Rhyolite Flows Research Articles

Postemplacement Uranium Mobility in Oligocene Ash Flow Tuff and Rhyolite Lavas from Hidalgo County, New Mexico: ABSTRACT

Investigations of the distribution of uranium and other trace and minor elements in a major welded ash flow tuff (the Gillespie Tuff) and in several rhyolite flow units (all Oligocene) included petrographic examination to detect characteristic textures produced during the process of crystallization, and comparison of the chemical composition of rocks of identical original composition, but crystallized by different processes. In all units investigated, unaltered vitrophyres were used to indicate initial content of uranium and elements. The major process of crystallization in these rocks involved growth of spherulitic and large crystal units in a supercooled magma, beginning at temperatures 200°C below the liquids and continuing at lower degrees of supercooling. This is not strictly devitrification because it probably occurs above the glass-transition temperature. Several distinctly different textures probably result from different crystallization conditions. Vapor-phase crystallization is rare in the Gillespie Tuff. Granophyric texture is common in both lavas and the Gillespie Tuff. In other areas, rocks displaying this texture may have lost some uranium, suggesting that volcanic rocks in Hidalgo County may have released considerable amounts during crystallization. End_of_Article - Last_Page 771------------

Geology of Sierra del Gallego Area: Comparison with Pena Blanca Uranium District, Mexico: ABSTRACT

A 1,000-m section of volcanic rocks overlying Lower Cretaceous limestone in the Sierra del Gallego area 200 km south of El Paso can be divided into four lithologic sequences. About 44 to 38 m.y. ago, at least five separate crystal-poor sanidine-bearing outflow units of rhyolitic tuff covered parts of the area. The tuffaceous activity was closely followed by lava flows of fritted feldspar-bearing tholeiitic andesite. These flows were followed about 37-35 m.y. ago by a sequence of rhyolite flows, flow domes, and intrusions. A 6-m.y. period of quiescence was followed about 29 m.y. ago by massive outpourings of basalt with minor associated rhyolite tuff. Parts of the oldest sequence are probably correlative with stratigraphic units in the Pena Blanca area in New Mexico. The P zos limestone conglomerate at the deposit has a counterpart in the Sierra del Gallego area. Petrographic similarities, thickness relations, geodes, and K-Ar dating directly link the H member in the lower part of the Liebres Formation in the Sierra del Gallego area with the Nopal Formation at Pena Blanca. The Pozos, Nopal, Escuadra, Pena Blanca, and Mesa Formations are, as a group, lithologically and chronologically similar to the oldest sequence (the Liebres Formation) of the Sierra del Gallego area. End_of_Article - Last_Page 755------------

Geology and Uranium Deposits Along Northeastern Margin of McDermitt Caldera Complex, Southern Malheur County, Oregon: ABSTRACT

The adjoining Aurora and Bretz uranium prospects are along the northeastern ring-fracture system of the Miocene McDermitt caldera. A series of block faults, constituting the ring-fracture system, divides the area into two contrasting terranes. The northern terrane, comprising the caldera wall and outflow facies, includes a series of mafic to silicic lavas and rhyolite ash-flow tuffs (Bretz Series). Rocks of the southern terrane (Aurora Series) represent infilling of the caldera after End_Page 770------------------------------ collapse. They include an enormously thickened pile of rhyolite ash-flow tuff, equivalent to the youngest Bretz Series ash-flows, intruded and overlain by rhyolite flow domes and lavas. These rhyolites are overlain by a sequence of intermediate lava flows, flow breccia, and pyroclastic breccia constuting the Aurora Lavas. Tuffaceous lacustrine sediment covers most of the southern terrane and much of the ring-fracture system. Hydrothermal alteration associated with mercury deposits at the old Bretz mine is clearly controlled by one of the ring faults, but uranium mineralization nearby is stratigraphically controlled and not directly associated with mercury. Uranium concentrations occur along several horizons, including (1) geologic contacts, unconformities, and redox boundaries in the Bretz Series; (2) a widespread horizon in the tuffaceous lake sediments; and (3) potentially commercial deposits along the flow boundaries and interflow breccias in the Aurora Lavas. The control of uranium by solution channelways along stratigraphic boundaries and lack of crosscutting veins, except in local areas of the Aurora Lavas, suggests that supergene mechanisms were important in ore formation. However, a combination of hydrothermal and supergene processes is favored to explain all the features observed, with the uranium in the lacustrine sediments precipitated from surficial hot springs active during mineralization. End_of_Article - Last_Page 771------------

