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Anais da Academia Brasileira de Ciências

Print version ISSN 0001-3765On-line version ISSN 1678-2690

An. Acad. Bras. Ciênc. vol.74 no.3 Rio de Janeiro Sept. 2002 



Instituto de Geociências, Universidade de São Paulo, 05508-900 São Paulo, SP, Brazil.


An integrated IDTIMS (isotope dilution thermal ionization mass spectrometry), EVTIMS (evaporation thermal ionization mass spectrometry) and SHRIMP (Sensitive High Mass Resolution Ion Microprobe) study of Archean orthogneiss reworked in the Brasiliano orogeny is discussed here. The sample comes from the Atuba Complex, near Curitiba, Paraná State.

Zircon age determination by three methods are 3055 ± 90 (IDTIMS upper concordia intercept), 3000 ± 40 Ma (EVTIMS 207Pb/206Pb plateau) and 3079 ± 23 Ma (SHRIMP). These results are in good agreement.

The SHRIMP analysis guided by cathodoluminescence (CL) also recognized younger age (e.g. ca. 2920Ma, 2200Ma and 800Ma) events in the growth/evolution of the zircons. The petrography of the zircons revealed by the CL images also clearly demonstrated that 3000-3100Ma zircon population is igneous oscillatory-zoned zircon, and is not an inherited component in a younger rock. Thus it is clear that MJ316 is a Mesoarchean rock.

The EVTIMS, "step heating", of one of the studied zircons showed that 207Pb/206Pb date rises from ca. 2350Ma at 1370oC to reach a plateau equivalent to ca. 3000Ma from 1420oC. This in accord with an event at ca. 2200Ma seen in the SHRIMP data, where there would have been not only development of partial overgrowths but also variable loss of some radiogenic Pb from "damaged" ca. 3000Ma zircon as well.

The IDTIMS study involved analysis of multigrain unabraded aliquots and also leached grains, now know from CL images to be structural complex. The upper concordia intercept (3055Ma) can only be considered as an "average", indicating an important component of Mesoarchean zircons. Two leaching phases yielded close to concordant dates at ca. 2390Ma - in agreement with the more interpretable SHRIMP indications of ca. 2200Ma. zircon growth/thermal disturbance.

It is considered that in this complex rock it is fortuitous that IDTIMS, EVTIMS and SHRIMP results give essentially the same protholith ages. In the other cases in the literature where there have been comparative EVTIMS/IDTIMS - SHRIMP studies of geologically complex samples with structurally complex zircons, dates by the methods do not always agree. However, when geologically simple samples are dated by the different methods the results are generally in good agreement. — (May 24, 2002).


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