Leached capping of the Dos Pobres orebody
Department of Geosciences, University of Arizona
The Dos Pobres orebody, of the Lone Star district of Arizona is characterized in its upper levels and exposures by oxidized iron and copper minerals. These characteristics are a subject of this study with the intent of developing criteria for recognition of Dos Pobres type ores in a region of arid climate. Ore grades (Langton and Williams 1982) are believed to be on the order of 0.7% copper in the hypogene zone and a complex weathering process is believed to have resulted in local copper grades in excess of 0.5%.
The surface of Dos Pobres is typical of that found across this region in copper deposits of comparable styles and compositions. In these deposits, a characteristic profile of weathering/oxidation, leaching, and copper precipitation occurs and results from effects of acid production from pyrite during the weathering of upper parts of the copper-iron sulfide systems. Acid produced from this weathering dissolves and alters minerals across the subjacent profile, transporting copper to deeper levels where it is believed contrasts in reduction/oxidation conditions results in precipitation of enriched copper sulfides, the enriched blankets that have given rise to the high value of these ores.
The supergene mineralogy at Dos Pobres indicates that we are viewing this surface at some level within the zone of leaching. The mineralogy is characterized by hematite, which often forms as a product of chalcocite weathering, is spatially widespread and most commonly found in thick veins and fractures with an envelope of acid alteration. Interpreted as the weathering product of chalcopyrite and pyrite, goethite also is locally abundance in the leached cap at Dos Pobres and can be found in association with or as a mixture with hematite. Neither sulfide minerals nor jarosite, an iron oxide common in low pH conditions associated with the weathering of pyrite, are found in abundance at the surface.
Neotocite and chrysocholla represent the major copper bearing minerals in the leached cap. Neotocite, a complex copper bearing silicate, on the surface is commonly found in areas with hematite abundance and is only slightly less spatially prolific as hematite. Chrysocholla, another copper bearing silicate, is also found at the surface but is limited to a few locations. The subsurface, as seen in locations with relatively deep exposures, may have chrysocholla in higher abundance.
Fractures existing prior to intrusion influenced intrusion geometry and are host to the products of primary sulfide mineral weathering. Oxidation and transport processes are also heavily influenced by fractures formed prior to, during and after mineralizing events. On the surface fracture abundance varies but higher fracture abundance tends to be associated with hematite rich areas.
Victor A Valencia1, Joaquin Ruiz1, Lucas Ochoa2 and Enrique Espinoza3
Here we present data that suggest that La Caridad, which is a porphyry
copper-molybdenum deposit and la Caridad Vieja, which is a high-sulfidation
epithermal system are related.
Acknowledgements: We are most grateful to Grupo Mexico for allowing
access to the mine operations and logistics and to the Arizona Geological
Society for generous financial support. In particular, we would like
to thank to Ing Remigio Martinez, Ing Jose Contla, Fernando Barra, Sergio
Castro and Dr Chris Eastoe for their help and for interesting discussions.
John Porter1 and Dana T. Griffen2
Barite (BaSO4) is a mineral commonly found in with hydrothermal ore
deposits, and other environments. Common barite colors include white,
blue, yellow and colorless. The cause of color in yellow barite, called
"golden barite", has been determined by electron spin resonance
spectroscopy (ESR) to be due to a color center. The color is bleached
when heated to 500° C for 20 minutes or more. In some samples the
color began to fade after 400° and the samples were all colorless
after being heated to 600°. Heated colorless samples were then irradiated
using Cu x-rays for three hours, inducing a pale blue color. The induced
color fades with exposure to uv light. Efforts to reintroduce the golden
color to heat-bleached samples have been unsuccessful. Esr spectra and
optical absorption spectra were taken at all three stages, golden (untreated),
colorless (heated) and blue (irradiated). The absorption spectra of
the samples varied widely with absorption peaks from 380 to 520 nm in
the untreated sample and a broad peak around 600 nm in the irradiated
sample. The ESR spectra confirmed that the color centers provide the
mechanism for coloring in these barites. Although experimental setup
was not adequate to identify the geometry of the hole centers, the ESR
spectra show some interesting differences. The ESR spectrum of the untreated
sample shows a much more complicated signal than that of the irradiated
sample. However, the peaks found in the signal of the irradiated sample
can be seen in the untreated spectrum, suggesting that the hole center
causing the blue color is present in the golden sample, but is masked
by the larger signal. Spectra of heated samples show no signal that
can be resolved from the background.
