DECIPHERING THE PETROGENESIS OF TWO A-TYPE GRANITES: THE MOUNT OSCEOLA AND THE CONWAY GRANITES, WHITE MOUNTAIN BATHOLITH, NEW HAMPSHIRE

A-type granites are alkaline, anhydrous and occur in anorogenic settings. These granites have high concentrations of K₂O, Na₂O, SiO₂, FeO_total, REE, and high field strength elements, but are low in Al₂O₃, MgO, CaO, and water contents. A-types granites have been classified as two main groups; A₁ and A₂. A₁ granites are emplaced in extensional environments, representing differentiation of mantle-derived magmas. These granites have trace element ratios (Yb/Ta, Y/Nb, Ce/Nb) that indicate derivation from mantle sources similar to those that produce ocean island basalts. The second group A₂, are post-collisional granites, with lower Nb/Y values than the A₁ type, having been derived from previously melted continental crust, or at least, contaminated with a significant crustal component.

The White Mountain Batholith (WMB) is a member of the White Mountain Plutonic-Volcanic Suite of New Hampshire. Granites, quartz syenites, and syenites comprise 97% of the batholith, with 3% consisting of volcanic rocks. The two largest units of the batholith are the Mount Osceola and the Conway granites. We compare and contrast these two granites to determine the differences/similarities in their petrogenesis.

Whole-rock elemental compositions confirm that these two granites are A₁-types. The rocks have high Ga/Al, and high Nb and Zr concentrations. Moreover, electron microprobe analysis of biotite are similar to other A-types with the Mount Osceola having especially Fe-rich biotite with Fe/(Fe+Mg) ~ 0.97 versus ~0.84 of the Conway biotite. Likewise, amphibole in Mount Osceola is also very Fe-rich (0.97). This granite has higher CaO, MREE to HREE abundances and FeO/MgO values, lower Na₂O, Th and U concentrations than the Conway Granite. It also has lower Nb/Y values, perhaps representing more crustal contamination of the Mount Osceola or different source rock compositions for the two granites. We will evaluate the amount of crustal contamination and/or differences in source rock compositions by obtaining in situ ẟ¹⁸O and Ɛ_Hf analyses of zircon to further explore the origin of these two WMB members, and to compare the WMB zircon data with similar zircon data from the young A-type rhyolites on the Yellowstone hotspot track.