a rapidly growing moraine dammed glacial lake on ngozumpa glacier nepal abstract moraine dammed glacial lakes are becoming increasingly common in the himalaya as a result of glacier mass loss causing concern about glacier lake outburst flood risk in addition to extant lakes the potential exists for many more to form as more glaciers ablate down to the level of potential moraine dams in this paper we document the recent rapid growth of a moraine dammed lake on ngozumpa glacier nepal using a combination of ground based mapping and sonar surveys aerial photographs

first cycle quartz arenites in the orinoco river basin venezuela and colombia modern first cycle quartz arenites are forming in the orinoco drainage basin by at least two distinct mechanisms common to both mechanisms is an environment of intense chemical weathering and extended time over which weathering can occur in the llanos andean foreland basin extended time is provided by temporary storage of orogenically derived sediments on extensive alluvial plains in lowland portions of the guayana shield long soil mineral residence times are produced by very low erosion and transport rates resulting from low relief and tectonic quiescence both weathering intensity and the duration of weathering are important in the modification of sand composition first cycle quartz arenites are produced in diverse tectonic settings and cannot be used by themselves to infer paleotectonic settings of ancient sequences furthermore although tectonic setting is of great importance in determining sand composition modifications during transport and deposition can overprint the effects of the tecto

mudstone petrology of the mesoproterozoic unkar group grand canyon u s a provenance weathering and sediment transport on intracratonic rodinia abstract the synthesis of mudstone petrology and interbedded sandstone detrital zircon geochronology provides insights into the provenance weathering regime hydrodynamic effects and diagenesis of the mesoproterozoic unkar group unkar mudstones were derived largely from the grenville orogen go of southwest texas and the adjacent yavapai mazatzal ym and southern granite rhyolite sgr terranes detrital zircon data indicate distinct pulses of go derived sediment during hakatai and dox formation deposition whereas shinumo sediment contains a larger component of ym and sgr material an increase in plagioclase and biotite abundances in the dox further suggest an orogenic pulse bulk chemistry including elevated cr and ni abundances and ree systematics implicate a heterogeneous provenance for unkar sediment best approximated by a mix of granite or granodiorite with some basalt weathering of unkar sediment as determined from mudstone and crystalline source bulk chemical trends is characterized as moderate cia values between 55 and 70 indicative of a temperate climate in an orogenic setting small less than 10 wt amounts of detrital kaolinite and chlorite are consistent with a moderate temperate weathering regime illite the dominant clay mineral in the unkar group is largely the 2m1 polytype and detrital in origin however it is estimated that 7 of the k in the escalante creek member results from k metasomatism and may form up to 15 wt authigenic 1m or 1md illite the clay size fraction and zircon are the chief contributors to the ree budget in unkar mudstones lithostratigraphic variations in the hree distributions can be attributed partially to variable zircon abundance in the silt size fraction a likely function of hydrodynamic sorting age constraints on the duration of unkar group sedimentation range from 30 to 100 my for the 1100 m conformable shinumo dox succession and result in compacted sedimentation rates of between 1 0 and 3 6 cm 103 yr these rates are comparable to mesozoic foreland basin settings

the candelaria punta del cobre iron oxide cu au zn ag deposits chile several iron oxide rich cu au zn ag deposits define an approximately 5 km wide and at least 20 km long belt along the eastern margin of the coastal batholith near copiapo chile this belt includes the large candelaria mine and a group of middle and small sized mines in the punta del cobre district which is located about 3 km northeast of the candelaria deposit estimated geologic resources of the belt are on the order of 700 to 800 million metric tons mt at 1 0 percent cu the ore occurs in veins breccia and stringer bodies and in replacement bodies that are roughly concordant to bedding the orebodies are hosted mainly by volcanic and volcaniclastic rocks of the punta del cobre formation and in places also occur in volcaniclastic intercalations in the lower part of the overlying early cretaceous chanarcillo group most of the larger orebodies in the belt are located where northwest trending brittle faults intersect the contact between massive volcanic and volcaniclastic rocks these northwest faults and a major northeast trending ductile shear zone control portions of the ore of the candelaria deposit chalcopyrite is the only hypogene cu mineral the cu au ore is characterized by abundant magnetite and or hematite and by locally elevated concentrations of ag zn mo and light rare earth elements the ore is hosted mainly in zones with biotite potassium feldspar calcic amphibole epidote alteration at candelaria in the punta del cobre district ore in the deeper parts of the deposits is similarly associated whereas at shallow levels it occurs in zones of biotite potassium feldspar or albite chlorite calcite alteration mineralization at candelaria punta del cobre took place under relatively oxidized conditions manifested by the formation of specular hematite in parts of the district the pseudomorphic replacement of early specular hematite by magnetite during the main iron oxide formation marks a shift toward more reduced conditions or higher temperatures the bulk of the magnetite probably formed at temperatures of about 500 to 600 c the main sulfide stage followed with formation of pyrite and chalcopyrite at temperatures of 470 to 328 c subsequent martitization of the magnetite points to a temperature decrease cooling of the hydrothermal system is also indicated by the homogenization temperatures of 236 c of saline fluid inclusions in late stage calcite oxygen isotope combined with microthermometric data suggest that magmatic fluids or nonmagmatic fluids equilibrated with magmatic silicates were dominant during the main copper mineralization relatively light oxygen isotope signatures of fluids in equilibrium with late calcite suggest mixing with a nonmagmatic fluid e g basinal brines or meteoric waters during the late stages of hydrothermal activity sulfur isotope ratios of chalcopyrite pyrite pyrrhotite and sphalerite from the bronce candelaria las pintadas santos and socavon rampa deposits range from 34scdt values of 0 7 to 3 1 per mil this narrow range of sulfur isotope ratios near 0 per mil is consistent with sulfur of magmatic origin anhydrite from the candelaria mine paragenetically overlaps with chalcopyrite fluid inclusions in this anhydrite homogenize between 340 and 470 c and it has 34scdt values between 14 5 and 17 5 per mil a sulfate sulfide value of 13 4 per mil for a sample with coexisting anhydrite and chalcopyrite is consistent with sulfide sulfate fractionation at temperatures on the order of 400 c ore lead isotope signatures are homogeneous and similar to those of least altered volcanic host rocks and nearby intrusive rocks radiometric ages including new 40ar 39ar ages for hydrothermal alteration at candelaria point to a main cu au mineralization event at candelaria punta del cobre at around 115 ma the ages indicate that ore formation was broadly coeval with batholithic granitoid intrusions and with regional uplift they further imply that the cu au zn ag deposits formed at shallow crustal levels 3 km the stable isotope data the presence of previously reported hypersaline co2 rich fluid inclusions in the main cu ore stage and saline fluid inclusions in late stage calcite the oxidized character of the first ore fluid pulses and the mineralization age coeval with nearby intrusive activity are consistent with but not unequivocally evidence of magmatic fluid contribution into the hydrothermal system

the paleoproterozoic upper gowganda formation whitefish falls area ontario canada subaqueous deposits of a braid delta the upper gowganda formation is part of the paleoproterozoic huronian supergroup ca 2 5 2 2 ga of the north shore of lake huron the upper gowganda formation rests with sharp conformable contact on glaciogenic rocks of the lower gowganda formation and is gradational with cross bedded sandstones of the overlying lorrain formation at the southern margin of the huronian fold belt in the whitefish falls area the upper gowganda formation is 380 750 m thick and consists of four coarsening upward cycles from 30 to 300 m in thickness each is comprised of the succession a laminated argillite deposited from suspension on the prodelta b argillite and cross laminated sandstone laid down on the delta front by normal fluvial input and flood episodes c fine to coarse grained cross bedded sandstone formed as distributary mouth sand sheets influenced by shallow marine processes abundant soft sediment deformation indicates rapid sedimentation and or contemporaneous fault related seismicity erosional co

