Rb-Sr isochron ages from east Milne Land, Scoresby Sund, East Greenland

Escher, A. & Watt, W. S. (edit.) Geology of Greenland, 304-339. Copenhagen: Geo!. Surv. Greenland. Eldholm, O. & Windisch, C. C. 1974: Sediment distribution in the Norwegian Greenland Sea. Bul/. geol. Soc. Arner. 85, 1661-1676. Featherstone, P. S., Bott, M. H. P. & Peacock, J. H. 1977: Structure of the continental margin of South-Eastern Greenland. Geophys. J. R. astr. Soc. 48, 15-29. Henderson, G. 1976: Petroleum geology. In Escher, A. & Watt, W. S. (edit.) Geology of Greenland, 488-506. Copenhagen: Geo!. Surv. Greenland. Hinz, K. & Schliiter, H. U. 1978: Der Nordatlantik Ergebnisse geophysikalischer Untersuchungen der Bundesanstalt fUr Geowissenschafien und Rohstoffe an nordatlantischen Kontinentalrandem Erdol Erdgas Z. 94,271-280. Johnson, G. L., McMilIan, N. J. & Egloff, J. 1975a: East Greenland continental margin. In Yorath, C. J. Parker, E. R. & Glass, D. J. (edit.) Canadas Continental Margin and offshore Petroleum Exploration. ean. Soc. petrol. geol. Mern. S, 205-244. Johnson, G. L., Sommerhoff, G. & Egloff, J. 1975b: Structure and morphology of the West Reykjanes Basin and the south-east Greenland continental margin. Mar. Geo/. 18, 175-196. Larsen, B. 1975: Marine geophysical survey ofthe East Greenland shelf south of Angmagssalik. Rapp. Grønlands geol. Unders. 75, 87-88. Larsen, H. C. 1975: Aeromagnetic investigations in East Greenland. Rapp. Grønlands geol. Unders. 75, 88-91. Larsen, H. C. 1978: Offshore continuation of East Greenland dyke swarm and North Atlantic Ocean formation. Nature 274, 220-223. Larsen, H. C. 1979: New bathymetric maps of the East Greenland continental margin. Internal GGU-report (in prep.) Phillips, J. D., Fleming, H. S. & Feden, R. H. 1973: Aeromagnetic study of the Greenland and Norwegian seas. Abs. geol. Soc. Arner. 5,767. Sur1yk, F. 1978: Jurassic basin evolution of East Greenland. Nature 274, 130-133. Talwani, M. & Eldholm, O. 1977: Evolution of the Norwegian-Greenland Sea. Bul/. geol. Soc. Arner. 88,969-999. Thorning, L. 1977: A brief description of the computer programs used by GGU in the treatment of aeromagnetic data. Rapp. Grønlands geol. Unders. 85, 37--45. Thorning, L. 1978: Project EASTMAR: a new aeromagnetic survey of the continental shelf of eastern Greenland. Rapp. Grønlands geol. Unders. 90, 113-115. Verna Cruise 27, 28, 1974: Underway Marine Geophysical Data in the North Atlantic June 1961 January 1971, Part F: Seismic Reflection Profiles. In Talwani, M. (edit.) Larnont-Doherty Survey of the World Ocean, 293 pp.

major plutonic rock types are found. Field investigations indicate the oldest plutonic rock to be a granodiorite, followed by a coarse-grained mafie, quartz syenite, and the youngest bodies to be of granitie composition. In the southern part a large area is eovered by Mesozoic sediments whieh eross the fault-line without displacement. Locally, outliers of Tertiary plateau basalts rest on the Mesozoie rocks ( fig. 33). The geology has been outlined by Henriksen & Higgins (1971, 1973 and preliminary age determinations were published by Hansen et al. (1972) and Hansen & Steiger (1976). This more extensive study deals with Rb-Sr whole-roek and mineral analyses on samples from two post-kinematie intrusions and from the metasediments in the northern area.

