Recent developments in the Understanding of the...

St. Lawrence University Geology Alumni Conference – Chiarenzelli and Regan

Recent developments in the Understanding of the

Geology of the Adirondack Lowlands

Isoclinally folded turbiditic rocks and peridotite (inset) along the Grasse River at Pyrites, New York. These outcrops are part of a dismembered ophiolite complex.

A field trip for alums prepared by J. Chiarenzelli (’81) and Sean Regan (’10) St. Lawrence University Geology Alumni Conference IX September 27th, 2015


Introduction

The goal of this trip is to introduce, or reintroduce, you to some of the amazing geology within a few tens of miles of our campus. The ability to take students to visit these classic localities makes our program special and unique. In addition, we hope to bring you up to date on some of the current thinking about the Adirondack Lowlands and its geologic evolution. When hired at SLU one of my goals was to reinstate the geology department to the forefront of Adirondack research. Sean Regan (’10) is carrying on this tradition by mapping in the eastern Adirondacks for the United States Geological Survey. He shares my interest in the region and Precambrian rocks in general. Finally, it would be just plain weird not to offer at least one field excursion at a geology conference in a setting this grand.

Field trip route, stops, towns, and major roadways. The trip begins and ends at St. Lawrence University in Canton.


Regional setting of the Adirondacks. ADK – Adirondack uplift; BLSZ – Black Lake sz; CCSZ – Carthage Colton sz; CMB – Central Metasedimentary Belt ; D – Dysart, Ontario; FT – Frontenac Terrane; GM – Green Mountain Grenville inlier; PLSZ – Piseco Lake sz; SA- Southern Adirondack tonalite belt. From Chiarenzelli et al. (2015).

Below, in bullet form, are some of the recent developments in our collective understanding of the Adirondack Lowlands. We will attempt to introduce as much of this as possible on the outcrops where the field relations can be observed, pondered, and discussed.

• The differences between the Adirondack Highlands and Lowlands include metamorphic grade, topographic relief, and the relative abundance of metasedimentary rocks. However, perhaps the most important difference is the lack of Ottawan (ca. 1090-1040 Ma) deformation and metamorphism in the Lowlands. The Lowlands were last deformed and metamorphosed during the Shawinigan Orogeny (ca. 1200-1150 Ma; McLelland et al., 1993; Heumann et al., 2006) while a pervasive thermal event at 1050 Ma appears to be restricted to the Highlands (McLelland et al., 2001). The boundary between the two terranes, the Carthage-Colton Shear Zone, has both an earlier (Shawinigan) ductile, compressional history, as well as, a late (Ottawan) brittle, reverse history (Selleck et al., 2005).

• Intrusive rocks in the Lowlands range from ca, 1200-1155 million years in age and track the tectonic evolution of the area from calc-alkaline arc magmatism to anorthosite-mangerite-charnockite-granite (AMCG) plutonism (Peck et al., 2013; Regan et al., 2014). Only the ca. 1170 Ma Hyde School Gneiss has a temporal equivalent in the Frontenac


Terrane, named the Rockport Granite. This suggests that docking of the Lowlands to the Frontenac Terrane occurred by ca. 1170 Ma, along a feature known as the Black Lake Shear Zone (Wong et al. 2011).

• Metasedimentary rocks in the Lowlands have a recognized stratigraphy which includes the Lower Marble, the Popple Hill Gneiss, and the Upper Marble. Recent work on the detrital zircons tracks the tectonic evolution of the basin these rocks were deposited in. Rocks representing an early rift-drift phase (Lower Marble), development of a foredeep (Popple Hill Gneiss), and, finally, initial compression and periodic isolation from the open ocean (Upper Marble) prior to the Elzivirian Orogeny (Chiarenzelli et al., 2015) have been identified. The age of the sequence is bracketed between ca. 1290-1250 Ma, identical to similar rocks of the Grenville Supergroup in the Central Metaedimentary Belt of Canada and the Franklin marble in the New Jersey Highlands (Peck et al., 2009).

