Crystallization Ages Research Articles

Mafic Alkaline Magmatism and Rare Earth Element Mineralization in the Mojave Desert, California: The Bobcat Hills Connection to Mountain Pass

AbstractOccurrences of alkaline and carbonatite rocks with high concentrations of rare earth elements (REE) are a defining feature of Precambrian geology in the Mojave Desert of southeastern California. The most economically important occurrence is the carbonatite stock at Mountain Pass, which constitutes the largest REE deposit in the United States. A central scientific goal is to understand the genesis of the carbonatite ore body in the context of widespread REE‐rich igneous activity. A swarm of mafic alkaline (shonkinite) dikes has been mapped and sampled at Bobcat Hills, 65 km southeast of the Mountain Pass mine. Whole‐rock geochemistry and zircon geochronology demonstrate a clear affinity to the ca. 1.4 Ga Mountain Pass intrusive system. Bobcat Hills dikes have comparably high REE concentrations (La ∼1,000× chondritic) and an error‐weighted mean 207Pb/206Pb zircon crystallization age of 1,426 ± 2 Ma (2σ). Unlike the alkaline intrusions at Mountain Pass, which have abundant inherited zircon from Paleoproterozoic basement rocks and crustally influenced oxygen isotope compositions (δ18Ozircon = 6.5–7.5‰), the Bobcat Hills dikes lack any evidence of crustal assimilation and have oxygen isotope values that overlap a mantle range (Bobcat Hills average δ18Ozircon = 5.6 ± 0.3‰). The dikes were a high‐temperature, early center of mafic alkaline magmatism in the Mojave Desert that serve as a snapshot of melt generation from a spatially extensive, metasomatized mantle source. We propose that modification of the crust over many tens of Myr at Mountain Pass created an environment that favored crustal assimilation and enabled ascent of late‐stage, REE‐rich carbonatite magmas.

Petrogenesis and tectonic implications of the carboniferous El Carmen Pluton, central Colombian Andes: insights from whole-rock and mineral geochemistry, in situ zircon Lu–Hf isotopes and U–Pb geochronology

ABSTRACT During the initial stages of Pangea amalgamation in the Carboniferous period, the Rheic Ocean basin experienced subduction prompted by the convergence between Gondwana and Laurasia, culminating in the basin’s closure that preceded the paleocontinent collision. This tectonic episode promoted the Carboniferous (Mississippian and Pennsylvanian) tholeiitic to calc-alkaline magmatism, represented in the Colombian Andes through the tonalite and diorite-gabbro facies of the El Carmen Pluton. We investigate the two main facies (felsic and mafic facies) of the plutonic body, which crop out in the Central Cordillera of the Colombian Andes. U-Pb zircon crystallization ages are 326 ± 1 Ma for the mafic facies and 313 ± 1 Ma for the felsic facies. Positive ɛHf(i) isotopes ranging from +5.4 to +10.9 suggest mantle-derived magma with low crustal assimilation. The progressive increase in Eu anomaly (Eu × 0.64 to 1.15) and high LREE/HREE ratios underscore a continental magmatic arc formation in congruence with a subduction context. Thermobarometric analyses of mineral geochemistry across both facies reveal the magmatic emplacement at temperatures spanning 753–910°C, pressures between 3 and 4.65 kbar, and depths approximating 9–14 km. The simultaneous presence of two magmatic facies bearing tholeiitic and calc-alkaline characteristics, separated by ~13 Ma crystallization gap, infers a magmatic recharge episode. The results highlight a slightly thickened crust due to the arc maturity, or a transition from shallow-dipping to steep-angle subduction, consistent with tectonic slab-rollback mechanisms. The modest crustal thickening, indicative of low arc maturation between 326 and 313 Ma, is concomitant with the closure of the western part of the Rheic Ocean basin.

New U-Pb and Hf data of the contact region between the Oriental and Cabo Frio terranes, Central Ribeira Belt, Brazil: Implications for the closure of the Ediacaran-Cambrian oceanic space

The Ribeira Belt represents a deeply eroded orogen which exposes rocks from the different stages of magmatic arcs and associated metasedimentary basins. This contribution focus on the last collisional episode between the Outer Magmatic Arc System (OMAS) and the Cabo Frio Terrane (Angola craton). The OMAS represents a long-lived magmatic arc that collided against the Inner Magmatic System (IMAS) at ca. 590 Ma (M1 metamorphism), and against the Cabo Frio Terrane at ca. 530 Ma (M2 metamorphism). The nature of the magmatism recorded in the first collisional episode, and the subduction polarity of the second is a current topic of debate. New U-Pb age and Lu-Hf analyses in zircon grains from units close to the contact are presented to propose a tectonic model for the closure of the oceanic space between the OMAS and the Cabo Frio Terrane. The results in combination with published data suggest that the metasedimentary rocks of the São Fidélis Group are in a back-arc position of the OMAS and were affected by both the M1 and M2 metamorphic episodes, reflected by the presence of metamorphic zircons with ages between ca. 592–553 Ma and 543–533 Ma. The orthogneiss of the Tinguí Complex intrusive in the back-arc metasedimentary units yielded a crystallization age of 578 ± 9 Ma, coeval with the syn-collisional stage M1. A younger metamorphic overprint at 529 ± 7 Ma was also detected (syn-M2). The orthogneiss of the Região dos Lagos Complex (Cabo Frio Terrane) rendered a crystallization age of 1968 ± 26 and a M2 metamorphic overprint at 522 Ma. Zircon analyses from folded leucosomes in the orthogneiss also indicated a crystallization age of 523 ± 10 Ma, which constrain the time of folding after thrusting. Undeformed post-collisional granitoids and pegmatites yielded crystallization ages between 496 and 499 Ma, closing the tectonic evolution of the belt with the orogen collapse. Our results corroborate with terrane accretion model for the Central Segment of the Ribeira Belt.

