The petrology and geochemistry of granitic gneisses from the East Arunta inlier, central Australia: implications for Proterozoic crustal development

Precambrian Research, 40/41 (1988) 233-259

233

Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THE PETROLOGY AND GEOCHEMISTRY OF GRANITIC GNEISSES FROM THE EAST ARUNTA INLIER, CENTRAL AUSTRALIA: IMPLICATIONS FOR PROTEROZOIC CRUSTAL DEVELOPMENT J.D. FODEN, I.S. BUICK* and G.E. MORTIMER Department of Geology and Geophysics, The University of Adelaide, Box 498, G.P.O., Adelaide, S.A. (Australia) (Received January 6, 1987; revision accepted December 24, 1987)

Abstract Foden, J.D., Buick, I.S. and Mortimer, G.E., 1988. The petrology and geochemistry of granitic gneisses from the East Arunta Inlier, central Australia: implications for Proterozoic crustal development. Precambrian Res., 40/41: 233259.

The Entia Gneiss Complex is an Early to Middle Proterozoic inlier composed of upper amphibolite facies orthogneisses and paragneisses in the eastern end of the Arunta Block in central Australia. It is separated from overlying, structurally distinct, compositionally different and probably younger, gneisses of the Harts Range Complex, by a subhorizontal detachment zone. This paper discusses the petrology of four major granitic gneiss bodies. One of these (the Bruna Gneiss) has intruded this detachment zone, the other three are components of the structurally underlying Entia Gneiss Complex. The Entia Gneiss Complex has a distinctive subhorizontaUy layered character with recumbent folds and flat-lying schistosity. The granitic gneisses that are components of this complex appear to have intruded an actively deforming terrain and are folded sheet-like bodies. The orthogneisses of the Entia Complex range from gabbro to true granite in composition and the granites are Itype on both geochemical and petrographic criteria. They are significantly different from the relatively alkali-rich granites of equivalent age which occur in some of the other northern Australian Proterozoic Blocks and are most like the granites of the modern cordilleran belts, particularly in the association of diorite and gabbro with granodiorite. It is suggested they are formed in narrow collisional belts formed as a result of closure of intracratonic rift basins. The high surface-area to volume ratio of these granites, which is a consequence of the deformational environment into which they were intruded, has resulted in considerable wall rock exchange. In the more differentiated end members of the suites this has led to contamination by a net gain of K20, Rb and Ba and loss of relatively compatible elements including the REE, Y, Nb and TiO2. Compared with the syn-tectonic Entia granites, the Bruna Granite Gneiss is post-tectonic and shows geochemical affinities with A-type or anorogenic Palaeozoic granites. Most of this granite is confined to the detachment zone, but the very latest phases also intrude the Harts Range Cover sequence. An important conclusion of this study is that detachment zones of the type described from this area of the eastern Arunta Block, exercise strong control on plutonism into the Early to Middle Proterozoic crust. As long as the very mobile style of Early Proterozoic tectonics continues, the intracrustal dislocation zone confines syn-tectonic granitic magrnatism to deeper levels.

*Present address: Department of Earth Sciences, Cambridge University, Cambridge.

234

Introduction Were the mechanisms responsible for crustal growth and development the same in the Proterozoic as those which have operated through the Phanerozoic? Uniformitarian models emphasize the role of ocean-continent collision, subduction and continental margin Alpine-style orogenesis. Non-uniformitarian models for the Proterozoic include Kroner's (1983) A-subduction, Wynne-Edwards' (1976) 'millipede' model and the extensional tectonic concepts of Wernicke and Burchfiel (1982), Etheridge et al. (1987) and Sandiford and Powell (1986). These imply ensialic orogeny, both intracrustal and lithosphere-asthenosphere delamination, and the generation of small and short-lived oceanic basins. In Australia, there is a growing body of evidence from nearly all of the Proterozoic domains that extensional tectonism is a very important mechanism in crustal evolution (e.g., Etheridge et al., 1987). In fact its continued importance to the present day has also been recognized! In the Australian Proterozoic there is also clear evidence (e.g., James and Ding, 1988) that extensional, basin formational periods were terminated by phases of compression. This recognition stems from structural 'sense of motion' studies as well as from observed rates of crustal thickening. Wyborn (1987, 1988) and Wyborn et al. (1987) have recognized that the distinctive style of Proterozoic tectonism in Australia, has resulted in the widespread occurrence of felsic magmas (granites and acid volcanics) of unusual geochemistry. These are anorogenic, rich in K20, Rb, Nb, Y and F, and Sr-poor. Volumetrically important as these 'anorogenic' granites are, this paper draws attention to another type of granite suite which, though of minor extent, may be important to our understanding of Proterozoic orogenesis.

The granites described in this paper come from the eastern Arunta Inlier (e.g., Plumb, 1979) and are exposed as gneisses in the Entia Dome, which is a window through a major detachment zone (James and Ding, 1988). In contrast to the 'anorogenic' granites these were emplaced in the upper surface of the lower plate in the detachment zone model (e.g., Etheridge et al., 1987; Fig. 1 ). They also differ in having been emplaced at the start of the phase of compressional tectonism that terminated extensional rift-basin formation. This tectonic setting is therefore more like a modern Cordilleran one and this is reflected in the geochemistry of the granites. This paper discusses the nature and origin of this syn-tectonic suite of Proterozoic granites and considers their tectonic implications.

Regional geology The Entia Domal Structure (EDS) was first described by Joklik (1955) who also mentioned two of the granite gneiss bodies discussed in this paper (the Huckitta and Inkamulla granite gneisses). More recent studies (Buick, 1983, 1985; Stewart, 1985; Sullivan, 1985) have contributed to the present understanding of the geology of this basement inlier. The EDS is at the eastern end of the Harts Ranges (see maps in Ding and James (1985) or James and Ding (1988)) in the eastern Arunta Inlier. Here rocks are regionally of amphibolite or granulite facies and have yielded maximum Rb-Sr and U - P b zircon ages of ~ 1800 Ma (Black et al., 1983; Cooper et al., 1988) and model N d - S m ages up to ~ 2000 Ma (Windrim and McCulloch, 1986). Investigation of the regional geology of the Harts Range area has been carried out by the Bureau of Mineral Resources ( Shaw and Stewart, 1975; Shaw et al., 1979, 1982; Stewart et al., 1984). The geology of the eastern Arunta Block is

235 TABLE I Representative analyses of the Huckitta Tonalite Gneiss (HTG) of the Entia Domal Structure, Arunta Inlier, central Australia a,b Sample H25 Si02 Ti02 A1203 Fe203d MnO MgO CaO Na20 c K20 P20~ Losse Total Sr Rb Y Zr Nb Ba Sc Ni V Cr Ce Nd Mg number Rb/Sr

825/10 808/71

H37

H27

H21

808/15

825/14 825/15 808/69

52.75 0.30 10.03 11.59 0.25 11.63 10.20 1.06 1.68 0.04 0.87

54.28 0.33 14.34 8.05 0.17 7.72 10.09 2.21 0.79 0.05 0.61

56.10 0.57 12.75 7.21 0.14 8.12 10.32 2.07 1.80 0.22 0.53

57.62 0.38 14.78 7.40 0.15 6.14 7.83 2.99 2.01 0.03 0.57

59.04 61.20 0.49 0.51 16.66 16.64 5.79 5.52 0.09 0.09 4.90 4.40 7.18 5.21 3.35 3.35 1.80 2.39 0.13 0.17 0.54 1.71

6 4 . 9 5 66.14 7 2 . 4 4 0.36 0.26 0.15 1 7 . 1 4 16.25 1 5 . 0 0 3.97 3.25 1.60 0.08 0.07 0.06 1.65 1.97 0.74 4.61 4.13 2.56 3.94 3.94 3.93 2.95 2.43 2.82 0.14 0.10 0.03 0.38 0.43 0.36

73.18 0.08 13.09 1.47 0.04 1.05 2.03 1.78 6.42 0.05 0.28

100.40

98.64

99.83

99.90

99.97 101.19 100.17 98.97 9 9 . 6 9

99.47

79 16.8 26.4 25.7 4.4 622 29.6 345 141 1119 50 25

323 12.3 15.3 29.7 0.1 266 46.4 101 165 442 33 19

425 40.2 19.8 50.0 4.2 1183 34.0 81.0 190 352 106 20

415 32.1 24.3 46.8 5.2 1380 29.3 71.0 111 309 75 36

530 44.5 13.1 89.0 4.0 1064 19.4 68.0 115 160 57 19

438 76.0 14.9 126 4.6 899 16.9 60 103 142 65 27

519 95.0 9.3 131 6.3 1098 7.6 5.5 55 9.0 69 23

480 90.0 9.5 114 6.0 724 8.6 17 43 35 68 26

369 73.0 2.1 77.0 3.8 921 3.2 4.0 16 -53 14

506 128 8.5 34.1 1.1 3960 6.0 14 20 30 23 7.0

73.5 67.9 71.3 69.6 70.1 68.8 47.8 57.2 50.4 61.2 0.213 0 . 0 3 8 0 . 0 9 5 0 . 0 7 7 0 . 0 8 4 0 . 1 7 4 0 . 1 8 3 0.188 0 . 1 9 8 0.253

"Analyses in Tables I-V were performed by XRF in the Department of Geology, University of Adelaide. bAll major elements are expressed in wt.% and trace elements in ppm. CNa20 values were determined by atomic absorption. dTotal Fe is expressed in Fe203. eLoss was determined by ignition at 1000°C. d o m i n a t e d b y a s e q u e n c e of a m p h i b o l i t e grade, m e t a p e l i t i c a n d m a f i c m e t a - i g n e o u s r o c k s (Sivell a n d F o d e n , 1985) w h i c h c o m p r i s e m o s t o f t h e H a r t s R a n g e G r o u p of S h a w et al. (1982). T h e base of this s e q u e n c e is d e f i n e d b y a spect a c u l a r series o f t h r u s t s a n d m y l o n i t e z o n e s ( D i n g a n d J a m e s , 1985; J a m e s a n d Ding, 1988) t h r o u g h w h i c h D i v i s i o n 1 ( S h a w et al., 1982), S t r a n g w a y s O r o g e n i c Belt ( J a m e s a n d Ding, 1988) ' b a s e m e n t ' inliers are exposed. T h e s e in-

clude t h e O o n a g a l a b i Inlier and, a c c o r d i n g to D i n g a n d J a m e s (1985), t h e E n t i a Inlier. W h e r e a s t h e O o n a g a l a b i Inlier is like t h e S t r a n g w a y s M e t a m o r p h i c C o m p l e x f u r t h e r to t h e west a n d is o f g r a n u l i t e facies ( W a r r e n , 1983 ), t h e E n t i a D o m e is of u p p e r a m p h i b o l i t e facies. T h e E n t i a Gneiss C o m p l e x is s e p a r a t e d f r o m t h e H a r t s R a n g e G r o u p (the ' c o v e r ' s e q u e n c e ) b y a major, low-angle, m y l o n i t e zone c o m p o s e d