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

fault assisted vertical pluton growth coastal cordillera north chilean andes abstract extensive tracts of magmatic arc plutonic suites in the mesozoic coastal cordillera of northern chile imply that large volumes of granitic to dioritic magmas were transferred from a lower crustal source to the upper crust space for single intrusions was created by interaction of vertical pluton growth and dip slip reactivation of arc parallel faults during episodic emplacement of subhorizontal compositionally distinct magma pulses cross sectional pluton shapes are broadly tabular in geometry and were controlled by differential subsidence of the pluton floors during fault reactivation and incremental assembly of subhorizontal sheets equal amounts of floor subsidence of the fault footwall and hanging wall during magma accommodation led to symmetrical intrusions whereas differential subsidence caused plutons to grow asymmetrically strongly asymmetrical plutons resulted when only the hanging wall was reactivated to accommodate floor subsidence late post intrusion contractional deformation resulted in open folding of some tabular plutonic units and local inversion of bounding faults which has modified but not obscured the original syn emplacement geometry and fault kinematics

petrology chemistry and texture of modern river sands amazon river system key idea following the spirit of p d krynine the sands of the world s largest tropical river system are studied to reveal much about the general origin of sand and sandstone thin section and chemical study of 95 sand samples from the amazon river system in brazil peru ecuador and colombia a river system with great contrasts in climate and rock types reveals that sands derived from the andes are lithic arenites whereas sands derived from precambrian terrains are much richer in quartz as are sands in rivers draining tertiary molasse sand becomes more mature downstream along the main stem of the amazon solimoes maranon so that a sample analyzed at the mouth does not accurately reflect headwater composition dilution by quartz rich tributaries and quartz rich tertiary outcrops possible differential elimination of rock fragments and even possible weathering on the modern flood plain of the amazon contribute to enhanced maturity downstream we speculate that although data on modern large subarti

geochemical and paleoenvironmental record of the early to early late aptian major episodes of accelerated change evidence from sierra del rosario northeast mexico abstract the lower to lower upper aptian succession of northern mexico documents the drowning of the shallow water cupido cupidito carbonate platform system followed by deposition of the deep water sediments of the la pena formation using 13 c stratigraphy geochemical and mineralogical information coupled with previous microfacies paleontological and total organic carbon toc data from a stratigraphic section which includes such lithological change this study identifies distinctive episodes of accelerated global environmental change and determines the paleoenvironmental conditions conducive to the deposition of toc rich intervals within the cupidito unit the oceanic anoxic event 1a oae 1a is recorded near the base of the section and the intra furcata negative excursion in the topmost beds of the unit the upper part of the section within the la pena formation is correlatable with the noire level organic carbon rich intervals occur in the lower and middle part of the oae 1a upper part of the cupidito unit base of the la pena formation and in the noire level equivalent reducing conditions within the sediment and oxic dysoxic at the seafloor locally controlled persisted both before and during oae 1a interval in the cupidito lagoon oxygen depleted conditions dysoxic anoxic were more permanent and stronger during the deposition of the base of the la pena formation and the noire equivalent level it is proposed here that deposition of the lower middle part of the oae 1a and the base of the la pena formation was influenced by climate controlled increases in detrital and accompanying nutrient influx that supplied especially biolimiting nutrients fe p fostering marine productivity and toc burial upwelling of nutrient rich deeper waters and minor arrival of nutrients from runoff both account for the domination of radiolaria and organic carbon burial during the noire level equivalent record of the oae 1a time equivalent facies in the cupidito lagoonal sediments confirms the diachronism of the cupidito la pena flooding heterozoan carbonate production in the cupidito ramp was uninterrupted during and after oae 1a periodic arrival of detrital components and nutrients during warmer humid periods accelerated platform drowning which peaked during the diachronic deposition of the la pena formation

high resolution clay mineral and major elemental characterization of a permian triassic terrestrial succession in southwestern china diagenetic and paleoclimatic paleoenvironmental significance abstract relatively high resolution clay mineralogical and chemical index of alteration cia analyses were conducted on the terrestrial lubei section in southwestern china to gain a better understanding of the paleoclimatic paleoenvironmental evolution on land during the permian triassic transition p tr transition clay mineral species include illite smectite mixed layers i s illite irregular chlorite expandable mixed layers c e chlorite kaolinite vermiculite hydroxyl interlayered mineral him and corrensite cia values are as high as approaching 100 in the permian and show a gradual and subtle decrease towards the lowermost triassic where a sudden decrease to persistently

early to middle tertiary foreland basin development and the history of andean crustal shortening in bolivia a 2 5 km thick succession of tertiary strata in the eastern cordillera of southern central bolivia consists of predominantly fluvial and lacustrine deposits the age of the base of the succession is middle paleocene and its upper part which is erosionally truncated is probably late oligocene early miocene the lower 50 130 m of the succession consists of interbedded fluvial sandstone and mudstone including minor paleosols of the santa lucia formation these strata are overlain by up to 50 m of pervasively pedogenically altered mudstone and sandstone in the upper part of the santa lucia formation and lower part of the impora formation that may represent much of late paleocene eocene time above the paleosol zone is a thin zone 10 m of lacustrine carbonate and clastic rocks in the uppermost impora formation and this in turn is overlain by a 2000 m thick upward coarsening succession of clastic fluvial deposits in the cayara camargo and suticollo formations paleocurrent data indicate that fluvial channels that deposited the santa lucia impora and cayara formations flowed mainly westward whereas channels responsible for the camargo and suticollo formations flowed generally eastward modal sandstone petrographic data show a long term evolution from subarkosic during santa lucia deposition to quartz arenitic during impora and cayara deposition to sublitharenitic during camargo and suticollo deposition compositions we argue that the lithofacies and sediment accumulation history of this succession are most consistent with deposition in an eastward migrating foreland basin system e mail decelles geo arizona edu critical to this interpretation is the zone of extreme stratigraphic condensation in the lower impora formation the inferred upper paleocene eocene part of the succession the abrupt decrease in sediment accumulation represented by this zone is difficult to reconcile with the leading alternative model in which continuous postrift thermal subsidence resulted in a gradual continuous decay of early tertiary sediment accumulation rates stratigraphic condensation is however consistent with passage of the forebulge through the region an interpretation that also can be reconciled with the record of foreland basin development in the altiplano to the west and gross sediment accumulation patterns in the subandean zone and modern foreland basin to the east simple flexural modeling and palinspastic reconstruction of the complete cenozoic migration history of the foreland basin system suggest that 1000 km of foreland lithosphere has been underthrust westward beneath the andean orogenic belt at the latitude of southern bolivia the amount of middle and lower crust that would have been added to the andean infrastructure is sufficient to explain the present crustal thicknesses in bolivia

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

extensional tectonics during late cretaceous evolution of the southern central andes evidence from the chilean main range at 35 s abstract the study of late cretaceous earliest paleogene series in the chilean andes at 35 s provides new constraints on the coeval evolution of the area and the southern central andes these correspond to the continental series of brcu and the overlying plan de los yeuques formation pyf zircon age determinations reveal that these represent an almost continuous record of deposition respectively from the cenomanian early campanian to the campanian danian according to paleogeographic reconstructions the brcu series corresponds to the coeval foreland basin deposits in a near arc position which was westerly bounded by the earliest late cretaceous orogen detrital zircon age spectra for this reveal only a cretaceous component thus indicating a depocenter disconnected to that represented by coeval synorogenic deposits in argentina the volcanic rocks of the pyf record an eastern migration of the magmatic loci into the area at least since 80 ma its outcrops display a series of syn sedimentary normal faults indicating extensional conditions throughout its deposition its volcanic rocks conform an apparent bimodal suite characterized by arc like affinities low pressure conditions for the corresponding magmas and juvenile isotopic signatures for all the compositional spectra comparison and modeling of such compositional characteristics indicate a derivation from parental magmas similar to those currently sourcing the southern segment of the andean southern volcanic zone 37 42 s altogether these characteristics suggest an extensional intra arc basin setting for the deposition of this unit probably framed by a normal to thin crust of 30 35 km the extensional conditions observed for this unit are also recorded in numerous late cretaceous arc deposits along the andean margin thus indicating that this corresponds to a regional scale feature this evidence records a change from late cretaceous from compressional to extensional conditions in the arc area which agrees well with major plate tectonic configuration proposed for this time span in the andean margin

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

late mesozoic to paleogene stratigraphy of the salar de atacama basin antofagasta northern chile implications for the tectonic evolution of the central andes abstract the salar de atacama basin the largest pre andean basin in northern chile was formed in the early late cretaceous as a consequence of the tectonic closure and inversion of the jurassic early cretaceous tarapaca back arc basin inversion led to uplift of the cordillera de domeyko cd a thick skinned basement range bounded by a system of reverse faults and blind thrusts with alternating vergence along strike the almost 6000 m thick upper cretaceous to lower paleocene sequences purilactis group infilling the salar de atacama basin reflects rapid local subsidence to the east of the cd its oldest outcropping unit tonel formation comprises more than 1000 m of continental red sandstones and evaporites which began to accumulate as syntectonic growth strata during the initial stages of cd uplift tonel strata are capped by almost 3000 m of sandstones and conglomerates of western provenance representing the sedimentary response to renewed pulses of tectonic shortening which were deposited in alluvial fan fluvial and eolian settings together with minor lacustrine mudstone purilactis formation these are covered by 500 m of coarse proximal alluvial fan conglomerates barros arana formation the top of the purilactis group consists of maastrichtian danian alkaline lava and minor welded tuffs and red beds cerro totola formation 70 64 ma k ar deposited during an interval of tectonic quiescence when the el molino yacoraite late cretaceous sea covered large tracts of the nearby altiplano puna domain limestones interbedded with the totola volcanics indicate that this marine incursion advanced westwards to reach the eastern cd slope cd shortening in the late cretaceous was accompanied by volcanism and continental sedimentation in fault bounded basins associated to strike slip along the north chilean magmatic arc to the west of the cd domain indicating that oblique plate convergence prevailed during the late cretaceous oblique convergence seems to have been resolved into a highly partitioned strain system where margin parallel displacements along the thermally weakened arc coexisted with margin orthogonal shortening associated with syntectonic sedimentation in the salar de atacama basin a regionally important early paleocene compressional event is echoed in the salar de atacama basin by a distinctive angular unconformity which separates paleocene continental sediments from purilactis group strata the basin also records the eocene early oligocene incaic transpressional episode which produced renewed uplift in the cordillera de domeyko and triggered the accumulation of a thick blanket of syntectonic gravels loma amarilla formation

