Valles caldera (New Mexico, USA) is the type example of a resurgent caldera and source of the Tshirege Member, Bandelier Tuff, which has been long recognized as a normally zoned sequence of ignimbrites. In this paper, we present geologic, stratigraphic, chemical, and mineralogical data from upper flow units of the Tshirege Member obtained at multiple sites within and east of Valles caldera showing that the upper part of the Tshirege Member is reverse-zoned in chemistry and mineralogy. The key to deciphering these compositional changes in zoning is recognition of flow unit Qbt4u, which we informally name the high Ti-Ba unit or HTBU. The HTBU is widespread within the resurgent dome area of the caldera, but is only found in a small area outside and east of the caldera. The HTBU is the most "mafic" unit in the Tshirege Member and is identified by chemical maxima in Ti, Ba, Sr, P, V, and Th, and by unusually high contents of anorthoclase, plagioclase, orthopyroxene, ilmenite, apatite and zircon with respect to other flow units. The HTBU also contains small (<= 1 cm) enclaves of quenched andesitic magma consisting primarily of plagioclase, orthopyroxene, clinopyroxene, glass and vesicles. Later erupted Tshirege flow units Qbt5l and Qbt5u are chemically more evolved and contain less plagioclase, orthopyroxene, ilmenite, apatite and zircon than the HTBU, and they are more evolved than flow units erupted before the HTBU (approximately 90% of the Tshirege Member). The HTBU is the host ignimbrite of a complex vent breccia exposed for over 3 km along the NW faulted face of Redondo Peak, the Valles caldera resurgent dome. Breccia clasts consist of angular to subrounded fragments :<= 1 m in diameter of Permian red beds, Miocene basin fill sediments, Miocene to Pliocene volcanics and Quaternary ignimbrite (Otowi Member, Bandelier Tuff) that underlie Valles caldera. Flow units Qbt5l and Qbt5u overlie the HTBU vent breccia but these later units do not contain unusual quantities of lithic fragments. Significantly, small mudstone lithic fragments (generally :<= 200 mu m in diameter) in the HTBU vent breccia contain reaction rinds of secondary garnet (andradite) along margins with ignimbrite host. We argue that the tiny andradite crystals (<= 50 mu m) formed by reaction of degassing volatiles in the vent area with relatively Fe-Ca-rich vent breccia fragments at temperatures approaching 800 degrees C and pressures less than a few MPa. Geologic relations and the restricted distribution of upper Tshirege flow units Qbt3t to Qtb5u indicate they were erupted in a short period of time (weeks to months) from a centralized vent near Redondo Peak, and from other buried vents as far east as the eastern ring fracture. We also conclude that late injection of andesitic melt into the residual Bandelier magma chamber stimulated the eruption of upper Tshirege Member pyroclastic flows from different locations and depths of the chamber resulting in a small but significant reversal in chemical and mineralogical zonation. (C) 2014 Elsevier B.V. All rights reserved.

Tertiary sedimentary rocks, in places over 200 m thick, occupy a series of elongate basins within and partly surrounding the Arunta crystalline basement in the Alice Springs region, central Australia. The best-known sedimentary succession is that of the Hale Basin, where four distinctive members constitute the poorly indurated Early Tertiary Hale Formation. The oldest, the Am• balindum Sandstone Member, rests on deeply weathered basement. It is overlain by the Delaney Mudstone Member, olive-green mudstone and siltstone, deposited probably under quiet-water reducing conditions in a series of large lakes. At this time there was low input of terrigenous detritus from the nearby MacDonnell , Strangways and Harts Ranges. In places, these fine-grained sediments grade upwards and laterally into a lignite and oil shale unit, the Ulgnamba Lignite Member, probably reflecting local development of shoals and peripheral swamps. A mid to Late Eocene age is likely for this member. The Ulgnamba Lignite Member or, where it is absent, the Delaney Mudstone Member, passes abruptly upwards into the poorly sorted, coarse-grained Tug Sandstone Member. This unit is likely to have been deposited rapidly on piedmont slopes flanking the margins of the uplands. The abrupt change in sedimentary characteristics suggests either uplift in the nearby ranges or a change in climate, or both. In the Hale Basin and other Cainozoic basins, the uppermost part of the Cainozoic succession, the Waite Formation, generally consists of green and red, silty sandstone, containing ferruginous pisoliths and a few massive chalcedonic calcrete beds. Erosion,

