Metasedimentary Units Research Articles

U–Pb detrital zircon ages and Sm–Nd isotopic features in low-grade metasedimentary rocks of the Famatina belt: implications for late Neoproterozoic–early Palaeozoic evolution of the proto-Andean margin of Gondwana

Abstract: The Famatina belt, Central Andes, is part of an ancient accretionary margin built along Western Gondwana in the early Palaeozoic. U–Pb ion microprobe analysis of detrital zircons and Sm–Nd whole-rock analysis of two early Palaeozoic low-grade metasedimentary units record the early evolution of this region. Detrital zircons in the Negro Peinado and Achavil formations have ages ranging from Palaeoproterozoic to Cambrian, consistent with derivation from Gondwanan sources. T DM ages suggest that the sedimentary rocks were derived from a composite source area, which separated from the mantle during the Palaeoproterozoic ( c . 1.8–1.6 Ga). Constraints from the youngest detrital grains indicate accumulation in a Mid- to Late Cambrian foreland basin adjacent to the inboard Pampean orogenic tract. The dominance of Cambrian ages in the Negro Peinado Formation suggests derivation principally from the eastern Pampean belt whereas the dominance of late Neoproterozoic ages in the Achavil Formation suggests that input from the Pampean belt was overwhelmed by older sources. The paucity of Palaeoproterozoic ages argues against direct input from older areas such as the Río de la Plata craton. The predominance of Meso- and Neoproterozoic ages over older sources suggests that a Brasiliano-age magmatic arc developed on a Mesoproterozoic basement, probably a southern extension of the Arequipa–Antofalla massif.

Tectonic evolution of forearc nappes of the active Banda arc-continent collision: Origin, age, metamorphic history and structure of the Lolotoi Complex, East Timor

An integrated multidisciplinary investigation of the Lolotoi Complex of East Timor (Timor Leste) indicates that it is part of the Banda forearc that was metamorphosed and rapidly exhumed during the Eocene and accreted to the NW Australian continental margin during Late Miocene to present arc–continent collision. Greenschist, graphitic phyllite, quartz–mica schist, amphibolite and pelitic schist dominate metamorphic rock types. Mineral, whole rock, and trace element geochemical analyses of metabasites indicate protolith compositions consistent with tholeiitic basalt and basaltic andesite with mixed MORB and oceanic arc affinities. Metapelite schist is mostly composed of metasedimentary units derived from mafic to intermediate rocks with oceanic to continental volcanic arc provenance. Thermobarometric calculations show peak metamorphic conditions of 530 °C to 680 °C for garnet–biotite pairs and amphibole, and peak pressures of 5 to 10 kbar for garnet–aluminosilicate–quartz–plagioclase assemblages. Peak metamorphism occurred at 45.36 ± 0.63 Ma, as indicated by Lu–Hf analyses of garnet. Detrital zircon grains have a U/Pb age distribution with spikes at 663, 120 and 87 Ma, which is typical of detrital zircon ages throughout the Great Indonesian Arc of Asia, but is distinct from Australian affinity units. These data indicate deposition and later metamorphism occurred after 87 Ma. Structural analyses of the metamorphic rocks and their sedimentary and volcanic cover units reveals 5–6 deformational phases of alternating shortening and extension. There is little to no evidence of strike-slip deformation. Phases 1–4 are inferred as pre-Oligocene from age determinations. Phases 5 and 6 are most likely related to latest Miocene to Pliocene nappe emplacement and Pliocene to present collisional deformation. Kinematic indicators show mostly top to the SE directed shortening and top to the south and SE extension. Structural mapping indicates that the Lolotoi Complex and some of its cover units are in thrust contact with underlying Gondwana Sequence rocks. Asian affinity volcanic and sedimentary cover units are found mostly in normal fault contact with metamorphic rocks. These data indicate that the Lolotoi Complex of Timor Leste is correlative with the Mutis Complex of West Timor and both form part of the Banda Terrane, which is composed mostly of dispersed fragments of the eastern Great Indonesian Arc. The study demonstrates the complex nature and deformational history of forearc basement.

