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USGS2013

Melts of garnet lherzolite: experiments, models and comparison to melts of pyroxenite and carbonated lherzolite

Phase equilibrium experiments on a compositionally modified olivine leucitite from the Tibetan plateau have been carried out from 2.2 to 2.8 GPa and 1,380–1,480 °C. The experiments-produced liquids multiply saturated with spinel and garnet lherzolite phase assemblages (olivine, orthopyroxene, clinopyroxene and spinel ± garnet) under nominally anhydrous conditions. These SiO 2 -undersaturated liquids and published experimental data are utilized to develop a predictive model for garnet lherzolite melting of compositionally variable mantle under anhydrous conditions over the pressure range of 1.9–6 GPa. The model estimates the major element compositions of garnet-saturated melts for a range of mantle lherzolite compositions and predicts the conditions of the spinel to garnet lherzolite phase transition for natural peridotite compositions at above-solidus temperatures and pressures. We compare our predicted garnet lherzolite melts to those of pyroxenite and carbonated lherzolite and develop criteria for distinguishing among melts of these different source types. We also use the model in conjunction with a published predictive model for plagioclase and spinel lherzolite to characterize the differences in major element composition for melts in the plagioclase, spinel and garnet facies and develop tests to distinguish between melts of these three lherzolite facies based on major elements. The model is applied to understand the source materials and conditions of melting for high-K lavas erupted in the Tibetan plateau, basanite–nephelinite lavas erupted early in the evolution of Kilauea volcano, Hawaii, as well as younger tholeiitic to alkali lavas from Kilauea.

Contributions to Mineralogy and Petrology↗

USGS1973

Oxidation during magmatic differentiation, Finnmarka Complex, Oslo area, Norway: Part 2, the mafic silicates

Electron-microprobe analyses are presented for pyroxene, amphibole, and biotite from monzonite, granodiorite, and granite at Finnmarka, Norway. Compositional trends measured in biotite, present in all three rock types, and in amphibole, present in the monzonite and granodiorite, are markedly atypical and are interpreted as reflecting crystallization under progressively more oxidizing conditions. The average Fe/Fe + Mg for biotites from successively more silicic rock types changes from 0.64 → 0.35 → 0.28, and for amphiboles changes from 0.58 in the monzonite to 0.29 in the granodiorite. Analyses of selected areas within amphibole grains in the granodiorite show marked chemical variations, although single-crystal X-ray photographs are sharp and do not reveal multiple phases. On the basis of 33 such analyses, four coupled substitutions are identified as operative; the most unusual finding is the relation of 1 Ti cation to 4 Al IV cations in the unit cell. Variations within individual amphibole grains of the granodiorite resemble changes noted in evolution of amphibole composition from monzonite to granodiorite and are interpreted as reflecting progressive oxidation. Consideration of these data for the mafic silicates, data for the opaque oxides, and the extensive formation of sphene in the granodiorite, has allowed development of schematic reactions and an overall picture of magmatic environment and evolution at Finnmarka. Crystallization apparently took place at PH 2 o of 1000 bars or less and a temperature of about 700 °C. The trend of oxidation during differentiation is more extreme than any heretofore reported. Amphiboles, as well as biotites, may participate in oxidation reactions and may reflect the oxidation-reduction processes that occurred during magmatic evolution.

Oslo↗

USGS2004

Neoproterozoic A-type granitoids of the central and southern Appalachians: Intraplate magmatism associated with episodic rifting of the Rodinian supercontinent

Emplacement of compositionally distinctive granitic plutons accompanied two pulses (765-680 and 620-550Ma) of crustal extension that affected the Rodinian craton at the present location of the central Appalachians during the Neoproterozoic. The dominantly metaluminous plutons display mineralogical and geochemical characteristics of A-type granites including high FeO t/MgO ratios, high abundances of Nb, Zr, Y, Ta, and REE (except Eu), and low concentrations of Sc, Ba, Sr, and Eu. These dike-like, sheet complexes occur throughout the Blue Ridge province of Virginia and North Carolina, and were emplaced at shallow levels in continental crust during active extension, forming locally multiple-intrusive plutons elongated perpendicular to the axis of extension. New U-Pb zircon ages obtained from the Polly Wright Cove (706??4Ma) and Suck Mountain (680??4Ma) plutons indicate that metaluminous magmas continued to be replenished near the end of the first pulse of rifting. The Suck Mountain body is presently the youngest known igneous body associated with earlier rifting. U-Pb zircon ages for the Pound Ridge Granite Gneiss (562??5Ma) and Yonkers Gneiss (563??2Ma) in the Manhattan prong of southeastern New York constitute the first evidence of plutonic felsic activity associated with the later period of rifting in the U.S. Appalachians, and suggest that similar melt-generation processes were operative during both intervals of crustal extension. Fractionation processes involving primary minerals were responsible for much of the compositional variation within individual plutons. Compositions of mapped lithologic units in a subset of plutons studied in detail define overlapping data arrays, indicating that, throughout the province, similar petrologic processes operated locally on magmas that became successively more chemically evolved. Limited variation in source-sensitive Y/Nb and Yb/Ta ratios is consistent with results of melting experiments and indicates that metaluminous granitoids of the supersuite likely were derived through melting of lower crustal sources. Mildly peralkaline rocks of the Robertson River batholith and Irish Creek pluton may be derived from more chemically primitive sources similar in composition to ocean-island basalts. Blue Ridge granitoids define a plutonic episode that occurred during an unsuccessful pulse of crustal extension which predated opening of Iapetus by more than 100 million years. Granitoid gneisses in New York were emplaced during an extension-related, dominantly mafic magmatic episode that ultimately led to development of Iapetus. ?? 2003 Elsevier B.V. All rights reserved.

