Basement Terranes Research Articles

Crustal Structure of the Hikurangi Subduction Zone Revealed by Four Decades of Onshore‐Offshore Seismic Data: Implications for the Dimensions and Slip Behavior of the Seismogenic Zone

AbstractFour decades of seismic reflection, onshore‐offshore and ocean‐bottom seismic data are integrated to constrain a high‐resolution 3‐D P‐wave velocity model of the Hikurangi subduction zone. Our model shows wavespeeds in the offshore forearc to be 0.5–1 km/s higher in south Hikurangi than in the central and northern segments (VP ≤ 4.5 km/s). Correlation with onshore geology and seismic reflection data sets suggest wavespeed variability in the overthrusting plate reflects the spatial distribution of Late Jurassic basement terranes. The crustal backstop is 25–35 km from the deformation front in south Hikurangi, but this distance abruptly increases to ∼105 km near Cape Turnagain. This change in backstop position coincides with the southern extent of shallow slow‐slip, most of which occurs updip of the backstop along the central and northern margin. These relationships suggest the crustal backstop may impact the down‐dip extent of shallow conditional stability on the megathrust and imply a high likelihood of near/trench‐breaching rupture in south Hikurangi. North of Cape Turnagain, the more landward position of the backstop, in conjunction with a possible reduction in the depth of the brittle ductile transition, reduces the down‐dip width of frictional locking between the southern (∼100 km) and central Hikurangi margin by up‐to 50%. Abrupt transitions in overthrusting plate structure are resolved near Cook Strait, Gisborne and across the northern Raukumara Peninsula, and appear related to tectonic inheritance and the evolution of the Hikurangi margin. Extremely low forearc wavespeeds resolved north of Gisborne played a key role in producing long durations of long‐period earthquake ground motions.

Multi-stage tectonic record of the Central Sakarya Terrane Basement (NW Turkey): Implications for the Paleozoic evolution of Rheic Ocean and northern Gondwanan margin

In northwestern Turkey, within the Central Sakarya Terrane, the Söğüt Metamorphics is one of the Variscan tectonic units preserved as a pre-Jurassic basement assemblage of the Sakarya Composite Terrane (SCT). It consists of para- and orthogneisses that host bands, lenses, boudins, and tectonic slices of amphibolites, and are altogether intruded by the Sarıcakaya Granitoid. The characterization and timing of the precursor magmatic events for the protoliths of this assemblage and the timing and conditions of metamorphism have been the topics of a long-lasting debate. Here we present new geochemical and geochronological data from the Söğüt orthogneisses, amphibolites, and the crosscutting Sarıcakaya Granitoid to provide further understanding of the geodynamic evolution of the SCT Basement and the larger-scale tectonic events that controlled its formation. Available geological and geochronological data indicate that the generation of the orthogneiss protoliths began in the Late Cambrian (ca. 485 Ma) and continued until the middle Silurian (ca. 430 Ma), accompanied by the subduction of the Iapetus Ocean and its progressive roll-back beneath northern Gondwana. The slab roll-back caused mantle-upwelling and Ordovician (ca. 465 Ma) continental rift-related mafic magmatism, eventually leading to the opening of the Rheic Ocean. Prior to the Variscan orogeny, a short-lived episode of middle Carboniferous (ca. 336−328 Ma) intra-oceanic arc magmatism occurred within the Rheic Ocean north of the SCT. Accretion of all this material and continuous subduction beneath the SCT led to a two-stage metamorphism during the late Carboniferous (ca. 326−324 Ma and ca. 318 Ma) and synchronous generation of granitic (ca. 327−324) and dioritic (ca. 319−317 Ma) rocks of the Sarıcakaya Granitoid during the Variscan orogeny, terminating the life cycle of the Rheic Ocean.

On the Billefjorden fault zone in Garmdalen, central Spitsbergen: implications for the mapping of major fault zones during geological fieldwork and for the tectonic history of Svalbard.

