Systematic Zircon Research Articles

Xenolith Petrochronology (San Luis Potosi, Mexico) Constrains Heat Sources for Cenozoic Ultrahigh‐Temperature Metamorphism in the Lower Crust

AbstractUltrahigh‐temperature (UHT; >900°C) metamorphism drives crustal differentiation and is widely recognized in the rock record, but its geodynamic causes are debated. Previous work on granulite‐facies metapelite xenoliths from San Luis Potosí, Mexico suggests the lower crust experienced a protracted UHT metamorphic event that coincided with the onset of regional extension. To determine the duration, conditions, and heat sources of UHT metamorphism recorded by these xenoliths, this study characterizes the major‐element, trace‐element, and U‐Pb isotopic systematics of quartz, rutile, feldspar, garnet, and zircon by in situ electron microprobe (EPMA) and laser‐ablation inductively coupled‐plasma mass spectrometry (LA‐ICP‐MS), and augments these data with detailed petrography, thermobarometry, phase equilibria modeling, and diffusion modeling. Thermobarometry and phase equilibria modeling suggest peak metamorphic conditions exceeded 0.7 GPa and 900°C. Zircon petrochronology confirms >15 Myr of UHT conditions since its onset at ∼30 Ma. A small population of zircon record elevated temperatures following transition from backarc compression to extension during the waning stages of orogenesis (60–37 Ma). Garnet preserves trace‐element zoning and mineral inclusions consistent with suprasolidus garnet growth and subsequent compositional modification by intracrystalline rare‐earth element diffusion during protracted heating, with diffusion chronometry timescales in agreement with zircon data, followed by fluid‐driven remobilization of trace elements along now‐healed fractures within ∼1 Myr of eruption. In sum, these data are most compatible with lithospheric mantle attenuation or removal as the dominant heat transport mechanism driving synextensional UHT metamorphism and crustal melting, which has bearing on models for crustal differentiation and formation of modern and ancient granulite terranes globally.

Using U-Pb detrital zircon systematics to constrain the architecture and provenance of the Ronda peridotites’ crustal hanging wall, S Spain

Detrital zircon geochronology is frequently used for sediment provenance analysis in orogenic belts, but it can also be applied to locate the boundary between basement and sedimentary cover in successions and tectonometamorphic units where the paleontological record is missing, and primary stratigraphic relationships have been erased by intense deformation and/or metamorphism. This is critical to differentiate metamorphic mineral associations, P-T conditions and deformational events associated with superposed orogenic cycles. In this work, detrital zircon geochronology has been applied to the Ronda peridotites’ crustal hanging wall (hinterland of the Alpine Betic Cordillera, southern Spain), apparently made up of two major tectonic units – the Jubrique unit and the Maláguide complex – separated by a major Alpine tectonic contact camouflaged in between low-grade metamorphic rocks.Our results show that pre-Paleozoic peaks (c. 650, 1000, 2000, and 2600 Ma) are common along the entire studied crustal sequence, and Paleozoic ages are only recorded in the upper part of the Jubrique unit (c. 290, 350, and 475 Ma) and in the Maláguide complex (c. 350 and 475 Ma). Thus, the Paleozoic rocks of the Maláguide complex, constituting the highest crustal unit overlying the Ronda peridotites, represent syn-to-late orogenic Variscan sediments deformed and slightly metamorphosed prior to the Alpine cycle. In contrast, the underlying Jubrique unit is made of a post-Variscan sedimentary cover deposited on a Variscan basement, with the contact between both (hidden unconformity?) located within low- to medium-grade schists. These data support the presence of a major tectonic contact between the Maláguide complex (Late Carboniferous maximum depositional ages) and the Jubrique unit (Early Permian maximum depositional ages), i.e. the Maláguide complex would have overthrust the Jubrique unit during the Alpine orogeny. These results indicate a Permian or younger exhumation/emplacement up to crustal levels of the Ronda peridotites subcontinental mantle slab.Altogether, these detrital zircon age spectra support a relatively, but not entirely local derivation of the studied crustal sequences (fragments of the Iberian Massif) and suggest some Mesozoic drifting with respect to its present position. Nonetheless, the different age distribution of the Paleozoic populations between the Jubrique (Alpujárride complex) and Maláguide samples suggests that the former deposited relatively close to other samples from the eastern-central Alpujárride complex and Nevado-Filábride complex, which, in turn, show strong similarities with the Cantabrian zone and NE Iberia (Iberian Massif). In contrast, the Maláguide samples were probably deposited close to the Ossa-Morena Zone (Iberian Massif) . Thus, Alpujárride/Nevado-Filábride and Maláguide complexes would have been juxtaposed during the Triassic-Cretaceous Neotethys rifting/drifting and/or the subsequent Alpine collisional events.

