Prebatholithic metasedimentary rocks east of El Marmol on the 30th parallel of Baja California include tightly folded Permian and Lower Triassic rocks metamorphosed to upper greenschist/amphibolite facies. These rocks are divided into one informal and three formal formations. The rocks of El Marmol (informal formation) have a minimum thickness of 2,000 m and consists of thin-bedded argillite, sandstone, and chert with sparsely fossiliferous carbonate rock interstratified with more thickly bedded lenses of calcareous quartzarenite, impure carbonate rock, and chert-quartzite-carbonate clast conglomerate. The rocks appear to have been variously deposited by sediment gravity flows and intervening quiet-water deposition in a hemipelagic setting. The rocks of El Marmol contains no datable fossil material but are Lower Permian or older. The Arroyo Zamora Formation has an estimated thickness of 500 m, conformably overlies the rocks of El Marmol, and is composed primarily of massive or crudely laminated silty metaargillite that locally exhibits fine-grained turbidite successions and pebbly mudstone. Slumps, boudined sheets, and channelized sediment, gravity-flow deposits composed of coarser carbonate and quartzose clasts, including carbonate blocks up to several meters, are scattered throughout. Fossils in both the matrix and clasts of these coarse-grained deposits include Lower Permian (Leonardian) fusulinids, bryozoa, brachiopods, and crinoid columnals. The Cerro El Volcan Formation has an estimated thickness of 1,400 m and is divided into three members. The lowermost member (A) consists of 1,000 m of rhythmically bedded, dark siliceous argillite that contains intervals of metamorphosed massive calcareous sandstone, sandy limestone, and thinly bedded carbonate rock with siliceous partings. The overlying member (B) consists of 50 m of metamorphosed conglomerate, pebbly quartzite, and calcareous sandstone. Angular blocks throughout this member commonly contain bioclastic debris. The uppermost member (C) consists of 350 m of laminated argillite containing widely spaced interbeds of metamorphosed argillaceous carbonate rock. It is considered Late Permian in age because it lies conformably on rocks of Early Permian age, and is overlain with possible disconformity by the Lower Triassic De Indio Formation. The De Indio Formation has an estimated thickness of 300 m and is divided into two members. The lower member (A) contains up to 50 m of metamorphosed chert-pebble conglomerate, quartzite, cross-bedded calcareous sandstone, and limestone. The upper member (B) consists of 250 m of carbonaceous, staurolite-bearing argillite. Ammonoids and conodonts from member A are of Early Triassic (Smithian) age. These four formations have lithologies and faunal assemblages similar to coeval rocks deposited along the outer edge of the Cordilleran miogeocline of western North America, and were probably deposited in a southern extension of this miogeocline.

ABSTRACT Upper Triassic and Lower to Middle Jurassic strata in the Plomosas uplift of central Chihuahua accumulated in backarc and rift settings, respectively. The succession, as much as ~3250 m thick, consists of four stratigraphic units. The Cerro El Carrizalillo Formation (Carnian–Norian), a volcanic-lithic shallow-marine succession deposited in the (newly named) El Carrizalillo backarc basin, is characterized by predominantly Triassic detrital zircon ages. The overlying Plomosas Formation consists of three members: (1) the Cerro de Enmedio Member (Hettangian–Toarcian), a succession of conglomerate, siltstone, and shallow-marine carbonate strata deposited during the onset of extension in Chihuahua; (2) the Cerro Nevado Ignimbrite Member (176 ± 1 Ma; late Toarcian), a widespread ash-flow tuff; and (3) La Sofía Member (Aalenian–Callovian?), consisting of alluvial-fan conglomerate, fluvial sandstone, tidal sandstone and siltstone, and delta-plain red beds characterized by rapid facies changes, lithic compositions, and diverse Proterozoic, Paleozoic, and Triassic detrital zircon ages characteristic of a rift-basin setting. The extensional basin in which the Cerro de Enmedio and La Sofía members accumulated is termed the Plomosas basin. Improved age control provided by U-Pb maximum depositional ages from detrital zircon and U-Pb zircon analyses of the ignimbrite indicates that the Cerro El Carrizalillo Formation is partly correlative with the Chinle Formation of the Colorado Plateau, and the Plomosas Formation is equivalent to eolianites of the Glen Canyon and San Rafael Groups of the Colorado Plateau. Detrital zircon ages and sandstone textures are consistent with both proximal and distal sediment sources along the Laurentia-Gondwana suture and adjoining Grenville basement of Laurentia, including sources in northern Mexico and the composite Appalachian orogen. Although the depositional setting of the Cerro El Carrizalillo Formation was not connected to fluvial systems of the Chinle Formation, subsequent eolian transport of voluminous sediment to the overlying Cerro de Enmedio and La Sofía members from the Colorado Plateau ergs is suggested by the composition and texture of some sandstone, thick siltstone accumulations, and detrital zircon characteristics that broadly resemble those of the Colorado Plateau eolianites. Thick siltstone in the upper part of La Sofía Member is interpreted as deflated fine-grained sediment that was transported downwind from a time-equivalent erg to accumulate in shallow-marine and coastal-plain settings of the Plomosas basin.