• At Pyrites, approximately 5 miles south of campus, peridotitic mantle rocks are overlain by turbiditic pyritic gneisses (Chiarenzelli et al., 2011). These rocks occur as part of an extensive 50 km long belt of highly dismembered gabbros, amphibolites, serpentinites, and deep-water, in part chemogenic, metasedimentary rocks. These rocks represent the upper mantle, ocean crust, and its thin sedimentary cover obducted when the back-arc basin they were deposited in closed during the Shawinigan Orogeny.

• The Upper Marble, which occurs as a belt of rock extending from Balmat to Parishville, hosts the Zn-Pb deposits and most of the local talc occurrences. Within the 16 subunits recognized in the Sylvia Lake synform (Brown and Engel, 1956; de Lorraine and Carl, 1993) there are three evaporate-ore couplets. The sequence consists mostly of silicified carbonate rocks and in at least one unit stromatolites have been recognized.

Stop 1. Selleck Road tremolite locality at West Pierrepont, NY

The Selleck Road tremolite locality has yielded fine specimens of green tremolite for more than a century. However it was originally known as an actinolite locality until Dr. George Robinson, SLU Geology Research Associate, found that the beautiful amphiboles exposed here had little to no iron! This locality is one of more than two dozen known to contain exceptionally large calc-silicate minerals such as tremolite or diopside in the Adirondack Lowlands (Hill et al., 2015). Other minerals typically associated with these deposits include calcite, quartz, tourmaline, phlogopite, scapolite, and apatite. Recent work has established that these fascinating mineral occurrences are, at least in part, intrusive.


Part of the Adirondack Sheet showing the major tectonic subdivision of the northwestern Adirondacks and surrounding terranes. The Carthage-Colton Shear Zone (CCSZ) separates the Adirondack Highlands and Lowlands. The Black Lake Shear Zone (BLSZ) separates the Adirondack Lowlands and the Frontenac Terrane.

Large tremolite collected at the Jenne Farm, Russell, NY. Others collected for the NYS Museum are over two feet in length. Over two dozen localities for calc-silicate minerals occur near the boundary between the Lowlands and Highlands.


Here at Selleck Road, green tremolite is well developed along the contact between Grenville Suoergroup quartz-rich rocks and, calc-silicate gneiss and marble. In fact, most of these deposits involve the same lithologies. Many also are also spatially associated granitic pegmatites than contain Ca-rich pyroxenes and/or amphiboles. Further the vast majority occur along or within a few kilometers of the trace of the Carthage-Colton Shear Zone (CCSZ). Those on the Lowlands side of the CCSZ yield ages that correspond to the end of the Shawinigan (ca. 1150 Ma), while those on the Highlands side yield Ottawan ages (ca. 1050 Ma). At this locality zircons were separated from a calc-silicate pegmatite that crosscuts a feldspathic quartzite with black heavy mineral bands. The majority of the zircons analyzed were metamict and yield discordant ages. One grain, with the lowest U-content, gave concordant age of ca. 1151 Ma. This is in excellent agreement with pegmatitic tourmaline-bearing dikes nearby at Powers Farm (1153 Ma). The feldspathic quartzite, cut by the pegmatite, yielded a detrital zircon population with the youngest concordant grains of ca. 1240 Ma.

Geochronological data from the Selleck Road tremolite locality (from Hill et al., 2015).