Zircon U–Pb ages and Hf and O isotope systematics of crustal zircons from Mesoproterozoic kimberlites of the Dharwar craton, India: Implications for Neoarchean craton assembly

Plate tectonic models for the amalgamation of the dominantly Paleo-Mesoarchean Western Dharwar Craton (WDC) and Neoarchean Eastern Dharwar Craton (EDC) remain inconclusive on the polarity of Neoarchean subduction. Here we present insitu U-Pb age and Hf-O isotopic compositions of xenocrystic zircons in ca. 1.4-1.1 Ga kimberlites and lamproites emplaced within the ca. 2.8-2.5 Ga EDC as deep probes of the crustal architecture across the WDC-EDC boundary. The different zircon populations yield wide range of ages extending back to ca. 3.6 Ga, including distinctly ‘kimberlitic zircons’ with a mean 206Pb/238U age of 1112 ± 42 Ma, consistent with previous age estimates for kimberlite magmatism. The presence of zircons with ages >3.33 Ga is surprising as this age group of zircons is atypical of magmatic rocks in the EDC, but wide-spread in the WDC. Also, the highest concentration of zircons with ages >3.0 Ga is found in the kimberlites closest to the notional boundary shear zone between the WDC and EDC. The Hf isotope systematics of a majority of magmatic zircons with ages between 2.75-2.50 Ga require melting of older (3.6-3.3 Ga) source rocks. O-isotope data for these Neoarchean zircons show a wide scatter, lie both above (δ18O to 13.5 ‰) and well below mantle values (δ18O to <2 ‰). δ18O values outside the mantle range are closely associated with low εHf. The data are interpreted as evidence for transportation of older upper crustal rocks to lower crust and mantle depths consistent with tectonic models invoking collisional under thrusting of WDC beneath EDC.

Lower crustal hot zones as zircon incubators: Inherited zircon antecryts in diorites from a mafic mush reservoir

Abstract Continental arcs are key sites of granitic magmatism, yet details of the origins of these magmas, including the role and contribution of mafic magma, the timing and location of initial zircon formation and how zircon isotopic signatures relate to granite formation, remain as challenges. Here we use U–Pb dating, trace elements and Hf isotopic systematics of zircon in mafic microgranular enclaves (MMEs), from the convergent plate margin Satkatbong diorite (SKD) in Korea to understand lower arc magmatism and zircon production. The host granitic body and MMEs display similar major element evolutionary trends and similar ranges of Sr, Nd and Hf isotopes, implying a cognatic relationship. Zircons show a large variability in ε Hf ( t ) ( c. 6 units) and age (&gt;30 Ma). We propose that the SKD and MMEs originated from the same, long-lasting, lower crustal mush reservoir, enabling long and variable residence times for zircons. Prolonged zircon ages, combined with the Hf isotope variability within a single pluton (SKD and its MME), indicate that not all zircons were instantaneously crystallized in a rapidly cooling shallow magma chamber but were continuously formed in a long-lasting hot source. A low-melt-fraction mush type reservoir in a deep crustal hot zone provides a viable model for the source setting. Continuous replenishment of mafic magmas acts as the main re-activator of the reservoir, and provide a critical role in spawning zircons that record a long age span, because (1) the magma adds Zr into the reservoir, enabling it to reach zircon saturation and (2) the generated zircon grains are transported upward as antecrysts by flow inside of the reservoir. This means that antecrysts with different ages may mix with each other in the ascending magma body. The significance of this model is that a conclusive time of intrusion cannot be constrained by such zircon ages, as these antecrysts constitute inherited grains.

Geochronology and geochemistry of basalts from the Yingchuan Formation, eastern Jiangnan Orogen: Implications for the Neoproterozoic tectonic evolution of the South China Block