Precambrian Research↗

USGS2017

Constraining the magmatic system at Mount St. Helens (2004–2008) using Bayesian inversion with physics-based models including gas escape and crystallization

Physics-based models of volcanic eruptions track conduit processes as functions of depth and time. When used in inversions, these models permit integration of diverse geological and geophysical data sets to constrain important parameters of magmatic systems. We develop a 1-D steady state conduit model for effusive eruptions including equilibrium crystallization and gas transport through the conduit and compare with the quasi-steady dome growth phase of Mount St. Helens in 2005. Viscosity increase resulting from pressure-dependent crystallization leads to a natural transition from viscous flow to frictional sliding on the conduit margin. Erupted mass flux depends strongly on wall rock and magma permeabilities due to their impact on magma density. Including both lateral and vertical gas transport reveals competing effects that produce nonmonotonic behavior in the mass flux when increasing magma permeability. Using this physics-based model in a Bayesian inversion, we link data sets from Mount St. Helens such as extrusion flux and earthquake depths with petrological data to estimate unknown model parameters, including magma chamber pressure and water content, magma permeability constants, conduit radius, and friction along the conduit walls. Even with this relatively simple model and limited data, we obtain improved constraints on important model parameters. We find that the magma chamber had low (<5wt%) total volatiles and that the magma permeability scale is well constrained at ~10-11.4 m2 to reproduce observed dome rock porosities. Compared with previous results, higher magma overpressure and lower wall friction are required to compensate for increased viscous resistance while keeping extrusion rate at the observed value.

Washington↗

USGS1971

Structure and petrology of the alpine-type peridotite at Burro Mountain, California, U.S.A.

