enriched grenvillian lithospheric mantle as a consequence of long lived subduction beneath laurentia geochemical and nd isotopic data from mafic and newly discovered ultramafic rocks in the adirondack lowlands suggest widespread enrichment of the lithospheric mantle under the grenville province incompatible element abundances and previously published hf t dm zircon depleted mantle model age and nd t dm ages from rocks of the anorthosite mangerite charnockite granite suite in the adirondack highlands document similar enrichment in the lower crust and its strong influence on subsequent magmatic events throughout the ontario quebec adirondack segment of the grenville province likely the consequence of long lived ca 1 4 1 2 ga northwest directed subduction along the southeast edge of laurentia previously proposed andean margin this enrichment is similar to that associated with the vast 240 000 km 2 ultrapotassic province of the western churchill province enrichment of the lithospheric mantle beneath orogenic belts is a predictable and important differentiation process that has operated on earth for at least the past 3 b y

lu hf and pbsl geochronology of apatites from proterozoic terranes a first look at lu hf isotopic closure in metamorphic apatite abstract the mineral apatite is characterized by elevated and highly variable lu hf ratios that in some cases allow for single crystal dating by the lu hf isotopic system apatites from the adirondack lowlands and otter lake area in the grenville province and from the black hills south dakota yield lu hf ages that are consistently older than their respective pb step leaching ages isotopic closure for the lu hf system therefore occurs before u pb system closure in this mineral in the adirondack lowlands where h2o activity was low lu hf systematics of cm sized apatite crystals remained undisturbed during upper amphibolite facies metamorphism 700 to 675 c at 1170 1130 ma the relatively old lu hf ages of 1270 and 1230 ma observed for these apatites correlate with decreasing crystal size in contrast apatite from the fluid rich otter lake area and black hills yields unrealistically low apparent lu hf closure temperatures implying that in these apatites fluids facilitated late exchange the lu hf ages for the metamorphic apatites were thus controlled either by the prevailing temperature and grain size or by fluid activity

trondhjemitic 1 35 1 31 ga gneisses of the mount holly complex of vermont evidence for an elzevirian event in the grenville basement of the united states appalachians a newly recognized suite of trondhjemite tonalite and dacitic gneiss forms a 10 km wide belt of rocks within the mount holly complex in the central part of the green mountain massif of vermont field relationships and chemistry indicate that these gneisses are calc alkaline volcanic and hypabyssal plutonic rocks older than the middle proterozoic regional deformation that affected the mount holly complex u pb zircon dates indicate ages as great as 1 35 ga for crystallization of the volcanic protoliths and for intrusion of crosscutting trondhjemite tonalitic plutonism continued until 1 31 ga map scale contacts between the trondhjemitic tonalitic dacitic gneisses and the paragneiss sequence of the mount holly complex are sharp suggesting that the volcanic rocks of the trondhjemite tonalite suite underlie the paragneiss units and do not intrude them these relationships suggest that the trondhjemite tonalite suite is either considerably older than and unconformable beneath the paragneiss cover rocks or

precise zircon geochronology in the adirondack lowlands and implications for revising plate tectonic models of the central metasedimentary belt and adirondack mountains grenville province ontario and new york new high precision single grain dating of leucogranites from the adirondack lowlands dated previously by multigrain zircon methods at ca 1416 ma wellesley island and ca 1285 1230 ma hyde sch

age field and petrological relationships of the hyde school gneiss adirondack lowlands new york criteria for an intrusive igneous origin alaskitic and tonalitic rocks constituting hyde school gneiss hsg occur in 14 domical bodies in the adirondack lowlands recent models have interpreted these bodies as metamorphosed rhyolitic and dacitic ash flow deposits forming the basal member of a regional stratigraphic package in contrast this paper presents criteria and evidence for an intrusive origin for hsg field evidence includes intrusion breccias and complex crosscutting relationships involving mafic layers resembling synplutonic dikes petrological constraints supporting an intrusive origin include 1 the common occurrence of quartz mesoperthite hypersolvus assemblages 2 magmatic features in tonalitic trondhjemitic and alaskitic facies 3 local occurrences of orthopyroxene in all facies of hsg and 4 the development of marginal garnet sillimanite gneiss with corundum spinel garnet sillimanite assemblages yielding paleotempera tures of 780 810 c and interpreted as restite remaining after anatexis of country rock metapelite by

