A rubidium-strontium chronology of the metamorphism and prehistory of central Australian granulites

A rubidium-strontium chronology of the metamorphism and prehistory of central Australian granulites Geology Department,

C. M. GRAY La Trobe University, Bundoora, Vie. 3083. Australia and

W. COMPSTON Research

School of Earth Sciences, Australian National P.O. Box 4. Canberra, ACT. 2600, Australia

(Ruceiced

University,

18 Apri/ I978 ; accepted in revised form I4 July

1978)

Abstract-Rubidium-strontium isotopic study of intermediate-pressure granulites at Mt. Aloysius. central Australia reveals total rock isochrons that either record the metamorphism or predate it. The gneisses involved, typically quartz + feldspar + orthopyroxene + garnet granulites, occur in five lithological units which outline a simple fold structure. The distribution of isotopic ages in a 25 km’ area is tested using 74 samples collected in groups of 2 to 4 both along and across strike in each of the units. Two total rock isochron ages of 1200 and 1550 Myr occur, and both are found at different sites in one unit. Mineral ages are younger and independent of location, with feldspars giving 800 Myr and biotites 730 Myr. The 1200 Myr isochrons show the features of outcrop-scale Sr isotopic homogenisation and are taken to record the time of metamorphism. Contemporaneous regional depletion of U, commonly associated with granulite facies metamorphism, confirms the interpretation. The 1550 Myr isochrons describe entire lithological units and are best assigned to the supracrustal genesis of the rocks. The preservation of two ages indicates that isotopic equilibration of anhydrous total rocks is incomolete even within the granulite facies. Careful interpretation is required to assign geological meaning ;o granulite isochrons.-

INTRODUCTION

THE ISOTOPIC geochemistry of deep-seated metamorphic rocks is the antithesis of the closed system models usually applied in Rb-Sr geochronology. To begin, the parent rocks may be of any degree of heterogeneity in original age and provenance. Secondly, to reset established total rock ages during metamorphism this heterogeneity must be cancelled by extensive isotopic mixing-the extent of this mixing is the crux of any study of metamorphic chronology. If equilibration does occur its relation to the petrologitally observed effects of metamorphism is unclear; it may reflect processes ranging from the first mineral reconstitution to the final decline in temperature of the terrain. The isotopic homogenisation of minerals in amphibolite facies rocks is well known (COMPSTON and JEFFERY, 1959), but the involvement of total rock samples under similar conditions seems unlikely as isotopic exchange between compositional bands in a gneiss has been shown to be limited to distances of centimetres (KROGH and DAVIS, 1969, 1970, 1973). However, PIDGEON (1967) inferred homogenisation within rock units on a very large scale to explain total rock ages for granulites, and ARRIENS and LAMBERT (1969) invoked regional movement of Sr (along with other elements) to account for a single isochron found throughout a granulite terrain. Mineral equilibration is justified readily because it postdates the original age of a rock as given by a total rock iso-

chron. Total rock equilibration is harder to prove as the single total rock isochron produced is dificult to distinguish from that of an undisturbed premetamorphic system. It must be stressed that, at present, the interpretation of total rock isochrons for metamorphic rocks generally assumes the equivalence of isotopic age and time of metamorphism. This study is a detailed isotopic examination of a single area of well-exposed granulite facies rocks in central Australia, where the geology is relatively simple and retrogressive metamorphism is virtually absent. Companion papers detail U-Pb isotopic measurements (GRAY and OVERSBY, 1972), major and trace element geochemistry (GRAY, 1977), and regional geochronology (GRAY, in prep.). These combined works lead to the identification of Rb-Sr isochrons which either predate the granulite facies metamorphism or record it. GEOLOGICAL

ENVIRONMENT

The Musgrave Block of central Australia is a Proterozoic high-grade metamorphic and igneous complex of east-west trend exposed over 120,000 km* in the Precambrian core of the continent. This study is located at Mt. Aloysius in the Tomkinson Ranges at the western end of the axial granulite terrain of the block. The regional geology is discussed by WILSON (1969). The geology of the Tomkinson Ranges has a tripartite subdivision into high grade metamorphic rocks, basic igneous intrusives, and volcanic and sedimentary cover rocks. The metamorphic rocks are predominantly anhydrous intermediate-pressure granulite

1735

1736

C. M.

GRAY

and W.

gneisses which are wetf-tayercd and have a granodioritic composition overall (GRAY. 1937); the area at Mt. Aloysius is typical. The layered basic intrusions comprise anorthositic. gabbroic and pyroxenitic members of great size (strike lengths of up to 50 km) and thickness (up to Loo mf (NFSHITTcr ai., f970). These sill-like bodies are transgressivr: to the folded layering of the granulites and were emplaced after the climax of metamorphism. Crystallization at high pressure (10-12 kbar) is a distinctive feature of some intrusions @Z~WDE and MOORE, 1975). A subsequent acid magmatic phase at $040 Myr was marked by granitic intrusions and rhyolitic volcanics, and was probably contemporaneous with a second foldLlg episade which deformed the basic intrusions (GRAY. 1971). The high grade rocks form a basement upon which the volcanics were emplaced unconformably; the volcanics themsctves arc overlain unconformably by Upper Proterozoic sandstones of the Officer Basin (DANIEL& 1971). Mount Aloysius consists of a folded succession of granulite gneisses in six tithological units disti~~u~sh~d by criteria given in the Appendix. The units outline a folded structure in which two fold styles with overprinting relations are identified (Fig. I). The major structure of similar style (F,) is the older, closing in the south-east as a synform plunging to the west. but with an outcrop trace that swings to north and then north-east, apparently reclosing in another, but sharper, westerly-plunging synform. Mesoscopic folds of this similar style are very common in the banded granulite unit. In a small area within the clasure of the major synform (south of station 61) these structures are overprinted by concentric folds (F,) which also ptunge steeply to the west. It is plausible to link the large scale folding of the F, synform which resulted in its double 610~ sure. with folding on an east.-west F2 axis. Subsequent to deformation two dolerite dyke suites. the more prominent of which is shown on Fig. I. were emplaced, and minor faulting with some local low grade alteration took place. Metamorphic grade is uniform throughout the area and corresponds to the intermediate-pressure granulite subdivision of GREEN and R~NGW~~D (1967). The physical conditions of metamorphism can be estimated by comparing the mineralogy of rocks of known chemistry with experimental studies on equivalent compositions. Comparison with dry experimental situations is aided by the anhydrous nature of the rocks: hydrous minerals are rare and the average water content is 0.3?,. The miner& assemblages and chemical analyses of the granutites studied are given in GRAY (1977). Petrological features indicative of metamorphic grade are listed below. (,I) Anhydrous basic gram&es have the mineral assemblage pfagioclase + orthopyroxene f clinopyroxene. The coexistence of plagioclase and orthopyroxene indicates an upper pressure limit defined by the reaction (GREEN and RINGWOOD. 1967). plagiociase + orthopyroxene * cIinopyroxene + garnet f quarts. (2) A dolerite dyke and some acidic granulites show a minor modification of the primary mineral assemblage in which garnet & quartz forms coronas about orthopyroxcne or opaques. Passage across the above reaction boundarq probably occurred at high pressure during the final cooling of the area; the reaction is erratically developed and is not correlated with bulk chemistry. (3) Pelitic compositions have a quartz + K-feldspar + garnet + sillimanite mineralogy which has a low pressure fimit of approximately 7 kbar from the work of HENSEN and GREEN (I 972). (4) Sillimanite is the stable Al$?.iO, polymorph. (5) Basic granulites with hornblende as a major phase show the breakdown reaction, hornblende * orthopyroxene + clinopyroxene

