Quartz Monzonite Pluton Research Articles

40Ar/39Ar age spectra for biotite and hornblende from plutonic rocks in the Victorville region, California

40Ar/39Ar data from plutonic rocks in the Mojave Desert near Victorville, California, provide preliminary insight into the complex thermal history of this region and suggest that some plutons here may be significantly older than any plutons in the Sierra Nevada to the north. Near Victorville, multiple deformation and metamorphism of platform-miogeoclinal strata is inferred to postdate deposition of Permian rocks and predate intrusion of monzonite plutons. Monzonite that intrudes multiply deformed Paleozoic marble and that is unconformably overlain by the Mesozoic (Triassic or Jurassic) Fairview Valley Formation has hornblende with an age spectrum typical of severe 40Ar loss. We interpret the age of the highest temperature increment (fusion), 233 ± 14 m.y., as a minimum age for this pluton. The total gas age for the hornblende is only 169.9 m.y. Biotite from the same rock exhibits a plateau age of 76.59 ± 1.24 m.y. The Fairview Valley Formation is overlain by volcanic rocks of the Sidewinder sequence. These rocks were folded and intruded by quartz monzonite that postdates all penetrative deformation in the region. Biotite-hornblende tonalite at Quartzite Mountain is part of one of these younger quartz monzonite plutons, and its biotite yields a plateau age of 73.16 ± 1.11 m.y. This age is the same at the 95% confidence level as the age of biotite from the older monzonite; thus, this younger thermal event was at least the partial cause for the argon loss pattern displayed by the monzonite hornblende. Hornblende gabbro that intrudes Paleozoic marble at Sidewinder Mountain and is in turn contact metamorphosed by later plutonic rocks has hornblende with a disturbed 40Ar/39Ar age spectrum. The 1025, 1050, and 1075 °C steps contain 44.64% of the total argon and yield a common age of 119.2 ± 3.4 m.y., which we interpret as the best estimate for the age of the hornblende.

Paleomagnetic documentation of the early, high-temperature zone of mineralization at Butte, Montana

Butte quartz monzonite, host rock to mineralization in the Butte district, Montana, was pervasively altered to a high-temperature K-silicate alteration assemblage during pre-Main Stage, porphyry-type mineralization within the deep levels of the district. Magnetization directions of steep, negative inclination (reverse polarity) are characteristic of Butte quartz monzonite within the zone of pervasive pre-Main Stage alteration and mineralization. Butte quartz monzonite in surrounding, less altered parts of the district and farther from Butte contains only steep, positive inclination characteristic directions (normal polarity). Magnetic, petrographic, and radiometric evidence together with the exact correlation of reversed polarity directions with high-temperature pre-Main Stage alteration assemblages suggest that, in the deep, central portion of the Butte district, the original normal polarity thermoremanent magnetization was completely unblocked by heating to temperatures in excess of 578 degrees C. A secondary thermoremanent magnetization was acquired in subsequent cooling in a field of reversed polarity prior to Main Stage, fracture-controlled mineralization. The detection of reversed polarity directions in a portion of the Butte quartz monzonite pluton serves as a method for delineating the extent of a high-temperature (greater than 578 degrees C) thermal perturbation during mineralization at Butte.

Two hundred and twenty million years of Archean evolution: a zircon U–Pb age stratigraphic study of the Uchi–Confederation Lakes greenstone belt, northwestern Ontario

Three metavolcanic units in the Uchi–Confederation Lakes greenstone belt have been dated by the zircon U–Pb method. Ages obtained reveal a time interval of [Formula: see text] separating the cycle I crystal tuff (2958.6 ± 1.7 Ma) from the cycle III rhyolite ([Formula: see text]). A cycle II rhyolite about 2794 Ma old lies between the other two units. Uranium-rich xenocrystic cores occur in the cycle I zircons and are indirect evidence of preexisting sialic material.A metamorphosed granodiorite, intrusive into the cycle III rhyolite, is about the same age as the rhyolite. A post-metamorphic quartz monzonite pluton is dated at 2729.6 ± 1.3 Ma. A period of dynamothermal metamorphism in this portion of the Superior Province indirectly dated at [Formula: see text] reflects the Kenoran orogeny.

A Claim of High Level Granitization Refuted

Poorly lithified Upper Devonian conglomerates do not exhibit a gradational granitized contact with the high level Belleoram quartz monzonite pluton in southern Newfoundland as previously maintained, and there is no prima facie case for previously suggested granitization by massive magmatic vapour transport.

Revised Rb–Sr Age of the Lac du Bonnet Quartz Monzonite, southeastern Manitoba

New whole-rock Rb–Sr isotopic data from the Lac du Bonnet Quartz Monzonite pluton, combined with previously published analyses, yield an increased date for that pluton, the latest stage of major intrusion in this section of the Archean, English River Gneissic Belt. The age of the pluton and the associated metamorphism is revised from 2495 ± 180 m.y. to 2680 ± 91 m.y.