Archean subaqueous felsic flows, Rouyn-Noranda, Quebec, Canada, and their Quarternary equivalents

Subaqueous rhyolite flows are dome-shaped or tabular masses, up to 10 km long, and up to 400 m thick, rising with a steep (10°–40°) flow front. Larger flows show a distinct facies change from the feeding fissure to the flow front. The proximal facies consists mainly of massive lava continuous into the filling of the feeder dyke; a thin capping of flow breccia is locally present. Laterally, it passes into a very heterogeneous median and distal facies, characterized by pods, lobes and tongues of rhyolite lava enveloped by rhyolite microbreccia in the median facies, by rhyolite breccia in the distal facies. The lava pods consist of a microcrystalline core zone. Their rim zone is flow-layered due to alternation of cm-thick bands of pumice, obsidian, and partly crystalline, poorly vesiculated rhyolite. The border zone is obsidian. The microbreccia consist of few (< 10%) lava fragments, loosely set in an abundant hyaloclastite matrix, whereas lava fragments predominate (> 50%) in the rhyolite breccia. Hyaloclastite is composed of granulated obsidian and, locally, pumice fragments. Lava tongues and lobes show foreset cross-bedding. Subaqueous rhyolite flows are, in many respects, similar to subaqueous basalt flows. The lava pods, lobes and tongues are equivalent to pillows, and were emplaced by plasticoviscous flow at the top and front of the actively moving flow. Hyaloclastite formed by granulation of rhyolite lava due to aqueous chilling; lava fragments of the breccia and microbreccia formed by the disintegration of lava lobes and tongues. Breccia and microbreccia were emplaced by avalanching and cascading of fragments at the active flow front.

Subglacial volcanics — On the formation of acid hyaloclastites

Several rhyolitic volcanic accumulations of subglacial origin in Iceland consist of two principal components: (1) hyaloclastite; and (2) ellipsoidal to irregular lobes averaging about 7 m in diameter. Two different types of hyaloclastite, reflecting major changes in the style of subglacial eruptions, occur either as separate layers or in intimate association. A pumice-bearing type is thought to have resulted from explosive events during an eruption. The second type, characterized by fragments of obsidian and lithic rhyolite, is genetically related to the lobes. The lobes generally consist (from the margin inwards) of an obsidian rind, a zone of flow-banded/flow-folded pumiceous material, and a central zone of columnar rhyolite. They are thought to represent remnants of disintegrated subglacial (subaquatic) rhyolitic flows or intrusions into the water-logged pumice-bearing hyaloclastite. Due to the rapid chilling of subglacial flows, dome-shaped bodies representing the initial stage of a growing lobe, develop on flow units. When the internal pressure of the lobe due to inflow of magma exceeds the tensional strength of the obsidian rind the latter shatters, and new lava is exposed to the quenching medium. As long as this process of build-up/ sudden release of pressure can take place during a relatively quiet phase of an eruption, the second type of acid hyaloclastite will form.

Surface folding and viscosity of rhyolite flows

Regularly spaced ridges on rhyolite flows are analyzed through the use of a surface-folding model that was first applied to ropy structures on pahoehoe basalt flows. The requirement that there be a strong folding instability to produce regularly spaced ridges places constraints on three dimensionless parameters related to the properties of the lava and the geometry of the channel: R > 35, S 28. R is the ratio of surface to interior viscosities, S is a ratio between the stress due to the weight of the lava and the compressive stress due to folding, and L dγ is a dimensionless form of the ridge spacing. Estimates of strain rates and measurements of ridge spacings and thicknesses of thermal boundary layers of flows allow these three parameters to be calculated independently for a given flow lobe. For the Big Glass Mountain rhyolitic obsidian flow in northern California, R ≅ 104, S 44. This compatibility between theory and observation supports the folding interpretation for ridges. Furthermore, the model allows calculation of the minimum viscosity of many flows for which such data are otherwise unavailable. The viscosities of a dacite flow in Chile and of a. possible lava flow on Mars are calculated as examples.

