9: PRE-OTTAWAN (1.09 GA) INFRASTRUCTURE AND TECTONICS OF THE HUDSON HIGHLANDS, NEW YORK
Nicholas M. Ratcliffe
U.S. Geological Survey
Reston VA 20192
ratclif@usgs.gov
John N. Aleinikoff
Denver, CO 80225
jaleinikoff@usgs.gov
INTRODUCTION
This trip focuses on some of the oldest Mesoproterozic rocks in the Appalachians and the nature of the pre-Ottawan (pre-1.09 Ga) infrastructure of the Hudson Highlands and Manhattan Prong. Zircon geochronology by Aleinikoff (Table 1) is revised from that given in Table 1 of Ratcliffe and Aleinikoff (2008) and is still preliminary. Ongoing geologic mapping east of the Hudson River in the Peekskill, West Point, Lake Carmel, Mohegan Lake, Brewster and Peach Lake quadrangles continues to lead to a reinterpretation and clarification of our former views (Ratcliffe and Aleinikoff, 2001, 2008) as well a new synthesis for the Hudson Highlands east of the Hudson River. After publishing the detailed geology of the Poughquag quadrangle (Ratcliffe and Burton, 1990) and the Oscawana Lake quadrangle (Ratcliffe, 1992), investigations were resumed in 2000 to prepare the previously mapped but unpublished quadrangles for publication.
This trip presents, in preliminary form, much of the recent, (post-2000) information, which helps clarify the age and distribution of Mesoproterozoic basement rocks of the Hudson Highlands and Manhattan Prong and is an abbreviated version of our 2008 NEIGC trip. Please note that stops have been changed and that the route is different.
A remarkable newly recognized aspect of Hudson Highlands geology covered in this trip is the widespread presence of orthogneisses and metavolcanic rocks belonging to the time period between 1400 Ma and 1200 Ma, or during Geochrons 14 and 13 of Rivers (1997). These rocks formed before the Grenville orogeny, defined by Rivers (1997), as ranging from 1.09 Ga to 0.98 Ga, and are coeval with crustal convergence in the Elzevir belt in Ontario and Quebec, at the western margin of the Grenville orogenic belt. We know little about the nature of the deformation that may have affected rocks of the Hudson Highlands during the Elzevirian orogeny except that some were transformed into gneissic rocks prior to about 1230 Ma and multiply deformed after that.
Igneous activity and deformation in the time period of about 1170 to 1145 my ago, at or near the time of the Shawinigan pulse of the Grenville orogeny of Rivers (1997) affected many the rocks of the Hudson Highlands and presumably of the Manhattan Prong although data there are sparse. Penetrative, regionally developed, Y F2 folds of the Hudson Highlands have been mapped in northern New Jersey (Dallmeyer, 1972) and West Point area of the Hudson River (Helenek, 1971; Helenek and Mose, 1984) eastward through the Hudson Highlands (Ratcliffe and Burton, 1990; Ratcliffe, 1992). This deformation resulted in redeformation of older rocks as well as deformation of hornblende granites of the Storm King type, which have an intrusive age of 1174 ± 8 Ma (Table 1). Folds of this generation are folded by the sigmoidal deformation zones associated with the intrusion of the Canopus pluton, and crosscut by mafic dike swarms. Acceptance of an intrusive age of 1143 ± 12 Ma for the Canopus pluton (Table 1) and for the associated mafic dikes, requires that the regional Y F2 dynamothermal event be older than the Ottawan phase of the Grenville orogeny. This is important because many workers think that Ottawan or the terminal phase of the Grenville orogeny was responsible for the major deformational features in Mesoproterozoic rocks of the Appalachians. This view is being reevaluated in the Appalachian outliers of the Grenville and in the Adirondacks, as the recognition of rocks and deformational events at about 1230 Ma and 1150-1134 Ma that are older than the Ottawan are discovered (Ratcliffe and others, 1991; Ratcliffe and Aleinikoff, 2001, 2008). In the Adirondack Highlands evidence for Shawingan migmatite in the age range of 1180-1160 Ma is widespread (Heumann and others, 2006; Bickford and others, 2007) Throughout the Hudson Highlands and in northern Appalachians, there is a relative scarcity of Ottawan igneous rocks, although ages of metamorphic overgrowths on zircon and of migmatization of older rocks attest to widespread Ottawan re-metamorphism.
Rocks intruded between 1134 ± 8 Ma and 1045 Ma, are foliated, as in adjacent areas, such as the New Milford massif (Walsh and others, 2004) and as seen on this trip. However there are abundant data which suggest that the deformational effects in the Hudson Highlands and elsewhere in the northern Appalachians are the result of multiple orogenic events, some predating the Ottawan. On this trip we will visit and summarize information concerning recently dated rocks that are critical to defining the pre-Ottawan history of the Grenville orogen here and elsewhere in the Appalachians of New York and New England.
In addition newly determined SHRIMP U-Pb zircon data, and Ar 40 /Ar 39 hornblende ages support late Ordovician to Silurian Taconian overprinting of the basement and cover rocks between 450 to about 443 my ago and suggest that Acadian effects were minimal by comparison.