Elizabeth Wilson and Spence Titley
Volcanic rocks and related plutons are common in southeastern Arizona and surrounding areas and relate to Laramide and mid-Tertiary regional magmatism. The oldest of these rocks were produced during the Laramide Orogeny, associated with the subduction of the Pacific Farallon plate beneath North America, approximately 80-50Ma. This extensive magmatism was followed by mineralization of plutons, forming porphyry copper deposits. Such deposits in Arizona, New Mexico, and Sonora, Mexico are extremely large, accounting for 10% of world copper production. Laramide plutons in this region were studied extensively over the course of the twentieth century due to their economic value but on an individual deposit or district-scale, and Laramide volcanics, direct precursors to porphyry copper deposits, have never been coherently studied on a regional scale. Stratigraphy, timing, and composition of individual volcanic units in Arizona are well known, however similar volcanics in New Mexico and Sonora, Mexico are not well understood and require extensive study.
This study provides a better understanding of 1) Laramide magmatic processes in the southwest; 2) porphyry copper deposit lifecycles; and 3) possible environmental impacts caused by the exposure of these volcanic rocks. By characterizing the geological and geochemical trends of Laramide volcanics on a regional scale, a survey of major, minor, trace, and rare earth elements coupled with stratigraphy provides a regional view of spatial and chemical trends in Laramide volcanics across the west. Laramide volcanic rocks within the study area are intermediate to silicic in composition, and occur largely as andesite to dacite or rhyolite sequences. Type localities for Laramide volcanics include the Salero Formation of the Santa Rita Mountains and the Silver Bell Formation in the Silver Bell Mountains. Many locations do not exhibit a complete volcanic sequence, thus volcanics across the region can be correlated using these type localities.
All samples plot within the calc-alkaline portion of the AMF diagram and trend towards the alkali end member. Of the alkalis, Na2O remains relatively constant over the range of silica, 48-79%, though K2O increases with silica content. All other major oxides demonstrate decreases as silica increases into rhyolitic compositions. MgO, Fe2O3, Al2O3, and TiO2 all decrease with increases in SiO2 content, with Fe2O3 and MgO having a sub-parallel relationship. Of the trace elements, Th, La, Nb, and Y concentrations are relatively constant for all samples, despite lithology variations. Though Hf concentrations occur within a small range, 0-5ppm, rhyolitic rocks contain greater amounts relative to andesitic-dacitic samples. Rb and Sr vary widely for all samples, however andesitic units have higher concentrations of Sr, up to 1600ppm. Ba is enriched in rhyolites compared to intermediate rocks. Rare earth element trends based on sample to chondrite ratios for Laramide volcanics reveal that magma compositions were enriched in all rare earths and especially LREE. Overall, a slight negative Eu anomaly exists. Additionally, when rare earth element concentrations are examined on a west to east projection, concentrations of LREE appear to fluctuate regularly.
By characterizing Laramide-age volcanics, a regional picture of magmatic
chemical behavior emerges. Chemical anomalies will also indicate if
weathering of these exposed rocks pose any adverse environmental impacts,
such as enriched Pb in soils or groundwater, to the Sonoran region.
Stacie Gibbins1, Spence Titley1 and K. Friehauf2
The Ertsberg Mining District is located in the Papuan Mobile Fold and Thrust Belt in the Central Ranges of Irian Jaya, Indonesia. The district is comprised of a number of carbonate- and intrusion-hosted mineralized centers, with the super-giant Grasberg Cu-Au porphyry deposit being the largest (2 Bt of 1.2%Cu and 1.2g/t Au) and (likely) the most well known. Mineralization in the district is spatially and temporally related to Pliocene intrusions, which are the result of continent-island arc collision between the Australian Craton and the Pacific Plate during the Miocene.
