Crustal Ages Research Articles

In-situ Rb-Sr dating of celadonite from altered upper oceanic crust using laser ablation ICP-MS/MS

Interaction of seawater with oceanic lavas at low temperatures (10's of °C) away from mid-ocean ridges has been suggested to play a significant role in setting the major element composition of seawater. One of the abundant, void-filling, secondary minerals that forms during this process is the K-rich phyllosilicate celadonite. Here, we apply a recently developed in-situ Rb-Sr dating method, using laser ablation tandem quadrupole inductively coupled mass spectrometry (LA-ICP-MS/MS), to determine the ages of celadonites in upper ocean crust lavas. We present an analytical method and data reduction scheme for in-situ Rb-Sr dating using methyl-fluoride (CH3F) as a collision reaction gas. The 87Sr/86Sr ratios were quantified by standard sample bracketing using NIST SRM612. The 87Rb/86Sr ratios were calibrated using the MPI-DING standards ATHO-G, T1-G and StHs6/80-G. We analysed veins and amygdales in 32 samples recovered by seafloor drilling of Cretaceous and Cenozoic aged oceanic crust. The data shows that ~80% of celadonite formed within the first 20 Myr after the crust accreted, with no systematic variation in the duration of celadonite formation with crustal age, sedimentation history or depth in the crust. There is no resolvable difference in the ages of celadonite precipitated in veins and amygdales suggesting infilling of both interconnected and more isolated pores within the same time interval. The fraction of celadonite formed over any time interval after crustal accretion closely matches the fraction of hydrothermal heat loss suggesting that heat flow fraction provides a good proxy for chemical exchange in these systems. Overall, we conclude that alteration conditions within the first 20 Myr after crustal accretion must largely control the bulk chemical exchange between seawater and the upper oceanic crust.

What Controls Effective Elastic Thickness of the Lithosphere in the Pacific Ocean?

AbstractThe effective elastic thickness () of the lithosphere is a proxy for mechanical strength and can be used to constrain lithospheric rheology and understand how surface deformation relates to deep Earth processes. Here, we map variations over the Pacific Ocean from the inversion of the admittance between free‐air gravity anomaly and bathymetry data calculated using a continuous wavelet transform, taking both surface and subsurface loads into account. The Pacific lithosphere show ranging between 0 and 80 km with a mean of 13.5 km and a standard deviation of 12.3 km. We find that is generally poorly correlated with plate loading age, crustal age, heat flow and Curie point depth, except for relatively young (<60 Ma) and warm lithospheres. Most oceanic plateaus and seamounts show km, with the lowest values (<5 km) around active spreading centers. The highest estimates (>30 km) are found along subduction zones and around the Hawaiian‐Emperor Seamount Chain (HESC). Taken together, these results support a temperature control on for small loads and warm lithosphere through steady‐state creep processes, but strain hardening operating at large plastic strain and low temperature could explain high associated with large‐amplitude and long‐wavelength loads (subduction zones and the HESC) and should be incorporated in yield strength models of oceanic .

Effect of crustal porosity on lunar magma ocean solidification

The lunar ferroan anorthosites, formed by plagioclase flotation from the crystallization of the lunar magma ocean, have an age span of over ~ 200 Ma. However, previous thermal models predicted a much shorter time range. We propose that a much smaller thermal conductivity of anorthositic crust due to its high porosity may have delayed the solidification of the lunar magma ocean. Our thermal simulation results, using the thermal conductivity of porous lunar crust, show that crystallization of a 1000 km deep magma ocean could be prolonged to tens of millions of years, and up to 180 Ma under some extreme conditions. The porous crust alone can’t explain the large crustal age span, however. Other circumstances must be taken into consideration, such as a thick lunar soil.

