State the petrographic characters of different types of anorthosites. Write a note on petrogenesis of anorthosites.

Petrographic Characters of Different Types of Anorthosites

Anorthosites are broadly classified based on their texture, mineral composition (specifically the An content of plagioclase), and the presence of minor phases. Here's a breakdown of the key types and their petrographic characteristics:

1. Massive Anorthosites

• Texture: Coarse-grained, equigranular, and typically massive. Plagioclase crystals are generally well-formed and interlocked.
• Mineralogy: >90% plagioclase (typically By-Ab50), with minor amounts of pyroxene (augite, hypersthene), olivine, and ilmenite.
• Plagioclase: Exhibits normal zoning, with calcium-rich cores and sodium-rich rims. Twinning is common (Albite and Carlsbad twins).
• Accessory Minerals: Apatite, magnetite, and occasionally quartz.

2. Layered Anorthosites

• Texture: Characterized by distinct layering or banding, often millimeter to centimeter scale. The layering can be compositional (varying plagioclase/pyroxene ratios) or textural (varying crystal size and orientation).
• Mineralogy: Similar to massive anorthosites, but with more pronounced variations in the abundance of mafic minerals (pyroxene, olivine) within the layers.
• Plagioclase: Zoning is often more complex in layered anorthosites, reflecting changes in magma composition during crystallization. Cumulate textures are common, where crystals have settled out of the melt.
• Accessory Minerals: Chromite can be present in association with the mafic layers.

3. Gabbronorites

• Texture: Similar to massive anorthosites, but with a more subequigranular texture due to the presence of significant amounts of mafic minerals.
• Mineralogy: Plagioclase (typically Ab50-An60) constitutes 50-70% of the rock, with 20-40% pyroxene (augite, orthopyroxene). Olivine and ilmenite are also present.
• Plagioclase: Zoning is less pronounced than in pure anorthosites.
• Accessory Minerals: Apatite, magnetite.

4. Proterozoic Anorthosites (e.g., Bushveld Complex, Stillwater Complex)

• Texture: Often exhibit a layered structure, with varying degrees of cumulate textures. Some layers may be massive.
• Mineralogy: Highly variable, ranging from nearly pure plagioclase anorthosites to gabbronorites and even troctolites (olivine-rich). Chromitite layers are common.
• Plagioclase: Anorthite content varies significantly, often showing reverse zoning in some layers.
• Accessory Minerals: Significant concentrations of platinum group elements (PGEs) are often associated with these anorthosites, making them economically important.

Petrogenesis of Anorthosites

The origin of anorthosites has been a long-standing debate in petrology. Several theories have been proposed, each attempting to explain the unusual plagioclase-rich composition.

1. Magma Series and Fractional Crystallization

This theory suggests that anorthosites are formed through extensive fractional crystallization of a basaltic or gabbroic magma. As the magma cools, plagioclase crystals preferentially crystallize and settle out of the melt due to their density. This process continues, leading to a residual melt that becomes increasingly enriched in iron, magnesium, and other incompatible elements. The resulting rock is a plagioclase-rich anorthosite. However, this model struggles to explain the large volumes of anorthosites observed in some regions.

2. Plume-Lithosphere Interaction

This model proposes that anorthosites form through the interaction of mantle plumes with the lower crust. The upwelling plume melts the lower crust, generating a magma that is rich in plagioclase. This magma then intrudes into the upper crust, forming anorthosite bodies. This theory can explain the association of anorthosites with large igneous provinces.

3. In-situ Crystallization of a Plagioclase-rich Magma

This theory suggests that anorthosites form through the in-situ crystallization of a magma that is already rich in plagioclase. This magma may be derived from partial melting of a plagioclase-rich source rock in the mantle. This model is supported by the observation that some anorthosites contain evidence of minimal transport.

4. Melt-Crystal Filter Press Mechanism

This model suggests that anorthosites form through a process of melt-crystal filter pressing. As plagioclase crystals crystallize, they form a network that impedes the flow of melt. This leads to a concentration of plagioclase crystals and a depletion of other minerals in the melt. The resulting rock is a plagioclase-rich anorthosite.

Currently, a combination of these mechanisms is considered most likely, with the specific processes involved varying depending on the geological setting and the scale of anorthosite formation.