9: PRE-OTTAWAN (1.09 GA) INFRASTRUCTURE AND TECTONICS OF THE HUDSON HIGHLANDS, NEW YORK

STOP 1. Reservoir Gneiss--amphibolite inclusions
I-84 Exit 17 Luddingtonville Rd. Poughquag quadrangle STAY OFF THE INTERSTATE PLEASE. Park by the overpass and walk along west entrance lane.
This belt of trondhjemitic gneiss forms the eastern end of a wide belt of Reservoir Gneiss (of Ratcliffe and Burton, 1990) that extends from near Cat Hill Stop 7 through alternate Stop 5b into the Poughquag quadrangle and the contiguous Brewster quadrangle (Figure ). A SHRIMP zircon age of 1333±6 Ma has been obtained from this outcrop. Ages of overgrowths on the zircon are 1.2 and 1.0 Ga indicative of Elzevirian and Ottawan zircon growth. Sweat outs of more granitic composition are common and are shown by coarser grain size in indefinite irregular pockets in which and older gneissosity is overprinted. Abundant mafic inclusions choke this rock in most exposures. Reaction rims around the mafic inclusions contain pyroxene, hornblende, and biotite and are gradational with the host rock. It is likely that these reaction zones and the pegmatitic sweat outs are the result of Ottawa metamorphism.
Normative An-Ab-Or compositions of some the felsic rocks belonging to Geon 14 and those of Geon 13 are plotted in Figure 8 and range from trondhjemite to granite. Fields for the Reservoir Gneiss at Peekskill (stop 9), at Cat Hill (Stop 8), in the Oscawana Lake area (Stop 5b) and here at Luddingtonville (Stop 1) show the range of compositions in the Reservoir Gneiss in the westernmost belt closest to the Canopus deformation zone.
These trondhjemites and granites all have distinctively high La/Ybcn values, steep middle to light rare earth slopes, and flat, negative to positive HREE slopes comparable to that shown for the Cat Hill Gneiss (Figure 10). The samples of the dated Fordham Gneiss have very similar patterns. Plots of Rb vs Nb+Y (Figure 9) show the similarities among the dated rocks as well. The filled dots are samples of Yrg in the Brewster area that are correlated with the main Reservoir Gneiss belt on the basis of continuity on the ground between the two areas. We can not be certain that all rocks shown as Reservoir Gneiss in the Brewster area are in fact the same as these rocks are not dated.
Mafic inclusions in this outcrop and in all stops in the rocks of Geon 14 may be inclusions and or dikes as will be discuss further at Stop 2. The amphibolites here show no clear indications of having been dikes but that cannot be ruled out. Note the felsic sweat outs and pegmatites near the wall of the "inclusions" as well as the pyroxene- hornblende reaction zones, which grade into the bordering rocks.

