Eastern Mojave Research Articles

Geochemical evolution of Jurassic diorites from the Bristol Lake region, California, USA, and the role of assimilation

Late Jurassic dioritic plutons from the Bristol Lake region of the eastern Mojave Desert share several geochemical attributes with high-alumina basalts, continental hawaiite basalts, and high-K are andesites including: high K2O concentrations; high Al2O3 (16–19 weight %); elevated Zr/TiO2; LREE (light-rare-earth-element) enrichment (La/YbCN=6.3–13.3); and high Nb. Pearce element ratio analysis supported by petrographic relations demonstrates that P, Hf, and Zr were conserved during differentiation. Abundances of conserved elements suggest that dioritic plutons from neighboring ranges were derived from similar parental melts. In the most voluminous suite, correlated variations in elemental concentrations and (87Sr/86Sr)i indicate differentiation by fractional crystallization of hornblende and plagioclase combined with assimilation of a component characterized by abundant radiogenic Sr. Levenberg-Marquardt and Monte Carlo techniques were used to obtain optimal solutions to non-linear inverse models for fractional crystallization-assimilation processes. Results show that the assimilated material was chemically analogous to lower crustal mafic granulites and that the mass ratio of contaminant to parental magma was on the order of 0.1. Lack of enrichment in 18O with differentiation is consistent with the model results. Elemental concentrations and O, Sr, and Nd isotopic data point to a hydrous REE-enriched subcontinental lithospheric source similar to that which produced some Cenozoic continental hawaiites from the southern Cordillera. Isotopic compositions of associated granitoids suggest that partial melting of this subcontinental lithosphere may have been an important process in the development of the Late Jurassic plutonic arc of the eastern Mojave Desert.

Source region of a granite batholith: evidence from lower crustal xenoliths and inherited accessory minerals

ABSTRACTLike many granites, the Late Cretaceous intrusives of the eastern Mojave Desert, California, have heretofore provided useful but poorly focused images of their source regions. New studies of lower crustal xenoliths and inherited accessory minerals are sharpening these images.Xenoliths in Tertiary dykes in this region are the residues of an extensive partial melting event. Great diversity in their composition reflects initial heterogeneity (both igneous and sedimentary protoliths) and varying amounts of melt extraction (from <10% to >70%). Mineral assemblages and thermobarometry suggest that the melting event occurred at T ≥ 750°C at a depth of about 40 km. Present-day Sr, Nd, and Pb isotopic ratios indicate a Mojave Proterozoic heritage, but unrealistic model ages demonstrate the late Phanerozoic adjustment of parent/daughter ratios. A link between these xenoliths and the Late Cretaceous granites, though not fully documented, is probable; in any case, they provide invaluable clues concerning a crustal melting event, recording information about nature of source material (heterogeneous, supracrustal-rich), conditions of melting (moderately deep, moderately high T, accompanied by partial dehydration), and melt extraction (highly variable, locally extensive).The Old Woman-Piute granites contain a large fraction of inherited zircon and monazite. A SHRIMP ion probe investigation shows that these zircons record a Proterozoic history similar to that which affected the Mojave region. Zonation patterns in zircons, and to a lesser extent monazites and xenotimes, document multiple phases of igneous, metamorphic, and sedimentary growth and degradation, commonly several in a single grain. Low Y in portions of the cores of inherited zircons and monazites and in monazites and outer portions of zircons from the xenoliths appear to indicate growth in equilibrium with abundant garnet.

Tertiary extension‐related volcanism, Old Woman Mountains area eastern Mojave Desert, California

