Timing of superposed volcanism in the Proterozoic Mount Isa Inlier, Australia

Precambrian Research, 21 (1983) 223--245

223

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

TIMING OF SUPERPOSED VOLCANISM IN THE PROTEROZOIC MOUNT ISA INLIER, A U S T R A L I A

R.W. PAGE

Bureau of Mineral Resources, Geology and Geophysics, Canberra, A.C.T. (Australia) (Received October 6, 1981; revision accepted November 30, 1982)

ABSTRACT Page, R.W., 1983. Timing of superposed volcanism in the Proterozoic Mount Isa Inlier, Australia. Precambrian Res., 21 : 223--245. Metamorphosed Proterozoic successions in the Mount Isa Inller contain a variety of volcanic suites interspersed at several intervals throughout the 30 kin-thick stratigraphic pile. The U--Pb isotopic systematics of zircons crystallised in superposed feisic volcanic units allow detailed definition of the chronology of this volcanism and, therefore, of the major depositional cycles. This supracrustal development spanned a 270 Ma interval in the early to mid-Proterozoic. New U--Pb zircon data from the earliest recognised volcanic cycle (Leichhardt Metamorphics) yield a pooled age for this event at 1867 ± 5 Ma. A far northern segment of the same basement succession contains a geochemically similar felsic sequence 1852 ± 7 Ma old. These results confirm the coevality between this felsic volcanism and nearby intrusive bodies of the same age. The second major cycle of volcanism is 60--80 Ma younger than the first, this interval representing the m a x i m u m duration of the intervening regional unconformity. Dacitic and mafic volcanic rocks (Bottletree Formation), which unconformably overlie the western flank of the 1870 Ma basement, are dated in two areas at 1790 ± 10 and 1808 ± ~. Rhyodacites (Argylla Formation) on the eastern side of the basement have a similar age, 1783 ± 5 Ma, so for purposes of correlation and crustal evolution, these two sequences flanking either side of the basement can be considered to comprise a single volcanic cycle. Subsequent cycles of feisic volcanism are recognised in stratigraphically younger sequences which include rhyolites with well resolved U--Pb zircon discordance patterns indicating ages of 1720 ± 7 Ma (Doherty Formation) and 1678 ± 3 Ma (Carters Bore Rhyolite). The youngest volcanic cycle (1603 +- 6 Ma) is evidenced by rhyolitic lavas in a part of the sequence previously considered to be much older, at least 1740 Ma. Recognition of such a young volcanic episode has an important economic bearing, for it implies that rocks equivalent in age to the prospective Pb--Zn-rich Mount Isa Group in the west (ca. 1670 Ma) could well exist in the eastern succession of the inlier.

INTRODUCTION

A variety of felsic and mafic volcanic suites is interspersed at several stratigraphic levels throughout the metamorphosed supracrustal successions (ca.

0301-9268/83/$03.00

© 1983 Elsevier Science Publishers B.V.

224 30 km thick) that compose the Mount Isa Inlier. Mapping within this meridionally-trending inlier, some 400 km long and 100 km wide, reveals an extensive mid-Proterozoic stratigraphic history. This has given rise to divergent tectonic interpretations, from evolution in a cratonic (? continental) margin environment (Wilson, 1978; Plumb et al., 1980), to that of an extensional rifting regime (Bultitude and Wyborn, 1982). The successions have suffered intense deformation, regional metamorphism, and intrusion of abundant granitic batholiths, some of which have coeval volcanic counterparts. The purpose of this study of felsic volcanic units is to refine and extend, stratigraphically and geographically, the geochronological framework for this volcanic evolution, initially established by Page (1978). Documentation of such an age framework provides essential data for framing evolutionary models. Although these rocks have been metamorphosed under varying degrees of greenschist- to amphibolite-facies conditions, relict igneous textures and minerals are partly retained. In particular, past experience shows that the U--Pb zircon systems in volcanic rocks can withstand isotopic perturbations due to low- to medium-grade post-crystallisation processes, and mostly provide precise, stratigraphically meaningful ages. An objective of the present study is to consider the stability of zircon U--Pb systems in the felsic volcanic sequences at somewhat higher grades of regional metamorphism. In general, the metamorphic grade is much higher in the southern than in the northern parts of the inlier. Given a suitably high metamorphic regime, the mineral phase zircon could: (1) recrystallise; (2) lose some radiogenic daughter product; (3) host new zircon growth on the pre-metamorphic nucleus; or (4) suffer intricate combinations of such episodes. Since there are independent geological constraints in the Mount Isa Inlier, as well as other relevant U--Pb zircon information at lower metamorphic grades, these perturbed zircon systems will, if they exist, be recognisable and possibly interpretable. ANALYTICALMETHODS Zircon was extracted from 50 kg samples of porphyritic felsic lavas by crushing and separation procedures involving a disc grinder, Wilfley concentrating table, heavy liquids and Frantz magnetic separator. The less magnetic zircon fractions, sized on a nylon mesh screen, were hand-picked to 100% purity under a binocular microscope, and washed in hot 7N HNO3, water and acetone to remove surface contamination. Digestions with HF(-HNO3) in teflon bombs, anion-exchange chemistry, and mass spectrometry followed procedures established by Krogh (1973). Lead isotopic measurements employing the silica-gel technique, and U measurements using Ta2Os as an activator were made on a 30 cm-radius, 60 ° sector Nuclide Analysis Associates mass spectrometer. Total Pb blank levels ranged from 0.1 to 0.5 ng, the average being 0.25 ng; the U blank is normally measured at < 0.1 ng. A modified York (1969) program was used to regress the U--Pb data, and