The alpine-type peridotite at Burro Mountain is a partially serpentinized harzburgite-dunite body approximately 2 km in diameter. It lies in a chaotic mélange derived from the Franciscan Formation (Upper Jurassic to Upper Cretaceous) of the southern Coast Ranges of California. The peridotite is bounded on the east by a vertical fault in the Nacimiento fault zone that brings sedimentary rocks of Taliaferro's (1943 b ) Asuncion Group (Upper Cretaceous) into contact with the peridotite. The peridotite appears to be one of a number of tectonic lenses, having a wide range in size, that make up the mélange . These lenses include metagraywacke, metachert, greenstone, amphibolite, and blueschist, as well as ultramafic rocks, and represent a wide range of pressure-temperature environments. The outer shell of the peridotite is a sheared serpentinite zone 10–15 m thick. The peridotite was tectonically emplaced at its present level as a cold solid mass and had little effect on the mineral assemblages of the Franciscan Formation. Local development of lawsonite and aragonite in shear zones may be related to the peridotite emplacement. Foliated harzburgite forms approximately 60 per cent of the peridotite. It is a lithologically uniform rock that has an olivine: orthopyroxene ratio of approximately 75:25. Accessory clinopyroxene and chromian spinel generally make up less than 5 per cent of the harzburgite. Dunite, composed of olivine, accessory chromian spinel (< 5 per cent), and trace amounts of pyroxene, makes up approximately 40 per cent of the peridotite and occurs as dikes, sills, and irregular bodies in the harzburgite. Olivine and pyroxene show small but significant compositional variations and chromian spinel shows a large range in the cation ratio Cr/(Cr+Al+ Fe 3+ ). The compositional variations in these minerals are related to original differences in bulk chemical composition. The following compositional ranges were determined for minerals in the harzburgite: olivine, Fo 91.1 −Fo 91.4 ; orthopyroxene, En 89.8 −En 91.1 ; clinopyroxene, Ca 47.0 Mg 50.0 Fe 3.0 −Ca 48.7 Mg 48.2 Fe 3.1 ; chromian spinel, Cr/(Cr+Al+Fe 3+ ) 0.37−0.55. The pyroxenes have a range in A1 2 O 3 content of 1.3−3.0 wt per cent. Olivine from dunite ranges from Fo 91 to Fo 92 7 and the chromian spinel has a range in the Cr/(Cr+Al+Fe 3+ ) ratio of 0.30−0.75. Although all the dunites are lithologically similar, three distinct types are recognized on the basis of composition of coexisting olivine and chromian spinel. Structural relations between the three types of dunite suggest three periods of emplacement (possibly overlapping) of dunite into harzburgite. The evidence indicates that the dunite, and probably also the harzburgite crystallized from an ultramafic magma, probably in the upper mantle. After the magmatic episode and crystallization, the peridotite was subjected to a deep-seated plastic deformation and recrystallization. The first phase of the deformation produced a pervasive, planar structural element (S 1 ) that crosscuts many harzburgite-dunite contacts. It is probable that some of the dunite sills were emplaced during this deformation. The foliation, S 1 , is defined by layers of different orthopyroxene content in harzburgite, and by discontinuous layers of chromian spinel in dunite. Flow or slip along S 1 produced slip folds in harzburgite—dunite contacts with axial planes parallel to S 1 . At a later stage, isoclinal folds developed in S 1 , and the present olivine microfabric was probably formed by recrystallization in the stress field that produced the isoclinal folding. In the olivine microfabric, X tends to be perpendicular to the axial planes (S 2 ) of the isoclinal folds and Y and Z tend to form double maxima in S 2 approximately 90° apart. Mg−Fe 2+ distribution between coexisting mineral pairs yields a calculated temperature of formation of approximately 1200 °C. Although this temperature is only a nominal value, it indicates that the mineral pairs equilibrated at a significantly high temperature. In view of the deformation and recrystallization, the calculated temperature possibly represents subsolidus re-equilibration of the minerals during this event. The deformation and recrystallization probably occurred shortly after crystallization while the peridotite was still at a high temperature. A later deep-seated deformation produced small scattered kink folds in S 1 that tend to disrupt the major olivine microfabric. The kink folding was accompanied or followed by the development of kink bands in olivine that reflect intragranular gliding on the system T = [ Okl ], t = [100]. The kink bands probably formed at a minimum temperature of 1000 °C. Following the deep-seated deformation, which probably took place in the mantle, the peridotite mass was tectonically detached and moved upward to its present level in the crust. Cleavages, joints, and faults provided channels for water to pervade the peridotite and allow alteration of the primary minerals.

California↗

USGS2006

U.S. Geological Survey and Bureau of Land Management Cooperative Coalbed Methane Project in the Powder River Basin, Wyoming

The Bureau of Land Management (BLM) Wyoming Reservoir Management Group and the U.S. Geological Survey (USGS) began a cooperative project in 1999 to collect technical and analytical data on coalbed methane (CBM) resources and quality of the water produced from coalbeds in the Wyoming part of the Powder River Basin. The agencies have complementary but divergent goals and these kinds of data are essential to accomplish their respective resource evaluation and management tasks. The project also addresses the general public need for information pertaining to Powder River Basin CBM resources and development. BLM needs, which relate primarily to the management of CBM resources, include improved gas content and gas in-place estimates for reservoir characterization and resource/reserve assessment, evaluation, and utilization. USGS goals include a basinwide assessment of CBM resources, an improved understanding of the nature and origin of coalbed gases and formation waters, and the development of predictive models for the assessment of CBM resources that can be used for such purposes in other basins in the United States (for example, the Bighorn, Greater Green River, and Williston Basins) and in other countries throughout the world (for example, Indonesia, New Zealand, and the Philippines). Samples of coal, produced water, and gas from coalbed methane drill holes throughout the Powder River Basin, many of which are adjacent to several active mine areas (figs. 1, 2), have been collected by personnel in the USGS, BLM Reservoir Management Group, and Casper and Buffalo BLM Field Offices. Sampling was done under confidentiality agreements with 29 participating CBM companies and operators. Analyses run on the samples include coal permeability, coal quality and chemistry, coal petrography and petrology, methane desorption and adsorption, produced-water chemistry, and gas composition and isotopes. The USGS has supplied results to the BLM Reservoir Management Group for their resource management needs, and data are released when the terms of the confidentiality agreements are completed and consent is obtained.

