Magmatic Zircons Research Articles

Significant Zr isotope variations in single zircon grains recording magma evolution history

Zircons widely occur in magmatic rocks and often display internal zonation finely recording the magmatic history. Here, we presented in situ high-precision (2SD <0.15‰ for δ94Zr) and high-spatial-resolution (20 µm) stable Zr isotope compositions of magmatic zircons in a suite of calc-alkaline plutonic rocks from the juvenile part of the Gangdese arc, southern Tibet. These zircon grains are internally zoned with Zr isotopically light cores and increasingly heavier rims. Our data suggest the preferential incorporation of lighter Zr isotopes in zircon from the melt, which would drive the residual melt to heavier values. The Rayleigh distillation model can well explain the observed internal zoning in single zircon grains, and the best-fit models gave average zircon-melt fractionation factors for each sample ranging from 0.99955 to 0.99988. The average fractionation factors are positively correlated with the median Ti-in-zircon temperatures, indicating a strong temperature dependence of Zr isotopic fractionation. The results demonstrate that in situ Zr isotope analyses would be another powerful contribution to the geochemical toolbox related to zircon. The findings of this study solve the fundamental issue on how zircon fractionates Zr isotopes in calc-alkaline magmas, the major type of magmas that led to forming continental crust over time. The results also show the great potential of stable Zr isotopes in tracing magmatic thermal and chemical evolution and thus possibly continental crustal differentiation.

Formation Age of the Sujiagou Komatiites in Western Shandong: Further Constraints from SHRIMP U‐Pb Zircon Dating on Granitic Dykes

AbstractKomatiites are presented as direct evidence for higher mantle temperatures during the Archean. In the North China Craton, komatiites with spinifex structure have been identified only at one locality, i.e. the Sujiagou area, western Shandong. They were considered as formed during the early Neoarchean mainly based on their association with supracrustal rocks considered to be that age. This study carried out SHRIMP U‐Pb zircon dating on metamorphosed trondhjemitic and monzogranitic dykes intruding the Sujiagou komatiites, and they have magmatic zircon ages of 2592 ± 12 Ma and 2586 ± 13 Ma respectively. This provides direct evidence that the komatiites formed during the early Neoarchean.

RETRACTED: Detrital zircon age distribution characteristics of Jurassic sandstone from the Southern Yili basin, China

The Central Asia orogenic belt contains a wide range of structural elements, including micro continent, back arc system, ocean island/plateau, ophiolite and subduction accretion complex. But its final closing time has been controversial. Based on the magmatic age of the surrounding orogenic belt, the source of this set of clastic sediments is determined, which provides new important evidence for the evolution of the South Tianshan orogenic belt. The results show that the Jurassic detrital zircons from the study area were mainly derived from magmatic zircons and are deposited in a proximal source. The detrital zircon age of the Lower Jurassic Badaowan and Sangonghe Formation are concentrated in 290–260 Ma, and in 350–290 Ma and 460–390 Ma, respectively. The detrital zircon age of the Middle Jurassic Xishanyao Formation concentrates in 370–320 Ma and 450–390 Ma. There are very few zircons from the Precambrian period. These ages are consistent with the timing, indicating these clastic sediments were mainly originated from the southern margin of the Yili - Middle Tianshan Block. The Late Permian - Middle Triassic detrital zircons almost do not exist, implying that there were no contemporary magmatism related to collision or post-collision in the South Tianshan district, its complex evolution and orogenic stage are still a challenging topic. In the ancient active plate margin, the sedimentary records in the pre-arc basin can provide more information about the magmatic arc and basin-orogen coupling than the present exposed arc itself. The rhyolite, trachyte, and trachyandesite of the Dahalajunshan Formation were widely developed in the Yili - Middle Tianshan Block during the Early Carboniferous. During the formation of Wulang Formation in the Early Permian, a large number of rhyolite were developed. The age data of 75 detrital zircons were obtained from the sandstone (J1s-5) of the Sangonghe Formation, of which 74 zircons have a concordance degree of over 90%, and their age data also fall on the harmonic curv. In addition, the age of the youngest zircons increased gradually from Early to Middle Jurassic, indicating that the sediments in this period had the feature of uncovering. Our study provides a good reference for the analysis of provenance and regional tectonic evolution.

The zirconium stable isotope compositions of 22 geological reference materials, 4 zircons and 3 standard solutions

We report stable Zr isotope compositions of four common natural zircon grains: Penglai, 91500, GJ-1 and Mud Tank, and 22 whole rock reference materials which include fifteen magmatic rocks, one metamorphic rock, six sediments or sedimentary rocks. In addition, the isotopic calibration of the three Zr standard solutions used in different publications (IPGP-Zr, SRM3169, NIST) has been determined. The data are reported as δ94/90ZrIPGP-Zr, the permille deviation of the 94Zr/90Zr ratio relative to the IPGP-Zr standard solution. The isotopic offsets among the three Zr standard solutions are small (less than 0.033 ± 0.033‰). The isotopic composition of the zircons agrees well with previous in-situ measurements, validating the in-situ measurements. On the other hand, all the samples analyzed in this study show a large range of Zr isotope compositions of 0.602‰. This confirms the fairly large natural Zr isotopic variations. The general variation of δ94/90ZrIPGP-Zr with magmatic evolution of the various igneous samples is in agreement with previous studies, with the most magmatically evolved samples being the isotopically heavier. The two magmatic zircons are isotopically lighter compared to the magmatic rocks, which is in agreement with the enrichment of heavy isotope in the evolved samples. This is ultimately controlled by the crystallization of isotopically light zircons.