Passive infrared (FTIR) and correlation spectrometer (COSPEC) measurements were conducted at Popocatépetl volcano during February 10 to 26, 1998 from sites 4 to 17 km distant from the summit. Volcano behavior was relatively quiet and SO2 flux averaged 1670±1420 t/day (51 measurements), relatively small for Popocatépetl. Concurrent HCl/SO2 and HF/SO2 ratios were 0.17±0.01 and 0.031±0.003, respectively, about the same as ratios measured from 1994 to 1997. The amount of CO2 in the volcanic plume was quantified using FASCODE in which atmospheric CO2 is numerically subtracted from the total infrared spectrum to obtain the residual magmatic CO2. Surprisingly, CO2/SO2 mass ratios rose dramatically to values as high as 140, about 30 times higher than typical values of 2 to 8 measured from 1994 to 1996. These excursions in high CO2/SO2 ratios were short-lived, lasting no longer than about 0.5 to 3.0 h but CO2 flux occasionally exceeded 100,000 t/day. We estimate that the average CO2/SO2 ratio for the period was about 23, yielding an average CO2 flux of roughly 38,000 t/day. Chemical and petrographic analyses of lava and pumice erupted during explosions on June 30, 1997 and January 1, 1998 show conclusively that Popocatépetl produces mixed products formed by injection of mafic magma into a more silicic chamber at temperatures and pressures of roughly 1040°C and 5 kbar. In addition, Popocatépetl eruptive products include xenoliths of metamorphosed carbonate rocks containing wollastonite and other calc-silicate minerals indicating reaction of magma with Cretaceous limestone underlying the volcano. Using a normal CO2/SO2 ratio of 4 for reference, we calculate an average excess CO2 production of 32,000 t/day for 17 days. This would require assimilation of only 5×10−4 km3 of limestone, an amount easily accessible in the 3-km-thick Cretaceous section beneath the volcano. We also examine two scenarios in which excess CO2 is produced by degassing of subjacent basalt magma, but these explanations seem less plausible to us. Because many other volcanoes are underlain by carbonate sequences, short-duration bursts of CO2 flux, and increased CO2/SO2 ratio, might be observed at other sites, if simultaneous, real-time measurements of major gas species are made.

This map and accompanying cross sections present an updated synthesis of the geologic framework of the Oasis Valley area, a major groundwater discharge site located about 15 km west of the Nevada Test Site. Most of the data presented in this compilation is new geologic map data, as discussed below. In addition, the cross sections incorporate new geophysical data that have become available in the last three years (Grauch and others, 1997; written comm., 1999; Hildenbrand and others, 1999; Mankinen and others, 1999). Geophysical data are used to estimate the thickness of the Tertiary volcanic and sedimentary rocks on the cross sections, and to identify major concealed structures. Large contiguous parts of the map area are covered either by alluvium or by volcanic units deposited after development of the major structures present at the depth of the water table and below. Hence, geophysical data provide critical constraints on our geologic interpretations. A companion paper by Fridrich and others (1999) and the above-cited reports by Hildenbrand and others (1999) and Mankinen and others (1999) provide explanations of the interpretations that are presented graphically on this map. This map covers nine 7.5-minute quadrangles in Nye County, Nevada, centered on the Thirsty Canyon SW quadrangle, and is a compilation of one published quadrangle map (O'Connor and others, 1966) and eight new quadrangle maps, two of which have been previously released (Minor and others, 1997; 1998). The cross sections that accompany this map were drawn to a depth of about 5 km below land surface at the request of hydrologists who are modeling the Death Valley groundwater system.

