The Stibnite-Yellow Pine district of central Idaho was mined from the early 1900s until the 1990s, extracting gold, antimony, tungsten, and mercury from veins and disseminated and replacement ores in mountainous terrain along the headwaters of the Salmon River. Mining during the two World Wars supplied critical antimony and tungsten to the war efforts. Recent exploration has delineated mineral resources of over 187 metric tons Au, 274 metric tons Ag, and 93,000 metric tons Sb. Mineralization is hosted in Cretaceous Idaho batholith granitic rocks and a sequence of Neoproterozoic to Paleozoic metasedimentary strata of carbonate and siliciclastic compositions. Historical studies outlined some of the complex paragenesis but debated the absolute age of mineralization. New petrographic and geochronologic work documents a sequence of five hydrothermal events in the Stibnite-Yellow Pine district. Event 1 is related to Cretaceous magmatic and hydrothermal activity and includes events ranging in age from 86 to 75 Ma, including sparse quartz-molybdenite veins dated at 86 Ma. Disseminated gold mineralization of event 2 is associated with sericitic alteration and sulfidation of igneous biotite and replacement of plagioclase by potassium feldspar, largely in granodiorite. Gold is present in zoned arsenian pyrite in both disseminated ores and in crosscutting carbonate-quartz veins containing pyrite and arsenopyrite. The large Yellow Pine deposit, localized at a dilatant bend in the Meadow Creek fault, hosts such disseminated and vein gold. Event 2 is interpreted as the major gold-forming event; 40Ar/39Ar ages of sericite and potassium feldspar alteration range, respectively, from 70 to 59 and 66 to 56 Ma. The long span is interpreted to reflect the age of gold mineralization and local overprinting by event 3. A narrower range from 66 to 61 Ma is interpreted to date the peak of gold mineralization and alteration. Event 3, tungsten mineralization with scheelite, is texturally later than event 2 gold and localized along the Meadow Creek structure. Event 3 scheelite has been dated by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb methods at 57 Ma. Event 4, best developed in the West End area, includes gold-silver-bearing quartz-carbonate-pyrite veins and breccias with epithermal textures and potassium feldspar alteration envelopes. Adularia from event 4 yields 40Ar/39Ar plateau ages of 52 to 51 Ma. Event 5 antimony and mercury mineralization consists of stibnite veins and breccia cements at the Yellow Pine and Hangar Flat deposits as well as cinnabar veins and replacements at the peripheral Fern and Hermes deposits; it is constrained by an LA-ICP-MS U-Pb date on scheelite (ca. 47 Ma) intergrown with stibnite. Minor propylitic and argillic alteration is evident in 47 Ma igneous dikes, which do not contain economic mineralization. The Au-Sb-W ores in the Stibnite-Yellow Pine mining district formed over an extended time period from about 70 to 45 Ma in multiple pulses that were localized along the Meadow Creek fault zone. Each event corresponds to episodes of magmatism and/or hydrothermal activity in the region. Insignificant event 1 skarn and molybdenum mineralization is similar in age to the Thompson Creek porphyry molybdenum deposit in central Idaho. Event 2 gold mineralization occurred during a magmatic gap in central Idaho but was synchronous with magmatism in the Bitterroot lobe further north; event 2 is similar in age to orogenic gold-arsenic mineralization at the Beartrack mine in eastern Idaho. Event 3 scheelite mineralization coincides with tungsten mineralization at the Quartz Creek deposit, late magmatism in the Bitterroot lobe, and rapid exhumation of the Atlanta lobe of the Idaho batholith. Event 4 gold mineralization is coincident with the onset of regional Challis magmatism and extension. Event 5 antimony and mercury mineralization is time-equivalent to epithermal gold mineralization in the nearby Thunder Mountain volcanic field and the peak of Challis magmatism.

ABS T R A C T The Nieve monogenetic volcanic cluster is located in the central -eastern region of the Michoac an -Guanajuato volcanic field, along the Huiramba fault zone, a relay ramp in the Morelia -Acambay fault system produced by oblique north-northwest transtension. This volcanic cluster includes at least 17 middle Pliocene to late Pleistocene lava domes, two small shield volcanoes, and two scoria cones. Between 4 and 3.8 Ma, two effusive eruptions built two small shield volcanoes overlying one another, with a magma volume of 3.93 km 3 . Between 2.9 Ma and 21.4 ka, 17 lava domes and two scoria cones were emplaced on the flanks of these volcanoes. The entire cluster resulted in a total erupted volume of 17 km 3 , covering an area of 326 km 2 and reaching a thickness of emplaced volcanic material of 1200 m, resulting in a magma eruption rate equivalent to 0.004 km 3 /ka. All the rocks associated with this cluster are within a relatively restricted range in composition, between 53.9 and 64.2 wt% SiO 2, from andesite enriched in silica to basaltic andesite. The presence of intrusive-rock xenoliths and xenocrysts with dissolution textures reveals that assimilation processes modified the magmas. Based on the regional geological record, we suggest that the establishment of the Nieve volcanic cluster has been controlled by tectonic structures and the basement of the region, which has allowed the chemical evolution of these magma batches that probably had sources in at least two deep reservoirs as reflected by the Nb/Th versus Ta/U ratio.

