late proterozoic granitoid magmatism in the state of sao paulo southeastern brazil abstract the precambrian basement in the state of sao paulo is made up of a series of metamorphic units exposed at various erosional levels and in part separated by large faults a strong overprint of the brasiliano cycle 700 500 ma is evident from isotopic data over most of the region nevertheless a large part of the metamorphic belts belong to older archaean to middle proterozoic reworked sequences about 30 of the total basement area consists of intrusive granitoids of late proterozoic age these can be grouped into six major groups on the basis of petrography geochemistry rock association associated manifestations and whenever possible age relationships gneissic mangerite charnockite hastingsite granite associations appear as folded tabular bodies within high grade regions the sao jose do rio pardo suite is chemically comparable to the aluminous a type granitoid series and its emplacement possibly signals a period of extensional tectonics prior to the onset of the brasiliano cycle 1000 ma a series of elongated batholiths 650 ma with predominant porphyritic hornblende biotite granitoids is present mainly in high grade areas the compositions are generally granitic to granodioritic but intermediate e g monzodioritic rocks are locally common these high k calc alkaline suites are in some aspects comparable to the cordilleran i type granitoids and their origin might be related to a subductional environment migmatitic granitoids constitute a peculiar association occurring as a series of small bodies within high grade allochthonous domains these polyphase heterogeneous often nebulitic granitoids are source related products of crustal anatexis syn to late orogenic brasiliano granitoids in the low to medium grade domains are dominantly porphyritic calc alkaline biotite granites and granodiorites reminiscent of caledonian i types in contrast a peraluminous association porphyritic biotite granitoid to equigranular two mica granite predominates in high grade domains which possibly reflects a different tectonic evolution for the two regions late to post orogenic brasiliano granitoids intruded after the major orogenic development of the region 610 590 ma i type calc alkaline and aluminous a type granitoids are geographically associated in the itu belt and appear to show some temporal and geochemical overlapping wiborgitic facies and some sn w mineralization are locally present in the a type suites a post orogenic extensional period is marked in the south of the area by the emplacement of shallow a type granitoids in the serra do mar belt

evidence for lithospheric detachment in the central andes from local earthquake tomography abstract data from three temporary seismic networks were merged for tomographic inversion although the deployments did not coincide in time spatial overlap was achieved by re occupying existing sites travel times and t operators of about 1600 earthquakes were inverted for 3d models of p p s and p wave attenuation q p 1 all three attributes provide a consistent image of the entire subduction zone on a lithospheric scale the tomographic images reveal low velocities and high attenuation in the crust and mantle underlying the western cordillera and most of the puna plateau indicative of weak rheology and mostly asthenospheric mantle in contrast forearc and eastern foreland are characterized by high q p values corresponding to cold temperatures in accordance with thermal models in the backarc between 23 s and 24 s a high velocity high q p structure beneath the eastern cordillera and eastern puna is interpreted as detaching continental lithosphere that has been thickened in the orogenic process south of this structure the mantle is characterized by low velocities high p s ratios and low q p values here it is believed that lithosphere originally underlying andean crust has already been removed this is supported by new estimates of crustal thickness and volcanic activity

elemental and sr nd isotope geochemistry of two neoproterozoic mangerite suites in se brazil implications for the origin of the mangerite charnockite granite series abstract neoproterozoic 625 ma mangerites are the dominant rock type in a 100 km long belt of deformed tabular bodies in the socorro guaxupe nappe sgn a high grade terrane southwest of the sao francisco craton southeastern brazil the mangerites form two suites sao pedro de caldas shows a wide compositional variation and abundant felsic rocks mangerite hornblende granite leucogranite initial 87 sr 86 sr of 0 707 and initial e nd of 4 to 5 divinol ndia is dominated by more mafic mangerites with minor charnockite granite and diorite and has initial 87 sr 86 sr of 0 710 and initial e nd of 10 to 12 the probable sources of the sao pedro de caldas mangerites were granulites depleted by previous melting events in radiogenic elements and the light rare earth element primary melts of sao pedro de caldas were generated at 950 c p 10 kbar from a biotite free and k feldspar bearing source with plagioclase as an important residual phase the amount of melting was low as required by low mg enrichment in nb and ta relative to potential sources and high nb ta and was probably controlled by exhaustion of k feldspar in the source contribution from infracrustal mafic melts cannot be ruled out and would help explain the weakly negative e nd and high nb but is not essential the divinol ndia rocks depart from typical within plate mangerites towards more mafic rocks with orogenic signatures e g higher mg sr ca lower zr radiogenic isotope data require sources with greater crustal residence time than those envisioned for sao pedro de caldas elemental data require that these sources were more depleted e g higher k rb granulites with these characteristics are exposed in the northern portion of the sgn these were probably melted at slightly higher temperatures than the sao pedro de caldas suite 970 c before a sufficient amount of melt was generated and the magmas could leave their sources in this case k feldspar was exhausted and plagioclase in amounts exceeding those of the granite minimum was dissolved

initial hf isotope compositions in magmatic zircon from early proterozoic rocks from the gawler craton australia a test for zircon model ages abstract we evaluate the validity of age information obtained from two stage hf model ages in zircon multiple isotope investigations of minerals from the kiana granite of the archean sleafordian complex from the southern gawler craton australia reveal a complex geologic evolution of the suite rubidium sr mineral ages define a tectono thermal event at 1673 4 4 2 ma associated with the kimban orogeny id tims u pb ages from single zircons yield an upper concordia intercept at 2348 21 ma which marks the time of magmatic emplacement whole rock nd model ages yield crustal residence ages of 2 8 ga for each single zircon grain the lu hf isotope systematic was determined by solution mc icpms two stage hf model ages for the magmatic zircons vary by as much as 500 myrs depending on the choice of lu hf for the source of the host rock putting further emphasis on the need to constrain the source lu hf of a rock used for model age calculations an average lu hf for mafic crust was assumed for the source reservoir of the rocks to calculate its hf isotope evolution through time which yields zircon model ages of 3 5 ga that coincide with the initial growth episode postulated for the gondwana supercontinent it is demonstrated that two stage zircon model ages deduced from initial hf isotopes can provide important age information for the source of igneous rocks and consequently on important crustal formation events even when conventional geochronologic investigations fail e g due to thermo tectonic overprints or mixing

pre grenvillian evolution and grenvillian overprinting of the parautochthonous belt in key harbour ontario u pb and field constraints the parautochthonous belt in the region of key harbour ontario is composed of early proterozoic migmatitic para and orthogneiss and mid proterozoic granitoids which were reworked during the grenville orogeny grenvillian deformation is localized into anastomosing arrays of high strain shear zones enclosing elongate bands and lozenges of rock subjected to lower and near coaxial strain crosscutting relationships preserved in the low strain domains document two pre grenvillian plutonic and tectonometamorphic events which are bracketed in age by u pb zircon geochronology a 1694 ma leucogranite intrudes and provides a minimum age for high metamorphic grade gneisses formed during an earlier tectonometamorphic event d1 m1 the leucogranite was intruded by mafic dykes deformed and metamorphosed at uppermost amphibolite facies during d2 m2 before the emplacement of mid proterozoic granitoids at ca 1450 ma following the emplacement of gabbro dykes and pods at ca 1238 ma the area was overprinted by

docking of the central metasedimentary belt to laurentia in geon 12 evidence from the 1 17 1 16 ga chevreuil intrusive suite and host gneisses quebec the chevreuil intrusive suite 1 17 1 16 ga represents a chronological field marker of regional extent that intruded the central metasedimentary belt in the western grenville province of quebec after peak metamorphism style and site of magma emplacement and extent of deformation of chevreuil plutons and dykes permit unravelling of the early grenvillian evolution of the belt with respect to cratonal north america the suite comprises a series of vertically layered gabbro stocks and monzonite diorite gabbro sheet intrusions and a swarm of microdiorite dykes that cut across gneisses the dykes display systematic variations in extent of deformation across the belt we targeted u pb geochronology on gneisses within the identified strain windows they preserve the record of a ca 1 20 ga high pressure temperature p t metamorphic event the sheet intrusions define magmatic corridors all along and concordant with the western northern and eastern tectonic boundaries of the belt the concordant and elonga

