Chemical Abrasion Isotope Dilution Thermal Ionization Mass Research Articles

Petrochronology of Zircon and Baddeleyite in Igneous Rocks: Reconstructing Magmatic Processes at High Temporal Resolution

Zircon (ZrSiO4) and baddeleyite (ZrO2) are common accessory minerals in igneous rocks of felsic to mafic composition. Both minerals host trace elements substituting for Zr, among them Hf, Th, U, Y, REEs and many more. The excellent chemical and physical resistivity of zircon makes this mineral a perfect archive of chemical and temporal information to trace geological processes in the past, utilizing the outstanding power and temporal resolution of the U–Pb decay schemes. Baddeleyite is a chemically and physically much more fragile mineral. It preserves similar information only where it is shielded from dissolution and physical fragmentation as an inclusion in other minerals or in a fine-grained or non-reactive rock matrix. It offers the potential for dating the solidification of mafic rocks with high-precision through its crystallization in small pockets of Zr-enriched melt, after extensive olivine and pyroxene fractionation. Zircon and baddelelyite U–Pb dates are, for an overwhelming majority of cases and where we can assume a closed system, considered to reflect the time of crystallization. The development of the U–Pb dating tool CA-ID-TIMS (chemical abrasion-isotope dilution-thermal ionization mass spectrometry) since 2005 has led to unprecedented precision of better than 0.1% in 206Pb/238U dates (Bowring et al. 2005). Increased sensitivity of mass spectrometers and low laboratory blanks due to reduction of acid volumes allow routine U–Pb age determinations of micrograms of material at sufficiently high radiogenic/common lead ratios (see Schoene and Baxter 2017, this volume). In situ U–Pb age analysis using laser ablation or primary ion beam sputtering allows analysis of sub-microgram quantities of zircon material from polished internal sections or zircon surfaces with spot diameters ranging from ~30 μm for laser-ablation, inductively coupled plasma mass spectrometry (LA-ICP-MS) to 10 μm for secondary ion mass spectrometry (SIMS), lateral resolutions of 2–5 μm for NanoSIMS …

Dodging snowballs: Geochronology of the Gaskiers glaciation and the first appearance of the Ediacaran biota

Abstract The snowball Earth hypothesis predicts that low-latitude glaciation lasted millions of years while CO2 built up to critical levels to culminate in catastrophic deglaciation in a supergreenhouse climate. The Gaskiers Formation of eastern Newfoundland (Canada) has been attributed to a snowball glaciation event, but the lack of robust paleomagnetic data and precise geochronological constraints has precluded tests of the hypothesis. Here we present high-precision U-Pb zircon geochronology (chemical abrasion–isotope dilution–thermal ionization mass spectrometry) from eight tuffs from multiple distant stratigraphic sections that bracket glacial diamictites and the first appearance of large Ediacaran fossils. Including internal error, deposition of the Gaskiers diamictite on the Avalon Peninsula is constrained to have been between 580.90 ± 0.40 and 579.88 ± 0.44 Ma, and the Trinity diamictite on Bonavista Peninsula was deposited between 579.63 ± 0.15 and 579.24 ± 0.17 Ma. Assuming approximately synchronous deglaciation, these results imply a maximum duration for deposition of the Trinity diamictite of ≤340 k.y.; this is inconsistent with the multimillion year duration predicted by the snowball Earth hypothesis. Our geochronologic data also constrain the first appearance datum of Ediacaran fossils to <9.5 m.y. after the Gaskiers glaciation. Thus, despite existing paleomagnetic constraints that indicate that marine ice sheets extended to low to middle latitudes, it appears that Earth narrowly escaped a third Neoproterozoic snowball glaciation just prior to the late Ediacaran expansion of metazoan ecosystems.

