Overgrowth Rims Research Articles

Application of Ti-in-zircon and Zr-in-rutile thermometers to constrain high-temperature metamorphism in eclogites from the Dabie orogen, central China

Ultrahigh-pressure (UHP) eclogites from the northern Dabie orogen, central China underwent a complex metamorphic evolution during Triassic continental deep subduction and subsequent exhumation. The eclogites were strongly affected by multiple decompression and re-crystallization processes during exhumation, thus making the determination of peak metamorphic conditions particularly challenging. The recently calibrated Ti-in-zircon and Zr-in-rutile thermometers provide new tools to estimate the peak and post-peak temperatures. Rutiles within UHP mineral-bearing domains of zircon have Zr contents of 1030−4310ppm and record a temperature range of 880−1080°C, with a peak at 970°C. Rutiles within high-pressure (HP) mineral-bearing domains contain Zr contents of 800−5800ppm and define a temperature range of 780−1030°C with an average value of 873°C. Rutiles in granulite-facies mineral-bearing rims of zircon yield a temperature of 845°C. In contrast, rutile inclusions within garnet have lower Zr contents of 100−800ppm, yielding lower temperatures of 600−850°C. The UHP and HP mineral-bearing mantle domains of zircon have Ti contents of 2.15–97.8ppm, yielding Ti-in-zircon temperatures of 620−1020°C. Ti contents in the rim domains of zircon are low (<7ppm), indicating a temperature of 650−700°C, except for two granulite-facies overgrowth rims which yield a temperature of 793−840°C. Thus, most of the temperatures for different domains of metamorphic zircons are <800°C and generally lie around 650±50°C. Few measures of temperature are the >900°C for the UHP and HP domains of zircon. Therefore, this study combined with the previously published data suggests that the eclogites in the region experienced a protracted high-temperature (HT) metamorphic evolution from UHP eclogite-facies to HP granulite-facies metamorphic stages. This long-lived HT evolution coupled with slow cooling and partial melting may explain the rare preservation of UHP traces in the area. HP/UHP mineral inclusions in garnet and zircon experienced variable degrees of decompression breakdown or retrogression. Therefore, also Ti concentrations in zircons and Zr concentrations in rutiles within garnet and zircon may have been strongly modified by re-crystallization or partial melting. As a result, only few zircons record their actual crystallization temperatures. In comparison, the rutile inclusions in zircon generally define higher temperatures. We therefore suggest that rutile inclusions in zircon are the most suitable candidates for HT estimates, especially in strongly retrogressed eclogites.

Earliest Paleoproterozoic supracrustal rocks in the North China Craton recognized from the Daqingshan area of the Khondalite Belt: Constraints on craton evolution

The Upper Wulashan “Subgroup” at Daqingshan in the Khondalite Belt of the Western Block of the North China Craton contains an Early Paleoproterozoic (2.5–2.45Ga) supracrustal sequence, incorporating banded iron formations, which we refer to as the Daqingshan Supracrustal Rocks. They contain rounded to elliptical zircon grains that commonly show core–rim or core–mantle–rim structures and precise SHRIMP U–Pb dating of these domains allows the discrimination of a series of tectono-thermal events that straddle the Archean/Proterozoic boundary. Detrital zircon cores with oscillatory zoning have formation ages ranging from 2.55 to 2.50Ga, indicating derivation from Late Neoarchean magmatic rocks. As a result of exhumation and erosion, these became incorporated in earliest Paleoproterozoic sediments that were deposited between 2.50 and 2.45Ga and then underwent high-grade metamorphism at 2.45–2.40Ga. This event variously recrystallized the cores into two main domains: a dark inner domain and a gray outer domain, as imaged in cathodoluminescence. The gray domains commonly show sector zoning and have Th/U ratios of 0.1–0.5, a feature commonly noted in zircons recrystallized under high-grade metamorphic conditions. Metamorphic mantles are generally more homogeneous in structure and have lower Th/U ratios (commonly <0.1) than the recrystallized domains, although they show similar age distributions. However, they are difficult to distinguish when only two domains are present in the zircon and they represent a continuum of recrystallization. The rocks then underwent a second episode of high-grade metamorphism in the Late Paleoproterozoic, as revealed by thin overgrowth rims that are homogeneous or show only weak zoning, have low Th/U ratios (commonly <0.1), and record ages of 1.95–1.90Ga. This is the first time that two tectono-thermal events of Early and Late Paleoproterozoic age have been identified in single rock samples from the Western Block of the North China Craton. The conclusion that unequivocal earliest Paleoproterozoic sediments are present in the North China Craton is also supported by evidence that the Daqingshan Supracrustal Rocks are cut by anatectic garnet granite, previously dated at 2.39Ga. Combined with evidence obtained here and elsewhere in the North China Craton, we conclude that the 2.45–2.40Ga metamorphism was not an extension of the well-established, craton-wide Late Neoarchean tectono-thermal event, but a previously unrecognized episode that followed a short quiescent period that allowed sedimentation of the Daqingshan Supracrustal Rocks.

