Cycles in the Warrawoona Group (3.47-3.4 Ga)

Cycles in the Warrawoona group (3.47-3.4 Ga)

Report of the results of the field work in the Coongan and Warrrawoona greenstone belts, Pilbara Block, Westem Australia.

Diana de Leeuw Faculty of Earth Sciences Department of Petrology and Isotope Geology Vrije Universiteit Amsterdam The Netherlands September 2000 Contents

page

Abstract 2

1 Introduction 3

2 Geological setting 5

3 Previous work 7

4 Coongan greenstone belt 11 4.1 Introduction 11 4.2 Cycli 11 4.3 Rock types 14 4.4 Interpretation 16

5 Warrawoona greenstone belt 19 5.1 Introduction 19 5.2 CycH 19 5.3 Rock types 21 5.4 Interpretation 22

6 Sampling 24 6.1 Introduction 24 6.2 Geochemistry samples 24 6.3 Zircon samples 25

7 Conclusion and discussion 26

Acknowledgements 28

References 29

Appendix la: Geological map lb: Cycle map 2 : Stratigraphical columns 3 : Pictures 4 : Sample list

Picture on front page shows the Duffer sequence and gives an impression of the field area.

1 Abstract

This report contains the results of the data that were obtained during field work in • the oldest recogmzed greenstone unit, the Warrawoona Group (3.47-3.4 Ga), in the Coongan greenstone belt and the Warrawoona greenstone belt, in the East Pilbara in Western Austraha. The rocks that constitute the greenstone belts are assumed to be deposited into a sedimentary basin. In both of the studied greenstone belts rock units can be grouped into cycles. A cycle mainly consists of a volcanic base with a sedimentary top or contains sediments only. In the studied part of the Coongan greenstone belt eight different cycles were recognized. In the studied part of the Warrawoona greenstone belt three different cycles were found. Facing direction in the Coongan greenstone belt is to the east, facing direction in the Warrawoona greenstone belt points to the southeast. On base of this younging direction and the occurrence of rather comparable cycles in both greenstone belts, the cycles of the Coongan and Warrawoona greenstone belts are assumed to be correlated with each other, with cycles in the Warrawoona greenstone belt lying on top of the cycles in the Coongan greenstone belt.

From cycle 1 in the Coongan greenstone belt up to cycle 3 in the Warrawoona greenstone belt the basin infill teUs the story of the basin. First a basin was formed into an extending basement. During the first cycles the basin was mainly filled with submarine basaltic/andesitic flows and sediments, after which the upper part was eroded. During beginning of repeated infill with coarse erosive material, the basin collapsed and growth faults occurred. In the north of the area a rhyodacitic volcano (dated at 3.467-3.468 Ga) is assumed to have been active at this titne. During the upper cycles the basin was mainly filled with sedimentary rocks. In the sedimentary package only one cycle is present with submarine mafic flows, m which volcanic exhalatives occur. In the upper sedimentary cycles slumps occur. In the Warrawoona area large erosion products were found in the cycle that is assumed to be lying on top of the sedimentary package of cycles in the Coongan area. During cycle 2 the sedimentary period changes to a period with mafic flows again in cycle 3. This whole basin evolution is assumed to be happened between about 3.47 (age Duffer) and 3.4 (age of felsic intrusives) Ga. Younger units are present in the Warrawoona greenstone belt, but have not been studied for this report. After deposition in the basin the total basin was tilted to the east by an unknown process. After that the batholiths are assumed to be intruded and several shear events took place.

Stratigraphy and facing direction in the studied parts of the basin do not support the diapiric model of Collins et al. (1998). Facing is directed throughout the studied Warrawoona group to the east or southeast and no symmetric repetition was found in the studied parts of the greenstone belts. Therefore the mid-crustal detachment model is preferred above the diapiric model. This implies that sub-horizontal forces were already active in the early Archaean.

2 1 Introduction

This report is the result of a field work project which is part of the larger Pilbara project which is coordinated by Prof. Dr. S.H. White. Research in the Pilbara project is concentrated on the tectonic-kinematic evolution of the early Archaean granitoid- greenstone terrain in the Pilbara Craton, Western Australia, with as mam goal to determine the tectonic processes in the early earth history. About those tectonic processes that were active in the early Archaean of the Pilbara craton contrasting ideas exist. One model prefers doming of granite bodies in ductile crust (Collins and Van Kranendonk (1999), Collins et al. (1998) while in the other model tectonic processes are mainly driven by sub-horizontal forces as would be expected by (an already cooled) relative cool and brittle upper crust and a more ductile middle crust (Kloppenburg et al. (submitted), Zegers (1996), Zegers et al. (1996)). In the diapiric model the differences between tectonic processes in the early crust and recent orogenic processes are emphasized. In the model with sub-horizontal forces (mid-crustal detachment model) the same tectonic processes are active throughout earth history and the different appearance of the early earth crust is interpreted to be a result of differences in lithology. The processes and force pattems in the latter model are comparable with these in the modem earth crust, which makes this model potential compatible with the plate-tectonic model, in contrast to the diapiric model. As a participant in the Pilbara Project my research was concentrated on the oldest recognized greenstone unit (Warrawoona group, dated at 3.47-3.4) to gain more information about the tectonic processes that were active during the early Archaean in order to shed hght onto the discussion between the two contradicting models. Field work was carried out in July and August 2000 in two key areas in the eastern part of the Pilbara craton. The main area was situated in the Coongan (NS-orientated) greenstone belt to the east ofthe Shaw bathoMth. The other area comprised parts of the Warrawoona (EW- oriented) greenstone belt northwest of the Corunna Downs batholith. In the two areas evidence was expected to be found that would be diagnostic for the diapiric model or for

Fig, 1. Location of the field work area in the Pilbara Block, Western Australia. Domain 1 to 6 represent tectonostratigraphic domains of the Archaean Pilbara Block as used in Krapez and Eisenlohr (1998; modified from Eriksson et al. (1994)). The field work areas are located in domain 2.

3 the mid-crustal detachment model. A global position of the field work areas is shown in figure 1. Main aims of the field work were to gain data about the general geology of the two areas, mostly about the stratigraphic and intrusive units, and to take samples for geochemistry (from intrusive and extrusive rocks) and for U/Pb-dating (on zircons from felsic intrusives and extrusives). For the field work areas in the Coongan greenstone belt and the Warrawoona greenstone belt global geological maps and stratigraphic columns were produced, both with sample locations. The obtained data can be very useful for further interpretation of greenstone units that he close to the studied area. Analysis of the sampled intrusive and extrusive rocks will give more precise information of basm evolution in addition to field data and on timing of different events. Geochemical analysis of the samples can also give more and crucial information of the tectonic setting in which the rocks have been in/extruded. The total amount of obtained data of the oldest recognized greenstone unit, the Warrawoona group, may give a better understanding of the processes that were active in the early Archean. In this report first the geological setting of the Pilbara Block wül be summarized, to give the reader a global idea where the field work has taken place (chapter 2). In chapter 3 previous research will be briefly discussed and explained where the recent field work fits in with the older data. Chapter 4 and 5 show field results followed by an interpretation ofthe Coongan greenstone belt and the Warrawoona greenstone belt respectively. A separate chapter in which the sampling is described is made for convenience for ftirther analysis of the collected samples (chapter 6). In the end (chapter 7) the data ofthe Coongan greenstone belt and the Warrawoona greenstone belt is compared, summarized and discussed.

