242-5 Crustal Evolution and Mineralized A-type Granites of the Massangana Complex, Rondônia Tin Province, Southwestern Amazonian Craton

Session: Crustal Petrology, Part I

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Abstract:

The Massangana Complex, located in the Rondônia Tin Province (RTP) of the southwestern Amazonian Craton, hosts cassiterite-bearing granites and associated greisen systems, providing key insights into Proterozoic crustal evolution and critical metal enrichment. The pluton comprises biotite-hornblende porphyritic granites, coarse to medium biotite granites and syenogranites, and fine-grained biotite granites, with late-stage albite- and topaz-granites, as well as exo- and endogreisens and pegmatites. Whole-rock geochemistry reveals a wide SiO₂ range (65.2–76.5 wt.%), negative correlations of SiO₂ with Ba, Sr, Zr, and Ce, and enrichment in Rb, Pb, Nb, and Sn toward residual liquids, consistent with extensive fractional crystallization. Chondrite-normalized REE patterns display strong negative Eu anomalies (0.12–0.60) and a characteristic “seagull” shape, with HREE enrichment in greisens, reflecting feldspar fractionation and accessory mineral control (allanite, monazite, zircon, xenotime), further enhanced by late magmatic-hydrothermal fluids.

High-precision U–Pb zircon dating by LA-ICP-MS (1.03–0.97 Ga) constrains the emplacement of these granites to the late Mesoproterozoic anorogenic to post-orogenic stage, linking magmatism to regional shear zones that facilitated hydrothermal fluid circulation. Integrated Lu–Hf LA-ICP-MS analyses yield εHf(t) values from –13.7 to –0.4 and +0.2 to +5, indicating derivation from predominantly Paleoproterozoic crustal sources with a minor juvenile input. The combination of U–Pb and Lu–Hf via LA-ICP-MS is fundamental for reconstructing the petrogenetic evolution of these A-type granites, clarifying the timing of magmatism, crustal reworking, and the episodic mantle contribution to fertile melts.

The integration of geochronology, isotopic constraints, and lithogeochemistry demonstrates that the Massangana magmatism records crustal reworking punctuated by minor mantle input, driving the generation of fertile A-type granites. Volatile-rich magmatic-hydrothermal stages, particularly fluorine-bearing fluids, played a critical role in modifying primary mineral assemblages through metasomatic processes such as albitization, topazification, and greisenization, ultimately enhancing cassiterite and critical metal mineralization. These results underscore the interplay between Proterozoic crustal processes, magmatic differentiation, volatile fluxes, and metallogeny, providing a framework to understand the evolution and critical metal fertility of tin-bearing granite systems.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9651

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