Stephen Sheppard

Isotopic dating of the migration of a low-grade metamorphic front during orogenesis

The southern margin of the Pilbara Craton in northwestern Australia underwent regional heating, folding, and thrusting as well as extensive fluid flow during collision in the Paleoproterozoic. However, the precise timing of this event is uncertain. Previous geochronologic studies of Archean basement rocks have yielded a wide range of younger radiometric dates (from ca. 2.4 Ga to ca. 2.0 Ga), interpreted to record multiple hydrothermal alteration events. In situ U-Pb geochronology of authigenic monazite and xenotime in very low grade Archean metasedimentary rocks from across the craton suggests that the Pilbara was affected by at least two cryptic thermotectonic events, the first between ca. 2430 Ma and ca. 2400 Ma, and the second between ca. 2215 Ma and ca. 2145 Ma. The older event is restricted to three localities in the west, and its cause is unknown. The younger event affected most of the craton (>100,000 km 2 ), spanning a 70 m.y. period from ca. 2215 Ma nearest the collisional margin in the south, to ca. 2145 Ma toward the craton interior in the north. The widespread geographic and stratigraphic distribution of ca. 2.2 Ga phosphates suggests that fluid flow was pervasive. This event was probably driven by the northward-advancing Ophthalmian fold-and-thrust belt that developed during protracted collision. The associated low-grade metamorphic front migrated over ∼350 km at an average rate of ∼5.0 mm/yr, placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings.

Tectonic implications of Palaeoproterozoic post-collisional, high-K felsic igneous rocks from the Kimberley region of northwestern Australia

Abstract Palaeoproterozoic high-K I-type granites, high-level porphyry intrusions, and felsic volcanic rocks of the Whitewater Volcanics dominate the Hooper and Lamboo Complexes in the Kimberley region of northwestern Australia. The granites, porphyries and volcanic rocks are gradational into each other in the field, and they have the same mineralogy, similar major and trace element abundances, and indistinguishable SHRIMP U–Pb zircon ages of 1865–1850 Ma. There is evidence of widespread mingling between the granites and coeval gabbros. Magma mixing may be important in the formation of some of the mafic granites, but most of the rocks probably formed from felsic parent magmas that underwent variable degrees of fractional crystallization. The felsic igneous rocks may have formed by partial melting of intermediate to felsic, calc-alkaline rocks along the southern and eastern margins of the Kimberley Craton, following accretion of various earlier Palaeoproterozoic terranes to the craton. Therefore, models for Palaeoproterozoic high-K granites in northern Australia that invoke intracratonic rifting of a stable Archaean craton may need to be revised.

Testing ore deposit models using in situ U-Pb geochronology of hydrothermal monazite: Paleoproterozoic gold mineralization in northern Australia

The inability to establish absolute ages for gold deposition in the Pine Creek orogen of northern Australia has led to conflicting ore deposit models, ranging from intrusion related, which predict that gold mineralization was synchronous with granite magmatism (ca. 1835–1820 Ma), to orogenic, which place ore deposition nearly 100 m.y. later. Here we present ion microprobe U-Pb geochronology for a mineralized quartz reef from Toms Gully mine, Mount Bundey, Northern Territory, Australia, and nearby granitic rocks and associated contact aureoles. Isotopic dating of zircon and monazite indicates that intrusion and contact metamorphism occurred ca. 1825 Ma, whereas hydrothermal monazite from the auriferous quartz reef gives a mean 207Pb/206Pb age of 1780 ± 10 Ma, interpreted as the time of gold mineralization. Mineralization therefore postdated intrusion by ∼45 m.y. and preceded a postulated ca. 1740–1730 Ma cratonwide orogenic gold event by ∼50 m.y. Hence, neither the intrusion-related model nor the recently proposed orogenic model is applicable. Combined with a reevaluation of age data from the nearby Goodall gold deposit, our data suggest that mineralization coincides with, and may be related to, an episode of regional low-grade metamorphism, deformation, and fluid circulation (Shoobridge event). Our results demonstrate the importance of high-precision in situ geochronology and detailed petrography for deciphering age relationships in ore deposits, and of testing the veracity of models for ore formation.

High- and low-K granites and adakites at a Palaeoproterozoic plate boundary in northwestern Australia

The Lamboo Complex is one of several Palaeoproterozoic terrains in northern Australia that is intruded by high-K, I-type granites. The oldest granites in the complex consist of the high-K 1865–1850 Ma Paperbark supersuite with initial εNd values between −2.7 and −4.0. Tonalite sheets of the c. 1850 Ma Dougalls suite have high Al2O3, Na2O, Sr, Sr/Y and La/Yb, and low Y and HREE, and initial εNd values between +1.4 and +0.7. The rocks have compositions similar to adakites that form by melting of mafic rocks in the deep crust with garnet in the residue. Granites of the 1835–1805 Ma Sally Downs supersuite range from high-K types through to crustally derived adakites, all with initial εNd values between +0.1 and −1.2. The two supersuites and the Dougalls suite were intruded into different tectonic settings. Previously all of the granites in the Lamboo Complex were interpreted as partial melts of a chemically and isotopically uniform underplate beneath a rifted Archaean craton. However, their spatial and temporal distribution, and their varied compositions, are best explained by large-scale horizontal plate motions and subduction of oceanic crust.

