Andrew H. Allibone

Granitoids of the Dry Valleys area, southern Victoria Land, Antarctica: plutons, field relationships, and isotopic dating

Abstract Detailed mapping throughout much of the Dry Valleys area indicates the region is underlain by 15 major granitoid plutons and numerous smaller plugs and dikes. Intrusive relationships of these plutons and dikes indicate repeated intrusion of superficially similar granitoids at different times. Sufficient internal lithologjc variation occurs within individual plutons, to allow correlation with several of the previously defined granitoid units based on lithologic character. Consequently, previous subdivision schemes based on lithology are no longer tenable and are here replaced with a subdivision scheme based on the identification of individual plutons. The elongate, concordant Bonney, Denton, Cavendish, and Wheeler Plutons, which range in composition between monzodiorite and granodiorite, are the oldest relatively undeformed plutons in the Dry Valleys area. Each pluton is characterised by flow alignment of K‐feldspar megacrysts, hornblende, biotite, and mafic enclaves. Field relationships and radio...

Plutonic rocks of the Median Batholith in eastern and central Fiordland, New Zealand: Field relations, geochemistry, correlation, and nomenclature

Abstract This paper provides a comprehensive description of all major plutonic rock units in Fiordland between Lakes Poteriteri and Te Anau, and the heads of Doubtful and George Sounds. Plutonic rocks comprise c. 80% of the basement in the area described, the remainder being metase dim entary and metavolcaniclastic rocks. The plutonic rocks, of which c. 50% are granitoids, were emplaced in three phases—at c. 492 Ma, between c. 365 and 318 Ma, and between 168 and 116 Ma. Correlatives of the Devonian Karamea Suite emplaced between c. 375 and 367 Ma, and the Triassic to Early Jurassic part of the Darran Suite emplaced between c. 230 and 168 Ma, are not present in the area described here. The strongly deformed Late Cambrian to Early Ordovician Jaquiery Granitoid Gneiss is one of the oldest plutonic rocks yet discovered in New Zealand and is of similar age to plutonic rocks within the Ross and Delamerian Orogens of Victoria Land and South Australia. Rocks emplaced between c. 365 and 318 Ma include Ridge Suite S‐type granitoids and closely related S/A‐type plutons, Foulwind Suite A‐type mafic and granitoid plutons, Tobin Suite I‐type granitoids, and several unassigned mafic plutons. Rocks emplaced between 168 and 116 Ma include extensive c. 168–128 Ma old calc‐alkaline LoSY gabbros, diorites, and granitoids of the Darran Suite, c. 165–135 Ma old hypersolvus perthitic syenogranites and peralkaline granitoids, c. 125 Ma gneissic diorite similar to the Western Fiordland Orthogneiss, and c. 123–116 Ma old quartz diorites and granitoids of the HiSY Separation Point Suite. Plutons from each suite tend to be concentrated in distinct NNE‐striking parallel belts up to 20 km wide and 110+ km long. These belts are one of the key features which define the regional structural grain of Fiordland basement geology. Their strike remains constant from the Carboniferous through to the Cretaceous. S, S/A, and A‐type plutons of the Carboniferous Ridge and Foulwind Suites are confined to a 125 km long but discontinuous belt in southern and central Fiordland, wholly within the areal extent of early Paleozoic metase dim entary basement. Volumetrically minor Carboniferous Tobin Suite I‐type granitoids are confined to the area east of exposed early Paleozoic metasedimentary basement. Much of eastern Fiordland is underlain by an extensive belt of heterogeneous Darran Suite rocks. Darran Suite rocks extend from Stewart Island to the Darran Mountains of northern Fiordland, forming a belt c. 15 km wide and 300 km long. Correlative Darran Suite rocks also occur further west where they intrude early Paleozoic metasediments, indicating that Jurassic to Early Cretaceous arc‐related plutonism and volcanism occurred inboard of the edge of early Paleozoic basement in some parts of the Median Batholith. Distinctive Jurassic, pink, hypersolvus syenogranite and alkalic granitoids form a narrow discontinuous belt within the wider calcalkaline Darran Suite. Cretaceous Separation Point Suite plutons form two major belts, one in easternmost Fiordland partially covered by Cenozoic sedimentary rocks, and the other stitching inboard and outboard parts of the Median Batholith in central Fiordland.