Permian to Triassic tectonic evolution of the Alxa Tectonic Belt, NW China: Constraints from petrogenesis and geochronology of felsic intrusions

The Alxa Tectonic Belt (ATB) is located in the southern Central Asian Orogenic Belt (CAOB), and its tectonic affinity and evolutionary history related to the closure of the Paleo-Asian Ocean are not fully understood. To refine our understanding of these issues, an investigation that includes petrographic observations, whole-rock geochemistry, zircon UPb dating and Hf isotopic analyses was carried out on four felsic intrusions in the Zongnaishan–Shalazha Zone (ZSZ) and the Nuoergong–Honggueryulin Zone (NHZ) within this region. Despite their lithological differences, the four intrusions are identified as metaluminous, calc-alkaline to high-K calc-alkaline I-type granitoids that were derived from partial melting of heterogeneous crustal basement without significant fractional crystallization and crustal contamination. Zircon LA-ICP-MS UPb dating results suggest that they were emplaced from the late Permian to the early Triassic (i.e., 247.7 ± 0.7 Ma for the Shalazha granite, 253.2 ± 0.9 Ma for the Shalazha granodiorite, 253.3 ± 0.6 Ma for the Nuoergong granite, and 251.6 ± 0.9 Ma for the Nuoergong tonalite). Geochemically, these granitoids show typical signatures of arc magma, such as enrichments in Rb, Th, K, and Pb, depletions in Zr, Nb, and Ta, and low Sr/Y and (La/Yb)N values. On tectonic discriminant diagrams, they plot in volcanic arc to post-collisional fields, indicating a tectonic transition from oceanic subduction to closure in the late Permian to early Triassic. Interestingly, granitoids from the ZSZ have positive εHf(t) values ranging from +1 to +7.9 with young two-stage crustal ages (TDMC) of 781–1209 Ma, in sharp contrast to the extremely negative εHf(t) values (−15.8 to −8.7) and old TDMC ages (1831–2272 Ma) of the granitoids in the NHZ. In addition, the zircon Hf isotopes of the ZSZ and NHZ granitoids show similarities to those of the magmatic rocks in the southern CAOB and western North China Craton, respectively, implying that the northern and southern ATB have different tectonic affinities. Thus, we propose that the Paleo-Asian Ocean lasted subduction in the late Permian and may close in the early Triassic, with subsequent accretion of the northern juvenile exotic microcontinent (ZSZ) to the southern old crustal basement (NHZ).

Nd isotope mapping of the Frontenac Terrane in southwestern Grenville Province

Nd isotope analyses are presented for granitoid rocks from the western part of Frontenac Terrane in the Grenville Province of Ontario. The depleted mantle model (TDM) ages show no correlation with the silica content of the rocks, but instead correlate with geographical location, suggesting that the TDM ages are indicative of regional crustal formation age and do not result from mixing between sources with different provenance ages. Based on these observations, we identify a new crustal age boundary that follows the Desert Lake – Canoe Lake fault and the Rideau Lake fault and, hence, a new juvenile crustal block (Westport domain). This domain is identified as part of the ensimatic back-arc rift zone that formed the juvenile segment of the Central Metasedimentary Belt in Ontario. However, additional sampling along the Ottawa River suggests that the juvenile Westport domain does not extend into Quebec. Instead, a narrower ensialic rift zone is represented by the Marble domain in Quebec. Based on comparison with the Taupo volcanic zone and the northern Red Sea as modern analogues, we suggest that the transition from a wide ensimatic rift zone in Ontario to a narrow ensialic rift in Quebec was accommodated by transtensional motion along a zone of diffuse shear east of Ottawa.

Microbe-mineral biogeography from multi-year incubations in oceanic crust at North Pond, Mid-Atlantic Ridge.

Subseafloor oceanic crust is a vast yet poorly sampled habitat for life. Recent studies suggest that microbial composition in crustal habitats is variable in space and time, but biogeographic patterns are difficult to determine due to a paucity of data. To address this, we deployed hundreds of mineral colonization experiments at and below the seafloor for 4-6 years at North Pond, a borehole observatory network in cool (<10°C) and oxic oceanic crust on the western flank of the Mid-Atlantic Ridge. The overall community composition of mineral incubations reveals that colonization patterns are site dependent, with no correlation to mineral type. Only a few members of the Thioalkalispiraceae and Thioprofundaceae exhibited a mineral preference pattern, with generally higher abundance on metal sulphides compared to silicates, while taxa of the Gammaproteobacteria and Deltaproteobacteria were common in the colonization experiments. In comparison to datasets from other crustal habitats, broader biogeographic patterns of crustal communities emerge based on crustal habitat type (surface-attached communities versus fluid communities), redox environment and possibly crustal age. These comparisons suggest successional biogeography patterning that might be used as an indicator of how recently permeable pathways were established within oceanic crust.

