Magmatic Zircon Crystals Research Articles

On the Morphology and Geochemistry of Hydrothermal Crypto- and Microcrystalline Zircon Aggregates in a Peralkaline Granite

Singular crypto- and microcrystalline hydrothermal zircon aggregates occur in peralkaline granites from the Corupá Pluton of “A-type” granites and syenites in Graciosa Province, Southern Brazil, and are herein characterized for their morphological, textural and geochemical (major, minor and trace elements, and Lu-Hf isotopes) properties. The aggregates were found to present a variety of habits, such as dendritic, oolitic, botryoidal and spherulitic, and they are associated with typical hydrothermal minerals (alkali-feldspars, quartz, fluorite, epidote-group minerals, phyllosilicates and Fe oxides) in micro-fractures and small miarolitic cavities in the host rock. They precipitated directly from a hydrothermal fluid and, compared to magmatic zircon crystals from the host, were found to contain relatively high abundances of the “non-formula” elements (e.g., Fe, Al, and Ca) and HFSEs (High-Field-Strength Elements), particularly the L- and MREEs (Light and Medium Rare Earth Elements), features most typical of hydrothermal zircon, as well as high Th/U ratios, whereas the Lu-Hf isotopic signatures were found to be similar. The formation of the zircon aggregates and the associated epidote-groups minerals was probably due to the interaction between an orthomagmatic, F-bearing, aqueous fluid transporting the HFSEs with the host-rock and/or with an external meteoritic fluid from the country rocks. The preservation of an amorphous-like Zr-silicate compound and crypto-to-microcrystalline zircon varieties is arguably related to the inefficient fluid flux and/or elemental diffusion in a low-temperature oxidizing environment.

AGE OF THE KINTEREP FORMATION OF THE NORTHWESTERN SALAIR: CHEMOSTRATIGRAPHY AND U-Pb ZIRCON DATING

A comprehensive study of the stratotype section of the Kinterep formation of the North-Western Salair was carried out, including geochemical and isotope (Sr, C, O) studies of carbonate rocks, and U-Pb dating (LA-ICP-MS) of igneous zircon crystals from tuffites. The studied carbonate rocks are pure limestones with Mg/Ca less than 0.007 and a low proportion of insoluble residue (average 5 %). Kinterep limestones are characterized by δ 18 O SMOW values from 19.8 to 23.8 ‰ and δ 13 С PDB values from –0.7 to +0.9. The Sr isotope composition in limestones of the Kinterep formation varies in a narrow range of 87 Sr/ 86 Sr values between 0.70851 and 0.70859. Comparison of the obtained isotopic characteristics ( 87 Sr/ 86 Sr and δ 13 С PDB ) of Kinterep formation limestones with the generalized global 87 Sr/ 86 Sr variation curve and δ 13 С PDB values in the pale-ocean suggests two equally probable interpretations of the time of accumulation of Kinterep formation limestones: 550–540 and 525–510 Ma. U-Pb dating of magmatic zircon crystals from tuffite, which forms an interlayer among limestones of the Kinterep formation, showed the age at the boundary of ~515 Ma. Thus, using a combination of isotope (Sr, C) chemostratigraphy of carbonate rocks and U-Pb dating of igneous zircon crystals from tuffites syngenetic to limestones, the age limit of 525–510 Ma was established for the time of the formation of the Kinterep formation of the Salair basin. Carbonate rocks of similar age and similar features of the isotopic composition are known in neighbouring areas (for example, the Kuznetsk Alatau) and in regions of the Siberian Platform and microcontinents of Central Asia (Tuva-Mongolian and Dzabkhan) spatially distant from the Salair basin.

Emergence of continents above sea‐level influences sediment melt composition

AbstractThe Archean‐Proterozoic transition heralded a number of fundamental changes on Earth, including the oxygenation of the atmosphere, a marked emergence of continents above sea‐level, and an increase in δ18O of felsic magmas. The potential drivers for the latter are changes in the composition of supracrustal material or increased crustal reworking. Although the onset of subduction‐induced continental collision and associated enhanced crustal recycling could produce high‐δ18O felsic magmas, temporally constrained zircon δ18O reveals an increase in δ18O at ~2.35 Ga that predates the oldest widely recognized supercontinent. In this work, we use the O and Hf isotope ratios of magmatic zircon crystals in Archean and Proterozoic sediment‐derived granitoids of the North China Craton to track the incorporation of supracrustal material into magmas. The results are consistent with a Paleoproterozoic increase of continental freeboard producing sedimentary reservoirs with comparatively elevated δ18O that subsequently partially melted to generate the granitoids.

Paleoproterozoic arc magmatism in the southern Amazonian Craton, Brazil: Constraints from geology, geochemistry, and geochronology of granitic rocks