4 2 Geological setting

The Pilbara Block in the northwest of Western Austraha has been part of the oldest known craton, called Ur (Rogers, 1996). The basement of this craton was stabilized at 3 Ga on which then shallow marine water sediments were be deposited. In the rest of the world shallow water depositions are aU younger than 2.5 Ma, which means that basements there had yet not stabilized at 3 Ga. In time the old craton has spht up. The five spht up segments that belonged to the oldest known (stable) craton are the Kaapvaal craton, Dharwar craton, Bhandara craton, Singbhum craton and the Pilbara craton. They are aU situated in the Eastern Gondwana part of Pangaea. The Pilbara Craton can be subdivided into two components: the Early-Mid Archaean rocks, which form the Pilbara Block, and younger overlying Late Archaean volcano-sedimentary rocks. The Archean granitoid-greenstone terrain of the Pilbara Block, which is formed between 3680 Ma and 2775 Ma (Krapez, 1998), is composed of large domal granitoid-gneiss bathohths that are separated by long-shaped greenstone belts (Barley and Bickle, 1982). This is best shown in the East Pilbara. The Archaean granitoid-greenstone terrain mainly crops out in the northem Pilbara. The younger sediments with minor volcanics, deposited in the late Archaean to early Proterozoic between 2770 Ma and 2400 Ma (Nelson et al., 1992), unconformably overhe the earlier Archaean rocks and form the infill of the Hamersley Basin. The Hamersley Basin occupies the southern two-thirds of the Pilbara craton. The currently exposed margins of the craton are defined by later, Proterozoic orogenic events, which developed along former continental margins (Tyler and Thome, 1990). In late Archaean times the craton has been estabhshed as a distinct continental plate (Myers, 1990). The Archaean Pilbara Block can be subdivided in six fault-bounded tectonostratigraphic domains, which are shown in figure 1. The chronological sequence of sedimentary and volcanic rocks, deposited in basins that now form greenstone belts and which are passably coherent within the tectonostratigraphic domains, can be arranged

V w vv W W Sl <^ % imfii; 'rttaiBSiï ilnêtude» mfi^*»» MsalX' Whim Creek Group A A ^ A A

Gcinj« OmK Crouo

calc-9l»ali(M wleonie» (itojiBBnMy itmmi flf U V V W-V vv V w-V ¥ k'Vv vvvvyvvvvv^-i 's>vvv vvvvvvvvvv; tnaiK V9l«anict («hMcs miiqiWsiiw i»mt cpit iwnof liCBKHtittit ly WWV V wwvvvVi

WarrowoMa Group

vvvwvvvvvvyv >/V VVV ¥V¥V VVtfV (Mde ••«leanlM jineliKSet MifjMs>itfl bawil) vv vW VVV vv V**

Fig. 2. Simplified stratigraphic column of greenstone sequences in the Pilbara Block showing major lithological associations (afl;er Hickman, 1980). Total thickness is probably less than 15 km. The Negri basin sequence on top of the Whim creek group is not shown.

5 according to the megacycle during which they were deposited. Krapez and Eisenlohr (1998) have determined that the crustal and supracrustal rocks correlate to four alkaline volcanic rocks. The succession is only present in domain 1, 2 and 3. The Gorge Creek sequence contains mainly sediments that are deposited in throughs on the Warrawoona sequence during the second megacycle (3325-3135 Ma). The Whim Creek sequence is deposited during the third megacycle (3135-2955 Ma) and contains sub-aerial tholeiitic to calc-alkalic volcanic rocks with associated sedimentary rocks. During the last megacycle (2955-2775) deposition took place in the Negri basin, with dominantly basalts and subvolcanic mafic-ultramafic layered intrusion. A simplified stratigraphic column of greenstone sequences in the Pilbara Block is shown in figure 2. The earliest granitic rocks comprise a 3.5-3.3 Ga granodioritic plutonic calc- alkaline suite intrusive into the earhest greenstones. Within this time span intrusions occurred during two phases, one about 3.46 and the other about 3.3 Ga (Zegers, 1996). Younger (ca 2.97 Ga) "Late-tectonic" granitoids were intruded synchronously with large scale diapiric doming ofthe Shaw batholith (Bickle, 1989). "Post-tectonic" granitoids were intruded at ca 2.85 Ga.

6 3 Previous work

A global map ofthe east Pilbara granite-greenstone terrain with the distribution of main tectonic and stratigraphic units is shown in figure 3. In this figure the Warrawoona group is further subdivided (from old to young) into the Taiga Taiga, Duffer and Salgash subgroups. The Warrawoona group has been reviewed by Barley (1993). Greenstone rocks older than the Warrawoona group are found by Buick et al. (1995), which indicates that the Warrawoona group rocks were deposited on pre-existing continental crust (Barley, 1997). Recent detailed work in the areas of concern for this report have concentrated on the one hand on the Shaw Bathohth and the Coongan greenstone belt (Zegers, 1996) and on the other hand on the Warrawoona greenstone belt and the Corunna Downs and Mt. Edgar Bathohths (Kloppenburg et al., submitted). More global mapping was done by the Geological Survey of Western Australia. Detailed field information for the area where the Coongan greenstone belt and the Warrawoona greenstone belt meet was missing. In this area the field work took place.

Fig. 3. Map of tlie east Pilbara granite-greenstone terrain with the distribution of main tectonic and stratigraphic units, as taken from Barley (1997).

7 m emo Ccm Qfctifl

WEI toöa Xaiji Sifefireu p-

|ES3 IMtlS!K>j<:f!)* «hèsf mi»

Fig. 4. Scliematic map of the Coongan greenstone belt showing the major stratigraphic subdivisions and dates. A-A' refers to the stratigraphic sequence in figure (taken from Zegers, 1996).

Figure 4 shows a schematic map of the Coongan greenstone belt showing the major stratigraphic subdivisions and dates (taken from Zegers, 1996). In figure 5, which is also taken from Zegers (1996) a more detailed hthological map of the Coongan greenstone belt is given, together with the vertical sections belonging to the three traverse hnes. In figure 6 a stratigraphic column for the northern traverse is shown. The cycle numbers that are used in this report are given with it for comparison. Figure 7 shows the extensional faults in the Duffer sequence, as mapped by Zegers (1996) more precisely. Available data for the concerned field area in the Warrawoona belt is summarized in figure 8 (taken from Kloppenburg et al., submitted).

8 Fig. 5. Detailed lithological map ofthe Coongan greenstone belt is given, together with the vertical sections belonging to the three traverse lines (after Zegers (1996)).

9 Oliiup? J

Fig. 6. (left top) Stratigraphic sequence for the northem traverse as mapped by Zegers (1996). Cycle numbers that are used in this report are given next to it for comparison.

Fig. 7. (right top) Extensional faults in the Duffer sequence, as mapped by Zegers (1996).

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'•aTïi r»hE i»*3P=iiK!ffina»

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Fig. 8. Geological map ofthe west part of the Warrawoona greenstone belt (after Kloppenburg et al. 2000).

10 4 Coongan greenstone belt

4.1 Introduction

The main area was situated around Shark Guhy Mining Center, which is situated in the Coongan (NS-orientated) greenstone belt at the east side of the Shaw bathohth. In the area three long traverses were made, about 4 kilometer in spacing, and several short stops, to get an idea about the geology in the area. Location of traverses are indicated on the produced geological map (appendix la). Besides the long traverses and some short stops the rest of the geological map is an interpretation by means of far-distance geology, aerial photograph interpretation (scale 1:25.000) and interpretation taken from the GSWA geological map sheet (compiled by A. Hickman and S. Lipple). On base of the different hthologies the area can be subdivided into cycles. A cycle is a series of rock units that can be grouped together and which may repeat itself. Because this is for major importance in understanding the area, the recognized cycles will be dealt with first (§ 4.2), before the rock types that were found in the area are mentioned (§ 4.3). An interpretation of the cycles and the rocks in the cycles is given in § 4.4.