236 TABLE II Representative analyses of the Huckitta Granodiorite Gneiss (HGG) of the Entia Domal Structure, Arunta Inlier, central Australia (notes to Table I apply) Samples H44

808/028 H48

Si02 TiO~ A1203 Fe203 MnO MgO CaO Na20 K20 P205 Loss

61.17 0.47 17.98 5.50 0.12 2.38 5.26 4.06 2.69 0.17 0.53

64.50 0.39 17.40 3.98 0.08 1.85 4.78 4.00 2.52 0.16 0.44

Total

100.33

100.10

Sr Rb Y Zr Nb Ba Sc Ni V Cr Ce Nd

518 87 18.6 192 7.7 990 14.7 14 75 7 95 33

Mg number 54.5 Rb/Sr 0.168

569 73 10.8 136 5.8 1109 9.0 3 56 11 81.4 26 56.2 0.128

65.21 0.35 17.35 3.88 0.07 1.62 4.30 3.95 3.02 0.13 0.31 100.19 529 98 10.4 143 6.3 1022 9.1 10 60 5 61 21 53.6 0.185

H33

H32

808/012 808/117 808/006 H47

67.65 68.83 0.21 0.22 16.88 16.82 3.06 2.56 0.08 0.04 1.13 0.86 4.28 3.79 4.12 4.49 1.91 1.93 0.10 0.09 0.31 0.37 99.73 532 44.4 10.9 104 3.8 1030 13.3 7 32 -43 13

100.00 544 51 3.9 92 3.2 987 3.5 5 25 -29 7

71.97 0.14 14.86 1.59 0.03 0.84 2.27 3.09 4.74 0.09 0.36

72.52 0.10 15.51 0.97 0.02 0.34 2.81 3.20 4.46 0.05 0.23

100.07

99.98

100.21

481 110 5.0 61 3.3 2060 6.5 6 18 5 13.6 0.9

50.5 48.2 0.083 0.094

partly of sheared Entia Gneiss lithologies which, at its upper surface has been intruded by an also partly mylonitic granite gneiss (the Bruna Granite Gneiss (BGG)). The Bruna Granite Gneiss is dated at 1745 Ma (Cooper et al., 1988) and its intrusion post-dates the recumbent folding of the EGC granite gneisses. This mylonite zone is a major detachment surface, referred to by James and Ding (1988) as the Harts Range Detachment Zone. It has been the site of both reverse and normal motions.

70.73 0.09 15.48 1.98 0.06 0.90 3.27 3.42 4.03 0.07 0.04

538 132 3.4 86 4.7 940 2.2 12 19 . . 48.6 4

55.7 59.4 0.229 0.245

554 106 1.5 78 1.6 2294 1.2 5 12 . 28.1 2.6 49.2 0.191

808/119

73.95 0.09 13.99 1.19 0.01 0.15 1.55 2.58 5.64 0.04 0.23

75.98 0.01 13.56 0.18 0.01 0.08 1.01 2.48 6.23 0.01 0.20

99.41

99.75

457 130 2.1 92 0.9 2255 0.3 1 17 . 67 13 25.8 0.284

580 155 0.1 117 0.4 1886 5.5 1 6.5 0.6 55.1 0.267

The geology of the Entia Domal Structure (EDS) The EDS is a major gneiss dome ~ 35 km in diameter. The assemblage of rocks exposed in this dome will be referred to collectively as the Entia Gneiss Complex (EGC). This is composed of two fundamental suites; a supracrustal association of orthogneisses and paragneisses and an intrusive association of amphibolites and felsic orthogneisses. The supracrustal suite is made up of both ortho- and para-amphibolite,

237 TABLE III Representative analyses of the Inkamulla Granite Gneiss (IGG) of the Entia Domal Structure, Arunta Inlier, central Australia (notes to Table I apply) Samples 825/03

825/52

SiO2 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P20~ Loss

68.63 0.26 15.86 2.86 0.05 1.50 3.59 4.68 1.39 0.10 0.31

71.74 0.21 14.51 2.04 0.06 0.72 2.39 3.51 3.93 0.06 0.34

Total

99.23

99.51

Sr Rb Y Zr Nb Ba Sc Ni V Cr Ce Nd Mg number Rb/Sr

311 63 11.2 94 10.0 161 6.2 8 38 5 118 44 53.6 0.203

442 86 11.0 92 7.8 -4.5 6 17 6 40 4 43.7 0.195

825/02

825/56

825/81

825/36

825/38

72.00 0.23 14.73 2.31 0.04 1.65 0.85 3.32 3.52 0.08 0.96

74.96 0.02 13.78 0.34 0.17 0.06 0.57 3.61 6.17 0.01 0.09

75.33 0.03 13.42 0.49 0.01 0.09 1.14 3.15 5.62 0.01 0.16

76.10 0.08 12.90 1.06 0.01 0.12 1.07 3.38 4.89 0.02 0.18

76.72 0.04 13.13 0.49 0.05 0.01 0.66 3.58 5.36 0.01 0.11

99.69

99.78

99.44

99.81

100.15

154 70 6.6 91 4.0 1382 3.7 6 20 7 26 6 61.1 0.455

calc-silicate, s o m e r a r e c a r b o n a t e rocks, t h i n metapelite-quartzite layers and grey biotite gneiss w h i c h c o u l d e i t h e r h a v e b e e n o f sedim e n t a r y or v o l c a n i c origin. T h e s e s u p r a c r u s t a l r o c k s are t h o u g h t to be o f rift-fill origin. T h e series o f s u p r a c r u s t a l r o c k s h a s b e e n int r u d e d b y v e r y v o l u m i n o u s m a g m a s o f largely i n t e r m e d i a t e to felsic c o m p o s i t i o n , b u t w h i c h also include g a b b r o a n d u l t r a m a f i c c u m u l a t e r o c k s ( T a b l e s I - V ) . T h e s e i n t r u s i o n s are c o n formably interlayered with the supracrustal gneisses, t a k i n g sill-like f o r m s . I t is a f e a t u r e o f the EGC that conformable igneous intrusion h a s t a k e n p l a c e o n all scales, w i t h o r t h o g n e i s s i c l a y e r s r a n g i n g in t h i c k n e s s f r o m m i l l i m e t r e s to

31.9 141 24.7 18.5 10.4 149 3.9 1 2 -22 5 27.9 4.42

89.0 99 19.8 54 14.6 195 0.9 -3 -8 3 28.7 1.11

126 98 3.8 68 2.2 467 1.3 2 8 -39 10 20.0 0.778

34.2 125 22.5 59 27.8 119 3.1 4 3 6 29 12 -3.65

t e n s o f m e t r e s , a c c o u n t i n g for a t l e a s t 60% of the total EGC. T h i s p a p e r is m a i n l y c o n c e r n e d w i t h t h e pet r o l o g y o f a few o f t h e larger a n d h e n c e m a p p able o r t h o g n e i s s bodies w i t h i n t h e E G C . T h e s e include t h e H u c k i t t a G r a n o d i o r i t e G n e i s s ( H G G ) (Buick, 1983 ) (Fig. 1 ), t h e p a r t i a l l y interleaved Huckitta Tonalite Gneiss (HTG) and t h e distinctive I n k a m u l l a G r a n i t e G n e i s s ( I G G ) (Fig. 2).

Structure and metamorphism T h e d e f o r m a t i o n a l h i s t o r y of t h e E G C c a n be d e s c r i b e d in t h e c o n t e x t o f t h r e e periods: (1)

238 TABLE IV Representative analyses of the Bruna Granite Gneiss (BGG) from the margin of the Entia Domal Structure, Arunta Inlier, central Australia (notes to Table I apply) Samples H064

83/104

808/100 H067

Si02 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P205 Loss

63.39 0.93 15.27 7.63 0.15 1.79 3.44 2.66 4.35 0.28 0.40

63.95 0.96 15.01 7.56 0.14 1.81 3.27 2.68 4.08 0.28 0.42

Total

99.89

9 9 . 7 4 100.25

Sr Rb Y Zr Nb Ba Sc Ni V Cr Ce Nd Ga Mg number Rb/Sr

218 156 40 365 16 1351 19 13 88 20 111 40 21

197 139 41 354 16 1226 17 12 101 20 131 55 21

64.15 0.90 15.18 6.92 0.13 1.69 3.22 2.71 4.60 0.28 0.47

196 158 38 353 15 1298 16 11.5 79 20 ----

64.48 0.72 15.99 5.15 0.09 0.84 2.68 3.02 6.30 0.20 0.31

82/65

H068 H065

66.35 0.65 15.40 4.99 0.09 1.10 2.47 2.84 5.98 0.19 0.24

69.10 0.59 13.49 4.50 0.06 0.82 2.37 3.01 4.75 0.18 0.28

9 9 . 4 7 100.06 98.87 175 172 38 590 18 2380 16 5 33 <5 106 41 19

164 171 39 463 15 1926 15 7 40 10 113 47 19

84 323 124 436 34 761 14 6 35 8 268 131 23

31.7 32.2 32.6 24.4 30.4 26.5 0.716 0 . 7 0 6 0 . 8 0 6 0.983 1.04 3.85

p r e - g r a n i t i c i n t r u s i o n , (2) p r e - j u x t a p o s i t i o n o f b a s e m e n t a n d cover, a n d (3) e v e n t s a f f e c t i n g t h e c o v e r a n d b a s e m e n t t o g e t h e r . T h e last o f t h e s e g r o u p s o f e v e n t s p r o d u c e d several genera t i o n s of u p r i g h t , t i g h t to v e r y o p e n folds. T h e d o m a l s t r u c t u r e o f t h e E n t i a I n l i e r is due to int e r f e r e n c e b e t w e e n t h e s e late, b r o a d N E - S W a n d N W - S E t r e n d i n g folds (Fig. 2). T h e p r e - g r a n i t e s t r u c t u r e s are difficult to define due to t h e wholesale i n v a s i o n o f this p a r t o f t h e c r u s t b y t h e g r a n i t e sheets. H o w e v e r , in s o m e rare i n s t a n c e s c o u n t r y rock x e n o l i t h s do s h o w fold a n d fabric d e v e l o p m e n t w h i c h are dis-

