Abstract

The partition coefficients of trace elements between clinopyroxene and basanite/tephrite matrices representative of magmatic melts have been obtained from ultramafic alkaline dikes related to Cretaceous magmatism on the South American Platform. The four studied dikes, related to the Ponta Grossa Arch (APG) and the Serra do Mar (SM-U, SM-PN, SM-B) alkaline provinces, are porphyritic melanocratic bodies with fine-grained to very fine-grained matrices and are classified as lamprophyres (monchiquite or camptonite), with the exception of SM-B, which is classified as an olivine basalt. The rocks have potassic to ultrapotassic affinity and are tephrite or basanite. The respective melts of the last stage of magmatic differentiation (fine-grained to very fine-grained matrices) are also tephrite or basanite. In general, the composition of the matrices differs from the respective whole-rock compositions by enrichment in normative felsic minerals and incompatible elements and by depletion in normative mafic minerals.

The forumla obtained lie within the range of values from the literature. In general, the studied elements are more incompatible in clinopyroxene from the APG dike than the Serra do Mar dikes. The Serra do Mar dikes exhibit negative forumla anomalies in the forumla diagram, a feature that is absent in the APG dike. A lattice strain model was applied to the results and forumla (REE and Y) values are 0.34 (APG), 0.89 (SM-PN), 1.21 (SM-U), and 1.57 (SM-B).forumla is positively correlated with the amount of Al in the melt and at the T site of clinopyroxene and is inversely correlated with the degree of polymerization of the melt. The chemical control on forumla by Al in the melt seems to be more independent of SiO2 saturation than Al at the clinopyroxene T site. Once the trace element content in the melt is not coupled to the degree of SiO2 saturation for the studied dikes, it is proposed that the obtained difference in melt composition among the dikes of Serra do Mar and Ponta Grossa Arch alkaline provinces can be related to the mantle source and the degree of partial melting. The higher CaO/Al2O3 (1.67) value and greater REE enrichment (ΣREE = 527 ppm) of APG suggest that the mantle source underwent more significant metasomatic processes than the mantle source of the dikes from the Serra do Mar province (CaO/Al2O3 = 0.60–0.84 and ΣREE = 310–435 ppm). The slightly more pronounced K-negative anomaly and higher (La/K)N of the APG melt could be due to a greater amount of residual phlogopite in the mantle source compared with the mantle source of dikes from the Serra do Mar province. Thus, more Al was preserved in the residue and a greater amount of incompatible elements was found in the melt of the APG dike. This study shows the importance of obtaining incompatible trace-element partition coefficients between clinopyroxene and alkaline primitive melts with similar composition of natural systems, because it shows that similar primitive alkaline melt compositions can present different incompatible trace element behaviors due to small heterogeneities in the metasomatized mantle source.

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