NO LATERAL GEOCHEMICAL VARIABILITY BETWEEN WESTERN AND EASTERN WHITE MOUNTAIN BATHOLITH DESPITE 10-15 MYR AGE GAP

The Jurassic White Mountain Magma Series (WMMS) of New Hampshire consists of a series of alkali syenites, syenites, and ferroan A-type granites. The most voluminous exposures of WMMS magmas are contained within the White Mountain Batholith (WMB). The WMB is split into two geographic regions, the Western and Eastern batholiths on the basis of field mapping and geochronology. The Western and Eastern batholiths both display a lithological progression from syenite to hornblende granite (Osceola Granite) to biotite granite (Conway granite), however the Western batholith granites pre-date those in the Eastern batholith by at least 10 Ma (Kinney et al., 2022). This study aims to use geochemical analysis of Conway and Osceola granites from both batholiths to determine whether the granites display a geochemical evolution in addition to their known chronological and geographical differences.

We measured whole rock major and trace elements compositions from 36 samples across the WMB granites. Granites from the Western and Eastern batholiths are indistinguishable in their major element ratios (MALI, ASI, AI, and Fe-Index) when plotted against SiO₂, with all of the granites having compositions typical of A-type granites (e.g., Frost & Frost, 2011). This west-east similarity continues into trace element geochemistry, where trace element ratios of Nb vs. Y, Yb/Ta vs. Y/Nb (e.g. Eby et al., 1992) are similar between the respective granite units in each batholith.

Our results indicate that the Western and Eastern batholith granites are nearly identical in major and trace element ratios, and that both batholiths likely fractionated from OIB-type basalts (e.g. Eby et al., 1992). Thus, we find no appreciable difference in source magma composition in the WMB between the two magmatic episodes. Our result implies that the source and trigger to the WMB magmas was a long-lived and consistent feature of the region as rifting was initiating along the eastern North American margin.