Jotunite

Jotunite: A member of the charnockitic rock series equivalent to orthopyroxene monzonorite in QAPF diagram. From Jotunheim, Norway.

Charnockites, Opx-bearing (or more rarely fayalite-bearing) granitic rocks, make up only a tiny fraction of the granitic rocks worldwide. Despite their comparative rarity, over the past twenty-five years these rocks have been the subject of a significant number of papers. The name charnockite was proposed by Holland (1900) to describe hypersthene-bearing granitic rocks. The name charnockite comes from Job Charnock, the founder of Calcutta, whose tomb is made of this rock.

Part of the confusion about charnockites is caused by the fact that the term charnockite refers both to a series of igneous rocks and a mineral assemblage that is similar to a metamorphic facies. Charnockites are characteristic of granulite terranes, just as the highest-grade portions of amphibolite facies are often associated with biotite or hornblende granites. As a result, for many workers the word charnockite has become synonymous with granulite, producing a wealth of terminological imprecision. Most of the terms (Fig.1) for individual rock types belonging to this series, including enderbite (Opx bearing tonalite), opdalite (Opx-bearing granodiorite), man gerite (Opx-bearing monzonite) and jotunite (Opx-bearing monzodiorite), are not widely used.

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Fig.1: QAP diagram showing the names of rocks in the charnockite series of igneous rocks. From B. Ronald Frost (2007).




Rogaland Igneous Province

The anorthosite-mangerite-charnockite(-granite) (AMCG or AMC) association forms huge igneous complexes that are accompanied by minor amounts of mafic to intermediate (jotunite) rocks. AMCG complexes form a broad belt of Proterozoic age (1.7–0.9 Ga), trending from eastern through northern Europe into eastern North America (e.g.Wiebe, 1992). The petrogenetic model generally accepted for these complexes includes anorthosite formation by extensive crystallization of a basaltic to picritic, mantle-derived magma in a chamber at or near the base of the continental crust, and diapiric uprise of the crystallization products up to higher crustal levels. In this scheme, it has also been argued that:

• The voluminous acidic plutons associated with anorthosites mostly formed by anatexis at the base of the crust;
• Jotunites found in AMCG complexes were residual liquids left after anorthosite crystallization;
• Contamination of the anorthosites and related rocks by crustal material took place in the early crystallization stage, in the deep-seated magma chamber, and possibly also up to final emplacement depth (usually in high-grade, granulite-facies gneisses).

The Rogaland Igneous Province is situated at the southwestern end of the exposed Sveconorwegian orogenic belt. The Sveconorwegian belt of Fennoscandia is generally correlated with the Grenvillian belt of Laurentia. Both belts formed during a polyphase orogenic event between 1.25 and 0.90 Ga, at the transition between the Meso- and Neoproterozoic.

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Fig.2: The Rogaland anorthosite province. Bs: Bøstølen intrusion; Bf: Pegmatitic norite of the Blåfjell deposit; Hg: Hogstad noritic intrusion; K: Koldal noritic intrusion; S: Storgangen deposit; T: Tellnes dyke; Vb: Varberg dyke; L: Lomland dyke; Vt: Vettaland dyke; Vs: Vaersland dyke. From Michot 1960.




The Rogaland Anorthositic Province is one of the AMCG (Anorthosite–Mangerite–Charnockite–Granite) suites, which are characteristic of the Proterozoic (Ashwal, 1993). It consists of three large massif-type anorthosites (Egersund-Ogna, Håland-Helleren, Åna-Sira), a layered intrusion (Bjerkreim-Sokndal) and two smaller leuconoritic bodies (Hidra and Garsaknatt).

The Egersund-Ogna massif anorthosite is essentially made up of granulated homogeneous plagioclase with some megacrysts of high-Al orthopyroxene and plagioclase. The Håland-Helleren massif consists of two distinct units: the Håland intrusion, a foliated anorthosite–leuconorite comparable to the foliated marginal zone of Egersund-Ogna body; the Helleren massif dominated by coarse-grained anorthosite.
The Bjerkreim-Sokndal layered intrusion comprises a lower part exhibiting a series of modally layered anorthosite–(leuco)norite–gabbronorite cumulates overlaid by essentially massive mangerites, quartz mangerites and charnockites.