Tertiary volcanism in the Old Woman Mountains area, eastern California overlapped temporally and spatially with lithospheric extension during the early Miocene. Field relations and restoration of movement along major faults indicate that the primary locus of magmatism probably lay along or flanked the axis of the Piute Mountains. The maximum age of volcanism is unknown but is probably about 20–21 Ma (after regional extension began). Most of the volcanism terminated after emplacement of the 18.5 Ma Peach Springs Tuff. Contrary to conventional models which suggest that extension‐related volcanism is basaltic or bimodal, the volcanic rocks of the Old Woman Mountains area represent a calc‐alkaline continuum dominated by mafic andesite; the most mafic rocks are mildly alkalic. Compositional variability within the suite is the result of mixing a mantle component with silicic crust, accompanied initially by minor olivine fractionation. Petrographic and field observations indicate that both magma mixing and assimilation of granitic crust have operated to produce the observed chemical variation. Trace elements and isotopes preclude binary mixing and suggest that mixing involved several crustal end members. This is consistent with the heterogeneous nature of both the exposed basement and a lower crustal xenolith suite sampled by a Tertiary dike. The mildly alkalic nature of the mafic lavas, the enrichment in large ion lithophiles and light rare earth elements, and the relatively radiogenic character of the most primitive uncontaminated basalt suggest a source similar to enriched mantle lithosphere. Alternatively, these geochemical and isotopic signatures could be attributed to contamination of asthenospheric basalt with mafic lower crust or low‐degree partial melts of this crust.

Lower Jurassic unconformity (J-0) from the Colorado Plateau to the eastern Mojave Desert: Evidence of a major tectonic event at the close of the Triassic

From southern Nevada to the eastern Mojave Desert, the Lower Jurassic basal unconformity (J-0) cuts down section from the Upper Triassic Chinle Formation to deformed Paleozoic carbonate rocks. The Chinle Formation is not present southwest of the central Spring Mountains. Stratigraphic relations suggest correlation of J-0 with unconformities in western Arizona and the Inyo Mountains. Close proximity of J-0 in time and space to (1) first appearance of western -derived volcanic clasts in lower Mesozoic cratonal stratigraphy, (2) early Mesozoic thrust faults, and (3) the J-1 cusp in the apparent polar wander path imply a major change in tectonic setting of the southwestern Cordillera at the Triassic/Jurassic boundary.

En echelon Miocene rifting in the southwestern United States and model for vertical-axis rotation in continental extension

Two areas of intense early Miocene crustal extension in the southwestern United States, the Colorado River trough and the central Mojave Desert, are separated by a weakly deformed area in the eastern Mojave Desert. We propose that these areas form a left-stepping en echelon rift system linked by a ductile detachment at depth. The en echelon geometry explains the southward loss of displacement in the central Mojave Desert and northward loss of coeval displacement in the Colorado River trough, and it incorporates seismic reflection evidence that, mid-crustal Tertiary extensional mylonites continue beneath the weakly deformed area. This geometry also explains clockwise paleomagnetic declination anomalies from lower Miocene rocks as recording thin-skinned, detached rotations; large- scale block rotations are not required. Obliquity of the northeast-trending crustal-extension vector to the east-west-trending early Miocene synextensional volcanic belt may have caused the en echelon pattern to develop.

The 40Ar/39Ar thermochronology of the eastern Mojave Desert, California, and adjacent western Arizona with implications for the evolution of metamorphic core complexes

Mesozoic thickening and Cenozoic extension resulted in the juxtaposition of upper and middle crustal rocks in the eastern Mojave Desert, southeastern California and western Arizona. The application of 40Ar/39Ar thermochronology to rocks in this region provides information about the timing and nature of thrusting, plutonism, metamorphism, denudation, and detachment faulting. The 40Ar/39Ar ages of 175 to 125 Ma from the Clipper, Piute, Turtle, Mohave, Bill Williams, and Hualapai Mountains are interpreted to be the result of a middle Mesozoic thermal event(s) caused by crustal thickening. The 40Ar/39Ar data from the Clipper and Piute Mountains suggest that this thermal event was followed by a period of cooling at rates of 1°–5°C/m.y. Orogenesis culminated during the Late Cretaceous when rocks exposed in the Old Woman‐Piute, Chemehuevi, and Sacramento Mountains attained temperatures >500°C which reset the K–Ar systems of minerals from Proterozoic rocks. High‐grade metamorphism in the Old Woman Mountains area was caused by the intrusion of the Old Woman‐Piute batholith at 73±1 Ma. Cooling rates following batholith emplacement in the Old Woman Mountains were ∼100°C/m.y. between 73 and 70 Ma and 5°–10°C/m.y. from 70 to ∼30 Ma. Between 65 and 25 Ma the entire eastern Mojave Desert underwent a period of cooling at a rate of 2°–10°C/m.y. By 30 Ma, rocks exposed in the Old Woman‐Piute, Marble, Ship, Clipper, and Turtle Mountains were below ∼100°C. The 40Ar/39Ar ages from the Sacramento Mountains suggest that mylonitization caused by the onset of regional extension occurred at 23±1 Ma. When extension started in the Chemehuevi Mountains, rocks exposed in the southwestern and northeastern portions of footwall to the Chemehuevi detachment fault were at ∼180°C and ∼350°C, respectively. This suggests that the exposed part of the Chemehuevi detachment fault initiated at a dip of 5°–30° or as a series of higher‐angle faults that cut to a depth of 10–12 km and were later rotated to their present dip. Unroofing of the footwalls to detachment faults in the Sacramento and Chemehuevi Mountains resulted in average cooling rates of 10°–50°C/m.y. between 22 and 15 Ma.