H062 82/67 H063

70.21 0.45 13.28 4.43 0.05 0.28 1.79 2.45 6.54 0.12 0.31

76.94 0.07 12.16 1.05 0.01 0.01 0.79 2.76 6.04 0.01 0.20

76.95 0.12 12.18 1.16 0.02 0.13 0.89 2.45 5.56 0.01 0.22

77.96 0.12 11.34 1.40 0.02 0.14 0.64 2.56 5.53 0.02 0.18

99.60 99.82 99.47 99.73 79 272 76 512 31 1343 8 5 10 <5 265 114 19

11 325 84 102 49 33 3 3 1 <5 95 39 18

25 299 113 143 27 115 3 2 2 <5 146 63 17

20 331 74 150 15 114 3 3 5 <5 181 80 16

11.1 1.9 1 8 . 2 16.5 3.44 29.5 1 2 . 0 16.6

o r i e n t a t e d with respect to t h a t o f t h e fabric o f t h e granites. T h e lit-par-lit style o f i n t r u s i o n of t h e g r a n i t e suggests t h a t t h e r e was a pervasive, s u b h o r i z o n t a l fabric a l r e a d y d e v e l o p e d p r i o r to t h e g r a n i t e i n t r u s i o n . T h i s m a y t h e n have also d e v e l o p e d quickly in t h e g r a n i t e s as t h e y intruded. J a m e s a n d D i n g (1988) suggest t h a t t h e onset o f r e c u m b e n t style folding o f t h e b a s e m e n t reflects t h e t r a n s i t i o n f r o m e x t e n s i o n a l to c o m p r e s s i o n a l t e c t o n i s m . T h e y also m a k e t h e observation t h a t t h e massive influx of granite t h a t t o o k place at this stage did so in r e s p o n s e to t h e

239 TABLE V Representative analyses of amphibolites from the Entia Domal Structure, Arunta Inlier, central Australia (notes to Table I apply) Samples 825/29

H56

H14

825/43 857/04 H9

Si02 Ti02 A1203 Fe203 MnO MgO CaO Na20 K20 P205 Loss

52.68 0.26 4.95 9.51 0.21 18.60 11.68 0.72 0.09 0.07 0.71

52.47 49.84 49.33 50.56 50.57 0.80 0.72 0.63 0.55 0.71 11.86 7.00 9.57 9.32 13.73 7.68 1 3 . 3 2 1 0 . 0 3 10.56 10.57 0.19 0.28 0.28 0.24 0.27 11.14 1 4 . 8 7 1 1 . 2 3 12.32 8.85 11.19 12.31 15.07 14.40 11.83 2.16 0.72 1.42 1.28 2.00 1.26 0.62 0.76 0.50 1.02 0.54 0.11 0.12 0.12 0.16 0.64 0.88 0.47 0.68

Total

99.48

99.93

Sr Rb Y Zr Nb Ba Sc Ni V Cr Ce Nd Mg number R b /S r

66.0 0.4 7.8 19.1 1.1 36 46 351 134 2145 15 8

318 16.1 22.2 294 19.9 677 45 191 173 922 90 35

81.2 80.0 0.006 0.051

100.67 33.9 2.8 27.5 77 3.3 36 72 81 223 187 39 28 75.5 0.083

reorientation of the strain ellipsoid at the stage when the deformational flow rate was minimal. The gneissic granites all possess the same relatively flat-lying schistosity as the host gneisses. In only rare cases can this be associated with large-scale fold closures, though it does often appear to be axial planar to small, hinge-thickened, intra-folial folds within the granitic gneisses. At least two further superposed phases of recumbent style deformation refold this fabric. The Inkamulla Granite Gneiss outcrop is at least partly the preserved hinge of one of these folds (Fig. 2), and the main fold involving the Huckitta Granodiorite Gneiss

98.91 232 12.2 17.9 52 10.8 98 86 54 273 105 41 24 71.1 0.053

99.85 209 11.5 22.6 51 3.4 173 84 61 226 89 47 33 76.3 0.055

H8

H57

50.47 57.40 0.57 0.81 15.92 9.75 12.13 6.88 0.22 0.24 5.41 9.94 1 1 . 1 6 11.79 2.05 2.11 1.16 0.79 0.12 0.46 0.89 0.42

100.39 100.10 100.59 443 9.8 16.5 52 5.3 332 60 41 270 53 49 22 69.8 0.022

667 10.5 12.9 88 4.4 246 25 12 333 -54 26

259 13.1 43.4 121 14.7 260 48 147 161 726 58 35

55.2 80.0 0.016 0.051

(Fig. 1 ) is another example. These recumbent fold phases pre-date the last movements on the Harts Range Detachment Zone (James and Ding, 1988). Even though other areas of the Strangways Orogenic Belt further to the west are of granulite facies (e.g., Warren, 1983; Windrim and McCulloch, 1986), the EDS appears to have reached only upper amphibolite grades. Mafic rocks are composed of hornblende, zoisite, diopsidic clinopyroxene, andesine and sphene. Pelitic rocks show signs of partial melting (migmatite development ) and are composed of quartz, K-feldspar, biotite, garnet, oligoclase,

240 I

I

I

GEOLOGY OF THE H U C K I T T A EAST A R U N T A BLOCK NORTHERN

AREA

I ~3°~

I

TERRITORY

¢,

LOCALITY MAP EAST ARUNTA eLOCK f

~Y

ENTIA

~I =11

~,RUNTAINLIER

,

/

N i <

/××\ ~,

×

'

xlp x x x

! //×

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× × : x × × ~

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×

. x

×

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x x x × × × ~×

Geological boundaries

~ : _ /

,

x

×

inferred

Faulting ~ observe_ ~-inferred Folding

x ....

observed

~,~" f~

overturned antiform antiform synform fold axial trace

Bedding

dip and strike of $1 schistosity in granite ~r~, Sl schistosity in host gneiss Lineation undifferentiated ~,~-~ with dip and strike of schistosity ~,; vergence of ~;,,

,'~, ~

, 'x 7'

~.

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' '\

:

: x

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-

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>

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x x

×× x

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.,,:,,:,

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minor

folds

~

TONALITE GNEISS

~

E"'.
[~]

ENTIA GNEISS

~

AMPHIBOLITE

::

. . . .

\

~

PEAK 'k

",

- ~ x ~ xxi

I

×, .~ >( × x

?

,'x '

×

f

;f

' ~ x x

,:

I

x x× x 'kxXX X × x

x

x x x ×

I

I

Fig. 1. A geological map of the Huckitta area of the Entia Domal Structure (EDS). See fig. 1 in James and Ding (1988) for a regional map of the Harts Range area and for the distribution of the Bruna Granite Gneiss.

241

-

.~> I

~o d_ ° =

~

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° ~o~

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.*

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%

,

++++(

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"++++,t< , t ~, /+++ I + + + \ ><+/ + + ~- + + + +~ ~,~ -" + + +t ~..'~.' + "i ! ~+ ..... • ,~'~ + + +) ¢ - + / , S + + + +

+/

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242 10 5 0 10

INKAMULLA GRANITE GNEISS I

I

I

I

I

]

60 7 ~ 1

HUCKITTA GRANODIORITE GNEISS

m 5

"5 0

I

HUCKITTA TONALITE GNEISS o>- 5 zuJ Ol 0

I

I

I

I

I

I

I

I

I

5O ENTIA AMPHIBOLITES

u. 15 10 5 0 20 15 10 5 0

I

I

50

I

i

I

ALL SAMPLES TOTAL - 131

44

50

SiO 2 wt.%

I

60

70

r

J

60

,

I

1

70

I

I

r__i__ ], I i I

I

J

u

I 6O

60

70

76

Fig. 3. Si02-frequency (where frequency means number of samples) histograms of those major orthogneiss units of the Entia Gneiss Complex discussed in this paper.

20

20 [] []o gneissHUCkitaHucki tonalite ta

15

[]

15

granodiorite gneiss

[]

O 10

% ~

50

several HGG

O

c cj

points

60 70 SiO2 wt.%

80

o

50

60 70 SiO 2 wt.%

i8I

E~3

[]

2

rn rT~ / ~. ~-'~bi&o

80

Entia [ : arnphibolite

~o

6

0

10

D

4

oo

o

[]

o

~ ~ J ~ 2

4 Na20 wt.%

2

4 Na20 wt.%

I 6

Fig. 4. Selected variation diagrams for the Huckitta Tonalite Gneiss ( [] ), the Huckitta Granodiorite Gneiss ( © ) and the Entia amphibolites ( • ) . These symbols represent the same sample groups in subsequent diagrams. In the MgO-SiO2 diagram, several samples of the HGG suite in the area of the small envelope have been omitted for clarity.

243 40

The granitic rocks: geochemistry and petrology

A Inkamulla granite gneiss o Huckitta granodiorite gneiss

3O

A E ~_ 20 >-

A

A A

o ° o

o

oA

°~O°~oo

o

~

o

oo

10

A

A

/vx

A A~ A

o % o%o°o z ~

66

71 SiO 2 wt.%

76

81

40

3O E A

20

AA A

A

10

oo

o

o

o

o8ooooo_% ~~

o

A

I

66

~ --

71

76

81

SiO 2 wt.%

Fig. 5. Y - S i 0 2 a n d N b - S i 0 2 variation of t h e H u c k i t t a Granodiorite Gneiss ( © ) a n d Inkamulla Granite Gneiss ( I G G ) suite ( A ) .

kyanite, cordierite, gedrite and sometimes muscovite. In some metapelitic rocks within the EDS, kyanite post-dates the biotite-defined fabric and occurs with gedrite which together are replaced by cordierite. These are very similar to assemblages described by Warren (1983) and attributed by her to Palaeozoic isobaric cooling and uplift. The Harts Range Group Cover sequence has had a distinctly different structural and metamorphic history to the EGC. The complex has also undergone upper amphibolite facies metamorphism, but unlike the EGC, the aluminosilicate involved is sillimanite, not kyanite. Migmatite development is also much less pervasive than in the basement.

The orthogneisses described below occur as conformable sheets and exhibit considerable strain variation. They each have subhorizontal gneissic fabric, parallel to that in the country rocks, but this varies in intensity. Sampling was undertaken in low-strain areas where igneous features were obvious. These include local lowangle discordance, angular xenolith entrainment and multiple intrusion and net veining. Examination of some examples of net veining and multiple intrusion indicates that igneous flow alignment and early gneissic strain fabrics form part of a continuum. Magmatic intrusion occurred at the peak of metamorphism. At this time the crustal zone into which these granitoids were emplaced was undergoing local partial melting to yield migmatite and pegmatite, and ductile flow. The variation in concentration of selected elements/oxides is depicted in Figs. 3-10.

Huckitta Tonalite Gneiss (HTG) The H T G is a complex intrusion ranging in composition from gabbro to granodiorite. The most mafic end-members of the series are nearly quartz-free. Plagioclase and quartz contents increase towards the more felsic end-members of the series, as does the relative proportions of biotite (Mg52_62) compared with hornblende (Ca30 Mg31_42 Fe29_40). The total amounts of biotite and hornblende decrease. The most mafic end-members of the series contain clinopyroxene. Plagioclase shows some normal zonation and covers a total analysed range from An7oto An3o. Levels of SiO2 range from 50 to 70% (Fig. 3 ), MgO from 15 to < 1%, CaO from 12 to 2% and K20 from 0.75 to 6.5% (Fig. 4). The most mafic end of the series is gabbroic, while the most felsic is adamellitic. With increasing SiO2 or decreasing MgO, the elements or oxides of the group FeO*, CaO, TiO2, Cr, Sc, Ni, Y and V all show trends of depletion. Elements or oxides of

244

2.0

8O +

1.5 -

+

60

+ ++

(~ 1.0 _

+

i=

[] rnq

E

[]

[]

-IF

+~++

g,

[]

20

0.5

100

200 V ppm

300

400

5

15

20

I 60 70 SiO2 wt.%

80

20

tonalite [] Huc)dtta gneiss

400

10 MgO wt.%

+ Moruya batholith 30O

15

200

10

[]

E

gc

+

. 100

[]

j

O

[] 5

[]

2

I

I

4 K20 wt.%

6

I 50

Fig. 6. The comparative compositional variation of the Huckitta Tonalite Gneiss suite ( [ ] ) and that of the Moruya Batholith ( + ) from the eastern Lachlan Fold Belt (Griffin et al., 1978).

