Old Model Ages Research Articles

Ca. 2.1 Ga low-δ18O gabbro-diorite association in southern North China Craton: Implications for an intraplate rifting

Although widespread ca. 2.2–2.1 Ga igneous activities were identified through the North China Craton (NCC), their petrogenesis and implications for the Paleoproterozoic tectonic regime of the NCC are still under hot debate. Here we present the 2.18–2.14 Ga gabbro-diorite plutons in the Xiong'ershan Taihua Complex, southern North China Craton. They are dominantly gabbros with minor as gabbroic-dioritic rocks. The samples exhibit transitional geochemical characteristics from tholeiitic to calc-alkali series with enrichment in LREEs (e.g., La/Yb N = 1.90–13.9) and LILEs (e.g., Rb, Ba and Sr) but depletion in HFSE (e.g., Nb, Ta and Ti), corresponding to affinity of arc-related magmatism. The high Fe 2 O 3 (8.90–14.91 wt%), CaO (6.78–9.81 wt%) contents, and Ti/Y ratios (179–635), low Th concentrations (0.11–4.08 ppm), and consistent La/Nb and Th/Nb ratios, preclude significant crustal contamination during magma emplacement. The samples show negative zircon ε Hf (t) (−7.3–−1.6) and whole-rock ε Nd (t) (−4.0–+0.9) values with much older model ages ( T DM Hf = 2811–2606 Ma and T DM Nd = 3069–2711 Ma) than their formation ages, along with high La/Nb (1.69–6.21) and low La/Ba (0.01–0.05) ratios, indicating a refertilized Archean lithospheric mantle source. Considering that zircons of 2.18–2.14 Ga gabbro-diorite in the Xiong'ershan area lack metamict features and exhibit originally low but homogeneous δ 18 O values in the mantle source (3.04‰ to 5.07‰, 4.21‰ on average), we prefer they derived from preexisting 18 O-depleted mantle source, which might be a highly heterogeneous metasomatized SCLM by subducted-fluid during the Archean cratonization. The consistent Sm/Yb and La/Sm ratios indicate ∼5–30% partial melting of the garnet-spinel dominant source region. Combing the low δ 18 O signature and within-plate (high Zr, Zr/Y, Th/Hf and Ta/Hf) geochemical features, we suggest a thermal anomaly linked to a rift setting could account for the mantle melting. The intraplate ca. 2.1 Ga low-δ 18 O magmatism possibly resembles those during the Neoproterozoic glaciations associated with major dispersal periods of Rodinia. To better understand the Paleoproterozoic tectonic evolution of the NCC, we have also comprehensively compiled all the published ages and geochemical data of the 2.2–2.1 Ga mafic-felsic igneous rocks. The mafic suites can be divided into two groups based on ages and geochemical affinities. Mafic rocks of Group 1 (2.19–2.12 Ga) belong to rift-related basalts, together with spatially associated 2.19–2.13 Ga A-type granitic rocks and volcano-sedimentary associations, likely indicating an intraplate rift setting. Whereas mafic rocks of Group 2 (2.15–2.06 Ga) show geochemical affinity of island arc calc-alkaline basalts, together with coeval 2.18–2.05 Ga I-S type granitic associations, implying an arc-related setting and recording a possible tectonic transition from extension to subduction-collision. The rifting events in the NCC during 2.2–2.1 Ga may coincide with the global lithospheric break-up of the supercontinent, and the subsequent tectonic transition could correspond to development of the globally orogenesis at 2.1–1.8 Ga. • The Paleoproterozoic low-δ 18 O gabbro-diorite plutons were formed at 2.18–2.14 Ga. • The mafic association was derived from an ancient metasomatized SCLM source. • They were formed in an intraplate rifting system. • The extensive 2.2–2.1 Ga magmatism in the NCC corresponds to global breakup event.