The original discovery in the mining district was the Gunung Bijih
orebody, a magnesium skarn with 33 Mt of 2.5% Cu and 0.8 g/t Au. The
Gunung Bijih orebody has been proposed to be a carbonate roof pendant
within the Ertsberg Intrusion. Subsequent exploration on the periphery
and within the Ertsberg Intrusion identified the Ertsberg East Skarn
System (EESS), a magnesium skarn deposit boasting 250 Mt resource of
1.06 %Cu and 0.81 g/t Au, and Dom, a calcic skarn with 30 Mt of 1.67%Cu
and 0.42 g/t Au. Thus while carbonate-hosted mineralization was recognized
as being associated with the Ertsberg Intrusion, intrusion-hosted mineralization
was not considered until the early to mid-1990's.
Cu and Au mineralization is deposited within early biotite-anhydrite-bornite(chalcopyrite
veins, and later quartz-bornite-chalcopyrite//green sericite-bornite-chalcopyrite
veins. Ore grade is contained almost exclusively within the pervasive
biotite-magnetite-anatase(?)+(k-feldspar-epidote) alteration zone. Mapping
mineralization/alteration as absent or present within recognized igneous
phases provides a time-space picture of the interplay and evolution
between magmatic and hydrothermal processes within an intrusive complex.
Steve Young and Joaquin Ruiz
Acid ferric sulfate dissolution experiments on Cu-(Fe)-S minerals and supergene chalcocite, Cu2S, ore reveals appreciable Cu isotope variability can occur as Cu is redistributed during weathering of porphyry copper deposits. Solutions collected from reaction columns constructed to model conditions of Morenci, AZ, copper ore heap leaching exhibit d65Cu variability as much as 3 as aqueous Cu2+ is progressively extracted in a two-stage dissolution process. The first stage of copper extraction exhibits near-linear dissolution kinetics as well as a rapid 2-3 decrease in d65Cu of derived copper-sulfate solutions. Parabolic reaction kinetics and gradually increasing d65Cu of solutions (approximately 1) characterize second-stage copper recovery. Except for increased reaction rates in mesophilic bacteria-mediated experiments, there is no resolvable chemical or isotopic difference between leachates collected from biotic or abiotic experiments.
Molecular models for chalcocite, covellite (CuS) and intermediate non-stoichiometric
copper sulfide minerals (1 < Cu:S < 2) show that Cu-S bonds occur
in both three- and four-fold coordinations. In addition, Cu-S bond geometry
changes as a function of Cu:S ratio. Features of Cu-S coordination number
and d65Cu curves, with respect to Cu concentration variation, are nearly
coincident, suggesting copper isotopes are fractionated between distinct
bonding environments within the crystal structure. The likelihood of
copper isotope fractionation between mineral and sulfate solution is
greatly reduced through recognition of the invariant d65Cu pattern obtained
by leaching chalcopyrite (CuFeS2), a mineral in which Cu is in tetrahedral-coordination
with S, exclusively. Dissolution experiments on pure chalcocite and
bornite (Cu5FeS4) mineral separates were used to assess relative contributions
from equilibrium and kinetic fractionation processes. Naturally-produced
variation in Cu isotopes and stable isotopes of other transition metals
have been well-documented through prior examination of geologic and
extraterrestrial materials. However, this is the first instance in which
observations permit construction of a model describing their fractionation
at a molecular scale in nature. Understanding mechanisms which fractionate
stable transition metal isotopes is ultimately important for describing
geochemical processes responsible for distributing chalcophile and siderophile
elements during terrestrial evolution.
