geochemical heterogeneities of metagraywackes from the sandur schist belt implications for active plate margin processes abstract metagraywackes found at four stratigraphic levels in different blocks of the sandur schist belt represent unmixed sediments exhibiting great diversity in petrography mineralogy major trace and rare earth element ree geochemistry most of these rocks have a low q f c ratio and compared to aucc average upper continental crust are depleted in incompatible elements like rb ba th u nb ta k na ca ce and sr and enriched in compatible elements such as mg cr ni and sc they are enriched in fe2o3 upto 25 and their mg k ratio varies from la sc th sc th yb and co th ratios along with several diagnostic geochemical twins such as u th hf zr and nb ta indicate that upper continental crust has contributed marginally to the composition of most of these graywackes in each block except one two types of ree patterns are found type i patterns have moderate ree la n yb n 3 and no significant eu anomalies on the other hand type ii patterns show lree enrichment resulting in significant increase in ree and la n yb n between 5 and 15 type ii patterns are common in the deogiri block type i patterns are similar to those found in modern deep sea forearc basins significant negative eu anomalies are found in a few cases of type ii graywackes where recycling of sediments such as micaceous quartzite is evident la th sc and co th th sc and la sc ratios show that most of these graywackes were supplied from a source that was mainly basic volcanic rocks banded iron formations and cherts however significant compositional variations are found from block to block and within the same block these are attributed to the proportional differences in their local sources the overall composition indicates that the graywackes of different blocks have been deposited in various environments of oceanic subduction complex and have not sampled much of the upper continental crust the compositional structural and metamorphic discontinuities observed in the belt are best explained by a small plate tectonic model involving accretion of diverse blocks to a pericontinental passive margin succession it is argued that even in the archaean volcanosedimentary processes at young undifferentiated arcs involving ocean floor ridge basalts aorb have generated turbidites which in many geochemical features resemble those found in modern forearc basins

white mica crystallinity finite strain and cleavage development across a large variscan structure nw spain x ray diffraction analyses were performed on white micas formed under strain and low grade metamorphic conditions from sandstones and siltstones along a transect through the narcea antiform a large structure that separates the external from the internal zones of the variscan belt of nw spain the results indicate a close spatial relationship between the onset of metamorphism and the cleavage front at the cleavage front strain is characterized by oblate ellipsoids that have x y z ratios from 1 to 1 5 the x y z ratio increases westward up to 3 0 as does the grade of metamorphism and is accompanied by an increase in the 9crystallinity9 index of the white mica from very low to low metamorphic grade this increase corresponds qualitatively to an increase in penetrativeness of the axial planar cleavage in the western part of the narcea antiform the major deformation event was the development of kilometre scale reverse shear zones because of polyphase deformation these rocks are not suitable for quantitative strain analysis but qualitatively finite strain can be observed to be prolate and invariably larger in this region than in the eastern sector the metamorphic grade is higher in the western part of the narcea antiform with local maxima occurring next to the base of the shear zones the b 0 cell parameter a semi quantitative geobarometer measured in the white mica is chaotic in the eastern sector showing a clear detrital inheritance in the western sector it is rational due to complete re equilibration during thermal resetting the inverse correlation between finite strain and white mica 9crystallinity9 is close showing that the anchizone epizone limit can be related to the increase of finite strain above values of r s 1 5 the close relation between white mica 9crystallinity9 and strain is suitable for tracking strain variations in extensive fine grained siliciclastic rocks where no appropiate strain markers are found

archean paleoproterozoic transition the indian perspective abstract india has perhaps the richest paleoarchean to paleoproterozoic crustal components on earth miller et al 2018 notwithstanding this much of the indian subcontinent remains unexplored compared to other ancient cratons in north america australia brazil and africa there are five archean cratonic nuclei in india the aravalli bundelkhand singhbhum bastar and the dharwar with well preserved proterozoic supracrustal sequences this paper critically reviews the paleoarchean to paleoproterozoic supracrustal record of the indian subcontinent with special emphasis on the archean paleoproterozoic transition which is generally placed at 2 5 ga based on the emplacement age of the great dyke of zimbabwe in general the archean paleoproterozoic transition in indian cratonic blocks is associated with high continental freeboard condition unlike north america south africa and western australia the indian cratonic blocks except bastar are devoid of paleoproterozoic glacial diamictite mazumder et al 2015 however similar to the pongola supergroup of south africa there are evidences of neoarchean glaciation in dharwar ojakangas et al 2014 in contrast to the global scenario of extensive development of bif across the archean paleoproterozoic transition the indian paleoproterozoic successions are devoid of banded iron formation bif except the 1 85 ga old bif in bundelkhand in contrast the majority of the indian bifs are of paleoarchean or neoarchean age thus the geodynamic and supracrustal data collected from five indian archean cratonic blocks do not correspond uniquely and precisely to a global change in tectonic style onset of neoarchean global magmatism and rifting or lithostratigraphic and environmental changes suggested across the archean paleoproterozoic transition van kranendonk et al 2012 therefore a revision and redefinition of the archean proterozoic boundary is outstanding this article is part of a special issue entitled archean earth processes edited by rajat mazumder and patrick eriksson

mid archaean evolution of the eastern indian craton geochemical and isotopic evidence from the bonai pluton abstract the archaean eastern indian craton consists of several granitoid batholiths intruding greenstone like sequences the bonai pluton one such body in the central sector of the craton consists of a dominant porphyritic and a subordinate equigranular variety ranging in composition from trondhjemite to granite xenoliths of massive trondhjemite migmatite and quartzite occur in both phases the granitoid xenoliths represent remnants of a sialic crust and are the oldest recognizable element in the bonai pluton these are high al 2 o 3 trondhjemites with a highly fractionated rare earth element ree pattern a pb pb isochron age of 3369 57 ma has been obtained from a few spatially related xenoliths the porphyritic and equigranular granitoids show similar petrographic characteristics and most of the samples show a calc alkaline trondhjemitic association a few granite samples however belong to a normal calc alkaline association the trondhjemitic samples are of the low al 2 o 3 type with fractionated light ree lree flat heavy ree hree and a marked negative eu anomaly granites of the calc alkaline association have an identical ree pattern pb isotopic data for all samples together yield a pb pb isochron age of 3163 126 ma the ree characteristics of the two trondhjemite suites can be generated by partial melting of a mafic source with variable residual phases at different depths on the other hand the normal calc alkaline granite may be generated by partial melting of trondhjemite similar in composition to that represented by the 3369 ma xenoliths isotopic ages and the chemical nature of the bonai pluton are similar to those identified in the adjacent singhbhum granite complex it is suggested that the entire eastern indian craton has a common mid archaean history the earliest sialic crust formed at 3400 ma and was followed by trondhjemitic addition at 3300 ma during this phase restricted reworking of the older sialic crust produced calc alkaline granites

conodont color and textural alteration an index to regional metamorphism contact metamorphism and hydrothermal alteration experimental and field data are used to extend the utility of conodonts as semi quantitative thermal indices into the regimes of regional and contact metamorphism as well as hydrothermal alteration an arrhenius plot of data from induced conodont color alteration by pyrolysis in air at 1 atm was used to generate the geologic temperatures for conodont color alteration indices cai above 300 c that is for cai values of 5 through 8 such cai values occur in very low to medium grade regionally metamorphosed contact metamorphosed and hydrothermally altered rocks the uniformity or variability of cai values within a sample together with conodont texture can help to distinguish grades and environments of metamorphism particularly in metacarbonate sequences induced cai by pyrolysis in a water methane mixture at kbar results in retardation of cai and in a disparate mixture of both low and high cai values within each experimental sample in this system color alteration processes above a cai of 2 to 3 seem to change from predominantly carbonization to predominantly loss of organic matter presumably by oxidation and volatilization of oxides these experiments approximate the type of cai mixture characteristically found in conodonts recovered from hydrothermally altered rocks these data indicate that cai values of 6 to 8 cannot be used to assess precise temperatures of hydrothermally altered rocks but may serve as useful indicators of potential mineralization

tectonothermal analysis of a major unit of the cantabrian zone the ponga unit variscan belt nw spain the paleothermometric evolution of the ponga unit at the core of the cantabrian zone the arcuate foreland fold and thrust belt of the variscan orogen in nw spain is analyzed using the conodont color alteration index and the kubler index of illite the results indicate that the distribution of the thermal maturity patterns is independent of the stratigraphy and the structure of the unit these include the occurrence of dominant diagenetic values and very low to low grade metamorphism in the southern area the main thermal event is interpreted to be related to hydrothermal fluids ascending through late variscan extensional faults the anomalous high heat flow in the southern sector of the ponga unit is part of a previously identified large band in the cantabrian zone where the common occurrence of low grade metamorphic rocks hydrothermal dolostones and ore mineralizations is related to an extensional orogenic episode that started near the carboniferous permian boundary soon after the closure of the asturian arc

geochemistry of lower vindhyan clastic sedimentary rocks of northwestern indian shield implications for composition and weathering history of proterozoic continental crust abstract the western margin of the 1600 ma vindhyan basin of the indian shield is marked by the presence of lower vindhyan group lvg comprising shales and sandstone with minor limestone conglomerate and mafic volcanic flows at the base major and trace element compositions of sandstones and shales of the lvg have been investigated to infer the nature of provenance weathering processes and tectonic setting provenance analysis using trace elements and petrography suggests that the archaean banded gneissic complex bgc of rajasthan is most likely the source of these clastic rocks the shale and sandstone comprising lower part of lvg namely khardeola formation are distinctive in having high lile such as th rb and k 2 o high ferromagnesian trace elements and also higher values of k 2 o na 2 o rb sr and la yb n ratios in comparison to succeeding sedimentary units comprising upper formations of the lvg the significantly different geochemical characteristics of khardeola formation suggest change in provenance with sediments in the lower formation being derived from proximal source and those of upper formations from distal source based on ree patterns and eu eu values the clastic sediments of khardeola formation can be modeled by a mixture of 60 berarch granite and 40 mafic volcanic rocks of hindoli belt on the other hand the upper formations are best modeled with a mixture of 60 bgc granitic gneisses 20 bgc mafic enclaves and 20 berach granite cia chemical index of alteration values and a cn k plot suggest that the source area experienced moderate to high degree of chemical weathering under warm and humid conditions the shift of sediment source from local in the beginning to distal during later period suggests sedimentation in a tectonically active basin

geothermal palaeogradients and metamorphic zonation from the conodont colour alteration index cai the conodont colour alteration index cal is potentially does not exist there are areas where cal and ki values are valuable in thermal history investigations but there are notably consistent for the anchizone boundaries however problems in its use an approach is proposed herein to anomalous correlations between cal and ki values have also overcome the problem of establishing reliable geothermal been found in some areas a cal anchizone or ancaizone is palaeogradients in areas with burial metamorphism based on defined in order to enable cal values to be used independently knowledge of the stratigraphic succession thickness the to establish a metamorphic zonation boundaries of chronostratigraphic units and the age and duration of deformation episodes although also problematical terra nova 13 79 83 2001 that a correlation between cal and kubler index ki values

geochemistry of the mesoproterozoic lakhanda shales in southeastern yakutia russia implications for mineralogical and provenance control and recycling abstract shales of the lakhanda group of late mezoproterozoic age 1050 1000 ma from the southeastern siberian craton in russia have been analyzed for major elements and a number of trace elements including the ree s shales along the maya river formed as platform sediments in a deeper shelf facies whereas shales along the belaya river formed in more active and open environments of an upper shelf carbonate ramp the log of most elemental compositions to al2o3 ratios are the same in the maya and belaya river samples suggesting a similar source rock composition for rocks in the two areas the log of sio2 mgo na2o k2o rb ba and ni to al2o3 ratios are significantly higher and the log of tio2 to al2o3 ratios are significantly lower in shales from the belaya river than the maya river sections the cia chemical index of alteration is thus significantly lower in the belaya shales than the maya shales suggesting less weathering of the in the belaya shales than the maya shales the icvs index of compositional variability fe2o3 k2o na2o cao mgo tio2 al2o3 of the lakhanda shales are less than 1 suggesting that they are compositionally mature and were likely dominated by recycling several samples have icv 1 suggesting some first cycle input the low k2o al2o3 ratios of these shales suggest that minimal first cycle alkali feldspar was present in the initial source most shales of the lakhanda plot parallel and along the a k line in a cn k plots suggestive of intense chemical weathering high cia and do not indicate any clear cut evidence of k metasomatism or direct weathering back to the original source if k metasomatism produced these rocks then they could have formed from tonalites to basalts if weathering produced these rocks then they could have been produced from varied amounts of mostly granodiorite to granite elemental ratios critical of provenance la sc la cr la co th sc th cr th co and eu eu are not significantly different between the maya river and belaya river shales and the ratios are similar to fine fractions derived from the weathering of mostly granitoids and not basic rocks the eu eu th sc and low k2o al2o3 ratios of most shales suggest weathering from mostly a granodiorite source rather than a granite source consistent with a source from old upper continental crust some samples at the bottom of the belaya river section contain very low eu eu 0 35 suggesting significant input of first cycle detritus from highly differentiated granitoids similar to those from the aldan shield

