Serpentinites and serpentinized mantle peridotites with various tectonic origins occur in the Franciscan Complex of the Northern California Cordillera, USA. Boron isotopes of serpentinites ...differentiate with fluid-mediated processes, and have great potential for key geologic markers in convergent margins. To understand boron isotope behavior within the Franciscan subduction zone system, we apply a newly developed ablation volume correction (AVC) method for in-situ isotope/elemental analyses using a laser-ablation multiple collector inductively-coupled-plasma mass spectrometry (LA-MC-ICPMS) on seventeen different Franciscan serpentinites (sensu lato) collected from eight separate areas. Boron abundances and isotope compositions of the studied serpentinites show large variations B = 1.6–239 μg·g−1, δ11B = −12.0 to +24.4‰, which allow to discriminate the serpentinites into two groups: (1) a lighter δ11B of −12.0 to +8.8‰ with a lesser B < ~56 μg·g−1 and (2) a heavier δ11B of +7.2 to +24.4% with a greater B ~34–239 μg·g−1. These groups lithologically correspond to the presence or absence of associated blueschist-facies metamorphic rocks, respectively. The blueschist-bearing and/or blueschist-associated serpentinites might have been affected by a deep forearc slab fluids in the depth of > ~2 GPa. Preferential partitioning of 11B into fluids released from the subducted slab at shallow leaves lighter δ11B in the slab resulting in lighter δ11B in the deep slab fluids. In contrast, the blueschist-absent serpentinites with heavier δ11B may have formed at a shallow environment where shallow slab or hydrothermal fluids with heavier 11B were present. Lesser versus greater amounts of B in the deep versus shallow serpentinites are also consistent with the dehydration profile of B from a slab. Our results show the versatility of boron isotopes and composition for identification of the origin of serpentinite in Pacific-type orogenic belts.
•B isotope compositions of Franciscan serpentinites (17 samples from 8 localities) were determined by LA-MC-ICPMS.•The B isotope composition discriminates the serpentinites into two groups.•Serpentinites associated with blueschists are characterized by low δ11B (−12 to +8.8‰).•Serpentinites without blueschists are characterized by high δ11B (+7.2 to +24.4‰).•The results show the versatility of B isotopes for identification of the origin of serpentinite.
The Ubendian Belt between the Archean Tanzania Craton and the Bangweulu Block, represents a Paleoproterozoic orogeny of these two constituents of the Congo Craton assembled at ~1.8 Ga, forming the ...Central African Shield, during the Columbia Supercontinent cycle and consolidated during the Gondwana assembly. Metagranitoids from the Southern and Northern Ufipa Terranes (Western Ubendian Corridor) and those of the Bangweulu Block are compositionally similar and are contemporaneous. The protolith of the Ufipa Terrane is originated from the collided crustal rocks of the Bangweulu Block. New LA-ICPMS zircon U–Pb age of metagranitoids and granoporphyries confirmed magmatic events from 1.89 to 1.85 Ga. The metagranitoids of the Western Ubendian Corridor and that of the Bangweulu Block cannot be distinguished by their trace element characteristics and ages. Geochemically, they belong to high-K calc-alkaline to tholeiite series. The 1.89–1.85 Ga metagranitoids and granoporphyries are characterized by evolved nature, which are common for slab-failure derived magmas. Such geochemical features and the presence of ~2.0 Ga eclogites suggest an Orosirian oceanic subduction and subsequent slab break-off. Melt derived from the mafic upper portion of torn slab led to the partial melting of crust which formed high-K and calc-alkaline, I- and S-type magmatism in the Bangweulu Block and the Ufipa Terrane. Zircons from two metagranites from the Northern Ufipa Terrane show Neoproterozoic (Ediacaran) overprints at ~570 Ma, suggesting the Bangweulu Block collided with the continental margin of the Tanzania Craton. However, we found non-annealed Orosirian apatites in metagranitoids from the Southern Ufipa Terrane and the Kate–Ufipa Complex, implying that areal heterogeneity of the Pan-African tectonothermal overprint in the Ufipa Terrane. All evidences suggest that the Bangweulu Block and the Ubendian Belt participated in the amalgamation of the Central African Shield as separated continents surrounded by oceanic crusts during the Paleoproterozoic Eburnean and the Neoproterozoic Pan-African orogenies.
