Lower crustal granulite xenoliths from the Arkhangelsk kimberlite pipes: petrological, geochemical and geophysical results

Lithos 51 Ž2000. 135–151 www.elsevier.nlrlocaterlithos

Lower crustal granulite xenoliths from the Arkhangelsk kimberlite pipes: petrological, geochemical and geophysical results A.J.W. Markwick, H. Downes

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Department of Geology, Birkbeck College, UniÕersity of London, Malet Street, London WC1E 7HX, UK Received 21 December 1998; accepted 1 September 1999

Abstract Five mafic garnet granulite xenoliths have been obtained from Devonian kimberlite hosts from the Pachuga field, Arkhangelsk, NW Russia. Whole rock major and trace element data, Sr and Nd isotopic analyses, mineral chemistry, calculated compressional wave velocities and densities of the xenoliths are reported here. The xenoliths are medium grained and are reasonably fresh with well-developed granoblastic fabrics. Primary mineralogy consists of almandine–pyrope-rich garnet, diopside, oligoclase plagioclase and scapolite; minor phases include rutile and ilmenite. One sample contains only garnet and metasomatic pargasitic amphibole, pseudomorphing primary diopside. All analysed samples have sub-alkaline basaltic compositions. REE patterns show variable LREE-enrichment ŽLa NrYb N s 13 to 1.4. and small negative and positive Eu anomalies Ž0.87–1.32., indicating that both plagioclase fractionation and accumulation have occurred. Present-day 87 Srr86 Sr and 143 Ndr144 Nd isotope ratios are very low Ž0.70272–0.70378 and 0.511736–0.512225, respectively., suggesting that both assimilation of Archaean continental crust and removal of Rb, possibly related to granulite-facies metamorphism, took place early in the history of these rocks. PrT estimates Ž670–7308C, 1.4–1.6 GPa. indicate equilibration depths of up to 50 km. Calculated compressional wave velocities range from 6.94 to 7.70 kmrs and densities from 3.2 to 3.5 Mgrm3, assuming PrT conditions of 5008C and 1.4 GPa. These values support an origin in the deep continental crust. TDM model ages are 1.7 to 1.9 Ga and may represent the time of extraction from the mantle during the mid-Proterozoic. These xenoliths may represent the in situ mafic lower crust beneath Arkhangelsk, formed as a result of basaltic underplating during the mid-Proterozoic. q 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Arkhangelsk; Granulite; Xenolith; Lower crust

1. Introduction The inaccessibility of the lower continental crust makes it a relatively poorly understood region of the Earth. However, studies of xenoliths offer a unique )

Corresponding author. Tel.: q44-171-380-7712; fax: q44171-383-0008; e-mail: [email protected]

glimpse into the chemical and physical properties of the in situ deep continental crust. A large number of geochemical and geophysical studies exist for lower crustal xenoliths that have been sampled through Phanerozoic crust and our understanding of crustal formation processes is significant in these areas ŽRudnick and Taylor, 1987; Downes, 1993; Rudnick and Fountain, 1995.. In contrast, studies of xenoliths,

0024-4937r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 Ž 9 9 . 0 0 0 7 8 - X

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A.J.W. Markwick, H. Downesr Lithos 51 (2000) 135–151

granulite xenoliths have been obtained from the drillhole core of a sub-surface diatreme ŽAnomaly 688., from the Pachuga kimberlite field. The Arkhangelsk kimberlites are considered to be Devonian–Middle Carboniferous in age ŽSinitsyn et al., 1992; Parsadanyan et al., 1996.. Beard et al. Žthis volume. report the age of a melilitite pipe from the nearby Onega Peninsula as 361 " 23 Ma, and assuming that this event was contemporaneous with the kimberlite magmatism, we can infer that the garnet granulite xenoliths from Arkhangelsk represent the in situ lower crust beneath the Archaean Kola–Kuloi craton ŽFig. 1. during Devonian times. The present-day crustal thickness is 38–45 km ŽMeissner et al., 1987; Ziegler, 1990; White et al., 1995..

3. Analytical techniques Fig. 1. Geological sketch map of the Baltic Shield Žadapted from Beard et al., this volume. showing the locations of the Arkhangelsk kimberlite field.

which have been sampled through Archaean or Proterozoic crust, are less common and as a consequence, much less is known about the lower crust of Proterozoic and Archaean areas. In this paper, we present new geochemical and mineralogical data for five lower crustal xenoliths sampled through Archaean crust from beneath the Arkhanglesk Kimberlite Province, which forms part of the northern Baltic Shield ŽFig. 1.. We show that the lower crust from this region is predominantly composed of mafic garnet granulites and may represent a periodrs of mafic underplating. We compare the data for the xenoliths with garnet granulites from Belarus ŽMarkwick et al., submitted., Finland ŽHoltta ¨ ¨ et al., 2000. and Kola ŽKempton et al., 1995; Kempton et al., submitted.. Compressional wave Ž Vp . velocities and density values for the xenoliths have been calculated and compare well with observed seismic velocities.

2. Geological setting The magmatism of the Arkhangelsk kimberlite province has been documented by Parsadanyan et al. Ž1996. and Beard et al. Ž2000.. Five mafic garnet

Modal estimates of minerals from the xenoliths were obtained using a 1000-point count ŽTable 1.. Electron microprobe analyses of mineral compositions were obtained using a Jeol 733 Superprobe ŽBirkbeck College. with an Oxford Instruments ISIS energy dispersion system ŽTable 2.. Analytical conditions were 15 kV accelerating voltage, a spot diameter of 1–2 mm and a count time of 100 s. Host rock was removed from the xenoliths by sawing and grinding to remove possible contaminants. Less than 10 g of whole rock powder were available for the major and trace element analyses of four samples.