Chronologic and isotopic framework for Early Proterozoic crustal evolution in the eastern Mojave Desert Region, SE California

The Early Proterozoic geologic evolution of the eastern Mojave Desert region, as defined by characteristics of its supracrustal rocks, granitoids, metamorphism, structural history, and Pb and Nd isotopic signature, contrasts sharply with other Proterozoic provinces of the southwestern United States. The oldest supracrustal rocks of the Mojave Desert region contain zircons over 2.0 Ga, corroborating Nd isotopic evidence for a much older crust here than elsewhere in the southwestern United States. Granitoids widely emplaced within these supracrustal rocks range from 1.76 to 1.64 Ga. The earlier plutons and surrounding supracrustal rocks were metamorphosed to granulite and high amphibolite facies throughout the province at about 1705 Ma in a migmatite‐producing event that we term (informally) the Ivanpah orogeny. Subsequent granitoids, emplaced from 1.69 to 1.67 Ga, were voluminous along a north trending belt in the middle of the Mojave province. Younger plutons were emplaced at about 1.66 Ga in several places and at about 1.64 Ga along the extreme southern part of the province. Commonalities between the Proterozoic evolutions of the Mojave and Arizona crustal provinces do not conclusively establish the time that the provinces were juxtaposed; the data only suggest that the juxtaposition occurred between about 1.76 and 1.64 Ga.

Elevated Nitrate Levels in Soils of the Eastern Mojave Desert

AbstractDeep cores taken on an uncultivated desert alluvial fan revealed two large volumes of coarse soil with high NO3 levels (20–208 mg L−1 NO3‐N in water saturation extracts). Nitrate levels were unpredictable both laterally and vertically and unrelated to alluvial strata. The large soil volumes with elevated NO3 levels were found 2.7 to 7.3 m deep in four cores at one 3 by 3 m site, and 8.8 to 21 m deep in two cores at another site. Isolated samples with elevated NO3 were found in three other cores and six cores contained no elevated NO3. Overall, nitrate was best correlated with EC, Na, SO4, and Cl (r = 0.68, 0.66, 0.67, and 0.73, respectively; P ≤ 0.05). Stronger correlations were found within the four individual sites and 15 cores, but the ions best correlated with NO3 varied, even between cores from the same site. Saturation extract NO3 was not correlated with bulk soil total organic C or total N. The NO3 distribution patterns found appear related to inorganic processes (salinization and leaching), but specific processes of N redistribution and primary N sources remain unclear. Elevated NO3 in deep soils appears to be a natural phenomena that reflects Pleistocene site conditions and possible geologic NO3 sources. Local volcanic rocks contain up to 73 mg NO3‐N kg−1 rock (water‐extractable). Naturally elevated NO3 levels in rocks and coarse alluvial fan soils are poorly understood and easily overlooked potential sources of groundwater NO3.

North-South Contraction of the Mojave Block and Strike-Slip Tectonics in Southern California

The Mojave block of southern California has undergone significant late Cenozoic north-south contraction. This previously unappreciated deformation may account for part of the discrepancy between neotectonic and plate-tectonic estimates of Pacific-North American plate motion, and for part of the Big Bend in the San Andreas fault. In the eastern Mojave block, contraction is superimposed on early Miocene crustal extension. In the western Mojave block, contractional folds and reverse faults have been mistaken for extensional structures. The three-dimensional complexity of the contractional structures may mean that rigid-block tectonic models of the region based primarily on paleomagnetic data are unreliable.

