Determining Relative Magma-Xenolith Rheology during Magmatic Fabric Formation in Plutons: Examples from the Middle and Upper Crust

Field observations, structural analysis and analytical calculations are utilized to evaluate the magma rheology during crystallization in a regional strain field. Two granodioritic plutons are examined, the sub-volcanic 98 Ma. Jackass Lakes pluton (JLP), Sierra Nevada, CA, and the voluminous middle crustal 442 Ma. Andalshatten pluton (AP), Norway. The AP example contains mm- to km-scale screens and xenoliths that display evidence for synmagmatic deformation after being isolated in the magma including fold reactivation and boudinage. Fabrics adjacent to the xenoliths are usually magmatic, with local, discontinuous zones of crystal-plastic deformation < 1 m from the xenolith contact. Well exposed mafic metavolcanic xenoliths in the JLP were strained prior to incorporation and then separated from the remaining host rock by brittle cracking. Once isolated, some of these xenoliths were intruded by veins fed by the in situ draining of melt from the surrounding crystal mush. The xenoliths then deformed ductiley at fast strain rates during which veins in the xenoliths were boudinaged or folded at the same time that a magmatic fabric formed in the surrounding JLP.

The behavior of these xenoliths/magmas suggests that late in the crystallization history, magmas in both settings behaved as a high strength crystal mush capable of transmitting deviatoric stresses, which drove both elastic and plastic deformation in the enclosed xenoliths. Simultaneously, intercrystalline melt/magma was drained from the host magma into the xenoliths.

Modeling based on geochemical data yields an effective viscosity of ~10⁴ Pa s and increased to ~10⁷ Pa s as cooling proceeded to 758 °C and crystal content approached 40% - viscosities too low to transmit strain into the xenoliths. Xenolith preservation is compatible with higher crystallinities and/or magma yield strengths. Estimated effective viscosities considering magma yield strength and measured density variables (melt and solid) are in the range of 10¹³ Pa s.