Zircon Rare Earth Element Research Articles

Himalayan-like Crustal Melting and Differentiation in the Southern North American Cordilleran Anatectic Belt during the Laramide Orogeny: Coyote Mountains, Arizona

Abstract The southern US and northern Mexican Cordillera experienced crustal melting during the Laramide orogeny (c. 80–40 Ma). The metamorphic sources of melt are not exposed at the surface; however, anatectic granites are present throughout the region, providing an opportunity to investigate the metamorphic processes associated with this orogeny. A detailed geochemical and petrochronological analysis of the Pan Tak Granite from the Coyote Mountains core complex in southern Arizona suggests that prograde metamorphism, melting, and melt crystallization occurred here from 62 to 42 Ma. Ti-in-zircon temperatures (TTi-zr) correlate with changes in zircon rare earth elements (REE) concentrations, and indicate prograde heating, mineral breakdown, and melt generation took place from 62 to 53 Ma. TTi-zr increases from ~650 to 850 °C during this interval. A prominent gap in zircon ages is observed from 53 to 51 Ma and is interpreted to reflect the timing of peak metamorphism and melting, which caused zircon dissolution. The age gap is an inflection point in several geochemical-temporal trends that suggest crystallization and cooling dominated afterward, from 51 to 42 Ma. Supporting this interpretation is an increase in zircon U/Th and Hf, a decrease in TTi-zr, increasing zircon (Dy/Yb)n, and textural evidence for coupled dissolution–reprecipitation processes that resulted in zircon (re)crystallization. In addition, whole rock REE, large ion lithophile elements, and major elements suggest that the Pan Tak Granite experienced advanced fractional crystallization during this time. High-silica, muscovite± garnet leucogranite dikes that crosscut two-mica granite represent more evolved residual melt compositions. The Pan Tak Granite was formed by fluid-deficient melting and biotite dehydration melting of meta-igneous protoliths, including Jurassic arc rocks and the Proterozoic Oracle Granite. The most likely causes of melting are interpreted to be a combination of (1) radiogenic heating and relaxation of isotherms associated with crustal thickening under a plateau environment, (2) heat and fluid transfer related to the Laramide continental arc, and (3) shear and viscous heating related to the deformation of the deep lithosphere. The characteristics and petrologic processes that created the Pan Tak Granite are strikingly similar to intrusive suites in the Himalayan leucogranite belt and further support the association between the North American Cordilleran anatectic belt and a major orogenic and thermal event during the Laramide orogeny.

Application of Onuma and lattice strain derived methods to calculate missing REE and Ce and Eu anomalies in magmatic zircons

The shape of chondrite normalized zircon rare earth element (REE) patterns, including the magnitude of the Cerium (Ce) and Europium (Eu) anomalies, provide valuable insights into the magmatic conditions under which a zircon formed. However, lanthanum (La) and praseodymium (Pr), which are essential for the calculation of Ce anomalies, are often present at concentrations close to or below the detection limit of most analytical methods. We propose two new methods to calculate missing REE, based on Onuma diagrams (Chondrite-Onuma) and the lattice strain theory (Chondrite-Lattice), but using chondrite normalized values instead of partition coefficients. We compiled a dataset of ~ 1500 zircons with known REE + Y concentrations and used it to test and calibrate these methods and demonstrate that they are more accurate than other previously published models, with the Chondrite-Onuma method performing better than the Chondrite-Lattice method. These methods require analyses of as few as five REEs to impute the missing REE data or to estimate La and Pr concentrations or Ce anomalies in magmatic zircons, which allows a reduction in the number of REE analysed, where desirable, or to impute missing REEs in legacy data. The imputeREE package for the R programming language was written with a set of tools to apply these methods. A companion app is available to calculate missing REE and Ce and Eu anomalies.

Paleogeographic Reconstruction of Precambrian Terranes Reworked by Phanerozoic Orogens: An Example Based on Detrital Zircon REE From Lhasa Terrane in Southern Tibet

AbstractPaleogeographic reconstruction of Precambrian terranes reworked by Phanerozoic orogens (e.g., the Tibetan Plateau) results in complex lithotectonic relations due to intracrustal reworking by tectonothermal events. Detrital zircon rare earth element (REE) databases at global (global major river sands) and regional (the Gangdese Mountains, southern Tibet) scales reveal trends in and Eu/Eu* that effectively record the crustal evolution of the source, including crustal thickness and redox state of the magma that generated the zircons. Regional comparisons of these chemical markers provide a new approach for paleogeographic reconstructions that we apply to study the origin of the Lhasa terrane, southern Tibet. Using Precambrian to early Paleozoic sedimentary and igneous rocks in the Lhasa terrane and compiling detrital zircon analyses from the northern margin of Gondwana, we show that the Lhasa terrane had an African affinity in the Rodinia–Gondwana supercontinent cycles (ca. 1.4–0.4 Ga).