Table 1 Estimated modal mineralogy for Arkhangelsk lower crustal xenoliths using a 1000-point count Mineral abbreviations: Ga s garnet, Plg s plagioclase, Cpx s clinopyroxene, Kf s K-feldspar, Ops opaques, Scapsscapolite, Ampsamphibole, Apsapatite, Bi s biotite, Alt salteration, Rt s rutile. Ilmenite occurs in varying amounts in all samples ŽAppendix A. but crystals were not large enough to be identified in the point count. Modal mineralogy

A1 A2 A3 A4 A5

Ga

Plg

Cpx Kf Op Scap Amp Ap Bi Alt

Rt

34.6 30.4 42.4 40.2 34.8

38.2 0 24.8 15.0 30.8

0 tr 32.8 44.3 33.4

2.0 0 tr 0.2 0.2

0 0 tr 0 0

1.4 23.8 0 0 tr 0 46.2 tr tr 0 0 tr 0.3 0 0 0 0.8 0 0 tr

tr 0 0 0 0

tr 23.4 0 tr 0

A.J.W. Markwick, H. Downesr Lithos 51 (2000) 135–151

Whole rock samples were analysed by X-ray fluorescence ŽRoyal Holloway, University of London., using glass discs for major elements ŽTable 3a.. A selection of trace elements were analysed using ICPMS ŽNERC ICPMS facility, University of London.. Results are given in Table 3a. Four garnet granulite xenoliths were analysed for REE and Sr and Nd isotopes ŽTable 3a and b. at Royal Holloway, University of London. Sr and Nd fractions were separated using standard ion-exchange procedures. Sr was run as metal on a single Ta filament, Nd was loaded onto a single Re filament and run as an oxide. REE were analysed using an isotope dilution method ŽThirlwall, 1982.. Analyses were made using a VG354 multicollector mass spectrometer in dynamic mode. 86 Srr88 Sr was normalised to a value of 0.1194 and 146 Ndr144 Nd to a value of 0.7219. Values for 87 Srr86 Sr and 143 Ndr144 Nd are recorded relative to the measured values of SRM987s 0.71024 and an internal Aldrich laboratory standard, which gave a value of 0.511415, equivalent to a value of 0.511856 for the La Jolla standard.

4. Results 4.1. Petrography, mineralogy and mineral chemistry Little is known about the composition of the lower crust beneath Arkhangelsk. The very small size Ž2–3 cm diameter after grinding. of the xenoliths made analysis difficult and limits our confidence that these rock samples are truly representative of the lower crust beneath Arkhangelsk. The suite consists of five relatively unaltered, fine to medium grained mafic garnet granulites. All samples exhibit polygonal granoblastic fabrics with well-developed triple junctions, characteristic of equilibrium textures developed under long-lasting granulite facies metamorphic conditions. All samples are garnetiferous Ž30–42%., and three also contain plagioclase and clinopyroxene. One sample ŽA1. contains scapolite Ž23%., plagioclase and garnet. Sample A2 contains abundant garnet, amphibole, and pseudomorphs after plagioclase. Modal proportions of the constituent minerals are given in Table 1. Mineral banding is observed only in one sample ŽA1.. However, lattice

137

preferred orientation exists in three samples ŽA1, A4 and A5.. A fuller account of petrographic and mineralogical observations is given in Appendix A. Garnets are pink, near equant and riddled with pressure release cracks that have been variably altered to a complex chlorite-type mineral assemblage. Very thin Ž; 75 mm. dark redrbrown cryptocrystalline chloriteramphibolerplagioclaserkelyphite rims occur around the garnets. Inclusions of small rounded scapolite, apatite, zoned ilmenite and diopside are present within the garnets. Exsolution of acicular rutile from the garnets occurs rarely. The garnets are unzoned almandine-rich solid solutions in the range 40–60% almandine, 19–38% pyrope, 17– 22% grossular, and are homogeneous within a single xenolith ŽTable 2; Fig. 2.. Embayment of garnet by clinopyroxene and plagioclase is very limited and may represent decompression reactions. Plagioclase is unzoned oligoclase with a narrow compositional range of An 12 –An 25 ŽTable 2.. Deformation twinning and undulose extinction are commonly found in plagioclase crystals, as is a weak development of micro-antiperthitic textures, often occurring as patchy blebs or lamellae. It is likely that the garnetramphibole-rich xenolith A2 originally contained primary plagioclase. However, saussuritisation has now produced mineralogically complex pseudomorphs after plagioclase ŽAppendix A.. Kfeldspar ŽOr90 . occurs rarely in only one sample ŽA5. as fresh, discrete anhedral crystals. Pyroxenes are pale green, slightly pleochroic, unzoned primary diopside and occur in abundance in three samples ŽTable 2.. Where inclusions of diopside occur, they have identical major element compositions to the diopside that forms part of the equilibrium mineral assemblage. Sample A2 has very rare relic diopside, the rest of the pyroxene having been pseudomorphed by brown pargasitic amphibole ŽTable 2.. Primary scapolite is present in only one sample ŽA1., where it forms part of the granulite facies paragenesis. It is also commonly found as rounded inclusions in garnets. Compositionally, it is close to the intermediate SO42y-rich mizzonite ŽTable 2.. Amphibole occurs in one sample ŽA2., forming abundant deep redrbrown crystals which pseudomorph diopside. It is pargasite and has a Mga s 0.75 ŽTable 2..

A.J.W. Markwick, H. Downesr Lithos 51 (2000) 135–151

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Minor phases include fluor-apatite, rutile, which is particularly abundant in sample A1, and ilmenite, which occurs as zoned inclusions in garnets in sam-

ple A4. Biotite is very rare being found in one sample where it is highly altered ŽTable 2.. A single zircon was found in sample A5.

Table 2 Electron microprobe analysis of constituent minerals in Arkhangelsk xenoliths, values are expressed in wt.% Feldspar A1 Plg SiO 2 63.80 Al 2 O 3 22.41 CaO 3.46 Na 2 O 9.08 K 2O 0.62 Total 99.37 Ab 79.6 An 16.8 Or 3.6 Cationsr eight oxygens Si 2.833 Al 1.172 Fe 0.001 Ca 0.165 Na 0.782 K 0.035 Amphibole A2 SiO 2 43.23 TiO 2 1.58 Al 2 O 3 13.65 FeO 9.41 MnO 0.00 Cr2 O 3 0.23 MgO 14.85 CaO 11.69 Na 2 O 2.93 K 2O 1.46 SO 3 Total 99.03 MgrŽMg q Fe. 0.75 Cationsr 23 oxygens TSi 6.232 TAl 1.768 CAl 0.549 CCr 0.026 CTi 0.171 CMg 3.191 CFe 2q 1.062 BFe 2q 0.072 BCa 1.806 BNa 0.122 ANa 0.697 AK 0.269

