Adirondack Highlands Research Articles

Mineral Zoning, P-T-X-M Phase Relations, and Metamorphic Evolution of some Adirondack Granulites, New York*

Detailed petrologic analysis of ten meta-anorthosites and related rocks from the Adirondack highlands, New York, has been used to constrain the P–T history of the region. Metamorphic orthopyroxene and clinopyroxene occur in aggregates of ∼1 mm diameter grains that are zoned concentrically with decreasing Al and Fe/(Fe+Mg) and increasing Ca towards the rim; moreover, zoning is not related to the presence of garnet. Zoning is interpreted to have been produced during growth by the reaction: igneous pyroxene + plagioclase = metamorphic Opx + metamorphic Cpx along a cooling path with a slope of 6 bars/°C starting at ∼850°C and 6.5–8 kbar. Garnet occurs as isolated clusters of 5–25 grains of ∼1 mm diameter crystals and as necklaces of ∼1 mm diameter crystals surrounding metamorphic pyroxenes or oxides. Plagioclase ± quartz typically separates garnet from pyroxene, but locally garnet embays metamorphic pyroxene, suggesting that garnet grew at the expense of pyroxene. Garnet clusters have slightly lower Fe/(Fe+Mg) than garnet in necklaces [Fe/(Fe+Mg)=0.73 and 0.75–0.78, respectively] and slightly higher grossular (Xgrs=0.20 vs 0.19). Neither is zoned in Mn and zoning towards increasing Fe/(Fe+Mg) is restricted to contacts with biotite and locally clinopyroxene. Biotite adjacent to garnet is zoned towards decreasing (Fe/(Fe+Mg). Production of garnet in necklaces is interpreted to be the result of the continuous Fe–Mg reaction Opx+Pl+Grt+Cpx+Qtz. P–T–X–M modeling of this reaction reveals a P–T path with a slope of 6 bars/°C. No evidence for early, premetamorphic peak garnet is observed, which rules out any possibility of early higher-pressure metamorphism. The P–T path is therefore inferred to be counterclockwise with loading to 6.5–8 kbar at a maximum temperature of ∼850°C occurring during or immediately following an influx of heat, presumably from early orogenic plutons.

The Grenville Orogenic Cycle (ca. 1350-1000 Ma): an Adirondack perspective

The Adirondack Mountains are characterized by three major events that took place during the interval ca. 1350-1000 Ma. The earliest of these is the arc-related Elzevirian Orogeny (ca. 1350-1185 Ma) during which substantial volumes of juvenile calc-alkaline crust were added to the Adirondacks as well as to the northwest segment of the Central Metasedimentary Belt. Data from the southwestern United States as well as from Ireland and Baltica indicate that Elzevirian magmatism and orogeny were of global dimensions. Within the southwestern sector of the Grenville Province, the Elzevirian Orogeny culminated at ca. 1185 Ma when accretion of all outboard terranes was completed. Compressional orogeny related to this convergence resulted in overthickened crust and lithosphere which subsequently delaminated giving rise to orogen collapse and AMCG magmatism that swept southeastward from the Frontenac Terrane into the Adirondack Highlands during the interval ca. 1180-1130 Ma. Localized compressional events within neighboring parts of the Grenville Province emphasize the continued existence of contraction during this interval, although crustal extension caused local in sedimentary basins in which were deposited the Flinton and the St. Boniface Groups. The Adirondacks have not yet provided any record of events within the interval ca. 1125-1100 Ma, although there is evidence of contraction elsewhere in the southwestern Grenville Province at that time. At 1100-1090 Ma the northern Adirondack Highlands were invaded by mildly A-type hornblende granites (Hawkeye suite) that are interpreted to be the result of local crustal thinning contemporaneous with rifting and mafic magmatism taking place in the Midcontinent rift. Immediately following, at ca. 1090 Ma, the global-scale continental collision of the Ottawan Orogeny was initiated. Strong convergence, deformation, and metamorphism continued to at least ca. 1070 Ma, and rocks older than this are profoundly affected by this event. During the waning stages of the Ottawan Orogeny, overthickened crust and lithosphere delaminated and the orogen underwent collapse. Large extensional faults such as the Carthage-Colton-Labelle shear zone developed and rapidly exhumed granulite facies rocks in the mobile core of the orogen which centers on the Adirondack-Morin terranes and extends southeastward into the New York-New Jersey Highlands. Extensional faulting along the Carthage-Colton mylonite zone dropped the amphibolite facies Lowlands down to the west and into juxtaposition with granulite facies rocks of the Highlands. UPb cooling ages from garnet, monazite, and titanite exhibit a sufficiently broad spectrum to accommodate an initial rapid rate of rebound-related cooling followed by a slower, erosion-controlled cooling history. During delamination, late- to post-tectonic granites of the Lyon Mt. Gneiss (ca. 1070-1045 Ma) were emplaced. The youngest member of this suite is an undeformed fayalite granite dated at ca. 1045 Ma which crosscuts all older rocks and fabric. High-potassium, post-tectonic granites of similar age are common in other parts of the southwestern Grenville Province. Renewed contraction and metamorphism at ca. 1030 Ma demonstrate that the Ottawan Orogen was still experiencing convergence well after the peak of orogeny. However, most of the manifestations involve reactivation of older thrust faults, including the Grenville Front Tectonic Zone. The intrusion of small bodies of anorthosite at ca. 1015 Ma (i.e., Labrieville) provide further evidence for the emplacement of these rocks within collisional orogens, albeit in their collapsing phase.

