38 Peri-Gondwanan Terranes of the Appalachians I

Friday, 26 March 2004
8:00 AM - 12:00 PM

USING EBSD ANALYSIS OF OMPHACITE TO EVALUATE ECLOGITE EXHUMATION MECHANISMS IN THE SOUTHERN APPALACHIANS

Paper number: 38-5
Program: THEME SESSIONS
Time: 9:10 AM – 9:30 AM

Authors

Abstract

Eclogite blocks appear to rise from depths of greater than 50 km in one of two ways. Mafic eclogite blocks that are part of regional eclogite facies terranes are incorporated into slabs of less-dense felsic rocks and make their way to shallower levels encased in the rising coherent sheets. Eclogite blocks in mélange terranes (e.g. Franciscan Complex) are carried upward as they are entrained in the flowing mud-matrix of the subduction complex. The eclogite in the western Blue Ridge of North Carolina share similarities with both of these types of eclogite. While the blocks’ large sizes suggest they may be part of an extensive eclogite facies terrane, the fact that they are surrounded by a metamorphosed subduction complex suggests they are more Franciscan-like. Franciscan-type exhumation, however, is restricted to eclogite bodies smaller than 25 m in diameter. The question we are left with is: Are the Blue Ridge eclogites part of a regional eclogite facies terrane? Or are they part of a Franciscan-type mélange, in which case the mechanism by which sizable eclogite bodies are exhumed within subduction zones needs to be reevaluated.

The Blue Ridge eclogite bodies consist of about seven discrete blocks ranging in size from 200 to 1000 m across and over twelve smaller blocks. They outcrop within an area of about 6 km2. Omphacite in the eclogite bodies has a pronounced c-axis lattice preferred orientation (LPO). Because omphacite is an eclogite-facies phase, we can use this fabric as a marker of eclogite-facies deformation. Using EBSD, we have measured omphacite LPO from five blocks. If the blocks were part of a coherent eclogite-facies terrane we would expect the c-axis orientations to be similar. If the blocks were exhumed in a mélange we would expect the blocks to have highly scattered c-axis orientations. Preliminary data show similar c-axis LPO among three of the larger eclogite bodies. This pattern suggests these large bodies are part of larger eclogite facies terrane. Although no eclogite facies minerals have been discovered in the rocks surrounding the eclogite bodies these phases may have been obliterated by later amphibolite facies metamorphism.