Showing papers in "The Journal of Geology in 1975"

Classification, Characteristics, and Origin of Ophiolites

Abstract: Tholeiitic (TH) series rocks occur in almost all types of tectonic settings, whereas calc-alkalic (CA) series rocks occur characteristically in island arcs and continental regions. Abyssal tholeiit...

Distribution of Sediment Transport and Shear Stress in a Meander Bend

Abstract: In a 1-m wide, meandering flume with movable sand bed, uniform flow was established at discharges of 20, 35, and 50 1/sec. Bed shear stresses, bed geometry, the distribution of sediment in transport, and the strength of secondary flow were determined at each discharge. The zones of maximum bed shear stress, \tau, and maximum sediment discharge per unit width, $$q_{s}'$$, coincide. They are on the point bar in the upstream part of the bend, cross the channel centerline in the middle or downstream part of the bend, and follow the concave or down-valley bank to the next point bar downstream. With increasing discharge, secondary currents increase in strength. Consequently, the zone of maximum \tau and $$q_{s}$$ remains closer to the inside bank across the point bar, and crosses the channel centerline somewhat lower in the bend. It appears that bed geometry is adjusted to provide, at each point on the bed, precisely the shear stress necessary to transport the sediment load supplied. For example, the gradual de...

Statistical Classification of Garnets from Kimberlite and Associated Xenoliths

Abstract: Cluster analysis of 352 garnets from kimberlites, associated xenoliths, and diamonds has allowed recognition of 12 chemically coherent groups, on the basis of $$TiO_{2}, Cr_{2}O_{3}, FeO, MgO$$, and CaO contents. A flow-sheet classification scheme was created for classification of new garnets from kimberlites. Other points discussed are: correlation between the various oxides (including MgO-FeO correlations and their meaning); the chemistry of garnets from deformed Iherzolites; and the recognition of one group of garnets that probably crystallized from the kimberlite magma, as opposed to most other kimberlite garnets which are chemically the same as those occurring in xenoliths.

Deformation-Induced Recrystallization Due to Progressive Misorientation of Subgrains, with Special Reference to Mantle Peridotites

Abstract: The variation with strain of the misorientation between adjacent subgrains has been investigated in the olivine crystals of mantle peridotites naturally deformed at high temperatures. It is found that the misorientation of subgrains increases with strain up to about 15°, when the subgrains can be considered as independent grains. Evidence of this recrystallization "in situ," which does not involve grain-boundary migration, has been found in equigranular recrystallized peridotites: it is possible to find local domains where the grains have orientations traceable to subgrains of former deformed grains.

Melting Reactions in the System NaAlSi_{3}O_{8}-KAlSi_{3}O_{8}-SiO_{2} to 35 Kilobars, Dry and with Excess Water

Abstract: Piston-cylinder apparatus was used to determine the univariant minimum melting reaction in the dry system Ab-Or-Qz to 30 kb, and to extend the corresponding curve with excess water from 20 kb to 35 kb. At 30 kb, temperatures of the two univariant curves are 1,235°C and 680°C, respectively. The new data and other published data are combined to produce a complete P-T diagram for univariant reactions in the system Ab-Or-Qz-H_2O above 500°C, and phase diagrams comparing the effect of pressure on field boundaries in the systems Ab-Or-Qz-H_2O and Ab-Or-Qz. The slopes (dP/dT) of the excess water reactions change from negative to positive at pressures where the feldspars are replaced by denser minerals: about 17 kb where jadeite is formed from albite, and 27-30 kb where sanidine is replaced by sanidine hydrate. The liquidus volume for the primary crystallization of quartz increases with pressure, and above 10 kb the effect is greater in the dry system than in the water-saturated system. This accounts for the increased melting interval of granites with increasing pressure, and the wider interval beneath the liquidus of granites for the crystallization of quartz (or coesite) without accompanying feldspar. Partial fusion of metamorphosed feldspathic sediments in deeply subducted oceanic crust would produce a liquid with only a few per cent normative quartz; the initial liquid would be closer to a trachyte than a normal rhyolite. The results are consistent with the conclusion from other experiments that primary granites and ryolites cannot be derived from subduction zones at depths of 100 km or so.