40Ar/39Ar and Nd‐Sr isotopic characteristics of mid‐Ordovician North American K‐Bentonites: A test of early Paleozoic Laurentia‐Gondwana interactions

Highly altered tephra layers (K‐bentonites) of mid‐Ordovician age are widespread throughout the midcontinent region of the United States and the Appalachian Taconic foreland basin. The numerous ash layers, their wide areal extent, and their large volume provide evidence that extensive volcanic activity accompanied Taconic orogenesis. The mineralogical and geochemical characteristics of these widespread Ordovician tephra layers can be used to constrain the nature of their source region, and thereby help test recent hypotheses that Gondwana collided with Laurentia in mid‐Ordovician time. Initial 143Nd/144Nd and 87Sr/86Sr ratios of two K‐bentonites that occur traced throughout the midcontinent are 0.51183–0.51187 and 0.7101–0.7120, respectively. Isotopic ratios for K‐bentonites within the Taconic basin are 0.51178–0.51201 and 0.70564–0.70905. The isotopic composition of the tephra layers and the ubiquitous presence of Proterozoic zircon xenocrysts within the K‐bentonites is consistent with an origin by crustal anatexis. The cooling history of the source is constrained by 842–996 Ma 40Ar/39Ar apparent ages of hornblende xenocrysts contained within two Taconic basin K‐bentonites. A possible South American source region of the K‐bentonites is the Arequipa massif of Peru. This region contains early Paleozoic arc‐related rocks, many of which contain mid‐Proterozoic zircon xenocrysts, and Early Proterozoic gneisses which were affected by Grenville‐age granulite grade metamorphism. Gneisses within the Adirondack Highlands of the Grenville Province have similar εNd(450 Ma) values and hornblende cooling ages to the K‐bentonites, consistent with this region as a source. Caradocian K‐bentonites in Sweden have Sr isotopic ratios similar to the North American beds. If the Swedish and North American K‐bentonites were produced from the same source, then Baltica and Laurentia were proximal in Ordovician time precluding a Gondwana‐Laurentia link. If Caradocian K‐bentonites with North American characteristics are found in western South America, support for a Laurentia‐Gondwana link would be established. If such K‐bentonites cannot be found, considerable doubt would be cast on this model.

The Parry Sound domain: a far-travelled allochthon? New evidence from U–Pb zicon geochronology

