Hf Isotope Ratios Research Articles

U-Pb ages and Hf isotope compositions of zircons in plutonic rocks from the central Famatinian arc, Argentina

The Famatinian arc formed around the South Iapetus rim during the Ordovician, when oceanic lithosphere subducted beneath the West Gondwana margin. We present combined in situ U–Th–Pb and Lu–Hf isotope analyses for zircon to gain insights into the origin and evolution of Famatinian magmatism. Zircon crystals sampled from four intermediate and silicic plutonic rocks confirm previous observations showing that voluminous magmatism took place during a relatively short pulse between the Early and Middle Ordovician (472–465 Ma). The entire zircon population for the four plutonic rocks yields coherent εHf negative values and spreads over several ranges of initial εHf(t) units (−0.3 to −8.0). The range of εHf units in detrital zircons of Famatinian metasedimentary rocks reflects a prolonged history of the cratonic sources during the Proterozoic to the earliest Phanerozoic. Typical tonalites and granodiorites that contain zircons with evolved Hf isotopic compositions formed upon incorporating (meta)sedimentary materials into calc–alkaline metaluminous magmas. The evolved Hf isotope ratios of zircons in the subduction related plutonic rocks strongly reflect the Hf isotopic character of the metasedimentary contaminant, even though the linked differentiation and growth of the Famatinian arc crust was driven by ascending and evolving mantle magmas. Geochronology and Hf isotope systematics in plutonic zircons allow us understanding the petrogenesis of igneous series and the provenance of magma sources. However, these data could be inadequate for computing model ages and supporting models of crustal evolution.

Evidence for hydrothermal alteration and source regions for the Kiruna iron oxide–apatite ore (northern Sweden) from zircon Hf and O isotopes

Zircon grains from the Kiruna iron oxide–apatite (IOA) ore bodies in northern Sweden are distinct in their hafnium and oxygen isotopic ratios compared to zircon grains from adjacent metavolcanic host rocks and related intrusions. Here, we combine these two isotopic systems on previously dated zircon grains to improve our understanding of these ore deposits with a long-debated origin. Contrasting theories for the formation of the Kiruna iron ores suggest either (1) emplacement through immiscible silicate–iron oxide melts or (2) transportation and deposition of iron by hydrothermal fluids. Zircon from the metavolcanic host rocks and intrusions have oxygen isotopic ratios (δ 18 O ∼3‰) that lie below typical magmatic compositions, which is evidence that roof rocks altered by meteoric water were digested into the magma. In contrast, the ores show an influence of a fluid that is higher in δ 18 O (∼7‰). Based on these findings, we propose the involvement of episodic magmatic-hydrothermal fluids in the ore genesis of the Kiruna iron ore deposits: (1) the first episode related to a deep-seated magmatism and to regional-scale metasomatic alteration, and (2) a later fluid event related to shallow intrusions and responsible for the ore formation. Distinct differences in the Hf isotopic ratios for host rocks and intrusions (e Hfi = −6 to −10, Archean crust) and ore (e Hfi = −5 to +3, depleted mantle) further allow us to screen possible fluid sources for their involvement in the ore process.

Late Paleozoic rift-related basalts from central Inner Mongolia, China

Late Paleozoic basaltic volcanism widely occurred in central Inner Mongolia, the southeastern part of the Central Asia Orogenic Blet (CAOB). Three volcanic pulses are identified by ion microprobe (SHRIMP II) zircon U-Pb analysis, which are Late Carboniferous (ca. 315Ma), the latest Carboniferous (ca. 303Ma) and early Permian (ca. 284Ma). Basalts in the earliest stage are characterized by flat HREE, depletion in Nb and Ta, MORB-like Th/Yb ratios and depleted mantle (DM)-like Hf isotopic ratios. These are similar to forearc basalts, that have been generated by melting of a fluxed depleted mantle in an extensional environment. The latest Carboniferous basalts (sample D1363) have no Zr-Hf anomalies, relatively radiogenic Hf isotopic ratios (εHf(t)=+12.1 to +19.7) and high δ18O values up to 8.5‰, VSMOW. These are likely to be magmas originated from decompressional melting of a depleted mantle that interacted with ocean floor pillow basalts during their ascending through the crust. The early Permian sample D1380 have variable 206Pb/238U zircon ages with an interpreted magma crystallization age of 284.4±2.5Ma. These zircons have scattered Hf isotope (εHf(t)=+5 to +14) and δ18O (1.8–11.0‰, VSMOW), which suggest the involvement of a relatively enriched mantle source. Overall the temporal evolution of the Late Paleozoic basaltic volcanism from central Inner Mongolia suggests a rift-related tectonic setting.