Paleomagnetism of the Teel basalts from the Zavkhan terrane: Implications for Paleozoic paleogeography in Mongolia and the growth of continental crust

A narrow extensional basin on the Zavkhan terrane of Mongolia exposes a >1.8-km-thick succession of basalt flows within the Teel Formation, along with rhyolites and interflow sediments. We present new U-Pb zircon ages of 446.03 ± 0.21 Ma (chemical abrasion–isotope dilution–thermal ionization mass spectrometry) on a rhyolite in the Teel Formation and 286 ± 5 Ma (laser ablation–inductively coupled plasma–mass spectrometry) on a nearby granitic intrusion (Tonkhil Complex). New paleomagnetic data yield a magnetite remanence that is likely primary, acquired during cooling of flows. The mean direction is statistically improved after tilt corrections; however, the tilt test significance is limited given the low variation in tilt between flows. We interpret a second remanence, held by hematite, as an overprint that was likely acquired later in the Paleozoic Era. The tilt-corrected magnetite direction implies a paleolatitude of ∼20°, while the hematite overprint is equatorial in both geographic and tilt-corrected coordinates. The ca. 446 Ma Teel remanence is consistent with an Ordovician paleogeographic position near Siberia; however, the hematite direction requires subsequent drift to the equator, indicating that these Mongolian terranes were not continuously connected to Siberia, which moved away from the tropics during the Paleozoic Era. This result is consistent with biogeographic constraints and a previously proposed model wherein Amuria traveled with North China during the Permian Period and collided with Siberia during the Jurassic to Triassic closure of the Mongol-Okhotsk Ocean. In this model, continental growth occurred through the collision and oroclinal buckling of a ribbon continent rather than long-lived accretion on the margin of a major craton.

Linking the thermal evolution and emplacement history of an upper-crustal pluton to its lower-crustal roots using zircon geochronology and geochemistry (southern Adamello batholith, N. Italy)

The Val Fredda igneous complex in the southern Adamello batholith (N. Italy) consists of dioritic to gabbroic sills and dykes that were injected at 6–10 km depth into partly crystallized tonalites and granodiorites. High-precision U–Pb age determinations by chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA–ID–TIMS) show very similar dispersion of zircon U–Pb dates over 90–200 ka and identical age distributions with a dominant mode at 42.5 Ma for six samples ranging in composition from gabbro to granodiorite. The co-variation of the probability density curves of zircon dates from mafic and felsic units suggests that they shared a common thermal history with periods of enhanced and reduced zircon growth, reflecting lowered and increased magma temperatures, respectively. However, trace element compositions, Ti-in-zircon temperatures and Hf isotopic compositions of zircon from mafic lithologies are distinctly different from those in felsic zircon and suggest their crystallization occurred in isotopically and chemically diverse magma batches. These magma batches formed in the lower crust from mingling and mixing of residual melts (derived from fractional crystallization of mainly amphibole from basaltic melt) with crustal partial melts at high temperatures above zircon saturation. Zircons crystallized during incipient cooling of these magmas and were entrained into the ascending melts, which were emplaced and rapidly solidified in the upper crust. The reported age dispersions imply that fractional crystallization and hybridization in the lower-to-middle crust, ascent into the upper crust and solidification did not last for more than 200 ka. The small magma volumes and flux also preclude significant zircon crystallization at the upper crustal emplacement level.

Linking rapid magma reservoir assembly and eruption trigger mechanisms at evolved Yellowstone-type supervolcanoes

The geological record contains evidence of volcanic eruptions that were as much as two orders of magnitude larger than the most voluminous eruption experienced by modern civilizations, the A.D. 1815 Tambora (Indonesia) eruption. Perhaps nowhere on Earth are deposits of such supereruptions more prominent than in the Snake River Plain–Yellowstone Plateau (SRP-YP) volcanic province (northwest United States). While magmatic activity at Yellowstone is still ongoing, the Heise volcanic field in eastern Idaho represents the youngest complete caldera cycle in the SRP-YP, and thus is particularly instructive for current and future volcanic activity at Yellowstone. The Heise caldera cycle culminated 4.5 Ma ago in the eruption of the ∼1800 km3 Kilgore Tuff. Accessory zircons in the Kilgore Tuff display significant intercrystalline and intracrystalline oxygen isotopic heterogeneity, and the vast majority are 18O depleted. This suggests that zircons crystallized from isotopically distinct magma batches that were generated by remelting of subcaldera silicic rocks previously altered by low-δ18O meteoric-hydrothermal fluids. Prior to eruption these magma batches were assembled and homogenized into a single voluminous reservoir. U-Pb geochronology of isotopically diverse zircons using chemical abrasion–isotope dilution–thermal ionization mass spectrometry yielded indistinguishable crystallization ages with a weighted mean 206Pb/238U date of 4.4876 ± 0.0023 Ma (MSWD = 1.5; n = 24). These zircon crystallization ages are also indistinguishable from the sanidine 40Ar/39Ar dates, and thus zircons crystallized close to eruption. This requires that shallow crustal melting, assembly of isolated batches into a supervolcanic magma reservoir, homogenization, and eruption occurred extremely rapidly, within the resolution of our geochronology (103–104 yr). The crystal-scale image of the reservoir configuration, with several isolated magma batches, is very similar to the reservoir configurations inferred from seismic data at active supervolcanoes. The connection of magma batches vertically distributed over several kilometers in the upper crust would cause a substantial increase of buoyancy overpressure, providing an eruption trigger mechanism that is the direct consequence of the reservoir assembly process.