The present contribution reexamines the geometry of a segment of a presumably long-lived fault in Svalbard, the Balliolbreen Fault segment of the Billefjorden Fault Zone, along which presumably two basement terranes of Svalbard accreted in the early-mid Paleozoic after thousands of kilometers strike-slip displacement. We performed structural fieldwork to Billefjorden in central Spitsbergen and interpreted satellite images. Field observations demonstrate that the Balliolbreen Fault formed as a top-west thrust fault in the early Cenozoic and that weak sedimentary units such as shales of the Lower Devonian Wood Bay Formation and coals of the uppermost Devonian-Mississippian Billefjorden Group partitioned deformation, resulting in significant contrast in deformation intensity between stratigraphic units. For example, tight early Cenozoic folds are localized in shales of the Wood Bay Formation and contemporaneous top-west brittle-ductile thrusts within coals of the Billefjorden Group, whereas Pennsylvanian deposits of the Hultberget (and/or Ebbadalen?) Formation are simply folded into gentle open folds. Rheological contrasts also resulted in the development of décollements locally, e.g., between tightly folded strata of the Wood Bay Formation and Billefjorden Group and flat-lying, brecciated limestone-dominated strata of the Wordiekammen Formation. Despite the limited quality and continuity of outcrops in the area, the eastward-thickening character (i.e., away from the fault) of Pennsylvanian deposits of the Hultberget, Ebbadalen, and Minkinfjellet formations suggests that the fault did not act as a normal fault in Pennsylvanian times. The study suggests that strain partitioning of early Cenozoic Eurekan contraction alone may explain the deformation patterns in Paleozoic rock units in central Spitsbergen, i.e., that Late Devonian Svalbardian contraction is not required, and that a major segment of the Billefjorden Fault Zone formed in the early Cenozoic. The present work illustrates the crucial need for interdisciplinary approaches and composite educational backgrounds in science.

The influence of the strength of pre-existing weak zones on rift geometry and strain localization

Continental rifts normally initiate within previously deformed lithosphere and thus their evolution and architecture can be largely controlled by inherited weak zones in the pre-rift crust. Here, we quantify the role of the strength and obliquity of pre-existing crustal-scale weak zones in the evolution of continental rift systems. We use a 3D numerical geodynamic model to assess strain localization, associated fault development, and rift segmentation during the early stages of tectonic extension. We find that both the strength and obliquity of the weak zones significantly influence the patterns of strain localization. A pre-existing very weak zone with low obliquity can promote the development of continuous and long-lived border faults parallel to the rift axis. Conversely, a comparatively strong weak zone with high obliquity leads to a staggered en-echelon rift geometry that lacks rectilinear laterally persistent strain localization. Furthermore, we find that rift obliquity and weak zone strength may modulate rift fault length, throw, and azimuth. These results provide new and compelling insights into the structure and evolution of natural active rifts that develop within orogenic basement terranes.

Satellite imagery reveals the nature of the Archaean granite-greenstone basement of the Kruger National Park, South Africa, including the newly discovered Olifants River Greenstone Belt

Abstract Archaean greenstone remnants comprising mainly metamorphosed basaltic lavas and lesser metasedimentary interlayers are components of the granite-gneiss-migmatite basement terrane of the central and northern parts of the Kruger National Park (KNP), South Africa’s premier wildlife nature reserve. Most of the greenstone remnants represent extensions into the Kruger Park of better developed and exposed greenstone belts that exist beyond the borders to the west, including the Giyani and Murchison greenstone belts. Geological mapping in the national park presents challenges as exposures are poor and discontinuous, with the dense bush and wildlife adding to the difficulties. Satellite remote sensing provided an ideal solution and in this study Google Earth imagery proved to be especially helpful. The imagery provides a synoptic overview of the terrane making it possible to establish the outline of greenstone remnants irrespective of the dense bush cover. Google Earth imagery furthermore provided a preliminary impression of a hitherto unknown, yet sizeable, greenstone belt remnant in a remote area south of the Olifants River in the central section of the KNP. Referred to as the Olifants River Greenstone Belt in this study the remnant occurs as a major Type 2 interference fold resembling a mushroom-shaped diapir with a broad bulbous core fringed by laterally flattened skirts. Images are presented that illustrate various components of this imposing structure. Google Earth images furthermore display the general character of the Archaean granite-greenstone basement and the Palaeo- and Mesoproterozoic intrusive dyke swarms on the northeastern flank of the Kaapvaal Craton.

Australian critical metal exploration for analogues of Chinese ionic-clay REE deposits

The clean energy transition has focused attention on the critical metals required for manufacture of new energy technologies. The extremely heterogeneous distribution of critical metal mineral deposits requires that countries must make new discoveries of key critical metals to avoid potential future geopolitical risks. Although Australia has REE resources, they are mainly of LREEs, so deposit styles with significant HREEs are key targets. The most obvious are the so-called ionic-clay REE deposits of southern China that are the major global suppliers of HREEs. Mineral exploration in Australia using the Chinese model is producing hitherto unrecognized REE concentrations, here termed regolith-hosted REE mineralization, in a variety of regolith types in several weathering environments, particularly in Western and South Australia. Amongst these has been the discovery of near-surface, regolith-hosted REE mineralization in the Albany-Fraser Orogen which has the potential to complement REE production from the giant Mount Weld carbonatite and other monazite-sand deposits in Western Australia. Widespread near-surface, regolith-hosted REE mineralization is present over an area of 12,000 km2 in the Esperance District. As much of this area is beneath barren Eocene-aged cover clays, all the new REE discoveries have been made, and are being delineated using shallow drilling, to depths of less than 80 m. The exploration implication is that the search space for regolith hosted REE deposits in Western Australia remains immature. These recently discovered deposits have both similarities and contrasts with geological features of ionic clay hosted REE deposits in China. Similarities include their apparent paragenesis and geometry, particularly their blanket morphology, sub-horizontal and gentle dips, and the common, but not exclusive, relationship with felsic basement terranes. However, Western Australian prospects are related to Upper Cretaceous to Eocene climates rather than more recent weathering under sub-tropical conditions. Whether the deposits can be developed to production remains unclear, with project economics tied closely to the efficacy of mineral processing technologies that target high REE extraction rates and recoveries.

