Harald H. Mehnert

Volcanic History of the San Juan Mountains, Colorado, as Indicated by Potassium–Argon Dating

Volcanic rocks in the San Juan Mountains constitute the largest erosional remnant of a once nearly continuous volcanic field that extended over much of the southern Rocky Mountains and adjacent areas in Oligocene and later time. Recent regional studies have shown that the gross petrologic evolution throughout the San Juan remnant of this field was relatively simple, with initial intermediate lavas and breccias, followed closely in time by more silicic ash-flow tuffs, and ending with a bimodal association of basalt and rhyolite. More limited data from other remnants of the original field indicate a similar evolution. In the San Juan field, voluminous early lavas and breccias—mainly alkali andesite, rhyodacite, and mafic quartz latite—were erupted from numerous scattered central volcanoes onto an eroded tectonically stable terrane. They formed mostly during the interval 35 to 30 m.y. ago, but some probably were erupted earlier and others up to several million years later. About 30 m.y. ago, major volcanic activity changed to explosive ash-flow eruptions of quartz latite and low-silica rhyolite that persisted until about 26 m.y. ago. Source areas for the ash flows are marked by large calderas in the central and western San Juan Mountains. Two groups of lavas and associated rocks of intermediate composition intertongue with the ash-flow sequence: (1) quartz latitic lavas that were erupted in and adjacent to caldera structures and are genetically related to the ash-flow activity; and (2) other, generally more mafic lavas and related rocks that are widely distributed without evident structural relation to the ash-flow eruptive centers. The second group apparently represents a continuation of the early intermediate activity into the period of major ash-flow eruption. In the early Miocene the character of volcanism changed notably. Whereas the Oligocene volcanics are predominantly intermediate lavas and related silicic differentiates, the younger rocks are largely a bimodal association of basalt and high-silica alkali rhyolite. Basalt and minor rhyolite were erupted intermittently through the Miocene and Pliocene, and at one time formed a widespread thin veneer over the older volcanic terrane. The marked contrast between the Oligocene intermediate to low-silica rhyolitic magmas and the later basaltic and rhyolitic magmas implies either different conditions of magma generation or processes of differentiation for the two suites. This petrologic change coincides approximately in time with nearby development of the Rio Grande depression, a major rift that is the local expression of widespread late Cenozoic crustal extension. Whatever the cause of the petrologic change, the progression from predominantly intermediate to bimodal basalt-rhyolite volcanism, approximately concurrent with initiation of late Tertiary crustal extension, appears characteristic of Cenozoic volcanism for much of the western interior United States.

New K-Ar dates and the late Pliocene to Holocene geomorphic history of the central Rio Grande region, New Mexico

The aerial distribution, dissection, and stratigraphic relations of Pliocene and younger mafic volcanic flows indicate that the Rio Grande in central New Mexico became a throughflowing drainage system during late Pliocene time (between about 3.0 and 4.5 m.y. ago). K-Ar dates indicate that the course of the Rio Grande was established more than 4.0 m.y. ago in central New Mexico and about 3.0+ m.y. ago in north-central New Mexico. The Ortiz geomorphic surface was probably graded to the ancestral Rio Grande about 3 m.y. ago and is considerably older than the Llano de Albuquerque surface. The central Rio Grande region has been warped and faulted since late Pliocene time. A complex fault block in the southeastern part of the Albuquerque-Belen Basin includes volcanic rocks more than 20 m.y. old that originated from southwestern New Mexico before rifting of the Rio Grande depression.

Geologic Evolution of the Sierra Nevada de Santa Marta, Northeastern Colombia

New geologic, petrographic, and radiometric evidence (52 ages) from the Sierra Nevada suggest that plate tectonics controlled the complex Mesozoic evolution of the Caribbean continental margin. The triangular Sierra Nevada massif is bounded by the Oca fault, Santa Marta–Bucaramanga fault, and Cesar lineament. During the Tertiary, dextral and sinistral movement of 65 and 110 km, respectively, occurred along the Oca and Santa Marta–Bucaramanga faults; subsequently, several thousand meters of uplift produced the present geomorphic setting. Three metamorphic terranes are present; they differ petrographically and geochronologically and are separated by the Sevilla and Cesar lineaments (geosutures). The youngest terrane consists of three northeast-trending regional metamorphic; belts (Permian-Triassic gneiss, Jurassic schist, and Cretaceous-Paleocene green schist) that formed in successive subduction zones northwest of the Sevilla lineament. Tertiary plutons intrude this terrane. Most of the Sierra Nevada massif consists; of l,300-m.y.-old granulite terrane overlair by unmetamorphosed Paleozoic and Permian(?)-Triassic rocks and intruded by four northeast-trending belts of plutons that filled successive dilational rifts. These plutonic belts become younger, shallower, and more potassic in a southeastward direction. Extensional disruption, with transform separations up to 46 km, culminated with Middle Jurassic emplacement of two belts of composite batholiths and extensive ignimbritic eruptions. These events are related to the same southeast-dipping subduction zone that produced the Jurassic schist in the youngest metamorphic terrane. The third metamorphic terrane consists of younger(?) Precambrian amphibolite-grade rocks overlain by Silurian phyllites and unmetamorphosed Paleozoic and Mesozoic rocks that are typical of the Cordillera Oriental.