The Neoproterozoic magmatism along the Jiangnan Orogen preserves significant information for understanding the tectonic evolution of the South China Block (SCB). We conducted a systematic zircon U–Pb geochronological and whole‐rock geochemical study on the basalts from the Yingchuan Formation in the eastern Jiangnan Orogen, SCB. The basalts yielded a weighted mean 206Pb/238U age of 817 ± 13 Ma, enriched in Rb, Th, U, LREE and HFSE (e.g., Zr and Ti). The basalts have positive εNd(t) (+1.9 to +2.1) and high (87Sr/86Sr)i (0.69853–0.70737), indicating a slightly depleted mantle source. The high La/Nb (2.14–2.35) and La/Ta (28.6–33.3) ratios indicate a lithospheric mantle source for the basalts. Their high Ba/Yb (7.85–44.3) and Th/Yb (0.59–0.74) ratios and high absolute Nb contents (7.64–11.6 ug/g) indicate that the lithospheric mantle source has been metasomatized by melts and fluids derived from a subducted slab. The basalts are characterized by (La/Yb) N ratios (3.0–3.5) and Sm/Nd ratios (0.24–0.26), comparable to intra‐continental back arc basin basalts (BABBs). Their Ta/Yb (0.14–0.16) and Th/Yb (0.59–0.74) ratios are similar to those of Okinawa Trough basalts and contemporary mafic rocks formed in the back‐arc basin in the Jiangnan Orogen. Combined with their source features, we propose that the basalts were formed in a back‐arc basin setting. The identification of BABBs from the Yingchuan Formation shows that there were Neoproterozoic back‐arc basins in the south‐eastern Yangtze Block, indicating that the final assembly between the Yangtze and Cathaysia blocks did not occur before ca. 817 Ma.

Zircon U-Pb Dating and Metamorphism of Granitoid Gneisses and Supracrustal Rocks in Eastern Hebei, North China Craton

Granitoid gneisses dominated by tonalitic–trondhjemitic–granodioritic (TTG) compositions, with metamorphic supracrustal rocks consisting of sedimentary and volcanic rocks, are widely exposed in the Eastern Hebei terrane, North China Craton (NCC). This study presents systematic zircon U–Pb geochronological and whole-rock geochemical data of the Neoarchean granitoid gneisses and supracrustal rocks in Eastern Hebei. Zircon U–Pb isotopic dating for the representative samples reveals that magmatic precursors of granitoid gneisses were emplaced between 2524 ± 7 and 2503 ± 12 Ma, and the protoliths of the pelitic granulites were deposited in the Late Neoarchean era. Both of them have been subjected to granulite facies metamorphism during 2508 ± 10 to 2468 ± 33 Ma, coeval with the intrusion of syenogranitic pegmatite (2488 ± 5 Ma). Zircon ages of 2.45–2.01 Ga obtained from the analyzed samples were considered mixed data from 2.53–2.48 Ga and 1.9–1.8 Ga and were chronologically meaningless. Paleoproterozoic metamorphic zircon ages of 1.9–1.8 Ga were usually neglected because of hardly being obtained from TTG gneisses and supracrustal rocks. The tectonic regime during the Neoarchean era was considered to be dominated by vertical tectonism in the Eastern Hebei terrane.

A review of U-Pb detrital zircon systematics from Cambrian- Lower Devonian rocks of the Moroccan Meseta

U-Pb geochronological analysis of detrital zircon grains is a powerful tool to decipher the pre-Mesozoic paleogeography. In this work, we present a sediment provenance study based on a compilation of 41 Cambrian-Early Devonian siliciclastic samples from the Moroccan Meseta. All of them are characterized by late Tonian-Ediacaran (c. 600 Ma, c. 50% of the data), Rhyacian-Orosirian (c. 2100 Ma, c. 20% of the data), and Neoarchean (c. 2500 Ma, c. 4% of the data)detrital zircon age populations indicating a strong West African Craton affinity. Minor populations are: (i) Cambrian-Ordovician (c. 500 Ma), particularly relevant in Lower Ordovician samples, which were probably sourced by local volcanic centers formed during an aborted rift; (ii) Stenian-early Tonian (c. 1000 Ma); and (iii) Orosirian-Statherian (c. 1800 Ma). These two latter populations were probably fed by paleocurrents from NE-African regions (e.g., Sahara Metacraton and Arabian-Nubian Shield) that reached only intermittently the studied area. Finally, the spatial and chronological distribution of the detrital zircon age populations suggests that the tectonic boundaries normally used to separate internal subdomains in northern Morocco did not imply far-travelled terranes during early-middle Paleozoic times.

Geochemistry and U-Pb-Hf zircon systematics of Cryogenian syn-rift magmatic rocks from the subsurface of the Tarim Craton: Implications for subduction-related continental rifting