Petrologic and geochemical characterization of rift-related magmatism at the northernmost Main Ethiopian Rift: Implications for plume-lithosphere interaction and the evolution of rift mantle sources

In this paper, we present petrography, K–Ar ages, whole-rock major- and trace-element concentrations, and Sr–Nd–Hf–Pb isotopic ratios of volcanic rocks from Debre Birhan area in the northernmost Main Ethiopian Rift (MER). The K–Ar ages of the mafic series range from 27 to 0.25Ma, and two felsic rocks yield ages of 0.93 and 0.23Ma. The mafic volcanics are classified into older and younger series based on their K–Ar ages. The Mg-numbers (Mg#<60) of both mafic series, along with low Ni and Cr contents, clearly indicate that these lavas have undergone fractionation en route to the surface. Geochemical and isotopic compositions of the older and younger mafic lavas indicate that crustal contamination did not play a significant role in the evolution of their magmas, hence reflect the geochemical characteristics of the sources. The older and younger mafic volcanic rocks display contrasting trace element and isotopic signatures. The older lavas have higher La/Nb, Zr/Nb, Ba/Nb and 87Sr/86Sri (0.70445–0.70681) and lower ԐHf (−2.58 to +6.01) and ԐNd (−1.25 to +3.43) and less radiogenic Pb isotopic ratios (206Pb/204Pbi=17.82–18.64, except TG-54=19.1) relative to the younger mafic lavas. Correlations among isotopic ratios and trace element concentrations provide evidence for the involvement of at least three major end-member components (C-1, C-2, and C-3) in the petrogenesis of these mafic lavas. The C-1 end-member component contributes dominantly to younger mafic lavas, and has isotopic composition similar to the common component ‘C’ deduced from oceanic basalts. The second end-member component (C-2) is represented by lavas near the transition between older and younger, and has isotopic composition similar to that of the seafloor basalts of the Red Sea and the Gulf of Aden. The third end-member component (C-3) is prominent in the older mafic series, and similar isotopically to Pan-African lithosphere. The younger felsic volcanic rocks generally have higher 87Sr/86Sri (0.70395–0.70689) than younger mafic series (0.70366–0.70454) but Nd, Hf and Pb isotopic ratios similar to those of the younger mafic lavas. The element concentrations and Sr, Nd, Hf and Pb isotopic compositions of the younger felsic rocks are explained by fractional crystallization of mafic melts in shallow-level magma chambers accompanied by limited assimilation of crustal material. Low degrees of partial melting of existing mafic underplate can account for the genesis of the least-differentiated (intermediate) lavas, but failed to explain highly evolved younger felsic rocks.

Short episodes of crust generation during protracted accretionary processes: Evidence from Central Asian Orogenic Belt, NW China

Accretionary orogens are major sites of generation of continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (∼540–270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ∼400 Ma and 280 Ma for the Altai segment, and ∼430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (∼20–40 Ma), and variations in magma volumes and in Nd–Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting. The Junggar segment is characterized by high and uniform Nd–Hf isotope ratios (εNd(t)=+5 to +8; zircon εHf(t)=+10 to +16) and it appears to have formed in an intra-oceanic arc system. In the Altai and Chinese Tianshan segments, the Nd–Hf isotope ratios (εNd(t)=−7 to +8; zircon εHf(t)=−16 to +16) are lower, although they increase with decreasing age of the rock units. The introduction of a juvenile component into the Chinese Tianshan and Altai granitic rocks appears to have occurred in continental arc settings and it reflects a progressive reduction in the contributions from old continental lower crust and lithospheric mantle. Within the long-lived convergent margin setting (over ∼200 Ma), higher volumes of magma, and greater contributions of juvenile material, were typically emplaced over short time intervals of ∼20–40 Ma. These intervals were associated with higher Nb/La ratios, coupled with lower La/Yb ratios, in both the mafic and granitic rocks, and these episodes of increased magmatism from intraplate-like sources are therefore thought to have been in response to lithospheric extension. The trace element and Nd–Hf isotope data, in combination with estimates of granitic magma volumes, highlight that crust generation rates are strongly non-uniform within long-lived accretionary orogens. The estimated crust generation rates range from ∼0.1 to ∼40 km3/km/Ma for the Paleozoic record of the CAOB, and only comparatively short (20–40 Ma) periods of elevated magmatic activity had rates similar to those for modern intra-oceanic and continental arcs.

The origin of the Palaeoproterozoic AMCG complexes in the Ukrainian shield: New U-Pb ages and Hf isotopes in zircon

The Ukrainian shield hosts two Palaeoproterozoic anorthosite-mangerite-charnockite-granite (AMCG) complexes (the Korosten and Korsun-Novomyrhorod complexes) that intruded Palaeoproterozoic continental crust in north-western and central parts of the shield, respectively. We report results of U-Pb zircon and baddeleyite dating of 16 samples from the Korosten plutonic complex (KPC), and 6 samples from the Korsun-Novomyrhorod plutonic complex (KNPC). Fifteen zircon samples from both complexes were also analysed for Hf isotopes. These new, together with previously published data indicate that the formation of the KPC started at c. 1815Ma and continued until 1743Ma with two main phases of magma emplacement at 1800–1780 and 1770–1758Ma. Each of the main phases of magmatic activity included both basic and silicic members. The emplacement history of the KNPC is different from that of the KPC. The vast majority of the KNPC basic and silicic rocks were emplaced between c. 1757 and 1750Ma; the youngest stages of the complex are represented by monzonites and syenites that were formed between 1748 and 1744Ma. Both Ukrainian AMCG complexes are closely associated in space and time with mantle-derived mafic and ultramafic dykes. The Hf isotope ratios in the zircons indicate a predominantly crustal source for the initial melts with some input of juvenile Hf from mantle-derived tholeiite melts.The preferred model for the formation of the Ukrainian AMCG complexes involves the emplacement of large volumes of hot mantle-derived tholeiitic magma into the lower crust. This resulted in partial melting of mafic lower-crustal material, mixing of lower crustal and tholeiitic melts, and formation of ferromonzodioritic magmas. Further fractional crystallization of the ferromonzodioritic melts produced the spectrum of basic rocks in the AMCG complexes. Emplacement of the ferromonzodioritic and tholeiitic melts into the middle crust and their partial crystallization caused abundant melting of the ambient crust and formation of the large volumes of granitic rocks present in the complexes.