-t- plagioclase.

COMPSTON

(6) The solidus for anhydrous adamellite compositions was not reached as there is no evidence of partial melting: migmatitic structures, pegmatitic pods and crosscutting acidic dykes are absent. The sum of these features limits metamorphic conditions to pressures of X-f 2 kbar and temperatures of 81xriooo”C (GRAY, 1971).

Some clues as to the primary nature of the granulites have survived metamorphism. The major element chemistry shows no sign of gross modi~catjon and demonstrates clearly that the siltimanite granulite is a metasediment, while the other units could have been feldspathic sandstones or more likely a volcanic suite (GRAY. 1977). The characteristic macroscopic layering is strikingly reminiscent of primary supracrustai layering: internally homogeneous horizons 5-f00m thick persist over strike lengths of up to 5 km: lithologies range in composition from basic to acidic to pelitic. and the discrete basic granulite horizons in the banded granulite can only be metamorphosed basalt Rows Centimetre-scak layering? which is due to variation in the proportions of minerals or gross chemical composition. generally pinches out within a single outcrop and is of problematic origin. It may represent bedding or be a product of metamorphic differentiation. In summary, the rocks of Mt. Aloysius initially formed a prominently layered sequence mairdy of tofcanics with lesser sediments. Subsequent metamorphism achieved intermediate-pressure granulite grade under very anhydrous conditions and was probably accompanied by the F, folding phase. The F, deformation significantly postdated metamorphism as it was recorded in ~st-metamorphjc basic intrusions to the east (NESRITTet al.. 1970). It. and later geological events did not significantly affect the granulites with the exception OF inducing deformational features: retrogressive metamorphism is absent. EXPERIMENTAL

PROCEDURE

Sample colfeetion of a suite of rocks for Rb-Sr geochronology must be consistent with the assumptions inherent in the isochron equation. namely (a) a uniform initial Sr isotopic composition for the suite, (b) the simultaneous closure of each member to the movement of Rb and Sr. and (c) no gain or loss of the elements due to subsequent geological events. A practical necessity (d) is variation in RbiSr ratio within a rock suite to define the eradient of the isochron. The crux of the sampling problem lies in the conflict between the desire to obtain a uniform initial isotopic ratio (a). and the need to have a spread in Rb/Sr ratio (d). A uniform initial ratio implies homogeneity in the rocks and leads to a small sampling area, whereas dispersicm in Rb/Sr ratio implies heterogeneity and often compels widespread collection. The following compromise was adop?ed. Sampling stations (identity by a field number) were tocated throughout the area according to the strategy outlined below so as to obtain a spread in Rb/Sr ratio. At each station two to four samples were taken within a Iam radius to provide a suite likely to possess a uniform initial ratio. The sampling programme attempted to:

(I) characterise lithological units by placing stations along and across strike; f2) examine atcme_strike variation by sampling a prominent horizon in the banded granuiite at intervals over a distance of 5 km (stations 64-67 and 95-99); (3) examine across-strike variation within the banded granulite by sequential sampling of horizons (stations 83-85); (4) estabfish whether folding affected isotopic systems by placing stations on the limbs and within the closure of the F, macroscopic fold (stations 86. 89, 92 and 61).

A rubidium-strontium

chronology

of central

Australian

1137

granulites

SILUMANITE

61

GARNET

MASSNE

GRANUUTE

A

BAND

BANDED

GRANUUTE

WITH

GARNET

GRANULITE

II

MASSIVE

\

GRANUUTE

GRANULITE

\

* 65” \

i\

\

-

0

V

Fig. 1. Geology

Analytical technique Isotope dilution analyses for Rb and Sr followed the procedure of COMPSTONet al. (1965), with the exception of independent dissolutions for the two elements. Strontium isotopic ratios were measured on a 60-degree sector 12-inch radius of curvature mass spectrometer (ARRIENS and COMPSTON,1968) using magnetic field switching. The

126’ 36’ ,

of Mt. Aloysius.

s’Sr/*%r ratios are relative to a value of 0.7081 for the Eimer and Amend SrCO, standard and have a two-sigma precision of better than +0.0005. The uncertainty in the s’Rb/%r ratio is + 1.0% (20). The linear regression of isochrons follows the method of MCINTYRE et al. (1966). modified by the removal of the t-multiplier. and their terminology is used to describe the quality of fit of data

1738

C. M. GRAY

station

Sample Number

and W. COMPSTON

Rb ppm

Sr ppm

*7Rb/86Sr

87Sr/86Sr

92.6 5.9 123.4 97.0 109.0 90.6 70.7 70.9 79.7 28.4 58.6

65.3 661.3 59.1 77.4 51.9 41.7 42.1 47.9 57.4 38.1 42.7

4.132 0.026 6.104 3.647 6.148 h.335 4.902 4.312 4.048 2.168 3.998

0.8036 0.7042 0.8381 0.7889 0.8391 0.8236 0.8080 0.8004 0.8007 0.7712 0.7935

122.2 10.7 91.4 137.5 75.9 55.0 13.4 53.6 111.8 34.2 4.0

267.6 663.5 203.8 246.6 190.0 240.3 294.9 232.1 322.1 563.2 51P.l

1.322 c.047 1.299 1.616 1.157 0.662 0.131 0.668 1.004 0.175 0.072

0.7344 0.7054 0.7373 0.7416 0.7314 0.7206 0.7107 0.7220 0.7279 0.7082 0.7054

127.4 li2.7 141.1 163.6 201.9 203.1 203.6 215.3 245.7 98.4 112.5 133.5 114.0 166.6 159.5 115.2 240.6 3.7 253.5 118.7 140.9 0.8 34.7 127.4 103.9 106.2 202.9 81.2

