The Eocene Big Timber Stock, south-central Montana; development ofextensive compositional variation in an arc-related intrusion byside-wall crystallization and cumulate glomerocryst remixing

Abstract

The Eocene Big Timber stock in the Crazy Mountains of south-centralMontana is an elliptical, 8 by 13 km, compositionally and texturallydiverse composite intrusion with a well-developed radial dike swarm. Asharp intrusive contact separates its two phases: the core of theintrusion is fine-grained quartz monzodiorite, and the volumetricallydominant remainder is composed of medium-grained diorite and gabbro.Differentiation-related major oxide variation within the stock isextensive and spatially nonsystematic. However, abundances of mosttrace elements were not strongly influenced by differentiation; latezircon and apatite fractionation caused moderate heavy and slightlight rare earth element abundance depletions, respectively. Mineralcompositions and assemblages indicate crystallization between nearlyequal 950 and 700 degrees C at a pressure of nearly equal 0.8 kbar (3km). Mixing models indicate that fractionation of varying amounts ofplagioclase, orthopyroxene, clinopyroxene, magnesio-hastingsite,hornblende, biotite, titanite, apatite, and magnetite (the stock'sprincipal constituents, with quartz and potassium feldspar) andremixing of these minerals and residual liquids controlledcompositional evolution in the reservoir. Crystals apparentlynucleated at the reservoir wall while residual silicate liquid wasdisplaced inward and remixed. Some crystals were plucked from thesolidification front, as indicated by glomerocrysts present throughoutthe stock, and also remixed with residual liquid. Solidification ofthe reservoir represented by the stock involved heat loss to enclosingwall rock, side-wall crystallization, and subsequent, variablyeffective, crystal-liquid remixing. This process is an importantvariant of conventionally invoked models pertaining to solidificationof intrusions and explains extensive, relatively nonsystematiccompositional variation. The genesis of compositional evolution inother intrusions characterized by extensive, spatially nonsystematicvariation may result from the important process documented herein.Compositional and geologic relationships are consistent with magmagenesis related to subduction and magmatic-arc processes inboard fromthe western edge of the early Cenozoic North American plate. Arcmagmatism in south-central Montana during Eocene time is consistentwith models pertaining to early Cenozoic southward sweep and westwardretreat of magmatism. Magmatism represented by the Big Timber stockprovides significant new support for steepening subduction, westwardretreat of the subduction hinge line, and development of anasthenospheric mantle wedge that fueled renewed magmatism beneath thewestern edge of the North American continent following early Cenozoicshallow subduction.

69 samples from this reference