Sequence Of Carbonate Rocks Research Articles

Petrographic Characteristics of the Proterozoic Vempalle Carbonates, Cuddapah Basin, India and their Implications

Abstract: The Vempalle Formation of the Proterozoic Cuddapah basin has a well developed sequence of carbonate rocks, which are interbedded with shales, siltstones and chert. The stromatolitic carbonates are conspicuous at many places but the oolitic carbonates are less prominent and are present only in some areas. All the carbonates are pervasively dolomitized. Petrographic examination of these carbonates revealed that they are predominantly made up of fine grained micrite with patchy development of sparite and chert/quartz. The stromatolitic carbonates show distinct banding of alternate carbonate and cherty layers. The latter are rich in organic matter indicating prevalence of profuse biogenic activity. The oolitic carbonates comprise of ooids showing both concentric and radial patterns and made up of carbonate/ chert and cemented by micro/mega quartz or carbonate itself. Diagenetic and post depositional features are reflected in cementation, recrystallization, compaction, stylolite formation and silicification processes. Various stages of cementing material are observed. Secondary vein fillings of carbonate or quartz traverse the carbonate/cherty groundmass. Intraclasts present suggest occasional erosional destruction of associated sediments, short lived transport and local redeposition. Accessory silicate minerals represent terrigenous influx during deposition. Dolomitization of the carbonates was fabric retentive and early diagenetic. The environmental conditions were characterised by low energy, within a shallow water zone, in occasional higher energy events and turbulence. The carbonates appear to have been deposited on a shallow water ramp within a tidal regime.

Crustal structure of Damavand volcano, Iran, from ambient noise and earthquake tomography

We investigated the shear wave velocity structure beneath the Damavand volcano, Iran, using analysis of Rayleigh wave empirical Green’s functions obtained from the cross-correlation of 1-year seismic ambient noise and 26 local earthquakes. After the observed phase, velocity dispersion curves were inverted to obtain phase velocity tomographic maps; the synthetic phase velocity dispersion curves were then estimated for each evenly spaced geographic grid point. Estimated synthetic phase velocities were inverted using the nonlinear damped least-squares inversion method to obtain a quasi-3D shear wave velocity for the study area. Analysis of obtained quasi-3D shear wave velocity model reveals the presence of three distinct low or high anomalies in the upper 5 km. A low-velocity layer (VS ~ 2.8 km/s) was observed in the upper ~2 km of the crust that includes the sedimentary sequence of carbonate, siliciclastic, and volcanic rocks. According to our results, there is an indication of a high-velocity layer (VS ≥ 3.0 km/s) at the depth range of 2 to 5 km, which may indicate the presence of dense, cooled magma. There is a clear indication of intrusion of the low-velocity anomaly/body into the higher velocity layer, in the depth range of approximately 3.0 to 4.5 km. The corresponding low-velocity body is interpreted as a hot magma chamber associated with the young eruption of Damavand. The chamber is located southwest of the crater, which stretches from the west to beneath the Damavand cone.

New evidence of an Ediacaran age for the Bambuí Group in southern São Francisco craton (eastern Brazil) from zircon U–Pb data and isotope chemostratigraphy

