PILLOWED METABASALTS AND METABRECCIAS FROM THE VALAISAN OCEAN-CONTINENT TRANSITION PRESERVED IN THE BREUIL VALLEY, AOSTA (ITALIAN WESTERN ALPS)

The Monviso metamorphic ophiolite, one of the best preserved relics of oceanic crust in the Western Alps, was formed during the opening of the Mesozoic Western Alpine Tethys and underwent metamorphism to eclogitic conditions during Alpine subduction. The Monviso ophiolite encompasses the whole lithological spectrum of the Piedmont-Liguria ophiolite rocks, with a basal unit of serpentinized peridotite in tectonic contact with the overlying metagabbros, eclogites and pillowed metabasalts. Slivers of serpentinized peridotite hosting banded eclogites and metagabbros divide these units from the overlying Forciolline Unit. The latter (formerly called Costa Ticino Series) is an overturned sequence of gabbros with pods of cumulate troctolite and lenses of serpentinized peridotite, overlain by massive and pillow metabasalts. A unit of massive metabasalts tops the tectonic stack. A body of jadeite-quartz bearing metaplagiogranite has been recently found in the Basal Serpentinite Unit near Verne, northwest of Sampeyre, Val Varaita. Zircon crystals recovered from the Verne metaplagiogranite have large domains with typical magmatic zoning with broad oscillatory bands. They have Th/U ratios in the range 0.3-0.7, as commonly observed in magmatic zircon. In situ ion microprobe (SHRIMP) U-Pb dating of the magmatic domains yielded a mean age of 152±2 Ma, which is interpreted as the crystallization age of the Monviso plagiogranite. Unzoned domains that crosscut magmatic zircon yielded younger, apparent ages which are most likely due to Pb loss during Alpine metamorphism. In conjunction with previous works on ophiolites from the Western Alps, Northern Apennines, and Alpine Corsica, the new data from Monviso suggest that the plutonic activity in the Piedmont-Liguria domain of the western Tethys may have lasted only 15 to 20 Ma, between ca. 170 and ca. 150 Ma. As shown by Radiolarian biostratigraphy, this is approximately the same time span encompassed by the extrusion of tholeiite basalts which cap both the gabbro plutons and their peridotite country rocks. The new data indicate that the plutonic activity recorded at Monviso was coeval with basalt extrusion and deep-sea sediment deposition in some Liguriantype ophiolite bodies of the Cottian Alps. This suggests that the oceanic crust preserved in the Monviso ophiolite may have formed later, and in a more central position of the basin, than the oceanic crust preserved in such Ligurian-type ophiolite bodies.

Lithological architecture and petrography of the Mako Birimian greenstone belt, Kédougou-Kéniéba Inlier, eastern Senegal

This paper describes lithostratigraphy and Alpine tectono-metamorphic evolution of the oceanic succession of the Monte Banchetta-Punta Rognosa unit (Italian Western Alps). The oceanic substratum consists of serpentinized peridotites covered by ophicarbonates, documenting therefore exhumation and exposure of the upper mantle at the seafloor in Jurassic times. Upwards, the sedimentary cover begins with polymictic metabreccias and intercalated siliciclastic sediments (considered Late Jurassic - Early Cretaceous in age), both containing oceanic and continental detritus, and interpreted as mass-flow deposits on sea floor. Then, the upper part of the cover consists of Cretaceous pelagic carbonate sediments (calcschists), lying over a main unconformity. The stratigraphic features and the architecture of the sedimentary cover suggest that this segment of the Piemonte-Liguria Ocean was in a proximal position with respect to the rifted margins. In a general context of the ocean-continent transition, source areas for continental detritus can be envisaged on the hyperextended part of the European margin or on its more proximal part, adjacent to structural highs made of oceanic mantle, as recorded by oceanic detritus. The combination of structural,petrographic and mineral chemistry data defined the Alpine prograde and retrograde metamorphic evolution of this oceanic segment. The metamorphic peak was reached during the D1 event at the transition between lawsonite- and epidote- blueschist facies conditions. Then, a first decompressional event D2 always at blueschist facies conditions was followed by a D3 event at green schist facies conditions.