Metamorphic P–T paths and New Zircon U–Pb age data for garnet–mica schist from the Wutai Group, North China Craton

The amalgamation of the North China Craton (NCC) is popularly viewed as a result of assembling between the Western and Eastern Blocks at ∼1.85Ga, forming the Trans-North China Orogen. Disputations still exist on the age and detailed tectonic processes involved in the assembling. Garnet–mica schist samples were collected from the lower part of Wutai Group in the central Trans-North China Orogen for determination of the metamorphic P–T path and age and for better understanding the tectonic evolution of the NCC. The garnet–mica schist generally contains mega-garnet porphyroblasts together with biotite, muscovite, quartz and ilmenite with or without plagioclase, epidote, ortho-amphibole, staurolite and kyanite. Garnet is mostly weakly zoned with core–rim increasing pyrope and decreasing grossular, but occasionally exhibits pronounced zoning involving different patterns in the core and mantle, suggesting two-stage growth. Pseudosections were calculated using THERMOCALC for three representative samples. The modeling results, together with the petrographic observations suggest that the garnet–mica schist may have experienced four metamorphic stages. The first-stage (S-I), which is revealed only from the infrequent garnet core zoning, is characterized by a flat P–T vector dominated by heating, closing to the low-pressure facies series. The second-stage (S-II) exhibits a steep P–T vector with increasing pressure and temperature to the pressure peak at >9.0kbar/615–645°C, being the medium-pressure facies series (kyanite type). The third stage (S-III) is the decompression with heating to the temperature peak at ∼7.5kbar/660°C. The fourth stage (S-IV) is the further decompression after the temperature peak, accompanied with considerable cooling. Metamorphic overgrowth rims of zircon and fully metamorphic crystals from two samples dated using SHRIMP and LA-ICP-MS methods yield new metamorphic ages of ∼1.95Ga, which were interpreted to represent the pre- or peak stages (i.e. S-II and S-III) of the kyanite type metamorphism for the dated zircon grains contain inclusions similar to the minerals in the host rocks and the REE patterns in zircon show metamorphic characteristics. This suggests that there would be a collision or crust thickening event coupled with the kyanite type metamorphism occurring at ∼1.95Ga, which is about 100Ma older than the popularly viewed collision age (∼1.85Ga). A probable interpretation is that there may be a long-lived orogenic process where the older metamorphic ages of ∼1.95Ga represent the main collision stage, and the younger ages of ∼1.85Ga indicate uplifting and cooling of high-grade terranes. The first stage (S-I) metamorphism of low-pressure facies series could be an indicator of crust extension before the collision event.

A ∼2.5 Ga magmatic event at the northern margin of the Yangtze craton: Evidence from U-Pb dating and Hf isotope analysis of zircons from the Douling Complex in the South Qinling orogen

The poorly studied Douling Complex is a crystalline basement that developed in the Neoproterozoic-Paleozoic weakly metamorphosed to non-metamorphosed strata at the South Qinling tectonic belt. Five banded dioritic-granitic gneiss samples from the Douling Complex were chosen for LA-MC-ICPMS U-Pb zircon dating, which yielded protolith emplacement ages of 2469 ± 22 Ma, 2479 ± 12 Ma, 2497 ± 21 Ma, 2501 ± 17 Ma and 2509 ± 14 Ma, respectively. An important peak age of ∼2.48 Ga was also obtained for a metasedimentary rock in the same region. These discoveries suggest the occurrence of magmatic activity of 2.51–2.47 Ga at the northern margin of the Yangtze craton. The age-corrected ɛHf(t) values obtained from in situ zircon Hf isotopic analysis are mainly between −5.5 and +0.3, and the two-stage zircon Hf model ages range from 3.30 to 2.95 Ga. Considering two important periods of ∼3.3–3.2 Ga and ∼2.95–2.90 Ga for the continental crustal growth in the Yangtze craton, we infer that the dioritic-granitic gneisses from the Douling Complex are the products of reworking of Paleo- to Mesoarchean crust at the northern margin of the Yangtze craton at ∼2.5 Ga. In addition, metamorphic ages of 837 ± 8 Ma and 818 ± 10 Ma were obtained for zircon overgrowth rims from a dioritic gneiss and a metasedimentary rock, indicating that the main phase amphibolite facies metamorphism of the Doulng Complex occurred during the Neoproterozoic, although its geological meaning remains ambiguous.

Zircon U[sbnd]Pb geochronology and Hf isotopes of major lithologies from the Yishui Terrane: Implications for the crustal evolution of the Eastern Block, North China Craton

As an important component of the Western Shandong Complex in the Eastern Block of the North China Craton, the Yishui Terrane consists of Neoarchean high-grade supracrustal rocks, granitoid gneisses and charnockites. LA-ICP-MS zircon UPb dating and Hf isotopic analyses on these lithologies have been carried out and the results provide new insights into the Neoarchean crustal evolution of the Eastern Block of the North China Craton. New zircon dating results reveal that the magmatic precursors of the supracrustal metapelites and granitoid gneisses were generated at 2.54–2.53Ga and 2.57–2.55Ga, respectively, and the charnockites were emplaced contemporaneously around 2.56–2.53Ga. Single metamorphic zircon grains and overgrowth rims from these rocks document consistent metamorphic ages at ~2.50Ga, suggesting that the Yishui Terrane experienced a high-grade metamorphic event at the end of the Neoarchean. Detrital zircons from pelitic gneisses yield 207Pb/206Pb ages of 2.89–2.65Ga, with a major age peak at ~2.53Ga and a subordinate age peak at ~2.70Ga, implying that the sedimentary protoliths of the pelitic gneisses may have been sourced from ~2.53Ga rocks in the Yishui Terrane and ~2.70Ga rocks in the adjacent Luxi Granite-Greenstone Terrane. Hf isotopic compositions show that the Neoarchean magmatic zircons have positive εHf(t) values ranging from +1.4 to +7.8 and depleted mantle model ages of 2.92–2.60Ga with a prominent peak at 2.8–2.7Ga, suggesting that the Neoarchean crust was derived mainly from juvenile sources and partly from the recycling of old continental crust in the Yishui area. Combined with previous data from the adjacent Luxi Granite–Greenstone Terrane and other complexes in the Eastern Block, it is concluded that the Neoarchean was an important period of crust accretion in the Eastern Block of the North China Craton.