The volcanic history of many of the western Central Andes volcanoes, including Hualca Hualca in the northern Ampato Volcanic Complex, remains poorly constrained. Based on an updated 1:50,000-scale geological map, new cross-sections of the Ampato Complex, a compilation of published dates, new geochemical data and Ar-40/Ar-39 ages, we present new insights into its volcanic evolution. Our study identifies 7 principal geological units (4 were not reported on the previous geologic map) and 8, mostly constructive, volcanic phases. The Ar-40/Ar-39 dating supports that Hualca Hualca began its formation during the Early Pleistocene (>1.6 Ma) with the establishment of an ancient stratovolcano. This volcano underwent a massive sector collapse to the north that created a U-shaped large amphitheater. This collapse likely resulted in the temporary damming of the Colca River, though no dates or associated deposits have been identified. Subsequent eruptions led to the formation of a smaller volcano along the amphitheater's scar that emplaced andesitic and dacitic lavas. This volcano dubbed modern Hualca Hualca holds the main summit. One of the longest lavas of this edifice reach the Colca valley and was previously dated at 610 ka. Sometime around 550 ka, volcanic activity migrated to the interior of the amphitheater forming the Nevado de Puye, Cruz del Condor, and Ahuashune domes that produced lavas of andesitic and dacitic composition. Around 164 ka a dacitic lava flow was emitted between Nevado del Puye and the base of the modern Hualca Hualca inside the horseshoe crater. The youngest known collapse of the Hualca Hualca complex involved parts of Nevado de Puye dome and the modern Hualca Hualca volcano, emplacing a debris avalanche that again dammed the Colca River and formed an upstream temporary lake that emplaced lacustrine and volcaniclastic deposits. After this collapse, no other volcanic deposit of the Hualca Hualca complex has been identified. Subsequently, glacial erosion during the local Last Glacial Maximum (17-16 ka) modified the volcanic landscape. Geochemical characteristics of Hualca Hualca suggest that their magmas were produced in a metasomatized mantle wedge and likely underwent crustal contamination during their ascent through the thick continental crust.

One of the most debated volcanic areas along the eastern coast of Mexico is Los Tuxtlas Volcanic Complex located in the south of the Veracruz state. This volcanic province stands out as an isolated massif between the Trans -Mexican Volcanic Belt to the NW and the Chiapanecan Volcanic Belt to the SE. Los Tuxtlas complex is sur-rounded by two large alluvial plains formed by the Papaloapan and Coatzacoalcos rivers, and include the active volcano San Martin Tuxtla, the last eruption of which occurred in 1793. The ages, distribution, morphology and geochemistry of the volcanic vents and lavas, particularly the co-existence of the alkaline and subalkaline suites, offer an excellent opportunity to understand, in space and time, the tectono-magmatic history of Los Tuxtlas in comparison with other similar volcanic complexes in the eastern Trans-Mexican Volcanic Belt. Most of the volcanic centers of Los Tuxtlas show NW-SE alignments. This vent arrangement is associated with a left-lateral fault system known as the Veracruz fault, the northwestern expression of which are the submarine volcanoes that form the Anegada High. This fault might be a reactivation of the ancient Tamaulipas-Oaxaca fault related to the opening of the Gulf of Mexico. Based on geomorphological, geochemical, petrographical, and 40Ar/39Ar data, we propose three different stages of volcanic activity for Los Tuxtlas: The older Montepio-El Vigia (7-1.4 Ma), the intermediate Santa Marta (1-0.5 Ma), and the younger San Martin Tuxtla (50 ka to present). The main focus region of this paper is the stratovolcanoes of the Sierra de Santa Marta, formed by at least four major NW-SE aligned volcanic edifices, which from NW to SE are Encanto (950 m.a.s.l; identified and named in this work for the first time), Yohualtajapan (1450 m.a.s.l), Santa Marta (1685 m.a.s.l), and San Martin Pajapan (1250 m.a. s.l). The geological map, the 40Ar/39Ar dates, and the geochemistry data indicate a NW-SE migration of the volcanic activity of the four stratovolcanoes that have created the sierra de Santa Marta. Petrography and chemistry of rocks suggest that the subalkaline rocks of the three magmatic stages were fed by subduction ac-tivity of the Cocos Plate. The alkaline suite is likely associated with asthenosphere upwelling and slab edge melting, facilitated by a slab tear of the south Cocos plate evolved since the Early Miocene.

