U-PB ZIRCON AND MONAZITE RESULTS FROM GRANITE AND CHARNOCKITE FROM THE MAZAGAN ESCARPMENT, OFFSHORE MOROCCO

The Mazagan escarpment forms the edge of the continental shelf in offshore Morocco. U-Pb LA-ICPMS analysis was carried out on zircon and monazite from two samples from the escarpment to help determine the affinity of the block. Data from 30 zircon grains from granite from the DSDP544 core yielded a ~560 Ma crystallization age with abundant inherited zircon cores as old as ~620 Ma and few with dates of ~2.67 Ga, ~1.78 Ga and ~1.20 Ga. A charnockite sampled by submersible ~30 km SSW of DSDP544, has previously yielded ~1.0-0.9 Ga K/Ar dates. Cores and rims of zircon in thin section from the same sample yielded similar ²⁰⁷Pb/²⁰⁶Pb dates between ~1.95 and ~1.72 Ga. The weighted mean of all ²⁰⁷Pb/²⁰⁶Pb dates is 1856±17 Ma (2σ uncertainty; n=34; MSWD = 4.7). Unzoned monazite grains yielded LA-ICPMS ²⁰⁷Pb/²⁰⁶Pb dates between ~1.82 and ~1.64 Ga, and the weighted mean of all ²⁰⁷Pb/²⁰⁶Pb dates is 1752±23 Ma (2σ uncertainty; n=15; MSWD = 4.3).

Rocks from the Mazagan escarpment may have originated from NW Africa, or from the eastern North American margin, if left behind after the breakup of Pangea. Between ~560 Ma and the formation of Pangea, only rifting occurred along the NW African margin. The ~560 Ma granite may have Pan-African, Avalonian or Ganderian origin. The presence of a ~1.20 Ga inherited zircon and the absence of ~2.2-2.0 Ga is not characteristic for northwest Africa, though the scarcity of inherited zircon makes this interpretation ambiguous. The charnockite may have a NW African origin and that the ~1.95 Ga oldest zircon date represents its crystallization age at the end of the Eburnean orogeny. All younger analyses are then a result of metamorphic zircon growth or Pb loss, perhaps as a result of widespread mafic magmatism that occurred between ~1.95 and ~1.64 Ga. The charnockite is unlikely to be part of Laurentian Paleoproterozoic basement, because such basement is not known to contain ~560 Ma granite. However, it may represent a fragment of Ganderian or Avalonian basement, even though such basement is not exposed in the Appalachians. Nd whole-rock model ages and inherited zircon of ~1.85 Ga are common for Ganderia, and less common for Avalonia. If rocks of the Mazagan escarpment have Ganderian or Avalonian origin, then the Pangean suture zone lies on the continental shelf southeast of the Mazagan escarpment.