Shaw Batholith Research Articles

40Ar/ 39Ar age constraints on tectonothermal events in the Shaw area of the eastern Pilbara granite–greenstone terrain (W Australia): 700 Ma of Archean tectonic evolution

The Shaw area in the eastern Pilbara Craton, consists of granitoids with intrusion ages ranging from 3470 Ma to 2850 Ma, and supracrustal rocks with ages ranging from 3515 Ma to 3100 Ma as determined by U–Pb and Pb–Pb dating methods. In this terrain several major deformation events have recently been recognized, including late strike-slip deformation, compression and early extension. In order to provide time constraints for these events, twenty-four samples from the Shaw area have been used for 40Ar/ 39Ar step heating experiments (twenty-two hornblendes, one muscovite, one biotite), resulting in spectra defining ages between 3520 and 2800 Ma. The late strike-slip event occurred prior to 2944±9 Ma. The argon cooling ages provide time constraints for the compressional event between 3325 Ma and 3200 Ma. Cooling ages between 3300 and 3200 Ma cannot be related to granitoid intrusion in this area and are interpreted as cooling after regional metamorphism related to the compressional event. The best time constraint from 40Ar/ 39Ar ages for the Split Rock Shear Zone, a major ductile detachment in the Shaw batholith associated with early extension, is that it was last active prior to 3222±13 Ma. A 3341±13 Ma plateau age of a metabasalt sampled in contact with granites of the northwestern Corunna Downs Batholith provides a minimum age for granitoid intrusion and a minimum age for the Warrery Shear Zone. Hornblendes from the Black Range Dolerite Dyke gave a plateau age of 2796±9 Ma, providing a constraint on the time of cratonization of the Pilbara granitoid–greenstone terrain. Most of the cooling ages, including a 3466±13 Ma age for the contact zone of the North Shaw Granodiorite, are related to intrusion of granitoids. In the Shaw Batholith, a large number, but significantly, not all of the sample population has been reset during the intrusion of late to post-tectonic adamellites between 3050 Ma and 2850 Ma. Preservation of such a large range (700 Ma) in argon isotopic ages in a relatively small area is unusual. The resetting of argon ages in the granitoid domain is best explained by a combination of preferential intrusion of younger granites and the difference in heat conductivity of granite versus basalt. In the greenstone domain the resetting effect is more localized into a network of fluid flow channels, mostly situated in shear zones.

Timing and tectonic significance of Late Archaean, sinistral strike-slip deformation in the Central Pilbara Structural Corridor, Pilbara Craton, Western Australia

A north-south striking zone of sinistral strike-slip shear (20 × 100 km) and associated folding and thrusting within the central part of the Pilbara Craton is described. This zone, referred to herein as the Central Pilbara Structural Corridor (CPSC), was formed during regional D3 deformation and overprints two earlier sets of structures associated with granitoid doming. It is bound to the west by a continuous, north-south striking, curviplanar fault at greenschist-facies. In the east it is bound by a series of NE-SW striking fault segments located along sheared fold limbs, and by a kilometre-wide zone of amphibolite-facies mylonite within the western margin of the Shaw Batholith. Faults vary from narrow, high-level, brittle structures and retrograde breccia zones in the north, to broader, more ductile structures with S C mylonitic foliations in the south, suggesting a southerly increase in exhumation. A set of kilometre-scale, en-échelon folds with NE-SW striking, upright axial planes and NE-plunging axes characterize the interior of the CPSC and are wholly contained within the bounding faults, thus indicating coeval development with faulting during sinistral wrenching ( σ 1 = NW-SE). Strike-slip deformation in the CPSC is interpreted to have resulted from northwesterly indentation of Shaw Batholith (and contiguous rocks to the east). Westerly-directed tectonic escape of the Strelley Granite laccolith and associated supracrustal rocks from in between Shaw Batholith and North Pole Dome, was followed by ramping up the concave eastern margin of the Yule Batholith and eastward tilting during the transition from westerly to northerly displacement. Continued northward translation of the Strelley Granite at the leading edge of the system was terminated by constriction between the Carlindi Batholith and the North Pole Dome. Trailing greenstone successions were crumpled into a series of NE-trending, en-échelon folds and related faults. Kinematic evidence and structural geometry indicate that sediments of the Lalla Rookh Basin (De Grey Group) were deposited in advance of the north-moving Strelley Granite within a basin bounded by faults of opposite displacement, rather than in a pull-apart basin as previously suggested. The age of the D3 deformation is indicated by resetting of the ArAr and RbSr isotopic systems to ca. 2950 Ma in the Western Shaw region, and by UPb ages of syn-kinematic granitoid rocks.

