genesis of gray monazites evidence from the paleozoic of belgium systematic mineralogical investigations carried out on panned stream concentrates from the northwestern part of the cambro ordovician stavelot massif belgium led to the discovery of nodular gray monazites these were later identified in situ in black slates associated with conglomeratic levels containing phosphatic material microscopic investigation shows that gray monazites are authigenic and predate cleavage to test the hypothesis of a sedimentary origin for rare earth elements samples of shales phosphatic conglomerates and monazite concentrates were submitted to sequential inductively coupled plasma analysis results show that the rare earth element pattern in gray monazites is a reflection of the abundance of these elements in the host rock this suggests a model invoking desorption of the rare earth elements from their clayey support and differential precipitation of rare earth phosphates in the sediment initiated by p mobility during diagenesis the genetic model for gray monazite mineralization assumes three essential and complementary phenomena 1 sedimentation of clays in a reducing marine environment 2 a phosphate anomaly in the same lithological sequence and 3 evolution to low grade epizonal metamorphic conditions these nonrestrictive genetic conditions clearly suggest that gray monazites have been generally overlooked and must be much more widespread than believed the economic interest of gray monazite lies in potential stream or beach placers their low th content and comparatively high eu content are interesting features in the scope of ore beneficiation

syntectonic monazite in low grade mudrocks a potential geochronometer for cleavage formation monazite paragenesis during low grade metamorphism is poorly understood the first unequivocal instance of the mineral having crystallized synchronously with compressional cleavage formation is reported compositionally these monazites resemble diagenetic forms but differ texturally by displaying syntectonic microfabrics their predominant association with organic walled fossils implies that organic material may play an important role in controlling the behaviour of light rare earth elements lree during low grade metamorphism probably by releasing diagenetically sequestered lree and organically bound phosphorus during thermal maturation devolatilization and or by promoting a favourable environment for monazite precipitation potentially such monazites could provide a powerful geochronometer for cleavage formation

geochronology of the sequence hosting the broken hill pb zn ag orebody australia robust stratigraphic control and constraints on the timing of events have been established for the paleoproterozoic metasedimentary and metavolcanic succession that hosts the broken hill pb zn ag orebody in western new south wales geologically consistent depositional intrusive and metamorphic ages are elucidated by means of shrimp u pb analyses of zircon and integrated field studies comparisons of the new shrimp results with those of previous studies clarify some previously untenable age interpretations a range of zircon provenance ages is found in albitic metasedimentary rocks from one of the oldest recognized parts of the willyama supergroup thackaringa group the zircons indicate maximum depositional ages of 1720 to 1730 ma and a minimum stratigraphic age for the lower thackaringa group is constrained by the 1704 3 ma intrusive alma gneiss the latter is the same age as gneissic granitoid rocks in the redan zone southern broken hill domain 1703 3 1705 3 ma felsic magmatism of this age has long been known in the contiguous olary domain south australia but has not been recognized previously at broken hill depositional ages in the broken hill group the upper part of which hosts the pb zn ag orebody indicate that the entire package was deposited within a period of ca 10 m y an age of 1685 3 ma for felsic metavolcaniclastic rocks in the hores gneiss spatially and stratigraphically associated with the ore horizon corroborates previous work this age is within error of that determined for the hanging wall and footwall unit rasp ridge gneiss of the orebody 1683 3 ma it suggests that felsic magmatism in the rasp ridge gneiss was coeval with that of the hores gneiss previous interpretations which suggested that the hores gneiss is not a single stratigraphic entity but comprises younger intrusions having ages from 1690 to1640 ma are not supported by our data and conclusions the stratigraphic integrity of the broken hill group excluding some mafic intrusions is further supported by results from an older formation near its base where tuffaceous metasedimentary rocks from the ettlewood calc silicate member allendale metasediments define an age of 1693 4 ma this is in good agreement with the age of 1693 5 ma for the immediately overlying parnell formation provenance ages obtained from detrital zircons in broken hill group psammites indicate a remarkable uniformity of paleoproterozoic and late archean source s generally matching provenance ages are found in the overlying sundown group psammopelites suggesting a stable flux of sediment fill as the basin deepened and matured similar provenance is indicated for upper paragon group sedimentary rocks and inherited zircons in most of the younger granitoids have age spectra similar to those of the detrital zircons the perpetuation of these age patterns in both clastic detritus and magmatic xenocrysts thus broadly fingerprints exposed and buried older crustal elements that provided material to produce the willyama supergroup basin fill tuffaceous metasiltstones in the middle paragon group bijerkerno metasediments are interpreted to have been derived partly from air fall volcaniclastic material and give consistent depositional ages of 1655 4 and 1657 4 ma in the northern broken hill and euriowie blocks respectively younger tuffaceous metasiltstones in the upper paragon group dalnit bore metasediments provide a stratigraphically consistent maximum depositional age of 1642 5 ma zircon ages from granitoid intrusions that bracket the deformation events suggest that the d 2 and d 3 deformations are inseparable within the interval 1597 3 to 1591 5 ma low th u zircon overgrowths are ubiquitous and irrespective of stratigraphic position record metamorphic ages of 1600 ma metamorphic rutile has the same age confirming that high grade metamorphic event s and d 2 and d 3 structural events were approximately synchronous we find no isotopic support for any pre 1600 ma high grade metamorphic events using newly acquired local age control for the galena pb isotope growth curve the new data support a stratigraphic syngenetic or diagenetic model of primary ore formation for the broken hill orebody at ca 1690 ma although the present distribution of the lodes likely reflects displacement as a result of later structural remobilization

early palaeozoic rift related magmatism in variscan europe fragmentation of the armorican terrane assemblage early palaeozoic bimodal rift related magmatism is widespread throughout much of the variscides of europe it is traceable from the polish sudetes to nw iberia granitic plutonism generally predates cambro ordovician bimodal magmatism in the n bohemian massif this early palaeozoic granitic plutonism was generated by partial melting of cadomian basement whereas contemporaneous alkali granites with a mantle component are typical of the nw iberian massif silurian devonian mafic magmatism in the n bohemian massif massif central and nw iberian massif is partly preserved as obducted ophiolites compositional diversity displayed by cambro ordovician mafic magmatism can be accounted for by interaction between a spreading centre and an upwelling mantle plume this indicates that combined tensional forces and mantle plume convection assisted the early palaeozoic dispersal of terranes from the n gondwana margin continued fragmentation resulted in development of an archipelago of related terranes separated by a network of seaways and formation of oceanic crust

horodyskia williamsii new species a mesoproterozoic macrofossil from western australia abstract a new species of abundant macrofossil previously described informally as a string of beads is here assigned to the genus horodyskia yochelson and fedonkin 2000 and formally named horodyskia williamsii it is known from mesoproterozoic siliciclastic rocks that are older than 1070 ma and younger than 1465 ma the backdoor formation of the collier group and the poorly time constrained but probably correlative stag arrow formation of the manganese group both part of the bangemall supergroup of the ashburton region of western australia the fossils are most commonly found as impressions on the sole of thin frequently ripple marked sandstones very rarely they are present as traces on the tops of sandstone beds at some localities there are thin silty partings separating the sandstone beds some strings are current oriented but commonly most strings crosscut current directions much of the observed variation appears to be taphonomic

1 8 1 5 ga links between the north and south australian cratons and the early middle proterozoic configuration of australia abstract the archaean and early middle proterozoic 1 8 1 5 ga terranes of the north australian craton and the south australian craton are separated by 400 km of ca 1 33 1 10 ga orogenic belts and phanerozoic sediments however there is a diverse range of geological phenomena that correlate between the component terranes of the two cratons and provide evidence for a shared tectonic evolution between approximately 1 8 and 1 5 ga in order to honour these correlations we propose a reconstruction in which the south australian craton is rotated 52 counterclockwise about a pole located at 136 e and 25 s present day coordinates relative to its current position this reconstruction aligns the ca 1 8 1 6 ga orogenic belts preserved in the arunta inlier and the gawler craton and the ca 1 6 1 5 ga orogenic belts preserved in the mount isa block and the curnamona province before 1 5 ga the south australian craton was not a separate entity but part of a greater proto australian continent which was characterised by accretion along a southward migrating convergent margin ca 1 8 1 6 ga followed by convergence along the eastern margin ca 1 6 1 5 ga after 1 5 ga the south australian craton broke away from the north australian craton only to be reattached in its current position during the ca 1 33 1 10 ga albany fraser and musgrave orogenies

