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229 records · Page 13Linked to original sources
USGS1952

Magmatic differentiation in tertiary and quaternary volcanic rocks from Adak and Kanaga Islands, Aleutian Islands, Alaska

Samples of 17 volcanic rocks of Tertiary and Quaternary age from Adak and Kanaga islands have been chemically analyzed and studied microscopically. Spectrograms have been made of 10 of them. The rocks from Adak represent one center of possibly older Tertiary age and two centers of younger Tertiary or Quaternary age. The rocks from Kanaga Island represent both a shield volcano of possibly Tertiary age, partly destroyed by the formation of a caldera, and a young cone of Quaternary age that has grown within the caldera. All the rocks are basalt or andesite. Modally, all are characterized by relatively large crystals of plagioclase more calcic than andesine, and by one or more of the following ferromagnesian minerals: olivine, hypersthene, augite, and hornblende. Apatite and iron ores are common, and late silica minerals and orthoclase occur interstitially in the groundmasses of some rocks . As analyses of no more than four samples are available for each center, the small differences between sets of analyses representing different centers are of doubtful significance. Consequently, the analyses representing all the centers have been plotted on each of the several diagrams used. The several types of variation diagrams show that the province is a calc-alkaline one. The alkali-lime index is in the neighborhood of 63. This very high value is comparable with that for Katmai and is only slightly less than the maximum for the Japanese volcanic rock series. The quantities of minor constituents present are not exceptional for the rock types analyzed; the rocks from Adak are apparently more strontium-rich than those from Kanaga . The chemical analyses of the more basic rock types, as compared with the average analysis of plateau basalt, suggest that the Aleutian parental magma could have been derived from a plateau basalt magma by the addition of plagioclase and the subtraction of pyroxene, iron ore, and some quartz. The distribution of the minor elements can be explained more easily by postulating that, at least on Kanaga Island , some sediments have been assimilated. The derivation of the analyzed rocks from the Aleutian parental magma is most easily explained by the hypothesis that the plagioclase remained in suspension while the ferromagnesian minerals were settling out.

Alaska↗
USGS1996

Recurrent eruption and subsidence at the Platoro caldera complex, southeastern San Juan volcanic field, Colorado: New tales from old tuffs

Reinterpretation of a voluminous regional ash-flow sheet (Masonic Park Tuff) as two separate tuff sheets of similar phenocryst-rich dacite erupted from separate source calderas has important implications for evolution of the multicyclic Platoro caldera complex and for caldera-forming processes generally. Masonic Park Tuff in central parts of the San Juan field, including the type area, was erupted from a concealed source at 28.6 Ma, but widespread tuff previously mapped as Masonic Park Tuff in the southeastern San Juan Mountains is the product of the youngest large-volume eruption of the Platoro caldera complex at 28.4 Ma. This large unit, newly named the “Chiquito Peak Tuff,” is the last-erupted tuff of the Treasure Mountain Group, which consists of at least 20 separate ash-flow sheets of dacite to low-silica rhyolite erupted from the Platoro complex during a 1 m.y. interval (29.5−28.4 Ma). Two Treasure Mountain tuff sheets have volumes in excess of 1000 km 3 each, and five more have volumes of 50–150 km 3 . The total volume of ash-flow tuff exceeds 2500 km 3 , and caldera-related lavas of dominantly andesitic composition make up 250-500 km 3 more. A much greater volume of intermediate-composition magma must have solidified in subcaldera magma chambers. Most preserved features of the Platoro complex-including postcollapse asymmetrical trap-door resurgent uplift of the ponded intracaldera tuff and concurrent infilling by andesitic lava flows-postdate eruption of the Chiquito Peak Tuff. The numerous large-volume pre-Chiquito Peak ash-flow tuffs document multiple eruptions accompanied by recurrent subsidence; early-formed caldera walls nearly coincide with margins of the later Chiquito Peak collapse. Repeated syneruptive collapse at the Platoro complex requires cumulative subsidence of at least 10 km. The rapid regeneration of silicic magmas requires the sustained presence of an andesitic subcaldera magma reservoir, or its rapid replenishment, during the 1 m.y. life span of the Platoro complex. Either case implies large-scale stoping and assimilative recycling of the Tertiary section, including intracaldera tuffs.

