Tải bản đầy đủ - 0 (trang)
Chapter 4.1 Paleoarchean Development of a Continental Nucleus: the East Pilbara Terrane of the Pilbara Craton, Western Australia

Chapter 4.1 Paleoarchean Development of a Continental Nucleus: the East Pilbara Terrane of the Pilbara Craton, Western Australia

Tải bản đầy đủ - 0trang

308



Chapter 4.1: Paleoarchean Development of a Continental Nucleus



Fig. 4.1-1. Simplified geological map of the Pilbara Craton, showing terranes, late tectonic basins of

the De Grey Supergroup, and Nd model age data. Inset shows craton in Western Australia.



by volcanic and sedimentary rocks of the ca. 2.78–2.45 Ga Mount Bruce Supergroup deposited in the Hamersley Basin (inset of Fig. 4.4-1: Arndt et al., 1991; Blake et al., 2004;

Trendall et al., 2004).

Hickman (1983, 1984) provided a regional lithostratigraphic interpretation of the ‘Pilbara Block’, based on reconnaissance geological mapping, and concluded there was a

relatively uniform stratigraphy across the craton and that the major tectonic structures

resulted from essentially solid-state granitoid diapirism. Subsequent geochronology and

mapping, however, showed that the greenstones are much more laterally variable in age

across the craton than previously thought (Horwitz and Pidgeon, 1993; Hickman, 1997;

Smith et al., 1998; Buick et al., 2002), and include exotic terranes accreted during the later

stages of tectonic development of the craton (Tyler et al., 1992; Barley, 1997; Smith et al.,

1998; Van Kranendonk et al., 2002, 2006a, 2007a; Hickman, 2004).



4.1-2. Geology of the Pilbara Craton



309



Recent compilations of stratigraphic, geochronological, structural, and isotopic data collected during a 12-year geoscience program by the Geological Survey of Western Australia,

Geoscience Australia and a variety of university-based researchers have been used to divide

the granite-greenstone rocks of the northern Pilbara Craton into five terranes and five late

tectonic, dominantly clastic sedimentary basins (Fig. 4.1-1: Van Kranendonk et al., 2002,

2006a, 2007a). Terranes have unique lithostratigraphy, granitic supersuites, structural map

patterns, geochemistry and tectonic histories, as outlined in more detail elsewhere (Hickman, 1997, 2004; Van Kranendonk et al., 2006a, 2007a; Smithies et al., 2007b).

Terranes include: the 3.53–3.165 Ga East Pilbara Terrane (EPT), representing the ancient nucleus of the craton; the 3.18 Ga Kurrana Terrane in the southeastern part of the

craton, which has EPT-type Sm-Nd model ages and may represent a rifted fragment of this

crust; and the 3.27–3.11 Ga West Pilbara Superterrane (WPS). The WPS is a collage of

three distinct terranes that include: the 3.27 Ga Karratha Terrane (KT), which may represent a rifted fragment of the EPT (Hickman, 2004) and which is juxtaposed against; ca.

3.20 Ga, MORB-type basaltic rocks of the Regal Terrane across a major thrust; the 3.13–

3.11 Ga Sholl Terrane in the southern part of the WPS, which is juxtaposed against the

other two terranes of the WPS to the north across the crustal-scale, strike-slip Sholl Shear

Zone, which has a long history of formation and re-activation from ca. 3.07 to 2.92 Ga

(Hickman, 2004).

The rocks of the WPS, EPT and Kurrana Terrane are separated by, and unconformably

overlain by, the dominantly clastic sedimentary rocks of the De Grey Supergroup that was

deposited across the whole of the craton between 3.02–2.93 Ga (Van Kranendonk et al.,

2006a, 2007a). The supergroup comprises the basal Gorge Creek Group (ca. 3.02 Ga),

the Whim Creek Group (3.01 Ga), the Croydon Group (2.99 or 2.97 Ga, through to

2.94 Ga) and the Nullagine Group ( 2.93 Ga). Major depositional basins include the Gorge

Creek Basin, which formed across the whole of the craton, the Mallina Basin between the

WPS and EPT, and the Mosquito Creek Basin between the EPT and the Kurrana Terrane