the emplacement of the granitic las tazas complex northern chile the relationship between local and regional strain abstract the coastal batholith of northern chile grew under transtensional extensional conditions that prevailed along the andean margin during the mesozoic the batholith hosts the atacama fault zone a major arc parallel fault system which was characterised by sinistral transtensional shearing during the early cretaceous the las tazas complex is a composite granitoid intrusion that was emplaced syntectonically along the atacama fault zone at 130 ma syntectonic emplacement is indicated by a consistent kinematic history between the complex and its wall rocks together with synchronous crystallisation and shearing ages in contrast to regional patterns the las tazas complex was emplaced during a local change from vertical east side down to dextral transcurrent displacement along the fault zone during intrusion strain was partitioned between non coaxial simple shearing within country rock mylonites and a flattening strain across the crystallising complex this combination indicates that the pluton was emplaced under temporary transpressive conditions that were localised around the pluton probably induced by magma emplacement such a difference between local and regional strain suggests that emplacement related structures should only be related to regional strain states with great care

provenance analysis of muddy sandstones abstract a new approach to determine the precursor grains of detrital and diagenetic clay in sandstones requires a reiterative use of optical microscopy and electron beam microanalysis excluding muddy sandstone successions from analysis of the siliciclastic fill of a basin may lead to incomplete if not erroneous provenance interpretation because of biased samping of the sediments to minimize problems inherent in muddy sandstones we have combined a two tier optical modal analysis with energy dispersive x ray analysis edxa to identify precursors of clay this is a new method of point counting and estimating the mineral compositions of muddy sandstones because we apply the principle of identifying the mineral under the cross hair albeit on a micron scale we call this the new or modified g zzi dickinson method calculations of mass balance type using the results of this new method may be performed to reconstruct the original composition of the parental detrital material we have used muddy siliciclastic rocks outcrop and core samples of siltstones and sandstones from the oligocene intra arc medicine lodge beds of southwestern montana usa for this study eocene volcanic rocks of the challis region and thrusts that have transported pre cenozoic sedimentary and metasedimentary rocks in this region were the sources of the siliciclastic sediments results of modal analysis by the standard gazzi dickinson method and by the new modified gazzi dickinson method using both optical and edxa on a micron scale plot in different clusters in a qfl diagram our results show that the standard gazzi dickinson method is not truly independent of grain size even in the context of provenance analysis on the other hand the modified version has the promise of achievi g truly mineralogical modal petrologic analysis of detrital rocks of any grain size

depositional environments of the early proterozoic espanola formation ontario canada the carbonate rich espanola formation forms part of the huronian supergroup deposited in early proterozoic time between about 2 5 and 2 1 ga ago the espanola formation overlies glacigenic diamict

extensional tectonics cretaceous andes northern chile 27 s in the andes of northern chile the sierra fraga puquios region 27 s exposes a complete record of superposed deformation that affected this part of the orogen during the mesozoic and cenozoic a major extensional event which probably occurred between the aptian and cenomanian generated a suite of structures remarkably similar to those observed in the extensional terranes of western north america four allochthons involving paleozoic and mesozoic strata are bounded by low angle normal faults that place younger rocks over older omitting as much as several kilometers of stratigraphic section in one area an extensional chaos is composed of domino and boudined blocks of neocomian limestone floating in a matrix of cataclastically deformed volcanics domino orientation boudin asymmetry sheath folds and mesoscopic sense of shear indicators identify two senses of movement on the detachments the first and less important was top to the northwest and the second major event was top to the northeast the parautochthonous upper paleozoic basement core of the extensional terrane differs from the metamorphic core complexes in that it is not highly metamorphosed or ductilely deformed we attribute this difference to the lack of a prior crustal thickening event in the andes and to the position of the sierra fraga puquios at the margin of the central chile aborted marginal basin the extensional events occurred during a global episode of rapid spreading which was accompanied along the andean margin by a substantial increase in magmatism

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

quantitative trace element modelling of the crystallization history of the kinojevis and blake river groups abitibi greenstone belt ontario the archean tholeiitic kinojevis suite is characterized by an iron enrichment trend and abundant fe ti oxides in its evolved basalts andesites and rhyolites the rare earth element ree patterns of the suite remain flat from the basalts through to the rhyolites with the development of small negative eu anomalies quantitative modelling of the trace elements from little altered samples is consistent with the mineralogy suggesting that the suite was produced through fractional crystallization of olivine pyroxene plagioclase and fe ti oxides the evolved rhyolites are interpreted as having developed by greater than 90 fractional crystallization in a high level magma chamber the calc alkaline blake river group conformably overlies the kinojevis rocks and is characterized by enrichment in alkalis and silica the ree patterns are light rare earth element lree enriched and the felsic rocks have prominent negative eu anomalies geochemical modelling shows that the suite could have developed either th

the productora cu au mo deposit chile a mesozoic magmatic hydrothermal breccia complex with both porphyry and iocg affinities the productora cu au mo deposit is hosted by a cretaceous hydrothermal breccia complex in the coastal cordillera of northern chile the current resource which includes the neighboring alice cu mo porphyry deposit is estimated at 236 6 mt grading 0 48 cu 0 10 g t au and 135 ppm mo local wall rocks consist of a thick sequence of broadly coeval rhyolite to rhyodacite lapilli tuffs 128 7 1 3 ma u pbzircon and two major intrusions the cachiyuyito tonalite and ruta cinco granodiorite batholith 92 0 1 0 ma u pbzircon previous studies at productora concluded the deposit had strong affinities with the iron oxide copper gold iocg clan and likened the deposit to candelaria based on new information we document the deposit geology in detail and propose a new genetic model and alternative classification as a magmatic hydrothermal breccia complex with closer affinities to porphyry systems hydrothermal and tectonic breccias veins and alteration assemblages at productora define five paragenetic stages stage 1 quartz pyrite cemented breccias associated with muscovite alteration stage 2 chaotic matrix supported tectonic hydrothermal breccia with kaolinite muscovite pyrite alteration stage 3 tourmaline pyrite chalcopyrite magnetite biotite cemented breccias and associated k feldspar albite alteration stage 4 chalcopyrite pyrite muscovite illite epidote and chlorite veins and stage 5 calcite veins the productora hydrothermal system crosscuts earlier formed sodic calcic alteration and magnetite apatite mineralization associated with the cachiyuyito stock main stage mineralization at productora was associated with formation of the stage 3 hydrothermal breccia chalcopyrite is the dominant hypogene cu mineral and occurs predominantly as breccia cement and synbreccia veins with pyrite the alice cu mo porphyry deposit is characterized by disseminated chalcopyrite and quartz pyrite chalcopyrite molybdenite vein stockworks hosted by a granodiorite porphyry stock alice is spatially associated with the silica ridge lithocap which is characterized by massive fine grained quartz altered rock above domains of alunite pyrophyllite and dickite rhenium os dating of molybdenite indicates that main stage mineralization at productora occurred at 130 1 0 6 ma and at 124 1 0 6 ma in the alice porphyry chalcopyrite and pyrite from productora have 34ssulfide values from 8 5 to 2 2 consistent with a magmatic sulfur source and fluids evolving under oxidizing conditions no significant input from evaporite or seawater sourced fluids was detected stage 3 tourmalines have average initial sr of 0 70397 consistent with an igneous derived sr source the productora magmatic hydrothermal breccia complex formed as a result of explosive volatile fluid release from a hydrous intrusive complex metal bearing fluids were of magmatic affinity and evolved under oxidizing conditions despite sharing many similarities with the andean iocg clan strong structural control regional sodic calcic alteration locally anomalous u fluid evolution at the productora cu au mo deposit is more consistent with that of a porphyry related magmatic hydrothermal breccia sulfur rich acid alteration assemblages and relatively low magnetite contents