Changes in magmatism and sedimentation along the late Neoproterozoic-early Paleozoic Ross orogenic belt in Antarctica have been linked to the cessation of convergence along the Mozambique belt during the assembly of East-West Gondwana. However, these interpretations are non-unique and are based, in part, on limited thermochronological data sets spread out along large sectors of the East Antarctic margin. We report new 40Ar/39Ar hornblende, muscovite, and biotite age data for plutonic (n = 13) and metasedimentary (n = 3) samples from the Shackleton–Liv Glacier sector of the Queen Maud Mountains in Antarctica. Cumulative 40Ar/39Ar age data show polymodal age peaks (510 Ma, 491 Ma, 475 Ma) that lag peaks in U-Pb igneous crystallization ages, suggesting igneous and metamorphic cooling following magmatism within the region. The 40Ar/39Ar ages are similar to ages in other sectors of the Ross orogen, but younger than detrital mineral 40Ar/39Ar cooling ages indicative of older magmatism and cooling of unexposed inboard areas along the margin. Detrital zircon trace element abundances suggest that the widespread onset of magmatism in outboard localities of the orogen correlates with a ~560–530 Ma decrease in crustal thickness. The timing of crustal thinning recorded by zircon in magmas overlaps with other evidence for the timing of crustal extension, suggesting that the regional onset of magmatism with subsequent igneous and metamorphic cooling probably reflects slab rollback that coincided with possible global plate motion changes induced during the final assembly of Gondwana.

Meso- to Neoproterozoic geological and paleomagnetic data support a direct connection between Baltica and Laurentia in both Nuna and Rodinia supercontinents, however in different relative configurations. Previous paleomagnetic data limit the time of break-up of Nuna core configuration and ca. 90 degrees rotation of Baltica relative to Laurentia between 1.27 Ga and 0.99 Ga. Despite the well documented relative motion of continents, the tectonic mode during the Meso- to Neoproterozoic has been questioned and the operation of single lid tectonics at 1.6-1.0 Ga during the Nuna-Rodinia supercontinent cycles has been proposed.In this study, new paleomagnetic and whole rock 40Ar-39Ar geochronological data from a basic dyke in Stugun central Sweden are combined with coeval data to produce a 1.22 Ga (1.244-1.200 Ga) moderate-quality paleomagnetic pole. This is done to better estimate the break-up time of the core of Nuna and explore the plate tectonics at the Mesoproterozoic. The new pole fills part of the 1.247-1.140 Ga gap in the paleomagnetic record of Baltica. By comparing apparent polar wander paths (APWPs) and calculated drift velocities, a break-up of Baltica and Laurentia at 1.12-1.04 Ga is suggested. Plate velocities calculated for Laurentia, Baltica and Siberia for the time of the Nuna supercycle are similar and low to moderately high corresponding with the present-day tectonic speeds. Furthermore, the obtained velocity peaks may be related with onset of the break-up of the Nuna supercontinent, the break-up of the direct Baltica-Laurentia connection in Nuna and nascent Rodinia. We suggest that the velocity peaks and large oscillating shifts in late Mesoproterozoic pole positions for Laurentia and Baltica result from a combination of relative plate motion and inertial interchange true polar wander (IITPW) events. Both IITPW events and relative plate motions argue for an operation of plate tectonics in the Meso- to Neoproterozoic.

Se presenta la estratigrafía volcánica de la porción suroeste del Lago de Cuitzeo, Michoacán dentro del sector central de la provincia de la Faja Volcánica Transmexicana, donde se registró una estructura caldérica tipo graben. Se identificaron seis secuencias volcánicas con edades que van del Mioceno temprano al Pleistoceno tardío que incluyen: 1) Secuencia Copándaro-Tarímbaro (SCoT): originada durante el Mioceno temprano, contiene intercalaciones de flujos de lavas de andesita basáltica e ignimbritas con alto grado de alteración hidrotermal, 2) Secuencia Tarímbaro (ST): fechada en 18.7 Ma, consiste de lavas andesítico basálticas acompañadas de traquiandesitas, 3) Secuencia de Ignimbritas Chucándiro (SICh): con edades de 16.88 ± 0.34, 16.72 ± 0.24 y 16.88 ± 0.22, que contiene tres litofacies de ignimbritas que consisten en mesas con notorio basculamiento ortogonal a las fallas NE-SO. En la base de las ignimbritas se observaron depósitos brechoides cuyos rasgos se interpretan como brechas de rezago, que siguen la traza del sistema de fallas Morelia-Acambay (SFMA). Estas cubren a la 4) Secuencia El Caracol (SEC): con edades de 13.73 ± 0.24 Ma a 3.02± 0.03 Ma; constituidas por cuatro unidades volcánicas de basaltos, andesitas y andesitas basálticas. Estos productos están intercalados con una unidad lacustres, donde se encontró un fósil índice del Mioceno tardío. Los depósitos lacustres sobreyacen a la 5) Secuencia Quinceo-Tetillas (SQT) fechada en 1.48 ± 0.016 Ma, conformada por depósitos de caída con intercalaciones de lahares (flujos hiperconcentrados); sobreyacidos por los productos de los volcanes Quinceo (1.5 ± 0.14 Ma) y Tetillas (0.4 ± 0.08 Ma). Finalmente, la secuencia 6) está representada por el cono cinerítico Chucándiro (CCh): con una edad de 0.161 ± 0.014 Ma, de composición andesítica, correspondiente al Campo Volcánico Michoacán Guanajuato (CVMG). Los análisis de geología estructural y geoquímica muestran que las secuencias Copándaro-Tarímbaro (SCoT), Tarímbaro (ST) e ignimbrita de Chucándiro (SICh) están estrechamente asociadas a eventos de transición entre el vulcanismo de la SMO y la FVTM. Con base en la geometría del Graben Chucándiro, la ubicación de los depósitos de brechas de rezago, las fallas secundarias de dirección NE-SO y la distribución de las Ignimbritas Chucándiro, se propone que estas ignimbritas se originaron en el Mioceno temprano a partir de una caldera tipo graben del mismo nombre y producto del régimen extensivo del SFMA, cuya edad inicial de la transtensión en esta porción central de la FVTM se asigna al Mioceno temprano, con edades mayores a 17.1 Ma. Nuestros resultados indican que el inicio de la FVTM dentro del área inició durante el Mioceno temprano con la conformación de un graben en donde se emplazaron volcanes monogenético seguido del relleno con sedimentación lacustre que continua en la actualidad.