Extensive carbonate–siliciclastic successions of the Bambuí Group, which overlie Neoproterozoic glaciogenic diamictites, cover most of the southern São Francisco craton (eastern Brazil). This group records sedimentation in a foreland setting related to the diachronic orogenic processes that formed the Brasília and Araçuaí marginal belts. The lowermost unit of the Bambuí Group, the Sete Lagoas Formation, comprises two shallowing-upward sequences of carbonate rocks with subordinated pelitic intercalations, overlying the glaciogenic diamictites in the southern São Francisco Craton. This study combines isotope chemostratigraphy (C, O, Sr) and U–Pb dating of zircon detrital grains retrieved from marls of the Sete Lagoas Formation. The basal sequence comprises low organic matter limestones and dolostones with δ13C values around 0‰, positioned above cap carbonates dated at around 740Ma (Pb–Pb whole-rock isochron). The U–Pb ages obtained for this sequence show several age peaks between 1270–870Ma and 625–550Ma. The upper sequence includes dark limestones with δ13C values as high as +10‰, best preserved 87Sr/86Sr ratios of around 0.7075 and U–Pb ages ranging from 625Ma to 550Ma. Our geochronological data suggest that the Araçuaí orogen is the main source of sediment for the Sete Lagoas Formation, and the youngest zircon population sets the maximum depositional age for its upper part at around 557Ma. This suggests that the studied section of the Sete Lagoas Formation is not related to either the Sturtian or the Marinoan glacial events. Also, the 87Sr/86Sr ratios obtained from Sete Lagoas carbonates contrast with Sr evolution curves available in the literature, especially with those for the Ediacaran–Cambrian limit, when ratios higher than 0.7080 would be expected. The same discrepancy is reported for other Ediacaran carbonate successions, pointing to local disturbances in Sr composition of marine basins rather than global processes. Interbasinal correlations and blind dating based on isotope chemostratigraphy should proceed carefully, especially for Ediacaran marine deposits located on the inner parts of large palaeocontinental regions, such as those found in western Gondwana.

Application of petrography, major and trace elements, carbon and oxygen isotope geochemistry to reconstruction of diagenesis of carbonate rocks of the Sanganeh Formation (Lower Cretaceous), East Kopet-Dagh Basin, NE Iran

Postdepositional history of the Sanganeh Formation (Lower Cretaceous), in eastern Kopet-Dagh Basin in northeast of Iran was considered. The Sanganeh Formation mainly composed of shale, marl, siltstone, and interbeds of limestone. The study of limestone interbeds led to recognizing several diagenetic processes including micritization, cementation (including isopachous rim, syntaxial overgrowth, equant mosaic spary, blocky, and poikilotopic cements), neomorphism, compaction (including physical and chemical), dolomitization, pyritization, fracturing, and vein filling. Limestone samples were analyzed for major (Ca, Mg) and minor (Na, Sr, Fe, and Mn) elements as well as oxygen and carbon isotopic compositions (δ18O and δ13C). Many of diagenetic processes such as cementation, neomorphism, dolomitization, and pyritization happen in several diagenetic environments. In order to differentiate them, geochemical data (including element and isotopic) have been used. In addition, blocky, mosaic spar, and poikilotopic cements show different luminescence under the cathodoluminescence microscope that is accordant to their geochemical contents. Therefore, the luminescence was a guide for recognizing cements and neomorphed lime muds of different diagenetic environments of studied samples. The neomorphed lime muds and lime muds were differentiated by geochemical data. The marine lime muds and micrites includes mean δ18O 0.04 ‰, δ13C 1.95 ‰, Na mean 1491.21 ppm, Sr mean 5233.68 ppm, Fe mean 61.34 ppm, and Mn mean 35.96. The meteoric cements and includes mean δ18O −1.02 ‰, δ13C 8.85‰, Na mean 126.34 ppm, Sr mean 300.03 ppm, Fe mean 2883.88 ppm, and Mn mean 382.88. The burial cements and neomorphed lime mud samples contained mean δ18O −9 ‰, δ13C −6.33‰, Na mean 308.29 ppm, Sr mean 731.72 ppm, Fe mean 816.02 ppm, and Mn mean 37.28. Paragenetic sequence of the Sanganeh Formation carbonate rocks was interpreted and depicted using integration of petrographic, major and trace elements, and isotopic data in three phases including (i) eogenesis, (ii) mesogenesis, and (iii) telogenesis.