mafic volcanic rocks on king island tasmania evidence for 579 ma break up in east gondwana abstract the eastern coast of king island in southeastern australia exposes a thick well preserved sequence of latest neoproterozoic volcanic and related shallow intrusive rocks these rocks are associated with shallow marine carbonates and siltstones and pass up into massive conglomerates representing a marine flooding event and unconformity during continental break up and subsequent volcanic passive margin formation unusual differentiated sills grimes intrusive suite with extreme internal variation wehrlite to andesite compositions intrude deformed proterozoic metasediments of the rodinian basement a thin basal tholeiitic basaltic volcanic unit city of melbourne volcanics is less contaminated than the underlying sills and preceded eruption of a thick sequence of highly depleted picritic pillows sub aerial flows and hyaloclastites shower droplet volcanics the picrite sequence is overlain by thick tholeiitic basalts and reworked volcanogenic conglomerates bold head formation that show a strong compositional similarity to enriched mid ocean ridge basalts both the picrites and the upper tholeiitic basalts are not crustally contaminated and have an nd sm isochron age of 579 16 ma with initial nd of 4 2 the lithostratigraphy and range of compositions represented are analogous to early magmatism associated with continental break up and volcanic passive margin formation including voluminous seaward dipping reflector sequences in the mesozoic north atlantic volcanic margins

isotopic dating of the migration of a low grade metamorphic front during orogenesis the southern margin of the pilbara craton in northwestern australia underwent regional heating folding and thrusting as well as extensive fluid flow during collision in the paleoproterozoic however the precise timing of this event is uncertain previous geochronologic studies of archean basement rocks have yielded a wide range of younger radiometric dates from ca 2 4 ga to ca 2 0 ga interpreted to record multiple hydrothermal alteration events in situ u pb geochronology of authigenic monazite and xenotime in very low grade archean metasedimentary rocks from across the craton suggests that the pilbara was affected by at least two cryptic thermotectonic events the first between ca 2430 ma and ca 2400 ma and the second between ca 2215 ma and ca 2145 ma the older event is restricted to three localities in the west and its cause is unknown the younger event affected most of the craton 100 000 km 2 spanning a 70 m y period from ca 2215 ma nearest the collisional margin in the south to ca 2145 ma toward the craton interior in the north the widespread geographic and stratigraphic distribution of ca 2 2 ga phosphates suggests that fluid flow was pervasive this event was probably driven by the northward advancing ophthalmian fold and thrust belt that developed during protracted collision the associated low grade metamorphic front migrated over 350 km at an average rate of 5 0 mm yr placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings

dating low grade metamorphic events by shrimp u pb analysis of monazite in shales although the u pb isotopic system is widely used to date magmatism and medium to high grade metamorphism dating low grade metamorphic events has been hampered by a scarcity of suitable minerals we show that monazite forms in shales during low grade metamorphism and can be dated by in situ u pb analysis providing precise ages for low temperature thermal events in proterozoic shales from the pine creek inlier of northern australia monazite and xenotime crystals that formed in a contact aureole yield ages synchronous with granite intrusion ca 1830 ma in the pilbara craton of northwestern australia monazite growth 2192 5 m y ago coincided with basaltic volcanism and rifting an event apparently responsible for hydrothermal iron oxide mineralization in a nearby giant iron ore deposit the mineralizing environment was highly oxic and bathed by meteoric fluids thus our results imply the existence of an oxygen rich atmosphere by 2200 ma given the abundance of shales in the stratigraphic record and the likely widespread distribution of monazite in such rocks in situ u pb dating of monazite may become an important chronometer for low grade metamorphic events

geochemistry and tectonic significance of metabasic suites in the gory sowie block sw poland gneisses of the gory sowie block of probable late precambrian early cambrian age contain numerous small metabasite bodies which may have originated as dykes or sills many metabasites have phase assemblages and textures which show that they suvered early granulite facies and later amphibolite facies metamorphism although some exhibit no evidence for the earlier event trace element geochemistry enabled four metabasite groups to be distinguished a a dominant meta tholeiite group sowie group probably incorporating more than one set of intrusive rocks b meta tholeiites characterized by higher nb y high nb y group c ti poor metabasites myslecin group that exhibit strongly depleted hfse d alkali metabasalts wloki group sowie group meta tholeiites were also divided according to their dominant host lithology granitoid gneiss migmatitic paragneiss or gneiss containing relict granulite facies rocks those in granitoid gneiss are compositionally restricted although they fall within the same compositional range as other sowie meta tholeiites and may comprise both early metabasites and a later intrusive group postdating granite emplacement both the latter metabasites and the granitoid gneiss lack phase assemblages indicative of granulite facies metamorphism this suggests that both were emplaced after the granulite facies metamorphic event but before or during amphibolite facies metamorphism an early ordovician emplacement age for syn metamorphic granitoid gneiss shows that the gory sowie block underwent an ordovician early caledonian metamorphic event following metabasite emplacement or was at this grade during the ordovician nonetheless the chemistry of all the metabasite groups and their geological setting is consistent with emplacement either through continental crust undergoing extension at a passive continental margin or during the formation of an intracratonic basin before subsequent metamorphism recorded a local compressional event

discovery of a microcontinent gulden draak knoll offshore western australia implications for east gondwana reconstructions analysis of dredged samples from the gulden draak knoll demonstrates that it is a submarine rifted continental fragment that lies at the boundary between the western perth abyssal plain and wharton basin indian ocean the knoll comprises a granulite facies basement including pelitic paragneiss and mafic orthogneiss with a cambrian granite inferred to intrude the other rocks boulders and cobbles of felsic gneiss with mesoproterozoic and cambrian protolith ages were also sampled likely reflecting a complex basement to variable sedimentary and volcanic rocks the u pb isotopic system in the archean and mesoproterozoic zircon is significantly disturbed reflecting cambrian orogenesis that affected all samples the protolith to garnet sillimanite biotite paragneiss has a maximum deposition age of 1163 24 ma and includes older detrital zircon grains with populations at c 2 65 ga and between 1 4 and 1 1 ga a younger population in this sample is interpreted as a mix of newly grown metamorphic zircon and isotopically reset zircon implying that the granulite facies metamorphism occurred at c 511 5 ma protracted cambrian orogenesis is indicated by a metamorphic age in the mafic orthogneiss of 530 6 ma and isotopic disturbance shortly following emplacement of granite c 540 ma with zircon ages disturbed to 509 7 ma and the protolith to the felsic orthogneiss c 528 ma with zircon ages disturbed to 510 3 ma xenocrystic zircon grains in the cambrian rocks include archean c 2839 9 ma and mesoproterozoic 1230 1370 ma populations also isotopically disturbed during cambrian orogenesis igneous cambrian zircon grains have less radiogenic hf isotope compositions hfi 0 281821 0 281367 than mesoproterozoic xenocrysts hfi 0 282267 0 281993 indicating limited involvement of the mesoproterozoic crust in granite production a more likely source includes archean crust represented by xenocrysts with hfi 0 281399 0 280863 the gulden draak knoll is reconstructed in gondwana leeuwin full fit model along strike of a major structure termed the indo australo antarctic suture iaas recently mapped from geophysical interpretations in wilkes land antarctica new isotopic data suggest that basement rocks from the gulden draak knoll have affinity to crust exposed either side of the iaas determining if this structure is a suture zone sensu stricto remains to be tested

scratching the surface a marine sediment provenance record from the continental slope of central wilkes land east antarctica the geology of wilkes land east antarctica is masked by kilometers of ice and remains largely unexplored defining the sediment provenance adjacent to this hidden region is important for distinguishing the proximal subglacial basement terranes and refining the dynamic regional glaciological history this study presents a detrital sediment provenance record spanning c 23 5 ka from the continental slope of central wilkes land sediment provenance was characterized using u pb geochronology and trace element geochemistry from detrital zircon titanite and apatite and pb isotopic signatures from detrital feldspar these data were compared with new feldspar pb isotopic signatures and existing u pb zircon datasets from rare nearby coastal outcrop a principally igneous source was revealed with dominant age populations between c 1360 1100 ma and c 1620 1490 ma characteristic of rocks of the proximal wilkes and banzare provinces respectively minor detritus was additionally sourced from the proximal nuyina province c 1450 1390 ma temporal variation in the climate and ice sheet configuration are likely responsible for subtle downcore changes observed in detrital sediment provenance high sedimentation rates during the glacial period suggest reworking of continental shelf sediments and downslope transport in debris flows during ice sheet advance glacial meltwater fluxes fed largely by the totten glacier were responsible for supplying detritus during deglaciation during interglacials detritus was derived from a broad coastal region and delivered to the slope via multiple glacial outlets these results present the first substantial offshore evidence to support recent interpretations that the subglacial crust of central wilkes land has a dominantly mesoproterozoic history

