Miocene Volcanism Research Articles

Campanian volcanism within the Asher-1 borehole

Asher Oil Corporation. 1964. Composite log. Compiled by Naphta Israel Petroleum Corporation. Folkman,Y. 1966. The lithostratigraphy and petrography of the Zalmon and Yagur formations in the Haifa– Jalame area. Geol. Surv. Isr. Rep. M.P. 179/66 (in Hebrew). Ilani, S., Kafri, U., Harlavan, Y. 2005. Miocene volcanism in the Western Galilee coastal plain. Isr. J. Earth Sci. 54: 47–53. Kafri, U. 1969. Geology and groundwater of the Cenomanian formations in Galilee, west of the watershed. Ph.D. thesis, Hebrew Univ., Jerusalem (in Hebrew). Kashai, E. 1958. A note on the revision of the stratigraphy of the southern Carmel. Bull. Res. Counc. Isr. 7G: 164–165. Sass, E. 1980. Late Cretaceous volcanism in Mount Carmel. Isr. J. Earth Sci. 29: 8–24. Segev, A., Sass, E., Ron, H., Lang, B., Kolodny, Y., McWilliams, M. 2002. Stratigraphic, geochronologic, and paleomagnetic constraints on Late Cretaceous volcanism in northern Israel. Isr. J. Earth Sci. 51: 297–309. Isr. J. Earth Sci.; 54: 179–181

Geology, geochronology and tectonic setting of late Cenozoic volcanism along the southwestern Gulf of Mexico: The Eastern Alkaline Province revisited

A NNW-trending belt of alkaline mafic volcanic fields parallels the Gulf of Mexico from the U.S. border southward to Veracruz state, in eastern Mexico. Previous studies grouped this volcanism into the so-called “Eastern Alkaline Province” (EAP) and suggested that it resulted from Gulf-parallel extensional faulting migrating from north to south from Oligocene to Present. On the basis of new geologic studies, forty-nine unspiked K–Ar and two 40Ar– 39Ar ages, we propose a new geodynamic model for the volcanism along the southwestern Gulf of Mexico. We studied in detail four of the six recognized fields of mafic alkaline volcanism in Veracruz state: 1) The lavas flows of Tlanchinol area (7.3–5.7 Ma), 2) the Alamo monogenetic field and Sierra de Tantima (7.6–6.6 Ma), 3) the Poza Rica and Metlatoyuca lava flows (1.6–1.3 Ma) and 4) the Chiconquiaco–Palma Sola area (6.9–3.2 Ma). Other two mafic volcanic fields may represent the continuation of alkaline volcanism to the southeast: the Middle Miocene lavas at Anegada High, offshore port of Veracruz, and the Middle to Late Miocene volcanism at the Los Tuxtlas. The existence of major Neogene extensional faults parallel to the Gulf of Mexico (i.e., ∼N–S to NNW–SSE) proposed in previous works was not confirmed by our geological studies. Elongation of volcanic necks, vent alignment, and faults mapped by subsurface data trend dominantly NE to ENE and NW to NNW. These directions are parallel to transform and normal faults that formed during the Late Jurassic opening of the Gulf of Mexico. Ascent of mafic magmas was likely facilitated and controlled by the existence of these pre-existing basement structures. Coupled with previous studies, our data demonstrate the occurrence of three magmatic episodes in Veracruz: 1) A Middle Miocene (∼15–11 Ma) episode in southern Veracruz (Palma Sola, Anegada, and Los Tuxtlas); 2) A Late Miocene to Early Pliocene (∼7.5–3 Ma) pulse of mafic alkaline volcanism throughout the study region; and 3) A Late Pliocene to Quaternary transitional to calc–alkaline volcanism in southern Veracruz (Palma Sola, Los Tuxtlas). Whereas the first and third episodes may be considered part of the subduction-related Trans-Mexican Volcanic Belt, the second pulse of mafic alkaline volcanism has a more complex origin. The absence of significant extensional faulting precludes a rift origin. We favor a model in which a transient thermal anomaly and melting of the mantle was triggered by the tearing and detachment of part of the subducted slab.

Patterns of speciation in endemic Mexican Goodeid fish: sexual conflict or early radiation?

