T. W Stern

Age of the Morton and Montevideo Gneisses and Related Rocks, Southwestern Minnesota

Granitic gneisses in the vicinities of Morton and Montevideo in the Minnesota River Valley are dated at 3550 m.y. ago and are the oldest rocks so far found in North America. The gneisses were altered in varying degree by younger events of which two have been dated at 2650 m.y. and 1850 m.y. old. The event which occurred 2650 m.y. ago was a high-grade metamorphism accompanied by the intrusion of a large volume of granitic magma. Only the U-Pb zircon and the Rb-Sr whole-rock ages survived this event, and both types are discordant. A two-stage model that explains the U-Pb discordant ages combines a primary discordance produced during the metamorphism of 2650 m.y. ago with a secondary discordance developed approximately 100 m.y. ago when uplift and erosion brought the rocks close to the surface. This secondary discordance is also shown by the zircon from granite near Sacred Heart (2650 m.y. old) and from a younger granitic pluton (1850 m.y. old) near Granite Falls. The discordance in the Rb-Sr whole-rock ages is attributed primarily to the loss of radiogenic Sr 87 that probably occurred largely during the metamorphism of 2650 m.y. ago. Some later loss, however, is indicated in the younger ages of biotite and K-feldspar. Granitic material introduced or mobilized during the metamorphism is also a complicating factor. The 1850-m.y.-ago event was a low-grade metamorphism that reset the K-Ar and Rb-Sr ages of biotite in the rocks between Granite Falls and Ortonville. A number of small plutons, ranging in composition from gabbro to granite, and basaltic dikes were emplaced in the gneisses at this time, but only the granitic pluton near Granite Falls has been dated by both U-Pb and Rb-Sr methods. The mineral ages show variations that are difficult to explain, and the low apparent ages of the biotite may be in some way related to epeirogeny and the stabilizing of the K-Ar and Rb-Sr systems. The southeastern part of the valley, underlain by the Morton Gneiss and the granite at Sacred Heart, was stabilized 2400 to 2600 m.y. ago, but the northwestern part, underlain by gneiss in the Granite Falls-Montevideo area and by granite in the Ortonville area, was not stabilized until 1700 to 1850 m.y. ago. The Morton Gneiss was formed by synkine-matic intrusions of trondhjemitic and granitic magmas, and the structure dates back to the time of the intrusions, 3550 m.y. ago. A similar origin as a synkinematic intrusion of granite is favored to explain the gneiss at Montevideo. The country rock appears to have been a layered series of basaltic lavas, sedimentary rocks, and possibly some sill-like masses of diabase or gabbro. The structure of the region probably was considerably modified during the high-grade metamorphism 2650 m.y. ago. The rock types that were involved in the Mortonian event 3550 m.y. ago are similar to more recent crustal rocks and do not represent a protocrust.

Effects of weathering on the UPb ages of zircon from the Morton Gneiss, Minnesota

Abstract Weathering has caused large losses of lead from the zircon in the residual clay derived from the Morton Gneiss of southwestern Minnesota, drastically reducing the206Pb/238U and the207Pb/235U ages. The207Pb/206Pb age probably has not been significantly affected. Loss of lead by leaching during weathering has not been adequately considered in explanation of discordant ages of zircon.

Ages of Batholithic Intrusions of Northern and Central Chile

Their stratigraphic setting demonstrates that three batholithic intrusions in northern and central Chile are respectively pre-Jurassic, Jurassic, and Cretaceous. Radiometric age determinations, by the lead-alpha (Larsen) method, indicate ages of 265 ± 30, 120 ± 15 to 125 ± 15, and 95 ± 10 to 130 ± 15 million years respectively for samples from the batholiths whose stratigraphic setting has been established or is inferred. With one exception, the radiometric ages are consistent with the stratigraphy. The exceptional determination of 130 ± 15 million years was for a sample of granite that intrudes Cretaceous rocks. Three other samples of Cretaceous granitic rocks were within the range of 95 ± 10 to 105 ± 10 million years. Angular unconformities whose ages are correlative with the intrusions suggest that the three batholiths can be related respectively to Hercynian, Late Jurassic, and middle Cretaceous orogenies.

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.

