fluid flow and mass transport at the valentine wollastonite deposit adirondack mountains new york state the valentine wollastonite skarn in the north west adirondack mountains new york is a seven million ton deposit which resulted from channellized infiltration of hzo rich silica bearing fluids the wollastonite formed by reaction of these fluids with non siliceous calcite marble the skarn formed at the contact of the syenitic diana complex and was subsequently overprinted by grenville age granulite facies metamorphism and retrograde hydrothermal alteration during uplift calcite marbles adjacent to the deposit have generally high 6 o values c 21 0 typical of grenville marbles which have not exchanged extensively with externally derived fluids carbon isotopic fractiona tions between coexisting calcite and graphite in the marbles indicate equilibration at 675 c consistent with the conditions of regional metamorphism oxygen isotopic ratios from wollastonite skarn are lower than in the marbles and show a 14 variation 1 to 13 some isotopic heterogeneity is preserved from skarn formation and some represents localized exchange with low4 0 retrograde fluids detailed millimetre to centimetre scale isotopic profiles taken across skarn marble contacts reveal steep 8i8o gradients in the skarn with values increasing towards the marble the gradients reflect isotopic evolution of the fluid as it reacted with high 6 o calcite to form wollastonite calcite in the marble preserves high 6 o values to within 5 mm of the skarn contact the preservation of high 6 o values in marbles at skarn contacts and the disequilibrium fractionation between wollastonite skarn and calcite marble across these contacts indicate that the marbles were not infiltrated with significant quantities of the fluid thus the marbles were relatively impermeable during both the skarn formation and retrograde alteration skarn formation may have been episodic and fluid flow was either chaotic or dominantly parallel to lithological contacts although these steep isotope gradients resemble fluid infiltration fronts they actually represent the sides of the major flow system because chromatographic infiltration models of mass transport require the assumption of pervasive fluid flow through a permeable rock such models are not applicable to this hydrothermal system and by extension to many other metamorphic systems where low permeability rocks restrict fluid migration pathways minimum time integrated fluid fluxes have been calculated at the valentine deposit using oxygen isotopic mass balance reaction progress of fluid buffering reactions and silica mass balance all three approaches show that large volumes of fluid were necessary to produce the skarn but silica mass balance calculations yield the largest minimum flux and are hence the most realistic

deformation of the proterozoic successions in the pranhita godavari basin south india a regional perspective abstract a number of proterozoic intracratonic basins namely cuddapah pranhita godavari pg abujhmar indravati ampani chattisgarh and khariar basins occur to the immediate west of the eastern ghats granulite belt eggb in india of these the pg basin hosts an axial outcrop of late paleozoic mesozoic gondwana rocks flanked on either side by proterozoic sedimentary successions among the latter the somanpalli group in the eastern part and the penganga and yellandlapad groups in the western part show contractional folds imbricate thrusts and cleavage these deformed rock groups and the pakhal group in the western belt include shallow to deep marine sedimentary successions stratigraphic relations indicate that the major episode of deformation in the pg basin occurred prior to deposition of the unconformably overlying sullavai group the latter is a neoproterozic undeformed fluvial sequence with some aeolian intercalation deformed successions also occur in other proterozoic basins of the south indian craton notably those in the nallamalai fold belt nfb along the eastern part of the cuddapah basin the major fold fault structures in the arcuate nfb vary in trend from nnw sse in the southern part to ne sw in the northern part at least one phase of folding and thrusting predates the 1000 ma old event in the eggb deformation associated with the grenville and later pan african tectonic mobility has imprints on the neoproterozic kurnool group in the palnad sub basin northern nfb the major structural trend in the pg basin is nw se except for the yellandlapad area in the southern part where late superposed folds trend e w the deformation along the eastern margin of the chattisgarh basin its southern extension in khariar ampani and indravati basins may also be related with the main grenville deformation in the eggb a regional mesoproterozic episode of basin closure and inversion is thus evident from the proterozoic cratonic successions west of the eastern ghats this occurred prior to a regional development of neoproterozoic successions at these sites

u pb dating of monazite and its application to geological problems monazite is an underutilized mineral in u pb geochronological studies of crustal rocks it occurs as an accessory mineral in a wide variety of rocks including granite pegmatite felsic volcanic a

seismic passive resistance in soils for negative wall friction in the presence of pseudo static seismic forces passive earth pressure coefficients behind retaining walls were generated using the limit equilibrium method of analysis for the negative wall friction angle case i e the wall moves upwards relative to the backfill with logarithmic spirals as rupture surfaces individual density surcharge and cohesion components were computed to obtain the total minimum seismic passive resistance in soils by adding together the individual minimum components the effect of variation in wall batter angle ground slope wall friction angle soil friction angle and horizontal and vertical seismic accelerations on seismic passive earth pressures are considered in the analysis the seismic passive earth pressure coefficients are found to be highly sensitive to the seismic acceleration coefficients both in the horizontal and the vertical directions the results are presented in graphical and tabular formats

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

geochemistry and u pb protolith ages of eclogitic rocks of the asis lithodeme piaxtla suite acatlan complex southern mexico tectonothermal activity along the southern margin of the rheic ocean recent data indicating that the piaxtla suite acatlan complex southern mexico underwent eclogite facies metamorphism and exhumation during the devono carboniferous suggest an origin within the rheic ocean rather than the iapetus ocean the asis lithodeme piaxtla suite consists of polydeformed metasediments and eclogitic amphibolites that are intruded by megacrystic granitoid rocks u pb zircon data indicate that the metasediments were deposited after c 700 ma and before intrusion of c 470 420 ma quartz augen granite the metasedimentary rocks contain abundant mesoproterozoic detrital zircons c 1050 1250 ma and a few zircons in the range of c 900 992 and c 1330 1662 ma their geochemical and sm nd isotopic signature is typical of rift related passive margin sediments derived from an ancient cratonic source which is interpreted to be the adjacent mesoproterozoic oaxacan complex megacrystic granites were derived by partial melting of a c 1 ga crustal source similar to the oaxacan complex amphibolitic layers exhibit a continental tholeiitic geochemistry with a c 0 8 1 1 ga source t dm age and are inferred to have originated in a rift related environment by melting of lithospheric mantle in the ordovician this rifting may be related to the early ordovician drift of peri gondwanan terranes e g avalonia from gondwana and the origin of the rheic ocean

geochemistry and provenance of the middle ordovician austin glen member normanskill formation and the taconian orogeny in new england the austin glen member of the upper middle ordovician normanskill formation is a sandstone shale ysch succession deposited in the foreland of the taconian orogen petrographic major and trace element and nd pb isotopic data provide substantial constraints on its provenance lack of k feldspar and paucity of plagioclase in addition to the dominance of sedimentary rock fragments indicate that the source was dominated by recycled sedimentary components major and trace element data support this conclusion and indicate that the provenance of both shales and sandstones was the same no evidence of an ophiolitic or volcanic component was observed interpretation of nd isotopic characteristics are complicated by a partial resetting of the nd isotope system at about the time of sedimentation but indicate that the provenance of the austin glen member had a long term history of light rare earth element lree enrichment average tdm 1a8 ga furthermore nd isotopic compositions are extremely homogeneous end 8a1 0a6 1 s d n 23 at 450 ma the approximate depositional age indicating either a single source or very well mixed sources 207pb 204pb ratios are variable but within the range of pb isotopic compositions typically described as grenvillian the range of 207pb 204pb is greater than expected for the range of 206pb 204pb and suggests an additional component of pb possibly introduced during diagenesis the immediate source of the austin glen member may have been the accretionary prism that developed as older sediments of the laurentian margin were scraped off the basin oor incorporated within the accretionary prism and shed into the basin no evidence indicating the arrival of an undifferentiated island arc or continental fragment during the taconian orogeny has been found the data acquired during this study can be explained almost exclusively by grenville province source components but with possible additional contributions from older laurentian terranes and late proterozoic rift volcanics that are not readily quanti ed but likely to have been minor accordingly we conclude that the taconian orogeny in new england involved either 1 a continental arc that involved exclusively laurentia 2 collision of a continental block with identical geochemical characteristics as laurentia or 3 essentially no detritus from any exotic colliding block island arc or continental fragment reached the foreland basin at the time of austin glen deposition