The Mojiawan I-type granite of the Kangding Complex in the western Yangtze Block: new constraint on the Neoproterozoic magmatism and tectonic evolution of South China

ABSTRACT Studies of Neoproterozoic granites near the western margin of the Yangtze Block provide new evidence on the mechanism leading to the Neoproterozoic break-up of the Rodinia Supercontinent. This contribution focusses on the structural geology, geochemistry, zircon U-Pb geochronology, and Hf-O isotope systematics of the poorly documented Mojiawan Monzogranite in the Kangding Complex, which gives us better constraints on the break-up of the supercontinent. Zircon U-Pb data show that two samples of the Mojiawan Monzogranite were emplaced during 797 ± 13 Ma and 785 ± 11 Ma (ca. 791 Ma). The granitic samples are Na-rich, calc-alkaline, peraluminous with A/CNK values of 1.13 and 1.25, and their zircons have positive εHf(t) values between +4.28 and +9.79 with crustal model ages TDM2 of 1424 to 1079 Ma, indicative of a juvenile source. The granitic samples also have low Rb/Sr and Rb/Ba ratios and high δ18O values of 5.42‰–8.97‰, consistent with derivation from the partial melting of the lower to middle crust. Combined with the geochemistry of other granitic plutons along the western margin of the Yangtze Block, we propose that the region records rifting at ca. 800 Ma due to the action of a mantle super-plume leading to the break-up of the Rodinia Supercontinent.

SEAFLOOR MAPPING OF THE ATLANTIC OCEAN BY GMT: VISUALIZING MID ATLANTIC RIDGE SPREADING, SEDIMENT DISTRIBUTION AND TECTONIC DEVELOPMENT

The study presents the insights of the tectonic development and geological settings of the Atlantic Ocean supported by cartographic visualization in Generic Mapping Tools (GMT). The aim is to study geologic situation and trends in the tectonic development of the Mid-Atlantic Ridge and Atlantic Ocean seafloor. The objective is to find out impact of various factors (such as volcanic, tectonic, hydrothermal and sedimentary processes) that sculpt seafloor geomorphology, and correlation between early history of crust formation, geological processes and present submarine landforms. Other assignments in this work refer to mutual comparison of raster grids on sedimentation, topography, geology, seafloor fabric and highlighting similarities among the landforms and sediment thickness. Asymmetry in crustal accretion is explained by the tectonic history of the lithosphere formation. Correlation between plate subduction and development of the submarine landforms is explained by the Earth's crust extension resulting in formation of cracks, elongations, faults, rifts. Ocean seafloor geomorphology is shaped by a variety of factors that impact its form at different scales. These drivers (tectonic evolution, oceanic currents, hydrology, sedimentation) have effects on geomorphic landforms of the seafloor in context of historic geological development and during Quaternary. Technical part of this work was performed by GMT scripting toolset with all maps plotted in American polyconic projection. The results are received by overlay, cartographic analysis and synthesis of the multi-source geodata through mapping and interpreting grids (ETOPO1, EGM96, GlobSed, crustal age). This work contributes to expand the knowledge on geological and tectonic development of the Atlantic Ocean seabed in order to complete the view of its submarine geomorphology

A Global Data Set of Present‐Day Oceanic Crustal Age and Seafloor Spreading Parameters

AbstractWe present an updated oceanic crustal age grid and a set of complementary grids including spreading rate, asymmetry, direction, and obliquity. Our data set is based on a selected set of magnetic anomaly identifications and the plate tectonic model of Müller et al. (2019, 10.1029/2018TC005462). We find the mean age of oceanic crust is 64.2 Myr, slightly older than previous estimates, mainly due to the inclusion of pockets of Mesozoic aged crust in the Atlantic and Mediterranean and improvements to the Jurassic Pacific triangle. This older crust is partly compensated by additional Cenozoic‐aged back‐arc basin crust not included in previous models. The distribution of spreading modes based on area of preserved crust is relatively equal between slow (20–55 mm/yr) and fast (75–180 mm/yr) spreading systems at 33% and 39%, respectively. Crust transitional between fast and slow, or intermediate systems (55–75 mm/yr), cover 20% of the preserved ocean floor with much smaller proportions of crust formed at ultraslow (5%) and super‐fast (3%) spreading systems. Slow and intermediate spreading systems exhibit the most stable behavior in terms of spreading asymmetry and obliquity, with the widest distribution of obliquities occurring at ultraslow spreading systems, consistent with present‐day observations. Our confidence grid provides a complementary resource for nonexperts to identify those parts of the age grid that are least well constrained. Our grids in 6, 2, and 1 arc min resolution as well as our python workflow, isopolate, used to compute our data sets are freely available in online repositories and on the GPlates data portal.