We used tephrochronology for upper Neogene deposits in the Española Basin and the adjoining Jemez Mountains volcanic field in the Rio Grande rift, northern New Mexico, to correlate key tephra strata in the study area, identify the sources for many of these tephra, and refine the maximum age of an important stratigraphic unit. Electron-microprobe analyses on volcanic glass separated from 146 pumice-fall, ash-fall, and ash-flow tephra units and layers show that they are mainly rhyolites and dacites. Jemez Mountains tephra units range in age from Miocene to Quaternary. From oldest to youngest these are: (1) the Canovas Canyon Rhyolite and the Paliza Canyon Formation of the lower Keres Group (ca. <12.4–7.4 Ma); (2) the Peralta Tuff Member of the Bearhead Rhyolite of the upper Keres Group (ca. 6.96–6.76 Ma); (3) Puye Formation tephra layers (ca. 5.3–1.75 Ma); (4) the informal San Diego Canyon ignimbrites (ca. 1.87–1.84 Ma); (5) the Otowi Member of the Bandelier Tuff, including the basal Guaje Pumice Bed (both ca. 1.68–1.61 Ma); (6) the Cerro Toledo Rhyolite (ca. 1.59–1.22 Ma); (7) the Tshirege Member of the Bandelier Tuff, including the basal Tsankawi Pumice Bed (both ca. 1.25–1.21 Ma); and (8) the El Cajete Member of the Valles Rhyolite (ca. 60–50 ka). The Paliza Canyon volcaniclastic rocks are chemically variable; they range in composition from dacite to dacitic andesite and differ in chemical composition from the younger units. The Bearhead Rhyolite is highly evolved and can be readily distinguished from the younger units. Tuffs in the Puye Formation are dacitic rather than rhyolitic in composition, and their glasses contain significantly higher Fe, Ca, Mg, and Ti, and lower contents of Si, Na, and K. We conclude that the Puye is entirely younger than the Bearhead Rhyolite and that its minimum age is ca. 1.75 Ma. The San Diego Canyon ignimbrites can be distinguished from all members of the overlying Bandelier Tuff on the basis of Fe and Ca. The Cerro Toledo tephra layers are readily distinguishable from the overlying and underlying units of the Bandelier Tuff primarily by lower Fe and Ca contents. The Tshirege and Otowi Members of the Bandelier Tuff are difficult to distinguish from each other on the basis of electron-microprobe analysis of the volcanic glass; the Tshirege Member contains on average more Fe than the Otowi Member. Tephra layers in the Española Basin that correlate to the Lava Creek B ash bed (ca. 640 ka) and the Nomlaki Tuff (Member of the Tuscan and Tehama Formations, ca. 3.3 Ma) indicate how far tephra from these eruptions traveled (the Yellowstone caldera of northwestern Wyoming and the southern Cascade Range of northern California, respectively). Tephra layers of Miocene age (16–10 Ma) sampled from the Tesuque Formation of the Santa Fe Group in the Española Basin correlate to sources associated with the southern Nevada volcanic field (Timber Mountain, Black Mountain, and Oasis Valley calderas) and the Snake River Plain–Yellowstone hot spot track in Idaho and northwestern Wyoming. Correlations of these tephra layers across the Santa Clara fault provide timelines through various stratigraphic sections despite differences in stratigraphy and lithology. We use tephra correlations to constrain the age of the base of the Ojo Caliente Sandstone Member of the Tesuque Formation to 13.5–13.3 Ma.

