A4: MESOPROTEROZOIC MAGMATISM OF THE ADIRONDACK LOWLANDS: THE RESULT OF CLOSURE OF THE TRANS-ADIRONDACK BACK-ARC BASIN
Sean P. Regan
Department of Geosciences
University of Massachusetts
W.H.Peck, B.W.Selleck, and M.S.Wong
Department of Geology
Colgate University
Hamilton, NY 13346
J.R.Chiarenzelli
St. Lawrence University
Canton NY 13617
INTRODUCTION AND REGIONAL BACKGROUND
The Mesoproterozoic Grenville orogenic belt is one of the world’s largest preserved Precambrian convergent margins, with a North American extent from Labrador to Texas. It records several tectonic, magmatic, and accretionary pulses typically referred to as the Grenville orogenic cycle (Ca. 1.30 – 1.0 Ga; Rivers, 2008). This sequence of punctuated crustal growth is thought to have culminated during a ca. 1.08 Ga terminal collision, which resulted in the final assembly of the supercontinent Rodinia (McLelland et al., 2001). The Adirondack Mountains in New York are a domical uplift forming the southern extension of the Canadian Shield into New York, and expose rocks that formed and evolved during this orogenic cycle. The Adirondack Highlands and Adirondack Lowlands are separated by the Carthage-Colton shear zone (Fig. 1; Selleck et al., 2005). The Adirondacks record sedimentation, plutonism, high-grade metamorphism and deformation during the Elzevirian and Shawinigan orogenies of the Grenville orogenic cycle, and are variably affected by terminal, Ottawan, deformation (Fig. 1b; McLelland et al., 1992; Chiarenzelli et al., 2011c;).
The Adirondack Lowlands are dominated by supracrustal, amphibolite-facies metasedimentary rocks arrayed in NE-SW striking belts (Wong et al., 2011; Baird and Shrady, 2012). The Lowlands contain relatively subordinate, variably deformed igneous intrusives (Carl and deLorraine, 1997; Wasteneys et al., 1999; Peck et al., 2013). In contrast, the Adirondack Highlands are dominated by granulite facies meta-igneous rocks, with minor supracrustal rocks (McLelland et al., 2004). Published data suggest that unlike the Highlands, the Adirondack Lowlands did not experience penetrative Ottawan deformation, and that the rocks of the Lowlands were likely at a higher crustal position during the Ottawan (McLelland et al., 1992; Selleck et al., 2005; Heumann et al., 2006). Thus, the Adirondack Lowlands are an ideal location to study earlier phases of Grenvillian orogenesis because of a lack of pervasive overprinting, such as is seen in the Highlands.
This field trip focuses on the major igneous rock suites that were emplaced into the metasedimentary units of the Adirondack Lowlands. Recent work on these rocks is presented in several contributions (Chiarenzelli et al., 2010b; Wong et al., 2011; Peck et al., 2013), which grew from a Keck Geology Consortium Project led by Colgate University during the summer of 2008. This project produced a new igneous whole rock geochemical dataset from many of the major intrusive suites exposed in the Lowlands. Accompanying these data, Sm-Nd isotopic analyses were acquired along with SHRIMP-RG U-Th-Pb analyses of zircon and titanite (Chiarenzelli et al., 2010b; Wong et al., 2011; Peck et al., 2013). This trip will visit localities where igneous bodies are in contact with surrounding meta-sediments, providing an opportunity to address the deformation history of the metasedimentary rocks. However, the central goal is to document how magmatism evolved during Shawinigan orogenesis, and the relationships between igneous rock geochemistry and regional tectonic history of the Grenville Province.
The Black Lake shear zone represents a major lithotectonic boundary within the Mesoproterozoic basement of New York and Ontario marking the NE extent of ca. 1180 – 1200 Ma subduction and the >1250 Ma ophiolite in the Lowlands (Chiarenzelli et al., 2010b; 2011a; Wong et al., 2011; Peck et al., 2013). This shear zone has recently been interpreted to represent magmatism and the approximate location of the rifted margin of Laurentia prior to Shawinigan orogenesis (Fig. 3; Chiarenzelli et al., 2010b). This interpretation suggests that stratigraphic correlations across the shear zone are possible, and would predict shallower facies assemblages (ie quartzites, psammites) consisting of clastic material to the northwest, grading into carbonate (limestone) and pelitic (mud) assemblages to the southeast. The gross stratigraphy preserved within the Adirondack Lowlands is interpreted to record a period of protracted opening and subsequent basin closure and restriction consistent with a back arc model (Carl et al., 1990; Chiarenzelli et al., 2010b; 2011b)
A maximum age on rifting is not well constrained, despite well understood stratigraphic and spatial relationships. Detrital zircon analyses from quartzites in the Frontenac terrane yield a maximum deposition age of 1306 Ma (Sager-Kinsman and Parrish, 1993). Detrital zircon geochronology three samples from the metasedimentary sequence indicates deposit prior to ca. 1250 Ma (Chiarenzelli et al., 2013), and likely represents silicilastic deposition during basin evolution from opening to closure. Deposition of carbonate and evaporitic rocks of the upper Marble likely occurred in isolated remnants of the collapsing basin restricted from the open ocean immediately prior to Elzevirian orogeneisis. Cessation of extension likely occurred during the onset of the Elzevirian orogeny within the Central Metasedimentary Belt to the NW (ca. 1220-1240 Ma) and stepped to the southeast into the Adirondack Lowlands (Chiarenzelli et al., 2011b).