This work presents new geological, geochemical, and geochronological data of Paleoproterozoic granites located in the south-central sector of the Amazonian Craton. This region falls within the Peixoto de Azevedo Domain in the southern Tapajós-Parima Tectonic Province. The results here presented are of the utmost importance in advancing geological knowledge of the Alta Floresta Gold Province. The study area encompasses granitic rocks of the Matupá Intrusive Suite, and of the newly individualized Teles Pires Intrusive Suite. The latter includes two granitic bodies that have been previously ascribed to the Matupá Intrusive Suite. The Matupá Intrusive Suite was divided into two facies: Facies 1, composed of syenogranites and monzogranites, and Facies 2, composed of granodiorites. Granodiorites of Facies 2 have chemical characteristics similar to that of magnesian and metaluminous granites, while syenogranites and monzogranites of Facies 1 are ferrous and slightly peraluminous. Petrographic and geochemical features demonstrate the calc-alkaline nature of the Matupá Intrusive Suite which is typical for I-type granites from of volcanic arc settings. Magmatic zircon crystals yielded U–Pb LA-ICP-MS ages at 1864 ± 19 Ma and 1859 ± 8 Ma for syenogranite and monzogranite of Facies 1, respectively, and an age of 1881 ± 5 Ma for granodiorite of Facies 2. These are the crystallization ages for intrusive bodies of the Matupá Intrusive Suite. The Teles Pires Intrusive Suite varies from syenogranite to monzogranite whose calc-alkaline signature can be linked to the setting of a mature magmatic arc. U–Pb zircon in situ dating with LA-ICP-MS yielded upper intercept ages at 1790 ± 6 Ma and 1793 ± 7 Ma which are interpreted as its magmatic crystallization. Besides these crystallization ages, inherited zircon crystals also provided an older intercept age of 1870 ± 14 Ma, which is correlatable with the Matupá Intrusive Suite. Crustal assimilation and/or contamination are suggested for both the Matupá and Teles Pires intrusive suites due to the presence of inherited zircon populations of ca. 1960 Ma and 1900 Ma, and ca 1896 Ma and 1880 Ma, respectively. These ages are coherent with the Creporizão (Creporizão orogenic) Intrusive Suite, Tropas Intrusive Suite (Tropas orogenic), and Parauari Intrusive Suite (Parauari orogenic), which are all part of the Tapajós Domain. The two intrusive suites outcropping in the study area are therefore attributed to the Matupá Intrusive Suite, Peixoto de Azevedo Domain, and to the Teles Pires Intrusive Suite, Juruena Domain, which in turn is part of the Rondônia-Juruena Tectonic Province.

Geochronological study of biotite aegirine‐augite syenite in the Bengge area, Western Yunnan: An example of zircon U–Pb dating for alkalic rocks

The Bengge biotite aegirine‐augite syenite (BAS) is located in the southern part of Yidun Arc, eastern margin of the Tibetan Plateau area. It is an integral part of the Bengge ultrapotassic rocks (UPR), an alkalic complex mainly consisting of BAS, biotite pyroxene syenite (BPS), and biotite syenite (BS). Geochronological studies including zircon LA‐ICPMS U–Pb dating and biotite 40Ar/39Ar dating have been conducted on the Bengge BAS to constrain its emplacement age. The zircon U–Pb dating yields a wide range of ages from 209 ± 2 Ma to 239 ± 2 Ma, with the concordia 206Pb/238U age being 228 ± 8.9 Ma (MSWD = 0.13). Meanwhile, the biotite 40Ar/39Ar dating yields a plateau age of 205.7 ± 1.5 Ma. The whole‐rock Zr‐saturation temperature (Tzir‐sat) of the Bengge BAS has shown to be very low (86 ± 13°C to 634 ± 13°C), which is much lower than Ti‐in‐zircon temperature (>758°C) of the zircons used for U–Pb dating. This indicates that the zircons in the Bengge BAS are inherited, and they were already present in the BAS melts before the melts became Zr‐saturated. Similarly, the Tzir‐sat of the Bengge BPS and BS are 346 ± 14°C ~ 549 ± 13°C and 281 ± 14°C ~ 525 ± 13°C, respectively, which is also too low for magmatic zircon crystallization in the melts. The inherited zircons of Bengge BAS have typical features of magmatic zircon, with high (Sm/La)N ratios (2.54–819), Ce/Ce* ratios (4.77–304) and low La concentration. Low εHf (T) values (−5.0 to 0.42) and high U/Yb (0.67–12.6) values imply that the inherited zircons (209–239 Ma) in the Benge BAS are derived from an early arc‐magma system of the Yidun Arc. The biotite 40Ar/39Ar dating constrains the Bengge BAS emplacement age at 205.7 ± 1.5 Ma, the time when the arc‐magma activity had ceased, and a regional geodynamic transition in the Yidun Arc from convergence to back‐arc spreading had started.

Geochronology and petrogenesis of the Late Silurian Shitoukengde mafic–ultramafic intrusion, NW China: Implications for the tectonic setting and magmatic Ni-Cu mineralization in the East Kunlun Orogenic Belt

