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USGS2008

Overview of the 2004 to 2006, and continuing, eruption of Mount St. Helens, Washington

Rapid onset of unrest at Mount St. Helens on September 23, 2004, initiated an uninterrupted lava-dome-building eruption that continues to the time of writing this overview (spring 2006) for a volume of papers focused on this eruption. About three weeks of intense seismic unrest and localized surface uplift, punctuated by four brief explosions, constituted a ventclearing phase, during which there was a frenzy of media attention and considerable uncertainty regarding the likely course of the eruption. The third week exhibited lessened seismicity and only minor venting of steam and ash, but rapid growth of the uplift, or welt, south of the 1980-86 lava dome proceeded as magma continued to push upward. Crystalrich dacite (~65 weight percent SiO 2 ) lava first appeared at the surface on October 11, 2004, beginning the growth of a complex lava dome of uniform chemical composition accompanied by persistent but low levels of seismicity, rare explosions, low gas emissions, and frequent rockfalls. Petrologic studies suggest that the dome lava is chiefly of 1980s vintage, but with an admixed portion of new dacite. Alternatively, it may derive from a part of the magma chamber not tapped by 1980s eruptions. Regardless, detailed investigations of crystal chemistry, melt inclusions, and isotopes reveal a complex magmatic history. Largely episodic extrusion between 1980 and 1986 produced a relatively symmetrical lava dome composed of stubby lobes. In contrast, continuous extrusion at mean rates of about 5 m 3 /s in autumn 2004 to <1 m 3 /s in early 2006 has produced an east-west ridge of three mounds with total volume about equal to that of the old dome. During much of late 2004 to summer 2005, a succession of spines, two recumbent and one steeply sloping and each mantled by striated gouge, grew to nearly 500 m in length in the southeastern sector of the 1980 crater and later disintegrated into two mounds. Since then, growth has been concentrated in the southwestern sector, producing a relatively symmetrical mound with steep gougecovered slabs on its east flank. Throughout the eruption, the position of the extrusive vent has remained more or less fixed. Lack of geodetic evidence for either volume increase or pressure increase in the deep magmatic system since about 1990 and geodetic modeling that can account for only 20 to 30 percent of the 2004-to-present dome volume puzzles geodesists. Better constraints on parameters such as magma-chamber volume, crustal properties, and magma compressibility are needed to improve the models. Development of the welt and the new dome bisected horseshoe-shaped Crater Glacier, which formerly wrapped around three sides of the 1980s dome, and fractured, compressed, and thickened the glacier’s surviving east and west arms. Doubling of ice thickness resulted in increased flow rate and advance of termini, although rapid infiltration of water into the highly porous glacier bed prevented substantial basal sliding. Overall, dome growth and disintegration has removed surprisingly little ice. The outcome of the ongoing eruption remains uncertain, but Mount St. Helens’ varied eruptive history suggests multiple possibilities. One dynamical model and several petrologic investigations regard the current eruption as an extension of 1980s dome building that may persist continuously or episodically for years to come.

Washington↗

USGS2019

Application of electron microscopy TEM and SEM for analysis of coals, organic-rich shales and carbonaceous matter

This paper provides a brief summary of the history, development and variety of applications of electron microscopy techniques to analyze coals, organic-rich shales, and carbonaceous materials. General construction and principles of operation of transmitted (TEM) and scanning electron microscope (SEM) are outlined, along with guidance on specimen preparation, and a brief overview of published TEM and SEM applications related to coal, shales, and carbonaceous materials. This work was accepted as the chapter 18.2 of International Committee for Coal and Organic Petrology Methods Handbook at the ICCP Plenary Session on September 27, 2018 in Brisbane (Australia).

International Journal of Coal Geology↗

USGS1981

Progradational sequences in Miocene shoreline deposits, southeastern Caliente Range, California

