A newly defined Late Ordovician magmatic‐thermal event in the Mt Painter Province,northern Flinders Ranges,South Australia     

M. A. Elburg  P. D. Bons  J. Foden  J. Brugger 《Australian Journal of Earth Sciences》2013,60(4):611-631

Magmatism,metamorphism and metasomatism in the Palaeoproterozoic‐Mesoproterozoic Mt Painter Inlier and overlying Neoproterozoic Adelaidean rocks in the northern Flinders Ranges (South Australia) have previously been interpreted as resulting from the ca 500 Ma Delamerian Orogeny. New Rb–Sr, Sm–Nd and U–Pb data, as well as structural analysis,indicate that the area also experienced a second thermal event in the Late Ordovician (ca 440 Ma). The Delamerian Orogeny resulted in large‐scale folding, prograde metamorphism and minor magmatic activity in the form of a small volume of pegmatites and leucogranites. The Late Ordovician event produced larger volumes of granite (the British Empire Granite in the core of the inlier) and these show Nd isotopic evidence for a mantle component. The high‐temperature stage of this magmatic‐hydrothermal event also gave rise to unusual diopside‐titanite veins and the primary uranium mineralisation in the basement, of which the remobilisation was younger than 3.5 Ma. It is possible that parts of the Mt Gee quartz‐hematite epithermal system developed during the waning stages of the Late Ordovician event. We suggest that the Ordovician hydrothermal system was also the cause of the commonly observed retrogression of Delamerian metamorphic minerals (cordierite, andalusite) and the widespread development of actinolite, scapolite, tremolite and magnetite in the cover sequences. Deformation during the Late Ordovician was brittle. The recognition of the Late Ordovician magmatic‐hydrothermal event in the Mt Painter Province might help to link the tectonic evolution of central Australia and the southeast Australian Lachlan Fold Belt.  相似文献   

SHRIMP U–Pb geochronology of authigenic xenotime and its potential for dating sedimentary basins     

I. R. Fletcher  B. Rasmussen  N. J. McNaughton 《Australian Journal of Earth Sciences》2013,60(5):845-859

SHRIMP (Sensitive High‐Resolution Ion MicroProbe) analytical procedures have been developed to enable dating of the small, early diagenetic xenotime overgrowths that commonly occur on zircons in siliciclastic sedimentary rocks. The method will be particularly useful in Precambrian terranes, where diagenetic xenotime dating could play a role equivalent to biostratigraphic dating in the Phanerozoic. Reliable ²⁰⁷Pb/²⁰⁶Pb data are more readily obtained than ²⁰⁶Pb/²³⁸U, which also favours application to the Precambrian. However, it is demonstrated that ²⁰⁶Pb/²³⁸U dating of larger overgrowths (>10 μm) is also viable and applicable to Phanerozoic samples. SHRIMP Pb/Pb geochronology of authigenic xenotime in an unmetamorphosed Palaeoproterozoic sandstone in the Kimberley Basin has constrained diagenesis to a precision of ± 7 Ma. In contrast, greenschist‐facies metasediments of the Archaean Witwatersrand Basin, South Africa, contain both authigenic and alteration xenotime that record a complex history of growth from early diagenesis to the last major thermal event to affect the basin.  相似文献   

Tectonic interpretation of metamorphic tectonites: integrating compositional mapping,microstructural analysis and in situ monazite dating     

M. L. WILLIAMS  M. J. JERCINOVIC 《Journal of Metamorphic Geology》2012,30(7):739-752

Interpreting tectonic histories from metamorphic tectonites requires an understanding of the linkages and feedbacks between deformation and metamorphism. Relationships between deformation and metamorphism can be divided into two broad groups: active and passive. Active relationships involve direct interactions whereby deformation directly influences metamorphic reactions or metamorphism directly affects the rate or style of deformation. One of the most important ‘active’ relationships is the role that deformation plays in helping to remove unstable reactant phases and to promote the growth of stable product phases. Passive relationships are correlations or linkages that allow the deformation history to be integrated with the metamorphic history. Compositional mapping of major and accessory phases and especially maps of larger thin section domains are particularly valuable for evaluating strain partitioning, scales of equilibrium, relationships between metamorphic textures and deformational fabrics, and in particular, for interpreting geochronological data. Petrological pseudosections are an increasingly utilized tool for interpreting microtextures and for linking deformation, metamorphism, and large‐scale tectonics. In situ geochronology and petrological analysis of chronometer phases (i.e. monazite, xenotime, titanite, allanite, etc.) are a critical part of tectonic analysis of metamorphic rocks. The electron microprobe plays an essential role in characterizing chronometer phases and placing them into the context of silicate fabrics and textures.  相似文献   

