Zircon Xenocrysts Research Articles

Archaean zircons in Miocene oceanic hotspot rocks establish ancient continental crust beneath Mauritius

A fragment of continental crust has been postulated to underlie the young plume-related lavas of the Indian Ocean island of Mauritius based on the recovery of Proterozoic zircons from basaltic beach sands. Here we document the first U–Pb zircon ages recovered directly from 5.7 Ma Mauritian trachytic rocks. We identified concordant Archaean xenocrystic zircons ranging in age between 2.5 and 3.0 Ga within a trachyte plug that crosscuts Older Series plume-related basalts of Mauritius. Our results demonstrate the existence of ancient continental crust beneath Mauritius; based on the entire spectrum of U–Pb ages for old Mauritian zircons, we demonstrate that this ancient crust is of central-east Madagascar affinity, which is presently located ∼700 km west of Mauritius. This makes possible a detailed reconstruction of Mauritius and other Mauritian continental fragments, which once formed part of the ancient nucleus of Madagascar and southern India.

Syntectonic emplacement of the Triassic biotite-syenogranite intrusions in the Taili area, western Liaoning, NE China: Insights from petrogenesis, rheology and geochronology

The North China Craton (NCC) is one of the oldest cratons in the world, and it recently becomes a hot study area because of large volumes of Mesozoic intrusions associated with lithospheric thinning contributing to cratonic destruction in late Mesozoic times. However, the timing of initial thinning and destruction is still controversial. The Taili area, western Liaoning Province, in the northeastern part of the NCC well exposes the Archean basement rocks and the Mesozoic magmatic rocks with variable plastic deformation. This study focuses on the syntectonic emplacement of the Triassic biotite-syenogranite intrusions, in order to understand their petrogenesis, timing as well as the geological significance. Zircon LA-ICP-MS U-Pb ages reveal that the biotite-syenogranites formed between 246 and 191Ma, and contain many ancient (2564–2317Ma) zircon xenocrysts. Geochemical data suggests that the biotite-syenogranites display an adakitic affinity with high Sr/Y=135–167 and (La/Yb)N=48–69, as well as negligible Eu anomalies (δEu=0.87–0.94), high negative zircon εHf(t) values (−15.5 to −21.5) and ancient TDM2 ages (2246–2598Ma). This data suggests that the parent magmas were generated from partial melting of thickened Archean lower crustal rocks probably due to the bidirectional amalgamation of the NCC with the NE China micro-blocks and the Yangtze Craton in its north and south, respectively. In the middle part of the Taili area, magmatic fabrics are well preserved in the biotite-syenogranite intrusion characterized by the strong preferred orientation of biotite and hornblende crystals, which parallel to the intrusion margin and are slightly oblique to the gneissosity of the sheared host Neoarchean granitic gneisses. The quartz grain size piezometer suggests that the paleo-differential stresses weaken toward to the central part of the intrusion, ranging from 21.40–22.22MPa to 16.74–19.34MPa, during quartz crystallization in the emplacement stage. This allow deduce much higher strain rates in the center (1.26×10−11–2.24×10−9s−1) than at the margin (9.07×10−12–1.31×10−9s−1) of the pluton. These observations are interpreted by the rheological behavior of magma during the magmatic “pipe” flow. The adakitic source melts ascended through the conduits along weak NE-trending sinistral shear zones, and emplaced at the shallower depth of ∼16km before Early Jurassic (∼190Ma). The biotite-syenogranites were still in a semisolid state, when garnet-bearing granitic aplites injected at ∼220Ma. This stage records elongate (constrictional) strain under the sinistral shear stresses, particularly in quartz grains occurring in the margin of intrusions. In combination with previous studies, an exhumation rate of the NCC’s Archean basement (from ∼25km to ∼11km in depth) is calculated as initial low exhumation rate of ∼4.0mm/kyr from Neoarchean to Late Triassic, and subsequent a rapid exhumation process of ∼63mm/kyr between Late Triassic to Early Cretaceous. All the results presented here allow us to consider the geodynamic evolution of the eastern NCC and constrain the onset of lithospheric thinning and cratonic destruction of the NCC as early as Middle Triassic (∼240Ma) triggered by the amalgamation of adjacent blocks. It developed prosperously since Late Triassic, due to the oblique subduction of the Paleo-Pacific Plate.

