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Abstract: We present a detailed case study of a strike-slip shear zone that cut homogeneous granite near the brittle–ductile transition. This zone contains three distinct strain facies: (1) a 2–3-km-wide belt of foliated granite, (2) a 100–300-m-wide belt of small localized shear zones, and (3) a 200–400-m-wide belt of ultramylonite that includes both homogeneous and local pinstripe ultramylonites. The foliated granite formed via dislocation creep of quartz and minor dissolution–precipitation creep and dislocation creep of feldspar under amphibolite-facies conditions shortly after granite crystallization. The localized shear zones and homogeneous ultramylonites formed directly from pseudotachylyte and cataclasite, and they deformed by granular flow—grain-boundary sliding coupled with dissolution–precipitation creep. These deformation features reveal a three-phase rheologic evolution in granite near the brittle–ductile transition. Phase 1 is recorded by the foliated granite, and the rheology was governed by dislocation creep of quartz. Phase 2 was a transient period of brittle deformation and represents a temporal strength maxima in the fault zone. Phase 3 was a long-lived period of ductile deformation, and the rheology was governed by the granular flow mechanism. Strain weakening in this zone was a direct result of grain size reduction and phase mixing during transient brittle deformation. [Copyright &y& Elsevier]

The article offers information on various bryophytes species from different countries. It states the barbilophozia atlantica was found in Portugal with good quantity and grows in granitic rock crevices. It mentions the bucklandiella microcarpa moss native in Hungary which grows on non-calcareous rocks. It adds the climacium dendroides located in wet and damp grasslands in the U.S.

Most typical rock outcrop plants of the eastern United States occur either on calcareous or on non-calcareous outcrops, but not both. Often this is because their growth is inhibited in soil from the non-native substrate, as shown in this study for the granite outcrop endemic sedge Cyperus granitophilus when grown on limestone soil. For those rock outcrop species that can exploit both calcareous and non-calcareous substrates, it might be expected that they would do so by substrate specialization, with each population growing better on its native substrate than the other substrate. However, in most previously tested species, populations from both substrates grow well on the same substrate and both are inhibited on the second substrate. Populations found on the second substrate, though inhibited, are nevertheless able to maintain good health and grow sufficiently to maintain themselves there. In this study we show that the widespread sedge Cyperus aristatus (=C. inflexus), a regular component of limestone, sandstone, granite, chert, and serpentine rock outcrop communities in the eastern United States, exploits both calcareous and non-calcareous substrates by substrate specialization, something previously shown in only one other eastern United States rock outcrop species. [ABSTRACT FROM AUTHOR]

Abstract: Mesoproterozoic (~1.1Ga) plutons of the eastern Llano Uplift, central Texas, USA contain two types of magmatic enclaves (<1% by vol.). Although volumetrically insignificant, the enclaves contain important petrogenetic information. Type I enclaves are felsic in composition (70–75wt.% SiO2), with mineral assemblages and chemical compositions comparable with the host granites, but typically display a finer grained texture. They are interpreted as partly chilled disrupted material from the margins and roof of the plutons. Type II enclaves are intermediate in composition (~56–69wt.% SiO2), with many elements defining trends continuous with the host granites. Both types of enclaves display sharp borders in contact with the host granite suggesting magma quenching with little or no physical exchange between host granite and enclave magma. Type II enclaves contained within the Marble Falls (MF) and Lone Grove (LG) plutons exhibit enrichments in Y, Nb, and Zr relative to their respective host granites. Enrichments in these incompatible trace elements at low SiO2, renders unlikely the possibility that the MF and LG Type II enclaves are the result of partial melting (anatexis) of mafic crustal rocks. Numerical modeling of fractional crystallization and simple mixing fails to explain the observed trace element trends. Because no coeval mafic to intermediate rocks are exposed in the uplift, characteristics of Type II enclave source magma(s) is uncertain. However, assuming source magmas similar to primitive continental arc basaltic andesite, trace-element trends (i.e., incompatible element enrichment and compatible element depletion) can be adequately replicated by a replenishment fractional crystallization (RFC) model. Chemistry of the MF and LG Type II enclaves suggest repeated replenishment of primitive magmas with only limited interaction with the host granitic magmas; the more primitive enclave magmas evolving in near chemical isolation by RFC processes. However, evidence from Type II enclaves in two other plutons in the Llano Uplift (Kingsland and Enchanted Rock) suggest that the isolation was non-ideal; i.e., some limited mixing may have occurred. Rapid quenching likely limited the potential for physical and chemical exchange between Type II enclaves and their host granite magmas. [Copyright &y& Elsevier]

Abstract: The gamma radiation emitted from a variety of commercial decorative granites available for use in U.S. homes has been measured with portable survey meters as well as an NaI(Th) gamma spectrometer. The 40K, U-nat, and 232Th activity concentrations were determined using a full-spectrum analysis. The dose rates that would result from two different arrangements of decorative granite slabs as countertops were explored in simulations involving an adult anthropomorphic phantom. [Copyright &y& Elsevier]

Abstract: Growing evidence supports the notion that plutons are constructed incrementally, commonly over long periods of time, yet field evidence for the multiple injections that seem to be required is commonly sparse or absent. Timescales of up to several million years, among other arguments, indicate that the dominant volume does not remain largely molten, yet if growing plutons are constructed from rapidly solidifying increments it is unlikely that intrusive contacts would escape notice. A model wherein magma increments are emplaced into melt-bearing but crystal-rich host, rather than either solid or crystal-poor material, provides a plausible explanation for this apparent conundrum. A partially solidified intrusion undoubtedly comprises zones with contrasting melt fraction and therefore strength. Depending on whether these zones behave elastically or ductilely in response to dike emplacement, intruding magma may spread to form sheets by either of two mechanisms. If the melt-bearing host is elastic on the relevant timescale, magma spreads rather than continuing to propagate upward, where it encounters a zone of higher rigidity (higher crystal fraction). Similarly, if the dike at first ascends through rigid, melt-poor material and then encounters a zone that is weak enough (poor enough in crystals) to respond ductilely, the ascending material will also spread because the dike tip ceases to propagate as in rigid material. We propose that ascending magma is thus in essence trapped, by either mechanism, within relatively crystal-poor zones. Contacts will commonly be obscure from the start because the contrast between intruding material (crystal-poorer magma) and host (crystal-richer material) is subtle, and they may be obscured even further by subsequent destabilization of the crystal-melt framework. Field evidence and zircon zoning stratigraphy in plutons of the Colorado River region of southern Nevada support the hypothesis that emplacement of magma replenishments into a crystal-laden host is important in pluton construction. The dominant granite unit of the Spirit Mountain batholith displays only subtle internal contacts. However, ages and elemental zoning in zircons demonstrate a protracted history of almost 2 million years, major fluctuations in T and host melt chemistry, and mixing of strongly contrasting populations of magmatic zircon in single samples. We interpret this to reflect reactivation of rigid sponge and mush and entrainment of earlier-formed crystals, and we infer that this was in response to granitic replenishment. Much of the smaller Aztec Wash pluton comprises interlayered cumulate-textured quartz monzonite and mafic sheets. The latest phase of pluton emplacement is marked by numerous thick, fine-grained granite “sills” that intruded the subhorizontal quartz monzonite sheets. Contacts between granite and quartz monzonite are “soft,” highly irregular on cm–dm scale with coarse xenocrysts from the quartz monzonite entrained in the fine-grained granite. We interpret the granite replenishments to have spread laterally within crystal-rich, melt-bearing quartz monzonite beneath rigid mafic sheets. In this case, clear evidence for the emplacement process is fortuitously preserved because the granite was emplaced in the waning stage of the thermal lifetime of the pluton, and because the mafic sheets enhance the strength contrast and make the geometry more visible. Similar “sills” of fine-grained granite were also preserved during the late stages of the history of the Spirit Mountain batholith. [Copyright &y& Elsevier]