This digital geologic map compilation presents new polygon (i.e., geologic map unit contacts), line (i.e., fault, fold axis, dike, and caldera wall), and point (i.e., structural attitude) vector data for the Thirsty Canyon NW 7 1/2' quadrangle in southern Nevada. The map database, which is at 1:24,000-scale resolution, provides geologic coverage of an area of current hydrogeologic and tectonic interest. The Thirsty Canyon NW quadrangle is located in southern Nye County about 20 km west of the Nevada Test Site (NTS) and 30 km north of the town of Beatty. The map area is underlain by extensive layers of Neogene (about 14 to 4.5 million years old [Ma]) mafic and silicic volcanic rocks that are temporally and spatially associated with transtensional tectonic deformation. Mapped volcanic features include part of a late Miocene (about 9.2 Ma) collapse caldera, a Pliocene (about 4.5 Ma) shield volcano, and two Pleistocene (about 0.3 Ma) cinder cones. Also documented are numerous normal, oblique-slip, and strike-slip faults that reflect regional transtensional deformation along the southern part of the Walker Lane belt. The Thirsty Canyon NW map provides new geologic information for modeling groundwater flow paths that may enter the map area from underground nuclear testing areas located in the NTS about 25 km to the east. The geologic map database comprises six component ArcINFO map coverages that can be accessed after decompressing and unbundling the data archive file (tcnw.tar.gz). These six coverages (tcnwpoly, tcnwflt, tcnwfold, tcnwdike, tcnwcald, and tcnwatt) are formatted here in ArcINFO EXPORT format. Bundled with this database are two PDF files for readily viewing and printing the map, accessory graphics, and a description of map units and compilation methods.

This report provides baseline geochemistry for soils (including fill), and for bedrock within three specific areas that are planned for use in the remediation of Material Disposal Area P (MDA-P) at Technical Area 16 (TA-16). The baseline chemistry includes leachable element concentrations for both soils and bedrock and total element concentrations for all soil samples and for two selected bedrock samples. MDA-P operated from the early 1950s to 1984 as a landfill for rubble and debris generated by the burning of high explosives (HE) at the TA-16 Burning Ground, HE-contaminated equipment and material, barium nitrate sand, building materials, and trash. The aim of this report is to establish causes for recognizable chemical differences between the background and baseline data sets. In many cases, the authors conclude that recognizable differences represent natural enrichments. In other cases, differences are best attributed to analytical problems. But most importantly, the comparison of background and baseline geochemistry demonstrates significant contamination for several elements not only at the two remedial sites near the TA-16 Burning Ground, but also within the entire region of the background study. This contamination is highly localized very near to the surface in soil and fill, and probably also in bedrock; consequently, upper tolerance limits (UTLs) calculated as upper 95% confidence limits of the 95th percentile are of little value and thus are not provided. This report instead provides basic statistical summaries and graphical comparisons for background and baseline samples to guide strategies for remediation of the three sites to be used in the restoration of MDA-P.

Forty years of geologic investigations at the Nevada Test Site (NTS) have been digitized. These data include all geologic information that: (1) has been collected, and (2) can be represented on a map within the map borders at the map scale is included in the map digital coverages. The following coverages are included with this dataset: Coverage Type Description geolpoly Polygon Geologic outcrops geolflts line Fault traces geolatts Point Bedding attitudes, etc. geolcald line Caldera boundaries geollins line Interpreted lineaments geolmeta line Metamorphic gradients. The above coverages are attributed with numeric values and interpreted information. The entity files documented below show the data associated with each coverage.

Volcanism in the Yucca Mountain region of southern Nevada in the last 5 m.y. is restricted to moderate‐to‐small volumes of subalkaline basaltic magmas, produced during at least 6 intervals, and spanning an age range from 4.6 Ma to about 125 ka. Where paleomagnetic evidence is available, the period of volcanism at individual eruptive centers apparently was geologically short‐lived, even where multiple eruptions involved different magma types. K‐Ar studies are consistent with most other geochronologic information, such as the minimum ages of exposure‐dating techniques, and show no evidence of renewed volcanism after a significant quiescence at any of the centers in the Yucca Mountain region. A volcanic recurrence interval of 860 ± 350 kyr is computed from a large K‐Ar data set and an evaluation of their uncertainties. Monte Carlo error propagations demonstrate the validity of uncertainties obtained for weighted‐mean ages when modified using the goodness of fit parameter, MSWD. Elevated 87Sr/86Sr initial ratios (Sri) in the basalts, nearly constant at 0.707, combined with low SiO2 and Rb/Sr ratios indicate a subcontinental, lithospheric mantle source, previously enriched in radiogenic Sr and depleted in Rb. Beginning with eruptions of the most voluminous eruptive center, the newly dated Pliocene Thirsty Mountain volcano, basaltic magmas have decreased in eruptive volume, plagioclase‐phenocryst content, various trace element ratios, and TiO2, while increasing in light rare earth elements, U, Th, P2O5, and light REE/heavy REE ratios. These time‐correlated changes are consistent with either increasing depths of melting or a decreasing thermal gradient in the Yucca Mountain region during the last 5 m.y.

