Volcanic Units Research Articles

Provenance and tectonic evolution of the forearc basin along the northwestern Central Qilian belt, northeastern Tibet

The age and properties of detrital zircon grains in a forearc basin provide crucial information on the history of convergent plate margins. These data can be used to constrain provenance dispersal patterns, establish source-to-sink relationships, and reveal the tectonic framework and subduction history. The Qilian orogen in the northeastern margin of the Tibetan Plateau records the tectonic history of the Proto-Tethys Ocean from its initial spreading and subduction to final closure. However, the timing of subduction initiation and the subduction polarity of the Proto-Tethys Ocean remain controversial. The Qilian orogen includes the North Qilian, Central Qilian, and South Qilian belts. A succession of sedimentary rocks along the northwestern margin of the Central Qilian belt, previously considered to be Precambrian in age, has been redefined based on detailed field investigation and geochronology. U-Pb analysis of detrital zircon grains from metasedimentary rocks of the Gongcha Group of the northwestern Central Qilian belt yielded Paleozoic ages between 530 and 470 Ma, with prominent Mesoproterozoic and Paleoproterozoic ages between 1800 and 1000 Ma. In contrast, the overlying Duoruonuoer Group yielded predominantly early Paleozoic ages between 465 and 434 Ma, with a few older age populations. This change in source is reflected in a substantial increase in volcanic detritus within the Duoruonuoer Group as compared to the Gongcha Group. The detrital zircon age spectra and lithologic characteristics indicate the Gongcha and Duoruonuoer Groups constitute the lower and upper volcanic-sedimentary strata in the forearc basin. Based on resemblances of statistical ages and εHf(t) values of zircon, we interpret that the early Paleozoic intrusive and volcanic units and Precambrian basement rocks of the Central Qilian belt were the main source of the Gongcha and Duoruonuoer Groups, verifying the southward subduction of the North Qilian Ocean, a portion of the larger Proto-Tethys Ocean, beneath the Central Qilian belt. The detrital zircon ages reveal that the initial subduction of the Proto-Tethys Ocean in the Qilian orogen occurred ca. 530 Ma and closed ca. 435 Ma. These forearc sedimentary units constitute the tectonic boundaries between the Central Qilian belt and North Qilian belt.

Early Paleogene alkaline magmatism in the Subandean Ranges of Jujuy Province

The Zapla Range (Jujuy province) represents the southern edge of the central Andean Subandean Ranges in Argentina. In the northernmost area of the Zapla Range, basaltic rocks are interbedded in the Mealla Formation. These volcanic units form part of the post-rift magmatism of the Salta rift basin, which mainly focused on the Lomas de Olmedo sub-basin to the east. In this paper, six of these basaltic occurrences are characterized on the basis of new geological, petrographic and geochemical data, in order to explore their emplacement, petrogenesis and possible tectonic setting during rift evolution. Peperites in the upper contact of sills evidence the interaction between basalts and unconsolidated or poorly consolidated wet sediments. This implies that there was concomitance between sedimentation and subsurface magma emplacement, suggesting a Paleocene age for the magmatic event according to zircon U-Pb data available for Mealla Formation rocks. Other basaltic units show hipocrystalline textures and lack peperites at the top, which points to their extrusive nature. Additionally, some of these lava flows are spatially associated with pyroclastic deposits that reveal the occurrence of explosive hydrovolcanic eruptions. The study rocks are classified as alkaline basalts and basanites with high Mg# and Ni- and Cr-rich compositions, close to primitive magmas. They show affinity with continental intraplate settings. The analyses of Ba/Nb, Nb/La, (La/Yb)N, (La/Sm)N, (Gd/Yb)N and (Dy/Yb)N ratios, as well as normalized trace and rare earth element patterns, suggests a genesis by low degrees of partial melting of a residual garnet-bearing mantle source. Therefore, basalts from this part of the Subandean Ranges evidence a magmatic episode probably linked to an extensional reactivation stage of the Salta Group rift system in the Lomas de Olmedo depocenter during the Paleocene (∼60 Ma).

