Masters Thesis

Kern knob pluton and other highly-evolved granitoids in east-central California

The Late Cretaceous (91.4 Ma) Kern Knob pluton (KKp) 1s a petrographic and geochemical oddity in the eastern portion of the Sierran batholith in east-central California. Several facies with distinctive compositions and textures are recognized within this pluton, including: (1) a leucocratic facies composed of subequal amounts of quartz, albite and microcline, and characterized by patches of Fe-Mn-Ti hydroxides; (2) a pegmatite facies consisting of amazonite, albite and muscovite; (3) marginal dikes which are modally similar to leucocratic facies samples, but which have pronounced protoclastic fabrics; and (4) mineralized veinlets characterized by the presence of fluorite and beryl. Chemical compositions are near the F-rich granite minima, and are characterized by (1) high contents of Na, Mn, F, Rb, Li, U, Th, Hf and Nb; (2) low contents of Fe, Ca, Sr and Ba; and (3) high Fe+3/Fe+2 (~ 7). REE patterns are flat with large negative Eu anomalies typical of highly-evolved granitic melts. Although KKp has highly-evolved chemical signatures, it is metaluminous with A/CNK ~ 0.94, presumably because of the abundance of Na. Initial 87Sr/86sr is high at 0.7491, with an ESr = +635, and δ180 = +7.74 %. Isotopic signatures, coupled with overall composition, suggest that the source of KKp was primarily in the crust, but small amounts of undifferentiated mantle material and/or fractionated subduction zone magma may be additional components. Evolution of the pluton was likely multistaged. The latest evolution was dominated by two-alkali- feldspar fractionation, and was followed, during the final pre-emplacement and post-emplacement stages, by activity of a halogen-rich aqueous phase which influenced the distribution of several elements among the various KKp facies. Seven Late Cretaceous to early Tertiary, highly-evolved granitoids in east-central California were sampled to provide a basis of comparison with Kern Knob pluton. These granitoids are: Boundary Peak adamellite, Birch Creek pluton, Papoose Flat pluton, Skidoo pluton, Hail Canyon pluton, Copper Queen alaskite and Noname Canyon alaskite. Modal data and major-element compositions of these plutons are relatively homogeneous and are reflective of a high degree of magmatic evolution. However, the heterogeneity of trace- and rare-earth-element contents and isotopic signatures between these plutons suggest that they were derived from different source materials and/or are representative of a continuum of compositions at various stages in the the evolution of highly-evolved melts.

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