Adirondack Highlands Research Articles

Evolution of layering in a migmatite sample: Implications for the petrogenesis of multidomain monazite and zircon

Abstract The timing of partial melting in high-grade metamorphic rocks is critical for constraining tectonic histories and processes. However, uncertainties exist about the behavior of monazite and zircon during partial melting, especially about the timing of crystallization with respect to melting reactions. This study is focused on a single sample (16TG143) of finely layered, migmatitic gneiss from the Adirondack Highlands, New York, interpreted to have undergone extensive biotite dehydration melting (i.e., Bt + Pl + Als + Qz = Grt + Kfs + melt). The rock contains one distinct leucosome layer. The non-leucosome (gray gneiss) portion of the migmatite has millimeter-scale sublayers with distinct differences in modes and mineralogy. The layers are interpreted to reflect the differential preservation of reactants and products formed during the forward and reverse progress of the melting reaction. Monazite and zircon modes, and to some degree, texture, composition, and geochronology all vary from layer to layer. Both minerals have up to three domains: ca. 1150 Ma anhedral cores, ca. 1050 Ma monazite mantles/fir tree textured zircon, and ca. 1030 Ma rims. The heterogeneous layered gray gneiss provides robust constraints on the timing of melting (ca. 1050 Ottawan orogenesis), melt crystallization, and post-melting retrogression, in addition to information about earlier metamorphic events. Early-formed monazite and zircon grains were largely dissolved during progressive melting, except where preserved as relicts or inclusions. Monazite mantles and fir tree zircon grains precipitated upon cooling during progressive melt crystallization between temperatures of 800 and 750 °C. Rims are interpreted to have precipitated during subsolidus, solid-state retrogression after ca. 1050 Ma. Correlations between the gneissic layering, melting reactions, and the character of geochronometers emphasize the importance of characterizing the layer-forming and chronometer petrogenesis processes as a critical part of deconvoluting the history of migmatitic gneisses.

Ultrahigh-temperature granulite-facies metamorphism and exhumation of deep crust in a migmatite dome during late- to post-orogenic collapse and extension in the central Adirondack Highlands (New York, USA)

AbstractThis study combines field observations, mineral and whole-rock geochemistry, phase equilibrium modeling, and U-Pb sensitive high-resolution ion microprobe (SHRIMP) zircon geochronology to investigate sillimanite-bearing felsic migmatites exposed on Ledge Mountain in the central Adirondack Highlands (New York, USA), part of an extensive belt of mid-crustal rocks comprising the hinterland of the Mesoproterozoic Grenville orogen. Phase equilibrium modeling suggests minimum peak metamorphic conditions of 960–1025 °C and 11–12.5 kbar during the Ottawan orogeny—significantly higher pressure-temperature conditions than previously determined—followed by a period of near-isothermal decompression, then isobaric cooling. Petrography reveals abundant melt-related microstructures, and pseudosection models show the presence of at least ~15%–30% melt during buoyancy-driven exhumation and decompression. New zircon data document late Ottawan (re)crystallization at ca. 1047 ± 5 to 1035 ± 2 Ma following ultrahigh-temperature (UHT) metamorphism and anatexis on the retrograde cooling path. Inherited zircon cores give a mean date of 1136 ± 5 Ma, which suggests derivation of these felsic granulites by partial melting of older igneous rocks. The ferroan, anhydrous character of the granulites is similar to that of the ca. 1050 Ma Lyon Mountain Granite and consistent with origin in a late- to post-Ottawan extensional environment. We present a model for development of a late Ottawan migmatitic gneiss dome in the central Adirondacks that exhumed deep crustal rocks including the Snowy Mountain and Oregon anorthosite massifs with UHT Ledge Mountain migmatites. Recognition of deep crustal meta-plutonic rocks recording UHT metamorphism in a migmatite gneiss dome has significant implications for crustal behavior in this formerly thickened orogen.

New interpretations of the ages and origins of the Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss, Adirondack Mountains, NY: Implications for the nature and timing of Mesoproterozoic plutonism, metamorphism, and deformation

The Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss are widespread Mesoproterozoic plutonic rocks that occur in the amphibolite- to granulite-facies Adirondack Highlands of northern New York, USA. The strongly deformed Hawkeye Granite Gneiss, previously dated by zircon multi-grain thermal ionization mass spectrometry (TIMS) U-Pb analyses at about 1100 Ma, was intruded by the weakly deformed Lyon Mountain Granite Gneiss. Previous sensitive high resolution ion microprobe (SHRIMP) analyses of Lyon Mountain Granite Gneiss zircon rims were considered to record the time of igneous emplacement at about 1.05 Ga, whereas the ages of zircon cores (~1.15 Ga) were interpreted as being inherited from nearby metaigneous country rocks. This interpretation has formed the basis of numerous models for the Mesoproterozoic structural and tectonic evolution of the Adirondacks Highlands. New U-Pb spot analyses (~15–20 µm diameter) by SHRIMP from four samples of Hawkeye Granite Gneiss and eight samples of Lyon Mountain Granite Gneiss challenge the historically accepted ages of the rocks. Using a combination of high-resolution CL imagery of oscillatory zoned cores and weakly zoned to unzoned rims, SHRIMP U-Pb geochronology, SHRIMP trace element geochemistry, and SEM petrography, we conclude that: (1) the Hawkeye Granite Gneiss was emplaced at about 1160–1155 Ma; (2) the vast majority of Lyon Mountain Granite Gneiss zircon cores yield ages of 1150–1145 Ma and are mainly primary, not inherited; and (3) the Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss zircon rims (~1080–1000 Ma) are metamorphic in origin, not igneous. Thus, both the Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss are considered herein to be late members of the mangerite-charnockite-granite (MCG) plutonic suite, emplaced during the waning stages of the Shawinigan orogeny. Zircon cores from two small plutons of largely undeformed fayalite granite of the Lyon Mountain Granite Gneiss are about 1142 Ma, providing a minimum age constraint for the termination of the Shawinigan. Zircon rim ages can be deconvoluted into multiple metamorphic events of the Ottawan and Rigolet tectonothermal events. These new interpretations have profound implications for structural, tectonic, and ore deposit models of the Adirondacks Highlands.

Gore Mountain Garnet Amphibolite Records UHT Conditions: Implications for the Rheology of the Lower Continental Crust during Orogenesis

Abstract The Gore Mountain Garnet Amphibolite (GMGA), part of the Mesoproterozoic Grenville Province in the Adirondack Highlands, NY, USA, is an iconic rock type known for hosting the world’s largest garnets (up to 1 m diameter). We present a new detailed petrographic study of these rocks. Field relations, whole-rock, and mineral major and trace element chemistry suggest that these rocks formed via a prograde hydration reaction of a metagabbro during an increase in pressure and temperature. Laser ablation inductively coupled plasma mass spectrometry U–Pb geochronology applied to zircon interpreted to be metamorphic in origin dates this reaction to 1053·9 ± 5·4 Ma (2σ; MSWD = 0·94), during the Ottawan Orogeny (1090–1020 Ma). Our results on peak metamorphic P–T conditions based on thermobarometry, diffusion models, and thermodynamic modelling indicate that these rocks formed at ultrahigh-temperature (>900 °C) conditions (P = 9–10 kbar, T = 950 ± 40 °C), significantly hotter than previously estimated. Diffusion models pinned by nearby cooling ages require the GMGA to initially cool quickly (9·1 °C Ma–1), followed by slower cooling (2·6 °C Ma–1). The two-stage cooling history for the GMGA could reflect initial advection-dominated cooling followed by conduction-dominated cooling once flow ceases. Our results suggest that the region was hot enough to undergo topography-driven lower crustal flow similar to that hypothesized for modern Tibet for 20–0 Ma (25–0 Ma when the effects of melt are included).

Integrated geophysical imaging of rare earth element-bearing iron oxide-apatite deposits in the Eastern Adirondack Highlands, New York

The eastern Adirondack Highlands of northern New York host dozens of iron oxide-apatite (IOA) deposits containing magnetite and rare earth element (REE)-bearing apatite. We use new aeromagnetic, aeroradiometric, ground gravity, and sample petrophysical and geochemical data to image and understand these deposits and their geologic framework. Aeromagnetic total field data reflect highly magnetic leucogranite host rock and major structures that likely served as fluid conduits for the hydrothermal system. Band-pass filtering of the aeromagnetic data reveals locations of individual deposits that were verified in the field or from historical records. A 3D inversion for magnetic susceptibility images these deposits at depth, allowing the inference of plunge directions and relative size. Radiometric data highlight variations in the surface geology and several large tailings piles that contain REE-bearing apatite. Within the host rock, eTh (equivalent Th), K, and the eTh/K ratio are variable with high eTh/K near several of the IOA deposits. Areas with elevated K or low eTh/K representing potassic alteration appear to be rare; instead, elevated eTh/K ratios likely reflect widespread sodic alteration that overprinted potassic alteration. Bouguer gravity anomalies show limited correspondence to the surface geology, radiometric data, or magnetic data, but they do exhibit approximately 10 km wide highs in areas where deposits are observed. Some 2D forward models of the gravity and magnetic data show that deeper dense material beneath the leucogranite is quantitatively feasible. If these dense rocks represent intrusions that were emplaced or still cooling at the time of mineralization, they may have served as a heat source that helped to drive the hydrothermal system. Combining data sets, we find that deposits occur toward the distal ends of major structures within the host leucogranite and mostly above gravity highs. The geophysical modeling thus suggests that IOA deposits formed in structural, thermal, and chemical traps near the distal ends of the hydrothermal system.