Early Paleozoic accretionary history of the Pearya terrane: New insights from igneous and detrital zircon signatures of the Kulutingwak Formation, Ellesmere Island, Nunavut, Canada

AbstractThe juxtaposition of the composite Pearya terrane and the northern Laurentian margin at Ellesmere Island, Nunavut, Canada, has significant ramifications for the Paleozoic tectonic history of the circum-Arctic region. Published tectonic models rely upon interpretation of the subduction-related Kulutingwak Formation as an indicator of Ordovician and/or Silurian accretion (Trettin, 1998). New igneous and detrital zircon U-Pb and Lu-Hf isotopic data from 16 samples collected in the Yelverton Inlet–Kulutingwak Fiord region of northern Ellesmere Island suggest that the Kulutingwak Formation of Trettin (1998) contains structural blocks derived from both the Pearya terrane and Silurian strata associated with the ancestral Laurentian margin. Data from this study demonstrate a complex provenance history for rocks within the Petersen Bay, Kulutingwak Fiord, and Emma Fiord fault zones, with age probability peaks of ca. 470 Ma, 650 Ma, and 960–980 Ma that suggest affinity with the Pearya terrane, and age probability peaks of ca. 1800 Ma and 2700 Ma that indicate connections to the Laurentian margin. The combination of these signatures in Kulutingwak Formation rocks suggests that the Pearya terrane was proximal to the northern Laurentian margin by Late Ordovician time. Silurian and younger strike-slip displacement on the major fault zones resulted in the incorporation of blocks derived from the Pearya terrane basement and Silurian clastic rocks into the Kulutingwak Formation. Silurian displacement along these strike-slip faults, which are integral components of the Canadian Arctic transform system, is recorded by syndepositional deformation structures in the Danish River Formation and prevented the transition from soft to hard collision of the Pearya terrane. The two-stage model for the Pearya terrane—accretion followed by significant translation—provides a process for developing complex steep terrane boundaries with contentious displacement histories that are common in accretionary orogens.

Tectonic and Crustal Processes Drive Multi-Million Year Arc Magma Evolution Leading up to Porphyry Copper Deposit Formation in Central Chile

Abstract Subduction zone magmatism is a major control of volcanism, the generation of modern continental crust and the formation of economically important porphyry Cu–(Mo–Au) deposits. Reading the magmatic record of individual arc segments and constraining the rates of magmatic changes are critical in order to fully understand and quantify the processes that drive magma evolution in subduction settings during arc growth. This study focuses on the San Francisco Batholith and the Rio Blanco-Los Bronces porphyry deposit cluster in central Chile, which provides an igneous rock record over ~13.5 Myr of arc evolution. We use whole-rock geochemistry, zircon geochronology and Hf isotope geochemistry to track changes in the crustal magmatic system of this arc segment during crustal thickening and porphyry Cu deposit formation. By combining the analytical dataset with Monte Carlo fractional crystallisation and assimilation fractional crystallisation modelling, we test a model for significant crustal involvement during magma evolution. Systematic and continuous increases in Dy/Yb, La/Yb, V/Sc and Sr/Y in the magmas over time indicate a transition in the main fractionation assemblage from plagioclase-dominated to amphibole-dominated that reflects deeper crystallisation and/or a higher meltwater content. Concomitant decreases in εHf and Th/La as well as increasing Ba/Th are best explained by assimilation of progressively deeper crustal lithologies from low (Chilenia) to high Ba/Th (Cuyania) basement terranes. Our study highlights that an increasingly hydrous magma and a deepening locus of crustal magma differentiation and assimilation, driven by crustal thickening contemporaneous with increased tectonic convergence and ingression of the aseismic Juan Fernandez ridge, can account for all investigated aspects of the multi-Myr magmatic evolution leading up to the formation of the Rio Blanco-Los Bronces porphyry Cu deposits. Our findings corroborate the importance of high-pressure differentiation of hydrous magma for the formation of Andean-style porphyry deposits. Once magmas favourable for porphyry Cu mineralisation were generated in the lower crust, multiple episodes of efficient magma migration into the upper crust fed several, discrete, shallow magmatic-hydrothermal systems over ~3.5 Myr to form the world’s largest known Cu resource at Rio Blanco-Los Bronces.

