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A geochemical survey of fluids in the Las Tres Vírgenes geothermal field (LTVGF) in Baja California Sur, Mexico, was carried out to describe their origins and evolution within the reservoir. Major and minor elements and noble gas isotopes (He, Ne, Ar, Kr, and Xe) were measured in fluids from three production wells, one injection well, and one fumarole (El Azufre). Stable isotopes of water (δD and δ18O) show mixing between Quaternary recharge, lighter than present-day rainfall, and a deep fluid of andesitic origin. The Na/Br and Cl/Br ratios indicate that deep brines from the LTVGF are seawater having leached evaporite deposits (i.e., halite). These deposits are presently absent in the area, but likely occurred during the Miocene, prior to the breakup of the Gulf of California, suggesting that the saline fluid end-member of the LTVGF is several million year old. Measured 3He/4He ratios of greater than 6.5 Ra (where Ra is the atmospheric ratio of 1.384 x10-6) show that LTVGF fluids are a mixture of meteoric waters and deep magmatic fluids, with the hotter and more pristine mantle fluids found in the southern part of the exploitation zone.

The Tepic-Zacoaico rift, a NW-trending corridor ~50 x ~250 km, is one arm of a triple-rift system in western Mexico. Together with the Colima rift and the Middle America Trench, it bounds the Jalisco block, a portion of western Mexico that may be moving independently of North America. The predominant basement rock types in the Tepic-Zacoalco rift are rhyolitic ash-flow tufts and lavas, which were previously assumed to be Oligocene-Miocene in age, related to the Sierra Madre Occidental volcanic province, or older. New [sup.40]Ar/[sup.39]Ar dates on 41 volcanic samples reveal a previously unrecognized, voluminous flare-up of rhyolitic ignimbrites between 5 and 3 Ma throughout the entire corridor of the Tepic-Zacoalco rift; they are often associated with Pliocene high-Ti basalts. The eruption rate during this Pliocene time period was an order of magnitude higher (hundreds of m/m.y.) than that documented in the Tepic-Zacoalco rift over the last 1 m.y. The Pliocene ash-flow tufts have been faulted along NW-trending lineaments, producing vertical offsets up to at least 500 m. The voluminous ignimbrite flare-up in the Tepic-Zacoalco rift at 5-3 Ma may reflect the initial stages of rifting of the Jalisco block away from North America, analogous to what occurred in the proto-gulf region at 12-6 Ma, prior to the transfer of Baja California from North America to the Pacific plate. Additionally, new [sup.40]Ar/[sup.39]Ar dates show that the Sierra Madre Occidental volcanic province extends across the entire width of the Tepic-Zacoalco rift and terminates abruptly at the northern boundary of the Jalisco block near the Rio Ameca. In contrast, Paleocene-Eocene basement from the Jalisco block extends northward into the Tepic-Zacoalco rift, where it is locally overlain by Sierra Madre Occidental rhyolites. Keywords: ignimbrite flare-up, lithospheric extension, plate transfer, [sup.40]Ar/[sup.39]Ar geochronology, Rivera plate, Sierra Madre Occidental.

Fault rocks from the classic Rocky Mountain foreland fold-and-thrust belt in southwestern Canada were dated by Ar analysis of clay grain-size fractions. Using X-ray diffraction quantification of the detrital and authigenic component of each fraction, these determinations give ages for individual faults in the area (illite age analysis). The resulting ages cluster around 72 and 52 Ma (here called the Rundle and McConnell pulses, respectively), challenging the traditional view of gradual forward progression of faulting and thrust-belt history of the area. The recognition of spatially and temporally restricted deformation episodes offers field support for theoretical models of critically stressed wedges, which result in geologically reasonable strain rates for the area. In addition to regional considerations, this study highlights the potential of direct dating of shallow fault rocks for our understanding of upper-crustal kinematics and regional tectonic analysis of ancient orogens. Keywords: Rocky Mountains, Canada, faults, orogeny, geochronology, gouge.

