Abstract
The Barth Island layered structure is an oval, 6 by 9 km body, consisting of rhythmically layered adamellitic rock in the center which grades outward through jotunite into troctolite. Farther outward the sequence repeats itself in reversed order, strongly reduced in magnitude and finer grained; the adamellitic zone is followed by jotunite which grades into coarse-grained leuconorite and into anorthosite of the Nain complex. The Barth Island structure, having an inverted conical base topped by a hemispherical depression, seems to represent a distorted sequence of rock layers with troctolite at the bottom, grading upward into adamellitic rocks which grade into anorthosite at the top. Trend-surface analysis demonstrates the regional variation of plagioclase and orthopyroxene compositions in the troctolite—adamellite sequence of the central part of the structure. The fits for the second- and third-degree surfaces are good and significant at the 99 percent level. The regression line for compositional variation in coexisting plagioclase and orthopyroxene in all analyzed rocks has a correlation coefficient of r2 = 0.78. The difference between the trends in the troctolite—adamellite sequence and the anorthosite—adamellite sequence is insignificant. The regression curve for compositional variation in coexisting orthopyroxene and olivine has a correlation coefficient of r2 = 0.98. The curve shows good correlation with the experimentally established partitioning curve of Medaris, which indicates that equilibrium conditions prevailed during formation of the olivine—ortho-pyroxene pairs. The results suggest that the troctolite—adamellite sequence and the anorthosite—adamellite sequence are products of fractional crystallization, possibly from the same parental magma.
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Deer, W. A., Howie, R. A., and Zussman, J., 1963, Rock forming minerals, v. 2, Chain silicates: John Wiley & Sons, New York, 379 p.
de Waard, D., and Mulhern, K., 1973, The Barth layered structure,in The Nain Anorthosite Project, Labrador, field report 1972, Univ. Massachusetts (Amherst), Geol. Dept. Contr. No. 11, p. 71–79.
de Waard, D., and Wheeler, E. P., 1971, Chemical and petrologic trends in anorthositic and associated rocks of the Nain massif, Labrador: Lithos, v. 4, no. 4, p. 367–380.
Esler, J. E., Smith, P. F., and Davis, J. C., 1968, KWIKR8, a FORTRAN IV program for multiple regression and geologic trend analysis: Kansas Geol. Survey Computer Contr. 28, 31 p.
Levendosky, W. T., 1975, The geology of the Barth Island layered intrusion: unpubl. masters thesis, Syracuse Univ., 73 p.
Medaris, L. G., 1969, Partitioning of Fe++ and Mg++ between co-existing synthetic olivine and orthopyroxene: Am. Jour. Sci., v. 267, no. 8, p. 945–968.
Morse, S. A., 1968, Revised dispersion method for low plagioclase: Am. Mineralogist, v. 53, nos. 1 and 2, p. 105–115.
Morse, S. A., 1969a, Layered intrusions and anorthosite genesis: New York Mus. Sci. Serv. Mem., v. 18, p. 175–187.
Morse, S. A., 1969b, The Kiglapait layered intrusion, Labrador: Geol. Soc. America Mem. 112, 204 p.
Morse, S. A., 1972, An alternative model for the anorthosite and associated rocks of the Nain massif, Labrador: Lithos, v. 5, no. 1, p. 89–92.
Morse, S. A., 1974, A comment on field relations at Ikkinikulluit, Khingughutik, and Dog Island,in The Nain Anorthosite Project, Labrador, field report 1973, Univ. Massachusetts (Amherst), Geol. Dept. Contr. No. 13, p. 78–79.
Morse, S. A., and Berg, J. H., 1972, Petrographic methods,in The Nain Anorthosite Project, Labrador, field report 1971, Univ. Massachusetts (Amherst), Geology Dept. Contr. No. 9, p. 69–71.
Mulhern, K., 1974, Petrography and structure of the northern margin of the Barth layered structure, Labrador: unpubl. masters thesis, Syracuse Univ., 48 p.
Rubins, C. C., 1972, The Barth Island troctolite body; granulite—adamellite—anorthosite relations at the northern margin,in The Nain Anorthosite Project, Labrador, field report 1971, Univ. Massachusetts (Amherst), Geology Dept. Contr. No. 9, p. 34–42.
Rubins, C. C., 1973, Structural and petrologic relations of rocks south of the Barth Island layered intrusion, Labrador: unpubl. doctoral dissertation, Syracuse Univ., 100 p.
Rubins, C. C., and de Waard, D., 1971, Granulite zones in anorthosite near Nain, Labrador: Proc. Kon. Ned. Akad. v. Wetensch. (Amsterdam), v. B. 74, p. 263–268.
Slemmons, D. B., 1962, Determination of volcanic and plutonic plagioclase using a three-or four-axis universal stage: Geol. Soc. America Sp. Paper 69, 64 p.
Wheeler, E. P., 1942, Anorthosite and associated rocks about Nain, Labrador: Jour. Geology, v. 50, no. 6, pt. 1, p. 611–642.
Wheeler, E. P., 1955, Adamellite intrusive north of Davis Inlet, Labrador. Geol. Soc. America Bull., v. 66, no. 8, p. 1031–1060.
Wheeler, E. P., 1960, Anorthosite—adamellite complex of Nain, Labrador: Geol. Soc. America Bull., v. 71, no. 12, p. 1755–1762.
Wheeler, E. P., 1969, Minor intrusives associated with the Nain anorthosite: New York Mus. Sci. Serv. Mem., v. 18, p. 189–206.
Yoder, H. S., 1973, Contemporaneous basaltic and rhyolitic magmas: Am. Mineralogist, v. 58, nos. 1 and 2, p. 153–171.
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de Waard, D., Mulhern, K. & Merriam, D.F. Mineral variation in anorthositic, troctolitic, and adamellitic rocks of the Barth Island layered structure in the Nain anorthosite complex, Labrador. Mathematical Geology 8, 561–574 (1976). https://doi.org/10.1007/BF01042994
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DOI: https://doi.org/10.1007/BF01042994