A summary of the geology, geochemistry, and geophysics of the Roosevelt Hot Springs thermal area, Utah

The Roosevelt Hot Springs thermal area is a newly discovered geothermal power prospect in Utah. Seven production wells have been drilled with a maximum per well flow capability averaging [Formula: see text] of combined vapor and liquid per hour at a shut‐in bottom hole temperature near 260°C. The thermal area is located on the western margin of the Mineral Mountains, which consist dominantly of a Tertiary granitic pluton 32 km long by 8 km wide. Rhyolitic tuffs, flows, and domes cover about [Formula: see text] of the crest and west side of the Mineral Mountains within 5 km of the thermal area. The rhyolitic volcanism occurred between 0.8 and 0.5 m.y. ago and constitutes a major Pleistocene thermal event believed to be significant to the evaluation of the Roosevelt Hot Springs thermal area. Thermal waters of the (now) dry spring, a seep, and the deep reservoir are dilute (ionic strength 0.1 to 0.2) sodium chloride brines.

Stratigraphy of the Lamington Volcanics in far Northeastern New South Wales

Abstract The stratigraphic nomenclature for the Lamington Volcanics in the southern part of the Tweed Shield Volcano is revised. An alkaline formation (the Kyogle Basalt) containing discontinuous rhyolitic‐agglomerate (the Homeleigh Agglomerate Member) and rhyolite flow (the Georgica Rhyolite Member) horizons is the basal unit of the volcanic succession. The Kyogle Basalt thins eastward where it is overlain by a subalkaline sequence comprising the Lismore Basalt, Nimbin Rhyolite, and Blue Knob Basalt respectively. The Lismore Basalt is composed predominantly of tholeiitic andesite with minor icelandite and sporadic flows of mildly alkaline rocks; tholeiitic andesites predominate in the Blue Knob Basalt. The Nimbin Rhyolite consists of rhyolite flows, and a relatively minor pyroclastic component (principally lapilli tuff).

Geophysical surveys of Yellowstone Lake, Wyoming

Reflection, refraction, and paleomagnetic data from Yellowstone Lake, Wyoming, were used to evaluate the Quaternary, tectonic, volcanic, and glacial history of this high mountain lake. Reflection data revealed a paleobasin beneath the main body of the lake containing up to 190 m of lake sediments deposited on intrusive or volcanic bedrock. Prominent reflectors are interpreted to correspond to glaciated layers, erosional surfaces developed during periods of desiccation, and sediments deposited during Recent lake impoundment by rhyolite flows. Volcanic units that outcrop at the south end of the lake are correlated with reflectors that plunge northward beneath the paleobasin, indicating at least 300 m of Recent subsidence of the Yellowstone caldera. Sedimentation rates of 10--13 cm/100 yr were deduced from paleomagnetic logs of shallow cores and suggest a maximum of 200,000 years of deposition within the paleobasin. Refraction data delineated a 1-km-deep, 5.0-km/s layer, interpreted as the top of a shallow crystalline intrusive or volcanic layer possibly associated with the Yellowstone caldera. A rapid decrease in the number of earthquakes occurring 4 km inside the geologically mapped caldera boundary corresponds to a zone of thermal gradients increasing from 200/sup 0/C/km to 800/sup 0/C/km and is interpreted to reflect the boundary ofmore » a deeper crustal structure, perhaps the edge of a cooling magma body. Contoured water depths obtained from high-resolution 7.5-kHz seismic reflection profiling provide a detailed bathymetric map of Yellowstone Lake.« less