The Effects of Pressure, Temperature, and $X_{H_{2}O}$ on Phase Assemblage in Four Synthetic Rock Compositions

Abstract: The variation in stable phase assemblage has been determined over a wide range of temperature, pressure, and water content for four compositions within the system $$CaAl_{2}Si_{2}O_{8}-NaAlSi_{3}O_{8}-KAlSi_{3}O_{8}-SiO_{2}$$. The compositions used were based on the average analyses of (1) hornblende-biotite granite; (2) hornblende-biotite adamellite; (3) hornblende-biotite granodiorite; and (4) hornblende-biotite tonalite as compiled by Nockolds. The variable parameters studied were pressure (2-8 kb), temperature (625°-1,200°C), and $$H_{2}O$$ content (0-20 wt %). The results are compared with recent vapor-saturated "rock-melting" experiments on similar natural materials, and the approximate position of the bivariant liquid which coexists with plagioclase, alkali feldspar, quartz, and vapor in the system $$CaAl_{2}Si_{2}O_{8}-NaAlSi_{3}O_{8}-KAlSi_{3}O_{8}-SiO_{2}-H_{2}O$$ determined. The study suggests that quartz resorption should commonly occur during the intrusion of granitic magmas in the absence of...

Surface Stone Movement and Scree Formation

Abstract: The accumulation of rockfall talus slopes has been studied for conditions in which stones are added relatively slowly, so that dynamic interactions between them may be ignored, and stone movements modeled "one-at-a-time." Laboratory scale models (1 m long) are compared with the results of mathematical and computer simulation. Mathematical analysis assumes that stone movement down the talus is equivalent to a "sliding-only" movement, and laboratory results support this assumption. Grain-size grading is explained in terms of the relative roughness of different material sizes to stones moving down the talus. Profile concavity is ascribed to the run-out of some material, corresponding to the tail of the frequency distribution for travel distances. Growth of the talus profile is modeled as a kinematic wave. The lower concavity obtained on both stochastic and scale models relative to deterministic models is thought to result from local accumulations of stones in the upslope parts of the talus which then produce...

Metagabbros from the Mid-Atlantic Ridge at 06°N: Contact-Hydrothermal-Dynamic Metamorphism beneath the Axial Valley

Abstract: Metagabbros with a transitional greenschist-amphibolite-facies mineralogy are exposed on the lower slopes of the eastern wall of the axial valley of the Mid-Atlantic Ridge at 06°N. These metagabbros are associated with metabasalts in the greenschist facies and with only mildly or nonmetamorphic basalts. Among the metagabbros some are finely banded, consisting of alternating amphibole-rich and plagioclase-rich zones, each about 1 mm thick. Specimens showing gabbro-basalt contacts are interpreted as representing the margins of basaltic dikes originally crossing through the gabbroic lower oceanic crust. The chemical composition of the metagabbros appears to be nearly unchanged from that of their parent rocks, which probably were gabbros with intermediate Fe/Mg ratio, and with olivine tholeiite normative composition and low K content characteristic of abyssal tholeiites. One of the metagabbros was dated at between 2 and 3 m.y. by the fission-track method on zircon crystals. The metamorphic processes which gav...

Distribution by Latitude of Phanerozoic Evaporite Deposits

Abstract: Most modern evaporites are accumulating within 50° of latitude of the equator. Within this evaporite belt the distribution pattern is bimodal; evaporite deposition is concentrated in the subtropical high-pressure zones of each hemisphere. Few evaporites accumulate in the low-pressure equatorial zone. This bimodality results from the arrangement of atmospheric circulation and is matched by similar bimodality in the distributions of hot deserts, average marine surface salinity, and the difference between evaporation and precipitation. When ancient evaporites are analyzed in terms of their assumed latitudes of original deposition they are found to follow the present-day distribution model quite closely-at least as far back as the Permian, and possibly as far back as the beginning of the Phanerozoic. This essential constancy of the evaporite regime is taken as an indication that atmospheric circulation and hence the distribution of deserts and of marine salinity have also adhered in general to the modern mode...

Ultramafic and Mafic Inclusions, Kanaga Island, Alaska, and the Occurrence of Alkaline Rocks in Island Arcs

Abstract: Dunite nodules, bearing minor amounts of chromite and clinopyroxene, occur as inclusions in an isolated outcrop of alkaline basalt (limburgite) near the northwest coast of Kanaga Island in the Aleutian arc. Gabbroic inclusions with very calcic plagioclase (up to An91) are found in a nearby outcrop on the Kanaga coast, but their geological relationship, if any, to the dunitic material is unknown. The dunite nodules are not typical lherzolites from an alkali-basalt xenolith suite. Instead, the dunite nodules have mineralogical affinities with dunitic members of Alaskan-type zoned ultra-mafic complexes. In particular, olivine compositions (Fo86) and clinopyroxene major-element (Ca47Mg48Fe5) and minor-element (0.10 wt% TiO2, 1.41 wt% Al2O3) compositions are very similar to those of Alaskan-type dunites. Clinopyroxenes in the host limburgite from Kanaga also fall on the trend of calcic augites and accord with previous arguments for a genetic association of Alaskan-type zoned complexes with alkaline magmas. The...

