Neoproterozoic magmatism in the Itremo Region, central Madagascar: Geochronology, geochemistry, and petrogenesis

Neoproterozoic (900–540 Ma) magmatism in the Itremo region of central Madagascar occurred during a period of major reorganization and realignment of Earth's tectonic plates. Detailed chemical, isotopic, and geochronologic study of Itremo intrusive igneous rocks provides insight into the geologic and tectonic events that occurred on the putative margin of East Gondwana during the breakup of one supercontinent (Rodinia), and the assembly of another (Gondwana).; Intrusive igneous rocks were emplaced in the Itremo region during two episodes in middle Neoproterozoic (804–778 Ma) and late Neoproterozoic to early Cambrian time (571–536 Ma), and these events bracket a period of regional deformation. Middle Neoproterozoic coeval gabbro-diorite and granitoid suites exhibit intrusive relationships with Itremo country rocks (late Archean tonalite gneiss and Mesoproterozoic metasedimentary rocks), all of which were deformed in late Neoproterozoic time prior to the emplacement of discordant granitoid plutons ca. 540 Ma.; Chemical and isotopic compositions for the ca. 800 Ma igneous rocks are consistent with production in a continental margin arc setting. In the petrogenetic model for this study, the gabbro-diorite rocks represent mantle-derived basaltic magmas that were variably contaminated by local late Archean crust (7–25% assimilation), whereas the granitoid rocks represent mixtures between fractionated mantle-derived magmas and anatectic crustal melts. The lack of intermediate rocks, or continuous spectrum of magma compositions typical of arc suites, is interpreted to result from a lack of substantial mixing between mafic and silicic magmas.; U-Pb isotopic ages for the Itremo intrusive suites help to place regional igneous, metamorphic, and deformational events into the larger context of plate tectonic interactions in Neoproterozoic time. In addition, detailed geochemical and isotopic analyses of the Itremo igneous rocks enable the development of petrogenetic models that can be compared with coeval intrusive events elsewhere in Madagascar, specifically along the axis of a proposed 800 Ma Andean-style magmatic arc in central and northern Madagascar. Magmatic events in the Itremo region can also be correlated with coeval formative events that occurred in neighboring cratons and can be used to test and constrain current models for Precambrian plate reconstructions.