Stop 2. Turbidities and Ultramafic Rocks at Pyrites, NY

The following is excerpted from a field guide for the NYSGA 2014 meeting in Alexandria Bay (Chiarenzelli et al., 2014). The rocks at Pyrites lie with the triangular core zone of a large winged structure defined by amphibolite and gabbroic rocks that extends northeast from Stellaville (former site of the largest pyrite mine in St. Lawrence County) nearly 15 km to Crary Mills. The general shape of the structure indicates it is folded and most of the amphibolitic rocks have a strong foliation and are highly disrupted by later magmatism. The belt of mafic and ultramafic rocks lies within

U-Pb Zircon Results Selleck Road, West Pierrepont


marble. Although the actual surface exposure of the ultramafic rocks is modest (~ 1 km2), the largest gravity anomaly in the region (Revetta and McDermott, 2003) extends from here southwest for about 10 kilometers. This suggests that it represents just a small part of the body and that a considerable mass of ultramafic rock, extending downward for some distance, lies parallel to the Carthage-Colton Shear Zone.

Geological map of the area around Pyrites, New York modified from Isachsen and Fisher (1970). Upper right: Concordia diagram of zircon separated from pyroxenite at Pyrites. Lower right: ultramafic classification diagram for ultramafic rocks collected at Pyrites based on modal mineralogy determined by whole rock chemistry.

At this stop we will examine rocks associated with one of several pyrite mines that was in production between ca. 1880 to 1920 (Prucha, 1957) as a source of sulfur for industrial processes and sulfuric acid feedstock. Two adits can be seen on the side of the mustard-colored hill as you approach the outcrop along the bank of the Grasse River. The adits intersect the steeply-dipping ore zone and the workings continued downward for as much as two hundred feet and extend underneath the river. On the bank, the rocks can be seen dipping steeply to the northwest and show differential weathering related to the variation in their mineral content and extent of weathering. Investigation at the top of the exposure shows the characteristic red coloration of pre-Potsdam weathering indicating that the unconformity was likely within a few feet of the current erosional level at the top of the hill.


The main ore zone consists of a ~2m thick planar layer of pyritic breccia. Examination of the rock at the outcrop, and in thin-section, indicates the ore is composed of rounded 1-2 cm fragments of breccia indicating enrichment of the ore during repeated fault movement. A black, rounded, 5 centimeter wide fragment composed of breccia within breccia can be seen at the foot of the hill within the primary ore zone. Careful examination of the ore and surrounding rock reveals highly weathered lath-like crystals of sillimanite. Other minerals within the sequence include quartz, feldspar, mica, chlorite, sericite, garnet, graphite (Figures 16-18), and a host of trace minerals including pyrrhotite, rutile, sphalerite, chalcopyrite, molybdenite, and chromite (Tiedt and Kelson, 2008). Tiedt and Kelson (2008) suggest a possible biogenic origin for the pyrite on the basis of sulfur isotopes, in line with the presence of graphite. However, a hydrothermal volcanic influence is also possible based on the variety of metals found and association with mafic and ultramafic rocks. In addition to a strong foliation and compositional layering, the sequence is cut by shallowly dipping veins which weather in positive relief. These veins are composed mostly of pyrite, minor pyrrhotite, and silica and represent a much later event in the history of the deposit. Early deformation is defined by compositional layering which is isoclinally folded. Thin seams of breccia less than a 1 cm wide can be seen cutting the foliation and/or folded layers in several areas in the exposure. Near the ore, within white feldspathic pods, relatively large, brown rutile crystals can be seen. Other non-sulfide minerals observed in the ore include chlorite, graphite, quartz, rutile, and monazite. Note the variation in resistance of the rocks to weathering, colors, and variation in layering as you walk upstream. Towards the end of the accessible outcrop area along the river the rocks change in character and color to a massive, knobby weathering green ultramafic rock. The actual contact between the ultramafic rocks and metasedimentary sequence is well exposed in the wall of the river bank. Whether the contact is sedimentary or tectonic, or both, can be debated here. Regardless of the nature of the contact, the layered metasedimentary rocks become progressively greener in color and more Mg-rich towards the contact. Given the occurrence of chromite noted by Tiedt and Kelson (2008) in these rocks, proximity to ultramafic rocks during deposition is likely. Depositional along a transform fault is one possibility that could account for the juxtaposition of these two rock types and lend additional credibility to a deep water turbidity origin for the pyritic gneisses. Several meters above the contact a large isoclinal fold can be seen. About the hinge of the fold, one can see both recessive and resistant cm-scale layering. The recessive layers consist of quartzofeldspathic gneiss with both sillimanite and garnet, whereas the more resistant layers consist of up to 85% or more quartz. This exposure provides insight into both the original nature of the layered metasedimentary rock and its metamorphic grade. Couplets of mud and sand are suggestive of a turbiditic sequence deposited in a relatively deep water setting. At some point, likely associated with the Shawinigan Orogeny (1150-1200 Ma), these rocks were intensely deformed and metamorphosed to Upper Amphibolite facies (garnet-sillimanite).


