A2: PRECAMBRIAN GEOLOGY OF THE EAGLE LAKE QUADRANGLE, ESSEX COUNTY, NY
Sean P. Regan, Phillip S. Geer, Gregory J. Walsh, and John N. Aleinikoff
United States Geological Survey
Graham B. Baird
Department of Earth and Atmospheric Sciences
University of Northern Colorado
Greeley, CO 80639
Peter M. Valley
Weatherford Labs
Houston, TX 77086
Michael L. Williams and Michael J. Jercinovic
Department of Geosciences
University of Massachusetts
Amherst, MA 01003
Timothy W. Grover
Castleton State College
Castleton, VT 05735
INTRODUCTION AND REGIONAL BACKGROUND
The Grenville Province of eastern North America has long been interpreted as the roots of an ancient orogenic system that formed approximately 1.0 billion years ago (Hoffman, 1988; McLelland et al., 2010). The entire region formed and evolved during punctuated phases of crustal growth during the Mesoproterozoic involving a variety of tectonic processes including accretion, arc magmatism, back-arc development and collapse, and unique to the Grenville, anorthosite-mangerite-charnockite-granite (AMCG) plutonism. These processes occurred over > 300 my and are collectively referred to as the Grenville Orogenic cycle (Fig. 1; McLelland et al., 1996; Rivers, 2008). The northeast Grenville Province has been shown to have a long history, extending back to ca. 1.35 Ga (McLelland et al., 1993), and contains evidence of long-lived active margin processes (Dickin and McNutt, 2007; Chiarenzelli et al., 2010a). However, the most extensive regional deformation has been interpreted as having formed during a culminating collision at ca. 1.07 Ga (Ottawan orogeny), which resulted in the final assembly of the supercontinent Rodinia (McLelland et al., 1992; 1996; 2001a; 2010; 2011; Rivers et al., 2008). The Grenville Province offers an excellent opportunity to study the growth and modification of mid to lower continental crust during these tectonic processes within the context of a long-lived active margin (Mezger, 1992).
The Adirondack Mountains of northern New York are a large domical uplift and are subdivided into the northwest Adirondack Lowlands and southeast Adirondack highlands separated by the Carthage- Colten shear zone (Streepey et al., 2001; Selleck et al., 2005). The Adirondack Highlands are dominated by granulite facies (meta)igneous rocks in contrast to the Lowlands, which are dominated by supracrustal rocks metamorphosed to amphibolite facies. Another important difference is the timing of deformation in each region. In the Lowlands the majority of deformation was the result of Shawinigan orogenesis (ca.1.17 Ga; Fig. 1b; Wasteneys et al., 1999; Heumann et al.,2006; Chiarenzelli et al., 2010b; Wong et al., 2011a), and evidence for Ottawan deformation (ca. 1.07 Ga; McLelland et al., 2001a) is lacking (Baird and Shrady, 2011). In contrast, the Adirondack Highlands are known to have experienced both Shawinigan and Ottawan phases of the Grenville Cycle. Here, differentiating the effects of these two phases remains a major challenge that is critical for understanding the role and importance of the Ottawan orogeny in the Grenville Province as a whole (Rivers, 2011).
This field trip focuses on the general Precambrian geology of the Eagle Lake quadrangle (ELq) in the eastern Adirondack Highlands. We will discuss recent 1:24,000 scale mapping of the ELq and the historical mining area Hammondville (Geer et al., 2015). The new mapping represents work by Regan, Geer, and Walsh. Provisional mapping by Walton (1960) has proven quite helpful, and provided a foundation for the new work. We will present a new structural nomenclature established during mapping that is regionally transformative and will hopefully be a useful guide for future analytical work. New forward petrologic modeling of the metamorphic assemblages that define a major structural fabric, geochronologic data that constrain the timing of both granite petrogenisis and anatexis of metasedimentary lithologies, and geochemical data from the historical mining district of Hammondville.
The Adirondack Highlands are predominately underlain by granulite facies (meta) igneous lithologies (McLelland et al., 1996). The most voluminous plutonic rocks exposed belong to the anorthosite-mangerite-charnockite-granite (AMCG) suite, which underlies approximately 60 % of the Adirondack Highlands (Hamilton et al., 2004). The petrogenisis of AMCG suites has been debated for decades, but detailed U-Pb geochronologic work presented in McLelland et al. (2004) and Hamilton et al. (2004) shows that granitoid lithologies (1158 Ma) are statistically older than anorthositic and other mafic members of the suite (1154 Ma), but overlap in age (Figure 5e). These data were interpreted as showing that AMCG suites are “… coeval, but not necessarily comagmatic” (McLelland et al., 2004). Geochemical and isotopic analysis performed by Regan et al. (2011) showed that coronitic metagabbros of the Adirondack Highlands (McLelland and Chiarenzelli, 1988) are permissible as the parental magma for the anorthosite. Quartz-bearing end-members were interpreted to have been derived from partial melting of lower continental crust where anorthosite formed via fractional crystallization of an asthenospherically derived gabbroic magma (McLelland et al., 2004; Bickford et al., 2008; Regan et al., 2011). The largest body of anorthosite is the Marcy Massif, which underlies the high peaks region of the Adirondack Mountains (Buddington, 1939). This suite of rocks has been interpreted as the result of lithospheric- mantle delamination at the waning phases of Shawinigan orogenesis (McLelland et al., 2004). Therefore, the deformation and metamorphism present throughout AMCG rocks has been interpreted as Ottawan in age.
The Lyon Mountain Granite Gneiss (LMG: nomenclature after Postel, 1952) is the youngest igneous rock exposed within the Adirondack highlands (Buddington, 1939; Postel, 1952; McLelland et al., 2001a,b,c; Selleck et al., 2005; Valley et al., 2009; 2010). It is a leucogranite that rims the Adirondack Highlands and is host to low-Ti magnetite deposits interpreted as forming via kiruna-type Fe – oxide, copper, gold (IOGC) mineralization processes (McLelland et al., 2001b,c; Valley et al., 2009). The LMG contains no consistent shape-preferred orientation (except in the East Adirondack shear zone; see Grover et al., trip B-2), and very little evidence of regionally significant subsolidus deformation, although the nature of layering is currently being debated (McLelland et al., 2001a). The LMG has been interpreted as accompanying orogenic collapse at the tail end of Ottawan orogenesis (McLelland et al., 2001a; Selleck et al., 2005).