Physical properties of deep crustal reflectors in southern California from multioffset amplitude analysis

Analysis of reflection waveforms before stack can constrain the physical properties of reflectors in the deep crust. To simplify this analysis, recorded amplitudes are assumed to be reflections from weak elastic heterogeneities. With these assumptions, trends in reflection amplitudes with offset may indicate whether the signs of a reflector’s density and rigidity contrast agree with or oppose the sign of its Lamé’s parameter contrast. The slope of the trend indicates the degree of Poisson’s ratio contrast. No attempt is made to invert for the individual modulus or density contrasts. By examining only gross amplitude‐versus‐offset (AVO) trends, deep reflections constrain some crustal properties. Two seismic reflection surveys in the Mojave Desert recorded deep reflections that show amplitude changes with offset. Both the 1985 Calcrust Ward Valley survey in the eastern Mojave and the 87 km COCORP Mojave line 3 in the western Mojave incorporate long offsets of 10 km or more. Prestack traces are equalized using a quantile technique assuming a constant noise level at large time, then corrected for spherical divergence. Gross AVO trends that are summarized for each survey in amplitude trend stacks suggest that the strongest reflectors in the middle and deep crust represent Poisson’s ratio contrasts of at least 10 percent. In the eastern Mojave, a transition to a basal‐crustal zone, at ∼23 km depth, may include an increase in Poisson’s ratio with depth. Poisson’s ratio may also increase at the Moho.

Faults in the Mojave Desert, California, as Revealed on Enhanced Landsat Images

Previously unknown strike-slip and normal faults in the central and eastern Mojave Desert have been revealed on Landsat Thematic Mapper images enhanced by four-component processing. This method provides color images on which lithologies are discriminated by their contrasting absorption and reflection, primarily at infrared wavelengths and particularly with regard to their ferric iron, ferrous iron, and hydroxyl contents, while retaining landform depiction. These discriminants represent a new type of geophysical display for geologic mapping in regions of well-exposed bedrock. Faults are revealed on the images by abrupt spectral and textural contrasts that coincide with aligned topographic features. The newly discovered faults form part of an extensive regional network of right shear that connects faults in the Death Valley region with the San Andreas fault system. They support a heterogeneous strain model for late Cenozoic tectonic evolution of the region. Regional structural relations indicate a westward migration of the locus of strain through time. Some of the newly identified faults bound blocks that have experienced contrasting rotational histories since early Miocene time.

Late Cretaceous ductile deformation, metamorphism and plutonism in the Piute Mountains, eastern Mojave Desert

Late Cretaceous deformation in the Piute Mountains involved ductile thrusting on a network of anastomosing northeast and southwest directed shear zones. The shear zones often separate Proterozoic lithologies with strong competency contrasts, suggesting that Proterozoic crustal anisotropies were important in controlling the geometry of the shear zone network. Shear zones now divide the crust into wedge and lozenge‐shaped blocks. The Piute Mountains can be separated into two kinematic domains, each characterized by a dominant sense of shear. Thrusting in the southwest directed kinematic domain ended at 85±7 Ma, the age of the late synkinematic East Piute pluton. Thrusting in the northeast directed kinematic domain could have lasted until 74±3 Ma, the age of the Lazy Daisy pluton. Postthrusting northwest‐southeast shortening produced a second generation of upnght open folds with a northeast striking subvertical crenulation cleavage. F2 strain increases toward the south and is concentrated around the margins of the Lazy Daisy pluton. This deformation was synchronous with pluton emplacement. Late Cretaceous peak metamorphism outlasted all ductile deformation. Peak metamorphic grade increases from upper greenschist facies in the north to upper amphibolite facies as the Lazy Daisy pluton is approached. Temperature and pressure of approximately 450°C and 2.5–4.0 kbar were achieved during ductile thrusting. Temperatures of 500°‐540°C existed at the onset of upright open folding, and peak temperatures as high as 620°C near the Lazy Daisy pluton occurred after F2 upright folding. All penetrative deformation ended before 72–71 Ma, by which time the area had cooled to below 300°C (Foster et al., 1989).