The thin, resistant, quartz-rich layers were sampled for detrital zircon geochronology and yield a sparse, but very homogeneous, population of detrital zircons. The zircons are between 25-100 microns in length, fairly euhedral, and zoned. Limited evidence of metamorphic rims or xenocrystic cores was found, despite BSE and CL imaging of cross-sectioned grains. The zircon yielded a near unimodal population of grains with a maximum depositional age of 1284+/-7 Ma. This result is compatible with a restricted source consisting of igneous zircon, perhaps derived from a sheet of rift-related volcanic rocks.

Probability histogram of over 100 U-Pb analyses from detrital zircons separated from quartz-rich layers in the pyritic gneiss at Pyrites, NY. Note unimodal population at ca. 1300 Ma. Upper right inset: Graded beds alternating for garent-sillimanite-rich muds to quartz-rich fine sandstones. Lower right inset: Zircons analyzed from Pyrites.

The massive green rock at the end of the outcrop is composed of a mass of hydrous secondary minerals including Mg-rich phases such as talc, tremolite, chlorite, serpentine, and phlogopite. While the vast majority of primary minerals are replaced, isolated core fragments of augite survive in addition to chromite. Consisting of ~40% SiO2 and nearly 33% MgO, it also contains substantial amounts of Cr and Ni (Chiarenzelli et al., 2011). Its knobby texture likely represents the pseudomorphic replacement of large okiocrysts. Further upriver the ultramafic rock varies from peridotite to pyroxenite to layered ultramafic rocks all highly altered and nearly completely replaced. Nonetheless, original layering is frequently preserved and no indication of ductile deformation is found in stark contrast to the adjacent metasedimentary lithologies,


which are isoclinally folded. Classification based on normative mineralogy determined by the CIPW method indicates that most ultramafic rocks are peridotites and a few pyroxenites, consistent with field observations. The lower grade assemblage and undeformed nature of the ultramafic rocks appears to be at odds with the intensely folded garnet-sillimanite gneisses they are in contact with. This can be explained either juxtaposition of the rocks after peak metamorphic conditions or the competent and hydrous nature of the ultramafic rocks which contain as much as 10% H2O. Several attempts were made to directly determine the age of the ultramafic rocks (Chiarenzelli et al., 2011). A range of rock types were analyzed for both Rb-Sr and Sm-Nd isotopes but unfortunately both systems appear to have undergone strong disturbance and provide isochron ages with large errors. Small zircons (~50-200 microns) were observed within thin mm-scale, calcite-rich veins in samples of both periodite and pyroxenite. Attempts to recover them were successful and isotopic analysis by laser ablation – inductively coupled plasma mass spectrometry (LA-ICP-MS) and Sensitive High Resolution Ion Microprobe (SHRIMP) were conducted. Zircons from the peridotite were dated by SHRIMP in Perth, Australia and yielded an age of 1140±7 Ma with a few older grains yielding an age of 1202±20 Ma. Zircon from the pyroxenite were dated by LA-ICP-MS at the Arizona Laserchron Center and yielded and age of 1197±5 Ma. Given the unlikely occurrence of zircon as a magmatic product in silica-undersaturated rocks, these ages are thought to represent the timing of zircon growth during metamorphic events and thus provide only a minimum age for the ultramafic rocks. The ages correspond to the known effects of the Shawinigan Orogeny in the Lowlands from previous studies (Heumann et al., 2006). Stop 3. Hermon Granitic Gneiss at Trout Lake

