Precambrian Research, 40/41 (1988) 509-541 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
509
PETROLOGY, GEOCHRONOLOGY AND ISOTOPE GEOCHEMISTRY OF THE POST-1820 Ma GRANITES OF THE MOUNT ISA INLIER: MECHANISMS FOR THE GENERATION OF PROTEROZOIC ANOROGENIC GRANITES L.A.I. W Y B O R N
and R.W. PAGE
Bureau of Mineral Resources, P.O. Box 378, Canberra, A.C.T., 2601 (Australia) M.T. McCULLOCH
Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra, A.C.T., 2601 (Australia) (Received June 6, 1987; revision accepted February 12, 1988)
Abstract Wyborn, L.A.I., Page, R,W. and McCulloch, M.T., 1988. Petrology, geochronology and isotope geochemistry of the post-1820 Ma granites of the Mount Isa Inlier: mechanisms for the generation of Proterozoic anorogenic granites. Precambrian Res., 40/41: 509-541. Four distinct episodes of granitic intrusion occurred in the Mount Isa Inlier at 1820-1800 Ma, 1760-1740 Ma, 17001670 Ma and 1560-1480 Ma. Combined, these granites cover at least 4700 km 2 and they contrast in composition and intrusive style with an earlier major felsic magmatic event which occurred between 1870 and 1840 Ma. Post-1820 Ma granite batholiths are of two types: large major coarse-grained anorogenic I-type intrusives, and small minor microgranites, which are probably S-type. The I-type granites are all pink to red colour and most contain fluorite; rapakivi textures are common. They have a high ratio of Fe/Mg, and are more enriched in Ti02, Zr, Nb, Y and U than the 1870-1840 Ma felsic igneous rocks. The post-1820 Ma I-types are interpreted to have formed by small degrees of partial melting of a mafic underplate. The smaller microgranites are relatively K20 enriched and are sometimes muscovite-bearing. They are interpreted to be derived from Proterozoic feldspathic metasediments. The first three episodes of granite magmatism are coeval with major extensional sedimentary sequences, but they are generally spatially displaced from the maximum area of surface rift development. This separation is predicted by the asymmetric detachment model of continental extension. The tectonic relationship of the youngest granite episode is not clearly defined. The 143Nd/144Nd initial ratios of the post-1820 Ma granites fall into two groups, one having positive, relatively primitive ~Ndvalues of + 1.8 to + 3.5, the other having negative ~Sd values ranging from --2.9 to --3.7. Samples with positive ~Ndvalues are interpreted as partial melts of mafic sources formed post-1870 Ma, whilst the group with negative eNd values are derived from significantly older sources, formed probably around 2300-2000 Ma. The sources of these granites are produced by significant underplating events that may occur during either extension or compression.
Introduction G r a n i t i c r o c k s in t h e e a r l y t o m i d d l e P r o t e r o zoic M o u n t I s a I n l i e r o f n o r t h w e s t e r n Q u e e n s -
l a n d w e r e e m p l a c e d o v e r n e a r l y 400 M a o f E a r t h h i s t o r y , b e t w e e n 1860 a n d 1480 M a . T h e y c o n s t i t u t e a l m o s t 14% b y a r e a o f t h e I n l i e r ( T a b l e I ) , a n d o c c u r in d i s t i n c t g e o g r a p h i c a l g r o u p -
510 TABLE I Area covered by major igneous units in the Mount Isa Inlier Unit
Kalkadoon Granodiorite Ewen Granite Leichhardt Volcanics Yeldham Granite Big Toby Batholith Big Toby Microgranite
Area (km 2) 1600 345 2685 20 24 7
Argylla Formation Magna Lynn Metabasalt Bottletree Formation
1520 420 170
Eastern Creek Volcanics Marraba Volcanics
3200 440
Wonga Batholith
600
Fiery. Creek Volcanics Carters Bore Rhyolite
150 8
Weberra Granite
20
Sybella Batholith
1600
Naraku Batholith
395
Williams Batholith
2100
Total granite outcrop
6711
Total outcrop area of the Mount Isa Inlier
51000
ings, most of which are elongate north-south (Fig. 1 ). Joplin and Walker (1961) first recognized the composite nature of the granite intrusions in the Inlier and subdivided them into eight different named granite bodies on petrological and geochemical properties. During 1 : 100 000 scale mapping of the Mount Iso Inlier by the Bureau of Mineral Resources and the Geological Survey of Queensland, many of the plutons within these geographically distinct areas were named (e.g., Derrick et al., 1978; Hutton and Sweet, 1980; Blake et al., 1981). However, most of the names used by Joplin and Walker (1961) have been retained and elevated to batholith status (e.g., Blake, 1987 ) as shown in Fig. 1.
Following Carter et al. (1961) the Mount Isa Inlier can be divided into three major stratotectonic units, which Blake (1987) has termed the Western Fold Belt, the central KalkadoonLeichhardt Belt and the Eastern Fold Belt (Fig. 1 ). Blake (1987) also subdivided the stratigraphy into four main units: a basement sequence overlain by three successively younger sequences of sedimentary and volcanic rocks named cover sequence 1, 2 and 3. Using the model of McKenzie (1978) for the development of sedimentary basins, the second and third cover sequences can be divided into a lower volcanogenic rift phase overlain by a thermal subsidence or sag phase. The relationship of the granites to these important basin-forming events and to major periods of deformation and metamorphism is schematically shown in Fig. 2. The oldest major granitic intrusions in the Mount Isa Inlier are part of the KalkadoonLeichhardt Association, comprising the Kalkadoon Granodiorite, Wills Creek, Woonigan and Ewen Granites, and their comagmatic subaerial extrusives, the Leichhardt Volcanics, of cover sequence 1. The rocks of this association form a uniform geochemical suite covering at least 5000 km 2 (Table I) dated between 1870 and 1840 Ma (Wyborn and Page, 1983). The intrusions of this suite rarely show any signs of fractional crystallization; their compositional diversity is attributed to restite unmixing (Wyborn and Page, 1983). Isotopically the unmetamorphosed members of this suite are characterized by low initial 87Sr/S6Sr ratios (Wyborn and Page, 1983) with eNd values between - 1.6 and - 2.7 (Table II; McCulloch, 1987). The T TM model source ages (calculated using depleted mantle parameters, see footnote to Table II) for these magmas are at least 200 Ma older than their emplacement age. The post-1820 Ma granites are petrographically and compositionally distinct from those of the Kalkadoon-Leichhardt Association. Wyborn and Page ( 1983 ) and Wyborn et al. (1987) showed that the major post-1820 Ma granite
511 137019 ,
139°30 , 18°00 '
:::.:.o i:il!.:..:/"
o 100 km
140030 ,
~\
19°00 ,
WEBERRA=,, \X
GRANITE
L
EWEN / 8ATHOLITH
MOUNT ISA INLIER Western
I
Fold Belt
K a l k a d o o n - L e i c h h a r d t Belt Eastern Fold Belt
I
NARAKU
Granite
KALKADOON
BATHOLITH 141000 , 20°30 '
SOUTH NICHOLSON BASIN
,
• ,,~
Ma~ Kathleen
oncurry
k
SYBELLA
BATHOLITH
-'1
16/O/161
139°00 `
22o07 ,
141oG0,
Fig. 1. Locality map of the major granite batholiths of the Mount Isa Inlier (based on Joplin and Walker (1961) and Blake (1987)).
batholiths tend to have a more restricted range in SiO2 values, a higher ratio of Fe 3 + / F e 2+, and
be more enriched in K20, TiO2, Zr, Nb, Y, Th and U compared with the pre-1820 Ma granites.
512 WESTERN
FOLD BELT
EASTERN
KALKADOONLEICHHARDT BELT
.~
SOUTHNICHOLSONGROUP
i~
156o~~' g 1480±2e 1508±7o
DEFORMATION o~ (7)
FOLD BELT
AND
METAMORPHISM
j
McNAMARAGROUP
1610 - 1550 M a ~
MTALOERTGROGP
MT ISA GROUP ? 1670+~!