This report presents results from 53 40 Ar/ 39 Ar and 51 U-Pb analyses of 73 sedimentary, intrusive, and volcanic rock samples collected in the south-central Tyonek Quadrangle of southern Alaska.These data provide new age control and provenance information for igneous and sedimentary rock units throughout the study area-some of which lack prior geochronologic age assignments.The samples were collected in concert with new geologic mapping of approximately 900 square miles encompassing parts of the Late Cretaceous-Paleogene magmatic arc and Cenozoic forearc basin in south-central Alaska.Detailed stratigraphic studies of the entire Cenozoic succession were undertaken at the same time.The widespread exposures of forearc strata in the map area are surface correlatives to hydrocarbon-hosting and commercially producing units in the adjacent subsurface of Cook Inlet basin to the southeast.The results inform structural and depositional models for the Cook Inlet forearc basin that are leveraged to assess tectonic events affecting the continental margin.Intrusive crystallization dates and geochemistry of igneous rocks define episodes of arc magmatism in the region at ca. 84-78 Ma, ca.74-71 Ma, ca.69-65 Ma, and ca.62-57 Ma.Mafic and intermediate volcanics with arc to intra-plate signatures are restricted to a period of reduced arc magmatism from about 57 to 47 Ma.Bedrock cooling from approximately 550˚C to below 150˚C occurred monotonically, but at varying rates.The cooling trends are consistent with thermal conduction after shallow melt emplacement followed by structurally controlled cooling during the early Paleocene.New dates of reworked tephras from Cenozoic forearc strata constrain the depositional age of the West Foreland Formation at a maximum of ca.47 Ma to ca. 38 Ma.The dates bracket renewed arc volcanism and middle Eocene syntectonic sedimentation at the western basin margin.More sparsely dated Tyonek and Beluga formations produced zircon U-Pb maximum depositional dates of ca.15.3-15.2Ma, and ca.11.2-9.7 Ma, respectively.These dates are used to anchor a large existing palynologic dataset from the area.New and previously published palynologic results indicate that the oldest Hemlock Conglomerate extends to middle Eocene and thus is partially contemporaneous with West Foreland deposition.Sediment accumulation rates calculated from dated stratigraphic intervals and revised stratigraphic age ranges of the West Foreland Formation and Hemlock Conglomerate suggest a pronounced decrease in basin subsidence during Hemlock deposition followed by an order of magnitude increase in sediment accumulation rates by the middle Miocene.Conglomerate

New U-Pb and 40Ar/39Ar ages integrated with geologic mapping and observations across the western Alaska Range constrain the distribution and tectonic setting of Cretaceous to Oligocene magmatism along an evolving accretionary plate margin in south-central Alaska. These rocks were emplaced across basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Triassic and Jurassic plutonic and volcanic rocks of the Peninsular terrane, and Jurassic and Cretaceous siliciclastic strata of the Kahiltna assemblage. Plutonic rocks of different ages also host economic mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related rare-earth elements. The oldest intrusive suites were emplaced ca. 104–80 Ma into the Peninsular terrane only prior to final accretion. Deformation of the northern Kahiltna succession and underlying Farewell terrane occurred at ca. 97 Ma, and more widespread deformation ca. 80 Ma involved south-vergent folding and thrusting of the Kahiltna assemblage that records collisional accretion of the Peninsular-Wrangellia terrane and juxtaposition of sediment wedges formed on the inboard and outboard terranes. More widespread magmatism ca. 75–55 Ma occurred in two general pulses, each having distinct styles of localized deformation. Circa 75–65 Ma plutons were emplaced in a transpressional setting and stitch the accreted Peninsular and Wrangellia terranes to the Farewell terrane. Circa 65–55 Ma magmatism occurred across the entire range and extends for more than 200 km inboard from the inferred position of the continental margin. The Paleocene plutonic suite generally reflects shallower emplacement depths relative to older suites and is associated with more abundant andesitic to rhyolitic volcanic rocks. Deformation ca. 58–56 Ma was concentrated along two high-strain zones, the most prominent of which is 1 km wide, strikes east-northeast, and accommodated dextral oblique motion. Emplacement of widespread intermediate to mafic dikes ca. 59–51 Ma occurred before a notable magmatic lull from ca. 51–44 Ma reflecting a late Paleocene to early Eocene slab window. Magmatism resumed ca. 44 Ma, recording the transition from slab window to renewed subduction that formed the Aleutian-Meshik arc to the southwest. In the western Alaska Range, Eocene magmatism included emplacement of the elongate north-south Merrill Pass pluton and large volumes of ca. 44–37 Ma andesitic flows, tuffs, and lahar deposits. Finally, a latest Eocene to Oligocene magmatic pulse involved emplacement of a compositionally variable but spatially concentrated suite of magmas ranging from gabbro to peralkaline granite ca. 35–26 Ma, followed by waning magmatism that coincided with initiation of Yakutat shallow-slab subduction. Cretaceous to Oligocene magmatism throughout the western Alaska Range collectively records terrane accretion, translation, and integration together with evolving subduction dynamics that have shaped the southern Alaska margin since the middle Mesozoic.