The Hermon granitic gneiss is one of several major intrusive units in the Adirondack Lowlands which cross-cuts metasedimentary rocks of the Grenville Supergroup. It is particularly and voluminous near the Popple Hill Gneiss which it commonly intrudes. Its large feldspar megacrysts are distinctive and allow it to be easily distinguished from other intrusive units of smaller grain-size. Recently the Hermon Granitic Gneiss at this outcrop was dated at 1182+/-7 Ma by U-Pb zircon techniques (Heumann et al., 2006). Its geochemical trends are transitional between the arc signature of the older Antwerp-Rossie suite and the anorogenic trend of the younger Hyde School Gneiss (Peck et al., 2013).

Equivalents to the Hermon Granitic Gneiss are not recognized to the north and west of the Black Lake Shear Zone in the Frontenac Terrane, suggesting that the Black Lake shear zone as a major discontinuity persisted until 1180 Ma. Possible equivalents in the Highlands include the megacrystic gneisses associated with the Piseco Lake Shear Zone (Chiarenzelli et al., 2014) Similarities in field relations with metasedimentary rocks, age, geochemistry, and Nd-systematics suggest they may well be related to the same or similar magmatic events.


Typical outcrop of the Hermon granitic gneiss. Note strong fabric and alignment of feldspar megacrysts.

Stop 4. Upper Marble at Balmat

The Upper Marble occurs in a NE-trending belt that extends from Balmat to Seven Springs between Parishville and Colton. It is the youngest sedimentary unit in the Lowlands and hosts the major zinc deposits mined at Balmat, Edwards, and West Pierrepont. Because of the intensive zinc exploration the Upper Marble in the Sylvia Lake Synform has been subdivided into 16 Units. These include a wide variety of metasedimentary rocks dominated by siliceous dolomitic marbles but including evaporates, pelitic and calc-silicate gneisses, talc-tremolite schists, and the sphalerite sedimentary exhalative ore, among others.

Here, outside of the entrance to the St. Lawrence Zinc Corporation, we can see Units 4 and 5. Unit 4 is a pyroxene-bearing, calc-silicate rock with structures that are thought to resemble stromatolites. Note that in this case the stromatolites appear to be upside down! This agrees with the field relations as we are on the overturned limb of the Sylvia Lake Synform. A sparse population of rounded, silt-sized zircon grains were obtained from the rock of Unit 4 here suggesting that they may well have been blown into shallow water. Study of other units in the


Upper Marble suggest a diverse provenance with a sampling of zircons from older Precambrian provinces throughout the Great Lakes and Mid-Continent region.

Note that the diopside is white to grey here and the black to green portions of the rock are serpentine, a common mineral in the Upper Marble.

Possible overturned stromatolites from Unit 4 of the Upper Marble. They are found on the overturned limb of the Sylvia Lake syncline near Balmat, New York. Photograph from http://www.frontenacpark.ca/pages/FrontNews60.html

Stop 5. Popple Hill Gneiss at Poplar Hill

The Popple Hill Gneiss was once considered to be a volcanic and/or volcaniclastic dominated sequence (Carl, 1988). Recent work suggests that volcanic input, if present, is relatively minor. Petrographic examination and geochemical analysis of drill core suggests that the Popple Hill Gneiss was composed primarily of silt to sand-sized sedimentary units (Chiarenzelli et al., 2013). In some sections the rock displays reverse grading as mud-rich units grew new minerals (garnet and sillimanite), whereas sand-rich portions changed relatively little.

http://www.frontenacpark.ca/pages/FrontNews60.html


Stratigraphic column of the Upper Marble in the Sylvia Lake Synform at Balmat, New York. Courtesy of William deLorraine, Principal Geologist, St. Lawrence Zinc Corporation.

