October 1992

Chinese earthquakes to rock Australian

buildings China and Australia signed a Memorandum of Understanding (MOU) in April 1990 committing Australian seismologists from AGSO and the Seismology Research Centre (SRC), and Chinese seismologists from the State Seismological Bureau (SSB), to undertake joint research into intraplate earthquakes to improve mitigation techniques and hazard assessments.

Knowledge of how strongly the ground shakes, for how long, and at what frequency, is the basic input required by structural engin•eers investigating the response of buildings and structures. This data is used in computer simulation and laboratory modelling of the effects of the earthquake on structures, so that better building codes and structural design rules can be developed. Civil engineering de•partments at Melbourne and Adelaide Univer•sities have large computer-controlled shaking tables on which they have modelled structures using a AGSO strong-motion record of a Tennant Creek aftershock.

One of the two joint programs involved coupling two SRC designed and manufac•tured digital recorders (Kelunji) to SSB accel•erometers and installing the accelerographs near Tang shan , China. One of these was set up by Peter Gregson (AGSO) and Gary Gibson (SRC) during their visit in OctoberlNovember 1991; the other by SSB seismologists a few months later. Tangshan was destroyed by a magnitude 7.6 earthquake in July 1976 and some 250 000 people are reported to have died. No measurement of the near-field ground motion was obtained.

In contrast to Australia, earthquakes occur more frequently in China and the period of recorded Chinese history is more than ten times longer. The tectonic setting (intraplate) and earthquake mechanisms in parts of China are similar. It is hoped that important record•ings of the near-field ground motion can be obtained more rapidly in China than Australia, and used in both shaking table experiments and revision of the Australian building code.

Recent earthquakes near Tangshan trig•gered the two Kelunji recorders, producing about thirty accelerograms. The largest of them, in February 1992, had a magnitude 4.4, and was less than 20 km from the recorder where the peak accelerations was about 0.03 g. In contrast to accelerograms re•corded in Australia from similar-s ized events at comparable distances, the Chinese records seem to have lower amplitudes, longer duration and lower frequencies . Detailed analysis must await receipt of the digital data.

For further information contact Kevin McCue, Australian Seismological Centre,AGSO.

AGSO Research Newsletter 17

Zircon U/Pb chronology, tectono-thermal and crust-forming events in the Tomkinson Ranges, Musgrave Block, Central Australia

The Musgrave Block is an important geological feature between the Proterozoic Arunta and Gawler Blocks, and the Paterson and Albany-Fraser belts. The dominant event in its evolution may be correlated with the global Grenville Orogeny (Fig. 1). Geochemical analysis and SHRIMP UlPb zircon dating of major rock units in the Tomkinson Ranges were carried out as part ofthe NGMA Musgrave Project. This study aims to define major tectono-thermal and magmatic events; to correlate between various rock types and units; and to define the age and genetic relationships between the layered mafic/ultramafic Giles Complex, regional granulite-facies metamorphism and widespread porphyritic and rapakivi granites. Finally, the geological relationships between the Musgrave Block and neighbouring blocks and the possible connection with the global Grenville Orogenic event are explored.

Clarke & others discuss the intrusion, deformation and recrystallisation history of the Giles Complex and host granulites (see their article in present issue for regional geol•ogy and geochronological events).

The present article is concerned with the following questions:

• What are the differences in age and style of crust formation between the Musgrave and its neighbouring blocks?

• Was the thermal pulse associated with the Giles Complex also responsible for the regional granulite-facies metamorphism?

• Is the rapakivi granite intruding the Hinck•ley Intrusion the result of mixing of por•phyritic granite and mafic rock?

• What are the genetic relationships be•tween different generations of mafic dykes and the Giles Complex, on one hand, and younger mafic and felsic igneous rocks, on the other?

• How widespread in the Musgrave Block are felsic volcanics and granites with chemistry similar to the Tollu Volcanics, which are highly enriched in K, Rb, Ba, Th, U, Nb, Zr, Y, and rare earth elements (e.g. La, Ce, Nd)? How were these A-type granites and volcanics generated?

SHRIMP U-Pb zircon ages New zircon ages, using the SHRIMP at

the Research School of Earth Sciences, ANU, confirm earlier studies using whole-rock Rb•Sr dating (e.g. Gray, 1978: Journal of the Geological Society of Australia, 25, 403-414). The ages of various events, mainly

..-•"..--/

/ /

•.... . " D.B - 1.4 Ga sedimentary rocks " ~. (Belt & equivalent)

D D,B-I,5Ga

D 1,5-2,IGa

V!\"/l > 2. 1 Ga

/ '

16/60 • Mafic complexes & anorthosites

Ophiolites (?)

s Suture (?)

