Igneous & Metamorphic Petrology - Exam 2

What processes are involved in the formation of andesites/dacites/rhyolites

Fractional crystallization of basaltic melt; assimilation of sialic crust by basaltic magma; mixing of magmas with different compositions; combination of all the above

Only melts (more evolved/less evolved) than a basalt have solidus temps low enough for hydrous minerals to crystallize

More evolved

What hydrous mineral can appear in andesite

Hornblende

What hydrous mineral can appear in more evolved rocks

Biotite

What happens to hydrous minerals at near surface conditions

Dehydration occurs and the hydrous minerals can only appear as phenocrysts, not as groundmass

Volcanic glass examples

Obsidian, vitrophyre, snowflake obsidian

How does water exist in andesite/dacite/rhyolite melts

It can be dissolved in the magma or exist as a separate phase

What happens to excess water in andesite/dacite/rhyolite magma

Excess water leads to a decrease in the solidus temp at increasing pressure

The amount of water dissolved in an andesite/dacite/rhyolite melt (increases/decreases) with pressure

Increases

What are island arcs

Arcuate volcanic island chains along subduction zones

How are these island arcs different from basaltic provinces

The composition in island arcs is more diverse, basalts are generally subordinate

What is the most common volcanic landform

Stratovolcanoes

Describe the petrological model for island arc basalts

Subducted oceanic crust is partially melted —> melts rise through overriding plate building volcanoes behind leading plate edge —> unlimited supply of oceanic crust to melt

Partial melts are more _______ than their source

Silicic

Why does subduction cause magmatism

Basalts/seeds of oceanic crust contain water —> amphiboles/chlorites/other hydrous minerals form as slab descends —> at higher temps, hydrous metamorphic minerals break down and release water rich fluids —> mantle rocks just above slab are hydrated, but melts do not form since temps aren’t high enough —> hydrated mantle is eventually dragged down with slab until melting point reached —> basalt formed by melting hydrated mantle rises, fractionates, and mixes with crustal components

Why are continental arc basalts more prone to contamination

Thick sialic crust contrasts with the mantle driven partial melts leading to more pronounced contamination effects

Why do continental arc basalts have greater potential for differentiation

Low density of the crust causes a slower ascent, leading to stagnation of magmas and more potential for differentiation

(High/low) melting point of the crust allows for partial melting and crustally derived melts

Low

Cenozoic/mesozoic plutonic rocks

Rock types range from gabbros to granites, but tonalites and grabodiorites are volumetrically most important

Describe the petrological model for continental arc basalts

Slab is dehydrated —> hydration/melting of mantle wedge and enriched SCLM —> crustal underplating of mantle derived melts and MASH processes as well as crystallization of underplates —> remelting of under plate to produce tonalitic magmas —> further differentiation and/or assimilation during ascent

MASH

melting - assimilation - storage - homogenization

SCLM

Subcontinental - lithospheric - mantle

Describe how degrees of partial melting affects Na/K levels

Magmas generated by small degrees of partial melting have higher Na/K than magmas generated by larger degrees of partial melting of same source rock

How does depth of melting control composition

The depth of melting shifts the eutectic of a magma

Partial melts become increasingly more alkaline as pressure (decreases/increases)

Increases

Mantle metasomatism

Rising water rich fluids carry alkali into SCLM, altering the original peridotite and enriching it in K/Na/LILE such as Rb. Later partial melting of old metasomatize cratonic SCLM may produce magmas that are enriched in these elements

T/F: carbonatites have always been believed to be of igneous origin

F: only recently were they believed to be of igneous origin

Describe carbonatites

Most occur as small intrusive bodies closely related with alkali rich silicate rocks

What minerals can be found in carbonatites

Calcite/dolomite/ankerite (Ca/Fe) most common, with subordinate amounts of pyroxene/amohiboke/biotite/apatite

Natro-carbonatite

Rarest carbonatite; composed of unusual Na-K carbonate minerals

What type of magma is Ol Doingo Lengai

Natro-carbonatite

How are carbonatites economically useful

They are important sources of REE and exploration targets for mining companies

Carbonatite rocks

Ijolite, urtite, souite, fenite

Ijolite

Plutonic rock composed mainly of nepheline and pyroxene

Urtite

Plutonic rock composed of more than 70% nepheline and pyroxene

Souite

Formerly calcite carbonate

Fenite

Alkali enriched alteration zones bordering alkaline/carbonate intrusions

What are some questions we have about carbonatites

Primary partial melts of mantle? Splitting of magma into immiscible liquid pairs? Ol Doinyo Lengai is unique?

Carbonatites nearly always appear to be the (first/last) magma to form

Last; they cut across other complexes and rocks

Batholiths

Large, shallow tabular granite intrusions

Describe the space problem regarding batholiths

What was there before they were emplaced? What happened to the rocks that were there?

How do granites form (extrusive/intrusive?)

