Hf Isotopic Model Ages Research Articles

The effect of sediment recycling in subduction zones on the Hf isotope character of new arc crust, Banda arc, Indonesia

A large portion of Earth's crust is formed at convergent plate boundaries that are accompanied by the subduction of sediments that can contain evolved crust-derived detritus. Partial melting of such sediments can strongly affect the trace element and isotope geochemistry of new arc rocks. Here, we present high-precision Lu–Hf–Zr concentration data and Hf isotope compositions for a series of volcanic rocks from the Banda arc, East Indonesia, to quantify the transfer of subducted Hf to the Banda arc crust and address the influence of recycled Hf in subduction zones on the Hf isotope systematics of arc rocks.Along-arc from NE to SW, the 176Hf/177Hf decreases from 0.28314 to 0.28268 ranging from predominantly mantle-like ratios towards more crustal signatures. Hf–Nd isotope co-variations require low Nd/Hf in the arc magma source, inconsistent with fluid addition to the arc melts, but in agreement with the involvement of partial sediment melts. The systematic decrease in Hf–Nd isotopes infers a NE–SW along-arc increase in the involvement of subducted continental material (SCM) in the arc magma source, consistent with δ18O and Nd–Sr–Pb isotope constraints. The along-arc decrease in Hf isotopes coupled with increasing Zr/Hf (from 31.9 to 36.1) and decreasing Lu/Hf suggests that the newly produced arc lavas contain crustal-derived Hf as a result of partial melting of SCM associated with the breakdown of zircon. Based on recent experimental estimates of temperatures required to achieve zircon breakdown, we infer that slab surface temperatures in the Banda arc region need to be as high as 925°C.As a consequence of the inheritance of non-radiogenic Hf from SCM, the juvenile Banda arc crust exhibits Hf isotope model ages biased by hundreds of millions of years. We conclude that crust-formation ages derived from Hf isotope ratios of convergent margin rocks and their constituent minerals (such as zircon) can be geologically meaningless mixing ages, even when they readily preserve low δ18O values (i.e., <6.5). These findings are discussed with respect to the inferred origin of Hadaean zircons at convergent plate boundaries, which appear consistent with an origin in a convergent margin setting.

Detrital zircon evidence for Hf isotopic evolution of granitoid crust and continental growth

We have determined U–Pb ages, trace element abundances and Hf isotopic compositions of approximately 1000 detrital zircon grains from the Mississippi, Congo, Yangtze and Amazon Rivers. The U–Pb isotopic data reveal the lack of >3.3 Ga zircons in the river sands, and distinct peaks at 2.7–2.5, 2.2–1.9, 1.7–1.6, 1.2–1.0, 0.9–0.4, and <0.3 Ga in the accumulated age distribution. These peaks correspond well with the timing of supercontinent assembly. The Hf isotopic data indicate that many zircons, even those having Archean U–Pb ages, crystallized from magmas involving an older crustal component, suggesting that granitoid magmatism has been the primary agent of differentiation of the continental crust since the Archean era. We calculated Hf isotopic model ages for the zircons to estimate the mean mantle-extraction ages of their source materials. The oldest zircon Hf model ages of about 3.7 Ga for the river sands suggest that some crust generation had taken place by 3.7 Ga, and that it was subsequently reworked into <3.3 Ga granitoid continental crust. The accumulated model age distribution shows peaks at 3.3–3.0, 2.9–2.4, and 2.0–0.9 Ga. The striking attribute of our new data set is the non-uniformitarian secular change in Hf isotopes of granitoid crusts; Hf isotopic compositions of granitoid crusts deviate from the mantle evolution line from about 3.3 to 2.0 Ga, the deviation declines between 2.0 and 1.3 Ga and again increases afterwards. Consideration of mantle-crust mixing models for granitoid genesis suggests that the noted isotopic trends are best explained if the rate of crust generation globally increased in two stages at around (or before) 3.3 and 1.3 Ga, whereas crustal differentiation was important in the evolution of the continental crust at 2.3–2.2 Ga and after 0.6 Ga. Reconciling the isotopic secular change in granitoid crust with that in sedimentary rocks suggests that sedimentary recycling has essentially taken place in continental settings rather than active margin settings and that the sedimentary mass significantly grew through addition of first-cycle sediments from young igneous basements, until after ∼1.3 Ga when sedimentary recycling became the dominant feature of sedimentary evolution. These findings, coupled with the lack of zircons older than 3.3 Ga in river sands, imply the emergence of large-scale continents at about 3.3 Ga with further rapid growth at around 1.3 Ga. This resulted in the major growth of the sedimentary mass between 3.3 and 1.3 Ga and the predominance of its cannibalistic recycling later.

