Barian Mica and Distribution of Barium in Metacherts and Quartzofeldspathic Schists, Southern Alps, New Zealand

Rodney H. Grapes
Research School of Earth Sciences, Victoria University of Wellington, New Zealand

Abstract: 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.

Keywords: mica • barium • metachert • schist • Southern Alps • New Zealand

Mineralogical Magazine; June 1993 v. 57; no. 387; p. 265-272; DOI: 10.1180/minmag.1993.057.387.09
© 1993, The Mineralogical Society
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