348.9 375.8 340.6 314.2 198.1 91.6 157.6 157.2 136.2 419.9 356.1 256.2 345.3 141.5 65.8 127.1 54.4 361.1 60.7 98.9 102.2 267.9 168.9 169.2 347.2 374.6 129.9 412.6

1.056 1.176 1.171 1.508 2.959 6.489 3.760 3.986 5.262 0.677 0.911 1.509 0.955 3.427 7.102 0.573 13.142 0.029 12.389 3.494 4.017 0.009 0.594 2.183 0.865 0.820 4.551 0.569

0.7259 0.7270 0.7282 0.7335 0.7640 0.8521 0.7864 0.7901 0.8092 0.7183 0.7233 0.7355 0.7241 0.7868 0.8553 0.7205 1.0068 0.7093 0.9852 0.7890 o.i970 0.7042 0.7244 0.7531 0.7230 0.7214 0.8047 0.7171

17.7 37.0 53.7 69.5 118.1 134.1 100.1

316.6 517.1 387.6 347.6 421.1 390.2 329.7 ZJ5.4

0.161 0.207 0.400 0.577 0.811 0.994 1.434 1.051

0.7111 0.7095 0.7144 0.7174 0.7215 0.7284 0.7347 0.7283

19.3 20.: 18.3 55.5 35.4

328.6 358.i 380.7 392.5 409.0

O.LiO O.l6i, 0.138 0.430 0.250

0.7085 0.7082 0.7086 0.7137 0.7103

Sillimanite Granulite 90 90 90 90 90 90 115 115 115 115 115

GA 2757 GA 2790 69-1237 69-1238 69-1313 69-1314 69-1239 69-1240 69-1241 69-1242 69-1315 Garnet Granulite A

86

86 a9 89 91 91 91 91 92 92 142

69-1245

69-1246 69-1319 69-1320 GA 2788 69-1252 69-1253 69-1254 69-1255 69-1256 GA 2791 Banded Granulite

54 54 54 54 61

61 61 61 61 64 65 66 67 83 83 83 84 04 84 85 85 85 93 93 95 96 98 99

GA 2785 69-1272 69-1276 69-1277 GA 2784 69-1233 69-1234 69-1235 69-1236 69-1243 69-1244 71-271 69-1280 69-1440 69-1441 69-1442 69-1282 69-1283 69-1292 69-1437 69-1438 69-1439 GA 3549A GA 35498 71-272 69-1278 69-1323 69-1324 Garnet Granulite B

43 43 44 44 44 45 45 45

69-1321 69-1322 GA 2797 69-1248 69-1249 GA 2798 69-1250 69-1251

1h3.4

Massive Granulite 138 139 140 141 141

GA GA GA GA GA

2794 2795 2796 2786 2789

A rubidium-strontium

chronology Table

of central

2. Regression

Age

(Myr)

Initial 87Sr/86Sr

MSWD

Model

1.7 7.6 92.0 6.4 49.1

1 3 2 1 2

Regressions Used in Age Determinations

Garnet Banded Banded Banded Banded Banded Garnet Garnet B.

1739

granulites

analysis

Number of Samples

A.

Australian

Granulite Granulite Granulite Granulite Granulite Granulite Cranulite Granulite

5 8 5 4 7 2 3 3

1607 1545 1606 1195 1549 1260 1210 1061

i + t * ? + ? i

28 36 300 115 50 48 122 110

0.7043 0.7033 0.6968 0.7076 0.7087 0.7137 0.7074 0.7129

t 0.0004 + 0.0008 ? 0.0172 i 0.0020 i 0.0005 + 0.0007 i-0.0011 t 0.0018

10 5 4 6 9 19 8 5

1055 958 1406 1650 1593 1533 1410 1331

? + L t t t i ?

155 135 50 84 74 60 153 146

0.7374 0.7416 0.7080 0.7042 0.7049 0.7023 0.7064 0.7055

t ? t ? * ? ? i

quoted

as

A2 - southern limb band - Station 61 - Station 54 - traverse - Station 93 B - Station 44 B - Station 45

0.04 7.2

1 1

Other Regressions Referred to in Text

Sillimanite Granulite Sillimanite Cranulitf - Station 115 Garnet Granulite A, Garnet Granulite A1 - with 69-1319 Garnet Granulite AL- without 69-1253, 69-1319 Banded Granulite - low initial ratio group Garnet Granulite B - all data Massive Granulite

Regression after the method of MCINTYREet al. (1966). All uncertainttes of *‘Rb is 1.42 x IO-” yr-‘. MSWD = Mean square of weighted deviates.

points to an isochron. All uncertainties are quoted at the two-sigma level and the decay constant for “Rb used is 1.42 x lo-” yr-’ (STEIGER and JLGER. 1977).

RUBIDIUMSTRONTIUM TOTAL ROCK MEASUREMENTS Rubidium-strontium total rock isochrons at Mt. Aloysius fall into three classes with 1550 Myr, 1200 Myr and uncertain ages. The data are presented in the following progression: garnet granulite A with an essentially 1550 Myr age; the banded grant&e with a dominantly 1550 Myr age, but some instances of 1200 Myr; garnet granulite B entirely 1200 Myr in age; and the sillimanite and massive granulites of indecisive age. The analytical data are listed in Table I and the details of regressed isochrons are given in Table 2.

Garnet

yranulitr

0.0087 0.0065 0.0005 0.0004 0.0010 0.0023 0.0018 0.0005

f2~.

143 53 6.5 21 10.8 38.7 24.9 2.5

Decay

constant

A---l 550 Myr

In traverses outward from the core of the Mt. Aloysius synform (i.e. away from the sillimanite granulite), three horizons can be recognised within the garnet granulite unit. Their relative thickness varies greatly and contacts are sometimes gradational, but they are persistent even though the unit as a whole pinches out on the eastern limb of the fold. The inner band. labelled garnet granulite A,, provides four samples from a single outcrop (station 91) and gives an age of 1406 k 50Myr with an initial ratio of 0.7080 + 0.0005 (Fig. 2). The central band is a massive quartz + plagioclase + orthopyroxene rock with a low Rb/Sr ratio which renders it unsuitable for dating (e.g. GA 2791-87Rb/8hSr = 0.022). The outermost horizon. garnet granulite A*, was sampled at three stations (86. 89, 92) dispersed around

076

I

GARNET

GRANULITE

2 2 1 2 3 3 3 1

A

I

1607*28 Myr 07043+0~004

0.72

Fig. 2. Total rock isochron diagram for garnet granulite A. Garnet Station 91. the massive band by station 142, and garnet granulite

granulite A, is represented A, by stations 86, 89 and

by 92.