Magmatic history and evolution of continental lithosphere of the Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica

The Sør Rondane Mountains, eastern Dronning Maud Land, East Antarctica, are situated within the Pan-African suture zone, between West and East Gondwana, and the timing of collision event is regarded as the late Neoproterozoic to early Cambrian. In order to understand the tectonothermal history and evolution of the continental lithosphere, geochemical studies were conducted and zircon U–Pb SHRIMP dating was performed on intrusive rocks with basaltic compositions and associated metamorphic rocks. The metamorphosed tonalite complex exposed in the southwestern part of the mountains comprises tonalite associated with microgabbros, occurring as magmatic enclaves and later dikes that have intruded both the host and magmatic enclaves. Geochemically, the microgabbros are classified as low-Ti and high-Ti types, corresponding to the magmatic enclaves and dikes, respectively. Moreover, the geochemical features of the low-Ti microgabbro resemble those of an oceanic-arc tholeiite, whereas the high-Ti microgabbro has features of a back-arc basalt. The apparent zircon U–Pb ages show c. 990Ma for the low-Ti microgabbro and c. 950Ma for the high-Ti microgabbro. The high-grade metamorphic rocks situated in the northern part of tonalite complex were metamorphosed between 640 and 620Ma, as constrained by zircon overgrowth rims of a migmatite leucosome (620±2Ma) and the previously reported age data. The timing of peak metamorphism corresponds to the early stage of the Pan-African suture event. Postdating the suturing event, unmetamorphosed minette dikes, dated 564±2 Ma, intrude the tonalite complex and high-grade gneisses. On the basis of geochemical investigation, including Sr and Nd isotopic systematics, the microgabbros are considered to have originated from a depleted mantle source, whereas the minette magma is derived from an enriched mantle source. Consequently, the source mantles of the mafic magmas in the Sør Rondane Mountains have fundamentally changed from a depleted source in the early Neoproterozoic to a more enriched source in the late Neoproterozoic. Geochemical and isotopic evidence suggest that this compositional change of source mantle would reflect the interaction between the depleted mantle and the enriched crustal materials, such as subducted crustal rocks, caused by the Pan-African suture event.

U–Pb ages and trace elements of detrital zircons from Early Cretaceous sedimentary rocks in the Jiaolai Basin, north margin of the Sulu UHP terrane: Provenances and tectonic implications

The Dabie–Sulu orogen in east-central China is the largest UHP belt in the world. Many studies have dealt with detrital minerals from sedimentary basins surrounding the Dabie belt in order to constrain the tectonic unroofing of this terrane. On the other hand, still few are the works focused on detritals from the basins associated with the Sulu belt. The Jiaolai basin, adjacent to the Sulu belt, is made of Mesozoic sedimentary rocks. Two sandstone samples collected from the Early Cretaceous Laiyang Group in the northern and southern margins of the Jiaolai basin were selected for zircon U–Pb dating and trace element analysis. The zircons show main U–Pb age clusters at ca. 2500–2400, 2000–1700, 850–700, and 330–130Ma, with a minor peak at ca. 1100Ma and a few Archean ages. Most zircons show texture features reflecting a magmatic origin, whereas several late Paleoproterozoic and Triassic zircons show homogeneous CL images and low Th/U ratios, indicating a metamorphic origin. The Neoproterozoic ages are consistent with the protolith ages of the metamorphic rocks in the Sulu terrane, implying that the Sulu tarrane may be a major source for this component. Whereas most Neoproterozoic zircons are euhedral to subhedral or angular in shape and are not surrounded by overgrowths, this implies that low-grade metamorphic Neoproterozoic metaigneous rocks ocurring along the northern margin of the Sulu terrane or metasediments overlying the HP–UHP rocks in the Sulu terrane might provide source materials to these detrital zircons. The late and early Paleoproterozoic age peaks are consistent with the thermal events occurred in the Jiaobei terrane, slightly younger than those in the Yangtze Block. These suggest that the Jiaobei terrane was a potential source for the Paleoproterozoic to Archean detrital zircon components. Only two early Paleozoic zircon grains were found in the studied samples, indicating the early Paleozoic arc did not extend into the Sulu terrane. According to the distribution of different kinds of rock adjacent to the Sulu terrane, the Early Cretaceous detrital zircons may be potentially derived from Cretaceous plutons from the Sulu belt. Eight detrital zircons including six metamorphic grians and two metamorphic overgrowth rims yield the Triassic ages, which might be derived from HP–UHP rocks from the Sulu terrane. A metamorphic overgrowth rim yields a metamorphic age of 246±3Ma, probably recording the timing of the prograde metamorphism of the Sulu terrane. One metamorphic grain yields an age of 212±2Ma, which is interpreted to record a retrograde metamorphic event. Based on the REE patterns, the remaining six Triassic analyses can be subdivided into two groups. Four of them record a weighted mean age of 230±6Ma and show nearly flat HREE patterns and slight negative or positive Eu anomalies. The last two grains are enriched HREE; considering their ages and the REE patterns, it is suggested that they may originate from other diverse UHP rocks. The Triassic metamorphic zircons were detected in the Laiyang Group, indicating that the Sulu UHP rocks were exposed to the surface before the Early Cretaceous.

