PLUTON EMPLACEMENT DURING NEVADAN OROGENESIS: THE ASHLAND PLUTON, KLAMATH MOUNTAINS, OR AND CA

The composite Ashland pluton (AP) intruded three Mesozoic terranes: Rattlesnake Creek, Western Hayfork, and Eastern Hayfork. Eastward tilting and erosion have exposed a zoned magma system, with gabbro to quartz diorite in the south and tonalite in the west. Quartz monzodioritic (QMD) rocks are exposed in the central zone and adjacent to the eastern contact; these QMD exposures are separated by a wide zone of granitic intrusions. Tonalite and QMD units display variably intense magmatic foliation that decreases in structurally higher levels.

New LA-SF-ICPMS zircon U-Pb ages indicate pluton emplacement from 160.9 to 151.9 Ma. The oldest unit is the westernmost biotite-hornblende tonalite (160.9 ± 0.6 Ma; all uncertainties at 2se) with NE-dipping foliation. After a period of quiescence, gabbroic to quartz dioritic rocks were emplaced in the southern part of the pluton (158.0 ± 0.7 Ma). The central quartz monzodiorite magmas were emplaced over ca. 3 m.y., from 156.9 ± 0.8 to 153.9 ± 0.7 Ma. This activity was coeval with emplacement of tonalite along the western part of the pluton (154.8 ± 0.6 Ma). Foliation in the QMD unit and younger tonalite dip west, nearly orthogonal to foliation in the older tonalite. Two monzogranite dikes that crosscut foliation in the QMD were dated. A dike in the SW part of the pluton was dated to 153.1 ± 1.0 Ma; another from Mt Ashland summit was dated to 151.9 ± 0.7 Ma. Scant antecrystic zircon (168–161 Ma) indicates that precursor magmatism began as early as ca. 168 Ma, making the total longevity of this trans-crustal system at least 16 m.y.

Early Nevadan orogenesis involved thrusting of AP host terranes over low-grade rocks of the outer Condrey Mountain Schist at ca. 157 Ma, with local deformation continuing to ca. 149 Ma. Thus, precursor AP magmatism began at least 10 m.y. before Nevadan deformation and continued during and into waning stages of the orogeny.

The ε_Hf values of primocrystic and antecrystic zircon are statistically identical, with averages of +7.9–9.2. These values, the presence of scant zircon xenocrysts (241–170 Ma), and oxygen isotope data attest to deep-seated assimilation of lower crustal rocks throughout AP magmatism.