Contact and shape of plutons

And some enigmatic features…

Contacts

• Sharp contacts

• Deformed/synfolial contacts

• Progressive contacts

Sharp contacts

Torres del Paine, S. Chile

2.7 Ga Kangaatsiaq granite, W. Greenland

Contact aureole

Flamanville, France

Chilled margin

Figure 4-29Figure 4-29. . Continuity of foliation across an igneous contact for a pre- or syn-tectonic Continuity of foliation across an igneous contact for a pre- or syn-tectonic pluton. From Compton (1962), pluton. From Compton (1962), Manual of Field GeologyManual of Field Geology. © R. Compton. . © R. Compton.

Breccias, dykes, transition zones

2.9 – 2.7 Ga granites

3.1 Ga granites & syenites

Moodies

Fig Tree

Onverwacht

Ca. 3.2 Ga TTG

Ca. 3.4 Ga TTG

« Ancient gneisses »(3.6 – 3.4 Ga)

Geology of the Nelshoogte pluton

5 km

Jamestown Schist belt

Nelshoogte pluton

Intrusive breccias

2.9 – 2.7 Ga granites

3.1 Ga granites & syenites

Moodies

Fig Tree

Onverwacht

Ca. 3.2 Ga TTG

Ca. 3.4 Ga TTG

« Ancient gneisses »(3.6 – 3.4 Ga)

Dyke swarms

Figure 4-28Figure 4-28.. Marginal foliations developed within a pluton as a result of differential motion Marginal foliations developed within a pluton as a result of differential motion across the contact. From Lahee (1961), across the contact. From Lahee (1961), Field Geology. © Field Geology. © McGraw Hill. New York. McGraw Hill. New York.

Deformed contacts

Sea Point Contact between granite and Malmesbury metasediments

Figure 4-27Figure 4-27.. Gradational border zones between homogeneous igneous rock (light) and Gradational border zones between homogeneous igneous rock (light) and country rock (dark). After Compton (1962), country rock (dark). After Compton (1962), Manual of Field GeologyManual of Field Geology. © R. Compton. . © R. Compton.

Progressive contacts

Contacts can be combinations of…

• Injection

• Deformation

• Melting/in situ reactions

Things to consider on Wednesday!

Enclaves

• Source-related– MME (microgranular mafic enclaves)– Restites (« surmicaceous » enclaves)

• Emplacement related– Xenoliths– Microgranular felsic enclaves (« chilled

margin »)

MME

Hoedjiespunt granite (540 Ma), Saldanha, RSA

Vredenburg granite (540 Ma), Britannia Bay, RSA

MME

Hoedjiespunt granite (540 Ma), Saldanha, RSA

Surmicaceous?

Hoedjiespunt granite (540 Ma), Saldanha, RSA

Surmicaceous?

Xenolith

Hoedjiespunt granite (540 Ma), Saldanha, RSA

Xenolith

Microgranular felsic

Hoedjiespunt granite (540 Ma), Saldanha, RSA

Table 18-1. The Various Types of Enclaves

Name Nature Margin Shape Features

Xenolith piece of country sharp to angular contact metamorphicrocks gradual to ovoid texture and minerals

Xenocryst isolated foreign sharp angular corrodedcrystal reaction rim

Surmicaceous residue of melting sharp, lenticular metamorphic texture Enclave (restite) biotite rim micas, Al-rich minerals

Schlieren disrupted enclave gradual oblate coplanar orientation

Felsic Micro- disrupted sharp to ovoid fine-granied granular Enclave fine-grained margin gradual igneous texture

Mafic Micro- Blob of coeval mostly ovoid fine-granied granular Enclave mafic magma sharp igneous texture

Cumulate Enclave disrupted mostly ovoid coarse-grained (Autolith) cumulate gradual cumulate texture

After Didier and Barbarin (1991, p. 20).

Table 18-1. Didier, J. and Barbarin (1991) The different type of enclaves in granites: Nomenclature. In J. Didier and B. Barbarin (1991) (eds.), Enclaves in Granite Petrology. Elsevier. Amsterdam, pp. 19-23.

