Cenozoic Age Research Articles

Peak metamorphic temperature and thermal history of the Southern Alps (New Zealand)

The Southern Alps orogen of New Zealand results from late Cenozoic convergence between the Indo-Australian and Pacific plates and is one of the most active mountain belts in the world. Metamorphic rocks carrying a polymetamorphic legacy, ranging from low-greenschist to high-grade amphibolites, are exhumed in the hanging wall of the Alpine Fault. On a regional scale, the metamorphic grade has previously been described in terms of metamorphic zones and mineral isograds; application of quantitative petrology being severely limited owing to unfavorable quartzofeldspathic lithologies. This study quantifies peak metamorphic temperatures (T) in a 300×20km area, based on samples forming 13 transects along-strike from Haast in the south to Hokitika in the north, using thermometry based on Raman spectroscopy of carbonaceous material (RSCM). Peak metamorphic T decreases across each transect from ≥640°C locally in the direct vicinity of the Alpine Fault to less than 330°C at the drainage divide 15–20km southeast of the fault. Thermal field gradients exhibit a degree of similarity from the southernmost to the northernmost transects, are greater in low-grade semischist than high-grade schist, are affected by folding or discontinuous juxtaposition of metamorphic zones, and contain limited information on crustal-scale geothermal gradients. Temperatures derived by RSCM thermometry are slightly (≤50°C) higher than those derived by traditional quantitative petrology using garnet–biotite thermometry and THERMOCALC modeling. The age of RSCM T appears to be mostly pre-Cenozoic over most of the area except in central Southern Alps (Franz Josef–Fox area), where the amphibolite facies schists have T of likely Cenozoic age. The RSCM T data place some constraints on the mode of exhumation along the Alpine Fault and have implications for models of Southern Alps tectonics.

Emplacement age of leucogranite in the Kampa Dome, southern Tibet

Himalayan leucogranite is an important rock to decipher the orogenic evolution of the Himalayan orogen. It has been recognized that the leucogranite occurred as two separate, but parallel belts within Himalaya. Leucogranite in the north belt is mainly exposed an intrusion component within the gneissic dome. Although most domes have been intensively studied, the Kampa Dome, located in the east of Southern Tibet, is less investigated due to its inaccessibility. In this study, we conducted a comprehensive set of U(–Th)–Pb zircon, monazite and xenotime dating and trace element composition analyses using laser ablation technique, in order to constrain the emplacement age and lithological nature of the Kampa leucogranite. Zircons from gneiss are mainly magmatic and define a protolith age of ca. 498Ma, but no Cenozoic age is documented. Zircons from leucogranite display inherited magmatic core overgrown by thin dark rim, which occasionally gives a much younger age of 27.7±1.0Ma. However, monazites from leucogranite yield a Th–Pb age of 25.7±0.5Ma to 26.8±0.4Ma, which is comparable to the U–Pb age of 24.8±0.4Ma obtained from xenotimes. For the granitic gneiss into which the leucogranite intruded, monazites and xenotimes give ages of 25.8±0.6Ma to 26.6±0.8Ma and 25.0±0.4Ma to 26.0±1.0Ma, respectively. These ages are comparable to those obtained from the leucogranite, and suggest that the metamorphic monazite and xenotime from gneiss formed during the leucogranite emplacement. Trace element analyses indicated that zircons from leucogranite have high P (up to 3721ppm) and REE concentrations, but have low in Th/U ratios (0.04–0.16). Similarly, monazites from leucogranite generally show extreme HREE depletion and significant negative Eu anomaly with some displaying significantly tetrad effect. These geochemical features indicated that the Kampa leucogranite was highly fractionated during its evolution.