We report U–Pb zircon ages for metaplutonic and metasedimentary rocks from three lithotectonic assemblages within the Parry Sound allochthon of the Central Gneiss Belt, southwestern Grenville Orogen: the basal Parry Sound, interior Parry Sound, and Twelve Mile Bay assemblages. Magmatic crystallization ages for granitic to tonalitic gneisses from the basal Parry Sound assemblage fall in the range 1400–1330 Ma. Younger intrusions include the Parry Island anorthosite dated at 1163 ± 3 Ma and a crosscutting mafic dyke bracketed between 1151 and 1163 Ma. Dated at [Formula: see text] a tonalitic gneiss from the overlying interior Parry Sound assemblage is slightly younger than the older group of rocks from the basal Parry Sound assemblage. 207Pb/206Pb ages for zircons from a quartzite of the basal Parry Sound assemblage range from 1385 Ma to the Neoarchaean. An absolute maximum age for this quartzite is 1436 ± 17 Ma. In contrast, detrital zircons from a quartzite of the Twelve Mile Bay assemblage constrain the age of deposition at post-ca. 1140–1120 Ma. We speculate that Grenvillian-age zircons within this quartzite were derived from rocks in the Adirondack Highlands and Frontenac terrane, implying that part of the Parry Sound domain and these terranes were contiguous during deposition of the quartzite. Our data support previous interpretations that the Parry Sound domain is allochthonous with respect to its surroundings, and suggest that the most likely source region of the basal Parry Sound domain lies southeast of the Central Gneiss Belt, within the Central Metasedimentary Belt boundary thrust zone or the Adirondack Highlands. This implies the possibility of 100–300 km of displacement of the domain. Emplacement of the Parry Sound domain into its present position must have occurred relatively late in the orogen's history, by about 1080 Ma.

The role of inclusions in U-Pb and Sm-Nd garnet geochronology: Stepwise dissolution experiments and trace uranium mapping by fission track analysis

The U-Pb and Sm-Nd dating of garnet are important tools for understanding rates of tectonometamorphic processes and have been widely applied in studies of metamorphic terranes. However, the budgets of uranium, lead, samarium, and neodymium in garnet separates from metamorphic rocks may be dominated by contributions from inclusions of monazite or zircon. A combined fission track and stepwise dissolution technique is proposed for evaluating the role of inclusions of monazite and zircon in the budgets of uranium, lead, samarium, and neodymium in garnets used for U-Pb and Sm-Nd chronology. Variations of Th/U, U/Nd, and Sm/Nd ratios between successive dissolution steps reveal the contributions of monazite and zircon inclusions. The stepwise dissolution procedure does not induce any apparent artifacts on 207Pb- 206Pb and Sm-Nd ages. The technique has been applied to garnets from three metamorphic terranes. Almandine garnet samples from the high grade Pikwitonei Granulite Domain (Manitoba, Canada) and the Wind River Range (Wyoming, USA) have the majority of their uranium, samarium, neodymium, and radiogenic lead (Pb*) hosted by micrometer-scale inclusions of monazite. Fission track densities reveal that uranium is 10 8 times more abundant in inclusions than in garnets. Stepwise dissolution of the samples shows that neodymium and uranium are associated with the dissolution of monazite. In contrast, grossularandradite garnets from Cascade Slide (Adirondack Highlands, NY, USA) show little variation in fission track density and similar isotopic ratios between dissolution steps, indicating that the budgets of uranium, samarium, neodymium, and Pb * are not significantly influenced by inclusions. The demonstrated success of these techniques leads us to recommend similar procedures as a routine matter in U-Pb and Sm-Nd garnet geochronology of metasedimentary rocks.

Moderate pressure metamorphism and anatexis due to anorthosite intrusion, western Adirondack Highlands, New York

Garnet-sillimanite-biotite gneiss near Port Leyden, in the western Adirondack Highlands, New York, contains mineral assemblages and textures that formed during high temperature metamorphism and anatexis at mid-crustal pressures. Evidence for melting includes thin, plagioclase-rich veins, sieve textures in biotite, and the presence of small, euhedral garnet neoblasts. Hercynite-silicate equilibria in combination with the solidus for biotite dehydration melting indicate metamorphic pressure was between 4 and 6.4 kbar at the temperature of melting (ca. 735° C). The gneiss is intruded by a small, discordant Fe-Ti oxide-apatite (nelsonite) dike. Reported field occurrences of nelsonite demonstrate its common association with anorthosite plutons. Although no anorthosite bodies are exposed in the Port Leyden region, the presence of nelsonite is evidence of anorthositic magmatism in the western Adirondacks. Post-intrusion metamorphism has caused partial apatite recrystallization and produced a weak foliated texture in the dike. U-Pb ages from zircon and monazite from both the gneiss and the nelsonite dike indicate that these rocks experienced a complex, polymetamorphic history that we interpret to reflect two thermal episodes. An older event is recorded by discordant zircons in the gneiss, which indicate a minimum age of 1129±6 Ma. A linear best fit to the data yields an upper intercept at 1166±53 Ma. This range of ages coincides with anorthosite-suite magmatism in the Adirondacks. A minimum zircon age of 1104±3 Ma was obtained from the nelsonite dike. Lead-loss or late zircon crystallization at about 1020 Ma affected the U-Pb systematics of zircon in the dike. Monazite ages from both rocks also indicate high temperature metamorphism (>700° C) between 1040 and 960 Ma. The older zircon ages and textural relations in the metapelite are viewed as evidence for anatexis at ca. 1150 Ma, and the presence of nelsonite suggests that the intrusion of anorthosite was coincident with partial melting in the gneiss. P-T estimates of metamorphism, therefore, imply that anorthosite was emplaced to about 15 km depth in the western Adirondack Highlands.

