Geology topics

R. E. Zartman

Publications and source records attributed to R. E. Zartman.

46 records · Page 3Linked to original sources

USGS1980

Allochthonous Jurassic ophiolite in northwest Washington

Fragments of Jurassic ophiolite having U-Pb zircon ages narrowly grouped at 160 to 170 m.y. are widespread over parts of northwest Washington. The Haystack thrust fault is inferred to mark the base of the ophiolite in the San Juan Islands and adjacent Cascade foothills; other bodies of mafic and ultramafic rock in the western Cascades may be klippen of the Haystack thrust plate. The Haystack thrust fault is probably the structurally highest and possibly most extensive thrust yet recognized within a family of Late Cretaceous thrust faults in northwest Washington. The ophiolite and its time of emplacement (bracketed between about 100 and 88 m.y.) suggest a similarity with the Coast Range thrust of California which thrust Upper Jurassic ophiolite and the Great Valley sedimentary sequence over the Franciscan assemblage. However, relations in the Cascades are complicated by the extraordinarily diverse character of lower plate rocks, of which very few resemble the Franciscan. We conclude that an original subduction system was modified by later tectonic activity so that a variety of terranes was juxtaposed as a family of rootless thrusts, with the ophiolite forming, at least in some areas, the uppermost structural unit. Perhaps the emplacement of Wrangellia, an allochthonous microcontinent west of the San Juan Islands, caused the thrusting.

Washington↗

USGS1979

U-Th-Pb geochronology of the Massabesic Gneiss and the granite near Milford, South-Central New Hampshire: New evidence for avalonian basement and taconic and alleghenian disturbances in Eastern New England

U-Th-Pb systematics for zircon and monazite from Massabesic Gneiss (paragneiss and orthogneiss) and the granite near Milford, New Hampshire, were determined. Zircon morphology suggests that the paragneiss may be volcaniclastic (igneous) in origin, and thus the age data probably record the date (minimum of 646 m.y.) at which the rock was extruded. A two-stage lead-loss model is proposed to explain the present array of data points on a concordia diagram. Orthogneiss ages range only narrowly and are clustered around 475 m.y. Data for the granite of Milford, New Hampshire, are scattered, but may be interpreted in terms of inheritance and modern lead loss, yielding a crystallization age of 275 m.y. This is the only known occurrence of Avalonian-type basement in New Hampshire and as such provides evidence for the location of the paleo-Africa-paleo- North America suture. The geochronology also further documents the occurrence of disturbances during the Ordovician and Permian. ?? 1979 Springer-Verlag.

Contributions to Mineralogy and Petrology↗

USGS1973

Uranium concentration and distribution in six peridotite inclusions of probable mantle origin

Fission-track activation was used to investigate uranium concentration and distribution in peridotite inclusions in alkali basalt from six localities. Whole-rock uranium concentrations range from 24 to 82 ng/g (1ng= 10 −9 g). Most of the uranium is uniformly distributed in the major silicate phases — olivine, orthopyroxene, and clinopyroxene. Chromian spinels may be classified into two groups on the basis of their uranium content, having either less than 10 ng/g or 100–150 ng/g U. In one sample accessory hydrous phases, phlogopite and hornblende, contain 130 and 300 ng/g U, respectively. The contact between the inclusion and the host basalt is usually quite sharp. Glassy or microcrystalline veinlets found in some samples contain more than 1μg/g(1 μg= 10 −6 g). Very little uranium is associated with microcrystals of apatite. Our results agree with some earlier investigators, who have concluded that suboceanic peridotites contain too little uranium to account for normal oceanic heat flow by conduction alone.

Earth and Planetary Science Letters↗

USGS1973

Lead concentration and isotopic composition in five peridotite inclusions of probable mantle origin

The lead content of five whole-rock peridotite inclusions (four lherzolites and one harzburgite) in alkali basalt ranges from 82 to 570 ppb (parts per billion). Approximately 30–60 ppb of this amount can be accounted for by analyzed major silicate minerals (olivine ≤ 10 ppb; enstatite 5–28 ppb; chrome diopside ∼400 ppb). Through a series of acid leaching experiments, the remainder of the lead is shown to be quite labile and to reside in either glassy or microcrystalline veinlets or accessory mineral phases, such as apatite and mica. The lead isotopic composition of the peridotites ( 206 Pb/ 204 Pb= 18.01–18.90; 207 Pb/ 204 Pb= 15.52–15.61; 208 Pb/ 204 Pb= 37.80–38.86) lies within the range of values defined by many modern volcanic rocks and, in particular, is essentially coextensive with the abyssal tholeiite field. In all but one instance, isotopic differences were found between the peridotite and its host alkali basalt. Two of the peridotites clearly demonstrated internal isotopic heterogeneity between leachable and residual fractions that could not simply be due to contamination by the host basalt. However, there is no evidence that these ultramafic rocks form some layer in the mantle with isotopic characteristics fundamentally different from those of the magma sources of volcanic rocks.