Late Neoarchean-early Paleoproterozoic K-rich granitoid rocks and mafic dykes from the Liaodong Bay Depression, Bohai Sea Basin: Implications for cratonization of the North China Craton

Late Neoarchean to early Paleoproterozoic potassic granites and mafic dykes are widely distributed across the Eastern Block (EB) of the North China Craton (NCC), which provide important information about the geodynamic origin and tectonic evolution of Archean terranes. Here, we are first to report the zircon U–Pb–Hf isotopic and whole-rock compositions of potassic granites and mafic dykes from the Liaoxi Rise of the Liaodong Bay Depression (LBD) in the offshore Bohai Sea Basin (OBSB) of the northeastern NCC. The potassic granites have crystallization ages of 2499–2460 Ma and are characterized by variable SiO2 content (63.2 to 72.3 wt%) and high K2O (3.27–6.88 wt%). In addition, they show geochemical features akin to post-collisional related granites and possess positive zircon εHf(t) values of 0.9 to 4.95, suggesting that they mainly originated from the partial melting of high-K mafic rocks and formed under the post-collisional setting. The mafic dykes have a crystallization age of 2485 Ma. They show high Nb (8.64–9.39 ppm) and high Nb/U ratio (10.3–14.8), as well as radiogenic initial zircon Hf isotopes (﹢1.26–﹢4.39). These values indicate affinities with Nb-enriched basalts and that they originated from mantle-derived magma under a subduction-related back-arc extensional setting. Combined with previous research results, we believed that magmatic rocks of the LBD area may have formed under remote effects of oceanic slabs being subducted from the northwest to southeast of the eastern Hebei–western Liaoning Province back-arc basin system during the late Neoarchean to early Paleoproterozoic. Meanwhile, the K-rich granitoid rocks and coeval mafic dyke sequences imply the increased maturity of the continental crust and late Neoarchean to early Paleoproterozoic basement initiation of cratonization in the NCC.

Paleoarchean to Neoproterozoic crust formation and migmatization events in the Borborema Province, NE Brazil: Implications for the growth and reworking of the continental crust

The Patos Shear System (PSS) is a 20–40 km wide domain of anastomosing shear zones intercalated with non-mylonitic segments at the core of the Borborema Province, a transpressive Neoproterozoic orogen in NE Brazil, part of the reworked São Francisco Craton. We provide new U-Pb, Lu-Hf and Sm-Nd analyses for non-mylonitic basement migmatitic orthogneisses. Four magmatic events (∼3.40, 3.26, 2.68–2.64 and 2.18 Ga), as well as three metamorphic/migmatization ones (∼3.0, 2.1–2.0 Ga and 570–575 Ma) were evidenced. Metamorphism and leucosome injection at ∼570–575 Ma attest the PSS was subjected to high heat from medium- to high-temperature dextral shearing. A distinct Paleoproterozoic (∼2.1–2.0 Ga) migmatization event is probably related to the amalgamation of Neo-Paleoarchean blocks and an older ∼3.0 Ga metamorphic event may represent crustal thickening induced by the collision between small Paleoarchean landmasses. Paleoarchean magmatic protoliths obtained here were emplaced concomitant with TTGs and high-K rocks regionally, and our Hf data from these rocks, after filtering for isotopic disequilibrium during anatexis, combined with a large compilation, point to an apparent mixing of magma derived from a CHUR-like reservoir with older crust, better reconciled, although not conclusively, with a stagnant-lid scenario dominated by plume activity. Hf data from 2.68 to 2.64 Ga old migmatites plot along a conspicuous Meso-Neoproterozoic “reworking array” and our preferred model is that magmatism between 2.9 and 2.6 Ga took place in an accretionary/collisional continental setting, following renewed juvenile additions between ∼3.2 and 2.9 Ga. At last, the crystallization age of ∼2.18 Ga of an orthogneiss is related to another Wilson Cycle. Our data point to reworking of Archean lithologies during the Paleoproterozoic, as evidenced by inherited Archean zircon grains, but ƐHf and ƐNd close to CHUR suggest involvement of more juvenile material as well, consistent with regional data, and the preferred scenario is that of subduction followed by collision.