Constraining the timing and character of crustal melting in the Adirondack Mountains using multi-scale compositional mapping and in-situ monazite geochronology

AbstractMigmatites are common in the hinterland of orogenic belts. The timing and mechanism (in situ vs. external, P-T conditions, reactions, etc.) of melting are important for understanding crustal rheology, tectonic history, and orogenic processes. The Adirondack Highlands has been used as an analog for mid/deep crustal continental collisional tectonism. Migmatites are abundant, and previous workers have interpreted melting during several different events, but questions remain about the timing, tectonic setting, and even the number of melting events. We use multiscale compositional mapping combined with in situ geochronology and geochemistry of monazite to constrain the nature, timing, and character of melting reaction(s) in one locality from the eastern Adirondack Highlands. Three gray migmatitic gneisses, studied here, come from close proximity and are very similar in microscopic and macroscopic (outcrop) appearance. Each of the rocks is interpreted to have undergone biotite dehydration melting (i.e., Bt + Pl + Als + Qz = Grt + Kfs + melt). Full-section compositional maps show the location of reactants and products of the melting reaction, especially prograde and retrograde biotite, peritectic K-feldspar, and leucosome, in addition to all monazite and zircon in context. In addition, the maps provide constraints on kinematics during melting and a context for interpretation of accessory phase composition and geochronology. More so than zircon, monazite serves as a monitor of melting and melt loss. The growth of garnet during melting leaves monazite depleted in Y and HREEs while melt loss from the system leaves monazite depleted in U. Results show that in all three localities, partial melting occurred during at ca. 1160–1150 Ma (Shawinigan orogeny), but the samples show high variability in the location and degree of removal of the melt phase, from near complete to segregated into layers to dispersed. All three localities experienced a second high-T event at ca. 1050 Ma, but only the third (non-segregated) sample experienced further melting. Thus, in addition to bulk composition, the fertility for melting is an important function of the previous history and the degree of mobility of earlier melt and fluids. Monazite is also a sensitive monitor of retrogression; garnet breakdown leads to increased Y and HREE in monazite. Results here suggest that all three samples remained at depth between the two melting events but were rapidly exhumed after the second event.

Microscopic Blue Sapphire in Nelsonite from the Western Adirondack Mountains of New York State, USA

Microscopic, non-gem quality, grains of blue sapphire (corundum) have been identified in a small (1–2 cm wide), discontinuous, dike of nelsonite hosted by aluminous feldspathic gneiss. The gneiss was excavated during the construction of a hydroelectric plant on the Black River at Port Leyden, NY (western Adirondack Highlands). The sapphire location is 250 m NE of the Port Leyden nelsonite deposit. The small dike may represent a separate intrusion of nelsonite or one sheared from the main nelsonite orebody during Ottawan (circa 1050 Ma) deformation and metamorphism. The sapphires range in size from 0.1 to 2.0 mm, and commonly show parting, pleochroism, and hexagonal oscillatory zoning (from deep blue to clear). Electron microprobe analysis shows comparable levels of Fe in both clear (0.71–0.75 wt. %) and blue (0.38–0.77 wt. %) portions of grains, but clear sections have significantly lower TiO2 levels (0.002–0.011 wt.%) compared to blue sections (0.219–0.470 wt. %). Cr2O3 abundances range from 0.006 to 0.079 wt. % whereas V2O3 abundances range from 0.010 to 0.077 wt. % in blue sapphires. Small amounts of MgO were detected in one of the clear corundum grains (0.013 wt. %) and two of the six blue grains (0.001–0.015 wt. %), but the remaining five grains were below the limit of detection. Ga2O3, however, was detected in five out of six blue-colored grains (0.026–0.097 wt. %) but was below the limits of detection for clear grains. Optical spectroscopic data collected on the blue sapphire grains show broad absorbance in the yellow, orange, and red part of the spectrum (~565–740 nm) consistent with intervalence charge transfer between the next nearest neighbor Fe2+ and Ti4+. A magmatic origin of the sapphire grains is supported by petrologic and trace element data from the blue sapphires, but Cr abundances are inconsistent with this interpretation. Sapphire in a nelsonite host rock represents a new type of occurrence.

Detrital zircon constraints on Grenville sedimentation at the margin of Laurentia