Improved age constraint for pre‐ and post‐Anglian temperate‐stage deposits in north Norfolk,UK, from analysis of serine decomposition in Bithynia opercula     

Rob Westaway 《第四纪科学杂志》2010,25(5):715-723

The ‘new glacial stratigraphy’ (NGS) of Britain postulates that deposits hitherto assigned to the Anglian glaciation in Marine Isotope Stage (MIS) 12 represent MIS 16, 12, 10 and 6. This controversial idea can be tested at Sidestrand in north Norfolk, where fossiliferous temperate‐stage deposits underlie the oldest and overlie the youngest of the glacial deposits. Previous work has considered biostratigraphy and amino acid dating of these temperate‐stage deposits, but did not achieve tight age constraint using the amino acid evidence alone. The total hydrolysable amino acid fraction in intra‐crystalline protein from Bithynia tentaculata opercula has previously been analysed for concentrations of serine and alanine. Statistical screening of these data gives alanine/serine ratios of 4.572 ± 0.114 for the Sidestrand Hall Member (beneath the glacial deposits) and 3.564 ± 0.091 for the Sidestrand Cliff Formation (overlying the glacial deposits). These ratios imply ages of MIS 13a and 11c, respectively, indicating the latest Cromerian and early Hoxnian interglacials and invalidating the NGS. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

SHRIMP zircon age for an Early Cambrian dolerite dyke: An intrusive phase of the Antrim Plateau Volcanics of northern Australia     

L. M. Hanley  M. T. D. Wingate 《Australian Journal of Earth Sciences》2013,60(6):1029-1040

The Antrim Plateau Volcanics, Australia's largest Phanerozoic flood‐basalt province, originally covered an area of at least 300 000 km² across northern Australia. Stratigraphic constraints indicate that the Antrim Plateau Volcanics are of Early Cambrian age (ca 545–509 Ma), although previous attempts to date the Antrim basalts by radiometric methods have been inconclusive. We present an ion microprobe U–Pb zircon age of 513 ± 12 Ma for the ～250 km‐long Milliwindi dolerite dyke in the west Kimberley. The dolerite is geochemically identical to basalts of the Antrim Plateau Volcanics, and was probably a feeder dyke for basalts that have since been eroded. It is suggested that the Antrim Plateau Volcanics extended hundreds of kilometres further to the west than recognised previously and may have once covered part of the Kimberley block.  相似文献   

Clay mineral,geochemical and Sr–Nd isotopic fingerprinting of sediments in the Murray–Darling fluvial system,southeast Australia     

M. Raetz  M. Krabbendam  A. G. Donaghy 《Australian Journal of Earth Sciences》2013,60(6):965-983

The Willyama Supergroup of the Broken Hill region in southern Australia consists of supracrustal sedimentary and magmatic rocks, formed between 1810 and 1600 Ma. A statistical analysis of nearly 2000 SHRIMP U–Pb zircon spot ages, compiled from published and unpublished sources, provides evidence for three distinct tectonostratigraphic successions and four magmatic events during this interval. Succession 1 includes Redan Geophysical Zone gneisses and the lower part of the Thackaringa Group (Cues Formation). These rocks were deposited after 1810 Ma and host granite sills of the first magmatic event (1710–1700 Ma). Succession 2 includes the upper Thackaringa Group (Himalaya Formation), the Broken Hill Group and the Sundown Group and was deposited between 1710 and 1660 Ma. These rocks all contain detrital zircons from the first magmatic event (1710–1700 Ma) and in some cases from the second magmatic event (1690–1680 Ma). The second magmatic event (1690–1680 Ma) was bimodal, resulted from crustal extension, and was coeval with deposition of the Broken Hill Group and deepening of the basin. With this event a mafic sill swarm focused in the Broken Hill Domain. Mafic sills lack any trace of inheritance, unlike the granitoids that commonly contain inherited zircons typical of the supracrustal sediments. Succession 3, the Paragon Group and equivalents were deposited after 1660 Ma, but before a regional metamorphic event at 1600 Ma. Metamorphism was closely followed by inversion of the succession into a fold‐and‐thrust belt, accompanied by a fourth late to post‐orogenic magmatic event (ca 1580 Ma) characterised by granite intrusion and regional acid volcanism (the local equivalents of the Gawler Range Volcanics in South Australia).  相似文献   