Detrital zircon evidence for the antiquity of Taiwan

In-situ U-Pb geochronology and Hf isotopic studies of zircons from the Pre-Tertiary basement complex and Eocene rocks of the Central Range of Taiwan were carried out to elucidate the history of detrital zircons older than 2.3 Ga. Zircons from the eastern Backbone Range and Eastern Central Range show Paleoproterozoic to late Neoarchean ages (2.3−≥2.5 Ga), whereas those from the Hsuehshan Range yield Neoarchean to Mesoarchean ages (2.6–3.2Ga). The ɛHf(T) for the Paleoproterozoic to late Neoarchean zircons varies from −7.4 to +5.1, implying both juvenile crustal growth and reworking of old crusts. This dual origin could extend back to Mesoarchean-Paleoarchean (3.0–3.4 Ga) based on Hf isotopic model ages. Consistent but narrower ɛHf(T) values and Hf crustal model ages for zircons from Taiwan compared to Cathaysia suggest a possible common origin for these zircons, although Taiwan has a generally younger crustal evolution history than SE China.

U–Pb and Hf isotopic study of detrital zircons from the Lüliang khondalite, North China Craton, and their tectonic implications

AbstractTwo types of metasedimentary rocks occur in the Trans-North China Orogen of the North China Craton. One type consists of highly metamorphosed supracrustal rocks with protoliths of mature cratonic shale, called khondalites, as found in the Lüliang Complex; rocks of the other type are also highly metamorphosed but less mature, as represented by the Wanzi supracrustal assemblage in the Fuping Complex. U–Pb isotopic data for detrital zircons from khondalites show a provenance dominated by 1.9–2.1 Ga Palaeoproterozoic rocks. These detrital zircons display a wide range of εHfvalues from −16.0 to +9.2 and give Hf isotopic model ages mostly around 2.3 Ga. The high positive εHfvalues approach those for the depleted mantle at 2.1 Ga, highlighting a juvenile crustal growth event in Palaeoproterozoic times. Hf isotopic data also imply thatc.2.6 Ga old crustal material was involved in the Palaeoproterozoic magmatic event. These data are similar to those for the khondalitic rocks from the interior of the Western Block of the North China Craton, suggesting a common provenance. In contrast, other metasedimentary rocks in the Trans-North China Orogen, such as the Wanzi supracrustal assemblage in the Fuping Complex, have a source region with both Palaeoproterozoic and Archaean rocks. Their detrital zircon Hf isotopic data indicate reworking of old crustal material and a lack of significant juvenile Palaeoproterozoic magmatic input. These rocks are similar to the coevally deposited meta-sedimentary rocks in the interior of the Eastern Block. We propose that the Lüliang khondalites were deposited on the eastern margin of the Western Block in a passive continental margin environment and were thrust eastward later during collision with the Eastern Block. Other metasedimentary rocks in the Trans-North China Orogen were deposited on the western margin of the Eastern Block in a continental arc environment. Our data support the eastward subduction model for the Palaeoproterozoic tectonic evolution of the North China Craton.

Zircon U–Pb and Hf isotope constraints on crustal melting associated with the Emeishan mantle plume

SHRIMP zircon U–Pb dates, combined with in-situ Hf isotopic data, provide new constraints on the petrogenesis and protolith of peralkaline, metaluminous and peraluminous intrusions and rhyolitic tuffs in the Emeishan large igneous province, with significant bearing on crustal melting associated with mantle plumes. Syenite and A-type granitic intrusions from Huili, Miyi and Taihe in the center of this large igneous province yield U–Pb dates at ∼260 Ma, consistent with the ages obtained for mafic layered intrusions in the same province. Zircon from these rocks exhibits a wide range of initial Hf isotope ratios ( ε Hf( t) = −1.4 to +13.4), with corresponding T DM1 of 400–900 Ma. The highest ε Hf( t) value is only marginally lower than that of depleted mantle reservoir at 260 Ma, suggesting that their source is primarily juvenile crust added during Emeishan volcanism, with incorporation of variable amounts of Neoproterozoic crust. The trigger of crustal melting is most likely related to advective heating associated with magmatic underplating. In contrast, the 255–251 Ma peraluminous granites from Ailanghe and 238 Ma rhyolitic tuff from Binchuan, have negative initial ε Hf values of −1.3 to −4.4, and of −7.7 to −14, respectively. Hf isotopic model ages and presence of inherited zircons indicate their derivation from Mesoproterozoic and Paleoproterozoic crust, respectively. Given the time lag relative to the plume impact (∼260 Ma) and insignificant mantle contribution to 255–238 Ma magmatism, conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas. The involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation, if the age of crust is inversely stratified, i.e., changes from Paleoproterozoic to Mesoproterozoic to Neoproterozoic to Permian with increasing depth. Such crust may have resulted from episodic, downward crustal growth during the evolution of the western Yangtze Craton.