On Grain Size from Thin Sections

Abstract: Grain-size distributions based on analysis of thin sections are affected by the methods used for (1) selecting grains, (2) assigning linear dimensions to each grain, (3) computing frequencies. Many combinations of methods have been proposed in the past, and they all produce valid size distribution, which may, however, be biased geometrically with respect to the desired distribution. The probabilistic relationship between the size distributions (by number) of spherical and ellipsoidal particles, and the corresponding apparent size distributions seen in thin section is discussed briefly with references to the extensive literature. Solutions to this problem exist for dilute particulate phases distributed in space according to a Poisson process, but for the commonly occurring geological case of densely packed grains in contact, no analytical solution exists. Consequently, only those sampling procedures which are geometrically equivalent to the desired standard distribution should be utilized in geological stu...

Oregon Continental Shelf Sedimentation: Interrelationships of Facies Distribution and Sedimentary Processes

Abstract: The nature and distribution of sedimentary facies on the Oregon continental shelf are controlled by several factors: (1) river discharge and sediment input, (2) estuarine circulation system, (3) wave dimensions and direction, (4) subsurface and bottom currents, (5) density stratification of the water column, and (6) reworking by benthic organisms. Most of these factors were probably operating during the Holocene transgression of the sea which deposited a basal sand facies over the shelf. As sea level approached its present position, mud (silt and clay) began to accumulate slowly on the mid-and outer-shelf. Through the reworking by benthic organisms, this mud was mixed into the underlying basal sand creating a mixed mud and sand facies. A mud facies developed in the vicinity of the Columbia, Umpqua, and Rogue Rivers where the rate of mud deposition exceeded the reworking activity. The drowned mouths of coastal estuaries produce a sediment trap for fluvial and marine sand and the silts and clays that come i...

Problems in Determining Mantle Geotherms from Pyroxene Compositions of Ultramafic Rocks

Abstract: The interpretation of lithologically complex rock sections from xenolith suites sampled by volcanic vents has been recognized as a major problem for some time. The recent use of inherent pyroxene thermometers and barometers to determine the apparent stratigraphic succession of mixed suites of ultramafic xenoliths has led to conclusions of far-reaching importance concerning the history of the lithosphere, and the technique has been extended to the interpretation of ultramafic massifs as well. While the pyroxene thermometer-barometer approach promises to become a powerful tool in the study of many mantle-derived rocks, we have encountered several problems in its application that require consideration. These are: (1) systematic compositional variations across xenoliths that contain both dikes and their wall rocks yield enormous apparent pressure differences in the same hand sample, (2) alpine peridotites show apparent pressure ranges far in excess of those appropriate to their thickness, and (3) xenoliths fr...

Geochemistry of the Cenozoic Volcanic Rocks of Ross Island and Vicinity, Antarctica

Abstract: The Cenozoic volcanic rocks of Ross Island and vicinity, Antarctica, are surface flows, tuffs, breccias, and small intrusives. The prominent rock is basanitoid. Alkali-basalt magma reached the surface over an appreciable length of time and was differentiated to produce a rock series : basanitoid → trachybasalt → phonolite. The alkalic, silica-undersaturated rocks are part of a larger petrologic province composed of volcanic centers in a belt nearly 2,000 km long roughly parallel to the Trans-antarctic Mountains. Two trends of magmatic differentiation are apparent. At depth the fractional crystallization of olivine, clinopyroxene, and opaque oxide minerals, all of which are abundant as phenocrysts in the basanitoid flows, produced trachybasalt magma. At crustal levels, fractional crystallization of clinopyroxene, apatite, opaque oxides, kaersutite, plagioclase, and anorthoclase developed the phonolitic rocks. Alkali enrichment is marked, and the end-member phonolites contain 11% Na₂O, 5.6% K₂O, and 57% SiO...