I SYBELLA I Surprise Creek Formation I BATHOLIT~1RRrJ+26 uuu--21 ~ 8 )ella Microgranite 1678±3 Carters Bore Rhyol 1670 Wonga Microgra~il 1671-+78 Weberra Granitl +28 1698_2~
I I I I ? _
LunchCreek 9araku Mierogranite 1754 ±25
+17 WONGA 1745-15 UATUOLITH I Quilalar Formation
/
~
L
Dohe~y Formation 1720±7
03 JMitakoodi Quartzite
lMarraball
Granite
~J Voicanics ~ Yeldham Granite 1800-1820
¢
Big Toby Microgranite 180, 1 ~,
--~176n+23
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I
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m
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~
Lower crustal mantle derived underplates (with age of underplating)
Mafic volcamcs -
-
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Recognised unconformity
g ~ z ~ , ~8~_~A~ DEFORMATION i ~ Kamerga
Plum Mtn Gneiss _ _ ----]
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May Downs 8 Suliemon Gnaissea St Roman Metamorphics
SOLDIERSCAP GROUP
Oouble Grossing Metamorphics
u~
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~,,=,
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Fig. 2. Schematic diagram of the relationship of the granites to major structural and stratigraphic events in the Mount Isa Inlier. Geochronological information is from Page ( 1983a, b ), Page and Bell (1986) and Wyborn and Page (1983).
513 TABLE II Sm-Nd isotopic data from Mount Isa Sample
Sm (ppm)
Nd (ppm)
147Sm/144Nd
143Nd/144Nda
9.8
92.7
0.09063
0.511580±6
6.32 7.95
53.9 70.8
0.1073 0.06782
4.78 11.45
62.8 94.4
8.87 10.17 11.12
(~Nd
TNd (Ma)
T v Pb~
+1.8
1790
1670
0.511750±5 0.511476 ± 4
+2.5 + 2.0
1820 1630
1754 1480
0.06831 0.07334
0.511585±5 0.511551±6
+3.5 +3.0
1530 1620
1480 1560
44.5 59.1 62.5
0.1206 0.1040 0.1077
0.511596±21 0.511439±39 0.511489±26
-3.7 -3.1 -2.9
2280 2160 2170
1740 1740 1740
13.0
65.8
0.1197
0.511655±20
-1.8
2170
1783±5
8.4 8.5
50.5 47.5
0.1007 0.1080
0.511355 ± 20 0.511415±20
- 2.3 -2.7
2210 2300
1865 ± 3 1886_+35
8.3
56.5
0.0888
0.511245 ± 20
- 1.6
2140
1862 ± 25
(Ma)
Post-1820 M a
Sybella Batholith: 72-514D Naraku Batholith: 72-251A 72-248 Williams Batholith: 5320 5322 Wonga Batholith: 72-237G c 72-241A c 72-242A c Argylla Formation: 74-360 d Pre-1820 M a
Leichhardt Metamorphics: 74-368A d 7920.5312 d Kalkadoon Granodiorite: 7920.5309 d
" '4:~Nd/144Nd ratios normalized to 144Nd/~46Nd =0.7219. bAnalysis from Maas et al. (1987). CAnalysis from McCulloch (1987). dU-Pb zircon ages from this study and Page (1978, 1983b). end= [ ( ' 4 3 N d / 1 4 4 N d ) T c H u a / ( ' 4 3 N d / ' 4 4 N d ) . . . . - 1 ] X 1 0 4 , where T = U - P b age. T N d = (1/ 2 )ln [ (14:~Nd/144Nd) DM-- ( 143Nd/'44Nd ) . . . . / ( 147Sm/'44Nd ) D M - - (147Sm/'44Nd) . . . . ) ], where 2 = 6.54 × 10- ,2 y r - 1 Depleted mantle parameters used for calculating T TM ages are ( 143Nd/144Nd)DM = 0.513163 and (]47Sm/144Nd)DM= 0.225. Present-day reference values are (~4aNd/ '44Nd)cHUR=0.512650 and (147Sm/~44Nd)cnwR=0.1967.
In this paper the petrology, geochemistry and geochronology of the post-1820 Ma granite intrusions are documented. Reference will be made to their association with structural, sedimentological and tectonic events taking place in the Mount Isa Inlier not only at the time of their intrusion, but also during that of the inferred time of emplacement of their sources. The granites will be described in two parts; those of the Western and those of the Eastern Fold Belts. Comparisons will also be made with compositionally similar granite suites emplaced during an equivalent time period, the so-called Proterozoic anorogenic suites (e.g., Emslie, 1978; Anderson et al., 1980; Nurmi and Haapala, 1986). Nomenclature
The I.U.G.S. nomenclature (Streckeisen, 1973 ) for igneous rocks is followed, and the term
granite is applied to the broad spectrum of granitic compositions, namely tonalite, granodiorite and granite. For felsic igneous rocks, the classification of I- and S-types will be followed, with particular reference to the recent refinement of the definition of I- and S-type granites from 'igneous' and 'sedimentary' to 'infracrustal' and 'supracrustal', respectively (White and Chappell, 1983; Chappell and White, 1984).
Analytical methods Zircon chemistry and U - P b mass spectrometry followed the procedures of Krogh (1973). The average Pb processing blank was 0.25 ng, having a composition: 2°6Pbff°4pb-- 17.97, 2°Tpbff°4pb-- 15.55, and 2°Spbff°4pb = 37.71. A modified York (1969) program was used to regress the U - P b data, and the resultant errors are quoted as 2a. Uncertainties in U / P b and
514 2°Tpb/2°6Pb are 0.8% and 0.2%, respectively (20). Rb-Sr procedures followed those described in Page et al. (1976), and isochron regression was that of McIntyre et al. (1966), giving 95% confidence limits to the determined ages. All U-Pb and Rb-Sr ages were calculated using the decay constants recommended by Steiger and Jager (1977). The Sm-Nd analytical techniques are similar to those described by McCulloch and Chappell (1982). All samples were dissolved using Teflon bombs and samples were totally spiked using 147Sm- and 15°Nd-enriched isotopes. The 143Nd/144Nd ratios have been corrected for mass discrimination by normalizing to 146Nd/144Nd= 0.7219 (McDonough and McCulloch, 1987). All geochemical work was carried out in the Bureau of Mineral Resources' laboratories. All major elements and most trace elements were analysed by X-ray fluorescence, Li, Co, Cr, Cu, Zn and Ni were determined by atomic absorption spectrophotometry and H20 +, H 2 0 - and CO2 by gravimetric techniques. G r a n i t e s o f t h e W e s t e r n Fold B e l t
Big Toby Batholith Geological setting and petrology The Big Toby Batholith is the westernmost intrusive in the Mount Isa Inlier and consists of two major plutons, as well as numerous small, scattered outcrops of granite. The northern pluton is essentially a grey, biotite __muscovite microgranite, and contrasts with the southern pluton which consists predominantly of coarsegrained, foliated, pink feldspar granite. Most of the geochemical and geochronological work has been undertaken on the northern microgranite, and in view of the petrological and geochemical contrasts, the results do not necessarily apply to the coarse-grained southern intrusion. The northern microgranite intrudes the Yaringa Metamorphics of the basement sequence in which a high-grade metamorphic event has been dated at 1890 +_8 Ma (Page and Williams,
1988). The microgranite is also overlain unconformably by the 1680 Ma Carters Bore Rhyolite and sediments of the McNamara Group of cover sequence 3 (Fig. 2). It is heterogeneous and in places is strongly foliated and ranges from biotite tonalite to monzogranite, with granodiorite being the most abundant composition. Quartz is recrystallized, and the K-feldspar is mainly microcline. Biotite usually forms large plates, although some decussate aggregates are present: allanite, apatite and zircon are common accessories. Muscovite occurs in altered plagioclase grains as well as forming large plates. Alteration mineral assemblages imply metamorphism to upper greenschist facies, comparable with retrograde metamorphic assemblages in the adjacent Yaringa Metamorphics, suggesting that emplacement of the microgranite post-dates the 1890_+ 8 Ma amphibolite grade metamorphic event in the adjacent Yaringa Metamorphics.
