Early Paleozoic plutonism in the central Virginia Piedmont consists of the igneous Lahore Complex and Ellisville Pluton. The Lahore Complex consists of a small altered mafic pluton intruded by the shoshonitic, alkalic monzonites of the Lahore Pluton (-450 Ma) that, in turn, is intruded by the calc-alkaline, granodioritic Ellisville Pluton (-440 Ma). These plutons were emplaced at about 760aC at a depth of 12 to 18 km within greenschist-facies rocks and are enclosed by contact-metamorphosed rocks.

The Idono Complex of west-central Alaska is a fault-bounded, approximately 300 km2 fragment of Early Proterozoic continental crust surrounded by overlap assemblages and younger terranes accreted in Mesozoic time. It is composed of granitic to dioritic orthogneiss, amphibolite, and metasedimentary rocks. Trace element compositions of the granitoids and amphibolite suggest rock formation in a subduction-related volcanic arc terrain. Nine zircon fractions from three samples of granitoid orthogneiss define a U-Pb discordia line intersecting concordia at 2062 +/- 7 Ma and 182 +/- Ma. The upper and lower intercepts are interpreted, respectively, as approximations of the time of granitoid crystallization and major episodic Pb-loss. The lower intercept age is similar to that indicated by some hornblende and biotite K-Ar ages. Other biotite, hornblende, and white mica K-Ar ages record later isotopic disturbance in Early Cretaceous time. Nd isotopic compositions at 2.06 Ga for tonalite orthogneiss (epsilon-Nd(t) = +2.8) and amphibolite (epsilon-Nd(t) = +1.8 and +2.3) indicate that Early Proterozoic crust formation in the Idono Complex involved significant additions of mantle-derived magma. These rocks yield T(Sm-Nd) (depleted mantle) model ages of 2.0 to 2.1 Ga, similar to the U-Pb zircon upper intercept age. Involvement of Archean crust in the formation of at least some rocks of the Idono Complex is indicated by a granitic orthogneiss (epsilon-Nd(t) = -4.6), which yields a T(Sm-Nd) model age of 2.5 Ga. The approximately 2.06-Ga age of the Idono Complex is similar to that indicated for provenance(s) of widely distributed sedimentary rocks in western North America and may represent a fragment of the North American craton displaced by northward movement of the Pacific plate. At such, rocks of the Idono Complex may provide important insights into both Early Proterozoic evolution along the craton margin, and subsequent displacements.

New U/Pb zircon ages from the Ray Mountains of central Alaska clarify the plutonic history of the Ruby geanticline and support earlier suggestions that the Ruby geanticline and southern Brooks Range were once parts of the same tectonostratigraphic terrane. U/Pb zircon ages of 109 to 112 Ma from the Ray Mountains pluton confirm previously reported mid-Cretaceous K/Ar ages and rule out the possibility that the earliest intrusive phase of the pluton is older than mid-Cretaceous. New U/Pb zircon ages from four granite gneiss samples in the Ray Mountains indicate a Devonian protolith age of 390 ± 12 Ma and suggest that the Ruby geanticline, like the southern Brooks Range, underwent a major plutonic event in mid-Paleozoic time.

Mississippian Fossils from Southern Appalachian Metamorphic Rocks and Their Implications for Late Paleozoic Tectonic Evolution

