Felsic Orthogneisses Research Articles

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The Southern Prince Charles Mountains (SPCM) are mostly occupied by the Archaean Ruker Terrane. The Lambert Terrane crops out in the northeastern part of the SPCM. New geochemical and zircon U–Pb SHRIMP ages for felsic orthogneisses and granitoids from both terranes are presented. Orthogneisses from the Ruker and Lambert terranes differ significantly in their major and trace-element compositions. Those from the Ruker Terrane comprise two distinct groups: rare Y-depleted and abundant Y-undepleted. U–Pb isotopic data provide evidence for tonalite−trondhjemite emplacement at 3392 ± 9 and 3377 ± 9 Ma, pre-tectonic granite emplacement at 3182 ± 9 Ma, metamorphism(?) at c. 3145 Ma, and thermal events at c. 1300(?) and 626 ± 51 Ma. The Lambert Terrane orthogneisses probably originated in a continental magmatic arc. Zircon dating shows a very different geological history: pre-tectonic granitoid emplacement at 2423 ± 18 Ma, metamorphism at 2065 ± 23 Ma, and syn-tectonic granitoid emplacement at 528 ± 6 Ma, syn-tectonic pegmatite emplacement at 495 ± 18 Ma. The Lambert Terrane can be correlated with neither the Meso- to Neoproterozoic Beaver Terrane in the Northern PCM, which differs in isotopic composition, nor with the Archaean Ruker Terrane, which differs in both granitoid chemical composition and the timing of major geological events. It represents a Palaeoproterozoic orogen which experienced strong tectonic re-activation in Pan-African times. The Lambert Terrane has some geochronological features in common with the Mawson Block, which comprises south Australia and some areas in East Antarctica.

Metamorphic events in the eastern Arunta Inlier, Part 2. NdSrAr isotopic constraints

In the Harts Ranges, the Central Province of the eastern Arunta Inlier is characterised by upper amphibolite facies ortho- and paragneisses. Previously published UPb dating of zircons, as well as new SmNd and RbSr data indicate that the intrusive ages of both mafic and felsic orthogneisses are in the range 1780-1740 Ma (Strangways Orogeny). These rocks had initial ϵ Nd values of 0 to − 3 and depleted mantle model ages of 2.5-2.1 Ga and they intruded supracrustal rocks which are now preserved as paragneisses with initial ϵ Nd of about − 15. SmNd and RbSr whole-rock isotope data indicate that the hornblende-rich amphibolitic gneisses and mylonites in the Harts Range Detachment Zone (Ding and James, 1989) were equilibrated at ∼ 1473 Ma (± 133) when the rocks experienced hydration, isotopic homogenisation and thrust faulting. This is the Anmatjira Event described elsewhere in the Arunta Inlier and is a predominantly structural influence with thrust faults and hydrous fluid flux. Accordingly the record of this event by radiogenic geochronometers has been diffuse, a problem also exacerbated by the probable influence on these rocks, of least one more subsequent lower-temperature amphibolite facies metamorphic event. In spite of the indications of ∼ 1470 Ma isotopic homogenisation of amphibolites in the Harts Ranges, RbSr isotopic analyses of all minerals and 40Ar 39Ar from hornblendes yield ages in the range 450-300 Ma. Using the SmNd system, a few garnet-hornblende pairs give 1400-1250 Ma ages, but several of these pairs also yield early Palaeozoic ages. These data suggest that the Central Province of the eastern Arunta Inlier also experienced an early Palaeozoic thermal event, having cooled from temperatures > 550°C during and immediately before the Carboniferous Alice Springs Orogeny. In the Southern Province, south of the Illogwa Shear Zone, micas record Mesoproterozoic ages (1500-1250 Ma) for the RbSr, KAr and ArAr systems, equivalent to the whole-rock ages obtained from the Central Province, to the north of the Illogwa Shear Zone. Clearly the exposed southern basement cooled to <350°C, presumably as a result of uplift and exhumation during the Anmatjira Event, when basement now exposed in the Central Province remained at depth, undergoing one or more additional amphibolite facies events. The spectacular contrast in cooling histories across the Illogwa Shear Zone emphasises that this is an important crustal scale tectonic structure.