STOP 2. Snake Hill - Granitic gneiss typical of Brewster belt and mafic dikes
Steeply dipping biotite granite gneiss typical of the vast majority of granitic gneiss and associated migmatitic lit par lit contact zone on Joes Hill. Gneissic granite like this is crosscut at the west end of Joe Hill by a monzonite dike that has a zircon Pb207/206 SHRIMP age of 1240±7 Ma. The granite gneiss here is tentatively assigned to the old gneiss belt and is judged to be 1330 Ma or older. Fine-grained foliated mafic dikes cross cut the gneissic structure and are fairly abundant on this hill and in areas to the east. Locally coarser grained diorite to gabbroic masses also cross cut the older gneisses and these rocks pass into finer grained rocks like the dikes seen here.
Figure 12 shows one of a series of meter-wide dikes petrographically like the small ones above. On Joes Hill these dikes cross cut high-grade gneissosity and intrude across mylonite zones. Although apparently straight walled, they are re-deformed by isoclinal folding. Both the dikes and the older gneissosity are cut by foliated granitic pegmatite of probable Mesoproterozoic age. Based on these observations, the mafic dikes, like that figured below must be Grenvillian and perhaps older than the Ottawan phase of the Grenville orogeny. In this regard these dikes resemble the dikes of the Canopus pluton (stop 6), which also cross cut an older gneissosity.
Chemically, the dikes in question are somewhat like those of the Canopus pluton as illustrated in Figure 13, and to dioritic rocks at stop 3. The diagram shows a similarity in chondrite LREE enrichment trends (but not in absolute values) and similar range in Ta -Yb ratios. The plots do not establish identity of the various rocks only suggest that possibility.
The dike in Figure 12 is a typical example. It cross cuts migmatitic gneiss that at this locality is intruded by a monzonitic granite that has a SHRIMP zircon age of 1240±7 Ma (Table 1). The age of these dikes is younger than 1240 Ma. They could be the same age as the diorites associated with the dated hornblende granite (1134±8 Ma) at stop 3 and with dikes of the Canopus pluton (Stop 6A, B) that are probably approximated by the Canopus monzonite zircon Figure 11. Two fine-grained mafic dikes cross the coarse ropy gneissosity of SHRIMP age of 1143±12 Ma. The mafic dikes of the Canopus and else-where in the Brewster area cross cut coarse gneissosity in the 1340-1330 Ma rocks; both are intruded by foliated pegmatite.
Alternately these dikes could be Neoproterozoic. However major and trace element characteristic differ from Neoproterozoic dikes found in the Hudson Highlands (Ratcliffe, 1978), which are typical transitional rift basalts.
The clearly cross cutting mafic dikes and mafic and isoclinally folded "inclusions" in the granitic gneisses of the Brewster area are indistinguishable chemically and petrographically from one another. This observation plus the fact that individual dikes like that in Figure 12, which cross cut a gneissosity can be traced within outcrops into isoclinally and cross foliated zones suggests that many of the inclusions or indefinite dikes like those at stop 1 may all be the same age. If so, there is a widespread mafic dike event at about 1143 to 1134 Ma. that post dates an earlier Shawinagan deformation.

STOP 3. Unnamed 1134±8 Ma hornblende granite gneiss
Dioritic gneiss and sulfidic schist screen, Brewster, N.Y.
Hornblende-biotite granite gneiss 140 feet wide, on the southeast end of this NW-SE vertical cut becomes very hornblende rich and passes into a 15-ft-wide pyroxene hornblende pegmatite near the small screen of sulfidic paragneiss. The hornblende granite has a 1134±8 Ma SHRIMP age. North of that dioritic gneiss about 35 ft thick passes northward into a hornblende-rich margin that grades into a second 40 ft-thick zone of granite gneiss. That granite gneiss grades upwards into diorite gneiss at northwest end of the exposure. The felsic and mafic components grade into each other at the margins and appear to be coeval. In this characteristic these rocks resemble rocks of the Canopus pluton (Stop 6A, B) and have essentially the same age as the felsic component of the Canopus which has a concordant zircon SHRIMP age of 1143±12 Ma.
We regard the felsic and more mafic rocks as part of the same igneous suite. Near contacts with sulfidic schists and calc-silicate rocks hornblende-pyroxene pegmatite and a white hornblende-spotted aplite carrying magnetite is common. This is the common association for the Brewster magnetic deposits, which are developed on strike to the southwest at the borders of xenoliths included in the hornblende granite and along the wall of this intrusion. All the rocks are foliated but tend not to be gneissic like the older rocks. Similar dioritic intrusives occur to the north in Brewster and preserve coarse diabasic texture and relict coarse - plagioclase flow structure. The mafic dikes we have seen may also be the same age.

Tilly Foster mines
Calc-silicate rock, marble and magnetite are developed here at the contact with the same hornblende granite gneiss dated at Stop 3.

