11: THE CLASSIC BARROVIAN METAMORPHIC SEQUENCE OF DUTCHESS COUNTY AND ITS STRUCTURAL AND STRATIGRAPHIC CONTEXT IN THE TACONIC OROGENY
Frederick W. Vollmer
Department of Geological Sciences
State University of New York at New Paltz
New Paltz, New York
Jeffrey Walker
Department of Earth Science and Geography
Vassar College
Poughkeepsie, New York
Introduction
Metamorphism in Dutchess County, NY has been considered a classic example of Barrovian-type metamorphism since early mapping of the region (Knopf, 1927; Balk, 1936; Barth, 1936). Interestingly enough, few workers have extended the work that Balk and Barth began. People have been willing to accept it as classic without further testing (notable exceptions are the work of Bence, Vidale, Whitney and their coworkers). In his 1998 NYSGA field guide to Dutchess County, Donnelly remarks that "The area remains one of the clearest Barrovian sequences in the world." And yet, there are unanswered questions about the metamorphism. Some questions arise because of the complexities of the stratigraphy and the timing of episodes of deformation, metamorphism, and fluid flow. Others arise because of new analytical techniques that can be brought to bear to help elucidate the complex metamorphic history of the rocks.
The metamorphic sequence in Dutchess County begins with chlorite grade rocks at the Hudson River (Figure 1; Stop 1 in this guide; all pelitic rocks east of the Shawangunk Ridge in Ulster County contain chlorite, although whether detrital or authigenic/metamorphic is not always clear) and progresses beyond the second sillimanite (sillimanite-orthoclase) isograd near the Connecticut border (Stop 8). Rocks now exposed over a distance of about 30 km were tectonically buried as much as 25 km (Bence and McLelland, 1976; Whitney et al., 1996b) during the Taconic Orogeny. The sequence was subsequently tilted to its present orientation during uplift to the east associated with the Acadian Orogeny. In addition to isograd mineral growth, the prograde sequence also demonstrates fabric development from phacoidal cleavage at the Hudson River, through phyllites and schists in the middle of the county, to gneissic fabric near the Connecticut border.
Balk (1936) and Barth (1936) believed that metamorphism in Dutchess County was due to "steeping" of the pelitic rocks in igneous fluids emanating from magmas deeper within the crust. They cited as evidence the fact that granitoid intrusions are found to the east and south (in Connecticut and Westchester County, NY, respectively), on strike with the increasing metamorphic gradient mapped in the county. Some of these granites, however, contain muscovite suggesting that they are peraluminous S-type granitoids formed by partial melting of a source dominated by sedimentary material (Winter, 2001, p. 350). Some outcrops in the sillimanite zone of Dutchess County contain qtz-fsp-mica veins which Barth (1936) interpreted as apophyses from the granitoid intrusions. These veins have also been interpreted as leucosomes from anatectic melting of the sediments (Bence and McLelland, 1976; Whitney, personal communication, 2009). Most workers now believe that the heat for metamorphism was generated by tectonic burial of the rocks during the Taconic Orogeny although fluid flow was probably an important heat-transfer mechanism (Whitney et al., 1996b).
The classic nature of the metamorphic sequence in Dutchess County, and its excellence for teaching, mean that a number of very good field guides are available for the area (Bence and McLelland, 1976; Donnelly, 1998; Whitney and Peck, 2004). Bence and McLelland's 1976 NYSGA field guide provided a thorough discussion of the sequence, and identified many of the outcrops that have since become almost mandatory on local field trips (Stops 2, 4, 6, 7 and 9 in this guide). They also included whole rock chemical analyses, and microprobe analyses of individual miner - als that are very helpful for understanding metamorphic conditions. Additional sources of whole rock analyses are Vidale (1974b), Whitney et al. (1996b) and Donnelly (1998). Whitney et al. (1996b) also published microprobe analyses of individual mineral grains. This field trip will follow Bence and McLelland (1976) and Whitney and Peck (2004) for the higher grade (chlorite grade and higher) outcrops. To this, we add several additional topics:
We will make stops in the lower part of the chlorite zone at which we discuss the evidence for low grade metamorphism and the difficulties in distinguishing metamorphic from detrital chlorite
We stop at exposures of the Taconic melange and discuss it's significance.
We comment on some recent proposals relevant to the sometimes confusing stratigraphic nomenclature.
We comment on the development of metamorphic fabrics across the county.
We provide photomicrographs for discussion of relationships that are difficult to examine in outcrop.
We include additional equations and AFM projections to illustrate possible reactions.

