Age Of Pluton Research Articles

Regional lithologic variations in the Patagonian batholith

The Patagonian batholith, in the southern Andes of Chile, represents a major addition to the Pacific margin of the South American plate due to semi-continual subduction since the Late Jurassic. Plutonic ages range from 166 to 12 Ma, with an apparent peak around 100-70 Ma. Most of the batholith is composed of I-type, metaluminous, calc-alkaline rocks ultimately related to partial melting of mantle wedge material and/or subducted material. Tonalite (plus minor quartz diorite) is the dominant lithology (36% of 585 samples); other lithologic groups (granite, granodiorite + quartz monzodiorite, and diorite + gabbro) make up roughly equal proportions of the samples (18–25% each). Minor amounts of peraluminous rock suggest some amount of crustal recycling (contamination of mantle magmas or melting of continental crust). Most plutons are undeformed; foliated rocks crop out in Andean orogenic zones and locally along lineaments. Local, protracted, felsic to mafic progressions are probably related to intrusion geometry and erosion level rather than evolution of a single magma. Similarities with the Coastal batholith of Peru include size, the period of intrusion (mainly Cretaceous-Tertiary), and the general I-type, tonalitic character. Differences include no unidirectional migration in time, thinner and younger pre-batholithic crust, deeper level of erosion (generally mesozonal), and significant primary biotite in mafic rocks. These differences probably reflect different mechanisms of magmatic evolution and arc construction related to subduction kinematics and crustal setting.

Geochronology of granite terranes in the Ford Ranges, Marie Byrd Land, West Antarctica

Abstract Rb-Sr whole-rock isochron and and K-Ar mineral ages are presented for Ford Granodioritemineral ages are presented for Ford Granodiorite (Devonian) and Byrd Coast Granite (Jurassic-Cretaceous) in the Ford Ranges, Marie Byrd Land, West Antarctica. K-Ar mineral ages of the Ford Granodiorite are 324–375 Ma, cocentrating a 355ȁ375 Ma and those < 360 Ma are mainly in the central Denfeld Mountains. Rb-Sr whole-rock isochron ages of plutons in Denfeld and Sarnoff Mountains are 355–375 Ma (Late Devonian) for earliest adamellite granodiorites and 340–360 Ma (Early Carboniferous) for younger biotite granodiorite and muscovite granite. Devonian plutonism in West Antarctica is restricted to western Marie Byrd Land but has correlatives in northern Victoria Land (East Antarctica), Tasmania and Victoria (Australia), and South Island (New Zealand). Rb-Sr whole-rock isochron ages of Byrd Coast Granite (Mesozoic) indicate two plutonic phases: (l),leucogranites 140–150 Ma (Late Jurassic - Early Cretaceous) in the Clark Mountains and at Mt Corey, with associated minor intrusives at Breeding and Stanford Nunataks; (2), widespread granodiorites, 110–115 Ma (mid Cretaceous), in the Allegheny and Sarnoff Mountains. Correlatives in the Fosdick Mountains are 95–100 Ma old. The older Jurassic plutonics are confined to the eastern margin of the Ford Ranges but mid-Cretaceous granites are part of a widespread belt along the Pacific coast of West Antarctica. Strontium initial ratios of Ford Granodiorite and Byrd Coast Granite are mostly 0.704–0.706, characteristic of I-type calcalkaline granite-granodiorite plutonism.

A new correlation of sapphirine granulites in the indo-antarctic metamorphic terrain: Late proterozoic dates from the eastern ghats province of India

Abstract Uranium-lead and thorium-lead ages were obtained from perrierite and zircons in two samples from the Eastern Ghats Province of South India, a sapphirine-bearing granulite from Anakapalle and a charnockite from Visakhapatnam. The isotopic data are concordant or close to concordant at 1 Ga, which is interpreted to be the age of granulite-facies metamorphism and charnockite plutonism in the vicinity of Anakapalle and Visakhapatnam, and provides the first report of a late Proterozoic granulite-facies event in the Eastern Ghats Province. In reassemblies of Gondwanaland based on DuToit's original proposal, the Eastern Ghats Province is juxtaposed against a high-grade terrain in East Antarctica affected by a granulite-facies event and charnockitic plutonic activity 800–1100 Ma ago. The ages reported here suggest that a portion of the Eastern Ghats Province represents an extension of the late Proterozoic terrain from Antarctica into India. The radiometric results on the Anakapalle rocks are the first evidence in South India for sapphirine formation during the late Proterozoic; other South Indian sapphirine localities appear to be Archaean in age. A review of sapphirine localities in the Indo-Antarctic metamorphic terrain indicates that sapphirine developed under a range of pressure-temperature conditions during the Archaean and Proterozoic. Evidence for a systematic change in peak metamorphic conditions with time is obscured by these regional variations. Characterization of the entire pressure-temperature path, in addition to estimates for the peak conditions, is needed to distinguish the Archaean and Proterozoic metamorphic regimes.