NO EXCESSIVE CRUSTAL GROWTH IN THE CENTRAL ASIAN OROGENIC BELT: FURTHER EVIDENCE FROM FIELD RELATIONSHIPS AND ISOTOPIC DATA

Abstract We provide new field observations and isotopic data for key areas of the Central Asian Orogenic Belt (CAOB), reiterating our previous assessment that no excessive crustal growth occurred during its ca. 800 Ma long orogenic evolution. Many Precambrian blocks (microcontinents) identified in the belt are exotic and are most likely derived from the northern margin of Gondwana, including the Tarim craton. Ocean opening in the Palaeo-Asian Ocean, arc formation and accretionary processes began in the latest Mesoproterozoic along the southern margin of the Siberian craton and continued into the Neoproterozoic, giving rise to tectono-metamorphic terranes distinct from the exotic microcontinents in that they include tectonically mixed ancient crust as well as juvenile, mantle-derived igneous rocks. Several previous assessments of crustal growth based on the distribution of oceanic and island arc complexes and on Nd isotopic data for post-accretion igneous rocks are questionable, and we show that such data, in combination with the occurrence of old zircon xenocrysts, frequently signify tectonic mixing of juvenile and ancient crustal components. The only truly juvenile terranes, including oceanic crust and intra-oceanic arcs, seem to occur in northeastern Kazakhstan, in the Altai-Sayan region of Siberia and in the Lake and Trans-Altai zones of Mongolia. The largest area of pre-CAOB continental crust forms a broad belt from northwestern Kazakhstan via the Kyrgyz North and Middle Tianshan to the Yili Block and Chinese Central Tianshan in NW China. Most arcs in the CAOB formed on older continental crust, or with substantial addition of old crustal material via sediment recycling, similar to the situation in the present Southwest Pacific in southern Indonesia, and we suspect that the volume of old material in the lower crust of the CAOB is considerable but largely unaccounted for because of lack of geophysical data. Comparing the lithospheric mantle domains as revealed by Os model ages, with ancient crust at least Mesoproterozoic in age and predating formation of the CAOB significantly reduces the volume of new juvenile crust generated during the orogeny. We conclude that the volume of truly juvenile crustal material in the CAOB is about 20%, similar to that in other accretionary orogens through Earth history, and considering the ca. 800 Ma history of the belt this is not anomalous.

TRACKING DEEP ANCIENT CRUSTAL COMPONENTS BY XENOCRYSTIC ZIRCONS OF PALEOZOIC FELSIC IGNEOUS ROCKS FROM THE ALTAI-EAST JUNGGAR TERRANE AND ADJACENT REGIONS AND ITS TECTONIC SIGNIFICANCE

The deep crustal continental components and architecture of the Central Asian Orogenic Belt have long been a matter of debate [Wang et al., 2009; Kroner et al., 2014; Xiao et al., 2015; Yang et al., 2017]. We present an integrated study of geochronological and Hf-inzircon isotopic data for xenocrystic zircons from the Paleozoic granitoid rocks and associated felsic volcanic rocks of the Chinese Altai, East Junggar and nearby regions. The aim is to trace the age spatial distribution of deep old crustal components in these key parts of the western Central Asian Orogenic Belt.

New insights into the crustal growth of the Paleoproterozoic margin of the Archean Kéména-Man domain, West African craton (Guinea): Implications for gold mineral system

The Baoulé-Mossi Paleoproterozoic domain on the West African Craton is known for its world-class orogenic gold deposits formed during the Eburnean orogeny between ca. 2.15 and 2.07Ga. New results of U-Pb zircon dating in combination with Hf-isotope geochemical data indicate that a suite of mafic and felsic potassic plutonites were emplaced in Guinea (in vicinity to the Archean Kéména-Man domain, KMD) between ca. 2.10 and 2.08Ga. All plutonites are characterized by (i) enrichment in LILE and LREE, (ii) strong positive Pb spikes and (iii) depletion in Ta-Nb-Ti, but based on zircon U-Pb-Hf isotope data two groups can be distinguished, each comprising Qtz-monzodiorites and Bt-granites. Bt-granites of group 1, sampled away from the craton are xenocryst-free despite similar or higher Zr-saturation temperatures (∼820°C) compared to group 2 rocks (∼780°C), and reveal mostly superchondritic ɛHf(2.1 Ga) up to +3.7, (TDM=2.8–2.4Ga). Rocks of group 2, sampled in close vicinity to the KMD, show highly negative ɛHf(2.1 Ga) of −16 to −10 (TDM=3.5–3.2Ga) and contains abundant zircon xenocrysts with ages between 3.71 and 2.78Ga (TDM=4.2–3.4), overlapping with zircon U-Pb-Hf data obtained from three granodiorites of the KMD, dated at ca. 3.09, 3.08 and 2.87Ga. This difference indicates that the potassic plutonites resulted from partial melting of different sources within the crust and mantle during the same magmatic event at ca. 2.10–2.08Ga. The Qtz-monzodiorites derived from metasomatized mantle sources enriched by subducted sediments that in case of group 1 were derived from relatively juvenile Paleoproterozoic island arcs, and in case of group 2 from the Archean KMD. Bt-granites of group 1 might result either from reworking of relatively juvenile Paleoproterozoic crust, or from hybridization between (fractionated) monzodiorites and hot crustal melts, whereas Bt-granites of group 2 can be explained by in situ reworking of Archean TTG crust. Contemporaneous partial melting of the different mantle and crustal domains perhaps happened in response to lithospheric delamination following the collision of the juvenile Paleoproterozoic domain with the KMD. We further propose that this process also caused remelting of residual Au-rich cumulates left in the deep lithosphere by previous arc magmatism or Au-enriched metasomatic veins in lithospheric mantle.