Geochronology and Mapping Constraints on the Time-Space Evolution of the Igneous and Hydrothermal Systems in the Taurus Cu-Mo District, Eastern Alaska

Abstract The Taurus porphyry Cu-Mo district contains four mineralized porphyry centers in the eastern interior of Alaska. All four centers were emplaced during a magmatic episode that spanned from ca. 72 to 67 Ma, with seven distinct igneous suites. Each igneous suite resulted in hydrothermal alteration and mineralization, with younger pulses overprinting older pulses. Each magmatic-hydrothermal system is not present at all four mineralized centers. Apart from the Dennison occurrence, each mineralized center records pulses of repeated intermediate-silicic magmatism and associated alteration and mineralization. Laser ablation-inductively coupled plasma-mass spectrometry U-Pb zircon crystallization ages indicate that an early quartz porphyry dike swarm ranges in age from ca. 71 to 70 Ma and is associated with potassic, sericitic, and propylitic alteration. Quartz latite intrusions were emplaced at ca. 69 Ma and exhibit early sodiccalcic alteration overprinted by potassic, sericitic, and propylitic alteration. The Taurus monzonite suite is cut by quartz latite but yielded an ca. 70 Ma emplacement age and exhibits the largest footprint of potassic and sericitic alteration. Feldspar porphyry dikes were emplaced ca. 69 Ma and have significant tourmaline-bearing potassic and sericitic alteration. This suite was followed by development of an igneous breccia with a monzonitic igneous matrix. Sodic-calcic alteration was associated with the igneous brecciation. A small stock of monzonite was emplaced at ca. 68 Ma causing locally pervasive sericite-tourmaline-pyrite alteration. The youngest suite of magmatism dated in the district is a series of granodiorite porphyry dikes with weak sodic-calcic and propylitic alteration that truncates earlier alteration assemblages. Mineralization in the district consists of chalcopyrite and molybdenite associated with sugary quartz veins with potassium feldspar and biotite alteration envelopes (A veins). Less common banded quartz-molybdenite veins (B veins) occur with potassium feldspar envelopes. Gold occurs throughout the district and is strongly correlated with copper grade. Sericitic alteration contains lower copper contents and is predominantly associated with quartz-pyrite veins with sericite envelopes (D veins). Pyrrhotite and local arsenopyrite are present in sericitic assemblages. Pyrrhotite also occurs as inclusions in pyrite within D veins. Magmas across the district exhibit oxidized characteristics, evidenced by the presence of abundant magnetite, rare titanite, and elevated Eu/Eu* and Ce/Ce* in zircon. Zircon Th/U and Yb/Gd compositions suggest a fractionation path controlled by apatite, titanite, and hornblende. Zircon rare earth element ratios and trace element data indicate two distinct batches of magma evolved from mafic parental compositions to monzonite and granodioritic compositions via fractional crystallization. In the early pulse of magma (ca. 72–69 Ma), fractional crystallization was key to ore formation. Earlier, better mineralized suites evolve to less negative Eu anomalies (Eu/Eu* > 0.7), indicating more oxidized and higher-water-pressure conditions evidenced by the suppression of plagioclase crystallization, compared to later, more poorly mineralized suites. The temporal and spatial evolution of the district was determined from mapping and U-Pb and Re-Os geochronology. Mapping of igneous and hydrothermal assemblages indicates that the locus of the intrusive suites and hydrothermal systems shifted spatially over time, based on the presence of high-temperature (K-silicate–dominant) alteration, which is coincident with the highest Cu and Au grades. The earliest hydrothermal system was centered at Bluff and East Taurus and transitioned to West Taurus during emplacement of the second magmatic suite. Emplacement of the third magmatic suite was centered back at East Taurus, and the fourth and fifth suites were centered at West Taurus. The latest suites were widespread without a core of high-temperature alteration marking a central locus. East Taurus contains the overlap of six of the seven magmatic and hydrothermal suites and has the highest intersected grades and tonnages in the district. The Bluff and Dennison occurrences exhibit fewer igneous suites and hydrothermal assemblages with weak mineralization. Sodic-calcic alteration, common on the deep and distal flanks of porphyry systems, is only present at West Taurus and is indicative of a localized source of high-salinity nonmagmatic fluids.

