Contact and shape of plutons And some enigmatic features…

Contacts Sharp contacts Deformed/synfolial contacts Progressive contacts

Sharp contacts Torres del Paine, S. Chile

Contact - Hoedjiespunt granite and Saldanha quartz porphyry Saldanha quartz porphyry (515Ma) Hoedjiespunt granite (552Ma)

2.7 Ga Kangaatsiaq granite, W. Greenland

Contact aureole Flamanville, France

Chilled margin

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

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

Nelshoogte pluton Jamestown Schist belt

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

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

Sea Point Contact between granite and Malmesbury metasediments

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

Contacts can be combinations of… Injection Deformation Melting/in situ reactions Things to consider on Wednesday!

Dome & keel structures in the Barberton granitoid-greenstone terrain Domes: 3.4 & 3.2 Ga gneissic TTG`s Concentric foliation patterns concordant with foliation in the surrounding supracrustals Keels: tightly folded synformal supracrustals with TPC`s. 5 km After Anhaeusser et al. (1981) Anhaeusser (2001).

Enclaves Source-related Emplacement related MME (microgranular mafic enclaves) Restites (« surmicaceous » enclaves) Emplacement related Xenoliths Microgranular felsic enclaves (« chilled margin »)

MME Hoedjiespunt granite (540 Ma), Saldanha, RSA

MME Vredenburg granite (540 Ma), Britannia Bay, RSA

Surmicaceous? Hoedjiespunt granite (540 Ma), Saldanha, RSA

Surmicaceous? Hoedjiespunt granite (540 Ma), Saldanha, RSA

Xenolith Hoedjiespunt granite (540 Ma), Saldanha, RSA

Xenolith

Microgranular felsic Hoedjiespunt granite (540 Ma), Saldanha, RSA

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 pluton, W. Greenland

amphibolites and Bt-gneisses A key outcrop Kangaatsiaq Granite Interstratified amphibolites and Bt-gneisses Orthogneiss Site jfm-089

Synthetic cross-section NW SE Flat pluton (subsequently folded)

Gravitationnal studies Depth of pluton floor Bouguer anomaly

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

Structures and Field Relationships Figure 4-30. 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 upper contact above the surface is approximated by the dashed line on the front plane. From Lahee (1961), Field Geology. © McGraw Hill. New York.

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

Sills Karoo dolerites

Most intrusions are rather flat

Wedge-shaped intrusions Closepet, 2.5 Ga, South India

« Flat » and « long » plutons

“Vertical” intrusions Figure 4-20. Schematic block diagram of some intrusive bodies.

Pluton shapes reflect strain pattern…

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!

“Vertical” intrusions Figure 4-20. Schematic block diagram of some intrusive bodies.

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.

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