Challenges of dating metasedimentary successions in collisional orogens: A case study of a Neoproterozoic passive margin in West Gondwana

Abstract

Metasediments in collisional orogens critically record tectonic conditions during sediment formation and the thermal evolution during orogenesis. However, Precambrian metasediments can be difficult to date to a reasonable precision. Maximum depositional ages based on detrital zircon can vary considerably from sample to sample, even within a single formation. Although associated igneous bodies can yield depositional ages, their relationship with associated metasediments can be hard to constrain in poly-deformed terranes, as thrusting leads to the imbrication of rocks of different origins and ages. We present new U-Pb zircon ages for metasediments and tectonically-intercalated basement rocks from the Neoproterozoic Southern Brasília Orogen, West Gondwana, and explore the challenges of dating metasedimentary sequences in collisional orogens more widely. Our approach has been to constrain a metasedimentary succession based on field and petrographic analyses and to consider the youngest detrital zircon age, of all samples, as the maximum depositional age. Most samples yield youngest ages older than the maximum depositional age and are thus referred to as “apparent” maximum depositional ages. This results from the age distributions in the source regions, where there is major peak magmatism at ∼ 2.2–2.1 Ga, and small volumes of rocks with younger ages. Hence, apparent maximum depositional ages of ∼ 2.1 Ga, common in the studied succession, appear to be related to source characteristics. Five metasedimentary rocks were analyzed here, three yielded apparent maximum depositional ages between 1.8 and 1.6 Ga, and two samples provided ages of ∼ 1.0 Ga, considered the maximum depositional age. The Archean-Paleoproterozoic basement of the Southern Brasília Orogen is the main source region for overlying Neoproterozoic metasedimentary successions, with some material derived from the adjacent Southern São Francisco Craton. P-T obtained for the studied succession is consistent with sedimentary facies and geochronological studies that suggest it is the passive margin of the Southern São Francisco Craton.