Hf Isotope Ratios Research Articles

Progressive Changes in Magma Transport at the Active Serreta Ridge, Azores

AbstractVolcanism in the Eastern Azores Plateau occurs at large central volcanoes and along subaerial and submarine fissure zones, resulting from a mantle melting anomaly combined with transtensional stresses. Volcanic structures are aligned WNW‐ESE and NW‐SE, reflecting two tectonic stress fields that control the direction of lateral melt transport. Terceira Island is influenced by both stress fields, dividing the island into an eastern and western part. Several submarine volcanic ridges with variable orientations are located west of Santa Bárbara, the youngest central volcano on Terceira. Major, trace element and Sr‐Nd‐Pb‐Hf isotope compositions from submarine lavas and glasses, in part associated with the 1998–2001 Serreta Ridge eruption, vary between different lava suites, suggesting a formation from different mantle sources. Submarine lavas are more primitive than those from Santa Bárbara volcano, indicating that they are not laterally connected with the shallow magma reservoir located in 2‐ to 5‐km depth beneath the central volcano. Mineral thermobarometric data suggest that the older Serreta magmas were laterally transported at depths >5 km from Santa Bárbara predominantly in WNW direction. We propose that lithospheric extension controls magma transport from the central volcano to Serreta Ridge. The youngest Serreta lavas differ from Santa Bárbara and other submarine ridges in having less radiogenic Pb and higher Hf isotope ratios representing a new magma pulse ascending from the mantle. We conclude that lateral magma transport and the morphology of volcanic ridges are controlled by tectonic stresses in the lithosphere, whereas vertical melt transport is initiated by processes in the mantle.

Crustal maturation through chemical weathering and crustal recycling revealed by Hf–O–B isotopes

Juvenile continental crust is dominantly formed at intra-oceanic arcs via subduction zone magmatism. However, it remains unclear how basaltic oceanic arcs convert to granodioritic continental compositions. Here we present zircon U–Pb and Hf–O isotope data, whole-rock B isotope compositions, combined with a synthesis of over 1100 geochemical analyses from magmatic rocks that span a wide range of emplacement ages (∼490–270 Ma), from the Junggar intra-oceanic arc of the southern Central Asian Orogenic Belt (CAOB). Geochemical data show that the Junggar evolved from a juvenile oceanic arc composition (low SiO2, K2O and Rb, and high MgO contents) to one closer to continental crust (high SiO2, K2O and Rb, and low MgO contents) at ca. 300 Ma. All samples show very high and uniform Hf isotope ratios with εHf(t) values from +10.6 to +14.4. Zircon δ18O values and whole-rock δ11B values, however, are highly variable. The Silurian – Carboniferous (pre-300 Ma) rocks have distinctly lower zircon δ18O values ranging from 5.16 to 6.72‰ with an average of 5.78‰, and high δ11B values (−7.5–+12.2‰), suggesting that they were derived from the asthenospheric mantle wedge, which supports the genesis of primitive intra-oceanic crust during that time interval. In contrast, the Early Permian (post-300 Ma) rocks display much higher zircon δ18O values (8.24 to 10.29‰) and lower δ11B values (−9.0 to −12.2‰), combined with the presence of Carboniferous inherited zircons, which requires a source component comprising young, weathered, volcanogenic sediments from the Carboniferous Junggar intra-oceanic arc. The Early Permian rocks were produced by low degrees of partial melting of all sources (ca. 10%) and element and Hf–O–B isotope mixing calculations indicate a contribution of more than ∼50% from weathered sediments in those sources. Thus, the evolution of the Junggar segment from a primitive basaltic intra-oceanic arc toward a young continent was related to the recycling of crustal residues of chemical weathering and the younger magmatism was generated by crustal melting of these weathering products after they were buried to lower crustal depths. Our study highlights that an intra-oceanic arc's own chemical weathering history promotes its transformation into continental crust during collisional events and clarifies the relationship between continental crust formation and intra-oceanic arcs through time.