Currently there is much interest in the potential for sexual selection or conflict to drive speciation. Theory proposes that speciation will be accelerated where sexual conflict is strong, particularly if females are ahead because mate choice will accentuate divergence by limiting gene flow. The Goodeinae are a monophyletic group of endemic Mexican fishes with an origin at least as old as the Miocene. Sexual selection is important in the Goodeinae and there is substantial interspecific variability in body morphology, which influences mate choice, allowing inference of the importance of female mate choice. We therefore used this group to test the relationship between sexual dimorphism and speciation rate. We quantified interspecific variation in sexual dimorphism amongst 25 species using a multivariate measure of total morphological differentiation between the sexes that accurately reflects sexual dimorphism driven by female mate choice and also used a mtDNA-based phylogeny to examine speciation rates. Comparative analyses failed to support a significant association between sexual dimorphism and speciation rate. In addition, variation in the time course of speciation throughout the whole clade was also examined using a similar tree containing 34 extant species. A constant rates model for the growth of this clade was rejected, but analyses instead indicated a decline in the rate of speciation over time. These results support the hypothesis of an early expansion of the group, perhaps due to an early radiation influenced by the key innovation of live bearing, or the prevalence of Miocene volcanism. In general, support for the role of sexual selection in generating patterns of speciation is proving equivocal and we argue that vicariance biogeography and adaptive radiations remain the most likely determinants of major patterns of diversification of continental organisms.

Impact of Grassland Radiation on the Nonmarine Silica Cycle and Miocene Diatomite

Abstract The Early Miocene rise of the grass-dominated ecosystem is a plausible trigger for a sharp Miocene increase in accumulation of nonmarine diatomaceous sediment as well as diversification of nonmarine diatoms. This grassland radiation introduced a biogeochemical mechanism for enhancing widespread and sustained mobilization of usable silica and other nutrients. Volcanism was probably responsible for episodic nonmarine diatomaceous sediments from the advent of the oldest known nonmarine diatoms in the Late Cretaceous through the Oligocene. Although prolific Miocene volcanism was undoubtedly still important in the development of many diatomites, feedback from grassland colonization of volcanic soils may explain why diatomaceous sedimentation surged in the Miocene following a more sparse pre-Miocene record. The initial rise of the grass-dominated ecosystem, increased nonmarine diatomite accumulation, and Early Miocene evolutionary radiations of nonmarine diatom taxa are at least approximately coeval. A...

Miocene volcanism in the Western Galilee coastal plain

Miocene volcanism in the Western Galilee coastal plain

Miocene volcanism of the Cserhát Mts (N Hungary): Integrated volcano-tectonic, geochronologic and petrochemical study

New K/Ar ages and paleomagnetic data connected with volcano-tectonic observations detected three intermediate (andesitic) and three acidic (dacitic-rhyolitic) magmatic phases. Cserhat magmatic activity occurred between 21-12 Ma. The timing of the initial and final acidic and intermediate phases may be connected with the Matra volcano situated to the east. During the Badenian (15-14 Ma) the volcano-tectonic evolution was relatively independent in the Cserhat Mts. The third acidic and intermediate volcanic phases, which developed in the Lower Sarmatian, show similar features as the final phases of the Matra volcano. Based on the major and trace element geochemistry the acidic rocks result from partial melting of the lower crust. Most of the intermediate volcanic rocks were generated from a rather homogeneous fluid-modified source (lithospheric), as triggered by an important heat transfer event. Initial melts sometimes experienced mixing or contamination in the lower or upper crust. This was a period of stro...

New paleomagnetic, rock magnetic, and petrographic results from the Valle Group, Baja California, Mexico: Exploring the causes of anomalously shallow paleomagnetic inclinations