STOP 4. Hornblende Granite gneiss like that at Stop 3
NOTE: on Figure 4 this location does not correctly show the hornblende granite which was discovered after the figure was published. See Figure 2 for a more accurate map. This illustrates an important point namely that telling the old 1340-1300 Ma granite gneisses from the 1134 Ma ones is not easy. I expect to complete mapping in the Lake Carmel quadrangle this Fall and undoubtedly contacts will shift.
Well lineated and foliated hornblende granite gneiss and aplite cuts gneissosity in older well layered biotite and hornblende-quartz-plagioclase gneisses. Both rocks are folded into tight folds containing axial surfaces of N 65 W 45 NE and a strong lineation plunging to the east and northeast. These relationships support the idea that the 1135 Ma hornblende granites cross cut older structure just as the Canopus pluton Stop 6A, B does.
Excellent exposures in the Croton Reservoir (Stop '08-4 of Ratcliffe and Aleinikoff, 2008) are no longer available as the reservoir has now been refilled and the exposures covered. Observations there showed that:
1. The hornblende granite is foliated but does not have the same coarse ropy- textured gneissosity of the older 1340 Ma gneisses seen at Stop 2.
2. Xenoliths of foliated gneisses occur within the younger hornblende granite gneisses and aplite
3. The coarse and fine grained variants of the hornblende granite and hornblende pegmatite occur at contacts with the country rocks and an older gneissosity is truncated by the younger granites.
4. Contacts with rusty sulfidic paragneisses developed abundant magnetite and sulfides just as the wall rocks of the larger bodies of hornblende granite do in the Brewster magnetite district do.

General geology.
We are in the center of the Lake Carmel quadrangle headed west across a particularly well developed sequence of Mesoproterozoic paragneiss, calc-silicate rock, marble, thin quartzites and schist. We will cross two of the thickest belts of amphibolite in the Hudson Highlands that are mappable into the Poughquag quadrangle to the north. These amphibolites are true amphibolites, unlike so many of the rocks mapped as amphibolites by myself and others in the Highlands. These are composed of >70% hornblende rather that the more common type of "dioritic" amphibolites which commonly contain equal amounts of plagioclase and hornblende. This belt of coarse to fine amphibolite is associated with pods of serpentinite, anthophyllite schist and gabbroic to dioritic igneous rocks. The serpentinites are in contact with paragneisses, diopside bearing quartzites, and carbonate containing ovoidal pods of what probably are serpentinized forsterite knots.

STOP 5A. Coarsest amphibolite
Coarsest amphibolite perhaps related to the Wiccopee pluton of the Oscawana Lake quadrangle (Ratcliffe, 1992) discussion of amphibolite chemistry

STOP 5B. (Optional) Reservoir Gneiss at its type locality
Discussion of regional tectonic of the Oscawana Lake-right lateral deformation zone (see Stops 1 and 7 for chemistry discussion)
These road cuts in the Reservoir Gneiss represent the type locality of the gneiss and were visited on Stop 6 of Ratcliffe and others (1985) and is in part reproduced from Figure 23 of Ratcliffe (1992). Aside from illustrating the type Reservoir Granite of Berkey and Rice (1921), these crops contain mylonite zones typical of the Paleozoic fabrics found to mark the larger shear zones at biotite and higher grades. Here evidence for reactivation as Mesozoic normal faults may be seen. A similar observation will be made at Stop 8.

General geology
We are traveling down the Peekskill Hollow-Nuclear Lake-shear zone that marks the eastern side of the large right-lateral sigmoidal deformation zone in the center of the Oscawana Lake quadrangle. Although the sigmoid is Mesoproterozoic (Ratcliffe, 1992), the California Hill and this shear zone are Paleozoic based on the retrogressive phyllonite there and offsets of Paleozoic rocks along their lengths and from dating of syntectonic biotite in the mylonite of the California Hill shear zone that has a 436 ± 3 Ma Ar/Ar age (see Figure 0 in Ratcliffe, 1992). The sigmoidal deformation of the gneisses however is Mesoproterozic based on the mapping of non rotated metadiabase dikes that have the chemistry of Neoproterozoic rift basalt. (Ratcliffe, 1992). Based on this reasoning, the California Hill shear zone has both a Mesoproterozoic and a Paleozoic history, as do the faults bordering the Canopus Valley to the west.