ABSTRACT The Shitoukengde mafic–ultramafic intrusion, located in the East Kunlun Orogenic Belt, is spatially and temporally associated with the Xiarihamu Ni-Cu sulphide deposit, the second-largest Ni deposit in China. This intrusion comprises mainly lherzolite, harzburgite, olivine websterite, and gabbroic rocks. The Shitoukengde mafic–ultramafic rocks are enriched in light rare earth elements (REE) relative to heavy REE and show pronounced negative Nb–Ta anomalies. Magmatic zircon crystals of the studied samples have high δ18O values (6.5 to 8.0‰) with a weighted mean value of 7.3‰ that is higher than the typical mantle value (~5.3‰). They are characterized by high εHf(t) values (–2.0 to 9.2) and variable εNd(t) values (–4.46 to 2.83). Olivine grains from the ultramafic rocks have high forsterite (Fo) values of 79.3–86.8 and variable Ni contents of 746–3347 ppm. Our calculations indicate that parental magma of the Shitoukengde intrusion had a high-Mg basaltic composition with 10.9 wt% MgO and 9.8 wt% FeOT. Fractional crystallization and significant crustal contamination triggered S saturation and sulphide enrichment in the magma. Zircon LA-ICPMS and SIMS U–Pb analyses reveal the consistent Silurian ages of 420–424 Ma for the gabbronorite and Ol websterite, which is coeval with the Xiarihamu Ni-Cu deposit. Based on these data and the regional tectonic setting, we suggest that the Shitoukengde mafic-ultramafic rocks formed in the post-collisional, extensional environment. The cessation of subduction may have resulted in break-off of the dense subducted slab, and triggered ascent of the hot convective asthenospheric mantle. The mantle wedge was modified by the subduction-related fluids and these enriched mantle components were incorporated into the ascending asthenospheric melts at relatively shallow depths, generating the high-Mg basaltic magmas that formed the compositionally variable mafic-ultramafic rocks (e.g., Shitoukengde and Xiarihamu intrusions) in the Eastern Kunlun Orogenic belt.

Structure and evolution of the Wairakei–Tauhara geothermal system (Taupo Volcanic Zone, New Zealand) revisited with a new zircon geochronology

The central part of the Taupo Volcanic Zone (TVZ), New Zealand, is notable for the number (23) and size of its high-temperature geothermal systems. Of these, the Wairakei-Tauhara geothermal system collectively represents the largest commercially developed example. Natural thermal output prior to utilisation, i.e. before 1950 was ∼530MW. Since 1950, drilling of over 300 closely spaced wells for power generation has provided geological data for a comprehensive stratigraphic framework. Within this framework, the strata are dominantly rhyolitic pyroclastic rocks and lava bodies, with interbedded fluvial and lacustrine deposits. Until now, no subsurface units had been directly dated and their ages were constrained only by their positions relative to the surficial 25.4 ka Oruanui ignimbrite, or to buried ignimbrite that is correlated with the 349 ka Whakamaru Group. This paper presents 33 suites of U-Pb age data and a limited number of reconnaissance U-Th disequilibrium age data from magmatic zircon crystals (zircons) to date the rocks of the Wairakei-Tauhara system. The oldest units dated are intermediate lavas from ca. 1.7 Ma, while the youngest locally vented eruptives of Tauhara dacite are tentatively dated to ca. 60 ka. Placing the age data within the established drillhole record allows for the recognition of major volcano-tectonic events and spatially clustered local volcanism that represents magmatic heat sources for geothermal activity within the Wairakei-Tauhara area. In contrast to the Rotokawa and Ngatamariki geothermal systems 10–20km to the northeast, the Wairakei-Tauhara area was affected by two episodes of caldera collapse associated with deposits dated at 349 ka and 310 ka, leading to major faulting and abrupt variations in thicknesses of units. The faulting and deposit thickness variations established in these two events remain major controls on present-day permeability of the geothermal system. The new age data and stratigraphy demonstrate development of distinct magma systems through time which may have stimulated previous periods of hydrothermal activity. However, from zircon age data from the rhyolite-andesite mixed Tauhara dacite, we propose that the current geothermal system was initiated at ca. 60 ka and is thermally maintained by continuing contributions from the same magmatic system that has sporadically vented distinctive rhyolite since.

280–310 Ma rift-related basaltic magmatism in northern Baoshan, SW China: Implications for Gondwana reconstruction and mineral exploration

The temporal and spatial variations of Late Paleozoic basaltic lavas in Baoshan, the northern part of a Gondwana-derived micro-continental block called Sibumasu, are important for Gondwana reconstruction. Magmatic zircon crystals from three selected dolerite dykes in the Baoshan region yield U-Pb ages from ~295 to ~310 Ma. These new ages, together with previous zircon U-Pb ages for this type of rock, define a protracted (~30 myr) episode of basaltic magmatism from ~310 to 280 Ma in a small area of this region, which is inconsistent with the typical temporal-spatial distribution of mantle plume magmatism. The trace element compositions of the Baoshan dolerite dykes and associated lavas are similar to arc basalts as well as continental flood basalts worldwide, showing light REE enrichments and negative Nb-Ta anomalies. Mixing calculations using the Sr-Nd-Hf isotope data of the Baoshan mafic rocks indicate that their intriguing trace element characteristics can be explained by contamination of mantle-derived magmas with crustal materials. Our new data, together with the lack of Late Paleozoic are-related calcalkaline rocks and granitoids in the Sibumasu block and other contemporaneous Gondwana-derived micro-continental blocks, strongly support the premise that the 310–280 Ma basalts and dolerites in the Baoshan region are the products of continental rift-related magmatism rather than arc magmatism. Based on the temporal correlation of the 310–280 Ma rift-related magmatism in several related Gondwana-derived micro-continents (Sibumasu, South Qiangtang, Lhasa and Himalaya), plus other independent constraints such as paleoclimate biotas and paleolatitudes from the literature, we provide an improved model for the configuration of the Gondwana supercontinent in the Early Permian. Based on the results from this study, we conclude that further investment in the exploration of magmatic Ni-Cu sulfide deposits associated with the 310–280 Ma mafic-ultramafic intrusions in northern Baoshan and the other related Gondwana-derived micro-continental blocks is warranted.

