Review of Greenland Avtivities 2001 - Geus

More documents

Recommendations

Info

chemical composition – may indicate P–T conditions within the diamond stability field. In the Palaeoproterozoic reworked Archaean terrane north of Søndre Strømfjord another kimberlitic dyke reported by exploration companies was visited (locality C, Fig. 2). The dyke consists of a 5–10 m wide, c. 500 m long train of kimberlitic boulders and outcrops in banded grey gneiss. Peridotitic and pyroxenitic xenoliths are abundant, ranging in size from less than 1 cm to around 10 cm. Dunitic (ol), harzburgitic (ol + opx) and lherzolitic (ol + opx + cpx ± gt) xenoliths were identified in the field. The Survey campaign located two further dykes north of Søndre Strømfjord (localities D and E, Fig. 2). The locality D dyke is vertical, 2–5 m wide and can be traced for c. 1.5 km. Xenoliths are much less abundant here than in the dykes south of Søndre Strømfjord, and their sizes range from less than 1 cm to about 5 cm. Macroscopically, the peridotitic xenolith types are dunite (ol) and lherzolite (ol + cpx + opx). The locality E dyke is a gently dipping (16–20°S), 30–50 cm thick sheet, and contains small xenoliths that are mostly less than 2 cm in diameter. Preliminary results Seven xenoliths from the locality A dyke have been analysed for major element mineral chemistry by electron microprobe at the Geological Institute, University of Copenhagen, and garnets in three of them have been subjected to reconnaissance laser ablation analyses at the Survey. Garnets in the same three xenoliths were subjected to high-precision analysis of Ni by electron microprobe, and the xenolith rock types (Table 2) have been determined from their mineral assemblages and estimated mineral proportions (e.g. LeBas & Streckeisen 1991). Generally, the major element compositions of minerals within individual xenoliths are homogeneous Cr 2 O 3 (wt%) 12 10 8 6 4 2 0 464002 464007.1 464006 464003 STD G10 464008.8 464008.10 464008.11 0 2 4 6 8 CaO (wt%) Fig. 6. Garnet compositions in xenoliths from the locality A dyke, analysed by electron microprobe. The G9–G10 boundary line is from Gurney (1984) and Gurney & Zweistra (1995), and STD is a reference garnet analysed along with microprobe standards. GGU 464xxx numbers refer to sample material in the files of the Geological Survey of Denmark and Greenland. (Table 2). The average forsterite (Fo) content of the olivines ranges from Fo87 to Fo92; garnet wehrlites contain the olivines with the lowest Fo, garnet harzburgites and garnet lherzolites the olivines with the highest Fo. Average Ni contents are within the range of 2900 ± 360 ppm reported by Ryan et al. (1996) for garnet peridotites. Orthopyroxenes contain less than 1.6 wt% CaO and have relatively low Al 2 O 3 , the lowest values being from ilmenite-free garnet lherzolites (0.49–0.53 wt%). Clinopyroxene Mg/(Mg + Fe 2+ ) ratios correlate positively with Cr 2 O 3 . Garnet Mg/(Mg + Fe 2+ ) ratios tend to correlate positively with Fo contents of olivine. CaO and Cr 2 O 3 contents of garnets show limited variation within individual xenoliths but large overall variations (Fig. 6), and the garnets from two harzburgite xenoliths clearly G9 Table 2. Calculated temperatures and pressures for xenolith minerals from the locality A dyke Mineral GGU sample 464002.2 464003 464006 464008.10 464008.11 analysed Rock type gt hzb gt lhz gt lhz gt lhz gt weh Minerals ol-opx-gt ol-opx-cpx-gt ol-opx-cpx-gt ol-opx-cpx-gt-ilm ol-cpx-gt cpx T (°C) * 1039 ± 30 1194 ± 30 1124 ± 30 1030 ± 30 cpx P (kbar) * 48.9 ± 2.3 57.8 ± 2.3 68.7 ± 2.3 52.5 ± 2.3 gt T Ni (°C) † 1143 ± 42 1101 ± 18 1221 ± 30 T, P and T Ni calculated according to: * Nimis & Taylor (2000), † Ryan et al. (1996). ol: olivine, opx: orthopyroxene, cpx: clinopyroxene, gt: garnet, ilm: ilmenite, hzb: harzburgite, lhz: lherzolite, weh: wehrlite. The GGU numbers refer to material from Greenland in the files of the Geological Survey of Denmark and Greenland. 62