Detrital zircon ages: Improving interpretation via Nd and Hf isotopic data

The bulk of sedimentary provenance studies use similarities in the detrital zircon age patterns or “barcodes” in sedimentary systems and potential source regions to make interpretations about palaeogeographic settings. While this “age-only” approach is generally considered to be effective, it is limiting because the timing of zircon growth events may not be unique to specific terrains and important rock forming events may be associated with little zircon growth. To a significant extent these limitations can be overcome by employing additional isotopic data sets such as Sm–Nd and Lu–Hf that provide information on the crustal evolution of the source region, and allow comparisons with sedimentary packages. As an example, the age spectra of detrital zircons in Palaeoproterozoic metasedimentary rocks in the eastern Gawler Craton in southern Australia are virtually identical to the dominant zircon growth timelines in adjacent older domains of the Gawler Craton, presenting a prima facie case that it was the source region. However, whole rock Nd isotopic data indicate that the pre-existing proto Gawler Craton was isotopically too crustally evolved ( ε Nd (1850 Ma ) − 10) to have supplied the bulk of the sediment to the relatively more juvenile metasedimentary units ( ε Nd (1850 Ma) − 1 to − 5). In addition, zircon Hf isotopic compositions from ca 2000 Ma detrital zircons in the metasedimentary rocks ( ε Hf (2000 Ma) + 2 to + 5) are significantly more juvenile than 2000 Ma rocks in the adjacent Gawler Craton ( ε Hf (2000 Ma) − 2 to − 5). The combination of bulk rock Nd and Hf zircon data suggest that the Gawler Craton is not a viable source region for the metasedimentary packages, despite the striking similarity between detrital zircon ages and zircon crystallisation events within the craton.

U-Pb zircon geochronology of Palaeozoic units in Western and Central Guatemala: insights into the tectonic evolution of Middle America

Abstract Precambrian and Palaeozoic basements are present in southern Mexico and Central America, where several crustal blocks are recognized by their different geological record, and juxtaposed along lateral faults. Pre-Mesozoic reconstructions must take into account the nature of such crustal blocks, their geological history, age and petrology. Some of those crustal blocks are currently located between southernmost north America (the Maya Block) and Central America (Chortís Block).To better understand the geology of these crustal blocks, and to establish comparisons between their geological history, we performed U–Pb dating of both igneous and metasedimentary key units cropping out in central and western Guatemala. In the Altos Cuchumatanes (Maya Block) granites yield both Permian (269±29 Ma) and Early Devonian (391±7.4 Ma) U–Pb ages. LA-ICPMS detrital zircon ages from rocks of the San Gabriel sequence, interpreted as the oldest metasedimentary unit of the Maya Block, and overlain by the Late Palaeozoic Upper Santa Rosa Group, yield Precambrian detrital zircons bracketed between c . 920 and c . 1000 Ma. The presence of these metasedimentary units, as well as Early Devonian to Silurian granites in the Mayan continental margin, from west (Altos Cuchumatanes), to east (Maya Mountains of Belize) indicates a more or less continuous belt of Lower Palaeozoic igneous activity, also suggesting that the continental margin of the Maya Block can be extended south of the Polochic fault, up to the Baja Verapaz shear zone. A metasedimentary sample belonging to the Chuacús Complex yielded detrital zircons with ages between c . 440 and c . 1325 Ma. The younger ages are similar to the igneous ages reported from the entire southern Maya continental margin, and show proximity of the Complex in the Middle-Late Palaeozoic. The S. Diego Phyllite, which overlies high-grade basement units of the Chortís Block, contains zircons that are Lower Cambrian ( c . 538 Ma), Mesoproterozoic ( c . 980 to c . 1150 Ma) and even Palaeoproterozoic ( c . 1820 Ma). Absence of younger igneous zircons in the San Diego Phyllite indicates that either its sedimentation took place in a close range of time, during the Late Cambrian, or absence of connection between Chortís and Maya Blocks during the Early–Mid-Palaeozoic. The Precambrian zircons could have come from southern Mexico (Oaxaca and Guichicovi Complexes), or from Mesoproterozoic Massifs exposed in Laurentia and Gondwana. Palaeogeographic models for Middle America are limited to post-Jurassic time. The data presented here shed light on Palaeozoic and, possibly, Precambrian relationships. They indicate that Maya and the Chortís did not interact directly until the Mesozoic or Cenozoic, as they approached their current position.