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•Ubendian and Bangweulu units amalgamated within the Central African Shield•Ufipa Terrane's protolith (Ubendian Belt) originated from the Bangweulu Block•~1.87 Ga granitoids came from slab failure magmatism and lower crust partial melting•Neoproterozoic (Pan-African) metamorphism affected unequally the Ufipa Terrane units
Abstract
In collision-type orogens, where high-pressure and ultrahigh-pressure (HP–UHP) metamorphism usually occurs, deeply subducted continental slabs with eclogitized mafic rocks often undergo ...recrystallization/overprinting with various geothermal gradients after the peak conditions at lower-to-middle-crustal levels. During the crustal stabilization, the transition from eclogite-to granulite-facies is common. We conducted metamorphic petrology and zircon geochronology on (1) bimineralic and (2) partially granulitized eclogites from the Neoproterozoic Ufipa Terrane (Southwestern Tanzania). Microtextural relationships and mineral chemistry define three metamorphic stages: eclogite metamorphism (M1), HP granulite-facies overprinting (M2), and amphibolite-facies retrogression (M3). The bimineralic eclogite has a basaltic composition and lacks M2 minerals. In contrast, the kyanite eclogite is characterized by a gabbro-dioritic whole-rock composition and contains inherited magmatic zircon. Although the matrix is highly granulitized, garnet and kyanite contain eclogite-facies mineral inclusions. Phase equilibria modeling revealed P–T conditions of 2.1–2.6 GPa and 650–860°C for the M1 stage and 1.4–1.6 GPa and 750–940°C for the M2 stage. Zircon with eclogite-facies mineral inclusions from the bimineralic eclogite lacks Eu anomaly in the REE patterns and yielded the M1 eclogite metamorphic age of 588 ± 3 Ma. Zircon overgrowths surrounding the inherited Paleoproterozoic magmatic cores in kyanite eclogite yielded 562 ± 3 Ma. A weak negative Eu anomaly in the REE patterns and the absence of eclogitic mineral inclusions suggest the zircon growths at the M2 HP granulite-facies metamorphic stage. These new data indicate an eclogite-to granulite-facies transition time of 26 ± 4 million years (Myr), suggesting a rate of HP rock exhumation toward a lower crustal level of 0.7–1.5 mm/year. Furthermore, the density evolution model indicates that buoyant host orthogneiss with low-density gabbro-dioritic eclogite plays an important role in carrying high-density basaltic eclogite. Our 2D thermomechanical modeling also suggests that a slab break-off with a lower angle subduction of <20° triggers the exhumation of the HP slab sliver with 20–30 Myr eclogite-to granulite transition time of large HP–UHP terranes in major collision zones.
White mica (phengite and paragonite) K-Ar ages of eclogite-facies Sanbagawa metamorphic rocks (15 eclogitic rocks and eight associated pelitic schists) from four different localities yielded ages of ...84-89 Ma (Seba, central Shikoku), 78-80 Ma (Nishi-Iratsu, central Shikoku), 123 and 136 Ma (Gongen, central Shikoku), and 82-88 Ma (Kotsu/Bizan, eastern Shikoku). With the exception of a quartz-rich kyanite-bearing eclogite from Gongen, white mica ages overlap with the previously known range of phengite K-Ar ages of pelitic schists of the Sanbagawa metamorphic belt and can be distinguished from those of the Shimanto metamorphic belt. The similarity of K-Ar ages between the eclogites and surrounding pelitic schists supports a geological setting wherein the eclogites experienced intense ductile deformation with pelitic schists during exhumation. In contrast, phengite extracted from the Gongen eclogite, which is less overprinted by a ductile shear deformation during exhumation, yielded significantly older ages. Given that the Gongen eclogite is enclosed by the Higashi-Akaishi meta-peridotite body, these K-Ar ages are attributed to excess
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Ar gained during an interaction between the eclogite and host meta-peridotite with mantle-derived noble gas (very high
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Ar/
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Ar ratio) at eclogite-facies depth. Fluid exchange between deep-subducted sediments and mantle material might have enhanced the gain of mantle-derived extreme
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Ar in the meta-sediment. Although dynamic recrystallization of white mica can reset the Ar isotope system, limited-argon-depletion due to lesser degrees of ductile shear deformation of the Gongen eclogite might have prevented complete release of the trapped excess argon from phengites. This observation supports a model of deformation-controlled K-Ar closure temperature.