the flin flon paleosol and the composition of the atmosphere 1 8 bybp within the 1800 to 1900 my old flin flon snow lake greenstone belt amisk group volcanics are overlain by missi group fluvial sediments several localities along the missi amisk contact the volcanics show evidence of subaerial weathering field relationships mineralogical evidence and chemical analyses confirm that this alteration zone is a paleosol pedogenic fabrics and mineralogy were somewhat obscured by greenschist grade metamorphism associated with the hudsonian orogeny 1750 my this is especially true in the upper meter of the paleosol where metamorphic paragonite and sericitic micas developed in a crenulated fabric this metamorphism did not however obliterate the imprint of weathering on the amisk volcanics features characteristic of well drained modern soils are evident in the paleosol corestones of spheroidally weathered pillow lavas occur at depth within the paleosol cr horizon the corestones decrease in size upward and eventually disappear into a hematite rich horizon at the top of the paleosol these macroscopic changes are accompanied by a decrease in cao and mgo and by an increase in al2o3 tio2 and total iron toward the paleosol missi contact ferrous iron decreases upward toward the contact feo was apparently oxidized to ferric iron and retained within the paleosol during weathering the oxidation and retention of iron within the flin flon paleosol indicates that po2 was probably or 10 2 p a l at the time of weathering the behavior of iron in the flin flon paleosol contrasts sharply with its behavior in the 2200 my hekpoort paleosol which is strongly depleted in iron this difference suggests that a significant increase in the ratio of po2 pco2 in the atmosphere took place between 2200 and 1800 mybp

deformation in neogene sediments of the sorbas and vera basins se spain constraints on simple shear deformation and rigid body rotation along major strike slip faults abstract detailed structural analyses are presented of the neogene sorbas basin adjacent to the e w striking gafarillos fault zone and the vera basin adjacent to the 020 striking palomares fault zone in southeastern spain a stress regime with an e w oriented subhorizontal maximum principal stress 1 existed in pre tortonian 11 3 ma time a strike slip regime with nw se oriented compression during tortonian and earliest messinian time caused dextral displacement along the e w trending gafarillos fault of approximately 10 km structural analysis indicates that most displacement took place in the early tortonian deformational patterns within the adjacent pull apart basin reflect a dextral simple shear zone of at least 500 m width kinematical analysis of folds in the sorbas basin suggests however that rotational effects are largely caused by rigid body rotation without much internal deformation sinistral strike slip displacements occurred along the palomares fault zone under the influence of the same stress regime an abrupt change in the orientation of the stress field to n s directed compression in earliest messinian time 6 5 ma caused the termination of displacements along the gafarillos fault zone whereas the 020 trending palomares fault zone continued to accumulate sinistral strike slip displacements of about 25 km volcanism occurred along splays of the fault zone a wider shear zone of a few kilometers width evolved in which considerable anti clockwise rotation of folds occurred kinematic analysis of these folds shows that these rotational effects are again dominantly rigid body rotations assuming rotations are merely caused by simple shear deformation overestimates the amounts of strain a better way to deal with simple shear deformation is to compare observed shortening caused by folding with the magnitude of rotation of fold hinges

geochemistry of shales from the archean 3 0 ga buhwa greenstone belt zimbabwe implications for provenance and source area weathering abstract phyllites from the archean 3 0 ga buhwa greenstone belt zimbabwe were deposited on a stable cratonic platform analyses of the phyllites generally define a single geochemical group based on major and trace element abundances the phyllites are strongly depleted in cao na2o and sr with respect to average archean upper crust by contrast k2o ba and rb are enriched several times relative to average archean upper crust reflecting basin scale k metasomatism transition metals are somewhat depleted whereas high field strength elements are typically enriched relative to average archean upper crust samples generally have fractionated lree patterns average cen smn 2 4 with small negative eu anomalies average eu eu 0 78 and generally flat hrees average gdn ybn 1 3 such geochemical characteristics suggest that the source dominantly consisted of tonalite with less common occurrences mafic volcanic rocks and granite mixing calculations which massbalance the rees and th sc suggest 70 tonalite 15 mafic volcanic rocks and 15 granite as possible proportions for the source of the phyllites a nearby early archean 3 5 ga continental nucleus preserves similar lithologies and is the probable source intense chemical weathering of the source terrane is indicated by premetasomatized chemical index of alteration values of 95 100 nearly complete depletion of cao and na2o and high al2o3 na2o ratios fine grained sediments of comparable age and presumed tectonic setting elsewhere in southern africa show similar geochemical characteristics implying that source area compositions and weathering intensities were similar these mature platformal deposits suggest a tectonically stable environment where intense chemical weathering took place at 3 0 ga across southern africa

sm nd isotopic and geochemical study of the archean tonalite amphibolite association from the eastern indian craton we report the results of a sm nd isotopic major element and rare earth element ree study of the older metamorphic group omg tonalite amphibolite association of the eastern indian craton the older metamorphic tonalite gneisses omtg have been previously dated to be 3 8 ga using sm nd isotope systematies and 3 2 3 4 ga by rb sr and pb pb dating the results of this study indicate that the protoliths of the omg amphibolites are 3 3 ga isochron age 3 30 0 06 ga nd 0 9 0 7 and therefore the omtg which intrude into the associated amphibolites cannot be any older than 3 3 ga the amphibolites display light ree enrichment ce yb n 2 2 6 7 la 30 100 x chondrite and nearly flat heavy ree patterns tb lu n 1 2 1 9 the basaltic parents of the amphibolites were probably generated by the partial melting of a spinel lherzolite mantle strong linear relationships between the amphibolites and tonalites in 147sm 144nd 143nd 144nd space isochron age 3 29 0 04 ga nd 0 8 0 8 imply that they are genetically related the tonalites display fractionated ree patterns la 100 300 x chondrite with moderate heavy ree depletions tb lu n 1 9 3 4 the isotopic major element and ree data are consistent with the derivation of the omtg from partial melting of omg amphibolites or equivalent rocks at amphibolegarnet stabilization depths an initial nd t value of 0 9 0 7 for the amphibolites indicates the presence of a slightly depleted mantle source at 3 3 ga with 147sm 144nd between 0 20 and 0 22 it is suggested that the growth of continental crust in the eastern indian craton occurred in response to magmatic underplating in a plume setting

metamorphic fluids and transtension in the cantabrian mountains of northern spain an application of the conodont colour alteration index conodont colour alteration index cai values from upper paleozoic rocks in the cantabrian zone of northern spain show that temperatures during hercynian metamorphism locally exceeded 300 c various temperature domains have been defined which are generally separated by fundamental structures these domains do not correspond with the tripartite subdivision based on stratigraphic analysis the observed cai values of conodonts are in general agreement with the mineral paragenesis areas with high cai values display extensive alteration and mineralization and where cai values exceed 4 4 5 200 c slaty cleavage has developed the cantabrian zone is an area of very low grade metamorphism where peak conditions were reached in upper carboniferous to lower permian times the characteristics of the metamorphism and its spatial relationship with major faults suggest that fluids were the main source for regional heating and that fluid transport was focussed along crustal scale structural features the overall deformation regime in this part of the variscan orogen of western europe is interpreted to be large scale transtension this is in agreement with earlier proposed models for the formation of upper palaeozoic basins in this area

correlations between the eastern block of the north china craton and the south indian block of the indian shield an archaean to palaeoproterozoic link abstract recent tectonometamorphic data reveal that the north china craton formed by amalgamation of an eastern block eb and a western block wb along the trans north china orogen tnco and the indian shield formed by amalgamation of the south indian block sib and the north indian block nib along the central indian tectonic zone citz the eb of the north china craton and the sib of the indian shield share many similar magmatic and sedimentary features the early archaean supracrustal sequences in both the eb and sib consist of 3 6 3 4 ga fuchsite quartzites pelitic gneisses calc silicates marbles banded magnetic quartzites and amphibolites called the caozhuang group in the eb and the older metamorphic group omg in the sib the middle archaean assemblages in both the blocks can be divided into an early 3 4 3 3 ga phase represented by the chentaigou supracrustal rocks and orthogneisses in the eb and the holenarsipur supracrustal rocks and the gorur orthogneisses in the sib and a late 3 1 2 9 ga phase represented by the qianan supracrustal rocks and the yangyashan orthogneiss in the eb and the sargur group and the peninsular gneiss in sib the late archaean taishan group and its equivalents in the eb are stratigraphically similar to the dharwar supergroup in the sib moreover both the eb and sib are characterised by near contemporaneity of late archaean granitoid intrusive and metamorphic events in addition dome and basin structures are dominant in both the eb and sib and anticlockwise p t paths charaterise the metamorphic evolution of the late archaean granulites in the two blocks the palaeoproterozoic sedimentary volcanic rocks in both blocks comprise lower clastic rich middle volcanic rich and upper clastic carbonate sequences the strong resemblance of the archaean to palaeoproterozoic magmatic and sedimentary records suggests that it is possible that the eb and sib are dispersed remnants of what was once a single continent a possible fit is proposed for the reconstruction of the eb and sib where the northern margin of the eb is placed adjacent to the western margin of the sib with the tnco and wb representing the continuations respectively of the citz and nib this fit suggests that the eb sib and the wb nib represent two discrete continental blocks that joined along the tnco citz orogen during a palaeoproterozoic collisional event fragmentation of the eb and sib may have occurred at the beginning of the mesoproterozoic and by 1 4 ga the eb and sib may have separated into two different continental blocks as evidenced by striking differences in the mesoproterozoic sedimentary successions and associated bimodal volcanics in the two blocks

potassic and sodic metasomatism in the southern province of the canadian shield evidence from the paleoproterozoic serpent formation huronian supergroup canada abstract major element geochemistry petrographic observations and microprobe analyses of the paleoproterozoic serpent formation huronian supergroup canada suggest that these strata experienced both potassium and sodium metasomatism major element data plotted in al2o3 cao na2o k2o compositional space form an array nearly identical to data from sub huronian paleosols reported elsewhere suggesting that serpent fluvial systems captured materials from entire weathering profiles potassium metasomatism was concentrated in shales and represents the illitization of kaolinite which is a residual product of plagioclase weathering sodium metasomatism principally affected silt and sand sized particles microprobe analyses of plagioclase grains indicate that they are nearly pure albite ab96 corresponding whole rock major element data plot in bulk albite space on an al2o3 k2o cao na2o ternary diagram whereas basement rocks plot in bulk andesine and oligoclase space end member compositions and the absence of luminescence when bombarded with an electron beam indicate that the albites are of secondary origin extremely pure albite cements ab99 commonly fringe detrital grains there has been little replacement of detrital k feldspars despite the complete albitization of plagioclase grains the timing of metasomatism at least the last stage is constrained to the period between 1700 and 1750 ma based on geochronologic data reported elsewhere when the southern province of the canadian shield experienced considerable felsic plutonism possibly related to arc collision this plutonism may have resulted in the heating of deep basinal brines stresses associated with orogenic build up or gravity driven groundwater circulation working in concert with the plutonism expelled these brines through the serpent formation and the entire huronian supergroup thoroughly metasomatizing the region such processes associated with collisional tectonics are akin to the development of mississippi valley type pb zn mineralization in the mid continent of the united states

archaean proterozoic transition geochemistry provenance and tectonic setting of metasedimentary rocks in central fennoscandian shield finland the central part of the fennoscandian shield in finland is composed of the palaeoproterozoic svecofennian domain and the archaean karelian craton with a palaeoproterozoic allochthonous and autochthonous cover a cryptic suture separating these areas and another tentative suture dividing the svecofennian into central and southern parts have been proposed the chemical composition of sedimentary rocks n 300 within the study area including the effects of palaeoweathering hydraulic sorting depositional environment and post depositional processes have been studied in order to delineate sediment source components the main proposed source components for the archaean sedimentary rocks are weathered 3 0 3 2 ga greenstone granite ttg and local 2 7 ga sources autochthonous 2 2 1 9 ga cover rocks were mainly derived from a mixture of chemically weathered palaeosol 2 2 2 35 ga sedimentary rocks derived from the palaeosol and mafic dykes and plateau volcanics mainly 2 2 2 1 ga although in places locally derived non weathered archaean sources dominated archaean crust and 2 0 1 92 low k bimodal rocks from a primitive island arc are the proposed source for the allochthonous western kaleva cover rocks these formed in a subsiding foredeep during initial collision from orogenic detritus in the same oblique collision zone the central svecofennian sedimentary rocks can be divided into local arc derived rocks 1 89 ga and older 1 91 ga rocks from a mixture of western kaleva sources and a 1 91 2 0 ga mature island arc active continental margin source rifting followed by increased subsidence during initial collision in the ne and subsequent arc reversal caused rapid erosion from the mountain belt exposing diverse source compositions as seen in the large variation of th sc 2 0 5 and deposition into an oblique hinterland basin further developing into a subduction related foredeep mature greywackes from the southern svecofennian in the study area resemble passive margin sediments with a source dominated by inferred alkaline affinity complexes and archaean rocks less mature rocks also occur and had sources dominated either by island arc active continental margin rocks or local picritic rocks in the sedimentary record the archaean proterozoic transition up to 2 1 ga was dominated by input of mainly mafic plateau type volcanic contribution to the archaean detritus palaeoproterozoic sediments having a crustal component 2 1 ga show higher th sc th cr and lower sm nd and eu eu relative to the archaean rocks but locally low th cr ratios complicate the situation ba depletion relative to k rb and th is a characteristic feature of the sedimentary rocks of the central fennoscandian shield indicating high amounts of ba lost from the clastic record during 2 3 1 9 ga and further recycled back to the mantle forming a subduction component and an enriched mantle component ba depletion seems to have been especially characteristic of chemical weathering during 2 35 2 2 ga under co2 rich and low o2 atmosphere whether this strong ba depletion is characteristic of the archaean proterozoic transition and quiet supercontinent stages in general remains to be determined