U-Pb geochronology and petrography of Neoproterozoic to early Cambrian volcanic rocks in basement crustal terranes beneath the deep-water Gulf of Mexico

Abstract Though the Gulf of Mexico has been studied for more than a century, the lithology, age, and origin of the basement crustal terranes remain poorly understood. New U-Pb zircon ages of a volcanic sample in the DeSoto Canyon 535 #1 Raptor well were obtained by in situ laser-ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) and resulted in a crystallization age of 509 ± 23 Ma, indicating the crustal terrane beneath the northeastern Gulf of Mexico is likely of peri-Gondwanan affinity. This basement may be associated with the felsic volcanism related to Neoproterozoic to Cambrian subduction along the supercontinent’s western margin. This subduction-related volcanism likely represents the last pulses of magmatic activity before the west coast of Gondwana transitioned into a passive margin ca. 500 Ma. The data presented here for the DeSoto Canyon 535 #1 Raptor trachyte represent the oldest radiometrically dated evidence of volcanism in the present-day deep-water Gulf of Mexico.

Tonian rifting coeval with the last stages of amalgamation of Rodinia: Evidence from southern Borborema province, northeastern Brazil

During the last stages of assembly of the Rodinia supercontinent at 1000-900 Ma, rifting processes were occurring in places that would be part of Gondwana in the late Neoproterozoic. In the Borborema Province, northeastern Brazil, this event is known as Cariris Velhos. It is represented in the Alto-Pajeú Domain from the Central subprovince by a 700 km-long, 70 km-wide NE-SW belt comprising bimodal metavolcanic rocks and granitic orthogneisses. Orthogneisses with similar ages were found in previous works in the western Pernambuco-Alagoas Domain from the Southern subprovince immediately to the south of the ESE-trending dextral West Pernambuco shear zone. The Airi Complex comprises several elongate bodies that extend for at least 80 km. It consists of a bimodal sequence of metafelsic (granitic orthogneisses) associated with layers and enclaves of metamafic (amphibolites) rocks. Seven orthogneiss samples dated by LA-ICPMS yielded zircon U–Pb crystallization ages of 950–1011 Ma. Zircon Hf analyses from two samples yielded εHf(t) values ranging from −1.4 to 2.1 and TDM model ages of 1.7–1.9 Ga. Geochemical analyses allow to distinct different source rocks for the protoliths of the orthogneisses. Orthogneisses derived from metasedimentary sources have modal muscovite ± garnet, are ferroan, alkali-calcic and predominantly peraluminous, exhibiting strong negative Eu anomalies in chondrite-normalized rare earth elements diagrams. Orthogneisses derived from high-K mafic rocks are calc-alkalic, slightly peraluminous, with small or absent negative Eu anomalies. Orthogneisses from tonalitic sources are magnesian, calc-alkalic to alkali-calcic, and slightly peraluminous, without Eu anomalies. There is no correlation between crystallization ages and source rock types. The results indicate that the Airi Complex originated through partial melting of ancient heterogenous crustal sources. Therefore, its geochemistry reflects the nature of these sources, not the tectonic setting of intrusion. Emplacement of the complex was broadly coeval with well-documented extensional processes in the São Francisco/Congo Craton and represents a failed attempt to break-up a larger paleocontinent encompassing this craton and the basement of the Borborema Province.

Baddeleyite microstructural response to shock metamorphism in three enriched shergottites and implications for U–Pb geochronology

Baddeleyite (monoclinic zirconia; m-ZrO2) occurs as a late-stage accessory mineral in shergottites and has been used to determine U–Pb igneous crystallization ages via in-situ secondary ion mass spectrometry (SIMS). During shergottite ejection from the surface of Mars, baddeleyite develops a range of microstructures primarily due to a series of shock-induced transformations to high pressure and temperature polymorphs. It remains poorly constrained to what extent U–Pb systematics in baddeleyite are sensitive to shock conditions experienced by shergottites. To investigate this, we examined baddeleyite in the enriched shergottites Jiddat al Harasis (JaH) 479, Northwest Africa (NWA) 10299, and NWA 12919, which bridge the gap in shock conditions represented in previous microstructural studies. Electron backscatter diffraction (EBSD) analysis reveals that although some baddeleyite grains retain magmatic microstructures (i.e. homogenous crystallographic orientations and twinning of igneous origin), there is widespread phase transformation to high-pressure orthogonal polymorphs (o-ZrO2) followed by reversion. JaH 479 contains more grains with preserved magmatic microstructures than the other two shergottites, suggesting that it experienced lower bulk shock pressures. Nanometer-scale reverted m-ZrO2 in NWA 10299 and NWA 12919 further points to insufficient post-shock temperatures; this contrasts with JaH 479 where greater variation in local temperature conditions enabled the development of µm-scale domains of reverted m-ZrO2. Individual grains that are separated into two distinct microstructural domains may reflect controls on shock propagation due to relative density contrast among the surrounding phases.SIMS U–Pb baddeleyite analysis yields igneous crystallization ages of 210 ± 9 Ma (JaH 479), 196 ± 11 Ma (NWA 10299), and 188 ± 11 Ma (NWA 12919). At the SIMS resolution, we find no clear evidence for significant Pb loss in the surveyed baddeleyite grains, suggesting that temperatures during the formation of both nm-scale and µm-scale reverted m-ZrO2 in the three shergottites were insufficient to cause significant Pb diffusion. Given the robust baddeleyite U–Pb isotope systematics in the majority of shergottites dated by SIMS methods thus far, we argue that shock conditions experienced by the bulk of shergottites were insufficient to introduce significant U–Pb isotopic mobility, which is limited to grains showing microstructural evidence for extensive post-shock heating and recrystallization. Our findings place new constraints on baddeleyite microstructural response to shock conditions of shergottite ejection and demonstrate that microstructural observations are critical when using baddeleyite as a chronometer in shocked planetary materials.