Metasediments
The main outerops oeeur in the north. They include quartzites, mica quartzites, mica sehists and marbles. The sediments are extremely well banded and rippie marks and eross-bedding are preserved.
The isolation of the east Milne Land metasedimentary sequenee from other sedimentary sequences in the inner fjord zone makes correlation difficult, but it is in some respects comparable to parts of the Krummedal supracrustal sequence. Six samples were taken from the northern mica schists to reveal the stratigraphy of the metasediments. The isotopic ratios are tabled and plotted (Table 7 and fig. 34). Five of the six samples plot along an isochron with a slope corresponding to an age of 444± 17 m.y. The initial ratio is O,7395±O.0008. It is believed this ref!ects a complete resetting during Caledonian times. The initial ratio is higher than the 0.725 ratio obtained for Caledonian reset sediments of the Krummedal supracrustal sequence (Hansen et al., 1978). Thus the sediments of east Milne Land might be correlated with the ones of the Krummedal supracrustal sequence, of which the first metamorphism has been dated IO 1122 m.y. There is some petrological evidence for an earlier metamorphism of Ihe east Milne Land sequence. The micas are very often arranged in polygonal arches around microscopic folds of alater phase indicating that there was recrystallization subsequent to folding (Higgins, personal communication). One sample (142381) has been omitted from isochron calculation because it is slightly chloritisised, which may have caused loss of radiogenic stn~ntium. Sample 166853 also falls slightly below the isochron.  A loss of radiogenie strontium might be related to the intrusion of an aplite (166852) at this loeation. The aplite shows no sign of metamorphism thus suggesting a whole-rock biotite age of 411±10 m.y. The age of the intrusion of the aplite and also a lower time limit for the last metamorphism of the mica sehists is shown.

Granodiorite
The granodiorite is the oldest of the plutonic rocks found in east Milne Land. It forms a thick sheet-like body and is bordered by a marble to the north and the south. The granodiorite is mesoeratic and medium-grained, eomprising hornblende, biotite, plagiociase dominant over microciine, and quartz. Mafie inciusions of igneous origin are regularly distributed, but are eoncentrated locally (?) in zones.  Six samples from within the granodiorite have been collected in the south and four in the north ( fig. 33). The Rb-Sr isotope date is tabled and plotted in Table 8 and fig. 35, Nine of the ten points lie on an isochron with a slope corresponding to an age of 453±23 m.y. and with an initial ratio of 0.7071±0.0002. This age is regarded as the time of intrusion during an early stage of the Caledonian orogeny. However, this contradicts a preliminary biotite age of 1460±20 m.y. (sample 135501) published by Hansen et al. (1972). The discrepancy was tested by separating a new biotite concentrate from the original specimen and the whole-rock biotite age obtained on this sample (135501) is 405±10 m.y., which is interpreted as a cooling age. This age is believed reliable, as further microscopic studies have shown that the first sample has contained a fine network of hydrothermal veins, thus the rock may not have acted as a closed system with respect to rubidium and strontium. The first biotite determination showed an unusually low content of rubidium. The position of the whole-rock data point for sample 135503 might also be related to open system behaviour. This sample, collected a few hundred metres further north, also shows fine hydrothermal veins and is therefore excluded from the isochron calculation. A lower time limit for the intrusion of the granodiorite is given by two whole-rock analyses on a discordant aplite giving an age of 425±28 m.y. (Table 8).

Pink biotite granite
Two different types of granites are found in east Milne Land. The first type in the north of the area is medium-grained, leucocratic biotite granite comparable to many of the major plutons of the Stauning Alper (Henriksen & Higgins, 1969). The second occurs in the south, mainlyas small circular plugs punching up through the granodiorite. One of the southern granite plugs, about 1500 m in diameter, has an inc1usion of basic rock near the centre. Samples from the granite and the basic inc1usion have been analysed and the isotopic data are tabled and plotted in Table 9 and fig. 36. The seven granite samples plot on an isochron with an age of 373±9 m.y. The figure is lower than a preliminary age published by Hansen & Steiger (1976) and c1early confirms that the plugs intruded at a very late stage of the Caledonian orogeny. An age of 373±9 m.y. is identical, within limits of error, to the K-Ar biotite ages of post-tectonic intrusive granites reported by Haller & Kulp (1962) and a biotite K-feldspar age of 386±5 m.y. for a pegmatite reported by Hansen & Steiger (1971).
Two samples from the basic inc1usion gave an age of 406±13 m.y. This body is now interpreted as a large inc1usion of older material, having undergone isotopic rehomogenization and recrystallization when the granite intruded.
The above results offer evidence that the main plutonic activity and metamorphic overprinting of the rocks in east Milne Land took place during Caledonian times.