An exceptionally well exposed marine-nonmarine transition in middle Miocene strata exists in the southeastern Caliente Range, California. About 50 individual progradational sequences form a succession that ranges in thickness from approximately 1000 m (where predominantly nonmarine) to more than 2500 m (where predominantly marine). Paleogreographic evidence in basalt flows near the top of the succession and in overlying fluvial deposists indicates that these middle Miocene strata were deposited across a north-northwest trending shoreline. A complete progradational sequence typically is several meters to a few tens of meters thick and includes strata that represent three intertonguing stratigraphic units. Individual sequences generally rest on a thin gravel deposit interpreted as a transgressive lag on an erosional surface. The gravel is overlain by structureless siltstone or fine-grained sandstone deposited at water depths where the rate of faunal mixing exceeded that of production of structures by physical processes. These rocks grade upward into bedded fine sandstone deposited closer to shore where physical processes exceeded bioturbation. Crossbedded lenses of coarse sand or fine gravel in the upper part of this facies suggest the presence of failry long-period surface waves. The bedded fine sandstone is sharply overlain by a crossbedded coarse sandstone facies that is interpreted as a combined offshore bar-rip channel-surf zone assemblage. Cross-strata dip dominantly offshore, suggesting substantial deposition from rip currents. A secondary, shore=parallel mode of cross-strata direction suggests longshore currents produced by surface waves from the northwest. The crossbedded coarse-grained sandstone grades upward into planar-bedded medium-grained sandstone that is interpreted as a beach foreshore. This facies grades upward through structureless medium-grained sandstone into nonmarine or lagoonal red and green mudstone of the Caliente Formation. The middle Miocene succession was deposited in a subsiding basin that was otherwise remarkably stable tectonically; the position of the strand line differed no more than a few kilometers through a period of 1 to 3 m.y. The average duration of the transgressive-regressive cycles, a few tens of thousands of years, together with their distribution in groups of three or four in the lower two-thirds of the succession, is consistent with the pattern of long-term climatic cycles produced by periodicity of the earth's solar orbit and may be related to eustatic sea level changes attendant to the development of the Antarctic ice cap. Changes in the pattern of progradation in the upper part of the succession and nearby basaltic eruptions may hav been precursors to the onset of movement along the San Andreas fault in this area 12-14 m.y. ago.

California↗

USGS2024

Structure and morphology of chars and activated carbons obtained from thermal treatment of coal and biomass origin materials, including their wastes: Results from the ICCP Microscopy of Carbon Materials Working Group

This paper describes the evaluation of petrographic textures in char and activated carbon derived from coal, coal by-products and biomass, formed during carbonization and activation processes. This work represents the results of interlaboratory exercises from 2016 to 2022 of the Microscopy of Carbon Materials Working Group in Commission III of the International Committee for Coal and Organic Petrology. The interlaboratory exercises were run on photomicrograph samples. For textural characterization of carbon materials, the existing American Society for Testing and Materials (ASTM) classification system for metallurgical coke was applied. Morphological differences were evaluated in 29 carbon material types, including 22 char samples, and 7 activated carbon (AC) samples obtained experimentally using conventional direct/indirect and microwave heating technologies. This approach gives an extended view on the identification of microporous carbons, and how a certain heat treatment develops a certain optical texture and structure in a raw material. The requested evaluation of carbon materials was related to their porosity, origin, extent, and characteristics, which are particular to each carbon material type. Because carbon matrices can form a wide range of optical textures during heat treatment it is important to demonstrate which carbon occurrences will have a crucial role in industrial applications dominated by adsorption phenomena. The interlaboratory exercises included 17 participants from 14 laboratories. Four sets of digital black and white and colour photomicrographs were distributed, which in total comprised 184 fields of different types of carbon material. The results were evaluated based on four levels: (i) optical texture (isotropic/anisotropic), (ii) optical type and size (punctiform, mosaic, fiber, ribbon, domain), iii) morphology (porous, non-porous/massive), and (iv) particle origin (precursor type). The statistical method applied to evaluate the results was based on “raw agreement indices”. Comparative analyses of the average values of the level of overall agreement showed homogeneity in the results, the mean value was 89%, with a minimum value of 87% and a maximum value of 91% for those who participated in at least three out of four exercises.

International Journal of Coal Geology↗

USGS1983

The mineralogy of the Snake Creek-Williams Canyon pluton, southern Snake Range, Nevada

The Snake Creek-Williams Canyon pluton of the southern Snake Range crops out over an area of about 30 km 2, about 60 km southeast of Ely, Nev. This Jurassic intrusion displays large and systematic chemical and mineralogical zonation over a horizontal distance of 5 km. Major-element variations compare closely with Dalyls average andesite-dacite-rhyolite over an SiO 2 range of 63 to 76 percent. For various reasons it was originally thought that assimilation played a dominant role in development of the Snake Creek-Williams Canyon pluton. However, based on modeling of more recently obtained trace element and isotopic data, we have concluded that the zonation is the result of in-situ fractional crystallization, with little assimilation at the level of crystallization. This report summarizes data available for each of the mineral species present in the zoned intrusion. Special attention has been paid to trends that might relate to the variation in the chemical petrology of the pluton.

