Alkalic Plutons Research Articles

Age and origin of Early Cretaceous magmatism in the southernmost Omineca Belt, northern U.S. Cordillera

We present results of a geochronological and geochemical study of the Kaniksu batholith, located in northern Idaho and Washington near the Canadian border. This batholith occurs at the southern end of the Omineca Belt and forms an integral part of the Priest River metamorphic complex. The Kaniksu batholith is compositionally diverse, but granitoids can be grouped into a volumetrically major peraluminous and metaluminous suite and a volumetrically minor alkalic suite based on lithology and major-element composition; additional compositionally similar alkalic plutons extend far to the south of the main footprint of the batholith. Laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb zircon dating revealed that most granitoids of the batholith formed in the Early Cretaceous between 120 Ma and 100 Ma, although magmatism continued until ca. 73 Ma. The dominance of Early Cretaceous ages closely matches magmatic age distributions elsewhere in the Omineca Belt. There is no clear relationship between age and suite assignment or other geochemical characteristics. Whole-rock Sr, Nd, and Pb isotope data along with zircon Hf isotope data suggest that the alkalic and metaluminous suites can largely be explained as mixtures of partial melts from the Mesoproterozoic Moyie sills and metasedimentary rocks of the Belt Supergroup, whereas a third component, perhaps Paleoproterozoic or Neoarchean meta-igneous basement, is required to explain the origin of the peraluminous suite. All of these granitoids formed well behind the Cordilleran arc front in an area characterized by multiple episodes of crustal thickening. As there is little evidence of contemporaneous mantle-derived magmatism, crustal thickening was probably the main driver of batholith formation.

Within-Plate Setting of Paleozoic Alkalic Suites in Southeastern New England, USA: Constraints from Chemical Abrasion–TIMS U-Pb Geochronology and Paleomagnetism

AbstractMid-Paleozoic alkalic plutons and associated volcanic rocks in a northeast-trending belt transecting the southeastern New England segment of “West” Avalonia are well known for their anoroge...

Fingerprints of late Neoproterozoic ridge subduction in the Pan-African Damara belt, Namibia

Subduction of mid-ocean ridges is a common feature in recent convergent margins, but is rarely documented in Proterozoic to Paleozoic orogenic belts. Here we describe evidence for ridge-trench interaction in the deeply eroded late Neoproterozoic Damara orogenic belt, central Namibia. The earliest interaction is indicated by primary intrusive contacts between amphibolite facies mid-ocean ridge metabasalts and trench metasediments. U-Pb zircon ages of 550–540 Ma from syntectonic granites in the forearc indicate the timing of partial melting and mafic underplating of the prism in response to ridge subduction. The thermal peak in the Damara belt, associated widespread granitic and alkalic plutonism, and hydrothermal activity coincide with the waning stages of tectonism at 530–520 Ma and are interpreted to indicate slab window widening and slab delamination. We suggest that the proposed two-stage thermal evolution of the Damara belt, comprising latest Neoproterozoic ridge subduction and early Cambrian slab delamination, represents a fingerprint of ridge subduction in ancient orogens.

The Northern Cordilleran Mid‐Cretaceous Plutonic Province: Ilmenite/Magnetite‐series Granitoids and Intrusion‐related Mineralisation

Abstract.Twenty‐five Early and mid‐Cretaceous (145–90 Ma) plutonic suites and belts are defined across Alaska and Yukon, in the northern North American Cordillera, on the basis of lithological, geochemical, isotopic, and geochronometric similarities. These features are combined with aeromagnetic characteristics, magnetic susceptibility measurements, and whole‐rock ferric: ferrous ratios to ascertain the distribution of magnetite‐ and ilmenite‐series plutonic belts. Magnetite‐series plutonic belts are dominantly associated with the older parts of the plutonic episode and comprise subduction‐generated metaluminous plutons that are distributed preferentially in the more seaward localities dominated by primitive tectonic elements. Ilmenite‐series plutonic belts comprise slightly younger, slightly peraluminous plutons in more landward localities in pericratonic to continental margin settings. They were likely initiated in response to crustal thickening following terrane collision. The youngest plutonic belt forms a small, but significant, magnetite‐series belts in the farthest inboard position, associated with alkalic plutons that were emplaced during weak extension.Intrusion‐related metallogenic provinces with distinctive metal associations are distributed, largely in accord with classical redox‐sensitive granite‐series. Copper, Au, and Fe mineralisation are associated with magnetite‐series plutons and tungsten mineralisation associated with ilmenite‐series plutons. However, there are some notable deviations from expected associations, as intrusion‐related Ag‐Pb‐Zn deposits are few, and significant tin mineralisation is rare. Most significantly, many gold deposits and occurrences are associated with ilmenite‐series plutons: these form the basis for the newly recognized reduced intrusion‐related gold deposit model.

