Cathodoluminescence Images Of Zircons Research Articles

Metamorphosed mélange in the eastern Himalayan syntaxis: constraint for the India–Asia collision in the Eocene

Tectonic mélanges are crucial for deciphering collision processes in orogenic belts. This study investigates the nature of the metamorphosed mélange in the Milin area of the eastern Himalayan syntaxis to shed light on the India–Asia collision in the region. The mélange exhibits a ‘block-in-matrix’ fabric in the lower and upper parts and a ‘lenticular-thrust-slices’ fabric in the middle. Quartzite blocks in the lower part are proposed to be Silurian or younger quartz arenites with a Tethyan affinity. Metacherts and amphibolites in the middle and upper parts may have originated from hydrothermal or terrestrial cherts and Nb-enriched basalts in the Asian forearc or syncollisional basin at ∼48–45 Ma. The diverse blocks/slices of the mélange, sourced from both the subducting and overriding plates, are suggested as having formed during Indian plate subduction. The protoliths of the amphibolite–metachert slice (∼45 Ma) in the mélange are distinctly different from the contemporary shallow sea (post-collisional) deposits but similar to the deep-water (initial collisional or pre-collisional) sediments in other parts of the suture zone. This implies that the amphibolite–metachert slice may represent the product of the initial India–Asia collision or even pre-collision and that (hard) collision likely lagged by ∼10 Myr in the eastern Himalaya syntaxis. Supplementary material: Analytical methods, Figures S1–S5 showcasing field occurrences, cathodoluminescence images of zircons, and geochemical characteristics, Table S1 – geochronology data, Table S2 – zircon Lu–Hf isotopic data, and Table S3 – whole-rock major and trace element results are available at https://doi.org/10.6084/m9.figshare.c.6896942 Thematic collection: This article is part of the Ophiolites, melanges and blueschists collection available at: https://www.lyellcollection.org/topic/collections/ophiolites-melanges-and-blueschists

Chronolgy and Geochemistry of the Sijiaying Iron Deposit in Eastern Hebei Province, North China Craton: Implications for the Genesis of High-Grade Iron Ores

The Sijiaying iron deposit is located in the Eastern Hebei area of the southern section of the northern margin of the North China Craton (NCC) and is the largest single iron deposit in China. The deposit contains many banded iron formations (BIFs) and was proven to have more than 3 million tons of high-grade iron ore resources. This study carried out geochemistry and zircon U–Pb analysis of normal-grade iron ore, high-grade iron ore, and wall rock (biotite–leptynite, chlorite–sericite schist) in the Sijiaying deposit and discussed the genesis and metallogenic age of high-grade iron ore. BIFs have low concentrations of Al2O3 and TiO2 and high field strength element (HFSE) depletion, indicating almost no contamination via terrestrial debris. The standardized post-Archean Australian shale (PAAS) rare earth element (REE) distribution pattern indicates that the iron formation exhibits positive Eu, Y, and heavy rare earth element (HREE) anomalies and lacks negative Ce anomalies, indicating that the Sijiaying BIF was enriched with iron sources via high-temperature hydrothermal fluids from the seabed and deposited in an anoxic ancient marine environment. In addition, geological field work identified two types of high-grade iron ore in the mining area: primitive sedimentary and hydrothermally altered high-grade iron ore. Further ore geochemical research showed that the primitive sedimentary-type iron ore is similar in geochemistry to the BIF. In addition to low Eu/Eu* values, the hydrothermally altered high-grade iron ore shows geochemical characteristics similar to those of the BIF, suggesting that they share the same iron source but did not form at the same time. The total large ion lithophile element (LILE) (Sr, Ba, Pb) contents in primitive sedimentary-type high-grade iron ore are higher than those in hydrothermally altered high-grade iron ore, indicating that LILEs are carried away via fluids during the hydrothermal alteration process in normal-grade iron ore. The geochemical characteristics of biotite–leptynite and chlorite–sericite schist include high contents of SiO2 and Al2O3, light rare earth elements (LREEs), LILE enrichment (Rb, Ba, Sr, Zr), and HFSE depletion (Nb, Ta, P, Ti), characteristics that are similar to island arc volcanic rocks. The reconstruction of the original rock indicates that the wall rock is a product of volcanic sedimentary cycles in an island arc setting. Zircon cathodoluminescence images and LA–ICP–MS zircon U–Pb dating can be divided into four age groups (3283 Ma, 2547 Ma, 2500 Ma, and 2407 Ma), which correspond to the earliest volcanic activity in eastern Hebei, the main mineralization age of the Sijiaying BIF (the mineralization age of primitive sedimentary high-grade iron ore), a regional tectonic–metamorphic event, and the occurrence of migmatization (the mineralization age of hydrothermally altered high-grade iron ore), respectively. Therefore, the Sijiaying BIF and primitive sedimentary high-grade iron ores were deposited and mineralized at 2547 Ma, and the iron orebody was later altered via the hydrothermal solution at 2407 Ma, forming large-scale high-grade iron ores.