sedimentary rocks and crustal evolution tectonic setting and secular trends a significant change in the composition of turbidites deposited at active tectonic settings is seen at the archean post archean transition in comparison to post archean active margin turbidites archean greenstone turbidites exhibit more uniform th sc ratios but with a similar average greater hree depletion gd n yb n 2 less eu depletion eu eu mostly 0 85 and an absence of low th u ratios th u 3 these data indicate that differing mantle sources and or conditions of mantle melting existed during the archean and that intracrustally differentiated rocks older cratons or young differentiated arcs were relatively unimportant as provenance components for archean turbidites in contrast some sedimentary rocks preserved in archean platformal sequences of high grade terranes display significant negative eu anomalies similar to the majority of post archean shales indicating that the general process of cratonization including intracrustal differentiation has taken place since at least 3

geochemistry of the 2 51 ga old rampur group pelites western himalayas implications for their provenance and weathering the late archean volcano sedimentary succession of the rampur group rg represents the oldest superacrustal rocks exposed in the himalayas the sedimentary component is pelite dominated in the lower part and siliciclastic with associated mafic volcanics dominated in the upper part indicating shallowing of the basin over time the bulk chemistry of the rg pelites shows illite chlorite control that gets diluted with increase in quartz content they have a general similarity with the post archean shales but a slightly higher average sio2 and lower al2o3 contents and severe depletion in cao na2o sr and ba relative to the north american shale composite ree patterns are highly fractionated with la yb n ratio ranging from 8 8 to 18 6 hree show slight fractionation average gd yb n b2 and all the samples have large negative eu anomalies average 0 61 la yb n decrease with increase in al2o3 na2o but lack any relationship with k2o thereby discounting clay mineral control hree have a positive correlation with tio2 th y and nb but not with p2o5 and zr the increase in hree seems likely due to the increased mobilization of hree enriched phase s during intense weathering high zr and th contents and th ni la sc and la yb n ratios as well as sc th sc relations favor a dominantly felsic source with only a minor contribution from mafic rocks the most likely source lithology compatible with the southerly paleocurrent direction and first cycle sediment textural features of the rg sediments appears to be the banded gneissic complex bgc of the northern indian shield the same conclusion is reached when rg data are plotted in al2o3 cao na2o k2o a cn k compositional space the data define a trend which indicates unweathered plagioclase k feldspar source composition similar to that of the bgc pre metasomatized high chemical index of weathering as well as the high plagioclase index of weathering for a majority of the samples conforms to the world wide warm moist climate prevalent during the time of their deposition 2001 elsevier science b v all rights reserved

geochemical and sm nd isotopic constraints on the petrogenesis and tectonic setting of the proterozoic mafic magmatism of the gwalior basin central india the influence of large igneous provinces on proterozoic crustal evolution

effects of chemical weathering and sorting on the petrogenesis of siliciclastic sediments with implications for provenance studies the bulk chemical compositions mineralogy and mineral proportions of sands and muds of the mallacoota basin in southeastern australia reflect the composition of weathering profiles mantling source rocks rather than bedrock muds contain abundant clay minerals that are virtually absent from the source rocks but abundant in the weathering profiles sands are strongly enriched in quartz relative to source rocks even in the headwaters of the fluvial system demonstrating that feldspar destruction occurs by in situ chemical weathering within profiles and before detritus enters the fluvial system k feldspar is proportionally more abundant in fluvial sands than in source rocks because plagioclase is more rapidly destroyed than either quartz or k feldspar in weathering profiles subsequent erosion and sorting produce sands enriched in quartz with high k feldspar plagioclase ratios relative to source rocks the composition of plagioclase incorporated into the fluvial sands is also controlled by chemical weat

geochemical evolution of the tertiary succession of the nw shelf bengal basin bangladesh implications for provenance paleoweathering and himalayan erosion abstract whole rock major trace and rare earth element ree compositions of paleogene to neogene sedimentary rocks of the nw shelf succession province 1 of bangladesh contain a record of interaction of the india and asian plates himalayan tectonism and climatic development analyses of 66 sandstones and mudrocks from the tertiary succession of bangladesh were made to examine provenance source weathering and the influence of paleoclimate and tectonism the sediments display linear geochemical trends due to quartz dilution and varying quartz clay ratios produced by hydrodynamic sorting chondrite normalized ree patterns for both sandstones and mudrocks from different groups are similar to upper continental crust with moderate to high lree enrichment lithotypes within formations average lan ybn 5 31 11 41 and marked negative eu anomalies eu eu 0 51 0 69 based on geochemical criteria the succession can be divided into three parts jaintia barail surma and dupi tila very high silica contents in jaintia tura sandstones and high chemical index of alteration cia indices in kopili mudrocks fe shales suggest derivation from a deeply weathered and stable cratonic source india the tura sandstones are interpreted as first cycle quartz arenites produced while the indian plate drifted across equatorial regions during the paleocene eocene thermal maximum petm the barail surma and dupi tila sediments were derived from a felsic orogen the himalaya the barail surma sediments were mainly derived from the trans himalayan batholith and associated granitoids with significant contribution from the lesser himalaya mafic input is also evident probably from intraoceanic arc material within the himalaya barail mudrocks have uniformly high cia values 92 95 suggesting intense steady state weathering of their proto himalayan source and warm and humid climate in contrast cia values of surma mudrocks range from 66 to 93 average 84 suggesting non steady state weathering related to active uplift in the himalaya the dupi tila sediments were derived from a more felsic lesser himalaya source with significant contribution from the trans himalaya and very little or no ophiolitic or arc material dupi tila mudrocks have cia ratios of 62 99 average 72 also indicating non steady state weathering in the rising himalayan source geochemical compositions of the nw shelf sediments are comparable to coeval successions in the surma basin province 2 of bangladesh and the siwaliks india indicating similar source evolution of the indian monsoon and associated high precipitation caused intense chemical weathering of the surma and dupi tila source despite rapid uplift the surma group thus bears the signature of evolution of the asian monsoon in the bengal basin at 21 ma simultaneous with the development of the east asian monsoon this supports proposals that both monsoon systems developed at the same time

depositional character of a dry climate alluvial fan system from palaeoproterozoic rift setting using facies architecture and palaeohydraulics example from the par formation gwalior group central india abstract the 20 m thick coarse grained clastic succession in the basal part of palaeoproterozoic par formation gwalior group has been investigated using process based sedimentology and deductive palaeohydraulics bounded between granitic basement at its base and shallow marine succession at the top the studied stratigraphic interval represents products of an alluvial fan and its strike wise co existent braided river system that possibly acted as a tributary for the fan detailed facies facies association analysis allowed identification of two anatomical parts for the fan system viz proximal and mid fan while thin proximal fan is represented by products of rock avalanche and hyperconcentrated flows with widely varying rheology the mid fan is represented by products of sheet floods and flows within streamlets the interpretation found support from palaeoslope estimation carried out on the fluvial part of the mid fan that plot dominantly within the alluvial fan field demarcated by blair and mcpherson 1994 dry climatic condition suggested from dominance of stream flow over mass flow deposition within the par alluvial fan strike wise the fan is discontinuous and juxtaposed with a braid plain system in contrast to the fluvial part of fan system the palaeoslope data from the braid plain system dominantly plot within the natural depositional gap defined by blair and mcpherson a raised palaeoslope for the river systems as suggested from proterozoic braid plain deposits around the globe is found valid for the par braid plain system as well from preponderance of granular and sandy sediments within the alluvial fan and braid plain systems and a pervasive north westward palaeocurrent pattern within the fluvial systems the present study infers a gently sloping bevelled source area in the south southeast of the basin with occurrence of steep cliffs only locally

207pb 206pb zircon ages and the evolution of the singhbhum craton eastern india an ion microprobe study1this paper is dedicated to prof a k saha one of the senior co authors who passed away after submission of the paper 1 abstract we used an ion microprobe to measure 207 pb 206 pb ages of single zircons from four key lithologies from the singhbhum craton of eastern india detrital zircons from the oldest recognized sedimentary sequence of the craton the older metamorphic group yielded 3 5 3 6 ga ages indicating that crustal formation had already been initiated by 3 6 ga in this region this data also established an older limit of 3 5 ga for the age of sediment deposition the first appearance of extensive continental crust in the singhbhum region marked by the emplacement of the older metamorphic tonalite gneisses is dated at 3 44 ga our data from both of these units reveal that a 3 2 ga event induced overgrowths within each of these pre existing zircon populations we determine an age of 3328 7 ma for the singhbhum granite phase ii the most widespread of this granite s three phases the emplacement of the two phases of the mayurbhanj granite marking the youngest major episode of acid plutonism in this craton took place in rapid succession at 3 1 ga marking the broad scale stabilization of this terrain

time constraints for the tectono thermal evolution of the cantabrian zone in nw spain by illite k ar dating abstract contrasting sedimentary facies and tectonic structures diversely distributed and affected by diagenetic to low grade metamorphic activity form a complex pattern in the northwestern spanish cantabrian area of the variscan orogen the distribution of these metamorphic features is based on field observations clay mineralogy mineral crystallinity indices vitrinite reflectance data and observation of rock microtextures k ar dating of fine grained illite rich size fractions provides a complete set of ages for the episodic deformation and thermal overprints in the southeastern cantabrian area the usual k ar isochron plotting was combined with the bench type age distribution to unravel the successive illite crystallization episodes several tectono thermal episodes occurred in the southeastern pisuerga carrion province initially during the early permian at 293 4 ma in the ruesga line valsurvio dome and sierra del brezo area a further mid permian event was detected at 268 6 ma in the palentine nappes and during the mid triassic at 243 5 ma again in the valsurvio and sierra del brezo areas and along the ruesga line a later reactivation of the ruesga line was detected during mid jurassic time at 175 6 ma

prediction of some weathering trends of plutonic and volcanic rocks based on thermodynamic and kinetic considerations abstract the exposed crust consists mainly of plagioclase 35 quartz 20 k feldspar 11 volcanic glass 12 biotite 8 and muscovite 5 quartz is a resistate thus feldspars and glass represent approximately 75 percent of the labile minerals the weathering characteristics of these constituents are summarized in the context of thermodynamic mass balance and kinetic considerations experimentally determined release rate constants were used to predict the proportions of ca na and k released by feldspars of plutonic rocks granites to gabbros to weathering solutions the chemical weathering trends of the weathered residues calculated from the kinetic data conform closely to the initial trends observed in some recent weathering profiles demonstrating the accuracy of the predictions since the weathering of feldspars is controlled by processes that should not change through geological time the relative release rates of ca na and k from the feldspars of granitic rocks can be calculated for future and past episodes of continental weathering experimentally determined release rate constants are not available for a wide range of volcanic glass compositions but the limited data indicate that compositional trends are predictable in weathering profiles developed on volcanic rocks the kinetic data available for rhyolitic glasses accurately predict the initial weathering trends observed in a recent rhyolite weathering profile

three dimensional geometry and interference of fault bend folds examples from the ponga unit variscan belt nw spain abstract examples of the three dimensional geometry and interference patterns between fault bend folds occur in the ponga unit of the variscan cantabrian foreland thrust and fold belt of nw spain the ponga unit consists of several e directed thrust sheets with a cumulative displacement of 93 km which were emplaced during upper westphalian stephanian time two major lateral ramps developed in the basal thrust of the ponga unit reaching 15 and 20 km in length respectively and some 3 km in height lateral structures are also common within the hangingwall thrust sheets the three dimensional staircase trajectories of thrust faults together with the tectonic superposition of thrust sheets has produced a complex interference pattern of fault related folds most map scale folds formed as fault bend folds related to interference between frontal and lateral ramps fold geometry and size varies depending on the geometry and size of the associated thrust ramp and flat the axial traces are grouped into lateral and frontal sets which are sub parallel or sub perpendicular to the tectonic transport direction respectively corner folds form with plunging axes at the intersections of frontal and lateral ramps and generally link pairs of frontal lateral folds they have relatively short axial traces that are oblique to the tectonic transport direction fold plunge varies due to the superposition of lateral and frontal structures of a thrust fault and also depends on the geometry of subsequent thrusts in the footwall the final geometry of frontal folds in the ponga unit are not only the result of the bending mechanism but have been modified by simple shear parallel to the emplacement direction major lateral structures have been reactivated as reverse faults and are locally overturned due to post emplacement north south compression