(Fig. 4.1-1). Recent models suggest deposition of the De Grey Supergroup (and intrusion

of contemporaneous granitic supersuites) mainly as a result of lithospheric extension following accretion of the WPS at 3.07 Ga (Van Kranendonk et al., 2007a). Extension was

viewed by these authors as the result of orogenic relaxation and breakoff of the slab of

oceanic crust that was subducted during the formation of the Whundo Group (Sholl Terrane). The latter stages of deposition accompanied compressional deformation that affected

the whole of the WPS and western half of the EPT at ca. 2.94–2.92 Ga, and the southeastern

part of the northern Pilbara Craton at 2.905 Ga, the latter during accretion of the Kurrana

Terrane with the EPT across the Mosquito Creek Basin (Van Kranendonk et al., 2007a).

Rocks of the WPS show lithostratigraphic, geochemical and structural features that are

distinctly different from the EPT and thus have been used to argue for a change in tectonic style, from crust formation processes dominated by mantle plumes prior to 3.2 Ga, to

processes dominated by plate tectonics after ca. 3.2 Ga (Smith et al., 1998; Hickman, 2004;

Van Kranendonk et al., 2006a, 2007a). A key element in this model is geologic, isotopic,

and geochemical data relating to the 3.13–3.11 Ga Whundo Group of the Sholl Terrane,

which is a fault-bounded, >10 km thick succession of juvenile (εNd values of ∼ +2 to



310



Chapter 4.1: Paleoarchean Development of a Continental Nucleus



+3 on volcanic rocks: Sun and Hickman, 1998), bimodal basaltic to felsic volcanic and

volcaniclastic rocks that show affinities with an oceanic island arc (Smithies et al., 2005a,

2007a). Geochemical data indicate that the Whundo Group has a complex, but compelling,

arc-like assemblage of rock types, including boninites (that have all of the compositional

hallmarks of modern boninites), interlayered tholeiitic and calc-alkaline volcanics, Nbenriched basalts, adakites, and rhyolites (Smithies et al., 2004b, 2005a, 2007b). Unusual

trends of increasing trace element enrichment (i.e., higher La/Sm), which measures increasing degrees of partial melting (e.g., increasing Cr, decreasing HFSE, HREE, etc.) in

the calc-alkaline rocks, can only be explained if the degree of melting is sympathetically

tied to metasomatic enrichment (i.e., flux melting) of a mantle source. Trends to higher

Th/Ba upsection in the Whundo Group stratigraphy reflect an increasing slab-melt component, and the calc-alkaline basalts and andesites eventually give way to adakites and Nbenriched basalts, which require a major slab-melt component in the petrogenesis. Thus, the

Whundo Group provides strong geochemical evidence for a subduction-enriched mantle

source that strongly supports existing geological relationships (e.g., exotic terrain, no continental sediments, arc-like volcanic assemblage) for modern-style subduction-accretion

processes at 3.12 Ga (Smithies et al., 2005a, 2007a, 2007b; Van Kranendonk et al., 2007a).



4.1-3. GEOLOGY OF THE EAST PILBARA TERRANE

The East Pilbara Terrane (EPT) is composed of rocks that formed between ca. 3.53 Ga

and 3.165 Ga, prior to the time of onset of common geological events across the craton at

3.07 Ga (Van Kranendonk et al., 2006a, 2007). Volcano-sedimentary rocks (greenstones)

of the EPT are assigned to the Pilbara Supergroup, which consists of four demonstrably

autochthonous groups deposited from ca. 3.53–3.19 Ga. These are distributed in arcuate

greenstone belts that wrap around domical, multiphase granitic complexes (Fig. 4.1-2).

Granitic rocks of the EPT include five granitic supersuites that were emplaced at discrete intervals from ca. 3.50 to 3.165 Ga. Granitic rocks are exposed in domical granitic

complexes and in syn-volcanic laccoliths. Periods of deformation and metamorphism accompanied each of the groups and emplacement of granitic rocks into the EPT prior to

deposition of unconformably overlying supracrustal rocks (De Grey Supergroup) and intrusion by younger granitic rocks, both of which relate to common geological events across

the craton between 3.07–2.83 Ga.

4.1-3.1. Lithostratigraphy in Greenstone Belts

The EPT is underlain by the generally well-preserved Pilbara Supergroup, which consists

of four demonstrably autochthonous volcano-sedimentary groups deposited between ca.

