SEARCH · Geology Reports

Results for “Andean Geology”

Search indexed USGS publications on groundwater, aquifers, geologic maps, mineral resources and earthquakes. Explore source records by subject and place.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

36 records · Page 2Linked to original sources

USGS2009

Detrital zircon evidence for progressive underthrusting in Franciscan metagraywackes, west-central California

We present new U/Pb ages for detrital zircons separated from six quartzose metagraywackes collected from different Franciscan Complex imbricate nappes around San Francisco Bay. All six rocks contain a broad spread of Late Jurassic-Cretaceous grains originating from the Klamath-Sierra Nevada volcanic-plutonic arc. Units young structurally downward, consistent with models of progressive underplating and offscraping within a subduction complex. The youngest specimen is from the structurally lowest San Bruno Mountain sheet; at 52 Ma, it evidently was deposited during the Eocene. None of the other metagraywackes yielded zircon ages younger than 83 Ma. Zircons from both El Cerrito units are dominated by ca. 100-160 Ma grains; the upper El Cerrito also contains several grains in the 1200-1800 Ma interval. These samples are nearly identical to 97 Ma metasedimentary rock from the Hunters Point shear zone. Zircon ages from this m??lange block exhibit a broad distribution, ranging from 97 to 200 Ma, with only a single pre-Mesozoic age. The Albany Hill specimen has a distribution of pre-Mesozoic grains from 1300 to 1800 Ma, generally similar to that of the upper El Cerrito sheet; however, it contains zircons as young as 83 Ma, suggesting that it is significantly younger than the upper El Cerrito unit. The Skaggs Spring Schist is the oldest studied unit; its youngest analyzed grains were ca. 144 Ma, and it is the only investigated specimen to display a significant Paleozoic detrital component. Sedimentation and subduction-accretion of this tract of the trench complex took place along the continental margin during Early to early-Late Cretaceous time, and perhaps into Eocene time. Franciscan and Great Valley deposition attests to erosion of an Andean arc that was active over the entire span from ca. 145 to 80 Ma, with an associated accretionary prism built by progressive underthrusting. We use these new data to demonstrate that the eastern Franciscan Complex in the northern and central Coast Ranges is a classic accretionary prism, where younger, structurally lower allochthons are exposed on the west, and older, structurally higher allochthons occur to the east, in the heavily studied San Francisco Bay area. ?? 2009 Geological Society of America.

Geological Society of America Bulletin↗

USGS1992

An Appalachian isochron: A kaolinized Carboniferous air-fall volcanic-ash deposit (tonstein)

The Fire Clay tonstein is a kaolinized, airfall volcanic ash bed that was deposited in a widespread late Carboniferous peat-forming mire. Eleven samples from Kentucky and West Virginia, spanning a distance of 200 km, and two samples from Tennessee and Virginia indicate a characteristic mineralogical signature, as compared with other Appalachian tonsteins, consisting of well-crystallized kaolinite, beta-quartz crystal paramorphs, sanidine, ilmenite, zircon, and brookite. Detrital illite and quartz are rarely present or are in very small amounts, which indicates rapid deposition in a mire. Several normal graded cycles in this tonstein suggest repeated episodes of pyroclastic activity that produced a composite ash layer. A high-silica alkalic rhyolitic source is suggested by the geochemistry of immobile elements and by electron-probe analyses of glass inclusions in volcanic quartz from the Fire Clay tonstein. The rare-earth-element plots (chondrite normalized) of the tonstein show a pronounced negative Eu anomaly and relatively high concentrations of Zr and Th, which are both indicative of a rhyolitic source. Probe analyses of the Fire Clay glass inclusions from four states indicate a chemically identical high-silica rhyolite with peraluminous affinities. 40 Ar/ 39 Ar sanidine plateau dating indicates an age of 312 ± 1 Ma for the Fire Clay tonstein, which is consistent with previous 40 Ar/ 39 Ar dates for this tonstein. This age is in agreement with a late Westphalian B age in the European Carboniferous chronostratigraphy on the basis of an age of 311 Ma for the Westphalian B/C boundary. A new isopachous map of the Fire Clay ash-fall deposit indicates an area of 37,000 km 2 and a probable source to the present-day southwest. The deposit has a minimum preserved compacted volume of 2.8 km 3 , which corresponds to an original uncompacted volume of about 20 km 3 . This preserved volume indicates an ultraplinian volcanic explosion. Pindell and Dewey (1982) proposed an Andean-type arc in this block during the late Carboniferous, prior to South American-North American plate collision. We hypothesize an associated back-arc caldera system in the Yucatan block to explain the high-silica, potassic rhyolitic ash that gave rise to the Fire Clay tonstein.