Fact Sheet↗

USGS2009

Low-fluorine Stockwork Molybdenite Deposits

Low-fluorine stockwork molybdenite deposits are closely related to porphyry copper deposits, being similar in their tectonic setting (continental volcanic arc) and the petrology (calc-alkaline) of associated igneous rock types. They are mainly restricted to the Cordillera of western Canada and the northwest United States, and their distribution elsewhere in the world may be limited. The deposits consist of stockwork bodies of molybdenite-bearing quartz veinlets that are present in and around the upper parts of intermediate to felsic intrusions. The deposits are relatively low grade (0.05 to 0.2 percent Mo), but relatively large, commonly >50 million tons. The source plutons for these deposits range from granodiorite to granite in composition; the deposits primarily form in continental margin subduction-related magmatic arcs, often concurrent with formation of nearby porphyry copper deposits. Oxidation of pyrite in unmined deposits or in tailings and waste rock during weathering can lead to development of acid-rock drainage and limonite-rich gossans. Waters associated with low-fluorine stockwork molybdenite deposits tend to be nearly neutral in pH; variable in concentrations of molybdenum (<2 to >10,000 ug/L); below regulatory guidelines for copper, iron, lead, zinc, and mercury; and locally may exceed guidelines for arsenic, cadmium, and selenium.

Open-File Report↗

USGS1991

A lead isotopic study of the Stillwater Complex, Montana: constraints on crustal contamination and source regions

Analyses of the Pb isotopic compositions of plagioclase from 23 samples covering the stratigraphic thickness of the Stillwater Complex indicate a narrow range of apparent initial isotopic compositions (206Pb/ 204Pb=13.95; 207Pb/204Pb=14.95-15.01; 208Pb/204Pb=33.6). The uniformity of our data is in contrast to, but not necessarily contradictory to, other recent investigations which give indications that the complex formed by repeated injection of magmas with at least two distinct compositions that were presumably derived from different source regions. Samples from the Basal series of the complex have consistently higher 207Pb/204Pb ratios, suggesting either minor contamination from adjacent country rocks or a slight distinction between parental magmas. Apparent initial Pb isotopic compositions of the complex are very radiogenic compared to Late Archean model-mantle values, but are nearly identical to initial Pb isotopic compositions found for the the adjacent, slightly older (2.73-2.79 Ga), Late Archean crustal suite in the Beartooth Mountains. Contamination of magmas parental to the Stillwater Complex by the Late Archean crustal suite is rejected for two reasons: (1) Th and U concentrations in Stillwater rocks and plagioclase are very low (about 0.08 and 0.02 ppm respectively), yet Th/U ratios are uniform at about 4, in contrast to the highly variable (2-26) but often high Th/U ratios found for the Late Archean crustal complex; (2) it seems improbable that any contamination process would have adjusted the isotopic compositions of the diverse magmas entering the Stillwater chamber to near-identical values. The preferred hypothesis to explain the Pb isotopic data for the Stillwater Complex and the associated Late Archean crustal suite involves a major Late Archean crust-forming event that resulted in a compositionally complex crust/mantle system with relatively homogeneous and unusual Pb isotopic compositions. The parental magmas of the Stillwater Complex were generated at different levels within this crust/mantle system, before isotopic contrasts could develop by radioactive decay within compositionally discrete reservoirs. This situation limits the utility of all isotopic tracer systems in discriminating among the various mantle and crustal reservoirs that may have affected the final isotopic character of the Stillwater magmas. The late Archean crustal complex and the Stillwater Complex melts were ultimately derived from the same distinct mantle without obvious direct interaction with the Middle to Early Archean crust present in the region. ?? 1991 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS2006

Protolith and metamorphic ages of the Haiyangsuo Complex, eastern China: A non-UHP exotic tectonic slab in the Sulu ultrahigh-pressure terrane

The Haiyangsuo Complex in the NE Sulu ultrahigh-pressure (UHP) terrane has discontinuous, coastal exposures of Late Archean gneiss with amphibolitized granulite, amphibolite, Paleoproterozoic metagabbroic intrusives, and Cretaceous granitic dikes over an area of about 15 km2. The U-Pb SHRIMP dating of zircons indicates that theprotolith age of a garnet-biotite gneiss is >2500 Ma, whereas the granulite-facie metamorphism occurred at around 1800 Ma. A gabbroic intrusion was dated at ???1730 Ma, and the formation of amphibolite-facies assemblages in both metagabbro and granulite occurred at ???340-460 Ma. Petrologic and geochronological data indicate that these various rocks show no evidence of Triassic eclogite-facies metamorphism and Neoproterozoic protolith ages that are characteristics of Sulu-Dabie HP-UHP rocks, except Neoproterozoic inherited ages from post-collisional Jurassic granitic dikes. Haiyangsuo retrograde granulites with amphibolite-facies assemblages within the gneiss preserve relict garnet formed during granulite-facies metamorphism at ???1.85 Ga. The Paleoproterozoic metamorphic events are almost coeval with gabbroic intrusions. The granulite-bearing gneiss unit and gabbro-dominated unit of the Haiyangsuo Complex were intruded by thin granitic dikes at about 160 Ma, which is coeval with post-collisional granitic intrusions in the Sulu terrane. We suggest that the Haiyangsuo Complex may represent a fragment of the Jiao-Liao-Ji Paleoproterozoic terrane developed at the eastern margin of the Sino-Korean basement, which was juxtaposed with the Sulu terrane prior to Jurassic granitic activity and regional deformation. ?? Springer-Verlag 2006.

