Magma and intrusive rocks

Magma

A molten mixture of rock-forming substances, gases, and water from the mantle.

Anatexis

The partial melting of rocks, especially in the formation of metamorphic rocks (migmatites).

Liquid melt and residual rock

Products of anatexis

Equilibrium melting

Occurs in a closed system where chemicals are neither added nor removed from the plutonic environment Requires the melt to remain in contact with the residual rock throughout the melting process.

Fractional melting

Solids and melt separate into isolated fractions that do not react to each other together during the melting process. Produces a melt that is more evolved than the parent.

Increasing temperature Decreasing pressure Influence of volatiles Partial melting and melt composition

Factors affecting anatexis and initial melt composition.

Geothermal gradient

The gradual increase in temperature with depth in the crust. Higher in hotspots and spreading centers.

25 degrees Celsius / km

Average geothermal gradient.

Decompression melting

Partial melting of hot mantle rock when it moves upward and the pressure is reduced to the extent that the melting point drops to the temperature of the body.

Adiabatic melting

Other name for decompression melting.

Flux

Agent that reduces the melting temperature of a substance.

Anatectic melts tend to be more enriched with low melting temperature components than the restite.

True.

Restite

Residual rock.

Light rare earth elements and large ion lithophile elements

Trace elements that are enriched in products of partial melting.

Closed-system diversification

The original melt evolves into one or more melts with a different composition, without material being exchanged with an external source.

Bowen's reaction series

Sequential, predictable, dual-branched pattern in which minerals crystallize from cooling magma.

Continuous series

One of the 2 branches of Bowen's reaction series concerning the crystallization of plagioclase minerals.

Discontinuous series

One of the 2 branches of Bowen's reaction series concerning the crystallization of ferromagnesian minerals.

Zoned crystals

Display systematic pattern of chemical variations from the periphery of the crystal towards its center, recording an incomplete continuous chemical reaction.

Reaction rims

Display a new mineral along crystal periphery that surrounds a partially resorbed core composed of a different mineral indicating incomplete discontinuous chemical reactions between crystal and melt.

Marginal accretion model

Crystallization along the walls of the magma chamber (in contact with the country rock) in which crystals preferentially form and adheres to the edges.

Roof, sidewall, and floor accretions

Types of marginal accretion based on position in the magma chamber.

Chilled margins

Finer-grained rocks found at the edges of plutons due to surrounding country rock cooling the pluton.

Gravitational separation process

Includes processes that occur when crystals develop with significantly different densities than the surrounding magma.

Crystal settling and floatation

Types of gravitational separation processes.

Gravity segregation processes are more common in viscous magmas.

False.

Convective flow

Occurs whereby liquids and crystals are segregated due to factors such as velocity, density or temperature.

Filter pressing

During the crystallization of a magma there is a point where crystals and liquid coexist as a slushy mass. As gravity settling takes place the weight of the crystals causes the remaining melt to be squeezed out forming a separate layer above. This liquid is depleted in the elements incorporated into the early formed crystals and enriched in felsic forming minerals.

Differential diffusion

Involves the preferential diffusion of select ions within the magma in response to compositional, thermal, or density gradients as well as water content.

Liquid immiscibility

Also called liquid-liquid fractionation. Occurs when magma separates into two or more distinct immiscible liquid phases.

Open-system diversification

Involves chemical changes in the system due to interaction of magma with surrounding country rock or with other magmas entering the system.

Assimilation

Occurs whereby the surrounding wall rock or country rock is intruded by and reacts chemically with the magma. Important process in thick crustal regions.

Stoping

Forceful injection of magma into country rock.

Xenolith

A relict of wall rock surrounded by intrusive rock. Product of stoping.

Xenocryst

A crystal foreign to the igneous rock in which it occurs.

Magma mingling

Occurs when two or more dissimilar magmas coexist, displaying contact relations but retaining their distinctive individual magma characteristics. Important in continental rifts and convergent environments.

Magma mixing

Implies thorough mixing do that the individual magma components are no longer recognizable. Important in continental rifts and convergent environments.

Alkaline magma

Have higher Na2O and K2O concentrations compared to SiO2 and are generally undersaturated with respect to SiO2. Less common. Most likely to form by limited melting (<10%) of a parent source under high pressure conditions, especially if the source rocks are relatively undepleted or even enriched in alkalis.

