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Petrology and geochemistry of the Lyngdal granodiorite (Southern Norway) and the role of fractional crystallisation in the genesis of Proterozoic ferro-potassic A-type granites

TL;DR: In this article, the petrography and geochemistry (major and trace elements, Sr-Nd isotopes) of the Lyngdal granodiorite and associated massifs are presented.
About: This article is published in Precambrian Research.The article was published on 2003-07-01 and is currently open access. It has received 74 citations till now. The article focuses on the topics: Anorthosite & Mafic.

Summary (5 min read)

1. Introduction

  • Granitoids associated with anorthosite massifs are classified in the ilmenite-series (Frost & Frost, 1997).
  • This granite has been linked to the rapakivi group by Dall’Agnol et al. (1999a) on the basis of geochemical arguments.
  • The AMC suite has already been the subject of a considerable attention (e.g. Longhi et al., 1999 and references therein) but modern data are lacking for the HBG suite.
  • The Lyngdal granodiorite is a huge massif (ca. 300 km2) that, together with small granitoid bodies (the Tranevåg massif and the Red Granite), forms the southernmost outcropping massifs of the HBG suite (Fig. 1).

2. Geological setting

  • The Sveconorwegian province forms the south-western part of the Baltic shield (inset of Fig. 1) and is the result of two important orogenies: the Gothian orogeny (1750–1500 Ma) and the Sveconorwegian orogeny (1250–950 Ma).
  • This province is made up of several terranes (Bingen et al., 2001) separated by major North-South faults and shear zones.
  • This phase of regional metamorphism shortly follows an important syn-kinematic calc-alkaline magmatism dated at 1050+2/−8 Ma (U–Pb on zircon) in the Rogaland-Vest Agder sector (Bingen and van Breemen, 1998a).
  • A high-temperature/lowpressure static metamorphism in granulitic facies is dated at 930–925 Ma (Bingen and van Breemen, 1998b).
  • This HBG suite is dominated in Vest Agder by large plutons (e.g. Lyngdal and Svöfjell intrusions, Fig. 1) stretching along the MandalUstaoset Line ( Vander Auwera et al., 2003).

3.1. Field relationships

  • The Lyngdal granodiorite (Fig. 2) and the associated granitoids (Tranevåg and the Red Granite) commonly display a syn-magmatic foliation well shown by the mafic minerals.
  • The very homogeneous Lyngdal granodiorite contains two kinds of enclaves: (1) angular gneissic enclaves (a few decimetres to several tens of metres in size) locally very abundant near the contacts and (2) oblate mafic microgranular enclaves with an average size of ca. 20 cm.
  • A porphyritic granite, called the Red Granite due to its coloured alkali feldspar, outcrops near the Tranevåg massif (Fig. 2).
  • Most enclaves are gneissic but some are also oblate mafic microgranular enclaves of metre size.

3.2.1. Lyngdal and Tranevåg massifs

  • Most of the rocks are porphyritic with plagioclase and alkali feldspar phenocrysts up to 2 cm.
  • The matrix consists of the same minerals plus quartz and aggregates of mafic minerals (amphibole, biotite, opaques, apatite, zircon, titanite, ±allanite).
  • Amphibole is a hornblende (magnesio-hastingitic, edenitic, ferro-edenitic hornblendes: Leake, 1978) and is anhedral to euhedral ( Table 2).
  • Plagioclase is the first tectosilicate to crystallise, K-feldspar and quartz being late in the sequence of crystallisation.

3.2.2. The Red Granite

  • This is a porphyritic granite with red K-feldspar (perthitic microcline) and plagioclase as phenocrysts ; myrmekites are widespread.
  • The matrix is composed of the same minerals with quartz and scarce biotite often chloritised.
  • Accessory minerals include opaques, apatite, zircon, allanite and fluorite.
  • Fluorite is usually interleaved with biotite and seems to replace it.

3.2.3. Mafic microgranular enclaves

  • The enclaves have an equigranular structure (average grain size 0.5 mm), with elongated anhedral to euhedral plagioclase, amphibole and biotite.
  • Amphibole and plagioclase are rich in oxides and apatite needles.
  • The mineral composition is similar to those of Lyngdal–Tranevåg massifs: plagioclase is weakly zoned (An25), XFe is around 0.38 in biotite and around 0.46 in amphibole.
  • Some euhedral plagioclases show an irregular overgrowth rim.
  • These microstructures (elongate grain shapes, apatite needles, plagioclase overgrowths) reflect an igneous origin (Vernon, 1991).

