Volcanic Units Research Articles

Forced subduction initiation within the Neotethys: An example from the mid-Cretaceous Wuntho-Popa arc in Myanmar

Abstract Despite decades of research, the mechanisms and processes of subduction initiation remain obscure, including the tectonic settings where subduction initiation begins and how magmatism responds. The Cretaceous Mawgyi Volcanics represent the earliest volcanic succession in the Wuntho-Popa arc of western Myanmar. This volcanic unit consists of an exceptionally diverse range of contemporaneously magmatic compositions which are spatially juxtaposed. Our new geochemical data show that the Mawgyi Volcanics comprise massive mid-oceanic ridge basalt (MORB)-like lavas and dikes, and subordinate island arc tholeiite and calc-alkaline lavas. The Mawgyi MORB-like rocks exhibit flat rare earth elements (REEs) patterns and are depleted in REEs, high field strength elements (except for Th) and TiO2 concentrations relative to those of MORBs, resembling the Izu-Bonin-Mariana protoarc basalts. Our geochronological results indicate that the Mawgyi Volcanics formed between 105 and 93 Ma, coincident with formation of many Neotethyan supra-subduction zone ophiolites and intraoceanic arcs along orogenic strike in the eastern Mediterranean, Middle East, Pakistan, and Southeast Asia. Combined with its near-equatorial paleo-latitudes constrained by previous paleomagnetic data, the Wuntho-Popa arc is interpreted as a segment of the north-dipping trans-Neotethyan subduction system during the mid-Cretaceous. Importantly, our restoration with available data provides new evidence supporting the hypothesis of a mid-Cretaceous initiation of this >8000-km-long subduction system formed by inversion of the ∼E-W–trending Neotethyan oceanic spreading ridges, and that this was contemporaneous with the final breakup of Gondwana and an abrupt global plate reorganization.

EOCENE VOLCANICLASTICS IN THE KARTLI BASIN, GEORGIA: A FRACTURED RESERVOIR SEQUENCE

In the broader Caucasus region, multiple extrusive volcanic units are present within the Jurassic, Cretaceous, Eocene and Miocene sedimentary successions. Partial reworking of volcanic material from various provenance areas into Eocene, Oligocene and Miocene reservoir units is commonly observed in the Eastern Black Sea and in the Rioni, Kartli and Kura Basins of onshore Georgia. Reservoir quality has in general been negatively affected by volcanic rock fragments which may have undergone complex diagenetic alteration. However, despite concerns regarding reservoir quality, oil at the Samgori field, the largest field in Georgia (∼200 MM brl recovered), is hosted in altered Middle Eocene volcaniclastic sandstones interbedded with deep‐water turbidites. Previous studies of core material from numerous wells in this field showed that most of the oil is contained in altered, microfractured, laumontite‐rich tuffs which have fracture and cavernous net porosities averaging 12% and average permeability of 15 mD. The laumontite tuffs comprise only up to 20% of a tuffaceous sandstone section and occur as isolated lenses or pods on a sub‐seismic scale (i.e. 5‐10 m thick), causing highly variable oil productivity from one well to another.The petrographic analysis of samples of Middle Eocene volcaniclastic sandstones from outcrops in the central part of the Kartli Basin around Tbilisi broadly confirms the main conclusions of studies completed some 30 years ago which were based on the analysis of subsurface samples. However, the surface samples analysed show that zeolitization events typically did not improve, but actually reduced, reservoir quality due to extensive zeolite cementation. The poor reservoir properties of the plug samples, which are age‐equivalent to the proven subsurface Middle Eocene reservoir interval, highlight fracturing as a key factor controlling the presence of exceptional producers (up to 9000 b/d) in the Samgori field complex. The study therefore underlines the critical role of fracturing of the Middle Eocene volcaniclastic reservoir sequence in the Kartli Basin.

Isotopic evolution of prehistoric magma sources of Mt. Etna, Sicily: Insights from the Valle Del Bove