We determined Ar/Ar eruption ages of eight extrusions from the Pleistocene Coso volcanic field, a long-lived series of small volume rhyolitic domes in eastern California. Combined with ion-microprobe dating of crystal ages of zircon and allanite from these lavas and from granophyre geothermal well cuttings, we were able to track the range of magma-production rates over the past 650 ka at Coso. In ≤230 ka rhyolites we find no evidence of protracted magma residence or recycled zircon (or allanite) from Pleistocene predecessors. A significant subset of zircon in the ~85 ka rhyolites yielded ages between ~100 and 200 Ma, requiring that generation of at least some rhyolites involves material from Mesozoic basement. Similar zircon xenocrysts are found in an ~200 ka granophyre. The new age constraints imply that magma evolution at Coso can occur rapidly as demonstrated by significant changes in rhyolite composition over short time intervals (≤10’s to 100’s ka). In conjunction with radioisotopic age constraints from other young silicic volcanic fields, dating of Coso rhyolites highlights the fact that at least some (and often the more voluminous) rhyolites are produced relatively rapidly, but that many small-volume rhyolites likely represent separation from long-lived mushy magma bodies. [ABSTRACT FROM AUTHOR]

Abstract  Deposition of quartz–molybdenite–pyrite–topaz–muscovite–fluorite and subsequent hübnerite and sulfide–fluorite–rhodochrosite mineralization at the Sweet Home Mine occurred coeval with the final stage of magmatic activity and ore formation at the nearby world-class Climax molybdenum deposit about 26 to 25 m.y. ago. The mineralization occurred at depths of about 3,000 m and is related to at least two major fluid systems: (1) one dominated by magmatic fluids, and (2) another dominated by meteoric water. The sulfur isotopic composition of pyrite, strontium isotopes and REY distribution in fluorite suggest that the early-stage quartz–molybdenite–pyrite–topaz–muscovite–fluorite mineral assemblage was deposited from magmatic fluids under a fluctuating pressure regime at temperatures of about 400°C as indicated by CO2-bearing, moderately saline (7.5–12.5 wt.% NaCl equiv.) fluid inclusions. LA-ICPMS analyses of fluid inclusions in quartz demonstrate that fluids from the Sweet Home Mine are enriched in incompatible elements but have considerably lower metal contents than those reported from porphyry–Cu–Au–Mo or Climax-type deposits. The ore-forming fluid exsolved from a highly differentiated magma possibly related to the deep-seated Alma Batholith or distal porphyry stock(s). Sulfide mineralization, marking the periphery of Climax-type porphyry systems, with fluorite and rhodochrosite as gangue minerals was deposited under a hydrostatic pressure regime from low-salinity ± CO2-bearing fluids with low metal content at temperatures below 400°C. The sulfide mineralization is characterized by mostly negative δ34S values for sphalerite, galena, chalcopyrite, and tetrahedrite, highly variable δ18O values for rhodochrosite, and low REE contents in fluorite. The Pb isotopic composition of galena as well as the highly variable 87Sr/86Sr ratios of fluorite, rhodochrosite, and apatite indicates that at least part of the Pb and Sr originated from a much more radiogenic source than Climax-type granites. It is suggested that the sulfide mineralization at the Sweet Home Mine formed from magmatic fluids that mixed with variable amounts of externally derived fluids. The migration of the latter fluids, that were major components during late-stage mineralization at the Sweet Home Mine, was probably driven by a buried magmatic intrusion. [ABSTRACT FROM AUTHOR]

In Phanerozoic circum-Pacific orogenic belts, most "postcollisional" S-type granites and associated high-temperature, low-pressure metamorphic complexes formed during early arc extension. The granites are part of a tripartite association consisting of (1) inboard S-type granite, (2) outboard oceanic arc, and (3) intervening, turbidite-filled backarc basin. S-type granites herald the formation of new outboard oceanic arc and extensional backarc systems, but thickening of a preexisting, sediment-dominated backarc basin is a prerequisite for their generation. In these environments, S-type plutonism is triggered by renewal of arc magmatism following thickening, when hot basaltic magmas are intruded into the thickened backarc crust once slab retreat is reestablished. With ongoing extension during retreat, the crust becomes progressively thinned, the sedimentary contribution is diminished, and the granites lose their S-type character. Such tripartite associations involving S-type granite are probably diagnostic of repeated slab-retreat episodes, and the Jurassic U.S. cordillera might be an example. [ABSTRACT FROM AUTHOR]

The >5 Moz (1 oz (troy) = 31.103 g) gold veins of the Liese Zone and nearby prospects in the Goodpaster Mining District of east central Alaska are spatially and temporally associated with late-Early to Late Cretaceous reduced granitoids that are divided into a granite suite, tonalite suite, and diorite suite in decreasing age. Synkinematic to postkinematic biotite ± hornblende granite, granodiorite, pegmatite, and two-mica granite with accessory garnet compose the granite suite (109–107 Ma). The tonalite suite (107–103 Ma) forms small to large bodies of postkinematic hornblende–biotite granodiorite to tonalite with rare granite. Intruding the granite and parts of the tonalite suites are aplite and pegmatite that grade through sugary pegmatoidal or aplitic quartz veins and finally into quartz veins along strike. The diorite suite (95.4–93.7 Ma) consists of small stocks of diorite to tonalite that intrude the older suites and are inferred to intrude the shallowly dipping auriferous quartz veins. Limited data indicate granitoid emplacement at 5–9 km depths, consistent with formation of the auriferous quartz veins based upon published fluid inclusion data. The weakly peraluminous granite and tonalite suites are distinguished by variable amounts of monazite and zircon whereas the diorite suite is metaluminous, contains <5% magnetite, and lacks monazite. All suites are subalkalic, calc-alkaline and have low magnetic susceptibilities, high large-ion lithophile element/high field-strength element (LILE/HFSE), and depleted Nb and Ti. The granite suite has higher 206Pb/204Pb values (19.4–19.6) than the diorite (19.1). Overall the granite and tonalite suites likely represent melts generated late during crustal thickening that intruded along shallowly dipping faults during exhumation, whereas the diorite suite represents postdeformation melts that underwent less interaction with the old silicic crust. [ABSTRACT FROM AUTHOR]