unusual lithospheric structure beneath the hyderabad granitic region eastern dharwar craton south india abstract using multiparametric geological and geophysical data the evolutionary history of the lithosphere beneath the late archean early proterozoic hyderabad granitic region hgr of the eastern dharwar craton of south indian shield has been attempted our study reveals that the entire granitic region and the surroundings 200 km 200 km is being neotectonically uplifted possibly due to a major intrusive body situated at sub crustal depth leading to changes in river courses and also erosion of several kilometers 10 km from its upper crustal column beneath this region the moho is elevated 32 33 km the reduced heat flow is of the order of 28 mw m2 and the asthenosphere is located at shallow depths of about 124 km i e far less than 200 400 km depth found in precambrian shield areas of the globe the cratonic mantle lithosphere beneath hgr contains a highly conductive 15 m hydrous and seismically anisotropic metasomatic zone between depth of 90 and 100 km where the estimated temperatures could be in the range of 900 1000 c the surface granitic layer containing unusually high radioactivity 5 25 w m3 appears very thin probably a kilometer or so beneath which the entire crustal column seems to be differentiated and made up of low radioactive granulite facies rocks of acid to intermediate composition persistent episodic thermal reactivations during the last 2 6 gega years and continuous uplifting seems to have played a major role towards making its crustal column unusual

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

late silurian early devonian slab break off beneath the canadian appalachians insights from the nashwaak granite west central new brunswick canada abstract late silurian early devonian granites are widespread in the miramichi highlands of new brunswick but their petrogenesis is controversial and hinders an understanding of the geological evolution of the canadian appalachians here we present a detailed geochemical and geochronological study of the nashwaak granite and compare it with coeval granites in this region to indicate a late silurian early devonian slab break off regime in the canadian appalachians the nashwaak granite consists of two mica granite biotite granite and biotite granite dyke swarms these rocks intruded at ca 420 ma and their compositions show a highly siliceous calc alkaline and peraluminous characteristic they show similar ree distribution patterns similar depletion of ba sr nb p and ti and similar nd isotopes as contemporaneous granites inboard of the salinic suture zone bamford brook fault together these data suggest that late silurian early devonian granites in the miramichi highlands may be derived from partial melting of the neoproterozoic to lower cambrian ganderian basement resembling the brookville terrane enriched subcontinental lithospheric mantle may have also been involved in the magma system where coeval gabbro occurs within the granite but a contribution from asthenospheric mantle is absent these linearly distributed and post collisional granites are most likely formed by salinic slab break off with corresponding uplift and extension recorded by 422 419 ma bimodal volcanic rocks unconformities in sedimentary rocks and extensional deformation d3 in the brunswick subduction complex of northern new brunswick

lead isotopic compositions of the western dharwar craton southern india evidence for distinct middle archean terranes in a late archean craton abstract lead and sr isotopic compositions of whole rocks and feldspar separates from several fielddefined suites of rocks from the western dharwar craton of southern india elucidate a complex history of the area the evolution of the craton earlier than 3 0 ga is difficult to determine because of thermal and metasomatic overprinting at 3 0 ga many whole rocks were completely reset then but feldspar separates preserve some of the earlier history apparently a series of terranes evolved from the mantle over the period 3 4 to 3 0 ga to form the western dharwar craton in each case metamorphism and anatexis of older terranes accompanied extraction of juvenile material from the mantle the oldest gneisses 3 4 ga had radiogenic pb and sr isotopic compositions when they formed and were derived from older crustal protoliths that were plausibly accreted to the crust at or even before 3 8 ga the old gneisses were greatly depleted in rb probably at 3 4 ga when the protoliths of younger magmatic rocks may have separated from the mantle younger gneisses apparently formed from these mafic intermediate protoliths in the age range of 3 2 to 3 0ga the 3 0 ga event in the central part of the craton was accompanied by intrusion of small diapiric trondhjemites formed by melting of older crust and penecontemporaneous metasomatism of country rocks by hydrous fluids carrying u and other lithophilic elements the resultant deep crust was greatly depleted in heat producing elements and the craton has been inactive since 3 0 ga except for production of minor granites from depleted source regions at 2 5 2 6 ga the lack of activity at 2 5 ga contrasts strongly with extensive magmatism and metamorphism in surrounding areas that had not undergone depletion at 3 0 ga lower crustal depletion also has caused present day reduced heat flow to be very low

anomalous crustal and lithospheric mantle structure of southern part of the vindhyan basin and its geodynamic implications abstract tectonically active vindhyan intracratonic basin situated in central india forms one of the largest proterozoic sedimentary basins of the world possibility of hydrocarbon occurrences in thick sediments of the southern part of this basin has led to surge in geological and geophysical investigations by various agencies an attempt to synthesize such multiparametric data in an integrated manner has provided a new understanding to the prevailing crustal configuration thermal regime and nature of its geodynamic evolution apparently this region has been subjected to sustained uplift erosion and magmatism followed by crustal extension rifting and subsidence due to episodic thermal interaction of the crust with the hot underlying mantle almost 5 6 km thick sedimentation took place in the deep faulted jabera basin either directly over the bijawar mahakoshal group of mafic rocks or high velocity high density exhumed middle part of the crust detailed gravity observations indicate further extension of the basin probably beyond nsl rift in the south a high heat flow of about 78 mw m2 has also been estimated for this basin which is characterized by extremely high moho temperatures exceeding 1000 c and mantle heat flow 56 mw m2 besides a very thin lithospheric lid of only about 50 km many areas of this terrain are thickly underplated by infused magmas and from some segments granitic gneissic upper crust has either been completely eroded or now only a thin veneer of such rocks exists due to sustained exhumation of deep seated rocks a 5 8 km thick retrogressed metasomatized zone with significantly reduced velocities has also been identified around mid to lower crustal transition

nature and evolution of the eastern margin of lapetus geochemical and isotopic constraints from siluro devonian granitoid plutons in the new brunswick appalachians in the gander zone ordovician to devonian metaluminous to slightly peraluminous plutons with within plate affinities intrude a lower paleozoic metasedimentary package during the siluro devonian end t and 18o values of the metasedimentary rocks would have been about 7 to 8 and 10 to 14 respectively granitoid end t values range from 0 3 to 4 2 18o values lie between 6 6 and 10 4 and published pb isotopic compositions are radiogenic 207pb 204pb 15 65 end t values of associated silurian gabbros are somewhat positive 1 2 to 2 3 some plutons exhibit evidence for comingling with this mafic component potential source materials include enriched mantle supracrustal and infracrustal sources although some constraints can be put on the first two sources the nature of infracrustal material is difficult to evaluate heterogeneous data obtained from silurian plutons can be explained by crust mantle mixing if some plutons contain a significant mantle component then mesoproterozoic o

