Epidote Amphibolite Research Articles

The petrology of the basic and intermediate rocks of upper Swat, Pakistan

SummaryA series of metamorphosed (amphibolite facies) basic and intermediate igneous rocks from upper Swat, Pakistan, is described, with seven chemical analyses. The rocks intrude ?Palaeozoic metasediments and are partially bordered by other, later, intrusive and volcanic rocks. The group, now represented by, from S–N, epidote amphibolites, amphibolites, noritic and hypersthene gabbros, and quartz diorites, is considered to be derived from a series of plagioclase hypersthenites, norites, hypersthene gabbros, and hypersthene diorites. Variation diagrams are used to show that the series forms a differentiated sequence; FMA and lime-alkali diagrams suggest that it is similar to the rocks of the Garabal Hill–Loch Fyne area of Scotland. Some of the mineralogy of the group is briefly discussed, with five chemical analyses. A K/Ar age determination gives 67 Ma; thus the rocks probably derive from an early Himalayan (Alpine) tholeiitic basalt magma.

METAMORPHIC ROCKS OF THE YAWATAHAMA DISTRICT, WESTERN SHIKOKU, JAPAN

The Sanbagawa metamorphic terrain of the Yawatahama district in western Shikoku is divided into three zones (I, II and II') on the basis of systematic variation of rock-forming minerals.Zone I: This comprises the area of Paleozoic and also Mesozoic formations. Zone I is characterized by the occurrence of prehnite and pumpellyite. But actinolite is rarely observed. This zone is corresponds to the prehnite-pumpellyite metagreywacke facies.Zone II: Prehnite and pumpellyite are not stable. The common minerals of basic rocks are glaucophane, actinolite, epidote, chlorite, white mica, albite and quartz. The physical conditions of this zone are intermediate between those of the glaucophane schist and greenschist facies.Zone II': The hornblende-oligoclase assemblage is characteristic of zone II'. Garnet and/or clinopyroxene are often associated with hornblende. This zone is restricted the area where the Kawamai tectonic mass is exposed. Probably zone II' is correlated to the epidote amphibolite facies. The meatamorphism of zones I, II and II' is progressive one, and then the metamorphic grade increases northwards. The retrogressive metamorphism, however, is observed in zone II'. In many basic rocks of zone II' the primary metamorphic constitures are replaced by chlorite, actinolite, glaucophane and epidote. The retrogressive metamorphism would have been caused by the change of the physical conditions owing to the upheaval movement of the Kawamai mass. It seems that the Sanbagawa metamorphism continued in the Lower Cretaceous age in the Yawatahama district.

ON THE OCCURRENCE OF ANHYDRITE IN THE IRATSU ULTRAMAFIC COMPLEX, CENTRAL SHIKOKU

From the Iratsu ultramafic complex, anhydrite has been newly found in such rock types as clinopyroxenite, hornblendite, epidote amphibolite and marbe. The anhydrite fills generally interstices between other minerals. In the paragenesis of these rock types, the mineral of sulphate is only represented by anhydrite. Although the origin of anhydrite in the Iratsu complex is not clarified, it can safely be said that the component SO3 in inert under the condition of recrystallization related to the formation of the present mineral assemblage of these rock types.

METAMORPHIC HORNBLENDES FROM AN OUTCROP IN THE HITACHI DISTRICT

Six hornblendes and their host amphibolites collected from an outcrop in the Hitachi district were studied chemically. The metamorphic grade of the host rocks corresponds to the higher temperature part of the epidote amphibolite facies. There is no regular relation between the alkali contents in the hornblendes and their host rocks. With increasing Ca/(Ca+Na) ratio of the host rock, this ratio of the hornblende remains nearly constant, although anorthite molecule in the coexisting plagioclase increases. Thus, the variation of Ca/(Ca+Na) ratio of the host rock is mainly reflected on the composition of the plagioclase, but not on that of hornblende.

AMPHIBOLITES ASSOCIATED WITH THE MIYAMORI ULTRAMAFIC INTRUSIVE IN THE KITAKAMI MOUNTAINS

In the Miyamori district of the Kitakami mountains, small amphibolite masses are located at the margin and within the ultramafic intrusive. They were probably formed under the conditions corresponding to the epidote amphibolite and amphibolite facies. They can hardly be interpreted to form at the present position, because they are not isofacial with their surrounding rocks. These amphibolites may be not the products of thermal metamorphism by the Miyamori ultramafic intrusive at deeper levels, but the fragments of the basement complex which is expected to be widely distributed beneath the Paleozoic formations in the Kitakami mountains.

