GSA Annual Meeting in Phoenix, Arizona, USA - 2019

Paper No. 198-9
Presentation Time: 9:00 AM-6:30 PM


MATOS, Javier, DORAIS, Michael J., CHRISTIANSEN, Eric H. and NELSON, Stephen T., Department of Geological Sciences, Brigham Young University, Provo, UT 84602

A-type granites are alkaline, anhydrous and occur in anorogenic settings. These granites have high concentrations of K2O, Na2O, SiO2, FeOtotal, 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 ƐHf 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.