In situ trace element and Sm-Nd isotope analysis of accessory minerals in an Eoarchean tonalitic gneiss from Greenland: Implications for Hf and Nd isotope decoupling in Earth's ancient rocks

Some of the Earth's oldest preserved continental crustal rocks, from southern West Greenland, contain contradictory radiogenic isotope signatures, whereby Hf isotope ratios are chondritic but Nd isotope ratios are distinctly super-chondritic. Models to explain this discrepancy are speculative and variously invoke deep magma ocean crystallisation, HfNd decoupling in subduction zones, or metamorphic disturbance during younger thermal events. Determining the cause of this discrepancy is essential for understanding Eoarchean crust-mantle differentiation. We employ, for the first time, micro-analysis of REE-rich accessory minerals to shed light on the Nd isotope evolution of a key tonalitic gneiss sample from southern West Greenland that displays the apparent HfNd isotope decoupling. The results show that the Sm-Nd isotope system was homogenized during a metamorphic event at ca. 2690 Ma. We suggest that metamorphic reactions involving consumption and re-crystallisation of REE-bearing phases were accompanied by LREE element mobility and the loss of unradiogenic Nd, shifting the bulk rock composition to a more radiogenic Nd isotope value. Our study provides the first direct evidence that the anomalous Nd isotope signatures in some Eoarchean gneisses are artefacts of the disturbance of the Sm-Nd isotope system, and not due to extensive differentiation of the bulk silicate Earth by magma ocean crystallisation or continental crust formation.

Discrepancy between bulk-rock and zircon Hf isotopes accompanying Nd-Hf isotope decoupling

Abstract Zircon is an important accessory mineral for studying the crust-mantle interaction and crustal growth through time because zircon crystals not only allow precise dating but also record initial Hf isotope ratios of the host magma. Our study on a suite of gabbronorite, mafic diorite cumulate, diorite and granite from the Kekeli Batholith in the North Qilian Orogenic Belt, Northern Tibetan Plateau, shows (1) a significant Hf isotope discrepancy between zircon and their bulk rocks; and (2) bulk-rock Nd-Hf isotope decoupling. These observations thus demonstrate that zircons do not always capture the full history of magmatic system. The significant positive correlation between bulk-rock Hf isotope ratios and TiO2 content (R2 = 0.94) indicates that Ti-rich minerals (e.g., ilmenite, amphibole) are likely important Hf hosts. The early-formed Ti-rich minerals possibly record different Hf isotopes from those of zircons crystallized subsequently, thus causing discrepancy between zircon and bulk-rocks and leading to bulk-rock Nd-Hf isotope decoupling. Correlations between bulk-rock TiO2 content, Mg isotopes and Hf isotopes indicate a mixing process, with granite and gabbronorite representing two compositional endmembers. Because Ti minerals have higher crystallization temperatures than zircons, when the mixing melts have contrasting isotopes (or from heterogeneous sources/have a strong crustal contamination), the bulk-rock and zircon Hf isotope discrepancies reflect mineral crystallization sequence during mafic and felsic magma mixing. It is thus imperative to consider early formed minerals such as Ti-rich minerals and the bulk rock composition, not just zircons, when using Hf isotopes to track melt evolution and precisely constrain mantle contribution to granitoid petrogenesis.

Distinguishing Plume and Metasomatized Lithospheric Mantle Contributions to Post-Flood Basalt Volcanism on the Southeastern Ethiopian Plateau

Abstract Magmatism in the East African Rift System (EARS) contains a spatial and temporal record of changing contributions from the Afar mantle plume, anciently metasomatized lithosphere, the upper mantle and the continental crust. A full understanding of this record requires characterizing volcanic products both within the rift valley and on its flanks. In this study, three suites of mafic, transitional to alkaline lavas, were collected over a northeast-southwest distance of ∼150 km along the southeastern Ethiopian Plateau, adjacent to the Main Ethiopian Rift. Specifically, late Oligocene to Quaternary mafic lavas were collected from Chiro, Debre Sahil and the Bale Mountains. New major element, trace element, 40Ar/39Ar ages and isotopic results (Sr, Nd, Pb, Hf, Os, He) show spatial and temporal variation in the lavas caused by dynamical changes in the source of volcanism during the evolution of the EARS. The trace element compositions of Oligocene and Miocene Chiro lavas indicate derivation from mildly depleted and nominally anhydrous lithospheric mantle, with variable inputs from the crust. Further south, Miocene Debre Sahil and alkaline Bale Mountains lavas have enriched incompatible trace element ratios (e.g. Ba/Nb = 12–43, La/SmN = 3·1–4·9, Tb/YbN = 1·6–2·4). Additionally, their 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf and 206Pb/204Pb values trend toward a radiogenic Pb (HIMU) component. Radiogenic 187Os/188Os in these lavas correlates positively with 206Pb/204Pb and trace element indicators consistent with ancient metasomatic enrichment of their mantle source. In contrast, transitional Miocene Bale Mountains lavas have lower incompatible trace element abundances, less enriched trace element ratios (Ba/Nb ∼7, La/SmN = 2·3–2·5) and less radiogenic isotopic signatures that originate from melting garnet-bearing, anhydrous lithospheric mantle (Tb/YbN = 2·5–2·9). Pliocene and Quaternary Bale Mountains basaltic lavas are chemically and isotopically similar to Main Ethiopian Rift lavas. Trace element and isotopic indicators in both of these suites denote an amphibole-bearing source distinct from that sampled by the older Bale Mountains lavas. Isotopically, Pliocene and Quaternary Bale lavas have notably less radiogenic Sr–Nd–Pb–Hf isotopic ratios. Quaternary Bale Mountains lavas have the strongest mantle plume contribution (3He/4He = 12·1–12·5 RA), while other Bale Mountains, Debre Sahil and Chiro lavas were derived dominantly by melting of lithospheric or upper mantle sources (3He/4He = 5·1–9·1 RA). A multi-stage, regional-scale model of metasomatism and partial melting accounts for the spatial and temporal variations on the southeastern Ethiopian Plateau. Early Debre Sahil and alkaline Bale Mountains mafic lavas are melts derived from Pan-African lithosphere containing amphibole-bearing metasomes, while later transitional Bale basalts are melts of lithosphere containing anhydrous, clinopyroxene-rich veins. These ancient metasomatized domains were eventually removed through preferential melting, potentially during thermal erosion of the lithosphere or lithospheric foundering. Pliocene and Quaternary Bale Mountains lavas erupted after tectonic extension progressed throughout Ethiopia and was accompanied by increased plume influence on the volcanic products.