Chemical and isotopic features of cold and thermal fluids discharged in the Southern Volcanic Zone between 32.5°S and 36°S: Insights into the physical and chemical processes controlling fluid geochemistry in geothermal systems of Central Chile

The Principal Cordillera of Central Chile is characterized by two belts of different ages and lithologies: (i) an eastern Mesozoic belt, consisting of limestone- and gypsum-rich sedimentary rocks at the border between Central Chile and Argentina, where the active volcanic arc occurs; and (ii) a western belt of Cenozoic age containing basaltic to andesitic volcanic and volcanoclastic sequences. This distinctive geological setting controls water chemistry of cold and thermal springs in the region, which are fed by meteoric water that circulates through deep regional structures. In the western sector of Principal Cordillera, water–rock interaction processes produce low TDS, slightly alkaline HCO3− dominated waters, although dissolution of underlying Mesozoic evaporitic rocks occasionally causes SO42− and Cl− enrichments. In this area, few Na+–HCO3− and Na+–SO42− waters occurred, being likely produced by a Ca2+–Na+ exchange during water–rock interactions. Differently, the chemical features of Ca2+–Cl− waters was likely related to an albitization–chloritization process affecting basaltic to andesitic rocks outcropping in this area. Addition of Na+–Cl− brines uprising from the eastern sector through the west-verging thrust faults cannot be excluded, as suggested by the occurrence of mantle He (~19%) in dissolved gases. In contrast, in the eastern sector of the study region, mainly characterized by the occurrence of evaporitic sequences and relatively high heat flow, mature Na+–Cl− waters were recognized, the latter being likely related to promising geothermal reservoirs, as supported by the chemical composition of the associated bubbling and fumarolic gases. Their relatively low 3He/4He ratios (up to 3.9 Ra) measured in the fumaroles on this area evidenced a significant crustal contamination by radiogenic 4He. The latter was likely due to (i) degassing from 4He-rich magma batches residing in the crust, and/or (ii) addition of fluids interacting with sedimentary rocks. This interpretation is consistent with the measured δ13C-CO2 values (from −13.2 to −5.72‰ vs. V-PDB) and the CO2/3He ratios (up to 14.6×1010), which suggest that CO2 mostly originates from the limestone-rich basement and recycling of subducted sediments, with an important addition of sedimentary (organic-derived) carbon, whereas mantle degassing contributes at a minor extent. According to geothermometric estimations based on the Na+, K+, Mg2+ and Ca2+ contents, the mature Na+–Cl− rich waters approached a chemical equilibrium with calcite, dolomite, anhydrite, fluorite, albite, K-feldspar and Ca- and Mg-saponites at a broad range of temperatures (up to ~300°C) In the associated gas phase, equilibria of chemical reactions characterized by slow kinetics (e.g. sabatier reaction) suggested significant contributions from hot and oxidizing magmatic gases. This hypothesis is consistent with the δ13C-CO2, Rc/Ra, CO2/3He values of the fumarolic gases. Accordingly, the isotopic signatures of the fumarolic steam is similar to that of fluids discharged from the summit craters of the two active volcanoes in the study area (Tupungatito and Planchón–Peteroa). These results encourage the development of further geochemical and geophysical surveys aimed to provide an exhaustive evaluation of the geothermal potential of these volcanic–hydrothermal systems.

Evolution of porewater composition through time in limestone aquifers: Salinity and D/H of fluid inclusion water in authigenic minerals (Jurassic of the eastern Paris Basin, France)

Past water circulations can significantly reduce the porosity and permeability of marine limestones. This is particularly the case in the Middle (Bathonian/Bajocian) to Upper (Oxfordian) Jurassic limestones from the eastern border of the Paris Basin. The knowledge of the timing, the temperature and composition of paleowaters is essential to model the hydrological evolution in this area where the Callovian–Oxfordian claystones are studied for the storage of nuclear wastes. In this way, fluid inclusions hosted in low-temperature (<60°C) authigenic calcite, quartz and celestite crystals were analyzed by Raman spectroscopy and mass spectrometry to determine the chlorinity and D/H ratios. Chlorinity measurements (mmol Cl per liter of water) in fluid inclusions trapped in authigenic crystals during the late Jurassic/early Cretaceous period revealed unexpected high values, up to 3800mmoll−1, indicating that brines were involved in some of the diagenetic crystallization processes. By contrast, fluid inclusions in calcite cements of Cenozoic age within the Oxfordian limestones have low Cl concentration (less than 150mmoll−1), thus showing that a dilution event caused by water infiltrations during the Cretaceous uplift of this part of the basin has flushed out the original saline porewater. By coupling δD of fluid inclusion with δ18O of calcite crystals, we estimate that calcite precipitation occurred at temperatures between 25 and 53°C. The hydrogen isotope composition of calcite-forming water is different between the Middle Jurassic (δD ranging from −20 to −35.8‰V-SMOW) and the overlying Oxfordian limestone (δD from −59.5 to −44.8‰V-SMOW). Present-day groundwaters are also of distinct composition on both sides of the Oxfordian claystones, indicating that limestone aquifers underwent independent hydrologic evolutions since the early diagenetic Jurassic cementation.