Provenance and magmatic-tectonic setting of Campanian-aged volcaniclastic sandstones of the Kannaviou Formation in western Cyprus: Evidence for a South-Neotethyan continental margin volcanic arc

Regionally developed late Cretaceous subduction-related magmatism in the eastern Mediterranean records progressive closure of the Southern Neotethys. In western Cyprus, the late Cretaceous (c. 90 Ma) Troodos ophiolite is depositionally overlain by volcaniclastic sandstones (up to 750 m thick) that are dominated by redeposited pyroclastic fallout, interbedded with both non-calcareous and calcareous radiolarian-bearing mudstones. The sands were mainly deposited by channelised mass-flow processes and to a lesser extent by turbidity currents in a deep-water forearc basin. The succession was folded and locally thrust-deformed related to latest Cretaceous emplacement of adjacent Mesozoic continental margin and oceanic units (Mamonia Complex). SIMS U-Pb analysis of euhedral to subhedral magmatic zircon crystals yielded a weighted mean 206Pb/238U age of 80.1 ± 1.1 Ma (Campanian). Sandstone petrography including compositional framework (Q12F21L67) suggests sediment supply from a volcanic arc. Subordinate continentally-derived detritus suggests a continental margin arc setting for the mafic to felsic fallout ash. SIMS analysis of little-altered volcanic glass indicates high Th/Nb and Th/La ratios that suggest the involvement of continental crust and/or subducted terrigenous sediments in magma genesis. Whole-rock chondrite-normalised REE patterns have comparable trends to modern and ancient forearc basin volcaniclastic sands and sandstones (e.g., northwest Pacific region). Trace element chemistry, although scattered towards the continental island arc fields on some geochemical diagrams (e.g., Th-Sc-Zr), mainly suggests an oceanic island arc source for the Kannaviou Formation sandstones, with variable enrichments in V-Cr-Ni-Sc, depletion in Nb-Ta, and relatively low trace element ratios (e.g., La/Co, Th/Co, La/Sc, Th/Sc). An explanation for this apparent discrepancy is that the volcaniclastic sandstones were derived from a relatively primitive arc constructed on previously depleted, rifted (thinned) Neotethyan continental crust. The inferred continental margin arc developed during early-stage northward subduction of the Southern Neotethys beneath a Tauride microcontinent to the north. Large volumes of volcanic ash were derived from continental margin arc volcanism, probably in the vicinity of the Kyrenia Range in northern Cyprus.

Zircon U–Pb, Lu–Hf and O isotopes from the 3414 Ma Strelley Pool Formation, East Pilbara Terrane, and the Palaeoarchaean emergence of a cryptic cratonic core

The detrital zircon record yields important information on crustal evolution that may be missing from extant magmatic rocks. The Palaeoarchaean to Neoarchaean East Pilbara Terrane (EPT), Western Australia, is the ancient core of the Pilbara Craton, and the archetypal granite-greenstone terrane. Magmatic zircon U–Pb crystallization ages from the EPT record crustal magmatism spanning 3.53–3.22 Ga. However, detrital zircons with ages as old as 3.65 Ga have been identified in EPT supracrustal sequences, which may provide key insights into the EPT’s early Archaean history. The Strelley Pool Formation (SPF), one of the earliest EPT siliciclastic sedimentary successions, is a critical unit in the field of Precambrian palaeobiology, containing multiple lines of evidence for some of Earth’s earliest life forms. The SPF was deposited on a continental shelf in a shallow water environment, upon perhaps the oldest preserved terrestrial erosion surface on Earth; it thus provides a record of a newly-emergent Palaeoarchaean continent. We report U–Pb, Lu–Hf and O isotope data from a suite of detrital zircon crystals sourced from the type locality of the SPF. Two hundred and five U–Pb detrital zircon analyses were undertaken via SIMS and LA–ICPMS. Four analyses define a young age peak of 3414 ± 34 Ma (2σ), which we interpret as a maximum depositional age. Zircon Hf isotope analyses yield εHfi of −3.5 to +5.2, clustering around chondritic values, with two-stage Hf crustal model ages of 3.9–3.5 Ga. Zircon O isotopes give a range in δ18O values from mantle-like (5.3‰) to more elevated (∼6.4‰). Taken together, the Hf and O isotope record, from new and published detrital and magmatic zircon crystals, implies a juvenile (mafic) source for evolved rocks of the EPT until ca. 3.3 Ga, after which time it experienced a period dominated by reworking of existing crust, possibly reflective of a change in geodynamics. The relationship between the cryptic core of the EPT, which is perhaps as old as 3.8–3.7 Ga, and the extant magmatic rocks that comprise the exposed Palaeoarchaean granite domes, remains uncertain.

Sub-arc mantle heterogeneity in oxygen isotopes: evidence from Permian mafic–ultramafic intrusions in the Central Asian Orogenic Belt