alkali metasomatism in the major gneiss northwest adirondacks new york open system or closed the thickest clastic unit in the northwest adirondacks new york undergoes gradual transition from amphibolite to granulite facies over a distance of 55 km from emeryville to colton this precambrian quartz biotite oligoclase k feldspar gneiss with migmatite characteristics has long been cited as having undergone progressive loss of k si fe2 ba and h2o toward colton and granulite facies conditions engel and engel 1958 1960a b chemical data obtained from new samples however cast doubt upon the interpretation of open system alkali metasomatism outcrop to outcrop chemical variation is shown to be of similar magnitude to the chemical variation recorded across the entire gneiss belt chemical differences between emeryville and colton are not those expected from progressive degranitization by loss of alkali bearing fluid ratios of krb and ca na k suggest that metamorphism was isochemical on the scale of outcrop these new data were obtained from mixtures of leucosome and fine grained paragneiss at each of 7 outcrops the sampling technique contrasts with that of a e j and c engel who analyzed leucosome separately from fine grained least altered gneiss

adirondack highlands northwest lowlands boundary a multiply folded intrusive contact with fold associated mylonitization the boundary between the adirondack highlands and the northwest lowlands near harrisville new york is a multiply folded intrusive contact between metamorphosed sedimentary rocks of the lowlands and metamorphosed intrusive rocks of the diana complex in the highlands this is shown by detailed geologic mapping and analysis of stratigraphy and minor structural features regionally developed protomylonitic foliation in the diana complex is an axial plane foliation formed during isoclinal folding of dry quartz syenite at granulite facies metamorphic grade lack of major post intrusive fault displacement along the diana complex northwest lowlands boundary is demonstrated by correlation of xenoliths in the diana complex with immediately adjacent metamorphosed sedimentary rock units of the northwest lowlands and by continuity of major folds across the boundary a stratigraphic succession consisting of seven rock units has been recognized in northwest lowlands rocks of the harrisville area rocks in these units include from oldest to youngest 1 garnet sillimanite cordierite gneiss 2 rusty pelitic gneiss with biotite gneiss pyroxene amphibolite and quartzite 3 graphitic calcite marble 4 siliceous marble 5 rusty calc silicate rocks 6 diopside gneiss and 7 calc silicate granulite four phases of folding are evident in these stratigraphic units but only the last three phases affect intrusive quartz syenite gneisses of the diana complex first phase isoclinal folds in bedding have a pervasive axial plane foliation which is the regional foliation associated regional lineations are parallel to first phase fold axes foliation and lineation formed during the first phase of folding were deformed by second phase isoclinal folds which have a weak to strong axial plane foliation or slip cleavage quartz syenite of the diana complex intruded folded metamorphosed sedimentary rocks early during second phase folding and was itself isoclinally folded somewhat later during the second phase isoclinal folding of quartz syenite gneiss is associated with development of protomylonitic to mylonitic axial plane foliation which is most intense in the northwest border zone of the diana complex due to lithic contrast with rocks northwest of the border third phase folds are isoclinal to open and are locally characterized by axial plane slip cleavage or fracture cleavage the axial surfaces of folds associated with the first three phases of deformation have a general northeasterly trend and northwesterly dip but their axes have varied plunges two dimensional heart and anchor and hook patterns are a product of interference of second and third phase folds fourth phase deformation resulted in northwest trending open folds with locally developed axial plane slip or fracture cleavage interference of third and fourth phase folds is manifest in dome and basin patterns that are elongate to the northeast and overturned to the southeast similar to dome and basin patterns throughout the northwest lowlands late stage ductile shearing produced very fine grained thin mylonite and ultramylonite in shear zones that transect the preexisting protomylonite foliation in quartz syenite gneiss later brittle deformation resulted in closely spaced fractures quartz and epidote veins and local minor faults however no evidence for major faulting has been observed along this part of the adirondack highlands northwest lowlands boundary

age and provenance of grenville supergroup rocks trans adirondack basin constrained by detrital zircons detrital zircons from quartzites constrain the timing of deposition provenance and basin evolution of the grenville supergroup in the adirondack lowlands three samples from the stratigraphic succession yield maximum depositional ages of 1284 16 ma to 1257 6 16 ma shifts in provenance from rifting unimodal volcanic carbonate platform laurentian interior deep water clastics southern adirondack arc to final basin fill laurentian interior are documented the trans adirondack backarc basin is one of several that opened at ca 1 30 ga filled with thick carbonate dominated sequences and closed during the elzevirian orogeny ca 1245 1220 ma terminating widespread extension across the leading edge of southeast laurentia