Late Neoarchean plume-induced crust-mantle interaction and crustal reworking: Evidence from the Jianping Complex, North China Craton

The Archean basement of the North China Craton (NCC) is predominantly composed of 2.6–2.5 Ga lithological assemblages, while the tectonic regime that governed the formation and evolution of these Neoarchean basement rocks has been a controversial issue. Based on field relationships, petrographical features, whole-rock chemistry and zircon Hf isotope data, diverse late Neoarchean granitoids exposed in the Jianping Complex of the NCC are categorized into TTGs, sanukitoids and potassic granites. Their petrogenesis are associated with partial melting of metabasalts, melting of metasomatized mantle, and melting of crustal lithologies including tonalite and metasediments, respectively. Zircon U-Pb results suggest that different types of Neoarchean granitoids in the Jianping Complex are nearly coeval with crystallization ages of ∼ 2.54–2.52 Ga and metamorphic ages of ∼ 2.49–2.48 Ga. New Hf isotopic data reveal that these granitoids show positive εHf(t) values, with the tonalitic gneisses of εHf(t) values closer to the depleted mantle than those of granodioritic gneiss, sanukitoids and potassic granite. This result reflects potentially more enriched source for other granitoids than the tonalitic gneisses, which is linked with increasing level of crustal-mantle interaction and crustal reworking the late Neoarchean. Integrating other evidence from geochemical, structural and metamorphic studies, it is inferred that the plume activities play an important role in the emplacement of late Neoarchean granitoids in the Jianping Complex, with TTGs formed through the partial melting of juvenile crust induced by plume activity, sanukitoids produced from melting of metasomatized mantle resulted from sagduction process, and potassic granites formed by reworking of earlier crustal lithology caused by sustained underplating of mantle material. Overall, we interpreted that the late Neoarchean crustal growth and crustal-mantle interaction in the Jianping Complex can be attributed to upwelling of mantle plume and potential sagduction process in a vertical tectonic regime.

A slowly cooled deep crust on asteroid 4 Vesta and the recent impact history of rubble pile vestoids recorded by diogenites

In this study, we investigate the 40Ar/39Ar systematics of nineteen diogenites thought to come from deep crustal levels of asteroid 4 Vesta. We applied both Electron Backscattered Diffraction (EBSD) and 40Ar/39Ar and methods to the unbrecciated diogenite LAP 031381. We obtained three plateau ages resulting in a combined weighted mean age of 4441 ± 15 Ma (P = 0.16). The EBSD analyses suggest that LAP 031381 displays minimal evidence of shock and, when combined with petrography observations, diffusion modelling and 40Ar/39Ar data, these results suggest that the crustal volume that initially contained this diogenite, reached a temperature of ca. 630 °C at ∼ 4.44 Ga. This corresponds to a linear cooling rate of ∼ 5 °C / Ma for a crystallization age of 4550 Ma. Independent thermal models suggest that these conditions were present at a depth of 60 to 65 km at 4.44 Ga.The other eighteen diogenites yielded 40Ar/39Ar results that indicate that they have been variously shocked by impact events and seven of them yielded plateau ages ranging from 2413 ± 189 Ma to 84 ± 162 Ma. We combined these results with 40Ar/39Ar ages from eucrites and howardites and propose that the HED (Howardite, Eucrite, Diogenite) meteorites recorded impact events at the surface of Vesta until ∼ 3.4 Ga when they were then ejected during a large collision. The eucrites, diogenites and howardites were then recombined into small rubble pile asteroids which probably make up a large part of the Vestoid family. After ejection, the K/Ar system in plagioclase crystals ceased in most cases to be fully reset by impact events as the temperature spikes reached during small impacts lack enough energy to trigger significant 40Ar* diffusion. On the other hand, ultra-transient and high-temperature – sensitive pyroxene crystals kept a more systematic record of small impacts until recent time. 38Arc cosmochron cosmogenic exposure ages on diogenites mostly range from 51 ± 7 Ma to 0 ± 1 Ma and when combined with other HED cosmochron ages, suggest that almost all the HED meteorites were continuously ejected from secondary rubble pile asteroids mostly between 50 Ma and present.

