Zoned barian muscovite (2.52–5.66% BaO) and unzoned biotite (0.99–1.77% BaO) occur in two amphibolite grade metacherts of the Alpine schists, Southern Alps, New Zealand. The Ba-micas are associated with quartz-chlorite-Mn-garnet-tourmaline-apatite-sulphide ± oligoclase ± rutile ± magnetite ± ankerite. Increasing Ba (core to rim) in the muscovite is accompanied by a decrease in Si, Ti, Fe + Mg, and K and an increase in ^[4]Al, ^[6]Al, and Na. The main substitution that accounts for entry of Ba into both micas is ^[A]Ba²⁺ + ^[4]A^[3+] ⇌ ^[A]K⁺ + ^[4]Si⁴⁺ and possibly ^[A]Ba²⁺ ⇌ ^[A]K⁺ + □. Compositional variation of the muscovite is also governed by the substitutions, ^[6]Al³⁺ + ^[4]Al³⁺ ⇌ ^[6](Mg,Fe)²⁺ + ^[4]Si⁴⁺, and ^[A]Na⁺ ⇌ ^[A]K⁺. The presence or absence of oligoclase, rutile, magnetite and Mg/(Mg + Fe) ratio of coexisting biotite control the Na, Ti, Fe and Mg contents of muscovite in the respective metacherts. Important variables controlling the occurrence of Ba-mica is the Ba-rich composition of the metacherts (1387 and 2741 ppm Ba) and metamorphic grade. In metacherts, siliceous and quartzofeldspathic schists with <1000 ppm Ba barium increases with increasing K₂O content indicating that in K-feldspar-absent rocks barium is mainly contained in micas (<0.70% BaO). In greenschist facies metacherts and siliceous schists with high Ba (>1000 ppm) and low K₂O, barian micas are not present and most of the Ba occurs in baryte ± barian carbonate with implication of a significant original hydrothermal-hydrogenous input. Although low grade illite/sericite/smectites containing Ba are the most likely precursor of the barian micas in the metacherts, strong marginal Ba enrichment in the muscovite indicates a later Ba-metasomatism resulting from the breakdown of baryte under reducing conditions during amphibolite facies metamorphism.