vertical extrusion and horizontal channel flow of orogenic lower crust key exhumation mechanisms in large hot orogens a large database of structural geochronological and petrological data combined with a bouguer anomaly map is used to develop a two stage exhumation model of deep seated rocks in the eastern sector of the variscan belt an early sub vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks these events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18 20 kbar to depths equivalent to 8 10 kbar and for coeval burial of upper crustal rocks to depths equivalent to 8 9 kbar following the folding and vertical extrusion event sub horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening fabrics that record the early vertical extrusion originated between 350 and 340 ma during building of an orogenic root in response to se directed saxothuringian continental subduction fabrics that record the later sub horizontal exhumation event relate to an eastern promontory of the brunia continent indenting into the rheologically weaker rocks of the orogenic root indentation initiated thrusting or flow of the orogenic crust over the brunia continent in a north directed sub horizontal channel this sub horizontal flow operated between 330 and 325 ma and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re equilibration the erosion depth as well as the degree of reworking decreases from south to north pointing to an outflow of lower crustal material to the surface which was subsequently eroded and deposited in a foreland basin indentation by the brunia continental promontory was highly noncoaxial with respect to the se oriented saxothuringian continental subduction in the early visean suggesting a major switch of plate configuration during the middle to late visean

preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend gneiss complexes are the world s most horrendously complexrocks to understand see cover of this issue yet they form a largeamount of the continents albeit they are normally hidden fromview by sedimentary sequences the last half century has seenenormous advances in understanding these rocks which form notonly a large amount of the continental crust but also hold much of earth s oldest rock record the archaean disciplines medicine and health sciences social and behavioral sciences publication details nutman a p 2014 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend precambrian research 255 part 3 771 773 this journal article is available at research online http ro uow edu au smhpapers 2672 preface to the special issue of precambrian research on the understanding of gneiss complexes in honour of clark r l friend

crystallization conditions and controls on trace element residence in the main minerals from the pedra branca syenite brazil an electron microprobe and la icpms study abstract major and trace element microanalyses of the main minerals from the 610 ma pedra branca syenite southeast brazil allow inferences on intensive parameters of magmatic crystallization and on the partition of trace elements among these minerals with important implications for the petrogenetic evolution of the pluton two main syenite types make up the pluton a quartz free syenite with tabular alkali feldspar laminated silica saturated syenite lss with na rich augite phlogopite hematite magnetite titanite apatite and a quartz bearing syenite laminated silica oversaturated syenite lso with scarce corroded plagioclase plus diopside biotite hornblende ilmenite magnetite titanite apatite both types share a remarkable enrichment in incompatible elements as k ba sr p and lree apatite saturation temperatures of 1060 1090 c are the best estimates of liquidus whereas the pressure of emplacement based on al in hornblende barometry is estimated as 3 3 to 4 8 kbar although both units crystallized under oxidizing conditions oxygen fugacity was probably higher in lss as shown by higher mg of the mafic minerals and higher hematite contents in hem ilmss in contrast with the ca bearing alkali feldspar from lso which hosts most of the whole rock sr and pb virtually ca free alkali feldspar from lss hosts 50 of whole rock sr and 80 of pb the remainder of these elements being shared by apatite pyroxene and titanite this contrast reflects a strong crystal chemical control whereby a higher proportion of an element with similar ratio and charge ca2 enhances the residence of sr and pb in the m site of alkali feldspar the more alkaline character of the lss magma is inferred to have inhibited zircon saturation zr hf remained in solution until late in the crystallization and were mostly accommodated in the structure of ca na pyroxene and titanite which are one order of magnitude richer in these elements compared to the same minerals in lso where most of zr and hf are inferred to reside in zircon the ree th and u reside mostly in titanite and apatite d ree tit ap raises steadily from 1 to 6 from la to tb then remains constant up to lu in the lso sample these values are about half as much in the lss sample where lower contents of incompatible elements in titanite are attributed to its greater modal abundance and earlier crystallization

shrimp u pb zircon ages of granitoid rocks in the luliang complex implications for the accretion and evolution of the trans north china orogen abstract the luliang complex is situated in the central segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of supracrustal and granitoid rocks of which the latter can be subdivided into pre tectonic ttg gneisses including the yunzhongshan guojiazhuang and chijianling guandishan gneisses syn tectonic gneissic granites represented by the huijiazhuang gneissic granite and post tectonic granites including the luyashan charnockite luchaogou porphyritic granite and tangershang guandishan massive granite the pre tectonic ttg gneisses are mostly calc alkaline and considered to have formed in a magmatic arc environment shrimp u pb analyses reveal that the yunzhongshan gneisses were emplaced at 2499 9 ma representing the earliest arc related magmatic event in the luliang complex this was followed by the intrusion of the guojiazhuang gneisses at 2375 10 ma the most widespread arc related magmatic event in the region was the emplacement of the chijianling guandishan granitoid gneisses of which the tonalitic granodioritic and monzogranitic gneisses were emplaced at 2199 11 ma 2180 7 ma and 2173 7 ma respectively metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207 pb 206 pb age of 1872 7 ma consistent with the metamorphic age range of 1880 1820 ma defined by metamorphic zircons from various high grade rocks in the tnco thus the evolution of the luliang complex involved emplacement of the yunzhongshan granitoids at 2499 ma the guajiazhuang granitoids at 2375 ma and the chijianling guandishan granitoids at 2199 2173 ma with the final collision between the eastern and western blocks in this area occurring at 1872 7 ma the whole series of magmatic events lasting nearly 650 ma this suggests that the trans north china orogen represents a long lived magmatic arc the new shrimp zircon data combined with structural and petrological studies also reveal the existence of both syn and post collisional granites in the luliang complex the former is represented by the huijiazhuang gneissic granite that was emplaced at 1832 11 ma whereas the latter include the 1815 5 ma luyashan charnockite 1807 10 ma luchaogou porphyritic granite 1798 11 ma guandishan massive granite and 1790 14 ma tangershan massive granite

thickening the altiplano crust by gravity driven crustal channel flow 1 in the central andes crustal thickness is not well correlated to upper crustal shortening only little shortening is documented in the upper crust of the 60 65 km thick altiplano plateau whose thickening and uplift were delayed with respect to earlier and greater thickening in the adjacent western and eastern cordilleras because crustal thickness variations induce horizontal stress gradients and cause crustal flow a thickness dependent channel flow is modeled here and applied to the central andes in situ thickening is assumed for both cordilleras while the altiplano crustal thickening is generated by lateral flow from these overthickened adjacent domains a 8 1018 pa s viscosity channel is predicted for crustal thicknesses exceeding 45 50 km to match the estimated topographic evolution of the central andes thickening in the cordilleras was sufficient to generate a flow of 6 109 m3 per unit length toward the initially 30 35 km thick altiplano

complex 3670 3500 ma orogenic episodes superimposed on juvenile crust accreted between 3850 and 3690 ma itsaq gneiss complex southern west greenland abstract the itsaq gneiss complex of the nuuk region southern west greenland is dominated by 3850 3690 ma tonalites intruded into and intercalated with lesser amounts of different 3850 3700 ma supracrustal units published whole rock sr and nd isotopic studies demonstrate that the tonalites are juvenile crustal additions from a depleted mantle source from our field studies and shrimp u pb zircon dating we argue that this juvenile crustal accretion was spread over ca 170 m yr 3850 3690 ma following 3850 3690 ma juvenile crustal accretion events the evolution of the itsaq gneiss complex continued with numerous crustal reworking events between 3670 and 3500 ma examples of these reworking events are as follows 1 there is intrusion of several generations of geochemically diverse granites sensu stricto with subordinate gabbros and diorites 2 there are superimposed multiple episodes of amphibolite to granulite facies metamorphism illustrated with akilia island samples by detailed dating of

