reply to n van wyck s comment on two paleoproterozoic statherian siliciclastic metasedimentary sequences in central wisconsin the end of the penokean orogeny and cratonic stabilization of the southern lake superior region precambrian res 157 2007 188 202

nd and sr isotopic systematics of central australian granulites chronology of crustal development and constraints on the evolution of lower continental crust nd and sr isotopic data are reported for a granulite terrain in the proterozoic arunta block of central australia sm nd data from a wide range of rock types define a crust formation age of 2 070 125 ma and provide further evidence for voluminous crustal growth in the proterozoic an nd value of 1 5 0 8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust field relationships geochemistry and rb sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism rb depletion at 1 800 ma rb sr data for strongly retrogressed granulites define an age of 1 700 ma which is interpreted as the time of retrograde biotite growth partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism sm nd and rb sr isotopic systematics of minerals indicate that the terrain cooled slowly did not experience significant uplift until 1 000 ma and remained at temperatures above 320 c until the late palaeozoic the mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain the granulite protoliths appear to have formed in a rift which closed within 280 ma of initial separation deformation and granulite facies metamorphism at 1 800 ma are interpreted to be a consequence of collision between the continental blocks which defined the rift regional retrogression and granitoid magmatism at 1 700 ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision subsequent events in the cooling and uplift history appear to have been controlled by the presence of long lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere the results of this study suggest that granulite terrains in general cannot be equated with lower continental crust but instead represent assemblages of mainly supracrustal rocks which in some instances have been involved in major collision events

structural outer rim of chesapeake bay impact crater seismic and bore hole evidence nine seismic reflection profiles and four continuous core holes define the gross structural and stratigraphic framework of the outer rim of the chesapeake bay impact crater the rim is manifested as a 90 km diameter ring of terraced normal fault blocks which forms a 320 m 1200 m high rim escarpment the top of the rim escarpment is covered by a 20 m 30 m thick ejecta blanket the escarpment encircles a flat floored annular trough which is partly filled with an 250 m thick breccia lens exmore breccia the exmore breccia overlies a 200 m 800 m thick interval of slumped sedimentary megablocks which in turn rests on crystalline basement rocks all postimpact strata upper eocene to quaternary sag structurally into the annular trough and most units also thicken as they cross the rim into the crater postimpact compaction and subsidence of the exmore breccia have created extensive normal faulting in overlying strata

banded amphibolites of the harts range meta igneous complex central australia an early proterozoic basalt tonalite suite abstract a multistage petrogenesis is proposed for banded metabasaltic amphibolite and tonalite tectonically interlayered on a fine centimetre scale within upper and lower amphibolite sequences of the harts range meta igneous complex eastern arunta block central australia the basaltic amphibolites exhibit only slight rare earth element ree fractionation ce n yb n 2 5 and can be related by plagioclase clinopyroxene dominated low pressure fractional crystallisation in the tholeiitic trend early olivine removal from a moderately aluminous trace element impoverished tholeiitic picrite parent magma caused significant ni depletion in the evolved residual liquids that gave rise to the basaltic amphibolites the primary picritic melt was generated by high degrees 30 of hydrous partial melting of a depleted mantle source selectively re enriched in large ion lithophile lil elements chondrite normalised ree patterns for low al 2 o 3 tonalitic layers within the basaltic amphibolite units are moderately fractionated ce n yb n 3 3 7 3 and show significant negative eu anomalies ree modelling indicates that the precursors to the tonalitic rocks could have been generated by small degrees 10 of partial melting of a light rare earth element lree enriched basaltic parent with plagioclase clinopyroxene and minor hornblende comprising coexisting residual phases melting at relatively low p h 2 o the amphibolite sequence is interpreted as a complexly folded and metamorphosed bimodal basalt tonalite suite formed at an early proterozoic rifted continental margin or intra cratonic rift

petrogenetic interpretation of granitoid rock series using multicationic parameters abstract granitoid rock compositions from a range of tectonic environments are plotted on a multicationic diagram devised by de la roche and his coworkers this shows that there is a systematic change through an orogenic cycle which leads progressively to the ultimate development of alkaline magmas possible source materials and mechanisms of magma generation are considered from analysis of mineral compositional vectors these suggest that most granitoid series result from a two stage process first fractional crystallisation of clinopyroxene olivine and calcic plagioclase from a basic source with tholeiitic affinities produces a magma of intermediate composition this magma then undergoes periodic mixing with a felsic magma followed by in situ fractionation to generate individual intrusions within granitoid series

