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Early Neoproterozoic Granulite Facies Metamorphism of Mafic Dykes From J. metamorphic Geol., 2014, 32, 1041–1062 doi:10.1111/jmg.12106 Early Neoproterozoic granulite facies metamorphism of mafic dykes from the Vestfold Block, east Antarctica X. C. LIU,1 W.-(R. Z.)WANG,1 Y. ZHAO,1 J. LIU1 AND B. SONG2 1Key Laboratory of Paleomagnetism and Tectonic Reconstruction of Ministry of Land and Resources, Institute of Geomechanics, Chinese Academy of Geological Sciences, Beijing 100081, China ([email protected]) 2Beijing SHRIMP Centre, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China ABSTRACT Proterozoic mafic dykes from the southwestern Vestfold Block experienced heterogeneous granulite facies metamorphism, characterized by spotted or fractured garnet-bearing aggregates in garnet- absent groundmass. The garnet-absent groundmass typically preserves an ophitic texture composed of lathy plagioclase, intergranular clinopyroxene and Fe–Ti oxides. Garnet-bearing domains consist mainly of a metamorphic assemblage of garnet, clinopyroxene, orthopyroxene, hornblende, biotite, plagioclase, K-feldspar, quartz and Fe–Ti oxides. Chemical compositions and textural relationships suggest that these metamorphic minerals reached local equilibrium in the centre of the garnet-bearing domains. Pseudosection calculations in the model system NCFMASHTO (Na2O–CaO–FeO–MgO– Al2O3–SiO2–H2O–TiO2–Fe2O3) yield P–T estimates of 820–870 °C and 8.4–9.7 kbar. Ion microprobe U–Pb zircon dating reveals that the NW- and N-trending mafic dykes were emplaced at 1764 Æ 25 and 1232 Æ 12 Ma, respectively, whereas their metamorphic ages cluster between 957 Æ 7 and 938 Æ 9 Ma. The identification of granulite facies mineral inclusions in metamorphic zircon domains is also consistent with early Neoproterozoic metamorphism. Therefore, the southwestern margin of the Vestfold Block is inferred to have been buried to depths of ~30–35 km beneath the Rayner oro- gen during the late stage of the late Mesoproterozoic/early Neoproterozoic collision between the Indian craton and east Antarctica (i.e. the Lambert Terrane or the Ruker craton including the Lam- bert Terrane). The lack of penetrative deformation and intensive fluid–rock interaction in the rigid Vestfold Block prevented the nucleation and growth of garnet and resulted in the heterogeneous granulite facies metamorphism of the mafic dykes. Key words: early Neoproterozoic; east Antarctica; granulite facies metamorphism; mafic dykes; P–T conditions; Vestfold Block. supercontinents (Fitzsimons, 2003; Harley, 2003; INTRODUCTION Yoshida et al., 2003; Zhao et al., 2003). Therefore, Old cratonic blocks and/or continental marginal the detailed investigation of the reworking processes basements may experience relatively young orogenic and mechanisms of each cratonic block is essential processes. In and near the late Mesoproterozoic/early for understanding the nature and role of the Rayner Neoproterozoic (i.e. Grenvillian) Rayner orogen orogeny. exposed in Kemp Land, MacRobertson Land and The Vestfold Block is an Archean/Palaeoprotero- Princess Elizabeth Land of east Antarctica, there zoic cratonic fragment located 15 km northeast of exist four Archean/Palaeoproterozoic cratonic blocks the Rauer Group. One of the most important that preserve different crustal histories (Harley, 2003; features of the Vestfold Block is the occurrence of Boger, 2011), including the Lambert Terrane in the several mafic dyke swarms of Proterozoic age (Black southern Prince Charles Mountains, the Napier Com- et al., 1991a; Lanyon et al., 1993). Furthermore, plex in Kemp Land and the Rauer Group and Vest- some of these mafic dykes experienced amphibolite fold Block in Prydz Bay (Fig. 1). Given that much of facies recrystallization and deformation (Collerson & the Rayner orogen was affected by late Neoprotero- Sheraton, 1986; Kuehner & Green, 1991; Passchier zoic/Cambrian (i.e. Pan-African) high-grade meta- et al., 1991), although the metamorphic age remains morphism and deformation, it remains a matter of unconstrained. However, careful petrographic obser- debate as to when these cratonic blocks were assem- vations indicate that mafic dykes from the southwest- bled (Harley et al., 2013; Liu et al., 2013, and refer- ern Vestfold Block underwent heterogeneous ences therein). This has led to different formation granulite facies metamorphism (Liu et al., 2013). This models of the proposed Rodinia and Gondwana provides an opportunity to unravel the behaviour of © 2014 John Wiley & Sons Ltd 1041 1042 X. C. LIU ET AL. o o o o 50 E Napier Complex 60 E 70 E 80 E Gondwana Reworked o Napier Complex 66 S 66o S Mawson Coast Vestfold Block Rayner Complex 1000 km Prydz Bay Fig. 2 Kemp Land Shelf Rauer Group Reworked in the late Neoproterozoic/Cambrian ? Enderby Land Northern Prince o 70 S Charles Mountains Mesoproterozoic/early Neoproterozoic Amery Ice 70o S Archean/Palaeoproteroz oic Fisher Terrane Clemence 0 200 km t Massif n n e Grove o m s Ruker Terrane p Mountains r w a a c M s Southern Prince E Charles