Fernando Barra, Lukas Zurcher and Victor Valencia
One of the most fundamental questions in metallogenesis is the age of mineralization and duration of hydrothermal systems. This problem has been addressed using different techniques such as K-Ar, U-Pb, Ar-Ar and more recently using Re-Os isotopes. The advantage of Re-Os over the other systems is that with this system we can directly date the sulfides and not the silicate assemblages that probably are unrelated to the mineralization event. Molybdenite has ppm levels of Re and no initial 187Os and hence has been used as a high-precision geochronometer. Although the system is reliable the suitability of the sample must always be assessed before analysis (McCandless et al., 1993). Here I use Re-Os systematics applied to molybdenite from four different deposits located in different tectono-stratigraphic terranes of northern Mexico. The Cuatro Hermanos prospect is located in the Caborca terrane (Campa and Coney, 1983) and has a Re-Os molybdenite age of 55.7 ± 0.3 Ma. The Malpica prospect in the Guerrero terrane shows a Re-Os age of 54.1 ± 0.3 Ma and the La Caridad deposit in the North American terrane has a Re-Os age of 53.8 ± 0.3 Ma. These results support the notion that during the interval between 50 - 60 Ma an important mineralization event occurred in northern Mexico and in southern Arizona as previously stated by McCandless and Ruiz (1993). These data are also in good agreement with previous geochronological information using the K-Ar isotopic system (Damon et al., 1983). The longevity of a porphyry system can also be assessed using this technique, and as shown for the Tameapa deposit (located in the Guerrero terrane) a protracted hydrothermal activity that spans from 57 Ma to 50 Ma is observed. These results add to the growing notion that some porphyry systems are the result of multiple intrusions and mineralization events as has been demonstrated for Bagdad in northern Arizona and El Teniente in Chile.
Campa MF, Coney PJ (1983) Tectono-stratigraphic terranes and mineral
resource distributions in Mexico. Canadian Journal of Earth Sciences
Victor A. Valencia 1, John Chesley 1, Kevin Righter 2, Joaquin Ruiz
1 and Luca Ferrari 3
Lower and middle crustal xenoliths are found in volcanic breccias
from Rincon de Parangueo maar, near Valle de Santiago, in the central
Trans-Mexican Volcanic Belt (TMVB). The major and trace element geochemistry
of the crustal xenoliths are more similar to those observed in arc environments
in South America than other xenoliths of rift environments of Mexico,
suggesting that these xenoliths are more representative of arc environments
The mafic xenoliths have higher fO2 than the other xenoliths within
the Rincon de Parangueo complex and than upper mantle xenoliths elsewhere
in Mexico, most likely the result of interaction of the lower crust
with slab-derived, oxidized fluids or melts.
William J. A. Stavast, Jeffrey D. Keith2, Eric H. Christiansen2 and
Michael J. Dorais2
Magmatic sulfides in 300 samples of volcanic and intrusive rocks from
the Tertiary Bingham (Cu-Au-Mo) and Tintic (Ag-Pb-Zn-Cu-Au) districts,
Utah were examined to help better understand preservation and removal
processes of magmatic sulfides. Our findings show that shallowly emplaced
dikes and sills have erratic, but locally high, concentrations of sulfides.
Sulfide concentrations appear to vary according to cooling rate and
inferred pressure at the time of quenching or crystallization of matrix.
Volcanic rocks from these districts typically have at least two orders
of magnitude fewer sulfides than very unusual quenched dikes. In addition,
sulfide concentrations vary dramatically across these dikes and sills;
for example, sulfide abundances in one sill in Castro Gulch, Bingham
district, increase from 9 ppm by volume in the center to more than 2000
ppm near the margin. This change in abundance is due to destruction
of sulfides in the center of the sill. Chalcophile metals such as copper
show corresponding changes in abundance. For example, the whole-rock
copper content of the same sill ranges from 23 ppm in the center to
35 ppm along the margins. The concentrations of base metals lost from
the center of the sill have ratios that are similar to the ratio of
the metals in the entire deposit (production and reserves). The textures
of sulfide grains (recrystallization, resorption, and degassing) even
in the most sulfide-rich samples commonly are indicative of alteration,
suggesting that no sample preserves all of its original magmatic sulfide
content. Immiscible liquids of monosulfide solid solution crystallized
as pyrrhotite, pyrrhotite and chalcopyrite, or pyrite and chalcopyrite
with declining temperature and pressure. These may recrystallize to
pyrite and chalcopyrite or to pyrite and an Fe-oxide. The alteration
and preservation textures change from sub-spherical sulfide blebs near
the margins of dikes and sills, to partially altered sulfides farther
in, to no remnant of the altered sulfides in the vast majority of intrusions
(except where small sulfides are completely enclosed by phenocrysts).
