Geology, Petrology and Geochemistry of Traveler Rhyolite and Katahdin Pluton (northcentral Maine)

The Traveler Rhyolite-Katahdin Pluton Igneous Complex of of the Lower Devonian age in Northcentral Maine consists of two penecontemporaneous but genetically independent magmatic episodes. The earlier, voluminous episode includes the Traveler Rhyolite and the Katahdin Granite, whereas the slightly younger episode is comprised of minor intrusions of Horserace Quartz Diorite, Harrington Lake Porphyry and Debsconeag Granodiorite. Electron microprobe analyses of mineral assemblages, as well as major element geochemistry, provide data that show the earlier episode being derived by partial melting of graphite bearing sediments at a depth af 25-30 km and at a temperature of 810 0 C. The pre-eruption temperature and P(H 2 0) for the Traveler Rhyolite is estimated at 800 0 C and 1100 bars respectively. Similar estimates for Katahdin Granite yield 710oC and 1200 bars. Both units were buffered at oxygen fugacities slightly below the QFM buffer (10-1 5 00 for rhyolites; and 10-17.25 for granites) as a consequence of the presence of graphite in the parental sediments. The younger episode, as evidenced by the trend on an AFM diagram, crystallized under oxidizing conditions. Trace element abundances are consistent with the derivation of the earlier episode from the Appalachian eugeosynclinal sediments. In particular, the unusually high Sc abundances (25-35 ppm) in the acidic rocks preclude a derivation by extensive fractional crystallization of a basaltic magma. Variations of trace element abundances in the Traveler Rhyolite and Katahdin Granite are best explained if the granites represent fractionated rhyolitic magma after removal ABSTRACT (continued)continued) of 25% crystals of alkali feldspar (35 wt%),plagioclase (25 wt%), quartz (31 wt%), biotite (8 wt%), and magnetite (1 wt%). Aplitic magmas require approximately 80-90% crystal removal of the same mineral proportions as granites. The calculated mean partition csefficients required by this model for Co, Rb, Hf, Sr, and Eu are 3.18, 0.49, 2.42, 2.08, and 2.41. The mean partition coefficient for Hf is largely due to a removal of accessory zircon. The similar values of partition coefficients for Eu and Sr are consistent with the estimated very low oxygen fugacities. The trace element data further show that the Horserace Quartz Diorite and Harrington Lake Porphyry are genetically independent from the earlier episode and that the Debsconeag Granodiorite is a result of mixing of 40% Traveler Rhyolite magma and 60% Harrington Lake Porphyry. Higher abundances of heavy rare earth elements (Gd-Lu) and Hf in aplites, relative to granites, suggest that zircon ceased to crystallize during the late stages of differentitation due to the gradual loss of zirconium via an exsolving vapor phase. Thesis Supervisor: F.A. Frey Associate Professor of Geochemistry Title: OBJECTIVE AND STUDY PLAN The goal of this study is to gain an understanding of the tectonic significance of the two igneous belts in northcentral Maine, the Piscataquis Volcanic Belt and the Greenville Plutonic Belt. As a first step in achieving this goal, this study is focused on the origin, evolution, and mode of emplacement of the Traveler Rhyolite Katahdin Granite