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Advanced Rock Concepts: Magmatic Differentiation and Metamorphic Facies

Magmatic differentiation and metamorphic facies are key concepts in understanding how rocks evolve through geological processes. Magmatic differentiation refers to the chemical changes magmas undergo as they cool, crystallize, and interact with their environment. These changes produce a sequence of magmas, known as a magma series, and result in diverse igneous rocks. Processes like fractional crystallization, assimilation, and magma mixing play major roles in altering magma composition. Metamorphic facies, on the other hand, describe groups of mineral assemblages that form under specific temperature and pressure conditions during metamorphism.

Figures (6)

Schematic diagrams showing the principles behind fractional crystallisation in a magma. While cooling, the magma evolves in composition because different minerals crystallize from the melt. 1: olivine crystallizes; 2: olivine and pyroxene crystallize; 3: pyroxene and plagioclase crystallize; 4: plagioclase crystallizes. At the bottom of the magma reservoir, a cumulate rock forms.
Triangular diagrams showing the aluminium (A), calcium (C) and iron (F) content of the main phases (dark dots) in metamorphic rocks in various facies. Thin grey lines are stable phase equilibria.
Triangular diagrams showing the aluminium (A), iron (F) and magnesium (M) content of the main phases (dark dots and, when the composition can vary, stripes). Thin grey lines represent equilibria between phases.
Olivine weathering to iddingsite within a mantle xenolith, demonstrating the principles of the Goldich dissolution series
Bowen's reaction series

This happens through mechanisms like fractional crystallization, where early-formed minerals are removed from the melt, changing the remaining magma’s makeup. A magma series is the sequence of magmas that results from this process, often becoming more silicic over time. Understanding whether a rock is a cumulate—formed from early-crystallized minerals—is key to tracing back to the original magma composition.

Each facies corresponds to a range on a temperature-pressure graph and is defined by its characteristic mineral assemblages. For example, the blueschist facies forms at high pressure and low temperature, typical of subduction zones, while the granulite facies forms at high temperature and medium pressure. These facies help link rocks to their tectonic settings and geological history.

To picture magmatic differentiation, imagine a cooling magma chamber where minerals form and settle out, leaving behind a melt that becomes richer in certain elements. For metamorphic facies, think of a rock undergoing heat and pressure, changing its minerals in predictable ways based on the environment. Both processes show how rocks evolve through time and space, shaped by the conditions they experience.

Key Points

  • Magmatic differentiation is an umbrella term for the various processes by which magmas undergo bulk chemical change during the partial melting process, cooling, emplacement, or eruption.
  • Fractional crystallization is the removal and segregation from a melt of mineral precipitates, which changes the composition of the melt.
  • Assimilation is a process where a mass of magma wholly or partially homogenizes with materials derived from the wall rock of the magma body.
  • Magma mixing is the process by which two magmas meet, comingle, and form a magma of a composition somewhere between the two end-member magmas.
  • A metamorphic facies is a set of mineral assemblages in metamorphic rocks formed under similar pressures and temperatures.

Terms

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