The Ross orogenic belt in Antarctica is one of several Neoproterozoic-early Palaeozoic orogens that crisscrossed Gondwana and are associated with Gondwana's assembly. We present new age data from the Queen Maud Mountains, Ross orogen, from areas that hitherto have lacked precise ages from the local plutonic rocks. The zircon U-Pb igneous crystallization ages (n = 7) and a hornblende 40Ar/39Ar cooling age (n = 1) constrain plutonism to primarily lie within the Cambrian to Ordovician. Cumulative zircon U-Pb crystallization age data yield polymodal age distributions (516 Ma, 506–502 Ma, and 488 Ma age peaks) that are similar to other areas of the Queen Maud-Horlick Mountains, consistent with regional magmatic flare-ups along the Pacific-Gondwana margin during these times. The ages of deformed plutons constrain deformation to the Cambrian (Series 2) to Ordovician (Lower), with some regions indicating a transition to post-tectonic magmatism and cooling at ~509-470 Ma. Collectively, the data indicate that the Queen Maud-Horlick Mountains share a similar petrotectonic history with other regions of the Pacific-Gondwana margin, providing new evidence that this tectonostratigraphic province is part of and not exotic to the larger igneous-sedimentary successions developed in the peri-Gondwana realm under a broadly convergent margin setting.

High-grade epithermal Au–Ag veins of the Vodorazdelnaya district in far eastern Russia are hosted by Early Cretaceous volcanic rocks of the Tytylveem belt. The largest deposit is Zone 37, a 1-km long, northeast-striking vein that is up to 35-m wide, containing at least 32 t Au. The main shoot consists of crustiform—colloform banded quartz-chalcedony-adularia-sulfide veins, of low-sulfidation style. The vein is cut by late-mineral rhyolite sills and dikes. Zone 37 vein is dated at 117 ± 2 Ma, overlapping within error with the cross-cutting rhyolites (121–119 Ma) and with intrusions of the nearby Ilirney granitic pluton (119–117 Ma). The volcanic host rocks are dated at 121–115 Ma. At September Northeast, 15 km west-northwest of Zone 37, mineralisation consists of veins up to 4 m wide, with crustiform banded quartz ± chalcedony ± adularia ± chlorite, and Zn-Pb-Cu sulfides, of intermediate sulfidation style, in a 1-km-long, north-northeast striking trend. High grades are associated with gold dendrites in fine-grained quartz bands and with minor late-stage tellurides. The veins were disrupted by intrusion of rhyolite dikes and associated phreatic breccias. Some breccias host mineralisation as vein clasts and minor matrix sulfides. The Tytylveem belt is preserved below a regional unconformity that forms the base to the overlying Late Cretaceous (106–77 Ma) Okhotsk-Chukotka Volcanic Belt (OCVB). Intrusive rocks attributed to OCVB magmatism were emplaced at about 96 Ma and are associated with overprinting hydrothermal alteration, sub-economic veins and thermal resetting of some vein adularia ages at Zone 37, to 91–97 Ma.

The Late Devonian Corner Bay pluton is a part of the Sitkoh alkaline plutonic complex on the Chichagof Island, southeastern Alaska, which is located in the Alexander terrane of the North American Cordillera. The intrusion, about 12 km long and 4 km wide, is composed predominantly of amphibole-biotite-bearing syenitic/monzonitic rocks with minor alkaline gabbros and has a U-Pb zircon age of similar to 367 Ma interpreted as the crystallization age of the pluton. The rocks are metaluminous and have silica contents ranging from 47 to 63 wt% with a silica gap at 52-58 wt% and have high contents of alkalis, typical of shoshonitic rocks. They are rich in large-ion-lithophile elements (including Rb, Sr, Ba, U) and their isotopic composition is characterized by relatively uniform epsilon(Nd)(t) values (+3.4 to +3.7), Neoproterozoic Nd depleted mantle model ages (650-700 Ma) and initial Sr-87/Sr-86 ratios (similar to 0.704). The rocks were generated by fractional crystallization of alkali basaltic magma formed by partial melting of an amphibole-phlogopite-bearing peridotite of the sub-arc lithospheric mantle. The mantle-source underwent a Neoproterozoic metasomatic enrichment event and does not show a contribution from juvenile Devonian mantle indicating that rifting associated with the magma emplacement was of limited extent as it did not replace the old lithospheric mantle. Several geothermobarometers document emplacement of the parental magma at mid-crustal levels (8.5-10 kb) and polybaric crystallization from similar to 1200 degrees C to solidus. Crystallization of amphibole took place at similar to 880-980 degrees C and 2.4-5.5 kb. Subsequently the rocks underwent recrystallization (feld-spars: 170 Ma-40Ar/39Ar age) and re-equilibration (biotite: 129 Ma (40Ar/39Ar) at similar to 600-650 degrees C and 2 kb). Zircon, rutile and apatite saturation temperatures provide consistent results (similar to 800 degrees C). The intrusion is a part of a metallogenic province of rare metals, which stretched across the islands in the southeastern most part of Alaska. (C) 2021 Elsevier B.V. All rights reserved.

