Ian S. Buick

Isotopic variations in S-type granites: an inheritance from a heterogeneous source?

Inherited zircons from S-type granites provide exceptionally good insight into the isotopic heterogeneity of their sources. Zircons from four samples (one granite, two granodiorites, one granodioritic enclave) of Pan-African S-type granite of the Cape Granite Suite (c. 540xa0Ma) have been the subject of a laser LA-ICP-MS zircon U/Pb study to determine emplacement ages and inheritance. Zircons from three of these samples (2 granodiorites and 1 granodioritic enclave) were also analysed for Hf isotopes by LA-MC-ICP-MS. Ages of inherited cores range from 1,200 to 570xa0Ma and show Hafnium isotope values (εHf,t) for the crystallisation age (t) of the different cores that range from −14.1 to +9.1. Magmatic zircons and magmatic overgrowth with concordant spot ages between ca. 525 and ca. 555xa0Ma show a similar range of εHf,t, between −8.6 and +1.5, whilst εHf values calculated at 540xa0Ma (εHf,540) for inherited cores range from −15.2 to +1.7. Thus, our results show that the time evolved εHf arrays of the inherited cores overlap closely with the εHf range displayed by the magmatic rims at the time of crystallisation of the pluton. These similarities imply a genetic relationship between magmatic and inherited zircons. Within the inherited cores, four main peak ages can be identified. This, coupled with their large Hf isotopic range, emphasises that the source of the granite is highly heterogeneous. The combination of the U/Pb zircon ages ranges and Hf isotope data implies that: (1) The source of S-type granite consists of crustal material recording several regional events between 1,200 and 600xa0Ma. This material records the recycling of a much older crust derived from depleted mantle between 1.14 and 2.02xa0Ga. (2) The homogenisation of Hf isotopic variation in the magma acquired through dissolution of the entrained zircon, via mechanical mixing and/or diffusion between within the granite was particularly inefficient. (3) This evidence argues for the assembly of the pluton through many relatively small magma batches that undergo rapid cooling from their intrusion temperature (ca. 850°C) to background magma chamber temperature that is low enough to ensure that much of the magmatic zircon crystallised rapidly (>80% by 700°C). (4) There is no evidence for the addition of mantle-derived material in the genesis of S-type Cape Granite Suite, where the most mafic granodiorites are strongly peraluminous, relatively low in CaO and K2O rich. Interpreted more widely, these findings imply that S-type granites inherit their isotopic characteristic from the source. Source heterogeneity transfers to the granite magma via the genesis of discrete magma batches. The information documented from the S-type CGS zircons has been recorded because the individual batches of magma crystallised the bulk of their magmatic zircon prior to mechanical or diffusional magma homogenisation. This is favoured by zirconium saturation in the magma shortly after emplacement, by partial dissolution of the entrained zircon fraction, as well as by the intrusion of volumetrically subordinate magma batches into a relatively cool pluton. Consequently, evidence recorded within inherited cores will most likely be best preserved in S-type granite plutons intruded at shallow depths. Other studies that have documented similar εHf arrays in magmatic zircons have interpreted these to reflect mixing between crustal- and mantle-derived magmas. This study indicates that such arrays may be wholly source inherited, reflecting mixing of a range of crustal materials of different ages and original isotopic signatures.

Fingerprints of late Neoproterozoic ridge subduction in the Pan–African Damara belt, Namibia

Subduction of mid-ocean ridges is a common feature in recent convergent margins, but is rarely documented in Proterozoic to Paleozoic orogenic belts. Here we describe evidence for ridge-trench interaction in the deeply eroded late Neoproterozoic Damara orogenic belt, central Namibia. The earliest interaction is indicated by primary intrusive contacts between amphibolite facies mid-ocean ridge metabasalts and trench metasediments. U-Pb zircon ages of 550–540 Ma from syntectonic granites in the forearc indicate the timing of partial melting and mafic underplating of the prism in response to ridge subduction. The thermal peak in the Damara belt, associated widespread granitic and alkalic plutonism, and hydrothermal activity coincide with the waning stages of tectonism at 530–520 Ma and are interpreted to indicate slab window widening and slab delamination. We suggest that the proposed two-stage thermal evolution of the Damara belt, comprising latest Neoproterozoic ridge subduction and early Cambrian slab delamination, represents a fingerprint of ridge subduction in ancient orogens.

