Role of melt in the formation of a deep‐crustal compressive shear zone: The MaClaren Glacier Metamorphic Belt, south central Alaska

Abstract

The Maclaren Glacier metamorphic belt is an exhumed portion of a deep‐crustal shear zone where hot, upper amphibolite facies rocks were emplaced over cooler, lower grade rocks. It is located in south central Alaska within the collision zone between terranes previously accreted to North America and the Wrangellia superterrane. Crosscutting relationships and orientations of thin granitoid sills within the hanging wall show that melt was repeatedly intruded into the shear zone during overall compression. In addition, a 1 km thick tonalite sill (the Valdez Creek tonalite) was emplaced into the shear zone while it was active; this conclusion is based on the presence of a well‐developed foliation within the sill which is concordant to the fabric of the shear zone, the alignment and tiling of plagioclase laths, the presence of highly strained mafic enclaves within a matrix which shows no evidence of any subsolidus deformation, and the preservation of a delicate magmatic texture which indicates that a magmatic fluid was still present after deformation in the sill had ceased. A one‐dimensional thermal model for sill‐shaped plutons of varying thicknesses, which are emplaced into country rocks at different initial temperatures, indicates that melt can be present for long periods of time in the deep crust (>1 m.y.). An important factor controlling the length of time that a sill remains molten is the temperature of the surrounding rocks which, in the lower crust, can be as high as the solidus temperature of granitoid melts. If the amount of time for melt to crystallize is sufficiently long, potentially large amounts of strain will be accommodated by zones containing melt due to the low strength of melt compared to rock. In the Maclaren Glacier metamorphic belt, the amount of time needed for the Valdez Creek tonalite to fully crystallize is calculated to be about 90,000 years. This is a minimum value since the original thickness of the sill is not known. Nevertheless, using the 90,000 year value and assuming that the bulk of convergence between North America and Wrangellia was concentrated within the Valdez Creek tonalite, a displacement of at least 10 km could have been accommodated across the sill while melt was present.