Earth and Planetary Science Letters↗

USGS1973

Geochronology of Precambrian rocks of the Teton Range, Wyoming

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. The oldest rocks in the Teton Range are complexly deformed interlayered biotite gneiss, plagioclase gneiss, amphibole gneiss, and amphibolite. Also, within these rocks, there are concordant bodies of strongly lineated quartz monzonite gneiss, here named the Webb Canyon Gneiss, which may be of volcanic origin. Coarse metagabbro, here named the Rendezvous Metagabbro, is intrusive into the layered gneiss sequence and was metamorphosed and deformed along with the enclosing rocks. These older rocks are cut by discordant plutons and swarms of undeformed dikes of quartz monzonite and associated pegmatite. The quartz monzonite, which makes up much of the central part of the Teton Range, is here named the Mount Owen Quartz Monzonite. The youngest Precambrian rocks are undeformed dikes of slightly metamorphosed tholeiitic diabase. A Rb-Sr whole-rock isochron on the Webb Canyon Gneiss and the Rendezvous Metagabbro indicates that these rocks were metamorphosed 2,875 ± 150 m.y. ago. The initial Sr. ratio of 0.700 suggests that the original rocks are probably not significantly older than the metamorphism. The Mount Owen Quartz Monzonite has a whole-rock isochron age of 2,495 ± 75 m.y. and an unusually high initial ratio of 0.732. Plagioclase-microcline isochrons from two samples of the quartz monzonite indicate partial re-equilibration of the Rb-Sr system during a thermal event 1,800 m.y. ago. The age of the diabase dikes has not been definitely determined, but biotite in the wall rocks of one major dike has a K-Ar age of 1,450 m.y. This suggests that the dike was emplaced during or prior to a thermal event 1,300 to 1,500 m.y. ago that was responsible for resetting many of the previously reported K-Ar mineral ages throughout the range. The geochronologic record in the Teton Range is very similar to that elsewhere in the Wyoming Precambrian province. Major metamorphic events with ages between 2,700 and 2,900 m.y. have been identified in the Bighorn, Beartooth, Little Belt, and Granite Mountains. Post-tectonic granitic rocks with ages of 2,500 to 2,700 m.y. have been found in the Wind River Range and the Granite Mountains. Later thermal events have affected Rb-Sr systematics of rocks in the Beartooth Mountains, Wind River Range, and Granite Mountains, as well as in the Teton Range at about the same time as major episodes of regional metamorphism in terranes flanking the Wyoming province in southwestern Montana and in the Front Range in Colorado.

Wyoming↗

USGS1973

Lead isotope systematics and uranium depletion in the Granite Mountains, Wyoming

Isotopic composition and concentration of lead in whole rock and microcline and concentration of uranium and thorium in whole-rock samples of granite from the Granite Mountains, Wyoming, have been determined. The lead isotopic composition in the whole rocks was found to be highly radiogenic with a range in Pb 206 /Pb 204 of 19.58 to 42.27; the corresponding range in microclines is 15.39 to 22.44. A Pb 206 /Pb 204 versus Pb 207 /Pb 204 plot of the whole-rock data yields an apparent isochron age of 2,790 ± 80 m.y. as the time of crystallization of the granite. Chemically determined values of U 238 /Pb 204 in the whole rocks lie between 3.3 and 18.4 and are too low to account for the amount of radiogenic lead observed. A material balance of lead, thorium, and uranium components indicates that an average of approximately 75 percent of the amount of uranium required to produce the radiogenic lead was removed from the rocks, whereas, on the average, there was no apparent loss of thorium. Loss of uranium from the granite is demonstrated to extend at least to a depth of 165 ft in a drill core. The average uranium loss from the samples analyzed represents about 20 g uranium per 1,000 kg of rock that apparently was removed during the Cenozoic and that probably constitutes the major source of uranium now in ore deposits in central Wyoming basins. The lead isotopic composition of the microclines indicates that lead was mobilized within the granite and was isolated in the feldspar during a thermal event about 1,640 + 120 m.y. ago. However, there is no evidence that the whole rocks themselves became open systems at that time. Whole-rock and microcline isochrons intersect at Pb 206 /Pb 204 and Pb 207 /Pb 204 of 13.77 and 14.86, respectively, indicating a characteristic U 238 /Pb 204 of 8.96 in the source region of the granite magma.