Volcanic rocks in the Narragansett basin, southeastern New England: petrology and significance to early basin formation

A suite of volcanic rocks in the northwest corner of the Narragansett basin includes at least four basalt flows, two rhyolite flows, and associated pyroclastic rocks. The volcanics are interbedded with non-marine, sedimentary rocks within the lower part of the Wamsutta Formation and, until recently, were considered Pennsylva- nian in age. An age of about 373 Ma has been reported for the rhyolite, based on U/Pb geochronology (Thompson and others, 1999). The basalt flows are typically 1 to 2 m thick and marked by pillows, sediment dikes, and magma/sediment commingling features. The two younger flows contain intergranular to subophitic clinopyroxene and sparse phenocrysts of plagioclase and pseudomorphed olivine in a pilotaxitic ground- mass. The two older flows are similar but contain no pyroxene. The rhyolite flows, from 3 to 20 m thick, are characterized by subhorizontal quartz seams that represent cooling cracks or zones of preferential vesiculation filled with late-stage quartz. Both rhyolite flows contain phenocrysts of anorthoclase in a granophyric, devitrified ground- mass with relict perlitic features. Identification of pyroclastic deposits beneath one rhyolite flow indicates that extrusion was preceded by explosive activity. Major and trace elements indicate that the volcanics are mildly alkaline, and geochemical trends suggest the basalt and rhyolite originated by partial melting of different sources, followed by limited fractional crystallization. Apparent restriction to early basin sediments suggests that the volcanics reflect a rifting event associated with the formation of the Narragansett basin. Though it is now apparent that the basin was active well before the Pennsylvanian, it is not clear whether the rift event was related to the extensional environment that prevailed through much of the Paleozoic, producing the alkalic plutonism prominent in this part of the Avalon Zone. Similarities to Devonian-Carboniferous bimodal suites in the Maritimes basin of Canada suggest possibly analogous origins.

Geochronology and radiogenic isotope geochemistry of plutonic rocks from the central Abitibi subprovince: significance to the internal subdivision and plutono-tectonic evolution of the Abitibi belt

Nine new U-Pb ages are reported for plutons of the central granite-gneiss zone of the Abitibi belt in Quebec. The large plutonic complex along Lithoprobe seismic reflection line 28 formed by multiple intrusion over at least 40 million years, synchronous with and postdating formation of adjacent volcanic sequences. Ages for the four principal plutons within the complex are: Mistaouac at 2726 ± 2 Ma, Boivin at 2713 ± 2 Ma, Rousseau at 2703 ± 2 Ma, and Paradis at 2686 ± 2 Ma. The latter also constrains deformation within the Laberge deformation zone to be at least in part younger than 2686 Ma. Inherited zircons in the Mistaouac pluton indicate that the oldest pluton formed in significantly older crust (>2.75 Ga), not presently exposed in the area. The La Reine and Waswanapi plutons have ages of ca. 2695 Ma similar to other tonalitic plutons in the area and elsewhere in the Abitibi belt. A syenite pluton deformed within the Douay fault zone, a late fault associated with the Casa Berardi zone, has an age of 2676+6-5 Ma, similar to alkalic plutons associated with the Destor-Porcupine and Cadillac-Larder Lake deformation zones of the southern Abitibi belt. Two samples from the Lac Case pluton yielded monazite ages of 2676 ± 3 and 2660 ± 3 Ma. Nd, Pb, and Sr isotopic compositions for central Abitibi belt plutons show dominantly juvenile sources with minor contributions of older crust in the Lac Case pluton. Although geochronological data for volcanic rocks has been used to suggest that the northern zone is older and magmatic activity youngs to the south, consideration of the ages for plutonic and volcanic rocks does not support such hypothesis. The available data indicate that magmatism occurred throughout the Abitibi subprovince from 2730 to 2685 Ma, permissive of a linked tectono-magmatic evolution for the northern and southern zones.