Petrogenesis of Quebrada de la Mina and Altar North porphyries (Cordillera of San Juan, Argentina): Crustal assimilation and metallogenic implications

We investigate the geology of Altar North (Cu–Au) and Quebrada de la Mina (Au) porphyry deposits located in San Juan Province (Argentina), close to the large Altar porphyry copper deposit (995 Mt, 0.35% Cu, 0.083 g/t Au), to present constraints on the magmatic processes that occurred in the parental magma chambers of these magmatic-hydrothermal systems. Altar North deposit comprises a plagioclase-amphibole-phyric dacite intrusion (Altar North barren porphyry) and a plagioclase-amphibole-biotite-phyric dacite stock (Altar North mineralized porphyry, 11.98 ± 0.19 Ma). In Quebrada de la Mina, a plagioclase-amphibole-biotite-quartz-phyric dacite stock (QDM porphyry, 11.91 ± 0.33 Ma) crops out. High Sr/Y ratios (92–142) and amphibole compositions of Altar North barren and QDM porphyries reflect high magmatic oxidation states (fO2 = NNO +1.1 to +1.6) and high fH2O conditions in their magmas. Zones and rims enriched in anorthite (An37–48), SrO (0.22–0.33 wt.%) and FeO (0.21–0.37 wt.%) in plagioclase phenocrysts are evidences of magmatic recharge processes in the magma chambers. Altar North and Quebrada de la Mina intrusions have relatively homogeneous isotopic compositions (87Sr/86Sr(t) = 0.70450–0.70466, εNd(t) = +0.2 to +1.2) consistent with mixed mantle and crust contributions in their magmas. Higher Pb isotopes ratios (207Pb/204Pb = 15.6276–15.6294) of these intrusions compared to other porphyries of the district, reflect an increase in the assimilation of high radiogenic Pb components in the magmas. Ages of zircon xenocrysts (297, 210, 204, 69 Ma) revealed that the magmas have experienced assimilation of Miocene, Cretaceous, Triassic and Carboniferous crustal rocks.Fluids that precipitated sulfides in the Altar deposit may have remobilized Pb from the host rocks, as indicated by the ore minerals being more radiogenic (207Pb/204Pb = 15.6243–15.6269) than their host intrusions. Au/Cu ratio in Altar porphyries (average Au/Cu ratio of 0.14 × 10−4 by weight in Altar Central) is higher than in the giant Miocene porphyry deposits located to the south: Los Pelambres, Río Blanco and Los Bronces (Chile) and Pachón (Argentina). We suggest that the increase in Au content in the porphyries of this region could be linked to the assimilation of high radiogenic Pb components in the magmas within these long-lived maturation systems.

Recurrent magmatic activity on a lithosphere-scale structure: Crystallization and deformation in kimberlitic zircons

Kimberlites are not only the most economically important source of diamonds; they also carry unique information encapsulated in rock fragments entrained as the magma traverses the whole thickness of the lithosphere. The Nurbinskaya pipe in the Siberian kimberlite province (Russia) is one of several intruded along the Vilyui Rift, a major terrane boundary. The pipe contains three populations of mantle-derived zircon xenocrysts: Archean (mean age 2709±9Ma), Devonian (mean age 371±2.3Ma), and a subset of grains with evidence of brittle deformation and rehealing, and a range of ages between 370 and 450Ma. The Hf-isotope, O-isotope and trace-element signatures of the last group provide a link between the Archean and Devonian events, indicating at least three episodes of magmatic activity and zircon crystallization in the lithosphere beneath the pipe. The emplacement of the Nurbinskaya pipe ca 370Ma ago was only the youngest activity in a magma plumbing system that has been periodically reactivated over at least 2.7 billion years, controlled by the lithosphere-scale structure of the Vilyui Rift.

Oxygen isotope evolution of the Lake Owyhee volcanic field, Oregon, and implications for the low-δ18O magmatism of the Snake River Plain–Yellowstone hotspot and other low-δ18O large igneous provinces