A detailed paleomagnetic, rock magnetic, and sedimentologic study was conducted on Cenomanian‐age strata of the Valle Group at El Campito, Baja California, to examine the burial compaction effects on the paleomagnetic inclination of marine sedimentary rocks. Alternating field and thermal demagnetization isolated a characteristic remanence at 16 of 18 sites with a mean of Dec = 342.6°, Inc = 19.1°, and α95 = 3.4°. The inclination is 38.7° shallower than that predicted by the reference paleopole for North America. Laboratory experiments indicate that compaction could account for only 7.5° of the inclination shallowing. Fold tests at different scales yield both positive and negative results, making a tight constraint on the remanence's age impossible. A primary postdepositional remanence is supported by bedding‐parallel foliations in the anisotropy of anhysteretic remanence fabric. Although primary anomalously shallow inclinations could indicate significant southerly, then northerly, paleolatitudinal offset, a more likely scenario is a late Cenozoic low‐temperature remagnetization, which is suggested by alteration along the edges of some detrital silicate grains and a bimodal magnetic grain size distribution revealed by isothermal remanence data. The synthesis of the data indicates that the Vizcaino terrane was contiguous with the Cordillera margin during remagnetization and underwent a subsequent 13.5 ± 5.4° clockwise rotation. We tentatively interpret that the remagnetization resulted from burial and/or Miocene volcanism, which caused an acquisition of thermochemical remanent magnetization (TCRM). This study reveals a complicated remanence that resists easy interpretation and suggests caution in studies of similar rocks.

Landslide inventory of northwestern Anatolia, Turkey

Turkey is heavily exposed to natural hazards such as earthquakes, landslides and floods. The total loss caused by landslides in terms of affected buildings, in a period of 35 years between 1959 and 1994, constitutes 27% of the entire loss from all natural hazards and is second after earthquakes. There are no other available data on either direct or indirect losses due to landslides on a national scale. The General Directorate of Mineral Research and Exploration (MTA) started the ‘Turkish Landslide Inventory Mapping Project’ in 1997 to improve understanding of regional and national landslide processes. The purpose of the project is to establish landslide inventory maps at medium (1:25,000), regional (1:100,000) and national (1:500,000) scales. Existing landslides are mapped on 1:25,000 scale topographic base maps by interpretation of aerial photographs and field investigations. The base maps are then digitized and stored in a geographic information system (GIS) database by the Geological Research Department of MTA. Hence, regional- and national-scale landslide maps will be available as well, as the work progresses. Landslides are classified as fall, topple, slide and flow and are broadly characterized as active or inactive. The landslides are also classified according to their relative depths, as shallow (depth<5 m) and deep-seated (depth>5 m). The present paper will attempt to describe the project standards and its application to the area of 1:500,000 scale Zonguldak quadrangle. The study area extends to 39,081 km 2, and 7.1% of the area was found to be affected by landslides. A total 10,007 landslides (392 shallow-seated, 8020 deep-seated active and 1595 deep-seated inactive) were mapped in the area covering 2768 km 2. Cretaceous flysch, Paleocene–Eocene flysch and Paleocene–Middle Miocene volcanics are the most landslide-prone units and constitute 27.8%, 29.9% and 7.2% of the all landslides, respectively.

Early Miocene andesite conglomerates in the Sierra de Famatina, broken foreland region of western Argentina, and documentation of magmatic broadening in the south Central Andes

A thick synorogenic megasequence, known as the Del Crestón Formation, crops out in the Famatina belt in western Argentina. Its coarsening upward arrangement, rotational syntectonic unconformities, and provenance analysis allowed considering this unit as an this unit was an Andean-related foreland succession, although its age and geological meaning have been largely controversial. It unconformably overlies Late Paleozoic (Paganzo Group) red beds and is covered by the Middle–Late Miocene Angulos Group in a faulted contact. No previous geochronologic data constrain this important synorogenic cycle bracketed between the Triassic and the late Neogene. This work provides a set of new ages for andesite boulders recorded in the lowermost volcanogenic conglomerate of the Del Crestón Formation. The andesite boulders provide evidence of backarc volcanism in the external foreland during the Early Miocene (ca. 17 Ma). This volcanic pulse, together with other evidence for Early Miocene volcanism in the Argentine Precordillera, support a previously unrecognized stage of magmatic broadening in the south Central Andes associated with the initiation of shallow subduction and changes in the plate convergence pattern at about 18 Ma. Major progressive unconformities and unroofing history from conglomerates indicate deformation at a proximal foreland depozone and basement involvement (broken foreland stage). This deformation could have occurred coevally with the deposition of distal facies in the Bermejo basin system to the south, suggesting markedly nonuniform Andean foreland development.