Trend lines

Fig. 1. Cartoon with generalised Precambrian features of part of Gondwana and North America, and their possible correlation. F for Fraser belt; G for Gawler craton. Modified and published with permission after Figure 3 of Moores (1991: Geology 19, 425-428).

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AGSO Research Newsletter 17

based on whole-rock Rb-Sr dating, are sum•marised in Table 1 of Clarke & others (this issue). SHRIMP U-Pb zircon dating provides more accurate ages, in favourable circum•stances, and can distinguish multiple thermal events. For example, zircons from felsic granulite between Michael Hills and Mt Davies (sample 69- 1304 from location 20 of Gray, 1978) give a near-concordant age of 1305 ± 8 Ma (Rb-Sr age 1358 ± 40 Ma) for the possibly volcanic protolith, whereas meta•morphic overgrowths of zircon are 1223 ± 7 Ma old. The Minno Granite from location 7 of Gray (1978), south of the Kalka Intrusion was emplaced before D2 deformation, and well-zoned, slender igneous zircons give a concordant age of 1198 ± 6 Ma (Rb-Sr age 1204 ± 17 Ma). Inherited zircon cores indicate an age of 1296 ± 10 Ma for the source rock (Fig.2). Zircons from a banded felsic granulite from Mount Aloysius suggest a protolith age of about 1530 Ma and granulite -facies metamorphic age of about 1200 Ma. In addi•tion, two age groupings of about 1300 Ma and about 1400 Ma are tentatively identified and may correspond to other metamorphic events.

A porphyritic granite, post-dating the Giles Complex from Champs de Mars (Fig. 1 in Clarke & others, this issue), contains abun•dant igneous zircons which define an age of 1185 ± 5 Ma. The emplacement age of the Giles Complex can thus be constrained to between granite intrusion at 1185 Ma and granulite facies metamorphism at 1200 Ma. To clarify the relationship between the Giles Complex and the granulite-facies metamor•phism, one future approach will be to date features formed directly by the Giles Com•plex, such as 'back intruded' felsic dykes and contact metamorphic rocks.

Zircon U/Pb dating ofthe Tollu Volcanics (related to the cauldron subsidence) of the Bentley Group (west of the area shown in Figure 1 of Clarke & others, this issue) and a 'granophyre' sample from the Bell Rock Intrusion have both a similar chemical com•position as well as a similar age of about 1060 Ma. This age is within the age uncer•tainty of Type B mafic dykes of the Musgrave Block (Table 1 of Clarke & others, this issue) .

0.25 Minno Augen Gneiss

:::>

"" .., N

~ 0.20 co <=I N

70-203

1000

Nd isotope model ages and crustal residence times

Sm-Nd model ages (T DM) of six samples (1770- 1890 Ma) reported by McCulloch (Geodynamic Series 17, AGU, 115- 130, 1987; see also for definition of the depleted mantle model age, T DM) indicate that the pro•toliths of the Musgrave Block had a younger crustal resident time (approximately an aver•age age for source rocks last separated from the mantle) than the Arunta Block (three analyses, 2120--2190 Ma). Additional new data from both Blocks confirm this earlier conclusion. Our extensive sampling coverage in the Tomkinson Range give a broader range ofNd TDM model ages (1460-1910 Ma) . The young model ages of 1460- 1570 Ma are rep•resented by the Tollu Volcanics, a 'grano•phyre' within the Bell Rock Intrusion (with a zircon age of about 1060 Ma) and related granites. Felsic granulites of possible volcanic origin from Mount Aloysius (proto lith age -1550 Ma) also have young T DM model ages of 1620- 1740 Ma. This suggests new crustal formation at -1550 Ma with only minor con•tribution from older sources, as opposed to melting of older basement rocks. Metasedi•ments from Mount Aloysius have T DM model ages ranging from 1610 Ma (meta-arkose of local origin) to 1900 Ma (sillimanite gneiss). Felsic granulites with protolith ages of about 1300 Ma from several locations south of the Hinckley Fault (Fig. 1 in Clarke & others, this issue) have TDM model ages of 1660-1780 Ma, similar to model ages for -1550 Ma Mount Aloysius granulites. Granites gener•ated during the 1200 Ma granulite-facies metamorphism at Mount Aloysius, Ewarara and Minno (south of the Kalka Intrusion) have Nd TDMmodel ages of 1680- 1860Ma, simi•lar to those of the country rocks.