Geochronology of Archean and Proterozoic Rocks in the Southern District of Keewatin

Rb–Sr and U–Pb dating techniques have been utilized to identify and date Archean supracrustal rocks within the Churchill structural province in regions where K–Ar age determinations have recorded only the effects of younger Hudsonian orogeny. The age of emplacement of Archean granodiorite has been established at 2550 m.y., a determination that also provides a minimum age for volcanic rocks intruded by the granodiorite.The overlying Proterozoic Hurwitz Group volcanic rocks have been dated for the first time at 1808 ± 35 m.y. (Upper Aphebian). A post-Hurwitz Group quartz monzonite pluton intruded the granodiorite gneiss at 1772 ± 22 m.y. and the age of the post-tectonic Nueltin Lake Granite has been established at 1700 ± 16 m.y. (Paleohelikian).It is concluded that the Hurwitz Group cannot be correlated with the Huronian succession in Ontario as the Hurwitz Group rocks are 300 to 400 m.y. younger than the Huronian strata.

O18/O16 ratios of the Johnny Lyon granodiorite and Texas Canyon quartz monzonite plutons, Arizona, and their contact aureoles

O18/O16 ratios have been measured for 29 quartz samples, 6 whole-rocks, 3 muscovites, and 1 K-feldspar from two adjacent granitic plutons of vastly different age (about 1660 m.y, and 70 m.y.) intruded into the same type of country rock, the Precambrian Pinal schist. Sample traverses were made across 3 different contact zones of these intrusive bodies. Except for 2 quartz veins with δO18=+11.0 and + 12.3, all quartz samples collected more than 15 cm from the margin of the Early Tertiary Texas Canyon pluton are isotopically exceedingly uniform with δO18=9.47±0.11. Four quartz samples collected more than 10 m from the margin of the Precambrian Johnny Lyon pluton have δO18=10.43±0.08. Compared with previous studies of this type, only relatively minor O18-enrichments have occurred in the border zones of the plutons. This is in part because the original δO18 differences between the metasedimentary rocks and the intrusives are relatively small (only 3 to 6‰), but is mainly due to the lack of H2O in the contact zones during intrusion as a result of the general impermeability and prior dehydration of the schist. There is no isotopic evidence for significant influx of “external” H2O into either of the plutons during their crystallization and cooling. However, in roof-zones where metasedimentary rocks overlie the plutons there is a strong δO18 lowering in the contact metamorphic aureoles, indicating upward expulsion of low-O18 “magmatic” H2O into these rocks.

Correlation of uranium, thorium, and potassium with aeroradioactivity in the Berea area, Virginia

In the Berea area, a small quartz monzonite pluton intrudes chlorite-actinolite schist and is overlapped by Coastal Plain sand and gravel deposits. A detailed aeroradioactivity survey of the area shows unusually high radioactivity (1,150 counts per second) over the quartz monzonite. A total of 22 auger samples was taken across all formations to a depth of 2 to 3 feet, and radioelement concentrations were determined by gamma-ray spectrometry. There is a general direct correlation of high radioactivity with increase in radioelement concentrations. Abundances of uranium and particularly thorium in the quartz monzonite are two and three times the average for rocks of this type; a comparison is made with other granitic rocks high in uranium and thorium.

Mesozoic Granitic Rocks in Northwestern Nevada: A Link between the Sierra Nevada and Idaho Batholiths

Extensive areas in northwestern Nevada are underlain by granodiorite and quartz monzonite plutons, as well as less common smaller bodies of quartz diorite. Twenty-six K/Ar age determinations on rocks from this suite range from about 175 to 85 m.y., but most of the plutons are between 105 and 85 m.y. old. This Late Cretaceous intrusive epoch extending from 105 to 85 m.y. ago is here named the Lovelock intrusive epoch. Twenty-three whole-rock chemical analyses show that the granitic rocks of northwestern Nevada form a homogeneous differentiation series with a narrow range in major element distribution. The granitic plutons of northwestern Nevada are chemically and petrographically indistinguishable from granitic intrusives of equivalent age in the Sierra Nevada and Idaho batholiths, and form a link between these two major batholiths.

Potassium-Argon dates on minerals from the Mt Falconer area, Lower Taylor Valley, South Victoria Land, Antarctica

Abstract In this note we report K-Ar dates on biotite and hornblende separated from igneous and metamorphic rocks of the Mt Falconer area (77° 34′ S, 163° 08′ E) of the Lower Taylor Valley, South Victoria Land. The study was undertaken to obtain the age of emplacement of the Mt Falconer quartz monzonite pluton as well as setting a limit on the age of metamorphism in this area.

