Elsevier

Lithos

Volume 125, Issues 1–2, July 2011, Pages 675-692
Lithos

The mafic–silicic layered intrusions of Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands), Armorican Massif: Gabbro–diorite layering and mafic cumulate–pegmatoid association

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Abstract

The Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands) Intrusive Complexes (hereafter referred to as SJIC and NGIC, respectively) are examples of mafic–silicic layered intrusions in the Armorican Massif. Both are characterized by the occurrence of (1) a basal/peripheral gabbroic unit interlayered with sheets (generally dioritic in composition, occasionally gabbroic) and crossed by leucocratic diapirs and pipes (from monzodioritic to Q-monzonitic in composition), (2) peripheral pegmatoids associated with mafic cumulates and (3) coeval granitoids. Beside these main similarities, some contrasted features lead us to propose two distinct models of formation. The Variscan SJIC includes tholeiitic mafic rocks (monzogabbro) that locally mingle and mix with leucocratic components (monzonite or Q-monzonite). The Cadomian NGIC is calc-alkaline. The SJIC sheet-bearing gabbro is homogeneous from a petrologic point of view, whereas the NGIC exhibits gabbroic macrorhythmic sequences with mineral layering. The Sr–Nd isotopic compositions of the SJIC gabbros are significantly different from those of the associated dioritic layers. This is not the case in the NGIC where the magmas could be cogenetic. We argue that the SJIC gabbro was a liquid that crystallized in situ without significant crystal settling. By contrast, the rhythmic sequences of the NGIC are consistent with crystal accumulation. Subsequently, both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition. We interpret the sheets in the SJIC as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below, at levels of neutral buoyancy. Injections and convection in the central part of the reservoir possibly resulted in spectacular mixing/mingling structures. In the NGIC, the emplacement of the dioritic sheets was rather controlled by pre-existing rhythmic cumulative structures. In both intrusions, late differentiated diapirs were extracted from the dioritic sheets. Associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase. This extraction might have been enhanced by the disruption of the peripheral cumulate stack, perhaps following pressure drops.

Research highlights

► The SJIC and NGIC are mafic–silicic layered intrusions in the Armorican Massif. ► At some stages, both were mafic reservoirs invaded by intermediate magmas. ► Layering can result from spreading of dioritic magma within a crystallizing gabbro. ► Extraction of late diapirs results in cumulative textures in dioritic layers. ► Associated pegmatoids correspond to liquids issued from a mafic intercumulus melt.

Introduction

The term MASLI (MAfic–Silicic Layered Intrusion), as first proposed by Wiebe (1993a, b), refers to plutonic complexes with interlayered mafic (gabbroic) and intermediate/felsic (dioritic to granitic) rocks (Wiebe, 1996, Franceschelli et al., 2005). MASLIs can be recognized by distinctive field relationships which always include (i) lobate contacts between the main gabbro-dioritic intrusions and surrounding or underlying granitic/granodioritic plutons and (ii) layers, sheets, diapirs, and/or veins of dioritic/silicic material into gabbroic units (Wiebe, 1996). Other features commonly (but not always) described in such complexes are: (iii) macrorhythmic gabbro-dioritic units, from less than one to several tens of meters thick, with chilled bases (Wiebe, 1993b, Wiebe, 1994, Wiebe, 1996, Waight et al., 2007), (iv) modal layering and feldspar lamination in the dioritic/silicic layers (Wiebe, 1996), (v) mafic/intermediate enclaves in the granites (Wiebe, 1994, Wiebe et al., 1997), and (vi) basaltic pillow-like chilled bodies within felsic units (Wiebe, 1974, Wiebe, 1993b, Wiebe et al., 2001). Although differences exist between the examples, Wiebe and others first proposed a comprehensive model to account for these features: repeated mafic injections into a felsic chamber, from dioritic to granitic composition, followed by fractional crystallization and complex liquid–liquid (and/or partially crystallized magma–magma) interactions including small scale diapirism and hybridization. More recently, MASLIs were recognized to result from multiple replenishments of mafic and felsic magmas, mingling and limited mixing, and rejuvenation of granite (Wiebe et al., 2007).

If the Cadomian layered gabbro–diorite complex of North-Guernsey (Channel Islands) was suspected by Wiebe (1996) to be a MASLI, the Variscan intrusion of Saint-Jean-du-Doigt (France) was never considered as such. However, both Armorican plutonic bodies display many of the features characteristic of mafic–silicic layered intrusions. Additionally, both complexes show spectacular pegmatoid (gabbroic pegmatite) occurrences systematically associated with coarse-grained mafic cumulates. Such a lithologic association has already been described in various gabbroic intrusions (Smartville intrusive complex, California: Beard and Day, 1986; Kraemer macrodyke, Greenland: Momme and Wilson, 2002; Mount Sheridan gabbro, Oklahoma: McEllen, 2006), but not specifically in MASLIs. In spite of their similarities, each Armorican intrusion shows distinctive petrological and geochemical (isotopic) characters.