Integrated zircon–olivine O–Hf isotope data have been successfully used to unravel the nature of the source mantle for the early Permian post-collisional mafic–ultramafic intrusive rocks in the southern margin of the Central Asian Orogenic Belt in NW China. Olivine crystals with forsterite (Fo) contents varying from 91 to 87 mol% from the Permian Pobei mafic–ultramafic complex in the region yield highly elevated δ18O from 6.0 to 7.2‰. These values are much higher than typical mantle values (~ 5.3‰) and are apparently at odds with the mantle-like eNd(t) values of whole rocks (4.9–5.4). Magmatic zircon crystals from troctolite and gabbroic rocks show divergent oxygen and hafnium isotopic compositions: mantle-like eHf(t) values from 5.1 to 11.9 and crust-like δ18O values from 7.6 to 10.1‰. The observed increase of δ18O values from olivine (an early crystallizing phase) to zircon (a late crystallizing phase) in the mafic–ultramafic rocks is generally consistent with an AFC process. However, this process cannot fully explain the highly elevated δ18O values (6–7‰) for the most primitive olivine containing Fo as high as mantle olivine (> 90 mol%) and the mantle-like Hf isotope composition of zircon. Mixing calculation indicates that such highly unusual isotope compositions can be explained by the previous source mantle contamination with subducted sediment-derived melts and slab-derived fluids. Our results show that the combination of zircon O–Hf isotopes and olivine oxygen isotopes is more effective than the data of zircon or olivine alone to distinguish the effect of AFC process from source contamination. The results from this study provide a new line of evidence that the sub-arc mantle is not homogeneous in oxygen isotopes.

Late Carboniferous–Early Permian high- and low-Sr/Y granitoids of the Xing'an Block, northeastern China: Implications for the late Paleozoic tectonic evolution of the eastern Central Asian Orogenic Belt

This study presents new zircon U-Pb geochronological, whole-rock geochemical, and zircon Hf isotopic data along with regional geological observations for five late Paleozoic granitic plutons within the Xing'an Block of northeastern China. These data provide insights into the petrogenesis of these granitoids and the late Paleozoic tectonic evolution of the region. The geochronological and geochemical data indicate that the intrusions were emplaced during two separate periods, with the resulting granites having distinct geochemical features. The early Late Carboniferous (321–310 Ma) granitic Lizishan (LZS), Xiaonangou (XNG), and Dabeigou (DBG) plutons have high Sr/Y values (33.15–126.73), with low concentrations of Y and heavy rare earth elements (HREE), indicating adakitic affinities. However, the XNG granodiorites contain higher concentrations of MgO, Cr, and Ni, and have higher Mg# values (57–59) than the other plutons. The latest Carboniferous–Early Permian (301–296 Ma) granitic Weilegesi (WLGS) and Wuyi (WY) plutons are high-K calc-alkaline, with low Sr/Y ratios (0.18–24.15), and show petrological and geochemical characteristics of I-type granites. In addition, the magmatic zircon crystals within these plutons yield positive εHf(t) values (+4.1 to +12.9) and juvenile two-stage model (TDM2) ages (1056–499 Ma). The timing of emplacement and geochemical and isotopic compositions of the LZS and DBG granitoids are indicative of derivation from magmas generated by the partial melting of thickened lower crustal material, whereas the XNG granitoids were most likely formed from magmas generated by the partial melting of delaminated lower crustal material that subsequently interacted with mantle material during ascent. The latest Carboniferous–Early Permian low-Sr/Y granitoids may have formed from magmas generated by the partial melting of a dominantly juvenile amphibolite-facies crustal source under relatively low-pressure conditions, with these magmas subsequently undergoing variable degrees of fractional crystallization. Combining these new data with the results of previous research indicates that the terminal collision between the Xing'an and Songliao blocks occurred between the late Early and early Late Carboniferous, and the voluminous Late Carboniferous to Early Permian granitoids in the Xing'an Block are most likely related to post-collisional delamination.

Permian bimodal magmatism in the southern margin of the Central Asian Orogenic Belt, Beishan, Xinjiang, NW China: Petrogenesis and implication for post-subduction crustal growth

Previous studies show that Permian bimodal magmatism is widespread in the Tianshan region, the southern part of the Central Asian Orogenic Belt in northern Xinjiang, NW China. We have found that such bimodal magmatism extended farther to the south in the Beishan region. Our new zircon U-Pb age data show that the dolerite and granite dykes in the Poyi area of the region were emplaced at ~270 Ma, >20 myr after subduction ended in the region. The dated dolerite dyke is characterized by pronounced negative Nb-Ta anomalies, high εNd(t) from +4.3 to +5.1 and low initial 87Sr/86Sr ratios from 0.7051 to 0.7060 for whole rocks, plus high εHf(t) from +8.96 to +13.0 and elevated δ18O from 5.8 to 6.5‰ for zircon crystals, which indicate a subduction-modified mantle source and moderate degrees of crustal contamination (<10 wt%). Whole-rock 1000Zn/Fe, Mn/Zn, Dy/Yb and La/Yb ratios of the Poyi dolerite dyke are consistent with a peridotite-dominant mantle source. Between two possible causes of mantle partial melting, such as mantle plume impingement and lithospheric delamination coupled with asthenosphere upwelling, we favor the latter because existing data indicate that the Permian mafic-ultramafic intrusive rocks in the region were emplaced over a long period (>25 myr) and show no age progression in a single direction, which are inconsistent with the results of mantle plume magmatism. Whole rocks from the Permian granite dyke in the Poyi area are characterized by strong negative Nb-Ta anomalies, negative εNd(t) from −2.4 to −1.6 and high initial 87Sr/86Sr from 0.7080 to 0.7088. Magmatic zircon crystals from the granite dyke have εHf(t) from +0.86 to +5.64 and δ18O from 6.5 to 7.2‰. The isotope compositions of the granite dyke are within the ranges of the pre-Permian crustal rocks in the region, indicating a predominantly crustal source. Based on these data, we propose that partial melting in the crust was induced by underplating of mantle-derived mafic magmas, some of which were emplaced in the upper parts of the crust to form the broadly coeval dolerite dykes and mafic-ultramafic intrusions. The results from this study support the view that post-subduction bimodal magmatism is important for continental crustal growth and differentiation.