Abstract: The varying geochemical and petrogenetic nature of A-type granites is a controversial issue. The oxidized, magnetite-series A-type granites, defined by Anderson and Bender [Anderson, J.L., Bender, E.E., 1989. Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States of America. Lithos 23, 19–52.], are the most problematic as they do not strictly follow the original definition of A-type granites, and approach calc-alkaline and I-type granites in some aspects. The oxidized Jamon suite A-type granites of the Carajás province of the Amazonian craton are compared with the magnetite-series granites of Laurentia, and other representative A-type granites, including Finnish rapakivi and Lachlan Fold Belt A-type granites, as well as with calc-alkaline, I-type orogenic granites. The geochemistry and petrogenesis of different groups of A-types granites are discussed with an emphasis on oxidized A-type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis. Oxidized A-type granites are clearly distinguished from calc-alkaline Cordilleran granites not only regarding trace element composition, as previously demonstrated, but also in their major element geochemistry. Oxidized A-type granites have high whole-rock FeOt/(FeOt +MgO), TiO2/MgO, and K2O/Na2O and low Al2O3 and CaO compared to calc-alkaline granites. The contrast of Al2O3 contents in these two granite groups is remarkable. The CaO/(FeOt +MgO+TiO2) vs. CaO+Al2O3 and CaO/(FeOt +MgO+TiO2) vs. Al2O3 diagrams are proposed to distinguish A-type and calc-alkaline granites. Whole-rock FeOt/(FeOt +MgO) and the FeOt/(FeOt +MgO) vs. Al2O3 and FeOt/(FeOt +MgO) vs. Al2O3/(K2O/Na2O) diagrams are suggested for discrimination of oxidized and reduced A-type granites. Experimental data indicate that, besides pressure, the nature of A-type granites is dependent of ƒO2 conditions and the water content of magma sources. Oxidized A-type magmas are considered to be derived from melts with appreciable water contents (≥4 wt.%), originating from lower crustal quartz-feldspathic igneous sources under oxidizing conditions, and which had clinopyroxene as an important residual phase. Reduced A-type granites may be derived from quartz-feldspathic igneous sources with a metasedimentary component or, alternatively, from differentiated tholeiitic sources. The imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct A-type granite groups. Assuming conditions near the NNO buffer as a minimum for oxidized granites, magnetite-bearing granites formed near FMQ buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced A-type granites and, respectively, magnetite-series and ilmenite-series granites is not always observed. [Copyright &y& Elsevier]

Revegetation on disturbed, low organic matter content, decomposed granite (DG) substrates are limited by low plant-available moisture and nitrogen. Data from a single DG site in northern California, USA, showed that a significant fraction of the ammonium from fertilizers or organic matter mineralization was fixed into silicate interlayer positions. To evaluate the broader relevance of NH fixation, the NH fixation capacities of 11 other drastically disturbed DG substrates throughout California were evaluated. The fixation capacities of the substrates were quite varied and increased as added NH application levels increased (124–1,670 kg NH ha−1). When amended with 124 kg NH ha−1, 7 of the 11 substrates fixed between 14 and 78% of the added NH. Analysis of particle size fractions of a typical material indicated that the very fine sand fraction had the highest fixation capacity and the clays and very coarse sands had the lowest, on a gravimetric basis. The overall fixation capacities showed no significant relation to potential predictive characteristics, including extractable K+, NH, or total N levels. Three methods of cation exchange capacity (CEC) measurement were tested for their ability to predict NH fixation. The Ba method which utilizes an indicator cation that is not subject to interlayer fixation was not a reliable indicator of NH fixation. The NH method had the strongest relation to NH fixation in the DG materials. The difference between the measured CEC of the NH method and the Ba method was found to be most predictive of NH fixation. [ABSTRACT FROM AUTHOR]

Initiation of the Cordilleran magmatic arc in the southwestern United States is marked by intrusion of granitic plutons, predominantly composed of alkali-calcic Fe- and Sr-enriched quartz monzodiorite and monzonite, that intruded Paleoproterozoic basement and its Paleozoic cratonal-miogeoclinal cover. Three intrusive suites, recognized on the basis of differences in high field strength element and large ion lithophile element abundances, contain texturally complex but chronologically distinctive zircons. These zircons record heterogeneous but geochemically discrete mafic crustal magma sources, discrete Permo-Triassic intrusion ages, and a prolonged postemplacement thermal history within the long-lived Cordilleran arc, leading to episodic loss of radiogenic Pb. Distinctive lower crustal magma sources reflect lateral heterogeneity within the composite lithosphere of the Proterozoic craton. Limited interaction between derived magmas and middle and upper crustal rocks probably reflects the relatively cool thermal structure of the nascent Cordilleran continental margin magmatic arc. [ABSTRACT FROM AUTHOR]

Abstract: In the western Churchill Province, Canadian Shield, Neoarchean supracrustal and plutonic rocks, intruded by Paleoproterozoic mafic dykes and granitic rocks, comprise the MacQuoid supracrustal belt and the structurally overlying Cross Bay plutonic complex. They form part of the northwestern Hearne subdomain that occupies an intermediate position between the continental Rae domain to the north and west, and the oceanic central Hearne subdomain to the south and east. New geological mapping and supporting geoscience are compatible with the presence of 2550–2500Ma, southeast-directed, mid-crustal, thick-skinned thrusting that juxtaposed the plutonic complex over the supracrustal belt. The structural contact between the MacQuoid supracrustal belt and the Cross Bay plutonic complex potentially represents a fundamental boundary between isotopically distinct crustal blocks. The ∼2190Ma MacQuoid mafic dyke swarm cuts across Neoarchean deformation fabrics, but records ∼1.9Ga, deep-crustal, regional metamorphism that affected both the supracrustal belt and the plutonic complex. Other Paleoproterozoic deformation events that occurred at ∼1850–1810Ma are of local extent and appear to be relatively minor manifestations of more important events elsewhere, related to the Trans-Hudson orogen. [Copyright &y& Elsevier]

Granitic materials represent a common erosive substrate in California and much of the western United States. When granitic rocks weather, they disintegrate into coarse textured, non-cohesive substrates, known generally as decomposed granite (DG). Because of low moisture and N availability, revegetation of these substrates for erosion control is difficult. If nitrate based fertilizers are used, they can be rapidly leached, while NH fertilizers may be sequestered mineralogically by interlayer fixation. In this study, we focus on the occurrence of NH fixation on a decomposed granitic substrate and show that the fixation capacities of these sandy saprolites are significant despite analyses indicating that the samples are predominantly quartz, have low clay contents and have low cation exchange capacities (CEC). At field loading rates equivalent to less than 300 kg N ha−1, 36–42% of added NH may become unavailable to plants due to interlayer collapse and fixation for an unknown period of time. Ammonium fixation did not vary significantly in relation to substrate weathering class in these samples. Oriented X-ray diffraction analysis revealed the presence of vermiculite in particle size fractions from clay to very coarse sands. While other studies have identified silt as the most active fraction, the relative fixation capacity of these granite saprolites was greatest in the fine and very fine sand fractions when considered on a gram for gram basis of each individual particle size. We found that the extractant cation also influenced the measured levels of NH fixation in these granite saprolites. At loading rates of 0–150 kg NH ha−1, extraction with KCl resulted in apparent NH fixation capacities that were twice as great as those found with NaCl extractions when tested at low NH concentrations and close to 35% greater at higher NH amendment loading. Estimation of available ammonium in the decomposed granite using conventional KCl extraction methods appears to cause fixation, rather than extraction of at least part of the substrate’s NH content. [ABSTRACT FROM AUTHOR]