constraints on deposition in the trans adirondack basin northern new york composition and origin of the popple hill gneiss abstract the popple hill gneiss phg records the transition from back arc to foreland basin sedimentation in the trans adirondack back arc basin between ca 1220 1245 ma composed of upper amphibolite facies gneisses with the assemblage biotite plagioclase potassium feldspar quartz garnet sillimanite and occasionally abundant k spar megacrysts and or leucosomes the phg occurs stratigraphically between the upper and lower marbles which were deposited in shallow and sometimes evaporitic marine waters best exposed and most intact in the adirondack lowlands correlative granulite facies rocks often mylonitic and extensively migmatized occur in the highlands in both areas the phg is intruded by ca 1170 1200 ma calc alkaline granitic rocks and those of the slightly younger amcg suite ca 1155 ma the phg is geochemically similar to ucc estimates has a calc alkaline trend and incompatible element patterns related to arc magmatism variable but low cia indices are indicative of a slightly weathered and or rapidly exhumed source the phg contains sand and silt sized zircon xenocrysts ranging in age from ca 1220 to 1400 ma lacks an archean population has rims related to shawinigan 1160 1180 ma and ottawan ca 1050 ma anatexis eastern adirondack highlands only and yields monazite populations of several ages including some in excess of 1220 ma these data are consistent with a rapidly exhumed felsic arc source for much of the unit likely derived from ca 1250 1350 ma rocks of the southern adirondacks the metasedimentary sequence in the adirondack lowlands records rifting and carbonate platform deposition associated with the opening of the trans adirondack back arc basin lower marble evolution to and rapid fill of a foreland basin flysch sequence phg and cyclic carbonate evaporite deposition upper marble as the basin filled and convergence continued just prior to collisional tectonism associated with the shawinigan orogeny ca 1160 1200 ma

geologic transect across the grenville orogen of ontario and new york revised cross sections of the western grenville province incorporate new geologic results and reprocessed seismic reflection data the geology is presented in terms of three tectonic elements 1 pre grenvillian laurentia and its margin with ca 1740 and 1450 ma continental arc plutons and associated supracrustal rocks 2 composite arc belt of allochthonous 1300 1250 ma volcanic arcs and sedimentary rocks and 3 frontenac adirondack belt characterized by supracrustal and granitoid rocks and anorthosites of uncertain affinity that may represent a distinctive part of the composite arc belt or an offshore micro continent rocks of the composite arc and frontenac adirondack belts were amalgamated with each other by ca 1160 ma were then thrust over laurentia during ca 1080 1035 ma and ca 1010 980 ma phases of convergence and were dissected and exhumed by 1040 ma normal faults penetrative deformation was restricted to that part of the pre grenvillian laurentian margin that lies to the southeast of the grenville front and parts of the accreted composite arc and frontenac adirondack belts the laurentian rocks in the grenville province are bounded to the northwest and southeast by southeast dipping ductile thrust and or normal shear zones the composite arc and frontenac adirondack belts to the southeast are bounded by ductile and brittle ductile thrust and or normal faults that separate domains with contrasting cooling histories despite a long pre grenvillian tectonic and plutonic history the present crustal architecture and much of the seismic reflectivity were acquired during 1080 980 ma phases of compression and extension