Mount Etna in NE Sicily occupies an unusual tectonic position in the convergence zone between the African and Eurasian plates, near the Quaternary subduction-related Aeolian arc and above the down-going Ionian oceanic slab. Magmatic evolution broadly involves a transition from an early tholeiitic phase (~ 500 ka) to the current alkaline phase. Most geochemical investigations have focussed on either historic (> 130-years old) or recent (< 130-years old) eruptions of Mt. Etna or on the ancient basal lavas (ca. 500 ka). In this study, we have analysed and modelled the petrogenesis of alkalic lavas from the southern wall of the Valle del Bove, which represent a time span of Mt. Etna’s prehistoric magmatic activity from ~ 85 to ~ 4 ka. They exhibit geochemical variations that distinguish them as six separate lithostratigraphic and volcanic units. Isotopic data (143Nd/144Nd = 0.51283–0.51291; 87Sr/86Sr = 0.70332–0.70363; 176Hf/177Hf = 0.28288–0.28298; 206Pb/204Pb = 19.76–20.03) indicate changes in the magma source during the ~ 80 kyr of activity that do not follow the previously observed temporal trend. The oldest analysed Valle del Bove unit (Salifizio-1) erupted basaltic trachyandesites with variations in 143Nd/144Nd and 87Sr/86Sr ratios indicating a magma source remarkably similar to that of recent Etna eruptions, while four of the five subsequent units have isotopic compositions resembling those of historic Etna magmas. All five magma batches are considered to be derived from melting of a mixture of spinel lherzolite and pyroxenite (± garnet). In contrast, the sixth unit, the main Piano Provenzana formation (~ 42–30 ka), includes the most evolved trachyandesitic lavas (58–62 wt% SiO2) and exhibits notably lower 176Hf/177Hf, 143Nd/144Nd, and 206Pb/204Pb ratios than the other prehistoric Valle del Bove units. This isotopic signature has not yet been observed in any other samples from Mt. Etna and we suggest that the parental melts of the trachyandesites were derived predominantly from ancient pyroxenite in the mantle source of Etna.

Hydrogeophysical Characterization in a Volcanic Context From Local to Regional Scales Combining Airborne Electromagnetism and Magnetism

AbstractIn volcanic islands, a crucial step in managing watershed water resources is the characterization of groundwater aquifers from local to regional scales. Airborne geophysical data provide high‐resolution images down to hundreds of meters below the surface, over large areas. Yet, the production of an accurate interpretation of regional geophysical imagery may be time consuming or limited by the low density of geological and hydrological field observations. Here, we propose an approach combining airborne electromagnetic and magnetic data in order to reduce geophysical ambiguities and provide a multiscale hydrogeophysical characterization of Piton des Neiges volcano (Réunion Island). With limited calibration data, this methodology produces a geological model more accurate than using airborne electromagnetic data alone. Through the continuous coverage of both methods, we demonstrate the influence of volcanic unit geometries on groundwater flows within the critical zone and we highlight major structures impacting groundwater flows at both local and regional scales.

Late Miocene to Plio-Pleistocene volcanic and paleoenvironmental history of the Cerro Domo and Ventisqueros Mesa area, northwest James Ross Island, northern Antarctic Peninsula

We present the description and interpretation of a James Ross Island Volcanic Group (JRIVG) succession around the Cerro Domo and Ventisqueros Mesa area. Our aim is to reconstruct the volcanic evolution and provide new palaeoenvironmental information for the northwest sector of James Ross Island, northern Antarctic Peninsula. We present representative stratigraphic logs that are combined with new chronological constraints provided by 40Ar/39Ar ages and 87Sr/86Sr isotope analyses that demonstrate a record of the late Miocene to earliest Pleistocene. The first volcanic activity registered is associated with tuff cone deposits that resulted from marine hydromagmatic eruptions, close to 6.18 ± 0.14 Ma. Subsequently, lodgement till deposits record a grounded glacial advance. Overlying the glacigenic deposits, fossil-bearing sediments record a marine or glacimarine environment following the ice sheet retreat, and an 87Sr/86Sr age of 6.02 (+0.09, −0.09) Ma dates ice-poor conditions. A lava-fed delta system, which yields an age of 5.37 ± 0.12 Ma, prograded over the previous deposits. Stacked pillow lavas, curvy columnar-jointed basalts and subaerial tabular lavas, with a 4.9 ± 0.10 Ma age, were emplaced after the lava-fed delta deposition. Another lava-fed delta located to the north of the previously volcanic units yielded an age of 3.57 ± 0.19 Ma. A glacial pavement carved into the top of these deposits records a second ice advance in the area. The final volcanic activity emplaced Cerro Domo, a large volcanic edifice which was the centre of explosive hydromagmatic eruptions. The presence of pristine, articulated bivalves in life position within the lower volcanic deposits of Cerro Domo, together with an 87Sr/86Sr age of 3.16 (+0.59, −0.43) Ma, are strong indicators of a marine environment and suggest ice-poor “interglacial” conditions at this time. Dykes cutting the volcanic deposits of Cerro Domo and considered coeval with the eruptions afford ages of 2.62 ± 0.08 and 2.72 ± 0.09 Ma. Our results provide new insights on the volcanic and glaciation history of James Ross Island, as well as supporting marine, ice-poor conditions around ~6 Ma, and near or right before the Plio-Pleistocene boundary, between ~3.2 and ~2.6 Ma.