rate of arunta inlier evolution at the eastern margin of the entia dome central australia abstract zircon dating in the entia dome of the arunta inlier helps to temporally define the multiphase successive strangways and harts range orogenies it appears that the entire activity of both orogenies took place between 1767 2 ma and 1731 1 ma with a major shearing and thrusting event associated with plutonic activity at 1747 2 3 ma these comparatively short time spans indicated are quite sufficient for the large horizontal movements required for the intra continental basin formation and deformation that has been postulated by james and ding the earlier of these events are later than those currently proposed for equivalent stages of other middle proterozoic inliers of the north australian orogenic province which raises difficulties with detailed correlations in grouping the inliers together monazite u pb muscovite rb sr and biotite rb sr isotopic systems had not closed until 360 ma 325 ma and 308 ma respectively indicating uplift and cover removal from 20 km to

geochemistry of metatholeiites from the harts range central australia implications for mantle source heterogeneity in a proterozoic mobile belt metatholeiite series from early middle proterozoic basement and cover sequences in the harts range central australia display marked differences in trace element abundances and ratios that reflect the chemical heterogeneity of their mantle sources the trace element data reveal a multistage geochemical evolution for the attendant proterozoic sub continental mantle early formed supracrustal tholeiites from the oonagalabi and entia basement gneiss complexes display slight to moderate light rare earth element lree enrichment ce yb n 1 8 3 9 unfractionated heavy ree distributions and chondritic zr nb and zr y ratios they were generated by variable degrees 15 30 of partial melting of a relatively undepleted mantle source in an extensional ensialic rift environment later formed metabasites comprise numerous sills that intrude the supracrustal sequence within the entia gneiss complex primary basaltic liquids for these rocks possessed more strongly fractionated ree levels ce yb n 3 0 12 3 large ion lithophile lil element enrichment and pronounced depletions in high field strength elements their high la nb ba zr and cr y ratios resemble those of convergent plate margin basaltic magmas derived from mantle sources that have been metasomatised as a result of subduction related processes metatholeiites from the harts range meta igneous complex in the cover sequence display high zr nb ratios reflecting a mantle source that was strongly depleted in incompatible elements during a previous melting event tholeiites from the lower part of the cover sequence show distinct lil element and lree depletion ce yb n 0 8 1 0 features resembling depleted n type mid ocean ridge basalts and some continental tholeiites tholeiites from the upper part of the cover sequence show enrichment of highly incompatible lil elements especially ba and k relative to ree and pronounced decoupling between high field strength elements and ree resulting in high la nb ratios they represent melts derived from a mantle source which has been depleted in a basaltic component and subsequently re enriched in incompatible elements e g via introduction of a subduction related component the inferred mantle evolution is compatible with a tectonic model that involves successive extensional and compressional episodes during repeated ensialic orogenesis as proposed for the harts range basement and cover sequences

strain and displacement in the harts range detachment zone a structural study of the bruna gneiss from the western margin of the entia dome central australia abstract a detailed structural and strain study has been carried out on the bruna granitic gneiss and its bounding thrusts from the western margin of the entia dome in the eastern harts ranges the zone containing the bruna gneiss has been described recently as comprising the harts range detachment zone hrdz a major low angle lithospheric detachment along which both major extensional and contractional movements have occurred the bruna gneiss contains a ductile fabric which varies from an augen streaky foliation through a ribbon gneiss to protomylonite and mylonite in what is predominantly a granitic gneiss conformably overlying the bruna gneiss is a distinctive dark coloured megacrystic gneiss layer which intruded independently from the bulk of the bruna gneiss and followed a different subsequent strain history principal stretch lineations are consistently north plunging or horizontal n s except where folded the bruna gneiss fabrics are complexly folded about axes in an asymmetric manner which suggests interruption of ductile slip and crumpling of the fabrics followed by continuing slip on other discrete surfaces and shear zones strain measurements of elongated feldspars and aggregates indicate moderate to high strains with predominantly plane strain symmetries strain is partitioned into planar parallel zones and suggests dominant heterogeneous simple shear as the main non coaxial flow mechanism flattening strains occur near the bases of two major zones and constrictional strains occur within some zones a strain integration profile across the detachment zone produces total displacements of only a few kilometres suggesting that much of the overall indicated displacement is partitioned into the discrete bounding thrusts asymmetric augen pressure shadows and shear bands provide kinematic indicators in the bruna gneiss most show reverse sense displacement of the overlying harts range cover in a southerly direction over the entia gneiss basement opposite extensional sense movements occur in some zones and especially at the margins of the detachment zone