Colorado↗
USGS1971

Boulder Creek batholith, Colorado part II: Isotopic age of emplacement and morphology of zircon

Zircon separated from six rocks whose compositions spanned the range of differentiation in the Boulder Creek batholith yielded a “discordia” age of emplacement of 1725 m.y., close to the average PB 207 /Pb 206 age 1720 m.y.) and indicating that the constituent rocks are cogenetic within approximately ± 20 m.y. Statistical studies show that from 20 to 80 percent of the zircon in any one sample (1) is no-neuhedral, (2) has lower (length/width) ratios than the associated euhedral zircon, and (3) in direct contrast to the euhedral, increases markedly interior of the batholith toward contacts with the older metasediments and internal zones of contamination; it is inferred to have been “inherited” via assimilation. Unlike the noneuhedral fraction the euhedral zircon shows a linear decrease in length/width ratio with an increase in SiO 2 content of the containing rocks; it is inferred to be magmatic in origin. Regardless of the relative abundance of inherited versus magmatic zircon, all samples closely fit a single discordia chord, indicating that both zircon fractions formed at about the same time. This conclusion is compatible with field relationships that indicate the emplacement of the syntectonic Boulder Creek rocks took place during a period of metamorphism notable for the widespread development of new minerals in the country rocks. (1) Zircon from a Silver Plume Granite dike intruding the Boulder Creek batholith, (2) zircon from Silver Plume correlatives immediately to the north (Tilton and co-workers), and (3) uraninite from a probable Silver Plume correlative in the Central City district, together yield a sharply defined discordia age of emplacement of 1415 m.y. The separate “discordia” chords for the Boulder Creek and Silver Plume zirconconverge close to their lower intercepts with “concordia” indicative of a one-step lead loss during the Laramide disturbance. The Silver Plume “thermal event” left no age imprint on the Boulder Creek zircon, presumably because insufficient time had elapsed to produce significant metamictization.

Colorado↗
USGS1973

Geochronology of Precambrian rocks of the Teton Range, Wyoming

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. The oldest rocks in the Teton Range are complexly deformed interlayered biotite gneiss, plagioclase gneiss, amphibole gneiss, and amphibolite. Also, within these rocks, there are concordant bodies of strongly lineated quartz monzonite gneiss, here named the Webb Canyon Gneiss, which may be of volcanic origin. Coarse metagabbro, here named the Rendezvous Metagabbro, is intrusive into the layered gneiss sequence and was metamorphosed and deformed along with the enclosing rocks. These older rocks are cut by discordant plutons and swarms of undeformed dikes of quartz monzonite and associated pegmatite. The quartz monzonite, which makes up much of the central part of the Teton Range, is here named the Mount Owen Quartz Monzonite. The youngest Precambrian rocks are undeformed dikes of slightly metamorphosed tholeiitic diabase. A Rb-Sr whole-rock isochron on the Webb Canyon Gneiss and the Rendezvous Metagabbro indicates that these rocks were metamorphosed 2,875 ± 150 m.y. ago. The initial Sr. ratio of 0.700 suggests that the original rocks are probably not significantly older than the metamorphism. The Mount Owen Quartz Monzonite has a whole-rock isochron age of 2,495 ± 75 m.y. and an unusually high initial ratio of 0.732. Plagioclase-microcline isochrons from two samples of the quartz monzonite indicate partial re-equilibration of the Rb-Sr system during a thermal event 1,800 m.y. ago. The age of the diabase dikes has not been definitely determined, but biotite in the wall rocks of one major dike has a K-Ar age of 1,450 m.y. This suggests that the dike was emplaced during or prior to a thermal event 1,300 to 1,500 m.y. ago that was responsible for resetting many of the previously reported K-Ar mineral ages throughout the range. The geochronologic record in the Teton Range is very similar to that elsewhere in the Wyoming Precambrian province. Major metamorphic events with ages between 2,700 and 2,900 m.y. have been identified in the Bighorn, Beartooth, Little Belt, and Granite Mountains. Post-tectonic granitic rocks with ages of 2,500 to 2,700 m.y. have been found in the Wind River Range and the Granite Mountains. Later thermal events have affected Rb-Sr systematics of rocks in the Beartooth Mountains, Wind River Range, and Granite Mountains, as well as in the Teton Range at about the same time as major episodes of regional metamorphism in terranes flanking the Wyoming province in southwestern Montana and in the Front Range in Colorado.