STOP 1. Route 11 east of Antwerp, NY
This outcrop is one of many exposures of the Antwerp-Rossie suite, and surrounding marble, exposed along this section of US-11. Interestingly, the granitoid appears to cross-cut an isoclinal, recumbent fold in the marble, and contains quartzite xenoliths. However, Shawinigan deformation is generally accepted to postdate 1200 Ma. An important question to consider at this outcrop is the origin of this layering. Is the folding related to deformation around the more competent granitoid, or does the granitoid cross cut an earlier phase of deformation?

STOP 2. The Steers Head locality
The steers head locality is one of the many classic field trip localities in the Adirondack Lowlands. The marble contains graphite-rich layers and dismembered calc-silicate gneisses with well-developed reaction rims along their margins. The reaction rims contain tremolite, diopside, phlogopite, quartz, and calcite, among others. Do you see any other minerals in the reaction rims? Also, try to find the folded tourmaline vein on the north end of the outcrop within the marble.
It is important to note that the ARS has a wide range of SiO2 contents. The rocks seen at Stop 1 and 2 are representative of the more felsic members of the ARS. Samples from this belt have relatively juvenile Sm-Nd systematics with no systematic relationship between εNd with SiO2, suggesting a minimal, if any, signal of crustal contamination.
This outcrop was host to an important stable isotope study by Cartwright and Valley (1991). Stable isotope ratios were measured in both metaigneous and metasedimentary country rocks across the intrusive contact to assess the importance of metamorphic fluid flow; essentially looking for a hydrologic ‘breakthrough curve’ caused by the difference in δ18O between these lithologies. No breakthrough curve is present, and the gradient in δ18O between lithologies contains an inflection point at the contact, with a several meter equilibration gradient from high δ18O values in the marble, to lower δ18O values in the granite. The preservation of these steep isotopic gradients and lack of offset of the inflection point suggest that minimal fluid flow occurred during regional metamorphism (Cartwright and Valley, 1991).

STOP 3. County Road 17 north of De Kalb Junction
The megacrystic Hermon Granite gneiss is a distinctive rock unit throughout the Adirondacks, with coarse K-spar augen. Here, the Hermon gneiss was emplaced into marble. With an age of 1182 Ma (Heumann et al., 2006), all deformation within the Hermon Granite gneiss is likely associated with Shawinigan tectonism. As noted above, this suite of rock shares many similarities to the ARS, but lacks a major Pb anomaly when normalized to primitive mantle and typically plots on field boundaries on tectonic discrimination diagrams (Pearce et al., 1984). Note the strong fabric development at this outcrop. Do the different minerals show varying amounts of deformation or recrystallization? What is the strike and dip of the gneissic layering?

STOP 4. South of Beaver Creek, County Route 7. Park on Childs Road.
BE CAREFUL!!! SMALL SHOULDER!
This outcrop contains meter to sub-meter concordant layers of the Hermon granite gneiss (HGG) within a fine grained sulfidic, pelitic gneiss. Both contain a well-developed, upright foliation, and a moderately plunging stretching lineation. Near the margin of the Hermon granite gneiss, there are sub-meter scale pelitic xenoliths aligned with the tectonic fabric (Figure 10). What is the strike and dip of the major fabric in the metapelite? The granitic gneiss? The foliation is fairly representative of the major structural fabrics throughout the Adirondack Lowlands and is well developed within the Hermon granite gneiss, suggesting that the NE-SW structural grain, predominate throughout the Adirondack Lowlands occurred after the emplacement of the Hermon granite gneiss.
Similarly to the ARS, the Sm-Nd systematics for the HGG form a fairly steep evolutionary curve. They plot between CHUR and contemporaneous depleted mantle at time of crystallization suggesting a juvenile source. The sample analyzed plots entirely within the evolutionary envelope for the ARS.