4.2 Cycli

In the studied Coongan greenstone belt eight cycles in basin development were recognized. A cycle mainly consists of a volcanic base with a sedimentary top. Less- perfect cycles may be present due to small variations in basin evolution and/or partial erosion of rock units in a cycle. Ordering of the rocks in the Coongan greenstone belt in cycles is as foUows:

Cycle 1: Sedimentarv-volcaniclastic sequence; The cycle consists of sedimentary rocks with compositional layering, cross- bedding, bed gradation and slump folds that are hornfelsed by contact metamorphism of the Shaw bathohth. Occurrence to about 2 km within the pluton. Within the pluton these rocks occur as migmatites. On the geological map (scale 1:250.000) some ofthe hornfelsed sediments are mapped as rhyolite dykes.

Cvcle 2: Volcano-sedimentary sequence: The cycle consists predominantly of basalts and andesites, banded iron formations and greywackes. The unit can be subdivided into a lower basalt-andesite subunit and an upper sedimentary unit. The basal unit contains some coarser, (vesicule rich) gabbroic layers and at the middle an andesitic layer with minor sedimentary layers. Some of the gabbroic layers are assumed to be basal parts of thick basalt flows. The basal part ofthe sequence is partly hornfelsed by the Shaw granodiorite. The upper sedimentary subunit contains mainly greywackes with minor chert, banded iron formation, dolomite and mafic sediments.

Cvcle 3; Volcano-sedimentary sequence; The cycle contains predominantly basalts and andesites, with major cherts and

11 quartz-rich sandstones. The quartz-rich sediments mainly occur in the upper part of the unit. At the top also (high Mg) basalts occur. The lower part of the unit is locaUy intruded by minor gabbro sills. Occurrence of the cycle is up to 1.5 km to the west of Shark GuUy Mining Center.

Cycle 4; Volcanic sequence; The cycle contains basalts, high Mg (?) basalts, andesites, mafic and quartz-rich sediments, chert and dolomite. The imit is intruded by granodiorite-tonahte at the lower part and gabbroic siUs in the upper part. Also basalt-andesitic dykes occur. The top of the cycle is eroded up to the intrusive gabbroic sills. The sequence occurs between Shark GuUy Mining Center and Shark weU. The high Mg (?) basalts at the base form hills, hke the gabbroic sills at the top of the sequence do.

Cycle 5; Duffer sequence; The sequence differs greatly from the north to the south of the studied area. Only in the northern part, north of Shark well, the cycle contains rhyodacites and volcanisedimentary (erosion) products at the base. On top agglomerates and tuffs are foUowed by conglomerates, sandstone, sUtstones, feldspar-rich sediments, jaspihte, dolomitic chert and chert. Some basaltic dykes and tonahte dykes occur at the base. In the middle part of the studied area minor and thin Volcanosedimentary rocks occur at the base of the sequence, foUowed by conglomerates, (clastic) sandstone, siltstones and minor cherts. In the south of the studied area the basal part of the sequence (where growth faults occur) consists of mafic conglomerate, sandstone and gridstone (picture 1 in appendix 3, taken at coordinates 07.72.922/76.22.971). On top of this a thick unit of feldspar-rich sediments (sandstone) occurs, foUowed by cross-bedded sandstones (picture 2 in appendix 3, taken at coordinates 07.74.329/76.23.717), breccia/conglomerate (picture 3 in appendix 3, taken at coordinates 07.74.333/76.23/727), jaspilite and dolomite-chert (with stromatohtes, picture 4 in appendix 3, taken at coordinates 07.74.429/76.23.771). Occurrence of the sequence is to the east of Shark weU. The feldspar-rich sediments in the center and south of the studied area form hiUs, as can be seen on the picture on front of this report. The dolomitic chert at the top of the sequence forms hiUs with steep slopes.

Cvcle 6; Volcano-sedimentary sequence: The cycle can be subdivided into a basaltic bottom, a central andesitic part and sediments (siltstone and sandstones) on top. The base of the cycle contains some thin layers of volcanic exhalative (brown colored) and andesites (at the base with glassy tuffacious xenohths), quickly giving way to piUow basalts. The boundary between the basaltic unit and the andesitic unit on top is marked with a chert layer. The andesite has a pillow base (picture 5 in appendix 3, taken at coordinates 07.75.555/76.23.512), foUowed by a massive part on top of that. In some parts the pUlows are leached (pictures 6a, b and c in appendix 3, taken at coordinates 07.74.313/76.31.725) and volcanic exhalative occurs (picture 7 in appendix 3, taken at coordinates 07.74.015/76.31.328). The sedmimentary part at the top of the sequence contains sanstones, tuff, siltstone, gridstone

12 and cherts. Roll and slump structures can be found in them (picture 8 in appendix 3, taken at coordinates 07.71.438/76.36.456). Occurrence of the unit is up to 1 km south of Glen Herring Gorge, and to the south from there.

Cvcle 7: BIF-chert sequence; The sequence contains mainly BIF's, cherts, sandstones and siltstones. The unit is well bedded. In parts extensive slumping with chaotic bedding is present (picture 9 in appendix 3, taken at coordinates 07.76.156/76.23.605). The sequence may be masked by a thick layer of silcrete (76.35-76.36) and ferrocrete (76.31-76.32) on top. Occurrence of the sequence is southeast of Glen Herring Gorge. It forms hills that are more than 400 meter in height, with steep slopes.

Cvcle 8 Sedimentary sequence; The sequence is formed by a turbidity sequence, made up of well-bedded layers of gravel, sand, siltstone, shale. On top cherts occur. Mega slumps (dm-100 meter scale) occur throughout the total sequence (picture 10a and b in appendix 3, taken at coordinates 07.76.091/76.34.600 and 07.70.911/76.34.307 respectively). Occurrence is from 2.5 to 5 km east of Glen Herring Gorge. The rocks form smooth hills.

Rocks in all above mentioned cycles roughly give steep east dipping surfaces. Facing direction on many features in the rocks (cross-bedding, scrape marks, grading, wave ripples, slump folds, basaltic flow stratigraphy) give all younging direction to the east. The cycles are ah roughly north-south orientated with the northern part bending more to the west (shown in appendix 1). Cycle 1 is formed by hornfelsed sediments at the eastern part of the Shaw granite, aU other cycles are to the east side of the granite. Only one well developed erosion surface was found on top of cycle 4, which produces coarse material at the base of cycle 5. In cycle 4 many growth faults occur (see the report of Kim Hein), which die out in cycle 5 and have their basal decollement in cycle 3. The growth fault planes are filled with recrystaUised, mainly mafic, material from the country rocks. In cycle 5 the northern part of the studied area consists of rhyodacites and their erosion product, while in the south (south of Shark Well) predominantly felspar-rich (white) sandstones occur. The rocks must have been deposited after the growth faults were active because the growth faults die out in it. The chertish layer on top of cycle five shows no breaking up at ah and is thus an important stratigraphic marker. Large extension faults occur in cycle 6 and 7, which indicates that they have to be younger than cycle 7, probably of Fortescue Group age. According to Kim Hein these faults show southeast block up, which gives them a sinistral expression on map scale and in horizontal outcrop. A compressional fault was found in the southern part of the area, at the base of cycle 5. This fault produces highly fohated rocks and kink bands that show east block up (picture 11 in appendix 3, taken at coordinates 07.72.950/76.20.250). On top of cycle 1 to 8 the Fortescue group is deposited. In the studied area basal conglomerates^reccias and andestitic tuffs were found. On top of this unconformably the highly vesicular Mount Roe basalt/andesite is deposited. Scree slope and hills of Archaean erosion products were found at many places in the studied area. They may indicate former hiU slopes.

13 OccuiTence of the different recognized cycles is shown in appendix lb. In appendix la the geological map of the studied area is presented. Appendix 2 shows stratigraphical columns of the section locations on appendix la and an ideahzed stratigraphical column.