Wyoming↗
USGS1969

Mesozoic California and the underflow of Pacific mantle

The Mesozoic evolution of California is interpreted as dominated by the underflow of oceanic mantle beneath the continental margin. Underflow during part of Late Cretaceous time of more than 2000 km of the eastern Pacific plate seems required by the marine magnetic data. Correspondingly, varied oceanic environments—abyssal hill, island arc, trench, oceanic crust, and upper mantle, perhaps also continental rise and abyssal plain—appear to be represented in the eugeosynclinal terranes of California. The rock juxtapositions accord with the concept that these materials were scraped off against the continent as the oceanic plate slid beneath it along Mesozoic Benioff seismic zones, which are now seen as serpentine belts separating profoundly different rock assemblages. The chaotic Franciscan Formation of coastal California consists of deep-ocean Late Jurassic to Late Cretaceous sedimentary, volcanic, crustal, and mantle materials. As open-ocean abyssal oozes and the oceanic crust beneath them were swept into the Benioff-zone trench at the continental margin, they were covered by terrigenous clastic sediments, and the entire complex was carried beneath the correlative continental-shelf and continental-slope deposits (Great Valley sequence) and the older Mesozoic complexes. The other eugeosynclinal terranes of California can be interpreted, albeit with less confidence, in similar terms of underflow of Pacific mantle. In the Klamath Mountains and northern Sierra Nevada, for example, Ordovician and Silurian ocean-floor materials, overlain by or juxtaposed against an Upper Silurian to Permian island arc, were swept in first to the continent, along with a large fragment of oceanic crust and mantle and another fragment of an old orogenic belt. This debris was followed by Permian and Triassic ocean-floor deposits. Late Triassic and Jurassic volcanic products from stocks and batholiths forming in the welded complexes lapped across both landward and oceanward sides of the region. Reversal of Cenozoic extension, strike-slip faulting, and volcanic crustal growth in the western United States reveals a Cretaceous tectonic pattern strikingly like the modern pattern of the Andes, so the paleotectonic setting of North America can be inferred from the South American present. The Mesozoic batholiths of North America, like the late Cenozoic volcanic belt of the central Andes, are products of the same rapid motion of oceanic plates that carried oceanic sediments against the continent to form eugeosynclinal terranes. Magmas generated in the Benioff zones formed the batholiths and the volcanic fields which initially capped them.

California↗
USGS1965

Structure, metamorphism, and plutonism in the south-central Klamath Mountains, California

In the south-central Klamath Mountains 50 miles of the the north-trending central metamorphic belt and adjacent parts of the eastern Paleozoic and western Paleozoic and Triassic belts have been mapped and studied in detail. Within the central metamorphic belt a sequence of three lithologically distinctive metamorphic units has been recognized (from bottom to top): (1) siliceous metasedimentary rocks and greenstones of the Stuart Fork Formation; (2) the Salmon Hornblende Schist; and (3) siliceous, calcareous, and amphibolitic rocks, predominantly metasedimentary, of the Grouse Ridge Formation. The age of these metamorphic rocks is uncertain; they are known only to predate intrusion of Late Jurassic (Nevadan) granitic rocks. Ultramafic rocks, mainly alpine-type peridotites, were emplaced before the granitic rocks and occur primarily in a single large sheetlike body which separates the central metamorphic belt from the eastern Paleozoic belt. Granitic plutons, including quartz diorites, trondhjemites, granodiorites, diorites, and gabbros, in decreasing order of abundance, range in size from less than 1 to about 80 square miles in area. Two orogenic phases in the central metamorphic belt have been distinguished by structural and textural features. A late deformation uniformly affected the metamorphic terrane and the ultramafic rocks but predated granitic rocks. It was accompanied by some metamorphism in the lower to middle greenchist facies and produced upright folds that trend south and plunge gently. An earlier phase affected the various rock units differentially; it produced widespread recumbent folding and upper greenschist- to amphibolite-facies metamorphism in Salmon and Grouse Ridge rocks, but involved the underlying Stuart Fork Formation less severely, producing at least local recumbent folding and lower greenschist-facies metamorphism. The preferred interpretation of this upward increase in structural complexity and metamorphic grade is that the Salmon-Grouse Ridge sequence is a thrust sheet which overrode the Stuart Fork rocks concurrently with emplacement of the ultramafic rocks during the culmination of early recumbent folding and metamorphism. Thrusting was then followed by upright folding during the waning stages of regional metamorphism. The first deformational phase, and possibly the second, occurred during late Paleozoic time as indicated by recent isotopic ages of Salmon Hornblende Schist.

