Magmatic Zircons Research Articles

Zircon U–Pb geochronology and Hf isotope geochemistry of magmatic and metamorphic rocks from the Hida Belt, southwest Japan

Zircon U–Pb and Hf isotope data integrated in this study for magmatic and metamorphic rocks from the Hida Belt, southwest Japan, lead to a new understanding of the evolution of the Cordilleran arc system along the ancestral margins of present-day Northeast Asia. Ion microprobe data for magmatic zircon domains from eight mafic to intermediate orthogneisses in the Tateyama and Tsunogawa areas yielded weighted mean 206Pb/238U ages spanning the entire Permian period (302–254 Ma). Under cathodoluminescence, primary magmatic growth zones in the zircon crystals were observed to be partially or completely replaced by inward-penetrating, irregularly curved featureless or weakly zoned secondary domains that mostly yielded U–Pb ages of 250–240 Ma and relatively high Th/U ratios (> 0.2). These secondary domains are considered to have been formed by solid-state recrystallization during thermal overprints associated with intrusions of Hida granitoids. Available whole-rock geochemical and Sr–Nd isotope data as well as zircon age spectra corroborate that the Hida Belt comprises the Paleozoic–Mesozoic Cordilleran arc system built upon the margin of the North China Craton, together with the Yeongnam Massif in southern Korea. The arc magmatism along this system was commenced in the Carboniferous and culminated in the Permian–Triassic transition period. Highly positive εHf(t) values (> +12) of late Carboniferous to early Permian detrital zircons in the Hida paragneisses indicate that there was significant input from the depleted asthenospheric mantle and/or its crustal derivatives in the early stage of arc magmatism. On the other hand, near-chondritic εHf(t) values (+5 to −2) of magmatic zircons from late Permian Hida orthogneisses suggest a lithospheric mantle origin. Hf isotopic differences between magmatic zircon cores and the secondary rims observed in some orthogneiss samples clearly indicate that the zircons were chemically open to fluids or melts during thermal overprints. Resumed highly positive zircon εHf(t) values (>+9) shared by Early Jurassic granitoids in the Hida Belt and Yeongnam Massif may reflect reworking of the Paleozoic arc crust.

Magmatism associated with lithospheric thinning, mantle upwelling, and extensional tectonics: Evidence from Carboniferous-Permian dyke swarms and granitoids from Inner Mongolia, Central Asian Orogenic Belt

The east-Ujimqin complex located north of the Erenhot-Hegenshan ophiolite belt in North China is composed of extensive Carboniferous and Permian magmatic rock suites, which preserve the records of an extensional tectonic regime following collision. These rocks provide important clues to the closure of the Hegenshan ocean and intraplate processes. Here we investigate the diorite dyke swarms in association with granitic rocks from the Xing'an-Mongolian Orogenic Belt (XMOB) through field investigations, petrology, geochemistry, zircon U-Pb and Lu-Hf studies, to gain insights into the timing, magma source and tectonic setting during Late Carboniferous and Early Permian. The diorite rocks display sub-alkaline, calc-alkaline, and high-K calc-alkaline features. Their extensional setting is suggested by plots in the fields of within plate basalt and active continental margin + alkaline oceanic arc. Zircon U-Pb data on magmatic zircons indicate that the granitoids crystallized at ca. 301 Ma, whereas the diorite dykes were emplaced at 286 Ma. Xenocryst zircon grains with 320 to 300 Ma ages in the dyke samples suggest incorporation from the surrounding granites. Zircon Lu-Hf isotopic data show consistently positive εHf(t) values (5.3 to 10.8) and plot between CHUR (Chondritic Uniform Reservoir) and depleted mantle lines suggesting that depleted mantle sources contributed significantly to the magma source. The computed model ages suggest that the magma source involved Neoproterozoic juvenile components.We propose a model where the magmatism occurred within an extensional tectonic setting caused by asthenosphere upwelling and lithospheric thinning. Our study also constrains the timing of Early Permian extension which suggests that the Hegenshan Ocean, as part of the Central Asian Orogenic Belt closed prior to 286 Ma.

Tectonic significance of the Variscan suture between Brunovistulia and the Bohemian Massif

The Velké Vrbno Dome crops out at the boundary between the Brunovistulian Terrane and the internal parts of the Bohemian Massif. Here, eclogite boudins occur within an Ediacaran volcano-sedimentary sequence. Strong Nb depletion (Nb/Nb* = 0.19–0.82) combined with moderately positive Nd isotopic compositions (εNd (i) = +3.89 – +5.77) is used to argue for emplacement of the eclogite protoliths in a transitional supra-subduction to continental-rift setting. Conversely, heterogeneously enriched large ion lithophile elements and highly radiogenic Sr isotopic ratios ( 87 Sr/ 86 Sr = 0.705–0.720) are interpreted to have been modified following fluid infiltration subsequent to eclogite-facies metamorphism. U–Pb laser ablation inductively coupled plasma mass spectrometry dating of magmatic zircon from the rift-type eclogite indicates Early Cambrian emplacement ( c. 535 Ma) following episodic Ediacaran volcanic arc activity. Moreover, a continental setting is emphasized by zircon dating of a mylonitic orthogneiss, revealing a fragment of Palaeoproterozoic ( c. 2000 Ma) basement, the first such finding within the Brunovistulian Terrane sensu stricto . The new data from eclogite confirm that rifting in this segment of Gondwana pre-dated the Ordovician opening of the Rheic Ocean and therefore that the suture between Brunovistulia and the rest of the Bohemian Massif likely represents the vestige of an older hyperextended basin or oceanic tract. Supplementary material: Previously unpublished single zircon evaporation ages from Ediacaran orthogneiss from the Velké Vrbno Dome (supplement A); detailed analytical methodology (supplement B); whole rock geochemical data (supplement C); and U-Pb LA-ICP-MS zircon data (supplement D) are available at https://doi.org/10.6084/m9.figshare.c.5233079