The Nlonako anorogenic complex (NAC) is located in the south western part of the Cameroon Line and is superposed on the N50E section of the central Cameroon shear zone. It is suggested to be a ring complex of 10 km diameter which was emplaced as sill intrusive body during the Tertiary. In order to do characterize the Nlonako complex shape and characterize the tectonic history in the study area and surrounding, spatial technology (Landsat 8 ETM+ and SRTM images) were used for geological mapping and structural reconstruction based on lineament analysis.
 Data extracted from Landsat 8 ETM and SRTM images show a sub-circular shape for the NAC and the superposition of field and petrographic data from our previous research works confirm the fact that the NAC is a ring or annular complex slightly elongated NNE-SSW, thus it is known as the Nlonako ring complex (NRC). Map of lineament synthesis and the SRTM image of the study area display NE-SW, NW-SE and N-S trend structures. (1) The dominant NE-SW trend mostly displayed by the NRC and its basement rock is parallel to (i) the main metamorphic foliation trend and (ii) to the central Cameroon shear zone regional fault, corresponds to the trending of the regional foliation, structures and the Ngondo pluton elongated shape. (2) The secondary N-S direction may correspond to the late deformational phase in the area because, the N-S-related structures transposing NE-SW structures towards a meridional direction in the NRC basement rock. (3) The NW-SE trend corresponds to the direction of NW-SE fault cross cutting the NRC.
 Field data and synthetic lineament map enable to identify: (1) a Compressive deformational phase D1 whose main markers are NNW-SSE to NW-SE S1 foliation; (2) an early left-lateral deformational phase D2 characterized by anticlockwise rotation of clasts and NE-SW sinistral transposition of early structures; (3) a NE-SW right-lateral deformational phase D3 marked by clockwise rotation and dextral transposition of preexisting structures as well as the NE-SW S3 foliation and (4) a late left-lateral deformational phase D4 marked by the N-S transposition of NE-SW dextral D3-related structures by late sinistral shear movement. This suggests a more complex tectonic history for the Pan-African Belt in Cameroon showing at least three shear phases, that is a right-lateral phase, sandwich by two left-lateral phases.

Remote sensing is a robust and useful tool for providing high‐resolution image data and enabling reliable geological mapping during the initial stage of mineral exploration. One of its main applications is the extraction of lineaments and to locate alteration areas to target gold exploration. It has been long used in the Pan‐African belt of Cameroon to identify a hydrothermal alteration and a great number of lineaments associated with mineralizations. The study area located in the Pan‐African belt hosts numerous alluvial gold deposits where the primary mineralization was still largely poorly unknown until now, due to deep weathering. Therefore, remote sensing combined with field data is useful for targeting potential zones of primary gold resources involved in the hydrothermal and lineament systems. In this study, remote sensing data from Landsat 8 imagery were selected to map the distribution of hydrothermal minerals, and gravity data were interpreted for highlighting structural patterns related to the control of high‐potential zone for gold mineralization, generating a mineral prospect map. The lineaments network shows directions ranging from ENE‐WSW to E‐W, with main direction N45° and a secondary striking N275°. Image enhancement/processing techniques included the application of band ratio and principal component analysis that were helpful to demarcate potential alteration zones marked by iron oxide/hydroxides in which haematite and pyrite are used as proximal alterations and hydroxyl‐bearing minerals in which sericite (muscovite) is used as a marker of proximal alteration, while chlorite, epidote, biotite, quartz, and calcite are used as distal alteration zone, as described by field and petrographic data. The identified alteration zones display a high consistency with the known locations of gold occurrences (mining sites) and closely concordant with large‐scale gold mineralization in the study area. This study presents an integrated approach of Landsat 8 imagery with gravity data and field data for discovering primary mineral resources in a deep weathering area.

