Chapter 4 Plutonic field relationships

Plutonic Rocks

Intrusive – non-specific types commonly called plutons or intrusions
Typically thought of as solidified magma bodies formed at depth within Earth that intrude “country rock”
Remnants of:
 crystallized magma-chambers, magma lenses
 magma conduits /feeders
 volcanic plugs
 deep zones of crustal melt
Also late-stage intrusions of granitic veins and pegmatites
Multiple: All phases of injection are of the same composition
Composite: More than one rock type is represented.

Plutonic rocks classified by:

Tabular - injected along planes of weakness (fractures) sheet-like dikes (discordant) and sills (concordant)
Discordant: Cross cutting the country rock body
Concordant: parallel to the country rock structure

Vast majority of dikes and sills form by injection and dilation

Types of tabular igneous bodies in bedded strata based on method of emplacement.

a. Simple dilation (arrows) associated with injection.

b. No dilation associated with replacement or stoping.

Dike discordant -crosscutting

Vein

Vein refers to a small tabular body, whether or not it is discordant or concordant

Swarms

Genetically related sets of numerous dikes or sills

Yhe
formation of ring dikes
and cone sheets.

Cross section of a rising pluton causing fracture and stoping of roof blocks.
b. Cylindrical blocks drop into less dense magma below, resulting in ring dikes.

c. Hypothetical map view of a ring dike with N- S striking country rock strata as might result from erosion to a level approximating X-Y in (b).
d. Upward pressure of a pluton lifts the roof as conical blocks in this cross section.
Magma follows the fractures, producing cone sheets. Original horizontal bedding plane shows offsets in the conical blocks.

Caldera formation

Plutonic rocks classified by: Non-tabular

Non-tabular – detailed form depends on depth, density, ductility of country rock
various shapes
stocks (< 100 km2): plugs, necks –often volcanic conduits
Batholiths- ( >100km2; often composite): Mostly discordant but some more conformable to regional structures
laccolith (arched roof)
lopolith (cone shaped basin)

Plutonic Bodies - Stocks

Smaller igneous bodies, generally steep sided (roughly cylindrical).
Many unrelated to larger batholiths
-These associated with alkaline mantle derived magmatism at continental rifts and hotspots
Not limited to being granitic

Some form in the roof zone of granitic batholiths and may be connected to a batholith at depth

Plutonic Bodies - Batholiths

-Largest plutonic rock body (>100km2).
-Smaller bodies called stocks
-Broadly granitic in composition.
-Typically form in orogenic belts
-Deep roots. For some debate exists about the 3- shape/deeper structure
-Magmas ascend at depth via plastic deformation of surroundings. At shallower depths ascend by stoping.
-Probably rise as diapir

-Amount thermal energy to form and maintain batholith as it rises is high.
-Most cases heat source is mantle-derived basaltic melts.
-Form in orogenic belts and some in continental hotspots & rifts

Volcanic plugs

A volcanic plug: Stocks representing the cylindrical conduit and magma chamber
beneath volcanoes.

Volcanic plugs

volcanic neck: exposed portion of a plug, commonly remaining after the more easily eroded volcanics of the cone removed

Ship Rock NM

Ship Rock NM the central feeder pipe of larger volcanic landform which has since
eroded away.

Concordant Plutons: Laccoliths and Lopoliths

Plutonic Bodies - Laccolith

-Sufficiently viscous (silicic) to limit magma flow along the horizon plane
-Shallow enough to lift the roof rock
-Much smaller than lopolith

Plutonic Bodies - Lopoliths

-Down warped (saucer-shaped) concordant mafic intrusion.
-Almost all Precambrian
-Much larger than laccolith
Bushveld complex in S. Africa largest at 300 km diameter and 8 km thick)
Took 200,000 years to cool
-Some associated with meteorite impact

Layered Mafic Intrusions: LMI form in regions of extensive magmatism

Characterized by igneous layering distinguishable by shifts in mineralogy, texture, or
composition

Many emplaced during the Precambrian, predominantly at the margins of ancient cratons during intervals of supercontinent accretion and destruction

Contacts of Plutons: Chilled margins:

Chilled margins: A margin of finer grain size resulting from rapid solidification of the pluton at the contact with wall rock (thermal effects)

Contacts of Plutons

Schelieren

Schelieren: Elongated or flattened mineral aggregates or ductile
heated xenoliths that produce disc-shaped mass

Marginal foliations

Marginal foliations developed within a pluton as a result of differential motion across the contact.
If the intruding magma is sufficiently viscous and the country rock hot enough, the contact shear can also affect the country rock, rotating the foliation outside the pluton
into parallelism with the contact.
Discordant to Concordant

Classification related to tectonic timing of intrusion

Related to Orogenic / subduction processes
Post-tectonic
After a metamorphic event, cross-cut deformation (lack deformational features ass. w/ met event) and overprint metamorphism

Syn-tectonic
During orogenic event.
Related to regional metamorphism foliation,continuous w/ emplacement
True granites associated with migmatites and gneiss

Pre-tectonic (rare)
Emplacement prior to an orogenic event. Imprinted by met.
MOR or Arc-related volcanism/plutonism

Additional Classification related to depth of intrusion

Pressure and Temperature of country rock

Epizonal : Brittle and Cool country rock
Shallow level < 8 Km (~3 kb, 0.3 Gpa)
Low T - <300 °C
Post-tectonic
Discordant – sharp contacts, tend to be smaller surface exposure
Contact metamorphic zone (high T low P)

Mesozonal – Low grade regional metamorphic rocks
Transitional (5-15 km, 300-500 °C)
Sharp-gradational contact; discordant or concordant
syn- or post- tectonics

Catazonal: Regional Metamorphism, hi-P and T (migmatites)
Deep Crustal levels >10 km
High T (> 400 -600°C)
Syn-tectonic, gradational contacts with no chilling margins
Conformable, deformation common

Magma Ascent and emplacement

Diapir

Diapir: a mobile mass that rises andmpierces the layers above it.
Diapiric Rise Model: Rising through ductile country rocks or rely on fractures or other weaknesses in the rocks