Braided Fluvial to Marine Transition: The Basal Lower Cambrian Wood Canyon Formation, Southern Marble Mountains, Mojave Desert, California

ABSTRACT The Lower Cambrian Wood Canyon Formation, recognized over a 39,000 km2 belt through the southern Great Basin and Mojave Desert, was examined in the southern Marble Mountains, eastern Mojave Desert, southeastern California. There it crops out as a well-exposed 90-130 m thick, feldspathic sandstone nonconformably overlying Proterozoic basement. Previous investigators have suggested a tide-dominated, nearshore, shallow marine origin for the entire unit in the eastern Mojave, based on broad-scale stratigraphic trends and localized fossil occurrences in the upper member of the formation. In contrast to a regional approach, the present study uses a detailed lithofacies analysis in a smaller study area to suggest that a fluvial to marine transition occurs within these strata. The middle and upper members of the Wood Canyon Formation are recognized in the southern Marble Mountains and are divisible into six distinct lithofacies that possess gradational contacts where exposed: cross-stratified sandstone (Facies A); mediumpebble conglomerate (Facies B); planar-stratified sandstone (Facies C); sandstone and thin mudstone (Facies D); mudstone with intercalated sandstone (Facies E); and horizontally laminated siltstone (Facies F). Facies A-D, contained within the middle member, comprise the bulk of Wood Canyon sediments and record the depositional characteristics of a transition zone from a distal alluvial braid plain to a tidally influenced, fluvial-dominated braid-delta complex. Cross stratification in the braid-plain sediments displays a strongly unimodal pal ocurrent pattern, suggestive of downstream-migrating sinuous-crested megaripples. Braid-delta sediments, however, show a more dispersed paleocurrent pattern and contain a sparse trace fossil assemblage. A change from fluvial-dominated to marine-dominated processes is recorded in upper member rocks (Facies E and F). Facies E contains many of the traits characteristic of a low- to mid-tidal flat sequence, such as coarsely interlayered bedding, flaser and lenticular bedding, microripple marks, and locally abundant Rusophycus and Planolites trace fossils. Facies F, which dominantly consists of horizontally laminated siltstone, is interpreted as the product of an estuarine lagoon that developed behind and within a discontinuous barrier of stranded braid bars.

Phytophagous Insect Faunas of the Desert Shrubs Bebbia juncea and Trixis californica in Southern California

Phytophagous insects representing seven orders, 24 families, 33 genera, and 48 species compose the insect fauna of Bebbia juncea (Bentham) Green (Asteraceae, Tribe Heliantheae, Subtribe Galinsoginae) in the Colorado and eastern Mojave deserts of southern California. Similarly, phytophagous insects representing six orders, 22 families, 27 genera, and 34 species compose the insect fauna of Trixis californica Kellogg (Asteraceae, Tribe Mutisieae). Most insects on these native shrubs were ectophagous sap- and foliage-feeding species. Twenty-nine (69%) of the 42 identified species of insects on B. juncea also were reported from one or more genera of Ambrosiinae, whereas 13 (50%) of 26 identified species from T. californica also feed on Ambrosinae. Fifteen (58%) of the 26 identified species on T. californica also occurred on B. juncea . Thus, an appreciable component of the insect faunas of both plants comprises a guild of unspecialized generalists that feed or breed to a greater or lesser degree on other plant species belonging to a different tribe or subtribe of Asteraceae. Host-plant architecture is suggested as one factor governing selection of B. juncea by these generalist insects.