The Snake River Plain–Yellowstone (SRP-Y) hotspot track represents the largest known low-δ18O igneous province; however, debate persists regarding the timing and distribution of meteoric hydrothermal alteration and subsequent melting/assimilation relative to hotspot magmatism. To further constrain alteration relations for SRP-Y low-δ18O magmatism, we present in situ δ18O and U–Pb analyses of zircon, and laser fluorination δ18O analyses of phenocrysts, from the Lake Owyhee volcanic field (LOVF) of east-central Oregon. U–Pb data place LOVF magmatism between 16.3 and 15.4 Ma, and contain no evidence for xenocrystic zircon. LOVF δ18O(Zrc) values demonstrate (1) both low-δ18O and high-δ18O caldera-forming and pre-/post-caldera magmas, (2) relative increases in δ18O between low-δ18O caldera-forming and post-caldera units, and (3) low-δ18O magmatism associated with extension of the Oregon-Idaho Graben. The new data, along with new compilations of (1) in situ zircon δ18O data for the SRP-Y, and (2) regional δ18O(WR) and δ18O(magma) patterns, further constrain the thermal and structural associations for hydrothermal alteration in the SRP-Y. Models for low-δ18O magmatism must be compatible with (1) δ18O(magma) trends within individual SRP-Y eruptive centers, (2) along axis trends in δ18O(magma), and (3) the high concentration of low-δ18O magmas relative to the surrounding regions. When considered with the structural and thermal evolution of the SRP-Y, these constraints support low-δ18O magma genesis originating from syn-hotspot meteoric hydrothermal alteration, driven by hotspot-derived thermal fluxes superimposed on extensional tectonics. This model is not restricted to continental hotspot settings and may apply to several other low-δ18O igneous provinces with similar thermal and structural associations.

Stratigraphy and tectonic setting of Laochang massive sulfide deposit in the North Qinling belt, central China

The Qinling orogenic belt in central China is the northernmost orogenic collage within the Tethyan domain, which records the evolution of the Paleo-Tethys Ocean. A suite of volcano-sedimentary rocks containing rare fossils and several VHMS deposits is exposed along the North Qinling belt. These units are separated into the Caotangou, Xieyuguan and Erlangping groups from west to east. Systematic studies on the facies and geochemistry demonstrate that the Caotangou Group represents a bimodal volcanic sequence formed in a backarc setting with massive sulfide horizons closely associated with a siliciclastic-felsic volcanic sequence. SHRIMP and LA-ICP-MS zircon U-Pb data from rhyolite and tuff of the Caotangou Group indicate that the volcanism took place between ca. 440–406Ma, which is similar to the dacite and andesitic basalt of the Xieyuguan Group. The Precambrian xenocrystic zircon grains suggest that the subduction-related crustal source of those volcano-sedimentary rocks in the North Qinling belt is closely related to the Qinling Group. Based on the combination of previous fossil ages, U-Pb dating of volcanic rocks and geochemistry of lavas and sulfides, we suggest that a southward-facing subduction-accretionary system developed along the southern margin of the North China plate during 490–410Ma. Subduction-related calc-alkaline magmatism continued until at least ca. 410Ma before collision of the arc with the Qinling terrane to the south during the Early Devonian led to the continent-continent collision between the Qinling terrane and North China craton in the Late Carboniferous.

The long (3.7–2.1 Ga) and multistage evolution of the Bug Granulite–Gneiss Complex, Ukrainian Shield, based on the SIMS U–Pb ages and geochemistry of zircons from a single sample

Abstract Multidisciplinary studies of zircons, rock-forming minerals and the whole-rock composition of granulite samples from the Bug Granulite–Gneiss Complex, Ukraine (including ion microprobe REE analysis, secondary ion mass spectrometry (SIMS) U–Pb and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) Lu–Hf analysis of zircons from a single sample) have revealed three major stages in the geological evolution of the complex. (i) At 3.66 Ga, a mafic intrusion contaminated with felsic rocks formed, as evidenced by 3.74 Ga zircon xenocrysts with inclusions of plagioclase, K-feldspar and quartz. (ii) At 3.59–3.55 Ga, high-temperature and high- to moderate-pressure granulite-facies metamorphism accompanied by migmatization and deformation resulted in the formation of mafic granulites. (iii) At 2.1–2.0 Ga, metamorphic overprinting occurred, and metatrachybasaltic dykes intruded at approximately 2.0 Ga. The metamorphic mineral assemblages recorded in the dykes formed at temperatures similar to those of the 3.59–3.55 Ga metamorphism but at pressures 2–3 kbar lower. This metamorphism disturbed the Sm–Nd whole-rock system, altered the Hf isotope system of the older zircons and resulted in Pb loss in small zircon grains. This complex event history recorded in zircons from a single rock corresponds to major stages of the geological evolution of both the Dniester–Bug Province and the entire Ukrainian Shield.

Precise U-Pb zircon ages and geochemistry of Jurassic granites, Ellsworth-Whitmore terrane, central Antarctica