terrane accretion and upward extrusion of high pressure granulites in the neoproterozoic nappes of southeast brazil petrologic and structural constraints the high grade crystalline nappes exposed southeast of the sao francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of neoproterozoic deep continental crust the socorro guaxupe nappe above derived from an arc terrane and the tres pontas varginha nappe below metamorphism in the tres pontas varginha nappe is characterized by the exceptional preservation of kyanite granulites 700 750 c 15 kbar and followed by limited retrogression maximum temperatures around 900 950 c were reached toward the base of the overlying socorro guaxupe nappe during the intrusion of charnockitic mangeritic magmas lower pressure metamorphism accompanied by anatexis prevailed at shallower crustal levels our petrological results document an inverted thermal structure with isobaric heating of the top of the high pressure granulite nappe both granulite nappes were transported more than 200 km eastward above lower nappes involving reworked basement and passive margin units both metamorphosed to high pressure but lower temperature conditions significant thinning and cooling of the two granulite nappes may have occurred before their emplacement onto the lower nappes the proposed geodynamic scenario considers that continental subduction took place westward underneath neoproterozoic oceanic lithosphere the two granulite units crystallised at 45 km depths under distinct paleogeotherms within this subduction zone around 630 ma the kyanite granulites were rapidely exhumed through the mechanism of low angle forced extrusion whereas syncollisional collapse affected the soft anatectic middle crust of the overlying arc terrane the final emplacement of the thinned nappe pile onto the cold sao francisco craton and its platform cover with at most anchizonal to greenschist facies metamorphism occurred around 600 ma

plate tectonics and orogenic research after 25 years a tethyan perspective abstract orogeny the process by which the earth s prominent mountain ranges are constructed has been a central topic of interest in the earth sciences since at least the end of the 18th century the recognition that strains and displacements of very considerable magnitude occur along all of the three dimensions within an orogenic belt during its evolution has grown gradually during the last two centuries emphasis on primary vertical movements dominated the ideas on the nature of orogeny during the first half of the 19th century whereas compression and consequent uplift across mountain belts were believed to be the main cause for their origin during the subsequent one hundred years or so that were spent under the dominance of the fixist contraction theory mobilist tectonicians realised that continental drift in places also required motion along the trend of orogenic belts but this view did not gain general acceptance recognition of significant strike slip motion parallel or subparallel with mountain ranges evolved independently and mostly within the fixist camp by the 1960 s presence of important motions both along and across mountain belts had become common knowledge but no theoretical basis existed to account for them all

formation of diatexite migmatite and granite magma during anatexis of semi pelitic metasedimentary rocks an example from st malo france petrological and geochemical variations are used to investigate the introduction formation of granite magma from diatexite migmatites derived from partial melting experiments show that protoliths of pelite metasedimentary rocks of pelitic to greywacke composition at st and greywacke composition yield up to 40 vol malo france anatexis occurred at relatively low temperatures granite melt over the temperature range 650 900 c and pressures 800 c 4 7 kbar principally through muscovite typical of anatexis of the continental crust conrad et dehydration melting biotite remained stable and serves as a tracer al 1988 vielzeuf holloway 1988 patino douce for the solid fraction during melt segregation the degree of partial johnston 1991 vielzeuf montel 1994 patino douce melting calculated from modal mineralogy and reaction stoichiometry harris 1998 therefore to make a granite magma was 40 vol there is a continuous variation in texture the melt fraction must be segregated from a solid fraction mineralogy and chemical composition in the diatexite migmatites of much greater volume and then collected together mesocratic diatexite formed when metasedimentary rocks melted the type of melt residuum separation process depends sufficiently to undergo bulk flow or magma flow but did not on the melt fraction present experience significant melt residuum separation mesocratic diatexite melt residuum separation at low degrees of partial that underwent melt segregation during flow generated 1 memelting f and consequently low melt fraction mf lanocratic diatexites at the places where the melt fraction was has been documented from metatexite migmatites in removed leaving behind a biotite and plagioclase residuum enriched these rocks melt segregation produced centimetre scale in tio2 feot mgo cao sc ni cr v zr hf th u and leucosomes distributed in a host composed of layers that ree and 2 a complementary leucocratic diatexite enriched in have melted and others that have not e g henkes sio2 k2o and rb where the melt fraction accumulated leucocratic johannes 1981 gupta johannes 1982 dougan 1983 diatexite still contained 5 15 vol residual biotite mg number sawyer 1987 tait harley 1988 barbey et al 1990 40 44 and 10 20 vol residual plagioclase an22 anatectic brown 1994 leucosomes represent small volumes of granite magma developed from the leucodiatexite first by further felsic magma formed when melt was squeezed out of the melt residuum separation then through fractional crystallization deforming matrix mclellan 1988 sawyer 1991 1994 most biotite in the anatectic granite is magmatic mg number brown 1994 brown et al 1995 and collected in low18 22 pressure sinks such as shear bands and inter boudin

synconvergent ductile flow in variable strength continental crust numerical models with application to the western grenville orogen 1 we present results from numerical models for a convergent orogen with laterally variable lower crustal strength representing a simplified orogenic system in which a strong craton flanked by progressively weaker terranes collides with another continent with progressive convergence crustal thickening and thermal relaxation lower crust becomes decoupled from upper and middle crust forming a ductile orogenic infrastructure beneath a stronger superstructure collision with strong external crust results in uplift and expulsion of ductile nappes from the orogenic core creating allochthonous terranes overlying a lower crustal indentor the extent of transport and exhumation of lower crustal nappes over the indentor reflects the amount of convergence and the erosion rate the western grenville orogen displays across strike variations in age tectonic history and protolith association suggesting a systematic variation in precollision crustal strength the laurentian craton margin and accreted terranes were variably reworked at synorogenic depths of 25 35 km during the ottawan orogeny deformation propagated from younger monocyclic rocks in the southeast into older polycyclic rocks flanking the craton on the northwest a comparison between numerical model results and crustal scale cross sections from the georgian bay and montreal val d or transects shows close correspondence between crustal structure and model geometry this indicates that the models produce geologically realistic results and provides a context for interpreting the tectonic evolution of the western grenville orogen contrasts between the results of homogeneous channel flow models and the present ductile nappe models suggest that the effects of different styles of ductile flow can be distinguished in the geologic record

lower crustal melting and the role of open system processes in the genesis of syn orogenic quartz diorite granite leucogranite associations constraints from sr nd o isotopes from the bandombaai complex namibia abstract the bandombaai complex southern kaoko belt namibia consists of three main intrusive rock types including metaluminous hornblende and sphene bearing quartz diorites allanite bearing granodiorites and granites and peraluminous garnet and muscovite bearing leucogranites intrusion of the quartz diorites is constrained by a u pb zircon age of 540 3 ma quartz diorites granodiorites and granites display heterogeneous initial nd and o isotope compositions end 540 ma 6 3 to 19 8 18o 9 0 11 6 but rather low and uniform initial sr isotope compositions 87sr 86srinitial 0 70794 0 70982 two leucogranites and one aplite have higher initial 87sr 86sr ratios 0 70828 0 71559 but similar initial end 11 9 to 15 8 and oxygen isotope values 10 5 12 9 the geochemical and isotopic characteristics of the bandombaai complex are distinct from other granitoids of the kaoko belt and the central zone of the damara orogen our study suggests that the quartz diorites of the bandombaai complex are generated by melting of heterogeneous mafic lower crust based on a comparison with results from amphibolite dehydration melting experiments a lower crustal garnet and amphibole bearing metabasalt probably enriched in k2o is a likely source rock for the quartz diorites the granodiorites granites show low rb sr 1 and are most likely generated by biotite dehydration melting of heterogeneous felsic lower crust all segments of the lower crust underwent partial melting during the pan african orogeny at a time 540 ma when the middle crust of the central damara orogen also underwent high t medium p regional metamorphism and melting geochemical and isotope data from the bandombaai complex suggest that the pan african orogeny in this part of the orogen was not a major crust forming episode instead even the most primitive rock types of the region the quartz diorites represent recycled lower crustal material