3.3. Estimation of intensive parameters from mineral equilibria

  • Many experimental and empirical calibrations have been done to estimate pressure with the Al-in-hornblende geobarometer (Anderson and Smith, 1995 for a review).
  • Studied granitoids contain the appropriate mineral assemblage to use these geobarometers (see Section 3.2).
  • Amphiboles from the Lyngdal granodiorite give a pressure of ca. 4 kbar with the experimental calibration of Johnson and Rutherford (1989) while the calibration of Schmidt (1992) gives pressure higher by 1.3 kbar due to the temperature effect discussed by Anderson and Smith (1995).
  • Amphiboles from the Tranevåg massif give a slightly lower pressure (P<3.6 kbar) than those from the Lyngdal granodiorite.
  • The stability of the assemblage titanite, magnetite and quartz implies that fO2 was at least NNO (Wones, 1989).

4. Analytical methods and selected samples

  • Analysed samples include the Lyngdal granodiorite, the Tranevåg massif, the Red Granite and two samples of mafic enclaves (VDA9912 and MB2002).
  • Minerals have been analysed with the Cameca SX50 electron microprobe of the CAMST (‘Centre d’Analyse par Microsonde pour les Sciences de la Terre’, Louvain-La-Neuve) and the Cameca Camebax electron microprobe of ‘Services Communs BRGM-CNRS-UO, Orléans’.
  • For the Cameca SX50, the accelerating voltage was 15 kV and the beam current was 20 nA.
  • The analytical procedure for Sr–Nd isotopic compositions is described below.

5.1. Results for granitoids

  • Whole rock analyses (major and trace elements) are listed in Table 4. Fig. 5B show that the samples define a calc-alkaline trend in the Peacock diagram but the FeOt/MgO ratio is too high to be characteristic of the calc-alkaline series.
  • The enrichment in Zr and REE gives an A-type character to these granitoids after the geochemical classification of Whalen et al. (1987).
  • Some trace elements (Fig. 9 and Fig. 10) distinguish the Lyngdal and Tranevåg massifs from the Red Granite, in particular Sr and Th that are significantly more abundant in the Red Granite.

5.2. Results for mafic microgranular enclaves

  • These enclaves have a major and trace elements (Fig. 8 and Fig. 9) composition close to the gabbronorites ( Demaiffe et al., 1990), with slightly lower Al2O3 and CaO content and higher Fe2O3t and Na2O content.
  • In a MORBnormalised diagram ( Fig. 10), mafic microgranular enclaves and gabbronorites have a smoother pattern than granitoids, with only a weak negative anomaly in Nb–Ta.

6. Sr–Nd isotopic compositions

  • Seven samples from the Lyngdal granodiorite, five samples from the Tranevåg massif, one sample from the Red Granite and one mafic enclave sample have been selected for Sr and Nd isotopic analyses.
  • The Red Granite and sample VDA9920 being out of the Lyngdal–Tranevåg trend for some elements (Fig. 9 and Fig. 10) as well as for the Nd isotopic composition (see the following), the isochron built with all these samples is questionable.
  • A major observation is the decoupling between Sr and Nd isotopic initial ratios when comparing the gabbronorites and the granitoids: between 50 and 73 wt.% SiO2, Sri varies from 0.7038 to 0.7058, the variation being linked to plutons and not to silica.
  • In contrast with Sri, Nd for the gabbronorites is positive (as for the MME) while the Nd for the granitoids is negative, as previously said.

7.1. The mafic microgranular enclaves

  • Field and petrographical observations suggest that the mafic microgranular enclaves have a magmatic origin.
  • Moreover, they have isotopic and geochemical compositions similar to the coeval gabbronorites ( Demaiffe et al., 1990).
  • Enclaves are, however, slightly richer in Na, Rb, K, Fe, Zn, Mn and lower in Ca and Al than the gabbronorites, indicating that chemical exchange may have occurred between the enclaves and its host magma, as often observed.
  • The presence of hydrous minerals in the enclaves could be due to a higher H2O-content of the magma before it mingled with the granitic magma.

7.2. The granitoids

  • Fig. 8, Fig. 9 and Fig. 10 show that the most felsic samples (above 63 wt.% SiO2) of the Lyngdal–Tranevåg trend are very similar to Proterozoic metaluminous A-type granites (rapakivi granites).
  • Authors studying Proterozoic rapakivi granites favour a crustal source ( Rämö and Haapala, 1995).
  • The protolith of the Jamon Granite, which is similar to the granitic samples of the Lyngdal–Tranevåg trend and which has similar fO2 and H2O content (Dall’Agnol et al., 1999b and Bogaerts et al., 2001), is believed to be an Archean quartz diorite ( Dall’Agnol et al., 1999c).
  • The Lyngdal complex differs from the other Proterozoic granitoids by the overwhelming proportion of granodioritic rocks over granites.