geology geochemistry and tectonic setting of archaean greywackes from karnataka nucleus india more than 80 of the area covered by the chitradurga and shimoga schist belts 3 0 2 6 ga of karnataka nucleus india is made up of greywackes feldspathic lithic and quartzose greywackes are deposited respectively along the western northern and the eastern margin of the chitradurga schist belt greywackes of the shimoga schist belt show a large compositional scatter between feldspathic and lithic types compositional differences indicate variable provenance for the different localities tonalite trondhjemite gneisses ttg mafic ultramafic schists basic intermediate acid volcanic rocks sedimentary rocks like banded iron formation quartzites and cherts and k granites in minor quantity were exposed in the source area volcanic debris was also contributed from contemporaneous volcanism in the basin greywackes in this region are invariably interbedded with fine grained biotite rich phyllites and schists marked compositional differences are observed between the greywackes and the interbedded phyllites the greywackes have k2o na2o 1 whereas phyllites show large variation in k2o na2o ratios ranging from 1 5 to 15 0 high cr ni content of the greywackes and phyllites of the chitradurga belt are indicative of an important contribution from ultramafic rocks light ree enrichment and negative eu anomalies are particularly pronounced in the mudpacks of the greywackes represented by phyllite the ree and other chemical data such as la th ti zr and k2 mgo ratios indicate that these greywackes were supplied from a juvenile continental nucleus and were laid down on a simatic and converging active continental margin an archaean miniplate model with low angle subduction and sedimentary basins of moderate size is proposed to explain the geochemical and tectonic features of the schist belts

orogenesis vs diagenesis can we use organic rich shales to interpret the tectonic evolution of a depositional basin abstract black shales from the southern appalachian basin and the southwest welsh basin have anomalous u pb and nd model ages suggesting syn and post depositional resetting of the sm nd and u pb isotopic systems this alteration to the primary detrital signature of these two shale sequences is indicative of black shale diagenetic depositional processes that obscure paleo environmental and provenance information recorded prior to and during deposition the trace element and isotopic signatures of these two shale sequences reveal a syn post depositional history that is de coupled from the coeval orogenic history of the region making it difficult to reconstruct the tectonic and oceanographic conditions present at the time of deposition both the ordovivian welsh basin and the devonian appalachian basin sequences host ree and u bearing diagenetic phosphate minerals that play a critical role in the whole rock ree and u budgets in the welsh basin shales early diagenetic apatite and a later monazite phase dominate the ree budget and cause the redistribution of ree early in the basin s history ca 460 ma this redistribution is recorded by the sm nd system 450 90 ma and the nd model ages that are anomalously old by as much as 20 t dm 2 0 ga this early history is complicated by a permo triassic fluid event affecting the whole rock u budget and resetting the u pb isotopic system at 193 45 ma the appalachian basin sequence appears to have a much less complicated history yet still records a significant disturbance in both the sm nd isotopic system 392 76 ma and the pb isotopic system 340 50 ma at about the time of deposition ca 365 ma these two sequences suggest a pattern of diagenetic disturbance common to black shales these processes are unique to black shales and must be considered when interpreting provenance and paleo environmental information from the black shale sequences although these rocks are susceptible to alteration the alteration may provide extensive information on the post depositional history of the basin while still retaining some primary depositional information if black shale processes are considered during the interpretation of isotopic and trace element signatures from organic rich shales it may be possible to recover an extensive basin history

a 2 5 ga porphyry cu mo au deposit at malanjkhand central india implications for late archean continental assembly abstract eight re os ages from six molybdenite samples representative of the cu mo au mineralization at the giant malanjkhand deposit in madhya pradesh were obtained using id ntims these data provide a clear late archean early paleoproterozoic age for this deposit and by implication for its calc alkaline granitoid host among other diverse models the origin of malanjkhand has been debated as orogenic lode style associated with development of a quartz reef in a shear zone or as porphyry style associated with a pink granitoid intrusion that is conspicuously present in the vicinity of the brittle ductile deformed quartz reef previous models included middle proterozoic ages for the cu mo au mineralization based on poorly constrained rb sr and k ar data direct dating of molybdenite shows that stringer mineralization in the quartz reef and disseminated and vein mineralization in the granite were contemporaneous at 2490 8 ma 2489 5 1 4 ma based on regression without uncertainty in the 187re decay constant mswd 0 5 n 5 additional molybdenite from the quartz reef dated at 2475 and 2450 ma represents at least two pulses of reworking that we suggest configured the elongate and arcuate quartz reef into its present position we suggest that the quartz reef is a high silica stockwork cap embedded in its primary potassic alteration halo and subsequently tilted so that it lies convex to the east with deeper levels exposed to the north petrographic evidence coupled with high re concentrations for molybdenites 400 650 ppm support this scenario and suggest that malanjkhand is a subduction related stockwork porphyry style cu mo au deposit involving mantle malanjkhand geology shares little affinity with three surrounding and tectonically juxtaposed terranes we suggest that the malanjkhand cu mo au deposit and the surrounding batholith containing multiple cu mo au prospects may be the vestiges of a microplate captured along the north margin of the nearly amalgamated southern indian craton at 2 5 ga as the southern craton began to converge with the northern indian craton or other protocontinents now embedded or consumed within the central indian tectonic zone citz this scenario could imply an early paleoproterozoic age for the earliest development of the citz at least in the region of the central indian shear suture cis whose marked ene trend imprints the northwestern part of the malanjkhand batholith and the cu mo au deposit based on a compilation of 2 5 ga events in both the southern and northern indian cratons we suggest that some units in the citz linked an extensive and continuous late archean early paleoproterozoic orogenic belt that may extend to the moyar bavali bhavani attur palghat and cauvery shear zones in southern india and include inboard units of the eastern ghats and the aravalli delhi orogenic belts robust geochronology that can be tied to the early history within these orogenic belts is scarce and extensive reworking during grenvillian and pan african time creates a pervasive overprint at some localities this late archean early paleoproterozoic belt preserves sutures within a coalesced late archean continent that included east antarctica napier and vestfold hills complexes at 2 5 ga

a review of the geophysical constraints in modelling the gondwana crust in india abstract the gondwanic lithosphere of india has evolved by suturing of three tectonic blocks namely the south indian block sib the bundelkhand block bb and the trans aravalli block tab the bb became separated from the russian platform during the opening of the proto tethys and joined the sib during the satpura orogeny around 921 ma the tab represents a younger crustal block than bb and is possibly a detachment of the arabian nubian shield which became separated along the owen fracture zone chaman fault during the himalaya collision tectonics around 10 ma the paleo plate margins of the microplates constituting the gondwanic lithosphere of india are defined by gravity and magnetic highs in regional and global geophysical data the crustal thickening as brought out by gravity data analysis has been controlled by thrust stacking the intent of this communication is to highlight the significance of geophysical constraints in paleo plate reconstructions and crustal growth

conodont alteration and tectonothermal evolution of a diagenetic unit in the iberian variscan belt ponga cuera unit nw spain tectonothermal analysis of a mainly carbonate unit located in the external part of the variscan orogen in nw spain is dealt with using the conodont colour alteration index cai and the study of textures of these microfossils the kubler index of the illite ki is used as a complementary method the area is characterized by a great uniformity in the cai values which in most cases are

heat production heterogeneity of the indian crust beneath the himalaya insights from the northern indian shield abstract crustal thermal structure of the himalaya depends partly on the thermal properties of the indian crust that has subducted beneath the himalayan range and partly accreted to it in most studies the indian crust is assigned uniform thermal properties mostly derived from the data from southern india we show here that this might actually not be a good assumption based on the in situ heat production measurements at 279 sites in the bundelkhand craton chottanagpur gneissic complex and singhbhum craton the dominant rocks granites and gneisses in the bundelkhand craton and chottanagpur gneissic complex are characterized by high heat production 3 93 6 52 w m 3 whereas the rocks in the singhbhum craton are low in heat production 1 41 3 27 w m 3 thereby suggesting a large variation of heat production in the northern indian shield the available surface heat flow values also show consistently large variation from 56 to 96 mw m 2 the crustal heat production heterogeneity within the indian crust has to be duly considered in the interpretation of thermobarometric and thermochronological data of the himalaya

tectonothermal evolution of a major thrust system the esla valsurbio unit cantabrian zone nw spain the tectonothermal evolution of a unit in the foreland fold and thrust belt of the iberian massif is established using the conodont colour alteration index cai the unit consists of two parts with different tectonothermal histories the esla nappe region and the valsurbio region separated by a synorogenic carboniferous basin guardo valderrueda basin the esla nappe region evolved in diacaizonal conditions corresponding to the diagenetic conditions of the pelites whose palaeotemperatures were controlled by rock burial maximum values were reached before the emplacement of the thrust nappes so tectonic superimposition is not registered by the cai overburial due to the emplacement of the thrust units was prevented by simultaneous intense erosion the geothermal gradient obtained for burial was c 35 c km 1 and the temperature reached by the older cambrian rocks was c 210 230 c the valsurbio region was affected by an extensional tectonothermal post orogenic event that gave rise to metamorphism with ancaizonal or epicaizonal conditions corresponding to anchizone or epizone of the pelites the most common maximum palaeotemperatures reached in this event fall within the range 305 415 c although higher palaeotemperatures could be reached locally as a consequence of contact metamorphism this event gave rise to subhorizontal cleavage that cuts the main variscan folds coal rank data indicate an increase in maximum palaeotemperatures eastwards from the esla nappe region to the valsurbio region through the guardo valderrueda basin

petrology and geochemistry of greywackes from the aravalli supergroup rajasthan india and the tectonic evolution of a proterozoic sedimentary basin the proterozoic aravalli supergroup in south central rajasthan western india is predominantly composed of coarse low grade meta arenites carbonates greywackes interbedded fine grained silty arenites and metapelites resting on an archaean basement complex of gneisses schists and high grade metasediments we have differentiated four lithofacies a b c and d within the greywacke suite each with characteristic qfl composition grain size and major trace and rare earth element concentrations tectonic setting for each facies type could only be discriminated using qmflt plots greywackes of facies a and d were derived from a recycled orogen and facies b was derived from arc related andesitic source rocks a continental block provenance is indicated for the silty arenites of facies c this petrofacies based interpretation shows good agreement with environmental reconstructions based on lithofacies and sedimentary structure analyses the only exception is facies b whose andesitic parentage as deduced from petrofacies analysis could not be demonstrated at its outcrop these provenance discriminations were tested through tectonic setting interpretations based on major trace and rare earth elements the geochemical data broadly confirmed the tectonic interpretations made on the basis of the petrological attributes use of these laboratory based methods in conjunction with field measurements of primary sedimentary structures allow us to discriminate provenance and tectonic setting fairly precisely a proterozoic plate model with subduction to the west has been proposed to explain the sedimentological variation in the aravalli supergroup

genesis and rank distribution of upper carboniferous coal basins in the cantabrian mountains northern spain abstract the cantabrian mountains located in the nw of the iberian peninsula constitute the most important coal mining district of spain anthracitic and bituminous coals high and medium rank coals have been mined in the area since the end of the nineteenth century and they currently account for about 70 of the total coal resources of the country the region forms part of the cantabrian and west asturian leonese zones of the iberian variscan fold belt and is strongly deformed by a set of imbricate thrusts coeval folds and high angle faults coal bearing successions are westphalian and stephanian pennsylvanian in age and are exposed in numerous coalfields of variable size arranged roughly parallel to the tectonic structures coal rank varies from medium rank bituminous d coals rr 0 5 to high rank anthracites a coals rr

paleothermicity in the central asturian coal basin north spain this research shows for the first time maps of vitrinite reflectances and paleotemperatures from the central asturian coal basin north spain which is a carboniferous pennsylvanian basin mainly of moscovian age vitrinite reflectance values decrease from north to south whereas volatile matter distribution increases from south to north vitrinite reflectance and volatile matter parameters indicate a coal rank ranging from high volatile bituminous coal in the north to semianthracite and anthracite in the south rock eval data show that the organic matter of this basin is type iii kerogen with a maturation ranging from oil to gas window paleotemperatures were calculated by diverse methods using vitrinite reflectance data for different durations of heating and rock eval results the calculated paleotemperatures and vertical paleotemperature gradients decrease from south to north the thermal gradient variation in the central asturian coal basin points to the influence of at least two heating events that affected the organic matter the first associated with a regular geothermal gradient operating over a long period of time and the second linked to a southern granitic event of short duration estimated by tectonic data the short thermal event was located at the end of sedimentation late moscovian and late westphalian d and after folding but before the overthrusting during the asturian tectonic phase located before the early kasimovian cantabrian and stephanian deposits finally a simulation of paleotemperatures around the granitic pluton was calculated and compared to maps of paleotemperatures obtained by various methods these maps refer to an initial depth of one or two km in accordance with the selected methods that are compatible with local erosion this approach was preferred in order to explain the metamorphism of coal rather than the hypothesis of hydrothermal fluid flow proposed for other foreland basins this regional thermal anomaly could be important for future coalbed methane assessment