Abstract: This review, in honor of Ilmari Haapala''s retirement, reflects on lessons learned from studies of three granitic systems in western North America: (1) Mesoproterozoic samples from west Texas and east New Mexico; (2) Laramide granitic systems associated with porphyry-copper deposits in Arizona; and (3) granites of the Colorado Mineral Belt. The studies elucidate relationships amongst tectonic setting, source material, and magma chemistry. Mesoproterozoic basement samples are from two different felsic suites with distinct elemental and isotopic compositions. The first suite, the “plutonic province”, is dominantly magnesian, calc-alkalic to alkali-calcic, and metaluminous. It has low K2O/Na2O and Rb/Sr, and Nd model ages of 1.56 to 1.40 Ga. The second suite, the “Panhandle igneous complex”, is magnesian, metaluminous, alkalic, and is part of the Mesoproterozoic belt of magmatism that extends from Finland to southwestern United States. Samples from the Panhandle igneous complex demonstrate three episodes of magmatism: the first pulse was intrusion of quartz monzonite at 1380 to 1370 Ma; the second was comagmatic epizonal granite and rhyolite at 1360 to 1350 Ma. Both of these rock types are high-K to slightly ultra-high-K. The third pulse at 1338 to 1330 Ma was intrusion of ultra-high-K quartz syenite. Nd model ages (1.94 to 1.52 Ga) are distinct from those of the “plutonic province” and systematically older than crystallization ages, implying a substantial crustal input to the magmas. At the Sierrita porphyry-copper deposit in the Mazatzal Province of southeastern Arizona, trace element, Sr, and Nd isotopic compositions were determined for a suite of andesitic and rhyolitic rocks (67 Ma) intruded by granodiorite and granite. Isotopic composition and chemical evolution are well correlated throughout the suite. Andesite has the least negative initial ε Nd (−4.3) and lowest 87Sr/86Sr i (0.7069). It is also the oldest and chemically most primitive, having low concentrations of Rb, SiO2, and high concentrations of transition elements. These parameters change through the system to the youngest unit (granite), which has the most negative ε Nd (−8.5), the highest 87Sr/86Sri (0.7092), and is chemically most evolved. Correlation between chemical and Nd isotopic evolution probably resulted from a continuous process of progressive assimilation, in which mafic magmas invade and incorporate continental crust. Deposits in Arizona with ε Nd values more negative than the −8.5 of Sierrita lie in the older Yavapai province in the northwestern part of the state. The difference in the most negative epsilon Nd implies that Nd isotopic signature is sensitive to the age of the Precambrian domain. The granites from the Colorado Mineral Belt were emplaced during the transition from Laramide convergence to mid-Tertiary extension. Three different groups of granites are recognized. The first is Laramide and was formed during assimilation-fractional crystallization involving lower crustal mafic source materials; the second and third groups are mid-Tertiary and represent intracrustal melting of heterogeneous sources. This change in source regions and melt regimes in transition from convergence to extension is fundamental to the Mesozoic and Cenozoic evolution of western North America. [Copyright &y& Elsevier]

Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of ∼0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent 207Pb/206Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of ∼2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of ∼2900 Ma crust that potentially underlies the Black Hills or as detritus derived from ∼2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at ∼2590–2600 Ma (SBH) and ∼2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of ∼1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between ∼2480 and ∼1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the ∼2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a ∼2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland. [Copyright &y& Elsevier]

Resonant Ultrasound Spectroscopy (RUS) uses normal modes of elastic bodies to infer material properties such as elastic moduli and Q. In principle, the complete elastic tensor can be inferred from a single measurement. For centimeter-sized samples RUS fills an experimental gap between low-frequency stress-strain methods (quasi-static up to a few kHz) and ultrasonic time-delay methods (hundreds of kHz to GHz). We use synchronous detection methods to measure the resonance spectra of homogeneous rock samples. These spectra are then fit interactively with a model to extract the normal-mode frequencies and Q factors. Inversion is performed by fitting the normal-mode frequencies. We have successfully applied this technique to a variety of isotropic and anisotropic samples, both man-made and natural. In this paper we will show in detail the procedure applied to a cylindrical core of Elberton granite. By means of a statistical fit of the measured normal modes and an independent laser ultrasonic measurement, the granite core was inferred to have orthorhombic symmetry. A 10 per cent P-wave anisotropy was measured in the plane perpendicular to the core axis. [ABSTRACT FROM AUTHOR]

A sinuous zone of gently southeast-dipping low-angle Tertiary normal faults is exposed for 100 km along the eastern margins of the Anaconda and Flint Creek ranges in southwest Montana. Faults in the zone variously place Mesoproterozoic through Paleozoic sedimentary rocks on younger Tertiary granitic rocks or on sedimentary rocks older than the overlying detached rocks. Lower plate rocks are lineated and mylonitic at the main fault and, below the mylonitic front, are cut by mylonitic mesoscopic to microscopic shear zones. The upper plate consists of an imbricate stack of younger-on-older sedimentary rocks that are locally mylonitic at the main, lowermost detachment fault but are characteristically strongly brecciated or broken. Kinematic indicators in the lineated mylonite indicate tectonic transport to the east-southeast. Syntectonic sedimentary breccia and coarse conglomerate derived solely from upper plate rocks were deposited locally on top of hanging-wall rocks in low-lying areas between fault blocks and breccia zones. Muscovite occurs locally as mica fish in mylonitic quartzites at or near the main detachment. The 40Ar/39Ar age spectrum obtained from muscovite in one mylonitic quartzite yielded an age of 47.2 + 0.14 Ma, interpreted to be the age of mylonitization. The fault zone is interpreted as a detachment fault that bounds a metamorphic core complex, here termed the Anaconda metamorphic core complex, similar in age and character to the Bitterroot mylonite that bounds the Bitterroot metamorphic core complex along the Idaho-Montana state line 100 km to the west. The Bitterroot and Anaconda core complexes are likely components of a continuous, tectonically integrated system. Recognition of this core complex expands the region of known early Tertiary brittle-ductile crustal extension eastward into areas of profound Late Cretaceous contractile deformation characterized by complex structural interactions between the overthrust belt and Laramide basement uplifts, overprinted by late Tertiary Basin and Range faulting. [ABSTRACT FROM AUTHOR]

