Fred W. McDowell

Timing of mid-Tertiary volcanism in the Sierra Madre Occidental between Durango City and Mazatlan, Mexico

The relatively flat-lying rocks of the upper volcanic supergroup exposed between Durango City and Mazatlan in western Mexico consist of two distinct sequences. Near Durango about 800 m of ignimbrites and associated lavas and tuffs were emplaced between 32.1 and 28.3 m.y. ago (23 K-Ar dates). About 100 to 200 km to the west, at least 1,000 m of ignimbrites, lavas, and tuffs were emplaced 23.3 ± 0.3 m.y. ago, as indicated by 8 K-Ar dates. The two calc-alkalic sequences have significant but slight chemical and mineralogic differences, the older and eastern one being more alkalic. In the Durango vicinity, distinctive alkali basalts were erupted during normal faulting about 12 m.y. ago. Tholeiitic basalt of Quaternary age is also widespread there. Neither the normal faulting nor the basalts are prominent to the west, except near the Gulf of California. The upper volcanic supergroup commonly rests upon an older volcanic-intrusive complex approximately 100 to 45 m.y. old. Between 45 and 32 m.y. ago little or no magmatism occurred in the Sierra Madre Occidental, but major rift-related alkalic volcanic rocks were erupted then in Trans-Pecos Texas. Voluminous rhyolitic activity occurred between 32 and 23 m.y. ago in western Mexico. Intense volcanism began in southern Baja California 20 m.y. ago. The shifting position, nature, and amount of volcanism may correspond to changes in lithospheric plate motions in the east Pacific region, and 45 m.y., 32 m.y., and 23 m.y. ago appear to be key times of plate-boundary reorganization.

Duration of Late Cretaceous–early Tertiary magmatism in east-central Sonora, Mexico

Late Cretaceous‐early Tertiary arc magmatism in east-central Sonora includes the Sonoran batholith and the volcanic rocks of the Tarahumara Formation. The Tarahumara Formation consists predominantly of propylitically altered andesitic to dacitic lava, agglomerate, and volcanic breccia, all of local derivation, along with subordinate felsic pyroclastic components. Relatively thin volcaniclastic intervals with intercalated freshwater limestone that contains Late Cretaceous plant fossils occur within the upper third of the formation. Six new U-Pb zircon ages from the Tarahumara Formation extend the known duration of Late Cretaceous‐early Tertiary magmatism in east-central Sonora. Four of the samples yield ages between 73 and 70 Ma, and the other two are 90 and 89 Ma (errors are generally ,1 m.y.). All of these ages are older than 65 to 55 Ma K-Ar and U-Pb ages for plutons of the Sonoran batholith in eastcentral Sonora determined in other studies. Combined, the ages indicate arc magmatism over at least 35 m.y. Initiation of magmatism in eastern Sonora by 90 Ma is much earlier than predicted by regional trends, which are largely based on geochronology of plutons. It appears that a full history of the regional evolution of this magmatic arc will require substantial age information from its volcanic rocks. A component of inherited zircon was detected in five of the six dated samples from the Tarahumara Formation. Although ages calculated from upper concordia intercepts are mostly imprecise, all are consistent with the presence of one or more Proterozoic components. In one sample, a persistent inherited component provided a precise upper-intercept age of 1448 Ma. These small cores surrounded by new zircon growth must represent material incorporated into the magma chamber prior to eruption. The results record the presence of Laurentian basement 200 km south and east of known outcrops in Sonora.

Interrelationship of sedimentary and volcanic deposits associated with Tertiary extension in Sonora, Mexico