monazite as a metamorphic chronometer south of the grenville front western quebec monazite was utilized as a chronometer to examine the effects of high grade metamorphism across the parautochthonous belt and allochthon boundary thrust of the grenville province in western quebec this study in addition to previous geo chronological studies indicates an archean component in the gneisses which is consistent with the presence of more than one set of peak metamorphic conditions single grain monazite analyses from metasedimentary gneisses from four locations within the parautochthonous belt yielded grenvillian u pb dates of 1000 5 to 1006 2 ma the location farthest to the northwest 45 km southeast of the grenville front included monazite with a distinct archean signature southeast of this point an archean signature was not detected in the monazite at 70 km southeast of the grenville front monazite yielded two discrete ages of 1005 2 and 1020 3 ma xenotime from one location indicated that the closure temperature of this mineral may be equivalent to that of monazite 725 25

syn collisional lower continental crust anatexis in the neoproterozoic socorro guaxupe nappe system southern brasilia orogen brazil constraints from zircon u pb dating sr nd hf signatures and whole rock geochemistry abstract the southern brasilia orogen occurs along the southernmost border of the sao francisco craton this orogen is structured as a pile of ediacaran syn metamorphic thicked skinned nappes that diachronically migrate toward the cratonic margin a magmatic arc environment is represented by the socorro guaxupe nappe sgn which is a thick segment of partially molten lower to middle continental crust the sgn comprises three main units a lower granulitic an intermediate diatexitic and an upper metatexitic high temperature metamorphic peak was reached at 625 ma when large volumes of high k diatexitic rocks were formed charnockitic and mangeritic magmatic rocks sao jose do rio pardo suite and in situ leucosomes of mafic composition are inferred to have formed under hornblende dehydration melting conditions i e t 850 c granitic diatexites pinhal type were also formed and zircon thermometry indicates t 850 c elemental and isotopic geochemistry indicate that these diatexites and in situ leucosomes derived from melting of lower to middle continental crust tims zircon u pb ages are presented for the lower and middle units u pb dating coupled to lu hf analysis la mc icp ms was performed on charno enderbitic in situ leucosomes from metatexitic rocks that also resulted from hornblende dehydration melting two different zircon typologies were recognized the first comprises bipyramidal prismatic grains with oscillatory zoning and high luminescence sometimes preserving low luminescence 670 ma cores the high luminescence grains show 19 concordant ages at 621 16 ma the second typology comprises isometric or soccer ball type grains these grains show sector zoning and a concordia age of 608 4 ma th u ratios from soccer ball grains vary from 1 253 to 2 107 whereas prismatic grains vary from 0 118 to 1 774 initial hf signatures between 13 and 21 in both typologies provide evidence of a reworked crustal reservoir the older prismatic grains may be related to the onset of metamorphic peak 625 ma whereas the isometric ones provide evidence of long lived high t conditions until 610 ma

crustal channel flows 1 numerical models with applications to the tectonics of the himalayan tibetan orogen 1 plane strain thermal mechanical numerical models are used to examine the development of midcrustal channel flows in large hot orogens in the models radioactive self heating reduces the viscosity of tectonically thickened crust and increases its susceptibility to large scale horizontal flow channels can be exhumed and exposed by denudation focused on the high relief transition between plateau and foreland we interpret the himalaya to have evolved in this manner channel flows are poorly developed if the channel has a ductile rheology based on wet quartz flow laws and well developed if there is an additional reduction in viscosity to 1019 pa s this reduction occurs from 700 c to 750 c in the models and is attributed to a small percentage of in situ partial melt melt weakening model ht1 provides an internally consistent explanation for the tectonic evolution of many features of the himalayan tibetan orogenic system erosional exhumation exposes the migmatitic channel equivalent to the greater himalayan sequence ghs between coeval normal and thrust sense ductile shear zones corresponding to the south tibetan detachment and the main central thrust systems outward flow of unstable upper crust rotates these shears to low dip angles in the model both the ghs and the lesser himalayan sequence are derived from indian crust with the latter from much farther south similar models exhibit a range of tectonic styles including the formation of domes resembling north himalayan gneiss domes model results are relatively insensitive to channel heterogeneities and to variations in the behavior of the mantle lithosphere beneath the model plateau

transect across the northwestern grenville orogen georgian bay ontario polystage convergence and extension in the lower orogenic crust the grenville orogenic cycle between 1190 and 980 ma involved accretion of magmatic arcs and or continental terranes to the laurentian craton a transect across the western central gneiss belt georgian bay ontario which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20 30 km offers some insights on the thermal and mechanical behavior of the lower crust during the development of the grenville orogen prior to grenvillian metamorphism this part of laurentia consisted largely of meso proterozoic 1450 ma granitoid orthogneisses granulites and subordinate mafic and supracrustal rocks grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed 1160 ma parry sound domain over the craton sometime between 1120 ma and 1080 ma this stage of transport was followed by out of sequence thrusting and further convergence along successively deeper foreland propagating ductile thrust zones a major episode of extension at 1020 ma resulted in southeast directed transport of allochthonous rocks along the midcrustal shawanaga shear zone the final stage of convergence involved deformation and metamorphism in the grenville front tectonic zone at 1000 980 ma peak metamorphism along most of the transect at 1065 1045 ma followed initial transport of allochthonous rocks over the craton by 15 35 m y regional cooling which postdated peak metamorphism by 70 m y was probably delayed by the combined effects of late stage extension and convergence transport of allochthons at least 100 km over the craton was accomplished along a weak migmatitic decollement further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement extensional deformation was associated with distributed ductile flow the formation of regional transverse folds with axes parallel to the stretching direction and reactivation of the allochthon parautochthon thrust boundary as an extensional decollement the extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the central gneiss belt

trans hudson orogen of north america and himalaya karakoram tibetan orogen of asia structural and thermal characteristics of the lower and upper plates 1 the trans hudson orogen tho of north america and the himalaya karakoram tibetan orogen hkto of asia preserve a paleoproterozoic and cenozoic record respectively of continent continent collision that is notably similar in scale duration and character in tho the tectonothermal evolution of the lower plate involves 1 early thin skinned thrusting and barrovian metamorphism 2 out of sequence thrusting and high t metamorphism and 3 fluid localized reequilibration anatexis and leucogranite formation the crustal evolution of the indian lower plate in hkto involves 1 early subduction of continental crust to ultrahigh pressure uhp eclogite depths 2 regional barrovian metamorphism and 3 widespread high t metamorphism anatexis and leucogranite formation the shallow depths of the high t metamorphism in hkto are consistent with early to mid miocene ductile flow of an indian lower plate midcrustal channel from beneath the southern tibetan plateau to the greater himalaya melt weakening of the lower plate in tho is not observed at a similar scale probably due to the paucity of pelitic lithologies tectonothermal events in the upper plate of both orogens include precollisional accretion of crustal blocks emplacement of andean type plutonic suites and high t metamorphism syncollisional to postcollisional events include emplacement of garnet biotite muscovite leucogranites anatectic granites and sporadic metamorphism up to 90 myr following the onset of collision in tho comparing the type and duration of tectonothermal events for tho and hkto supports the notion of tectonic uniformitarianism for at least the later half of dated earth history and highlights the complementary nature of the rock record in an older exhumed orogen compared to one undergoing present day orogenesis

shrimp u pb zircon geochronology of the huai an complex constraints on late archean to paleoproterozoic magmatic and metamorphic events in the trans north china orogen the huai9an complex is situated in the northern segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete archean eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of six distinct lithologic units the huai9an ttg gneisses the manjinggou high pressure mafic granulites the khondalite series the dongjiagou granitic gneiss the huai9an charnockite and the dapinggou k feldspar granite shrimp u pb geochronology combined with th and u data and cathodoluminescence cl imaging of zircons enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the tnco cl images reveal the coexistence of magmatic and metamorphic zircons in most lithologies of the huai9an complex of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating oscillatory zoned magmatic zircon cores shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic protoliths of the huai9an ttg gneisses were emplaced at 2515 20 ma 2499 19 ma and 2440 26 ma respectively much earlier than the emplacement of the dongjiagou granitic gneiss dated at 2036 16 ma however their metamorphic zircons yield similar concordant 207pb 206pb ages of 1847 17 ma 1842 10 ma and 1847 11 ma for the tonalitic trondhjemitic and granodioritic gneisses respectively and 1839 46 ma for the dongjia granitic gneiss these ages demonstrate that the huai9an complex underwent a regional metamorphic event at 1850 ma which is further supported by a mean 207pb 206pb age of 1848 19 ma for metamorphic zircons in the manjinggou high pressure mafic granulite and 1849 10 ma and 1850 17 ma for igneous zircons in the anatectic huai9an charnockite and dapinggou garnet bearing s type granite respectively the timing of late archean to paleoproterozoic magmatism and regional metamorphism in the huai9an complex is in general agreement with recent shrimp zircon data for other metamorphic complexes in the tnco these data prove that the high grade gneiss complexes were not the basement to the low grade granite greenstone terranes in the tnco furthermore the lithologies of the orogen are considered to have developed as a long lived magmatic arc that was subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western blocks at 1 85 ga leading to final assembly of the north china craton

