Zircon Grains Research Articles

Ti‐in‐zircon and Ti‐in‐quartz thermometry using electron probe microanalysis: A promising approach in retrieving thermal conditions of granulite facies metamorphism and felsic magmatism in the Sandmata Complex, Aravalli Craton (NW India)

The titanium (Ti) concentrations in zircon and quartz and corresponding thermometers have been regarded as powerful monitors that constrain petrogenetically meaningful temperature conditions of crystallization and metamorphism. The precise measurement of trace elements by electron probe microanalyzer (EPMA) is quite challenging as their x‐ray intensities are only slightly higher than the Bremsstrahlung background, leading to significant errors while quantifying the elemental abundances (e.g., Ti‐in‐zircon). We present EPMA‐based protocols for analysing Ti concentrations in zircon and quartz by considering recent analytical and instrumental improvements. High counting times at peak and background positions, simultaneous acquisition of Ti in multiple (three) spectrometers and data processing using the non‐central chi2 test for risk determination in sub‐counting method were employed in Ti‐in‐zircon and Ti‐in‐quartz protocols. Applying these protocols to the Mongolian garnet, San Carlos olivine and NIST 610 glass standards yielded Ti concentrations of 0.60 ± 0.02 wt%, 18 ± 3 ppm and 437 ± 26 ppm (in ±2σ), respectively. The Ti concentrations obtained from these standards are consistent with those published based on high‐precision analytical methods. We use the existing Ti‐in‐zircon and Ti‐in‐quartz thermometers to understand the thermal evolution of various lithologies in the Sandmata Complex, Aravalli Craton (north‐western India). The investigated samples include garnet–biotite gneiss, migmatite gneiss, mafic granulite and Anjana granite. The Ti‐in‐zircon temperatures calculated from the patchy‐zoned zircon cores yield HT–UHT conditions (892–959°C) for garnetiferous granite gneiss, migmatite gneiss and mafic granulite, whereas the zircon overgrowths yield temperatures of 725–871°C. The homogeneous zircon cores from Anjana granite yield temperatures (914–984°C) comparable to zirconium saturation conditions (920–960°C), indicating zircon crystallization from “hot” granitic melt. The oscillatory overgrowths on the zircon core obtain variable temperature conditions for inner overgrowth (811–877°C) and outer overgrowth (714–782°C), suggesting the episodic growth of zircon grains from different magma pulses. Additionally, the application of pressure‐dependent Ti‐in‐quartz thermometry yields quartz recrystallization conditions for garnet–biotite gneiss (430–556°C), migmatite gneiss (423–593°C), mafic granulite (430–679°C) and Anjana Granite (444–533°C). The combination of Ti‐in‐zircon thermometry, Ti‐in‐quartz and conventional thermobarometry demonstrates cooling histories for various high‐grade metamorphic and magmatic rocks in the Sandmata Complex. Based on this study, we emphasize that trace element thermometers can decipher high‐temperature metamorphism, which is otherwise erased in conventional thermometers due to diffusional readjustments during long‐lasting near‐peak and post‐peak metamorphism.

Duluth Complex FC1 Apatite and Zircon: Reference Materials for (U‐Th)/He Dating?

The accuracy and validation of geo‐ and thermochronological dating hinges on the availability of well‐characterised age reference materials. The Mesoproterozoic gabbroic anorthosite FC1 from the Duluth Complex, Minnesota is a reference material for zircon U‐Pb and a suggested reference material for apatite fission‐track dating. We evaluate FC1 as (U‐Th)/He reference material, and determine its apatite U‐Pb, and zircon and apatite (U‐Th)/He age. Our dating results constrain the thermal history of FC1, showing that fast cooling occurred between ~ 1099 and 1040 Ma from ≥ 600 °C to ~ 200 °C. The zircon (U‐Th)/He data from air‐abraded grains give a robust isochron age of 1037 ± 25 Ma (2s) without overdispersion. The within‐grain homogeneity of U and Th, the availability of FC1 zircon, and the absence of radiation‐damage effects on the (U‐Th)/He age support its use as reference material. Unabraded zircon grains give lower and more dispersed ages, highlighting the usefulness of air abrasion to control for α‐ejection in (U‐Th)/He dating. Our apatite (U‐Th‐Sm)/He single‐grain ages vary between 180 and 300 Ma. Their wide dispersion argues against the use of FC1 apatite as (U‐Th‐Sm)/He reference material and makes the interpretation of their low‐temperature thermal history complicated.

The temporal evolution of subduction initiation in the Samail ophiolite: High‐precision U–Pb zircon petrochronology of the metamorphic sole