Zircon U-Pb geochronology and Nd-Pb isotope geochemistry of Blue Ridge basement in the eastern Great Smoky Mountains, U.S.A.: Implications for the Proterozoic tectonic evolution of the southeastern Laurentian margin

The Mesoproterozoic to Paleozoic history of the southeastern Laurentian margin involved repeated collisional and accretionary tectonomagmatic events that reworked and recycled older continental crust of preceding events. The Great Smoky Mountains Basement Complex (GSMBC) within the southern Appalachian Blue Ridge exposes complexly deformed orthogneiss and paragneiss that preserve a record of Laurentian margin evolution from ca. 1.9 Ga to 450 Ma. The GSMBC consists primarily of: (1) 1.34 to 1.31 Ga (pre-Elzevirian) granodioritic orthogneiss and entrained mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (correlated with pre-Elzevirian crustal components in the Adirondack, Green Mountains, New Jersey Highlands, and French Broad massifs), (2) ca. l.15 to 1.05 Ga augen and granitic orthogneiss produced during Shawinigan and Ottawan phases of Grenville-age magmatism and metamorphism, respectively, and (3) paragneiss derived from protoliths with either Grenville-age (1.1–1.0 Ga) or post-Grenville (Neoproterozoic) depositional ages based on presence/absence of ca. 1.0 Ga metamorphic zircon and 1.9 to 1.1 Ga detrital zircon. All lithologies experienced Taconian metamorphism and variable migmatization. Pre-Ottawan paragneiss exhibits major detrital zircon ages modes at 2.0 to 1.6 and 1.4 to 1.3 Ga that require a component of older Proterozoic crust in the sediment source region. Detrital zircon grains in post-Ottawan paragneiss exhibits the full spectrum of Grenville-age modes that correlate with the five phases of Grenville magmatic/metamorphic events in eastern Laurentia. These paragneiss samples also contain scattered 750 to 600 Ma detrital zircon grains that constrain their maximum depositional age to late Neoproterozoic. The sedimentary protoliths of the latter paragneiss consist largely of detritus from exhumation of all Grenville crustal age components during post-orogenic exhumation and crustal extension leading up to Late Neoproterozoic breakup of Rodinia. Most zircon U-Pb age systematics exhibit variable discordance that can be attributed to disturbance of the U-Pb system and/or new zircon growth during either high-grade Ottawan (ca. 1.04 Ga) or Taconian (ca. 0.46 Ga) regional metamorphism and migmatization. Neodymium TDM model ages for granodioritic orthogneiss and paragneiss range from 1.8 to 1.6 Ga, indicating that the rocks were derived from recycling of Proterozoic crust (that is, they are not juvenile), consistent with the 1.9 to 1.6 Ga detrital zircon grains in pre-Ottawan paragneiss and with 1.8 to 1.7 Ga inherited zircon in some 1.33 Ga orthogneisses and a 1.35 Ga xenolith. Whole rock Pb isotope compositions of GSMBC rocks overlap the field of compositions characteristic of Amazonian crust and of other basement rocks from the south-central Appalachians. The Pb isotopes and geochronology in orthogneiss, mafic xenoliths, and pre-Ottawan paragneiss are consistent with a correlation of the GSMBC with the Mars Hill terrane within the French Broad massif and with the greater Grenvillian south-central Appalachian basement (SCAB) that is considered exotic to Laurentia, and transferred during Rodinian collision prior to ca. 1.2 Ga. Similarities in protolith ages and Pb isotopes point to the Paragua terrane of Amazonia (southwestern Brazil and eastern Bolivia) as a likely match for SCAB. Initial Amazonia-Laurentia collision occurred at ca. 1.35 to 1.32 Ga with final transfer of SCAB to Laurentia occurring after 1.20 Ga within the sinistral oblique collision zone between Laurentia and Amazonia defined by previous workers.

