Increase In Metamorphic Grade Research Articles

Notes on the Petrology and Mineralogy of the Barrovian Metamorphic Zones

AbstractThe antipathetic relation between chloritoid and biotite in the lower metamorphic grades of the Barrovian zones and the genesis of the index minerals in the higher metamorphic grades are briefly discussed. The observed mineral assemblages of 13 new analyses of peltic schists are compared with the assemblage required by the mineralogical phase rule; good agreement exists provided allowance is made for magnesia acting as a separate component. Six new biotite analyses covering all of the Barrovian zones show an increase in the degree of replacement of Si**** by Al*** with increase in metamorphic grade. The ratio of FeO: MgO varies according to the associated ferromagnesian minerals and shows no regular relation to metamorphic grade.

Calcium-poor garnet in relation to metamorphism

1. (1) The writer has carried out a systematic study of pyralspites (calcium-poor garnets) in pelitic metamorphic rocks of the Gosaisyo-Takanuki district. With the increase in metamorphic grade of their host-rocks, the MnO content of the pyralspites decreases, while the FeO content increases. Accompanying the variations in composition, the edge length of the unit cell of the minerals shortens and the refractive index becomes generally higher. The associated biotite also varies in composition and proportion with advancing metamorphism. 2. (2) The MnO content of pyralspites in ordinary politic metamorphic rocks generally decreases with the increase in metamorphic grade. When we use main mineral facies as a scale of metamorphic grade, however, the MnO contents of pyralspites in ordinary politic metamorphic rocks of the same mineral facies differ in different districts, owing to the difference in character of metamorphism in different districts. Thus, the MnO contents of pyralspites in ordinary pelitic metamorphic rocks are a useful indicator of the character of metamorphism (Fig. 5). 3. (3) When we treat pyralspites not only from ordinary pelitic metamorphic rooks, but also from rocks of a much wider range of composition, the minerals show a progressive enlargement of their composition field from Mn-rich compositions to Fe-rich ones, and further to Mg-rich ones, with advancing metamorphism, instead of the above-mentioned gradual decrease in MnO content (Fig. 7). 4. (4) The progressive enlargement of the composition field in pyralspite with advancing metamorphism can be explained in terms of crystal chemistry. Mn +2 ions are the most appropriate in size to enter the crystal structure of pyralspite, and so only pyralspite rich in Mn +2 can be formed at very low temperatures. With the increase in the temperature of its formation, ions whose ionic radii are less appropriate to the crystal, such as Fe +2 and further Mg +2, become increasingly free to enter the crystal, and so the composition field of pyralspite becomes progressively larger with advancing metamorphism of the host-rocks. Such cryatallo-chemical properties of pyralspite reveal themselves also in the distribution of Mn +2, Fe +2, and Mg +2 in pyralspites and associated ferromagnesian minerals.