Diagenetic Model of Carbonate Rocks of Guri Member of Mishan Formation (Lower to Middle Miocene) SE Zagros Basin, Iran

Abstract In order to understand the post-depositional history of carbonate rocks of Guri Member (Lower to Middle Miocene), three stratigraphic sections were selected in north Bandar-Abbas in southeast of Iran. Sampling was carried out, analyzed for selective parameters such as oxygen and carbon isotopic compositions (δ18O and δ13C) and interpreted in the present study. We recognized several diagenetic processes including micritization, cementation, neomorphism, compaction, dissolution, silicification, dolomitization, fracturing and vein filling. Some of the diagenetic processes occurred at different conditions, so in order to achieve precise interpretation, samples from different carbonate components such as, micrite, fracture cement, solution pore cement, intergranular cement, and some biotic allochems were analyzed. In this study micrite samples were subdivided into two groups including micro-spary and micrite. They were recognized under Cathodoluminescence microscope. In addition, micrite samples were classified into five groups based on their depositional environments: supratidal, lagoon, coral bar, open sea, and open basin. There were minor changes in stable isotope ratios based on the sedimentary environments, stratigraphy successions, and micro-spary or micrite properties. In this study, similar calcite cements in petrography studies were differentiated by stable isotope data. Those calcite cements have formed in different diagenetic environments such as meteoric and burial cements. Paragenetic sequence of carbonate rocks were interpreted by integration of petrographic and isotopic studies. We have reconstructed diagenetic models of Guri Member into four stages including marine, meteoric, burial, and uplifting.

Mineralogical study of sediment-hosted gold deposits in the Yangshan ore field, Western Qinling Orogen, Central China

The Yangshan gold ore field is located in the southern subzone of the Western Qinling Orogen. Mineralization is confined by the east–west-striking Anchanghe thrust fault zone. These subparallel faults constitute a branch of the regional Mianlue structural zone, crosscutting Middle Devonian carbonaceous carbonate and clastic rock sequences, an ore-bearing unit locally named the Sanhekou Formation. The metasedimentary clastic and carbonate rocks containing fine-grained sulfides are the main host rocks of the deposit, with minor mineralization occurring as coarse-grained pyrite–quartz veinlets in black shale and as dissemination in some plagiogranite dykes. Electron microprobe (EMPA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses show that arsenian pyrite and arsenopyrite are the major hosts for gold with tens of ppm up to weight percent levels of Au, and the Au contents in arsenopyrite are one order of magnitude higher than those in pyrite. A negative correlation of As and S in arsenian pyrite is consistent with the substitution of As for S in the mineral. Both arsenian pyrite and arsenopyrite in the Yangshan ore field show chemical zonations with middle parts (mantle) enriched in As and Au relative to cores and the outermost rims, reflecting the chemical evolution of ore-forming fluids. High resolution transmission electron microscopy (HRTEM) analysis failed to identify any nanoparticle of native gold even in the highest Au parts of arsenopyrite. This observation combined with the relatively homogenous distribution of Au, a positive correlation of As and Au, and Au/As ratios below the solubility limit of gold in arsenian pyrite and arsenopyrite, suggests that invisible gold is likely present as structurally bound Au+1 in sulfides, although our work cannot exclude the existence of Au nanoparticles in arsenian pyrite as identified in American Carlin-type gold deposits. Submicron native gold may be much more easily found in arsenian pyrite than in arsenopyrite, because the later has larger capacity of enrichment of Au+1 than the former due to the substitution of As for S. The mechanism for nanoparticle formation is likely exsolution of native gold from metastable arsenian pyrite caused by later hydrothermal event, which was recorded by the outermost As-, Au-poor overgrowths both in arsenian pyrite and arsenopyrite in the Yangshan gold ore field.

