METAMORPHOSED ORDOVICIAN IRON-RICH ROCKS IN SOUTH-CENTRAL MAINE: FROM PERI-GONDWANAN SEDIMENTATION THROUGH ACADIAN METAMORPHISM

The Wilson Cove Member of the Cushing Formation is a thin (up to 120 meters thick) but distinctive metamorphosed Fe-rich unit exposed discontinuously over a distance of > 100 km in southern Maine.  In south-central Maine the unit shows considerable lithologic variability but is dominated by dark gray to black, sulfide-rich, garnet-grunerite gneiss and granofels.  This study employs a multidisciplinary approach to unraveling the history of this unit, from original deposition through amphibolite facies metamorphism.

Cathodoluminescence imaging of zircon grains in the unit reveal distinct low U/Th detrital cores with high U/Th overgrowths.  U-Pb (SHRIMP)core ages are variable, ranging from 463 to 2058 Ma, with a strong peak in the late Neoproterozoic-Early Cambrian.  The distribution of the core ages is consistent with a Gondwanan source region and a Late Ordovician depositional age.  Zircon rims have a uniform ²⁰⁶Pb/²³⁸U age of 373±4 Ma, consistent with growth during late Acadian metamorphism.

Whole rock geochemistry reveals the rocks are metamorphosed ironstones with total iron contents ranging from 23 to 43 wt.% in 11 of the 12 samples analyzed.  In addition, many samples contain elevated abundances of MnO (up to 11 wt.%), P₂O₅ (up to 3 wt.%), Ba (up to 2400 ppm), and heavy metals (e.g., As, Cr, Co, Cu, Ni, Zn).  Rare earth element patterns are nearly flat with enrichments of 0.6 to 2.0 relative to NASC and LREE enriched relative to chondrites.  The geochemistry suggests the protoliths were mixtures of chemical precipitates (hydrothermal exhalatives) and terrigenous sediment deposited in a marine environment.

The most common mineral assemblages in the meta-ironstones are grunerite + garnet ± quartz ± biotite ± hornblende and are consistent with relatively low pressure (< 4.0 kbar), high temperature (~ 600^oC) metamorphism.  Highly manganiferous rocks contain the assemblage grunerite + fayalite + garnet and all three phases contain considerable MnO (garnet = 1.19 pfu, fayalite = 0.38 pfu, grunerite = 0.72 pfu).  It seems likely that the addition of Mn has shifted the stability of the garnet+olivine reaction to lower pressures and temperatures.  Finally, arsenic-bearing minerals arsenopyrite, cobaltite, and loellingite have been identified in many samples and indicate the unit is a naturally occurring source of arsenic in the region.