Mineral metastability and effective bulk composition: The effect of grain sizes and modal mineral amounts

VERDECCHISA S.O, ; RECHE J., ; BALDO E. G.,; SEGOVIA DIAZ E., ; MARTINEZ F. J.

Praga

Conferencia; Goldschmidt Conference; 2011

Large staurolite porphyroblasts (up to 2 cm) wererecognized in quartz-mica schist of the Sierra de Ancasti(west-central Argentina) [1] with the mineral association: Crd-And-St-Grt-Bt-Ms-Pl-Qtz-Chl-Op. Textural relations suggestthat the assemblage St-Grt-Bt was present in the metamorphicpeak. Growth of late And or Crd porphyroblast is recognizedas well. Two post-peak textural domains were indentified: a)St being replaced by And without Crd, and b) Crd richdomains lacking And. A P-T pseudosection in theMnNCKFMASH system using XRF bulk compositionallowed to estimate conditions of 590°C and 5.2kb for the St-Grt-Bt assemblge but post-peak textures and mineralcompositions where not predictable. We assumed that theoriginal XRF bulk composition was not valid during post-peakconditions not only because the cores of metastableporphyroblasts such as staurolite could have been isolated andexcluded of the effective bulk composition [2] (EBC), butalso, because heterogeneous distribution of staurolite couldhave affected the local EBC. Accordingly, post-peak texturalrelations (~580°C-3.5kb) developed during decompressionwere modeled using two bulk compositional domains definedon the basis of observed differences in staurolite mode. Twoextreme cases were modelled: a) St poor - Crd rich domainslacking And; b) And + St rich domains lacking Crd. Resultsapproximate real compositions and modal amounts observedin both post-peak textural domains. Thus, variation of EBCand generation of EBC domains through the PT path is acritical factor to consider for correct interpretation ofmetamorphic mineral and textural evolution and particularlyfor discrimination between polymetamorphic and singlemetamorphic episodes.