Composition and Structure of Zircon from Hydrothermal Uranium Occurrences of the Litsa Ore Area (Kola Region, Russia)

Zircon grains from various metagranitoids (plagio- and monzo-granites, gneisses, metasomatic rocks, and pegmatoid veins) from the Skal’noe and Dikoe sites of the Litsa uranium ore area (Kola Region, Russia) were studied in order to reconstruct the sequence and timing of events in the area and to observe effects of hydrothermal process related to uranium mineralization on structure and composition of zircon. Individual zircon grains were studied by means of laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS), ion microprobe and Raman spectroscopy. Isotopic LA–ICPMS data for the Skal’noye and Dikoe ore occurrences suggest the following age sequence of events in the area: intrusion of plagiogranites—2829 ± 12 Ma, formation of magmatic protolith of gneisses—2781 ± 17 Ma, metamorphism of plagiogranites—2636 ± 34 Ma; intrusion of monzogranites and pegmatoid veins—2549–2526 Ma, hydrothermal event with uranium input—2276 ± 21 Ma, last metamorphism of plagio- and monzo-granites—1892–1696 Ma. Ore-bearing rocks in the area are pegmatoid veins and quartz–feldspar metasomatites which contain uraninite. During a 2.3 Ga hydrothermal process, newly formed zircon rims grew simultaneously with the precipitation of uraninite in the veins and metasomatites. These rims are characterized by high U and rare earth elements (REE) contents (up to 6560 and 8760 ppm, respectively), dark cathodoluminescence, low Th-U ratios (0.1–0.007) and a flat LREE-enriched pattern, in some cases inherited from minerals, dissolved during a hydrothermal event (magmatic plagioclase and probably monazite). Hydrothermal zircon rims grew with partial dissolution of the magmatic zircon, as evidenced by the rounded and curved shapes of zircon cores. The degree of alteration caused by hydrothermal events depends on the uranium content in the pre-existing zircon. The effects of zircon alteration and newly formed zircon composition reflect the redistribution of uranium in rocks.

Persistence of partial melting in the southern North China Craton: Evidence from Paleoproterozoic migmatites of the Taihua Complex

The Taihua Complex is exposed in the southern North China Craton (NCC), it has experienced a long and complicated history of multiple crustal growth, magmatism, metamorphism and deformation during the early Precambrian. Abundant leucosomes are widely distributed within the migmatites of the Taihua Complex, which occurred as irregular lenses, blocks, layers or dikes with several thickness of millimeters to tens of centimeters wide. Meanwhile, former presence of melt and typical microstructures related to partial melting also can be recognized in the thin sections under the microscope, those provide compelling evidences that the Taihua Complex has extensively suffered partial melting, and can be regarded as the best target to decipher multistage anatexis during metamorphism as well as assembly of the whole NCC. However, the timing and nature of the partial melting in the Taihua Complex have not been well studied up to now. In this contribution, we present an integrated in situ zircon U-Pb ages and Hf isotope compositions for the Paleoproterozoic migmatites from the Taihua Complex in the southern NCC. Both the metasedimentary and metamorphized plutonic rocks of the Taihua Complex have extensively suffered partial melting. The ca. 2.50 Ga magmatic zircons or inherited magmatic cores from the migmatites and leucosomes present similar Hf isotope composition to those of the contemporaneous depleted mantle at ca. 2.50 Ga, suggesting that ca. 2.50 Ga is a major period of crustal growth in the southern NCC as well as the whole NCC. The ages of metamorphic zircons obtained from leucosomes, melanosomes and migmatites show a large time interval of 1909–1798 Ma, indicating that the Taihua Complex has experienced a persistence of partial melting event during Late Paleoproterozoic that lasted more than 100 Ma. The crustal anatexis in the southern NCC probably occurred during the post-peak upper amphibolite- to granulite facies stage with melting temperature of at least 652–753 °C. The decompression melting and widespread migmatization of the Taihua Compelx are probably due to extension related to exhumation of the previously thickened crust during Paleoproterozoic collision between the Eastern and Western block, or between the Southern Archean Block and other micro-continents. The main collision-related assembly and crustal thickening events in the southern NCC likely occurred at ca. 1.95 Ga, rather than ca. 1.85 Ga as previously believed.