Conference Paper↗

USGS2007

Petrology and tectonics of Phanerozoic continent formation: From island arcs to accretion and continental arc magmatism

Mesozoic continental arcs in the North American Cordillera were examined here to establish a baseline model for Phanerozoic continent formation. We combine new trace-element data on lower crustal xenoliths from the Mesozoic Sierra Nevada Batholith with an extensive grid-based geochemical map of the Peninsular Ranges Batholith, the southern equivalent of the Sierras. Collectively, these observations give a three-dimensional view of the crust, which permits the petrogenesis and tectonics of Phanerozoic crust formation to be linked in space and time. Subduction of the Farallon plate beneath North America during the Triassic to early Cretaceous was characterized by trench retreat and slab rollback because old and cold oceanic lithosphere was being subducted. This generated an extensional subduction zone, which created fringing island arcs just off the Paleozoic continental margin. However, as the age of the Farallon plate at the time of subduction decreased, the extensional environment waned, allowing the fringing island arc to accrete onto the continental margin. With continued subduction, a continental arc was born and a progressively more compressional environment developed as the age of subducting slab continued to young. Refinement into a felsic crust occurred after accretion, that is, during the continental arc stage, wherein a thickened crustal and lithospheric column permitted a longer differentiation column. New basaltic arc magmas underplate and intrude the accreted terrane, suture, and former continental margin. Interaction of these basaltic magmas with pre-existing crust and lithospheric mantle created garnet pyroxenitic mafic cumulates by fractional crystallization at depth as well as gabbroic and garnet pyroxenitic restites at shallower levels by melting of pre-existing lower crust. The complementary felsic plutons formed by these deep-seated differentiation processes rose into the upper crust, stitching together the accreted terrane, suture and former continental margin. The mafic cumulates and restites, owing to their high densities, eventually foundered into the mantle, leaving behind a more felsic crust. Our grid-based sampling allows us to estimate an unbiased average upper crustal composition for the Peninsular Ranges Batholith. Major and trace-element compositions are very similar to global continental crust averaged over space and time, but in detail, the Peninsular Ranges are slightly lower in compatible to mildly incompatible elements, MgO, Mg#, V, Sc, Co, and Cr. The compositional similarities suggest a strong arc component in global continental crust, but the slight discrepancies suggest that additional crust formation processes are also important in continent formation as a whole. Finally, the delaminated Sierran garnet pyroxenites have some of the lowest U/Pb ratios ever measured for silicate rocks. Such material, if recycled and stored in the deep mantle, would generate a reservoir with very unradiogenic Pb, providing one solution to the global Pb isotope paradox.

Earth and Planetary Science Letters↗

USGS1987

Sedimentary processes on the northwestern Iberian continental margin viewed by long-range side-scan sonar and seismic data

The effects of an eastern boundary current in the North Atlantic have been mapped from about 39° north latitude along the Iberian margin to as far north as 43°30 north latitude at the western margin of Galicia Bank. The geostrophic current has produced sediment drifts that are covered with bedforms. The sediment drifts are difficult to detect on Gloria long-range side-scan sonar data but are easily resolved on seismic-reflection records as anomalously thick accumulations of sediment banked against either buried or outcropping basement highs. The bedforms ornamenting the drift surfaces were subdivided into 1,000-m water-depth intervals, and their dimensions were tabulated. There are few bedforms in water depths less han 2,000 m, but from depths between 2,000 and 4,000 m they are numerous and have a mean wavelength of 695 m. Bedforms from depths greater than 4,000 m have a mean wavelength of 999 m. The different wavelengths from different water depths suggest two distinct and separated boundary flows. The wave heights of all bedforms found in water depths greater than 2,000 m are less than 10 m. In order to investigate the continuity of sediment drifting through geological time, the stratigraphic section drilled at DSDP Site 398 was reinterpreted and, using seismic-reflection profiles, was traced throughout the northern Iberian margin. Together, the lithostratigraphic and seismic data indicate that sediment drifting developed along this margin in the Eocene. The lithofacies of the Eocene section is t e oldest to have numerous layers of sand and silt. An unconformity separates the Eocene section from the latest Miocene-Pliocene section. The unconformity is interpreted to be the result of the initial pulses of Mediterranean outflow that followed the Messinian desiccation events. A second period of sediment drifting commenced during the Pliocene once the Mediterranean basin filled and the flow out of the Strait of Gibraltar resumed.