The geographic distribution of elevated groundwater As concentrations in a fractured silicate bedrock aquifer in central New Hampshire correlates with the presence of pegmatites which border late Devonian granites and intrude metasedimentary rocks. As concentrations in the pegmatites averaged 9.6 mg/kg, which is much higher than the associated granites (0.24 mg/kg) and metasedimentary rocks (0.8 mg/kg). As was concentrated in these pegmatites by partial melting of calcareous metapelites and subsequent recrystallization as granites with low As concentrations and pegmatites with high As concentrations. Arsenic behaves similarly to B, which was also concentrated into these late stage fluids. Arsenopyrite (FeAsS) with an oxidation reaction rim of scorodite (FeAsO4·2H2O) was observed in aquifer materials. Elevated As concentrations have been observed to occur in other New England locations in pelitic metasediments intruded by anatectic plutons. It is proposed that pegmatite formation from partial melting of pelitic metasediments may be an important mechanism for concentrating As in crystalline aquifer materials, which can then cause local As enrichment of groundwaters. Groundwater As concentrations ranged from 26 nmol/l to 5300 nmol/l with a median value (210 nmol/l) that is more than 30 times higher than the median for groundwaters from all of New Hampshire (6.5 nmol/l). High chloride concentrations (>1 mmol/l), resulting from road salt contamination of recharge waters, suggest that groundwaters are most likely young (<50 years). A systematic relationship was observed between pH and dissolved As and Fe concentrations. All waters with elevated As concentrations (>670 nmol/l or ∼50 μg/l) have very low Fe concentrations (<18 μmol /l) and high pH (>7); samples with low As (<340 nmol/l) have variable Fe concentrations, but all occur at low pH (<7). pH-dependent Fe oxyhydroxide adsorption of As oxyanions is consistent with the observations. At pH>∼7, Fe oxyhydroxides form rapidly and have a neutral or negative net surface charge that does not readily adsorb As. At pH <∼7, Fe oxyhydroxide formation is slow and depends on dissolved O2 availability, however the resultant Fe oxyhydroxides have a positive net surface charge, and appear to adsorb As readily. [Copyright &y& Elsevier]

Granitic mylonites from the Pinalen˜o Mountains metamorphic core complex in southeastern Arizona record general shear deformation. The mean kinematic vorticity number (Wm) was estimated using (1) the strain ratio (Rs) and the angle (Θ) between the long axis of the strain ellipsoid with respect to the high-strain zone boundary and (2) the porphyroclast hyperbolic distribution method. Wm for protomylonites and mylonites ranged from 0.6 to 0.9. Ultramylonites record lower Wm values (0.1–0.3) suggesting that the incremental vorticity changed during deformation. Three-dimensional strain analysis indicates that deformation in protomylonites and mylonites approximates plane strain and has a monoclinic symmetry. The vorticity path followed by rocks tectonically exhumed in the footwall of major extensional fault systems may evolve from a significant pure shear component early in the deformation towards simple shear as overburden load decreases during uplift. [Copyright &y& Elsevier]

We propose that the internally zoned, Li-bearing Tin Mountain pegmatite in the Harney Peak granite-pegmatite system of the Black Hills, South Dakota, crystallized from fluidrich, compositionally complex melts at ∼400-350 °C. The low crystallization temperatures resulted from the combined fluxing effects of Li, B, P, H[sub 2]O, and carbonate anions. The presence of hydrous silicate melts at temperatures of ∼350 °C is revealed by microthermometric data on primary fluid inclusions cogenetic with crystallized-melt inclusions. Mineral assemblages of the crystallized-melt inclusions and chemistry of bulk-fluid leachates indicate that the melts and fluids contained significant amounts of Li and Na as carbonates and/or borates that acted as powerful fluxes. The low temperatures give a new perspective on the stability of silicate melts in the crust. [ABSTRACT FROM AUTHOR]

Determines the petrochemical evolution of granitic rocks in the Mount Barcroft area in California. Geologic mapping; Average mineral modes and grain sizes of intrusive rocks; Major and trace element analyses of bulk-rock; Magmatic history; Origin of chemical variation.

The North Fork Poudre River in Phantom Canyon has a pool-riffle sequence formed in granitic bedrock. We address two questions: What controls downstream pool spacing? Within a pool, what factors control pool dimensions? We hypothesize that both pool spacing and pool geometry could be governed by available flow energy or by substrate characteristics. These hypotheses were addressed using channel-bed and water-surface gradients surveyed under moderate flow, Selby rock-mass strength at vertical outcrops forming lateral pool constrictions and at other vertical outcrops, and joint density measured from high-resolution digital images of the bedrock outcrop along each of 10 pools. The downstream length between pools is quite variable (mean [m] = 117 m, standard deviation [SD] = 106 m), as is the cumulative drop between pools (mean = 1.3 m, SD = 0.8 m). Total upstream bed gradient and approach bed gradient vary by an order of magnitude. Thus, pools do not have a uniform longitudinal or gradient distribution along the study reach, indicating that the downstream spacing of pools does not reflect systemwide available flow energy. The mean of 10 rock-mass strength measurements at pool constrictions (m = 73), and 10 measurements elsewhere (m = 72), indicate no significant differences in rock strength at pool locations. However, pools are almost always associated with lateral constrictions where the active channel intersects a bedrock ridge. The downstream spacing of these ridges reflects regional patterns of structure and weathering, and the local jointing of each outcrop influences the formation of constrictions. The downstream spacing of pools thus appears to reflect substrate controls. Both flow energy and substrate appear to exert an influence on individual pool geometry. Pools with stronger lateral constrictions are deeper and have a shallower approach gradient. More-constricted pools decrease upstream sediment transport capability through a backwater effect. This... [ABSTRACT FROM AUTHOR]

Examines the ilmenite grains from weathering profiles developed on granite and ultramafic chlorite schist in the Georgia Piedmont for evidence of morphological and chemical alteration. Sample sites and processing; Bulk properties of the weathering profiles; Textural and chemical properties of the ilmenite.

The Ireteba pluton is a ~66 Ma biotite ± muscovite ± garnet granite emplaced at the eastern edge of the Cordilleran plutonic belt in southeastern Nevada. In common with other Cordilleran peraluminous granites, its mineralogy, major element chemistry, isotopic composition, and abundant Proterozoic zircon inheritance document crustal origin. Distinctive trace element chemistry, field relations, and inherited components further constrain its genesis. High Sr concentrations, low heavy rare earth elements, and absence of negative Eu anomalies indicate that the Ireteba magma was extracted from a residue relatively rich in garnet and poor in feldspar; rounded quartz is probably a resorbed, high-P liquidus phase or restite. The granite shares with adakites (slab-derived arc magmas) and Archean granitoids the Sr-rare earth element signature of deep-source origin. Nd-Sr isotopic compositions indicate a dominantly crustal origin for the granite, but it is less mature than the underlying ancient Mojave crust. The granite is apparently a hybrid derived primarily from the ancient crust but with a less mature component as well: either Jurassic igneous rock, as suggested by sparse 150-170 Ma zircon cores, or juvenile mafic magma, as implied by abundant synplutonic mafic rocks, or both. Influx of basaltic magma during the waning stages of Cordilleran convergence may have triggered melting in the deep, thickened crust, with basaltic magmas being trapped beneath the less dense crustal melts. Like other relatively young peraluminous granites of the Cordilleran Interior, including the Idaho-Bitterroot batholith, the Ireteba pluton may reflect changing conditions during the waning stages of plutonism. [ABSTRACT FROM AUTHOR]

Examines the mafic injections, in situ hybridization and crystal accumulation in Pyramid Peak granite, California. Formation of intramagmatic flows by injections of mafic magma; Gradation of diorite to granite with decreasing abundance of mafic enclave; Solidification of magma by accumulation of crystal.