GRADE DISTRIBUTION OF THE GIANT OK TEDI Cu-Au DEPOSIT, PAPUA NEW GUINEA

The Ok Tedi porphyry Cu-Au deposit in Papua New Guinea is a world-class mineral deposit. In its pre-mining stage, it contained 5.5 Mt Cu and 18.1 Moz Au; skarn accounts for ~80% of the current 1.9 Mt Cu and 7.7 Moz Au in total resources. Two composite felsic stocks intruded a tectonostratigraphic sequence of clastic and carbonate rocks at ~1.16 ± 0.10 Ma, in a postcollisional tectonic setting. Ensuing magmatic-hydrothermal mineralization occurred within a geologically short time span of ~200 k.y. On the mine scale, heterogeneous metal grades (0–2.5% Cu) reflect contrasting styles of mineralization, conforming to classic skarn- and porphyry-type mineralization. A three-dimensional geologic model, alteration distribution, mineralization styles, and grade distribution indicate that fluids, exsolved from a deeper magma chamber, used shallow stocks as their preferred permeable pathway upward. Where limestone was present at depth, high-grade skarn formed, with the permeable stock having very low (<0.1% Cu) grade. In contrast, where the intrusions were encapsulated solely by impermeable silicified siltstone, economic grade (avg 0.6% Cu) developed within the intrusion, showing concentric patterns interpreted to represent individual magmatic pulses within the stock. The rare opportunity to visualize this in three dimensions and interact with the model is provided in the interactive electronic supplementary material.

The Long Canyon Deposit: Anatomy of a New Off-Trend Sedimentary Rock-Hosted Gold Discovery in Northeastern Nevada

Abstract The Long Canyon deposit is a new sedimentary rock-hosted gold discovery located in northeastern Nevada, over 150 km east of the well-explored Carlin trend. Unlike most examples of this type, the deposit is hosted in a sequence of Cambro-Ordovician carbonate rocks deposited in a periodically emergent, platform to shelf-edge environment. The strata were strongly deformed and weakly metamorphosed during a mid-Mesozoic orogenic event that segmented and extended a brittle, 80-m-thick dolomite layer into a series of elongate, N- to NE-trending boudin blocks, while the enclosing limestone underwent ductile deformation. This unusual structural architecture focused subsequent contractional deformation, as well as Tertiary extensional brittle deformation, forming pathways for meteoric and hydrothermal fluids, which formed and enlarged dissolution cavities and dissolution breccias along boudin margins, boudin necks, and normal faults. Gold occurs primarily in zones of polyphase dissolution breccias that are localized in minor faults and fold hinges in the structurally complex areas along and adjacent to dolomite block margins. Two general phases of brecciation are present, including calcareous breccias and later, Fe ± As oxide-rich breccias, which carry the highest gold grades. Breccias are accompanied by variable pervasive silicification. Alteration assemblages include zoned dickite + white mica + hematite in the cores of mineralized zones, with peripheral kaolinite + sericite and goethite + hematite and, ultimately, more distal illite + smectite. Local preservation of fine-grained sulfides encapsulated in calcite and silica suggests that the iron oxide assemblages may represent oxidation of fine-grained pyrite + marcasite + arsenopyrite assemblages. As currently defined, the Long Canyon deposit extends for 3 km in a northeasterly direction and consists of several subparallel zones of mineralization focused on boudin block margins, boudin necks, and incipient boudins. It contains a combined (measured, indicated, and inferred) estimated resource comprising 2.2 million ounces of gold, at an average grade of ~2.3 g/t gold. Metallurgical work indicates that the deposit is highly amenable to cyanidation by heap leaching or milling. The Long Canyon deposit is open to further exploration and resource additions, and is shallowly buried, relatively high grade, and thoroughly oxidized. The stratigraphic position of the Long Canyon deposit on the Paleozoic shelf and its location well east of the majority of identified major trends indicate significant potential for new discoveries in this underexplored area of eastern Nevada.