3.53–3.19 Ga (Fig. 4.1-3: Van Kranendonk et al., 2006a, 2007a). Detailed geological mapping and extensive SHRIMP geochronology has revealed no stratigraphic repetitions in

any of the greenstone belts in the EPT (Van Kranendonk et al., 2004, 2006a).



4.1-3. Geology of the East Pilbara Terrane

311



Fig. 4.1-2. Geological map of the East Pilbara Terrane, showing the dome-and-keel architecture and the distribution of major units of the Pilbara

Supergroup.



312



Chapter 4.1: Paleoarchean Development of a Continental Nucleus



Fig. 4.1-3. Lithostratigraphic column of the Pilbara Supergroup.



4.1-3. Geology of the East Pilbara Terrane



313



The maximum preserved thickness of the Pilbara Supergroup is ∼20 km. However, contacts of the lower part of the supergroup are everywhere with intrusive granitic rocks and

the upper parts of each of the groups has been eroded beneath unconformably overlying

groups or formations, such that the original thickness of any of the component groups, and

the supergroup as a whole, is unknown. Furthermore, there is good evidence from changes

in the dip of bedding between groups, and from the nature of the unconformities between

groups, that the supergroup was deposited as a laterally accreting succession within developing synclinal basins on the flanks of amplifying granitic domes, during periods of

extension (Hickman, 1984; Van Kranendonk et al., 2004, 2007).

4.1-3.1.1. Warrawoona Group

The stratigraphically lowest Warrawoona Group is at least 12 km thick and was deposited

over 100 My as a result of continuous volcanism from ca. 3.53–3.43 Ga (Fig. 4.1-3: Van

Kranendonk et al., 2002, 2006a, 2007a). The bulk of this succession comprises wellpreserved pillow basalt, komatiitic basalt, and komatiite (Fig. 4.1-4(a,b)). Lesser felsic

volcanic intervals are typically restricted to thin tuffaceous horizons a few 10’s of metres

thick, but dacite dominated deposits locally reach a few kilometers thick in the Coucal,

Duffer and Panorama Formations, where coarse volcaniclastic breccias are a common

lithofacies (Fig. 4.1-4(c): see Smithies et al., this volume).

Sedimentary units in the Warrawoona Group include a variety of lithology, such as volcaniclastic rocks, quartz-rich sandstone, and carbonates, but mainly consist of grey, white,

blue-black, and red layered cherts derived from silicified carbonates (Fig. 4.1-4(d): Buick

and Barnes, 1984; Van Kranendonk, 2006). An unusual sedimentary unit is the 3470±2 Ma

Antarctic Creek Member of the Mount Ada Basalt, which contains thin beds with sandsize grains of altered quench-textured impact spherules (Lowe and Byerly, 1986; Byerly

et al., 2002). Many chert units in the Warrawoona Group, including the jaspilitic cherts

of the ca. 3.47 Ga Marble Bar Chert Member of the Duffer Formation, are well layered

at a millimetre to centimetre scale, indicating deposition under quiet water, deep marine

conditions, consistent with evidence from underlying and overlying pillow basalts. In contrast, the chert-barite unit of the ca. 3.49 Ga Dresser Formation in the North Pole Dome

shows evidence of deposition under tectonically active conditions, as indicated by rapid

lateral facies variations, beds of olistostrome breccia, and internal erosional unconformities (Nijman et al., 1998; Van Kranendonk, 2006; Van Kranendonk et al., 2006c, 2007c).

The local preservation of desiccation cracks and the more widespread occurrence of rippled carbonate sedimentary rocks and stromatolites in this unit indicate periods of shallow

water deposition, including subaerial exposure (Lambert et al., 1978; Walter et al., 1980;

Groves et al., 1981; Buick and Dunlop, 1990; Van Kranendonk, 2006). Stromatolites are

well developed in this unit and vary in morphology, suggesting diverse assemblages of

micro-organisms at this early stage in Earth history (see Van Kranendonk, 2006, this volume, Chapter 7.2). Whereas original studies suggested deposition of the Dresser Formation

in a restricted shallow marine basin, more recent studies suggest deposition within a felsic

volcanic caldera affected by syn-depositional growth faults and voluminous hydrothermal



314

Chapter 4.1: Paleoarchean Development of a Continental Nucleus



Fig. 4.1-4. Representative rock types of the Warrawoona Group: (A) Pillow breccia, Mount Ada Basalt; (B) Pyroxene spinifex-textured komatiitic basalt, Mount Ada Basalt; (C) Coarse volcaniclastic breccia, Duffer Formation; (D) Typical laminated grey and white chert, Dresser

Formation; (E) Carbonate-altered, cross-bedded volcaniclastic sandstone (basal surge deposit?), Panorama Formation.