Estado del conocimiento de la durofagia en el registro fósil: interacción depredador-presa en moluscos marinos (Clases Gastropoda y Bivalvia)

Predation as an evolutionary mechanism of diversification is a topic of great interest. Durophagy is the feeding behavior of preying on hard-shelled or exoskeleton-bearing organisms. On this matter, because of their worldwide distribution, abundance, wide range of habitats, and a remarkable preservation frequency, mollusks are of particular interest in the study of predation in the fossil record. In the present work, papers published in international scientific magazines focusing on the issue of durophagy on fossil marine bivalve and gastropods as potential prey were analyzed. A total of 101 papers on this subject were reviewed. Six-kinds of evidence of durophagy were identified: drilling, repaired and fragmented shells, bites, punctures and coprolites. Most items are of Cenozoic age and the majority of the records available correspond to the tropics. Most Paleozoic potential predators were not identified; this situation extends to the Mesozoic, but predation of gastropods and bivalves by carnivorous gastropods, arthropods, fishes and reptiles was recognized. Predation by naticid and muricid gastropods prevails during the Cenozoic, also was recognized predation by arthropods, fishes and mammals.

The age and petrogenesis of alkaline magmatism in the Ampasindava Peninsula and Nosy Be archipelago, northern Madagascar

The Ampasindava alkaline province consists of a series of circular and elliptical intrusions, lava flows, dyke swarms and plugs of Cenozoic age emplaced into the Mesozoic-Cenozoic sedimentary rocks of the Antsiranana basin (NW Madagascar) and above the crystalline basement. The magmatism in the Ampasindava region is linked to a NW-SE trending extensional tectonic setting. New 40Ar/39Ar age determinations on feldspar separate of alkali granites and basaltic dykes yielded ages of 18.01 ± 0.36 Ma and 26 ± 7 Ma, respectively. Alkali basalts and basanites, nepheline syenites and phonolites, and silica saturated-to-oversaturated syenites, trachytes, granites and rhyolites are the main outcropping lithologies. These rocks have sodic affinity. The felsic rocks are dominant, and range from peraluminous to peralkaline. The mantle-normalized incompatible element patterns of the mafic lavas match those of Na-alkaline lavas in within-plate rift settings. The patterns are identical in shape and absolute concentrations to those of the Bobaomby (Cap d’Ambre) and Massif d’Ambre primitive volcanic rocks. These geochemical features are broadly compatible with variable degrees of partial melting of incompatible element-enriched mantle sources. The mineralogical and geochemical variations are consistent with fractional crystallization processes involving removal of olivine, feldspar, clinopyroxene, amphibole, Fe-Ti oxides and apatite. Removal of small amount of titanite explains the concave upward lanthanide pattern in the evolved nepheline syenites and phonolites, which are additionally rich in exotic silicates typical of agpaitic magmas (eudialyte, F-disilicates).

Present-day structure and stages of tectonic evolution of Wrangel Island, Russian eastern Arctic Region

Present-day structure of Wrangel Island was formed during two main stages of Mesozoic-Cenozoic deformation. The general fold-thrust structural grain of the island, characterized by northern vergence and complicated by NW-trending right-lateral strike-slip faults, originated in the post-Triassic. Mesostructural data indicate a near-meridional orientation of regional compression. This deformation stage was related to the orogeny in the New Siberian-Chukchi Fold System, which occurred at the end of Neocomian in the pre-Aptian. The next stage was characterized by a near-meridional and NNW-SSE extension established by superposition of normal, right-lateral strike-slip, and pull-apart kinematics upon the former fold-thrust structure. Comparison of the structural and the published seismic data allows us to suggest the later stage of compression to be in the Late Cretacous-Paleocene, which is correlated to the Mid-Brookian angular unconformity in Arctic Alaska and North Chukchi Basin. Accordingly, the Cenozoic age (since Paleocene-Eocene) of the main extension and right-lateral transtension most likely corresponds to opening of the South Chukchi (Hope) Basin localized immediately to the south of the Wrangel-Herald Arch (High). The difference in structural patterns of the Silurian-Lower Devonian and the Upper Devonian (?)-Triassic rocks is evidence for deformation related to the Ellesmerian Orogeny in the Middle-Late Devonian.