The Amphibole Peridotite-Metagabbro Complex, Finero, Northern Italy: Reply

Abstract: The Finero complex is a lens of interlayered peridotite and hornblende-garnet gabbro, occurring in a regionally metamorphosed basement. From a regional geological study and from gravity surveys it has previously been interpreted as an alpine-type peridotite or as an upthrust slice of uppermost mantle. The geochemical data presented here are not compatible with either model, and it is suggested that the complex formed by fractional crystallization from a hydrous magma. Pyroxene and amphibole analyses from the peridotite suggest conditions of crystallization of 800°-1,100°C (depending on the method of calculation) and approximately 5 kb water pressure. The unusual chemistry of the hornblende-garnet gabbros may also be interpreted as the result of crystal fractionation with accumulation of amphibole, plagioclase, and calcic pyroxene.

Polyphase Deformation in Phanerozoic Rocks of the Central Himalayan Gneiss, Northwest India: a Reply

Abstract: Multiple deformation in gneisses and schists of the Chandra Valley, Himachal Pradesh, can be divided into three phases of folding. The earliest recognizable deformation, D₁, is characterized by isoclinal folds, F₁, with a strong axial-surface foliation, S₁, containing a mineral lineation, L₁, which may lie in the kinematic a direction. The F₂ folds deform S₁ and L₁, and vary from open to very tight structures with rounded hinges, crenulation lineations, and a weak axial-surface crenulation cleavage. The consistent sense of asymmetry of F₂ folds southwest of the central crystalline axis indicates strong overthrusting from northeast. The third phase of deformation, D₃, produced the regional broad antiforms and synforms which plunge 5° NW. Calc-schistsnear Tandi contain marine fossils of Jurassic age. These calc-schists are deformed by the three deformations recognized in the gneisses, and are thus part of the central crystalline zone. The fossiliferous Spiti section is a para-autochthon tectonically overlyi...

Permo-Triassic Sea-Level Change

Abstract: Shallow seas covered approximately 35% of the continents during the Middle Permian, and less than 15% at the end of the Permian. A similar change in the area of modern shallow seas would result from a 210-m drop in sea level. If the rate of sea-floor spreading were reduced by 5 cm a year for 8 m.y., the average depth of the ocean would increase by about 200 m. During a period of slower spreading, cooling sea floor would sink closer to ridges, and increase the volume of the ocean basins. Changes in the rate of sea-floor spreading are considered a reasonable mechanism causing the eustatic change in sea level associated with the Permo-Triassic faunal crisis. "During much of Paleozoic and Mesozoic time ... land surfaces were much lower than they are today. An appreciable rise in sea level was sufficient to flood large areas; a drop of a few feet caused equally large areas to emerge ...." (Newell 1963, p. 92).

Stratigraphic Implications of Coarse-Grained Gravel Deposited as Overbank Sediment, Southern Illinois

Abstract: Coarse-grained gravel lifted from the channel of Sexton Creek during flash flooding was deposited on the floodplain surface in three distinct lobes. The overbank gravel was laid down passively on the floodplain and no scouring of that surface preceded the depositional event. The deposits have a flat lower surface and are convex-up in shape. Similar deposits are exposed as gravel lenses in the stratigraphic sequence underlying the floodplain. Failure to recognize the overbank origin of the gravel lenses could lead to incorrect interpretations of the paleoenvironment.

Compositions of Glasses from St. Paul's Peridotite Partially Melted at 20 Kilobars

Abstract: St. Paul's peridotite with 5.7% H_₂O (considered to be derived from the upper mantle) was partially-melted at 20 kb in Ag_(75)Pd_(25) and Pt capsules with run durations up to 17 hours. Run products were analyzed for Fe, Mg, Ca, and Al using an electron microprobe. FeO loss from sample to capsule wall increased with temperature and run duration and was as much as 90% in Pt capsules at 1,250°C and 4 hours, and 73% at 1,150°C and 17 hours in Ag_(75)Pd_(25) capsules. Iron loss was lower in larger Pt capsules with a higher sample/platinum ratio. The amount of liquid produced in runs increased as a function of run duration, confirming the nonequilibrium condition. It was difficult to measure the composition of glass in narrow seams and wedges between mineral boundaries, and the problem was compounded by the presence of numerous minute olivine grains from the mylonite starting material and of quench amphibole in the glass. Glass compositions were estimated from analyses of glassy areas and quench amphiboles, and the compositional trend between 1,100°C and 1,250°C was plotted on variation diagrams. Expressed in terms of Al_₂O_₃ and CaO only, the glass analyses are similar to andesite and tholeiite. Our estimated glass compositions are compared with published data for measured glasses from partially melted peridotites of similar composition. In terms of the four elements that we analyzed the compositions of glasses vary considerably from one set of experimental conditions to another. We conclude that this is due in part to the experimental problems outlined above. The best results that can be achieved represent a compromise between runs long enough to ensure reaction and short enough to minimize the changing composition of the reacting system through iron loss.