4.3 Rock types

Rock types that occur in the studied area are:

Sedimentary rocks: Sedimentary rocks occur in all cycles. Rock types belonging to this group are: conglomerate, breccia, gridstone, sandstone (quartz- and feldspar-rich), siltstone, shale, dolomite, chert, BIF, greywacke, mafic sediments, volcaniclastic sediment, agglomerate and tuff.

Basalts and andesites: Basaltic and andestic submarine extrusives occur in cycle 2, 3 and 6. Primary features that these rocks show are piUows, hyaloclastites, columnar jointing, segregation, varioles, flow textures, ropy textures, and vesicles. Thick flows may show doleritic to gabbroic parts. Basaltic/andesitic dykes and sills occur in cycle 4 and 5.

Gabbros; Gabbroic intrusives occur in cycle 2, 3 and 4. In cycle 2 vesicles were found in the gabbro, which indicates that the gabbro in this cycle might be a thick extrusive lava flow.

Ultramafics: Ultramafics occur only in cycle 4. Features are pyroxenites (pyroxene and ohvine) and spinifex textures.

Granodiorite-tonalite: Granodiorite-tonahtes occur in cycle 4 and 5. In cycle 4 an extensive granodiorite- tonahte sill occurs. In cycle 5 a tonahte dyke was found.

Rhyodacite; Rhyodacites occur in cycle 5. A top flow of the rhyodacite was found, which indicates that it is in part extrusive.

Shaw granodiorite: The Shaw granodiorite bathohth is intruded into cycle 1. It contains approximately 20 % biotitie (1-6 cm), 40 % quartz (< 1 cm) and 40 % feldspar (plagioclase). Flow ahgnment of minerals is present and is pointed to the southeast.

14 legenda Shaw granodiorite granodio rite-tü nalite I; I rhyodacite [ ' I sediiments 1 basa it/a rides its

[ I ulti-amaf iG

Fig. 9. Global geological map of the Coongan greenstone belt.

15 A global geological map with the above used simplification in rock types is shown in figure 9. AU rock types except for the Shaw granodiorite and the sedimentary rocks have been sampled for geochemistry. If useful, also samples for zircon dating were taken. In order to stay with rock types and cycles in basin development, the rocks in the cycles in the Coongan greenstone belt are first interpreted. After that the field data and interpretation of the Warrawoona greenstone belt is dealt with (chapter 5) before sampling in both areas is described (chapter 6).

4.4 Interpretation

In this section the rocks in the cycles are described in successive order, is described what characteristics in these rocks mean for basin infiU and is an interpretation given for their occurrence. In the end a large overview of basin development is given.

On base of the occurrence of mainly submarine basaltic/andesitic and sedimentary rocks in the studied area, it is assumed that the rocks of the Warrawoona group have been deposited in a basin environment. This indicates that there must have been a basement which extended to form the basin in. Most cycles start off with basalts and have a sedimentary package on top of that. The rocks in cycle 1 are only sedimentary, which implies that something on the basement must have been eroding to form the sediments. Layering and slump folds indicate that there must have been a slope from which the slumping could start from. Cycle 2 is a normal cycle which starts off with basalts and andesites that are assumed to be extruded into the basin, on top of the cycle 1 layered sediments. They contain smaU sedimentary layers. On top of the basalts/andesites hes a 500 meter thick sedimentary package, made up of greywackes, thin fine sandstone layers and chert layers. Because layering and slump folds were seen in the chert, the chert layers are assumed to have been deposited deep in a basin, with relatively quieter periods than when the greywackes and sandstones have been deposited into the basin. It is assumed that the sediments only mainly could have been deposited after mafic extrusion deceased, because the mafic and sedimentary units are not totaUy mixed and form their own units. Cycle 3 is like cycle 2 a normal cycle. It starts off with basalts/andesites and has a sedimentary package on top. In the basaltic/andesitic part a pronounced chert layer occurs that is assumed to have been deposited in a quiet period. The sedimentary unit at the top of cycle 3 starts off with a chert layer too. The sediments here are extremely white and contain much quartz and feldspar. Cycle 4 is mainly volcanic and is the only cycle that contains more mafic constituents. It starts off with high Mg (?) basalts, some sediments, andesites, an alternation of basalt/andesite with sediments, and on top ultramafics. The mix of basalts/andesites with sedmientary layers is only found in this cycle. Together with the more mafic constituents in this cycle it indicates that things are changing compared to the rather similar cycles 2 and 3. On top of these maf ics sediments have probably been deposited, like in the cycles below. These can not be found anymore because they have been eroded away. At the time the cycle is being deposited also gabbros are intruded. Some time after gabbro intrusion, erosion took place, up to the gabbros. To be able to

16 % \ \ —.-w—.— ^—4^ tycMJ

Fig. 10. Schematic drawing of the listric and synthetic faults in cycle 4. The orientation of the basin is drawn as would be expected from back rotation by use of facing dkections.

erode, the basin must have been inverted by a compressional phase. The compressional phase is foUowed by coUapse. Rock material is deposited again in the sinking basin in cycle 5. Because rock parts that occur at the base of this cycle have far different compositions than that are found in cycles 1 to 4 it is assumed that much has been eroded of cycle 4. At the beginning of the renewed infill of the basin in cycle 5 a hstric normal fault, accompanied by synthetic normal faults was formed. The synthetic normal faults mainly occur in cycle 4 and die out in the basal part of cycle 5. This means that at the time the basal part of cycle 5 was deposited the faults have been active. The major hstric fault on which the synthetic faults slide is situated in cycle 3. A schematic drawing of the hstric and synthetic faults is shown in figure 10. For more information on these faults Kim Hein needs to be contacted. In the area south of the major hstric faults at Shark well, cycle 5 is mainly formed by conglomerates and feldspar-rich sediments. To the north of Shark weU rhyodacites and volcanosedimentary rocks occur (both with mafic and felsic constituents) instead of the feldspar rich sediments. On top agglomerates and tuffs occur. It is assumed that a rhyodacitic volcano was situated somewhere in the northern part of the studied area. The top of cycle 5 in whole the area is constituted by bedded sandstone, breccia/conglomerate (picture 3 in appendix 3) and dolomite/chert with stromatohtes in it. This is the first form of hfe seen so far. The dolomitic chert and the stromatohtes require a quite period in basin evolution. It is assumed that the granodiorite-tonahte in cycle 4 is intruded during cycle 5, because these rocks are crosscut by dykes that are assumed to be feeder dykes for cycle 6. Cycle 6 is a normal cycle again with a mafic extrusive base and a sedimentary package on top. It starts with thin (10-50 cm) ferrogenous chertish and dolomitic layers and pillow basalts. After a chert layer a package of pillow andesites, massive andesites and a chert layer foUows, before the sedimentray package starts with one or more cherts. It is assumed that intrusive basaltic dykes in cycle 4 and 5 are the feeder system for the basalt/andesites in this sequence. Those mafic dykes are thus be assumed to be intruded at this time. North of Shark Well the piUow andesites have been leached by assumed volcanic exhalative processes. The roundish pUlow shape has remained (picture 6b in appendix 3), but the outer sides of the piUows have been cracked (picture 6a and c in appendix 3). Between the pillows the same dolomitic/chertic material occurs as in banks of 1 to 2 meter occur in this area (picture 7, appendix 3). It is assumed that a stockwork

17 by which the andesites are leached is situated close to cooridinates 07.73.800/76.31.600. Massive sulphides may also be present on the contact of the basalt to the andestite in cycle 6, or at the base of this cycle. Normahy massive sulphides are mostly closely associated with volcaniclastic rocks and many orebodies overlay the explosive products of rhyolite domes. They are mostly found at a horizon with changes in composition ofthe volcanic rocks, a change from volcanism to sedimentation or a pause in volcanism (Evans, 1996). Cycle 7 and 8 are both sedimentary, without extrusive mafic material. Cycle 7 contains a weU bedded sequence of BIF's, cherts, sandstones and siltstones, cycle 8 consists of a turbidity sequence of mainly sandstones and siltstones. In cycle 7 smah scale slumps occur (dm-m scale), in cycle 8 besides small scale slumps also major slumps occur (up to several 100 meters in scale; picture 10b in appendix 3).

