A U‐Pb zircon study of the Mesoproterozoic Charleston Granite,Gawler Craton,South Australia

The Charleston Granite from the Gawler Craton, South Australia, has been dated by the ion‐microprobe U‐Pb zircon method at 1585 ± 5 Ma (2σ). This confirms previous interpretations of population‐style U‐Pb zircon analyses which record a slightly older age due to the presence of inherited zircon. Inherited cores are present in many zircon crystals, and while the age of some cores can not be accurately determined due to extreme loss of radiogenic Pb, others have ages of ～ 1780, ～ 1970, and > 3150 Ma. These cores record a diverse crustal heritage for the Charleston Granite and indicate that ancient crustal material (> 3150 Ma) is present at depth in the Gawler Craton. This is also suggested by available Nd isotopic data for both the Charleston Granite and other Gawler Craton Archaean rocks. The Rb‐Sr and K‐Ar biotite ages from the Charleston Granite of 1560 to 1570 Ma are close to the U‐Pb zircon crystallization age and suggest that the granite has not experienced sustained thermal disturbance (> 250° C) since emplacement and cooling. However, a much younger Rb‐Sr total‐rock age of 1443 ± 26 Ma probably reflects low‐temperature disturbance to the Sr isotope system in feldspar.     

Timing of the progress granite,Larsemann hills: Additional evidence for early Palaeozoic orogenesis within the east Antarctic Shield and implications for Gondwana assembly

The Progress Granite is one of numerous S‐type granitoid plutons exposed in the Larsemann Hills region, southwest Prydz Bay, east Antarctica. The granite was emplaced into a migmatitised pelitic to felsic paragneiss sequence during a regional high‐grade transpressional event (D₂) that pre‐dates high‐grade extension (D₃). SHRIMP (II) U‐Pb dating for two occurrences of the Progress Granite from D₂ and D₃ structural domains gives ²⁰⁶Pb/²³⁸U ages of 516.2 ± 6.8 Ma and 514.3 ± 6.7 Ma, respectively. These ages are interpreted as crystallisation ages for the Progress Granite and confirm Early Palaeozoic orogenesis in the Larsemann Hills region. This orogen appears to have evolved during continental convergence and is probably responsible for widespread radiogenic isotopic resetting and the near‐complete exhumation of the adjacent northern Prince Charles Mountains which evolved during a ca 1000 Ma event. The identification of a major Early Palaeozoic orogen in Prydz Bay allows tentative correlation of other domains of Early Palaeozoic tectonism both within the east Antarctic Shield and other, once contiguous, Gondwana fragments and illustrates the potential complexity inherent within intercratonic mobile belts. One such possibility, tentatively offered here, suggests a continuous belt of Early Palaeozoic tectonism from Prydz Bay eastward to the West Denman Glacier region and into the Leeuwin complex of Western Australia.     

Late Ordovician stratigraphy,zircon provenance and tectonics,Lachlan Fold Belt,southeastern Australia

Ordovician quartz turbidites of the Lachlan Fold Belt in southeastern Australia accumulated in a marginal sea and overlapped an adjoining island arc (Molong volcanic province) developed adjacent to eastern Gondwana. The turbidite succession in the Shoalhaven River Gorge, in the southern highlands of New South Wales, has abundant outcrop and graptolite sites. The succession consists of, from the base up, a unit of mainly thick‐bedded turbidites (undifferentiated Adaminaby Group), a unit with conspicuous bedded chert (Numeralla Chert), a unit with common thin‐bedded turbidites (Bumballa Formation (new name)) and a unit of black shale (Warbisco Shale). Coarse to very coarse sandstone in the Bumballa Formation is rich in quartz and similar to sandstone in the undifferentiated Adaminaby Group. Detrital zircons from sandstone in the Bumballa Formation, and from sandstone at a similar stratigraphic level from the upper Adaminaby Group of the Genoa River area in eastern Victoria, include grains as young as 453–473 Ma, slightly older than the stratigraphic ages.The dominant detrital ages are in the interval 500–700 Ma (Pacific Gondwana component) with a lessor concentration of Grenville ages (1000–1300 Ma). This pattern resembles other Ordovician sandstones from the Lachlan Fold Belt and also occurs in Triassic sandstones and Quaternary sands from eastern Australia. The Upper Ordovician succession is predominantly fine grained, which reflects reduced clastic inputs from the source in the Middle Cambrian to earliest Ordovician Ross‐Delamerian Fold Belts that developed along the eastern active margin of Gondwana. Development of subduction zones in the Late Ordovician marginal sea are considered to be mainly responsible for the diversion of sediment and the resulting reduction in the supply of terrigenous sand to the island arc and eastern part of the marginal sea.     