U-Pb age of autochthonous paleoproterozoic charnockite in the Aldan Shield

Autochthonous and parautochthonous charnockites in granulite facies of the Aldan Shield (the Aldan River upper flow) were dated. According to the geological observation data, the autochthonous and parautochthonous granite formation included successive development of nebulite (Lc1), its melting product such as early diatectite (Lc3), later “layer-by-layer” migmatite (Lc4), and diatectite (Lc5). The concordant ages of Lc1 and Lc3 were estimated at 2436 ± 10 and 2453 ± 14 Ma. The age of Lc5 was estimated by the upper concordia crossing at 1960 ± 8 Ma likely corresponding to the diatectic melt crystallization period. The process is accompanied by repeated high-temperature alterations of nebulite, diatectite, and their zircons yielding a concordant age of 1945 ± 13 Ma. This zircon making up the overgrowth rims is characterized by remarkable enrichment in uranium and thorium. The granulite facies metamorphism is confirmed by dating of monazite from migmatite after metapelite (1947.7 ± 8.7 Ma). The two main stages of the autochthonous and parautochthonous charnockite formation initiated the development of the crust magmatic chambers. The first stage (2430–2450 Ma) was synchronous to allochthonous high-K alkali granite in the Olekma granite-greenstone region. The second stage (1900–1960) implied the formation of autochthonous and parautochthonous charnockites under the granulite facies conditions and development of allochthonous charnockite and granite in the central part of the granulite areal.

Decoding multiple tectonothermal events in zircons from single rock samples: SHRIMP zircon U–Pb data from the late Neoarchean rocks of Daqingshan, North China Craton

A complex sequence of tectonothermal events shaped the Neoarchean–Paleoproterozoic crustal evolution in the North China Craton (NCC). Here we present a systematic SHRIMP zircon U–Pb investigation on a suite of metamorphic rocks from the Hademengou area of Daqingshan within the Khondalite Belt of the Inner Mongolia Suture Zone. We analyzed zircons from two meta-volcanic rocks of the Sanggan “Group” and four meta-igneous rocks of late Neoarchean to early Paleoproterozoic. Zircons from these rocks display typical core–rim or core–mantle–rim structures. The relict magmatic zircons record crystallization ages of ca. 2.5Ga followed by variable Pb loss, showing a range of ages in the concordia up to 2.1Ga. The recrystallized domains of zircons show sector zoning or are composed of inner dark domains, and outer lighter domains. The overgrowth mantles are homogeneous. The recrystallized zircons and overgrowth mantles show similar age data distribution along a concordia from ~2.5 to ~2.1Ga. The overgrowth rims are poorly developed but record ages (1.96–1.84Ga) similar to those displayed by metamorphic zircons in the late Paleoproterozoic “Upper Wulashan Subgroup” as well as the charnockites and UHT granulites reported in previous studies from this region. A meta-syenogranite intruding the Sanggan “Group” and Hypersthene dioritic gneiss contains 1.95Ga magmatic and 1.85Ga metamorphic zircons. Magmatic zircons of the late Neoarchean–early Paleoproterozoic metavolcanic rocks have εHf(t) of 1.2 to 7.0, tDM1(Hf) model ages of 2687 to 2532Ma and tDM2(CC)(Hf) model ages of 2837 to 2559Ma. The recrystallized zircons and mantle domains display Hf isotope composition similar to that of the magmatic domains. Our study is the first report from this region that identifies multiple tectonothermal events ranging from late Neoarchean through early to late Paleoproterozoic from zircons within single rock samples in the NCC. A strong overprint of late Paleoproterozoic metamorphism traced commonly in all the rock units indicates that these rocks were involved in the tectonothermal event associated with the final amalgamation of the NCC.

Zircon Trace Element and O–Hf Isotope Analyses of Mineralized Intrusions from El Teniente Ore Deposit, Chilean Andes: Constraints on the Source and Magmatic Evolution of Porphyry Cu–Mo Related Magmas