Doubling Sensitivity in Multicollector ICPMS Using High-Efficiency, Rapid Response Laser Ablation Technology

The introduction of rapid response laser ablation cells and sample transport technologies to laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) has enabled signal pulse durations for a single laser ablation shot of less than 10 ms. These developments have resulted in marked improvements in analytical throughput, resolution, and sensitivity vital for the generation of large, highly spatially resolved elemental maps. The focus on mapping, particularly bioimaging, has obscured the possibility of applying the sensitivity advantage of rapid response technologies to other LA-ICPMS applications, such as high-precision isotope ratio analysis on multicollector (MC) ICPMS. In this work a commercially available rapid response sample transport system and a conventional configuration were compared for LA-MC-ICPMS analysis. Ablation of known reference materials demonstrated "sensitivity" or sample ion yield of 7-9% using the rapid response sample transport system, more than double that for the conventional setup. This increase in efficiency was demonstrated to improve precision for the Pb isotope ratio analysis of the MPI-DING reference glasses and improve the spatial resolution of Hf isotope ratio analysis of reference zircons.

Upper Cretaceous trench deposits of the Neo-Tethyan subduction zone: Jiachala Formation from Yarlung Zangbo suture zone in Tibet, China

The history of convergence between the India and the Asia plates, and of their subsequent collision which triggered the Himalayan orogeny is recorded in the Yarlung Zangbo suture zone. Exposed along the southern side of the suture, turbidites of the the Jiachala Formation fed largely from the Gangdese arc have long been considered as post-collisional foreland-basin deposits based on the reported occurrence of Paleocene-early Eocene dinoflagellate cysts and pollen assemblages. Because magmatic activity in the Gangdese arc continued through the Late Cretaceous and Paleogene, this scenario is incompatible with U-Pb ages of detrital zircons invariably older than the latest Cretaceous. To solve this conundrum, we carried out detailed stratigraphic, sedimentological, paleontological, and provenance analyses in the Gyangze and Sajia areas of southern Tibet, China. The Jiachala Formation consists of submarine fan deposits that lie in fault contact with the Zongzhuo Formation. Sandstone petrography together with U-Pb ages and Hf isotope ratios of detrital zircons indicate provenance from the Gangdese arc and central Lhasa terrane. Well preserved pollen or dinoflagellate cysts microfossils were not found in spite of careful research, and the youngest age obtained from zircon grain was ~ 84 Ma. Based on sedimentary facies, provenance analysis and tectonic position, we suggest that the Jiachala Formation was deposited during the Late Cretaceous (~88–84 Ma) in the trench formed along the southern edge of Asia during subduction of Neo-Tethyan oceanic lithosphere.