At Adams Corner we will turn west and cross the end of the Annsville syncline
At Adams Corner we will turn west and cross the end of the Annsville syncline which probably has a Mesozoic fault long its northwestern flank. See discussion Stop 7.

General geology
Two miles south on Rt. 20, in front of the Putnam Valley Elementary School, there are superb outcrops of Mesoproterozoic shear zone rocks invaded by aplites that are also highly deformed and crossed by Mesoproterozoic pegmatites supporting intense Mesoproterozoic deformation east of and parallel to the main Canopus shear zones. This is approximately on the California Hill fault zone and supports Mesoproterozoic inheritance here.

Canopus Valley
Continue on Cimmaron Rd. down the hill into the Canopus Valley that is underlain by coarse phlogopitic- feldspathic-scapolite dolomite marble, quartzite and retrograde diopside calc- silicate rock. This is the thickest belt of Mesoproterozoic marble and calc-silicate rocks in the Hudson Highlands of NY.

Old magnetite mine developed in marble and calc-silicate rock
Immediately above and on the south side of Indian Hill Road is an old magnetite mine developed in marble and calc-silicate rock just off the north end of the Canopus pluton. Here aplite and pegmatite that makes the border phase of the Canopus , contain large 1-2 cm clots of magnetite where it intrudes hornblende granite gneiss like the Storm King Granite. This lit par lit border zone will be seen at Stop 6b where it is caught in the retrogressive shear zone along the west side of the Canopus Valley at the east edge of the pluton.

STOP 6A. Canopus pluton diorite, dikes and screen of Mesoproterozoic gneiss
At the end of the driveway are excellent new exposures of the dioritic rocks of the pluton on the right and left sides of the cut. Note the well defined areas of monzodiorite in diorite in the block to the left (S), the diorite - a screen of isoclinally folded country rocks exhibiting the YF2 structure and the flow layered diorite on the right (N). Two mafic dikes cross cut both the gneiss inclusion and the flow layered diorite. The observations here point to composite intrusion of the dioritic phases of the pluton. Dike swarms in the vicinity of the pluton cross cut YF2 structures in the gneisses. Some dikes are as much a 0.5 km long and are present as many as 2 km west of the pluton.These dikes are chemically and petrographically like the dikes in the Brewster area seen at Stop 2.