Calc-alkaline and tholeiitic magma

Subdivisions of the sub-alkaline series.

Calc-alkaline magma

Have moderate to high Na2O, K2O, and SiO2 concentrations. Consists largely of andesite, rhyolite, as well as alumina basalt. Dominates convergent margin environments where batholiths and volcanoes develop above subduction zones.

Tholeiitic magma

Have low Na2O, K2O, and SiO2 concentrations but high FeO and CaO concentrations. Dominate extensional environments such as ocean ridges and continental rifts; can also occur at hotspots in intraplate settings and in the frontal portions of immature volcanic arcs characterized by thin volcanic arc crust.

Variation diagrams provide a means to concisely and clearly display rock chemistry variations between samples collected in an area with an eye to establishing their origins and relationships.

True.

Bivariate variation diagram

Harker diagram. Vertical ordinate represents the weight percents of major mand minor oxide compounds such as FeO, MgO, CaO, Na2O, K2O, or TiO2. Horizontal abscissa represents weight percent SiO2 content. The objective is to concisely display variations in major and minor oxide concentrations with respect to SiO2.

Spider diagram

Trace element concentrations are plotted on the abscissa relative to a standard reference for each element on the ordinate. Useful in demonstrating the enrichment or depletion of compatible vs incompatible elements (LREE vs HREE).

Ternary variation diagram

Plotted on an AFM (alkali-iron-magnesium) diagram in order to determine the suite where the rock belongs.

Bimodal suite

Characterized by the voluminous occurrence of silicic and basic rocks with few intermediate rocks. Related to continental rifts.

Concordant bodies

Parallel to the pre-existing layering in the surrounding country rock which can be bedding, metamorphic foliation, volcanic layers, or other structural forms, or the Earth's surface if no discernible form.

Discordant bodies

Oriented at a sharp angle to the country rock layering and cut across it.

Sills Laccoliths Lopoliths Veins Phacoliths

Types of concordant bodies.

Dikes Cone sheets Bysmaliths Stocks Batholiths Chonoliths Neck and diatreme

Types of discordant bodies.

Sill

Tabular, concordant pluton that parallels country rock. Have two mutually perpendicular long dimensions and a short dimension perpendicular to the other two.

Laccolith

A blister-like concordant pluton characterized by a flat floor and domed roof. May have begun as a sill that was inflated.

Lopolith

Dish-shaped to funnel-shaped concordant plutons that resemble a champagne glass in cross-sectional view. Due to an astrobleme, normal faulting and crustal melting, or sill-like structure from upwelling magma from a conical feeder. Enriched in Ni, Cu, Cr, Pt, and Pd.

Vein

Form as hot fluids flow through fractures and then cool and crystallize. Can be either concordant or discordant to host rock.

Vein swarm

Display random or preferred orientations which can be parallel, sub-parallel, orthogonal, en echelon, or sigmoidal.

Neck

Cylindrical pluton dikes exposed at the surface by subsequent erosion. Represent ancient conduit pipes that funneled magma upward to a volcano that has long since been removed by erosion.

Diatreme

Carrot-shaped, cylindrical pipes that can extend at depths of 200km. Develop via explosive intrusions that original deep within the mantle; explosiveness is due to the high volatile content which propels magma and xenolith fragments to the surface. Associated with kimberlites and tuff cones.

Dike

Tabular intrusions that cross-cut country rock layers. Commonly form topographically elevated ridges. Can occur individually or in dike sets/swarms.

Dike swarm

Consist of multiple dikes that can occur either in parallel, sub-parallel, radiating, concentric, or random orientations.

Radial dike

Typically produced when vertical forces related to rising magma can produce fractures that radiate outward from the central vent. Form when magma from the central vent is injected outward into the radial structures.

Ring dike

Nearly vertical in cross-section and circular in map view. Constitutes network if interrelated, discordant bodies associated with concentric fracture sets.

Cone sheet

Circular in map view and resemble ring dikes except that cone sheet dikes converge at depth. Constitutes network if interrelated, discordant bodies associated with concentric fracture sets.

En echelon dike

Dike sets consisting of parallel, offset dikes that form in response to shear. Form perpendicular to extension. Common in rift environments and sheeted dikes.

Sheeted dike

Steeply inclined, composed of gabbro, diabase, and basalt. Overlain by pillow basalts. Form by cooling and contraction of magma as it is injected into extensional fractures in oceanic rift valleys