Zircon petrochronology reveals the timescale and mechanism of anatectic magma formation

Igneous rocks of intermediate to acidic composition commonly exhibit considerable degrees of isotope variability preserved at the crystal and sub-crystal scale, as well as a significant U–Pb age range, reflecting protracted timescales of zircon crystallization and long magma residence times. The association of high-precision U–Pb zircon dates with stable and radiogenic isotope data represents a powerful tool to unravel the petrological evolution of granitic rocks, hence allowing a better understanding of the processes that led to the formation and reworking of the continental crust.In this case study, we combine high-precision U–Pb dates with stable and radiogenic isotope data from zircon crystals in the Larderello–Travale (Italy) shallow-level granites. These rocks are peraluminous two-mica, cordierite-bearing granites and represent pure crustal anatectic magmas, generated in a post-collisional extensional setting. As such, they are ideal candidates to investigate the timing, rates and mechanisms of melt production during anatectic magma formation, giving insights into the process of intracrustal differentiation. Magmatic zircon crystals from the Larderello–Travale granites contain δ18O values ranging from 8.6 to 13.5‰ and crystals from individual samples exhibit inter- and intra-grain oxygen isotope variability exceeding 3‰. The analysed crystals have εHf values ranging between −7.4 and −12.4, with moderate, intra-sample εHf isotope variability. All CA-ID-TIMS (chemical abration isotope-dilution thermal ionization mass spectrometry) 206Pb/238U zircon ages range from 4.5 to 1.6 Ma and suggest four pulses of magmatic activity at ∼3.6, 3.2, 2.7 and 1.6 Ma. More importantly, zircon crystals from individual samples typically exhibit an age spread as large as 300–500 ka. This age dispersion suggests that most of the zircon did not crystallize at the emplacement level but in the middle crust and were subsequently recycled and juxtaposed during ascent and emplaced at shallow level. When plotted against age, stable and radiogenic isotope data suggest the co-existence of multiple and isotopically distinct magma batches produced by partial melting of different crustal domains. This requires coeval magma batches that are physically separated and evolve independently for hundreds of thousands of years before coalescing during ascent and emplacement. The involvement of multiple sources in the production of crustal anatectic magmas reflects the inherent heterogeneous nature of the continental crust and result from the interplay between the rise and evolution of the geotherms through the crust and the composition of the fertile source rocks. Finally, the isotopically diverse zircon-bearing magma batches mixed and assembled into shallow-level intrusions generated during the four major magma pulses.

New Zr–Hf Geothermometer for Magmatic Zircons

A geothermometer equation $$T = \frac{{1531}} {{\ln K_d + 0.883}}$$ , where $$K_{\dot d} = \frac{{X_{Zr}^S X_{Hf}^m }} {{X_{Zr}^m X_{Hf}^s }}$$ [X is the concentration (in ppm) of component i in phase j] is the Zr and Hf distribution coefficient between melt and zircon, and T is temperature in K, was derived by thermodynamic processing of literature experimental data on Zr and Hf distribution between acid melts (m) and zircon (s) and on the solubility of zircon and hafnon in the melts with variable silica content. In calculations with this equations, we assumed the Zr concentration in zircon to be constant: 480000 ppm. It is shown that the commonly observed increase in Hf concentration from the cores to margins of magmatic zircon crystals is caused by the fractional crystallization of zircon. For differentiated acid magmatic series, the initial crystallization temperature of zircon in the least silicic cumulates should be evaluated using the cores of large zircon grains with the highest Zr/Hf ratio. Application of the geothermometer for mafic and intermediate rocks may be hampered due to simultaneous crystallization of zircon with some other ore and mafic minerals relatively enriched in Zr and Hf. The newly derived geothermometer has some advantages over other indicators of the crystallization temperature of magmatic zircon based on the zircon saturation index (Watson and Harrison, 1983; Boehnke et al., 2013) and on Ti concentration in this mineral (Ferry and Watson, 2007) as it does not depend on the major-oxide melt composition and on the accuracy of the estimated SiO2 and TiO2 activities in the melts. Calculations of the Zr and Hf fractionation trends in the course of zircon crystallization in granitoid melts allow one to evaluate the temperature at which more evolved melt portions were segregated.

Pliocene granodioritic knoll with continental crust affinities discovered in the intra-oceanic Izu–Bonin–Mariana Arc: Syntectonic granitic crust formation during back-arc rifting

A widely held hypothesis is that modern continental crust of an intermediate (i.e. andesitic) bulk composition forms at intra-oceanic arcs through subduction zone magmatism. However, there is a critical paradox in this hypothesis: to date, the dominant granitic rocks discovered in these arcs are tonalite, rocks that are significantly depleted in incompatible (i.e. magma-preferred) elements and do not geochemically and petrographically represent those of the continents. Here we describe the discovery of a submarine knoll, the Daisan–West Sumisu Knoll, situated in the rear-arc region of the intra-oceanic Izu–Bonin–Mariana Arc. Remotely-operated vehicle surveys reveal that this knoll is made up entirely of a 2.6 million year old porphyritic to equigranular granodiorite intrusion with a geochemical signature typical of continental crust. We present a model of granodiorite magma formation that involves partial remelting of enriched mafic rear-arc crust during the initial phase of back-arc rifting, which is supported by the preservation of relic cores inherited from initial rear-arc source rocks within magmatic zircon crystals. The strong extensional tectonic regime at the time of intrusion may have allowed the granodioritic magma to be emplaced at an extremely shallow level, with later erosion of sediment and volcanic covers exposing the internal plutonic body. These findings suggest that rear-arc regions could be the potential sites of continental crust formation in intra-oceanic convergent margins.