Exotic REE behaviors of zircon in the Koktokay No. 3 granitic pegmatite, Xinjiang, northwest China

Rare earth elements (REEs) can be incorporated into zircon at significant concentrations that are controlled by the temperature, pressure, and composition of their growth environment. However, the distribution of REEs between zircon and melt does not always conform to the currently available partitioning data. This study presents evidence of exotic REE behaviors in zircon from the marginal zone of the Koktokay No. 3 granitic pegmatite in Xinjiang, northwest China. The behavior of the Koktokay REEs represents by large negative Ce anomalies, flat heavy REE patterns, and lanthanide tetrad effects. Two generations of zircon (Zrn-I and Zrn-II) are identified, which were formed by dissolution and re-precipitation processes. Zrn-I represents relic zircon domains with dissolution textures that formed from pegmatite melt during the late stage of the magmatic–hydrothermal transition. Zrn-II occurs as overgrowths and infilling fractures around or within Zrn-I, which formed by hydrothermal re-precipitation. From Zrn-I to Zrn-II, the zircon exhibits increasingly negative Ce anomalies and greater enrichment in light REEs and other non-stochiometric elements (e.g., Ca, Al, Fe, and Mg). Given the lack of Ce depletion in the host granite, the negative Ce anomalies in zircon can be attributed to Ce being sequestered into hydrothermal minerals (i.e., cerianite and/or Fe–Mn oxyhydroxides) as tetravalent Ce. In addition, the flat heavy REE patterns of both Zrn-I and Zrn-II likely resulted from the crystallization of heavy REE-enriched minerals, such as zircon and garnet, and were further enhanced by hydrothermal alteration. The two generations of zircon display identical M-type lanthanide tetrad effects in REE patterns, with similar TE3 values. This ubiquitous M-type tetrad effect of the REEs in garnet, zircon, and the host granite can be attributed to the exsolution and subsequent interaction with magmatically derived fluids containing F–aquo-complexes. Therefore, we conclude that such anomalous REE incorporation into zircon from a hydrous pegmatitic melt is controlled by the evolution of the hydrous melt and dissolution–re-precipitation processes. Our results suggest that the reliability of zircon REE partitioning data should be revaluated, and that further experimental studies are needed to fully understand the behavior of REEs in hydrothermal systems.

Detrital Zircon Geochronology and Tectonic Evolution Implication of the Middle Jurassic Zhiluo Formation, Southern Ordos Basin, China

The Ordos Basin’s southern part is a composite zone made up of numerous continental blocks and has long been influenced by surrounding tectonism. However, only a few studies have investigated the existence of southern provenance supply and the basin’s southern boundary in the Middle Jurassic Zhiluo Formation. Based on sandstone detrital zircon U-Pb dating and zircon rare earth element analyses, net-sand ratio maps, paleocurrent direction and the field outcrop survey, this study establishes the source area and boundary of the Zhiluo Formation in the southern basin, and discusses the tectonic events. The study shows that the four main age peaks in the detrital spectra occurs at 2283 Ma, 1788 Ma, 432.5 Ma and 218.7 Ma, with a few of the zircons dated at 794.5–1235.2 Ma. The North Qilian orogenic belt (N-QLOB), the western part of the North Qinling orogenic belt (NQOB), and the southern margin of the North China Block (SNCB) contributed to the provenance. According to an integrated analysis of the provenance and tectonic background of continental blocks in the basin’s southern margin, the boundary of the basin in the depositional period of the Zhiluo Formation should reach the N-QLOB in the southwest, the NQOB in the south, and the Sanmenxia–Lushan fault belt in the southeast. On the basis of the aforementioned findings, the tectonic evolution of the continental blocks at the southern periphery of the Ordos Basin was restored.

A New Method for Relating Zircon Crystallisation to Petrogenetic Events

Abstract The trace element contents of zircon can provide unique insights into tectonothermal events, however, interpreting these data and identifying correlations with specific magmatic/metamorphic events can be challenging. This limits our ability to construct temporally constrained petrogenetic histories of complex metamorphic terranes. Unlocking the information that the rare earth element (REE) patterns of zircon contain is difficult because of the need to quantify differences. We have parametrised the shape of zircon REE patterns in terms of three independent parameters: average abundance, slope, and curvature. Quantifying REE patterns using independent shape parameters is similar to the use of REE ratios but is an improvement as (1) it uses information from all 14 REE rather than just two; (2) the use of two independent parameters (e.g. slope and curvature) is a more robust discriminant than the use of a single ratio; and (3) subtle variations in shape are easily distinguished enabling trends in the REE patterns of large datasets to be identified. Quantitative models were constructed showing how the shapes of the REE patterns of zircon change due to the co-crystallisation of other metamorphic minerals (monazite, apatite, and garnet). Diagnostic changes in shape enable the REE contents of zircon crystals or crystal zones to be accurately related to the growth of specific minerals and hence metamorphic events. The results were used to interpret the REE patterns of zircons from high-grade metamorphic terranes, which have experienced multiple deformation events (Val Malenco, Italy; Betic Cordillera, Spain; Seram, Indonesia; Lewisian Gneiss Complex, Scotland; Napier Complex, East Antarctica) and clearly identified zircon that crystallised in the presence of garnet. Quantitative comparison enabled zircon that crystallised prior to, synchronously with, or after garnet to be identified. Similar models can be used to interpret the REE patterns of monazite. This allows the relative timing of the growth of these minerals to be accurately constrained, which given the importance of zircon for geochronology and garnet for geobarometry has the potential to provide insights into the evolution of a metamorphic event.