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

extraction of absolute p velocity from receiver functions summary receiver functions constitute an important tool in regional and global studies of crust and upper mantle structure and are frequently employed to constrain crustal thickness and bulk vp vs ratio the methods commonly used to compute these parameters assume a known crustal p velocity however this quantity trades off with crustal thickness we demonstrate that p velocity and vp vs ratio can be readily computed through solution of a linear system of equations incorporating traveltimes of direct conversions and free surface reverberations representing a range of horizontal slowness determination of crustal thickness follows trivially we apply this approach to data from station hyb on the indian craton to determine bulk vp vs ratio p velocity and thickness of 1 79 0 007 6 1 0 13 km s 1 and 30 5 0 8 km respectively the addition of average crustal p velocity to the suite of parameters accessible from receiver function data may prove useful in constraining bulk crustal composition and its secular evolution

geochemistry of the arvonia formation chopawamsic terrane virginia implications for source area weathering and provenance the metasedimentary arvonia formation in the piedmont province of central virginia ranges primarily from slates to garnet schists which unconformably overly middle ordovician metavolcanic rocks of the chopawamsic arc terrane the formation has typically been interpreted as late ordovician based on poorly preserved fossils perhaps having formed as a successor basin following accretion of the chopawamsic arc to laurentia however alternative interpretations have been proposed including deposition outboard of laurentia on chopawamsic arc crust that is prior to accretion furthermore the origin of the chopawamsic arc as peri laurentian or peri gondwanan is an unresolved issue we report whole rock major and trace element compositions as well as nd isotopic compositions of the arvonia formation and use these results to evaluate further its origin samples collected along the length of the outcrop belt show similar major element compositions which are typical of shales chemical index of alteration values 68 89 corrected for k metasomatism indicate intermediate to extreme levels of source weathering a majority of the samples show uniform rare earth element ree patterns that are light ree enriched with negative eu anomalies a few samples display anomalous ree patterns indicative of some disturbance of the light ree at approximately the time of deposition a variety of trace element values or ratios indicate that arvonia sediments are similar to shale composites thus upper continental crust nd isotopic compositions for eight samples are similar with end 5 6 to 8 9 400 ma collectively these results are consistent with derivation of arvonia sediments from a single source or well mixed sources of typical upper crustal composition the isotopic data are compatible with a laurentian provenance however they are equally compatible with a strictly chopawamsic crustal source based on available nd isotopic data

sm nd isotopic systematics as tectonic tracers an example from west avalonia in the canadian appalachians abstract the tectonothermal evolution of west avalonia in mainland nova scotia appalachian orogen provides an excellent example of the application of sm nd isotopic analyses to tectonic studies regional syntheses indicate that west avalonia originated in the neoproterozoic as one of a number of terranes collectively known as peri gondwanan terranes along the gondwanan margin these terranes are characterized by late neoproterozoic voluminous arc related magmatism west avalonia became detached from gondwana in the early ordovician was accreted to laurentia by the silurian and was deformed by post accretionary strike slip tectonics in the late devonian and carboniferous associated with the amalgamation of pangea all stages of this evolution can be recognized in the sm nd isotopic signature in various neoproterozoic and paleozoic volcanic and sedimentary successions in mainland nova scotia crustally derived felsic volcanic rocks of late neoproterozoic ca 615 ma early cambrian and early silurian ages define an envelope that provides gross constraints on the sm nd isotopic composition of the underlying basement source e nd values range from 0 78 to 2 92 in the neoproterozoic rocks from 2 0 to 5 0 in the cambrian rocks and from 0 1 to 2 5 in the silurian rocks t dm model ages for each of these suites are remarkably similar they typically range from 0 9 to 1 2 ga suggesting that the sm nd isotopic composition of these suites was profoundly influenced by repeated melting of the same basement source this sm nd envelope provides a reference by which other suites may be compared and interpreted for example turbidites in a late neoproterozoic volcanic arc basin have sm nd compositions that lie within this envelope suggesting that they are derived from the coeval volcanic rocks that flank the basin the genetic significance of the model ages can be investigated by the analysis of felsic and metasedimentary rocks that pre date the main arc phase two felsic complexes have compositions that lie within the envelope and have similar model ages the presence of this signature in early avalonian rocks suggests that the model ages represent a genuine tectonothermal event at ca 1 0 to 1 2 ga that formed juvenile proto avalonian crust by implication the main arc phase in avalonia recycled this crust sm nd isotopic analyses of the metasedimentary sequences are incompatible with previous interpretations of deposition in a passive margin setting that pre dated avalonian arc activity instead the data indicate derivation from two distinct sources ancient gondwanan basement and juvenile avalonian arc suggesting deposition in a western pacific type back arc environment this re interpretation has important implications for neoproterozoic reconstructions the sm nd isotopic composition of west avalonian basement contrasts with that of other peri gondwanan terranes for example similarly aged felsic rocks in the cadomian terrane of nw france are characterized by more negative e nd values and older t dm model ages 1 0 1 9 ga these data together with detrital u pb data suggest that west avalonia was developed along the amazonian margin whereas cadomia was developed along the west african margin of gondwana sm nd isotopic analyses of early silurian clastic rocks in mainland nova scotia show fundamental differences with unconformably underlying neoproterozoic and early paleozoic avalonian successions these data have e nd values ranging from 5 7 to 6 1 with t dm model ages of about 1 4 to 1 7 ga indicating that the clastic rocks cannot have been derived from underlying successions a variety of geologic data and regional constraints suggests that they may have been derived from the caledonide orogen of western europe thereby providing a minimum age for the accretion of west avalonia to laurentia baltica sm nd isotopic analyses from late devonian early carboniferous continental clastic rocks deposited along the southern flank of avalonia in mainland nova scotia also show little influence from avalonian rocks and are compatible with derivation from the meguma terrane immediately to the south these data are consistent with regional synthesis which indicate uplift of the meguma terrane relative to west avalonia during dextral transpression

the southern high grade margin of the dharwar craton the amphibolite facies to granulite facies transition at the southern margin of the dharwar craton has been studied in the krishnagiri satnur halaguru and kushalnagar areas of southern india in all three areas the transition appears to be a progressive metamorphic overprint on cratonal gneisses and their mafic and metasedimentary enclaves without major structural interruption in the kabbal satnur br hills section of karnataka a high grade charnockite massif with pronounced rb depletion is the culmination of an apparently continuous increase of metamorphic grade southward in this and the kushalnagar areas increase of paleopressure from near 6 to near 8 kbar with increasing grade indicates a depth zone relationship of the amphibolite and granulite facies incipient charnockite replacing peninsular gneiss first appears along n s shears parallel to the regional grain of the craton low p h 2 o high co 2 vapors were instrumental in the creation of orthopyroxene introduction from a deep so

geochemistry of the archean low to high grade transition zone southern india the transition zone between archean low and high grade rocks in southern india represents eroded crustal levels representative of 15 20 km it is comprised chiefly of tonalitic gneisses with some varieties showing incipient charnockitization and of minor amounts of granitic gneiss and charnockite both of which appear to have developed from the tonalitic gneisses

thorium and uranium contents of gneisses and trondhjemites in the western dharwar craton india widespread metasomatic addition of u has apparently occurred in archean tonalitic trondhjemitic gray gneisses in the western dharwar craton of southern india gneisses of 3200 3000 ma age near the holenarasipur schist belt contain average concentrations of about 10 ppm th and 3 5 ppm u the u is considerably more abundant than average u 1 5 ppm in archean gray gneisses of the superior province of the canadian shield and consequently the th u ratios are low in the indian gneisses comparatively massive trondhjemitic plutons associated with a 3000 ma event in the dharwar craton are very depleted in both th and u and we propose that the metasomatism of the gneisses occurred while the plutons were being emplaced and were expelling fluids from the magmas there is no evidence for metasomatism 2500 ma ago the age of extensive granulite metamorphism in southern india