AbstractHigh‐precision dating of the metamorphic sole of ophiolites can provide insight into the tectonic evolution of ophiolites and subduction zone processes. To understand subduction initiation beneath a young, well‐preserved and well‐characterized ophiolite, we performed coupled zircon laser‐ablation inductively coupled mass spectrometry trace element analyses and high‐precision isotope dilution‐thermal ionization mass spectrometry U–Pb dating on 25 samples from the metamorphic sole of the Samail ophiolite (Oman‐United Arab Emirates). Zircon grains from amphibolite‐ to granulite‐facies (0.8–1.3 GPa, ~700–900°C), garnet‐ and clinopyroxene‐bearing amphibolite samples (n = 18) show systematic trends of decreasing heavy rare earth element slope (HREE; Yb/Dy) with decreasing Yb concentration, reflecting progressive depletion of the HREE during prograde garnet growth. For half of the garnet‐clinopyroxene amphibolite samples, Ti‐in‐zircon temperatures increase, and U–Pb dates young with decreasing HREE slope, consistent with coupled zircon and garnet growth during prograde metamorphism. In the remaining samples, there is no apparent variation in Ti‐in‐zircon temperature with decreasing HREE slope, and the combined U–Pb and geochemical data suggest zircon crystallization along either the prograde to peak or prograde to initial retrograde portions of the metamorphic P–T–t path. The new data bracket the timing of prograde garnet and zircon growth in the highest grade rocks of the metamorphic sole between 96.698 ± 0.094 and 95.161 ± 0.064 Ma, in contrast with previously published geochronology suggesting prograde metamorphism at ~104 Ma. Garnet‐free amphibolites and leucocratic pods from lower grade (but still upper amphibolite facies) portions of the sole are uniformly HREE enriched (Yb/Dy > 5) and are ~0.5–1.3 Myr younger than the higher grade rocks from the same localities, constraining the temporal offset between the metamorphism and juxtaposition of the higher and lower grade units. Positive zircon εHf (+6.5 to +14.6) for all but one of the dated amphibolites are consistent with an oceanic basalt protolith for the sole. Our new data indicate that prograde sole metamorphism (96.7–95.2 Ma) immediately predated and overlapped growth of the overlying ophiolite crust (96.1–95.2 Ma). The ~600 ky offset between the onset of sole metamorphism in the northern portion of the ophiolite versus the start of ophiolite magmatism is an order of magnitude shorter than previously proposed (~8 Ma) and is consistent with either spontaneous subduction initiation or an abbreviated period of initial thrusting during induced subduction initiation. Taken together, the sole and ophiolite crust preserve a record of the first ~1.5 Myr of subduction. A gradient in the initiation of high‐grade metamorphism from the northwest (96.7 Ma) to southeast (96.0–95.7 Ma) may record propagation of the nascent subduction zone and/or variations in subduction rate along the length of the ophiolite.

SOURCES OF THE LATE CENOZOIC DEPOSITS FROM THE AEOLIAN “SANDS” MASSIF IN THE CHARA BASIN OF THE BAIKAL RIFT ZONE: FIRST RESULTS OF U–TH–PB (LA-ICP-MS) GEOCHRONOLOGICAL STUDY OF DETRITAL ZIRCON

The results of U–Th–Pb (LA-ICP-MS) geochronological studies of detrital zircon (fraction more than 50 µm) from the Late Cenozoic deposits of the large aeolian “Sands” massif of the Chara Basin of the eastern flank of the Baikal Rift Zone show that they are dominated by detrital zircon with ages of 263–265 and 1861–1864 Ma. Archean ages have been obtained only for individual zircon grains. Quite possibly, this is due to the fact that zircon of this age is present in “small” fractions, the study of which is not possible due to the technological features of the method of geochronological studies used. Based on the geological situation, only granites of the Kodar complex can be considered as the only source of detrital zircon of the Early Proterozoic age. The source of detrital zircon with an age of 263–265 Ma remains a mystery. Granitoids of this age are not known on the territory of the Aldan Shield. They are widespread in its western frame.

The Fundamental Role of H2O in the Generation of Coeval Sodic and Potassic Granitoids at Continental Arcs: An Example from the Yangtze Block, South China

AbstractThe bulk rock composition of granitoids reflects the composition of their source and the conditions of partial melting, which are functions of the geodynamic setting in which they formed. Granitoids in active continental margins (continental arcs) are dominated by calc-alkaline rocks with subordinate alkaline compositions, although how these different magma compositions formed is not well understood. Neoproterozoic magmatic rocks are widely distributed along the western margin of the Yangtze Block in South China to form the >1000-km long Panxi continental arc system, which is dominated by granitoids with minor mafic–ultramafic and intermediate plutons. The granitoids are subdivided into sodic and potassic variants that occur as belts along the western and eastern sides of the continental arc, respectively. Sodic granitoids from the western part consist of tonalite, granodiorite, and monzogranite with crystallisation ages ranging from 870 Ma to 740 Ma. They have low K2O/Na2O ratios (0.1–1.0) and high Na2O contents (3.5–6.7 wt%), high but variable SiO2 (61–75 wt%) concentrations, and negative to positive whole-rock εNd(t) values (−1.7 to +2.9). Zircon grains from the sodic granitoids have εHf(t) values ranging from +0.3 to +9.6 and δ18O from 3.90‰ to 7.71‰. The potassic granitoids from the eastern side consist of monzogranite and syenogranite with crystallisation ages from 820 Ma to 790 Ma. They have high K2O/Na2O ratios (0.6–2.2), K2O (2.6–5.9 wt%) and SiO2 contents (69–78 wt%), but whole-rock εNd(t) (−0.9 to +2.9) and zircon εHf(t) (+1.8 to +12.9), and δ18O values (2.98‰ to 6.41‰) similar to those of the sodic granitoids. The isotopic compositions of both the sodic and potassic granitoids are similar to those of spatially- and temporally-related mantle-derived (mafic to ultramafic) rocks, and are considered to have been derived from juvenile mafic continental crust. Phase equilibrium modelling shows that the H2O content of the granitoid source rocks played a key role in their petrogenesis, both in lowering solidus temperatures and in controlling the compositions of the derived partial melts. Our results indicate that calc-alkaline sodic granitoids can be formed by water-fluxed melting of juvenile mafic crust at 750–900°C and 9–12 kbar in which the required H2O was derived from the dewatering of underplating mafic arc magmas. By contrast, the potassic granitoids were generated by fluid-absent (H2O-undersaturated) partial melting of a similar juvenile mafic source at 725–900°C and 6–9 kbar. We conclude that the sodic granitoids were derived from partial melting of the newly-formed mafic lower crust in the continental arc, whereas the potassic granitoids were likely generated in the back-arc setting induced by upwelling of asthenospheric mantle.