Zircon recycling and crystallization during formation of chromite- and Ni-arsenide ores in the subcontinental lithospheric mantle (Serranía de Ronda, Spain)

The ultramafic massifs of the Serranía de Ronda (namely Ronda, Ojén and Carratraca) are portions of Proterozoic (∼1.2–1.8Ga) subcontinental lithospheric mantle (SCLM) affected by partial melting and infiltration of melts. The latter of these events was broadly coeval with the tectonic emplacement of the peridotites into the continental crust in the Early Miocene. This resulted in the formation of chromite and Ni-arsenide ores (Cr-Ni) associated with orthopyroxenites and cordieritites. Six zircons recovered from a massive chromitite sample from the Ronda massif yield both concordant and discordant ages between 2309±37Ma and 109±15Ma, and δ18O between 8.3‰ and 9.4‰. Two Proterozoic ages obtained for zircons of this population (1815±9Ma and 1794±17Ma) are identical, within error, to those of zircons reported previously in the garnet pyroxenites of Ronda (1783±37Ma). Similarly, concordant Early Jurassic (192±13Ma) and Cretaceous ages (109±15Ma) obtained from the core and rim, respectively, of a single zircon from the chromitite are also consistent with the ages (180±5Ma, 178±6Ma, and 131±3Ma) already reported for magmatic zircons from corunudum-bearing garnet pyroxenites in the Ronda massif. The observation that chromitites and garnet-pyroxenites contain similar populations of zircons suggests that the parental melts of chromitites inherited zircons from their protolithic garnet pyroxenites, representing relics of oceanic/arc crust recycled in the mantle. Eleven zircons recovered from a massive cordieritite associated with chromitite in the Ronda massif yield scattered concordant and discordant ages between 568Ma and 21Ma, with correspondingly variable δ18O (4.8–13.5‰) and unradiogenic Hf-isotope ratios (εHf(t) from −12.36 to −4.43). The youngest age is concordant at 21.18±0.4Ma and matches the ages of zircons from the chromitite (weighted average age of 20.4±0.87Ma, n=4) and a plagioclasite dyke (scattering between 20.1±0.2Ma and 17.9±0.1Ma; n=11) associated with the Cr-Ni mineralization in the Ojén massif. These zircons show similar unradiogenic Hf-(εHf(t) between −14.5 and −7.6) and heavy O-isotope compositions (δ18O=11.3–12.4‰). A sample of the massive cordieritite hosting the chromitites contains abundant zircons that yield scattered concordant, sub-concordant and discordant U-Pb ages varying from 33.8±1Ma to 781±10Ma; these zircons (n=21) have variable U-contents (105–13900ppm) and Th/U ratios (0.003–0.8). On the basis of O- and Hf-isotope compositions, these zircons define three populations independently of their ages: (1) grains with consistent high δ18O (6.1–12.7‰) and negative εHf(t) (from −14.42 to −6.88); (2) grains with high δ18O (7.6–11.1‰) and positive εHf(t) (3.10–4.84); and (3) grains with δ18O<5.5‰ typical of mantle values. We suggest that zircons from this cordieritite with U-Pb ages older than Miocene are inherited, and were incorporated physically into the SCLM by fluids or melts produced during dehydration-melting of the crustal rocks wrapping the peridotite massifs. The population of Early Miocene zircons found in the chromitites and associated cordieritites and the plagioclasite dyke in the mineralization of the Ojén massif date the crustal emplacement of the peridotites and, therefore, the formation of the Cr-Ni ores. We propose a model in which the unique Cr-Ni mineralizations found in the ultramafic rocks of the Serranía de Ronda were formed as a result of contamination of the SCLM with crustal components.

Hydrothermal alteration of magmatic zircon related to NaCl-rich brines: Diffusion-reaction and dissolution-reprecipitation processes

Magmatic zircon from altered gabbros adjacent to the ∼1.07 Ga Lala Fe-Cu deposit, SW China was modified by NaCl-rich brines related to the Fe-Cu mineralization. The modified zircon grains are composed mostly of both inclusion-free and porous domains, and some grains also have overgrowth/rims. The inclusion-free domains, commonly overgrown by the porous domains, are roughly homogeneous under BSE imaging but display oscillatory or sector zoning under CL imaging. In contrast, the porous domains are distinctly mosaic-like under CL imaging, and contain abundant pores and mineral inclusions such as thorite, xenotime, REE-rich phases, actinolite, albite, biotite and calcite. The inclusion-free domains have concentrations of “non-formula” elements (for example Al, Ca, and Fe) much higher than the magmatic zircon, and are thus interpreted to be products of interaction between magmatic zircon (possibly metamictized) and the fluids via a diffusion-reaction process. However, these resultant domains have retained the Th and U contents, REE patterns, U-Pb ages and Hf isotopes of the precursor, magmatic zircon. On the other hand, the porous domains have stoichiometric end-member compositions (that is lowered Th, U, REE, Y, and P), and given that they are porous and inclusion-rich, we proposed that they have formed from the precursor, element-rich inclusion-free domains or magmatic zircon via a fluid-induced dissolution-reprecipitation process. It is notable that the porous domains retained the 177Hf/176Hf ratios of the precursors, but possess modified and meaningless U-Pb ages. Instead, a meaningful U-Pb age (1006 ± 62 Ma), similar to the timing of the Lala deposit, is well recorded by the overgrowth/rims with distinctly high LREEs (La = 50–700 ppm) and elevated Hf isotopic ratios. Our new results reveal that in the presence of the NaCl-rich brines, the nature of the precursor, magmatic zircon (that is high element budgets and/or metamictization) plays a key role on hydrothermal alteration of zircon.

