Resumo

MARENSSI, Sergio A; SANTILLANA, Sergio N; NET, Laura I  e  RINALDI, Carlos A. Heavy mineral suites as provenance indicators: La Meseta Formation (Eocene), Marambio (Seymour) Island, Antarctic Peninsula. Rev. Asoc. Argent. Sedimentol. [online]. 1998, vol.5, n.1, pp.9-19. ISSN 1853-6360.

Transparent heavy mineral suites from forty two samples of the Eocene La Meseta Formation, Marambio (Seymour) Island, Antarctica, were studied in order to obtain new data of heavy mineral detrital modes and to assess the source rocks and tectonic setting of the provenance area. This unit is the youngest formation exposed in the James Ross Basin. It developed at the rear of an active magmatic arc and holds some 6-7 km of Cretaceous-Paleogene clastic sedimentary rocks. The magmatic arc was related to oblique subduction of the Pacific and associated plates below the Antarctic Plate. Calc-alkaline magmatism migrated to the northwest and by Eocene times the arc would has been located some 150 km to the north-northwest of the Marambio area. The La Meseta Formation represents deltaic, estuarine and shallow marine sediments deposited mostly within a NW-SE incised valley cut down into Late Cretaceous and Paleocene sedimentary rocks.  The 720 meters thick sedimentary column has been subdivided into six allomembers. Heavy mineral suites are dominated by green-brown hornblende, green-colorless augite-diopside?, garnet, opaques and epidote with minor amounts of hyperstene, zircon, apatite, kyanite, staurolite, zoicite, sphene and tourmaline. They were determined to represent those originally present in the parent rocks namely the metasedimentary Trinity Peninsula Group (TPG) and the calc-alkaline igneous (mainly volcanic) Antarctic Peninsula Volcanic Group (APVG) cropping out along the Antarctic Peninsula. Although garnets would also come from reworked Cretaceous and Paleocene rocks, textural evidences were not able to demonstrate it. Moreover, the unconsolidated nature of most of the rocks of the basin precludes the preservation of sedimentary rock fragments in the light mineral fraction. The relative abundances of MF, MT and GM mineral assemblages (Nechaev & Isphording, 1993) has permitted the characterization of the source area as a convergent continental margin. This is in agreement with most other data from the James Ross Basin. On the other hand, published data of QFL detrital modes from La Meseta Formation showed an upward increase in quartz and feldspar content interpreted as an unroofing trend. However, the heavy mineral suites do not record the same increase in basement participation, but they keep a high proportion of minerals belonging to the MF association. Non opaque heavy minerals from the La Meseta Formation have provided new data not recorded through the study of light minerals. In this particular case, they allowed to challenge earlier hypothesis of a simple uroofing trend and to envisage a more complex scenario with a still active volcanism for the Eocene of the northern Antarctic Peninsula.

Palavras-chave : Heavy mineral suites; Provenance; Eocene; Antarctica.

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