Metamorphic basement rocks of Yap Islands, Western Pacific: possible oceanic crust beneath an island arc

The basement rocks of the Yap Islands are composed principally of greenschist and amphibolite, unconformably overlain by a breccia bed with mid-Tertiary foraminifera and by a deeply weathered andesitic agglomerate. The metamorphic grade increases eastwards from the greenschist to the amphibolite facies, but the epidote amphibolite facies appears to be absent; the metamorphism is of low pressure type. Chemically the rocks of basic igneous origin are characterized by high MgO and low K 2O contents and a high ratio of Na 2O/K 2O, similar to oceanic tholeiitic basalts, but are distinguished from the Tertiary basalts of island arc tholeiitic series in Guam by lower K 2O and somewhat higher TiO 2 contents. The basic metamorphic rocks may represent part of a lower layer of oceanic crust or have formed as the earliest manifestation of island arc development.

A part of the Ubendian Orogenic Belt in northern Malawi and Zambia

The Ubendian Orogenic Belt of southern Tanzania continues across the strike into northern Malawi where it is represented by the Misuku Belt. The Misuku Belt has been examined in two areas on the international boundary between northern Malawi and Zambia; the Mafingi Hills area and the western escarpment of the Nyika Plateau. In each area the Misuku Belt has been divided into three distinctive rock units: 1. the Misuku Gneisses which were metamorphosed in the epidote amphibolite to amphibolite facies, 2. the Jembia River Granulites, metamorphosed under conditions near to the triple point between the cordierite-granulite, granulite and amphibolite facies and 3. the post-tectonic Nthonga and Nyika Granites of the Mafingi Hills and Nyika Plateau respectively. Evidence is provided to show that the two metamorphic units are stratigraphic equivalents which were deformed and metamorphosed in different environments and subsequently placed in juxtaposition. The results of radiometric age determination made on the Misuku Belt provide no evidence of the true age of the granites nor of the time of metamorphism. The apparent ages are considered the result of overprinting during a later tectono-thermal event.

Distribution of iron and magnesium in coexisting biotite, garnet, and cordierite

By S. K. Saxena, N. B. Hollander. Equilibrium of cordierite close to ideal solid solution in epidote amphibolite facies conditions, disequilibrium in high amphibolite or granulite facies, Sweden-Finland

Petrologic comparison of the franciscan and sanbagawa metamorphic terranes

Mafic schists in California carry the typical blueschist facies assemblage of glaucophane—crossite + lawsonite 4 sphene + chlorite ± aragonite or calcite ± pumpellyite. Albite, epidote and actinolite are not common in “in situ” metabasalts. In contrast, typical Sanbagawa schists contain crossite or actinolite + albite + epidote + chlorite ± calcite; the highest-grade rocks generally contain garnet and blue—green hornblende, lower grade rocks may contain pumpellyite and/or stilpnomelane, rarely lawsonite. Franciscan metaclastic rocks show three principle assemblages, all of which contain the phases quartz 4 white mica + chlorite + sphene ±lawsonite ± stilpnomelane: albite 4 calcite, albite 4 aragonite; and jadeitic pyroxene ± aragonite ± glaucophane. Quartzose Sanbagawa metaclastic rocks represent the albite ± calcite type compatibilities; lawsonite, aragonite and jadeitic pyroxene are either absent or are very rare. Many Franciscan siliceous metasediments contain, in addition to quartz, the phases riebeckite, stilpnomelane and/or spessartine. In contrast, many Sanbagawa siliceous schists carry abundant piemontite ± magnesioriebeckite in addition to spessartine. Correlation of the above mineral assemblages with experimental data suggests that blueschist-facies metamorphism took place in the Franciscan terrane at high fluid pressures, and involved temperatures of 200–300°C and lithostatic pressures of 6.5–8 kbar. Apparently Sanbagawa metamorphism reflected physical conditions transitional between the blueschist, greenschist and albite—epidote—amphibolite facies, approximating 250–400°C at 5–7 kbar lithostatic pressure.