Mineralogy and Geology of the Maungaparerua Halloysite Deposit in New Zealand

AbstractThe Maungaparerua halloysite deposit is located on the North Island of New Zealand some 210 km north of Auckland. The halloysite deposit was formed by hydrothermal alteration of rhyolite flow rocks. Superimposed on the hydrothermally altered halloysite is deep intensive surficial weathering resulting from the humid climate on the extreme tip of the North Island. The deposit has been drilled and the cores have been analyzed mineralogically and chemically. Drilling has shown that the deposit is circular or ovate and covers about 350 acres. The altered rocks contain about 50% quartz and fine amorphous silica and 50% of a mix of halloysite, kaolinite, and allophane with a small amount of plagioclase feldspar in the coarse fraction. The upper 8–30m, with an average of 15m, of the halloysite alteration deposit consists of relatively soft clay. Below this soft clay, which is the result of intense surficial weathering, the clay becomes hard and dense. The halloysite and kaolin from this deposit are mined and beneficiated using wet process techniques of dispersion, centrifugation, leaching, dewatering, and drying. The products are used as filler in paper, paint, and plastics and as a ceramic raw material for producing whiteware and fine china. This deposit is unique in that it contains a very high proportion of halloysite in the clay fraction. The detailed geology, mineralogy, geochemistry, and the physical properties of the clay will be discussed.

Timber Mountain–Oasis Valley caldera complex of southern Nevada

The Timber Mountain–Oasis Valley caldera complex lies within a volcanic field in southern Nevada that once covered 11,000 km2. The caldera complex, active from 16 to 9.5 m.y. ago, was the source of nine voluminous rhyolitic ash-flow sheets and numerous smaller rhyolitic tuffs and lava flows. Several centers of basaltic and related volcanism were active before the complex formed, continued around its periphery during caldera activity, and have since overlapped the caldera complex. Extensional normal faulting and perhaps deep-seated right-lateral deformation preceded, accompanied, and followed evolution of the caldera complex and its surrounding volcanic field. The youngest major structure of the complex is the Timber Mountain resurgent caldera, 25 by 30 km across. Several stages of its development can be documented: (1) magmatic insurgence accompanied by gentle tumescence, formation of a ring-fracture zone, and minor rhyolitic volcanism; (2) eruption about 11 m.y. ago of a voluminous ash-flow sheet, caldera collapse during the eruption, and postcollapse caldera infilling by sediments and rhyolite flows; (3) renewed ash-flow eruptions and further caldera collapse; (4) resurgent doming of the cauldron block; and (5) postcollapse rhyolitic volcanism and filling of the caldera by sediments. Only parts of the older calderas are preserved, but they can be interpreted in terms of evolutionary cycles similar to that of the Timber Mountain caldera. Major tectonic intersections appear to have controlled the locations and certain structural features of each major volcanic source area. Differentiation at high crustal levels of the silicic magmas related to the caldera complex produced compositionally zoned ash-flow sheets. High-level differentiation also is represented at most of the basaltic centers of the field. Each caldera cycle probably represents a separate batch of rhyolitic magma that rose high into the crust, differentiated in place, and partly erupted to the surface. Each of these magmas probably rose independently through the crust, but all of them were related ultimately to a single magmagenetic system, as were the basaltic magmas of the field. The silicic magma bodies consolidated to form large shallow granitic plutons, and the caldera complex now overlies a small composite granitic batholith.

Obsidian hydration dating and correlation of Bull Lake and Pinedale Glaciations near West Yellowstone, Montana

The ages of the last two glaciations near West Yellowstone, Montana, can be calculated by obsidian hydration techniques that are calibrated by K-Ar dating of obsidian-bearing lava flows. The average age of glacial abrasion of obsidian in the Pinedale terminal moraines is about 30,000 yr, with most age measurements between 20,000 and 35,000 yr. For the Bull Lake moraines, it is about 140,000 yr, with most measurements between 130,000 and 155,000 yr. This age for the Bull Lake moraines is also supported by geologic relations that show that the moraines are older than a rhyolite flow dated by K-Ar as 114,500 ± 7,300 yr old (lσ). These obsidian hydration ages of the Pinedale and Bull Lake Glaciations correlate well with the last two cold intervals of the marine record. The age determined for the Bull Lake Glaciation near West Yellowstone antedates the last interglaciation of the marine record, which is commonly correlated with the Sangamon Interglaciation. Our results suggest correlation of the Pinedale Glaciation near West Yellowstone with much or all of the Wisconsin Glaciation and of the Bull Lake with the late Illinoian. This differs with the commonly accepted correlation of the Pinedale with the late (“classical”) Wisconsin and of the Bull Lake with the early Wisconsin. The correlation of Bull Lake with late Illinoian appears equally or more compatible with traditional criteria for correlation, namely comparative soil development and degree of preservation of morainal morphology.

