An integrated thermochronology, organic maturation and provenance study in the South Portuguese Zone and Algarve Basin (South Portugal)

摘要

Organic maturation levels (vitrinite reflection) were combined with low-temperaturethermochronometers (apatite fission track analysis and (U-Th)/He in apatite) to constructtemperature-time paths for the Upper Palaeozoic rocks of the South Portuguese Zone and theMesozoic rocks of the Algarve Basin. Detrital zircon ages were also studied in order toconstrain the sedimentary source regions and basin fill history of the Baixo Alentejo FlyschGroup from South Portuguese Zone and the Upper Triassic sandstones from the AlgarveBasin. The maximum temperature attained in the South Portuguese Zone was 326 ºC andoccurred at 320 Ma. All the samples from the South Portuguese Zone have been totally resetafter deposition and yield apatite fission track ages ranging from the Cretaceous (111 to 59.9Ma) to the Upper Jurassic (157.9 Ma). The fission track ages from the Upper Triassicsandstone did not reset after de deposition, however, the intrusion of the Monchique AlkalineComplex (MAC) and related swarm of dikes had local influence upon organic maturation.Inverse modelling on apatite samples from the MAC indicate that it cooled very fast frominitial magmatic temperatures to around 70ºC. The rock sample studied stayed at a constanttemperature close to the top of the apatite partial annealing zone from 68.7 Ma to 5 Ma. Since5 Ma until the present the MAC underwent an exhumation of ca. 300 m/Ma.The U-Pb detrital zircon and apatite ages from the Mértola and Mira formations suggestsedimentary provenance areas from a detrital intrabasinal source (South Portuguese Zone) andfrom areas associated with the magmatic arc related rocks located at the southern border ofthe Ossa Morena Zone. The U-Pb detrital zircon ages obtained for the Brejeira Formationsuggest a source area related to the Avalonian/Meguma terranes and also from recycledclastic sediments associated to the South Portuguese Domain. The ages from the AlgarveBasin Upper Triassic sandstones suggest that they are derived from multiple sources terraneswith Permian zircons ages more abundant in the samples located in the eastern part of thebasin.