19 17 15

o

o./ oo /

/

~3

°o ° ~oo°

.o. "L

FIELD OF I-TYPE I GRANITES FROM |

900 8OO 70O

]

E 600

o T.ELAO.,AN FOLD BELT

o

,~00'°

/

o~ oo

z 9~

I

/ ~ o

4O0

o Huckitta granodiodte gneiss A Inkamulla granite gneiss

Ooo ~ o ~ %

oo~Oo°

o~sO~o~ Ooa

"~'~.

°o

oo

A

300

"~-Z~.

200 3#

/ff 0.5

°

1.0

o,

,

100 I

1.5 2.0 2.5 3.0 MgO wt.%

I

I

3.5 4.0

180~-

8

160[-

7-

140F

2

65

t

67

I

i

I

69 71 73 SiO 2 wt.%

~

O Of

I

O

O

77

I

79

o o

C

8o[~.~,,_~

75

o

,001--oo oO oO. o<2 °o

5

4

~3

I

63

9 -

FIELD OF I-TYPE GRANITES FROM THE LACHLAN FOLD BELT

~ 4 I

I

4.5

.....~-oo~o ,,..

40120t-

o

z~

O,~ ~

~ '~

o°

I

I

I

[

I

I

I

[

I

I

I

1

2

3

4 5 6 K 2 0 wt.%

7

8

9

62

64

66

I

I

I

68 70 72 SiO 2 wt.%

I

I

I

74

76

78

Fig. 7. Selected inter-element variations for the Huckitta Granodiorite Gneiss and Inkamulla Granite Gneiss suites for the Entia Domal Structure. The fields shown on the Ni-MgO and Na20-K20 diagrams are those of all Lachlan Fold Belt I-type granites (from Chappell and White, 1982 ) and the lines in the Sr-Si02, and Rb-Si02 diagrams are the regressed trends for these elements for the Moruya Batholith from Griffin et al. (1978).

245

6.75 6.00

A

Inkamulla

o

Huckitta

granite

gneiss

granodiorite

t

gneiss

100

ISA

MT

/

4.50

~ 3,75 ©

/

o

/

/

o

t

ENTIA i

GRANITE GNEISSES t

i

J

r

t

F

i

T

2.1"825/02}825/81nkamu a granite gneiss

~ LJ-...~._~

3.. 808//17; }Huckitta

tonalite

5. 808/28 /. Huckitta 6, 808/12 Jgneiss

granodiorite

gneiss

5o

o°~°

6

o

~ A

o ~o%O o o

/

/ ~,~,/~

1.50

T

4

~/

/

/

2.25

3 i

!

/

3.00

OO

/

/

i

5 %

FIELD OF


5.25

i

SiO 2

i <)

75.3

~, lO c~

o o

56.1 5

0.75

2

54.3 64,5 720

i/

7o:7 100

200

300

400

500 St" ppm

600

700

800

900

I I La Ce

I

I

Nd

I

I I I Sm EU Gd

I

I Dy

[

Fir

I

I Yb

I

REE ATOMIC NUMBER

90O

800

Fig. 9. Chondrite-normalised rare earth element analyses of representative samples from the IGG, HGG and HTG series. Wt.% Si02 values for each sample are indicated. Analyses were done by isotope dilution using the method described by Sun and Nesbitt (1978). Normalization values are also those used by Sun and Nesbitt (1978). Values are given in Table IV.

700 °

60O

o

E

o

o

o

~ 500

o o

°°°

o

o

c~ 4 0 0 F 3 0 0 I--

z~ ~

/

2°°I-

~

.~'--'---~. ~ ~

~

<~ ~

50

100

~

150

200

--

\

FIELD OF KALKADOON SUITE MT ISA

'

\

250 Rb ppm

, 300

, 350

, 400

, 450

Fig. 8. Selected major- and trace-element variations amongst members of the Huckitta Granodiorite and Inkamulla Gneisses (symbols as in previous figures). The outlined field is that of the Kalkadoon suite of early-middle Proterozoic age from the Mount Isa area (Wyborn and Page, 1983).

probably hornblende gabbros of cumulate origin. The HREE are depleted towards the felsic end of the suite (Table VI ) and this is reflected by the Ce/Y (N) ratios which are < 10 in the most mafic rocks and > 30 in the felsic ones.

The Huckitta Granodiorite Gneiss (HGG) the group A12Q, P205, Nb, La, Ce, Nd, Sr and Zr all show initial enrichment and then depletion, while K20, Na20, Rb and Ba show continuous enrichment. The most common rock type in the suite is quartz diorite which has a silica content of 60%. Samples of this type are represented by H27 and H21 (Table I). Rocks at the most mafic end of the spectrum of this suite (e.g., H25, Table I) occur as nebulites or screens in the more mixed parts of the intrusive sheet. Compositionally they have very high MgO and CaO contents, high ( 100 M g ) / M g + Fe ratios (up to 75 ) and have such high concentrations of compatible trace elements (V, Cr, Ni and Sc) that it is unlikely that they were liquids. They were

This body is a composite intrusive. The earliest ('host') phase is a grey, medium-grained, granodioritic gneiss. This is intruded by thin planar bodies of coarser granodioritic gneiss containing aggregates of biotite and hornblende set in a coarse mosaic of quartz, plagioclase and minor K-feldspar. Both these phases are intruded by a fine- to medium-grained aplitic phase as an anastamosing network. The earliest grey granite gneisses component has stronger foliation than the cross-cutting phases. In this, deformation and recrystallization has produced elongate quartz grains, which show undulose extinction developed at high angle to their direction of elongation. More rarely,

246 OHuckitta

tonalite

20

z O i< b3

gneiss

Increasing Plagioclase (~Biotite)

o Huckitta granodiorite gneiss o

% CRYSTALLISATION 20 e

[]

35 0

[]

C 10

,, \-

16

orite

Granodiorite

.<

[]

~ ' ~ T o n a l i t-

12 []

~

I

t

4

l

6

I

8

I

I

I

10 12 Wt.% CaO

I

16

14

0 --

Plag:Amph ~--- 65:35

\

8

2

\iorite

~

-

50 25

e

~ X

"

--

o [ i 2:

-- 25

50|~

--

2; o (..) <

CPX+Amph:Plag ~4:1

I

18 I

6

I

10 Wt.% CaO

I

14

18

• Entia amphibolites

Fig. 11. A summary of the postulated fractionation processes capable of generating the type of geochemical variation in Fig. 10. This illustrates the results of least-squares modelling of the type shown in Table VII. At the mafic end of the suites, fractionation is dominated by plagioclase-poor pyroxene-hornblende assemblages.

22 20

[] O <

16 14 []

[]

~

~o

• °

[]

°~

•

•

edP

~

TABLE VI

•

6

Rare earth element analyses of selected granitic gneisses I

2

I

4

I

6

1

8

I

I

10 12 Wt.% CaO

1

14

I

16

I

18

Fig. 10. Wt.% A1203-CaO variation of the Huckitta Tonalite and Granodiorite Gneiss suites and the Entia amphibolite suites (symbols represent the same groups as in previous figures).

quartz has been reduced to strain-free finergrained aggregates. Feldspars are commonly fractured, but seldom show deformational twinning. The tendency towards granoblastic polygonal mosaic development is consistent with grain boundary readjustment under relatively highgrade metamorphic conditions (White, S., et al., 1980, 1982). The Huckitta Granodiorite Gneiss suite is composed of plagioclase (An4o, 35-70% ), quartz (15-30%), K-feldspar (1-20%), hornblende (Caso Mg47 Fe2s, 0-25%) and biotite (Mgso, 0-20%). Accessory phases include apatite, sphene, zircon, magnetite, clinozoisite-epidote, allanite and clinopyroxene. Biotite tends to be far more abundant than hornblende in all but the most mafic end-members of the series and

from the Entia Domal Structure, Arunta Inlier, central Australia ".b HTG c

HGG c

IGG ~

808/71 825/10 808/28 808/12 825/02 825/81 La 31.0 12.0 51.0 9.34 Ce 64.0 27.9 94.3 17.5 Nd 33.7 14.6 32.7 6.16 Sm 6.37 3.45 5.24 0.87 Eu 1.91 0.81 1.01 0.34 Gd 3.51 1.67 4.42 0.79 Dy 2.90 1.98 1.90 0.98 Er 1.84 1.22 1.09 0.60 Yb 1.46 1.07 0.92 0.64

9.44 22.0 11.5 1.99 0.75 1.48 1.39 0.75 0.69

1.67 5.05 4.71 1.66 0.48 2.46 3.55 2.49 2.08

"All abundances in ppm. bAll analyses by isotope dilution following the method described by Sun and Nesbitt {1978). CHTG = Huckitta Tonalite Gneiss; HGG = Huckitta Granodiorite Gneiss; IGG = Inkamulla Granite Gneiss.

is developed at the expense of hornblende in the more sheared margins of the intrusion. Allanite is clearly of igneous origin and its cores have become metamict and their high REE and U content has been redistributed to much later epidote growth.

247 Rocks from the HGG range from 60 to 76% SiO2 (Fig. 3). The main or 'host' phase of this multiple intrusive pluton is granodioritic to tonalitic in composition (e.g., H48 or H32, Table II). It is a calcic granite (CaO 6.0-0.75%) with high, and relatively invariant Sr content (about 550 ppm; Fig. 7). It has some quite mafic characteristics, with Ni contents ranging from 19 ppm to < 3 ppm and MgO between 2.5 and ~0.1%. REE are depleted with increasing silica content (Fig. 9). Likewise, P205, Nb, Ti, Y and to a lesser extent Zr all show marked depletion towards the more felsic end of the series (Fig. 5). The LIL element group including Ba, Rb and K20, on the other hand, shows enrichment with increasing silica. Like the most felsic tonalites (Huckitta Tonalite suite), the K20 and Ba contents of the most felsic members of the Huckitta Granodiorite suite (e.g., 808/119 or H47, Table II) are very high. The Huckitta Tonalite Gneiss suite extends to much more mafic, silica-poor compositions than does the Huckitta Granodiorite Gneiss. At their most felsic, the two suites are alike and the minimum silica values of the Huckitta Granodiorite suite ( ~ 6 2 % ) coincide with a marked inflection in the variation of the Huckitta Tonalite suite. At this silica value, CaO and A1203 in the HTG suite change from negative to positive correlation (Fig. 10) and with increasing SiO2 the rate of CaO and MgO depletion decreases (Fig. 4). A silica value of 62% also marks a change in the correlation of K20 and Na20 from positive to negative (Figs. 4 and 7). The incompatible element patterns of both the Huckitta Granodiorite and Tonalite suites are very similar with marked relative Nb depletions and Ba, K and LREE enrichments. Both groups also have relative depletions of P and Ti and the Huckitta Granodiorite suite in particular has very low concentrations of Y (Table II).

