Emplacement and hypersolidus fabric development in the Wooley Creek batholith, Klamath Mountains, California



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The Wooley Creek batholith is a late Jurassic mid-crustal pluton exposed in the Klamath Mountains of northern California and southwestern Oregon. Metamorphic assemblages in the aureole of the pluton indicate it has been tilted to the southwest, exposing approximately 9 km of structural relief [Barnes et al., 1986b]. Structural mapping in the interior of the pluton combined with previous geochemical and geochronology studies has shown that the Wooley Creek batholith was emplaced in two stages between 158 – 156 Ma. Diapirs of basaltic and andesitic magma began intruding into the accreted terranes of the western Paleozoic and Triassic belt around 158 Ma. Prolonged magmatism allowed these intrusions to amalgamate into a large plutonic body, forming the lower Wooley Creek batholith. The intrusion of these magma batches compressed first the surrounding host rock, forming a narrow structural aureole, then older batches, forming a steeply dipping magmatic foliation. The resulting plutonic mass was both texturally and compositionally highly variable. In the waning stages of emplacement, magmatism reinitiated with an influx of dacitic magma. The dacitic magma intruded through the lower Wooley Creek batholith, pooling at the rheological interface between the partially molten lower Wooley Creek batholith and overlying host rocks. As magma continued to feed into the chamber, it began convecting and stoped upwards. Repeated injection of basaltic magmas throughout this time collected at the floor of the expanding chamber, adding heat and facilitating localized mixing and mingling between the upper and lower Wooley Creek batholith. Chamber-wide convection within the upper Wooley Creek batholith formed margin parallel magmatic fabrics and caused the intrusion to rapidly cool. Subsequent foundering of the upper Wooley Creek batholith deformed the underlying lower Wooley Creek batholith immediately adjacent at hypersolidus conditions. Field relationships suggest that the Wooley Creek batholith is decoupled from host rock structures, and internal fabrics are entirely emplacement-related.