The multiply deformed foreland fold-thrust belt of the Balkan orogen, northern Bulgaria

The generally east-west–trending Balkan orogen (eastern Europe) consists of a northern belt of folded and thrusted Mesozoic and Cenozoic strata that forms the external fold-thrust belt of late Mesozoic and early Cenozoic age, and a southern belt that consists of deformed igneous and metamorphic rocks overprinted by Cenozoic extensional basins. Unlike most foreland fold-thrust belts, wherein deformation commonly migrates toward the foreland, the fold-thrust belt within the Balkan orogen is marginal to the Moesian Platform to the north, but was deformed in at least three events related to three different dynamic systems caused by changes in plate interactions. The earliest event of late-Early to early-Late Cretaceous deformed strata deposited within the Moesian continental margin and within a continental rifted belt containing deep-water flysch of Late Jurassic–Early Cretaceous age, a probable eastward extension of oceanic troughs from the Southern Carpathians. The shortening was a consequence of south or southwest synthetic subduction within the Vardar zone along the southern margin of the Balkan orogen. In Late Cretaceous time a backarc and/or intraarc rift zone developed along the southern margin of the fold belt, terminating shortening. The backarc and/or intraarc basin closed in Late Cretaceous–early Paleocene time, deforming the fold-thrust belt for a second time, but antithetically to north or northeast subduction in the Vardar zone. North- and northwest-vergent subduction within the Vardar zone caused magmatism, metamorphism, and deformation within the Rhodope area of southern Bulgaria south of the foreland thrust belt. In Paleogene time the southern part of the Balkan orogen became extensional with development of extensional basins and abundant magmatism due to trench rollback. The time of the final foreland fold-thrust belt deformation was late Eocene extending into Oligocene or early Miocene, contemporaneous with the extension to the south. The deformation within the fold-thrust belt was caused by a transfer of transpressional right shear within north Bulgaria and the Southern Carpathians as crustal units were translated northward west of the Moesian foreland crust and moved northeast and eastward into the eastern Carpathian west-dipping subduction zone. During the third event of deformation crustal units were molded around the Moesian foreland crust. The shortening ceased by early Miocene time and the right shear west of Moesian foreland crust was manifested by discrete right-slip faults to the present. During this third event southern Bulgaria was in an extensional regime that dominated the south- to southwest-vergent Hellenide orogen throughout the Cenozoic, thus dividing the Balkan orogen into two different deformational regions.

Cylindrichnus concentricus Toots in Howard, 1966 (trace fossil) in its type locality, Upper Cretaceous, Wyoming

Cylindrichnus concentricus is a wide spread trace fossil in shallow marine sedimentary rocks of Mesozoic and Cenozoic age. This paper clarifies the ichnotaxonomy of C. concentricus by of fering an emended diagnosis of the ichnogenus and a new diagnosis of the ichnospecies. The broad, bow-shaped architecture of C. concentricus with two openings at the sediment surface suggests that the trace maker was either a filter-feeding animal that captured suspended food particles from the water column or else a surface deposit feeder that employed tentacles or an eversible pharynx to collect nutritious sediment from the surface around one or both of the burrow openings. Although C. concentricus has been reported in a variety of different sedimentary environments around the world, in its type locality in the Cretaceous of Wyoming the trace fossilis found in low-diversity or monoichnospecific beds that exhibit hummocky cross stratification. This occurrence suggests the burrower’s tolerance (or perhaps preference) for an episodic high-energy hydrodynamic environmentre lated to major storms.