The InkamuUa Granite Gneiss (IGG) This granite gneiss occurs as a very distinctive, pink, leucocratic sill-like body (Fig. 2). The IGG outcrop forms the hinge zone of a very large recumbent folded sheet. The IGG is composed of quartz, microcline, plagioclase, biotite and hornblende, with accessory apatite, magnetite, allanite, zircon and an epidote-group mineral. By comparison with the HGG suite this granite is much more quartz(25-50%) and microcline-rich (25-50%). The more mafic end-members contain both biotite and hornblende, while the felsic end-members are very poor in ferromagnesian minerals and contain only biotite. This body is much more felsic than the Huckitta Tonalite and Granodiorite Gneisses (69-77% SiO2, Table III, Fig. 3). The very silicic rocks in the suite have low CaO and Sr and those with > 73% SiO2 show enrichment in Y and Nb. These characteristics make the IGG more like the common alkali-rich Proterozoic granites discussed by Wyborn et al. (1987). Molar A120ff (CaO + Na20 + K20) values are quite constant through the whole suite with values between 1.0 and 1.05. The other granite suites also all have A12OffCaO + Na20 + K20 values < 1.1 and in this respect are like the Itype granites of Chappell and White (1982).

Entia Amphibolites (Table V) There are two groups of amphibolitic rocks, one a component of the supracrustal suite of the Entia Gneiss Complex and the other a younger suite of sills and small layered intrusions. The first of this group may be metabasalts as they are interlayered with metasediments. Some of the intrusive group of amphibolites (Table V) must have been cumulate metapyroxenites (825/29). Some others may have represented hornblendites or unfractionated gabbros and (as illustrated in Figs. 10 and 11 )

248 there is a continuum between the mafic end of the Huckitta Tonalite suite and this amphibolite series. This association of silicic as well as intermediate to mafic and ultramafic intrusive rocks is very like that commonly observed in modern cordilleran settings (e.g., Snoke et al., 1981; Honegger et al., 1982). The Bruna Granite Gneiss (BGG) The BGG is mainly confined to the Harts Range Detachment Zone and is younger than the other gneiss bodies discussed so far (Cooper et al., 1988). Some late apophyses do appear to intrude the Irindina Gneiss in the overlying Harts Range Group. The BGG ranges from granodiorite to granite and is a coarse-grained, porphyroblastic, Kfeldspar-rich augen (1-3 cm) gneiss. The microcline augen show some undulose and patchy extinction and minor recrystallization to equant, unstrained subgrains. Together with Kfeldspar, the BGG is composed of quartz, plagioclase (oligoclase), biotite, hornblende, garnet, sphene and magnetite. Apatite and zircon are abundant minor phases. Quartz shows some ribbon development with subsequent subgrain formation. The hornblende-biotite-plagioclase-quartz component of the rock is much finer grained than the K-feldspar augen, and tends to form a matrix which envelopes these. The BGG (Table IV) is compositionally very distinct from the granitic rocks of the Entia Dome. It is more Fe- and K20-rich, and less CaO- and Na20-rich. It is much more TiO2-rich and also has much higher concentration of high field-strength elements (Zr, Nb and P2Q). The Y contents are much higher which probably indicates generally higher HREE concentrations, while Sr levels are lower. The compositional distinction between the granites of the Entia Dome and the BGG is somewhat akin to that ascribed by Brown et al. (1984) as resulting from increasing maturity of magmatic arc sources. It is also a convergence towards A-type granite geochemistry (e.g., Collins et al., 1982).

Petrogenesis of the Entia Granitic Gneisses

The Entia granitic gneisses each show some compositional features which, if of primary magmatic origin are inconsistent with evolution by the restite unmixing mechanism proposed by White and Chappell (1977). Elemental variations in coeval members of each of the suites undergo inflection. For instance, A1203 shows initial enrichment in the Huckitta Tonalite suite, followed by depletion (Figs. 10 and 11 ). In the Huckitta Granodiorite suite, the HREE, Y and Nb all show continuous depletion (Fig. 5), but the LREE group initially remain constant or increase slightly, and then show depletion. This behaviour is consistent with changing liquidus mineralogy or progressive change in bulk distribution coefficients, both effects due to cooling. Where the inflected trends lead to an increase in the rate of enrichment of incompatible elements such as K20 or Ba, over and above the normal enrichment explicable by fractionation, then this effect may also be due to contamination. A series of least-squares mass balance calculations (Table VII) has been performed in order to determine if the trends shown by the Huckitta Tonalite and Huckitta Granodiorite suites could be reproduced and by what mineral assemblages. Mineral compositions used were taken from compilations of microprobe data from these rocks by Buick ( 1985 ). In the Huckitta Tonalite suite these calculations indicate that evolution from mafic (e.g., H37, Table I) to felsic magmas (e.g., 825/15, Table I) could result from plagioclase-hornblende crystallization with some biotite involvement at the felsic end of the series. Clinopyroxene was also required to achieve good solutions at the mafic end of the suite. The proportion of plagioclase relative to ferromagnesian phases increases from ~ 16% to > 50% and this change is responsible for the inflection in the CaO-A12Q variation (Figs. 10 and 11). These results are very similar to those cal:

249 T A B L E VII (a) Least-squares modelling of the Huckitta Tonalitic Gneiss series Model 1:H37-H21

Si02 Ti02 A1203 FeO* MnO MgO CaO Na20 K20 P20~

Model 2:H21-825/14

Model 3:825/14-825/15

A

B

C

A

B

C

A

B

C

D

57.82 0.38 14.83 6.68 0.15 6.16 7.86 3.00 2.02 0.03

61.24 0.51 16.65 4.97 0.09 4.40 5.21 3.35 2.39 0.17

61.23 0.32 16.63 4.97 0.10 4.41 5.24 3.50 2.55 0.04

61.25 0.51 16.65 4.97 0.09 4.40 5.21 3.35 2.39 0.17

66.66 0.26 16.38 2.95 0.07 1.99 4.16 3.97 2.45 0.10

66.66 0.34 16.49 3.03 0.08 2.07 3.94 3.82 2.01 0.24

66.62 0.26 16.37 2.95 0.07 1.98 4.16 3.97 2.45 0.10

72.31 0.15 14.97 1.44 0.06 0.74 2.56 3.92 2.81 0.03

72.29 0.19 15.06 1.78 0.06 0.41 2.52 3.78 2.79 0.14

48.79 0.54 10.00 11.34 0.30 10.93 14.99 1.67 0.58 n.d.

M o d e l 1: parent magma = H37 modelled daughter magma = H21 % crystallization = 27 cumulate = 41.6% amphibole, 16.1% plagioclase, 39.9% clinopyroxene, 2.4% magnetite R 2 = 0.099

M o d e l 2: parent m a g m a = H21 modelled daughter magma-- 825 / 14 % crystallization = 28 cumulate = 47.5 % amphibole, 25.9% biotite, 26.6% plagioclase R2=0.317

Model 3: parent magma--825/14 modelled daughter magma = 825/15 % crystallization = 28 cumulate = 34.5 % amphibole, 9.7% biotite, 55.7% plagioclase R2=0.271

A-- parent magma composition; B = daughter composition to be matched; C = calculated daughter composition; D = cumulate composition produced by model 1; n.d. = not determined; *total Fe expressed as FeO. (b) Least squares modelling of the Huckitta Granodioritic Gneiss series Model 1 : H 4 4 - H 3 3

Si02 TiO2 A1203 FeO* MnO MgO CaO Na20 K20 P20s

Model 2 : H 3 3 - H 3 2

Model 3:808/006-808/119

A

B

C

A

B

C

A

B

C

61.02 0.47 17.94 4.94 0.12 2.37 5.25 4.05 2.68 0.17

67.58 0.21 16.86 2.75 0.08 1.13 4.28 4.12 1.91 0.10

67.56 0.22 16.82 2.65 0.10 1.12 4.49 4.14 2.24 --

67.58 0.21 16.86 2.75 0.08 1.13 4.28 4.12 1.91 0.10

68.57 0.22 16.76 2.29 0.04 0.86 3.78 4.47 1.92 0.09

68.56 0.20 16.87 2.40 0.07 0.69 3.81 4.16 1.97 0.04

71.76 0.10 15.35 0.86 0.02 0.34 2.78 3.17 4.41 0.05

75.56 0.01 13.49 0.16 0.01 0.08 1.00 2.47 6.20 0.01

75.66 0.09 13.22 0.11 0.00 0.11 1.14 2.46 5.44 0.00

Model 1: parent magma-- H44 modelled daughter magma-- H33 % crystallization = 34 cumulate-- 15.6% amphibole, 51.0% plagioclase, 31.3% biotite, 2.1% apatite R 2 -- 0.253

M o d e l 2: parent m a g m a = H33 modelled daughter magma-- H32 % crystallization = 6 cumulate = 59.9% amphibole, 37.8% plagioclase, 2.3% apatite R2=0.159

Model 3: parent magma = 808/006 modelled daughter magma = 808/119 % crystallization = 21 cumulate = 14.3% amphibole, 78.4% plagioclase, 1.3 % apatite, 5.9% epidote R 2= 0.688

A = parent magma composition; B = daughter composition to be matched; C = calculated daughter composition; *total Fe expressed as FeO.

250 culated by Arth et al. (1978) for a series of gabbros, diorites, tonalites and granodiorites from the Scandinavian Svencokarelides. Likewise, Atherton ( 1984 ) (Peruvian Batholith ), Whalen (1985) (New Britain, Tertiary M-type granitoids) and Perfit et al. (1980) (Captains Bay pluton, Alaska) also model the fractionation of tonalite and granodiorite from dioritic parental magmas with similar results. Whalen (1985) suggests that M-type granitoid evolutionary processes may contrast with those applied to Lachlan Fold Belt suites in the active role of fractional crystallization rather than restite unmixing. The resultant cumulate rocks in these cases are mafic to ultramafic in character. The calculated bulk composition of the cumulate formed to generate tonalite H37 from diorite H21 (Table VII) is very like that of some of the Entia amphibolites (857/04, Table V). At SiO2 values of < 70%, the Huckitta Granodiorite suite is likewise modelled by fractionation of an assemblage mainly composed of plagioclase-hornblende, with little biotite. Granites of this suite with SiO2 > 70%, were more difficult to relate to each other on a closed system basis. For instance, 808/119 (Table VII) can be generated quite well from 808/006, except that there is a significant shortfall in the K20 content of the calculated daughter magma. Furthermore, the achievement of the lowest residuals in these calculations always requires the inclusion of epidote, which is present as a significant minor phase.