New evidence for the Baltican cratonic affinity and Tonian to Ediacaran tectonic evolution of West Avalonia in the Avalon Peninsula, Newfoundland, Canada

Provenance studies of quartzite clasts and enclosing sandstone units in the Avalon Peninsula, Newfoundland, Canada, were conducted to test the Baltican or Gondwanan cratonic affinities of West Avalonia and examine the stratigraphic archives of Neoproterozoic arc evolution. Four cobble-sized quartzite clasts from a syn-orogenic, Ediacaran fluvial unit mostly yield ca. 1220, 1510, and 1900–2200 Ma detrital zircon age maxima and were probably derived from sub-arc basement units. Detrital zircon U-Pb-Hf isotope statistical assessments of the clasts indicate provenance ties with Proterozoic strata of the Sarmatian craton and Timanian passive margin of northern Fennoscandia, which corroborates models for proto-Avalonia to have originated near southern or northeastern Baltica, respectively. Analogous quartzite basement units in Nova Scotia and New England also have Baltican cratonic affinities and reflect provenance from southern Fennoscandia. Ediacaran exhumation of arc and basement infrastructure was accommodated by transpressive faults during the Avalonian orogeny and resulted in the deposition of Signal Hill Group terrestrial strata. Eleven Signal Hill Group sandstone samples are correspondingly dominated by Tonian to Ediacaran detrital zircon age fractions that constrain the crustal evolution of West Avalonia. The composite dataset includes 818–777 Ma detrital zircon grains that record the rifting of a proto-Avalonia sliver or ribbon arc from Baltica. Tonian igneous rocks are rare in West Avalonia, but ca. 760 Ma and 730 Ma detrital zircon grains indicate both crust and mantle contributions to the oceanic Burin arc, its tectonic interactions with proto-Avalonia, and subsequent arc magmatism across the composite terrane. Cryogenian detrital zircon ages substantiate the timing of 700–670 Ma magmatism and 670–640 Ma tectonic interactions with Baltica or another block. The detrital zircon U-Pb-Hf isotope record of the 640–600 Ma magmatic phase includes 6–9 Myr-long, fluctuating cycles that indicate the frequency of West Avalonian arc processes in the Mirovoi Ocean prior to Avalonian orogenesis.

Redefinition of the Petersburg batholith and implications for crustal inheritance in the Dinwiddie terrane, Virginia, USA

Field relations as well as geochemical and petrologic studies of meta-igneous rocks assigned to the Pennsylvanian–Permian Petersburg batholith identify at least two distinct rock types: foliated metagranitoid gneiss and massive to porphyritic granite. Foliated metagranitoid gneiss of mostly granodioritic composition is geochemically distinct from associated massive and porphyritic granitic rocks. These gneissic rocks yield radiometric ages from ca. 425 Ma to ca. 403 Ma and document that many of the rocks assigned to the late Paleozoic Petersburg batholith are 100 m.y. older than the youngest portions of the composite batholith and are part of an earlier infrastructural terrane. Two samples of massive equigranular granite southwest of Petersburg, Virginia, yield ages of ca. 321 Ma and ca. 317 Ma, which are 15–20 m.y. older than ca. 300 Ma ages for porphyritic granite, massive granite, and monzodiorite near Richmond, Virginia. Geologic mapping shows that the Early Pennsylvanian granite southwest of Petersburg is separated from Late Pennsylvanian to early Permian granite near Richmond by a map-scale septum of Silurian–Devonian foliated metagranitoid gneiss, referred to herein as the informal Pocoshock Creek gneiss. Laser ablation–inductively coupled plasma–mass spectrometry data from one sample of a quartz-muscovite felsic schist xenolith show a peak age mode of ca. 529 Ma that we interpret to be the maximum depositional age. Inherited zircons from foliated metagranitoid gneiss and massive equigranular granite range from ca. 631 Ma to ca. 376 Ma, but many are Cambrian. Neoproterozoic–Cambrian quartz-muscovite felsic schist and amphibolite, Silurian–Devonian Pocoshock Creek gneiss, and Pennsylvanian–Permian granite comprise a fault-bounded terrane referred to herein as the Dinwiddie terrane. Ages of inherited cores in zircon from igneous rocks and limited detrital zircon geochronology suggest the terrane is of peri-Gondwanan affinity. U/Pb ages of healed fractures in zircon grains from foliated metagranitoid gneiss indicate low-grade deformation of the gneiss at ca. 378–376 Ma, while ca. 320–280 Ma rims on many grains record intrusion of late Paleozoic granite. The temperature-time-deformation history of the Dinwiddie terrane is distinct from the adjacent Goochland and Roanoke Rapids terranes. Orogen-scale dextral transpression likely translated the Dinwiddie terrane southward during the Alleghanian orogeny, at which time they were intruded by Pennsylvanian to Permian granite.