Zircon in mantle eclogite xenoliths: a review

AbstractVery few zircon-bearing, kimberlite-hosted mantle eclogite xenoliths have been identified to date; however, the zircon they contain is crucial for our understanding of subcratonic lithospheric mantle evolution and eclogite genesis. In this study, we constrain the characteristics of zircon from mantle eclogite xenoliths based on existing mineralogical and geochemical data from zircons from different geological settings, and on the inferred origin of mantle eclogites. Given the likely origin and subsequent evolution of mantle eclogites, we infer that the xenoliths can contain zircons with magmatic, metamorphic and xenogenic (i.e. kimberlitic zircon) origins. Magmatic zircon can be inherited from low-pressure mafic oceanic crust precursors, or might form during direct crystallization of eclogites from primary mantle-derived melts at mantle pressures. Metamorphic zircon within mantle eclogites has a number of possible origins, ranging from low-pressure hydrothermal alteration of oceanic crustal protoliths to metasomatism related to kimberlite magmatism. This study outlines a possible approach for the identification of inherited magmatic zircon within subduction-related mantle eclogites as well as xenogenic kimberlitic zircon within all types of mantle eclogites. We demonstrate this approach using zircon grains from kimberlite-hosted eclogite xenoliths from the Kasai Craton, which reveals that most, if not all, of these zircons were most likely incorporated as a result of laboratory-based contamination.

Preservation of Xiuwacu W-Mo deposit and its constraint on the uplifting history of Eastern Tibetan Plateau

Thermochronology has become an increasingly effective tool to reveal the post-mineralization uplift and exhumation history of ore deposits. In this paper, we studied the exhumation history and preservation conditions of the Xiuwacu tungsten-molybdenite deposit (resource: 24,000 t Mo and 23,000 t WO3), which is located on the southern Yidun Terrane (SYT), Eastern Tibetan Plateau. At Xiuwacu, both the eastern and western ore zones contain quartz-vein-type orebodies in the K-feldspar, greisen, and silicic alteration zones. The orebodies distributed in a northwestward trend along regional faults, with ore minerals including mainly molybdenite, scheelite, chalcopyrite, and pyrite.The mineralization at Xiuwacu occurred in 86–82 Ma, coeval with the emplacement of the ore-causative Xiuwacu porphyry monzogranite. The deposit likely underwent rapid cooling from 750 − 800 °C (magmatic zircon U-Pb age) to ~ 300 °C during 90–86 Ma (molybdenite Re-Os mineralization age). After that, the temperature slowly and continuously cooled to ~ 180 °C from the Late Cretaceous to 25 Ma, and then rapidly cooled to ~ 125 °C in 25–17 Ma, and eventually to 125–60 °C during 17–5 Ma (apatite fission-track ages; n = 8).The thermochronologic history of the Xiuwacu intrusive complex was reconstructed by inverse modelling based on the apatite fission-track ages and lengths. There was 6426–6915 m (average 6650 m) uplift, while only 1982–2471 m (average 2200 m) exhumation. Considering the depth of current exploration and typical depth of granitoid-related W-Mo mineralization, we infer that there is still great potential to discover concealed W-Mo orebodies beneath the current mining level at Xiuwacu.

Provenance analysis of Late Triassic turbidites in the eastern Songpan–Ganzi Flysch Complex: Sedimentary record of tectonic evolution of the eastern Paleo-Tethys Ocean

The Xikang Group represents the main stratigraphic sequence of the Songpan–Ganzi complex (SGC), a flysch basin with a wide distribution and monotonous lithology. Strong tectonic deformation and accumulation of folds have led to destruction of the original sedimentary sequence and sedimentary structure of the strata, such that there exists no reasonable sedimentary model that can explain the massive flysch. Herein, we investigated in detail the sedimentary structures, paleocurrent direction, particle sizes, major and trace elements, and magmatic zircon and detrital zircon U–Pb ages of the typical sections of the Xikang Group in the eastern SGC. Our analysis led to the following findings: 1) The U–Pb zircon ages from the Galazi (215.6 ± 1.1 Ma), Sudi (214.5 ± 1.5 Ma and 221.1 ± 1.5 Ma), and Gulu (221.3 ± 1.2 Ma) plutons, which intrude the Xikang Group, and detrital zircon samples from the uppermost part of the Xinduqiao Fm. indicated the youngest mean age (YC1σ[2+]) of 222.4 ± 2.5 Ma. This in turn indicates a pre-221 Ma depositional age for the Xikang Group. 2) The provenance was derived from the North China Block and Qiangtang Terrane, Qinling–Dabie orogenic belt, South China Block, and East Kunlun–Qaidam, and the construction of the flysch in the SGC belongs to the multi-source or line source provided by the peripheral orogenic belt. 3) Trace elements suggested that the Xikang Group may have been deposited in a sedimentary basin associated with a continental island arc. 4) The sedimentary environment in this area belonged to the distal fan of a submarine, and the new sedimentation age and sedimentation rate indicate that the Carnian pluvial event may have been the trigger mechanism. 5) The formation of the flysch deposition in SGC related to the Paleo-Tethys Ocean underwent a scissor-like closure from east to west.

