IMPLICATIONS OF BONINITES FOR CAMBRO-ORDOVICIAN ARC DEVELOPMENT IN THE NE APPALACHIANS

Boninites are high Mg andesites that form in suprasubduction zone tectonic environments as primary melts from refractory mantle. Boninites can constrain tectonic models for ancient terranes because “modern” boninites have only been recognized in unequivocal forearc settings. Tectonic models for “modern” boninite genesis include subduction initiation (“infant” arc), forearc spreading, and the forearc side of intra-arc rifting. These models can be differentiated by the relative age of the boninites and to a lesser degree, geochemistry.

The distinctive geochemistry of boninites promotes their recognition in ancient terranes. Numerous mafic terranes in the northeastern Appalachians contain boninites; these terranes were situated on both sides of Iapetus. The characteristics of these boninites can be used to constrain tectonic models of the evolution of the northeastern Appalachians.

On the Laurentian side of Iapetus, infant arc boninites apparently were not produced ubiquitously during the Cambrian subduction initiation. Only boninites from the >496 Ma Hawley Formation and the >488 Ma Betts Cove Ophiolite could be associated with Late Cambrian subduction initiation. The possibility that the Cambrian subduction initiation on the Laurentian side was characterized by both “infant arc” boninitic arc construction (perhaps Hawley) and “normal” arc construction (Mt. Orford) is consistent with the suggestion that the pre-collisional geometry of the Laurentian margin was complex. The Bay of Islands Complex and Thetford Mines ophiolite boninites are likely associated with forearc/intra-arc spreading during the protracted evolution of the Cambrian arc system. The relatively young boninites in the Bronson Hill Arc suggest that the Taconic continuous eastward subduction tectonic model is less tenable than other models.

On the Gondwana side of Iapetus, the Tea Arm boninites of the Exploits Group stratigraphically rest on arc and MORB volcanics. This stratigraphy, and the relatively young age of the boninites (486 Ma), compared to assumed subduction initiation age, suggest that the boninites may be more consistent with forearc spreading/intra-arc spreading; however, an “infant arc” model cannot be dismissed.