Formation Schist Research Articles

Linking sedimentary provenance, climate and tectonics in the Neoproterozoic Seridó Belt, Borborema Province (NE Brazil)

The Seridó Belt, at the extreme northeast of Brazil, is one of the several Neoproterozoic mobile belts related to the Brasiliano/Pan-African Orogeny in Western Gondwana. The lithostratigraphy of the Seridó Belt comprises basal quartzites and metaconglomerates (Equador Formation) overlain by hornblende-bearing paragneisses, calc-silicate rocks and marbles (Jucurutu Formation) and biotite-schists (Seridó Formation). We investigated the sedimentary provenance of these rocks based on an integration of compiled whole-rock geochemistry, Sm-Nd isotopes, U-Pb detrital zircon ages and a new set of whole-rock trace elements analyses. Equador Formation quartzites have major and trace elements patterns that resemble cratonic sandstones, with a typical felsic provenance consistent with the isotopic composition of plutonic Archean to Paleoproterozoic basement rocks. Geochemical proxies suggest that these rocks were deposited in fresh waters under anoxic conditions and moderate to intense weathering conditions (CIA = 58–79). A shift in provenance led to an increase in intermediate-mafic sources during the deposition of the Jucurutu and Seridó formations. Trace elements from rock types in these units are nearly identical, yet they differ on the contents of some high field-strength elements (e.g., Sc, V and Ni), which are higher in the Seridó Formation than average pelites. Geochemical proxies show similar depositional environments, with brackish to marine conditions for the Jucurutu and Seridó formations under anoxic to euxinic conditions. Weathering conditions are consistently lower in the Jucurutu Formation (CIA = 58–66) in comparison with the Seridó Formation (CIA = 60–88). Compiled U-Pb data suggest maximum depositional ages of 633 ± 11 Ma and 633 ± 6 Ma for Jucurutu and Seridó formations, respectively, suggesting deposition following the ca. 645–635 Ma Marinoan glaciation. We compared the geochemistry of the Seridó Formation schists with currently exposed basement rocks and showed a strong affinity with Neoproterozoic arc-related intermediate rocks, which is in agreement with their slightly radiogenic signature (average εNd(630Ma) = –1.68). When comparing with other pelitic sequences on the western Borborema Province, most of them fit the same sources and weathering conditions of the Seridó Formation schists and Jucurutu Formation paragneisses. We propose that the Seridó Belt represents an intracontinental rift basin (Equador Formation) that was overlain by fine-grained magmatic arc-related sediments (Seridó and Jucurutu formations) during the post-Marinoan deglaciation and sea level rise. Similar pelitic deposits in western Borborema Province suggest a large marine-influenced continental depositional system that was mostly sourced from magmatic arcs of the West Gondwana Orogen.

