The Pomarinho enclave swarm: structural and petrological evidence of repeated quenching, splitting and magmatic erosion at the sidewalls of conduits (Iberian Variscan belt)

Magmatic microgranular enclaves of variable size and texture commonly occur as swarms within I-type batholiths. These enclave swarms provide a valuable opportunity to test competing hypotheses about enclave origin, as enclaves with variable textural features, mainly grain size, are spatially associated with new magmatic pulses related to a common pluton. In this study, the Pomarinho enclave swarm hosted within an I-type (328 ± 2.4 Ma) granodiorite of the Évora gneiss dome has been investigated. The Pomarinho enclave swarm is particularly well suited for this purpose, as quarrying has exposed the swarm in three dimensions, creating a natural laboratory to better understand relationships among enclaves and host granite, as well as among different enclave types, including single and double enclaves. Based mainly on field evidence but also using petrographic, mineralogical, geochemical, and isotopic data, it was possible to infer an origin of enclaves. Field, petrographic, and geochemical relations indicate that enclaves most likely derived from quenched magma conduit sidewalls rather than from fragmented synplutonic magma injections. We propose that shear induced by magma ascent physically eroded the sidewalls of the quenched conduit during during magma recharge, thereby facilitating the formation, transport, and rounding of enclaves.