PRIMITIVE BASALTS OF THE POISON LAKE CHAIN: ISOTOPE RATIOS DISTINGUISH DIVERSE SOURCE COMPOSITIONS IN THE SOUTHERN CASCADES

Primitive basalts from monogenetic cones can be used to characterize the role of heterogeneous mantle sources vs active processes in arc systems; leading to a better picture of crustal evolution behind the arc. Re-Os used in conjunction with other isotopes is ideal for investigating diverse mantle sources and interaction of young mantle melts with variably evolved crust in the arc. The Poison Lake chain (PLC), a series of monogenetic cones and flows, located east of Lassen Volcanic Center in the southern Cascade Arc, presents a unique opportunity to examine deep arc processes using coeval (~100 ±10ka) basalts erupted in a small area (<30 km²) just behind the arc. Six compositionally distinct groups of primitive, calc-alkaline basalts that erupted in the PLC have Mg# = 59-76, Ta and Nb depletions, Sr and Pb enrichments, La/Yb = 4.17-21.88, and limited ranges of Sr, Nd and Pb isotope ratios that are more radiogenic than MORB. These basalt groups cannot be related via simple fractionation or crystallization and represent individual batches of mantle melts that have been variably modified by subduction components. New Os isotopic analyses are elevated relative to MORB mantle and range from initial ¹⁸⁷Os/¹⁸⁸Os of 0.139-0.230 (ƔOs: 9.5-81.2).

Although the PLC lavas are relatively primitive, they display a range of Os isotopic values that is similar to that of less primitive magmas in the Cascades. Combined with Sr, Nd and Pb isotopes and trace elements, these results confirm that individual PLC groups are mantle-derived melts with diverse isotopic compositions not just variable degrees of assimilation+fractional crystallization (AFC). High Os values and the primitive nature of PLC basalts suggest that local mantle sources have higher more diverse Os isotopic compositions, that are then variably modified by young lower arc crust and/or young slab, and lesser degrees of AFC processing. Geochemical variability in the small area of the PLC is similar to that observed on a larger scale throughout the Cascades. Diverse PLC primitive basalts likely result from their location on the eastern edge of the Cascades where nearby extension allows small batches of magma to reach the surface without contamination or modification, thus providing a sampling of the heterogeneity of the sub-arc mantle.