Constraints on the evolution of the eastern margin of the Hearne craton: New data from the southwestern Rottenstone Domain, Trans-Hudson Orogen, Saskatchewan

The Rottenstone Domain (RD) is a complex lithotectonic supracrustal belt of the Paleoproterozoic Trans-Hudson Orogen in Saskatchewan, Canada, located between the Archean Hearne craton and the juvenile Paleoproterozoic Reindeer Zone (RZ). Its evolution and crustal affinity have been under considerable debate in the past few decades, with most interpretations considering it to be part of RZ, but some hinting that certain elements might have affinities to the Archean hinterland. On the basis of bedrock mapping, U-Pb, Sm-Nd and lithogeochemical data, the rocks of the southwestern RD were subdivided into several lithodemic entities, which were used to reconstruct a plate tectonic framework for the western part of the Trans-Hudson Orogen during Orosirian time.A U-Pb zircon crystallization age of 2518 ± 6 Ma and depleted mantle model age (TDM) of 2.8 Ga for an augen granite confirmed that the western part of RD is part of the Hearne craton. A newly discovered structural inlier at the eastern boundary of RD consists of a central unit of 2045 ± 9 Ma augen granite, with TDM ages between 2.8 and 2.6 Ga, that was intruded into gneissic granite with a TDM of 3.3 Ga. The augen granite was interpreted as representing a rift-related granite, emplaced into Archean rocks of the Hearne craton. Two lithotectonic assemblages, one dominated by feldspathic quartzite and psammite the other by migmatitic psammite-psammopelite, are in structural contact with the granitic inliers and have model ages of 2.6–2.4 Ga. One of the feldspathic quartzite samples contains detrital zircon age modes at 1910 and 1860 Ma, and lesser Archean modes. Both assemblages were likely deposited in a forearc basin to the 1.865–1.850 Ga Wathaman continental arc.Recognition of Archean components and metasedimentary assemblages with older model ages than those in the RZ to the east, suggest that this southern part of RD is more closely affiliated with the Hearne craton. The eastern edge of RD coincides with a mylonite zone (Howard Lake shear zone), which likely represents a fundamental, although multiply reactivated, structural break separating Archean granitoids and structurally overlying metasedimentary rocks from rocks of the more juvenile Reindeer Zone.

Nd and Hf isoscapes of the Yilgarn Craton, Western Australia and implications for its mineral systems

The Yilgarn Craton, Western Australia is endowed with significant gold, nickel‑copper, Cu-Pb-Zn volcanic-hosted massive sulphide (VHMS) and iron ore deposits. Detailed knowledge of the spatial relationships between mineral systems and the crustal framework of the Yilgarn Craton may be powerful tools in the predictive exploration of undiscovered mineral systems. Both Nd and Hf isotopes have the potential to image crustal residence, mantle tapping structures, and juvenile input over both space and time. This study uses a recent compilation of Hf (1236 zircon analyses) and Nd (462 whole rock analyses) isotopes to image the architecture of the crust and compare this to the distribution of gold, nickel‑copper-platinum group elements (Ni-Cu-PGEs), Cu-Pb-Zn VHMS and iron ore deposits. Spatial correlation plots reveal a statistically significant relationship between Nd and Hf isotopic domains and the occurrence of gold, Ni-Cu-PGE, base metals, and iron confirmed by the coefficient of determination (R2) which measures how differences in metal occurrences can be explained by differences in the isotopic signature. Nickel occurrences show the strongest antithetic spatial correlation (R2 = 0.80) with Hf model ages, followed by gold (R2 = 0.58) and base metals (R2 = 0.35), which implies higher metal endowment in areas of more juvenile crust. In contrast, there is a significant positive spatial correlation (R2 = 0.49) between iron ore occurrence and older model ages. Isotopic data can also be plotted as Hf or Nd time-slices, where the strongest spatial correlation between gold, nickel, and base metal occurrence and juvenile input occurs at ca. 2700–2550 Ma. Generally, a model can be proposed where (1) base metals are more prevalent in juvenile crust because of its thinner nature, greater mantle input, high-heat flow, increased water-magma interaction; (2) iron ore is concentrated in regions of old, mature, and stable crust; (3) komatiite-hosted Ni-Cu-PGE systems are focused in juvenile rift zones at the edge of old crustal blocks; and (4) gold is also localised in juvenile crust at craton edges where several factors are localised, including orogenesis, occurrence of previously fertilised crust (i.e. Fe-rich lithologies, sanukitoids), favourable structures (potential inversion of rift structures), and ‘late’ alkaline magmatism. In gold, Ni-Cu-PGE, and base metal mineral systems, the ability of magmas to access higher crustal levels unimpeded appears vital in regional prospectivity and is itself a function of crustal configuration.