11: THE CLASSIC BARROVIAN METAMORPHIC SEQUENCE OF DUTCHESS COUNTY AND ITS STRUCTURAL AND STRATIGRAPHIC CONTEXT IN THE TACONIC OROGENY

STOP A. (Optional) Austin Glen Formation
Graywacke beds here show sedimentary structures indicative of deep marine turbidity deposits, including ripple marks, graded beds, and sole marks. Although these sedimentary structures show that the beds are upright, cleavage dips more gently than the upright bedding indicating downward facing structures (Figure 5). This illustrates the complex structural relationships found in this belt due to refolding and probable post-cleavage block rotation in front of the Taconic Allochthon.

STOP B. (Optional) Austin Glen Fm. at Johnson Iorio Park
Outcrops of Middle Ordovician Austin Glen Formation thick bedded graywacke and shale show sedimentary structures typical of these deep marine turbidity current deposits that were shed to the west off the rising Taconic Mountains (Figure 6). These deposits were subsequently deformed as the Taconic Allochthon overrode them from the east. Across the Hudson River can be seen outcrops of the Taconic melange, the main Taconic thrust zone (Stop 1), and the Taconic Range to the east. The rock in these cuts is particularly well exposed because of the high percentage of sandstone, more common shale rich exposures may be more representative of a protolith for metapelite outcrops visited at subsequent stops.

Excellent exposures of the Middle Ordovician Austin Glen Fm.
Road cuts from here to the Mid-Hudson Bridge show excellent exposures of the Middle Ordovician Austin Glen Formation thick bedded graywacke and shale. The outcrop shows the variation in bed thickness and shale content, as well as numerous faults. See the descriptions above for similar exposures at Optional Stops A and B.

STOP 1. Taconic Melange at Kaal Park
Taconic melange at Kaal Park is the type locality of the Poughkeepsie Melange of Fisher. The Taconic melange is well exposed here, although large sections are covered with poison ivy. The phacoidal, or scaly, cleavage is typical of melange fabric world-wide. The shale breaks into centimeter-scale lens-like phacoids whose surfaces are polished and striated. Blocks within the melange are mainly graywacke here, and probably represent a disrupted bedded sequence similar to exposures of Austin Glen on the west side of the river at Optional Stop A. Locally, however, limestone blocks occur within the melange suggesting a sedimentary olistostromal component. The Taconic melange is a thick (kilometer-scale) tectonostratigraphic unit that occurs along the western margin of the Taconic Allochthons in the northern Appalachians, and represents a complex syn-sedimentary fault zone associated with allochthon emplacement. To the north, the Taconic melange is unconformably overlain by the Silurian Helderberg Group (Vollmer,1981), establishing it's Taconian age.
Many exposures of "Normanskill" or "Snake Hill" shale in the Poughkeepsie area show clasts suggestive of melange, and the Taconic melange is suggested to be a potential protolith for some of the metapelites (such as Walloomsac) examined at subsequent stops.

STOP C. (Optional) Wappinger Group Carbonates
For optional stop C, park on Henmond Road to the right just past the outcrop and return on foot.
Outcrop of Wappinger Group carbonates, possible protolith for metacarbonates to be examined at Stop 9.