plot within the G10 field of Gurney (1984) and Gurney & Zweistra (1995). G10-class pyrope garnets are considered to be strongly indicative of conditions favourable for diamond stability. Equilibration temperatures (T) and pressures (P) for the clinopyroxene-bearing xenoliths were calculated using the Nimis & Taylor (2000) single clinopyroxene thermobarometry equations (Table 2), and these temperature estimates are comparable to T Ni values calculated using the high-precision Ni analyses and the empirical Ni-in-garnet thermometer of Ryan et al. (1996). Variations of Zr vs. Y in garnets of three xenoliths from the locality A dyke are shown in Fig. 7, and averaged chondrite-normalised REE patterns for garnet and clinopyroxene in each xenolith in Fig. 8. The very low Zr and Y contents in garnets of a garnet harzburgite xenolith (sample GGU 464002.2) are consistent with their low- Ca character (Fig. 6) and indicate that this xenolith represents original lherzolite mantle depleted by partial melting (Griffin et al. 1992). The garnet in sample GGU 464002.2 is more depleted in the middle and heavy REE than the garnets in lherzolites from samples GGU 464003 and 464006. The sinuous pattern resembles that of low- Ca garnet in a harzburgite xenolith from the Sarfartoq area (Garrit 2000). The low REE contents support the idea that the garnet is hosted in a rock depleted by partial melting, whereas the sinuous shape may indicate postdepletion metasomatic enrichment processes (Hoal et al. 1984) or disequilibrium garnet growth (Shimizu & Sobolev 1995). Garnet lherzolites from samples GGU 464003 and 464006 have much higher Zr and Y concentrations in the garnets, and their chondrite-normalised REE patterns are more like those of typical LREE-depleted and MREE- to HREE-enriched garnets of primitive mantle (e.g. Haggerty 1995). Garnets from samples GGU 464003 and 464006 plot within fields indicating phlogopite and melt metasomatism of previously depleted mantle, respectively (Fig. 7). Phlogopite occurs in the xenoliths as part of kelyphitic rims on garnets, and in sample GGU 464003 also as a minor phase away from garnets. Ti contents of garnets, which follow Zr in some metasomatic processes (Griffin et al. 1999), show no significant zoning. This may indicate that if metasomatic processes affected garnet compositions, there was sufficient time for equilibration with the metasomatising agents. Geophysical properties of kimberlites Airborne geophysical surveys play an important role in diamond exploration, as kimberlites are often hidden under surficial deposits but also often have magnetic properties that make them distinguishable from the country rocks (e.g. Keating 1995; Macnae 1995). Since 1996 several helicopter-borne surveys have been commissioned by exploration companies to cover their licence areas in the Maniitsoq and Kangerlussuaq regions (Olsen et al. 1999). Data from the latest helicopterborne survey in the Kangerlussuaq region in 2000 were used to outline targets for the drilling that led to the Fig. 7. Laser ablation ICP-MS analyses of Zr and Y in garnets from xenoliths in the locality A dyke (this study, data from GEUS), garnet lherzolite and garnet harzburgite xenoliths from the Sarfartoq area (Garrit 2000, data from Memorial University, Canada) and in garnet separates from the Sarfartoq area (brown dots: Garrit 2000, data from Macquarie University, Australia). The coloured fields are from Griffin & Ryan (1995) and Griffin et al. (1999), and arrows indicate core-to-rim zonation direction in metasomatised garnets of Griffin et al. (1999). Y (ppm) 60 50 40 30 20 10 Undepleted garnets (high geotherm) Melt metasomatism Phlogopite metasomatism 0 0 100 200 300 400 Depleted garnets Zr (ppm) Gt SEP 464002.2 464006 464003 Lh-1 Lh-2 Hz 63
Page 1:
GEOLOGY OF GREENLAND SURVEY BULLETI
Page 4:
Geology of Greenland Survey Bulleti
Page 7 and 8:
Contents Numbers of articles corres
Page 9:
■ ■ ■ ■ ■ ■ ■ ■ ■
Page 12 and 13:
Canada Pituffik O 1 Thule Air Base
Page 14 and 15: Publications A complete list of geo