The Mojave Extensional Belt of southern California

It is proposed that the regional pattern of early Miocene low‐ and high‐angle normal faulting and dike swarm emplacement in the western and central Mojave Desert is the result of regional extension developed within a roughly east trending zone, named here the “Mojave Extensional Belt”. This zone of extension, although now disrupted by late Cenozoic right‐slip faulting, can be traced from the intersection of the San Andreas and Garlock faults to the eastern Mojave Desert near Bristol Lake and the Granite Mountains fault. Restoration of post‐Oligocene movement along the San Andreas fault system places the now buried western part of the Mojave Extensional Belt opposite a similar age‐extended area of west central California. Surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the Mojave Extensional Belt was low‐angle, normal sense, simple shear (detachment faulting). Strain within the Mojave Extensional Belt is partitioned between four domains (Edwards, Waterman, Daggett, and Bullion terranes) that each consist of one or more half‐grabens. Each half‐graben is composed of tilted, normal fault‐bounded blocks that lie above a rooted, low‐angle, brittle‐ductile normal sense shear zone. Tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping (∼80°) planar to slightly curviplanar normal faults. At least one generation of high‐angle faults postdates the formation of the detachments. Differential extension between the domains was accommodated by strike‐slip faults (transfer zones). Similarly, the lack of early Miocene extensional elements north of the western segment of the Garlock fault suggests that the Garlock may have been active in the early Miocene and served as the northern transfer boundary of the Mojave Extensional Belt. The Mojave Extensional Belt initially began to open about 22 m.y. ago, with the major phase of extension occurring between ∼22 and 20 Ma. Local, high‐angle, normal faults cut older detachments and were active between ∼20 and 17 Ma. Kinematic indicators suggest that the western part of the Mojave Extensional Belt opened in a NE–SW direction, whereas the eastern portion extended in an ENE–WSW direction (present‐day reference frame). Rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the Mojave Extensional Belt was originally oriented ∼N–S and was rotated as much as 50° (clockwise) between ∼20 and 18 Ma (Early Miocene).

Soil evolution along an altitudinal transect in the Eastern Mojave Desert of Nevada, U.S.A.

Six soil profiles were examined along an altitudinal transect, from 840 to 2170 m, in the eastern Mojave Desert of southern Nevada. The soils formed on gravelly limestone alluvium which varied in age from Holocene to mid-Pleistocene. Climate and vegetation varied from an arid, desert scrub community at the low elevation to a sub-humid, coniferous forest at the highest elevation. Silicate clay mineralogy was similar to that of the parent material in all the Holocene soils but differed with elevation in the mid-Pleistocene soils, these being dominated by palygorskite at 1400 m and smectite at 2150 m. Pedogenic carbonate content of the Bk horizons and morphologic stage of carbonate accumulation increased with soil age with the exception of the 2150 m site where excess moisture leaches carbonate out of the profile. DCB-extractable iron increased with both age and elevation whereas soil organic C and CO 2 increased primarily with elevation, reflecting the gradient in moisture regime and biologic productivity. Aeolian dust may have a strong role in the formation of the vesicular, platey surface horizons at many of the sites.

Age, Inheritance, and Uplift History of the Old Woman-Piute Batholith, California and Implications for K-Feldspar Age Spectra

Ion probe U-Pb zircon analyses yield a crystallization age of $$74 \pm 3 Ma$$ for both peraluminous and metaluminous plutons within the Old Woman-Piute batholith. These analyses reveal that about 50% of the zircons from the peraluminous Sweetwater Wash granite are restitic. Inherited zircons from this batholith yield ages ranging from older than 1800 Ma to ~1100 Ma, and most are similar in age to Proterozoic igneous rocks in the eastern Mojave Desert. $$^{40}Ar/^{39}Ar$$ hornblende ages from the metaluminous granodiorite are $$73 \pm 2 Ma$$, and mica ages from both plutonic series are $$70 \pm 2 Ma$$. K-feldspar age spectra yield age gradients with initial steps between 60 and 67 Ma and rise to plateau ages of $$70 \pm 2 Ma$$. Several K-feldspars from the granodiorite yield elevated "plateau" ages after 50% $$^{39}Ar$$ released that result from excess argon. Uplift rates as high as 4 mm/yr between 74 and 70 Ma are required by the narrow age range for all minerals.

A comparison of soil climate and biological activity along an elevation gradient in the eastern Mojave Desert.