Petrogenesis of the Despen Volcanic Rocks of the Middle–Late Ordovician Volcanoplutonic Association of the Tannu-Ola Terrane (Southwestern Tuva)

Abstract ––We have studied the structure and composition of a volcanic unit in the valley of the Despen River, on the southern slope of the East Tannu-Ola Ridge. The unit was earlier assigned to the Lower Devonian Kendei Formation. The new geological and geochronological data show that it resulted from explosive volcanism at 460–450 Ma. The Despen volcanic rocks formed in association with granitoids of the Argolik complex at the end of the accretion–collision stage of evolution of the Altai–Sayan region, in particular, the Tannu-Ola terrane. These are predominantly felsic ferroan metaluminous and weakly peraluminous nappe volcanic rocks resulted from the differentiation of tholeiitic basalts. Their REE patterns, like those of the Argolik granitoids, are flat in the HREE, show a distinct Eu anomaly, and suggest magma generation at shallow depths in the upper crust. The magmatic source was of subduction origin, as evidenced by the negative Ta–Nb anomalies in the multielement patterns and by εNd(T) = +3.1 to +5.6, and has a Neoproterozoic model age, TNd(DM-2st) = 0.94–0.69 Ga.

Characterization of the Candidate Landing Region for Tianwen‐1—China's First Mission to Mars

AbstractThis paper presents our efforts to characterize the candidate landing region (109°–133°E, 23°–30°N) for Tianwen‐1, China's first mission to Mars, in terms of engineering safety and scientific significance. Topographic analysis reveals that the region has a low elevation around −4,230 m, and 98% of the region have slopes smaller than 8°. The geomorphological mapping and analysis show that the region has an average crater density of about 28 craters (≥200 m in diameter) per 100 square kilometers, with several clusters of high crater densities distributed around the center of the region. There are also pitted cones distributed mainly in the southern part of the region, with a density of approximately 6.6 cones per 100 square kilometers in specific local areas. The region has rock abundances ranging from 1% to 23%, with local clusters of low and high rock abundances. The region comprises four main geological units, including a lowland unit formed in the Late Hesperian and a volcanic unit formed in the Amazonian and Hesperian period. Their specific surface ages are estimated through the analysis of crater size‐frequency distribution. Combining the engineering constraints on surface slopes, crater density, cone density, and rock abundance, a hazard map of the candidate landing region is generated for landing site evaluation and safety assessment. Based on the results, we further discuss the potential scientific outcomes from the exploration in this region. The findings will be helpful for the mission planning and maximization of the scientific return from Tianwen‐1, and complement existing Martian scientific research.

A geochemical study of the Crown Formation and Bird Member lavas of the Mesoarchaean Witwatersrand Supergroup, South Africa

Abstract The ca. 2.97 to 2.80 Ga Witwatersrand Supergroup, South Africa, represents the oldest intracontinental sedimentary basin of the Kaapvaal craton. Two volcanic units occur in this supergroup: the widespread Crown Formation lavas in the marine shale-dominated West Rand Group and the more geographically restricted Bird Member lavas, intercalated with fluvial to fluvio-deltaic sandstone and conglomerate of the Central Rand Group. These units remain poorly studied as they are rarely exposed and generally deeply weathered when cropping out. We report whole-rock major and trace elements, Hf and Nd-isotope whole-rock analyses of the lavas from core samples drilled in the south of the Witwatersrand basin and underground samples from the Evander Goldfield in the northeast. In the studied areas, both the Crown Formation and Bird Member are composed of two units of lava separated by sandstone. Whereas all the Crown Formation samples show a similar geochemical composition, the upper and lower volcanic units of the Bird Member present clear differences. However, the primitive mantle-normalized incompatible trace element concentrations of all Crown Formation and Bird Member samples show variously enriched patterns and marked negative Nb and Ta anomalies relative to Th and La. Despite the convergent geodynamic setting of the Witwatersrand Supergroup suggested by the literature, the Crown Formation and Bird Member are probably not related to subduction-related magmatism but more to decompression melting. Overall, the combined trace element and Sm-Nd isotopic data indicate melts from slightly to moderately depleted sources that were variably contaminated with crustal material. Greater contamination, followed by differentiation in different magma chambers, can explain the difference between the two signatures of the Bird Member. Finally, despite previous proposals for stratigraphically correlating the Witwatersrand Supergroup to the Mozaan Group of the Pongola Supergroup, their volcanic units are overall geochemically distinct.