magmatic and metamorphic evolution of metagabbros in the munchberg massif n e bavaria the central gneiss complex of the munchberg massif consists of the liegendserie at the base and the hangendserie at the top metagabbros are found in the liegendserie eclogites occur in the hangendserie new isotope data revive the discussion whether a genetic relationship exists between metagabbros and eclogites in the munchberg massif it is possible to relate the two types of rocks to the same protoliths by variable degrees of crustal contamination and magmatic accumulation therefore a common magmatic origin may be assumed both the metagabbros and the eclogites were affected by amphibolite facies metamorphism the amphibolitization of the metagabbros was a prograde metamorphic event increasing temperature is indicated by inverse zonation of recrystallized plagioclase and increasing mg fe ratios in garnet from core to rim geothermobarometry yields a temperature of 600 c and a pressure of about 11 kbar for the peak of metamorphism in contrast the eclogites underwent a first high pressure stage at a minimum pressure of 14 kbar and a temperature estimated at 600 c and were subsequently overprinted under amphibolite facies conditions at 10 kbar 700 c a common magmatic origin of metagabbros liegendserie and eclogites hangendserie of the munchberg massif can no longer be discarded however the converging p t t paths reflect a different geodynamic evolution of the liegendserie and the hangendserie after magmatism and before amphibolite facies metamorphism

constraints and deception in the isotopic record the crustal evolution of the west musgrave province central australia abstract the hf and nd isotopic evolution of the musgrave province central australia is used to constrain the timing of crust formation and lithospheric organisation of proterozoic australia the dataset from this region challenges two widely held tenets of hf and nd isotope systematics namely that crust formation events can only be identified as periods when crystallisation ages correspond to model ages and that linear hf evolution arrays away from depleted mantle along crustal lu hf or sm nd slopes reflect reworking of the same source hf isotopes in musgrave province zircon crystals indicate two major crust formation events at c 1900 ma and at 1600 1550 ma although no juvenile rocks or crystals are known from c 1900 ma radiogenic addition into the crust at this time is required to account for consistent nd and hf evolution patterns which show no indication of an initially heterogeneous source oxygen isotopes in zircon grains confirm that much of the c 1900 ma hf isotopic signal is not compromised by mixtures furthermore the correspondence between mantle extraction and the commencement of reworking of archean material supports new crust generation at c 1900 ma and a coupling between lower and upper crustal processes the c 1900 ma timing of juvenile addition is dissimilar to that in the albany fraser and arunta orogens and may reflect continental arc development on the margin of a southern continent the general hf isotopic evolution trend of the musgrave province apparently reflects reworking from a dominant c 1900 ma source with some additional unradiogenic and radiogenic input through time however in the 1220 1050 ma interval this apparent isotopic evolution contrasts with geological observations that indicate input of voluminous mantle derived material intracontinental rifts and other regions with sustained very high temperature crustal recycling processes generate magmatic provinces with extreme hfse enrichment this can have a profound influence on isotopic evolution trends suppressing typical juvenile addition patterns isotopic mixture modelling indicates that a significant volume of mantle derived material can be accommodated within hfse enriched magmas without diverging isotopic signatures from apparent reworking trends in the musgrave province the crust had become so hfse enriched during the prolonged musgrave orogeny 1220 1150 ma that it was insensitive to mantle input which is estimated to have been as much as 85 during this event

erratum superimposed low grade metamorphism in the mount fisher area southeastern british columbia implications for the east kootenay orogeny middle proterozoic 1500 1350 ma belt purcell strata exposed in the purcell and southwestern rocky mountains were affected by at least three distinct episodes of deformation and regional metamorphism the oldest episode 1300 1350 ma apparently terminated belt purcell sedimentation and involved foiding regional metamorphism and granitic intrusion the second episode 800 900 ma occurred during deposition of the windermere supergroup and involved uplift block faulting and low grade regional metamorphism mesozoic cenozoic metamorphism deformation and plutonism overprinted the results of the earlier deformation and metamorphism illite crystallinity and muscovite polymorph ratios indicate that purcell strata in the mount fisher area are in the lower greenschist to prehnite pumpellyite facies of regional metamorphism in the steeples and fisher blocks this metamorphism is related to structures that formed during the late cretaceous paleocene deformation however in the sand creek block the regional metamorphism is related to the development of a spaced cleavage that is folded by a late cretaceous pdeocene nappe regional considerations suggest that this cleavage formed during the 1300 1350 ma episode of deformation and metamorphism the east kootenay orogeny as currently defined embraces the two older episodes of tectonism it is proposed that the term east kootenay orogeny be restricted to designate the 1300 1350 ma episode and that the term goat river orogeny designate the 800 900 ma episode of tectonism the east kootenay and goat rivet orogenies appearto be correlative with the racklan and hayhook orogenies recognized in the northern canadian cordillera

proterozoic evolution and tectonic setting of the northwest paterson orogen western australia abstract the proterozoic igneous deformation and metamorphic histories of the palaeoproterozoic rudall complex in the northwestern paterson orogen can be linked to those of the arunta inlier in central australia and in part with the capricorn orogen in central western australia the similarities in deformation and metamorphic histories for these widely separated regions indicate a palaeoproterozoic continent continent collisional event between the palaeoproterozoic west australian and north australian cratons between c 1830 and 1765 ma in the paterson orogen this palaeoproterozoic collisional event resulted in the yapungku orogeny which included thrust stacking of clastic sedimentary and volcanic rocks deposition of the protoliths for the c 1790 ma siliciclastic paragneiss succession contemporaneous with granitic intrusion and metamorphism up to granulite facies during this 65 million year period the arunta inlier and capricorn orogen were deformed metamorphosed at medium to high grades and intruded by granitoids during the strangways orogeny in the arunta inlier and the capricorn orogeny in the capricorn orogen the neoproterozoic tarcunyah throssell and lamil groups are clastic sedimentary sequences that were deposited after 1070 ma in the northwestern paterson orogen and deformed by the miles orogeny before 678 ma the miles orogeny produced a northwesterly trending fold and fault system of tight to isoclinal upright and overturned folds and thrust faults the orogeny may have been coincident with the c 750 720 ma areyonga tectonic movement affecting the arunta inlier and the lower neoproterozoic part of the amadeus basin in central australia at c 550 ma the paterson orogeny which is most likely equivalent to the petermann orogeny in the musgrave complex of central australia deformed the northwestern paterson orogen and was preceded by local intrusion of granites the similarities of styles and timing of deformation in the northwestern paterson orogen arunta inlier and capricorn orogen indicate that these three regions were probably linked during most of the proterozoic

the cottons breccia ediacaran and its tectonostratigraphic context within the grassy group king island australia a rift related gravity slump deposit abstract the cottons breccia is part of the upper neoproterozoic ediacaran 586 572 ma grassy group exposed on king island tasmania this formation has been widely interpreted as an unambiguous tillite in support of tectono climatic models that invoke catastrophic latest neoproterozoic global glaciation however dissenting opinions interpret the cottons breccia as a tectonic breccia in this paper we review the tectonostratigraphy of the cottons breccia and the enclosing grassy group we find no unambiguous evidence for a direct glacial origin as postulated by some early workers an interpretation of the cottons breccia as a deposit within half graben in the shoulder regions of an actively developing latest neoproterozoic volcanic rifted margin is consistent with all the available sedimentological structural geochronological and tectonic evidence

u pb ages from sw poland evidence for a caledonian suture zone between baltica and gondwana faunal and palaeomagnetic evidence suggests the existence of a c 3000 km wide tornquist sea between gondwana and baltica in early ordovician times which narrowed to 50 2 ma an unmetamorphosed ophiolite gives a 420 20 2 ma age whilst in two other terranes amphibolite facies island arc lavas have fossiliferous lower ludlow 424 415 ma sequences all are unconformably overlain by unmetamorphosed upper devonian conglomerates a sinistral transpressive regime is observed in regionally extensive mylonite zones titanite and zircon ages 338 2 3 and 339 4 ma record variscan magmatism the data suggest considerable narrowing of the tornquist sea during the ordovician continuing ocean floor and island arc activity in the silurian and final sinistral transpressive closure by the mid devonian