Volcanic and deformation history of the Bodrum resurgent caldera system (southwestern Turkey)

The volcanic rocks of the Bodrum Peninsula, in SW Turkey and NE of the Hellenic Arc, outcrop over an area of 138 km 2. A monzonitic intrusion is exposed in the western part of the peninsula. Upper Miocene volcanism is represented by high-K (HK)-andesitic, andesitic lava flows and pillows, sparse HK-andesitic and dacitic lava domes and associated block-and-ash flows. A HK-andesitic ignimbrite sequence with two stratigraphic units is associated with the collapse of a complex caldera system. Breccias, formed as a result of slumping of the caldera walls are observed inside the caldera. Post-caldera activity is represented by HK-andesitic, HK-basaltic andesitic lava flows, domes and associated block-and-ash flows. Numerous dykes, HK-andesitic and shoshonitic in composition cut all volcanic units. The structure of the Bodrum caldera was investigated using SPOT image, digital elevation model (DEM), aerial photographs as well as field data. The Bodrum caldera is a NE–SW-elongated, semi-elliptical, deeply eroded caldera with dimensions of 18.7×7.7 km. It is partly submerged in the SW part. The complex caldera system can be described in terms of two structural domains. The collapse of the Dagbelen domain is interpreted as a piston type subsidence, while the Karakaya domain represents a piecemeal collapse. Both domains exhibit two separate resurgence events. The elongation of the caldera may be related to pre-existing regional tectonic structures. The caldera is also affected and cut by late stage faults related to regional extensional events. Moreover, pre-caldera volcanism is dispersed and cannot be related to a pre-existing stratovolcano. Bodrum volcanism is therefore interpreted as a complex ignimbritic shield volcano.

Miocene volcanic seamounts on northern Lord Howe Rise: Lithology, age and origin

Multibeam sonar swath‐mapping has revealed small submarine volcanic cones on the northeastern Lord Howe Rise, a submerged ribbon continent. Two such cones, aligned north‐northwest and 120 km apart, were dredged at 23–24°S. Water depth is about 1150 m nearby: the southern cone rises to 750 m and the northern to 900 m. Volcanic rocks dredged from the cones are predominantly highly altered hyalodastites with minor basalt. The clasts are mostly intensely altered vesicular brownish glass with lesser basalt, set in zeolitic, clayey, micritic, or ferruginous cement. Lavas and hyalodastites contain altered phenocrysts of olivine and plagioclase, and fresh clinopyroxene. The latter have compositions between acmite and Ti‐augite, matching clinopyroxene phenocrysts in undersaturated intraplate basanitic mafic lavas. Interbedded micrites in the volcaniclastics represent calcareous ooze that was deposited with (or later than) the volcanic pile. Foraminifers indicate that the micrite is dominantly Early Miocene (ca 16 Ma), and that the ooze was deposited in cool water. Upper Miocene to Pliocene micrites, presumed to be later infillings, all contain warm‐water forms. This evidence strongly suggests that both cones formed in pelagic depths in the Early Miocene. Ferromanganese crusts from the two cones are up to 7 cm thick and similar physically, but different chemically. The average growth rate is 4 mm/106 y. Copper, nickel, and cobalt content are relatively high in the north, but copper does not exceed 0.08 wt%, nickel 0.65%, and cobalt 0.25%. The Mn:Fe ratio is high in the south (average 13.7) suggesting strong hydrothermal influence. Such small volcanic cones related to intraplate hot‐spot‐type volcanism may occur in extensive fields on the Lord Howe Rise, like those off southern Tasmania. On the Lord Howe Rise, the known small volcanic cones coincide with broad gravity highs in areas of shallow continental basement. The highs are aligned parallel to the Lord Howe Rise margins, and it is probable that the Miocene volcanism is related to jostling on older faults. Other fields of small volcanic seamounts may occur on the Lord Howe Rise on such fractured gravity highs.