Geological Society of America Bulletin↗

USGS2020

Evidence for primitive magma storage and eruption following prolonged equilibration in thickened crust

In continental arcs, the exposure of primitive eruptive products at the surface is typically a result of rapid magmatic transfer through the crust. As a result, the initially primitive magma experiences minimal crustal residence and thus insignificant differentiation towards more evolved products. This rapid transfer of primitive magma through thickened crust is commonly recorded from smaller, monogenetic cinder cones. Manantial Pelado (35.5° S) is a long-lived stratocone in the Southern Andean Volcanic Zone (SVZ) overlying thick continental crust (45–50 km) that produces almost exclusively mafic material. As Manantial Pelado is surrounded by extensive silicic volcanism, the study of its mafic exposure as a stratocone can be used to further understand magmatic origins of long-lived volcanic systems. Our study uses textural, geochemical, and geochronological data from lavas collected from Manantial Pelado to characterize its magmatic petrogenesis, assess the primitive nature, and explain processes in the crust within the SVZ. A geologic description of the volcano reveals a mostly monotonous eruptive history of basaltic andesites that are now accessible through glacially carved valleys. New 40 Ar/ 39 Ar dating constrains most of the volcano’s cone constructing phase to last from ~ 220 to 190 ka. At ~ 30 ka, small-volume activity and different petrography of more intermediate magmas were present reflecting a change in the volcano’s character. A combination of the whole-rock and mineral-scale data reveals that basaltic andesites at Manantial Pelado are among the most primitive magmas in the thickened crust of the SVZ. Evidence for this primitive signature consists of textural and zonation patterns in olivine, the presence of Cr-spinel in olivine cores, and elevated Fo and Ni content within olivine cores. This data combined with elemental diffusion modeling provides evidence for a primitive signature for these lavas. Intermediate Fo olivines with uniform core compositions (Fo 80–84 ) suggest that basaltic andesites reside in the crust in quasi-closed system environments for extended storage prior to eruption (~ 25–6000 years). Diffusive equilibration in those intermediate Fo olivines masks the primitive nature of the magmas. These results suggest that mafic magmas can have a protracted storage history in the crust that does not significantly alter their primitive bulk composition before reaching the surface. We argue that these are important processes in understanding the magmatic origin of long-lived systems and the presence of compositionally homogenous olivines at intermediate Fo content may represent cryptic evidence for recharge with primitive magmas that experienced prolonged crustal storage.

Manantial Pelado↗

USGS2003

Tectonic controls of Mississippi Valley-type lead-zinc mineralization in orogenic forelands

Most of the world's Mississippi Valley-type (MVT) zinc-lead deposits occur in orogenic forelands. We examine tectonic aspects of foreland evolution as part of a broader study of why some forelands are rich in MVT deposits, whereas others are barren. The type of orogenic foreland (collisional versus Andean-type versus inversion-type) is not a first-order control, because each has MVT deposits (e.g., Northern Arkansas, Pine Point, and Cevennes, respectively). In some MVT districts (e.g., Tri-State and Central Tennessee), mineralization took place atop an orogenic forebulge, a low-amplitude (a few hundred meters), long-wavelength (100-200 km) swell formed by vertical loading of the foreland plate. In the foreland of the active Banda Arc collision zone, a discontinuous forebulge reveals some of the physiographic and geologic complexities of the forebulge environment, and the importance of sea level in determining whether or not a forebulge will emerge and thus be subject to erosion. In addition to those on extant forebulges, some MVT deposits occur immediately below unconformities that originated at a forebulge, only to be subsequently carried toward the orogen by the plate-tectonic conveyor (e.g., Daniel's Harbour and East Tennessee). Likewise, some deposits are located along syn-collisional, flexure-induced normal and strike-slip faults in collisional forelands (e.g., Northern Arkansas, Daniel's Harbour, and Tri-State districts). These findings reveal the importance of lithospheric flexure, and suggest a conceptual tectonic model that accounts for an important subset of MVT deposits-those in the forelands of collisional orogens. The MVT deposits occur both in flat-lying and in thrust-faulted strata; in the latter group, mineralization postdated thrusting in some instances (e.g., Picos de Europa) but may have predated thrusting in other cases (e.g., East Tennessee).

Mineralium Deposita↗

USGS1998

Ordovician K-bentonites in the Argentine Precordillera: Relations to Gondwana margin evolution

