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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    Silurian intermediate-felsic complex in the Xiangtaohu area of central Qiangtang, northern Tibet: Evidence for southward subduction of the Longmuco-Shuanghu Prototethys oceanic plate: Comment

    Li-Zhi ShiZhuo-Zhuo Wang
    3页
    查看更多>>摘要：We disagree that, as suggested by Liu et al. (2021) (Lithos, v. 404-405, 106,465), the newly discovered Silurian (438-429 Ma) intermediate-acid plutons in the Xiangtaohu area of central Qiangtang, northern Tibet, marked southward subduction of the Shuanghu Tethyan ocean and divergent double subduction occurred in the Shuanghu Tethyan ocean during the Paleozoic. A review indicates that these Silurian magmatic rocks are located to the northern edge of the ~100-km-wide central Qiangtang accretionary complex of the Shuanghu suture zone. They, along with Paleozoic fore-arc sediments and arc magmatic rocks in the southernmost margin of the northern Qiangtang terrane, indicate the northward subduction of the Shuanghu Tethyan ocean during the Paleozoic. Furthermore, no evidence of magmatic association and geochemistry supports divergent double subduction in the Shuanghu Tethyan ocean during the Paleozoic.
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    Onset of the anorogenic alkaline magmatism in the Nigerian Younger Granite province: Constraints from the Daura and Dutse complexes

    Victor Ikechukwu VincentLian-Xun WangYu-Xiang Zhu
    29页
    查看更多>>摘要：The Younger Granite province of Niger and Nigeria exemplifies a classical anorogenic alkaline ring granite belt with southward age migration from ca. 470 Ma to 140 Ma. However, the precise age transition from the older Nigerien Younger Granite (NrYG) to the younger Nigerian Younger Granite (NaYG) is poorly constrained. The Daura and Dutse complexes, located at the northernmost tip of the NaYG province, represent a temporal and spatial link between the NrYG and NaYG provinces. The Daura complex is composed of amphibole syenite and biotite granite, while the Dutse complex consists of fayalite syenite and arfvedsonite aegirine granite. Similar to other alkaline rocks of the NrYG and NaYG provinces, the Daura and Dutse samples all chemically show typical A1-type granite features, such as high alkalis (K2O + N2O = 8.33-11.27 wt%.), Ga/Al ratios, Zr + Nb + Ce + Y contents (527-2345 ppm) and FeOt/(FeOt+MgO) ratios (0.03-0.70) as well as elevated HFSEs. The Nd-Hf and Pb isotopic systematics show that all the syenite-granites yield eNd (t) = -2.9 to +1.5, eHf (t) = -3.8 to +16.7, and ~(206)Pb/~(204)Pb - 17.818 to 19.555, suggestive of magmas derived from a slightly depleted mantle source with OIB imprints. A reduced to slightly oxidized redox state is invoked for the parental melt based on estimations from mineral chemistry. The Daura amphibole syenite is composed of >60% alkali feldspar, show high Eu/Eu* (>1), Ba/Zr (>3.5) values and cumulate textures, resembling an alkali feldspar cumulate rock. The other alkaline granitic rocks were likely generated from variable fractional crystallization of the residual melt with distinct crustal contaminations. Zircon U-Pb dating reveal that the Daura amphibole syenite and biotite granite were emplaced at 324 ± 1 Ma and 321 ± 3 Ma, respectively, whereas the Dutse granitoids showed episodic intrusive periods of ca. 264 Ma and 232 Ma. All these results predate other anorogenic alkaline complexes of the NaYG and update the oldest age (ca. 214 Ma in previous references) recorded in the alkaline granite province. To the north, it connects well with ages from the Zinder complex (-330 Ma) in the southernmost NrYG province, reflecting a significant link between the NrYG and NaYG provinces. From a broad view, a linear age migration from north to south in the Niger-Nigeria Younger Granite province remains well preserved, although temporal and spatial intervals exist in local region. This emplacement pattern can be traced to the periodic and repeated reactivation of lithospheric shear zones and transcurrent faults.
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    The Quaternary monogenetic Bayuda Volcanic Field, Sudan - Insights into mantle and crustal processes during magma petrogenesis