Multiple ages of mid-Tertiary mineralization and alteration in the western San Juan Mountains, Colorado

Ore deposits in the western San Juan Mountains formed intermittently in middle to late Tertiary time, from about 30 to 10 m.y. ago, during essentially the same span as that of associated igneous activity, as indicated by 31 new K-Ar and fission-track ages. Mineralization occurred recurrently during the waning stages of evolution of several precaldera central volcanoes and also after formation of the Uncompahgre, San Juan, Silverton, and Lake City calderas. The richest ore deposits are associated with structures of the Silverton caldera, but they were emplaced 5 to 15 m.y. after the caldera formed. This mineralization appears genetically unrelated to evolution of the caldera and its associated magmatic system but seems closely related in space and time to volumetrically minor intrusions of quartz-bearing silicic porphyry. This association is common in many mining districts in the Rocky Mountains region.

Radiometric Ages and Stratigraphic Sequence of Volcanic and Plutonic Rocks, Southern Nye and Western Lincoln Counties, Nevada

The geochronology of Tertiary igneous events at the Nevada Test Site and adjacent area is outlined by 36 recently determined K-Ar ages, together with other published K-Ar ages. The first evidence of Tertiary igneous activity is the ash-fall bedded tuffs in the Horse Spring Formation. One such tuff has been dated as 29 m.y. old (late Oligocene). Other ash-flow tuffs and lavas formed during the Miocene and Pliocene, according to radiometric age determinations. The youngest ash-flow tuff in this area is about 6 m.y. old. Great volumes of ash and lava were spewed forth 13 to 11 m.y. ago to form the Paintbrush and Timber Mountain Tuffs. Sixteen replicate age determinations on minerals from four densely welded ash-flow tuffs from these formations gave a pooled standard deviation of about ± 2 percent error, provided anomalous ages were rejected on the basis of rock alteration or analytical difficulties. In the Air Force Gunnery Range, just north of the test site, K-Ar ages suggest that the oldest ash flows, the Monotony Tuff, were emplaced 27.6 m.y. ago (late Oligocene) and were followed by outpourings of lava and ash throughout most of the Miocene. Youngest dated lava is about 13 m.y. old. In the southern Egan and northern Seaman Ranges of central Nevada, the Needles Range (?) Formation has an averaged K-Ar age of about 30 m.y., which compares closely with 29.2 m.y., the average of four earlier K-Ar ages determined by other investigators on known Needles Range Formation in eastern Nevada and western Utah. K-Ar ages given by micas from two exposed plutons in the Nevada Test Site suggest emplacement of these plutons at about 93 m.y. ago (early Late Cretaceous), although earlier emplacement in the Mesozoic would be more consistent with Pb-α ages.

Relation of peralkaline magmatism to heterogeneous extension during the middle Miocene, southeastern Nevada

Volcanism migrated southward in the northern Basin and Range province in the Oligocene and early Miocene to produce voluminous calcalkaline silicic ash flow tuffs. Alkaline volcanism became dominant by middle Miocene (17–14 Ma) as smaller volumes of rhyolite-trachyte-basalt suites were erupted from the relatively small Kane Springs Wash caldera complex including the Narrow Canyon, Boulder Canyon, and Kane Springs Wash calderas in southeastern Nevada. Only minor extension affected the Kane Wash area before the end of calcalkaline activity, but extension expressed by rate of progressive stratal tilt peaked (15–13.5 Ma) with peralkaline magmatism (14.7–14.4 Ma). Variations in distribution, degree, style, and timing of deformation demonstrate heterogeneous extension in the Kane Wash area. Only minor extension and tilting persisted post-middle Miocene (<12 Ma). All major eruptive sources overlap domains of rapid extension. Most of the eruptive volumes from the two oldest calderas of the complex apparently pooled within their calderas, creating outflow deficits. Denudation faulting associated with magmatic tumescence may have followed preexisting active extensional fault systems to unload magma chambers, thus triggering eruptions into structural depressions. Evolution of alkaline magmas is demonstrated by progressive increases in peralkalinity and high field strength elements such as Zr, Y, and Nb. Nd, Pb, and Sr isotopic compositions provide evidence that significantly less crustal interaction affected middle Miocene peralkaline magmas than pre-middle Miocene calcalkaline magmas. eNd values are −5 to −7 for peralkaline magmas and −7 to −11 for calcalkaline magmas; 208Pb/204Pb ratios are 38.2–38.6 for peralkaline magmas and 38.5–38.9 for calcalkaline magmas. Regional cooling, short duration of magmatism, small volumes of magma, and local extension caused less crustal interaction in peralkaline Kane Wash magmas than in earlier magmas. North of the Kane Wash area, older more voluminous calcalkaline magmas intruded hotter crust for a longer period and thus interacted with the crust to a greater degree in spite of synvolcanic extension.