A2: PRECAMBRIAN GEOLOGY OF THE EAGLE LAKE QUADRANGLE, ESSEX COUNTY, NY
S.P.Regan, P.S.Geer, G.J.Walsh, J.N.Aleinikoff, G.B.Baird, P.M.Valley, M.L.Williams, M.J.Jercinovic, T.W.Grover

Ticonderoga McDonalds Parking Lot
S.P.Regan, P.S.Geer, G.J.Walsh, J.N.Aleinikoff, G.B.Baird, P.M.Valley, M.L.Williams, M.J.Jercinovic, T.W.Grover

STOP 1. Roadcut on NY-74, west of Ticonderoga, NY
The easternmost rock is a poikiloblastic garnet amphibolite that contains garnet-clinopyroxene, plagioclase, and hornblende. Fe-Ti oxides are present as well, though can only be seen in thin section. The garnet crystals can reach upwards of 3 cm in diameter, the best examples are concentrated in the westernmost part of the garnet amphibolite. Schistosity, mostly defined by aligned hornblende and appears to both traverse and wrap around the garnet poikiloblasts, suggesting garnet growth was syn-kinematic.
Further west along the outcrop is a transition zone, marked by a vegetated slope, containing some layers of marble and a rusty weathering paragneiss. West of the transitions zone the vast majority of the outcrop is composed of a garnet-biotite-plagioclase-perthitic microcline-quartz ± sillimanite migmatitic paragneiss. Leucosome proportion is estimated to be 30-50% and preferentially contains garnet, while biotite is concentrated in the melanosome. Though some texturally late leucosome contains biotite, this can be shown to grade into garnet-bearing leucosome. Both leucosome and melanosome layers are deformed and define a foliation parallel to a schistosity defined by aligned biotite. Quartz and feldspar are granoblastic in both the leucosome and melanosome.
Within the migmatitic paragneiss a number of garnet rich boudins occur, these are possibly deformed and metamorphosed dikes. The boudins indicate stretching in all directions parallel to gneissic layering suggesting a flattening finite strain. Also present within the migmatitic paragneiss are bodies of amphibolite similar to what is present at the east end of the outcrop. Three, 5 to 30 meters wide, unstrained, amoeboid-shaped, pegmatitic granitic plutons also occur in the migmatitic paragneiss.
Mineralogy of the pegmatites includes feldspar, quartz, and biotite. Most feldspar appears to have exsolution lamellae indicating alkali feldspar, but locally plagioclase has been observed. Due to the coarse grain size (upwards of 30 cm for feldspar and quartz and 50 cm. for biotite) the exact lithology has not been constrained, but it is thought to be an alkali-feldspar granite. Locally associated with the pegmatites in the host migmatite are cm. books of graphite.
Fabric orientation is essentially parallel among the garnet amphibolite, the migmatitic paragneiss, and the transitional contact zone between the two. The foliation in the garnet amphibolite and eastern portions of migmatitic paragneiss is approximately 062°, 43° but changes to about 103°, 45° by the westernmost end of the outcrop. Lineation is reasonably parallel to the outcrop face and ranges from a horizontal to 30° plunge, tending 100°. The lineation is defined by aligned hornblende, biotite, or sillimanite, depending on location within the outcrop. Asymmetric tails on garnet poikiloblasts in the garnet amphibolite suggest south side (top) to the west transport.