Mineral Texture Classification Using Deep Convolutional Neural Networks: An Application to Zircons From Porphyry Copper Deposits

AbstractThe texture and morphology of igneous zircon indicates magmatic conditions during zircon crystallization and can be used to constrain provenance. Zircons from porphyry copper deposits are typically prismatic, euhedral, and strongly oscillatory zoned which may differentiate them from zircons associated with unmineralized igneous systems. Here, cathodoluminescence images of zircons from the Quellaveco porphyry copper district, Southern Peru, were collected to compare zircon textures between the premineralization Yarabamba Batholith and the Quellaveco porphyry copper deposit. Quellaveco porphyry zircons are prismatic, euhedral, and strongly oscillatory zoned, whereas the batholith zircons are subhedral‐anhedral with weaker zoning. We adopt a deep convolutional neural network (CNN) approach to demonstrate that a CNN can classify Quellaveco porphyry zircons with high success. We trial several CNN architectures to classify zircon images: LeNet‐5, AlexNet and VGG, including a transfer learning approach where we used the weights of a VGG model pretrained on the ImageNet data set. The VGG model with transfer learning is the most effective approach, with accuracy and receiver operating characteristic‐area under curve (ROC‐AUC) scores of 0.86 and 0.93, indicating that a Quellaveco porphyry zircon CL image can be ranked higher than a batholith zircon with 93% probability. Visualizing model layer outputs demonstrates that the CNN models can recognize crystal edges, zoning, and mineral inclusions. We trial implementing trained CNN models as unsupervised feature extractors, which can empirically quantify crystal textures and morphology. Therefore, deep learning provides a tool for the extraction of information from large, imaged‐based petrographic data sets which can facilitate petrologic and provenance studies.

Petrogenesis and tectonic significance of Neoarchean (~2.6 Ga) alkaline ultrapotassic granitic gneisses from the southeastern margin of the North China Craton: Constraints from U-Pb dating, Hf isotope and petrogeochemistry

The Wuhe complex (WC), exposed at the southeastern margin of the North China Craton (NCC), is an important constituent of the Eastern Block of the NCC. In order to better understand the Precambrian crustal composition and evolution of this region, a comprehensive investigation on zircon geochronology and Hf isotopes, as well as whole-rock geochemistry and Sr-Nd-Pb isotopes was conducted on alkaline ultrapotassic granitic gneisses from the WC at Mashan. These gneisses are considered as meta-igneous rocks based on geochemical and mineralogical criteria (with special reference to zircon cathodoluminescence images). They are characterized by modally abundant alkali feldspar (more than 60%) and quartz (more than 30%) while plagioclase is rare. Mafic minerals are sodic amphiboles such as arfvedsonite and eckermannite, aegirine and rare biotite. Minor constituents are rutile, muscovite, apatite and barite. Major elements geochemistry shows high SiO2 (69.85%–74.51%) and K2O + Na2O (9.67%–12.17%) contents, high K2O/Na2O (7.41–22.53) ratios, and relatively low MgO (0.37%–0.87%) and CaO (0.10%–0.23%) contents. Trace elements geochemistry shows significant depletions of Nb, Y, Ce, Ga, and REE (rare earth elements) relative to anorogenic granites. These features suggest that the magmatic protoliths of the studied gneisses belong to ultrapotassic silica-saturated alkaline series from an extensional background, perhaps in a subduction-related rifting environment. As concerning isotope geochemistry, their 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios are 16.6297–17.1877, 15.4454–15.5066, and 36.6036–38.0304, respectively, and εNd(t) values vary from −1.7 to +9.1. Two samples yielded zircon 207Pb/206Pb ages of 2615 ± 4 Ma and 2617 ± 5 Ma, respectively, representing their precursor ages. The zircon igneous core domains exhibit oscillatory growth zoning with positive εHf (t) values (+2.5 − +6.6). These data, coupled with chondrite-normalized and primitive-mantle-normalized element patterns, suggest that the precursors of the studied granitic gneisses were mainly derived from a subduction-modified ultrapotassic syenitic parental magma, and may be considered as a particular group of A-type granites, involved in an important crustal growth and reworking event at ~2.6 Ga. These rocks experienced a granulite-facies metamorphic stage accompanied by partial melting, as testified by clinopyroxene + rutile + K-feldspar + spinel + quartz + apatite inclusions in zircon metamorphic domains which were dated at ~2.5 Ga. The occurrence of this metamorphic stage is also supported by the lower ΣREE contents, negative Eu anomalies, and high Ti-in-zircon temperatures (>800 °C) of metamorphic zircon mantles dated at ~2.5 Ga. Eventually, the studied rocks suffered a later ~1.85 Ga metamorphic overprinting, possibly related to the Paleoproterozoic collisional orogeny recorded in the region.