Discovery of Early Mesozoic Magmatism in the Northern Muglad Basin (Sudan): Assessment of Its Impacts on Basement Reservoir

The crystalline basement in the Central African Rift System (CARS) has been confirmed as a good unconventional reservoir in the surrounding basins of the Muglad area, Sudan. This study has recognized at least five Early Mesozoic intrusions for the first time at the northern margin of the Muglad Basin. Field and petrographic investigations show that they are mainly granite, syenite, diorite, and monzonite, with dominant alkali feldspar, plagioclase, quartz, and minor hornblende, biotite, and magnetite. Zircon internal structures, Th/U ratios, and rare earth element (REE) patterns indicate their magmatic origin. A total of 210 zircons from five samples were analyzed using LA-ICP-MS and they yielded U-Pb ages from 195.0 ± 6 to 225.0 ± 3 Ma, belonging to the Late Triassic-Early Jurassic. These results suggest that the Muglad Basin basement is not universally Precambrian as previously considered. The older basement was intruded and modified by Early Mesozoic magmas as evidenced by field contacts and our dating results. These magmatic events produced diverse Mesozoic igneous rocks prior to the formation of the Muglad Basin (Early Cretaceous), becoming new members of the crystalline basement. This composite basement then experienced prolonged weathering and fracturing for 65–95 Ma, resulting in porous and permeable weathered layers or fractured zones, making the basement a good unconventional reservoir for oil and gas.

Paleo-Mesoarchean crustal evolution in the Coorg Block, southern India: Insights from magmatic and metamorphic records in mafic granulites

Lower crust mafic granulites are key components in understanding the formation and growth of the continental crust. Here we investigate the mafic granulites and associated rocks from the Paleo-Mesoarchean Coorg Block in southern India with a view to gain insights into the crust-mantle dynamics and the subduction–accretion‐collision history. We present petrology, mineral chemistry, conventional thermobarometry and computed pseudosections, whole‐rock geochemistry including major, trace element, and rare earth element (REE), and zircon U–Pb and Lu-Hf isotopes to trace the magmatic evolution and metamorphism of these rocks. The petrological features, mineral chemistry data and phase equilibria modelling suggest that the rocks underwent HP metamorphism at peak temperatures up to 900 °C and pressures up to 9.5–10 kbar along a clockwise P–T path followed by near isobaric cooling. Zircon U–Pb data from the mafic granulites place the peak emplacement time of the protolith at 3.1 Ga with a minor group of younger zircons indicating a post-collision thermal event at 2.7 Ga. Zircon REE patterns suggest the involvement of continental crust components in the magma source. Zircon Lu-Hf analysis yield positive εHf(t) values from 0.5 to 4.1 with model ages (TDM) of 3.3 Ga for rocks from the northern margin of the block and the bordering Mercara suture zone while samples from the southern margin display slightly older (3.4 Ga) ages. Our data indicate that the parent magmas were derived from both juvenile and reworked Paleoarchean sources. The mafic granulites record Mesoarchean subduction-related magmatism and crust building in the Coorg block. Our study contributes to the understanding of geodynamic processes and continent formation in the Early Earth.

Metamorphic evolution of a Tonian eclogite associated with an island arc of the southern Brasiliano Orogen

The early evolution of the southern Brasiliano Orogen is recorded in the São Gabriel Terrane, which consists of two Neoproterozoic (Tonian and Cryogenian) juvenile arcs, ophiolite complexes and supracrustal rocks. This study aims at providing new insights into the geodynamics processes and subduction process of this orogen by investigating the first Tonian eclogite (named Três Vendas eclogite) discovered in the southern Brasiliano Orogen. To determine the tectono-metamorphic evolution, we integrated field work with petrography, scanning electron microscopy, electron microprobe analyses of minerals and in situ LA-ICP-MS U-Pb geochronology with Lu-Hf and rare earth elements in zircon. The preserved high-grade metamorphic assemblage in these rocks has garnet, rutile, phengite, and omphacite, indicating that these rocks are (retro) eclogites. The presence of omphacite with up to 50% jadeite indicates minimum burial pressures of ≥15 kbar that are consistent with forward modeling of the garnet compositions. Omphacite was altered to fine-grained symplectites of hornblende, oligoclase, and diopside or augite with minor quartz during rapid exhumation. The second stage of retrogression involved reaction of garnet and diopside to plagioclase and hornblende symplectites (kelyphites) at amphibolite facies conditions. Kelyphitic hornblende-garnet pairs indicate peak temperatures of 710–760 °C. Whole rock composition and Zr–Y characteristics are consistent with tholeiitic island arc basalts, while their garnet compositions point to subduction zone eclogites. U-Pb ages of zircon cores yielded 909.5 ± 5.3 Ma and metamorphic rims 891.9 ± 8.2 Ma. Zircon age-corrected εHf isotope compositions range between +4 and +10, reflecting the moderately juvenile nature of these rocks. Zircon cores are distinguished by higher rare earth element (REE) contents and steeper middle REE to heavy REE (MREE-HREE) patterns, higher Th/U ratios and higher Sr and Ca contents than the metamorphic rims, and the rims are likely coeval with the kelyphites. The combined data indicate that the Três Vendas eclogites belong to a Tonian arc and were buried to >45 km, most likely in a subduction channel. The data from the first eclogite in southern Brazil have significant consequences for the interpretation of the Tonian evolution of the Brasiliano Orogen and, by extension, the Rodinia framework.