The aim of this study is to reexamine the published models of MASLI formation through these two new examples from the Armorican Massif, which display both shared and contrasted features. This leads us to propose distinct processes of construction emphasizing the possibility that replenishment of mafic chambers by intermediate partially crystallized magmas explains some features of MASLIs.

Section snippets

Analytical techniques

Compositions of mineral phases were obtained with a Cameca SX50 automated electron microprobe (Microsonde Ouest, Brest). Analytical conditions were 15 kV, 15 nA, counting time 6 s, correction by the ZAF method. Concentrations of < 0.3% are considered qualitative.

Major and trace element compositions (Table 1) were measured at the University of Brest on whole rock powders by inductively coupled plasma atomic emission spectrometry (ICP-AES). Analytical methods are described in Cotten et al. (1995).

Geology

The Saint-Jean-du-Doigt Intrusion (SJIC) is located between the Trégor Cadomian domain, mainly composed of volcanoclastic formations overlying a Palaeoproterozoic (Icartian) gneissic basement (c. 2.0 Ga, Auvray et al., 1980), and the Léon Hercynian metamorphic domain (Fig. 1a). The SJIC is composite and includes four main units: the Primel cumulate–pegmatoid association, the Saint-Jean-du-Doigt monzogabbro–(Q-)monzonite mingled/mixed rocks, the Poul Rodou layered gabbro–diorites and several

Geology

Guernsey (Channel Islands), about 50 km off the French coast, is part of the Armorican Massif (Fig. 1a). The island is divided into two main geological domains. The Southern Metamorphic Complex mainly consists of Palaeoproterozoic gneisses (including the Icart orthogneiss, with a magmatic U–Pb/zircon age of 2061 ± 2 Ma, Samson and D'Lemos, 1998), which were later intruded by the deformed Neoproterozoic Perelle Q-diorite (Samson and D'Lemos, 1999). The Northern Neoproterozoic Intrusive Complex –

Geochemical sections through gabbro–diorite units

Vertical variations of MgO, TiO2 and Al2O3 through four gabbro-dioritic sections are shown in Figs. 8 (SJIC) and 9 (NGIC). The chemical composition of the Poul Rodou gabbro is relatively constant, above and below the diapir-producing leucocratic sheets, for all the sections (Fig. 8). Only TiO2 varies significantly, from 1.18 (SJ34g) to 1.91 wt.% (SJ43f). This relative chemical homogeneity of the mafic body is a rather original feature with respect to many other MASLIs (e.g., Wiebe, 1996). By

MASLI-type reservoirs: the models

Wiebe and others have defined the concept of MASLI and studied many examples of such intrusions, especially in Maine. A MASLI petrologically corresponds to a composite layered sequence of alternating gabbroic and dioritic units situated between or below granitic bodies (Chapman and Rhodes, 1992, Wiebe, 1996, Wiebe and Collins, 1998). Variants are intrusions of bimodal gabbro–diorite (e.g., Ingonish, Cape Breton Island: Wiebe, 1974) or diorite–granite (e.g., Nord-Forez, Massif Central, France:

Conclusions

  • (1)

    The Saint-Jean-du-Doigt (SJIC) and the North-Guernsey (NGIC) Intrusive Complexes are two MAfic–Silicic Layered Intrusions (MASLI) of the Armorican Massif. Both are triple component complexes (gabbro, diorite, and granite) characterized by the occurrence of pegmatoids in association with cumulates and of gabbroic units including diapir-producing dioritic sheets. The NGIC is Cadomian and calc-alkaline. Mafic rocks of the Hercynian SJIC are tholeiitic.

  • (2)

    Pegmatoids are interpreted as liquids

Acknowledgements

We are grateful to P.-M. Le Dantec for field assistance (Primel and Roc'h Louet cartography). Detailed and constructive comments by Dr. P. Barbey and an anonymous reviewer helped us to improve the manuscript. Pertinent suggestions of A. Brink are gratefully acknowledged. We also thank Dr. C.G. Barnes for discussions and Dr. N. Eby for editorial assistance.

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    1

    Present address: Department of Geosciences, Texas Tech University, Lubbock, TX 79409, USA.

    2

    Present address: IPGP, Laboratoire de Géosciences Marines, 4, Place Jussieu 75252 Paris cedex 05, France.

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