Offshore Bars along the Alaskan Arctic Coast

Abstract: Well-developed offshore bars along the 1,355-km-long Alaskan Arctic coast possess three distinctive forms: (1) multiple parallel bars containing inner and outer bar systems; (2) long, parallel bars occurring in series but attached to the shore at their updrift ends; and (3) snorter en echelon bars attached to the shore. Some short en echelon bars are migrating to the west at a rate of $$70 m yr^{-1}$$ and moving sediment on the order of $$4 \times 10^{5} m^{3} yr^{-1}$$. Bars attached to the beaches generate pronounced shoreline rhythms consisting of horns and bays with amplitudes up to 200 m and lengths of 4-5 km. Low beach-ridge and swale systems are preserved on the horns and destroyed in the bays. The bays, where the beach is narrowest, tend to coincide with locations of inlets or erosion of the tundra hinterland. Field evidence strongly indicates that variation in bar forms is related to the angle of dominant wave approach to the shore; bars tend to be shorter and more transverse to the shore where w...

A Laboratory Study of Wave-Cut Platform Formation

Abstract: In order to understand wave-cut platform initiation at a time of stationary sea level, a wave flume experiment was conducted. A steep model cliff was exposed to three different types of waves: standing, breaking, and broken waves. Results showed that breaking and broken waves formed platforms, while standing waves did not. In the development of these platforms, impulsive pressure was predominant in the physical action of breaking waves, while shearing force was dominant in broken waves. Erosive forces of breaking, broken, and standing waves decreased in that order if their deep water wave characteristics were the same. The platform produced by breaking waves was located lower than that made by broken waves.

Simulation of Developmental Process of Interfluvial Slopes with Reference to Graded Form

Abstract: A mathematical model of slope development based on the combined effects of the subduing and recessional coefficients is reliable when viewed with regard to the characteristics of the graded form that is derived from it. The coefficients express the net effects of wash, creep, and weathering which play an important role in slope development. A series of developmental processes of interfluvial slopes is simulated, on the basis of the model with the boundary condition corresponding to river-bed elevation. The results show that the slope development is separated into the following three phases: (1) an irreversible process, in which an equilibrium state tends to be established, (2) a steady (equilibrium) state, in which a graded (time-independent) form is maintained; and (3) an irreversible process, in which the equilibrium state and the graded form are destroyed and the slope is worn down to a final peneplanation stage.

Phanerozoic Diversity Patterns: Tests for Randomness

Abstract: Patterns of origination within taxa, and patterns of dominance diversity among taxa are examined to distinguish nonrandom from random components. Numbers of first appearances per stage are sporadic and tend to be distributed nonrandomly through time. However, many taxa show a randomly distributed order of stages containing first appearances. A stochastic model of origination only partially reflects the patterns seen in the fossil record. Levels of dominance diversity among many taxa vary synchronously and at the same rates. The Phanerozoic is characterized by a succession of groups of taxa. Such associations are not predicted by a random model of diversity change. The fossil record is examined for evidence of an evolutionary equilibrium with constant taxonomic turnover. Fossil extinction and origination rates do not support the assumption of a static evolutionary equilibrium. Taxonomic and morphological evidence is consistent with a model of progressive specialization through the Phanerozoic.

Sediment Distribution and Deposition by the Western Boundary Undercurrent: The Great Antilles Outer Ridge

Abstract: The Greater Antilles Outer Ridge in the southwestern North Atlantic Ocean is a thick, topographically isolated prism of acoustically transparent sediment. Cores from the ridge contain finegrained, homogeneous brownish sediments with cyclic variations in carbonate content (generally 0%-30%). The sediment is enriched in organic carbon (up to 2%) and in the clay minerals, chlorite and illite, suggesting that the sediment was derived from the northeastern continental margin of North America and transported south to the outer ridge by the Western Boundary Undercurrent. Pleistocene and Holocene sediments have accumulated at rates up to 30 cm/1,000 years on the western outer ridge and 6 cm/1,000 years on the eastern sector in areas where the Western Boundary Undercurrent is presently active. Sedimentation rates are much lower in adjacent areas not affected by the abyssal currents. The distribution of suspended sediment in the currents suggests that it is the immediate source for sediment deposited on the outer r...

