Internal architecture and Fe–Ti–V oxide ore genesis in a Variscan synorogenic layered mafic intrusion, the Beja Layered Gabbroic Sequence (Portugal)

Abstract The Beja Layered Gabbroic Sequence (LGS) intruded the SW border of the Ossa Morena Zone at ca. 350 Ma during the early stages of the Variscan oblique continental collision in Iberia. It represents an exceptional case-study for the petrogenesis and metallogenesis of the rare class of synorogenic layered intrusions. LGS oxidised parental magmas (+ 1.7 ± 0.5 ΔFMQ) evolved to relatively reduced ferro-basaltic magmas (± 0.5 ΔFMQ) after extensive differentiation (SB I–SB II–ODV I Series: Fo88-54) and gave rise to ODV I Series oxide-rich ferro-gabbros and massive oxide ores, that represent a potential economic resource for Fe, Ti and V. Vanadium enrichment in ODV I melt peaked at the formation of basal ores and was followed by heterogeneous depletion at prevailing high Fe–Ti contents within other oxide-rich domains of the Series. Geological, geophysical and geochemical evidence suggest that density driven processes were fundamental to oxide accumulation and segregation within ODV I basal domains. The SB I–SB II–ODV I suite was followed by monotonous gabbroic Series (ODV II, III, BRG I, II) developed from repeated magma replenishments with similar initial composition (Fo78). The prevailing oxidising conditions (Fo76-66 ΔFMQ + .5 to +1.0) and the buffering effect of recurrent replenishments constrained the Fe–Ti enrichment within a typical calc-alkaline differentiation trend; decreasing recharge/crystallisation rates locally produced oxide accumulations at ≈ +1Δ FMQ. The coupled ƒO2 decreasing with Fe-enrichment throughout LGS is consistent with crystallisation under conditions closed to oxygen exchange. It is inferred that the large amount of oxides that precipitated throughout ODV I Series required transient, episodic openings of the system favoured by the tectonically active character of the magma chamber. Modelling of melt compositions supports that LGS parental magmas were high-alumina, primitive basalts (#Mg0.63), which after some differentiation gave rise to SB II Series (#Mg0.59) preserved as a chilled margin. The SB I parental magmas may account for the monotonous Series compositional spectrum, whereas ODV I Series model melts may be derived as residual magmas from SB II Series. ODV I melts show strong Fe–Ti enrichment and Si depletion, consistent with a Fenner trend, wherein the formation of extreme differentiation products was precluded by the arrival of new magma batches related with the monotonous Series.