Sr–Nd isotopic composition of lamprophyre dykes from Queen MaudLand, East Antarctica

A. M. Dayal and S. M. Hussain

National Geophysical Research Institute, Hyderabad 500 007, India

Alkali basaltic-lamprophyric dykes intrude Precambrian gneisses in the Schirmacher Oasis, Queen Maud Land, East Antarctica. Rb–Sr and Sm–Nd isotopic data on two lamprophyre dykes are reported from the Schirmacher Oasis. The lamprophyre dykes have given Rb–Sr biotite/whole rock isochron age of 439 ±  10 Ma (2s ). Large variations in e Sr and e Nd indicate higher degree of crustal assimilation. Higher e Sr (+54 to +72) and lower e Nd (–7 to –13) of these dykes also suggest that the source was contaminated by older crust. Biotite/whole rock Rb–Sr isotope data on lamprophyre dykes show that the terrain was
affected by thermal event during early Paleozoic which is correlated with the end of Pan-African orogeny.

EARLY Paleozoic dykes of alkaline affinities intrude high-grade gneiss terrain. These dykes are widespread in the Vestfold hills, Prince Charles Mountains, Enderby Land and Droning Maud Land1,2. Metamorphism, tectonism and plutonism during the lower Paleozoic are well documented from many parts of East Antarctic shield3. These dykes provide useful time stratigraphic data which can be used to understand the relationships between crustal units of different ages4. Geochemical and isotopic studies of these dykes provide information about their petrogenesis, emplacement time, geochemical and isotopic heterogeneity within their mantle source regions and amount of crustal contamination due to wall rock assimilation and diffusion5.

The Schirmacher Oasis (SO) (lat: 71° 44¢ to 70° 47¢ S; long: 11° 22¢ to 11° 55¢ E) located in Queen Maud Land, East Antarctica is composed of Precambrian crystalline rocks mainly garnet biotite gneiss, augen gneiss, leuco gneiss and pyroxene granulites. These are intruded by basic rocks such as gabbro, dolerite, basalt, amphibolite and lamprophyre in the form of dykes and sills. Geological and geochemical studies have been carried out on these rocks earlier6–9.

The intrusion of dykes indicates recurring episodes of crustal extension since Archaean. Reliable isotopic data and ages for these intruding younger litho-units are meagre. To understand the chronology of the geological events in Queen Maud Land, East Antarctica, Rb–Sr isotopic studies were carried out on different types of rocks. Leuco gneiss (latest phase) of SO yielded an age of 779 ±  26 Ma (Rb–Sr)10 and the lamprophyre dykes of west central part of SO gave ages of 455 ±  12 Ma and 458 ±  6 Ma (ref. 11). K–Ar ages for some basaltic dykes of this area range from 290 to 302 Ma (ref. 12). We try here to understand the emplacement of two lamprophyre dykes east of Indian permanent station ‘Maitri’ and NE part of SO using Rb–Sr and Sm–Nd isotopic systematics and REE geochemistry.

The lamprophyre dykes trending NE–SW (Figure 1) intrude the garnet biotite gneisses and are exposed in the northeastern part of SO. Medium to coarse-grained lamprophyre dyke is 200 m in length and 3.5 m in width

 

Figure 1.  Outcrop map of Schirmacher Oasis. The inset shows the geology and location of dykes studied.

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while the fine-grained lamprophyre dyke is 150 m in length and 1.5 m in width, both are melanocratic greenish black in colour. They show panidiomorphic texture and consist mainly of two generations of biotite phenocrysts in a ground-mass of potash feldspar (orthoclase). Euhedral and subhedral prisms of diopside and hornblende also occur. The ground mass consists of crypto-crystalline microcline and orthoclase. Occasional grains of interstitial quartz and plagioclase occur. Opaques are associated with biotite. Apatite inclusions are noticed in the biotite. Xenocrysts of potash feldspar also occur. Calcite, apatite and iron oxides occur to a lesser extent. Except variations in grain size and biotite/amphibole modal percentage, the mineral assemblage is almost similar in both the dykes. Based on their mineral assemblage both lamprophyre dykes have been classified as ‘minette’ (mica lamprophyre) using the nomenclature of Streckeisen13.

Trace and rare earth elements (Table 1) were analysed by ICP-MS following the techniques described by

Figure 2.  Chondrite normalized REE diagram for both (triangle, coarse grained and circle fine grained) lamprophyre dyke samples. Chondrite values after Sun and McDonough24.

 

Figure 3.  Primordial mantle normalized spider diagram illustrating geochemical comparison among the lamprophyre dykes. Normalization factors after Sun and McDonough24.

Balaram et al.14. The overall composition of both lamprophyre dykes is similar (Table 1). Large ion lithophile (LIL) elements (Rb, Sr and Ba) are highly enriched with respect to primitive mantle (PM). The high field strength (HFS) elements (Y, Zr, Nb) also show slight enrichment compared to PM (Figure 2). Both lamprophyre dykes exhibit identical REE patterns and have almost similar REE contents (total REE 297.6 and 369.4 respectively) with slightly higher REE contents in fine-grained one. Chondrite normalized data show fractionated REE (Figure 3) (Cen/Ybn = 34.8 and 47.3). Fractionation in LREE (Lan/Smn = 5.9 and 5.1) and HREE (Gdn/Ybn = 6.6 and 8.6) is observed (LREE/HREE = 14.3 and 15.9 respectively) with no prominent Eu anomalies (Eu/Eu* = 1.03 and 0.99).

