Calciferous Amphiboles Research Articles

Qualitative thermobarometry of inverted metamorphism in the Pelona and Rand Schists, southern California, using calciferous amphibole in mafic schist

Abstract The Rand and Pelona Schists consist of eugeoclinal rock types overlain by continental basement along the Vincent‐Chocolate Mountains (VCM) faults. Both schists display inverted metamorphic zonation, defined in part by a systematic variation in composition of calcic to sodic‐calcic amphibole in mafic schist structurally upward. The compositional progressions include increase of total A1, A1IV and Ti, but decrease in the ratios of Na/(Na + Ca) to A1/(A1 + Si), and NaM4 to (A1VI+ Fe3++ Ti). These variations imply that structurally high rocks belong to a lower‐pressure metamorphic fades series than those at depth. This result is consistent with previous views that the inverted metamorphic zonations represent intact structural sequences.Amphibole composition is dependent not only on structural position (i.e. P‐T), but also upon bulk‐rock composition. The important controls are whole‐rock Mg/(Mg + Fe2++ Mn) and Fe3+/Fe2+. The greatest impact of these factors, however, is on the absolute values of Na and Al, rather than their ratio. Thus, interpretation of facies series is not seriously hindered by compositional variability.Sodic amphibole in epidote blueschists from the Rand Schist is extensively replaced by sodic‐calcic amphibole. Sodic‐calcic amphibole in the Rand Schist and Pelona Schist is, itself, rimmed by actinolitic amphibole. Similar blueschist to greenschist transitions in other metamorphic terranes are typically attributed to exhumation. In the Rand and Pelona Schists, the sequence probably formed during burial.

Twin Sisters dunite: Petrology and mineral chemistry

Petrologic study of the Twin Sisters dunite, an alpine-type ultramafic body about 90 km 2 , has led to the establishment of three broad stages in the recrystallization of the body. The first stage is represented by “primary,” now highly strained, large olivine crystals, which were subsequently partly recrystallized and/or mylonitized in the upper mantle. The development of chromite pods and lenses occurred in this first stage. The second stage, separated from the first by deformation, is manifest by the segregation, probably also in the upper mantle, of at least two generations of olivine-bearing orthopyroxenite and clinopyroxenite veins. The third stage is represented by the generation of metamorphic minerals exemplified by the following reactions: enstatite + diopside + H 2 O = forsterite + tremolite; enstatite + H 2 O = talc + forsterite; chromite + forsterite + enstatite + H 2 O = ferritchromite + chlorite. Electron microprobe study of coexisting minerals indicates that olivine, orthopyroxenes, and clinopyroxenes are remarkably homogeneous. Minor but systematic variations in mineral compositions do exist. Olivine in chromite pods and lenses is more forsteritic and contains nearly twice as much NiO than olivine in the main dunite and pyroxenite veins. Chromite compositions show two distinct but parallel trends — one for chromite present in the main dunite and in veins and the other for chromite in pods and lenses. As in olivine, X Mg in chromite pods and lenses is higher than that in chromite from the main dunite. Unlike the trends for the Kilauean chromites, total Fe increases with increasing Cr 2 O 3 content, whereas Al 2 O 3 and MgO decrease. Calcic amphiboles are heterogeneous, and important end members in different samples are pargasite, edenite, and tremolite. Anthophyllite coexists with calciferous amphiboles. Magnesian cummingtonite was also found. A consistent apparent equilibration temperature of 1100 to 1200 °C, derived from the olivine-chromite geothermometer, is 138 to 333 °C higher than that derived from the Ca content of orthopyroxene and clinopyroxene pairs. Temperature of unmixing in pyroxenes is about 115 °C lower than that indicated by their bulk composition.

Iron- and manganese-rich minor intrusions emplaced under late-orogenic conditions in the proterozoic of South Greenland

