Hostname: page-component-cd9895bd7-lnqnp Total loading time: 0 Render date: 2024-12-26T05:18:21.358Z Has data issue: false hasContentIssue false
  • English
  • Français

Geochemistry of orbicular diorite from the Grenville Front zone, eastern Labrador

Published online by Cambridge University Press:  05 July 2018

J. Victor Owen
J. Victor Owen*
Affiliation:
Department of Geology, Saint Mary's University, Halifax, N.S. Canada B3H 3C3
Get access Rights & Permissions [Opens in a new window]

Abstract

Orbicules in diorite from the Grenville Front zone of eastern Labrador consist of biotite- and/or hornblende-studded, dioritic cores enclosed by fine-grained shell structures alternately enriched and depleted in biotite. The orbicules occur in a mesocratic, quartz-bearing matrix. Epidote of inferred magmatic origin occurs in all parts of the rock. Plagioclase in the matrix is relatively sodic, and biotite more ferroan than in the orbicules, suggesting that the matrix material has the most evolved composition, and crystallized last.

The diorite is unusually aluminous (orbicules: 24.9-27.4 wt.% Al2O3; matrix: 22.4-23.6% Al2O3) and calcic (orbicules: 7.0-8.4 wt.% CaO; matrix: 6.0-6.9% CaO); it shows a positive Eu anomaly, and has elevated Sr concentrations (1800-2500 ppm Sr), demonstrating that, compositionally, it resembles a plagioclase cumulate. Mass-balance calculations suggest that the orbicule cores had a crystal/melt ratio of ≤5. This accounts for the extreme fractionation of the rock (e.g., in orbicules, Zr <5 ppm). Compared with fractional crystallization patterns, variation diagrams show counter-trends (e.g. the siliceous matrix contains elevated TiO2) or scatter for several components, suggesting that the crystal/melt ratio governed some of the geochemical characteristics of the diorite.

The presence of coarse mafic clots containing primary epidote, biotite and/or hornblende testify to an elevated water content in the orbieule cores. The shell magma apparently formed as a result of the interaction of supercooled orbicule core fluids with the matrix magma, and tended to serve as a reservoir for alkalis and Fe. Alkalis and Ca diffused in opposite directions, possibly as a result of a temperature gradient at the orbicule/matrix interface. This, however, requires decoupling of the thermodiffusional behaviour of alkalis and femic components in hydrated intermediate magma, which contrasts with documented Soret diffusion in mafic systems.

The solidification of the shell magma prior, to the orbicule cores and matrix is attributed to dewatering, consistent with the fine grain size of the shell structures. Except where remobilized core material has disrupted the shells, the cores crystallized in isolation from the matrix, which fractionated toward a more evolved composition.

Keywords

dioriteorbiculegeochemistryGrenville FrontLabrador
Type
Research Article
Information
Mineralogical Magazine , Volume 56 , Issue 385 , December 1992 , pp. 451 - 458
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1992

Access options

Get access to the full version of this content by using one of the access options below. (Log in options will check for institutional or personal access. Content may require purchase if you do not have access.)

References

Bowen, N. L. (1921) Diffusion in silicate melts. J. Geol., 29, 295–317.CrossRefGoogle Scholar
Elliston, J. N. (1984) Orbiculcs: An indication of the crystallisation of hydrosilicates, 1. Earth-Sci. Rev., 20, 265–344.CrossRefGoogle Scholar
Helz, R. T., Kirschenbaum, H. and Marinenko, J. W. (1989) Diapiric transfer of melt in Kilauea Iki lava lake, Hawaii: A quick, efficient process of igneous differentiation. Geol. Soc. Amer. Bull., 101, 578–94.2.3.CO;2>CrossRefGoogle Scholar
Hildreth, W. (1981) Gradients in silicic magma chambers: Implications for lithospheric magmatism. J. Geophys. Res., 86, 10153-92.CrossRefGoogle Scholar
Leveson, D. J. (1963) Orbicular rocks of the Lonesome Mountain area, Beartooth Mountains, Montana and Wyoming. Geol. Soc. Amer. Bull., 74, 1015-40.CrossRefGoogle Scholar
Moore, A. C. (1984) Orbicular rhythmic layering in the Palaboracarbonatite, South Africa. Geol. Mag., 121, 53–50.CrossRefGoogle Scholar
Owen, J. V. (1991) Significance of epidote in orbicular diorite from the Grenville Front zone, eastern Labrador. Mineral. Mag., 55, 173–81.CrossRefGoogle Scholar
Owen, J. V. Dallmeyer, R. D., Gower, C. F. and Rivers, T. (1988) Metamorphic conditions and 4Ar/39 geochronological contrasts across the Grenville Frong zone, coastal Labrador, Canada. Lithos, 21, 13–35.CrossRefGoogle Scholar
Philpotts, A. R. (1982) Compositions of immiscible liquids in volcanic rocks. Contrib. Mineral. Petrol., 80, 201–18.CrossRefGoogle Scholar
Walker, D. and DeLong, S. E. (1982) Soret separation of mid-oceanic ridge basalt magma. Ibid., 79,231-40.Google Scholar