Regional magnetic and gravity structures and distribution of mineral deposits in Central Iran: Implications for mineral exploration

Regional-scale magnetic and gravity data have been enhanced to identify major structures and distinguish geophysical blocks in order to evaluate the distribution of major mineral deposits and prospects in Central Iran. Major structures and coherent magnetic blocks have been defined from magnetic data enhanced using a combination of upward continuation and edge enhancement filters. Broad-scale regional structural architecture and coherent density blocks have been defined from gravity data enhanced using the tilt angle. Crustal thickness was calculated from Bouguer gravity data. Integration of the structures, magnetic, density and depth characteristics derived from the complementary geophysical techniques have been used to correlate and interpret the regional distribution of mineral deposits and occurrences to assist future exploration. Porphyry Cu deposits in the Kerman metallogenic zone are shown to be closely associated with major structures in high magnetic and gravity blocks that are coincident with the Tertiary volcanic units, and in areas where the crustal zone was interpreted to be more than 50 km thick. Pb–Zn deposits in Central Iranian Microcontinent are shown to be spatially associated with major structures, predominantly outside of the high magnetic response blocks, and commonly overlay the low gravity and thick crustal zones. In the north of the study area, one group of Fe deposits display a close correlation to blocks with both a high magnetic and high gravity response. Along strike to the southeast, a second group cluster is seen in an area with a moderate magnetic and a low gravity response.

The Serrinha granite-greenstone terrane revisited: A reevaluation of its evolution based on the current U–Pb age database

This paper aims to summarize the current knowledge of the Serrinha Nucleus and add new U–Pb geochronological data for this area. New ages were obtained for granitoids and a metadacite belonging to the Felsic Volcanic Unit of the Rio Itapicuru Greenstone Belt (RIGB). The age of 3185 ± 9 Ma obtained at the Pedra Alta granodiorite-tonalite pluton corresponds to the interval of emplacement (2.9–3.2 Ga) of the TTG granites in the Serrinha Nucleus during the Archean. As expected for granites belonging to the Paleoproterozoic TTG/calc-alkaline group, Barrocas and Lagoa dos Bois plutons yielded U–Pb zircon ages of 2168 ± 12 Ma and 2158 ± 13 Ma, respectively. These 2.15–2.17 Ga ages are attributed to a syn-orogenic evolution of the calc-alkaline felsic volcanism of the RIGB during the Rhyacian period, which is related to the closure of the Itapicuru basin. A sample of a biotite-granite dyke collected at the Araci TTG Pluton, which also is part of the Archean TTGs group of granites, yields an age related to the post-orogenic Morro do Lopes magmatism. Its age of 2076 ± 6 Ma also coincides with those attributed to the hydrothermal event that resulted in the gold mineralization at the RIGB and reinforces the widespread efects of this thermal post-orogenic alkaline magmatic event. The metadacite age of 2147 ± 38 Ma is considered the minimum crystallization age for the felsic volcanic arc of the RIGB, being compatible with regional ages obtained for other metavolcanic rocks and granitoids from this area. The ages obtained in the present work, added to the geochronological database of Serrinha Nucleus, show that the closure of the Itapicuru basin occurred between approximately 2.15 and 2.17 Ga, period corresponding to the first deformation phase that affected the orogen. The age obtained for the intrusive Barrocas Pluton (2168 ± 12 Ma), which outcrops at the southern tip of the greenstone, suggests that the volcanic-sedimentary rocks of Itapicuru should be older and highlights a new model for the evolution of Serrinha Nucleus and the concept of the “Rio Itapicuru Greenstone Belt”.

Eruptive history and &amp;lt;sup&amp;gt;40&amp;lt;/sup&amp;gt;Ar∕&amp;lt;sup&amp;gt;39&amp;lt;/sup&amp;gt;Ar geochronology of the Milos volcanic field, Greece