Proterozoic‐Cambrian detrital zircon and monazite ages from the Anakie Inlier,central Queensland: Grenville and Pacific‐Gondwana signatures

The Anakie Metamorphic Group is a complexly deformed, dominantly metasedimentary succession in central Queensland. Metamorphic cooling is constrained to ca 500 Ma by previously published K–Ar ages. Detrital‐zircon SHRIMP U–Pb ages from three samples of greenschist facies quartz‐rich psammites (Bathampton Metamorphics), west of Clermont, are predominantly in the age range 1300–1000 Ma (65–75%). They show that a Grenville‐aged orogenic belt must have existed in northeastern Australia, which is consistent with the discovery of a potential Grenville source farther north. The youngest detrital zircons in these samples are ca 580 Ma, indicating that deposition may have been as old as latest Neoproterozoic. Two samples have been analysed from amphibolite facies pelitic schist from the western part of the inlier (Wynyard Metamorphics). One sample contains detrital monazite with two age components of ca 580–570 Ma and ca 540 Ma. The other sample only has detrital zircons with the youngest component between 510 Ma and 700 Ma (Pacific‐Gondwana component), which is consistent with a Middle Cambrian age for these rocks. These zircons were probably derived from igneous activity associated with rifting events along the Gondwanan passive margin. These constraints confirm correlation of the Anakie Metamorphic Group with latest Neoproterozoic ‐ Cambrian units in the Adelaide Fold Belt of South Australia and the Wonominta Block of western New South Wales.     

Relationship between volcanism and marine sedimentation in northern Austral (Aisén) Basin, central Patagonia: Stratigraphic, U–Pb SHRIMP and paleontologic evidence

The northernmost part of the oil-producing Austral Basin, known as Aisén Basin or Río Mayo Embayment (in central Patagonian Cordillera; 43–46°S), is a special area within the basin where the interplay between volcanism and the initial stages of its development can be established. Stratigraphic, paleontologic and five new U–Pb SHRIMP age determinations presented here indicate that the Aisén Basin was synchronous with the later phases of volcanism of the Ibáñez Formation for at least 11 m.yr. during the Tithonian to early Hauterivian. In this basin marine sedimentary rocks of the basal units of the Coihaique Group accumulated overlying and interfingering with the Ibáñez Formation, which represents the youngest episode of volcanism of a mainly Jurassic acid large igneous province (Chon Aike Province). Five new U–Pb SHRIMP magmatic ages ranging between 140.3 ± 1.0 and 136.1 ± 1.6 Ma (early Valanginian to early Hauterivian) were obtained from the Ibáñez Formation whilst ammonites from the overlying and interfingering Toqui Formation, the basal unit of the Coihaique Group, indicate Tithonian, early Berriasian and late Berriasian ages. The latter was a synvolcanic shallow marine facies accumulated in an intra-arc setting, subsequently developed into a retro-arc basin.     

Datation U–Pb des deux faciès du granite de Soultz (Fossé rhénan,France)

Deep boreholes drilled in the basement of the Rhine Graben at Soultz-sous-Forêts have shown the presence of an ubiquitous monzogranite. Borehole GPK-2, with a total depth of 5090 m, also intersected a more leucocratic fine-grained two-mica granite, locally present below 4860 m depth and continuously found between 5047 and 5090 m. Thanks to trace elements and in particular to REE (rare-earth elements), it could be shown that the leucocratic rock is a differentiated expression of the potassic magma that was at the origin of the monzogranite pluton. This model agrees with Sr–Nd isotope data, even though a slight contribution from the pre-existing basement should be considered as well. Use of the U–Pb method on monzogranite from EPS-1 – after zircon dissolution – has yielded an emplacement age of 334.0+3.8/?3.5 Ma $\text{(2}\text{σ)}$. Point dates obtained by SHRIMP II on the rare zircons from the fine-grained granite showed that it was emplaced in a basement with very heterogeneous ages, ranging from Early Proterozoic to Silurian. The estimated crystallization age of the last granite is 327±7 Ma $\text{(2}\text{σ)}$, slightly later than, or sub-contemporaneous with, the emplacement of the common monzogranite, in agreement with structural constrains. To cite this article: A. Cocherie et al., C. R. Geoscience 336 (2004).     

New age constraints for Grenville-age metamorphism in western central Dronning Maud Land (East Antarctica), and implications for the palaeogeography of Kalahari in Rodinia