A3 Plg

A3 Or

A4 Plg

A5 Plg

A5 Or

61.49 23.82 5.11 8.21 0.60 99.23 71.8 24.7 3.5

59.95 23.46 1.25 1.66 12.26 98.58 15.9 6.6 77.5

64.86 21.54 2.54 9.70 0.61 99.25 84.3 12.2 3.5

62.43 23.34 4.80 8.78 0.32 99.67 75.4 22.8 1.8

64.25 18.49 0.06 1.11 14.98 98.89 10.1 0.3 89.6

2.749 1.254 0 0.245 0.712 0.039 Scapolite A1 48.85 24.16

0.61 14.10 5.31 5.62 98.65

2.786 1.284 0 0.062 0.150 0.727

2.878 1.125 0 0.121 0.834 0.035

2.775 1.222 0 0.229 0.757 0.018

2.990 1.013 0 0.003 0.100 0.889

A.J.W. Markwick, H. Downesr Lithos 51 (2000) 135–151

139

Table 2 Žcontinued. Garnet A1 SiO 2 39.85 TiO 2 0.03 Al 2 O 3 22.14 FeO 21.74 MnO 0.00 MgO 9.63 CaO 6.62 Total 100.01 Alm 45.9 And 0.2 Gross 17.7 Pyr 36.2 Spess 0.0 Cationsr 12 oxygens Si 3.018 Al IV 0.000 AlVI 1.975 Fe 3q 0.000 Ti 0.002 Cr 0.003 Fe 2q 1.377 Mg 1.087 Mn 0.000 Ca 0.537

A2 39.73 0.06 21.78 21.23 0.00 10.09 6.64 99.53 44.3 0.7 17.2 37.8 0.0 3.017 0.000 1.947 0.004 0.003 0.002 1.344 1.142 0.000 0.540

A3 39.76 0.00 22.26 18.55 0.00 9.77 9.39 99.73 38.2 1.1 24.1 36.6 0 2.998 0.002 1.974 0.023 0.000 0.000 1.147 1.098 0.000 0.758

A4

A5

38.33 0.13 21.1 27.39 0.58 4.84 7.52 99.89 58.0 0.67 20.8 19.2 1.3

42.62 0.19 22.72 21.91 0.00 5.86 6.29 99.59 54.3 0 19.9 25.8 0.0

3.008 0.000 1.950 0.013 0.008 0.000 1.784 0.566 0.039 0.632

Pyroxene A2 SiO 2 52.33 TiO 2 0.20 Al 2 O 3 3.14 FeO 5.42 MgO 14.35 CaO 22.29 Na 2 O 1.44 Total 99.17 Wo 48.0 En 43.0 Fs 9.0 Cationsr six oxygens Si 1.921 AlIV 0.079 M1Al 0.057 M1Ti 0.006 M1Fe 3q 0.106 M1Fe 2q 0.039 M1Cr 0.007 M1Mg 0.785 M2Fe 2q 0.021 M2Ca 0.877 M2Na 0.103

A3

A4

A5

51.56 0.70 7.66 5.12 12.19 20.80 2.27 100.30 49.8 40.6 9.6

52.45 0.44 4.62 11.5 9.97 18.18 2.97 100.13 44.3 33.8 21.9

53.01 0.23 4.92 8.28 12.94 17.50 2.45 99.33 41.7 42.9 15.4

1.869 0.131 0.196 0.019 0.052 0.071 0.004 0.659 0.033 0.808 0.160

1.944 0.056 0.145 0.012 0.099 0.192 0.000 0.551 0.065 0.722 0.213

1.951 0.049 0.164 0.006 0.047 0.073 0.000 0.710 0.135 0.690 0.175

3.300 0.000 2.072 0.000 0.011 0.000 1.419 0.676 0.000 0.522

A.J.W. Markwick, H. Downesr Lithos 51 (2000) 135–151

140

Table 3 Ža. Major Žwt.%., trace Žppm. and REE Žppm. abundances obtained from Arkhangelsk xenoliths ŽMga s Mg 2qrŽMg 2qq Fe 2q ., assuming Fe 2qs 0.85 Fe 2 O 3 .; Žb. Sr and Nd isotopic data for Arkhangelsk garnet granulites; Žc. Normative mineralogy for xenoliths from Arkhangelsk ŽCIPW norms are calculated after Irvine and Baragar Ž1971.. Ža.

A1

A3

A4

A5

SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 MgO CaO MnO Na 2 O K 2O P2 O5 Loss on ignition Total Mga Rb Sr Y Zr Ba Hf Th La Ce Nd Sm Eu Gd Dy Er Yb Lu La N rYb N

47.84 1.087 19.42 11.52 8.92 6.08 0.166 2.76 0.758 0.163 3.26 98.71 0.65 5.7 339 6.9 37.4 298 1.0 0.5 13.26 26.34 11.353 1.894 0.764 1.67 1.40 0.77 0.68

46.64 0.661 16.14 10.54 11.23 12.49 0.153 1.73 0.311 0.101 1.11 100.00 0.72 3.5 307 9.4 18.7 94 0.8 0.3 2.8 8.74 6.511 1.644 0.576 1.85 2.13 1.33 1.29

13.1

1.4

47.47 1.744 13.23 16.11 7.95 10.37 0.332 2.33 0.268 0.192 0.63 100.00 0.54 2.0 210 33.3 87.6 101 2.1 0.6 7.76 20.55 14.214 3.987 1.30 5.10 6.46 3.90 3.83 0.57 1.4

48.34 1.563 15.25 13.86 7.85 8.74 0.347 2.49 1.144 0.432 1.15 100.01 0.56 6.9 460 17.7 45.3 311 1.4 1.3 17.05 35.85 17.905 3.831 1.131 3.90 3.86 2.14 1.870 0.252 6.9

Žb.

87

A1 A3 A4 A5

0.704019 " 11 0.702889 " 10 0.703219 " 11 0.703728 " 10

U

Srr

86

U

Sr

´ Sr

143

y6.8 y22.9 y18.2 y10.9

0.511736 " 5 0.512225 " 14 0.512476 " 4 0.511921 " 5

Ndr

144

Nd

´ Nd

TNd ŽGa.

y17.6 y8.1 y3.2 y13.9

1.73 = 10y9 1.95 = 10y9 1.85 = 10y9 1.96 = 10y9

Žc.

