TEMPORAL VARIATION IN PLUTON-WALLROCK INTERACTION IN THE SIERRAN ARC

Zircon and whole rock compositions of the Grant Grove and Giant Forest plutons of Sequoia National Park provide insight into pluton formation at two different times during the evolution of the Sierran Arc. Both plutons include distinct margins with mineralogy that suggests incorporation of metasedimentary wallrock. However, differences in mineralogy and geochemistry may reflect differences in pluton-wallrock interactions with time.

The Grant Grove Pluton consists of granite with K-feldspar phenocrysts. Granite along the margin contains garnet. A new SHRIMP-RG zircon U/Pb data for the interior granite yields an age of ca. 113 Ma, revising the previously published ages of 106 and 128 Ma (Chen and Moore, 1982). Compositions of two zircons indicate significant fractionation and very high temperatures which may reflect the presence of inherited zircon. Cathodoluminescence (CL) images of zircons show homogeneous magmatic zonation and few inclusions. Whole rock compositions shift from interior to margin, with increasing SiO₂ and Rb/Sr and decreasing CaO and Na₂O/K₂O, consistent with an increase in incorporation of wallrock at the margin.

The interior of the Giant Forest Pluton is granodiorite with euhedral hornblende. The margin consists of granite with abundant quartz and K-feldspar phenocrysts. New U/Pb dating of interior and margin yields ages of ca. 100 Ma, consistent with the previously published age range of 102-97 Ma (Chen and Moore, 1982). Small Eu anomalies in zircon from the pluton margin may reflect incorporation of plagioclase-bearing wallrock. Zircon CL images identify rounded cores surrounded by magmatic rims in pluton-margin rocks, whereas pluton-interior rocks lack the rounded cores. Whole rock compositions similarly shift from interior to margin, however variation in composition is less pronounced than in the Grant Grove rocks suggesting a smaller component of incorporated wallrock in the Giant Forest margin rocks.

The higher SiO₂ content, presence of inherited zircon cores in the pluton-interior, and more extreme compositional variation from interior to margin suggest that the older Grant Grove Pluton interacted with a greater amount of pre-existing crust than the younger Giant Forest Pluton which intruded into an already well established continental magmatic system.