From subduction to collision: Thermal overprint of HP/LT meta-sediments in the north-eastern Lepontine Dome (Swiss Alps) and consequences regarding the tectono-metamorphic evolution of the Alpine orogenic wedge

The Cenozoic-age metamorphic structure of the Alps consists of a throughgoing pressure-dominated belt (blueschists and eclogites) that strikes parallel to the orogen and was later truncated by two thermal domes characterised by Barrow-type metamorphism (Lepontine dome and Tauern window). This study documents for the first time that relics of Fe-Mg carpholite occur also within meta-sedimentary units that are part of the north-eastern Lepontine structural and metamorphic dome, where so far exclusively Barrovian assemblages were found. They occur in meta-sediments of both Valais Oceanderived Lower Penninic Bündnerschiefer and structurally lower Europe-derived Sub-Penninic cover nappes and slices. These high-pressure units were subsequently overprinted by a thermal event, as is documented by the growth of new minerals typical for Barrovian metamorphism. We present evidence for a two-stage metamorphic evolution in the northern part of the Lepontine dome: (1) Early subduction-related syn-D1 (Safien phase) HP/LT metamorphism under blueschist facies conditions (350–400 °C and 1.2–1.4 GPa) was immediately followed by “cold” isothermal (or cooling) decompression during D2 nappe-stacking (Ferrera phase). (2) Collisionrelated Barrovian overprint (500–570 °C and 0.5–0.8 GPa) postdates the D3 nappe-refolding event (Domleschg phase) and represents a late heating pulse, separated by D2 and D3 from the D1 high-pressure event. It occurred before and/or during the initial stages of D4 (Chiéra phase) representing a second nappe-refolding event. In discussing possible heat sources for the late Barrow-type heating pulse it is argued that heat release from radioactive decay of accreted material may play an important role in contributing much to heat production. Based on the field evidence, we conclude that heat transfer was essentially conductive during these latest stages of the thermal evolution.

The Zn-Pb deposits of Casario (Ligurian Alps, NW Italy): Late Palaeozoic sedimentary-exhalative bodies affected by the alpine metamorphism

In the Ligurian Alps (South-Western Italian Alps), Zn-Pb deposits occur within late Palaeozoic meta-sedimentary units belonging to the Briançonnais Zone near Casario (Tanaro valley). Different types of sulphide-rich, lens-shaped mineralizations are recognized: sphalerite-galena massive sulphide bodies, pyrite-rich lenses and sulphide-rich quartz–carbonate-chloritoid granofels. Sulphide lenses and host rocks are affected by at least three ductile deformation phases and by a polyphase alpine metamorphism, whose climax conditions are estimated, based on P-T pseudosection calculations, at T = 300-325 °C and P = 0.55-0.60 GPa. In all the mineralized lenses the ore minerals are represented, in variable amount, by Fe-poor sphalerite, galena, pyrite and arsenopyrite (± tetrahedrite, chalcopyrite and pyrrhotite); the gangue consists of quartz, carbonate (sideritemagnesite ± rhodochrosite s.s.), Fe-chloritoid, muscovite-phengite and chlorite. The mineralizations are associated with chloritoid – carbonate micaschists displaying a finely bedded texture, with sharp between-bed compositional contrast, which suggests their exhalative origin. In spite of the tectono-metamorphic overprint, some pre-metamorphic features of the hydrothermal system are still recognized, like relics of the hydrothermal feeding system, primary growth textures and sulphide-rich microbreccias. These massive sulphide lenses, which share many characters with the SEDEX deposits, testify to the occurrence of an exhalative event of Upper Carboniferous age previously unrecognized in the Ligurian Briançonnais Unit.