Isotopic Ages of Minerals from the Pre-Cambrian Complex of the Malverns

THE Malvern Hills, Worcestershire, form one of many Pre-Cambrian inliers of the Welsh Borderland1; rocks of Pre-Cambrian age are divided into Malvernian ‘schists and gneisses’ and the overlying Warren House volcanic group. The analyses reported in the following communication are from minerals occurring within the supposed metamorphic rocks and do not include any from the volcanic group, no suitable material having been obtained from the latter. The ‘schists and gneisses’ were identified as Malvernian more than a century ago2, but the origin of their structures remained a puzzle, and it was a matter of some controversy as to whether the rocks were igneous or metamorphic3,4. However, most of the rocks seen by us are igneous, usually hydrothermally altered and sheared, but still retaining igneous textures and occasionally igneous mineralogy. Such schistose rocks as have been observed are either in shear zones or in fault blocks, and true gneisses, in the usual sense of high-grade, banded, metamorphic rocks, are absent. The commonest rocks are of intermediate igneous type, classifiable as mafic diorite or tonalite. Amphibolites of various kinds, usually with igneous textures, are another common rock type. Ultramafic rocks are usually rich in tremolite or actinolite, with biotite or chlorite. Granitic rocks are rare, but syenitic veins and pegmatites frequently occur. The usual mineral assemblages are those of the greenschist or epidote–amphibolite metamorphic facies. It is generally agreed that the alteration of the igneous rocks of the Malvernian occurred in the Pre-Cambrian, as no equivalent hydrothermal or metamorphic activity has affected the Cambrian or later strata.

Common hornblendes in metagabbro and amphibolites of Takozu district, Kitakami Mountainland

The ultramafic masses in this district intrude into the green rocks of the Motai formation and the slate of the Tobigamori formation. The green rocks of the Motai formation are partly composed of the glaucophane rocks. Three common hornblendes were separated from the metagabbro and amphibolites, which associate with the serpentain masses, and analysed chemically. The chemical analyses and the atomic ratios are shown in Table 1. The results plotted on the Hallimond's triangle diagram indicates that the hornblendes of the Takozu district are similar in composition to that of the Miyamori and Abukuma districts. Optical and X-ray powder data are shown in Table 2 and 3. The observed mineral parageneses of these rocks are Common hornblende-albite-epidote-biotite-sphene Common hornblende-oligoclase-epidote-chlorite-sphene Lastly, petrogenesis of metagabbro and amphibolite is briefly discussed from the views of mineral paragenesis and mineralogical properties of the hornblendes. It may be that these rocks were retrogressively formed from gabbroic rocks in the condition of epidote amphibolite and amphibolite facies.

On the amphibolitic rocks of the upper course of Syumbetsu River, Hidaka, Hokkaido

In this paper is presented a detailed subdivision of socalled gabbro-amphibolite zone of the Hidaka metamorphic belt, the axis of island of Hokkaido. It is composed of four tectonic units which are arranged in the following order from west to east, as is shown in figure I. 1) green schist zone, 2) epidote amphibolite zone, 3) green hornblende schistose amphibolite zone and 4) saussurite gabbro zone. Concerning zones 1), 2) and 3), their original rock facies are not clearly determinable. The zone of saussurite gabbro is composed of three varieties of amphibolite, a) hornblende schist facies, b) coarse grained amphibolite (=metasomatic) facies and c) compact amphibolite facies, all of which are derived from saussurite gabbro. In such a diverse metamorphic process, the different characters of tectonic forces and combined metasomatic effects have been the chief determinants.

GEOLOGY OF THE WABUSH LAKE AREA, SOUTHWESTERN LABRADOR AND EASTERN QUEBEC, CANADA

Wabush Lake, Labrador, is situated a few miles from where the unmetamorphosed younger Precambrian strata of the northwest-trending Laborador geosyncline are truncated by the southwest-trending structures of the Grenville orogenic province. Field mapping and lithologic comparisons show that the metasedimentary rocks exposed within this portion of the Grenville province can be correlated with the unmetamorphosed strata of the Labrador geosyncline. The distinctive stratigraphic units of the Wabush Lake area are quartzite, marble, muscovite and kyanite schists, and iron formation of carbonate, silicate, and oxide facies. The principal ores of the iron formation are martite-magnetite quartz rock and specular hematite quartz rock. Facies of regional metamorphism range from epidote amphibolite near the postulated “Grenville front” to granulite and migmatite farther south. Superimposed folding throughout the area can be explained as the impression of Grenville province deformation upon earlier structures of the Labrador geosyncline.

Schistose rocks from St. Heliers Bay, Auckland

Summary Epidote amphibolites occurring as accidental blocks in the tuff of the St. Heliers Bay volcano and at Taylor's Hill, Auckland, are described. They are regarded as the product of the retrogressive metamorphism of a gabbroid intrusion into the basement rocks.

