Zircon Oxygen Isotope Research Articles

Transcrustal magmatic systems in NE China: Insights from Early Cretaceous metaluminous–peraluminous–peralkaline rock associations in the southern Great Xing’an Range

The generation and evolution of crustal-scale magmatic systems are important in revealing the continental crust differentiation. In the Bayangeer-Aliwula area of Inner Mongolia, metaluminous–peraluminous–peralkaline rock assemblages provide a rare opportunity to evaluate Early Cretaceous magmatic systems. This study presents new geochemical data, including whole-rock and zircon geochemistry, zircon U–Pb dating, and Hf–O isotopic analysis of rocks formed between 132 and 121 Ma. The metaluminous rocks exhibit low SiO2 (55.35–65.65 wt%), low Rb/Sr (0.08–0.24) and (La/Yb)N (4.88–7.58) ratios, and positive εHf(t) values (+4.9 to +8.7). These features, along with enrichment in large-ion lithophile elements and depletion in Nb and Ta, suggest formation via partial melting of fluid-metasomatised lithospheric mantle. By contrast, peraluminous rocks have high SiO2 (69.37–80.58 wt%), a high differentiation index (DI = 88–95), and Fe-index (0.84–0.95), resembling highly fractionated I-type granites. They show high Rb/Sr (0.77–3.35) and (La/Yb)N (5.79–22.75) ratios, and positive εHf(t) values (+6.7 to +10.8), combined with the modelling results, indicating origin from partial melting of K-rich mafic lower crust followed by magma fractionation. Peralkaline rocks display typical ferroan A-type granite characteristics (Zr + Nb + Ce + Y = 906–4292 ppm; Fe-index = 0.96–0.99), with high SiO2 (74.02–77.50 wt%), high Rb/Sr (6.17–121.38), and (La/Yb)N ratios (2.19–17.72), and positive εHf(t) values (+3.1 to +9.6). Zircon geochemistry characteristics suggest that peralkaline and peraluminous felsic melts are different batches extracted from the same magma reservoir. Further analysis, including hyperbola diagrams and zircon oxygen isotope compositions, suggests peralkaline magma formation through the mixing of altered oceanic crust fluids and peraluminous melts after melt extraction. During Early Cretaceous, the transcrustal magmatic system provides a reasonable explanation for the petrogenesis of various contemporaneous rocks in the study area in southern Great Xing’an Range.

A rapid transition from subduction to Barrovian metamorphism: geochronology of mafic–ultramafic relicts of oceanic crust in the Central Alps, Switzerland

Relicts of subducted oceanic lithosphere provide key information for the tectonic reconstructions of convergent margins. In the Central Alps, such relicts occur as isolated mafic–ultramafic lenses within the migmatites of the southern Adula nappe and Cima-Lunga unit. Analysis of the major-, minor-, and accessory minerals of these ophiolitic relicts, combined with zircon and rutile U–Pb ages and zircon oxygen isotopes, allows the reconstruction of different stages of their complex evolution. The mafic–ultramafic suite in Valle di Moleno consists of chlorite-harzburgites associated with metarodingites and retrogressed eclogites. Relic omphacite and kyanite in retrogressed eclogites provide evidence for subduction-related metamorphism. Increasing XPrp in the garnet mantle towards the rim documents heating during high-pressure metamorphism up to 800–850 °C. Polyphase inclusions and chemical zoning in garnet suggest fluid-assisted melting during high-pressure metamorphism dated at 31.0 ± 0.9 Ma. In Val Cama, chlorite-harzburgites, metarodingites and calcsilicate-metasediments occur. Detrital zircon ages in the metasediment suggest a Mesozoic deposition. The metarodingite-metaperidotite-metasediment association and the low δ18O signatures of zircon (δ18O 3.0–3.7‰), inherited from seafloor metasomatism of the protoliths, show that the rocks are derived from former altered oceanic crust. Amphibolite facies metamorphism related to the Central Alps Barrovian evolution in Val Cama occurred at 28.8 ± 1.5 Ma. The combined data from Moleno and Cama indicate a rapid transition (~ 2 Ma) from subduction to collisional metamorphism with corresponding exhumation rates of 3–6 cm/year. Fast exhumation tectonics may have been favored by slab break-off or slab extraction. U–Pb dating of rutile from both localities yields ages of ~ 20 Ma, suggesting that these rocks remained at amphibolite-facies conditions for about 10 Ma and underwent a second fast exhumation of 3 cm/year associated with vertical movements along the Insubric line.