Ordovician K-bentonites have now been recorded from >20 localities in the vicinity of the Argentine Precordillera. Most occur in the eastern thrust belts, in the San Juan Limestone and the overlying the Gualcamayo Formation, but a few ash beds are known also from the central thrust belts. The oldest occur in the middle Arenig I. victoriae lunatus graptolite ( Oe. evae conodont) Zone, and the youngest in the middle Llanvirn P. elegans ( P. suecicus ) Zone. Mineralogical characteristics, typical of other Ordovician K-bentonites, include a matrix of illite/smectite mixed-layer clay and a typical felsic volcanic phenocryst assemblage: biotite, beta-form quartz, alkali and plagioclase feldspar, apatite, and zircon, with lesser amounts of hornblende, clinopyroxene, titanite and Fe-Ti oxides. The proportions of the mineral phases and variations in their crystal chemistry are commonly unique to individual (or small groups of) K-bentonite beds. Glass melt inclusions preserved in quartz are rhyolitic in composition. The sequence is unique in its abundance of K-bentonite beds, but a close association between the Precordillera and other Ordovician sedimentary basins cannot be established. The ash distribution is most consistent with palaeogeographical reconstructions in which early Ordovician drifting of the Precordillera occurred in proximity to one or more volcanic arcs, and with eventual collision along the Andean margin of Gondwana during the mid-Ordovician Ocloyic event of the Famatinian orogeny. The Puna-Famatina terrane northeast of the Precordillera might have served as the source of the K-bentonite ashes, possibly in concert with active arc magmatism on the Gondwana plate itself.

Geological Society Special Publication↗

USGS2025

Pre- and post-eruptive geochemical and isotopic fingerprints of rhyolites parental to volcano-sedimentary lithium brine and clay resources in the western USA & central Andes

Lithium is a high-demand, critical element used not only in lightweight rechargeable lithium-ion batteries, but also in nuclear applications and industries producing ceramics, aluminum, and medical products. It is extracted primarily from pegmatites and volcano-sedimentary brines and clays in arid, closed lacustrine or caldera basins. Lithium brines of the central Andean salars in the AltiplanoPuna Plateau contain around ~70% of the world’s lithium resources. In contrast, Clayton Valley, Nevada is the only current producer of lithium brines in the United States and accounts for ~6% of current global lithium production. Clayton Valley hosts a newly defined Li-clay resource where locally exposed rhyolite tuffs have been proposed as a lithium source. Identifying magma evolution processes and determining the importance of syn- and post-eruptive processes on the source, mobility, and distribution of lithium is an ongoing area of research. These two regions illustrate distinctive magmatic-tectonic regimes for volcano-sedimentary lithium enrichment and therefore represent ideal regions to explore the key geological processes critical to the enrichment of lithium resources.

Clayton Valley, Puna Plateau↗

USGS1985

Origin and tectonic evolution of the Maclaren and Wrangellia terranes, eastern Alaska Range, Alaska

Major portions of the eastern Alaska Range, south of the Denali fault, in the McCarthy, Nabesna, Mount Hayes, and eastern Healy quadrangles, consist predominantly of the Maclaren and Wrangellia tectono-stratigraphic terranes. The Maclaren terrane consists of the Maclaren Glacier metamorphic belt and the regionally deformed and metamorphosed East Susitna batholith. The Maclaren Glacier metamorphic belt is composed of argillite, metagraywacke, and sparse andesite flows that are progressively regionally metamorphosed from lower greenschist facies to middle amphibolite facies near the East Susitna batholith. The East Susitna batholith is composed of gabbro, quartz diorite, granodiorite, and sparse quartz monzonite. Isotopic ages are as old as a K-Ar hornblende age of 87.5 m.y., possibly reset, and a U-Pb zircon age of 70 m.y. The batholith is intensely deformed and regionally metamorphosed under conditions of the middle amphibolite facies. The Wrangellia terrane is divided into two subterranes: (1) the Slana River subterrane, composed of late Paleozoic andesite to dacite flows, tuff, limestone, and argillite, unconformably overlying massive basalt flows of the Triassic Nikolai Greenstone, Late Triassic limestone, and younger Mesozoic flysch; and (2) the Tangle subterrane, a deeper-water equivalent of the Slana River subterrane, composed of late Paleozoic and Early Triassic aquagene tuff, chert, minor andesite tuff and flows, limestone, unconformably overlying pillow basalt and massive basalt flows of the Triassic Nikolai Greenstone, and Late Triassic limestone. Both subterranes are intruded by locally extensive gabbro and diabase dikes and by cumulate mafic and ultramafic sills. Less extensive terranes (two) are the Clearwater terrane, a sequence of intensely deformed chlorite schist, muscovite schist, marble, and greenstone of Late Triassic age; and an unnamed terrane of ultramafic and associated rocks of presumable Paleozoic or Mesozoic age. Each terrane or subterrane generally has (1) a distinctive time-stratigraphic sequence reflecting a unique geologic history; (2) a missing provenance for bedded sedimentary or volcanic rocks; and (3) bounding thrust or strike-slip faults, interpreted as accretionary sutures. The Maclaren and Wrangellia terranes are juxtaposed along the Broxson Gulch thrust, which consists of an imbricate series of north-dipping thrust faults. Paralleling the Broxson Gulch thrust, a few kilometres to the south, is the north-dipping Eureka Creek thrust, along which are juxtaposed the Slana River and Tangle subterranes. The Maclaren terrane is correlated with the Kluane Schist and the Ruby Range batholith in the southern Yukon Territory, which represent the northward extension of the Taku and Tracy Arm terranes. If correct, this correlation defines a minimum displacement of the Maclaren terrane along the Denali fault of ∼400 km. The Maclaren terrane is interpreted to have formed in a synorogenic Andean-type arc setting on the west margin of Mesozoic North America in the middle to late Mesozoic and early Cenozoic. The Wrangellia terrane is interpreted to have initially formed in an island-arc setting during the late Paleozoic. Subsequently in the Late Triassic, the Wrangellia terrane underwent rifting near the paleoequator, with formation of the Nikolai Greenstone and associated mafic and ultra-mafic igneous rocks. In the middle and late Mesozoic, Wrangellia migrated toward, and was accreted during, the middle Cretaceous to the Maclaren terrane along the Broxson Gulch thrust. Subsequent dispersion of both the Maclaren and Wrangellia terranes along the Denali fault and the Broxson Gulch thrust commenced during the early Tertiary and continues through the present.

