TEMPORAL RECORD OF MAGMATISM DURING THE RIFT TO DRIFT TRANSITION ALONG THE NEWFOUNDLAND-IBERIA CONJUGATE MARGINS

The absence of significant volumes of melts during the rift to drift transition in magma poor rifts remains an enigma. Two hypotheses have been put forward to explain this absence: (1) magma is produced during extension but is dominantly emplaced as plutons in a thick lithospheric lid with only minor volumes erupted at the surface, or (2) magma is not produced because of low mantle temperature and/or fertility.

We present eight high-precision U-Pb zircon CA-IDTIMS dates from two drill sites (ODP Leg 173 Site 1070 and Leg 210 Site 1277) that sampled gabbro within the exhumed mantle that composes the ocean continent transition (OCT) along the Newfoundland-Iberia conjugate margins. Two dates from site 1070 on the Iberia margin are ca. 124 Ma and are in good agreement with the proposed magnetic polarity chron (M3-M1: 124.58-121.54 Ma) during which this crust was emplaced, while six dates from site 1277 on the Newfoundland margin are ca. 115 Ma and are significantly younger than the date of crustal accretion (M1-M0: 123.51-120.95 Ma). This data supports a complex magmatic history for the OCT that includes both focused magmatism during mantle exhumation as well as younger ‘off-axis’ magmatism. The focused magmatism may contribute to the symmetric, albeit weak, magnetic anomalies along these margins, suggesting that more plutonic rocks are present at depth. If the OCT is composed of exhumed subcontinental lithospheric mantle, then it would have both suppressed melt production through its low fertility and acted as a rigid lid through which melts could not easily traverse, thereby explaining the absence of significant volcanics during its formation. The ‘off-axis’ magmatism at site 1277 on the Newfoundland margin is more puzzling. It is similar in age to a basin-wide seismic reflector composed of mass wasting deposits recently interpreted to represent final separation of subcontinental lithospheric mantle. We suggest that the pulse of ‘off-axis’ magmatism recorded at site 1277 is related to this tectonic event.