The Mulgandinnah Shear Zone; an Archean crustal scale strike-slip zone, eastern Pilbara, Western Australia

A large part of the deformation in the Archean Pilbara granitoid-greenstone terrain is localized in relatively narrow shear zones. The Mulgandinnah shear zone (MSZ) is a major one of these, with a width up to 8 km, that can be followed for over 70 km along strike in the Shaw Batholith in the eastern Pilbara. It forms part of the Mulgandinnah Lineament, that can be traced to the Lalla Rookh Basin and the Carlindi Batholith in the north, giving it a total length of over 150 km. The MSZ contains both mylonites and ultramylonites, both of which have foliations that are subvertical to steeply dipping, with the ultramylonitic foliation overprinting the mylonitic foliation to form more localized zones of high strain. Stretching lineations are mostly subhorizontal to shallow south plunging for both the mylonites and ultramylonites, but can vary between 50°N and 50°S. Both foliations formed under upper-greenschist to amphibolite grade conditions. The MSZ displays a consistent sinistral sense of shear, as indicated by σ and δ-type porphyroclasts, SC fabrics and rotated pegmatite veins. The mylonites and ultramylonites are interpreted to have developed as part of a progressive deformation event leading to strain localization in the ultramylonites. Timing constraints indicate that the MSZ has been active at ca. 2930 Ma, but it is likely to have been a long lived structure that was reactivated at that stage, when it acted as a conjugate shear during a NW-SE crustal shortening in the eastern Pilbara. The final juxtaposition of the various domains in the Pilbara granitoid-greenstone terrain occurred during this deformation phase

Evidence for multiphase deformation in the Archean basal Warrawoona Group in the Marble Bar area, East Pilbara, Western Australia

The results of a structural analysis of the Marble Bar Greenstone Belt in the McPhee Reward area of the eastern Pilbara Block in Western Australia indicate that the area has undergone a multiphase deformation on shears which approximately parallel the trace of stratigraphy (cf. primary igneous layering) around the Talga Talga Anticline. Early extensional movements were associated with the formation of the Warrawoona Group (chiefly the Duffer Formation) and indicate a WNW-ESE extensional direction similar to the syn-Warrawoona extension found in the area of the North Shaw Batholith. Extension was followed by thrusting with transport to the east. The Talga Talga anticline is a culmination which formed during this event by thrust stacking. A major shear in the area, the Duffada Shear, was reactivated during granodiorite emplacement with transport to the NE. In their present geometry, the shears have thrust movements on the south side of the Talga Talga anticline but normal movements on the north side. After the reactivation came a NW-SE shortening which was accompanied by conjugate shear activity on north-trending sinistral and west-trending dextral shears, and constrained to ∼2950 Ma in the North Shaw area. The final movements were associated with a NS shortening which resulted in an EW trending thrust, which cuts the northern flank of the Talga Talga anticline, and NW and NE trending conjugate faults. There is no evidence of radially oriented stretching lineations or slickensides which would be expected if the granitoid bodies which make up the Mount Edgar Batholith were emplaced by a solid-state diapiric process. The net result of deformation was tectonic disruption of the stratigraphic stacking within the type ara of the Talga Talga Subgroup in the Warrawoona Group. This may offer an additional explanation for anomolously young dates of the Talga Talga Subgroup with respect to the Duffer Formation, and also indicates that the type section for the Talga Talga Subgroup is a lithotectonic sequence.

Formula omitted] laserprobe ages of metamorphic hornblendes from the Coongan Belt, Pilbara, Western Australia