C. M. GRAY

1740

/ 080 i-

BANDED

GRANULITE

STATION

61

/

37’2 i_..__..-.-.._ 0

and W.

10

?5 8iRb

/

/a5

15

2;

r

Fig. 3. Totai rock isochron diagrams for stations in the handed granuhte--A station 61; B station 54. the major fold closure; the six analyses outline a poor isochron with an age of 1650 $ 84 Myr. A discordant sample (69-l 319) is unique in containing one percent biotite. as the mineral is an accessory in all similar rocks. This evidence of unusual low grade modification justifies its removal from the regression. which then yields an age of 1607 5 28 Myr, initial ratio 0.7043 & 0.0004 (Fig. 2). The position of samples in the synform is not reflected in the data and garnet granulitc A, has essentially a single age throughout. The significant difference between the ages and initial ratios of garnet granulites A, and A2 is surprising as the rock types are identical and closely related. The disagreement is mainly due to one sample from the former (69-1253) and if it is rejected and all data combined the overall age becomes 1593 &- 74 Myr. The scatter in this isochron exceeds experimental error, but nonetheless the data are coherent and it seems that garnet granulite A, has a partially disturbed 1600 Myr isotopic system. Bauded gram&e--l

550 and 1200 Myr

The great areal extent of the banded gram&e requires that it be considered in terms of individual localities before the data are assembled for an overall view. Unless otherwise noted the rocks analysed are acidic gneisses with quartz and alkali and plagioclase feldspars as the major phases. (a) Fold closure--station 61. A 1600 Myr age is indicated within the fold closure. but the scatter of data points is great (Fig. 3A): the regressed age is 1600 $- 300 Myr. AlI rocks are typical quartz _t Kfeldspar + plagioctase t_ orthopyroxene granulites.

COMPSTON

(b) ‘~~~~~~~~ ~j~~~rff~io~ 54. A contrasting younger age in this area, 1195 i: It 5 Myr, initial ratio 0.7076 & 0.0020. has a large uncertainty because of the limited variation in Rb/Sr ratio in the rocks (Fig. 3B). (c) Northern lirn&-across-strike trucerse. The oIder age recurs in rocks taken from a traverse across three acidic granulite and two intervening basic granulite bands (stations 83.-85). The acidic granulites have a good spread in Rb/Sr ratio and define a 1549 + 50 Myr isochron and initial ratio of 0.7087 I_ 0.0005 (Fig. 4A); removal of the one divergent point (69-1441) does not produce a s~gni~cant reduction in uncertainty. The two basic granulites are isotopically distinct. One. 69-1442. has the mineral assemblage plagioclase + orthopyroxene + clinopyroxene + hornblende + biotite, the relatively high “Rb/‘%r ratio of 0.57, and is consistent with the above isochron. The other, 69- 1439 with a plagioclase + orthopyroxene + clinopyroxene + hornblende mineralogy, has a low a7Rb/8”Sr ratio of 0.009 and a distinctly lower initial ratio of 0.7040 & 0.0005. (d) Southern limb hmd. A prominent acidic granulite band 10m wide can be traced for 5 km in the southern limb of the fold (Fig. 1). Sampling stations (64-67 and 95-99) are spaced at intervals of 0.5 km along its length and the rocks obtained are very uniform in felsic mineralogy and RbjSr ratio; mafic minerals as orthopyroxene or less frequently garnet are in accessory amounts. The age recorded is 1545 ) 36 Myr with an initial ratio of 0.7033 i 0.0008 (Fig. 4B). (e) Banded specir?lertstation 93. A banded hand specimen was cut to provide material from adjacent layers of basic and acidic composition-samples GA 3549A and GA 3549B with the mineral assemblages plagiodase + orthopyroxene + opaque and quartz + K-feldspar + plagioclase + orthopyroxene respectively. The stabs of rock were 2 cm thick (8 x 3 x 2 cm) and directly opposed across a very sharp lithological contact. The connecting line between the two data points of Figure 8 gives an age of 1260 2 48 Myr and initial ratio of 0.7137 t 0.0007. (f) Compiled hmded grmulite data. Figure 4C displays all measurements on banded granulites. Two groupings of initial ratios can be discerned. The first (0.708) is found in the traverse rocks and characterizes the banded granulite near its contact with garnet granulite A. The remaining data. from closer to the contact with garnet granulite B. fall near an isochron of low initial ratio which regresses at 1533 If: 60 Myr, initial ratio 0.702 & 0.002. Gamer

g~~f~ff~jte i?-- 1200 Myr

Garnet granulite B has quite distinct Rb/Sr ratios at three sites due to modal variation in alkali feldspar even though the unit appears uniform and massive in the field (Fig. 5). Station 43 has no internal dispersion in Rb/Sr ratio and a spread in measured 87Sr/8hSr ratios exceeding experimental error.

A rubidium

strontium

BANDED

chronology

of central

Australian

granulitcs

GRANULITE

TRAVERSE

;:2/ 070;

A

1439’283 0

, 2

8

6

i

IO

12

87Rb/86Sr

0 85

BANDED

GRANULITE

SOUTHERN 080 -

LIMB

BAND

1545*36 My, 0 7033 * 0 ooa3

87s/86sr

---

!-

-1

:oo;

+’ BANDED ALL

+/ I

GRANULITE

DATA

0301

0

L

2

0

87Rb/

Fig. 4. Total

rock isochron diagrams

c /

b-1 6

IO

for the banded rranuliteP A across-strike limb band: C compiled data.

Stations 44 and 45 are located 250 m apart across strike and their data are described best by parallel isochrons. one for each station, giving model one ages of 1210 k 120Myr and 1060 f 1lOMyr and initial ratios of 0.7074 _t 0.0011 and 0.7129 k 0.0018 respectively. The two results pool as a single age of 1128 + 80 Myr. Alternatively. if all samples from the unit are combined in a single isochron the result is 1410 i 150Myr. initial ratio 0.7064 + 0.0018. with scatter about the line exceeding experimental error.

12

It.

/%,r

traverse:

The first interpretation is preferred are taken to record the same age.

B southern

and both stations

Analyses of sillimanite granulites from two stations (90, 115) are scattered on an isochron diagram (Fig. 6). The only distinctive feature of the rocks aside from the pelitic composition (GRAY. 1977) is slight alteration of alkali feldspar. Little weight can be attached to the age indicated (1060 + 150 Myr, initial ratio

C.

