International Geologiical Congress - Oslo 2008

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MPN-01 General contributions to metamorphic petrology

 

Dioctahedral mixed K-Na-micas and paragonite in low-temperature metamorphic terrains: The effect of bulk rock composition and kinetics

 

Péter Árkai, Institute for Geochemical Research, Hungarian Academy of Sciences (Hungary)
Péter Horváth, Institute for Geochemical Research, Hungarian Academy of Sciences (Hungary)
Kenneth JT Livi, The Morton K. Blaustein Department of Earth and Planetary Sciences, Johns Hopkins University (United States)
 

 

Metamorphic mineral assemblages in low-T metaclastic rocks often contain paragonite (Pg) and/or its precursor metastable phase (mixed K-Na-white mica). Relationships between the bulk rock major element chemistries and the formation of Pg at several localities of low-T metamorphic clastic rocks were studied. Considerable overlap between the projected bulk compositions of the Pg-free and Pg-bearing metaclastic rocks indicate significant differences between the "as analyzed" and effective bulk chemical compositions in low-T metaclastic rocks. Excluding inherited and inert phases/constituents, it was found that a decrease in Na/(Na+Al*) and in K/(K+Al*) ratios of rocks favors the formation and occurrence of Pg and its precursor phases. In contrast to earlier suggestions, enrichment in Na and/or an increase in Na/K ratio by themselves do not lead to formation of Pg. Bulk rock chemistries favorable to the formation of Pg and its precursor phases are characterized by an enrichment in Al and depletion in Na, K, Ca. Such bulk rock chemistries are characteristic of chemically "mature" (strongly weathered) source rocks of the pelites and may also be formed by syn- and post-sedimentary hydrothermal activity.

Representative K-white mica-bearing samples, with or without Pg and Ab, were chosen for modeling the phase relations in the NKFMASH. In some of the samples Pg breaks down to Ab (or vice versa) via continuous reactions in the assemblage Chl-Ms-Pg-Ab, while in the other ones Pg is not present, so Ab is replaced by glaucophane or jadeite. For the Pg-out line, there is a general shift in pressure from 4.66 to 6.53 kbar at 300oC and 1.51 to 8.95 kbar at 400oC correlating with increasing bulk rock Na/(Na+Al) ratio. A simultaneous decrease in the Na/(Na+K) ratio of co-existing muscovite is also observed (from 0.32 to 0.06). The present study provides bulk-chemical and phase equilibria explanations to the contrasting phyllosilicate assemblages related to different geotectonic settings outlined earlier by Merriman.
Traditionally, the Pg-in reaction was assumed to involve kaolinite-Ab contacts reflecting a condition of local equilibrium. However, observations of metaclastics with maximum metamorphic temperatures between 100 and 300oC do not support this, but rather a model where grain boundary transport distributes Na on a much wider scale. Here, incipient Pg or brammallite nucleates as nanodomains on or within preexisting sheet silicates or in crystals with metastable compositions within the Pg-Ms miscibility gap. These Na-micas are found throughout the matrix and not at kaolinite-Ab contacts. Thus, the network of relatively fast transport in the grain boundary medium, along with the difficulty of Pg to homogeneously nucleate, contradicts the simplistic notion of local equilibrium at low metamorphic grades. This work was financially supported by the Hungarian National Science Fund (OTKA, grant number T049454 to PA).

 

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