XENOLITHS, ENCLAVES, AND SYNPLUTONIC DIKES IN THE WOOLEY CREEK BATHOLITH, NORTHERN CALIFORNIA

Mafic magmatic and metasedimentary enclaves were important in development of I- and S-type terminology. The origins of peraluminous metasedimentary enclaves (xenoliths versus refractory residue of partial melting) are particularly vexing. The Wooley Creek batholith (WCb) provides an example in which enclave origins and modification can be studied in detail. The WCb is a tilted plutonic complex intruding three host terranes: a structurally lower ophiolitic mélange, an overlying volcanogenic metasedimentary unit, and a structurally highest chert-argillite mélange and quartz wacke. The lower zone of the batholith (159.22 ± 0.10 Ma) ranges from gabbro to tonalite, whereas the upper zone (158.22 ± 0.14 Ma) ranges from quartz diorite to granite. A central zone represents the co-magmatic interface between these two zones. Xenoliths encompass the rock types found in the host terranes and many are migmatitic. On the basis of rock type, trace elements, and isotopic compositions, most xenoliths can be correlated to one of the three host terranes. The largest concentrations of xenoliths are near the lower-to-central zone boundary or within one km of the intrusive contact. Most xenoliths are chemically refractory compared to their protoliths but few were in textural or mineralogical equilibrium with the host magma. We conclude that melt escaped from these xenoliths but the xenoliths were not invaded by surrounding melt. Mafic magmatic enclaves (mme) are also widespread and are most abundant in the central and adjacent upper zones, with enclave swarms giving way upward to isolated mme. The lower zone has few mme. Instead, this zone is characterized by synplutonic mafic dikes, which become more abundant and more deformed upward to the central zone. Most mme are ‘porphyritic’, with large hornblende and plagioclase crystals derived from the host magma and matrix crystals that grew from a hybrid. In rare instances, poikilitic K-feldspar crystals grew across mme–host boundaries. These features indicate that the mme are also not restite, but represent blobs of mafic magma that were variably hybridized with the host. Hybridization involved ‘kneading’ of phenocrysts into the enclave and infiltration of melt. In the WCb, mafic and metasedimentary enclaves provide information about magmatic process but do not reveal magma sources.