palaeozoic terranes and their lithospheric boundaries within the trans european suture zone tesz a review the trans european suture zone tesz is a broad and complex zone of terrane accretion separating ancient lithosphere of the baltic shield and east european craton eec from the younger lithosphere of western and southern europe there is debate about the number of terranes involved and their origins the most significant terrane boundaries originally oceanic sutures are poorly exposed and are defined using faunal provinciality prominent geophysical lineaments may represent sub terrane boundaries within composite terranes the avalonia composite terrane was amalgamated to baltica in latest ician time the provenance of the malopolska and moravo silesian terranes whose neoproterozoic basement is suspect with respect to baltica but with which they nonetheless share faunal affinities is still debated the extent of the variscide rhenohercynian zone defines the southern limit of the early palaeozoic accreted terranes terranes within the bohemian massif exhibit hp metamorphism recording subduction driven collision prior to incorporation in the tesz collage itself associated with a hp event in romania the saxothuringian zone and rheic suture are directly juxtaposed with the eec and the crustal structure of the tesz has been much modified by post palaeozoic tectonism deep seismic reflection data where available show that the oceanic sutures are frequently associated with inclined reflectivity zones irz in the lithospheric mantle here inferred to mark relict subduction zones interpreting the age of subduction is complicated by offset of originally coplanar mantle and crustal segments of the irz resulting from early orogenic lithospheric delamination at the moho interface and or post orogenic reordering processes the mantle irz appear to indicate a n directed polarity for early palaeozoic subduction apparently incompatible with the geological evidence which indicates subduction of the tornquist sea beneath avalonia and a large amount of underthrusting of avalonia by baltica possible explanations for these apparently incompatible observations are evaluated

a structural model for the western central sudetes a deformed stack of variscan thrust sheets a model based on new structural and geochemical data is presented it unifies the structural history of the izera rudawy janowickie and kaczawa complexes with the fore sudetic block despite their current separation by the intra sudetic and marginal sudetic faults above the granitoid izera kowary and w droze gneisses at the base of the structural sequence the ductile kowary shear zone marks the basal decollement of the wierzawa thrust sheet comprising often highly strained metasediments associated with enriched tholeiitic and alkaline metabasites above it the ductile kaczorow shear zone corresponding to the main mylonitic zone within the leszczyniec shear zone in the rudawy janowickie complex marks the base of the dobromierz thrust sheet characterized by voluminous morb like meta tholeiites and minor metasediments in the higher parts of the rudawy janowickie and kaczawa complexes and the pyszczynska hill area of the fore sudetic block in the east the sleza ophiolite and the gory sowie block override the entire nappe stack kinematic fabrics in the major and related shear zones indicate d 1 compressional transport towards the northwest followed by minor d 2 extensional movements the thrust stack was deformed during d 3 by southwest verging folds was subsequently intruded by post orogenic granites and later disrupted by the intra sudetic marginal sudetic and associated faults

efficient regional scale 3d potential field geophysical modelling to redefine the geometry of granite bodies beneath prospective geologically complex northwest tasmania in this study a regional model that defines the three dimensional geometry of the subsurface geology beneath the complex prospective northwestern tasmania has been developed this has been achieved using a series of potential field inversions constrained by surface geology geological sections seismic interpretations and a newly extended petrophysical dataset three major episodes of granitic magmatism are preserved in tasmania in the neoproterozoic cambrian and devonian granite bodies are hence considered important indicators of mineralization for explorers in an area of challenging vegetation topography and cover sequences forward modelling and property based inversions of the pre existing geological model show that the previously interpreted subsurface geometry is not compatible with potential field data four sub regions displayed a large discrepancy between calculated and observed data this study redefines the subsurface geometries of these sub regions through individual geometry inversions the density and magnetic susceptibility ranges of units are further refined through property inversions the modified geometry of the devonian granites in the four sub regions may be summarized as follows 1 the housetop granite is relatively thin 5 km thickness whereas 2 the heemskirk and meredith granites are very thick and granite extends to a shallower depth between these bodies than previously interpreted this region between plutons is thus a more prospective region than previously thought 3 for the first time an intrusive body underlying the eastern part of the rocky cape group has been identified its petrophysical properties are similar to that of a granite and its top is interpreted at a depth of 3 km this interpreted low density granitic unit may be either neoproterozoic or devonian 4 a new non magnetic low density cambrian granite with a minimum burial depth of 1 km is also modelled in 3d within the mount read volcanics in the south of the study area our approach whereby sub regions are identified for more detailed modelling enables new constraints to be introduced in a computationally efficient way and has general application to refining the geometry of key structures in prospective regions

electron microprobe dating of monazite because monazite is extremely rich in u and th radiogenic pb pb accumulates very quickly and reaches in about 100 ma a level where it is possible to analyse it with the electron probe assuming that common pb is negligible and that partial loss of pb has not occurred the simultaneous measurement of u th and pb allows to obtain a geologically meaningful age from a single electron probe analysis here we present the results of two years of systematical investigations aiming to define both the limits and potential of this method a specific statistical method to deal with the large number of data which can be obtained on a single sample is described and several guidelines illustrated by examples are suggested to optimize the method electron probe measurements carried out on samples of known age from 200 ma to 3 1 ga yield ages that always fall inside the confidence interval of the isotopically determined age demonstrating that this method is reliable the younger age limit is approximately 100 ma although it can be younger in some favourable cases in old monazites extremely high pb contents have been found up to 5 wt indicating that monazite can tolerate high radiation doses without experiencing lead loss the final precision on the age for a normal monazite is 30 50 ma for a total counting time of 600 s a complete dating procedure can be completed in less than 1 h first results indicate that old ages can be preserved in monazite either in small relict cores in crystals or by the coexistence of several generations of monazites in a sample this method has all the advantages of the electron probe it is non destructive has an excellent spatial resolution monazites as small as 5 i m can be dated and because it is possible to work on normal polished thin sections the petrographical position of the dated crystal is known this method offers a large number of geologists access to an in situ dating technique at moderate cost

evidence for a lost ocean in variscan terranes of the western massif central france plate tectonics is now commonly invoked to explain the variscan orogeny1 3 but only the presence of ophiolites can provide real evidence for an early variscan ocean none has been described so far in south western europe with the possible exception of a few scattered lenses4 within the low grade moeche unit in cabo ortegal spain and the chamrousse ophiolite5 6 french alps which displays no clear relationships to documented variscan terranes to fill this gap eclogites with mid ocean ridge basalt morb compositions have been tentatively ascribed to an oceanic origin7 and various serpentinites and amphibolites have been inferred to derive from ophiolites1 3 but many geologists are still reluctant to admit that the variscan orogeny resulted from the closure of a true ocean here we describe the remains of an ophiolite found in high grade crystalline rocks in the european variscan fold belt in limousin western french massif central this ophiolite comprises diopside bearing harzburgites depleted harzburgites dunites wehrlites and morb type gabbros and has structural petrological and geochemical characteristics typical of oceanic lithosphere thereby providing evidence for a lower palaeozoic ocean

redistribution of rare earth elements during diagenesis of turbidite hemipelagite mudrock sequences of llandovery age from central wales

mesoproterozoic tasmania witness to the east antarctica laurentia connection within nuna most recent paleogeographic reconstructions of the supercontinent nuna juxtapose the north australian craton mawson continent south australia east antarctica and laurentia between 1 6 ga and 1 3 ga but differ in their relative positioning the 10 km thick siliciclastic rocky cape group of tasmania was deposited in an opening marine basin on the margin of east antarctica during nuna breakup based on a similar detrital zircon signature and depositional age the rocky cape group has been correlated with the upper belt purcell supergroup in laurentia thus representing a key tie point within nuna here the detrital zircon age signature of mesoproterozoic rocky cape group quartzites is investigated by comparing new detrital zircon u pb hf isotopic data to an extensive compilation of zircon isotopic data from australia east antarctica and laurentia our analysis demonstrates that the rocky cape group is unlikely to have been sourced from any geological terrane exposed in presentday australia instead zircon u pb hf isotopic data from basement terranes in laurentia and east antarctica show striking similarities to the rocky cape group detrital signature paleocurrent data indicate that the majority of sediment in the rocky cape group was sourced from laurentia which was to the southeast present day coordinates of tasmania supporting a sweat like southwest united states east antarctica configuration for nuna we suggest that rifting left a thinned continental connection between east antarctica and laurentia onto which the lower middle rocky cape group was deposited between 1 45 and 1 30 ga

age mapping and dating of monazite on the electron microprobe deconvoluting multistage tectonic histories high resolution x ray mapping and dating of monazite on the electron microprobe are powerful geochronological tools for structural metamorphic and tectonic analysis x ray maps commonly show complex th u and pb zoning that reflects monazite growth and overgrowth events age maps constructed from the x ray maps simplify the zoning and highlight age domains microprobe dating offers a rapid in situ method for estimating ages of mapped domains application of these techniques has placed new constraints on the tectonic history of three areas in western canada age mapping has revealed multiphase monazite with older cores and younger rims included in syntectonic garnet microprobe ages show that tectonism occurred ca 1 9 ga 700 m y later than mylonitization in the adjacent snowbird tectonic zone in new mexico age mapping and dating show that the dominant fabric and triple point metamorphism occurred during a 1 4 ga reactivation not during the 1 7 ga yavapai mazatzal orogeny in norway monazite inclusions in garnet constrain high pressure metamorphism to ca 405 ma and older cores indicate a previously unrecognized component of ca 1 0 ga monazite in all three areas microprobe dating and age mapping have provided a critical textural context for geochronologic data and a better understanding of the complex age spectra of these multistage orogenic belts