In an investigation termed the Skagway Traverse, a geologic map was made of an area about 70 km long and l0 to 15 km wide transecting the Coast batholith (Coast Plutonic Complex of most Canadian workers) from Haines to Skagway, Alaska, to Log Cabin and Tutshi Lake, British Columbia. The batholith here consists of (l) Cretaceous or older migmatitic orthogneiss of quartz dioritic and tonalitic compositions and minor metasedimentary rocks; (2) minor, Late Cretaceous or older nonmigmatitic orthogneiss of tonalitic composition; (3) Late Cretaceous (zircon U-Pb age : 72 m.y.) feldspar-porphyritic biotitehornblende granite-the granite of Log Cabin; (4) Eocene (zircon U-Pb age : 54 m.y.) massive plutons of hornblende-biotite tonalite and granodiorite-the tonalite of Skagway; and (5) Eocene (zircon U-Pb ages are 53 and 48 m.y.) massive plutons of near-minimummelt biotite leucogranite-the granites of Clifton and Summit Lake. The orthogneisses and tonalite of Skagway show typical calc-alkaline, Pacific-margin compositions:59-7lo/o SiOr, 18.6-15.4o/o Al2Or,6-30/oCaO,l.5-3.10/o IlO, 830-480 ppm Sr, 65-45 ppm Rb, and chondrite-normalized rare-earth-element (REE) patterns of moderate slope and small negative or no Eu anomaly. The granites of Clifton and Summit Lake have higher SiOr(73.9-77.2o/o),KrO (4.74.2o/o), and Rb (85-219 ppm), less AlrO3 (14.012.3o/o), CaO (1.6-1.10/o) and Sr (210-85 ppm), and large negative Eu anomalies and flat slopes for heavy REE. The granite of Log Cabin is transitional between these two types. Initial 87Sr/t6Sr ratios for most plutonic rock units are 0.7052 to 0.7058; Summit Lake Granite is 0.7049 and 0.7060. They show no systematic variation related to rock type or location. The orthogneisses and tonalite ofSkagway show a rough trend on the normative quartzplagioclase-orthoclase diagram from near dioritic composition toward granodiorite and granite. These rocks probably originated largely by fractionation ofplagioclase, hornblende, and other phases. However, oxygen-isotope ratios indicate either exchange of the magmas with a high 'tO/I6O fluid or minor assimilation of high-6'tO crustal rocks. The granites of Log Cabin, Clifton, and Summit Lake, however, probably were derived largely by crustal melting of graywacke (flysch) or siliceous to intermediate igneous rocks.

In the Eastern United States, Precambrian rocks are exposed in the Adirondack massif and in the Appalachian orogen.Rocks dated at 1,300-1,000 m.y.occur as outliers of the Grenville province of Canada.These rocks constitute a western basement to the Appalachian orogen and appear in the Adirondack massif, in anticlinoria along the Blue-Green-Long axis, and in gneiss domes farther east.The Chain Lakes massif in Maine, about 1,500 m.y.old, may represent a different block of continental crust.Rifting of the Grenville continental mass, accompanied by anorogenic igneous activity, began about 820 m.y.ago.An eastern basement (750-650 m.y.old) has been identified in the Avalonian zone of the Appalachian orogen in southeastern New England.It is intruded by calc-alkaline granitic rocks roughly 600-640 m.y.old that are nonconformably overlain by Lower and Middle Cambrian strata.Extensive Precambrian stratified rocks (mostly younger than 610 m.y.but possibly including rocks as old as 725 m.y.) crop out in the Carolina volcanic slate belt and probably in the higher grade metamorphic belt east of that.These rocks include large volumes of felsic volcanic rocks and probably were deposited on a continental crust.The eastern terranes were sutured to ancestral North America during the Appalachian orogenic events that closed the lapetus Ocean basin.The location of the suture is uncertain, but it probably lies just east of the gneiss domes containing rocks older than 1 b.y.'Synthesized stratigraphic columns to which the isotopic ages are tied are identified by circled capital letters on pi. 1. Related stratigraphic columns are indicated by subscripts.The generalized geographic locality of every constructed or synthesized stratigraphic column is shown on pi. 2 but because of space constraints not all are shown on pi. 1.

Several suites of plutonic rocks have been identified by geologic mapping in the polydeformed and metamorphosed (amphibolite-grade) Piedmont near Fredericksburg, Va.Two of these suites were dated by U/Pb (zircons) and Rb/Sr (whole-rock) methods.The oldest suite is the Falls Run Granite Gneiss, a coarse-grained, strongly foliated, and highly metamorphosed rock that ranges in composition from granite to monzonite.The chief mass of Falls Run Granite Gneiss (formerly called the Berea pluton) is intrusive into the Holly Corner Gneiss of Early Cambrian(?)age.Both these gneisses are allochthonous remnants of the inverted limb of a recumbent fold; subsequent deformation formed a type-2 interference fold.U/Pb and Rb/Sr studies indicate that the Falls Run is 410 million years old and has an initial 87 Sr/86 Sr of 0.7070.Younger granitoid plutons, dikes, and sills, assigned to the Falmouth Intrusive Suite, are widespread in the area.These plutons are abundant in the eastern part of the area but are rare west of the Quantico Formation.Rocks of the Falmouth consist of strongly to weakly foliated: A, biotite adamellite and granodiorite having Rb/Sr less than 0.2, and B, muscovite-biotite adamellite and granite having Rb/Sr greater than 0.4.Concordant zircon ages and two whole-rock isochrons indicate that both groups are 300-325 million years old.The initial 87 Sr/86Sr of group A is 0.704, which suggests a lower crust or mantle source, whereas that of group B is 0.7088, similar to the 0.7070 for the Falls Run, and suggests crustal involvement in the magma generation.Recently reported ages from North and South Carolina are similar to those of the Falmouth.Thus, an extensive belt of 300-325 million-yearold plutons is present in the eastern Piedmont.

