MULTIPLE MAGMA BATCHES IN THE TILTED WOOLEY CREEK BATHOLITH, KLAMATH MOUNTAINS, CALIFORNIA

The Wooley Creek batholith (WCb), situated in the Klamath Mountains, is a tilted intrusion that was emplaced between 160 and 155 Ma. Roof dikes of similar chemical composition to the intrusive rocks represent snapshots of magma that escaped the system (Barnes et al, 1990). The WCb was originally thought to consist of a pyroxene-bearing lower unit of gabbro to tonalite and a hornblende-bearing upper unit of tonalite to granite, without no visible contact between them. The two parts differ in terms of chemical and isotopic compositions but original U-Pb ages indicated emplacement at ~161 Ma (Barnes et al, 1987). Pyroxene and hornblende-bearing roof dikes were thought to sample the lower part and the upper part of the intrusion, respectively. New laser ablation ICP-MS trace element data on amphibole and pyroxene as well as new U-Pb SHRIMP data on zircon have shown that the evolution of the magmatic system is more complex. U-Pb SHRIMP ages on zircon for the lower part of the system range from 160.1 ± 1.6 Ma to 158.0 ± 1.3 Ma for the northern diorite and northern tonalite respectively whereas in the upper part ages range from 156.6 ± 1.3 Ma on granodiorite to 154.2 ±2.1 Ma on late-stage granite. Trace elements in zircon support the division of the intrusion in two main parts, with zircon from the lower part having larger negative Eu anomalies and lower Yb/Gd ratios compared to zircon from the upper part of the system. Pyroxene from roof dikes show evidence of oscillatory zoning and high Ti and low Cr concentrations compared to pyroxene from the lower part of the WCb which are normally zoned, implying a different origin for the roof dike pyroxenes. Euhedral hornblende crystals from both roof dikes and the upper part of the system have similar compositions, suggesting that some of the roof dikes came from the upper part of the system; however, late poikilitic reversely zoned amphiboles, have been found in the upper tonalite unit, indicating a more complex history, locally involving recharge and mixing. New data show that despite the lack of sharp internal intrusive contacts, the gradational change from lower to upper parts of the system conceals a 4 m.y.-long record of magma emplacement.