Roof-to-floor exposures of mid-Miocene plutons in tilt blocks south of Las Vegas, NV, reveal distinct but strongly contrasting magma chamber statigraphy. The Searchlight and Aztec Wash plutons are well-exposed, stratified intrusions that show a similar broad range in composition from ∼45–75 wt.% SiO2. Homogeneous granites that comprise about one-third of each intrusion are virtually identical in texture and elemental and isotopic chemistry. Mafic rocks that are present in both plutons document basaltic input into felsic magma chambers. Isotopic compositions suggest that mafic magmas were derived from enriched lithospheric mantle with minor crustal contamination, whereas more felsic rocks are hybrids that are either juvenile basaltic magma+crustal melt mixtures or products of anatexis of ancient crust+young (Mesozoic or Miocene?) mafic intraplate.Despite general similarities, the two plutons differ markedly in dimensions and lithologic stratigraphy. The Searchlight pluton is much thicker (∼10 vs. 3 km) and has thick quartz monzonite zones at its roof and floor that are absent in the Aztec Wash pluton. Isotopic and elemental data from Searchlight pluton suggest that the upper and lower zones are cogenetic with the granite; we interpret the finer grained, slightly more felsic upper zone to represent a downward migrating solidification front and the lower zone to be cumulate. In contrast, the upper part of the Aztec Wash pluton is granite, and a heterogeneous, mafic-rich injection zone with distinct isotopic chemistry forms the lower two-thirds of the intrusion. Similar mafic rocks are relatively sparse in Searchlight pluton and do not appear to have played a central role in construction of the pluton. Large felsic and composite dikes that attest to repeated recharging and intrachamber magma transfer are common in the Aztec Wash pluton but absent in the Searchlight pluton. Thus, although both intrusions were filled by similar magmas and both developed internal stratification, the two intrusions evolved very differently. The distinctions may be attributable to scale and resulting longevity and/or to subtle differences in tectonic setting. [Copyright &y& Elsevier]

Examines the use of sensitive high resolution ion microprobe for dating ordovician granites and tonalites in the central Appalachian Piedmont, U.S. Utilization of cathodoluminescence images of zoning in the zircons to select the areas for dating; Application of isotope dilution thermal ionization mass spectrometry; List of dated ares.

Examines the geology and geochemistry of mafic and felsic plutonic rocks in the Yoshemite Valley, California. Example of coeval mafic and felsic magmatism in a continental-margin arc; Relation of mafic rocks to the El Capitan Granite; Similarities on the isotopic composition of mafic and felsic rocks.

Examines the chemical and isotopic study of Laramide granitic belt of northwestern Mexico. Sequence of Paleozoic eugeoclinal rocks overlain by Late Triassic clastic units; Characteristics of middle to late Mesozoic island-arc-related volcanic and sedimentary rocks of the Gurrero terrane; Derivation of juvenile mantle melts with partial melts.

The 1.43-Ga Sherman batholith of southeastern Wyoming and northeastern Colorado is a texturally and geochemically heterogeneous intrusion that comprises rocks derived from at least four different sources. The coarse-grained, metaluminous, biotite-hornblende Sherman Granite is volumetrically the most significant unit in the batholith. It has geochemical characteristics at the extreme end of A-type suites with high Fe# > 88, high K[sub2]O wt. % (generally > 5%), molar Na/K generally < 1, and high abundances of incompatible elements. The source for the Sherman Granite is constrained to be a Fe-rich, low oxygen fugacity source mafic material by initial isotopic ratios (ε[subNd] = -0.8-1.1 and initial [sub87]Sr/[sub86]Sr = 0.7024-0.7126), low oxygen fugacity (-0.1 to -0.5 log units below the fayalite-magnetite-quartz buffer reaction) and water activity (∼ 0.7), comparisons to experimental melt compositions, and mineralogical and geochemical similarities to monzonitic intrusions from the Laramie anorthosite complex (LAC). The medium-grained, peraluminous Lincoln granite, which is volumetrically subordinate to the Sherman Granite, has less extreme A-type geochemical characteristics. Initial isotopic ratios (ε [subNd] = ∼-1 and initial [sup87]Sr/[sup86]Sr = 0.7189 and 0.7238), comparisons to experimental melt compositions, and geochemical similarities to intrusions from the peraluminous Silver Plume suite favor a parental magma for the Lincoln granite derived from intermediate to felsic crustal rocks from the Colorado province, which forms the basement to the Sherman batholith. A suite of mineralogically heterogeneous biotite-hornblende porphyritic quartz monzonites and granites has geochemical characteristics that also fit the A-type classification but are less extreme than that for the Sherman Granite, with distinctly lower Fe# (< 88) and molar Na/K (generally 1). Field and geochemical observations are consistent with many of the... [ABSTRACT FROM AUTHOR]

The 100 km² Horse Creek anorthosite complex, which was emplaced between 1770 ± 3 and 1755 ± 2 Ma, is the older of two anorthosite complexes located in the southern Laramie Mountains, southeastern Wyoming. The complex is composed of four lithologic units, which are in order of decreasing age the Ragged Top granodiorite, Horse Creek granite, Horse Creek anorthosite, and Horse Creek monzonite. Nd, Sr, and Pb isotopic compositions of all units indicate that they are dominated by sources such as contemporary depleted mantle and crustal rocks with short crustal residence ages. However, the involvement of a small amount of Archean material is required by the Pb isotopic data. Both the Horse Creek anorthosite complex and the 1.43-Ga Laramie anorthosite complex were emplaced along the Cheyenne belt, which marks the suture between the Archean Wyoming province and Proterozoic Colorado province. The time of emplacement of the Horse Creek anorthosite complex coincides with late stages of the Medicine Bow orogeny, in which Proterozoic island arcs collided with the Wyoming province. The Horse Creek anorthosite complex was emplaced in a zone of local transtension formed as a result of oblique collision of the Colorado province along the southern margin of the Wyoming craton. [ABSTRACT FROM AUTHOR]

Presents petrographic evidence and phase equilibria results indicating magmatic anhydrite in Late Cretaceous rocks in the Cajon Pass Deep Scientific Drillhole in California. Geology of granitic rocks in the Cajon Pass Well; Occurrence of anhydrite.

