Strain gradients and melt pathways, Twin Sisters complex, Washington State

The Twin Sisters complex in the North Cascades of Washington state is a large (~6 by 16 km), virtually unaltered ultramafic body that provides information about the relationships between the formation of compositional layering, structural fabrics and the formation of inferred melt pathways in naturally deforming peridotites. Compositional layering is largely defined by alternating layers of orthopyroxene-absent dunite (>95% olivine) and orthopyroxene-present (~15% orthopyroxene; ~85% olivine) harzburgite aligned parallel to a roughly N-S striking and steeply dipping foliation. Orthopyroxene- and clinopyroxene-bearing dikes occur throughout the Twin Sisters and crosscut the host dunite and harzburgite layering. Orthopyroxene dikes range in thickness from 1 cm to >1 m and are variably oriented and may be folded. Clinopyroxene-bearing dikes are thinner, more consistently oriented (~N-S), and generally more tabular than the orthopyroxene dikes. In the Twin Sisters, cm- to m-scale zones of porphyroclastic dunite cross-cut the main dunite-harzburgite compositional layering and display a variety of relationships with pyroxene dikes in the region. These porphyroclastic dunite bands locally contain single olivine grains >10 cm and likely represent former pathways of melt migration. Transect mapping along an E-W traverse across the Twin Sisters massif reveals that these inferred melt channels form at various angles relative to the main dunite-harzburgite layering. In the west, porphyroclastic olivine layers form at low angle to the main foliation and compositional layering. These zone form at systematically higher angles across the structural section of the Twin Sisters massif to the east and commonly form conjugate cross-cutting sets at high-angle to the main N-S dunite-harzburgite layering. This change in band angle correlates broadly with changes in the intensity of folding of orthopyroxene-bearing dikes, with more intensely deformed dikes in the west to more planar dikes on the eastern side of the massif. Assuming that all of the porphyroclastic dunite bands (inferred melt channels) formed subsequent to the main dunite / harzburgite layering and at initially high angles as seen on the eastern end of the transect, then the trend to lower inter-band angles likely represents a strain gradient from east to west across the massif. Field relations indicate that the dunite bands (melt channels) formed after the prominent harzburgite/dunite layering and dike injection and did not act as shear zones. However, deformation may have partitioned into the porphyroclastic dunite bands or adjacent to their margins, as suggested locally by offset orthopyroxene dikes within them and zones of cataclasis at some of their margins.