A1

A3

A4

A5

C Or Ab An Di Hy Ol Mt Il Ap

3.47 4.58 23.89 29.77 0.00 25.24 7.62 2.85 2.11 0.36

0.00 1.85 14.77 35.65 21.10 1.89 20.61 2.50 1.27 0.22

0.00 1.61 19.98 25.17 21.15 15.32 8.86 4.02 3.36 0.42

0.00 6.84 21.31 27.35 11.18 15.58 10.16 3.48 3.00 0.95

A.J.W. Markwick, H. Downesr Lithos 51 (2000) 135–151

Fig. 2. Ternary plot of representative garnet compositions from Arkhangelsk Žthis paper. and from lower crustal granulites from Kola ŽKempton et al., 1995., Belarus ŽMarkwick et al., submitted., Finland ŽHoltta ¨ ¨ et al., 2000. and Lesotho ŽGriffin et al., 1979..

4.2. Major and trace element geochemistry The four analysed granulite xenoliths from Arkhangelsk are basic in composition ŽTable 3a. and have sub-alkaline affinities ŽFig. 3.; they are predominantly olivine–hypersthene normative, but

Fig. 3. Discrimination diagram using whole rock SiO 2 vs. ZrrTiO 2 ŽWinchester and Floyd, 1977. for garnet granulite xenoliths from Arkhangelsk Žthis paper., Kola ŽKempton et al., 1995., Belarus ŽMarkwick et al., submitted., Lesotho ŽGriffin et al., 1979. and Mg-basalts from the Pechenga–Varzuga Belt ŽSmolkin and Skufin, 1995..

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xenolith A3 is strongly olivine normative ŽTable 3c.. Little variation in SiO 2 content Ž47–48 wt.%. occurs, but MgO and CaO range from 8 to 11 wt.% and 6 to 12 wt.%, respectively. Loss on ignition is low Ž0.6–1.2 wt.%. in three of the xenoliths, but high in the scapolite-rich xenolith A1. Total alkalis reach 3.6 wt.% and Na 2 OrK 2 O ratios vary from 2.2 to 8.7. Magnesium numbers vary from 0.54 to 0.72 ŽTable 3a.. Poor negative correlations exist between MgO and SiO 2 , CaO and Al 2 O 3 and MgO and Fe 2 O 3 , for all xenoliths. The high Al 2 O 3 and Na 2 O concentrations in sample A1 correlate well with the much higher modal percentage of plagioclase and scapolite in this rock. The REE abundances for the Arkhangelsk xenoliths, when normalised to chondrite, show variable concentrations and patterns ŽFig. 4.. A1 and A5 are LREE-enriched ŽLa N rYb N s 13 and 7, respectively., but A5 has much higher concentrations of all REE and a small negative Eu anomaly ŽEurEuU s 0.90., whereas the scapolite-rich xenolith ŽA1. has a positive Eu anomaly ŽEurEuU s 1.32.. A3 and A4 display relatively flat patterns ŽLa N rYb N s 1.4, Fig. 4.. However, A4 has considerably higher REE concentrations and a small negative Eu anomaly ŽEurEuU s 0.87. and A3 has a maximum at Nd. Trace element patterns, normalised to N-MORB ŽFig. 5., are complex with little resemblance to melt-like compositions. The Eu anomaly of A1 is supported by its

Fig. 4. Chondrite normalised REE patterns for whole rock garnet granulites from Arkhangelsk and selected samples from Kola ŽKempton et al., submitted. and Belarus ŽMarkwick et al., submitted.. Normalisation coefficients from Nakamura Ž1974..

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A.J.W. Markwick, H. Downesr Lithos 51 (2000) 135–151

1985.. 87 Srr86 Sr values are low, ranging from 0.703 to 0.704 with the highest value being from the scapolite-rich xenolith A1. The 143 Ndr144 Nd values are also low and vary from 0.5117 to 0.5122, the lowest value being from sample A1. TDM model ages have been calculated using a present-day 143 Ndr144 Nd of 0.513114 and 147 Smr144 Nd of 0.222 for depleted mantle ŽMichard et al., 1985. and indicate extraction from a mantle reservoir during the mid-Proterozoic Ž1.7–1.9 Ga., assuming that SmrNd ratios have not been altered by either fractionation or contamination ŽRudnick, 1990.. 87 Srr86 Sr and 143 Ndr144 Nd values for Arkhangelsk xenoliths appear to fall on a trend between mantle compositions Žlow 87 Srr86 Sr and high 143 Ndr144 Nd. and Baltic Shield lower crust represented by Kola and Belarus granulite xenoliths Žhigh 87 Srr86 Sr and low 143 Ndr 144 Nd.. 4.4. Pressure and temperature determinations Fig. 5. N-MORB normalised trace element abundances for whole rock garnet granulite xenoliths from Arkhangelsk and Mg-basalts from the Pechenga–Varzuga Belt ŽSmolkin and Skufin, 1995.. Normalisation coefficients from Saunders and Tarney Ž1984..

Temperatures and pressures have been estimated ŽTable 4. using the convergence of plotted data for the Fe 2qrMg equilibria geothermometers in garnet–clinopyroxene ŽPowell, 1985; Krogh, 1988. and

small positive Sr anomaly. However, the large positive Sr peak in xenolith A3 has no positive Eu counterpart, suggesting that the parent rock was Srrich and that this anomaly is not entirely due to plagioclase accumulation. This observation may also be explained as a consequence of the parental magma having a higher oxygen fugacity. All samples have low Zr, Hf, Ti and Y concentrations, which may be due to residual garnet in their source andror attributed to fractional crystallisation of ilmenite and zircon. 4.3. Sr and Nd isotope geochemistry Present-day 87 Srr86 Sr and 143 Ndr144 Nd values ŽTable 3b. for the four Arkhangelsk xenoliths are plotted in Fig. 6, along with fields for lower crustal xenoliths from Kola ŽKempton et al., submitted. and Belarus ŽMarkwick et al., submitted., Lesotho ŽRogers and Hawkesworth, 1982., Tanzania ŽCohen et al., 1984. and Snake River Plain ŽLeeman et al.,

Fig. 6. Variation of present-day Sr and Nd isotope ratios for lower crustal garnet granulite xenoliths from Arkhangelsk, Belarus ŽMarkwick et al., submitted., Kola ŽKempton et al., submitted., Lesotho ŽRogers and Hawkesworth, 1982. and Snake River Plain ŽLeeman et al., 1985..

A.J.W. Markwick, H. Downesr Lithos 51 (2000) 135–151 Table 4 Calculated Pr T, Vp and r values for Arkhangelsk garnet granulite xenoliths

A1 A2 A3 A4 A5

PrGPa

T r8C

Velocity Žkmrs.