Clastic sedimentary deposits and associated volcanic rocks record the progress of Tertiary extension in the Mexican state of Sonora. These deposits accumulated within basins located throughout the eastern two-thirds of the state. The rocks are slightly indurated conglomerates and sandstones; clast types reflect the local highland exposures. Volcanic units that bound and are interlayered with the sedimentary rocks provide stratigraphic and time markers of basin evolution and, by inference, of marginal fault development. This record has been examined in an east-west belt across south-central Sonora. The volcanic rocks occur in three distinct associations. At the base and interbedded within the lower portions of the sedimentary sections are lava flows of dominantly basaltic andesite composition. In most localities the overlying clastic sedimentary strata are conformable with these lava flows, and we conclude that the magmatism was triggered by early faulting along the basin margins. A second association present near the base of the sections comprises massive lava domes and flows of intermediate composition that contain distinctive phenocrysts of dark brown amphibole. The domes apparently formed when viscous lava welled up along developing marginal basin faults. In one case a transition was observed over a short distance from a structureless dome to a thick lava flow that is conformably interbedded with, and sheds clasts laterally into, the sedimentary section. A third volcanic association comprises layers of rhyolitic lava flows and ignimbrites that overlie the coarsest and thickest lower portions of the sedimentary sections. K-Ar ages for volcanic rocks in the two oldest of these settings indicate that each Tertiary basin had a distinct history with no apparent regional geographic pattern. In the Rio Yaqui basin, where exhumation by the major river of Sonora has exposed a full section of the volcanic and clastic sequences, extension was possibly as old as 27 Ma. Elsewhere, deposition of sediments was underway in most basins by 24 Ma, and in all basins by 20 Ma. K-Ar ages of the younger rhyolitic volcanic rocks fall within a narrow range from 12.8 to 10.5 Ma. These ages provide a younger time limit to deposition of the coarser and thicker portions of the clastic sedimentary sections. The mafic lava flows, dominantly basaltic andesite, are similar in major-element composition to mafic lava flows that cap felsic sections within the Sierra Madre Occidental volcanic field of western Mexico and the mid-Tertiary Datil-Mogollon volcanic field in southwestern New Mexico. They are higher in silica and lower in total alkalies than younger capping mafic lava flows within the Gulf of California extensional province of western Sonora and Neogene basaltic lava flows in the Datil-Mogollon field. They are similarly distinct from basaltic dikes and lava flows that are clearly associated with Basin and Range faulting in Trans-Pecos Texas and in northern Durango state. A convergent-margin magmatic arc was active in the present-day Gulf of California region between 24 and 11.5 Ma. The earliest extension in south-central Sonora was therefore occurring in a back-arc setting. Continued sedimentation and rotation of volcanic strata indicate that extension continued throughout the region after 10 Ma with little coeval volcanism.

Climate forcing by iron fertilization from repeated ignimbrite eruptions: The icehouse–silicic large igneous province (SLIP) hypothesis

During middle Eocene to middle Miocene time, development of the Cenozoic icehouse was coincident with a prolonged episode of explosive silicic volcanism, the ignimbrite flare-up of southwestern North America. We present geochronologic and biogeochemical data suggesting that, prior to the establishment of full glacial conditions with attendant increased eolian dust emission and oceanic upwelling, iron fertilization by great volumes of silicic volcanic ash was an effective climatic forcing mechanism that helped to establish the Cenozoic icehouse. Most Phanerozoic cool-climate episodes were coeval with major explosive volcanism in silicic large igneous provinces, suggesting a common link between these phenomena.

K-Ar and U-Pb zircon chronology of Late Cretaceous and Tertiary magmatism in central Chihuahua State, Mexico

Magmatism in central Chihuahua occurred from at least 68 to 27.5 Ma, a duration that can be separated into two periods by an order of magnitude increase in the intensity of magmatism starting at 46 Ma. Activity prior to 46 Ma was Volumetrically minor and possibly intermittent. The oldest rocks (68 Ma) are a localized, very thick, intermediate-composition sequence of lava flows, debris flows, and related sediments. From 62 until at least 52 Ma, numerous small intrusives and felsic tuffs were emplaced locally. Between 46 and 27.5 Ma, the area was inundated with a quasi-steady supply of volcanic products. The post-46 Ma activity evolved through four distinct stages that produced two sequences of felsic ash-flow tuffs separated by a sequence of massive coarsely porphyritic lavas and tuffs of intermediate to felsic composition. These three calc-alkaline stages were gradually succeeded at about 31 Ma by mildly alkaline basaltic andesite lavas and rhyolitic tuffs with peralkaline characteristics. All volcanic activity terminated abruptly at 27.5 Ma. The most significant events in the evolution of this magmatism are its onset, its termination, and the profound increase in activity beginning at 46 Ma. In its timing and subdued intensity, the pre-46 Ma magmatism fits a setting within the interior of the Laramide Cordilleran orogen. A convergent magmatic are had advanced eastward to central Chihuahua by at least 68 Ma. The oldest rocks are tilted, but volcanic rocks 46 Ma and younger were not deformed. The abrupt increase in magmatism at 46 Ma may be a manifestation of diminished compressional stress related to a decrease in the rate of plate convergence along western North America. The regional stress field remained in compression until about 31 Ma, which corresponds approximately to the time of transition from calc-alkaline to mildly alkaline mafic and peralkaline felsic volcanism. Magmatism ended at 27.5 Ma, prior to extensional faulting in the area, but generally coincident with the end of plate convergence along part of the continental margin of western Mexico.