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

structural setting of the candelaria fe oxide cu au deposit chilean andes 27 30 s the candelaria fe oxide cu au ore deposit lies within the thermal aureole of the lower cretaceous magmatic arc plutonic suite in the candelaria punta del cobre district atacama region northern chile 27 30 s economic mineralization is present within a synplutonic ductile shear zone candelaria shear zone containing foliated and potassically altered volcanic and volcaniclastic rocks of the punta del cobre formation in the candelaria shear zone there was a complex interplay in space and time between ductile and brittle deformation during mineralization at high structural levels brittle moderately to steeply dipping extensional faults curve downward into gently dipping ductilely deformed rocks in the shear zone textural and structural characteristics of the ore and the deformed rocks indicate that the dilation of the sulfide bearing vein systems occurred at the same time as ductile deformation and implies that ductile brittle cycling occurred in a cooling hydrothermal system in the thermal aureole of the arc plutonic rocks the la brea and the san gregorio plutonic complexes are exposed in the plutonic arc immediately adjacent to the candelaria orebody the intrusions are interpreted to be flat lying tabular bodies emplaced by roof uplift floor depression mechanisms during regional extensional deformation an 40 ar 39 ar age of 119 6 1 2 ma and five k ar ages between 123 and 117 ma were obtained for the la brea pluton and an 40 ar 39 ar age of 111 5 0 4 ma was obtained for the san gregorio plutonic complex 40 ar 39 ar ages of 111 0 1 4 and 110 7 1 6 ma obtained from syntectonic biotite from the candelaria shear zone are interpreted to be the age of the mineralization these are indistinguishable from the age of the san gregorio plutonic complex and therefore heat from this pluton could have promoted ductile deformation during emplacement of the orebody this result together with known isotopic similarities between candelaria ore and the plutonic rocks implies that the san gregorio plutonic complex was the most probable source of the mineralizing fluids at candelaria although the critical intrusive unit may not be exposed the ore deposit is located where a roof uplift fold in the side wall of the san gregorio plutonic complex provided a dilatational site which may have drawn hydrothermal fluids out of the adjacent fractionating magmas into a structural trap in the punta del cobre formation below the carbonate rocks of the chanarcillo group

geochemistry of fine grained clastic sediments in the archean abitibi greenstone belt canada implications for provenance and tectonic setting abstract archean sandstone and shale samples from turbiditic tf and alluvial facies af in the southern volcanic zone of the abitibi greenstone belt canada have been analyzed for major and trace elements the sandstones from tf are mainly greywackes whereas those from af are lithic arenite sio 2 ranges from 51 to 68 geochemically the clastic sediments cluster into mafic element mep and low mafic element lmep populations the mep sediments which include turbiditic facies sediment samples from the larder lake group in the kirkland lake area are characterized by elevated abundances of mgo 5 ni 200 ppm cr 400 ppm sc 22 ppm and feo 7 5 abundances lower contents of incompatible elements such as th u ta hf zr and total rees n ratios are 4 5 with eu eu 1 the lmep sediments which include samples from the beatty dome and three nations lake formations in timmins the kewagama group noranda area and timiskaming group in the kirkland lake area however show reverse abundance distributions the geochemical gaps shown by the major and trace elements define two populations of sediments suggesting contrasting provenances for them the mep sediments require a dominantly mafic and ultramafic 90 source with 10 felsic contribution whereas the lmep sediments require a mixture of felsic 75 and mafic 25 rocks this compositional distribution of the sediments is closely related to their tectonic environments the mep was formed in basins proximal to or within an active oceanic rift consequently having ultramafic and mafic rocks as the major source for the lmep calc alkaline volcanic sequences formed in an island arc like setting were the major source these two contrasting volcanic plutonic terranes were tectonically juxtaposed during late archean collisional tectonics

distribution timing and causes of andean deformation across south america the andean orogeny in south america has lasted over 100 ma it comprises the peruvian incaic and quechuan phases the nazca and south american plates have been converging at varying rates since the palaeocene the active tectonics of south america are relatively clear from seismological and global positioning system gps data horizontal shortening is responsible for a thick crust and high topography in the andes as well as in se brazil and patagonia we have integrated available data and have compiled four fault maps at the scale of south america for the mid cretaceous late cretaceous palaeogene and neogene periods andean compression has been widespread since the aptian the continental margins have registered more deformation than the interior for the peruvian phase not enough information is available to establish a tectonic context during the incaic phase strike slip faulting was common during the quechuan phase crustal thickening has been the dominant mode of deformation to investigate the mechanics of deformation we have carried out 10 properly scaled experiments on physical models of the lithosphere containing various plates the dominant response to plate motion was subduction of oceanic lithosphere beneath continental south america however the model continent also deformed internally especially at the margins and initial weaknesses

the role of true polar wander on the jurassic palaeoclimate from the late carboniferous until the middle jurassic continents were assembled in a quasi rigid supercontinent called pangea the first palaeomagnetic data of south america indicated that the continent remained stationary in similar present day latitudes during most of the mesozoic and even the palaeozoic however new palaeomagnetic data suggest that such a scenario is not likely at least for the jurassic in order to test the stationary versus the dynamic continent model we studied the jurassic apparent polar wander paths of the major continents that is eurasia africa and north america that all in all show the same shape and chronology of the tracks with respect to those from south america we thus present a master path that could be useful for the jurassic pangea one of the most remarkable features observed in the path is the change in pole positions at 197 ma early jurassic which denotes the cessation of the counter clockwise rotation of pangea and commencement of a clockwise rotation that brought about changes in palaeolatitude and orientation until the end of the early jurassic 185 ma here we analyse a number of phenomena that could have triggered the polar shift between 197 and 185 ma and conclude that true polar wander is the most likely in order to do this we used morgan s tectonophysics 94 123 139 1983 grid of hotspots and performed absolute palaeogeographical reconstructions of pangea for the late triassic and jurassic the palaeolatitudes changes that we observe from our palaeomagnetic data are very well sustained by diverse palaeoclimatic proxies derived from geological and palaeoecological data at this time of both the southern and northern hemispheres

contrasting styles of superposed deformation in the southernmost andes brittle fault arrays regional structural patterns and marine multichannel seismic reflection profiles across the magallanes basin near 53 s latitude show contrasting styles of superposed extensional contractional and transtensional deformation below the northeastern end of a ne sw transect of the basin seismic reflection profiles show deepening levels of pre late jurassic basement and upper jurassic to lower cretaceous volcanic horizons toward the southwest this trend is associated with apparent normal fault offsets that may be attributed to a widespread late jurassic early cretaceous extensional event contrastingly below the southwestern end of magallanes basin subsurface depths of basement and volcanic horizons shallow to the southwest integration of seismic reflection data with published seismic refraction data geologic mapping and satellite gravity anomaly data suggest that this shallowing trend is controlled structurally by craton vergent basement involved contractional deformation this style of shortening contrasts with thin skinned styles of thrusting in the northern and central parts of the basin on tierra del fuego structural relationships stratigraphic data and previously published metamorphic data indicate that basement involved contraction occurred during latest cretaceous to early tertiary time overprinting all contractional structures steep tertiary strike slip and oblique slip faults in the central and southern parts of the magallanes basin display oblique normal mostly down to the southwest offsets across wnw ese trending segments of the south american scotia transform plate boundary on the south american continent newly discovered features along the plate boundary segment include sediment filled grabens and half grabens up to 7 km wide that occur between subparallel fault splays in the magallanes fault zone kinematic analyses of fault slip data indicate that sinistral transtensional motion occurred within this fault zone between the eastern arm of the straits of magellan and central tierra del fuego together these data indicate that 1 the southernmost andes were under compression during rapid cooling and exhumation of unique high pressure 8 kbars basement rocks now exposed in the darwin metamorphic complex dmc near 55 s latitude and 2 final exhumation of basement rocks in the dmc occurred during a shift away from compression toward transtension during late tertiary crustal relaxation we suggest that preexisting structural anisotropies formed during basement involved contractional deformation and the initiation of seafloor spreading south of south america during the oligocene controlled late tertiary sinistral transtensional patterns in the magallanes fault zone