A technique for improving pseudo-synthetic seismograms generated from neutron logs in gas-saturated clastic rocks

In clastic and carbonate rock sequences, the neutron and sonic log curves usually deflect in a similar fashion. Moreover, in some cases the two curves can be overlain and they generally appear to mimic each other, with variations between them only in the amplitudes of the two curves. This descriptive correlation is the basis of direct cross-plot techniques used to convert a neutron log into a pseudo-sonic log, which can then be combined with a density log to create a pseudo-synthetic seismogram. Unfortunately, the seismograms produced in this way may not match the standard synthetic seismograms produced from the sonic and density logs if the ?gas effect? is not taken into account. In order to correct for the gas effect, the inter-log correlations between the compensated neutron log (CNL) and the borehole-compensated (BHC) sonic log curves from a well in Taiwan were carefully examined. Then, we developed a technique for transforming the CNL log into a pseudo-BHC log by splicing together several continuous sandstone intervals in which the gas effect could be identified from the scattered data on the cross-plot of neutron porosity versus sonic interval transit time. Based upon our results, application of the new composite transform method yields a pseudo-synthetic seismogram that better matches the standard synthetic seismogram (made from the sonic and density logs) according to frequency, amplitude and polarity. This gas correction technique may be particularly useful in oil and gas exploratory and development areas where neutron logs are more prevalent than sonic logs or where sonic logs are scarce.

Characteristics and evolution of Cenozoic sediments in the Liyue Basin, SE South China Sea

The Liyue Basin in southeastern South China Sea is considered to be a Cenozoic basin superimposed on the Mesozoic Erathem. The present account is based on comprehensive analysis of seismic profiles, dragnet and core samples. Cenozoic deposits in the basin are characterized by a sequence of marine detrital and carbonate rocks. There are two regional unconformities (T70 and T40), corresponding to the commencement and termination of sea-floor spreading in the South China Sea, in the Late Oligocene and late Early Miocene respectively. The Cenozoic sedimentary evolution of the basin recorded different sedimentary responses to sea-floor spreading in the South China Sea, and reflected differences in the basin structure, paleogeographic pattern and sedimentary characteristics before 30Ma (Late Oligocene) and after 30Ma. The evolution of sea-floor spreading, changes in sea level and sediment sources controlled the Cenozoic sedimentary infill.

Middle and late Permian foraminifers from the Chichibu Belt, Takachiho Area, Kyushu, Japan: implications for faunal events

The middle and late Permian to Triassic sequence of carbonate rocks in the Takachiho area, central Kyushu, Japan, originated as a Panthalassan seamount and formed parts of far-traveled accreted terranes. This succession is divided into the Permian Guadalupian Iwato and Lopingian Mitai formations, and the Triassic Kamura Formation. The Iwato Formation is further into three zones, the Neoschwagerina, Yabeina higoensis, and Neoendothyra permica zones in ascending order; the Mitai Formation includes the Nanlingella suzukii Zone. Biotic turnovers are recorded twice in the Iwato and Mitai formations. The first and more remarkable biotic event is the total disappearance of fusulines at the top of the Yabeina higoensis Zone. The Neoendothyra permica Zone is characterized by black fine-grained micritic limestones having only sparse non-fusuline foraminifers. The second biotic event is the first appearance of Wuchiapingian fusulines by which the Nanlingella suzukii Zone is defined. The Capitanian–Wuchiapingian boundary is drawn between the Neoendothyra permica and Nanlingella suzukii zones. Above this boundary both the numbers of individual foraminifers and diversity of species gradually increase. The turnovers recorded in the Takachiho area were the result of late Capitanian–Wuchiapingian environmental changes which were sufficient to cause the disappearance of large, multi-chambered fusulines and allow the new appearance of an ‘ultimate’ Paleozoic foraminiferal assemblage. Ten species of foraminifers, including Yabeina higoensis and Yabeina Columbiana, are described.

