Peraluminous Plutons Research Articles

A comparative 40Ar/39Ar conventional and laserprobe study of muscovite from the Port Mouton pluton, southwest Nova Scotia

We report 40Ar/39Ar age spectrum and laserprobe data for primary magmatic or fluido-magmatic muscovite minerals from the Port Mouton pluton, one of several weakly peraluminous peripheral plutons in the Meguma terrane, southwestern Nova Scotia. Laserprobe data from the cores of thin grain fragments suggest that this pluton cooled rapidly following intrusion at 373 ± 1 Ma, the U–Pb monazite age. The rims of thicker more complete grains record ages of 315–325 Ma, even in cases where there have been no apparent changes in grain rim chemistry and where deformation is minimal. The observed age gradients may be the result of prolonged reheating during the Late Carboniferous Alleghanian Orogeny or, alternatively, the result of rapid cooling at this time to temperatures below the closure value for muscovite rims. Conventional age spectra obtained from muscovite separates record neither the older intrusion age nor the younger reset–cooling age. Instead, these intermediate ages appear to reflect the averaging of intragrain (core–rim) age variations in thick grains and thus have no chronological significance. For these Port Mouton muscovite minerals, the record of initial cooling appears to reside only in certain limited regions of a given grain, a record that can be recovered by the laserprobe technique applied to carefully prepared subgrain fragments. A contrast in the early tectono-thermal histories of plutonic rocks in southwestern Nova Scotia relative to those in the northeast may be the result of perturbation by a mantle plume.

Regional geochemical and isotopic variations of northern New England plutons: Implications for magma sources and for Grenville and Avalon basement-terrane boundaries

Examination of 36 Devonian to Pennsylvanian plutons within and bordering the Central Maine terrane of north-central New England shows that the plutons display regional variations in mineral and bulk-rock compositions and in oxygen and sulfur isotopic values. These variations principally correspond to the regional structural trends of the Central Maine terrane. Plutons located on that terrane9s southeast flank adjacent to the Massabesic Gneiss complex, on the terrane9s northwest flank adjacent to the Bronson Hill anticlinorium, and in the northeastern part of the Northeast Kingdom batholith of Vermont have biotites with relatively low Al concentrations (≤3.0 cations p.f.u. [per formula unit]), feldspars with higher An contents (up to An 55 ), and lower δ 18 O (7‰–9‰) and higher δ 34 S (average of 0‰) bulk-rock values compared to plutons within the central parts of the Central Maine terrane. These characteristics are most compatible with metaigneous or metavolcaniclastic sources. As shown by others, the plutons of Vermont and northwestern Maine preserve Grenvillian signatures whereas those of southeastern New Hampshire (specifically, the granite at Milford, New Hampshire) and Maine (Sebago, Phillips) have Avalonian signatures. Within the interior of the Central Maine terrane of New Hampshire and western Maine and in the Connecticut Valley trough of Vermont are distinct regions characterized by peraluminous plutons with high δ 18 O values (>11‰), low δ 34 S values (−25‰), Al-rich biotites (>3.3 cations p.f.u.), and An-poor plagioclases ( 30 ). These data are most consistent with metasedimentary sources, or, at a minimum, the plutons were heavily contaminated by Central Maine terrane or Connecticut Valley trough metasedimentary rocks. These peraluminous plutons do not have the unique signatures of the underlying Precambrian basements. Several metaluminous plutons in the Central Maine terrane along the New Hampshire–Maine border separate two regions of peraluminous plutons. These metaluminous plutons have biotite and isotopic compositions that are similar to those of the plutons along the flanks of the Central Maine terrane, indicative of less Paleozoic metasedimentary input compared to the peraluminous plutons in the interior of the Central Maine terrane. The metaluminous plutons also have trace element characteristics identical to those of plutons in Vermont and northwestern Maine that have a known Grenville basement-terrane source. We suggest that the metaluminous plutons also had sources in the underlying Grenville basement and that the Grenville-Avalon boundary cuts diagonally across northern New England. Because the plutons with Grenvillian signatures are adjacent to plutons with Avalonian signatures—the Sebago batholith and Phillips pluton—we suggest that there is no Medial New England basement in this region of New England. Some peraluminous, two-mica plutons have trace element and stable isotopic characteristics that are compatible with partial melting of metasedimentary rocks, the components of which were deposited in an anoxic basin such as the Smalls Falls Formation of the Central Maine terrane, but many other plutons have compositions that are inconsistent with this model. We suggest that the compositions of many New Hampshire Plutonic Suite plutons indicate greater heat input than that available from U-enriched metasedimentary rocks and instead require emplacement of mafic magmas from the mantle or lower crust.