Retrograde pumpellyite was newly found in garnet blueschist that is Mg–rich equivalent of late Paleozoic retrograde eclogite of the Yunotani Valley in the Omi area, Hida–Gaien Belt. The pumpellyite ...with high Al/(Al + Mg + Fe) occurs in pressure shadows around garnets; it is associated with secondary glaucophane, epidote, chlorite, titanite, phengite, albite, and quartz, which all characterize a retrograde blueschist–facies mineral assemblage after peak eclogite–facies mineral assemblage. This feature is comparable with retrograde pumpellyite in late Paleozoic garnet blueschist (with relict eclogite–facies mineral assemblage) in the Osayama area of the Chugoku Mountains. Equilibrium phase calculation confirmed that the pumpellyite is stable at a low temperature and pressure portion of the lawsonite–blueschist–facies. T–bulk–composition (Mg) pseudosection suggests that pumpellyite appears preferentially in high Mg/(Mg + Fe) bulk composition. The limited occurrence of retrograde pumpellyite in the Yunotani garnet blueschist and retrograde eclogite would be explained by Mg–rich bulk compositions. Also, the limited occurrence in pressure shadows around garnets suggests that the fluid trapped in the pressure shadows might have enhanced growth (or precipitation) of pumpellyite. This finding provides a strong evidence that the deeply subducted (eclogite–facies) metabasaltic rocks both in the Hida–Gaien Belt and the Chugoku Mountains were subjected to a very similar blueschist–facies overprinting locally reached the pumpellyite stability field. The ‘Franciscan–type’ cooling path suggests a ‘steady–state’ underflow of the paleo–Pacific oceanic plate in late Paleozoic at a convergent margin of the South China Craton.
The Kitomyo Schist from Kurosegawa Belt, Shikoku, has been long considered as the oldest records of subduction metamorphism in Japan, based on an early 1970s K–Ar dating of white mica. The schist ...consists of mafic and pelitic layers and occurs as a tectonic block within serpentinite. Reappraisal of the schist confirmed the schist is characterized by an epidote-amphibolite peak metamorphic facies. The mafic portion is characterized by zoned amphibole + epidote + chlorite + titanite ± phengite ± rutile. The presences of relict rutile surrounded by titanite and the barroisitic cores of zoned amphibole suggest a high-pressure intermediate type metamorphism at the metamorphic peak (P = ~0.8–1.5 GPa and T = ~500–570 °C). The presence of Mn-rich garnet and the lack of biotite, oligoclase and paragonite also support high-pressure intermediate type metamorphism that eliminate the possibility of a typical blueschist-facies metamorphism. New SHRIMP and LA-ICPMS zircon U–Pb geochronology on a pelitic sample show detrital grains of Mesoproterozoic and Early Paleozoic ages, suggesting a maximum deposition age for the trench-fill sediment of ~440 Ma. Also the U–Pb data confirmed ~360 Ma overgrown rims that might have formed during the subduction zone epidote-amphibolite facies metamorphism. Reappraisal revealed that the Kitomyo Schist is not the oldest high-pressure type schist in Japan and rather comparable to the Late Paleozoic Renge Metamorphic Rocks and their equivalents in the Kurosegawa Belt. The Devono–Carboniferous high-pressure metamorphic rocks in Japan might have been paired with their coeval batholiths along the ‘Greater South China’ margin that was extensively eroded during later tectonic processes.