paleoclimatic control on the composition of the paleoproterozoic serpent formation huronian supergroup canada a greenhouse to icehouse transition unlike other feldspathic arenites in the 12 km thick huronian supergroup strata of the 250 350 m thick serpent formation contain abundant plagioclase deposited in a distal alluvial setting the serpent formation consists of arenites with minor shale and siltstone the plagioclase abundance could reflect unroofing of a unique source relative to other huronian units or could be due to less intense chemical weathering major trace and rare earth element ree geochemistry of the serpent formation are used to evaluate these two possibilities serpent formation source rocks form part of the archean superior province which mainly consists of tonalitic granitic and supracrustal assemblages and their metamorphosed equivalents in al2o3cao na2ok2o acnk space shales define a linear array which indicates that the unweathered source contained plagioclase and k feldspar in the ratio of 5 1 this particular array indicates that serpent fluvial systems tapped a variably weathered carapace developed on the superior province rather than purely fresh bedrock a modelled mixture of 80 tonalite and 20 granite group rocks reproduces ree and trace element characteristics of the serpent formation sandstones and shales average eu eu 0 87 lan smn 4 4 gdnybn 1 8 and th sc 1 65 mass balance considerations indicate that less than 5 supracrustal group rocks can be added at the expense of the granite group a unique source for serpent detritus is rejected because underlying and overlying units have approximately the same provenance composition plagioclase k feldspar ratios in serpent sandstones range from 2 1 to 1 2 which achieves mass balance among source mudstone and sandstone compositions when plotted in acnk space achievement of mass balance implies that sandstones and mudstones were derived from the same weathering profiles these findings indicate that plagioclase preservation in the serpent formation is related to less intense compared with other huronian arenites paleoweathering conditions probably a harbinger of the widespread continental glaciation recorded in the overlying gowganda formation

the role of detrital composition and climate on the diagenetic evolution of continental molasses evidence from the cambro ordovician guaritas sequence southern brazil abstract the cambro ordovician molassic guaritas sequence camaqua basin southern brazil comprises alluvial fan and braided fluvial sandstones and conglomerates with intercalated aeolian and lacustrine deltaic deposits and andesitic lava flows the sediments display a complex detrital composition derived from plutonic gneissic acidic volcanic and low grade metamorphic source rocks this detrital assemblage was strongly modified by semi arid continental near surface diagenesis early cementation by hematite smectite quartz and calcite and the relatively limited burial prevented strong compaction and preserved some primary macroporosity in most of the sandstones whereas the absence of early cements and or abundance of ductile grains promoted substantial porosity destruction by compaction and the inhibition of further diagenetic modifications the diagenetic dissolution and replacement of volcanic rock fragments and detrital feldspars by clays and albite changed the original framework composition as well as the tectonic provenance classification of the sandstones detailed quantitative petrographic study allowed the reconstruction of the original detrital compositions and the distinction of six different pathways of diagenetic evolution of the sandstones attesting to the efficiency of this method for diagenetic modelling and provenance analyses of ancient sandstones

crustal growth during back arc closure cretaceous exhumation history of cordillera darwin southern patagonia the cordillera darwin metamorphic complex is unique in the andes in exposing kyanite staurolite schist north of the beagle channel in southern patagonia garnet in amphibolite facies pelitic schists from bahia pia has patchy textures whereby some grains consist of clear grossular rich garnet with fine grained s1 inclusion trails truncated by regions of turbid spessartine pyrope rich garnet with biotite muscovite plagioclase and quartz inclusions micron scale aqueous inclusions in turbid garnet are consistent with recrystallization facilitated by fluid ingress s2 inclusion trails indicate this was broadly contemporary with the growth of kyanite and staurolite in the matrix pseudosection modelling in na2o cao k2o feo mgo al2o3 sio2 h2o tio2 fe2o3 nckfmashto is used to infer a p t path dominated by decompression from 12 to 9 kbar at t 620 c coupled with garnet mode decreasing from 5 to 1 u th pb in situ dating of s2 monazite indicates that staurolite and kyanite growth and thus exhumation was underway before 72 6 1 1 ma contact aureoles developed adjacent to late granite intrusions include sillimanite bearing migmatites formed at p 6 kbar after 72 ma metamorphism of southern cordillera darwin induced by continental underthrusting beneath the arc related to closure of the rocas verdes back arc basin was terminated by thrusting controlled exhumation with the rocks at p 9 kbar by c 73 ma and 6 kbar by c 70 ma

mid cretaceous polar standstill of south america motion of the atlantic hotspots and the birth of the andean cordillera article i nfo a paleomagnetic study on mid cretaceous rocks from the san bernardo foldbelt patagonia yields high unblocking temperature and high coercivity magnetizations the results indicate absence of relative vertical axis rotations during development of the foldbelt with the associated pole position being highly concordant with coeval poles from brazil and patagonia taken together mid cretaceous poles derived from studies in widely distributed localities provide supportive evidence that south america was essentially motionless with respect to the paleomagnetic axis from ca 125 to at least 100 ma the paleolatitudes of south america are not consistent with the occurrence of mid cretaceous true polar wander suggesting that the previously observed discrepancy between the paleomagnetic and the fixed indo atlantic hotspot reference frames be related to motion of the atlantic hotspots in agreement with this the discrepancy is diminished by half when the cretaceous poles of the americas are observed in a moving hotpots reference frame with the residual offset being comparable to that seen for younger time intervals the south american paleopoles and the moving hotspot framework provide a kinematic scenario that allows relating the extensional tectonics in the early stages of andean evolution with episodic divergence between the trench and the continental interior likewise the beginning of contractional events correlates with model predicted westward acceleration of south america in the late cretaceous suggesting that the continent episodically overrode the andean trench by those times we argue that this change in andean tectonic regime is associated to major plate reorganization at ca 95 ma

late stage evolution of the chemehuevi and sacramento detachment faults from apatite u th he thermochronometry evidence for mid miocene accelerated slip the unique low temperature sensitivity of the apatite u th he ahe technique provides valuable new information relating to the late stage thermal evolution of metamorphic core complexes in the southern basin and range province in all the ranges studied the data show greater complexity than previously seen from other thermochronometers in the chemehuevi and northern sacramento mountains a trend of decreasing ages toward the northeast applies only for the structurally shallowest parts of the footwall this is followed by essentially age invariant segments of 15 1 ma reflecting an increase in the rate of detachment fault movement the timing of the change suggested by the data agrees with recently published ahe results from the nearby harcuvar mountains located 125 km to the southeast suggesting that an underlying regional mechanism triggered this change plate reconstructions indicate that the pioneer mendocino fracture zone migrated northward through the area at this time leaving a slab window in its wake we suggest that the additional heat applied to the base of the lithosphere from the slab window resulted in mechanical weakening of the area and that this combined with the extensional strain regime present resulted in an increased rate of slip within the actively extending metamorphic core complexes this agrees well with timing constraints for extension onset in the central basin and range province of southern nevada and suggests that weakening of the southern basin and range province at 15 1 ma was partially responsible for the onset of extension in the central basin and range these findings demonstrate that improved constraints on changes in extension rate within the basin and range province should be possible through the application of the ahe technique to other metamorphic core complex footwalls

the late jurassic to present evolution of the andean margin drivers and the geological record 1 uncommonly long lived subduction and variable plate geometry along the south american andean plate margin resulted in diverse relationships between magmatic flux and extensional and contractional deformation as recorded by the overriding continental plate convergence velocities absolute overriding plate velocities and subducting slab ages were resolved along the trench from 170 ma to the present using a recently developed kinematic global plate model to identify any relationship between subduction conditions deformation style and magmatic features in the overriding plate key correlations reflect the dependence of macroscopic crustal strain style on subduction mechanism and relative plate vectors extensional back arc basins involving mafic oceanic crust developed only when the overriding plate velocity of south america was directed away from the trench and the modeled age of the subducting slab was older than 50 myr the development of fold and thrust belts and uplift of major plateaus was accompanied by trench normal convergence velocities in excess of 4 cm yr parameters investigated in this study revealed no correlation with the timing of major magmatic events nor was any correlation observed with the structural style of fold and thrust belts

a u pb age for mineralized nipissing diabase gowganda ontario concordant baddeleyite and rutile analyses from mineralized nipissing diabase at gowganda s castle mine yield a u pb age of baddeleyite is a primary phase of the diabase therefore the age dates the magmatic emplacement the genesis of rutile is less certain rutile could have formed during an episode of alteration veining and ag mineralization in which case the u pb age would also date the secondary event and demonstrate that ag mineralization was related to the nipissing magmatism however a primary magmatic origin of the rutile cannot be completely excluded for now thus the u pb age may not constrain the time of ag mineralization further research is needed to clarify this point

continental underthrusting and obduction during the cretaceous closure of the rocas verdes rift basin cordillera darwin patagonian andes 1 the patagonian andes record a period of cretaceous neogene orogenesis that began with the compressional inversion of a late jurassic rift called the rocas verdes basin detrital zircon ages from sediment that filled the southern part of the basin provide a maximum depositional age of 148 ma suggesting that the basin opened approximately simultaneously along its length during the late jurassic structural data and u pb isotopic ages on zircon from granite plutons near the beagle channel 55 s show that basin inversion involved two stages of shortening separated by tens of millions of years an initial stage created a small 60 km wide thrust wedge that placed the basaltic floor of the rocas verdes basin on top of adjacent continental crust prior to 86 ma structures and metamorphic mineral assemblages preserved in an exhumed middle to lower crustal shear zone in cordillera darwin suggest that this obduction was accompanied by south directed subduction of the basaltic crust and underthrusting of continental crust to depths of 35 km beneath a coeval volcanic arc a subsequent stage of out of sequence thrusting culminating in the paleogene shortened basement and upper jurassic igneous rock in the internal part of the belt by at least 50 km forming a bivergent thrust wedge this latter period coincided with the exhumation of rocks in cordillera darwin and expansion of the fold thrust belt into the magallanes foreland basin this orogen provides an important example of how orogenesis initiated and led to continental underthrusting and obduction of basaltic crust during closure of a quasi oceanic rift basin