The magmatic plumbing beneath the Wudalianchi volcanic region, northeast China, is recharged by magma reservoirs under Keluo volcano

The intraplate magmatic systems of the Wudalianchi potassic volcanic region in northeast China have historically exhibited eruption activity, and there is evidence that they retain the potential for future eruptions. However, the characteristics of this complex volcanic region remain debated and poorly understood due to the relatively low resolution of existing crustal tomography. Here we present a high-resolution shear-wave velocity model of the crust and uppermost mantle obtained by applying full-waveform ambient noise tomography to data recorded at 106 recently deployed broadband seismic stations. Our results reveal two vertical low-velocity anomalies discovered beneath the Nuominhe and Keluo volcanoes, which appear to be magma ascending pathways connecting a mantle source and crustal reservoirs. A horizontally interconnected low-velocity zone in the middle-lower crust indicates a magmatic migration pathway transporting potassium-rich melts directly among the potassic volcanic units. In addition, the Wudalianchi volcano is likely recharged by deeper and larger crustal magma reservoirs under the Keluo volcano. The unique geometries of the magmatic plumbing systems provide valuable seismic evidence for the volcanic eruption dynamics and potential volcanic hazards.

A 2‐Billion‐Year History of Water‐Alteration in Terra Sirenum, Mars: Volcanism's Influence on Aluminum Clay Formation and Chemically Distinct Waters Forming Sulfates and Chlorides Into the Amazonian

AbstractTerra Sirenum, a region of Noachian highlands southwest of the Tharsis volcanic complex, is unique in the number, proximity, and diversity of orbital detections of secondary minerals, as the sole region found to date hosting large‐scale deposits of all of Mars' major salts (chlorides, sulfates, carbonates) as well as diverse hydrated silicates. We combine mineralogical information, high‐resolution imagery, and elevation models to investigate the geologic context of these secondary minerals to understand the sources of water and ions for each type of deposit and their spatial/temporal relationships. Carbonates, where present, are part of Noachian basement rocks exposed through cratering and do not appear associated with evaporative sequences. Numerous small detections of the acid sulfate minerals alunite and jarosite mirror the dominant clay cation in the localities they are found—Al phyllosilicates and Fe phyllosilicates, respectively—suggesting in situ formation. We interpret a previously discovered kaolinite‐rich unit overlying Fe/Mg clays across northeast Terra Sirenum as remnants of a widespread ash unit rather than a pedogenic weathering sequence. Sulfate and chloride detections are decoupled, with sulfates in topographic lows likely precipitated from volcanism‐associated groundwaters, while chloride detections are consistent with surface water runoff, in some instances clearly post‐dating volcanic units capping sulfate detections. Volcanic resurfacing of craters in the region is progressively younger from west to east, and crater statistics‐based ages indicate localized sulfate‐ and chloride‐forming processes continue to occur from ∼3.5 to ∼1.4 Ga. We hypothesize that their decoupling points to disconnected, episodic surface and groundwater reservoirs, perhaps separated by a permafrost layer.

Miocene Petit-Spot Basanitic Volcanoes on Cretaceous Alba Guyot (Magellan Seamount Trail, Pacific Ocean)

New data obtained from core samples of two boreholes and dredged samples from the Alba Guyot in the Magellan Seamount Trail (MST), Western Pacific, including the 40Ar/39Ar age determinations of basanite, and the mineralogy of basanite, tuff, tuffite, mantle-derived inclusions in basanite and tuff (lherzolite xenolith and Ol, Cpx, and Opx xenocrysts), and calcareous nannofossil biostratigraphy, have implications for the guyot′s development and history. Volcanic units in the upper part of the Alba Guyot main edifice and its Oma Vlinder satellite, at sea depths between 3600 and 2200 m, were deposited during the Cretaceous 112 to 86 Ma interval. In the following ~60 myr, the Alba Guyot became partly submerged and denuded with the formation of a flat summit platform while the respective fragment of the Pacific Plate was moving to the Northern Hemisphere. Volcanic activity in the northeastern part of the guyot summit platform was rejuvenated in the Miocene (24–15 Ma) and produced onshore basanitic volcanoes and layers of tuff in subaerial and tuffite in shallow-water near-shore conditions. In the Middle-Late Miocene (10–6 Ma), after the guyot had submerged, carbonates containing calcareous nannofossils were deposited on the porous surfaces of tuff and tuffite. Precipitation of the Fe-Mn crust (Unit III) recommenced during the Pliocene–Pleistocene (<1.8 Ma) when the guyot summit reached favorable sea depths. The location of the MST guyots in the northwestern segment of the Pacific Plate near the Mariana Trench, along with the Miocene age and alkali-basaltic signatures of basanite, provide first evidence for petit-spot volcanism on the Alba Guyot. This inference agrees with the geochemistry of Cenozoic petit-spot basaltic rocks from the Pacific and Miocene basanite on the Alba Guyot. Petit-spot volcanics presumably originated from alkali-basaltic melts produced by decompression partial melting of carbonatized peridotite in the metasomatized oceanic lithosphere at the Lithosphere–Asthenosphere Boundary level. The numerous volcanic cones with elevations of up to 750 m high and 5.1 km in basal diameter, discovered on the Alba summit platform, provide the first evidence of voluminous Miocene petit-spot basanitic volcanism upon the Cretaceous guyots and seamounts of the Pacific.