Intergranular diffusion kinetics of Fe and Mg during retrograde metamorphism of a pelitic gneiss from the Adirondack Mountains

Pelitic granulite gneiss from the Irving Pond Formation in the southern Adirondack Highlands, New York, contains chemical zoning in Fe and Mg in both individual mineral grains and over a distance of millimeters in the polycrystalline matrix. Garnet porphyroblasts are zoned with increasing X Alm and decreasing X Prp towards crystal rims. Compositions of individual biotite grains in the matrix vary systematically with distance from the garnet porphyroblasts up to about 1 mm, with the highest Mg concentrations in grains adjacent to the garnet and progressive Fe-enrichment in grains further away. We have constructed numerical models of the distribution of Fe and Mg components obtained during the retrograde reaction and diffusion history of this rock based on the mineral assemblage, modes, textural information, and the preserved zoning trends. These simulations indicate that the effective rate of intergranular transport of Fe and Mg components through the polycrystalline matrix was only one order of magnitude faster than the rate of volume diffusion in garnet. The relatively slow intergranular kinetics indicate that grain boundaries did not provide rapid diffusion pathways. Absence of an interconnected fluid phase along grain edges and grain boundary annealing likely inhibited intergranular diffusivities during cooling. Our models also suggest that the mineral assemblage experienced metamorphic temperatures in excess of 700°C, followed by rapid cooling. This T-t evolution is quite different than determined for the ca. 1060-1000 Ma regional granulite-facies metamorphism in the Adirondack Highlands. Fe Mg distribution in the gneiss likely reflects and earlier metamorphic episode.

Preservation of premetamorphic oxygen isotope ratios in granitic orthogneiss from the adirondack mountains, New York, USA

The Adirondack Mountains, New York, expose a diverse group of Proterozoic igneous rocks that were metamorphosed to granulite facies conditions during the Ottawan phase of the Grenville orogeny. Oxygen isotope data for seventy whole rock samples of gabbroic to granitic meta-igneous rocks, primarily from the charnockite suites from the Tupper and Saranac sheets in the central Adirondacks, demonstrate a correlation between δ18Owr and major element composition within continuous, mappable, meta-plutonic units. No such relationship is seen among nonconsanguineous granitoids. Variations in mineral δ18O values and large differences in δ18O between rocks with nonconsanguineous protoliths but similar bulk composition demonstrate that these rocks were not infiltrated by, or isotopically equilibrated through, a pervasive metamorphic fluid. Values of δ18O for mineral separates preserve generally high temperature fractionations indicative of dominantly closed-system retrograde exchange.Values of δ18Owr in Adirondack orthogneisses were not significantly shifted during granulite facies metamorphism and were dominantly controlled by processes active during premetamorphic magmatism. Values of δ18Owr may thus serve as a petrogenetic indicator, allow discrimination of rock units, and serve as a source constraint for meta-igneous rocks. Fayalite meta-granites with low δ18Owr values can be discriminated from surrounding granitoids having high δ18Owr values. A record of assimilation in the evolution of differentiated granitic units is preserved. The preservation of primary igneous δ18Owr values in granulite facies orthogneiss imposes constraints on the synmetamorphic and postmetamorphic fluid history of the Adirondack Highlands. Oxygen isotopic compositions at the hand-sample scale have been preserved through granulite-facies metamorphism. Fluid absence or low fluid/rock ratios on a regional scale are indicated.