2. A College as Big as All Outdoors

2. A College as Big as All Outdoors Bill Broyles (bio) High school had been an ordeal for Ronald and undoubtedly for his family, but for him education could be found in the hills around his childhood home and in the mountains of the West. Following his graduation in June of 1928, he broke free of the East and his family almost as soon as he received his diploma. His university would be the great outdoors. Ronald explained his decision this way: "I headed west to Boulder when I was 18. It was a long way from the Friends meeting house and a long way from shrinking female relatives. Each one had a different story of what God demanded. I could never get them to agree on anything. They were overly-devout and we had a great deal of religious disagreement. I didn't like the East particularly probably because it had, let's say, too much Jesus and not enough good sense."100 Not only did Ronald explore academics, but he also explored new [End Page 264] territory. Ronald's interest in geology apparently grew from his childhood in Pennsylvania, New York, and New Jersey. At age nine he was somewhat interested in Chestnut Hill outside Philadelphia. It had exposures of mica and schist, and just north of the city was a marble quarry. He said that he got interested by watching the new cuts in the Pennsylvania railroad. They reduced the grade on the Chestnut Hill branch and went through a whole series of Wissahickon Formation schists and red sandstone quartzites. I watched it being dug out with steam shovels, footprints in sandstone and things like that. It was three blocks from our house. Just outside of Philadelphia, a few miles from where we lived, they had marble quarries from which parts of Independence Hall were taken.101 Too, back when I was in high school I found a lead line in Ellenville, New York, about 45 miles from Montclair. I rode out there on my bicycle and collected, I guess, 50 pounds of galena, one of the crystals used in radios. I broke the galena up into quarterinch tubes and sold them for $.25 a piece. In those days, that was wealth. All through high school I made a reasonable income from the sale of crystals for radios until about 1928 when I graduated, and by then crystals were on their way out and vacuum tubes were coming in.102 Glaciation drew his interest when he visited a glacier-carved lake near Paterson, New Jersey, at age 10.103 Later his 1937 master's thesis was about glaciation (in 1947 he earned a second master's degree in geography). His father was not interested in geology to any particular extent, but Ronald asked questions of everyone, a form of inquiry that he practiced the rest of his life. Even as a boy he was comfortable conversing with adults on an academic level. And he was intrigued by the laval palisades along the Hudson River. It was heady stuff for a 10-year-old. It is nice to think of a young Ives seeing rocks and learning about deposition, eruption, and rock sequences, much like an early Robert Chambers or Charles Darwin. That experience plus a book helped lure him to the Pinacate region of northwest Sonora; he had read it when he was starting grade school. "Back in about 1915, I picked up a copy of W. T. Hornaday's book Camp-fires on Desert and Lava.104 He was the first Anglo to go in the area and write any publication about it. I read the book and was interested. I had a chance to get down there, and I went down and took a look. That was 1931. Obviously, Hornaday hadn't gotten it [End Page 265] all. So I've been planning to get more ever since. I think I've raised more problems than I've solved, which is standard."105 When asked what launched his first scientific interest in the Pinacate, he replied, "Volcanics. It is lava beds with desert on both sides. Besides the lava beds, you have a shield volcano...

Tectonometamorphic evolution of Seram and Ambon, eastern Indonesia: Insights from 40Ar/39Ar geochronology

The island of Seram, eastern Indonesia, experienced a complex Neogene history of multiple metamorphic and deformational events driven by Australia–SE Asia collision. Geological mapping, and structural and petrographic analysis has identified two main phases in the island's tectonic, metamorphic, and magmatic evolution: (1) an initial episode of extreme extension that exhumed hot lherzolites from the subcontinental lithospheric mantle and drove ultrahigh-temperature metamorphism and melting of adjacent continental crust; and (2) subsequent episodes of extensional detachment faulting and strike-slip faulting that further exhumed granulites and mantle rocks across Seram and Ambon. Here we present the results of sixteen 40Ar/39Ar furnace step heating experiments on white mica, biotite, and phlogopite for a suite of twelve rocks that were targeted to further unravel Seram's tectonic and metamorphic history. Despite a wide lithological and structural diversity among the samples, there is a remarkable degree of correlation between the 40Ar/39Ar ages recorded by different rock types situated in different structural settings, recording thermal events at 16Ma, 5.7Ma, 4.5Ma, and 3.4Ma. These frequently measured ages are defined, in most instances, by two or more 40Ar/39Ar ages that are identical within error. At 16Ma, a major kyanite-grade metamorphic event affected the Tehoru Formation across western and central Seram, coincident with ultrahigh-temperature metamorphism and melting of granulite-facies rocks comprising the Kobipoto Complex, and the intrusion of lamprophyres. Later, at 5.7Ma, Kobipoto Complex rocks were exhumed beneath extensional detachment faults on the Kaibobo Peninsula of western Seram, heating and shearing adjacent Tehoru Formation schists to form Taunusa Complex gneisses. Then, at 4.5Ma, 40Ar/39Ar ages record deformation within the Kawa Shear Zone (central Seram) and overprinting of detachment faults in western Seram. Finally, at 3.4Ma, Kobipoto Complex migmatites were exhumed on Ambon, at the same time as deformation within the Kawa Shear Zone and further overprinting of detachments in western Seram. These ages support there having been multiple synchronised episodes of high-temperature extension and strike-slip faulting, interpreted to be the result of Western Seram having been ripped off from SE Sulawesi, extended, and dragged east by subduction rollback of the Banda Slab.