Glaucophane schists and ophiolites of the northern California Coast Ranges: Isotopic ages and their tectonic implications

The geology of a strip of 15′ quadrangles across the Franciscan Complex in the Coast Ranges of northern California is the basis for interpretation of the significance of potassium-argon and fossil ages of glaucophane schists, ophiolitic materials, and associated sedimentary rocks. Whole-rock and mineral separates of Franciscan rocks were analyzed, and 69 potassium-argon ages were obtained. White micas and actinolites from blueschist blocks in melange give apparent ages clustering about 142 and 153 m.y., but they probably represent a continuum. Blue amphibole ages from blocks range from 98 to 151 m.y.; the younger ages are from the eastern portion of the central melange belt. Published U-Pb ages from the ophiolite belt give crystallization ages from 153 to 165 m.y.; K-Ar ages extend from 143 to 166 m.y. Whole-rock ages from the South Fork Mountain Schist range from 113 to 158 m.y. but cluster around 124 m.y. The association of older K-Ar ages with more coarsely crystalline schist suggests that metamorphism occurred prior to 124 m.y. ago. Fossil evidence indicates that the basal sediments of the Great Valley Sequence were deposited upon the disrupted Coast Range Ophiolite within 5 to 10 m.y. of the oldest crystallization ages for ophiolite. Fossils within melange units, broken formations, and unsubducted trench-slope deposits provide evidence for melange development by the Early Cretaceous, with youngest melange units generally lying to the west. Available chronologic evidence is compatible with a tectonic model involving melange formation in a restricted flow channel beneath the accretionary wedge of the subduction complex. Upflow of melange returned blocks of coarsely crystalline metamorphic rocks that were accreted during the initial stages of subduction when the hanging wall was still hot. The viability of this and other proposed tectonic models can be assessed by continued careful field documentation of the nature of contacts between key lithotectonic units and by detailed application of temperature-sensitive geochronologic tools.

Age and extent of potassic volcanism on the Colorado Plateau

Abstract Six K-Ar ages on phlogopite from minettes in and near the Buell Park diatreme establish that minette and coeval kimberlitic volcanism occurred 25 m.y. ago. Two altered phlogopites yield younger ages, while on phlogopite sample with an age of 34 m.y. may contain excess argon. Published apatite fission-track ages for inclusions in the kimberlitic tuff at Buell Park are too old, perhaps due to partial retention of pre-eruption tracks. Published ages for other minette localities on the Colorado Plateau are insufficient to precisely define the duration of volcanism, but some eruptions may be as old as 30 m.y. Potassic volcanic rocks similar to the felsic minettes at Buell Park erupted at about the same time at Chino Valley, Arizona, in the transition zone of the Colorado Plateau. The correlation between the two localities of unusual potassic rocks is evidence that these rocks may reflect mantle conditions associated with the stability of the Plateau, and that these conditions contrasted with those in adjacent tectonic provinces at least as early as 25 m.y. ago.

Paleogene Extension in the Southern Basin and Range Province of Mexico: Syndepositional Tilting of Eocene Red Beds and Oligocene Volcanic Rocks in the Guanajuato Mining District

The Guanajuato region, located in the southern part of the Basin and Range tectonic province, is characterized by a rhombohedral fault pattern. Extensional faulting-as well as related uplift, erosion, and deposition of continental clastic deposits-was in progress at 49 Ma, as indicated by K-Ar dating of intercalated mafic volcanic lavas. Earliest extension is registered in the lower member of the Guanajuato conglomerate and produced NW-trend-ing, down to the southwest, normal faults and northeast tilting of the hanging-wall clastic sequence. Beds in the upper member of the Guanajuato conglomerate display lateral thickness variations, changes in size and composition of the clasts, and ENE and SE inclination, all of which suggest activity along a second set of NE-trending younger faults. Extension continued also after emplacement of the overlying Bufa ignimbrite (∼37 Ma, K-Ar). Dip differences between the conglomerate and the Calderones Formation ( 30 Ma), which is part of the volcanic sequence that...