Detrital zircon work on this unit (Heumann et al., 2006) indicates a strong Shawinigan component associated with leucosome production but the rock bears essentially no Ottawan zircons in the Lowlands. Leucosome is relatively widespread and indicates temperatures and pressures permission of partial melting. Study of the unit suggests metamorphic conditions in the Lowlands reached mid-Amphibolite facies near Balmat but progressed to granulite facies near Colton. Detrital zircons have a restricted source, 1300-1400 Ma in age, and are thought to have been derived from tonalites of this age that make up a portion of the southern and eastern Adirondacks (McLelland and Chiarenzelli, 1990; Ratcliffe et al., 1991) and Green Mountains of Vermont.


Study of drill core and field relations indicates that there are some substantial up-section changes in the Popple Hill Gneiss (Chiarenzelli et al., 2013). Amphibolitic, as seen here in the Poplar Hill cut, and gabbroic layers are voluminous in the base of the section and virtually absent from the mid and upper sections. In addition, the proportion of quartz increases and the thickness of quartzite layers increases towards the top of the section. The transition upwards into the Upper Marble appears to be conformable in the Sylvia drill core examined. Low-grade alteration in the core was especially pervasive, however, the age of this is currently unknown.

Drill core recovered from the Popple Hill Gneiss from the Sylvia Lake synform Balmat, New York. Note the garnetiferous layers (euhedral crystals) grading to thin quartzite layers (whitish) best developed in the upper length of core.

On the west side of the road cut a 1 cm wide crack filled with rounded sand grains cuts the Popple Hill Gneiss. It is difficult to spot and has a carbonate cement. Infillings of sandstone have been documented many hundreds of meters below ground in the Balmat zinc mines indicating substantial fracture networks on the Precambrian paleosurface. General map trends indicate that the Potsdam sandstone filled in paleovalleys several hundred feet in depth. Many hematitic ore and/or cherty breccias can be found along the unconformity indicating substantial paleoweathering.


Stop 6. Antwerp-Rossie Suite and Lower Marble at Hailesboro

The lowermost, continuous stratigraphic unit currently recognized in the Lowlands is the Lower Marble (Wiener et al., 1984). Although efforts have been made to subdivide it into several members it is barren in terms of zinc and has received less detailed attention. However, it has been widely used as a building stone with much of the rock coming from quarries in the Gouverneur area. Recently, pure white dolomitic varieties have been quarried as a raw material for a wide range of products including ag lime, paints, plastic products, landscaping stone, and road metal. Many of the brilliant white driveways you see are a consequence of this activity; however, it must be problematic if you have dark carpets!

At this outcrop, appropriately named the Steer’s Head, you can see the complex intrusive relationship between the Antwerp-Rossie suite (Chiarenzelli et al., 2010) and the Lower Marble and spectacular reaction rims between the two rocks. Note that the layering in the Lower Marble is clearly cross-cut by the A-R suite and in some areas isoclinal folds are readily truncated. Since the A-R suite is ca. 1200 Ma, the fabric it cuts in the Lower Marble must predate the Shawinigan Orogeny. This suggest that the fabric may belong to an even older event known as the Elzivirian Orogeny (1220-1240 Ma), best known from the Central Metasedimentary Belt of Ontario. During this event fragments of the 1300-1400 Ma tonalitic arc rifted from the margin of Laurentia are believed to have returned. This would have closed the basins in which the Grenville Supergroup was deposited and set the stage for later continent-continent collision during the Shawinigan.