Cross Wappinger Creek

Cross Sprout Creek

STOP 2. Chlorite Zone at Noxon Road
These outcrops are within the Taconic Allochthon, and show considerable structural complexity. The rock here is chlorite grade, and exhibits slaty cleavage, abundant chevron folds, and crenulation cleavage that folds an earlier foliation subparallel to bedding. Thin sections show that cleavage formation is dominated by pressure solution, a likely source of quartz veins (Figure 8 and 9). Witney et al. (1996) suggest that the chlorite forms from clay minerals. These reactions release silica that may also contribute SiO2 to the formation of quartz veins. Four distinct lithologic units are present within these outcrops: graywacke and dark shale, green slates, red slates, and slate with interbedded micrite. The contacts do not trace directly across the road, and are either part of steeply plunging structures or are discontinuous. Fisher and Warthin (1976) and Bence and McLelland (1976) assign the graywacke unit to Mount Merino (at the far west end), and the other three to the Indian River Formation. Structurally this outcrop lies within the Giddings Brook Slice, the the structurally lowest slice of the Taconic Allochthon (Stanley and Ratcliffe, 1985).
On the south side of the road, west of the main outcrops, are dark crenulated slates and folded graywacke beds. Bottom structures exposed in a meter-scale synform indicate the folds are upward facing. This unit is in contact with green slates further east on the north side of the outcrop, where quartz veining indicate faulting along the contact. It seems likely that this unit is equivalent to the Austin Glen/Pawlet, and represents flysch deposited on the continental rise and subsequently carried with the allochthon (see main text).
The bulk of the main outcrop consists of highly folded and crenulated green slate with some interbedded chert and siltstone, however on the south side of the road is a large section of red slates of typical Indian River aspect. Bence and McLelland (1976) report hematite is present in the red slates, and absent in the green slates. The presence of hematite in Indian River has been attributed to both primary and secondary origins (e.g., Bird and Dewey, 1970; Bence and McLelland, 1976), and may represent erosion of a weathered horizon exposed as a foreland bulge during the Taconic Orogeny. If the graywacke unit is Austin Glen/Pawlet, then the Mount Merino may be represented by the green slates with cherty interlayers.
At the eastern end of the outcrop on the north side, a unit of banded gray micrites and interlayered slate is in contact with the green slate. The contact between the two units is complex, and disruption of the micrite beds is suggestive of soft sediment deformation with a strong tectonic overprint. This unit is remarkably similar to thinly bedded to brecciated micrites at Schodack Landing, also within the Giddings Brook slice, which have been interpreted as slump deposits on the continental slope (Bird and Dewey, 1975; Friedman, 1979). This unit likely has a similar origin.
As we drive east from this stop we will leave the Giddings Brook slice, and shortly enter the structurally higher Everett slice. The remaining metapelite stops, except possibly Stop 5, are below the Everett slice (Stanley and Ratcliffe, 1985).

Cross Jackson Creek

STOP 3. Biotite Zone at Perkins Lane
Black slates in biotite zone, although biotite is not visible in hand sample. These rocks are assigned to the Walloomsac Formation and are believed to be deposited on Wappinger Group shelf carbonates, which underlie Clove Valley just to the east. Thus, tectonically, these are exposed in a window and represent flysch overridden by the allochthon, or are brought up with basement by late reverse faults.
The main cleavage in the outcrop is relatively low angle, suggestive of high shear strains associated with allochthon emplacement. Numerous folded veins are related to shortening across the cleavage (Figure 10). Whitney, et al. (1996b) propose that biotite could form by the reaction:
phengite + chlorite = biotite + muscovite + quartz + H2O
An AFM projection for the upper biotite zone shows that at this grade biotite is stable for a wide range of pelitic compositions (Figure 11A).

Lunch Stop - Tymor Park.
South of the picnic area a dirt road runs through a spectacular gorge in the Wappingers Group dolostones to Furnace Pond. The pond was named either for a lime mill that stood on its shore (on Lime Mill Road), or for the Clove Springs Ironworks whose furnaces processed up to 700 tons of iron ore from Union Vale mines during the 19th century. Remains of the ironworks can be seen south of the park on Furnace Road.