Bayesian modelling the retreat of the Irish Sea Ice Stream     

Richard C. Chiverrell  Ian M. Thrasher  Geoffrey S. P. Thomas  Andreas Lang  James D. Scourse  Katrien J. J. van Landeghem  Danny Mccarroll  Chris D. Clark  Colm Ó Cofaigh  David J. A. Evans  Colin K. Ballantyne 《第四纪科学杂志》2013,28(2):200-209

We present an 8000‐year history spanning 650 km of ice margin retreat for the largest marine‐terminating ice stream draining the former British–Irish Ice Sheet. Bayesian modelling of the geochronological data shows the ISIS expanded 34.0–25.3 ka, accelerating into the Celtic Sea to reach maximum limits 25.3–24.5 ka before a collapse with rapid marginal retreat to the northern Irish Sea Basin (ISB). This retreat was rapid and driven by climatic warming, sea‐level rise, mega‐tidal amplitudes and reactivation of meridional circulation in the North Atlantic. The retreat, though rapid, is uneven, with the stepped retreat pattern possibly a function of the passage of the ice stream between normal and adverse ice bed gradients and changing ice stream geometry. Initially, wide calving margins and adverse slopes encouraged rapid retreat (～550 m a^?1) that slowed (～100 m a^?1) at the topographic constriction and bathymetric high between southern Ireland and Wales before rates increased (～200 m a^?1) across adverse bed slopes and wider and deeper basin configuration in the northern ISB. These data point to the importance of the ice bed slope and lateral extent in predicting the longer‐term (>1000 a) patterns and rates of ice‐marginal retreat during phases of rapid collapse, which has implications for the modelling of projected rapid retreat of present‐day ice streams. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Structural kinematics,metamorphic P–T profiles and zircon geochronology across the Greater Himalayan Crystalline Complex in south‐central Tibet: implication for a revised channel flow     

J. M. Wang  J. J. Zhang  X. X. Wang 《Journal of Metamorphic Geology》2013,31(6):607-628

A specific question about the Himalayas is whether the orogeny grew by distributed extrusion or discrete thrusting. To place firm constraints on tectonic models for the orogeny, kinematic, thermobarometric and geochronological investigations have been undertaken across the Greater Himalayan Crystalline Complex (GHC) in the Nyalam region, south‐central Tibet. The GHC in this section is divided into the lower, upper and uppermost GHC, with kinematically top‐to‐the‐south, alternating with top‐to‐the‐north shear senses. A new thrust named the Nyalam thrust is recognized between the lower and upper GHC, with a 3 kbar pressure reversion, top‐to‐the‐south thrust sense, and was active after the exhumation of the GHC. Peak temperature reached ～749 °C in the cordierite zone, and decreased southwards to 633–667 °C in the kyanite and sillimanite‐muscovite zones, and northwards to greenschist facies at the top of the South Tibetan Detachment System (STDS). Pressure at peak temperature reached a maximum value in the kyanite zone of 9.0–12.6 kbar and decreased northwards to ～4.1 kbar in the cordierite zone. Zircon U‐Pb ages of a sillimanite migmatite and an undeformed leucogranite dyke cutting the mylonitized rocks in the STDS reveal a long‐lived partial melting of the GHC, which initiated at 39.7–34 Ma and ceased at 14.1 Ma. Synthesizing the obtained and collected results, a revised channel flow model is proposed by considering the effect of heat advection and convection by melt and magma migration. Our new model suggests that distributed processes like channel flow dominated during the growth of the Himalayan orogen, while discrete thrusting occurred in a later period as a secondary process.  相似文献   

The petrogenesis and implications of deformed late Caledonian–early Hercynian granites in the Wuyi area,South China          下载免费PDF全文