Abstract Volcanic plumbing systems emplaced in sedimentary basins may exert significant mechanical and thermal effects on petroleum systems. The last decade of research has evidenced that igneous intrusions may enhance thermal maturation of organic matter in source rocks and lead to both small- and large-scale structures that can deeply impact fluid migration or trapping. This contribution describes how the emplacement of a whole intrusive complex generated a dome structure of the overburden, which is the main trapping structure of a large producing oilfield. Our case study is the lower Miocene Cerro Bayo de la Sierra Negra (CBSN) intrusive complex, Neuquén Basin, Argentina, associated with the El Trapial oilfield where the main trapping structure is a large domal antiform centred on the CBSN complex. This study integrates the large subsurface dataset produced during the development of the El Trapial oilfield. More than 1200 vertical wells (producers and injectors) have been drilled in the flanks of CBSN complex. In addition, five 3D seismic cubes have been acquired over the years that have been merged and reprocessed into a single volume. Such a dataset allows a detailed characterization of both the structure affecting the Mesozoic strata and the geometry of the intrusive complex. Igneous rocks have been recognized along the entire stratigraphic section. Sill intrusions appear to concentrate in the shale units and the stacking of them has a direct impact on the doming structure generation. Our study allowed us to establish a direct correlation between the distribution of the intrusions and the extent, amplitude and style of doming, showing that the dome structure results from the emplacement of the intrusive complex. We also show that part of the doming is related to intrusions emplaced in the Mesozoic formations of the Neuquén Basin, whereas the other part of the doming is related to deeper structures not imaged on the geophysical data. We estimate that the amplitude of the doming reaches up to c. 500 m. The voluminous subsurface data, combined with exposed outcrops, makes the CBSN complex a world-class case study for showing how the shallow plumbing system of a volcanic complex may control the growth of large-scale trapping structures for various fluids, such as drinkable water, geothermal fluids and hydrocarbons.

Abstract The distribution and chemistry of late Oligocene to Recent central Andean magmatic rocks and mineral deposits between 22° and 33° S latitude reflect changes in the dip of the subducting Nazca plate and the thickness of the overlying lithospheric mantle and crust. Correlations of major magmatic and tectonic events at ca. 18 to 16 Ma, ca. 10 Ma, ca. 7 to 5 Ma, and ca. 2 Ma with previously proposed Andean-wide pulses support external causes for major events and regional geometric control on local style. Evolving magmatic and tectonic patterns indicate that the slab has shallowed beneath the modern Chilean flat-slab region (28° to 33° S), steepened beneath the modern northern Puna plateau (ca. 25° and 22° S), and remained in a transitional state beneath the intervening modern southern Puna. Shallowing in the Chilean flat-slab region is indicated by eastward migration of subduction-related magmatism and deformation, termination of main-arc andesitic volcanism by ca. 10 Ma, and the virtual cessation of volcanic activity by ca. 5 Ma. Shallowing was accompanied by crustal thickening, lithospheric thinning and hydration, and substantial loss of the asthenospheric wedge. Steepening of the slab below the northern Puna is indicated by widespread deformation and basin formation associated with virtual volcanic quiescence in the late Oligocene to middle Miocene, followed by westward contraction of the middle Miocene to Recent volcanic arc. A westward-shifting focus of giant late Miocene to Pliocene ignimbritic eruptions reflects massive melting caused by the introduction of a thickening asthenospheric wedge above a steepening subduction zone and below a thinned hydrated lithosphere. A contemporaneous eastward shift in the major zone of thrusting to the sub-Andean belt can be explained by compressional collapse of the hot, ductile crust beneath the plateau. Lithospheric thickening accompanied deformation above the steepening slab. A persistent, intermediate dip of the slab beneath the intervening southern Puna is supported by the lack of a volcanic gap, and by a transitional, magmatic, and tectonic history compared to that of the north and south. Extreme crustal thickening over the intermediately dipping slab resulted in instabilities in eclogitic lower crust that led to Pliocene continental lithospheric foundering (delamination). Evidence for delamination comes from Pliocene to Recent eruptions of the Cerro Galán ignimbritic center, a concentration of primitive mafic lavas associated with normal and strike-slip faults, high average regional elevation, and seismic evidence for a thin underlying lithosphere and an abnormally hot subducting slab. Temporal variations in mantle-derived mafic magma chemistry indicate Neogene mantle enrichment by introduction of crustal material during the subduction process. Within this framework, major central Andean Neogene Au and Cu deposits in the greater El Teniente (ca. 32°–34° S), greater El Indio (ca. 29°–31 ° S), and Maricunga (26°–28°) belts formed as crustal thicknesses reached 45 to 50 km over the shallowing and cooling subduction zone. The general southward younging of these deposits reflects a southward pattern of crustal thickening. Emplacement of the deposits took place in the waning stages of arc volcanism as the arc front migrated eastward or extinguished. Mineralization occurred as geochemically inferred, hydrous, hornblende-based, residual mineral assemblages that were in equilibrium with erupted magmas dehydrated to yield high-pressure, garnet-bearing assemblages.