Geochronology and geochemistry of the Tabaquito batholith (Frontal Cordillera, Argentina): geodynamic implications and temporal correlations in the SW Gondwana margin

The Tabaquito batholith (Frontal Cordillera, western Argentina), is mainly composed of shallowly emplaced granodiorite to minor monzogranite with abundant mafic microgranular enclaves. New sensitive high-resolution ion microprobe U–Pb zircon ages of c. 337 Ma (biotite granodiorite) and c. 284 Ma (mafic dyke) along with previously published geochronological data suggest that a long-lived magmatic system formed through at least two magmatic pulses at c. 337 and c. 322 Ma with later superimposition of Permian magmatism. The Tabaquito granitoids are metaluminous, calc-alkalic and magnesian with I-type affinity. Elevated Th/Nb, Y/Nb and La/Nb ratios along with negative Nb–Ta and positive Pb anomalies are consistent with a continental arc setting. Hf, Nd and Sr isotopic composition of the Tabaquito granitoids suggests that their source could result from mixing of an old felsic crustal component and a juvenile mafic to intermediate component. New geochronological and geochemical data together with published data reveal a continuous arc setting from the Carboniferous to the Permian in Argentina, and important magmatic compositional variations through time and space controlled by episodic fluctuations in the subduction angle of the oceanic plate. Reported and compiled data allow us to infer the continuity of the Carboniferous magmatic arc along the west margin of Gondwana. Supplementary material: Detailed petrography, analytical methods and data, zircon cathodoluminescence images and supplementary figures are available at https://doi.org/10.6084/m9.figshare.c.4763993

Early Precambrian tectono-thermal events in Southern Jilin Province, China: implications for the evolution of Neoarchean to Paleoproterozoic crust in the northeastern North China Craton

The early Precambrian basement of the North China Craton, China, records a complex history during the Neoarchean and Paleoproterozoic. Southern Jilin Province is one of the best regions to evaluate the early Precambrian crustal growth of the Eastern Block of the North China Craton. We herein present new zircon cathodoluminescence images, U–Pb dates, trace element and Lu–Hf isotope data for three late Neoarchean tonalitic–trondhjemitic–granodioritic (TTG) gneisses and one Paleoproterozoic mafic dyke from Tonghua region, southern Jilin Province. These results lent convincing support to the occurrence of multiple tectono-thermal events in southern Jilin Province during the Archean and Paleoproterozoic, and shed light on the formation and evolution of continental crust in the northeastern Eastern Block. Zircon LA–ICP–MS U–Pb isotopic analyses indicated three episodes of magmatism. Inherited (or captured) zircons with ages of ca. 2.6 Ga provided strong evidence for a pre-2.5 Ga magmatic event in this region. The second magmatic episode occurred at 2556–2522 Ma, as evidenced by extensive exposures of TTG gneisses. The third episode occurred at ca. 2200 Ma and is recorded by several mafic dykes that intruded the Archean TTG gneisses. Metamorphic zircons yielded consistent ages of 2493–2465 Ma, indicating a regional metamorphic event immediately after the late Neoarchean magmatism in southern Jilin Province. Zircon Hf isotope data of the TTG gneisses indicated that the main phase of crustal growth occurred in the late Neoarchean, with a minor input of Mesoarchean continental crustal materials. The combined geochronological, geochemical, and geological data suggested that the three episodes of crustal growth in the northeastern Eastern Block occurred at 2.8–2.7, 2.6–2.5, and 2.2 Ga.

