HAFNIUM AND OXYGEN ISOTOPIC COMPOSITION OF ZIRCON FROM TWO A-TYPE GRANITES: THE MOUNT OSCEOLA AND THE CONWAY GRANITES, WHITE MOUNTAIN BATHOLITH, NEW HAMPSHIRE

A-type granites are typically alkaline, anhydrous, and occur in anorogenic settings. These granites usually have high concentrations of K₂O, Na₂O, SiO₂, FeO_total, REE, and high field strength elements, but are low in Al₂O₃, MgO, CaO, and water content. A-types granites have been classified as two main groups; A₁ and A₂ (Eby 1992). A₁ granites are emplaced in extensional environments, apparently representing differentiation of mantle-derived magmas. The second group A₂, are post-collisional granites, hypothetically derived from previously melted continental crust, or at least, contaminated with a significant crustal component. The White Mountain Batholith is a member of the White Mountain Plutonic-Volcanic Suite of New Hampshire. The two largest units of the batholith are A-type granites - the Mount Osceola and the Conway granites. We compare and contrast these two granites to constrain their petrogenesis. Electron microprobe analyses of biotite in both granites are similar to that in other A-types with the Mount Osceola having especially Fe-rich biotite. Likewise, amphibole in Mount Osceola is also very Fe-rich. Both granites are classified as A₁, but the Mount Osceola has lower Nb/Y than the Conway, suggesting more crustal contamination, or different source rock compositions for the two granites. In situ ẟ¹⁸O and Ɛ_Hf analyses of zircon show significant crustal contamination in the both granites. The ẟ¹⁸O values for the Mount Osceola are between 8.0-8.9‰, and the Conway Granite are 7.0-7.9‰ (+/- 0.3‰). The values for both granites are distinct from mantle zircon values (+5.3 +/- 0.3‰). Additionally, Ɛ_Hf for the Mount Osceola zircon ranges from -1.1 to +3.9, and those from the Conway Granite range from -1.8 to 4.6; both isotopic systems indicate a considerable amount of continental crust in the sources of granites. Although both granites have A₁ compositions suggesting a mantle derived contribution to the granites, this contribution is not recorded by the zircons. Either the A₁-A₂ classification scheme does not always accurately determine the petrogenesis of A-type granites, or more likely, these zircons crystallized after considerable crustal contamination of mantle-derived A₁ magmas, and missed capturing the signature of that mantle component.