Three small intrusions in Ketilidian gneisses near Julianehaab comprise sheets and veins of olivine-magnetite-grunerite or magnetite-amphibole rocks partly surrounded by garnetiferous hornblende-biotite granitoid rock. The latter “skin” locally widens out into diffuse bodies of fayalite-orthopyroxene-quartz syenite or monzonite and biotite “granite”, which show layering similar to that resulting from gravity settling of crystals. Near the intrusions the country rocks lose their foliation and have been partially melted. Intrusion probably occurred at the close of regional metamorphism ∼ 1,750–1,780 m.y. ago, just prior to emplacement of the rapakivi granite suite of South Greenland. The mafic minerals of the intrusions are markedly enriched in iron and, in the case of olivine, orthopyroxene, grunerite and garnet, in manganese as well: olivine Fa90Te5Fo5; orthopyroxene (inverted pigeonite) Ca2Fe77Mn6Mg15; calciferous amphiboles are typically hastingsitic; biotites generally have Fe/Fe+Mg ratios of 0.8; garnets are almandine-grossularite-spessartine mixtures; essentially pure magnetite is the dominant oxide mineral and ilmenite is only moderately manganiferous. Crystallization of the mafic rocks appears to have followed the trend of the quartz-fayalite-magnetite buffer curve from perhaps ∼800°C to <550°C at pressures, calculated from thermodynamic considerations, of 4 to 5 kb. However, the presence of Mn makes estimates of pressure and temperature uncertain. Comparison with other late- to post-orogenic intrusions—the South Greenland and Finnish rapakivi granite suites, the Labrador “adamellite” complex and the Pikes Peak batholith of Colorado—reveals both similarities and differences, particularly with respect to mineral parageneses, depth of emplacement and manganese enrichment.

A NEW GRAPHIC REPRESENTATION FOR CALCIFEROUS AND SUBCALCIFEROUS AMPHIBOLES SOLID SOLUTION

A new graphic method for calciferous and subcalciferous amphiboles is proposed on the basis of the electric neutrality of a crystal. The nine basic types of the cation-substitutions are discussed with the diagram which is constructed by the three axes of (A1++X1+), (Y3++Z3+)/4 and (X2++Y2++Z4+)/15. The solid solution of calciferous and subcalciferous amphiboles can be derived from the triply composite substitution constructed by 2 (X2+, Y2+)_??_(A1+, X1+)+Y3+, Z4+_??_(A1+, X1+)+Z3+ and (X2+, Y2+)+Z4+_??_Y3++Z3+, mainly. In the new graphic representation, there exist compositional gaps in the actinolitehornblende solid solution and in the subcalciferous amphibole solid solution under metamorphic conditions. When X2++Y+Z=15, calciferous amphiboles with Y3++Z3+=1.34-1.50 do not appear in nature. The compositional gap does not exist between calcic and subcalcic hornblende solid solution, but the gap between calcic and subcalcic actinolite solid solution may exist. On the basis of the consideration by the new graphic method, if the host rock compositions are in isochemical, metamorphic amphiboles change the variation trend of their compositional range according to the progressive mineral facies changes.

Petrology of the alkali syenites of the Mundwara magmatic suite, Sirohi, Rajasthan, India

SummaryThe alkali syenites of the Mundwara magmatic suite, Sirohi district, Rajasthan, in north-western India, are chiefly developed in the Musala hill of the complex. They comprise three principal varieties representing different stages of crystallization, viz. nepheline-sodalite syenite, nepheline-barkevikite microsyenite, and nepheline poor leucosyenite. The dominant constituent of these hypersolvus syenites is alkali feldspar, an orthoclase microperthite showing a variable degree of exsolution. The chief mafic mineral is a distinctly green and zoned sodic pyroxene, close to aegirine-augite in composition. A brown calciferous amphibole, identified as a barkevikite, is developed in addition in the microsyenites. Reddish brown biotite of the phlogopite-annite series is common to all the syenites. The petrography and mineralogy of the syenites are discussed, and a comparative study of the Mundwara syenites with similar rocks of the Deccan volcanic province is presented. Chemistry and mineralogy of the syenites of the Mundwara suite suggest that they are related to those associated with the Deccan volcanic province.

On aluminous and edenitic hornblendes

SummaryBased on nearly 1500 published amphibole analyses the maximum possible Alvi in hornblendes is shown to increase with increase of Aliv. New analyses of hornblendes from amphibole-corundum rocks, with and without anorthite, are given and after critical examination of the available data it is concluded that the maximum verified Alvi-rich calciferous amphibole that approaches the closest to hypothetical tschermakite comes from a kyanite-bearing aluminous high-pressure-crystallized schist from Lukmanier, Switzerland. Pure natural edenite or ferroedenite is unknown, but a new analysis of the nearest known natural edenite, from Mysore, India, agrees with the postulated view that extraordinarily low temperatures are needed for edenite-ferroedenite crystallization, much below that possible in magmas and only rarely achieved in metamorphic rocks containing amphiboles. The limit of the approach of igneous hornblendes to edenite-ferroedenite and tremolite-ferroactinolite is outlined.At least 1100 °C is required for complete expulsion of water from some amphiboles.