225 t h e r e s u l t a n t age u n c e r t a i n t i e s o f c o n c o r d i a i n t e r s e c t i o n s are given at t h e 2-0 level; this uses e s t i m a t e d 2-o e r r o r a s s i g n m e n t s o f 0.7% f o r U / P b ratios, 0.2% f o r 2°TPb/2°6Pb ratios, a n d a c o r r e l a t i o n c o e f f i c i e n t o f 0 . 9 6 derived f r o m t h e t r e a t m e n t o f L u d w i g ( 1 9 8 0 ) . All ages r e f e r r e d t o in this p a p e r are c a l c u l a t e d using d e c a y c o n s t a n t s r e c o m m e n d e d b y t h e I U G S S u b c o m m i s s i o n o n Geoc h r o n o l o g y (Steiger a n d J~iger, 1 9 7 7 ) . SAMPLE LOCATIONS Details o f s a m p l e l o c a t i o n s are listed in T a b l e I. TABLE I Sample locationlist Unit

Sample No.

Field No.

1:100 000 sheet, grid reference

P P Daj Du Du My

Prospector Prospector Dajarra Duchess Duchess Myally

807407 799420 683890 712587 585510 714513

Prospector Marraba Merlin

818388 204853 250102

B.M.R.

A.N.U.

Leichhardt Metamorphics

7320.5128 7320.5129 7920.5312 7920.5317 7720.5011 7920.5305

74-367 74-368 -----

Argylla Formation

7320.5121 7920.5307 7920.5321

74-360 P 10.61.2 --Met 4.18.C166

Bottletree Formation

7720.5013 7920.5318

---

Du 2.8098.M14 Duchess 515540 Du 2.87.1 Mary Kathleen 503779

Doherty Formation

7920.5319

--

Sel 2.73.C148

Tommy Creek area rhyolite microgranite

7920.5308 7720.5010

---

9.13.6 9.13.7 8.46.C42 6.16.E91 3.8132.M13 12.84.1

---

Selwyn

887148

Marraba Marraba

143041 234075

G E O L O G I C A L RELATIONSHIPS, EXISTING ISOTOPIC A G E C O N S T R A I N T S , A N D SPECIFIC OBJECTIVES

T h e M o u n t Isa Inlier (Figs. 1 a n d 2) is a w e l l - e x p o s e d p a r t o f t h e e a s t e r n m a r g i n o f t h e A u s t r a l i a n P r e c a m b r i a n Shield. I t c o m p r i s e s t h r e e f u n d a m e n t a l b l o c k s : (1) a n o r t h - t r e n d i n g c e n t r a l b a s e m e n t igneous succession, t h e Kalkad o o n - - L e i c h h a r d t b l o c k , s e p a r a t i n g (2) a y o u n g e r e a s t e r n succession, a n d (3) an a p p r o x i m a t e l y e q u a l l y y o u n g w e s t e r n succession. This b r o a d sub-division ( C a r t e r e t al., 1 9 6 1 ; D e r r i c k e t al., 1 9 7 7 ) has r e c e n t l y b e e n q u e s t i o n e d (Bul-

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227

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(1979).

titude et al., 1977; Blake, 1980, 1981). In the present paper, objective evaluation of these divergent geological interpretations is sought by attempting to resolve relatively small age differences within specific parts of the volcanic pile. The relevant geological and isotopic constraints are outlined below. Volcanic enclaves of the Kalkadoon--Leichhardt basement block

A belt, > 4 km thick, of deformed and metamorphosed felsic volcanic and volcaniclastic rocks constitutes the oldest known unit (Leichhardt Metamor-

228 phics--Tewinga Group, Carter et al., 1961) east of Mount Isa (Figs. 1 and 3). This is part of the Kalkadoon--Leichhardt basement block (Derrick et al., 1977). An earlier U--Pb zircon study (Page, 1978) showed that volcanics of the Leichhardt Metamorphics in the northern part of the block were erupted 1865 + 3 Ma ago, and were intruded by coeval granitic rocks (Kalkadoon Granite) whose pooled U--Pb age was found to be 1862 _~+ ~7 Ma. In the southern part of the Kalkadoon--Leichhardt block, and broadly on strike with the above isotopically studied sequence, Bultitude et al. (1978) and Blake (1980) report further felsic (and some mafic) volcanic sequences, informally termed 'Standish volcanics'. These are metamorphosed to the greenschist facies and were considered to be much younger than the volcanism of the Leichhardt Metamorphics, because of an interpreted unconformable relationship with the Kalkadoon Granite. A further unit, delineated by Bultitude et al. (1978) as 'Undivided Tewinga Group', also lies within the Kalkadoon--Leichhardt basement block. This consists of greenschist- to amphibolite-grade volcanic and sedimentary rocks. Most of the former are i-(z:

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229

felsic types, but are extensively recrystallised to a gneissic (migmatitic) fabric and have only sparse textural evidence of their original volcanic nature. Argylla Formation