<p>The heterogeneous presence of ephemeral magmatic systems below the ridge axis and their complexity mostly account for the heterogeneous character of the oceanic crust accreted at (ultra) slow-spreading ridges. In order to better understand the magmatic processes involved in slow-spreading lower oceanic crust formation, we studied a drilled section of an oceanic core complex (OCC) interpreted as an exhumed portion of lower crust close to the ridge axis. We focused on ODP Hole 735B which presents the most primitive lithologies sampled at Atlantis Bank OCC (Southwest Indian Ridge) in a ~250 m thick section previously interpreted as a single crustal intrusion.</p><p>We combined detailed structural and petrographic data with whole-rock and <em>in situ</em> mineral analyses to determine the processes of emplacement and differentiation of melts within this section. The lower half of the unit is comprised of alternating troctolites and olivine gabbros showing intrusive contacts, and both magmatic and crystal-plastic fabrics. Such features are lacking in the upper half, rather uniform, gabbroic sequence. Whole-rock compositions highlight the cumulative character of both lower and upper units, and a great compositional variability in the lower sequence, whereas the upper sequence is rather homogeneous and differentiates up-section. <em>In situ</em> analyses of mineral phases document magma emplacement processes and provide evidence for ubiquitous reactive porous flow during differentiation. Comparison between both units' geochemistry also led us to strongly favor a model of formation of the reservoir that genetically links melts from the lower and the upper unit.</p><p>We show that the whole section, and related geochemical units, likely constitutes a single magmatic reservoir, in which the lower unit formed by emplacement of primitive sills related to the continuous recharge of primitive melts. Recharge led to partial assimilation of the crystallizing primitive mush, and related hybridization with interstitial melts. Hybrid melts were progressively collected in the overlying mushy part of the reservoir (upper unit), whereas the sills' residual melt differentiated by reactive porous flow processes under a predominantly crystallization regime. Similarly, hybrid melts’ evolution in the upper unit was governed by upward reactive porous flow and progressive differentiation and accumulation of evolved melts at the top of the reservoir. Our results provide the first integrated model for magma reservoir formation in the lower slow-spreading oceanic crust, and have potential implications regarding the lower crust structure and the composition of MORBs.</p>

A habitat evaluation of the Weir property, an approximately 200-acre in holding of private property within the Little Pend Oreille National Wildlife Refuge (Refuge), was conducted using the Habitat Evaluation Procedures (HEP) methodology. The Weir property consists of two separate parcels, an upper unit of 40 acres and a 160-acre lower unit. Evaluation species were ruffed grouse and white-tailed deer. Life requisites evaluated were available browse for white-tailed deer and winter food and fall-to-spring cover for ruffed grouse. Field data were collected on October 16, 17, and 21, 1997. Approximately 37 acres of the lower 160-acre unit are currently grasslands with no shrub or tree cover, and therefore do not provide suitable ruffed grouse or white-tailed deer cover. They excluded this acreage from the HEP calculations for current conditions. This acreage was included in the HEP calculations for ruffed grouse after future management strategies were factored in. It was not included in projections for white-tailed deer. The entire property was stratified into 6 stands (2 in the upper unit and 4 in the lower unit) for data collection. Data were collected at 10 points, spaced 20 paces (approximately 16 m) apart along one randomly selected transect in each stand, formore » a total of six transects. A circular quadrat (.004 ha) was used at each sampling point. Within this quadratwe counted all deciduous, coniferous, and shrub stems {ge} 0.9 m in height and made an ocular estimate of shrub (< 1.5 m in height) canopy cover. We measured the height of the closest (to the center of the quadrat) three deciduous trees, conifer trees, deciduous shrubs, and lowest conifer branch. We estimated the distance to 20 aspen trees at three points along each transect. For a ruffed grouse a Habitat Suitability Index (HSI) was calculated for each stand for each variable. Therefore, two HSIs were calculated for each stand, one for winter food and one for fall-to-spring cover. Weighted HSI scores were calculated by taking the lower of the two HSIs for each stand and multiplying by the stand acreage. The overall HSI is equal to the sum of the weighted HSI scores divided by the total area of all cover types. For white-tailed deer the mean canopy cover was calculated for the entire property and used to determine the overall HSI.« less

