Sm-Nd Garnet Ages Research Articles

Metamorphic evolution for the Inyoni shear zone: Investigating the geodynamic evolution of a 3.20 Ga terrane boundary in the Barberton granitoid greenstone terrane, South Africa

Abstract The Inyoni shear zone represents an important tectonic boundary between (i) the ca. 3.45 Ga high-pressure amphibolite facies, granite-greenstone domain south of the Barberton greenstone belt, termed the Stolzburg terrane, and (ii) the ca. 3.29 to 3.23 Ga rocks of the trondhjemitic Badplaas pluton to the west. The Stolzburg terrane is separated from the greenschist facies rocks of the rest of the Barberton greenstone belt by the Komati fault, which records >10 km uplift of the Stolzburg terrane relative to the lower-grade rocks of the greenstone belt at ca. 3.23 Ga. A number of studies within the Stolzburg terrane have documented high-pressure amphibolite facies metamorphism that occurred concurrently with exhumation, with the lowest apparent geothermal gradients documented in the Inyoni shear zone, where strong constraints on the age of metamorphism are most limited. In addition, different studies on Inyoni metamorphism have produced significantly different temperature estimates. This study utilizes garnet Sm-Nd geochronology in combination with P-T modelling to directly date the metamorphism and re-evaluate the P-T conditions of the Inyoni shear zone. Two petrologically distinct samples produce similar P-T evolutions. A heterogeneous sample with both garnet-bearing and garnet-absent domains gives up-P evolutions reaching conditions of 550 to 675°C and 7 to 10 kbar, whereas a homogenous sample containing garnet and clinopyroxene produces a similar dominantly up-P evolution reaching peak conditions of 650°C and 8 to 10 kbar. Sm-Nd garnet ages of 3 201.6 ± 4.7 Ma (MSWD = 1.02) and 3 200.3 ± 5.3 Ma (MSWD = 0.44) were obtained from two samples of the homogenous garnet and clinopyroxene-bearing amphibolite. The Sm-Nd garnet geochronology provides accurate ages for the metamorphism of the Inyoni shear zone, with age results suggesting activity on the Inyoni shear zone may have continued after the regional metamorphism at ca. 3.23 Ga previously established by zircon U-Pb geochronology. However, 147Sm decay constant uncertainty leaves open the possibility that Inyoni garnet growth could have coincided with the previously recognized 3.23 Ga regional metamorphism.

The subduction and exhumation history of the Voltri Ophiolite, Italy: Evaluating exhumation mechanisms for high-pressure metamorphic massifs

The problem of how dense high- (HP) and ultrahigh-pressure (UHP) metamorphosed oceanic mafic and ultramafic rocks are exhumed from deep within subduction zones is crucial to understanding processes occurring at the interface between the subducting slab and the overlying plate and mantle wedge. In this study, we use the P-T-t evolution of metamorphosed FeTi gabbros hosted in meta-serpentinite from the Voltri Ophiolite (Ligurian Alps, Italy) to evaluate potential exhumation mechanisms and assess whether HP mafic rocks were exhumed as blocks within a complex serpentinite-hosted mélange or as coherent sections of oceanic lithosphere. A number of observations provide evidence for subduction and exhumation of the central portion of Voltri Ophiolite as a coherent package of ultramafic and mafic material: (1) a similar evolution in mineral assemblages, textures and compositions suggesting peak metamorphism in the eclogite-facies, followed by a retrograde evolution through blueschist-, amphibolite- and greenschist-facies conditions; (2) uniform P-T conditions of both initial garnet growth (~475–480 °C and 2.0–2.2 GPa) and peak metamorphism (~500–525 °C and 2.3–2.5 GPa); and (3) bulk SmNd garnet ages that reveal a very tight grouping of ages for four well-equilibrated foliated FeTi meta-gabbros clustering at 39.98 ± 0.84 Ma. A massive undeformed gabbro also from the central Voltri area gave a slightly younger age of 38.15 ± 0.89 Ma, which is interpreted to reflect kinetic overstepping and delayed growth of garnet. By contrast, an SmNd garnet age from the NW portion of the Voltri Ophiolite indicates peak eclogite-facies metamorphism occurring ~10–12 Ma earlier at 49.68 ± 0.35 Ma. A compilation of the current geochronological data suggests that the NW portion of the Voltri Ophiolite underwent peak eclogite-facies metamorphism first at ~50 Ma, the sedimentary portion of the Voltri Unit (the Voltri-Rossiglione meta-sediments) experienced peak metamorphism around ~47–44 Ma, whilst subduction and peak metamorphism of the central part of the ophiolite occurred later at 41–38 Ma. Alongside recent studies of other Alpine ophiolites (Zermatt Saas, Monviso, Alpine Corsica), this study suggests that detachment and buoyant exhumation of large kilometric-scale coherent sections of slab material, rather than exhumation within a chaotic serpentinite mélange, is the dominant mechanism of preserving and returning eclogite-facies metamorphic rocks to the surface in the Western Alps. In the case of the Voltri Ophiolite, which lacks any associated continental massif, this exhumation was likely aided by the buoyancy of the serpentinite-dominated oceanic lithosphere, rather than by coupling to continental crust.