Geochronology The sample worked on in the U-Pb zircon study and included in the Rb-Sr study, is a mildly recrystallized biotite granodiorite. The clear, euhedral zircon population includes a minority of grains that have older cores surrounded by finely zoned euhedral selvages. Most grains are, however, structureless. Although the common Pb in the suite is moderately high (Table III), the linear discordant trend ( M S W D = 11.6) indicates the zircon crystallization age at 1804_+15 Ma (lower intercept 187+68 Ma) (Fig. 3). The discordia fit is not perfect and suggests geological complexity other than simple Pb loss. However, the averaged age of 1804 _+15 Ma given by these zircon analyses is consistent with the petrological and field constraints and is considered to date the time of igneous crystallization. Rb-Sr total-rock data from the same locality (six samples) are grouped with two previously published analyses (GA 530.3382, Farquharson and Wilson, 1971) from the Big Toby microgranite in Table IV and Fig. 4. The model 3 fit
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Fig. 3. U-Pb concordia diagram showing data for five zircon fractions (solid error boxes) from the Big Toby microgranite and the fitted discordia line between 1804± 15 Ma and a lower intersection towards 187 ± 68 Ma. Two zircon (solid error boxes) and four xenotime (open error boxes, xen.) analyses from the Yeldham Granite are also shown, all with 2a error uncertainties. Six zircon fractions from the Weberra Granite ( + ) define a younger discordia trajectory between 1698_+~ and 143 _+185 Ma. of this isochron indicates geological scatter (MSWD -- 6.2 ), and the young apparent age of 1707 + 46 Ma (initial STSr/S6Sr 0.7085 + 0.0016 ) reflects post-emplacement disturbance of the R b - S r system. The mineral isotopic systems show further internal discordance. Two plagioclase fractions plot above the total-rock isochron, and the subparallel tie lines to their respective total rocks suggest disturbance at 1470 Ma. This is in accord with the biotite R b - S r age of 1460 Ma (Richards, 1966), whereas the K - A r biotite age reflects an even younger cooling temperature at 1411 Ma (recalculated from Richards et al., 1963). It is not clear whether the 1707_ 46 Ma age relates to any particular geological event. This age is similar to the age of most of the magmatic events associated with cycle 3 in the Western Fold Belt (Fig. 2) including the felsic volcanic which overlies the Yaringa Metamorphics (Page, unpublished data). Alternatively, it may represent a partial update of the primary 1804 + 15 Ma intrusion age in response to one
or more of the deformation events documented in this region between 1610 and 1510 Ma by Page and Bell (1986).
Geochemistry Four samples were analysed from the Big Toby microgranite and their average composition is listed in Table V. Relative to other post1820 Ma felsic igneous rocks, the microgranite contains high Sr and A1203, and low K20, Ti02, Zr, Nb, Y and U (Fig. 5, Table II ) and is similar to rocks from the 1870-1840 Ma KalkadoonLeichhardt Association, lacking the enrichment in incompatible elements that is typical of the post-1820 Ma felsic melts (Fig. 5). Although not markedly peraluminous, the abundance of metasedimentary xenoliths suggests that this microgranite is S-type.
Yeldham Granite Geological setting and petrology The Yeldham Granite crops out ~ 200 km north of M o u n t Isa in the elongate Kamarga
518 TABLE IV Rb-Sr data on whole-rock and mineral samples from the Big Toby and Naraku Batholiths, Mount Isa Inlier. Sample prefix 7220. Two 'GA' data from Farquharson and Wilson (1971). la uncertainties in SVRb/S6Sr0.5%, and STSr/S6Sr0.01% Rb (ppm)
Sr (ppm)
SVRb/S6Sr
STSr/S6Sr
172.6 182.8 94.0 37.7 113.9 52.6 179.4 209.5 183.4 165.0
164.8 205.6 258.2 442.9 255.0 427.1 168.0 163.4 183.7 240.9
3.046 2.585 1.054 0.246 1.294 0.356 3.107 3.737 2.870 1.976
0.78218 0.77220 0.73464 0.71734 0.73955 0.72004 0.78441 0.80039 0.7797 0.7575
244.0 453.3 287.8 40.3 18.7 97.1 312.2 190.4
58.6 50.5 49.2 152.6 43.8 63.4 51.8 41.3
12.381 27.551 17.605 0.765 1.233 4.468 18.159 13.751
1.0065 1.3608 1.1341 0.73098 0.73710 0.80872 1.1489 1.0356
255.0 295.5 314.6 219.6 115.1 225.3
85.0 70.5 48.8 128.1 206.9 29.5
8.816 12.408 19.306 4.997 1.612 23.032
0.89086 0.96378 1.0887 0.81152 0.74390 1.1643
Big Toby Microgranite 5078 A B C C plagioclase D D plagioclase E F GA.530 GA.3382
Naraku Microgranite 5028 B C E 5029 A C D 5030 A C
Naraku Main Phase 5031 B C D 5032 A A plagioclase B
D o m e . It covers < 20 k m 2 a n d is a fine- to medium-grained, equigranular muscovite-rich m o n z o g r a n i t e to alkali-granite. It i n t r u d e s t h e K a m a r g a Volcanics, a s e q u e n c e of lower g r e e n s c h i s t grade b a s a l t s a n d f e l d s p a t h i c s a n d s t o n e s of u n k n o w n age; it is u n c o n f o r m a b l y overlain by t h e ~ 1670 M a M c N a m a r a G r o u p ( S w e e t a n d H u t t o n , 1982).
078
~o
1707'+46
Ma
0 . 7 0 8 5 + 0.001 6
/
Geochronology A n altered m u s c o v i t e - c h l o r i t e granite sampled c o n t a i n s a c c e s s o r y x e n o t i m e a n d sparse zircon, sufficient for o n l y two a n a l y s e d fractions. T h e U - P b d a t a for t h e s e m i n e r a l s are given in T a b l e I I I a n d Fig. 3. T h e less discord a n t coarser zircon has a 2°Tpb/2°6pb age of 1816
09/[3s[3
0 74
070
I
I
I
I
!
2
3
4
87Rb/SSSr
16/Q/166
Fig. 4. Rb-Sr isochron plot of whole-rock ( [] ) and plagioclase ( • ) data from the Big Toby microgranite.
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Fig. 5. Harker variation diagrams for the granites of the Western Fold Belt. Broken lines enclose the field for the 1870-1840 Ma Kalkadoon-Leichhardt Association, solid lines enclose the field for the 1810-1780 Ma Argylla and Bottletree Formations (data from Bultitude and Wyborn ( 1982 ), Wilson ( 1978,1983 ) and Wyborn and Page (1983) ).
522 Ma, which establishes the minimum age of crystallization. The finer zircon fraction has lost about half its radiogenic Pb, but if the two-point discordia has validity, the extrapolated upper intercept is ~ 1820 Ma. The xenotime analyses are variably discordant, dependent on their degree of alteration, and approximately related to the high U (and presumably Th) content, from 2460 to 3170 ppm. However, they have a very close grouping in 2°Tpb/2°6pb and a mean age of 1796_+3 Ma (1 mean). This younger age could be explained as a result of minor earlier Pb loss, followed by substantial recent Pb loss from the xenotime. Alternatively, the xenotime minimum 2°Tpb/2°6pb age may be the time of igneous crystallization, and the 20 Ma older zircon age may be related to inheritance. Despite these uncertainties, the results indicate that the Yeldham Granite is comparable in age to the Big Toby microgranite, and is probably 18201800 Ma old.
Geochemistry The only sample analysed is the most peraluminous sample of the Western Fold Belt (Table V, Fig. 5 ). It has high K20, but is low in incompatible elements, like the Big Toby microgranite and other pre-1820 Ma granites in the Mount Isa Inlier (Fig. 5).