STOP 6B. Canopus pluton 1143 ± 12 Ma
Biotite monzonite and hypersthene biotite diorite composite intrusive.
This spectacular and demonstrative new driveway cut exposes the transition from the Canopus shear zone into the Canopus pluton. The Canopus pluton is intruded between two Mesoproterozoic right lateral shear zones. The eastern one has been reactivated at greenschist grade and is exposed here. The western one is not reactivated and contains stringers of monzodiorite- monzonite and cross cutting composite diorite- monzonite dikes of the main pluton and abundant pegmatite, all of which crosscut the mylonitic host rocks, or as much as 2 km north of the pluton. Internal structure in the pluton consists of mutually crystallizing igneous phases and a well developed and folded igneous flow layering that defines right lateral deformation of the pluton during crystallization. At the walls igneous rocks of the pluton cross cut the bordering mylonite zones of the faults and fill internal shears within the pluton. Isoclinal folds in the mylonitic country rocks plunge nearly vertically down the mylonite structure, as do the axes of the internal and folded igneous flow structure in the pluton. This has been interpreted to mean intrusion of the igneous rocks into a subvertical active shear zone in the Mesoproterozoic. Early attempts to date the igneous rocks using Rb/Sr whole rock techniques by Dick Armstrong (Ratcliffe and others, 1972) and using multigrain zircon techniques by Aleinikoff gave ages of about 1.0 Ga and were erroneous. Subsequent SHRIMP zircon dating by Aleinikoff gives a more precise concordant age of 1143±12 Ma with overgrowth ages of about 1 Ga. That is, during the end stages of the Shawinigan pulse of the Grenville orogeny (Rivers, 1997) as well as Ottawan overgrowths.
Starting from the road level some of the significant features to be seen are:
1. Mylonite, consisting of lit par lit veins of aplite , isoclinal steeply plunging folds in the mylonite.
2. Zones of pegmatite saturating and postdating the mylonite structure
3. The wall of the monzonite with fine grained monzonite in the mylonite
4. Tabular K-spar defining an igneous flow structure cut by aplite dikes and by intermediate diorite dikes
5. A pillowed zone consisting of monzodiorite ellipsoidal segregations in the monzonite and turbulent flow structure in the monzonite around the "pillows"
6. Ductile shear zones in the monzonite which deform the flow layering into lineated tectonites and undeformed granitic segregations which saturate the lineated zones
Some features described above are shown in Figure 19.
These features and other similar observations made during mapping of the shear zones and the igneous rock (Ratcliffe, 1971; Ratcliffe and others, 1972) support the concept of syntectonic intrusion and document right-slip faulting at about 1143 Ma. To the east the right sigmoidal deformation in the area of Wiccopee diorite pluton indicates this type of tectonism in a zone as much as 10 km wide (Ratcliffe, 1992). Mafic dioritic to hornblendic dikes are common within this sheared zone cutting the fault fabrics and folds associated with the strike-slip events. This is very similar to the dikes and igneous rocks associated with stops 2 and 3 today in the Brewster area where a similar age for intrusion of 1134±8 Ma has been established for comagmatic granitic and mafic rocks.
Mafic dikes commonly less than several meters thick, crosscut high grade shear zones on both sides of the Canopus shear zone and have compositions similar to the dioritic and most mafic part of the pluton. Locally ultramafic hornblendite plugs and radiating dikes are found near Fort Hill. Importantly, dioritic dikes and segregations within the pluton crosscut the igneous flow structure (Figure 6), and all rocks of the pluton lack a strong Mesoproterozoic deformational structure that is present in the country rocks. This is also true of the granites and dioritic rocks of the same age at Stops 3 and 4 and of the mafic dikes and gabbro-diorite plutons in the Brewster area. We may speculate that the deformed dikes seen in the tonalite –trondhjemite and at Stop 1 today might also be dikes of the same age.

The dated monzonite
The dated monzonite came from just north of this intersection on the west side of the road.

STOP 7. Cat Hill Gneiss 1333 ±7 Ma trondhjemite gneiss
And Mesozoic cataclastic structure on the north limb of the Annsville syncline
On the south side of the road are limestone crops of the Cambrian and Ordovician Wappinger Group here barely at biotite grade. North of the road before the Cat Hill Gneiss are crops of the Lower Cambrian Poughquag Quartzite then cataclastic gneiss of the Cat Hill Gneiss. In the 1970's, I mapped an unconformity between Paleozoic rocks of the Annsville syncline and the basement as Walter Bucher had. New cuts and reexamination of the cover rocks along the Annsville syncline to the east now support high angle normal faulting and brecciation of the kind associated with Mesozoic tectonics and the Newark basin.
The Cat Hill Gneiss is less cataclastic in the outcrops behind the house but even these are not typical because of the extensive fracturing. A few hand samples from the dated locality less tan 0.5 miles to the northeast demonstrate the difference. To get to the sample locality follow the directions to Stop 11 in Ratcliffe and Aleinikoff (2008)
The Cat Hill Gneiss is a white-weathering biotite-oligoclase-quartz gneiss of trondhjemitic and tonalitic composition. It is typical of a series of similar massive biotite poor garnet bearing gneisses that occur in the Reservoir Gneiss of Berkey and Rice (1923) and which closely resemble parts of the Losee gneiss of the New Jersey Highlands. It also is very similar to tonalitic gneisses of the Green Mountains of Vermont, which formed as calc-alkaline intrusives between about 1390 Ma to about 1320 Ma (Ratcliffe and others, 1991). The rock is very massive, contains almost no inclusions. It does pass into a more garnet-rich phase and also hosts abundant white aplitic granites at its contacts with amphibolites and has brown weathering spots suggestive of hypersthene but none has been found here. These bordering aplites produce a spectacular intrusive breccia with inclusions of hornblende gneisses, amphibolites. Because the Cat Hill Gneiss contains migmatitic zones of similar aplite and granite it may be that the breccia is a product of mobilization and intrusion along the walls resulting from secondary mobilization in an event younger than 1337 Ma. The purple brittle fracture zones contain minute riebeckite and are very similar to fracture zones along the Ramapo and other faults along the Newark basin in New Jersey
Figure 20 summarizes the axial traces of Mesoproterozoic folds in the Hudson Highlands of NewYork. Folds related to the Canopus- California hill deformation zone document a wide area of right sigmoidal folding here interpreted to be of Pre-Ottawan at approximately 1142 Ma and contemporaneous with intrusion of the Canopus pluton and its ferrodiorite dikes. Folds west of the shear zone, identified as Yf3 folds are Ottawan and folds identified as Yf2? to the east may be also be.In the Brewster quadrangle (Stop 2) and on Joes Hill similar dioritic dikes cross cut the granitic gneisses and intrude along steeply dipping high-grade shear zones. I tentatively correlate the igneous rocks of the Canopus area with the dikes in the Brewster area and with the dated granite - and diorite at stops 3 and 4. If this is correct, there may be evidence for a broad zone of right lateral shearing and igneous activity in the northern Hudson Highlands near the end of the Shawinigan phase of the Grenville orogeny. This raises the possibility that some of the amphibolite inclusions in these older rocks (Stops 1, 8 and 10) could be dikes, masking as xenoliths in the older granitic to tonalitic rocks! The more I learn about these rocks the more that interpretation seems likely to me.