Remobilization and eruption of an upper crustal cumulate mush at the Singkut caldera (North Sumatra, Indonesia)

Understanding the conditions and timescales of storage and remobilization of magma bodies in the upper crust is key to interpreting the signals of potential reawakening at active volcanoes. In this paper, we provide the first volcanological and petrochronological characterization of the Singkut caldera, a young volcanic system located in northern Sumatra (Indonesia), in close proximity to Medan, one of the country's most popoluous cities. Singkut formed at ∼44 ka during a VEI 6 explosive eruption that deposited at least ∼26 km3 of tephra (dense rock equivalent, DRE). The cataclysmic eruption was preceded by >200 ky of mostly effusive pre-caldera activity and followed by effusive to mildly explosive post-caldera activity. The lavas and pumices have high crystallinity (up to 62% crystals) with andesitic to dacitic bulk-rock composition and rhyolitic glass. Mineral textures and matrix glass compositions indicate resorption of quartz, plagioclase and zircon. Zircon crystallization ages show a complete overlap with the eruption ages in pre-caldera lavas, while a time gap in zircon crystallization (>50 ky) is identified in the caldera-forming tuff and post-caldera lavas. Ti-in-zircon thermometry shows that the Singkut magma body experienced a temperature increase starting approximately upon eruption of the pre-caldera lavas (∼254 ka). Such thermal perturbation determined progressive melting of mineral phases in the cumulate crystal mush, caused the resorption of the youngest zircon domains before the caldera-forming eruption, and hampered zircon crystallization between the caldera-forming eruption and the effusion of the post-caldera lavas (∼16 ka). Our data demonstrate how cumulate melting processes played a key role in leading the volcanic system towards a caldera-forming eruption and controlled the transitions in eruptive style between the effusive phases and the explosive climactic eruption.

O–Hf isotope ratios of Alvand S-type granite, western Iran, reveal crustal melting in an extensional regime

There are a few intrusive bodies in the central Sanandaj-Sirjan Zone (C-SaSZ), which is parallel to the Zagros suture zone, in western Iran. The Alvand batholith in the Hamedan area is one of these bodies and consists of rock types with large variation, in particular, two-mica garnet-bearing granite with an S-type signature. Zircon UPb dating gives crystallization ages of 170–163 Ma for the S-type granite. The 176Hf/177Hf ratios in the zircon grains vary from 0.282001 to 0.282780 (rim), with low 176Lu/177Hf ratios (0.002 to 0.018). The εHf(t) values (rim) are clustered into two main groups: a negative εHf(t) group (−6.9 to 0.0, n = 39), with some highly negative εHf(t) values (−25.8 to −21.9, n = 3), and slightly positive εHf(t) values (+0.3 to +2.9, n = 14) of inherited grains. The δ18O values for the zircon grains are positive and range from +10.4‰ to +11.8‰. The high δ18O (positive) and mostly negative εHf(t) values are more consistent with continental crustal sources for these rocks, and the Hf model ages (TDM2) show a wide range from 2.7 to 0.8 Ga, suggesting the heterogeneity of the crustal components. The long incubation time (2089 to 544 Ma) indicates a long history of crustal residence and crustal reworking. The incubation age of the zircon grains, >300 Ma, confirms the recycling of crustal material by partial melting. The simultaneous occurrence of S-type granites with mafic bodies, such as gabbro and diorite members with tholeiitic signatures, suggests that the injection of mafic magma increased the geothermal gradient from the bottom and sides along the major faults, which was the trajectory for the injection of hot mafic melts to shallow depths. These processes abnormally increased the geothermal gradients locally, and the partial melting of crustal and supracrustal materials produced S-type granites and migmatization among the hot blade-shaped mafic bodies. Therefore, the partial melting of crustal material in an extensional tectonic regime and/or in a thin continental crust occurred in a continental rift and/or in the Neo-Tethys passive margin. The present work concludes that the passive margin can be suggested as the tectonic setting for the generation of S-type granites, without the collisional regime that has been widely considered for the sources of this type of granite in the world.

Apatite geochronology and geochemistry of Gucheng granites: Implications for petrogenesis and REE metallogenesis in South China

Regolith-hosted rare earth element (REE) deposits are common in South China, and occurred dominantly in regolith of granite. The Gucheng deposit in western Guangdong Province is one example of HREE-dominated hosted in weathered coarse-grained biotite granites (CGBG). We conducted petrographic observations, in-situ U-Pb dating, EPMA, and LA-ICP-MS in situ compositions analyses on apatite grains from the CGBG to gain insights into the petrogenesis of CGBG and implications for REE mineralization at Gucheng. U-Pb geochronology of 50 apatite grains yields 206Pb/238U age of 102.1 ± 3.1 Ma, representing the crystallization age of CGBG, which suggests the CGBG was formed in the late Early Cretaceous.LA-ICP-MS data show that the CGBG apatite is characterized by enrichment of LREE ((La/Yb)N = 6.3 to 11.4) and obvious Eu anomalies (δEu = 0.12 to 0.16), indicating crust-mantle mixing in the magma source. Meanwhile, the mantle source was indicated to be metasomatized by both sediment melting and slab melting components triggered by the subducted Paleo-Pacific slab. In conjunction with LREE enrichment, no Eu anomaly and ratios (e.g., (Sm/Nd)N, (La/Sm)N, (La/Yb)N and Sr/Y) in apatite, the CGBG is classified as I-type granite. Meanwhile, although apatite is low in the CGBG (0.6 mod. %), it is an important REE-carrier mineral in the CGBG (5194 to 6255 ppm) and contains 30.5 times higher REE concentrations than those of the CGBG. Moreover, it contribute 19.5 % of REE concentrations for total REE of the CGBG, and its decomposition during weathering has a great contribution for REE mineralization in regolith. However, the decomposition of apatite and release of REE cannot account for the HREE-dominated mineralization in regolith of CGBG that are characterized by enrichment of LREE.