The relationship between tectonism and composition of granitoid magmas, Yarlarweelor Gneiss Complex, Western Australia

Abstract The northwestern edge of the Archaean Yilgarn Craton in Western Australia was intensely deformed and metamorphosed during the Palaeoproterozoic Capricorn Orogeny (D 1n and D 2n ) to form the Yarlarweelor Gneiss Complex. High-grade metamorphism and crustal thickening in the complex at ca. 1812 Ma during D 1n was accompanied by voluminous veins and sheets of I-type granite and pegmatite. Most of these rocks are leucocratic, and are characterised by low Rb, FeO T +MgO, Y, Zr, Th, U and total rare earth elements (∑REE), and high SiO 2 , Ba, K/Rb and Ba/Rb, and chemically resemble leucosomes from stromatic migmatites (metatexites). The granite and pegmatite formed during compression and represent low to moderate degree melts of late Archaean and Palaeoproterozoic granitic rocks. At ca. 1800 Ma, dextral strike-slip faulting during D 2n followed exhumation of the Yarlarweelor Gneiss Complex, and was accompanied by intrusion of dykes and steeply dipping sheets of I-type granites. These granites have high K 2 O/Na 2 O ratios, high Rb, Zr, ∑LREE, Y, Th and U contents, and low K/Rb ratios similar to high-K granite melts and diatexite migmatites derived by large-scale biotite dehydration melting of igneous rocks. The source of the syn-D 2n granites was Palaeoproterozoic meta-igneous rocks. The change in granite chemistry between ca. 1812 and ca. 1800 Ma reflects a change in the nature of melting and melt extraction, which were in turn controlled by the style of deformation.

Syntectonic granite in the southern margin of the Palaeoproterozoic Capricorn Orogen, Western Australia

Along the southern margin of the Capricorn Orogen the Narryer Terrane of the Archaean Yilgarn Craton was pervasively intruded by granite during ductile deformation and two stages of meta‐morphism at 1820–1800 Ma. Medium‐ to high‐grade metamorphism was closely followed by greenschist‐facies metamorphism, indicating a period of uplift which was coincident with dextral strike‐slip faulting.

Geochronological constraints on tectonic models for Australian Palaeoproterozoic high-K granites

High-K granites extensively intruded all the Palaeoproterozoic terrains of northern Australia, and have been interpreted as the culmination of widespread intracratonic orogeny. One of these terrains, the Lamboo Complex, developed between the Kimberley and North Australian Cratons. The complex is divided into three terranes, referred to as the Western, Central, and Eastern zones, and was intruded by a variety of I-type granites and gabbros. High-K granites of the 1865–1850 Ma Paperbark supersuite are restricted to the Western zone. Tonalite sheets of the Dougalls suite, and unassigned leucogranite sheets, intruded the Central zone at c. 1850 Ma. The 1835–1805 Ma Sally Downs supersuite is composed of tonalite and high-K granite that mainly intruded the Central zone. However, high-K granites of the Sally Downs supersuite intruded all three zones from c. 1820 Ma onwards, and mark the amalgamation of the terranes. The youngest granites in the complex are high-K granites of the 1805–1790 Ma San Sou suite, which intrude the Eastern and Central zones. The temporal and spatial distribution of the granites is not easily reconciled with previous intracratonic models, but is consistent with tectonic processes similar to those operating at present.

U–Pb dating of metamorphic monazite establishes a Pan-African age for tectonism in the Nallamalai Fold Belt, India

The Nallamalai Fold Belt comprises late Palaeoproterozoic to Mesoproterozoic sedimentary rocks deformed into a fold-and-thrust belt along the eastern side of Peninsular India. The age of thin-skinned thrusting, folding and low- to medium-grade metamorphism in the belt is unclear, with estimates ranging from Palaeoproterozoic to early Palaeozoic. A possible Pan-African age for thrusting has previously been inferred from Rb–Sr dating of muscovite in shear zones from the adjacent Krishna Province (501 – 474 Ma) but these structures are separated from the Nallamalai Fold Belt by a major thrust. Here, we present in situ U–Pb dating of metamorphic monazite within a low-grade metasedimentary rock in the Nallamalai Fold Belt at the Mangampeta barite mine. Our date of 531 ± 7 Ma for the monazite is the first direct evidence that west- to NW-directed nappe stacking, folding and low-grade metamorphism in the fold belt are related to Pan-African incorporation of India into the Gondwana supercontinent.

Role of geochronology in our present-day understanding of the Proterozoic: an Australian perspective

Geochronology has proven pivotal to our present-day understanding of the dynamic processes that have shaped the Earth. The proliferation of high-precision geochronology worldwide in the last 15 years, in particular that generated by sensitive high-resolution ion microprobe (SHRIMP), has revolutionised the geological communitys view of the Proterozoic Eon, and the implicit assumptions that form the basis of most work conducted on rocks of that age. Although most of this dating has concentrated on U–Pb zircon ages in felsic rocks, an impressive record of Proterozoic crust-forming events and high-grade tectonic reworking has been identified. In some instances, the dating has also defined the duration of orogenic activity. Our present-day understanding is that: (i) many Proterozoic orogens mark ancient collision zones, and as a corollary, plate tectonics has operated throughout the Proterozoic; (ii) Proterozoic orogens were commonly the sites of repeated reworking or reactivation; (iii) the Proterozoic saw the assembly and dispersal of at least two supercontinents; (iv) the Proterozoic was punctuated by the activity of numerous large igneous provinces; and (v) Proterozoic basin formation and evolution were driven largely by plate-tectonic processes. Although there are large areas yet to be dated, there is no reason to suggest that the processes identified there will be any different. The primary question has evolved from ‘Did plate tectonics operate?’ to ‘What form did plate tectonics take?’ One of our major challenges is to extend the geochronological framework of the Proterozoic by routinely dating other uranium-bearing accessory phases, following the successful application of zircon geochronology.