Multi-scale, open-system magmatic and sub-solidus processes contribute to the chemical and isotopic characteristics of the Jurassic Guadalupe Igneous Complex, Sierra Nevada, California, USA

Abstract The chemical and isotopic characteristics of a solidified pluton represent the integration of magmatic and sub-solidus processes operating across a range of spatial and temporal scales during pluton construction, crystallization, and cooling. Disentangling these processes and understanding where chemical and isotopic signatures were acquired requires the combination of multiple tools tracing processes at different time and length scales. We combine whole-rock oxygen and Sr-Nd isotopes, zircon oxygen isotopes and trace elements, and mineral compositions with published high-precision U-Pb zircon geochronology to evaluate differentiation within the bimodal Guadalupe Igneous Complex, Sierra Nevada, California (USA). The complex was constructed in ~300 k.y. between 149 and 150 Ma. Felsic magmas crystallized as centimeter- to meter-sized segregations in gabbros in the lower part of the complex and as granites and granophyres structurally above the gabbros. A central mingling zone separates the mafic and felsic units. Pluton-wide δ18O(whole-rock), δ18O(zircon), and Sr-Nd isotopic ranges are too large to be explained by in situ, closed-system differentiation, instead requiring open-system behavior at all scales. Low δ18O(whole-rock) and δ18O(zircon) values indicate assimilation of hydrothermally altered marine host rocks during ascent and/or emplacement. In situ differentiation processes operated on a smaller scale (meters to tens of meters) for at least ~200 k.y. via (1) percolation and segregation of chemically and isotopically diverse silicic interstitial melt from a heterogeneous gabbro mush; (2) crystal accumulation; and (3) sub-solidus, high-temperature, hydrothermal alteration at the shallow roof of the complex to modify the chemical and isotopic characteristics. Whole-rock and mineral chemistry in combination with geochronology allows deciphering open-system differentiation processes at the outcrop to pluton scale from magmatic to sub-solidus temperatures over time scales of hundreds of thousands to millions of years.

Mantle-like to low oxygen isotopes in zircon from the mid-Cretaceous high-silica granites reveal unweathered basement recycling along the present coastal area of SE China

Zircon oxygen isotope compositions can provide potential constraints on the origin of granitic magmas. In this work, we report new data of zircon UPb dating, trace elements and HfO isotopes for two types of mid-Cretaceous high-silica granites from Zhoushan archipelago in Zhejiang Province, SE China. The in-situ zircon data are carefully screened for radiation damage and post-magmatic alteration. The analyzed magmatic zircon grains indicate that the Zhoushan calc-alkaline granites (ZCAG; 101–92 Ma) have mantle-like δ18O values of 5.29–5.88 ‰, in contrast to the Zhoushan peralkaline granites (ZPAG; 91–89 Ma) with relatively low δ18O of 4.42–4.6 ‰. The slight decreases in δ18O and εHf(t) from zircon center to periphery of both groups suggest that they may have consistent oxygen isotopic compositions in their parental magma sources and have gone through assimilations to hydrothermally altered wall rocks in the shallow crust. The mantle-like δ18O values in zircon are widespread in the Cretaceous felsic rocks in the present coastal area of SE China, which, in combination Hf isotopes, indicates the possible existence of a buried unweathered Proterozoic basement in the eastern part of the Cathaysia Block. This work highlights the role of deep crustal magma sources, assimilation and post-magmatic radioactive damage on the formation of mantle-like to low δ18O signals in zircons, and has fundamental implications for understanding the formation of widespread mid-Cretaceous felsic rocks along the present coastal area of SE China.

Bimodality in zircon oxygen isotopes and implications for crustal melting on the early Earth

Zircons from the oldest dated felsic crust, the Acasta Gneiss Complex, Canada, provide key information that may help understand the generation of crust on our nascent planet. When screened to eliminate grains with secondary alteration by measuring relative hydration (Δ16O1H/16O), primary ≥ 3.99 Ga zircon cores show δ18O of 5.88 ± 0.15 ‰, at the extreme upper (heavy) range for mantle values. Another early (≥3.96 Ga) zircon component indicates distinctly different, primary light δ18O values (δ18O ≤ 4.5 ‰). This bimodality in ancient zircon oxygen isotopes implies partial melting of both deep (lower crustal) and shallower (near surface) source rocks, responsible for felsic crust production on the early Earth. A similar bimodality in zircon δ18O is recognised in data from other ancient cratons, albeit at different times. Although alternative (uniformitarian) interpretations may also satisfy the data, the tempo of this bimodality matches models of planetary high-energy impact flux, consistent with a fundamental role for bolide impacts in the formation of crustal nuclei on the early Earth.