The Ellsworth-Whitmore Mountain terrane of central Antarctica was part of the early Paleozoic amalgamation of Gondwana, including a 13,000 m section of Cambrian−Permian sediments in the Ellsworth Mountains deposited on Grenville-age crust. The Jurassic breakup of Gondwana involved a regional, bimodal magmatic event during which the Ellsworth-Whitmore terrane was intruded by intraplate granites before translation of the terrane to its present location in central Antarctica. Five widely separated granitic plutons in the Ellsworth-Whitmore terrane were analyzed for their whole-rock geochemistry (X-ray fluorescence), Sr, Nd, and Pb isotopic compositions, and U-Pb zircon ages to investigate the origins of the terrane magmas and their relationships to mafic magmatism of the 183 Ma Karoo-Ferrar large igneous province (LIP). We report high-precision (±0.1 m.y.) isotope dilution−thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from granitic rocks from the Whitmore Mountains (208.0 Ma), Nash Hills (177.4−177.3 Ma), Linck Nunatak (175.3 Ma), Pagano Nunatak (174.8 Ma), and the Pirrit Hills (174.3−173.9 Ma), and U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from the Whitmore Mountains (200 ± 5 Ma), Linck Nunatak (180 ± 4 Ma), Pagano Nunatak (174 ± 4 Ma), and the Pirrit Hills (168 ± 4 Ma). We then compared these results with existing K-Ar ages and Nd model ages, and used initial Sr, Nd, and Pb isotope ratios, combined with xenocrystic zircon U-Pb inheritance, to infer characteristics of the source(s) of the parent magmas. We conclude that the Jurassic plutons were not derived exclusively from crustal melts, but rather they are hybridized magmas composed of convecting mantle, subcontinental lithospheric mantle, and lower continental crustal contributions. The mantle contributions to the granites share isotopic similarities to the sources of other Jurassic LIP mafic magmas, including radiogenic 87Sr/86Sr (0.706−0.708), unradiogenic 143Nd/144Nd (eNd < −5), and Pb isotopes consistent with a low-µ source (where μ = 238U/204Pb). Isotopes and zircon xenocrysts point toward a crustal end member of predominantly Proterozoic provenance (0.5−1.0 Ga; Grenville crust), extending the trends illustrated by Ferrar mafic intrusive rocks, but contrasting with the inferred Archean crustal and/or lithospheric mantle contributions to some basalts of the Karoo sector of the LIP. The Ellsworth-Whitmore terrane granites are the result of mafic rocks underplating the hydrous crust, causing crustal melting, hybridization, and fractionation to produce granitic magmas that were eventually emplaced as post-Ferrar, within-plate melts at higher crustal levels as the Ellsworth-Whitmore terrane rifted off Gondwana (47°S) before migrating to its current position (82°S) in central Antarctica.

Eastern Ancient Terrane of the North China Craton

AbstractBased on the spatial distribution of ancient rocks and zircons, three ancient terranee older than ca. 2.6 Ga have recently been identified in the North China Craton, namely the Eastern, Southern, and Central Ancient Terranee. The Eastern Ancient Terrane is the best studied and understood of the three ancient terranes. It has a long geological history back to ca. 3.8 Ga ago and includes the areas of Anshan‐Benxi, eastern Hebei, eastern Shandong and western Shandong. In Anshan‐Benxi, several different types of 3.8 Ga rocks were discovered together with 3.1–3.7 Ga rocks, whereas 2.9–3.0 Ga K‐rich granites and 2.5 Ga syenogranite occur on larger scales. In eastern Hebei, 3.0–3.4 Ga rocks and older detrital and xenocrystic zircons were identified. In eastern Shandong, there are a large volumes of 2.7 Ga and 2.9 Ga rocks. In western Shandong, early Neoarchean (2.6–2.7 Ga) intrusive and supracrustal rocks are widely distributed. Whole‐rock Nd and zircon Hf isotope data suggest that both mantle additions and crustal recycling played important roles within the Eastern Ancient Terrane during almost every tectono‐magmatic event. Most BIFs in the North China Craton are late Neoarchean in age and are distributed on continental crust along the western margin of the Eastern Ancient Terrane, probably suggesting that a stable environment was one of the key factors for the formation of large‐scale BIFs.

Bi-directional subduction of the South Tianshan Ocean during the Late Silurian: Magmatic records from both the southern Central Tianshan Block and northern Tarim Craton

The subduction polarity of the South Tianshan Ocean (STO) is a matter of debate, primarily in that the Paleozoic structures of Tianshan orogenic belt have been strongly overprinted by the Cenozoic intra-continental deformation. Indentifying the arc-related magmatic rocks may provide a convincible clue for understanding the closure process of the STO. In this study, whole-rock geochemistry, zircon U-Pb dating and Hf isotope were presented on the andesite and monzonite from the Bayanbulak area of the southern Central Tianshan Block (CTB) and on the quartz diorite from the Ouxidaban area of the northern Tarim Craton. Geochemically, all the samples are Na-rich, enriched in light rare earth elements and large ion lithophile elements (Rb, Ba, U, K and Pb), and depleted in high strength field elements (Nb, Ta and Ti), like most arc-type igneous rocks. The Bayanbulak andesite samples display high MgO, Fe2O3T, TiO2 and Mg# values, and positive εHf(t) values, indicating magma source from the wedge mantle. But the existence of xenocrystic zircons implies that continental crust material were involved during magma ascend, suggesting a continental arc setting for the Bayanbulak andesite. The Bayanbulak monzonite and the Ouxidaban quartz diorite samples display relatively higher SiO2 contents, and lower MgO, Fe2O3T and TiO2 concentrations, indicating crustal sources. But the Mg# values of the Bayanbulak monzonite and the Ouxidaban quartz diorite are 48.76–51.85 and 50.31–53.73, and the εHf(t) values are −2.5 to 8.7 and −1.7 to 4.1, indicating that their magma sources were also mixed by mantle-derived components. LA-ICP-MS zircon U-Pb dating results reveal that the Bayanbulak andesite, the Bayanbulak monzonite and the Ouxidaban quartz diorite were formed at 423, 424Ma, and 421Ma, respectively. The age and geochemical data indicate that both the southern CTB and northern Tarim Craton were active continental margins during the Late Silurian, favoring a bi-directional subduction model for the evolution of the STO during that period.