Constraining silica diagenesis in methane-seep deposits

Silicified fossils and authigenic silica are common in ancient seep limestones. Silicification of calcareous fossils facilitates the preservation of even fine details and is therefore of great interest to paleontologists, permitting a reliable taxonomic identification of the chemosynthesis-based taxa that lived at hydrocarbon seeps. Four methane-seep limestones of Paleozoic, Mesozoic, and Cenozoic age with abundant silica phases are compared in this study; one, an Eocene seep deposit on the north shore of the Columbia River at Knappton, western Washington State, USA, is described for the first time. Its lithology and fabrics, negative δ13Ccarbonate values as low as −27.6‰, and 13C-depleted biomarkers of archaea involved in the anaerobic oxidation of methane (AOM) reveal that the carbonate rock formed at a methane seep. The background sediments of the studied Phanerozoic seep limestones contain abundant siliceous microfossils, radiolarian tests in case of the Carboniferous Dwyka Group deposits from Namibia and the Triassic Graylock Butte deposits from Oregon (USA), as well as diatom frustules in case of the Eocene Knappton limestone and an Oligocene seep deposit from the Lincoln Creek Formation (Washington State, USA). These microfossils are regarded as the source of dissolved silica, causing silicification and silica precipitation. Silica cements formed after AOM-derived cements ceased to precipitate but before equant calcite formed. Numerical experiments using the computer code PHREEQC confirmed that (1) AOM increases the pH of pore waters and that (2) this pH increase subsequently mobilizes biogenic silica, (3) followed by the re-precipitation of the dissolved silica in the periphery of the AOM hotspot. The experiments revealed that degassing of carbon dioxide has the potential to significantly increase the local pH of pore waters, exerting an even stronger control on the local pH and silica dissolution than the rate of AOM alone.

LA-ICP-MS mineral chemistry of titanite and the geological implications for exploration of porphyry Cu deposits in the Jinshajiang – Red River alkaline igneous belt, SW China

The Jinshajiang–Red River alkaline igneous belt in the eastern Indian–Asian collision zone, of southwestern China, hosts abundant, economically important Cu–Mo–Au mineralization of Cenozoic age. Major- and trace-element compositions of titanites from representative Cu-mineralized intrusions determined by LA-ICP-MS show higher values for Fe2O3/Al2O3, ΣREE + Y, LREE/HREE, Ce/Ce*, (Ce/Ce*)/(Eu/Eu*), U, Th, Ta, Nb and Ga, and lower values for Al2O3, CaO, Eu/Eu*, Zr/Hf, Nb/Ta and Sr than those for titanites from barren intrusions. Different ΣREE + Y, LREE/HREE, U, Th, Ta and Nb values of titanites between Cu-mineralized and barren intrusions were controlled mainly by the coexisting melt compositions. However, different Sr concentrations and negative Eu anomalies of titanites between Cu-mineralized and barren intrusions were most probably caused by different degrees of crystallization of feldspar from melts. In addition, different Ga concentrations and positive Ce anomalies of titanites between Cu-mineralized and barren intrusions were most likely caused by different magmatic fO2 conditions. Pronounced compositional differences of titanites between Cu-mineralized and barren intrusions can provide a useful tool to help discriminate between ore-bearing and barren intrusions at an early stage of exploration, and, thus, have a potential application in exploration for porphyry Cu deposits in the Jinshajiang – Red River alkaline igneous belt, and to other areas.

New contributions in Baffin Bay/Labrador Sea petroleum exploration and development geoscience

Baffin Bay and Labrador Sea encompass a vast marine region straddling the international jurisdictions of Canada and Greenland (Fig. 1). Together, these marine bodies encompass more than 1 500 000 km2 and they are bounded by a vast and geographically-complicated shoreline characterized by long fiords and deep channels. Sea-ice forms in much of Baffin Bay in winter, while icebergs originating from glaciers descending from the Greenland icecap create a constant hazard across the marine region during summer. Although much of the surrounding coastline is precipitous, a significant extent of continental shelf is present in these marine basins, characterized by accumulations of clastic strata, principally of late Mesozoic to Cenozoic age. Recent assessments of petroleum potential of the region have suggested that Baffin Bay may hold 1555 MMbbl of oil and 9.3 TCF of gas (Gautier et al., 2011), while shelf areas of the Labrador Sea may contain recoverable resources of 4.2 TCF of natural gas and 123 MMbbl of oil (Government of Newfoundland and Labrador, 2000). Figure 1 Map of Baffin Bay/Labrador Sea region showing principal geographic features and areas of focus of the contributions in the Special Issue. BI = Bylot Island; DI = Disko Island. While onshore exposures of Mesozoic-Cenozoic strata are extremely rare along the Labrador coast, significant onshore successions are found in the Baffin Bay region on Nuussuaq Peninsula and Disko Island and adjacent areas of West Greenland, and on Bylot Island and adjacent areas of northeast Nunavut. These successions provide accessible outcrops that serve as analogues for stratigraphic sequences preserved in the offshore. While extensive study of the Greenland succession has been undertaken recently, and a comprehensive litho- and biostratigraphic framework for these rocks established (Dam et al., 2009), similar studies of the Canadian side of the Baffin Bay are vintage 1970s. As well, some …