Tectonic evolution of the Reynolds-Anmatjira Ranges: a case study in terrain reworking from the Arunta Inlier, central Australia

Abstract The Reynolds-Anmatjira Range region forms part of the Arunta Inlier in central Australia and has undergone four tectonothermal cycles that span an interval of c. 1450 Ma. The first two cycles were the Stafford Tectonic Event c. 1820 Ma, and the Strangways Orogeny c. 1770–1780 Ma, both of which were associated with regional low-pressure high-temperature metamorphism up to granulite grade that was coeval with the emplacement of voluminous sheet-like granites. The subsequent Chewings Orogeny occurred at around 1590–1570 Ma and was a long-lived event that produced regional low-pressure greenschist to granulite facies metamorphism without obvious associated magmatism. During the mid-Palaeozoic Alice Springs Orogeny (400–300 Ma), the terrain was dissected by a system of sub-greenschist to mid-amphibolite facies shear zones. In the Reynolds Range, the Proterozoic events produced a single regional foliation that is axial planar to simple large-scale folds. The composite regional Proterozoic foliation increases in grade smoothly from northwest to southeast, producing a pattern of isograds that is remarkably similar to those that formed during the mid-Palaeozoic Alice Springs Orogeny. Despite this simple pattern, the isograds reflect the superimposed metamorphic effects of four unrelated tectonothermal cycles. Without geochronological and stratigraphic information, the degree of terrain reworking in the Reynolds-Anmatjira Range region could have been largely obscured by the apparent simplicity of many of the structural and metamorphic relationships.

Paleo- to Mesoarchean polymetamorphism in the Barberton Granite-Greenstone Belt, South Africa: Constraints from U-Pb monazite and Lu-Hf garnet geochronology on the tectonic processes that shaped the belt

The Barberton Granite-Greenstone Belt (BGGB) of South Africa is an exceptionally well preserved Meso-Paleoarchean metamorphic supracrustal belt, one of only a few in the world. Studies of metamorphism in the BGGB have considerable potential to advance our understanding of tectonic processes in the Archean crust. Two current hypotheses persist to explain the origin of amphibolite-facies metamorphism in the southern BGGB. The first interprets these rocks to be the consequence of accretionary tectonics, while the second proposes a “dome-and-keel” vertical tectonic process driven by sinking of greenstone layers and the doming of the underlying granitoid crust. In this study, metamorphic pressure-temperature ( P-T ) analysis has been combined with garnet Lu-Hf and monazite U-Pb geochronology to directly date the amphibolite-facies metamorphism within the Stolzburg terrane of the BGGB. A garnet-biotite-chlorite–bearing sample yields a Lu-Hf garnet age of 3233 ± 17 Ma and a garnet-staurolite-kyanite–bearing sample produces a U-Pb monazite age of 3191 ± 9 Ma, whereas an andalusite-kyanite–bearing sample produces a U-Pb monazite age of 3436 ± 18 Ma. Phase diagrams and garnet compositional modeling produce a clockwise P-T evolution, with rocks reaching peak P-T conditions of 8.5 kbar and 640 °C for the ca. 3200 Ma event and minimum peak P-T conditions of ∼4.5 kbar and 550 °C for the ca. 3435 Ma event. The duration of metamorphism for the ca. 3200 Ma event is estimated to be ∼50–20 m.y. based on differences in age between U-Pb and Lu-Hf systems and durations needed to fit models of diffusionally modified garnet chemical zoning. Similarly shaped P-T paths over the Stolzburg terrane indicate that the metamorphism occurred in response to crustal thickening due to an accretionary tectonic process. Thus, the Stolzburg terrane constitutes an orogenic core, exhumed along the Komati fault.

Successive midcrustal, high-grade metamorphic events provide insight into Mid-Archean mountain-building along the SE margin of the proto-Kaapvaal craton

In this study, we investigate the evolution of continental crust at the Paleoarchean to Mesoarchean boundary by documenting the deposition, deep burial, and partial melting of metasedimentary gneisses along the SE margin of the proto–Kaapvaal craton. Successive high-grade metamorphic events recorded by the gneisses between ca. 3.23 and 3.07 Ga not only coincided with the timing of subduction-accretion in the adjacent Barberton greenstone belt at ca. 3.23–3.22 Ga, but also with discrete pulses of potassic granitic magmatism during differentiation and consolidation of the newly assembled lithosphere. Mineral equilibria modeling demonstrates that the granulites evolved along pressure-temperature ( P - T ) paths similar to those documented for metamorphism in modern collisional orogens. High-temperature deformation at ca. 3.11–3.07 Ga during uplift of the terrane was coaxial with the main NE-SW–trending structural grain of the Barberton greenstone belt and reflects the regional NW-SE shortening and NE-SW orogen-parallel extension exhibited by the younger potassic granites. We reconcile these features with Mesoarchean terrane assembly in Barberton via two NW-dipping subduction zones. The trace of the first is represented by the main terrane boundary within the Barberton greenstone belt; the second resulted in accretion, burial, and high-temperature metamorphism along the SE margin of the Kaapvaal craton, with the granulites providing valuable insight into the mid- to lower-crustal response to what appears to have been a protracted accretionary orogenic event.

