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Título

REWORKING OF MESOARCHEAN BASEMENT DURING PALEOPROTEROZOIC OROGENESIS IN THE SOUTHERN BACAJÁ DOMAIN, SE AMAZONIAN CRATON

Texto do resumo

The Bacajá domain is situated in the Maroni-Itacaiúnas province in the SE portion of the Amazonian Craton. It is related to a Rhyacian collisional event that took place during the Transamazonian Orogeny (ca. 2.2-1.95 Ga), which reworked minor Mesoarchean to Neoarchean and Siderian crust and induced significant crustal growth during the Rhyacian. New fieldwork, associated with additional petrographic, geochronologic, and Nd-Hf isotopes data on the Mesoarchean basement of the southern portion of the Bacajá Domain, near its boundary with the Carajás Province, allowed us to better understand the diversity and evolution of the Archean basement of the domain. The Mesoarchean basement of the southern Bacajá domain was designated as the Cajazeiras Complex and comprises mainly clinopyroxene-amphibole-biotite and amphibole-biotite tonalitic to granodioritic orthogneisses with lenses and boudins of metamafic rocks. Minor paragnaisses also occur. A well-developed penetrative foliation, displaying a WNW-ESE to NW-SE trend with moderate to steep dips towards both NE and SW associated with shallow plunging mineral stretching lineation towards W is imprinted in these rocks. Zircon U-Pb geochronology (LA-ICP-MS) in the zircon cores furnished a minimum crystallization age for the igneous protolith of 2975±9 Ma. The ages obtained in the rims of zircon crystals, interpreted as formed during a metamorphic event, spread along the concordia and range from 2136±14 to 2074±14 Ma (apparent 207Pb/206Pb age). Lu-Hf isotopes on zircon magmatic cores reveal positive εHf(t) values of +0.5 to +3.1 and Mesoarchean two-stage model ages (Hf-TDMc) varying from 3.11 to 3.19 Ga, while data from the metamorphic rims provided negative εHf(t) values between −8.9 and -8.0, and Mesoarchean Hf-TDMc from 3.14 to 3.17 Ga. Nd whole-rock isotopes provided positive εNd(t) values (+0.64) and Mesoarchean TDM ages (3.06 Ga). The absence of orthopyroxene in the orthogneisses precludes granulite facies conditions and suggests a metamorphic peak at upper amphibolite facies. Previous data coupled with our new data reveals that the Archean basement of the Bacajá domain is diverse in age and isotopic behavior. Four Archean age sets were recognized: (1) at 2.5 Ga (São José complex); (2) at 2.60-2.63 Ga [εNd(t) = -3.08 to -2.95; εHf(t) = -1.7 to -0.3, Aruanã complex]; (3) at 2.67 Ga [εNd(t) = +2.66, tonalitic gneiss from Manelão area]; and (4) at 3.0-2.97 [εNd(t) = +0.64; εHf(t) = +0.5 to +3.1, Cajazeiras complex]. The rocks of the two younger Neoarchean age groups show crustal signatures and are concentrated in the northern and western sectors of the studied area. In contrast, the groups with older Neoarchean and Mesoarchean ages display juvenile signatures and are located in its central and southern portions, near the boundary of the Bacajá Domain with the Carajás Province.

Palavras Chave

U-Pb geochronology; Nd-Hf isotopes; Archean basement; Amphibolite facies;