Open-File Report↗

USGS1984

Comparative geochronology in the reversely zoned plutons of the Bottle Lake Complex, Maine: U-Pb on zircons and Rb-Sr on whole rocks

The Bottle Lake Complex is a composite granitic batholith emplaced into Cambrian to Lower Devonian metasedimentary rocks. Both plutons (Whitney Cove and Passadumkeag River) are very coarse grained hornblende and biotite-bearing granites showing petrographic and geochemical reverse zonation. Two linear whole rock Rb/Sr isochrons on xenolith-free Whitney Cove and Passadumkeag River samples indicate ages of 379??5 m.y. and 381??4 m.y., respectively, in close agreement with published K-Ar ages for biotite from Whitney Cove of 377 m.y. and 379 m.y., and for hornblende 40Ar/39Ar determinations from Passadumkeag River which indicate an age of 378??4 m.y. The initial Sr isotopic ratio for Whitney Cove is 0.70553 and for Passadumkeag River is 0.70414. A whole-rock isochron on a suite of xenoliths from the Passadumkeag River granite indicates a whole rock Rb-Sr age of 496??14 m.y., with an initial Sr isotopic ratio of 0.70262. Two types of zircon exhibiting wide petrographic diversity are evident in variable proportions throughout the batholith. One of these types is preferentially found in a mafic xenolith and it is widely dispersed in the host granites forming discrete grains and probably as inclusions in the other type of zircon. U-Pb analyses of zircons give concordia intercept ages of 399??8 m.y. for Whitney Cove, 388??6 m.y. for Passadumkeag River, 415 m.y. for a mafic xenolith in Passadumkeag River, and 396??32 for combined Whitney Cove and Passadumkeag River granite. The zircons show a spread of up to 20 m.y. in the 207Pb/206Pb ages. Omitting the finest zircon fraction in the Passadumkeag River results in a concordia intercept age of 381??3 m.y., in better agreement with the whole-rock Rb-Sr and mineral K-Ar ages. For the Whitney Cove pluton, exclusion of the finest fraction does not bring the zircon age into agreement with the Rb-Sr data. Age estimates by the whole rock Rb-Sr, mineral K-Ar and Ar-Ar methods suggest that the crystallization age of the plutons is about 380 m.y., slightly younger than the U-Pb zircon intercept ages. A possible reason for this discrepancy is that the zircons contain inherited lead. Thus, zircon U-Pb ages might represent a mixture of newly developed zircon and older inherited zircon, whereas the Rb-Sr whole rock age (380 m.y.) reflects the time of crystallization, and the argon ages result from rapid cooling after emplacement. ?? 1984 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS2015

The effect of pressurized magma chamber growth on melt migration and pre-caldera vent locations through time at Mount Mazama, Crater Lake, Oregon

The pattern of eruptions at long-lived volcanic centers provides a window into the co-evolution of crustal magma transport, tectonic stresses, and unsteady magma generation at depth. Mount Mazama in the Oregon Cascades has seen variable activity over the last 400 ky, including the 50 km 3 climactic eruption at ca. 7.7 ka that produced Crater Lake caldera. The physical mechanisms responsible for the assembly of silicic magma reservoirs that are the precursors to caldera-forming eruptions are poorly understood. Here we argue that the spatial and temporal distribution of geographically clustered volcanic vents near Mazama reflects the development of a centralized magma chamber that fed the climactic eruption. Time-averaged eruption rates at Mount Mazama imply an order of magnitude increase in deep magma influx prior to the caldera-forming event, suggesting that unsteady mantle melting triggered a chamber growth episode that culminated in caldera formation. We model magma chamber–dike interactions over &sim;50 ky preceding the climactic eruption to fit the observed distribution of surface eruptive vents in space and time, as well as petrologically estimated deep influx rates. Best fitting models predict an expanding zone of dike capture caused by a growing, oblate spheroidal magma chamber with 10–30 MPa of overpressure. This growing zone of chamber influence causes closest approaching regional mafic vent locations as well as more compositionally evolved Mazama eruptions to migrate away from the climactic eruptive center, returning as observed to the center after the chamber drains during the caldera-forming eruption.

Oregon↗

USGS2012

Petrologic evolution of divergent peralkaline magmas from the Silent Canyon caldera complex, southwestern Nevada volcanic field