Eclogite-Bearing Amphibolites from the Appalachian Mobile Belt, Northwest Newfoundland: Dry versus Wet Metamorphism

Abstract: Eclogite-bearing amphibolites from the western marginal metamorphic zone of the Appalachian mobile belt (the orthotectonic zone), northwest Newfoundland, were derived from basaltic dikes metamorphosed in situ under conditions of the upper greenschist to lower amphibolite facies. Where they occur in "dry" basement, the basic dikes have been recrystallized to eclogite, and in the "wet" sedimentary cover to a more typical amphibolite assemblage, thus, reflecting only differing $$P_{H_{2}O}$$ under similar pressure and temperature conditions. The eclogites do not, therefore, represent the eclogite facies of metamorphism sensu stricto. This is compatible with the (disputed) extrapolation of experimentally determined phase boundaries which suggests that eclogite mineralogy may be stable in dry basaltic rocks along normal geothermal gradients in continental crust. It is further suggested that kelyphitic rims around the garnets are primary progressive metamorphic textures, explicable perhaps in terms of pre-eutec...

Statistical Tests for Preferred Orientation

Abstract: Most previously proposed tests for preferred orientation in fabric diagrams, including all tests based on contouring, are unsatisfactory, as we indicate in a critical review. We present tests for maxima and girdles, with a simple "Schnitteffekt" correction procedure for applications to inequant grains such as mica flakes or to planar elements such as rock joints. Previous corrections were not intended (and not convenient or even valid) for statistical use. Valid rotationally invariant tests involve lengthy computation. We have preferred a method convenient for hand use. We divide the hemisphere or disk into 16 cells and find the largest and smallest numbers of points per cell. Our tables and formulas evaluate the significance of the results for any number of grain points. Use of one standard cell pattern avoids the "retrospective fallacy."

Support and Refinement of the Bruun Rule on Beach Erosion

Abstract: If a beach and nearshore profile is at equilibrium, as sea level rises, the Bruun Rule on beach erosion indicates that foreshore erosion will take place in order to provide sediments to the nearshore so that the nearshore profile can be elevated in direct proportion to rising sea level. This study supports the concept that rising sea level can cause beach erosion. At Terry Andrae State Park, Wisconsin, which borders Lake Michigan, a field study conducted from spring to fall of 1971 shows that when wave energy was reasonably constant the beach eroded by about 23 feet as lake water level rose by 1 foot. Of the 23 feet about 6 feet was lost solely due to the drowning effect of rising water level. Also, for a given wave energy level, the foreshore zone increases in elevation in direct proportion to the increase in the elevation of the water level and the foreshore zone retreats parallel to itself with rising water level.

Ultrabasic Breccias in Layered Intrusions: The Rhum Complex

Abstract: Two breccias in the southwest part of the ultrabasic Rhum complex are considered. Aspects of field relations are discussed along with questions regarding the petrography of the matrix. Attention is given to textures and chemical mineralogy, the mechanism of brecciation, matrix magmas, and the possible implications of the findings. It is concluded that the Harris Bay and Ard Mheall ultrabasic breccias formed by brecciation due to the intrusion of feldspathic peridotite magmas.

$^{40}Ar/^{39}Ar$ Release Spectra of Biotite and Hornblende from the Cortlandt and Rosetown Plutons, New York, and Their Regional Implications

Abstract: Biotite and hornblende from the western portion of the Cortlandt complex record undisturbed $$^{40}Ar/^{39}Ar$$ incremental-release spectra with average total-gas dates of 420 m.y. (hornblende) and 390 m.y. (biotite). Biotite from the younger phase of the Rosetown pluton also records undisturbed release spectra with an average total-gas date of 420 m.y. Coexisting hornblendes display more complex spectra (a result of excess argon contamination); however, they have similar high-temperature "plateaus" at 450 m.y. Both biotite and hornblende from the older portion of the Rosetown have disturbed release spectra, possibly a result of excess argon contamination (hornblende) and partial postcrystallization gas loss (biotite). The disturbed spectra preclude a definitive estimate of absolute age but the data do suggest a minimum date of ~485-500 m.y. Because field criteria (Ratcliffe 1968, 1971) clearly indicate that both the Cortlandt and the younger Rosetown postdate a regionally pervasive progressive metamorphi...