Petrogenesis of the peralkaline Dutsen Wai and Ropp complexes in the Nigerian younger granites: implications for crucial metal enrichments

ABSTRACT The Dutsen Wai and Ropp complexes are parts of the over 50 within-plate alkaline ring complexes in north-central Nigeria. These complexes consist mainly of A-type granites that were emplaced between 175 Ma (in Dutsen Wai) and 149 Ma (in Ropp). They are characterized by moderately negative zircon ԑHf(t) values (–9.8 to – 5.86) with Paleoproterozoic (1597–1884 Ma) crustal model ages (TDMc), suggesting the mantle-derived parental magmas have been substantially modified by assimilation of crustal materials. The granites exhibit elevated HFSEs (e.g., Nb and Zr) and LILE (e.g., Rb and REE) concentrations, with higher values in the peralkaline than in the aluminous granites, but are depleted in compatible elements (e.g., CaO, MgO, Ba and Sr). This is typical of highly differentiated granites associated with rare metal mineralization. The peralkaline plutons show REE patterns with strong negative Eu anomalies, significant M-type lanthanide tetrad effect (TE1,3 = 1.16–1.32), and non-CHarge and Radius Controlled (CHARAC) behaviour of some HFSEs. The positive correlation of Na2O vs. Nb (r2 = 0.92) and Nb vs. TE1,3 (r2 = 0.7), increasing degree of albitization and shift in eutectics of granite minimum towards Ab-Qtz sideline indicate that Na-rich fluid plays an important role in the complexation between F and other incompatible elements in open system processes. Fluorine also favoured the retention of these crucial metals in the peralkaline melt until late magmatic-stage during which the breakdown of the F-HFSEs and F-REE complexes was accompanied by crystallization of pyrochlore and REE-bearing accessory minerals. We proposed that the emplacement of these A-type granites was probably linked to the prevailing transtension regime before the fragmentation of Gondwana during the Mesozoic period which reactivated some deep-seated Pan-African transcurrent faults. This, in turn, caused pressure release and channelled fluid, which triggered partial melting of the lower part of the lithospheric mantle.

Petrogenesis and geodynamic setting of granitoids at Machangqing Cu–Mo (Au) deposit, western Yangtze craton, southwestern China: constraints from zircon U–Pb and molybdenite Re–Os geochronology, Lu–Hf isotopes, and geochemistry

The Machangqing Cu–Mo (Au) deposit is located in the central part of the Jinshajiang – Red River belt in the Sanjiang orogen, which lies across the Qiangtang terrane and western Yangtze craton, southwestern China. Zircon U–Pb dating constrains that the granite porphyry and porphyritic granite emplacements occurred at 35.92 ± 0.31 Ma and 34.92 ± 0.31 Ma, respectively. The Re–Os model ages of molybdenite are 34.94 ± 0.38 Ma. The new ages presented here, along with previously published data in the region, define a short duration of potassic magmatism and mineralization from 37 Ma to 34 Ma in the Jinshajiang – Red River belt. Zircon Ce4+/Ce3+ values of the porphyritic granite and granite porphyry vary from 50.32 to 1579.20 (averaging 481.01) and 33.18 to 1511.80 (averaging 452.98), respectively, and the log(fo2) values vary from –6.66 to −23.86 and −9.88 to −25.18, respectively, which plot within the range of the fayalite–magnetite–quartz buffer curve to the magnetite–hematite buffer curve, indicating an oxidized magma source, which may have facilitated the Cu–Au enrichment. Zircons from granitoids show εHf(t) values ranging from −0.75 to +2.33 and crustal model ages between 0.9 and 1.1 Ga. The features of Lu–Hf isotopes and wide range of Mg#, Cr, and Ni contents imply that the magmas of the Machangqing granitoids were probably derived from partial melting of juvenile lower crust and mixed with some mantle melts. Combined with the features of the Machangqing granitoids, the following evolution process are concluded. During the Cenozoic, the India–Asia continental collision triggered upwelling of hot asthenosphere and underplating of the thickened juvenile lower crust, which caused the formation of mafic and felsic magmas. Those magmas ascended, mixed, crystallized, and formed Machangqing ore-bearing granitoids in an intracontinental extension setting.