conodont palaeothermometry of contact metamorphism in middle ordovician rocks from the precordillera of western argentina the yerba loca formation middle upper ordovician exposed in the western precordillera san juan argentina is made up of clastic carbonate turbidites and basic ultrabasic rocks it is affected by regional siluro devonian very low grade metamorphism that locally reaches greenschist facies at ancaucha creek 45 conodont samples were taken from two sections that include 30 to 50 m thick sills in order to analyse the thermal alteration patterns produced by these intrusive bodies conodont colour alteration index cai is contrasted with optical petrography and x ray diffraction analyses of clay minerals the intrusions are dated as post darriwilian da2 as determined by conodont biostratigraphy of the host rock which indicates the paroistodus horridus subzone of the lenodus variabilis zone the distribution of cai values defines a thermal aureole of about 2 5 times intrusion thickness that prevailed over the later very low grade metamorphism metasomatism at ancaucha creek is recorded by cai values of 4 to 7 particularly restricted to a few layers close to the intrusions as indicated by conodont textures and rock fabric one dimensional thermal computer simulation conforms to empirical data indicating temperatures greater than 600 c for the contact zone although it points out slightly narrower thermal aureoles the clay mineral assemblage of most of the analysed samples chlorite illite smectite and i s mixed layers is complex and probably derives from several superimposed processes thus representing non equilibrium assemblages in turn ki values 0 27 0 32 indicate anchizone metamorphism in agreement with regional cai values of 4 consequently the occurrence of smectite and i s probably resulted from retrograde diagenesis processes

proterozoic rotation of the peter lake domain in the trans hudson orogen of saskatchewan canada deduced from paleomagnetism abstract the peter lake domain is a northeast trending 200 km long cigar shaped terrane on its southeast side is the 1854 11 ma wathaman batholith that marks the suture zone for the juvenile terranes of the trans hudson orogen tho and on its northwest side is the wollaston fold belt that defines the margin of the archean slave rae hearne srh craton two well defined remanent magnetization components are carried by hematite magnetite and pyrrhotite in a range of metamorphic rock types the a component was probably acquired during the d1 regional metamorphic event at 1869 ma and gives a direction of d 302 i 40 95 10 k 16 n 15 its pole position at 3 n 52 w dp 7 dm 12 locates the peter lake domain along the margin of the srh craton at the time of metamorphism and was later rotated 32 14 counterclockwise probably when the abutting southwestern arm of the wathaman batholith was similarly rotated during oroclinal deformation the b component is found preferentially in the parker lake gneisses but is also found in minor shear zones in the peter lake complex and wathaman batholith and it replaces the a component by remagnetization b has a direction of d 92 i 83 95 8 k 19 n 20 that gives a pole at 54 n 80 w dp 15 dm 15 the b remanence is thought to have been acquired at about 1780 ma during the d3 shearing event this event produced the needle falls and parker lake shear zones that separate the peter lake domain from the wollaston fold belt and wathaman batholith respectively the b component likely defines the end of the trans hudson orogenic event during which the 5000 km wide manikewan ocean closed thorough evaluation of the andean collisional model for the tho will require much more paleomagnetic data

ion microprobe 207pb 206pb ages of zircons from the bundelkhand massif northern india implications for crustal evolution of the bundelkhand aravalli protocontinent abstract 207pb 206pb ages of zircon from gneissic rocks and granitoids representing all major lithological units of the bundelkhand massif northern india have been determined using an ion microprobe the presence of early archaean granitic crust in this region is established from these ages ages for the kuraicha and the mahoba gneisses and associated metabasic enclaves suggest the oldest partial and metamorphic melting event in this region occurred at 3 3 ga the presence of younger gneisses with ages of 2 7 and 2 5 ga suggests the existence of multiple protolith age components within the massif three major granitoid suites hornblende biotite leuco granitoids were emplaced at 2 5 ga within a time span of a few tens of millions of years felsic volcanism also occurred around this time an age of 2 49 ga for the youngest phase of leucogranitoid marks the time of overall stabilization of this massif the geochronological history of the bundelkhand massif appears to be remarkably similar to that of the basement complex of the aravalli delhi orogenic belt another prominent archaean crustal block to the west of this massif geochemical features of the different rock types in these two regions are also very similar we propose that these two apparently discrete blocks evolved as a single crustal unit with stabilization taking place at 2 5 ga

effects of diagenesis on the nd isotopic composition of black shales from the 420 ma utica shale magnafacies abstract the utica black shales were deposited in the taconic foreland basin 420 ma ago the organic matter in these shales is of marine origin and the timing of deposition of these shales has been constrained by graptolite biostratigraphy and bentonite chronostratigraphy rare earth element ree concentrations and nd isotopic ratios were measured in whole rock black shales with different grades of thermal maturity from the utica shale magnafacies of quebec immature ontario mature and new york post mature these measurements were also made in the organic carbonate and sulfide fractions of the shales that were isolated by sequential leaching the purpose of this study is to understand how diagenesis thermal maturation may affect the nd isotopic compositions of different components as well as the whole rock black shales bulk rock ree concentrations of the utica black shales are similar to average shales however these rocks show large range in 147 sm 144 nd 0 08 0 19 indicating diagenetic mobilization of the light ree lree although thermal maturation of black shales may mobilize all lree our data indicate that mobility of la relative to sm is greater in the mature shales compared to the post mature shales in post mature shales nd is more mobile relative to sm immature shales however show wide ranges in sm nd and la sm that cannot be explained solely by diagenesis implying source heterogeneity whole rock black shales and their isolated components show overlapping but characteristically different nd isotopic compositions if the present day nd isotopic compositions of black shales are mostly an artifact of diagenesis thermal maturation which alters the sm nd ratio of the rock it can be argued that the different components of black shales are affected to different degrees for a given diagenetic process or different diagenetic processes affect the different components alternatively the varying nd isotopic compositions of the shale components may reflect differences in provenance of the different shale components nd isotopic composition of mature and post mature black shales of the utica magnafacies are controlled by thermal maturation of these rocks as indicated by the correlation of e nd 0 with various lree ratios such as la sm and sm nd however the observed variations in the nd isotopic composition of immature shales dominantly reflect source heterogeneity this is also supported by the correlation of e nd 0 and immobile heavy ree ratios such as dy yb which reflect source compositions the average e nd 0 compositions of the whole rocks and separated components vary with increasing maturity the post mature samples showing the least radiogenic nd and the mature samples with the most radiogenic nd isotopic compositions the initial nd isotopic compositions at 420 ma of the utica black shales overlap with the global seawater nd isotopic composition at that time this similarity is observed for the samples with sm nd ratios similar to average shales suggesting no diagenetic imprint and that the taconic foreland basin hosting the utica shale magnafacies was probably connected to the global ocean during the time of deposition depleted mantle nd model ages of the utica black shales and their different components increase with increasing sm nd however these model ages range from 1 6 to 2 3 ga average 1 9 ga for the samples with sm nd similar to average shales these ages indicate that the taconic basin sediments of the utica shale magnafacies were sourced from pre grenville age rocks

the cr th ratio in precambrian pelites from the kaapvaal craton as an index of craton evolution the cr th ratio in precambrian pelites from the kaapvaal craton in southern africa correlates with pelite th sc and co th ratios and appears to monitor changes in source composition it is a more sensitive indicator of provenance than other commonly used ratios including eu eu la yb and th u pelite cr th ratios are controlled most strongly by the effects of local provenance and local tectonics secular decreases in this ratio the most prominent of which occurred at 2 4 2 3 ga in pelites of the kaapvaal craton record rapid uplift exposing granite decreases in the ratio reflect increases in the abundance of komatiite basalt in sediment sources in response to arc collisions rift hotspot volcanism or intracratonic uplift of older greenstones relatively constant cr th ratios appear to require widespread uplift or and efficient sediment mixing during tectonic stability changes in cr th ratio of kaapvaal pelites reflect a complex tectonic history of the craton from about 3 3 ga to 1 8 ga and do not favor single stage cratonization in the late archean

a paleoweathering profile from griqualand west south africa evidence for a dramatic rise in atmospheric oxygen between 2 2 and 1 9 bybp a core drilled near wolhaarkop in griqualand west south africa intersected highly oxidized kuruman iron formation below red beds of the gamagara formation the lateral equivalents of the kuruman iron formation in this drill hole consist largely of siderite ankerite magnetite greenalite and quartz the oxidation of the kuruman iron formation in wol 2 occurred almost certainly during weathering prior to the deposition of the gamagara formation the date of this weathering episode is bracketed between about 2 2 and 1 9 bybp by the age of the ongeluk lavas in the transvaal sequence below the unconformity and by the age of the hartley lavas in the olifantshoek group above the unconformity the ratio of iron to sio2 in the several facies of the weathered kuruman iron formation in wol 2 is nearly the same as that in their unweathered equivalents since sio2 loss during weathering was almost certainly minor the similarity of the fe sio2 ratio in the weathered and unweathered bif indicates that nearly all the feo in the kuruman iron formation was oxidized and retained as feo3 during weathering such a high degree of iron retention is best explained by an o2 content of the atmosphere or 0 03 atm at the time of weathering such an o2 pressure is very much greater than that suggested by the composition of paleosols developed on basalt or 2 2 bybp but is consistent with the highly oxidized nature of the 1 85 by flin flon paleosol the new data suggest that po2 rose dramatically from about 1 percent pal present atmospheric level to or 15 percent pal between 2 2 and 1 9 bybp

on the geochemical evolution of sedimentary rocks the average major element compositions of clastic sedimentary rock assemblages i e carbonates evaporites and iron formations not considered for various times in earth history archean early proterozoic late proterozoic phanerozoic are reported based largely on the compilations of f l schwab uncertainties in the averages are considerable there is little evidence of secular change in the average composition of sedimentary rocks during post archean times such data do not support the idea of a gradual change in composition of the upper continental crust from mafic to felsic composition trace element patterns in sedimentary rocks particularly ree and th distributions support the idea of constancy of upper crustal composition during the post archean the average composition of archean clastic sedimentary rocks differs significantly from the post archean average being enriched in na mg and ca and depleted in si and k such differences are consistent with the archean upper crust having a more mafic composition than post archean upper crust the trace element data further support this idea it is noted that the average composition of post archean clastic sedimentary rocks predicts to within a factor of 1 2 the composition of the exposed continental crust for the elements si al fe k mg and ti the abundances of ca and na are depleted in the sedimentary rocks due to the fact that the compositions of carbonates evaporites and seawater were not considered the average composition of archean clastic sedimentary rocks does not compare favourably with an independent estimate of archean upper crust composition being enriched in si k and depleted in ca fe mg and ti such a comparison indicates that the archean upper crust may have had a more felsic major element composition than previously thought

trace element and nd isotopic evidence for sediment sources in the mid proterozoic vindhyan basin central india abstract ancient sediments reflect the compositions of old eroded continental crustal material and preserve geochemical signatures of paleo tectonic and volcanic events geochemical study of such sediments provides insight into tectonic events that shaped the face of the earth nd isotopes are best suited for such studies because rare earth elements ree specifically sm and nd are not fractionated during post depositional processes such as erosion transportation and diagenesis of organic poor sediments and hence reflect the source composition of these sediments sedimentation in the proterozoic vindhyan basin of central india with a 4 km thick succession of unmetamorphosed and mostly undeformed sediments lasted over 1000 ma from 1630 ma up to the precambrian cambrian boundary these sediments provide an ideal opportunity to study the evolution of ancient continental crust as well as the tectonic evolution of peninsular india the occurrence of porcellanites in the lower part of the basin indicates active volcanism during deposition of the vindhyan sediments these tuffs show large ion lithophile element lile enrichment as well as depletions in nb and ta in primitive mantle normalized plots characteristic of an arc volcanic source in combined trace element ratio plots these volcanic rocks indicate mixing between mantle derived melts and the continental crust and overlap with the composition of global arcs their initial nd at 1630 ma ranging from 5 7 to 0 9 is similar to those observed in andean type continental margin arcs trace element concentration patterns as well as nd isotopic compositions of the clastic and non clastic sediments of the vindhyan basin from one of the type localities of the vindhyan stratigraphy in the son valley are vastly different from those of the bundelkhand granite which is the basement for these sediments indicating that these granitic rocks were not a major source of sediments for the vindhyan basin our nd isotopic study clearly indicates that sedimentation in the vindhyan basin which lasted over 1 ga was not continuous the boundaries between the semri and kaimur groups as well as the rewa and bhander groups are characterized by abrupt changes in the nd isotopic compositions indicating changes in provenance with sediments in the kaimur and bhander groups being derived from more juvenile sources compared to the semri and rewa groups respectively this inference is also supported by changes in t dm and f sm nd for the sediments across these boundaries the strikingly different trace element patterns of the rewa group sediments compared to the underlying kaimur sediments indicates that sediment sources also changed across the kaimur rewa boundary multiple changes in provenance in the vindhyan sediments indicate that this basin was tectonically active during the entire span of sedimentation while there is some uncertainty in determining the exact sources of sediments for the vindhyan basin we argue on the basis of our geochemical in particular nd isotopic ratios data and available paleocurrent data that majority of the sediments were derived from a now extinct andean type arc source in the south that resulted from a southerly dipping subduction prior to the collision of the bhandara and bundelkhand cratons according to this reconstruction the vindhyan basin was initiated as a foreland basin after the collision of these two cratons

thickness composition and evolution of the indian precambrian crust inferred from broadband seismological measurements abstract we present seismological constraints to model the evolution of indian precambrian crust through receiver function analysis of teleseismic waveforms recorded on 22 broadband seismographs along a n s corridor from southern granulite terrain to delhi on the aravalli craton archean terrains eastern dharwar bastar bundelkhand and aravalli cratons are underlain by crust with felsic intermediate composition vp vs 1 65 1 75 in contrast to mafic composition vp vs 1 78 of crust beneath vindhyan basin godavari basin and the narmada son lineament nsl of proterozoic age the moho depth varies between 33 and 43 km with no definite relationship with age of the crust duplex moho with highly mafic underplated lower crust is characteristic feature of the nsl signifying its paleo rift character present day felsic character of the indian archean crust could be a consequence of post formation modification of the initial mafic lower crust through the process of lithospheric delamination this major tectonic event also created layered lithosphere beneath parts of india imprinted as positive velocity contrast at a depth of 80 km usually referred to as hales discontinuity the upper part of the lithosphere has shear velocity vs 4 52 km s and the lower one 4 77 km s we speculate that such a layered lithosphere could be due to a low density depleted spinel peridotite with archean affinity lying over higher density fertile garnet peridotite of proterozoic age