tectonic setting and u pb zircon dating of the plutonic socorro complex in the transpressive rio paraiba do sul shear belt se brazil the precambrian rio paraiba do sul shear belt comprises a 200 km wide anastomosing network of ne sw trending ductile shear zones extending over 1000 km of the southeastern coast of brazil granulitic gneissic migmatitic and granitoid terrains as well as low to medium grade metavolcano sedimentary sequences are included within it these rocks were affected by strong contractional tangential tectonics due to west northwestward oblique convergence of continental blocks subsequent transpressional tectonics accomodated large dextral orogen parallel movements and shortening the plutonic socorro complex is one of many deformed granites with a foliation subparallel to that of the shear belt and exposes crosscutting relationships between its tectonic magmatic and metamorphic structures these relationships point to a continuous magmatic evolution related to regional thrusts and strike slip ductile shear zones the tectonic and magmatic structural features of the serra do lopo granite provide a model of emplacement by sheeting along shear zones during coeval strike slip and cross shortening of country rocks geochronological data indicate that the main igneous activity of socorro complex spanned at least 55 million years from the late stage of the northwestward ductile thrusting 650 ma through right lateral strike slip 595 ma deformation the country rocks yield discordant age data which reflect a strong imprint of the transamazonian tectono metamorphic event 1 9 to 2 0 ma we propose a model for the origin of calc alkaline granites of the ribeira belt by partial melting of the lower crust with small contributions of the lithospheric mantle during transpressional thickening of plate margins which were bounded by deep shear zones the transpressional regime also seems to have focused granite migration from deeper into higher crustal levels along these shear zones

crustal flow modes in large hot orogens abstract crustal scale channel flow numerical models support recent interpretations of himalayan tibetan tectonics proposing that gravitationally driven channel flows of low viscosity melt weakened middle crust can explain both outward growth of the tibetan plateau and ductile extrusion of the greater himalayan sequence we broaden the numerical model investigation to explore three flow modes homogeneous channel flow involving laterally homogeneous crust heterogeneous channel flow involving laterally heterogeneous lower crust that is expelled and incorporated into the mid crustal channel flow and the hot fold nappes style of flow in which mid lower crust is forcibly expelled outward over a lower crustal indentor to create fold nappes that are inserted into the mid crust the three flow modes are members of a continuum in which the homogeneous mode is driven by gravitational forces but requires very weak channel material the hot fold nappe mode is driven tectonically by for example collision with a strong crustal indentor and can occur in crust that is subcritical for homogeneous flows the heterogeneous mode combines tectonic and gravitationally driven flows preliminary results also demonstrate the existence and behaviour of mid crustal channels during advancing and retreating dynamical mantle lithosphere subduction an orogen temperature magnitude t m diagram is proposed and the positions of orogens in t m space that may exhibit the flow modes are described together with the characteristic positions of a range of other orogen types

migmatites formed by water fluxed partial melting of a leucogranodiorite protolith microstructures in the residual rocks and source of the fluid abstract the opatica subprovince in the canadian shield is a late archaean 2761 2702 ma plutonic arc formed above a north dipping subduction zone anatexis 2690 2677 ma of leucogranodiorite and leucotonalite orthogneisses in the opatica generated migmatites in an area of north vergent back thrusts visible at the surface and in l ithoprobe seismic profile 48 schollen diatexite migmatites occur in the thrusts and metatexite migmatites between them the modal mineralogy microstructure and whole rock major trace and oxygen isotope compositions of the protolith and migmatites were investigated to 1 determine the melting reaction 2 find microstructural criteria for identifying residual rocks in leucocratic systems where there is no melanosome and 3 to determine the source of the fluid involved in anatexis partial melting of the protolith did not change the mineral assemblage but the abundance of quartz and microcline both declined and plagioclase and biotite increased in the residual rocks quartz plagioclase and microcline show evidence for dissolution and biotite does not thus water fluxed melting of quartz plagioclase microcline occurred a mass balance indicates 25 30 partial melting the melting reaction consumed the microcline and created essentially monomineralic domains of plagioclase extraction of 80 90 of the melt left a thin film of melt on the grain boundaries and crystallization of these in the plagioclase domains created diagnostic microstructures microcline fills the last remaining pore space and forms high aspect ratio crystals between plagioclases or triangular crystals at grain junctions quartz shows a range of morphologies from high aspect ratio films through the string of beads to isolated rounded grains as the microstructure progressively equilibrated after crystallisation most accessory phases including zircon remained in the residuum however almost all the schollen migmatites have high contents of th u nb ta and ree relative to the protolith due to contamination by accessory phases derived from mafic rocks disaggregation of the mafic rocks may have been facilitated by the high strain in the back thrusts where the schollen diatexites formed average whole rock 18 o for the protolith and migmatites are similar ca 8 2 and the small difference between melt rich 8 6 and residuum rich rocks 8 0 is consistent with fractionation thus the fluid that caused melting was probably of metamorphic origin with 18 o similar to the protolith the seismic profile shows several reflectors extending to a present depth of 20 km ca 40 km in the late archaean under the migmatites these are the paths along which the metamorphic fluid migrated and generated the migmatites now at the surface a new type of neosome reported in this study may have formed along fractures that the fluids migrated along however these are peripheral pathways in the metatexites adjacent to the back thrusts and schollen diatexites

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

constraining timing and p t conditions of continental collision and late overprinting in the southern brasilia orogen se brazil u pb zircon ages and geothermobarometry of the andrelândia nappe system abstract the andrel ndia nappe system ans is an important component of the neoproterozoic southern brasilia orogen sbo in se brazil and its evolution is related to the convergence and subsequent collision between the passive margin of the sao francisco paleocontinent and paranapanema block during the brasiliano orogeny the ans is mainly composed of a pile of deformed meta sedimentary rocks that record an inverted metamorphic pattern ranging respectively from amphibolite facies at the bottom to high p granulite conditions at the top the ans is here simplified into the lower middle and upper nappes four samples from different tectonic settings within the ans were collected for zircon isotopic investigation in addition geothermobarometry modelling were performed on seven samples from the middle nappe retro eclogite slices associated to basement gneissic rocks occur along the contact between the upper and middle nappes and have zircon grains with metamorphic rims grown in high p conditions of 12 16 kbar with temperatures of 700 800 c during the continental subduction dated here by u pb shrimp in zircon at 625 6 ma a period of nappe emplacement accompanied by melting in the middle nappe is evidenced by a garnet muscovite staurolite kyanite schist with leucosome parallel to the main foliation dated by u pb shrimp in zircon at 618 5 ma re equilibrium at pressures of 4 7 kbar with temperatures of 500 700 c is ascribed to a second heat pulse under lower pressure conditions the period of melting associated to this event in the middle nappe occurred at 586 9 ma and is marked by an anatectic tourmaline garnet muscovite bearing s type granite this granite body is part of a ne sw belt of granites with ages between 605 and 563 ma and covering both the central ribeira orogen cro and the sbo the age and the alignment of this granite belt justify the interpretation that it is genetically related to the cro thus the new data provide evidence that tectonothermal events related to the collisional front of the cro extended up to 200 km overlapping the consolidated sbo at ca 580 ma