magmatism and metamorphism in the ladakh himalayas the indus tsangpo suture zone abstract ladakh india provides a complete geological section through the northwestern part of the himalayas from kashmir to tibet within this section the magmatic metamorphic and geotectonic evolution of the northern himalayan orogeny has been studied using petrographic geochemical and isotope analytical techniques the beginning of the himalayan cycle was marked by large basaltic extrusions panjal trap of permian to lower triassic age at the northern margin of the gondwana continent indian shield these continental type tholeiitic basalts were followed by a more alkaline volcanism within the triassic to jurassic lamayuru unit of the gondwana continental margin lower jurassic to cretaceous oceanic crust and sediments ophiolitic melange s s accompany the triassic to cretaceous flysch deposits within the indus tsangpo suture zone the major structural divide between the indian shield high himalaya and the tibetan platform so far no relic of paleozoic oceanic crust has been found subduction of the tethyan oceanic crust during upper jurassic and cretaceous time produced an island arc represented by tholeiitic and calc alkaline volcanic rock series dras volcanics and related intrusives accompanied by volcaniclastic flysch deposits towards the tibetan continental margin subsequent to the subduction of oceanic crust large volumes of calc alkaline plutons trans himalayan or kangdese plutons intruded the tibetan continental margin over a distance of 2000 km and partly the dras island arc in the ladakh region the collision of the indian shield and tibetan platform started during the middle to upper eocene and caused large scale still active intracrustal thrusting as well as the piling up of the himalayan nappes the tectonically highest of these nappes is built up of oceanic crust and huge slices of peridotitic oceanic mantle spongtang klippe in the high himalayas the tectonic activity was accompanied and outlasted by a barrovian type metamorphism that affected triassic sediments of the kashmir nun kun synclinorium up to kyanite staurolite grade and the deeper seated units up to sillimanite grade cooling ages of micas are around 20 m y muscovite and 13 m y biotite towards the indus tsangpo suture zone metamorphism decreases with no obvious discontinuity through greenschist prehnite pumpellyite to zeolite grade remnants of possibly an eo himalayan blueschist metamorphism have been found within thrust zones accompanying ophiolitic melange in the suture zone

geochemical and nd isotopic systematics of granites from the arunta inlier central australia implications for proterozoic crustal evolution geochemical and sm nd isotopic results are reported for granites from the proterozoic arunta inlier of central australia these combined with new geochronological data for the granites zhao and bennett 1995 allow important constraints to be placed on proterozoic tectonic and crustal evolution in central australia granites from the arunta inlier can be divided into three major geochemical groups a calcalkaline trondhjemitic group cat a high heat production group hhp and a volumetrically significant main group the cat group which occurs only in the southern margin of the inlier is characterised by high na2o nak sr krb and sry and relatively low k2o rb rbsr th u ree nb and y analogous to calc alkaline suites occurring in modern convergent plate margins the hhp group which occurs mainly in the interior of the arunta inlier and is spatially associated with the main group is characterised by high k rb th u rbsr and rbzr and relatively low sr ba nak krb barb mgo cr and ni the main group which occurs throughout the arunta inlier is geochemically intermediate between the cat and hhp groups it is geochemically analogous to the 1880 1850 ma old barramundi igneous association recognised in other proterozoic terrains of central australia and can be further subdivided into four age subgroups 1820 1780 1750 1650 and 1615 1600 ma respectively all three groups show similar negative nb anomalies on the trace element normalized diagrams despite the geochemical diversity of the arunta granites no correlations between the nd isotopic and geochemical signatures are observed the ranges of initial nd values and nd depleted mantle model ages tnddm for the three groups of granites overlap with each other overall there are two groups of tnddm ages the most common ranging from 2 33 to 1 96 ga and the other from 1 83 to 1 72 ga although initial nd values of the granites show large variations there is a general trend for the nd values to increase i e become less negative with decreasing crystallization ages the large geochemical and isotopic variations of the arunta granites reflect considerable heterogeneity in the sources of the granites and do not support a uniform 2 3 2 1 ga old barramundi type underplate source for the origin of the granites the combined geochemical and nd isotopic data suggest the sources of the granites contain at least two main components an palaeoproterozoic mantle derived component and an older crustal component the older crustal component was incorporated into the source region of the granites either through magma assimilation of crustal materials at lower crustal levels or more probably via sediment subduction and mantle wedge metasomatism the increasing nd values with decreasing crystallization ages may be a result of progressively increasing proportions of newly accreted island arc materials being incorporated in the subducted sediments alternatively a juvenile mantle derived component may have been added to the source region of the younger granites it is apparent that the nd isotopic signatures in the granites has resulted from complex processes and neither simple mixing nor simple two stage protolith models can as yet satisfactorily explain the observations the petrogenesis of the three geochemical groups of granites can be interpreted in a plate tectonics scenario it is considered that the cat group was formed by fractionation of arc type magmas and or partial melting of arc related intrusions or underplates in a subduction related continental margin setting whilst the formation of the main group may involve partial melting of fractionated and or modified arc related underplates but not necessarily in a subduction environment the younger hhp group was probably generated by remelting of the coexisting main group during subsequent tectonothermal events

