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Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex, southern West Greenland: Nature of the parent magma. Reply to a Comment by P. Henderson
Gmch~micu a C‘uvnuchmico Acla Vol. 48, pp. 417-418 0 Pergamon PressLtd.1984.Printed in U.S.A.
~X~Ih-70i7/R4/$3.00
AUTHORS’
+ 00
REPLY
Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex, southern West Greenland: Nature of the parent magma. Reply to a Comment by P. Henderson BARRY L. WEAVER,
Department
of Geology,
JOHN TARNEY University
and BRIAN F. WINDLEY
of Leicester.
Leicester
LEI 7RH. U.K.
(Received October 12, 1983; accepted in revised,form November 9. 1983)
with La, = 17, LuN = 0.7 and Zr = 11-15 ppm. HENDERSON et al. (I 976) also propose that this liquid may have had a positive Eu anomaly. Such trace element characteristics are highly unrealistic for any primary basaltic liquid. A second issue concerns the effects of metamorphic reequilibration on the mineral chemistry and, here, the work of HUMPHRIESand CLIFF (1982) on Sm-Nd dating of basic granulites from the Lewisian of N.W. Scotland is relevant. Sm-Nd whole-rock dating of both amphibolite and granulite facies Lewisian gneisses gives an age of 2.92 b.yr. (HAMILTON et a/., 1979), whereas the Rb-Sr and PbPb isotope systematics record metamorphic ages which are ca. 100-200 m.yr. younger (e.g. CHAPMAN and MOORBATH, 1977). These ages are similar to the 2.85 b.yr. age recorded by a Pb-Pb isochron for the final granulite-facies metamorphic event in the Fiskenaesset area (BLACK et al.. 1973), and the tectonic and thermal histories of these two portions of the Archaean North Atlantic craton are comparable. However, HUMPHRIES and CLIFF (1982) demonstrated that Sm-Nd isochrons for mineral separates from gabbroic granulites record ages 430 m.yr. younger than the Sm-Nd whole rock age. This they attributed to diffusion of Sm and Nd during slow uplift and cooling of the complex, although clearly such re-equilibration occurred only over small distances. The effect of such metamorphic reequilibration is to produce enhanced Sm/Nd ratios in the phases garnet, hornblende and clinopyroxene and a diminished Sm/Nd ratio in plagioclase, which is in accord with our interpretation of the REE patterns of mineral separates from rocks of the Fiskenaesset complex. In this case REE data for mineral separates from layered igneous complexes which have been slowly uplifted and cooled from high metamorphic grade clearly cannot be used to infer the composition of coexisting equilibrium liquids. However, as the scale of diffusion of the REE is limited, whole rock data (with appropriate assumptions) can be used for such modelling. In conclusion we feel that there is considerable evidence to supportour contention that the REE in the Fiskenaesset complex have re-equilibrated on a grain-site scale, and that the parental liquid to the complex had a slightly LREE depleted REE pattern.
WE ARE GRATEFUL for Dr. Henderson’s comments in that they are useful in illustrating the problems inherent in dealing with the geochemistry of cumulate rocks, particularly when these rocks have been metamorphosed to high grade. We are aware of the contradictions in our respective but contrasting methods for estimating the REE patterns of liquids in equilibrium with the cumulates of the Fiskenaesset Complex, but believe that there is considerable evidence to support our arguments for an unfractionated parental magma and for metamorphic re-equilibration of the REE on a grain-size scale which invalidates the approach used by Henderson. The Fiskenaesset anorthosite complex has been metamorphosed to upper amphibolite to granulite facies assemblages (WINDLEY et al., 1973) and much of the vital primary mineralogical and textural evidence for the cumulus or intercumulus nature of phases has been eradicated. Thus we do not find it possible to state unequivocally that sample GGU 132033 has the attributes of an orthocumulate, nor to estimate the cumulate or intercumulate proportions of mafic phases in rocks of gabbroic to anorthosite composition. However, rather than replying in detail to all of the individual points raised by Henderson we would like to cite two additional pieces of evidence which support our previous contention. Firstly, a subsequent paper (WEAVER et al., 1982) provided good evidence for a comagmatic relationship between the cumulates of the Fiskenaesset anorthosite complex and the bordering metavolcanic amphibolites. The basaltic amphibolites have low Ce,/Yh ratios of 0.6 to 1.2 and the least evolved samples (which have CeN/YbN < I) have major and trace element compositions consistent with their representing the primary liquids of the anorthosite complex as modelled by WEAVER et al. (1981). Moreover, the range of trace element abundances in the basaltic amphibolites matches that in the successive equilibrium liquids for the complex derived by WEAVER ef al. (198 1). In contrast, adoption of the modelling used by Henderson yields an unrealistic parental liquid composition. Accepting that CGU 132023 is an orthocumulate, an equilibrium liquid with LaN = 30 and LuN = 2 is proposed by Henderson. The data of WEAVER et al. (1981, Table 3) indicate that 132023 contains I 1 ppm Zr, which yields an equilibrium liquid concentration of 22-30 ppm Zr following the logic of Henderson. Sample 132023 is from a stratigraphically high zone (the chromitite zone) in the complex and would have crystallised from an evolved liquid; extrapolating this composition back to a parental composition for the complex (such as done by WEAVER et al., 1981) yields a liquid