The majority of the sulfides along the quenched margins of these dikes
and sills occurs in the matrix. These data suggest that slow cooling
coupled with removal of magmatic volatiles, including sulfurous gases
(H2S, SO2, etc.), allows the resorption or degassing of magmatic sulfides
to occur almost universally during final crystallization of a magma.
This removes greater than 90 percent of the original endowment of magmatic
sulfides. This probably explains the low magmatic sulfide abundances
(1-15 ppm) of slowly cooled, large porphyritic intrusions. Most importantly,
this removal process allows metals and sulfur to participate in the
formation of porphyry deposits.
Geographic Information Systems are an extremely powerful analysis tool that is under utilized in the Geosciences but is becoming more prevalent in research today. Due to the varied spatial nature of geologic data, GIS are more and more able to process and analyze data and graphically reproduce it into interactive dynamic maps, which enable the creator to represent multiple data layers of a single system. The power of a GIS comes from its ability to integrate and overly many different data sets all linked by the same spatial coordinates allowing the user to analyze and gather important information about relationships along with many other important functions.
Some of the more universal applications of GIS as applied to Hydrogeologic and Geomorphologic studies include database construction, conceptual modeling and map creation. Examples include groundwater flow, surface water flow, depositional environments and morphology as well as landscape evolution. The use of GIS for conceptual groundwater modeling and hydrogeologic studies will be reviewed in some detail. An Assured Water Supply study for the City of Oro Valley was performed using GIS for model development, database development, map creation, and analysis of spatial data. Examples of multi-layered sets of spatial data will be examined for there impact on the water supply study as well as other related groundwater studies. GIS applications for well impact studies and infiltration studies will also be reviewed. Other possible applications of GIS to different geological fields will be discussed along with the benefits of spatial analysis. The power and versatility of Geographic Information Systems will prove to be an asset to almost any geologic endeavor.
Daniel C. Berman
Background: The discovery of hillside gullies on Mars, interpreted by Malin and Edgett  to be evidence for recent water release, has caused much excitement and debate. At current atmospheric pressure and surface temperatures, liquid water is unstable, and scientists had previously assumed that water was not currently active in geologic processes on Mars. The giant outflow channels observed in early photographs gave rise to the theory that the climate of Mars had undergone some change in the past, and that liquid water had at some point been stable. Before the Mars Global Surveyor (MGS) mission, no evidence for recent water activity had been observed.
Since the beginning of its mission, the MGS Mars Orbiter Camera (MOC) has captured images of hundreds, if not thousands, of examples of apparently young, gully-like features. The gullies are mainly observed on the sloping walls of large craters or channels. These features commonly share three major characteristics: head alcoves, downslope channels, and debris fans. There is also a geographic correlation; most of the channels are poleward of 30 degrees latitude (most in the southern hemisphere) and usually are seen on pole-facing slopes.
As of this date, 112,000 MOC images have been publicly released. I have begun a comprehensive survey of the images to catalogue all that contain gullies. I have so far catalogued over 350. The latitudinal trend in the initial observations has held; the majority of the gullies are found between 30 and 60 degrees, primarily in the southern highlands. Also, most are found on the pole-facing slopes of craters and channels.
Crater counts show these features to be extremely young. Few, if any intact craters are observed on the slopes. Although it is difficult to draw conclusions from crater populations on a strongly sloping surface, the sheer lack of any observable craters, intact or not, is meaningful. Further, the debris fans of many of the gullies are seen to be underlying patches of transient dunes, which furthers the argument that they are very young features.
