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Testing ore deposit models using in situ U-Pb geochronology of hydrothermal monazite Paleoproterozoic gold mineralization in northern Australia [查看] BirgerRasmussen、StephenSheppard、IanR.Fletcher

The inability to establish absolute ages for gold deposition in the Pine Creek orogen of northern Australia has led to conflicting ore deposit models, ranging from intrusion related, which predict that gold mineralization was synchronous with granite magmatism(ca. 1835–1820 Ma), to orogenic, which place ore deposition nearly 100 m.y. later. Here we present ion microprobe U-Pb geochronology for a mineralized quartz reef from Tom’s Gully mine, Mount Bundey, Northern Territory, Australia, and nearby granitic rocks and associated contact aureoles. Isotopic dating of zircon and monazite indicates that intrusion and contact metamorphism occurred ca. 1825 Ma, whereas hydrothermal monazite from the auriferous quartz reef gives a mean 207Pb/206Pb age of 1780 6 10 Ma, interpreted as the time of gold mineralization. Mineralization therefore postdated intrusion by ;45 m.y. and preceded a postulated ca. 1740–1730 Ma cratonwide orogenic gold event by ;50 m.y. Hence, neither the intrusion-related model nor the recently proposed orogenic model is applicable. Combined with a reevaluation of age data from the nearby Goodall gold deposit,our data suggest that mineralization coincides with, and may be related to, an episode of regional low-grade metamorphism, deformation, and fluid circulation (Shoobridge event). Our results demonstrate the importance of high precision in situ geochronology and detailed petrography for deciphering age relationships in ore deposits, and of testing the veracity of models for ore formation.

SHRIMP U–Pb zircon ages of pyroclastic rocks in the Bansong Group,Taebaeksan Basin, South Korea and their implication for the Mesozoic tectonics [查看] RaeheeHan、Jin-HanRee、Deung-LyongCho、Sung-TackKwon、RichardArmstrong

The Bansong Group (Daedong Supergroup) in the Korean peninsula has long been considered to be an important time marker for two wellknown orogenies, in that it was deposited after the Songnim orogeny (Permian–Triassic collision of the North and South China blocks) but was deformed during the Early to Middle Jurassic Daebo tectonic event. Here we present a new interpretation on the origin of the Bansong Group and associated faults on the basis of structural and geochronological data. SHRIMP (Sensitive High-Resolution Ion MicroProbe) U–Pb zircon age determination of two felsic pyroclastic rocks from the Bansong Group formed in the foreland basin of the Gongsuweon thrust in the Taebaeksan Basin yielded ages of 186.3T1.5 and 187.2T1.5 Ma, respectively, indicating the deposition of the Bansong Group during the late Early Jurassic.Inherited zircon component indicates ca. 1.9 Ga source material for the volcanic rocks, agreeing with known basement ages. The Bansong Group represents syntectonic sedimentation during the late Early Jurassic in a compressional regime. During the Daebo tectonic event, the northeast-trending regional folds and thrusts including the Deokpori (Gakdong) and Gongsuweon thrusts with a southeast vergencedeveloped in the Taebaeksan Basin. This is ascribed to deformation in a continental-arc setting due to the northwesterly orthogonal convergence of the Izanagi plate on the Asiatic margin, which occurred immediately after the juxtaposition of the Taebaeksan Basin against the Okcheon Basin in the late stage of the Songnim orogeny. Thus, the Deokpori thrust is not a continental transform fault between the North and South China blocks,but an “intracontinental” thrust that developed after their juxtaposition.

Sensitive high-resolution ion microprobe U-Pb ages of thelatest oligocene-early miocene rift-related hidaka high-temperaturemetamorphism in Hokkaido, northern Japan [查看] TADASHIUSUKI、HIROSHIKAIDEN、KEIJIMISAWA、KAZUYUKISHIRAISHI

In order to define the timing of granulite facies metamorphism, sensitive highresolution ion microprobe (SHRIMP) U-Pb analyses were performed on zircons of three pelitic granulites from the lower metamorphic sequence of the Hidaka Metamorphic Belt,southern central Hokkaido, Japan. Both rounded and prismatic zircons were found in the granulite samples. The rounded zircons had thin (10–20 μm) concentric overgrowth rims on detrital cores, while the prismatic zircons did not have detrital cores. Both the overgrowth rims on the rounded zircons and the entire prismatic zircons were formed under granulite facies metamorphism and consistently yield Latest Oligocene–Early Miocene ages (23.7 ± 0.4 Ma to 17.2 ± 0.5 Ma; 206Pb/238U ages (n = 31) with low Th/U ratios, mostly <0.1).The internal structure of zircons and their SHRIMP U-Pb ages provide strong evidence in support of the granulite facies event occurring during the Latest Oligocene-Early Miocene. The detrital cores of rounded zircons show a huge variety of ages; Mesoarchean to Paleoproterozoic, Paleozoic to Mesozoic and Paleogene. The interior and marginal portions of the Eurasian continent including cratonic areas are suggested for their source provenances. These wide variations in age suggest that the protolith of the granulites of the lower metamorphic sequence were deposited near the trench of the Eurasian continental margin during Paleogene. The protolith of the lower metamorphic sequence of the Hidaka metamorphic belt was thrust under the upper metamorphic sequence, which had already been metamorphosed in early Paleogene. The Latest Oligocene-Early Miocene Hidaka high-temperature metamorphic event is presumed to have been caused by asthenospheric upwelling during back-arc rifting of the Kuril and Japan basins.