ON PALEOTECTONIC BELONGING OF THE SUVANYAK METAMORPHIC COMPLEX (SOUTHERN URALS) FROM THE U-Th-Pb DATING OF DETRITAL ZIRCON GRAINS

U-Th-Pb isotope dating of grains of detrital zircon from quartzites of the Suvanyak metamorphic complex, which forms the Suvanyak tectonic unit that forms the western part of the Uraltau uplift, located in the east of the West Ural megazone in the Southern Urals. The results of isotope dating of grains of detrital zircon from quartzites of the southern part of the Suvanyak metamorphic complex (samples G18-1 and R14-396) show that numerous populations of Late Neoproterozoic–Early Cambrian detrital zircon grains suggest a Peri-Gondwanan origin of the primary sources of detrital material for the protolith of the studied rocks.The structure of the Suvanyak tectonic unit involves metamorphic formations of different ages – Early Paleozoic in the south and Late Precambrian in the north, which are now formally united into a single Suvanyak metamorphic complex. Their differentiation requires additional research.For the Late Paleozoic southeastern margin of the Baltica (at that time already involved in the structure of the composite continent Arct-Laurussia), according to the results of isotope-geochronological study of detrital zircon from the sedimentary and metasedimentary sequences of the Southern Urals, a number of the following tectonic structures were identified. Near the southeastern edge of the margin, there was a Late Neoproterozoic–Early Cambrian oceanic basin, within which a volcanic arc or arcs were active during 650–520 Ma. The structure of the southeastern edge of the margin included the Peri-Gondwanan terrane or terranes (? Cadomian type), as well as thick Riphean-Early Paleozoic sedimentary sequences, autochthonous to the Baltica.

AGE OF GRANITOIDS OF THE BEKCHIUL PLUTON (LOWER AMUR REGION)

The Bekchiul pluton is located in the Lower Amur region and is a large granitoid body of complex structure within the Zhuravlevka-Amur terrain of the Sikhote-Alin orogenic belt. On the northwestern flank of the Bekchiul pluton there is the Mnogovershinnoe gold-silver deposit. To determine the formation time of this pluton, U/Pb dating was performed on zircons from the second-phase granodiorite of the Bekchiul complex and the third-phase granite. Granodiorite yielded U-Pb age of 73.8±0.4 Ma, and granite – 66.2±0.3 Ma. Some zircon grains found in granite have a U-Pb age of 75.6±0.6 Ma which is close to that of granodiorites. Magmatic pulses of about 76–73 Ma and about 66 Ma are synchronous with the stages of ore formation of the Mnogovershinnoe deposit, corresponding to earlier-obtained K-Ar age of adularia from ore zones. The formation of granitoids and the associated mineralization probably occurred due to subduction of the Izanagi Plate beneath the continental margin.

Detrital zircon geochronology of quartzites from the Central Zone of the Limpopo Complex in southern Africa: Implications for different models and the timing of Zimbabwe Craton–Kaapvaal Craton amalgamation

U–Pb geochronological analyses have been conducted on 765 detrital zircon grains from 11 quartzite samples in the Central Zone of the Limpopo Complex, southern Africa, for constraining the depositional age and provenance of the protolith quartz-sandstone and evaluating the effect of high-grade metamorphism on zircon age data in polymetamorphic terranes. The age data have also been used to evaluate two contrasting tectonic models for the evolution of the complex; collision of the Zimbabwe and Kaapvaal Cratons and diapir-driven intracratonic orogeny, both during the Neoarchean. Magmatic cores of the zircons from the Beit Bridge, Phikwe, and Motloutse Complexes in the Central Zone show a wide age range from 3828 Ma to 1995 Ma. Ages from the Beit Bridge Complex in Zimbabwe (ca. 3.6–3.1 Ga) and South Africa (ca. 3.5–2.4 Ga) are dominantly Paleoarchean to Neoarchean with minor Paleoproterozoic ages. Similar Eoarchean to Neoarchean ages were also obtained from the Phikwe Complex (ca. 3.8–2.6 Ga), whereas ages from the Motloutse Complex are slightly younger (early Mesoarchean to Paleoproterozoic; ca. 3.0–2.4 Ga). Although the age patterns from the three complexes are different, the distribution patterns of discordant and concordant data suggest Eoarchean to Mesoarchean (ca. 3.8–3.1 Ga) ages for magmatic cores of detrital zircon grains. Homogeneous zircon rims around magmatic cores gave metamorphic ages of ca. 2.61 Ga and ca. 2.27–2.02 Ga, probably formed by Neoarchean and Paleoproterozoic high-grade metamorphisms. Similar Neoarchean (ca. 2.7–2.6 Ga) and Paleoproterozoic concordant ages obtained from magmatic zircon cores are interpreted as reset ages due to Pb loss during the metamorphic events. The smaller peak of the Paleoproterozoic age population compared to that of the Neoarchean age data suggests the dominant high-grade metamorphism of the Central Zone took place at ca. 2.7–2.6 Ga, either by collision of the Zimbabwe and Kaapvaal Cratons or by diapir-driven intracratonic orogeny. Therefore, cratonization of the Azanian (Kalahari) Craton probably occurred before ca. 2.6 Ga. Based on the age patterns, the depositional age of the protolith quartz-rich sandstone is constrained as 3.1–2.7 Ga. The provenance of the Paleoarchean to Mesoarchean (3.6–3.1 Ga) detrital zircons has been inferred as orthogneisses and granitoids of the southern Zimbabwe Craton (e.g., Tokwe Segment). The oldest Eoarchean detrital zircon (3828 Ma) obtained from the Phikwe Complex suggests the presence of unknown older crust in this provenance.