We identify two piercing point pairs along a ~500 km transect of the arcuate strike‐slip Denali fault to document long‐term slip partitioning. Geochemical and isotopic similarity between Foraker and Panorama‐Schist Creek‐Nenana Plutons suggest ~155 km of right‐lateral displacement on the western Denali fault since 37 Ma at a rate of ~4.2 mm/year. The eastern Denali fault Maclaren‐Cottonwood Terrane geochronology correlation establishes ~305 km of displacement on the eastern Denali fault since 33 Ma at a rate of ~9.2 mm/year. The ratio of Pleistocene‐Holocene slip rates between the western (5.3 mm/year) and eastern (12.9 mm/year) Denali fault is 0.41 and our new constraints yield a Late Eocene‐Holocene ratio of 0.46. Hence, we interpret that the overall arcuate geometry of the Denali fault master strand was established by 33 Ma. We infer that the persistent long‐wave geometric stability of the Denali fault and other highly slip partitioned fault systems are related to long‐term highly oblique transpressive environs.

The Paleocene-Oligocene Kootznahoo Formation in SE Alaska provides a glimpse of sedimentation and transform tectonics during and after the transition from subduction to a transform margin. The Kootznahoo Formation consists of distinctive fluvial deposits of cross-bedded sandstone, gravel and conglomerate up to several kilometers thick, and lies 10-80 km west of the Coast Mountains. Paleocurrent indicators consistently show paleoflow to the SW, indicating the sediment was mainly derived from the high topography of the Coast Mountains, consistent with provenance work. There are about a dozen small depocenters and two larger depocenters, which we refer to as the Angoon and Kake basins. These two basins (1) have strata that dip moderately to the SE, (2) are equidimensional and about 10-15 km across, and (3) have late Paleocene to late Oligocene strata, although the Kake basin is dominantly Paleocene and the Angoon basin is dominantly Oligocene. These two basins are asymmetric, with a wedge-shaped geometry that thickens to the SE, consistent with being ‘trap door’ or stepover basins within a nascent right-lateral fault system. We infer NW-striking dextral faults defining the basin margins, with one or more NE-striking normal faults connecting them in the stepover region and creating accommodation space for deposition. The NW-striking dextral faults are truncated by the Chatham Strait fault, and thus these faults were active prior to Neogene activity on the Chatham Strait – eastern Denali faults. Subsequent late Oligocene-Miocene volcanism and magmatism of the Tkope-Portland Peninsula belt were focused along the inferred basin-bounding dextral faults. Kootznahoo deposition began before the end of Coast Mountains batholith magmatism at 55 Ma, but was primarily after ~50 Ma when SE Alaska became a transform margin. Some Kootznahoo deposition and associated faulting was coeval with deposition in basins along the Denali fault in the Yukon and central interior Alaska. Cessation of Kootznahoo deposition is coincident with the rise of the Alaska Range and the collision of the Yakutat terrane into southern Alaska. We infer a large-scale reorientation of crustal stresses related to Yakutat collision, and dextral faulting focused on the modern Queen Charlotte-Fairweather fault resulted in cessation of Kootznahoo deposition.

The Corner Bay pluton is part of the Sitkoh alkaline complex that locally has high rare earth element (REE) contents on Chichagof Island in southeast Alaska. The alkaline complex is part of the Alexander terrane, which is composed in part of Neoproterozoic and Paleozoic metavolcanic and metasedimentary rocks that have previously documented oceanic arc geochemical and isotopic signatures. Late Silurian-Early Devonian juxtaposition of oceanic arcs resulted in an orogeny with associated metamorphism, calcalkaline plutons, pillow basalt, conglomerate, and redbeds. This orogenic belt hosts Upper Devonian alkaline basalt and rhyolite dike swarms, flows, and the Sitkoh alkaline complex. The Corner Bay pluton is composed of amphibole-biotite metaluminous syenite, monzonite and minor gabbro, and has a ~367 Ma U-Pb zircon age. These rocks have 46 to 63 weight percent silica, and high alkali and large-ion-lithophile element (Rb, Sr, Ba, U) contents. Main accessory minerals include apatite, fluorapatite, titanite and allanite that contain elevated concentrations of REE. The rocks have relatively uniform ƐNd(t) values (+3.4 to +3.6), Neoproterozoic Nd model ages of 650-700 Ma, and initial 87Sr/86Sr ratios of ~0.704. We interpret generation of the intrusive rocks by fractional crystallization of an alkali basaltic parent magma formed by partial melting of an amphibole-phlogopite-bearing peridotite of the sub-arc lithospheric mantle. Geothermobarometers document emplacement of the parental magma at mid-crustal levels (8.5-10 kb/31-37 km) and polybaric crystallization from ~ 1,200°C to solidus. Crystallization of amphibole took place at ~880-980°C and 2.4-5.5 kb under hydrous conditions. Zircon, rutile, and apatite provide consistent saturation temperatures at ~800°C. The chemical and isotopic composition of the Corner Bay pluton is consistent with emplacement in a rift. Nd isotopic data suggest the source of mafic rocks was Neoproterozoic sub-arc lithospheric mantle with no contributions from juvenile Devonian mantle, implying that rifting was of limited extent and sub-arc lithospheric mantle was unaffected by upwelling juvenile asthenosphere beneath the rift. The Late Devonian alkaline belt that contains the Corner Bay pluton is part of a metallogenic province of rare metals in the Alexander terrane.