Reconciling deep seismic refraction and reflection data from the grenvillian-appalachian boundary in western New England

The Grenvillian-Appalachian boundary is characterized by pervasive mylonitic deformation and retrograde alteration of a suite of imbricated allochthonous and parautochthonous gneisses that were thrust upon the Grenvillian continental margin during the lower Paleozoic. Seismic reflection profiling across this structural boundary zone reveals prominent dipping reflectors interpreted as overthrust basement slices (parautochthons) of the Green Mountain Anticlinorium. In contrast, a seismic refraction study of the Grenvillian-Appalachian boundary reveals a sub-horizontally layered seismic velocity model that is difficult to reconcile with the pronounced sub-vertical structures observed in the Green mountains. A suite of rock samples was collected from the Green Mountain Anticlinorium and measured at high pressures in the laboratory to determine the seismic properties of these allochthonous and parautochthonous gneisses. The laboratory-measured seismic velocities agree favorably with the modelled velocity structure across the Grenvillian-Appalachian boundary suggesting that the rock samples are reliable indicators of the rock mass as whole. Samples of the parautochthonous Grenvillian basement exposed in the Green Mountains have lower velocities, by about 0.5 km/s, than lithologically equivalent units exposed in the eastern Adirondack Highlands. Velocity reduction in the Green Mountain parautochthons can be accounted for by retrograde metamorphic alteration (hydration) of the paragneisses. Seismic anisotropies, ranging from 2 to 12%, in the mylonitized Green Mountain paragneisses may also contribute to the observation of lower seismic velocities, where the direction of ray propagation is normal to the foliation. The velocity properties of the Green Mountain paragneisses are thus insufficiently different from the mantling Appalachian allochthons to permit their resolution by the Ontario-New York-New England seismic refraction profile.

Sm-Nd and U-Pb Isotopic Evidence of Juvenile Crust in the Adirondack Lowlands and Implications for the Evolution of the Adirondack Mts.

Sm-Nd investigations of Hyde School Gneiss yield $T_{DM}$ ages only slightly older than U-Pb zircon emplacement ages thus indicating substantial addition of juvenile, calcalkaline crust to the Adirondack lowlands at ca. 1300 Ma. Juvenile crust of similar age has been recognized within the Adirondack highlands, the Elzevir terrane further to the northwest in Canada, and in the Green Mts. of Vermont suggesting the presence of an extensive magmatic arc at that time. While the precise nature of the arc remains uncertain, an Indonesian analogue is favored. U-Pb zircon ages of ca. 1150-1160 Ma are reported for lowlands samples of syenitic and granitic rocks belonging to the anorthosite-mangerite-charnockite-granite (AMCG) suite that also occurs in the highlands, as well as in the Canadian sector of the Frontenac terrane. The regional occurrence of these anorogenic intrusions indicates that the Adirondack highlands and Frontenac terrane, including the Adirondack lowlands, constituted a single, contiguous entity by ca. 1160 Ma. Monazite ages from AMCG rocks and metamorphic zircons from Hyde School Gneiss indicate that the last high grade metamorphism (i.e., upper amphibolite facies) in the Adirondack lowlands occurred at ca. 1150 Ma. The presence of 1050 Ma granulite facies assemblages east of the Carthage-Colton mylonite zone (CCMZ) in the highlands indicates the presence of a discontinuity across the CCMZ, which may manifest ca. 1050 Ma downfaulting of the lowlands block along the northwest dipping CCMZ in response to verthickened crust during the Ottawan orogeny.

Granulite facies metamorphism, palaeo‐isotherms and disturbance of the U‐Pb systematics of zircon in anorogenic plutonic rocks from the Adirondack Highlands