Crustal growth event in the Cathaysia Block at 2.5 Ga: evidence from chronology and geochemistry of captured zircons in Jurassic acidic dykes

AbstractSix acidic dykes were discovered surrounding the Laiziling pluton, Xianghualing area, in the western Cathaysia Block, South China. A number of captured zircons are found in two of these acidic dykes. By detailed U–Pb dating, Lu–Hf isotopes and trace-element analysis, we find that these zircons have ages clustered atc.2.5 Ga. Two acidic dyke samples yielded upper intersection point206U/238Pb ages of 2505 ± 42 Ma and 2533 ± 22 Ma, and weighted mean207Pb/206Pb ages of 2500 ± 30 Ma and 2535 ± 16 Ma. The majority of these zircons have high (Sm/La)N, Th/U and low Ce/Ce* ratios, indicating a magmatic origin, but some grains were altered by later hydrothermal fluid. Additionally, the magmatic zircons have high Y, U, heavy rare earth element, Nb and Ta contents, indicating that their host rocks were mainly mafic rocks or trondhjemite–tonalite–granodiorite rock series. Equally, their moderate Y, Yb, Th, Gd and Er contents also indicate that a mafic source formed in a continental volcanic-arc environment. These zircons have positive ϵHf(t) values (2.5–6.9) close to zircons from the depleted mantle, withTDM(2565–2741 Ma) andTDM2(2608–2864 Ma) ages close to their formation ages, indicating that these zircons originated directly from depleted mantle magma, or juvenile crust derived from the depleted mantle in a very short period. We therefore infer that the Cathaysia Block experienced a crustal growth event atc.2.5 Ga.

Timing and origin of multi-stage magmatism and related W–Mo–Pb–Zn–Fe–Cu mineralization in the Huangshaping deposit, South China: An integrated zircon study

The Huangshaping deposit (South China) is closely related to hypabyssal granites (e.g., quartz porphyry, granophyre, and granite porphyry), developing massive skarn-type W–Mo–Fe polymetallic mineralization, skarn-type Pb–Zn–Cu mineralization, and vein-type PbZn (Cu) mineralization. To better constrain the geochronology, fluid composition, magma source, metallogenic affinity, and source of ore-forming materials in the Huangshaping deposit, morphological, textural, in situ geochronological, compositional, and LuHf isotopic studies on zircons hosted in granitoids, skarns (ores), and wall rocks have been carried out. Overall, five types of zircons are identified: low-U magmatic (abbreviated as LUMZ), high-U magmatic (HUMZ), hydrothermally-altered metamict (HAZ), inherited, and detrital types. Round-shaped inherited and detrital zircons with low trace element compositions are rarely captured in the granite porphyry and largely hosted in sulfide skarn ores and wall rocks. The HUMZ are mainly observed in the granophyre and are easily distinguished from LUMZ by their much higher U, Th, and Y contents, black CL images, and stronger Eu anomalies. In addition, the crystallization time of HUMZ was later than that of LUMZ. The HAZ uniquely exists in granite porphyry and genetic-related skarns with cracked and porous textures and strong enrichment of many trace elements. These zircons were originally HUMZ that have undergone rapid partial metamictization and subsequent fluid-zircon interaction. This study demonstrates that the granophyre is a two-stage intrusion with ages of 179 ± 0.3 Ma and 163 ± 0.7 Ma, whereas the granite porphyry yields younger ages of 157 ± 0.5 Ma. The discovery of Triassic inherited zircons in the latter leads us to propose the presence of a concealed Triassic pluton (~220 Ma). The zircon populations of W−Mo−Fe mineralized garnet skarns are mainly composed of ~160 and 180 Ma magmatic zircons, whereas magnetite skarn ores contain both magmatic zircons (~50%) and inherited zircons (~50%). In contrast, inherited zircons are the dominant zircon population of the skarn Pb−Zn ores. This may indicate that the granite porphyry (the foremost one) and granophyre mostly contributed to the skarn-type W−Mo−Fe mineralization, whereas the Carboniferous and Devonian carbonate strata significantly contributed to the skarn-type Pb−Zn (Cu) mineralization. Fluids separated from the granite porphyry that modified the zircons are likely the principal ore-forming fluids, which are enriched in F, U, Th, Y, REE, Nb, Ta, Hf, Ca, P, Ti, Pb W, Mo, and Fe. Zircon Hf isotopes suggest that these magmas, at least the granophyre and granite porphyry, were generated by reworking of Mesoproterozoic crustal rocks without significant input of mantle materials. This study highlights the use of different types and generations of zircons from variable rock types to reveal fluid compositions and mineralization potentials in complex metallogenic systems.