Oligocene basaltic volcanism of the Northern Rio Grande Rift: San Luis Hills, Colorado

The inception of the Rio Grande rift in northern New Mexico and southern Colorado was accompanied by voluminous mafic volcanism preserved in part as erosional remnants on an intrarift horst within the current axial rift graben of the San Luis Valley. Oligocene (∼26 Ma) volcanic rocks of the Hinsdale Formation at San Luis Hills range from 49 to 57 wt % SiO2 and include nepheline and hypersthene normative lavas. A mildly alkalic series consisting of trachybasalt, basaltic trachyandesite, and trachyandesite is volumetrically dominant, olivine tholeiites are subordinate, and xenocrystic trachyandesites containing abundant quartz and plagioclase xenocrysts occur only locally. Relative to the San Luis Hills olivine tholeiites which have La/Smn ∼ 2, the more alkaline series are enriched in light rare earth elements (LREE) and have La/Sm ratios that increase in the trachybasalt-basaltic trachyandesite suite (La/Smn ∼ 3) to xenocrystic trachyandesites that are the most LREE enriched (La/Smn ∼ 4). Chondrite-normalized, trace element patterns for the lavas in the San Luis Hills are similar in shape within the mildly alkaline to transitional series; they have characteristic Nb and Ta depletions and high K and Th relative to Ta, Nb, and LREE. Major and trace element constraints support a petrogenetic model of fractionation plus lower crustal assimilation for petrologic suites within the San Luis Hills rocks, although the model cannot relate lavas for the entire series to a common parent. Most mafic lavas of the San Luis Hills were evolved (Mg # <60) and contaminated by LREE-enriched silicic partial melts of granulitic lower crust depleted in Rb, Th, and U. Pb isotopes are the most sensitive indicators of crustal contamination, whereas shifts in Nd and Sr isotope ratios are associated with large amounts of assimilation. However, relatively noncontaminated lavas can be identified and indicate at least two mantle source regions were involved.

Sm-Nd, K-Ar and petrologic study of some kimberlites from eastern United States and their implication for mantle evolution

We provide new data on Sm-Nd systematics, K-Ar dating and the major element chemistry of kimberlites from the eastern United States (mostly from central New York State) and their constituent mineral phases of olivine, clinopyroxene, garnet, phlogopite and perovskite. In addition, we report Nd-isotopes in a few kimberlites from South Africa, Lesotho and from the eastern part of China. The major element compositions of the New York dike rocks and of their constituent minerals including a xenolith of eclogite are comparable with those from the Kimberley area in South Africa. The K-Ar age of emplacement of the New York dikes is further established to be 143 Ma.We have analyzed the Nd-isotopic composition of the following kimberlites and related rocks: Nine kimberlite pipes from South Africa and Lesotho, two from southern India; one from the U.S.S.R., fifteen kimberlite pipes and related dike rocks from eastern and central U.S. and two pipes from the Shandong Province of eastern China. The age of emplacement of these kimberlites ranges from 1300 million years to 90 million years. The initial Nd-isotopic compositions of these kimberlitic rocks expressed as ɛNdIwith respect to a chondritic bulk-earth growth-curve show a range between 0 and +4, with the majority of the kimberlites being in the range 0 to +2. This range is not matched by any other suite of mantle-derived igneous rocks. This result strengthens our earlier conclusion that kimberlitic liquids are derived from a relatively primeval and unique mantle reservoir with a nearly chondritic Sm/Nd ratio.

Geologic Framework of the Kuluncak-Sofular Area, East-Central Turkey, and K-Ar Ages of Igneous Rocks

The Kuluncak-Sofular area, located about midway between Sivas and Malatya in east-central Turkey is underlain by a variety of sedimentary, volcanic, and intrusive rocks. The sedimentary rocks have been deposited on a pre-Campanian serpentinite basement and include Cretaceous conglomerate, graywacke, tuff, and limestone; Eocene arkosic sandstone, conglomerate, and limestone; and Miocene limestone and dolomite. K-Ar ages determined for volcanic and intrusive rocks from the same area are 75.5 m.y. for alkalic diabase that intrudes the Upper Cretaceous sedimentary rocks; 74.3 and 71.1 m.y. for trachyte that partly overlies and partly intrudes the same Upper Cretaceous sequence; 65.2 m.y. for alkalic syenite that intrudes Upper Cretaceous limestone; 18.7 to 16.8 m.y. for andesite and basalt that overlie middle to late Eocene sedimentary rocks; and 14.1 m.y. for a dacite plug that cuts Miocene limestone.

Radiometric Ages of Intrusive Rocks in the Little Belt Mountains, Montana

Radiometric ages indicate that most, if not all, of the major intrusions in the Little Belt Mountains, central Montana, were emplaced during the Eocene epoch, between 48 and 54 m.y. ago. In the Hughesville area, igneous activity continued, or was episodic until 42 m.y. ago. As a result of the continued igneous activity, radiometric ages in the Hughesville area can be interpreted either as primary ages or as reset ages.