SHORT EPISODES OF CRUST GENERATION DURING PROTRACTED ACCRETIONARY PROCESSES: EVIDENCE FROM CENTRAL ASIAN OROGENIC BELT, NW CHINA

Abstract Accretionary orogens are major sites of generation of continental crust but the spatial and temporal distribution of crust generation within individual orogens remains poorly constrained. Paleozoic (∼540–270 Ma) granitic rocks from the Alati, Junggar and Chinese Tianshan segments of the Central Asian Orogenic Belt (CAOB) have markedly bimodal age frequency distributions with peaks of ages at ∼400 Ma and 280 Ma for the Altai segment, and ∼430 Ma and 300 Ma for the Junggar and Chinese Tianshan segments. Most of the magma was generated in short time intervals (∼20–40 Ma), and variations in magma volumes and in Nd–Hf isotope ratios are taken to reflect variable rates of new crust generation within a long-lived convergent plate setting. The Junggar segment is characterized by high and uniform Nd–Hf isotope ratios ( e Nd ( t ) = + 5 to + 8 ; zircon e Hf ( t ) = + 10 to + 16 ) and it appears to have formed in an intra-oceanic arc system. In the Altai and Chinese Tianshan segments, the Nd–Hf isotope ratios ( e Nd ( t ) = − 7 to + 8 ; zircon e Hf ( t ) = − 16 to + 16 ) are lower, although they increase with decreasing age of the rock units. The introduction of a juvenile component into the Chinese Tianshan and Altai granitic rocks appears to have occurred in continental arc settings and it reflects a progressive reduction in the contributions from old continental lower crust and lithospheric mantle. Within the long-lived convergent margin setting (over ∼200 Ma), higher volumes of magma, and greater contributions of juvenile material, were typically emplaced over short time intervals of ∼20–40 Ma. These intervals were associated with higher Nb/La ratios, coupled with lower La/Yb ratios, in both the mafic and granitic rocks, and these episodes of increased magmatism from intraplate-like sources are therefore thought to have been in response to lithospheric extension. The trace element and Nd–Hf isotope data, in combination with estimates of granitic magma volumes, highlight that crust generation rates are strongly non-uniform within long-lived accretionary orogens. The estimated crust generation rates range from ∼0.1 to ∼40 km3/km/Ma for the Paleozoic record of the CAOB, and only comparatively short (20–40 Ma) periods of elevated magmatic activity had rates similar to those for modern intra-oceanic and continental arcs.

Contrasting geochemical signatures of fluid-absent versus fluid-fluxed melting of muscovite in metasedimentary sources: The Himalayan leucogranites

Abstract Most of the Himalayan Cenozoic leucogranites are products of partial melting of metapelite sources. In the Malashan-Gyirong area (southern Tibet), the geochemical compositions of leucogranites define two groups with distinct whole-rock major elements, large ion lithophile elements, rare earth elements, high field strength elements, and Sr and Hf isotope ratios. Based on published experimental results that define generalized melting reactions of metapelitic sources, we infer that these leucogranites are the products of two different types of crustal anatexis: fluid-fluxed melting and fluid-absent melting of muscovite in metasedimentary sources. As compared to the leucogranites derived from fluid-absent melting, those from fluid-fluxed melting have relatively higher Ca, Sr, Ba, Zr, Hf, Th, and light rare earth element concentrations, and Zr/Hf, Eu/Eu*, and Nd/Nd*, but lower Rb, Nb, Ta, and U concentrations, Rb/Sr and 87Sr/86Sr ratios, and εHf(t). The geochemical differences can be explained by melting behaviors of major (muscovite, feldspar) and accessory minerals (zircon and monazite) during different modes of crustal anatexis. The systematic elemental and isotopic signatures of different types of crustal anatexis and, in particular, the coupling of major and trace elements that results from common influences on rock-forming and accessory mineral behaviors provide tools with which to refine our understanding of the nature of crustal anatexis.