Age of emplacement of the Okanogan gneiss dome, north-central Washington

Contact relations and internal fabric suggest that penetrative metamorphism and deformation of parental rocks of the Okanogan gneiss dome culminated in their mobilization and diapiric intrusion into the country rocks. The gneiss dome cuts both plutonic rocks and eugeosynclinal low-grade metamorphic rocks, including rocks as young as Late Triassic, but not adjacent sedimentary deposits and associated volcanic rocks of Eocene age. The gneiss dome is itself cut by the Swimptkin Creek pluton, an epizonal quartz monzonite body that yields concordant biotite and hornblende K-Ar ages of 48.0 and 48.2 m.y. (Eocene), respectively. Thus, the field relations bracket the age of emplacement of the gneiss dome between Late Triassic and Eocene time. A total of 21 age determinations were made using samples from seven localities within the gneiss dome. Zircon dated by U-Th-Pb methods gave the oldest ages, including Pb 206 /U 238 and Pb 207 /U 235 ages of 87.3 and 100.0 m.y., respectively. Fission-track ages of 66 m.y. (sphene), 63 m.y. (epidote), 59 m.y. (allanite), 53 m.y. (apatite), and 51 m.y. (apatite) were obtained, along with K-Ar ages ranging from 58 m.y. (hornblende) to 46 m.y. (biotite). These data suggest that the gneiss dome was emplaced in Late Cretaceous time — probably between roughly 87 and 65 m.y. ago — then cooled slowly through the successive temperature thresholds for sphene, epidote-allanite, hornblende, and finally apatite-muscovite-biotite, below which either loss of argon or erasure of fission tracks ceased.

Boulder Creek Batholith, Colorado Part II: Isotopic Age of Emplacement and Morphology of Zircon

Zircon separated from six rocks whose compositions spanned the range of differentiation in the Boulder Creek batholith yielded a “discordia” age of emplacement of 1725 m.y., close to the average PB 207 /Pb 206 age 1720 m.y.) and indicating that the constituent rocks are cogenetic within approximately ± 20 m.y. Statistical studies show that from 20 to 80 percent of the zircon in any one sample (1) is no-neuhedral, (2) has lower (length/width) ratios than the associated euhedral zircon, and (3) in direct contrast to the euhedral, increases markedly interior of the batholith toward contacts with the older metasediments and internal zones of contamination; it is inferred to have been “inherited” via assimilation. Unlike the noneuhedral fraction the euhedral zircon shows a linear decrease in length/width ratio with an increase in SiO 2 content of the containing rocks; it is inferred to be magmatic in origin. Regardless of the relative abundance of inherited versus magmatic zircon, all samples closely fit a single discordia chord, indicating that both zircon fractions formed at about the same time. This conclusion is compatible with field relationships that indicate the emplacement of the syntectonic Boulder Creek rocks took place during a period of metamorphism notable for the widespread development of new minerals in the country rocks. (1) Zircon from a Silver Plume Granite dike intruding the Boulder Creek batholith, (2) zircon from Silver Plume correlatives immediately to the north (Tilton and co-workers), and (3) uraninite from a probable Silver Plume correlative in the Central City district, together yield a sharply defined discordia age of emplacement of 1415 m.y. The separate “discordia” chords for the Boulder Creek and Silver Plume zirconconverge close to their lower intercepts with “concordia” indicative of a one-step lead loss during the Laramide disturbance. The Silver Plume “thermal event” left no age imprint on the Boulder Creek zircon, presumably because insufficient time had elapsed to produce significant metamictization.

Geochronology of the St. Kevin Granite and Neighboring Precambrian Rocks, Northern Sawatch Range, Colorado

Radiometric ages have been measured on rocks of a crystalline terrane that includes ancient gneisses and migmatites, two granitic batholiths (St. Kevin Granite and granite of Cross Creek), and various minor intrusive rocks. A whole-rock Rb-Sr isochron age on the St. Kevin Granite establishes it as 1390 ± 60 m.y. old. Mineral ages on the St. Kevin and numerous other rocks are either about the same as the St. Kevin whole-rock age or younger by as much as 200 m.y., even where the relative age is known to be older. Some minor Precambrian intrusive masses that are probably younger than St. Kevin Granite yield mica ages within analytical error of the St. Kevin age, indicating that these rocks can be younger than the granite by only a few tens of millions of years. The mica ages, both Rb-Sr and K-Ar, are thought to be minimal, but a K-Ar age of 2020 m.y. on horn-blende probably reflects excess argon. Mica ages from all rocks known geologically to be older than St. Kevin Granite are low and are interpreted as heating ages reflecting intrusion of the granite, in some cases modified further by heating during Laramide time. In this area, Precambrian intrusion and deformation had largely ended by 1200 or 1300 m.y. ago. Plutonism, represented here by the St. Kevin Granite and elsewhere by the Silver Plume and other granites, probably accounts for the numerous mineral ages of about 1300 m.y. previously reported from Colorado although weak regional metamorphism may also have been a factor.

