LTD-Thellier paleointensity of 1.2 Ga Nova Floresta mafic rocks (Amazon craton)

摘要

Absolute paleointensity at 1.2 Ga was obtained on basalts and gabbros from Nova Floresta Formation ( Amazon craton, Brazil). Magnetic carriers in these rocks are single-domain to multi-domain pure ( to nearly pure) magnetite. They present a strong decrease in remanence after low-temperature demagnetization (LTD) accompanied by an increase in median destructive fields. Incorporation of LTD to Thellier double-heating measurements improved paleointensity estimates and enabled paleofield recovery for otherwise sterile rock samples, giving twenty seven successful determinations for six sites (45% success rate). Site-mean paleofield values are within 4.90 mu T and 15.55 mu T (mean at 8.37 +/- 4.07 mu T). These results correspond to a mean VDM of 2.08 +/- 0.94 x 10(22) Am-2, thus increasing the number of low-field determinations that dominate the ( still scarce) Precambrian database. Our study exemplifies the need for alternative approaches in the study of ancient rocks.