Stable isotopic signatures of superposed fluid events in granulite facies marbles of the Rauer Group, East Antarctica

ABSTRACTThe role of volatiles in the stabilization of the lower (granulite facies) crust is contentious. Opposing models invoke infiltration of CO2‐rich fluids or generally vapour‐absent conditions during granulite facies metamorphism. Stable isotope and petrological studies of granulite facies metacarbonates can provide constraints on these models. In this study data are presented from metre‐scale forsteritic marble boudins within Archaean intermediate to felsic orthogneisses from the Rauer Group, East Antarctica.Forsteritic marble layers and associated calcsilicates preserve a range of 13C‐ and 18O‐depleted calcite isotope values (δ13C= ‐9.9 to ‐3.0% PDB, δ18O = 4.0 to 12.1% SMOW). A coupled trend of 13C and 18O depletion (∼2%, ∼5%, respectively) from core to rim across one marble layer is inconsistent with pervasive CO2 infiltration during granulite facies metamorphism, but does indicate localized fluid‐rock interaction. At another locality, more pervasive fluid infiltration has resulted in calcite having uniformly low, carbonatite‐like δ18O and δ13C values. A favoured mechanism for the low δ18O and δ13C values of the marbles is infiltration by fluids that were derived from, or equilibrated with, a magmatic source. It is likely that this fluid‐rock interaction occurred prior to high‐grade metamorphism; other fluid‐rock histories are not, however, ruled out by the available data. Coupled trends of 13C and 18O depletion are modified to even lower values by the superposed development of small‐scale metasomatic reaction zones between marbles and internally folded mafic (?) interlayers. The timing of development of these layers is uncertain, but may be related to Archaean high‐temperature (&gt;1000d̀C) granulite facies metamorphism.

Sapphirine granulites from the Vestfold Hills, East Antarctica: geochemical and metamorphic evolution

A varied suite of sapphirine-bearing and quartz-undersaturated granulites, the Taynaya Paragneiss, occur as boudins and enclaves within the c. 2500 Ma old felsic orthogneisses of northern Vestfold Hills. Highly magnesian varieties with XMg (=100x(Mg/(Mg+Fe)) near 95 preserve the assemblage sapphirine + enstatite + spinel, whereas sapphirine + cordierite + sillimanite + corundum occurs in aluminous and feldspathic types with XMg near 90. Phase equilibria and relative thermometry based on Al2O3 solubility in enstatite indicate equilibration of these assemblages at c. 830–880°C and 0.35–0.85 GPa. There is no evidence for the extreme temperatures (1000–1100°C) previously proposed for early metamorphism in the Vestfold Hills, and no indication in the metamorphic assemblages of isobaric cooling prior to 2500 Ma. Two types of metasomatism have altered the bulk rock compositions near boudin and enclave margins. Cordierite rinds locally formed on corundum-sillimanite granulites reflect interaction with magmatic precursors to the enclosing felsic gneisses, as supported by the isotopic and chemical compositions of cordierite channel volatiles. More extensive metasomatism producing schistose phlogopite + sapphirine rinds on all boudins involved infiltration of a LILE-enriched fluid which introduced K2O, H2O, Fe, Rb, Ba and minor Sr along the boudin margins. Whole rock geochemistry of Taynaya Paragneiss unaffected by this metasomatism is consistent with their derivation from evaporitic mudstones, and implies the existence of a basement older than the dominant 2500 Ma orthogneisses.