Provenance Analysis of the Ojén Nappe and Its Implication for the Geodynamic History (Alpujárride Complex, Betic Cordilleras, Spain)

The Ojén nappe, an allochthonous unit that underlies the Ronda peridotites (Betic Cordilleras), is composed of Triassic marbles overlying a metapelitic sequence. In order to carry out an LA-ICP-MS U-Pb age determination and provenance analysis, detrital zircon grains from two quartzites interlayered within marbles and metapelites were extracted. The obtained results yield the youngest zircon population of 254 ± 3 Ma (Late Permian) with a mean Th/U ratio of 0.34 that supports both the Permian–Triassic age of the marble member and the felsic magma nature for zircon-bearing protoliths. The Permian zircons exhibit tight age peaks in three main populations of 292 ± 2 Ma, 278 ± 3 Ma, and 254 ± 3 Ma, in agreement with the main age populations reported from rhyolites and shallow crustal basaltic–andesite subalkaline rocks emplaced in transtensional Permian basins of the Variscan Orogen during the break-up of Pangea. Moreover, the analyzed detrital zircon grains yield age distributions with clear Tonian–Stenian (ca. 950–975 Ma), Ediacaran–Cryogenian (ca. 560–615 Ma), Ordovician (ca. 460–465 Ma), and Cisuralian peaks (ca. 280–290 Ma) and smoothed Mesoproterozoic peaks that point to a weak and residual Cadomian peri-Gondwanan terrane inheritance. The identified Middle Ordovician detrital zircon populations (460–465 Ma) strengthen the hypothesis that the Alborán domain would be located along the southern passive margin of the European Hunic superterrane.

Multiple-Stage Neoproterozoic Magmatism Recorded in the Zhangbaling Uplift of the Northeastern Yangtze Block: Evidence from Zircon Ages and Geochemistry

The Yangtze Block records Neoproterozoic magmatism and sedimentation related to the breakup of Rodinia and is an important piece in the reconstruction of the supercontinent. However, the tectonic setting and position of this block in Rodinia remain a subject of debate. In the present study, we report the zircon U-Pb ages and Hf isotopic composition of zircon and geochemical and Nd-Pb isotopic compositions for meta-volcanic rocks exposed in the Zhangbaling uplift of the NE Yangtze Block. The volcanic rocks, dominated by rhyolite and dacite, belong to the calc-alkaline series and show geochemical characteristics of arc rocks. Zircon U-Pb isotopic ages show that volcanic rocks in the Xileng Formation formed at ca. 790 Ma and ca. 760–700 Ma peaking at ~740 Ma. The late-stage volcanism was widely exposed in the uplift, characterized by a temporal-spatial trend becoming younger southwards. The old volcanic rocks have low initial εNd (−11.0) and εHf (−19.7 to −8.2) values and low Pb isotopic ratios, likely indicating an origin from ancient basement rocks underneath the Yangtze Block. The younger ones, being similar to continental arc andesite in trace element compositions, have relatively high initial εNd (mostly −4.6 to 0.5) and εHf (−0.4 to 8.8) values and high Pb isotopic ratios. These isotopic features point to an origin from the partial melting of juvenile crustal rocks. Sedimentary rocks of the Xileng Formation and the overlying strata also contain numerous zircon grains of ~700 Ma to ~630 Ma. The volcanic rocks in the Zhangbaling uplift might demonstrate long-lasting subduction along the northeastern margin of the Yangtze Block, probably active until ca. 700 Ma.

Insights into the Crustal Evolution and Tungsten Mineralization of the West Cathaysia Block: Constraints from the Inherited Zircons from the Mesozoic Dengfuxian and Paleozoic Tanghu Plutons, South China

The formation and evolution of the ancient continental crust are crucial issues in solid-earth geology which are commonly associated with global tectonic events and the formation of economically valuable magmatic-hydrothermal ore deposits. The Cathaysia Block, one of the ancient continents in Southeast Asia, can be subdivided into two parts: the West Cathaysia Block and the East Cathaysia Block. Unlike the East Cathaysia Block, no Precambrian rocks are exposed in the West Cathaysia Block, constraining further understanding of the formation and evolution of this block. In this study, a total of four hundred and thirty-three zircon U-Pb dating analyses and two hundred and eighteen Lu-Hf isotopic analyses on zircon grains from the Jurassic Dengfuxian granites and Ordovician Tanghu granites, Nanling Range, were carried out. LA-ICP-MS zircon U-Pb dating yields mean average 206Pb/238U ages of 152.6 ± 2.2 Ma (MSWD = 1.6) and 442.4 ± 1.7 Ma (MSWD = 3.8), which are regarded as the rock-forming age for the Jurassic Dengfuxian granites and Ordovician Tanghu granites, respectively. The 207Pb/206Pb ages of the inherited zircons from the Jurassic Dengfuxian granites and Ordovician Tanghu granites range from 522 Ma to 3395 Ma, hosting two major peaks at the 0.9–1.0 Ga and 2.4–2.5 Ga. In contrast to the East Cathaysia Block, the West Cathaysia Block lacks the age peak of 1.8–1.9 Ga, indicating that the West Cathaysia Block was not influenced by the assembly of the Columbia supercontinent in the Paleo-Proterozoic. In combination with the Lu-Hf isotopes, we proposed that the crust evolution of the West Cathaysia Block in Archean is dominated by juvenile crustal growth events, and dominated by the crustal reworking since the Proterozoic. The long duration of crustal reworking in the West Cathaysia Block resulted in the enrichment of lithophile elements (e.g., W, Sn, Nb, and Ta) in the crust of that region. Therefore, the Jurassic granites in the Nanling Range, which are mainly derived from the partial melting of Proterozoic basement rocks, became associated with large-scale tungsten polymetallic mineralization.