40Ar/39Ar ages for plutons of the Monteregian Hills, Quebec: Evidence for a single episode of Cretaceous magmatism

40Ar/39Ar data for biotite and amphibole from six plutonic complexes of the Monteregian petrographic province in Quebec, Canada, are presented. Generally uncomplicated and concordant incremental heating spectra are observed, although there is evidence for minor excess 40Ar at several localities. Biotites and amphiboles are concordant, consistent with rapid cooling of the high-level complexes. Ages for different phases of individual complexes are analytically indistinguishable and suggest that the various principal lithologies are cogenetic. The 40Ar/39Ar ages of six Monteregian complexes (St. Hilaire, Rougemont, Johnson, Shefford, Brome, Megantic) all fall within the very restricted range of 124 ± 1 m.y. It is suggested that all of the Monteregian plutons, with the exception of Oka, were formed within a short interval (1–2 m.y.) during the Cretaceous, consistent with paleomagmetic results. The new ages, along with geochemical characteristics, support derivation of the alkaline magmas by a fixed mantle hot spot which also produced broadly similar complexes in New England and the chain of New England Seamounts.

Seismic imaging of the Elberton Granite, Inner Piedmont, Georgia, using COCORP southern Appalachian data

Seismic imaging of the Elberton granite in the Georgia Inner Piedmont was undertaken to define its shape and to determine its relation to the southern Appalachian thrust. The recent Consortium for Continental Reflection Profiling (COCORP) lines in the southern Appalachians suggest a major thrusting of a crystalline block over sediments, although the extent of this thrust is subject to interpretation. The fine‐grained Elberton, of Late Devonian age, is the only major pluton crossed by the seismic traverse and is not well defined on the unmigrated common depth point (CDP) seismic section. Migration by beam‐forming reveals considerable variation in the direction of returning signal and provides a clearer image of the laminated reflections, ubiquitous in the southern Appalachians, which have been interpreted as the equivalents of the Valley and Ridge sediments beneath an allochthonous sheet. The upper 5 km portions of the migrated and CDP sections lack events, probably because of the necessary stripping (mute) of a large‐amplitude, early arrival from the seismic records before further processing. Filtering, and transformation of the data to the intercept‐slowness domain, to eliminate this interfering arrival are attempted to enhance the upper portion of the seismic sections Although these approaches do not give a consistent picture of the shallow section, a pattern of scatterers located at depths of 2–3.5 km emerges; these may correlate with the base of the intrusive body. This identification of the granite base, along with the presence of the thrust‐related reflectors beneath the pluton, suggests a prethrust or synthrust emplacement of the granite. However, tectonic interpretations of the Inner Piedmont based on the Elberton granite are severely limited by assumptions: that the pluton age corresponds to time of cooling, that the pluton is not fed by a narrow (seismically elusive) feeder, and that the thrust has not been reactivated after emplacement of the intrusive body. If all these assumptions hold, then the Late Devonian age of the granite is the earliest possible time of thrusting.

Ages of some intrusive rocks of southwestern Maine, U.S.A.

Whole-rock Rb–Sr and U–Pb zircon age measurements on intrusive rocks in southwestern Maine indicate igneous activity at 400, 340, and 320 Ma. These plutonic rocks were emplaced into deformed Ordovician to Devonian(?) (and perhaps Hadrynian) rocks of the Shapleigh and Merrimack Groups. Folding of the Shapleigh and Merrimack Group rocks is interpreted to have occurred during the Acadian event, or before. The [Formula: see text] age of the Webhannet pluton in southwestern Maine sets a minimum time for Acadian deformation in this region.The 320 Ma age of the Lyman two-mica granite pluton of this study coupled with the reported ages of the Milford two-mica granite (275 Ma) and Lake Sunapee two-mica granite (323 Ma) of New Hampshire suggests a spectrum of Hercynian igneous activity in northern New England similar to that of the well established Hercynian intrusive events in the southern Appalachians and western Europe.

Rb‐Sr geochronology and Sr isotopic composition of Devonian granitoids, eastern Tasmania

Abstract Biotite igneous ages and well‐defined isochron ages of plutons from the composite Blue Tier Batholith and the Coles Bay area in northeastern Tasmania range from 395 to 370 Ma. The older limit of this range, for the George River granodiorite, is considerably older than any age previously recorded for NE Tasmania. The ages of the youngest plutons (Mt Paris and Anchor granites), which host cassiterite ores, record pervasive hydrothermal alteration events. The initial 87Sr/80Sr ratios of the granitoids range from 0.7061 to 0.7136 and suggest different protolith compositions, consistent with mineralogical and geochemical characteristics of each pluton. The S‐type garnetbiotite granites (Ansons Bay and Booby alia granites) have initial ratios greater than 0.7119, indicative of enriched, high Rb/Sr ratio, crustal source‐rocks of Proterozoic age (1700–800 Ma). The S‐type biotite granites (Poimena and Pearson granites) have relatively high initial 87Sr/86Sr ratios (0.7070, 0.7105) but overlap with those of the I‐type granodiorites (George River, Scamander Tier, Pyengana and Coles Bay granodiorites) which are in the range of 0.7061 to 0.7073. The initial ratios of the enriched altered plutons are poorly constrained, and on both hand‐specimen and thin‐section scales, reveal open‐system Sr isotopic patterns. Isochron ages for the arenite‐lutite and lutite sedimentary associations of the Mathinna Beds, which are intruded by the granitoids, reflect an approach to Sr isotopic equilibrium during regional metamorphism. The metamorphic age (401 ± 7 Ma) of the early Pragian arenite‐lutite association indicates a relatively small time interval between deposition, regional metamorphism and granitoid intrusion. The isotopic age for the lutite sedimentary association (423 ± 22 Ma) is tentatively correlated with a Benambran‐age burial metamorphic event that has not previously been recorded in Tasmania.

