Geochemical signature of high to ultra-high pressure metamafic rocks from Ceará Central Domain, Borborema Province (NE Brazil)

The definition of the geotectonic setting of crystallization for the igneous protoliths of high- and ultrahigh-grade metamorphic rocks must acknowledge that the high mobility of many elements commonly used in widely accepted tectonic discrimination diagrams makes them unreliable in such cases. Unlike igneous rocks, there are no unequivocal formulas for assigning geotectonic settings to high- or ultrahigh-grade metamorphic rocks, each occurrence must be evaluated on a case-by-case basis, integrating whole-rock rare earth elements and trace elements distribution information, with the support of isotopic data (U–Pb, Sm/Nd and Lu–Hf, for example). In this study, the main Precambrian occurrences of high-grade and UHP metabasites from the Ceará Central Domain (CCD, Borborema Province, NE Brazil) are shown to have formed during the tenure of extensional events, rather than from collisional magmatism. Protholiths of the two recognized UHP-type metabasites from the CCD, namely the coesite-bearing retrograde eclogites of Forquilha and Jerimum, are indicated to have crystallized during the Rhyacian and Calymmian extensional events, respectively. Other high-grade occurrences relate mainly to the Calymmian extensional event and show geochemical similarities to mid ocean ridge basalt. Tonian metabasites are recognized as registers of events that led up directly to the Neoproterozoic amalgamation of West Gondwana supercontinent and are represented by metabsites hosted by metasedimentary Independência Group and by the Lagoa Caiçara Unit anatexites. The first set of rocks also present a geochemical signature that suggests crystallization in an oceanic basin, while the second set of metabasites has evident crustal component in its geochemistry (crustal contamination). Although the superposition fabrics of these three extensional events in the CCD rocks are recognizable, their exact duration and development within the context of Wylson Cycles are not fully constrained, with exception of the last event (Tonian).