timing of anatexis in the eastern adirondack highlands implications for tectonic evolution during ca 1050 ma ottawan orogenesis u pb sensitive high resolution ion microprobe shrimp analyses of zircons from migmatitic metapelites in the eastern adirondack highlands demonstrate that substantial anatexis took place ca 1050 ma during the late but still high grade ca 1090 1050 ma ottawan orogeny these results contrast with data from metapelites of the southwestern adirondack highlands and adirondack lowlands which indicate that anatexis occurred ca 1200 1160 ma during the shawinigan orogeny and subsequent ca 1155 ma anorthosite mangerite charnockite granite amcg magmatism ca 1180 1150 ma zircons from this western regime do not contain ca 1050 ma ottawan metamorphic overgrowths the absence of ca 1050 ma ottawan anatexis and overgrowths in the adirondack lowlands is explained by the observation that consistent with titanite cooling ages post 1150 ma temperatures did not exceed 700 c in the southwestern adirondack highlands the absence of ca 1050 ma anatexis can be accounted for by earlier dehydration of metapelites during ca 1180 1150 ma shawinigan amcg metamorphism the occurrence of ca 1050 ma anatexis and formation of metamorphic zircons in the eastern adirondacks cannot be explained by higher temperatures because geothermometry indicates that all of the adirondack highlands reached a nearly uniform 750 800 c during this period accordingly the occurrence of ca 1050 ma ottawan anatexis in the eastern regime is interpreted to be the result of 1 infl uxes of hydrous fl uids at ca 1050 ma or 2 decompression melting during late extensional exhumation a recently recognized low angle late ottawan ca 1050 ma fault system may have provided both fl uid conduits and footwall decompression the sense of displacement along the shear zone has not yet been unequivocally determined but preliminary investigations of kinematic indicators demonstrate normal displacement thus this structure may be an analogue of the ca 1050 ma northwest dipping carthage colton zone normal fault system that juxtaposed the adirondack lowlands against the adirondack highlands in this context these fault zones form a quasi symmetrical core complex or gneiss dome within which the high grade core of the adirondack highlands was tectonically exhumed a similar east dipping along strike normal fault in quebec tawachiche shear zone exhumed the eastern morin and lac taureau terranes at ca 1050 ma and may merge with the eastern adirondack shear zone described here

detrital zircon constraints on grenville sedimentation at the margin of laurentia abstract the southern grenville province is made up of metaigneous and metasedimentary rocks that were formed and metamorphosed as part of the mesoproterozoic active margin of laurentia grenville supergroup quartzites from the adirondack highlands new york the morin terrane quebec and new jersey highlands were analyzed to determine detrital and metamorphic zircon u pb ages and to help constrain mesoproterozoic tectonics these grenville quartzites have broadly similar detrital age populations as each other suggesting similar sedimentary source regions and depositional settings in back arc basin environments at the laurentian margin during geon 12 all samples contain detrital zircon with 1 45 1 25 ga ages consistent with proximal grenville source regions they also contain detrital ages that point to derivation from the mazatzal 1 7 1 6 ga yavapai 1 8 1 7 ga and penokean 1 9 1 8ga orogens of the north american midcontinent and have a diverse population of archean ages these results and published data from adirondack lowlands metasediments chiarenzelli et al 2015 2017 show distinctly laurentian sources and the related and likely connected nature of the sedimentary histories of the adirondack highlands and lowlands morin terrane and new jersey highlands metamorphic zircon formation is common in high grade quartzites and ages are variable between samples depending on the metamorphic history of each terrane and the melt productivity of different rocks peck et al 2010a adirondack highlands samples experienced metamorphic zircon grown during the shawinigan orogeny and anorthosite suite emplacement 1 19 1 14 ga morin terrane samples are from the morin shear zone which was active and caused metamorphic zircon growth during the ottawan phase of the grenvillian orogeny 1 09 1 02 ga the new jersey highlands sample has metamorphic zircon formed during the rigolet phase of the grenvillian orogeny 1010 980 ma

oxygen isotope constraints on terrane boundaries and origin of 1 18 1 13 ga granitoids in the southern grenville province granitic rocks related to 1 18 to 1 13 ga anorthosite mangerite charnockitegranite plutonism stitch three terranes in the southwestern grenville province adirondack highlands morin terrane frontenac terrane elzevir terrane because of the refractory nature of zircon zrn analysis of oxygen isotope ratios of dated igneous zircon from these rocks allows calculation of 18 o values of original magmas even if the rocks were subjected to late magmatic assimilation postmagmatic alteration or metamorphism documented variability in 18 o zrn for these granitic rocks corresponds to their geographic location seven plutons from the central frontenac terrane ontario have a high average 18 o zrn 11 8 1 0 which corresponds to 18 o magma values of 12 4 14 3 in contrast twenty seven other plutons and dikes of this suite new york ontario and quebec average 18 o zrn 8 2 0 6 with a typical igneous range of 8 6 to 10 3 for 18 o magma values high 18 o values in the frontenac terrane are some of the highest magmatic oxygen isotope ratios recognized worldwide but these plutons are not unusual with respect to whole rock chemistry or radiogenic isotope compositions such high 18 o values can result from mixing between paragneiss 18 o 15 and hydrothermally altered basalts and or oceanic sediments 18 o 12 in the source region we propose that high 18 o hydrothermally altered basalts and sediments were subducted or underthrust to the base of the frontenac terrane during closure of an ocean basin between the frontenac terrane and the adirondack highlands at or prior to 1 2 ga