Provenance and age delimitation of Quadrila´tero Ferrı´fero sandstones based on zircon U–Pb isotopes [查看] Le´oA.Hartmann、IssamuEndo、MarcosTadeuF.Suita、Joa˜oOrestesS.Santos、Jose´CarlosFrantz、Maurı´cioA.Carneiro、NealJ.McNaughton、MarkE.Barley

The Quadrila´tero Ferrı´fero has some of the largest iron and gold deposits in the world and is a major geotectonic unit of the Sa˜o Francisco Craton in Brazil. U–Pb zircon SHRIMP geochronology of six detrital sedimentary and metasedimentary rocks (114 zircon crystals, 118 spot analyses) has improved the understanding of the sedimentary processes and provenance ages of both rocks and the associated iron formation. The age of deposition of the iron formation is constrained between 2.58 and 2.42 Ga. The presence of an old Paleoarchean crust is dated in detrital zircon crystals, including the oldest zircon in South America (3809G3 Ma). Only high-Th/U, magmatic zircon crystals are present in the dated sedimentary rocks, and these indicate that the crust of the region was formed mostly during the Jequie´ cycle (six age peaks between 3055 and 2635 Ma). This time span ofw420 m.y. is similar to the duration of a long-lived Wilson cycle. Most of the Rio das Velhas Basin was filled during approximately 30 m.y. between 2746 and 2717 Ma, though volcanism probably started earlier. The youngest detrital zircon age from the Minas Supergroup indicates that the intracratonic basin fill, including the iron formation, was deposited after 2580 Ma. Therefore, the crust was cratonized shortly after the intrusion of minor granitic bodies at around 2.62–2.58 Ga. A large gap in orogenic activity is indicated by the absence of zircon ages of 2580–2182 Ma.

Palaeomagnetism and geochronology of mafic dykes in south Siberia, Russia the first precisely dated Early Permian palaeomagnetic pole from the Siberian craton [查看] S.A.Pisarevsky、D.P.Gladkochub、T.A.Donskaya、B.DeWaele、A.M.Mazukabzov

New palaeomagnetic and geochronological data from mafic dykes emplaced into the basement of the Siberian Craton on the southwestern coast of the Baikal Lake are present. The concordant zircon U–Pb SHRIMP age of these dykes is 275 ± 4 Ma. Palaeomagnetic study of the dykes isolated a stable primary remanence with steep upward direction supported by a positive baked contact test. The palaeomagnetic pole (50.5◦N, 121.4◦E, A95 =16.7◦) is the first precisely dated Siberian pole between ∼360 and 260 Ma. It falls near the interpolated Siberian APWP and suggests that Siberia had not joined Eurasia by 275 Ma. The studied dykes are the first reported evidence of Permian magmatic activity in the cratonic part of southern Siberia. They are probably related to the prominent 300–260 Ma magmatic event recognized in Trans-Baikalia.The comparison of our new result with recently published palaeomagnetic data from Trans-Baikalia suggests that differential movements between the Siberian Craton and blocks of Trans-Baikalia around 275Maare unlikely. This confirms tectonic models considering Trans-Baikalia as an Andean-type active margin of the Siberian continent in Permian times.

New SHRIMP geochronology for the Western Fold Belt of the Mt Isa Inlier developing a 1800-1650 Ma event framework [查看] N.L.NEUMANN、P.N.SOUTHGATE、G.M.GIBSON、A.MCINTYRE