Geochronology of the northern Idaho batholith and the Bitterroot metamorphic core complex: Magmatism preceding and contemporaneous with extension

Granitic plutonism and extension are broadly contemporaneous in many metamorphic core complexes. However, the relationship between magmatism and extension is rarely unambiguous. The northern Idaho batholith (Idaho-Bitterroot batholith), Montana and Idaho, composes the footwall for most of the Bitterroot metamorphic core complex and thus is an ideal area for assessing the relationships between magmatism and extension. We analyzed zircon from six samples of granitic rock from the Idaho-Bitterroot batholith using the SHRIMP (II) ion microprobe. Three samples of mylonitic granite from the Bear Creek pluton, Lost Horse Canyon, give a weighted mean 206 Pb/ 238 U age of 54.3 ± 0.7 Ma. A protomylonitic granite from the central part of the Bitterroot core complex (also Bear Creek pluton) gives a similar 206 Pb/ 238 U age of 54.6 ± 0.8 Ma. Mylonitic megacrystic granite from Sweathouse Canyon yields an age of 63.6 ± 0.6 Ma. A granite sample from the Lochsa Canyon, in the central Idaho-Bitterroot batholith, gives an age of 56.7 ± 1.0 Ma. Inherited zircon from the granitoids ranges in age from 800 to 1820 Ma, but the majority of grains have formation ages of 1750–1800 Ma. This suggests that Paleoproterozoic crust dominates the source region of the Idaho-Bitterroot batholith. Hornblende 40 Ar- 39 Ar age spectra for mafic dikes intruded during late-stage crystallization of main-phase granite in the central Idaho-Bitterroot batholith suggest crystallization of the main-phase plutons in this area at ca. 57 Ma. New and previously published 40 Ar- 39 Ar and K-Ar apparent ages of biotite and muscovite from the Lochsa River area and the western and central Bitterroot core complex are 50 to 47 Ma. Younger mica ages (46–43 Ma) are restricted to the vicinity of the Bitterroot mylonite zone. These results indicate that the cessation of main-phase magmatism within the Bitterroot metamorphic core complex migrated east with time, and that most of the plutons in the core complex were intruded during the Paleocene and early Eocene. When the regional tectonic setting changed from compression to extension at ca. 50 Ma, the late stages of mid-crustal, peraluminous plutonism appear to have been localized within the Bitterroot core complex. The presence of the youngest mid-crustal plutons in this area may have focused extensional deformation leading to the thick mylonite zone, as a consequence of rheological contrasts with cooler areas to the east and west. A progression of K-Ar and 40 Ar- 39 Ar cooling ages from west to east within the core complex part of the batholith is consistent with top-to-the-east shear indicators in the mylonite zone. Thermochronology indicates that the western part of the Bitterroot metamorphic core complex was below ≈350°C at the same time as the last stage of granite emplacement and metamorphism in the east. Therefore, the transition from mylonitization to brittle deformation to inactivity of the shear zone was progressive from west to east across the core complex from ca. 50 to 44 Ma. These features offer an explanation for the previously enigmatic occurrence of amphibolite facies ductile deformation in the eastern part of the core complex coincident with emplacement of epizonal, alkali-feldspar granite plutons along the western side of the complex.