Episodic crustal growth and reworking at the southeastern margin of the North China Craton: evidence from zircon U–Pb and Lu–Hf isotopes of Archean tonalite–trondhjemite–granodiorite gneisses in the Bengbu-Wuhe area

The cratonization history of the North China Craton (NCC) and the nature of tectonothermal events are still highly controversial. Tonalite-trondhjemite-granodiorite (TTG) gneisses, as the dominant lithological assemblages in Archean metamorphic terranes, can provide significant clues to the magmatic and metamorphic evolution of Precambrian crust. This study presents zircon laser-ablation inductively-coupled-plasma mass spectrometry U–Pb ages, trace-element, and in-situ LA-MC-ICP-MS zircon Hf isotope data for the TTG gneisses from the Bengbu-Wuhe area on the southeastern margin of the NCC. Cathodoluminescence images and trace elements indicated that magmatic zircons display the characteristics of euhedral-subhedral crystals with oscillatory growth zoning structures, high ΣREE contents, marked Ce positive anomalies, and Pr–Eu negative anomalies. The metamorphic zircons display the spherical-oval crystals with distinct core-rim structures, high and homogeneous luminescent intensity, lower ΣREE, Nb, Ta, Hf contents, relative flat REE patterns, weak Ce positive anomalies, and Pr-Eu negative anomalies. The Ti–in–zircon geothermometer data indicate that the crystallization temperature of the TTG gneiss ranged from 754 to 868 °C. Zircon U–Pb ages indicate that the TTG gneisses formed at 2.79–2.77 Ga and 2.50 Ga and underwent metamorphism at 2.57–2.52 Ga. The Hf isotopic data indicate that the magmatic zircons exhibit high, positive eHf(t) values close to those of the coeval depleted mantle, whereas the metamorphic zircons exhibit negative or nil eHf(t) values. This implies that the TTG gneisses were derived from the partial melting of the ~ 2.9–2.6 Ga juvenile crustal sources mixed with ~ 3.0–2.8 Ga ancient crustal materials. Combined with the regional tectonic evolution, we propose that the metamorphic basement at the southeastern margin of the NCC underwent episodic crustal growth at ~ 2.7 and ~ 2.5 Ga and subsequently underwent crustal reworking or re-melting of the ancient crust during the Neoarchean. The Neoarchean TTG gneisses might have been derived from the partial melting of lower crustal materials related to plate subduction.

Paleoproterozoic (1.96–1.86 Ga) granites in Xinyang record zoned deep crustal structure and multi-stage reworking beneath the southern North China Craton

As one of the oldest cratons on Earth, the North China Craton (NCC) is a frequent area for geological research due to its complex crustal evolution. Regarding the Paleoproterozoic crustal evolution of the craton, most studies have focused on continental amalgamation (i.e. horizontal crustal evolution), but few on the vertical crustal evolution. Here we report whole-rock geochemistry, zircon U-Pb geochronology and Lu-Hf isotope geochemistry for the Xinyang granites to constraint the vertical crustal structure and evolution of the southern NCC. The studied granites comprise two-mica K-feldspar granite and garnet-bearing K-feldspar granite. All the rocks have high SiO2 and total alkali, but low CaO, MgO and Fe2O3T contents. Their A/CNK values (1.05–1.28) suggest that they are peraluminous rocks. These granites are enriched in LILE (Rb, Th and K), depleted in Sr, P and HFSE (Nb and Ti). The two-mica K-feldspar granites show obviously fractionated REE patterns ((La/Yb)N = 18.3–387) with negative or positive Eu anomalies (δEu = 0.47–1.03, 4.85–6.72), while the garnet-bearing K-feldspar granites have flat REE patterns ((La/Yb)N = 1.69–4.89) with extraordinary negative Eu anomalies (δEu = 0.22–0.42). The magmatic zircons of the two-mica K-feldspar granites yield crystallization ages ranging from ~ 1.86 Ga to ~ 1.90 Ga, and those of the garnet-bearing K-feldspar granites give crystallization ages of ~ 1.90 Ga. Previous work also reported a biotite K-feldspar granite pluton formed at ~ 1.96 Ga in the Xinyang area. Taken together, three major episodes of Paleoproterozoic granitoid events (~1.86 Ga, ~1.90 Ga and ~ 1.96 Ga) are recognized in the area. Combined with regional geochronological investigations, the peraluminous granitic magmatic activities occurring during 1.96–1.86 Ga in the Xinyang area could be linked to the collision between the Eastern and Western Blocks of the NCC. Alternatively, zircon Hf isotopic compositions suggest that these rocks derived from partial melting of pre-existing crust with heterogeneous compositions. Zircons from the ~ 1.86 Ga two-mica K-feldspar granites yield εHf(t) values of −5.2–1.7 with Tcrust of 2.40–2.86 Ga, compared to zircons from the ~ 1.90 Ga two-mica K-feldspar granites which yield wide range of εHf(t) values of −13.5–1.0 with Tcrust of 2.50–3.42 Ga. In contrast, the ~ 1.90 Ga garnet-bearing K-feldspar granites have homogeneous negative zircon εHf(t) values ranging from −11.8 to −10.4, with Tcrust ranging from 3.23 to 3.30 Ga. Moreover, zircons from the biotite K-feldspar granites have εHf(t) values of −0.6–2.6, corresponding to Tcrust of 2.44–2.77 Ga. Based on abovementioned geochemical signals, we propose that the Paleoproterozoic crust in the southern NCC likely consisted of two parts in vertical, the upper Middle-Late Archean crust and the lower early Paleoproterozoic crust with Eo-Paleoarchean crustal remnants. During the Paleoproterozoic, the southern NCC experienced multi-stage crustal reworking, indicating that a stable continental crust likely had been formed by the end of Archean.

