i) Zonal Classification
The metamorphic terrane of the northern Kanto Mountains is divided into four zones I, IIa, IIb and III. Spatial distribution of metamorphic mineral assemblages and zones are indicated in Fig.III-1. The mineralogical variations of basic and pelitic rocks are shown in Fig. III-2. Zones in the order of increasing metamorphic grade are defned by the mineral assemblages in the basic rocks as follows.
Fig. III-1. Zonal map and distrinbution of mineral assemblages in basic rocks in the Kanto Mountains. |
- Zone I:
- This zone is characterized by the prehnite+pumpellyite+chlorite assemblage. Neither actinolite nor sodic amphibole are observed in the basic, politic and psammitic rocks.
- Zone IIa:
- This zone is characterized by disappearance ofprehnite and appear ance of pumpellyite+ actinolite +chlorite assemblage in basic rocks. Sodic amphibole is common in basic rocks.
- Zone IIb:
- This zone is characterized by the assemblage of actinolite +epidote + glaucophane+chlorite and appearance of omphacite+albite+quartz assemblage in basic rocks. Lawsonite is present but not common in basic rocks.
- Zone III:
- This zone is characterized by the disappearance of pumpellyite, sodic amphibole, lawsonite and omphacite and by the appearance of the actinolite+epidote+chlorite+barroisitic amphibole in basic rocks.
The boundaries between the zones are roughly in harmony with the stratigraphi cal boundaries. Zone I comprises the southern marginal zone of this area, where the Kamiyoshida and Mamba formations are exposed. Zones IIa and IIb run through the area where the Chichibu group and the Upper Sambagawa formation are ex posed, respectively. Zone III comprises the northern marginal area of this district, where the Lower Sambagawa formation is exposed.
ii) Mineral Facies
Zone of the Prehnite-pumpellyite facies
Zone I.
Basic rocks.
1) Prehnite+pumpellyite+chlorite+calcite+albite+quartz
2) Pumpellyite + chlorite + albite + qu artz
3) Chlorite + calcite + albite + quartz
4) Chlorite + epidote + albite + quartz
Assemblages 2) and 3) are most common in basaltic lava and pyroclastic rocks, in which relic augites are common and scarcely altered to chlorite. Assemblages 1) and 3) are common in the basaltic rocks in which subophitic texture is found. Veins and amugdules filled by the assemblages 1), 2) and 3) are common.
Pelitic and psammitic rocks.
1) Muscovite+chlorite+albite+calcite+quartz+sphene.
In zone I, mudstone and sandstone are scarcely metamorphosed.
Zones of the Glaucophane schist facies.
Zone IIa.
Basic rocks.
1) Chlorite + pumpellyite + actinolite + magnesioriebeckite + aegirine-augite + talc + epidote
2) Chlorite +pumpellyite + actinolite + magnesioriebeckite + aegirine-augite + epidote
3) Chlorite+ pumpellyite + calcite
4) Chlorite + actinolite + epidote + calcite
5) Chlorite+magnesioriebeckite+calcite
6) Chlorite + calcite
The assemblages 1) and 2) are common in weakly metamorphosed basaltic rocks containing many relic titaniferous augites and small amounts of calcite, albite, quartz and sphene. Weakly metamorphosed glassy basalts contain assem blages of types 3) and 5), whereas in doleritic rocks the assemblage 4) is common. Metamorphosed basaltic tuff contains assemblages 5) and 6) with abundant mus covite, albite, quartz and calcite. Veins and amydgules in the glassy basaltic rocks are filled by the chlorite+albite+quartz, and pumpellyite + chlorite + albite + quartz assemblages.
Pelitic and psammitic rocks.
1) Chlorite + muscovite + albite + quartz + calcite + sphene + tourmaline + stilpnomelane + graphite.
2) Chlorite + muscovite + albite + quartz + calcite + sphene + graphite + tour maline.
The assemblage 2) is common in this zone. In muddy chert alkali amphibole is sometimes found.
Zone IIb.
Basic rocks.
1) Chlorite + actinolite + alkali amphibole + epidote
2) Chlorite + actinolite + epidote
3) Chlorite + alkali amphibole + epidote
4) Chlorite + actinolite + pumpellyite + epidote
5) Chlorite + actinolite + lawsonite + epidote
6) Chlorite+omphacite+actinolite+ epidote
7) Chlorite + epidote
8) Chlorite
The assemblages 1), 2) and 4) are common and coexist with albite, quartz, sphene and small amounts of calcite and muscovite. The assemblages 3) and 7) are also fairly common, and contain abundant calcite, albite, and quartz, and small amounts of muscovite. The assemblages 3) and 7) are also fairly common, and contain abundant calcite, albite, and quartz. The assemblages 6) and 8) are rare, and the former assemblage contains ferri-phengite, albite and quartz, but the latter does contain only sphene and muscovite. The assemblage 5) is rarely found and is restricted to the gabbroic rocks. Relict augite occurs in the rocks with assemblages 1), 2), 4) and 5), and it is partly altered to aegirine-augite, and actinolite. In rocks with assemblage 4), pumpellyite shows lenticular aggregate of fine grains, being surrounded by films of chlorite. Compositional banding is common in the schists with assemblages 1), 2), 3) and 4).