Late Paleozoic assembly of the Alexander-Wrangellia-Peninsular composite terrane, Canadian and Alaskan Cordillera

Late Paleozoic assembly of the Alexander-Wrangellia-Peninsular composite terrane is recorded by two phases of regional deformation, metamorphism, and magmatism within basement complexes of the Alexander (Craig and Admiralty subterranes), Wrangellia, and Peninsular terranes in the Canadian and Alaskan Cordillera. New secondary ion mass spectrometry (SIMS) and chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) zircon U-Pb ages, whole-rock major- and trace-element and Nd-Sr isotope geochemical compositions, and geological field observations of late Paleozoic igneous rocks were used to identify the precise timing and significance of this tectonism in the Saint Elias Mountains region of southwestern Yukon and eastern Alaska. Middle to Late Pennsylvanian (301–307 Ma) igneous rocks, herein assigned to the Barnard Glacier suite, were preferentially emplaced along the Wrangellia-Craig subterrane boundary and mainly comprise syenitic plutons that intrude Paleozoic country rocks with evidence of Pennsylvanian or older (D1) deformation. We propose that Barnard Glacier suite magmatism was produced by a slab breakoff event after the consumption of a narrow backarc ocean basin and early Pennsylvanian collision between the Wrangellia-Peninsular arc and Craig subterrane passive margin. Early Permian (284–291 Ma) dioritic to granodioritic rocks, herein assigned to the Donjek Glacier suite, comprise the vestiges of an extensive magmatic system within the Craig subterrane of southwestern Yukon and southeastern Alaska. The available data suggest that the Donjek Glacier suite represents part of a short-lived, Early Permian arc that initiated along the outboard margin of the Craig subterrane–Wrangellia–Peninsular block after Pennsylvanian collision and slab breakoff. At two field localities in southwestern Yukon, Paleozoic country rocks with D1 fabrics are also intruded by sills and dikes of the Donjek Glacier suite that show evidence of ca. 285 Ma regional deformation and metamorphism (D2). Field evidence for Early Permian tectonism in the Saint Elias Mountains implies direct connections with coeval deformation and metamorphism in the Admiralty subterrane, a microcontinent in the Admiralty Island region of southeastern Alaska that developed separately from the Craig subterrane prior to the Early Permian. D2 tectonism was likely related to the entry of the Admiralty subterrane margin into the Early Permian subduction zone, which resulted in collision and final amalgamation of the Alexander-Wrangellia-Peninsular composite terrane. Our tectonic scenarios require the currently accepted configuration of the Alexander terrane (composite of the Craig and Admiralty subterranes) to have only existed after the Early Permian collision between the Admiralty subterrane and the previously assembled Craig subterrane–Wrangellia–Peninsular terrane. Biogeographic and other geological data suggest that the two-part assembly of the Alexander-Wrangellia-Peninsular composite terrane took place along a convergent margin to the north of the Cordilleran pericratonic arc terranes (Yukon-Tanana, Quesnellia, and others), in between the paleo–Pacific Ocean and paleo–Arctic Ocean realms, to the northwest of the supercontinent Pangea. The assembly of the Alexander-Wrangellia-Peninsular composite terrane might have been associated with the Early to Middle Permian subduction polarity flip recognized in the Cordilleran pericratonic realm, which led to the closure of a backarc ocean basin and Late Permian arc-continent collision along the western margin of North America.