Three chemically distinct types of granite (''high Sr/low Y'' calc-alkaline, Palaeoproterozoic ''normal'' and ''enriched'') are present in the Palaeoproterozoic Arunta Inlier of central Australia. Twenty-one representative samples were selected for Nd isotopic analysis to complement geological and geochemical data. Most samples analysed have depleted-mantle Nd isotope model ages (T-DM, following McCulloch, 1987) of 2.3 to 2.1 Ga, similar to most of Palaeoproterozoic granites of northern Australia (McCulloch, 1987). These ages do not show a younging trend from the Northern to Southern provinces, although some post similar to 1750 Ma granites from the Central and Southern provinces, with typical felsic crust Sm-147/Nd-144 ratios (0.09-0.11), have younger model ages (less than 2.0 Ga). Post 1700 Ma, ''normal-type'' granites from the Southern Province have chemical compositions similar to older granites (1880-1760 Ma) of the same suite, even though they have younger model ages (1960-1930 Ma). This implies an increase in a younger component within southern source regions through melting of new underplate and/or additional mantle input. The syn- or post-tectonic granites (1720-1140 Ma) of the ''enriched-type'' (high heat producing granites rich in Th, U and K) have similar T-DM model ages to older granites of the ''normal-type'' from the same areas. The ''enriched-type'' granite from the Teapot Granite Complex of the Southern Province are characterised by low Cs, Sr and high Rb/Cs (30-50), Th/U (7-20) ratios and high Y contents. These chemical characteristics are consistent with magma being formed through anatexis of ''normal-type'' granite sources. In contrast, an 1820 Ma, S-type granite from the Harverson suite in the Northern Province has an older T-DM model age (2.32 Ga) than the T-DM 2.18 Ga age of the nearby Aileron Metamorphics, suggesting that the source rock of the granite may contain a significant Archaean component. The Nd isotope data, when integrated with geological and chemical information, are compatible with the recycling of Archaean crustal material. Such a process might have taken place very early in the history of the Arunta Inlier through subduction along a continental margin made up of a stretched, dismembered, thin Archaean basement. Magmatic underplating may have taken place shortly before 1850 Ma and continued episodically during subsequent tectono-thermal events. Crustal melts of mixed source rocks, formed during these events, could contain different amounts of an Archaean component.

Revision of lithological logs for boreholes penetrating the volcanic center at Pahute Mesa, Nevada, has led to a thorough review of the volcanic stratigraphy and geologic structure. We have combined this review with a compilation of old and newly acquired gravity and seismic travel time data, producing a unified interpretation along a northwest to southeast profile. The analysis supports a new interpretation of the Silent Canyon caldera complex. The caldera is found to be more asymmetric than previously suggested, with the southeastern boundary formed by linear, high‐angle normal faults and a more gently sloping northwestern boundary. The total thickness of volcanic units within the caldera complex does not appear to exceed 5 km. The shallow structure at Pahute Mesa could have a profound effect on the seismic response for regional and teleseismic signals from this nuclear test site. The Silent Canyon caldera complex is actually a set of nested calderas first filled by thick (>1 km) postcaldera lavas and subsequently buried by outflow sheets of the Timber Mountain caldera to the south. Thick, postcaldera lavas filled a half‐graben structure formed west of the West Greeley fault, dropping the tops of the youngest caldera‐forming units to depths in excess of 2 km. Therefore the western boundary of the caldera complex is poorly defined. East of the West Greeley fault, two overlapping calderas are defined, and stratigraphic data suggest the presence of even older calderas. The youngest caldera, the calc‐alkaline Area 20 caldera, is well defined from drill hole data. The Area 20 caldera overlaps the 13.6 Ma peralkaline Grouse Canyon caldera, which is less well defined, but apparently collapsed in trap‐door style along the Almendro fault. For both these calderas, collapse continued after the main caldera‐forming eruption, concurrent with the accumulation of thick (>1 km) lavas within the peripheral collapse zones. The geophysical interpretation indicates that the major structural boundary of the caldera complex corresponds to the NNE trending Scrugham Peak and Almendro faults, which offset the pre‐Tertiary contact more than 1 km but have less than 200 m offset in rocks of 11 Ma age. Drill hole data show that offsets along these faults increase systematically within older (up to 15 Ma) units, which are commonly rotated eastward in a style similar to units at the surface. Abrupt changes in the subsurface thickness of the caldera‐forming units occur across the faults, indicating that these linear features served as caldera boundaries.