Garnet effect on Nd-Hf isotope decoupling: Evidence from the Jinfosi batholith, Northern Tibetan Plateau

The initial Nd and Hf isotope ratios of a 420Ma post-collisional dioritic-granitic batholith from the Northern Tibetan plateau define a negative trend above and orthogonal to the ԐHf(t)-ԐNd(t) terrestrial array. This uncommon trend offers an insight into the origin of the puzzling Nd-Hf isotope decoupling in the crustal rocks. On this trend, samples depleted in heavy rare earth elements (HREEs, i.e., [Dy/Yb]N≫1) deviate most from the terrestrial array whereas samples with flat HREEs (i.e., [Dy/Yb]N≥1) deviate less or plot within the terrestrial array, pointing to the controlling effect of garnet in the magma source. Ancient garnet-bearing residues after melt extraction will have elevated Lu/Hf ratios and can evolve with time to produce high ԐHf(t) at a low ԐNd(t) value. Mixing of melts derived from such source lithologies (high Lu/Hf) with melts possessing a within-terrestrial array Nd-Hf isotopic composition (low Lu/Hf) best explains the observed trend orthogonal to the terrestrial array. The samples from the Jinfosi batholith with the most decoupled Nd-Hf isotope compositions require a larger degree (>40%) and ancient (i.e., ≥1.8Gyr) previous melt extraction from their source. It follows that the ancient melts with depleted HREEs complementary to those garnet-bearing residues should have low ԐHf values and plot below the terrestrial array, which is indeed shown by some Archean/Paleoproterozic TTGs.

Orogeny processes of the western Jiangnan Orogen, South China：Insights from Neoproterozoic igneous rocks and a deep seismic profile

The Jiangnan Orogen is a collisional suture belt between the Yangtze and Cathaysia Blocks in South China, with many unanswered questions regarding its tectonic evolution. Using the basement structure of the Jiangnan Orogen, we investigate the granite and dacite exposed along the western Jiangnan Orogen and present new LA-ICP-MS zircon U-Pb ages, Hf isotopes, and whole rock geochemistry data. The results suggest that the granite plutons belong to the calc-alkaline series and are typical S-type granites. It yields a mean U-Pb age of 854±2 Ma, which is determined from the core of zircon and possibly inherited from its source or wall rocks. The initial emplacement age of granite may be 826-805 Ma, whereas the dacites yield an age of 805±1.6 Ma and belong to the shoshonite series. The initial Hf-isotope ratios (176Hf/177Hf) in the granite sample are mostly negative εHf(t), with a few of positive value with 1.38–1.6 Ga TDM and 1.67–2.06 Ga TDM2, whereas the dacite samples have mostly positive εHf(t), with a 0.78–1.6 Ga TDM and 0.83–2.2 Ga TDM2t. A comparison of the εHf(t) and TDM2t with the corresponding intruded strata, helps illustrate the origin of the magma and the finals stages ofcollision. Based on our results, we conclude that the western Jiangnan Orogen was a back-arc foreland basin that developed on the margin of the Yangtze continent and collided with the Cathaysia Block, forming a continent-arc-continent accretionary orogeny between 860 and 800 Ma.

Primitive arc magma diversity: New geochemical insights in the Cascade Arc

The origin of the large range of compositional diversity encompassed by primitive arc magmas and the respective roles of mantle heterogeneity versus slab-derived contributions are the center of ongoing debate. The Cascade Arc of western North America is a global endmember hot subduction zone and an ideal setting in which to examine fundamental questions about the origin of arc magmas. Cascade Arc primitive magmas define several distinct compositional endmembers, the most widespread of which are high alumina olivine tholeiite (HAOT), calc-alkaline basalt (CAB), and intraplate-type basalt (IPB). New high precision Sr-Nd-Hf-Pb isotopic and trace element data for 49 of the most primitive magmas from seven major High Cascades volcanic centers are used to assess whether the basalt groups are derived from geochemically distinct mantle sources. Along with recently published data for the Garibaldi Volcanic Belt (GVB), the northern and other major segment of the Cascade Arc, and Mt. Rainier, the new data expand the high-precision isotopic coverage to seventeen volcanic centers spanning the ~1300km length of the arc. In all investigated isotopic systems, the new High Cascades data allow for finer resolution with much less scatter than previous data. HAOT mantle sources are systematically more depleted in incompatible elements than CAB sources, yet High Cascades CABs and HAOTs are isotopically indistinguishable, defining a remarkable linear Pb isotopic array consistent with a single mantle endmember that is similar to the source of Juan de Fuca mid-ocean ridge basalts. This mantle has been variably modified by contributions from a single subducting sediment endmember that is a close isotopic match to average Northern Cascadia basin sediment. Although Astoria Fan sediment was previously used as a proxy for the subducting sediment composition in the Cascadia subduction zone, the isotope ratios of Cascades basalts record no input from Astoria Fan sediment. High Cascades CABs and HAOTs both contain subducting sediment input in the form of a melt, but CABs also contain melts of subducting oceanic crust which is most apparent in the southern Cascades high-Sr/P suite and in the GVB. The least sediment input is recorded by GVB basalts, which also become progressively lower in 208Pb*/206Pb* and Hf isotope ratios to the north, reflecting the influx of an isotopically distinct enriched mantle component that generates IPBs at the northern slab edge. Enriched mantle with a composition trending towards HIMU is also recorded by a distinct isotopic array that is defined by IPBs from the Mt. Adams-Simcoe back arc region. We propose that slab gaps permit the influx of sub-slab asthenospheric mantle in both the northern GVB and Mt. Adams-Simcoe back-arc. Otherwise, western North America is underlain predominantly by isotopically homogeneous, MORB-type depleted mantle from which both the CAB and HAOT basalt groups are derived. Our results from the Cascades imply that most of the compositional heterogeneity encompassed by primitive arc magmas is not related to the presence of multiple mantle components but to variability in the composition and quantity of slab contributions. This study highlights the importance of high precision Sr-Nd-Hf-Pb isotope and trace element data in obtaining new insights on the petrogenesis of arc magmas.