earliest paleoproterozoic high grade metamorphism and orogenesis in the gawler craton south australia the southern cousin in the rae family neoarchean earliest paleoproterozoic rocks of the gawler craton preserved within the mulgathing complex south australia record metamorphism and deformation during an earliest paleoproterozoic tectonothermal event known as the sleafordian orogeny we present metamorphic data from pelitic gneisses of the mulgathing complex from three locations challenger mt christie and blackfellow hill in order to constrain the pressure temperature time p t t evolution of this orogeny in situ u pb dating via laser ablation inductively coupled plasma mass spectrometry shows that monazites within garnet porphyroblasts have ages ca 2470 ma providing an estimate of the timing peak p t conditions monazite crystallisation from melt occurred at ca 2460 ma in the presence of peritectic garnet either late on the prograde path during biotite dehydration melting or immediately following peak t lobate and patchy rims in leucosome hosted monazite suggest a dissolution reprecipitation process occurred during cooling across the elevated dry solidus at ca 2440 ma monazite younger than ca 2430 2420 ma that is located at grain boundaries between matrix minerals likely reflects monazite crystallisation during cooling across the dry solidus and or subsolidus mineral breakdown during the waning stages of the thermal event phase diagram modelling of pelitic gneisses suggests peak metamorphic conditions at the three localities overlap at 7 kbar and 850 c with apparent thermal gradients in the order of 125 135 c kbar 35 45 c km metamorphism involved a tight anticlockwise p t t path based on mineral assemblage paragenesis and indicates a slow shallow decompressive cooling path sub parallel to the apparent thermal gradient in the order of few degrees per myr a protracted magmatic history involving bimodal magmatism associated with a thinned and underplated extensional basin in the mulgathing complex was likely a thermal precursor to the high thermal gradient metamorphism and convergent transpressive deformation a significant period of post 2 4 ga quiescence in the gawler craton subsequent to basin inversion suggests that this orogenesis may have occurred interior to a plate margin the similarity in timing and style between the sleafordian orogeny in the gawler craton and the 2 5 2 28 ga arrowsmith orogeny of the rae craton supports the notion that these terranes together with the broader gawler adelie craton of the mawson continent the north australia craton elements of the north china sino korean and dharwar cratons formed part of a family of neoarchean terranes described as the rae family of cratons with a distinctive tectono metamorphic record as compared with other archean cratons we suggest the mawson continent is the southern member of this family dominated as it is by terranes now present in the northern hemisphere

structural and u pb geochronological evidence for 1 47 ga rifting in the belt basin western montana structural mapping and u pb dates from mafic sills in the perma culmination of west central montana constrain the timing of a major rifting event in the early history of the belt basin magmatic zircon from the plains and paradise mafic sills within the prichard formation yielded u pb ages of 1469 2 5 and 1457 2 ma respectively the older sill invaded unconsolidated wet sediments at a depth of less than 1 6 km and its emplacement is coeval with widespread soft sediment folding mud diapirism and bathymetric collapse of the basin between members e and f of the prichard formation the youngest sill intruded near the base of the exposed section after significant accumulation of sediment had occurred in the basin correlation of these data with recent geochronologic results from the moyie sills in british columbia and purcell lava in montana suggests that the rifting and subsidence event was basin wide and that at least two thirds of the deposition of the belt supergroup occurred between 1470 and 14

constraining the vandieland microcontinent at the edge of east gondwana australia abstract using airborne magnetic and marine gravity data the geological subdivisions of western tasmania have been interpreted north across bass strait into victoria the three westernmost tasmanian zones the king island rocky cape and burnie zones are inferred to form the largely concealed selwyn block in victoria the eastern tasmania zone correlates with the victorian tabberabbera zone thus the tasmanian tamar fracture zone corresponds with the victorian governor fault the victorian ceres gabbro is correlated with magnetic rocks west of king island that are tentatively considered to be neoproterozoic most of the cambrian felsic volcanic rocks of the tasmanian mount read volcanics lie above the burnie zone as do the similar rocks exposed in the jamieson licola and glen creek windows in central victoria reinterpretation of a victorian deep seismic reflection line indicates burnie zone equivalent rocks were thrust south west over rocky cape zone equivalents a link between western tasmania and central victoria is evident from upper devonian granites intruded into the selwyn block region the eastern end of the upper devonian cobaw complex and the warburton granodiorite contain calcsilicate enclaves interpreted to be derived from a northern equivalent to the smithton basin the mount disappointment granodiorite has high ni and cr contents and pseudomorphs after orthopyroxenes consistent with having been partly sourced from the underlying basaltic rocks like those on the eastern margin of the king island zone the magnetic responses under the strathbogie complex the cerberean caldera and the lysterfield granodiorite are attributed to metamorphism of part of an extension of the smithton basin probably equivalents to the 580 ma spinks creek volcanics quartzite cobbles in a devonian conglomerate in the south eastern melbourne zone may be derived from rocky cape group equivalents when integrated with the geological interpretation of tasmania we provide a stratotectonic map of the vandieland micro continent

age constraints on the hydrothermal history of the prominent hill iron oxide copper gold deposit south australia the mesoproterozoic prominent hill iron oxide copper gold deposit lies on the fault bound southern edge of the mt woods domain gawler craton south australia chalcocite bornite chalcopyrite ores occur in a hematitic breccia complex that has similarities to the olympic dam deposit but were emplaced in a shallow water clastic carbonate package overlying a thick andesite dacite pile the sequence has been overturned against the major steep east west hangingwall fault beyond which lies the clastic to potentially evaporitic blue duck metasediments immediately north of the deposit these metasediments have been intruded by dacite porphyry and granitoid and metasomatised to form magnetite calc silicate skarn pyrite chalcopyrite the hematitic breccia complex is strongly sericitised and silicified has a large sericite chlorite halo and was intruded by dykes during and after sericitisation this paper evaluates the age of sericite formation in the mineralised breccias and provides constraints on the timing of granitoid intrusion and skarn formation in the terrain adjoining the mineralisation the breccia complex contains fragments of granitoid and porphyry that are found here to be part of the gawler range volcanics hiltaba suite magmatic event at 1600 1570 ma this indicates that some breccia formation post dated granitoid intrusion monazite and apatite in fe p ree albite metasomatised granitoid paragenetically linked with magnetite skarn formation north of the hangingwall fault grew soon after granitoid intrusion although the apatite experienced u pb lree loss during later fluid mineral interaction this accounts for its calculated age of 1544 39 ma to the south of the fault within the breccia 40ar 39ar ages yield a minimum age of sericitisation cu fe ree of dykes and volcanics of 1575 ma firmly placing prominent hill ore formation as part of the gawler range volcanics hiltaba suite magmatic event within the olympic cu au province of the gawler craton

insights into the metamorphic evolution of the belt purcell basin evidence from lu hf garnet geochronology we have determined lu hf garnet ages from spatially separated garnet bearing localities in northern idaho the lu hf ages are diverse and reflect a progression of mesoproterozoic metamorphic events the oldest lu hf garnet age determined in this study is 1463 24 ma for garnet within a kyanite schist exposed in the core zone of the boehls butte metamorphic complex a garnet schist from the priest river complex yields a well defined age of 1379 8 ma a garnet staurolite schist a garnet mica schist and a gem grade idaho star garnet sample all from the general vicinity of clarkia idaho yield ages of 1064 10 1081 20 and 1078 17 ma respectively a garnet amphibolite also collected from the boehls butte metamorphic complex yields an age of 1151 41 ma for garnet porphyroblasts and 137 25 ma for diffuse overgrowths the hf in all of the proterozoic garnets is extraordinarily radiogenic with ehf values ranging from 1210 to 10546 which attest to both their high lu hf ratios and their great