Alaska↗

USGS2022

U-Pb isotopic evidence for the accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield.

This area includes three of the main tectonic units of the Arabian Shield: the Afif continental terrain, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrain. U/Pb zircon data from a pelitic garnet-sillimanite gneiss show that the Kabib formation in the S of the Afif terrain may be as old as 1770 m.y. Pb and Rb/Sr isotopic data in the Zalm region reveal a change in the nature of the underlying crust, from continental basement in the NE to less radiogenic marginal arc rocks in the SW. Miogeosynclinal continental shelf facies of the Siham group lie unconformably over the Kabid formation. U/Pb zircon age determinations show that this 'Andean' continental margin developed before approx 720 m.y. and the emplacement of calc-alkaline plutonic rocks continued until approx 690 m.y. During the period 685-640 m.y. the continental Afif microplate collided with the Asir terrain as part of the Nabitah orogeny. At approx 640 m.y. age the Najd strike-slip faulting commenced, with a dextral phase that controlled emplacement of granite plutons as well as the development of large pull-apart grabens. Some of the latter were floored by new oceanic crust and filled with volcanosedimentary rocks of the Bani Ghayy group.-R.A.H.

Journal of the Geological Society↗

USGS2019

The Zn–Pb mineralization of Florida Canyon, an evaporite-related Mississippi Valley-type deposit in Bongará district, northern Peru

The Florida Canyon evaporite-related Zn–Pb sulfide deposit, in northern Peru, is one of the largest Mississippi Valley-type (MVT) deposits in South America. Triassic carbonate and former evaporite-bearing rocks of the Pucará Group host the ore bodies that comprise two different styles: (i) predominantly stratabound ore associated with hydrocarbon-rich porous dolostones and evaporite dissolution breccias; and (ii) high-grade ore associated with evaporite breccias representing diapiric injections along faults. A dome structure that controls the location of the ore deposit was defined by drillhole spatial data; the dome likely resulted from halokinetic processes during Andean deformation. NNE-trending steeply dipping secondary faults linked to major NW structures appear to control the distribution of ore grades in the deposit. Mineralization post-dated hydrocarbon migration and accumulation. Strontium, carbon, and oxygen data isotopic signatures allow distinction between pre-and syn-mineralization carbonate stages. The sulfur isotopic composition of sulfides in the deposit suggests they precipitated as the result of mixing of a metal-rich fluid with resident hydrogen sulfide in the dome. Local thermochemical sulfate reduction (TSR) may have contributed to the reduced sulfur budget during mineralization.

Economic Geology↗

USGS1986

U-Pb isotopic evidence for accretion of a continental microplate in the Zalm region of the Saudi Arabian Shield

The region covered by this work includes three of the main tectonic units of the Arabian Shield: the Afif continental terrane, the Nabitah suture with its associated mobile belt, and the Asir ensimatic arc terrane. The geology of the area is well understood, and this geochronologic and isotopic study confirms that the Afif terrane was a continental microplate in the late Proterozoic. The study also provides a time frame for the crustal evolution of this part of the Shield. U-Pb zircon age data from a pelitic garnet-sillimanite gneiss of the Kabid formation shows that this part of the continental basement in the Afif terrane may be as old as 1830 Ma. Isotope data indicate that lead from the Kabid gneiss resided in the upper continental crust for a long period before 1830 Ma, and require that Archean rocks exist at depth beneath the Afif terrane. Lead isotope data reveal a change in the nature of the underlying crust, from continental basement in the northeast, to less radiogenic, marginal arc rocks in the southwest. This change is coincident with both aeromagnetic data, and a fades change within a pre-collision marginal basin. Miogeosynclinal continental shelf fades of the Siham group lie unconcormably over the Kabid formation, and are in the area of continental lead signatures. Eugeosynclinal deep water sediments and volcanics, in association with ultramafic rocks, occur in the area of marginal arc signatures. U-Pb zircon age determinations show that this "Andean" continental margin developed before about 720 Ma, and emplacement of calc-alkaline plutonic rocks continued until about 690 Ma. During the period 690-640 Ma, the continental Afif microplate collided with the Asir terrane as part of the Nabitah orogeny. At approximately 640 Ma ago, the Najd strike-slip orogen commenced with a dextral phase that controlled the emplacement of granitic plutons as well as the development of a series of large pull-apart grabens. Some of these grabens were floored by new oceanic crust and were filled with volcano-sedimentary rocks of the Bani Ghayy group. Subsequently, the Najd fault system changed to sinistral strike slip motion at about 620 Ma ago.