Soil temperature, moisture, and CO2 were monitored at four sites along an elevation transect in the eastern Mojave Desert from January to October, 1987. Climate appeared to be the major factor controlling CO2 partial pressures, primarily through its influence of rates of biological reactions, vegetation densities, and organic matter production. With increasing elevation, and increasing actual evapotranspiration, the organic C, plant density, and the CO2 content of the soils increased. Between January and May, soil CO2 concentrations at a given site were closely related to variations in soil temperature. In July and October, temperatures had little effect on CO2, presumably due to low soil moisture levels. Up to 75% of litter placed in the field in March was lost by October whereas, for the 3 lower elevations, less than 10% of the litter placed in the field in April was lost through decomposition processes.

Hydrothermal alteration associated with the Iron Hat iron skarn deposit, eastern Mojave Desert, San Bernardino County, California

The Iron Hat deposit, which occurs in the Marble Mountains, San Bernardino County, California, is a typical example of Mojave Desert iron skarn deposits. Three episodes of skarn formation in calcareous wall rocks adjacent to granitic intrusions have been recognized:1. Early contact skarn formation generated iron-poor garnet, pyroxene, and minor vesuvianite, with retrograde epidote, actinolite, calcite, chlorite, and late-stage vein quartz + or - specular hematite at temperatures < or = 600 degrees C; X (sub CO 2 ) varied between <0.001 and 0.1. Prograde fluids were NaCl-H 2 O-rich, with maximum salinities near 50 equiv wt percent NaCl. Some prograde contact skarn growth was associated with boiling or condensing fluids. Isotopic analyses of vein and residual calcite in contact skarn and adjacent marble can be explained by decarbonation of the marble and interaction with an infiltrating fluid in equilibrium with the pluton during skarn formation and by superimposed low-temperature exchange with evolved meteoric fluids.2. Intermediate-age skarn formed around felsic dikes and produced epidotized felsic dikes with alteration envelopes of iron-rich garnet and pyroxene, minor wollastonite, and retrograde actinolite, chlorite, calcite, quartz, and specular hematite.3. Magnesian-magnetite skarn was the last skarn formed and occurs as fault-controlled clinohumite-magnetite-serpentine bodies at contacts between dolomitic marble and granite. Fluid involved in the precipitation of magnetite skarn was enriched in CO 2 and KCl relative to that which formed contact skarn. Relatively CO 2 -rich conditions (X (sub CO 2 ) approximately 0.1) during this skarn-forming event are consistent with delta 18 O analyses of magnetite and calcite in ore zones, which suggest that these minerals precipitated from 18 O-enriched fluids. This is explicable through mixing of H 2 O-rich fluids in equilibrium with the pluton with CO 2 -rich fluids produced during devolatilization of the adjacent marble. Minimum temperatures of 375 degrees C are suggested by mineral assemblages in ore zones and by isotopic fractionation between coexisting magnetite and calcite.Two distinct alteration events are recorded in the associated pluton:1. Pervasive albitization of granite and felsic dikes occurred penecontemporaneously with contact skarn and dike skarn development but prior to iron ore deposition. Fluid inclusion and isotopic data suggest that the fluid responsible for albitization had a large magmatic component or that it isotopically equilibrated with the pluton at temperatures as high as 600 degrees C.2. Late-stage quartz + or - specular hematite alteration is found in all three types of skarn and in all igneous rocks. Temperatures of 200 degrees to 250 degrees C and NaCl-CaCl 2 -H 2 O + or - KCl fluids with salinities of 0 to 22 equiv wt percent NaCl are indicated by fluid inclusion studies. Drastic whole-rock 18 O depletions associated with this event suggest that the fluid responsible for alteration was largely meteoric in origin.

Relationship between climate and vegetation and the stable carbon isotope chemistry of soils in the eastern Mojave Desert, Nevada

The relationship between the stable C-isotope composition of the soil environment and modern climate and vegetation was determined empirically along a present-day climatic transect in the eastern Mojave Desert. The δ 13C of the soil CO 2 and carbonates decreased with increasing elevation and plant density, even though plant assemblages at all elevations were isotopically similar. Several factors, including differences in the ratios of pedogenic of limestone calcite and differences in past vegetation, were considered as explanations of this trend, However, it appears that in the sparsely vegetated Mojave Desert, the δ 13C of pedogenic carbonate is controlled by differences in plant density and biological activity. This relationship may provide a tool for assessing past vegetational densities, as long as the vegetation is isotopically homogeneous.