7.2.1. Origin of the Lyngdal–Tranevåg trend

  • The linear or pseudo-linear trends displayed by the granitoids could have been produced by: (1) restite unmixing, (2) mixing between two magmas, (3) partial melting, (4) crystal fractionation.
  • 2.1.1. Restite unmixing Mafic microgranular enclaves and relics of pyroxene in amphiboles have been interpreted as evidence of restites in granitoids by White and Chappell (1977) and Chappell et al. (1987).
  • The mafic microgranular enclaves have the composition of coeval mafic magmatic rocks , which suggests a magmatic origin.
  • Secondly, the compositions of the ferromagnesian minerals, and more precisely of clinopyroxene, have been reproduced in a series of crystallisation experiments performed on two samples from Lyngdal ( Bogaerts et al., 2001).

7.2.2. Modelling of a liquid line of descent

  • In order to better test and quantify the fractional crystallisation process, mass-balance calculations were performed using the least-squares method (e.g. Martin, 1987).
  • There are two possible models for stage 1.
  • The sums of the squared residual are low (Table 6) and the percentage of crystallisation is similar for the two models (32%).
  • The fractionating minerals are plagioclase, hornblende, magnetite, ilmenite and apatite for Model 1, the same minerals plus biotite in Model 2 and plus biotite and clinopyroxene in Model 3.
  • Major and trace elements thus show that the Lyngdal and Tranevåg granitoids belong to the same liquid line of descent.

7.2.3. Origin of the Lyngdal and Tranevåg granitoids

  • The Lyngdal granodiorite has already been investigated by Pb–Nd–Sr isotopic studies but without an associated geochemical investigation by major and trace elements (Weis, 1986; Demaiffe et al., 1986; Pedersen and Falkum, 1975 and Menuge, 1988) and related petrographical observations.
  • Duchesne and Demaiffe (1978) suggested that the Lyngdal granodiorite could have a genetic link with the anorthosites and would represent residual liquid from the differentiation of a jotunitic magma.
  • Demaiffe et al. (1986) and Weis (1986) stressed that if an origin by partial melting of the crust is a viable hypothesis, a mantle origin is also possible provided that some contamination occurs.
  • Vander Auwera et al. (2003) suggest that the HBG granitoids are derived from gabbronorites by fractional crystallisation with some assimilation.
  • The small differences in Sri between Lyngdal and Tranevåg could be explained by two distinct batches of gabbros evolving along similar liquid line of descent.

7.2.4. The Red Granite

  • The authors have only a limited number of Red Granite samples; hence the following interpretation will be only qualitative.
  • It cannot be considered as being an end product of the liquid line of descent modelled above.
  • The origin of fluorite in granitic magmas is subject to debate: some authors consider it as crystallising from a fluid and others from the melt itself (e.g. Collins et al., 1982 and King et al., 1997).
  • The petrographical observations indicate a hydrothermal alteration episode (sericitised plagioclase and chloritised biotite) but except for the geochemical differences described above, the trace element pattern of the Red Granite is similar to Lyngdal–Tranevåg one.
  • The higher LREE-Th–Rb–Sr and lower Nd content in the Red Granite is probably due to a different initial magma composition.

8. Conclusions

  • The Lyngdal granodiorite, and associated plutons (Tranevåg and the Red Granite) are ferropotassic A-type granitoids and belong to the post-collisional HBG suite of Southern Norway (see Vander Auwera et al., 2003).
  • This kind of association is similar to the rapakivi granitoids associated with anorthosite massifs and charnockites (the AMCG complexes: Emslie, 1991).
  • The Lyngdal pluton is indeed a huge masse of homogeneous granodiorite (SiO2 ranges between 60 and 65 wt.%), while the rapakivi are dominantly granitic in composition (>65 wt.%).
  • With Sr–Nd isotopes and major and trace elements modelling, their study shows that the granites of the Lyngdal–Tranevåg trend are derived by fractional crystallisation (without significant crustal assimilation) from the quartz monzodiorites.
  • These results can be extended to the whole HBG suite.

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Q1. What are the contributions mentioned in the paper "Petrology of the lyngdal granodiorite (southern norway) and the role of fractional crystallization in the genesis of proterozoic rapakivi-like granites" ?

In south-western Norway, the Sveconorwegian orogenic thickening ( 1024–970 Ma ) is followed by an important post-collisional magmatism ( 950–930 Ma ), divided in two suites ( Vander Auwera et al., 2003 ): the Anorthosite–Mangerite–Charnockite suite ( AMC suite ) and the Hornblende–Biotite Granitoids suite ( HBG suite ). This paper presents the petrography and geochemistry ( major and trace elements, Sr–Nd isotopes ) of the Lyngdal granodiorite and associated massifs ( Tranevåg and Red Granite massifs ) which belong to the HBG suite, although being very close to the anorthosite massifs. Mafic microgranular enclaves ( MME ), resulting from magma mingling, can be abundant and probably correspond to the parent magma of the studied plutons. This study shows that ferro-potassic A-type granitoids can be derived by fractional crystallisation from mafic magmas.