proterozoic crustal evolution of central east antarctica age and isotopic evidence from glacial igneous clasts and links with australia and laurentia abstract rock clasts entrained in glacial deposits sourced from the continental interior of antarctica provide an innovative means to determine the age and composition of ice covered crust zircon u pb ages from a suite of granitoid clasts collected in glacial catchments draining central east antarctica through the transantarctic mountains show that crust in this region was formed by a series of magmatic events at 2 01 1 88 1 85 1 79 1 57 1 50 1 41 and 1 20 1 06 ga the dominant granitoid populations are ca 1 85 1 45 and 1 20 1 06 ga none of these igneous ages are known from limited outcrop in the region in addition to defining a previously unrecognized geologic history zircon o and hf isotopic compositions from this suite have 1 mantle like 18o signatures 4 0 4 5 and near chondritic hf isotope compositions ehf 1 5 for granitoids of 2 0 ga age 2 mostly crustal 18o 6 0 8 5 and variable hf isotope compositions ehf 6 to 5 in rocks with ages of 1 88 1 85 1 79 and 1 57 ga in which the 1 88 1 79 ga granitoids require involvement of older crust 3 mostly juvenile isotopic signatures with low mantle like 18o 4 5 and radiogenic hf isotope signatures ehf 6 to 10 in rocks of 1 50 1 41 ga age with some showing crustal sources or evidence of alteration and 4 mixed crustal and mantle 18o signatures 6 0 7 5 and radiogenic hf isotopes ehf 3 to 4 in rocks of 1 2 ga age together these age and isotopic data indicate the presence in cratonic east antarctica of a large composite igneous province that formed through a punctuated sequence of relatively juvenile proterozoic magmatic events further they provide direct support for geological correlation of crust in east antarctica with both the gawler craton of present day australia and proterozoic provinces in western laurentia prominent clast ages of 2 0 1 85 1 57 and 1 45 ga together with sediment source linkages provide evidence for the temporal and spatial association of these cratonic elements in the columbia supercontinent abundant 1 2 1 1 ga igneous and metamorphic clasts may sample crust underlying the gamburtsev subglacial mountains indicating the presence of a mesoproterozoic orogenic belt in the interior of east antarctica that formed during final assembly of rodinia

dating diagenetic monazite in mudrocks constraining the oil window authigenic monazite nodules increasingly recognized in pre mesozoic mudrock successions provide a means for dating diagenesis by exploiting trace uranium within the monazite structure but dating is difficult in practice because of zonation and abundant host rock inclusions we report here a refined technique that has yielded a date of 419 14 ma 2 mswd 1 2 for a 17 point pb pb regression age from a telychian silurian late llandovery mudrock from central wales shrimp analyses of monazite nodules from the same horizon are consistent with this and additionally demonstrate concordance of the u pb and th pb systems these results open the door to providing robust dates for a phase of burial related diagenesis which in wales seems approximately coincident with and perhaps was related to hydrocarbon expulsion

in situ u pb dating and element mapping of three generations of monazite unravelling cryptic tectonothermal events in low grade terranes abstract in situ u pb dating of monazite and xenotime in sedimentary rocks from the mid archean soanesville group in the pilbara craton yields ages for provenance diagenesis and multiple low grade metamorphic events detrital monazite and xenotime grains give dates 3250 ma whereas diagenetic xenotime provides a new minimum age of 3190 10 ma for deposition of the basal soanesville group previously constrained between 3235 ma and 2955 ma metamorphic monazite provides evidence for three episodes of growth at 2 88 2 16 and 1 65 ga element mapping of monazite for la sm y and th reveals distinct cores and rims in some crystals that were used to guide the placement of analytical spots during in situ u pb dating by sensitive high resolution ion microprobe shrimp specifically la and sm distributions closely correlate with different generations of monazite the presence of two generations in single monazite crystals highlights the need for characterizing mineral chemistry prior to geochronology it also shows the importance of using in situ dating techniques rather than methods that rely on the analysis of entire potentially multi aged crystals the ages recorded by metamorphic monazite span more than one billion years and are interpreted to record cryptic tectonothermal events within the craton the 2 88 ga age coincides with a phase of regional deformation metamorphism and gold mineralization along a major crustal lineament whereas the most common monazite age population at 2 16 ga corresponds with the migration of a foreland fold and thrust belt across the craton the youngest age 1 65 ga coincides with an episode of tectonic reworking in the capricorn orogen along the southern pilbara margin the prolonged history of monazite growth may in part relate to channelized fluid flow during reactivation of long lived n to ne trending crustal structures that transect the craton despite repeated episodes of metamorphism the isotopic system in each generation of monazite remained unperturbed yielding precise dates the ability of monazite to record three separate events and in some instances two events in a single crystal distinguishes it from most other low temperature mineral chronometers which are readily reset during metamorphic overprinting low temperature monazite geochronology can provide a detailed isotopic history of cryptic thermal events and reveal the temporal and spatial patterns of far field fluid flow related to tectonic processes the previously unrecognized history of crustal fluid flow in the pilbara craton has implications for chemical mineralogical and isotopic studies seeking to understand conditions on the early earth

inverse modeling constrained by potential field data petrophysics and improved geologic mapping a case study from prospective northwest tasmania the heazlewood luina waratah area is a prospective region for minerals in northwest tasmania australia associated with historically important ore deposits related to the emplacement of granite intrusions and or ultramafic complexes the geology of the area is poorly understood due to the difficult terrain and dense vegetation we have constructed an initial high resolution 3d geologic model of this area using constraints from geologic maps and geologic and geophysical cross sections this initial model is improved upon by integrating results from 3d geometry and physical property inversion of potential field gravity and magnetic data petrophysical measurements and updated field mapping geometry inversion reveals that the devonian granites in the south are thicker than previously thought possibly connecting to deep sources of mineralization in addition we identified gravity anomalies to the northeast that could be caused by near surface granite cupolas a newly discovered ultramafic complex linking the heazlewood and mount stewart ultramafic complexes in the southwest also has been modeled this implies a greater volume of ultramafic material in the cambrian successions and points to a larger obducted component than previously thought the newly inferred granite cupolas and ultramafic complexes are targets for future mineral exploration petrophysical property inversion reveals a high degree of variation in these properties within the ultramafic complexes indicating a variable degree of serpentinization sensitivity tests suggest maximum depths of 2 3 km for the contact aureole that surrounds major granitic intrusions in the southeast whereas the heazlewood river complex is likely to have a deeper source up to 4 km we have demonstrated the value of adding geologic and petrophysical constraints to 3d modeling for the purpose of guiding mineral exploration this is particularly important for the refinement of geologic structures in tectonically complex areas that have lithology units with contrasting magnetic and density characteristics

u pb zircon geochronology and geochemistry from ne vietnam a tectonically disputed territory between the indochina and south china blocks volcanoplutonic complexes in ne vietnam have recently been interpreted as intraplate products of the emeishan plume alternatively mafic ultramafic rocks have been considered as dismembered palaeotethyan ophiolites juxtaposed along a tectonic melange zone new u pb zircon geochronological and geochemical datasets presented here suggest a complex geological history that records collision between the indochina south china blocks mafic ultramafic rocks exposed within a tectonic melange song hien tectonic zone include sub alkaline pillow basalts that define two geochemically distinct ophiolitic suites sh 1 n morb like sh 2 transitional e morb like both suites have geochemical signatures suggestive of crustal contamination compatible with a volcanic passive margin rift setting we suggest that sh 1 basalts may correlate with the devonian carboniferous jinshajiang ailaoshan song ma branch of the palaeotethys and form part of the associated dian qiong belt whereas sh 2 basalts are co magmatic with middle late permian mafic ultramafic intrusive rocks dolerites gabbros peridotites that developed in a rift basin most likely on the margin of the down going south china plate during west vergent subduction beneath indochina during continental orogenesis and thrust stacking these ophiolitic rocks were juxtaposed with other lithotectonic blocks within the song hien tectonic zone post collisional relaxation led to the development of a rift basin song hien rift comprising late permian triassic volcano sedimentary strata including 270 265 ma terrigenous sandstones 252 ma mudstones and c 254 248 ma felsic effusives granites and granodiorites were emplaced across ne vietnam between c 252 and 245 ma in a syn to post collisional setting the late permian early triassic felsic magmatic rocks best correlate with coeval rocks in sw guangxi and the central and western ailaoshan fold belts china and the truong son fold belt vietnam together they signal the final to post collisional stages of indochina south china collision we demonstrate that the analysed magmatic rocks in the lo gam song hien domains of ne vietnam are not genetically linked to the emeishan large igneous province in the yangtze block of south china as has been previously widely proposed

nanosized authigenic magnetite and hematite particles in mature paleosol phyllosilicates new evidence for a magnetic enhancement mechanism in loess sequences of china