Geology of the plutonic basement rocks of Stewart Island, New Zealand

Abstract Exposures of basement rocks on Stewart Island provide a c. 70 km long by 50 km wide map of part of the Median Batholith that spans the margin of the Western Province. Because of their distance from the present plate boundary, these rocks are relatively unaffected by Cenozoic tectonism, allowing examination of unmodified Carboniferous‐Cretaceous relationships within the Median Batholith. Thirty individual plutons (>c. 20 km2) have been mapped along with numerous relatively small intrusions (<c. 5 km2). The large plutons form 85–90% of the Median Batholith on Stewart Island while the many smaller intrusions comprise 10–15%, mostly in the north. Lithologies include: biotite ± minor hornblende granodiorite, granite and leucogranite with accessory titanite ‐ magmatic epidote and allanite (c. 50%); biotite ± muscovite ± garnet granite with S‐type affinities (c. 10%); alkaline quartz monzonite, granite, and alkali feldspar granite with rare aegirine and blue‐green amphibole (c. 3%); quartz monzodiorite and diorite with hornblende > biotite (c. 23%); gabbro and anorthosite (c. 12%) and ultramafic rocks (c. 2%). U‐Pb zircon and monazite dating indicates that c. 12% of these plutonic rocks were emplaced during the Carboniferous between 345 and 290 Ma, c. 20% in the Early‐Middle Jurassic at c. 170–165 Ma, c. 30% in the latest Jurassic to earliest Cretaceous between 152 and 128 Ma, and c. 38% in the Early Cretaceous between 128 and 100 Ma. The distribution of Pegasus Group schists and peraluminous granitoid rocks indicates that the northern limit of extensive early Paleozoic Western Province basement is located either within the Gutter Shear Zone or at the Escarpment Fault, 10–15 km south of the Freshwater Fault System previously thought to mark this boundary. Carboniferous and Middle Jurassic magmatism extended plutonic basement northwards as far as the Freshwater Fault System, while further magmatism during the latest Jurassic and earliest Cretaceous produced the basement north of the Freshwater Fault System. The focus of Early Cretaceous plutonism then returned southwards into the Western Province, although the older basement in this area was only involved in the genesis of subordinate peraluminous plutonism at this time and not the more extensive metaluminous rocks. The Escarpment Fault disrupted this c. 40 km wide section across the margin of the Western Province at c. 110–100 Ma.

Granitoids of the Dry Valleys area, southern Victoria Land: geochemistry and evolution along the early Paleozoic antarctic craton margin

Abstract Field relationships and geochemistry indicate granitoid plutons of the Dry Valleys area comprise at least three petrogenetically distinct suites. The older Dry Valleys 1a (DV1a) suite, comprising the Bonney, Catspaw, Denton, Cavendish, and Wheeler Plutons and hornblende‐biotite orthogneisses, and Dry Valleys 1b (DV1b) suite, comprising the Hedley, Valhalla, St Johns, Dun, Calkin, and Suess Plutons, biotite granitoid dikes and biotite orthogneisses, were emplaced before prominent swarms of Vanda mafic and felsic dikes. Both the DV1a and DV1b suites are time transgressive, with older intrusions in each suite being emplaced during the later stages of deformation of the Koettlitz Group. Younger granitoids that postdate the majority of the Vanda dikes include: the Dry Valleys 2 (DV2) suite, comprising the Pearse and Nibelungen Plutons plus several smaller, unnamed plugs; and the Harker, Swinford, Orestes, and Brownworth Plutons with identical field relationships and enclaves but distinct chemistries. ...

Age, Correlation, and Provenance of the Neoproterozoic Skelton Group, Antarctica: Grenville Age Detritus on the Margin of East Antarctica

Detrital zircon U‐Pb ages constrain the age and provenance of the Skelton Group in southern Victoria Land, one of several Proterozoic‐Cambrian metasedimentary units that form basement to the Ross Orogen in East Antarctica. The age of the youngest detrital zircons combined with previous dating of crosscutting intrusive rocks indicates deposition of the northern and southern parts of the Skelton Group between ca. 1050–535 and ca. 950–551 Ma, respectively. Many zircons in the northern part of the Skelton Group crystallized after partial melting during upper amphibolite facies metamorphism at ca. 505–480 Ma, although older ca. 550‐Ma metamorphic zircon rims indicate an earlier episode of high‐grade metamorphism. Detrital zircon ages from the Skelton Group are dominated by ca. 1300–950‐Ma ages similar to those in the Beardmore Group in East Antarctica and the Adelaidean succession of South Australia, suggesting that these rocks are generally correlative. Zircons that crystallized at ca. 1050 Ma form the major age population of the northern Skelton Group, while a broader range of Neoproterozoic zircons form significant components in other sediments deposited on the margin of East Antarctica–Australia at this time, indicating a close proximity to exposed Grenville age crust. Inferred basement rocks of Grenville age beneath the Ross Orogen in East Antarctica (represented by a potential \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Isotopic character of Cambro‐Ordovician plutonism, southern Victoria Land, Antarctica