Depositional age and provenance of the Wutai Group: Evidence from zircon U–Pb and Lu–Hf isotopes and whole-rock geochemistry

The Hengshan–Wutai–Fuping area is situated in the middle segment of the Trans-North China Orogen and consists of the upper amphibolite- to granulite-facies Hengshan and Fuping Complexes and the intervening greenschist- to lower amphibolite-facies Wutai Complex. The Wutai Complex is composed of Neoarchean to Paleoproterozoic granitoids and low-grade metamorphosed supracrustal rocks, of which the latter are further divided into the Wutai and Hutuo Groups. Geochemistry of the meta-sedimentary samples from the Wutai Group indicates weak to moderate source weathering, dominantly compositional variation trend and major felsic with few mafic source rocks. On the tectonic discrimination plots, most of the samples display continental island arc and active continental margin features. U–Pb ages of detrital and xenocrystic zircons from the Wutai Group yield two major age populations of 2565–2465Ma and 2729–2676Ma and a subordinate age population of 2372–2331Ma. The major ∼2522Ma and ∼2699Ma zircons were most likely derived from the Wutai meta-volcanic rocks. The minor ∼2348Ma zircons are only found from the upper Gaofan Subgroup and most likely sourced from the 2360–2330Ma orthogneisses in the Hengshan Complex. The maximum depositional ages of 2535–2516Ma from the meta-sedimentary rocks are consistent with the eruption ages of 2548–2513Ma from meta-volcanic rocks in the lower Shizui and middle Taihuai Subgroups, indicating coeval sedimentation and volcanism in a fore-arc or intra-arc basin. This favors a tectonically juxtaposed contact, rather than a layer-cake model. However, the maximum depositional age of 2348Ma and the 2176–2161Ma felsic intrusions suggest a much younger formation age for the Gaofan Subgroup.

Constraints of volcanic rocks of the Wutai Complex (Shanxi Province, Northern China) on a giant late Neoarchean intra-oceanic arc system in the Trans-North China Orogen

The Wutai Complex is located in the central part of the Trans-North China Orogen. The complex consists of Neoarchean to Paleoproterozoic granitoids and low-grade metamorphosed volcanic and sedimentary rocks of the Wutai Group. The group is subdivided into the Shizui, Taihuai and Gaofan Subgroups. Meta-volcanic rocks from the Taihuai Subgroup have igneous zircon U–Pb ages of 2548–2538Ma and εHf(t) values of −4.10 to +8.84, with xenocrystic zircons of 3.6–2.7Ga. Geochemically, the meta-volcanic rocks can be classified into four groups. Group 1 low-Ti tholeiites have the highest (Nb/La)PM ratios and the lowest (La/Sm)CN ratios, flat to slightly light REE-depleted chondrite-normalized patterns and lower TiO2 and Yb contents compared to V. They were most likely derived from partial melting of depleted mantle in a fore-arc setting. Group 2 island arc tholeiites displays flat to mildly right-declined REE patterns and moderately negative Nb, Ta, Zr, and Ti anomalies. Group 3 mature arc calc-alkaline basalts display strongly fractionated chondrite-normalized REE patterns and evidently negative Nb, Ta and Ti anomalies. Group 2 and 3 basalts formed by partial melting of sub-arc depleted mantle wedge modified by different degrees of slab-derived melts. Group 4 meta-andesites and rhyolites have high La and low Yb contents and strongly fractionated REE patterns, and may have formed by fractionational crystallization of magma of Group 3 basalts with clinopyroxene as a major mineral phase. The 2.55–2.52Ga Wutai Group represents an intra-oceanic arc underlain in part by a ∼2.7Ga continental nucleus-derived fragment.