Mechanisms for generating elevated zircon δ18O in Archean crust: Insights from the Saglek-Hebron Complex, Canada

The Saglek-Hebron Complex (northern Labrador, Canada) is a polymetamorphic terrane preserving felsic crustal rocks as old as 3.87 Ga. Crustal rocks of this age are rare and offer opportunities to interrogate how Earth's earliest continent forming processes may have differed from the modern plate tectonic regime. Oxygen isotope ratios (δ18O) in primary crystalline zircon are strongly affected by the incorporation of supracrustal materials during magmatism and can provide useful information about the character of early crustal reworking events. While zircon δ18O signatures >+6.4 ‰ indicate the incorporation of supracrustal material into the source of crustal anatexis, questions remain regarding the volumetric scale, sources, and processes by which supracrustal and magmatic mixing occurred during the Archean. Here, we present 271 zircon in-situ oxygen isotope analyses for a suite of seven Saglek-Hebron granitoids emplaced between 3.87 and 2.79 Ga in age, significantly expanding the temporal and lithologic diversity of the magmatic SHC zircon δ18O record. Collected from carefully characterized grain domains, primary zircon δ18O values range from 5.0 to 8.4 ‰ and include some of the highest igneous Archean zircon oxygen isotope ratios recorded to date. Leveraging an integrated workflow for identifying preserved zircon geochemistry, and correlating our new data with previous information from zircon Hf and whole-rock 142Nd/144Nd isotopes, we identify three periods of contribution from high-δ18O supracrustal material to crustal melts during reworking events in the SHC at 3.63, 3.33, and 2.79 Ga. Our work points towards repeated episodes of interaction between a maturing surface environment and magmatism from 3.6 Ga onwards at this locality.

Identifying crustal contributions in the Patagonian Chon Aike Silicic Large Igneous Province

The volcanic rocks of the Chon Aike Silicic Large Igneous Province (CASP) are recognized as magmas dominantly produced by crustal anatexis. Investigating the zircon of the CASP provides an opportunity to gain further insight into geochemical and isotopic differences of the potential magmatic sources (i.e., crust versus mantle), to identify crustal reservoirs that contributed to the felsic magmas during anatexis, and to quantify the contributions of the respective sources. We present a combined zircon oxygen and hafnium isotope and trace element dataset for 16 volcanic units of the two youngest volcanic phases in Patagonia, dated here with LA-ICP-MS U–Pb geochronology at ca. 148–153 Ma (El Quemado Complex, EQC) and ca. 159 Ma (western Chon Aike Formation, WCA). The EQC zircon have 18O-enriched values (δ18O from 7 to 9.5‰) with correspondingly negative initial εHf values (− 2.0 to − 8.0). The WCA zircon have δ18O values between 6 and 7‰ and εHf values ranging between − 4.0 and + 1.5. Binary δ18O-εHf mixing models require an average of 70 and 60% melt derived from partial melting of isotopically distinct metasedimentary basements for the EQC and WCA, respectively. Zircon trace element compositions are consistent with anatexis of sedimentary protoliths derived from LIL-depleted upper continental crustal sources. The overlap between a high heat flux environment (i.e., widespread extension and lithospheric thinning) during supercontinental breakup and a fertile metasedimentary crust was key in producing voluminous felsic volcanism via anatexis following the injection and emplacement of basaltic magmas into the lower crust.

Petrogenesis and Geodynamic Evolution in the Northern Westfjords, Iceland, Elucidated by Iceland's Oldest Silicic Rocks

AbstractIceland's oldest silicic rocks provide unique insight into the island's early crustal evolution. We present new zircon U‐Pb ages bolstered with zircon trace element and isotopic compositions, and whole rock Nd, Hf, and Pb isotope compositions, from three silicic magmatic centers—Hrafnsfjörður, Árnes, and Kaldalón—to understand the petrogenesis of large silicic volcanic centers in the northern Westfjords, Iceland. Our data confirm Hrafnsfjörður as the oldest known central volcano in Iceland (∼14 Ma) and establish an older age for Árnes (∼13 Ma) than previously estimated. We also report the first U‐Pb zircon dates from Kaldalón (∼13.5 Ma). Zircon oxygen isotope compositions range from δ18O∼+2 to +4‰ and indicate involvement of a low‐18O component in their source magmas. Hrafnsfjörður zircon Hf (mean sample εHf ∼ +15.3–16.0) and whole rock Hf and Nd (εHf = +14.5 to +15; εNd = +7.9 to +8.1) isotopic compositions are more radiogenic than those from Árnes (zircon sample εHf ∼ +11.8–13; whole rock εHf = +12.8 to +15.1; εNd = +7.3 to +7.7), but Hrafnsfjörður whole rock Pb isotope compositions (208/204Pb = 37.95–37.96; 206/204Pb = 18.33–18.35) are less radiogenic than those from Árnes (208/204Pb = 38.34–38.48; 206/204Pb = 18.64–18.78). Kaldalón has zircon Hf isotope compositions of εHf∼+14.8 and 15.5 (sample means). These age and isotopic differences suggest that interaction of rift and plume, and thus the geodynamic evolution of the Westfjords, is complex. Isotopic compositions of Hrafnsfjörður and Árnes support involvement of an enriched mantle (EM)‐like mantle component associated with a pulsing plume that resulted in variable spreading rates and magma fluxes and highlight the heterogeneity of the Icelandic mantle.