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Structural controls on gold mineralization at the Ashanti Deposit, Obuasi, Ghana

\end{document} ‐Ma orthogneiss), Paleozoic cover in eastern Australia, and ice in Marie Byrd Land in West Antarctica are potential sources for the Grenville age component in these Neoproterozoic sedimentary rocks.

Petrology and geochronology of the volcaniclastic and volcanogenic Mesozoic Loch Burn Formation in eastern Fiordland, New Zealand

Abstract This paper provides a comprehensive description of the plutonic rocks of western Fiordland between Breaksea and Sutherland Sounds. The area is dominated by the Early Cretaceous Western Fiordland Orthogneiss (WFO), but also includes smaller bodies of Paleozoic and Cretaceous granitoid. Plutonic rocks of western Fiordland intrude metasediments of the Western Province, many of whose age and terrane affinities remain undefined. Paleozoic granitoids in western Fiordland include the Pandora Orthogneiss (c. 500 Ma) and widespread related sills within Paleozoic metasedimentary rocks; the All Round Pluton (c. 340 Ma); the Deas Cove Granite (c. 372 Ma); and possibly the Straight River Granite. The Pandora Orthogneiss is one of the oldest plutons yet found in the Median Batholith. Correlatives include the Jaquiery Granite Gneiss in central Fiordland and orthogneiss in Doubtful Sound. Plutonism of Ross/Delamarian age is therefore widespread in those parts of Fiordland where Cambrian or older Western Province metasedimentary rocks form basement. The All Round Pluton and Deas Cove Granite are correlatives of the S‐type Ridge and A/I‐type Foulwind Suites, respectively. The c. 125–116 Ma WFO includes at least seven major dioritic and monzodioritic plutons in western Fiordland, one in central Fiordland, and one in central Stewart Island. Plutons which compose the WFO are distinguished by differences in their age, petrography, structural and metamorphic histories, and geochemistry. The WFO in northern Fiordland and the correlative Walkers Pluton on Stewart Island were emplaced in the mid crust (4–9 kbar) at depths comparable with some Separation Point Suite plutons of similar age. WFO plutons in southern Fiordland were emplaced at greater depths (10–18 kbar). WFO plutons have been variably recrystallised to eclogite; omphacite‐, garnet‐, two‐pyroxene‐, and hornblende‐granulite; and hornblende‐amphibolite facies assemblages, reflecting different PTX conditions during metamorphism of each body. Some parts of the WFO remain undeformed and unmetamorphosed. Evidence of up to c. 6 kbar loading after emplacement is limited to WFO plutons in northern Fiordland and adjacent country rocks. Extensional ductile shear zones previously shown to locally separate the WFO from adjacent rocks are discontinuous later features, commonly localised along earlier intrusive contacts between WFO plutons and metasedimentary country rocks. They do not form a regionally extensive detachment between the upper and lower plates of a metamorphic core complex. The WFO has previously been included in the Separation Point Suite since both units share a high Sr/Y (HiSY) chemistry and were emplaced at broadly the same time. However, the WFO and Separation Point Suite have distinct chemistries. Separation Point Suite rocks generally contain greater Sr, Na, and Al, and have lower Sr/Rb ratios, rare earth element and Y contents, than WFO rocks with comparable amounts of SiO2. Many aspects of the WFO chemistry (aside from its HiSY character) are similar to that of the older Darran Suite rather than the Separation Point Suite. This may reflect a greater amount of partial melting during generation of the SiO2‐poor WFO than the SiO2‐rich Separation Point Suite. Alternatively it may indicate derivation of the WFO and Separation Point Suite from different sources, albeit at depths greater than those where residual plagioclase is stable. Relatively large variations in the major element chemistry of the Separation Point Suite reflect fractionation and/or accumulation of plagioclase, whereas the more limited variability in the major element chemistry of the WFO reflects minor fractionation and/or accumulation of hornblende and/or clinopyroxene.