U–Pb detrital zircon provenance of metasedimentary rocks from the Ceará Central and Médio Coreaú Domains, Borborema Province, NE-Brazil: Tectonic implications for a long-lived Neoproterozoic active continental margin

U–Pb geochronological analyses have been acquired on detrital zircon grains collected from 14 samples in the Ceará Central (CCD) and Médio Coreaú (MCD) tectonic domains, separated by the Transbrasiliano-Kandi Lineament (TKL) on the Borborema Province. To the West of this lineament, the basement of the MCD has a tectonic affinity with the West African and São Luis Cratons, where Paleoproterozoic rocks predominate. Within this area, three samples of the São Joaquim quartzite yielded Paleoproterozic and Archean ages only, with the youngest 206Pb/207Pb ages at ca. 1750Ma. However, the Goiabeira Formation schist, in the same domain, yielded younger 207Pb/235U ages around 720Ma, with predominance of zircons within the 750–1100Ma interval. To the East of the TKL, the Borborema Province correlates well with the Transaharan Belt of West Africa, where the Paleoproterozoic-Archean basement was affected by a strong tectonic imprint of the Neoproterozoic Brasiliano-Pan African orogeny. Three samples from the region between the TKL and the Tamboril-Santa Quitéria granitic–migmatitic Complex (TSQgmC) yielded younger 207Pb/235U concordant ages (ca. 660–700Ma), which are 20–60Ma younger than those of the Goiabeira Formation. Further east within the CCD, eastern of the TSQgmC, six samples yielded two distinct detrital patterns, with some samples showing younger concordant ages (ca. 900 and 750Ma), while others demonstrate a strong Paleoproterozoic source component with zircons older than ca. 1500Ma. Finally, one sample of the post-collisional extensional Jaíbaras Trough, within the main axis of the TKL, yielded a maximum deposition age of ca. 540Ma, with a strong source component ranging from 540 to 640Ma, derived mainly from the TSQgmC. Evidence of a large ocean basin prior to 800Ma, along the vicinity of the TKL, is well established in Central Brazil and West Africa. We believe that the strong source component at around 800–700Ma, and possibly also the older ones at ca. 1000Ma, were shed from magmatic arc systems developed at the margins of the ancient continent which makes up the basement of the Borborema Province. Abrupt cessation of the detrital zircons input at ca. 650Ma, suggests a change in the tectonic regime at this time, marking the onset of collisional tectonics and main metamorphism in the province. This fact is supported by the ages spanning the 640–590Ma interval found in the metamorphic domains of the studied zircons which are partially synchronous with the development of the main tectonic and magmatic stages of the TSQgmC.

Neoproterozoic structural evolution of SE Sinai, Egypt: I. Re-investigation of the structures and deformation kinematics of the Um Zariq and Malhaq Formations, northern Wadi Kid area

The Wadi Kid area of the SE Sinai comprises Neoproterozoic arc metavolcanics and metasediments bordered to the N, W and S by post-kinematic granitoid plutons. The structure and tectonics of the Kid area remain controversial despite more than two decades of investigations. In this study the structural history and characteristics of the northern part of the Kid area (Um Zariq and Malhaq Formations) are described. The earliest tectonic event, D1, produced upright folds with probably originally WNW–ESE trending hinges. The F1 folding weakens to the north. The main deformation, D2, produced regionally penetrative shallow-dipping S2 foliations and semi-recumbent tight to isoclinal F2 folds. S2 cleavage is a product of pure flattening strain that involved pressure solution deformation mechanism, rather than a shear foliation that involved mylonitization as previously reported. Macroscopic F2 folds are dissected by top-to-SSE low-angle thrusts and mylonite zones that emplaced the high T Um Zariq Formation schists over the low-grade metavolcanics in the southern part of the Kid area. The SSE direction of thrusting is parallel to stretching lineations on the S2 cleavage planes. Later folds include D3 WNW–ESE to E–W trending upright gentle folds and D4 upright gentle N–S to NE–SW trending folds. In contrast to the extensional histories recently proposed this study finds the deformation events in the northern Kid area to comprise a compressional folding and thrusting history.