California↗
USGS1998

Structural and kinematic evolution of the Yukon-Tanana upland tectonites, east-central Alaska: A record of late Paleozoic to Mesozoic crustal assembly

The Yukon-Tanana terrane, the largest tectonostratigraphic terrane in the northern North American Cordillera, is polygenetic and not a single terrane. Lineated and foliated (L-S) tectonites, which characterize the Yukon-Tanana terrane, record multiple deformations and formed at different times. We document the polyphase history recorded by L-S tectonites within the Yukon-Tanana upland, east-central Alaska. These upland tectonites compose a heterogeneous assemblage of deformed igneous and metamorphic rocks that form the Alaskan part of what has been called the Yukon-Tanana composite terrane. We build on previous kinematic data and establish the three-dimensional architecture of the upland tectonites through kinematic and structural analysis of more than 250 oriented samples, including quartz c-axis fabric analysis of 39 samples. Through this study we distinguish allochthonous tectonites from parautochthonous tectonites within the Yukon-Tanana upland. The upland tectonites define a regionally coherent stacking order: from bottom to top, they are lower plate North American parautochthonous attenuated continental margin; continentally derived marginal-basin strata; and upper plate ocean-basin and island-arc rocks, including some continental basement rocks. We delineate three major deformation events in time, space, and structural level across the upland from the United States-Canada border to Fairbanks, Alaska: (1) pre-Early Jurassic (>212 Ma) northeast-directed, apparent margin-normal contraction that affected oceanic rocks; (2) late Early to early Middle Jurassic (>188-185 Ma) northwest-directed, apparent margin-parallel contraction and imbrication that resulted in juxtaposition of the allochthonous tectonites with parautochthonous continental rocks; and (3) Early Cretaceous (135-110 Ma) southeast-directed crustal extension that resulted in exposure of the structurally deepest, parautochthonous continental rocks. The oldest event represents deformation within a west-dipping (present coordinates) Permian-Triassic subduction zone. The second event records Early to Middle Jurassic collision of the arc and subduction complex with North American crust, and the third event reflects mid-Cretaceous southeast-directed crustal extension. Events one and two can be recognized and correlated through southern Yukon, even though this region was affected by mid-Cretaceous dextral shear along steep northwest-striking faults. Our data support a model of crustal assembly originally proposed by D. Tempelman-Kluit in which previously deformed allochthonous rocks were thrust over parautochthonous rocks of the attenuated North American margin in Middle Jurassic time. Approximately 50 m.y. after tectonic accretion, east-central Alaska was dissected by crustal extension, exposing overthrust parautochthonous strata.

Alaska↗
USGS1971

Basin and range structure: A system of horsts and grabens produced by deep-seated extension