Density ŽMgrm3 .

U

U

U

U

1.55 1.55 1.35

730 700 670

6.94 7.10 7.55 7.70 7.44

3.18 3.44 3.46 3.49 3.51

the garnet q clinopyroxene q plagioclase q quartz equilibria geobarometers ŽNewton and Perkins, 1982; Powell and Holland, 1988.. As quartz is absent from the xenoliths, the calculated pressures are maximum values. Re-calculation of Fe 3q was made on the basis of mineral stoichiometry using the MINPET program and PrT values were obtained from the PTMAFIC program ŽSoto and Soto, 1995.. Estimated errors are "258C for temperature determinations and "0.15 GPa for pressure. Results suggest that these rocks experienced low to medium grade granulite facies metamorphic conditions with temperatures between 6708C and 7308C and pressures of 1.4–1.6 GPa ŽTable 4.. However, calculating for Fe T s Fe 2q significantly increases the equilibration temperatures by up to 1008C. Pearson et al. Ž1991. also found such differences. The

143

absence of quartz in these xenoliths results in these pressures being maximum estimates. 4.5. Compressional waÕe and density estimates Direct measurements of compressional wave velocities Ž Vp . could be made because of the very small size of the Arkhangelsk xenolith samples ŽChristensen and Fountain, 1975; Kern and Schenk, 1985.. Therefore, we have calculated bulk Vp values ŽTable 4. using modal mineral proportions ŽTable 1. and individual mineral Vp values ŽFurlong and Fountain, 1986; Christensen, 1989. that correspond most closely to the mineral compositions in these samples ŽTable 5.. Temperature profiles of the Baltic Shield obtained by thermal modelling indicate present-day lower crustal temperatures of between 4708C and 5308C ŽBalling, 1995., hence PrT conditions for calculations were chosen as 5008C and 1.4 GPa. For details of the assumptions and limitations of the method, see Markwick et al. Žsubmitted.. Estimated densities, adjusted to 1.4 GPa and 5008C ŽTable 4., were made using mineral modal estimates and published values of bulk Ž K s . moduli and their pressure derivatives ŽBass, 1995. and thermal expansion coefficients ŽFei, 1995., which most closely match the compositions found in the Arkhangelsk xenoliths, and an Anderson–Gruneisen parameter of 5.5 ŽFei,

Table 5 Experimental elastic moduli and their temperature and pressure derivatives, density and thermal expansion coefficients used in this study Vp values without superscript were taken from Furlong and Fountain Ž1986. and references therein.

Garnet Diopside Plagioclase Scapolite Amphibole Rutile Opaque K-feldspar Apatite Biotite a

Vp Žkmrs.

ydVp rdT Žkm sy1 Ky1 .

dVprd P Žkm sy1 GPay1 .

r Žkgrm3 .

K s ŽGPa.

d K srd P

aŽT.

8.17 a 7.85 6.166 5.63 b 7.04 a 9.253 7.4 5.556 6.7 5.26 a

3.93 = 10y4a 6.33 = 10y4 0.16 = 10y3 0.16 = 10y3 6.33 = 10y4 0.943 = 10y3 0.943 = 10y3 0.16 = 10y3 0 6.33 = 10y4

7.84 = 10y2a 2.04 = 10y1 13.67 = 10y2 13.67 = 10y2 2.04 = 10y1 7.60 = 10y2 7.60 = 10y2 13.67 = 10y2 0 2.04 = 10y1

4.160 3.289 2.640 2.800 3.150 4.260 5.206 2.560 3.146 2.900

177.0 114.0 62.0 65.3 93.3 215.5 161.0 55.9 212.3 58.2

5.43 9.6 62.0 65.3 9.6 6.76 161.0 6.4 6.4 9.6

3.1311 = 10y5 0.3330 = 10y4 2.5697 = 10y5 2.3617 = 10y5 3.1770 = 10y5 0.2890 0.5013 = 10y4 2.2287 = 10y5 y0.0440 = 10y4 0.0994

Values obtained from Christensen Ž1989.. Value was calculated from elastic constants from Bass Ž1995.. All Vp values have been calculated from elastic constants using the Voight–Reuss–Hill averaging method ŽBass, 1995.. Density Ž r . values were obtained from Bass Ž1995. except scapolite, which was obtained from Christensen Ž1989.. Bulk modulus Ž K s ., its pressure derivative and the thermal expansion coefficients Ž aŽT. s a 0 q a 1T q a 2 Ty2 . are from Fei Ž1995.. The Anderson–Gruneisen parameter of 5.5 Žfor olivine. was used in all calculations. b

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1995.. Where mineral density data was not available values for the most similar minerals were used ŽTable 5.. Calculated bulk rock Vp values ŽTable 4. for the Arkhangelsk xenoliths range from 6.94 to 7.70 kmrs. Generally, the higher Vp values are obtained for more garnet-rich, plagioclaseq scapolite-poor rocks. Density values range from about 3.2–3.5 Mgrm3 ŽTable 4..

5. Discussion 5.1. Comparison with other lower crustal xenoliths 5.1.1. Petrology and mineralogy The dominant granoblastic fabric in this suite of xenoliths is commonly found in rocks that have equilibrated within a typically homogeneous stress field under granulite facies metamorphic conditions in the lower crust ŽGriffin et al., 1979; Kempton et al., 1995.. As with many other garnet granulite xenolith suites, both mineralogically banded and unbanded rocks are present and when mineral segregation is not apparent, lattice preferred orientation within constituent minerals has been found. The disparity in sample A1 between modal abundance of rutile ŽTable 1. and whole rock analysis for TiO 2 ŽTable 3a. suggests that metamorphic banding is more pronounced than is seen in thin section. Errors inherent in the point counting method could be significant especially for finer grained samples. The xenoliths contain garnets which are almandinerpyrope-rich solid solutions. They fall into two groups which differ markedly in their Fe, Mg and Ca concentrations. Garnets in xenoliths A4 and A5 have higher proportions of almandine and a lower pyrope content than garnets in xenoliths A1 and A3. However, the garnets mainly fall into the field for granulites ŽMottana, 1986; Fig. 2. as do most garnets in garnet granulites from Kola ŽKempton et al., 1995., Finland ŽHoltta ¨ ¨ et al., 2000. and Belarus ŽMarkwick et al., submitted.. The compositional range for garnets in garnet granulites and eclogite xenoliths from Lesotho are more varied ŽGriffin et al., 1979; Fig. 2.. Retrogressive decomposition to kelyphite rims in the Arkhangelsk xenoliths is much less extensive than in Kola and Belarus. Exsolution of acicular rutile