Discovery of widely available abyssal rock patches reveals overlooked habitat type and prompts rethinking deep-sea biodiversity

Habitat heterogeneity and species diversity are often linked. On the deep seafloor, sediment variability and hard-substrate availability influence geographic patterns of species richness and turnover. The assumption of a generally homogeneous, sedimented abyssal seafloor is at odds with the fact that the faunal diversity in some abyssal regions exceeds that of shallow-water environments. Here we show, using a ground-truthed analysis of multibeam sonar data, that the deep seafloor may be much rockier than previously assumed. A combination of bathymetry data, ruggedness, and backscatter from a trans-Atlantic corridor along the Vema Fracture Zone, covering crustal ages from 0 to 100 Ma, show rock exposures occurring at all crustal ages. Extrapolating to the whole Atlantic, over 260,000 km2 of rock habitats potentially occur along Atlantic fracture zones alone, significantly increasing our knowledge about abyssal habitat heterogeneity. This implies that sampling campaigns need to be considerably more sophisticated than at present to capture the full deep-sea habitat heterogeneity and biodiversity.

Petrogenesis of the meta-igneous rocks of the Sierra El Arco and coeval magmatic rocks in Baja California: Middle Jurassic-Early Cretaceous (166-140 Ma) island arc magmatism of NW México

ABSTRACT The Jurassic-Early Cretaceous Vizcaíno and Alisitos terranes are juxtaposed in the central part of the Baja California Peninsula, México. The southernmost outcrops of the Alisitos terrane are located in the Sierra El Arco, which consists of greenschist-facies meta-igneous rocks. Further east in the Calmallí area, amphibolite and paragneiss dominate. The meta-igneous rocks constitute a magmatic system ranging from peridotite to granodiorite, with associated basaltic to dacitic volcanic rocks. The suite displays negative εSrt (average −8.4, n = 23) and positive εNdt values (average +5.8, n = 23), suggesting mantle-derived melts with no discernible assimilation of continental crustal material. Geochemical and isotopic data of the amphibolite suggests a protolith similar to the meta-igneous rocks. Paragneiss samples display εSrt of +166 and εNdt around −7, with Nd crustal residence ages of 1.5 Ga. The isotopic data of the paragneiss suggest provenance from continental sources. Similar to Triassic-Middle Jurassic metasedimentary rocks from the Baja California Peninsula and western México. Middle Jurassic magmatism on Cedros Island (Vizcaíno terrane) and the Sierra El Arco (Alisitos terrane) define an εSrt vs. εNdt evolutionary trend that is mostly within the mantle array, suggesting a common origin within the same island arc subduction setting. Jurassic magmatism of continental affinity, defined by positive εSrt (average +39, n = 15) and negative εNdt values (average −4.2, n = 15), is widespread in the Baja California Peninsula and western México. Our Sr-Nd isotope mixing curves suggest that the assimilation of Late Triassic metasedimentary rocks or a lower crust could explain the isotopic composition of the continental affinity Jurassic magmatism.

Early Neoproterozoic granitic gneisses in the Amdo micro-continent, Tibet: petrogenesis and geodynamic implications

ABSTRACT The palaeogeographic locations of ancient continental blocks in the Tibetan Plateau, especially of micro-continental blocks during the Neoproterozoic supercontinent cycle, remain enigmatic. To clarify the petrogenesis of Neoproterozoic granitic gneisses and the early evolution of the Amdo micro-continent, we conducted an integrated study of the petrography, geochronology, and geochemistry of biotite monzonitic gneisses from the Precambrian metamorphic basement. The biotite monzonitic gneiss sample yielded a weighted mean zircon U–Pb age of 820 ± 11 Ma, indicating an early Neoproterozoic magmatic event. All gneiss samples show enrichment in light rare-earth elements and depletion in high-field-strength elements in N-MORB-normalized spider diagrams, which suggests that their precursor magma was generated in a subduction-related active continental margin. In addition, these rocks have mostly positive εHf(t) values (−1.0 to 7.0) and corresponding crustal model ages of 1.3 − 1.8 Ga. Zircon Hf isotopes and whole-rock geochemistry indicate that the gneisses were derived from partial melting of juvenile crust material. Recently published age spectra of detrital zircons from different blocks suggest that the Amdo micro-continent had a closer affinity to the Lhasa terrane than to the Qiangtang terrane prior to and during the break-up of the Rodinian supercontinent. However, the gneisses are geochemically distinct from back-arc magmatic rocks in the Lhasa terrane and similar to coeval Andean-type magmatic rocks in blocks that constituted the northwestern margin of Rodinia (including Madagascar, Seychelles, and Malani). This study therefore provides new constraints on the Neoproterozoic palaeogeographic location and evolution of the Amdo micro-continent.