The Acoculco caldera complex (ACC) is located in the eastern part of the Trans-Mexican Volcanic Belt in the northern part of the State of Puebla. The complex sits at the intersection of two regional fault systems with NESW and NW-SE orientations. The ACC was built atop Cretaceous limestones, the Zacatan basaltic plateau of unknown age, early Miocene domes (similar to 12.7-10.98 Ma), and Pliocene lava domes (similar to 3.9-3 Ma). Detailed field mapping and stratigraphy studies complemented by 40Ar/39Ar and C-14 dating allowed the division of the ACC volcanic succession into 30 volcanic units. Based on the new results and previous studies, the ACC eruptive chronology was grouped in four eruptive phases: syn-caldera, early post-caldera, late post-caldera, and extracaldera. Inception of the ACC volcanism began around 2.7 Ma with the dispersion of an andesitic ignimbrite followed by the collapse of the magma chamber roof as attested by the presence of a lithic breccia in isolated parts of the caldera rim. The collapse produced a 18 x 16 km caldera depression which was partly filled by the ignimbrite (total volume of similar to 127 km(3)) followed by the establishment of an intracaldera lake of unknown total extension. Early post-caldera collapse activity (2.6-2.1 Ma) was restricted within the caldera producing 27 km(3) of lava flows and domes dominantly of basaltic trachyandesite to basaltic composition. Late post-caldera collapse activity (2.0- < 0.016 Ma) migrated dominantly to the caldera rim and periphery emplacing 90 km(3) of magma as rhyolitic domes, lava flows, scoria cones, and two younger ignimbrites. The 1.2 Ma Encimadas ignimbrite (26 km(3)) was vented through the eastern margin of the caldera and dispersed to the northeast, and the 0.6-0.8 Ma Tecoloquillo ignimbrite and dome (11 km(3)) erupted from the southwestern margin of the caldera. The most recent eruption of this phase was vented close to the southeastern caldera rim producing the Cuatzitzingo (< 16,710 +/- 50 years BP) scoria cone. Extra-caldera activity (2.4-0.19 Ma) of the Apan-Tezontepec volcanic field produced scoria cones and lava flows of basaltic trachyandesite to basaltic andesite composition that are interbedded with the products of the caldera complex. Aeromagnetic data further constrains the edge of the caldera rim and is consistent with the presence of at least four intrusive bodies at depths of > 1 km hosted in the Cretaceous limestones. These bodies might represent a series of horizontal mafic intrusions located at different depths that provide the energy that maintains the Acoculco geothermal system active. The Acoculco caldera complex (ACC) is located in the eastern part of the Trans-Mexican Volcanic Belt in the northern part of the State of Puebla. The complex sits at the intersection of two regional fault systems with NESW and NW-SE orientations. The ACC was built atop Cretaceous limestones, the Zacatan basaltic plateau of unknown age, early Miocene domes (similar to 12.7-10.98 Ma), and Pliocene lava domes (similar to 3.9-3 Ma). Detailed field mapping and stratigraphy studies complemented by 40Ar/39Ar and C-14 dating allowed the division of the ACC volcanic succession into 30 volcanic units. Based on the new results and previous studies, the ACC eruptive chronology was grouped in four eruptive phases: syn-caldera, early post-caldera, late post-caldera, and extracaldera. Inception of the ACC volcanism began around 2.7 Ma with the dispersion of an andesitic ignimbrite followed by the collapse of the magma chamber roof as attested by the presence of a lithic breccia in isolated parts of the caldera rim. The collapse produced a 18 x 16 km caldera depression which was partly filled by the ignimbrite (total volume of similar to 127 km(3)) followed by the establishment of an intracaldera lake of unknown total extension. Early post-caldera collapse activity (2.6-2.1 Ma) was restricted within the caldera producing 27 km(3) of lava flows and domes dominantly of basaltic trachyandesite to basaltic composition. Late post-caldera collapse activity (2.0- < 0.016 Ma) migrated dominantly to the caldera rim and periphery emplacing 90 km(3) of magma as rhyolitic domes, lava flows, scoria cones, and two younger ignimbrites. The 1.2 Ma Encimadas ignimbrite (26 km(3)) was vented through the eastern margin of the caldera and dispersed to the northeast, and the 0.6-0.8 Ma Tecoloquillo ignimbrite and dome (11 km(3)) erupted from the southwestern margin of the caldera. The most recent eruption of this phase was vented close to the southeastern caldera rim producing the Cuatzitzingo (< 16,710 +/- 50 years BP) scoria cone. Extra-caldera activity (2.4-0.19 Ma) of the Apan-Tezontepec volcanic field produced scoria cones and lava flows of basaltic trachyandesite to basaltic andesite composition that are interbedded with the products of the caldera complex. Aeromagnetic data further constrains the edge of the caldera rim and is consistent with the presence of at least four intrusive bodies at depths of > 1 km hosted in the Cretaceous limestones. These bodies might represent a series of horizontal mafic intrusions located at different depths that provide the energy that maintains the Acoculco geothermal system active.