This unit conformably overlies, but interfingers near its base with the Magna Lynn Metabasalt which, in turn, unconformably overlies the 1865 + 3 Ma Leichhardt Metamorphics volcanic succession (Fig. 3). Argylla Formation rhyodacites in the northern part of the Mount Isa Inlier give a zircon U--Pb age of 1777 + 7 Ma (Page, 1978). The Duck Creek Anticline (Figs. 2 and 3) contains further felsic volcanic components considered equivalent to the isotopically-dated (1777 Ma) sections of ArgyUa Formation. Page (1978) reported anomalously young Rb--Sr total-rock results from these rocks. The present paper attempts to reconcile their stratigraphic standing by means of U--Pb zircon measurements on relatively l o w , f a d e felsites in the northern part of the Duck Creek Anticline. and amphibolite-grade (recrystallised) felsites from the eastern limb, 75 km further south in the same anticlinal structure. Bottletree Formation

This volcanic sequence (Bultitude et al., 1977, 1978; Blake, 1980) unconformably overlies the western edge of the Kalkadoon--Leichhardt basement block and was equated by Derrick et al. (1977) to the Argylla Formation sequence on the eastern edge. In contrast, Blake (1980) proposed that the Bottletree Formation significantly pre-dates the eastern volcanic sequence and has no exposed eastern equivalent. The age of the Bottletree Formation volcanic rocks has an important bearing, not only on these problematic correlations, but also on the question of the maximum age of the 6--18 kinthick Haslingden Group, which conformably overlies them. Doherty and Corella Formations (Mary Kathleen Group)

The Doherty Formation (Blake et al., 1981) consists of rocks in the southeastern part of the Mount Isa Inlier which were previously mapped as CoreUa Formation (Carter et al., 1961). They consist predominantly of amphibolite-grade calcareous metasediment and breccia, minor schist, and some metarhyolite and metabasalt. The porphyritic rhyolite bodies are considered to be extrusive and, based on supposed Corella Formation equivalence, their isotopic age would be expected to be 1730--1740 Ma or older (Page, 1983). Corella Formation -- felsic volcanic rocks in the T o m m y Creek area

Another region of felsic and mafic volcanic and volcaniclastic rocks of the CoreUa Formation (Mary Kathleen Group) occurs in the T o m m y Creek area,

230

~30 km west of Cloncurry (Fig. 2). This sequence includes porphyritic rhyolite, ignimbritic tuff, and rhyolitic agglomerate interbedded with mafic volcanic and metasedimentary rocks. It is intruded by numerous high-level felsic bodies (Tommy Creek Microgranite). The age of this volcanism should provide a further direct time datum for the Corella Formation. ZIRCON U--Pb DATA

Kalkadoon--Leichhardt basement block

Two rhyodacites (7920.5312, 5317) of the 'Standish volcanics' (Blake, 1980) exemplify the felsic volcanic component in the south of the Kalkadoon--Leichhardt basement block. These two rhyodacites, separated by ~70 km (Fig. 2), are partly recrystallised and similar in most stratigraphic and petrographic respects to the 1865 + 3 Ma volcanic component of the Leichhardt Metamorphics to the north. U--Pb data for 11 zircon fractions from the two samples are listed in Table II. Zircon U values (290--360 ppm) are broadly comparable to those in the Leichhardt Metamorphics zircons, and there is equivalent overlap in terms of Th/U ratios, as inferred from radiogenic 2°Spb (Th-daughter) abundances (Fig. 4). All 11 fractions have unusually high proportions of nonradiogenic (common) Pb, exceeding the analytical common Pb contribution by a factor of 10--100. I

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231

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The zircon isotopic data (Fig. 5) are some 10% discordant, but neither set is sufficiently well aligned (regressions summarised in Table III) to justify protracted statistical comparisons. The - 1 5 0 NMO fraction (6) from sample 5317 is clearly aberrant with respect to the remaining 10 data points, which approximately fit a trajectory intercepting the concordia curve at 1875 -+ ~ Ma. The data are only moderately well fitted to this chord {Mean Square of Weighted Deviates, M.S.W.D. = 6.9) indicating that isotopic perturbations, in addition to that accounted for experimentally, are affecting the results. This could be due to a small inheritance c o m p o n e n t of the kind that has severely affected the anomalous fraction 6, which has a Pb--Pb apparent age some 20 million years older than the rest of the population. I

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2o7pb/235 u Fig. 5. Concordia plot of U--Pb zircon data from the Leichhardt Metamorphics--'Standish volcanics' (small crosses), Ewen area volcanic rocks (large crosses), and metavolcanic sample 7720.5011 (circles). Labelled data points referred to in Table II and text. Best-fit regression lines and upper intercept ages and 2-o age errors shown. Fractions B,C,D not included in the regression of sample 7720.5011.

The similarity in age between these 'Standish-type' felsic volcanic sequences in the south of the inlier and the Leichhardt Metamorphics sequences (1865 + 3 Ma) in the north is mirrored by petrographic and geochemical evidence (Bultitude and Wyborn, 1982). This coherence justifies a pooled treatment of the zircon U--Pb data, which results in a better defined discordancy line with upper and lower concordia intercept ages of 1867 + 5 Ma and 327 • 78 Ma. The 1867 Ma result is considered to be the best regional estimate for the age of magmatic crystallisation associated with the earliest recognised volcanic cycle in the Mount Isa Inlier.

233 TABLE III Summary of zircon U--Pb ages on some Mount Isa Inlier igneous rocks Unit Leichhardt Metamorphics North (Page, 1978) South 'Standish' (7920.5312) South 'Standish' (7920.5317) South 'Standish' (both) Above, combined Ewen block (7920.5305) 7720.5011

Number of

Discordia

Upper intercept

Lower intercept

fractions

M.S.W.D.

age (Ma)

age (Ma)

9 5 5 10 19 4 4

1.6 6.4 5.9 6.9 4.3 1.4 0.32

1865 1886 1852 1875 1867 1852 1866

370 528 138 413 327 123 350

A r g y l l Formation 74--360 7920.5307, Nth. Duck Ck. 7920.5321, Sth. Duck Ck.