Open-File Report↗

USGS2005

Petrology, mineralogy and geochemistry of mined coals, western Venezuela

Upper Paleocene to middle Miocene coal samples collected from active mines in the western Venezuelan States of Táchira, Mérida and Zulia have been characterized through an integrated geochemical, mineralogical and petrographic investigation. Proximate, ultimate, calorific and forms of sulfur values, major and trace element, vitrinite reflectance, maceral concentrations and mineral matter content have been determined for 16 channel samples from 14 mines. Ash yield generally is low, ranging from < 1 to 17 wt.% (mean = 5 wt.%) on a dry basis (db). Total sulfur content is low to moderate, ranging from 1 to 6 wt.%, db (average = 1.7 wt.%). Calorific value ranges from 25.21 to 37.21 MJ/kg (10,840–16,000 Btu/lb) on a moist, mineral-matter-free basis (average = 33.25 MJ/kg, 14,300 Btu/lb), placing most of the coal samples in the apparent rank classification of high-volatile bituminous. Most of the coal samples exhibit favorable characteristics on the various indices developed to predict combustion and coking behavior and concentrations of possible environmentally sensitive elements (As, Be, Cd, Cr, Co, Hg, Mn, Ni, Pb, Sb, Se, Th and U) generally are similar to the concentrations of these elements in most coals of the world, with one or two exceptions. Concentrations of the liptinite maceral group range from < 1% to 70 vol.%. Five samples contain > 20 vol.% liptinite, dominated by the macerals bituminite and sporinite. Collotelinite dominates the vitrinite group; telinite was observed in quantities of ≤ 1 vol.% despite efforts to better quantify this maceral by etching the sample pellets in potassium permanganate and also by exposure in an oxygen plasma chamber. Inertinite group macerals typically represent < 10 vol.% of the coal samples and the highest concentrations of inertinite macerals are found in distantly spaced (> 400 km) upper Paleocene coal samples from opposite sides of Lago de Maracaibo, possibly indicating tectonic controls on subsidence related to construction of the Andean orogen. Values of maximum reflectance of vitrinite in oil ( R o max ) range between 0.42% and 0.85% and generally are consistent with the high-volatile bituminous rank classification obtained through ASTM methods. X-ray diffraction analyses of low-temperature ash residues indicate that kaolinite, quartz, illite and pyrite dominate the inorganic fraction of most samples; plagioclase, potassium feldspar, calcite, siderite, ankerite, marcasite, rutile, anatase and apatite are present in minor or trace concentrations. Semiquantitative values of volume percent pyrite content show a strong correlation with pyritic sulfur and some sulfide-hosted trace element concentrations (As and Hg). This work provides a modern quality dataset for the western Venezuela coal deposits currently being exploited and will serve as the foundation for an ongoing coal quality research program in Venezuela.

International Journal of Coal Geology↗

USGS2007

Organic petrology of Paleocene Marcelina Formation coals, Paso Diablo mine, western Venezuela: Tectonic controls on coal type