Abstract In the Adirondack Highlands of New York State, the effect of granulite facies metamorphism on the physical and isotopic characteristics of zircon from anorogenic plutonic rocks has a distinct geographical pattern. The location of zircon populations which appear to have been altered describes a roughly circular area where metamorphic palaeotemperatures have been determined to be in excess of 750° C. Zircons from anorogenic plutonic rocks outside this area were undisturbed during metamorphism and yield well constrained ages.Granitic, charnockitic and mangeritic anorogenic plutonic rocks peripheral to the Marcy anorthosite massif have large, euhedral, prismatic zircons that display fine, internal, magmatic growth zonations and abundant, randomly orientated, mineral inclusions. Co‐genetic zircon fractions yield linear discordant arrays and well constrained upper intercepts of 1125–1157 Ma. Metamorphic zircon is limited to sporadically developed and volumetrically insignificant, clear, low‐U overgrowths or protuberances.In marked contrast, zircons from petrographically and geochemically identical rocks adjacent to, or within, the Marcy anorthosite massif are typically large, limpid, anhedral to subhedral crystals or crystal fragments lacking internal features except for tubular cavities and CO‐2‐rich inclusions. Co‐genetic zircon fractions yield nearly concordant, non‐linear clusters with 207Pb/206Pb minimum ages of 1073–1095 Ma. Metamorphic overgrowths cannot be readily identified by optical or cathodoluminscence techniques; however, many grains show complex and unusual external boundaries suggestive of post‐crystallization modification.These data indicate that temperatures as low as 750° C, in combination with other factors, may have been sufficient to facilitate recrystallization, and diffusion of radiogenic Pb from the zircon crystal structure, during the complex, protracted metamorphism of the Adirondack Highlands.

Closure temperatures of the Sm—Nd system in metamorphic garnets

Garnet-whole rock and garnet-mineral isochrons were determined on granulite facies gneisses and amphibolites from the Archean Pikwitonei Granulite Domain of the Superior Province, and the Proterozoic Central Gneiss Belt and Adirondack Highlands of the Grenville orogen. The Sm—Nd ages obtained from Archean garnets 0.1–0.5 cm in length are 30–110 Ma younger than the U—Pb ages obtained on the same garnets and also younger than the time of the last regional metamorphism, as determined by the growth ages of the youngest metamorphic garnets and zircons. Similarly, the Sm—Nd ages obtained from Proterozoic garnets with a diameter of 0.1–5 cm are younger than the time of the last regional metamorphism and similar or younger than cooling ages obtained on sphenes from the same sample or from the same geologic setting. Only the core of a garnet with a diameter of ca. 30 cm and without abundant inclusions may record the time of garnet growth. Comparison of the Sm—Nd ages with other geochronologic data and temperature estimates leads to the conclusion that the closure temperature for the Sm—Nd system in garnets analyzed in this study is ca. 600 ± 30°C. Only garnets with radii much larger than 5 cm may record Sm—Nd growth ages in upper amphibolite facies rocks from slowly cooled terranes. Garnets from higher grade terranes yield cooling ages that define the retrograde history of metamorphic terranes.

The Carthage-Colton Mylonite Zone (Adirondack Mountains, New York): The Site of a Cryptic Suture in the Grenville Orogen?

U-Pb ages were determined on metamorphic sphenes and monazites from the Late Proterozoic Adirondack Highlands and Lowlands in the vicinity of the Carthage-Colton mylonite zone. Monazites were extracted from metapelites, and sphenes were separated from marbles, calc-silicate gneisses, and granite gneisses in order to determine the timing and the duration of metamorphism as well as the cooling histories for rocks on either side of the mylonite zone. Monazite ages from the Lowlands range from 1171-1137 Ma; sphene ages in the Lowlands range from 1156-1103 Ma, those from the Highlands immediately to the east of the mylonite zone range from 1050-982 Ma. The ages indicate that the last high-grade metamorphism in the Highlands is ca. 100 m.y. younger than in the Lowlands and that both terranes had separate cooling histories at least until ca. 1000 Ma. Sphenes from within the Carthage-Colton mylonite zone yield ages of about 1098 Ma, which are distinct from sphene ages on either side of the shear zone. The mineral...