Miocene volcanism in the Lhasa block, Tibet: spatial trends and geodynamic implications

Miocene ultrapotassic (UP), shoshonitic (P) and calc-alkaline (CA) volcanism is recognized widely over the Tibetan plateau. This volcanism occurred after the end of India’s oceanic lithosphere subduction beneath the plateau. Ambiguities about the distribution and significance of this volcanism arise in part from a paucity of geochronologic and geochemical data. We have sampled two volcanic fields in the Lhasa block: (1) the eastern edge of the Zabuye Salt Lake (central part of the block), (2) the southwestern edge of the Yangbajin Graben. Major and trace elements display high K 2O (6.06–6.54%) over a wide range of SiO 2 (75.03–55.30%), Na 2O (1.32–3.30%) and LREE/HREE enrichment with a significant rare earth element fractionation (26<(La/Yb) n <42) as well as negative Nb, Ta, and Ti anomalies. These features indicate that our samples are chemically identical to the post-collision Miocene UP to P volcanic rocks already identified all over the Tibetan plateau and the southern plateau specifically but distinct from the CA rocks found in the Lhasa block. The Miocene volcanics found in the Lhasa block present greater chemical variation than the rest of the Tibetan plateau. 40Ar/ 39Ar ages on both sanidine and biotite gave ages ranging from 16.16±0.12 Ma to 16.01±0.12 Ma and 10.60±0.14 Ma to 10.88±0.17 Ma (2σ, excluding systematic errors, relative to Fish Canyon sanidine at 28.02 Ma) for Zabuye Salt Lake and Yangying geothermal field volcanics respectively. These results are consistent with the range of previously published ages from this part of the Tibetan plateau. The presence of N–S-trending dikes and the location of the volcanic fields suggest that local E–W extension in the Lhasa block was coeval. We propose that the synchronous UP, P and CA volcanism in the Lhasa block between 23 and 8 Ma is the result of progressively deepening northward subduction of Indian continental lithosphere producing asthenospheric upwelling. Convective thinning (delamination) of lithospheric mantle fueled by the hot asthenosphere induced partial melting of an enriched sub-continental mantle lithosphere as well as eclogitic lower crust. This mechanism could also explain the local E–W extension observed in the same period (18–10 Ma). We also propose that lateral migration of the deepening Indian continental lithosphere mantle slab could explain the observed younger volcanism in the eastern part of the Lhasa terrane.

Cenozoic volcanism, tectonism and stream derangement in the Snowy Mountains and northern Monaro of New South Wales

Studies of Cenozoic lavas and associated sediments in the Kiandra‐Cabramurra and Adaminaby‐Cooma areas identify and date tectonic deformations responsible for differential uplift and drainage development of the region. Volcanic activity on the northern Monaro was mainly Eocene‐Oligocene but in the extreme north there are Early Miocene sediments and lavas. Volcanic activity and folding began to rearrange the drainage in the Eocene‐Oligocene. The headwaters of the Murrumbidgee River originally flowed south into the Eucumbene River but Early Miocene folding and faulting uplifted the Monaro Range and created a large lake near Adaminaby. Lake overtopping rerouted the drainage east and then south along the basalt‐filled valley of an old north‐flowing tributary, the ‘Adaminaby River’, forming the present‐day Murrumbidgee River. The folding also produced a 300 m height difference between the Berridale and Adaminaby Plateaus and formed a section of the Great Divide. This fold displacement ranks with the largest Cenozoic fault displacements. In the Kiandra area tectonism associated with Early Miocene volcanism rearranged the drainage and tilted the Kiandra area and Kosciuszko Block to the north.

Cenozoic volcanism, tectonism and stream derangement in the Snowy Mountains and northern Monaro of New South Wales

Studies of Cenozoic lavas and associated sediments in the Kiandra–Cabramurra and Adaminaby–Cooma areas identify and date tectonic deformations responsible for differential uplift and drainage development of the region. Volcanic activity on the northern Monaro was mainly Eocene–Oligocene but in the extreme north there are Early Miocene sediments and lavas. Volcanic activity and folding began to rearrange the drainage in the Eocene–Oligocene. The headwaters of the Murrumbidgee River originally flowed south into the Eucumbene River but Early Miocene folding and faulting uplifted the Monaro Range and created a large lake near Adaminaby. Lake overtopping rerouted the drainage east and then south along the basalt‐filled valley of an old north‐flowing tributary, the 'Adaminaby River', forming the present‐day Murrumbidgee River. The folding also produced a 300 m height difference between the Berridale and Adaminaby Plateaus and formed a section of the Great Divide. This fold displacement ranks with the largest Cenozoic fault displacements. In the Kiandra area tectonism associated with Early Miocene volcanism rearranged the drainage and tilted the Kiandra area and Kosciuszko Block to the north.