Abstract. High-resolution geochronology is essential for determining the growth rate of volcanoes, which is one of the key factors for establishing the periodicity of volcanic eruptions. However, there are less high-resolution eruptive histories (&gt; 106 years) determined for long-lived submarine arc volcanic complexes than for subaerial complexes, since submarine volcanoes are far more difficult to observe than subaerial ones. In this study, high-resolution geochronology and major-element data are presented for the Milos volcanic field (VF) in the South Aegean Volcanic Arc, Greece. The Milos VF has been active for over 3 Myr, and the first 2 × 106 years of its eruptive history occurred in a submarine setting that has been emerged above sea level. The long submarine volcanic history of the Milos VF makes it an excellent natural laboratory to study the growth rate of a long-lived submarine arc volcanic complex. This study reports 21 new high-precision 40Ar/39Ar ages and major-element compositions for 11 volcanic units of the Milos VF. This allows us to divide the Milos volcanic history into at least three periods of different long-term volumetric volcanic output rate (Qe). Periods I (submarine, ∼ 3.3–2.13 Ma) and III (subaerial, 1.48 Ma–present) have a low Qe of 0.9 ± 0.5 × 10−5 and 0.25 ± 0.05 × 10−5 km3 yr−1, respectively. Period II (submarine, 2.13–1.48 Ma) has a 3–12 times higher Qe of 3.0 ± 1.7 × 10−5 km3 yr−1. The Qe of the Milos VF is 2–3 orders of magnitude lower than the average for rhyolitic systems and continental arcs.

Central volcanoes and caldera collapses in the late Miocene – Late Pleistocene Tibesti Volcanic Province, northwest Chad

The Tibesti Volcanic Province (TVP) in northwest Chad represents the second largest of the five Gharyan–Tibesti volcanic provinces and covers an area around 29,000 km2. The other four provinces are in Libya, but all five provinces are from late Miocene to Quaternary and may have a common mantle source. The TVP, however, differs from the other four as regards volcano-tectonic processes, eruption style, and production of volcanic materials. The volcanic products of the TVP were erupted from the end of Miocene to late Pleistocene, range from basaltic to acidic, and suggest a double magma source – a shallow chamber fed by a deeper and larger reservoir. More specifically, field observations and numerical modelling results suggest that the basaltic magmas forming scoria cones, primarily at the periphery of the TVP, came from a deeper magma reservoir in the lower crust while the rhyolite and ignimbrites were fed by a shallow crustal magma chamber. By contrast, the volcanic products of the four volcanic provinces in Libya are primarily basalts and fed directly from single deep reservoirs. In the period from 8 Ma to 7−5 Ma, the evolution of the TVP was characterised by the formation of central volcanoes. Subsequently, in the period from 7−5 Ma to 0.43 Ma, the TVP was subject to caldera collapses that produced large-volume ignimbrites (>100 km3). Here we present numerical models with plausible loading conditions to understand better the volcano-tectonic evolution of the TVP and the spatial and temporal distribution of its major volcanic units. Our results suggest that the normal local stress field encouraged the formation of a central-volcano edifice (a volcanic cone) fed by radial dykes and inclined sheets and mostly erupting small volumes. By contrast, stress fields generated as a result of small crustal uplift or doming, due to the accumulation of magma in a deep-seated reservoir, triggered the formation of a ring-fault and the injection of a ring-dyke above the lateral margins of a shallow crustal magma chamber. Subsequently, the piston-like caldera subsidence helped to squeeze magma out of the shallow chamber resulting in large eruptions.

Earliest Arikareean (later early Oligocene) Iniyoo local Fauna from Chilapa Formation of Santiago Yolomécatl area in northwestern Oaxaca, southern Mexico