the cadomian orogeny in saxo thuringia germany geochemical and nd sr pb isotopic characterization of marginal basins with constraints to geotectonic setting and provenance abstract the cadomian basement and the cambro ordovician overstep sequence in saxo thuringia is characterized by clastic sedimentation from the late neoproterozoic to the ordovician magmatism in the avalonian cadomian arc preserved in saxo thuringia occurred between ca 570 and 540 ma peri gondwanan basin remnants with cadomian to early palaeozoic rocks are exposed as very low grade metamorphosed rocks in six areas schwarzburg anticline berga anticline doberlug syncline north saxon anticline lausitz anticline and elbe zone a hiatus in sedimentation between 540 and 530 ma cadomian unconformity is related to the cadomian orogeny a second gap in sedimentation occurred during the upper cambrian 500 to 490 ma and is documented by a disconformity between lower to middle cambrian rocks and overlying tremadocian sediments major and trace element signatures of the cadomian sediments reflect an active margin continental arc those of the ordovician sediments a passive margin the cambrian sediments have inherited the arc signature through the input of relatively unaltered cadomian detritus initial nd and pb isotope data from the six saxo thuringian areas demonstrate that there is no change in source area with time for each location but that there are minor contrasts among the locations 1 cadomian sediments from the lausitz anticline the doberlug syncline and the elbe zone have lower 207pb 204pb than all other areas 2 the core of the schwarzburg anticline which is overprinted by greenschist facies conditions and detached is isotopically heterogeneous one part of its metasedimentary units has less radiogenic nd than sediments from other low grade units of similar age in the same area 3 cadomian sediments from the schwarzburg anticline show an input of younger felsic crust 4 the rothstein group shows distinct input of young volcanic material also 5 cadomian sediments from the lausitz anticline the elbe zone and parts of the north saxon anticline are characterized by input from an old mafic crust nd isotope data of the remaining areas yield average crustal residence ages of the sediment source of 1 5 1 9 ga which suggests derivation from an old craton as found for other parts of the iberian armorican terrane collage similarly the pb isotope data of all areas indicate sediment provenance from an old craton the rapid change of lithologies from greywacke to quartzite from the late neoproterozoic cadomian basement to the ordovician does not reflect changes in sediment provenance but is essentially due to increased reworking of older sediments and old weathering crusts that formed during various hiatus of sedimentation this change in sediment maturity takes its chemical expression in lower overall trace element contents in the quartzite dilution effect by quartz and relative enrichment of some trace elements zr mree hree due to detrital zircon and garnet the rb sr systematics of the quartzites and one ordovician tuffite was disturbed most likely during the variscan orogeny which suggests a lithology controlled mobility of alkali and calc alkali elements by comparison with available data it seems unlikely that only nd tdm model ages are useful to distinguish between west african and amazonian provenance nd tdm model ages of 1 5 to 1 9 ga in combination with paleobiogeographic aspects age data from detrital zircon and palaeogeographic constraints especially through tillites of the saharan glaciation in the hirnantian strongly indicate a provenance of saxo thuringia from the west african craton

sm nd and u pb isotopic evidence of juvenile crust in the adirondack lowlands and implications for the evolution of the adirondack mts sm nd investigations of hyde school gneiss yield t dm ages only slightly older than u pb zircon emplacement ages thus indicating substantial addition of juvenile calcalkaline crust to the adirondack lowlands at ca 1300 ma juvenile crust of similar age has been recognized within the adirondack highlands the elzevir terrane further to the northwest in canada and in the green mts of vermont suggesting the presence of an extensive magmatic arc at that time while the precise nature of the arc remains uncertain an indonesian analogue is favored u pb zircon ages of ca 1150 1160 ma are reported for lowlands samples of syenitic and granitic rocks belonging to the anorthosite mangerite charnockite granite amcg suite that also occurs in the highlands as well as in the canadian sector of the frontenac terrane the regional occurrence of these anorogenic intrusions indicates that the adirondack highlands and frontenac terrane including the adirondack lowlands constituted a single contiguous entity

topographic architecture and drainage reorganization in southeast china zircon u pb chronology and hf isotope evidence from taiwan abstract the uplift of tibet plateau and the marginal sea spreading have had important influence on the tectonic landform and drainage system in east asia although the marginal sea spreading in shaping the topography and drainage reorganization in east asia has been still controversial here we present u pb age and hf isotopic composition of detrital zircon grains from cenozoic sedimentary rocks in taiwan to understand how the provenance and river systems evolved our data show that the u pb age spectra of detrital zircon grains in paleogene sandstones are dominated by yanshanian 180 67 ma zircon grains and with subordinate or nil proterozoic and archean zircon grains these results are in contrast to those in miocene rocks that are dominated by the indosinian 257 205 ma zircon grains together with yanshanian proterozoic and archean population the initial hf isotope ratios ehf t of the zircon grains also display systematic change in paleogene and neogene strata our data demonstrate that the hsuehshan range and western foothills in taiwan have the same sedimentary sources the source region of paleogene strata was mainly located at the coast in southeast china and migrated inland over time the source might have reached the lower yangtze region during early miocene although the mechanism of transport of sediments from the lower yangtze region to taiwan is unclear we speculate that the minjiang river might have been larger in early miocene than the present and might have delivered inland material along the boundary of yangtze and cathaysia blocks to taiwan these were then captured by the yangtze river systems at some time after late miocene this change corresponds to the time of the drainage reorganization in east tibet such as yangtze river and the regional subsidence resulting from the opening of marginal sea the combined effects of tibet uplift and opening of marginal sea might have shaped the topography and river system reorganization in east tibet the evolution of topography and drainage systems in southeast china seems to be mainly controlled by the opening of marginal sea

upwarped high velocity mafic crust subsurface tectonics and causes of intraplate latur killari m 6 2 and koyna m 6 3 earthquakes india a comparative study abstract a unique attempt is made to understand the genesis of intraplate seismicity in the latur killari and koyna seismogenic regions of india through derived crustal structure by synthesizing active and passive seismic magnetotelluric gravity and heat flow data it has indicated presence of relatively high velocity density intermediate granulite and amphibolite facies rocks underneath the deccan volcanic cover caused mainly due to a continuous geodynamic process of uplift and erosion since precambrian times these findings have been independently confirmed by detailed borehole geological geochemical and mineralogical investigations the crystalline basement rock is found to contain 2 wt of carbon di oxide fluid components the presence of geodynamic process associated with thermal anomalies at subcrustal depths is supported by a high mantle heat flow 29 36 mw m2 beneath both regions although some structural and compositional variations may exist as evidenced by p and s wave seismic velocities we suggest that the stress caused by ongoing uplift and a high mantle heat flow is continuously accumulating in this denser and rheologically stronger mafic crust within which earthquakes tend to nucleate these stresses appear to dominate over and above those generated by the india eurasia collision the role of fluids in stress generation as advocated through earlier studies appears limited

whole rock pb and sm nd isotopic constraints on the growth of southeastern laurentia during grenvillian orogenesis the conventional view that the basement of the southern and central appalachians represents juvenile mesoproterozoic crust the final stage of growth of laurentia prior to grenville collision has recently been challenged new whole rock pb and sm nd isotopic data are presented from mesoproterozoic basement in the southern and central appalachians and the granite rhyolite province as well as one new u pb zircon age from the granite rhyolite province these data combined with existing data from mesoproterozoic terranes throughout southeastern laurentia further substantiate recent suggestions that the southern and central appalachian basement is exotic with respect to laurentia sm nd isotopic compositions of most rocks from the southern and central appalachian basement are consistent with progressive growth through reworking of the adjacent granite rhyolite province however pb isotopic data including new analyses from important regions not sampled in previous studies do not correspond with pb isotopic compositions of any adjacent crust the most distinct ages and isotopic compositions in the southern and central appalachian basement come from the roan mountain area eastern tennessee western north carolina the data set indicates u pb zircon ages up to 1 8 ga for igneous rocks inherited and detrital zircon ages 2 0 ga sm nd depleted mantle model t dm ages 2 0 ga and the most elevated 207 pb 204 pb observed in southeastern laurentia the combined u pb geochronologic and sm nd and pb isotopic data preclude derivation of southern and central appalachian basement from any nearby crustal material and demonstrate that grenville age crust in southeastern laurentia is exotic and probably was transferred during collision and assembly of rodinia these new data better define the boundary between the exotic southern and central appalachian basement and adjacent laurentian granite rhyolite province