role of melt during deformation in the deep crust deformation in the deep crust is strongly influenced by the presence of melt injected melt or magma weakens the crust because strain will tend to localize where melt is present the amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate for plutons that intrude into rocks which are near the solidus temperature of the melt crystallization times can be quite long 1myr partial melting of deep crustal rocks can lead to melt enhanced embrittlement this occurs because the volume change for most melting reactions is positive therefore when the rate of melt production outpaces the rate at which melt can leave the system the melt pressure increases eventually the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the central gneiss belt canada this suggests that melt enhanced embrittlement occurred in these rocks an expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived assuming that migmatite has a small tensile strength differential stresses are 20 mpa in migmatitic rocks at the time melt enhanced embrittlement occurs the occurrence of melt enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting furthermore repeated episodes of melt enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains

thermal mechanical model for crustal thickening in the central andes driven by ablative subduction the thick up to 65 km crust of the high elevation regions of the andean orogenic belt results from structural thickening of the felsic upper crust of south america the great thickness implies shortening or loss of several hundred kilometers of lithospheric mantle and lower crust through numerical simulation of the thermal and mechanical processes of crustal thickening we test a model of lithospheric mantle removal by ablative subduction the model combines kinematic assumptions regarding mantle motion with dynamic calculations of crustal deformation predictions include the formation of an outwardly growing orogenic plateau analogous to the altiplano that results from high temperature low viscosity flow of the lower crust due to thickening of the high heat production upper crust

resorption growth solid state recrystallisation and annealing of granulite facies zircon a case study from the central erzgebirge bohemian massif zircon crystals have been investigated from a gneiss area in the central erzgebirge bohemian massif where small occurrences lenses of granulites and sometimes eclogites are located within amphibolite facies gneisses geological relationships indicate that leucocratic quartzofeldspathic rocks within the granulite boudins represent melts whereas garnet rich melanocratic rocks are considered restites derived through multiple extraction of the leucocratic melts the morphology of zircon crystals is quite different in these two types of granulites with rounding pointing to higher zircon resorption in garnet rich rocks due to multiple interaction with melts extensive new zircon growth can be demonstrated within granulitic boudins on the basis of zircon morphology internal zircon textures and age data the newly formed zircon grains and rims are non cathodoluminescent and yield an age of about 340 ma secondary ionisation mass spectrometry sims and zircon evaporation raman microprobe investigations revealed heterogeneous metamictisation within zircon grains and pronounced damage of the zircon structure in u rich non cathodoluminescent zircon rims the common reverse discordance of such non cathodoluminescent zircon rims may be explained as a result of preferential pb sputtering during sims analysis leading to apparent pb excess which seems to occur principally in the most damaged zircon domains it is proposed from field relationships that surrounding gneisses although equilibrated under amphibolite facies conditions experienced the same metamorphic pt history as the granulites these amphibolite facies rocks contain zircon distinct from the granulite zircon populations with morphologies and internal textures typical for their protolith rocks implying only slight zircon changes due to hp overprint new zircon growth is here mostly limited to the appearance of very rare new grains or more commonly to formation of thin overgrowths the internal texture of these zircon crystals shows clear signs of solid state recrystallisation leading in most cases to a partial resetting of their u pb ages mainly due to pb loss in addition u contents are slightly higher in cl dark rims that we interpret to have formed mainly due to solid state recrystallisation most evaporation and sims ages vary between protolith rock age and 340 ma so that the interpretation of the 340 ma age is not unambiguous it could be explained as the age of zircon crystallisation from a u rich and th depleted melt or alternatively as a resetting age due to solid state recrystallisation resulting from melt or fluid interaction with old zircon it is difficult to determine at which stage of metamorphic evolution new zircon growth occurred in the granulitic boudins and remains unclear that zircon growth occurred at peak metamorphic conditions

u pb geochronology from tonagh island east antarctica implications for the timing of ultra high temperature metamorphism of the napier complex abstract ion microprobe u pb zircon geochronology of an orthopyroxene bearing felsic orthogneiss from central tonagh island enderby land east antarctica provides insight into the chronological metamorphic evolution of the archaean napier complex the details of which have been the source of debate for over two decades the orthogneiss crystallised at 2626 28 ma predating peak ultra high temperature uht metamorphism and development of an intense regional s1 gneissosity two subsequent episodes of zircon growth resetting can be identified a minor period of zircon growth occurred at 2546 13 ma the regional significance and geological nature of which is unclear this was followed by an episode of abundant zircon growth as mantles on 2626 ma cores and as anhedral grains partly characterised by high th u 1 2 at 2450 2480 ma this age coincides with both lower and upper concordia intercept ages from other u pb zircon studies and several rb sr and sm nd whole rock isochron ages from the napier complex we conclude that uht metamorphism occurred at 2450 2480 ma and find no compelling evidence that uht occurred much earlier as has been postulated the zircon u pb data from this study also indicates a lower intercept age of 500 ma which coincides with the emplacement of early palaeozoic pegmatite swarms and synchronous infiltration of aqueous fluids into the southwestern regions of the napier complex

the neoproterozoic southern passive margin of the sao francisco craton insights on the pre amalgamation of west gondwana from u pb and hf nd isotopes abstract sedimentary basins are in many cases the only preserved record of the geological processes active through geological time these deposits preserve age isotopic and geochemical signatures of the source rocks providing insights into tectonic setting in this context we investigate the lithostratigraphy provenance and tectonic environment of the carrancas group a meta sedimentary sequence characterised by a basal psammitic unit lower unit passing up into interbedded psammo pelites intermediate unit which are overlain by a further psammitic unit upper unit this succession along with the canastra paranoa and vazante groups brasilia orogen comprises a mature passive margin developed at the western southern margin of the sao francisco craton during the neoproterozoic detrital zircon u pb ages and nd hf isotope data enable revised stratigraphic correlations across the basin highlighting variations in the source of detritus deposition began after 0 92 ga with the sedimentation of the lower unit with input of juvenile 2 1 ga detritus endt 0 1 to 3 2 derived from the sao francisco craton a marine transgression characterises the transition to the intermediate unit which gradually covered over juvenile 2 1 ga terrains and rendered them unavailable as possible source rocks for detrital zircons evolved archaean palaeoproterozoic basement rocks the espinhaco supergroup and evolved neoproterozoic granitoids all belonging to the sao francisco craton acted as sources for the intermediate and upper units u pb 2 7 ga to 0 9 ga endt 10 4 to 7 1 the goias massif might also have acted as a source providing juvenile mesoproterozoic detritus the sediments of the upper unit register a significant shift in source with incoming of juvenile mesoproterozoic and crustal 1 9 1 7 ga detritus this change in the ages of the detrital material may be related to exhumation of the espinhaco supergroup during the evolution of the passive continental margin basin the whole rock nd tdm model ages become progressively younger from the base to the top of the sequence nd tdm 2 6 1 8 ga the collisional events between amazonian and sao francisco cratons and the paranapanema block set the minimum age of deposition along this passive continental margin at around 0 67 ga

lithotectonic elements and geological events in the hengshan wutai fuping belt a synthesis and implications for the evolution of the trans north china orogen the hengshan wutai fuping belt is located in the middle segment of the trans north china orogen a palaeoproterozoic continental collisional belt along which the eastern and western blocks amalgamated to form the north china craton the belt consists of the medium to high grade hengshan and fuping gneiss complexes and the intervening low to medium grade wutai granite greenstone terrane and most igneous rocks in the belt are calc alkaline and have affinities to magmatic arcs previous tectonic models assumed that the hengshan and fuping gneiss assemblages were an older basement to the wutai supracrustal rocks but recent studies indicate that the three complexes constitute a single long lived neoarchaean to palaeoproterozoic magmatic arc where the wutai complex represents an upper crustal domain whereas the hengshan and fuping gneisses represent the lower crustal components forming the root of the arc the earliest arc related magmatism in the belt occurred at 2560 2520 ma marked by the emplacement of the wutai granitoids which was followed by arc volcanism at 2530 2515 ma forming the wutai greenstones extension driven by widespread arc volcanism led to the development of a back arc basin or a marginal sea which divided the belt into the hengshan wutai island arc japan type and the fuping relict arc at 2520 2480 ma subduction beneath the hengshan wutai island arc caused partial melting of the lower crust to form the hengshan tonalitic trondhjemitic granodioritic ttg suites whereas eastward directed subduction of the marginal sea led to the reactivation of the fuping relict arc where the fuping tonalitic trondhjemitic granodioritic suite was emplaced in the period 2360 2000 ma sporadic phases of isolated granitoid magmatism occurred in the hengshan wutai fuping region forming 2360 ma c 2250 ma and 2000 2100 ma granitoids in the hengshan complex the c 2100 ma wangjiahui and dawaliang granites in the wutai complex and the 2100 2000 ma nanying granitoids in the fuping complex atc 1920 ma the hengshan wutai island arc underwent an extensional event possibly due to the subduction of an oceanic ridge leading to the emplacement of pre tectonic gabbroic dykes that were subsequently metamorphosed together with their host rocks to form medium to high pressure granulites at 1880 1820 ma the hengshan wutai fuping arc system was juxtaposed intensely deformed and metamorphosed during a major and regionally extensive orogenic event the l uliang orogeny which generated the trans north china orogen through collision of the eastern and western blocks the hengshan wutai fuping belt was finally stabilized after emplacement of a mafic dyke swarm at 1780 1750 ma