Orosirian (ca. 1.96 Ga) mafic crust of the northwestern São Francisco Craton margin: Petrography, geochemistry and geochronology of amphibolites from the Rio Preto fold belt basement, NE Brazil

The bulk of the Atlantic shield basement in the eastern South American platform is made of Rhyacian calc-alkaline magmatic rocks (∼2.2–2.15 Ga) affected by granulite-facies metamorphism at around 2.06 Ga, as part of a Paleoproterozoic orogenic system which joined Archean crustal fragments together. Orosirian (2050–1800 Ma) rocks, on the other hand, are scarce in all four major provinces of eastern South America (the Borborema, São Francisco, Tocantins and Mantiqueira provinces). Recently, however, sparse occurrences of Orosirian-aged rocks have been described, particularly in the northern São Francisco Craton margin and adjoining fold belts that compose the Borborema Province further north. In this paper, we present new geochemical, geochronological and isotopic data of amphibolites which compose the basement of the Rio Preto fold belt, in the northwestern São Francisco Craton margin. Geochemical data suggest tholeiitic gabbro protoliths intruded in a subduction-related setting, as suggested by tectonic discrimination diagrams and differentiation trends. U–Pb analyses of magmatic zircon crystals yielded a 1958.3 ± 16 Ma Concordia age. Slightly negative to positive εNd(1.96) = (−0.3 to +1.0) and εHf(1.96) from −0.76 to −4.55 suggest variable mixing of a depleted mantle source and older continental crust. We interpret the amphibolite body as a remnant of a subduction-related crustal fragment developed in the Orosirian, around 1.96 Ga ago, possibly in a continental back-arc setting. This crustal fragment was further reworked as part of the basement of the Rio Preto fold belt during the Brasiliano Orogeny (∼600–550 Ma). The Cristalândia do Piauí Complex gneisses further north might represent fragments of a coeval continental magmatic arc. These rocks could be the first well documented representatives of an Orosirian tectono-magmatic event which has been, up to now, broadly ignored within this portion of West Gondwana, and thus present major implications for the Proterozoic crustal evolution of South America and for Paleoproterozic supercontinent reconstructions in general.

Zircon Geochronology and Trace Element Geochemistry from the Xiaozhen Copper Deposit, North Daba Mountain: Constraints on Albitites Petrogenesis

AbstractAlbitite often accompanies with various metal and gem mineral deposits and a large number of occurrences have been reported globally, including the South Qinling orogen, China. The Xiaozhen copper deposit is a typical deposit in the North Daba Mountain area of the South Qinling orogen whose distribution is controlled by albitite veins and fractures. As there are few studies on the petrogenesis of albitite in Xiaozhen copper deposit, this paper focuses on the petrogenesis of albitite and its mineralization age. Detailed fieldwork and mineral microscopic observations initially suggest that albitite from the Xiaozhen copper deposit is igneous in origin. Further zircon trace element geochemistry studies indicate that these zircons have high Th/U ratios (&gt;0.5), low La content, high (Sm/La)N and Ce/Ce* values, and a strong negative Eu anomaly, which are commonly seen in magmatic zircons. The chondrite–normalized rare earth element (REE) patterns are consistent with magmatic zircons from throughout the world, and they fall within or near the field of magmatic zircons on discriminant diagrams. The calculated average apparent Ti–in–zircon temperature for young zircons is 780°C, consistent with magmatic zircon crystallization temperatures. Therefore, zircon geochemistry indicates that the albitite origin is magmatic. SIMS U–Pb dating on nine magmatic zircons yielded a concordia age of 154.8±2.2 Ma, which represents the formation of albitite and the metallogenic age. More importantly, it is consistent with the ages of Yanshanian magmatism and metallogenesis in the South Qinling orogen, so formation of the Xiaozhen copper deposit may be a closely related geological event.

Unraveling the metamorphic history at the crossing of Neoproterozoic orogens, Sør Rondane Mountains, East Antarctica: Constraints from U–Th–Pb geochronology, petrography, and REE geochemistry