Complexity of In-situ zircon U–Pb–Hf isotope systematics during arc magma genesis at the roots of a Cretaceous arc, Fiordland, New Zealand

Zircons from seventeen samples of Western Fiordland Orthogneiss (WFO) diorites and three samples of country rock (two schists and one Darran Suite diorite) from the lowermost exposed sections of the Median Batholith, Fiordland, New Zealand, were analysed for in-situ U–Pb and Hf-isotopes. The WFO represents the deeper levels of Early Cretaceous continental arc magmatism on the Pacific margin of Gondwana, marking the final stage of long-lived arc magmatism on the margin spanning the Palaeozoic. The WFO plutons were emplaced at high-P (mid to deep crust at c. 8–12kbar) between 124 and 114Ma. Minor very high-P (c. 18kbar) WFO eclogite and omphacite granulite facies orthogneiss (Breaksea Orthogneiss) are inferred to have crystallised in the base of thickened crust at c. 124Ma. Zircons from the Breaksea Orthogneiss are considered to be variably affected by Pb-loss due to emplacement of the adjacent (Malaspina) Pluton at c. 114Ma. By identifying Pb-loss, magmatic ages were able to be inferred in respect to apparent Pb-loss ages. Hf isotope data for the WFO define an excursion to less radiogenic Hf isotope ratios with time, reflecting increased recycling of an old source component. Peaks at c. 555, 770 and 2480Ma, determine the age spectra of inherited populations of zircons within the WFO. This contrasts with detrital zircon patterns in country rocks of the Takaka terrane, which include peaks at c. 465Ma, and 1250–900Ma that are absent in the WFO inheritance pattern. These results indicate a previously unrecognised Precambrian lower crustal component of New Zealand. Recycling of this lower crust became increasingly important as a source for the final stage or Mesozoic arc magmatism along this segment of the palaeo-Pacific margin of Gondwana.

Petrology and geochemistry of Early Cretaceous A-type granitoids and late Mesozoic mafic dikes and their relationship to adakitic intrusions in the lower Yangtze River belt, Southeast China

ABSTRACTThe relationship among magmatism, large-scale metallogenesis of Southeast China, and subduction of the Pacific plate has long been debated. The lower Yangtze River belt (LYRB) in the northeastern edge of Southeast China is characterized by intense late Mesozoic magmatism and associated polymetallic mineralization such as copper, gold, iron, tungsten, molybdenum, etc. The copper-related adakitic rocks (148–130 Ma) in this belt are the oldest episode of magmatism and intruded as small intermediate-acid intrusive bodies. The Huayuangong granitoids (HYG), located in the southern part of this belt, however, are copper-barren. Three granitoid samples from this pluton give zircon U–Pb ages of 126.4 ± 1.6 Ma, 125.9 ± 1.9 Ma, and 126.2 ± 1.2 Ma, respectively. The HYG has A-type affinity with metaluminous to weakly peraluminous, high FeOT/(FeOT+MgO) ratios, and high Zr+Nb+Ce+Yb contents. Meanwhile, 10 late Mesozoic mafic samples from the LYRB exhibit similar trace element characteristics to those of ‘continental arc andesite’ (CAA) and suggest an enriched lithospheric mantle source with depletion in high field strength elements (e.g. Nb, Ta, Zr, Hf, and Ti) and enrichment of large ion lithophile elements (e.g. Rb, Th, U, and Pb). Although the HYG exhibits similar Sr–Nd isotope composition with the mafic dikes, distinct whole-rock Pb isotope ratios imply that the granitoids and mafic magmas originated from heterogeneous mantle sources. Compared with coeval Baijuhuajian A-type rocks that are exposed along the Jiang–Shao fault of Southeast China, the HYG shows enriched Hf isotope ratios of zircon with εHf(t) values ranging from −4.8 to −11.1. In the Yb/Ta versus Y/Nb diagram, being different from the major asthenospheric mantle-origin Baijuhuajian pluton, a large range of and high Y/Nb ratios as well as high Zr contents of the HYG pluton suggest a magmatic source of mixing between the asthenospheric and enriched crustal component in the LYRB. Compared with early-stage copper-related adakitic rocks (148–130 Ma) with subduction-related affinities and high oxygen fugacity, the copper-barren HYG has with-plate A-type affinities and lower oxygen fugacity. Summarizing, the production of early-stage (i.e. subduction related) adakitic rocks followed by late-stage A-type granitoids in the LYRB is ascribed to the rollback of the Palaeo-Pacific plate beneath Southeast China and associated with asthenospheric upwelling and lithospheric thinning during the late Mesozoic era.