4.1-3. Geology of the East Pilbara Terrane



315



Fig. 4.1-4. (Continued.)



circulation (Nijman et al., 1998; Van Kranendonk, 2006; Van Kranendonk et al., 2006c,

2007c).

The stratigraphy of the Warrawoona Group varies across the EPT, but dating confirms stratigraphic correlations across most of the terrane. The Warrawoona Group is

uplifted and eroded away in the west, were the oldest components of the group (the

Coonterunah Subgroup) are exposed and unconformably overlain by the Kelly Group

(see Fig. 17 in Van Kranendonk and Pirajno, 2004). The thickest (12 km) and most complete section through the group is exposed in the Marble Bar greenstone belt (Hickman,

1983), although the lowermost part of the stratigraphy is excised by intrusive granitic

rocks. A distinctive feature of the group in this belt is the great thickness (4 km) of

volcaniclastic rocks of the Duffer Formation (Hickman, 1983) and the presence of a vo-



316



Chapter 4.1: Paleoarchean Development of a Continental Nucleus



luminous mafic dyke swarm that feeds the Apex Basalt (Van Kranendonk et al., 2006b).

To the west and south of the Marble Bar area, most of the group is preserved, except

that the Apex Basalt is missing and the Panorama Formation lies directly on the Duffer

Formation across an erosional unconformity (Di Marco and Lowe, 1989b: see Fig. 11

in Van Kranendonk et al., 2004). The youngest Panorama Formation is present in all

greenstone belts except in the far western part of the EPT, where it has been eroded

away.

Detailed mapping and extensive SHRIMP U-Pb zircon geochronology suggest that the

volcanic rocks were erupted as eight (ultra)mafic through felsic volcanic cycles of ca.

15 My duration (Hickman and Van Kranendonk, 2004), each of which was capped by thin

sedimentary rock units silicified by syn-depositional hydrothermal fluids (Van Kranendonk, 2006). Warrawoona Group volcanism closed with Panorama Formation andesitic to

rhyolitic volcanism that was developed in several stratigraphically and compositionally distinct centres across the terrane (Smithies et al., 2007b, this volume) from 3.458–3.426 Ga,

deposited under shallow water to subaerial conditions (Fig. 4.1-4(e): Di Marco and Lowe,

1989a).

4.1-3.1.2. Kelly Group

Deposition of the Warrawoona Group was followed by a 75 My hiatus in volcanism, during

which time the terrane was uplifted and eroded under at least locally subaerial conditions

(Buick et al., 1995). Sedimentary rocks of the 30–1000 m thick Strelley Pool Chert at the

base of the Kelly Group were deposited on the Warrawoona Group during this time interval, across a regional unconformity (Fig. 4.1-3: Van Kranendonk et al., 2006a, 2007a).

This formation comprises a lower unit of fluviatile to shallow marine conglomerates and

quartzite, a middle unit of stromatolitic marine carbonates, and an upper unit of coarse

clastic rocks (Lowe, 1983; Hofmann et al., 1999; Van Kranendonk et al., 2003; Van Kranendonk, 2006; Allwood et al., 2006a), and was deposited on a carbonate platform that

extended across the EPT (Van Kranendonk, this volume, Chapter 7.2).

The conformably overlying Euro Basalt consists of a 1.5 km thick basal unit of komatiite and up to 5 km of overlying, interbedded komatiitic basalt and tholeiitic basalt

that was erupted in ∼25 My, from 3.35–3.32 Ga (Fig. 4.1-5(a,b)). This was followed by

eruption of ca. 3.325 Ga high-K rhyolites of the Wyman Formation (Fig. 4.1-5(c)) that

was accompanied by the emplacement of genetically related, voluminous, monzogranitic

plutons of the Emu Pool Supersuite (see 3.2 Granitic Rocks, below). Basaltic volcanism

continued with eruption of the conformably overlying, but undated, Charteris Basalt, which

is locally 1000 m thick (Hickman, 1984).