STOP 5. County Road 76, north of Cooper Rd, south of Turner Rd.
BEWARE ANTS
This outcrop is representative of the Hyde School Gneiss (HSG). The HSG has been interpreted alternatively as having an intrusive, or an extrusive (ash flow) protolith. However, substantial work has confirmed its plutonic origin (McLelland et al., 1992; Wasteneys et al., 1999).This outcrop displays rhythmic layering characteristic of the HSG, with amphibolite lenses to continuous layers that are relatively thin here. On a larger scale, the HSG forms km-scale domical lenses within surrounding metasedimentary packages, and contains a composite igneous/metamorphic fabric that parallels its margin. Medium grained in this locality, the HSG ranges from coarse leucogranite to tonalite, to finer grained aplitic layers. Geochemically, it shares many of the same characteristics as the older magmatic suites. It contains minor enrichments with respect to its LILEs and minor depletions in its HFSE when normalized to primitive mantle (Sun and McDonough, 1989), and plots from alkali-calcic to calc-alkalic with constant MALI with increasing SiO2. Furthermore, unlike the ARS and Hermon Gneiss, the HSG samples plot as largely ferroan with regard to their Fe-index, with a subset plotting in the magnesian field (Peck et al., 2013; Frost and Frost, 2008).

STOP 6. Route 58 north of junction with Acres Road outside Pope Mills, NY.
The youngest igneous suite within the Adirondack Lowlands is a series of ferroan rocks collectively referred to as the Edwardsville pluton (Buddington, 1929). Ranging from syenitic to subordinate gabbro, this suite of rocks is both texturally and compositionally heterogeneous. Recent single grain SHRIMP analysis (Peck et al., 2013) has corroborated older multi-grain TIMS work (1164 Ma; McLelland et al., 1993), which yield an age of 1149 ±22 and 1161±16 Ma from a monzonitic member of the Edwardsville pluton and a granite from the Honey Hill syenite, respectively (Peck et al., 2013). These ages correspond to anorthosite-mangerite-charnockite-granite (AMCG) magmatism within the Adirondack Highlands (McLelland et al., 2004; Hamilton et al., 2004), and the similarly ferroan magmatic event in the Frontenac terrane to the northwest (1180-1160 Ma).
Although texturally similar to the Hermon Gneiss, can you see a mineralogical difference? Is there a strong tectonic fabric? Lastly, is this outcrop homogeneous, or are there texturally distinct rocks types, and, if so, what is the relationship between them?

STOP 7. Rossie Diorite, Split Rock Road.
Very close to private property, please be courteous!
The Rossie diorite represents a more mafic component of the ARS suite. It is important to note that we are very close to the Black Lake shear zone, an important structural and lithologic discontinuity separating the Adirondack Lowlands from the Frontenac Terrane (Wong et al., 2011). Although slightly more mafic than its southern counterparts, the Rossie diorite displays similar geochemical signatures. However, despite these similarities, the belt of dioritic rocks exposed along the eastern margin of the Black Lake shear zone contain the lowest εNd(T) values, and yield the oldest TDM ages, approximately as much as 400 My older than ARS samples analyzed further south. These data suggest a major source of contamination by, or interaction with, older more evolved continental lithosphere (n=3 samples along the northwestern extent of the ARS).

STOP 8. Rockport Granite; US 12.
These exposures are on the NW side of the Black Lake shear zone (Wong et al., 2011). We are now in the Frontenac Terrane. This outcrop exposes the voluminous Rockport granite (ca. 1170 Ma; Wasteneys et al., 1998), which extends across the Thousand Islands region and into Ontario, Canada. Here, the Rockport Granite is in contact with strongly deformed calc-silicate (metasedimentary) gneisses. There is a faint, near-vertical gneissic layering within the Rockport Granite that is shared by the adjacent calc-silicate gneisses. This suite of granitic rocks is time equivalent to the domical Hyde School gneisses in the Adirondack Lowlands. However, the Rockport Granite has a much different intrusive style, and is hosted most commonly by quartzite and quartzite-calc-silicate country rock. The Rockport granite is generally slightly to moderately deformed, and doesn’t occur as strongly deformed domical bodies like its Hyde School gneiss counterpart. Furthermore, no exposures of ARS or Hermon Granite gneiss have been identified on this side of the Black Lake shear zone.