Pelitic and psammitic rocks.
1) Chlorite + epidote + muscovite + stilpnomelane + tourmaline
2) Chlorite + epidote + muscovite + tourmaline
3) Chlorite + calcite + tourmaline + muscovite
The assemblages 2) and 3) are common, and assemblage 1) is also found, as sociated with quartz, albite, sphene, graphite and apatite.
Zone of the greenschist facies.
Zone III.
Basic rocks.
1) Barroisite + epidote + chlorite + biotite
2) Barroisite + epidote + chlorite
3) Actinolite+ epidote + chlorite
The assemblage 2) is common in this area, and it is associated with actinolite, albite, quartz and sphene. Barroisite shows compositional zoning from barroisite to actinolite. The assemblage 1) occurs in the highest grade rocks of this zone which comprises the axial part of anticline at Tsukiyoshi. The assemblages 1), 2), and 3) are associated with quartz, albite, sphene and small amount of muscovite and apatite.
Pelitic and psammitic rocks.
1) Chlorite + epidote + garnet + sphene + graphite
2) Chlorite+epidote+sphene
3) Chlorite+garnet+sphene
The most common assemblages are 3) and 4), and they are associated with muscovite, quartz, albite, graphite and small amounts of calcite and apatite. The assemblages 1) and 2) are also common.
iii) Chemical Equilibria Defining Isograds
Isograd between zones I and IIa:
The representative mineral assemblages in basic composition are pumpellyite +prehnite +chlorite +calcite and pumpel- lyite+chlorite+actinolite+magnesioriebeckite in zones I and IIa, respectively. Thus, the isograd between them is defined by the following univariant equilibrium relation;
71Ca2Al2Si3O10(OH)2+7Mg7Al4Si4O15(OH)12+8H2O
=3Ca2Mg5Si8O22(OH)2+34Ca4MgAl5Si6O23(OH)32H2O+12SiO2,
prehnite +chlorite +H2O = actinolite+pumpellyite+quartz.
Modal amounts of calcite decrease abruptly, and prehnite disappears in zone IIa basic rocks, whereas in zone I, modal amounts ofpumpellyite are very low, thereby showing the reality of this equation as the isograd between zones I and IIa.
Isograd between zones IIa and IIb:
The representative assemblages of zones IIa and IIb basic rocks are epidote + chlorite + actinolite + glaucophane and pumpellyite + chlorite + actinolite + magnesioriebeckite assemblages, respectively. The isograd between these zones is defined by the following univariant chemical equilibrium;
34Ca4MgAl5Si6O23(OH)32H2O + Mg16Al4Si10O30(OH)24 + 32SiO2
= 10Ca2Mg5Si8O22(OH)2 + 58Ca2Al3Si3O12(OH) + 90H2O,
pumpellyite + chlorite + qu artz = actinolite + clinozoisite + H2O.
From the substitution of Fe for Mg in actinolite, pumpellyite, and chlorite, and the mixing of CO2 and H2O, this equation becomes multi-variant in the basic composition, and hence, the boundary between zones IIa and IIb defined by this equation is broad. Otherwise, modal amounts of pumpellyite decrease abruptly in zone IIb, whereas those of actinolite and epidote increase. Moreover, chemical composition of epidote and chlorite becomes rich in Al2O3-content with increasing grade. Chlorite has the composition of (MgFe)12Al6Si7O25(OH)20 in zone IIa basic rocks, and thus the Al/Mg ratio of this chlorite is higher than that of chlorite in equation mentioned above.
Isograd between zones IIb and III:
The mineral assemblages are chlorite+ actinolite + glaucophane + epidote and barroisite + epidote + chlorite assemblages in zone IIb and zone III basic rocks, respectively. The composition of barrosite is approximated as the mixture of actinolite and Ts50Gl50* components. Therefore, the following divariant chemical equation relates these two assemblages;
6Ca2Al3Si3O12(OH) + 12NaAlSi3O8 + Mg36Al6Si23O65(OH)52 + 7SiO2
=6Ca2Al4Mg3Si6O22(OH)2 • 6Na2Mg3Al2Si8O22(OH)2 + 17H2O,
clinozoisite + albite + quartz + chlorite = Ts50Gl50 + water.
This equation gives the solubility limit of Ts50Gl50 component in calciferous amphibole. In zone III basic rocks, the Ts-Gl content in amphibole is consider ably high as compared with that in zone IIb basic rocks. Further, chlorite becomes rich in Al2O3 in zone III. Thus, the boundary between zones IIb and III may be well denned by this equation at the fixed PH2O.