geochemistry of precambrian carbonates iv early paleoproterozoic 2 25 0 25 ga seawater in an effort to narrow down the first and second order variations in isotopic composition of the early paleoproterozoic 2 25 0 25 ga seawater we report a comprehensive study of mineralogy chemistry and isotopic composition of the malmani dolomite transvaal supergroup south africa duck creek dolomite wyloo group australia and bruce limestone member of the espanola formation huronian supergroup canada based on trace element data their diagenetic rank increases in the order duck creek bruce malmani the interpolation of alteration trends to best values yields an estimate of 0 70550 for 87sr86sr comparable to our previously published value of 0 70496 for 13c the measured range of 0 1 5 pdb is similar to that observed for phanerozoic marine carbonates in contrast the best 18o value for dolostones is 5 pdb depleted in 18o relative to phanerozoic counterparts but comparable to the estimates obtained by similar approach for archean facies the isotope geochemistry and mineralogy of bruce limestone member is consistent with the proposition that the sequence may have been deposited in a lacustrine environment possibly related to the recession of the brace glaciers if such interpretation proves unacceptable from a sedimentological standpoint the origin of the unusual isotopic signatures must be sought despite the above trace element interpretation in as yet unspecified post depositional alteration phenomena in contrast to phanerozoic carbonates the early paleoproterozoic as well as the previously described archean counterparts are enriched in mn and fe

the gowganda formation in the southern part of the huronian outcrop belt ontario canada stratigraphy depositional environments and regional tectonic significance abstract the early proterozoic huronian supergroup of the north shore of lake huron comprises a lower part that is largely of continental origin and a more widespread dominantly marine upper succession the gowganda is the lowest formation of the upper huronian in the southern part of the huronian outcrop belt it is in conformable but commonly erosional contact with the underlying serpent formation the basal part of the gowganda formation unit 1 is stratified diamictites boulder and cobble orthoconglomerates and subordinate thin turbiditic sandstones these rocks are largely resedimented but glaciogenic the basal diamictite complex is overlain by unit 2 a thick argillite formed in deep water during a period of glacial retreat unit 3 begins with a discontinuous glaciomarinite this is overlain by resedimented boulder and cobble conglomerates that pass up into sandy turbidites down to basin movements on syn sedimentary north trending normal faults led to deposition of coarsening upward wedges of eastward transported debris there is a thick stratified diamictite complex at the top of unit 3 in the eastern part of the study area it includes a 50 m thick argillite the diamictite complex is overlain by a series of coarsening upward sequences formed by prograding deltas these comprise unit 4 of the gowganda formation the lowest deltaic sequence includes a thick diamictite in the east and a more widespread thin diamictite in its upper part the glaciogenic and resedimented nature of the rocks at the base of the gowganda formation suggests that basin deepening was tectonic rather than due to eustatic sea level rise glaciation in the huronian supergroup may have been initiated by uplift of rift shoulders and propagated by influx of marine waters into the interior of the continent the regional extent of the gowganda formation its unconformable relationship with older huronian rocks in the north and its largely resedimented nature all indicate an episode of regional subsidence possibly after continental separation similar tectonic and climatic events have been postulated for early proterozoic rocks of southeastern wyoming and possibly elsewhere lending support to earlier correlations based mainly on stratigraphic similarities glaciogenic diamictites are present above the breakup unconformity in many early and late proterozoic successions suggesting a possible causative relationship

geochemistry and origin of archean felsic metavolcanic rocks central noranda area quebec canada felsic magma petrogenesis was studied by analyzing 24 stratigraphically controlled archean andesite to rhyolite lava flows of both tholeiitic and calc alkalic affinity from the upper noranda subgroup quebec using instrumental neutron activation and x ray fluorescence techniques the lavas have moderate values of la yb n 0 9 3 8 and low values of 100 th zr 1 according to calculations following batch partial melting and rayleigh fractional crystallization models both the calc alkalic and tholeiitic felsic volcanic rocks are probably products of shallow level fractional crystallization of mafic parental magmas formed respectively by lower 7 for calc alkalic and higher 14 for tholeiitic degrees of partial melting of a primitive mantle source contribution to the magma genesis from plausible crustal materials was negligible a back arc type diapirism is geochemically suggested for the tectonic model of origin of noranda felsic magmas in conformity with geological observations felsic volca

constraints on the development of orogenic style gold mineralisation at mineral de talca coastal range central chile evidence from a combined structural mineralogical s and pb isotope and geochronological study mineral de talca is a rare occurrence of mesozoic gold bearing quartz vein mineralisation situated within the coastal range of northern chile quartz veins controlled by nnw sse trending faults are hosted by devonian carboniferous metasediments of greenschist facies and younger undeformed granitoid and gabbro intrusions the principal structural control in the area is the easterly dipping nnw sse trending el teniente fault which most likely developed as an extensional normal fault in the triassic but was later reactivated as a strike slip fault during subsequent compression a dilational zone in the el teniente fault appears to have focussed fluid flow and an array of nw se trending veins is present as splays off the el teniente fault mineralised quartz veins typically up to a metre thick occur in three main orientations 1 parallel to and within nnw sse trending e dipping faults throughout the area 2 along nw se trending ne dipping structures which may also host andesite dykes and 3 rarer e w trending subvertical veins all mineralised quartz veins show evidence of multiple fluid events with anastomosing and crosscutting veins and veinlets some of which contain up to 3 5 vol base metal sulphides mineralogically au is present in three textural occurrences identified by 3d ct scanning 1 with arsenopyrite and pyrite in altered wall rock and along the margins of some of the veins 2 with cu pb zn sulphides within quartz veins and 3 as nuggets and clusters of native au within quartz fluid inclusion work indicates the presence of co2 ch4 bearing fluids with homogenisation temperatures of 350 c and aqueous fluids with low moderate salinities 0 4 15 5 wt nacl eq with homogenisation temperatures in the range of 161 321 c the presence of au with arsenopyrite and pyrite in structurally controlled quartz veins and in greenschist facies rocks with evidence of co2 bearing fluids is consistent with an orogenic style classification for the mineralisation however the significant amounts of base metals and the moderate salinity of some of the fluids and the proximity to felsic granitoid intrusions have raised the possibility of an intrusion related origin for the mineralisation vein sulphides display s isotope signatures 34s 2 1 to 4 3 that are intermediate between the host rock metasediments 34s 5 3 to 7 5 and the local granitoids 34s 1 3 to 1 4 indicating a distinct crustal source of some of the s in the veins and possibly a mixed magmatic crustal s source the local granite and granodiorite give u pb zircon ages of 219 6 1 and 221 3 2 8 ma respectively lead isotopic compositions of galena in the veins are consistent suggesting derivation from a homogeneous source differences however between the isotopic signatures of the veins and igneous feldspars from nearby intrusions imply that these bodies were not the source of the metals though an igneous source from depth cannot be discounted the triassic age of the granitoids is consistent with emplacement during regional crustal extension with the el teniente fault formed as an easterly dipping normal fault the change to a compressional regime in the mid jurassic caused reactivation of the el teniente fault as a strike slip fault and provided a structural setting suitable for orogenic style mineralisation the intrusions may however have provided a structural competency contrast that focused the mineralising fluids in a dilational jog along the el teniente fault to form wnw trending veins as such the mineralisation is classified as orogenic style and the identification of the key mineralogical isotopic and structural features has implications for exploration and the development of similar deposits along the coastal range

exceptions to the relationship between plate tectonics and sandstone composition abstract routine use of qfl qplsmlvm and lmlslv plate tectonic provenance diagrams has led to the recognition of error populations for these sandstones the interpretation of tectonic setting based on data exclusive of petrography does not coincide with the interpretation of tectonic setting based on their location on compositional diagrams the anomalous sandstones can be separated into at least four categories i sandstones deposited during the transition between tectonic regimes may be derived in part from relict source rocks ii the relative enrichment of sandstones in detrital quartz by weathering and or depositional reworking may lead to an inaccurate interpretation of tectonic setting from compositional data iii sandstones deposited in tectonic settings as yet unrepresented on he provenance diagrams may plot between the provenance fields or overlap existing fields iv the role of detrital carbonate rock fragments directly affects the location of data points on provenance diagrams and is an unresolved problem the first two categories represent permanent problems in the use of sandstone composition as an indicator of plate tectonic setting whereas the last two may only be temporary problems that can be solved by more data and a consensus on determining detrital modes recognition of anomalous sandstones in the rock record will reduce the probability of erroneous interpretations of tectonic setting and enhance the use of sandstone petrology as an indicator of plate tectonic setting