Middle Miocene rocks of the southwestern Nevada volcanic field (SWNVF) lie across the projection of the Walker Lane belt within the Basin and Range province and thus provide an interesting opportunity to test for late Cenozoic vertical‐axis rotation. Paleomagnetic data from individual ash flow sheets document no significant relative vertical‐axis rotation among localities within central SWNVF, an area of relatively low stratal tilts and widely spaced faults. A time‐averaged mean paleomagnetic direction (D = 351.4°, I = 52.7°, α 95 = 4.5°) calculated from data from numerous separate rock units suggests that the central SWNVF underwent minimal counterclockwise vertical‐axis rotation (R = −7.1° ± 6.6°) with respect to the North American craton. No clockwise vertical‐axis rotation is found to support projection of dextral faults of the Walker Lane beneath the central SWNVF. Clockwise rotation of variable magnitude is common at numerous sites from southern and western margins of the field. These clockwise rotations probably reflect dextral shear strain developed at the interface between the little extended central SWNVF block and more strongly extended areas to the south and southwest of the field. Negligible rotation of 11.45‐Ma to 13.25‐Ma tuffs relative to the central SWNVF was found at the southeast margin of the field where 90° clockwise rotation at the northwest termination of the Las Vegas Valley shear zone had been postulated. Any clockwise rotation in this area must predate 13.25 Ma, and thus dextral shear within this part of the Walker Lane belt was not synchronous or connected across the southern margin of the field. Small counterclockwise vertical‐axis rotation relative to the craton, as found for the central SWNVF block, might be a regional feature in the western Great Basin.

The middle Miocene southwestern Nevada volcanic field (SWNVF) is a classic example of a silicic multicaldera volcanic field in the Great Basin. More than six major calderas formed between >15 and 7.5 Ma. The central SWNVF caldera cluster consists of the overlapping Silent Canyon caldera complex, the Claim Canyon caldera, and the Timber Mountain caldera complex, active from 14 to 11.5 Ma and centered on topographic Timber Mountain. Locations of calderas older than the Claim Canyon caldera source of the Tiva Canyon Tuff are uncertain except where verified by drilling. Younger peralkaline calderas (Black Mountain and Stonewall Mountain) formed northwest of the central SWNVF caldera cluster. We summarize major revisions of the SWNVF stratigraphy that provide for correlation of lava flows and small-volume tuffs with the widespread outflow sheets of the SWNVF. New laser fusion Ar-40/Ar-39 isotopic ages are used to refine and revise the timing of eruptive activity in the SWNVF. The use of high-sensitivity mass spectrometry allowed analysis of submilligram-sized samples with analytical uncertainties of approximately 0.3% (1 sigma), permitting resolution of age differences as small as 0.07 Ma. These results confirm the revised stratigraphic succession and document a pattern of episodic volcanism in the SWNVF. Major caldera episodes (Belted Range, Crater Flat, Paintbrush, Timber Mountain, and Thirsty Canyon Groups) erupted widespread ash-flow sheets within 100-300 k.y. time spans, and pre- and post-caldera lavas erupted within 100-300 k.y. of the associated ash flows. Peak volcanism in the SWNVF occurred during eruption of the Paintbrush and Timber Mountain Groups, when over 4500 km3 of metaluminous magma was erupted in two episodes within 1.35 m.y., separated by a 750 k.y. magmatic gap. Peralkaline and metaluminous magmatism in the SWNVF overlapped in time and space. The peralkaline Tub Spring and Grouse Canyon Tuffs erupted early, and the peralkaline Thirsty Canyon Group tuffs and Stonewall Flat Tuff erupted late in the history of the SWNVF, flanking the central, volumetrically dominant peak of metaluminous volcanism. Magma chemistry transitional between peralkaline and metaluminous magmas is indicated by petrographic and chemical data, particularly in the overlapping Grouse Canyon and Area 20 calderas of the Silent Canyon caldera complex. Volcanism in the SWNVF coincided with the Miocene peak of extensional deformation in adjoining parts of the Great Basin. Although regional extension was concurrent with volcanism, it was at a minimum in the central area of the SWNVF, where synvolcanic faulting was dominated by intracaldera deformation. Significant stratal tilting and paleomagnetically determined dextral shear affected the southwestern margin of the SWNVF between the Paintbrush and Timber Mountain caldera episodes. Larger magnitude detachment faulting in the Bullfrog Hills, southwest of the central SWNVF caldera cluster, followed the climatic Timber Mountain caldera episode. Postvolcanic normal faulting was substantial to the north, east, and south of the central SWNVF caldera cluster, but the central area of peak volcanic activity remained relatively unextended in postvolcanic time. Volcanism and extension in the SWNVF area were broadly concurrent, but in detail they were episodic in time and not coincident in space.