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•The putative oldest high-pressure schist in Japan metamorphosed at ~360 Ma•This schist shows the typical Late Paleozoic metamorphic ages found in Renge belt•360 Ma HP rocks in Japan are coeval with batholiths•Renge HP and batholiths possibly formed a metamorphic paired belt
The Mongol-Okhotsk Belt is the youngest segment of the Central Asian Orogenic Belt, which is the venue of the massive juvenile crust emplacement, and its formation and evolutions are still pending ...problems. This paper presents the first up-to-date U–Pb zircon ages, Hf-in-zircon isotope, geochemical and whole-rock Nd isotope data from igneous rocks of the Khangay-Khentey basin, Central Mongolia. The U–Pb zircon ages indicate three groups of magmatism at ~296 Ma, ~280 Ma, and ~230 Ma. The ~296 Ma magmatic rocks are characterized by negative εHf(t) and εNd(t) values and old Hf and Nd model ages suggesting their derivation by the melting of the crustal source. The ~280 Ma rocks are A2-type monzonites, granitoids, and rhyolites show positive εHf(t) and εNd(t) values and Neoproterozoic Hf and Nd model ages. The geochemical and isotope data suggest that ~280 Ma magmatism derived by the melting of a crustal source, induced by mantle upwelling. The ~230 Ma rock assemblage includes granitoids and volcanic rocks. The I-type calc-alkaline granitoids are enriched in K, Rb, U, and Th. The geochemical characteristics suggest that they have formed by the melting of a hornblende-bearing crustal source with the participation of fluids separated from the subducting slab. The positive εHf(t) and εNd(t) ~230 Ma rocks suggest partial melting of a depleted lower crustal material with the contribution of ancient crustal material. The ~296 Ma granitoids possess coherent/coupled Nd–Hf isotopic compositions supporting their origin from the ancient crust. Although the number of ~296 Ma samples are small, we suggest that they were probably emplaced at an active continental setting, ~280 Ma samples could have formed in a setting of local extension environment, ~230 Ma granitoids were also formed at an active continental margin. These magmatic rocks formed during the subduction of the Mongol-Okhotsk oceanic plate beneath the Central Mongolia-Erguna Block.
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•Granitoids in the youngest segment of CAOB can be divided into three groups•~296 Ma granitoids are I-type and emplaced along active continental margin setting•~280 Ma rocks belong to the A-type and formed in an extension setting•~230 Ma rocks are related to the subduction of the Mongol–Okhotsk Ocean
•UHP minerals in orogenic granulite, in ophiolitic chromitite, and in mantle xenolith.•Recycling of crustal materials+organic carbon through subduction, mantle upwelling, and to the ...surface.•Crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds.•New ophiolite-type diamond+highly reduced mineral and alloy formed at deep mantle.
Newly recognized occurrences of ultrahigh-pressure (UHP) minerals including diamonds in ultrahigh-temperature (UHT) felsic granulites of orogenic belts, in chromitites associated with ophiolitic complexes, and in mantle xenoliths suggest the recycling of crustal materials through deep subduction, mantle upwelling, and return to the Earth’s surface. This circulation process is supported by crust-derived mineral inclusions in deep-seated zircons, chromites, and diamonds from collision-type orogens, from eclogitic xenoliths in kimberlites, and from chromitities of several Alpine–Himalayan and Polar Ural ophiolites; some of these minerals contain low-atomic number elements typified by crustal isotopic signatures. Ophiolite-type diamonds in placer deposits and as inclusions in chromitites together with numerous highly reduced minerals and alloys appear to have formed near the mantle transition zone. In addition to ringwoodite and inferred stishovite, a number of nanometric minerals have been identified as inclusions employing state-of-the-art analytical tools. Reconstitution of now-exsolved precursor UHP phases and recognition of subtle decompression microstructures produced during exhumation reflect earlier UHP conditions. For example, Tibetan chromites containing exsolution lamellae of coesite+diopside suggest that the original chromitites formed at P>9–10GPa at depths of >250–300km. The precursor phase most likely had a Ca-ferrite or a Ca-titanite structure; both are polymorphs of chromite and (at 2000°C) would have formed at minimum pressures of P>12.5 or 20GPa respectively. Some podiform chromitites and host peridotites contain rare minerals of undoubted crustal origin, including zircon, feldspars, garnet, kyanite, andalusite, quartz, and rutile; the zircons possess much older U–Pb ages than the time of ophiolite formation. These UHP mineral-bearing chromitite hosts evidently had a deep-seated evolution prior to extensional mantle upwelling and partial melting at shallow depths to form the overlying ophiolite complexes. These new findings together with stable isotopic and inclusion characteristics of diamonds provide compelling evidence for profound underflow of both oceanic and continental lithosphere, recycling of surface ‘organic’ carbon into the lower mantle, and ascent to the Earth’s surface through mantle upwelling. Intensified study of UHP granulite-facies lower crustal basement and ophiolitic chromitites should allow a better understanding of the geodynamics of subduction and crustal cycling.
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