36 Rates, Dates, and Plates: Petrochronological Approaches to Unraveling Tectonometamorphic Histories

This session welcomes contributions  applying modern approaches in metamorphic petrology, geochronology, and structural geology to unravel the dynamics of convergent margin processes from the Appalachians and beyond. We encourage submission from Early Career participants.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Room 22/23

The Bear’s Den Granite: a peraluminous pegmatite from the Bronx Zoo, New York City.

Paper number: 36-4
Program: Technical Sessions
Time: 9:05 AM – 9:25 AM

Authors

Abstract

Recent mapping in Northern Manhattan and the Bronx (New York City) has identified a previously unmapped, 2 square km pegmatitic granite colloquially named “the Bear’s Den Granite”. This unit, mostly exposed near the bear exhibit at the Bronx Zoo, is distinct in texture coarse plagioclase, quartz, muscovite, and garnet and large, single crystal K-feldspar up to 30 cm in length. The K-feldspar are poikilitic and commonly include apatite, zircon, titanite, and garnet. Major element chemistry show this is a peraluminous granite with Al2O3/(CaO+Na2O+K2O) of 1.80. Zircon crystals are large (up to 2 mm in length), euhedral with a high aspect ratio, and dark brown to black in color. Zircons contain large inclusions of apatite, thorite, and uraninite, typical of pegmatitic zircons. Due to extremely high U concentrations (2000-5000 ppm) in the zircon, it is highly metamict and difficult to date. Nevertheless, U-Pb laser ablation dates from four grains suggests intrusion in the Early Devonian (~400 Ma). Additional analyses are required to confirm the age. Inherited zircons are common, with most being Mesoproterozoic and one being Archean.  The Bear’s Den Granite records a general enrichment in REEs relative to chondrite, a steep Light Rare Earth Element (LREE) pattern with a relatively flat heavy rare earth (HREE) trend, and a negative Eu anomaly. Isotopically, the granite is highly evolved, with a 143Nd/144Nd value of 0.512264 (eNd -3), and an 87Sr/86Sr value of 0.720315. 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios are 18.9211, 15.5970, and 38.6552.

The simplest explanation for these geochemical trends is derivation from either a) something akin to an A-type trace element budget or (b) a source (S-, I- or A-type) that has seen significant fractionation to concentrate said elements and isotopes. It is also important to point out that these preliminary ages from the BD should be considered in terms of tectonic history of the region, as the Bear’s Den zircon crystallization ages postdate peak metamorphism from the Taconic Orogen (440-460 Ma), and is in between the canonical age of Acadian (420 Ma) and NeoAcadian (385 Ma).

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411307

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