ABSTRACTGranites and their associated comagmatic felsic volcanic rocks occur in most Proterozoic provinces of Australia. Using multi-element, primordial-mantle-normalised abundance diagrams and various petrological characteristics, Australian Proterozoic granites can be subdivided into five groups: (i) I-type, Sr-depleted, Y-undepleted, restite-dominated, (ii) I- type, Sr-depleted, Y-undepleted, fractionated, low in incompatible elements, (iii) I-type Sr-depleted, Y-undepleted, enriched in incompatible elements (anorogenic granites), (iv) I-type, Sr-undepleted, Y-depleted, (v) S-type, Sr-depleted, Y-undepleted. The four Sr-depleted groups dominate, and group (iv) is of very limited extent. A comparison of these Proterozoic granites with Australian and Papua New Guinean granites of other time periods shows that these characteristic Sr-depleted Y-undepleted patterns are also dominant in early Palaeozoic granites. They are significantly different from those of granites in modern island arcs associated with subduction, and with most granites from Archaean terranes, where the multi-element diagrams are dominated by Sr-undepleted, Y-depleted patterns.The Sr-depleted, Y-undepleted patterns are thought to indicate source regions that contained plagioclase but not garnet, whilst the Sr-undepleted, Y-depleted patterns are taken to correspond with the presence of garnet, but not plagioclase, in the source rocks. The Sr-depleted, Y-undepleted patterns also only occur in regions where the lower crustal structure is dominated by an underplated mafic layer with a P-wave velocity of 7·2-7·-4 km/s. In contrast, in regions where the granites are dominated by Sr-undepleted, Y-depleted patterns, such as in the Archaean and in Cainozoic island arcs, this intermediate velocity layer is not present, and the crust-mantle boundary is very sharp.Two other distinctive compositional changes have been noted among the I-type granites of different age. Firstly, Na is highest in Archaean and Cainozoic granites, and lowest in early Proterozoic granites; Palaeozoic and Mesozoic granites have intermediate values. Secondly, late Archaean and Proterozoic granites are the most enriched in K, Th and U, while the Cainozoic and early Archaean tonalites are the most depleted; Palaeozoic and Mesozoic granites again contain intermediate amounts of those elements.