the link between western boundary currents and equatorial undercurrents abstract a constant potential vorticity model is used to investigate the relation between the properties of an equatorward directed western boundary current and the formation of the equatorial undercurrent euc in the western equatorial oceans it is shown that the value of the bernoulli function at the equator undetermined in earlier purely inertial euc models is fixed by the bifurcation latitude of the western boundary current that feeds the euc this is an additional example of the nonlocal forcing of the euc by the subtropical gyre the constant potential vorticity model yields reasonable values for the initial euc core velocities for a bifurcation latitude on the order of 6 from the equator

the central metasedimentary belt grenville province as a failed back arc rift zone nd isotope evidence abstract nd isotope data are presented for granitoid orthogneisses from the central metasedimentary belt cmb of the grenville province in order to map the extent of juvenile grenvillian age crust within this orogenic belt that is composed mostly of older crustal terranes the data reveal a 150 km wide belt of juvenile crust in ontario but this belt contains a block of pre grenvillian crust containing the elzevir pluton which yields an estimated crustal formation age of 1 5 ga the recognition of an older block within the cmb has profound implications for its structure and tectonic evolution because it implies that juvenile grenvillian crust apparently forming a wide ne sw belt is in fact distributed in two narrower segments with approximately n s strike we suggest that the cmb comprises an en echelon series of ensimatic rift segments created by back arc spreading behind a continental margin arc these rift segments extend southwards in the subsurface into the northeastern unites states the rift segments contain abundant marble outcrops consistent with marine incursion into the rift zone and these deposits also continue northwards into a marble domain of the cmb in quebec however crustal formation ages in the latter domain are largely pre grenvillian implying that the quebec rift segment was ensialic hence we interpret the cmb in ontario and quebec as the northern termination of a failed back arc rift zone

popple hill gneiss as dacite volcanics a geochemical study of mesosome and leucosome northwest adirondacks new york new chemical data from a widespread precambrian stratigraphic unit the popple hill gneiss in the northwest adirondacks have meaning for depositional setting and correlation of grenville metasediments average chemistry of the predominant rock type is appropriate for dacite volcanics rather than the previously proposed elastic sediments particularly unsuitable for shale are the high total alkali na 2 o and sio 2 content slightly peraluminous composition and low index of chemical alteration this gneissic unit and leucogneisses of probable ash flow tuff origin lower in the section hyde school gneiss are interpreted as massive volcanic outpourings within a platformal type sedimentary sequence a recently proposed caldera complex in anorogenic or rift related environments is an appropriate setting but vulcanism interrupted as well as preceded carbonate deposition popple hill and hyde school gneisses belong to a common depositional sequence with associated metasedimentary units and should be placed in the same stratigraphic group popple hill leucosomes are variable in shape and texture and most likely in age and mode of origin most are foliated and show grain size reduction others cut each other and are isoclinally folded physical appearance seems unrelated to chemical composition confmement of leucosomes to popple hill gneiss and chemical correlations with host mesosome suggest in situ origin and equilibration with mesosome many are enriched over mesosomes in k 2 o sr ba and rb have low total ree and zr and have positive eu anomalies restite is not readily observed such leucosomes could have formed throughout various phases of folding given the dry state of metamorphism as proposed elsewhere for the gneiss metamorphic differentiation is a likely mode of origin other leucosomes with negative eu anomalies and ree content similar to those of mesosome may be partial melts that formed during early grenvillian metamorphism