Using 87Sr/86Sr Ratios to Date Fossil Methane Seep Deposits: Methodological Requirements and an Example from the Great Valley Group, California

AbstractMethane seep carbonates preserve information about the history of methane seepage and of the fauna inhabiting these ecosystems. For this information to be useful, a reliable determination of the carbonates’ stratigraphic ages is required, but this is not always available. Here we investigate the using strontium isotope stratigraphy to date fossil methane seep carbonates via detailed petrographic and geochemical investigation of the different carbonate phases in biostratigraphically well-dated seep carbonates of Paleozoic, Mesozoic, and Cenozoic age. The best results are obtained from banded, botryoidal rim cements from carbonate phases showing a weak or no cathodoluminescence signal, an oxygen isotope signature close to that of seawater, and the lowest Mn concentrations. We then applied the method to a presumably late Jurassic seep carbonate from the Great Valley Group in California. Strontium isotope ratios of the least diagenetically altered carbonate phases indicate a Tithonian (late Jurassic) ...

Stratigraphy and reservoir quality of the turbidite deposits, western sag, Bohai bay, China P.R.

Stratigraphic and subtle reservoirs such as pinchouts, sand lenses and unconformities have been discovered in Bohai basin. These reservoirs occur in sub-basins and sag structures called depressions. A prolific depression is the Liaohe depression that has been filled with rapidly changing mixed alluvial fan deposit of the Cenozoic age. Attempts made at recovering residual hydrocarbon from the subtle reservoir have necessitated the re-evaluation of available data to characterize and model the prolific Shahejie Formation turbidite deposit occurring as pinchouts and sand lenses for hydrocarbon assessment, reservoir quality and possible recovery through enhanced methods. Methods employed covered well logs analysis, clustering analysis for electrofacies and fuzzy logic analysis to predict missing log sections. Stratigraphic and structural analysis was done on SEGY 3D seismic volume after seismic to well tie. Stochastic simulation was done on both discrete and continuous upscaled data. This made it possible to correctly locate and laterally track identified reservoir formation on seismic data. Petrophysical parameters such as porosity and permeability were modeled with result of clustering analysis. Result shows that electrofacies converged on 2 rock classes. The area is characterized by the presence of interbeded sand-shale blanket formations serving as reservoir and seal bodies. The reservoir quality of the formations as seen on the petrophysical analysis done is replicated in simulation volume results. Reservoir rocks have porosity between 0.1 and 0.25, permeability between 1 and 2mD and hydrocarbon saturation as high as 89%. Lithofacies are observed to be laterally inconsistent, sub-parallel to dipping and occurring as porous and permeable continuous beds or pinchouts hosting hydrocarbon. The stochastic stratigraphic model depicts rock units in associations that are synsedimentary. The prevalent configuration gotten from the model gave an insight into exploring and developing the field for enhanced oil recovery of the heavy hydrocarbon of this area.