u pb geochronology geochemistry and provenance of the grenvillian huiznopala gneiss of eastern mexico abstract the huiznopala gneiss is the smallest 25 km2 of four exposures of grenvillian granulites in eastern and southern mexico the gneiss comprises three major lithologic units 1 a main series of orthogneisses 2 an anorthosite gabbro complex and 3 a layered paragneiss sequence thirty one u pb zircon ages from seven samples all lie within a restricted range of 200 million years and the geochronological results are interpreted to reflect principally a two stage history the earliest stage was arc magmatism that extended from 1200 to 1150 ma the second stage included granulite facies metamorphism which peaked at about 725 50 c and 7 2 1 0 kbar and perhaps emplacement of the anorthosite gabbro complex at 1000 ma a pegmatite with an age of 988 3 ma post dates ductile deformation the timing of ductile deformation and granulite facies metamorphism of the huiznopala gneiss corresponds closely to that of final thrusting and deformation in the grenville province thus the huiznopala gneiss may be in the core of the orogen formed during the final thrusting event recognized in the grenville orogeny gondwanan faunal assemblages are found in lower paleozoic sedimentary rocks that overlie grenvillian granulites elsewhere in mexico however the pb isotope compositions of samples from the huiznopala gneiss overlap with those of the basement from laurentia adirondack highlands and lowlands texas and they are distinct from those of the gondwanan arequipa antofalla craton of southern bolivia and northern chile the magmas parental to the huiznopala arc and those parental to the laurentian crust may have shared a common isotope reservoir and the pb isotope compositions are consistent with the paleomagnetic results that place southern mexico adjacent to ontario quebec at about 950 ma

seismic imaging of the proterozoic cuddapah basin south india and regional geodynamics abstract cuddapah basin has been believed to be one of the largest intra cratonic proterozoic sedimentary basins of india situated in the eastern part of the dharwar craton of the south indian shield which is magmatically infested and contain thick column of sediments its overall sedimentary thickness as well as the nature of crustal evolution has remained enigmatic based on deep seismic sounding and other geological studies in the past it was perceived that this basin may contain as much as 10 12 km thick sediments the results of our present analysis derived up to a depth of 12 km reveals five layered upper crust associated with velocities i 4 50 km s ii 5 20 5 30 km s iii 5 50 5 80 km s iv 5 85 6 00 km s and v 6 40 km s out of which second and third layers correspond to upper and lower cuddapah sediments the results suggest the presence of only 4 0 km thick sediments in the deepest part of the basin below the nallamalai fold belt which has its implications in the developmental history of the basin a thermal driving force was invoked by earlier workers to account for estimated 10 12 km thick sediments however the present estimate of only 4 km basement depth in the cuddapah basin shows that the role of the thermal driving force may be marginal particularly in the deeper eastern cuddapah as isostatic subsidence due to sedimentary accumulation alone is enough to explain the basin depth further a basement sag of about 10 km would have logically needed lateral extension of the order of several hundreds of kilometers however as our present estimate of the sediment thickness gets reduced from earlier 10 km to only 4 km the size of the basin 44 500 km 2 would be in conformity with isostatic subsidence due to sedimentary accumulation hypothesis the structural features derived from present analysis like maximum depth observed near the thrust suture on the basin margin from where it decreases away from it its association of shallow marine sediments the arcuate shape of the basin along with its areal dimension resembles foreland basin between continent continent collisions

precambrian continental crust of india and its evolution the indian precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes the bulk of this crust was formed prior to 2600 m y ago and remobilized at least twice between 2600 2000 m y ago early proterozoic mobile belt epmb and 2000 1500 m y ago middle proterozoic mobile belt mpmb three early precambrian nucleii karnataka kn jeypore bastar jbn and singhbhum sn appear to have survived in the craton and are characterized by low grade supracrustals and tonalitic trondhjemite gneisses formed 3800 2600 m y ago the epmb event involved sedimentation amphibolite granulite facies metamorphism and co 2 k metasomatism and produced amphibolite facies rocks and k granites in the north and charnockite and other granulite facies rocks in the south gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the epmb event k granites form a garland around the central dharwar craton sugg

lithospheric mantle deformation beneath the indian cratons abstract the nature of deformation of the deep continental roots beneath the archean early proterozoic terrains opens the question whether these ancient terrains have had stable roots since the precambrian or whether recent plate motions have deformed them in view of this we make an attempt to study the thermal structure beneath the cratonic regions of the indian shield which vary in lithospheric thickness from 65 km in the singhbhum craton to 148 km in the archean dharwars the average depth of 104 km to the top of the underlying asthenosphere is consistent with other termination methods and is in fact less than half the 200 400 km depth found in other stable areas of the earth similarly the average reduced heat flow of about 35 mw m2 and moho temperature of about 550 c range 400 730 c for the indian cratons are also much higher than their counterparts elsewhere our study indicates a large scale deformation of the cratonic mantle lithosphere beneath the indian shield since the mesoproterozoic c

imprints of a proterozoic tectonothermal anomaly below the 1 1 ga kimberlitic province of southwest cuddapah basin dharwar craton southern india summary a detailed 2 d modelling of the seismic structure and other geological and geophysical signatures across the cuddapah basin of the southern indian shield suggests upwarping of highvelocity and high density layers which are observed close to the surface below the southwestern part of the basin this anomalous feature is constrained by i a strong gravity high anomaly of about 55 mgal ii a 100 km wide high conductivity anomaly resistivity 100 m extending from surface to a minimum depth of 50 km in the mantle lithosphere and iii large scale massive intrusive activity these features are interpreted to be an expression of a thermal anomaly which may have acted like a plume during the proterozoic and could well correspond to a 1 1 ga kimberlitic activity below this region the thin granitic gneissic crust is underlain by well differentiated high velocity layers possibly due to underplating and densification of much of the crust by extruded magma

seismogenesis in the stable continental interiors an appraisal based on two examples from india characteristics of two recent moderate earthquakes from india 1993 killari mw 6 1 and 1997 jabalpur mw 5 8 are presented in this paper to illustrate the influence of geologic and tectonic setting on the nature of seismicity in stable continental regions scrs the killari event which occurred in the middle of a cratonic region of low background seismicity long recurrence interval and poorly developed neotectonic features may be qualified as a common type of scr event this earthquake is also characterized by a shallow focal depth and a long sequence of aftershocks in contrast the jabalpur earthquake shows a spatial association with a well defined tectonic structure with significant background seismicity a remarkable feature of this earthquake is its deep focus not commonly observed in scr seismicity this event was associated with fewer aftershocks unlike most other scr earthquakes analogous events are found in other shield regions as well based on their general characteristics a broad classification of moderate scr earthquakes is attempted here two broad groups have been identified in this study 1 a set of deep focus earthquakes that are related to intracratonic paleorifts and 2 shallow focus earthquakes associated with discrete faults in the less deformed precambrian terrains a synthesis of earthquake data from different geologic environments will improve our understanding of the seismogenesis in the shield regions 1999 elsevier science b v all rights reserved