Provenance of rifted continental crust at the nexus of East Gondwana breakup

The Bruce Rise, a prominent bathymetric feature offshore the Bunger Hills (East Antarctica), has basement of unknown crustal affinity and age. In East Gondwana, the Bruce Rise is reconstructed near the Naturaliste Plateau (offshore SW Australia) and microcontinents now submerged in the eastern Indian Ocean. New zircon U-Pb-Hf data from two c. 1150 Ma granite samples dredged from the eastern escarpment of the Bruce Rise demonstrate that these rocks are dominated by xenocrystic cargo. Mesoproterozoic xenocrystic cores show textural evidence of melt-mediated coupled dissolution-precipitation to form rim domains with apparent ages that skew towards c. 1150 Ma. The zircon U-Pb-Hf signatures from the xenocrysts in the Bruce Rise granites, and from c. 1230 to 1180 Ma felsic-intermediate granites and orthogneisses from the conjugate Naturaliste Plateau basement, suggest late Mesoproterozoic magmatism occurred at a transition in the regional tectonic architecture between a reworked Archean cratonic margin and Proterozoic juvenile crust. On the basis of plate reconstructions, exhumation and thinning of the basement to the Bruce Rise/Naturaliste Plateau occurred predominantly during rifting of India (prior to c. 120 Ma). Minor further thinning likely occurred leading up to the onset of seafloor spreading between Australia/Naturaliste Plateau and Antarctica/Bruce Rise (from c. 90 to 84 Ma).

Plata semirrefinada para los plateros de la Edad del Hierro en el Mediterráneo: un mecanismo para identificar la plata ibérica

Un fragmento de lingote de plata descubierto recientemente en el yacimiento fenicio de La Rebanadilla (Málaga) ha sido investigado mediante isotopos de plomo y análisis elemental.El lingote recuperado en los niveles inferiores del yacimiento, se fecha potencialmente entre fines del siglo XI y el IX a.C., situándose en cronología similar a algunos depósitos de hacksilver del área del levante mediterráneo.La edad de la corteza calculada a partir de los isotopos de plomo y la composición señalan que el lingote fue obtenido de minerales de Edad Hercínica con concentraciones altas de bismuto.Esta signatura es compatible con la de la Faja Pirítica del suroeste de la península ibérica, en particular con la de las antiguas minas de la zona de Riotinto.Se propone que la plata de este lingote fue obtenida de las jarositas argentíferas de Riotinto, donde sufrió solo un primer refinado mediante copelación, conservando un alto contenido en plomo antes de ser comercializado hacia La Rebanadilla, que pudo ser un lugar potencial para su transporte hacia los territorios fenicios en el Mediterráneo oriental. Las implicaciones del transporte de plata sin refinar son discutidas en relación al comercio de la plata por los fenicios durante la Edad del Hierro en el Mediterráneo y la dificultad de identificar la plata ibérica en el registro arqueológico.

Age of the Canada Basin, Arctic Ocean: Indications From High‐Resolution Magnetic Data

AbstractThe origin and history of the Amerasia basin are long‐running debates, which hinder our knowledge of the Mesozoic tectonic configurations and geodynamic processes in the Arctic. This lack of knowledge is due in part to the paucity of accurate magnetic data in the ice‐covered basin. Here, we identify the crustal age of the Canada Basin, a major part of the Amerasia Basin, through high‐resolution deep‐tow and sea‐surface magnetic data. The best fit of the four pairs of magnetic lineations revealed by the high‐resolution magnetic data is 139.5–128.6 Ma (or 142.4–132.8 Ma, depending on the geomagnetic polarity timescale). The crustal age provides crucial constraints on the evolution of the circum‐Arctic tectonic features and generally supports the hypothesis that the opening of the Amerasia Basin is related to the subductions during the closure of the South Anyui Ocean.

