Geologia, geoquímica e geocronologia do diopsídio-norito Pium, Canaã dos Carajás, Província Carajás

Abstract

The Diopsídio-norito Pium that occurs between Vila Feitosa and Cedere III localytis, Canãa dos Carajás district, on the southeast of Amazonian Craton, consists an elongated body with approximately 35 km of length, parallel with the E-W regional foliation, it is formed by diverse gabbroic rocks in composition (norites, gabbronorites, hornblende gabbronorites, hornblende gabbro, quartz-gabbro), including varieties riches in quartz (enderbite).The field relationships show that the noritic variety occurs as angular enclaves and with sudden contacts into the quartz-gabbro and hornblende gabbro rocks,indicating high viscosity contrast between the two magmas, could mean that the norite are slightly older.The geochemystri data show that such rocks have a subalkaline tholeiitic nature. Tectonical setting diagrams using Zr and Y as parameter shows that these samples have geochemistry affinities whith intra-plate basalts. Field relationships, geochemical and petrographic data, suggest a magmatic evolution process from the norites, passing by quartz-gabbros until the enderbites. This evolution was commanded by a magmatic differentiation process, enriching the liquid in SiO2, K2O, P2O5, TiO2, Ba, Sr, Rb, Zr, Nb, and Y, or by different partial melting grades, as it is suggested by field relationships between those varieties. The cumulatic rock shows a strong increase of MgO and Fe2O3, as well as a strong decrease of Al2O3, CaO e Na2O contents, being clear a compositional gap between this variety and the others. It suggests that the cumulatic variety was generated by accumulation of pyroxene crystals, as evidenced by modal data and by its strong Eu negative anomaly, indicating the unexpressive participation of plagioclase at the formation process of this variety. The magmatic evolution for the others varieties is marked by light rare earth elements enrichment regarding the heavy rare earth elements, besides the increase of the La/Yb ratios on the SiO2-richer rocks. The new geochronological dates obtained in this work displayed completely distinguished from the dates available in the literature. The ages obtained by single zircon Pb-Pb-evaporation method were 2745.2 ± 1.2 Ma for diopsidio-norite facies (RDM-06), 2744.5 ± 0.8 Mafor hornblende gabbronorite facies (RDM10) and 2744.2 ± 1.2 Ma for quartz-gabbro facies (CP-01A). Coexistences relationship between these varieties in plastic state reinforcing the idea of this body have formed in a single magmatic pulse. The crystallization ages obtained for the rocks of Diopside-norite Pium(2.74Ga) suggest that they were formed before those Planaltos granites formed at 2736-2710 Ma. The Diopside-norite Pium characteristics also suggest an extensional ambient associated to Carajás Basin opening stage. The analysis Sm-Nd yielded TDM model ages between 3.14 and 3.06 Ga suggesting that they were derived from mantle sources at about 3.1Ga, while εNd (t = 2.74 Ga) values between –2.78 and –1.58, indicated the involvement of crustal sources in its genesis. The model ages confirma mesoarchean period of crust formation in Carajas Province.