Quantitative palaeogeographical reconstruction of the North China Block during the Carboniferous and Permian transition: Implications for coal accumulation and source rock development

The Carboniferous-Permian strata of the North China Block (NCB) contain significant economic reserves of coal and gas. In this paper, a comprehensive dataset comprising 401 stratigraphic sections encompassing the Carboniferous-Permian transition in the NCB is compiled from the OneStratigraphy and Geobiodiversity databases. We conducted quantitative analyses of this dataset using ArcGIS and GPlates software, and utilized an up to date plate motion model, to perform a dynamic palaeogeographical reconstruction. The spatio-temporal distribution of depositional facies were also analysed, including basement terranes, fluvio-lacustrine settings, swamps, shorelines, tidal flats, and carbonate platforms. Our reconstructions reveal that the NCB was flanked by basement areas of the Yinshan-Yanshan landmass in the north and the Qinling-Dabie landmass in the south. Peat (coal) deposition occurred across a range of terrestrial to coastal environments, including fluvio-lacustrine settings, deltas, shorelines and tidal flats. Carbonaceous and organic-rich clastic rocks also developed in these areas while organic-rich carbonate rocks were primarily deposited in carbonate platform. The conditions conducive to peat (coal) formation were intricately connected to eustatic fluctuations, which resulted in frequent transgressions and regressions. The sea-level fluctuations during the Pennsylvanian might be, in part, a response to both the waxing and waning of continental ice sheets and tectonic process in the northern margin of the NCB. In contrast, sea-level changes during the early Permian were probably controlled by the tectonic process only. This paper provides a state-of-the-art review of palaeogeographical evolution of the NCB during the Carboniferous and Permian transition, in response to eustatic and palaeoclimate change, and sheds light on resource accumulation based on quantitative analyses.

Reconstructing the thermal history of the Morondava Basin, Madagascar, after Gondwanan breakup as resolved through detrital zircon (U–Th)/He and U–Pb geochronology

Formation of the Phanerozoic basins of Madagascar coincided with the initial stages of Permian break-up of Gondwana. The largest of these, the Morondava Basin, records the tectonic history of rifting, drifting, and uplift of the Malagasy terrane over the last 300 Myr. Herein, we have applied detrital zircon U–Pb geochronology and (U–Th)/He low-temperature thermochronology to resolve the thermal history of the basin as Madagascar separated from Africa and subsequently India. Coarse-grained siliciclastic samples were taken along two transects parallel to the Morondava River in the central basin, including from the Permo-Triassic failed rift basin and from post-Jurassic passive margin sequences. Detrital zircon U–Pb age distributions are defined by common Neoarchean and Neoproterozoic populations which suggests input from the basement terranes of the Malagasy Central Highlands (Antananarivo domain). Samples also contain Paleo-to Mesoarchean and Cambrian populations likely associated with metamorphic terranes amalgamated during the Pan-African orogeny. Single crystal zircon (U–Th)/He dates are 503 ± 17 Ma to 40 ± 1 Ma with effective uranium (eU) values ranging from 35 to 1760 ppm, which exhibit a strong negative date-eU relationship that indicates partial resetting of the He systematics in zircon throughout the basin. Inverse and forward thermal history models from the data reveal separate thermal histories for strata on either side of basin-dividing Illovo Fault. Under an elevated geothermal gradient, temperatures did not exceed 180 °C either during post-Gondwanan burial of Jurassic rift strata or during Late Cretaceous burial of Early Cretaceous passive margin strata. Intrusion of Turonian-age dikes and sills related to the Marion hot spot likely promoted subtle yet widespread heating. The time of latest cooling during the Paleogene recorded in the basin is poorly constrained by the detrital zircon thermochronology.

Capitana mine, Tignamar district, a unique tin – bismuth concentration in the Chilean Central Andes, 250 km west of the Bolivian Tin Belt: Link to the Proterozoic-Paleozoic basement Belen inlier and the Arequipa-Antofalla terrane, a probable remnant of Laurentia