The southern Grenville Province is made up of metaigneous and metasedimentary rocks that were formed and metamorphosed as part of the Mesoproterozoic active margin of Laurentia. Grenville Supergroup quartzites from the Adirondack Highlands (New York), the Morin terrane (Quebec), and New Jersey Highlands were analyzed to determine detrital and metamorphic zircon U-Pb ages and to help constrain Mesoproterozoic tectonics. These Grenville quartzites have broadly similar detrital age populations as each other, suggesting similar sedimentary source regions and depositional settings in back-arc basin environments at the Laurentian Margin during Geon 12. All samples contain detrital zircon with 1.45–1.25 Ga ages, consistent with proximal Grenville source regions. They also contain detrital ages that point to derivation from the Mazatzal (1.7–1.6 Ga), Yavapai (1.8–1.7 Ga), and Penokean (1.9–1.8Ga) orogens of the North American Midcontinent, and have a diverse population of Archean ages. These results and published data from Adirondack Lowlands metasediments (Chiarenzelli et al., 2015, 2017) show distinctly Laurentian sources, and the related and likely connected nature of the sedimentary histories of the Adirondack Highlands and Lowlands, Morin terrane and New Jersey Highlands. Metamorphic zircon formation is common in high-grade quartzites, and ages are variable between samples depending on the metamorphic history of each terrane and the melt productivity of different rocks (Peck et al., 2010a). Adirondack Highlands samples experienced metamorphic zircon grown during the Shawinigan orogeny and anorthosite-suite emplacement (1.19–1.14 Ga). Morin terrane samples are from the Morin Shear Zone, which was active and caused metamorphic zircon growth during the Ottawan phase of the Grenvillian orogeny (1.09–1.02 Ga). The New Jersey Highlands sample has metamorphic zircon formed during the Rigolet phase of the Grenvillian orogeny (1010–980 Ma).

Syn-collisional exhumation of hot middle crust in the Adirondack Mountains (New York, USA): Implications for extensional orogenesis in the southern Grenville province

Abstract Extensional deformation in the lower to middle continental crust is increasingly recognized and shown to have significant impact on crustal architecture, magma emplacement, fluid flow, and ore deposits. Application of the concept of extensional strain to ancient orogenic systems, like the Grenville province of eastern North America, has helped decipher the structural evolution of these regions. The Marcy massif is a ∼3000 km2 Mesoproterozoic anorthosite batholith in the Adirondack Mountains (New York, USA) of the southern Grenville province. Bedrock geology mapping at 1:24,000 scale paired with characterization of bedrock exposed by recent landslides provides a glimpse into the structural architecture of the massif and its margin. New data demonstrate granulite- to amphibolite-facies deformational fabrics parallel the margin of the batholith, and that the Marcy massif is draped by a southeast-directed detachment zone. Within the massif, strain is localized into mutually offsetting conjugate shear zones with antithetic kinematic indicators. These relationships indicate that strain was coaxial within the Marcy massif, and that subsimple shear components of strain were partitioned along its margin. In situ U–Th–total Pb monazite analysis shows that deformation around and over the Marcy massif occurred from 1070 to 1060 Ma during granulite-facies metamorphism, and monazite from all samples record evidence for fluid-mediated dissolution reprecipitation from 1050 to 980 Ma. We interpret that rocks cooled isobarically after accretionary orogenesis and emplacement of the anorthosite-mangerite-charnockite-granite plutonic suite at ca. 1160–1140 Ma. Gravitational collapse during the Ottawan phase of the Grenville orogeny initiated along a southeast-directed detachment zone (Marcy massif detachment zone), which accommodated intrusion of the Lyon Mountain Granite Gneiss, and facilitated substantial fluid flow that catalyzed the formation of major ore deposits in the Adirondack Highlands.

Geochemistry and Geophysics of Iron Oxide-Apatite Deposits and Associated Waste Piles with Implications for Potential Rare Earth Element Resources from Ore and Historical Mine Waste in the Eastern Adirondack Highlands, New York, USA

Abstract The iron oxide-apatite (IOA) deposits of the eastern Adirondack Highlands, New York, are historical high-grade magnetite mines that contain variable concentrations of rare earth element (REE)-bearing apatite crystals. The majority of the deposits are hosted within sodically altered Lyon Mountain granite gneiss, although some deposits occur within paragneiss, gabbro, anorthosite, or potassically altered Lyon Mountain granite gneiss. The IOA deposits and the waste and/or tailings piles associated with them have potential as an unconventional resource for REEs. Reprocessing of these piles would have the advantage of partial recycling of the waste material to produce a set of critical elements. Thirty-four ore, nine rock, 25 waste-pile, and four tailings-pile samples were collected and analyzed for major, minor, and trace elements. At the tailings- and waste-pile sites, composite samples were collected by combining 30 to >50 subsamples randomly distributed over each pile. The total REE content of the waste and tailings piles varied from approximately 10 to 22,000 ppm, whereas the ore sample concentrations ranged from approximately 15 to 48,000 ppm total REEs. A positive correlation exists between the total REE content of ore and its associated waste pile. Median light REE/heavy REE values were 2.14 for waste/tailings piles and 2.25 for ore, which is a substantial relative enrichment in the heavy REEs in comparison to many developed REE mines, such as the mined carbonatites of Bayan Obo, China, and Mountain Pass, California. Importantly, the ore and waste samples are significantly enriched in both Y and Nd compared to other REEs in the samples. Other minor components such as Th are also elevated. Airborne radiometric surveys show large positive eTh and eU anomalies corresponding to tailings piles. Although it is a limited data set, geochemical data of unaltered and altered host rocks suggest a speculative new model for IOA ore formation in the Adirondack Highlands that is consistent with the geology and previously published data. The ferroan ore-hosting Lyon Mountain granite gneiss underwent localized potassic alteration that enriched the altered rock in Fe, REEs, Th, and other metals. A later sodic alteration event affected the previously potassically altered Lyon Mountain granite gneiss, which increased rock porosity and remobilized Fe, REEs, and other elements from the host rock into the iron ore seams. The sodic fluids responsible for ore formation were enriched in F and Cl.