Late Paleoproterozoic post-collisional bimodal magmatism in the North China Craton: Insights from the Miyun gabbro-granite suite

Following the final cratonization of the North China Craton (NCC) through the assembly of major crustal blocks, the craton witnessed post-collisional magmatism during late Paleoproterozoic with the formation of diverse types of magmatic suites in the western and eastern margins of the Trans-North China Orogen (TNCO). Here, we investigate the anorogenic granitoid and associated gabbro from the Miyun complex located at the eastern margin of the TNCO. We present petrology, geochemistry, and zircon U-Pb, and Lu-Hf data on these rocks. The granite pluton is composed of a rapakivi phase with almost totally digested mafic magmatic enclaves, whereas the granite phase hosts gabbroic enclaves. The granitoids show high SiO2 content (71.28 to 78.83 wt%), with peraluminous A2 type affinity and within plate (rift-related) features. The magmatic zircon grains from the granite yielded 207Pb/206Pb weighted mean age of 1696 Ma. A similar crystallization age of 1683 Ma for the gabbro indicates coeval bimodal magmatism. Both rocks carry zircon xenocrysts with older ages that are consistent with the magmatic and metamorphic events recorded from the basement rocks. The εHf(t) values of the zircon grains from the granitoid are markedly negative (−7.9 to −5.0) with two-stage Hf crustal model ages (TC DM) ranging from 2688 Ma to 2855 Ma, indicating the involvement of Mesoarchean to Neoarchean crustal components in the magma source. In contrast, the zircon grains from gabbro show both positive and negative εHf(t) values, suggesting magma derivation from depleted mantle sources with minor juvenile crustal component. We envisage that lower crustal melting induced by mantle-derived mafic magmas during post-collisional rifting generated felsic magmas, into which the mafic magma was injected, resulting in magma mixing and mingling. In conjunction with the information from post-collisional magmatic suites in the adjacent locations including gabbro-anorthosite complex, and similar aged magmatic complexes within the TNCO, we propose a major post-collisional magmatic event in the NCC which involved extensive mantle upwelling and lower crustal melting.

U–Pb detrital zircon constraints on active margin magmatism and sedimentation after the Grampian Orogeny in western Ireland

Early Ordovician collision of the Lough Nafooey Arc (part of the Baie Verte Oceanic Tract) with the passive continental margin of Laurentia peaked at c. 475 Ma in Scotland and Ireland and was followed by subduction polarity reversal. We examined Upper Ordovician–Silurian sedimentary rocks from western Ireland to see whether collision was followed by renewed arc magmatism. Despite the scarcity of dated igneous intrusions between the Grampian ( c. 470 Ma) and Acadian ( c. 420 Ma) orogenies in Ireland, detrital zircons show a continuity of activity peaking at 480–440 Ma, implying no hiatus in regional magmatism. Differences in zircon U–Pb age spectra highlight the isolation of basins in the southern Killary Harbour area from those north of the South Mayo Trough. These latter rocks were largely derived by erosion from Moine and Upper Dalradian sources. By contrast, the Killary Harbour Basin shows a decreasing influence from the Dalradian after c. 436 Ma and an increasing influence of contemporaneous magmatic zircons. These were transported from sources along-strike from the present NE, probably at the southern end of the Scandian Mountains in SE Greenland. The western Irish basins formed as pull-apart basins in a forearc setting and are analogous to Cenozoic pull-apart basins in Sumatra. Supplementary material: U-Pb zircon analytical data is available at a decreasing influence from the Dalradian after c. 436 Ma and an increasing influence of contemporaneous https://doi.org/10.6084/m9.figshare.c.5209849

Hualgayoc mining district, northern Peru: Testing the use of zircon composition in exploration for porphyry-type deposits

The Hualgayoc district in the Cajamarca region, northern Peru, hosts several deposits, including the Cerro Corona Au + Cu porphyry deposit (0.55 Mt Cu, 99 t Au) and the Tantahuatay high-sulfidation epithermal Au deposit (74 t Au), plus other Au, Cu, Ag and base metal prospects. Previous studies have documented that zircon in igneous rocks associated with porphyry systems elsewhere in the world shows large positive Ce anomalies and small negative Eu anomalies, which reflect both the oxidized and hydrous nature of their parental magmas. This study presents the trace element compositions of magmatic zircon from igneous rocks associated with porphyry-style mineralization, as well as barren stocks, sills and volcanic rocks in the district, in order to investigate the usefulness of zircon composition in exploration for porphyry-type deposits. The compositions of the zircon suggest that all parental magmas in the Hualgayoc district are oxidized, and that the magnitude of Ce and Eu anomalies is independent of the spatial and/or temporal association of igneous rocks with mineralization. Although zircon composition is not sufficient to recognize specific intrusions as fertile to generate hydrothermal ore deposits, the composition, properly applied, may be useful to target areas containing oxidized igneous rocks for further detailed exploration.