ABSTRACT The western Cerros El Amol near Altar, northwest Sonora, Mexico, is underlain by a 5- km-thick clastic sedimentary sequence, of which, the upper part has been metamorphosed into greenschist facies. The stratigraphic column includes, in ascending order, the Bisbee Group, El Chanate Group and the Altar Formation. The first two groups are late Early and Late Cretaceous siliciclastic sequences, respectively, that represent the most northwestern deposition in the Bisbee Basin of Sonora. Several fining upward cycles with intercalated fossiliferous carbonate strata suggest deposition in a back-arc basin that experienced several transgression and regression cycles. A few volcanic and volcaniclastic horizons are supposedly related to the mid-Cretaceous activity of the Alisitos magmatic arc. The coarsening-upward cycles, the geometric pattern of conglomeratic wedges, their thicknesses and abundance in the Altar Formation suggest deposition at the foot of a range or a topographic prominence. A weak metamorphism is present in all units, but it is pervasive in the stratigraphically higher units which show a greenschist facies. A low-angle fault juxtaposes part of the Bisbee Group on El Amol member of the Altar Formation.

Abstract The Miocene Cabo de Gata volcanic arc in SE Spain comprises a wide variety of volcanic facies and eruptive styles in subaqueous to subaerial environments. In the SW sector of the area, 5–100 m-thick, NNW–SSE-orientated dykes feed and intrude submarine hyaloclastite deposits. We analysed the anisotropy of magnetic susceptibility (AMS) of six dykes and five hyaloclastite sites from three volcanic units: the Cerro Cañadillas, Los Frailes, and El Barronal formations. The main magnetic minerals are primary low-Ti titanomagnetite and magnetite. The AMS ellipsoids in the dykes are generally oblate-triaxial in shape, with magnetic foliations subparallel to the dyke walls. Kinematic field evidence supports the inferred flow directions deduced from magnetic lineation and imbrication of magnetic foliation. The geometric relationships between dyke margins and AMS axes indicate that dykes at El Barronal were emplaced via prevalent subvertical upward magma flow. The inferred flow directions are reproduced well by analogue models of AMS simulating magma migration in dykes with a diapiric geometry. The other dykes were emplaced by lateral magma propagation. Conversely, hyaloclastite shows a large scatter of the AMS axes reflecting different degrees of fragmentation. We observe a gradual increase in scatter in the AMS from confined dykes to fragmented hyaloclastite.

Abstract Porphyry deposits arguably represent the most economically important class of nonferrous metallic mineral resources. These magmatic-hydrothermal deposits are characterized by sulfide and oxide ore minerals in vein-lets and disseminations in large volumes of hydrothermally altered rock (up to 4 km 3). Porphyry deposits occur within magmatic belts worldwide and are spatially, temporally, and genetically related to hypabyssal dioritic to granitic intrusions that are porphyritic and that commonly have an aplitic groundmass. The preponderance are Phanerozoic and most typically Cenozoic in age, which reflects the dominance of magmatism related to subduction tectonics and preservation in young rocks. Porphyry deposits are here grouped into five classes based on the economically dominant metal in the deposits: Au, Cu, Mo, W, and Sn. For each porphyry class, the major metal concentration is enriched by a factor of 100 to 1,000 relative to unmineralized rocks of a similar composition. The mass of porphyry deposits ranges over four orders of magnitude, with the mean size of a deposit ordered Cu > Mo ~ Au > Sn > W. Hydrothermal alteration is a guide to ore because it produces a series of mineral assemblages both within the ore zones and extending into a larger volume (>10 km 3) of adjacent rock. The typically observed temporal evolution in porphyry ores is from early, high-temperature biotite ± K-feldspar assemblages (potassic alteration) to muscovite ± chlorite assemblages (sericitic alteration) to low-temperature, clay-bearing assemblages (advanced argillic and intermediate argillic alteration), which is consistent with progressively greater acidity and higher fluid-to-rock ratios of fluids, prior to their eventual neutralization. Although advanced argillic alteration is relatively late in the deposits where it is superimposed on ore and potassic alteration, in the deposits where advanced argillic alteration (especially as quartz + alunite) is preserved spatially above ore and commonly extending to the paleosurface, it can form early, broadly contemporaneous with potassic alteration. In contrast, assemblages of Na plagioclase-actinolite (sodic-calcic alteration) and albite-epidote-chlorite-carbonate (propy-litic alteration) form from a fluid with low acidity and commonly lack ore minerals. Geologic, fluid inclusion, and isotopic tracer evidence indicate magmatic fluids dominate acidic alteration associated with ore and non-magmatic fluids dominate sodiccalcic and propylitic alteration. Veins