Journal of Sedimentary Petrology↗

USGS1991

Age and petrology of the Tertiary As Sarat volcanic field, southwestern Saudi Arabia

Harrat As Sarat forms the second smallest and southernmost of the basalt fields of western Saudi Arabia and is part of a voluminous Red Sea rift-related continental alkali basalt province. The rocks of the As Sarat were emplaced during the first stage of Red Sea rifting and represent the northernmost extension of the Tertiary Trap Series volcanics that occur mainly in the Yemen Arab Republic and Ethiopia. The field consists of up to 580 m of basalt flows, that are intruded by basaltic plugs, necks, minor dikes, and highly evolved peralkaline trachyte intrusions. K-Ar ages indicate that the As Sarat field formed between 31 and 22 Ma and contains an eruption hiatus of one million years that began about 25 Ma ago. Pre-hiatus flows are primarily hypersthene normative intersertal subalkaline basalt, whereas the majority of post-hiatus flows are nepheline normative alkali basalt and hawaiite with trachytic textures. Normative compositions of the basalts are consistent with their genesis by partial melting at varying depths. Trace element abundances in the basalt indicate that varying degrees of partial melting and fractional crystallization (or crystal accumulation) had major and minor roles, respectively, in development of compositional variation in these rocks. Modeling indicates that the pre-hiatus subalkaline basalts represent 8–10 percent mantle melting at depths of about 70 km and the post-hiatus alkali basalts represent 4–9 percent mantle melting at depths greater than 70 km.

southwestern Saudi Arabia↗

USGS2025

Confocal laser-scanning microscopy (CLSM)-based thermal maturity of Tasmanites and progress in standardization of fluorescence microspectrometry

Evaluation of thermal maturity in vitrinite-free or vitrinite-deficient sediments via fluorescence microspectrometry can provide relevant information related to petroleum exploration and thermal history assessment. However, variation in spectral fluorescence properties of alginite macerals with increasing thermal maturity is largely underexplored. Here, authors of this study have applied confocal laser-scanning microscopy (CLSM) in conjunction with fluorescence microspectrometry to a maturity series of marine Upper Devonian Tasmanites algae from the Ohio Shale (Huron Member) and a single sample from the Marcellus Formation of the Appalachian Basin. Spectral fluorescence properties of Tasmanites were evaluated in relation to orientation, measurement location, and the number of measurements per sample, and were compared to published literature. Emission spectra of Tasmanites from continuous wave laser excitation (405 nm) were acquired from sections perpendicular and parallel to bedding and at different positions within individual Tasmanites bodies. The results showed a progressive red-shift in emission maxima (λ max ) in a large sample sized maturity series ( N = 19), e.g., 493 to 578 nm for the perpendicular section at middle position. Further, blue-shifted apex and mineral-adjacent positions within sections perpendicular to bedding were observed, with the latter being reported here for the first time. While blue-shift at apex positions can be attributed to mechanical deformation-induced reorientation of photoselected fluorophores, the blue-shifted mineral-adjacent positions could result from strain loading and development of a plastic deformation region at the mineral contact zone with Tasmanites . A decrease in standard deviation with increasing number of measured emission maxima is well-observed, and 15 to 20 individual measurements per sample appears sufficient for low standard deviation and coefficient of variance. CLSM-derived thermal maturity parameters indicated that a moderate positive correlation of red/green quotient (Q 650/500 ; R 2 = 0.67) with solid bitumen reflectance (BR o in %) exists. For reproducible results, the determination of λ max and Q 650/500 should be conducted exclusively in the middle position at perpendicular and parallel sections of the polished whole-rock pellets, where the lowest standard deviation in measurement was observed. These results strengthen the suitability and relevance of the CLSM technique in thermal maturity studies of dispersed organic matter (DOM) and contribute to the standardization of fluorescence microspectrometry methods in organic petrology investigation.

