Petrology of the high-Mg tonalites and dioritic enclaves of the ca. 2130 Ma Alto Maranhão suite: Evidence for a major juvenile crustal addition event during the Rhyacian orogenesis, Mineiro Belt, southeast Brazil
Résumé
Combined field observations, petrography, mineral chemistry, geochemistry, Nd isotopes and U-Pb zircon data are presented for the ca. 2130 Ma Alto Maranhão suite. The suite, composed by allanite-bearing biotite hornblende tonalites and commingled dioritic mafic magmatic enclaves (MME), occupies an estimated area of >300 km2 in the southern Quadrilátero Ferrífero region, Minas Gerais, Brazil. Tonalites and commingled MME are medium-K rocks, with high-Mg (Mg-number ≥ 0.46) and high-Cr (≥55 ppm) contents. They are also enriched in large-ion lithophile elements (LILE, Sr, Ba), some high-field-strength elements (HFSE, Th, U), and light rare earth elements (REE), but depleted in Nb, Ta, and heavy REE. TDM model ages for tonalites and MME are similar and range from 2.3 to 2.4 Ga. The ɛNd(t) values are grouped around chondritic values (=0 ± 1.0). Field relations and geochemical data indicate that tonalites and commingled dioritic MME crystallized as synchronous independent magma pulses, with limited mixing. Both magmas are compositionally equivalent to high-Mg andesites in modern subduction zones, implying that the suite was issued from the partial melting of a mantle wedge metasomatised to different degrees by slab-derived melts. Geobarometry (aluminium-in-hornblende) indicates crystallization at mid-crustal depths (i.e. 0.59-0.42 GPa). The igneous suite evolved by fractional crystallization of hornblende, biotite and accessory minerals (zircon, apatite, allanite and Fe-Ti oxide). The ubiquitous occurrence of hornblende and allanite in tonalites and dioritic MME suggests that this mineral pair was largely responsible for the observed changes of light REE/heavy REE and Eu/Eu* ratios within the members of the suite. The geographical extent (>1000 km2) and Paleoproterozoic ages of the Alto Maranhão suite (ca. 2130 Ma) and associated granitoids of the Mineiro Belt (2.1-2.2 Ga) constitute a major event of juvenile addition to the continental crust in the context of the southern São Francisco craton, and the ca. 2.2-2.1 Ga Rhyacian orogenesis of the South American platform. The age and general geochemical characteristics of the Alto Maranhão suite and its emplacement after a ca. 2.35 Ga juvenile, tholeiitic-source derived high-Al TTG suite, indicate a tectonic setting akin to that of Late-Archaean high-Mg granitoids.