Fig. 4.1-5. Representative volcanic rocks of the Kelly Group: (A) Olivine spinifex-textured komatiite, Euro Basalt: top of flow unit to right (width of view is 30 cm across); (B) Large, lobate pillows,

Euro Basalt; (C) Columnar rhyolite, Wyman Formation.



4.1-3. Geology of the East Pilbara Terrane



317



318



Chapter 4.1: Paleoarchean Development of a Continental Nucleus



4.1-3.1.3. Sulphur Springs Group

The 3.27–3.23 Ga Sulphur Springs Group was deposited across an unconformity on

older greenstones in the western part of the EPT (Figs. 4.1-2, 4.1-3: Van Kranendonk,

2000; Buick et al., 2002). In the type area in the Soanesville greenstone belt, this group is

up to 4000 m thick and consists of basal sandstone and felsic volcaniclastic rocks of the

3.27–3.25 Ga Leilira Formation (Buick et al., 2002; sample 178045 in GSWA, 2006), up

to 2000 m thick of komatiite to komatiitic basalt of the ca. 3.25 Ga Kunagunarrina Formation (geochronology from sample 160957 in GSWA, 2006), and up to 1500 m thick

of andesite-basalt through to rhyolite of the 3.245–3.235 Ga Kangaroo Caves Formation,

which is capped by <30 m of silicified epiclastic and siliciclastic rocks (Van Kranendonk,

2000; Buick et al., 2002). Felsic volcanic rocks of the formation thin laterally away from

the thickest part of the group and pass into 500 m of banded iron-formation (Pincunah

Member of the Kangaroo Caves Formation: Van Kranendonk et al., 2006a). The banded

iron-formation, along with panels of silicified epiclastic sediments and large blocks of

rhyolite, are incorporated within a unit of coarse olistostrome breccia at the top of the

formation, over the apex of the Strelley Monzogranite (Fig. 4.1-6).

The group is intruded by the shallow level, syn-volcanic Strelley Monzogranite, a

K2 O-rich subvolcanic laccolith with rapakivi textures (Brauhart, 1999; Van Kranendonk,

2000, 2006). Heat from this intrusion drove hydrothermal circulation that precipitated volcanogenic massive sulphide (Cu-Zn) deposits (Fig. 4.1-6: Morant, 1998; Vearncombe et

al., 1998; Huston et al., 2001; Van Kranendonk, 2006). Eruption of the Sulphur Springs

Group was coeval with widespread monzogranite plutonism of the 3.275–3.225 Ga Cleland Supersuite across the northern and western parts of the EPT.

4.1-3.1.4. Soanesville Group

Disconformably overlying the Sulphur Springs Group is the 3.235–3.19 Ga succession

of clastic sedimentary rocks, basalt, and banded iron-formation of the Soanesville Group

(Figs. 4.1-2, 4.1-3: Buick et al., 2002; Van Kranendonk et al., 2006a: Rasmussen et al.,

2007). This group includes, from base to top: shale of the Cardinal Formation; the Corboy

Formation of dominantly sandstone; the Paddy Market Formation of shale and siltstone;

a 1150 m thick succession of interbedded high-Mg and tholeiitic pillow basalts known

as the Honeyeater Basalt; and the Pyramid Hill Formation of banded iron-formation and

ferruginous shale (Van Kranendonk et al., 2006a). The Honeyeater Basalt is associated with

the Daltons Suite of layered mafic-ultramafic intrusions that range in composition from

dunite, through pyroxenite, to dolerite (with local granophyre: Van Kranendonk, 2000).

These rocks lie conformably, or with an onlapping lower contact, on silicified epiclastic rocks of the Sulphur Springs Group (Van Kranendonk, 2000). The lower sedimentary

formations of the group are linked to waning activity of the Sulphur Springs Group on



Fig. 4.1-6. Schematic evolution of deposition of the Sulphur Springs Group during emplacement of

the Strelley Monzogranite, in a felsic volcanic lacco-caldera setting.



Tài liệu bạn tìm kiếm đã sẵn sàng tải về

Chapter 4.1 Paleoarchean Development of a Continental Nucleus: the East Pilbara Terrane of the Pilbara Craton, Western Australia

Tải bản đầy đủ ngay(0 tr)

×