Late Carboniferous porphyry copper mineralization at La Voluntad, Neuquén, Argentina: Constraints from Re–Os molybdenite dating

The La Voluntad porphyry Cu–Mo deposit in Neuquen, Argentina, is one of several poorly known porphyry-type deposits of Paleozoic to Early Jurassic age in the central and southern Andes. Mineralization at La Voluntad is related to a tonalite porphyry from the Chachil Plutonic Complex that intruded metasedimentary units of the Piedra Santa Complex. Five new Re–Os molybdenite ages from four samples representing three different vein types (i.e., quartz–molybdenite, quartz–sericite–molybdenite and quartz–sericite–molybdenite ± chalcopyrite–pyrite) are identical within error and were formed between ~312 to ~316 Ma. Rhenium and Os concentrations range between 34 to 183 ppm and 112 to 599 ppb, respectively. The new Re–Os ages indicate that the main mineralization event at La Voluntad, associated to sericitic alteration, was emplaced during a time span of 1.7 ± 3.2 Ma and that the deposit is Carboniferous in age, not Permian as previously thought. La Voluntad is the oldest porphyry copper deposit so far recognized in the Andes and indicates the presence of an active magmatic arc, with associated porphyry style mineralization, at the proto-Pacific margin of Gondwana during the Early Pennsylvanian.

The oldest known rocks in south–western China: SHRIMP U–Pb magmatic crystallisation age and detrital provenance analysis of the Paleoproterozoic Dahongshan Group

The Paleoproterozoic Dahongshan Group outcrops along the present day southwest margin of the Yangtze Craton. It is the oldest rock found in the region and forms the basement for the late Mesoproterozoic sedimentary successions. The Dahongshan Group is a sequence of volcanic and sedimentary rocks which have undergone structural deformation and metamorphism to upper greenschist–lower amphibolite facies. Some of the original sedimentary features, such as unimodal cross-bedding in quartzite and rounded conglomerate clasts, are still preserved, suggesting deposition in a fluvial environment. Abundant carbonate rocks, also found within the sedimentary sequence, indicate periods of shallow marine or lagoonal conditions. A weighted mean SHRIMP 206Pb/ 207Pb age of 1675 ± 8 Ma for a tuffaceous schist unit confirms the Paleoproterozoic formation age of the group. SHRIMP and LAM ICP-MS U–Pb analysis of detrital zircon grains from metasedimentary units indicate a late Archean to Paleoproterozoic (ca. 2780–1860 Ma) source region. The age spectrum of the detrital zircons is similar to that of the late Archean and early Paleoproterozoic basement rocks previously identified in the Yangtze Craton, but includes a population of Paleoproterozoic grains (ca. 2400–2100 Ma) which have no known source region in the exposed Yangtze Craton.

West Gondwana amalgamation based on detrital zircon ages from Neoproterozoic Ribeira and Dom Feliciano belts of South America and comparison with coeval sequences from SW Africa

Abstract Neoproterozoic–Cambrian amalgamation of West Gondwana involved the collision of several terranes of older crust that are now in eastern South America and western Africa. U–Pb (SHRIMP) detrital zircon ages from representative metasedimentary units of the Ribeira and Dom Feliciano belts (South America) and Gariep and Damara belts (Africa) provide constraints on the possible sediment source areas across probable suture zones. Ribeira detrital zircons are Palaeoproterozoic and Archaean. For the Dom Feliciano Belt, a contribution of Meso- and Neoproterozoic zircons is present, which definitely indicate Neoproterozoic sedimentation. It is proposed that the inflow of material to the Ribeira basin was essentially derived from the Paranapanema and Rio de la Plata cratons, whereas for the Damara and Gariep–Rocha belts source areas were from the Namaqua Belt. The Dom Feliciano Belt received sediments from the South American side and to a lesser degree from African sources. These results highlight the differences in the detrital zircon signatures across a proposed West Gondwanan suture, with those in the west being derived from distinctive South American basement sources and those in the east from distinctive African sources.

Synthetic model testing and distributed acquisition dc resistivity results over an unconformity uranium target from the Athabasca Basin, northern Saskatchewan

The dc resistivity method has become a preferred reconnaissance mapping tool for uranium exploration targets in the Athabasca Basin, in northern Saskatchewan, Canada. Regionally, uranium deposits can occur beneath 1 km thick sandstone cover rocks, are commonly associated with deeper basement graphitic metasedimentary units and are also often accompanied by clay-alteration zones in the sandstones. As a result, deep-penetrating electromagnetic and electrical geophysical techniques are ideally suited for indirect exploration of these types of deposits. A variety of electrode configurations are being used, however the pole-pole array is currently favoured in the Athabasca Basin due to its high signal levels, its deep penetration and its anomaly resolution. More recently, however, other technologies such as audio-magnetotellurics (AMT/MT) and 24-bit A/D distributed acquisition systems (DAS) have been introduced to extend the depth of exploration below 1 km. In the example presented here, a DAS acquisition system was used to acquire dc resistivity data, using a variety of electrode arrays, to examine the response parameters from the different configurations along a single line located along a known conductive trend at the M-Zone on the Wheeler River property.