Summarized, the basal part of the sequence cycle 2, 3 and 4 is mainly mafic, with in each cycle mafic extrusives foUowed by a sedimentary package. Cycle 5, 7 and 8 are sedimentary, with cycle 6 again extrusive mafics with sediments on top. There is thus a clear change in basin evolution, by infiU of the basin in 8 cycles from mafic to sedimentary. Cycle 4 is deposited during major changes in the basin development. After deposition of the other cycles foUowing on top of cycle 8 (which have not been studied in the field), the basin has been tilted. After this major event the Shaw granodiorite is assumed to have been intruded. This has to be taken place after tUting of the basin, because the contact metamorphic cycle 1 is also tilted. The Fortescue Group starts unconformibly with basal conglomerates/breccias, foUowed by andesitic tuffs, siltstones and Mount Roe basalts. After deposition the rocks have been tilted slightly (about I5O).

18 5 Warrawoona greenstone belt

5.1 Introduction

The other area comprised part the Warrawoona (EW-orientated) greenstone belt, which is situated north-west of the Corunna Downs bathohth. In the area short traverses were made and several short stops, to get an idea about the geology in the area. Locations of traverses are indicated on the produced geological map (appendix la). Besides the short traverses and some looks the rest of the geological map is an interpretation by means of far-distance geology, aerial photograph interpretation (scale 1:25.000) and interpretation taken from the GSWA geological map sheet (compiled by A. Hickman and S. Lipple). Like in the Coongan area, on base of the different lithologies the area can be subdivided into cycles. A cycle is a series of rock units that can be grouped together and which may repeat itself. Because cycles in the Coongan area have been recognized, the rock units in the Warrawoona area will, like in the previous chapter, also be described first in recognized cycles, although there are only three recognized cycles in the studied area. Recognition of the cycles is of major importance for understanding the area. The recognized cycles wül be dealt with in § 5.2. The rock types that were found in the area and which constitute the cycles are described in § 5.3. An interpretation of the cycles and the rocks in the cycles is given in § 5.4.

5.2 Cycli

In the studied Warrawoona greenstone belt three cycles in basin development have been recognized. A cycle mainly consists of a volcanic base with a sedimentary top. Less-perfect cycles may be present due to smaU variations in basin evolution and/or partial erosion of rock units in a cycle. Ordering of the rocks in the Warrawoona greenstone belt in cycles is as foUows:

Cvcle 1; Sedimentary sequence: The sequence consists of badly sorted sediments. It contains boulder porphyritic conglomerates, gridstones, sandstones, siltstones and cherts. The base of the cycle is not shown in the studied area. Occurrence of the sequence is south of Camel creek.

Cycle 2: Mafic volcanisedimentary sequence; The sequence consists predominantly of mafic sediments, basaltic/andesitic flows with some minor quartz/feldspar-rich sediments. The sequence can be subdivided into a mafic lower unit and a mafic/quartz-feldspar-rich sedimentary upper unit. The lower mafic unit consists of andesites, hyaloclastites, tuff, greywacke and mafic sediments. In the middle part a chert occurs. The boundary between the lower and the upper unit is a black-white chert layer. The upper mafic/quartz-feldspar-rich sedimentary unit contains a layered mix of mainly mafic sediments, with minor quartz-plagioclase rich sand- to gridstones, dolomite, sUtstone and greywackes. In the total sequence some andesitic flows may occur. The occurrence of the sequence is south of Camel creek, to the south of

19 cycle 1 in this area.

Cycle 3; Mafic volcanic sequence; The sequence comprises an andesitic, variohtic basaltic/andesitic unit (hilly basalt) with on top a thin sedimentary unit of siltstones and a chert layer. The upper part has not been studied. At the base of the lower unit mm-sized rounded, black, glasy tuffacious xenohths were found in the andesite. In the lower part of the lower unit the varioles in the andesite clump together to form large dm-sized varioles (shown on picture 12 in appendix 3). More to the top of the lower unit the varioles become smaller. Hyaloclastites and pillow-shapes are more common in the top of the sequence than on the bottom. Between the lower and the upper unit a volcanic exhalative layer occurs. The sequence occurs south of cycle 2, south of Camel creek.

Minor intrusives into these three sequences are the Camel creek granite, that is assumed to be the oldest recognized rock in the studied area, and columnar jointed quartz-porphyritic granites. The Camel creek granite is intrusive in cycle 1, and was found at coordinates 07.87.789/76.39.736. Quartz-porphyries were found in cycle 1 and 2. They show the largest outcrop at coordinates 07.87.330/76.39.918 and 07.82.400/76.38.000. Only the lirst mentioned quartz-porphyry contains perfect colunmar jointing (picture 13 in appendix 3), the second one shows pseudo-columnar jointing. Rocks in the three recognized cycles roughly give steep southeast dipping surfaces. Facing direction, determined on flow stratigraphy in the hilly basalt, points to the southeast. The cycles are all roughly east-west orientated and wrap around the Coruna Downs bathohth (shown in appendix 1). In the studied area Kim Hein found a 060 fohation (flattening only, no shear) that is overprinted and crosscut by a 000 fohation. The 000-fohation is formed by north-south orientated shearing in two major shears (see appendix la). The southern ends ofthe shears are mainly sinistral strike slip. The northern part of the shears (at coordinates 76.39.500) are thrusts with southeast block up movement. Intersection of both fohations produces an intersection lineation. An east-west orientated shear zone at Camel creek (to the east fohowing the Broekman Hay cutting) crosscuts the 060 and 000-fohations, and is thus younger in age. The rocks in the area are overprinted by a conjugate fracture cleavage (340-060). On top of cycle 1 to 3 the Fortescue Group is unconformably deposited. In the studied area a thin (several meters) basal shale layer was found, with on top the highly vesicular Mount Roe basalt/andesite. Scree slope and hills of Archaean erosion products were found at many places in the studied area, which may indicate former hill slopes.

Occurrence of the different recognized cycles is shown in appendix lb. In appendix la the geological map of the studied area is presented. Appendix 2 shows an idealized stratigraphical column of the section locations that are shown in appendix la.

20 5.3 Rock types

Rock types that occur in the studied area are:

Sedimentary rocks; Sedimentary rocks occur in all cycles. Rock types belonging to this group are: boulder conglomerate, grid- to gravelstone, sandstone (quartz/feldspar-rich), siltstone, shale, dolomite, greywacke, mafic sediments, volcanoclastic sediment, tuff and chert.

Basalts/ andesites; Basaltic/ andestic extrusives occur in cycle 2 and 3. Primary features that these rocks may show are flow textures, pillows, hyaloclastites, vesicles, segregation and varioles. Thick flows may show doleritic to gabbroic parts and columnar jointing.

Camel creek granite; The Camel creek granite only occurs in cycle 1. Characteristic are coarse, cm-sized, feldspars.

Quartz-feldspar porphyritic granite; A granite with quartz-feldspar porphyroclasts occurs in cycle 1 and 2. The granite shows (pseudo to real) columnar jointing (picture 13 in appendix 3, taken from coordinates 07.87.330/76.39.918), which indicates that it is intruded at a subvolcanic level.

"hi

^•ediraents

Fig. 11. Global geological map of the studied area in the Warrawoona greenstone belt.