Eclogitic rocks of the D'Entrecasteaux Islands, Papua New Guinea, are of three types: true eclogites of omphacite-garnet-rutile; retrogressed or S-stage eclogites in which some omphacite has altered to symplectites of albite and less-jadeitic clinopyroxene (or amphibole); and rocks that are eclogites in all respects except that clinopyroxene is jadeitic diopside (Jd<20) rather than omphacite. The rocks of the third group equilibrated in eclogite facies P-T conditions and, we conclude, are Na-poor eclogites, rather than granulites; i.e., low Na in the bulk rock is the reason for low jadeite content of clinopyroxene. Bulk rock chemical data confirm low Na and Si. Other prograde phases in the ecologitic rocks are kyanite, quartz, epidote group minerals and phengite and, in the low-Na group, orthopyroxene. Post-eclogite phases are amphibole, epidote group minerals, phengite and albite and, in the Na-poor eclogites, late phlogopite, calcic plagioclase, rare scapolite, and sulfides. The eclogitic rocks occur as lenticular boudins and small concordant tabular bodies within a 2–3 km thick sequence of migmatitic gneisses and, less commonly, in granodiorite. The gneiss sequence is bounded by detachment faults above and by younger granodiorite below, and is folded into broad antiforms. The three types of eclogite equilibrated at temperatures ranging from 530 to 840°C and pressures of 12 to 24 kbar. The metamorphic complex developed during Early Cenozoic subduction and arc-continent collison, and was elevated and exposed during Mid and Late Cenozoic crustal extension. The thermal gradient during subduction averaged 10°C/km and remained low during initial uplift, increasing to 18°C/km subsequently. Uplift averaged about 1 mMa-1 from 60 to 5 Ma, then about 4 mMa-1.

This report is a compilation of petrographic and mineral chemical data for stratigraphic units at Yucca Mountain. It supports a possible peer review of Yucca Mountain drill core by summarizing the available data in a form that allows comparison of stratigraphic units in drill holes with surface outcrops of the same units. Petrographic and mineral chemical data can be used in conjunction with other geologic and geophysical information to determine if stratigraphic relations in Yucca Mountain drill core are geologically reasonable and compare well with relations known from extensive surface studies. This compilation of petrographic and mineral chemical data is complete enough for most stratigraphic units to be used in a peer review of Yucca Mountain drill core. Additional data must be collected for a few units to complete the characterization. Rock units at Yucca Mountain have unique petrographic and mineral chemical characteristics that can be used to make accurate stratigraphic assignments in drill core samples. Stratigraphic units can be differentiated on the basis of petrographic characteristics such as total phenocryst abundances, relative proportions of phenocryst minerals, and type and abundances of mafic and accessory minerals. The mineral chemistry of phenocrysts is also an important means of differentiating among stratigraphic units, especially when used in conjunction with the petrographic data. Sanidine phenocrysts and plagioclase rims have narrow compositional ranges for most units and often have well-defined dominant compositions. Biotite compositions are useful for identifying groups of related units (e.g., Paintbrush Tuff Members vs Crater Flat Tuff Members) and for providing an important check on the consistency of the data. 21 refs., 12 figs., 2 tabs.

Protoliths of metamorphites in the Proterozoic Reynolds Range region, northern Arunta Block, central Australia, included mafic and felsic igneous rocks, pelites, calcareous rocks and quartzites. Metamorphism began with an early high-T, low-P facies stage, followed by early rehydration in higher grade areas, isobaric cooling, and late high-T, low-P facies hydration in and near shear zones. Metamorphic grade increases southeastwards: shown by the change from andalusite in metapelites in the northwest to sillimanite in the southeast, and by the incoming of cordierite, cordierite-biotite, cordierite-K-feldspar ±sillimanite, and cordierite-garnet. Migmatites, kornerupine, quartz-spinel, and orthopyroxene-K-feldspar assemblages occur in the highest grade rocks. Downwarping at near-peak temperatures is shown by garnet-sillimanite replacing cordierite, and by sillimanite pseudomorphing andalusite. Early in the near-isobaric cooling that followed peak temperatures, freezing of partial melts released water, and biotite formed. The hydrated retrograde assemblages, formed after temperatures had fallen considerably, are also zoned from northwest to southeast, with andalusite-, sillimanite- or kyanite-bearing assemblages developed in appropriate bulk compositions. The early high-T metamorphism is best explained by tectonic models based on extension, crustal delamination and underplating; the hydration and deformation were caused by processes unrelated to the early history, occurring at least 200 Ma later, possibly as late as the mid-Palaeozoic.