8 7 7

Bottletree Formation 7920.5318, Nth. 7720.5013, Sth.

5 3

Doherty Formation Tommy Creek rhyolite Tommy Creek mierogranite

6 8 4 2

1.2 20.5 -0.94 0.20 4.3 2.3 27.4 --

±3 ±~ ±~ ±~ ±5 ±7 ±5

± 50 ± 315 ± 380 ± 240 ± 78 ± 33 ± 20

1783 ± 5 1766 ± 23 19 21713 - 1780

416 ± 52 316 ± 100 --

1790 ± '~ 1808 ± ~

380 ± 192 327 ± 187

1720 1603 1624 1600

336 63 544 427

±7 ±6 ± ~0 ±~

± 30 ± 32 ± 1280 s30 ± 143

Z i r c o n d a t a h a v e also b e e n o b t a i n e d f r o m a n o t h e r p a r t o f t h e b a s e m e n t b l o c k ( U n d i v i d e d T e w i n g a G r o u p , 15 k m s o u t h w e s t o f s a m p l e 7 9 2 0 . 5 3 1 7 ) w h e r e m e t a m o r p h i c r e c r y s t a l l i s a t i o n o f t h e s e q u e n c e is f a r m o r e a d v a n c e d . T h e m e t a v o l c a n i c u n i t s a m p l e d ( 7 7 2 0 . 5 0 1 1 , Fig. 2) has a w e a k schistose fabric, consisting d o m i n a n t l y o f a recrystaUised aggregate o f q u a r t z , plagioclase, K - f e l d s p a r , b i o t i t e , chlorite, e p i d o t e a n d t r e m o l i t e . S o m e zircons are w e l l - f o r m e d e u h e d r a l prisms, b u t t h e m a j o r i t y are b r o k e n , s h o w e v i d e n c e o f m i l d r e s o r p t i o n a n d c o r r o s i o n , a n d c o n t a i n small d a r k inclusions w h i c h c o u l d n o t b e r e m o v e d f r o m t h e seven (7) a n a l y s e d f r a c t i o n s ( T a b l e II, Fig. 5). On the concordia diagram the 7 analysed fractions (A--G) do not define a single line, i m p l y i n g a c o m p l e x h i s t o r y since initial crystallisation. T h e 3 c o a r s e f r a c t i o n s ( A , E , G ) a n d t h e - 6 0 m i c r o n f r a c t i o n (F) are, h o w e v e r , perf e c t l y aligned (M.S.W.D. = 0.3) o n a t r a j e c t o r y ( n o t s h o w n ) d e f i n i n g an u p p e r i n t e r c e p t age o f 1 8 6 6 + 5 Ma. This is v i r t u a l l y i d e n t i c a l t o t h a t defining t h e p r e v i o u s l y c o n s i d e r e d z i r c o n d a t a , w h i c h was i n t e r p r e t e d as giving t h e p o o l e d initial c r y s t a l l i s a t i o n age o f v o l c a n i s m in t h e l o w e r g r a d e areas. I t w o u l d s e e m highly likely t h a t t h e 4 e q u a l l y o l d z i r c o n s f r o m t h e schistose m e t a v o l c a n i c t e r r a i n ( 5 0 1 1 ) are r e t a i n i n g a c o m p l e t e i s o t o p i c m e m o r y o f this s a m e initial m a g m a t i c event.

234 A porphyritic rhyolitic ignimbrite studied from the Ewen block in the far northern part of the Mount Isa Inlier (7920.5305, Fig. 1) is from a lower greenschist-facies terrain and only slightly recrystallised. Four zircon fractions define a model 1 Pb-loss trajectory with an upper intercept age of 1852 + 7 Ma (Fig. 5). The result is interpreted as the crystallisation age of this part of the Ewen block volcanic sequence, and is consistent with the fact that the latter is intruded by the 1840 Ma-old Ewen Granite (Wyborn and Page, 1983). Although this is some 15 Ma younger than the pooled age of felsic volcanics in the Leichhardt Metamorphics in the south: it supports the view that both volcanic sequences are part of the same ancient block.