    Francois J.P. LotterNils LenhardtLothar Viereck
    20页
    查看更多>>摘要：The Holocene Bayuda Monogenetic Volcanic Field (BMVF) is located at the Great Bend of the river Nile in the north of Sudan. The analysed volcanic rocks are of basanitic to hawaiitic composition, characterising the BMVF as a sodic-alkaline volcanic field. Petrographic evidence documents olivine and clinopyroxene as the main fractionating phases as well as FC processes that were accompanied by assimilation of mantle and crustal rocks in addition to mingling and mixing processes. Utilising systematic differences in trace element variations, two primary magmas of Series A and B can be distinguished. Uniform normalised incompatible element patterns, e.g. LREE enrichment accompanied by HREE depletion, negative anomalies of K and Pb, and modelling based on partition coefficients of La, Dy and Yb in mantle residues, suggest partial melting of an amphibole-garnet lherzolite. The presence of amphibole provides a constraint, implying that the melt was derived from a meta-somatised lithospheric mantle. The presence of the Pb-depletion (documented by Ce/Pb ratios of >30) suggests that all magmas are of HIMU-OIB type. This is supported by their uniform ~(87)Sr/~(86)Sr (0.703010-0.703347), ~(143)Nd/~(144)Nd (0.512869-0.512995), ~(206)Pb/~(204)Pb (19.787-19.895), ~(207)Pb/~(204)Pb (15.656-15.653) and ~(208)Pb/~(204)Pb (39.560-39.678) isotope characteristics. Additionally, we can describe the HIMU character of the NE African mantle with respect to ~(176)Hf/~(177)Hf (0.283006-0.283026). The mantle source of the BMVF magmas appears to have the composition of the common mantle ''C'' with an overprint by a HIMU component, resulting in higher radiogenic ~(206)Pb/~(204)Pb ratios above 19.5 (FOZO-HIMU-OIB type). We attribute this to the metasomatic character of the lithospheric mantle. The Hf/Nd isotope characteristics of the BMVF suggest that this metasomatism in the mantle source is most probably late Pan-African in age. When compared to circum-Mediterranean and regional volcanic fields, the high radiogenic Pb character of the BMVF indicates that their genesis cannot be related to magmatism associated with the activity of the Afar plume event and consequential magmatism along the Main Ethiopian Rift and the Red Sea Rift, as they are all of low radiogenic character.
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    Tectonic evolution of Southwest Japan at the Cretaceous time inferred from the zircon U-Pb geochronology along the Maana Belt, western Shikoku, Japan

    Kenta KawaguchiYasutaka HayasakaTomoyuki Shibata
    23页
    查看更多>>摘要：The Mesozoic geological history of proto-Japan holds a crucial role in understanding the tectonic evolution during this time in East Asia, as a whole. In this context, the evolutionary history of the Median Tectonic Line (MTL) is important but still under debate. Most of the studies on the MTL were carried out on well-preserved clear zonally distributed areas such as the central Shikoku Island, Southwest Japan. However, these clear zonal arrangements are absent and several tectonic units including the MTL appear in close proximity at the westernmost Shikoku Island and the Maana Belt occurs here in a restricted area. Such unclear arrangements of lithounits pose a difficulty for a comprehensive understanding of the large-scale tectonic evolution of Southwest Japan. Hence, the study on and along the Maana Belt is crucial to unravel the holistic tectonic evolution of the MTL. The zircon age dating of the tonalites, geochemically of volcanic arc affinity, revealed 470.6 ± 3.2 and 468,3 ± 2.7 Ma as a magmatic age, which confirmed them as members of the Kurosegawa tectonic zone. In contrast, the scarce chemically-altered leucocratic mylonite shows the zircon age of 151.8 ±1.8 Ma as a possible timing of magmatism. This may be the exotic block because its age is known as a magmatic hiatus in Southwest Japan. The detrital zircon dating of three (meta-)sedimentary rocks along the Maana Belt confirmed that they can be attributed to the Northern Chichibu Belt or the Kurosegawa tectonic zone. Newly characterized rock unit (= the Northern Chichibu Belt with the Kurosegawa tectonic zone) is in contact with the mid-Cretaceous igneous province of the Oshima metamorphic rocks. The suggested juxtaposition in the present study area is a remnant of the original settings including the initial phase of the MTL but is completely disrupted by the exhumation of the Sambagawa Belt in major parts of the Shikoku Island. Identifying diis original setting provides not only the initial arrangement of different lithounits but also a clear understanding of the preexisting large-scale tectonic modification systems at the margin of the East Asian continent.
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    Reassessing the chronostratigraphy and tempo of climate change in the Lower-Middle Permian of the southern Sydney Basin, Australia: Integrating evidence from U-Pb zircon geochronology and biostratigraphy