episodic growth of the gondwana supercontinent from hafnium and oxygen isotopes in zircon it is thought that continental crust existed as early as 150 million years after planetary accretion but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records for example the striking global peaks in juvenile igneous activity 2 7 1 9 and 1 2 gyr ago imply rapid crustal generation in response to the emplacement of mantle super plumes rather than by the continuous process of subduction yet uncertainties persist over whether these age peaks are artefacts of selective preservation and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle these model ages are only meaningful if the host magma lacked a mixed or sedimentary source component but the latter can be diagnosed by oxygen isotopes which are strongly fractionated by rock hydrosphere interactions here we report the first study that integrates hafnium and oxygen isotopes all measured in situ on the same precisely dated detrital zircon grains the data reveal that crust generation in part of gondwana was limited to major pulses at 1 9 and 3 3 gyr ago and that the zircons crystallized during repeated reworking of crust formed at these times the implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts

temperatures of granulite facies metamorphism constraints from experimental phase equilibria and thermobarometry corrected for retrograde exchange this study assesses temperatures of formation of common granulites by combining experimental constraints on the p t stability of granulite facies mineral associations with a garnet orthopyroxene grt opx thermobarometry scheme based on al solubility in opx corrected for late fe mg exchange we applied this scheme to 414 granulites of mafic intermediate and aluminous bulk compositions our findings suggest that granulites are much hotter than traditionally assumed and that the p t conditions of the amphibolite granulite transition portrayed in current petrology textbooks are significant underestimates by over 100 c for aluminous and intermediate granulites mean corrected temperatures based on our method are 890 17 and 841 11 c respectively uncertainties reported as 95 confidence limits on the mean consistent with minimum temperatures for orthopyroxene production by fluid absent partial melting in these bulk compositions in contrast mean temperatures based on grt opx fe mg exchange equilibria using the same thermodynamic data are 732 22 and 723 11 c respectively well below the minimum temperatures for opx stability for mafic granulites the mean corrected temperature using our method is 816 12 c similar to the mean temperature of 793 13 c from fe mg exchange reasons for the differences between the mafic granulites and aluminous intermediate granulites are unclear but may be due to the lower al concentrations in opx in the mafic rocks and possible deficiencies in the thermodynamic modelling of these low concentrations we discuss a number of well known granulite terrains in the context of our findings including the adirondacks the acadian granulites of new england the incipient charnockites of southern india and sri lanka and the kerala khondalite belt our findings carry implications for thermotectonic models of granulite formation a computer program to perform our thermobarometry calculations rclc is available from the journal of petrology website at http www petrology oupjournals org or from the authors at http www geo ucalgary ca pattison drm pattison rclc htm

timing and duration of melting in the mid orogenic crust constraints from u pb shrimp data muskoka and shawanaga domains grenville province ontario the central gneiss belt in the grenville province ontario exposes metaplutonic rocks orthogneisses and minor paragneisses that were deformed and metamorphosed at crustal depths of 20 35 km during the mesoproterozoic grenvillian orogeny we present sensitive high resolution ion microprobe shrimp u pb zircon data from eight samples of migmatitic orthogneiss granite and pegmatite from the muskoka and shawanaga domains that constrain the age and duration of partial melting in the mid orogenic crust our results support earlier interpretations that the protoliths to these migmatitic orthogneisses formed at ca 1450 ma emplacement and crystallization of granite and pegmatite in the shawanaga domain took place at ca 1089 ma apparently coevally with deformation and high grade metamorphism leucosomes in the muskoka and shawanaga domains yield ages of 1067 and 1047 ma respectively interpreted as the ages of melt crystallization the geochronological data and field observations suggest that melt was pr

kinematical and rheological evolution of a crustal scale ductile thrust zone central metasedimentary belt grenville orogen ontario reply the central metasedimentary belt boundary thrust zone is a 10 km thick 200 km long stack of crystalline thrust sheets enclosed by an anastomosing network of ductile thrust zones formed at mid

an integrated microtextural and chemical approach to zircon geochronology refining the archaean history of the napier complex east antarctica integrated textural and chemical characterisation of zircon is used to refine the u pb geochronology of the archaean ultra high temperature napier complex east antarctica scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon garnet and orthopyroxene are integrated to place zircon growth in an assemblage context thereby providing tighter constraints on the timing of magmatic and metamorphic events data indicate that magmatism occurred in the central and northern napier complex at ca 2 990 ma a regional relatively low pressure metamorphic event occurred at ca 2 850 2 840 ma mineral ree data from garnet bearing orthogneiss indicate that ca 2 490 2 485 ma u pb zircon ages provide an absolute minimum age for the ultrahigh temperature uht foliation preserved in this rock internal zircon zoning relationships and estimated zircon garnet dree values from paragneiss suggest that an absolute minimum age of ultra high temperature metamorphism is ca 2 510 ma but that it is more likely to be older than ca 2 545 ma we suggest that the high proportion of published zircon u pb data with ages between ca 2 490 2 450 ma reflects late post peak zircon growth and does not date the timing of peak uht metamorphism

time of metamorphism beneath the central metasedimentary belt boundary thrust zone grenville orogen ontario accretion at 1080 ma new u pb zircon and titanite data from the muskoka domain grenville province ontario provide protolith and metamorphic ages for the southwestern central gneiss belt discordant analyses from a migmatitic orthogneiss and its leucosome form a linear array with an upper intercept age of 1457 6 ma and a lower intercept age of 1064 18 ma u pb analyses on metamorphic zircon from an amphibolite yield a set of concordant analyses with an average 207pb 206pb age of 1079 3 ma a weakly migmatitic granitoid rock and a transecting charnockitic vein in the immediate footwall of the central metasedimentary belt boundary thrust zone yielded a discordant array of analyses wth an upper intercept age of 1394 13 ma and a lower intercept age of 1066 8 ma the charnockitic vein yielded concordant zircon ages of 1077 2 ma the upper intercept ages are interpreted in terms of protolith crystallization and the concordant and lower intercept ages as grenviilian high grade metamorphism and associated anatexis we h

crustal volume balance during the india eurasia collision and altitude of the tibetan plateau a working hypothesis a working hypothesis is developed for the two stages of evolution of the tibetan plateau during the continental collision between india and eurasia the first phase was dominated by crustal shortening and thickening in what is now tibet while indochina was expelled southeastward away from eurasia at the end of this phase at 17 ma the estimated tibetan crustal thickness was 51 km the second phase has been dominated by thickening of tibet by the influx of crustal material from india with no associated crustal shortening the crustal thickening that is inferred to have occurred by this process is only feasible because leakage of crust from beneath the plateau is restricted by neighboring archean cratons that block lower crustal flow this leakage to the east and south from tibet to beneath yunnan is estimated as 20 30 of the influx at present but was much smaller at the start of this second phase of deformation it is suggested that at 11 ma the force exerted against the tibetan upper crust by this flowing material first exceeded the threshold that this brittle layer could resist and tibet began to extend eastward any satisfactory working hypothesis for the tibetan plateau must explain its dimensions its 17 ma ending of shortening and 11 ma start of extension and volcanism the eastward slip sense on its active normal and strike slip faults and the ratios of their displacements to present day slip rates which imply that their slip rates have gradually increased this working hypothesis accounts for these features and predicts a smooth and continuing increase in altitude