METAMORPHIC ROCKS OF THE HITACHI DISTRICT IN THE SOUTHERN ABUKUMA PLATEAU

The Paleozoic rocks of the Hitachi district in the southern Abukuma Plateau suffered metamorphism ranging from the greenschist facies to the amphibolite facies through the epidote-amphibolite facies. This area can be divided into the following five zones by the stability relations of some metamorphic minerals. Zone I: chlorite zone Zone II: biotite zone Zone III: epidote-hornblende-sodic plagiocalse zone Zone IV, V: sillimanite zone The metamorphism is characterized by the behavior of the anorthite content in plagioclase, the chloritoid-Fe-chlorite-anorthite assemblage, the optical property of calciferous amphibole, the assemblage of cummingtonite-epidote-sodic plagioclase and the association of Al2SiO5 polymorphism. The association of andalusite-kyanite-sillimanite shows the following relation, These minerals probably grew metastably close by the triple point of these polymorphs. The chloritoid-Fe-chlorite-anorthite assemblage can be interpreted by the Po2 condition at the various stages of the metamorphism; chloritoid and Fe-chlorite are sensitive to Po2 and Fe-chlorite-chloritoid-magnetite assemblage occurs at the low Po2 condition. And as a result of dehydration reaction at the low Po2 condition, anorthite did not decompose to epidote but reacted to Fe-chlorite to produce chloritoid, ilmenite, quartz and calcite. The assemblage of cummingtonite-epidote-sodic plagioclasc indicate the higher PH2O condition of metamorphism; under the metamorphic condition such as the epidote-amphibolite facies, anorthite is unstable and the reaction between anorthite mole and vapor may begin easily to produce zoisite, so that under the higher PH2O and Po2 condition the reaction of producing cummingtonite-epidote-oligoclase may happen with progressive increasing of the metamorphic grade. The anorthite content in plagioclase is generally slower incrcasing with the rising of the metamorphic grade on account of the nature of metamorphism itself and peristerite may occur at a part of the lower metamorphic grade in this district. The optical property of calciferous amphibole is characterized by the small optic axial angle and the wide range of refractive index. Some amphiboles are plotted in field M by Miyashiro's classification. Reflecting on these characters, the metamorphism in the Hitachi district can be explained to be single progressive metamorphism without “polymetamorphism”. The Hitachi metamorphic facies series is of rather higher pressure and/or lower temperature than the central Abukuma Plateau and is characterized by heterogeneous PH2O or Po2 condition of the metamorphism.

Paragenetic types of calciferous amphiboles of metamorphic rocks

A statistical study of about 550 chemical analyses of amphiboles in the hornblende-actinolite series makes it possible to recognize their paragenetic types. The similarities or differences in the above types have been demonstrated by applying Student's t-test and Fischer's test. Typical hornblendes of regional metamorphic rocks are stable within the range 900 to 550°C. Actinolite is characteristic of greenschist facies rocks that formed at temperatures below 550°C. The contents of AlIV and alkalies decrease with decreasing temperatures and the content of AlIV increases with significant increase of pressure. The Al content also increases (due to AlvI) during the transition from clinopyroxene bearing rocks to garnet bearing rocks. Correlations revealed some peculiarities in isomorphism which confirmed the scheme Fe+3AlIV → MgSi, but failed to confirm the replacement of AM AlIv by MgSi. The role of a glaucophane component has been shown. Curves showing the relationship of Ng and f in the various paragenetic types of amphiboles have been derived using regression equations.

Calciferous Amphibole rich in Sodium from Jadeite-bearing Albitite of Kotaki, Niigata Prefecture

Calciferous Amphibole rich in Sodium from Jadeite-bearing Albitite of Kotaki, Niigata Prefecture

Aluminium replacing silicon in some silicate lattices

The purpose of this account is to review, from the literature, figures for the replacement of silicon by aluminium in biotites, calciferous amphiboles, and clinopyroxenes. Atomic ratios to be quoted have been calculated on a basis of 24(O,OH,F) in biotites, hornblendes, and pyroxenes alike. This facilitates comparison and the higher factor magnifies differences. It may be emphasized, to avoid possible confusion, that in this discussion it is the amount of silicon replaced by aluminium that will be under review. The ratio of tetrahedrally co-ordinated Al to octahedrally co-ordinated A1 will not be considered, although for the sake of completeness, figures for AI in Y groups will be furnished to show total A1 values. Machatschki's group notation is employed. Comparison between biotites, hornblendes, and clinopyroxenes, for present purposes, may best be made between minerals that crystallized under the same pressure-temperature conditions, to reduce variables. Few analyses of specimens of these three minerals, that have crystallized together in the same rock, are available however. One such paragenesis is quoted below. This association occurs as gneiss-like inclusions in calcite, taken from the Tiree marble.