Leucite from East Greenland: a New Petrographic Sub-Province of the Tertiary North Atlantic Province C

Leucite from East Greenland: A new petrographic sub-province of the Tertiary North Atlantic province C. K. BROOKS, J. G. R0NSBO and T. F. D. NIELSEN f T\/^J? Brooks, C. K., Rønsbo, J. G. and Nielsen, T. F. D.: Leucite from East Greenland: a new dI MJKJM. petrographic sub-province of the Tertiary North Atlantic province. Bull. geol. Soc. Den- S mark, vol. 24, pp. 93-98. Copenhagen, November, 19th., 1975. ]YL 5* CJ A new occurrence of ultramafic alkaline rocks in the Kangerdlugssuaq district of East k^3f Greenland is briefly described and more detailed penological and mineralogical data are presented for a leucite ankaratrite, which consists of diopside, leucite, nepheline, alkali feldspar, magnetite and phlogopite. The presence of potassic rocks in this area provides important additional evidence for the rifted origin of the area. This is the first well- substantiated report of leucite from Greenland. C. K. Brooks, J. G. Rønsbo and T. F. D. Nielsen, Geologisk Centralinstitut, Øster Vold gade 5-7, DK-1350 København K., Denmark. March 20th, 1975. Leucite is a characteristic mineral of potas this led Brooks (1973) to postulate that the sium-rich volcanic and sub-volcanic rocks fjord is the non-spreading arm of a triple rift and* although it is well known and abundant system. in certain areas, it is otherwise a rare mineral. Recent discoveries show that the rocks In this note, we report an occurrence of leu are considerably more undersaturated beyond cite from East Greenland and this is the first the head of the fjord, where a completely dif reported occurrence not only from Greenland ferent petrographic province of unknown ex but from the entire Tertiary North Atlantic tent crops out from under the inland ice. Thus, province. For this reason and also because its Brooks & Rucklidge (1974) have described un occurrence here lends weight to an earlier dersaturated, alkaline rocks as erratic blocks interpretation of the tectonics of the area which are believed to originate in this area (Brooks, 1973; Brooks & Rucklidge, 1974) we and Frisch & Keusen (in press) report a cir feel it is advisable to publish a preliminary cular intrusion, named Gardiner intrusion (see note at this stage even though the available fig. 1), which consists of dunites, pyroxenites, specimens are few and the field relations poor uncompahgrites (melilite rocks) and possibly ly known. Furthermore, the intrusion from carbonatites. which these samples originate is extremely in The samples described here were taken from accessible and it is doubtful when a new visit an intrusion, for which the name 'Batbjerg can be made to the locality. intrusion' is proposed, which is located on the innermost nunatak on the north-east side of the Kangerdlugssuaq Glacier (68°40'N, 28° Geological setting 5CW, see fig. 1). It is probably also circular in plan, but a substantial part is hidden under The fjord of Kangerdlugssuaq is best known glacier ice. for the Skaergaard intrusion, which lies close About twenty rock samples from the Bat to its mouth in an area of extensive Lower bjerg intrusion have been examined by us and Tertiary tholeiitic plateau lavas which are re although the number of samples is too small garded as being formed around the time of to give a reliable picture of the intrusion it continental break7up in this part of the North would appear that it is made up predomi Atlantic (Brooks,'1973). Rocks of alkaline af nantly of olivine and pyroxene rocks. The py finities occur along the fjord (see fig. 1) and roxenites are medium-grained, vary from 94 Brooks et al.: Leucite from East Greenland GEOLOGY OF KANGERDLUGSSUAQ EAST GREENLAND FELSIC PLUTONICS MAFIC ond ULTRAMAFIC PLUTONICS GARDINER INTRUSION VOLCANICS [ | SEDIMENTS | ] PRECAMBRIAN GNEISSES Fig. 1. Geological sketch map of the Kangerdlugssuaq district, East Greenland, showing the location of the Batbjerg, Gardiner and other intrusions. olive green to almost black in hand specimen lection is a porphyritic ultramafic type with and tend to be fresh and rather friable. The phenocrysts of clinopyroxene (ca. 4 mm in dunitic rocks are dark-coloured and many spe size) in a fine-grained groundmass of clino cimens are sheared and serpentinized, probably pyroxene, phlogopite and ore with a total ab due to faulting in a north-easterly direction, sence of light minerals. Also present are sy as reported by Polegeg & Kock (pers. comm., enite dikes and a fenitized zone in the sur 1971). Phlogopite is abundant in many of rounding Precambrian gneisses. these rocks and variants enriched in apatite These rocks from the Batbjerg intrusion are and magnetite also occur. Felsic minerals are strongly reminiscent of those reported from the often completely absent, but in others nephe- Gardiner intrusion (Frisch & Keusen, in