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

seismic uplift capacity of inclined strip anchors uplift capacities of inclined strip anchors in soil with a horizontal ground surface are obtained under seismic conditions limit equilibrium approaches with a logarithm spiral failure surface and pseudostatic seismic forces are adopted in the analysis the results are presented in the form of seismic uplift capacity factors as functions of anchor inclination embedment ratio angle of internal friction of the soil and horizontal and vertical seismic acceleration coefficients the uplift capacity factors are worked out separately for cohesion surcharge and density components use of the principle of superposition for calculating anchor uplift capacity is validated the vertical seismic acceleration coefficient always reduces the uplift capacity whereas the horizontal seismic acceleration coefficient reduces the uplift capacity in most cases the roles of anchor embedment ratio soil friction angle and anchor inclination in determination of the seismic uplift capacity are also discussed comparisons of the proposed method with available theories in the seismic case are also presented the present study gives the minimum seismic uplift capacity factors compared with the existing theory

imaging the lithosphere asthenosphere boundary of the indian plate using converted wave techniques 1 the lithospheric structure of the indian plate has been investigated using converted wave techniques p and s receiver functions and a novel stacking analysis technique without using deconvolution applied to a large seismological data set from permanent and temporary broadband seismic stations we observe coherent energy from at least two seismic discontinuities i e the crust mantle moho and lithosphere asthenosphere boundary lab in the uppermost mantle here we provide a novel seismic image of the indian lithosphere showing definitive evidence of its flexure which is interpreted to be primarily caused by the hard collision at 55 myr resulting in the world s highest mountain chain the himalayas and the tibetan plateau results from geoidal and gravity studies do suggest postcollisional flexuring of the indian plate however the flexure lacks observational constraints the observed wavelength of the flex is 1000 km with the thickness of the indian shield lithosphere varying from 70 km to 140 km such a low value for a continent implies that the indian plate has been reworked in the past the plate deepens in the himalayan region to a depth of 170 km further the converted phases are interpreted to be resulting from the bottom of the lithosphere we clearly demonstrate that these are distinct and different from the midlithospheric discontinuity for a large number of stations the midlithospheric discontinuity and lab are clearly separated in depth our observations suggest that the archaean lithosphere is no longer intact and is prone to deformation

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

petrogenesis of the ultrapotassic lincoln syenite maine late silurian early devonian melting of a source region modified by subduction driven metasomatism the late silurian early devonian 418 1 ma lincoln syenite exposed discontinuously over a distance of 75 kilometers in south central and mid coastal maine represents one of the most unusual igneous rocks in the appalachian orogen detailed field and petrographic study reveals that most of the exposed syenites have been variably affected by post magmatic recrystallization however a small proportion of the syenites in south central maine preserve original igneous minerals and textures and allow for an unobstructed study of the igneous petrogenesis the syenites are characterized by large dark colored alkali feldspar megacrysts set in a finer grained matrix dominated by alkali feldspar clinopyroxene orthopyroxene biotite oxides the rocks are intermediate in terms of sio2 content 55 59 wt yet are ultrapotassic 6 7 wt k2o and relatively primitive in terms of their mgo mg 67 cr 400 ppm and ni 150 ppm concentrations additionally the rocks exhibit large ion lithophile element enrichment relative to the high field strength elements along with elevated concentrations of light rare earth elements 200 300 chondritic abundances the syenites show limited radiogenic isotopic variability with relatively high initial 87sr 86sr values 0 707 to 0 710 relatively low initial end values 4 8 to 6 7 and initial pb isotopic compositions characteristic of igneous rocks in central maine for example 207pb 204pb 15 577 to 15 639 the geochemical characteristics of these rocks are most consistent with a model involving the partial melting of a mantle wedge previously metasomatized by subduction derived fluids crystallization of this magma was influenced by the presence of significant amounts of water that suppressed plagioclase feldspar crystallization facilitated the crystallization of mg rich biotite and ultimately resulted in the nucleation and growth of the alkali feldspar megacrysts correlation with the geochemically similar parks pond pluton in eastern maine extends the distribution of late silurian early devonian ultrapotassic magmatism in maine to an orogen parallel distance of over 150 km comparisons with rocks of similar composition in known tectonic environments suggest the lincoln syenite and by association the parks pond pluton formed as a result of lithospheric extension superimposed on a sub continental lithospheric mantle source region previously modified by subduction driven metasomatism the timing and structural position of this ultrapotassic magmatism is consistent with a geodynamic model involving slab break off or the convective removal of lithospheric mantle delamination in association with acadian convergence both processes facilitate a brief period of crustal extension and decompression melting of source rocks previously modified by subduction related processes

crustal structure of the southeastern grenville province northern new york state and eastern ontario the grenville province exposes an oblique cross section through mid lower crustal lithologies that were pervasively deformed and subjected to regional thermal overprinting during the grenvillian orogeny 1 1 ga the southeastern grenville province is divided into two subterranes by the carthage colton mylonite zone a 110 km long lineament characterized by intense ductile shear and igneous intrusion which separates the amphibolite facies metasediments of the central metasedimentary belt to the west from the granulite facies metaplutonics of the central granulite terrane to the east the recognition of distinct lithotectonic domains separated by zones of intense ductile shear in the grenville province raises questions concerning the deep structure of these subterranes and in particular the means by which the mid lower crustal rocks exposed in the grenville province were emplaced seismic refraction wide angle reflection data were acquired to investigate the deep structural interrelationships within the southeastern grenville province a travel time inversion for velocity and interface depth was applied to the seismic data together with constraints from amplitude modeling to produce a seismic velocity model of the crust in the southeastern grenville province in the central metasedimentary belt the upper crust is characterized by velocities in the range 6 3 6 4 km s and a poisson s ratio of 0 26 0 01 which are attributed to quartzofeldspathic rocks farther east in the central granulite terrane upper crustal velocities of 6 55 km s and a poisson s ratio of 0 28 0 01 are associated with the marcy anorthosite the seismic homogeneity of the upper crust in the region of the carthage colton mylonite zone suggests that this boundary is a shallow feature limited to the upper 2 3 km of the crust the deep crustal structure of the southeastern grenville province is characterized by two discrete and laterally discontinuous seismic interfaces in the central metasedimentary belt the top of the lower crust is delineated by an eastward dipping interface at 24 28 km depth in the central granulite terrane prominent en echelon reflections referred to as the tahawus complex form a gently arched dome at 17 22 km depth interpretation of the tahawus complex as a zone of layered mafic cumulates is supported by its high velocity 7 1 km s and poisson s ratio 0 27 0 02 the lower crust is characterized by a velocity of 7 0 7 2 km s and an anomalously high poisson s ratio of 0 30 0 02 which are representative of pyroxene garnet granulites in contrast velocities of 6 8 7 0 km s are modeled beneath the central granulite terrane and appear to signify a lateral change in composition the moho lies at 44 45 km depth and is characterized by pronounced en echelon reflection segments suggesting compositional interlayering around the crust mantle boundary the velocity of the upper mantle is 8 0 8 2 km s an anomalous upper mantle layer with a reversed velocity of 8 6 km s dips eastward from 50 to 60 km depth beneath the southeastern grenville province our results indicate that remnants of magmatic intrusions that mobilized and thickened the crust during the grenvillian orogeny are preserved in the mid lower crust as a layered cumulate body tahawus complex and in the upper mantle as an eclogitic lens possibly delaminated from the overthickened crust during uplift of the southeastern grenville province

proterozoic felsic volcanism in the pranhita godavari valley india its implication on the origin of the basin abstract the proterozoic sedimentary rocks in the pranhita godavari valley pg valley south india were deposited in diverse environments ranging from continental shallow shelf to deep basin and occur as several unconformity bound sequences the sedimentary rocks also include felsic pyroclastics in different formations and at different stratigraphic levels repeated occurrences of intrabasinal pyroclastic rocks of similar composition suggest that the detritus might have been erupted from a common chamber which was activated at different times throughout the basin filling episode it is postulated that the mature proterozoic crust of indian craton acted as a heat lens resulting in extensive partial melting at the lower crustal levels localized stretching and rifting of the craton leading to the formation of the intracratonic basin the sedimentological attributes also support extensional setting and the hypothesis may be extended for several meso neoproterozoic cratonic basins in the indian shield the mechanism operated over the time span of 1 6 0 7 ga and overlaps with the age of development of an extensive granite rhyolite province in north america the process of accumulation of heat at the lower crustal levels generating a heat lens is typical of the stationary phase of supercontinental cycles and the origin of the cratonic basins the origin of the pg valley basin may be related to the fragmentation of a supercontinent of which the indian craton was a part