Correlation of Pinnacles and Neenach Volcanic Formations and Their Bearing on San Andreas Fault Problem

The Pinnacles Volcanic Formation (San Benito County, California) is an early Miocene (23.5 m.y.B.P.) sequence of calc-alkaline andesite, dacite, and rhyolite flows interbedded with pyroclastic and volcaniclastic rocks. Structurally the volcanic rocks form a west-dipping homocline which is truncated on the northeast by the Chalone Creek fault. The Chalone Creek fault is interpreted to be an early Miocene trace of the San Andreas fault. The Neenach Volcanic Formation (Los Angeles County, California) is an early Miocene (23.5 m.y.B.P.) sequence of calc-alkaline andesite, dacite, and rhyolite flows interbedded with pyroclastic and volcaniclastic rocks. Structurally the rocks form a northwest-dipping homocline which is broken by many faults. The formation is truncated on the southwest by the San Andreas fault. The Pinnacles Volcanic Formation and the Neenach Volcanic Formation lie respectively west and east of the San Andreas fault zone and are separated 315 km from each other in a right-lateral sense. Ten rock types with nearly identical field, petrographic, and chemical characteristics are present in each formation in essentially the same stratigraphic order: vitric lapilli-tuff, flow-banded rhyolite, perlite, pumice lapilli-tuff, hypocrystalline hypersthene andesite, augite-olivine andesite, andesite tuff, porphyritic dacite, agglomerate, and porphyritic rhyolite. The Pinnacles Volcanic and Neenach Volcanic Formations were contiguous across the San Andreas fault at the time of their formation and subsequently have been offset 315 km along the San Andreas fault. For several reasons the Pinnacles-Neenach correlation provides some of the most conclusive documentation of large-scale, lateral displacement on the San Andreas fault yet discovered in central California: (1) a precise distance of separation can be determined because both areas are directly adjacent to traces of the fault, thus eliminating the necessity of interpretive projection into the fault; (2) the age of the formations can be determined accurately because the rocks are volcanic and suitable for radiometric dating; (3) the correlation is based on 10 different rock types, each with distinctive petrologic features, thus reducing the likelihood of chance occurrence. Other correlations of pre-Miocene rocks along the San Andreas fault show offsets of the same magnitude as the Pinnacles-Neenach rocks suggesting that 315 km may be the maximum offset along the San Andreas fault proper.

The place of sulfide mineralization in the volcanic sequence at Noranda, Quebec

Copper-zinc massive sulfide ore deposits in Noranda, Quebec, occur in a thick pile of Archean andesites and rhyolites that form the upper part of the Blake River Group. The flows are weakly metamorphosed and over a large part of the area have only shallow dips. Flows are extensive, often thick, sheets, and several features imply their extrusion in a deep-water setting. Subsidence during the buildup of the pile maintained relatively subhorizontal surfaces over large areas. Five zones of rhyolitic lavas mark five successive periods of rhyolitic activity.The ore deposits, which overlie chloritic pipes of alteration, occur at or near the top of rhyolitic formations and many are associated with primary volcanic features such as lava domes and explosive breccias. They show a zoning of copper-zinc ratios and evidence of fragmentation of some massive sulfides prior to their having been covered by later flows of andesite or rhyolite. They are attributed to submarine volcanogenic processes forming sulfide sinters over hot springs.Two main types of deposit, copper-zinc and zinc-rich, are recognized as occurring, respectively, in the third and fourth rhyolitic zones, while only a massive pyrite body is known in the fifth zone.Deposits in the third zone are divided into three groups which lie at different contacts due to the wedging out of several formations. The north group, often associated with rhyolite domes, formed on the Waite Rhyolite and may be placed precisely in a narrow time-span in the succession. They were deposited during the waning stage of the third period of rhyolitic activity. This waning was concurrent with the onset of the eruption of the overlying Amulet Andesite. Deposits of the center group occur at or above the upper contact of the Amulet Rhyolite, an older formation of the third zone. A wider time-span for their formation is possible but, as several deposits of this group occur in the Amulet Andesite and are in part associated with rhyolite domes, they are interpreted as having formed contemporaneously with those in the north group. Though deposits of the south group overlie rhyolites equated with the Amulet or Waite Rhyolites, they are overlain by rhyolites of the fourth zone and similarly were deposited in the same narrow time span as those of the other groups.Zinc-rich deposits formed with renewed hydrothermal activity higher in the pile, completing a cycle of copper to zinc with time, a feature also noted in most deposits.Ore deposits are restricted to a small part of the pile, that in the center of the area which is probably that over the magma chamber whose position is reflected by one of the granitic bodies and evidence of subsidence.