Late Neoarchean crust–mantle interaction and tectonic implications in western Shandong Province, North China Craton: Evidence from a granodiorite pluton and associated magmatic enclaves

This study presents original data on the microstructure, mineralogy, whole–rock geochemistry, and isotopes of a Late Neoarchean granodiorite pluton and associated mafic microgranular enclaves (MMEs) in western Shandong Province of the eastern part of the North China Craton (NCC). The MMEs hosted in the granitic pluton are more mafic, and amphibole and biotite are the predominant mafic minerals. The sharp contacts and fine–grained textures of the MMEs indicated a rapid quenching process during a magma mingling event. Zircon and titanite UPb dating indicated that the host granodiorite pluton formed at circa 2546 Ma, and the MMEs formed at approximately 2538 Ma. The dating results indicated a crust–mantle interaction. Geochemically, the host pluton has high silica contents (SiO2 = 67.68–68.51 wt%), Mg# values (51–53) and low Aluminum Saturation Index ratios (A/CNK = 0.92–0.95), which are indicative of I–type granitic affinity. Moreover, they show arc–type trace element features characterized by enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs) (e.g., Cs, Rb, K, and Pb), but depletion in high field strength elements (HFSEs) (e.g., Nb, Ta, P, and Ti). In contrast, the MMEs have lower silica contents (SiO2 = 52.07–56.03 wt%), but higher Mg# values (56–68) and Cr, Ni and Cu contents of 454–633, 159–225 and 98.3–207 ppm, respectively, corresponding to a mantle component. Similar to the host pluton, the MMEs show enrichment of LREEs and LILEs, but depletion of HFSEs, reflecting an arc–type magma origin. Isotopically, the host granodiorite and MMEs both have relatively depleted NdHf isotopic compositions, with εNd(t) values of −0.67–1.22 and − 0.31–1.89, and εHf(t) values of 5.63–2.03 and − 0.18–4.48, respectively. Zircon oxygen isotopes reveal that the host granodiorite and MMEs have slightly higher oxygen isotopes than the normal mantle zircon (5.3‰), with mean δ18O values of 6.35‰ and 6.81‰, respectively, indicating assimilation of minor supracrustal materials in the magma source. However, a combination of Nd–Hf–O isotopes suggests that the MMEs haveslightly enriched isotopic compositions than the host granodiorite pluton, suggesting a “reversed isotope” feature. This important finding also enlightens that “reversed isotope” phenomenon represented magma mingling between isotopically relatively enriched mafic and relatively depleted felsic magmas. Furthermore, the Ce4+/Ce3+ ratios in zircon from the host granodiorite and MMEs indicated a low oxygen fugacity, with mean values of 24.25 and 39.69, respectively. Although the host granodiorite and MMEs have the similar oxygen fugacity ranges, the MMEs have high Cu concentrations of 98.3–207 ppm (mean value = 151 ppm), indicating a Cu–rich magma source. Integrating the petrogenesis of the Late Neoarchean granodiorite pluton and associated MMEs, it is inferred that the host granodiorite pluton was derived from partial melting of mafic lower crust with minor insignificant ancient materials, and the MMEs were derived from partial melting of depleted mantle metasomatized by sulfur–rich sediment melts/fluids (chalcophile element). Tectonically, our samples reveal an affinity with an arc in a syn–collisional setting. Together with regional rock assemblages, we suggest that the onset of plate tectonics (subduction) was to occur by the end of Neoarchean in the NCC.

Did transit through the galactic spiral arms seed crust production on the early Earth?

Abstract Although there is evidence for periodic geological perturbations driven by regular or semi-regular extra-terrestrial bombardment, the production of Earth's continental crust is generally regarded as a function of planetary differentiation driven by internal processes. We report time series analysis of the Hf isotopic composition of zircon grains from the North Atlantic and Pilbara cratons, the archetypes of Archean plate tectonic and non-plate tectonic settings, respectively. An ~170–200 m.y. frequency is recognized in both cratons that matches the transit of the solar system through the galactic spiral arms, where the density of stars is high. An increase in stellar density is consistent with an enhanced rate of Earth bombardment by comets, the larger of which would have initiated crustal nuclei production via impact-driven decompression melting of the mantle. Hence, the production and preservation of continental crust on the early Earth may have been fundamentally influenced by exogenous processes. A test of this model using oxygen isotopes in zircon from the Pilbara craton reveals correlations between crust with anomalously light isotopic signatures and exit from the Perseus spiral arm and entry into the Norma spiral arm, the latter of which matches the known age of terrestrial spherule beds. Our data support bolide impact, which promoted the growth of crustal nuclei, on solar system transit into and out of the galactic spiral arms.