40 Ar/ 39 Ar chronostratigraphy and geochemistry of Harrats Ishara and Kura in the Hamd–Jizl Basin, Madinah, Saudi Arabia

An extensive system of mafic dykes, volcanic centers, and provincial flood basalts (harrat) have erupted within the Arabian flank of the Red Sea rift since the early Oligocene. The age, spatial distribution, and composition of these features correlates with distinct changes in rift flank structural architecture as the margin progressed through successive deformational phases. A widespread pre-rift dyke swarm in the proto-Red Sea region c. 25 Ma created thermomechanical crustal conditions favourable for diffuse continental rifting and inboard extensional strain markers like the Hamd–Jizl Basin (HJB) originated amagmatically c. 23 Ma. Six million years later, basalt flows from Harrats Ishara and Kura entered the HJB at several points along strike during a 4.5 million year-long eruptive phase. Two distinct volcanic units developed in Harrat Ishara: a predominantly alkali-olivine basalt with an age range of 18.0–14.3 Ma (Ishara A) and a mostly olivine transitional basalt with an age range of 16.6–12.1 Ma (Ishara B). The contemporaneous eruption of these units indicates that multiple magma sources operated simultaneously during the middle Miocene. Both units generally decrease in alkalinity and rare earth element abundances up section, signaling either ongoing melt fractionation processes or output from different magma chambers with time.

Continental rift evolution and drainage reorganization along the Dead Sea rift since the Miocene

The Dead Sea fault is a section of the Arabian-African plate boundary. Widespread field relations indicate that three major drainage systems (stages) occupied the landscape west of the Dead Sea fault since its initiation at ca. 20 Ma. Specifically, (1) an early to middle Miocene drainage system, only minorly reconfigured by the fault (all sediments of this system belong to the Hazeva Formation); (2) a late Miocene to early Pleistocene fault-parallel drainage system named Paran-Neqarot (all sediments of this system belong to the Arava and Zehiha Formations; and (3) the early Pleistocene to present drainage configuration. The temporal and spatial frameworks of drainage stage 1 are generally constrained by radiometric dating of interfingering volcanic units, and the onset and temporal and spatial frameworks of drainage stage 3 are well constrained by cosmogenic 10Be surface exposure ages. The timing and longevity of the rift-parallel drainage (stage 2) have until now been evasive to direct dating. The overall time gap between stages 1 and 3 is ∼12−13 million years. Thus, an early age (within this time gap) of stage 2 would imply an immediate response of drainage reorganization to rift tectonics, while a later age of this drainage system would imply a delayed response. We present 11 10Be-26Al cosmogenic burial ages of alluvial and colluvial units related to the fault-parallel drainage system (stage 2), which collectively constrain the time of deposition of the Arava Formation sediments in the central Negev to ca. 8 Ma. The general lack of stratigraphic order, together with the large dispersion of ages both across and within the sampling sites, attests to significant recycling of sediments from drainage stage 1 into the Arava Formation deposits. The termination of Arava and Zehiha Formation sediment deposition at ca. 1.8 Ma was determined previously using cosmogenic exposure ages of desert pavements that cover the formations. Combining the previously published data with our new data, we established the longevity and character of the Paran-Neqarot drainage system. This framework highlights the temporal aspect of drainage system build-up and collapse as it responded to transform and extensional plate boundary tectonics during the Neogene.

Late Devonian felsic magmatism in southern New Brunswick and its association with a large igneous province that may have contributed to the Frasnian-Famennian extinction