Basin and Range structure can be interpreted as a system of horsts and grabens produced by the fragmentation of a crustal slab above a plastically extending substratum. According to this view, the extension of the substratum causes the basal part of the slab to be pulled apart along narrow, systematically spaced zones which in turn cause the downdropping of complex horizontal prisms (grabens) in the brittle upper crust. The grabens form valleys at the surface; the intervening areas are horsts, or tilted horsts. Not all geologists have agreed, however, that Basin and Range structure consists of a system of horsts and grabens. Instead, the structure is commonly considered to consist of tilted blocks in which the upslope part of an individual block forms a mountain and the downslope part a valley. Recent detailed studies, including geophysical work, suggest that the horst and graben model may be more generally applicable. Many of the valleys in the Great Basin are bounded on both sides by faults that drop the valley block down; these faults are exposed at the surface or can be inferred from steep gravity gradients indicative of steep faulted subsurface bedrock slopes. Some areas that were thought to represent a typical series of tilted blocks may be a series of highly asymmetrical grabens in which one side of a valley is marked by a master fault and the other side by valleyward tilt. With present knowledge, most, or perhaps all, of the major valleys in the Great Basin can plausibly be considered to be grabens, and most or all of the mountains can be considered to be horsts or tilted horsts. The grabens, and the underlying inferred deep zones of extension that cause them, are systematically distributed in the Great Basin. They are generally north-trending features spaced 15 to 20 mi apart. Locally, the pattern is more complex, and individual grabens divide and trend away from each other at acute or high angles. In a few places, the pattern may even be roughly polygonal. The distribution pattern of the grabens and the related deep zones of extension resemble crack patterns in small-scale tensional systems, and both patterns may be mechanically related. By analogy with the small-scale systems, the areas of generally north-trending and parallel grabens require east-west extension, whereas the areas with a possible polygonal pattern of grabens must extend radially. The geometry of block faulting related to Basin and Range structure requires sizable east-west extension, estimated at about 1.5 mi on the average for each major valley and at about 30 to 60 mi across the entire Great Basin. Most of this extension has taken place in the last 17 m.y., or perhaps even in the last 7 to 11 m.y., indicating a rate of extension in the range of 0.3 to 1.5 cm/yr.

Arizona, California, Idaho, Nevada, Oregon, Utah↗
USGS1989

Significance of new potassium-argon ages from the Goldens Ranch and Moroni Formations, Sanpete-Sevier Valley area, central Utah

Exposures of volcanic-sedimentary strata are widely distributed within central Utah. We believe that these volcanic and stratified sedimentary rocks, known by different formational names in different parts of this region, are, in fact, segments of one and the same suite of rocks that formed during the early and middle Tertiary. The volcanic-sedimentary complex is exposed on both sides of a north-trending lowland formed by the collinear Juab and Sevier Valleys. West of the lowland, the complex has been named the "Goldens Ranch Formation" east of the lowland, it has been called the "Moroni Formation."; Both formations are stratigraphically alike in that each consists of a lower unit composed predominantly of water-laid, variably cemented sediments and sedimentary rocks with some tuff beds near the base, and an upper unit of intermediate-composition volcanic rocks, chiefly ash-flow tuffs, and volcanic breccias. Both formations contain abundant exotic clasts of andesite, tan and purple quartzite, and dark blue limestone and dolomite. Both formations are folded and faulted along with the underlying sedimentary units. Potassium-argon ages indicate that both the Goldens Ranch and Moroni Formations formed during the late Eocene to middle Oligocene. The geochronology and stratigraphic relations are strong evidence that the Goldens Ranch and Moroni Formations are correlative, and that they are one and the same depositional unit. During the latest Oligocene-earliest Miocene, minor monzonitic bodies intruded sedimentary units in the area. The new K-Ar data bear on the matter of the origin of the complex structural deformation in central Utah. Different workers have attributed the singular deformation either to recurrent episodes of compression stemming from the Sevier orogeny, or to repeated episodes of salt diapirism. We recognize two sequences of repeated deformation: one that occurred prior to deposition and consolidation of the Goldens Ranch and Moroni Formations, and a second that occurred after these formations were emplaced, in essence, after early Oligocene time. The Sevier orogeny ended in Paleocene time; thus, the compression and thrusting stemming from the Sevier orogeny could be responsible for the structural complexity that marks pre-Paleocene units. These same orogenic forces do not seem to be viable explanations for the broad flexures and monoclinal downwarps that mark the Goldens Ranch, Moroni, and younger formations. In our view, multiple episodes of salt diapirism more reasonably explain the structural complexity in central Utah.