Weberra Granite Geological setting and petrology The Weberra Granite is a small pluton covering only 20 km 2 (Table I) which intrudes the Myally Subgroup and the carbonate-bearing Quilalar Formation of cover sequence 2 some 150 km north of Mount Isa (Fig. 1). Several rhyolite and trachyte dykes emanating from it appear to be comagmatic with the Fiery Creek Volcanics from the rift phase of cover sequence 3. The pluton itself is medium- to coarse-grained and ranges from a syenogranite to an alkali feldspar granite and is slightly more mafic near the perimeter. It contains quartz and K-feldspar with accessory interstitial biotite, zircon,
sphene and hematite and has been albitized in places.
Geochronology The sample analysed from this pluton has euhedral, colourless zircons which contain inclusions and cracks especially in the coarser fractions; these probably account for the very high common Pb in the analytical data (Table III, Fig. 3 ). The three non-magnetic, finer fractions have the same U content (239 ppm) and are more concordant than the remaining three, which also have little spread in U ( ~ 275 ppm). Despite the high common Pb, all six analyses are closely grouped in 2°Tpb/2°~pb, with an age range of 1691-1676 Ma (mean 1681+5 Ma). The best-fit discordia (MSWD = 6.3 ) indicates the zircon crystallization age at 169 8 +28 -2, Ma, with Pb loss at 143 __185 Ma, indistinguishable from present-day Pb loss. This 1698 Ma age for the Weberra Granite is not statistically different from crystallization ages in the Carters Bore Rhyolite ( 1678 _+3 Ma, Page (1978)) of the rift phase of cover sequence 3, and the Sybella Batholith (1671+8, 1668_+24 Ma, Page and Bell (1986)).
Geochemistry The five analysed samples from the Weberra Granite have a restricted SiO2 range from 74.37 to 78.62 wt.% and their average is listed in Table V. One sample was albitized and is aberrant on the plots. The granite is characterized by very high K20 values (Fig. 6), a high ratio of Fe2Off FeO, and high values of Zr, Nb, Y and Rb, and low Sr, similar to other post-1820 Ma granites (Table V, Fig. 5), although relative to these granites, Th, U, La and Ce are low. From the chemistry and mineralogy we suggest that the Weberra Granite is a fractionated I-type. These high K20 values are also characteristic of the Fiery Creek Volcanics, the Carters Bore Rhyolite, and dykes associated with the granite (Bultitude and Wyborn, 1982; Wilson, 1983), which also show comparable values for other elements, e.g., TiO2 and T h (Fig. 6). This sug-
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gests that the Weberra Granite may be comagmatic with the Fiery Creek Volcanics, and hence its age also dates the major volcanic components of cover sequence 3. Although these rocks clearly have altered feldspars, and the plots of some elements show considerable scatter (e.g., A1203, Fig. 6), the high K20 values are characteristic of the igneous rocks only and not of the associated country rocks, inferring that although alteration may have enhanced the K20 values of these rocks, the primary values were also high.
Sybella Batholith Geological setting and petrology The Sybella Batholith consists of elongate plutons which extend meridionally for 180 km and cover 1600 km 2 (Table I). It intrudes older basement units and the Haslingden Group of
the rift phase of cover sequence 2 (Fig. 2 ). Five main phases can be recognized: a main phase, fl-quartz phase, microgranites, pegmatites, and Kahko phase. The Kahko phase, which is similar to the Kalkadoon-Leichhardt Association, is of minor occurrence in the southern part of the batholith and has been described by Blake et al. (1984). The Sybella Batholith has been regionally metamorphosed and deformed (Wilson, 1972) and was emplaced at ~ 1670 Ma, some 60 Ma prior to the three main deformation and metamorphic events that affected the Western Fold Belt at 1610_+13 Ma, 1544_+12 Ma and 1510 _+13 Ma (Page and Bell, 1986). Greenschist grade rocks occur only in the north and northwest whilst most of the remainder of t h e batholith and its country rocks are amphibolite grade (Wilson, 1972; Bultitude, 1982; Bultitude et al., 1982 ). Despite the metamorphic im-
524
print, the original primary rock types can be discerned. The main phase ranges from a granodiorite to an alkali-feldspar granite and is even grained to porphyritic, with coarse K-feldspar augen up to 30 mm in length. Particularly in the felsic compositions, rapakivi textures are common, with the K-feldspars having albite or oligoclase rims. The fl-quartz phase in the northwest is more felsic and less metamorphosed and still retains primary phenocrysts of fl-quartz and K-feldspar, the latter commonly having albite rims. Plagioclase, biotite, hornblende (ferrohastingsite), apatite and sphene are common to both phases: fluorite is ubiquitous. Microgranites are most common in the northeastern part of the Batholith, where they contain abundant metasedimentary xenoliths. The microgranites are fine- to medium-grained, and range from granodiorite to alkali feldspar granite. They are hornblende-free and contain more K-feldspar and less ferromagnesian minerals than the main phase or the fl-quartz phase.
Kalkadoon-Leichhardt Association, which are characterized by negative {~Ndvalues ( - 1.6 to --2.7), and model source ages of between 2300 and 2100 Ma (Table II, Fig. 7).
Geochemistry Average analyses of the granitic phases of the Sybella Batholith are listed in Table V. The main phase and fl-quartz phase can be distinguished in that at the same SiO2 levels, the flquartz phase has higher Ba, Sr, MnO, Nb, La, Ce, Zr and Y, and lower A1203, Th, Rb and Pb. Both have higher TiO2, Fe, K20, P205, Th, U, Zr, Nb, Y, La and Ce, and lower A1203 and Sr contents (Fig. 5) than the pre-1820 Ma Mount Isa granites. Also some elements, e.g., Th (Fig. 5 ), F (Fig. 8), Rb and U show marked increases with increasing SiO2 content, relative to the Kalkadoon-Leichhardt Association, suggesting that the Sybella Batholith evolved by fractionation. Rapakivi textures may have developed as a result of increasing F in the melt, as such an increase would cause the ternary
Geochronology A Sm-Nd measurement on a sample of the fl-quartz phase has an eNd value of + 1.8 and a depleted model age of 1790 Ma (Table II). These values are different from those of the 8 • 6
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525 minimum during crystallization to move towards albite (Manning, 1981; Pichavant and Manning, 1984). From mineralogy and chemistry we infer that the main and fl-quartz phases of the Sybella Batholith are fractionated I-types. The greater chemical variation within the Batholith contrasts with that of the Kalkadoon-Leichhardt Association, which is thought to have crystallized by a process involving the separation of restite from a minimum-melt liquid (Wyborn and Page, 1983 ). Crystallization of these phases of the Sybella Batholith, on the other hand, were dominated by fractional crystallization from a predominantly liquid magma. The chemical differences are consistent with the Nd isotopic evidence that the melts are from completely different source materials. The microgranites of the Sybella Batholith are petrographically and geochemically distinct from the main and fl-quartz phases and are unlikely to be related to them. They are more potassic, as noted by Joplin and Walker (1961), and have lower Ti02, total Fe, MnO, CaO, Ba, Sr and Zn, and higher A1203, Na20, K20, Rb, Th and U (Fig. 5 ). Relative to the KalkadoonLeichhardt Association, these microgranites are enriched in incompatible elements, a feature common to post-1820 Ma granites, but not to the microgranites of the Big Toby Batholith and the Yeldham Granite. G r a n i t e s of the E a s t e r n Fold Belt
Wonga Batholith Geologicalsetting and petrology The Wonga Batholith covers 600 km 2 and consists of series of elongate plutons emplaced in a N-S trending belt 180 km long and up to 20 km wide (Fig. 1 ). All plutons are metamorphosed, and some are strongly deformed to augen gneisses. Like the Sybella Batholith, the Wonga Batholith is a composite collection of plutons and is broadly of three types. The main type is characterized by coarse microcline me-
gacrysts, quartz, plagioclase, biotite, hornblende (ferrohastingsite) with sphene, apatite, fluorite and zircon as common accessories. This type is commonly associated with coeval tholeiitic intrusions. Microgranites form the second type of granite and, like those in the Western Fold Belt, contain abundant xenoliths. Mineralogically they contain microcline, quartz, biotite, plagioclase, sphene, apatite and zircon, and most contain muscovite and allanite. The third and least voluminous type of granite in the Wonga Batholith is believed to be comagmatic with the felsic volcanics of the Argylla Formation (Blake et al., 1984).