We find that site effects on weathered granodiorite bedrock at Piñon Flat in southern California are highly variable at distance scales of the order of tens of meters. We base this conclusion on 111,632 power spectral estimates obtained from P and S waveforms from 251 local earthquakes recorded in the 1990 Piñon Flat, high-frequency array experiment. Spectral estimates were obtained from each of the 60 three-component stations of this array for each observed P and S phase using the multitaper spectral method. For each event we calculated a median spectrum for the entire surface array from each component and the total power on all three components. We then calculated spectral ratios for each channel and the total power relative to the array median. We found variations of amplitude of these spectral ratios between different stations that exceeded a factor of 10 at some frequencies. Comparison of the entire population of spectral ratios demonstrated that variations seen in the data are unquestionably statistically significant. We display the results using a three-dimensional visualization tool that displays the spectral ratios for the entire array simultaneously, allowing for the observation and interpretation of patterns of spectral variation across the array. By scrolling through frequency we see large variations in spectral amplitude on different components that vary systematically across the array. We also find that the spectral responses of stations in the array systematically differ with event approach azimuth. The scale of these patterns is consistent with Fresnel zone lengths of diffractions induced by scattering within the weathered layer. We compare borehole data to surface data and use synthetic seismograms to show that the high-frequency spectra of the surface stations are extremely sensitive to near-surface velocity structure, as well as surface sensor depth. We suggest that the amplitude variations we see are due to the characteristic,... [ABSTRACT FROM AUTHOR]

Investigates the titranite and fluorite stability and equilibria in A-type granite from Mount Scott, Oklahoma. Higher modal abundance of plagioclase and hornbelende due to an increase of fluorine during crystalization; Presence of primary fluorite indicates high magmatic fluorine; Presence of titanite without fluorite indicated low initial fluorine.

Analyzes the orogeny of the leucogranites in the Black Hills of South Dakota. Heat source for producing leucogranite magma; Consistency of the model with isotopic data for the granite; Shear heating as a viable mechanism for leucogranite generation.

The ∼ 1.08-Ga Pikes Peak composite batholith of central Colorado is a type example of an A-type granitic system. From the 1970s through the 1990s, details of the field relations, mineralogy, major and trace element compositions, and isotopic geochemistry of Pikes Peak rocks were documented, and they reveal the existence of two chemical groups, a potassic and a sodic series. The potassic series (∼ 64-78 wt % SiO[sub2]) includes the Pikes Peak Granite, which is mostly coarse-grained biotite ± hornblende syenogranite and minor monzogranite that dominates the batholith. The potassic series also includes fine- to medium-grained biotite granite found in numerous, small, late-stage plutons throughout the batholith. The sodic series is found in seven plutons comprised of a wide range of rock types (∼ 44-78 wt % SiO[sub2]), including gabbro, diabase, syenite/quartz syenite, and fayalite and sodic amphibole granite. Differences in petrologic and geochemical characteristics between the sodic and potassic series indicate different petrogenetic histories. Major and trace element and strontium and oxygen isotopic data were used by some workers to hypothesize that mantle-derived alkali basalt underwent crystal fractionation and reaction with lower crustal rocks to generate syenitic magmas of the sodic series, which subsequently underwent further fractionation to produce sodic granites. Recent studies involving estimates of oxygen fugacities, along with additional trace element and neodymium isotopic data, also support a basalt fractionation model for the sodic series, but suggest only minor crustal involvement. Gabbros and diabase dikes associated with the sodic series appear to have been derived from mantle sources that previously had been affected by a subduction event, based on neodymium isotopic and trace element data. Some workers propose that the potassic series also formed by fractionation of syenitic and/or basaltic magmas coupled with reaction with... [ABSTRACT FROM AUTHOR]

We present major element, trace element, and Nd isotopic analyses from cuttings and core samples for three subsurface terranes in west Texas and eastern New Mexico. The most northerly is the Panhandle volcanic terrane, which represents a large part of the Mesoproterozoic southern granite-rhyolite province. This terrane is comprised of undeformed rhyolite, ignimbritic tuff, granite, and diabase. The Panhandle terrane is split by the Debaca terrane, which consists of intercalated metasedimentary and metavolcanic rocks intruded by olivine gabbro, ferrogabbro, and diabase. Mildly to strongly deformed intermediate and felsic intrusive rocks of unknown affinity make up the third terrane, called here the crystalline terrane; it is located south and southeast of the Panhandle and Debaca terranes. Intermediate-to-felsic rocks of the terranes can be subdivided on the basis of their geochemistry into those with: (1) K[sub2]O/Na[sub2]O > 1 and A-type trace element characteristics; and (2) K[sub2]O/ Na[sub2]O < 1 and I-type trace element characteristics. All but a few samples from the Panhandle terrane, both north and south of the Debaca terrane, are A-type. Depleted mantle model ages for granites and rhyolites from the northern Panhandle terrane range from 1.50-1.69 Ga. TWo samples from the southern Panhandle terrane have model ages of 1.74 Ga, and a third sample's model age is 1.57 Ga. These model ages are older than the four reported crystallization ages of 1.37 -1.4 Ga, indicating that: (1) A-type rocks of the Panhandle terrane contain a significant crustal component; and (2) Panhandle terrane is underlain by gt; 1.7-Ga crust. The southern edge of Laurentia, therefore, is farther south than previously inferred. A diabase from the Panhandle terrane has a T[subDM] of 1.44 Ga. If this model age is close to the crystallization age, then diabase in the Panhandle terrane is approximately coeval with the granite and rhyolite. The model age for the gabbro from the Debaca... [ABSTRACT FROM AUTHOR]

Sugarloaf Peak is one of seven sodic plutons that lie within or adjacent to the ca. 1.08-Ga Pikes Peak batholith in central Colorado. This report represents the first study of the Sugarloaf pluton. Major element and modal analyses from the other six plutons (Lake George, Tarryall, Rampart Range, West Creek, Mt. Rosa, and Spring Creek), together with data presented here, indicate that sodic and potassic rocks from all of them were produced by fractional crystallization of mantle-derived basaltic magmas. The Sugarloaf pluton is composed of fine-grained, medium-grained, coarse-grained, and pegmatitic syenite. The syenites lack quartz and are dominated by perthitic feldspar and ferro-richterite amphibole. The fine-grained syenite intrudes the medium- and coarse-grained syenites. The Sugarloaf pluton is surrounded by coarse-grained Pikes Peak Granite, which is the predominant rock type in the batholith. The linear arrangement of six of the seven sodic plutons parallel to major Precambrian fault trends suggests that the emplacement of Sugarloaf pluton may be rift-related. The Sugarloaf syenites have high total alkalis, high FeO (total), low CaO, and low MgO concentrations. They are also enriched in rare earth elements (REE) and high field strength elements (HFSE). Pronounced trace element variation among the Sugarloaf syenites can be explained partially by models of fractional crystallization. A plot of Ba versus Sr shows that compositions of the syenites closely follow modeled fractionation vectors for potassium feldspar. The fractional crystallization trends show that fine-grained syenite is the most chemically evolved, consistent with field relations that show the fine-grained syenite intruded the medium-and coarse-grained syenites. Accessory mineral fractionation, release of volatiles, or removal of pegmatitic fluids also may have influenced geochemical variations among the Sugarloaf syenites. [ABSTRACT FROM AUTHOR]