andror clinopyroxene are indications of the very great depths from which these rocks have been derived. Pyroxenes in these granulites also form two groups, A1 and A3 have similar compositions to pyroxenes in both Belarus ŽMarkwick et al., submitted. and Kola ŽKempton et al., 1995. xenoliths, whereas A4 and A5 have significantly higher concentrations of FeO and lower CaO content. However, pyroxenes in these xenoliths are all diopsidic similar to many other garnet granulite xenolith suites. It is rare to find large amounts of orthopyroxene in high pressure granulite xenoliths ŽToft et al., 1989; Kempton et al., 1995. and no orthopyroxene has been found in the Arkhangelsk xenoliths. Alteration of pyroxenes to small crystal aggregates of pargasitic hornblende by metasomatic fluids has been observed in some xenolith suites ŽKempton et al., 1995; Markwick et al., submitted.. However, similar metasomatic events appear not to be so widespread in the Arkhangelsk xenoliths, as only one sample contains amphibole which has been interpreted as metasomatic Žsee Appendix A.. This may of course be a consequence of the small number of samples available for analysis. Commonly, the most variable mineral composition found in mafic granulite rocks is that of plagioclase. Lower crustal xenoliths have compositional ranges from labradorite to albite and occasionally more than one plagioclase composition may be found in the same rock ŽKempton et al., 1995.. Plagioclase compositions found in the Arkhangelsk xenoliths are extremely similar to those found in Lesotho ŽGriffin et al., 1979., but more sodic than compositions from both Finland ŽHoltta ¨ ¨ et al., 2000. and Belarus ŽMarkwick et al., submitted., consistent with the higher estimated pressure for Arkhangelsk xenoliths. Accessory mineralogy is also quite limited in this suite of xenoliths and includes scapolite, apatite, ilmenite, magnetite, biotite and rutile. Rutile is a good indication of the high confining pressures at which these rocks equilibrated. Scapolite has been recorded in many suites of deep-seated xenoliths ŽLovering and White, 1964; Griffin et al., 1979; Kempton et al., 1995. and the Arkhangelsk xenolith suite is no exception ŽTable 1.. The association of mizzonite with oligoclase in granulite-facies terrane rocks was observed by von Knorring and Kennedy Ž1958. and similarly occurs in this rock. Scapolite is

A.J.W. Markwick, H. Downesr Lithos 51 (2000) 135–151

stable under lower crustrupper mantle conditions if PCO 2 is high ŽAitken, 1983. and its frequent occurrence as a primary mineral in granulite-facies lower crustal xenoliths supports the premise that such rocks could act as reservoirs for CO 2 and SO42y ŽNewton and Goldsmith, 1975; Goldsmith and Newton, 1977.. 5.1.2. Geochemistry Arkhangelsk lower crustal xenoliths are mafic and are represented only by sub-alkaline compositions, unlike xenolith suites from Belarus ŽMarkwick et al., submitted., Kola ŽKempton et al., 1995. and Lesotho ŽGriffin et al., 1979., which also contain more alkaline samples ŽFig. 3.. There are many interpretations for the origin of lower crustal xenoliths, from melt compositions, residue after partial melting, cumulates and metamorphic mineral segregation, but the possibility that changes have occurred in the concentrations of the more mobile major and trace elements in such old metamorphosed rocks makes the use of many discrimination diagrams problematic. However, Kempton and Harmon Ž1992. were able to distinguish melt-like compositions from cumulates with some success using Mga and SiO 2rAl 2 O 3 values. Using similar arguments for the Arkhangelsk, Kola and Belarus xenoliths, very little fractionation appears to have taken place, suggesting that these rocks are very close to melt compositions ŽFig. 7.. The slight shift of sample A1 towards lower

Fig. 7. Mg-number vs. SiO 2 rAl 2 O 3 for lower crustal granulite xenoliths from Arkhangelsk, Kola ŽKempton et al., 1995., Belarus ŽMarkwick et al., submitted. and Mg-basalts from the Pechenga– Varzuga Belt ŽSmolkin and Skufin, 1995..

145

SiO 2rAl 2 O 3 would be expected for this plagioclase q scapolite-rich rock. Although it is often the case that lower crustal mafic xenolith suites contain similar minerals with limited compositional ranges, trace element data tend to be far more varied Že.g., Rogers and Hawkesworth, 1982.. Certainly, the REE and trace element patterns of the Arkhangelsk rocks are difficult to interpret ŽFigs. 4 and 5.. They show a wide range of concentrations and patterns in just four samples. A1 has a very similar REE pattern to a granulite xenolith By5x from Belarus ŽMarkwick et al., submitted., which has been interpreted as either a plagioclase cumulate or a metamorphic rock that has segregated plagioclase. In contrast, A5 bears a resemblance to xenolith N43 from Kola peninsula ŽKempton et al., 1995., which has a melt-like pattern and has been interpreted as being of near melt composition. In this case, the small negative Eu anomaly may indicate plagioclase fractionation. Flat REE patterns, shown by A4 and A3, are much less common in granulite xenoliths. The trace element patterns show fewer similarities, reflecting possible variations in either crystal fractionation extents in their protoliths andror variations during high grade metamorphism. These chemical characteristics are not uncommon in flood basalts ŽCadman et al., 1995; White and McKenzie, 1995.. Isotope systematics clearly demonstrate that the Arkhangelsk xenoliths have considerably lower 87 Srr86 Sr ratios compared to both Belarus and Kola xenoliths. The very low Sr ratios suggest that early depletion of Rb occurred and this event may be related to the granulite facies metamorphism that has affected these rocks. This is supported by the exceptionally low Rb concentrations found in these rocks ŽTable 3a.. However, the possibility that such low 87 Srr86 Sr ratios is a primary igneous feature of these rocks cannot be ruled out. The generally higher Sr isotope ratios found in xenolith suites from Kola ŽKempton et al., submitted. and Belarus ŽMarkwick et al., submitted. may in part be explained by the more widespread metasomatic events that have affected these rocks. However, the Arkhangelsk xenoliths fall into the same quadrant as some xenoliths from Lesotho ŽRogers and Hawkesworth, 1982., Tanzania ŽCohen et al., 1984. and Snake River Plain ŽLeeman et al., 1985. ŽFig. 6.. The low 143 Ndr144 Nd

