PRESENCE OF EXOTIC LAURUSSIAN CRUSTAL BLOCKS WITHIN, AND SOUTH OF, THE RHEIC SUTURE ZONE IN SW IBERIA

The Devonian Pulo do Lobo Zone (PDLZ) in southern Iberia is classically interpreted as a polydeformed accretionary complex developed between the allochthonous South Portuguese Zone (SPZ, a fragment of Laurussia) and the Ossa Morena Zone (OMZ, Gondwanan para-autochthon) during the late Paleozoic closure of the Rheic Ocean, a major event in the development of the Variscan orogen in SW Europe. U/Pb LA-ICPMS analysis of detrital zircons from late Paleozoic clastic units in the SPZ and PDLZ yield contrasting age populations and attest to the exotic nature of both zones relative to the adjacent OMZ.

Late Devonian SPZ samples are dominated by 0.5-0.7 Ga and 1.8-2.3 Ga zircon populations that mimic those found in the Meguma terrane of the Appalachians. In contrast, olistostromal quartzite clasts, quartzite matrix and quartzitic metasedimentary rocks from the PDLZ have an abundance of 1.0-1.5 Ga and subordinate 1.6-1.9 Ga zircon populations and lack the Neoproterozoic and Paleoproterozoic detrital zircons that are typical of either Gondwana, peri-Gondwanan terranes (e.g. Meguma) or the late Devonian continental clastics of the SPZ. The PDLZ data are instead consistent with derivation from Laurentia or recycled from early Silurian deposits, which had a Laurentian and or Baltican source (e.g. the Southern Uplands terrane (SUT) of the British Caledonides).

These data are consistent with lateral tectonic transport of a crustal fragment correlative with SUT between the SPZ (a Meguma terrane correlative) and the Gondwana paraautochthon as a result of a Devonian collision between an Iberian promontory of Gondwana with Laurussia. This collision resulted in the lateral intrusion of a crustal block derived from the Iapetus suture into the Rheic suture zone. Our data constrain the timing and the processes involved in the amalgamation of Pangea in this part of the orogen and demonstrate that accretionary complexes may have long-lived and complex paleogeographic and geologic histories.