A practical guide to terminology for kimberlite facies: A systematic progression from descriptive to genetic, including a pocket guide

Siliceous breccias with accompanying massive to banded metachert have been found hosted in metasandstones of the Fortuna Formation, the lower unit of the siliciclastic, mostly psammitic, Aguapeí Group. This group developed mainly in a late Mesoproterozoic aborted continental rift or aulacogen of the Sunsás-Aguapeí Province (1.2-0.95 Ga), whose evolution led to the stabilization of the southwestern fringe of the Amazonian Craton, promoting its final consolidation. Regionally, the breccias and related metachert form four main linear, NW-SE strike-parallel trends that lie symmetrically about the hinge zone of synclinal folds. These trends are known to occur for a length of 25 km and a width of up to 4 km, so that the main breccia occurrences are distributed over an area of ca. 100 km2 herein termed the Serra da Borda Breccia Field after the hills where they have been found in SW Mato Grosso state, western Brazil. The breccias display a multitude of textures and structures, e.g., massive, roughly tabular, bank-like outcrop morphology, dense to disperse packing and jig-saw to mosaic to chaotic clast distribution. They usually contain metachert fragments set in a fine-grained siliceous-clastic material, but fragments of crystalline quartz aggregates and of metasandstone are present as well. Stratified breccias with a clearly clastic sandy matrix also occur. Associated with the breccias are massive to banded metachert as well as sinter-like vuggy siliceous rocks with vugs following banding. This picture suggests strongly that the breccias constitute strata-bound syn-sedimentary bodies. On grounds of their outcrop and textural features, the breccias and associated metachert are provisionally interpreted as hydrothermal phreatic products of syn-sedimentary eruptive hot-spring activity completed by debris flow deposition, with the involvement of rift-related growth faults.

Research Article| October 01, 1973 Fusion Relations in the System NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O and Generation of Granitic Magmas in the Sierra Nevada Batholith D. C. PRESNALL; D. C. PRESNALL 1Institute for Geological Sciences, University of Texas at Dallas, Dallas, Texas 75230 Search for other works by this author on: GSW Google Scholar P. C. BATEMAN P. C. BATEMAN 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1973) 84 (10): 3181–3202. https://doi.org/10.1130/0016-7606(1973)84<3181:FRITSN>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. C. PRESNALL, P. C. BATEMAN; Fusion Relations in the System NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O and Generation of Granitic Magmas in the Sierra Nevada Batholith. GSA Bulletin 1973;; 84 (10): 3181–3202. doi: https://doi.org/10.1130/0016-7606(1973)84<3181:FRITSN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Chemical analyses of 167 typical specimens indicate that about 95 percent of the intrusive rocks of the central Sierra Nevada contain more than 79 percent normative Ab + An + Or + Qz. If the composition of the lower continental crust is similar to or slightly more felsic than andesite, as seems likely, the system NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O provides an excellent chemical model for testing various schemes of fusion of the lower crust and crystallization of the resulting magmas. From consideration of this system in conjunction with field and petrographic data, we conclude that the intrusive rocks are best explained by repeated episodes of equilibrium fusion corresponding to magmatic sequences defined by field, petrologic, chemical, and geochronologic data. Fractional crystallization of the crystal-liquid mush generated by equilibrium fusion, coupled with periodic upward or lateral movement of the less crystallized central part of the magma, would produce the characteristic mafic to felsic sequence of intrusion; each mafic to felsic sequence corresponds to a separate equilibrium fusion event. In contrast, a close approach to fractional fusion of the lower crust is inadequate for obtaining most of the plutonic rocks, because rock compositions capable of being produced by this process do not match those observed. Normal amounts of conductive heat from the mantle and from radioactive decay in the crust may have been capable of causing fusion in the deepest parts of a thickened crust under the central part of the Sierra Nevada without the aid of a transient heat source from the mantle, but would have been inadequate where the crust was thin in the western Sierra Nevada. However, upward transport of andesitic and basaltic magmas generated along a Mesozoic subduction zone dipping beneath the Sierra Nevada would have provided sufficient additional heat to make fusion of the lower crust unavoidable. This implies that a major portion of the present batholith must have been derived from the lower crust. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

