U-PB DETRITAL ZIRCON DATA FROM THE LATE PALEOZOIC PULO DO LOBO AND SOUTH PORTUGUESE ZONES, SOUTHERN IBERIA: REMNANTS OF AN IAPETAN ACCRETIONARY COMPLEX ALONG A RHEIC SUTURE?

The Devonian Pulo do Lobo Zone (PDLZ), which crops out immediately north of the allochthonous South Portuguese Zone (SPZ) in Southern Iberia, is classically interpreted as a polydeformed accretionary complex developed along the southern margin of the Gondwanan para-autochthon, during the late Paleozoic closure of the Rheic Ocean. This closure was a major event during the development of the Variscan orogen and the amalgamation of Pangea. U/Pb laser ablation inductively coupled mass spectrometry 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.

A sample from the late Devonian continental clastic strata of the SPZ yields detrital zircon ages dominated by Neoproterozoic (ca. 0.5-0.7 Ga) and Paleoproterozoic (1.8-2.3 Ga) populations. In contrast, olistostromal quartzite clasts and quartzite matrix from the PDLZ both have an abundance of Mesoproterozoic zircons (ca. 1.0-1.5 Ga), and a subordinate Paleoproterozoic (ca. 1.6-1.9 Ga) population. PDLZ samples also lack the Neoproterozoic (ca. 0.6-0.9 Ga) and Paleoproterozoic (ca. 2.0-2.5 Ga) detrital zircons that are typical of late Paleozoic sedimentary rocks derived from Gondwana (upper plate), peri-Gondwanan terranes or the late Devonian continental clastics (PQ) of the SPZ (lower plate).

The polydeformed PDLZ metasediments are instead consistent with derivation from Baltica, Laurentia or recycled early Silurian deposits along the Laurentian margin which themselves were derived sediment from either a Baltica or Laurentia source. An example of one such deposit that could yield the appropriate zircon populations is the Southern Uplands terrane (SUT) of the British Caledonides. Taken together, these data can be reconciled by a model involving tectonic transport of a crustal fragment that was laterally equivalent to the SUT between the allochthonous SPZ and the Gondwana paraautochthon as a result of an early Devonian collision between an Iberian indenter (Gondwana) with Laurussia. Our data provide important insights into the timing of terminal collision between Gondwana and Laurussia in the late Paleozoic and demonstrate that, in some cases, apparent accretionary complexes may have long-lived and complex paleogeographic and geologic histories.