1742

1128*80

M. GRAY

and W.

COWSTON

Myr

STATIONS

I

070

I . 4

I

0

05

10

43 44 45

15

20

87Rb/86Sr

Fig. 5. Total rock isochron diagram for garnet grantilite B.

0751

Fig. 6. Total rock isochron diagram for the sillimanitc granulite. A nominal isochron is shown. 0.737 f 0.009) as it is greatly dependent

on sample 69-1242. If the two stations are taken individually, 115 yields an age of 960 1: I30 Myr with an initial ratio of 0.742 + 0.007. while 90 is at best crudely consistent with it. A distinctive band within the unit (GA 2790--quartz + plagioclase + orthopyroxene) is very unradiogenic and has an initial s7SrishSr value of 0.7037 + O.OtXE.Ages for individual sillimanite-bearing samples calculated using this initial ratio range from 1500 to 1700Myr.

The massive granulite samples come from dispersed sampling sites (I 38-f 41) and only justify a reconnaissance study because of poor enrichment in radiogenic Sr (Fig. 7). The age of 1330 + 150Myr. initial ratio 0.7055 + O.0005. has a high uncertainty and can be regarded only as an approximation because spot samples have no guarantee of a common initial ratio.

The isotopic ages (Table 2) are taken to be bimodal about pooled means of 1578 1 20 Myr and 1222 i: 39 Myr. The former is developed in precise isochrons from garnet granulite A and several parts of the banded granulite. The latter is more subjective and is based upon well-fitted though imprecise isochrons for single stations (44, 45. 54, 93). The lack of precision of these isochrons is attributed to small dispersion in Rb/Sr ratios and the vagaries of the Sr

Fig. 7. Total

rock isochron diagram granulite.

for the massive

migration involved in their formation. Their coherence is shown in Fig. 8 which compares the younger lines with a 1550 Myr reference. The mean is confirmed by the addition of four other results between 1170 and 1220 Myr from nearby areas, to produce an overall pooled age of 1189 rt_ 9 (GRAY, in prep.).

A rubidium-strontium

chronology

of central

Austrahan

1143

granuhtes

1

1

Fig.

8. Total

rock Station

isochron diagram 93 is the location

comparing 1200 Myr-isochrons with a 1550 Myr of the banded specimen from the banded granulite.

The alternative interpretation of a continual history from 1600 to IOOOMyr is unlikely to be correct because the few ages outside of the two populations are either derived from badly scattered data (e.g. sillimanite granulite) or liable to criticism in detail (e.g. garnet granulite A 1, massive granulite). A geographic distinction can be made between the two age groups; the older age is concentrated around the fold closure in garnet granulite A, while the younger is found in the vicinity of garnet granulite B. It is suggested that two age domains occur on Mt. Aloysius. There is no obvious relationship between the isotopic age distribution and either lithology or geological structure. The ages are independent of the position of stations in the major synform. and the only possible structural effect is the badly scattered 1600-Myr isochron at station 61 in the banded granulite, which is located in a complex fold closure. RUBIDIUM-STRONTIUM

reference.

The feldspar ages arc derived from isochrons which join coexisting alkali and plagioclase feldspars via the appropriate total rock. All lines fit to within experimental error and three stations (45, 61. 92) yield essentially the same result (Fig. 9) which pools as a common age of 816 + 14 Myr. The younger value for station 54 of 741 & 38 Myr may be related to retrogressive metamorphism associated with minor move-

OXL 372 I

0701 0

I 3

i

MINERAL

AGES

Feldspar and biotite ages are significantly younger than the total rock determinations and are independent of lithology or location (Table 3).

Fig. 9. Feldspar lsochron diagram for single rocks from garnet granulite A. station 92 (69-1255). banded granulite, station 61 (GA 2784). banded granulite. station 54 (GA 2785). garnet granulite B. station 45 (GA 2798). KF = Kfeldspar. TR = total rock, PL = plagioclase.

Table 3. Analytical data and ages for mineral

separates

/ Station

45

54

61

92

83 139 143

Sample Number

GA 2798 W ,I " TR 1, I1 PL GA 2785 KP " " TR 11 4' PL GA 2784 KF II I* TR II !' PL 69-1255 KF 81 " TR ,I 1f PL 69-1442 BI GA 2795 BI 69-179 BI

5

Rb PP~

277.0 134.1 39.9 317.4 127.4 29.1 429.8 201.9 21.0 213.3 111.8 8.8 391.8 410.4 418.0

KF = K-feldspar: TR = Total Rock:

Initial 87sr/86Sr

Sr PPrn

515.7 390.2 438.6 540.6 348.9 226.8 305.0 198.1 92.4 487.9 322.1 73.6 14.7 63.0 34.7

PL = Plagioclase:

0.7165 i 0.0006

0.7144 5 0.0006

812 i 16

0.7296 t 0.0006

844 i 52

0.7160 I 0.0007

723 ? 9 740 i 15 728 i ii

BI = Biotite

C. M. GKAY and W. COMPSTON

1744

16 6

16 0

17 0

17 2

17L

176

‘“-Pb. “‘“Pb ““‘Pbs”‘4Pb model diagram comparing granulites from Mt. Aloysius (solid dots) with the dis(ribution of 1050 Myr-old rocks (ZARTMANand WASSERRURC.1969). The granulite data arc corrected for irl siru decay of U subsequent to I200 Myr. The straight lines are primary isochrons and the curves are growth curves for “‘U ‘““Pb of X.2 and X.5. Fig. IO.

ment on the nearby fault zone; mylonitised granulite from the fault is the only rock in the area in which epidotc occurs as a reaction product on plagioclase. The age is comparable to those obtained on biotites. Biotite is a rare phase at Mt. Aloysius owing to the high metamorphic grade and anhydrous nature of the rocks. It occurs in the primary mineralogy of some thick basic granulite bands (69-179, 69-1442). and in some quartz-bearing rocks as uncommon, very minor alteration to mafic minerals (GA 2795). Three age determinations arc in good agreement at 730 Myr and arc virtually independent of the assigned initial ratio as calculated from the total rock samples. CRANIUM-LEAD