The integration of detrital and magmatic U– Pb zircon SHIRMP geochronology with facies analysis has allowed the development of a chronostratigraphic framework for the Leichhardt and Calvert Superbasins of the Western Fold Belt, Mt Isa Inlier. This new event chart recognises three supersequences in the Leichhardt Superbasin: the Guide, Myally and Quilalar Supersequences. The Guide Supersequence spans the interval ca 1800 – 1785 Ma and includes the Bottletree Formation and the Mt Guide Quartzite. Sequence relationships suggest that this sedimentary package represents an asymmetric second-order cycle, recording a thickened transgressive suite of deposits and a comparatively thin second-order highstand. The overlying Myally Supersequence spans the interval ca 1780-1765 Ma and includes the Eastern Creek Volcanics and syndepositional Lena Quartzite, and the Myally Subgroup. This package represents a second-order supersequence cycle in which mafic volcanism was initiated during a phase of east – west extension. Following the cessation of volcanism, transgression led to the deposition of the Alsace Quartzite and deeper water Bortala Formation. An increase in the rate of sediment supply over accommodation resulted in progradation and deposition of the Whitworth Quartzite and redbed playa facies of the Lochness Formation as accommodation closed. The Quilalar Supersequence spans the interval ca 1755 – 1740 Ma. Sequence analysis in the eastern part of the Leichhardt River Fault Trough identifies a transgressive suite of facies at the base of this supersequence.Black shales from the upper part of the transgressive deposits characterise the condensed section for this supersequence. Facies analysis indicates that deposition took place in a series of storm-, tide- and wave-dominated shelfal marine depositional systems. Although there are no new depositional age constraints for the younger Bigie Formation, field relationships suggest that it is coeval with, or immediately preceded, the ca 1710 Ma Fiery magmatic event. Therefore, a separate supersequence is defined for the Bigie Formation, the Big Supersequence, even though it may be more genetically related to the Fiery magmatic event. The Big Supersequence, together with the ca 1690 Ma Prize Supersequence, comprise the Calvert Superbasin. The evolution of the Leichhardt and Calvert Superbasins are temporally and spatially related to magmatism. In particular, the new maximum depositional ages for the Guide and Myally Supersequences refine the age of the Eastern Creek Volcanics to ca 1780-1775 Ma. The new age for the Weberra Granite is within error of the age for the Fiery Creek Volcanics, indicating that they are both part of the ca 1710 Ma Fiery event. New ages for the Sybella Granite confirm that magmatism associated with this magmatic event is refined to 1680-1670 Ma, and is followed by deposition of the Gun Supersequence. Combining the new geochronological constraints with previous work now provides a detailed stratigraphic event framework between 1800 and 1575 Ma for the Western Fold Belt of the Mt Isa Inlier, and allows detailed comparisons and correlations with the Eastern Fold Belt and other Proterozoic terranes.

Miocene to Holocene landscape evolution of the western Snake River Plain region Idaho Using the SHRIMP detrital zircon provenance record to track eastward migration of the Yellowstone hotspot [查看] LukeP.Beranek、PaulKarlLink、C.MarkFanning

We report new U-Pb detrital zircon sensitive high-resolution ion microprobe (SHRIMP) age data (702 grains) from 13 samples collected from Miocene to Holocene sedimentary deposits in the western Snake River Plain region. These samples effectively show that modern stream sediments of the Snake River system reliably and repeatedly record the detrital zircon age populations that are present as sources in their drainage basins across the Cordilleran thrust belt and Basin and Range Province. We use this framework and the provenance of Neogene sedimentary rocks in the region to test the effect of the migrating Yellowstone hotspot on regional drainage patterns in southern Idaho since the middle Miocene. Our results indicate that Neogene paleodrainages were fi rst directed radially away from the tumescent Yellowstone highland, then subsequently reversed their fl ow toward the subsiding Snake River Plain basin. This occurred in east-progressing time-constrained intervals starting at 16 Ma.In northern Nevada, the drainage divide is represented by a northeast-trending, southeast-migrating crest of high topography.Specifi cally, middle to late Miocene (16–10 Ma) sedimentary deposits of the western Snake River Plain and Oregon-Idaho graben contain early to middle Eocene (52–42 Ma) detrital zircon populations sourced in Challis magmatic rocks north of the Snake River Plain. Middle Jurassic (160 Ma) and middle to late Eocene (42–35 Ma) detrital zircons,sourced from rocks in northern Nevada, are not present. Late Eocene detrital zircons from Nevada are present in two younger than 7 Ma sedimentary units of the Idaho Group along the Oregon-Idaho border.This indicates that by the late Miocene,southeastward headward erosion of the paleo–Owyhee River into the Owyhee Plateau had captured drainage from north-central Nevada and directed it northwestward toward the subsiding western Snake River Plain. The modern Owyhee Plateau is still a topographic high, in contrast to the modern Snake River Plain, suggesting that lowering of the regional Snake River Plain base level,rather than crustal subsidence, drove stream capture. By the late Pliocene (3 Ma), Middle Jurassic detrital zircons are recorded in the Glenns Ferry Formation and Tuana Gravel of the central Snake River Plain, suggesting that surface subsidence reversed the flow direction of paleo–Salmon Falls Creek from southward into Nevada to northward toward Idaho.Miocene strata of the western Snake River Plain lack recycled Proterozoic detrital zircons that are ubiquitous in sedimentary rocks of the central and southeast Idaho thrust belts. Such detrital zircons appear on the central and western Snake River Plain in early Pliocene to Holocene (4–0Ma) deposits. This records capture of drainage from the eastern Snake River Plain. The Yellowstone hotspot controlled the east-migrating continental divide, in the wake of which formed the western-draining, and progressively eastward-collecting, Snake River system.