Oligocene or Younger Thrust Faulting in the Ruby Mountains, Northeastern Nevada

A klippe of unmetamorphosed Devonian carbonate rocks rests on the Harrison Pass intrusive body south of Toyn Creek in the central Ruby Mountains, Elko County, Nevada. This klippe and other klippen of Carboniferous strata—first mapped by R. P. Sharp (1942)—that rest on lower Paleozoic strata are believed to represent a once-continuous thrust sheet that developed after emplacement of the intrusive body. This intrusion of coarse-grained granodiorite to quartz monzonite is exposed over an area of about 45 square miles. Potassium-argon and lead-alpha age determinations on four samples of the intrusive body establish Oligocene or younger age for the thrusting. Potassium-argon age determinations on biotites from the four samples range from 29 to 36 m.y. with a possible analytical error of ±10 percent. Lead-alpha age determinations (all with a possible analytical error of ±10 m.y.) on zircon showed 40 m.y. for three of the samples and 30 m.y. for the fourth. Lead-alpha and potassium-argon dates on a fifth sample collected by R. R. Coats also fall within these ranges. The agreement of the radiometric dates indicates a lack of thermal activity subsequent to the emplacement of the intrusion and establishes a maximum age of Oligocene for the thrust faulting. The present distribution of thermally metamorphosed Paleozoic rocks in the Ruby Mountains seems to preclude the possibility that the thrust plate originated within the Ruby Mountains. Sharp suggested a western source with displacement of from 7 to 10 miles.

Boulder Creek Batholith, Colorado Part III. Fingerprinting Discordant Zircon Ages in a Complex Intrusion

The apparent ages (32 lead/alpha and 6 Pb 206 /U 238 ) of zircon as plotted on an isochron map of the Boulder Creek batholith define the following pattern: (1) very high ages (1600 to 1900 m.y.) within the outermost border zone on the southwest, south, and southeast; (2) transitional high ages (1300 to 1600 m.y.) within an inner border zone on the east and widening to the south and west to include about one-third of the batholith; (3) transitional low ages (1000 to 1300 m.y.) throughout much of the interior of the northern half; and (4) very low ages (1000 m.y. or less) limited to a small area within the northeast corner. The area of minimum age is shown to be part of the reduced-age aureole surrounding the 77 m.y. hornblende granodiorite stock at Jamestown that intrudes the Silver Plume Granite of the Longs Peak-St. Vrain batholith in the region immediately to the north of the minimum-age area of the Boulder Creek batholith. A southeastward elongation of the area of minimum age is attributed to channelway control of the solutions responsible for the recrystallization of the zircon by those northwest-trending breccia reefs that are cut by, or strike toward, the Laramide intrusion. Statistical studies of five zircon separates used for isotopic work showed that the frequency of grains having partial, or complete, rims of colorless zircon on purple to semiopaque zircon cores increased inversely with measured Pb 206 /U 238 age along a smooth curve that, when extrapolated, connected the point representing age of emplacement (0 percent rims) and the point representing the approximate age of re-crystallization (100 percent rims). Consequently, in the Boulder Creek zircon rim frequency gives a useful estimate of the amount of lead lost relative to uranium and thorium from a given sample during its recrystallization. The microstudy indicated: (1) the surface separating core and rim is a major discontinuity; (2) the greater part of the rims appear to be true overgrowths; and (3) the highest frequency of rims is found in the most metamict zircon. However, in any one sample a significant fraction of the most metamict zircon has been sheltered from reaction, presumably by inclusion within relatively impervious minerals, and remains free of rims. These observations coupled with the map evidence of selective channelway control point to warm solutions rather than dry heat as the agent of recrystallization and lead loss.