We present an original geological map of the north-eastern Serre Batholith (southern Italy), together with new field and petrographic data of its main five granitoid units. Our study provides an overall picture of a c. 13-thick floor-to-roof batholith exposure, focusing on the relationships between the magmatic units, which were sequentially emplaced at depth from c. 23 to c. 6 km, in a time interval from c. 297 Ma to c. 292 Ma. Indeed, this composite and zoned batholith, with its crustal scale exposure and large compositional and structural diversity, is a real natural laboratory where to test models of granitoid magma production and batholith construction. The new geological map and related field and petrographic data provide a valuable addition to the existing knowledge of the Serre Batholith and, at the same time, a new starting point for further in-depth multidisciplinary investigations aimed to better understand its architecture and build-up mechanisms.

The Guerrero terrane is composed of Middle Jurassic–Lower Cretaceous arc assemblages that were rifted from the North American continental mainland during Late Jurassic–Early Cretaceous back-arc spreading within the Arperos Basin, and subsequently accreted back to the continental margin in the late Aptian. The Sierra de los Cuarzos area is located just 50 km east of the Guerrero terrane suture belt and, therefore, its stratigraphic record should be highly sensitive to first-order tectonic changes. Two Upper Jurassic–Lower Cretaceous clastic units were recognized in the Sierra de los Cuarzos area. The Sierra de los Cuarzos Formation is the lowermost exposed stratigraphic unit. Petrographic data and U-Pb zircon ages suggest that the Sierra de los Cuarzos Formation was derived from quartz-rich sedimentary and igneous sources within the North American continental mainland. The Sierra de los Cuarzos Formation is overlain by the Pelones Formation, which is composed of volcanoclastic sandstones derived from a mix of sources that include the mafic arc assemblages of the Guerrero terrane and quartz-rich sedimentary and volcanic rocks exposed in the continental mainland. The provenance change documented in the Sierra de los Cuarzos area suggests that the Pelones Formation was deposited when the Arperos Basin was closed and the Guerrero terrane was colliding with the North American continental mainland. Based on these data, we interpret the Pelones Formation as the syn-tectonic stratigraphic record associated with the accretion of the Guerrero terrane.

The Messinian marine to nonmarine gypsums of Jumilla (Northern Betic Cordillera, SE Spain): Isotopic and Sr concentration constraints on the origin of parent brines

Late Pliocene to early Pleistocene depositional patterns of upper Etchegoin and lower San Joaquin Formation sand and shale units in the Buena vista field area were controlled by changes in clastic input, eustatic sea level, structural growth, and circulation patterns in the south end of the San Joaquin Valley. Wireline and drill-strip logs, core depositions, paleontology, and petrographic data from these units suggest the interpretation of a series of shallow to marginal marine deposits with distinctive morphologic features and production characteristics. Late Pliocene marginal marine drainage systems transported clastics from southerly sources as structural and/or eustatic changes shoaled the southern area. An erosional hiatus and shallow marine transgression marked the extent of Plio-Pleistocene shoaling and rapid early Pleistocene foundering. Later Pleistocene changes in sediment supply and structural growth isolated the area from marine conditions as the basin filled with nonmarine sediments. Early field development was influenced by the areal distribution and reservoir characteristics of these sands as well as by the timing of such development activity. Depositional models derived from these data are useful in constructing paleogeographic models with regional hydrocarbon significance.

Geochemistry and genesis of Austrian talc deposits