Abstract Intrusive rocks related to porphyry copper mineralization are part of the wide diversity of subduction-related, mantle-derived, igneous rocks generated in convergent margin settings. What differentiates them from barren igneous rocks results ultimately from the multi-component and multi-stage processes that condition magma composition in these settings. Unfortunately, the petrogenetic history is largely obscured by the pervasive alteration that affects rocks in these deposits. We address this issue through the study of zircon grains from El Teniente, one of the largest known porphyry Cu–Mo deposits in the world. El Teniente belongs to the Miocene–Pliocene Cu–Mo belt of the Central Chilean Andes, which formed in a short timespan during the Cenozoic constructive period of the orogen. Previously U–Pb dated zircon grains were selected for re-examination of their morphological characteristics and in situ analysis of chemical (rare earth element, Hf, Y and Ti contents) and isotopic (Hf, O) composition. They are from six intermediate to felsic syn- to late-mineralization, intrusive units covering a timespan of ∼1·6 Myr. The El Teniente zircons have compositional and morphological characteristics indicating crystallization from a series of cogenetic melts. However, a minor hydrothermal imprint is documented in the presence of crystals with mottled surfaces that correspond to thin high U–Th overgrowth rims (low-luminescent features in cathodoluminescence images). In terms of any other chemical and isotopic characteristic, these are indistinguishable from the main mineral populations. Zircons define morphological and chemical trends reflecting an evolution towards more differentiated magma compositions, lower crystallization temperatures and increased cooling rates with decreasing age of intrusion. Hf and O isotopic compositions are remarkably uniform at grain, sample and deposit scale. This, together with the general absence of older inherited zircon components, the lack of correlations between isotopic signature and whole-rock composition and high initial εHf values (total average 7·4 ± 1·2; 2σ), rules out involvement of any significant crustal contamination in the genesis of the El Teniente magmas. The Hf isotopic composition indicates a relatively juvenile source, but with some crustal residence time. The δ18OZrc weighted mean of 4·76 ± 0·12‰ (2σ; 61 analyses) is at the lower limit of the normal mantle zircon range of 5·3 ± 0·6‰ (2σ), and might reflect crystallization from low-18O magmas. Hf isotopic compositions have a restricted range in initial εHf values between +6 and +10, identical to preceding Cenozoic barren magmatic activity in Central Chile. These igneous rocks are the product of nearly 25 Myr of subduction-related magmatic activity, developed under contrasting tectonic regimes and margin configurations. This suggests a primary control of the isotopic signature by a stable long-lived MASH-type (melting, assimilation, storage and homogenization) reservoir in the deep lithosphere. In the context of the Cenozoic evolution of Central Chile we argue that dehydration melting in the enriched MASH reservoir occurred as a consequence of increasing crustal thickness, and was prompted by a high-temperature thermal regime resulting from long-lasting preceding magmatism. This process can also fractionate O to generate low-18O magmas. At the time of El Teniente formation, dehydration melting occurred coevally with arc migration, which probably influenced the fertility of the magmas by increasing the melt component derived from this process relative to the component derived from primary basalt differentiation. At a regional scale, such reactions are expected to occur as a consequence of progressive crustal thickening during the constructive period of the Andes, and can explain the simultaneous generation of porphyry deposits in the Miocene–Pliocene Cu–Mo belt of Central Chile.

New SHRIMP U–Pb zircon ages and CHIME monazite ages from South Harris granulites, Lewisian Complex, NW Scotland: Implications for two stages of zircon formation during Palaeoproterozoic UHT metamorphism

Sensitive high-resolution ion microprobe (SHRIMP) U–Pb zircon age dating was applied to several types of granulites and gneisses from the Lewisian Complex in South Harris, NW Scotland, to clarify the timing of Palaeoproterozoic tectono-thermal events. For comparison, monazite was dated using the chemical Th–U–total Pb isochron method (CHIME). The obtained U–Pb zircon age data were evaluated under the constrains of the modal amount of constituent minerals and whole-rock compositions of analysed samples, the zircon internal texture based on Th/U ratios, and the monazite CHIME ages. Zircons in three samples, characterised by zoned overgrowth rims, sector-zoned grains, and structureless high-U cores, yielded concordant ages of approximately 1900Ma (1909±3Ma, 1899±10, and 1888±15Ma, respectively). The age of 1899±10Ma, obtained from melanocratic granulite (garnet and kyanite/sillimanite: 76–91vol.%), is interpreted to represent the timing of zircon crystallisation prior to the formation of restitic garnet and kyanite/sillimanite. Therefore, the age of ∼1900Ma is proposed to represent the timing of early metamorphism, close to the thermal peak (M1). In the leucocratic gneiss (sample 93921-7), the age population at 1868±35Ma obtained from low-U and low-Th overgrowth rims represents a second stage of zircon growth from crystallising anatectic melt (M3), whereas the high-U cores represent the timing of zircon growth and recrystallisation prior to anatexis. Monazites from two leucocratic gneisses yield a relatively narrow range of CHIME ages: 1858±7Ma and 1840±7Ma. These ages probably represent the timing of zircon and monazite crystallisation from partial melt and resetting of the U–Th–Pb system. These results suggest that the timing of zircon formation was controlled by metamorphic processes, such as restite formation and crystallisation from partial melt.

A granulite record of multistage metamorphism and REE behavior in the Dabie orogen: Constraints from zircon and rock-forming minerals