Geochronology Most of the Wonga Batholith was probably emplaced at around 1740 Ma. U - P b zircon ages for plutons of the main granite type include: Burstall Granite 1745+17-15 Ma, rhyolite dykes associated with the Burstall granite 1737_+ 15 Ma (Page, 1983a), and an unpublished date of 1740 Ma on a coarse-grained pluton at Wonga Waterhole, northwest of Mary Kathleen. Two U - P b zircon determinations have been made on the microgranite type; 1671 +s Ma (Page, 1983a) for one pluton near Mary Kathleen, and ~ 1700 Ma for a microgranite 20 km to the north (Page, unpublished data). The Rb-Sr age determinations include 1621+28 Ma and 1629 + 25 Ma for the coarse-grained, main type at Wonga Waterhole, and 1700 + 20 Ma on the microgranite near Mary Kathleen. The 1670 Ma U - P b zircon granite age and the 1700 Ma RbSr determination are statistically equivalent to the volcanic events in the rift phase of cover sequence 3 in the Western Fold Belt, although there are no positively recognized sedimentary equivalents of this sequence in the Eastern Fold Belt. Two samples of the Burstall Granite have eNd values of --2.9 and - 3 . 1 and T Nd model ages of 2160 and 2170 Ma (Maas et al., 1987). An associated rhyolite (72-237G) has similar characteristics with eNd------3.7 and T Nd. These
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model ages are similar to those from the pre1820 Ma magmatism (Table II).
Chemistry As in the Sybella Batholith, fractional crystallization processes were dominant in the main type of the Wonga Batholith and produced a suite of chemically distinct, yet coeval plutons. Fractionation in some plutons produced high levels of F, Y, Rb, Th and U, especially in rocks with SiO2 values > 75 wt.% (Figs. 8 and 9). Despite having similar Nd isotopic characteristics to the pre-1820 Ma suites, the main type of granite in the Wonga Batholith is compositionally more similar to the post-1820 Ma granites, being enriched in the high field strength elements such as Th, U, Zr, Nb and Y, and depleted in A1203 and Sr. However, relative to the main phase of the Sybella Batholith, these Wonga plutons have less Ti02 and P205 and more A1203, La and Ce. In contrast to the main phase of the Wonga Batholith, the microgranites contain higher K20 and Rb, and lower Fe, Y, Zr and Nb, although the absolute values are still more typical of the post-1820 Ma felsic igneous rocks (Fig. 9).
Naraku Batholith Geological setting and petrology The Naraku Batholith occurs northwest of Cloncurry, and intrudes amphibolite grade assemblages of the Soldiers Cap Group, and the Mitakoodi Quartzite and Corella Formation of cover sequence 2 (Fig. 2). Although there are only 395 km 2 of outcrop (Table I), the batholith is believed to be at least three times larger, as aeromagnetic data indicate that it underlies the Mesozoic and Cenozoic sediments of the Eromanga Basin. The batholith consists of two distinct types of granite: microgranites and a main coarse-grained type. The microgranites form small plutons which pre-date the main regional metamorphism, and are most common where the country rock is in upper amphibolite
facies. The main coarse-grained type forms two large post-tectonic undeformed plutons. The microgranites are mineralogically similar to those previously described. They are strongly recrystallized and are in metamorphic equilibrium with the rocks that they intrude. The two post-tectonic plutons of the Naraku Batholith differ from one another. The northwest pluton is fairly mafic and varies from a hornblende-biotite tonalite at the margins to a biotite_+ hornblende monzogranite in the centre. The larger pluton to the southeast is more felsic and ranges from a biotite monzogranite at the rims to an alkali feldspar granite in the core. Both plutons are only weakly recrystallized.
Geochronology Zircon in a sample of microgranite near Cloncurry consists of clear to light brown euhedra, peppered with minute haematitic inclusions. It is commonly highly cracked, indicative of high U, although the whole-rock sample only contains 8 ppm U. Once again common Pb (Table III, Fig. 10) is relatively high, and all six fractions have suffered considerable and variable Pb loss, with 2°Tpb/2°~Pb ages of 1588-1496 Ma. The most discordant fraction ( - 75 M3) is an outlier to a well-fitted discordia (MSWD-- 2.5 ) given by the other five points. The extrapolated U - P b age, 1754+25 Ma (lower intercept 477 _+32 Ma), is considered to be the igneous crystallization age. U - P b zircon data from the main southeastern pluton of the Naraku Batholith (sample 77205032 ) have less common Pb correction, but have comparably high U and somewhat greater Pb loss than the microgranite (Table III, Fig. 10). The whole-rock sample also contains much higher U, 28 ppm. The non-linear discordant array indicates some degree of inheritance a n d / or multiple Pb-loss events, but it is not possible to distinguish these factors or interpret a firm primary U - P b age. The high uncertainty of the best-fit discordia line, 1508 + 70 Ma (lower intercept 245+ 108 Ma), expresses these reser-
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in the R b - S r systems. The microgranite near Cloncurry has a best-fit model 4 R b - S r isochron age of 1667 + 40 ( M S W D = 142, initial STSr/S6Sr 0.708 + 6). The high initial ratio and discrepancy with the zircon age may mean that this R b - S r age does not record a primary igneous event, and may have no particular meaning. However, it may be significant that this R b Sr isochron is statistically equivalent to the U Pb zircon age of one of the microgranites from the Wonga Batholith, and it also, within experimental error, correlates with the age of the main magmatism of the cover sequence 3 of the Western Fold Belt and the R b - S r isochron on the Big Toby microgranite. This suggests that there was a significant thermal event throughout the M o u n t Isa Inlier at ~ 1670 Ma, which either produced crustal melting, or reset R b - S r systematics in various localities. Five whole rocks and a plagioclase from the main Naraku type define a younger isochron ( M S W D = 1 3 . 3 , model 2) with an age of 1390 _+40 Ma (0.712 _ 2) (Fig. 10). This R b - S r isochron correlates with two K - A r biotite ages in the Naraku Granite of 1415 Ma and 1455 Ma
529 (recalculated from Richards et al., 1963) and may reflect the post-emplacement recrystallization seen in thin section. The S m - N d results also indicate two different granite types. The microgranite, 72-251A, has an eNd value of + 2.5 with a T TM source age of 1832 Ma. The main phase, 72-248, whilst having a similar eNd value of 2.0, has a younger source ge of 1637 Ma (Table II, Fig. 7).
Geochemistry The two petrographically and isotopically distinct types of granite can also be differentiated geochemically. Relative to the main phase, at similar SiO2 values, the microgranite has distinctively high P205, U and Y, and overall is enriched in incompatible elements as is typical for the major post-1820 Ma felsic igneous rocks (Table VI, Fig. 9). Both the abundance of hornblende and the total-rock geochemistry suggest the main phase is I-type. As also indicated by the isotopic data, the main phase is distinctly different from older I-type granites in the Mount Isa Inlier, as it contains more Na20 and lower K20 than for other older granites (Tables V and VI), although it is similar to values in the coeval Williams Batholith. This main phase contains distinctly high Zr relative to all granites in the Mount Isa Inlier (Table VI) and is also enriched in incompatible elements, with T h and Rb showing sharp increases with increasing SiO2 content. Ba first increases with increasing SiO2 content to ~ 70 wt.%, and then decreases (Fig. 9), inferring crystallization from a liquid (Chappell et al., 1988).