21 In the field special attention was asked for two felsic dykes, with respectively a north and northeast trend, and one of them fohated. None of the two dykes were found m the area. Besides the (pseudo-columnar jointed) quartz-feldspar porphyry and the Camel creek granite no intrusive felsics were found in the studied area. The only recognized rocks with felsic composition are badly sorted gravelstones that occur in cycle 2 as layers in mafic sediments. The (pseudo-columnar jointed) quartz-feldspar porphyry is assumed to be one ofthe youngest rock in the studied area and this rock is also fohated. In some parts this rock shows less fohation, which is assumed to be caused by internal rheological differences.

Because the two different felsic dykes were not found in the area and interest (for dating) went out for them, only one sample was taken for geochemistry and two for dating. In order to stay with rock types and cycles in basin development, the rocks are first interpreted (§ 5.4) before sampling in both the Warrawoona and Coongan areas are described (chapter 6). A global geological map with the above used simphfication in rock types is shown in figure 11.

5.4 Interpretation

In this section the rocks in the three cycles are described in successive order, is described what characteristics in these rocks mean for basin infill and is an interpretation given for their occurrence. In the end a summary of basin development is given.

On base of the occurrence of mainly submarine basaltic/andesitic and sedimentary rocks in the studied area, it is assumed that, like in the Coongan greenstone belt, the rocks ofthe Warrawoona group in this area have been deposited in a basin environment. The rocks in cycle 1 are only sedimentary, with badly sorted quartz-rich component. This imphes that something close to the basm was of felsic material and that this has been eroded to form the sediments. The particle size is highly variable and coarse grained (up to meter sized boulders), which indicates that the environment at that tune was highly energetic. Cycle 2 is a mafic volcanosedhnentary sequence that predominantly consists of mafic sediments, basaltic/andesitic flows interbedded with some quartz/feldspar-rich sediments in the upper part of the sequence. Occurrence of tuffacious layers and hyaloclastites in the lower part and mafic and quartz/feldspar-rich sediments and dolomites in the upper part hnplies a shallow basin environment. Cycle 3 is a mafic volcanic sequence with mainly variohtic basalts/andesites. This implies a deeper basin level in which mafic volcanic rocks extruded. More to the top of the sequence pillows and hyaloclastites occur, which indicate eruption on the bottom of the basin. The siltstones and chert on top of it indicate a quite period in which volcanic eruptions have stopped for a moment. Summarized, from cycle 1 to cycle 3 the basin environment in which the rocks have been deposited changes from shallow and sedimentary (quartz-rich) with no volcanic

22 activity, to infill of the basin with mainly mafic volcanic activity. Cycle 2 is intermediate with mafic sediments, some andesitic flows and some quartz/feldspar rich layers in the upper half. After deposition of the other cycles foUowing on top of cycle 3 (which have not been studied in the field), the basin has been tilted. After this major event the Coruna Downs bathohth is assumed to be intruded. The Fortescue Group starts unconformably with shales which are foUowed by Mount Roe basalts. After deposition the rocks have been tilted shghtly (about 15^).

23 6 Sampling

6.1 Introduction

For both field work areas samples are taken for geochemical analysis and zircon dating. In appendix 4 sample number, coordinates and rock name is given for aU samples. In appendix la the samples are plotted on the geological map and in appendix 2 the stratigraphic height where the samples are taken from is marked. For the Warrawoona area there is only one geochemistry sample (AOO/1) and two zircon samples (WA07 and WA09). All other samples are firom the Coongan area. Below is described briefly which rocks have been sampled for geochemistry (§ 6.2) and which for zircon sampling (§ 6.3). For the zircon samples also the reason is given why dating of the samples are important and what the age imphes.

6.2 Geochemistry samples

From cycle 1 nothing is sampled for geochemistry in the Coongan greenstone belt, because the rocks are ah hornfelsed. This would not give the original composition of the rocks. From cycle 2 two samples were taken. An amphibolitic basalt was taken from the lower part, which is assumed to be hornfelsed and thus geochemically altered, and a doleritic basalt from the upper part of the sequence. From cycle 3 one sample was taken, which is a doleritic basalt. From cycle 4 thirteen samples were taken. Three samples are from a basalt, of which two are from the top and base in the north of the area and one at the base in the south. Two assumed high Mg basalts are taken, one at the base and one at the top of the cycle. Two gabbros are sampled which are taken from the base in the north, and the middle part in the central area. A dolerite (probably the base of a thick basaltic flow) is sampled from the top of the cycle in the north, a pyroxenite is sampled from the top of the cycle in the south and an andesite is sampled in the south of the area. Two granodioritic-tonahtic intrusive sills are sampled in the north and in the south of the area and one andesitic dyke is sampled at the base of the sequence in the north. From cycle 5 six samples were taken. Two rhyodacites from the top and center in the northern part of the area, a tonahtic dyke in the north, two basaltic dykes in the north and center of the area and porphyritic boulders in a conglomerate from the base of the cycle in the center. From cycle 6 three samples were taken. A basalt with black xenohths (probably tuffacious) from the base of the cycle, a normal basalt and an andesite from top of that. From cycle 7 and 8 no samples were taken in the Coongan greenstone belt, because the cycles were both totaUy sedimentary and only intrusive and extrusive rocks were sampled for geochemistry. From the Warrawoona belt one sample of cycle 1 is taken from a quartz-porphyritic boulder in a conglomerate. A geochemistry sample of an andesitic tuff is taken from the Fortescue basalt.

24 6.3 Zircon samples

From the Coongan greenstone belt the rhyodacite (cycle 5) and the intrusive granodiorite/tonahte (cycle 4) are sampled for Zr-dating. The rhyodacite is the oldest rock that could be sampled for Zr-dating in the Coongan greenstone belt. It is situated in the Duffer sequence and will probably give the same age as the dacite that was dated by Pigeon (3.467 + 5 Ma). The intrusive granodiorite-tonahte intrudes the andesitic-basaltic unit. The age wiU thus be a minimum age for the basin. Growth faults crosscut the rock, and the date will thus give a maximum age for the activity of the growth faults. Because the growth faults were active at the beginning of the Duffer sequence (see § 4.2) the tune when de growth faults were active will be between the age of the granodiorite-tonahte and the age of the Duffer (about 3.47 Ga). The age is thus a maximum age for growth fault activity. The granodiorite-tonahte is crosscut by the 000 fohation. The age of the rock wih thus be a maximum age for formation of this fohation. From the Warrawoona greenstone belt the Camel creek granite and a quartz/feldspar porphyry in a boulder conglomerate, both form cycle 1 there, are sampled for Zr-dating. The Camel creek granite is sampled because it looked very old and is highly foliated. The quartz/feldspar porphyry in the boulder conglomerate must give a date from the basement where it was eroded from. This has to be the maximum age of cycle 1 in the Warrawoona area.