genesis of metapelitic migmatites in the adirondack mountains new york oxygen isotope ratios and rare earth element ree concentrations provide independent tests of competing models of injection v anatexis for the origin of migmatites from amphibolite and granulite facies metasedimentary rocks of the adirondack mountains new york values of 18o and ree profiles were measured by ion microprobe in garnet zircon pairs from 10 sample localities prior u pb sims dating of zircon grains indicates that inherited cores 1 7 1 2 ga are surrounded by overgrowths crystallized during the grenville orogenic cycle 1 2 1 0 ga cathodoluminescence imaging records three populations of zircon i featureless rounded whole grains interpreted as metamorphic or anatectic and rhythmically zoned igneous cores truncated by rims that are either ii discordant rhythmically zoned igneous or iii unzoned metamorphic or anatectic these textural interpretations are supported by geochronology and oxygen isotope analysis in both the amphibolite facies nw adirondacks and the granulite facies se adirondacks 18o zrc values in overgrowths and whole zircon are highly variable for metamorphic zircon 6 1 13 4 n 95 10 m spot in contrast garnet is typically unzoned and 18o grt values are constant at each locality differing only between leucosomes and corresponding melanosomes none of the analysed metamorphic zircon garnet pairs attained oxygen isotope equilibrium indicating that zircon rims and garnet are not coeval furthermore ree profiles from zircon rims indicate zircon growth in all regions was prior to significant garnet growth thus petrological estimates from garnet equilibria e g p t cannot be associated uncritically with ages determined from zircon the unusually high 18o values 10 in zircon overgrowths from leucocratic layers are distinctly different from associated metaigneous rocks 18o zrc 10 indicating that these leucosomes are not injected magmas derived from known igneous rocks surrounding melanosomes have similarly high 18o zrc values suggesting that leucosomes are related to surrounding melanosomes and that these migmatites formed by anatexis of high 18o metasedimentary rocks

nature and timing of anatexis in the eastern and southern adirondack highlands metapelitic rocks in the eastern and southern adirondack highlands contain two feldspar quartz leuco somes that pre date most deformational fabric as well as granulite facies recrystallization whole rock xrf analyses of these leucosomes yield cipw norms which correspond closely with experimental melts in the system ab or qt an h 2 o thus indicating that the leucosomes are anatectic in origin derived p t values for anatexis indicate high temperatures and low pressures suggestive of widespread contact metamorphism the heat source is believed to have been provided by anorthositic gabbroic magmas postulated to have been anorogenically emplaced prior to the grenville orogeny

shrimp u pb zircon geochronology of the bondy gneiss complex evidence for circa 1 39 ga arc magmatism and polyphase grenvillian metamorphism in the central metasedimentary belt grenville province quebec

timing of anatexis in metapelites from the adirondack lowlands and southern highlands a manifestation of the shawinigan orogeny and subsequent anorthosite mangerite charnockite granite magmatism anatectic leucogranites are common in metapelites within both the highlands and lowlands terranes of the adirondack mountains of northern new york state the formation of these igneous bodies which are folded in the lowlands and commonly mylonitized in the highlands has been widely considered an event accompanying the ca 1050 ma ottawan orogeny during which metamorphic grade reached granulite facies in the highlands while the lowlands experienced amphibolite facies metamorphism sensitive high resolution ion microprobe shrimp analyses of zircons separated from leucosomes and melanosomes in both the southern highlands and the lowlands indicate that primary anatexis occurred ca 1180 1160 ma and is thus a manifestation of heating during the earlier shawinigan orogeny ca 1210 1160 ma and associated anorthosite mangerite charnockite granite amcg magmatism ca 1165 1150 ma the absence of ottawan overgrowths on shawinigan zircons in these leucosomes suggests that by ottawan time the rocks were too dry for further melting or zircon growth to occur however electron microprobe analyses of monazites from the southern highlands reveal multiple age zones including cores with ages of ca 1170 1180 ma consistent with primary growth during shawinigan orogenesis complex zones formed ca 1140 1155 ma during amcg magmatism and ca 1050 1020 ma formed during ottawan orogenesis and high grade metamorphism throughout the adirondacks leucosomes and melanosomes contain older ca 1320 ma zircons that are considered to be remnant detrital zircons derived from arc rocks of the elzevirian terrane the apparent absence of archean detrital zircons suggests that the protoliths of the metapelites were deposited in restricted basins that did not receive detritus from the superior craton