the atnarpa igneous complex southeast arunta inlier central australia implications for subduction at an early mid proterozoic continental margin abstract three distinctive igneous suites have been identified from the atnarpa igneous complex southeast margin of the arunta inlier central australia the 1879 11 10 ma gabbro diorite tonalite trondhjemite suite sio2 49 78 shows both calc alkaline and trondhjemitic affinities the overall depletion of nb ti y k and rb in this suite indicates that its parental magma must have been depleted in these elements it is the oldest zircon age so far obtained in the arunta inlier the 1873 11 10 ma low al tonalite trondhjemite granodiorite suite is also depleted in k and rb however unlike either the 1879 ma or the 1751 ma suites it has lower al2o3 and sr at comparable sio2 levels and display flat hrees and negative eu anomalies the 1751 12 ma younger tonalite suite is high in sr sr y and al2o3 and low in zr ti nb y th and u and displays a strongly fractionated ree pattern with hree depletion typical of archaean high al tonalite trondhjemite granodiorite suites such geochemical features contrast with the 1850 1880 ma old k rb ree th enriched na sr depleted barramundi igneous association widespread in proterozoic terrains of northern australia the parental magma of the 1879 11 ma suite is considered to be derived by subduction related partial melting of a metasomatised hybridised mantle wedge with phlogopite retained in the residue the parental magma of the 1873 11 ma low al suite by relatively low pressure partial melting of a mafic source with residual plagioclase left in the residue and the 1751 12 ma younger tonalite suite by partial melting of a subducted oceanic crust at mantle depth with residual garnet amphibole these processes contrast with models of restite unmixing of a geochemically uniform but fractionated preexisting mantle derived underplate previously proposed for the k rich barramundi igneous association combining geochemical features of the atnarpa igneous complex with reported data from the harts range and the alice springs areas we propose that the southeast margin of the arunta inlier represents a convergent continental margin during the early to middle proterozoic which has undergone at least two episodes of subduction related magmatism 1860 1880 ma and 1730 1770 ma such continental margin subduction could provide additional driving force for crust mantle delamination and ensialic a subduction within the preexisting continental crust further north both within plate vertical underplating and marginal lateral accretion could be important processes for proterozoic crustal evolution

amphibolites from the entia gneiss complex eastern arunta inlier geochemical evidence for a proterozoic transition from extensional to compressional tectonics geochemically distinct metabasic amphibolite suites within the dominantly quartzo feldspathic entia basement gneiss complex egc harts range central australia imply two separate magmatic episodes of contrasting tectonic affinity that contributed to the early mid proterozoic crustal evolution of the eastern arunta inlier the metabasic suites directly reflect a distinctive style of proterozoic tectonics involving sequential mantle plume generated extension crustal thinning and subsequent compression crustal thickening that took place on a lithospheric detachment zone during ensialic orogenesis metabasic rocks from an early formed supracrustal association were generated in an intra continental rift environment they have the geochemical characteristics of continental tholeiites similar to metabasites in other eastern arunta basement gneiss terranes younger basic to ultrabasic amphibolites comprise thick massive deformed and metamorphosed sills and small layered igneous bodies that intruded the earlier formed supracrustal sequence trace element characteristics of these rocks e g high la nb and zr nb typify subduction related magmas the data imply an arc or collisional type environment this regime post dating the tensional i e rifting stage a metacumulus origin is proposed for many of these rocks their continuous geochemical trends imply extensive pyroxene dominated fractionation of mantle derived parental magmas in differentiating sill complexes emplaced at depths in the range 20 25 km incompatible element distributions and continuous overlapping major element trends indicate that the cumulate metapyroxenite bodies are related to the early fractionation of voluminous tonalite granodiorite orthogneiss intrusives that comprise a significant portion of the entia dome and which closely resemble intrusive suites commonly observed in cordilleran settings anomalously high k zr rb zr and ba zr ratios for these entia amphibolites together with variations in sio2 ce yb n and k rb ratios indicate contamination of some of the entia magmas by continental crust most likely by small degrees of melting of easily fusible felsic gneiss component aided by high surface area to volume ratio of the sills

