ORIGIN AND TECTONIC SIGNIFICANCE OF UNUSUAL FE-MN RICH ROCKS, EASTERN MAINE

Recent mapping of the Miramichi terrane in eastern and east-central Maine reveals a unique sequence of Ordovician rocks enriched in Fe and Mn far beyond levels of associated metavolcanic and metasedimentary strata. Field and petrographic examination suggest a fine grained clastic sedimentary origin but whole-rock geochemistry and Energy Dispersive X-ray analyses reveal extremely depleted alkalis and elevated levels of Fe, Mn, and Ba, along with significant trace amounts of Cu, Pb, Zn. Association with arc-related Ordovician volcanic rocks and the unusual chemistry suggests a complex paragenesis involving hydrothermal precipitation and extensive alteration of volcaniclastic rocks, possibly in a back-arc basin. Small opaque spherules typically concentrated in dark brown nodules prove to be composite Fe and Mn oxides and hydroxides, including specular hematite. Translucent spherules from white layers and laminae are zoned ankerite-siderite assemblages.

The only similar rocks in the region are Miramichi ironstones in the Bathurst mining district of northern New Brunswick and the Wilson Cove Formation of the Cushing Formation (Wilson Cove Member) in coastal Maine. Nearly identical chemistries from these areas suggest that these rocks are correlative. The Wilson Cove rocks lie across the Norumbega fault system from the Fe-Mn rocks reported here, supporting conclusions that this fault system does not separate exotic blocks.