Age and provenance constraints on seismically-determined crustal layers beneath the Paleozoic southern Central Asian Orogen, Inner Mongolia, China

We present 110 ages and 51 in-situ δ18O values for zircon xenocrysts from a post-99Ma intraplate basaltic rock suite hosted in a subduction–accretion complex of the southern Central Asian Orogenic Belt in order to constrain a seismic profile across the Paleozoic Southern Orogen of Inner Mongolia and the northern margin of the North China Craton. Two zircon populations are recognized, namely a Phanerozoic group of 70 zircons comprising granitoid-derived (ca. 431–99Ma; n=31; peak at 256Ma), meta-granitoid-derived (ca. 449–113Ma; n=24; peak at 251Ma) and gabbro-derived (436–242Ma; n=15; peaks at 264 and 244Ma) grains. Each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation. The Precambrian population (2605–741Ma; n=40) exhibits a prominent age peak at 2520Ma (granulite-facies metamorphism) and four small peaks at ca. 1900, 1600, and 800Ma. Our new data, together with literature zircon ages, significantly constrain models of three seismically-determined deep crustal layers beneath the fossil subduction zone–forearc along the active northern margin of the North China Craton, namely: (1) an upper arc crust of early to mid-Paleozoic age, intruded by a major Permian–Triassic composite granitoid–gabbroic pluton (8–20km depth); (2) a middle crust, predominantly consisting of mid-Meso- to Neoproterozoic felsic and mafic gneisses; and (3) a lower crust composed predominantly of late Archean granulite-facies rocks. We conclude that the Paleozoic orogenic crust is limited to the upper crustal level, and the middle to lower crust has a North China Craton affinity. Furthermore, integrating our data with surface geological, petrological and geochronological constraints, we present a new conceptual model of orogenic uplift, lithospheric delamination and crustal underthrusting for this key ocean–continent convergent margin.

Deciphering protoliths of the (U)HP rocks in the Makbal metamorphic complex, Kyrgyzstan: geochemistry and SHRIMP zircon geochronology

The (ultra)high-pressure ( U ) HP Makbal metamorphic complex (MMC) belongs to a chain of early Paleozoic ( U ) HP terranes to the south of the Paleo-Kazakhstan continent, a major component of the Central Asian Orogenic Belt. We present geochemical characteristics, reconstructions of geodynamic settings and constraints on the emplacement and depositional ages of metamorphosed intrusive and sedimentary rocks of the MMC. The MMC comprises variably metamorphosed juxtaposed packages of mica schists, hosting boudins of metamorphosed mafic rocks, metaquartzites and metacarbonates in a tectonic melange. The rocks, metamorphosed under ( U ) HP conditions, are situated in the central part of the terrane. The UHP garnet–talc–chloritoid schists are believed to have formed from immature pelitic sediments with maximum depositional ages of 700–650 Ma. The ( U ) HP metaquartzites comprise mature sediments derived from a Paleoproterozoic continental source. They were formed in a passive-margin environment and have a maximum depositional age of ca . 1600 Ma. The chemical composition of the MMC metabasic rocks is compatible with subalkaline tholeiitic basalts with E-MORB compositions; however, some samples have N-MORB and OIB affinities. The ages of zircon xenocrysts from the metabasic rocks show that the mafic intrusions are probably younger than ca . 700 Ma. The metabasic rocks represent dikes and sills emplaced into immature pelitic sediments in a rift-related or continental-slope environment at the final stages of the Rodinia break up (700–600 Ma) when shallow mantle melting produced relatively primitive tholeiitic compositions. The whole sequence, including the quartzites, was later juxtaposed with other passive-margin sediments and metamorphosed under ( U ) HP conditions in a Caledonian subduction zone between 500 and 475 Ma. The ages of detrital zircon grains from the MMC rocks indicate derivation from a Paleoproterozoic continental source and match the age pattern characteristic for the Northern Tien Shan, with major peaks at 1.95–1.75 Ga and smaller peaks between 2.7 and 2.5 Ga and at ca . 1.0 Ga. The ages between 1.65 and 1.4 Ga are uncharacteristic for the Kyrgyz Northern Tien Shan, instead may be correlated with recently documented voluminous early Mesoproterozoic arc magmatism in the Central Tien Shan Arc Terrane in China. The resetting of the U–Th–Pb system and the growth of new zircon in the MMC rocks were probably defined by the permeability to aqueous fluids. An age of 456 ± 4 Ma, obtained for the rims of zircon xenocrysts from a small amphibolite boudin in the southern part of the MMC, corresponds with the age of an adjacent Ordovician granite and indicates that Paleozoic intrusions could have provided sufficient heat to cause resetting and/or growth of new zircon in the MMC rocks.

Age of the Siberian craton crust beneath the northern kimberlite fields: Insights to the craton evolution

Comprehensive studies of zircon xenocrysts from kimberlites of the Kuoika field (northeastern Siberian craton) and several kimberlite fields of the eastern Anabar shield, along with data compilation on the age of kimberlite-hosting terranes, reveal details of the evolution of the northern Siberian craton. The age distribution and trace element characteristic of zircons from the Kuoika field kimberlites (Birekte terrane) provide evidence of significant basic and alkaline–carbonatite magmatism in northern Siberia in the Paleozoic and Mesozoic periods. The abundance of 1.8–2.1Ga zircons in both the Birekte and adjacent Hapchan terranes (the latter hosting kimberlites of the eastern Anabar shield) supports the Paleoproterozoic assembly and stabilization of these units in the Siberian craton and the supercontinent Columbia. The abundance of Archean zircons in the Hapchan terrane reflects the input of an ancient source other than the Birekte terrane and addresses the evolution of the terrane to west (Magan and Daldyn terranes of the Anabar shield). The present study has also revealed the oldest known remnant of the Anabar shield crust, whose 3.62Ga age is similar to that of another ancient domain of Siberia, the Aldan shield. The first Hf isotope data for the Anabar shield coupled with the U–Pb systematics indicate three stages of crustal growth (Paleoproterozoic, Neoarchean and Paleoarchean) and two stages of the intensive crustal recycling in the Paleoproterozoic and Neoarchean. Intensive reworking of the existing crust at 2.5–2.8Ga and 1.8–2.1Ga is interpreted to provide evidence for the assembly of Columbia. The oldest Hf model age estimation provides a link to Early Eoarchean (3.7–3.95Ga) and possibly to Hadean crust. Hence, some of the Archean cratonic segments of the Siberian craton could be remnants of the Earth's earliest continental crust.