Diffusion Flow Laws in Metamorphic Rocks

Rocks which deform by pressure solution obey a diffusion flow law with a linear viscous or Newtonian stress to strain-rate relation. Undeformed relics of original grains preserved within newly grown crystals at grain boundaries under tension and presolved surfaces, together with accumulation of inert particles at grain boundaries under compression, are diagnostic evidence of a diffusion flow law. At a given stress, strain-rate is inversely proportional to the grain size to a power of two or three. A geologically useful plot has inverse temperature versus the logarithm of grain size as coordinates. Such a graph is separated into fields by three boundaries which meet at a triple point; within each field, either lattice diffusion, grain-boundary diffusion, or a dislocation flow law is predominant. It may be possible to calibrate this graph from naturally deformed rocks. Photomicrographs of isoclinally folded greenschist-grade quartzites and rhyolitic flows from the South Mountain–Blue Ridge area in Maryland demonstrate a diffusive mass transfer deformation mechanism, but estimates of effective diffusion coefficient compared to currently available laboratory diffusion data are insufficient to identify the diffusion path with certainty. However, the comparatively low ratio of metamorphic temperature to melting temperature and the physical nature of grain boundaries in metamorphic rocks, particularly concentrations of low-density impurities at grain boundaries, suggest the grain-boundary diffusion flow law.

Tertiary Volcanic Center, West-Central Nevada

The Clan Alpine Mountains, Churchill County, Nevada, contain two sequences of Tertiary rhyolite tuffs and flows which are contemporaneous but markedly different in structure and thickness. The sequence in the northern part of the range consists of about 500 m of well-stratified ash-flow sheets of age range 22 to 30 m.y. The southern part of the range contains an assemblage of lava flows, domes, and ash-flow and epiclastic tuff beds which is here subdivided into five mappable units. Radiometric ages are 22 to 30 m.y. Based on gravimetric analysis, the thickness of the southern sequence is at least 3,000 m and perhaps 5,000 m. The contact of the two rhyolite sequences, which is now largely eroded, was probably an abrupt gradation close to the present northern margin of the southern sequence. Structural evidence indicates that the voluminous eruptions of the southern sequence were probably from local sources and were concurrent with faulting such that the rocks were deposited in one or more volcano-tectonic depressions. Recognition of the north wall of at least one caldera within the southern sequence seems clear. Age relations indicate that the vents for the southern sequence were probably the source of at least some of the ash flows of the northern sequence. Similar age and structural relations between different sequences of rhyolite tuffs and flows in the next range east, the Desatoya Mountains, suggest that the volcano-tectonic feature of which the southern sequence is a part extended east of the Clan Alpine Mountains.

Paleomagnetism and Potassium-Argon Ages of the Sonoma Volcanics, California

Paleomagnetic data and potassium-argon ages indicate that the Sonoma Volcanics was erupted during the Pliocene Gilbert reversed and Gauss normal polarity epochs. The Gilbert reversed epoch is represented in the Howell Mountains east of Napa and east of St. Helena, in the mountains immediately east of the Valley of the Moon, and on the hill just north of Santa Rosa. The Gauss normal epoch is represented by the rocks from Mount St. Helena and possibly by the flows from the upper part of Sonoma Mountain and the rhyolite flows north of Sonoma. The age of the volcanic rocks ranges from 5.3 m.y. to about 2.9 m.y. Volcanic rocks approximately 11.8 m.y. old occur at Burdell Mountain northwest of Novato. This volcanic sequence is probably part of the Tolay Volcanics of the Petaluma area.

