Unlocking the Enigma: Exploring the Origins and Occurrence of Shoshonites in Petrology and Earth Science

Introduction to the Shoshonites

Shoshonites are a type of volcanic rock belonging to the alkaline series of igneous rocks. They are characterized by a specific geochemical composition that distinguishes them from other volcanic rocks, such as andesites. Shoshonites are named after the Shoshone Mountains in Nevada, USA, where they were first identified and extensively studied. These rocks are of great interest to petrologists and geoscientists because of their unique properties and the insights they provide into geological processes beneath the Earth’s surface.

Composition and Properties of Shoshonites

Shoshonites are classified as intermediate to high silica volcanic rocks that fall within the potassium-rich section of the alkaline basalt-trachyte series. They typically have a porphyritic texture, meaning that they contain distinct larger crystals, called phenocrysts, embedded in a fine-grained matrix or groundmass. The phenocrysts in shoshonites are commonly composed of potassium feldspar, plagioclase feldspar, and biotite or amphibole minerals.

One of the key characteristics of shoshonites is their elevated potassium content, which distinguishes them from other volcanic rocks. They also have high concentrations of other incompatible elements such as rubidium, barium, and thallium. In terms of chemical composition, shoshonites have relatively high levels of silica (SiO2), ranging from 52 to 63 percent by weight. They typically contain moderate amounts of aluminum, sodium, and calcium, while magnesium and iron are relatively low.

Shoshonite Formation and Petrogenesis

Shoshonites are primarily associated with subduction-related volcanic environments where oceanic lithosphere is consumed beneath an overriding tectonic plate. They are commonly found in volcanic arcs, back-arc basins, and within continental volcanic belts. The formation of shoshonites is closely related to the interaction between mantle-derived magmas and the surrounding crustal rocks.

The petrogenesis of shoshonites involves the interaction of a mantle-derived basaltic magma with a crustal component, such as the assimilation of continental crustal rocks or the incorporation of partial melts from subducted oceanic crust. These processes contribute to the enrichment of incompatible elements, including potassium, in the parent magma. The resulting shoshonitic magma then rises to the surface and erupts as volcanic rocks.

Conditions favoring shoshonitic magmatism over andesitic magmatism

Although shoshonites share some similarities with andesites, there are certain conditions that favor the generation of shoshonitic magmas over andesitic magmas. The primary controlling factors can be summarized as follows:

1. Potassium enrichment: Shoshonitic magmas form in tectonic settings where there is a relatively high degree of potassium enrichment. This enrichment can occur by various processes, such as recycling of potassium-rich sediments and fluids during subduction, or metasomatism of the mantle wedge by fluids derived from subducted slabs.

2. Water-rich environments: Shoshonitic magmas typically form in environments that have a higher water content than those that favor andesitic magmas. The presence of water facilitates melting of the mantle and lower crust, resulting in the formation of shoshonitic magmas with their distinctive geochemical signatures.

3. Lower oxygen fugacity: Oxygen fugacity, or the availability of oxygen during magma generation, plays a role in determining the composition of the resulting magmas. Shoshonitic magmas tend to form under relatively reduced conditions compared to andesitic magmas. This lower oxygen fugacity favors the stability of the iron- and magnesium-rich minerals characteristic of shoshonitic rocks.

4. Mantle Source Characteristics: The composition and characteristics of the mantle source also influence the generation of shoshonitic magmas. The presence of a mantle source enriched in potassium and other incompatible elements, as well as the involvement of metasomatized mantle regions, can favor the production of shoshonitic magmas.

In summary, shoshonites are unique volcanic rocks with distinct geochemical characteristics, including elevated potassium content and high concentrations of incompatible elements. They form in subduction-related volcanic environments and are generated under specific conditions such as potassium enrichment, water-rich environments, lower oxygen fugacity, and specific mantle source characteristics. The study of shoshonites provides valuable insights into the petrogenesis of volcanic rocks and geodynamic processes within the Earth’s mantle and crust.

FAQs

What are Shoshonites? What conditions make them occur instead of andesites?

Shoshonites are a type of volcanic rock that belongs to the potassium-rich series of magmas. They are characterized by their high concentrations of potassium (K), iron (Fe), and magnesium (Mg). Shoshonites are intermediate in composition between basalt and andesite.

Shoshonites occur under specific geological conditions that are different from those that give rise to andesites. They are typically associated with subduction zones, where one tectonic plate is forced beneath another. The conditions that favor the formation of shoshonites include:

Enrichment in potassium: Shoshonites are formed from magmas that have undergone enrichment in potassium, which can occur through processes such as fractional crystallization or assimilation of potassium-rich rocks.

High water content: The presence of water in the subduction zone facilitates the melting of the mantle, resulting in the formation of magmas that give rise to shoshonites.

Lower silica content: Shoshonites have a lower silica content compared to andesites. This lower silica content is attributed to the partial melting of the mantle wedge above the subducting plate.

Increased iron and magnesium content: Shoshonites contain higher amounts of iron and magnesium compared to andesites. This is due to the presence of mafic minerals in the mantle source, which contribute to the higher iron and magnesium content in shoshonitic magmas.

In summary, shoshonites are formed under specific conditions characterized by potassium enrichment, high water content, lower silica content, and increased iron and magnesium content. These conditions differ from those that favor the formation of andesites.