ZIRCON TRACE ELEMENT GEOCHEMISTRY AS AN INDICATOR OF MAGMA FERTILITY IN IRON OXIDE COPPER-GOLD PROVINCES

Abstract Extrusive and intrusive felsic magmas occur throughout the evolution of silicic-dominated large igneous province magmatism that is temporally related to numerous economically significant iron oxide copper-gold (IOCG) deposits in southern Australia. We investigate zircon trace element signatures of the felsic magmas to assess whether zircon composition can be related to fertility of the volcanic and intrusive suites within IOCG-hosted mineral provinces. Consistent with zircon forming in oxidizing magmatic conditions, the rare earth element (REE) patterns of zircon sourced from both extrusive and intrusive magmatic rocks are characterized by light REE depletions and a range of positive Ce and negative Eu anomalies. The timing of the major phase of IOCG mineralization overlaps with the early part of the first phase of Lower Gawler Range Volcanics magmatism (1593.6–1590.4 Ma) and older intrusive magmatism of the Hiltaba Suite (1593.06–1590.50 Ma). Zircon in these mineralization-related intrusives and extrusives is distinguished from zircon in younger, mineralization-absent rocks by higher Eu/Eu*, Ce/Ce*, and Ti values and separate magma evolution paths with respect to Hf. These zircon characteristics correspond to lower degrees of fractionation and/or crustal assimilation, more oxidizing magmatic conditions, and higher magmatic temperatures, respectively, in magmas coeval with mineralization. In this respect, we consider higher oxidation state, lower degrees of fractionation, and higher magmatic temperatures to be features of fertile magmas in southern Australian IOCG terrains. Similar zircon REE characteristics are shared between magmas associated with southern Australian IOCG and iron oxide-apatite (IOA) rhyolites from the St. Francois Mountains, Missouri, namely high Ce/Ce* and high Dy/Yb, indicative of oxidized and dry magmas, respectively. The dry and more fractionated nature of the IOCG- and IOA-associated magmas contrasts with the hydrous and unfractionated nature of fertile porphyry Cu deposit magmas. As indicated by high Ce/Ce* ratios, the oxidized nature is considered a key element in magma fertility in IOCG-IOA terrains. In both IOCG and IOA terrains, the trace element compositions of zircon are able to broadly differentiate fertile from nonfertile magmatic rocks.

Evidence of Carboniferous arc magmatism preserved in the Chicxulub impact structure

Abstract Determining the nature and age of the 200-km-wide Chicxulub impact target rock is an essential step in advancing our understanding of the Maya Block basement. Few age constraints exist for the northern Maya Block crust, specifically the basement underlying the 66 Ma, 200 km-wide Chicxulub impact structure. The International Ocean Discovery Program-International Continental Scientific Drilling Program Expedition 364 core recovered a continuous section of basement rocks from the Chicxulub target rocks, which provides a unique opportunity to illuminate the pre-impact tectonic evolution of a terrane key to the development of the Gulf of Mexico. Sparse published ages for the Maya Block point to Mesoproterozoic, Ediacaran, Ordovician to Devonian crust are consistent with plate reconstruction models. In contrast, granitic basement recovered from the Chicxulub peak ring during Expedition 364 yielded new zircon U-Pb laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) concordant dates clustering around 334 ± 2.3 Ma. Zircon rare earth element (REE) chemistry is consistent with the granitoids having formed in a continental arc setting. Inherited zircon grains fall into three groups: 400–435 Ma, 500–635 Ma, and 940–1400 Ma, which are consistent with the incorporation of Peri-Gondwanan, Pan-African, and Grenvillian crust, respectively. Carboniferous U-Pb ages, trace element compositions, and inherited zircon grains indicate a pre-collisional continental volcanic arc located along the Maya Block's northern margin before NW Gondwana collided with Laurentia. The existence of a continental arc along NW Gondwana suggests southward-directed subduction of Rheic oceanic crust beneath the Maya Block and is similar to evidence for a continental arc along the northern margin of Gondwana that is documented in the Suwannee terrane, Florida, USA, and Coahuila Block of NE México.