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ratios found in the Arkhangelsk xenoliths suggest that LREE-enrichment occurred early in the history of the xenoliths. The Sr–Nd isotope systematics for the Arkhangelsk xenoliths may be explained in a variety of ways. One possibility is that the original mantle-derived basaltic magmas assimilated pre-existing old continental crust. A similar argument has been used by Halliday et al. Ž1993. to explain the observed isotopic ratios of Scottish granulite xenoliths. If this is the case, then T DM model ages could represent an average age of mixing components and, hence, must be considered with some caution ŽArndt and Goldstein, 1987.. 5.2. Interpretation of geothermobarometric data The geothermobarometers used in this study are those preferred for mafic granulite facies rocks. They tend to give lower temperature estimates than older, less suitable methods Že.g., Ellis and Green, 1979.. The values obtained for the Arkhangelsk xenoliths suggest high equilibration pressures Žca. 1.5 GPa. at relatively low temperatures Žca. 7008C. for granulite facies rocks, similar to those obtained for Lesotho granulite xenoliths ŽGriffin et al., 1979.. This may suggest that these xenoliths have equilibrated within a crustal area of low heat flux, which is consistent with the kimberlite nature of the host rocks. However, in all calculations of in situ temperatures and pressures one must always be aware of the limitations of such methods caused by unknown Fe 3qrFe 2q ratios, which can markedly alter estimates obtained. Also, one must consider the interpretation of results carefully. Geothermobarometric calculations rely on chemical equilibria, which are controlled by diffusion rates and, hence, diffusion blocking conditions for the minerals being analysed. This is particularly important for ion-exchange equilibria used in geothermometry. It is possible that values of temperature and to a much lesser extent pressure, given the required volume changes ŽEssene, 1989., may indicate values reached on closure to diffusion of ions and not the ‘real’ conditions of paragenesis. Other unknown factors that may influence results are the presence of accessory minerals, which may not be fully integrated into equilibrium expressions, and dissolved fluids such as H 2 O, and in particular CO 2 for granulites, these may also need

to be included in expressions for mineral equilibria being used. 5.3. Significance of Vp and density Õalues As with many mafic rocks that have suffered high temperature and pressure metamorphism, these rocks have high Vp values Ž6.94–7.70 kmrs. which correspond well with wide angle Vp determinations of the deep crust ŽHolbrook et al., 1992; Christensen and Mooney, 1995; Rudnick and Fountain, 1995.. This strengthens the premise that these are indeed rocks that have equilibrated in the deepest lower crust. Fig. 8a shows how Vp varies with modal plagioclase q scapolite for Arkhangelsk and Kola ŽMarkwick, unpublished data. xenoliths. It is clear that in these rock suites compressional wave velocities are directly proportional to abundance of plagioclaseq scapolite and in this sense the possibility of layering, either magmatic or metamorphic, into plagioclaseq scapolite-rich and plagioclaseq scapolite-poor layers may influence the observed highly laminated appearance of the lower crust ŽHolbrook et al., 1992., certainly, there would be large impedance contrasts between layers. The generally higher values of Vp for Arkhangelsk xenoliths ŽFig. 8a. may be in part due to differences in modal garnet between the xenolith suites and this in turn may be reflected in the slightly higher pressures obtained for the Arkhangelsk xenoliths. Certainly, the evidence from the variety of xenolith compositions supports the possibility of a layered source region. Fig. 8b shows that a positive correlation exists between density and Vp as expected and is directly related to the mineralogy of the rocks ŽRudnick and Jackson, 1995., and that a similar general trend from mafic granulite to eclogite fields described by Mengel and Kern Ž1992. and earlier by Arculus et al. Ž1988. occurs. The high whole rock densities Ž r . and Vp values calculated for these rocks are in reasonable agreement with their mineral compositions and estimated depths of origin Ž; 50 km, using r s 3.0 Mgrm3 .. From these calculations and the estimated pressures Ž1.4–1.6 GPa. it is very likely that a significantly thick high velocity layer, composed of mafic garnet granulite, forms the lowest part of the Arkhangelsk lower crust. The calculated values are consistent with measured values from seismic refrac-

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are closer to the age of the upper sequence of the Pechenga–Varzuga Belt in the adjacent Kola peninsula ŽSharkov and Smolkin, 1997., which erupted at 2.0–1.9 Ga. Magma compositions ranging from tholeiitic to iron-rich basalts and Mg-basalts were erupted. Comparison of major and trace element data for basalts from the Pechenga–Varzuga Belt ŽSmolkin and Skufin, 1995. with the Arkhangelsk lower crustal xenoliths supports a possible petrogenetic link ŽFigs. 3, 5 and 7..

6. Concluding remarks

Fig. 8. Plot of: Ža. estimated modal plagioclaseqscapolite vs. calculated compressional wave velocities Ž Vp . for garnet granulites from Arkhangelsk, Kola ŽMarkwick, unpublished data. and Belarus ŽMarkwick et al., submitted.; and Žb. calculated whole rock densities vs. whole rock Vp values for Arkhangelsk, Kola ŽMarkwick, unpublished data. and Belarus ŽMarkwick et al., submitted. xenoliths.

tion values of between 6.8 and 7.4 kmrs for the Northern Baltic shield ŽAnsorge et al., 1992..

Petrological, geochemical and geophysical evidence obtained from the Arkhangelsk granulite xenoliths support the following conclusions. Ž1. These mafic garnet-rich granulites have PrT, calculated whole rock Vp , r , and mineral chemistries consistent with a lower crustal origin. Ž2. Geochemical evidence indicates an origin as a mafic underplate of tholeiitic flood basalt, which may be related to events that produced the mid-Proterozoic Pechenga–Varzuga belt volcanism. Ž3. The chemical and physical similarities between the Arkhangelsk lower crustal xenoliths and other xenoliths which have been sampled from beneath ArchaeanrProterozoic crust, in particular the Kola ŽKempton et al., 1995. and Belarus ŽMarkwick et al., submitted. xenolith suites, suggest that these rocks may have had similar origins and subsequent metamorphic histories. Differences in geochemistry and the much younger TDM model ages of the Arkhangelsk xenoliths compared to the Kola xenoliths ŽKempton et al., submitted. suggest an unrelated formation at a very different time.