Felsic rocks of the Piskahegan Group and coeval plutons form a major part of a Late Paleozoic epicontinental caldera in southwestern New Brunswick, Canada. The caldera forms an elliptical structure with a length of ∼34 km and a width of ∼13 km. It hosts a significant polymetallic deposit of tin, molybdenum, indium and bismuth associated with mid-sequence granitic intrusions. The caldera formed in the aftermath of the late Early to Middle Devonian Acadian Orogeny during the opening of the late Paleozoic Maritimes Basin in the Northern Appalachian Belt. Its surrounding successions are largely composed of bimodal igneous rocks that were derived from two distinct sources: upper mantle-derived mafic magma and lower crust-derived felsic magma. The rocks that marked the beginning of volcanic activity in the caldera are no younger than 374.2 ± 2 Ma, whereas the uppermost dated volcanic unit in the complex yielded an age of 364.6 ± 0.7 Ma. The latter is conformably overlain by a succession of red beds and undated basalts that also overlie penecontemporaneous felsic volcanic rocks adjacent to the caldera. The post-orogenic felsic rocks are fractionated, peraluminous A-2 type rocks with silica contents ranging from ∼70 to ∼79 wt% along with high K2O contents and ɛNd(t) ranging from −0.10 to +1.05. They were generated by the melting of lower crustal basement rocks linked to a lithospheric plate that was metasomatized during the Neoproterozoic. Based on available geochemical, geophysical and structural data on Late Paleozoic rocks of southeastern Canada, the melting may have been triggered by the injection of profuse mafic magma that eventually formed a thick underplating at the base of the crust in association with heat derived from an underlying mantle plume and conveyed by transtensional structures. Basal rocks of the Piskahegan Group (the Intracaldera Sequence) and penecontemporaneous volcanic rocks in Nova Scotia are interpreted as small erosional remnants of a Large Igneous Province (LIP) based on the extensive exposure of large coeval plutons throughout southeastern Canada. The new geochronological data suggests that profuse late Frasnian magmatism in this LIP could have contributed to the environmental deterioration that led to a significant extinction at the Frasnian-Famennian boundary (the Kellwasser Event).

A Preliminary Study: Physicochemical Characteristics and the Recommendations for Industrial Application of the Mlaten Clay Materials, Mojokerto, East Java, Indonesia

Abstract Clays are industrial minerals that are widely utilized for various industrial fields. Eastern Java has a high potential of clay minerals (±348 million tons), including Mojokerto. The research location is in Mlaten and its surroundings, Mojokerto, East Java and covers an area of ± 16 km2. The massive utilization and potential in this area has not been comprehensively followed by research and exploration of clay minerals. In this case, the potential of clay minerals needs to be investigated so that prospective locations can be determined along with recommendations for optimal utilization. To determine the physical properties, which are plasticity test, grain size test, and forming water test, were conducted on 3 samples of raw clay and 1 sample of industrial clay. Furthermore, the geochemical analysis is to determine the major oxide compounds and was conducted on 8 raw clay samples. The lithology units of the study area can be divided into 2 units, consisting of sand-clay unit and weathered upper Quaternary volcanic unit. The Mlaten clay deposits are found in both units. Physical characteristics of clay in Mlaten are consisting of 2,34% - 2,47% clay grain size and having of moderate to high plasticity. The recommendation for forming water in clay utilization ranges from 15-43%. On the other side, Mlaten clay is geochemically characterized by high content of SiO2 (50-59,6%), Fe2O3 (7,24-10,15%), and Al2O3 (16,9-19,9%), but low content of K2O (0,63-1,04%), Na2O (1,44-2,49%), MgO (1,04-1,96%), and CaO (2,42-4,87%). Based on the physicochemical characteristics, Mlaten clay are recommended as raw materials for the manufacture of earthenware ceramic industry, cement industry, and as absorbents in the palm oil industry.

Oligocene magmato-tectonic events influential in the development of Pb-Zn, Ag, and Au mineralization at the Plomosas deposit, southwestern Sierra Madre Occidental, Mexico

ABSTRACT The Plomosas deposit is the product of a sequence of overlapping events of Pb–Zn, Ag, and Au low sulfidation mineralization that occurred in Mexico’s southwestern Sierra Madre Occidental (SMO). Field observations, detailed petrography, and U–Pb age dating of the entire stratigraphic sequence were used to characterize the magmatic evolution of the district and to obtain new insights into the evolution of the SMO. The stratigraphy at Plomosas is distinct from other areas to the north as it lacks Jurassic marine units, Cretaceous continental volcanic successions as well as Cretaceous and Palaeocene batholiths. The local basement is composed of undeformed Late Jurassic volcaniclastic units and coeval felsic intrusions. Early to late Oligocene silicic ignimbrites, intermediate lava flows, and occasional basaltic flows unconformably overlie the Jurassic basement. Several Oligocene intermediate to felsic porphyry bodies and rhyolitic domes crosscut the stratigraphy. The upper volcanic sequence is separated into two packages by an angular unconformity. Mineralization produced an early Pb–Zn-mineralized hydrothermal fault breccia, followed by Cu–Ag veins and late Au veins. The Pb–Zn mineralized breccia is hosted by ~30 Ma Oligocene volcanic units that form the basal sequence. The Cu–Ag and Au-rich veins overlap and crosscut the early hydrothermal features and cut the late Oligocene volcanic cover that unconformably overlie the basal sequence. Our results highlight the importance of episodes of Oligocene magmatism to the genesis of mineralization in the southwestern SMO specifically through a genetic link between Pb–Zn veins and intermediate Oligocene intrusive bodies, and of Ag and Au veins with late Oligocene volcanism.