A combined study of mineral inclusions, U–Pb ages and trace elements was carried for zircon and coexisting minerals from granulite in the North Dabie Terrane (NDT) of the Dabie–Sulu ultrahigh-pressure metamorphic (UHP) zone, east-central China. The results provide insights into the exhumation history of NDT and into rare earth element (REE) behavior during retrogression. Besides inherited cores and one magmatic rim, zircons separated from the granulite record three episodes of metamorphism under different P–T conditions: (1) 223.8±2.3Ma for domains that contain Grt+Cpx±Rt±F−Ap±Aln inclusions without plagioclase and show flat HREE patterns without negative Eu anomalies, representing peak eclogite-facies event; (2) 213.3±2.1Ma for domains that contain Pl±Cpx±Grt±Qtz±Ap inclusions and show rather flat HREE patterns with negative Eu anomalies, corresponding to granulite-facies retrogression; (3) 199.9±3.3Ma for domains that contain Amp±Pl±Qtz±Ap inclusions and show high REE contents with steep HREE patterns and remarkable negative Eu anomalies, representing amphibolite-facies overprinting. Therefore, the UHP eclogite in NDT experienced decompression heating during the initial exhumation, with local hydration in the late stage of the Triassic continental collision.Garnet in the granulite is composed of a corroded core with embayed outline and spongy texture and an overgrowth rim. There is equilibrium distribution of HREE between garnet rim and granulite-facies zircon domain, confirming the geological interpretation of 213.3±2.1Ma for the granulite-facies metamorphism. There is the prograde HREE depletion in porphyroblastic garnet from core to rim and the continuous decrease of HREE from the eclogitic to granulitic zircons, suggesting that the metamorphic transformation from eclogite-facies to granulite-facies took place in a closed system. On the other hand, the amphibolitic zircons show steep HREE patterns and significantly elevated REE contents, suggesting that the amphibolite-facies metamorphism took place in an open system with introduction of external fluids containing more REE to the granulite-facies rocks. Residual clinopyroxene partially replaced by amphibole has remarkably elevated REE contents relative to euhedral primary clinopyroxene. There is equilibrium distribution of REE between the residual clinopyroxene and the amphibole, marking fluid action during the amphibolite-facies overprinting.

U–Pb geochronology and Hf–Nd isotopic geochemistry of the Badu Complex, Southeastern China: Implications for the Precambrian crustal evolution and paleogeography of the Cathaysia Block

The oldest rocks of the Cathaysia Block, South China, comprise the Badu Complex and Paleoproterozoic granites in the Wuyishan area (southern Zhejiang and northwestern Fujian Provinces). New zircon U–Pb ages, Hf isotopes and trace elements for metamorphic rocks from the Badu Complex, and bulk Nd-isotope compositions of these rocks and granites in the Wuyishan area provide important constraints on the Precambrian crustal evolution of the Cathaysia Block. Inherited cores of zircon grains from the metamorphic rocks are of magmatic origin, predominantly formed at ca 2.5Ga, while overgrowth rims reflect two episodes of granulite–facies metamorphism related to collisional orogeny at 1.89–1.88Ga and 252–234Ma. The unimodal age distribution (∼2.5Ga) of detrital zircons and the positive ɛHf(t) of most Neoarchean zircons suggest that the detritus of these sedimentary protoliths of the Badu Complex came from a proximal volcanic arc, and that they were deposited in an arc basin synchronously with ∼2.5Ga volcanism. Zircon U–Pb ages and Hf-isotopes with whole-rock Nd isotopes suggest that the juvenile crust of the eastern Cathaysia Block was generated mainly at 2.5Ga and 2.8Ga, and minor at 3.5–3.3Ga with some evidence for the generation at 3.7–3.6Ga and ∼4.0Ga. A strong ∼1.9Ga orogeny and the 3.3–3.0Ga thermal event only involved the reworking of older crust material. Paleoproterozoic (1.89–1.86Ga) granitic magmatic activity and high-grade metamorphism in the eastern Cathaysia Block were synchronous with the assembly of the Columbia supercontinent. Using integrated geochronological, Hf–Nd isotopic and petrologic data as a “barcode”, we compare the Cathaysia Block with other Paleoproterozoic orogens worldwide, and argue that its closest affinity is with the South Korean Peninsula and the Lesser Himalaya of NW India. Consequently, the eastern Cathaysia block, and the South Korean massif (as the united Cathay-South Korea terrane) was close to the Lesser Himalaya terrane in the Paleoproterozoic configuration of the Columbia supercontinent. The spatial linkage was maintained for ca 1Ga, until the fragmentation of the Rodinia supercontinent during Neoproterozoic time.

Zircon U–Pb constraints on the origin of UHP meta-supracrustal rocks in the Southern Sulu orogen, eastern China

Most zircons in marble and quartzite, typical meta-sedimentary rock from southern Sulu orogenic belt, have a core-rim structure, including an inherited detrital core and a metamorphic overgrowth rim. A few zircons are completely metamorphic. Most detrital cores have relatively high REE contents and Th/U ratios and yield highly variable 206Pb/ 238U ages ranging from 2828 to 211 Ma, with major age populations at ∼1800 Ma and ∼800 Ma for quartzites and ∼1300 Ma for marbles. In contrast, the metamorphic zircon rims have relatively low REE contents and low Th/U ratios, and yield consistent Triassic metamorphic ages with a weighted mean age of 231 Ma. These results indicate that the marble and quartzite, together with eclogite and orthogneiss from the southern Sulu orogenic belt, have been subjected to a coeval Triassic UHP metamorphism. The protoliths of the quartzites may have been formed in the Neoproterozoic, whereas the protoliths of the marble are probably in the Mesoproterozoic. The source regions of the UHP supracrustal rocks underwent Neoproterozoic to Paloproterozoic tectonic–thermal events, indicating that the protoliths of the Sulu UHP rocks were derived from the Yangtze craton, instead of the North China craton.

