PETROLOGY OF THE SALT CREEK PLUTON, NORTHERN SALT SPRING HILLS, CA

The Salt Creek Pluton (SCP), in the central Salt Spring Hills, CA (UTM 11 S 0565640E, 3943480N, NAD 27) intrudes a scapolite bearing pyroxene hornfels metamorphic suite (Yip & Erickson, 1999). The SCP has an early biotite hornblende granodiorite phase (EGP), and a later muscovite biotite granite phase (LGP). The EGP delta O18 of 9.5, A/CNK=0.96, and 9% hornblende classify it as a metaluminous I type pluton. One exposure of EGP contains 6x20 cm, fine-grained, mafic enclaves. EGP cooling age is 149.65+0.78 Ma by Ar/Ar on biotite. EGP has a strong LREE enrichment with Eu/Eu*=0.86. Pierce discrimination diagrams indicate a volcanic arc origin for it. The hornblende geothermobarometer (Ernst & Liou, 1998) in EGP give 850o C and 2-3 kbar, matching andalusite in the aureole. EGP plagioclase (Pl) has an An40 embayed core with a thick euhedral to subhedral An70 rim, overgrown successively by 2 rims with sharp compositional jumps of An40 and An25. Extremely rare, euhedral, epidote and sparse allanite are present in EGP. EGP also has common coarse epidote with exsolved sphene and deuteric clinozoisite abundant in Pl. LGP has sparse primary muscovite, typical of S type granite, and An35-An0 Pl with simple normal zoning.

This is our idealized model of evolution for the SCP. First, magmatic epidote shows EGP is emplaced in the lower crust at >6 kbar. The EGP delta O18 suggests an igneous source that assimilated at least 30% by volume of sediment rich wall rock; few inclusions are present in the pluton indicating thorough assimilation. Primary Pl of An40 composition forms at this stage. Second; a mafic body intrudes, heats EGP by magma mixing, enriches the magma in Ca, and forms mafic enclaves. An40 Pl is strongly resorbed and extensive An70 overgrowths form. Third, EGP rises in the crust reducing magma pressure and resulting in first boiling that produces a sharply bounded An40 overgrowth around the An70 rim. Fourth, anhydrous quartz and K-spar crystallize and cause second boiling that results in growth of a thin, sharp An25 rim. Biotite and hornblende form at this time, followed by quartz and K-spar. The small Eu anomaly in EGP portrays minimal Pl fractionation. Fifth, deeper EGP fractionates and S-type LGP forms. Simple normal zoned Pl in LGP is in accord with slow stationary cooling.