Argylla Formation The most concordant zircon fraction of the 7 defining the 1777 +- 7 Ma discordancy line reported by Page (1978) has a much higher analytical uncertainty than the other 6 fractions, and is anomalously young by ~ 1 0 Ma with respect to the rest of the population. In an endeavour to clarify this and improve the precision, two further fractions were analysed (Table IV). These fall within analytical error on the chord defined by the 6 original analyses of Page (1978), and the resultant model 1 fit of the combined 8 data points (Fig. 6) has an upper concordia intercept age of 1783 + 5 Ma. This result, although not markedly different from the previously published age, is considered more reliable as it is based on additional precise data. The age of 1783 -+ 5 Ma is now the best available determination of the timing of volcanism and zircon crystallisation in the Argylla Formation. Further Argylla Formation zircon data from a partly recrystallised porphyritic rhyolite in the northern part of the Duck Creek anticlinal structure (7920.5307, Fig. 2) are given in Table IV and Figs. 6 and 7. These 7 zircons do not fall within analytical error of a single discordancy line. The best-fit chord intersects the concordia at 1766 + ~] Ma. Considering the relatively high degree of discordance, the result is in good agreement with the age of the main belt of the Argylla Formation volcanism (1783 +- 5 Ma), supporting stratigraphic and geochemical arguments (Derrick et al., 1971) that the two units are coeval. These zircon U--Pb data further demonstrate the futility of Rb--Sr whole-rock dating of such felsic volcanic units for primary crystallisation ages. This is because one of the most anomalous Rb--Sr results from this area (72-476) recognised by Page (1978) gave the spurious age of 1390 + 284 Ma, with an apparent initial aTSr/a6Sr ratio of 0.717. Seventy-five km to the south, on the eastern limb of the Duck Creek Anticline (7920.5321, Fig. 2), the same felsic sequence has been metamorphosed to amphibolite-grade, as evidenced by the mineralogy of nearby mafic dykes and, in the felsic metavolcanics, the strongly recrystallised mosaic of quartz-feldspar, green amphibole, epidote, sphene and apatite. This relatively low-U zircon suite is more concordant than the other Duck Creek Anticline zircons (5307), but the data define an area rather than a line (Fig. 6). This is not

235 I

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Fig. 6. Concordia plot of U--Pb zircon data from (1) the ArgyUa Formation, sample 74--360 (dotted circles), 7920.5307 (crossed squares), 7920.5321 (open circles), and (2) Bottletree Formation, sample 7920.5318 (small crosses), 7720.5013 (big crosses). New data points for sample 74--360 are labelled 8,9 (Table IV); remainder taken from Page (1978). Inset shows 2-a error envelopes and same regression lines as in main figure, for samples 74--360, 7920.5318, 7720.5013.

amenable to statistical assessment, and graphically, and from the general distribution of 2°TPb/2°6Pb ages (1713--1780 Ma), it is evident that the data are reflecting variable disturbance. The effects of an amphibolite-grade metamorphic imprint (ca. 1650 Ma), coupled with the recognised relatively recent Pb-loss event (ca. 400 Ma) preclude precise dating of this suite, but for purposes o f regional correlation it can be reasonably inferred that the sequence has the same general age as the remainder of the Argylla Formation. Bottletree Formation

Two sampled outcrops of porphyritic metadacites, ~ 2 5 km apart in this western volcanic belt, occur as steeply
236

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238

crop (5318) are also aligned within analytical error defining a crystallisation age of 1790 + 10 Ma. The linearity of the data sets and broad geological agreement of the t w o results suggest that the zircon~s have not been perturbed during the lower-amphibolite grade conditions that prevailed when this sequence was folded and metamorphosed ~ 150--200 million years after its deposition.

Doherty Formation (Corella Formation, southeast) Magmatic zircons obtained from this partly recrystallised porphyritic rhyolite (7920.5319} have moderately high U and a variable discordance, broadly related to U concentration (Table IV, Fig. 8). The resultant chord has intercept ages of 1720 + 7 Ma and 336 + 30 Ma. The upper intercept is again interpreted as the crystallisation age of this zircon suite and also, therefore, as the age of this felsic volcanic episode in the Doherty Formation. Approximation to linearity of the discordant U--Pb zircon systems and the stratigraphic credibility of the result (see p. 229) demonstrate that the zircon systematics have not been significantly modified during the amphibolitegrade regional metamorphism that has been documented in this region (Blake et al., 1979; Jaques et al., 1982). The Palaeozoic age indicated by the lower intercept is not discussed in detail here. It is of similar magnitude to the other lower intercept ages (Table III), and is related to the discordance-producing alteration mechanism considered, on the basis of broadly equivalent fission-track apatite ages (Page, 1978, and unpublished data), to have taken place in response to mid-Palaeozoic regional uplift (cf. Goldich and Mudrey, 1972).

Corella Formation, north (Tommy Creek area) Flow-banded porphyritic rhyolites (7920.5308) are part of a steeplydipping sequence of volcanic and metasedimentary rocks 33 km west-southwest of Cloncurry (Fig. 2). In the same area massive leucocratic microgranites form a series of sill-like bodies ( T o m m y Creek Microgranite, sample 7720.5010) which intrude the Corella Formation. Isotopic data for 10 zircon fractions from the rhyolite define a sublinear trend (M.S.W.D. = 8.5), the extrapolation of which intersects concordia at 1601 -+ 2117Ma and 65 _+ 100 Ma. Interpretation of this unexpectedly young result may be more complex than first appears, not only because the suite is relatively discordant, but also because the apparent lower intercept age is unusually low. Multi-stage Pb-loss complexities are not, however, evident from the isotopic systematics, since 8 of the 10 analytical points (deleting fractions 4 and 5) are essentially aligned on a model-1 discordance trajectory. This closely defined chord has upper and lower intercept ages of 1603 + 6 Ma and 63 + 32 Ma, and the former must be considered as the crystallisation age of this volcanic sequence.

