolite facies. Large pink K-feldspar porphyroclasts are visible (former igneous phenocrysts) in a finer-grained matrix, representing a relict igneous texture. The matrix has white plagioclase, bluish quartz, K-feldspar, and a black mixture of biotite and epidote. Uthaug, Sør Trøndelag, Norway.
Figure 8.4 A more severely deformed gneiss that was originally probably like that in Figure 8.3.
The relict igneous phenocrysts (porphyroclasts in 8.3) are now augen: eye-shaped K-feldspar porphyroclasts that have been recrystallized on their margins and smeared out into the foliation plane (see Chapter 17). Two augen are indicated by red arrows, but many more of pink K-feldspar and white plagioclase are visible. Hejnskjel Island, Hagaskjera, Sør Trøndelag, Norway.
Figure 8.5 Strongly deformed, layered tonalitic gneiss. The layering represents smeared out
inhomogeneities that include dikes and xenoliths, which are recognizable in less deformed parts of this same outcrop. This is an example of deformational effects seen in Figures 8.1A-B and C-D. Baksteinen Peninsula, Sør Trøndelag, Norway.
Figure 8.6 Strongly deformed, layered tonalitic gneisses, with thick white and gray layers. The white
layers are poor in biotite and have very little internal layering. The thick gray layers are themselves made up of alternating, thin medium-gray and white layers. Many of the thin white layers are made entirely of quartz, and so are deformed quartz veins. Other white layers seem to be tonalitic, like the thick white layers, and so may be interlayered volcanics or strongly deformed dikes. Some of the thin tonalitic white layers are slightly crosscutting (one example indicated by a red arrow) and so are apparently deformed dikes (an example of the deformational effects illustrated in Fig. 8.1A-B). From this outcrop the geologic sequence seems to have been: 1) formation of the gray rock, 2) emplace- ment of the quartz veins, 3) emplacement of the white tonalite as dikes, and 4) strong deformation. Råkvåg, Sør Trøndelag, Norway.
Figure 8.7 Coarse-grained tonalitic gneiss that has thin, discontinuous felsic layers (yellow arrows) and
several small mafic enclaves, the largest of which is pointed out with a red arrow. The thin felsic layers may be deformed pegmatite dikelets, and the mafic enclaves may be deformed xenoliths. The cross- cutting pinkish body (black arrow) is fine-grained and granular, and looks on close examination like a deformed aplite dike. This is an example of the deformational effects seen in Figures 8.1A-B and C-D, but deformation was not severe enough to turn the enclaves into outcrop-spanning thin layers, or to make the pink dike nearly parallel to the other elongate features. Kjørsvik, Møre og Romsdal, Norway.
Figure 8.8 Charnockitic gneiss, cut by a basaltic dike (outlined in red). Charnockites are granites that
84 A pictorial guide to metamorphic rocks in the field
Figure 8.8 (Continued) unweathered, broken surfaces are gray. Orthopyroxene on exposed surfaces
weathers quickly, releasing Fe2+. The iron is oxidized and precipitated as the brown limonite staining
seen on many of the flat joint surfaces. The gray to brown coloration of charnockitic gneiss outcrops helps differentiate them from other granitoid gneisses that are more typically white, light-gray, or pink in this part of the world. This rock has undergone partial melting, but the amount of melting was small and the melt segregations are almost the same color as the host rock, and so hard to see at this scale. Whitehall, eastern Adirondacks, New York, USA.
Figure 8.9 Granitic gneiss with small augen of pink K-feldspar and white plagioclase. There is a fine-
grained layer under the toe of the boot, possibly a deformed aplite dike, and three light- colored, possibly pegmatitic layers or segregated local partial melts. All have been deformed so that the layers are parallel to themselves and to the rock foliation, though they are unlikely to have been parallel originally. Vingan, Sør Trøndelag, Norway.
Figure 8.10 Migmatitic granitic gneiss, with somewhat diffuse, patchy leucosomes (light-colored material,
Figure 8.10 (Continued) Deformation tends to turn inhomogeneities like these into parallel layers,
so this outcrop is intermediate between the undeformed and deformed melt segregations shown in Figures 8.1E and F, respectively. Warrensburg, Adirondack Mountains, New York, USA.
Figure 8.11 Complexly deformed gneiss containing boudins of eclogite (red arrows), amphibolite
(black arrow), and lots of white material that is mostly highly deformed, crystallized segregations of partial melt, akin to Figure 8.1F. Ræstad, Otrøy, Møre og Romsdal, Norway.
86 A pictorial guide to metamorphic rocks in the field
Figure 8.12 Gray granitic gneiss with vertical layering that contains many black amphibolite
boudins. Some of the amphibolites are folded (red arrows) with steep axial surfaces approximately parallel to the layering. The gneiss and amphibolites were intruded by granitic liquid, forming the white rock (yellow arrows) along sub-vertical channelways. Intrusion of the white rock disrupted some of the amphibolites even more than they had been previously. Putnam, eastern Adirondacks, New York, USA.
Figure 8.13 Migmatitic, garnet-rich granitic gneiss. The white partial melt segregations have clearly
been folded (numerous fold hinges visible, two highlighted with red lines), indicating that melting pre- ceded at least the last episode of folding. This is an example of the deformational effects illustrated in Fig. 8.1E-F. Juvika, Gossa Island, Møre og Romsdal, Norway.
Figure 8.14 Aluminous gneiss containing abundant garnet and small amounts of sillimanite and
biotite, but no muscovite. This rock may have been derived from a compositionally layered shale or feldspathic siltstone, but here, metamorphosed to upper amphibolite facies conditions, all of the muscovite has dehydrated. The rock is migmatitic, having partially melted, and the numerous thin white layers are strongly deformed partial melt segregations like those shown in Figures 8.1E and F. About two-thirds of the way up the rock face the garnet-rich gneiss is cut by a white and pink granitic dike (red arrow), which has been deformed to be almost parallel to layering. Ticonderoga, eastern Adirondacks, New York, USA.
88 A pictorial guide to metamorphic rocks in the field
Figure 8.15 Complexly folded, migmatitic granitic gneiss. Visible are white and pink layers, some
of which are apparently partial melts derived from the gneiss, and some are younger, crosscutting pegmatites presumably from outside of this outcrop (the largest, crosscutting pegmatitic body is indicated by a red arrow). There was likely more than one generation of partial melting that affected this outcrop. To the upper right are two amphibolite boudins (A). Unraveling the lithologic age and deformational and metamorphic history of outcrops like this takes careful observation in the field, thoughtful selection of samples, and a lot of lab work. It is from such evidence however, that the history of mountain belts is divined. Dryna, Midsund, Møre of Romsdal, Norway.