Zuloaga et al., pseudosections

Zuloaga et al., pseudosections - Bulk Rock Compositional...

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Pseudosections (MnNaCaKFMASH system) were constructed for an average biotite zone pelite and for a sample of the staurolite-andalusite zone of the Waterville Formation, south central Maine using THERMOCALC and the thermodynamic data of Holland and Powell (1998). Activity models were those of Holland and Powell (1998) and those supplied with THERMOCALC, except that some solutions were extended to include Mn. Pseudosections indicate: i) the relevance of these diagrams for predicting parageneses and ii) the control that bulk rock composition exerts on the parageneses. Predicted mineral assemblages were compared with observed parageneses. Most results from pseudosections are in good agreement with natural assemblages. The biotite-in reaction calculated for the average biotite pseudosection indicates biotite growth at 420 o C and 3.5 kbars. Ferry (1984) estimated that biotite formed ca. 410 o C and 3.5 kbars, in very good agreement with the pseudosection. The most striking discrepancy is that observed staurolite+cordierite is not predicted by the pseudosections. The pseudosections predict staurolite stability above 4.0 and 4.3 kbars, higher than cordierite and/or andalusite stability. A pseudosection was constructed by proportionally modifying the average biotite zone composition: alumina increased from 16.88% to 22. .35%. Comparisons of the three pseudosections show that small variations in bulk rock composition lead to significant changes in mineral assemblages. The biotite-in reaction shifts to higher T and lower P with increased alumina. Increase in alumina concentration drives paragonite stability to lower P (as low as 1 kbar), increases the P-T range of staurolite stability (mainly T range, P range remains above 4 kbars), and causes growth of aluminum silicate before biotite at P < 4 kbars. The average biotite zone and 980A pseudosections demonstrate the usefulness of pseudosections for predicting mineral assemblages with good (biotite stability) to moderate (staurolite) agreement between natural and predicted assemblages. Results from variation of alumina concentration demonstrate the compositional dependence of metamorphic mineral assemblages. AFM - TERNARY DIAGRAM Projected from H 2 O, Qtz, and Mu A ACN - TERNARY DIAGRAM Projected from H 2 O, Qtz, and Mu C N A 100 75 50 25 High Al pelite Average Biot. Zone 980A (St. Zone) SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 MnO MgO CaO Na 2 O K 2 O Biotite zone 60.78 0.79 16.88 7.63 0.13 3.44 1.21 1.65 3.70 High Al Pelite 58.36 22.35 7.33 0.13 3.30 1.16 1.58 3.55 980A (St zone) 52.43 1.01 20.78 9.36 0.16 3.60 2.70 2.59 4.68 MOLE PROPORTIONS Si Al Fe Mn Mg Ca Na K Biotite zone 1.05 0.17 0.10 0.002 0.09 0.02 0.03 0.04 High Al Pelite 0.97 0.22 0.09 0.002 0.08 0.02 0.03 0.04 980A (St zone) 0.90 0.21 0.12 0.002 0.09 0.05 0.04 0.05 NORMALIZED MOLE PROPORTION (volatile free) Biotite zone 69.89% 11.44% 6.60% 0.127% 5.90% 1.49% 1.84% 2.71% High Al Pelite 66.98% 15.12% 6.33% 0.125% 5.65% 1.43% 1.76% 2.60% 980A (St zone) 61.24% 14.31% 8.23% 0.158% 6.27% 3.38% 2.93% 3.49% WEIGHT PERCENT OXIDES The table and two triangular diagrams show the pelite compositions used in our modeling. The first data set corresponds to an average of 8 samples in the biotite zone of the Waterville Formation (Maine) published by Ferry
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Zuloaga et al., pseudosections - Bulk Rock Compositional...

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