The U–Pb and Lu–Hf zircon record of the Neoproterozoic tectonomagmatic evolution of the Ventania System basement, central-eastern Argentina: The southernmost exposure of the Dom Feliciano Belt?

In the Ventania System of central-eastern Argentina, Neoproterozoic igneous rocks of the Sauce Chico Complex (SCC) crop out. They consist of low-grade mylonitized granites and rhyolitic rocks. Metasedimentary rocks are represented by metapelite and metacarbonate xenoliths hosted in Neoproterozoic granites. This work presents lithogeochemical, and coupled LA-MC-ICP-MS U–Pb and Lu–Hf zircon data of Neoproterozoic rocks of the SCC in order to reinforce the already proposed link between these rocks and the Brasiliano/Pan-African Dom Feliciano Belt of southern Brazil and eastern Uruguay. Novel U–Pb results yielded crystallization ages of 623 ± 3 Ma and 608 ± 4 Ma in granite samples, and 545 ± 3 Ma in a tuff sample. Considering these new U–Pb ages and those published in previous works, Neoproterozoic rocks of the SCC can be grouped in Tonian (ca. 784–777 Ma) and Ediacaran (ca. 623–607 Ma) granites, and Ediacaran (ca. 577–544 Ma) volcanic and pyroclastic rocks. Xenocrystic/inherited ages span from the Ediacaran (ca. 555 Ma) to the late Neoarchean (ca. 2535 Ma). Ninety-four spots on zircon grains gave the following U–Pb and Lu–Hf results: ca. 2040–1700 Ma (n = 6, εHf(t) = -14.6/+6.3, TDM = 3.5–2.1 Ga); ca. 1500–850 Ma (n = 9, εHf(t) = -26.3/+4.5, TDM = 3.3–1.9 Ga); ca. 830–700 Ma (n = 26, εHf(t) = -40.5/+10.7, TDM = 4.1–0.9 Ga); and ca. 650–530 Ma (n = 53, εHf(t) = -38.8/+2.9, TDM = 3.9–1.3 Ga). When the Neoproterozoic rocks of the SCC are compared with those of the Punta del Este Terrane of the Dom Feliciano Belt, lithological, geochemical, geochronological, and isotopic similarities are evident. Both regions have Tonian orthoderived rocks (ca. 800–760 Ma) intruded by Ediacaran granites (ca. 630–570 Ma), and late Ediacaran plutonic and volcano-sedimentary rocks (<ca. 580 Ma). Tonian rocks of the Punta Mogotes Formation (Mar del Plata city area, southeasternmost tip of the Tandilia System) might represent the connection between the Punta del Este Terrane and the Ventania System basement. Additionally, the presence of Brasiliano/Pan-African rocks along the southern border of the Río de la Plata Craton effectively eliminates the possibility of the craton extending further south/southwest of the system.

Unravelling the genetic relationship between pegmatites and granites in the Jiajika Li-Be polymetallic district, Songpan-Ganze Orogenic Belt, Southwestern China: Insights from monazite U-Pb geochronology and trace element geochemistry

The Jiajika Li-Be Polymetallic District is located in the Songpan-Ganze Orogenic Belt, Southwestern China. The Li-Be mineralization is genetically, temporally, and spatially, related to pegmatites in the Jiajika District. Hundreds of pegmatite dikes were emplaced surrounding the adjacent Majingzi granite pluton. Drill core samples from the 3,200 m deep Jiajika Scientific Drilling project (JSD-1) were acquired to determine the vertical extent and the petrogenesis and metallogeny of the granites, and the pegmatites, respectively, and to obtain a better understanding of their genetic relationship. The vertical zonation of the pegmatites from shallow to deep is as follows: shallow albite pegmatites; albite-microcline pegmatites; microcline pegmatites; and deeper more albite pegmatite. Spodumene occurs mainly in the shallow albite pegmatites (55–105 m depth), and disseminated beryl, columbite-tantalite, and cassiterite, mainly occur in pegmatites at depths less than 700 m. Eight samples were selected for monazite U-Pb geochronology and trace element geochemistry analyses from pegmatites and granites at different depths. LA-ICPMS monazite U-Pb geochronology for the pegmatites indicates two episodes of magmatic activity; one at 211–205 Ma (2σ, 523–2,494 m depth) and another at 193–192 Ma (2σ, 3,170–3,211 m depth). The crystallization age of the older pegmatites is in good agreement with the monazite U-Pb geochronology of the adjacent Majingzi granite (208–205 Ma, 2σ, 1,265–2,568 m depth) within analytical uncertainty, indicating their close temporal relationship. The younger albite pegmatite (193–192 Ma) is associated with the columbite-tantalite Nb-Ta rare-metal mineralization. Compared to those in the deep pegmatites (excluding the younger albite pegmatite in 3,170–3,211 m depth), and the adjacent granites, monazites in the shallow pegmatites and the adjacent granites are relatively enriched in Li, Th, and U, indicating the continuous evolution of a felsic magmatic system; with the residual melt more enriched in various incompatible rare-metal elements. Hence, the monazite U-Pb geochronology and trace element geochemistry indicate that a highly evolved felsic magma plays an important role in the petrogenesis of the Jiajika Li-Be pegmatites.