Petrogenesis and magma fertility of the Heishishan skarn deposit, East Kunlun, NW China: Insights from geochronology, mineralogy, geochemistry, and Sr-Nd-Hf isotopes

The Heishishan skarn Cu-Pb-Zn deposit is located in the Wulonggou area, in the middle part of the East Kunlun Orogenic Belt. The ore body occurs at the contact between the Heishishan monzogranite and marble of the Paleoproterozoic Jinshuikou Group. U-Pb analysis of zircon grains shows that the monzogranite and granodiorite were formed at 241.7±1.0 Ma and 243.1±0.9 Ma, respectively. Bulk-rock analyses show that the monzogranite has similar chemistry characteristics to those of adakitic rocks, being rich in silicon, aluminum, potassium, sodium, and poor in iron and magnesium, with high Mg# values of 33.95–54.81. The monzogranite samples are enriched in large-ion lithophile elements such as Rb, Ba, U, K, and Sr, and depleted in high-field-strength elements such as Nb, Ta, Ti, and P, with a Sr to Y ratio of 41–63. The granodiorite samples are rich in silicon, potassium and sodium, poor in magnesium, and have a relatively low Mg# values of 30.88–32.61. The granodiorite is enriched in Rb, Ba, and U, and depleted in Th, Nb, Ta, Ti, and P. The whole-rock (87Sr/86Sr)t ratios of the monzogranite and granodiorite range from 0.709607 to 0.709738 and from 0.707084 to 0.707102, respectively; the whole-rock εNd(t) values range from –5.0 to –4.9 and from –3.1 to –3.0, respectively, and the zircon εHf(t) values range from –2.8 to +2.2 and from –1.8 to +1.2, respectively. The geochemistry and Sr-Nd-Hf isotope results imply that the monzogranite originated from crust-mantle magma mixing, whereas the granodiorite originated from partial melting of the lower crust, and both types were formed in a subduction tectonic setting. Estimates of magma fertility using mineral composition show that the monzogranite magma has a high oxygen fugacity, along with high water, S and Cl contents, thus showing strong metallogenic ability. By contrast, the granodiorite magma has a low oxygen fugacity and S content, thus showing weak metallogenic ability. The SO3 contents of the apatite from the monzogranite are much higher than those of the granodiorite, while the Cl contents show the opposite trend. This suggests that this is related to the exsolution of monzogranite magmatic fluid in the shallow crust. Thus, while the SO3 and Cl contents of the apatite are good indicators of the Cu fertility of the magma, the influence of fluid exsolution should also be considered. Determination of monzogranite as the ore-forming rock is achieved by using the spatial contact relationship between the orebody and the intrusive rock, along with the geochemistry and magma fertility. The formation time of the monzogranite (241.7 Ma) approximately represents the metallogenic epoch, which is under the subduction tectonic setting of the Paleo-Tethys Ocean. The metallogenic materials and fluids are mainly derived from the monzogranite magma. Additionally, the partial melting of the enriched mantle generated high fertility magma, which is critical to the formation of Heishishan skarn deposit.

Neoproterozoic amphibolite-facies metamorphism of the Douling complex in the northern Yangtze Craton and its tectonic implications: Constraints from petrology and zircon U-Pb-Hf-O isotopes

Neoproterozoic evolution of the Yangtze Craton has long been debated. The Douling complex, representing one of the oldest crystalline basements, records Neoproterozoic high-grade metamorphism and may provide pivotal clues into a reliable Neoproterozoic configuration of the Yangtze Craton. However, its metamorphic evolution and tectonic significance have not been well constrained. In this contribution, we conduct an integrated study of petrography, zircon U-Pb age and Hf-O isotopic compositions for two garnet amphibolites and one gneiss from the Douling complex. Phase equilibrium modeling and conventional geothermobarometry calculation define peak P-T conditions of 9–10 kbar and 670-690 °C, and shape a clockwise P-T path. Zircon grains from the gneiss mostly reveal core-rim structure. The cores display oscillatory zoning, high Th/U ratios and trace element contents, as well as HREE-enriched patterns, high 176Lu/177Hf ratios and δ18O values, consistent with a magmatic origin. They yield a weighted mean 206Pb/238U age of 868 ± 9 Ma, which is taken as the protolith age. The cores have strongly positive ƐHf(t) values of 7.0–15.6 and young two-stage Hf model ages of ca. 1310–760 Ma, indicating that the protolith was dominantly sourced from prompt reworking of Meso-Neoproterozoic juvenile crust. On the other hand, the zircon overgrowth rims in the gneiss exhibit no zoning patterns, low Th/U ratios and trace element contents, flat HREE patterns and pronouncedly negative Eu anomalies, variable 176Lu/177Hf ratios and comparable ƐHf(t) values to the magmatic cores, indicating their coeval formation with garnet and plagioclase under amphibolite-facies metamorphism and might be generated by dissolution-reprecipitation. Zircon crystals from the two amphibolites perform weak zoning patterns, low Th/U ratios and trace element contents, high formation temperatures, flat to weakly enriched HREE patterns, insignificantly and/or clear negative Eu anomalies, and low 176Lu/177Hf ratios and ƐHf(t) values, suggesting that they may form near the peak stage of amphibolite-facies metamorphism. Metamorphic zircon in the three samples yield homogeneously high δ18O values (10.81 ± 0.17‰, 11.45 ± 0.40‰, 10.92 ± 0.08‰), indicating the participation of external fluid originated from surrounding metasedimentary rocks. These metamorphic zircon grains together yield a weighted mean age of 817 ± 4 Ma (MSWD = 0.63), which is taken as the best estimated age for the upper amphibolite-facies metamorphism in the Douling complex. Considering the widespread contemporaneous magmatism in the northern Yangtze Craton, we propose that the Douling complex recorded a tectonic regime switch from compression to extension at ca. 830–820 Ma, in response to outboard oceanic subduction on the periphery of the Yangtze Craton and then tectonic extension and collapse of the thickened lithosphere.