Isotope chemostratigraphy of marbles in northeastern Mozambique: Apparent depositional ages and tectonostratigraphic implications

Marbles are minor but characteristic components of metasedimentary units within nappes in the Pan-African Mozambique Belt in NE Mozambique. Metasedimentary units remain largely undated, and carbon and strontium isotope stratigraphy of marbles has been used for indirect dating of the depositional history in this part of the Mozambique Belt. Sixty-nine samples from nine occurrences of dolomite, calcite and magnesite marbles in the Montepuez, Xixano, Lalamo, Ocua and Nampula metamorphic complexes were analysed for major and trace elements, and a subset of 39 samples for C, O and Sr isotopes. The least altered δ 13C values range from −3.5 to +7.1‰ (V-PDB) and 87Sr/ 86Sr ratios from 0.70504 to 0.70671. These values are considered as the best proxy to seawater composition at the time of deposition. The apparent deposition ages, derived from available seawater evolution curves, range from c. 1250 to c. 660 Ma. An age of 1250–910 Ma is obtained from a tripartite marble unit in the Montepuez Complex which is exposed in the Montepuez quarries. Five other age-groups are represented by marble units with apparent depositional ages of 800–750 Ma (Xixano North), 800–660 Ma (Montepuez West), c. 750 Ma (Nampula), c. 740 Ma (Xixano South and Lalamo), and 740–670 Ma (Montepuez East). The data suggest that: ( i) Pan-African nappes in NE Mozambique include Neoproterozoic and probable Mesoproterozoic sediments; ( ii) Neoproterozoic rocks of the Xixano and Nampula complexes might have different ancestry and were tectonically juxtaposed during the Neoproterozoic Pan-African orogeny.

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

This paper provides a first description of all major plutonic rock units between Resolution Island and Lake Poteriteri in southwest Fiordland. Plutonic rocks, of which c. 95% are granitoids, comprise c. 60% of the basement in southwest Fiordland. Approximately 50% of the plutonic rocks were emplaced between c. 355 and 348 Ma, 5% at c. 164 Ma, 25%between c. 140 and 125 Ma, and 20% between c. 125 and 110 Ma. These episodes of plutonism occurred in response to terrane amalgamation, continental thickening, and subduction along the convergent margin of Gondwana. Correlatives of Devonian plutonic rocks which occur in Nelson are absent from the area described here. A wide variety of plutonic rocks were emplaced at c. 355–348 Ma. These include relatively small plutons of K‐ and Rb‐rich gabbro‐diorite and members of at least three distinct suites of granitoids. Plutons of two‐mica ± garnet granodiorite, granite, and minor tonalite share affinities with the S‐type Ridge Suite and are the most widespread c. 355–348 Ma old granitoids in southern Fiordland. Plutons rich in Ca, Fe and Zr, depleted in K and Na, and containing quartz diorite, tonalite, and minor granodiorite with the unusual assemblage red‐brown biotite, garnet ± hornblende ± clinopyroxene also occur widely in southern Fiordland. These plutons are similar to peraluminous A‐type granitoids, indicating A as well as I and S‐type plutonism occurred in the Western Province at this time. The Newton River and Mt Evans Plutons have no correlatives amongst c. 355–348 Ma granitoids in southern Fiordland, but their chemistry is similar to that of the older Karamea Suite. Three regional‐scale metasedimentary units—locally fos‐siliferous Fanny Bay Group Buller Terrane rocks in southern Fiordland, Edgecumbe and Cameron Group Takaka Terrane rocks in south‐central Fiordland, and undifferentiated Deep Cove Gneiss high‐grade metasedimentary rocks of western Fiordland—are all stitched by c. 355–348 Ma old plutons, indicating they have been in close proximity since at least c. 355–348 Ma. In south‐central Fiordland, c. 355–348 Ma old plutons cut across fabrics defined by upper amphibolite facies mineral assemblages, indicating low pressure/high temperature metamorphism in this area before this time. The c. 164 Ma old leucocratic Lake Mike Granite is a unique pluton in southwest Fiordland with no obvious correlatives. Plutons emplaced between c. 140 and 125 Ma are similar to the Rahu Suite, although isotopic data are required to confirm this correlation. Rahu Suite plutonism may therefore have begun by c. 140 Ma, rather than c. 120 Ma as previously suggested. Plutons emplaced between c. 125 and 110 Ma have high Sr/Y ratios comparable with the Separation Point Suite. They occur in both an outboard location around Lake Poteriteri and an inboard location around the western end of Dusky Sound. The c. 115 Ma two‐mica garnet granites of the Anchor Island Intrusives #2 probably formed by partial melting of adjacent ortho‐ and paragneisses, indicating that upper amphibolite facies metamorphism in western Dusky Sound occurred during the Early Cretaceous. The Dusky Fault does not pass directly out to the coast through outer Dusky Sound as previously mapped. Instead it merges with the major northeast‐striking Lake Fraser Fault at Cascade Cove, which crosses the outer coast near West Cape. The Last Cove Fault is a minor structure which cannot be traced beyond Last Cove rather than a major fault of regional extent as has been previously suggested.