5.4. Possible origin of the xenoliths

Acknowledgements

The T DM model ages Ž1.7–1.9 Ga. of the Arkhangelsk xenoliths suggest that their formation post-dates that of the Kola lower crust. The Kola lower crustal xenoliths are considered to have formed as part of the 2.4 Ga Large Igneous Province ŽSharkov et al., 1997; Kempton et al., submitted.. In contrast, the model ages of the Arkhangelsk samples

We are grateful to Ivan Mahotkin for supplying the xenolith samples and Andy Beard for help with microprobe analysis. Kym Jarvis is thanked for trace element analysis which was carried out at the NERC ICPMS facility. XRF and radiogenic isotope facilities are supported by ULIRS. We are grateful to Matthew Thirlwall for assistance in the isotope labo-

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ratory. Discussions with David Price about geophysical calculations were extremely helpful. Thanks are due to P. Holtta, ¨ ¨ K. Mengel and H.G. Stosch for their constructive and very helpful reviews. This paper is a result of UK–Russian collaboration in the EUROPROBE SVEKALAPKO PROJECT.

Appendix A A1. A fine grained scapolite-rich garnet granulite showing weak banding due to mineral segregation. A granoblastic polygonal equilibrium fabric exists. Plagioclase and scapolite form well-defined bands with a strong lattice preferred orientation, while garnet forms poorly defined bands and discrete sub-equant mineral aggregates, possibly pseudomorphing an earlier phase Žorthopyroxene?.. Small Ž0.2–0.5 mm. pink garnets are highly fractured with thin kelyphitic rims. Cracks are altered to a chlorite-type mineral assemblage. Inclusions of small rounded scapolite are common in garnets. Scapolite forms generally larger Ž0.2–1.0 mm. equant crystals having well-developed triple point junctions and are often in intimate contact with plagioclase. Inclusions of rounded small apatites are often found in scapolite crystals. Plagioclase forms similar sized untwinned crystals with rare evidence of micro-antiperthite. Inclusions of small rounded apatite are common whereas small zircons are very rarely found in plagioclase. Rutile occurs as extremely small exsolution needles in garnets and also more commonly as larger irregularshaped crystals often near to garnet-rich areas in the rock. Quartz Ž- 0.5%. is abundant only in one area of the sample where it forms equant grains showing strained extinction. Small rounded crystals of ilmenite exist in both garnet-rich and scapolite-rich bands. Very rare zircon occurs in an ilmenite-rich vein in one area of this rock. Vestigial biotiter phlogopite Žtrace. clearly shows late-stage kink banding. However, this mineral is now totally altered to a smectiterserpentine mineral assemblage. The absence of secondary amphibole suggests that much of the alteration is sub-greenschist facies. A2. A medium to coarse grained rock containing predominantly garnet and amphibole. Garnets are large Ž1–2 mm in size. pink equant crystals with rare exsolution of acicular rutile and numerous pressure

release cracks. Kelyphitic rims are very thin and composed of a chloriteq clay q white mica assemblage. Amphiboles are pargasites and reach 4 mm in size, often with altered rims of chloriteq white mica. Extremely rare relic diopside was found almost completely pseudomorphed by amphibole. Apatite is found in crystal aggregates often with microscopic pyrite. A mineral assemblage containing white mica pseudomorphs a rhomb-shaped primary phase, possibly plagioclase. Ilmenite occurs as very small anhedral crystals within amphiboles. A3. An unaltered medium grained garnet granulite exhibiting a polygonal granoblastic texture. Relatively large Ž2–4 mm. pink near equant garnets contain rare exsolution of acicular rutile. Pressure release cracks are numerous and altered to chlorite. Kelyphitic rims are very thin and occasionally absent. Garnets have inclusions of small rounded apatite and rarer diopside. Very pale green diopsides form equant crystals Ž1 mm. many showing rim alteration to chlorite. Pressure release cracks are rare. Rarely diopside crystals contain small rounded inclusions of rutile. Inequant plagioclase crystals reach 1 mm in size and show deformation twinning and undulose extinction. Rare K-feldspar was found as patchy exsolution lamellae within plagioclase. Minor phases include rounded and prismatic rutiles up to 0.5 mm in size and trace amounts of small rounded fluor-apatite. A4. A fine to medium grained garnet granulite showing moderate alteration. A well-developed polygonal granoblastic fabric exists and a rudimentary mineral banding can be seen, which consists of plagioclase and diopside in the largest bands and garnet in smaller poorer defined bands. A weak lattice preferred orientation is evident in the plagioclase bands. Pink garnets form near equant crystals Ž1–2 mm. with very thin kelyphitic rims. Chlorite also occurs as an alteration product within pressure release cracks. Several garnets have inclusions of zoned ilmenite, with MnO-rich cores Žup to 13% MnO and 50% TiO 2 . and TiO 2-rich rims Žup to 2% MnO and 55% TiO 2 .. Also small rounded inclusions of diopside are abundant. Exsolution of acicular rutile are rarely observed. Green equant diopside crystals occur as highly fractured yet relatively fresh crystals and reach 1 mm in size. Plagioclase forms 0.2- to 1-mm sized inequant crystals that commonly

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show deformation twinning and undulose extinction. Micro-antiperthite exists in all plagioclase crystals as patchy blebs. Accessory rutile and ilmenite occur throughout the sample as anhedral crystals and reach 2 mm in size. A5. This is a fine grained Žcrystals range in size between 2 and 4 mm. garnet granulite exhibiting a near equant polygonal granoblastic fabric. Pink garnets Ž1 mm. are fractured due to pressure release and rims are altered to very thin kelyphite. Chlorite and white mica are common as alteration phases along crack boundaries. Garnets contain inclusions of small rounded plagioclase, larger ilmenite and occasionally rounded diopside. Diopside crystals range in size from 0.5 to 1 mm in size and occur as highly fractured crystals with crack boundaries commonly altered to chlorite. Many contain inclusions of small rounded ilmenite and larger, rarer apatite. Plagioclase Ž; 1 mm. demonstrates deformation twinning andror undulose extinction, evidence of a late stage stress field. Rare micro-antiperthites occur as patches. Several plagioclase crystals have prismatic and rounded apatite crystals. K-feldspar is rarely found as anhedral crystals often in contact with plagioclase or as alteration of plagioclase along microfractures. Relatively large Žup to 0.3 mm. ilmenite crystals exist throughout the rock, occasionally producing complex exsolution intergrowths with haematite.

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