AGE AND ORIGIN OF THE BOIL MOUNTAIN OPHIOLITE AND CHAIN LAKES MASSIF,MAINE - IMPLICATIONS FOR THE PENOBSCOTTIAN OROGENY

The Boil Mountain ophiolite complex of west-central Maine is widely in terpreted to mark the Lower Ordovician Penobscottian suture between th e Dunnage, Chain Lakes, and Gander terranes. The ophiolite consists of two distinct volcanic groups, including a lower island-are tholeiite sequence and an upper mid-ocean-ridge basalt sequence. A new Middle Or dovician 477 +/- 1 Ma U-Pb age on a tonalite sill that intrudes the lo wer volcanic-gabbroic sequence is younger than other ca. 500 Ma age co nstraints for the ophiolite and represents a maximum age for the ophio lite prior to final emplacement over gneissic rocks of the Chain Lakes massif. A comparison of ages and paleogeography of the Boil Mountain ophiolite with ophiolitic sequences in Quebec and Newfoundland indicat es that the Taconian and Penobscottian orogenies and ophiolite obducti on occurred simultaneously, although on different margins of the Iapet us Ocean. The Taconian ophiolite sequences were obducted onto the Appa lachian margin of Laurentia during its collision with the Notre Dame - Bronson Hill belt in the Middle Ordovician, whereas the Boil Mountain ophiolite was obducted onto the Gander margin of Gondwana during its collision with the Exploits subzone - Penobscot are of the Dunnage ter rane in the Lower-Middle Ordovician. We suggest that the lower volcani c-gabbroic sequence of the Boil Mountain ophiolite represents the fore -are ophiolitic basement to the Penobscot are. Middle Ordovician rifti ng of the Penobscottian orogenic collage on the Gander margin formed a new volcanic sequence (Popelogan are) in front of a growing back-are basin, and erupted the upper tholeiitic sequence of the Boil Mountain ophiolite in a back-are-basin setting. The tonalite sill formed during this event by partial melting of the lower volcanic-gabbroic sequence . Spreading in this back-are basin (Tetagouche basin) brought a fragme nt of the Gander margin (Chain Lakes massif), along with an allochthon ous ophiolitic cover (Boil Mountain complex) across Iapetus, where it collided with the Taconic modified margin of North America in the Late Ordovician and was then intruded by the Ashgillian Attean pluton.