Magmatic pulses of the Tatun Volcano Group, northern Taiwan, revisited: Constraints from zircon U-Pb ages and Hf isotopes

The initiation and duration of magmatism in the Tatun Volcano Group (TVG), likely resulting from extensional collapse of the northern Taiwan mountain belt, remains an important issue of debates in the Taiwan orogeny. Here we report a detailed study of U-Pb ages determined by SIMS and LA-ICPMS, together with Hf isotopic data, of zircon separates from a total of 22 extrusive rocks from the TVG lava flow and additional two andesites from Keelungyu (KLY), a volcanic islet off NE Taiwan. Magmatic zircons from the TVG are characterized by uniform and high Hf isotopic ratios, with εHf(T) mostly from +20 to +10. Their overall 206Pb/238U ages suggest that the TVG magmatism was initiated ∼3 Ma but more active ≤0.8 Ma with a flare up ≤0.35 Ma. Magmatic zircons from the two KLY samples reveal identical 206Pb/238U ages (0.41 Ma), coupled with Hf isotopes [εHf(T): +25 to +9] comparable to those of the TVG. Thus, KLY is proposed as part of the TVG. Consequently, we argue that the main body of the TVG formed ≤0.8 Ma by post-collisional volcanism, but the mechanism resulting in the magmatic episodes ≥1.0 Ma remains obscure due to the scarcity of rock exposures.

Petrogenesis of Mid-Eocene granites in South Sakhalin, Russian Far East: Juvenile crustal growth and comparison with granitic magmatism in Hokkaido and Sikhote-Alin

This study presents new whole-rock geochemical and Sr-Nd isotopic data, together with zircon U-Pb ages and Hf isotope ratios, of granitic rocks and associated dikes from the Okhotsk and Aniva plutons in South Sakhalin. The two plutons emplaced in the Mid-Eocene show markedly contrasting geochemical characteristics. The Okhotsk granites (44–42 Ma) are mildly peraluminous (A/CNK = 1.02–1.08), ferroan and alkali-calcic, and have transitional I- and A-type granite features. This granite suite contains rare inherited zircons. The Aniva granites (40 Ma) are peraluminous (A/CNK = 1.08–1.21), magnesian and calc-alkaline, typical of S-type features. This suite, essentially biotite-cordierite bearing, has abundant inherited zircons dated from 48 to 2446 Ma. Whole-rock Sr-Nd and zircon Hf isotopic data indicate that the genesis of the two plutons involved partial melting of mixed sources with dominant juvenile crustal components (newly accreted accretionary complexes or newly underplated basaltic materials) and magma mixing between underplated mantle-derived magmas and crustal partial melts. The Okhotsk granites have εNd (T) values of +3.1 to +3.7, initial 87Sr/86Sr ratios of 0.7047–0.7048, and zircon εHf (T) values of +11 to +16. The Aniva granites have εNd (T) values of +0.5 to +0.9 and initial 87Sr/86Sr ratios of 0.7052 to 0.7055, and zircon εHf (T) values of +4 to +12. According to Sr-Nd and Nd-Hf isotopic mixing calculations, the proportion of the juvenile component (=mantle-derived materials) is estimated to be >70%, and that of the upper crust component is less than 30% in the sources of the Okhotsk and Aniva plutons. We further argue that, based on Nd and Hf isotopic constraints, the Okhotsk and Aniva granites are correlated to the Mid-Eocene granites in the western and eastern parts, respectively, of the Hidaka terrane in Hokkaido. From the Sikhote-Alin area to the Sakhalin and Hokkaido Islands, the increase in juvenile sources and the decrease in recycled crustal materials are exhibited in the Nd isotopic signatures of the granitoids from early Cretaceous to Miocene. The granitic magmatism in South Sakhalin and Hokkaido may be the most prominent example of juvenile crustal growth among these areas.

Isotopic constraints on contamination processes in the Tonian Goiás Stratiform Complex

The Tonian Goiás Stratiform Complex (TGSC, Goiás, central Brazil), is one of the largest mafic-ultramafic layered complexes in the world, emplaced during the geotectonic events that led to the Gondwana accretion. In this study, we present trace elements and in-situ U/Pb-Lu-Hf analyses of zircons and 87Sr/86Sr ratios of plagioclases from anorthosites and gabbros of the TGSC.Although formed by three isolated bodies (Cana Brava, Niquelândia and Barro Alto), and characterized by a Lower and Upper Sequence (LS and US), our new U/Pb zircon data confirm recent geochemical, geochronological, and structural evidences that the TGSC has originated from a single intrusive body in the Neoproterozoic.New Hf and Sr isotope ratios construe a complex contamination history for the TGSC, with different geochemical signatures in the two sequences. The low Hf and high Sr isotope ratios of the Lower Sequence (εHf(t) from −4.2 down to −27.5; 87Sr/86Sr = 0.706605–0.729226), suggest the presence of a crustal component and are consistent with contamination from meta-pelitic and calc-silicate rocks found as xenoliths within the Sequence. The more radiogenic Hf isotope ratios and low Sr isotope composition of the Upper Sequence (εHf(t) from 11.3 down to −8.4; 87Sr/86Sr = 0.702368–0.702452), suggest a contamination from mantle-derived metabasalts in agreement with the occurrences of amphibolite xenoliths in the US stratigraphy. The differential contamination of the two sequences is explained by the intrusion of the TGSC in a stratified crust dominated by metasedimentary rocks in its deeper part and metavolcanics at shallower levels. Moreover, the differential thermal gradient in the two crystallizing sequences might have contributed to the preservation and recrystallization of inherited zircon grains in the US and total dissolution or magmatic overgrowth of the LS zircons via melt/rock reaction processes.