Sixty-two samples from well-established comagmatic granitoid sequences and certain unassigned formations and plutons of the central part of the Sierra Nevada batholith between latitudes 37° and 38° N. have been dated by the isotopic U-Pb method on zircon.The U-Pb ages indicate the following age distribution of the granitoids: (1) The axial part of the batholith is occupied by Cretaceous granitoid sequences that are progressively younger eastward over a 37-m.y.interval extending from about 125 m.y. to about 88 m.y.ago.(2) A single, but extensive, Triassic sequence with an optimum average age of about 210 m.y. is present in the east side of the batholith.(3) Plutons and granitoid sequences of Jurassic age, most of them with U-Pb ages between 186 and 155 m.y., occur in both margins and locally in the interior of the batholith.The distribution of Jurassic ages suggests that prior to the emplacement of the Cretaceous granitoids, Jurassic granitoids were widely distributed across the central Sierra Nevada but were not emplaced in a west-to-east succession as were the Cretaceous granitoids.Few of our ages fall between 155 and 125 m.y.However, a U-Pb age of 144 m.y. has been reported on the Sage Hen Flat pluton in the White Mountains, and U-Pb ages between 134 and 128 m.y.have been reported on remnants of older granitoids farther south in the Sierra Nevada, which are associated with roof pendants and septa.Also, numerous K-Ar ages on hornblende in the range of 152 to 131 m.y.have been reported on samples collected farther north along the west side of the batholith.The distribution of U-Pb ages is consistent with the interpretation that in the central Sierra Nevada, a belt of Cretaceous granitoids trending about N. 20° W. crosses a belt of Jurassic granitoids trending about N. 40° W.However, the U-Pb ages provide little support for the existence of five cyclic intrusive epochs for California and western Nevada.Comparison of the U-Pb ages on zircon with the K-Ar ages on biotite and hornblende shows generally good agreement for the younger granitoids but decreasing agreement for increasingly older granitoids.Most of the K-Ar ages on biotite and many on hornblende from older granitoids appear to have been reduced as a result of reheating by younger plutons.The dispersion of K-Ar ages reflects the complex structural and thermal history of the batholith.

The RbSr and UPb methods were used to study gneisses in the 712-minute Lake Helen quadrangle of the Big Horn Mountains, Wyoming. Two episodes of magmatism, deformation and metamorphism occurred during the Archean. Trondhjemitic to tonalitic orthogneisses and amphibolite of the first episode (E-1) are cut by a trondhjemite pluton and a calc-alkaline intrusive series of the second episode (E-2). The E-2 series includes hornblende-biotite quartz diorite, biotite tonalite, biotite granodiorite and biotite granite.