contain a large percentage of ore minerals in porphyry deposits and include high-temperature sugary-textured quartz veinlets associated with ore minerals and biotitefeldspar alteration and moderate-temperature pyritic veins with sericitic envelopes. The compositions of igneous rocks related to porphyry deposits cover virtually the entire range observed forpresentday volcanic rocks. Mineralizing porphyries are intermediate to silicic (>56 wt % SiO 2) and their aplitic-textured groundmass represents crystallization as a result of abrupt depressurization of water rich magma; however, small volumes of ultramafic to intermediate rocks, including lamprophyres, exhibit a close spatial and temporal relationship to porphyry ore formation in some deposits. The understanding of porphyry systems depends critically on determination of the relative ages of events and correlation of ages of events in different locations, which in part depends on exposure. Systems with the greatest degree and continuity of exposure generally have been tilted and dismembered by postmineralization deformation. Most porphyry intrusions associated with ore are small-volume (<0.5 km 3) dikes and plugs that were emplaced at depths of 1 to 6 km, though some were emplaced deeper. Deposits commonly occur in clusters above one or more cupolas on the roof of an underlying intermediate to silicic intrusion. Altered rocks extend upward toward the paleosurface, downward into the granitoid intrusion from which the porphyry magma and aqueous fluids were generated, and laterally for several kilometers on either side of a deposit. The underlying magma chambers operated as open systems via mafic magma recharge, wall-rock assimilation, crystallization, and intrusion, but mineralizing intrusions did not erupt. Present-day distributions of hydrothermally altered rock and sulfide-oxide ore minerals are time-integrated products of fracture-guided fluid flow. We distinguish three spatial configurations characteristic of all five classes of porphyry deposits, the first of which has two variants: (1a) sericitic alteration largely lies above and beside potassic alteration in a bell- or hood-shaped volume that narrows upward, as at Chorolque, Henderson, and San Manuel-Kalamazoo; (1b) sericitic alteration is present with advanced argillic alteration, and the latter in some cases forms a broader zone at higher levels in the system, as at Batu Hijau, Cerro Rico, and El Salvador; (2) intense sericitic and local advanced argillic alteration cuts through enclosing potassic alteration near ore but also extends above potassic alteration in an upwardly expanding zone with an overall geometry of a funnel, as at Butte, Chuquicamata, and Resolution; (3) sodic-calcic, in addition to potassic, alteration is widespread in the center of the system and has an inverted cup-shaped volume under potassic alteration, with fingerlike projections of sodic alteration extending up through the overlying orebody, as at Yerington. Metal grades are directly related to where ore minerals originally precipitate and the degree of subsequent remobilization. Precipitation of metals is a function of multiple variables, typically including temperature, acidity, and iron and sulfide availability. Hence, the shape of an orebody depends on the number and positions of mineralizing versus barren intrusions; the proportions, shapes, and orientations of veins, lodes, or breccias; and pressure-temperature changes and wall-rock reactions that govern ore mineral stability. Geochronology and thermal models suggest that durations of hydrothermal activity of 50,000 to 500,000 yr are common, but several large porphyry Cu deposits include multiple events spanning several million years. Crosscutting relationships, including offset veins, provide definitive evidence for the relative ages of hydrothermal events at a particular spatial location. Intrusive contacts that cut off older veins and are in turn cut by younger veins provide time lines that permit correlation of spatially separated events. Most porphyry deposits exhibit multiple intrusions, each associated with a series of hydrothermal veins formed over a declining temperature interval. The high-temperature starting point of hydrothermal fluid compositions varies systematically between porphyry classes and must reflect magma composition and chemical partitioning between melt, mineral, and aqueous fluid. Although the data are sparse, the magmas and associated high-temperature ore fluids vary such that oxidation state, sulfidation state, and total sulfur content are highest for porphyry Cu and Au classes, slightly lower for Mo, lower yet for Sn, and lowest for W. Nearly all classes and subclasses, however, have examples that diverge to low a K+ / a H+ and high sulfur fugacity at lower temperature to produce advanced argillic alteration and high-sulfidation state ore minerals. Just as with the spectrum of global magmatism, the breadth of porphyry mineralization shares fundamental processes yet maintains distinctive geologic characteristics. In spite of a century of study and economic impact, many questions remain unanswered.