The isotopic composition (Table 2) together with Rb, Sr, Sm and Nd contents for these dykes were determined on a VG-354 multicollector mass-spectrometer following the techniques described by Anil Kumar et al.15. Repeated analyses of the SRM 987 (Sr) and La Jolla (Nd) standards provided mean values of 87Sr/86Sr = 0.710225 ± 30 (2s ) and 143Nd/144Nd = 0.511865 ±  5 (2s ), n = 8 respectively. The decay constants used for 87Rb and 147Sm are 1.42 ´  10–11 a–1 and 6.54 ´  10–12 a–1 respectively16. Williamson17 and Provost18 programs were used for line regression and age computation.

The measured whole rock (WR) 87Sr/86Sr ratio ranges from 0.70925 to 0.71008 and 143Nd/144Nd ratio ranges from 0.511684 to 0.512023. As there is no spread in the whole rock Rb–Sr and Sm–Nd isotopic data, WR isochron age could not be obtained for these dykes. However, four biotite using leaching technique19 and two whole rock analysis of two samples (AM-301 and AM-305) gives an isochron of 439 ±  10 (2s ) Ma (Figure 4) with an initial strontium ratio of 0.70943 ±  62. e Srt and e Ndt ranges from 54.22 to 72.37 and –6.97 to –13.44 respectively (T = 439 Ma). TDM ranges from 1.00 to 1.609 Ga (Table 3).

The LREE enrichment is due to abundance of garnet and up to certain extent hornblende and pyroxene in the source residue. It suggests that magma source of these dykes has been derived by small degree of partial melting with residue enriched in HREE20. The high Cen/Ybn ratios (34.8 and 47.3) and LREE/HREE ratios (14 and 15.9) reflect the fractionation of these dykes. Low HREE values (4–20 times of chondrite) are due to retention of these elements in the residual magma as a result of fractionation of garnet in the source. The absence of Eu anomalies (Eu/Eu* = 1.03 to 0.99) is due to insignificant role of plagioclase fractionation. The enrichment of LIL and HFS elements is due to high contents of biotite, potash feldspar and apatite in these dykes. Zr/Nb ratio for the coarse-grained lamprophyre dykes is higher (16.4) than the fine grained dykes (4.5) due to higher Zr contents in the former.

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Figure 4.  Rb–Sr isochron for lamprophyre dykes.

The four-leached biotite separates define a linear array with a very good spread in 87Sr/86Sr ratio (Figure 4). Since the biotite macrocrysts are isolated systems and the plot of 87Sr/86Sr vs 1/Sr is not linear, this array is not a mixing line but an isochron corresponding to an age of 439 ±  10 Ma and the initial Sr ratio (Sri) of 0.70943 ±  62 (2s ). The leaching of biotite macrocrysts from lamprophyres prior to their Rb–Sr analysis removes trace amount of carbonate that may be entrained in cleavage planes or grain boundaries of biotite grains. Removal of carbonates by leaching facilitates interpretation of the Rb–Sr analyses of leached biotite in terms of their emplacement time.

The high 87Sr/86Sr and low 143Nd/144Nd ratios of these rocks suggest that continental crust has contributed to these elemental variations. Crust has e Nd = –15 and upper mantle has e Nd = +12. TCHUR ages vary from 1044 to 1572 Ma. Negative e Ndt suggest that the magma for these dykes to have been derived from assimilated rocks older than 1000 Ma. The high 87Sr/86Sr and low 143Nd/144Nd ratios of these lamprophyres indicate that crustal rocks within the mantle contaminated the source magma of this lamprophyre. Custal xenoliths occur in the lamprophyres. The variations found in the isotopic and trace element systematic can also be due to crustal contamination during the ascent of primary magma by older continental crustal rocks21.

Similar (WR) ages of 439 ±  13 Ma and 476 ±  22 Ma (Ar–Ar) for dolerite dykes from Untibba of Sor Rondane Mountains of East Antarctica have been reported22. Calc-alkaline lamprophyres (~470 Ma) occur in South Victoria Land and Charles mountains (413–430 Ma) of Antarctica12 and dolerites of similar age (K–Ar age of 477 ±  24 Ma) has been reported from Untibba area of Sor Rondane mountains22. Paleozoic ultrapotassic dykes in adjacent parts of East Antarctica indicate their derivation from older contaminated mantle15,23. The geochemical characteristics of these lamprophyre dykes suggest that the source magma has been generated by partial melting of upper mantle that was metasomatized prior to/or during melting by crustal component.

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ACKNOWLEDGEMENTS.  We thank Director, NGRI for the
encouragement and permission to publish this work. Discussions
and constructive comments with Dr V. Divakara Rao and Dr B. L. Narayana were very useful. We also thank Dr D. V. Subba Rao for the discussions in the field with Dr SMH during 8th Indian Scientific Expedition to Antarctica.

Received 14 May 1999; revised accepted 24 September 1999