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The small elliptic Rechla granite pegmatite complex belongs to the c.525 Ma Rare Metal Granite (RMG) province of the Laouni terrane of the Pan-African Tuareg Shield (Hoggar). It intrudes a porphyritic biotite-granite and is particularized by a rim of Quartz, K-feldspar and Zinnwaldite pegmatite. The centre is occupied by a medium-grained granite, with quartz, albite (An01), rare microcline, topaz, lepidolite (≤ 8% MnO), wolframowodginite and Hf-zircon. The pegmatite rim comprises, toward the intrusion (i) thick K-feldspar lenses (palissadic crystals ≥ 50 cm), (ii) a laminated quartz-zinnwaldite-(beryl) sequence, described as a unidirectional solidification texture (UST), and (iii) a discontinuous band of fine-grained granite, with quartz, albite, topaz, lepidolite, titanowodginite and beryl. The laminated sequence overprints the K-feldspar lenses. It comprises thick (≤ 20 m) quartz lenses cross-cut by 10 cm-sized alternating bands of euhedral quartz and Mn-zinnwaldite (≤ 6.5% MnO). At the boundary with the fine-grained internal band, euhedral quartz crystals are projecting toward the inner wall. The chemical composition of the medium-grained granite is typical of a low-P peraluminous RMG deriving from highly potassic calcalkaline suites (A2 type) enriched in Ta (165 ppm, Ta/Nb between 2.4 and 2.6), expressedas columbo-tantalite and Mn-wodginite, with low P2O5 (0.05%) and ∑ REE (23 ppm) contents, with a pronounced tetrad effect and <0 Eu anomaly in the REE pattern. The fine-grained granite is equally fractionated with Ta 240 ppm (Ta/Nb = 2.4) and Be 500 ppm. The surrounding porphyritic biotite-granite is representative of the evolved magmas of the A2-type Taourirt suite in the nearby terranes. Geochemical modeling, using the filter press model, shows that the main Rechla magma is likely the fractionated product of this already differentiated magma, mainly involving quartz and Kfs. The pegmatite rim is interpreted as the result of the sequential crystallization of a Rechla-type melt, with late individualization of a Fe-rich magmatic-hydrothermal phase responsible for the quartz-zinnwaldite assemblage, leaving a strongly Be-enriched residual liquid (the fine-grained granite). As demonstrated by the Rechla occurrence, Ta concentration at levels similar to those in Beauvoir-type high-P peraluminous granites may be reached in the low-P low-Ta A2 suites, provided that extreme fractionation processes are established.

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Petrogenesis of the Rechla Rare Metal Granite-Pegmatite Complex from the Laouni Terrane, Central Hoggar, Algeria

  • 章节

  • 发表日期：

    2025年

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  • Mokrane Kesraoui,
  • Ahmed Hamis,
  • Christian Marignac

摘要

The small elliptic Rechla granite pegmatite complex belongs to the c.525 Ma Rare Metal Granite (RMG) province of the Laouni terrane of the Pan-African Tuareg Shield (Hoggar). It intrudes a porphyritic biotite-granite and is particularized by a rim of Quartz, K-feldspar and Zinnwaldite pegmatite. The centre is occupied by a medium-grained granite, with quartz, albite (An01), rare microcline, topaz, lepidolite (≤ 8% MnO), wolframowodginite and Hf-zircon. The pegmatite rim comprises, toward the intrusion (i) thick K-feldspar lenses (palissadic crystals ≥ 50 cm), (ii) a laminated quartz-zinnwaldite-(beryl) sequence, described as a unidirectional solidification texture (UST), and (iii) a discontinuous band of fine-grained granite, with quartz, albite, topaz, lepidolite, titanowodginite and beryl. The laminated sequence overprints the K-feldspar lenses. It comprises thick (≤ 20 m) quartz lenses cross-cut by 10 cm-sized alternating bands of euhedral quartz and Mn-zinnwaldite (≤ 6.5% MnO). At the boundary with the fine-grained internal band, euhedral quartz crystals are projecting toward the inner wall. The chemical composition of the medium-grained granite is typical of a low-P peraluminous RMG deriving from highly potassic calcalkaline suites (A2 type) enriched in Ta (165 ppm, Ta/Nb between 2.4 and 2.6), expressedas columbo-tantalite and Mn-wodginite, with low P2O5 (0.05%) and ∑ REE (23 ppm) contents, with a pronounced tetrad effect and <0 Eu anomaly in the REE pattern. The fine-grained granite is equally fractionated with Ta 240 ppm (Ta/Nb = 2.4) and Be 500 ppm. The surrounding porphyritic biotite-granite is representative of the evolved magmas of the A2-type Taourirt suite in the nearby terranes. Geochemical modeling, using the filter press model, shows that the main Rechla magma is likely the fractionated product of this already differentiated magma, mainly involving quartz and Kfs. The pegmatite rim is interpreted as the result of the sequential crystallization of a Rechla-type melt, with late individualization of a Fe-rich magmatic-hydrothermal phase responsible for the quartz-zinnwaldite assemblage, leaving a strongly Be-enriched residual liquid (the fine-grained granite). As demonstrated by the Rechla occurrence, Ta concentration at levels similar to those in Beauvoir-type high-P peraluminous granites may be reached in the low-P low-Ta A2 suites, provided that extreme fractionation processes are established.