Geology and timing of dextral strike-slip shear zones in Danmarkshavn, North-East Greenland Caledonides [查看] C.SARTINI-RIDEOUT、J.A.GILOTTI、W.C.MCCLELLAND

The North-East Greenland eclogite province is divided into a western, central and eastern block by the sinistral Storstrømmen shear zone in the west and the dextral Germania Land deformation zone in the east. A family of steep, NNW-striking dextral mylonite zones in the Danmarkshavn area are geometrically and kinematically similar to the ductile Germania Land deformation zone, located 25 km to the east. Amphibolite facies deformation at Danmarkshavn is characterized by boudinage of eclogite bodies within quartzofeldspathic host gneisses, pegmatite emplacement into the boudin necks and subsequent deformation of pegmatites parallel to gneissosity, a widespread component of dextral shear within the gneisses, and localization of strain into 10–50 m thick dextral mylonite zones. The gneisses and concordant mylonite zones are cut by a swarm of weakly to undeformed,steeply dipping, E–W-striking pegmatitic dykes. Oscillatory-zoned zircon cores from two boudin neck pegmatites give weighted mean 206Pb/238U sensitive, high mass resolution ion microprobe (SHRIMP) ages of 376±5 Ma and 343±7 Ma. Cathodoluminescence images of these zircons reveal complex additional rims, with ages from ranging from c. 360 to 320 Ma. Oscillatory-zoned, prismatic zircons from two late, cross-cutting pegmatites yield weighted mean 206Pb/238U SHRIMP ages of 343±5 Ma and 332±3 Ma. Zircons from the boudin neck pegmatites record a prolonged growth history, marked by fluid influx, during amphibolite facies metamorphism beginning at c. 375 Ma. The cross-cutting pegmatites show that dextral deformation in the gneisses and ductile mylonite zones had stopped by c. 340 Ma. Ultrahigh-pressure metamorphism in the eastern block at 360 Ma requires that the Greenland Caledonides were in an overall contractional plate tectonic regime. This, combined with 20% steep amphibolite facies lineations in the eclogites, gneisses and mylonites suggests that dextral transpression may have been responsible for a first stage of eclogite exhumation between 370 and 340 Ma.

Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monaz [查看] JohnN.Aleinikoff、WilliamS.Schenck、MargaretO.Plank、LeeAnnSrogi、C.MarkFanning、SandraL.Kamo、HowellBosbyshell

High-grade rocks of the Wilmington Complex,northern Delaware and adjacent Maryland and Pennsylvania, contain morphologically complex zircons that formed through both igneous and metamorphic processes during the development of an island-arc complex and suturing of the arc to Laurentia.The arc complex has been divided into several members, the protoliths of which include both intrusive and extrusive rocks. Metasedimentary rocks are interlayered with the complex and are believed to be the infrastructure upon which the arc was built.In the Wilmington Complex rocks, both igneous and metamorphic zircons occur as elongate and equant forms. Chemical zoning,shown by cathodoluminescence (CL),includes both concentric, oscillatory patterns,indicative of igneous origin, and patchwork and sector patterns, suggestive of metamorphic growth. Metamorphic monazites are chemically homogeneous, or show oscillatory or spotted chemical zoning in backscattered electron images. U-Pb geochronology by sensitive high resolution ion microprobe (SHRIMP) was used to date complexly zoned zircon and monazite. All but one member of the Wilmington Complex crystallized in the Ordovician between ca. 475 and 485 Ma; these rocks were intruded by a suite of gabbro-to-granite plutonic rocks at 434 ± 5 Ma. Detrital zircons in metavolcanic and metasedimentary units were derived predominantly from 0.9 to 1.4 Ga (Grenvillian) basement, presumably of Laurentian origin. Amphibolite to granulite facies metamorphism of the Wilmington Complex, recorded by ages of metamorphic zircon (428 ± 4 and 432 ± 6 Ma) and monazite (429 ± 2 and 426 ± 3 Ma), occurred contemporaneously with emplacement of the younger plutonic rocks. On the basis of varying CL zoning patterns and external morphologies,metamorphic zircons formed by different processes (presumably controlled by rock chemistry) at slightly different times and temperatures during prograde metamorphism. In addition, at least three other thermal episodes are recorded by monazite growth at 447 ± 4, 411 ± 3, and 398 ± 3 Ma.

copper deposit new evidence for an Early Cretaceous porphyry-related metallogenic epoch in the Coastal Cordillera of northern Chile [查看] VictorMaksaev、FranciscoMunizaga、MarkFanning、CarlosPalacios、JoséTapia

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.

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