25 7 Conclusion and discussion

Rocks that constitute the greenstone belts are assumed to be deposited into sedimentary basins, as was interpreted in chapter 4 and 5. In both of the greenstone belts the rocks were mapped as units, which can further be grouped into cycles. A cycle mainly contains of a volcanic base with a sedimentary top. Less-perfect cycles may be present due to variations in basin evolution and/or partial erosion of rock units. In the Coongan greenstone belt eight different cycles were recognized. In the Warrawoona greenstone belt three different cycles were found. The cycles of the Coongan and Warrawoona greenstone belts are assumed to be connected with each other, with cycles in the Warrawoona greenstone belt lying on top of the cycles in the Coongan greenstone belt. Evidence for above hypothesis comes partly from facing direction in both the Coongan greenstone belt (facing to the east) and the Warrawoona greenstone belt (facing to the southeast). Cycle 1 in the Warrawoona greenstone bek is assumed to be the top of cycle 8 in the Coongan greenstone belt. Because the eastern side of cycle 7 in the Coongan greenstone belt is not studied south of Blue Bar mining center, evidence is not that hard. Further research here and to the southeastern part of the studied area m the Warrawoona greenstone belt needs to be done to give clear evidence for this hypothesis. From cycle 1 in the Coongan greenstone belt up to cycle 3 in the Warrawoona greenstone belt the basm infill tehs the story of the basin. First a basin was formed into a basement which was extended. During cycle 1, 2, 3 and 4 the basin was filled with basaltic/andesitic flows and sediments. During deposition in cycle 4 also mafics and granodiorite/tonahtes intruded into cycle 4. After intrusion erosion took place, by which the upper part of cycle 4 was eroded away. It is thus assumed that the basin was exposed above sea level. During the begmning of the renewed infill with coarse erosive material, the basin coUapsed and growth faults were formed. In the north of the area a rhyodacitic volcano is assumed to have been active at this time. The basin was filled up again with mainly sedimentary rocks during the rest of cycle 5, 6, 7 and 8. Only cycle 6 consists mainly of mafic flows, in which volcanic exhalatives occur. In cycle 7 and 8 slumps occur. In cycle 1 in the Warrawoona area large erosion products are found. During cycle 2 the sedimentary period changed to a period with mafic flows again in cycle 3. This whole basin evolution is assumed to be happened between about 3.47 Ga (age Duffer) and 3.3 Ga (age of felsic intrusives). Younger units ^e present in the Warrawoona greenstone belt, but have not been studied for this report. After deposition in the basin the total basin was tilted to the east by an unknown process. After that the bathohths are assumed to be intruded and several shear events took place.

Correlation of the data of the Coongan greenstone belt with older data gives some problems on ages. Already dated samples from the rhyodacite in cycle 5, the Duffer sequence, are ah around the age of 3.47 Ga. Dated samples of the north side of the Shaw granodiorite (North Shaw Suite) give also approximately this date, as shown in figure 4.2 and 4.3 in Zegers (1996). The rocks which are dated may have been contact metamorphosed rocks of cycle 1. If so the dates then will probably not give the age of the Shaw granodiorite, but of the cycle 1 sediments. If the date is the real date of intrusion of the Shaw granodiorite, timing is strange because the rocks of cycle 1 are subvertical, and thus first tilted with the rest of the basin infill (so after deposition of cycle 8) before it

26 could be contact metamorphosed by the granodiorite. Redatmg of the Shaw granodiorite is advised. The maps for the Coongan greenstone belt as were mapped by Zegers (1996, and figures 4 to 7 in this report) are very useful and of good quahty for ftirther research. Figure 6, which is based upon the northem part of the Coongan greenstone belt in the studied area, the Taiga Taiga subgroup is subdivided in this report into cycle 2, 3 and 4, the Duffer formation is in this report cycle 5, the Salgash subgroup is cycle 6, Gorge creek is cycle 7 and Lalla Rookh sandstone is cycle 8. Only the rhyolite from the Wyman formation is not recognized. The map for the Warrawoona greenstone belt as produced by Kloppenburg (figure 8) was not very usefiall because the unit with felsic volcanics (inch Wyman Formation)/minor felsic, mafic and ultramafic intrusives was not recognized hke that, and the Broekman Hay shear was not found, although that part would be in the studied area. Major parts of the area were covered with scree slopes. The unit of Kloppenburg with felsic volcanics is in this report a boulder conglomerate with other quartz-rich sediments (cycle 1) foUowed by cycle 2 that predominantly consists of mafic sediments, basaltic/andesitic flows (hyaloclastite layers, tuff) uiterbedded with some quartz/feldspar- rich sediments and dolomites in the upper part of the sequence. Where the extremely wide Broekman Hay shear zone is situated in figure 8 no mylonites were found, and the rocks in this report are ordered as cycle 3 (hilly basalt).

From facing direction and stratigraphy in the studied parts of the basin the diapiric model of CoUins and Van Kranendonk (1999) and CoUins et al. (1998) does not seem right. Facing is directed throughout the studied Warrawoona group to the east or southeast and no symmetric repetition was found in the greenstones, that would advocate the diapiric model (CoUins et al. (1998). The model of CoUins et al. (1998) is mainly based upon the Warrawoona greenstone belt. Because the field work was focussed on the Coongan greenstone belt, and only a smaU part of the Warrawoona greenstone was studied, no weU-considered opinion can be made on this point. This part of the FUbara therefore needs to be studied too. Also the cormection between the Coongan greenstone belt to the east of the studied area and the Warrawoona belt needs to be studied better. UntU then, with recent knowledge, the mid-crustal detachment model is preferred above the diapiric model. This implies that horizontal forces were already active in the Archaean. The problem is not solved yet. More research in this area and in the areas around it is StiU needed!

27 Acknowledgements

Above all I appreciate the opportunity that Prof. Dr. Stan White and Dr. Jan Wijbrans gave me to do geological field work in Austraha for the Pilbara Project for a second time, to work in a total different environment as I have worked before, to experience new geological features and for meeting and working with new people. Thank you! Dr. Kim Hein is thanked for teaching me geology and other essential things that are useful in the field (like siu-viving, four-wheel driving, bush-cooking, etc.) and for being my field partner. Dr. Schürmann Fonds, Stichting Molengraaff-Fonds and VU subsidie are thanked for financial support. Hugo, Fernando, ICike, Pieter, Geert and the persons akeady mentioned above are thanked for company once in a while during the field period.

28 References

Barley, M.E. (1997) The Pilbara Craton, chapter 3.13 from de Wit, M., Greenstone belts. Oxford University press. Oxford. Barley, M.E. and Bickle, M.J. (1982) Komatiites in the Pilbara Block, Western Australia, from Komatiites, Amdt, N.T. and Nisbet, E.G. Barley, M.E., Dunlop, J.S.R., Glover, J.E. and Groves, D.I. (1979) Sedmientary evidence for an Archaean shaUow-water volcanic-sedimentary facies, eastern Pilbara Block, Western Australia. Earth and Planetary Science Letters 43, 74-84. Bickle, M.J., Betternay, L.F., Chapman, H.J., Grove, D.I, McNaughton, M.J., CampbeU, I.H. and de Laeter, J.R. (1989) The age and origin of younger granitic plutons of the Shaw Batholith in the Archaean Pilbara Block. Contributions to Mineral. Petrol. 101, 361-376. Buick, R., Thornett, J.R., McNaughton, N.J., Smith, J.B., Barley, M.E., and Savage, M.D. (1995) Record of emergent continental crast 3.5 bilhon years ago in the Pilbara craton of Australia, Nature 375, 574-577. Colhns, W.J., Van Kranendonk, M.J. (1996) Model for the development of kyanite during partial convective overturn of Archean granite-greenstone terranes: the Pilbara Craton, Austraha, Journal of metamorphic geology 17 (2), 145-156. Colhns, W.J., Van Kranendonk, M.J., Teyssier, C. (1998) Partial convective overturn of Archaean crast in the east Pilbara Craton, Western Austraha: driving mechanisms and tectonic miplications. Journal of stractural geology 20 (9-10), 1405-1424. Eriksson, K.A., Krapez, B., Gralick, P.W. (1994) Sedimentology of Archean greenstone belts; signatures of tectonic evolution, Earth Science Review 37, 1-88. Evans, A.M. (1996) Ore geology and industrial minerals, an introduction, BlackweU Science. Hickman, A.H. (1980) Archaean geology of the Pilbara Block, Excursion Guide, Second International Archaean Symposium, Perth. Kloppenburg, A., White, S.H., Zegers, T.E. (submitted to Precambrian Research), Stractural evolution of the Warrawoona Greenstone Belt and adjoining granitoid complexes, Pilbara Craton, Austraha: implications for Archaean tectonic processes. Krapez, B., Eisenlohr, V. (1998) Tectonic settings of Archaean (3325-2775 Ma) crustal-supracrastal belts in the West Pilbara Block. Precambrian Research 88, 173-205. Myers, J.S. (1990) Precambrian tectonic evolution of part of Gondwana, Southwestern Australia. Geology 18, 537-540. Nelson, D.R., Trendall, A.F., de Laeter, J.R., Grobler, N.J., Fletcher, I.R. (1992). A comparative study of the geochemical and isotopic systematics of late Archaean flood basalts from the Pilbara and Kaapvaal Craton. Precambrian Research 54, 231-256. Rogers, J.J.W. (1996) A history of continents in the past three Billion years. The journal of Geology 104,91-107. Tyler, I.M. and Thome, A.M. 1990. The northern margin of the Capricorn Orogen, Western Austraha- an example of an early Proterozoic coUision zone. Journal of stractural geology 12, 685-701. Zegers, T.E. (1996) Stractural, kinematic and metallogenic evolution of selected domains of the Pilbara granitoid-greenstone terrain. PhD Thesis Ufrecht University, pp.208. Zegers, T.E., White, S.H., de Keijzer, M., Dirks, P. (1996) Extensional stractures during deposition ofthe 3460 Ma Warrawoona Group in the eastern Pilbara Craton, Westem Austtalia, Precambrian Research 80, 89-105.