Intraplate deformation during Gondwana breakup: a study of the Jurassic units of the Cañadón Asfalto Basin (extra-Andean Patagonia, Argentina)

SUMMARY In this study, we present the results of palaeomagnetic research conducted on Jurassic units of the Cañadón Asfalto Basin (CAB) in Patagonia, formed during Gondwana breakup. This basin is a key locality for understanding intraplate deformation within Patagonia during the Jurassic. The nature of this basin has been a subject of debate, based on the dynamics of the blocks that constitute its depocentres. In this context, the palaeomagnetic study of the Jurassic units of this basin provides a unique methodology to characterize the tectonic motions of its crustal blocks during its formation and development. To achieve this, we collected 350 samples from 53 sites in the sedimentary units of Las Leoneras (ca. 189 Ma) and Cañadón Calcáreo Formations (ca. 160–157 Ma), as well as the volcanic Lonco Trapial Group (ca. 185–172 Ma). The palaeomagnetic results from the sedimentary units show a regional remagnetization due to hydrothermal activity that obliterated the original remanence and overprinted a new one, simultaneously imprinting a secondary remanence in the volcanic units of the Lonco Trapial Group. When comparing the direction of the palaeomagnetic pole obtained from the remagnetized units with respect to average poles of equivalent ages, it is observed that the remagnetization must have occurred during the Late Jurassic (ca. 145 Ma). The age range in which this process occurred (Oxfordian to Aptian) and the direction of the calculated pole dispute a monster polar shift postulated for Late Jurassic to Early Cretaceous times. In addition, the primary magnetization recorded in the units of the Lonco Trapial Group indicates a counterclockwise rotation of the studied crustal blocks between 21° and 11°, which, in line with previous studies, refutes large-scale dextral motion along the Gastre Fault System since the Jurassic. Similar counterclockwise rotations of equivalent magnitudes are found along the units overlying the Palaeozoic Central Patagonian Igneous–Metamorphic Belt, which represents the opposite shear sense compared to the Jurassic units beyond this belt. This is interpreted as a reactivation of the Palaeozoic belt structures in the opposite sense, from transpressive during the Palaeozoic to transtensive during the Mesozoic.

Sulfur isotopic variations in the products of the 1895 CE eruption at Zao volcano (NE Japan): Implications for connecting eruption source and syn-eruptive magmatic-hydrothermal processes

Sulfur isotopic ratio in sulfate and sulfide in subvolcanic hydrothermal systems is a valuable tracer to study the magmatic-hydrothermal processes from the magma source through to volcanic eruptions. Zao volcano is among the most active volcanoes in NE Japan, with historical explosive eruptions occurring during the last thousand years and unrest episodes since 2013. This necessitates a detailed assessment of the potential risk of future volcanic hazards. We investigated the magmatic-hydrothermal processes that occurred during the 1895 CE eruption sequence at Zao volcano by conducting mineralogical and sulfur isotope analyses in the exposed well: (i) six volcanic units (Layers 1–6) of the 1895 CE eruption products (clayish ash deposits with andesitic bombs, lapilli of scoria, and minor altered lithic fragments) deposited on the rim of Okama crater lake; and (ii) clay-altered and silicified rocks from the Nigorikawa alteration zone (NGA) surrounding the Goshikidake cone. Mineralogical data show that the samples mainly consist of alunite, pyrite, and gypsum. Alunite and pyrite occur as fine crystal mixtures associated with mineral assemblages of both advanced argillic alteration (i.e., those of cristobalite and kaolinite) and silicification (i.e., those of cristobalite, tridymite and native sulfur). Gypsum typically appears as isolated euhedral crystals of several millimeters in size. Samples of the 1895 CE eruption products have a narrow range of δ34S values from +3 ‰ to +5 ‰ for gypsum, from +9 ‰ to +13 ‰ for alunite, and approximately −10 ‰ for pyrite. For the NGA samples, the δ34Sgypsum, δ34Snative sulfur, and δ34Spyrite values range from −12 ‰ to −9 ‰, whereas for alunite, these range from +8 ‰ to +18 ‰. This indicates that alunite and pyrite in the 1895 CE eruption products were derived from the advanced argillic alteration and silicification zones that developed under Okama crater, which is exposed as the NGA. Estimated alteration temperatures based on the sulfur isotopic equilibrium between alunite and pyrite pairs are 200 °C–300 °C. By contrast, δ34Sgypsum values in the 1895 CE products are significantly higher than those in the NGA (which are derived from oxidation of pyrite or H2S, or both), ranging between an estimated parental fluid of δ34Sbulk-initial = ca. +1 ‰ and the Quaternary volcanic rocks of the Japan arc. This suggests that gypsum in the 1895 CE eruption products derived from magmatic vapor condensate (anhydrite) formed in the volcanic conduit during the eruption, thus becoming replacement of anhydrite by gypsum after or during the tephra deposition on the Zao summit surface. Our results on sulfur-bearing minerals provide new clues for better understanding (and monitoring) the syn-eruptive processes of volcanic eruptions focused on subvolcanic hydrothermal systems.