Neoproterozoic rifting within the supercontinent Rodinia is generally linked to superplume activity, although the mechanisms are not well constrained at the supercontinent’s outer margins. The Tarim Craton, on the northern margin of Rodinia, is a key area for evaluating the process. We present zircon U-Pb-Hf isotopic data and whole-rock geochemistry from Neoproterozoic granitoids from drill cores in the Tarim Craton. Zircon SHRIMP geochronology yielded U-Pb ages of 744 ± 4 Ma, 750 ± 4 Ma, 755 ± 9 Ma and 734 ± 8 Ma for granitoid emplacement. The granitoids are enriched in light rare earth elements (LREE), depleted in high field strength elements (HSFE) including Nb, Ta, Sr, P and Ti, and most samples show weak negative Eu anomalies (Eu* = 0.56–1.02). Elemental data and negative zircon εHf(t) values (-18.72 to -10.23) indicate derivation from reworked continental crust. Compiled data suggest that ca. 760–730 Ma magmatism was widespread and marks the onset of extensive rifting throughout the Tarim Craton. The early Cryogenian granitoids are mainly slab failure-related and formed in a subduction-induced rift setting. Our findings consequently suggest that active continental rifting on the Tarim outer margin of Rodinia was triggered by subduction processes.

Combined U-Pb ages and Lu-Hf systematics of detrital zircons from Early Cambrian Gondwanan siliciclastic rocks in S Turkey: Provenance and correlations with coeval successions in peri-Gondwanan terranes

Detrital U-Pb age peaks and εHf(t) and TDM(Hf) of peri-Gondwanan units of S Turkey indicate that they may have been derived from Neoproterozoic igneous suites in the Sinai Peninsula (Egypt), Israel (Elat) and Eastern Desert (Egypt) part of the Nubian Shield. Early Cambrian siliciclastics in the peri-Gondwanan units of S Turkey consist of quartzites in the Menderes Massif of the Tauride-Anatolide Platform (TAP), meta-quartz arenites in the different Alpine tectonic slivers of the TAP-margin and quartz-arenites in the Southeast Anatolian Autochthon Belt (SAAB). Geochemical data indicate that these sediments are likely to be derived from sedimentary recycling processes in oxic depositional basins within a continental back-arc tectonic setting. They were opened after the formation of the Avalonian-Cadomian magmatic arc. In the TAP, the youngest detrital zircon 238U/206Pb ages are range from 530.6 + 6.3/-12.0 Ma to 549.4 ± 10 Ma and from 551.9 + 7.9/-8.4 Ma to 545.8 ± 11 Ma in the SAAB verifying the Early Cambrian maximum depositional ages for the sampled siliciclastic successions. In the TAP, meta-quartz arenites, Neoproterozoic detrital zircons make up about 75% of the total grains, whereas they constitute about 88 % in the SAAB sedimentary rocks. Tonian zircons in the TAP (>40 %) and SAAB (<32 %) reveal key populations to determine their provenance and the paleotectonic positions of the respective terranes. The combination of U-Pb age and Lu-Hf systematics in the detrital zircons of the TAP and SAAB show that age groups at 535 to 562 Ma and 888 to 3370 Ma are mainly represented by the zircons derived from evolved sources with a limited juvenile component. The predominant zircon age groups between 577 and 854 Ma in the TAP and SAAB, on the other hand, were derived from juvenile to evolved sources. Non-parametric Kolmogorov-Smirnov tests suggest that Early Cambrian successions in the TAP (except Kocaosman-Isparta) may have been located in similar paleogeographic locations with coeval successions in the SAAB during the Early Cambrian. U-Pb geochronology and Lu-Hf systematics of the Early Cambrian successions in the TAP and SAAB indicate that the dominant detrital zircon age groups were mainly sourced and transported to continental back-arc fluvial to shallow marine sedimentary environments from igneous rocks of the Nubian Shield and the Saharan Metacraton by a combination of the glaciers, eolian events and multiple-stage erosional and sedimentary recycling processes during the Early Cambrian (ca 520 Ma). The provenance of the Early Cambrian successions in the TAP and SAAB correlate with the provenance areas of the coeval siliciclastic sequences in Israel and Jordan (Arabian Shield), Iraq and Iran and the peri-Gondwanan Carpathians, rather than the Early Cambrian successions in the central and southern European peri-Gondwanan terranes.

U-Pb zircon and titanite age of the Christiansø granite, Ertholmene, Denmark, and correlation with other Bornholm granitoids