Williams Batholith Geological setting and petrology The Williams Batholith covers ~ 2100 km 2 and contains at least two different ages of granite intrusion. The older intrusions, as yet undated, consist of small microgranite bodies which pre-date the major deformation event that affects the Eastern Fold Belt (Blake et al.,
1984). The younger intrusions dominate the Williams Batholith. They were intruded after the main deformation of the inlier and cut across the major N - S structures. However, the granite is strongly foliated near major N N W - S S E or N N E - S S W trending shear zones. This foliation is defined by regrowth of biotite, muscovite and actinolite, indicating that this young deformation was of at least upper greenschist metamorphic grade. Heterogeneities within this batholith, as with the Sybella and Wonga Batholiths, are believed to be due to fractional crystallization, producing a suite of coeval, but compositionally distinct plutons. There is a north to south gradation in the plutons, as was first noted by Joplin and Walker (1961), and deeper levels of the plutonic system appear to be exposed in the south. In the northern part of the batholith, large felsic, essentially homogenous syenogranites to alkali-feldspar granites dominate, and contrast more mafic plutons in the south. These more felsic phases are often associated with small aplite and albitite bodies. Some of the more southerly plutons are coarse-grained zoned plutons which range from tonalite to granodiorite margins through to cores of alkalifeldspar granite and aplite. The plutons in the north are ubiquitously associated with the extensive development of breccias in the country rock. In places the granite is pervasively brecciated itself, veined with aplite, and shows extensive development of haematitic alteration. The breccias extend over a strike length of at least 100 km in a belt that varies from 5 to 20 km in width. They are always associated with either the more felsic, fractionated, granitic plutons or the small aplitic and albitite bodies. No breccias have been found associated with the more mafic plutons in the south, suggesting that a large proportion, but not all, of the breccias in the area south of Cloncurry were formed by fracturing and brecciation possibly caused by second boiling and decompression (Burnham and Ohmoto, 1980; Burnham, 1985) above large, high-level frac-
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532 tionated magma chambers. Geophysical evidence suggests that the Williams Batholith extends beneath the breccias in the area south of Cloncurry, and is continuous with the Naraku Batholith (Wellman, 1986). Two important variables control the petrology of this Batholith: (1) the primary magmatic variation, and (2) chemical interaction with the adjacent country rocks, particularly alteration where granite intrudes brecciated calc-silicate rocks. In the 'uncontaminated' granites, the more mafic types contain hornblende + biotite + K-feldspar + quartz + oligoclase and up to 5 wt.% magnetite, with accessory allanite, apatite, calcite, chlorite, epidote and zircon. Only more fractionated phases, particularly those that are more sodic, develop fluorite. Where the granite intrudes calc-silicate or evaporite-derived rocks, an assemblage of albite, clinopyroxene and red-brown euhedral sphene develops and haematite becomes the dominant opaque. K-rich alteration can have a characteristic red colour and is often referred to as 'red-rock' alteration.
Geochronology The Wimberu and Yellow Waterhole Granites in the northwestern and southern parts of the batholith, respectively, were sampled for U Pb zircon work. The Wimberu Granite contains euhedral, clear to translucent zircon with minor dark inclusions. The seven analysed fractions (Table III) have negligible common Pb, and, although they have a narrow range in 2°Tpb/2°SPb ages (1502-1476 Ma), there is a suggestion of minor inheritance in the array (Fig. 10). The relatively low U zircons of the Wimberu Granite are more concordant than the higher U zircons from the more felsic samples of the Yellow Waterhole Granite and the main Naraku Batholith. However, the variation in 2°Tpb/2°~pb and small dispersion in U / P b cause large extrapolated errors in the U-Pb age of the Wimberu Granite, which is 1560+~ ° Ma (lower intercept 407 + 300 Ma). Because of the likely inherited component,
1560 Ma is considered to be a maximum value for the zircon crystallization age, and an appropriate minimum (assuming no gross inheritance) can be obtained from the oldest 2°7pb/ 2°6Pb age, 1500 Ma. A biotite K-Ar age from close to the same locality is 1439 Ma (recalculated from Richards et al., 1963), considerably younger, possibly due to later metamorphism. A non-porphyritic biotite granite containing 16 ppm U was sampled from the Yellow Waterhole Granite. The zircon is generally euhedral, unzoned and has minor Fe staining. The U-Pb data show a high U, very discordant zircon population (Table III, Fig. 10). In terms of the high U content and low common Pb, the present data are inseparable from the data of the main phase of the Naraku Granite data, and overlap on the concordia plot (Fig. 8). The four-point regression for the data on the Yellow Waterhole Granite indicates zircon crystallization at 1480_+28 Ma (MSWD=7.1; lower intercept 220 _+40 Ma). The 1480 Ma zircon age is within error of Richards et al. (1963) K-Ar biotite age of 1465 Ma (recalculated to new constants) from another sample close to the same exposure, and this is also within error of a Rb-Sr whole-rock isochron of 1509 _+22 on the Mount Dore Granite some 15 km to the north (Nisbet et al., 1983). The ~ 1500 Ma zircon crystallization ages, given by the highly discordant data sets from the main type of granite from the Naraku Batholith and the Yellow Waterhole Granite, are based on the assumption of only one Pb-loss period. However, zircons in both plutons have very high U, as do the total rocks from which these samples come, and they could have sustained additional recent Pb loss, rendering the apparent ~ 1500 Ma ages too young. The zircons are too discordant and have too small a range of U / P b to resolve this, but if they had seen Palaeozoic as well as recent Pb loss, their true crystallization ages could be older, perhaps closer to the 1560 Ma age interpreted for the Wimberu Granite. This interpretation is supported by the whole-rock geochemistry, which
533 suggests that the three granite plutons dated are from the one suite, and would be expected to have similar ages. It is significant that the younger ages come from fairly high SiO2 samples with whole-rock U contents of 16 and 28 ppm, whilst the Wimberu Granite sample is relatively mafic (66.7 wt.% SiO2) and has 4 ppm U. The initial 143Nd/144Nd ratios correspond to eNd values of + 3 for sample 5322 from the Wimberu Granite and + 3.54 for sample 5320 of the Yellow Waterhole Granite. These positive end values are similar to those of the main phase of the Naraku Batholith, implying that the sources of these granites must also be new crustal additions from the mantle which are significantly different from any other granite batholiths of the Mount Isa Inlier. Both of the samples from the Williams Batholith are strongly LREE enriched, consistent with LREE fractionation during partial melting and therefore the T TM model ages of 1530 and 1620 Ma are minimum estimates. A more realistic estimate of the source age can be obtained by assuming a 147Sm/144Nd ratio in the source of ~0.14. This multistage model gives T TM ages of 1630 and 1720 Ma, respectively.
Geochemistry The Williams Batholith is predominantly metaluminous and on mineralogical grounds would be I-type (Chappell and White, 1974). There is far greater diversity both within and between the individual plutons than is found in other granites from the Mount Isa Inlier. Table VI lists the average of individual plutons. Like the Naraku Batholith, the Williams Batholith has higher Na20 and lower K20 relative to the older granites. In addition, the Cu, Pb and Zn values are lower, whilst Nb, Sr and P205 are higher than for any other granites in the Mount Isa Inlier (Tables V and VI; Fig. 9). However, like the other post-1820 Ma granites, the Williams Batholith contains high levels of incompatible elements (Table VI, Fig. 9 ), although it does not show a similar enrichment in F (Fig.
8 ). The samples that have interacted with calcsilicate rocks have noticeably higher Fe2OJFeO ratios and Na20, and lower K20, Rb and Ba. Discussion Summarizing the data section, it appears that two types of granite were emplaced post-1820 Ma in the Mount Isa Inlier: major anorogenic I-type granite intrusions which dominate the Sybella, Wonga, Naraku and Williams Batholiths, and the Weberra Granite and much less voluminous microgranites which are possibly Stype. The anorogenic I-type batholiths all have a crustal pre-history and cover quite large areas. Geophysical data suggest that although the surface extent of these post-1820 Ma granites is at least 4700 km 2 (Table I), this exposure probably represents only one third of the actual dimensions of the batholiths, and that the subsurface extent of these major post-1820 Ma batholiths is in fact a minimum of three times the actual exposure at the surface (Wellman, 1986). This has important implications for petrogenesis as significant heating of the lower crust is required to generate these large batholiths, and the question then becomes: what tectonic processes are capable of not only generating and emplacing these granites, but also their sources?