A New HP–UHP Eclogite Belt Identified in the Southeastern Tibetan Plateau: Tracing the Extension of the Main Palaeo-Tethys Suture Zone

Abstract The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is an important link between the Longmu Co–Shuanghu suture (LCSS) in the northern Tibetan Plateau and the Chiang Mai–Inthanon and Bentong–Raub sutures in Thailand and Peninsular Malaysia. These belts and sutures are generally regarded as containing the remnants of the oceanic crust of the Palaeo-Tethys that formed by seafloor spreading as a result of the separation of Gondwana- and Eurasia-derived blocks during the Middle Cambrian. In this paper we report the first discovery of abundant unaltered and retrograde eclogites that occur as irregular lenses and blocks in metasedimentary rocks of the CMOB, and these eclogites form an elongate and almost north–south-trending high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic belt that is ∼200 km long and ∼50 km wide. The newly discovered phengite/talc/epidote–glaucophane eclogites, lawsonite–talc–phengite eclogites, dolomite/magnesite–kyanite eclogites and phengite–kyanite-bearing retrograde eclogites have enriched (E-) and normal mid-ocean ridge basalt (N-MORB)-like affinities and mainly positive as well as some negative whole-rock εNd values (–4·34 to +7·89), which suggest an enriched and depleted oceanic lithosphere source for their protoliths. Magmatic zircons separated from the epidote–glaucophane, magnesite–kyanite and (phengite–kyanite-bearing) retrograde eclogites gave protolith ages of 317–250 Ma, which fit well within the time frame of the opening of the Palaeo-Tethys during the Middle Cambrian and its closure during the Triassic. Abundant metamorphic zircons in the eclogites indicate a Triassic metamorphic event related to the subduction of the Palaeo-Tethys oceanic crust from 235 to 227 Ma. Taking into account previous isotopic age data, we now establish the periods of Early–Middle Triassic (246–227 Ma) and Late Triassic (222–209 Ma) as the ages of subduction and exhumation of the Palaeo-Tethyan oceanic crust, respectively. Thermodynamic modelling revealed that the eclogites record distinct HP–UHP peak metamorphic conditions of 23·0–25·5 kbar and 582–610 °C for the phengite–glaucophane eclogites, 24·0–25·5 kbar and 570–586 °C for the talc–glaucophane eclogites, 29·0–31·0 kbar and 675–712 °C for the dolomite–kyanite eclogites, and 30·0–32·0 kbar and 717–754 °C for the magnesite–kyanite eclogites. These P–T estimates and geochronological data indicate that the Palaeo-Tethys oceanic slab was subducted to different mantle depths from 75 km down to 95 km, forming distinct types of eclogite with a variety of peak eclogite-facies mineral assemblages. The eclogites consistently record clockwise metamorphic P–T–t paths characterized by a heating–compression prograde loop under a low geothermal gradient of 5–10 °C km–1, indicating the rapid subduction of cold oceanic crust at a rate of 4·5–6·0 km Ma–1, followed by isothermal or cooling–decompressive retrogression and exhumation at an average rate of 3·2–4·2 km Ma–1. The newly discovered eclogites of the CMOB with their signatures of ocean-crust subduction are petrologically, geochemically and geochronologically comparable with those of the LCSS, providing powerful support for the idea that a nearly 2000 km long HP–UHP eclogite belt extends from the northern Tibetan Plateau to the southeastern Tibetan Plateau, and that it represents the main boundary suture of the Palaeo-Tethyan domain. These results have far-reaching implications for the tectonic framework and complex metamorphic evolution of the Palaeo-Tethyan domain.

Comparative studies on two phases of Archaean TTG magmas from different blocks of the North China Craton: petrogenesis and constraints on crustal evolution

AbstractWhole-rock major and trace elements and Hf isotopes of magmatic zircons of tonalite–trondhjemite–granodiorite (TTG) rocks with different ages (2.9, 2.7 and 2.5 Ga) from the three blocks (the Eastern Block, Western Block and Trans-North China Orogen) of the North China Craton were compiled to investigate their respective petrogenesis, tectonic setting and implications for crustal growth and evolution. Geochemical features of the 2.5 Ga TTGs of the Eastern Block require melting of predominant rutile-bearing eclogite and subordinate garnet-amphibolite at higher pressure, while the source material of the 2.7 Ga TTGs is garnet-amphibolite or granulite at lower pressure. The 2.5 Ga TTGs have high Mg#, Cr and Ni, negative Nb–Ta anomalies and a juvenile basaltic crustal source, indicating derivation from the melting of a subducting slab. In contrast, features of the 2.7 Ga TTGs suggest generation from melting of thickened lower crust. The 2.5 and 2.7 Ga TTGs in the Trans-North China Orogen were formed at garnet-amphibolite to eclogite facies, and the source material of the 2.5 Ga TTGs in the Western Block is most likely garnet-amphibolite or eclogite. The 2.5 Ga TTGs in the Trans-North China Orogen and Western Block were generated by the melting of a subducting slab, whereas the 2.7 Ga TTGs in the Trans-North China Orogen derived from melting of thickened lower crust. The Hf isotopic data suggest both the 2.5 and 2.7 Ga TTG magmas were involved with contemporary crustal growth and reworking. The two-stage model age (TDM2) histograms show major crustal growth between 2.9 and 2.7 Ga for the whole North China Craton.