Deconvolving the pre-Himalayan Indian margin – Tales of crustal growth and destruction

The metamorphic core of the Himalaya is composed of Indian cratonic rocks with two distinct crustal affinities that are defined by radiogenic isotopic geochemistry and detrital zircon age spectra. One is derived predominantly from the Paleoproterozoic and Archean rocks of the Indian cratonic interior and is either represented as metamorphosed sedimentary rocks of the Lesser Himalayan Sequence (LHS) or as slices of the distal cratonic margin. The other is the Greater Himalayan Sequence (GHS) whose provenance is less clear and has an enigmatic affinity. Here we present new detrital zircon Hf analyses from LHS and GHS samples spanning over 1000 km along the orogen that respectively show a striking similarity in age spectra and Hf isotope ratios. Within the GHS, the zircon age populations at 2800–2500 Ma, 1800 Ma, 1000 Ma and 500 Ma can be ascribed to various Gondwanan source regions; however, a pervasive and dominant Tonianage population (∼860–800 Ma) with a variably enriched radiogenic Hf isotope signature (εHf = 10 to −20) has not been identified from Gondwana or peripheral accreted terranes. We suggest this detrital zircon age population was derived from a crustal province that was subsequently removed by tectonic erosion. Substantial geologic evidence exists from previous studies across the Himalaya supporting the Cambro-Ordovician Kurgiakh Orogeny. We propose the tectonic removal of Tonian lithosphere occurred prior to or during this Cambro-Ordovician episode of orogenesis in a similar scenario as is seen in the modern Andean and Indonesian orogenies, wherein tectonic processes have removed significant portions of the continental lithosphere in a relatively short amount of time. This model described herein of the pre-Himalayan northern margin of Greater India highlights the paucity of the geologic record associated with the growth of continental crust. Although the continental crust is the archive of Earth history, it is vital to recognize the ways in which preservation bias and destruction of continental crust informs geologic models.

Rates of generation and growth of the continental crust

Models for when and how the continental crust was formed are constrained by estimates in the rates of crustal growth. The record of events preserved in the continental crust is heterogeneous in time with distinctive peaks and troughs of ages for igneous crystallisation, metamorphism, continental margins and mineralisation. For the most part these are global signatures, and the peaks of ages tend to be associated with periods of increased reworking of pre-existing crust, reflected in the Hf isotope ratios of zircons and their elevated oxygen isotope ratios. Increased crustal reworking is attributed to periods of crustal thickening associated with compressional tectonics and the development of supercontinents. Magma types similar to those from recent within-plate and subduction related settings appear to have been generated in different areas at broadly similar times before ∼3.0 Ga. It can be difficult to put the results of such detailed case studies into a more global context, but one approach is to consider when plate tectonics became the dominant mechanism involved in the generation of juvenile continental crust. The development of crustal growth models for the continental crust are discussed, and a number of models based on different data sets indicate that 65%–70% of the present volume of the continental crust was generated by 3 Ga. Such estimates may represent minimum values, but since ∼3 Ga there has been a reduction in the rates of growth of the continental crust. This reduction is linked to an increase in the rates at which continental crust is recycled back into the mantle, and not to a reduction in the rates at which continental crust was generated. Plate tectonics results in both the generation of new crust and its destruction along destructive plate margins. Thus, the reduction in the rate of continental crustal growth at ∼3 Ga is taken to reflect the period in which plate tectonics became the dominant mechanism by which new continental crust was generated.