variscan exhumation of a subducted paleozoic continental margin the basal units of the ordenes complex galicia nw spain a structural and metamorphic study was carried out in the basal units of the ordenes complex in spain thought to represent a subducted part of the paleozoic margin of gondwana according to their metamorphic evolution this part of the margin was subducted at the onset of the variscan orogeny becoming part of an accretionary complex developed below a colliding element built previously variations in the pt conditions of the first high pressure metamorphic event along the units indicate a polarity of the subduction to the west subsequent underthrusting of more continental material blocked the subduction and triggered the ascent and exhumation of the basal units whereas the convergence continued recumbent folds and thrusts developed along with successive normal detachments compressional and extensional structures were synchronous or alternated in time and together induced the thinning and tapering of the orogenic wedge and its lateral spreading the unroofing took place locally under an inverted temperature gradient caused by a detachment which carried a part of the hot mantle wedge above the subduction zone over the subducted units

the string of beads fossil horodyskia in the mid proterozoic of tasmania abstract horodyskia has been found at a single tasmanian locality in the cassiterite creek quartzite ca 1300 800 ma part of a thick proterozoic mildly deformed low greenschist facies marine shelfal siliciclastic succession known as the rocky cape group the rock hosting the fossils is thinly interbedded and interlaminated dark grey slaty shale and quartzose siltstone the sharp based graded siltstone layers are interpreted as distal storm surge deposits on an outer marine shelf the strings of beads are mostly preserved at the base of the siltstone layers in concave hyporelief external moulds on the soles of the event beds and as convex epirelief casts on the tops of the underlying shale beds the casts comprise shale identical to the underlying bed the beads average 1 7 mm in diameter and the gap between the borders of adjacent beads tends to be approximately equal to the bead diameter occasionally the fossils are preserved within shale as wholly flattened beads delineated by a subtle darkened halo the tasmanian strings of beads have most of the morphological attributes of previously described horodyskia including regularity of size and spacing of beads in any one string lack of branching and in some instances haloes and casts with apical depressions the strings on at least one bedding plane have a strong n s preferred orientation of unknown origin tectonic deformation has resulted in 30 shortening in a sw ne direction the morphologic similarity but differing mode of preservation of the tasmanian horodyskia to the two previously described mesoproterozoic species is strong evidence for a biologic origin for the string of beads phenomenon after morphological and morphometric comparisons with other species of horodyskia the tasmanian specimens are assigned to horodyskia williamsii

early deep magnetite fluorapatite mineralization at the olympic dam cu u au ag deposit south australia the olympic dam iron oxide copper gold iocg uranium silver deposit south australia is hosted in the large olympic dam breccia complex within the 1 59 ga roxby downs granite this breccia complex formed through multiple stages of hydrothermal activity and texturally destructive brecciation that affected the granite the deepest diamond drill hole to date rd2773 end at 2 329 m intersected weakly altered in situbrecciated granite 370 2 329 m and a quartz phyric felsic unit 2 010 2 265 m these two rock units host coarse grained hydrothermal minerals from 2 150 m to the end of the drill hole the main minerals in this assemblage are magnetite hematite pyrite fluorapatite and quartz with minor disseminated chalcopyrite sericite chlorite rare earth element ree fluorcarbonates monazite uraninite thorite galena sphalerite anhydrite schorl rutile and pyrrhotite the assemblage is cut by abundant multiphase veinlets and calcite fluorite barite veins a zircon u pb age for the felsic unit 1591 11 ma implies that this unit is broadly coeval with the granite whereas u pb ages for hydrothermal uraninite 1593 5 5 1 ma fluorapatite 1583 3 6 5 ma and hematite 1592 15 ma indicate that deposition of the u ree rich hydrothermal magnetite fluorapatite pyrite quartz assemblage and replacement of magnetite by hematite occurred soon after emplacement of the granitic host rocks sm nd dating of ubiquitous calcite veins suggests formation at 1 54 ga the deep 1 59 ga magnetite fluorapatite pyrite quartz assemblage at olympic dam resembles those characteristic of iron oxide apatite deposits and many other sensu stricto iocg deposits this study confirms that the 1 59 ga event involved significant and widespread iocg mineralization in the olympic cu au province

testing ore deposit models using in situ u pb geochronology of hydrothermal monazite paleoproterozoic gold mineralization in northern australia 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 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

provenance of rifted continental crust at the nexus of east gondwana breakup the bruce rise a prominent bathymetric feature offshore the bunger hills east antarctica has basement of unknown crustal affinity and age in east gondwana the bruce rise is reconstructed near the naturaliste plateau offshore sw australia and microcontinents now submerged in the eastern indian ocean new zircon u pb hf data from two c 1150 ma granite samples dredged from the eastern escarpment of the bruce rise demonstrate that these rocks are dominated by xenocrystic cargo mesoproterozoic xenocrystic cores show textural evidence of melt mediated coupled dissolution precipitation to form rim domains with apparent ages that skew towards c 1150 ma the zircon u pb hf signatures from the xenocrysts in the bruce rise granites and from c 1230 to 1180 ma felsic intermediate granites and orthogneisses from the conjugate naturaliste plateau basement suggest late mesoproterozoic magmatism occurred at a transition in the regional tectonic architecture between a reworked archean cratonic margin and proterozoic juvenile crust on the basis of plate reconstructions exhumation and thinning of the basement to the bruce rise naturaliste plateau occurred predominantly during rifting of india prior to c 120 ma minor further thinning likely occurred leading up to the onset of seafloor spreading between australia naturaliste plateau and antarctica bruce rise from c 90 to 84 ma

tourmaline rich features in the heemskirk and pieman heads granites from western tasmania australia characteristics origins and implications for tin mineralization distinctive magmatic hydrothermal tourmaline rich features have developed in the heemskirk and pieman heads granites from western tasmania australia they are categorized as tourmaline rich patches orbicules cavities and veins based on their distinctive morphologies sizes mineral assemblages and contact relationships with host granites these textural features occur in discrete layers in the roof zone of granitic sills within the heemskirk and pieman heads granites tourmaline patches commonly occur below a tourmaline orbicule rich granitic sill tourmaline filled cavities have typically developed above the tourmaline quartz orbicules in the upper layer of the white phase of the heemskirk granite tourmaline quartz veins penetrate all exposed levels of the granites locally cutting tourmaline orbicules and cavities the tourmalines are mostly schorl fe rich and foitite with an average end member component of schorl 45 dravite 6 tsilaisite 1 uvite 0 fe uvite 3 foitite 31 mg foitite 4 olenite 10 element substitutions of the tourmalines are controlled by femg 1 al y x square r2 na 1 and minor alo al y r2 oh 1 where r2 fe2 mg2 mn2 exchange vectors several trace elements in tourmaline have consistent chemical evolutions grouped from tourmaline patches through orbicules and cavities to veins there is a progressive decrease of most transition and large ion lithophile elements and a gradual increase of most high field strength elements these compositional variations in the different tourmaline rich features probably relate to element partitioning occurring in these phases due to volatile exsolution and fluxing of aqueous boron rich fluids that separated from the granitic melts during the emplacement of s type magmas into the shallow crust 4 to 5 5 km tourmalines from the heemskirk granite are enriched in fe na li be sn ta nb zr hf th and rare earth elements relative to the tourmalines from the pieman heads granite but depleted in mg mn sc v co ni pb sr and most transition elements these results imply that bulk compositions of the host granites exert a major control on the chemical variations of tourmalines the trace element compositions of tourmalines from the sn mineralized heemskirk granite are different from those of the barren pieman heads granite trace element ratios e g zn nb co nb sr ta and co la and sn concentrations in tourmaline can distinguish the productive heemskirk granite from the barren pieman heads granite

u pb dating of an early paleozoic bimodal magmatism in the french massif central and of its further metamorphic evolution in the marvejols area southern french massif central the gneissic marvejols supergroup is overthrust on the metasedimentary serie du lot deposited in part prior to 540 ma the allochtonous terranes are characterized by the occurrence of a leptyno amphibolitic group a complex association of mafic and felsic rocks of igneous and sedimentary derivation a 480 10 ma age has been obtained by u pb dating of zircons for the crystallization of both mafic and felsic meta igneous rocks these rocks were emplaced during an important extensional tectonics relics of eclogites pyrigarnites coronite gabbros and hp trondhjemites are clear evidence for a further hp ht event dated at 415 6 ma on zircons from a hp trondhjemite subsequently the marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345 10 ma