Pu–Xe, U–Xe, U–Pb chronology and isotope systematics of ancient zircons from Western Australia

The presence of xenon isotopes from in-situ spontaneous fission of short-lived 244Pu has been confirmed in a suite of 16 Hadean detrital zircons from Western Australia. In order to investigate the effects of xenon loss on estimates of the inferred Pu/U ratio we have irradiated the zircons with thermal neutrons to generate Xe from 235U neutron fission. 131Xe/ 134Xe and 132Xe/ 134Xe ratios have been used to calculate the relative contributions from spontaneous fission of 244Pu and 238U and neutron fission of 235U and hence compare nominal Pu/U ratios and xenon retention ages. U–Xe ages are typically lower than the Pb–Pb ages, indicating that xenon loss is common. We show how the ternary mixing diagram can be used to place constraints on the timing and extent of this loss and to generate a corresponding Pu–U–Xe isochron. Although the zircons investigated in this study were extracted from the same metasedimentary unit, the timing of xenon loss is variable. This suggests that the loss may be the result of variable degrees of metamictization from grain to grain. Inferred (Pu/U) o ratios show a general decrease with the discordancy between Pb–Pb and U–Xe ages. For the least discordant samples we infer (Pu/U) o ∼ 0.008 which is close to the widely adopted chondritic value. While we cannot completely exclude the effects of Pu/U fractionation in magmatic and other processes between formation of the Earth and crystallisation of the zircons we conclude that they have been relatively small (< factor 2?) and reflect the compatible behaviour of Pu and U in the 4+ valence state.

PLATINUM-GROUP ELEMENTS IN ORDOVICIAN MAGMATIC Ni Cu SULFIDE PROSPECTS IN NORTHEAST SCOTLAND

Previous work on early Paleozoic mafic–ultramafic intrusions in northeast Scotland identified two targets for Ni–Cu–PGE exploration. The first prospect is at Arthrath, near the town of Ellon, and the second, in the southeastern corner of the Knock intrusion, near the town of Huntly, around the farms of Littlemill and Auchencrieve. Both prospects occur within a group of mafic–ultramafic rocks known as the Younger Basic intrusions, emplaced broadly synchronously with the later stages of Ordovician amphibolite-facies metamorphism of metasedimentary units of the Dalradian Supergroup. We have re-examined the available samples from the Ni–Cu exploration program in terms of their PGE potential, placing particular emphasis on the less-well-known Arthrath prospect. PGE abundances of up to 418 ppb Pt (Littlemill) and 458 ppb Pd (Arthrath) associated with zones of Cu–Ni–Fe sulfide mineralization have been identified. At Arthrath, the Pd is hosted primarily in merenskyite enclosed in base-metal sulfide; the occurrences of platinum-group minerals define a bimodal distribution. At Littlemill, the occurrences of platinum-group minerals involve mobilization and redeposition of precious metals in sheared rocks enclosed within the base-metal sulfide zones. When the mineral compositions of all the rocks associated with the zones of Cu–Ni–PGE mineralization are considered relative to the known differentiation series present within these intrusions, a model involving the mixing and mingling of relatively late, primitive magma and pre-existing differentiates is favored for both prospects. In particular, a cumulate orthopyroxenite unit at Arthrath closely associated with a zone of Cu–Ni–PGE mineralization is directly interpreted as the result of the influx into a magma chamber of a late, primitive magma. Previous work had suggested crustal contamination as the mechanism of sulfide immiscibility. Although evidence of crustal contamination is present, it is not favored as the direct mechanism for the formation of the sulfide-rich zones that comprise the principal targets of Ni–Cu–PGE exploration.