STOP 4. Garnet Zone at Wingdale Mountain Road
Garnet zone with chloritoid. At this point we have crossed the garnet isograd, and garnets are now present in hand sample, although not always easy to find. Downhill from the parking area are biotite-bearing dark schists and metagraywackes (Figure 12). Garnet is found in the more fine-grained beds, which are presumably more aluminous than the graywackes. Whitney et al. (1996b) propose the reaction:
chlorite + muscovite = biotite + quartz + H2O
Across from the pull out, near a gated entrance, are chloritoid-bearing schists (Figures 13 and 14), suggesting the reaction:
chloritoid + biotite + quartz = garnet + muscovite + H2O
Bence and McLelland (1976) attribute the mineralogical difference between these two outcrops to a more aluminous protolith, and the separation of the two bulk compositions by the garnet-chlorite tie line in an AKFM projection (Figure 11B). Note that the tie line switch in the biotite (Stop 3) to garnet (Stop 4) zone transition (Figures 11A and 11B) allows garnet to occur in typical pelitic compositions.
In thin sections from the lower outcrop, biotite is found growing across a crenulation cleavage, suggesting post-tectonic growth (Figure 12). Thin sections from the chloritoid-bearing outcrop show clear rotation of chloritoid and garnet porphyroblasts associated with shear strain (Figures 13 and 14). These porphyroblasts have overgrown an early foliation which has then been rotated during continued or later deformation.

STOP 5a. Staurolite Zone (Alternate stop with better parking)
Alternate stop location with better parking than Stop 5 without requiring permission. Pull out is on left at hair pin turn. It is suggested that cars turn around below and return to park on right, facing up hill. Outcrops are across from pull out and up hill on right.

STOP 5. Staurolite Zone at Beebe Hill Road
Coarse biotite-muscovite schists with garnet and staurolite porphyroblasts. Rocks here are now well within the staurolite zone, and contain centimeter scale garnet and staurolite porphyroblasts in the muscovite schist. This is the upper staurolite zone of Whitney, et al. (1996b), and because there is no chlorite in the rocks, the likely reaction is:
chlorite + muscovite = staurolite + biotite + quartz + H2O
A tie line switch from garnet-chlorite to staurolite-biotite occurs at this grade, making staurolite stable over a wide range of pelitic compositions (Figure 11D).

STOP D. (Optional) Lower Staurolite Zone at Still Road
Please DO NOT HAMMER on outcrop. Chloritoid is partially replaced by staurolite, and porphyroblasts are difficult to see in hand sample. As noted by Bence and McLelland (1976), five coexisting ferromagnesian phases, garnet-chloritoid-staurolite-biotite- chlorite, can be identified in thin section, a disequilibrium assemblage violating the mineralogical phase rule (Figures 11C and 15). Three different reactions are possible here (Whitney et al., 1996b):
chloritoid + quartz = staurolite + garnet + H2O
chlorite + garnet + muscovite = staurolite + biotite + quartz + H2O
garnet + chloritoid + chlorite + muscovite + ilmenite = biotite + staurolite
Careful study is needed to determine which reaction(s) are most likely because garnet may be either a product and a reactant. Figure 11C shows the chloritoid-out reaction, and the phase stability triangles for the five phases.

STOP E. (Optional) Poughquag Quartzite
The Poughquag Quartzite is the basal unit overlying Precambrian gneisses here, and is correlative with the Potsdam sandstone and similar Cambrian basal units. On NY 55 to the left (east) the unconformity of the Poughquag Quartzite with underlying Precambrian gneisses is crossed proceeding uphill. These basement rocks were carried up on reverse faults, presumably later in the orogeny, and along steeper faults, when the continental basement eventually became involved in the collisional event.