A granitic sample from the Danish island of Christiansø in the Ertholmene island group north of Bornholm is described petrographically and geochemically, and dated using U-Pb in zircon and titanite. Zircon systematics in the sample are complicated by abundant Pb loss and a large population of zircons interpreted as being inherited. Removal of highly disturbed zircons, imprecise analyses, and assumed inherited zircons yield an upper intercept date of 1500 ± 18 Ma (MSWD = 13, n = 58). Removal of zircons with high common Pb from this population yields an identical result of 1500 ± 22 Ma (MSWD = 8, n = 34). Zircons that are ≤3% discordant give a weighted average 206Pb/238U age of 1458 ± 12 Ma (MSWD = 3.0, n = 18), and a weighted average 207Pb/206Pb age of 1495 ± 14 Ma (MSWD = 4.7, n = 19). Titanites from the sample yield a lower intercept age of 1448 ± 15 Ma (MSWD = 6.8, n = 45). The sample contains a significant number of inherited grains indicative of ages around 1.7–1.8 Ga. The relatively large MSWDs for these age determinations indicate geological complexity, likely reflecting Pb loss, and the possible presence of inherited zircons which suffered major Pb loss during incorporation in the granitic magma. The zircon and titanite dates agree reasonably well with previous age determinations on felsic lithologies from the Bornholm mainland, as well as from the Blekinge Province of southern Sweden. Petrographically and geochemically, the Christiansø granite is indistinguishable from, and can be correlated with, the A-type granites and gneisses which occur on Bornholm. The high abundance of disturbed and inherited zircons (c. 1.7–1.8 Ga) may indicate that the granite was intruded into and assimilated a nearby region of unexposed Transscandinavian Igneous Belt rocks. The somewhat altered nature of the rock, and overall disturbance of U-Pb zircon systematics, suggest alteration associated with fluid-flow along nearby faults defining the northern margin of the Sorgenfrei–Tornquist Zone.

Forging isotopically juvenile metamorphic zircon from and within Archean TTG gneiss: Whole-rock Sr-Nd-Pb and zircon U-Pb-Hf-REE constraints

The Winnipeg River terrane is one of three plutonic-gneiss terranes that contains the oldest rocks in the Archean Superior Province and is integral to understanding the evolution of Earth's largest Archean craton. We evaluate the evolution of the Winnipeg River terrane using whole-rock Sr-Nd-Pb isotope data for a suite of 17 samples of the Cedar Lake gneiss, along with U-Pb-Lu-Hf isotopes and trace elements in zircon grains from one sample. Most whole-rock Sr-Nd-Pb isotope data trend along 3.25 Ga reference isochrons, which overlap with the dominant population of ca. 3.25 Ga zircon, and are interpreted to mark the igneous crystallization of the protolith to the Cedar Lake gneiss. An older population of zircon aged ca. 3.5–3.4 Ga indicate either Hadean or Eoarchean crust was reworked at ca. 3.25 Ga. At ca. 2.7 Ga, new zircon grew with limited dissolution of older grains resulting in near-chondritic time-integrated Hf isotope signatures and low Th/U ratios relative to the older grains. As a result of the overlap between whole-rock reference isochrons at 3.25 Ga and the dominant zircon population ca. 3.25 Ga, the radiogenic Hf isotope signature of the ca. 2.7 Ga zircon grains imply Hf was recycled from non-zircon constituents within the rock rather than from external input. Additional support is drawn from elevated Nb and Ta concentrations in ca. 2.7 Ga zircon, hornblende and biotite. Our data highlights a complexity in the LuHf isotope systematics of zircon, where metamorphic zircon inherited radiogenic Hf isotope compositions as a result of internal redistribution of Hf from non-zircon phases in the rock rather than juvenile input.

U-Pb Age and Hf Isotope Systematics of Zircon from Eclogite Xenoliths in Devonian Kimberlites: Preliminary Data on the Archaean Roots in the Junction Zone between the Sarmatian and Fennoscandian Segments of the East European Platform

The results of a laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U–Pb dating and a Hf isotope study of zircon crystals separated from small eclogite xenoliths found in Devonian kimberlites within the Prypyat horst, Ukraine, have been reported. The studied area is located in the junction zone between the Sarmatian and Fennoscandian segments of the East European Platform. Four laser ablation sites on two zircon grains yielded Paleoproterozoic U–Pb ages between 1954 ± 24 and 1735 ± 54 Ma. In contrast, three of four Hf sites revealed negative εHf values and Paleoarchean to Mesoarchean model ages, excluding the possibility that the eclogite xenoliths represented segments of a juvenile Paleoproterozoic subducted slab or younger mafic melts crystallized in the subcontinental lithospheric mantle. A single laser ablation Hf spot yielded a positive εHf value (+3) and a Paleoproterozoic model age. Two models for eclogite origin can be proposed. The first foresees the extension of the Archean lower-crustal or lithospheric roots beneath the Sarmatia–Fennoscandia junction zone for over 200 km from the nearest known outcrop of Archean rocks in the Ukrainian Shield. The second model is that the Central Belarus Suture Zone represents a rifted-out fragment of the Kola–Karelian craton that was accreted to Sarmatia before the actual collision of these two segments of Baltica.