Spatial and temporal links between shawinigan accretionary orogenesis and massif anorthosite intrusion, southern grenville province, New York, U.S.A.

Synthesis of new and existing field, structural, geochemical, isotopic, and geochronologic data in the Adirondack Highlands of the southern Grenville Province indicate a suite of arc plutonic rocks (Piseco Lake granite suite) was intruded into basement rocks from ca. 1205–1180 Ma due to northward subduction beneath the Laurentian margin. Although rocks of similar age and origin are found in the Adirondack Lowlands and elsewhere in the Highlands, they occur in the greatest volume and are most intensely deformed in the Piseco Lake Shear Zone (PLSZ). This structure separates the Adirondack Highlands from ca. 1350-1300 Ma tonalitic gneisses in the southern Adirondacks. Tectonites in the PLSZ record the deformation of a continental arc batholith during collision with an outboard terrane which was accreted to Laurentia during the Shawinigan Orogeny. Internal to the PLSZ, a 10–15 km wide, subvertical zone of subhorizontally lineated L-S and L-tectonites (Piseco Lake gneisses) was derived primarily from megacrystic arc plutonic protoliths and documents sinistral relative motion. Intrusion of the arc granitoids, deformation and anatexis (ca. 1180–1160 Ma) throughout the entire Adirondack region immediately preceded and overlapped the intrusion of the anorthosite-mangerite-charnockite-granite (AMCG) suite at ca. 1165-1145 Ma in the Adirondack Highlands. We hypothesize that delamination or slab detachment, in combination with an irregular Laurentian margin, triggered disruption of the underlying lithosphere, allowed ascent of enriched asthenosphere, and facilitated the intrusion of voluminous AMCG rocks in the Adirondacks north of the PLSZ. Other large anorthosite massif complexes in the southern Grenville Province, including the Morin and Lac St. Jean bodies near the Laurentian margin, have also been linked to Shawinigan convergent structures and were an important part of accretionary processes documented in the Grenville Orogen.

Calcite twinning strain variations across the Proterozoic Grenville orogen and Keweenaw-Kapuskasing inverted foreland, USA and Canada

We report the calcite twinning strain results of a traverse across the Grenville orogen from Parry Sound, Ontario (NW) to Ft. Ann, New York (SE), including the younger, adjacent Ordovician Taconic allochthon. Fifty four carbonates (marbles, calcite veins, Ordovician limestone) were collected resulting in 68 strain analyses on mechanically twinned calcite (n = 2337 grains) across the Central Gneiss Belt (CGB; 3 samples), the Central Metasedimentary Belt (CMB; 27 samples), the Central Granulite Terrane (CGT; Adirondack's; 13 samples) and the Ottawan Orogenic Lid (OOL; 11 samples). Twinning strains in the greenschist-grade OOL marbles preserve N–S shortening and U-Pb titanite ages (∼1150 Ma; n = 4) document these marbles formed during the Shawinigan (1190–1140 Ma) part of the Grenville orogen. From northwest to southeast, the Ottawan (1095–1020 Ma) twinning strain is dominantly a layer-parallel shortening fabric oriented N–S (Parry Sound), then becomes parallel to the Grenville thrust direction (NW–SE) across the CMB to the Adirondack Highlands where the sub-horizontal shortening strain becomes margin-parallel (SW–NE). Within the regional sample suite there are two areas studied in detail, the Bancroft shear zone (n = 11) and a roadcut on the southeast side of the Adirondack Mountains (Ft. Ann, NY; n = 8). Marbles from the Bancroft shear zone contain calcite grains with 2 sets of twin lamellae (e1 and e2). The better-developed e1 sets (n = 406) record a horizontal fabric oriented NW–SE whereas the younger e2 lamellae (n = 146) preserve a margin-parallel (SW–NE) horizontal fabric. Both the e1 and e2 strains record an overprint vertical shortening strain (NEV), perhaps related to extensional orogenic collapse. We also report an Ottawan orogen-aged granoblastic mylonite (1093 Ma, U-Pb zircon; 1102 Ma Ar-Ar biotite) in the Keweenaw thrust hanging wall 500 km inboard of the Grenville front and interpret the relations of Grenville-Keweenaw far-field dynamics.

