subducting lithosphere in Figure 11.18, would provide
a larger window of opportunity for slab melting. An
alternate mechanism for TTG magma production
involves not subduction of dense oceanic lithosphere
but rather thrust stacking of hydrated buoyant crust,
building a pile of basaltic rock thick enough to partially
melt under the requisite pressures (Figure 19.10). In
either scenario, the oceanic basaltic crust was hydrated
by advecting seawater on the ocean floor and, accord-
ing to isotopic data, partial melting of the hydrated
basalt occurred within a few hundred Ma of the time
basalt magmas were generated from mantle peridotite.
By whatever mechanism, production of TTG mag-
mas waned in the cooling Archean–Proterozoic Earth
while generation of arc magmas in the mantle-wedge
above subducting oceanic lithosphere waxed stronger.
This transition likely occurred at different times at
different places (diachronously) over the Earth over a
long period of time, but geochemical data compiled by
Martin (1994) suggest a transition near 2500 Ma—the
time boundary between the Archean and Proterozoic.
This accounts for the restriction of andesitic rocks
possessing typical arc signatures to younger Archean
and Proterozoic greenstone sequences.
19.4.2 High-Mg Granitoids
Some younger Archean granitoids have unusually high
Mg numbers, Mg/(Mg Fe) 0.4, precluding an
origin by partial melting of a basaltic source and
suggesting, instead, a peridotitic source. The less
evolved dioritic rocks that have Mg numbers 0.55
resemble boninites of modern island arcs that are gen-
erated from silica- and incompatible-element-enriched
harzburgite in the mantle wedge overlying subducting
oceanic lithosphere (Section 13.5). Kerrich et al. (1998)
describe metamorphosed boninites from the 2.7 Ga
Abitibi greenstone belt in Canada, while Evans and
Hanson (1997) and Rapp (1997) describe a suite of
variably evolved high-Mg granitoids from the Superior
Province of Canada. They suggest that hybridization of
magmas from two sources—subducted basalt crust and
metasomatized overlying mantle peridotite—created a
spectrum of late Archean granitoids.
19.4.3 Granite
Large volumes of K-rich granite were emplaced as plu-
tons several millions of years or more after greenstone
belt volcanism, TTG plutonism, and widespread meta-
morphism (Sylvester, 1994). Most were emplaced after
about 3.1 Ga. Granite plutons are only moderately
foliated to isotropic, medium- to coarse-grained rocks
that retain primary magmatic fabric and mineral com-
positions. Metaluminous, strongly peraluminous, and
relatively younger alkaline granites occur in roughly
equal proportions across broad areas of Archean
cratons, unlike the narrow belts of predominantly
granodiorite–granite in Mesozoic arcs of western South
and North America (Figure 9.16). Isotopic data are
sparse and modes of origin and evolution of these
granitic magmas remain speculative. However, a pla-
gioclase residue in a source within the continental crust
is implied by the depletion in Eu in the rocks. Hence,
production of these granite magmas can be considered
as the earliest partial melting and recycling of older
continental rocks.
19.5 MID-PROTEROZOIC TECTONISM
AND MAGMATISM
During the middle Proterozoic and virtually only then
two distinct magmatic associations were emplaced into
the continental crust; these are massif-type anorthosite
and rapakivi granite. Because most of these rocks are
spatially and temporally related to the evolution of
the Grenville Province, this collisional terrane will be
discussed next, followed by a discussion of the two
middle Proterozoic magmatic associations.
19.5.1 Grenville Province
Several Archean terranes, including the Superior, were
amalgamated in the Early Proterozoic from 1980 to
1790 Ma, forming a complex collage of sutured terranes
—the Laurentian Craton (Figure 19.27; see also
Hoffman, 1989; Rivers, 1997). The southeastern mar-
gin of this craton was subsequently partially reworked
by episodes of Andean-style orogeny and magmatism
accompanying northwest-directed plate convergence,
adding progressively younger arc terranes between
about 1700 and 1200 Ma. Finally, continent–continent
collision during the Grenvillian Orogeny between
about 1190 and 980 Ma added still more real estate to
Laurentia. The Grenville Orogen extends into Texas
and Mexico but only the Canada segment is considered
here. A Scandinavian segment, along with much of
the colliding continent, now possibly South America,
was rifted away sometime in the late Proterozoic. The
Keweenawan mid-continent rift that contains as much
as 10 km of basalt formed farther into Laurentia during
the Grenville orogeny at about 1100 Ma.
The Grenville Orogen (Figures 19.28 and 19.29)
is notable for ductile deformation during thrusting
and contemporaneous, widespread, high-grade meta-
morphism that implies exposure of deeply eroded con-
tinental crust. Granulite-facies rocks were equilibrated
at depths of about 30–40 km (P 8–11 kbar), while
local eclogites equilibrated at P as much as 16 kbar
and T 700–800°C indicate much deeper burial,
probably during stacking of thrust slices accompanying
continent–continent collision (Indares, 1993).
These thrust sheets moved northwesterly on south-
east-dipping high-grade ductile shear zones, now mylo-
nite. Scattered anorthosite suite magmas (discussed
636 Igneous and Metamorphic Petrology