USGS Science⌕ Search

Geology topics

E-an Zen

Publications and source records attributed to E-an Zen.

At least 19 recordsLinked to original sources
USGS1981

Metamorphic mineral assemblages of slightly calcic pelitic rocks in and around the Taconic Allochthon, southwestern Massachusetts and adjacent Connecticut and New York

The mineral assemblages from metamorphosed slightly calcic pelitic rocks of the Taconic Range in southwestern Massachusetts and adjacent areas of Connecticut and New York were studied petrographically and chemically. These rocks vary in metamorphic grade from those below the chloritoid zone through the chloritoid and garnet zones into the kyanite-staurolite zone. Microprobe data on the ferromagnesian minerals show that the sequence of increasing Fe/ (Fe+Mg) value is, from the lowest, chlorite, biotite, hornblende, chloritoid, staurolite, garnet. Hornblende, epidote, garnet, and plagioclase are the most common minerals that carry significant calcium. Biotite is persistently deficient in alkali but is abnormally rich in octahedral aluminum to such an extent that the overall charge balance can be ascribed to an AI=K+ (Fe,Mg) diadochy. Muscovite contains small though persistent amounts of iron and magnesium in octahedral positions but has a variable K/Na ratio, which is potentially useful as a geothermometer. One low-grade muscovite is highly phengitic, but the white micas in rocks from metamorphic grades higher than chloritoid zone do not contain significant phengite components. Chlorite is persistently high in aluminum and so its ratio of divalent ions to aluminum is approximately that of garnet. Many garnets show pronounced zoning in manganese and less pronounced zoning in calcium. Garnet coexisting with hornblende contains a high proportion of the grossularitic component. The calcium content is significant in all the analyzed garnets, except those from a cummingtonite-bearing sample that is free of muscovite. This suggests that in slightly calcic pelitic rocks, calcium-free garnet cannot coexist with muscovite. Most of the mineral assemblages formed in the presence of excess quartz and muscovite. The phase-petrologic analysis, made with the aid of an eight-phase multisystematic model, shows the following major points: 1. Chloritoid and staurolite coexist in a definite interval of prograde metamorphism. 2. Biotite-chloritoid does not constitute an alternative assemblage to garnet-chlorite-muscovite, because the former combination is found predominantly in the presence of the latter combination. Because the garnet contains lime, all five phases are stable together in lowlime pelitic rocks. 3. The first appearance of staurolite in the area does not correspond to the reaction leading to the first intrinsic stable existence of this phase. Inasmuch as the first appearanc,e of staurolite is always in chlorite-bearing as semblages, I suggest that the mapped staurolite zone marker corresponds to a reaction whereby staurolitechlorite becomes stable. The probable lower grade chemical equivalent, for example, chloritoid-aluminum silicate, however, has not been found in the area of study. Several staurolite-forming reactions discussed in the literature are ruled out because of the relative siderophility of the minerals. A second staurolite isograd involves the reaction, chloritoid+chlorite+muscovite= staurolite+biotite. A third isograd involves staurolite+ chlorite=biotite+kyanite; this reaction is postulated on the basis of the observed assemblage biotite-kyanitesta urolite-garnet-muscoviteplagioclase-quartzilmenite. 4. In low-grade rocks, epidote is stable considerably before the first appearance of chloritoid. The nature of the high-aluminum phase in low-grade rocks that leads to the formation of chloritoid remains obscure. The epidote is always rich in ferric iron (pistacite content of about 1/ 4 to 1/3). Garnet-bearing assemblages (with or without epidote) are formed next as metamorphic grade increases. The next more calcium-rich silicate is hornblende, and despite the meager data on assemblages that include hornblende, the first intrinsic appearance of this phase has probably been recorded. At highstaurolite grade, the most calcium-rich assemblage in pelitic rocks is hornblende-garnet-biotite-plagioclase (bytownit

Professional Paper↗
USGS1975

Quantitative determination of dawsonite in Green River Shale by powder-sample X-ray diffraction; effect of grinding