The Capitana mine, Tignamar District, is a unique Sn-Bi concentration in the highlands of Arica, in the Chilean Central Andes. Tin deposits associated with Triassic to Pliocene age igneous rocks are plentiful in the Tin Belt of Peru, Bolivia, and NW Argentina, but have not been reported in Chile, where contemporaneous rocks formed metallogenic belts rich in Fe, Cu, Mo, Ag and Au. We describe vein ore from Capitana mine, a small but exceptional Sn-bearing Pb-Bi-Ag-Cu-Sb-As epithermal deposit in the Tignamar (or Ticnamar) district in the high Andes of Arica, Chile, ca. 250 km west of the Bolivian Tin Belt. Fluid inclusions in vein quartz trapped a non-boiling, low salinity (<∼4 wt% NaCl equiv.), moderate T fluid (minimum T ∼ 220 °C) prior to metal precipitation. Flashing of this fluid, likely due to rapid changes in confining P, was synchronous with metal deposition; sulfosalt textures show evidence of syn-deformational growth. Although located within a district considered a porphyry copper system of Early Miocene age, it is likely that the Capitana deposit formed later, mainly as a result of regional hydrothermal activity related to compressive and transpressional tectonics as young as Pliocene. Common lead is 206Pb/204Pb 18.18; 207Pb/204Pb 15.61; and 208Pb/204Pb 38.53, significantly less radiogenic than that of Bolivian Sn deposits, and has a Paleozoic model lead age. These values point to affinities with a tectonically-emplaced inlier of crystalline basement of Proterozoic – Early Paleozoic age known as the Belen Metamorphic Complex, and with the Arequipa-Antofalla basement terrane, a probable remnant of Laurentia (North America), left behind during the dismemberment of the Rodinia Supercontinent. The complex mineralogy of Capitana vein, unlike other Chilean deposits, includes various sulphides and sulphosalts with Cu, Fe, Zn, Pb, Bi, Sn, Ag, As, Sb, Ga and In, and is exceptionally rich in U.

TWO STAGES OF MINERALIZATION IN THE SOUTHERN TIEN SHAN Au-W METALLOGENIC BELT: ISOTOPIC U-Pb DATING (LA-ICP-MS METHOD) OF ZIRCON FROM INTRUSIVE ROCKS AT THE JILAU W-Au DEPOSIT (TAJIKISTAN)

The paper presents new isotopic U–Pb zircon data (LA–ICP–MS method) on the intrusive rocks from the Chinorsai massif, which is spatially and probably genetically related to the large Jilau tungsten-gold deposit. This deposit, together with the other large Au deposits (Muruntau, Zarmitan, Kumtor, etc.) is part of the largest Au (Au–W) metallogenic belt of Tien Shan. The deposit is represented by small zones of scheelite-bearing skarn and later (overprinting) large stockwork of veins and veinlets with scheelite-gold mineralization that occurs within and near the intrusion. The concordant isotopic zircon U-Pb data (301.0 ± 2.6 Ma; MSWD = 2.6) are older than the isotopic zircon U–Pb data previously reported (about 288 Ma). This indicates a substantial extent of the magma emplacement and crystallization process that probably corresponded to several intrusive events or phases, which is a prerequisite for the formation of associated intrusive-related Au deposits. Also, the isotopic age determined highlights the earlier formation of gold-polymetallic-tungsten deposits as compared to Mo–W and Sn-W deposits in the region, which are related to the later Early Permian intrusions. In addition, zircon xenocrysts with much older age (from ca. 970 Ma to ca. 2200 Ma) have been identified; they probably represent the age of the orogenic basement including that of the basement terranes of the Tarim and Karakum cratons.

A tectonic reconstruction model for Aotearoa-New Zealand from the mid-Late Cretaceous to the present day

ABSTRACT We present a mid-Late Cretaceous to present day tectonic reconstruction model for Aotearoa-New Zealand. Our GPlates model comprises 50 rigid crustal blocks grouped into regions with common deformation histories set within a well-defined Australia-Pacific-Antarctica plate circuit tied to a published global paleomagnetic absolute reference frame. Within the model, four distinct periods of deformation are recognised from both near- and far-field observations. A key model assumption is the continuity of basement terranes between North and South Zealandia prior to Middle Eocene rifting and Late Oligocene initiation of transform motion. To complement the rigid crustal block model, continuously closing polygons show a ∼25% decrease in plate boundary area since the Middle Eocene that has been compensated for by corresponding increases in crustal thickness. A kinematic fault-propagation fold model demonstrates the plausibility of post-Late Oligocene asymmetric oroclinal folding on both sides of the evolving transform boundary. ‘Missing’ areas of map section common to previous tectonic reconstructions can be reconciled through contraction, elongation and vertical-axis rotation of continental crust within the deformation zone ahead of northward propagation of the Alpine Fault. This tectonic reconstruction model provides an open, accessible, and testable foundation for current and future paleogeographic and tectonic studies across Zealandia.