Once I have examined all MOC images for gullies, I will then make statistical comparisons between their locations and factors such as water depth, altitude, and insolation. These correlations will help us better understand where the water is on Mars, and how and when it is reaching the surface.
Hector R. Hinojosa1 and William F. Manley2
GIS technology is becoming an essential tool in the effort to understand
the processes of global change. Glaciers in Alaska have experienced
record warming for the past 50 years, and for their topographic and
meteorological reasons, they are a good source of climate change indicators
in Alaska. Because one benchmark glacier in each glaciated region regulates
the physical processes that govern glacier evolution and stream flow
in that region (Fountain et al, 1997), every glaciated range in Alaska
should contain a benchmark glacier that can help understand adjacent
glaciers, or the less intensively studied glaciers, providing information
on the range of the glacier variation (Fountain et al, 1997). I explored
the assessment of labeling the McCall Glacier, Eastern Brooks Range
(EBR), Alaska, as a benchmark glacier based upon glacier area; basin
area; perimeter; compactness; slope angle; aspect; median elevation;
and hypsometric curves values. I used the Student's t-Test to statistically
assess whether such glaciological parameters could make the glacier
representative from the EBR. Based upon the Student's t-Test, the McCall
Glacier is not a representative one from the EBR. However, the McCall
Glacier and the EBR Elevation Line Altitude (ELA) elevation value share
almost the same value; this suggests that the McCall's Glacier ELA elevation
value might be representative for the EBR; and, in terms of previous
and intense mass balance studies, the McCall Glacier is representative
for the EBR.
Stephen B. DeLong and Jon D. Pelletier
Stream aggradation due to Quaternary climate-change induced variation in hillslope sediment delivery is widely recognized on a variety of timescales in both tectonically active and inactive terranes. The sedimentary fills that result from this cyclic aggradation tend to be preserved as spatially and temporally complex fan and river terraces within geologic basins. Unraveling what controls the deposition of these terraces necessitates detailed field, chronologic and modeling approaches. Here we present initial field observations from the topographically and tectonically complex Cuyama River basin, which lies at the junction of the Coast Ranges and the Transverse Ranges near the "Big Bend" of the San Andreas Fault Zone in southern California.
Cuyama Valley contains an ephemeral through-going river that in its middle reaches drains two asymmetrical WNW-trending mountain ranges. Extensive steep coarse-grained alluvial fans slope from the 1700-m north-facing Sierra Madre range, filling approximately 80% of the lateral distance between ranges. These fans contain multiple Pleistocene terrace surfaces which are testimony to cyclic aggradation in an overall downcutting fluvial regime. The modern piedmont channels draining this range appear generally underfit and lack extensive Holocene terraces. In contrast, the south-facing 1500-m Caliente Range lacks an extensive suite of Pleistocene fan terraces, but has a more-developed suite of Holocene fan terraces and currently aggrading range-front fans. The Caliente Range appears to have supplied sediment through the late Holocene in quantity sufficient to cause main-stem Cuyama River aggradation.
This out-of-phase geomorphic response is interpreted as evidence for hillslope vegetation density being the primary transmitter of climatic perturbation to channels. Slope aspect and orographic precipitation are the primary controls on modern vegetation density. Although the north-facing Sierra Madre range receives greater precipitation at higher elevations, it appears the lower and drier Caliente range has a higher rate of Holocene sediment supply to the fluvial system due to greater hillslope sensitivity to low-amplitude Holocene climate variation. In contrast, the large-scale valley asymmetry likely results from millennial and longer time-scale sediment delivery being higher from the Sierra Madre range, owing to their higher relief. This higher relief allowed for more dramatic hillslope response to large-amplitude Pleistocene climate variation.
Aaron J. Martin , P. Jonathan Patchett, George E. Gehrels, Peter G.
