METAMORPHIC TEMPERATURE–XCO2 CONDITIONS IN MARBLES OF THE SEQUOIA REGION, SIERRA NEVADA, CA

In and adjacent to Sequoia National Park, metasedimentary rocks of the Jura-Triassic Kings Sequence form a regionally extensive network of roof pendants in the Sierra Nevada batholith. As part of an ongoing investigation of magmatic and metamorphic fluid flux from the Sierran arc, we report findings from a regional isotopic and phase equilibria study of contact metamorphic conditions recorded in the calcite (Cc) marbles commonly found in the Sequoia pendants.

In outcrop, marbles dip steeply (vertical ± 10°) and vary considerably in appearance (fine to very-coarse grained; blue-grey to bleached-white color). Marbles typically contain a few percent of Ca-Mg silicates indicating minor dolomite and silica in protolith limestones. Peak temperatures varied considerably as revealed by appearance of tremolite, diopside, and occasionally forsterite, which indicate metamorphic conditions of at least 350°C in all rocks, given regional pressures estimates of 3–4 kbar based on phase equilibria in pelites and igneous barometry (Ague and Brimhall 1988, GSA Bull.). Skarns containing calcic garnet, diopside, and wollastonite occur locally at contacts of marbles and granitic plutons.

Calcite in 22 marble samples varies considerably in δ¹⁸O (15.9 to 23.0‰, VSMOW) and δ¹³C (–3.8 to 3.5‰, PDB), with high values typical of Jurassic marine limestones, and lower values suggesting metamorphic exchange. Overall, no pattern of δ¹⁸O(Cc) and δ¹³C(Cc) change emerges with degree of metamorphism, however skarns have δ¹⁸O(Cc) values of 13.5–14.1‰ (n=2), evidence of infiltration by magmatic fluid. In several samples, δ¹³C of both calcite and crystalline graphite were measured, which yields peak temperatures of 619±35°C by the calcite-graphite thermometer of Kitchen and Valley (1995, J. Met. Geo.).

Given this peak temperature, the occurrence of wollastonite in skarns indicates that infiltrating magmatic fluids were water-rich (XCO₂ < 0.15); however, most marbles experienced higher XCO₂ conditions and generally record closed-system devolatilization reactions that were internally buffered and relatively impervious to magmatic fluids. In contrast, magmatic fluid flow was focused along marble and pendant margins.