Timing Constraints of Gold Mineralization along the Carlin Trend Utilizing Apatite Fission-Track, 40Ar/39Ar, and Apatite (U-Th)/He Methods

Apatite fission-track analysis is used to demonstrate that the timing of gold mineralization at the Carlin East and Betze-Post deposits is 37.3 ± 1.5 Ma, based on the weighted mean of measured fission-track ages, and 41.6 ± 1.6 Ma, based on the weighted mean of the ages of the oldest fission tracks retained. Regional measured apatite fission-track ages cluster between 28 and 41 Ma to the south of the Carlin East deposit whereas measured ages are greater than ~56 Ma to the north. The regional trend of the apatite fission-track ages and the pattern of gravity and magnetic anomalies suggest that the thermal center for gold mineralization along the Carlin trend was south of the Carlin East deposit, near the 37 Ma Welches Canyon stock. Modeling of apatite fission-track-age data on both sides of the Post fault indicates that the Little Boulder stock was approximately 35°C cooler at 40 Ma relative to similarly positioned samples in the Goldstrike stock. It is unclear whether this thermal history difference across the Post fault is due to post-40 Ma differential exhumation or differential heating across the fault at the time of gold mineralization. Comparison of apatite fission-track and whole-rock 40Ar/39Ar ages from altered dikes in the ore zone at Carlin East demonstrates that 40Ar/39Ar ages do not reflect the age of gold mineralization, whereas the apatite fission-track ages were completely reset at the time of hydrothermal activity. (U-Th)/He ages are younger than the inferred mineralization age and may reflect weak hydrothermal activity associated with Miocene volcanism.

Geology of the Ivanhoe Hg-Au District, Northern Nevada: Influence of Miocene Volcanism, Lakes, and Active Faulting on Epithermal Mineralization

Geology of the Ivanhoe Hg-Au District, Northern Nevada: Influence of Miocene Volcanism, Lakes, and Active Faulting on Epithermal Mineralization

Geology of the Ivanhoe Hg-Au District, Northern Nevada: Influence of Miocene Volcanism, Lakes, and Active Faulting on Epithermal Mineralization

The mercury-gold deposits of the Ivanhoe mining district in northern Nevada formed when middle Miocene rhyolitic volcanism and high-angle faulting disrupted a shallow lacustrine environment. Sinter and replacement mercury deposits formed at and near the paleosurface, and disseminated gold deposits and high-grade gold-silver veins formed beneath the hot spring deposits. The lacustrine environment provided abundant meteoric water; the rhyolites heated the water; and the faults, flow units, and lakebeds provided fluid pathways for the hydrothermal fluids. A shallow lake began to develop in the Ivanhoe area about 16.5 Ma. The lake progressively expanded and covered the entire area with fine-grained lacustrine sediments. Lacustrine sedimentation continued to at least 14.4 Ma, and periodic fluctuations in the size and extent of the lake may have been responses to both climate and nearby volcanism. The eruption of rhyolite and andesite flows and domes periodically disrupted the lacustrine environment and produced interfingered flows and lake sediments. The major pulse of rhyolitic volcanism took place between 15.16 ± 0.05 and 14.92 ± 0.05 Ma. High-angle faulting began in the basement about 15.2 Ma, penetrated to and disrupted the paleosurface after 15.10 ± 0.06 Ma, and largely ceased by 14.92 ± 0.05 Ma. Ground motion related to both faulting and volcanism created debris flows and soft-sediment deformation in the lakebeds. Mercury-gold mineralization was coeval with rhyolite volcanism and high-angle faulting, and it took place about 15.2 to 14.9 Ma. At and near the paleosurface, hydrothermal fluids migrated through tuffaceous sediments above relatively impermeable volcanic and Paleozoic units, creating chalcedonic, cinnabar-bearing replacement bodies and sinters. Disseminated gold was deposited in sedimentary and volcanic rocks beneath the mercury deposits, although the hydrologic path between the two ore types is unclear. Higher-grade gold-silver deposits formed in massive rhyolites and Paleozoic quartzites at deeper levels, and these mineralized zones possibly represent the feeder zones for the higher-level deposits. Fluctuations in the ground-water table locally produced hydrothermal oxidation of the near-surface mercury and disseminated gold deposits. The locus of mineralization shifted with time, moving south and east from its inception point in the west-central part of the district. Thus, although mineralization in the district took place during a span of 300,000 years, the duration of mineralization at any one place probably was much shorter. The low-sulfidation deposits of the Ivanhoe district formed at the same time and under similar conditions as those in the nearby Midas district, 15 km to the northwest, which includes the large, high-grade Ken Snyder gold-silver epithermal vein deposit. The exposures in the Ivanhoe district are interpreted to represent the near-surface example of the paleosurface that originally was present above the Midas mineralizing system. The resulting combined Ivanhoe-Midas model provides an exploration guide for epithermal deposits in similar geologic environments in northern Nevada.