Eclogites and basement terrane tectonics in the northern arm of the Grenville orogen, NW Scotland

The presence of eclogites within continental crust is a key indicator of collisional orogenesis. Eclogites within the Eastern Glenelg basement inlier of the Northern Highland Terrane (NHT) have been re-dated in order to provide more accurate constraints on the timing of collision within the northern arm of the Grenville Orogen. The eclogites yield dates of ca. 1200 Ma which are interpreted to record the onset of continent–continent interaction, and the NHT as a whole is thought to represent the lower plate in successive 1200–1000 Ma collision events. The Eastern Glenelg basement inlier is viewed as a fragment of the leading edge of the NHT continental basement that was partially subducted along a suture and then exhumed back up the subduction channel. Differences in ages of igneous protoliths and intrusive histories, and metamorphic events (this paper) between the NHT basement and the Laurentian foreland, suggests that they were separate crustal blocks until after ca. 1600 Ma. We therefore suggest that: (1) the NHT represents a fragment of Archean–Paleoproterozoic crust that was reworked within the ca. 1.7–1.6 Ga Labradorian-Gothian belt, although whether it was derived from Laurentia or Baltica is uncertain, and (2) amalgamation of the NHT with the Laurentian foreland did not occur until the terminal stages of the Grenville collision at ca. 1000 Ma.

Crustal basement terranes under the Taupō Volcanic Zone, New Zealand: Context for hydrothermal and magmatic processes

Continental magmatic arcs commonly have heterogeneous and anisotropic metamorphic basement foundations hosting their plutonic roots and associated geothermal systems. Quaternary volcanic and volcaniclastic rocks of New Zealand's Taupō Volcanic Zone (TVZ) lie above and between the Torlesse and Waipapa Composite Terranes, two of New Zealand's major basement geological units. Deep geothermal and magma systems beneath TVZ interact with and/or are hosted in these Mesozoic metasedimentary crustal basement terranes rather than in Quaternary volcanic and volcaniclastic fill. New mineralogical, whole rock geochemical, O, Sr, Nd and Pb isotopic, and U-Pb zircon age data from lava-hosted xenoliths and ignimbrite-hosted lithics imply that the terrane boundary is not sub-vertical as previously thought, but instead dips gently west. Most of the non-igneous crust under the TVZ is thus composed of Torlesse Composite Terrane metasedimentary rocks, which have higher quartz, mica and SiO2 abundances, higher 87Sr/86Sr, 206Pb/204Pb and δ18O ratios, lower chlorite and MgO contents and lower 143Nd/144Nd ratios than their Waipapa Composite Terrane counterparts. This new interpretation provides important new constraints to refocus geodynamic understanding, petrological modelling, and future deep geothermal drilling.

Transcrustal and source processes affecting the chemical characteristics of magmas in a hyperactive volcanic zone

Arc magma generation and evolution are affected by transcrustal processes, such as assimilation and fractional crystallisation, and source processes like slab influx into the mantle wedge. However, the relative contribution of these mechanisms remains contentious. The Pleistocene to Holocene Taupo Volcanic Zone (TVZ) is unusually dominated by felsic volcanism with more than 95% of the total eruptive volume corresponding to rhyolitic magmas. Despite the relatively thin crustal basement (∼16–30 km), combined wall rock assimilation and fractional crystallization (AFC) of primary basalt has previously been presented as a necessary means to generate the chemical characteristics of these magmas. This petrogenetic model is here assessed using a new, internally consistent set of major and trace element concentrations, and Sr-Pb isotope data from a suite of basaltic to rhyolitic samples that are representative of the entire TVZ with respect to geographical distribution and age. The dataset was modelled for AFC using the energy- and mass-constrained Magma Chamber Simulator (MCS) and the local Permian to Early Jurassic Torlesse composite terrane basement used as the principal assimilant. Regardless of the intensive parameters employed, the energy- and mass-constrained models presented here fail to realistically reproduce the combined major oxide, trace element, and isotope systematics of the sample set. The Pb isotope ratios of TVZ basalts to rhyolites reveal a broadly linear trend ranging from an unradiogenic (206Pb/204Pb: 18.77) to a more radiogenic crustal endmember (206Pb/204Pb: 18.84). This trend can be reproduced by a composite mélange dominated by the Torlesse terrane (representative of tectonically eroded forearc crust), with some contribution of subducting sediments (GLOSS-II), interacting with the sub-arc mantle to generate the source of the magmas observed in the TVZ. This model is also consistent with Sr-Pb isotope systematics. At New Zealand’s active margin, tectonic erosion has been reported based on geophysical data. Therefore, it is proposed that slab input can have a stronger relevance in the magma generation of the TVZ than previously thought. The broad compositional variations in the TVZ may result from source contamination by a subduction mélange, with differentiation processes within the overriding crust being subordinate.