Microtextural and mineral chemical analyses of andesite–dacite from Barren and Narcondam islands: Evidences for magma mixing and petrological implications

Andesite and dacite from Barren and Narcondam volcanic islands of Andaman subduction zone are composed of plagioclase, orthopyroxene, clinopyroxene, olivine, titanomagnetite, magnesiohornblende and rare quartz grains. In this study, we use the results of mineral chemical analyses of the calc-alkaline rock suite of rocks as proxies for magma mixing and mingling processes. Plagioclase, the most dominant mineral, shows zoning which includes oscillatory, patchy, multiple and repetitive zonation and ‘fritted’ or ‘sieve’ textures. Zoning patterns in plagioclase phenocrysts and abrupt fluctuations in An content record different melt conditions in a dynamic magma chamber. ‘Fritted’ zones (An55) are frequently overgrown by thin calcic (An72) plagioclase rims over welldeveloped dissolution surfaces. These features have probably resulted from mixing of a more silicic magma with the host andesite. Olivine and orthopyroxene with reaction and overgrowth rims (corona) suggest magma mixing processes. We conclude that hybrid magma formed from the mixing of mafic and felsic magma by two-stage processes – initial intrusion of hotter mafic melt (andesitic) followed by cooler acidic melt at later stage.

Prolonged magmatism, juvenile nature and tectonic evolution of the Chinese Altai, NW China: Evidence from zircon U–Pb and Hf isotopic study of Paleozoic granitoids

Paleozoic granitoid magmatism played an important role in the tectonic evolution of the Chinese Altai, and zircon U–Pb and Hf isotopic compositions have been determined for samples from eleven granitic plutons/batholiths. The Jiadengyu gneissic granitic pluton yielded a zircon U–Pb age of 479 Ma, and thus it does not represent Precambrian basement as suggested previously. Our results and published data demonstrate that voluminous granitoids were continuously emplaced over more than 30% area of the Chinese Altai during the period from 447 Ma to 368 Ma with a climax at ca. 400 Ma. Ages for zircon overgrowth rims demonstrate additional thermal events at ca. 360 and 280 Ma, respectively. Positive ε Hf( t) values (0 to +9) of normal magmatic zircons suggest that the granitoid magmas were derived from juvenile sources. Xenocrystic zircon cores are 543–421 Ma old and also give positive ε Hf( t) values (+2.5 to +12), suggesting their origin as early crystallized minerals in the magma chambers or as inherited cores from newly-accreted meterials. The strong magmatism at ca. 400 Ma significantly changed the Hf isotopic composition of the magma source by substantial input of juvenile material in a relatively short period. Geophysical, geological and geochemical data support that ridge subduction was a possible mechanism for the strong magmatism ca. 400 Ma and the above mentioned change of Hf isotopic composition in the magma source.

Mineralogy and geochemistry of gold-bearing arsenian pyrite from the Shuiyindong Carlin-type gold deposit, Guizhou, China: implications for gold depositional processes

Arsenian pyrite in the Shuiyindong Carlin-type gold deposit in Guizhou, China, is the major host for gold with 300 to 4,000 ppm Au and 0.65 to 14.1 wt.% As. Electron miroprobe data show a negative correlation of As and S in arsenian pyrite, which is consistent with the substitution of As for S in the pyrite structure. The relatively homogeneous distribution of gold in arsenian pyrite and a positive correlation of As and Au, with Au/As ratios below the solubility limit of gold in arsenian pyrite, suggest that invisible gold is likely present as Au1+ in a structurally bound Au complex in arsenian pyrite. Geochemical modeling using the laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of fluid inclusions for the major ore forming stage shows that the dominant Au species were Au(HS)2− (77%) and AuHS(aq)0 (23%). Gold-hydroxyl and Gold-chloride complexes were negligible. The ore fluid was undersaturated with respect to native Au, with a saturation index of −3.8. The predominant As species was H3AsO30(aq). Pyrite in the Shuiyindong deposit shows chemical zonation with rims richer in As and Au than cores, reflecting the chemical evolution of the ore-bearing fluids. The early ore fluids had relatively high activities of As and Au, to deposit unzoned and zoned arsenian pyrite that host most gold in the deposit. The ore fluids then became depleted in Au and As and formed As-poor pyrite overgrowth rims on gold-bearing arsenian pyrite. Arsenopyrite overgrowth aggregates on arsenian pyrite indicate a late fluid with relatively high activity of As. The lack of evidence of boiling and the low iron content of fluid inclusions in quartz, suggest that iron in arsenian pyrite was most likely derived from dissolution of ferroan minerals in the host rocks, with sulfidation of the dissolved iron by H2S-rich ore fluids being the most important mechanism of gold deposition in the Shuiyindong Carlin-type deposit.

The 390 Ma high-T metamorphic event in the Chinese Altai: A consequence of ridge-subduction?

High-grade rocks occur in the Chinese Altai, but the timing of metamorphism is poorly constrained, which hinders our understanding of the thermo-tectonic history of the region. Representative high-grade samples from the sillimanite zone extending from Hanas to Fuyun were selected for zircon U-Pb dating and temperature estimation. LA-ICP-MS analyses of zircon overgrowth rims and recrystallized domains give consistent ages of ∼390 Ma, which is interpreted to record a regional metamorphic event. Temperature (T) estimations using the amphibole-plagioclase-quartz (Amp-Pl-Qtz) and garnet-biotite (GB) geothermometers give relatively high temperatures ranging from 650 to 700 °C. The zircon metamorphic rims yield temperature estimates of ∼720 °C by using the Ti-in-zircon thermometer. These data suggest that a high-temperature metamorphic event took place in the Chinese Altai in the Middle Devonian, and may imply a tectonic environment involving an unusually elevated heat flux from a deep-seated source. Our data support possible ridge subduction around 390 Ma that caused upwelling of the hot asthenosphere and triggered the high-T metamorphism. This model can also account for coeval volcanic activity with a range of geochemical characteristics, diverse mafic intrusions and extensive hydrothermal mineralization in the Altai orogen.