Targeting porphyry Cu deposits in the Chahargonbad region of Iran: A joint application of deep belief networks and random forest techniques

Mineral prospectivity modeling (MPM) is a valid and progressively accepted predictive tool for mapping reproducible potential mineral exploration targets. In this study, a hybrid approach combining unsupervised deep belief networks with supervised random forest (DBN-RF) is performed to delineate potential exploration targets for porphyry Cu deposits in the Chahargonbad region of Iran. Firstly, a mineral system model for porphyry Cu deposits is established, and relevant targeting criteria are delineated based on comprehensive exploration datasets. Subsequently, within this hybrid framework, the DBN extracts deep implicit feature information, which is then utilized as input for the RF. The comparative results on the performance of the hybrid model and the RF model trained by the primary targeting criteria, in terms of the improved prediction-area plot, demonstrate that the DBN-RF prospectivity model outperformed the RF-generated model with an overall efficiency of 0.53. This hybrid model accurately identified 81.97 % of known Cu deposits within an investigation area of 18.03 %, with primary trends aligned with the primary faults and volcanic units of the region. This study demonstrates effective performance of DBN-RF in identifying exploration targets for porphyry Cu deposits at regional scale and also highlights the potential of deep learning-based methods for successful MPM.

New details on the volcanic and tectonic evolution of the Mesoarchaean Dominion Group with special reference to the Syferfontein Formation in the Ottosdal area (South Africa)

Abstract The ca. 2.96 Ga Dominion Group (DG) preserves the first subaerial volcano-sedimentary succession on the Kaapvaal Craton. Based on field and core observations, this study provides revised stratigraphic logs and geological maps from the area around Ottosdal that refine the understanding of the stratigraphic and tectonic evolution of these ancient rocks. Fluvial sandstone and conglomerate of the Rhenosterspruit Formation (RsF) were deposited on granite basement, followed by andesitic to basaltic volcanic rocks of the Rhenosterhoek Formation (RhF). These amygdaloidal andesitic to basaltic lava units show fragmentation around lava flows that may represent an indication of prevalent subaerial environment. However, local hyaloclastite and pillow lava units indicate periodic aqueous conditions. The Syferfontein Formation (SF) has the most extensive exposures in the Ottosdal area and represents the youngest volcanic unit of the DG. The porphyritic and spherulitic volcanic rocks tell a story of subaerial volcanism interspersed with periods of lacustrine deposition of sandstone and shale. The Witwatersrand Supergroup (WSG) overlies the DG along an angular unconformity. Folding affected the succession of the DG, WSG and the Ventersdorp Supergroup (Rietgat Formation). This event is reflected in small-scale folds and mullion structures in the central part of the study area and by larger scale north-northwest–south-southeast-striking anticlines and synclines. Folding was accompanied by northwest-southeast-striking thrust faulting, either during or shortly after the formation of the Ventersdorp Supergroup. In the study area, post-Ventersdorp deformation is restricted to east-northeast–west-southwest-striking faults.

