Petrology of volcanic rocks from Sao Miguel and Faial, Azores Islands

Abstract

Volcanic rocks from Sao Miguel form two distinct alkaline (potassic) magma series. Recent eruptives comprise Ne-normative basalts and derivatives (Main Series). Older basalts associated with the extinct stratovolcano Povoacao are either Ne or Hy-normative (Povoacao Series ) and have lower P/Zr, Ba/Zr and Sr/Zr ratios. Faial volcanic rocks form a single series, basaltic compositions being Ne-normative. Differentiates on both islands range through intermediate compositions to metaluminous and peralkaline trachytes. The latter are strongly enriched in the incompatible elements (eg Th, Rb, Ta, Nb, Zr and the rare-earths) and are depleted in Mg, Ni, Cr, V, Ti, P, Sr, Eu and Ba. Fractional crystallization of the observed phenocryst assemblages can account for most chemical variations, incompatible element abundances indicating that the trachytes result from some 60--85% solidification of a parental basalt.

Mixed-magma lavas occur on both Faial and Sao Miguel. They form two distinct groups. (1) M-type lavas. These have high Ni, Cr and MgO and low CaO, Sr, P₂ O₅, TiO₂ and V. Modelling suggests they are a mix of trachyte (70%) and MgO-rich basalt (30%). (2). AM-type lavas. They show similar geochemical features but also have high Ba and Eu and large K/Rb ratios. They are best modelled by mixing hawaiite, trachyte and alkali feldspar. AM-type lavas and geochemically heterogeneous pumice deposits suggest compositionally zoned (basalt-intermediate-trachyte) magma chambers beneath both islands.

Compositional variations in recent pumice deposits from Agua de Pau volcano show temporal control. Trends up the succession include the depletion of Sr, Ba and Eu coupled with stepwise enrichment in the incompatible elements. These suggest the deposits represent successive samples of an evolving body of trachytic magma. The lack of similar variations in the Furnas pumice succession is attributed to a high magma viscosity, inhibiting crystal/liquid fractionation.

Pre-eruptive temperatures for trachytic pumice deposits, from coexisting magnetite and ilmenite, ranged between 960°C--880°C. fO ₂ varied between 10-10.9--10-13.4. Thermodynamic-based estimates of magnetic water contents were between 6.5--7.2 wt.%. These values are compatible with field data. Estimates of S₂, H₂S, SO₂, SO₃, H₂, CO₂, HCl and HF suggest they are several orders of magnitude less abundant than H₂O.

Lastly, Azores basalts exhibit inter and intra-island compositional heterogeneity. Relative to N type m.o.r.b., they are characterized by high La/Yb, Ba/La, 87Sr/86Sr ratios and low 143Nd/ 144Nd, Zr/Nb, Zr/Ta and K/Rb ratios. Sao Miguel shows a systematic increase in 87Sr/86Sr ratios from west to east (up to 0.70522). Most of these features represent source characteristics and suggest a complexly heterogeneous mantle.