Tectonic implications of the granite geochemistry Etheridge et al. (1987), Wyborn et al. (1987) and Wyborn and Page (1983) have examined various aspects of felsic magmatism throughout many of the Australian Proterozoic domains. They recognize that several cycles of felsic magmatism occurred during the early and mid-Proterozoic. The granites which characterize these widespread events are I-type in character but differ from Phanerozoic (and es-

pecially Lachlan Fold Belt) analogues in having higher LIL and LREE and Nb, Y and Zr contents and lower Sr, CaO, MgO and Na20 concentrations. In Figs. 6 and 7, various Entia granitic gneiss suites are compared with analyses of some Siluro-Devonian I-type granites from the Lachlan Fold Belt in southeastern Australia. It is clear that the Entia granitic gneisses and these Palaeozoic granites have very similar MgO, CaO, Ni, Na20 and K20 concentration ranges. More specifically, the Huckitta Granodiorite and Huckitta Tonalite suites have CaO, MgO, V and Rb concentrations that are virtually identical to those in the Moruya suite (Fig. 6) of Griffin et al. (1978). This similarity also extends to the extension of the Moruya suite to low SiO2 quartz diorites and gabbros very like the more mafic end-members of the Entia Huckitta Tonalite Gneiss suite (Figs. 6 and 7). The Moruya suite is unusual in the Lachlan Fold Belt both because it has such high proportions of gabbroic and dioritic end-members and because it has the lowest initial 87Sr/S~Sr and highest 143Nd/144Nd ratios of all the Lachlan Fold Belt granitoids (Chappell, 1984 ). In these characteristics it tends to show strong similarities with M-type or arc-type granitic suites, like those described from New Britain (Whalen, 1985). The implication is that the Moruya suite has minimal involvement of evolved continental crust and is wholly derived from mafic or ultramafic source. This analogy is most appropriate to the Huckitta Tonalite and Granodiorite Gneisses. Recent investigation of the compositional variation amongst Phanerozoic granitic rocks has led to attempts to use differences in the chemistry of these rocks as a discriminant of tectonic setting (e.g., Bowden et al., 1984; Brown et al., 1984; Pearce et al., 1984; Batchelor and Bowden, 1985; Harris et al., 1986). This type of approach is more difficult to apply to granitic rocks than it is to basaltic ones. It requires a satisfactory distinction to be made

251

between the different settings into which granitic magmas intrude. As the majority of granitic rocks occur in what are broadly convergent plate margins, to be useful these subdivisions must distinguish between the progressive stages of collision and between the different types of collision zone (continent-continent or continentocean). Additional complications result from the compositional convergence of acid magmas approaching the liquidus minimum. This is particularly so for the major element chemistry. It is also likely that, compared with basalts, granites are a much poorer reflection of their source-rock compositions. This is particularly so for the trace elements, and is due to the high and variable distribution coefficients of a wide range of elements with respect to common residual minerals (particularly accessory phases, e.g., Green and Pearson (1984) and Sawka et al. (1984) ). Geochemical compositions of granites are therefore complex functions of both source-rock mineralogy and chemistry and of the precise nature of the melting process. The qualified success of the schemes of granitic discrimination proposed by Pearce et al. (1984) or Harris et al. (1986) suggests that the above variables are at least under some sort of loose tectonic control. Whether such discrimination, formulated as it is for Phanerozoic rocks and tectonic settings, may be meaningfully applied to the Proterozoic, is another question. The attitude taken in this study has been that if (as an example) the discriminant diagrams assign a granite suite to an early syn-collisional environment, then the same ingredients which contributed petrogenetically to the formation of the modern granites are also present in the Proterozoic. However, this has then not been taken as direct evidence of the analogy of the tectonic setting, allowing that a given juxtaposition of petrogenetic factors is achievable under different and possibly unique Proterozoic circumstances. In Fig. 12 the Arunta data are plotted on two Pearce et al. (1984) diagrams. In the Nb-Y diagram, the Huckitta Granodiorite and Tonalite

Gneiss suites and the Inkamulla Granite Gneiss plot in the combined field assigned to volcanic arc (VAG) and syn-collisional (SYN-COLG) granites. The Bruna Granite Gneiss falls in the within-plate granite (WPG) field. The Inkamulla Gneiss overlaps the field of Huckitta Granodiorite rocks, but also extends beyond the field of these rocks towards the Bruna group in the within-plate field. The Huckitta Tonalite suite shows rather limited overlap with the Huckitta Granodiorite field and as a group is displaced towards the so-called ocean ridge granite (ORG) field, which is the mantle-derived granite field (equivalent to M-type granites). The R b - ( Y + N b ) diagram (Fig. 12) distinguishes between the volcanic arc and the syn-collisional fields and in this the Inkamulla and Huckitta Granodiorite and Tonalite suites plot in the volcanic arc field. In the same way as the Nb-Y diagram, the Inkamulla suite projects from the Huckitta Granodiorite field towards that of the Bruna Gneiss in the withinplate field. As in the Nb-Y diagram, the Huckitta Tonalite suite projects towards the ocean ridge granite field. The assignments made in Fig. 12 are also reinforced by the R1-R2 variation (based on major elements; de la Roche et al. (1980)) illustrated in Fig. 13. In this figure the fields are those of Batchelor and Bowden ( 1982 ). Again, the Huckitta Tonalite suite shows origins in the field of mantle derivatives, its more felsic endmembers overlapping the Huckitta Granodiorite suite in the pre-plate°collision field. In this diagram, the Bruna and Inkamulla groups plot in the late-orogenic to anorogenic fields, consistent with the implications of their positions in Fig. 12. Harris et al. (1986) define four granite groups; pre-collisional or arc-type, syn-orogenic (including the two-mica or leucogranites of the Himalayas; e.g., Le Fort, 1986), a postorogenic calc-alkaline suite and post-collisional alkaline intrusives. Though the first and third of these groups are difficult to distinguish without Hf and Ta data, the Huckitta Grano-

252

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Fig. 12. Discriminant diagrams for granitic rocks as determined by Pearce (1984), with the fields of the East Arunta suites discussed in this paper plotted. The designated fields are as follows; VAG = volcanic arc granite, SYN-COLG = syn-collisional granites, ORG = ocean ridge granites, WPG = within-plate granites. The two smaller diagrams at the right-hand side illustrate the compositional evolutionary direction produced by contamination by low T crustal melts and the inferred direction of evolution of bulk crustal sources. In the lower diagram (Rb versus Y + Nb ), the dashed boundaries are a suggestion of the possible general sense of movement of the fields back into the Early Proterozoic. The two small insets in the Rb- (Y + Nb ) diagram show the contrast between the incompatible element patterns of granites derived from: (1), (lower) more mantle dominated sources; (2), (upper) more crustally influenced sources.

diorite and Tonalite suites are most like the precollision granites on the basis of Rb/Zr versus SiO2 variations. When specific comparisons are made between particular modern granitoid suites and the Huckitta Tonalite and Granodiorite suites, the best analogues are to be found in the cordilleran belts of North and South America and in the pre-collisional intrusives of the trans-Hi-

malayan Batholith (Honegger et al., 1982). An important factor in this analogy is the association of granodiorite and tonalite with diorite and gabbro or gabbro-norite and the presence of cumulate rocks in which the cumulate phases are dominated by plagioclase, hornblende and clinopyroxene with or without olivine and orthopyroxene. Good examples of this association are af-

253 3000 R2 /

2000 Or /

.

~

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1000

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Fig. 13. An R1-R2 diagram (de la Roche et al., 1980) showing the fields of the East Arunta granitoids and representative examples of the Moruya Batholith suite from the Lachlan Fold Belt (Griffin et al., 1978). The numbered fields and trends are those of Batchelor and Bowden (1985). ( 1 ) Mantle fractionates; (2) pre-collision granites; (3) postcollision uplift granites; ( 4 ) late orogenic granites; (5) anorogenic granites. The dashed arrow shows the source trend through an orogenic cycle.

forded by the Peruvian Coastal Batholith (e.g., Pitcher, 1974, 1984; Thorpe and Francis, 1979; Atherton, 1984). In North America, dioritic rocks are particularly common to the west of the 'quartz diorite line' which runs N-S through the Sierra Nevadas and confines plutons of low STSr/S6Sr ratio to the west. Brown (1977) has argued strongly on the basis of these and low Sr isotopic ratios elsewhere in other cordilleran belts (e.g., some of the British Caledonides ) for a significant mantle input to this form of granite generation. In the southern Californian, Peninsular Ranges Batholith a wide range of mafic, often cumulate-textured bodies occur in general association with diorite, tonalite, granodiorite and granite (Smith et al., 1983). Many of the mafic and ultramafic bodies described by Snoke et al. (1981) from the Californian Sierras may also be part of this same association. Again there is a close analogy between these mafic bodies, often with clinopyroxene-hornblende-dominated assemblages, and some of the Entia amphibolites. A particularly strong analogue of the Huckitta Granodiorite and Huckitta Tonalite suites

is provided by the Upper Cretaceous to Tertiary trans-Himalayan plutons which transgress the Indus-Tsanpo suture zone (Honegger et al., 1982). These were intruded during the late stages of the closure of the Tethyan ocean separating the Indian and Eurasian plates and include cumulate-textured gabbros, hornblende gabbros and norites, as well as quartz diorite, tonalite, granodiorite and granite. Rare earth and other trace element data presented by Honegger et al. (1982) are the same as that from the Huckitta Granodiorite and Tonalite suites, with their low HREE and Y concentrations and high Sr and Ba. The conclusion is that the Entia granitic gneiss suites show strong affinities with plutonism which now takes place in arc-type or cordilleran collision zones where oceanic crust is underthrusting youthful continental margins.