Towards accurate numerical calibration of the Late Triassic: High-precision U-Pb geochronology constraints on the duration of the Rhaetian

Research Article| July 01, 2014 Towards accurate numerical calibration of the Late Triassic: High-precision U-Pb geochronology constraints on the duration of the Rhaetian Jörn-Frederik Wotzlaw; Jörn-Frederik Wotzlaw * 1Section of Earth and Environmental Sciences, University of Geneva, 1201 Geneva, Switzerland *E-mail: joern.wotzlaw@unige.ch. Search for other works by this author on: GSW Google Scholar Jean Guex; Jean Guex 2Institute of Earth Sciences, University of Lausanne, 1015 Lausanne, Switzerland Search for other works by this author on: GSW Google Scholar Annachiara Bartolini; Annachiara Bartolini 3Muséum National d’Histoire Naturelle, 75005 Paris, France Search for other works by this author on: GSW Google Scholar Yves Gallet; Yves Gallet 4Equipe de Paléomagnétisme, Institut de Physique du Globe de Paris, 75238 Paris, France Search for other works by this author on: GSW Google Scholar Leopold Krystyn; Leopold Krystyn 5Institute of Paleontology, University of Vienna, 1090 Vienna, Austria Search for other works by this author on: GSW Google Scholar Christopher A. McRoberts; Christopher A. McRoberts 6Geology Department, State University of New York Cortland, Cortland, New York 13045, USA Search for other works by this author on: GSW Google Scholar David Taylor; David Taylor 7Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA Search for other works by this author on: GSW Google Scholar Blair Schoene; Blair Schoene 8Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA Search for other works by this author on: GSW Google Scholar Urs Schaltegger Urs Schaltegger 1Section of Earth and Environmental Sciences, University of Geneva, 1201 Geneva, Switzerland Search for other works by this author on: GSW Google Scholar Author and Article Information Jörn-Frederik Wotzlaw * 1Section of Earth and Environmental Sciences, University of Geneva, 1201 Geneva, Switzerland Jean Guex 2Institute of Earth Sciences, University of Lausanne, 1015 Lausanne, Switzerland Annachiara Bartolini 3Muséum National d’Histoire Naturelle, 75005 Paris, France Yves Gallet 4Equipe de Paléomagnétisme, Institut de Physique du Globe de Paris, 75238 Paris, France Leopold Krystyn 5Institute of Paleontology, University of Vienna, 1090 Vienna, Austria Christopher A. McRoberts 6Geology Department, State University of New York Cortland, Cortland, New York 13045, USA David Taylor 7Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA Blair Schoene 8Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA Urs Schaltegger 1Section of Earth and Environmental Sciences, University of Geneva, 1201 Geneva, Switzerland *E-mail: joern.wotzlaw@unige.ch. Publisher: Geological Society of America Received: 26 Feb 2014 Revision Received: 08 Apr 2014 Accepted: 10 Apr 2014 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (7): 571–574. https://doi.org/10.1130/G35612.1 Article history Received: 26 Feb 2014 Revision Received: 08 Apr 2014 Accepted: 10 Apr 2014 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jörn-Frederik Wotzlaw, Jean Guex, Annachiara Bartolini, Yves Gallet, Leopold Krystyn, Christopher A. McRoberts, David Taylor, Blair Schoene, Urs Schaltegger; Towards accurate numerical calibration of the Late Triassic: High-precision U-Pb geochronology constraints on the duration of the Rhaetian. Geology 2014;; 42 (7): 571–574. doi: https://doi.org/10.1130/G35612.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Numerical calibration of the Late Triassic stages is arguably the most controversial issue in Mesozoic stratigraphy, despite its importance for assessing mechanisms of environmental perturbations and associated biologic consequences preceding the end-Triassic mass extinction. Here we report new chemical abrasion–isotope dilution–thermal ionization mass spectrometry zircon U-Pb dates for volcanic ash beds within the Aramachay Formation of the Pucara Group in northern Peru that place precise constraints on the age of the Norian-Rhaetian boundary (NRB) and the duration of the Rhaetian. The sampled ash bed–bearing interval is located just above the last occurrence of the bivalve Monotis subcircularis, placing this stratigraphic sequence in the uppermost Norian, perhaps ranging into the earliest Rhaetian. Zircon U-Pb dates of ash beds constrain the deposition age of this interval to be between 205.70 ± 0.15 Ma and 205.30 ± 0.14 Ma, providing precise constraints on the age of the NRB. Combined with previously published zircon U-Pb dates for ash beds bracketing the Triassic-Jurassic boundary, we estimate a duration of 4.14 ± 0.39 m.y. for the Rhaetian. This ends a prolonged controversy about the duration of this stage and has fundamental implications for the rates of paleoenvironmental deterioration that culminated in the end-Triassic mass extinction. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Integrating 40Ar/39Ar, U-Pb, and astronomical clocks in the Cretaceous Niobrara Formation, Western Interior Basin, USA