Cretaceous basalt-andesite sequence in the Southern Pamir: arc—back-arc architecture at the Pamir Plateau genetically related to the northward flat subductions of the Neo-Tethys Ocean

The Pamir-Karakorum is one of the most important regions for our understanding the Tethys evolution. Although it is commonly known that the northward subduction of the Neo-Tethys Ocean induced the formation of the magmatic arc in Southern Pamir, the temporal-spatial configuration of the arc—back-arc during the Neo-Tethys evolution remains unclear. In this study, we report sedimentary feature, petrography, zircon U Pb ages and Hf O isotope compositions as well as bulk-rock and mineral geochemistry of the well-preserved Cretaceous volcanic sequence in Chinese Wahan Corridor at South Pamir. The volcanic sequence is mainly composed of phenocrysts- bearing basalt- andesites with minor rhyolite and volcaniclastic rock. Zircon U Pb ages reveal the basalt-andesite sequence erupted at 100–98 Ma. Bulk-rock geochemistry defines their potassic calc-alkaline signature and both their rock association and elemental geochemistry share most features of the continental arc volcanic sequences. On the other hand, they have variably enriched Nd isotope compositions with ε Nd (t) varying from −5.9 to −9.6, coupled with their enriched zircon Hf isotope signature with ε Hf (t) between −13 to −8. Whole-rock geochemistry and zircon oxygen isotope compositions (average δ 18 O = 7.74 ± 0.14) demonstrate that the primitive magma of the basalt-andesites were derived from partial melting of a metasomatized sub-continental lithospheric mantle and underwent variable assimilation fractional crystallization. Late Jurassic to Cretaceous volcanic-sedimentary sequences and coeval voluminous arc-like granodiorite and granites are widely distributed along the Southern Pamir. In the Central Pamir and further north, late Jurassic to Cretaceous sequences resemble features of back-arc basins. Taken together, we suggest that a northward low-angle or flat subduction of the Neo-Tethys Ocean along the Shyok zone was geodynamically related to the formation of the broad magmatic arc in South Pamir-Karakorum and the back-arc basins north of the South Pamir. • Arc-like Cretaceous basalt-andesite sequence in Southern Pamir (SP) erupted at ~100 Ma. • Architecture of the late Jurassic-Cretaceous arc-back-arc at Pamir was constructed. • Flat subduction of the Neo-Tethys Ocean produced the Cretaceous igneous rocks in SP.

Whole-rock geochemistry and zircon O-Hf isotope compositions of ca. 2.35 Ga strongly peraluminous granites: Implications for increase in zircon δ18O values during the Paleoproterozoic

Zircon oxygen isotope ratios have been used to trace the incorporation of sedimentary rocks into magmas. The dramatic increase in maximum zircon δ18O values in the Paleoproterozoic observed in global databases coincides with changes in surface environments (e.g., the rise of subaerial and oxidative weathering), implying a connection between elevated zircon δ18O and these changes. Zircon δ18O between 2.5 and 2.2 Ga, however, is relatively under-constrained owing to limited available data in this age range. To augment data from this critical time period and understand potential causes for the elevated zircon δ18O values, we report U-Pb zircon ages and δ18O values of zircon, as well as, whole-rock major and trace element geochemistry of Paleoproterozoic strongly peraluminous granites (SPGs) from the southwestern margin of the Yangtze Block (China). Our geochronological data demonstrate that these SPGs crystallized at ∼2.35 Ga and that inherited zircon with ages of 2428–2721 Ma are present in these granites, indicating the source rocks of these granites were deposited, subsequently metamorphosed, and partially melted between 2.43 and 2.35 Ga. Synmagmatic zircon from samples dated in this study have εHf(t) values of −6.4 to −0.9 and high δ18O values of 7.6–9.9‰, elevated above the maximum value observed in Archean zircon (∼7‰). These granites can be divided into two groups based on whole-rock geochemistry. Both Group 1 and Group 2 granites were derived from a similar high δ18O, metapelitic source, but were generated by dehydration melting and hydrous melting, respectively. Our results demonstrate that the fine-grained sedimentary rocks from which the SPGs were derived had relatively high δ18O (as compared to older sedimentary rocks) by 2.43–2.35 Ga. The depositional time interval of the high-δ18O sedimentary sources for SPGs studied here coincides with the emergence of continental crust above sea level and the Great Oxidation Event. Supporting the findings of previous studies, the contemporaneity of our dataset with these changes in Earth’s surface environments suggests that subaerial and potentially oxidative weathering contributed (at least partially) to the elevation of δ18O of fine-grained sedimentary rocks. Recycling of these high-δ18O sedimentary rocks into magmas contributed to the dramatic change in δ18O of magmatic zircon in the earliest Paleoproterozoic. In addition, although this study is focused on a single locality, our results suggest that the abrupt shift observed in global zircon δ18O data sets likely occurred by 2.35 Ga. Last, a literature compilation of zircon δ18O data from SPGs suggested that zircon δ18O values may have also experienced a stepwise increase in the Neoproterozoic to Phanerozoic from 12 to 14‰. The coincidence of these increases in zircon δ18O values with global oxygenation events suggests that atmospheric oxygenation may have contributed to the increase in δ18O of sedimentary rocks.