K-Ar dating of Late Miocene volcanics distributed in the northwestern part of the Iizaka district, Fukushima City, Japan

K-Ar dating of Late Miocene volcanics distributed in the northwestern part of the Iizaka district, Fukushima City, Japan

Exploration at the Kuscayiri Au (Cu) prospect and its implications for porphyry-related mineralization in western Turkey

The Kuscayiri Au (Cu) deposit is localized along EW-trending linear silica-quartz structures or pods within altered Lower Miocene volcanics that are underlain by medium to high level Mesozoic granodiorite intrusions. The deposit is characterized by disseminated or fracture-controlled pyrite (up to 12%), variable amounts of silicification, hematite–pyrite matrix breccias produced by intense fracturing of siliceous rocks and moderate to weak saccharoidal to crystalline quartz stringer stockwork veining in small breccia pipes. An early quartz–sericite alteration event was focused along the EW-trending structures and resulted in the precipitation of pyrite. A later, more spatially extensive, advanced argillic alteration event overprinted the quartz–sericite event. The highest gold content is restricted to the high-temperature portion of the alteration zone characterized by stockwork or replacement quartz associated with sericite–pyrite–alunite–pyrophyllite. Trace gold and associated magnetite, arsenopyrite, realgar, specular hematite and chalcopyrite also occur within the smectite–illite/smectite–kaolinite–chlorite alteration facies. During the Tertiary, widespread subaerial to subaqueous arc volcanism developed in western Turkey, and numerous occurrences of high/low sulfidation epithermal systems are associated with major NW–SE extensional deformation. The resultant NE–SW-trending grabens appear to be highly favorable host environments for the formation of porphyry-related gold–copper systems. The Kuscayiri mineralization may represent a lithocap with Au (Cu), the top of one such system yet to be discovered.

Geochemical Diversity of Late Miocene Volcanism in Southern Baja California, Mexico: Implication of Mantle and Crustal Sources during the Opening of an Asthenospheric Window

Five main petrologic and geochemical groups can be identified among the Middle to Late Miocene lavas from the western part of southern Baja California: (1) calc‐alkaline and K‐rich andesites emplaced between 15.5 and 11.7 Ma; (2) adakites and (3) associated niobium‐rich basalts erupted between 11.7 and 8.5 Ma in the Santa Clara volcanic field, Vizcaino Peninsula; (4) 10.6–9.2 Ma tholeiitic basalts and basaltic andesites that form large tabular plateaus near San Ignacio; and (5) magnesian and basaltic andesites of adakitic affinity whose emplacement started at 11.7 Ma south of San Ignacio and between 9.7 and 8.8 Ma near La Purisima. These lavas, although spatially and temporally related, display very different geochemical signatures. Their trace elements and isotopic characteristics suggest that three different magma sources were involved in their genesis. Partial melts of subducting altered oceanic crust produced the adakites when erupted directly at the surface. These magmas were eventually trapped in the mantle wedge where they reacted with ultramafic lithologies. Such slab‐melt‐metasomatized mantle could then melt to produce niobium‐rich basalts or magnesian andesites, depending on the pressure that controlled the stability of garnet into the mantle wedge. The melting of fluid‐metasomatized mantle wedge led to the emplacement of andesites. In southern Baja California, the opening of a slab window following active ridge subduction resulted in the additional contribution of partial melts from the suboceanic mantle uprising through the tear in the slab. This process might be responsible for the occurrence of tholeiitic basalts and basaltic andesites near San Ignacio. The studied association can be considered as a modern analog of high‐thermal‐regime Archean subductions.