To constrain timing of geologic events recorded in the outcrop, samples of the migmatitic paragneiss and of pegmatite were processed for U/Pb zircon geochronology. For the paragneiss, given the scale of leucosome and melanosome interlayering, it was not possible to separate the two upon processing, so separated zircon is from both components. Such zircons are euhedral to subhedral, stubby to elongate, yellow to amber to brown in color, and are 100 µm to 500 µm in length.
Cathodoluminescence (CL) of polished grain cross sections show that zircons have a wide variety of internal zoning patterns from oscillatory to irregular zoning (Figure 11 inset). No consistent core-rim zoning pattern (or of any other type) could be generalized for the separated zircon grains. Results of 21 SHRIMP-RG analyses of all types of zircons and all CL zones reveal what it interpreted as a unimodal age distribution. 16 of 21 analyses define a chord with an upper-intercept of 1186 ± 25 Ma, which is interpreted to be the age of migmatization for the paragneiss (Figure 11).
Zircon from one pegmatite body is euhedral to subhedral, equant to stubby, and light pink to purple in color. Zircon size is poorly constrained as most grains are fragments of grains greater than approximately 500 µm. Cathodoluminescence of polished grain cross sections reveal all grains possess oscillatory or sector zoning (Figure 12 inset). No convincing core-rim relationships were found. Fourteen SRHIMP-RG analyses produce what is interpreted as a unimodal distribution of ages. Six if these are nearly concordant and produce a 207Pb/206Pb weighted mean age of 1051 ± 25 Ma that is thought to best represent the age of pegmatite formation (Figure 12).
S.P.Regan, P.S.Geer, G.J.Walsh, J.N.Aleinikoff, G.B.Baird, P.M.Valley, M.L.Williams, M.J.Jercinovic, T.W.Grover

STOP 2. Transposed Grenville Supergroup
This is an exposure of metamorphosed and folded meta-siliciclastics of the Grenville Supergroup (Davidson, 1998). Notice the very well layered nature of the rock, the related compositional variability, and the gradational transitions between various layers. Also note several asymmetric folds that are particularly easy to see within leucosome. This suite of rocks saw the following events: 1) deposition, 2) partial melting (M1), and 3) folding of this migmatitic gneissosity, and the development of axial planar fabric (S2). These relationships suggest that anatexis and high-grade metamorphism likely accompanied S1, which is interpreted to be Shawinigan based on the geochronology discussed at stop 1. This suite of rocks is common throughout the ELq, and represents some of the oldest rocks preserved within ELq.

STOP 3. Contact between mangeritic gneiss and the Grenville Supergroup.
Several small outcrops are present to the southwest along Old Furnace Road. We will start by the parked cars. In stark contrast to the rocks seen at the previous outcrop, this rock is fairly homogenous. It is a mangeritic gneiss, with hbl ± grt composing the tails around primary opx (McLelland et al., 1980).
Do you see evidence of more than one phase of deformation within this lithology? As we move south, we will see a quick transition back into the Grenville Supergroup. This produces an interesting map pattern, a dome structure that is cored by monocyclic mangeritic gneiss, engulfed by meta-siliciclastic rocks of the Grenville Supergroup, with the predominate fabric running parallel to the axial trace of (F2?). However, we know from the previous stop that the predominate fabric we see in the paragneisses is an aggregate of two planar fabrics. The big question is; can we see S1 in the mangeritic gneiss?