Kentucky, Ohio, Virginia, West Virginia↗

USGS1986

Guidelines to classification and nomenclature of Arabian felsic plutonic rocks

Well-defined procedures for classifying the felsic plutonic rocks of the Arabian Shield on the basis of petrographic, chemical and lithostratigraphic criteria and mineral-resource potential have been adopted and developed in the Saudi Arabian Deputy Ministry for Mineral Resources over the past decade. A number of problems with conventional classification schemes have been identified and resolved; others, notably those arising from difficulties in identifying precise mineral compositions, continue to present difficulties. The petrographic nomenclature used is essentially that recommended by the International Union of Geological Sciences. Problems that have arisen include the definition of: (1) rocks with sodic, zoned or perthitic feldspar, (2) trondhjemites, and (3) alkali granites. Chemical classification has been largely based on relative molar amounts of alumina, lime and alkalis, and the use of conventional variation diagrams, but pilot studies utilizing univariate and multivariate statistical techniques have been made. The classification used in Saudi Arabia for stratigraphic purposes is a hierarchy of formation-rank units, suites and super-suites as defined in the Saudi Arabian stratigraphic code. For genetic and petrological studies, a grouping as ‘associations’ of similar and genetically related lithologies is commonly used. In order to indicate mineral-resource potential, the felsic plutons are classed as common, precursor, specialized or mineralized, in order of increasing exploration significance.

Journal of African Earth Sciences↗

USGS2021

Proportions, timing, and re-equilibration progress during the 1959 Summit Eruption of Kīlauea: An example of magma mixing processes operating during OIB petrogenesis

Petrographic and chemical analysis of scoria samples collected during the 1959 Kīlauea summit eruption illustrates the progress of thermal and chemical homogenization of the melts, and the gradual growth and/or re-equilibration of olivine phenocrysts, over the course of the eruption. Glass compositions show that thermal equilibration was largely complete within the span of the eruption, whereas chemical homogenization was a work in progress. The olivine phenocryst population, known to contain conspicuous antecrystic components, is also hybrid within the euhedral population. The bulk of the olivine reached the level of the erupting magma on November 18–19, 1959. Zoning patterns in olivine phenocrysts show that initially unzoned grains developed normal zoning by the end of the eruption. Reverse zoning in relatively Fe-rich olivine phenocrysts (interpreted as cognate to the stored magma) was progressively eliminated from November 21 to December 19, 1959, by diffusive re-equilibration between crystals and melt. Toward the end of the eruption, the only olivine composition in direct contact with the melt was Fo 84–86 , with the original rim compositional heterogeneity gone in 4–5 weeks’ time. Activity in December 1959 differed from that in November, as high fountaining events were more closely spaced and almost all samples were picritic, with bulk MgO ≥16·5 wt%. Three different levels were in play during the 1959 eruption: a deep source for high-MgO melts and forsteritic (Fo 87–89 ) olivines, an intermediate source for the bulk of the stored magma, and a shallower source for the most differentiated magma. This model is consistent with geophysical, petrological and chemical observations. Comparison of the 1959 eruption with results from older explosive deposits suggests that stored and recharge melts and olivine from the deeper parts of Kīlauea’s plumbing are similar in composition to those observed or inferred in the 1959 eruption, so they behave similarly during extrusive and explosive periods alike.

Hawaii↗

USGS1997

Comagmatic A-type granophyre and rhyolite from the Alid volcanic center, eritrea, northeast Africa

Granophyric blocks within late-Pleistocene pyroclastic flow ejecta from the Alid volcanic center, northeast Africa, are the rapidly crystallized, intrusive equivalent of pumice from the pyroclastic flow. Phenocryst compositions and geochemical characteristics of the pumice and granophyre are virtually identical. Silicate melt inclusions and other geochemical and geological constraints reveal those processes leading to development of the granophyric texture. Rhyolitic (A-type) magma with ∼2.6 wt % dissolved H 2 O and a temperature near 870°C was intruded to within 2–4 km of the surface, causing deformation and structural doming of shallow marine and subaerial strata. Eruptions of crystal-poor rhyolite from this shallow magma chamber caused degassing, which forced undercooling and consequent granophyric crystallization of some of the magma remaining in the intrusion. The most recent eruption from Alid excavated the crystallized granitic wall of the magma chamber, bringing the granophyric clasts to the surface.

Journal of Petrology↗

USGS2017

Petrology and geochronology of Mesoproterozoic basement of the Mount Rogers area of southwestern Virginia and northwestern North Carolina: Implications for the Precambrian tectonic evolution of the southern Blue Ridge province