The Copper Hill Cu-Ag-Sb deposit, Picuris Range, New Mexico; retrograde mineralization in a brittle-ductile trap

The strata-bound copper-silver-antimony deposit at Copper Hill, New Mexico, developed in a distinctive structural here termed a brittle-ductile trap, during regional retrograde metamorphism of Early Proterozoic metasedimentary rocks. The deposit is localized near the contact between massive orthoquartzite (Ortega Formation) and the overlying schists (Rinconada Formation). Peak regional metamorphism at 4 kbars and 500 degrees C occurred near the end of a regional shortening event that formed the upright, west-plunging Copper Hill anticline. At Copper Hill, late metamorphic, vertical, north-south-striking faults and fractures cut the Ortega Formation quartzite and terminate against folded Rinconada Formation schists. Synmetamorphic quartz veins, several centimeters to 1 m thick, fill many of the fractures and terminate against the overlying schists. At the quartzite-schist contact, many of the veins merge into mushroom-shaped bodies that project along the contact. Fracture-controlled and disseminated Cu-Ag-Sb minerals occur within the quartz veins and in quartzite near the veins. The mineral assemblage includes malachite, chrysocolla, stibiconite, cuprite, and minor chalcocite and covellite, but some of these are late-stage alteration-oxidation minerals derived from an originally sulfide- or oxide-dominated suite. A syn- to late metamorphic age for the primary deposit is indicated by: (1) the crosscutting nature of the mineralized veins, (2) local replacements of kyanite and staurolite by economic minerals, and (3) evidence for postmineralization annealing of the veins and quartzite. During retrograde metamorphism, the quartz veins and massive quartzite behaved brittlely whereas the overlying schists deformed ductilely. SiO 2 -bearing and then Cu-Ag-Sb-bearing metamorphic fluids migrated through the fractured quartzite and ponded below the folded impermeable schists. Although the Copper Hill deposit is not currently economically viable, it presents a model for retrograde metamorphic mineralization in rocks with varying mechanical properties that may be important in other regions.

The Sandamap Noord Gold Prospect, Central Namibia: Discovery of a New Style of Turbidite-Hosted Gold Mineralization

Early Paleozoic metaturbidite-hosted Au-As-W-B mineralization, believed to be of a style previously unknown in southern Africa, has been discovered in the ensialic inland branch of the Pan-African Damara Orogen. A 2500-m-long zone of gold mineralization is hosted by altered Kuiseb Formation schist, quartz veins, and mylonite rocks in the aureole of a leucogranite and rare element pegmatite dome on the farm Sandamap Noord, west of Usakos, central Namibia. Au/As fluids either mobilized from the metasediments by and/or expelled from the stanniferous pegmatites and leucogranite were concentrated in NNE-trending Damaran D4 structures. Rb-Sr whole-rock dates and Pb-Pb model ages indicate that the gold mineralizing event occurred at 469-464 Ma. The three main types of alteration/mineralization are: a kaolinite-alunite rock (possibly advanced argillic alteration) that contains < 1 g/t Au, a jarosite schist that contains up to 46 g/t Au, and a massive grunerite rock that contains up to 0.5 g/t Au. Associated alter...

The Petrophysics, Geophysics and Structure of the Koongarra Site, Northern Territory

