Petrography and geochemistry of aplites from the Seridó Pegmatite Province, NE Brazil: Petrogenetic Implications

• Published in: Journal of South American earth sciences Vol. 148; p. 105145
• Main Authors: Dalan, Clarissa A., Sallet, Ricardo, Souza Neto, João A., Sales, Marcos C.C., Hollanda, Maria H.B.M., Rosa, Maria L.S.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 15-11-2024
• Subjects: Anatexis; Aplite; Pegmatite; Petrography; Trace elements; Aplite; Trace elements; Petrography; Pegmatite; Anatexis
• ISSN: 0895-9811
• DOI: 10.1016/j.jsames.2024.105145

In the Seridó Pegmatite Province (SPP) there are thousands of granitic pegmatite intrusions, including unzoned and barren, and zoned and mineralized ones. The whole rock chemical analysis of pegmatite is usually unfeasible because of its large grain size. However, fine-grained aplites syn crystallized with pegmatites should allow them to be used as proxies to assess pegmatite melt differentiation. We selected 15 representative aplite occurrences within unzoned pegmatite intrusions, including 7 intrusions with the grayish muscovite-aplite type and 8 with the reddish magnetite-aplite type. We also used for new trace element analysis samples of medium-to coarse-grained facies of host unzoned pegmatites. These pegmatites occur as NE-SW and N-S dykes and sills intruding mica schist and paragneiss from the Seridó Belt. Both aplite types contain along with quartz and feldspars accessory apatite, zircon, monazite, and xenotime. Tourmaline, muscovite, and garnet occur in the grayish aplites, whereas biotite and opaque minerals occur in the reddish aplites. The grayish aplites are slightly to strong peraluminous, while the reddish aplites are slightly metaluminous to slightly peraluminous. The reddish aplites and their host pegmatites are enriched in Ba, Sr, U and Th, while the grayish aplites and their host pegmatites are enriched in B, Li and Cs. The K/Rb ratios of the aplites varies from 11 to 44 with similar ranges in both reddish and grayish aplites. Normalized to the average upper continental crust composition, both aplite types present positive anomalies of Rb, Pb, U, and Hf, and negative anomalies of Ba, Sr and Ti. K, Nb and Ta show variable enrichment and depletion. They usually show HREE enrichment in relation to LREE and negative Eu anomaly, but positive Eu anomaly and tetrad patterns occur in few samples. In the normative QAP diagram the aplites show a wide range of compositions with a trend from quartz-plagioclase-rich tonalite to quartz-poor K-feldspars-rich alkali granite. Such trend is inverted in relation to the characteristic trends of melt differentiation by crystal fractionation. The albite/K-feldspar ratio of aplites shows a wide distribution range that would be related to the compositional variety of the partially melted protoliths and/or the variation in melting rate during prograde anatexis. The trace element compositions of the medium-to coarse-grained facies of the unzoned pegmatites are very similar to those of the coexisting aplites. The obtained data using aplites as a proxy for their host pegmatites precludes a unique line of melt descent at the origin of the SPP aplites and unzoned pegmatites and thus strongly support an anatectic origin from different sources.
•Petrography and Geochemistry of Aplites and Pegmatites in the Seridó Pegmatite Province.•Aplites used as a proxy to assess the petrogenesis in the Seridó Pegmatite Province.•Aplites and pegmatites derived from the anatexis of various sources.