Hotspot origin of the Mississippi embayment and its possible impact on contemporary seismicity

Previous authors have related the Late Cretaceous/early Tertiary subsidence of the Mississippi embayment to the opening of the Gulf of Mexico, but the Gulf opened earlier in Triassic/Jurassic time. We offer an alternative hypothesis that development of the embayment was coeval with the passage of the Mississippi Valley graben system over the Bermuda hotspot about 90 Ma. Several lines of evidence of significant uplift of the embayment axis accompanying mid-Cretaceous magmatism and prior to Late Cretaceous subsidence support this proposal. First, reactivation of the Pascola arch in the northern embayment is recorded by flanking deposits of basal Upper Cretaceous gravel. Second, beneath a regional mid-Cretaceous unconformity, subcrops of Jurassic and Early Cretaceous strata define a pronounced southwest-plunging arch in the southern embayment. This arch is collinear with an arch revealed in Paleozoic rocks after restoration to mid-Cretaceous structural geometries. Third, a deep weathering profile on mid-Cretaceous alkalic plutons along the western embayment margin is nonconformably overlain by Paleocene sediments, and rapid mid-Cretaceous cooling of these intrusions has been interpreted from apatite fission tracks. Moreover, exploratory holes along the embayment axis encountered similar weathered alkalic intrusions nonconformably overlain by basal Upper Cretaceous strata. Fourth, there was an anomalous influx of clastic sediment into the northern Gulf of Mexico during mid-Cretaceous time, and subsequent clastic facies patterns suggest the Mississippi River drainage began to enter the Gulf in the Late Cretaceous. Passage of the Mississippi Valley graben over the Bermuda hotspot during elevated hotspot activity of Cretaceous time may have significantly weakened the previously rifted lithosphere. Rifted continental margin at Charleston, South Carolina, also passed over this hotspot in latest Cretaceous time. Similarly, the St. Lawrence rift system passed over the Great Meteor hotspot during the Cretaceous. It is important to note that these rift systems are the principal loci of strong seismicity in eastern North America, and thus weakening by increased Cretaceous hotspot activity may be an important common factor for these seismic rift zones.

Conflicting tectonics? Contraction and extension at middle and upper crustal levels along the Cordilleran Late Jurassic arc, southeastern California

Effects of mid-Mesozoic contraction followed closely in time by extension are present in mid- to upper-crustal plutonic rocks in the Chuckwalla Mountains of the eastern Transverse Ranges, California. Late Jurassic movement along a steeply dipping, right-lateral mylonitic shear zone is bracketed between 159 and 147 Ma via U-Pb dated plutons. Depth of emplacement vs. time data based on hornblende geobarometry and U-Pb geochronology of Mesozoic plutons indicate that a period of dramatic uplift affected the Chuckwalla Mountains during the Late Jurassic, contrasting sharply with data from the (then) nearby San Gabriel Mountains. Subsequent latest Jurassic extensional tectonics is suggested by alkalic plutonism, nearly synchronous intrusion of the Late Jurassic Independence dike swarm, and possibly by deposition of the McCoy Mountains Formation. We conclude that both contraction and extension were significant at upper- and mid-crustal depths in the Chuckwalla Mountains region during the Late Jurassic, and speculate that the combined influence of oblique convergence and the opening of the Gulf of Mexico may have caused the contrasting and nearly contemporaneous tectonic modes.