metamorphic events in the eastern arunta inlier part 1 metamorphic petrology abstract in the entia dome of the eastern arunta inlier kyanite facies metamorphism is recorded over a substantial area at least 160 km 2 and thus contrasts with the regionally extensive belts of high temperature low pressure andalusite sillimanite facies metamorphism documented elsewhere in the arunta inlier this amphibolite facies event appears to postdate regional granulite facies metamorphism evidence for which is almost totally obliterated in the entia dome prograde kyanite occurs in meta basites in comparatively rare associations involving hornblende staurolite and anorthite and in other rocks in association with gedrite and hornblende or garnet and biotite these kyanite bearing assemblages equilibrated at intermediate temperatures 570 710 c at peak pressures of 7 kbar and are overprinted by lower pressure assemblages including hornblende gedrite cordierite and sillimanite while this sequence of reactions may be simply interpreted in terms of a single clockwise orogenic cycle textural evidence and the available mineral isotopic data suggest a more complex history involving multiple heating and cooling episodes during the mesoproterozoic and possibly the palaeozoic the occurrence of regional retrogressive kyanite facies assemblages in the eastern arunta inlier provides evidence that the widely held perception that proterozoic crustal evolution in australia involved a distinct ensialic orogenic style characterised by high geothermal gradients etheridge et al 1987 loosveld and etheridge 1990 sandiford and powell 1991 is not universally true rather petrological evidence presented here with isotopic constraints described by foden et al 1995 suggests that the harts range region has experienced substantial crustal reworking at elevated temperatures during both the proterozoic and palaeozoic under thermal regimes more familiar to modern continental collision zones

two paleoproterozoic statherian siliciclastic metasedimentary sequences in central wisconsin the end of the penokean orogeny and cratonic stabilization of the southern lake superior region abstract statherian siliciclastic sedimentary rocks in the southern lake superior region are key to understanding the evolution and stabilization of paleoproterozoic crust in the midcontinent deposited during an important period of significant crustal growth controversy exists regarding the conditions of sandstone deposition and the relations between magmatism and sedimentation in central wisconsin the hamilton mounds inlier although restricted in areal extent is significant in that it reveals the existence of two separate statherian units an 1800 1760 ma immature meta arkose and a post 1760 ma baraboo interval supermature quartzite the meta arkose is crosscut by 1762 ma granite and contains abundant penokean age detrital monazite grains the younger quartzite contains detrital monazite as young as 1750 ma and was deposited unconformably upon these older units all units experienced 1630 ma low grade recrystallization and isotopic disturbance associated with the mazatzal orogeny in addition the supermature quartzite experienced k metasomatism and brecciation at 1470 ma ascribed to regional scale hydrothermal alteration related to the influence of the nearby 1470 ma wolf river batholith collectively these data suggest an early statherian episode of arkosic deposition and magmatism during yavapai orogenesis followed by a longer interval of deposition of supermature quartz arenite associated with crustal stabilization the existence of two separate statherian units reconciles apparent conflicts in depositional models and provides a greater understanding of the paleoproterozoic evolution of the midcontinent

tectonic evolution of proterozoic australia proterozoic australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic this paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes the assembly was completed between 1300 and 1100 ma when the crustal fragments were combined as an early component of the rodinian supercontinent rifting and fragmentation of archaean continents began in the late archaean and continued into the proterozoic passive margin deposits such as those of the hamersley basin accumulated on isolated fragments of archaean crust these numerous fragments were subsequently assembled into three cratons by 1830 ma a west australian craton was established by collision of the archaean pilbara and yilgarn cratons which were joined along the capricorn orogen similarly a south australian craton developed by amalgamation of the proto gawler and proto curnamona cratons along the kimban orogen a north australian craton appears to have formed by accretion of numerous crustal fragments including the kimberley pine creek lucas and altjawarra cratons with sutures marked by the king leopold halls creek tennant creek and proto isan orogens the southern margin of the north australian craton was the site of repeated terrane accretion and orogenic activity between 1880 ma and 1400 ma this included an orogenic event at 1880 1850 ma the strangways 1780 1730 ma argilke 1680 1650 ma and chewings 1620 1580 ma orogenies and the intracratonic anmatjira uplift 1500 1400 ma intracratonic rifting at 1750 to 1710 ma and 1640 to 1600 ma produced the mcarthur basin and related minor basins parts of which were deformed by the isan orogeny at 1600 and 1530 ma rifting along the line of the capricorn orogen led to deposition in the overlying intracratonic bangemall basin between 1630 and 1300 ma along the eastern margin of the south australian craton the 1670 to 1600 ma olarian orogeny marks interaction with now obscured continental crust to the east tectonic activity between 1300 and 1100 ma led to the assembly of proterozoic australia as an early component of the supercontinent of rodinia this first involved the amalgamation of the west australian and north australian cratons followed by collision with the south australian craton the centralian superbasin developed over the junction of the north south and west australian cratons between 830 and 750 ma rifting to the east formed the adelaide geosyncline at 830 ma this was followed by the breakup of rodinia with the rifting apart of laurentia and gondwanaland along the eastern margin of proterozoic australia at 750 ma and the subsequent formation of the palaeo pacific ocean after the breakup of rodinia a series of northeast southwest compressional events followed by periods of relaxation reflect the assembly of a new supercontinent old lines of weakness were reactivated culminating in the intracratonic king leopold paterson petermann ranges and pinjarra orogenies between 620 and 540 ma subsequent reactivation continued into the phanerozoic with the widespread eruption of continental flood basalts and the formation of intracratonic basins 540 530 ma