The northbrae rhyolite of Berkeley (California, USA) constrains motion of the proto-Hayward Fault

ABSTRACT Right-lateral transform motion associated with the Pacific-North American plate boundary in the modern-day San Francisco Bay Area occurs across a series of sub-parallel fault zones. Much of this motion is accommodated east of the San Andreas Fault by the faults of the East Bay fault system. A major tool for reconstructing the spatial and temporal history of fault motion is the correlation of offset Neogene volcanic rocks. These Neogene volcanics within the California Coast Ranges formed in association with the slab gap that grew as the Mendocino Triple Junction migrated northward. Some of the volcanic centres have been variably offset by subsequent strike-slip faulting. A felsic volcanic unit exposed in Berkeley, CA, known as the Northbrae rhyolite has variably been interpreted to be one of these Neogene volcanic units or to be a Mesozoic volcanic unit associated with the Coast Range ophiolite. A new U-Pb zircon date of 11.10 ± 0.09 Ma confirms the Neogene volcanic interpretation. This date is indistinguishable from previously published Ar/Ar dates from the Burdell Mountain volcanics of the North Bay region as well as a new U-Pb zircon date of 11.07 ± 0.10 Ma. In addition to the indistinguishable ages, similarities in bulk lithology, zircon crystallization/dissolution textures, and zircon trace element geochemistry are consistent with these rhyolites being associated with the same volcanic centre. This correlation implies that 40 ± 5 km of right-lateral offset occurred to the west of the modern-day position of the Hayward-Rodgers Creek fault zone. This offset represents ∼ 20% of the total offset along the East Bay fault system. A proto-Hayward Fault with a different geometry than that of the present-day played a significant role in the evolution of the fault system. This result highlights the dynamic spatiotemporal variability of strike-slip faults along transform margins.

A temporal framework for the Carrapateena Iron Oxide Copper-Gold (IOCG) deposit, Eastern Gawler Craton, South Australia

The Carrapateena Iron Oxide Copper-Gold (IOCG) deposit is located on the eastern margin of the Gawler Craton, approximately ∼475 km NE of Adelaide, South Australia. The deposit is hosted within a granite belonging to the Donington Suite and contains various polylithic breccias and intrusive volcanic units. Multiple stages of alteration/mineralisation are present throughout the deposit, which includes: (1) pre-mineralisation alteration; (2) hematite I; (3) Cu-Au mineralisation; (4) hematite II & REE mineralisation; (5) uranium mineralisation; (6) post-mineralisation alteration/veining. In order to establish a temporal framework, multi-mineral U–Pb geochronology was conducted on a variety of lithologies throughout the deposit and across multiple stages of alteration/mineralisation. Zircons from the host granite are predominantly discordant, with two concordant analyses from two samples producing 207Pb/206Pb ages of ca. 1880–1861 Ma, consistent with the emplacement of the Donington Suite (1860–1850 Ma). Apatite U–Pb geochronology performed on the granite-bearing samples produced ages of ca. 1775–1767 Ma, indicating the presence of a moderate temperature (≥350 °C) thermal event approximately 100 Ma after the crystallisation of the host rock. Zircon U–Pb geochronology conducted on a porphyritic volcanic unit and two polylithic breccia samples produce bi-modal age populations at ca. 1880–1860 Ma & ca. 1590 Ma. The ca. 1880–1860 Ma age reflects zircons inherited from the host granite, while the younger ca. 1590 Ma population represents the crystallisation age of the porphyritic volcanic unit and the maximum formation age of the polylithic breccias. U–Pb geochronology conducted on apatite, hematite and xenotime associated with various stages of mineralisation produce ages of ca. 1585 Ma, consistent with a major Fe-Cu-Au metallogenic event throughout the Gawler Craton. U–Pb geochronology performed on uraninite, apatite located within a late vein, and “vuggy” hematite records a second hydrothermal event from ca. 600–500 Ma, reflecting a period of minor deformation and deposit-wide fluid circulation.

Thermal Baking, Induration and Physical Volcanic Features in Deccan Volcanic Units and Their Suitability for Excavation, Sculpturing as well as Restoration of Geoheritage Sites in Western India

Thermal Baking, Induration and Physical Volcanic Features in Deccan Volcanic Units and Their Suitability for Excavation, Sculpturing as well as Restoration of Geoheritage Sites in Western India

Mineral chemistry and P-T conditions of the Karakaya volcanites at Kırka-Afyon-Isparta volcanic province, Afyon, Turkey