Sprout Brook valley
Sprout Brook valley is a continuation of the Canopus Valley that extend south to the Hudson River and which is floored by highly mylonite sillimanite containing garnet gneisses, as well as and calc- silicate rocks. Biotite grade phyllites of Ordovician age correlative with the Wallomsac Formation occur along the east side of the valley where they rest on Mesoproterozoic rocks along the east dipping Annsvile fault which I originally interpreted was an Ordovician thrust fault but which I now regard as a Mesozoic normal fault traceable to branches of the main Ramapo fault to the south.

Exposures of Wallomsac Formation
Exposures of Wallomsac Formation in contact with Mesoproterozoic gneiss along the extension of the Annsville fault. At these outcrops Walter Bucher documented a conglomeratic fine grained dolomite with clasts of gneiss which he interpreted as a basal units of the Wappinger Group resting unconformably on the basement. However, the large pebble- to cobble- size inclusions within the dolomite are all fragments of retrograded calc-silicate rock in an ultrafine grained mylonite or foliated dolomitic cataclasite that derived from Mesoproterozoic rock.

Poughquag Quartzite to the left are unconformable on Reservoir Gneiss
Crops facing consist of Cambrian Poughquag Quartzite to the left are unconformable on Reservoir Gneiss. Large continuous crops of the Reservoir Gneiss continue for a mile to the south in high road cuts.

STOP 8. Reservoir granite of Berkey and Rice (1921) at Peekskill
The exposures sampled for dating of zircon are at the east end of the large road cut on Rt. 9 and 202 below the Peekskill Inn. The exposures extend about 1000 feet to the north in high road cuts. Numerous thin to thick amphibolite gneisses and more biotitic gneisses form septae in the granodioritic gneiss. The rock is remarkably homogeneous and is interpreted as an intrusive rock. At one locality near the middle of the cut a pod of amphibolite is included in the granodiorite. A concordant SHRIMP zircon age of 1338 ± 6 Ma has been determined for the granodiorite. Zircon overgrowth ages are about 1.26 and 0.98 Ga.