Peraluminous leucogranites generation during different stages of the Brasiliano Orogeny in the Borborema Province, NE Brazil

The Mamanguape pluton comprises muscovite, biotite syeno-to monzogranite intruded in metasedimentary rocks. Geochemically, the studied samples of the Mamanguape pluton peraluminous granites are ferroan and alkali-calcic. Furthermore, trace element distribution diagrams are characterized by troughs at Ba, Nb, Sr, P and Ti. The REE patterns are fractionated showing (Ce/Yb)N ratios ranging from 12 to 15, and deep negative Eu anomalies, with Eu/Eu* ratios varying between 0.18 and 0.25. The geochemistry of the two-mica granites of the Mamanguape pluton shows similarities with peraluminous leucogranites from the Borborema Province with crystallization ages related to the contractional (640–600 Ma) and transcurrent (590–545 Ma) stages of the Brasiliano/Pan-African Orogeny. Peraluminous, two-mica and biotite-leucogranites were emplaced through all Borborema Province with ages within 640–600 Ma and 590–545 Ma intervals. Their generation is interpreted as the result of partial melting of metasedimentary rocks involving muscovite dehydration. The crystallization age (574 ± 2 Ma) of the Mamanguape two-mica granites suggest intrusion related to the transcurrent stage of the Brasiliano Orogeny. Zircon grains extracted from two samples of a two-mica leucogranites of the Jurema and Jupi Plutons, Pernambuco-Alagoas Domain, show a large variation of εHf(t) values (−45 to −5) and Hf TDM model age between 1.2 and 3.9 Ga. Archean Hf TDM model ages were recorded in Cryogenian/Ediacaran zircon grains as well as Paleoproterozoic grains, suggesting contribution of an Archean component in the source of the granites. One sample of the Mamanguape Pluton, Transversal subprovince, have εHf(t) values ranging between −13.7 and + 1.4 and Hf TDM model age between 1.4 and 2.2 Ga, suggesting that the magma was generated by partial melting of metasedimentary rocks that received a large contribution of Neoproterozoic and Paleoproterozoic igneous sources. Small amount of mantle melt contributing to the source of the two-mica granites is supported by the presence of mafic enclaves and positive εHf(t) values.

New evidence for the presence of the Inner Tauride Ocean: Lithological, geochronological and P-T correlations with the Tavşanlı and Afyon zones of Central Anatolia (Türkiye)

The Tavşanlı and Afyon zones, which are well defined in western Türkiye, represent high pressure-low temperature (HP-LT) metamorphic rocks formed in a subduction zone. The Korumaz and Hınzır mountains (northeast of Kayseri) between the Central Anatolian Crystalline Complex and the Eastern Tauride Belt are represented by a HP-LT metamorphic sequence, which is thought to have formed between the Carboniferous and the Cretaceous. Here we present 40Ar/39Ar data from white micas within the Lower-Middle Triassic calc-phyllites from Korumaz Mountain, which yielded an age of 87 ± 2 Ma. Therefore, based on new mineralogical and geochronological data we suggest that this low-grade (HP-LT) metasedimentary sequence correlates with the Afyon Zone. Tectonically emplaced on-top of the HP-LT metasedimentary units of the Korumaz and Hınzır mountains is a meta-accretionary complex consisting of serpentinite, listvenite, marble, phyllite, calc-phyllite, metabasite, and meta-plagiogranite. Petrographic and mineral chemical studies show that the meta-accretionary complex was subjected to metamorphism under the HP-LT conditions that are similar to those of the Tavşanlı Zone in western Türkiye. Zircon Hf isotopic compositions from the meta-plagiogranite in the Korumaz and Hınzır mountains exhibit a depleted mantle signature with positive epsilon Hf(t) values of +5.6 to +13.1, and yielded a U-Pb crystallization age of 101.14 ± 0.84 Ma, which is the oldest age determined in oceanic crustal rocks of the ophiolites from the Tauride Belt. Geochemically, the metabasites and meta-plagiogranites from the meta-accretionary complex show subduction-related characteristics. White micas from phyllites and listvenite, which dominate the matrix of the meta-accretionary complex, yielded 40Ar/39Ar metamorphic ages ranging from 78 to 60 Ma. Our new geological and geochronological data suggest that metamorphic units of the Korumaz and Hınzır mountains are the eastern continuation of the Afyon and Tavşanlı zones. This indicates that the Inner Tauride Ocean, the existence of which has been controversial in the literature, was indeed present between the Central Anatolian Crystalline Complex and the Eastern Tauride Belt.

