(blueschist, prehnite–pumpellyite, and locally eclogite and
zeolite facies), (2) intermediate-P facies series (greenschist,
amphibolite, and, locally, granulite facies) of Barrovian zones
generally coincident with the magmatic arc, and (3) low-P
rocks of the same facies associated with biotite-cordierite-
andalusite Buchan zones and syntectonic granitoids.
Metamorphic field gradients The locus of collective peak
P–T conditions (Figure 14.36) recorded in equilibrated min-
eral assemblages in different parts of a regional metamorphic
terrane or contact metamorphic aureole.
Metamorphic fluid Volatile phase liberated by metamorphic
mineral reactions.
Metamorphic grade Generally refers to the mineralogical
character of rocks linked to their T of metamorphism,
whether low, intermediate, or high. Although rock fabric may
change with respect to grade, the most diagnostic indicator of
grade is the mineral assemblage; in rocks of similar chemical
composition, ones metamorphosed at higher grade (higher
T) contain fewer volatile-bearing minerals or minerals that
contain less volatiles. In a terrane of increasing grade, suc-
cessive metamorphic zones can be delineated by isograds of
constant grade.
Metamorphic mineral assemblage Two or more minerals
equilibrated under a restricted range of intensive variables in
a particular rock system. Because of sluggish kinetics of solid-
state mineral reactions during changing metamorphic condi-
tions, the mineralogical composition of a metamorphic rock
does not necessarily constitute its stable mineral assemblage.
Metamorphic sole The dynamothermally metamorphosed
rocks developed beneath a large thrust slab of hot ophiolite
obducted onland. The sole has an inverted metamorphic
zonation.
Metamorphic zones Systematic spatial variation in some
distinctive fabric attribute or mineral assemblage within wide
contact aureoles and regionally metamorphosed terranes; re-
sult from variation in metamorphic conditions. Mineralogical
zones result from a gradient in intensive variables and are
delineated by isograd lines marked by the first appearance
(or disappearance) of a particular index mineral, or mineral
assemblage, in rocks of the same bulk chemical composition.
Barrovian metamorphic zones develop during regional meta-
morphism at intermediate pressures; delineating isograds are
marked by the index minerals biotite, garnet, staurolite,
kyanite, and sillimanite in pelitic rocks. Buchan metamorphic
zones develop in regional terranes at relatively low P and are
delineated by phase assemblages that contain the critical min-
erals andalusite and cordierite.
Metamorphism Subsolidus equilibrating adjustments in
composition and fabric of a rock that result from significant
changes in the state of the rock system, its defining P, T,
chemical composition, and state of stress; adjustments usually
take place in the presence of a volatile fluid and there may be
interacting deformation. During metamorphism, rocks are
hot enough to recrystallize but not hot enough to experience
widespread melting.
Metasomatism A type of open-system metamorphism in
which the nonvolatile chemical composition of a rock is
modified as a result of percolation of liquid through the
permeable rock body. Modification can be in the types and
amounts of minerals in modal metasomatism, or inconspicu-
ous changes in composition of original solid-solution minerals
in cryptic metasomatism, or a combination of these.
Metastable See Equilibrium.
Mg number Atomic 100 Mg/(Mg Fe) where Fe is total
iron.
Miarolitic In granitic rocks, texture defined by irregularly
shaped cavities (vugs) into which euhedral vapor-phase crys-
tals have grown; cavities are generally many grain diameters
apart.
Microcrystalline Texture that consists of a mosaic of crys-
tals that are visible only under a microscope.
Micrographic See Graphic.
Microlites Microscopic crystals that display optical bire-
fringence in polarized light; larger than crystallites.
Microlithon See Slaty cleavage.
Microstructure Alternate term for texture.
Migmatite Composite heterogeneous rock on the outcrop
scale that consists of relatively mafic metamorphic rock
mingled with felsic rock in the form of planar to folded and
contorted layers, criss-crossing veins, and irregular pods.
Felsic leucosome can be separated from the mesosome (a
descriptive term for intermediate rock) or paleosome (a gen-
etic term for the “average” protolith), by a more mafic selvage,
or melanosome. Migmatites originate by (1) local partial
melting and segregation of minimum-T-composition melts
that solidify in place within the more refractory host rock,
(2) injection of small volumes of felsic magma from external
sources, or (3) subsolidus metamorphic differentiation.
Migmatite dome See Mantled gneiss dome.
Mimetic recrystallization During metamorphism, new
grains have essentially replicated or reproduced pre-existing
fabric, as in growth of lepidoblastic micas from aligned clay
mineral grains during prograde metamorphism of shale.
Mineral assemblage See Metamorphic mineral assemblage.
Mineral association Two or more minerals that coexist
stably in a rock, such as quartz, feldspar, and biotite in a
granite. Compare Metamorphic mineral assemblage.
Mineral paragenesis Mineral assemblage in a rock and the
conditions responsible for its formation.
Mineralogical composition Types and chemical composi-
tions of minerals constituting a rock.
Mineralogical phase rule See Goldschmidt’s mineralogical
phase rule.
Mineralogical zone See Metamorphic zone.
Minette A type of lamprophyre composed of abundant
phenocrystic and matrix biotite–phlogopite together with
alkali feldspar plagioclase, clinopyroxene, and possibly
olivine in the matrix.
Mingled Refers to a hybrid rock or the magma from which
it formed that has compositionally contrasting parts pro-
duced by physically merged dissimilar magmas.
Miscibility gap Compositional range between stably coexist-
ing phases across a solvus at a particular P and T.
704 Igneous and Metamorphic Petrology