47 Tectonic Evolution of Eastern North America: Insights from Geology and Geophysics (Posters)

Tuesday, 22 March 2022
8:00 AM - 12:00 PM
Freedom Hall A

THE BALTIMORE MAFIC COMPLEX: KEY RECORD OF EARLIEST CONVERGENCE IN THE CENTRAL APPALACHIANS

Paper number: 47-8
Program: Theme Sessions
Time: 8:00 AM – 12:00 PM

Authors

Abstract

The Baltimore Mafic Complex (BMC) crops out over a ~25 km x 50 km area of western Baltimore City and adjoining counties. Rocks of the BMC define an incomplete ophiolite pseudostratigraphy that includes lithospheric mantle (Hollofield Ultramafite), a gabbro section (Mt Washington Amphibolite) and a volcanic–sedimentary succession (James Run Formation).

Mafic units of the BMC have whole-rock major, trace and rare Earth element (REE) chemistry (n = 35) that match three magmatic associations expected for a suprasubduction zone (SSZ) ophiolite: fore-arc (mid-ocean ridge-like) basalts; boninites/low-Ti basalts, and calc-alkaline basalts. Up section through the James Run Formation (from the Druid Hill Amphibolite to amphibolite lenses within the Carroll Gneiss), mafic volcanic units transition from high-Ti/low-Mg to low-Ti/high-Mg/REE-depleted in character. This volcanic chemostratigraphy is interpreted to reflect geodynamic evolution from seafloor (fore-arc?) spreading to subduction (and associated magmatism). The BMC appears to record Cambrian–Ordovician subduction initiation within the Iapetus Ocean.

In order to constrain the timing of earliest convergence in the central sector of the Appalachians, units of the BMC with differing chemistry (and interpreted geodynamic origin) have been targeted for zircon U–Pb dating. Dates for the Carroll Gneiss, Cold Spring Gneiss, Druid Hill Amphibolite and Relay Felsite, obtained by laser ablation inductively-coupled-plasma mass spectrometry (LA–ICP–MS), are in the range c. 490–460 Ma. The youngest of these (c. 460 Ma) is interpreted to date high-temperature metasomatism of basalt (metamorphic zircon after baddeleyite). The ~20 Ma (~4%, 2SE) uncertainty on the individual zircon U–Pb dates obtained by LA–ICP–MS does not allow exact timing of putative subduction initiation to be pinpointed within the c. 490–470 Ma window of potential igneous activity. Efforts to obtain more precise dates (by thermal ionization mass spectrometry) are underway.

Geological Society of America Abstracts with Programs. Vol. 54, No. 3

doi: 10.1130/abs/2022NE-375242

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