Construction of P–T–t paths for eclogite in the Tongbai orogen by combining phase equilibria modelling with zircon inclusion composition

AbstractThe interpretation of metamorphic ages for zircon domains in eclogite remains challenging because of the uncertainties in interpreting their rare earth element (REE) patterns and mineral inclusions. This has seriously hindered the construction of robust pressure (P)–temperature (T)–time (t) paths for eclogites from collisional orogens. In this study, a composite approach is used to obtain a reliable P–T path for eclogite from the Tongbai orogen and quantitatively attribute zircon U–Pb ages to specific P–T intervals. An integrated study using isochemical phase diagram section (pseudosection) modelling, multi‐equilibrium thermobarometry, and the Zr‐in‐rutile thermometry with petrographic observations indicates that the target eclogite experienced multistage metamorphic evolution. The evolution is characterized by a clockwise P–T path with compressional heating for peak eclogite facies metamorphism at 588 ± 20°C and 26.7 ± 2.8 kbar and two stages of isothermal decompression from an early stage of amphibole eclogite facies metamorphism at 15–10 kbar to a late stage of amphibolite facies metamorphism at pressures of <10 kbar. Metamorphic zircon domains in eclogite and quartz vein show relatively flat patterns of heavy REE without notable Eu anomalies but different cathodoluminescence responses and middle REE (MREE) contents. This indicates their growth in different stages of eclogite facies metamorphism, which are categorized into two groups. The first group is composed of bright rims or homogenous grains that are characterized by low MREE contents, suggesting the existence of abundant amphibole during the metamorphism. The compositional isopleths of amphibole inclusions indicate that this group of zircon domains would form by hydration during crustal exhumation to amphibole eclogite facies at 240 ± 2 Ma. The second group consists of dark sector mantles and shows high MREE contents, indicating the prograde to peak metamorphism with significant decomposition of amphibole or lawsonite during crustal subduction to eclogite facies at 245 ± 2 Ma. Therefore, the combination of the isopleth thermobarometry of mineral inclusions with the Zr‐in‐rutile thermometry and the zircon REE partitioning is an effective means to link the metamorphic P–T conditions to the ages of metamorphic zircons in the different stages of collisional orogeny.

Petrogenesis and tectonic evolution of the Palaeozoic to Mesozoic Niuxinshan granitoids in the North Qilian orogen,NWChina

Numerous granitoid suites exposed in the Qilian orogen, NE Tibetan Plateau preserve important records of the tectonic evolution of this orogen. Here we present zircon U–Pb, zircon rare earth elements (REEs), and whole‐rock geochemical and Sr–Nd–Hf isotopic data on Niuxinshan granitoids in the North Qilian orogen, with a view to constrain their petrogenesis and timing of emplacement, as well as to gain new insights into the Early Palaeozoic tectonic evolution of the Qilian orogen. Zircon U–Pb data show multiple concordant ages of ~517, 455–425, 397–389, and 351–319 Ma for syenogranite, and ages of ~758 and 724 Ma together with some scattered Early Mesozoic discordant ages for porphyritic granite, suggesting episodic magmatism from Neoproterozoic to Early Mesozoic. Zircon ∑REEs range from 276.43 to 4,582.32 ppm, and are characterized by depletion of light REEs (LREEs) and enrichment heavy REEs (HREEs) with negative Eu anomalies, indicating a mixture of magmatic and hydrothermal zircons. Whole‐rock geochemical data show similar variation of LREEs, large‐ion lithophile elements (LILEs), HREEs, and high‐field‐strength elements (HFSEs) with zircon REEs, with Eu, Ba, P, and Ti negative anomalies and Rb, U, and Pb positive anomalies. Based on their formation ages, two groups of granitoids are identified. Group I rocks (Early Palaeozoic) are represented by I‐type granites whereas Group II rocks (Early Mesozoic) are of S‐type. Whole‐rock Sr–Nd–Hf isotopes display (87Sr/86Sr)iratios of 0.707085–0.709591, εNd(t) values of −5.3 to −2.3, εHf(t) values of −2.2 to 0, and two‐stage model (Nd and Hf) ages of 1.6–1.4 Ga for Group I rocks, and of 0.728156–0.734113, −9.4 to −9.3, −10.6 to −6.9, and 2.1–1.9 Ga for Group II rocks. Accordingly, we infer that the magma for Group I was sourced from the partial melting of basaltic protoliths in the mafic lower crust and underwent fractional crystallization. Group II was derived from metasedimentary protoliths through reworking of ancient Palaeoproterozoic crustal components. Integrating the results from this study with those from previous studies on Niuxinshan granitoids, we argue that the southward subduction model of the North Qilian Ocean beneath the Qilian Block during the Early Palaeozoic might be a more reasonable scenario. However, the Early Mesozoic magmatic event reported here remains equivocal in the North Qilian orogen.