Charnockites and granites of the western Adirondacks, New York, USA: a differentiated A-type suite

Abstract Granitic rocks in the west-central Adirondack Highlands of New York State include both relatively homogeneous charnockitic and hornblende granitic gneisses (CG), that occur in thick stratiform bodies and elliptical domes, and heterogeneous leucogneisses (LG), that commonly are interlayered with metasedimentary rocks. Major- and trace-element geochemical analyses were obtained for 115 samples, including both types of granitoids. Data for CG fail to show the presence of more than one distinct group based on composition. Most of the variance within the CG sample population is consistent with magmatic differentiation combined with incomplete separation of early crystals of alkali feldspar, plagioclase, and pyroxenes or amphibole from the residual liquid. Ti, Fe, Mg, Ca, P, Sr, Ba, and Zr decrease with increasing silica, while Rb and K increase. Within CG, the distinction between charnockitic (orthopyroxene-bearing) and granitic gneisses is correlated with bulk chemistry. The charnockites are consistently more mafic than the hornblende granitic gneisses, although forming a continuum with them. The leucogneisses, while generally more felsic than the charnockites and granitic gneisses, are otherwise geochemically similar to them. The data are consistent with the LG suite being an evolved extrusive equivalent of the intrusive CG suite. Both CG and LG suites are metaluminous to mildly peraluminous and display an A-type geochemical signature, enriched in Fe, K, Ce, Y, Nb, Zr, and Ga and depleted in Ca, Mg, and Sr relative to I- and S-type granites. Rare earth element patterns show moderate LREE enrichment and a negative Eu anomaly throughout the suite. The geochemical data suggest an origin by partial melting of biotite- and plagioclase-rich crustal rocks. Emplacement occurred in an anorogenic or post-collisional tectonic setting, probably at relatively shallow depths. Deformation and granulite-facies metamorphism with some partial melting followed during the Ottawan phase of the Grenville Orogeny, yielding the present migmatitic granitic and charnockitic gneisses.

Wagnerite with isokite from the Benson Mines, west-central Adirondack Highlands, New York

AbstractThe rare fluophosphate minerals wagnerite, ideally Mg2(PO4)F, and isokite, ideally CaMg(PO4)F, are intimately associated with magnetite-hematite deposits in sillimanite-, garnet-, and pyroxene-rich paragneisses and migmatites at the Benson Mines, near Star Lake in the west-central Adirondack Highlands of New York State. Coarsely crystalline wagnerite occurs in lenticular masses, typically 4 × 8 cm, delineated by sharply cross-cutting, sinuous, 2 cm-wide veins of fine-grained, fibrous to platy isokite and granular fluorapatite. These also penetrate transverse fractures across wagnerite lenses. Isokite formed from the introduction of Ca- and O-rich hydrothermal solutions into wagnerite. Both minerals are monoclinic: wagnerite crystallises in space group P21/a with a = 11.945, b = 12.717, c = 9.70 Å, β = 108.18°, V = 1400.2 Å3, D(calc) = 3.291 g/cm3 for Z = 16; isokite crystallises in space group A2/a with a = 6.909, b = 8.746, c = 6.518 Å, β = 112.20°, V = 364.7 Å3, D(calc) = 3.248 for Z = 4. Optical properties for wagnerite are: α = 1.5845, β = 1.5875, γ = 1.6010, 2V = 51°(calc.) disp = r < v weak, absorption α < β > γ with α = col., β = pale yel., γ = v. pale yel. For isokite only a mean index of refraction, n = 1.598, could be measured. Wet chemical analysis of wagnerite containing a calculated 11.4% of isokite as fine lamellae, gave the formula:

The Petrology and Geochemistry of Mafic Igneous Rocks in the Anorthosite-Bearing Adirondack Highlands, New York

The Petrology and Geochemistry of Mafic Igneous Rocks in the Anorthosite-Bearing Adirondack Highlands, New York

Crustal structure of the western New England Appalachians and the Adirondack Mountains