Petrogenesis of the Nashwaak Granite, West-Central New Brunswick, Canada: Evidence from Trace Elements, O and Hf Isotopes of Zircon, and O Isotopes of Quartz

The petrogenesis of the Pridoli to Early Lochkovian granites in the Miramichi Highlands of New Brunswick, Canada, is controversial. This study focuses on the Pridoli Nashwaak Granite (biotite granite and two-mica granite). In situ trace elements and O and Hf isotopes in zircon, coupled with O isotopes in quartz, are used to reveal its magmatic sources and evolution processes. In the biotite granite, inherited zircon cores have broadly homogenous δ18OZrc ranging from +6.7‰ to 7.4‰, whereas magmatic zircon rims have δ18OZrc of +6.3‰ to 7.2‰ and εHf(t) of −0.39 to −5.10. The Hf and Yb/Gd increase with decreasing Th/U. Quartz is isotopically equilibrated with magmatic zircon rims. The biotite granite is interpreted to be solely derived by partial melting of old basement rocks of Ganderia and fractionally crystallized at the fO2 of 10−21 to 10−10 bars. The two-mica granite has heterogeneous inherited zircon cores (δ18OZrc of +5.2‰ to 9.9‰) and rims (δ18OZrc of +6.2‰ to 8.7‰), and εHf(t) of −11.7 to −1.01. The two-mica granite was derived from the same basement, but with supracrustal contamination. This open-system process is also recorded by Yb/Gd and Th/U ratios in zircon and isotopic disequilibrium between magmatic zircon rims and quartz (+10.3 ± 0.2‰).

Neoarchean seafloor hydrothermal metamorphism of basalts in the Zanhuang ophiolitic mélange, North China Craton

Epidosites are suggested to form in up-flow zones located at the base of ore-forming oceanic hydrothermal systems, and are commonly preserved in ophiolitic sequences. However, occurrences of epidosites in Archean tectonic mélanges are rarely documented. In this study, we report the presence of late Neoarchean epidosites in the Zanhuang ophiolitic mélange in the North China Craton using meter-scale litho-structural and petrologic analysis, zircon U-Pb dating, major and trace element, and oxygen and strontium isotopic geochemistry. Field studies show that the epidosites are mainly preserved as the cores of meta-pillow lavas, and are distributed as blocks in different matrices within the tectonic mélange. The pillow rims are used as protolith to estimate the element mobility during the alteration process. The epidosites display strong depletion in Na2O (0.01–0.27 wt%), MgO (0.43–3.01 wt%), depletion in large ion lithophile elements (LILE) (e.g. Rb, Ba, Cs, U) and transition metals (e.g., Co, Sc, Cu, Ni, Zn), and show enrichment of CaO (13.35–21.0 wt%), Sr and Pb. The high field strength elements (HFSE) (e.g. Zr, Hf, Ta, Th) of epidosites are varied but rare earth elements (REE) are unchanged. The amphibolitic pillow rims are characterized by enrichment of LILE, flat heavy rare earth elements (HREE) patterns, and depletion in HFSE with negative Ti anomalies and positive Pb and Ba anomalies. Oxygen and strontium isotopic ratios of epidosites are 5.0–8.6‰ and 0.70284–0.70551, respectively, both of which are greater than those of fresh basalts. The major and trace elements, oxygen and strontium isotopic values and extensive element migration indicate that the protolith was altered by a submarine hydrothermal system in the absence of continental influence. Magmatic zircons from two samples of epidosites yield 207Pb/206Pb ages of 2512 ± 13 Ma (MSWD = 0.51) and 2542 ± 13 Ma (MSWD = 1.4), confirming that the epidosites were generated at ca. 2.51–2.54 Ga. Based on geochemical data and field observations, we suggest that the protolith of the epidosites formed in an arc-related tectonic setting during the late Neoarchean. This study shows that hydrothermal activity operating in the late Neoarchean was similar to that of the modern ocean floor.

Lithostratigraphic evolution of the Bandamian Volcanic Cycle in central Côte d’Ivoire: Insights into the late Eburnean magmatic resurgence and its geodynamic implications

Late- to post-collisional volcanism in the southern portion of the West African Craton (sWAC) has been described along the paleo-craton margin between the Paleoproterozoic Baoulé-Mossi domain and Archean Kénéma-Man nucleus. New lithostratigraphic and isotopic data for central Côte d’Ivoire, as well as revised regional stratigraphic correlations across the western sWAC, indicate that this volcanic resurgence is not only restricted to the paleo-craton margin, but extends to most greenstone belts of the western half of the sWAC. This late volcanic event, historically referred to as the ca. 2117–2080 Ma Bandamian Volcanic Cycle, has received little attention since its identification in 1996. Detailed geological investigation of the Bouaflé granite-greenstone belt in Côte d’Ivoire, located 150 km east of paleo-craton margin, along with newly acquired U-Pb geochronological data indicate a peak in igneous activity showing evidence for bimodal magmatism affiliated to the Bandamian Volcanic Cycle in central Côte d’Ivoire between 2108 ± 5 Ma and 2105 ± 4 Ma. Whole-rock geochemistry and Lu-Hf isotopic measurements on magmatic zircons from the Bouaflé region show that these Bandamian rocks are marked by a juvenile signature with little degree of crustal contamination. Collectively, the data collected in this study show that the Bandamian Volcanic Cycle represents an important thermal event that affected the sWAC during the late stages of the Eburnean orogeny, spanning the incremental change from folding and thrusting to dominantly transcurrent tectonics along major faults and shear zones. This paper discusses the possible geodynamic settings that may account for such a peak in thermal activity during the late stages of the Eburnean orogeny.