The Silent Canyon volcanic center consists of a buried Miocene peralkaline caldera complex and outlying peralkaline lava domes. Its location has been corroborated by geophysical data and more than 50 drill holes. Two widespread ash flow sheets, the Tub Spring and overlying Grouse Canyon members of the Miocene Belted Range Tuff, were erupted from the caldera complex and have volumes of 60–100 km 3 and 200 km 3 , respectively. Eruption of the ash flows was preceded by widespread extrusion of precaldera comendite domes and was followed by extrusion of postcollapse peralkaline lavas and tuffs within and outside the caldera complex. Lava flows and tuffs were also deposited between the two major ash flow sheets. Rocks of the Silent Canyon center vary significantly in silica content and peralkalinity. The most mafic rocks are precollapse and postcollapse trachytes (65–69% SiO 2 ). Low-silica comendites (69–73% SiO 2 ) were erupted as the mafic upper part of the chemically zoned Grouse Canyon Member and as postcollapse lavas. The lower part of the Grouse Canyon Member and the underlying rhyolite of Split Ridge are moderately peralkaline comendite (PI is molar ratio Na + K/Al is 1.17–1.26). These comendites have major element characteristics and trace element enrichments approaching those of pantellerites. The Tub Spring Member, by contrast, is a weakly peralkaline chemically unzoned silicic comendite (75–76% SiO 2 ) ash flow tuff. Weakly peralkaline silicic comendites (PI 1.0–1.1) are the most abundant precaldera lavas. Postcollapse lavas range from trachyte to silicic comendite; some have anomalous light rare earth element (LREE) enrichments. Silent Canyon rocks follow a common petrologic evolution from trachyte to low-silica comendite; above 73% SiO 2 , compositions of the moderately peralkaline comendites diverge from those of the weakly peralkaline silicic comendites. These contrasting differentiation paths are shown in the behavior of Fe and other transition metals, Al, Na, K; the trace elements Ba, Zr, Nb; and probably F and Cl. Weakly peralkaline silicic comendites show a LREE/heavy REE crossover in early erupted/late erupted rocks; moderately peralkaline comendites are enriched in all REE. The development of divergent peralkaline magmas, toward both pantelleritic and weakly peralkaline compositions, is unusual in a single volcanic center.

Journal of Geophysical Research Solid Earth↗

USGS2010

Multi-stage origin of the Coast Range ophiolite, California: Implications for the life cycle of supra-subduction zone ophiolites

The Coast Range ophiolite of California is one of the most extensive ophiolite terranes in North America, extending over 700 km from the northernmost Sacramento Valley to the southern Transverse Ranges in central California. This ophiolite, and other ophiolite remnants with similar mid-Jurassic ages, represent a major but short-lived episode of oceanic crust formation that affected much of western North America. The history of this ophiolite is important for models of the tectonic evolution of western North America during the Mesozoic, and a range of conflicting interpretations have arisen. Current petrologic, geochemical, stratigraphic, and radiometric age data all favor the interpretation that the Coast Range ophiolite formed to a large extent by rapid extension in the forearc region of a nascent subduction zone. Closer inspection of these data, however, along with detailed studies of field relationships at several locales, show that formation of the ophiolite was more complex, and requires several stages of formation. Our work shows that exposures of the Coast Range ophiolite preserve evidence for four stages of magmatic development. The first three stages represent formation of the ophiolite above a nascent subduction zone. Rocks associated with the first stage include ophiolite layered gabbros, a sheeted complex, and volcanic rocks vith arc tholeiitic or (roore rarely) low-K calc-alkaline affinities. The second stage is characterized by intrusive wehrlite-clinopyroxenite complexes, intrusive gabbros, Cr-rich diorites, and volcanic rocks with high-Ca boninitic or tholeiitic ankaramite affinities. The third stage includes diorite and quartz diorite plutons, felsic dike and sill complexes, and calc-alkaline volcanic rocks. The first three stages of ophiolite formation were terminated by the intrusion of mid-ocean ridge basalt dikes, and the eruption of mid-ocean ridge basalt or ocean-island basalt volcanic suites. We interpret this final magmatic event (MORB dikes) to represent the collision of an active spreading ridge. Subsequent reorganization of relative plate motions led to sinistral transpression, along with renewed subduction and accretion of the Franciscan Complex. The latter event resulted in uplift and exhumation of the ophiolite by the process of accretionary uplift.

California↗

USGS1989

Subaqueous grain flows at the head of Carmel Submarine Canyon, California

The head of Carmel Submarine Canyon lies in 15 m of water about 200 m off a coarse-sand beach in the southeast corner of Carmel Bay, California. Very coarse sand is the predominate material on the beach, adjacent shelf, and upper canyon-head slopes, while silt and clay cover the surface below a water depth of about 35 m. Along a shore-normal transect, median grain size decreases between the beach and canyon rim but increases down the upper canyon-head slope. On angle-of-repose slopes in the upper canyon head, downslope-coarsening deposits, which are everywhere there is active sand movement, are similar to a type of sediment gravity flow deposit formed by grain flows (sand avalanches). Using three sand fractions that were dyed different fluorescent colors, scuba divers generated sand avalanches that produced deposits similar to the natural deposits. The dyed-sand deposits, which extended as far as 25 m below the initiation point, were inversely graded and increased in grain size downslope. Inverse grading was well developed within two m of the initiation point, though the thicknesses of the three layers varied in a nonsystematic manner both across and down slope. The minor amount of native material found within the dyed-sand layers showed that entrainment of the underlying sand was minimal.