Strata containing the Iniyoo Local Fauna (LF) crop out in the outskirts of Santiago Yolomécatl in the Yolomécatl Subbasin of the southeastern Tlaxiaco Basin in northwestern Oaxaca, southern Mexico. The assemblage is from a 325-m-thick fluviolacustrine unit previously assigned to the Yanhuitlán Formation and then the Yolomécatl Formation, which was restricted to the Yolomécatl Subbasin and its immediate vicinity. However, recent field investigations and laboratory analyses indicate the unit is, instead, a marginal facies of the dominantly lacustrine Chilapa Formation, which unconformably overlies the Yanhuitlán Formation where intervening volcanic units are absent. Consequently, Yolomécatl Formation is abandoned in favor of Chilapa Formation on the bases of synonymy and priority. Similarly, San Marcos Andesite and Nicananduta “Group” were applied to the same exposures of an andesitic unit interfingering with the Chilapa Formation. Accordingly, Nicananduta “Group” is abandoned and replaced by San Marcos Andesite for the same reasons.The Iniyoo LF is earliest Arikareean (Ar1) (i.e., later early Oligocene) in age, not latest Uintan to late Chadronian (i.e., later early middle to latest Eocene), based on the first appearance datum for the amphicyonid Mammacyon and the last appearance datums of the tayassuid Perchoerus probus and the rhinocerotid Subhyracodon. That age assignment is supported by (1) corrected potassium-argon (K–Ar) dates of 28.939 ± 0.6 and 30.376 ± 0.6 Ma for the Yucudaac Andesite's lowermost flow or lower tongue, which underlies the Chilapa Formation north of the Yolomécatl Subbasin, (2) a corrected 40argon/39argon (Ar/Ar) date of 27.829 ± 0.716 Ma for the correlative Nicananduta “Group” (= San Marcos Andesite), which overlies the Chilapa Formation in the Yolomécatl Subbasin, and (3) uranium-lead (U–Pb) maximum depositional ages of 30.6 ± 0.77 and 30.62 ± 0.67 Ma for detrital zircon grains from sandstone beds occurring above the bottom and top, respectively, of the nearly 88.4-m-thick vertebrate fossil-bearing interval. The K–Ar and Ar/Ar dates further constrain the age of the Iniyoo LF to an interval spanning about 27.83–28.94 Ma ago.A corrected Ar/Ar date of 40.561 ± 1.009 Ma for potassium-feldspar crystals in a bed originally identified as a silicified tuff sheet and later determined during the current investigation to be a sandstone bed was bracketed by the two sandstone beds subjected to U–Pb dating analyses. However, the last Ar/Ar age is much too old when compared with the two U–Pb dates for the Chilapa Formation and the three K–Ar and Ar/Ar for volcanic units bracketing the formation. Presumably, the dated material represented volcanic debris reworked from a substantially older unit. Consequently, the Ar/Ar date and any land mammal age assignment based on it are considered highly unreliable.

Geology Controlling Factors of the Top Reservoir; Muara Laboh Geothermal System Case Studies

Top of reservoir (TOR) determination is one of the resource keys features of the geothermal resource and significantly impacts the well’s productivity (Casing Point Determination). It is defined from the conceptual model process and refined by the well temperature data. This paper will describe the geologic controls on the top of reservoir geometries of the Muara Laboh (ML). ML has located in step over-pull apart basins of the Great Sumatra Fault Zone segment. The features then accommodate the emplacement of quaternary to tertiary volcanic and intrusion. The geoscientific data from the existing conceptual model, well cuttings, cores, MeB test, and image log data were combined with the drilling parameter data (mud temperature & mud loses) to characterize the geology of the TOR. Drilling results show that wells in the NE sector have a shallower TOR. The TOR is located near the quaternary Patah Sembilan volcanic unit’s contact with the undifferentiated silicic formation. The NE sector TOR corresponds with the change of alteration type from smectite rich argillic zone (<180°C) to the transitional zone (Chlorite, Chlorite-Illite, Chlorite-Smectite formed at 180°C to 240°C). The epidote – chlorite rich propylitic zone (>240°C) identified underlaying the NE reservoir sector’s transition zone. MeB Index below 10 seems to be corresponding with the base of conductive of 5 Ohm, and the NE TOR location. The SW sectors TOR sits a deeper elevation (∼300msl), lies within the intercalated dacite-andesite volcanic unit, higher temperature propylitic alteration zone, with epidote, epidote-adularia-quartz, and open space veins were observed below the TOR. Similar to the NE sector, the drop of the MeB index to less than ten, which is also related to the lithology contact, shows the possibility that the TOR has dropped in the SW sector. Below the low MeB zone, calcite infilled the fractures and sometimes encapsulated the pre-existing epidote vein; calcite then decreases with increasing epidote vein and epidote-adularia vein occurrence. The observation then suggests a rapid boiling process in the SW sectors due to the dropping of water levels, allowing the late-stage calcite vein to seal off the permeability within the shallow reservoir and not allowing the circulation of geothermal fluids within the interval.

Volcanostratigraphic Approach and its Implication for Geothermal Evaluation in Talang Volcano West Sumatra

The desk studies are a fundamental preliminary step in any geologic investigation of volcanic regions. These studies are very important approaches used in the reconnaissance survey stage for the exploration of volcanic geothermal systems. Volcanostratigraphic study has been done in Talang volcano and nearby regions, where the two topographic maps on the scale of 1:100000 and 1:50000 were utilized by drawing the patterns of drainage, ridges and flows without intersecting these flows. The first scale employed to distinguish eruption product units based on their respective eruptive centers (Crown), as well as a map on 1:50000 scale to find detailed parasitic distributed unit (s) product such as (Hummock). The challenge based on this approach is to distinguish different volcanic products based on their origins without ground truthing. Thus, the correlation with existing published geological map was employed. One volcanic unit were identified (Talang Brigade) which consists of three eruption centers (The Talang Bawah, Talang Batino and Talang Jantan Crowns) and three Hummocks with different ages based on age of eruption. The volcanostatigraphic units, is bounded by a large Talang Brigade caldera with a diameter more than 2 km. Based on analyses of the dimension, maturity, stress regimes, and an estimation of the thermal resource base of its magmatic heat source, it was concluded that Talang volcano as a geothermal potential that deserves further detailed studies before embarking to development phase. For verifying desk work studies, the detailed volcanostratigraphic mapping is recommended to understand the physical- and chemical properties of the volcanic rocks and provide the history of volcanism and validation of the potentially geothermal boundaries.