Potential contaminant transport in the regional Carbonate Aquifer beneath Yucca Mountain, Nevada, USA

Yucca Mountain, Nevada is the site of the proposed US geologic repository for spent nuclear fuel and high-level radioactive waste. The repository is to be a mine, sited approximately 300 m below the crest of the mountain, in a sequence of variably welded and fractured mid-Miocene rhylolite tuffs, in the unsaturated zone, approximately 300 m above the water table. Beneath the proposed repository, at a depth of 2 km, is a thick sequence of Paleozoic carbonate rocks that contain the highly transmissive Lower Carbonate Aquifer. In the area of Yucca Mountain the Carbonate Aquifer integrates groundwater flow from north of the mountain, through the Amargosa Valley, through the Funeral Mountains to Furnace Creek in Death Valley, California where the groundwater discharges in a set of large springs. Data that describe the Carbonate Aquifer suggest a concept for flow through the aquifer, and based upon the conceptual model, a one-layer numerical model was constructed to simulate groundwater flow in the Carbonate Aquifer. Advective transport analyses suggest that the predicted travel time of a particle from Yucca Mountain to Death Valley through the Carbonate Aquifer might be as short as 100 years to as long 2,000 years, depending upon the porosity.

Base level change and depositional filling response of Jurassic in the Qiangtang Basin of Tibet

The base level during the deposition of Jurassic in the Qiangtang Basin shows a complete cycle from rising to falling. The base level change is closely connected with tectonic evolution of the basin, especially connected with Bangonghu-Nujiang ocean evolution process in the formation and evolution of the basin. It is also affected by climate. The Jurassic strata correspond to a long-term base level cycle sequence. The sequence is in fact a non-complete symmetrical cycle, consisting of rising hemicycle and falling hemicycle. It can be divided into 6 intermediate-term base level cycle sequences, including 2 carbonate sequences, 3 mixture sedimentary sequences of carbonate and clastic rocks and one clastic sedimentary sequence. Depositional filling characteristics during base level change show that Bangonghu-Nujiang ocean spreads in Toarcian-Bajocian ages, and is at the height of spreading of Bangonghu-Nujiang ocean in Bathonian-Oxfordian ages. In that process, sea area became smaller because of the dry climate. Eventually, marine depositional filling is ended with the subduction and collision of Bangonghu-Nujiang ocean.

Algues et microfossiles problématiques du Dévonien Moyen du « Fondry des Chiens » (bord sud du Synclinorium de Dinant, Belgique) : implications paléobathymétriques

This study is concerned with the distribution of algae and microproblematica observed in a regressive sequence of carbonate rocks that accumulated open marine to lagoonal palaeoenvironments. The basal algal microflora is reduced to a few chlorophytes (Udoteacean, Dasycladales), but increases rapidly in number and diversity in the flanks of the bioconstructions, then explodes in the restricted facies (“Pseudo-udoteaceans”). An attempt to estimate the bathymetry is based on the palaeoecological constraints of the identified algae. We conclude that the maximum water depth did not exceed the twenty meters. The regressive sequences being made of around sixty meters (of reefal, perireefal and lagoonal carbonates), subsidence is believed to be the main mechanism controlling accommodation space.

Superimposed folding and oblique structures in the palaeomargin-derived units of the Central Betics (SW Spain)

The Subbetic units consist of a deformed wedge of sedimentary rocks deriving from the South Iberian palaeomargin, in the westernmost segment of the Alpine–Mediterranean orogenic belt. In the central Betics, shortening produced folding, thrusting and nappe-stacking. The structural trend is NE–SW to ENE–WSW, and the internal structure of the thrust sheets, which shows no preferred vergence, is strongly controlled by the rheology of the décollement level, a viscous layer of Triassic evaporites, and of the overlying carbonate rock sequence. The relationships of piggyback basin Miocene sediments with the main structures show that the latter developed during the Aquitanian–Burdigalian transition. Superimposed folding is described for the first time in the study area. The origin of the interference cannot be determined at present, but may be related to initially oblique structures refolded during progressive deformation. A revision of previously published palaeomagnetic data in the light of the data presented in this paper, in conjunction with a model of the structural evolution of the Subbetic units of the western Betics, permits us to establish milestones of the tectonic evolution of the external zones in the northern branch of the Gibraltar Arc.

