Upper Cambrian to Upper Ordovician peri-Gondwanan Island arc activity in the Victoria Lake Supergroup, Central Newfoundland: Tectonic development of the northern Ganderian margin
Аннотация:The Exploits Subzone of the Newfoundland Appalachians comprises remnants of Cambro-Ordovician peri-Gondwanan arc and back-arc complexes that formed within the Iapetus Ocean. The Exploits Subzone experienced at least two accretionary events as a result of the rapid closure of the main portion of the Iapetus tract: the Penobscot orogeny (c. 480 Ma), which juxtaposed the Penobscot Arc (c. 513 --486 Ma) with the Gander margin, and c. 450 Ma collision of the Victoria Arc (c. 473 --454 Ma) with the Annieopsquotch Accretionary Tract that juxtaposed the peri-Laurentian and peri-Gondwanan elements along the Red Indian Line. The newly recognized Pats Pond Group forms a temporal equivalent to other Lower Ordovician intra-oceanic complexes of the Penobscot Arc. The Pats Pond Group (c. 487 Ma) has a geochemical stratigraphy that is consistent with rifting of a volcanic arc. An ensialic setting is indicated by low εNd values (εNd 0.3 to -0.5) near the stratigraphic base and its abundant zircon inheritance (c. 560 Ma and 0.9 --1.2 Ga). The spatial distribution of Tremadocian arc --back-arc complexes indicates that the Penobscot arc is best explained in terms of a single east-dipping subduction zone. This model is favored over west dipping models, in that it explains the distribution of the Penobscot arc elements, continental arc magmatism, and the obduction of back-arc Penobscot ophiolites without requiring subduction of the Gander margin or subduction reversal. The newly recognized Wigwam Brook Group (c. 454 Ma) disconformably overlies the Pats Pond Group and records the youngest known phase of ensialic arc volcanism (εNd --4.1) in the Victoria Arc, which is also related to east-dipping subduction. Thus the Penobscot and the overlying Victoria Arc are reinterpreted in terms of a single, relatively long-lived east-dipping subduction zone beneath the peri-Gondwanan microcontinent of Ganderia. The cessation of arc volcanism towards the top of the Wigwam Brook Group and the subsequent syn-tectonic sedimentation in the Badger Group constrain the arrival of the leading edge of Ganderia with the ensialic arc complexes to the Laurentian margin to c. 454 Ma.