Geology of the Bunger Hills area, Antarctica: implications for Gondwana correlations

The Bunger Hills area of the East Antarctic Shield consists of granulite-facies felsic orthogneiss, with subordinate paragneiss and mafic granulite. The igneous precursors of granodioritic orthogneiss were emplaced 1500-1700 Ma ago, and late Archaean (2640 Ma) tonalitic orthogneiss occurs in the nearby Obruchev Hills. Peak metamorphism (M1) (at about 750-800°C and 5-6kb) occurred 1190 ±15 Ma ago (U-Pb zircon age), and was accompanied by the first of three ductile deformations (D1). Emplacement of voluminous, mainly mantle-derived plutonic rocks, ranging from gabbro, through quartz monzogabbro and quartz monzodiorite, to granite, followed between 1170 (during D3) and 1150 Ma. Intrusion of abundant dolerite dykes of four chemically distinct suites at about 1140 Ma was associated with shear zone formation, indicating at least limited uplift; all subsequent deformation was of brittle-ductile type. Alkaline mafic dykes were emplaced 500 Ma ago. Marked geochronological similarities with the Albany Mobile Belt of Western Australia suggest that high-grade metamorphism occurred during collision between the Archaean Yilgarn Craton of Australia and the East Antarctic Shield about 1200 Ma ago.

SmNd, UPb, and RbSr geochronology and lithostructural relationships in the southwestern Rae province: constraints on crustal assembly in the western Canadian shield

The Rae province is an area of Archean crust bounded by the Early Proterozoic Thelon orogen to the northwest and mylonites of the Snowbird Tectonic Zone to the southeast. The southern part of the province in Saskatchewan is a highgrade complex of ultramafic, mafic and felsic orthogneisses with interlayered supracrustal units. The oldest rocks in the area were derived from layered mafic plutonic complexes and supracrustal units. The earliest deformation (D 1 ) resulted in development of a layer-parallel foliation preserved in small lensoid pods of anorthosite, gabbro, and pyroxenite. A pyroxenite yields a SmNd T DM age of ∼ 3.5 Ga and is isotopically similar to a mafic granulite (metabasalt?) associated with early supracrustal units. These rocks were intruded by a sequence of mafic and tonalitic orthogneisses that yield a SmNd isochron age of 3.32±2.87 and T DM ages of ∼3.2−3.1 Ga. The mafic-tonalitic gneisses were subsequently migmatized producing anatectic net veins of tonalitic to granitic pegmatite. These were deformed during D 2 deformation. The terrane was then intruded by minor granodioritic to tonalitic plutonic sheets and a swarm of mafic dikes that were deformed by the last major period of inhomogeneous deformation (D 3 ) which ended at ∼2.0 Ga. The Clearwater Complex, a large 24×8 km long anorthosite massif, intruded the gneiss complex following D 3 , and yields a SmNd mineral-wholerock isochron age of 1.85 Ga. SmNd, RbSr, and UPb SHRIMP data record multiple thermotectonic events and provide constraints on crustal evolution of the Rae province. UPb SHRIMP data for metamorphic zircons separated from an early anorthosite associated with units that give pre-3.2 Ga Nd model ages show considerable variation in Th/U ratio. Zircons yield virtually concordant data that record Archean (2.9 Ga) and several Early Proterozoic (between 2.3 and 1.85) thermotectonic events. SmNd data are generally poorly correlated possibly as a result either of slight remobilization of REE's during granulite facies metamorphism or that rocks with distinct origins were compared. RbSr whole-rock and mineral data show substantial re-equilibration in mafic-tonalitic gneisses at ∼2.8 Ga, and in supracrustal units at ∼2.3 Ga. Also, a discordant granitic dike and biotite separated from a metapelite yield RbSr T DM ages of ∼1.9 Ga. These metamorphic ages are interpreted to reflect thermotectonism associated with formation and assembly of juvenile Early Proterozoic crust in the Talston-Thelon plutonic zone to the west, and with terminal collision in the Trans-Hudson orogen to the east.