Nd T DM model ages of porphyritic and rapakivi granites in the Champs de Mars area are of special interest for testing models of magma mixing. Two samples of porphyritic granite, having T DM model ages of 1877 and 1905 Ma, are among the oldest in the Tomkin•son Range, with low initial eNd values of -3 .7 and -4.2 at 1185 Ma emplacement time, whereas four samples of rapakivi granite have

1198 ±6 Ma (26)

0.15 L ________ ---1 _________ -L ________ '::.6/:..:6.:..J1

1.5 2.0 2.5 3.0 207 Pbl 235 U

Fig. 2. Concordia diagram for zircons from Minno granite gneiss, emplaced during granulite facies metamorphism. 1198 ± 6 Ma is the time of granite emplacement whereas 1296 ± 10 Ma the age of the protolith.

10

October 1992

considerably younger model ages of 1675-1787 Ma and higher initial eNd values of -1.4 to -3 .1. In addition to the possibility that rapakivi granites had an origin independent of porphyric granites, these data may be inter•preted either in terms of mixing between a porphyritic granite magma and an evolved mafic magma, or partial melting of a mafic source with a young model age and a higher initial eNd value. Both interpretations are consistent with the rapakivi granites being associated with and intruding the Hinckley mafic/ultramafic Intrusion, and being gener•ally more mafic (consistent with assimilation) than the porphyritic granites.

Giles Complex and mafic dykes Ballhaus & Glikson discussed in the April

1992 issue of BMR Research Newsletter the magmatic -metamorphic evolution and eco•nomic potential of the Giles Complex. They suggested that individual intrusive bodies had similar tholeiitic parent magmas in the mantle, but experienced different poly baric fractionation paths. On the basis of the chem•istry of some 'chilled' samples and potential feeder dykes, as well as model calculations using cumulate rocks, the parent magma com•position with an MgO content of 12% (see Table 1) was estimated. It is characterised by low Ti02 -0.8%, P20 S -0.09%, La -7 ppm, Nb -3 ppm, Sr -200 ppm, and Zr -70 ppm•indicating magma generation by a large de•gree of mantle melting at shallow depths (30-50 km?). The less abundant contemporary, or slightly younger mafic dykes have higher contents of these elements (e.g. 1.6% Ti02, 150 ppm Zr), indicating that they were gener•ated by smaller degrees of melting . Sr and Nd isotopic analyses of samples from Kalka and Wingellina Intrusions show considerable variation of initial 87 Srj86Sr (0.704 to 0.708) and eNd values (+2 to -2), suggesting differ•ent extents of crustal assimilation by the magma. The estimated parent magma compo•sition of the Giles Complex and contempo•rary mafic dykes are similar to basalts gener•ated in many modem continental rifts.

Chemical similarity of -1060 Ma meta•morphosed, NW-trending Type B mafic dykes (Table 1 of Clarke & others, this issue), and the Mummawarrawarra Basalt of the Bentley Group (Table 1) suggests that they may be genetically related.

Rock chemistry: implications for tectonic environment and melting

Chemical data (Table 1) and Nd isotope model ages, discussed earlier, plilce con•straints on the tectonic environment and con•ditions of melting. Compositions of basalts and mafic dykes (Table 1) of various ages from the Tomkinson Range are consistent with an origin in an intraplate environment rather than a convergent margin. Nd isotope model ages of the source rocks of about 1300 Ma felsic protoliths and 1200 Ma granites are similar to those of 1550 Ma sedi•mentary and igneous protoliths of the felsic granulites . Thus, these 1300 and 1200 Ma rocks could simply have been derived by remelting of -1550 Ma crust.

Chemical compositions of the 1185 Ma porphyritic granites and the 1060 Ma Tollu

October 1992 AGSO Research Newsletter 17

Table 1. Chemical composition of representative meta-igneous rocks and mafic dykes, Tomkinson Range area (major elements-weight %; trace elements-ppm)

1550 Ma 1185 Ma granites Mount Alo}'sius Estimated Giles Cham~s de Mars

mafic felsic Parental magma por-phyritic rapakivi mafic Volc_ Volc_

Si02 47 73 Ti02 1.5 0.2 A120 3 15.5 15 .3 IFeO 13.0 1.0 MnO 0.18 0.02 MgO 8.0 0.3 CaO 11.4 1.8

Na20 (1.2) 4.7 K20 (0.7) 3.6

P20S 0.13 0.03

Zr 77 110 Nb 4 2 Y 32 5 La 11 19 Ce 20 32 Nd 14 9 Th 4 U 0.5 Ga 21 23

Volcanics, as well as related granophyres and granites, show affinities to A-type granites, with high Zr, Nb, Y, REE, Th, U contents, in addition to high GaiAI ratios. However, they have quite different Nd model ages and char•acteristic element ratios. Porphyritic granites in the Champs de Mars area have the oldest Nd isotope model ages of the samples ana•lysed, consistent with an origin by melting of refractory crust after granulite facies meta•morphism (1200 Ma). The generation ofthese porphyritic granites may have been caused by the thermal anomaly associated with the Giles Complex, if one assumes a similar age. The Tollu Volcanics, in contrast, have consider•ably younger Nd isotope model ages (1460-1570 Ma) and thus cannot have been derived from pre-existing felsic crust. They have very high Nb and Th contents (up to 100 ppm) and Nb/Y and Nb/Ce ratios, features that are char•acteristic of differentiates of intraplate basalt (e.g. Eby, 1992: Geology, 20, 641-644), and positive initial eNd values similar to contem-