the mechanical and thermal structure of mercury s early lithosphere 1 insight into the mechanical and thermal structure of mercury s early lithosphere has been obtained from forward modeling of the largest lobate scarp known on the planet our modeling indicates the structure overlies a thrust fault that extends deep into mercury s lithosphere the best fitting fault parameters are a depth of faulting of 35 to 40 km a fault dip of 30 to 35 and a displacement of 2 km the discovery rupes thrust fault probably cut the entire elastic and seismogenic lithosphere when it formed 4 0 gyr ago on earth the maximum depth of faulting is thermally controlled assuming the limiting isotherm for mercury s crust is 300 to 600 c and it occurred at a depth of 40 km the corresponding heat flux at the time of faulting was 10 to 43 mw m 2 this is less than old terrestrial oceanic lithosphere but greater than the present heat flux on the moon

hot orogens tectonic switching and creation of continental crust many granulite terrains were too hot to have formed during continental collision rather along with many high grade metamorphic terrains that typify continental crust most formed in accretionary orogens during tectonic switching when prolonged lithospheric extension was interrupted by intermittent transient contraction based on modern and ancient examples tectonic switching occurs when slab retreat induces upper plate extension causing arc splitting formation of microcontinent slivers and backarc basins then intermittent arrival of buoyant oceanic plateaus induces transient flat subduction or slab flip and crustal thickening during slab retreat basaltic magmas produced from decompressed asthenosphere advect into the extending orogen causing granulite facies metamorphism and granite generation but subsequent thickening during flat subduction cools the region thickening is focused in the thermally softened backarc region and if sediment filled a hot short lived 10 m y narrow 50 100 km orogenic belt forms such thickening is often misleadingly ascribed to arc or microcontinent collision once slab retreat mode is reestablished lithospheric extension recommences and a new arc backarc system forms generally outboard arrival of another plateau will reverse the procedure and another short lived hot orogen will form within the orogenic system cycles of tectonic switching efficiently produce continental crust

shrimp u pb zircon geochronology of the huai an complex constraints on late archean to paleoproterozoic crustal accretion and collision of the trans north china orogen

40ar 39ar thermochronology along a western quebec transect of the grenville province canada ar ar thermochronology on 24 hornblendes 3 biotites 2 muscovites and 2 k feldspars collected along a 400 km long nw se geotraverse through the grenville province in western quebec is employed to provide time constraints on the intermediate and low temperature stages of cooling of part of the grenville orogen in the grenville front zone the c 1000 ma time of exhumation previously established from thermobarometric and isotopic studies is supported by the hornblende age data presented here from 60 km to 160 km se of the front reworked archaean migmatites of the parautochthonous reservoir dozois terrane rdt 1004 ma old metamorphic monazites contain hornblendes with 972 950 ma cooling ages assuming metamorphic geotherms between 25 and 30 c km 1 calculated cooling and unroofing rates are about 6 c ma 1 and 0 33 km ma 1 in the p t range 725 c 800 mpa and 450 c 400 mpa hornblendes from monocyclic rocks of the mont laurier and morin terranes mlt and mt monazite ages c 1165 ma give ages of about 1040 and 1010 ma respectively calculation of cooling unroofing rates from peak metamorphic conditions in this area is hampered by thermal perturbations associated with the still poorly dated grenville collision which took place approximately between 1060 and 1020 ma cooling ages of c 900 ma for muscovite and biotite and 860 810 ma for k feldspar show that cooling rates decreased to around 1 5 c ma 1 under retrograde greenschist facies conditions in the mlt on a time vs distance diagram the hornblende data define several distinct age ranges suggesting that each terrane had a characteristic thermal history thus cooling was diachronous and probably non homogeneous throughout this segment of the grenville orogen the time lag between the cooling history of the parautochthon 972 950 ma and the allochthons 1040 1010 ma is compatible with an earlier pre 1040 ma peak of metamorphism in the allochthons the reservoir cabonga allochthon was transported toward the nnw from its probable root zone in the mlt during the 1060 1020 ma grenvillian collision as a partially cooled slab the remobilization of the archaean parautochthon is attributed to this collision in the grenville front zone slightly older cooling ages and cooling rates initially faster than in the remaining part of the parautochthon are probably as a result of rapid tectonic exhumation shortly after collision the minor delay 20 30 ma in unroofing of the mt compared to the adjacent mlt is most likely related to post 1040 ma extensional displacement along the labelle shear zone in terranes like those described above where metamorphism is diachronous determination of cooling rates and the history of exhumation may be meaningless without a firm control on the regional structure however identification of contrasting cooling histories contributes to unravelling the independent movement of terranes

p t t evolution of orogenic belts and the causes of regional metamorphism barrow 1893 introduced three important ideas that furthered understanding of metamorphic processes i the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism ii the concept of progressive metamorphism and iii the concept of magmatic advection of heat as a possible cause of regional metamorphism this article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the p t t paths of metamorphic rocks and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum t and maximum p orogenic belts characterized by clockwise p t paths achieved maximum p before maximum t the metamorphic peak normally post dated early deformation within the belt and additional heating above the normal conductive flux has been related to the amount of overthickening by contrast orogenic belts characterized by counterclockwise p t paths achieved maximum t before maximum p the metamorphic peak normally pre dated or was synchronous with early deformation within the belt and additional heating above the normal conductive flux has been related to the emplacement of plutons techniques used to constrain portions of p t t paths include the use of mineral inclusion suites in porphyroblasts and reaction textures thermobarometry the use of fluid inclusions thermodynamic approaches such as the gibbs method radiogenic isotope dating fission track studies and numerical modelling we can utilize specific mineral parageneses in suitable rocks to determine individual p t t paths and a set of p t t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism recent advances are reviewed with reference to collision metamorphism high temperature low pressure metamorphism granulite metamorphism and subduction zone metamorphism and some important directions for future work are indicated

titanite petrochronology of the southern brasilia orogen basement effects of retrograde net transfer reactions on titanite trace element compositions abstract combining u pb geochronology and trace element characteristics of accessory minerals more reactive during metamorphism and deformation than zircon such as titanite catisio5 is a powerful tool for elucidating pressure temperature time p t t evolution of orogenic basement complexes this study reports in situ titanite u pb and trace element data from upper amphibolite facies metatexitic orthogneisses of the pouso alegre complex in the basement of the neoproterozoic southern brasilia orogen se brazil the titanite data is combined with thermodynamic modeling major phase compositions and u pb zircon dating and used to constrain the p t t path of the complex during the neoproterozoic orogenesis the investigated garnet amphibole gneisses record peak metamorphic conditions of 750 775 c and 10 5 12 5 kbar associated with partial melting and dated at ca 630 620 ma based on u pb analyses of hree y depleted titanite cores inferred to have crystallized in the presence of garnet near the peak p t conditions extensive melt crystallization at ca 616 ma is constrained by u pb dating of leucosome zircons the exhumation of the complex and re equilibration to ca 550 625 c at 6 9 kbar led to widespread generation of biotite epidote clinozoisite partially replacing amphibole garnet along the main foliation trace element signatures of these retrograde net transfer reactions are recorded in the titanite rims which are hree y enriched strontium depleted and were dated at 605 7 6 2 ma the average zirconium contents of the titanite rims are higher than cores suggesting disequilibrium partitioning of zirconium into titanite during the retrograde path resulting in overestimated zr in titanite temperatures for the rims the exhumation of the complex was concomitant with the intrusion of folded and recrystallized leucogranite dykes dated at 607 0 5 7 ma attesting the persistence of deformation after their intrusion assuming that the pouso alegre complex is part of the sao francisco paleocontinent reworked during the brasilia orogeny the data constrain the minimum age for the collision between the sao francisco paleocontinent and the paranapanema block at ca 620 630 ma and agree with the idea of long lived metamorphism and deformation in the southern brasilia orogen