the reappearance of the huronian in wyoming rifting and drifting of ancient continents striking stratigraphic and sedimentological similarities between the early proterozoic huronian supergroup of the canadian shield and the snowy pass supergroup of wyoming suggest that they were deposited in a single broad epicratonic basin developed atop a large archean continent that included the superior and wyoming geological provinces breakup of the continent after the 2 2 ga intrusion of widespread gabbro sheets and dykes resulted in the separation of the archean superior and wyoming cratons and their early proterozoic covers these crustal fragments were subsequently reassembled during early proterozoic 1 85 ga orogenesis the end result being the present 2000 km separation of the huronian and snowy pass supergroups and their archean basements

on the mechanism of crustal block rotations in the central andes abstract the results of 62 paleomagnetic studies of jurassic and younger rock units from the andes of argentina bolivia chile and peru are examined for indications of the timing and mechanism of crustal block rotations rotations and their 95 confidence intervals are recalculated in pole space using a new south american reference apw path as observed by many investigators the arica deflection the change in topographic and tectonic trends located near arica northernmost chile divides the data into a counterclockwise rotated set north of the deflection and a clockwise rotated set to the south rotations range from 75 counterclockwise to nearly 50 clockwise and show substantial differences between neighboring crustal blocks south of arica there is a weak indication of a decrease in amount of rotation with distance from the plate margin but no such correlation is observed north of arica rotation increases with age both north and south of arica for rock units of 90 ma for older rock units no correlation between age and amount of rotation exists unlike the western margin of north america no significant margin parallel displacements accompany the rotations several models for rotation are examined it is concluded that a combination of late cenozoic oroclinal bending and post mid cretaceous local block rotations in response to shear driven sub crustal ductile flow can explain most of the paleomagnetic and geologic observations

paleomagnetism and tectonics of the southern atacama desert 25 28 s northern chile we report paleomagnetic results for 131 sites from the modern forearc of northern chile 25 s and 28 s remanent magnetization in volcanic and intrusive rocks is mostly primary while a secondary magnetization is observed in sedimentary rocks comparison of locality mean directions with expected paleomagnetic directions indicates vertical axis rotations from 7 3 21 6 counterclockwise to 52 7 17 clockwise jurassic to early cretaceous rocks from the coastal cordillera and cretaceous to paleocene rocks from the central depression show similar magnitude 30 clockwise rotations while more variable rotations occur in mesozoic to eocene rocks of the precordillera clockwise rotations in mesozoic and paleogene rocks occur in the chilean frontal cordillera south of 27 30 s paleomagnetic results in three large miocene ignimbrite sheets overlying rotated and nonrotated older rocks in the precordillera and pre andean depression which show no relative rotation between sites indicate that most rotations within the study area occurred prior to 18 ma early miocene and likely during and after the incaic tectonic event which affected large tracts of the central andes the postulated onset of rotations in the north chilean forearc was contemporaneous with the beginning of horizontal shortening and uplift of the eastern cordillera in bolivia and northwestern argentina rotation of the chilean forearc enhancement of the curvature of the central andes and the formation of the bolivian orocline seem to be for the most part closely linked to the evolution of the eocene oligocene tectonics of the eastern cordillera

timing kinematics and displacement of the taltal fault system northern chile implications for the cretaceous tectonic evolution of the andean margin

magmatic arc fault systems deformation partitioning and emplacement of granitic complexes in the coastal cordillera north chilean andes 25 30 s to 27 00 s three fault systems were responsible for permian to late cretaceous deformation of the overriding plate of the andean convergent margin in the coastal cordillera of northern chile 25 30 to 27 00 s displacements were linked to crustal growth expressed by the emplacement of a sequence of magmatic arcs the tigrillo fault system active from triassic to early cretaceous time was characterized by arc normal extension with increasing left oblique extension transtension from early jurassic to early cretaceous time stretching of the crust created space for triassic early jurassic and early cretaceous arc basins where epiclastic volcaniclastic and volcanic sequences accumulated in continental to shallow marine environments tabular plutonic complexes were emplaced by roof uplift floor subsidence that allowed a vertical transfer of material in the crust without significant horizontal extension the atacama fault system was initiated at c 132 ma as a mainly left strike slip fault during left oblique extension of the margin elongate tabular plutonic complexes were emplaced within the atacama fault system between c 132 and c 106 ma again by roof uplift floor subsidence mechanisms ductile brittle transitions in synplutonic mylonitic rocks of the atacama fault system provided the setting for kiruna type fe apatite and fe oxide with cu and or au ores the chivato fault system was active as an extensional fault system at the eastern side of the coastal cordillera during displacement on the tigrillo fault system and later between c 125 and c 93 ma as a partitioned left oblique extensional fault system in post early cretaceous time the chivato fault system was inverted by left oblique contraction transpression when nw trending transfer faults some probably reactivated lateral ramps in the tigrillo fault system accommodated clockwise vertical axis rotations of 35 45 contraction inverted the atacama fault system and tigrillo fault system and was responsible for west vergent thin skinned fold thrust deformation in stratified rocks throughout the margin

what is the structure of the forearc region in the central andes of northern chile an approach from field data and 2 d reflection seismic data abstract the tectonic evolution of the andean forearc in the central andes is understood by the superimposition of deformational events associated with different tectonic regimes which control the main morpho tectonic units of northern chile varied interpretations have opened a great discussion about the deep structure of the andean forearc and its implications in the evolution of the central andes these ideas are commonly accompanied by the lack of regional tectonic models that explain the geometry and kinematics of first order structures the main limitations are due to the unknown origin of the structural styles in the substrate and their relationships with those exposed in the field in order to solve this problem we present a new structural model of the forearc region of northern chile specifically along the central depression 22 23 s supported by field observations and the structural interpretations of 2d seismic data four major tectono sequences have been recognized related to a a paleozoic triassic basement rocks b a jurassic lower cretaceous synrift deposits c upper cretaceous paleocene volcano synorogenic deposits and d a late cenozoic covered gravel the first order structural styles identified correspond to inverted half graben basement structures related to the jurassic tarapaca basin the hanging wall blocks of the master faults are usually composed of large inversion anticlines along which jurassic and lower cretaceous siliciclastic deposits were expelled other basement contractional structures consist of short cut and by pass faults that modified the previous geometry of extensional systems the second order thin skinned fold and thrust exposed in the region accommodated the shortening experienced by the mesozoic synrift deposits during the basin inversion the upper cretaceous paleocene deposits usually unconformably overlie the folded mesozoic synrift deposits this structural feature is first order evidence of the positive reactivation of the mesozoic rift basins and therefore is a regional marker of the onset of the andean uplift in the region

rare earth element patterns in archean high grade metasediments and their tectonic significance abstract metasediments from two contrasting types of archean high grade terrains are interpreted as being derived from distinct tectonic settings the kapuskasing structural zone canada represents the deep roots of a typical greenstone belt whereas the limpopo province southern africa and western gneiss terrain australia mainly consist of shelf sediments deposited on a granitic basement and then buried to the depths required for granulite fades metamorphism upper amphibolite to granulite fades paragneisses from the kapuskasing structural zone have ree patterns similar to those of greenstone belt sediments except where partial melting has occurred forming restites with eu enrichment and melts with eu depletion except in this latter instance metamorphism has not affected the primary ree patterns ree patterns in archean upper amphibolite to granulite facies metasediments from the central limpopo province and western gneiss terrain show wide differences ranging from patterns which resemble those in post archean terrigenous sediments to typical archean sedimentary rock patterns the diversity in ree patterns for these shallow shelf metasediments is interpreted as resulting from derivation from local provenances samples with post archean patterns displaying eu depletion are interpreted as being derived from k rich granitic plutons which were portions of small stable early archean terrains precursors of the widespread late archean proterozoic episode of major cratonic development

geologic evidence for a mantle superplume event at 1 9 ga 1 both preserved and restored areal distributions of proterozoic marine intracratonic passive margin and platform sediments show a prominent peak at 1 9 ga indicating that shallow marine sediments were widespread on the continents and that sea level was high at this time the chemical index of alteration in shales deposited at this time was high suggesting warm climates possibly due to enhanced co2 levels in the atmosphere high sea level and warms climate may also explain an abundance of black shale banded iron formations and shallow marine phosphate deposits and an increase in the number of occurrences and diversity of stromatolites in general and microdigitate stromatolites at 1 9 ga all of these observations are consistent with a 1 9 ga superplume event the occurrence of only a minor positive carbon isotope shift in marine carbonates at 1 9 ga indicates that the relative rates of burial of organic and oxidized carbon remained about the same as at present slightly low 87sr 86sr isotopic ratios in seawater at 1 9 ga reflect increased mantle input of sr from the proposed superplume event whereas higher ratios at 1 85 1 75 ga may reflect increased input of continental sr from a growing supercontinent the first massive sulfate evaporites in the geologic record at 1 8 1 6 ga follow the possible 1 9 ga superplume event this may reflect an increase in both oxidation state and carbonate deposition in the oceans as plume related volcanism wanes