A Palaeoproterozoic diamond-bearing lithospheric mantle root beneath the Archean Sask Craton, Canada

The recently recognised Sask Craton, a small terrane with Archean (3.3–2.5 Ga) crustal ages, is enclosed in the Paleoproterozoic (1.9–1.8 Ga) Trans Hudson Orogen (THO). Only limited research has been conducted on this craton, yet it hosts major diamond deposits within the Cretaceous (~106 to ~95 Ma) Fort à la Corne (FALC) Kimberlite Field. This study describes major, trace and platinum group element data, as well as osmium isotopic data from peridotitic mantle xenoliths (n = 26) from the Star and Orion South kimberlites.The garnet-bearing lithospheric mantle is dominated by moderately depleted lherzolite. Equilibration pressures and temperatures (2.7 to 5.5 GPa and 840 to 1250 °C) for these garnet peridotites define a cool geotherm indicative of a 210 km thick lithosphere, similar to other cratons worldwide. Many of the peridotite xenoliths show the major and trace element signatures of carbonatitic and kimberlitic melt metasomatism. The Re–Os isotopic data yield TRD (time of Re-depletion) model ages, which provide minimum estimates for the timing of melt depletion, ranging from 2.4 to 0.3 Ga, with a main mode spanning from 2.4 to 1.7 Ga. No Archean ages were recorded. This finding and the complex nature of events affecting this terrane from the Archean through the Palaeoproterozoic provide evidence that the majority of the lithospheric mantle was depleted and stabilised in the Palaeoproterozoic, significantly later than the Archean crust. The timing of the dominant lithosphere formation is linked to rifting (~2.2 Ga - 2.0 Ga), and subsequent collision (1.9–1.8 Ga) of the Superior and Hearne craton during the Wilson cycle of the Trans Hudson Orogen.

Age and geological context of the Barby Formation, a key volcanic unit in the Mesoproterozoic Sinclair Supergroup of southern Namibia

Abstract Volcanic and sedimentary rocks of the Sinclair Supergroup occur in the Konkiep Terrane of Southern Namibia. Three volcanic and sedimentary cycles are recognised. In this work we describe and date volcanic rocks of the Barby Formation, a key unit in the Sinclair area. The coeval Spes Bona Syenite and the Tiras Granite Gneiss are also described and dated. The rock types in the Barby Formation are rhyolites, basaltic trachyandesites, trachybasalts and trachydacites as well as volcanoclastic rocks. The rocks are largely undeformed and partly altered by deuteric and contact metamorphic processes but not regionally metamorphosed. Our samples represent both the calc-alkaline and alkaline trends documented in previous work. U-Pb ion probe and laser ablation inductively coupled plasma (LA-ICP) multicollector mass spectrometer Lu-Hf microbeam analyses were made of zircon and baddeleyite grains from four samples. A felsic tuff sample from the base of the Barby Formation has a 207Pb/206Pb zircon age of 1214 ± 5 Ma (2σ). A rhomb porphyry sample from the top of an 8.5 km-thick stratigraphic section gives a 207Pb/206Pb baddeleyite age of 1217 ± 2 Ma. The Spes Bona Syenite which intrudes the top of the Barby Formation has a 207Pb/206Pb baddeleyite age of 1217 ± 3 Ma and an indistinguishable LA-ICP collision cell mass spectrometer Rb-Sr biotite isochron age of 1238 ± 20 Ma, showing that there was no &amp;gt;350°C regional metamorphic event. Multi-element diagrams for the calc-alkaline samples show a dominant signature of reworked crust which is superimposed on a possible subduction signature. However the alkaline samples contain clear subduction signatures which are not seen in the underlying 1.37 Ga Kumbis rhyolite. The Barby Formation samples and coeval Spes Bona Syenite have Lu-Hf crustal residence ages between 1682 and 1873 Ma, suggesting that both of these units formed from a mixture of juvenile mantle-derived and older crustal material. The Barby Formation is considered to have originated due to a subduction event which took place during the assembly of the Rodinia supercontinent. The duration of the Barby magmatic episode is constrained to a maximum 9 m.y. period between 1219 and 1210 Ma, and during this period the Konkiep Terrane was an active continental margin. The 1204 ± 9 Ma Tiras Granite Gneiss is slightly younger than the Barby Formation and intruded across the Lord Hills Shear Zone, which is the suture between the hardly metamorphosed Konkiep Terrane and the highly metamorphosed Grunau Terrane of the Namaqua-Natal Province. Its intrusion reflects the end of subduction-related volcanism, due to the collision of Namaqua terranes with the Konkiep Terrane.