Page 16 and 17: Canada Greenland Greenland Inland I
Page 18 and 19: northern Nagssugtoqidian orogen (SN
Page 20 and 21: western to the north-eastern corner
Page 22 and 23: group consists of conjugate sets of
Page 24 and 25: y pure shear. Coincidence of the or
Page 26 and 27: The Precambrian supracrustal rocks
Page 28 and 29: 68°25′ 52° 68°25′ BIF 0 3 km
Page 30 and 31: Fig. 4. Dolomitic marble in the sou
Page 32 and 33: Bugt (Fig. 1; Henderson 1969). The
Page 34 and 35: Keto, L. 1962: Aerial prospecting b
Page 36 and 37: ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲ ▲
Page 38 and 39: increases towards the south. We obs
Page 40 and 41: Kalsbeek, F. & Taylor, P.N. 1999: R
Page 42 and 43: Canada Nuussuaq 0 50 km PROTEROZOIC
Page 44 and 45: deposit. Descriptions of the differ
Page 46 and 47: Pegmatites The gneisses throughout
Page 48 and 49: tacts with quartzo-feldspathic coun
Page 50 and 51: Geological correlation of magnetic
Page 52 and 53: Fig. 2. The hand-held magnetic susc
Page 54 and 55: Susceptibility distribution (%) Sus
Page 56 and 57: have a reducing effect, which hinde
Page 58 and 59: ancy between the susceptibility val
Page 60 and 61: 67° Sisimiut • 53° 52° 51° 50
Page 62 and 63: Table 1. Range of Nb concentrations
Page 66 and 67: Concentration/C1 chondrite 100.00 1
Page 68 and 69: Menzies, M.A. & Hawkesworth, C.J. (
Page 70 and 71: 72°30´ Aeromag 2001 50 km Upernav
Page 72 and 73: 56° 54° 52° 72° 72° nT 71°30
Page 74 and 75: Conclusions The aeromagnetic survey
Page 76 and 77: 53° 51° Inland Ice Greenland 72°
Page 78 and 79: On the south coast of Nuussuaq, wes
Page 80 and 81: Fig. 5. Blocks and boulders in the
Page 82 and 83: 2000 SW NE 1500 Surface trace 1985
Page 84 and 85: 1979). Along other sections of the
Page 86 and 87: Petroleum geological activities in
Page 88 and 89: Seismic acquisition during summer 2
Page 90 and 91: Other petroleum geological work Alt
Page 92 and 93: A multidisciplinary study of the Pa
Page 94 and 95: Seismic interpretation The grid of
Page 96 and 97: Middle Eocene Lower Eocene Upper Pa
Page 98 and 99: Chalmers, J.A., Dahl, J.T., Bate, K
Page 100 and 101: 95-06 ▲ ▲ 58° 95-12 95-10 56°
Page 102 and 103: 70° 71° 60° 2000 2000 2400 400 1
Page 104 and 105: Storey, M., Duncan, R.A., Pedersen,
Page 106 and 107: 24° 100 km 30° 27° 21 ° Jameson
Page 108 and 109: Oligocene Krabbedalen Fm Eocene Bop
Page 110 and 111: grading. Bivalve shell material is
Page 112 and 113: References Birkenmajer, K. 1972: Re
Page 114 and 115:
B A 72° Palaeogene basalts Gl 50 k
Page 116 and 117:
a b c d e f g h 20 µm i j k l Fig.
Page 118 and 119:
offshore South-East Greenland, onsh
Page 120 and 121:
▲ Albert Heim Bjerge Wordie Gletc
Page 122 and 123:
T D A CW NS placed the lowermost c.
Page 124 and 125:
is abundant and highly diverse, and
Page 126 and 127:
The Heimbjerge Formation comprises
Page 128 and 129:
Late Permian carbonate concretions
Page 130 and 131:
m 25 20 15 B3 L2 B2 Fig. 3. Simplif
Page 132 and 133:
(2001), who pointed out the restric
Page 134 and 135:
Piasecki, S. 1984: Preliminary paly
Page 136 and 137:
66° 68° • • • • • • 7
Page 138 and 139:
absent at Tikeraussaq where a postu
Page 140 and 141:
72° 70° 68° 66° Sample locality
Page 142 and 143:
Table 2. Summary of selected elemen
Page 144 and 145:
Concluding remarks Observations dur
Page 146 and 147:
Lake-catchment interactions with cl
Page 148 and 149:
lakes. As in earlier reports (e.g.
Page 150 and 151:
Discussion Global climate models pr
Page 152 and 153:
Glaciological investigations on ice
Page 154 and 155:
Fig. 2. The new one-mast design of
Page 156 and 157:
A B N 20 km 1993 1995 Fig. 5. Satel
Page 158 and 159:
Krabill, W., Frederick, E., Manizad
Page 160 and 161:
2001/47: Calibration of stream sedi
Page 162 and 163:
Hanghøj, K., Kelemen, P., Bernstei
Page 165 and 166:
Danmarks og Grønlands Geologiske U
Page 167:
GEOLOGY OF GREENLAND SURVEY BULLETI
show all

Review of Greenland Avtivities 2001 - Geus

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?