Zircon xenocrysts obscured the zircon date for the lower Koras Group, southern Africa

Abstract The Koras Group is a bimodal volcanosedimentary group located in post-tectonic grabens in a foreland thrust complex in the Kaaien Terrane of the Mesoproterozoic Namaqua-Natal Province of southern Africa. It contains two sequences of mafic and felsic volcanic rocks with an unconformity between them, only the lower sequence being slightly folded. The Koras Group was long regarded as having formed at the end of the 1 210 to 1 000 Ma Namaqua Orogeny, because it lacks the severe deformation and metamorphism of the underlying rocks, with igneous minerals preserved in many samples. Following years of unsuccessful attempts to precisely date the volcanic rocks, the first two ion probe U-Pb zircon studies both reported ages of ~1 172 Ma for the Swartkopsleegte Formation felsic lava in the slightly folded lower sequence (based on relatively few dated zircons) and ~1 100 Ma for the Leeuwdraai Formation rhyolite in the undeformed upper sequence. Thus a major 70 m.y. hiatus seemed apparent between the lower and upper sequences despite their similar geochemistry and rift-related setting. This gave rise to models which envisaged the Kaaien Terrane being unaffected by the syn- to late-tectonic deformation, migmatisation and granite intrusions, documented between 1 200 and 1 150 Ma in the adjoining Namaqua-Natal terranes to the west. A high-pressure (10 kbar) metamorphic event, recognised in the Kaaien Terrane basement just south of hardly-deformed Koras Group exposures and dated at 1 150 Ma, is inconsistent with such models. A re-investigation and microbeam dating campaign on the Koras Group confirms the 1 101 ± 2 Ma (n = 6) age for felsic volcanic rocks of the upper sequence, but establishes a new reliable age of 1 114 ± 4 Ma for the lower one (n = 2). The 1 170 ages obtained in the earlier two studies were revisited and are now considered to reflect the age of zircon xenocrysts from the source rocks, which dominate the zircon population of some Swartkopsleegte Formation samples. Several criteria to distinguish autocrystic (magmatic) from antecrystic (age-overlappping xenocrystic) data points were investigated. One sample had high Th levels in only the younger zircons, but histograms of sufficiently precise 207Pb/206Pb ages provided the main criterion. Calculations of zircon crystallisation temperature intervals were not useful in predicting the abundance or proportions of magmatic and antecryst zircons. A multi-episode model of magmatic generation and crystallisation events is probably appropriate. In cases when felsic volcanic samples yield few zircons, care must be taken to avoid the problem exposed in this study. The Koras Group sediments have similar detrital zircon U/Pb age distributions to those of the Rehoboth Basement Inlier. This supports the concept that the Kaaien Terrane originated as the southern part of the Rehoboth Province.

Petrogenesis and tectonic significance of two bimodal volcanic stages from the Ediacaran Campo Alegre-Corupá Basin (Brazil): Record of metacratonization during the consolidation of Western Gondwana

The Ediacaran Campo Alegre-Corupá Basin in South Brazil developed in two stages, the synorogenic passive rift (Basin Stage ∼605–590 Ma) and the post-collisional caldera volcano (Caldera Stage ∼583–577 Ma), respectively. Volcanic rocks from the Basin Stage show a bimodal compositional spectrum with dominant basalt and subordinate silicic rocks. The basaltic rocks are transitional to mildly alkaline, exhibiting Ocean Island Basalt-like (OIB-like) trace element enrichment patterns, with depletion in Nb and Ta, however, and crustal-like Sr-Nd isotopic signatures, suggesting that they were derived from low degrees (∼5%) of partial melting of an enriched lithospheric mantle source. The silicic rocks are transitional to mildly alkaline trachydacites associated with subordinate rhyolites, exhibiting trace element compositions typical of A2-type granitoids, produced by fractional crystallization of the coeval basalts in the Moho. Volcanic rocks from the Caldera Stage are constituted mainly by alkaline trachytes and rhyolites, occurring primarily as pyroclastic sequences coupled to minor effusive lava flows and domes, also exhibiting trace element compositions typical of A2-type granitoids. They are associated with subordinated effusive transitional to mildly alkaline basalts with Island Arc Basalt-like (IAB-like) trace element signatures. Compared to the Basin Stage, the basalts from the Caldera Stage result from higher degrees (∼15 %) of partial melting of possibly the same enriched lithospheric-mantle sources during the lithospheric root collapse of a cratonic terrane. The silicic rocks from the Caldera Stage are also derived from the coeval basalts by fractional crystallization in the Moho. However, an additional stage of differentiation in the upper crust is required to explain their silica-enriched compositions and eruptive styles. Results from this study support a connection between the silicic volcanic rocks from the Caldera Stage and the plutonic bodies from the nearby A-type Graciosa Province. Lu-Hf isotopes from detrital zircon suggest an Andean arc-type tectonic setting during the Paleoproterozoic (∼2,185 Ma) history of the Luis Alves Terrane (LAT) basement. This tectonic setting was responsible for the arc-like signatures of the intraplate lithospheric-derived rocks of both bimodal volcanic sequences. Crustal-like Sr-Nd-Hf isotopic characteristics result from a protracted isotope evolution of their enriched mantle sources, and each tectono-magmatic stage results from a different extensional setting, which has implications for the metacratonization of the LAT.