Nature and evolution of lithospheric mantle beneath the southern Ethiopian rift zone: evidence from petrology and geochemistry of mantle xenoliths

Mantle xenoliths hosted in Quaternary basaltic lavas from the Dillo and Megado areas of the southern Ethiopian rift are investigated to understand the geochemical composition and associated processes occurring in the lithospheric mantle beneath the region. The xenoliths are comprised of predominantly spinel lherzolite with subordinate harzburgite and clinopyroxenite. Fo content of olivine and Cr# of spinel for peridotites from both localities positively correlate and suggest the occurrence of variable degrees of partial melting and melt extraction. The clinopyroxene from lherzolites is both LREE depleted (La/Sm(N) = 0.11–0.37 × Cl) and LREE enriched (La/Sm(N) = 1.88–15.72 × Cl) with flat HREEs (Dy/Lu(N) = 0.96–1.31 × Cl). All clinopyroxene from the harzburgites and clinopyroxenites exhibits LREE-enriched (La/Sm(N) = 2.92–27.63.1 × Cl and, 0.45 and 1.38 × Cl, respectively) patterns with slight fractionation of HREE. The 143Nd/144Nd and 176Hf/177Hf ratios of clinopyroxene from lherzolite range from 0.51291 to 0.51370 and 0.28289 to 0.28385, respectively. Most of the samples define ages of 900 and 500 Ma on Sm–Nd and Lu–Hf reference isochrons, within the age range of Pan-African crustal formation. The initial Nd and Hf isotopic ratios were calculated at 1, 1.5, 2 and 2.5 Ga plot away from the trends defined by MORB, DMM and E-DMM which were determined from southern Ethiopian peridotites, thus indicating that the Dillo and Megado xenoliths could have been produced by melt extraction from the asthenosphere during the Pan-African orogenic event. There is no significant difference in 87Sr/86Sr ratios between the depleted and enriched clinopyroxene. This suggests that the melts that caused the enrichment of the clinopyroxene are mainly derived from the depleted asthenospheric mantle from which the xenoliths are extracted. Largely, the mineralogical and isotopic compositions of the xenoliths show heterogeneity of the CLM that could have been produced from various degrees of melt extraction, followed by metasomatism.

Multiple Mixing and Hybridization from Magma Source to Final Emplacement in the Permian Yamatu Pluton, the Northern Alxa Block, China

A combined study of zircon U–Pb ages, whole-rock chemistry, Sr–Nd isotopes and in situ Lu–Hf isotopic ratios in zircons was carried out on Permian monzogranites and mafic microgranular enclaves (MME) and coeval massive gabbros in the northern Alxa block, North China. The data obtained were used to constrain magma sources and petrogenetic processes involved in the generation of this igneous suite. Zircon U–Pb dating yields ages of 271 ± 1 Ma, 270 ± 1 Ma and 276–270 Ma for the monzogranites, MME and massive gabbros, respectively. Two populations of MME, gabbroic (SiO 2 <49 wt %) and dioritic (SiO 2 >53 wt %) enclaves, are identified. They represent mafic to intermediate magmas quenched in a partially crystallized granitic (sensu lato) host; evidence to support this conclusion includes their fine-grained textures, sinuous margins and diffuse contacts with the host monzogranites. Back-veins and xenocrysts of quartz and plagioclase, as well as various disequilibrium textures and mineral assemblages, indicate mingling or mixing processes. The two magma systems, mafic and felsic, have broadly similar isotopic characteristics with whole-rock initial 87 Sr/ 86 Sr(i) ratios ranging from 0·7075 to 0·7077 in the monzogranite host and from 0·7067 to 0·7069 in the MME, with eN d (t) values ranging from –10·2 to –12·4 in monzogranites and from –8·2 to –9·9 in the MME. Zircon eHf(t) values of the monzogranites, gabbroic enclaves and massive gabbros show a wide range and significant overlap from –1·2 to –15·2, –4 to –13·3 and –5·4 to –19·5, respectively; the maximum frequency value of the Hf model age is almost coincident with the whole-rock Nd model age. Mafic, gabbroic rocks similar in composition to some enclaves form layered synplutonic intrusions several metres in thickness and more than 100 m in lateral extent. A mixing test based on mass balance for whole-rock major element compositions reveals that mixing was an efficient process between two coeval magmas. The fraction of felsic magma involved in the hybrid rocks ranged from 0·19–0·29 in the dioritic enclaves to 0·84 in granodiorite and good linear fits (r 2 > 0·9) are obtained using the average composition of gabbroic enclaves and the most felsic monzogranite as end-members. Although the monzogranites and enclaves may be derived from distinct magma sources, they share similar isotopic signatures, pointing to interaction processes in the source region. The combination of information from geochronology, petrology, whole-rock geochemistry and isotopic compositions leads us to conclude that the processes of hybridization and magma mixing were effective at both the level of emplacement in the shallow crust and at depth in the magma source region. Hybridization in a source region within the lithospheric mantle, involving mantle and crustal source rocks, produced magma bimodality with strong geochemical affinities between end-members and clear calc-alkaline arc signatures, compatible with a subduction setting. A plausible subduction erosion plus relamination model, which differs from classical models based on mafic magma underplating, is proposed. Accordingly, the Yamatu monzogranites are argued to have been generated from granitic melt segregated and ponded in buoyant silicic diapirs, which formed by melting of subducted melanges in the lithospheric mantle and eventually relaminated to the lower crust. They represent the melts that metasomatized the mantle region during a pre-Permian subduction event. The massive gabbros were formed by decompression melting of the previously metasomatized mantle. The gabbroic enclaves and gabbro layers of the Yamatu pluton are interpreted as magmas formed by decompression melting of this modified hydrated mantle. The dioritic enclaves represent hybrid liquids generated from reaction between granitic melts, which were derived from subducted melanges, and the hydrated mantle or melts derived from it.