A cross correlation method for chemical profiles in minerals, with an application to zircons of the Kilgore Tuff (USA)

Magmatic crystals are characterised by chemical zonation patterns that reflect the thermal and chemical conditions within magma reservoirs in which they grew. Crystals that exhibit similar patterns of zonation are often interpreted to have experienced similar conditions of growth. These patterns of zonation may represent continuous processes such as cooling, or more instantaneous events such as magma injection, and provide an insight into the structure and evolution of a magmatic system, both temporally and spatially. We have developed an algorithm that is objectively able to quantify the similarity within and between suites of magmatic crystals from different samples. Significantly, the algorithm is able to identify correlation that occurs between the interiors of two crystals, but does not extend to the rim, which provides an opportunity to understand the long-term evolution of magmatic systems. We develop and explain the mathematical basis for our algorithm and introduce its application using cathodoluminescence images of zircons from the Kilgore Tuff (USA). The results allow us to correlate samples from two different outcrops that are found over 80 km apart.

The tectonic evolution of the Bogda region from Late Carboniferous to Triassic time: evidence from detrital zircon U–Pb geochronology and sandstone petrography

AbstractField-based mapping, sandstone petrology, palaeocurrent measurements and zircon cathodoluminescence images, as well as detrital zircon U–Pb geochronology were integrated to investigate the provenance of the Upper Carboniferous – Upper Triassic sedimentary rocks from the northern Bogda Mountains, and further to constrain their tectonic evolution. Variations in sandstone composition suggest that the Upper Carboniferous – Lower Triassic sediments displayed less sedimentary recycling than the Middle–Upper Triassic sediments. U–Pb isotopic dating using the LA-ICP-MS method on zircons from 12 sandstones exhibited similar zircon U–Pb age distribution patterns with major age groups at 360–320 Ma and 320–300 Ma, and with some grains giving ages of > 541 Ma, 541–360 Ma, 300–250 Ma and 250–200 Ma. Coupled with the compiled palaeocurrent data, the predominant sources were the Late Carboniferous volcanic rocks of the North Tianshan and Palaeozoic magmatic rocks of the Yili–Central Tianshan. There was also input from the Bogda Mountains in Middle–Late Triassic time. The comprehensive geological evidence indicates that the Upper Carboniferous – Lower Permian strata were probably deposited in an extensional context which was related to a rift or post-collision rather than arc-related setting. Conspicuously, the large range of U–Pb ages of the detrital zircons, increased sedimentary lithic fragments, fluvial deposits and contemporaneous Triassic zircon ages argue for a Middle–Late Triassic orogenic movement, which was considered to be the initial uplift of the Bogda Mountains.

The source of Dalradian detritus in the Buchan Block, NE Scotland: application of new tools to detrital datasets

Detrital zircons from four samples of upper Dalradian metasedimentary rocks from the Buchan Block in the NE Grampian Highlands of Scotland were analysed by laser ablation inductively coupled plasma mass spectrometry to establish their U–Pb age and trace element composition. The analysed grains (magmatic cores) mainly yield concordant ages ranging from Neoproterozoic to Eoarchaean. Kernel density plots of the data show pronounced peaks in the late Mesoproterozoic, Palaeoproterozoic and Neoarchaean eras. The data are indistinguishable from detrital zircon age spectra from Dalradian rocks elsewhere, an interpretation supported by application of a non-parametric multidimensional scaling algorithm, and are consistent with a Laurentian source. Similar to existing studies from other Dalradian rocks, the age spectra from the Buchan Block reveal an increase in the relative proportion of older detritus with time, suggesting derivation from late Mesoproterozoic (Grenville) then Palaeoproterozoic orogens before widespread exposure and denudation of their Archaean basement rocks. Application of a novel approach to estimate the most likely time of radiogenic-Pb loss indicates that some detrital zircon grains were affected by element mobility around 470–450 Ma as a result of Grampian orogenesis. Supplementary material: Laser ablation inductively coupled plasma mass spectrometry U–Pb and trace element data, a matrix showing results of Kolmogorov–Smirnov (K–S) test, cathodoluminescence imaging of zircons and selected trace element plots are available at http://doi.org/10.6084/m9.figshare.c.3153931