急速風化と成熟石英アレナイト堆積を支持する ウイスコンシン州 原生代初期珪岩の砕屑ジルコン年代

late paleoproterozoic climate tectonics and metamorphism in the southern lake superior region and proto north america evidence from baraboo interval quartzites abstract red supermature quartzites of the baraboo interval of the lake superior region contain detrital zircon that ranges in age from 1782 to 1712 ma deposition clearly occurred after the geon 18 penokean orogeny these late paleoproterozoic sedimentary rocks consist largely of quartz kaolinite or pyrophyllite and hematite detrital feldspar and muscovite are rare or absent their chemical index of alteration ranges from 96 8 to 98 6 among the most chemically mature clastic sediments in the geological record the quartzites are underlain by mature feldspar free paleosols accounting for the absence of feldspar in the overlying sediments and indicating the presence of first cycle quartzose detritus such physical and chemical characteristics imply that late paleoproterozoic deposition in the lake superior region occurred in a stable tectonic setting with subdued topographic relief in a warm humid climate folding and low grade metamorphism of the quartzites is thought to reflect 1630 ma foreland

the petrology and geochemistry of granitic gneisses from the east arunta inlier central australia implications for proterozoic crustal development the entia gneiss complex is an early to middle proterozoic inlier composed of upper amphibolite facies orthogneisses and paragneisses in the eastern end of the arunta block in central australia it is separated from overlying structurally distinct compositionally different and probably younger gneisses of the harts range complex by a subhorizontal detachment zone this paper discusses the petrology of four major granitic gneiss bodies one of these the bruna gneiss has intruded this detachment zone the other three are components of the structurally underlying entia gneiss complex the entia gneiss complex has a distinctive subhorizontally layered character with recumbent folds and flat lying schistosity the granitic gneisses that are components of this complex appear to have intruded an actively deforming terrain and are folded sheet like bodies the orthogneisses of the entia complex range from gabbro to true granite in composition and the granites are i type on both geochemical and petrographic criteria they are significantly different from the relatively alkali rich granites of equivalent age which occur in some of the other northern australian proterozoic blocks and are most like the granites of the modern cordilleran belts particularly in the association of diorite and gabbro with granodiorite it is suggested they are formed in narrow collisional belts formed as a result of closure of intracratonic rift basins the high surface area to volume ratio of these granites which is a consequence of the deformational environment into which they were intruded has resulted in considerable wall rock exchange in the more differentiated end members of the suites this has led to contamination by a net gain of k2o rb and ba and loss of relatively compatible elements including the ree y nb and tio2 compared with the syn tectonic entia granites the bruna granite gneiss is post tectonic and shows geochemical affinities with a type or anorogenic palaeozoic granites most of this granite is confined to the detachment zone but the very latest phases also intrude the harts range cover sequence an important conclusion of this study is that detachment zones of the type described from this area of the eastern arunta block exercise strong control on plutonism into the early to middle proterozoic crust as long as the very mobile style of early proterozoic tectonics continues the intracrustal dislocation zone confines syn tectonic granitic magmatism to deeper levels