The intrusion breccia in the valley of Roaring Brook, Giant Mountain, Adirondack Highlands, New York: A modern interpretation

An intrusion breccia exposed along Roaring Brook on the west face of Giant Mountain in the Adirondack Highlands (eastern USA) has been reinterpreted as a magma conduit. It lacks metasedimentary xenoliths, as previously proposed, and field, petrographic, and geochemical evidence point to an igneous origin for the vast majority of the enclaves. A mechanism involving magma mixing is proposed for the enclaves and explains their textural features (e.g., crescumulate and comb textures and mantled feldspars), compositional layering, and geochemical trends. Rock types ranging from charnockite to pyroxenite occur in the exposed section and document the variety of magmatic processes and compositional variation of magmas composing the Marcy Anorthosite massif. The bulk of the enclaves in the intrusion breccia can be explained by a batch of more felsic magma (charnockite-mangerite?) that intruded into a partially crystallized zone of anorthositic, gabbroic, and dioritic rock. This resulted in disruption, partial mixing and hybridization, and chilling of mafic magma within, and against, the more silicic melts.

Intragrain oxygen isotope zoning in titanite by SIMS: Cooling rates and fluid infiltration along the Carthage‐Colton Mylonite Zone, Adirondack Mountains, NY, USA

AbstractOxygen isotopes are an attractive target for zoning studies because of the ubiquity of oxygen‐bearing minerals and the dependence of mineral 18O/16O ratios on temperature and fluid composition. In this study, subtle intragrain oxygen isotope zoning in titanite is resolved at the 10‐μm scale by secondary ion mass spectrometry. The patterns of δ18O zoning differ depending on microstructural context of individual grains and reflect multiple processes, including diffusive oxygen exchange, partial recrystallization, grain‐size reduction, and grain growth. Using the chronological framework provided by structural relations, these processes can be related to specific events during the Grenville orogeny. Titanite was sampled from two outcrops within the Carthage‐Colton Mylonite Zone (CCMZ), a long‐lived shear zone that ultimately accommodated exhumation of the Adirondack Highlands from beneath the Adirondack Lowlands during the Ottawan phase (1090–1020 Ma) of the Grenville orogeny. Titanite is hosted in the Diana metasyenite complex, which preserves three sequentially developed fabrics: an early NW‐dipping protomylonitic fabric (S1) is crosscut by near‐vertical ultramylonitic shear zones (S2), which are locally reoriented by a NNW‐dipping mylonitic fabric (S3). Texturally early titanite (pre‐S2) shows diffusion‐dominated δ18O zoning that records cooling from peak Ottawan, granulite‐facies conditions. Numerical diffusion models in the program Fast Grain Boundary yield good fits to observed δ18O profiles for cooling rates of 50 ± 20 °C Ma−1, which are considerably faster than the 1–5 °C Ma−1 cooling rates previously inferred for the Adirondack Highlands from regional thermochronology. High cooling rates are consistent with an episode of rapid shearing and exhumation along the CCMZ during gravitational collapse of the Ottawan orogen at c. 1050 Ma. Texturally later titanite (syn‐S2) has higher overall δ18O and shows a transition from diffusion‐dominated to recrystallization‐dominated δ18O zoning, indicating infiltration of elevated‐δ18O fluids along S2 shear zones and continued shearing below the blocking temperature for oxygen (~≤500 °C for grain sizes at the study site). The texturally latest titanite (post‐S3) has growth‐dominated δ18O zoning, consistent with porphyroblastic grain growth following cessation of shearing along the Harrisville segment of the CCMZ.

Geochemistry and geochronology of the 1.3 Ga metatonalites from the Central Metasedimentary Belt boundary thrust zone in southern Ontario, Grenville Province, Canada

Studies of pre-Grenvillian (1.4–1.3 Ga) plutons offer insight into the dynamics of arc amalgamation and backarc rifting prior to continental collision during the Ottawan orogeny. The Central Metasedimentary Belt boundary thrust zone (CMBbtz) is a northeast-southwest–trending thrust zone consisting of metaplutonic thrust sheets enveloped in gneissic tectonites and calcitic-dolomitic marble. Tonalitic CMBbtz thrust sheets (Dysart and Redstone), located in the southern Ontario Grenville Province (Canada) are made up of upper amphibolite facies, foliation-concordant metatonalite (+ amphibole ± biotite ± accessory zircon and titanite) and amphibolite (± biotite ± clinopyroxene). These thrust sheets are thought to have formed and amalgamated onto the Laurentian margin prior to Ottawan orogeny. Major and trace element analyses show that the metatonalite rocks have calc-alkaline affinity and amphibolite rocks have both calc-alkaline and tholeiitic affinities, suggesting an arc environment. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) geochronology of zircon from the two thrust sheets yield igneous ages of ca. 1350–1300 Ma for diorite and granodiorite, ca. 1150–1100 Ma ages from Dysart tonalite interpreted to record metamorphic zircon growth, and a ca. 1086 Ma Ottawan metamorphic age from a Dysart amphibolite. The ca. 1150–1100 Ma metamorphic event has not been previously documented within these thrust sheets of the CMBbtz, but correlates well with thermal events in the allochthonous Parry Sound domain to the west, as do ca. 1350 Ma igneous ages of tonalite in both areas. These data support the hypothesis that the CMBbtz and Parry Sound domain may have been initially linked. Widespread ca. 1350 Ma crust along with distinct 1460–1400 Ma depleted mantle model ages (TDM) are also consistent with a shared genesis with the Dysart–Mount Holly suite in New York and Vermont, and support the correlation between the CMBbtz thrust sheets and the Adirondack Highlands–Mount Holly belt as a rifted arc. Alternatively, the CMBbtz thrust sheets and the Adirondack Highlands–Mount Holly belt may represent contemporaneous arc development at different parts of the convergent margin; however, we support the correlation between the CMBbtz thrust sheets and the Adirondack-Highlands–Mount Holly belt.