sw grenville province canada the case against post 1 4 ga accretionary tectonics abstract seven accretionary sutures formed between 1 16 and 1 03 ga have been identified by different authors in the ontario quebec adirondack oqa segment of the mesoproterozoic grenville orogen in canada with one exception the inferred accretionary terrane boundaries lie within or at the margins of the central metasedimentary belt cmb located between the central gneiss belt and the adirondack highlands central granulite terrane however geological geochronological and petrological data suggest that the grenville orogen on both sides of the proposed terrane boundaries sutures preserves a common 1 4 1 03 ga tectonomagmatic history inconsistent with its origin as a post 1 4 ga collage of exotic tectonic blocks features which straddle the proposed 1 16 1 03 ga sutures from the central gneiss belt via the adirondack highlands to the mauricie area include 1 mesoproterozoic continental crust 1 5 1 4 ga forming the host and or basement to younger magmatic and supracrustal suites 2 a 1 35 1 3 ga continental arc remnants of which occur from the cmb boundary zone cmbbz in ontario to the appalachians in the united states built on the 1 5 1 4 ga continental crust 3 intrusions of 1 17 1 13 ga age in the central gneiss belt mafic suite and the adirondack highlands and their quebec extension amcg suite i e anorthosite massifs and related granitoids 4 relics of 1 18 1 14 ga sedimentary basins in the northwestern cmb and the mauricie area we propose that an alternative model can adequately account for the observed geology of this part of the grenville orogen wherein the rocks of the oqa segment were part of an andean type margin between 1 4 and 1 2 ga at 1 35 1 3 ga a continental magmatic arc was built upon the southeastern margin of laurentia represented by the 1 5 1 4 ga mesoproterozoic continental crust the arc split at 1 3 ga forming an ensialic back arc basin relics of which now occur in the northwestern part of the cmb and the back arc basin was flanked to the southeast by an active 1 28 1 25 ga arc collision between the laurentian margin and another continent amazonia occurred at 1 2 ga resulting in closure of the back arc basin and initiation of thrusting along the cmbbz post collisional lithospheric shortening led to convective removal of thickened subcontinental lithosphere upper mantle melting and extension of the overlying crust resulting in widespread magmatic activity at 1 17 1 13 ga including emplacement of the amcg massifs crustal extension generated sedimentary basins now represented by the st boniface sediments in the mauricie area 1 18 to between 1 15 and 1 09 ga and the penecontemporaneous flinton group in the northwestern cmb renewed post collisional granulite facies shortening commenced at 1 12 ga manifested as nappes in the central gneiss belt and thrusting in the mauricie area continued post collisional shortening at 1 08 1 05 ga was more localised resulting in reactivation of thrusting in the cmbbz and initiation of the kinematically compatible tawachiche shear zone along the eastern border of the quebec extension of the adirondack highlands the characteristics of the oqa segment of the grenville orogen can all be accounted for in the context of 1 a 1 4 1 2 ga southeast facing andean type margin to a laurentian upper plate associated with northwest dipping subduction 2 continental collision at 1 2 ga and 3 subsequent continued post collisional shortening without invoking accretion of exotic terranes between 1 4 and 1 0 ga

the black lake shear zone a boundary between terranes in the adirondack lowlands grenville province abstract the black lake shear zone has been proposed as a potentially important tectonic boundary between the frontenac and adirondack lowlands terranes based on geologic and isotopic evidence we present new structural geochronologic and anisotropy of magnetic susceptibility ams results from the black lake area that constrain the location kinematics and timing of deformation in the region both structural and ams results document a northeast trending zone of high strain between black lake and the saint lawrence river within this zone strain is heterogeneous and includes a strongly developed sub vertical northeast trending foliation less commonly two orthogonal lineation sets are present one that is sub horizontal and another that is down dip the foliation and granitic dikes are locally folded forming tight to isoclinal northeast trending folds kinematic indicators show thrust deformation in areas of down dip lineation but are mixed in areas of sub horizontal lineation with a slight majority suggesting left lateral shear these results suggest major northwest directed shortening accommodated by flattening and ductile thrust faults possibly with a component of sub horizontal shear the two lineations may have formed during discrete episodes or synchronously during transpression u pb shrimp geochronology on igneous titanite from deformed dikes in combination with previous geochronology show that significant deformation occurred from ca 1170 1100 ma this deformation likely occurred during the late stages of the shawinigan orogeny and may record accretion of the adirondacks to the margin of laurentia during terrane assembly

organization of storm track anomalies by recurring low frequency circulation anomalies abstract from previous studies it is known that anomalous momentum fluxes by bandpass eddies are important in maintaining long lasting tropospheric flow anomalies evidence is presented that suggests that these anomalous fluxes do not occur at random but happen because the structure of storm track activity is modified by the presence of prominent low frequency large scale circulation anomalies this behavior is noted in an extended integration of a perpetual january simulation with a general circulation model gcm because nonlinear feedbacks between low and high frequency variability make it difficult to establish causal relationships between these two ranges of variability a model is constructed that approximates the storm track activity that would be expected to accompany a given low frequency without including the feedback of the high frequencies onto the low frequency state this model is based on the linearized primitive equations and uses a series of short integrations from random initial condi