Geochronology, geochemistry and Hf–Sr–Nd isotopic compositions of Huziyan mafic xenoliths, southern Hunan Province, South China: Petrogenesis and implications for lower crust evolution

The Huziyan basalts, occurring in the southern Hunan province, contain abundant mafic xenoliths. These xenolith and the basalt samples offer a unique opportunity to investigate the origin and nature of the lower crust beneath the inland area of the South China Block and to shed new light on the study of the early crustal evolution in the region. We carried out combined SHRIMP U–Pb geochronology and Hf isotope analyses on zircons from both the Huziyan mafic xenoliths and the host basalts. We have also carried out mineral chemistry, whole-rock geochemical, and Sr–Nd isotopic analyses on these mafic xenoliths. Our data show that these xenoliths were once seated in the lower crust according to petrographic examination and P– T estimation. We discovered one of the oldest zircon ( 207Pb/ 206Pb age = 3.73 Ga, and T DM = 4.08 Ga) in the Cathaysia Block in the Huziyan basalts, which implies the existence of a crustal remnant as old as 3.7 Ga. Three representative xenoliths yield consistent U–Pb ages of about 220 Ma (220 ± 9 Ma for H4-2 determined through two baddeleyites, 230 ± 8 Ma for H4-3 and 233 ± 5 Ma for H4-5-2). Some of the zircons with ages of about 220 Ma display depleted mantle-like Hf isotopic compositions. It is thus deduced that some juvenile crust formed at 220 Ma when the mafic melt, derived from depleted mantle, underplated the lower crust. Such a thermal event in the deep crust might have induced intensive crust–mantle interaction and contributed to the formation of contemporaneous Indosinian granitoids in peripheral areas. Another thermal event leading to crustal growth at about 800 Ma can also be identified both from U–Pb ages (770–850 Ma) of xenocrystic zircons in the basalts and xenoliths, and from upper intercept ages of discordia in two xenoliths (933 ± 65 Ma and 828 ± 80 Ma). Episodic crustal growths might also have taken place at 2.7 Ga and 1.9 Ga. This is implied by Hf isotope model ages of some of the zircons. Large variation in Hf isotopic compositions among zircons of the same age ( ɛ Hf( t) = − 5.4 to 20.8 for 220 Ma zircons, and − 8.3 to 10.9 for 800 Ma zircons) suggests mixing and interaction between DM-derived melt and old crustal components at both of these times. Furthermore, petrographic and geochemical characteristics suggest that these xenoliths were formed through accumulation of pyroxene and plagioclase, and were then metamorphosed in the lower crust. Multiple episodes of juvenile growth, crustal reworking, and mafic underplating resulted in complexity and heterogeneity in the lower crust, as demonstrated by diversities in bulk geochemistry, Sr–Nd isotopic compositions and zircon Hf isotopic composition of these xenoliths.

Hf isotopes of the 3.8 Ga zircons in eastern Hebei Province, China: Implications for early crustal evolution of the North China Craton

Zircon U-Pb dating indicates that the fuchsite quartzite in eastern Hebei Province was derived from weathering and erosion of the 3.6–3.8 Ga granitic rocks.In-situ zircon Hf analyses show that the Lu-Hf isotopic system remained closed during later thermal disturbances. Zircons with concordant ages have Hf isotopic model ages of about 3.8 Ga, suggesting a recycling of this ancient crust. The ∼3.8 Ga zircons have similar Hf isotopic compositions to those of chondrite, indicating that their source rocks (granitic rocks) were derived from partial melting of the juvenile crust which originated from a mantle without significant crust-mantle differentiation. Therefore, it is proposed that there was no large-scale crustal growth before ∼3.8 Ga in eastern Hebei Province. Considering zircon Hf isotopic data from other areas, it is concluded that the most ancient crust in the North China Craton probably formed at about 4.0 Ga, and the possibility to find crust older than 4.0 Ga is very limited.

LuHf geochronology applied to dating Cenozoic events affecting lower crustal xenoliths from Kilbourne Hole, New Mexico

Melt-extraction or crystal accumulation events that affected garnet-bearing, granulite xenoliths from Kilbourne Hole, New Mexico, have been dated using the LuHf isotope system. Two garnet-bearing granulites from Kilbourne Hole have extreme 176 Lu 177 Hf ratios of 0.95 and 1.3 ( Lu Hf ≈ 28 and 39 × chondritic), but relatively ‘normal’ ϵ Hf values (+ 5, and + 12) necessitating either garnet accumulation or melt-extraction from a garnet-bearing protolith in the Cenozoic. Hf isotope evolution curves for these two samples intersect those of depleted mantle and Proterozoic crust at high angles and at similar times, demonstrating the potential of Hf isotope model ages to yield true age significance even if the initial Hf isotope composition is not well constrained. A third garnet-granulite xenolith (CKH63; 176 Lu 177 Hf = 0.025 ) contains zircon, which buffered this sample against changes in Lu Hf during the Cenozoic differentiation event. The three garnet granulites lie closely about a 25 Ma LuHf reference line, demonstrating the potential of the LuHf system for detecting garnet-controlled differentiation events in the Cenozoic; given the range in Lu Hf ratios measured, events as young as 5 Ma may be detected using the Lu Hf system. Conventional UPb zircon data from sample CKH63 reveal both a ca. 1.4 Ga inherited component and a component of recent Pb-loss (or new zircon growth), supporting the Cenozoic event documented by Hf isotope model ages.