STOP 4. Hill No. 8
Hill No. 8 is in the northern part of the historic mining town of Hammondville. On the north eastern base is the remains of a great furnace used to melt and separate the ore mined here in the late 19th century (next stop). The remaining slag is the blue stone used on many of the roads in the area that you can see on the drive in and out.
Hill No. 8 was the northern most extent of the mining operations of Hammondville and included two major pits: the Hammond Pit and Number 8 Mine. The number 8 mine is the deepest in Hammondville, reaching depths of up to 1000 ft (Penfield museum, personal correspondence), however the exposure in the Hammond pit is better and more easily accessible.
Magnetite seams in the Hammond pit range from centimeter scale to roughly 2 meters in thickness. The seams display a discrete wavy contact with the host LMG and split into multiple seams in places. They are concordant to the local fabric (038°, 27°) which is much stronger within the vicinity of mineralization. The ore is >70% Fe2O3 with slightly elevated REE levels. A thin section of a seam contained beautiful S-C fabrics indicating a reverse sense shear with top to the southeast, Further investigation into the relationship between the timing of deformation and magnetite mineralization is underway. The host LMG immediately surrounding mineralization seems to be very gneissic and contains albite, quartz, amphibole, and pyroxene. It has an Fe2O3 content of less than 7%, Na2O of more that 6.5% and REE levels slightly higher than that of the ore.
A sample of LMG was also collected roughly 100 m from the mine on the western side of Hill #8 and is elevated in REEs and contains a potassium content an order of magnitude higher than the immediate host. It also records subsolidus deformation, perhaps related to the massive magnetite deposits, however the reverse shear is top to the NW. Again, constraining timing for this is underway.

STOP 5. Old Furnace.
Old Furnace locality. Depart by turning left onto Old Furnace Road.

STOP 6. Schofield magnetite mine.
WARNING: Route 74 is a busy road with lots of heavy truck traffic. Walk single file and stay well to the side of the road.
Walk west down the road for approximately a quarter mile until you reach a brown and yellow sign marking the boundary of the Adirondack Park. Proceed into the forest for approximately 75 meters to the base of a cliff.
This stop examines the relationship between magnetite ores and the Lyon Mountain granite (LMG) that hosts the ore. Most of the main ore seam has been mined out and is covered by large blocks of mine waste. However there are places where the relationship between the ore and the host granite can be observed. The main ore seam starts here at the base of the cliff and gradually climbs up the hill to the east parallel to the main foliation in the granite. The host rock on either side of the ore seam is albitized granite and is the result of fluid alteration. The amount of albite in the rock gradually diminishes as distance from the mineralized ore zone increases and perthitic feldspar becomes common. The ore is comprised of magnetite and apatite ± quartz. The apatite is generally reddish in outcrop and is extremely enriched in light rare earth elements and high field strength elements (especially Y, Ce and Nd). The mineralogy of the “granite” varies from perthitic feldspar + quartz + magnetite ± clinopyroxene, biotite, and amphibole in the least altered rocks, to quartz + albite + magnetite ± apatite and chlorite in the most altered zones. The contact between the ore and the granite is generally sharp but a large boulder here at the base of the cliff shows numerous mm-thick layers of magnetite. Zircon grains from the ore have been dated at 1000.9 ± 9.2 (2σ) (Valley et al., 2009).
Proceed uphill along the ore seam approximately 75 meters to the east by following an animal trail and scrambling over boulders. Here the ore is perfectly exposed in a large out of place slab and is where the geochronology sample was collected. Even though a reliable age for the host LMG could not be obtained at Skiff Mountain, the age of the zircon in the ore here is at least 34 my younger than that of the granite. This is based on the age of zircon rims in the LMG that was successfully dated near here at Eagle Lake (see EAGLE LAKE STOP; Figure 13).
Continue following the ore seam uphill to the east. Watch for a small outcrop of hematized breccia. The ore seam will disappear under mine waste rock and forest. At this point head directly uphill until reaching a relatively flat area. You will see what appears to be a ditch, sometimes filled with water, backed by a 3-4 m high outcrop. This is the top of the upper ore seam. Follow this relatively flat area to the west. At 43.886056, 73.637028, the top of the upper ore seam is visible. The ore seam is approximately 0.5 m thick with a second 2-3 cm thick vein just above the main ore. The adjacent granite is typically lacking in disseminated magnetite in the vicinity of the veins, but disseminated magnetite reappears where veins are lacking and distal to the ore vein. Near the top of this outcrop small lozenge shaped quartz-feldspar-magnetite pegmatites are present (∼20-30 cm long). Follow the upper ore seam west for approximately 60 m. Here 1 m below the ore body, there are two amphibolite layers 10 to 20 cm thick. Recent geologic mapping suggests that these are mafic “screens” that were incorporated in the LMG during intrusion. Zircon grains from one of these layers have been dated at 1046 ± 11 Ma (2σ) (Valley et al., 2011) with rare zircon cores that are ∼1150 Ma. These layers and the host LMG have been overprinted by Na fluid alteration providing a maximum age for Na fluid alteration. The increase in albitization around the ore suggests Na metasomatism is coeval with zircon growth and ore mineralization. It is probable that the U/Pb age from zircon in the amphibolite layer is the product of metamorphism or possibly a fluid alteration event that is older than the fluids associated with the growth of zircon in the ore at Skiff Mountain.