This study revises and improves the chronostratigraphic framework for late Turonian through early Campanian time based on work in the Western Interior U.S. and introduces new methods to better quantify uncertainties associated with the development of such time scales. Building on the unique attributes of the Western Interior Basin, which contains abundant volcanic ash beds and rhythmic strata interpreted to record orbital cycles, we integrate new radioisotopic data of improved accuracy with a recently published astrochronologic framework for the Niobrara Formation. New 40Ar/39Ar laser fusion ages corresponding to eight different ammonite biozones are determined by analysis of legacy samples, as well as newly collected material. These results are complemented by new U-Pb (zircon) chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages from four biozones in the study interval. When combined with published radioisotopic data from the Cenomanian-Turonian boundary, paired 206Pb/238U and 40Ar/39Ar ages spanning Cenomanian to Campanian time support an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. Stage boundary ages are estimated via integration of new radioisotopic data with the floating astrochronology for the Niobrara Formation. The ages are determined by anchoring the long eccentricity bandpass from spectral analysis of the Niobrara Formation to radioisotopic ages with the lowest uncertainty proximal to the boundary, and adding or subtracting time by parsing the 405 k.y. cycles. The new stage boundary age determinations are: 89.75 ± 0.38 Ma for the Turonian-Coniacian, 86.49 ± 0.44 Ma for the Coniacian-Santonian, and 84.19 ± 0.38 Ma for the Santonian-Campanian boundary. The 2σ uncertainties for these estimates include systematic contributions from the radioisotopic measurements, astrochronologic methods, and geologic uncertainties (related to stratigraphic correlation and the presence of hiatuses). The latter geologic uncertainties have not been directly addressed in prior time scale studies and their determination was made possible by critical biostratigraphic observations. Each methodological approach employed in this study—new radioisotopic analysis, stratigraphic correlation, astrochronology, and ammonite and inoceramid biostratigraphy—was critical for achieving the final result.

Batch-wise assembly and zoning of a tilted calc-alkaline batholith: Field relations, timing, and compositional variation

The Wooley Creek batholith is a tilted, zoned, calc-alkaline plutonic complex in the Klamath Mountains, northern California, USA. It consists of three main compositional-temporal zones. The lower zone consists of gabbro through tonalite. Textural heterogeneities on the scale of tens to hundreds of meters combined with bulk-rock data suggest that it was assembled from numerous magma batches that did not interact extensively with one another despite the lack of sharp contacts and identical ages of two lower zone samples (U-Pb [zircon] chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages of 158.99 ± 0.17 and 159.22 ± 0.10 Ma). The upper zone is slightly younger, with 3 samples yielding ages from 158.25 ± 0.46 to 158.21 ± 0.17 Ma, and is upwardly zoned from tonalite to granite. This zoning can be explained by crystal-liquid separation and is related to upward increases in the proportions of interstitial K-feldspar and quartz. Porphyritic dacitic to rhyodacitic roof dikes have compositions coincident with evolved samples of the upper zone. These data indicate that the upper zone was an eruptible mush that crystallized from a nearly homogeneous parental magma that evolved primarily by upward percolation of interstitial melt. The central zone is a recharge area with variably disrupted synplutonic dikes and swarms of mafic enclaves. Central zone ages (159.01 ± 0.20 to 158.30 ± 0.16 Ma) are similar to both lower and upper zones crystallization ages. In the main part of the Wooley Creek batholith, age data constrain magmatism to a short period of time (

New constraints on the Jurassic–Cretaceous boundary in the High Andes using high-precision U–Pb data