A comment on ultrahigh-temperature metamorphism from an unusual corundum + orthopyroxene intergrowth bearing Al–Mg granulite from the Southern Marginal Zone, Limpopo Complex, South Africa, by Belyanin et al.

Studies on the Southern Marginal Zone (SMZ) of the Limpopo Belt have generally concluded that this terrain was affected by a single granulite-facies metamorphic event at 2.67–2.66 Ga (Kroner et al. 2000; Kreissig et al. 2000; Zeh et al. 2005; Elington and Armstrong 2004; Stevens and Van Reenen 1992a, b; Barton and van Reenen 1992; Barton et al. 1992; Van den Berg and Huizenga 2001; Rigby et al. 2008), with peak metamorphic conditions of 7.5–9.5 kbar and 800–850 C. In their recent paper, Belyanin et al. (2012) propose an ultrahigh-temperature (UHT) metamorphic event in the SMZ, with peak metamorphic conditions in excess of 1,000 C at approximately 12 kbar. Similar conclusions were proposed by earlier work on the same rock (Belyanin et al. 2010). The proposed UHT conditions are based on Al-rich orthopyroxene and ternary feldspar thermometry using reintegrated feldspar compositions. This evidence is derived from a single outcrop containing unusual aluminous layers within metasedimentary granulites, although Tsunogae et al. (2004) used Al-in-orthopyroxene and ternary feldspar thermometry to argue for a peak metamorphic temperature of [950 C from more representative metapelitic granulites of the Bandelierkop formation. Both these studies have proposed that UHT metamorphism affected the SMZ generally and thus that the relevant PT path of all rocks traversed UHT conditions. This interpretation is problematic because it is difficult to reconcile with the partial melting history of the rocks and with the peak metamorphic assemblage in most rocks. In essence, both the metasedimentary granulites of the Bandelierkop formation and the trondhjemitic to tonalitic Bavianskloof gray gneisses contain a significant proportion of biotite, which defines a syn-peak metamorphic fabric (Fig. 1). Within the metasediments of the Bandelierkop formation, compositional banding at the outcrop scale (Fig. 1) exists that is considered to represent primary differences in composition between different sedimentary units (van Reenen 1983). Anatexis of these rocks has occurred via three reactions (Stevens and van Reenen 1992a): Ms ? Qtz ? Pl = Sil ? Melt; Bt ? Sil ? Qtz ? Pl = Grt ? Melt; and Bt ? Qtz ? Pl = Opx ? Crd ? Melt. The first two reactions exhausted muscovite and sillimanite, respectively, while the third reaction had just begun to occur in the chemically most favorable rocks at the conditions of peak metamorphism. Pseudosection modeling of metapelitic granulites from the Bandelierkop quarry, which have undergone anatexis and that represent restitic, melt-depleted compositions, indicates that biotite in such rocks would not have survived temperatures significantly above 900 C and that the mineral compositions in such rocks are consistent with peak metamorphic temperatures of 820–860 C at 8–10 kbar (Fig. 2). At the peak metamorphic conditions proposed by Belyanin et al. (2012), these restitic granulites would have become Communicated by T. L. Grove.

The Donkerhuk batholith, Namibia: A giant S-type granite emplaced in the mid crust, in a fore-arc setting

The mainly S-type Donkerhuk batholith intruded the accretionary prism in the Pan-African Damara Belt of Namibia. The batholith is elongate parallel to the regional SW–NE structural grain and the Okahandja Lineament Zone, which forms the accretionary prism backstop and the magma feeder zone. Over 5000u2005km3 of granitic magmas were emplaced as thousands of sheets, over perhaps 20u2005myr, with sheet orientations determined by the regional stress field and wall-rock anisotropies. Some of the magma source rocks are inferred to have been cordierite-bearing, suggesting an upper pressure limit of 600u2005MPa (6u2005kbar). Calculated phase relations suggest magma emplacement at 450u2005MPa (4.5u2005kbar), corroborated by pseudosection modelling of the phase assemblage in a migmatitic wall rock. The magmas were initially highly H2O-undersaturated and at around 850°C, indicating fluid-absent conditions in the source rocks. This suggests intraplating of mantle magmas to provide heat and that the granitic magmas ascended only to the mid crust. The late-tectonic emplacement in a fore-arc shares similarities with younger and similarly large batholiths such as the Cretaceous Kodiak batholith in Alaska, but the internal architecture and age structure of the batholiths differ markedly, suggesting that different processes can trigger voluminous near-trench plutonism. Supplementary material: A Microsoft Excel spreadsheet, in xlsx format, showing the analytical data relevant to this paper, is available at https://doi.org/10.6084/m9.figshare.c.3470880

Petrogenesis of the granitic Donkerhuk batholith in the Damara Belt of Namibia: protracted, syntectonic, short-range, crustal magma transfer