Detrital zircon geochronology and provenance of the Chubut Group in the northeast of Patagonia, Argentina

The Chubut Group constitutes the most widespread sedimentary unit in NE Patagonia, characterized by variable-energy fluvial deposits. U–Pb analysis of detrital zircons from two sections of the Chubut Group constraint the age of the oldest sedimentary rocks in the northeast of the Somuncurá – Cañadón Asfalto Basin. In the Cañadón Williams area, at San Jorge section, 20 km NW of Telsen locality, dating of 56 detrital zircons from a medium to coarse sandstone indicated a maximum depositional age of 109 ± 1 Ma (n = 4). These sandstones were interpreted to represent shallow channels, associated with a lacustrine system. In the Telsen locality, a laser ablation analysis of 115 detrital zircons from a medium to coarse-grained sandstone, from fluvial channel facies, yielded a maximum depositional age of ca. 106 ± 1 Ma (n = 8). Both ages are consistent with volcanic events of the Barremian to Albian age in the central Patagonian Andes Region. Cathodoluminescence images of zircons from the San Jorge sample suggest an igneous origin, which is further supported by Th/U values above 0.5 in most of the grains. The distribution of the statistical modes of the main age populations of detrital zircons for the two samples [182, 185 and 189 Ma for Telsen sample (T2S) and 181 ± 1 Ma for San Jorge sample (SJS)] matches the age of the volcanic Marifil Formation. The rocks of the Marifil Formation of these ages are exposed NE to SE of the study area. The abundance of zircons of similar Jurassic ages (n = 52 for SJS and n = 105 for T2S) and the external morphology of the zircons in the sample SJS, implies a close proximity of the source area. Suggestion that the Marifil Formation was the main provenance source is also supported by northeast–southeasterly paleocurrents measured at the San Jorge and Telsen sections.

Petrogenesis and tectonic significance of Paleoproterozoic meta-mafic rocks from central Liaodong Peninsula, northeast China: Evidence from zircon U–Pb dating and in situ Lu–Hf isotopes, and whole-rock geochemistry

The central Liaodong Peninsula is located in the northwestern part of the Jiao-Liao-Ji Belt in the Eastern Block of the North China Craton. Surface rocks in this region are voluminous meta-mafic rocks, which mainly comprise amphibolite and metamorphic gabbro or diabase. We present zircon cathodoluminescence images, U–Pb dates, and Lu–Hf isotope data, as well as whole-rock geochemical data for these rocks, in order to constrain the tectonic nature and evolution of the Jiao-Liao-Ji Belt. Zircons from these meta-mafic rocks can be divided into two groups, with one having a typical magmatic origin and the other a metamorphic origin. Zircon U–Pb dating and Lu–Hf isotopic results show that the magmatic zircons have two peak ages at 2503 and 2154Ma, with the former have ɛHf=+7.1 to +8.1 and single-stage model ages (THf)=2.50–2.55Ga, indicating that these captured or inherited zircons formed at ca. 2.50Ga by partial melting of coeval juvenile crust. The younger magmatic zircons have ɛHf=+3.2 to +9.6, mainly varying from +7.4 to +9.6 (average=+8.7), and THf=2.15–2.34Ga (peak at 2.19Ga), suggesting that the meta-mafic rocks were emplaced at ca. 2.15Ga and derived by partial melting of depleted mantle. The metamorphic zircons yield a peak age of 1897Ma and have ɛHf=−4.5 to +6.8. These zircons can be further divided into three groups on the basis of Hf isotope data (i.e., ɛHf=−4.5 to −2.3, −0.6 to +1.0, and +3.1 to +6.8; THf=2.39–2.54, 2.26–2.39, and 2.03–2.11Ga, respectively), indicating they were derived by reworking of Neoarchean–Paleoproterozoic ancient crust and some Paleoproterozoic juvenile crust. Thus there existed two crustal growth events at 2.50 and 2.15Ga and one reworking event at ca. 1.90Ga in the Eastern Block. Geochemical data show that these meta-mafic rocks are characterized by: SiO2=47.3–49.8wt.%; TiO2=0.97–1.59wt.%;K2O=0.39–1.95wt.%; Na2O/K2O=1.03–4.85; Mg#=39–52; Cr=29–247ppm; and Ni=24.4–73.4ppm. These features, combined with low total rare earth element concentrations (ΣREE), relative enrichment in light REE, significant positive Rb, Ba, K, and P anomalies, and depletions in high-field strength elements (e.g., Nb, Ta, and Ti), indicate derivation by partial melting of depleted lithospheric mantle that had been metasomatized by subduction-derived fluids, with little or no crustal contamination during magma ascent. Given our dating results and the regional geological framework, we propose that the subduction in the eastern Longgang Block took place at least in age of 2.15Ga, and that the meta-mafic rocks in the central Liaodong Peninsula may have formed in a back-arc basin. However, the tectonic location of this magmatic arc and its relationship with the Nangrim Block need to be further studied to confirm this hypothesis.

