COMPARISON BETWEEN ROOF DIKES AND EQUIVALENT INTRUSIVE ROCKS FROM THE WOOLEY CREEK BATHOLITH AND THE SLINKARD PLUTON, KLAMATH MOUNTAINS, CALIFORNIA: A LASER ABLATION ICP-MS STUDY OF AUGITE

The Wooley Creek batholith and the Slinkard pluton are parts of a tilted magmatic complex that was emplaced 161 Ma ago (Barnes et al., 1986). The system is composed of two-pyroxene hornblende gabbro and diorite at the bottom to biotite hornblende tonalite and granodiorite toward the top. Pyroxene-bearing dikes, ranging from basaltic to dacitic compositions escaped from the system. The aim of the study is to compare trace elements of augite from the dikes and those from their intrusive equivalent. A previous study indicated that (Mg/Mg+Fe) of augite from dikes and intrusive rocks are similar (Barnes, 1987). Lower Ca content of the augite from the roof dikes compared to the intrusive ones were interpreted as being induced by faster cooling. New REE data obtained by laser ablation ICP-MS show that roof dike augite are more depleted in LREE and globally richer in Ti than their equivalent in the intrusive rocks. Reversed zoning is observed in several roof dike phenocrysts, in which phenocryst mantles are poorer in REE and richer in Cr than cores and rims. Such zoning is indicative of recharge by mafic magmas. Augite from mafic magmatic enclaves found in the center part of the intrusive suite have lower Sc and Ti than roof dike augite suggesting that the magma escaping the system had a different composition than the mafic magmatic enclaves. Their REE pattern is also steeper with a negative slope. Ultrabasic (predominantly pyroxenite) blocks in the intrusion contain augite with very low Sc and high Cr. These compositions do not fit a cumulate origin, but instead suggest that the blocks were ultramafic xenoliths that underwent magmatic metasomatism after incorporation into the host magma. Variations of augite trace element contents in this system indicate a complex history of incremental emplacement, recharge, and that magma escaped during pluton construction.