Geological relationships in high‐grade basement gneiss of the northern Prince Charles Mountains, East Antarctica

High‐grade gneiss in the northern Prince Charles Mountains, East Antarctica, has a complex intrusive and deformational history. Outcrop is dominated by homogeneous felsic orthogneiss, which encloses boudinaged mafic and ultramafic units. These boudins preserve structures (S1, F2) not seen in the host gneiss, and are interpreted as transposed and boudinaged dykes. A sedimentary protolith is inferred for less homogeneous felsic gneiss interlayered with semi‐pelite, calc‐silicate and rare pelite. These basement lithologies were deformed into a series of flat‐lying structures (S3, F4) consistent with progressive horizontal shear, and then into a series of upright structures (F5) culminating in the development of regional synforms, antiforms and monoclines (F6) separated by zones of intense upright fabric (S6). The D3 to D6 time interval was associated with several episodes of partial melting which produced discordant Ieucogneiss bodies, and with the emplacement of mafic dykes and charnockite plutons correlate...

Rapid production and evolution of late Archaean felsic crust in the Vestfold Block of East Antarctica

The aim of this study is to present a definitive chronostratigraphy, based for the first time on Ub zircon ages, for the Vestfold Block of East Antarctica. This region consists of four principal rock units, three of which are composed of felsic orthogneisses, and form the subject of this article. These orthogneiss units comprise the previously defined Crooked Lake and Mossel gneisses, and a newly proposed felsic unit, the Grace Lake granodiorite. The entire gamut of significant felsic igneous activity, the two intense tectonothermal events which preceded the cratonisation of the Vestfold Block, and significant post-tectonic uplift were all accomplished during an approximately 50 Ma interval at the end of the Archaean. This conclusion is in marked contrast to a previously proposed model that such activity had occurred episodically over several hundred million years. The Mossel gneiss (predominantly tonalitic orthogneiss) has the oldest igneous precursors, with a significant age spread, from 2526±6 Ma (in the type area) to 2501±4 Ma. Precursors of the compositionally varied Crooked Lake gneiss (predominantly dioritic, monzodioritic and monzonitic rocks) also show a distinct range of crystallisation ages, from 2501±4 Ma in the type area, through 2493±5 Ma to 2484±6 Ma. Both units appear to show a general northwards migration of igneous activity. The Grade Lake granodiorite, which is defined here on field and isotopic criteria, yields a pooled mean age (from two rocks) of 2487±6 Ma. An undeformed, post-granulite-facies quartz diorite dyke was emplaced at 2477±5 Ma. The new isotopic data constrain the age of the first major granulite-facies deformation (D 1) to have affected the Vestfold Block to lie between 2501±4 Ma and 2487±6 Ma, and that of the second granulite-facies deformation (D 2) to 2487±6 Ma. There is evidence of older precursors for some of the late Archaean gneisses. Zircon cores and xenocrysts together with SmNd model ages indicate that the Grace Lake granodiorite was derived from crustal sources at least 2800 Ma old. The presence of inherited zircon in one of two analysed samples of the Mossel gneiss, and a spread of SmNd model ages, implies derivation by the melting of at least two source components, one at least 2700 Ma old. In contrast, the apparent lack of inherited zircon in the Crooked Lake gneiss samples is consistent with derivation of their precursors either exclusively from the mantle, or by a brief two-stage melting process. The Vestfold Block chronology is completely different from that of the adjacent Rauer Islands region, indicating that they were not juxtaposed during Archaean times. Neither do any nearby Archaean terranes appear to have comparable chronostratigraphy with the Vestfold Block. At our present state of knowledge, only the Napier Complex of Enderby Land, and possibly the southern Prince Charles Mountains of MacRobertson Land contain any rocks of similar age. This makes the Vestfold Block a potentially useful element for theoretical reconstruction of Gondwanaland.

