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I found spherical or globular structures, structures in basalts of a greenstone belt which underwent alteration and deformation. However, in thin section, these structures have undeformed pyroxene and olivine. Below is a field picture of the structure. Would anyone know what these structures are called and how they form?

Globular Structure in Basalt.

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  • $\begingroup$ look like oxidation-reduction zones to me, but I'm not sure $\endgroup$
    – haresfur
    Aug 2, 2019 at 3:06
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    $\begingroup$ Can you add photographs of the thin sections? $\endgroup$
    – Gimelist
    Aug 3, 2019 at 0:41

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These globular structures are called varioles, and there is not complete agreement as to how they are formed. The most likely theory is that the basalt has been hydrothermically altered by millions of years of water infiltration bringing in tiny amounts of other minerals, but the jury is still out.

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  • $\begingroup$ My recollection was also that there isn't consensus on this topic. I know it can occur in rhyolite as well as other rocks that I can't recall right now.. $\endgroup$
    – Tim Nevins
    Jan 29, 2020 at 22:37

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