Jotunitic rocks mostly occur as dikes or small intrusions: Eia-Rekefjord, Lomland (L), Tellnes (T), Varberg (Vb), Vaersland (Vs), Vettaland (Vt) crosscutting the massive anorthosites or form chilled margins of the leuconoritic bodies (Hidra and Garsaknatt) and the Bjerkreim-Sokndal intrusion. The Jotunitic dykes vary in thickness from a few decimetres to several tens of metres, and extend up to 20 km along strike.

The Jotunite suite

Proterozoic massif anorthosites are usually associated with variable amounts of a characteristic suite of intermediate rocks. The least evolved rocks of this suite are enriched in mafic minerals (low- and high-Ca pyroxenes, Fe-Ti oxides, apatite), and in some cases very high concentrations of these phases give rise to extremely mafic-rich rocks. Different names, including ferrodiorite, monzonorite, jotunite, Fe-Ti-P-rich rocks (FTP) or oxide-apatite gabbronorite, have been used.
However, Vander Auwera et al. (1998) referred to them by the collective term jotunite (Fe-Ti-Prich hypersthene monzodiorite). Evolved rocks of the suite include mangerite (hypersthene monzonite), quartz mangerite (hypersthene quartz monzonite) and charnockite (hypersthene granite).

The origin of jotunites has been the subject of considerable debate, despite their similar textural and geochemical characteristics from one anorthositic complex to another. Several hypotheses, not mutually exclusive, have been proposed:

• Jotunites are residual liquids after anorthosite crystallization
• Jotunites are products of partial melting of the lower crust with necessary heat produced by anorthosite emplacement
• Jotunites are transitional rocks in a comagmatic sequence from anorthosite to mangerite
• Jotunites are derived by fractionation of mafic magmas unrelated to the anorthositic
• Jotunites are immiscible liquids conjugate to mangerites

The Rogaland jotunites, first briefly described by Michot (1960), were extensively studied by Duchesne, and co-workers. Their petrography can be summarized as follows: the rocks are usually fine- to medium-grained (from 0.1mm to a few millimetres). They contain plagioclase (usually antiperthitic), Ca-poor and Ca-rich pyroxenes, ilmenite, magnetite, and apatite (plus some perthitic potassium feldspar, as well as minor quartz).

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Late-stage charnockite dykes cutting anorthosite, Rogaland, Norway. From Wikipedia.




Sample kindly given to me by Robert Gill.

Bibliography



• O.Bolle; D.Demaiffe, J.C. Duchesne (2003): Petrogenesis of jotunitic and acidic members of an AMC suite (Rogaland anorthosite province, SW Norway): a Sr and Nd isotopic assessment. Precambrian Research 124 (2003) 185–214
• J.V. Auwera; J.Longhi; J.C. Duchesne (1998): A liquid line of descent of the jotunite (hypersthene monzodiorite) suite. J. Petrol. 39, 439-468.


Photo
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Pyroxene, ilmenite and plagioclase crystals in a Jotunite. Norway. PPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. PPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase crystals in a Jotunite. Norway. PPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase crystals in a Jotunite. Norway. PPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase and apatite crystals in a Jotunite. Norway. PPL image. 2x (Field of view = 7mm)
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Pyroxene (Opx in gray), ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite, apatite and plagioclase and apatite crystals in a Jotunite. Norway. PPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite, apatite and plagioclase and apatite crystals in a Jotunite. Norway. PPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite, apatite and plagioclase crystals in a Jotunite. Norway. PPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene (with Exsolution Lamellae), ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene (with Exsolution Lamellae) and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene (with Exsolution Lamellae) and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene, ilmenite, apatite and plagioclase crystals in a Jotunite. Norway. PPL image. 2x (Field of view = 7mm)
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Pyroxene (with Exsolution Lamellae) and plagioclase with Antiperthite unmixing (the darker spots) crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene (with Exsolution Lamellae) and plagioclase crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene (with Exsolution Lamellae) and plagioclase crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Pyroxene (with Exsolution Lamellae) and plagioclase crystals in a Jotunite. Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 2x (Field of view = 7mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (note the difference in relief) in a Charnokite, Rogaland Province, Norway. PPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 20x (Field of view = 1mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 20x (Field of view = 1mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 20x (Field of view = 1mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 20x (Field of view = 1mm)
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Antiperthite unmixing in Plagioclase (note that the anthiperthite have within them perthite unmixing) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(29).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
antipertite(34).jpg

Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)
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Antiperthite unmixing in Plagioclase (the darker spots) in a Charnokite, Rogaland Province, Norway. XPL image. 10x (Field of view = 2mm)