Implications of Eocene-age Philippine Sea and forearc basalts for initiation and early history of the Izu-Bonin-Mariana arc

Whole-rock isotope ratio (Hf, Nd, Pb, Sr) and trace element data for basement rocks at ocean drilling Sites U1438, 1201 and 447 immediately west of the KPR (Kyushu-Palau Ridge) are compared to those of FAB (forearc basalts) previously interpreted to be the initial products of IBM subduction volcanism. West-of-KPR basement basalts (drill sites U1438, 1201, 447) and FAB occupy the same Hf-Nd and Pb-Pb isotopic space and share distinctive source characteristics with εHf mostly > 16.5 and up to εHf = 19.8, which is more radiogenic than most Indian mid-ocean ridge basalts (MORB). Lead isotopic ratios are depleted, with 206Pb/204Pb = 17.8–18.8 accompanying relatively high 208Pb/204Pb, indicating an Indian-MORB source unlike that of West Philippine Basin plume basalts. Some Sr isotopes show affects of seawater alteration, but samples with 87Sr/86Sr < 0.7034 and εNd > 8.0 appear to preserve magmatic compositions and also indicate a common source for west-of-KPR basement and FAB. Trace element ratios resistant to seawater alteration (La/Yb, Lu/Hf, Zr/Nb, Sm/Nd) in west-of-KPR basement are generally more depleted than normal MORB and so also appear similar to FAB. At Site U1438, only andesite sills intruding sedimentary rocks overlying the basement have subduction-influenced geochemical characteristics (εNd ∼ 6.6, εHf ∼ 13.8, La/Yb > 2.5, Nd/Hf ∼ 9). The key characteristic that unites drill site basement rocks west of KPR and FAB is the nature of their source, which is more depleted in lithophile trace elements than average MORB but with Hf, Nd, and Pb isotope ratios that are common in MORB. The lithophile element-depleted nature of FAB has been linked to initiation of IBM subduction in the Eocene, but Sm-Nd model ages and errorchron relationships in Site U1438 basement indicate that the depleted character of the rocks is a regional characteristic that was produced well prior to the time of subduction initiation and persists today in the source of modern IBM arc volcanic rocks with Sm/Nd > 0.34 and εNd ∼ 9.0.

Tectonic affinities of the accreted basalts in southern Taiwan

Tectonic affinities of accreted basalts provide constraints on mass transport in convergent boundaries, improving our understandings on the evolution of regional geology. In this study, nineteen accreted basalts from the southernmost tip of Taiwan Island, which is on the convergent boundary between the Eurasian and Philippine Sea Plates, were analyzed for element concentrations as well as Sr, Nd, Hf, and Pb isotope ratios to investigate their tectonic affinities. All the samples contain > 3% LOI, reflecting post-magmatic alteration. LOI and Nb variation diagrams together with comparisons to oceanic basalt compositions indicated that the concentrations of most major elements and Rb, Sr, and Ba were modified by post-magmatic processes to varying extents, while P2O5, REE and HFSE remained immobile. Although some samples show Pb loss, most samples have Pb concentrations not affected by post-magmatic processes. Isotope ratios of Pb, Nd and Hf, generally reflect the mantle source characteristics. The εNd–εHf relationship and trace element abundance ratios indicated that the LREE-depleted samples were mostly scraped off the subducting South China Sea floor, reflecting the volumetric dominance of N-MORB on ocean floors. The overriding Philippine Sea Plate contributed both N-MORB and E-MORB to the accretionary prism. The tectonic affinities of the LREE-enriched samples, however, could not be unambiguously determined for the large geochemical variability of OIB from both subducting and overlying slabs. Based on our results, it is proposed that the tectonic affinity of the basalts in an accretionary prism can indicate the subduction polarity of the associated convergent boundary, providing a constraint for regional geology evolution.

Partial melting of oceanic sediments in subduction zones and its contribution to the petrogenesis of peraluminous granites in the Chinese Altai

AbstractLate Carboniferous magmatism in the Chinese Altai provides an important view of geodynamic processes active during crustal growth in the Central Asian Orogenic Belt (CAOB). In this study, five representative peraluminous granite plutons from the Chinese Altai were selected for systematic geochronological, geochemical and Sr–Nd–Hf isotopic analyses (Table 1). These granites were emplaced between 449 and 327 Ma in an active subduction zone, and have moderate to high SiO2 (66.54–76.13 wt%), moderate Na2O+K2O (6.27–7.66 wt%), and high Al2O3 contents (12.43–16.18 wt%). All granite samples in this study showed significant decoupling of the Nd and Hf isotope systems. Results show negative εNd(t) values (−3.3 to −0.9), and predominantly positive εHf(t) values (+0.24 to +8.01, n=57) except for a few negative εHf(t) values (−7.44 to −0.03, n=9), high Mg# values (28.69–53.33), high Nd/Hf ratios (4.26–43.57), and enrichment of large-ion lithophile elements (LILEs; e.g. Pb, Th, and U), suggesting that the granites were derived from the partial melting of oceanic sediments and the associated mantle wedge, with fractionation of plagioclase, K-feldspar and biotite. In situ zircon Hf isotopic analyses yield negative εHf(t) values from −30.6 to −13.7 for the zircon xenocrysts. The U–Pb ages and Hf isotopic ratios of these zircon xenocrysts were probably inherited from oceanic sediments. Zircon saturation temperatures suggest that these peraluminous granites were emplaced at 537–765°C. We propose that: (1) the Nd isotopic system more faithfully reflects the source of peraluminous magmas in the Chinese Altai than the Hf isotopic system, and (2) the oceanic sediment recycling was an important process during continental growth in the CAOB.