STOP 6. Kyanite Zone
Kyanite is present as small 1-2 centimeter blue blades, but is difficult to find. It appears on weathered surfaces as white sprays or rosettes on foliation planes (Figure 16), apparently with no preferred linear alignment. DO NOT HAMMER WEATHERED SURFACES, samples may easily be obtained from fresh surfaces on existing road outcrops.
In thin section the assemblage kyanite-sillimanite-staurolite-garnet has been observed (Figures 17 and 18). The kyanite appears resorbed (Figure 17), and the presence of sillimanite as fibrolite (Figure 18) in the rocks suggests a captured divarient reaction, or disequilibrium (see Figures 3, 11E and 11F). Fibrolite has been observed in close proximity (millimeters) to the kyanite. One possibility is that linked reactions are occurring in adjacent local domains separated by a chemical potential gradient as proposed by Carmichael (see Blatt et al., 2006):
kyanite + quartz + K+ + H2O = muscovite + H+
muscovite + H+ = sillimanite + quartz + K+ + H2O
This allows the reaction to proceed without direct contact of the polymorphs. However, in the sample described here, this requires chemical potential gradients at a millimeter scale. Another possible reaction to explain the presence of fibrolite (Whitney et al., 1996b) is:
staurolite + muscovite + quartz = biotite + sillimanite + H2O
Again, this seems to require a strong chemical potential gradient and extremely limited fluid mobility.
Whitney et al. (1996) noted that fibrolite first occurs in the kyanite zone. They discuss this in detail, and present several hypotheses to explain this. Variation in Mg content between layers and a reaction involving staurolite is ruled out by them, as they did not find evidence of staurolite in fibrolitic sillimanite-bearing layers. They suggest instead that the fibrolite may have grown at a later time (later Taconic or possibly Acadian), and that the reaction may be related to fluid infiltration, with fibrolite growth controlled by differences in local permeability and possible metasomatic Al-enrichment.
As discussed in the main text, two metamorphic events have been identified in Dutchess County (Sutter et al., 1985), and may be related to these perplexing apparent disequilibrium assemblages.

STOP 7. Sillimanite Zone at Quaker Hill Road
Park on right using CAUTION DUE TO NARROW SHOULDER. Hazard blinkers and flag person recommended. Alternate parking is just below hairpin curve at bottom of hill (UTM 18T 0619799 4602388).
Prismatic sillimanite crystals are evident in thin section (Figure 19), but not visible in outcrop. Small pegmatite dikes containing tourmaline-muscovite- potassium feldspar-quartz intrude the sillimanite-garnet-muscovite schists, and provide evidence of anatectic melting (as opposed to quartz veins formed by lower temperature pressure solution dissolution and precipitation). In thin section (Figure 19) there is evidence for the reaction:
muscovite + quartz = sillimanite + potassium feldspar
Note that the sillimanite between the muscovite and potassium feldspar has a coarser prismatic form than that between the quartz and potassium feldspar. A possible explanation is that ion mobility was greater along the muscovite- potassium feldspar interface than the quartz-potassium feldspar interface due to easier fluid migration, so growth was favored over nucleation.

Alternate parking for Stop 7.

STOP 8. Sillimanite-Potassium Feldspar Zone at Burgess Road
Manhattan schist here displays more pronounced gneissic banding, and tourmaline-muscovite-potassium feldspar-quartz pegmatite dikes. Whitney and Peck (2004) assign this outcrop to the Sillimanite-Potassium Feldspar zone.

STOP 9. Metacarbonates of the Briarcliff Dolostone
Outcrops of diopside-tremolite- phlogopite marble that represent the metamorphism of carbonates to metamorphic grades approaching the sillimanite zone. One layer in particular contains a high concentration of diopside tablets and sprays (Figures 9 and 10). The diopsidic composition of these unusual forms was confirmed by thin section and X-ray diffraction analysis. Remnants of the less ductile silica-bearing beds within the marble record extreme ductile folding and boudinage, testifying to the high temperature deformation they enjoyed.