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

the archean proterozoic transition evidence from the geochemistry of metasedimentary rocks of guyana and montana abstract chemical and isotopic compositions of ancient sedimentary rocks have in some cases provided evidence of first order changes yin the composition and extent of continental crust from the archean to the proterozoic we reversed the typical sampling pattern comparing metapelites and carbonates from proterozoic greenstone belts with those from an archean continental margin sequence and found the reverse of the typical age composition relations metapelites and volcaniclastic metagreywackes from the early proterozoic greenstone belts of northern guyana resemble those of many archean greenstone belts in ree trace and major element compositions mean ratios la sm 5 0 la th 5 1 eu eu 0 80 th u 3 4 median k 2 o na 2 o 0 8 schists from an archean continental marginal sequence in montana have compositions like many post archean metapelites derived from continental sources mean ratios la sm 6 8 la th 2 7 eu eu 0 67 th u 5 2 median k 2 o na 2 o 1 7 other archean continental basin sediments also resemble their post archean counterparts implying that upper continental crusts of these eras may not have differed significantly in these respects however temporal change may have occurred in upper crustal contents of ni and cr many archean clastic metasedimentary rocks of both continental and greenstone belt provenance have contents of these compatible elements higher than in their post archean equivalents dolomitic limestone from a greenstone belt of northern guyana has a very low 87 sr 86 sr ratio of 0 7007 such low values may reflect the local influence of mantle derived volcanic rocks during carbonate deposition diagenesis or metamorphism and they may not be reliable indicators of the evolution of sr in seawater archean dolomitic marbles from the continental marginal sequence of montana have 87 sr 86 sr ratios of 0 7065 and higher significantly more radiogenic than many accepted values for carbonates of archean greenstone belts

coal basins in the cantabrian mountains northwestern spain abstract the cantabrian mountains containing about 70 of the total coals of the country and 95 of the anthracitic and bituminous resources is the most important coal mining district of spain coal bearing successions are late carboniferous in age and their deposition took place in a syn orogenic context during the hercynian activity mainly in three different time successive locations foreland basins intrathrust basins and intramontane fault controlled basins foreland deposition occurred in strongly asymmetrical basins located ahead of thrust sheets units intrathrust basins resulted from folding and tightening of these units during or after their emplacement intramontane and fault controlled basins were developed along lineaments with strike slip movements and as a result of fold and fault reactivation peat mires in foreland and intrathrust coal basins mainly occupied broad areas on abandoned delta and fan delta lobes coals in intramontane and fault controlled basins were related with alluvial fan and lacustrine environments coal ranks vary from high volatile bituminous coals to anthracite depending on the structural location of the coalfields hydrothermal activity seems to be the most important factor in coal evolution

the ciw index a new chemical index of weathering abstract a new chemical index of weathering that can be applied to modern soils and precambrian paleosis is proposed ciw al2o3 the value of this index increases as the degree of weathering increases and the difference between ciw index values of the silicate parent rock and soil or sediment reflects the amount of weathering experienced by the weathered material

geochemistry and mineralogy of sediments from the ventersdorp and transvaal supergroups south africa cratonic evolution during the early proterozoic approximately 100 pelite and 12 quartzite samples from the ventersdorp 2 7 ga and transvaal supergroups 2 6 2 1 ga have been analyzed to monitor the early proterozoic evolution of the kaapvaal craton southern africa from oldest to youngest pelites were sampled from the ventersdorp bothaville bot transvaal selati sel black reef br timeball hill th strubenkop str and silverton sil formations paleocurrent measurements in transvaal quartzites indicate sources lying predominantly to the north and east relative to the bot sel br pelites from the th str sil are enriched in heavy ree lile zr hf nb and ta depleted in k2o mgo ni and cr and have lower cr zr sc th k2o na2o and k rb ratios compared to sel br bot th str sil pelites have higher light ree contents and la yb ratios and lower eu eu ratios 0 61 0 66 relative to nasc north american shale composite thstr sil pelites are enriched in light ree th u ta nb sc cs have higher la yb ratios and are depleted in k2o and mgo bot sel br pelites are enriched in k2o mgo cr and ni have higher k2o na2o sc th and eu eu ratios and are depleted in th u heavy ree and high field strength elements hfse relative to nasc compositions of th str sil pelites suggest a provenance similar to average phanerozoic uppercontinental crust this source is more evolved than that of bot sel br pelites indicating a transformation from primitive mafic rich to evolved felsic rich upper crust at 2 2 ga this transition follows earlier primitive to evolved trends in moodies pongola 3 3 3 0 ga and witwatersrand 2 8 ga successions these data suggest that several cycles of changing upper continental crust occurred in the kaapvaal craton between 3 3 2 1 ga

provenance and tectonic settings of the late paleozoic sandstones in central inner mongolia ne china constraints on the evolution of the southeastern central asian orogenic belt abstract late paleozoic sedimentary strata outcrop extensively in central inner mongolia and are a key to understanding the tectonic evolution of the southeastern central orogenic belt a combined analysis of petrography whole rock major and trace element and nd isotope is carried out on representative sandstones from the late paleozoic sedimentary strata 420 270 ma the sandstones are mainly wackes and litharenites in lithology with low sio2 al2o3 of 2 85 9 47 averagely 5 22 and poor textural and compositional maturities implying short sediment transportation between the depositional basins and provenances the trace element compositions are generally comparable to that of the average upper continent crust ucc with negatively sloping chondrite normalized rare earth element distribution patterns la yb n 3 43 11 averagely 6 94 and flat ucc normalized trace element distribution patterns the nd isotopic compositions show great variation nd t 5 01 to 5 35 with depositional time of the sandstones and coincide well with the arc magmatic phases in central inner mongolia the geochemical signatures of the sandstones indicate that the dominant provenances are intermediate to felsic arc magmatic rocks that have ages approximating the deposition although old recycled sediments may have made a minor contribution an active continental arc setting during the late paleozoic in central inner mongolia controlled by the northward subduction of the paleo asian oceanic slab was the most likely depositional tectonic setting of the sandstones this active continental arc setting continued to at least 270 ma implying that the final closure of the paleo asian ocean along the solonker suture zone most likely occurred sometime during the late permian to early triassic the northward subduction of the paleo asian ocean is likely of west pacific style in which the present day baolidao arc has a close genetic link with the south mongolian microcontinent and likely the former originally formed as the arc margin of the latter

calculation of the conodont color alteration index cai for complex thermal histories abstract a simple model for the arrhenius reaction conodont color alteration index cai that can be implemented easily in a computer program is introduced the model named to as easycai envisages the overall process of conodont alteration as a series of 12 parallel pseudo first order reactions our approach is especially amenable to a spreadsheet program and can solve complex geological histories involving variable heating and cooling rates with easycai a profile of cai values versus time can be obtained for a given stratigraphic level if the time temperature history for that level has been estimated easycai can provide additional constraints to optimize thermal history models by computing profiles of cai values with depth and time thus providing a new tool for quantitative cai palaeothermometric analysis

geology and geochemistry of arenite quartzwacke from the late archaean sandur schist belt implications for provenance and accretion processes abstract detailed geological petrological and geochemical studies have been carried out on an arenite quartzwacke suite of rocks constituting a part of the late archaean sandur schist belt in dharwar craton southern india for understanding the nature of provenance for these sedimentary rocks the arenite quartzwacke consists of rounded to sub rounded and angular fragments of monocrystalline polycrystalline quartz quartzite and chert embedded in a fine grained matrix of quartz and sericite while arenites are more siliceous sio 2 80 92 wt the quartzwacke have relatively lower silica content ca 69 78 wt the arenites and quartzwackes have cia values ranging from 76 to 96 which suggest intense chemical weathering this is further corroborated by the positive correlation between al 2 o 3 and tio 2 in both these rock types the a cn k modeling of arenites and quartzwackes show evidence for addition of k 2 o during later metasomatic alteration in the a cnk fm ternary diagram all the samples plot along a mixing line between chlorite and sericite indicating alteration during k metasomatism and the presence of mafic rocks in the source the high concentration of hfse such as zr hf nb and ta and the trace element ratios th sc la sc th u and ce th in the arenite quartzwacke indicate a mixed provenance the rare earth element modeling of quartzwackes considering tonalite granite and amphibolite end members in the provenance suggests equal proportions of mafic and felsic end members a composition comprising of 25 tonalite 25 granite 50 amphibolite in the provenance appears to match with the observed range of ree patterns of quartzwackes the presence of higher proportions of granite in the provenance is evidenced by the large negative eu anomalies in these sediments field evidence and structural discordance suggest that the arenite quartzwacke suite is an allochthonous part of the sandur schist belt

the mechanisms of petrogenesis of the archaean continental crust comparison with modern processes the petrographic and chemical composition of magmatic rocks generated during the archaean appears to be different from that of post archaean rocks komatiites are widespread before 2 5 ga and rarely occur afterwards in addition the archaean continental crust is primarily ttg tonalitic trondhjemitic and granodioritic in composition exhibiting typical trondhjemitic differentiation trends whereas modern equivalents are granodioritic to granitic following classical calc alkaline differentiation trends this distinction becomes more prominent when rare earth elements ree are taken into account archaean ttg are yb poor ybn 8 5 and have high layb ratios 5 layb n 150 in comparison the post 2 5 ga granitoids emplaced in subduction zone geodynamic environments have high yb content 4 5 ybn 20 with very low layb n ratios 20 theoretical calculations and experimental petrology have shown that the ttg can be produced by partial melting of an archaean tholeiite transformed into garnet bearing amphibolite consequently the low heavy ree content of the ttg is explained by the influence of both residual garnet and hornblende in their source after 2 5 ga the role of these minerals in calc alkaline magma genesis becomes progressively less important which is interpreted in terms of a cooling earth model in modern subduction zone environments the subducted oceanic slab is relatively old and cold and the geothermal gradient along the benioff plane in low ca 10 c km consequently the down going lithosphere undergoes dehydration before partial melting is able to occur the liberated fluids are light ree and lile enriched and ascend into the overlying mantle wedge where they induced partial fusion the produced magmas separate from their mantle source region leaving a residue mainly composed of olivine and pyroxenes mantle derived magmas typically exhibit high yb contents due to low kdyb values for olivine and pyroxenes during the archaean the subducted lithosphere was relatively young and hot providing high geothermal gradients along the benioff zone thus partial melting of the subducted slab was possible at lower temperatures before dehydration would take place garnet and hornblende are the main residual phases accounting for the low yb contents of the archaean ttg this model can be tested using a modern analogue of archaean like subduction processes in south chile an oceanic ridge has subducted and all thermodynamic calculations indicate that this creates locally high geothermal gradients along the benioff zone thus in very small areas archaean like environments may be simulated in modern subduction zones the modern andesites produced in this environment show archaean geochemical characteristics with low ybn 5 whereas the majority of andesites along the andean arc have modern patterns with ybn ranging from 8 to more than 17 this conclusion was generalised to all young subducted lithospheres all over the world in conclusion it appears that since the archaean there has been a change in the site of continental crust genesis the location of calc alkaline magma source in subduction zone environments has migrated through time from the subducted slab to the mantle wedge this is a direct consequence of the progressive cooling of the earth

tectonothermal evolution of a foreland fold and thrust belt the cantabrian zone iberian variscan belt nw spain analysis of the conodont colour alteration index and the kubler index of illite allowed us the characterization of four types of very low or low grade metamorphism in the cantabrian zone cz and determination of their regional and temporal distri bution these types are 1 an orogenic variscan metamorphism present only in restricted areas of the western and north western parts of the cz where epizonal conditions are reached 2 a burial metamorphism that appears in the basal part of some nappes where anchizonal conditions are sometimes achieved the thermal peak preceded emplacement of the nappes 3 a late variscan metamorphism in the southern and south eastern parts of the cz a cleavage cutting most of the variscan folds is associated with this metamorphism which has been related to an extensional episode 4 a contact metamor phism and hydrothermal activity associated with minor intru sive bodies the extension continued after the variscan deformation giving rise to hydrothermal activity during permian times