Unraveling the origins and P-T-t evolution of the allochthonous Sobrado unit (Órdenes Complex, NW Spain) using combined U–Pb titanite, monazite and zircon geochronology and rare-earth element (REE) geochemistry

Abstract. The Sobrado unit, within the upper part of the Órdenes Complex (NW Spain) represents an allochthonous tectonic slice of exhumed high-grade metamorphic rocks formed during a complex sequence of orogenic processes in the middle to lower crust. In order to constrain those processes, U–Pb geochronology and rare-earth element (REE) analyses of accessory minerals in migmatitic paragneiss (monazite, zircon) and mylonitic amphibolites (titanite) were conducted using laser ablation split stream inductively coupled plasma mass spectrometry (LASS-ICP-MS). The youngest metamorphic zircon age obtained coincides with a Middle Devonian concordia monazite age (∼380 Ma) and is interpreted to represent the minimum age of the Sobrado high-P granulite facies metamorphism that occurred during the early stages of the Variscan orogeny. Metamorphic titanite from the mylonitic amphibolites yield a Late Devonian age (∼365 Ma) and track the progressive exhumation of the Sobrado unit. In zircon, cathodoluminescence images and REE analyses allow two aliquots with different origins in the paragneiss to be distinguished. An Early Ordovician age (∼490 Ma) was obtained for metamorphic zircons, although with a large dispersion, related to the evolution of the rock. This age is considered to mark the onset of granulite facies metamorphism in the Sobrado unit under intermediate-P conditions, and related to intrusive magmatism and coeval burial in a magmatic arc setting. A maximum depositional age for the Sobrado unit is established in the late Cambrian (∼511 Ma). The zircon dataset also record several inherited populations. The youngest cogenetic set of zircons yields crystallization ages of 546 and 526 Ma which are thought to be related to the peri-Gondwanan magmatic arc. The additional presence of inherited zircons older than 1000 Ma is interpreted as suggesting a West African Craton provenance.

Tectonic exhumation of the Central Alps recorded by detrital zircon in the Molasse Basin, Switzerland

Abstract. Eocene to Miocene sedimentary strata of the Northern Alpine Molasse Basin in Switzerland are well studied, yet they lack robust geochronologic and geochemical analysis of detrital zircon for provenance tracing purposes. Here, we present detrital zircon U–Pb ages coupled with rare-earth and trace element geochemistry to provide insights into the sedimentary provenance and to elucidate the tectonic activity of the central Alpine Orogen from the late Eocene to mid Miocene. Between 35 and 22.5 ± 1 Ma, the detrital zircon U–Pb age signatures are dominated by age groups of 300–370, 380–490, and 500–710 Ma, with minor Proterozoic age contributions. In contrast, from 21 Ma to ∼ 13.5 Ma (youngest preserved sediments), the detrital zircon U–Pb age signatures were dominated by a 252–300 Ma age group, with a secondary abundance of the 380–490 Ma age group and only minor contributions of the 500–710 Ma age group. The Eo-Oligocene provenance signatures are consistent with interpretations that initial basin deposition primarily recorded unroofing of the Austroalpine orogenic lid and lesser contributions from underlying Penninic units (including the Lepontine dome), containing reworked detritus from Variscan, Caledonian–Sardic, Cadomian, and Pan-African orogenic cycles. In contrast, the dominant 252–300 Ma age group from early Miocene foreland deposits is indicative of the exhumation of Variscan-aged crystalline rocks from the Lepontine dome basement units. Noticeable is the lack of Alpine-aged detrital zircon in all samples with the exception of one late Eocene sample, which reflects Alpine volcanism linked to incipient continent–continent collision. In addition, detrital zircon rare-earth and trace element data, coupled with zircon morphology and U∕Th ratios, point to primarily igneous and rare metamorphic sources. The observed switch from Austroalpine to Penninic detrital provenance in the Molasse Basin at ∼ 21 Ma appears to mark the onset of synorogenic extension of the Central Alps. Synorogenic extension accommodated by the Simplon fault zone promoted updoming and exhumation the Penninic crystalline core of the Alpine Orogen. The lack of Alpine detrital zircon U–Pb ages in all Oligo-Miocene strata corroborate the interpretations that between ∼ 25 and 15 Ma, the exposed bedrock in the Lepontine dome comprised greenschist-facies rocks only, where temperatures were too low for allowing zircon rims to grow, and that the Molasse Basin drainage network did not access the prominent Alpine-age Periadriatic intrusions located in the area surrounding the Periadriatic Line.

Tracking hydrothermal events using zircon REE geochemistry from the Carajás Mineral Province, Brazil