Reconstruction of a volcano-sedimentary environment shared by the Porongos and Várzea do Capivarita complexes at 790 Ma, Dom Feliciano Belt, southern Brazil

This work investigates the pre-collisional (before ca. 650 Ma) history of the Dom Feliciano Belt in southernmost Brazil through geochronological and zircon oxygen isotope study. U–Pb SHRIMP dating of two orthogneiss samples from the Várzea do Capivarita Complex and one metarhyolite sample from the Porongos Complex yielded crystallisation ages of 786 ± 5 Ma, 780 ± 10 Ma and 787 ± 5 Ma, respectively. The mean oxygen isotope values calculated for the ca. 790 Ma zircon cores from the orthogneisses are 8.41 ± 0.13‰ and 8.68 ± 0.14‰, and 8.75 ± 0.72‰ for the metarhyolite. Such values suggest that zircon crystallised in the more evolved magmas, either from the melting of host rocks and sediments or assimilation of crustal material by mantle-derived magmas. The detrital zircon population was analysed in one additional paragneiss sample from the Várzea do Capivarita Complex, and most of the values cluster at 790–750 Ma. The data spread is centred at ca. 790 Ma, which is the crystallisation age of the interleaved orthogneisses. In our interpretation, such dataset suggests a syn-volcanic origin of the paragneiss protolith and, therefore, a volcano-sedimentary origin of the Várzea do Capivarita Complex. The correspondence of geochronological data and zircon oxygen isotope values for the studied meta-igneous samples suggests that the Várzea do Capivarita and Porongos complexes have shared the same igneous history. Therefore, the samples probably represent one magmatic event at different levels of a single basin at ca. 800–770 Ma. Such results bring first-order information about the meaning of tectonic limits in this Gondwana-related belt and implications for reconstructing the pre-collisional history of the orogen.

Oceanic Zircon Records Extreme Fractional Crystallization of MORB to Rhyolite on the Alarcon Rise Mid-Ocean Ridge

Abstract The first known occurrence of rhyolite along the submarine segments of the mid-ocean ridge (MOR) system was discovered on Alarcon Rise, the northernmost segment of the East Pacific Rise (EPR), by the Monterey Bay Aquarium Research Institute in 2012. Zircon trace element and Hf and O isotope patterns indicate that the rhyolite formed by extreme crystal fractionation of primary mid-ocean ridge basalt (MORB) sourced from normal to enriched MOR mantle with little to no addition of continental lithosphere or hydrated oceanic crust. A large range in zircon ɛHf spanning 11 ɛ units is comparable to the range of whole rock ɛHf from the entire EPR. This variability is comparable to continental granitoids that develop over long periods of time from multiple sources. Zircon geochronology from Alarcon Rise suggests that at least 20 kyr was needed for rhyolite petrogenesis. Grain-scale textural discontinuities and trace element trends from zircon cores and rims are consistent with crystal fractionation from a MORB magma with possible perturbations associated with mixing or replenishment events. Comparison of whole rock and zircon oxygen isotopes with modeled fractionation and zircon-melt patterns suggests that, after they formed, rhyolite magmas entrained hydrated mafic crust from conduit walls during ascent and/or were hydrated by seawater in the vent during eruption. These data do not support a model where rhyolites formed directly from partial melts of hydrated oceanic crust or do they require assimilation of such crust during fractional crystallization, both models being commonly invoked for the formation of oceanic plagiogranites and dacites. A spatial association of highly evolved lavas (rhyolites) with an increased number of fault scarps on the northern Alarcon Rise might suggest that low magma flux for ~20 kyr facilitated extended magma residence necessary to generate rhyolite from MORB.

Disparities in oxygen isotopes of detrital and igneous zircon identify erosional bias in crustal rock record

The detrital zircon oxygen isotope record has been used to describe 4.4 billion years of crustal evolution and maturation. Implicit in the application and use of large databases of oxygen isotope ratio (δ18O) from detrital zircon is that such data are more representative of the history of the continental crust than are exposed rocks. Here we show that the δ18O of igneous zircon in preserved rocks varies greatly from the detrital zircon record over time, both in terms of the magnitude of δ18O and the secular variation. Starting from 4.0 Ga and lasting throughout the Archean, the detrital zircon record shows consistently higher δ18O values than the rock record. In post-Archean times, detrital zircon δ18O is conspicuously higher during periods associated with supercontinent amalgamation. Differences between detrital and igneous zircon δ18O records imply that the igneous source rocks from which high δ18O detrital zircon are derived have been entirely eroded and are not represented in the known geological record. We postulate that the ‘missing’ record of high δ18O magmatic rocks consists of upper crustal sedimentary-derived melts from collisional belts, such as that found in the Himalayan orogen. The observation of higher δ18O in Archean detrital zircon further requires a revision of the current paradigm for Archean magmatism. More broadly, recognition of a ‘missing’ component of the upper crustal rock record calls into question the degree to which the detrital zircon record accurately reflects the composition of the bulk continental crust.