Zoning patterns and petrologic processes in peraluminous magma chambers: Hall Canyon pluton, Panamint Mountains, California

Zoning patterns of mineral, rock, and isotopic compositions in the Hall Canyon pluton in southeastern California are used to determine and discuss the petrologic processes that operate in magma chambers that solidify to form peraluminous plutons. The pluton is biotite-muscovite granodiorite at lower levels and grades to a 150-m-thick upper zone of muscovite granite, capped by a 10- to 20-m-thick zone of pegmatite, aplite, and quartz veins at the roof of the pluton. Mineral and whole-rock compositions are essentially homogeneous in the >600 m of exposed lower zone, whereas the thinner upper zone/roof zone is systematically zoned to more evolved compositions roofward. Compositions of garnet, plagioclase, and muscovite in the upper zone/roof zone track bulk-rock compositions and, in the case of muscovite, range beyond those that have generally been considered magmatic, suggesting that compositional criteria for distinguishing magmatic from secondary muscovite cannot be drawn without consideration of bulk-rock composition. Initial ϵ Nd values of −18 to −19 for the pluton indicate that the magma sources were entirely intracrustal and lower Proterozoic in age. The lower zone could represent melt formed by dehydration melting of a biotite-rich metaigneous or metapsammitic source rock. Upper zone/roof zone rocks are too evolved to have been unmodified crustal melts. Even the least evolved was a product of fractional crystallization, and further in situ fractionation of the upper zone/roof zone produced compositional zoning that defines whole-rock Sr and Nd isochron ages of ca. 72 Ma. Slight differences in initial isotopic ratios indicate that lower zone magma and magma parental to the upper zone/roof zone were derived from different source rocks and remained separate during most of the subsequent fractionation of the upper zone/roof zone. Melt separation was inefficient, leaving behind melt-rich mushes that formed rocks that do not bear strong chemical signatures of being cumulate rocks. We suggest that the upper zone/roof zone was static while it fractionated by loss of residual liquid upward, whereas the thicker lower zone may have been capable of convection that stirred the magma as it crystallized, preventing it from becoming zoned. Late in the crystallization of the pluton, meter-scale layering of the granite formed when biotite muscovite granitic magma was injected as sills into the partly crystalline margin of the chamber near the upper zone–lower zone boundary. We suggest that peraluminous plutons are more heterogeneous isotopically and usually have less regular zoning patterns than metaluminous plutons, because the small effective heights of the magma bodies inhibit convection. This results, in part, from the sill-like form of many peraluminous plutons, but also from melt production rates in zones of purely intracrustal melting that are so low that there is significant cooling of magma in a chamber between arrival of successive melt batches. If resident magma has crystallized to the point of “lock-up,” it cannot mix with new arrivals and does not contribute to the height of magma capable of convection. Systems in which melting is largely the result of advection of heat by mantle-derived basalt might be expected to have larger magma supply rates, because the volume of partial melt produced is not so strictly limited by the abundance of hydrous phases and by the approach of quartz and feldspar to cotectic proportions in the source, and because some of the mafic magma is incorporated into the chamber. Convection would be favored by the resulting greater thickness of magma capable of flow.

Nature and evolution of the eastern margin of lapetus: geochemical and isotopic constraints from Siluro-Devonian granitoid plutons in the New Brunswick Appalachians

