THE ROLE OF ENRICHED-MANTLE IN THE PETROGENESIS OF THE POST-OROGENIC ITU GRANITIC PROVINCE, SE BRAZIL: PETROLOGY OF THE PIRACAIA MONZODIORITE

Identifying mantle and crust components in the petrogenesis of A-type granites is still a difficult task, as many key geochemical signatures cannot unambiguously be related to a single reservoir. Most of the granite plutons from the ~590-570 Ma post-orogenic Itu Granitic Province (IGP) in SE Brazil show structures indicative that mantle-derived magmas mingled and locally mixed extensively with the predominant silicic magmas: mafic microgranular enclaves; small bodies of biotite-rich diorite; complex textures such as rapakivi, anti-rapakivi and hornblende and/or biotite-mantled rounded quartz. These basic magmas seem to exert strong influence on the physico-chemical characteristics of the different granite suites; apparently their character and abundance may even be responsible for generating contrasts in granite typology (e.g., reduced versus oxidized A-type or high-K calc-alkaline). The ~570 Ma Piracaia Monzodioritic Pluton is the most extensive occurrence (~30 km2 exposed area) of mantle-derived rocks in the IGP, and comprises a wide compositional range (diorite to quartz syenite, with predominant monzodiorite; SiO2= 45-70 wt%). Detailed mapping shows that the pluton was built incrementally through the emplacement of several magma pulses, in a general mafic to felsic sequence. An enriched-mantle signature (high K, Ba, La/Yb, eNd570= -7 to -10) is common to all facies, but significant differences in trace-element chemistry and redox state point to a variety of parent melts. Some reduced felsic (syenite-quartz syenite) units, with low Sr and CaO and relatively high HFSE (Zr, Nb), are comparable to A-type granites. On the other hand, the predominant high La/Nb, Ba/Rb, Sr-rich more oxidized monzodiorites are similar to the mantle-derived end-member inferred to be present in the IGP high-K calc-alkaline plutons and to a lesser extent also in some metaluminous oxidized A-type plutons.