In search for the missing arc root of the Southern California Batholith: P-T-t evolution of upper mantle xenoliths of the Colorado Plateau Transition Zone

Xenolith and seismic studies provide evidence for tectonic erosion and eastward displacement of lower crust-subcontinental mantle lithosphere (LC-SCML) underlying the Mojave Desert Region (i.e. southern California batholith (SCB)). Intensified traction associated with the Late Cretaceous flattening of the subducting Farallon plate, responsible for deforming the SW U.S., likely played a key role in “bulldozing” the tectonically eroded LC-SCML ∼500 km eastwards, to underneath the Colorado Plateau Transition Zone (CPTZ) and further inboard. The garnet clinopyroxenite xenoliths from two CPTZ localities, Chino Valley and Camp Creek (central Arizona), provide a rare glimpse of the material underlying the CPTZ. Thermodynamic modeling, in addition to major and trace element thermobarometry, suggests that the xenoliths experienced peak conditions of equilibration at 600-900°C and 12-28 kbar. These peak conditions, along with the composition of the xenoliths (type “B” garnet and diopsidic clinopyroxene) strongly suggest a continental arc residue (“arclogite”), rather than a lower plate subduction (“eclogite”), origin. A bimodal zircon U-Pb age distribution with peaks at ca. 75 and 150 Ma, and a Jurassic Sm-Nd garnet age (154 ± 16 Ma, with initial εNd value of +8) overlaps eastern SCB pluton ages and suggests a consanguineous relationship. Cenozoic zircon U-Pb ages, REE geochemistry of zircon grains, and partially re-equilibrated Sm-Nd garnet ages indicate that displaced arclogite remained at elevated PT conditions (>700°C) for 10s of Myr following its dispersal until late Oligocene entrainment in host latite. With a ∼100 Myr long thermal history overlapping that of the SCB and the CPTZ, these assemblages also contain evidence for late-stage hydration (e.g. secondary amphibole), potentially driven by de-watering of the Laramide slab.In light of these results, we suggest that the CPTZ arclogite originates from beneath the eastern half of the SCB, where it began forming in Late Jurassic time as mafic keel to continental arc magmas. The displacement and re-affixation of the arclogites further inboard during the Late Cretaceous flat slab subduction, might have contributed to the tectonic stability of the Colorado Plateau relative to adjacent geologic provinces through Laramide time and likely preconditioned the region to Cenozoic tectonism, e.g. present-day delamination beneath the plateau, high-magnitude extension and formation of metamorphic core complexes.

Sm-Nd garnet ages for granulite and eclogite in the Breaksea Orthogneiss and widespread granulite facies metamorphism of the lower crust, Fiordland magmatic arc, New Zealand

Sm-Nd garnet ages for granulite and eclogite in the Breaksea Orthogneiss and widespread granulite facies metamorphism of the lower crust, Fiordland magmatic arc, New Zealand

Combined garnet and zircon geochronology of the ultra-high temperature metamorphism: Constraints on the rise of the Orlica-Śnieżnik Dome, NE Bohemian Massif, SW Poland