Early Permian subduction-related Ni-Cu sulfide mineralization in the Central Asian Orogenic Belt: A case of the Halatumiao deposit

The recently discovered Halatumiao magmatic Ni-Cu sulfide deposit is located in the western part of the Baolidao terrane, which represents the middle-eastern segment of the Central Asian Orogenic Belt (CAOB), Inner Mongolia, NE China. This deposit is an important case of a magmatic sulfide deposit related to Early Permian subduction magmatism in an arc setting. In this contribution, we report in-situ zircon U-Pb ages, whole-rock major and trace element and PGE data, and Os and S isotope compositions from the Halatumiao ore-bearing intrusion to constrain its genesis and relationship to the Early Permian subduction-related magmatism. The Halatumiao mafic-ultramafic intrusion is composed of wehrlite, olivine websterite and gabbro. Semi-massive and disseminated sulfide mineralization with a pentlandite + pyrrhotite + chalcopyrite assemblage occurs in both mafic and ultramafic rocks of the intrusion. In-situ dating of magmatic zircon grains separated from an ultramafic rock sample yielded a U-Pb age of 292.3 ± 1.5 Ma. This age is very close to the age of the Erbutu Ni-Cu sulfide deposit (294.2 ± 2.7 Ma) located in the western part of the northern margin of the North China craton. The high Ni/Cu ratios (6.7 in average, except one sample) of ore samples indicate that the parental magma to the Halatumiao deposit was a primitive basalt. The good positive correlation between IPGEs (Ir vs. Os, Ru and Rh) indicates that fractional crystallization of MSS is the key factor for the PGE tenors. The positive γOs(t) values (from +46 to +68) and mantle-like S isotopic compositions (δ34S mostly between −1.5 and 0.5‰), The trace element characteristics show that a minor to moderate degree of crustal contamination (5–10%) has occurred in the magma. Fractional crystallization and crustal contamination were likely the key factors for sulfide saturation in the Halatumiao mafic magma. The relatively high Ni tenors of sulfides (avg. 6.4 wt%) indicate that the Late Paleozoic mafic-ultramafic intrusions abounding in the Xing’an-Inner Mongolia region have a good potential for hosting economic Ni-Cu sulfide mineralization.

Unravelling the P-T-t history of three high-grade metamorphic events in the Epupa Complex, NW Namibia: Implications for the Paleoproterozoic to Mesoproterozoic evolution of the Congo Craton

The Epupa Complex of northern Namibia constitutes the south-western margin of the Archean Congo Craton of central Africa. We present new petrological and geochronological data for metasedimentary migmatites that decode the poorly-known Paleoproterozoic to Mesoproterozoic evolution of this remote part of the craton. Detrital magmatic zircons with concordant 207Pb/206Pb ages between 1898 and 1774 Ma are interpreted to demonstrate the formation of the metasedimentary protoliths through reworking of Paleoproterozoic granitoids of a magmatic arc in a back-arc basin. SHRIMP U-Pb zircon analyses of metamorphic rims around the detrital grains linked with constraints from pseudosection modelling revealed three distinct high-grade metamorphic events in the migmatites. Late Paleoproterozoic regional HT-LP metamorphism between 1740 and 1720 Ma constitutes the oldest event and only affected the rocks of the northernmost part of the Epupa Complex (Eyao Unit). Heating to mid-amphibolite facies peak P-T conditions of 720 °C and 4 kbar caused the partial replacement of early biotite-sillimanite by garnet-cordierite assemblages during melt-producing reactions in metapelites. The near-isobaric heating-cooling P-T paths and the high geothermal gradient (60 °C/km) are consistent with HP-LP metamorphism in a back-arc basin setting with up-rising basic melt as heat source. Early Mesoproterozoic (1530 Ma) HT-LP metamorphism is recorded by rare Mg-rich Opx-Crd rocks that are restricted to the westernmost part of the Eyao Unit. Near-isobaric P-T paths culminate at granulite facies peak-conditions of 830 °C and 2 kbar. This local HP-LP event is interpreted to record contact metamorphism related to the coeval emplacement of early-Mesoproterozoic granitoids or the emplacement of intimately associated gabbros. Metapelitic Grt-Bt-Sil migmatites record the third high-grade metamorphic event that was active at ca. 1330 Ma and is restricted to the southern part of the Epupa Complex (Orue Unit). Sillimanite pseudomorphs after kyanite and late cordierite coronas around garnet indicate a clockwise P-T path during regional upper-amphibolite facies metamorphism that peaks at lower crustal conditions of 770 °C and 7.5 kbar. The clockwise P-T path is interpreted to reflect crustal thickening through magmatic accretion of basic melts that produced the coeval Kunene Intrusive Complex, the largest massif-type anorthosite in the world. The metamorphic events detected in the migmatites record a change of the geotectonic position of the Epupa Complex from a convergent continental margin during the late Paleoproterozoic to an intracratonic position in the Mesoproterozoic. The migmatites of the Eyao Unit were affected by an intense hydrothermal alteration during the Pan-African orogeny (ca. 520 Ma) that also caused the partial re-setting of the U-Pb zircon isotope system.