Abstract: Precise U-Pb monazite and [sup.40]Ar/[sup.39]Ar hornblende ages have been obtained from three locations in the high-grade Archean core of the Wind River Range, Wyoming. Monazites from metapelites in the [...]

Late Miocene-Pliocene (8-3Ma) olivine basalt lavas, dated in this study by the [sup.40]Ar/[sup.39]Ar method, have been faulted and tilted on both the east and west sides of the Warner Range of NE California, which is itself a tilted block rising to 2960 m at its crest that is composed of Miocene-Oligocene lavas and volcaniclastic rocks. The late Miocene-Pliocene lavas, distinctively poor in [K.sub.2]O and rich in MgO, are called low-K olivine tholeiites and have a different mantle source region than that of the older subduction-related lavas of the main Warner Range. Hays Canyon Range (max. elev. 2400 m) lies to the east of the Warner Range, and the broad Surprise Valley separates the two fault-bounded ranges. Middle Miocene (ca. 15 Ma) basic lavas, with a small easterly dip, cap the Hays Canyon Range and overlie Oligocene silicic ash-flow deposits and a basaltic andesite spatter volcano. Middle Miocene basic lavas also form the crest of the Warner Range and its westerly dip slope (~15[degrees]). Nearly horizontal basic lavas of the same age are also found on both sides of the Warner Range, and it is a plausible conclusion that these middle Miocene basalts were a contiguous group before faulting and uplift of the Warner Range. Derived estimates of uplift rates (~1 mm/ yr) of the Warner Range indicate that uplift could have been initiated at ca. 4 Ma, a period of the most voluminous eruption of low-K olivine tholeiite lavas. If the slower Cretaceous exhumation rate of the Sierra Nevada (0.5-1.0 mm/yr) is applied to the total offset of the Warner Range (4270 m), and it did not vary with time, then the uplift of the Warner Range was initiated at ca. 8 Ma, which coincides with the age of the oldest low-K olivine tholeiite lava (8 Ma). Low-K olivine tholeiites require a hot shallow asthenospheric source, and it is the rise of this hot mantle that is presumed to have caused the uplift of the Warner Range. Whether or not the widespread eruption of small volumes of Pliocene low-K olivine tholeiites in central and eastern Oregon is associated with crustal uplift is unknown. Keywords: Surprise Valley, Basin and Range, spatter volcano, low-potassium olivine tholeiites, geochrontology.

[sup.40]Ar/[sup.39]Ar geochronology is coupled with quantitative volume determinations (utilizing field mapping, digital elevation models, orthophotos, and geographic information system [GIS] software) to constrain magma eruption rates at the Ceboruco-San Pedro volcanic field (1600 [km.sup.2]) in the western Trans-Mexican arc. Ages are reported for 40 volcanic units, including Volcan Ceboruco (an active, andesitic stratovolcano), peripheral domes, shields, cinder cones, and fissure-fed flows. After a hiatus of ~3 m.y., volcanic activity recommenced to produce 80.5 [+ or -] 3.5 [km.sup.3] of magma at a rate of 63 [m.sup.3]/[km.sup.2] per year over the past 0.8 m.y. However, 75% of this volume erupted in the past 100 k.y., including the 51 [+ or -] 2.5 [km.sup.3] of Volcan Ceboruco, equivalent to an eruption rate of ~377 [m.sup.3]/[km.sup.2] per year. There have been at least two stages of cone-building activity at Volcan Ceboruco. The main edifice is composed of ~38 [km.sup.3] of precaldera andesites, the youngest dated at 45 [+ or -] 8 ka. Their eruption was followed by a hiatus, interrupted by a Plinian eruption at 1 ka. The Plinian eruption and subsequent lava flows are andesite to dacite in composition and constitute ~13 [km.sup.3] of the total volume of Volcan Ceboruco. Overall, the relative proportions of lava types erupted in the past 0.8 m.y. are 18%-19% basaltic andesite, 56%-60% andesite, 18%-22% dacite, and 3% rhyolite. The peripheral lavas are each of small volume, geochemically diverse, and show little evidence of prior storage in an upper-crustal chamber. The eruptive sequence, proportions of lava types, phenocryst assemblages, textures, and geochemistry imply that the lavas do not reflect the differentiation of a single parental liquid in a long-lived magma chamber. The distinct geochemical signatures were present prior to magma emplacement in the upper crust, whereupon subsequent degassing and crystallization led to variable phenocryst abundances and assemblages. Keywords: eruption rates, [sup.4]Ar/[sup.39]Ar geochronology, GIS, magma evolution, Mexican volcanism.