ISOTOPIC

MEASUREMENTS

A detailed discussion of the I!-Pb isotopic system of granulites from Mt. Aloysius is contained in GRAY and OVERSBY(1972). Aspects of that work are relevant to the Rl+Sr data and are reconsidered in the light of the recent change in U decay constants. The samples analysed were very similar rocks from areas with 1550 and 1200Myr Rb-Sr total rock ages [garnet granulite A, (samples 69-1255. 69-132& 1550Myr) vs garnet granulite B (samples GA 2797. GA 2798-I 200 Myr) and banded granulite southern limb band (samples 71-271. 71-272p1550Myr) vs banded granulite station 54 (samples GA 2785 (= 71-273). 69-IY--1200 Myr)]. Their Pb isotopic compositions are independent of rock type or Rb-Sr age group: Pb in both total rocks and K-feidspars is extremely unradiogenic and measured isotopic ratios approximate initial values. Clearly, the granulites lost substantial amounts of U in the past and the initial Pb isotopic compositions refer to that time. The U-Pb method has two applications in this in238” ,$04Pb_ >ohPb 2”4Pb stance. The first employs a isochron approach to total rocks and feldspars.

attempting to date the time at which the mineral phases of single rock samples were last isotopically homogenized. The low 238U:‘“JPb ratios in the rocks (less than two) result in large uncertainties in the ages of individual samples, but the mean value is 1300 + 110 Myr (see GRAY and OVERSI~Y.1972). The second approach utilises the ‘model lead’ system to obtain a model age for the initial Pb of total rocks and minerals. Given the ‘age of the Earth’ (4550 Myr). the initial isotopic composition of the Earth (meteorite troilite values--TATsuMoro et u/.. 1973) and the decay constants of 235U and 238U (STEIGER and JAGER. 1977). the isotopic evolution of Pb in the Earth can be described as a function of time. Any Pb isotopic composition has a theoretical age in the context of this scheme. The model system is depicted on a ‘“‘Pb;‘“4Pb”‘hPb/-O”pb diagram where isotopic evolution is shown by growth curves appropriate to particular 23XU!204Pb (p) values and contemporaneous samples lit on straight hnes or primary isochrons. The current U decay constants result in Proterozoic or younger samples having model ages much lower than true ages (OV~RSBY, 1974). As an example. the central Australian analyses are plotted in Fig. IO--the mean model age is 858 F 10 Myr. However. the model system can be modified to obtain a better estimate of true ages. An empirical calibration starts by determining the range of initial Pb isotopic compositions in rocks of known age. ZARTMAN and WASXRBURC;(1969) have done this for rocks with a mean age of 105OMyr. Comparison of the rocks from Mt. Aloysius with this distribution establishes an age of approx 12OOMyr for the granulites (Fig. 10). Alternatively. STACEY and KRAM~RS (1975) have considered a two-stage evolution for terrestrial Pb in order to optimise the agreement between model ages and true ages for conformable Pb ore bodies. On that basis the ‘modified model age’ of the granulites is I285 Myr.

A rubidium-strontium

chronology

The U-Pb isotopic system records the time of pronounced loss of U from the granulites as approximately 1250 Myr. and this can be equated with the 1220 k 40 Myr Rb-Sr total rock age; an older 1550 Myr event is not detected. INTERPRETATION Mount Aloysius displays two Rb-Sr total rock ages, yet is clearly a geological entity without major structural breaks or intrusive relationships. Consequently, the age difference cannot be due to the addition of new material at 1200 Myr to a pre-existing 1550 Myr terrain. The younger age is characterized by a series of parallel isochrons, one per sampling site, with small dispersion in Rb/Sr ratio and initial ratios ranging to relatively high values (0.7074.714). This association can be interpreted in terms of the Sr isotopic homogenisation of 1550 Myr-old rocks at 1200 Myr. Consider a 1550 Myr isochron defined by two stations each of which supplies a number of samples and where one station has a distinctly higher Rb/Sr ratio than the other. The internal Sr isotopic homogenisation of each station at 1200 Myr would produce two internal isochrons of zero gradient that would record the time of homogenisation. The new isochrons would pivot about the point for the mean Rb/Sr ratio of the stations and lie rn r&Jon along the trend of the 1550 Myr isochron; their initial ratios would be proportional to the mean Rb/Sr ratios. The 12OOMyr isochrons at Mt. Aloysius provide just such a distribution in stations (44, 45) 250m apart across strike in garnet granulite B (Fig. 8). The scale of internal isotopic homogenisation must be less than 250 m and is more probably less than outcrop dimensions (10 m) as measurements at station 43 scatter in excess of experimental error. Station 54 in the banded granulite reinforces this view with a third isochron situated 600 m from station 45. The hypothesis of regional isotopic homogenisation (PIDGEON, 1967; ARRIENS and LAMBERT.1969) can be rejected unequivocally for this environment. Isotopic homogenization is detected most readily along the interface between chemically distinct systems in which a small degree of mixing causes a large deviation from undisturbed isotopic ratios. This phenomenon can be seen in the juxtaposed basic and acidic layers of specimen GA 3549 from the banded granulite (Fig. 8). It clearly records an updating event even though nearby total rock measurements do not. and shows that the updating phenomenon occurred throughout Mt. Aloysius. The 1200 Myr ages attributed to isotopic homogenisation would be conventionally equated with the time of metamorphism. Supporting evidence is found in a 1204 i 17 Myr result for an augen gneiss at Minno (50 km to the east of Mt. Aloysius) interpreted to be of anatectic derivation during the granulite metamorphism (GRAY, in prep.). The U-Pb system

of central Australian

granulitcs

1745

records the same event throughout Mt. Aloysius in isotopic homogenisation of Pb between feldspars and through the removal of much of the U originally present in the rocks. The latter is almost certainly a metamorphic phenomenon (L.&MWKT and HEIER. 1967, 1968) and is evident in most rocks sampled (GRAY. 1977). Isochrons that give I550 Myr ages have relatively large ranges in Rb,/Sr ratio, characterisc large areas or entire rock units and show a modcrate range in initial ratio which includes low values (0.703.-0.709). These features are not diagnostic of any particular origin for the isochrons and do not conform to the pattern of outcrop-scale isotopic homogenisation found in 1200 Myr systems. If the 1550 Myr ages record a high grade metamorphism. the accornpanying movement of Sr must have occurred on the scale of kilometres to give uniformity of isotopic composition throughout lithological units. Furthermore, migration of Sr must have occurred along strike and within individual units as each has a characteristic initial ratio: garnet granulite A and the southern limb band of the banded granulite with initial ratios of 0.7043 and 0.7033 respectively, are separated by an horizon within the banded granuhte with the higher initial ratio of 0.7087. The large scale htrmogenisation of acidic granulites within the banded granuiite should have resulted in identical initial ratios in the interlayered basic granulites. Of the two basic bands investigated one (69-1442). which contains biotlte as a major phase. is in isotopic equilibrium with its felsic neighbours-initial ratio 0.7087 at I550 Myr, Nearby, the other (69-1439) with an initial ratio of 0.704 shows no sign of equilibration and separates t&o acidic bands with initial ratios of 0.7087. To explam these observations as a metamorphic effect require.5 the postulation of a very pronounced amsotropy to channel Sr along lithological layers. Such an anisotropy is unlikely to develop under granuhte facles metamorphism of anhydrous rocks, and must have been destroyed subsequently. Because across-strike transfer of Sr is equally probable on the scale of Mt. Aloysius. the metamorphic interpretation is unlikclq to bc correct. The low initial ratios relative to Rh?jr ratios of some of the rocks would require that metamorphism occur shortly after their supracrustai genesis unless large amounts of unradiogenic Sr arc introduced into some units. units of The isotopic integrity of granuiite 1550 Myr age is matched by that of volcanic or sedimentary strata. A well-mixed lava flow of conslderable extent may form a single isotopic system. as can succeeding lavas with quite different isotopic properties. Sedimentary mlxmg processes also induce uniformity in individual horizons. The low initial ratios (-0.704) restrict any pre-1550 Myr history to less than 1OOMyr and are consistent with the suggested volcanic parent rocks. Consequently. the Sr isotopic data provide a restrained argument for a supracrustal origin of the 1550 Myr age.