The Jurassic–Cretaceous (J–K) boundary is poorly constrained, and is the only Phanerozoic system boundary that lacks an internationally accepted reference stratigraphic section (GSSP). Precise radio-isotopic U–Pb data are unavailable for the earliest stage of the Cretaceous— the Berriasian. The age of the Jurassic–Cretaceous boundary was based on several assumptions, including the relative duration of ammonite zones, the constant spreading rates of magnetic anomalies, and the extrapolation of Rb–Sr or K–Ar isotopic data. This paper discusses a site in an Andean Basin of Western Gondwana showing the J–K boundary interval with geographically widespread nannofossil markers which are here uniquely combined with precise radiometric dates. The recent finding of a sequence of marine black shales in the High Andes of Argentina, interbedded with ash-fall tuffs, provides important constraints on this boundary. This succession bears calcareous nannofossils and ammonites, which allow correlation with well-established Tethyan floras and faunas in the northern hemisphere. The Tithonian–Berriasian transition in the Andes was recognized on the basis of ammonite zones and nannofossil bioevents for the first time in the southern hemisphere. The new ages obtained are 137.9±0.9Ma by sensitive high-resolution ion microprobe (SHRIMP), and 139.55±0.09/0.18Ma by chemical-abrasion isotope-dilution thermal ionization mass spectrometry (TIMS) near the base of the Berriasian. These new ages can be interpreted in two different ways. The first alternative would indicate that the present geological time table is correct and the fossil levels should be late Berriasian. The second alternative is that the J–K boundary is 5Ma younger than the recently published geological time scale. The authors support the last alternative and propose that the J–K boundary should be close to 140Ma.

New high-precision U–Pb dates from western European Carboniferous tuffs; implications for time scale calibration, the periodicity of late Carboniferous cycles and stratigraphical correlation

We present three new high-precision U–Pb zircon chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) dates from western European Carboniferous diagenetically altered volcanic ash layers (bentonites and tonsteins) that occur interbedded with cyclic siliciclastic sedimentary rocks. The dates constrain the average periodicity of western European late Carboniferous (Arnsbergian to early Langsettian) cyclic sedimentation to between 89 and 124 ka per cycle. This is consistent with the 100 ka ‘short’ eccentricity Milankovitch cycle and supports previous suggestions that the cycles are the product of glacio-eustasy. Cyclostratigraphic tuning of Namurian cycles to the 100 ka eccentricity cycle revises the timing of the global Mississippian–Pennsylvanian boundary to c . 323.9 Ma. CA-ID-TIMS dates also facilitate correlation between different facies belts where biostratigraphical correlations are difficult. Correlation between eastern and western European sequences confirms that the base of the Moscovian stage is located within the Duckmantian (Westphalian B) western European regional substage. Supplementary materials: Zircon separation and concentration procedures, U–Pb zircon CA-ID-TIMS methods and data table, average periodicity calculations for western and central European Namurian and Westphalian sedimentary cycles, and procedures for recalibrating legacy 40 Ar/ 39 Ar dates are available at www.geolsoc.org.uk/SUP18547 .

Rates and mechanisms of Mesoarchean magmatic arc construction, eastern Kaapvaal craton, Swaziland

The mechanisms and time scales of magmatism and deformation during orogenesis are important for developing models of lithospheric growth. We summarize the first detailed study of a well-preserved Mesoarchean crustal section from the oldest portion of the Kaapvaal craton, southern Africa, which records the complex interactions between deformation and magmatism during craton assembly ca. 3.3–3.2 Ga. We use chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb zircon geochronology and apatite thermochronology in conjunction with geological mapping to show that the tonalitic to granitic Usutu magmatic suite intruded into the ca. 3.66–3.45 Ga Ancient gneiss complex over a period of ∼16 Ma ca. 3236–3220 Ma as discrete pulses of magma with variable intrusive styles. Usutu rocks retain magmatic fabrics that preserve a history of NW-SE regional shortening, consistent with synchronous deformation recorded in the adjacent Barberton greenstone belt. U-Pb zircon dates of ca. 3.28–3.23 Ga and apatite cooling dates from the Nhlangano gneiss SE of the Ancient gneiss complex reveal that they represent an older, largely undocumented period of crustal growth and modification. Synchronicity in magmatism and similarity in kinematics of deformation north and south of a previously suggested continental suture in the Barberton greenstone belt lead us to propose doubly vergent subduction zones that were active from at least ca. 3.28 to 3.22 Ga. Geochemistry of the Usutu suite reveals differences in magma sources from north to south, which, when combined with Nd isotopic signatures, are consistent with their production in a subduction zone. The combination of <1 Ma precision on crystallization dates with field observations and geochemistry allows us to track the evolution of this magmatic system with temporal resolution unprecedented for Archean rocks.