large magnitude continental extension an example from the central mojave metamorphic core complex the central mojave metamorphic core complex is defined by a belt of miocene brittle ductile extension and coeval magmatism the brittle ductile fault zone defines a basin and dome geometry that results from the interference of two orthogonal fold sets that we infer to have formed by mechanically independent processes one fold set contains axes that lie parallel to the extension direction of the shear zone and has a maximum characteristic wavelength of about 10 km the axial surfaces of these folds can be traced from the footwall mylonites through the brittle detachment and into hanging wall strata however folds of mylonitic layering have smaller interlimb angles than those of the brittle detachment suggesting that the folds began to form during ductile shearing and continued to amplify in the brittle regime possibly after movement across the fault zone ceased mesoscopic fabrics related to the transport parallel fold set indicate that the folds record true crustal shortening perpendicular to the extension direction we interpret these folds to form in response to elevated horizontal compressive stress perpendicular to the extension direction and suggest that this stress regime may be a natural consequence of large magnitude extension the other fold set has axes perpendicular to the extension direction and a characteristic maximum wavelength of about 50 km mesoscopic fabrics related to these folds include northwest striking joints kink bands and en echelon tension gash arrays these fabrics formed after mylonitization and record both layer parallel extension and northeast side up subvertical shear the postmylonitic fabrics are kinematically compatible with rolling hinge style isostatic rebound of the footwall following tectonic denudation the relative timing of extension related magma intrusion and ductile deformation varies through the central mojave metamorphic core complex on the scale of the small mountain ranges that make up the central mojave metamorphic core complex no correlation was observed between either shear zone thickness or intensity of ductile deformation and either the proximity or relative volume of extension related igneous rocks this suggests that models that invoke a single upper crustal genetic relationship such as magmatism triggering extension or vice versa do not apply to the central mojave metamorphic core complex systematic variation in the relative timing of dike emplacement and mylonitization suggests that at the time of dike emplacement rocks in the mitchel range were below the brittle ductile transition while those in the hinkley hills were above it the hinkley hills and mitchel range are separated by 2 km in the dip direction of the fault zone which suggests that the vertical thickness of the brittle ductile transition probably was between 100 and 950 m

extension and basin formation in the southern andes caused by increased convergence rate a mid cenozoic trigger for the andes the southern andes between 33 and 45 s latitude are characterized by a series of complex basins that spanned the contemporaneous active continental margin which itself was characterized by volcanic activity the basins are filled with thick up to 3000 m accumulations of interbedded sedimentary and volcanic strata of late oligocene early miocene age we interpret that these basins developed during a phase of moderate extension within the plate margin system triggered by an increased rate of convergence of the farallon nazca and south american plates between 28 and 26 ma this history is inconsistent with models of convergence that link high rates of convergence of a continental margin and an oceanic plate to strong compressional coupling although extensional basins of this age are only well described in the southern andes the convergence history and volcanic chronology are similar farther north in the central andes 18 33 s leading to the speculation that extension may have characterized the late oligocene early miocene interval regionally we hypothesize that this extension was a necessary condition to subsequent building of the modern andes mountains

geochronology and mass transfer in gulf coast mudrocks south central texas u s a rbsr smnd and ree systematics whole rock shale and 0 5 m sized clay separates from a well spanning 3500 m of burial in south central texas have been analyzed for there ree sm nd and rb sr contents clay rb sr and sm nd hcl leachate residue pairs yield ages which decrease with depth implying that these samples have not experienced a punctuated diagenetic event rb sr ages from the most deeply buried clay samples are younger than their depositional ages but sm nd leachate residue ages are always equal to or greater than depositional age leachate sr appears to be in isotopic equilibrium with present day sandstone formation waters implying that pore fluids readily pass between the two lithologies during and after diagenesis the ree are redistributed and fractionated on a local mineral grain scale during burial diagenesis the hcl soluble ree reservoirs decrease from 20 to 8 in whole rock and from 35 to 7 in 0 5 m clay and become progressively enriched in the middle ree relative to the light and heavy ree conversely the hcl insoluble silicate reservoirs become progressively enriched in the light and heavy ree relative to the middle ree this fractionation which is more pronounced in the clay than in the whole rock appears to be a result of authigenic illite and phosphate formation during burial diagenesis although the ree are redistributed on a local scale there is little evidence that the ree are gained or lost by the mudrocks during diagenetic redistribution combined nd and sr isotopic data show that the whole rock shale system may be modelled as a three component system composed of young volcanic derived and old craton derived detritus and a seawater derived component these three components are partially homogenized during diagenesis and as a result the sr nd provenance information originally held in the silicate fraction of whole rock shale becomes obscured

cenozoic tectonics of the high andes of west central argentina 30 36 s latitude abstract the structure of the central andes shows three distinctive segments characterized by different geometries these geometries are superimposed on the present large scale plate tectonic setting characterized by distinct subduction segments the northern la ramada segment is a thick skinned fold and thrust belt formed by tectonic inversion of a late triassic rift the central aconcagua segment consists of a thin skinned fold and thrust belt while the southern malargue segment like the first one is a thick skinned fold and thrust belt developed by tectonic inversion of a late triassic early jurassic rift system during late cenozoic times the amount of shortening gradually decreases from north to south as indicated by the crustal roots of the central andes the different geometries along the principal cordillera controlled the abrupt changes in the shortening among segments the structure of precordillera and sierras pampeanas has also been considered in order to account for the total shortening in the la ramada segment the main shortening occurred in the precordillera in the aconcagua segment in the principal cordillera while in the malargue segment the shortening is widely distributed in a broader principal cordillera because south of the flat slab subduction segment the precordillera and sierras pampeanas are missing

methods of provenance determination tested with discriminant function analysis abstract currently popular approaches to provenance identification commonly use mineralogic and chemical compositions of siliciclastic sediments or of detrital clastic grains for graphical solutions we have statistically tested three deterministic models using the raw data that were originally used to construct classifications for determining the provenance of sands and sandstones these models are based on 1 detrital modes of sandstones dickinson et al 1983 2 undulosity and polycrystallinity of detrital quartz basu et al 1975 and 3 chemical compositions of muddy sediments and rocks van de kamp and leake 1985 our results show that these models can successfully classify no more than 85 of their own data we suggest that additional variables such as heavy minerals the combinat on of different methods and statistical analysis of data are necessary to improve the reliability of provenance interpretation

regional cross section of the southern province adjacent to lake huron ontario implications for the tectonic significance of the murray fault zone the southern province in the lake huron area of ontario and the penokean fold belt of minnesota wisconsin and michigan have the earmarks of a collisional orogen the huronian supergroup a southerly facing passive margin sequence was deposited during early aphebian early proterozoic crustal stretching and ocean basin formation along the southern margin of the superior province archean craton it accumulated as four unconformity bounded northerly tapering onlapping clastic wedges megacycles the thickness and facies variations reflect syndepositional down to the basin south normal faulting that controlled the accumulation and preservation of the lower three huronian megacycles these are overlapped northward by the youngest megacycle an extensive sheet of clastic sediments recording post stretching regional subsidence of the cratonic margin due to cooling and thermal contraction soft sediment folds in the rocks of the youngest megaycycle to the extreme south probably indicate southward slu

trace element geochemistry of archean volcanic rocks and crystal growth in southwestern abitibi belt canada trace element geochemistry on 176 controlled samples in three adjoining abitibi 2 7 b y volcanic piles confirms the fundamental concept of archean volcanic cycles composed of lower tholeiitic and upper calc alkalic parts the volcanic piles each as much as 16 km thick are either unicyclic or multicyclic in the latter case the cycles display secular geochemical trends all the volcanic rocks studied are indicated to have come from mantle sources a fundamental feature of abitibi magma genesis is the simultaneous presence of both depleted and undepleted mantle sources abitibi tholeiitic basalts closely resemble modern mid ocean ridge basalts abitibi calc alkalic andesites and alkalic rocks are very similar to modern oceanic island arc andesites and to some modern volcanic arc high k rocks respectively the principal constraints to geodynamic processes are interpreted in terms of pulsating migrating mantle diapirism a type of hot spot tectonics mechanism involving a layered mantle responsible for the early tholeiitic depleted mantle source and later calc alkalic undepleted mantle source parts respectively of the volcanic cycles