U-Pb Geochronological Constraints On The Evolution Of The ASPY Terrane, Cape Breton Island: Implications For Relationships Between ASPY And BRAS D'OR Terranes And Ganderia In The Canadian Appalachians

New U-Pb zircon ages from nine samples of igneous and sedimentary rocks in the Aspy terrane, Cape Breton Island, show that Neoproterozoic rocks form a major part of the terrane and confirm that the terrane was affected by a major Silurian-Devonian tectonothermal event. A rhyolitic crystal tuff, a leucotonalite pluton and a felsic sheet yield ages of 618.8 ± 0.6 Ma, 619.7 ± 0.9 Ma and 573.5 ± 2.7 Ma, respectively. A metasedimentary unit contains zircon ranging in age from ca. 546 to ca. 1520 Ma. A diorite has an age of 428.6 ± 1.9 Ma, and another diorite unit and a quartz porphyry also have probable Late Silurian ages. A third diorite yields an age of 373.0 ± 0.5 Ma, part of a widespread bimodal igneous event represented by both volcanic and plutonic rocks throughout the Aspy region. Geochronological results and field relationships indicate that the Neoproterozoic rocks are similar to those in the Bras d9Or terrane and form the basement to the (Ordovician-)Silurian rocks in the Aspy terrane, and that the Aspy terrane is probably correlative with rocks in the Hermitage flexure of southern Newfoundland where Silurian metasedimentary and metavolcanic rocks lie unconformably on similar Neoproterozoic rocks. Likely correlative rocks also occur in the Kingston terrane of southern New Brunswick. The results of this study support the idea that the Aspy-Kingston terrane is part of Ganderia and that Neoproterozoic rocks in the Bras d9Or terrane and correlative rocks in southwestern Newfoundland and elsewhere represent exposed basement to Ganderia in the Canadian Appalachians.

THE METASILICICLASTIC-CARBONATE SEQUENCE OF THE ACQUADOLCE UNIT (EASTERN ELBA ISLAND): NEW PETROGRAPHIC DATA AND PALEOGEOGRAPHIC INTERPRETATION

The Acquadolce Unit (upper part of Trevisan’s Complex II) consists of a phyllitic-quartzitic succession with calcschist and metagraywacke intercalations and a serpentinite wedge at the top. This unit was generally correlated to the Dogger-Eocene metasediments of the Tuscan Paleozoic-Jurassic succession of the Apuan Alps but, more recently, has been attributed to the Ligurian or Piedmontese Domain (e.g. the “Schistes Lustres” of the Gorgona Island). In this paper we present a detailed petrographic study of both the Acquadolce Unit and the Metasedimentary Unit of the Gorgona Island. In particular, we found significant mineralogical analogies between the lithotypes and tectono-metamorphic imprint of the two successions. The hypothesis that the Acquadolce Unit can be ascribed to a “Schistes Lustres”-type succession is also supported by the relatively high-pressure (probably >8kbar) estimated for the D1 deformation phase of the Acquadolce Unit from the K-white mica composition. The Acquadolce Unit is attributed to the metamorphic Ligurian-Piedmontese successions, allowing a redefinition of the main geological boundaries in the Tuscan Archipelago.

