Bedrock Geology of the Adirondack Region

Precambrian rocks of Adirondack Region were part of a global system of mountains whose formation approximately one billion years ago led to the assembly of a supercontinent called Rodinia. In New York State, the eroded remnants of these enormous mountains extend beneath the Paleozoic cover rocks on the edge of the Adirondack topographic dome to form the basement rocks of New York State and connect, through exposures in the Thousand Islands Region, to the bulk of the contiguous Grenville Province of the Canadian Shield. Similar rocks are exposed in basement windows along the spine of the much younger Appalachian Mountains and can be traced into Mexico and beyond. Like other areas in the Grenville Province, the High Peaks region of New York is underlain by a large intrusive body of massif anorthosite, a rock composed of exceptionally large crystals of plagioclase feldspar. Rocks in the Adirondacks range in age from approximately 1350 to 1000 million years old and record as many as three or four tectonic events which were part of the Grenville Orogenic Cycle. The net results of these events were high-grade metamorphism, strong deformation, and the widespread overprinting of original relationships and primary textural features. Younger Paleozoic rocks include Cambrian and Ordovician sandstones, limestones, and shales deposited on the eroded metamorphic and igneous basement. These sedimentary rocks are found in fault-bounded outliers within the Adirondack massif and around the Adirondack margins. The current topography of the Adirondacks is related to doming which began about 180 million years ago, when the Atlantic Ocean opened; although the reason(s) for this doming remain to be fully elucidated. Doming has stripped away the younger Paleozoic rocks and exposed the roots of the mountains, which at one time were deformed and metamorphosed deep in the crust

Age and Origin of Monazite Symplectite in an Iron Oxide-Apatite Deposit in the Adirondack Mountains, New York, USA: Implications for Tracking Fluid Conditions

Monazite crystals, intergrown with allanite, fluorapatite, and quartz from the Cheever Mine iron oxide-apatite (IOA-type) deposit in Essex County, New York, USA, display rare symplectite textures. Electron probe wavelength-dispersive spectrometry (WDS) mapping and major and trace element characterization of these features reveal a natural experiment in fluid-mediated monazite recrystallization. Two types of monazite with symplectite intergrowths have been recognized (Type I and II). Both types of symplectite development are associated with a decrease in HREE, Si, Ca, Th, and Y, but an increase in both La and Ce in monazite. Electron microprobe Th-U-total Pb analysis of Type I monazite with suitable ThO2 concentrations yielded a weighted mean age of 980 ± 5.8 Ma (MSWD: 3.3), which is interpreted as the age of monazite formation and the onset of symplectite development. Both types of monazite formed during a series of reactions from fluorapatite, and possibly britholite, to produce the final assemblage of monazite, allanite, and fluorapatite. Monazite formation was likely a response to evolving fluid conditions, which favored monazite stability over fluorapatite at ca. 980 Ma, possibly a NaCl brine. A subsequent transition to a Ca-dominated fluid may have then promoted the consumption of monazite to produce another generation of allanite and fluorapatite. Our results indicate that recrystallized monazite formed during fluid-mediated processes that, over time, trended towards an increasingly pure end-member composition. Regionally, these data are consistent with a magmatic-origin followed by fluid-mediated remobilization of select phases at subsolidus conditions for the Adirondack IOA deposits

Contaminant Concentrations in Adirondack Soil and Sediment

Paired soil and sediment samples were collected along a transect across northern New York from Lake Ontario to the Tug Hill Plateau to Indian Lake in the Central Adirondack Mountains. The samples were analyzed for four classes of compounds including polychlorinated biphenyl.s, polyaro­matic hydrocarbons, select chlorinated pesticides, and inorganic elements and metals with different origins, usage history, and chemical properties. In general, the concentrations of these contaminants were higher in soil than sediment but relatively low compared to other more developed areas, consistent with the remoteness of the Adirondack region and limited permanent population. Trends in the data suggest that the concentrations of contaminants, particularly high molecular weight organic compounds, are generally greater closer to Lake Ontario. The reason(s) for this remain unclear, but are likely related to the large amounts of lake-effect precipitation the Tug Hill area receives and scavenging of contaminants by rain and snow. Levels reported for contaminants investigated in this study are generally lower than those reported elsewhere in the United States, and likely reflect the low population density of the region and less intensive !and use