Metamorphism chlorites

This has been confirmed in the present study for chlorites from six rocks from the Belt series in northern Montana (Harrison and Jobin, 1963) and three samples from the outer zones in the Alpine chain. From these analyses, it is apparent that metamorphic chlorites which have crystallized with muscovite have low silica contents and a rather limited alumina content (27 3% A1 ions). The relatively large variation in chlorite Fe-Mg ratio from rock to rock suggests that bulk rock composition is more important in determining the chlorite composition than is the case of chlorites from clay mineral facies. 

Thus we see that there is less grain-to-grain variation in metamorphic chlorites than in chlorites from clay mineral assemblages. 

Compositions. Berthierines are ferrous almost to the total exclusion of Fe (Brindley, 1982). Diagenetic and metamorphic chlorites contain... 

In thermal metamorphism, biotite occurs in the clorite-sericite facies as a dispersed phase in the argillitic matrix and is stable up to low-grade cornubianites. In regional metamorphism, biotite is typical of argillitic and pelitic rocks up to the staurolite-garnet facies (biotite, biotite-sericite, biotite-chlorite, and albite-biotite schists, and garnet-staurolite micaschists). 

In metamorphic rocks, muscovite occurs in low-grade terrains of the regional metamorphism (albite-chlorite-sericite schists). It must be noted here that the term sericite identifies fine-grained white micas (muscovite, paragonite). 

Biotite Gabbros, norites, diorites, granites, pegmatites Phyllites, chlorite-sericite schists, biotite facies of the regional metamorphism, gneisses ... 

The diagnostic mineralogy of sub-greenschist metamorphism and propylitic alteration of altered plutonic and volcanic rocks associated with porphyry Cu-Mo deposits in the Collahuasi district is summarized in Table 1. Chlorite and epidote originating from regional meta-... 

Retrograde metamorphic processes, linked to hydrothermal fluid circulation, finally produced a new mineral assemblage constituted mainly by tremolite-actinolite> epidote + chlorite + quartz + sericite + titanite + hematite. The amphibole appears as green fibrous crystals over clinopyroxene and other anhydrous minerals. 

Microlithofacial classification of the sandstones is based on Dott s classification modified by Pettijohn et al. (1972). They are mostly arenites and subarkose and quartz wackes (rare sublithic, sporadically lithic and arkosic). Quartz is the main component of the sandstones (about 60-70 vol. percent). Feldspars (6 vol. percent) are mostly represented by potassium feldspars with plagioclases in lesser amounts. Some micas (muscovite and biotite) and chlorites are observed. Mica content of arenites reaches 3 vol. %, but is higher in the wackes. Heavy minerals present include zircon, sphene, rutile and apatite. Magmatic rocks (volcanic more than Plutonic) are predominant among lithoclasts (about 2 vol. %), but some metamorphic and sedimentary clasts being present too. 

Typically illite-rich sediments develop the assemblage chlorite-illite-quartz or chlorite-illite-biotite-quartz upon epi-metamorphism (Maxwell and Hower, 1967 Dunoyer de Segonzac, 1969). 

Figure 11 indicates the necessary change in composition which a muscovite would need to become stable under conditions in a sedimentary rock where chlorite is present (x to y). The solid solution for mica-illites is delimited by the shaded area which represents a much larger variation than is possible under metamorphic or igneous conditions. The detrital muscovite (composition x) is in itself stable if the bulk composition of the sediment as projected into the coordinates is found at x. 

First we will consider the chlorite known as berthierine. These are the pelletal ocean bottom 7 8 chlorites. Those reported in the literature (Velde, et al,, 1974 Leone, et al., 1975) and several new analyses from Alpine samples indicate a homogeneous composition throughout the pellet. These samples have undergone metamorphism and the minerals now have a 14 8 polymorph. This may well explain their relative compositional homogeneity. The analyses done on many single grains (with microprobe) show some scatter of compositions but they all lie within the 7 8 chlorite... 

Figure 31b. Compositions of chlorites in the mixed-layered mineral facies of pelitic rocks (circles) and from the illite-chlorite facies (barred circles). Shaded area shows chlorite compositions from muscovite-chlorite metamorphic rocks.

It appears, using a limited number of samples, that low "metamorphic" or diagenetic grade in the clay mineral facies produces chlorites of... 

The compositional variation from grain to grain is greater in low temperature 14 8 chlorites and this scatter decreases as metamorphic grade increases. Low temperature chlorites tend to be ferrous and aluminous. 

It appears that increasing metamorphic grade tends to give a 30% aluminum atomic content in chlorites and often to decrease the iron content (Figure 31b). 

Most likely, the chemical system remains closed, as far as the other components in the silicate phases are concerned, as diagenesis or low grade metamorphism becomes more evident. Although there may be transfer of calcium, it seems, from bulk chemical analysis, that there is no systematic increase in potassium nor decrease in sodium content of argillaceous sediments. The transfer of Na and K is between the two size fractions—clay and coarse fraction—or between phyllosilicates and tectosilicates. Albitization of argillaceous rocks should be a common phenomenon where mixed layered phases are predominant in clay assemblages and especially evident in the illite-chlorite zone. 

DUNOYER DE SEGONZAC (G.) and HEDDEBAUT (C.), 1971. Paleozoique anchi-metamorphism a illite, chlorite, pyrophyllite, allevardite et paragonite dans les Pyrenees basques. Bull. Carte ser. Geol. Als. Lorr. 24, 277-90. 

AMPHIBOLITE. The amphibolites form a large group of rather important rocks of metamorphic character. As the name implies they are made up very largely of minerals of the amphibole group. There may be also a variety of other minerals present, such as quartz, feldspar, biotite, muscovite, garnet, or chlorite in greater or lesser amounts. 

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