The Kırka-Afyon-Isparta Volcanic Province (KAIVP) is one of the best known regions in Turkey for the origin and petrological evolution of the high potassium volcanic activity. The temporal and spatial variability of volcanic rocks in the region exerts significant control over their geochemical diversity. Alkaline and ultrapotassic volcanic rocks of the Afyon volcanism are the first products of asthenospheric origin after the orogenesis in western Anatolia. We have determined the mineralogical and petrographic properties of the Karakaya volcanites surrounding Afyon with the help of microprobe analyses. Estimated thermobarometers are calculated. The Karakaya volcanites have been grouped into four different units according to their their mineralogical, petrographic and geochemical characteristics: Seydiler ignimbirite, basaltic trachyandesite, trachyandesite, trachyte and lamproite. Most samples display hypocrystalline porphyritic texture, whereas samples of lamproite unit have a holocrystalline texture. Generally, volcanic units also exhibit some textural evidence of disequilibrium crystallisation, such as sieve texture and corrosion in plagioclase phenocrysts, zoning and inclusions in clinopyroxene phenocrysts. Mineral thermobarometric estimations in all suites were tested on clinopyroxene and feldspar compositions, considering different authors’ approaches. Values of temperatures and pressure range from 1105 to 1273ºC and 5.6 to 12.2kbar, respectively. The temperature and pressure values calculated from the mineral-melt associations in the volcanics suggest that the Afyon Volcanites were affected by magma mixing processes and crystallised at different depths during the transport of magma.

Volcanic crustal structure of the western Hikurangi Plateau (New Zealand) from marine seismic reflection imaging

Seamounts and basaltic basement can influence deformation and mass fluxes within subduction zones. We examined seamounts and volcanic units across the western Hikurangi Plateau, near the Hikurangi subduction margin, New Zealand, with seismic reflection images. Volcanism at the Hikurangi Plateau occurred in at least three phases that we attribute to (1) Early Cretaceous large igneous province formation, the top of which is marked by laterally continuous and dipping wedges of reflections that we interpret as lava flows; (2) Late Cretaceous seamounts and volcaniclastics that erupted onto the crust of the Hikurangi Plateau and make up the majority of seamount volume and basement relief; and (3) late-stage, Pliocene volcanics that erupted through and adjacent to Cretaceous seamounts and younger sediments of the north-central Hikurangi Plateau. The Pliocene volcanoes do not appear to be strongly welded to the plateau basement and may be petit spot volcanoes that are related to the displacement and accumulation of hydrous transition zone melts. Large seamounts and volcaniclastic units are evenly distributed across most of the Hikurangi Plateau near the Hikurangi margin but are absent from the Pegasus Basin. Although faults are imaged throughout the basement of the Pegasus Basin, contemporary normal faulting of the Hikurangi Plateau is uncommon, except for a zone of Quaternary normal faults near the Pliocene volcanics. These trends indicate that the Hikurangi megathrust may be more influenced by volcanic structures in the north and central Hikurangi margin, where plateau rifting and voluminous seamount eruptions have more substantially overprinted the original Early Cretaceous basement.

Cenozoic Basin Evolution During Alternating Extension and Shortening in the Southern Central Andes Along the Chile-Argentina Border, 37–38°S

The south-central Chile and Argentina margin experienced a regional phase of extensional tectonics during the Oligocene–early Miocene, forming several basins across the forearc, Andean Cordillera, and retroarc regions. These basins accumulated thick successions of volcanic and sedimentary rocks. Subsequently, Neogene contractional tectonics led to the development of the current Andean Cordillera and the deposition of synorogenic clastic deposits in foreland basins. Traditionally, the Cura Mallín Formation, comprising a lower volcanic unit (CMV) and an upper sedimentary unit (CMS), has been interpreted to have formed during the Oligocene–early Miocene extensional phase. However, some studies propose deposition of the CMS in a foreland basin during the early–late Miocene. To unravel the transition from extensional to contractional tectonics in the Andes of south-central Chile and Argentina, we conducted new geochronological analyses (U-Pb, LA-ICP-MS) and integrated these results with structural, stratigraphic, and sedimentological observations in key sections within the CMS and the overlying Trapa-Trapa Formation in the Principal Cordillera along the Chile-Argentina border (37°–38°S). Our findings indicate that only the lower part of the CMS was deposited in an extensional setting, as evidenced by the presence of an inverted extensional wedge dated at ∼20 Ma. The middle-upper CMS (∼19 to 9 Ma) and contemporaneous units to the east exhibit evidence of syncontractional deformation, suggesting deposition in a foreland basin generated by shortening of the western Principal Cordillera. Around 9 Ma, uplift of the Agrio and Chos Malal fold and thrust belts, east of the Principal Cordillera, led to segmentation of the foreland basin. The Trapa Trapa Formation was deposited in a hinterland basin, with sediment sourced from the east. After ∼6.5 Ma, major contractional deformation shifted westward, resulting in intense folding of the CMS and Trapa Trapa Formation and subsequent thrusting of the western Principal Cordillera over the Central Depression. Our study suggests that deformation progressed toward the eastern foreland during the early to late Miocene and then shifted toward the western forearc during the late Miocene to Pleistocene.