Middle Paleozoic Rhyolite of the Gorny and Rudny Altai: Geochronology and Composition

The paper reports the results of geological, geochemical, and isotope–geochronological studies of subvolcanic rhyolites of NW Gorny Altai and Rudny Altai, which belong to the large Hercynian volcanic systems: Altai-Minusa and Altai–Salair, respectively. U–Pb zircon dating revealed two age groups: ~410–405 and 390–381 Ma. The isotope–geochemical characteristics of the rhyolites demonstrate high eNd(Т) from +2.6 to 6.0 with relatively young model ages T(DM) = 851–966 Ma in Rudny Altai and older model ages, up to 1266 Ma, in NW Gorny Altai. The rhyolites show transitional geochemical signatures between within-plate and island-arc felsic magmas. The results obtained are consistent with two-stage evolution of volcanism and its migration from the continent to the ocean.

Reconstructing subduction polarity through the geochemistry of mafic rocks in a Cambrian magmatic arc along the Gondwana margin (Órdenes Complex, NW Iberian Massif)

In the allochthonous complexes of the NW Iberian Massif, the Upper Units have been interpreted as a section of a peri-Gondwanan magmatic arc active from Middle Cambrian to Early Ordovician times. The main plutonic bodies intruding the arc metasediments are the Monte Castelo gabbronorites and the Corredoiras orthogneisses, which include minor metagabbronorite bodies, both dated at ca 500 Ma. The geochemical features of Monte Castelo metagabbronorites indicate a tholeiitic affinity, with negative Nb anomaly; its 143Nd/144Nd ratios are high (0.5143119–0.513019), whereas initial 87Sr/86Sr ratios are low (0.702562–0.703174), with positive eNdi values (+7.8 to +5.4). The geochemistry of Corredoiras metagabbronorites indicates a calc-alkaline affinity, also with negative Nb anomaly, low 143Nd/144Nd (0.512575–0.512436) and high initial 87Sr/86Sr (0.705082–0.706684), eNdi values ranging between −0.65 and +1.83. In the eNd versus age diagram, Monte Castelo samples show compositions equivalent to the contemporaneous depleted mantle. Corredoiras metagabbros have much lower eNdi values compared with Monte Castelo samples, with older model ages ranging between 1165 and 1291 Ma, suggesting contamination of the original mafic mantle-derived magmas with an older continental crust. These geochemical features can be linked to the setting of a mature volcanic arc, in which Monte Castelo metagabbros were located close to the trench, while Corredoiras metagabbros would be in a relatively distant position from the trench, thus indicating subduction polarity.

Flood-flipped boulders: In-situ cosmogenic nuclide modeling of flood deposits in the monsoon tropics of Australia

During the Quaternary, extreme floods along the Durack River, in the Kimberley, northern Australia, dislodged, transported, and stacked massive meter-sized boulders from the underlying bedrock channel floor. Field evidence identified a population of the boulders to have been overturned after detachment. We measured in situ cosmogenic 10 Be and 26 Al concentrations in six imbricated boulders to constrain the timing of flood events. We present a simple numerical model that simultaneously solves the expressions for the predicted nuclide concentrations from the exposed and hidden surfaces of a flipped boulder to calculate the time since it was overturned. The ability of the model to unequivocally discern whether a boulder was overturned depends on boulder thickness and the site-specific steady-state erosion rate. Of the six boulders sampled, our model successfully determined four finite flip ages, whereas the other two boulders indicated steady state and were either not flipped or flipped sufficiently long ago for the nuclide profile to have returned to steady state. While the two older model ages (ca. 150 ka and ca. 260 ka) are strongly sensitive to assumptions made for the local erosion rate correction, the two younger flip ages, 5.6 ± 1.0 ka and 10.3 ± 1.9 ka, are robust against such corrections. Early to mid-Holocene major floods have been recorded in other parts of northern Australia. We suggest that similar Holocene floods occurred in the Kimberley and that such extreme events may have been widespread in northern Australia in the late Quaternary. Our boulder-flip model can be applicable to similar deposits associated with other extreme events such as paleo-tsunamis.