The dunite–wehrlite–clinopyroxenite–gabbro massif in Eastern Chukotka, a key object for geodynamic reconstructions of the Vel’may terrane, which represents one of the segments of the southern border of the Chukotka folded system (Chukotka microcontinent, or Arctic Alaska–Chukotka microplate), is investigated. Mineralogical and petrological–geochemical studies of rocks of this massif are carried out. A comparative analysis of the primary mineralogy and formation conditions of cumulative rocks of dunite–wehrlite–pyroxenite–gabbro assemblages from modern island-arc systems, mantle transition zones, and crustal sections of ophiolites and ancient island arcs shows that the rocks studied are cumulates crystallized from a tholeiitic melt in an intraoceanic island arc at a moderately high pressure. The 40 Ar/ 39 Ar age of magnesian hornblende from gabbro indicates the massif was formed no later than in the Early–Middle Jurassic. The petrological and geochemical modeling suggests that the analyzed olivine clinopyroxenites and gabbros are probable plutonic comagmates of the Late Triassic island-arc basalts and dolerites of the Vel’may terrane. The arc segment represented by these rocks of the Vel’may terrane was probably part of the system of island arcs which had been reconstructed in this region for the age interval of 163 to 230 Ma. In addition, there is a tendency for the rejuvenation of the Middle Triassic–Late Jurassic island-arc magmatism in the direction from west to east, namely, from the South Anyui terrane of Western Chukotka through the Vel’may terrane of Eastern Chukotka to the Angayucham terrain of Alaska.

Interactions between volcanic and tectonic processes affect the distribution, morphology, and volume of eruptive products in space and time. The Querendaro area in the eastern Michoacan-Guanajuato Volcanic Field affords an exceptional opportunity to understand these relationships. Here, a Pleistocene lava plateau and 20 monogenetic volcanoes are vented from an active ENE-striking segment of the Morelia-Acambay fault system. Thirteen scoria cones are aligned along this structure, vented from an extensional gap in between two rotated hanging wall blocks of a listric fault. A new geological map, volcanic stratigraphy, and 40Ar/39Ar dating indicate that this lava plateau and volcanic cluster were emplaced from 0.81 to 0.25 Ma by 11 intermittent eruptive epochs separated by ca. 0.05 Ma, emplacing a total magma volume of 5 km(3). Petrography and chemistry of rocks suggest that all volcanic structures were fed by three different magma batches but vented from independent feeder dikes. Our results indicate that preexisting faults exert a strong influence on volcanic spatial and temporal distribution, volcanic morphology, magma volume, and eruptive dynamics in this area. ENE-breached and ENE-elongated scoria cones indicate parallel subsurface fissure and feeder dikes. Additionally, points of maximum fault dilation at depth related to a transtensive state of stress coincide with less fragmented deposits and larger magma volumes. Furthermore, this study raises important questions on the geodynamics of volcano-tectonic interactions possible in similar monogenetic volcanic alignments worldwide.

We review the current knowledge of the Pleistocene Modern Chiapanecan Volcanic Arc (MCVA). This arc is related to the subduction of the Cocos plate beneath the North American plate in the State of Chiapas, southeastern Mexico. The MCVA consists of large intrusive bodies, domes, eroded volcanic landforms, and the active El Chichón, which produced the disastrous 1982 eruption, the deadliest in Mexico’s recorded history. The available geological knowledge, and new geological and aeromagnetic data on the arc, reveals a system composed of a sizeable intrusive body called the Santa Fe diorite, and small-size volcanoes such as El Chichón and Catedral, and extinct volcanoes associated with volcaniclastic deposits. A 3D-inversion of the aeromagnetic anomalies indicates that the Santa Fe diorite is a large intrusive body (27 km long, 4 km wide with a minimum volume of 1662 km3) while small volcanoes such as El Chichón have small-size magma chambers (~ 7 km3). Interestingly, our models of the causative bodies for the aeromagnetic anomalies suggest that the El Chichón volcano, as well as of other volcanic areas in the region, are not linked directly to the Santa Fe intrusive. However, new 40Ar/39Ar dates for samples from the Santa Fe intrusive (2.2 Ma), the Catedral volcano (1.6 Ma), and a mafic enclave (1.09 Ma) hosted in 1982 Chichón deposits, along with the aeromagnetic anomalies and geochemical data confirm that these extrusive and intrusive structures belong to the MCVA. The chemistry of these structures suggests that magmas generated in the upper mantle by the subduction system evolved through different processes, such as crustal contamination for the Santa Fe diorite and Catedral volcano, and crystal fractionation for El Chichón volcano.