Journal of Sedimentary Petrology↗

USGS2022

Pore systems and organic petrology of cretaceous Mowry and Niobrara source-rock reservoirs, Powder River Basin, Wyoming, USA

The Powder River Basin (PRB) is a world-class oil province, in large part thanks to contributions from premier source rocks, Cretaceous Mowry and Niobrara shales. Both formations are also unconventional reservoirs. A critical aspect of evaluating production potential and finding sweet spots is the nature of the pore systems in these fine-grained source-rock reservoirs. Variation by stratigraphic interval is important for selecting optimum target zones for horizontal wells. Understanding variation in pore type, size, and connectivity and relationships with mineralogy and fabric help in determining prospectivity in different parts of the basin. Deciphering controls on pore-system development helps predict intervals and locations of optimum reservoir quality. Imaging of Niobrara and Mowry samples from a range of thermal maturities provided observations and data on pore systems, organic matter (OM) types and associations with mineralogy and fabric, wettability, and microporosity associated with both diagenetic and detrital clays. Imaging techniques included scanning electron microscopy, organic petrography and correlative scanning electron microscopy, and mapping of mineralogy through energy dispersive spectroscopy. Mean solid bitumen (BR o ) and vitrinite reflectance (VR o ) values indicate all samples are in the oil window with values ranging from 0.52 to 1.15%. Organic fluorescence is prominent in amorphous OM, solid bitumen and some vitrinite in the early oil window. The fluorescence is extinguished at higher thermal maturity. Carbonate pellets (in Niobrara) mainly contain migrated solid bitumen and residual live oil and little or no terrigenous OM (vitrinite and inertinite). However, terrigenous OM is common in siliceous/argillaceous laminae in both formations, where it occurs with amorphous OM, some of which has converted in situ to a solid bitumen petroleum residue. One key finding is the widespread presence of migrated OM at very early oil window maturity. Distribution of such OM and associated wettability alteration is fabric-controlled, at all levels of thermal maturity studied. Clay morphology and abundance and supporting rigid mineral grain framework strongly influence pore development, preservation, and connectivity in both formations. Carbonate content is a good proxy for reservoir quality in Niobrara intervals due to association of porous solid bitumen with calcareous fecal pellets. High recrystallized microquartz content is associated with the best reservoir intervals in the Mowry.

Wyoming↗

USGS1960

Minor element abundance in a part of the Brazilian shield

Thirty-nine samples of granitic rocks from the Quadrilátero Ferrifero, Minas Gerais, Brazil, representing "mixed" ages and at least three distinct Pre-Cambrian ages were analysed for seventeen minor elements. The rocks are placed in five groups:, (1) 2400 million year old group;, (2) Bacao complex younger, a group of mixed rocks derived from, (1) and, (3);, (3) 1350 million year old group;, (4) a 500 million year old gneissic group;, (5) a 500 million year old silicic group, a later differentiate of a 500 million year old magma. Compared to granitic rocks elsewhere, these Brazilian rocks are about average in Ba, So, Zr, Pb, Nb, Co, Ni, Be and Mo; above average in La and Y; and below average in Cu, Sr, Ga, Sn, V and Cr. Compared to metasedimentary rocks elsewhere in the world., metasedimentary rocks of this region are about average in all seventeen minor elements. Sr has its greatest abundance in the 2400 million year old rocks; Ba, V, Co, Sc, Cr, Ni and Cu in the 1350 million year; and La, Nb, Sn, Zr, Y and Ga in the 500 million year silicic. These abundances are in accord with the petrology and chemistry of the rocks. The 1350 million year rocks are the most mafic of the group and should logically concentrate Ba, V, Co, So, Or, Ni and Cu which are most abundant in earlier crystallates. The 500 million year old silicic rocks are the most felsic and should concentrate La, Nb, Sn, Zr, Y and Ga, which are all most abundant in the latest fractions of granitic magna crystallization. The trace elements also lend support to the idea that the Bacao complex rocks developed by anatexis of 2400 million year rocks during the 1350 -pillion year orogeny. Cu, Sc, Ga, Zr, Sn and Nb have about the same magnitude in the Bagao and the 2400 million year rocks and Sr, V, Co, Or and Ni are lower in the BagAo than either the 2400 or the 1350 million yoar old rocks. This lower abundance is a probable effect of "debasif-cation" which is also brought out by the analysis of major constituents. The effects of a geochemical migration of the elements in this part of the earth's crust may be indicated by the increased abundance of Y in successivol y younger rock types. The increase of La, Nb, Pb, Sn, Y and Zr in the 1350 million year rocks compared to the 2,100 million year rocks might also be an effect of a secular migration.