In an effort to evaluate the method of quantitative analysis by X-ray diffraction as a means of determining dawsonite abundances in Green River" oil shale, we performed a series of grinding experiments. Weighed mixtures of dawsonite + quartz and dawsonite + quartz + shale were ground for preset lengths of time, and the following X-ray diffraction peaks were measured for intensity: dawsonite (110) at 15.6 2 θ and (211) (002) at 32.1° 2 θ , and quartz (100) at 20.9° 2 θ and (101) at 26.6° 2 θ . Heights and areas were measured. Intensity ratios were plotted as functions of grinding time with a calculated probable error. Intensity ratios generally decrease as grinding time increases. The intensity of the 15.6° peak, D 15.6° , is most affected, and the most drastic reduction occurs during the first 20 min of grinding. The D 32.1° peak is least sensitive to grinding but is subject to interference by other minerals in the shale. We conclude that the X-ray diffraction method is not well adapted to the routine quantitative determination of dawsonite in oil shale of the Green River Formation.

Journal of Research of the U.S. Geological Survey↗
USGS1974

Prehnite- and pumpellyite-bearing mineral assemblages, west side of the Appalachian metamorphic belt, Pennsylvania to Newfoundland

Prehnite- and/or pumpellyite-bearing meta-igneous rocks are found on the west side of the Appalachian metamorphic belt (1) near Jonestown, south-eastern Pennsylvania; (2) on Rensselaer Plateau, eastern New York; (3) near Quebec City, Quebec; and (4) at Little Port, Humber Arm, western Newfoundland. The assemblages critical to determining the conditions of metamorphism are (1) chlorite-epidote-hematite-pumpellyite-prehnite; actinolite-chlorite-hematite-pumpellyite-stilpnomelane; (2) actinolite-chlorite-epidote-stilpnomelane; chlorite-epidote-pumpellyite-stilpnomelane; chlorite-epidote-hematite-pumpellyite; (3) chlorite-epidote-hematite-pumpellyite-stilpnomelane; chlorite-epidote-pumpellyite-prehnite; and (4) chlorite-epidote-prehnite; chlorite-prehnite-stilpnomelane; chlorite-epidote-pumpellyite-prehnite. One pumpellyite-bearing rock from western Newfoundland shows a later vein of analcime-calcite. All the assemblages also include quartz, sphene, calcite, K-mica, and albite. Analysis of the mineral assemblages by the Schreinemakers method for the phases actinolite-chlorite-epidote-hematite-prehnite-pumpellyite-stilpnomelane shows that the different localities can be assigned different metamorphic grades. Though the detailed results of the Schreinemakers analysis depend on the assumed source of ferric iron in epidote, the major conclusions are not affected. The thermodynamic role of calcite is more problematic, but it appears that CO 2 did not behave as a boundary-value component during metamorphism. If calcite is treated as an excess phase, the Schreinemakers bundle decomposes to a net in a multisystem. Plotted on such a net, the various localities again occupy different parts signifying different metamorphic grades. The occurrence of pumpellyite-bearing assemblages on the west flank of the northern Appalachian metamorphic belt might suggest that these assemblages, contrary to the ideas of Miyashiro and of others, do not indicate high-pressure and low-temperature type of metamorphism. These assemblages, however, are compatible with an alternative interpretation as remnants of a high-pressure, low-temperature Taconic metamorphic regime, whose imprint within most of the Appalachian metamorphic belt has been obliterated by later events. Such a reconstruction is compatible with the suggestion that this zone, lying near the margin of the early Paleozoic craton, was an active subduction zone during the Taconic orogeny.

Appalachian metamorphic belt↗
USGS1968

Middle Pennsylvanian plant fossils: Problematic occurrence in the Bronx

A possible glacial boulder of undeformed and unmetamorphosed siltstone containing Middle Pennsylvanian plant fossils was recovered from the Bronx. The rock cannot be explained by known geologic relations and suggests the possibility of undetected outliers of Pennsylvanian rocks in the Hudson valley.

New York↗