DeCelles and Clark Isachsen
Our recent work in the central Nepal Himalaya suggests the need to relocate the boundary between Greater and Lesser Himalayan rocks, the Main Central Thrust. Ever since geologic research in the Himalaya began seventy years ago, researchers have debated the location of the Main Central Thrust (MCT). Confusion about the location stems in part from a compound definition of the MCT. One part of the definition treats the MCT as a lithotectonic boundary between Upper Proterozoic Greater Himalayan (GH) metasediments to the north and Lower Proterozoic Lesser Himalayan (LH) metasediments to the south. Another part of the definition treats the MCT as a structural feature: a ~5 km thick zone of ductile shear that accommodated top-to-the-south displacement of GH rocks onto LH rocks. This compound definition obfuscates the fact that the lithotectonic boundary need not be a thick zone composed of rocks different from GH or LH rocks. Since GH rocks are usually gneisses and LH rocks are usually schists, field workers typically place the MCT at the boundary between gneiss and schist. Our work indicates, however, that the contact between Greater and Lesser Himalayan rocks lies south of the gneiss-schist boundary, within ultra-deformed fine-grained rocks (phyllonites). The lithotectonic boundary is sharp and thin (less than 100 m thick). Thus at least two faults involve GH rocks: one fault between GH and LH rocks, and one fault within GH rocks at the gneiss-schist boundary. This is the first recognition that the gneiss-schist thrust involves Greater Himalayan rocks both in its hangingwall and in its footwall.
In addition to adding to shortening estimates across the Himalaya, the recognition of other faults near the MCT has profound implications that challenge the paradigm of Himalayan inverted metamorphism, at least within LH rocks and the lowermost GH rocks. An inverted metamorphic sequence is defined as an undisrupted package of rocks that exhibits increasing pressure and/or temperature with decreasing structural elevation. The recognition of faults within a package of rocks previously interpreted to exhibit inverted metamorphism obviates the need to call upon any unusual metamorphic or tectonic process to explain geologic observations. Instead, successively structurally higher thrust sheets carry higher-grade rocks, as is the norm in fold-thrust belts around the world.
We use Neodymium isotopes on whole rocks and detrital zircons from
quartzites to accurately locate the position of the MCT, defined as
a lithotectonic boundary. Because LH and GH rocks are different in age
and provenance, they have different geochemical characteristics. Lower
LH rocks contain zircons with ages between about 1600 and 2550 Ma, and
have very negative eNd(0) values (average -24.5). GH rocks contain zircons
as old as 3200 Ma and as young as 650 Ma, and have less negative eNd(0)
values (average -15.5). Detailed mapping and tight sampling in five
transects allows us to accurately locate the MCT and to map it across
60 km in the Annapurna Range.
Andrew C. McCarthy and Mihai N. Ducea
The upper mantle is one of the driest volumes of the solid earth. The
common upper mantle (~ 40-70 km) minerals olivine, orthopyroxene and
clinopyroxene are considered "nominally anhydrous"; i.e.,
H2O or OH are not written as part of their mineral formulae. However,
water is present in these "anhydrous" minerals at small but
measurable concentrations. In upper mantle peridotite, typical whole-rock
water solubilities are ~100 - 200 ppm by weight. Olivine, the dominant
upper mantle mineral (~ 60% by volume), can incorporate at least 135
ppm H2O at upper mantle temperatures and pressures of 1100° C and
2.5 GPa. Under the same conditions the solubility of water in orthopyroxene
is similar to olivine at 133 ± 12 ppm. Solubility in clinopyroxene
is much higher: 1080 ppm H2O has been measured in clinopyroxene from
a natural eclogite sample. Water content of minerals in xenoliths sampling
the upper mantle varies from the maximum solubility limits down to <5
ppm as measured by infrared spectroscopy. Spatial and temporal variations
in upper mantle bulk water content may be due to partial melting (water
is highly incompatible in upper mantle minerals), or the flux of fluids
(e.g. melts or water-based fluids) through the mantle volume in question.
