Niobium Mineralogy of Pliocene A1-Type Granite of the Carpathian Back-Arc Basin, Central Europe

Monika Huraiová,Patrik Konečný,Vratislav Hurai

doi:10.3390/min9080488

Monika Huraiová, Patrik Konečný + Show 1 more

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https://doi.org/10.3390/min9080488

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Abstract

A1-type granite xenoliths occur in alkali basalts erupted during Pliocene–Pleistocene continental rifting of Carpathian back-arc basin (Central Europe). The Pliocene (5.2 Ma) peraluminous calc-alkalic granite contains unusually high concentrations of critical metals bound in Nb, Ta, REE, U, Th-oxides typical for silica-undersaturated alkalic granites, and syenites: columbite-Mn, fergusonite-Y, oxycalciopyrochlore, Nb-rutile, and Ca-niobate (fersmite or viggezite). In contrast, it does not contain allanite and monazite—the main REE-carriers in calc-alkalic granites. The crystallization of REE-bearing Nb-oxides instead of OH-silicates and phosphates was probably caused by strong water deficiency and low phosphorus content in the parental magma. Increased Nb and Ta concentrations have been inherited from the mafic parental magma derived from the metasomatized mantle. The strong Al- and Ca-enrichment probably reflects the specific composition of the mantle wedge modified by fluids, alkalic, and carbonatitic melts liberated from the subducted slab of oceanic crust prior to the Pliocene-Pleistocene rifting.

Highlights

Niobium belongs to the group of incompatible high-field-strength elements (HFSE) concentrated in carbonatites and peralkalic granites, where it is commonly accompanied by tantalum, yttrium, and rare earth elements (REE)
The A1 -subgroup is diagnostic of the trace element signature similar to that of ocean-island basalts (OIB) and the offset from the OIB field towards the island-arc basalt-derived magmas (IAB) typical of the A2 -subgroup reflects the variable degrees of crustal contamination
The assemblage of HFSE-bearing minerals dominated by pyrochlore, fergusonite, samarskite, and columbite is typical of alkalic granites [58,59]

Summary

Introduction

Niobium belongs to the group of incompatible high-field-strength elements (HFSE) concentrated in carbonatites and peralkalic granites, where it is commonly accompanied by tantalum, yttrium, and rare earth elements (REE). A-type granites and genetically associated metasomatic rocks and pegmatites are potential resources of critical high-tech metals [2,3]. The original definition of A-type granites [4] included only REE- (excluding Eu), mildly alkalic metaluminous granitoids with low CaO, high FeOtot /MgO, prevalence of K2 O over Na2 O, increased contents of rare-earth (excluding Eu) and other incompatible elements (Nb, Ta, Zr, Hf), and low concentrations of compatible elements (Ba, Sr, Ti, P) originated by the fractional crystallization of alkali basalts at a low oxygen fugacity in intra-plate tectonic settings. The A1 -subgroup is diagnostic of the trace element signature similar to that of ocean-island basalts (OIB) and the offset from the OIB field towards the island-arc basalt-derived magmas (IAB) typical of the A2 -subgroup reflects the variable degrees of crustal contamination

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