Try to sketh two different contacts:

• A shallow intrusive, emplaced in a low strain situation

• A deep intrusive, syntectonic

Studying pluton shapes

• Map pattern

• Internal structures

• Geophysics

Kangaatsiaq

Kangaatsiaq pluton, W. Greenland

Kangaatsiaq

Site jfm-089

Granite

Orthogneiss

Interstratified amphibolites and Bt-gneisses

A key outcrop

NW SE

Synthetic cross-section

Flat pluton (subsequently folded)

Gravitationnal studies

Bouguer anomaly Depth of pluton floor

Internal structures

Coupling different approaches: Cabeza de Araya, Spain

Cabeza de Araya

Depth of pluton floor vs. Lineation plunge

Cabeza de Araya

Link with chemical composition

Shape of plutons

• Mostly flat (contrarily to what you might think…)

• Sometimes wedge shaped

• Rare « vertical » intrusions

• First order approximation: they reflect emplacement stress field

Figure 4-30Figure 4-30.. Block diagram several kilometers across, illustrating some relationships with Block diagram several kilometers across, illustrating some relationships with the country rock near the top of a barely exposed pluton in the epizone. The original the country rock near the top of a barely exposed pluton in the epizone. The original upper contact above the surface is approximated by the dashed line on the front plane. upper contact above the surface is approximated by the dashed line on the front plane. From Lahee (1961), From Lahee (1961), Field Geology. © Field Geology. © McGraw Hill. New York. McGraw Hill. New York.

Structures and Field Relationships

Septum RoofPendant

OffshootCupola

2.9 – 2.7 Ga granites

3.1 Ga granites & syenites

Moodies

Fig Tree

Onverwacht

Ca. 3.2 Ga TTG

Ca. 3.4 Ga TTG

« Ancient gneisses »(3.6 – 3.4 Ga)

Flat plutons

3.1 Ga Mpuluzi batholith, Barberton

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Laccoliths

a b

Sills

Karoo dolerites

Most intrusions are rather flat

Wedge-shaped intrusions

Closepet, 2.5 Ga, South India

« Flat » and « long » plutons

Figure 4-20Figure 4-20. . Schematic block diagram of some intrusive bodies. Schematic block diagram of some intrusive bodies.

Volcanic Neck

Sill

Stock

Sill Dike

Dike

Stock

Radial Dikes

Ring Dike

Batholith

“Vertical” intrusions

Pluton shapes reflect strain pattern…

1

3

Batholiths

• Plutons associations

• Definition not too clear

• Old litterature: « primordial » granitic level, somewhere down.

• Modern conceptions: associations of individual plutons, each with its shape and structures

• Probably rather flat as well!

Figure 4-20Figure 4-20. . Schematic block diagram of some intrusive bodies. Schematic block diagram of some intrusive bodies.

Volcanic Neck

Sill

Stock

Sill Dike

Dike

Stock

Radial Dikes

Ring Dike

Batholith

“Vertical” intrusions

An “old” view of a batholith

Figure 17-16. Schematic cross section of the Coastal batholith of Peru. The shallow flat-topped and steep-sided “bell-jar”-shaped plutons are stoped into place. Successive pulses may be nested at a single locality. The heavy line is the present erosion surface. From Myers (1975) Geol. Soc. Amer. Bull., 86, 1209-1220.

Figure 4-36Figure 4-36.. Diagrammatic cross section of the Boulder Batholith, Montana, prior to Diagrammatic cross section of the Boulder Batholith, Montana, prior to exposure. After Hamilton and Myers (1967), The nature of batholiths. exposure. After Hamilton and Myers (1967), The nature of batholiths. USGS Prof. PaperUSGS Prof. Paper, , 554-C, c1-c30. 554-C, c1-c30.

Modern conceptions: flat batholiths

10 kilometers

SSW NNE

Gneissic Country Rocks

Boulder Batholith

Coeval Volcanics