Cooling history of the Bay of Islands Complex sub-ophiolitic metamorphic sole constrained by new mica 40Ar/39Ar thermochronology: Tectonic implications and comparison with the Semail sole

Abstract

Metamorphic soles, including the Bay of Islands Complex (BOIC) sole in Western Newfoundland, are commonly observed welded to the base of large, obducted, flat-slab ophiolites throughout the world. The soles are characterized by similar inverted metamorphic field gradients with decreasing pressure-temperature (P-T) conditions recorded structurally-downward. Where studied in detail, they show that initial subcretion of near-peak P-T upper sections of the metamorphic soles is close in age to the formation of overlying ophiolite and that HT-MP portions of the soles cool from ∼800–900 °C to ∼300℃ within several million years (Myr). Previously the BOIC metamorphic sole has been interpreted to be generated near the age of ophiolite obduction. Recently, however, this interpretation has been challenged based on near peak-temperature U-Pb zircon age results of upper sole amphibolites. Here we present the first lower temperature ⁴⁰Ar/³⁹Ar biotite and muscovite cooling ages (2σ) of 479.8 ± 2.1 (± 7.2) Ma and 478.7 ± 2.2 (± 7.4) Ma, respectively, for a biotite-garnet schist within amphibolite facies portions of the BOIC HT sole. These ages, combined with recent peak-temperature U-Pb dating results of zircon (ca. 489 Ma) and titanite also in HT amphibolites, indicate an initial apparent fast-cooling rate of ∼65–78℃/Myr followed by gradually deceasing rates, which, although slightly different, are comparable to other metamorphic soles, such as the extensively studied Semail ophiolite sole. The results show that the BOIC metamorphic sole cooled from initial HT subcretion (i.e., underplating) at ∼820℃ to ∼335℃ within ∼10 Myr (ca. 489–479 Ma). The subcretion of slab material and relatively rapid subsequent cooling of the BOIC HT-MP to LT-LP sole dominantly occurred during peri‑Laurentian Iapetus intra-oceanic subduction and well prior to 1) the initiation of ophiolite obduction at ca. 470 Ma, 2) the age of LT-HP eclogite formation in the subducted Laurentian continental crust at ca. 464 Ma, and 3) final ophiolitic allochthon emplacement by ca. 460–458 Ma. Because previously existing and self-sustaining subduction beneath the central Newfoundland Notre Dame arc was ongoing prior to the synchronous formation of the BOIC forearc ophiolite and sole, no new, large-scale, spontaneous subduction initiation event is required. A simpler ridge-trench-trench triple-junction with upper plate high-angle forearc spreading centers led to incremental induced subduction initiation beneath the thin BOIC forearc lithosphere as the ophiolite and HT sole formed near-synchronously. Lower temperature sole underplating occurred during progressive exhumation of the slab interface by slab flattening. This scenario is consistent with modern ophiolitic-type forearc analogs characterized by near-trench, high-angle forearc spreading centers with steeper initial slab interface dips and consequent near-trench forearc basalt and boninite magmatism lasting ∼10–15 Myr.