THE TIMING AND SEQUENCE OF MAGMA EMPLACEMENT OF THE OSCEOLA GRANITE, WHITE MOUNTAIN BATHOLITH, NEW HAMPSHIRE (Invited Speaker)

The Jurassic White Mountain Magma Series is composed of alkaline syenites and ferroan granites that outcrop in a N-S trend over ~150 km of present-day New Hampshire. While much of the distributed magmatism occurs in a series of small satellite intrusions, the most volumetrically significant magmatism occurs in the White Mountain Batholith (WMB). Mapping and geochronology show that the WMB is not a monolithic pluton, but is divided into a Western and an Eastern batholith, each composed of porphyritic syenites (Lafayette and Garfield quartz syenites in the west, and Albany quartz syenite in the east), Na-Fe-rich rhyolite volcanics (the Moat Volcanics), hornblende granite (Osceola Granite), and biotite granite (Conway Granite). Recent high precision U-Pb zircon ages show that the syenites and Conway granite in the Western batholith were emplaced from ~198 - 190 Ma; whereas the same lithologies measured in the Eastern batholith were emplaced between ~184 - 180 Ma (Kinney et al., 2022). Thus, it appears that despite the apparent lithological similarity between the W and E portions of the batholith, they were constructed by entirely separate magmatic events.

The Osceola granite is the second-most voluminous lithology in the WMB. On the basis of field-mapping and whole rock Rb-Sr isochron ages, Creasy (1974) postulated that the Osceola was emplaced synchronously in the W and E, representing the final phase of W magmatism and the first phase of E magmatism. We present new CA-ID-TIMS U-Pb zircon geochronology based on weighted mean ²⁰⁶Pb/²³⁸U ages (2σ internal errors on the order of 0.03%) from the Osceola granite from the Western and Eastern batholith in order to test this hypothesis.

A sample of Osceola granite in the Western Batholith (Loon Mtn) gives a zircon age of 195.0 Ma, within the range of 198 - 190 Ma for the Western Batholith. A sample of Osceola granite in the Eastern Batholith (Middle Mtn), gives an age of 181.6 Ma, placing it in the range of 184 - 180 Ma for magmatism in the Eastern Batholith. Two additional LA-ICPMS zircon measurements on Osceola granite give ages of ca.196.0 Ma in the west, and ca. 183.4 Ma in the east. We conclude that the Osceola granite follows the lithologic progression in each portion of the batholith and is not a single widespread magmatic episode across the entire WMB.