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Complex patterns of fluid flow during wollastonite formation in calcareous sandstones at Laurel Mountain, Mt. Morrison Pendant, California

Jade Star Lackey^,1 and John W. Valley^,1

¹ Department of Geology and Geophysics, 1215 West Dayton Street, University of Wisconsin, Madison, Wisconsin 53706, USA

Late Cretaceous contact metamorphism at Laurel Mountain, east-central California, resulted in growth of wollastonite in calcareous metasandstones during infiltration of water-rich [X(CO₂) = 0.03–0.14] fluids. Fluids originated from the Laurel Mountain pluton and other magmatic sources. The mapped distribution of wollastonite versus calcite + quartz in the aureole reveals geometrically complex fluid flow paths at a range of scales. An inner aureole, concentric to the pluton, extends up to 500 m from exposed contacts and is characterized by ubiquitous wollastonite. An outer aureole has irregular distribution of wollastonite, chiefly at faults and stratigraphic contacts.

Fluid flow was largely vertical during metamorphism, and permeability varied systematically. As fluids migrated up from crystallizing magmas into pendant rocks, they were variably channeled along a hierarchy of structural features. Faults in the aureole transported the largest fluid fluxes as recorded by ¹⁸O(Wo) values (7–8) that approach equilibrium with the pluton. These faults distributed fluids up and away from the Laurel Mountain pluton. Of the total fluid flow, 2–4% was across layers, which mixed low-¹³C carbon (–25) from pelites into wollastonite metasandstones. Textural and isotopic data indicate that mineral reaction in the inner aureole was controlled by availability of fluids, permeability, and likely temperature; reaction in the outer aureole was driven mostly by access to water-rich fluids. Magmatic fluids in the outer aureole were isotopically shifted farther from magmatic values than fluids in the inner aureole and are inferred to have traveled from deeper sources.

Isotope disequilibrium is widespread at Laurel Mountain. Values of ¹⁸O(Qt–Cc) in calcareous sandstones (1.78 ± 0.3) indicate exchange, but not equilibration at peak metamorphic temperatures (560 °C). Most ¹⁸O(Qt–Wo) values (2.0–6.5) indicate kinetically controlled exchange and disequilibrium during fluid-rock interaction. Minor isotopic resetting in the aureole was caused by polymetamorphism related to younger plutonism in the eastern Sierra Nevada. This study highlights the complexities of modeling fluid flow during contact metamorphism in composite batholith settings. These complexities include multiple fluid and heat sources, plus variable permeability, temperature, mineral-fluid exchange, and flow path.

Key Words: contact metamorphism • wollastonite • fluid flow • Sierra Nevada • stable isotopes • oxygen isotopes • carbon isotopes

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