STOP 9. Cortlandt complex monzonorite and contact with the Walloomsac Formation
We are at the southeastern margin of the Cortlandt complex that consists of igneous norite, diorite, pyroxenite that cross cuts Taconic deformed metasediments of the Manhattan prong as well as the Laurentian basement. It was intruded following Taconic deformation and metamorphism and is one of the key observations in determining the timing of the Taconian orogeny. The igneous rocks are folded and intruded by the Devonian Peekskill Granite. A discussion of the contact relations, chemistry, and petrogenesis of the complex is beyond the scope of the present trip. Several papers and guidebooks provide that information (Bender, 1982; Ratcliffe and others, 1982, 1983).
The central basin of the complex consists of inward dipping sheets of biotite norite, hornblende and biotite poikilitic norite and locally at the very border a monzodiorite that contains zircon. Pockets of interstitial k-feldspar- quartz and radiating biotite define slightly pinkish areas in the norite adjacent to the contact with the Walloomsac Formation. Zircon from this rock yielded a concordant SHRIMP age of 444±3 Ma almost identical to the concordant SHRIMP zircon age from monzodiorite of the Peach Lake pluton (447±5 Ma).
Near the contact igneous rocks and mobilized schist form an intrusive breccia In the breccia are rotated fragments of foliated and folded calc- silicate rock, and calc- schist characteristic of the basal part of the Walloomsac Formation (formerly termed Manhattan A of Hall, 1968). A breccia zone like this was previously recognized along this contact but none of the exposures are as spectacular as this one. Just west of this location and in the cuts for the exit ramp from Rt. 9W dikelets of mobilized schist containing garnet sillimanite-spinel- and corundum back intrude the chill norite and cut across the intrusive flow layering in the igneous rocks (see Stop 6 of Ratcliffe and others, 1983). Evidently the schists of the contact aureole were mobilized not as melts but as volatile-rich pneumatic injections, driven by contact-induced decarbonization and dehydration reactions along the walls.
Thin sections of the contact schist here contain garnet- biotite- staurolite- plagioclase and quartz and lack either kyanite or sillimanite both of which are present in contact rocks not far from here. Schist outside the aureole to the south on Torment Hill also contains abundant staurolite and lacks either kyanite or fibrolite.

STOP 10. Granitic Fordham Gneiss 1351±6 Ma and amphibolitic gneisses
The first exposures by the RR tracks are dark-gray biotitic gneisses, amphibolite gneisses and some sulphidic schists of the Fordham gneiss, which are in contact with the granite gneiss from the dated locality westward to this point. The biotite- hornblende gneiss resembles the inclusions we will see in the granite gneiss on Oscawana Island. The amphibolitic gneiss is layered and grades into schists near the south end of the RR exposure. This suggests that the Fordham Gneiss contains paragneisses that are older than 1351 Ma. Hornblende from these exposures yielded a 40Ar/39Ar cooling age of 450 Ma (Dietsch and others, 2004).
Walk northwest to the island. Take bridge over tracks and follow trail to end of island. Walking south then west along the shore we will see excellent exposures of the dated gneiss and inclusions in it. Some of the tabular to irregular-shaped inclusions are layered, but the gneissosity seems to be shared by the granite. Note this is unlike the relations of the 1134 Ma intrusive rocks at Stops 3 and 4. I have not seen evidence that the 1350-1330 Ma rocks cross cut an older gneissosity.
Here we might consider how the zircon age and intrusive relationships should be interpreted. Does the zircon age date the time of the intrusion of granite, or could the intrusive relations we deduce been produced by migmatization of a pile of felsic and mafic volcanic rocks? This is a difficult question not easily answered, either from the zircon zoning patterns or the field observations. I think we can conclude safely however that the granitic gneiss and the inclusions were present at 1351 Ma, but in what exact form is left undetermined. We have chosen to interpret the zircon age as the emplacement and crystallization age. If this is correct then the biotite hornblende gneisses in the RR cut might be older than the granite and some of the Fordham Gneiss older than 1351 Ma. Summary diagrams of dike chemistry Figures 22 and 23 give a summary of some of the chemical characteristics of the mafic dikes and amphibolites we have seen on this trip. The data are consistent with the mysterious dikes of the Brewster area being correlative with 1143 Ma Canopus plutonism rather than Ordovician, as the field observations indicate. The data also allow for some of the amphibolite "inclusions" in the ∼ 1340 Ma granitoids to be deformed dikes, as field observations do indicate for some, perhaps in the same 1143-1134 Ma Shawinigan event. Notable are the coarse amphibolites (Stop 5) of the Nimham belt which are distinct outliers of low TiO2 mafic rocks. Ta/Yb of the Brewster dikes overlap those of the Canopus dikes although the centroid of the former suggests a more depleted mantle source rather than more MORB-like source for the latter. The quite alkaline nature of the Cortlandt-age dike swarm suggests an enriched mantle source at about 450 Ma., and the high-TiO2 – low MgO metadiabase dikes of probable Neoproterozoic age (Ratcliffe, 1987) suggest low degrees of partial melting in a rift event at about 570 to 600 Ma, although the age of these dikes has not been determined.