A magmatic age and four successive metamorphic events recorded in zircons from a single meta-anorthosite sample in the Central Zone of the Limpopo Belt, South Africa

Abstract: Ion microprobe U–Th–Pb analyses combined with cathodoluminescence imaging of zircons from a meta-anorthosite from the Central Zone of the Limpopo Belt record five distinct events. The most concordant ages of cores of zircon yielded a weighted mean 207 Pb/ 206 Pb age of 3344 ± 3.6 Ma, representing the protolith age. The unzoned overgrowths yielded four discrete groups with weighted mean 207 Pb/ 206 Pb ages of 3150 ± 15 Ma, 2614 ± 4 Ma, 2488 ± 22 Ma and 2009 ± 28 Ma, representing four possible metamorphic events in the Central Zone. This is the first time that a succession of several metamorphic events has been demonstrated in a single rock-type from the Limpopo Belt. Supplementary material: Analytical methods and data are available at http://www.geolsoc.org.uk/SUP18368 .

Application of cathodoluminescence to zircon in FEG-ESIM

Cathodoluminescence (CL) image analysis of zircon is a key step of mineral micro-area compositional investigation and U-(Th)-Pb dating. We introduce a system consisting of field emission ESEM and high performance cathodoluminescence. This system has obvious advantages in imaging resolution and CL spectrum. We acquired some zircon CL images and spectra to demonstrate the ability of this system. Our results show that this system has a great potential application to characterize and identify the genesis of zircon.

Idades SHRIMP U-Pb do Complexo Sertânia: implicações sobre a evolução tectônica da Zona Transversal, Província Borborema

The Sertânia complex in the Alto Moxotó Terrane, Zona Transversal of the Borborema Province, Northeast Brazil, is a metasedimentary sequence with a subordinate metavolcanic component, metamorphosed under upper amphibolite facies conditions. Several previous regional geological surveys have included this rock assemblage in an undivided gneissic-migmatitic complex, due to the lack of a tectonostratigraphic model for this unit. This paper presents the first geochronological zircon U-Pb ages (SHRIMP U-Pb ages) and the first Sm-Nd isotopic data for this complex. Field and microscopic research reveals the existence of a volcanoclastic fraction of andesitic and dacitic composition, besides the dominant clastic components in these supracrustals. Furthermore, cathodoluminescence images of zircons show well formed little rounded crystals, thus suggesting reduced sediment transport prior to deposition. The calculated ages means with a 95% confidence level, based on a group of zircons of the same generation plot around 2.0 Ga, representing the episode of primary zircon crystallization. Some observed overgrowths testify to an additional Brasiliano overprint (ca. 0.6 Ga). Geochronological data on the supposed meta-plutonic rocks of the same complex point to the same Paleoproterozoic time interval, thus indicating contemporaneity with the sedimentation of the supracrustals. The obtained Nd model ages and µNd(t) parameters also confirm a contribution of Archean and Paleoproterozoic sources to these supracrustals rocks. These data clearly demonstrate contrast between the supracrustal rock assemblages of the Alto Moxotó Terrane and the supracrustals of other terranes of the Zona Transversal, which display Meso to Neoproterozoic ages, and imply that this crustal segment represents an independent lithotectonic domain.