C. M. GRAY and W. COMPSTON

I746

The Pb isotopic results are decisive even though they do not register a 1550 Myr event. The common Pb in the granulites appears to have evolved in a normal crustal environment until 1200 Myr as its isotopic composition falls into the expected spectrum of Pb of this age. Therefore, the geological activity at 1550 Myr cannot have been such as to significantly change U/Pb ratios. Granulite facies and probably lower grade metamorphism can be rejected as possibilities because the accompanying depletion in U (LAMBERT and HEIER, 1967, 1968) decreases U/Pb ratios greatly. The supracrustal formation of the rocks is less likely to cause substantial changes in U/Pb ratios and is more in keeping with the isotopic evidence. It is postulated that the original sedimentary and/or volcanic genesis of the granulites occurred at I550 Myr. The I550 Myr-old Sr isotopic systems are still those of the original supracrustal rocks. The effects of the later 1200 Myr metamorphism were minimal and are seen in a few erratic data points influenced by small scale isotopic exchange. The mineral results can be interpreted in conventional terms whereby an age is initiated on the cooling of a mineral through its Sr diffusion threshold. Studies of contact metamorphism (HART et al., 1968) indicate that the critical temperatures are 50@55O”C in feldspars and 20@25o”C in biotites. hence these temperatures were reached at 820 and 730 Myr respectively. Moderately high temperatures continued for some 400Myr after the climax of metamorphism and closure of the biotites reflected the tectonic uplift of the region (ARMSTRONG, 1966). DISCUSSION

The updating of anhydrous total rocks must be extraordinarily difficult because intermediate-pressure granulite grade of metamorphism was required to initiate the resetting of RbSr total rock systems at Mt. Aloysius. The extent to which the migration of Sr can cause effective isotopic homogenisation is determined by the physical nature of a rock. For example. local structural or textural features may facilitate the movement of elements. More potent is the chemical uniformity of a lithological unit, as the registration of a metamorphic age requires communication between rocks of different isotopic composition. Thus chemically layered lithologies are more liable to record disturbance whereas rock units with large volumes that are isotopically uniform are difficult to update because Sr must move over great distances to alter the isotopic composition of any one site. Accordingly. the extent of development of metamorphic total rock ages at Mt. Aloysius may be a function of different scales of layering. This is not believed to be the case generally because as far as could be judged in the field the various sampling sites were litholoaically comparable. However, indecisive age determinations in the sillimanite granulite might be due to incomplete isotopic mixing governed by local factors.

Isotopic homogenisation is recognised on several scales, between mineral grains (millimetres), across lithological contacts (centimetres) and between total rocks (metres). It is proposed that as temperature increases during prograde metamorphism, the range of Sr diffusion increases. firstly exceeding the dimensions of mineral grains and leading in the granulite facies to ranges of metres. At this stage isotopic communication between total rock systems is possible and preexisting ages are erased. Moreover. a distinct diffusion threshold will cause a relatively sharp demarcation between equilibrated systems and nearby areas of slightly lower temperature with unaffected total rock ages. On this simple model Mt. Aloysius represents an area where metamorphic temperatures reached, but did not exceed the threshold level; metamorphic petrology gives 850°C as an appropriate temperature. On the decline of metamorphism. the cessation of isotopic mixing will be correspondingly sharp fixing relatively precise total rock ages. Successively, the smaller isotopic systems of the minerals will close leading to a geochronological hierarchy of closure: total rocks first at approx 850°C. feldspars at SOO’C and biotites at 250-C. Total rock RbSr isochrons for granulites, regardless of precision, need not register the time of metamorphism. The possibility of preserved premetamorphic ages causes a single isochron to be of limited value. Given a number of isochrons interpretation may be clarified by diagnostic features such as the parallel isochrons characteristic of isotopic homogenisation. Failing this independent temporal control on the metamorphism is required for an isotopic age to be meaningful. Ack,lowlrdge/t~rilfs_.M.G. gratefully acknowledges the financial support of a C.S.I.R.O. Postgraduate Studentship at the Australian National University. Dr. J. VEIZER provided invaluable assistance in the field and M. J. VERNON, D. J. MILLAR and M. COWAN gave technical instruction. Dr. P. A. ARRIENS wrote the computer programs that greatly facilitated the reduction of mass spectrometer data. Drs. R. C. PRICE, P. M. HURLEY, J. M. MATTINSON and G. R. TILTON kindly read and criticized the manuscript. HELENE ENGLISH and JANE. MORRIS are thanked for typing the manuscript. REFERENCES ARMSTRONG R. L. (1966) K-Ar dating of plutonic and volcanic rocks in erogenic belts. In Potassium Argorl Daling (eds. 0. A. Schaeffer and J. Zahringer). pp. 117-133. Springer. ARRIENS P. A. and COMPS~ON W. (1968) A method for isotopic ratio measurement by voltage peak switching. and its application with digital output. Int. J. Mass Specrrom. Ion Phys. 1, 471481. ARRIENS P. A. and LAMBERT 1. B. (1969)

On the age and

strontium isotopic geochemistry of granulite-facies rocks from the Fraser Range, Western Australia. and the Musgrave Ranges. central Australia. Spec. Pub/s geol. SOC. Aust. 2. 377-388. COMPSTON W. and JEFFERY P. M. (1959) Anomalous

mon strontium’ COMPSTON

W.,

in granite.