Xilin Zhao  Yang Jiang  Guangfu Xing  Minggang Yu  Kai Liu  Zhihong Chen  Shengyao Yu 《Island Arc》2017,26(5)

Precambrian basement rocks have been affected by Caledonian thermal metamorphism. Caledonian‐aged zircon grains from Precambrian basement rocks may have resulted from thermal metamorphism. However, Hercynian ages are rarely recorded. Zircon U–Pb Sensitive High Resolution Ion Microprobe (SHRIMP) dating reveals that zircon ages from the Huyan, Lingdou, and Pengkou granitic plutons can be divided into two groups: one group with ages of 398.9 ±5.3 Ma, 399 ±5 Ma, and 410.2 ±5.4 Ma; and a second group with ages of 354 ±11 Ma, 364.6 ±6.7 Ma, and 368 ±14 Ma. The group of zircon U–Pb ages dated at 410–400 Ma represent Caledonian magmatism, whereas the 368–354 Ma ages represent the age of deformation, which produced gneissosity. The three plutons share geochemical characteristics with S‐type granites and belong to the high‐K calc‐alkaline series of peraluminous rocks. They have (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.710 45–0.724 68 and εNd(t) values of ?7.33 to ?10.74, with two‐stage Nd model ages (T_DM2) ranging from 1.84 Ga to 2.10 Ga. Magmatic zircon εHf(t) values range from ?3.79 to ?8.44, and have T_DMC ages of 1.65–1.93 Ga. The data suggest that these granites formed by partial melting of Paleoproterozoic to Mesoproterozoic continental crust. A collision occurred between the Wuyi and Minyue microcontinents within the Cathaysia Block and formed S‐type granite in the southwest Fujian province. The ca 360 Ma zircon U–Pb ages can represent a newly recognized period of deformation which coincided with the formation of the unified Cathaysia Block.  相似文献   

Footwall dip of a core complex detachment fault: thermobarometric constraints from the northern Snake Range (Basin and Range,USA)     

F. J. COOPER  J. P. PLATT  R. ANCZKIEWICZ  M. J. WHITEHOUSE 《Journal of Metamorphic Geology》2010,28(9):997-1020

Low‐angle detachment faults are common features in areas of large‐scale continental extension and are typically associated with metamorphic core complexes, where they separate upper plate brittle extension from lower plate ductile stretching and metamorphism. In many core complexes, the footwall rocks have been exhumed from middle to lower crustal depths, leading to considerable debate about the relationship between hangingwall and footwall rocks, and the role that detachment faults play in footwall exhumation. Here, garnet–biotite thermometry and garnet–muscovite–biotite–plagioclase barometry results are presented, together with garnet and zircon geochronology data, from seven locations within metapelitic rocks in the footwall of the northern Snake Range décollement (NSRD). These locations lie both parallel and normal to the direction of footwall transport to constrain the pre‐exhumation geometry of the footwall. To determine P–T gradients precisely within the footwall, the ΔPT method of Worley & Powell (2000) has been employed, which minimizes the contribution of systematic uncertainties to thermobarometric calculations. The results show that footwall rocks reached pressures of 6–8 kbar and temperatures of 500–650 °C, equivalent to burial depths of 23–30 km. Burial depth remains constant in the WNW–ESE direction of footwall transport, but increases from south to north. The lack of a burial gradient in the direction of footwall transport implies that the footwall rocks, which today define a sub‐horizontal datum in the direction of fault transport, also defined a sub‐horizontal datum at depth in Late Cretaceous time. This suggests that the footwall was not tilted about the normal to the fault transport direction during exhumation, and hence that the NSRD did not form as a low‐angle normal fault cutting down through the lower crust. Instead, the following evolution for the northern Snake Range footwall is proposed. (i) Mesozoic contraction caused substantial crustal thickening by duplication and folding of the miogeoclinal sequence, accompanied by upper greenschist to amphibolite facies metamorphism. (ii) About half of the total exhumation was accomplished by roughly coaxial stretching and thinning in Late Cretaceous to Early Tertiary time, accompanied by retrogression and mylonitic deformation. (iii) The footwall rocks were then ‘captured’ from the middle crust along a moderately dipping NSRD that soled into the middle crust with a rolling‐hinge geometry at both upper and lower terminations.  相似文献