Systematics of detrital zircon U–Pb ages from Cambrian–Lower Devonian rocks of northern Morocco with implications for the northern Gondwanan passive margin

The systematic acquisition of U–Pb geochronological data from detrital zircon grains has become an essential tool in tectonic studies focused on reconstructing the pre–Variscan geography of the northern Gondwanan passive margin. New detrital zircon ages for 16 samples from the Cambrian–Lower Devonian succession of the Moroccan Mesetas (northern Morocco) are reported here. The results, combined with previously published data, reassert the strong West African Craton affinity of the Paleozoic sedimentary rocks, characterized by dominant Cadomian/Pan–African (c. 850–540 Ma) and Eburnean (c. 2.2–1.9 Ga) detrital zircon populations and a minor Leonian/Liberian (c. 2.5 Ga) population. Primary sources of these zircon grains are well established as the West African Craton located just to the south, but also in the Precambrian basement that locally crops out in the Moroccan Mesetas themselves. During the Cambrian–Early Ordovician, erosion preferentially dismantled Cadomian (c. 590–540 Ma) arc–derived rocks of the Gondwanan continental margin, while later, the slightly older Pan–African (c. 650–600 Ma) basement became the main sediment source. In the studied samples, irregularly present minor detrital zircon populations suggest additional sediment provenance from secondary sources such as: (i) remote northeastern African cratons (e.g., Saharan Metacraton and/or Arabian–Nubian Shield) that likely could have provided the c. 1.1–0.9 Ga and, possibly, the c. 1.9–1.7 Ga zircon grains, and (ii) rift–related Cambrian–Early Ordovician volcanic centers in the Moroccan Mesetas that supplied heterogeneously distributed – although locally dominant in small areas – sedimentary detritus before rift abortion and burial underneath the overlying passive margin sedimentary succession.

Beach placers of southwestern India: An archive of Precambrian supercontinent growth histories

The southwestern coast of India is known for its rich beach placer deposits. So far, there have been no substantial attempts to determine the provenance of these deposits on a regional scale. Here we present for the first time geochronology, trace element chemistry and Hf isotope systematics of zircon and monazite from representative beach placer deposits along the southwestern coast of India to constrain provenance. Detrital zircon grains display a wide variety of ages, with modes in the ranges of 650–450 Ma, 1100–650 Ma, 2300–1600 Ma, 2800–2300 Ma and 3500–2800 Ma. A high proportion of ages fall within the Neoproterozoic–Late Cambrian range (1066–490 Ma). Growth zoning in zircon grains shows magmatic growth of many generations with ages affected by Neoproterozoic Pb loss, along with overgrowths typical of those formed during high-T metamorphism. Grains of metamorphic zircon, along with grains of monazite from the same samples of sediment, have strong modes between 570 and 470 Ma, indicating derivation from sources in the Southern Granulite Terrane affected by a major Late Neoproterozoic to Cambrian tectonothermal event. Mineral geochemistry of zircon grains indicates continental derivation, especially with older zircon from TTG-type sources. The geochemistry of late Neoproterozoic–Cambrian zircon and monazite are consistent with derivation from amphibolite to granulite-facies source rocks. Zircon ages and Hf isotope systematics can be ascribed to provenance from various source terranes exposed along the Western Ghats. This young rift-flank escarpment formed along the coast at a high angle to the distribution of different Precambrian terranes in the Indian Peninsula. Erosion promoted by a tropical climate and a steep coastal geomorphology, in combination with strong southward along-shore ocean currents, led to the concentration and distribution of heavy minerals from multiple sources along southwestern coast of India. The relative intensity of age modes in detrital zircon differs between localities, demonstrating differing inputs from proximal sources (from rivers dissecting steep terrain) versus distal sources from more northerly cratons and terranes brought to deposition sites by along-shore drift.

U-Pb-Hf isotopic systematics of zircons from granites and metasediments of southern Ouaddaï (Chad), implications for crustal evolution and provenance in the Central Africa Orogenic Belt