Tertiary Volcanic Stratigraphy in the Goldfield and Superstition Mountains, Arizona

Recent work provides the basis for establishing a stratigraphic column for units within the Superstition cauldron of central Arizona. Two new formations are defined. The Geronimo Head Formation is a Miocene rhyolite tuff containing rhyolite, quartz latite, and basalt lava flows. The Superstition Tuff is a Miocene quartz latite welded tuff composed of three members. The Siphon Draw Member, Dogie Spring Member, and Canyon Lake Member were emplaced in order of decreasing age. The Dogie Spring Member intertongues with the Geronimo Head Formation. Numerous radiometric and fission track dates support the field determination of the stratigraphic sequence.

Paleomagnetism, Geochemistry, and Potassium-Argon Ages of the Rio Grande de Santiago Volcanics, Central Mexico

Seventy-two oriented cores of 2.5 cm diameter have been drilled from seven bodies comprising part of a sequence of ignimbrite sheets interdigitated with basalts and basaltic andesites, in the Grand Canyon of the Rio Grande de Santiago of the Mexican Cordilleran Volcanic Province. Field observations and whole-rock analyses reveal two groups: an olivine basalt-high alumina basalt-basaltic andesite series, and a sequence of rhyolitic ignimbrite sheets, with some rhyolitic and obsidian lava flows. Crystal separation, partial melting of oceanic tholeiite, and simple mixing of basalt and oceanic sediments or granodiorite cannot acceptably explain the observed fractionation trends. The favored origin is a partial melting of a second or third stage product, such as a basalt or amphibolite enriched in K, Rb, Th, and Pb, relative to oceanic tholeiite. The observed fractionation trends are parallel to those of the andesite-dacites of the Mexico City region, but the Rio Grande de Santiago rocks have a systematically higher K 2 O/SiO 2 ratio, which can be explained by a greater magma generation depth. The paleomagnetic polarity sequence with decreasing age is R,R,N,R,R,N,N, providing easily measurable markers of high potential stratigraphic value. The mean virtual geomagnetic pole is not significantly different from the present spin axis. The potassium-argon analyses show that the sampled section contains two distinct volcanic episodes, from approximately t = 4.6 to 5.5. m.y. and from approximately t = 8.7 to 9.5 m.y. The polarities recorded by the youngest volcanic cycle are consistent with the predicted polarity time scale of Heirtzler and others (1968), while the older cycle polarities can be explained by minor modification of the predicted polarity scale, to include a short period of reversed polarity between 9.0 and 9.5 m.y., within the normal polarity anomaly 5 (t = 8.79 to 9.94 m.y.) of the time scale.

Evolution of Mt. St. Joseph—an Archaean Volcano

A 6650-m thickness of volcanic rocks at Lake St. Joseph is here interpreted as an Archaean composite strato-volcano. Despite severe deformation, primary mesoscopic fabrics are well-preserved. They permit the inference of a physical eruptive history, to which the chemistry can be related.The lowermost 2700-m of flows, pillow breccias, and autobreccias are exclusively basaltic, and indicate quiet, probably semi-continuous activity. Interruption of this activity is shown by an intraformational conglomerate developed on a metadiorite, which has thermally metamorphosed argillite lenses intercalated with the flows. Subsequently, activity became more episodic and violent, yielding, first, 750-m of mixed flows and fragmental rocks, substantially andesitic; and then 3200-m of fragmental rocks, substantially salic. Basaltic dikes ramify through the volcanic pile. Several rhyolitic or dacitic flows are regarded as flank eruptions.Generally, these volcanic rocks have trends somewhat similar to trends for other Archaean volcanics. However, there is an iron enrichment trend which we suggest indicates an initially low pO2 increasing as the volcano ages. This increase may indicate an increase in total gas pressure, compatible with the change from quiet to violent eruptive mode.