M ountains Lambert Terr ane Princess Elizabeth Land o East Antarctica MacRobertson Land 74 S o o o o 74o S 50 E 60 E 70 E 80 E Fig. 1. Geological sketch map of Kemp Land, MacRobertson Land and Princess Elizabeth Land in east Antarctica (modified after Mikhalsky et al., 2001; Fitzsimons, 2003; Liu et al., 2014). Inset shows the location of this region in Gondwana at c. 500 Ma. the Vestfold Block during late Mesoproterozoic to Mossel Gneiss, comprising highly magnesian, silica- Cambrian orogenies and establish its relationship undersaturated sapphrine-bearing lithologies. The with other granulite terranes in the Rayner orogen. Tryne Mafic Gneiss generally occurs as layered two- In this article, we describe the petrological character- pyroxene granulite and also as nodular ultramafic istics of metamorphosed mafic dykes from the Vest- xenoliths within younger units. The Mossel Gneiss fold Block, estimate their metamorphic P–T consists predominantly of tonalitic orthogneiss, with conditions based on pseudosection calculations, and subordinate trondhjemitic, granodioritic and granitic determine the ages for the granulite facies metamor- orthogneiss, which intruded the aforementioned three phism using sensitive high-resolution ion microprobe constituent units between 2526 Æ 6 and 2501 Æ 4 (SHRIMP) U–Pb zircon dating. These data are then Ma (Black et al., 1991b). The Crooked Lake Gneiss used to discuss the tectonic evolution of the Rayner is the youngest unit and comprises medium- to orogen in the context of the assembly of the Indian coarse-grained orthogneiss of mostly dioritic, grano- craton and east Antarctica. dioritic, granitic and less commonly, gabbroic compo- sition. Emplacement ages inferred for the Crooked Lake Gneiss range from 2501 Æ 4 to 2484 Æ 6Ma REGIONAL GEOLOGY AND FIELD (Black et al., 1991b). Two episodes or a protracted RELATIONSHIPS period of high-grade metamorphism and deformation The basement of the Vestfold Block is dominated by took place from 2520 to 2450 Ma (Collerson et al., granulite facies orthogneiss with subordinate parag- 1983; Black et al., 1991b; Snape et al., 1997; Clark neiss (Fig. 2a). The basement rocks can be classified et al., 2012). into five units according to composition and relative Several mafic dyke swarms cut the high-grade base- age with respect to metamorphic and deformational ment of the Vestfold Block. On the basis of geo- events. These units are the Chelnok Paragneiss, Tay- chemistry, isotopic dating, cross-cutting relationships naya Paragneiss, Tryne Mafic Gneiss, Mossel Gneiss and orientation, at least four generations have been and Crooked Lake Gneiss (Collerson et al., 1983; recognized (Collerson & Sheraton, 1986; Black et al., Black et al., 1991b; Harley, 1993; Snape & Harley, 1991a; Lanyon et al., 1993; Dirks et al., 1994; Seitz, 1996; Snape et al., 1997, 2001). The Chelnok Parag- 1994; Hoek & Seitz, 1995; Snape et al., 2001). Early neiss consists mainly of garnetiferous pelite and high-Mg mafic dykes are generally E- or NE- semipelite, with less abundant psammite, quartzite, trending, with emplacement ages of 2241 Æ 4and calcsilicate and banded iron formation. Protoliths 2238 Æ 7 Ma (Lanyon et al., 1993). Group I high-Fe were deposited between 2575 and 2520 Ma (Clark mafic dykes have a NW trend and have been dated et al., 2012). The Taynaya Paragneiss locally crops at 1754 Æ 16 Ma (Lanyon et al., 1993). Group II out as a discontinuous layer or boudin train in the high-Fe mafic dykes are NNW-trending and have © 2014 John Wiley & Sons Ltd METAMORPHISM OF MAFIC DYKES IN ANTARCTICA 1043 (a) (b) Fig. 2. Simplified geological map of the Vestfold Block (a) and the Mule Peninsula of the southwestern Vestfold Block (b) (modified after Snape et al., 2001; Clark et al., 2012). The localities of the studied samples are indicated. been dated at 1380 Æ 7 Ma (Lanyon et al., 1993). (Fig. 3b), and is probably, part of the Group III The youngest Group III high-Fe mafic dykes are high-Fe mafic dykes. All these dykes are 2–8m in NNE- to N-trending and have been dated at width, and dip at a steep angle of ~80° towards 1248 Æ 4 and 1241 Æ 5 Ma, respectively (Black the east for the N-trending dykes and towards the et al., 1991a; Lanyon et al., 1993). In addition, alka- southwest for the NW-trending dykes. Most dykes line and ultramafic lamprophyric dykes were em- are massive, and only some show weak deforma- placed before the intrusion of Group II or III mafic tional features, as evident from oriented fine- dykes. In the northern part of the Vestfold Block, the grained mineral assemblages (Fig. 3c). The granulite dykes are un-deformed and display chilled margins facies metamorphism of most mafic dykes is hetero- and baked contacts, but in the southeastern part of geneous, characterized by a spotted texture pro- the Vestfold Block, some dykes are metamorphosed duced by pink garnet-rich aggregates in a grey to and locally deformed (Collerson & Sheraton, 1986; dark groundmass (see Fig. 3b,c). Some dykes are Passchier et al., 1991).
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