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Anorthosites are of enormous geologic interest, because it is still not fully understood how they form. Most models involve
Anorthosite on Earth can be divided into five types:
Of these, the first two are the most common. These two types have different modes of occurrence, appear to be restricted to different periods in
Proterozoic anorthosites typically occur as extensive
Major occurrences of Proterozoic anorthosite are found in the southwest U.S., the
Many Proterozoic anorthosites occur in spatial association with other highly distinctive, contemporaneous rock types: the so-called 'anorthosite suite' or 'anorthosite-
These rock types can include:
Since they are primarily composed of plagioclase feldspar, most of Proterozoic anorthosites appear, in
Most anorthosite plutons are very
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
Proterozoic anorthosites are typically >90% plagioclase, and the plagioclase composition is commonly between An40 and An60 (40–60%
Proterozoic anorthosites often have significant
The trace-element chemistry of Proterozoic anorthosites, and the associated rock types, has been examined in some detail by researchers with the aim of arriving at a plausible genetic theory. However, there is still little agreement on just what the results mean for anorthosite genesis; see the 'Origins' section below. A very short list of results, including results for rocks thought to be related to Proterozoic anorthosites,[
Some research has focused on
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.
The origins of HAOMs are debated.
One possible model suggests that, during anorthosite formation, a mantle-derived melt (or partially-crystalline mush) was injected into the lower crust and began crystallizing. HAOMs would have crystallized out during this time, perhaps as long as 80–120 million years. The HAOM-bearing melt could then have risen to the upper crust. This model is supported by the fact that aluminum is more soluble in orthopyroxene at high pressure. 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 suggest that the HAOMs may have formed in the lower crust independent of the anorthosite source-magma. Later, the anorthosite source-magma may have entrained pieces of the HAOM-bearing lower crust on its way upward.
Other researchers consider the chemical compositions of the HAOM to be the product of rapid crystallization at moderate or low pressures, eliminating the need for a lower-crustal origin altogether.
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
It was suggested early in the history of anorthosite debate that a special type of magma, anorthositic magma, had been generated at depth, and emplaced into the crust. However, the
The discovery, in the late 1970s, of anorthositic
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
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.