Eocene rhyolites from Saipan are unique in the history of the Izu-Bonin-Mariana (IBM) system in that they erupted during the earliest stages of subduction, but have "mature arc" major element, trace element, and isotopic compositions. The protoarc rhyolites on Saipan have (super 40) Ar/ (super 39) Ar ages of 46-45 Ma, indicating eruption 2-3 million years later than the boninitic lavas at DSDP Sites 458 and 459 and on the islands of Chichijima and Palau and 1-2 million years earlier than the less boninitic lavas on Guam. Relatively unaltered rhyolites have 77-78 wt. % SiO (sub 2) and are characterized by low rare earth element (REE) concentrations and flatter REE patterns compared to most high SiO (sub 2) rhyolites from continental arcs. These rhyolites are also enriched in Zr and Hf over middle REE, and are depleted in Nb and Ta with respect to the light REE. These trace element patterns differ significantly from those of other Mariana protoarc lavas, such as the boninite series lavas on Guam, which are characterized by enrichments in light REE over heavier REE (La/Yb = 2-3), and a slight depletion in the middle REE producing an overall U-shape to the REE pattern. Pb isotopes for the rhyolites fall at the low radiogenic intersection of two arrays toward Pacific volcaniclastic sediments and bulk sediments and imply a minor sediment component and that some Pb may have been derived from the subducting altered oceanic crust of the Pacific plate. Hf and Nd isotopic compositions are derived from the subarc mantle and plot well within the Indian mantle domain. Fractionation modeling suggests that the Saipan rhyolites were most likely produced by fractionation of tholeiitic-series magmas that are present on Guam a few million years later. This fractionation may have been accompanied by assimilation of boninitic lavas with low concentrations of REE and large enrichments in LILE. A possibly more intact and thicker section of crust beneath Saipan may have promoted deeper and less silicic mafic magmatism and would have allowed the arc-tholeiitic magmas longer residence times and greater crystal fractionation.