Generation and emplacement of the post-1820 Ma granites The two episodes of granite intrusion at 18201800 Ma and 1700-1670 Ma appear to be coeval with, but at the same time laterally displaced from the main locus of sedimentation or volcanic activity. For example, the Big Toby and Yeldham Granites are coeval with, but quite remote from, the extensive bimodal volcanic activity of the Argylla and Bottletree Formations and Magna Lynn Metabasalt, whilst the Sybella Batholith is geographically separate from coeval sedimentary sequences of cycle 3 (Fig. 2). In addition, some of the volcanics in the
534 sedimentary sequences are of the same approximate age as Rb-Sr metamorphic ages found in basement areas. For example, the anomalously young Rb-Sr isochrons for the microgranites of the Big Toby and Naraku Batholiths are consistent with the age of volcanic rocks in the rift phase of cycle 3. The generation of granites in the Wonga Batholith between 1760 and 1740 Ma appears to be synchronous with the development of a mid-crustal detachment surface (Holcombe et al., 1987; Pearson et al., 1987), although there are no sedimentary sequences of this age as yet recognized in the Mount Isa Inlier. It thus seems that granites generated between 1820 and 1670 Ma in the Mount Isa Inlier are related to extensional episodes, and that during them there is evidence of high heat flow throughout the inlier. The lateral separation of thermal anomalies with respect to the main surface development of the rift basins is predicted by the asymmetrical model of continental extension (e.g., Wernicke, 1981, 1985; Coward, 1986). With this model, which has been applied to the Mount Isa Inlier (e.g., Passchier, 1986; Holcombe et al., 1987; Pearson et al., 1987), the thinnest lithosphere, and hence the greatest thermal anomaly, can occur some 50 km away from the main area of surface extension. During thinning of the lithosphere, convecting asthenosphere will rise, and may result in underplating of the lower crust by mantle material (Lister et al., 1986, in press), which in turn could lead to crustal melting. It follows that during an extensional event there will be widespread heating, metamorphism, and melting of the crust, provided that there is suitable source material available to form felsic melts. The granites will most likely be emplaced above the area of thinnest lithosphere, which is away from the area of maxim u m surface rift development, creating the appearance that they are developing independent of any tectonic process, i.e., they are 'anorogenic', even though they are coeval with extensional events. Figure 12 shows the possi-
ble interrelationship between these extensional processes and granite magmatism. The relationship of the main phases of the Williams and Naraku Batholiths to tectonic events is unclear, in part, because of the problem of interpreting the U - P b systematics. On field evidence the granites clearly post-date major compressional deformational events between 1610 and 1550 Ma, which are characterized by low-P-high-T amphibolite grade metamorphism (Hill et al., 1975; Derrick et al., 1977; Jaques et al., 1982). Because of the uncertainty in the U - P b zircon data, it is not clear whether these granites were generated during this deformation, or whether they are younger than the deformation by 50 Ma, and hence unrelated to it. It is quite possible that they may be associated with a younger extensional event, which is not yet documented within the Mount Isa Inlier itself, although sedimentary sequences of an appropriate age have been recorded from the Lawn Hill Platform some 200 km to the northeast (Blake, 1987).
Source of granites emplaced post-1820 in the Mount Isa Inlier Composition o[ the source o[ the post-1820 Ma anorogenic I-type batholiths Because the processes of fractionation produce a diverse series of granite compositions within each batholith, we cannot model the precise composition of the source regions of these major post-1820 Ma anorogenic I-type batholiths, other than to infer that it was mafic, as implied by the abundance of amphibole in the more mafic end-members. The major post-1820 Ma I-type granites are characterized by high levels of incompatible elements and in most cases abundant fluorite. Experimental work on enriched, F-bearing felsic igneous rocks suggests that they are formed from relatively high temperature, water undersaturated, completely molten (restite-free) magmas (Clemens et al., 1986). We suggest that the change to more enriched compositions in
535 Core complex; upld2 may torm small local microgranites which intrude high grade metamorphics
Rift-fill sediments v Felsic volcanics
~ ......
Granites Mafic volcanics
~
Gabbros and other mantle derived intrusives •
ls/e/16s-1
Dolerite dyke
Fig. 12. Postulated relationship between granite generation and underplating during asymmetric continental extension (based on Listerr et al., 1986, in press).
felsic igneous rocks post-1820 Ma reflects a change in tectonic process, with the extensional tectonics having the capacity to generate the high temperatures on a regional scale (see Sandiford and Powell, 1986) necessary for the melting of these sources. Thus, although the source age for the main type of granite in the Wonga Batholith pre-dates both the orogenic event between 1880 and 1870 Ma and the granite event between 1870 and 1840 Ma, this source possibly did not melt at these times, presumably because temperatures reached during these thermal events were not sufficiently high to generate melts from this particular source.
Formation of the source of the major post1820 Ma I-type batholiths The S m - N d isotopic evidence can be interpreted to infer that the granites are derived from sources that are up to 400 Ma older than the
age of granite emplacement. However, Ardnt and Goldstein (1987) have suggested that these model source ages may in fact provide only an estimate of the average time that the material has been resident in the continental crust, and suggest that these model ages can be interpreted only as the time of crust-mantle separation if supported by other geological evidence. In the Mount Isa Inlier, each discrete granite event has a particular composition, and with decreasing age of emplacement there is a progressive decrease in model source age and an increase in the ENd values. There is no increase in model age in a particular geographical direction, which in other studies has been interpreted to mean an increasing mixing of an Archaean component with new mantle-derived Proterozoic material towards an Archaean craton (e.g., Chauvel et al., 1987). Because each major episode of granite for-
536 mation has produced a unique composition, we doubt that they are produced by variable mixture of two end-member components, an Archaean source and early Proterozoic mantlederived mafics. We conclude that the source of each granite is derived during separate, older Proterozoic geological events, during which large volumes of mantle-derived mafic material underplated the lower crust. The large area of these batholiths, combined with the fact that the granites are derived by small degrees of partial melting, implies that the source areas for these granites must be very voluminous in the lower crust. We suggest that there are at least four tectonic/geological processes, three of which can be observed in the Mount Isa Inlier, which could emplace sufficiently large volumes of mantle-derived mafic material into the lower crust to subsequently form the source of these major I-type anorogenic granites. The first possible underplating process occurs in association with the emplacement of continental tholeiites. Cox (1980) has suggested that only in areas of exceptional crustal thinning will parental picritic magmas rising from the mantle reach the surface. The most common scenario is that these magmas will intrude the base of the crust as a series of sills which will differentiate into upper gabbroic and lower ultramafic portions. There are significant tholeiitic events in the Mount Isa Inlier (e.g., Ellis and Wyborn, 1984), particularly those associated with cycle 2 between 1800 and 1760 Ma (including the Eastern Creek and Marraba Volcanics, the Magna Lynn Metabasalt and dolerite dyke swarms which cover at least 3600 km 2 (Table I)). It is probable that during these a considerable portion of the lower crust in the Mount Isa Inlier was underplated, in order to form the subcrustal magma chambers necessary for the generation of these enriched continental tholeiites; these subcrustal magma chambers may have subsequently become the source of the later major I-type granites.