Results from new geologic mapping, SHRIMP U-Pb geochronology, and petrologic studies indicate that Mesoproterozoic basement in the northern French Broad massif near Mount Rogers consists of multiple, mostly granitic plutons, map- and outcrop-scale xenoliths of pre-existing crustal rocks, and remnants of formerly overlying meta-sedimentary lithologies. Zircon and titanite ages demonstrate that these rocks collectively record nearly 350 m.y. of tectonic evolution including periods of igneous intrusion at ca . 1190 to 1130 Ma (Early Magmatic Suite) and ca . 1075 to 1030 Ma (Late Magmatic Suite) and three episodes of regional metamorphism at ca . 1170 to 1140, 1070 to 1020, and 1000 to 970 Ma. The existence of ca . 1.3 Ga age crust is indicated by (1) orthogranofels of ca . 1.32 Ga age in a map-scale xenolith, (2) inherited zircons of ca . 1.33 to 1.29 Ga age in Early Magmatic Suite plutons, and (3) ca . 1.36 to 1.30 Ga age detrital zircons in meta-sedimentary lithologies. Mineral assemblages developed in amphibolites and granofelses indicate that metamorphism during both Mesoproterozoic episodes occurred at upper amphibolite- to lower granulite-facies conditions. Syn-orogenic Early Magmatic Suite plutons emplaced at ca . 1190 to 1145 Ma are characterized by high-K, variably magnesian, dominantly calc-alkalic compositions, and have trace-element characteristics indicative of continental-arc magmatic origin involving melting of thick continental crust. In contrast, ca . 1140 Ma age quartz syenite displays A-type features indicating derivation from depleted crustal sources with increased mantle input during waning stages of regional contraction. Plutons of the compositionally bimodal Late Magmatic Suite include (1) ca . 1060 Ma meta-granite with geochemical characteristics transitional between silicic rocks of arc systems and post-collisional granites of A-type lineage, and (2) ca . 1055 Ma monzodioritic rocks with A-type compositional characteristics that likely reflect derivation from fertile, mafic sources in the lower crust. Collectively, these data suggest that Mesoproterozoic rocks of the study area preserve evidence of multiple orogenic episodes that likely involved continental-arc development and deformation at ca . 1150 Ma followed by crustal thickening at ca . 1060 Ma. Field relations and geochronologic data indicate that regional uplift and sedimentation occurred at ca . 1100 Ma between the two episodes of overlapping magmatism and orogenesis. The nature and timing of Mesoproterozoic events recorded in basement rocks of the study area illustrate significant differences in the lithologic assemblages and geologic history preserved by Mesoproterozoic basement of the adjacent Shenandoah and French Broad massifs, suggesting that the Blue Ridge massifs occupied different locations within the regional Grenville-age orogen until about 1070 Ma when the effects of Ottawan-age tectonics began to affect both areas. The near ubiquity of Ottawan-age orogenic activity recorded in Mesoproterozoic rocks of the Blue Ridge, other Appalachian inliers such as the New Jersey Highlands, and the Grenville province of Canada, including the Adirondacks, suggests that these formerly disparate terranes were amalgamated to form a common, regional orogen by this time.

North Carolina, Virginia↗

USGS1979

Petrology, composition, and age of intrusive rocks associated with the Quartz Hill molybdenite deposit, southeastern Alaska

A large porphyry molybdenum deposit (Quartz Hill deposit) was recently discovered in the heart of the Coast Range batholithic complex about 70 km east of Ketchikan, southeastern Alaska. Intrusive rocks associated with the mineral deposit form two composite epizonal to hypabyssal stocks and many dikes in country rocks. The stocks are characterized by a variety of textural rock types varying from equigranular or weakly seriate biotite granite to porphyries with aphanitic or very fine grained and aplitic groundmasses. These rocks contain about equal amounts of quartz, albitic plagioclase, and microperthitic microcline and less than 2.5% biotite. Unaltered rocks contain between 0.2 and 1% CaO, less than 1.7% combined Fe 2 O 3 , FeO, and MgO, and 74.4 to 77.7% SiO 2 . Total alkalis are between 8 and 9%, and K 2 O/Na 2 O is about 1.1. The range of major-oxide variation is small, but it is systematically related to lithology. Many trace-elements, including B, Pb, Sn, and Li have low concentrations. Intrusive rocks associated with the Quartz Hill deposit are more albitic and possibly trace-element depleted compared to some other rocks associated with porphyry molybdenum deposits.All observed metallization and alteration is within the Quartz Hill stock. Molybdenite forms fracture coatings and occurs in veins with quartz. Alteration is widespread and includes development of secondary quartz, pyrite, K-feldspar, biotite, white mica, chlorite, and zeolite. Field relations indicate that the stocks were emplaced after regional uplift and erosion of the Coast Range batholithic complex, and K–Ar data show that intrusion and alteration took place in late Oligocene time, about 27 to 30 Ma ago. Data from the Ketchikan quadrangle indicate that porphyry molybdenum metallization in the Coast Range batholithic complex is associated with regionally extensive but spotty, middle Tertiary or younger, felsic magmatism.

Alaska↗