Mechanical stratigraphic controls on fracture patterns within carbonates and implications for groundwater flow

Groundwater flow in low matrix-permeability carbonate rocks is largely controlled by fracture networks. The stratigraphic features that control fracture initiation and termination within a sequence of sedimentary rock strata define the mechanical stratigraphy of the sequence. We investigate the effectiveness of various types of stratigraphic horizons in terminating opening-mode fractures in two different carbonate rock sequences: a relatively homogeneous dolomite sequence, in Door County, WI and an interbedded chalk and marl sequence within the Austin Chalk, TX. Additionally, we present analog and numerical modeling results that delineate the specific mechanisms that facilitate fracture termination. The combination of model results and empirical relationships between observed sedimentary features and mechanical stratigraphy shows: (1) fractures terminate at weak contacts (e.g. thin organic layers), shallowly buried contacts or thick fine-grained units adjacent to thin fractured beds, (2) fractures propagate across strong contacts (e.g. intracycle contacts between different lithology) and thin fine-grained units adjacent to thick fractured beds and (3) fractures step-over at moderate strength contacts. We use these guidelines to predict fracture network from sedimentary stratigraphy by qualitatively assessing the mechanical stratigraphy of a portion of the relatively complex Cretaceous shelf-margin sequence at Sant Corneli, Spain. This predictive demonstration illustrates the utility of assessing the mechanical stratigraphy of subsurface strata within which fractures are not directly observable. We conclude that for a variety of carbonate mechanical stratigraphic sequences, dominant fluid flow characteristics, such as horizontal high flow zones and flow compartmentalization, can be evaluated using fracture spacing and connectivity within fracture networks that is predicted from sedimentary stratigraphy. Although the resulting heterogeneous flow networks do not rely on every fracture present, they are highly dependent on the mechanical stratigraphy.

Structure and timing of transpressional deformation in the Shackleton Glacier area, Ross orogen, Antarctica

Basement of the Transantarctic Mountains comprises the Ross orogenic belt, a Neoproterozoic to Ordovician mobile belt located along the palaeo-Pacific margin of Gondwana. Our structural analysis of deformation in the Liv Group, a sequence of Cambrian volcanic, volcaniclastic, clastic and carbonate rocks, and nearby plutonic rocks indicates that the Shackleton Glacier area has a polyphase deformation history that includes development of both contractional and strike-slip structures. We describe evidence for synchronous contraction and strike-slip movement, and suggest that the structural suite in this area developed, at least in part, within a sinistral transpressive kinematic regime. Deformation in the Shackleton Glacier area is inferred to be at least in part younger than 505 Ma, based on the deformation of the well-dated Taylor Formation. New 40 Ar/ 39 Ar cooling ages from metamorphosed igneous and sedimentary rocks range from c. 500 to 470 Ma and are interpreted to date orogenic cooling. Based on our structural results, and on consideration of the tectonic environment represented by the Liv Group, we propose that shortening across a subduction-related volcanic arc system caused deformation of the Liv Group and associated plutonic rocks. Oblique convergence and thus, transpression, may have ultimately been related to left-oblique plate subduction.

A Formação Água Clara na Região de Araçaíba - SP: registro U-Pb de uma bacia mesoproterozóica