Utah↗
USGS2000

Late Holocene Peléan-style eruption at Tacaná volcano, Mexico and Guatemala: past, present, and future hazards

Tacaná volcano, located on the border between Mexico and Guatemala, marks the northern extent of the Central American volcanic chain. Composed of three volcanic structures, it is a volcanic complex that has had periodic explosive eruptions for at least the past 40 k.y. The most recent major eruption occurred at the San Antonio volcano, the youngest volcanic edifice forming the complex, about 1950 yr ago. The Peléan style eruption, issued from the southwest part of the dome, and swept a 30° sector with a hot block and ash flow that traveled about 14 km along the Cahoacán ravine. Deposits from this event are well exposed around the town of Mixcun and were therefore given the name of that town, the Mixcun flow deposit. The Mixcun flow deposit is, in the channel facies, a light gray, massive, thick (>10 m), matrix-supported unit with dispersed lithic clasts of gravel to boulder size, divisible in some sections into a variable number of flow units. The overbank facies is represented by a thin (<1 m), massive, matrix-supported unit. In both of these facies the deposit has disseminated charcoal, fumarolic pipes, and juvenile lithics with cooling joints. The Mixcun flow deposit contains clasts of (1) light gray, dense andesite, (2) dark gray, glassy and banded andesite, and (3) minor altered red andesite from the edifice, set in a matrix of sand and silt. The Mixcun flow deposit covers an area of at least 25 km 2 and has a minimum estimated volume of 0.12 km 3 . Basaltic-andesite inclusions (54% SiO 2 ) and various signs of disequilibrium in the mineral assemblage of the two-pyroxene andesitic products (60%–63% SiO 2 ) suggest that magma mixing may have triggered the eruption. Following deposition of the Mixcun flow deposit andesitic to dacitic (62%–64% SiO 2 ) lava flows were extruded and a dacitic dome (64.4% SiO 2 ) at the San Antonio summit formed. Syn-eruptive and posteruptive lahars flooded the main drainages of the Cahoacán and Izapa-Mixcun valleys in the area of the present city of Tapachula (population 250000) and the pre-Hispanic center of Izapa. Three radiocarbon ages date this event between A.D. 25 and 72 (range ±1σ, 38 B.C.–A.D. 216), which correlates with a halt in construction at Izapa (Hato phase of ca. 50 B.C.–A.D. 100), probably due to temporary abandonment of the city caused by lahars. Another similar event would produce extensive damage to the towns (population of about 68,000 people) now built upon the Mixcun flow deposit. The main summit of Tacaná volcano continues to show signs of fumarolic activity; the most recent period of activity in 1985–1986 culminated in a minor phreatic explosion.

Tapachula↗
USGS1962

Distribution and composition of sulfide minerals at Balmat, New York

In the Balmat area in northern New York , tabular deposits of sulfide minerals parallel the layering in folded, siliceous magnesian marbles of a metamorphic complex commonly referred to as the Precambrian Granville Series. Sphalerite, pyrite, and, locally, pyrrhotite and galena have replaced the carbonate minerals in parts of the marble units. The contacts between ore and marble are, in general, ill-defined; scattered grains of sulfides are present from several inches to hundreds of feet from the massive portions of ore. Access to the ore is provided through the Balmat No. 2 and No. 3 mines. The isotopic composition of lead from primary galena is uniform within an individual mine. The model age of this lead agrees with the age of the mineralization determined by other means - about a billion years. The isotopic composition of the leads in the marble is not uniform today, and calculations indicate that it was probably not uniform a billion years ago. Unless the lead in the ores is a uniform mixture of lead isotopes from an isotopically poorly mixed source, it is doubtful that the lead in the ores was derived from the surrounding marbles. Cobalt and nickel concentrations in pyrite from grains disseminated in the metasedimentary rocks away from the ore bodies are each greater than 200 ppm. Most samples of pyrite from the ore bodies at the No. 2 and No. 3 mines contain less than 50 ppm each of cobalt and nickel. Therefore the author believes it unlikely that the pyrite of the ores is genetically related to the pyrite in the metasedimentary rocks. Textural relationships suggest that pyrrhotite formed after most of the sphalerite, which in turn formed after most of the pyrite in the ore bodies. By use of the experimentally determined systems FeS-ZnS and FeS-FeS 2 , it is inferred from the amounts of iron in sphalerite and sulfur in pyrrhotite that the bulk of the sulfide minerals in the No. 2 mine formed above 320° C. The absolute temperature of formation of pyrrhotite indicated by the FeS-ZnS system is about 150° higher than that indicated by the FeS-FeS 2 system. The former system probably gives the more reliable estimate. The concentrations of individual minor elements in sphalerite and pyrite range considerably among specimens of the same sulfide mineral from the same level and ore body. An exception is cadmium in sphalerite which has a narrow concentration range around 1400 ppm in both the No. 2 and No. 3 mines. The ratio of the concentrations of minor elements between sphalerite-pyrite pairs varies considerably also. This variation probably indicates that exchange of minor elements between pyrite and sphalerite durin g the formation of the ores was very slow and incomplete.