The complexities of Mesoarchean to late Paleoproterozoic magmatism and metamorphism in the Qixia area, eastern North China Craton: Geology, geochemistry and SHRIMP U-Pb zircon dating

Qixia is a typical area of early Precambrian basement in eastern Shandong Province, eastern North China Craton. Many TTG (tonalite-trondhjemite-granodiorite) assemblages were once considered to be supracrustal rocks (the Jiaodong Group), and the formation ages of the rocks have only been determined in a few outcrops as shown on the early geological map of the area. We carried out geological mapping, geochemical study and SHRIMP U-Pb zircon dating in order to determine the temporal and spatial distribution and origins of the TTG rocks. In the newly compiled geological map (1:50,000), the main rock types of the Archean basement are ∼2.9 Ga, ∼2.7 Ga and ∼2.5 Ga tonalitic gneisses with local occurrences of trondhjemitic gneisses, granodioritic gneisses, (quartz) dioritic gneisses and meta-gabbro showing the same age range. Supracrustal rocks with ages of ∼2.9 Ga and ∼2.5 Ga are locally identified. All rocks broadly extend in a NW-SE direction as a result of strong tectonothermal events of the late Neoarchean and late Paleoproterozoic. Although the late Paleoproterozoic tectonothermal event strongly influenced all 2.7 to 2.9 Ga rocks in the area, metamorphic zircon ages are not widely recorded in these rocks because the high-grade metamorphism at ca. 2.5 Ga caused the older 2.7 to 2.9 Ga rocks to become relatively dry systems. The three generations of TTG rocks are similar in major element composition, characterized by high Na2O and low K2O, except for the late Neoarchean granodioritic gneisses, which locally occur and are relatively high in K2O. All the TTG rocks of different ages commonly have zircon O isotopic compositions within the range determined by Valley and others (2005) for Archean magmatic zircon. The ∼2.9 Ga TTG rocks show large Sr/Y and La/Yb variations and depletion in whole-rock Nd and zircon Hf isotopic compositions. The ∼2.7 Ga TTG rocks are similar in Nd-Hf isotopic compositions to the ∼2.9 Ga TTG rocks but have low Sr/Y and La/Yb ratios. The ∼2.5 Ga TTG rocks are similar in trace element composition to the ∼2.9 Ga TTG rocks, showing large variations in Sr/Y and La/Yb ratios. They can be further subdivided into two types in terms of Nd-Hf isotopic compositions with the depleted type mainly including tonalitic gneisses [eNd(t) = +1.86 to +4.59, eHf(t) = +1.0 to +8.7] and the enriched type including trondhjemitic and granodioritic gneisses [eNd(t) = −2.38 to −0.06, eHf(t) = −1.6 to −2.9]. It is concluded that the ∼2.9 Ga TTG rocks were formed in an oceanic environment (oceanic plateau or intra-ocean subduction), and the 2.7 Ga TTG rocks were formed by mantle underplating that resulted in partial melting of lower crustal mafic rocks under relatively low pressure conditions. More ancient continental materials played a role in the ∼2.5 Ga magmatic process, but more work is required to determine the tectonic environment (underplating or arc magmatism).

New geochronological and geochemical data of the Longsheng mafic-ultramafic suite in northern Guangxi, China, and their implications in Rodinia breakup

The Longsheng mafic-ultramafic suite consists of more than 100 mafic and ultramafic intrusions and are extensively distributed within the Neoproterozoic Danzhou Group along the Jiangnan Orogenic Belt in northern Guangxi, China. The suite has been a hotly debated subject in the context of emplacement age, source of magmas, and tectonic setting for several decades. In this article, we present newly obtained LA-ICP-MS zircon U-Pb age data, whole-rock major and trace elemental data, and Sr-Nd and zircon Lu-Hf isotopic data. These data support a middle Neoproterozoic intrusive emplacement mechanism as a result of asthenospheric underplating and subsequent lithospheric thinning, extension, and rifting. This integrated process might well correspond to the early stage of the Rodinia breakup. For example, five mafic-ultramafic rock samples yielded an emplacement age of 774 ± 2 Ma. The samples contain a large number of inherited zircon xenocrysts dated at 821 ± 3 Ma, 1.4–2.1 Ga, and 2.3–2.8 Ga, indicative of crustal contamination. The mafic-ultramafic rocks are characterized by calc-alkaline to shoshonitic associations; their Zr/Nb, Th/Yb, La/Nb, Ba/La ratios, trace, and REE patterns show an OIB affinity. Their initial 87Sr/86Sr values range from 0.697721 to 0.708579 with an average of 0.702352. They have relatively low initial 143Nd/144Nd values of 0.511530–0.511859 with an average of 0.511661. Sixteen magmatic zircon grains from diabase samples show 176Hf/177Hf values of 0.281326–0.282638, eHf(t) values of − 34.48–11.89, and TDM2 of 925–3819 Ma. Their Nb/U and Th/Ta ratios support crustal contamination as well. It is concluded that the Longsheng mafic-ultramafic suite was originated from partial melting of the enriched upper mantle (EM-I type) due to asthenospheric underplating and lithospheric extension during the incipient Rodinia breakup, and that the magmas were contaminated with crustal material during ascent.