structural evolution of the harts range area and its implication for the development of the arunta block central australia abstract a division of the high grade metamorphics of the arunta block in the harts range area into two main groups on the basis of lithology structure metamorphism and igneous activity is suggested as a result of detailed mapping the lower group is regarded as a basement terrain and comprises major layered felsic or granitic gneiss minor amphibolite and mafic granulite while the structurally overlying group is regarded as a cover sequence irindina supracrustal assemblage and comprises predominantly metasediments pelitic and semi pelitic gneiss calc silicate and quartzite and amphibolite anorthosite and ultramafic rock of the harts range meta igneous complex the cover sequence might have been deposited in an aulacogen or adjacent to a continental margin which developed by rifting of a pre existing basement the two groups both show early major ductile tectonothermal stages with four major fold generations and a later discrete shear event in the basement and two major fold generations and a later series of stacked nappes developing in the cover the basement s early orogenic evolution with associated granulite facies metamorphism is typical of intensely reworked early precambrian terrains and developed spatially and or temporally separated from the cover cover deformation closely resembles continental margin cordillera type overthrusting and thus thin skinned tectonics associated with large scale crustal shortening during the later harts range remobilization the basement and cover were juxtaposed by displacement on a fundamental crustal dislocation recumbent contraction fault mylonites repeatedly developed along the dislocation which was also isoclinally folded intruded by a major megacrystic granitoid bruna gneiss sheet and repeatedly rejuvenated the history of development of the dislocation is documented and described as a process of underthrusting of basement beneath cover which led to the final termination of the orogenic activity

petrology geochemistry and origin of a major australian 1880 1840 ma felsic volcano plutonic suite a model for intracontinental felsic magma generation in australia a major period of felsic volcanism and granite emplacement occurred between 1880 and 1840 ma towards the end of the early proterozoic these volcanics and granites covering at least 37 000 km2 are compositionally uniform over large areas the felsic magmas are predominantly i type being derived from infracrustal sources which have never been exposed to surficial weathering processes when compared with early archaean and phanerozoic i type granites the 1880 1840 ma suite has distinctively high levels of k2o rb zr la ce th u and low mgo and cao contents even at relatively low sio2 levels of 60 65 the 1880 1840 ma rocks are characterized by low initial 87sr 86sr ratios and negative nd values chemical and isotopic modelling on these volcanics and granites suggest that they formed from mantle derived sources that are around 2300 2000 ma old there is no evidence of a major archaean component in these granites the sources are interpreted to have formed as a result of polygonal small scale mantle convection which led to extensive underplating of archaean crust during this underplating event large volumes of incompatible element enriched mantle derived material accreted to the base of the lower crust and fractionated significantly the more fractionated part of this source was remelted between 1880 1840 ma to produce the voluminous felsic volcanics and comagmatic granites present in nearly every proterozoic domain of australia the actual underplating event between 2300 2000 ma is believed to have triggered the formation of early proterozoic sedimentary basins which occur in a polygonal pattern and appear to wrap around archaean nuclei the composition of these felsic melts and the tectonic setting operating during their emplacement precludes genesis by contemporaneous subduction of oceanic crust all are generated by processes of vertical accretion in an intracontinental environment

granulite facies metamorphism in the mallee bore area northern harts range implications for the thermal evolution of the eastern arunta inlier central australia abstract the mallee bore area in the northern harts range of central australia underwent high temperature medium to high pressure granulite facies metamorphism individual geothermometers and geobarometers and average p t calculations using the program thermocalc suggest that peak metamorphic conditions were 705 810c and 8 12 kbar partial melting of both metasedimentary and meta igneous rocks forming garnet bearing restites occurred under peak metamorphic conditions comparison with partial melting experiments suggests that vapour absent melting in metabasic and metapelitic rocks with compositions close to those of rocks in the mallee bore area occurs at 800 875c and 9 10 kbar the lower temperatures obtained from geothermometry imply that mineral compositions were reset during cooling following the metamorphic peak the rocks underwent local mylonitization at 680 730c and 5 8 7 7 kbar after mylonitization ceased garnet retrogressed locally to biotite which was probably caused by fluids exsolving from crystallizing melts these three events are interpreted as different stages of a single continuous clockwise p t path the metamorphism at mallee bore probably occurred during the 1745 1730 ma late strangways orogeny and the area escaped significant crustal reworking during the anmatjira and alice springs events that locally reached amphibolite facies conditions elsewhere in the harts ranges