New York↗
USGS1960

Geology of the Mayagüez area, Puerto Rico

The Mayagüez area forms the southwestern corner of Puerto Rico , west of 67° W. and south of 18° 15' N. One-third of the 640 square kms is covered by thick alluvium. Unconformities separate a basal complex, two sequences of highly folded igneous and sedimentary rocks, and a younger sequence of gently dipping sedimentary rock. The basal Bermeja complex contains serpentinite, silicified porphyritic volcanic rock with some sedimentary rock, and minor spilite, amphibolitized spilite, and amphibolite. It is exposed chiefly in some anticlinal cores in southwestern Puerto Rico . Limestone, mudstone, andesite, and basalt form the older folded sequence. The Río Loco formation, bronzite andesite porphyry in part with pillow structures, was extruded perhaps in the Cenomanian. The Mayagüez group includes most of the rocks in southwestern Puerto Rico : the Yauco mudstone, Parguera limestone, Brujo limestone, Melones limestone, Maricao basalt, Sabana Grande andesite, and El Rayo volcanic rocks. The maximum possible age range is Turonian to Maestrichtian. The group ranges in thickness from about 800 m in the south to 3800 m in the north, and it varies in lithology from limestone in the south to mudstone and volcanic rock in the north, indicating a volcanic center to the north during that time. The second folded sequence contains andesitic volcanic rock, bedded tuff, and massive limestone. The San Germán formation (Maestrichtian) includes andesite, the Cabo Rojo agglomerate member, and the Cotui limestone member. The Jicara formation, massive limestone and bedded tuff, is Paleocene; there is one exposure of an unnamed ? Eocene marl. Post-Eocene limestone and conglomerate are also exposed in the area . The structure of the basement complex is obscured by its massiveness and by the cover of younger rocks. Two major deformations have affected the rocks of southwestern Puerto Rico since Cenomanian to Santonian time. In the Maestrichtian, the first of these formed folds with a N. 60° W. trend, asymmetric or overturned to the south. Near the south coast the folding of thin Mayagüez group rocks was probably influenced by trends in the Bermeja complex which caused deviations in the regional trends and also some overturning to the north. The San Germán formation, deposited unconformably on the eroded surface of the folded Mayagüez group, contains large allochthonous blocks of older and contemporaneous rocks. These blocks, up to 2 km by 1 km in exposure, were deposited by slumping or sliding due to gravity within and at the base and top of the San Germán formation near Lajas and San Germán. Most rocks in the blocks are extremely contorted and contain deformed Foraminifera. The San Germán and Jicara formations and perhaps the ?Eocene marls were deformed into gentle open folds trending east in the area covered by this report. Oligocene, Miocene, and younger sedimentary rocks have been tilted and uplifted. Large east-west left-lateral transcurrent faults cross the area , offsetting and offset by two sets of transverse faults (N. 45° E., N. 20° W.): most faults are probably Maestrichtian to Oligocene, although minor faulting has continued to the present. Dikes and sills of quartz diorite porphyry and mica-quartz dacite porphyry intrude the ?Maestrichtian San Germán formation and older units. A diorite plug cuts the Bermeja complex, and a granodiorite plug intrudes the Mayagüez group.