In the Gander Zone, Ordovician to Devonian, metaluminous to slightly peraluminous plutons, with within-plate affinities, intrude a lower Paleozoic metasedimentary package. During the Siluro-Devonian, εNd(T) and δ18O values of the metasedimentary rocks would have been about −7 to −8 and +10 to +14[Formula: see text], respectively. Granitoid εNd(T) values range from −0.3 to −4.2, δ18O values lie between +6.6 and +10.4[Formula: see text], and published Pb isotopic compositions are radiogenic (207Pb/204Pb > 15.65). εNd(T) values of associated Silurian gabbros are somewhat positive (+1.2 to +2.3); some plutons exhibit evidence for comingling with this mafic component. Potential source materials include enriched-mantle, supracrustal, and infracrustal sources. Although some constraints can be put on the first two sources, the nature of infracrustal material is difficult to evaluate. Heterogeneous data obtained from Silurian plutons can be explained by crust–mantle mixing. If some plutons contain a significant mantle component, then Mesoproterozoic or older crust is required. However, mixing is potentially complex and significant Neoproterozoic contributions cannot be excluded. The more compositionally homogeneous Devonian plutons probably represent partial melts of hybridized lower crust. The crustal component is interpreted as reflecting the seismically defined Central Crustal Block. The Gander Zone was probably deposited on Avalonian Brookville belt basement when it formed part of the Amazonian craton. Gander Zone clastic sediments, which are characterized by pre-Neoproterozoic isotopic signatures, were derived from this composite Gondwanan craton. We attribute Siluro-Devonian plutonism to collision and lithospheric delamination which accompanied, and followed, accretion of Avalonian belts to Laurentia during its collision with Gondwana.

The Butler Hill Caldera: a mid-Proterozoic ignimbrite-granite complex

Abstract Uplift along rift-related faults in the eastern St. Francois igneous terrane of southeastern Missouri exposes a high-level anorogenic igneous complex consisting of 1.5 km of intracaldera ignimbrites, a zoned central pluton, and late-stage ring plutons. A collapse structure, named the Butler Hill Caldera (BHC), formed in response to eruption of comenditic ignimbrites of the Lake Killarney Formation which attains an intracaldera thickness of 400 m. Continued peralkaline volcanism resulted in an additional 1000 m of Grassy Mountain Ignimbrite within the caldera and a thin, but widespread, outflow sheet. Following degassing of the magma chamber, the central pluton forcefully intruded the intracaldera volcanic sequence producing arcuate folds concentric to the caldera axis. Remnants of the original magma chamber roof are not preserved. The final igneous event related to the BHC involved intrusion of amphibole-granitoids in an annular pattern within, and concentric to, the caldera walls. The latter bodies probably merge, with depth, into a single ring pluton. The petrographic zoning exhibited by the mildly peraluminous central pluton of the BHC consists of: (1) a roof-zone of fine-grained, brick-red, hypersolvus granophyre with abundant miarolitic cavities which is nearly devoid of hydrous minerals; (2) a transitional zone of pink fine- to medium-grained seriate granite in which granophyric intergrowths are common and plagioclase and biotite are present in trace amounts only; and (3) a border zone of medium- to coarse-grained pink to grey subsolvus biotite granite with rapakivi texture. The paradoxical association of peralkaline and peraluminous suites with hypersolvus and subsolvus petrographic traits at the same structural level is recognized in other high-level anorogenic complexes. The common factor in these terranes, regardless of their age, appears to be that initial magmas were hot, relatively dry, low in CaO and high in F. These compositional features favor ascent of hypersolvus magmas to subvolcanic levels where saturation is accomplished by influx of meteoric water. Field, chemical, isotopic, and petrographic evidence indicate that BHC rocks have experienced at least two periods of subsolidus metasomatism. The resulting metasomatic features are typical of anorogenic ignimbrites and high-level granitoids elsewhere and must be quantified before existing chemical data can be applied to questions of igneous petrogenesis.