Neogene tectonics and fault-related folds in the Gulf of Hammamet area, Tunisian offshore

The western part of the Gulf of Hammamet area lies in the Pelagian Platform, eastward of the Atlas front. This region shows large changes in structural style compared to the Pelagian Platform.During the Neogene, the Gulf of Hammamet recorded the convergence of the Eurasia and Africa plates. During the Pliocene to early Quaternary, the area recorded an NW–SE shortening related to the Atlas contraction followed by an NNE–SSW extension linked to the Pantelleria-Malta graben opening.Subsurface geology of the Gulf of Hammamet area allows us to propose a tectonic sketch, where the Neogene deformation is characterized by (1) Cenozoic age detachment level that located in the Mio-Pliocene and Quaternary series; (2) the growth of fault-related folds (mainly fault-propagation folds) and (3) a total shortening of about 6km.The thin-skinned thrusting is late Miocene age and is synchronous to the Africa–Europe collision. During the upper Miocene (Tortonian), fault-propagation folds developed above inherited normal faults. The Pliocene age was tectonically calm period. Finally, post-Villafranchian contraction reactivated these structures.Despite the large number of hydrocarbon discoveries, the Gulf of Hammamet remains under-explored. This new interpretation of Neogene tectonic events and folding models could lead to a new play idea for the hydrocarbon prospectively in this proven oil province.

Stable isotope record of hydrothermal sulfate, sulfide and silicate minerals in the Darreh-Zar porphyry copper deposit in Kerman, southeastern Iran: Implications for petrogenesis and exploration

The Darreh-Zar porphyry copper deposit is found together with other porphyries such as Sar-Cheshmeh and Sungun in the Cenozoic age Urumieh–Dokhtar magmatic belt that is related to the subduction of the Arabian plate beneath Central Iranian microcontinent. The deposit is associated with Miocene granodiorite porphyry intrusions into Eocene basaltic volcanic rocks. The principal hydrothermal facies of the area are a core zone of potassic alteration enclosed by a peripheral zone of propylitic alteration. Chlorite-sericite, sericitic, advanced argillic and intermediate argillic alteration zones cut the upper part of the potassic zone and are developed near the interface between the potassic and propylitic zones. Chalcopyrite, pyrite or local bornite, and associated anhydrite are the chief hypogene sulfide minerals, and constitute <0.1 to 0.8wt.% Cu ores. A ~50-m-thick supergene zone containing 0.8 to 1.5wt.% Cu, characterized by covellite and chalcocite, underlies an oxidized and partially leached ~20-m-thick zone containing chrysocolla, malachite and azurite.The hydrogen isotopic composition of biotite from Darreh-Zar ranges from δD of −89 to −101‰, and the calculated aqueous fluids (δD of −66 to −78‰) in equilibrium with biotite are consistent with a large component of magmatic fluid. Epidote varies from δD of −56 to −53‰, and yields calculated fluids responsible for the propylitic alteration that range from −22 to −19‰. Oxygen isotopic data for the studied quartz, biotite, anhydrite and epidote vary between δ18O of 0.8 to 9.0‰. The calculated δ18O values for aqueous fluids in equilibrium with the biotite, quartz (from barren type and A veins), anhydrite (from B veins) and epidote are 7.8±0.4, 5.1±0.6, 2.6±0.5 and −1.1±0.1‰, respectively. This marked decrease likely reflects a change from magmatic-derived fluids in central potassic zone to meteoric or sedimentary-derived fluids in the outer propylitic zone. Sulfur isotope compositions are 11.8 to 14.0‰, for anhydrite (n=3), 2.4‰ for molybdenite (n=1), 1.7 to 3.9‰ for pyrite (n=17), and 1.6 to 2.9‰ for chalcopyrite (n=2). The sulfide data, alone, suggest a conventionally ‘magmatic’ value of about 1.6 to 2.9‰ for Darreh-Zar sulfur. However, the fairly oxidized granitic parental magma shows relatively heavy bulk sulfur (δ34SΣS≈+5‰) and sulfate–sulfide sulfur isotopic fractionation is consistent with an approach to isotopic equilibrium at calculated temperatures of 520±50°C for most of the coexisting anhydrite–pyrite pairs (n=4). An exploration implication is that the identification of isotopically high sulfur isotopic compositions of gypsum from near-surface samples determines a hydrothermal system that contains deeper hypogene anhydrite, a common indicator of large porphyry Cu-Mo deposits. In contrast, isotopically low sulfur isotopic values similar to and derived from weathering of pyrite and other sulfides are common in many hydrothermal environments.