Zircon‐bearing, crystallized melt inclusions in peritectic garnet from the western Adirondack Mountains, New York State, USA

AbstractCrystallized melt inclusions (MI) occur in garnet from metapelitic gneiss extracted from a hydroelectric plant on the Black River at Port Leyden, NY (western Adirondack Highlands). The garnet host grains are small (0.5–1.0 mm), euhedral neoblasts that formed peritectically during biotite melting accompanying the Ottawan phase of the Grenville Orogeny. The crystallized MI are typically 10–25 μm in diameter and occur randomly in garnet. Nearly all of the crystallized MI observed contain a relatively large (3–15 μm), often euhedral, zircon grain surrounded by finer‐grained (some <1 μm) phases with energy dispersive X‐ray spectra consistent with biotite, K‐feldspar, quartz, and albite. The inclusions are similar in size and composition to those described in garnet from other migmatites and are similarly interpreted as crystallized anatectic melt. Low Zr solubility in silicate melt indicates that the large zircon grain in each inclusion must be a trapped solid. It is inferred that micrometer‐scale droplets of anatectic melt chemically adhered to occluded zircon grains in biotite while peritectic garnet grew around them.

MINERALOGY, CLASSIFICATION, AND TECTONIC SETTING OF THE GRANITIC PEGMATITES OF NEW YORK STATE, USA

Pegmatites in New York State are largely restricted to areas of exposed Precambrian basement rocks, including the Adirondack Mountains and the Hudson Highlands. In the Adirondacks, they were emplaced into rocks that were metamorphosed during the Grenville orogeny to either the upper amphibolite facies (Adirondack Lowlands) or the granulite facies (Adirondack Highlands). Their mineral assemblages range from simple to complex, with localized occurrences of: (a) Be-, Al-, and B-rich species such as beryl, chrysoberyl, sillimanite, dumortierite, and tourmaline-supergroup minerals, (b) rare-element- and rare-earth-element-bearing minerals such as columbite-(Fe), uranopolycrase, fergusonite-(Y), uraninite, xenotime-(Y), and monazite-(Ce), (c) Ca-, F-, and K-dominant amphiboles, (d) phosphates (fluorapatite, isokite, wagnerite), (e) sulfides (bismuthinite, molybdenite), and (f) tungstates (scheelite). The pegmatite bodies from southern New York State and from some of the southern Adirondack locations display mineralogical zoning, typically with a quartz-rich core. Post-emplacement metamorphic features are observed in pegmatites in the northwestern Adirondack Lowlands, but are negligible or obscured in most of those from the southern and eastern Adirondacks. On the basis of the metamorphic grade of the host rocks, all New York pegmatites belong to the Abyssal class, whereas on the basis of mineralogy and inferred tectonic setting, they show affiliations to the NYF and mixed NYF–LCT pegmatite families. Pegmatites in the Adirondack Lowlands are related to the calc-alkaline arc magmatism of the Antwerp–Rossie granite suite and yield U–Pb zircon crystallization ages that indicate intrusion during the late Shawinigan orogeny (~1195 Ma); currently there are no known pegmatites related to the Ottawan and Rigolet orogenies in the Lowlands. Pegmatites in the Highlands yield U–Pb zircon crystallization ages that correspond to: (a) emplacement during the late Elzevirian (~1222 Ma), and metamorphism during the late Shawinigan (~1178 Ma) orogenies; (b) emplacement during the early (1098 Ma) and mid to late Ottawan orogeny (~1062–1025 Ma); (c) intrusion during the early Rigolet orogeny (1009–1003 Ma), and finally, (d) intrusion related to a thermal pulse at 949 Ma, possibly associated with the Cathead Mountain leucocratic dike swarm (935 ± 9.2 Ma). Few, if any, granitic pegmatites were intruded between 1178 and 1098 Ma in the Adirondack Highlands. The main pegmatite bodies in the Adirondack Highlands occur in association with extensional A-type granites such as the Hawkeye and Lyon Mountain Granite suites. These rocks were emplaced in response to orogenic collapse following the Ottawan orogeny. At present, relatively little can be concluded about the tectonic setting of pegmatites in southern New York.

