Geology topics
Harald H. Mehnert
Publications and source records attributed to Harald H. Mehnert.
Evolution of volcanic rocks and associated ore deposits in the Marysvale volcanic field, Utah
A geological account on the igneous activity and associated mineral deposition in the volcanic field of Marysvale in Utah is presented. Three episodes (34-22 Ma, 22-14 Ma and 9-5 Ma) involved in the volcanic rock eruption and associated mineralization are described. The first episode is believed to have occurred during the time of tectonic convergence when two contrasting suites of rocks, Mount Dutton Formation and Bullion Canyon Volcanics, erupted concurrently. Mineralization during this period was sparse. In the second episode, change from intermediate to bimodal volcanism occurred. During the third episode, basaltic compositions did not change. Although major element constituent had rhyolites similar to that of the second episode, rhyolites had a marked radiogenic isotope characteristic difference.
Potassium-argon ages from the Mount Taylor Volcanic Field, New Mexico
Fourteen new K-Ar dates for volcanic rocks of the Mount Taylor field, New Mexico, indicate that most activity occurred between 4.3 and 1.5 m.y. (million years) ago. Peak activity was at about 3.0-2.5 m.y., both on the central andesite-rhyolite shield volcano and on the surrounding alkali basalt-trachyte volcanic plateau, and occurred concurrently with an episode of NNE-trending basin-range faulting. The K-Ar dates also indicate that the regional Ortiz pediment surface, graded to the ancestral Rio Grande, existed in the Mount Taylor area as recently as 3 m.y. ago and that 250-400 m of erosional downcutting has occurred in subsequent time. Growth of the Mount Taylor field was also concurrent with peak volcanic activity along the northeast-trending Springerville-Raton zone, a major late Cenozoic volcanic belt that is considered to reflect the presence of a regional structural discontinuity of Precambrian age in the North American craton.
Blue Ribbon Lineament, an east-trending structural zone within the Pioche mineral belt of southwestern Utah and eastern Nevada
The Blue Ribbon lineament is an east-west structural zone that is about 25 kilometers wide and passes through the Pioche mineral belt at about 38°10' N. It is best known in Utah, where it is at least 190 km long, and extends from the southern Sevier Plateau in the High Plateaus westward and across southern Mountain Home (Needle) Range in the Great Basin. It probably continues westward an additional 170 km into Nevada, where it connects with the eastern end of the 230-km Warm Springs lineament. The Blue Ribbon lineament is defined by range terminations and east-trending valleys, alinement of eruptive centers of middle Miocene (20 million years) to Pliocene(?) (5-1.8 m.y.) alkalic rhyolite, alinement of areas of middle Miocene to Pliocene mineralized rocks (mostly fluorine, uranium, tungsten) and hydrothermally altered rocks, east-trending magnetic highs and interruptions of magnetic anomalies, and east-striking basin-range faults of late Tertiary and Quaternary age. Mountains south of the lineament are topographically and structurally lower than those to the north. North-striking Quaternary basin-range faults, the Thermo hot springs area, several warm springs and former hot springs, and numerous dacitic to andesitic volcanic centers of early to middle Miocene age (26-20 m.y.) occur along the lineament. The Blue Ribbon lineament is believed to be a deep crustal fault zone dating from at least middle Miocene time and possibly much earlier. It thus developed generally coincident with northerly trending classical basin-range faults. Its fracture system was an important, long-lived conduit for mineralizing fluids, and it should be an attractive target for minerals exploration in the future. The lineament could be due to an east-trending warp in the subducting mantle plate, or it could be part of a past or present intracontinental transform fault that locally gets younger eastward and dies out eastward in the western Colorado Plateaus province.
Age and tectonic setting of lower Paleozoic alkalic and mafic rocks, carbonatites, and thorium veins in South-central Colorado
Alkalic igneous rocks were emplaced into heterogeneous terrane of Precambrian X and Precambrian Y rocks about 570 m.y. ago (Cambrian or upper Precambrian) in the Powderhorn area in Gunnison County and about 520 m.y. ago (Cambrian) in the McClure Mountain, Gem Park, and Democrat Creek areas in the northern Wet Mountains, Fremont and Custer Counties. The radiometric ages are based upon studies by K-Ar, Rb-Sr, and fission-track methods. Associated with these alkalic rock complexes are numerous thorium-bearing veins and red syenite dikes; some, if not all, in the northern Wet Mountains were formed about 495 m.y. ago. Diabase, gabbro, and other mafic dikes appear to be slightly younger than the thorium veins in the Powderhorn area and both older and younger in the Wet Mountains region. In the Powderhorn district, an older group of syenites also intruded Precambrian rocks as plugs or small stocks about 1350-1400 m.y. ago. The various dike rocks and the thorium veins were formed in extensive, deep fractures, indicating a condition of tension or shear in this part of the crust during Cambrian or very late Precambrian to Ordovician time. The localized alkalic magmatisin may reflect melting spots in the mantle. In the two principal areas, 135 km apart, the episodes of alkalic magmatism differ in age by about 50 m.y., the younger toward the east. The age relations might be explained by migration of the sites of localized melting or volatile enrichment in the mantle or westward movement of the continent above a single site.
K-Ar ages of plutonic rocks in the Lassiter Coast area, Antarctica
Numerous middle Cretaceous stocks and small batholiths intruded Middle and Upper Jurassic sedimentary and volcanic rocks in the Lassiter Coast area of the southern Antarctic Peninsula. To establish the age of the quartz diorite and granodiorite plutonic events, five plutons were dated by the K-Ar method. The results indicate a timespan of about 20 m.y. for the emplacement of the plutons and a minimum age of 120 m.y. for the deformation of the sedimentary (Latady Formation) and volcanic rocks. Three of the more silicic plutons have concordant biotite and hornblende ages of 100 and 106 m.y.; two discordant ages of about 118 m.y. were obtained on the more mafic plutons. Petrographically, chemically, and geochronologically, the plutons of the southern to central Lassiter Coast are similar to the calc-alkaline suite of igneous rocks of the Andes of Patagonia, the Antarctic Peninsula, and eastern Ellsworth Land.
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. © 1974 Geological Society of America.
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.
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. © 1970, The Geological Society of America, Inc.
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