We present the first combined U-Pb ages and Lu-Hf isotope data of zircon grains from the southern Ouaddaï area in Eastern Chad in the Central Africa Orogenic Belt. Zircon xenocrysts from granites and detrital zircons from quartzites of the Ouaddaï document crustal accretion from depleted mantle sources at 3.0 ± 0.5 Ga, rarely up to 3.8 Ga, and for subsequent reworking during four magmatic events, during the Neoarchean at ca. 2.6 Ga (εHft = −7.0 to 4.6), Paleoproterozoic at ca. 1.9–2.1 Ga (εHft = −17.7 to 5.3), Mesoproterozoic at ca. 1.1 Ga (εHft = −15.3 to 5.9), and Neoproterozoic/early Cambrian at 635–540 Ma (εHft = −31.1 to 1.4). Superchondritic εHft values point to new crust addition during all four events. Metamorphic zircon rims yield a Concordia age of 600 ± 2 Ma, and provide evidence that greenschist to amphibolite-facies metamorphism affecting the quartzites occurred between syn-tectonic (635 Ma) and post-kinematic (540 Ma) granite intrusions. Our new set of zircon data, combined with field relationships, also indicate that the quartzite precursor rocks were deposited between 1045 and 635 Ma, possibly after 670 Ma in a basin that subsequently became the Central Africa Orogenic Belt. The detritus was supplied from surrounding cratons, although the sources of the Mesoproterozoic zircons remain ambiguous. Comparison of age-εHft data from worldwide sources suggests sediment supply from the Irumide, Katanga and Mozambique belts, located to the south-east of the Congo Craton. Paleogeographic reconstructions further require sediment transport prior to final closure of several oceanic basins surrounding Central and Northern Africa during the Neoproterozoic, comprising the Khomas and Mozambique oceans

Precise initial abundance of Niobium-92 in the Solar System and implications for p-process nucleosynthesis

The niobium-92-zirconium-92 (92Nb-92Zr) decay system with a half-life of 37 Ma has great potential to date the evolution of planetary materials in the early Solar System. Moreover, the initial abundance of the p-process isotope 92Nb in the Solar System is important for quantifying the contribution of p-process nucleosynthesis in astrophysical models. Current estimates of the initial 92Nb/93Nb ratios have large uncertainties compromising the use of the 92Nb-92Zr cosmochronometer and leaving nucleosynthetic models poorly constrained. Here, the initial 92Nb abundance is determined to high precision by combining the 92Nb-92Zr systematics of cogenetic rutiles and zircons from mesosiderites with U-Pb dating of the same zircons. The mineral pair indicates that the 92Nb/93Nb ratio of the Solar System started with (1.66 ± 0.10) × 10-5, and their 92Zr/90Zr ratios can be explained by a three-stage Nb-Zr evolution on the mesosiderite parent body. Because of the improvement by a factor of 6 of the precision of the initial Solar System 92Nb/93Nb, we can show that the presence of 92Nb in the early Solar System provides further evidence that both type Ia supernovae and core-collapse supernovae contributed to the light p-process nuclei.

U–Pb geochronology and Hf isotope systematics of detrital zircon from the late Ediacaran Kalyus Beds (East European Platform): palaeogeographic evolution of southwestern Baltica and constraints on the Ediacaran biota

A maximum depositional age (MDA) of 551.2 ± 4.2 Ma for the black argillites of the Kalyus Beds, Nagoryany Formation (East European Platform, northeastern Republic of Moldova) is reported based on the youngest cluster of detrital zircons dated by LA-ICP-MS. Their source can be linked to an arc-related volcanism that fed the tuffite layers interbedded in the basal part of the Kalyus Beds and reasonably support our inference that the reported MDA is consistently compatible with the true depositional age (TDA) of the Kalyus Beds. The volcanic event that led to ash deposition for the tuffites may be related to the active margin processes which developed adjacent to the Baltica palaeocontinent. Hf isotope values of two detrital zircons from the youngest cluster suggest mantle input, and a third detrital zircon was derived from felsic rocks that originated from re-melting of the older crustal rocks. The older detrital zircon clusters correlate well with known published ages of the different lithotectonic units from the East European Platform. The precise geochronological U–Pb ages, including the Kalyus detrital zircon age, offer outstanding data to calibrate the sedimentary and volcanic events, and the evolution of the Ediacaran biota in the Moldova-Podillya Basin, specifically the Miaohe biota biozone, for which a few newly identified taxa significantly increase its taxonomic diversity. The anoxic conditions together with the associated phosphorites that controlled the deposition of the Kalyus argillites represent a major depositional event in this area of the Baltica palaeocontinent, and can be correlated with other worldwide anoxic events during the late Ediacaran.