Zircon Trace Element Geochemistry and Ti-in-Zircon Thermometry of the Ngazi-Tina Pan-African Post-Collisional Granitoids, Adamawa Cameroon

In Cameroon, the Ngazi-Tina region belongs to the Adamawa-Yade domain of the Pan-African Central African Fold Belt (CAFB). It is composed of two petrographic types: quartz-monzonites (majority) and nepheline syenites. Two morphological types, prismatic and pyramidal, were recognized in the zircon grains samples. These zircon types display internal structures typical of magmatic zircons. Zircons separated from the Ngazi-Tina samples contain higher abundances of Hf (close to 8000 ppm) and moderate trace elements (Y, Th, U, Nb, Ta) and REE contents, suggesting a variable degree of magmatic evolution. The chondrite-normalized REE patterns of zircons are characterized by LREE depletion relative to HREE with positive Ce and negative Eu anomalies, typical of magmatic zircons. The high Hf content together with high Ce/Ce*, Th/U, Zr/Hf ratios suggest magma crystallization under variable oxidation and oxygen fugacity. The application of Ti-in-zircon thermometer reveals crystallization temperatures ranging from 678°C to 811°C and 658°C to 768°C for quartz monzonites and nepheline syenites respectively. These features indicate probably a partial melting of continental crust as the source of these zircons grains and emplacement in the magmatic-arc setting.

High Ba–Sr adakitic charnockite suite from the Nagercoil Block, southern India: Vestiges of Paleoproterozoic arc and implications for Columbia to Gondwana

The Nagercoil block is the southernmost crustal segment of the Southern Granulite Terrane (SGT) in India and is mainly composed of charnockitic rocks and felsic gneisses (charnockite suite). In this study, we present petrologic, geochemical, zircon U–Pb, REE, and Hf isotopic studies on the charnockites and leucogneiss from the Nagercoil block. Based on field investigations and petrologic studies, the charnockites can be divided into garnet-bearing and garnet-absent anhydrous granulite facies rocks with orthopyroxene. The charnockites and leucogneiss show transition from adakites to non-adakitic magmatic rocks, with enrichment in LREEs (light rare earth elements) and LILEs (large ion lithophile elements), and depletion in HREEs (heavy rare earth elements) and HFSEs (high field strength elements). Some of the charnockites and the leucogneiss show typical HSA (high silica adakite) characters, (high SiO2, Al2O3, Ba–Sr, La/Yb, and Sr/Y). The HSA is considered to have formed from the interaction of slab derived melts and peridotitic mantle wedge. The high Ba–Sr features were possibly inherited from subducted oceanic crust melting under high thermal gradient during Precambrian. The magmas were underplated and subjected to fractional crystallization. Zircon grains from the charnockite and leucogneiss show zoned magmatic cores surrounded by structureless metamorphic rims. Magmatic zircon grains from the charnockites show ages ranging from 1983 ± 8.8 Ma to 2046 ± 14 Ma, and the metamorphic domains show an age range of 502 ± 14 Ma to 547 ± 8.7 Ma. Zircon from the leucogneiss yielded magmatic and metamorphic ages of 1860 ± 20 Ma and 575.6 ± 8.8 Ma. Both charnockites and leucogneiss show two prominent age peaks at 1987 Ma and 568 Ma. The REE data of the zircon grains show LREE depletion and HREE enrichment, with the metamorphic grains showing more depletion in HREE. Zircon Hf isotopic data of the magmatic cores of zircon grains from the charnockite yielded εHf(t) values from −1.17 to 0.46 with TDM and TDMC and age peaks at 2392 Ma and 2638 Ma, suggesting Neoarchean to Paleoproterozoic juvenile sources. We suggest that the high Ba–Sr adakitic charnockite suite from the Nagercoil block formed in a Paleoproterozoic magmatic arc setting during the assembly of the Columbia supercontinent, and underwent high-grade metamorphism associated with the amalgamation of the Gondwana supercontinent during the late Neoproterozoic–Cambrian. Our study provides new insights into the vestiges of Columbia fragments within the Gondwana assembly with two distinct cycles of crustal evolution.