Natural Experiment on the Extraction and Quenching of Rapakivi-like Magmas: Traces of Interaction with the Mafic Melts and Their Derivatives, Salmi Batholith (Karelia, Russia)

The origin of the anorthosite-rapakivi granite-bearing rock complexes was a topic of research for geologists for more than a century. Magmatic systems that produce these complexes were widely developed in the Precambrian and have no direct analogues in modern times. The main goal of this study was to characterize the conditions under which primitive granitoids formed and to shed light on the history of ore matter transport during these stages. The object of the research was granite porphyry dikes—a subeffusive analogue of rapakivi granite from the Salmi batholith in the Fennoscandian Shield. Characteristic solid-phase inclusions of a mineral paragenesis of mafic rocks, as well as inclusions of sulfide melts, were found in zircon from this type of rock. These sulfide inclusions were homogenized by heating, and subsequently, the trace element composition was determined by the LA-ICP-MS method. The geochemistry of zircons was studied by the SIMS method, and the temperature of their formation was determined using a Ti-geothermometer. The temperature of the studied zircon grain formation varied from 925 to 765 °C. The values of ƒO2 for the early stages of the rock formation were in the region below the boundary of the FMQ buffer. The presence of two sulfide melts at the time at which the sulfide inclusions were captured by the zircon, pyrrhotite-chalcopyrite, and sphalerite-chalcopyrite compositions was established. The capture of sulfide inclusions from the pyrrhotite-chalcopyrite composition occurred above 841 °C, while in sphalerite-chalcopyrite, they occurred at a lower temperature of 765 °C. The connection between the formation of granite porphyry with mafic melts that form rocks of the gabbro and anorthosite types was established.

Genesis of Makeng-type Fe-polymetallic deposits in SE China: New constraints by geochronological and isotopic data from the Dapai–Makeng metallogenic system

The southwestern Fujian depression belt (SFDB) is an economically important Mesozoic Fe metallogenic belt in South China and is renowned for its Makeng-type Fe deposits, in which stratified skarn Fe orebodies generally occur in or near the contact zone between late Paleozoic carbonate sequences and Mesozoic granites. However, the genesis and geodynamic setting of these deposits remain unclear because the characteristics of the widely distributed Pb–Zn–Cu and Mo orebodies in these deposits and the temporal, spatial, and genetic relationships between magmatism and mineralization are poorly defined. The Dapai Fe polymetallic deposit in the SFDB is a typical example of Makeng-type Fe deposits but also has regional significance, whereby the stratified skarn Fe orebodies have overprinted the stratabound Pb–Zn–Cu mineralization followed by final fissure-filling by vein-disseminated Mo mineralization. A detailed geological investigation suggests that episodic magmatic–hydrothermal events were involved in the formation process of the Dapai Fe polymetallic deposit.Pyrite and sphalerite from the Pb–Zn–Cu orebodies yield an Rb–Sr isochron age of 175.5 ± 3.3 Ma, which is regarded as the timing of Pb–Zn–Cu mineralization. Zircon grains from Fe-mineralized granodiorite porphyry and Mo-mineralized monzogranite yield weighted-mean 206Pb/238U ages of 146.3 ± 0.9 Ma and 131.7 ± 0.4 Ma, interpreted as the timings of Fe and Mo mineralization, respectively. Six zircons from granodiorite also yield a 206Pb/238U model age cluster of ∼184 Ma, which coincides reasonably with the timing of Pb–Zn–Cu mineralization and implies the existence of an unidentified ore-related intrusion in the Dapai deposit. Five further zircons from the porphyritic granodiorite yield an age cluster of ∼150 Ma, consistent with the timing of Fe mineralization. Galena, pyrite, and sphalerite from the Dapai and Makeng deposits have similar S–Pb isotopic compositions and suggest a magmatic source. Combining our results with published isotopic data for the SFDB, we suggest that the Pb–Zn–Cu mineralization in this area was derived from crustal magmas that mixed with mantle-derived magma prior to emplacement. The δ56Fe and δ57Fe values of magnetite from Dapai and Makeng are both slightly lower than those of the ore-related granites, suggesting that Fe in the initial fluid in both deposits was derived mainly from coeval granitic rocks. The Fe isotopic variation between intrusions and skarn Fe orebodies is interpreted as resulting from mass fractionation that occurred during fluid exsolution from melt. Contents of Re in molybdenite from published data for the SFDB indicate crust–mantle mixed sources of Mo and Re. The Makeng-type Fe polymetallic deposits formed as a result of three magmatic–hydrothermal episodes, generating Pb–Zn–Cu mineralization at 185–160 Ma, Fe–Mo mineralization at 150–140 Ma, and Mo–Fe mineralization at 135–130 Ma. The different metal associations formed during multiple stages of magmatism caused by ongoing subduction and rollback and/or retreat of the paleo-Pacific Plate.