U-Pb geochronologic constraints on the age of thrusting, crustal extension, and peraluminous plutonism in the Little Rincon Mountains, southern Arizona

The Little Rincon thrust fault is a mylonitic shear zone that juxtaposes Middle Proterozoic Continental Granodiorite over metasedimentary rocks of Proterozoic and early Paleozoic age. This fault is structurally beneath the San Pedro detachment fault and associated ductile deformational fabrics, which formed during early Oligocene to early Miocene time. A syntectonic leucogranite sill within the Little Rincon shear zone yields a U-Pb concordia-intercept age 66{plus minus}10 Ma for zircon and a concordant age 51{plus minus}2 Ma for fractions composed of monazite and xenotime. This demonstrates that compressional deformation in the Catalina and Rincon mountains is generally coeval with Laramide thrust faults that extend at least from southeastern California to southeastern Arizona. A peraluminous granite pluton that truncates the shear zone but displays extension-related fabrics yields a lower-intercept age 24{plus minus}12 Ma for zircon and an age of 30{plus minus}6 Ma for monazite. This indicates that some peraluminous plutons in the region were emplaced during regional crustal extension.

Hepburn intrusive suite: Peraluminous plutonism within a closing back-arc basin, Wopmay orogen, Canada

Research Article| March 01, 1989 Hepburn intrusive suite: Peraluminous plutonism within a closing back-arc basin, Wopmay orogen, Canada André E. Lalonde André E. Lalonde 1Ottawa-Carleton Geoscience Centre, Department of Geology, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information André E. Lalonde 1Ottawa-Carleton Geoscience Centre, Department of Geology, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1989) 17 (3): 261–264. https://doi.org/10.1130/0091-7613(1989)017<0261:HISPPW>2.3.CO;2 Article history First Online: 02 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 André E. Lalonde; Hepburn intrusive suite: Peraluminous plutonism within a closing back-arc basin, Wopmay orogen, Canada. Geology 1989;; 17 (3): 261–264. doi: https://doi.org/10.1130/0091-7613(1989)017<0261:HISPPW>2.3.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 SocietyGeology Search Advanced Search Abstract Within the Hepburn metamorphic-plutonic internal zone of the Wopmay orogen (Northwest Territories, Canada) there are two chronologically and petrologically distinct plutonic associations. The more voluminous of the two, the older 1.885 Ga Hepburn intrusive suite, includes rocks ranging in composition from gabbro to granite, peraluminous granite dominating. The younger 1.855 Ga neighboring Bishop intrusive suite (also gabbro to granite) represents the waning stages of a well-documented calc-alkaline arc, the Great Bear magmatic zone. The petrological distinctions between the two suites are all late-acquired features imposed primarily by contrasting environments of emplacement. Hepburn magmas were intruded within a closing, dominantly sedimentary, back-arc basin. Magma emplacement was synchronous with crustal imbrication, regional metamorphism, and translation of the basin-fill units onto Archean crust. Significant assimilation of sedimentary host rocks by the rising Hepburn magmas occurred, whereas the postregional metamorphism emplacement of the Bishop magmas precluded similar assimilation. The gabbroic contribution observed in the Hepburn intrusive suite is interpreted to reflect a mantle-derived precursor inherited from the back-arc rifting event that immediately preceded emplacement of the suite. 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.

The Role of Manganese in the Paragenesis of Magmatic Garnet: An Example from the Old Woman-Piute Range, California

Strongly peraluminous plutons of the Old Woman-Piute Range, southeastern California, display the differentiation sequence: muscovite-biotite granite $$\rightarrow$$ muscovite-garnet-biotite granite $$\rightarrow$$ garnet-muscovite granite $$\rightarrow$$ garnet-muscovite aplite and pegmatite. The garnets show no evidence of reaction, but biotites in garnet-bearing rocks are rimmed by muscovite. Metamorphic assemblages in the country rocks suggest pressures of emplacement less than 4-5 kb. The plutons become relatively enriched in manganese with differentiation. Whole rock ratios of molar Mn/(Fe + Mg) increase from 0.01 in least felsic, garnet-free rocks to 0.3 in most felsic, garnet-rich rocks. MnO concentrations show a corresponding rise in biotites (0.4 to 1.8 wt %) and, less clearly, in garnets (9 to 25%). The differentiation sequence, textural relations, and mineral and whole rock compositions suggest that garnet paragenesis was controlled by the reaction: $$biotite + Mn-rich liquid \rightarrow garnet...