Zircon inheritance from long-lived sources of Late Triassic post-orogenic plutons, High Andes, Central Chile (~30°S): Magmatic feedbacks and petrogenetic implications

A comprehensive understanding of the magmatic system evolution and petrogenetic environment is crucial for the correct interpretation of magmatic zircon age spectra. In the Central Andes, a protracted Late Paleozoic period of collisional to post-collisional magmatism was followed by Late Triassic emplacement of post-orogenic plutons and associated mantle precursors. Some of these magma products are crowded with inherited zircons with a large age span at the sample scale (>25 Myr). This work deals with two contrasting inheritance-rich plutons: the peraluminous cordierite-bearing Los Tilos pluton (LTp) and the peralkaline hypersolvus Ferro-edenite/hedenbergite-bearing Monte Grande pluton (MGp). Both plutons were derived from melting of the crust: the peraluminous S-type granite represents derivation from mixed materials, whereas the peralkaline A-type leucogranite embodies nearly eutectic melts enriched in incompatible elements. Rapid cooling of entrained assemblages suggests that segregation and subsequent ascent was rapid, following emplacement-site isobaric cooling at ca. 3.5 and 2 kbar, for LTp and MGp respectively. Most zircons were inherited; their disparate chemistry originates from contrasting sources, unveiling an open-system behavior.The effects of variations in magma temperature, emplacement pressure, and water content were evaluated in light of zircon entrainment and dissolution potential, within a thermodynamically consistent framework. Relatively dry magmas (<3 H2O wt%) readily preserve inherited zircons whereas similar size intrusions in excess of water most probably dissolve zircons in less than 12 kyr. In this geological context, early zircon armoring and source enrichment is vital to enhance zircon preservation at high temperatures.We suggest that the large age span observed in zircon samples of both plutons reflects a long-lived source that fed arc-related granites in the Early to Middle Triassic, culminating in the Late Triassic with the final extraction of highly enriched crustal melts, in a post-orogenic context. Dry high-silica (>75 SiO2 wt%) magmatic flare-ups, in extensional settings, fulfill the inheritance requisites presented here. Contrastingly, a broad zircon age span in large calc-alkaline batholiths emplaced at similar depths, reflect not source processes but rather magmatic erosion, crystal armoring and recycling of previous magma batches, and a complex crystallization history, spanning the whole duration of batholith construction.

Mesozoic juvenile crustal formation in the easternmost Tethys: Zircon Hf isotopic evidence from Sumatran granitoids, Indonesia

Abstract Prior to the collision of India with Asia, the evolution of island arcs and resultant crustal formation in the now-disrupted easternmost Tethys are poorly constrained. Here, we report for the first time zircon U-Pb and Hf isotopic data from Mesozoic granitoids in Sumatra, Indonesia. Our analyses identified three magmatic episodes at 214–201 Ma, 148–143 Ma, and 102–84 Ma, respectively, with a drastic change in magmatic zircon εHf(t) values from −13.1 to +17.7 in the Late Triassic granitoids, which reveals a fundamental restructuring of the arc system in Sumatra. Subsequently, all Jurassic to Late Cretaceous granitoids have exclusively positive zircon εHf(t) values (+17.7 to +10.2), consistent with juvenile arc development owing to subduction of the easternmost Tethyan lithosphere beneath Sumatra. Such highly positive zircon εHf(t) values of the Sumatran granitoids, in general accordance with those of the Gangdese arc system in South Asia, are markedly higher than those (+13.7 to −14.7) of broadly contemporaneous Cordilleran arcs in Americas and Zealandia. Our findings from the easternmost Tethys provide new insights into not only the tectono-magmatic evolution of eastern Tethys, but also its crucial role in global juvenile crustal growth.