Extraction of meaningful information on the timing of fault activity from clay gouges using radiometric dating methods, such as those based on the K-Ar system, can be challenging. One of the factors complicating interpretation of the radiometric dating results is the presence of multiple K-bearing components in the gouge material. In the current study, an attempt was made to develop a new interpretative method for K-Ar and 40Ar-39Ar dating, capable of handling a three-component mixture. In addition, the mineral composition of clay gouges from the Tatra Mountains (Poland), which has not been investigated before, is reported. The mineral compositions of the bulk clay gouge material and separated size fractions were determined by X-ray diffractometry and Fourier-transform infrared spectroscopy. The gouge samples were composed of quartz, dioctahedral mica (as a discrete phase and as a component of mixed-layered illite-smectite), and chlorite, commonly with plagioclase and more rarely with K-feldspar, dioctahedral smectite, calcite, anatase, or trace kaolinite. One feldspar-free sample containing three mica polytypes (1Md, 1M, and 2M1) was chosen for dating with the 40Ar-39Ar method. The results of 40Ar-39Ar dating were interpreted using three concepts: Illite Age Analysis (IAA), a method based on the MODELAGE software, and a newly developed three-component concept. The age values obtained with IAA were −14 Ma ± 31 Ma and 180 ± 91 Ma for authigenic (1Md) and inherited (1M + 2M1) components, respectively. The MODELAGE-based approach returned –4 ± 40 Ma and 165 ± 62 Ma. The three-component approach returned age values of polytypes as follows: 1Md, 15 ± 37 Ma; 1M, 135 ± 57 Ma; 2M1, 121 ± 56 Ma based on the medians and the interquartile ranges of non-normal distributions of Monte Carlo-simulated age values. The results obtained indicated that the 1Md polytype was probably formed during the most recent stage of fault activity, while 1M and 2M1 polytypes are of equal age, roughly. [ABSTRACT FROM AUTHOR]

Author(s): Ben A. van der Pluijm (corresponding author) [1]; Chris M. Hall [1]; Peter J. Vrolijk [2]; David R. Pevear [2]; Michael C. Covey [2] Direct dating of ductile shear [...]

The argon isotope cooling ages of the hornblendes of the Robertson Lake shear zone, Grenville orogen, Canada, show that the area has had uniform unroofing since 950 Ma. The orogen separates the Mazinaw and Sharbot Lake areas. The Mazinaw area underwent slow cooling after maximum metamorphosis, followed by rapid cooling and then slow cooling. The Sharbot Lake area underwent slow cooling after metamorphosis followed by rapid cooling. The hanging wall adjacent to the shear zone was unroofed from the deep crust before the wall away from the shear zone.

A method for dating clays is important for studies of weathering, diagenesis, hydrocarbon migration, and the formation of major metalliferous deposits. However, many attempts have produced imprecise or inaccurate results. Data from shales show that, contrary to expectations, the 40Ar-39Ar dating technique can be successfully used to determine the diagenetic age of ancient sediments because 39Ar losses during irradiation are controlled by release from low retentivity sites in illite equivalent to those that have lost radiogenic 40Ar in nature, rather than by direct recoil as is generally assumed., Formation of clays involved in weathering, diagenesis, or hydrothermal alteration has been difficult to study because such low temperature nonequilibrium processes are difficult to simulate experimentally. Clay mineral diagenesis has [...]