THE CATOCTIN FORMATION: NEW GEOCHEMICAL CONSTRAINTS FOR THE BLUE RIDGE ANTICLINORIUM OF VIRGINIA, MARYLAND AND PENNSYLVANIA

The metamorphosed Neoproterozoic-age Catoctin Fm. crops out in the Blue Ridge anticlinorium (BRa), a macroscale, overturned, and shallowly NE-plunging fold, that extends from southern Pennsylvania to southern Virginia. Deformation of the Catoctin Fm., and construction of the BRa, is a consequence of the late Paleozoic Alleghanian orogeny.

Published petrographic and geochemical studies of the Catoctin Fm. have focused solely on the western limb of the BRa. These data have been used in previous interpretations of the Catoctin Fm. to describe the entire geographic extent, but no comparable results have been obtained from either the hinge or eastern regions of the BRa. This study provides new metamorphic mineral assemblage and whole-rock geochemical data from Catoctin Fm. samples collected from the western, eastern, and hinge regions for the BRa to compare with published western limb data. These data are used to evaluate whether the western limb geochemical signature is different from the hinge and eastern limb regions, and to evaluate if any discernable differences in metamorphic grade could be evaluated throughout the BRa.

Catoctin Fm. metabasaltic rocks mineral assemblage is: albite + chlorite + epidote + actinolite + opaque minerals ± calcite ± quartz. A majority of the metabasaltic rock samples contain relict porphyritic or amygdaloidal textures. Intercalated lavender phyllites are very fine grained and consist mainly of white mica + quartz + opaque minerals. Based upon mineral assemblage, these samples fall within the greenschist facies and, although actinolite size coarsens toward the hinge region, there is no evidence for amphibolite facies conditions.

Thirty-five (35) samples were analyzed for major, trace and rare-earth elements (REE). The metabasaltic rock data plot in the “MORB within-plate tholeiites” field on various discriminatory diagrams. LREE enrichment (20-100X), both positive and negative Eu-anomalies and flat HREE (10-40X) characterize the metabasaltic rocks, but the phyllites are more evolved (LREE ~200-400X). In general, trace and rare earth elements are enriched compared to chondrite, N-MORB, and primordial mantle, and depleted compared to upper continental crust. Overall, the western limb rocks are more enriched than the eastern limb and least enriched in the hinge region of the BRa.