Zircon xenocryst resorption and magmatic regrowth at El Chichón Volcano, Chiapas, Mexico

El Chichón volcano is the only active volcano located within the Chiapanecan Volcanic Arc in southern Mexico, which lies between the Trans-Mexican Volcanic Belt and the Central American Volcanic Arc. Previous studies have shown that ~12 eruptions have occurred at El Chichón within the last 8000years, forming a complex of lava domes with a central crater and surrounding pyroclastic deposits. Here, we report the discovery of zircon in Holocene El Chichón rocks, which were analyzed by high spatial resolution imaging (color cathodoluminescence CCL) and isotopic (secondary ionization mass spectrometry SIMS) methods to resolve core and rim crystallization ages. Pumice samples from five proximal pyroclastic flow and fall-out deposits were collected based on published stratigraphy. Two of the samples were further (re-)classified by new 14C dates. In addition, we sampled two lavas from the 1982 eruption and from remnants of the older Somma lava complex. Zircon crystals were dated using 230Th/238U disequilibrium (U–Th) and U–Pb geochronology. U–Th zircon ages fall between near eruption ages and ca. 84ka, with overlapping ages in all samples. By contrast, zircon core U–Pb ages range between ca. 290Ma and 1.9Ga. These ages are consistent with xenocrystic origins and their heterogeneity indicates derivation from clastic country rocks. Strong age contrasts between inherited xenocrystic and young magmatic domains in individual zircon crystals are evidence for arrested assimilation of crustal rocks where initially zircon-undersaturated magmas cooled rapidly to form a crystal mush or subsolidus amalgamate as a crustally contaminated boundary layer. This layer contributed zircon crystals to eruptible magma during episodic recharge events followed by partial melt extraction, mixing and homogenization. Zircon overgrowths are significantly older than major minerals whose U-series ages and sharp zonation boundaries suggest crystallization only within a few ka before eruption. This implies that zircon can detect magmatic longevity which is obscured in the major mineral record.

Tracing ancient events in the lithospheric mantle: A case study from ophiolitic chromitites of SW Turkey

New major-, minor- and trace-element data on high-Cr chromites from several ophiolitic podiform chromitites from Lycian and Antalya peridotites in southwestern Turkey reveal a polygenetic origin from a range of arc-type melts within forearc and back-arc settings. These forearc and the back-arc related high-Cr chromitites are interpreted to reflect the tectonic juxtaposition of different lithospheric mantle segments during the obduction. The diversity of the γOs(t=0) values (−8.28 to +13.92) in the Antalya and Lycian chromitite PGMs and their good correlations with the sub- to supra-chondritic 187Os/188Os ratios (0.1175–0.1459) suggests a heterogeneous mantle source that incorporated up to 40% recycled crust, probably due to subduction processes of the orogenic events. The few model ages calculated define two significant peaks in TRD model ages at 1.5 and 0.25Ga, suggesting that the chromitites are younger than 0.25Ga and include relics of an at least Mesoproterozoic or older (>1.0Ga) mantle protolith. Eight of the nine zircon grains separated from the chromitites, are interpreted as detrital and/or resorbed xenocrystic relics, whilst a significantly less reworked/resorbed one is considered to be of metasomatic origin. In-situ U–Pb dating of the xenocrystic zircon grains yielded a spread of ages within ca 0.6–2.1Ga, suggesting recycling of crustal rocks younger than 0.6Ga (Late Neoproterozoic). The notable coincidence between the lower age limit of the older zircons (ca 1.6Ga) and the oldest Os model age peak (ca 1.5Ga) from the PGM may suggest a Mesoproterozoic rifting stage. These findings imply a Paleoproterozoic sub-continental lithospheric mantle (SCLM) protolith for the SW Anatolian mantle which was later converted into an oceanic lithospheric mantle domain possibly following a rifting and continental break-up initiated during Mesoproterozoic (ca 1.5–1.0Ga). The single metasomatic zircon of ca 0.09Ga age coinciding with the initiation of the contraction of the Neotethys may be interpreted to date the timing of a metasomatic event beneath the intra-oceanic subduction zone.