Avalonian perspectives on Neoproterozoic paleogeography: Evidence from Sm-Nd isotope geochemistry and detrital zircon geochronology in SE New England, USA

Arc-related Northern Appalachian Avalonian terranes developed in the ocean surrounding Rodinia beginning around 760 Ma, so paleogeographic constraints on these terranes have implications for the margins of Rodinia itself in Neoproterozoic time. The first Sm-Nd isotopic data together with new detrital zircon ages from SE New England suggest that at least this segment of Avalonia contains rocks with contributions from crustal sources in Baltica, rather than Amazonia as commonly argued. Eight samples from 610 to 600 Ma Dedham, Dedham North, Milford, Fall River, Esmond, and Westwood granites in SE New England yield e Nd (600) values from −1.2 to +2.2 and depleted mantle model ages ranging from 2.2 Ga to 1.0 Ga. The younger model ages, reflecting largely juvenile magmatic components, are typical of other Avalonian terranes in Atlantic Canada. The oldest model ages can be produced by mixing juvenile mantle extractions with 10%-15% Baltic crust. The isotopic composition of Baltic crust in these calculations is proxied by Svecofennian metasedimentary rocks with detrital zircon suites comparable to those obtained from younger than 912 Ma quartzite of the Westboro Formation intruded by the Dedham Granite north of Boston (Massachusetts). An Avalonia-Baltica connection is also consistent with available paleomagnetic data placing both areas at moderate south paleolatitude between ca. 615 Ma and ca. 575 Ma during Ediacaran deposition of their respective Squantum-Gaskiers and Varanger glacial deposits.

The New England Batholith of eastern Australia: Evidence of silicic magma mixing from zircon 176Hf/ 177Hf ratios

Zircon Hf isotopic data from six granite supersuites recognised in the New England Batholith of eastern Australia indicate that magma mixing is present in all plutons. Apart from the S-type supersuites, inherited zircons are rare to absent, suggesting that initial melt temperatures were above zircon saturation for the M-, I- and A-type granites. In all supersuites, the range of zircon Hf model ages calculated relative to arc mantle varies greatly, with younger model ages somewhat less than the Permian and Triassic age of crystallisation, and older model ages indicating derivation from source rocks that may be as old as Neoproterozoic. The older model ages are consistent with late Precambrian rocks in the lower crust that may be elements of the Lachlan Fold Belt underlying the New England Fold Belt. Whole-rock δ 18O and εNd values, 87Sr/ 86Sr initial ratios (Sr i) and oxidation state (Fe 2O 3/FeO) are summarised for the five named supersuites, and for a previously unnamed group of Triassic plutons situated east of the main Batholith for which we propose the name Carrai supersuite. Supersuites with higher δ 18O, higher Sr i, lower δ 7Li and lower Fe 2O 3/FeO ratios indicate a metasedimentary component input. The pattern of zircon Hf isotopic variation reflects the other isotopic and geochemical indicators closely, the additional value being that it preserves a magmatic crystallisation record of Hf isotopic variation, that is, a measure of magma mixing from magma generation to final crystallisation. Using the above criteria, at least three distinct crustal components are considered necessary to explain the compositional and isotopic diversity within the Batholith. The crustal components indicated are: 1) a metasedimentary component of Carboniferous and Devonian age for the Hillgrove, Bundarra and Uralla supersuites; 2) a lower crustal K-rich (and Sr-rich) I-type component for the Moonbi, Carrai and possibly Uralla supersuites that could be as old as Proterozoic; and 3) a lower crustal K-poor I-type component of uncertain age for the Clarence River supersuite. In all supersuites, there is strong evidence of an M-type component based on the presence of zircons with high + εHf values.