40Ar 39 Ar laser step-heating analyses of metamorphic amphiboles and a whole rock sample from metavolcanic rocks of the Archaean Coongan Greenstone Belt in the south-eastern Pilbara in Western Australia provide evidence that these rocks, with no previous age constraints, belong to the ca. 3460 Ma old Warrawoona Group. The western margin of the belt is formed by an east-dipping ductile shear zone; the eastern margin is undeformed and shows a narrow contact metamorphic zone. The Coongan Belt shows a metamorphic gradient from amphibolite facies at the western margin to greenschist facies in the centre of the belt. The rocks in the centre of the belt are transected by north-south striking retrograde shear zones, one of which has been constrained at ca. 2940 Ma and may be related to the ca. 2940 Ma intrusive event in the eastern Pilbara. Foliated granodiorites from the ductile shear zone along the western margin of the belt yield dates of ca. 3170 Ma, which are interpreted as the age of recrystallisation after deformation. Deformed blue-green hornblendes from the adjacent foliated amphibolites, however, give younger and inconsistent dates of ca. 3090 Ma and ca. 3020 Ma. These dates cannot be interpreted as meaningful ages and indicate an inhomogeneous distribution of argon in the shear zone. Age spectra from blue-green hornblendes from the western part of the Coongan Belt constrain the age of a regional metamorphic event at 3240 Ma, which compares well with the metamorphic hornblende ages of ca. 3200 Ma obtained in the Western Shaw Belt by Wijbrans and McDougall. The same event is indicated by complex age spectra of partially metamorphosed mafic volcanic rocks, which yield ca. 3260 Ma maximum ages in the higher temperature steps. The age spectrum of contact metamorphic hornblendes from the eastern margin indicates that the cooling age of the undeformed granite in the south-western Corunna Downs Batholith is ca. 3400 Ma, which is 70–90 Ma younger than the North Shaw granodiorites in the Shaw Batholith, but considerably older than previously assumed, that is ca. 3300 Ma. All obtained ages fall within the expected range of ca. 3.5-2.94 Ga, and generally increase from west to east through the Coongan Belt. The effects of excess argon on the age spectra seems to be rather considerable in some of the experiments.

Extensional structures during deposition of the 3460 Ma Warrawoona Group in the eastern Pilbara Craton, Western Australia

Structures in the granite-greenstone terrain of the eastern Pilbara were mapped in both the Coongan Belt greenstones and Shaw Batholith granitoids. Important structures that are coeval with an early magmatic event, in both the granitoids and the greenstones, were identified. In the Coongan greenstones a brittle extensional fault array is syn-kinematic with felsic volcanics of the Duffer Formation, dated at 3452 ± 16 Ma (Pidgeon, 1978a). In the underlying Shaw Batholith a major ductile shear zone was mapped at the contact with the Coongan Belt. Although the foliation in the shear zone forms part of a domal structure, the stretching lineations constantly plunge to the ENE. The sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the EW-trending part and dextral movement on the NS-trending part. A wedge-shaped granitoid intruded into the shear zone during deformation. This granitoid is part of the North Shaw Suite which has been dated at 3467 ± 6 Ma (McNaughton et al., 1988). The structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels. The structural/kinematic, magmatic, metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry. In turn, this implies that the domal geometry of the Shaw Batholith was initiated, at mid crustal levels, at a very early stage (namely during deposition of the Warrawoona Group) in the evolution of the Pilbara craton. As extension occurred during intrusion and extrusion of large volumes of magma, the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry. The initial domal shape of the Shaw Batholith has been enhanced during later deformation phases, which caused steepening of NS-trending structures.

Origin of the 3500-3300 Ma calc-alkaline rocks in the Pilbara Archaean: isotopic and geochemical constraints from the Shaw Batholith

Calc-alkaline plutonic rocks, intruded at ∼3450Ma, comprise a major component of the Shaw Batholith in the Archaean east Pilbara Block, Western Australia. New whole-rock Pb isotopic geochronology confirms the extent of these rocks, but a minor plutonic phase is dated at 3338±52 Ma and represents a second plutonic event of the same age as much of the nearby Mt Edgar Batholith. The Sm-Nd isotopic systematics of the ∼3450Ma rocks imply their derivation from a heterogeneous source, which probably included a slightly older crustal component as well as a depleted mantle component. The 3338±52 Ma pluton includes components derived from crustal sources older than 3600 Ma. The geochemistry and SmNd isotopic systematics of these rocks are consistent with crustal growth in the early Archaean from upper mantle sources as depleted as the modern upper mantle. The Shaw Batholith calc-alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc-alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation. A suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals, principally hornblende, and possibly sphene, magnetite and epidote, and into a leucocratic component containing quartz, plagioclase and K-feld-par. These Archaean calc-alkaline plutonic rocks, in common with rocks from many other Archaean calc-alkaline provinces, exhibit very fractionated REE patterns with depleted HREE contents, a feature considered to result from equilibrium with garnet at depth in lower crustal regions. The geochemistry of the Pilbara Archaean calc-alkaline rocks is identical to the subset of modern continental-margin calc-alkaline plutonic rocks with fractionated REE patterns, such as those from the central and eastern Peninsular Ranges Batholith, western USA. The tectonic setting in which the Archaean calc-alkaline rocks formed is still not known. This reflects both uncertainty associated with the petrogenesis and environments of modern calc-alkaline rocks, as well as the limited knowledge of the precise timing and relationships of plutonic, depositional and tectonic events in the Pilbara Archaean.