Garnet and zircon geochronology combined with trace element partitioning and petrological studies provide tight constraints on evolution of the UHT-(U)HP terrain of the Orlica-Śnieżnik Dome (OSD) in the NE Bohemian massif. Lu-Hf dating of peritectic garnet from two mesocratic granulites constrained the time of its initial growth at 346.9±1.2 and 348.3±2.0Ma recording peak 2.5GPa pressure and 950°C temperature. In situ, U-Pb SHRIMP dating of zircon from the same granulite gave a younger age of 341.9±3.4Ma. Ti-in-zircon thermometry indicates crystallization at 810–860°C pointing to zircon formation on the retrograde path. Lu partitioning between garnet rim and zircon suggest equilibrium growth and thus U-Pb zircon age constrain the terminal phase of garnet crystallization which lasted about 6Ma.All Sm-Nd garnet ages obtained for mesocratic and mafic granulites are identical and consistently younger than the corresponding Lu-Hf dates. They are interpreted as reflecting cooling of granulites through the Sm-Nd closure temperature at about 337Ma.The estimated PTt path documents the ca. 10Ma evolution cycle of the OSD characterized by two distinct periods: (1) 347 - >342Ma period corresponds to nearly isothermal decompression resulting from crustal scale folding and vertical extrusion of granulites, and (2) at >342–337Ma which corresponds to a fast, nearly isobaric cooling.

Late Carboniferous high-pressure metamorphism of the Kassan Metamorphic Complex (Kyrgyz Tianshan) and assembly of the SW Central Asian Orogenic Belt

High-pressure (HP) metamorphism of the Kassan Metamorphic Complex (KMC) in the western Kyrgyz Tianshan has been related to either late Ordovician or late Carboniferous-Permian subduction processes. We report Sm-Nd ages for retrogressed eclogite samples and 40Ar/39Ar cooling ages for enclosing garnet-muscovite samples from the KMC as new age constraints on HP metamorphism and rock exhumation. These data will be used for an upgraded paleogeographic model for late Paleozoic crustal consolidation in the southwestern Central Asian Orogenic Belt. The retrogressed eclogite samples have transitional alkaline to tholeiitic affinity and trace-element patterns consistent with protoliths derived from garnet-bearing mantle sources at rifting plate margins. Geothermobarometric data for a retrogressed eclogite sample indicate peak-metamorphic conditions of 540 ± 30 °C at 1.6 ± 0.1 GPa. Samples from different lithotectonic units of the KMC provide coherent Sm-Nd garnet-whole rock ages of 317 ± 4 Ma and 316 ± 3 Ma (2σ). The prograde major-element zoning in the mm-sized garnets in combination with the moderate peak-metamorphic temperature, support our interpretation of the Sm-Nd garnet ages as unambiguous evidence for late Carboniferous HP metamorphism. The Sm-Nd garnet growth ages overlap within-error with the 40Ar/39Ar mica cooling ages of 314 ± 2 Ma and 313 ± 2 Ma (2σ) indicating rapid uplift of the subduction complex after peak metamorphism. The ca. 317-313 Ma HP-exhumation event of the KMC is contemporaneous with those of the Atbashi and Akeyazi (ca. 500 km east in NW China) HP complexes and implies similar collision histories at the South Tianshan Suture to the east and west of the Talas-Fergana Fault (TFF). The exhumation of the KMC and Atbashi HP complexes overlaps with the initiation of the TFF (Rolland et al., 2013) suggesting incipient separation of the Chatkal and Atbashi complexes during rock exhumation and early plate collision.

U-Pb garnet chronometry in high-grade rocks?case studies from the central Damara orogen (Namibia) and implications for the interpretation of Sm-Nd garnet ages and the role of high U-Th inclusions