Crustal evolution in the East African Orogen: a neodymium isotopic perspective

The East African Orogen (EAO) is one of Earth's great collision zones, where East and West Gondwana collided to form the supercontinent `Greater Gondwana' or `Pannotia' at the end of Neoproterozoic time. There is now sufficient Nd isotopic data for basement rocks of the EAO to yield a useful summary. A total of 449 samples were gleaned from the literature, recalculated to a common value for the La Jolla Nd standard, and entered in Excel spreadsheets. This data set was filtered to exclude samples with 147 Sm/ 144 Nd> 0.165, considered to yield unreliable model ages, leaving 413 suitable data. The crust of the Arabian–Nubian Shield, including Egypt east of the Nile, Sudan east of the Keraf suture, Sinai, Israel, Jordan, most of Arabia, Eritrea, and northern Ethiopia yields overwhelmingly Neoproterozoic model ages. Crust to the east, in the Afif terrane of Arabia, Yemen, Somalia, and Eastern Ethiopia yields much older model ages, averaging 2.1 Ga, demonstrating an abundance of reworked ancient crust. This provides an isotopic link with Madagascar (mean age of 2.4 Ga), which in pre-Jurassic reconstructions lies on the southern extension of this older, remobilized tract. Crust in the far southern extreme of the EAO in Tanzania also yields ancient model ages, averaging 2.3 Ga. The central EAO, in southern Ethiopia and Kenya, yields intermediate ages (mean 1.1–1.2 Ga), interpreted to indicate extensive mixing between Neoproterozoic mantle-derived melts and ancient crust. The Nd isotope data indicate that the northern EAO is composed of juvenile Neoproterozoic crust sandwiched between reworked older crust, whereas the EAO farther south is progressively dominated by ancient crust reworked during Neoproterozoic time. The distribution of juvenile and reworked ancient crust suggests that the most intense collision between East and West Gondwana occurred in the southern part of the EAO.

Neodymium and strontium isotope systematics of eclogite and websterite paragenesis inclusions from single diamonds, Finsch and Kimberley Pool, RSA

Garnet inclusions of the eclogite paragenesis from large single diamonds (1.8 to 4.2 carats) from the 120 Ma Finsch kimberlite have Nd model ages ranging from 1443 to 2408 Ma. Ages of 1443 ± 166 and 1657 ± 77 Ma in two samples with 87Sr86Sr = 0.7042 may represent the age of diamond formation and are within error of a 1580 ± 50 Ma Sm-Nd isochron age on clinopyroxene and garnet reported by Richardson et al. (1990). Older model ages in three other samples (1824, 2183, and 2408 Ma) correspond with unsupported radiogenic Sr (87Sr86Sr = 0.7095 to 0.7158) and anomalously high MnO (0.7 to 1.2 wt%.) suggestive of multistage evolution. Carbon isotopic compositions of the diamonds range from −3.6 to −7.7%., and do not correlate with isotopic or chemical features of the inclusions. Compared to world-wide compositional ranges of most eclogite paragenesis garnet inclusions in diamonds, four of five analyzed here are anomalously enriched in Fe and Mn, possibly a characteristic feature of the Finsch eclogitic diamond paragenesis suite.A large diamond with a websterite inclusion assemblage (clinopyroxene, orthopyroxene, garnet) from the Kimberley Pool (85 Ma emplacement age) has clinopyroxene with a Nd model age of 2111 ± 120 Ma. In contrast to the high Sm/Nd and radiogenic Nd of the eclogite garnets, the websterite cpx has low Sm/Nd and unradiogenic Nd more akin to that in garnet of harzburgite association diamonds from the Finsch kimberlite. However, neither the eclogite nor the websterite group diamonds can be related to the harzburgite suite. Analysis of inclusions from single diamonds eliminates the problems inherent in compositing possibly unrelated inclusions from large numbers of stones, but the large diamond size required to yield large inclusions may represent only a specific and possibly rare component of the general diamond population. The samples analyzed here may have been derived ultimately from subducted oceanic crust.