Zircon geochemistry and geochronology are powerful tools in the study of multistage cratonic settings, yielding information on ages and distinguishing terranes. The Carajás Mineral Province (CMP) SE of the Amazonian Craton is renowned for its complex tectonic evolution and notable mineral deposits endowment, such as the giant iron-oxide-copper-gold (IOCG)-type Sossego and Salobo. Tectonic, magmatic, metamorphic, and hydrothermal event sequences imprint their records regionally and deposit scales that, in most cases, are difficult to identify. Basement and wall-rock samples from the southern portion of the CMP's, Sossego area were selected and analyzed using sensitive high-resolution ion microprobe (SHRIMP). Cathodoluminescence images guided the morphological and textural characterization and spot location. U-Pb and rare earth elements (REE) analyses using SHRIMP provided ages and trace element contents leading to classification of zircon type as magmatic, hydrothermally altered magmatic, or hydrothermal zircon. Hydrothermal zircon is characterized by anomalous morphology and REE and U contents, and these features have prompted the identification of an Archean hydrothermal event in the province. These zircon classifications indicate a circa 2.54 Ga hydrothermal event in the southern region of Carajás, likely associated with main shear zone movements (Carajás Fault). The 2.54 Ga event is well constrained to the Carajás northern zone and has not been indicated in the southern region. The magmatic and hydrothermally altered zircons indicate ages around 2.95 Ga and 2.74 Ga, coinciding with the zone's known ages, with better resolution achieved using SHRIMP. The use of zircon type classification in cratonic terranes to track hydrothermal events offers original possibilities and represents new frontiers in mineral exploration.

Detrital zircon U\u2013Pb geochronology and geochemistry of the Riachuelos and Palma Sola beach sediments, Veracruz State, Gulf of Mexico: a new insight on palaeoenvironment

Zircons are abundant in the beach sediments. In this study, surface microtexture, mineralogy, bulk sediment geochemistry, trace element composition and U–Pb isotopic geochronology of detrital zircons collected from the Riachuelos and Palma Sola beach areas, southwestern Gulf of Mexico were performed to infer the sediment provenance and palaeoenvironment. The zircon microtexture was categorized as mechanically- and/or chemically-induced features. The weathering index values for the Riachuelos (~ 72–77) and Palma Sola (~ 71–74) beach sediments indicated moderate weathering of both of the two source areas. The major and trace element data of bulk sediments suggested passive margin settings for the two areas. The trace elemental ratios and chondrite-normalized rare earth element (REE) patterns of bulk sediments revealed that the sediments were likely sourced by felsic and intermediate igneous rocks. And the zircon Th/U ratios (mostly more than 0.2) and zircon REE patterns (with negative Eu and positive Ce anomalies) suggested a magmatic origin for both of the beach sediments from these two areas. Two distinct zircon age peaks respectively belonging to the Paleozoic and the Cenozoic were identified both in the Riachuelos and Palma Sola beach sediments. Zircon geochronology comparison research between the Riachuelos–Palma Sola beach sediments and potential source areas in SW Gulf of Mexico revealed that the source terrane supplied the Paleozoic zircons of this study was identified as the Mesa Central Province (MCP), and the Cenozoic zircons were transported from the nearby Eastern Alkaline Province (EAP). Moreover, although the Precambrian zircons were very few in the studied sediments, their geochronology and geochemistry results still could infer that they were contributed by the source terranes of Grenvillian igneous suites in the Oaxaca and the Chiapas Massif Complexes.

Zircon U[sbnd]Pb age constraints on the mid-Cretaceous Hkamti amber biota in northern Myanmar

Hkamti amber contains exquisite fossils, but the age of the amber biota is unclear. In this study, we documented the characteristics of fossils hosted in the amber, and we present zircon UPb ages of tuff layers from the Kachin and Hkamti sites (i.e., the “new mine”). We also present a compilation of arthropod and echinoderm syn-inclusions from the mid-Cretaceous Hkamti amber of Myanmar, indicating a humid tropical or subtropical forest paleo-environment. Zircon analyses (n = 143) of four tuff samples that host the amber deposit in different pits yielded a weighted-mean age of 109.7 ± 0.4 Ma. Zircon rare earth element data demonstrate the zircons are of igneous origin. Therefore, the age of the Cretaceous amber biota from Hkamti is ca. 110 Ma. This means that the Hkamti assemblage should be treated as distinct from other Burmese amber collections, as it represents a significantly older source within a different depositional environment. The commercial sources of Burmese amber are variable and encompass multiple mining regions, which makes maintaining distinctions between deposits important if their faunal contents are to be studied in a proper temporal or ecological context. The refined understanding of amber deposits in Myanmar highlights the importance of distinguishing sources, but it also offers an opportunity to study a series of deposits from ~110 to ~72 Ma, instead of just one snapshot of terrestrial ecology around ~99 Ma.

Key concepts in interpreting the concentrations of the rare earth elements in zircon

Because of the great potential for zircon to provide insights into crustal melting and evolution, much work has been done to develop tools to interpret its trace element concentrations. There are several complexities to zircon/melt partitioning that appear to be underappreciated, and these are illustrated using a combination of new and existing data. First, sector zoning causes significant dispersion in the population of zircon compositions even for those grown from a homogeneous melt. Second, many analyses of natural crystals contain trace elements at unrealistic concentrations: there are multiple potential sources for this apparent contamination. Third, new experimental data show that Henry's Law is obeyed by rare earth elements in zircon, despite recent suggestions to the contrary. Fourth, zircon/melt partition coefficients are functions of temperature, melt composition and in some cases oxygen fugacity, rather than being constants.