About 7??Mt of high volatile bituminous coal are produced annually from the four coal zones of the Upper Paleocene Marcelina Formation at the Paso Diablo open-pit mine of western Venezuela. As part of an ongoing coal quality study, we have characterized twenty-two coal channel samples from the mine using organic petrology techniques. Samples also were analyzed for proximate-ultimate parameters, forms of sulfur, free swelling index, ash fusion temperatures, and calorific value. Six of the samples represent incremental benches across the 12-13??m thick No. 4 bed, the stratigraphically lowest mined coal, which is also mined at the 10??km distant Mina Norte open-pit. Organic content of the No. 4 bed indicates an upward increase of woody vegetation and/or greater preservation of organic material throughout the life of the original mire(s). An upward increase in telovitrinite and corresponding decrease in detrovitrinite and inertinite illustrate this trend. In contrast, stratigraphically higher coal groups generally exhibit a 'dulling upward' trend. The generally high inertinite content, and low ash yield and sulfur content, suggest that the Paso Diablo coals were deposited in rain-fed raised mires, protected from clastic input and subjected to frequent oxidation and/or moisture stress. However, the two thinnest coal beds (both 0.7??m thick) are each characterized by lower inertinite and higher telovitrinite content relative to the rest of Paso Diablo coal beds, indicative of less well-established raised mire environments prior to drowning. Foreland basin Paleocene coals of western Venezuela, including the Paso Diablo deposit and time-correlative coal deposits of the Ta??chira and Me??rida Andes, are characterized by high inertinite and consistently lower ash and sulfur relative to Eocene and younger coals of the area. We interpret these age-delimited coal quality characteristics to be due to water availability as a function of the tectonic control of subsidence rate. It is postulated that slower subsidence rates dominated during the Paleocene while greater foreland basin subsidence rates during the Eocene-Miocene resulted from the loading of nappe thrust sheets as part of the main construction phases of the Andean orogen. South-southeastward advance and emplacement of the Lara nappes during the oblique transpressive collision of the Caribbean and South American tectonic plates in the Paleocene was further removed from the sites of peat deposition, resulting in slower subsidence rates. Slower subsidence in the Paleocene may have favored the growth of raised mires, generating higher inertinite concentrations through more frequent moisture stress. Consistently low ash yield and sulfur content would be due to the protection from clastic input in raised mires, in addition to the leaching of mineral matter by rainfall and the development of acidic conditions preventing fixation of sulfur. In contrast, peat mires of Eocene-Miocene age encountered rapid subsidence due to the proximity of nappe emplacement, resulting in lower inertinite content, higher and more variable sulfur content, and higher ash yield.

International Journal of Coal Geology↗

USGS1984

The Loma Seca tuff and the Calabozos caldera: a major ash-flow and caldera complex in the southern Andes of central Chile

A 26 × 14-km composite ring-structure caldera of late Pleistocene age has been discovered and mapped near the Andean crest in central Chile (35°30′S). Rhyodacitic to dacitic zoned ash-flow sheets, each representing 150 to 300 km 3 of magma, were emplaced 0.8, 0.3, and 0.15 m.y. ago; the youngest of the associated collapses was closely followed by resurgent doming of the caldera floor and development of a longitudinal graben. Postcaldera eruptions of dacite and andesite have persisted into Holocene time, and active hot springs are abundant along caldera-marginal and resurgent fault systems, suggesting a significant geothermal-energy resource. The Pleistocene eruption rate of this district and the abundance of older Quaternary to Miocene ash-flow remnants in the 33°S to 36°S segment of the glaciated southern Andes indicate that ash-flow magmatism has been no less important here than in the arid central Andes (16°S–28°S), where ash-flow sheets are far better preserved.

Geological Society of America Bulletin↗

USGS1978

Igneous and metamorphic petrology of the southwestern Dana Mountains, Lassiter Coast, Antarctic Peninsula

The southwestern Dana Mountains of the southern Antarctic Peninsula are underlain by the western part of a composite concentrically zoned Upper Cretaceous batholith consisting largely of granodiorite. The granodiorite (felsic phase) was intrusive into older heterogeneous gabbro-diorite (mafic phase) which makes up the margin of the batholith. Flat-lying pegmatite bodies and abundant inclusions, roof pendants, and septa of metasedimentary rocks and gabbrodiorite suggest that the granodiorite lies near the roof of the batholith. Petrography, chemistry, field relations, and radiometric age dates indicate that the batholith is part of the Andean intrusive suite, a suite of calc-alkaline plutonic rocks that crops out extensively throughout the Antarctic Peninsula and Chilean and Patagonian Andes. Textural features, mineralogy, and chemical data indicate that both phases of the batholith in the southwestern Dana Mountains have been contaminated by assimilation of the Latady Formation, an Upper Jurassic sequence of shale, siltstone, and sandstone that underlies large parts of the southern Antarctic Peninsula. Composition of the granodiorite also has been modified by assimilation of lesser amounts of the gabbro-diorite phase. Such contaminated plutonic rocks occur in several places in the northern Antarctic Peninsula; the batholith in the southwestern Dana Mountains represents the southernmost exposures of contaminated igneous rocks known from the Antarctic Peninsula. Hornblende andesite dikes cut both the mafic and felsic phases of the batholith. Chemical data imply that the dikes represent small amounts of mafic material remobilized by intrusion of the granodiorite. During the austral summers of 1969