d}'ke 49 68 65 51 0.8 0.5 1.3 0.9 14 14.5 13.1 16 11 3.4 6.3 11

0.17 0.06 0.11 0.16 12 0.7 1.2 6.7 11 2.0 2.9 -10

1.6 3.4 2.7 2.4 0.3 5.1 4.6 0.7

0.09 0.08 0.6 0.16

70 470 560 120 3 27 32 5

18 140 90 26 7

17 (280) (200) (25) 10 140 100

100 30 8 2

13 21 21

porary basalts of the Bentley Group. The Tollu Volcanics were, therefore, probably de•rived through extensive fractionation of ba•saltic magma in stable intraplate magma chambers.

Broader tectonic implications Crust-forming episodes at 1550 and

1300 Ma, observed in the Musgrave Block, have no counterparts in the neighbouring early Proterozoic Arunta and Gawler Blocks, suggesting an independent geological evolu•tion (Shaw & Black, 1991: Australian Jour•nal of Earth Sciences, 38, 307-332). On the other hand, 1300 and 1200 Ma magmatic and metamorphic events are well documented in the Albany-Fraser Province (Fig. 1) to the southwest of the Musgrave Block (e.g. Black & others, in press: Precambrian Research). A connection to the same tectono-thermal events experienced in Albany-Fraser belts can be suggested. Moores (1991: Geology,

14

18

-1060 Ma Bentle}, Grou~ -800 Ma

Mumawa- Tollu mafic dyke rrawarra Volc_

51 72 50 1.3 0.4 1.7

13.5 12.5 14.7 11 3.2 12.0

0.15 0.09 0.17 6.8 0.1 6.5

-10 1.5 11 2.3 3.6 2.2 1.3 5.5 0.3

0.18 0.03 0.16

150 950 105 6 80 5

29 180 27 10

(30) (280) 26 18 140 18

45 6

18 35 16

19,425-428) recently proposed that the Al•bany- Fraser Province was part of a Grenvil•lian fold belt (1300- 1100 Ma), which ex•tended from the Appalachian and Antarc•tica, into the Albany- Fraser Province (Fig . 1). Moores postulated thatthis Grenvil•lian belt extended farther to the south of the exposed Musgrave Block (Fig. 1). This would explain the apparently contrasting tectonic environments of the Musgrave Block (in•traplate) and Albany-Fraser province (colli•sion) even though both are of the same age, i.e. 1200 Ma. This proposal, however, needs closer examination. Available geological and geophysical (gravity and magnetic) data from central and western Australia re•quire a more complicated Grenvillian tec •tonic evolution than shown in Figure 1. Ad•ditional research is required.

For further information, contact Dr. Shen-su Sun or Dr. John Sheraton (Minerals and Land Use Program) at AGSO.

Aquifers at risk: towards a national groundwater quality

The Australian National University's Centre for Continuing Education is sponsoring a conference in Canberra, 15-17 February 1993, in co-operation with the Australian Geological Survey Organization (formerly B.M.R.), to provide a scientific and community forum on a range of groundwater quality issues. This conference will be the ninth in the series of Issues in Water Management.

High-salinity groundwaters have always been a constraint on water supply development in Australia. Where groundwaters contain high concentrations of naturally occurring sub•stances, such as nitrate, fluoride, and heavy

perspective . metals, consumption of these groundwaters may constitute a significant health hazard.

Numerous questions arise: Where are the major occurrences of these substances and what can be done for remediation of small•scale, local water supplies? In regard to sev•eral important inland and aquifers undergoing increasing salinity due to excessive with•drawal of groundwater, how best can these systems be managed in the face of increasing water consumption and potential sealevel rise caused by global warming?

Several important regional groundwater systems are known to be polluted by sewage and agrichemicals. Some aquifers are used for

domestic purposes, whereas others infiltrate sensitive surface waters. What modifications are needed in waste management, land use, and agricultural practice to control this pollu•tion of both subsurface and surface waters? Furthermore, an increasing number of local aquifers are polluted by toxic chemical and other wastes, and petroleum products. How can these problems be managed: can polluted aquifers and land be effectively rehabilitated? Do we have adequate health and environ•mental protection standards?

The processes involved in groundwater quality deterioration, and the management, remediation and protection of the nation's

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