The Priest pluton is part of the suite of 1.4-Ga granitic plutons in North America. The pluton is a corundum normative, peraluminous, K-feldspar megacrystic granodiorite to quartz monzonite. It is characterized by: (1) high Mg/Mg + Fe; (2) high abundances of Al[sub2]O[sub3], CaO, and large ion lithophile elements (e.g., Sr, Ba); and (3) low abundances of high field strength elements (e.g., Zr, Y). Major and trace element modeling suggests that variations in composition were produced by accumulation of K-feldspar and plagioclase, localized accumulation of plagioclase, biotite, and accessory minerals, and late-stage crystal fractionation of the residual melt. Plutons in the suite of 1.4 Ga, so-called "anorogenic" granitic plutons have been classified as A-type granites. Major and trace element abundances suggest that the Priest pluton is not A-type, but has more in common with S-type granites. In addition, typical tectonic classification techniques suggest that the pluton is either a volcanic-arc/syn-collisional or late orogenic granite. The pluton's intrusion was accompanied by highly partitioned, compressional deformation that is not typically associated with anorogenic, A-type plutons. The Priest pluton also is distinct from the nearby Sandia pluton, although the two plutons are nearly the same age. The Priest pluton has higher Mg number, alumina saturation index, and large ion lithophile element abundances as well as lower abundances of high field strength elements. The Priest pluton contains only biotite as its mafic phase, whereas the Sandia pluton contains biotite and hornblende. Differences in geochemistry between the Priest and Sandia plutons and the rest of the suite of 1.4 Ga granitic plutons suggest that the lower crust is a heterogeneous source region. Therefore, geochemical signatures that commonly are associated with particular tectonic settings may instead reflect heterogeneities in the source. [ABSTRACT FROM AUTHOR]

Proterozoic plutonism in Colorado and Wyoming was initatied ∼ 1.8 Ga with scattered tholeiitic mafic complexes coeval with widespread synorogenic bimodal volcanism. Limited Nd and Sr isotopic data for the metavolcanic rocks show derivation from depleted mantle. Major Paleoproterozoic granitic plutonism followed at 1.67-1.77 Ga. Most of the earliest plutons are distinctly calc-alkaline; they range largely from quartz diorite to granodiorite to trondhjemite in composition, and have trace-element signatures similar to plutons within magmatic arcs related to subduction zones. Later Paleoproterozoic plutons at 1.71 Ga include increased volumes of felsic rock types and, independent of silica, are shifted to more peraluminous and iron-rich compositions. The earliest appearance of anorthosite occurs with the 1.76-Ga Horse Creek anorthosite complex of Wyoming, and the earliest occurrence of A-type granite includes the late-kinematic, 1.66-Ga Garell Peak batholith of southern Colorado. Elemental and isotopic compositions of the younger Early Paleoproterozoic granitic plutons are consistent with a systematically increasing crustal component as a function of age in waning orogenic stages of crust formation in the region. After a 200 m.y. hiatus, renewed granitic plutonism occurred at 1.36-1.44 Ga. Plutonism was associated with emplacement of over a dozen Mesoproterozoic A-type granite batholiths and many smaller intrusions as part of a global "anorogenic" mid-Proterozoic event that commonly includes associated intrusions of anorthosite and charnockite. Across the former Laurentia supercontinent, three geographic and petrologic subprovinces merge in Colorado and Wyoming. An ilmenite-series granitic province, which includes the Sherman Granite and associated Laramie anorthosite complex of Wyoming, extends northeastward through Wisconsin to Labrador and the classic rapakivi granite-anorthosite intrusions of the Baltic region. A magnetite-series granite subprovince ranges... [ABSTRACT FROM AUTHOR]

Intrusion and crystallization of the Harney Peak Granite and associated plutons and pegmatites were the culminating events of the Trans-Hudson orogeny as expressed in the Black Hills. The granite was emplaced as thousands of sills and dikes at 1715 Ma, following an approximately 45 m.y. period of regional metamorphism and deformation of now exposed Proterozoic metasedimentary rocks. Isotopic ratios of neodymium and lead indicate that parts of the granite were derived from source rocks with Archean model T[subDM] extraction-ages, whereas other parts were derived from sources with only 100-300 m.y. crustal residence times. However, oxygen isotope ratios and trace element concentrations suggest that both sources were metapelites or metagraywackes analogous to the country rocks. Boron and TiO[sub2] concentrations suggest that the granites were generated either by muscovite or muscovite + biotite dehydration-melting reactions. Published thermobarometric and argon cooling-ages show that the country rocks cooled and decompressed from conditions at which garnet and staurolite grew at ∼ 7 kbar to less than 500°C and pressures of 3.5-4 kbar at the time of granite emplacement. We conducted a numerical simulation of metamorphism and generation of the Harney Peak Granite that is constrained by available data. The model involves shear-heating along a thrust to produce temperatures sufficiently high for melting of thrusted sedimentary rocks at relatively shallow levels in a thickened crust that is undergoing unroofing. The model successfully reproduces the metamorphic and magmatic events that occurred in the Black Hills segment of the Trans-Hudson orogen. [ABSTRACT FROM AUTHOR]

Analyzes the dating of single zircon grains from granite cuttings from southeastern Georgia. Age of the granite; Composition; Implications of the new data to the Suwannee terrane subcrop age province boundaries.

Focuses on a study conducted on fragments of comagmatic granophyric granite following the eruption of the Fish Canyon Tuff, Colorado at 27.8 Ma from the La Garita caldera in 1976. Identification of the study; Interpretation of the granophyric textures; Observations of the textures.

Presents a study on an undocumented belt of Upper Cretaceous, primary muscovite- and garnet-bearing granites, in the Mojave Desert, California. Location of where the granite belt occurs; What the granite belt correlates with.

Presents a working hypothesis for the tectonic setting of the 1.4 Ga magmatic event in western United States. Deformation and metamorphism in and adjacent to 1.4 Ga plutons; Examination and compilation of structural and kinematic features in 1.4 Ga granite; Consistent orientation to magmatic foliation, cogenetic dikes and synmagmatic shear zones.

Examines changes in the geochemistry of quartz-mica schist in Harney Peak Granite, Black Hills, South Dakota as evidence of widespread fluid flow and dispersion of granite-derived elements. Geology in the region; Previous studies of metasomatism in the region; Nature of the vein fluid.

Decomposed granite (DG) is often difficult to revegetate because of its low nitrogen (N) content and poor physical properties. Use of soluble fertilizers on DG sites increases plant-available N but may encourage invasive annuals to grow rapidly and exclude perennial species. This study evaluates the effect of N availability on two potential DG revegetation species: an invasive, exotic annual grass (Bromus mollis) and a native perennial grass (Elymus glaucus). Plants in 10-L pots filled with DG were irrigated with all essential elements except N, which was provided in treatments ranging from zero to 1000 μM NO3-N. Shoot biomass and root distribution were measured in monocultures and in mixtures of annual and perennial plants, both when the two species were seeded simultaneously and when the perennials were seeded 50 days prior to the annuals. At the higher N treatments, growth of annuals exceeded that of perennials. At solution N concentrations lower then the 50–100 μM treatments, however, growth of the perennial grasses equaled or exceeded that of the annuals. When seeded simultaneously, both species showed reduced biomass in mixtures to an extent similar to that when each species grew alone. When the perennials were already established, the biomass of annuals was reduced proportionately more than that of perennials, even at the highest N treatment. At low and medium N treatments, root placement of the perennial was deeper than that of the annual. At high N treatments, however, root distribution of both species was similar, and the deep rooting characteristic of the perennial was no longer observed. [ABSTRACT FROM AUTHOR]