Research Article| September 01, 1976 Age of emplacement of the Okanogan gneiss dome, north-central Washington K. F. FOX, JR.; K. F. FOX, JR. 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar C. D. RINEHART; C. D. RINEHART 1U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar J. C. ENGELS; J. C. ENGELS 2348 Waverly Street, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar T. W. STERN T. W. STERN 3U.S. Geological Survey, Reston, Virginia 22092 Search for other works by this author on: GSW Google Scholar Author and Article Information K. F. FOX, JR. 1U.S. Geological Survey, Menlo Park, California 94025 C. D. RINEHART 1U.S. Geological Survey, Menlo Park, California 94025 J. C. ENGELS 2348 Waverly Street, Menlo Park, California 94025 T. W. STERN 3U.S. Geological Survey, Reston, Virginia 22092 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1976) 87 (9): 1217–1224. https://doi.org/10.1130/0016-7606(1976)87<1217:AOEOTO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation K. F. FOX, C. D. RINEHART, J. C. ENGELS, T. W. STERN; Age of emplacement of the Okanogan gneiss dome, north-central Washington. GSA Bulletin 1976;; 87 (9): 1217–1224. doi: https://doi.org/10.1130/0016-7606(1976)87<1217:AOEOTO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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 Pb206/U238 and Pb207/U235 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Research Article| June 01, 1971 Boulder Creek Batholith, Colorado Part III. Fingerprinting Discordant Zircon Ages in a Complex Intrusion GEORGE PHAIR; GEORGE PHAIR U. S. Geological Survey, Washington, D. C. 20242 Search for other works by this author on: GSW Google Scholar THOMAS W STERN; THOMAS W STERN U. S. Geological Survey, Washington, D. C. 20242 Search for other works by this author on: GSW Google Scholar DAVID GOTTFRIED DAVID GOTTFRIED U. S. Geological Survey, Washington, D. C. 20242 Search for other works by this author on: GSW Google Scholar Author and Article Information GEORGE PHAIR U. S. Geological Survey, Washington, D. C. 20242 THOMAS W STERN U. S. Geological Survey, Washington, D. C. 20242 DAVID GOTTFRIED U. S. Geological Survey, Washington, D. C. 20242 Publisher: Geological Society of America Received: 24 Jul 1970 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1971, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1971) 82 (6): 1635–1656. https://doi.org/10.1130/0016-7606(1971)82[1635:BCBCPI]2.0.CO;2 Article history Received: 24 Jul 1970 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation GEORGE PHAIR, THOMAS W STERN, DAVID GOTTFRIED; Boulder Creek Batholith, Colorado Part III. Fingerprinting Discordant Zircon Ages in a Complex Intrusion. GSA Bulletin 1971;; 82 (6): 1635–1656. doi: https://doi.org/10.1130/0016-7606(1971)82[1635:BCBCPI]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The apparent ages (32 lead/alpha and 6 Pb206/U238) 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 Pb206/U238 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. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Research Article| June 01, 1971 Boulder Creek Batholith, Colorado Part II: Isotopic Age of Emplacement and Morphology of Zircon THOMAS W STERN; THOMAS W STERN U. S. Geological Survey, Washington, D.C. 20242 Search for other works by this author on: GSW Google Scholar GEORGE PHAIR; GEORGE PHAIR U. S. Geological Survey, Washington, D.C. 20242 Search for other works by this author on: GSW Google Scholar MARCIA F NEWELL MARCIA F NEWELL U. S. Geological Survey, Washington, D.C. 20242 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1971) 82 (6): 1615–1634. https://doi.org/10.1130/0016-7606(1971)82[1615:BCBCPI]2.0.CO;2 Article history received: 24 Jul 1970 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation THOMAS W STERN, GEORGE PHAIR, MARCIA F NEWELL; Boulder Creek Batholith, Colorado Part II: Isotopic Age of Emplacement and Morphology of Zircon. GSA Bulletin 1971;; 82 (6): 1615–1634. doi: https://doi.org/10.1130/0016-7606(1971)82[1615:BCBCPI]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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 PB207/Pb206 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 SiO2 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

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.

Research Article| September 01, 1970 Radiometric Ages and Stratigraphic Sequence of Volcanic and Plutonic Rocks, Southern Nye and Western Lincoln Counties, Nevada RICHARD F MARVIN; RICHARD F MARVIN U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar F. M BYERS, JR.; F. M BYERS, JR. U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar HARALD H MEHNERT; HARALD H MEHNERT U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar PAUL P ORKILD; PAUL P ORKILD U.S. Geological Survey, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar T. W STERN T. W STERN U.S. Geological Survey, Washington, D.C. 20242 Search for other works by this author on: GSW Google Scholar Author and Article Information RICHARD F MARVIN U.S. Geological Survey, Denver, Colorado 80225 F. M BYERS, JR. U.S. Geological Survey, Denver, Colorado 80225 HARALD H MEHNERT U.S. Geological Survey, Denver, Colorado 80225 PAUL P ORKILD U.S. Geological Survey, Denver, Colorado 80225 T. W STERN U.S. Geological Survey, Washington, D.C. 20242 Publisher: Geological Society of America Received: 25 Aug 1969 Revision Received: 09 Mar 1970 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1970, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1970) 81 (9): 2657–2676. https://doi.org/10.1130/0016-7606(1970)81[2657:RAASSO]2.0.CO;2 Article history Received: 25 Aug 1969 Revision Received: 09 Mar 1970 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation RICHARD F MARVIN, F. M BYERS, HARALD H MEHNERT, PAUL P ORKILD, T. W STERN; Radiometric Ages and Stratigraphic Sequence of Volcanic and Plutonic Rocks, Southern Nye and Western Lincoln Counties, Nevada. GSA Bulletin 1970;; 81 (9): 2657–2676. doi: https://doi.org/10.1130/0016-7606(1970)81[2657:RAASSO]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.