The Pokiok batholith: a contaminated Acadian intrusion with an anomalous Rb/Sr age

Samples of the stratigraphically placed (&lt; 400 Ma) Pokiok batholith of New Brunswick give a Rb/Sr total-rock age of 431 ± 9 Ma. This age is some 40 Ma older than the estimated age of crystallization and is interpreted to result from the mixing of variable amounts of crustal strontium with mantle-like Sr at or about 390 Ma. The existence of inherited or crustal isochrons in the Pokiok batholith implies that all uncorroborated total-rock Rb/Sr ages for granitic plutons in the Appalachians of eastern Canada should be carefully appraised in view of their importance in estimating the timing and duration of the Acadian Orogeny.

Paleomagnetism and age of Mafic Plutons, Wichita Mountains, Oklahoma

A paleomagnetic reconnaissance (43 samples) of the Raggedy Mountain Gabbro Group, which is comprised of a layered complex of anorthosites, anorthositic gabbros, and massive gabbro, shows: (1) stable remanent magnetization and (2) consistent directions which differ significantly from those of the nearby Cambrian Wichita Granite Group, with exception of one site near the granite contact. These results suggest a Precambiran age for the gabbro group. The difference in pole positions between the granitic and mafic groups demonstrates that the plutonic episodes responsible for the mafic and granitic magmas are significantly separated in time. Models of formation of the Wichita aulacogen should be altered to account for the difference in time between the mafic and silicic plutonism.

Cretaceous volcanism, Excelsior Mountains, Nevada

Rb-Sr isochrons obtained for locally extruded siliceous volcanic rocks in units in the Excelsior and Pilot Mountains, western Nevada, indicate ages, respectively, of 103 ± 5.7 m.y. and 142 ± 17 m.y. Both dated units occur in a complex of terrigenous and volcanic rocks previously thought to be entirely of early Mesozoic age. K-Ar ages of granitic rocks of the Excelsior Mountains region indicate that plutonism occurred mainly between about 70 and 100 m.y. B.P. The newly dated volcanic rocks of the Excelsior Mountains provide the first recognition of a volcanic center of Cretaceous age in the Great Basin and evidence that at least some of the thrusting and folding in the Excelsior Mountains region occurred in the Cretaceous or later time. Together, the two new Rb-Sr ages indicate a more prolonged history of Mesozoic sedimentation and extrusion than previously realized. The dated volcanic rocks of the Excelsior Mountains are generally contemporaneous with recently discovered Cretaceous extrusive rocks in the central Sierra Nevada, suggesting that volcanism may have been widespread in the Cretaceous in eastern California and western Nevada between 37° and 38°N. K-Ar ages of granitic plutons in the Excelsior Mountains region suggest that the Cretaceous volcanic rocks vented near the beginning of a 20-m.y.-long pulse of intrusion. Moreover, the pluton ages indicate a time lag between Cretaceous volcanism and plutonism similar to that in the Sierra Nevada.

U-Pb isotope ages of granitic plutons near Cananea, Sonora

The Cananea crops out in the Cananea mining district, which includes some of the most important known copper deposits in Mexico. The area is in northern Sonora, approximately 100 km southeast of the twin cities of Nogales, Arizona-Sonera, on the international border.By means of isotopic analyses of U-Pb systems in cogenetic suites of zircon from two different phases of the pluton, its interpreted age has been firmly established at 1,440 + or - 15 m.y. Cananea granite, actually a quartz monzonite, is lithologically characteristic of a major Precambrian suite of anorogenic, consanguineous plutons of porphyritic granodiorite to granite. Constituents of this previously recognized series, which crop out throughout the southwestern U. S. and into northern Mexico, consistently yield zircon radiometric ages within the interval 1,425-1,475 m.y.Major younger intrusions in and nearby the Cananea district include Cuitaca granodiorite and Chivato monzodiorite. These bodies yield apparent zircon ages of 64 + or - 3 and 69 + or - 1 m.y., respectively. Although more precise geochronologic relationships between young plutons, the adjacent layered sequence, and cross-cutting, brecciated, quartz porphyry plugs are needed, correlation exists between major plutonism and mineralization.The Precambrian age of the Cananea granite precludes its chronological association with the younger mineralized plugs and should be taken into account in future planning of exploration.

Age of plutonism and tectonism and a new minimum age limit on the Glenarm Series in the Northeast Virginia Piedmont near Occoquan

Age of plutonism and tectonism and a new minimum age limit on the Glenarm Series in the Northeast Virginia Piedmont near Occoquan