The volcanic section at Nazas, Durango, Mexico, and the possibility of widespread Eocene volcanism within the Sierra Madre Occidental

Descriptions of volcanic rocks in the Sierra Madre Occidental of western Mexico have thus far emphasized the widespread and nearly continuous cover of ash flow tuffs and other units attributed to the Oligocene ignimbrite flare-up. However, much less attention has been given to the pre-Oligocene volcanic sequence beneath this ash flow blanket. At Nazas, Durango, on the eastern flank of the Sierra Madre Occidental, a well-exposed section includes voluminous felsic volcanic rocks of both Eocene and Oligocene age. The oldest igneous rocks at Nazas include a small exposure of intermediate volcanic breccias and lava flows that overlie Cretaceous limestones and apparently were deformed with them during Laramide tectonism. More commonly, the limestones are overlain by Tertiary volcanic rocks and continental clastic deposits. The Tertiary volcanic section is about 800 m thick, not including intercalated fanglomerates. The section records three distinct magmatic episodes: from 51 to 40 Ma, comprising felsic ash flow tuffs and intermediate lava flows and domes; a pulse at about 30 Ma, comprising voluminous felsic ash flow tuffs with an aggregate thickness of about 500 m; and from 24 to 20 Ma, comprising alkalic basalts. The Ahuichila Formation, a molasse-type conglomerate up to 200 m thick that underlies the Tertiary volcanic section, was deposited during or just after the Laramide deformation. The Santa Ines Formation is a widespread fanglomerate, up to 110 m thick, that underlies the basalts and apparently accumulated during normal faulting. Eocene volcanism in the Nazas area is characterized by interfingering felsic ash flow tuffs and intermediate lava flows and domes. A similar sequence of Eocene volcanic rocks has been mapped in central Chihuahua state, about 350 km to the northwest of Nazas. Felsic tuffs interbedded with intermediate volcanic rocks older than 40 Ma are also exposed at Tayoltita, Durango, about 200 km southwest of Nazas. These occurrences suggest that a wider spectrum of compositions and volcanic styles existed during the Eocene than during the Oligocene in the Sierra Madre Occidental. Eocene volcanism was more typical of orogenic magmatic belts developed at continental margins, whereas the Oligocene activity was dominated by voluminous felsic ash flow tuffs erupted during a transition of tectonic setting from subduction along a continental margin to intraplate extension. In addition to these three areas, there are several other localities in western Mexico having volcanic rocks with reported ages between 40 and 53 Ma. It is possible that the volcanic field during the Eocene in Mexico was comparable in extent to that of the Oligocene. The Eocene volcanism in western Mexico was in part contemporaneous with the Challis volcanic episode of northwestern United States and its extension into western Canada. However, Eocene magmatism in the Pacific Northwest apparently developed in an extensional tectonic setting, whereas in Mexico no evidence for Eocene extensional deformation is known.

Evolution of waning, subduction-related magmatism, northern Sierra Madre Occidental, Mexico

The stratigraphic record at the Tomochic volcanic center, Chihuahua, documents a change from andesitic to rhyolite-dominated volcanism, with late extrusion of mafic lavas, during the final ∼10 m.y. of subduction-related activity in the northern Sierra Madre Occidental. Andesitic volcanism, which started before 38 Ma and continued until ∼35 Ma, was followed by eruption of large-volume rhyolite ash flows from the Las Varas (34.1 Ma) and Tomochic (31.8-31.4 Ma) calderas. Basaltic andesite, the most mafic rock type in the area, was extruded near the end of calcalkalic volcanism at ∼30 Ma. The change in style of volcanism at the Tomochic volcanic center also occurred in other parts of the northern Sierra Madre Occidental and is interpreted to reflect the evolution of waning, subduction-related magmatism, as convergence between the North American and Farallon plates slowed before it eventually ceased. Slowed plate convergence resulted in a decreased flux of mafic melts from the mantle to the crust, leading, in turn, to the stagnation and coalescence of intermediate-composition magmas similar to those that fed early andesitic eruptions. Subsequent differentiation led to growth of zoned magma reservoirs from which the rhyolite ash flows were extruded. Slowed convergence was accompanied by the onset of crustal extension, which possibly aided the coalescence of andesitic magmas and which allowed the ascent and eruption of mafic melts near the end of silicic volcanic activity. Although minor differences exist, the history of mid-Tertiary magmatism and tectonism in the northern Sierra Madre Occidental was broadly similar to that in parts of the south-western United States.