Puerto Rico↗
USGS2008

The Ellsworth terrane, coastal Maine: Geochronology, geochemistry, and Nd-Pb isotopic composition - Implications for the rifting of Ganderia

The Ellsworth terrane is one of a number of fault-bounded blocks that occur along the eastern margin of Ganderia, the western-most of the peri-Gondwanan domains in the northern Appalachians that were accreted to Laurentia in the Paleozoic. Geologic relations, detrital zircon ages, and basalt geochemistry suggest that the Ellsworth terrane is part of Ganderia and not an exotic terrane. In the Penobscot Bay area of coastal Maine, the Ellsworth terrane is dominantly composed of bimodal basalt-rhyolite volcanic sequences of the Ellsworth Schist and unconformably overlying Castine Volcanics. We use new U-Pb zircon geochronology, geochemistry, and Nd and Pb isotopes for these volcanic sequences to constrain the petrogenetic history and paleotectonic setting of the Ellsworth terrane and its relationship with Ganderia. U-Pb zircon geochronology for rhyolites indicates that both the Ellsworth Schist (508.6 ?? 0.8 Ma) and overlying Castine Volcanics (503.5 ?? 2.5 Ma) are Middle Cambrian in age. Two tholefitic basalt types are recognized. Type Tb-1 basalt, present as pillowed and massive lava flows and as sills in both units, has depleted La and Ce ([La/Nd]N = 0.53-0.87) values, flat heavy rare earth element (REE) values, and no positive Th or negative Ta anomalies on primitive mantle-normalized diagrams. In contrast, type Th-2 basalt, present only in the Castine Volcanics, has stightly enriched LREE ([La/Yb]N = 1.42-2.92) values and no Th or Th anomalies. Both basalt types have strongly positive ??Nd (500) values (Th-1 = +7.9-+8.6; Th-2 = +5.6-+7.0) and relatively enriched Pb isotopic compositions (206Ph/204Pb = 18.037-19.784; 207/204Pb = 15.531-15.660; 2088Pb/204Pb = 37.810-38.817). The basalts have compositions transitional between recent normal and enriched mid-ocean-ridge basalt, and they were probably derived by partial melting of compositionatly heterogeneous asthenosphenc mantle. Two types of rhyolite also are present. Type R-1 rhyolite, which mostly occurs as tuffs interlayered with basalt in the Ellsworth Schist, is calc-alkaline and characterized by relatively low REE, Zr, and Hf contents, enriched LREE ([La/Yb]N ???3-6), positive Th and negative Th anomalies, ??Nd (500) values near zero (+0.5 to -0.9), and relatively unradiogenic Ph isotope values (206Pb/204Pb = 18.845; 207Pb/ 204Pb = 15.625; 208Pb/204Pb = 38.626). The data suggest that R-1 rhyolite magma was Likely derived by mixing of basalt with melts from a relatively depleted crustal source. Type R-2 rhyolite, which mostly occurs as lava flows and domes in the Castine volcanics, is tholeiitic and characterized by enriched REE with flat patterns ([La/Yb]N = 1-2.5), moderate negative Eu anomalies (Eu/Eu* = 0-34.5), enriched Th, small negative Th anomalies, and ??Nd (500) (+5.8-+7.5) and Ph isotope (206Pb/204Pb = 19.175-19.619; 207Pb/204Pb = 15.605--15.649; 208Pb/204Pb = 38.834-38.851) values that overlap those of the tholeiitic basalts. The data suggest that R-2 rhyolite magma was derived by the partial melting of hydrothermally altered basalt with the addition of a small amount of an enriched component, probably R-1 rhyolite. The geololic, geochemicai, and isotopic characteristics of the bimodal volcanic sequences strongly suggest that the Ellsworth terrane did not evolve as an extensional back-arc basin behind an active arc, but rather it evolved as a proto-oceanic rift petrogenetically similar to Cenozoic rifts like the Gulf of California-Salton mrough and Red Sea-Gulf of Aden rift systems. Such a setting is supported by the presence of serpentinized mantle and zinc-copper-rich massive sulfide deposits in the Ellsworth terrane. We conclude that the Ellsworth terrane developed as a Mid

Geological Society of America Bulletin↗