Depth estimates of anomalous subsurface sources using 2D/3D modeling of potential field data: implications for groundwater dynamics in the Ziway-Shala Lakes Basin, Central Main Ethiopian Rift

Quantitative analysis of potential field data are made in the Ziway-Shala lakes basin over an area bounded by 38°00′ E - 39°30′ E and 7°00′ N - 8°30’ N. Most previous geophysical studies in the region under consideration focus on mapping the deep crustal structures and undulation of the Moho depth. Only few studies are targeted at mapping the shallow subsurface structures. The main focus of this paper is mapping geometries of the major lithological and structural units of the shallow subsurface using gravity and magnetic data. The ultimate objective of the research is to understand the hydrogeological dynamics of the region through mapping interfaces geometries. Automatic inversions, 2D joint forward modeling and 3D inversion are the major techniques employed. The 2D Werner de-convolution based on both gravity and magnetic data along the rift axis showed source depths tending to deepen northwards. Source depths estimates determined by Source Parameter Imaging also showed similar tendency. This is further strengthened by the joint 2D forward modeling of gravity and magnetic data which showed the top of the basement is sloping northwards. The result of the 3D gravity interface inversion agrees with results of the above mentioned depth estimation techniques. Finally, the gravity power spectral analysis resulted in two depth estimates, 1.53 km and 2.87 km which approximate the positions of two density interfaces. The shallow depth interface is thought to presumably delineate the low density Fluvio-lacustrine sediments including the rift floor volcanic units and crystalline basement. Our investigation results agree with the results of previous seismic studies which identified low velocity (“sediment-volcanic”) horizon in the rift floor with low resolution. The information obtained with regard to water balance of the basin, salinity level of the lakes and the conceptual hydrological flow model appears to reveal that the groundwater flow in the study region is controlled by subsurface structures, particularly, the mapped interface topographies.

Regional Geochemical Exploration for Cu-Au Deposit Based on Self-Organizing Map (SOM) in Valezir Area, Meshginshahr, NW of Iran

Since most of the geochemical data analysis procedures require preliminary assumptions that lead to constraint and errors in the true nature of the data causing reduction in the effectiveness of the adopted methods. Therefore, a model-based clustering method as self-organizing map (SOM) were employed with the aim of recognizing Cu-Au mineralized zones by establishing an optimized exploration tool in Valezir area, NW of Iran. SOM as a dimension reduction method was introduced to recognize geochemical distribution patterns of Cu-Au with higher certainty while preserving the originality of the data. Subsequent to data preprocessing and testing different SOM architectures, an appropriate structure with a pattern containing six clusters was selected. Accordingly, the related elements distribution model was extracted and interpretation of the geochemical system represents two significant sets of elements in clusters (i.e. 1st, 2nd and 6th clusters) to anticipate the mechanism of distribution: 1- Copper and pertaining trace elements formation from intermediate to acidic hydrothermal solutions, which are localized in the northern part of the area and emplaced in the quartz monzo-dioritd intrusive body. 2- Au Anomalies and its associated elements As, Hg and Bi depicted in 2nd cluster. The Au anomalies follow geochemical pattern with Bi, Sb, As, and W that are mostly elongated from NW to SW of the area. It seems relatively the low enrichment of gold has occurred during the intrusion of the igneous body into older volcanic units that caused extensive alterations, remobilization and localization of Au and related elements. To assess the SOM results, a comparative study was carried out with the results obtained from hierarchical clustering analysis (HCA). The results illustrated higher performance by SOM approach in characterizing geochemical system and detecting the elements paragenetic sequence in the area for locating the exploration targets.