geochemical and petrological evidence for subduction accretion processes in the archean eastern indian craton the composition of sandstones often provides key evidence about the tectonic weathering and transport processes operating on the surface at the time of deposition petrologic and geochemical analyses of little metamorphosed middle late archean sandstones from the eastern indian craton show that the sedimentary rocks were derived from dominantly 3 3 ga old amphibolites of the older metamorphic group omg and tonalites of the older metamorphic tonalite gneisses omtg the two oldest lithologic units of this craton chondrite normalized rare earth element ree patterns of the sandstones show a light ree enriched signature with la sm n varying from 5 2 to 6 7 with no eu anomaly while the heavy rees display flat patterns with gd lu n values of 0 9 1 6 primitive mantle normalized incompatible and compatible trace element plots of these sandstones demonstrate an overall similarity with global proterozoic archean sandstones including strong nb ta negative anomalies in an fsm nd vs ond 0 diagram the sandstones plot precisely between the regional amphibolites and tonalites we infer from the ree abundances and the fsm nd vs ond 0 plot that the sandstones represent a bimodal mechanical mixture of omg and omtg the low ce pb ratios of these rocks of 1 4 indicate a variably pb enriched archean crust and that the ce pb ratio acquired the continental crustal signature distinctly different from those of the bulk silicate earth and mantle values at least as early as mid archean strong nb ta depletion relative to the primitive mantle suggests the sandstones were derived from subduction related magmatic arc sources this latter suggestion is strongly supported by the low nb ta and high zr sm ratios of these sandstones identical to archean tonalite trondhjemites that require based on recent trace element partitioning results their protoliths to have formed by subduction melting of lowmagnesium amphibolites or metamorphosed hydrous basalts 1 2 the average nd model ages of the sandstones are greater than the sm nd crystallization ages of the omg and omtg at v3 3 ga the geochemical data presented here can be collectively interpreted to suggest the presence of subduction accretion processes operational in the midarchean eastern indian craton because the omg and omtg the source rocks of the sandstones formed in a subduction related arc setting the basement rocks upon which this arc was constructed must have been older this observation and the depleted mantle nd model ages tdm of the sandstones ranging from 3 6 to 4 0 ga strongly indicate the presence of continental crust in this eastern indian craton older than 3 3 ga and possibly as old as

transition from diagenesis to metamorphism near the front of the variscan regional metamorphism cantabrian zone northwestern spain abstract distribution of paleotemperatures has been studied in paleozoic rocks of the variscan fold and thrust belt of the nw cantabrian zone spain by a combined application of illite crystallinity ic and conodont color alteration index cai results show that the diagenetic metamorphic grade increases with the increase in rock age as marked by decreasing ic values and increasing cai values moreover the cai isogrades roughly reflect the structural traits of the area indicating that maximum temperatures were attained during sedimentary burial previous to folding the geothermal paleogradient obtained suggests conditions near the boundary between low and middle p t conditions there is evidence of at least two thermal events during the tectonothermal development of the belt the main stage is interpreted as the result of the burial of the rocks characterized by late diagenesis conditions that occurred under a geothermal gradient of 35 c km 1 the narrow zone with low anchizonal conditions in the w

processes controlling the distribution of ti and al in weathering profiles siliciclastic sediments and sedimentary rocks abstract the ratio of ti to al varies greatly in primary igneous source rocks these elements are considered to be relatively immobile in most weathering regimes so that tio2 al2o3 ratios have been used to investigate the provenance of sediments and sedimentary rocks in modern weathering profiles that have undergone moderate weathering ti al ratios remain fairly constant so that siliciclastic sediments derived from them should have ti al ratios like that of the source materials under extreme weathering conditions however the upper most weathered parts of profiles commonly show a marked increase in ti al ratio that is thought to be due to preferential translocation of al rich phases under steady state weathering conditions one particular level of the profile may act as the main source of detrital material and may thus control the ti al ratio of sediments produced from it in a glacio fluvial setting minimum weathering moderate sorting there is considerable variation in ti content but al is nearly constant this is due to concentration of ti bearing mafic minerals such as biotite in the fine fraction in modern sediments derived from moderately weathered and sorted material the ti al distribution reflects mixing of al and ti enriched fine grained materials with sands that are depleted in these elements when a plot of tio2 vs al2o3 is constructed for a suite of such sands and muds the resultant trend differs significantly from that shown by a similar suite of unweathered glacio fluvial sediments comparison of these results with data from sedimentary rock suites suggests that trends shown by plots of tio2 vs al2o3 may provide insight into weathering and depositional history in addition to their use as provenance indicators

archean chemical weathering at three localities on the canadian shield abstract sapropelite developed on a granodiorite basement below the archean steeprock group in the western superior structural province of canada weathered materials from a 3 15 ga granitic basement to the archean yellowknife supergroup in the slave structural province and weathered detritus from contemporaneous komatiites and gneissic basement of the archean prince albert group in northern churchill structural province have been analysed the deviation of the weakly metamorphosed weathered material from the chemical composition of their precursors is comparable to that seen in more recent weathered profiles from similar source materials this includes the enhancement of ferric to ferrous iron ratios and potash to soda lime and magnesia ratios there is also an increase in lighter ree and a depletion of heavier ree quartz rich sediments immediately above the unconformity derived from the basement granitoids may be enriched in aluminum and iron and yield chloritoid under proper metamorphic conditions sediments formed from mixed sources such as weathered gneissic basement and contemporaneous komatiitic volcanics yield anomalous resistate sedimentary mixtures such as chromiferous quartzites based on the data given here there is no need to postulate that different weathering and or ground water systems were operative during the archean compared to more recent times on the other hand the complicated weathering and diagenetic history that altered material undergoes below a subsiding unconformity relies as much or more on the buffering capacity of the enclosed rock mass as on the atmospheric conditions prevalent at the time of weathering hence weathering profiles old and new are more a response to local environmental conditions within the rock than the composition of the atmosphere

simultaneous use of thermal indicators to analyse the transition from diagenesis to metamorphism an example from the variscan belt of northwest spain

depositional environments provenance and paleoclimatic implications of ordovician siliciclastic rocks of the thango formation spiti valley tethys himalaya northern india abstract recently published findings indicate that the ordovician period has been much more dynamic than previously anticipated thus making this period significant in geological time the ordovician of india can best be studied in the spiti region because the spiti basin records the complete uninterrupted history of excellent marine sedimentary rocks starting from cambrian to paleogene which were deposited along the northern margin of india due to these reasons the geochemical data on the ordovician rocks from the spiti region is uncommon the present geochemical study on the ordovician thango formation sanugba group is mainly aimed to understand the provenance and the paleoclimatic conditions the sandstones are the dominant lithology of the thango formation with intercalations of minor amount of shales detailed petrographic and sedimentological analysis of these rocks suggest that three major depositional environments viz fluvial transitional and marine prevailed in the basin representing transgressive and regressive phases the major and trace element ratios such as sio2 al2o3 k2o na2o and la th sc discrimination diagram suggest that these rocks were deposited in passive margin tectonic settings various geochemical discriminants and elemental ratios such as k2o na2o al2o3 tio2 la sc th sc cr th zr sc gd yb n and pronounced negative eu anomalies indicate the rocks to be the product of weathering of post archean granites the striking similarities of the multi elemental spider diagrams of the studied sediments and the himalayan granitoids indicate that sediments are sourced from the proterozoic orogenic belts of the himalayan region chemical index of alteration cia values of the studied sediments 55 72 suggest that the source rocks underwent low to moderate degree of chemical weathering the span of the cia values 55 72 recorded in the sediments from the spiti region may have resulted from varying degrees of weathering conditions in the source area this in turn reflects variable climatic conditions prevailing during the ordovician period which is consistent with the global studies the high cia values observed by numerous studies on siliciclastic sediments may imply warm paleoclimate attributed to an increase in atmospheric carbon dioxide and or methane levels inducing an acidic weathering environment and enhanced silicate weathering while the low cia values indicates prevalence of arid glacial conditions where negligible chemical alteration is possible this study documents the noteworthy events like fluctuating rise and fall atmospheric co2 and sea levels during the ordovician period a conclusion which is consistent with the other such studies on the phanerozoic glaciation or mass extinction events on gondwana supercontinent

geology and major and trace element chemistry of late archean weathering profiles in the fortescue group western australia implications for atmospheric po2 abstract we have identified two thick weathering profiles between flows of the mt roe basalt unit of the fortescue group in the pilbara area of western australia the age of weathering is constrained by u pb zircon dating to lie between 2765 and 2715 ma the profiles herein referred to as the mt roe 1 and 2 paleosols were mapped in detail for 1500 m and 760 m along strike respectively samples were collected along traverses across both profiles where they were well developed and well exposed the profiles which may be structurally offset parts of the same weathering horizon have well developed upper sericite zones 1 15 m thick these grade downward into chlorite rich zones which are chemically and physically heterogeneous the chlorite zones grade downward into unweathered basalts the mt roe 1 profile is overlain by 2 6 m of bedded probably lacustrine sediments these in turn are covered by succeeding lava flows evidence of a surficial origin for the paleosols includes mud cracks ripple marks and rip ups of the paleosol material in the post paleosol sediments soft sediment deformation and probable dewatering structures are present along the paleosol sediment contact the mt roe 2 profile is overlain directly by the succeeding lava flow with little or no intervening sediments some brecciation and pillowing is observed along the base of the overlying basalt flow in both cases probably due to emplacement of lava into a shallow lake or onto wet soil in the sericite zones of both profiles nearly all of the fe mn mg and zn and a substantial fraction of the ca pb na ni and si have been leached the chlorite zones have elevated fe pb ni and zn contents and appear to be repositories of elements lost from the sericite zone al ti zr and th were immobile in the mt roe 1 paleosol and are concentrated by factors of 2 5 2 6 over the unweathered basalt v u and cr were also nearly immobile major element and trace element data indicate that weathering took place under conditions of low atmospheric oxygen k is strongly enriched toward the upper contact of the sericite zone and was introduced during post weathering metasomatism together with rb and sr na and ca are also enriched near the top of each profile and small amounts of each may have been introduced during regional low grade metamorphism of the hamersley basin at 2168 10 ma

neoproterozoic active margin in the northwestern yangtze block south china new clues from detrital zircon u pb geochronology and geochemistry of sedimentary rocks from the hengdan group the yangtze block in south china constitutes an important precambrian landmass in the present east asian continent the neoproterozoic sedimentary successions of the hengdan group in the nw yangtze block record essential information for deciphering the neoproterozoic tectonics along the nw margin however its depositional age provenance and tectonic properties remain uncertain here a combined analysis of detrital zircon u pb dating and geochemistry is performed on representative samples from the hengdan group concordant dating results of samples from the bottom and upper parts constrain the maximum depositional age at c 720 ma detrital zircon age patterns of samples reveal a uniformly pronounced age peak at c 915 720 ma which is consistent with the magmatic pulses in domains at the nw end of the yangtze block in addition these samples display left sloping post archaean australian shale paas normalized rare earth element patterns and variable trace element patterns resembling sediments accumulated in a basin related to an active continental margin geodynamic setting provenance analysis reveals that the main sources featured intermediate to felsic components which experienced rapid erosion and sedimentation these integrated new investigations along with previous compilations indicate that the hengdan group might have been deposited in a fore arc basin controlled by subduction beneath the bikou terrane thus such interpretation further supports proposals for subduction related tectonics along the western margin of the yangtze block during the early neoproterozoic

nature of continental crust along the narmada son lineament inferred from gravity and deep seismic sounding data abstract the narmada son lineament nsl is a very prominent feature on the tectonic map of india and divides the indian peninsula into two parts the northern part characterised by lower elevations and more positive bouguer gravity anomalies as compared to the southern part the lineament is characterised by a gravity high practically all along its length four published deep seismic sounding dss profiles across the nsl give vital information concerning its origin an analysis of the bouguer gravity field along these profiles has been carried out using all available geological and velocity density information the analysis reveals that the areas immediately south of the nsl are characterised by higher density crust in comparison to the areas north of it this increase in density could have taken place during mesozoic to tertiary times when major tectonic volcanic activity took place over peninsular india the presence of deep seated faults sediments and anomalous crust shows that the lineament has some characteristics of a rift valley

geochemistry and provenance of sediments from the pongola supergroup south africa evidence for a 3 0 ga old continental craton abstract the 3 0 ga pongola supergroup of south africa is one of the oldest well preserved supracrustal successions the sequence is divided into the nsuze chiefly volcanics quartzites and pelites and the overlying mozaan groups chiefly quartzites and pelites relative to north american shale composite nasc most pongola pelites have similar la yb and eu eu ratios and large ion lithophile elements lile high field strength elements hfse ree v and sc contents ni 30 300 ppm and cr 100 800 ppm contents are greater than those of nasc while cr ni ratios are generally higher than shales and pelites of all ages these trends may reflect residual cr enrichment during intense chemical weathering of komatiitic source rocks local populations of nsuze pelites based on major and trace element data may result from source heterogeneities and or contemporaneous volcanic input into the sedimentary basin pongola quartzite compositions and detrital modes are similar to those of sandstones from phanerozoic passive margin and basement uplift tectonic settings ree lile and hfse contents in pelites suggest that pongola source rocks were remarkably similar in composition to average phanerozoic upper continental crust trace element geographic and isotopic constraints favor 3 2 to 3 0 ga hood granites as a major source for pongola detritus moderate contributions from mafic sources are also required whereas relatively minor contributions from tonalite and komatiite are allowed these results indicate the existence of a chemically evolved felsic craton in southern africa by 3 0 ga this craton appears to have been more evolved than sources for either older moodies 3 3 ga or most of the younger witwatersrand sediments 2 8 2 7 ga pelites from the kaapvaal craton record at least two mafic to felsic source area compositional cycles in pelites deposited from the mid to late archean additional cycles may be masked by a depositional or erosional hiatus between the moodies and pongola sequences these sedimentary patterns suggest that crustal development in southern africa was episodic in nature during the early precambrian