large variation of curie depth and lithospheric thickness beneath the indian subcontinent and a case for magnetothermometry summary a first order estimate of curie depth and lithospheric thickness variation beneath the indian region has been obtained using the available geothermic data the curie depth is found to vary from 18 to 68 km and the lithospheric thickness from 38 to 186 km these estimates conform well with the mags at and seismological findings both the parameters exhibit covariancy and suggest the possibility of approximating heat flow values and lithospheric thickness in geothermally uncharted areas using curie point geotherm depths derived from air satellite borne magnetic anomalies interestingly the areas with shallow curie depth and lithospheric thickness are found to be associated with a thin crust higher heat flow higher geothermal gradients and positive gravity anomalies and vice versa large thickness estimate of magnetic crustal layer extending below the moho demands a new conceptual understanding the study has brought out a good case for wider use of magneto thermometry and provides understanding of the geodynamic evolution of the indian subcontinent

the purana basins of southern cratonic province of india a case for mesoproterozoic fossil rifts abstract since its cratonization in the palaeoproterozoic southern peninsular india witnessed the development of a number of large intracratonic sedimentary basins traditionally referred to as purana basins spanning in age from late palaeoproterozoic through neoproterozoic the localization of these intracratonic basins namely pranhita godavari pg chattisgarh and cuddapah basins is apparently controlled by pre existing sutures and or weak zones the pg basin and the cuddapah basin host unconformity bound thick sediment dominated successions attesting to several cycles of fluvial shallow marine to shelf slope basin sedimentation deposition was punctuated by block uplifts resulting in local hiatuses and or volcanic upheavals leading to intercalation of thin but persistent basaltic flows and acid tuffs and ignimbrites basin margin deep faults apparently played a role in the facies distribution in these basins based on these features we propose that these basins initiated as continental rifts which however never opened up into a full fledged ocean basin but links with open seaway are evident from frequent occurrence of deposits representing tidal and storm influence particularly in upper part of the chattisgarh succession spatial distribution of facies and sediment thickness in the cuddapah and pg basins suggest that an open seaway existed to the east of the south indian cratonic province during the mesoproterozoic while similar criteria point to the existence of an open seaway north of the chattisgarh basin development history including nature of inversion suggest that the southern cratonic province of india existed as a single large continental mass since the mesoproterozoic in spite of episodes of supercontinent build up and fragmentation involving india and east gondwana during the proterozoic

moderate pressure metamorphism and anatexis due to anorthosite intrusion western adirondack highlands new york garnet sillimanite biotite gneiss near port leyden in the western adirondack highlands new york contains mineral assemblages and textures that formed during high temperature metamorphism and anatexis at mid crustal pressures evidence for melting includes thin plagioclase rich veins sieve textures in biotite and the presence of small euhedral garnet neoblasts hercynite silicate equilibria in combination with the solidus for biotite dehydration melting indicate metamorphic pressure was between 4 and 6 4 kbar at the temperature of melting ca 735 c the gneiss is intruded by a small discordant fe ti oxide apatite nelsonite dike reported field occurrences of nelsonite demonstrate its common association with anorthosite plutons although no anorthosite bodies are exposed in the port leyden region the presence of nelsonite is evidence of anorthositic magmatism in the western adirondacks post intrusion metamorphism has caused partial apatite recrystallization and produced a weak foliated texture in the dike u pb ages from zircon and monazite from both the gneiss and the nelsonite dike indicate that these rocks experienced a complex polymetamorphic history that we interpret to reflect two thermal episodes an older event is recorded by discordant zircons in the gneiss which indicate a minimum age of 1129 6 ma a linear best fit to the data yields an upper intercept at 1166 53 ma this range of ages coincides with anorthosite suite magmatism in the adirondacks a minimum zircon age of 1104 3 ma was obtained from the nelsonite dike lead loss or late zircon crystallization at about 1020 ma affected the u pb systematics of zircon in the dike monazite ages from both rocks also indicate high temperature metamorphism 700 c between 1040 and 960 ma the older zircon ages and textural relations in the metapelite are viewed as evidence for anatexis at ca 1150 ma and the presence of nelsonite suggests that the intrusion of anorthosite was coincident with partial melting in the gneiss p t estimates of metamorphism therefore imply that anorthosite was emplaced to about 15 km depth in the western adirondack highlands

zircon u pb geochronology and nd pb isotope geochemistry of blue ridge basement in the eastern great smoky mountains u s a implications for the proterozoic tectonic evolution of the southeastern laurentian margin the mesoproterozoic to paleozoic history of the southeastern laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events the great smoky mountains basement complex gsmbc within the southern appalachian blue ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of laurentian margin evolution from ca 1 9 ga to 450 ma the gsmbc consists primarily of 1 1 34 to 1 31 ga pre elzevirian granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in appalachian grenville massifs correlated with pre elzevirian crustal components in the adirondack green mountains new jersey highlands and french broad massifs 2 ca l 15 to 1 05 ga augen and granitic orthogneiss produced during shawinigan and ottawan phases of grenville age magmatism and metamorphism respectively and 3 paragneiss derived from protoliths with either grenville age 1 1 1 0 ga or post grenville neoproterozoic depositional ages based on presence absence of ca 1 0 ga metamorphic zircon and 1 9 to 1 1 ga detrital zircon all lithologies experienced taconian metamorphism and variable migmatization pre ottawan paragneiss exhibits major detrital zircon ages modes at 2 0 to 1 6 and 1 4 to 1 3 ga that require a component of older proterozoic crust in the sediment source region detrital zircon grains in post ottawan paragneiss exhibits the full spectrum of grenville age modes that correlate with the five phases of grenville magmatic metamorphic events in eastern laurentia these paragneiss samples also contain scattered 750 to 600 ma detrital zircon grains that constrain their maximum depositional age to late neoproterozoic the sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all grenville crustal age components during post orogenic exhumation and crustal extension leading up to late neoproterozoic breakup of rodinia most zircon u pb age systematics exhibit variable discordance that can be attributed to disturbance of the u pb system and or new zircon growth during either high grade ottawan ca 1 04 ga or taconian ca 0 46 ga regional metamorphism and migmatization neodymium tdm model ages for granodioritic orthogneiss and paragneiss range from 1 8 to 1 6 ga indicating that the rocks were derived from recycling of proterozoic crust that is they are not juvenile consistent with the 1 9 to 1 6 ga detrital zircon grains in pre ottawan paragneiss and with 1 8 to 1 7 ga inherited zircon in some 1 33 ga orthogneisses and a 1 35 ga xenolith whole rock pb isotope compositions of gsmbc rocks overlap the field of compositions characteristic of amazonian crust and of other basement rocks from the south central appalachians the pb isotopes and geochronology in orthogneiss mafic xenoliths and pre ottawan paragneiss are consistent with a correlation of the gsmbc with the mars hill terrane within the french broad massif and with the greater grenvillian south central appalachian basement scab that is considered exotic to laurentia and transferred during rodinian collision prior to ca 1 2 ga similarities in protolith ages and pb isotopes point to the paragua terrane of amazonia southwestern brazil and eastern bolivia as a likely match for scab initial amazonia laurentia collision occurred at ca 1 35 to 1 32 ga with final transfer of scab to laurentia occurring after 1 20 ga within the sinistral oblique collision zone between laurentia and amazonia defined by previous workers