Wyoming↗

USGS1969

The isotopic composition of lead in potassium feldspars from some 1.0-b.y. old North American igneous rocks

The isotopic composition of lead and the uranium, thorium and lead concentrations in potassium feldspars are determined for more than 30 1.0-b.y. old North American igneous rocks. Samples representing a broad spectrum in petrographic type and mode of occurrence were chosen; an effort was made to include only rocks having well-documented ages from 950 to 1140 m.y. and showing minimal evidence of subsequent metamorphism. Most samples, including those from extensive terranes of contemporaneous age, have limited lead isotope variations ( Pb 206 Pb 204 = 16.74–17.38 "> Pb206Pb204= 16.74–17.38 ; Pb 207 Pb 204 = 15.39–15.59 "> Pb207Pb204= 15.39–15.59 ; Pb 208 Pb 204 = 36.38–37.10 "> Pb208Pb204= 36.38–37.10 ), which yield model ages close to the radiometric ages. Granite, pegmatite, and rhyolite from within the Grenville province of Canada and age-equivalent rocks of New York, Virginia, Texas, and Colorado and granophyric units associated with the Duluth Gabbro Complex of Minnesota are among the materials yielding this main isotopic spectrum. Several samples were encountered which had isotopic compositions very different from the above group. Lead showing a marked deficiency in radiogenic isotopes was found in two granitic bodies associated with older Labrador Trough rocks from Quebec, in a rapakivi granite from southern Nevada, and in a small granite stock from Mellen, Wisconsin. These occurrences all involve small intrusions of granite which lie near considerably older areas of basement rock. Model ages calculated from the Pb 206 Pb 204 "> Pb206Pb204 ratio are older than the age of the intrusions and approach the age of the host basement rock. Several possible interpretations are offered to explain the isotopic behavior encountered in this study. In particular, a “vertically differentiated crust” model is proposed which will account for both the main spectrum and the anomalous lead. The significance of lead isotopic studies in understanding crustal structure in continental regions is discussed.

Geochimica et Cosmochimica Acta↗

USGS1968

A lead isotope study of galenas and selected feldspars from mining Districts in Utah

The leads in feldspars from the main Tertiary intrusive bodies in each of three mining regions in Utah are isotopically similar to the lead deposits immediately associated with them. These deposits are the largest and also the least radiogenic in each region, whereas the smaller deposits are more radiogenic. Throughout each region the 'ore leads appear to be mixtures in various proportions of lead derived from the intrusive magma and a radiogenic lead component derived from the upper crustal rocks through which the mineralizing fluids passed. Linear relationships are exhibited between the lead isotope ratios within each mining region, and these enable the ages of the upper crustal basement rocks to be determined. A simple tvo-stage model theory for the development of the lead ores yields the folloving data: Oquirrh Mountains region, t 1,650 4-150 m.y., p-Cottonwood-Park City region, t = 2,415 4-30 m.y., p- = 8.87. Tintlc region, t 2,075 4-30 m.y., Milford region, t 1 765 4-70 m.y., p-T ( Us ) = , = 8.83. An alternative "mixing" model is also proposed which requires only 1,650-m.y. and 2,400-m.y. events in these regions. In either case these ages confirm and supplement our present geochronological knowledge of the basement rocks of Utah .

Utah↗

USGS1966

Isotopic study of galenas from the upper Mississippi Valley, the Illinois-Kentucky, and some Appalachian Valley mineral districts

Analyses of lead isotopes in galena in carbonate rock ore deposits collected from several mineralized districts in the Central and Eastern Interior of the United States support previous studies in establishing that the galena is anomalously radiogenic ("J"type")- This study , using care-fully selected samples to test some géologie relationships, shows region-wide and distrietwide systematic variations in isotope ratios. The genetic implications of thèse variations are discussed. Two mechanisms of origin are considered to explain thèse variations in isotopic ratios. One involves mixing of leads from multiple sources, the other calls for variable lead-isotope ratios originating from a single source. Several source materials for the lead ores have been suggested; whether a single source has predominated, or two or more main sources have predominated is still unresolved. Slopes from Pb206/Pb204 versus Pb207/Pb204 plot are compatible with a contribution of lead by the 1300 ± 300 m.y. basement rocks that underlie the deposits. The spatial relationships of alkaline igneous rocks to several of the minerai districts in the Mississippi Valley région suggest a possible mag-matic source for the lead. Isotopic analyses of galenas closely associated with several of the intrusive rocks are similar in isotopic composition to adjacent ore leads, but genetic implications are ambiguous. Galena oc-curring in shales in the Mississippi Valley région outside mineralized districts contains ordinary lead. Théories that call upon such shales to be the major source of the lead in the depoaits of the Mississippi Valley type must explain this distinct différence in isotopic compositions. Galena collected from districts within the Appalachian Ridge and Valley Province contains notably less radiogenic lead than that from the Mississippi Valley minerai districts , and the lead is of the ordinary type.

Illinois, Kentucky↗

USGS1964

Ancient granite gneiss in the Black Hills, South Dakota

Granite gneiss, with an age of approximately 2.5 billion years, in the Black Hills, South Dakota , provides a link betweeen ancient rocks in western Wyoming and Montana and in eastern North and South Dakota and Minnesota. The discovery suggests that early Precambrian rocks covered an extensive area in northcentral United States and were not restricted to several small nuclei.

South Dakota↗