New SHRIMP zircon data from Gjelsvikfjella and Mühlig–Hofmann–Gebirge (East Antarctica) indicate that the metamorphic basement is composed of Grenville-age rocks that are most likely part of the north-eastern continuation of the Namaqua–Natal–Maud Belt. Crystallisation ages of meta-igneous rocks range between ca. 1,150 to 1,100 Ma, with little inheritance recorded. Metamorphic zircon overgrowth during high-grade metamorphism is dated between ca. 1,090 to 1,050 Ma. Both, the crystallisation ages and the metamorphic overprint are similar to U–Pb data from a number of areas along a ca. 2,000-km stretch from Natal in South Africa to central Dronning Maud Land. The basement underwent in part strong high-grade reworking during the collision of East and West Gondwana at ca. 550 Ma. The timing of Grenville-age metamorphism has important implications for the position of Kalahari in Rodinia. It also questions that Coats Land is part of the Maud Belt because the undeformed volcanic rocks of Coats Land are older than the main metamorphism within the Maud Belt and, therefore, must rest on older basement. This interpretation explains why the pole of Coats Land at ca. 1,110 Ma differs from the Kalahari poles by 30°, i.e. Coats Land had not yet amalgamated to Kalahari. On the other hand, the palaeopoles from Coats Land and Laurentia at 1,110 Ma are identical within error. Thus, Coats Land could have been part of Laurentia prior to the final amalgamation of Rodinia, the Namaqua–Natal–Maud Belt could have been a part of the Grenville Belt and the entire Kalahari Craton could indeed have opposed Laurentia on its eastern side.     

SHRIMP U–Pb dating of the Antucoya porphyry copper deposit: new evidence for an Early Cretaceous porphyry-related metallogenic epoch in the Coastal Cordillera of northern Chile

The Antucoya porphyry copper deposit (300 Mt at 0.45% total Cu) is one of the largest deposits of a poorly known Early Cretaceous porphyry belt in the Coastal Cordillera of northern Chile. It is related to a succession of granodioritic and tonalitic porphyritic stocks and dikes that were emplaced within Jurassic andesitic rocks of the La Negra Formation immediately west of the N–S trending sinistral strike-slip Atacama Fault Zone. New zircon SHRIMP U–Pb data indicate that the porphyries of Antucoya crystallized within the time span from 142.7 ± 1.6 to 140.6 ± 1.5 Ma (±2 σ), and late, unmineralized, NW–SE trending dacite dikes with potassic alteration and internal deformation crystallized at 141.9 ± 1.4 Ma. The Antucoya porphyry copper system appears to be formed after a change of stress conditions along the magmatic arc from extensional in the Late Jurassic to transpressive during the Early Cretaceous and provides support for an Early Cretaceous metallogenic episode of porphyry-type mineralization along the Coastal Cordillera of northern Chile.     

The source of granitic gneisses and migmatites in the Antarctic Peninsula: a combined U–Pb SHRIMP and laser ablation Hf isotope study of complex zircons

Zircons gneisses and migmatites collected from the Antarctic Peninsula have different core–rim hafnium isotope ratio relationships depending on whether evidence for zircon dissolution is present or absent. Two samples contain inherited zircon that is partially dissolved. In these samples, the ¹⁷⁶Hf/¹⁷⁷Hf rations of the inherited zircon and new magmatic zircon rims are, on average, indistinguishable and consistent with in situ melting. In such cases the hafnium isotopic composition of the melt was probably strongly influenced by the dissolved zircon component at the source. Variation in ¹⁷⁶Hf/¹⁷⁷Hf within the magmatic zircon rims from grain to grain suggests that Hf isotopes were only partially homogenized during melt migration; alternatively, zircon growth may have taken place within small volumes of partial melt. Other samples do not preserve textural evidence for zircon dissolution during melt generation; in these samples the ¹⁷⁶Hf/¹⁷⁷Hf values of the inherited zircon and new magmatic zircon rims are different. The zircon rims apparently suggest a source of less evolved hafnium than that contained within the inherited zircon. Whether this relates to a separate juvenile source or, alternatively, is derived from minerals other than zircon at the source, cannot be resolved. Inherited zircon, irrespective of age, has been strongly influenced by the reworking of a juvenile Late Mesoproterozoic source, suggesting that such crust underlies the Antarctic Peninsula. Our results therefore suggest that Hf isotope analyses provide great potential for future studies investigating the source and processes involved in the generation of crustal melts.     

Zircon megacrysts from basalts of the Venetian Volcanic Province (NE Italy): U–Pb ages, oxygen isotopes and REE data

Mafic alkaline lavas from the Venetian Volcanic Province (NE Italy) contain orange–brown zircon megacrysts up to 15 mm long, subhedral to subrounded and showing equant morphology, with width-to-length ratios of 1:2–1:2.5. U–Pb ages of zircon (51.1 ± 1.5 to 30.5 ± 0.51 Ma) fit the stratigraphic age of the host lava (Middle Eocene and Oligocene) and their oxygen isotope composition (δ¹⁸O = 5.31–5.51‰) is similar to that of zircon formed in the upper mantle. Cathodoluminescence images and crystal chemical features, e.g. depletion of incompatible elements such as REE, Y, U and Th at constant Hf content, indicate that centre-to-edge zircon zoning is not consistent with evolution of the melt by fractional crystallization. All the above features, together with the fact that zircon and host basalts are coeval, indicate that the studied Zr megacrysts crystallised from a primitive alkaline mafic magma, which later evolved to the less alkaline host magma.