Ubiquitous post‐peak zircon in an eclogite from the Kumdy‐Kol, Kokchetav UHP‐HP Massif (Kazakhstan): Significance of exhumation‐related zircon growth and modification in continental‐subduction settings

AbstractU–Pb geochronological, trace‐element and Lu–Hf isotopic studies have been made on zircons from ultrahigh‐pressure (UHP) mafic eclogite from the Kumdy‐Kol area, one of the diamond‐facies domains of the Kokchetav Massif (northern Kazakhstan). The peak eclogitic assemblage equilibrated at &gt; 900 °C, whereas the bulk sample composition displays light rare‐earth element (LREE) and Th depletion evident of partial melting. Zircons from the eclogite are represented by exclusively newly formed metamorphic grains and have U–Pb age spread over 533–459 Ma, thus ranging from the time of peak subduction burial to that of the late post‐orogenic collapse. The major zircon group with concordant age estimates have a concordia age of 508.1 ±4.4 Ma, which corresponds to exhumation of the eclogite‐bearing UHP crustal slice to granulite‐ or amphibolite‐facies depths. This may indicate potentially incoherent exhumation of different crustal blocks within a single Kumdy‐Kol UHP domain. Model Hf isotopic characteristics of zircons (εHf(t) +1.5 to +7.8, Neoproterozoic model Hf ages of 1.02–0.79 Ga) closely resemble the whole‐rock values of the Kumdy‐Kol eclogites and likely reflect in situ derivation of HFSE source for newly formed grains. The ages coupled with geochemical systematics of zircons confirm that predominantly late zircon growth occurred in Th–LREE‐depleted eclogitic assemblage, that experienced incipient melting and monazite dissolution in melt at granulite‐facies depths, followed by amphibolite‐facies rehydration during late‐stage exhumation‐related retrogression.

High-Potassium Volcanism Approximately 640 Ma in the Southwestern Siberian Platform (Biryusa Uplift, Sayan Region)

On the basis of petrographic and mineralogical studies, we have established the presence of clastic rocks with a strong predominance of K-feldspar among the rock-forming fragments within the Late Precambrian sedimentary sequence in the southwestern part of the Siberian Platform. Two types of mineralogical occurrence of K-feldspars are determined: (1) huge zonal crystal clasts with increased Ba concentrations in the central parts of the grains and (2) the main mineral phase in the form of a decrystallized glassy mass. In both cases, low concentrations of Na (lower than 0.1 wt %) are detected. K-feldspars of the second type contain intergrowths of idiomorphic rhombic dolomite with a high ankerite component. Dolomite grains contain inclusions of K-feldspar. The prevailing accessory minerals are F-apatite (with high concentrations of REEs), zircon (with high concentrations of Th), magnetite, rutile, monacite, and sinchizite. Encasement minerals with an idiomorphic shape are identified, with K-feldspar being located in the center, while the middle shell is formed by apatite with a high REE content, and the outer shell is formed by apatite without rare earth elements. These rocks are products of high-potassium volcanic activity. The age of this event has been established on the basis of U–Pb zircon dating to about 640 Ma. The Lu–Hf zircon systematics for these rocks indicates the connection of volcanism with igneous events of mantle genesis within its range. The products of explosive eruption, which are widespread within the Biryusa uplift of the Siberian Platform, were erroneously considered earlier as Riphean sedimentary rocks of the Karagas Series.

Inherited or not inherited: Complexities in dating the atypical ‘cold’ Chopok granite (Nízke Tatry Mountains, Slovakia)

Zircon U-Pb SIMS dating combined with in-context (in thin section) monazite and xenotime U + Th-total Pb dating was used to clarify the Palaeozoic evolution of the ‘cold’ Chopok granite (Nízke Tatry Mountains, Slovakia). Four distinct zircon, monazite and xenotime age domains testify to a prolonged evolution from igneous formation to multi-stage metasomatism and hydrothermal overprinting. The geological interpretation of age patterns from ‘cold’ granites, expected to have low zircon saturation temperatures (<800 °C) and relatively high amounts of zircon inheritance, requires special care, especially for what concerns proper attribution of zircon inheritance and igneous growth ages. These issues can be resolved using zircon saturation temperatures (TZrn) as proxy for the amount of zircon inheritance in combination with the temperature differences between TZrn and the granite solidus. In this respect, the Chopok granite is an atypical ‘cold’ granite. Due to TZrn being substantially lower (ca. 80 °C ± 50 °C) than the granite solidus temperature, practically no zircon inheritance was found. The zircon age data indicates that the Chopok granite is a product of an Early Ordovician (475.8 ± 3.3 Ma) magmatic event, corresponding with the widespread Early Palaeozoic magmatism recorded throughout the European Variscan belt. This is further corroborated by phosphate mineral ages. The post-magmatic activity recorded in the U-Pb systematics of zircon and phosphates overgrowths can be related to the different phases of the evolution of the Variscan orogen: Early Carboniferous (ca. 352 Ma) metasomatism documents the main Variscan orogenic event, whereas the Permo-Triassic age (ca. 255 Ma) reflects thermo-tectonic activity associated with large-scale crustal extension, contemporaneous with the initial continental leading to the break-up of Pangea.