A word about the Antwerp-Rossie Suite is appropriate here as its geochemistry is suggestive of an arc plutonic origin. Particularly intriguing is the variation in Nd model age as one approaches the Black Lake Shear Zone (BLSZ). In general, Nd model ages increase from about 1300 Ma to more than 1600 Ma northwestward. This has been interpreted to suggest a greater influence of older Frontenac Terrane rocks as one approaches the BLSZ. This in turn emphasizes the importance of the BLSZ, not only as a dividing line for the occurring of early plutonic suites but as a terrane boundary or, more likely, a rifted margin

For you Pleistocenophiles, check out the beautiful striations on the opposite of the highway (use care crossing – heavy traffic and many of them have been drinking). For you mineral lovers check out the base of the outcrop for large grains of orange chondrodite, a common accessory humite mineral in the Lower Marble, and a tourmaline vein.


Left: Classic Steers Head outcrop on the east side of Highway 58 south of Gouverneur. Right: Isoclinal fold truncated by granitoid of the Antwerp-Rossie Suite along Route 11 near Antwerp. Note truncation of the fabric in the marble by the ca. 1203 Antwerp-Rossie rocks.

Stop 7. Neoproterozoic Dike in Black Lake Member of Lower Marble at Gouverneur

At this stop we will have an opportunity to observe the lowest member of the Lower Marble known as the Black Lake Member for exposures south and east of Black Lake. It commonly contains large quartz knots and scapolite, and both are abundant here. Note the black to grey squarish, elongate prisms and their abundance in some sections of the road cut.

Intruding the marble and cutting across the road is a meter-wide basaltic dike Such dikes occur throughout the Adirondacks into the Frontenac Terrane and are believed to represent basaltic magmatism associated with the opening of the Iapteus Ocean. Their enriched chemistry and Nd isotopic systematics supports such an interpretation. Recently a trachytic dike, part of a swarm of basaltic dikes at Dannemora yielded a U-Pb zircon age of 643+/-4 Ma. Work on other samples is on-going.

Stop 8. Tourmaline-bearing arkosic gneiss at Richville

Another interesting stratigraphic feature of the Lower Marble is the inclusion of a 50 km long belt of tourmalinites and tourmaline-rich gneisses (Brown and Ayuso, 1985) in contact with marble. These rocks occur northwest of Rt. 11 and can contain more than 50% tourmaline. Some recent work has attributed a chemogenic origin to these tourmalinites.


At Richville a meter-thick, quartz-rich layer was sampled for detrital zircon analysis. The rock yielded a complex population of zircons with ages spanning much of the Proterozoic; consistent with a mid-continent source for the sediment (Chiarenzelli et al., 2015). The present of rounded detrital zircon grains indicates that the rock is not chemogenic. Its overall arkosic composition suggests deposition along block faults active punctuating carbonate deposition. The source of the boron in these rocks and throughout the Lowlands awaits isotopic investigation.

Probability histogram for U-Pb analyses from detrital zircon recovered from a tourmaline-bearing arkose in tourmaline-rich gneisses near Richville, NY. Note that no zircons younger than ca. 1270 Ma where found and that this unit is within the stratigraphic section of the Lower Marble. Upper right inset: Detrital zircons separated from the rock. Lower right inset: Quartz-rich layer sampled just above Roselyne Laboso (’14).


Detrital zircon results from the Grenville Supergroup in the Adirondack Lowlands. Note the change in provenance as deposition within the basin evolved over time in response to tectonic events.

Stop 9. Leucogranitic gneiss with amphibolite xenoliths at Canton

Perhaps no rocks in the Adirondack Lowlands have generated more discussion about their origin than the alaskitic or leucogranitic gneisses. Originally interpreted as phaccoliths intruded into the core of domal folds by Buddington (1929), Carl and Van Diver (1975) presented evidence that they represent ash flow tuffs that occur at the base of the Lower Marble, and hence are the lowest stratigraphic unit in the Lowlands. Revisiting earlier geochronological data of McLelland et al., (1992), Wasteneys et al. (1999) reinterpreted zircon cores as xenocrysts and outermost rims as igneous overgrowths. The rims yield an age of 1172 Ma which corresponds to the timing of Shawinigan Orogenesis and the age of the Rockport Granites in the Frontenac Terrane. On the basis of geochemical grounds they were interpreted as part of the widespread AMCG suite by Peck et al. (2013).