Low-δ18O mantle-derived magma in Panjal Traps overprinted by hydrothermal alteration and Himalayan UHP metamorphism: Revealed by SIMS zircon analysis

We report two generations of low-δ18O zircons from the Himalayan eclogites and their host gneisses. In situ SIMS δ18O analyses on single zircon crystals (with known age and Hf isotope ratios) from two populations of chemically distinct zircons demonstrate a complex history: (1) an early low-δ18O mantle-derived magma, (2) followed by post-emplacement high-temperature meteoric-water alteration and finally (3) crystallization of new, low-δ18O minerals during the ultrahigh-pressure metamorphism. Magmatic zircon (269Ma) in Group I eclogites yielded δ18O values from 1.9 to 4.6‰ VSMOW with an average value of 4.0±0.2 (n=35, the error is 2SD analytical precision and “n” represents number of analyzed spots), which is lower than the typical mantle values (5.3±0.6, 2SD). In contrast, metamorphic zircons (45Ma) in Group II eclogites preserve unusually low, negative δ18O values from −3.9 to −2.7‰ (average: −3.4±0.4, n=35, 2SD). Zircons in felsic gneiss that surround Group II eclogites have inherited magmatic cores (ca. 260Ma) with δ18O values of ca. 2.9‰, which decrease to −0.1‰ in metamorphic (ca. 45Ma) rims. These zircons preserve lower δ18O values than would be equilibrated with typical mantle. The low-δ18O values in magmatic zircons suggest that the mafic protolith to these eclogites formed from a hydrothermally altered subducted oceanic crust and the negative δ18O values in metamorphic zircons indicate hydrothermal alteration after crystallization of the mafic magmas but before growth of metamorphic zircons. This study reports evidence for melting of subducted low-δ18O ocean crust to form low-δ18O mantle-derived mafic magmas as previously proposed by Cartwright and Valley (1991, Geology 19, 578–581) for Proterozoic Scourie Dikes.

Three‐Dimensional Evolution of the Early Paleozoic Western Laurentian Margin: New Insights From Detrital Zircon U‐Pb Geochronology and Hf Isotope Geochemistry of the Harmony Formation of Nevada

AbstractUranium‐lead (U‐Pb) geochronology and Hafnium (Hf) isotope geochemistry of detrital zircons of the Harmony Formation of north central Nevada provide new insights into the tectonic evolution of the Late Paleozoic western Laurentian margin. Using laser‐ablation inductively coupled plasma mass spectrometry, 10 arenite samples were analyzed for U‐Pb ages, and 8 of these samples were further analyzed for Hf isotope ratios. Three of the sampled units have similar U‐Pb age peaks and Hf isotope ratios, including a 1.0–1.4 Ga peak with εHf values of +12 to −3 and a 2.5–2.7 Ga peak with εHf values of +7 to −5. The remaining seven samples differ significantly from these three, but are similar to one another; having age peaks of 1.7–1.9 Ga with εHf of +10 to −20 and age peaks of 2.3–2.7 Ga with εHf of +6 to −8. The data confirm the subdivision of the Harmony Formation into two petrofacies: quartzose (Harmony A) and feldspathic (Harmony B). The three samples with 1.0–1.4 and 2.5–2.7 Ga peaks are the Harmony A, which originated in the central Laurentian craton. The other seven samples are the Harmony B, which originated in eastern Alberta‐western Saskatchewan, north of the Harmony A source. We propose that all Harmony Formation strata were deposited near eastern Alberta and subsequently tectonically interleaved with Roberts Mountains allochthon strata. We interpret that the entire package was tectonically transported south along the western Laurentian margin and then emplaced eastward onto the craton during the Late Devonian‐Early Mississippian Antler orogeny.