Paleomagnetism and magnetic mineralogy of Grenville metamorphic and igneous rocks, Adirondack Highlands, USA

Paleomagnetic samples were collected from an east–west traverse across the Highlands region of the Adirondack Mountains, northern New York, which forms part of the southern most exposure of the Grenville Province of North America. Granulite facies metamorphism (T>650°C) at ∼1050Ma completely reset pre-existing magnetic directions in sampled microcline gneisses and metamorphosed anorthosites. Fourteen sites of microcline gneiss yield a mean direction of I=−62.8°, D=289.2° with α95=7.6° and a corresponding pole at 18.4°S and 151.1°E. Metamorphic anorthosites and associated rocks (N=14) show a direction of I=−67.3°, D=283.9° with α95=7.7° and a corresponding pole at 25.1°S and 149.0°E. Post-metamorphic fayalite granites possess a statistically different direction of I=−75.8° and D=297.0° (α95=3.9°, N=8 sites) and a pole of 28.4°S and 132.7°E. Both normal and reverse polarities are recorded in all units, with reverse sites occurring on the eastern and western ends of the traverse, and normal polarities restricted to the central part of the massif. The remanence is carried by ilmeno-hematite in the microcline gneisses. In the metamorphosed anorthosites, and the unmetamorphosed fayalite granites, hemo-ilmenite and magnetite occur, though magnetite is the predominant oxide. Using cooling curves established for the Highlands, and blocking temperatures determined for ilmeno-hematite, hemo-ilmenite and magnetite, the age of remanence is determined to be ∼990Ma for the magnetite-bearing Wanakena granite, ∼970Ma for the metamorphosed anorthosite and related rocks, and ∼960Ma for the ilmeno-hematite rich microcline gneiss. The pole data from the Adirondacks, as well as selected studies from other areas of the Grenville Province on units with similar mineralogy and some age control, helps define the southerly part of the Grenville loop of the apparent polar wander path. The three units from the Adirondacks indicate counter-clockwise motion of the APWP between 990 and 960Ma.

Reconnaissance geochronology and geochemistry of the Mont-Tremblant gneiss of the Morin terrane, Grenville Province, Québec

The Mont-Tremblant gneiss is a granulite-facies quartzofeldspathic orthogneiss unit in Morin terrane of the southwestern Grenville Province. These gneisses are the oldest rocks in the Morin terrane and country rocks to intrusions of the 1.15 Ga anorthosite-mangerite-charnockite-granite (AMCG) suite. The Mont-Tremblant gneiss may also be basement to the Grenville Supergroup, but until now it has not been directly dated. Sensitive high-resolution ion microprobe–reverse geometry (SHRIMP-RG) geochronology of zircon from two penetratively deformed samples of Mont-Tremblant gneiss yields igneous ages of 1324 ± 38 Ma and 1333 ± 32 Ma (2σ), and one sample shows metamorphic zircon growth during the Shawinigan orogeny at 1159.4 ± 15.6 Ma. Geochemically, Mont-Tremblant gneiss samples are calc-alkaline, granitic, and show hallmarks of an evolved arc environment. These characteristics are consistent with the Mont-Tremblant gneiss being correlative with the Geon 14 Lacoste and Bondy suites in the Central Metasedimentary Belt of Quebec to the west, but contrast with the more juvenile tonalitic Geon 14 calc-alkaline suites in the Adirondack Highlands and other Grenville outliers in the Appalachian Mountains. This transition may reflect differences in the lithosphere along strike of the Laurentian margin during Geon 14 subduction.

Variation in river multi-element chemistry related to bedrock buffering: an example from the Adirondack region of northern New York, USA

Water from four north-flowing rivers (Oswegatchie, Grasse, Raquette, and West Branch of the St. Regis) traversing three distinct geologic terranes (Adirondack Highlands, Adirondack Lowlands, St. Lawrence Valley), originating in the acidified northern Adirondack region, display rapid spatial changes in downriver multi-element chemistry and physical parameters. Downriver, most soluble elements increase (Ba, Ca, Cu, K, Mg, Na, Rb, S, and Sr) while statistically significant decreases in insoluble elements (Al, Ce, Dy, Er, Fe, Gd, La, Mn, Nd, Pr, and Y) also occur. Lithium, Si, and Zr did not show a consistent increasing or decreasing trend. Concentrations of most elements measured on June 5, 2008 were greater than 7 weeks later; however, greater discharge and lower pH enhanced Al concentrations occurred during the later sampling date. Elemental ratios track changes in lithology, anthropogenic influence, and pH. The Raquette River shows the least variation in elemental concentrations because of storage in numerous hydropower reservoirs and has non-detectable concentrations of some redox sensitive elements such as Co, Ni, and V. Oswegatchie tributaries display similar geochemical trends dependent upon their location and local bedrock and show geochemical trends along the trunks of major rivers that are also evident in smaller drainage basins. Comparison with analytical results from the Western Adirondack Stream Survey also indicates acidification is prevalent in the Adirondack Highlands, particularly in the Oswegatchie headwaters during periods of high flow, but acidity is rapidly buffered downriver due to interaction with marble in the Adirondack Lowlands.