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Supplement C: Optical petrographic observations of thin
sections from volcanic samples
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Detailed petrographic observations from thin sections show different degrees of
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alteration of the samples. Sample NL1.2 from group A-E1d-L is an andesitic lava flow
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with porphyritic texture. Phenocrysts are dominated by euhedral to subhedral
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plagioclase (2 to 10 mm in size), which has been variably replaced by epidote and
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clay minerals (Figure S6a and b). The matrix is mainly composed of quartz,
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K-feldspar and albite. The accessory minerals are titanomagnetite, apatite and rutile.
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Slight alteration of plagioclase to reddish hematite is also observed (Figure S6a).
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Samples NL22.7, NL27.4 from group C-E1d-L and NL40.8 from group E-E2n-L are
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pinkish andesitic tuff which show evidence for hydrothermal alteration. They have a
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pyroclastic texture with feldspar/quartz lapillis and lithic fragments distributed in a
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silicic matrix (Figure S6c-i). Alteration of plagioclase and quartz to sericite and
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calcite is very common (Figure S6c-i). K-feldspar phenocrysts in samples are often
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replaced by albite and iron oxides. Accessory minerals include biotite, epidote, rutilite
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and iron oxides. Biotite is strongly exsolved/altered, and is often replaced by
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pseudomorphs of sericite, chlorite and iron oxides (Figure S6c-f). The opaque
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minerals (mostly iron oxides) are common as accessory phases in all samples. They
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consist of two phases: one with large independent crystals distributed randomly
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within the matrix, and a second that is a later alteration product of biotite and feldspar,
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distributed around and within the host minerals or along cracks. In general,
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carbonatization, sericitization, albitization and epidotization of the samples is very
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common, indicating a pervasive low-temperature hydrothermal alteration.
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