Effects of crustal assimilation and magma mixing on zircon trace element relationships across the Peninsular Ranges Batholith

Magmatic zircon tends to exhibit characteristic trends in trace element contents in response to magma cooling and fractionation, such that zircon may provide a window into melt evolution not accessible by whole rock geochemistry. The Peninsular Ranges Batholith of southern California and Baja California consists of older rocks of oceanic arc affinity in the west. Younger rocks to the east been interpreted via whole rock chemistry to reflect (Transition Zone) deeper origins and (Eastern Zone) deeper origins with significant crustal assimilation compared to the Western Zone. The youngest magmas in the Eastern Zone form large, typically zoned plutons and are high in Sr/Y. We present zircon trace element and oxygen isotope measurements in selected Western and Transition Zone samples and in many units from the high-Sr/Y La Posta and San Jacinto plutons. Most Western and Transition Zone populations exhibit typical shallow fractionation-related trace element trends, while one Eastern Transition Zone sample and most samples from the high-Sr/Y Eastern Zone lack Eu/Eu* evidence for plagioclase fractionation. Oxygen isotopes, zircon P contents, and Ce-based redox estimates suggest that the most evolved unit (2-mica granodiorite) of the La Posta pluton is significantly contaminated by sediments, with other high-Sr/Y units displaying zircon oxygen isotope and redox estimates falling between the 2-mica granodiorite and the relatively uncontaminated Transition Zone in these parameters. However, most of these zircons lack unambiguous markers for supracrustal contamination that could be interpreted even in a detrital context. The contrasting behavior of zircon U/Yb and Eu/Eu* during shallow fractionation and their similar response to melting depth variations can be used to distinguish the effects of shallow fractional crystallization, depth of melting, and introduction of exotic materials by either crustal assimilation or magma mixing. On the basis of U/Yb vs Eu/Eu* relationships, we interpret the behavior of Western Zone and most Transition Zone zircons as reflecting mainly shallow fractionation. One Eastern Transition Zone tonalite appears to result from mixing of magma from different depths. We interpret the high-Sr/Y Eastern Zone magmas as resulting from mixing of magmas that have assimilated varying amounts of metasediment. Moderate levels of sediment assimilation may complicate magma depth interpretations which are based on zircon (Th, U)/Yb plots or detrital zircon Eu/Eu* – without yielding clear zircon geochemical evidence for supracrustal assimilants. Magma mixing is less problematic for regional-scale detrital zircon Eu/Eu* studies and may be easier to detect based on the relationships among (Th, U)/Yb and Eu/Eu*.

Intracontinental rift-related magmatism in the eastern Emeishan Large Igneous Province traced by zircon oxygen isotopes

Rift systems associated with mantle plume activity are potential conduits for the transfer of voluminous magmas and fluids, and also contribute to ore formation in magmatic and hydrothermal systems. The Panxi (Panzhihua-Xichang) rift within the ~260 Ma Emeishan Large Igneous Province (LIP) is a typical case where world-class Fe-Ti-V oxide and relatively small Cu-Ni sulfide deposits are hosted in mafic-ultramafic intrusions. However, whether similar rifts exist in other parts of the Emeishan LIP remains unclear. Here we carry out an integrated SIMS analysis of zircon U-Pb-O isotopes from volcanic tuffs from the eastern part of the Emeishan LIP. Our results indicate that magmatic zircons with U-Pb ages coeval with the timing of the Emeishan magmatism have δ18O values lower than that of mantle, and as low as +2.94‰. In conjunction with previously reported evidence from sedimentary units, we propose that the silicic magmas with relatively low δ18O values in the eastern Emeishan LIP were derived from crustal recycling of hydrothermally altered rocks within an extensional setting associated with mantle plume activity. Our results combined with previous data suggest that the rifts triggered by the mantle plume activity in the eastern Emeishan LIP may be the potential locales for the exploration of magmatic sulfide deposits.

The onset of deep recycling of supracrustal materials at the Paleo-Mesoarchean boundary.