Marine geological maps of the Campania region (Southern Tyrrhenian Sea, Italy): Considerations and contributions to a different scale of geological survey

Marine geological maps of the Campania region have been constructed both to a 1:25.000 and to a 1:10.000 scale in the frame of the research projects financed by the Italian National Geological Survey, focusing, in particular, on the Gulf of Naples (Southern Tyrrhenian Sea), a complex volcanic area where volcanic and sedimentary processes strongly interacted during the Late Quaternary and on the Cilento Promontory offshore. In this paper, the examples of the geological sheets n. 464 “Isola di Ischia” and n. 502 “Agropoli” have been studied. The integration of the geological maps with the seismo-stratigraphic setting of the study areas has also been performed based on the realization of interpreted seismic profiles, providing interesting data on the geological setting of the subsurface. The coastal geological sedimentation in the Ischia and Agropoli offshore has been studied in detail. The mapped geological units are represented by: i) the rocky units of the acoustic basement (volcanic and/or sedimentary); ii) the deposits of the littoral environment, including the deposits of submerged beach and the deposits of toe of coastal cliff; iii) the deposits of the inner shelf environment, including the inner shelf deposits and the bioclastic deposits; iv) the deposits of the outer shelf environment, including the clastic deposits and the bioclastic deposits; v) the lowstand system tract; vi) the Pleistocene relict marine units; vii) different volcanic units in Pleistocene age. The seismo-stratigraphic data, coupled with the sedimentological and environmental data provided by the geological maps, provided us with new insights on the geologic evolution of this area during the Late Quaternary.

Tephrostratigraphy and Magma Evolution Based on Combined Zircon Trace Element and U-Pb Age Data: Fingerprinting Miocene Silicic Pyroclastic Rocks in the Pannonian Basin

We present a novel approach to use zircon as a correlation tool as well as a monitor for magma reservoir processes in silicic volcanic systems. Fingerprinting eruption products based on trace element content and U-Pb dates of zircon offers a promising, previously underestimated tephra correlation perspective, particularly in cases where the main minerals and glass are altered. Using LA-ICP-MS analyses, a rapid and cost-effective method, this study presents U-Pb dates and trace element concentration data of more than 950 zircon crystals from scattered occurrences of early to mid-Miocene silicic ignimbrites in the northern Pannonian Basin, eastern-central Europe. This magmatic phase produced &amp;gt;4000 km3of erupted material, which provide unique stratigraphic marker horizons in central and southern Europe. The newly determined zircon U-Pb eruption ages for the distal pyroclastic deposits are between 17.5 and 14.3 Ma, comparable with the previously published ages of the main eruptive events. Multivariate discriminant analysis of selected trace element concentrations in zircon proved to be useful to distinguish the main volcanic units and to correlate the previously ambiguously categorized pyroclastic deposits with them. Using the zircon trace element content together with published glass data from crystal-poor ignimbrites, we determined the zircon/melt partition coefficients. The obtained values of the distinct eruption units are very similar and comparable to published data for silicic volcanic systems. This suggests that zircon/melt partition coefficients in calc-alkaline dacitic to rhyolitic systems are not significantly influenced by the melt composition at &amp;gt;70 wt% SiO2at near solidus temperature. The partition coefficients and zircon trace element data were used to calculate the equilibrium melt composition, which characterizes the eruption products even where glass is thoroughly altered or missing. Hence, our results provide important proxies for tephrostratigraphy in addition to yielding insights into the complex processes of silicic magma reservoirs.

Mineralogical, geospatial, and statistical methods combined to estimate geochemical background of arsenic in soils for an area impacted by legacy mining pollution

The estimation of geochemical background is complex in areas impacted by point sources of atmospheric emissions due to unknowns about pollutant dispersion, persistence of pollutants on the landscape, and natural concentrations of elements associated with parent material. This study combined mineralogical analysis with conventional statistical and geospatial methods to separate anthropogenically impacted soils from unimpacted soils in the Yellowknife area, Northwest Territories, Canada, a region that was exposed to 60 years of arsenic (As)-rich atmospheric mining emissions (1938–1999) and that hosts natural enrichments of As. High concentrations of As (up to 4700 mg kg−1) were measured in publicly accessible soils near decommissioned roaster stacks in the region and strong relationships between As and distance from the main emission sources persisted in surface soils and soils at depth in the soil profile more than 60 years after the bulk of mining emissions were released. Mineralogical analysis provided unambiguous evidence regarding the source of As minerals and highlighted that most As in surface soils within 15 km of Yellowknife is hosted as anthropogenic arsenic trioxide (As2O3), produced by roaster stack emissions. Statistical protocols for the estimation of geochemical background were applied to an existing database of till geochemistry (N = 1490) after removing samples from mining impacted areas. Results suggested geochemical background for the region is 0.25–15 mg kg−1 As, comparable to global averages, with upper thresholds elevated in volcanic units (30 mg kg−1 As) that often host sulfide mineralization in greenstone belts in the region.