My students and I study the metamorphic products of subduction: rocks called eclogite and blueschist. These rocks begin their lives as basalt the igneous rock that makes up oceanic crust....think Hawaiian volcanoes) and are then subducted, or pulled under another tectonic plate. Although most of this material plunges deep into the mantle, a small volume of eclogite and related rocks is sometimes exhumed along the top of the subducting slab. When these rocks reach the surface, they become the only material that has actually been subducted but is now available for study. Because of this, subduction metamorphic rocks offer a unique window into subduction processes, and their study is one of the most exciting aspects of metamorphic petrology today.
The Junction School eclogite is located near Healdsburg, California. Garnets from this rock contain inclusions that document prograde metamorphism from blueschist to eclogite. This steep, cool prograde P-T path is in contrast to tectonic theories that suggest all high-grade blocks formed early in the history of this subduction zone with relatively warm prograde histories.
In cm-scale zones in this rock nearly euhedral garnets (Fig. 1) preserve a dramatic record of resorption and regrowth. Inclusions in the garnet rims record a return to blueschist-facies conditions.
A paper on the thermobarometry of this rock was published in Contributions to Mineralogy and Petrology.
Fig. 1 False color back-scattered electron (BSE) image of a garnet from the Junction School eclogite. Color warmth corresponds to average atomic number (Z). The central inclusion-rich (black spots) zone of the garnet records prograde zoning primarily in Mn yielding decreasing Z (red to yellow to green). This prograde garnet was resorbed during a thermal or fluid event and neoformed blueschist-facies garnet filled in the resorbed regions, oscillating in its Mn, Mg, Fe and Ca compositon (blue to yellow/green and back to blue/green).