Garnets from different migmatites and granites from the Damara orogen (Namibia) were dated with the U-Pb technique after bulk dissolution of the material. Measured 206Pb/204Pb ratios are highly variable and range from ca. 21 to 613. Variations in isotope (208Pb/204Pb, 206Pb/204Pb) and trace element (Th/U, U/Nd, Sm/Nd) ratios of the different garnets show that some garnets contain significant amounts of monazite and zircon inclusions. Due to their very low 206Pb/204Pb ratios, garnets from pelitic migmatites from the Khan area yield Pb-Pb ages with large errors precluding a detailed evaluation. However, the 207Pb/206Pb ages (ca. 550–500 Ma) appear to be similar to or older than U-Pb monazite ages (530±1–517±1 Ma) and Sm-Nd garnet ages (523±4–512±3 Ma) from the same sample. It is reasonable to assume that the Pb-Pb garnet ages define growth ages because previous studies are consistent with a higher closure temperature for the U-Pb system in garnet relative to the U-Pb system in monazite and the Sm-Nd system in garnet. For igneous migmatites from Oetmoed, Pb-Pb garnet ages (483±15–492±16 Ma) and one Sm-Nd garnet whole rock age (487±8 Ma) are similar whereas the monazite from the same sample is ca. 30–40 Ma older (528±1 Ma). These monazite ages are, however, similar to monazite ages from nearby unmigmatized granite samples and constrain precisely the intrusion of the precursor granite in this area. Although there is a notable difference in closure temperature for the U-Pb and Sm-Nd system in garnet, the similarity of both ages indicate that both garnet ages record garnet growth in a migmatitic environment. Restitic garnet from an unmigmatized granite from Omaruru yields similar U-Pb (493±30–506±30 Ma) and Sm-Nd (493±6–488±7 Ma) garnet ages whereas the monazite from this rock is ca. 15–25 Ma older (516±1–514±1 Ma). Whereas the monazite ages define probably the peak of regional metamorphism in the source of the granite, the garnet ages may indicate the time of melt extraction. For igneous garnets from granites at Oetmoed, the similarity between Pb-Pb (483±34–474±17 Ma) and Sm-Nd (492±5–484±13 Ma) garnet ages is consistent with fast cooling rates of granitic dykes in the lower crust. Differences between garnet and monazite U-Pb ages can be explained by different reactions that produced these minerals at different times and by the empirical observation that monazite seems resistant to later thermal re-equilibration in the temperature range between 750 and 900 °C (e.g. Braun et al. 1998). For garnet analyses that have low 206Pb/204Pb ratios, the influence of high-μ inclusions is small. However, the relatively large errors preclude a detailed evaluation of the relationship between the different chronometers. For garnet with higher 206Pb/204Pb ratios, the overall similarity between the Pb-Pb and Sm-Nd garnet ages implies that the inclusions are not significantly older than the garnet and therefore do not induce a premetamorphic Pb signature upon the garnet. The results presented here show that garnet with low 238U/204Pb ratios together with Sm-Nd garnet data and U-Pb monazite ages from the same rock can be used to extract geologically meaningful ages that can help to better understand tectonometamorphic processes in high-grade terranes.

Dating granulite-facies structures and the exhumation of lower crust in the Moldanubian Zone of the Bohemian Massif

Deformation of granulite-facies rocks in the Moldanubian Zone of the southern Bohemian Massif is expressed in two intersecting planar fabrics – steeply disposed (S1) and flat-laying (S2) – which correspond to two deformation stages (D1) and (D2). The existing Sm-Nd garnet ages from banded granulite gneisses, new U-Pb zircon data from deformed granite intrusions within the granulite gneisses, and the P–T and field structural relations constrain the ages and P–T conditions of the two deformation phases. The early deformation (D1) was associated with a HP-HT metamorphic stage with a minimum age of ca. 354 Ma which was followed by a near-isothermal decompression. A concordant U-Pb zircon age of 318±1 Ma dates the emplacement of intrusions of deformed granite into the granulite gneisses and constrains deformation phase (D2). This phase was associated with an LP–HT metamorphism dated in the region at ca. 340–330 Ma. The available structural and isotopic data indicate that granulites in the southern Bohemian Massif were exhumed from lower to middle crust during compression. The structural relations and P–T–t data for the studied granulites are consistent with their exhumation by near-vertical extrusion of the softened orogenic root.

Nd isotopic ages of crust formation and metamorphism in the Precambrian of eastern and southern Mexico

Sm-Nd ages for garnets in the three Precambrian exposures of eastern and southern Mexico demonstrate that they belong to the Grenville tectonothermal event. The Sm-Nd garnet ages of 0.95 Ga for the Oaxacan Complex and 0.90 Ga for the Huiznopala Gneiss, Molango and the Novillo Gneiss, Ciudad Victoria, are postdated 75 Ma by Rb-Sr ages on biotites. Both sets of data document a cooling history following Grenville metamorphism at or before 1.0 Ga ago. Our garnet data are consistent with a blocking temperature for Sm-Nd in that mineral around 600° C suggested by Humphries and Cliff (1982).

Xenoliths from southern Africa: a perspective on the lower crust

Summary Granulite-facies xenoliths from southern African kimberlites yield temperatures from 780–900°C and pressures from 10–15 kb. They have Nd model ages of 1.4–3.7 Ga and retain Rb-Sr mica and Sm-Nd garnet ages from 0.14–1 Ga, significantly older than kimberlite emplacement ages. It is argued that low abundances of U, K and Rb in lower crustal rocks can be either primary features resulting from plagioclase-dominated cumulate processes, or secondary effects caused by the granulite-facies breakdown of mica in wet intermediate-acid rocks. Combined trace element and isotope arguments suggest that contamination with lower crustal material does not significantly affect the geochemistry of flood basalts from the Karoo, the Parana and the Deccan.