LITHIUM ENRICHMENT IN ANATECTIC PEGMATITES: NEW INSIGHTS FROM THE THERMOMETAMORPHIC AUREOLE OF THE ADAMELLO BATHOLITH
In the contact aureole of the collisional metaluminous Adamello pluton (Italy), pelites reached hornfels facies conditions locally undergoing partial melting, forming Crd-bearing and Crd-absent migmatities. This small-scale migmatitic field is spatially associated with barren and Lithium-Cesium-Tanatalum (LCT) pegmatitic dykes of granitic composition, suggesting a direct link between low-pressure partial melting and generation of Li-enriched felsic melts. Phase equilibria modelling reveals that the Crd-bearing metapelitic migmatites produced around 20 vol.% melt fraction through fluid-assisted melting reactions, while the Crd-absent stromatic metatexites produced ca. 35 vol.% of melt during fluid-rich partial melting. Peak P-T metamorphic conditions were calculated at 690-700°C and 300 MPa for both rock types. Because of the steep contact thermal gradient, the underlying and older conglomerates of the Verrucano Formation, of which the pelitic succession is part of, as well as rock of the Collio Formation experienced comparably lower temperature metamorphism. Their mica-poor mineral compositions prevented them from melting and influencing the local anatectic processes. Crd-bearig migmatites leucosomes are compositionally similar to anatectic melts obtained by phase equilibria modelling, connected to fluid-assisted melting and defined as unfractionated, while the leucosomes dominated by Kfs-Bt cumulus structures and extremely enriched in K2O, Sr and Ba are connected to fluid-rich melting and Crd-absent migmatites. Major and trace element modelling suggests that part of the melt formed in the latter case was extracted from the source and that the composition of this evolved anatectic magma is compatible with the chemical composition of the migmatites-hosted LCT pegmatites. Whole rock Sr-Nd isotopic data confirm that the Li-rich pegmatites of the Forcel Rosso area are indeed crustally derived, marking them as produced by direct anatexis of metapelites. They also show that they were formed by two melt batches of different trace element composition originated by differential in-source fractionation. Conversely, isotopic data characterizes the pluton-hosted pegmatites as mantle-derived and directly linked to the Adamello batholith, clearly marking the lack of direct relationship existing between this magmatic intrusion and pegmatites featuring Li enrichment in its contact aureole. The data from this study shows that cordierite, or the absence of it, plays a key role in regulating the lithium transfer during partial melting in low pressure migmatites. Zircon U-Pb ages for all pegmatites in the area, point to the same age of the Adamello pluton intrusion, ca. 38 Ma, and trace elements compositions helped to outline the differences in the geochemical signature between pluton-derived and metapelite-derived pegmaties. The Ti-in-zircon geothermometer confirms that anatectic pegmatites crystallized at the peak metamorphic temperature calculated through phase equilibria modelling. Once defined that Li-enriched pegmatites are anatectic, tourmaline was used to study the continous evolution of the metapelitic system from unmetamorphosed up to the conditions of pegmatite crystallization. This mineral is the main boron carrier among the rock forming minerals and it’s obiquitous in the studied rocks. Studying the major element composition and boron isotopic signature several changes and trends are identified highlighting how this mineral evolves through the metamorphic history of metapelites, revealing a seemingly more complex picture compared to what is reported in the published literature data. Also, the data show how this mineral records variations during pegmatites crystallization processes making it valuable in the interpretation of their complex internal evolution processes. This PhD project proved that Li-enriched pegmatites may be anatectic and originate through fluid present melting of metapelites, even in the thermometamorphic aureole of plutons. We identify the mineral cordierite as the main regulator of lithium liberation during low pressure anatexis of suitably Al-rich pelitic rocks. We highlight that in-source and en_route melt fractionation play an extremely important role each time magma is extracted from its source and that this process is the one ultimately responsible for the formation of Li-enriched melts. Pegmatitic zircons, although complex, can be used as tracers in order to help define the origin of pegmatitic dykes, particularly in cases in which these bodies are too old for Sr isotopic determination. The studied area shows that small scale variations in the pre-anatectic abundance and presence of aqueous fluids influences melt production and melt composition of different migmatitic areas. Finally, the metamorphic and magmatic history of the studied tourmaline crystals highlights how this mineral may record almost every boron-involving reaction and help unravel the complex history of migmatites, pegmatites and Tur-bearing plutons.
lithium, cesium, tantalum, pegmatite, anatectic, fluid, present, melting, phase equilibria, tourmaline, Settore GEO/08 - Geochimica e Vulcanologia
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