REFERENCES BLACK L. P., MOORBATH S., PANKHURSTR. J. and WINDLEY B. F. (1973) *07Pb/2”Pb whole-rock age of the Archaean
417
418
B. L. Weaver, J. Tamey, and B. F. Windley
granu!ite facies event in West Greenland. ~ff~ure~~~~, Sci 244, 50-53. H. J. and MOORBATHS. (1977) Lead isotope measurements from the oldest recognised Lewisian gneisses of north-west Scotland. Nature 268, 4 l-42. HAMILTONP. J., EVENSENN. M., O’NIONSR. K. and TARNEY J. (1979) Sm-Nd systematics of Lewisian gneisses: implications for the origin of gram&es. Nature 277, 25-28. HENDERSONP., FISHLOCYK S. J., LAIJLJ. C., COOPER T. D., CONRAD R.L., BOYNTON W.V.and SCHMITT R.A.(1976) Rare earth element abundances in rocks and minerals from the Fiskenaesset complex, West Greenland. Earth Planet. Sci. Left. 30, 37-49. HUMPHRIESF. J. and CLIFF R. A. (1982) Sm-Nd dating and
CHAPMAN
cooling history of Scourian granulites, Sutherland. Nufure 295, 515-517. WINDLEYB. F., HERD R. K. and BOWDENA. A. (1973) The Fiskenaesset complex, West Greenland, Part I. A preliminary study of stratigraphy, petrology and whole rock chemistry from Qeqertarssuatsiaq. Bull. Gwz1and.s Geol. Under. 106, 80 pp. WEAVERB. L., TARNEYJ. and WINDLEYB. F. ( 198 1) Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex, southern West Greenland: nature of the parent magma. Geochim. Cosmochim. Acta 45, 7 1l-725. WEAVER B. L., TARNEY J., WINDLEY B. F. and LEAKE B. E. (1982) Geochemistry and petrogenesis of Archaean metavolcanic amphibolites from Fiskenaesset, SW. Greenland. Geoehim. Co~mocj~rn. Acts 46, 2203-22 15.
~X~Ih-70i7/R4/$3.00
AUTHORS’
+ 00
REPLY
Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex, southern West Greenland: Nature of the parent magma. Reply to a Comment by P. Henderson BARRY L. WEAVER,
Department
of Geology,
JOHN TARNEY University
and BRIAN F. WINDLEY
of Leicester.
Leicester
LEI 7RH. U.K.
(Received October 12, 1983; accepted in revised,form November 9. 1983)
with La, = 17, LuN = 0.7 and Zr = 11-15 ppm. HENDERSON et al. (I 976) also propose that this liquid may have had a positive Eu anomaly. Such trace element characteristics are highly unrealistic for any primary basaltic liquid. A second issue concerns the effects of metamorphic reequilibration on the mineral chemistry and, here, the work of HUMPHRIESand CLIFF (1982) on Sm-Nd dating of basic granulites from the Lewisian of N.W. Scotland is relevant. Sm-Nd whole-rock dating of both amphibolite and granulite facies Lewisian gneisses gives an age of 2.92 b.yr. (HAMILTON et a/., 1979), whereas the Rb-Sr and PbPb isotope systematics record metamorphic ages which are ca. 100-200 m.yr. younger (e.g. CHAPMAN and MOORBATH, 1977). These ages are similar to the 2.85 b.yr. age recorded by a Pb-Pb isochron for the final granulite-facies metamorphic event in the Fiskenaesset area (BLACK et al.. 1973), and the tectonic and thermal histories of these two portions of the Archaean North Atlantic craton are comparable. However, HUMPHRIES and CLIFF (1982) demonstrated that Sm-Nd isochrons for mineral separates from gabbroic granulites record ages 430 m.yr. younger than the Sm-Nd whole rock age. This they attributed to diffusion of Sm and Nd during slow uplift and cooling of the complex, although clearly such re-equilibration occurred only over small distances. The effect of such metamorphic reequilibration is to produce enhanced Sm/Nd ratios in the phases garnet, hornblende and clinopyroxene and a diminished Sm/Nd ratio in plagioclase, which is in accord with our interpretation of the REE patterns of mineral separates from rocks of the Fiskenaesset complex. In this case REE data for mineral separates from layered igneous complexes which have been slowly uplifted and cooled from high metamorphic grade clearly cannot be used to infer the composition of coexisting equilibrium liquids. However, as the scale of diffusion of the REE is limited, whole rock data (with appropriate assumptions) can be used for such modelling. In conclusion we feel that there is considerable evidence to supportour contention that the REE in the Fiskenaesset complex have re-equilibrated on a grain-site scale, and that the parental liquid to the complex had a slightly LREE depleted REE pattern.