Petrografia e geoquímica das rochas metavulcânicas máficas e ultramáficas do Complexo Metamórfico Brusque, região da Serra da Miséria, Itapema, SC

Brusque Metamorphic Complex is one of main units of the Tijucas belt in Santa Catarina. In Itapema region this complex is composed by metasedimentary association with subordinate presence of mafic and ultramafic metavolcanic rocks. The metavolcanic units occour as four bodies of metric to decametric thickness interlayered with metapelites, quartzites and calc-silicate schists. The mafic rocks are metabasalts and amphibole schists and the ultrama- fic rocks are magnesian schists, all interpreted as interlayered mafic flows. Blastoporphyritic and blastoamigdaloidal textures are recognized in metabasalts. Geochemically is identified one volcanic suite composed by tholeitic basalts of hy-normative type and ultramafic rocks of cumulatic origin. Chemical composition and normalized mantelic trace elements display typical features of continental tholeiitic basalts and suggests crustal elements-enriched mantelic sources. Field relations, geochemistry and protolith composition indicates that the mafic and ultramafic bodies were probably originated at the same tectonic environment. These bodies represent volcanic episodes generated in a continental rift of meso to neoproterozoic age. Considering the regional context, the formation of volcanogenic and metasedimentary units of Brusque Metamorphic Complex marks a period of fragmentation of ancient paleoproterozoic continental crust represented in this region by gneisses of Santa Catarina Granulitic Complex and Camboriu Complex.

Zircon geochronology from the Kangaatsiaq–Qasigiannguit region, the northern part of the 1.9–1.8 Ga Nagssugtoqidian orogen, West Greenland

The Kangaatsiaq–Qasigiannguit region in the northern part of the Palaeoproterozoic Nagssugtoqidian orogen of West Greenland consists of poly-deformed orthogneisses and minor occurrences of interleaved, discontinuous supracrustal belts. Laser ablation ICP-MS 207Pb/206Pb analyses of detrital zircons from four metasedimentary rocks (supplemented by ion probe analysis of one sample) and igneous zircons from six granitoid rocks cutting metasedimentary units indicate that the supracrustal rocks in the Kangaatsiaq–Qasigiannguit (Christianshåb) region are predominantly Archaean in age. Four occurrences of metasedimentary rocks are clearly Archaean, two have equivocal ages, and only one metasedimentary unit, from within the Naternaq (Lersletten) supracrustal belt, is demonstrably Palaeoproterozoic and readily defines a large fold complex of this age at Naternaq. The 2.9–2.8 Ga ages of detrital Archaean grains are compatible with derivation from the local basement orthogneisses within the Nagssugtoqidian orogen. The detrital age patterns are similar to those of metasediments within the central Nagssugtoqidian orogen but distinct from age patterns in metasediments of the Rinkian belt to the north, where there is an additional component of pre-2.9 Ga zircons. Synkinematic intrusive granitoid rocks constrain the ages of some Archaean deformation at 2748 ± 19 Ma and some Palaeoproterozoic deformation at 1837 ± 12 Ma.

Influence of volcano-sedimentary facies architecture on strain partitioning during the evolution of an orogenic-gold lode system, Stawell, western Victoria

The localised distribution of gold in a series of ore shoots adjacent to the western flank of the Cambrian Magdala Basalt is controlled by formation and partial reactivation of faults and shear zones that closely follow the geometry of the basalt. The recognition of the controls on the geometry of the basalt and overlying metasedimentary units is therefore vital for exploration in the region. Detailed structural mapping around one of the steep, west-dipping basalt sheets that comprise the Magdala Basalt, as well as the overlying metasedimentary units, has been used to compare the recorded structural history in the basalt relative to that in the metasediments to determine how strain has partitioned during the evolution of the deposit, and if the geometry is a product of primary volcanic processes or early ductile deformation. The results of this work show that much of the deformation history at the Magdala mine is recorded within the metasediments, while the basalt itself appears only affected by the late brittle deformation. The lack of evidence for early ductile deformation in the basalt and the partitioning of strain into the Albion Formation early in the deformation process suggests that the variable geometry of the basalt is a product of primary volcanic processes, rather than ductile deformation. The irregular geometry of basalt flows along the southwestern flank of the Magdala Basalt may be related to the dominance of distal volcanic facies, primarily pillow basalt, the variability of which influences localised deformation and gold mineralisation during an episode of low displacement east – west compression.

The origins and histories of metasedimentary units in the core of the Himalaya

The origins and histories of metasedimentary units in the core of the Himalaya