Amphibole Pairs, Epidote Minerals, Chlorite, and Plagioclase in Metamorphic Rocks, Northern Sierra Nevada, California
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Tremolite-hornblende relationships are reported for high grade and relatively low grade zones within the Barrovian type Skagit Suite, using rocks that range widely enough in composition to qualify as potential hosts of any calcic amphibole from practically A1-free to A1-rich types. Out of over 100 samples analysed by microprobe, 22 representative analyses are listed, with end-member calculations. In the sillimanite grade core of the Skagit Suite, calcic amphiboles from various, commonly metasomatized metamorphic ultramafics and genetically related hornblendites, from amphibolites, schists and gneisses, and from some metamorphosed impure dolomites show continuous solid solution between tremolite and highly aluminous hornblendes ranging from almost Fe-free to moderately Fe-rich types. A1total/A1IV is nearly constant and approximates 1·44. The second group of calcic amphiboles studied is from the lower-medium range of the epidote amphibolite facies (comprehensively defined), that is, from near and above the oligoclase isograd. Host rocks are variously metasomatized meta-peridotites, and amphibolites and schists. There is a large compositional gap between analyses of tremolites and of moderately to highly aluminous hornblendes. A1total/A1IV approximates 1·73 both in the tremolite and hornblende fields, as against 1·44 at high grade. Fe-poor hornblendes, such as are stable at high grade, were not found at the lower grade. At both grades, Al shows good overall correlation with Na+K, with A occupancy, and with Ti (with more Ti at high grade). The compositional gap between tremolite and hornblende analyses from the lower grade rocks does not necessarily define a solvus because no tremolite-hornblende pairs were found. Rather, the analyses provide outer limits on the possible width of a solvus at this grade (T). However, certain data suggest that a true miscibility gap not only exists but probably is not a great deal narrower than the gap between the actual analyses. Besides, the proposed restriction on A1/Fe ratios at this grade would reduce the probability of finding tremolite-hornblende pairs.
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Hydrothermal investigation of the bulk composition 2CaO·Al2O3·l/2Fe2O2·3SiO2+excess H2O (Ps 33 +excess H2O) has been conducted using conventional apparatus and solid oxygen buffer techniques. Coarse-grained epidotes (over 150 microns in some cases) were readily synthesized from oxide mixtures with a 98 per cent yield as well as from their high temperature equivalents at 600–700 °C and 5 kb Pfluid and over a range of oxygen fugacities. Electron microprobe analyses show that maximum Fe+3 content of synthetic epidotes varies as a function of fo2. Epidote is most iron-rich (Ps 33 ± 2) at high (HM and CCO) oxygen buffers and becomes progressively more aluminous (Ps 25 ± 3) with decreasing fo2 values and temperatures. Such variation is consistent with the change of refractive indices and cell dimensions. The mean refractive indices and cell dimensions for synthetic epidote (Ps 33) are Nα = 1.745 ± 0.005, Nγ = l.786±0.005, a = 8.920±0.005 Å, b = 5.645±0.004 Å, c = 10.190 ű0.006 Å, and β = 115° 31′±4′ and for epidote (Ps 25) are Nα = 1.735±0.005, Nγ = 1.775±0.005, a = 8.891±0.005 Å, b = 5.625±0.004 Å, c = 10.177±0.006 Å, and β = 115° 30′±3′. Mössbauer spectra indicate synthetic epidotes are relatively disordered. Garnets of intermediate composition in the grossular-andradite series were synthesized and the cell dimensions and refractive indices vary linearly with composition. With successive decrease in fo2, garnet synthesized on the Ps 33 bulk composition moves toward the grossular end member with simultaneously increasing almandine component; concomitantly the hercynite component of the coexistent magnetite increases. The fo2-T-Pfluid relations were determined by employing mineral mixtures of synthetic epidote and its high temperature equivalent in subequal proportions. Equilibrium was demonstrated for the reactions (1) epidote (Ps 33) = anorthite+grandite+FeOx+quartz + fluid, and (2) epidote (Ps 25) (+quartz) = garnet38+anorthite+magnetitc+fluid. With fo2 defined by the HM buffer, epidote (Ps 33) is stable up to 748 °C, 5 kb, 678 °C, 3 kb, and 635 °C, 2 kb Pfluid. With fo2 defined by the NNO buffer, the epidote (Ps 25) high temperature stability limit is reduced about 100 °C at 5kb Pfluid. At slightly lower fo2, than defined by the QFM buffer, epidote is not stable at any temperatures; the assemblage hedenbergite+anorthite+garnet38+fluid replaces epidote in the presence of excess quartz. Combined with previously determined equilibria for prehnite, andradite, and hedenbergite, isobaric fo,-T relations were further investigated by chemographic analysis interrelating the phases prehnite, epidote, grandite, hedenbergite, wollastonite, anorthite, and magnetite in the system CaO-Fe2O3-Al2O3-SiO2-H20. Such analysis allowed the construction of a semi-quantitative petrogenetic grid applicable to natural parageneses in low μCO2 environments, and the delineation of the low temperature stability limit of epidote as a function of fo2. Enlargement of the epidote stability range toward both high and low temperatures with increasing fo2, is consistent with widespread occurrences of epidote in low- and mediumgrade metamorphic rocks.
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