Mineral water content, more than any other single factor, controls the
rheology of the upper mantle. The addition of ~100 ppm H/Si to olivine
has been observed to enhance dislocation mobility by a factor of ~100,
significantly reducing the creep strength of the mineral. The macroscopic
result of such a weakening of minerals is to increase the strain rate
at much lower stresses and/or temperatures (i.e., lower the viscosity
of the rock). Additionally, intragranular water appears to control the
development of mantle anisotropy. Here we will present results from
xenoliths brought to the surface in two disparate geologic environments:
a hotspot situated below oceanic lithosphere (Hawai'i), and a continental
Robinson Cecil and Clement G. Chase
Until recently, the majority of the geologic community has been in
agreement that the Sierra Nevada mountain range experienced middle Miocene
or later uplift. This hypothesis was supported principally by geomorphic
evidence, regardless of the fact that magmatic activity had come to
an end in the Cretaceous. The concept of late uplift of the Sierra Nevada
has come under fire as of late because of the emergence of new geophysical,
geochemical and paleoaltimetric data. This data suggests that the Sierra
Nevada, despite the association between regions of high topographic
relief and rapid rates of erosion, remained at or near its present elevation
through the Cenozoic. The use of new paleobotanical studies will make
it possible to determine Eocene elevation of the Sierra Nevada and,
therefore, to decide between early and late uplift models. Paleobotanically-based
paleoaltimetry will allow for the placement of numerical constraints
on the elevation of the Sierra Nevada through the Tertiary, thereby
providing essential constraints on the geophysical model of the range's
evolution. Knowledge of the topographic history of mountains belts is
very difficult to ascertain, and the Sierra Nevada are particularly
puzzling because they are presently at a high average elevation, yet
a significant amount of time (~70 Ma) has passed since their last major
formational event. Could the Sierra Nevada have remained high, in spite
of high rates of erosion, or were they worn down and later uplifted
by enigmatic upper mantle/lower crustal processes? Paleobotanical research
is one of the best means of solving such a riddle, as the data it yields
is independent of geomorphic evidence that is susceptible to many alternative
Sarah Shoemaker1, Mihai Ducea1, J. Garver2, P. Reiners3, M.F. Campa4
and Joaquin Ruiz1
The southern slopes of the Cenozoic Sierra Madre del Sur Mountains,
located in southern Mexico, comprise the Xolapa complex; a Mesozoic-Cenozoic
magmatic arc reaching 50-100 km inland to the north and stretching 600
km from Zihuatenjo in the west to Salina Cruz in the east. There are
no quantitative data to constrain either the timing and amount of Cenozoic
uplift or the possible relationships between rock exhumation and plate
kinematics in the region. In order to address this problem, 38 samples
of undeformed and slightly deformed plutons were collected from six
N-S transects along the length of the Xolapa complex (Zihuatenejo, Atoyac,
Acapulco, Pinotepa N., P. Escondido, and P. Angel), and analyzed for
apatite fission track (AFT) thermochronometry; in addition, 20 (U-Th)/He
ages were obtained from the Acapulco and Puerto Angel transects. AFT
and (U-Th)/He data do not show consistent age-elevation correlations
along any of the sampled transects, but suggest slow average exhumation
rates of 0.19 mm/yr since 25 Ma in the west, increasing eastward to
0.34 mm/yr since 12 Ma. These rates suggest that mountain building commenced
in the region in the Miocene or, more probably, later. The proposed
eastward passage of the Cocos-Caribbean-North American triple junction
leading to a plate boundary change from transform (no uplift) to subduction
(uplift) in the Eocene would predict an eastward propagating Eocene
uplift event, which is not apparent in our data. There is little change
in exhumation rates in response to postulated reorganization of the
plate boundary west of the Sierra Madre del Sur (currently the Acapulco
trench). The complicated age data could be the result of extended duration
in the partial annealing (AFT) or retention ((U-Th)/He) zones, or may
reflect cooling too shallow to be recorded by known thermochronologic
systems. The data could also be influenced by the presence of unrecognized
late Cenozoic faults that may exist along the analyzed transects.
Shundong He and Andrew Leier