Comment on “Dynamic topography in South America” by Federico M. Dávila & Carolina Lithgow-Bertelloni

In their article Dávila and Lithgow-Bertelloni (2013) propose that the Andes have been isostatically uncompensated throughout the Cenozoic and that additional forces induced by mantle flow were required to explain the observed topographies. Although this hypothesis seems plausible, they provide a regional model of “the Bermejo-Pampas foreland of Argentina” which implies that the deposition of the Los Llanos Formation (in La Rioja, NW Argentina) occurred during Miocene. However, this age is incongruent with the presence of a neosauropod nesting site at Sanagasta and a Cretaceous faunal assemblage in Tama both in Los Llanos Formation and well documented in recent publications. Therefore, the proposed model for “the Bermejo-Pampas foreland of Argentina” appears incorrect. Moreover, the Cretaceous exposures at Sanagasta and Tama foster the need of revising the alleged Cenozoic age of the Los Llanos Formation in La Rioja and neighboring provinces, and the tectonic models associated with this formation.

The 36–18 Ma Central Nevada ignimbrite field and calderas, Great Basin, USA: Multicyclic super-eruptions

The 36–18 Ma Central Nevada ignimbrite field and calderas, Great Basin, USA: Multicyclic super-eruptions

The effect of inherited paleotopography on exhumation of the Central Andes of NW Argentina

Differential exhumation in the Puna Plateau and Eastern Cordillera of NW Argentina is controlled by inherited paleostructures and resulting paleotopography related to the Cretaceous Salta Rift paleomargins. The Ceno zoic deformation front related to the development of the Andean retro-arc orogenic system is generally associated with >4 km of exhumation, which is recorded by Cenozoic apatite fi ssion-track (AFT) and (U-Th-[Sm])/He ages (He ages) in the Eastern Cordillera of NW Argentina. New AFT ages from the top of the Nevado de Cachi document Oligocene (ca. 28 Ma) cooling, which, combined with existing data, indicates exhumation of this range between ca. 28 Ma and ca. 14 Ma. However, some of the highest ranges in the Eastern Cordillera preserve Cretaceous ages indicative of limited Cenozoic exhumation. Samples collected from an similar to 3-km-elevation transect along the northern part of the Sierra de Quilmes paleorift fl ank (Laguna Brava) show AFT ages between ca. 80 and ca. 50 Ma and He ages between ca. 45 and ca. 10 Ma. Another set of samples from an similar to 1-km-elevation transect farther to the southwest (La Quebrada) shows Cretaceous AFT ages between ca. 116 Ma and ca. 76 Ma, and mainly Cretaceous He ages, in agreement with AFT data. Analysis of existing AFT and He ages from the area once occupied by the Salta Rift reveals a pattern characterized by Cretaceous ages along paleorift highs and Cenozoic ages within paleorift hanging-wall basins and later foreland basin depocenters. This pattern is interrupted by the Sierras Pampeanas at similar to 28 degrees S, which record mid-Cenozoic ages. Our data are consistent with a complex inherited pattern of pre-Andean paleostructures, likely associated with paleotopography, which was beveled by the Cenozoic regional foreland basin and reactivated during the late Neogene (ca. <10 Ma), strongly controlling the magnitude of Cenozoic uplift and exhumation and thus cooling age distribution. This, combined with variable lithologic erodibility, resulted in an irregular distribution of themochronological ages.

Distribution of porphyry deposits in the Eurasian continent and their corresponding tectonic settings

In the Eurasian continent there are three huge metallogenic belts of Cu and Mo porphyry deposits, comprising the Paleozoic Central Asian Ore Belt in the north, the Tethyan Eurasian Ore Belt of Jurassic to Cenozoic age in the southwest, and the East Margin Ore Belt of the Eurasian Continent of Jurassic to Cretaceous age in the east. The latter is considered to be part of the vast Circum-Pacific ore belt. Some of the main features of the spatial–temporal distribution of Cu and Mo porphyry systems and related geodynamic processes of the three metallogenic belts are described. In particular, the key role of post-subduction – related porphyry ore systems is emphasized, comprising collisional and post-collisional Cu–Mo porphyry deposits during the geological history of the Eurasian continent. The recurrent feature of these ore systems and related felsic rocks is their derivation from partial melting of stagnant or residual oceanic slabs, and mixing with a variable amount of crustal material during magma ascent to shallower levels.