Granitic m a g m a t i s m and the g e o c h e m i c a l evolution of the crust in the Proterozoic It is reasonable to question whether the bulk composition of the continental crust has changed significantly since the Early Proterozoic and if this is reflected by the change in granite compositions. By comparison with the I-type granites of the Lachlan Fold Belt or with some Tertiary volcanic arc granites, the Huckitta Granodiorite and Tonalite suites are depleted in HREE, Y and Zr and to some extent Nb. Do these differences reflect source compositional differences, or differences in fractionation processes? In Fig. 12 it is clear that the Huckitta Tonalite series has higher Y contents than and similar Nb contents to the more felsic Huckitta Granodiorite suite. If the Huckitta Tonalite suite is considered as partly of mantle origin and thus providing a net Nb and Y gain to the crust, then the lower concentrations of these elements in the more felsic HGG suite suggests that there must be a crustal reservoir of these elements. The within-plate granites are post-orogenic, younger and have higher Nb and Y levels pos-

254

sibly reflecting the net enrichment of the crust in these elements during its evolution. This implies that the volcanic arc/syn-collisional field in Fig. 12 may have expanded away from the origin through time. Yet, the most fractionated members of the Huckitta Granodiorite and Tonalite series (e.g., H47 or 808/119, Table II and 808/69, Table I) have very low Y, Ti, P, Nb and HREE contents (Figs. 5, 9 and 12) and high Rb, Ba and K20 levels. This suggests that low T melts under the conditions prevalent during the collisional phases of magmatism are evolving under conditions where bulk distribution coefficients for the REE and high field-strength elements, are large. It would also appear that the total system is saturated with Rb, Ba and K20. If these conditions persisted during subsequent remelting events, it would not be possible to generate the granites rich in Y, Nb and REE which fall in the within-plate field on Fig. 12. This implies two types of behaviour of minimum melts, one in which accessory phases that retain REE, Y, Nb and Zr are saturated and the other (the A- or WPG-type) in which they have higher solubility (cf. White et al., 1982a). Evidence of the transition between these two types of behaviour is to be found in the Inkamulla Granite Gneiss suite. In this suite, the most siliceous end-members of the series undergo major enrichment in Nb and Y (Fig. 5).

rier against which the pre- to syn-tectonic plutons 'pond'. Only when the flow rates in the crust diminish and the detachment zone becomes inactive, do post-tectonic plutons intrude to higher crustal levels (Fig. 14). When magma bodies are sheets there is maximum opportunity for wall-rock-magma exchange, particularly as the magmas cool and PLUTONISM INTO A MOBILE AND ACTIVELY DEFORMING CRUST WITH MAJOR DETACHMENT ZONE

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The role of wall-rock-granite-magma exchange The overwhelming distinction between the EDS granites and some Lachlan Fold Belt analogues, is their intrusive form. The sheet-like Entia granites result from the very mobile, actively deforming environment into which intrusion took place. By contrast many high-level plutons, including those of the Lachlan Fold Belt, have been emplaced into static (brittle?) crust. This distinction leads to very big differences in the surface-area/volume ratios of the resultant plutons. It is also likely that the major detachment zone in the Arunta provides a bar-

F //

OrogenicGranite body

/Sf~---c~,Post

Fig. 14. Diagrammatic representation of plutonism in the Harts Range Mobile Zone. (SOB) Strangways Orogenic Belt (basement); (EGC) Entia Gneiss Complex; (HRC) Harts Range Complex (cover). Cartoons A-G represent a time sequence (A is oldest). The black bodies shown in the detachment zone are fragments of higher pressure lithologies carried from depth.

255 crystallize, releasing their heat of crystallization. This type of process has been described as combined assimilation and fractional crystallization (AFC) (De Paolo, 1981), and in the granite gneisses from the EDS its effects may be observed in the most fractionated members of the Huckitta Granodiorite and Tonalite suites. Both of these include felsic samples with > 72% SiO2, with very high K20 ( > 6.0% ) and Ba (up to 7400 ppm) contents which clearly cannot be simple fractionates of the diorites and tonalites (808/69, Table I; 808/006, H47 and 808/119 in Table II). In the Huckitta Tonalite suite for instance, in order to achieve the required three-fold enrichment of K20 from magmas of the quartz dioritic-tonalitic compositions (with ~ 2.0% K20 ) would require at least 70% crystallization even if K were perfectly incompatible. This is not borne out by the still relatively high MgO, CaO, Sr, Ni, Sc, Cr and V contents of these rocks (e.g., 808/69). These samples represent batches of fractionating tonalite magma which have been significantly contaminated by locally derived wall-rock melts which have high Rb, Ba and K20 contents. In the Rb versus ( Y + N b ) diagram on Fig. 12, contamination has produced a major offshoot of the Huckitta Granodiorite field to high Rb values. This contaminant has virtually no Y and Nb and is suggested to be a minimum melt phase produced in the local gneisses which host the intrusions (migmatites). It is probable that this exchange process also operates in the opposite direction, with withdrawal of elements for the melt to mineral sites in the wall-rock gneisses. The example of Y variation in the HGG series is a good illustration (Fig. 5). In this regard it is relevant to note that though some LFB granodiorites (e.g., Griffin et al., 1978; Hine et al., 1978) have significantly higher Y, Ti and HREE than the granodiorites of the Proterozoic EDS, the distinction is much less marked at the more mafic end of the suites.

Diorite from the Moruya suite has very similar Y contents (about 20 ppm) to diorite of the Huckitta Tonalite suite, yet granodiorite from both the HTG and HGG suites with > 65% SiO2 has < 10 ppm Y by comparison with Moruya or Kosciusko batholiths where equivalent rocks have > 20 ppm Y. In addition, although the Huckitta Granodiorite suite shows very marked depletion of Y with further fractionation (Figs. 5 and 12) the Palaeozoic suites show constant, or less strongly depleted Y contents. This is explicable if Y, HREE and high field-strength elements are being extracted from the melt and are entering minor phases and hornblende in the wall rocks. Under tectonic conditions where sheet intrusion is common the crustal levels where these occur will be efficient traps for relatively compatible trace elements (REE, Nb, Ti, Y and Zr). These may be relinquished to locally derived subsequent melts. If the tectonic conditions that favour sheet-like intrusion are more common in the Proterozoic than the Phanerozoic, then factors discussed in this section may lead to systematic differences in granite chemistry. Discussion and conclusions The Huckitta Tonalite and Granodiorite Gneisses are like Phanerozoic cordilleran M- or I-type granitoid suites. They are very different from the volumetrically superior and widespread anorogenic style of felsic magmatic suite so well represented in the Australian Proterozoic. However, in this Entia Dome region of the eastern Arunta Block, there is a transition towards this more 'typical' type of Australian Proterozoic felsic magmatism in the intrusion of the Bruna Granite Gneiss into the Harts Range Detachment Zone at 1745 Ma. The high CaO, MgO, Sr, Cr and Ni contents of many of the Huckitta Tonalite and Granodiorite gneisses and their extension to mafic compositions indicates that the source of these granitoid magmas is either mafic or ultramafic and therefore unlikely to be evolved Protero-

256 zoic continental crust. T h a t these magmas are juvenile additions to the crust is also supported by U - P b zircon geochronology which dates the Huckitta Granodiorite at ~ 1765 Ma and by ion microprobe investigation which indicates that there are no inherited zircon populations (G.E. Mortimer, personal communication, 1987). This is compatible with the structural arguments of James and Ding (1988) that the Harts Range Detachment Zone was undergoing extension prior to the intrusion of the Entia orthogneisses and was involved with contemporary rift basin formation. It is possible that the Entia supracrustal rocks were deposited in this rift basin. The Entia orthogneisses were then intruded into this sequence at the inception of the compressional phase that James and Ding (1988) recognize to have terminated basin formation. This model regards the rocks exposed in the Entia Inlier as an interplate, suture-fill sequence between cratons (polygons?) of older Proterozoic continental crust. The processes involved may be rather like those involved in the opening and closure of marginal seas/backarc basins in the Phanerozoic. It is probable that the more voluminous anorogenic style of Proterozoic magmatism is generated away from the central axis of rift formation, in the 'upper plate' of the extension-detachment model (see fig. 3 in Wyborn, 1987) and are derived both from melting of evolved crust and newly added underplate. The Entia Gneiss Complex is located on the footwall of the detachment zone at the top of the lower plate (Fig. 14). The location of the detachment zone is partly determined by the high ductility of this level of the crust at that time, with abundant local development of partial melts (migmatite and pegmatite) in relatively hydrated upper amphibolite facies rocks. The strong subhorizontal fabric developed at this level during the active motion of the detachment zone dictated the development of both magmatic flow and strain fabrics in the orthogneisses. It also governed the geometric form of

the intrusions as sheets. The high surface-area to volume ratio of these, aided magma-wall-rock exchange and yielded anomalously high K, Rb and Ba and anomalously low Y and HREE concentrations in some magmas. The apparent rarity of these syn-tectonic M-/ I-type granite suites in the Australian Proterozoic suggests that the rift-collision zones in which they were formed were narrow compared with the area of the cratons they separate. However, the Atnarpa Complex in the Giles Creek Syncline (Stewart et al., 1984) to the south of the Entia Dome is also a diorite-tonalite-granodiorite suite like the Huckitta Granodiorite. To the north of the Entia Dome in the Jervois Range, the Jinka Granite is also a CaO-, St-rich granodiorite with low Y. Both of these have very low initial STSr/S6Sr ratios (Black et al., 1983) and it is possible that together with the Entia Gneiss, they form a single eastern Arunta collisional suture province. James and Ding (1988) regarded the Entia Gneiss Complex as part of the Division 1, Strangways Orogenic Belt (SOB). This conclusion may be inconsistent with the petrological arguments sited in this paper, and with geochronological data presented by Cooper et al. (1988). Windrim and McCulloch (1986) provide N d - S m geochronological data for the Strangways Metamorphic Complex (100 km to the west). Here they have good evidence for average crustal age of ~ 2000 Ma and for granulite facies metamorphism at ~ 1760 Ma. This is the same age as the Entia granitic gneisses (Cooper et al., 1988) and it is possible that this metamorphism is sited above the displaced thermal anomaly due to the same rifting event in which the Entia supracrustal rocks and granitoid magmas were formed. In other words, perhaps the Strangways Orogenic Belt terrain formed the continent which was rifted to provide a depositional site for the Entia supracrustal rocks.

Acknowledgements The authors thank Dr. P.R. James, Mr. J. Stanley, Mr. P. McDuie, Dr. L. Wyborn, two

257

other reviewers of an earlier version of this manuscript, and Ms. S. Proferes and Mr. R. Barratt for various forms of assistance in the production of this paper. The project has been funded by the Australian Research Grants Scheme and by a Commonwealth Postgraduate Scholarship (to I.S.B.). References Arth, J.G., Barker, F., Peterman, Z.E. and Friedman, I., 1978. Geochemistry of the gabbro-diorite-tonalitetrondhjemite suite of southwest Finland and its implications for the origin of tonalitic and trondhjemitic magmas. J. Petrol., 19: 289-316. Atherton, M.P., 1984. The coastal batholith of Peru. In: R.S. Harmon and B.A. Barreiro (Editors), Andean Magmatism. Chemical and Isotopic Constraints. Shiva Geology Series, Cheshire, pp. 168-179. Batchelor, R.A. and Bowden, P., 1985. Petrogenetic interpretation of granitoid rock series using multicationic parameters. Chem. Geol., 48: 43-55. Black, L.P., Shaw, R.D. and Stewart, A.J., 1983. Rb-Sr geochronology of Proterozoic events in the Arunta Inlier, central Australia. BMR J. Aust. Geol. Geophys., 8: 129-137. Bowden, P., Batchelor, R.A., Chappell, B.W., Didier, J. and Lameyre, J., 1984. Petrological, geochemical and source criteria for the classification of granitic rocks: a discussion. Phys. Earth Planet. Inter., 35: 1-11. Brown, G.C., 1977. Mantle origin of cordilleran granites. Nature, 265: 21-24. Brown, G.C., Thorpe, R.S. and Webb, P.C., 1984. The geochemical characteristics of granitoids in contrasting arcs and comments on magma sources. J. Geol. Soc. London, 141: 413-426. Buick, I.S., 1983. The geology, petrology and geochemistry of the Huckitta granodioritic gneiss and associated granito;,~s, Harts Range, central Australia. Honours Thesi.~, University of Adelaide (unpublished). Buick, I.S., 1985. The petrology of granitic rocks from the Entia Domal structure, Eastern Arunta Block. M.Sc. Thesis, University of Adelaide (unpublished). Chappell, B.W., 1984. Source rocks of I- and S-type granites in the Lachlan Fold Belt, southeastern Australia. Philos. Trans. R. Soc. London, Ser. A, 310: 693-707. Chappell, B.W. and White, A.J.R., 1982. I- and S-type granites in the Lachlan Fold Belt, southeastern Australia. In: Geology of Granites and their Metallogenetic Relations. Nanjing, China, pp. 87-101. Collins, W.J., Beams, S.D., White, A.J.R. and Chappell, B.W., 1982. Nature and origin of A-type granites with particular reference to southeastern Australia. Contrib. Mineral. Petrol., 80: 189-200.