southeast Brazil↗

USGS1993

Pressure increases, the formation of chromite seams, and the development of the ultramafic series in the Stillwater Complex, Montana

This paper explores the hypothesis that chromite seams in the Stillwater Complex formed in response to periodic increases in total pressure in the chamber. Total pressure increased because of the positive δ V of nucleation of CO 2 bubbles in the melt and their subsequent rise through the magma chamber, during which the bubbles increased in volume by a factor of 4–6. By analogy with the pressure changes in the summit chambers of Kilauea and Krafla volcanoes, the maximum variation was 0⋅2–0⋅25 kbar, or 5–10% of the total pressure in the Stillwater chamber. An evaluation of the likelihood of fountaining and mixing of a new, primitive liquid that entered the chamber with the somewhat more evolved liquid already in the chamber is based upon calculations using observed and inferred velocities and flow rates of basaltic magmas moving through volcanic fissures. The calculations indicate that hot, dense magma would have oozed, rather than fountained into the chamber, and early mixing of the new and residual magmas that could have resulted in chromite crystallizing alone did not take place.

Journal of Petrology↗

USGS1995

Porphyry copper and other intrusion-related mineralization in Mexico

Intrusion-related copper-bearing ore deposits in Mexico span a wide-range of deposit types and geological settings and formed from the mid-Mesozoic through the Holocene. These deposits include world-class copper porphyry and skarn deposits as well as a continuum of similar skarn, porphyry, vein, and replacement deposits that contain variable quantities of molybdenum, zinc, silver, lead, iron, gold, tungsten, tin, fluorine, and beryllium. Based on a new compilation, this paper reviews data on the full spectrum of intrusion-related deposits, concentrating on copper-rich systems, and attempts to place them in a generalized geological and petrological context. In Mexico, intrusion-related mineral deposits are primarily Mesozoic to middle Tertiary in age. Three broad periods are prominent in the mineralization record: the late Mesozoic, the Laramide, and the middle Tertiary. Jurassic to Late Cretaceous calc-alkalic batholiths with sparse volcanic rocks occur along the Pacific margin mainly on eugeoclinal crust, although locally on continental crust (for example, in Sonora). Latest Cretaceous to Early Tertiary ('Laramide') calc-alkalic batholithic, subvolcanic, and volcanic centers occur in an overlapping but somewhat more easterly band that extends with diminished intensity and somewhat younger ages into the Sierra Madre Oriental. Mid-Tertiary volcanism and local intrusive centers are widely developed, with the greatest abundance of calcalkalic felsic volcanics in the Sierra Madre Occidental and more mafic middle to late Tertiary arc volcanics in the Sierra Madre del Sur in southern Mexico and as a fringe of alkalic volcanic' and sub volcanic centers in northeastern Mexico. Over 600 copper-rich intrusion-related systems can be inferred from the literature; about 100 can be documented with some confidence. Copper-rich deposits occur with both intermediate (dioritic) and felsic (granodioritic) intrusive centers and show a corresponding variety of associated metals and alteration types. Styles include porphyry-type disseminated or stockwork mineralization, skarn, breccia pipes, and pegmatites. Multiple styles commonly occur in the same district. Porphyry copper deposits are best developed in association with the Laramide intrusive centers of northern Mexico and the mid-Tertiary intrusions in southern Mexico. Other intrusion-related deposit types occur within the same magmatic framework, but they have different temporal and spatial correlations related to their igneous composition and exposure level. The continuum of intrusion-related mineralization in Mexico can be divided by geological associations, metal contents, and styles of alteration. Although more than 1,500 intrusion-associated mineral deposits are known, the scarcity of data requires a simplified approach focusing on major districts. We distinguish the following overlapping groups of deposits based on their metal contents and igneous compositions: (1) porphyry or skarn Cu(-Mo-Zn) associated with intermediate to felsic granitoids, (2) porphyry or skarn Cu (-Au-Fe) associated with intermediate intrusions, (3) greisen, skarn, or pegmatite W(-Mo) associated with intermediate to felsic granitoids, (4) replacement or skarn Zn-Pb-Ag(-Cu-F) deposits associated with felsic intrusions, (5) volcanic-hosted vein Ag-Au(-Zn-F-Sn) deposits associated with hypabyssal felsic intrusions, (6) vein ± replacement Ag-Au(-Cu-Zn-Pb) deposits associated with intermediate stocks, (7) volcanichosted Au-Ag(-Cu) systems, (8) rhyolite-related F(-Sn-Be) deposits, (9) diorite-related Fe(-Au-Cu) skarns, and (9) rhyolite-related Fe deposits. Some inferences can be drawn from examination of these patterns: • Igneous compositions vary in time and space in Mexico, but multiple compositions commonly were emplaced at different times in the same region. Temporal variations (as in Sonora) are as important as differences in province (as between Sonora and southern Mexico). • Alteration and metal differences between alkaline and sub alkaline, felsic and mafic magma suites can be partly rationalized from equilibria among igneous minerals (for example, in terms of a Al 2 O 3 vs a CaO [vs a SiO 2 ]), fluid chloride and sulfur contents, and magmatic metal contents which reflect province and process. • Exposure and preservation filter observed Mexican metallogeny. Erosion of the Mesozoic arc superstructure in the west leaves mainly tungsten-skarns, burial of the Laramide arc in central Mexico interrupts porphyry copper patterns, and minimal exhumation of mid-Tertiary intrusive centers preserves distal vein or replacement systems. • The superimposed metallogenic patterns in Mexico have parallels with metallogenic patterns in the western United States in terms of the effects of preservation, process, and province. Future work should focus on increasing the basic geological data on mineral deposits and igneous rocks. Geochronology, petrology, and geochemistry would help better define the temporal, spatial, and compositional interrelationships between tectonism, magmatism, and mineralization.