U–Pb and Hf isotopic study of zircons of the Helanshan Complex: Constrains on the evolution of the Khondalite Belt in the Western Block of the North China Craton

The Helanshan Complex is located at the westernmost segment of the Khondalite Belt, a continent–continent collisional belt along which the Yinshan Block in the north collided with the Ordos Block in the south to form the Western Block of the North China Craton. The complex consists of S-type granites and high-grade pelitic granulite/gneiss, felsic paragneiss, quartzite, calc-silicate rock and marble, together called the Khondalite Series rocks. LA-Q-ICP-MS U–Pb geochronology and LA-MC-ICP-MS Lu–Hf isotopic data of zircons, combined with cathodoluminescence (CL) imaging, enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the Khondalite Belt in the Western Block of the North China Craton. CL images reveal the coexistence of magmatic-type detrital zircons and metamorphic zircons in most of the Khondalite Series rocks, of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic-type detrital zircon cores. LA-Q-ICP-MS U–Pb analyses on magmatic-type detrital zircons reveal two distinct age populations, with one in the Archaean (2.85–2.53Ga) and the other in the Palaeoproterozoic (2.2–2.0Ga), suggesting that the sedimentary protoliths of the Khondalite Series rocks in the Helanshan Complex must have been deposited at some time after 2.0Ga. The Hf analyses show that the Archaean detrital zircons possess negative εHf(t) values from −7.64 to −0.15 with depleted mantle model ages ranging from 3.34 to 3.10Ga. This implies the existence of a Paleo-Mesoarchaean (3.34–3.10Ga) crust in the Western Block, which underwent a recycling event in the period 2.85–2.53Ga. Nearly all Palaeoproterozoic (2.2–2.0Ga) detrital zircons possess positive εHf(t) values (0.86–9.38) with depleted mantle model ages of 2.45–2.15Ga, suggesting a significant crustal growth event in the Western Block in the Palaeoproterozoic. Metamorphic zircons yield two groups of age, with one at ~1.95Ga and the other at ~1.87Ga, of which the former is considered as the timing of the collision between the Yinshan and Ordos Blocks to form the Western Block, whereas the latter is consistent with the age of ~1.86Ga from two S-type granites which are considered to have formed by the partial melting of pelitic granulites/gneisses at the stage of the exhumation of the Helanshan Complex.

The Magmatic Evolution of the Whakamaru Supereruption, New Zealand, Constrained by a Microanalytical Study of Plagioclase and Quartz

TheWhakamaru eruption is the largest-volume eruption known to have originated from the hyper-productive Taupo Volcanic Zone, New Zealand. Major, minor and trace element concentrations of plagioclase crystals and cathodoluminescence images, used as a proxy for Ti concentrations in quartz crystals, have been used to explore their chemical zonation. Three plagioclase populations are identified. Group 1 crystals are characterized by inherited cores of composition An45^60, Ba 115^650 ppm and La 3^9 ppm, rims of c. An30, Ba 450^800 ppm and La 7^10 ppm and the presence of a thin overgrowth rim on several crystals cores. Group 2 crystals are oscillatory-zoned plagioclases of composition An30^40, Ba 450^730 ppm and La 8·5^9·5 ppm. Group 3 plagioclase crystals have cores of An25^35 and rims of An20^25 and low Sr contents (280^480 ppm). From the chemical composition of these plagioclase crystals, four physicochemically distinct rhyolitic melts are identified: (1) an andesitic progenitor melt in which the cores of Group 1 crystals crystallized; (2) a greywacke melt or greywacke protolith melt responsible for narrow overgrowth rims on Group 1 crystal cores; (3) melt derived from the rejuvenation of a mature crystal mush body from which Group 3 plagioclase crystals crystallized; (4) a final, rhyolitic melt created by the amalgamation of varying proportions of the andesitic, greywacke-derived and rejuvenated melts with subsequent, open-system fractional crystallization of a plagioclasedominant crystal assemblage. Cathodoluminescence imaging of quartz crystals reveals complex zonation, the result of a dynamic crystallization history from potentially polygenetic sources. Diffusion modelling of the greyscale intensity of cathodoluminescence images (as a proxy forTi content) for a selection of bright core^rim interfaces of quartz crystals suggests that renewed quartz growth at the rim zones occurred5300 years (peak likelihood 50^70 years) prior to and continued towards the climactic eruption. This is consistent with timescales of5280 years determined from core^rim interfaces of Group 1 plagioclase crystals, suggesting that the magma chamber was ephemeral, derived from mixing of magmas from multiple sources shortly prior to eruption. This study adds to a growing body of evidence for the ephemeral nature and geologically rapid mixing and mobilization of liquid silicic magma bodies leading to supereruptions, compared with the timescales of hundreds of thousands of years required to accumulate the precursor magma and crystals.