upb pbpb and smnd dating of authigenic monazite implications for the diagenetic evolution of the welsh basin abstract authigenic monazite nodules from silurian turbiditic rocks in the central welsh basin are zoned with respect to nd isotope composition the cores are relatively radiogenic and the rim to core trend projects toward the composition of associated very early authigenic apatite concretions these authigenic phases therefore equilibrated with changing fluid composition as they formed the apatites and the monazite nodule cores may have included significant nd from organic matter or seawater whereas the rees in the outer parts of the monazite nodules were derived from clastic detritus the apparent sm nd age obtained from these phosphates is a mixing line which gives an ordovician age for these silurian rocks the u pb isotope system does not show similar trends and shows promise as an alternative method for dating such phosphates a date of 417 11 ma 2 mswd 107 agrees well with recent estimates of the time of the diagenetic smectite illite transition in the rocks of this study a pb pb isochron gave an age of 422 24 ma 2 mswd 0 9 this shows that the pb is in isotopic equilibrium and is undisturbed and that the elevated mswd of the u pb regression is caused by slight subsequent mobility in the u pb system the event dated by the monazite is probably compactional dewatering during accelerated mid silurian sediment loading in the central welsh basin

variscan oceans ages origins and geodynamic implications inferred from geochemical and radiometric data abstract a review of the magmatic associations providing evidence for the existence of extensional regimes during the palaeozoic evolution of variscan europe is presented the available geochronological data allow recognition of two major rifting events one in the early ordovician and the other in the middle late devonian scarce relicts of ophiolitic complexes indicate that both extensional episodes reached a true ocean spreading stage it is suggested that the variscan belt resulted from a complex two stage collision process through successive suturing in the silurian and the early carboniferous of these oceanic domains the superposition of two distinct opening closure cycles involving geographically separate oceanic basins may partly account for the unusual width of the variscan orogen

formation of authigenic monazite ce to monazite nd from upper carboniferous graywackes of the drahany upland roles of the chemical composition of host rock and burial temperature abstract irregular aggregates of monazite ce to monazite nd up to 150 m in diameter have been found in graywackes from the protivanov formation drahany upland czech republic monazite aggregates enclose a variety of detrital and authigenic minerals textural evidences suggest that they are of authigenic origin and grew during very low grade metamorphism at a maximum temperature 200 300 c in relation to a deep burial rarely they enclose relics of detrital monazite with a chemical composition typical for high grade monazite intensive fluid related ree remobilization occurred in relation to this very low grade metamorphism authigenic monazite appears to form only in ca poor rocks in ca rich graywackes which underwent the same low temperature metamorphic overprint authigenic synchysite ce or parisite ce are common instead in the overlaying rozstani and myslejovice formations t 150 200 c remobilization of rees was low and authigenic monazite did not form even in the ca poor graywackes authigenic monazite shows low concentrations of th u ca y hree and a low eu anomaly the central parts of the aggregates are enriched in nd mree whereas the outer parts are enriched in la and ce zoning pattern suggests an evolution in fluid chemistry during diagenesis very low grade metamorphism or it relates to different solubility of the individual ree phosphates the major source of rees in the pore fluids are probably detrital minerals mainly monazite other minor sources of rees probably include fe oxide hydroxide and or clay minerals which could have contributed to the nd mree rich cores in the authigenic monazite

authigenic monazite and detrital zircon dating from the proterozoic rocky cape group tasmania links to the belt purcell supergroup north america abstract the oldest known rocks in tasmania occur in the proterozoic rocky cape group a 10 km thick quartzarenite siltstone pelite dominated succession previously constrained to have been deposited between 1450 ma and 750 ma the rocky cape group contains the enigmatic fossil horodyskia string of beads and has the potential to place tasmania within supercontinent reconstructions detrital zircon and authigenic monazite grains dated via u pb laser ablation inductively coupled plasma mass spectrometry la icpms analysis yield a deposition window between c 1450 ma youngest zircon populations and c 1330 ma oldest authigenic monazite population for the 9 km thick lower middle units pedder river siltstone lagoon river quartzite balfour subgroup which hosts horodyskia detention subgroup the upper units 1 km include the irby siltstone which is younger than c 1310 ma this unit is likely separated from both the lower middle units and the overlying c 1900 ma grains were identified authigenic monazite age distributions are complex with multiple age domains within most samples the common pb corrected 206u 238pb ages defined by oldest grains in each sample identify three statistically significant groups 1 1358 1292 ma inclusive of two sigma errors lagoon river quartzite and pedder river siltstone 2 1283 1239 ma cowrie siltstone and balfour subgroup and 3 1085 9 ma detention subgroup we suggest monazite was precipitated during episodic fluid flow events at these three stages in various parts of the basin the original source for ree bearing fluids could be detrital monazite which is rarely preserved and or organic matter from the interbedded carbonaceous shales the rocky cape group has a shared provenance with the higher grade metasediments surprise bay and fraser formations of nearby king island the newly derived depositional ages also overlap and support the correlation of these rock associations on the basis of current datasets there are no obvious correlations that can be made with mesoproterozoic basins preserved in mainland australia instead an overlap in the timing of deposition similarities in detrital zircon signatures and analogous depositional environment suggests the c 1 45 1 37 ga upper belt purcell supergroup missoula and lemhi groups of western north america constitutes a plausible correlation with the tasmanian mesoproterozoic succession if the unexposed palaeoproterozoic basement of tasmania correlates with the transantarctic mountains region of east antarctica as previously proposed we suggest that the overlying mesoproterozoic sequences were deposited during rifting of the supercontinent nuna between proto australia including the mawson craton of antarctica and laurentia as predicted by the most recent palaeogeographic reconstructions both the tasmanian and western laurentian packages were affected by episodic post depositional fluid flow events between c 1 35 and 1 05 ga possible thermotectonic imprints of the subsequent assembly of rodinia

textural chemical and isotopic insights into the nature and behaviour of metamorphic monazite monazite is a mineral of choice for dating metamorphism in amphibolite and granulite grade metapelites however there exist a number of difficulties that complicate the interpretation of monazite geochronological data and prevent its application to many geological problems the two main obstacles addressed in this contribution are firstly the minor but significant e g 1 30 ma dispersal in duplicate isotope dilution thermal ionisation mass spectrometry id tims u pb age data commonly recorded from a single rock and secondly the difficulty of attaching monazite age data to pressure and temperature information through a multidisciplinary approach utilising tims and laser ablation multicollector inductively coupled plasma mass spectrometry la mc icpms isotope data quantitative and qualitative emp chemical analyses of monazite and textural studies we assess the significance of pb loss older components and continuous and episodic monazite growth in the generation of dispersed age data three samples from the canadian cordillera and one sample from the himalaya of pakistan are examined each sample exhibits an age dispersion of between 1 and 12 ma for single crystal and multi grain tims u pb monazite age determinations consideration of the closure temperature for pb diffusion in monazite and the metamorphic temperatures experienced by these samples suggests diffusive pb loss did not play a significant part in generating this age dispersal the la mc icpms study indicates that an older component 100 ma older than the tims ages contributed to the age dispersal in three of the four samples in all the samples however chemical analyses identified that the majority of monazites examined exhibited significant intra crystalline zoning in y content the la mc icpms analysis of one sample that was constrained to zones of distinct y content indicates that these zones are of distinct age we suggest that monazite grown before the appearance of garnet and during garnet breakdown is relatively rich in y whereas monazite grown after garnet is relatively poor in y a combination of these chemical data with textural observations suggests that once monazite had entered the mineral assemblage it grew or recrystallised episodically throughout the prograde and retrograde paths of the metamorphic event this behaviour contributes to and in one of the samples controls the observed age dispersal this recognition allows the generation of pressure temperature time points by combining textural and chemical information of monazite with in situ age determinations and pressure temperature information from garnet thus the episodic growth of compositionally distinct monazite throughout a metamorphic event provides the geochronologist with a very valuable chronological tool

temporal and spatial evolution of the neogene age breiðdalur central volcano through 39ar 40ar and u pb age determination abstract the volcanic sequences in eastern iceland provide a cross section of magmatic activity from the iceland rift system throughout the miocene this study focuses on a high precision age determination campaign of a single miocene volcanic system the brei dalur central volcano and associated dyke swarm in order to quantify its evolution the central volcano consists of a lower flank of mafic lavas erupted onto a basement of plateau basalts an upper flank of intermediate and mafic lavas an 8 10 km wide caldera with associated infill formations and a summit sequence of lavas and tuffs this study indicates that the central volcano had a lifespan of around 1 my from 10 2 9 1 ma and started with mafic magmatism intermediate magmatism began around 10 04 ma and the caldera formed between 9 8 and 9 5 ma evolving via multiple nested caldera collapses effusive silicic magmatism was predominantly in the youngest sequences between 9 5 and 9 1 ma after 9 1 ma the activity at the central volcano effectively ceased the volcano became buried by plateau basalt sequences predominantly derived from eruptions at the associated brei dalur dyke swarm activity on the dyke swarm began around 9 9 ma and continued to until 7 8 ma an additional 1 3 my