29 Idealized stratigraphical column Coongan greenstone belt

(pCd-i^sJtcm ucrA^ ch^

IT.

'9 oo

5-1

J5i ^ V/ 4^)

-V sT

t Coongan greenstone belt Stratigraphic colunm taken at section ABC

SOiÊsi \ • S'O. ooo

Sar^ljAfi.'

5-) sz. Coongan greenstone belt Stratigraphic column taken at section DEFG

^dMi\&nJt aJr base i4_

<:edX/TO/xtoxy unit

KfTV Coongan greenstone belt Stratigraphic column taken at section HIJK

6rf - ch&xt, S^mps

"TJTrwmTÏTTJii \1-

*9 ba^soOi, base g/zjsy to^omus mmk ao

ofl és

legenda I ~l quartz/feldspar-rich porphyry i I Shaw granodiorite I granodiorite/tonalite I j rhyodacite • gabbro/pyroxenite j ^1 high Mg (?) basalt 1^ ~1 basalt I andesite I I volcanoclastic sediment I I feldspar-rich sediment I I clastic sediment I I coarse sediment (conglo/breccia) |v /) dolomite chert 1 BIF Idealized stratigraphical column okrrv ikm Warrawoona greenstone belt : I

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legenda I 1 quartz/feldspar-rich porphyry I I Shaw granodiorite I granodiorite/tonalite I rhyodacite I I gabbro/pyroxenite j I high Mg (?) basalt I I basalt I andesite I volcanoclastic sediment I I feldspar-rich sediment I I clastic sediment II coarse sediment (conglo/breccia) r~ I dolomite chert i BIF Appendix 3: Pictures

Picture 1: Coarse erosion material at the base of cycle 5 (Duffer sequence). Boulder size is 0.5-1 meter, smaller particle are 10 cm m size. It contains gabbro, coarse sandstone to gridstone, (variohtic) basalt, volcanoclastics (mainly mafic), mafic conglomerate and fine layered sandstone. The picture is taken at coordinates 07.72.992/76.22.971.

Picture 2: Cross bedding in sandstones from the top of cycle 5 (Duffer sequence). Facing direction is to the east (top picture). The picture is taken at coordinates 07.74.329/76.23.717. Picture 3: Breccia/ conglomerate at the center of cycle 5 (Duffer sequence), coordinates 07.74.333/ 76.23.727.

Picture 4: Stromatohtes at the center of cycle 5 (Duffer sequence), coordinates 07.74.429/ 76.23.771.

Picture 5: Pillow andesites at the base of cycle 6. The picture is taken from coordinates 07.75.555/ 76.23.512. Picture 6a, b and c: "Felsic" pillows: leached andesitic pillows at the center of cycle 6. Note the cracks by which fluids are assumed to have been moved. Pictures are taken from coordinates 07.74.313/ 76.31.725. Picture 7: Volcanic exhalative in breciated pillows at center of cycle 6. Coordinate: 07.74.015/7 6.31.328.

Picture 8: Roll strucures (slumps) in sandstones at the top of cycle 6. Coordinate: 07.71.438/7 6.36.456.

Picture 9: Slump structure in a chert at coordinates 07.76.156/ 76.23.605, at the base of cycle 7. Facing direction is to the east (top picture). Picture 10a and b: slump folds in cycle 8. Coordinates 07.76.091/76.34.600 (a) and 07.75.911/ 76.34.307 (b). Note highly variable fold axes.

Picture 11: Kink bands in sediments at the base of cycle 5 at coordinates ± 07.72.950/ 76.20.250. The kinks are produced by thrusting with east block (left side picture) up. Picture 12: Variolitic basalt. Size of the stripes of the hammer is 5 cm each. When varioles clump together large varioles are formed of which one is shown above the hammer. The picture is taken from coordinates 07.82.832/76.36.387.

Picture 13: Columnar joints in quartz porphyry, which is a constituent ofthe Wyman formation. The picture is taken at coordinates 07.87.330/76.39.918. Appendix 4: Sample list

Geochemistry samples;

Name Coordinates Rock type Cycle AOO/1 07.87.330 quartz-porphyry Cl 76.39.918 AOO/5 07.69.623 doleritic basalt 3 76.27.751 AOO/6 07.70.387 (high Mg?) basalt 4 76.28.016 AOO/7 07.70.592 basalt 4 76.28.001 AOO/8 07.69.297 doleritic basalt 2 76.25.248 AOO/10 07.68.353 amphibolitic basalt 2 76.24.585 (hornfels?) AOO/11 07.71.027 granodiorite/ tonahte 4 76.27.817 AOO/17 07.75.682 andesite 6 76.23.543 AOO/19 07.74.890 basalt 6 76.23.655 AOO/20 07.74.756 basalt with black 6 76.23.726 xenohths AOO/22 07.72.507 (high Mg?) basalt 4 76.24.713 AOO/26 07.72.800(+) pyroxenite 4 76.21.000(+) AOO/27 07.72.397 andesite 4 76.20.566 AOO/28 07.72.088 granodiorite 4 76.20.536 AOO/29 07.71.666 basalt 4 76.20.476 AOO/34 07.70.977 andesitic dyke (intrudes 4 76.26.778 AOO/11) AOO/39 07.72.279 rhyodacite 5 76.32.438 AOO/48 07.71.034 rhyodacite 5 76.32.460 AOO/49 07.70.930(±) tonahte dyke 5 76.32.350(+) AOO/50 07.70.804 basalt dyke 5 76.31.936 Name Coordinates Rock tvne Cvcle AOO/51 07.70.780 dolerite 4 76.31.391 AOO/52 07.70.255 basalt 4 76.31.092 AOO/53 07.68.508 gabbro 4 76.30.664 AOO/54 07.66.630 andesitic tuff F 76.31.532 (Fortescue) AOO/63 07.71.856 gabbro 4 76.24.236 AOO/66 07.73.222(±) porphyry in 5 76.25.444(+) conglomerate AOO/68 07.72.639 basaltic dyke 5 76.27.971

Zircon samples:

Name Coordinates Rock type Cvcle AOO/11 07.71.027 granodiorite/tonahte 4 76.27.817 AOO/39 07.72.279 rhyodacite 5 76.32.438 WA07 07.87.789 granite Cl 76.39.736 WA09 07.81.867(+) Qz-feldspar-porphyry in Cl 76.39.254(+) conglomerate

N.B. Geochemistry sample AOO/1 and zircon samples WA07 and WA09 are taken from the Warrawoona area. AU other samples are taken from the Coongan area.