A new concordia age for the 'forearc' Bay of Islands Ophiolite Complex, Western Newfoundland utilizing spatially-resolved LA-ICP-MS U-Pb analyses of zircon

Advancements of zircon U-Pb dating techniques using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) provide an opportunity to precisely constrain the age of the Bay of Islands Ophiolite Complex (BOIC) in the Newfoundland Appalachians. New LA-ICP-MS U-Pb dates are presented for a large population of relatively simple magmatic zircon grains from trondhjemites using selective domain analyses and a 3% discordance filter in five samples collected from a single ophiolite massif, with four of the samples from a single pluton. All samples were derived from plagiogranite plutons intruded just below the gabbro-sheeted dike contact of the Blow Me Down Mountain massif, the massif sampled for three of the four prior single sample ID-TIMS age studies of the ophiolite. A highly consistent series of LA-ICP-MS concordant ages from the five samples collected yield a composite concordia age of 488.3 ± 1.5 Ma, which we propose for the revised igneous age of the ophiolite massif. This new age for the BOIC is based on 127 individual LA-ICP-MS zircon analyses and is slightly older and with lower uncertainty than the previous consensus age. We review previous pioneering ID-TIMS U-Pb dates, ages, intercept assumptions, and uncertainties for the BOIC in light of this new formation age. Legacy ages and uncertainties differ somewhat when data are reduced with modern recommended techniques and without geological interpretations that fix lower intercepts. The ca. 488.3 Ma BOIC age indicates its formation along a younger supra-subduction forearc trench-orthogonal spreading center that rifted the older Coastal Complex forearc ophiolitic assemblages reported to be formed at ca. 500-508 Ma. This rifted forearc is analogous to many modern ophiolitic forearcs. The BOIC age is highly correlated with similar ages of the peri-Laurentian Notre Dame arc-proximal Betts Cove and Point Rousse ophiolites, indicating temporal BOIC spreading center linkages within the Baie Verte Oceanic Tract.

Rapid cold slab subduction of the Paleo-Tethys: Insights from lawsonite-bearing blueschist in the Changning–Menglian orogenic belt, southeastern Tibetan Plateau

The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is considered as the main suture of the Paleozoic Paleo-Tethys that separates Gondwana-derived continental fragments from Eurasia-derived ones. Understanding the evolutionary history of this orogenic belt is of critical importance in the reconstruction of the tectonic history of Paleo-Tethys. The CMOB preserves well-exposed blueschist facies rocks, albeit their tectonometamorphic history and protolith signatures remain poorly constrained. Here we present, for the first time, results from a detailed investigation of lawsonite-bearing blueschist rocks, including epidote-magnesioriebeckite schist and garnet-ferroglaucophane schist from the CMOB and involving petrological, mineralogical, thermodynamic modeling, whole-rock geochemical, and geochronological studies. The epidote-magnesioriebeckite schist samples and garnet-ferroglaucophane schist samples display OIB- and E-MORB-like geochemical affinities, respectively, and have whole-rock εNd (t) values in the range of −5.4–+4.4, suggesting that their protoliths were mainly oceanic crust with limited degree of crustal assimilation. Magmatic zircon grains from the garnet-ferroglaucophane schist samples yield protolith ages of 253–250 Ma. Combined with previous data, our data represent the youngest ages of rock formation in the Paleo-Tethyan Ocean reported to date. The epidote-magnesioriebeckite schist samples show a peak assemblage of magnesioriebeckite + lawsonite + augite-aegirine + phengite + titanite + allanite, with peak P–T conditions of 12.4–16.8 kbar and 350–406 °C. In contrast, the garnet-ferroglaucophane schist samples preserve a peak assemblage of garnet + ferroglaucophane + omphacite + lawsonite + phengite + titanite/ilmenite/rutile ± allanite and yield peak P–T conditions of 19.5–22.6 kbar and 490–510 °C. The epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists record a nearly complete clockwise P–T loop characterized by a steep prograde P–T path with a low thermal gradient of ~5–8 °C/km followed by cooling or overprinting by isothermal decompression. Reconstruction of the metamorphic P–T evolution, together an evaluation of previous age data, allows us to propose rapid subduction of cold oceanic lithosphere to depths of 50–75 km at a rate of ~6.3–9.3 km/Myr during Early-Middle Triassic (248–240 Ma), followed by exhumation in the Late Triassic (231–214 Ma). The short time lag (<10 Ma) between the protolith generation and the high-pressure metamorphic peak further supports a rapid subduction process. Our results suggest that the young oceanic slab was dragged down by the thicker, long-lived, and cold downgoing Paleo-Tethyan Ocean plate, where it experienced blueschist-facies metamorphism. The co-occurrence of lawsonite-bearing epidote-magnesioriebeckite schists and garnet-ferroglaucophane schists and eclogites further points to a cold thermal structure at the convergent plate interface, leading to the interpretation that the CMOB represents a typical oceanic subduction-accretion belt. The results presented in this study provide important insights into the geodynamic evolution of the Paleo-Tethys.