Genesis of the southern Abitibi greenstone belt, Superior Province, Canada: Evidence from zircon Hf isotope analyses using a single filament technique

Hafnium isotopic analyses, together with U-Pb geochronology, have been performed on zircons from the main units of the Late Archean southern Abitibi greenstone belt in the Superior Province. A new analytical method in which Hf is loaded and run with a mixture of Ir-Mo and C on single filaments was used. The results indicate a relatively homogeneous isotopic composition of the greenstone belt, with ϵ Hf values of 6.3–4.2 for the 2730-2700 Ma preorogenic assemblages and 4.7-3.6 for the 2690-2675 Ma synorogenic intrusions. Hafnium isotopes and U-Pb ages indicate that the greenstone belt formed in a relatively short time interval within an ensimatic setting, with minimal interaction with older sialic crust. Primary magmas were derived from mantle sources with a depleted isotopic signature of ϵ Hf ≈ 5.5 ± 0.5, corresponding to published ϵ Nd ≈ 2.5, 87Sr 86Sr ≈ 7011, 207Pb 204Pb ≈ 14.4 , and 206Pb 204Pb ≈ 13.2. Slightly lower ϵ Hf values in some rocks may record moderate degrees of source heterogeneity; in the case of the alkalic plutons, the lower ϵ Hf can be interpreted in terms of mantle wedge contamination by subducted components isotopically similar to sedimentary rocks in the adjacent Pontiac Subprovince. The consistent, positive ϵ Hf signature confirms that the Archean mantle was already depleted prior to formation of 3.0–2.7 Ga Superior Province crust.

Remarkable isotopic and trace element trends in potassic through sodic Cretaceous plutons of the Yukon‐Koyukuk Basin, Alaska, and the nature of the lithosphere beneath the Koyukuk terrane

During the period from 110 to 80 m.y. ago, a 450‐km‐long magmatic belt was active along the northern margin of Yukon‐Koyukuk basin and on eastern Seward Peninsula. The plutons intruded Upper Jurassic(?) and Lower Cretaceous volcanic arc rocks and Cretaceous sedimentary rocks in Yukon‐Koyukuk basin and Proterozoic and lower Paleozoic continental rocks in Seward Peninsula. Within Yukon‐Koyukuk basin, the plutons vary in composition from calc‐alkalic plutons on the east to potassic and ultrapotassic alkalic plutons on the west. Plutons within Yukon‐Koyukuk basin were analyzed for trace element and isotopic compositions in order to discern their origin and the nature of the underling lithosphere. Farthest to the east, the calc‐alkalic rocks of Indian Mountain pluton are largely tonalite and sodic granodiorite, and have low Rb (average 82 ppm), high Sr (>600 ppm), high chondrite‐normalized (cn) Ce/Yb (16–37), low δ18O (+6.5 to +7.1), low initial 87Sr/86Sr (SIR) (0.704), and high initial 143Nd/144Nd (NIR) (0.5126). These rocks resemble those modelled elsewhere as partial melts and subsequent fractionates of basaltic or gabbroic metaigneous rocks, and may be products of melting in the deeper parts of the Late Jurassic(?) and Early Cretaceous volcanic arc. Farthest to the west, the two ultrapotassic bodies of Selawik and Inland Lake are high in Cs (up to 93 ppm), Rb (up to 997 ppm), Sr, Ba, Th, and light rare earth elements, have high (Ce/Yb)cn (30, 27), moderate to low δ18O (+8.4, +6.9), high SIR (0.712, 0.710), and moderate NIR (0.5121–0.5122). These rocks resemble rocks of Australia and elsewhere that were modelled as melts of continental mantle that had been previously enriched in large cations. This mantle may be Paleozoic or older. The farthest west alkalic pluton of Selawik Hills is largely monzonite, quartz monzonite, and granite; has moderate Rb (average 284 ppm), high Sr (>600 ppm), high (Ce/Yb)cn (15–25), moderate δ18O (+8.3 to +8.6), high SIR (0.708–0.712), and moderate NIR (0.5121–0.5122). These rocks may be the product of interaction of magma derived from old continental mantle and magma derived from old continental crust. Plutons between eastern and western extremes show completely gradational variations in the concentration of K and Rb and in the isotopic compositions of Sr, Nd, and O. These plutons probably originated either by melting in a mixed source composed of a Paleozoic or older continental section (mantle + crust) overlain by Mesozoic mafic arc rocks, or by mixing of ultrapotassic to potassic magmas from continental sources (mantle + crust), and tonalitic magmas from arc sources. We infer from these results that the northwest portion of Yukon‐Koyukuk basin is underlain by a substantial continental basement of Paleozoic or greater age. This basement probably thins out to the east. There is no geochemical evidence for continental basement east of about longitude 157°, or along a belt of at least 50 km width flanking Ruby Geanticline as far to the southwest as about longitude 161°. These areas are probably underlain by oceanic and Mesozoic arc rocks.