The second process is underplating that occurs in the area of greatest lithospheric extension (Fig. 12). This underplating also places mafic mantle-derived material at the base of the lower crust, which can also be the source for I-type melts. The third underplating process which is likely to have occurred in the Mount Isa Inlier, is associated with the two main compressional deformations between 1890 and 1870 and 1610 and 1550 Ma (Fig. 2). Both are characterized by high-T, low-P prograde andalusite-sillimanite metamorphism, which requires some heat input from the mantle (e.g., England and Richardson, 1977). Houseman et al. (1981) have argued that during compression, not only does the crust thicken but the underlying mantle lithosphere shortens and thickens also, causing the submersion of cold, dense material into the surrounding lithosphere. They calculated that the layer that forms the transition from stable lithosphere to the convecting asthenosphere may become unstable and stretch, causing the colder lithosphere to sink and to be replaced by hotter asthenospheric material. This would then place relatively primitive mantle material into the lower crust. Two consequences of this are that an abnormally high geothermal gradient would be created within the crust during deformation (which is actually observed during both major deformations ) and, yet again, more mantle material would be emplaced into the lower crust to form the source of I-type granite batholiths. The fourth possibility is not actually evident in the Mount Isa Inlier itself, but has been proposed for other Proterozoic domains in Australia. Etheridge et al. (1987) and Wyborn (1988) have argued that underplating associated with small-scale mantle convection between 2300 and 2000 Ma triggered off the formation of the widespread early Proterozoic basins. Most of this source melted at around 1870-1840 Ma during a compressional orogenic event to produce widespread comagmatic granites and volcanics. However, some (perhaps less
537 fractionated) parts did not melt until later extensional events (e.g., the Wonga Batholith). Any of the four processes outlined above will emplace significant amounts of mafic mantlederived material into the lower crust which may subsequently become the source of I-type granites. This supports the idea of major unique crustal additions, which when subsequently remelted will give granites of specific compositions. It is tempting to note that the T Nd age for the Sybella Batholith is coincident with the rift phase of cycle 2 and that continental tholeiites from this rift phase in the Western Fold Belt (Eastern Creek Volcanics) are the most enriched in incompatible elements in the Mount Isa Inlier (Bultitude and Wyborn, 1982). In order to produce these tholeiites, there must have been underplating of the crust in this part of the inlier (Cox, 1980), not all that distant from where the Sybella Batholith was ultimately emplaced. Another coincidence occurs with the T TM ages for the Williams and Naraku Batholiths which are coeval with the high-T-low-P metamorphism associated with the main, young compressional deformation in the Eastern Fold Belt. This implies that during this deformation, new mantle-derived material was emplaced into the lower crust, as predicted by the model of Houseman et al. ( 1981 ). This resulted not only in the formation of new I-type granite sources, but also produced a high geothermal gradient in the Eastern Fold Belt. However, because the model source ages are only an approximation, we cannot argue definitely which process ultimately formed each distinctive granite source. Nonetheless, we contend that: within the geological record preserved in the Mount Isa Inlier in the post-1820 Ma period there is evidence for there being several tectonic a n d / o r geological processes which would generate mantle-derived underplated layers in the lower crust. Evidence of an underplated lower crust comes from seismic refraction data which suggest that there is a lower crustal mafic layer in the Mount
Isa Inlier. A high-velocity lower crustal layer up to 15 km thick has been indicated by seismic refraction results (Drummond, 1982; Finlayson, 1982). D r u m m o n d (1982) suggested that this layer is made up of at least 45% gabbroic material. From some of this it would be possible to derive the major post-1820 I-type batholiths by small degrees of partial melting.
Comparison with other post-1820 Ma Proterozoic granite suites Wyborn et al. (1987) have shown that the distinct granite events that dominate in the Mount Isa Inlier between 1820 and 1450 Ma occur throughout the Australian continent, and are similar in composition and emplacement style, e.g., the Williams Batholith is coeval with and chemically comparable with a major granite province in the Stuart Shelf of South Australia, whilst the 1670 Ma Sybella Batholith has compositional similarities to 1660 Ma granites in the T e n n a n t Creek Inlier of the Northern Territory. These 1670 Ma ages are also similar to ages of high-grade metamorphism in the Arunta Inlier, suggesting that there may be systematic major crustal thermal anomalies throughout the Proterozoic which could be related to continent-wide extensional and compressional events. These major I-type granites from the Mount Isa Inlier are also similar to the major Proterozoic anorogenic granite suites described from the U.S.A. (e.g., Emslie, 1978; Anderson et al., 1980), Finland (e.g., Nurmi and Haapala, 1986) and Sweden (e.g., Lindh and Gorbatschev, 1984), some of which are characterized by rapakivi textures. The relationship of these granites to rifting has also been suggested by Emslie (1978). Nelson and DePaolo (1985) also record a similar change to more positive ENd values in similar granite suites. It could be that on a global scale there are comparable and coeval granite types, suggesting that perhaps t h e s e 'anorogenic' granite types are the end product of a sequence of tectonic processes that are related to global mantle activity.
538
Source of the post-1820 Ma microgranites The identification of the microgranites as Stype is tenuous, as they are only weakly peraluminous to metaluminous and, although some have muscovite, they do not have cordierite or andalusite, which are so characteristic of the Stypes of the Lachlan Fold Belt (White and Chappell, ]983). The weakly peraluminous character may be due to their sources being fairly feldspathic sediments which have undergone only limited sedimentary recycling. The post-1820 Ma Mount Isa microgranites can be subdivided into two groups: (a) the Yeldham Granite and the Big Toby microgranite, both of which have depleted compositions similar to those of the Kalkadoon-Leichhardt Association; and (b) microgranites of the Sybella, Wonga and Naraku Granites, which have enriched compositions comparable with the major post-1820 Ma granites. Suitable sources for these microgranites are only found in the quartzo-feldspathic sediments of basement units (such as the May Downs Gneiss and Yaringa Metamorphics) and cover sequence 2 (including parts of the Argylla, Bottletree and Yappo Formations, and the Ballara and Mitakoodi Quartzites). Although preliminary geochemistry suggests that the Big Toby microgranite cannot be derived from the adjacent Yaringa Metamorphics, as they do not contain enough Ca, Na, Sr of Pb (indicating insufficient feldspar), compositionally suitable feldspathic sediments do occur in the May Downs Gneiss to the east. In contrast, the younger microgranites can be derived chemically from the sediments of the Bottletree Formation and the Ballara and Mitakoodi Quartzites, which tend to be much more enriched in incompatible elements than the basement quartzo-feldspathic sediments. Conclusions
(1) Two types of granites were intruded into the Mount Isa Inlier between 1820 and 1450 Ma. The most significant consisted of large anoro-
genic I-type granites which were emplaced at 1760-1740 Ma, 1700-1670 Ma and 1560-1480 Ma. The second were small volumes of S-type microgranites emplaced at 1820-1800 Ma, 1760-1740 and 1670 Ma. (2) All of the granites have isotopically recognizable pre-histories which infer derivation from lower crustal sources emplaced between 2.3 and 1.5 Ga. (3) With the exception of the two oldest microgranites, the post-1820 Ma granites contrast in composition with the major granite batholith emplaced between 1870 and 1840 Ma. (4) The generation of both the granites and their sources can be related to recognizable tectonothermal regimes. Most of the granites were coeval with known extensional events, whilst the sources were formed either during underplating associated with extension or that associated with major compressional deformational events. (5) Finally, in a pedantic way, we question the use of the term 'anorogenic' to describe these major granite suties. The 'Glossary of Geology' (Bates and Jackson, 1980 ) defines 'anorogenic' as "not orogenic, lacking or unrelated to orogenic disturbance". Yet in both their emplacement in the upper crust and generation of their sources in the lower crust these granites were related to significant tectonothermal (=orogenic? ) events. In fact, without these, it would be impossible to obtain the heat required to cause melting on such a large scale, not only for the generation of these melts, but also for the prior formation of major I-type sources within the lower crust. Acknowledgements
We acknowledge D. Wyborn, G.M. Derrick, I.H. Wilson, D.H. Blake, J. Pye and K. Mitchell for helpful discussion and assistance in the early stages of field work. Painstaking reviews by D.H. Blake, I. Fletcher, J. Sheraton and D. Wyborn are gratefully acknowledged. T.K. Zapasnik, N.C. Hyett, D.B. Guy and L.A. Keast
539 prepared the mineral separates, and M.J. Bower u n d e r t o o k t h e U - P b c h e m i c a l p r o c e d u r e s . J. Pyke, W. Pappas and J.A. Haldane carried out t h e g e o c h e m i c a l a n a l y s e s , w h i l s t M. O w e n w r o t e t h e c o m p u t e r p r o g r a m s u s e d in a s s e s s i n g t h e data.
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