239

Four zircon fractions from a leucocratic microgranite sill of the T o m m y Creek Microgranite, 10 km to the northeast of the above rhyolite, have been isotopically examined in an a t t e m p t to provide a further minimum age for the sequence, and also to examine the possibility that the sills may be comagrnatic with the felsic volcanic rocks. The analysed zircons are clear and euhedral, b u t are pervasively cracked. They are n o t similar to those from the rhyolite, as the former have much higher U and inferred Th/U (Table IV, Fig. 7). I

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The microgranite zircon data (Fig. 8) define an area rather than a line, indicating a multi-stage Pb--U evolution. However, the degree of discordance is desirably small, and 2°Tpb/2°6Pb ages lie in the relatively narrow interval from 1566--1590 Ma. This pattern does n o t lend itself to statistical treatment as it appears that the t w o coarsest fractions (B and C) may have had a different and earlier U--Pb evolution than the other fractions. This suggests that a measurable c o m p o n e n t of radiogenic Pb present in the coarse fractions could be inherited from some prior crustal history. If the finer fractions are n o t significantly affected by inheritance, their apparent U--Pb age of 1600 -+ ~I Ma is the best estimate for the age of emplacement.

240 I

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2O7pb/235 U Fig. 8. Concordia plot of U-Pb zircon data from the Doherty Formation (dotted circles), Tommy Creek rhyolite (small open circles), and Tommy Creek microgranite (crosses). Labelled points referred to in Table IV and text.

RESPONSE OF U--Pb ZIRCON SYSTEMS TO R E G I O N A L METAMORPHISM

Arguments based on zircon morphology, regularity of data arrays of cogenetic zircons, and agreement with geological controls, in general demonstrate that the interpreted zircon ages represent magmatic crystallisation of the host rocks. This has implications for the stability of zircon U--Pb systems in felsic volcanic sequences at different grades of regional metamorphism. Previous Rb--Sr evidence (Page, 1978) has demonstrated two main periods of metamorphism in the Mount Isa Inlier, one at 1620--1670 Ma and another at ~ 1 5 0 0 Ma. It is clear from other work (e.g., Nunes and Thurston, 1980; Page, 1978) and from the present data that greenschist-facies metamorphic conditions do n o t perturb U--Pb systems in relatively low-uranium zircons from felsic igneous rocks. It is also known that highly radiationdamaged zircon lattices are prone to open-system behaviour, including Pb loss, at greenschist-facies temperatures of 350--400°C (Gebauer and Griinenfelder, 1976). However, the isotopic response of zircon systems to various amphibolite-facies conditions remains ambiguous. In one example from this study (7920.5321) the discordant zircon data are markedly disturbed in a

241

terrain which has been metamorphosed to only low amphibolite-grade temperatures (ca. 550°C) approximating the staurolite isograd. In contrast to this, zircon isotope systematics in the Doherty Formation rhyolite have maintained their igneous integrity in a higher amphibolite-grade metamorphic environment in which regional isograds (approximately sillimanitegrade) indicate temperatures close to 600°C (Jaques et al., 1982). In the case of 7720.5011 (schistose metavolcanic associated with rocks showing evidence of partial melting and migmatisation) the coarser zircons faithfully portray their primary volcanic parentage at 1866 + 5 Ma. Likewise, zircons in the rather sheared amphibolite-grade felsic rocks of the Bottletree Formation have survived these events and retained primary isotopic characteristics to provide precise ~tratigraphically meaningful ages. Any relationship between accumulated radiation damage in zircons and their susceptibility to metamorphic disturbance also remains unresolved from the present data. The disturbed zircon systems evident at site 7920.5321 are a low-U population, whereas the Bottletree zircons, which are equally low in U and have higher Th/U (Fig. 7), appear little affected under similar amphibolite-grade conditions. The degree of U--Pb isotopic disturbance in zircons thus depends not just on P--T conditions or radiation damage, but also on a complexity of factors including the nature of interacting metamorphic fluids, volume expansion of the crystals, physical cracking, and surface leachability (cf. Krogh, 1982a,b). In conclusion, zircon U--Pb systems in felsic volcanic rocks at Mount Isa have withstood greenschist and most amphibolite-facies alteration events which occurred some 100--200 million years after their initial formation. In some amphibolitefacies terrains, isotopic disturbance results in non-linear data arrays which, however, can still be geologically interpreted if sufficient corroborating data are available. OVERVIEW

O F U--Pb Z I R C O N R E S U L T S A N D S T R A T I G R A P H I C I M P L I C A T I O N S

The U--Pb results (Table III) from several felsic volcanic and high-level intrusive suites enable the chronology of major supracrustal sequences in the Mount Isa Inlier to be closely quantified. The ages reported by Page (1978) for the oldest known episodes of volcanism (Leichhardt Metamorphics) in the northern part of the inlier have been validated and further extended to include metavolcanic sequences in the same basement block in the southern part. These rhyodacites reflect greenschist- and amphibolite-facies metamorphic conditions, yet the pooled data from throughout the inlier yield a consistent pattern interpreted as giving the volcanic crystallisation age of 1867 + 5 Ma. The felsic volcanic rocks in the far northern Ewen block basement are slightly younger (1852 + 7 Ma) but constitute part of the same early volcanic cycle. Dacitic and mafic volcanic rocks of the Bottletree Formation, and predominantly rhyodacitic rocks of the Argylla Formation exemplify the second