Zircon U–Pb ages and O–Hf isotopes of Quaternary trachytes from the East Sea: Implications for the genesis of low-δ18O magmas

Quaternary intraplate magmatism formed several volcanic islands and seamounts, including Dokdo (DD), Ulleungdo (UD), Simheungtack (ST), Anyongbok, and Isabu in the southwest of the East Sea back-arc basin. In this study, we present whole-rock geochemical, zircon U–Pb age, and in situ O–Hf isotope data for the submerged volcanic rocks from DD, UD, and ST to provide new insights into the eruption timing of these volcanoes and constrain the magma evolution processes. All samples used in this study were trachytes and exhibited ferroan, alkalic, and metaluminous to weakly peraluminous characteristics. They showed light rare earth element (REE)-enriched patterns with (La/Yb)N ratios of 25.3–31.7 and mostly negative Eu anomalies in a chondrite-normalized REE plot. In addition, they were enriched in large-ion lithophile elements and high field strength elements; they exhibited positive Pb anomalies and strongly negative Ba, Sr, P, and Ti anomalies. The zircons yielded a weighted-mean 206Pb/238U age of 2.61, 0.348–0.704, and 2.76–2.94 Ma for the DD, UD, and ST trachytes, respectively. All zircons exhibited lower δ18O values than normal depleted mantle values, regardless of the crystallization age and spatial distribution of volcanoes. The δ18O values showed no correlation with U contents or Th/U ratios, indicating that the low δ18O signatures were of primary magmatic origin. The Hf isotopic compositions of the zircons were relatively heterogeneous but predominately characterized by positive εHf values. Binary O–Hf mixing modeling revealed that low-δ18O rocks with positive εHf values from the UD and ST volcanoes were derived from a hybrid source of recycled juvenile crustal materials with low-δ18O and positive εHf signatures and an enriched mantle source with normal δ18O and negative εHf values. The juvenile oceanic crust in the source was likely metasomatized by seawater at high temperatures prior to melting. In contrast, the felsic magma that formed the DD volcanoes may have assimilated with regional basement rocks (Triassic–Jurassic granitoids), resulting in increased δ18O values and decreased εHf values relative to those of the UD and ST volcanoes. Our study highlights the significant contribution of recycled oceanic crust materials to the generation of the Quaternary magmas.

Devonian to Early Carboniferous Retreating—Advancing Subduction Switch in the Northwestern Patagonia Accretionary Orogen: U‐Pb and Lu‐Hf Isotopic Insights

AbstractIn this contribution, we present new early middle Devonian igneous and metaigneous units with a major juvenile magmatic source input in the North Patagonian Massif, which were discovered through U‐Pb and Lu‐Hf zircon analyses. Afterward, we assessed their tectonic implications for northwestern Patagonia and then for southern South America, combining our results with available database information consisting of igneous crystallization ages and isotopic data of the Devonian to early Carboniferous magmatic units, tectonic‐metamorphic analyses, and thermochronologic record. This study allows for distinguishing retreating and advancing subduction switching in northwestern Patagonia (38°30′ to 44°S) and a contrasting coetaneous evolution for basement outcrops exposed further north (27°30′ and 37°30′S). The early middle Devonian (400–380 Ma) northwestern Patagonian magmatism is characterized by widespread magmatism and positive εHf–εNd linked to forearc and backarc magmatism that evolved within a retreating subduction stage. A tectonic switching toward advancing orogeny stage began in the late Devonian, evidenced by a lull in magmatic activity with a negative εHf–εNd trend, possibly contemporaneous with the first tectonic‐metamorphic event in western Patagonia. An early Carboniferous magmatic gap, followed by the subsequent development of the main foliation in the basement during the Carboniferous‐Permian period, denotes the acme of this contractional stage. In contrast, the Devonian period in the northern segment is characterized by mostly negative εHf–εNd values, reverse shear zone activity in the foreland, and an inboard magmatism migration, evidencing a compressive tectonic setting that changed to an extensional configuration in the early Carboniferous with widespread arc magmatism development.

Geochemistry and zircon U–Pb dating of amphibolite in the Menderes Massif (western Türkiye): first record of Late Devonian extension

ABSTRACT The timing of extensional events in northern Gondwana remains elusive. In relations to this event, a new zircon U-Pb age and Hf isotopic compositions, and whole-rock geochemistry provided here for the amphibolites in the basement unit of the Menderes Massif (MM) within the Anatolide-Tauride Block (ATB). LA-ICP-MS U-Pb zircon dating of one sample yielded a crystallization age of 367.0 ± 2.5 Ma, providing the first record of Late Devonian (Famennian) mafic magmatism in the Menderes Massif. This sample contains inherited zircon grains with ages spanning from 588 Ma to 541 Ma, likely sourced from late Neoproterozoic country rocks. Chondrite-normalized rare earth element patterns exhibit enrichment in light rare earth elements and flat patterns for heavy rare earth elements, suggesting affinities with tholeiitic EMORB and ocean island basalt. The geochemical data, coupled with the positive εHf(t) values ranging from 0.5 to 7.8, indicate that the amphibolite likely originated from juvenile melts generated by partial melting of the asthenospheric mantle. This melt was subsequently modified by crustal contamination and fractional crystallization. Contemporaneous EMORB- and OIB-like mafic rocks are identified in the eastern Taurides and Iran, respectively. Both regional geological and geochemical evidence proposes that Late Devonian magmatism in the Anatolide-Tauride Block occurred in a within-plate tectonic setting. This magmatic activity might have been triggered by the northward subduction of the Paleotethys beneath the Pontides during the Devonian, a process that would produce a slab-pull force, creating an extensional tectonic regime along the northern margin of Gondwana.