A Geochemical and Pb, Sr Isotopic Study of the Evolution of Granite-Gneisses from the Bastar Craton, Central India

Abstract Preliminary Pb-Pb and Rb-Sr geochronology of granitic and gneissic rocks from the Sukma area of the Bastar craton, Central India, provides important constraints on crustal evolution, Much of the craton is made up of felsic orthogneisses and younger granitic intrusives, compositionally ranging from tonalite to granite. Pb-Pb isotopic data suggest the presence of ca. 3.0 Ga old gneisses. Younger granitic intrusives have been dated at ca. 2.6 Ga which represents a widespread resetting and/or emplacement event. Comparison of the Pb-Pb and Rb-Sr whole rock ages suggests that the latter were more perturbed after the gneiss-forming or emplacement events. All rock suites show significant geological scatter of isotopic data probably because of sampling on a regional scale, and reflect multi-stage isotopic evolution in a complex terrain. The present isotopic data indicate the presence of Archaean rocks in the Bastar craton and suggest temporal similarity with the oldest crustal rocks in the Singhbhum and Dharwar cratons.

Geochemistry of exposed granulite facies terrains and lower crustal xenoliths in Mexico

The trace element compositions of 13 samples from exposed granulite facies terrains and 17 granulitic lower crustal xenoliths were determined using an inductively coupled plasma‐mass spectrometer to constrain the composition and evolution of the Mexican lower crust. Most xenoliths found are pyroxeneplagioclase orthogneisses, although quartzo‐feldspathic paragneisses form up to 50% of the xenolith population in some areas. Lithologies in the exposed terrains consist of pelitic and carbonate‐rich paragneisses, charnockites, and less abundant basic, intermediate, and felsic orthogneisses. Both xenoliths and exposed granulites generally have low abundances of incompatible elements such as K, Rb, Th, and U. The xenoliths, however, often have lower abundances of these elements than the exposed granulites. High K/Rb ratios, coupled with an inverse correlation between K2O and K/Rb, indicate that most samples from the Mexican lower crust are depleted in both K and Rb. Furthermore, Th/U ratios similar to upper crustal materials and high La/Th ratios suggest depletion in both Th and U relative to average crustal material. Removal of a melt phase, either after the crystallization of cumulates or following a partial melting event, can explain many of the chemical features observed and may be responsible for the elemental depletions. Some samples, however, appear to have been unaffected during their residence in the lower crust and chemically resemble their presumed protoliths. Available geobarometric data suggest that xenoliths equilibrated near the crust/mantle boundary (∼10 kbar) and represent the lowermost crust, while the exposed granulites equilibrated at shallower crustal levels (∼7 kbar). Thus the differences observed between the xenoliths and exposed granulites indicate that there may be chemical and lithologic zonation in the Mexican lower crust. The lowermost crust in Mexico consists of crystallized basaltic liquids, cumulates, and/or residue after the removal of a partial melt as well as garnet‐rich metasediments.

Origin and U–Pb geochronology of amphibolite-facies metamorphic rocks, Miramichi Highlands, New Brunswick

The Miramichi Highlands of New Brunswick are underlain by subgreenschist- to greenschist-facies sedimentary and volcanic rocks of the Cambro-Ordovician Tetagouche Group and by amphibolite-facies paragneisses, amphibolites, and felsic orthogneisses of the Trousers Lake and Sisson Brook suites. New field, geochemical, and geochronologic data for the amphibolites and felsic orthogneisses suggest that they are high-grade metamorphic equivalents of the Tetagouche volcanic rocks and their associated intrusions.Amphibolites in the Miramichi Highlands occur as striped and unstriped varieties that possess chemical characteristics indicative of an igneous origin. However, the two types are compositionally distinct: the striped amphibolites resemble volcanic-arc tholeiites, whereas the unstriped amphibolites are like within-plate tholeiites. The geochemically inferred tectonic origin of these amphibolites is compatible with a recently proposed intracontinental back-arc tectonic setting for the Tetagouche Group.Felsic orthogneisses (Fox Ridge augen granite and Trousers Lake felsic orthogneiss) exhibit concordant contacts with the unstriped amphibolites. U–Pb zircon ages for the Fox Ridge augen granite [Formula: see text] and Trousers Lake felsic orthogneisses [Formula: see text] indicate a Late Ordovician intrusive event. Thus, there is no evidence for Precambrian granite and orthogneiss in the Miramichi Highlands, as had been previously inferred from a correlation with purported Precambrian rocks in the Gander Zone of Newfoundland. The age of the unstriped amphibolites is interpreted as being the same as that of the felsic orthogneisses because these two rock types always exhibit close relationships in the field. The age of the striped amphibolites is less certain, although a correlation with Ordovician basalts of the Tetagouche Group is consistent with their field relationships and tectonic setting.