caterpillar tectonics in the harts range area a kinship between two sequential proterozoic extension collision orogenic belts within the eastern arunta inlier of central australia abstract we propose that in the harts range area of the arunta inlier of central australia a basement and cover relationship previously described by us represents a thick complexly deformed contact zone between an early middle proterozoic fold belt the strangways orogenic belt sob and a younger middle proterozoic orogenic belt the harts range orogenic belt both of which contain their own supracrustal sequences rocks of the sob were deposited and possibly metamorphosed to a maximum grade of granulite facies at 1800 ma during an early phase of crustal extension they were subsequently intruded by a significant proportion of acidic magmas at 1767 ma and compressed to form a distinctively nne ssw trending orogenic belt a wnw ese trending shallowly north dipping zone of intense and repeated deformation termed the harts range detachment zone hrdz formed initially during a period of nne ssw crustal extension as a major low angle normal shear detachment zone within and crosses at a high angle to the trend of the basement rocks of the sob the hrdz has been intermittently active as a zone of crustal weakness since its initiation the first extensional movement on this zone began the formation of the harts range mobile belt hrmb which displays many of the characteristics of modern tectonic zones activated by plate movement a supracrustal cover sequence the harts range cover hrc formed in the major geosynclinal basin which developed on the subsiding upper plate of the hrdz continued crustal extension juxtaposed the cover sequence which was then metamorphosed to a maximum grade of amphibolite facies and granulite facies basement of the hrdz before a fundamental change in movement sense led to crustal contraction and a collision related thrusting phase the hrmb which includes the hrdz and the hrc was then internally deformed over a relatively very short time span 15 20 ma before voluminous granitic magma intruded mainly along the hrdz during the harts range orogeny hro the intrusion of this major granitoid at 1747 ma precisely fixes the time scale of evolution for the earlier phases of the hro the hro is documented in detail as a peculiar orogenic style of crustal contraction which was largely taken up by reverse sense discrete thrusting and shearing on faults and shear zones associated with the hrdz displacement on the hrdz was a stick slip process with repeated punctuation of thrust slip movement by pinning at points located progressively to the south each period of cessation of thrust slip movement was followed by folding of discrete thrusts of captured basement lobes within the hrdz and of the overlying supracrustal sequence the individual discrete movement phases and folding episodes are each named and described as is a model of this unusual type of proterozoic tectonic activity

light rare earth element zoning in sphene and allanite during granitoid fractionation sphene and allanite rare earth element ree and whole rock light ree concentrations have been found to increase during granitoid fractionation the ree concentrations in both sphene and allanite crystals are zoned with relative depletion occurring at the crystal rims the ree concentration variability and zoning in these minerals can be explained by changes in the melt structure that occur during differentiation and final solidification of the granitoid our data suggest that crystal melt separation controls light ree enrichment or depletion in granitoids and that the end result depends on liquidus accessory and or restite phases present during the crystal melt separation

metamorphic and tectonic evolution of granulites arunta block central australia petrographical and chemical evidence presented here indicate a prolonged metamorphic evolution of the granulite terrain in the northern strangways range arunta block central australia nsr in fig 1 beginning with migmatization followed by dry granulite metamorphism at high temperatures t and moderate pressures p estimated at 850 920 c 8 1 kbar1 this occurred at 1 800 myr refs 2 3 very shortly after deposition of the protolith4 a bimodal volcanic suite5 and associated sediments granulite metamorphism was followed by isobaric cooling during which there was an episode of pervasive but partial hydration later retrogression and hydration localized along shear zones occurred at the normal geothermal gradient in the kyanite stability field a tensional rift like regime best describes the geological environment in which the protolith formed was deeply buried metamorphosed with high heat flow and then remained in isostatic equilibrium metamorphic parageneses recording uplift may be matched with mid palaeozoic tectonic events in central australia if this correlation is correct the rocks of the northern strangways range may have remained deeply buried for 1 400 myr during which the shear zones were reactivated several times fig 2

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

detrital zircon ages from early proterozoic quartzites wisconsin support rapid weathering and deposition of mature quartz arenites abstract the ages of more than 200 detrital zircons from two proterozoic quartzites the baraboo and hamilton mounds quartzites from central wisconsin have been measured utilizing a laser ablation icpms technique both quartzites contain penokean 1 83 1 89 ga and archean components as well as a younger post penokean component the 69 detrital zircons from hamilton mounds contain major pre penokean age populations at 3 2 2 9 and 2 55 ga penokean ages at 1895 1853 and 1811 ma and a post penokean population at 1772 ma most detrital zircon ages are similar to previously measured ages of locally underlying gneisses and intrusive rocks of the marshfield terrane the 147 detrital zircons from two samples of the baraboo quartzite show a grossly similar age distribution with populations at 1754 and 1850 ma and 2 55 ga however the zircon populations in the baraboo quartzite differ from those of hamilton mounds in having a large population at 1920 ma a broad distribution of ages from 2 0 to 2 4 ga a