Parking for Stop 7

STOP 7. The Lyon Mountain Granite Gneiss (after Postel, 1956)
From the cars, walk east about 0.3 miles to an outcrop on the right (south) side of the road. At this locality a granitic pegmatite dike crosscuts the fabric of the LMG. The LMG has been altered by K-rich fluid alteration that has been overprinted by minor Na alteration. The crosscutting dike has also experienced minor Na alteration. Both the dike and the granitic hosts have extreme concentrations of potassium for granitic rocks (∼8 wt.% for both rocks). It is not clear if the dike is high in potassium because it intruded an already metasomatized rock and thus was enriched in K2O by a relatively closed system, or if both the dike and the host were altered by potassium bearing fluids together in an open system. Both the dike and the granite experienced subsequent minor sodic alteration. The dike is comprised of coarse microcline, quartz, and biotite, with minor plagioclase, zircon, apatite, and magnetite ± clinopyroxene and muscovite. Plagioclase is secondary and is present interstitially and in “patch” perthite which crosscuts microcline grains.
A sample from the dike was collected for U/Pb zircon geochronology and was dated by Secondary Ion Mass Spectrometry (SIMS). Zircon crystals from this sample are elongate (300-500 µm long), clear with patchy zonation in BSE images, and typically contain large inherited cores of both AMCG (∼1150 Ma) and LMG (∼1060 Ma) age. The zircon rims from grains with relict cores and grains without cores have a concordant age of 1030.4 ± 1.8 Ma (2σ) (Valley et al. 2011). The aforementioned field relations imply that Na alteration has to be younger than the U-Pb zircon age of the dike. They also suggest that the forces responsible for fabric development within the LMG must have ended by this time.
A sample of the host rock was collected within in a few meters of the dike discussed above in order to constrain the timing of Skiff Mt. LMG intrusion. Zircons separated from the sample are euhedral to subhedral, stubby to elongate, typically 200-500 µm long, and are amber to brown in color.
Cathodoluminescence (CL) of polished grain cross sections reveal that most zircons have oscillatory zoned cores and prominent dark rims with subtle and limited oscillatory or patchy zoning (Figure 14). A few grains appear to possess a core, mantle, and rim. In such grains, rim CL is consistent with the rims of other grains, with the mantle and core both having oscillatory zoning. Twenty U/Pb SHRIMP-RG analyses of all zones demonstrate that the cores (or mantle, if three zones are present) are 1086 ± 34 Ma (2σ 207Pb/206Pb weighted average, 5 analyses; Figure 14), while rims cluster at 1043 ± 9 Ma (2σ) 207Pb/206Pb weighted average, 5 analyses, Figure 14). For grains with three zones, only two cores were analyzed and produced 2σ 207Pb/206Pb ages of 1169 ± 38 Ma and 1138 ± 28 Ma. These older core ages are interpreted to be inherited, while the c. 1086 Ma age is interpreted to be the magmatic crystallization age of the Skiff Mt. LMG body, with the c. 1043 Ma rim age representing the timing of fluid alteration. Though the timing of fluid alteration age is slightly older than the age of the altered pegmatite dike at the limits of error, we believe that pegmatite dike intrusion and fluid alteration were in close temporal proximity.

STOP 8. Highly deformed augen gneiss
We are in the eastern portion of the quadrangle, which is dominated by large amplitude, upright folds that contain a calculated β-axis of 10 to 112 (Figure 6). There is no identified axial planar fabric associated with this fold generation (F3). The rock exposed here is a strongly deformed augen granitic gneiss (Yggn on new 7.5’ quadrangle map). Any one exposure typically contains both pegmatitic (mesoperthitic) varieties that are weakly deformed as well as biotite-megacrystic varieties that are strongly deformed and contain >5% modal garnet (vol%). The latter unit is isoclinally folded (F2), and contains a very strong S2 here. Can we see a lineation? Is there evidence of folding at the outcrop scale (F1)?

Parking for Stop 9

STOP 9. Penfield Pond – granite pegmatite cross cutting amphibolitic gneiss
We are in the hinge of an F3 fold. We have walked through the LMG the entire way here. This outcrop contains amphibolitic gneiss, which is locally migmatitic, and a concordant lens of mesoperthite granite. The mesoperthite granite is typically associated with the augen gneiss seen at Stop 8, despite the lack of tectonic fabric within the granite. At one location in the outcrop (Fig. 5a,b), the granite cross cuts the amphibolite gneiss at a low angle. This truncation suggests that migmatization predates the mesoperthite granite, and that migmatitic layering is transposed to be nearly parallel to S2, but may have occurred during S1.