The age and origin of younger granitic plutons of the Shaw Batholith in the Archaean Pilbara Block, Western Australia

The whole-rock Pb-Pb method has been used to date four of the younger, mainly adamellite, late-tectonic plutonic phases within the ca. 3.5 Ga Shaw Batholith of the Archaean east Pilbara Block. Three suites give ages within error of 2966 Ma (Porphyritic Granites at 2948±50 Ma, Leuco-adamellites at 2943±46 Ma and Garden Creek Adamellite at 3007±48 Ma). The post-tectonic Cooglegong Adamellite gives an age of 2847±34 Ma. The Sm-Nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca. 3200 and 3600 Ma in age. The sources for these four younger plutonic phases were heterogeneous and, although exhibiting some isotopic characteristics of the older (3.5−3.3 Ga) calc-alkali plutonic suites, were more depleted in the LIL elements Rb, U and Th. In addition, the Garden Creek Adamellite and the Cooglegong Adamellite lack the very fractionated and HREE-depleted REE patterns characteristic of both the older calc-alkali plutonic rocks and the Porphyritic Granites and Leuco-adamellites. The crust underlying the Shaw Batholith at ca. 2950 Ma must have been both markedly heterogeneous and variably depleted, a conclusion consistent with the complex tectonic and plutonic evolution of this region.

On the metamorphic history of an Archaean granitoid greenstone terrane, East Pilbara, Western Australia, using the 40Ar/ 39Ar age spectrum technique

Age spectrum analyses of blue-green hornblendes from amphibolites from the Western Shaw Belt, East Pilbara, Western Australia, indicate an age of at least 3200 Ma for early regional metamorphism. Ages on hornblende and muscovite from the narrow contact zone with the adjacent Yule Batholith probably date updoming of the granitoid gneiss terranes at 2950 Ma. Hornblendes from within the Shaw Batholith and from a contact zone of a post-tectonic granitoid yield ages of 2840–2900 Ma, indicating either prolonged high temperatures within the granitoid gneiss terranes or a separate thermal pulse associated with the intrusion of post-tectonic granitoids. The preservation of very old hornblendes in a narrow greenstone belt surrounded by massive granitoid gneiss domes indicates that remarkable contrasts in metamorphic geotherms existed over short distances during the Late Archaean, suggesting that updoming occurred during a period of rapid tectonism.

A 3500 Ma plutonic and volcanic calc-alkaline province in the Archaean East Pilbara Block

Variably foliated, predominantly granodioritic plutonic rocks from the northern part of the Shaw Batholith in the east Pilbara Archaean craton are dated at 3,499±22 Ma (2σ errors) by a whole-rock Pb-Pb isochron. These rocks intrude the surrounding greenstone sequence, and their age is indistinguishable from that sequence. High strain grey gneisses which occupy much of the western and southern Shaw Batholith are chemically and isotopically similar to the North Shaw suite and are inferred to have been derived from this suite by tectonic processes. Felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc-alkaline volcanic and plutonic province at ∼3,500 Ma. This volcanic and plutonic suite is similar to modern calc-alkaline suites on the basis of major element, rare earh element and most other trace element contents. The Archaean suite contrasts with modern equivalents only in having lower concentrations of HREE and higher concentrations of Ni and Cr.

Horizontal tectonic interaction of an Archean gneiss belt and greenstones, Pilbara block, Western Australia

The Archean Pilbara block, Western Australia, is cited as a type example of a granite-greenstone terrane. New field evidence reveals that the internal structure of one of the major batholiths, the Shaw batholith, resembles an Archean gneiss belt. Greenstone intercalations within the gneiss belt can be traced into the surrounding greenstone sequence and were incorporated during subhorizontal thrusting and recumbent folding episodes. These early tectonic episodes preceded solid-state diapiric uprise of an overthickened crust. Granitic plutonism and partial melting of the sialic crust appear to have played an important role only in the later stages of the tectonic evolution of the Shaw batholith. Deformation and metamorphism are not a direct consequence of the emplacement of granitic magmas, contrary to the widely accepted magmatic model of batholith formation.

Lead isotopes and ages of Galenas from the Pilbara region, Western Australia

Abstract One of six galena samples from the Pilbara of Western Australia, located in a ‘greenstone’ sequence, appears from its lead‐isotope ratios to be of great age. ‘Linear Model III’ ages of Cumming & Richards (1975) agree with available geological evidence for this, and for one other younger sample located in the Shaw Batholith. Two of the other samples, also from Archaean granitic host rock, appear to be significantly younger than the host; the other two, from within the Lower Proterozoic Fortescue Group, suggest that its age is not yet well known. Previously‐published age information has been adapted to the newly‐accepted values for the decay constants throughout this discussion.