Arc magmatism witnessed by detrital zircon U-Pb geochronology, Hf isotopes and provenance analysis of Late Cretaceous-Miocene sandstones of onshore western Makran (SE Iran)

The present work aims to contribute to the Cenozoic tectonic setting of the western part of the Makran Accretionary Wedge in SE Iran. We determine the provenance of both deep marine turbiditic and deltaic-shelf Late Cretaceous-Miocene sandstones, describe the sandstone modal framework and heavy minerals and report a new geochronological and isotopic study including 2307 detrital zircons U-Pb ages and 204 in-situ Hf isotopic analyses. Modal sandstone framework compositions indicate that a magmatic arc and recycled accreted sediments were the main sources of Eocene-Oligocene and Miocene sandstones. Cr-spinel and heavy mineral assemblages indicate ultramafic rocks, likely ophiolitic mantle, as a subsidiary source. Detrital zircon U-Pb ages cluster in five main age groups: (1) Neoproterozoic grains suggesting a continental crust provenance within the Central Iran blocks, (2) Jurassic grains with Hf isotopic compositions of continental crust, suggesting a rifting related magmatic provenance, (3) Late Cretaceous and (4) Eocene grains, with Hf isotopic compositions typical of continental crust and non-depleted mantle, suggesting a continental magmatic arc provenance, and (5) Early Miocene grains. The new U-Pb age and Hf isotopic ratios correspond to those obtained in the east Iranian Makran. They fit tectonic reconstructions with Middle Jurassic intracontinental rift, Early Cretaceous to Eocene subduction below Central Iran forming a continental arc to the north of Makran and closure of the related oceanic in the Paleogene. Erosional products of the corresponding magmatic arc are found in the Makran Basin. Our data disprove that provenance characteristics of the Makran sedimentary rocks are consistent with derivation from the Himalayan sources.

Strike-slip tectonics during the Neoproterozoic–Cambrian assembly of East Gondwana: Evidence from a newly discovered microcontinent in the Indian Ocean (Batavia Knoll)

The first dredge samples recovered from the Batavia Knoll in the eastern Indian Ocean demonstrate it is a rifted continental fragment that now lies submerged in 2km of water between the Perth Abyssal Plain and Wharton Basin, ~1600km offshore Western Australia. This knoll is the second microcontinent to have been discovered in this region, lying ~180km to the NE of Gulden Draak Knoll; both knolls rifted from the Indian margin at c. 104–101Ma during the breakup of East Gondwana. Dredged rocks include granite, granodiorite, charnockite and quartz-rich pegmatite, preserving igneous microstructure and biotite-schlieren. The composite nature of many samples suggests derivation from a felsic igneous complex. This basement is at least partly covered by Late Cretaceous sedimentary strata. Zircon grains from five representative granitoid samples primarily display modified oscillatory zoning and U-Pb data that spread along concordia over ~50–70m.y., suggesting protracted magmatism and Pb-loss during the latest Neoproterozoic–early Cambrian (c. 580–530Ma). Despite the span in U-Pb zircon ages, the primary igneous populations exhibit a relatively narrow range of initial Hf isotopic ratios (Hfi=0.281290–0.281814; εHfi=−41.40 to −23.32). This evolved nature of the Hf signature suggests derivation of melts from ancient crust (TDM2=4.05–2.92Ga) with very limited juvenile input. Plate models based on sparse paleomagnetic data imply significant relative motion between India and Australia during the late Neoproterozoic–Cambrian. We suggest the new isotopic data from the Batavia Knoll granitic rocks, coupled with other regional geological constraints, are consistent with magmatic intrusion and subsequent disturbance of U-Pb systems along a late Neoproterozoic–Cambrian sinistral strike-slip margin related to Gondwana assembly.

Radiogenic isotopes in enriched mid-ocean ridge basalts from Explorer Ridge, northeast Pacific Ocean

Extreme gradients in topography related to variations in magma supply are observed on the Southern Explorer Ridge (SER), part of the northern Juan de Fuca ridge system. We report radiogenic isotope (Pb, Sr, Nd, Hf) and geochemical data for twenty-four basalt whole-rock and glass samples collected from the length of the SER and from Explorer Deep, a rift to the north of the SER. Lavas from the SER form a north-south geochemical gradient, dominated by E-MORB at the northern axial high, and range from T-MORB to N-MORB towards the southern deepest part of the ridge. Linear relationships between incompatible element ratios and isotopic ratios in MORB along the ridge are consistent with mixing of magmas beneath the ridge to generate the geographic gradient from E- to N-MORB. The E-MORB have high Sr and Pb, and low Nd and Hf isotopic ratios, typical of enriched mantle that includes a FOZO or HIMU isotopic component. The West Valley and Endeavour segments of the northern Juan de Fuca ridge also include this isotopic component, but the proportion of the FOZO or HIMU component is more extreme in the SER basalts. The FOZO or HIMU component may be garnet-bearing peridotite, or a garnet pyroxenite embedded in peridotite. Recycled garnet pyroxenite better explains the very shallow SER axial high, high Nb/La and La/Sm, and the “enriched” isotopic compositions.