preferential rifting of continents a source of displaced terranes lithospheric rifting while prevalent in the continents rarely occurs in oceanic regions to explain this preferential rifting of continents we compare the total strength of different lithospheres by integrating the limits of lithospheric stress with depth comparisons of total strength indicate that continental lithosphere is weaker than oceanic lithosphere by about a factor of 3 also a thickened crust can halve the total strength of normal continental lithosphere because the weakest area acts as a stress guide any rifting close to an ocean continent boundary would prefer a continental pathway this results in the formation of small continental fragments or microplates that once accreted back to a continent during subduction are seen as displaced terranes in addition the large crustal thicknesses associated with suture zones would ironically make such areas likely locations for future rifting episodes this results in the tendency described as the wilson cycle of new oceans to open along the suture where a former ocean had closed

the phillips pluton maine usa evidence of heterogeneous crustal sources and implications for granite ascent and emplacement mechanisms in convergent orogens the phillips pluton age of 403 8 1 3 ma was assembled at a crustal level below the contemporary brittle plastic transition during regional dextral reverse transpressive deformation the pluton is composed dominantly of medium to coarse grained leucogranite sensu lato s l but within its bounds includes decametric massive outcrop of fine to medium grained granodiorite s l in places the leucogranite contains centimetric enclaves apparently of the granodiorite granodiorite is host to more biotite than muscovite and more calcic oscillatory zoned plagioclase compared to the leucogranite pegmatitic granite and composite pegmatite aplite occur as metric sheets within the pluton and as larger bodies outside the pluton to the sw magmatic fabrics defined by biotite schlieren occur locally in the leucogranite the attitude of these fabrics and layering within the leucogranite are concordant with the ne striking steeply dipping country rock foliation k2o contents rb sr ratios rb sr and ba covariations and chondrite normalized rare earth element ree patterns of leucogranite are consistent with high to moderate a h2o muscovite dehydration equilibrium eutectic melting of a predominantly pelite source similar to metasedimentary rocks of the surrounding central maine belt cmb the ree patterns and rb sr ratios of granodiorite also suggest derivation from a metasedimentary source but more likely by moderate to low a h2o muscovite biotite dehydration equilibrium eutectic to non eutectic minimum melting of a protolith dominated by greywacke in which garnet and plagioclase were residual phases both granite s l types have heterogeneous initial nd isotope compositions samples of granodiorite define a range in end 404 ma of 1 8 to 0 1 0 3 2 uncertainty and samples of leucogranite define a range in end 404 ma of 8 0 to 5 3 0 3 2 uncertainty this bimodal distribution suggests that melts were derived from a minimum of two sources the data are consistent with these sources being cmb metasedimentary rocks end 404 ma 4 for the granodiorite the range of nd isotope compositions within each granite type most likely reflects isotopic heterogeneity inherited from the source these data imply that the integrity of individual melt batches was maintained during ascent and that extensive mixing of melt batches during emplacement at this level in the pluton did not occur although centimetric enclaves have intermediate nd isotope compositions consistent with small scale interactions between magmas we infer that the phillips pluton represents the root of a larger pluton and that what remains of this larger pluton is the feeder constructed from multiple melt batches arrested during waning flow of granite magma through a crustal scale shear zone system

tectonic evolution of the grenville orogen in the central appalachians abstract remnants of the grenville orogeny a globally pervasive tectonothermal event resulting from the amalgamation of the supercontinent rodinia between ca 1 3 and 1 0 ga are exposed throughout eastern north america for ca 3000 km from labrador to alabama although tectonic models for the development of the grenville have historically been based on observations from the large and well exposed canadian grenville province recent investigations suggest that initial continental collision occurred proximal to the appalachian blue ridge mesoproterozoic rocks within this portion of the orogen are exposed within a series of allochthonous terranes that were extensively reworked during paleozoic orogenesis to better understand the tectonic history of this critical region we utilized garnet and monazite geochronology to reconstruct the metamorphic history of eleven samples of ortho and paragneiss from the appalachian blue ridge province the resultant dates together with new results from trace element thermobarometry define a prolonged record of high temperature metamorphism coincident with the timing of continent continent collision as recognized in the northern portion of the orogen initial garnet and monazite growth commenced between 1100 and 1065 ma with peak granulite facies conditions 750 900 c 0 55 0 85 gpa occurring from 1050 to 1010 ma burial and heating of the paragneisses progressed rapidly with peak metamorphism occurring 20 30 myr after deposition of the sedimentary protoliths local hydrothermal alteration followed peak metamorphism and occurred at 1000 990 ma with later cooling through 660 c between 980 and 935 ma 3 2 c myr the metamorphic history outlined here together with insights from previous studies is consistent with formation of the blue ridge rocks within a continental suture zone that had an orogenic architecture comparable to that of the himalayan tibetan orogen

the dharwar craton and the assembly of peninsular india peninsular india can be separated into five discrete crustal areas the bhandara singhbhum and aravalli bundelkhand cratons the eastern ghats and a block in southern india containing the dharwar craton and adjoining granulite terrain these areas are separated by four major joins eastern ghats front godavari rift mahanadi rift sukinda thrust and narmada son lineament and associated thrusts all joins except the godavari rift are former orogenic belts where thrusts juxtapose rocks of different metamorphic grade these thrusts and the absence of clear correlation of suites among different cratons suggest that the indian shield formed by accretion of separate continental fragments epicontinental late archean early pro terozoic sediments throughout india however may indicate that the joins are intracontinental formed within a coherent block the term dravidian shield is used for the southernmost of the five crustal blocks including all of india south of the deccan basalts and godavari rift

age and regional relationships of granitoid rocks of the adirondack highlands approximately 55 of the adirondack highlands is underlain by granitic to tonalitic rocks that can be subdivided into five groups based on age composition and field occurrence 1 early calcalkaline tonalites ca 1300 ma and associated granitic rocks ca 1250 ma of the southern and eastern highlands 2 older anorogenic plutonic rocks 1145 1160 ma that occur throughout the highlands 3 younger anorogenic plutonic rocks ca 1130 ma that envelop anorthosite massifs in the central highlands 4 younger granitic rocks 1095 1105 ma of the northwest highlands and 5 late leucogranitic rocks 1060 1080 ma that occur throughout the northern highlands the data presented here are in good agreement with zircon ages from the green mountains of vermont as well as from southern ontario and quebec and suggest similar geologic histories for these regions these results document a protracted and complicated evolution for this portion of the grenville province

radioactivity heat flow and rifting of the indian continental crust the relationship between heat flow q and heat production a in india is quite different for the northern and southern parts of the peninsula in the north locations in a variety of tectonic settings including gondwana rifts show a linear relationship between q and a with a common reduced heat flow q about 40 mwm 2 in southern india the heat production of surface rocks is greater than is expected for the measured heat flow relative to other continental shields g is high in northern india and about average in the south apparently the indian crust evolved with a content of radioactive elements higher than in most shields and the southern part of the peninsula has undergone regional depletion in the lower crust and possibly upper mantle this depletion may coincide with a regional granulite event about 2 500 m y ago the high heat flow and heat production in northern india may explain several unique characteristics of the indian peninsula one feature is the continued reactivation of rif

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

u pb zircon geochronology of the adirondack mountains and implications for their geologic evolution u pb zircon studies of metamorphosed igneous rocks in the adirondack mountains have yielded preliminary ages within the range 1420 990 ma several geochronologically and geochemically distinct episodes of igneous intrusion and at least one pre granulite facies dynamothermal metamorphic event are documented this information is consistent with recent field and geochronological studies throughout the grenville province and suggests that a complex sequence of events occurred in the adirondack mountains prior to the widespread deformation and metamorphism commonly attributed to the approximately 1100 1000 ma ottawan phase of the grenvillian orogenic cycle