composition and weathering conditions of paleoproterozoic upper crust of bundelkhand craton central india records from geochemistry of clastic sediments of 1 9ga gwalior group abstract the 1 9 ga gwalior group represents one of the major paleoproterozoic depositional basin of bundelkhand craton and is comprised of 1 km thick unmetamorphosed and undeformed sedimentary succession with basal clastic sequence of sandstone shale association par formation and an upper chemogenic bif rich sequence morar formation the geochemical examinations of the basal clastic rocks are the subject of present study the compositional maturity and extreme quartz enrichment in sandstones of gwalior group suggest slow upliftment and high in situ weathering of basement terrains in a stable platformal setting these clastic sediments are characterized by strong depletion of mobile constituents like ca sr and na high plagioclase index of alteration pia values and strong positive correlation between al 2 o 3 and tio 2 indicating severe weathering of upper crust positive correlation between feo t and tio 2 suggests fe retention in paleoweathering profiles and is attributed to oxygenated atmosphere post depositional potash metasomatism is indicated in a cn k systematics of these sediments and k enrichment from near zero to as high as 38 is inferred with most of the shale samples show 30 extraneous k 2 o additions depleted transition element ni cr co sc contents elevated large ion lithophile element lile k rb ba th light rare earth elements lree contents fractionated rare earth element ree patterns strong negative eu anomalies in these sediments point toward k rich intracrustal granitic source component steeper ree patterns avg la yb n 12 57 compared to basement granites and conspicuous depletion of y over other liles in auc normalized spidergram also suggest a heavy rare earth element hree depleted ttg source component minor contribution from tholeiitic mafic rock is evidenced from th sc sc la sc sc th systematics and positive correlation of transition elements with feo t and tio 2 mixing calculations on the ree data suggest that upper continental crust of bundelkhand craton was constituted of 40 granite 40 ttg and 20 mafic rocks this is in contrast with the present day composition of bundelkhand granite gneiss complex bggc that contains 90 high k granite the data suggest unroofing of granite batholiths by means of erosion and upliftment of supracrustal ttg cover

sedimentation history of the paleoproterozoic singhbhum group of rocks eastern india and its implications abstract this paper reviews the sedimentological geochemical and stratigraphic characteristics of the paleoproterozoic chaibasa and dhalbhum formations the singhbhum group of eastern india and presents a comparative study with other paleoproterozoic lithostratigraphic units of india both the formations are entirely siliciclastic and deformed and metamorphosed generally at greenschist to upper amphibolite facies the older chaibasa formation consists of sandstone shale and sandstone shale interbanded heterolithic facies it conformably overlies the late archean paleoproterozoic dhanjori formation of terrestrial alluvial fan fluvial origin the lower chaibasa member formed in a marine setting the shale and heterolithic facies formed in a continental shelf setting below and above the storm wave base respectively the sandstone facies formed in a subtidal setting during sea level fall in contrast the upper chaibasa member formed in a shallow marine setting both the shale and heterolithic facies formed above the storm wave base in significant contrast the overlying dhalbhum formation is dominated by finer clastics with much lower proportion of sandstones the base of the terrestrial dhalbhum formation is a sequence boundary unconformity sedimentary facies analysis clearly shows two broad facies association of terrestrial origin fluvial and aeolian the aeolian facies association overlies the fluvial facies association the dhalbhum sandstones show typical ree pattern of quartz dilution with lower concentrations compared to the mudstones the dhalbhum finer clastics mimic typical ree patterns resembling abundances in the continental crust rare earth element plots display a good match with post archean australian shales paas including similar concentrations steep negative slope for lrees negative eu anomaly and nearly flat hrees the singhbhum paleoproterozoic succession is devoid of both chemical sediments and paleoproterozoic glacial deposits in contrast to neighboring cratons

geochemistry of mesoproterozoic sedimentary rocks of upper vindhyan group southeastern rajasthan and implications for weathering history composition and tectonic setting of continental crust in the northern part of indian shield abstract the upper vindhyan succession of southeastern rajasthan is divisible into kaimur rewa and bhander groups the major and trace element including rare earth elements data of the upper vindhyan shales and sandstones are investigated to determine the weathering history composition and tectonic setting of mesoproterozoic continental crust cia chemical index of alteration values a cn k plot a al2o3 cn cao na2o k k2o and depletion in u na2o cao sr and ba suggest that the source area experienced moderate to high degree of chemical weathering under warm and humid conditions provenance modeling indicates that the kaimur sandstones are best modeled with a mixture having 40 granitic gneiss 20 tonalite trondhjemite granodiorite ttg 20 mafic enclaves and 20 berach granite of the banded gneissic complex bgc a mixture of 60 granitic gneiss 20 mafic enclaves and 20 berach granite of the bgc can model the rewa and bhander groups it is suggested that the upper vindhyan sedimentation commenced at the time of delhi sausar orogeny at about 1100 1000 ma the orogenic movements uplifted the parts of old continental crust in the bgc terrain creating positive areas which exposed older crustal blocks containing ttg as important component the debris of kaimur sandstone probably derived from these uplifted blocks as indicated by palaeocurrent data the rewa and bhander formations were derived from bundelkhand granitic gneiss complex bggc occurring to the north of the basin and or the chotanagpur granitic gneiss complex cggc of eastern indian shield the derivation of lower and upper groups of vindhyan succession from different source terrains of identical composition suggests that at the time of vindhyan sedimentation the bgc of rajasthan the bggc of central india and the cggc of eastern india had similar lithological composition this implies that well before the origin of the vindhyan basin these discrete terrains evolved as a single unit that existed as a mesoarchaean nucleus in the northern part of the indian shield

tectonic setting and provenance of the neoproterozoic uinta mountain and big cottonwood groups northern utah constraints from geochemistry nd isotopes and detrital modes abstract the neoproterozoic uinta mountain group was deposited in an east trending intracratonic rift bounded on the north by an active fault system and opening into a shallow sea on the west where the big cottonwood group was deposited in an estuary although this rift may have been associated with the early stages in the breakup of rodinia it was not an aulacogen geochemical nd isotope and detrital mode studies indicate that uinta mountain group sediments were derived from mixed archean and paleoproterozoic sources with the former dominating big cottonwood group sediments appear to have been derived predominantly from paleoproterozoic sources the archean sediment source is the wyoming craton and source rocks comprised dominantly granites enriched in th u y zr hf and ree the relative abundance of enriched granite implied by sedimentary rocks of the uinta mountain group indicates that the wyoming craton is anomalous compared to other archean cratons cia values and a cn k relationships in shales of the uinta mountain and big cottonwood groups indicate high degrees of weathering of sources probably in subtropical to tropical climates supporting a near equatorial location for southwestern laurentia at about 800 ma differences in the nd isotopic composition between the big cottonwood group and neoproterozoic sedimentary rocks in western utah and northeast nevada suggest a northwest striking uplift in northwest utah possibly ancestral to the paleozoic toole uinta arch

the nature of the archean upper crust as revealed by the geochemistry of the proterozoic shales of the kaladgi basin karnataka southern india abstract major trace and rare earth element geochemical studies of the shales belonging to the 1 8 ga proterozoic kaladgi supergroup of southern india shows that they are derived from strongly chemically weathered archean upper crust this has resulted in depletion of na2o cao sr and u in these shales the shales of lower stratigraphic intervals are enriched in mgo fe2o3 cr ni co and sc relative to the upper shales the upper shales by contrast show enrichment in elements such as ba th and zr these characteristics and the barium cobalt ratios show that the early sediments were derived from a more mafic provenance and the contribution of granitoids increased towards the later stages of kaladgi sedimentation rare earth element ree modelling shows that the average kaladgi shale represents a contribution from a crust with a mafic to felsic rock ratio of 60 40 the post depositional introduction of potassium and lree in these shales has been inferred based on the overall geochemical characteristics

quartz arenites review and interpretation abstract quartz arenites orthoquartzites are exceptional sands pettijohn et al 1972 that in the past have formed extensive and commonly thick formations they are somewhat enigmatic in that they do not appear to be forming at the present time this paper examines the origin of quartz rich sands documents the range of environments in which they are found and suggests a model for their genesis quartz sand is most commonly formed by tropical weathering of coarse grained quartz bearing rocks such as granite it may be deposited as texturally immature fluvial sandstones generally under a humid climate with drying of the climate quartz rich bauxitic soils and sandy alluvium can be a substantial source of poorly rounded dune sands for major deserts quartz sand when transported into shallow marine environments becomes very well rounded through reworking by tidal currents such sand may spill into deep oceanic environments and by sedimentary or tectonic transport may come to rest in both stable and active tectonic settings quartz rich sandstones can also be formed by diagenetic destruction of labile framework grains of immature sandstones precambrian orthoquartzites are commonly thick and widespread when compared to younger examples the thinness of post silurian quartz arenites may be largely due to the trapping effect of land vegetation and solution of quartz by organic acids in weathering profiles there is an abundance of shallow marine and fluvial quartz arenites in the early paleozoic and the cretaceous this is due to widespread continental flooding and a humid world climate following the disintegration of supercontinents the shallow marine examples are commonly associated with bituminous sometimes phosphatic shale glauconite and possibly chamositic ironstone there is an abundance of eolian quartz arenites in the permo triassic this is related to a widespread dry continental climate during assembly of the pangean supercontinent

tectono thermal evolution in a region with thin skinned tectonics the western nappes in the cantabrian zone variscan belt of nw spain the palaeotemperature distribution in the transition from diagenesis to metamorphism in the western nappes of the cantabrian zone somiedo la sobia and aramo units are analysed by conodont colour alteration index cai and illite crystallinity ic structural and stratigraphic control in distribution of cai and ic values is observed both cai and ic value distributions show that anchizonal conditions are reached in the lower part of the somiedo unit a disruption of the thermal trend by basal thrusts is evidenced by cai and ic values there is an apparent discrepancy between the ic and cai values in carboniferous rocks of the aramo unit the ic has mainly anchizonal values whereas the cai has diagenetic values discrepant ic values are explained as a feature inherited from the source area in the carboniferous rocks of the la sobia unit both ic and cai indicate diagenetic conditions the anchimetamorphism predated completion of emplacement of the major nappes it probably developed previously and or during the early stages of motion of the units temperature probably decreased when the metamorphosed zones of the sheets rose along ramps and were intensely eroded in the context of the iberian variscan belt influence of tectonic factors on the metamorphism is greater in the internal parts where the strain and cleavage are always present than in the external parts cantabrian zone where brittle deformation and rock translation are dominant with an increasing role of the burial on the metamorphism

petrogenesis of archaean trondhjemites tonalites and granodiorites from eastern finland major and trace element geochemistry

provenance weathering conditions and tectonic evolution history of the cambrian meta sediments in the zhuguangshan area cathaysia block abstract sandstones and mudstones collected from the cambrian sedimentary sequences in the zhuguangshan area were analyzed to trace their provenance weathering conditions depositional environment and precambrian tectonic evolution in the cathaysia block south china detrital zircon geochronology defines at least five populations 2658 2370 ma 1866 1500 ma 1390 1268 ma 1250 858 ma and 830 700 ma the 2648 2370 ma population may correspond to the neoarchean global continental growth the 1816 1510 ma population may represent a subduction event caused by the assembly of the columbia supercontinent the 1390 1268 ma and 1250 858 ma population reflect the tectono thermal events associated with the assembly of the rodinia supercontinent while the 830 700 ma population may be associated with the breakup of the rodinia supercontinent detrital zircon hf isotopic compositions indicate that the mechanism involves multiple juvenile inputs and the reworking of older crustal materials the sandstones and mudstones display k2o enrichment but they are moderately to extremely depleted in na2o cao and sr the major and trace elements reveal that the sandstones and mudstones were derived from recycled felsic rocks and deposited in the passive continental margin the compositions of the protoliths are inferred equivalent to andesite to granodiorite the petrographic and geochemical features indicate that the sandstones and mudstones have undergone medium to intensive chemical weathering conversion of aluminous clay minerals to illite is inferred to be the predominant post depositional potash metasomatism

eastern indian 3800 million year old crust and early mantle differentiation samarium neodymium data for nine granitic and tonalite gneisses occurring as remnants within the singhbhum granite batholith in eastern india define an isochron of age 3775 89 x 10 6 years with an initial 143 nd 144 nd ratio of 0 50798 0 00007 this age contrasts with the rubidium strontium age of 3200 x 10 6 years for the same suite of rocks on the basis of the new samarium neodynium data field data and petrologic data a scheme of evolution is proposed for the archean crust in eastern india the isotopic data provide evidence that parts of the earth s mantle were already differentiated with respect to the chondritic samarium neodymium ratio 3800 x l0 6 years ago