Early Paleozoic alkalic plutonism in east-central Idaho

U-Th-Pb analyses of zircon suites document the presence of three lower Paleozoic alkalic plutons in east-central Idaho. The igneous bodies range in composition from alkali-feldspar syenite to alkali-feldspar granite. The epizonal Arnett Creek pluton contains a morphologically bimodal suite of anhedral and euhedral zircon, which combined gives an upper intercept age on a concordia diagram of 492 ± 39 m.y. Zircon analyses from the Deep Creek pluton plot near the 500-m.y. locus on concordia, but the pattern suggests the presence of a small component of inherited radiogenic lead. Limited Rb-Sr whole-rock data for the Arnett Creek and Deep Creek plutons are compatible with an Early Ordovician age but are too imprecise to refine interpretations. Zircon from the epizonal Beaverhead pluton, for which the enigmatic Ordovician-Silurian K-Ar and Rb-Sr mineral ages have long been debated, yields an approximate age of 483 m.y. The tectonic setting of these intrusions is poorly known. The coincidence of early Paleozoic plutonism and independently documented syndepositional faulting with the coeval topographic high, known variously as the Lemhi or Salmon River Arch, tentatively suggests a mutual tectonic control. Further geochronologic, stratigraphic, and especially petrogenetic studies, however, are required to evaluate this possibility.

Cambrian-Ordovician (500 Ma) Alkalic Plutonism in Southwestern New Mexico; U-Th-Pb Isotopic Data From the Florida Mountains

Cambrian-Ordovician (500 Ma) Alkalic Plutonism in Southwestern New Mexico; U-Th-Pb Isotopic Data From the Florida Mountains

Cambrian-Ordovician (500 Ma) alkalic plutonism in southwestern New Mexico; U-Th-Pb isotopic data from the Florida Mountains

Cambrian-Ordovician (500 Ma) alkalic plutonism in southwestern New Mexico; U-Th-Pb isotopic data from the Florida Mountains

Cratonic Sedimentation during the Proterozoic: An Anorogenic Connection in Wisconsin and the Upper Midwest

Certain Proterozoic quartzites (including, Baraboo, Barron, and Sioux) and associated metasedimentary rocks in the upper midwest represent a distinct epicontinental depositional environment. New data indicate that argillite, chert, iron formation, micaceous conglomerate, and rhyolite are much more abundant and significant in this sequence than previously reported. The entire group of rocks was deposited during one period in a series of tectonic episodes (magmatism and uplift) related to epeirogeny. None of these episodes included overt rifting. Sedimentation began on subsiding continental crust during the waning stages of anorogenic magmatism 1760 m.y. ago. Following deposition, a regional event 1630 m.y. ago is interpreted as a time of uplift and mild deformation with no magmatic expression in the region. Epeirogenic tectonism was culminated in the widespread intrusion of 1500 m.y.-old alkalic plutonism, which metamorphosed and further deformed the sedimentary rocks and surroundings.

Early alkalic plutonism in the calc-alkalic batholithic belt of California

Four alkali-rich plutons exposed in southern and eastern California are chemically distinct from the quartz-rich, calcalkalic batholithic rocks that typify the region. These predominantly monzonitic plutons are older (175 to 230 m.y.) and located farther east than the majority of typical batholithic rocks, but there is spatial and temporal overlap. Most of the monzonites contain minor quartz, but one unit is feldspathoidal. Na 2 O, K 2 O, and Sr are abundant, and SiO 2 is low relative to the calcalkalic rocks. Total REE is high, and patterns have steep negative slopes. The chemical data suggest an upper mantle source—perhaps eclogitic—for the monzonitic magmas. These plutons, together with other early Mesozoic alkali-rich intrusions in California, Washington, and British Columbia, may represent a previously unrecognized phase of alkali-rich magmatism associated with the early stages of continental margin orogeny.