WE ARE GRATEFUL for Dr. Henderson’s comments in that they are useful in illustrating the problems inherent in dealing with the geochemistry of cumulate rocks, particularly when these rocks have been metamorphosed to high grade. We are aware of the contradictions in our respective but contrasting methods for estimating the REE patterns of liquids in equilibrium with the cumulates of the Fiskenaesset Complex, but believe that there is considerable evidence to support our arguments for an unfractionated parental magma and for metamorphic re-equilibration of the REE on a grain-size scale which invalidates the approach used by Henderson. The Fiskenaesset anorthosite complex has been metamorphosed to upper amphibolite to granulite facies assemblages (WINDLEY et al., 1973) and much of the vital primary mineralogical and textural evidence for the cumulus or intercumulus nature of phases has been eradicated. Thus we do not find it possible to state unequivocally that sample GGU 132033 has the attributes of an orthocumulate, nor to estimate the cumulate or intercumulate proportions of mafic phases in rocks of gabbroic to anorthosite composition. However, rather than replying in detail to all of the individual points raised by Henderson we would like to cite two additional pieces of evidence which support our previous contention. Firstly, a subsequent paper (WEAVER et al., 1982) provided good evidence for a comagmatic relationship between the cumulates of the Fiskenaesset anorthosite complex and the bordering metavolcanic amphibolites. The basaltic amphibolites have low Ce,/Yh ratios of 0.6 to 1.2 and the least evolved samples (which have CeN/YbN < I) have major and trace element compositions consistent with their representing the primary liquids of the anorthosite complex as modelled by WEAVER et al. (1981). Moreover, the range of trace element abundances in the basaltic amphibolites matches that in the successive equilibrium liquids for the complex derived by WEAVER ef al. (198 1). In contrast, adoption of the modelling used by Henderson yields an unrealistic parental liquid composition. Accepting that CGU 132023 is an orthocumulate, an equilibrium liquid with LaN = 30 and LuN = 2 is proposed by Henderson. The data of WEAVER et al. (1981, Table 3) indicate that 132023 contains I 1 ppm Zr, which yields an equilibrium liquid concentration of 22-30 ppm Zr following the logic of Henderson. Sample 132023 is from a stratigraphically high zone (the chromitite zone) in the complex and would have crystallised from an evolved liquid; extrapolating this composition back to a parental composition for the complex (such as done by WEAVER et al., 1981) yields a liquid
REFERENCES BLACK L. P., MOORBATH S., PANKHURSTR. J. and WINDLEY B. F. (1973) *07Pb/2”Pb whole-rock age of the Archaean
417
418
B. L. Weaver, J. Tamey, and B. F. Windley
granu!ite facies event in West Greenland. ~ff~ure~~~~, Sci 244, 50-53. H. J. and MOORBATHS. (1977) Lead isotope measurements from the oldest recognised Lewisian gneisses of north-west Scotland. Nature 268, 4 l-42. HAMILTONP. J., EVENSENN. M., O’NIONSR. K. and TARNEY J. (1979) Sm-Nd systematics of Lewisian gneisses: implications for the origin of gram&es. Nature 277, 25-28. HENDERSONP., FISHLOCYK S. J., LAIJLJ. C., COOPER T. D., CONRAD R.L., BOYNTON W.V.and SCHMITT R.A.(1976) Rare earth element abundances in rocks and minerals from the Fiskenaesset complex, West Greenland. Earth Planet. Sci. Left. 30, 37-49. HUMPHRIESF. J. and CLIFF R. A. (1982) Sm-Nd dating and
CHAPMAN
cooling history of Scourian granulites, Sutherland. Nufure 295, 515-517. WINDLEYB. F., HERD R. K. and BOWDENA. A. (1973) The Fiskenaesset complex, West Greenland, Part I. A preliminary study of stratigraphy, petrology and whole rock chemistry from Qeqertarssuatsiaq. Bull. Gwz1and.s Geol. Under. 106, 80 pp. WEAVERB. L., TARNEYJ. and WINDLEYB. F. ( 198 1) Geochemistry and petrogenesis of the Fiskenaesset anorthosite complex, southern West Greenland: nature of the parent magma. Geochim. Cosmochim. Acta 45, 7 1l-725. WEAVER B. L., TARNEY J., WINDLEY B. F. and LEAKE B. E. (1982) Geochemistry and petrogenesis of Archaean metavolcanic amphibolites from Fiskenaesset, SW. Greenland. Geoehim. Co~mocj~rn. Acts 46, 2203-22 15.