Nature

‘com-

184, 1792~-1793.

LOVERING J. F. and VERNON M. J. (1965)

The rubidium-strontium

age of the Bishopville

aubrite

A rubidium-strontium

chronology

and its component enstatite and feldspar. Geochim. Cosmochim. Acta 29, 1085-1099. DANIELS J. L. (1971) Australia 1:250.000 Geological Series Sheet SG52-IO, Cooper. Geological Survey of Western Australia. GCI~DE A. D. T. and MOORE A. C. (1975) High pressure crystallization of the Ewarara, Kalka and Gosse Pile Intrusions, Giles Complex, central Australia. Contrih. Mineral. Petrol. 51, 77-97. GRAY C. M. (1971) Strontium isotopic studies on granulites. Unpublished Ph.D. thesis, Australian National University. GRAY C. M. (1977) The geochemistry of central Australian granulites in relation to the chemical and isotopic elTects of granulite facies metamorphism. Contrib. Mineral. Petrol. 65, 79989. GRAY C. M. and OVERSBY V. M. (1972) The behaviour of lead isotopes during granulite facies metamorphism. Geochim. Cosmochim. Acta 36, 939-952. GREEN D. H. and RINGW~CJD A. E. (1967) An experimental investigation of the gabbro to eclogite transformation and its petrological applications. Geochim. Cosmochim. Acta 31, 767-833. HART S. R., DAVIS G. L., STEIGER R. H. and TILTON G. R. (1968) A comparison of the isotopic mineral age variations and petrologic changes induced by contact metamorphism. In Radiometric Dating for Geologists (eds. E. I. Hamilton and R. M. Farquhar), pp. 733110. Interscience. HENSEN B. J. and GREEN D. H. (1972) Experimental study of the stability of cordierite and garnet in pelitic compositions at high pressures and temperatures-II. Compositions without excess alumino-silicate. Contrih. ‘Mineral. Petrol. 35. 331-354. KR~CH ‘I’. E. and DAVIS G. L. (1969) Old isotopic ages in the northwestern Grenville Province, Ontario. Grol. Assoc. Can. Spec. Pap. 5, 189-192. KR~GH T. E. and DAVIS G. L. (1970) Paragneiss studies in the Georgian Bay area 90 km Southeast of the Grenville Front. Carnegie Inst. Wash. Yearb. 69, 339-341. KROGH T. E. and DAVIS G. L. (1973) The effect of regional metamorphism on U-Pb systems in zircon and a comparison with Rb-Sr systems in the same whole rock and its constituent minerals. Carnegie Inst. Wash. Yearh. 72. 601-610. LAMBERT I. B. and HEIER K. S. (1967) The vertical distribution of uranium. thorium, and potassium in the Continental Crust. Geochim. Cosmochim. Acta. 31, 3777390. LAMBERT I. B. and HEIER K. S. (1968) Geochemical investigations of deep-seated rocks in the Australian Shield. Lithos I, 3c-53. MCINTYRE G. A., BRINKS C., COMPSTON W. and TUKEK A. (1966) The statistical assessment of RbSr isochrons. J. Geophys. Res. 71, 5459-5468. NESBITT R. W., GDDDE A. D. T., MOORE A. C. and HOPWOOD T. P. (1970) The Giles Complex, central Australia: a stratified sequence of mafic and ultramafic intrusions. Spec. Pubis. Geol. Sot. S. Afi. 1. 547-564. OVERSBY V. M. (1974) New look at the lead isotope growth curve. Nature 248, 132-133. PIDGEON R. T. (1967) A rubidium-strontium geochronological study of the Willyama Complex, Broken Hill, Australia. J. Petrol. 8, 283-324. STACEY J. S. and KRAMERS J. D. (1975) Approximation of terrestrial lead isotope evolution by a two-stage model. Earth Planet Sci. Lett. 26. 207-221. STEIGER R. H. and J;~GER E. (1977) Subcommission of geochronology: convention on the use of decay con-

of central

Australian

1747

granulites

stants in geo- and cosmochronology. Earth Plunet. Sci. Lett. 34. 359-362. TATSUMOTO M.. KNIGHT R. J. and ALLEGRO C. J. (1973) Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206. Science 180, 1279-1283. WILSON A. F. (1969) Granulite terrains and their tectonic setting and relationship to associated metamorphic rocks in Australia. Spec. Publs. Geol. Sot. Ausl. 2. 243-258. ZARTMAN R. E. and WASSERBURG G. J. (1969) The isotopic composition of lead in potassium feldspars from some I.0 Byr-old North American igneous rocks Geochim. Cosmochim. Acta 33, 901-942.

APPENDIX LITHOLOGICAL UNITS MAPPED AT MT. ALOYSHJS Sillimanite granulite The unit has a pelitic composition and a quartz + Kfeldspar + garnet + sillimanite + plagioclase mineralogy. Hand specimens show centimetre-scale layering and a pronounced lineation defined by prisms of sillimanite. Outcrops are buff-coloured and blocky. Garnet granulite A Garnet granulite A comprises three horizons, two garnet granulites separated by a massive granulite band. The garnet granulites have subacidic to intermediate compositions and a mineralogy based on varying proportions of quartz + K-feldspar + plagioclase + garnet + orthopyroxene. They are uniform in outcrop with garnet porphyroblasts in wavy lines defining the foliation. The central massive granulite band has a plagioclase + orthopyroxene k quartz mineralogy and an intermediate composition. It outcrops as smooth chocolate-coloured boulders which show occasional quartz schlieren. Thin horizons of a comparable lithology are common within the garnet granulitcs. Banded granulite The characteristic feature of the unit is interlaying of bands of acidic and basic composition with the former predominating. The felsic rocks are of quartz + K-feldspar + plagioclase k orthopyroxene k garnet mineralogy and outcrop as uniform layers up to five metres thick. Basic granulite occurs on two scales: layers (plagioclase + orthopyroxene) one centimetre thick in acidic granulite form a distinctive banded rock type; horizons (plagioclase + orthopyroxene + clinopyroxene + hornblende f biotite) five metres thick outcrop as jet-black boulders, The proportion of acidic to basic lithologies is difficult to estimate but is of the order of 20: 1. Contacts are invariably sharp. Mesoscopic similar folds are common in thinly banded rocks. Garnet granulite B The unit is similar uniform composition.

to garnet

granulite

A. but has a more

Massive granulite A very homogeneous quartz + plagioclase + orthopyroxene & K-feldspar k garnet lithology which outcrops as irregularly-shaped smooth brown boulders: it has a granodioritic composition.