Geochemistry of Bashibos-Bajrambos metasedimentary unit, Serbo-Macedonian massif, North Macedonia: Implications for age, provenance and tectonic setting

Metasediments of the Paleozoic Bashibos-Bajrambos unit, comprising the western perimeter of the Serbo-Macedonian Massif in SE Republic of North Macedonia, were investigated by employing whole-rock geochemistry and detrital zircon geochronology to put constraints of age, provenance and depositional tectonic setting. The metasedimentary unit consists of polymetamorphic and highly altered pelitic mica-schists and phyllites and metamafic rocks. The first limited data from the whole-rock composition of the metapelites and zircon Th/U ratios suggest derivation of detrital material from felsic and recycled older upper crustal sources. At the same time, mafic samples indicate an origin from a volcanic source without a significant contribution from ultramafic rocks. Whole-rock data and the detrital zircon record suggest that deposition of the sequence took place in collision-related, foreland basin, formed by the merging of Pelagonian and the Serbo-Macedonian Massifs. The population of the youngest recorded zircons indicates Early Permian sedimentation. The detrital zircon record speaks for significant sedimentary input of old crustal material from the Serbo-Macednonian Massif, while younger Carboniferous-Permian magmatic zircon populations support the significant influence of the Pelagonian Massif.

Mesoarchean accretionary mélange and tectonic erosion in the Archean Dharwar Craton, southern India: Plate tectonics in the early Earth

Accretionary orogens are hallmarks of subduction tectonics along convergent plate margins. Here we report a sequence of low-grade metasediments carrying exhumed blocks of ultramafic, mafic and felsic rocks from Sargur in the Western Dharwar Craton in India. These rocks occur along the southern domain of the Chitradurga Suture Zone, which marks the boundary between the Western and Central Dharwar Cratons and thus provide a window to explore Archean convergent margin processes. We present zircon U-Pb and Lu-Hf data from Sargur metasediments including quartz mica schist, fine-grained quartzite, and pelitic schist, as well as from blocks/layers of trondhjemite, garnet amphibolite, and chromite-bearing serpentinite occurring within the metasedimentary accretionary belt. The detrital zircon grains from the metasediments show multiple age groups, with the oldest age as 3482 Ma and an age peak at 2862 Ma. Magmatic zircons in trondhjemite show 207Pb/206Pb weighted mean age of ca. 2972 Ma, whereas those in the chromite-bearing serpentinite display multiple age populations of ca. 2896, 2750, 2648, 2566 and 2463 Ma, tracing zircon crystallization in an evolving mantle wedge adjacent to a subducting oceanic plate. Metamorphism is dated as ca. 2444 Ma from zircon grains in the garnet amphibolite. Zircon εHf(t) in the mafic-ultramafic rocks and trondhjemite are mostly positive, suggesting a juvenile (depleted mantle) source. The detrital zircon Lu-Hf data suggest that the sediment source involved Paleoarchean juvenile and reworked components. Based on our findings, we propose that the Sargur sequence represents an accretionary mélange which forms part of a major Mesoarchean accretionary orogen that witnessed multiple stages of tectonic erosion at least during three periods at ca. 3200–3000 Ma, 3000–2800 Ma and 2800–2500 Ma removing a large part of the accretionary prism along the convergent margin. We correlate the processes with prolonged subduction-accretion cycle culminating in the final collision between the Western and Central Dharwar cratonic blocks.

U–Pb age and Hf isotopic composition of detrital zircons from the modern river of the South Tianshan orogenic belt and their implications for Palaeozoic tectonic and crustal evolution

The U–Pb and Lu–Hf isotopic research of detrital zircons was carried out in southern modern rivers of the South Tianshan (STS) orogenic belt to understand the accretionary processes and crustal evolution in this region. Detrital zircons from river sands yield two dominant age populations at 460–390 Ma and 310–260 Ma with several Precambrian age groups. Such age patterns imply that the significant magmatic events occurred during the Palaeozoic in the STS. Comparative studies on U–Pb age of the detrital zircons from northern and southern river sands in the STS belt provide constraints on the tectonic evolution of the STS belt and its surrounding blocks. Abundant 500–460 Ma detrital zircons in northern river sands probably reflect a bidirectional subduction of the Palaeozoic Central Tianshan Ocean under the Yili Block and the Central Tianshan Block. The 460–390 Ma zircon population in both northern and southern river sands likely implies a bidirectional subduction of the South Tianshan Ocean. A lack of the 380–320 Ma zircon grains in the southern river sands may indicates the northward subduction of the South Tianshan Ocean. A final collision between the Tarim Craton and the south‐western Central Asian Orogenic Belt possibly took place in the Late Carboniferous; the 310–260 Ma zircon population in both northern and southern river sands suggests syn‐ and post‐collisional magmatism. Based on the Hf isotopic signatures in the detrital zircon and magmatic zircons, we propose new constraints on the Palaeozoic crustal evolution of the STS. The continent formation in the STS belt was characterised by reworking of the ancient crust in a compressional environment during the Ordovician (ca. 460–440 Ma). Abundant significant juvenile materials progressively added to the crust through time in an extensional tectonic regime since Late Ordovician to Devonian (ca. 440–390 Ma). The crustal formation is mainly dominated by reworking of the Precambrian basement rocks with limited contribution of juvenile components input into magma formation during Late Carboniferous to Early Permian (ca. 310–260 Ma).