Antarctic Peninsula, Dana Mountains, Lassiter Coas↗

USGS1973

Trans-Andean geophysical profile, southern Colombia

Negative Bouguer anomalies (−80 mgals) near the Pacific coast of southern Colombia define the position of the Tertiary Bolívar trough. Values increase eastward to a huge positive anomaly (+75 mgals) over Mesozoic “eugeosynclinal” rocks of the western Andes. This anomaly is part of the West Colombian gravity high, which extends from Panamá into western Ecuador and is caused by shallow mafic crust. Bouguer anomalies are strongly negative (−220 mgals) over pre-Mesozoic(?) metamorphic rocks, Mesozoic(?) granitic bodies, and Tertiary to Holocene volcanic rocks of the central Andes between Pasto and Ipiales. The steep gravity gradient between the West Colombian gravity high and the negative anomaly of the central Andes represents the transition between mafic crust to the west and continental crust to the east. This zone parallels the Romeral-Cauca megashear system. East of the Andes, Bouguer anomalies range from −50 to −120 mgals over a Mesozoic-Tertiary basin of the Putumayo district, indicating that the crust there is thinner or denser than it is beneath the central Andes. Models derived from gravity data suggest that the crust is about 45 km thick under the south-central Colombian Andes. If this is correct, the crust must thicken southward along the strike of the Andes, as thicknesses of 70 km have been reported in the Andes of southern Perú, Bolivia, and northern Chile by Lomnitz (1962) and James (1971a). Such differing crustal thicknesses may reflect different intensities of tectonic activity, greater crustal thickness indicating more intense or rapid growth of the volcano-plutonic arc or foreshortening of an existing crustal section.

Geological Society of America Bulletin↗

USGS2006

Upper Cretaceous to Holocene magmatism and evidence for transient Miocene shallowing of the Andean subduction zone under the northern Neuquén Basin

Evidence for a Miocene period of transient shallow subduction under the Neuquén Basin in the Andean backarc, and an intermittent Upper Cretaceous to Holocene frontal arc with a relatively stable magma source and arc-to-trench geometry comes from new 40 Ar/ 39 Ar, major- and trace-element, and Sr, Pb, and Nd isotopic data on magmatic rocks from a transect at &sim;36°–38°S. Older frontal arc magmas include early Paleogene volcanic rocks erupted after a strong Upper Cretaceous contractional deformation and mid-Eocene lavas erupted from arc centers displaced slightly to the east. Following a gap of some 15 m.y., ca. 26–20 Ma mafic to acidic arc-like magmas erupted in the extensional Cura Mallín intra-arc basin, and alkali olivine basalts with intraplate signatures erupted across the backarc. A major change followed as ca. 20–15 Ma basaltic andesite–dacitic magmas with weak arc signatures and 11.7 Ma Cerro Negro andesites with stronger arc signatures erupted in the near to middle backarc. They were followed by ca. 7.2–4.8 Ma high-K basaltic to dacitic hornblende-bearing magmas with arc-like high field strength element depletion that erupted in the Sierra de Chachahuén, some 500 km east of the trench. The chemistry of these Miocene rocks along with the regional deformational pattern support a transient period of shallow subduction that began at ca. 20 Ma and climaxed near 5 Ma. The subsequent widespread eruption of Pliocene to Pleistocene alkaline magmas with an intraplate chemistry in the Payenia large igneous province signaled a thickening mantle wedge above a steepening subduction zone. A pattern of decreasingly arc-like Pliocene to Holocene backarc lavas in the Tromen region culminated with the eruption of a 0.175 ± 0.025 Ma mafic andesite. The northwest-trending Cortaderas lineament, which generally marks the southern limit of Neogene backarc magmatism, is considered to mark the southern boundary of the transient shallow subduction zone.

Neuquen Basiin↗

USGS1991

Controls on the composition of fluvial sands from a tropical weathering environment: Sands of the Orinoco River drainage basin, Venezuela and Colombia

Fluvial sands in the Orinoco River drainage basin fall into three main compositional groupings: (1) sands of subarkose and arkose composition, from high-relief parts of the Guayana Shield, where crystalline rocks are exposed; (2) sands of litharenite and sub-litharenite composition from the active orogenic belt at the western and northwestern margins of the drainage basin, and in those parts of the Llanos (Andean foreland basin) proximal to the mountain belt; and (3) sands of quartz-arenite composition, widespread throughout the remainder of the basin. Multicycle sands of quartz-arenite composition are produced from platform cover on parts of the elevated shield and from uplifted foreland-basin fill in the eastern Llanos. First-cycle sands of quartz-arenite composition are produced from granitic rocks on low-relief regions of the Guayana Shield and from reworked Holocene alluvium in parts of the western Llanos distal to the orogenic terranes. Erosion in the Orinoco River drainage basin may be described in terms of transport-limited and weathering-limited denudation regimes. In active orogenic terranes and in parts of the elevated shield, transport processes can remove weathered material as rapidly as it is produced by chemical weathering. Thin soils and short soil-mineral residence time result in sands that are incompletely chemically weathered and accurately reflect source-rock composition. In the orogenic terranes, subtle variations in source-rock lithology are preserved in sand composition. In contrast, in low-relief parts of the Guayana Shield and on flat erosion surfaces of the upland shield, weathering exceeds the rate at which transport processes can remove weathered material. Thick soils accumulate, soil-mineral residence time is long, and detritus is highly altered chemically. On much of the lowland Guayana Shield, upper soil layers consist of nearly pure quartz sand that erodes to produce first-cycle fluvial sand of quartz-arenite composition. Chemically weathered orogenically derived sand enters the Orinoco River on the left bank, while feldspathic shield-derived sand enters on the right bank. This geometry is responsible for the nearly total lack of longitudinal variation in sand composition along the 1,400-km length of the Orinoco River mainstem. Except in the upper 100 km of the Orinoco River mainstem, cross-channel heterogeneity in sand composition is also modest. Nevertheless, a weighted linear least-squares modeling approach suggests that sand moves down the Orinoco River mainstem in distinct pulses, perhaps corresponding to times of accelerated erosion in headwater regions. Because rivers that head in the orogenic terranes and traverse the Llanos contribute more than 99% of the sand in the lower Orinoco River mainstem, the composition of this sand is dominated by chemically weathered sands from the Llanos. The Orinoco River—the third largest river in the world—delivers first-cycle sands of quartz-arenite composition to its delta.