242 major cycle of volcanism, some 60--80 Ma younger than the Leichhardt volcanism. This interval represents the maximum duration of the intervening regional unconformity. There is no recognisable time difference between ArgyUa formation volcanism (1783 -+ 5 Ma) and volcanism in the northern outcrop of the Bottletree Formation (1790 -+ 10 Ma), b u t there is a small difference between the former and the more southerly Bottletree Formation (1808 -+ 2217Ma). This suggests that volcanism to the west (Bottletree Formation) in part preceded and, in part, was synchronous with volcanism to the east (ArgyUa Formation). For purposes of correlation and crustal evolution, no major time break between the Bottletree and Argylla Formations can be inferred, and the felsic rocks of the t w o formations can be considered as parts of a single volcanic cycle. Mafic components of the Bottletree Formation volcanism may be equivalent to the Magna Lynn Metabasalt which conformably underlies and interfingers with lower Argylla Formation rhyodacites. The age of the mafic volcanic sequence in the Haslingden Group (Eastern Creek Volcanics) can also be inferred from the younger measured age of the Bottletree Formation. Thus, the maximum age of the Haslingden Group is 1790 -+ 10 Ma, and its minimum age, based on stratigraphic extrapolation between the overlying Quilalar Formation and Corella Formation (Derrick et al., 1980), is ~ 1 7 4 0 Ma (Page, 1983). This geochronological framework does n o t support a recent interpretation by Blake (1980) that the Haslingden Group substantially predates successions in the east, such as the Magna Lynn Metabasalt and Argylla Formation. A third cycle of felsic volcanism, 1720 -+ 7 Ma, is recognised in the stratigraphically younger Corella Formation equivalent (Doherty Formation). This is broadly consistent with regional geochronological constraints which elsewhere (Page, 1983) imply a minimum age for the Corella Formation of 1730--1740 Ma. Younger cycles of felsic volcanism are evident in higher stratigraphic units, as volcanic and volcaniclastic deposits within the Carters Bore Rhyolite and overlying Urquhart Shale (Mount Isa Group). U--Pb zircon study of the former unit defined an age of 1678 + 3 Ma (Page, 1978). Concordant tuff horizons in the mineralised Urquhart Shale yielded a stratigraphically consistent U--Pb age interpretation, not as precisely defined, but still the best estimate of the depositional age of the stratiform base-metal deposit, of 1670 -+ 201~Ma (Page, 1981). The youngest apparent age (1603 + 6 Ma) of any volcanic sequence in the Mount Isa Inlier is given by rhyolitic lavas interbedded with metasediments mapped as CoreUa Formation in the T o m m y Creek area. This result conflicts with earlier mentioned constraints indicating that the Corella Formation is at least 1730 Ma old. It can only be interpreted if either the sediments and volcanics in question belong to a stratigraphicaUy much younger unit than the Corella Formation, or the rhyolite in question represents a high-level intrusive sill. The latter is unlikely, given the petrographic evidence and the

243 occurrence of mafic volcanic rocks and other felsic volcaniclastic components in this sequence. Recognition of 1600 Ma volcanism, therefore, implies that this sequence is much younger than previously considered, and is younger than the Mount Isa Group in the western succession of the Mount Isa Inlier. A corollary is that as yet unrecognised Mount Isa Group equivalents (ca. 1670 Ma) may well have been deposited in the eastern succession, as originally considered by Carter et al. (1961). CONCLUSIONS U--Pb zircon results from feisic volcanic sequences in the Mount Isa Inlier mostly define linear or sub-linear arrays that can be explained as a result of an approximately two-stage process of initial volcanic crystallisation and later (mid-Palaeozoic to Mesozoic) Pb loss. The following conclusions can be emphasised. (1) Volcanic crystaUisation ages interpreted from the zircon data define an evolutionary framework spanning 270 Ma in the interval 1870--1600 Ma ago. All the known Proterozoic supracrustal sequences in the inlier were deposited within this interval. (2) In general, the zircon ages are in accord with known stratigraphic constraints (cf. Table III, Fig. 3). (3) The elongate volcanic belts which can be traced for more than 300 krn, represent geochronologically definable cycles of volcanism, within which ages are remarkably constant, varying by only 10--20 Ma. (4) A precise, but unexpectedly young age of 1603 +- 6 Ma was found for a metarhyolite in the eastern succession of the inlier. This Tommy Creek sequence must now be considered as being much younger than hitherto believed, and it follows that highly prospective Mount Isa Group correlatives (ca. 1670 Ma) may well exist in the eastern part of the Mount Isa Inlier. (5) Zircon U--Pb systems in felsic volcanic rocks can withstand greenschist- and most amphibolite-facies alteration events, even to temperatures of 600°C.

ACKNOWLEDGEMENTS I a m indebted to colleagues D.H. Blake, R.J. Bultitude,G.M. Derrick and I.H. Wilson who have carriedout and reported most of the detailedmapping to which the geochronological questions are addressed. The painstaking mineral separations were performed by T.K. Zapasnik, D.B. Guy and N.C. Hyett, and M.J. Bower undertook preparation of clean reagents and isotope dilutionchemistry. This paper has benefited from criticismsof the draft by L.P. Black, H.L. Davies, A.Y. Glikson and I.S. Williams. It is published with the permission of the Director, Bureau of Mineral Resources, Geology and Geophysics.

