Relationships between contact metamorphism and magnetite formation and destruction in a plutonʼs aureole, White-Inyo Range, eastern California

Abstract

Magnetic-petrologic studies on metasedimentary rocks from the low-pressure–high-temperature contact-metamorphic aureole of the Joshua Flat–Beer Creek pluton at the southern rim of the Inyo batholith, California, reveal the effect of contact metamorphism on magnetic minerals. The regional metamorphic biotite zone and three different contact-metamorphic zones (andalusite- cordierite, transition, and cordierite–K- feldspar) in the aureole are identified by silicate petrology and their rock magnetic behavior. Phengite barometry yields pressure data in the range of 0.2–0.3 GPa, and Na-in-cordierite thermometry indicates a peak temperature of ∼730 °C for the highest-temperature contact-metamorphic zone. Compositional changes of iron-magnesium silicates and iron oxides correspond to changes in magnetic susceptibility, temper ature dependence of magnetic susceptibility, and AF (alternating field) demagnetization of NRM (natural remanent magnetization) and can be correlated with the metamorphic history of the aureole. Magnetic susceptibility ranges from 0.2 × 10 –3 to 0.5 × 10 –3 SI in the biotite zone and is highly variable in the andalusite-cordierite and transition zones, for which the values range from 0.35 × 10 –3 to 28 × 10 –3 SI. In prograde metamorphism, the assemblage ilmenite + rutile ± hematite transforms to magnetite and ilmenite-hematite solid solutions, producing an increase in magnetic susceptibility that can be correlated with the cordierite-in isograd. In the cordierite–K-feldspar zone and in samples having undergone retrograde metamorphism, prograde magnetite reacts to ilmenite and hematite. This breakdown is associated with decreased magnetic susceptibility (0.2–0.8 × 10 –3 SI) and blurs the magnetite isograd that formed during prograde metamorphism in the outer contact aureole. Retrograde metamorphism in the contact aureole is defined by chlorite composition [ T : 280 and 420 °C, log f (O 2 ):−35 and−28]. Contact-metamorphic aureoles, as long as they are not strongly altered, may contribute significantly to magnetic anomalies around magnetite- series plutons. This fact has to be considered when interpreting aeromagnetic data of granitoid rocks.