The Sør Rondane Mountains in East Antarctica preserve two stages of tectonothermal events, at c. 650–600Ma and c. 560–550Ma, each of which is also widely preserved elsewhere in Gondwana within tectonic suture zones/orogens, although both ages of tectonothermal events are only rarely documented together in these other parts of Gondwana. A geochronological and petrogenetic study of highly retrogressed garnet–sillimanite–biotite-bearing pelitic gneiss, along with three generations of associated sub-concordant to discordant leucocratic felsic veins, which together are exposed in the southern Austkampane area in the central part of the Sør Rondane Mountains, provides important new constraints on the geological and age relationships between these two regional tectonothermal events. U–Pb SHRIMP dating of zircon grains isolated from the host garnet–sillimanite–biotite gneiss yields at least four distinct zircon age populations of c. 2800–720Ma, c. 700Ma, 640–630Ma, and ∼500Ma. Chondrite-normalized REE patterns indicate that a pooled age of 637±6Ma represents the timing of recrystallization of metamorphic HREE-depleted zircon, during which the zircon was in equilibrium with garnet. The earliest emplaced leucocratic vein (LV-1), which is partly intermingled with the host garnet–sillimanite–biotite gneiss (and enclosing garnet-bearing pelitic enclaves), contains a population of zircon grains that yielded a crystallization age of 635±4Ma, which is almost identical to the age of zircons found within the host pelitic gneiss. The second-generation leucocratic vein (LV-2) occurs as a part of a set of sub-concordant veins/leucosomes comprising mostly biotite and muscovite. Although these particular leucosomes do not contain any zircon, they were found to contain abundant monazite, which yielded U–Pb ages of 640–630Ma along with some older (relict) ∼700Ma ages. The third-generation leucocratic vein (LV-3) was sampled from a suite of pegmatitic veins that discordantly crosscut both the host pelitic gneisses and the older two sets of leucocratic veins. These third-generation veins contain magmatic zircons that exhibit HREE-enriched patterns typical of magmatic zircon crystallization, and which yielded a crystallization age of 550±3Ma. The data imply that a major high-grade metamorphic event (possibly at granulite facies) took place in this region along with a subsequent late-stage re-hydration event, all within a relatively narrow time interval of <10m.y. at ∼635Ma. A subsequent hydration event that took place much later on is recorded by the intrusion of a suite of discordant pegmatitic veins at ∼550Ma that are observed to crosscut and alter both the pre-existing pelitic gneiss and the other two older sets of felsic veins. These two ages of tectonothermal events in East Antarctica are coeval with the two major stages of Gondwana assembly. Hence, these results provide new insights into the development of metamorphic-fluid regimes within these major continent-transecting Neoproterozoic orogens.

Zircon and muscovite ages, geochemistry, and Nd–Hf isotopes for the Aktyuz metamorphic terrane: Evidence for an Early Ordovician collisional belt in the northern Tianshan of Kyrgyzstan

The Aktyuz metamorphic terrane in the Kyrgyz northern Tianshan consists of granitoid gneisses and migmatites with subordinate paragneisses, greenschists, presumed meta-ophiolites, and garnet amphibolite dykes that contain HP eclogite relicts. The gneisses and migmatites were previously considered to be Archaean and Palaeoproterozoic in age on the basis of α-Pb and U–Pb multigrain zircon dating. Zircons from a post-tectonic granite were previously dated at 692±15Ma, constraining the time of main deformation and metamorphism in the Aktyuz terrane to the Precambrian.The chemical characteristics of most granitoid samples are consistent with melting of chemically evolved crustal material, which is supported by Nd and Hf isotopic data. Zircon U–Pb SHRIMP ages were obtained for the main varieties of metamorphic rocks, for a gabbro of a low-grade ophiolite complex and for several post-kinematic igneous rocks. In addition, metamorphic muscovite was dated by the 40Ar–39Ar method, and whole-rock Sm–Nd isotopic systematics were obtained on several granitoid rocks. Our magmatic zircon crystallization ages for granitoid gneisses in the Aktyuz and Kemin Complexes range from 778±6 to 844±9Ma which we interpret to reflect the time of magmatic emplacement of the gneiss protoliths. These rocks reflect an episode of Neoproterozoic granitoid magmatism, which is also documented in southern Kazakhstan, the Kyrgyz Middle Tianshan, the Chinese Central Tianshan and the Tarim Craton. Nd and Hf isotopic systematics show these rocks to be derived from Mesoproterozoic to Archaean sources. The calc-alkaline composition of these rocks seems compatible with a subduction setting, but is most likely inherited from the source, therefore the tectonic scenario for emplacement of the gneiss protolith remains unknown. Two ages of 562±7 and 541±3Ma and negative εNd(t)-values for granitoid gneisses document a later crustal melting episode. Muscovite 40Ar/39Ar ages of ca. 470Ma for Aktyuz gneisses constrain the main fabric-forming metamorphism to the Early Palaeozoic. A migmatitic paragneiss, which was previously interpreted as Palaeoproterozoic, contains detrital zircons with an age spectrum from 503 to 1263Ma; the youngest grain suggests a maximum Cambrian age of protolith deposition. An ophiolitic metagabbro of the Kemin Complex yielded an Early Cambrian age of 531±4Ma, which is close to the age of ophiolites in the adjacent Djalair–Naiman belt of Kazakhstan, suggesting a possible genetic link. Two samples of quartz diorite from the post-kinematic Dolpran pluton yielded Early Ordovician zircon ages of 471.9±3.5 and 472.0±3.1Ma. The presence of a 783±7Ma xenocrystic zircon points at Precambrian crust at depth, which may explain an earlier, discordant apparent age obtained by multigrain zircon dating of this pluton. Undeformed rhyolite and basalt in the East Kyrgyz Range, previously classified as Neoproterozoic and Cambrian, yielded Late Ordovician ages of 451.9±4.6 and 448.9±5.6Ma respectively.Our data imply that the Aktyuz terrane is not a single segment of continuous Precambrian continental crust but represents a complex amalgamation of Neoproterozoic continental and Early Palaeozoic ophiolitic slivers, which were stacked together in a subduction and collisional setting. Large fragments of continental crust were subducted to HP eclogite-facies conditions, which led to eclogite metamorphism in the mafic dykes. The main deformational event occurred during the latest Cambrian to earliest Ordovician between 503 and 472Ma.