We present an interpretation of the crustal velocity structure of the New England Appalachians and the Adirondack Mountains based on a seismic refraction/wide‐angle reflection experiment in eastern North America extending from the Adirondacks in New York State through the northern Appalachians in Vermont and New Hampshire to central Maine. Modeling of the eastern portion of the profile within the New England Appalachians shows a subhorizontal layered crust with upper crustal velocities ranging from 5.5 to 6.2 km/s, a midcrustal velocity of 6.4 km/s, and a lower crustal velocity of approximately 6.8 km/s. Crustal thickness increases from 36 km beneath Maine to 40 km in Vermont. Little evidence is seen for structures at depth directly related to the White Mountains or the Green Mountains. A major lateral velocity change in the upper and mid crust occurs between the Appalachians and the Adirondacks. This boundary, projecting to the surface beneath the Champlain Valley, dips to the east beneath the Green Mountains and extends to a depth of ∼25 km below the eastern edge of the Connecticut Valley Synclinorium in Vermont. The Tahawus Complex, a series of strong horizontal reflections at 18–24 km depth beneath the Adirondack Highlands, is seen to dip eastward beneath Vermont. Upper crustal rocks in the Adirondack Mountains have Poisson's ratios of 0.28±0.01 that can be correlated with the Marcy Anorthosite. Pois son's ratios of 0.24±0.01 calculated for rocks of the Connecticut Valley Synclinorium indicate a siliceous upper crust in Vermont. The lower crust is considered to be best represented by intermediate to mafic granulites; a high Poisson's ratio (0.26–0.27) tends to support a mafic lower crust in the New England Appalachians. This seismic refraction/wide‐angle reflection experiment provides further evidence for the obduction of the allochthonous western Appalachian units onto Grenvillian crust above a zone of detachment that penetrates at least to midcrustal depths and was the locus of successive Paleozoic thrusting.

Age and Regional Relationships of Granitoid Rocks of the Adirondack Highlands

Approximately 55% of the Adirondack highlands is underlain by granitic to tonalitic rocks that can be subdivided into five groups based on age, composition, and field occurrence: (1) early calcalkaline tonalites (ca. 1300 Ma) and associated granitic rocks (ca. 1250 Ma) of the southern and eastern highlands, (2) older anorogenic plutonic rocks (1145-1160 Ma) that occur throughout the highlands, (3) younger anorogenic plutonic rocks (ca. 1130 Ma) that envelop anorthosite massifs in the central highlands, (4) younger granitic rocks (1095-1105 Ma) of the northwest highlands, and (5) late leucogranitic rocks (1060-1080 Ma) that occur throughout the northern highlands. The data presented here are in good agreement with zircon ages from the Green Mountains of Vermont as well as from southern Ontario and Quebec and suggest similar geologic histories for these regions. These results document a protracted and complicated evolution for this portion of the Grenville Province.

Juvenile Middle Proterozoic crust in the Adirondack Highlands, Grenville province, northeastern North America

Nd isotope data indicate that minimal amounts of significantly older crust have contributed to the genesis of the oldest (ca. 1.3-13.5 Ga) plutons in the Adirondack Highlands. These are magmatic arc tonalites with positive initial {epsilon}{sub Nd} values and Sm-Nd depleted mantle model ages (t{sub DM}) that are within 70 m.y. of the time of their crystallization. Granitoids of the anorthosite-mangerite-charnockite-granite suite, dated at 1,156-1,134 Ma, as well as the 1,100-1,050 Ma plutons, associated with the Ottawan phase of the Grenvillian orogenic cycle, also have positive initial {epsilon}{sub Nd} values and t{sub DM} ages similar to the tonalites. Derivation of both groups of granitoids by crustal melting of the magmatic arc is consistent with the available isotopic and geochemical data. Juvenile late Middle Proterozoic crust that formed during or just prior to the Grenville cycle appears to dominate the southwestern Grenville province as well as the Grenville inliers to the south. In contrast, most of the contiguous Grenville province in Canada comprises largely reworked older crust.

Regional AI-Fe mafic magmas associated with anorthosite-bearing terranes

PROTEROZOIC anorthosite massifs are nearly monomineralic accumulations of plagioclase feldspar, which must result from unusual conditions of crystallization and emplacement1,2. Their chemistry is compatible with derivation from a high-alumina-basalt parent of mantle origin1, and mafic magmas rich in aluminium and iron have long been suspected of having an important connection to anorthosites and their parent magmas1–4. Regional examples of mafic magmas clearly compatible with an anorthosite lineage have not been generally recognized3, in part because of the paucity of candidates within the anorthosite massifs themselves. Samples of such magmas should also be sought in the country rocks surrounding the massifs. Here we report the widespread occurrence of Al–Fe mafic magmas in the anorthosite-bearing Adirondack Highlands of New York state. They are chemically distinct from typical high-AI basalt, Fe basalt and most common magma types. Identification of similar Al–Fe basaltic magmas in other areas suggests that such magmas are characteristic of anorthosite-bearing terranes.