Pelitic Sedimentary Rocks Research Articles

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.

Ein Beitrag zur Geochemie des Lithiums in Sedimentgesteinen

A method for the determination of lithium in silicate materials by atomic absorption photometry is described. The sensitivity of this method is 1 ppm Li and the relative standard deviation in the 10–100 ppm Li range is 3 per cent. Sixteen minerals and 96 sedimentary rocks have been analysed for lithium. Micaceous rocks (sandstone, graywackes) and the clay minerals of shales usually contain the highest lithium concentration of sedimentary deposits. The average of sandstones (12 samples) is 20 ppm Li and of graywackes is 58 ppm Li. Two sandstones from the lower Permian (Ober-Rotliegendes) have notably high lithium content (930 and 1190 ppm). Their quartzes contain 600 and 575 ppm Li respectively. The following lithium data are averages for pelitic sedimentary rocks: 60 ppm Li (8 samples) in shales from the continental shelf and 90 ppm Li (11 samples) in deep sea clays (Atlantic ocean). A comparison between marine and freshwater sediments shows an enrichment of lithium in the marine deposits (117 ppm Li in 16 specimens) compared to samples of freshwater origin (72 ppm Li in 9 specimens). A lithium determination helps in discriminating a marine from a freshwater environment of clay formation. The average lithium content of carbonate rocks is 7,5 ppm (10 samples).