Precambrian geological relationships in high‐grade gneisses of the Rauer Islands, east Antarctica

Gneisses from the Rauer Group, near Davis Base, east Antarctica, preserve a complex structural‐metamorphic and igneous history. Mafic granulites and paragneisses, preserved as relics and rafts within Proterozoic felsic orthogneisses, were affected by two early deformation episodes (D1 and D2) under granulite facies conditions. Partial melting of the mafic rocktypes occurred subsequent to these events and partly synchronous with the intrusion of syntectonic (D3) felsic orthogneisses. D3, a major episode which resulted in close to isoclinal folding of most of the gneisses in the area, progressed under granulite facies conditions of greater than 800°C and 7–8 kbar. Preliminary isotopic data indicate that D3 can be no older than 1400 Ma. A period of near‐isothermal decompression to 4 kbar at 700∘C late in and following D3 resulted in the production of leucocratic melts from metasedimentary gneisses, prior to the intrusion of discordant basic dykes of probable Late Proterozoic age (1200–1000 Ma). Further deformation, under amphibolite facies conditions, caused refolding of all previous structures about open to tight southeast‐plunging folds with wavelengths of up to 2 km. This D4 deformation may be correlated with the major tectonothermal event which caused folding of dykes elsewhere in the Rayner Complex, and probably occurred at 1000–960 Ma. This scheme for the geological evolution of the Rauer Group differs from that of the Proterozoic Rayner Complex in Enderby Land and MacRobertson Land in the recognition of a major Late Proterozoic tectonothermal episode, here termed D3, which pre‐dates the intrusion of basic dykes, which were later deformed in a less intense event.

Character of the Alleghanian orogeny in the southern Appalachians: Part I. Alleghanian deformation in the eastern Piedmont of South Carolina

The eastern Piedmont Province in South Carolina contains a sequence of Cambrian volcanic and sedimentary rocks that was penetratively deformed (D1) and regionally metamorphosed (M1) to the greenschist facies during the early and/or middle Paleozoic. The eastern Piedmont was subsequently affected by late Paleozoic (Alleghanian) polyphase deformation (D2–D4) and regional metamorphism. The earliest Alleghanian event (D2) is associated with amphibolite facies regional metamorphism and felsic plutonism in a mid-crustal infrastructure at ca. 295–315 Ma. The gradational interface between infrastructure and overlying suprastructure contained a steep M2 metamorphic gradient (between amphibolite facies below and greenschist facies above), numerous sheets of felsic orthogneiss, and a deformation front marking the upper limit of intense D2 penetrative deformation. D3 is represented by northwestward-vergent folding of S1 and S2 foliations and M2 isothermal surfaces. The Kiokee belt in South Carolina is interpreted as a D2 infrastructure exposed within the core of a D3 antiform. This interpretation suggests that an Alleghanian infrastructure may be present in the subsurface beneath much of the Piedmont and Coastal Plain. The polygenetic Modoc zone forms the northwest boundary of the Kiokee belt. It is interpreted to represent an interface between D2 infrastructure and suprastructure which was rotated into a steep, northwest-dipping attitude during development of the Kiokee belt antiform. Between ca. 267 and 290 Ma, portions of the Kiokee belt, Modoz zone, and Carolina slate belt were overprinted by ductile deformation (D4) along steeply dipping, northeast-trending dextral shear zones.