Constraints on the timing of magmatism and rare-metal mineralization in the Fangzheng Rb deposit, Altai, NW China: Implications for the spatiotemporal controls on rare-metal mineralization

Amazonite granites are potential Rb resources given the abundance of Rb-bearing minerals they contain. The Altai Orogen, located in the southwestern portion of the Central Asian Orogenic Belt, is the world's most famous rare-metal metallogenic belt, containing numerous Li–Be–Nb–Ta–Cs deposits (e.g., the Koktokay No. 3 pegmatite deposit). The Fangzheng deposit, however, is the only known independent Rb deposit in this belt, with a proven Rb2O reserve of ∼12,940 tons at an average grade of 0.07%. The deposit is hosted by the Jiangjunshan pluton, which is composed largely of two-mica alkali feldspar granite and lesser amazonite granite on the peripheries, with amazonite pegmatite veins cutting the pluton and its wall-rocks. Although the pluton has been dated using various geochronological techniques, the magmatic and the rare-metal mineralization ages remain ambiguous given the complexity in zircon genesis and evolution, and their high Th and U contents. In this contribution, U–Pb geochronology of zircon, monazite, cassiterite, and columbite-group minerals, as well as their trace-element characteristics, are utilized to constrain the timing and evolution of magmatism and mineralization in the Fangzheng Rb deposit. Based on morphological and geochemical features, and cathodoluminescence images, zircon grains from different lithofacies are classified into three types — magmatic zircon, hydrothermally overprinted magmatic zircon, and hydrothermal zircon. The first two types of zircon grains are characterized by low Th/U ratios (0.22–4.00, with an average of 0.84) and ΣLREE contents (8.66–466, with an average of 90.6), indicative of a magmatic origin, whereas the third type is characterized by high Th/U ratios (0.22–7.26, with an average of 2.26) and ΣLREE contents (1154–4944 ppm, with an average of 2905 ppm), indicative of a hydrothermal origin. The magmatic and hydrothermally overprinted magmatic zircon grains from the two-mica alkali feldspar granite and amazonite granite yield U–Pb ages of 272.7±1.4 Ma and 272.5±1.7 Ma, respectively. Therefore, the earliest age of magmatism of the Jiangjunshan pluton is approximately 272 Ma (early–middle Permian). Based on the internal structure and geochemical characteristics of accessory minerals (monazite, cassiterite, and columbite-group minerals) selected from the ore bodies (amazonite granite and amazonite pegmatite), it is suggested that they crystallized during the magmatic–hydrothermal transition. The U–Pb ages of hydrothermal zircons (253.2±17.0 Ma), monazite (254.4±0.9 Ma), and columbite-group minerals (254.9±1.4 Ma) from the amazonite granite, as well as the U–Pb ages of monazite (253.3±0.7 Ma), cassiterite (250.4±1.4 Ma), and columbite-group minerals (254.5±0.8 Ma) from the amazonite pegmatite indicate that the age of rare-metal mineralization in the Fangzheng deposit is approximately 255–250 Ma (i.e., close to Permian/Triassic boundary). Combined with previous geochronological results and interpretations of tectonic evolution, it is evident that i) the middle Permian to Late Triassic is an important period of granitic magmatism and related rare-metal mineralization in the Chinese Altai, and ii) the rare-metal deposits formed in an intracontinental extensional setting after the tectonic transition from subduction to extension.

Melt-mediated re-equilibration of zircon produced during meltdown of the Chernobyl reactor

Abstract The mineral zircon is used widely to constrain the age of rocks and the processes that formed them. Although zircon is robust to a range of physical and chemical processes, it may show evidence for rapid re-equilibration that is generally considered to reflect interaction with hydrous fluids. Here, we show that zircon grains that crystallized from melt produced during the catastrophic meltdown of the Chernobyl nuclear reactor exhibit re-equilibration textures that occurred in an environment without free water. The process of re-equilibration involved a melt-mediated interface-coupled dissolution-reprecipitation that took place over a few days to produce textures that are commonly observed in igneous and anatectic systems. Thus, the composition of zircon can be modified even in the absence of hydrous fluids in a short time frame. Through this process, zircon crystals may track the timing of the last silicate melt they interacted with.

Hf isotopes in detrital zircon grains of the Sierra Albarrana Domain (SW Iberian Massif): Eburnean v. Archean basement signatures

This study presents Lu–Hf data on detrital zircon grains from the Lower Paleozoic metasedimentary successions of the Sierra Albarrana Domain (SW Iberian Massif). We provide new information about their origin, record of continental crustal evolution and geological affinity. Previous detrital zircon U–Pb data in this terrane reveal two main age populations, with age peaks at c. 595 Ma and c. 1.90 Ga. The Ediacaran events are interpreted to represent a magmatic arc with input of juvenile magmas intruding into the Eburnean basement of Gondwana, and probably mixing with it. The different evolutionary stages of the arc were probably linked to the Cadomian Orogeny during Neoproterozoic–earliest Cambrian times. The Paleoproterozoic zircon population corresponds to the Eburnean Orogeny. The magmas derived from an Eburnean depleted mantle partly intruded an older basement, leading to an incipient mixing process. ε Hf isotopic compositions indicate a possible affinity with the Central Iberian Zone, suggesting a common geological setting during Ediacaran–Cambrian times but different settings during the Paleoproterozoic. Supplementary material: A dataset of Lu/Hf analyses and a regional comparison of the ε Hf( t ) dataset with published data for metasedimentary rocks of the Iberian Massif are available at https://doi.org/10.6084/m9.figshare.c.6420215