Petrographic description
Under petrographic microscope, the sample is formed by phenocrysts mostly of olivine, clinopyroxene, plagioclase and in a smaller proportion quartz and orthopyroxene in a fine matrix (Figs. 2 and 3). The texture is porphyritic. Amygdules are scarce in the sample and are filled by calcite. The olivine grains are embayed and partially reabsorbed. Pyroxene crystals show complex zoning patterns, range from concentric to patchy. Plagioclase is partially altered and twins are common. The quartz crystals are anhedral.
The matrix shows microcrysts of pyroxene, plagioclase and ilmenite in glassy material. Rutile, ilmenite and zircon appear as accessory minerals. Barite appears in the alteration zones. The sample has an orthopyroxene core that has been transformed to clinopyroxene to the rim.
The clinopyroxenes zonation, the reabsorption of the olivines and the pyroxenes overgrowth indicate a complex evolution of the magma, together with the quartz grains occurrence. These suggest disequilibrium during magma crystallization (Figs. 4 and 5).
The sample has a fine rim, mainly formed by quartz crystals in a calcitic matrix and, in a minor proportion, by biotite, feldspar and pyroxene grains (Fig. 6).
The size of olivine crystals range 0.6-1 mm, 400-700 μm for pyroxenes, plagioclase and quartz, and the matrix is below to 70 μm. Plagioclase phenocrysts, pyroxenes and the altered olivines represent, respectively, 20, 5 and 5% of the sample.
Figure 6 highlights the complex evolution of the magma, with different phenocrysts and mineral transformations. Some phenocrysts show reaction rims, indicating transformation after crystallization. Fe-Ti oxides are common in the matrix.
Figure 7 shows the EDS analyses and indicates that olivine is Mg-rich (Fo= 0.85). Clinopyroxene composition corresponds to augite, with Ca= 0.9 apfu (atoms per formula unit), a proportion of magnesium Mg# = 0.72 and moderate Al = 0.49 apfu. Orthopyroxene has Mg#= 0.75. Plagioclase is highly altered, partially transformed to albite and K-feldspar, which composition is respecrively
XAb = 0.75 and XOr = 0.14.
The rock can be classified as porphyritic basalt (Le Maître et al., 2005). However, the crystals have complex zonation patterns and the presence of olivine and quartz (not in equilibrium), which could indicate a magma mixing.
Mineralogy and textural properties of samples were studied on (30m-thick) polished thin sections using transmitted and reflected light microscopy at LPA (UA) and UGR.
Photomicrographs were performed by using:
o Petrographic microscope: ZEISS Assioskop (LPA-UA).
o Digital camera: Photometrics CoolSNAPcf
o Image capture Software: RS ImageTM v.1.8.6
Samples (polished thin-sections) also were studied under scanning electron microscope (Research Technical Services – UA). Minerals were located and identified using BSE mode on Hitachi S3000N SEM coupled with an X-ray detector Bruker XFlash 3001 for microanalysis (EDS) and mapping.
Semi-quantitative EDS X-ray microanalysis was performed using EDS spot analysis with acquisition time 60s and maximum process time to achieve best resolution of peaks in spectra.
ANNEX II. REFERENCES
Le Maître, R.W. (Editor), Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., Bellieni, G., Dudek, A., Efremova, S., Keller, J., Lameyre, J., Sabine, P.A., Schmid, R., Sørensen, H. & Woolley, A.R. 2005. Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks. Cambridge University Press. Second Edition, revised, 236 p. Cambridge, UK.