press), line is present and with increasing nepheline in that they belong to an alkaline ultramafic content the pyroxenites pass into melteigitic association similar to that reviewed by Upton and ijolitic types. One dike rock in the col (1967) and, as such, are the only known rep- Bulletin of the Geological Society of Denmark, vol. 24 1975 95 resentatives of this association in the Tertiary Table 1. Leucite-bearing rock from the Batbjerg intru North Atlantic province. The main difference sion, Kangerdlugssuaq: chemistry and mineralogy. between the two intrusions appears to be the s;o2 45.75 Trace elements 8.92 absence of melilite and perovskite at Batbjerg, A1203 Fe203 8.04 Rb 128 both of which are important constituents of FeO 4.98 Ba 76 the Gardiner rocks. On the other hand, leu- MgO 9.30 Sr 1970 cite, the subject of this note, does not appear CaO 13.45 Y 7 Na20 3.50 Zr 32 to occur in the Gardiner intrusion. Whether K20 3.25 Nb 0 the differences are real or apparent can only MnO 0.19 Cu 122 Ti02 1.01 Co 54 be determined when more work can be car P2O5 0.97 Ni 73 ried out, especially on the Batbjerg intrusion. HaO+ 0.68 V 290 Cr 194 It is probable that these rocks are the in sum 100.04 trusive equivalents of lavas similar to the C.l.P.W. weight norm Mode (volume percent) nephelinite described by Brooks & Rucklidge (calculated) with analyti (1974), just as the • ijolitic and carbonatitic cal values and with leucite • 10 intrusive core of the Napak Volcano, Uganda Fe203/FeO adjusted nepheline 12 to 0.44 feldspar 4 (King, 1965) is regarded as the intrusive equi clinopyroxene 52 valent of the surrounding nephelinitic lavas or 15.66 4.70 phlogopite 3 lc 2.78 11.37 opaque ore 10 and pyroclastics (Middlemost, 1974). ne 15.05 15.05 apatite 1 The field relationships of the Batbjerg intru ac 1.61 1.61 alteration 8 sion does not allow a more precise dating than di 47.37 48.95 (nepheline + ol 1.88 8.31 feldspar) post-Precambrian, but the Gardiner intrusion mt 10.85 4.77 no. of points: cuts the plateau basalts which have been dated il 1.92 1.92 ca. 4000 ap 2.25 2.25 . to the interval 55-60 m.y. by the K-Ar meth od (Beckinsale et al., 1970) and, in view of Major element analysis: Geological Survey of Green the petrographic similarity between the intru land, Geochemical Laboratories. Trace element analyses: Rb, Sr, Y, Zr & Nb by sions, a similar age is assumed for the Bat J. Bailey, Institute of Petrology, Copenhagen University bjerg intrusion. (X-ray fluorescence). Ba, Cu, Co, Ni, V, Cr by Haldis Bollingberg, Institute of Petrology, Copenhagen Uni versity (optical spectrography). The leucite-bearing sample line. Also present are titaniferous magnetite, Leucite is confined to a single sample, but, phlogopite and apatite as minor constituents. in view of the rather dull appearance of this These phases have been investigated using the rock, there is no reason to believe it has been Hitachi XMA-5B microprobe at the Institute specially sampled. Unfortunately, the precise of Mineralogy, University of Copenhagen and field relationships of this rock are not known. the experimental techniques described by Pe It is a dark green, melanocratic rock, in which dersen et al. (in press). The approximate modal white spots of leucite, up to 1 cm in size, may composition is shown in table 1. be seen on the weathered surface. In thin sec Leucite, which shows the characteristic tion, the leucite phenocrysts are seen to be complex twinning, has a composition very accompanied by nepheline, up to 0.5 cm in close to the ideal leucite composition with size, set in a matrix of clinopyroxene prisms only 1.1 mole percent of the soda-leucite oriented to give a weak fluxion structure. (NaAlSi2Oe) component. It is stoichiometric, Pools of clear, untwinned alkali feldspar and unlike the leucites analysed by Carmichael a turbid alteration product, the latter also oc (1967) from the Leucite Hills of Wyoming. curring as rims on the leucite, are also pre The twinning, which was absent in Carmi- sent. Where nepheline is enclosed by the al chael's samples, would seem to indicate fairly kali feldspar it is perfectly euhedral. The al slow cooling from above the cubic-tetragonal teration product was identified as a finely- inversion temperature of ca. 600°C (Faust, divided mixture of alkali feldspar and nephe- 1963). 96 Brooks et al.: Leucite from East Greenland Table 2. Composition of clinopyroxene in the leucite- bearing rock from the Batbjerg intrusion, Kangerd- lugssuaq. Average of 1 complete Cations on basis of 4 P„ o=1000kg/cm point analyses cations and 6 oxygens s;o2 52.6 Si 1.946 1 Ti02 0.55 Al 0.039 > 2.000 + A1203 0.91 Fe3 0.015 J Fe.,03* 4.60 NoAISijtV \KAISi308 FeO* 2.64 Fe»+ 0.113 MnO 0.14 Ti 0.015 MgO 14.4 Fe2+ 0.082 \KAISi206 CaO 22.84 Mn 0.004 1.998 Na20 1.24 Mg 0.789 K20 0.03 Ca 0.905 Na 0.089 sum 99.95 K.

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