MIDPLEISTOCENE LAVAS FROM THE SEGUAM VOLCANIC CENTER, CENTRAL ALEUTIAN ARC - CLOSED-SYSTEM FRACTIONAL CRYSTALLIZATION OF A BASALT TO RHYODACITE ERUPTIVE SUITE

In contrast to adjacent volcanic centers of the modern central Aleutia n arc, Seguam Island developed on strongly extended arc crust. K-Ar da tes indicate that mid-Pleistocene, late-Pleistocene, and Holocene erup tive phases constitute Seguam. This study focuses on the petrology of the mid-Pleistocene, 1.07-.07 Ma, Turf Point Formation (TPF) which is dominated by an unusual suite of porphyritic basalt and basaltic andes ite lavas with subordinate phenocryst-poor andesite to rhyodacite lava s. Increasing whole-rock FeO/MgO from basalt to dacite, the anhydrous Plag + Ol + Cpx +/- Opx +/- Mt phenocryst assemblage, groundmass pige onite, and the reaction Ol + Liq = Opx preserved in the mafic lavas in dicate a tholeiitic affinity. Thermometry and comparison to published phase equilibria suggests that most TPF basalts crystallized Plag + Ol + Cpx +/- Mt at greater-than-or-equal-to 1160-degrees-C between about 3-5 kb (+/- 1-2% H2O), andesites crystallized Plag + Cpx + Opx +/- Mt at greater-than-or-equal-to 1000-degrees-C between 3-4 kb with 3-5% H 2O, and dacites crystallized Plag + Cpx +/- Opx +/- Mt at 1000-degrees -C between 1-2 kb with 2-3% H2O. All lavas crystallized at f(02) close to the NNO buffer. Mineral compositions and textures indicate equilib rium crystallization of the evolved lavas; petrographic evidence of op en-system mixing or assimilation is rare. MgO, CaO, Al2O3, Cr, Ni, and Sr abundances decrease and K2O, Na2O, Rb, Ba, Zr, and Pb increase wit h increasing SiO2 (50-71%). LREE enrichment [(Ce/Yb)n = 1.7 +/- 0.2] c haracterizes most TPF lavas; total REE contents increase and Eu anomal ies become more negative with increasing SiO2. Relative to other Aleut ian volcanic centers, TPF basalts and basaltic andesites have lower K2 O, Na2O, TiO2, Rb, Ba, Sr, Zr, Y, and LREE abundances. Sr-87/Sr-86 rat ios (0.70361-0.70375) and ratios of Pb-206/Pb-204 (18.88-18.97), Pb-20 7/Pb-204 (15.58-15.62), Pb-208/Pb-204 (38.46-38.55) are the highest me asured for any suite of lavas in the oceanic portion of the Aleutian a rc. Conversely, epsilon-Nd values (+ 5.8 to + 6.7) are among the lowes t from the Aleutians. Sr, Nd, and Pb ratios are virtually constant fro m basalt through rhyodacite, whereas detectable isotopic heterogenity is observed at most other Aleutian volcanic centers. Major and trace e lement, REE, and Sr, Nd, and Pb isotopic compositions are consistent w ith the basaltic andesitic, andesitic, dacitic, and rhyodacitic liquid s evolving from TPF basaltic magma via closed-system fractional crysta llization alone. Fractionation models suggest that removal of approxim ately 80 wt% cumulate (61% Plag, 17% Cpx, 12% Opx, 7% Ol, and 3% Mt) c an produce 20 wt% rhyodacitic residual liquid per unit mass of parenta l basaltic liquid. Petrologic and physical constraints favor segregati on of small batches of basalt from a larger mid-crustal reservoir trap ped below a low-density upper crustal lid. In these small magma batche s, the degree of cooling, crystallization, and fractionation are funct ions of the initial mass of basaltic magma segregated, the thermal sta te of the upper crust, and the magnitude of extension. Tholeiitic magm as erupted at Seguam evolved by substantially different mechanisms tha n did calc-alkaline lavas erupted at the adjacent volcanic centers of Kanaga and Adak on unextended arc crust. These variable differentiatio n mechanisms and liquid lines of descent reflect contrasting thermal a nd mechanical conditions imposed by the different tectonic environment s in which these centers grew. At Seguam, intra-arc extension promoted eruption of voluminous basalt and its differentiates, unmodified by i nteraction with lower crustal or upper mantle wallrocks.