Book chapter↗

USGS1984

Metamorphic history of a high-grade blueschist exotic block from the Franciscan complex, California

A high-grade blueschist tectonic block from the Franciscan Complex of the northeast Diablo Range shows evidence of three episodes of retrograde blueschist facies metamorphism ± deformation developed under progressively declining P-T conditions. The first retrograde metamorphism involved formation of an outer rind of actinolite + chlorite + rutile ± phengite, and the growth of coarse-grained chlorite + pumpellyite within the block. During the second event the rind and outer edge of the blueschist were folded, sheared and fractured, and primary glaucophane within the blueschist was replaced by albite, medium-grained chlorite, and glaucophane-crossite. The third retrograde metamorphism was marked by the pseudomorphic replacement of rind actinolite by aragonite and quartz. Aragonite also crystallized extensively within th block, accompanied by lawsonite, chlorite, jadeitic pyroxene, and crossite; this last mineral assemblage is identical to that of the surrounding Franciscan metasedimentary rocks. Features characteristic of the first and second retrograde metamorphic events are readily observed in other high-grade tectonic blocks of the Franciscan Complex and the correlative Otter Point Formation of Oregon. In contrast, evidence of a third retrograde metamorphism, matching that of presently associated Franciscan and Otter Point rocks, has been found in some but not all blocks examined so far. The high P/T conditions of prograde metamorphism and the available metamorphic age determinations suggest that the tectonic blocks originally formed in a pre-Franciscan subduction zone setting. Fragments of blueschist and eclogite from this metamorphic terrain were tectonically incorporated in a serpentinite diapir, and they developed alteration rinds through interaction with the enclosing ultramafic rock. The available data suggest that the examined exotic block and at least some others were transferred to the Franciscan as detritus from a body of serpentinite that reached the earth's surface. Such blocks were then resubducted and metamorphosed along with their presently associated sedimentary sequences.

Journal of Petrology↗

USGS1971

Petrologic and geophysical nature of serpentinites

Mineralogically, serpentinites consist predominantly of lizardite, clinochrysotile, and antigorite. Recent work has shown that these minerals are not polymorphs. Chrysotile is the only mineral recognized as a synthetic product in experimental studies of the system MgO-SiO 2 -H 2 O. Antigorite seems to be stable at higher temperatures than lizardite or chrysotile. The density of individual serpentine species is dependent on their morphology; the low-density serpentinites (<2.55g/cc) consist predominantly of clino-chrysotile. Seismic velocities and magnetic susceptibilities of serpentinites are related to the degree of serpentinization. The transition of massive serpentinites from ductile to brittle behavior in laboratory experiments at high confining pressures and temperatures above 300°C has been related to dehydration which may provide a mechanism for developing deep-focus earthquakes along Benioff zones. Serpentinite is formed by direct hydration of ultramafic protolith in the crust. The most common ultramafic protoliths are harzburgite, dunite, and Iherzolite. The assemblage generally developed from these is lizardite + chrysotile + brucite + magnetite. In areas of high-grade metamorphism, antigorite is the predominant serpentine mineral. The common, large, alpine-type serpentinized ultramafic masses contain brucite and have MgO/SiO 2 ratios similar to those of their protolith, resulting in volume increase during serpentinization. Metamorphic serpentinites and some highly sheared alpine-type serpentinites have lower MgO/SiO 2 ratios than their protolith, lack brucite, and appear t o have been formed by volume-for-volume replacement with concomitant loss of magnesium or addition of silica. Many large, young masses of peridotite appear to be slabs of oceanic mantle over-thrust onto continental edges. Subsequent sedimentation, serpentinization, and tectonism have greatly modified these original slabs so that their recognition in older orogenic zones is equivocal. The concept of the tectonic evolution of ultramafic rocks from oceanic crust-mantle slabs invading continental margins and being incrementally serpentinized and moved by later tectonic events provides a working hypothesis that allows a better explanation of the many peculiar and varied occurrences of serpentinite. The evidence does not support Hess' suggestion that the third layer of the oceanic crust consists of partly serpentinized mantle peridotite.