Using geophysics to follow and model the Precambrian basement terranes beneath the Caledonian nappes in Finnmark, northern Norway: A case study

The bedrock geology of Finnmark, northern Norway, consists primarily of rock complexes dating back to the Precambrian and early Mid Palaeozoic eras and comprises (1) a mid-crustal lithospheric basement which is characterised by granitoid and greenstone complexes dating from Neoarchean to late Paleoproterozoic in age; (2) the Caledonide Orogen, characterised by a series of nappes and thrust sheets. Finnmark forms part of the northern margin of the Fennoscandian Shield, which extends across northern Finland into northwestern Russia. Precambrian rock complexes are also found as thrust slices in Caledonian nappes at several places in Finnmark and Troms, as well as in tectonic windows.Recently acquired high-resolution aeromagnetic data have provided spectacular and confirmatory evidence for the continuation of the diverse Precambrian greenstone belts and granulite terranes beneath the magnetically transparent Caledonian nappes.This study aims at analysing this subsurface continuation of the terranes using potential field data and estimating the depths of the main magnetic sources beneath the nappes. In this study, several geophysical methods are used, including 3D Euler deconvolution and 2D Werner deconvolution. We have focused specifically on the depth-to-basement beneath nappes and purposefully excluded anomalies associated with shallow structures, such as dykes and faults. Different methods have produced similar results and indicate an appreciable increase in the depth of the Precambrian basement rocks from south to north. In the south, Precambrian basement rocks are either exposed on the surface southeast of the Caledonian front or reappear within tectonic windows. A prominent positive magnetic anomaly on the Porsanger Peninsula is thought to represent an extension of the anomaly ascribed to the greenstone belt of the Repparfjord Tectonic Window. A 3D model of this has also been provided. According to estimates, the depths-to-basement below nappes close to the Caledonian front range from less than 500 to 2000 m, whilst in the northern part of the study area the depths generally reach 3000 to 4000 m, and are locally estimated to range between 5000 m and 6000 m.

Paleoproterozoic metagabbro xenolith in the southeastern Dom Feliciano Belt, southern Brazil: A new piece in the West Gondwana assembly

The eastern portion of the Dom Feliciano Belt in southernmost Brazil, known as Pelotas Terrane, is composed of Neoproterozoic rocks presenting metamorphic xenoliths in granitoid rocks, described and interpreted as “basement inliers”. Based on novel petrographic, mineral/whole rock/Sr–Nd geochemistry and U–Pb geochronological data, this paper presents the first record of a Paleoproterozoic metagabbro xenolith in the Pelotas Terrane. The studied xenolith exhibits a sharp and angular contact with the host rock and a predominant granoblastic texture, with sub-vertical mm-to cm-thick metamorphic banding, marked by the felsic and mafic bands, composed respectively of bytownite (An85) ± quartz, and diopside ± tremolite ± epidote. Zircon, apatite, and iron/titanium oxides occur as accessory minerals. The peak metamorphism for this rock is estimated at amphibolite facies conditions. The whole-rock chemistry of the metagabbro suggests a basic derivation (e.g., SiO2 52 wt%; Al2O3 12 wt%; Ni 33 ppm). The spidergrams demonstrate high LILE and HFSE concentrations. Chondrite-normalized REE patterns show enrichment in LREE. The Sr–Nd isotopic data show 87Sr/86Sr(i) = 0.701024, εNd(i) = 5.44, and Nd-TDM = 2.1 Ga. The metagabbro yielded a U–Pb Concordia age with analytical points arranged in a Discordia line. The upper intercept was interpreted as a magmatic crystallization age of 2.1 Ga (zircon cores), and the lower intercept indicated a resetting age of 616 Ma (zircon rims). The obtained Archean age (2.8 Ga) is interpreted as inherited. Gabbro's metamorphism likely reached the amphibolite facies, elemental and isotopic geochemistry. The remote zircon inheritance of xenoliths that have had polycyclic survival is used to link the occurrence of a gabbro xenolith, in the Pelotas Terrane (Dom Feliciano Belt; Southern Brazil), whose magmatic crystallization age is Paleoproterozoic, with the basement terranes before the Namaquan orogeny, in the Rehoboth region (Gariep Belt and Bushmanland/Namaqualand; Namibia). These units could be related to the evolution of an ancient Paleoproterozoic magmatic arc formed by the convergence of two Archean continents during the evolution of the Atlantica Supercontinent.