This paper reports on part of a natural analogue study on the migration of radionuclides in the weathered zone at the Koongarra site in the Northern Territory of Australia. The unmined but intensively drilled Koongarra uranium deposit in the Northern Territory of Australia has been studied to evaluate the processes and mechanisms involved in the alteration of the primary uranium mineralisation and to model the formation of the secondary uranium orezone and the dispersion fan. The work has resulted in solid phase and groundwater databases, and hydrological data from field and laboratory measurements. These are being used to test a range of geochemical, hydrological and transport models. The Koongarra uranium deposit is situated in the Alligator Rivers Uranium Province of the Northern Territory of Australia, about 200 km east of Darwin. The uranium ore is located in steeply dipping, physically anisotropic, quartz chlorite Cahill Formation schists of early Proterozoic age and is near the base of a prominent southeast-facing sandstone escarpment. The upper 25 m of the schist is weathered. An upper clayey zone of intense weathering separates deeper partly weathered schist from the overlying Quaternary sands, silts and ferricretes. Directly to the northwest of the ore deposit, a major reverse fault, the Koongarra Fault, has upfaulted the schist against the middle Proterozoic Kombolgie Sandstone. Hydraulic properties of the fault zone have not been tested directly, but the fault was previously considered to be a barrier or partial barrier to lateral flow of groundwater. The site is currently undisturbed by mining, but the steel casings of over 100 exploratory drillholes proved to be a source of noise in magnetic and electrical surveys and the condition of the holes precluded the use of drillhole and cross-hole geophysics in the site studies. A number of different approaches have been made to characterise the site and to try to understand: how groundwater has moved and is moving through the orezone region; the source of the water ? natural recharge or flow across the fault; how the groundwater drains out of the site; whether fluid flow is through fractured or porous media or both; and in particular, how this has influenced the transport of uranium and the formation of a prominent dispersion fan.

Geological and physical characteristics of some magnetic sources in lower proterozoic metasediments pine creek geosyncline N.T.

Regional aeromagnetic surveys in the Pine Creek Geosyncline, show that much of the region has a fairly flat magnetic field, and few deep seated magnetic anomalies are evident. The most prominent features are zones of narrow curvilinear anomalies which occur over Lower Proterozoic metasediments. The geological and physical characteristics of the sources of some of the magnetic anomalies over the Lower Proterozoic metasediments have been investigated by: detailed interpretation of magnetic anomalies, geological mapping, ground magnetic surveys, laboratory physical property measurements, and mineralogical studies of rock samples and drill core. In the central part of the Pine Creek Geosyncline most of the prominent curvilinear anomalies result from strongly remanently magnetised iron sulphides in the Golden Dyke Formation sediments. In the east, in the Alligator Rivers Uranium Field, the prominent anomalies result from inductively magnetised magnetite in Cahill Formation schists. In the west, in the Litchfield Block, prominent anomalies result from inductively magnetised iron oxides in unnamed schists. The regional distribution of magnetic iron facies in the Lower Proterozoic metasediments might be a useful tool to assist in formulating hypotheses regarding the geological history of the Pine Creek Geosyncline.

Structure and relationships of late mesozoic schists of NW california and SW Oregon

Abstract Cross sections through the South Fork Mountain Schist of north-western California and the Colebrooke Formation schist of south-western Oregon are described and the regional relationships of the rocks outlined. The schists are of the so-called blueschist facies, and have undergone intense synmetamorphic and less intense post-metamorphic folding and cataclasis. High pressure-low temperature conditions of metamorphism apparently preceded and then overlapped dynamic metamorphism. Three textural subzones result from progressive increase in deformation; mineral zonation related to increase in P-T conditions is less distinct. Axial planes of folds dip easterly and most of the structures probably “face” westerly towards the Pacific Ocean. Relations between mineral and textural isogradic surfaces are obscure, but they are almost certainly non-parallel. Textural zones in the South Fork Mountain Schist are apparently inverted and mineral isograds steepened by thrust-faulting, but neither is directly related to the faults as now exposed. K/Ar dates reveal a dose relationship between metamorphic grade and age of schists in an apparently progressive sequence. The inferred macroscopic structures, the textural and mineral isograd relationships, and the age-grade relation are attributed to regional metamorphism deep in the crust, accompanied by localised deformation on deep shear zones. A mechanism is proposed whereby movements during deformation elevated older schists beyond the limit of argon equilibration, and at the same time generated newer schist at depth in and beneath the inclined shear zones; at upper crustal levels the elevation resulted in erosion of older eugeosynclinal sediments and in local deposition of conglomerates with derived clasts.