Petrogenesis of coeval sodic and potassic alkaline magmas at Spanish Peaks, Colorado: Magmatism related to the opening of the Rio Grande rift

Approximately coeval, relatively primitive (∼5–10% MgO with exception of a trachyandesite) alkaline mafic dikes and sills at or near Spanish Peaks, CO are divided into relatively sodic and potassic varieties on the basis of K2O/Na2O. Many of these dikes are true lamprophyres. In spite of variable alkali element ratios, the alkaline rocks share a number of geochemical similarities: high LIL element contents, high Ba and similar Sr, Nd and Hf isotope ratios near that of Bulk Earth. One important difference is that the potassic rocks are characterized by lower Al2O3 contents, typically less than 12wt.%, than the sodic dikes/sills which typically have more than 13wt.% Al2O3, and this difference is independent of MgO content. We attribute the distinct Al2O3 contents to varying pressure during melting: a mica-bearing, Al-poor vein assemblage for the potassic magmas melted at higher pressure than an aluminous amphibole-bearing vein assemblage for the sodic magmas. Remarkable isotopic and trace element similarities with approximately contemporaneous, nearby Rio Grande rift-related basalts in the San Luis Valley, indicate that the magmatism at Spanish Peaks was rift-related, and that lithosphere sources were shared between some rift magmas and those at Spanish Peaks. High Zn/Fe ratios in the Spanish Peaks mafic rocks point to a clinopyroxene- and garnet-rich source such as lithosphere veined by pyroxenite or eclogite. Lithospheric melting was possibly triggered by foundering of cool, dense lithosphere beneath the Rio Grande rift during the initiation of rifting with the potassic parent magmas generated by higher pressure melting of the foundered lithosphere than the sodic parent magmas. This process, caused by gravitational instability of the lithosphere (Elkins-Tanton, 2007) may be common beneath active continental rifts.

Detrital zircon U-Pb geochronology and Hf isotope geochemistry of the Roberts Mountains allochthon: New insights into the early Paleozoic tectonics of western North America

Detrital zircon U-Pb geochronology and Hf isotope geochemistry provide new insights into the provenance, sedimentary transport, and tectonic evolution of the Roberts Mountains allochthon strata of north-central Nevada. Using laser-ablation inductively coupled plasma mass spectrometry, a total of 1151 zircon grains from six Ordovician to Devonian arenite samples were analyzed for U-Pb ages; of these, 228 grains were further analyzed for Hf isotope ratios. Five of the units sampled have similar U-Pb age peaks and Hf isotope ratios, while the ages and ratios of the Ordovician lower Vinini Formation are significantly different. Comparison of our data with that of igneous basement rocks and other sedimentary units supports our interpretation that the lower Vinini Formation originated in the north-central Laurentian craton. The other five units sampled, as well as Ordovician passive margin sandstones of the western Laurentian margin, had a common source in the Peace River Arch region of western Canada. We propose that the Roberts Mountains allochthon strata were deposited near the Peace River Arch region, and subsequently tectonically transported south along the Laurentian margin, from where they were emplaced onto the craton during the Antler orogeny.

Temporal magma source changes at Gaua volcano, Vanuatu island arc

Gaua Island (also called Santa Maria), from the central part of the Vanuatu arc, consists of a large volcano marked by a caldera that hosts the active Mount Garet summit cone. In this paper, a geochemical study including Sr, Nd, Pb and Hf isotopic compositions of 25 lavas emitted since 1.8Ma is presented, with a focus on the volcanic products that preceded (old volcanics, main cone and pyroclastic series) and followed (Mount Garet) the caldera forming event.All lavas show an island arc signature with enrichment in LILE and depletion in HFSE. Post-caldera lavas define a medium-K calc-alkaline trend, whereas lavas from the former main cone have high-K calc-alkaline compositions. Compared to the pre-caldera volcanic suite, the Mount Garet lavas have similar Th/Nb (~1.5), 143Nd/144Nd (~0.51295) and 176Hf/177Hf (~0.28316) ratios, but higher Ba/La (~42 vs. ~27) and 87Sr/86Sr (0.70417 vs. 0.70405) ratios and lower Ce/Pb (~2.7 vs. ~4.6), La/Sm (~2.5 vs. ~4.0) and 206Pb/204Pb (18.105 vs. 18.176) ratios. High Th/Nb and low Nd and Hf isotopic ratios compared to N-MORB suggest the contribution of ~2% of subducted sediment melt to the mantle source of Gaua magmas. Most of the observed differences between pre- and post-caldera lavas can be accounted for by the involvement of at least two portions of the mantle wedge, metasomatized by different slab-derived aqueous fluids. In addition, the lower La/Sm (at a given 143Nd/144Nd) ratios of Mount Garet lavas suggest a higher degree of partial melting (~10–15%) compared to the pre-caldera lavas (~5%). The Santa Maria Pyroclastic Series (SMPS) eruption probably triggered the caldera collapse, in response to emptying of the magmatic chamber. This event may have allowed new access to the surface for a geochemically distinct batch of magma issued from a separate magma chamber, resulting in the birth and construction of Mount Garet within the caldera. As both magmatic suites were emitted over a very short time, the storage of their parental magmas beneath the volcano is still possible.