Magnetite in the carbonatites from the Jacupiranga Complex, Brazil

Abstract

Electron microprobe analyses of magnetites from five carbonatite intrusions (C_1, oldest, to C_5, youngest) constituting the carbonatite plug in the Jacupiranga complex confirm previous results from Jacupiranga giving compositions in the magnesioferrite-magnetite series very close to Fe_3O_4. Magnetites from other carbonatites are similar with somewhat more Ti and less Mg. MgO in Jacupiranga magnetites reaches no more than 10 wt.%. All analyzed grains are zoned, with Fe_3O_4 increasing toward the rim. In magnetites from C_2 to C_5, Fe_2O_3 replacement is mainly by Al_2O_3 and less by TiO_2; in C_1 magnetites TiO_2 replacement is more important. Despite their limited range of compositions, the cores of magnetites in each of the five intrusions are chemically distinct and distinguishable from each other as indicated by projections from within the Cr-free spinel prism, MgFe_2O_4-Mg_2TiO_4-MgAl_2O_4-Fe_3O_4-Fe_2TiO_4-FeAl_2O_4, and a plot of Mn ys. Mg. Magnetites from special locations such as dikes, banded reaction zones between carbonatite and jacupirangite, and in intergrowths with pyrite, are chemically related but distinct from the carbonatite magnetites. The systematic chemical variation and zoning of magnetites in the five carbonatite intrusions indicate magmatic origin. Magnetite crystals nucleated throughout the crystallization interval of the carbonatites, but most of them show evidence of marginal resorption. The oldest carbonatite, C_1, was probably derived from a magma somewhat different chemically from those producing carbonatites C_2 through C_5. The precipitation of carbonatite C_2 probably went to completion independently of C_3 through C_5, whereas carbonatites C_3 through C_5 probably were precipitated from successive batches of magma representing a continuum in time and magmatic evolution.

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