Geology and timing of mineralization at the Cangshang gold deposit, north-western Jiaodong Peninsula, China

The Cangshang gold deposit of the north-western Jiaodong Peninsula contains reserves of greater than 50 tonnes (t) and is developed by the largest open pit gold mine in China. This deposit is a Jiaojia-style (i.e. disseminated-and-veinlet) deposit. It is controlled by the San-Cang fault zone, which trends ~040° and dips 40–75°SE at the mine site. The main (no. 1) orebody lies between a hanging wall of Precambrian metamorphic rocks (mainly amphibolite) of the Fenzishan Group and a footwall composed of the Mesozoic Linglong granitoid. The ore zone is mainly composed of pyritized, sericitized and silicified granitoid, which has undergone variable degrees of cataclasis. SHRIMP U–Pb dating of zircon indicates that the protolith of the hanging wall amphibolite was formed at 2530±17 Ma and underwent metamorphism at 1852±37 Ma. The footwall granodiorite has been dated at 166±4 Ma, whereas zircons from the ore zone yield a younger age of 154±5 Ma. Cathodoluminescence images of zircons from the granodiorite and ore zone show oscillatory zonation indicative of an igneous origin for both and the ages of these zircons, therefore, are all interpreted to be representative of magmatic crystallization. Dating of sericite by 40Ar–39Ar has been used to directly determine the timing of formation of the Cangshang deposit, providing the first time absolute age on formation of the Jiaojia-style gold deposits. The well-defined age of 121.3±0.2 Ma provides the precise timing of gold mineralization at the Cangshang deposit. This age is consistent with those of Linglong-style (vein type) gold mineralization, also from the north-western Jiaodong Peninsula, at between 126 and 120 Ma. Therefore, our work indicates that both styles of gold deposits in the Jiaodong Peninsula were formed during the same mineralization event.

Ultrahigh-pressure mineral inclusions in zircons from gneissic core samples of the Chinese Continental Scientific Drilling Site in eastern China

The Chinese drillhole CCSD-PP2 penetrated to a depth of 1028.28 m near Donghai in the southwestern Sulu terrane. Core samples consist mainly of para- and orthogneisses with minor intercalated layers of amphibolite and eclogite. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified using Raman spectroscopy and electron microprobe analysis as inclusions in zircon separates from para- and orthogneiss samples. Matrix minerals in the coesite-bearing para- and orthogneiss samples are amphibolite-facies assemblages of quartz + feldspar + epidote + biotite ± amphibole ± garnet ± titanite. In most cases, zircons, however, preserve multi-stage assemblages in different domains; the parageneses and compositions of these included minerals yielded P-T paths consistent with those deduced previously from eclogite and garnet peridotite outcrops in Donghai. In generally, quartz inclusions were identified in zircon cores and rims, whereas coesite and other UHP mineral inclusions occur in the mantles (intermediate zircon growth zone between core and rim) of the same zircons. The occurrence of coesite inclusions in zircon indicates that eclogites together with their country rock gneisses experienced UHP metamorphism and retrogression. Coesite inclusions are ubiquitous in paragneiss and orthogneiss core samples in CCSD-PP2 drillhole and in outcrop samples from the southwestern Sulu terrane, indicating that coesite-bearing lithologies are widespread in the Sulu terrane, requiring subduction of a large terrane to mantle depths. Cathodoluminescence images of zircons from gneissic rocks display distinct zoning from core to rim, that can be genetically correlated with inherited (especially in the orthogneisses) or prograde, UHP, and retrograde mineral inclusion assemblages. These images reveal irregular zoning patterns and various thickness of cores, mantles, and rims. The abundance of inclusions complicates conventional U-Pb age dating, therefore the SHRIMP microspot U-Pb analyses are essential for protolith and metamorphic age dating.

Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project

The pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quartzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe+Omp+Grt+Phe, Coe+Jd+Phe+Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814–852 °C and pressures of ≥28 kbar, presenting the P– T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OG1) and non-UHP (OG2). In OG1 orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz+Phe+Ab+Ksp+Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengite-bearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz+Phe+Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event.