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).