The main objective of this work is to present U-Pb isotopic data obtained from zircons of metabasic rocks and subordinate acid and associated rocks of the Agua Clara Formation in the Aracaiba region - Sao Paulo. The Agua Clara Formation is predominantly constituted by a sequence of carbonate rocks (pure to impure marbles), calc-silicate rocks, calc schists, mica schists, amphibole schists, quartzites, garnet-chlorite-biotite schists, metacherts, basic and intermediate metatuffs, metabasites, amphibolites and cornubianites. The metabasic bodies are usually a few km long with widths of hundreds of m. They are greenish gray, fine- to medium-grained, and usually present nematoblastic texture. More isotropic terms also occur, showing granoblastic texture. They are composed of pyroxenes (diopside or augite), amphiboles (actinolite and hornblende), and plagioclases (andesine/oligoclase). The most common accessories are apatite, magnetite, epidote, titanite, and rare zircon. In thin section, preserved ophitic and subophitic textures are observed, indicating a probable igneous origin. The geochemical data suggest compositions similar to enriched midoceanic ridge basalts (E-MORB) tending to oceanic island basalts (OIB). The subalkaline, tholeiitic characteristics, similar to E-MOR basalts tending to OI basalts, suggest distension or back-arc environments as the geotectonic settings for the basic magmatism. The geochronologic and geochemical characteristics and structural patterns described for the Agua Clara Formation metabasic lithotypes (probably sills and/or dikes) suggest that periodic volcanism with associated sedimentation occurred during the Mesoproterozoic at about 1500 Ma. This event represents an important marker in the volcano-sedimentary evolution of the Apiai Domain. The Neoproterozoic values obtained refer to zircon recrystallization, to neoformation processes which affected the metabasics and amphibolites, to the emplacement of granitic and acid volcanic rocks, and to regional cooling of the Agua Clara Formation.

Origin of the Arabian Plate Structures: Amar Collision and Najd Rift

ABSTRACT The regularly spaced, N-trending Summan Platform, Khurais-Burgan Anticline, En Nala (Ghawar) Anticline and Qatar Arch in the eastern part of the Arabian Plate appear to have formed during the Precambrian Amar Collision between about 640 and 620 million years ago. This collision occurred along the N-trending Amar Suture (that bisects the Arabian Peninsula at about longitude 45 E) when the Rayn micro-plate in the east was fused to the western part of the Arabian Craton. The great anticlines are bounded by the NE-trending Wadi Batin Fault and NW-trending Abu Jifan Fault that converge on the Amar Suture. Deep wells drilled into these anticlines intersected deformed metasediments that are dated as syn-collisional. The Amar Collision was followed by a widespread extensional collapse of the Arabian-Nubian Shield between about 620 and 530 million years ago. During the final extensional stage, between about 570 and 530 million years ago, the NW-trending Najd Fault System dislocated the Arabian Shield left-laterally by about 250 to 300 kilometers. This dislocation appears to complement NE-oriented intra-continental rifts in Oman, Pakistan, Zagros Mountains and the Arabian Gulf. In these rift basins there accumulated thick sequences of clastic and carbonate rocks and salt, such as the Ara Group in Oman, Salt Range Formation in Pakistan, and Hormuz Series in the Arabian Gulf and Zagros Mountains. During the extensional collapse, the N-trending anticlines probably remained elevated as elongated horsts bounded by normal faults. The intervening subsiding grabens accumulated syn-rift sediments including the Hormuz Salt, and form an inter-fingering pattern between the great N-trending anticlines. The Precambrian anticlines, together with the syn-rift salt form the foundations of most of the hydrocarbon traps in the Arabian Plate.

Gold-bearing sulfide and gossan mineralisation systems of the Myriophyto Region, northern Macedonia, Greece

Circum Rhodope Belt marbles, thrusted over the Vertiskos mica gneiss basement and the Mesozoic Myriophyto granite, develop complex shear and fault structures, incorporating fragmented sequences of metamorphosed mafic and ultramafic rocks. The strongly sheared marble-gneiss contact zones and the post-shear subvertical faults and related breccia zones host gold-bearing sulfide and gossan mineralisations and accompanied silicification, dolomitization and ankeritization. Gold forms numerous Ag and Bi-bearing grains in arsenopyrite-pyrrhotite-pyrite-chalcopyrite assemblages and finely dispersed microscopic grains in limonite gossans. The restricted areal and volumetrical development of carbonate rock sequences is unabre to host economic deposits, though the later supergene system shows a potential Au-enrichment development.