Bulletin of the Geological Society of America↗

USGS2017

Thermal and petrologic constraints on lower crustal melt accumulation under the Salton Sea Geothermal Field

In the Salton Sea region of southern California (USA), concurrent magmatism, extension, subsidence, and sedimentation over the past 0.5 to 1.0 Ma have led to the creation of the Salton Sea Geothermal Field (SSGF)—the second largest and hottest geothermal system in the continental United States—and the small-volume rhyolite eruptions that created the Salton Buttes. In this study, we determine the flux of mantle-derived basaltic magma that would be required to produce the elevated average heat flow and sustain the magmatic roots of rhyolite volcanism observed at the surface of the Salton Sea region. We use a 2D thermal model to show that a lower-crustal, partially molten mush containing < 20 – 40 % "> <20–40% interstitial melt develops over a ∼ 10 5 "> ∼105 -yr timescale for basalt fluxes of 0.008 to 0.010 m 3 / m 2 / yr "> 0.010m3/m2/yr (∼0.0008 to ∼0.001 km / 3 yr "> /3yr injection rate) given extension rates at or below the current value of ∼0.01 m/yr ( Brothers et al., 2009 ). These regions of partial melt are a natural consequence of a thermal regime that scales with average surface heat flow in the Salton Trough, and are consistent with seismic observations. Our results indicate limited melting and assimilation of pre-existing rocks in the lower crust. Instead, we find that basalt fractionation in the lower crust produces derivative melts of andesitic to dacitic composition. Such melts are then expected to ascend and accumulate in the upper crust, where they further evolve to give rise to small-volume rhyolite eruptions (Salton Buttes) and fuel local spikes in surface heat flux as currently seen in the SSGF. Such upper crustal magma evolution, with limited assimilation of hydrothermally altered material, is required to explain the slight decrease in δ 18 O "> δ18O values of zircons (and melts) that have been measured in these rhyolites.

California↗

USGS1988

Origin of metaluminous and alkaline volcanic rocks of the Latir volcanic field, northern Rio Grande rift, New Mexico

Volcanic rocks of the Latir volcanic field evolved in an open system by crystal fractionation, magma mixing, and crustal assimilation. Early high-SiO2 rhyolites (28.5 Ma) fractionated from intermediate compositionmagmas that did not reach the surface. Most precaldera lavas have intermediate-compositions, from olivine basaltic-andesite (53% SiO2) to quartz latite (67% SiO2). The precaldera intermediate-composition lavas have anomalously high Ni and MgO contents and reversely zoned hornblende and augite phenocrysts, indicating mixing between primitive basalts and fractionated magmas. Isotopic data indicate that all of the intermediate-composition rocks studied contain large crustal components, although xenocrysts are found only in one unit. Inception of alkaline magmatism (alkalic dacite to high-SiO2 peralkaline rhyolite) correlates with, initiation of regional extension approximately 26 Ma ago. The Questa caldera formed 26.5 Ma ago upon eruption of the >500 km3 high-SiO2 peralkaline Amalia Tuff. Phenocryst compositions preserved in the cogenetic peralkaline granite suggest that the Amalia Tuff magma initially formed from a trace element-enriched, high-alkali metaluminous magma; isotopic data suggest that the parental magmas contain a large crustal component. Degassing of water- and halogen-rich alkali basalts may have provided sufficient volatile transport of alkalis and other elements into the overlying silicic magma chamber to drive the Amalia Tuff magma to peralkaline compositions. Trace element variations within the Amalia Tuff itself may be explained solely by 75% crystal fractionation of the observed phenocrysts. Crystal settling, however, is inconsistent with mineralogical variations in the tuff, and crystallization is thought to have occurred at a level below that tapped by the eruption. Spatially associated Miocene (15-11 Ma) lavas did not assimilate large amounts of crust or mix with primitive basaltic magmas. Both mixing and crustal assimilation processes appear to require development of relatively large magma chambers in the crust that are sustained by large basalt fluxes from the mantle. The lack of extensive crustal contamination and mixing in the Miocene lavas may be related to a decreased basalt flux or initiation of blockfaulting that prevented pooling of basaltic magma in the crust. ?? 1988 Springer-Verlag.

Contributions to Mineralogy and Petrology↗