The recycling of supracrustal materials, and in particular hydrated rocks, has a profound impact on mantle composition and thus on the formation of continental crust, because water modifies the physical properties of lithological systems and the mechanisms of partial melting and fractional fractionation. On the modern Earth, plate tectonics offers an efficient mechanism for mass transport from the Earth's surface to its interior, but how far this mechanism dates back in the Earth's history is still uncertain. Here, we use zircon oxygen (O) isotopes to track recycling of supracrustal materials into the magma sources of early Archean igneous suites from the Kaapvaal Craton, southern Africa. The mean δ18O values of zircon from TTG (tonalite–trondhjemite–granodiorite) rocks abruptly increase at the Paleo-Mesoarchean boundary (ca. 3230 million years ago; Ma), from mantle zircon values of 5‰–6‰ to approaching 7.1‰, and this increase occurs in ≤3230 Ma rocks with elevated Dy/Yb ratios. The 18O enrichment is a unique signature of low-temperature water–rock interaction on the Earth's surface. Because the later phase was emplaced into the same crustal level as the older one and TTG magmas would derive from melting processes in the garnet stability field (>40 km depth), we suggest that this evident shift in TTG zircon O isotopic compositions records the onset of recycling of the mafic oceanic crust that underwent seawater hydrothermal alteration at low temperature. The onset of the enhanced recycling of supracrustal materials may also have developed elsewhere in other Archean cratons and reflects a significant change in the tectonic realm during craton formation and stabilization, which may be important processes for the operation of plate tectonics on early Earth.

Evolution of the Lower Permian Rochlitz volcanic system, Eastern Germany: reconstruction of an intra-continental supereruption

Extensional tectonics in the Late Paleozoic Central Europe was accompanied by rift magmatism that triggered voluminous intracontinental caldera-forming eruptions. Among these, the Lower Permian Rochlitz Volcanic System (RVS) in the North Saxon Volcanic Complex (Eastern Germany, Saxony) represents a supereruption (VEI 8, estimated volume of 1056 km3) of monotonous rhyolites followed by monotonous intermediates. Mapping, petrography, whole-rock geochemistry along with mineral chemistry and oxygen isotopes in zircon display its complex eruption history and magma evolution. Crystal-rich (> 35 vol%), rhyolitic Rochlitz-α Ignimbrite with strong to moderate welding compaction erupted in the climactic stage after reheating of the magma by basaltic injections. Due to magma mixing, low-volume trachydacitic-to-rhyolitic Rochlitz-β Ignimbrite succeeded, characterized by high Ti and Zr-values and zircon with mantle δ18O. Randomly oriented, sub-horizontally bedded fiamme, and NW–SE striking subvolcanic bodies and faults suggest pyroclastic fountaining along NW–SE-oriented fissures as the dominant eruption style. Intrusion of the Leisnig and the Grimma Laccoliths caused resurgence of the Rochlitz caldera forming several peripheral subbasins. In the post-climactic stage, these were filled with lava complexes, ignimbrites and alluvial to lacustrine sediments. Significant Nb and Ta anomalies and high Nb/Ta ratios (11.8–17.9) display a high degree of crustal contamination for the melts of the RVS. Based on homogenous petrographic and geochemical composition along with a narrow range of δ18O in zircon Rochlitz-α Ignimbrite were classified as monotonous rhyolites. For the Rochlitz-β Ignimbrites, underplating and mixing with basic melts are indicated by Mg-rich annite–siderophyllite and δ18O < 6.0 in zircon. The wide spectrum of δ18O on zircon suggests an incomplete mixing process during the formation of monotonous intermediates in the RVS.

Raman spectroscopy-based screening of zircon for reliable water content and oxygen isotope measurements

Abstract Water content and oxygen isotopes in zircon provide crucial constraints on magma source and process, yet they can be significantly modified by zircon metamictization, which causes secondary water absorption into the zircon crystal and the concomitant oxygen isotope changes. Therefore, it is imperative to develop a screening scheme to select the least-metamict zircons for the analyses. We propose a screening scheme based on our study on the Suzhou A-type granite (South China) through integrating Raman spectroscopy, water and trace element measurements, and oxygen isotope analysis. The results show that the primary water content is retained in zircon when the full-width at half maximum (FWHM) is &amp;lt;8 cm−1 or the Raman shift is &amp;gt;1007 cm−1 of ν3(SiO4) vibration band, while the primary δ18O is preserved at &amp;lt;10 cm−1 FWHM or &amp;gt;1005.5 cm−1 Raman shift. Changes in trace element concentrations in Suzhou zircons are different from previous observations in metamict zircons but most likely related to magma evolution, which implies that trace elements are insensitive to metamictization. Primary δ18O in Suzhou zircons (4.5–6.0‰) fall into the mantle range, indicating a dominant mantle contribution to Suzhou granites. Primary water content was estimated at ca. 650–1400 ppm, significantly higher than those of typical I-type granite (400–736 ppm) and upper mantle-derived zircons (81–177 ppm). The high primary zircon water content was not controlled by the sub-solidus process, temperature, pressure, and cation charge balance but considered to reflect the high-water content in melts. This suggests a hydrous origin for the Suzhou A-type granite, which challenges the conventional view of anhydrous petrogenesis for A-type granites.