Age Proterozoic anorthosites were emplaced during the Proterozoic Eon (c. 2,500–542
Ma), though most were emplaced between 1,800 and 1,000 Ma. in northern Labrador, Canada. Major occurrences of Proterozoic anorthosite are found in the southeast U.S., the
Appalachian Mountains (e.g., the Honeybrook Upland of eastern Pennsylvania), eastern
Canada (e.g., the Grenville Province), across southern
Scandinavia and eastern
Europe. Mapped onto the
Pangaean continental configuration of that eon, these occurrences are all contained in a single straight belt, and must all have been emplaced
intracratonally. The conditions and constraints of this pattern of origin and distribution are not clear. However, see the Origins section below.
Related rocks Many Proterozoic anorthosites occur in spatial association with other highly distinctive, contemporaneous rock types: the so-called 'anorthosite suite' or 'anorthosite-
mangerite-
charnockite-granite (AMCG) complex'. These rock types can include: •
Mangerite: a pyroxene-bearing monzonite intrusive igneous rock •
Charnockite: an orthopyroxene-bearing quartz-feldspar rock, once thought to be intrusive igneous, now recognized as metamorphic • Iron-rich
felsic rocks, including
monzonite and
rapakivi granite • Iron-rich
diorite,
gabbro, and
norite •
Leucocratic mafic rocks such as
leucotroctolite and
leuconorite Though
co-eval, these rocks likely represent chemically-independent magmas, likely produced by melting of
country rock into which the anorthosites intruded.
Physical characteristics intrusion (1.29 to 1.35 billion years), Labrador. Polished slab; blue color is
labradorescence. Since they are primarily composed of plagioclase feldspar, most of Proterozoic anorthosites appear, in
outcrop, to be grey or bluish. Individual plagioclase crystals may be black, white, blue, or grey, and may exhibit an
iridescence known as
labradorescence on fresh surfaces. The feldspar variety labradorite is commonly present in anorthosites. Mineralogically, labradorite is a compositional term for any calcium-rich plagioclase feldspar containing 50–70 molecular percent anorthite (An 50–70), regardless of whether it shows labradorescence. The mafic mineral in Proterozoic anorthosite may be
clinopyroxene,
orthopyroxene,
olivine, or, more rarely,
amphibole.
Oxides, such as
magnetite or
ilmenite, are also common. Most anorthosite plutons are very
coarse grained; that is, the individual plagioclase
crystals and the accompanying mafic mineral are more than a few centimetres long. Less commonly, plagioclase crystals are megacrystic, or larger than one metre long. However, most Proterozoic anorthosites are
deformed, and such large plagioclase crystals have
recrystallized to form smaller crystals, leaving only the outline of the larger crystals behind. While many Proterozoic anorthosite plutons appear to have no large-scale relict igneous structures (having instead post-emplacement deformational structures), some do have
igneous layering, which may be defined by crystal size, mafic content, or chemical characteristics. Such layering clearly has origins with a
rheologically liquid-state
magma.
Chemical and isotopic characteristics Proterozoic anorthosites are typically >90%
plagioclase, and the plagioclase composition is commonly between An40 and An60 (40–60%
anorthite). Some research has focused on
neodymium (Nd) and
strontium (Sr)
isotopic determinations for anorthosites, particularly for anorthosites of the Nain Plutonic Suite (NPS). Such isotopic determinations are of use in gauging the viability of prospective sources for magmas that gave rise to anorthosites. Some results are detailed below in the 'Origins' section.
High-alumina orthopyroxene megacrysts (HAOMs) Many Proterozoic-age anorthosites contain large crystals of orthopyroxene with distinctive compositions. These are the so-called high-alumina orthopyroxene megacrysts (HAOM). HAOM are distinctive because 1) they contain higher amounts of Al than typically seen in orthopyroxenes; 2) they are cut by numerous thin lathes of plagioclase, which may represent exsolution lamellae; and 3) they appear to be older than the anorthosites in which they are found. In this model, the HAOM represent lower-crustal cumulates that are related to the anorthosite source-magma. One problem with this model is that it requires the anorthosite source-magma to sit in the low crust for a considerable time. To solve this, some authors eliminating the need for a lower-crustal origin altogether.
Origins of Proterozoic anorthosites The origins of Proterozoic anorthosites have been a subject of theoretical debate for many decades. A brief synopsis of this problem is as follows: The problem begins with the generation of magma, the necessary precursor of any igneous rock. Magma generated by small amounts of partial melting of the
mantle is generally of
basaltic composition. Under normal conditions, the composition of basaltic magma requires it to crystallize between 50 and 70% plagioclase, with the bulk of the remainder of the magma crystallizing as mafic minerals. However, anorthosites are defined by a high plagioclase content (90–100% plagioclase), and are not found in association with contemporaneous ultramafic rocks. However, the dykes were later shown to be more complex than was originally thought. In summary, though liquid-state processes clearly operate in some anorthosite plutons, the plutons are probably not derived from anorthositic magmas. Many researchers have argued that anorthosites are the products of basaltic magma, and that mechanical removal of mafic minerals has occurred. Since the mafic minerals are not found with the anorthosites, these minerals must have been left at either a deeper level or the base of the crust. A typical theory is as follows: partial melting of the mantle generates a basaltic magma, which does not immediately ascend into the crust. Instead, the basaltic magma forms a large magma chamber at the base of the crust and
fractionates large amounts of mafic minerals, which sink to the bottom of the chamber. The co-crystallizing plagioclase crystals float, and eventually are emplaced into the crust as anorthosite plutons. Most of the sinking mafic minerals form
ultramafic cumulates which stay at the base of the crust. This theory has many appealing features, of which one is the capacity to explain the chemical composition of high-alumina orthopyroxene megacrysts (HAOM). This is detailed below in the section devoted to the HAOM. However, on its own, this hypothesis cannot coherently explain the origins of anorthosites, because it does not fit with, among other things, some important isotopic measurements made on anorthositic rocks in the Nain Plutonic Suite. The Nd and Sr isotopic data show the magma which produced the anorthosites cannot have been derived only from the mantle. Instead, the magma that gave rise to the Nain Plutonic Suite anorthosites must have had a significant crustal component. This discovery led to a slightly more complicated version of the previous hypothesis: Large amounts of basaltic magma form a magma chamber at the base of the crust, and, while crystallizing, assimilating large amounts of crust. This small addendum explains both the isotopic characteristics and certain other chemical niceties of Proterozoic anorthosite. However, at least one researcher has cogently argued, on the basis of geochemical data, that the mantle's role in production of anorthosites must actually be very limited: the mantle provides only the impetus (heat) for crustal melting, and a small amount of partial melt in the form of basaltic magma. Thus anorthosites are, in this view, derived almost entirely from lower crustal melts. == Lunar anorthosite ==