Compositional Variations in Apatite and Petrogenetic Significance: Examples from Peraluminous Granites and Related Pegmatites and Hydrothermal Veins from the Central Iberian Zone (Spain and Portugal)

Abstract

Apatite can be used as an archive of processes occurring during the evolution of granitic magmas and as a pegmatite exploration tool. With this aim, a detailed compositional study of apatite was performed on different Variscan granites, pegmatites and quartz veins from the Central Iberian Zone. Manganese in granitic apatite increases with increasing evolution degree. Such Mn increase would not be related to changes in the fO2 during evolution but rather to a higher proportion of Mn in residual melts, joined to an increase in SiO2 content and peraluminosity. In the case of pegmatitic apatite, the fO2 and the polymerization degree of the melts seem not to have influenced the Mn and Fe contents but the higher availability of these transition elements and/or the lack of minerals competing for them. The subrounded Fe-Mn phosphate nodules, where apatite often occurs in P-rich pegmatites and P-rich quartz dykes, probably crystallized from a P-rich melt exsolved from the pegmatitic melt and where Fe, Mn and Cl would partition. The low Mn and Fe contents in the apatite from the quartz veins may be attributed either to the low availability of these elements in the late hydrothermal fluids derived from the granitic and pegmatitic melts, or to a high fO2. The Rare Earth Elements, Sr and Y are the main trace elements of the studied apatites. The REE contents of apatite decrease with the evolution of their hosting rocks. The REE patterns show in general strong tetrad effects that are probably not related to the fluids’ activity in the system. On the contrary, the fluids likely drive the non-CHARAC behavior of apatite from the most evolved granitic and pegmatitic units. Low fO2 conditions seem to be related to strong Eu anomalies observed for most of the apatites associated with different granitic units, barren and P-rich pegmatites. The positive Eu anomalies in some apatites from leucogranites and Li-rich pegmatites could reflect their early character, prior to the crystallization of feldspars. The increase in the Sr content in apatite from Li-rich pegmatites and B-P±F-rich leucogranites could be related to problems in accommodating this element in the albite structure, favoring its incorporation into apatite. The triangular plots ΣREE-Sr-Y and U–Th–Pb of apatites, as well as the Eu anomaly versus the TE1,3 diagram, seem to be potentially good as petrogenetic indicators, mainly for pegmatites and, to a lesser extent, for granites from the CIZ.

Apatite as an archive of pegmatite-forming processes: an example from the Berry-Havey pegmatite (Maine, USA)

“…The chemistry of apatite may also be a good indicator of the fractionation attained by the pegmatites. This has been previously described for apatite associated with pegmatites and leucogranites from the Central Iberian Zone (Spain and Portugal) showing different evolution degrees (Roda-Robles et al 2022). In the U-Th-P and Y-REE-Sr ternary plots (Fig.…”

“…At the Berry-Havey pegmatite, the REE content in apatite decreases from the wall zone inwards, i.e., with magmatic evolution, as it has been previously reported in other localities (e.g., Jolliff et al 1989;Bromiley, 2021;Roda-Robles et al 2022). Incorporation of REE into apatite occurs via two main substitution mechanisms that mainly involve Ca and, to a lesser extent, P, Si and Na: REE 3+ 1 Na + 1 Ca 2+ -2 and REE 3+ 1 Si 4+ 1 P 5+ -1 Ca 2+ -1 (Sha and Chapell, 1999;Pan and Fleet, 2002;Webster and Piccoli, 2015;Harlov, 2015).…”

Geochemical Evidence for Genesis of Nb–Ta–Be Rare Metal Mineralization in Highly Fractionated Leucogranites at the Lalong Dome, Tethyan Himalaya, China

“…Meanwhile, Zr/Hf = 26 could be used to distinguish magma and hydrothermal granitic systems [69]. Highly fractionated granites are characterized by the decreasing concentrations of REEs, more negative Eu anomalies, and REE tetrad effects [70]. In the Lalong Dome, two-mica granite, muscovite granites, albite granites, and pegmatites are peraluminous.…”

U–Pb Dating, Gemology, and Chemical Composition of Apatite from Dara-e-Pech, Afghanistan

“…In this section, apatite from various localities and generation environments are compared with the Afghanistan apatite, including AP (probably from Madagascar) [50], Otter Lake (Canada) from calcite [50], NW-1 (Ontario) from alkaline carbonatite [50], Slyudyanka (Lake Baikal) from metamorphic siliciclastic carbonate phosphogypsum [50], Durango (Mexico) from ion deposits [50], UWA-1 (Bancroft, Ontario) [50], Mud Tank (Austalia) from carbonatite [50], McClure Mountain (Colorado) from anorthite [50], SDG (Inner Mongolia, China) from alkaline ultramafic rocks [50], MADB (Madagascar) [35], and Moro (Anemzi, Morocco) [36]. Pegmatitic apatite from Madagascar (MAD) [38], the Central Iberian zone (CIZ) [51], and Xinjiang (Altay No.3 pegmatite) [52,53] are also used for origin identification.…”