The areally extensive (>5000xa0km2), syn-tectonic, ca. 520xa0Ma, mainly S-type Donkerhuk batholith was constructed through injection of thousands of mainly sheet-like magma pulses over 20–25xa0Myr. It intruded schists of the Southern Zone accretionary prism in the Damara Belt of Namibia. Each magma pulse had at least partly crystallised prior to the arrival of the following batch. However, much of the batholith may have remained partially molten for long periods, close to the H2O-saturated granite solidus. The batholith shows extreme variation in chemistry, while having limited mineralogical variation, and seems to be the world’s most heterogeneous granitic mass. The Nd model ages of ~2xa0Ga suggest that Eburnean rocks of the former magmatic arc, structurally overlain by the accretionary wedge, are the most probable magma sources. Crustal melting was initiated by mantle heat flux, probably introduced by thermal diffusion rather than magma advection. The granitic magmas were transferred from source to sink, with minimal intermediate storage; the whole process having occurred in the middle crust, resulting in feeble crustal differentiation despite the huge volume of silicic magma generated. Source heterogeneity controlled variation in the magmas and neither mixing nor fractionation was prominent. However, due to the transpressional emplacement régime, local filter pressing formed highly silicic liquids, as well as felsic cumulate rocks. The case of the Donkerhuk batholith demonstrates that emplacement-level tectonics can significantly influence compositional evolution of very large syn-tectonic magma bodies.

Fluid-rock interaction in the Reynolds Range, central Australia:. superimposed, episodic, and channelled fluid flow systems

Abstract The Reynolds Range, central Australia, is a polymetamorphic Proterozoic terrain within the Arunta Inlier. The terrain comprises a diverse sequence of metasedimentary rocks (including pelites, psammites, quartzites, marls and marbles) intruded by with two generations of granites. The Reynolds Range preserves evidence of undergoing several metamorphic events, including: phases of contact metamorphism at 1.82 Ga and 1.78 Ga; regional metamorphism at 1.6 Ga that varied in grade from greenschist facies (c. 400°C) to granuite facies (750–800°C) at 400–500 GPa; and metamorphism at up to amphibolite facies (550–600°C at 500–600 MPa) in the Alice Springs Orogeny at c. 334 Ma, the affects of which are recorded mainly within shear zones. Fluid flow occurred during both contact metamorphic events, cooling from the peak of regional metamorphism at 1.57–1.58 Ga, and additionally during Alice Springs shearing. By contrast, there is little fluid-rock interaction that can be attributed to the prograde stages of regional metamorphism, implying that fluids generated by metamorphic devolatilization at that time escaped relatively rapidly and did not interact with the rocks as a whole. Fluid flow changed the mineralogy and stable isotope ratios of rocks, and locally caused extensive metasomatism. During cooling from the peak of regional metamorphism, the fluids were derived from crystallization of partial melts and reflect internal fluid recycling. However, at least some fluid flow during contact metamorphism and shearing involved external fluids. In both contact metamorphic episodes, igneous and locally surface-derived fluids interacted with the country rocks adjacent to the granite plutons. During Alice Springs shearing, large volumes of surface-derived fluids infiltrated the middle crust. Much of the fluid flow was channelled on scales ranging from hecto- to millimetres as a result of variations in intrinsic permeability caused by deformation or reaction enhancement.

Erratum to: Post-orogenic shoshonitic magmas of the Yzerfontein pluton, South Africa: the ‘smoking gun’ of mantle melting and crustal growth during Cape granite genesis?

The post-orogenic Yzerfontein pluton, in the Saldania Belt of South Africa was constructed through numerous injections of shoshonitic magmas. Most magma compositions are adequately modelled as products of fractionation, but the monzogranites and syenogranites may have a separate origin. A separate high-Mg mafic series has a less radiogenic mantle source. Fine-grained magmatic enclaves in the intermediate shoshonitic rocks are autoliths. The pluton was emplaced between 533xa0±xa03 and 537xa0±xa03xa0Ma (LA-SF-ICP-MS U–Pb zircon), essentially synchronously with many granitic magmas of the Cape Granite Suite (CGS). Yzerfontein may represent a high-level expression of the mantle heat source that initiated partial melting of the local crust and produced the CGS granitic magmas, late in the Saldanian Orogeny. However, magma mixing is not evident at emplacement level and there are no magmatic kinships with the I-type granitic rocks of the CGS. The mantle wedge is inferred to have been enriched during subduction along the active continental margin. In the late- to post-orogenic phase, the enriched mantle partially melted to produce heterogeneous magma batches, exemplified by those that formed the Yzerfontein pluton, which was further hybridised through minor assimilation of crustal materials. Like Yzerfontein, the small volumes of mafic rocks associated with many batholiths, worldwide, are probably also low-volume, high-level expressions of crustal growth through the emplacement of major amounts of mafic magma into the deep crust.