Precambrian geochronology in the Unaweep Canyon, west-central Colorado

Precambrian rocks exposed in the Unaweep Canyon, west-central Colorado, include gneisses of probable metasedimentary origin, a batholith of coarse porphyritic granodiorite, a pluton of quartz monzonite, a pluton of granite, and dikes of granitic pegmatite and aplite. Field relations show that the batholith intruded the gneisses, that the two plutons intruded the batholith, and that all these units were intruded by the dikes. The following isochron ages have been obtained by Rb-Sr analyses of whole-rock samples: batholith of quartz monzonite porphyry, 1694±108 m.y.; biotite-muscovite granite pluton, 1472±57 m.y.; muscovite quartz monzonite pluton, 1479±60 m.y.; pegmatite dikes, 1408±52 m.y. These ages are consistent with numerous reports of igneous activity occurring at these times in Colorado and elsewhere in the southwestern United States.

Geology of the Mt Falconer Pluton, Lower Taylor Valley, South Victoria Land, Antarctica

Abstract The pre-Beacon Group rocks of the Mt Falconer area (about 77° 30'S, 163° E), Antarctica, consist of, in decreasing order of age: (a) Skelton Group metasedimentary rocks (amphibolite facies); (b) granodiorite gneiss; (c) diorite “hybrid”, microdiorite, and granophyre dikes; (d) Mt Falconer epizonal quartz monzonite pluton (about 2 square miles in outcrop area); and (e) camptonite dikes. The pluton and the dikes of units (c) and (e) cut off the foliation and gneissosity of units (a) and (b) rocks, but there is no apparent deflection of the earlier structural trends. Dikes of unit (c) are cut off at a low angle by the pluton, which is in turn cut by dikes of unit (e) along the same structural trend as the earlier dikes. It is thought that the dikes of units (c) and (e) and the Mt Falconer pluton were passively emplaced along joints normal to fold axes in the Skelton Group rocks. The maximum depth of emplacement was probably no greater than 8–10 km. Textural and field relationships and the lack of co...

Geology of the Harold D. Roberts Tunnel, Colorado: Station 468+49 to East Portal

The Harold D. Roberts Tunnel is a 23.3-mile water-diversion tunnel that extends through the western part of the Front Range between Grant and Dillon, Colorado. This report concerns the geology of the southeastern segment of the tunnel, which is partly in the Montezuma stock—a Laramide quartz monzonite pluton—but is mainly in Precambrian basement rocks. The Precambrian metamorphic rocks consist of relatively thick, complexly folded units of amphibolite, migmatite, microcline gneiss, and biotite gneiss. These rocks were invaded by plutons of Boulder Creek Granite and Silver Plume Granite, regarded as syntectonic and late tectonic, respectively, in relation to Precambrian deformation and metamorphism. Most of the basement rocks are distinctly foliated. Laramide deformation resulted in elevation of the core of the Front Range along northward-trending marginal upthrusts and in development of the Front Range mineral belt, a broad northeast-trending zone of strike-slip faults, veins, and igneous intrusions that extends diagonally across the range. The Montezuma stock was emplaced approximately at the junction of the mineral belt with the western margin of the range relatively late in the deformation. The stock is massive and shows little indication of flow fabric. Analysis of fault data obtained in the tunnel indicates two complementary sets of strike-slip faults, trending northeast and northwest, apparently related to wrench-fault movement along the mineral belt. Minor faults are randomly oriented but tend to be concentrated in north-trending dip-slip sets approximately parallel to the marginal upthrusts. The fracture pattern in the Montezuma stock conforms to the regional fault pattern. The major joint sets and fault sets are parallel to one another and probably formed concurrently. The fracture pattern in the Precambrian rocks is unlike that in the Montezuma stock. Minor faulting was controlled largely by the rock fabric. Joints bear no obvious relation to the faults or rock fabric and are believed to have formed mainly later than the faults. The anomalous rupture pattern in the foliated basement rocks is attributed to their anisotropy and to a probable transition from pseudoductile to brittle behavior as the core of the range was uplifted.

AGE OF THE INTRUSIVE ROCKS OF THE SOUTHEASTERN SAN GABRIEL MOUNTAINS, CALIFORNIA

Dating of the intrusive rocks of the southeastern San Gabriel Mountains by the Rb-Sr and by the K-A methods indicates two periods of plutonic intrusion. A concordant quartz diorite pluton, dated as 83–105 m.y., was emplaced during the Cretaceous orogeny. Small discordant quartz monzonite plutons, dated as 17–26 m.y., were emplaced during the Miocene orogeny. These dates permit a correlation of the geologic history of the southeastern San Gabriel Mountains with that of the Los Angeles basin.