Cooper, J.A., Mortimer, G.E. and James, P.R., 1988. Rate of Arunta Inlier evolution at the eastern margin of the Entia Dome, central Australia. Precambrian Res., 40/ 41: 217-231. De la Roche, H., Leterrier, J., Grand Claude, P. and Marchal, M., 1980. A classification of volcanic and plutonic rocks using R1-R2 diagrams and major element analyses - - its relationships with current nomenclature. Chem. Geol., 29: 183-210. De Paolo, D.J., 1981. Trace element and isotopic effects of combined wallrock assimilation and fractional crystallisation. Earth Planet. Sci. Lett., 53: 189-202. Ding, P. and James, P.R., 1985. Structural evolution of the Harts Range area and its implication for the development of the Arunta Block, central Australia. Precambrian Res., 27: 251-276. Etheridge, M.A., Rutland, R.W.R. and Wyborn, L.A.I., 1987. Orogenesis and tectonic process in the early to middle Proterozoic of Northern Australia. In: A. Kroner (Editor), Proterozoic Lithospheric Evolution. Am. Geophys. Un., Geodynamics Series, 17: 131-147. Green, T.H. and Pearson, N.J., 1984. Stability of REE-acceptor minerals at high pressures and temperatures. In: Geoscience in the Development of Natural Resources (Abstr.). Geol. Soc. Aust., 12: 197-199. Griffin, T.J., White, A.J.R. and Chappell, B.W., 1978. The Moruya Batholith and geochemical contrasts between the Moruya and Jindabyne suites. J. Geol. Soc. Aust., 25: 235-247. Harris, N.B.W., Pearce, J.A. and Tindle, A.G., 1986. Geochemical characteristics of collision-zone magmatism. In: M.P. Coward and A.C. Ries (Editors), Collision Tectonics. Geological Society Special Publication, 19: 67-81. Hine, R., Williams, I.S., Chappell, B.W. and White, A.J.R., 1978. Contrasts between I- and S-type granitoids of the Kusciusko Batholith. J. Geol. Soc. Aust., 25: 219-234. Honegger, K., Dietrich, V., Frank, W., Gansser, A., Thoni, M. and Trommsdorff, V., 1982. Magmatism and metamorphism in the Ladakh Himalayas (the IndusTsangpo suture zone). Earth Planet. Sci. Lett., 60: 253292. James, P.R. and Ding, P., 1988. 'Caterpillar tectonics' in the Harts Range area: a kinship between two sequential Proterozoic extension-collision orogenic belts within the eastern Arunta Inlier of central Australia. Precambrian Res., 40/41: 199-216. Joklik, G.F., 1955. The geology and mica fields of the Harts Ranges, central Australia. Aust. Bur. Miner. Resour., Bulletin 26. KrSner, A., 1983. Proterozoic mobile belts compatible with the plate tectonic concept. Geol. Soc. Am. Mem., 161: 59-74. Le Fort, P., 1986. Metamorphism and magmatism during the Himalayan collision. M.P. Coward and A.C. Ries (Editors), Collision Tectonics. Geological Society Special Publication, 19: 159-172.

258 Pearce, J.A., Harris, N.B.W. and Tindle, A.G., 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J. Petrol., 25: 956-983. Perfit, M.R., Brueckner, H., Lawrence, J.R. and Kay, R.W., 1980. Trace element and isotopic variations in a zoned pluton and associated volcanic rocks, Unalaska island, Alaska: A model for fractionation in the Aleutian calcalkaline suite. Contrib. Mineral. Petrol., 73: 69-87. Pitcher, W.S., 1974. The Mesozoic and Cenozoic batholiths of Peru. Pacific Geol., 8: 51-63. Pitcher, W.S., 1984. Phanerozoic plutonism in the Peruvian Andes. In: R.S. Harmon and B.A. Barreiro (Editors), Andean Magmatism; Chemical and Isotopic Constraints. Shiva Geology Series, pp. 152-167. Plumb, K.A., 1979. The tectonic evolution of Australia. Earth Sci. Rev., 14: 205-249. Sandiford, M. and Powell, R., 1986. Deep crustal metamorphism during continental extension: modern and ancient examples. Earth Planet. Sci. Lett., 79: 151-158. Sawka, W.N., Chappell, B.W. and Norrish, K., 1984. Light rare earth element zoning in sphene and allanite during granitoid fractionation. Geology, 12:131-134. Shaw, R.D. and Stewart, A.J., 1975. Arunta Block - - regional geology. In: C.L. Knight (Editor), Economic Geology of Australia and Papua New Guinea, Monograph 5. Shaw, R.D., Langworthy, A.P., Offe, L.A., Stewart, A.J., Allan, A.R., Senior, B.R. and Clarke, D., 1979. Geological report on 1 : 100,000 scale mapping of the southeastern Arunta Block, Alice Springs 1:250,000 sheet area, Northern Territory. Aust., Bur. Miner. Resour., Rec. 1979/47. Shaw, R.D., Freeman, M.J., Offe, L.A. and Senior, B.R., 1982. Geology of the Illogwa Creek 1 : 250,000 sheet area, central Australia-preliminary data, 1979-80 surveys. Aust., Bur. Miner. Resour., Rec. 1982/23. Sivell, W.J. and Foden, J.D., 1985. Banded amphibolites of the Harts Range metaigneous complex, central Australia: a bimodal basalt tonalite suite. Precambrian Res., 28: 223-252. Smith, T.E., Huang, C.H., Walawender, M.J., Cheung, P. and Wheeler, C., 1983. The gabbroic rocks of the Peninsular Ranges batholith, southern California: cumulate rocks associated with calcalkaline basalts and andesites. J. Volcan. Geotherm. Res., 18: 249-278. Snoke, A.W., Quike, J.E. and Bowman, H.R., 1981. Bear Mountain igneous complex, Klamath Mountains California: an ultrabasic to silicic calcalkaline suite. J. Petrol., 22: 501-522. Stewart, K.P., 1985. The petrological significance of calcsilicate and associated gneisses, Inkamulla Bore area, Entia Dome, Harts Range, Eastern Arunta Block. Honours Thesis, University of Adelaide (unpublished). Stewart, A.J., Shaw, R.D. and Black, L.P., 1984. The Arunta Inlier: a complex ensialic mobile belt in central Australia. Part 1: stratigraphy, correlations and origin. Aust. J. Earth Sci., 31: 445-455.

Sullivan, S.J., 1985. A detailed geological investigation of the Entia Gneiss and leucocratic gneiss intrusive, northern Entia Dome, Harts Range, Eastern Arunta Block. Honours Thesis, University of Adelaide. Sun, S.S. and Nesbitt, R.W., 1978. Petrogenesis of Archaean ultrabasic and basic volcanics: evidence from rare earth elements. Contrib. Mineral. Petrol., 65: 301-325. Thorpe, R.S. and Francis, P.W., 1979. Petrogenetic relationships of volcanic and intrusive rocks of the Andes. In: M.P. Atherton and J. Tarney (Editors), Origin of Granite Batholiths: Geochemical Evidence. Shiva Publishing, Cheshire, pp. 90-105. Warren, R.G., 1983. Metamorphic and tectonic evolution of granulites, Arunta Block, central Australia. Nature, 305: 300-302. Whalen, J.B., 1985. Geochemistry of an island-arc plutonic suite: the Uasilau-Yau Yau Intrusive Complex, New Britain, P.N.G.J. Petrol., 26: 603-632. Wernicke, B. and Burchfiel, B.C., 1982. Modes of extensional tectonics. J. Struct. Geol., 4: 105-115. White, A.J.R. and Chappell, B.W., 1977. Ultrametamorphism and granitoid genesis. Tectonophysics, 43: 7-22. White, S., Burrows, S., Carreras, J., Shaw, M. and Humphreys, F., 1980. On mylonites in ductile shear zones. J. Struct. Geol., 1: 175-187. White, A.J.R., Collins, W.J. and Chappell, B.W., 1982a. Influence of melt structure on the trace element composition of granites. In: Geology of Granites and their Metallogenetic Relations. Nanjing, China. White, S., Evans, D.J. and Zhong, D.L., 1982b. Fault rocks of the Moine Thrust Zone: microstructures and textures of selected mylonites. Textures and Microstructures, 5: 33-61. Windrim, D.P. and McCulloch, M.T., 1986. Nd and Sr isotopic systematics of central Australian granulites: chronology of crustal development and constraints on the evolution of lower continental crust. Contrib. Miner. Petrol., 94: 289-303. Wyborn, L.A.I., 1987. Extensional magmatism in the Australian Proterozoic and its metallogenic implications. Abstracts for the 1987 BMR Research Symposium on Applied Extension Tectonics. Aust., Bur. Miner. Resour., Rec. 1987/51: 189-196. Wyborn, L.A.I., 1988. Petrology, geochemistry and origin of a major Australian 1880-1840 Ma felsic volcano-plutonic suite: a model for intracontinental felsic magma generation. Precambrian Res., 40/41: 37-60. Wyborn, L.A.I. and Page, R.W., 1983. The Proterozoic Kalkadoon and Ewen Batholiths, Mt. Isa Inlier, Queensland: source, chemistry, age and metamorphism. BMR J. Aust. Geol. Geophys., 8: 53-69. Wyborn, L.A.I., Page, R.W. and Parker, A.J., 1987. Geochemical and geochronological signatures in Australian Proterozoic igneous rocks. In: T.C. Pharaoh, R.D. Beckinsale and D.T. Rickard (Editors), Geochemistry and Mineralization of Proterozoic Volcanic Suites.

259 Geological Society Special Publication, 377-394. Wynne-Edwards, H.R., 1976. Proterozoic ensialic oroge-

nesis: the millipede model of ductile plate tectonics. Am. J. Sci., 276: 927-953.