At this outcrop, in front of the Tallman House, near the railway overpass just outside of Canton one can see the Canton body. The Canton body is one of 14 elliptical domes of leucogranite in


the Lowlands. The Canton body is one of the largest bodies and displays the characteristic comma-like, hook shape suggestive of refolding. In addition, this outcrop displays disrupted amphibolite layers rimmed by a white alteration zone. The origin of the amphibolites has been attributed to mafic flows or intrusive dikes depending on one’s preference for the leucogranitic bodies themselves.

The amount of leucogranitic rocks in the Lowlands is great and it is entirely possible that more than one origin for them is possible. Our preferred interpretation is that at least some of these rocks represent arkosic gneisses related to opening of the basin which eventually was filled by the Lower Marble, Popple Hill Gneiss, and Upper Marble. If they are rift related they may well also have a volcanic or volcaniclastic component. However, resolution of this issue will require much additional work.

Portion of the Adirondack Sheet with major intrusive suites of the Lowlands highlighted. Note the spatial association of the ca. 1182 Ma Hermon granitic gneiss (dark pink) with the Popple Hill Gneiss (yellow with diagonal rule) and the lack of both the Hermon granitic gneiss and Antwerp-Rossie granitoids (orange) northwest of the Black Lake Shear Zone. Note however that the ca. 1172 Hyde School Gneiss and Rockport granite (light pink) are found on both sides.

BLSZ

CCSZ


Concluding Remarks

Over the last decade or so our understanding of the Adirondack Lowlands has increased as precise U-Pb zircon (McLelland et al., 1988), isotopic, and geochemical data has become available to establish boundary conditions. Much of this work has been done by students and faculty at St. Lawrence University. Some of it has been funded by the James Street Fund. We are grateful to you, the alumni, for supporting us and allowing us to offer our students exceptional research opportunities!

Hillary Hagen-Peter and Thomas Lockwood class of 2014 operate the laser and ICP-MS at the Arizona Laserchron Center in the geology building on the University of Arizona campus in Tucson.


References

Buddington, A. F., 1929, Granite phacoliths and their contact zones in the northwest Adirondacks: New York State Mus. Bull. No. 281, p. 51-107.

Brown, C.E. and Ayuso, R.A., 1985, Significance of tourmaline-rich rocks in the Grenville Complex of St. Lawrence County, New York. U.S. Geol. Surv. Bull. 1626-C, p. 1-33.

Brown, J. S. and Engel, A. E. J., 1956. Revision of Grenville Stratigraphy and Structure in the Balmat-Edwards District, Northwest Adirondacks, New York: Geological Society of America Bulletin, v. 67, p. 1599-1622.

Carl, J. D., 1988, Popple Hill Gneiss as dacite volcanic: A geochemical study of mesosome and leucosome, northwest Adirondacks, New York: Geological Society of America Bulletin, v. 100, p. 841-849.

Carl, J. D., and Van Diver, B. B., 1975, Precambrian Grenville alaskite bodies as ash flow tuffs, northwest Adirondacks, New York: Geol. Soc. America. Bulletin, v. 86, p. 1691-1707.

Chiarenzelli, J., Kratzmann, D., Selleck, B., and de Lorraine, W., 2015. Age and provenance of Grenville Supergroup rocks, Trans-Adirondack Basin, constrained by detrital zircons: Geology, v. 43; no. 2; p. 183–186.

Chiarenzelli, J., Lupulescu. M., and Bailey, D., 2014. Ultramafic and mafic rocks of the Pyrites Complex. Trip B-2. 86th Annual New York State Geological Association Field Conference Guidebook, p. 133-166.

Chiarenzelli, J., Valentino, D., and Regan, S., 2014. Origin of granitic protoliths of mylonites of the Piseco Lake Shear Zone: Geological Society of America Abstracts with Programs, Vol. 46, No. 2, p.121.

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