They form in place (in situ)

How do migmatites form

They form through analexis of pre existing country rock

Stoping

Space created through detachment of roof rocks

Diapinic upwelling

Less dense, more viscous, hotter magma rises upwards

K feldspar megacrysts

Large euhedral crystals, contain euhedral mineral inclusions, usually twinned

Rapakivi granite

Higher anorthite content over kspar

What are some granitic textures

Kspar megacrysts, overgrowth, rapakivi granite

S type granite

Remelting of sed rock, chem/mineralogy strongly suggests interaction between magma and sed country rock (high aluminum)

S type granite tectonic setting

Subduction zone

I type granite

Remelting of previous igneous rock

I type granite tectonic setting

Subduction zone

A type granite

Anorogenic, more granitic

A type granite tectonic setting

Rifts

M type granite

Mantle origin, extreme fractional crystallization

M type granite tectonic setting

Subduction zone

Granitoid formation processes

Extreme fractional crystallization of basalt; partial melting of lower crust; assimilation of quartz feldspathic material by more primitive basalt; combination of the above

Europium anomaly

Eu will partition between minerals and melt differently than other REE; mildly compatible in plag; fractionation of plag will deplete melt in Eu

Heat problem

Heat necessary to melt crust and to sustain larger magmatic chambers, but heat of crystallization offsets heat of fusion

What are examples of pyroclastic rocks in mafic eruptions

Fire fountains, welded splatter, peles tears and hair, scoria/cinders, bombs

Pyroclastic rock classification

Collectively called tephra regardless of size, depending on clast size can be classified as accidental or juvenile

Volcanic ash

Glass shards can form from disruption of bubbles formed by exsolving volatiles in magma

3 stages of explosive eruption

Fragmentation of magma —> gas/fragments discharged through vent —> ejected material forms plume in atmosphere

Plinian eruptions

Ash and gas column extending into the stratosphere, pumice/blast eruptions, pyroclastic flows, ash fall deposits (intermediate to silicic)

Surtseyan eruptions

Basaltic magma interacts with external water, pyroclastic eruptions driven by steam, basaltic glass fragments from quenching, palagonite tuff and basaltic tephra

Phreatic eruptions

All steam, groundwater percolates down and flashes to steam, can fragment previously existing rock (accidental material)

Outline the key observations that discriminate between Andesite and dacite

Andesite contains the essential minerals plagioclase and/or mafic minerals while dacite consists of sodic plag, quartz, and alkali feldspar. Andesites are fine-grained mesocratic rocks while dacites are fine-grained leucocratic rocks. Andesites also contain oscillatory-zoned plag phenocrysts with significant amounts of orthopyroxene in thin section.

Outline the key observations that discriminate between andesite and trachydacite

Trachydacites have slightly higher silica content (~65%) and alkali content with more alkali feldspar and quartz phenocrysts. Andesites have lower silica and alkali content with more plag phenocrysts. 

Outline the key observations that discriminate between Basaltic andesite and shoshonite

Shoshonite is a high-potassium alkaline rock, whereas basaltic andesite has lower alkali content and is subalkaline to mildly alkaline. Shoshonite contains significant amounts of K-feldspar along with clinopyroxene, whereas basaltic andesite is dominated by plagioclase, pyroxene, and olivine. Shoshonite may appear darker and richer in mafic minerals due to its alkaline composition, while basaltic andesite has a more intermediate appearance.

Outline the key observations that discriminate between Calc alkaline and tholeiitic fractionation paths

Tholeiitic magmas show early iron enrichment due to fractionation of Fe-poor minerals, whereas calc-alkaline magmas suppress iron enrichment due to early crystallization of magnetite. Calc-alkaline rocks have higher alkali contents than tholeiitic rocks. Tholeiitic rocks often contain clinopyroxene ± orthopyroxene, whereas calc-alkaline rocks are richer in amphibole and biotite.

Outline the key observations that discriminate between Dacite and rhyolite under the microscope

Dacite has a microcrystalline to felsitic groundmass, while rhyolite often has a glassy or spherulitic texture. Rhyolite has a higher quartz and alkali feldspar content, while dacite has more plag and minor quartz.

What is a partition coefficient? How does this control which elements will go into early melts?

The partition coefficient describes how an element is distributed between a solid mineral and coexisting melt where D = concentration of element in solid/concentration of element in melt. If D is greater than 1, then the element is compatible and will stay in the solid phase. If D is less than 1, the element is incompatible and will prefer to stay in the melt. Incompatible elements (like LREE) preferentially enter the melt, while compatible elements (like HREE in garnet) remain in the residual solid.

A depletion in HREE is indicative of melt derivation from which mantle source region? Why?

A depletion in HREE indicates that the melt was derived from a garnet-bearing mantle source. This happens because garnet strongly retains HREE, preventing them from entering the melt. In contrast, at shallower depths where spinel is stable instead of garnet, the HREE are not retained as effectively, leading to a melt with a more balanced REE profile.

Europium (Eu) is indicative of crystallization of which mineral within a melt? Why?

Europium anomalies are linked to the crystallization of plagioclase. This is because Eu can exist as Eu2+ under reducing conditions, and plagioclase preferentially incorporates Eu2+ into its crystal structure over other REE, which are typically in the 3+ oxidation state. As a result, when plagioclase crystallizes from a melt, it removes Eu from the melt, leading to a depletion in Eu in the residual melt. Conversely, melts derived from plagioclase-rich sources may show a positive Eu anomaly.