Origin of Mesozoic Porphyritic Rocks and Regional Magmatic Evolution in the Zijinshan Ore Field of Fujian Province, China: Hf-O Isotope Characteristics of Magmatic Zircons

Mesozoic porphyritic rocks from the Zijinshan area, southwestern Fujian Province, China, are andesitic to rhyolitic in composition. The whole-rock SiO2 contents of these rocks are between 62.5% and 78.1%. Magmatic zircon from the Mesozoic porphyritic rocks was determined via secondary-ionization mass spectrometry (SIMS) for the U-Pb age and Hf and O isotopes. The zircon U-Pb ages could be mainly divided into three age groups: Group 1: ~138.8 Ma; Group 2: 109.2~107.4 Ma; and Group 3: 99.7~98.2 Ma. The εHf(t) and δ18O values of the porphyritic zircons showed that the porphyritic rocks in Group 2 were more affected by mantle-derived magma. Combined with previous research results, the medium-acidic magmatism in the southwestern Fujian Province can be divided into eight periods: Paleoproterozoic, Mesoproterozoic, Middle Neoproterozoic, Silurian to Lower Devonian, Permian to Triassic, Middle Jurassic to early Lower Cretaceous, late Lower Cretaceous, and late Lower Cretaceous to early Upper Cretaceous. The Paleoproterozoic crust was the predominant magmatic source for the subsequent Mesoproterozoic to Jurassic magmatism, but the only melts that were closely related to mineralization were derived from partial melting of the Mesoproterozoic crust and a more depleted upper mantle.

Cordilleran Subduction Initiation: Retroarc Timing and Basinal Response in the Inyo Mountains, Eastern California

Abstract Subduction zones drive plate tectonics on Earth, yet subduction initiation and the related upper plate depositional and structural kinematics remain poorly understood because upper plate records are rare and often strongly overprinted by magmatism and deformation. During the late Paleozoic time, Laurentia’s western margin was truncated by a sinistral strike-slip fault that transformed into a subduction zone. Thick Permian strata in the Inyo Mountains of central-eastern California record this transition. Two basins that were separated by a transpressional antiform contain sedimentary lithofacies that record distinct patterns of shoaling and deepening conditions before and during tectonism associated with subduction initiation. Sandstone petrography and lithofacies analysis show that rocks in a southeastern basin are dominated by carbonate grains derived from adjacent carbonate shelves, whereas sandstones in a northwestern basin are predominantly quartzose with likely derivation from distant ergs or underlying strata. Detrital zircon spectra from all but the youngest strata in both basins are typical of Laurentian continent spectra with prominent peaks that indicate ultimate sources in Appalachia, Grenville, Yavapai/Mazatzal, and the Wyoming or Superior cratons. The first Cordilleran arc-derived detrital zircon grains appear in the uppermost strata of the northwestern basin and record Late Permian (ca. 260 Ma) Cordilleran arc magmatism at this approximate latitude, and a possible source area is suggested by geochemical similarities between these detrital zircons and broadly coeval magmatic zircons in the El Paso Mountains to the southwest. Deformation responsible for basin partitioning is consistent with sinistrally oblique contraction in the earliest Permian time. The data presented from the Inyo Mountains shed more light on the nature of Cordilleran subduction initiation and the upper-crustal response to this transition.

Geochronology and geochemistry of Devonian magmatism in the Frontal cordillera (Argentina): geodynamic implications for the pre-Andean SW Gondwana margin

ABSTRACT Whereas north of 33°S Devonian calcalkaline magmatism is notably absent, a Devonian arc is well developed to the south of 33°S. The Carrizalito, Pampa de los Avestruces, and Papagayos plutons are located in the Frontal Cordillera between 34° and 35°S. U-Pb zircon SHRIMP and LA-MC-ICP-MS data from the plutons yield Early Devonian ages of 389 ± 3 and 409 ± 3 for Carrizalito, 413 ± 2 Ma for Pampa de los Avestruces, and 346 ± 3, 393 ± 5 and 412 ± 3 Ma for Papagayos. The younger zircon age from the Papagayos pluton suggests that the Devo nian magmatism continued during the Early Carboniferous. Carrizalito and Pampa de los Avestruces plutons display metaluminous to slightly peraluminous granodiorite composition, with SiO2 ranging from 65.1 to 67.8 wt.% and from 67.9 to 69.7 wt.%, respectively, and define a magnesian calc-alkalic trend. The Papagayos pluton is metaluminous to slightly peraluminous and slightly more evolved in composition (felsic granodiorite to monzogranite), with a SiO2 content that varies between 68.4 to 70.5 wt.%, defining a magnesian with ferroan enrichment and calc-alkalic to alkali-calcic trend. These geochemical characteristics are similar to those reported for the Carboniferous arc granitoids in the Frontal Cordillera and Western Sierras Pampeanas at 27°–30°S and the archetypical calc-alkaline granitoids of the Peruvian Coastal batholith, indicating an Early Devonian magmatic arc. Rare earth element patterns ([La/Yb]N = 10–20) with mostly slight to moderate negative Eu anomalies (Eu/Eu* = 0.62–0.80) and geochemical ‘spider-diagrams’ are similar to arc granitoids. Combined whole-rock Nd and Sr isotope data from the largest Carrizalito and Pampa de los Avestruces plutons (εNdt = −4.3 and −4.6; 87Sr/86Srt = 0.70704 and 0.70762; TDM = ca. 1.5 Ga) together with in situ U–Pb and Hf isotope data from magmatic zircon (εHft ranging from −16.5 to −5.4; TDM = ca. 1.7 Ga), suggest a common older continental lithosphere source. The presented data indicate tectonic segmentation in the pre-Andean margin controlling the presence or absence of Devonian subduction-related magmatism. This segmentation is best explained by changes in the tectonic plate configuration occurred in a long-lived convergent margin.