Calc-silicate Gneisses Research Articles

The pre-Ketilidian history of the gneisses north-west of Frederikshåbs Isblink, Fiskenæsset region

The geometry and field relationships of the gneisses and their associated rocks in the area immediately north-west of the Frederikshåbs Isblink reflect a complex intrusive, metamorphic and deformational history. All the rocks have been affected to a greater or lesser extent by amphibolite facies metamorphism and they comprise predominantly dark grey biotite and hornblende gneiss, together with thin anorthosites (mostly brecciated to white agmatites), intercalated striped green and dark grey amphibolite (sometimes garnetiferous), black hornblendites, gamet-kyanitesillimanite gneisses, sillimanite quartzites and pegmatitic pale creamy brown marbles and calc-silicate gneiss and fine-grained leucocratic gneisses. Early discordant layers include hornblendites, amphibolites, peridotites, silico-carbonatites and aplitic rocks.

Autochthonous granodiorites north-west of Frederikshåbs Isblink, Fiskenæsset region

The gneiss complex immediately north-west of Frederikshåbs Isblink is composed mostly of amphibolite facies, grey, streaky, quartzo-feldspathic, biotite and hornblende gneisses. Subordinate amphibolites, homblendites, thin anorthosites, peridotite lenses up to 200 m across, rare concordant marble bands and associated calc-silicate gneisses and kyanite and sillimanite gneisses make up the bulk of the remainder, and the whole has been complexly deformed. Occupying the higher ground within the complex, and comprising the largest single lithological unit there is a considerable volume of massive weathering, porphyroblastic granodiorite and granodiorite gneiss. Field relationships show this to be unquestionably autochthonous and almost certainly of palingenetic origin and while on a small scale the 'contact' with the surrounding gneisses may be locally discordant, overall it follows closely the large scale structure and this suggests its control by preexisting lithology.

The Petrology of Moinian Calc-silicate Gneisses from Fannich Forest, and their Significance as Indicators of Metamorphic Grade

The Petrology of Moinian Calc-silicate Gneisses from Fannich Forest, and their Significance as Indicators of Metamorphic Grade

Regional Metamorphism, Metasomatism, and Partial Fusion in the Northwestern Part of the Okanogan Range, Washington

The northwestern part of the Okanogan Range, Washington, (in the vicinity of 49°00 ′N., 120°00′W.) comprises regionally metamorphosed rocks, largely of supracrustal origin, plus igneous rocks ranging in composition from quartz diorite to quartz monzonite. Metamorphic rocks include quartz dioritic to trondhjemitic orthogneisses, biotitic and hornblendic schists, gneisses and hornfelses, amphibolites, and calc-silicate gneisses. Mineral assemblages indicate that the region was metamorphosed more or less uniformly under conditions equivalent to the sillimanite-cordierite-orthoclase-almandine subfacies (cordierite amphibolite facies) with possible local transition to granulite facies. The presence of andalusite plus sillimanite indicates metamorphism of the Abukuma type (andalusite-sillimanite facies series) in contrast to the kyanite-sillimanite facies series in the Northern Cascade Range to the west. A single cycle of synkinematic metamorphism followed by a phase or separate cycle of static recrystallization in essentially the same P-T field is shown by rock textures and mineral assemblages. The parent material of the metamorphic rocks included mafic flows, sills and dikes, intermediate composition volcanic rocks and graywackes, minor amounts of carbonate-rich and pelitic sedimentary rocks plus an indeterminate amount of leucocratic igneous rock. Prior to metamorphism the average composition of the parent material was approximately tonalitic. The igneous rocks include intrusive granodioritic and quartz monzonitic rocks (average composition is granodiorite) of the Cathedral batholith (Daly, 1912) with an apparent age of 94.0 ± 2.8 m.y. (K/Ar date on biotite) and older trondhjemitic to leucogranodioritic rocks. The older igneous series comprises partly gneissose syntectonic intrusive rocks, and directionless late to post-tectonic rocks which lack intrusive contacts. The latter are interpreted as anatectites formed by (partial) fusion of rocks of quartz dioritic to trondhjemitic composition during regional metamorphism. Field and petrographic data indicate that the anatectites have not been deformed. The average chemical composition of the older igneous series is trondhjemitic. A small layered gabbroic pluton, characterized by iron-rich olivine and pyroxene, has intruded the metamorphic series. Its age relative to the Cathedral batholith is not known. The metamorphism was pre-late Cretaceous and, by analogy with surrounding areas, probably was pre-mid Jurassic but post-Paleozoic. The age of the eugeosynclinal rocks, which were parent materials for the metamorphic rocks, is unknown, but a mid- to late-Paleozoic age is suggested by comparison with rocks of known age in adjacent areas. These eugeosynclinal rocks were definitely sialic, their average composition being tonalitic. A comparison between the average chemical composition of the metamorphic rocks and syn-metamorphic plutons and the average composition of eugeosynclinal rocks of the Pacific rim, such as graywackes, suggests that the rocks of the Okanogan Range may have been formed by nearly isochemical metamorphism of eugeosynclinal sedimentary and volcanic rocks.

NEPHELINE GNEISSES OF THE WOLFE BELT, LYNDOCH TOWNSHIP, ONTARIO: I. STRUCTURE, STRATIGRAPHY, AND PETROGRAPHY

The Wolfe Belt is a sequence of banded gneisses characterized by nepheline and other minerals with abnormally high sodium contents. Two main units can be recognized. On the north side of the belt and extending along its entire mapped length are leurocratic feldspathic gneisses, generally nepheiine-bearing. These gneisses form a unit up to 350 ft thick and over 4 miles long in which also occur bands of amphibolite, crystalline limestone, and quartzite. The southern half of the belt consists of calc-silicate gneiss up to 230 ft thick with intercalated horizons of metamorphic pyroxenite and 'iron-formation'.These lithotogical units are interlayered and contain within them well-developed banding, which is attributed to relict supracrustal layering. Nepheline may be present in all of the units except the quartzite, but its distribution is irregular, although influenced to a large degree by the stratigraphy. The species and compositions of the associated mafic minerals are related to the nepheline content of the rocks as well as to the bulk rock composition.The Wolfe Belt is interpreted as being of metasedimentary origin, the parent rocks having been metasomatized during the Grenville orogenic period. The only igneous phase present is leucocratic alkali–syenite, which intrudes the nepheline gneisses and modifies them.

Scheelite in the Precambrian gneisses of Colorado

Scheelite of Precambrian age has been found at several localities in Colorado and Wyoming in recent years. Most of it is disseminated in regionally metamorphosed rocks, principally in calc-silicate gneiss, but also in amphibolite. Small amounts are found in pegmatites and in Au- or Cu-quartz veins of Precambrian age. The scheelite in most of the deposits contains Mo, and much of it is accompanied by discrete powellite (CaMoO 4 ). The scheelite deposits vary in age with the rocks that contain them and show no consistent genetic associations. Many of the deposits are products of some phase of the major plutonic stage that included regional metamorphism, migmatization, and emplacement of syntectonic granites. A few deposits may be related to younger Precambrian granites, and some are related to late diorite and pegmatite. W originally present in minor concentration in the sedimentary rocks that gave rise to the gneisses was evidently redistributed and recrystallized through successive Precambrian plutonic episodes. Reworked Precambrian deposits may have been a source of the W in the economically important Tertiary deposits, and possibly also of Mo. The Precambrian scheelite deposits found thus far have proved to be of minor economic importance, but others are still being found, and many more can be presumed to exist; among these may be some of importance.