244 REFERENCES Blake, D.H., 1980. The early geological history of the Proterozoic Mount Isa Inlier, northwestern Queensland: an alternative interpretation. B.M.R.J. Aust. Geol. Geophys., 5: 243--256. Blake, D.H., 1981. The early geological history of the Proterozoic Mount Isa Inlier, northwestern Queensland: an alternative interpretation. Reply to discussion. B-M.R.J. Aust. Geol. Geophys., 6 : 272--274. Blake, D.H., Jaques, A.L. and Donchak, P.J.T., 1979. Precambrian geology of the Selwyn region, northwestern Queensland -- preliminary data (unpubl.). Aust. Bur. Miner. Resour., Record 1979/86. Blake, D.H., Bultitude, R.J. and Donchak, P.J.T., 1981. Summary of new and revised stratigraphic nomenclature in the Precambrian of the Duchess and Urandangi 1:250 000 sheet areas, northwestern Queensland. Queensl. Govt. Min. J., 82: 580-589. Bultitude, R.J. and Wyborn, L.A.I., 1982. Distribution and geochemistry of volcanic rocks in the Duchess--Urandangi region, Queensland. B~VI.R.J. Anst. Geol. Geophys., 7: 99--112. Bultitude, R.J., Gardner, C.M. and Noon, T.A., 1977. A recently discovered unconformity near the base of the Proterozoic Cloncurry Complex south of Mount Isa, northwestern Queensland. B_M.R.J. Aust. Geol. Geophys., 2: 311--314. Bultitude, R.J., Blake~ D.H. and Donchak, P.J.T., 1978. Precambrian geology of the Duchess 1:100 000 Sheet area, northwestern Queensland -- preliminary data (unpubl.). Aust. Bur. Miner. Resour., Record 1978/112. Carter, E.K., Brooks, J.H. and Walker, K.R., 1961. The Precambrian mineral belt of north-western Queensland. Aust. Bur. Miner. Resour. Geol. Geophys. Bull. 51,344 pp. Bur. Miner. Resour. J. Aust. Geol. Geophys., 6: 267--271. Derrick, G.M., Wilson, I.H., Hill, R.M and Mitchell, J.E., 1971. Geology of the Marraha 1:100 000 sheet area Queensland (unpubl.). Aust. Bur. Miner. Resour., Record 1971/ 56. Derrick, G.M., Wilson, I.H., Hill, R.M., Glikson, A.Y. and Mitchell, J.E., 1977. Geology of the Mary Kathleen 1:100 000 sheet area, northwest Queensland. Aust. Bur. Miner. Resour. Geol. Geophys. Bull. 193,114 pp. Derrick, G.M., Wilson, I.H. and Sweet, I.P.,1980. Quilalar and Surprise Creek Formations -- new Proterozoic units from the Mount Isa Inlier: their regional sedimentology and application to regional correlations. B_M.R.J. Aust. Geol. Geophys., 5: 215--223. Gebauer, D. and GrUnenfelder, M., 1976. U--Pb zircon and Rb--Sr whole-rock dating of low-grade metasediments example: Montagne Noire (Southern France). Contrib. Mineral. Petrol., 59: 13--32. Goldich, S.S. and Mudrey, M.G., Jr., 1972. Dilatancy model for discordant U--Pb zircon ages. In: A.I. Tugarinov (Editor), Contributions to Recent Geochemistry and AnalyticalChemistry (Vinogradov Volume). Nauka Press, Moscow, pp. 415--418. Jaques, A.L., Blake, D.H. and Donchak, P.J.T., 1982. Regional metamorphism in the Selwyn Range area, northwest Queensland. BMI.R.J. Aust. Geol. Geophys., 7: 1 8 1 - 196.

Krogh, T.E., 1973. A low contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochim. Cosmochim. Acta, 37: 485--494. Krogh, T.E., 1982a. Improved accuracy of U--Pb zircon dating by selection of more concordant fractions using a high gradient magnetic separation technique. Geochim. Cosmochim. Acta, 46: 631--635. Krogh, T.E., 1982b. Improved accuracy of U--Pb zircon ages by the creation of more concordant systems using an air abrasion technique. Geochim. Cosmochim. Acta, 46: 637--649.

245 Ludwig, K.R., 1980. Calculation of uncertainties of U--Pb isotope data. Earth Planet. Sci. Lett., 46: 212--220. Nunes, P.D. and Thurston, P.C., 1980. Two hundred and twenty million years of Archean evolution: a zircon U--Pb age stratigraphic study of the Uchi---Confederation Lakes greenstone belt, northwestern Ontario. Can. J. Earth Sci., 17: 710--721. Page, R.W., 1978. Response of U--Pb zircon and Rb--Sr total-rock and mineral systems to low,grade regional metamorphism in Proterozoic igneous rocks, Mount Isa, Australia. J. Geol. Soc. Aust., 25: 141--164. Page, R.W., 1981. Depositional ages of the stratiform base metal deposits at Mount Isa and McArthur River, Australia, based on U--Pb zircon dating of concordant tuff horizons. Econ. Geol., 76: 648---658. Page, R.W., 1983. Chronology of magmatism, skarn formation and uranium mineralization, Mary Kathleen, Queensland, Australia. Econ. Geol., 78(5), in press. Plumb, K.A., Derrick, G.M. and Wilson, I.H., 1980. Precambrian geology of the McArthur River--Mount Isa region, northern Australia. In: R.A. Henderson and P.J. Stephenson (Editors), The Geology and Geophysics of Northern Australia. Geol. Soc. Aust, Queensl. Div., pp. 71--88. Steiger, R.H and Jiiger, E., 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth Planet. Sci. Lett., 36: 359--362. Wilson, I.H., 1978. Volcanism on a Proterozoic continental margin in northwestern Queensland. Precambrian Res., 7 : 205--235. Wyborn, L.A.I. and Page, R.W., 1983. The Proterozoic Kalkadoon and Ewen Batholiths, Mount Isa Inlier, Queensland: source, chemistry, age, and metamorphism. BMI.R. J. Aust. Geol. Geophys., in press. York, D., 1969. Least-squares fitting of a straight line with correlated errors. Earth Planet. Sci. Lett., 5: 320--324.