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Título

KAMAFUGITE-HOSTED MANTLE XENOLITHS FROM PARAÚNA REGION, SW MARGIN OF THE SÃO FRANCISCO CRATON, BRAZIL

Texto do resumo

The Goiás Alkaline Province (GAP), located in central-western Brazil, contains rocks with kamafugitic affinity, which occur as dikes, plugs, and lava flows. In the southern portion of GAP, the Paraúna region, flows of kamafugitic lavas host mantle xenoliths ranging from 0.6 to 3.2 cm. This study employs petrography and geochemistry (major, minor, and trace elements) studies on these mantle xenoliths to understand possible metasomatic processes in the lithospheric mantle beneath the GAP. The petrographic description was done using petrographical microscopes at the Mineralogy and Petrography Laboratory of the Federal University of Pelotas (UFPel). The classification of the ten xenoliths was determined by point counting (2.000-5.000 points per thin section) using Jmicrovision software. Mineral chemistry analyses to determine the composition of major and minor elements in silicates and oxides were performed using an electron microprobe. To obtain trace and rare earth elements (REE) in clinopyroxene, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used. Both methods were performed at the Institute of Mineralogy, University of Münster, Germany. The xenoliths were classified as two orthopyroxene-free wehrlites and eight spinel lherzolites. They exhibit protogranular texture, with the presence of kink band extinction in some olivine crystals. The orthopyroxenes near to the contact between the xenolith and the host lava exhibit resorption texture, while clinopyroxenes have reaction (spongy) rims. Thus, we conclude that both types of orthopyroxenes are formed by the interaction with the kamafugitic magma. Mineral analyses show that the olivines are forsterites, with Mg# ranging between 0.89 and 0.91, coupled with high NiO (0.31-0.48 wt.%). The orthopyroxene is enstatite-rich (En88-89, Fs9-11, Wo0-2), with Mg# values (0.89-0.91) similar to those observed in olivine and high Al2O3 contents ranging from 3.61 to 4.83 wt.%. The clinopyroxene is diopside-rich (En45-57, Fs4-6, Wo43-50), with Mg# ranging from 0.89 to 0.92. Their CaO (18.73-23.55 wt.%), Na2O (0.69-1.99 wt.%), TiO2 (0.15-1.73 wt.%) and Al2O3 contents display significant variations, with the Ca/Al ranging from 3.23 to 8.06. Spinel occurs with Mg# ranging from 67.56 to 79.15 and Cr# from 7.22 to 12.99, except in sample XP4C, which has a higher Cr# (16.91-22.05). Based on the REE diagram, Paraúna mantle xenoliths can be divided in two groups. Group 1 shows La/YbN ratios ranging from 0.01-3.43, with higher HREE values (e.g. YbN: 18.70-26.55; LuN: 2.76-3.77). Group 2 also shows low La/YbN ratios ranging from 0.02-0.15, but lower concentrations of HREE when compared to Group 1 (e.g YbN: 1.77-2.18; LuN: 0.24-0.32). Both groups display positive anomalies in Cs, Rb, and Li and negative anomalies in Ba, Pb, Zr, and Ti. Group 1 also shows negative anomaly of Sr, and samples of Group 2 show Li negative anomaly. At the edges of the xenoliths, the clinopyroxenes in contact with the host magma or near to percolating kamafugite veins are enriched in Na2O, TiO2 and Al2O3. The textures observed in the clinopyroxene crystals suggest a significant interaction between the host kamafugitic magma. However, the low LREE/HREE and the trace element data do not agree with a metasomatism caused by an enriched magma as kamafugitic. Next step of this study is to investigate what kind of process caused such pattern in the clinopyroxenes from Paraúna mantle xenoliths.

Palavras Chave

Mantle xenoliths; Paraúna; Kamafugite; Goiás Alkaline Province.