GRANITIZATION, MIGMATIZATION, AND FUSION IN THE NORTHERN ENTIAT MOUNTAINS, WASHINGTON

A tabular quartz diorite complex extends along the Entiat Mountains, which form a southeast-trending spur of the Northern Cascade Mountains. The country rocks of the complex are biotite gneiss, probably derived from arkosic sedimentary rocks, and hornblende schist, probably derived from basic volcanic rocks. Their present mineral composition is typical of the upper epidote amphibolite and amphibolite facies of regional metamorphism. During regional metamorphism the biotite gneiss was granitized to biotite-quartz diorite gneiss and the hornblende schist to hornblende-quartz diorite gneiss. The metamorphic origin of these quartz diorite gneisses involving relatively little replacement is shown by their chemical and mineralogical similarity to the original biotite gneiss and hornblende schist, by their enclosing long, thin, undisturbed layers of country rocks, and by their association with migmatites formed by metamorphic processes. Here and there in the quartz diorite gneisses massive rocks with hypidiomorphic granular “igneous” textures have formed principally by recrystallization, and these features are associated with small-scale swirling of the foliation and some intrusive features, which show that the gneiss was rendered plastic and mobile during granitization. Migmatites containing replacement bodies and secretions of leucocratic quartz diorite occur in the biotite gneiss and in most of the rocks of the complex—particularly in the biotite-quartz diorite gneiss. During the formation of the complex, the felsic material contained in the leucocratic rocks, principally sodium, potassium, and silicon, was generated by metamorphic differentiation. During the granitization, anatectic magmas were formed by fusion of hornblende-quartz diorite gneiss, and some similar magma intruded from below. Melting without metamorphic differentiation is indicated by the chemical similarity between the anatectic quartz diorite and the parent hornblende-quartz diorite gneiss. Smearing out of inclusions and crystals and the tabular, sill-like nature of the bodies indicate that fusion may have been triggered by differential movement and shearing. The anatectic rocks contain inclusions of hornblende schist that survived both the granitization to hornblende-quartz diorite and the fusion. Crystallization differentiation of the anatectic magmas yielded potassium-rich pegmatites and local granodioritic masses. Regional metamorphism continued after the anatectic magmas in the sills had solidified; felsic metamorphic differentiates occur in leucocratic quartz diorite that partly replaces the borders of the sills. The leucocratic material was plastic and facilitated differential movement between the sill and its walls.

TRONDHJEMITE BATHOLITH NEAR BIDWELL BAR, CALIFORNIA

Bald Rock batholith, California, is one of a chain of small plutons that lies 20 miles west of the main Sierra Nevada pluton and parallels the N.15°W. structural trend of the northern Sierra. The country rocks are mainly late Paleozoic (?) basic volcanic rocks cut by small masses of basic plutonic rocks prior to late Jurassic time and folded and dynamothermally metamorphosed in the Nevadan orogeny. Foliation in the pre-batholith terrain was vertical, and its mineral assemblages were, with local anomalies, typical of greenschist facies. After regional metamorphism, gabbroic, dioritic, and granitic rocks were emplaced south and east of the site of Bald Rock batholith. The basic rocks of this sequence are in part magmatic, but the granitic bodies show gradational borders and internal structures that indicate static metasomatic growth, perhaps related to magmatic cores. Bald Rock batholith was a forceful intrusion, as is shown by the abrupt concordant swing of bedding around it. However, local crosscutting relations show that about a fourth of its area at the exposed level was gained by other means. Large concentric outliers and a hull of injection migmatite suggest stoping. Inclusions are too scarce to account for the missing wall rocks, but gradational zoning of the batholith from a trondhjemite core through granodiorite to a heterogeneous tonalite rim suggests that basic stoped rock contaminated an originally trondhjemitic magma. This is supported by comparing a computed chemical composition of the granodiorite and tonalite with chemical analyses of two trondhjemites. The benign nature and pattern of flow structures shows that the different granitic rocks could not have been intruded separately. In contrast to the wall rock, plagioclase of the granitic rocks shows pronounced oscillatory zoning, indicating thorough reworking of the stoped rock. The reactions required for the changes were presumably exothermic, but the broad mixing shown here must have required additional heat. Contact-metamorphic rocks of epidote amphibolite to possible pyroxene hornfels facies form an aureole that had an area nearly as great as the original kinetic intrusion. Squeezing and vertical stretching of these rocks is indicated by pebble and cyrstal lineation, most pronounced near the contact. Post-kinetic replacement veins of tonalite occur sporadically throughout the inner part of the aureole. The rise in anorthite content of plagioclase in metabasaltic rocks is gradual toward the batholith and sharp near the tonalite veins. The obvious fracture control of these high-temperature metamorphic structures indicates that heat addition to the aureole was largely through fluid transfer rather than simple thermal diffusion.