Geochemistry and geochronology of the gneisses east of the Southern Rocky Mountain Trench, near Valemount, British Columbia

Petrographic, geochemical, and geochronological data are presented on the gneisses of the Bulldog Creek block, the Mount Blackman block, and the Hugh Allan Creek block, which lie to the east of the Southern Rocky Mountain Trench (SRMT) south of Valemount, British Columbia.Petrographical and geochemical data, especially immobile-trace-element ratios (Nb: Y, Ti: Zr), and CaO versus Y and AFM plots are used to deduce the probable origins and protoliths of the gneisses. The Mount Blackman block consists of a psammitic paragneiss, probably derived from an immature arkosic sedimentary protolith, intruded by sills of tholeiitic basalt, now amphibolites. The Bulldog Creek block consists of felsic orthogneisses of calc-alkaline affinity, which are structurally concordant with mafic orthogneisses of possible tholeiitic basalt parentage. The Hugh Allan Creek block consists of a felsic orthogneiss with a probable alkaline granite protolith.Rb–Sr, and some U–Pb analyses show that each block has experienced a separate geological history. The Mount Blackman block psammitic paragneisses are the only analysed gneisses east of SRMT with a probable Archean Rb–Sr model crustal residence age. U–Pb analyses on zircons from these gneisses give a 1950 Ma minimum source rock age, and Rb–Sr whole-rock analyses suggest a 1860 ± 50 Ma age for amphibolite-facies metamorphism of both paragneisses and amphibolites. The Bulldog Creek block gneisses have a metamorphic age of at least 640 Ma, but their Rb–Sr systematics have been extensively disturbed, possibly during Mesozoic retrogressive metamorphism. The Hugh Allan Creek block gneisses have a Rb–Sr model crustal residence age of ~900 Ma and a metamorphic age of 805 ± 11 Ma. It is not possible to correlate any of these lithologies or events across the SRMT with the Malton block, and it is concluded that the SRMT is the site of a major fault or faults at this latitude.

Geochemical evolution of Archaean granulite-facies gneisses in the Vestfold Block and comparisons with other Archaean gneiss complexes in the East Antarctic Shield

The Vestfold Block, like other Archaean cratons in East Antarctica and elsewhere, consists predominantly of felsic orthogneiss (Mossel and Crooked Lake gneisses), with subordinate mafic granulite (Tryne metavolcanics) and paragneiss (Chelnok supracrustals). Two major periods of continental crust formation are represented. The Mossel gneiss (metamorphosed about 3,000 Ma ago) is mainly of tonalitic composition, and is similar to much of the roughly coeval Napier Complex in Enderby Land. The Crooked Lake gneiss was emplaced under high-grade conditions about 2,450 Ma ago and comprises a high proportion of more potassic rocks (monzodioritic and monzonitic suites), as well as tonalite and minor gabbro and diorite. Both Mossel and Crooked Lake gneisses are depleted in Y and have moderate to high Sr, Ce/Y, and Ti/Y, consistent with melting of a mafic source (?subducted hydrated oceanic crust) leaving major residual hornblende (± garnet). Most Crooked Lake gneisses are more enriched in incompatible elements (P, Sr, La, Ce, and particularly Rb, Ba, and K) than Mossel gneisses, suggesting derivation from a more enriched mafic source. The Vestfold Block contains few orthogneisses derived by melting of older felsic crustal rocks, in marked contrast to the Archaean Napier Complex and, in particular, southern Prince Charles Mountains. Both Mossel and Crooked Lake tonalites are strongly depleted in Rb, K, Th, and U, and have very low Rb/Sr and high K/Rb; more potassic orthogneisses are depleted in Th, U, and, to lesser extents, Rb. Tryne metavolcanics are depleted in Th and Rb, but appear to have been enriched in K (and probably Na), possibly during early low-grade alteration.