Banca de DEFESA: SALVIANO PEREIRA DA SILVA

Uma banca de DEFESA de MESTRADO foi cadastrada pelo programa.
STUDENT : SALVIANO PEREIRA DA SILVA
DATE: 05/12/2022
TIME: 14:00
LOCAL: ambiente virtual (remotamente)
TITLE:
KEY WORDS:

Eclogite; blueschist; oceanic plateau; rollback; slab breakoff

PAGES: 153
BIG AREA: Ciências Exatas e da Terra
AREA: Geociências
SUMMARY:

Most of what we know about oceanic plateaus comes from fragments accreted onto active margins. Although, numeric models suggest that under certain circumstances they may subduct, reports of eclogites or blueschists which protolith was originated from oceanic plateaus are lacking. Here are presented geochemical, geochronological, and metamorphic constraints of blueschists and eclogites from a high-pressure (HP) ophiolite that evidences their oceanic plateau origin debating the metamorphic signature imprinted by the subduction of oceanic plateaus.

The study focused on the Raspas Metamorphic Complex, SW of Ecuador. It is made up by eclogites, metaultramafic, and blueschists with minor greenschists and garnet amphibolites that record the onset of an important HP event on Early Cretaceous. They exhibit a N- to E-MORB signature with trace element ratios pointing to a heterogeneous mantle source (Zr/Nb – 7.69-112.66). Moreover, they show evidences of a change on the source depth (Sm/Yb ­ 0.94-1.95) coupled by an increase of a depleted component (La/Yb ­ 0.76-6.86), typical of an ascending mantle plume.

The HP event had onset registered by the formation of the assemblage winchite + paragonite/phengite + epidote/clinozoisite + quartz ± chlorite at 490 °C/1.15 GPa on the blueschists. Followed by an isothermal compression during the collision of the plateau with the Andean margin at 523 °C/1.68 GPa observed by the growth of garnet cores and by the eclogitic assemblage omphacite + katophorite + clinozoisite + garnet + quartz ± phengite between 598-661 °C/1.43-1.63 GPa. Due their greater thickness the portion at the center took longer to be fully heated creating a thermal zoning inside the plateau, translated in the temperature difference between eclogites and blueschists. Moreover, the eclogitization of the plateau increase the density favoring slab-pull forces. In this scenario a rollback took place which coupled by an astenospheric flow thinned the plateau. Which is evidenced by an isobaric heating during the formation of atoll garnets on eclogites at 709 °C/1.61 GPa and by the small decompression at the metamorphic peak garnet_rim + phengite + clinozoisite + wihchite + omphacite + quartz at 570-590 °C/1.4-1.7 GPa on the blueschists.

Eclogitic phengite Ar/Ar ages of 129 Ma indicate fast exhumation rates (1.25-0.5 cm/yr) soon after the HP event occurred at 133 Ma. During exhumation the metamorphic path exhibit an isothermal decompression during the formation of pargasite + Mg-hornblende + epidote + albite + chlorite at 450-550 °C and < 0.6 GPa during a slab breakoff.

COMMITTEE MEMBERS:
Externo à Instituição - RAFAEL GONÇALVES DA MOTTA - UFRN
Presidente - 2363175 - CARLA JOANA SANTOS BARRETO
Interno - 2983231 - LAURO CEZAR MONTEFALCO DE LIRA SANTOS