Geological Society of America Bulletin↗

USGS1990

The Ag-Mn-Pb-Zn vein, replacement, and skarn deposits of Uchucchacua, Peru; studies of structure, mineralogy, metal zoning, Sr isotopes, and fluid inclusions

Uchucchacua is an Ag-Mn-Pb-Zn vein, replacement, and skarn mineral district in the central Andes of Peru. Host rocks are massive Jumasha Formation shelf limestones of Turonian age that have been folded into an asymmetric northeast-verging anticline of Andean trend. Strata near the fold crest are cut by minor dacitic intrusions and have been displaced by a conjugate set of steep wrench faults that strike northwest-southeast and northeast-southwest. Most ore occurrences are restricted to host rocks that lie below marly limestone at the top of the middle Jumasha Formation. Vein ores located along the fracture system have formed by fissure infill and by replacement of limestone wall rocks. Larger sheetlike replacement orebodies are parallel and adjacent to a large fault. Replacement is in zones of brecciation adjacent to fault bends which were dilatent during sinistral slip of the master fracture. Such fracture belts may have been subject to paleokarst solution processes before mineralization.Four paragenetic stages have been identified. Fe, Mn, and Si were introduced at the exoskarn stage (I) as the anhydrous silicates ferroan tephroite, johannsenite, rhodonite, and bustamite. During the early main stage (II) ferroan tephroite was replaced by friedelite and magnetite under oxidizing conditions at a relatively low pH. Pb, Zn, Fe, Cu, and B were introduced; principal sulfides are pyrrhotite, Fe-rich sphalerite, Mn-rich wurtzite, alabandite, galena, chalcopyrite, and tetrahedrite. Pyrrhotite was replaced by other sulfides during later stage II. Main gangue minerals were calcite, kutnohorite, rhodochrosite, and quartz. Ag, As, and Sb were introduced during the late stage (III) in the form of sulfosalts, principally pyrargyrite. Redistribution of metals introduced at stage II resulted in the growth of Fe-poor sphalerite and alabandite accompanied by calcite gangue. Decreasing Fe contents of alabandite and sphalerite during late stage II and stage III, together with the appearance of pyrite, indicate an increase in sulfur fugacity and/or decrease in temperature over this period. The supergene stage (IV) affects the upper 30 to 150 m of most veins and involves the growth of Mn hydroxides, goethite, orpiment, marcasite, cerussite, and siderite.Distribution patterns of metal ratios and high metal values define ore bands, with a succession of antiforms and synforms. Ore-band locations are determined by vein width, itself a function of wall-rock reactivity, and fracture permeability. Metal ratio distributions in the Luz vein show symmetric dispersion of metals at right angles to the ore-band axis. Ag shows the least dispersion with progressively greater dispersion of Pb and Zn.Geologic considerations indicate mineralization at a minimum depth of 1,600 m. Most primary fluid inclusions in calcite of probable stage II origin were trapped over a temperature range of about 200 degrees to 337 degrees C (hydrostatic pressure) or about 225 degrees to 362 degrees C (lithostatic). Fluid inclusions from late stage II quartz were trapped at about 165 degrees to 205 degrees C (hydrostatic) or 190 degrees to 230 degrees C (lithostatic). Salinity ranged from 0.5 to 29.7 equiv wt percent NaCl-CaCl 2 with little relation to temperature. Calcite 87 Sr/ 86 Sr values of 0.70743 to 0.71122 extend well outside the range of values both determined for magmatic rocks of the region and estimated for the host limestones but are probably compatible with older sedimentary rocks of the mine district. Salinity levels and NaCl/CaCl 2 ratios are similar to basinal brines associated with Mississippi Valley-type deposits. It is concluded that basinal brine, expelled from the deforming Mesozoic sequence, was a significant component of the ore fluid. This fluid was probably heated by dacitic magmas or cooling dacitic intrusions; a polygenetic high-salinity magmatic-basinal fluid then rose along existing strike-slip fractures toward the mineralization site where it mixed with heated low-salinity ground water.

Uchucchacua↗