    G.R. ShiAllen P. NutmanSangmin Lee
    21页
    查看更多>>摘要：The Permian of the southern Sydney Basin (Australia) comprises both marine and non-marine sedimentary sequences with evidence of glacial deposition and syndepositional magmatism represented by tuffs, lava flows and hypabyssal intrusions. This succession has been well-studied in lithostratigraphy and sedimentology, but the Lower-Middle Permian chronostratigraphy remains poorly constrained due to a lack of radiometric isotope ages and strong faunal endemism. In this study, we report the first suite of reconnaissance U-Pb zircon ages from this succession, comprising two Wandrawandian Siltstone (WS) tuffs (Green Point tuff 272.6 ± 6.5 Ma population, Callala Bay tuff >=290 Ma Permian populations) and magmatic zircon ages from three discrete syndepositional intrusions: the Termeil Complex intruding the Snapper Point Formation dated 278.9 ± 3.7 Ma (95% confidence), the Milton Complex intruding the Snapper Point Formation and likely the WS dated 275.5 ± 4.3 Ma, and a dyke/ sill at Kinghorn Point intruding the WS dated 275.6 ± 4.3 Ma. In light of the new preliminary U-Pb ages, combined with stratigraphic and biostratigraphic evidence, the age of the Wasp Head Formation is reassessed to be late Asselian-Sakmarian, Pebbley Beach Formation Artinskian, Snapper Point Formation early Kungurian, Wandrawandian Siltstone late Kungurian-Roadian, Nowra Sandstone and Berry Siltstone both confined within early Wordian, and Broughton Formation from mid-Wordian to early Capitanian. The ages and duration of the Permian glacial events are also revised, with the first episode (PI) lasting from basal Permian to early Sakmarian (~299-293 Ma), P2 spanning Artinskian (~290-283 Ma), P3, possibly the longest of all Permian glacial intervals, extending from mid-Kungurian to early Capitanian (~278-263 Ma), and P4 from mid-Capitanian to late Wuchiapingian (~260-254.5 Ma). The >290 Ma Permian zircon populations in the Callala Bay tuff reflect either an early Permian volcanism or older zircons recycled from elsewhere.
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    Highly variable petrophysical properties in felsic high-pressure rocks of the continental crust

    Sascha ZertaniJan PleugerHem B. Motra
    18页
    查看更多>>摘要：Seismic wave velocities and anisotropy can potentially discern lithologies and their structural configuration at depth. An increasing amount of data exists that indicates seismic anisotropy can be a valuable marker to identify internal deformation witiiin deeply buried crust. Here, we present petrophysical properties of variably deformed felsic rocks that have been metamorphosed at blueschist and eclogite-facies conditions, representative of the bulk composition of the mid continental crust. The samples were collected from the Monte Mucrone area (Sesia Zone, Western Alps) and the Serra di Pigno, Farinole, and Tenda areas (Corsica). The mineral assemblages of all samples indicate that fluids were abundant during re-equilibration at HP conditions. Petrophysical properties were derived from ultrasonic measurements as well as calculated from crystallographic preferred orientations obtained by neutron texture goniometry. The results show that crystallographic preferred orientations are weak and yet the magnitude of seismic anisotropy produced by metamorphism of the felsic continental crust is highly variable, ranging from 1 to 12% for P waves and 1-14% for S waves depending on the strength of deformation fabrics. At the same time, absolute wave velocities remain low compared to more mafic rocks that have equilibrated at HP conditions. These results demonstrate that, other than their mafic equivalents, felsic HP rocks cannot be distinguished from their protoliths by wave velocities because their increase during HP metamorphism is too small. However, fluid availability during metamorphism and deformation causes strong variations in the seismic anisotropy of such lithologies, that might be distinguishable at depth.
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    Early cretaceous tectonic setting of eastern Australia: Evidence from the subduction-related Morton Igneous Association of Southeast Queensland

    Robert HendersonC. SpandlerE.K. Foley
    13页
    查看更多>>摘要：Early Cretaceous magmatic rocks of the central east coast of Australia are recognized as a lithologically, geochemically, and geochronologically distinctive rock package herein named the Morton Igneous Association. The association embraces small plutons and hypabyssal intrusions within the Jurassic and Lower Cretaceous Nambour and Maryborough basins. It also includes volcanics of the Grahams Creek Formation, basal infill of the Maryborough Basin with an indicated volume exceeding 0.25 M km3, representing a much more substantial igneous assemblage. Plutonic rocks vary from hornblende-rich gabbro to granodiorite, and syenite. The Grahams Creek Formation consists of volcanic to volcaniclastic rocks of basaltic to dacitic composition. The association is U-Pb zircon dated at 145-140 Ma, and is characterized by distinctive subduction-related trace element compositions typical of magmatic rocks from continental arc settings. Oxygen isotope and εHf compositions obtained from zircon are consistent with a largely mantle source for the host magmas. Trace element data for the 130-95 Ma Whitsunday Igneous Province, of more northerly location on the east Australian coast, are consistent with a continental back-arc association. The MIA and Whitsunday Igneous Province represent small parts of an extensive Cretaceous active margin system developed for east Gondwana, mainly located on continental crust comprising northern Zealandia which is now separated from east Australia, largely submerged and little documented. East Australia records both older (145-140 Ma) continental arc magmatism and younger (132-95 Ma) continental back-arc magmatism attributed to progressive Cretaceous roll-back of a consuming plate boundary to east Gondwana.
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    Unveiling the occurrence of transient, multi-contaminated mafic magmas inside a rhyolitic reservoir feeding an explosive eruption (Nisyros, Greece)

    E. BraschiF. MastroianniS. Di Salvo
    20页
    查看更多>>摘要：The investigation of heterogeneous magma systems enhances the understanding of magma differentiation and transfer processes in active volcanoes, thus constraining the dynamics driving the eruptions and the related hazard. Magma heterogeneity is generally preserved in the coeval juvenile products of explosive eruptions, as it occurs in the Upper Pumice sequence, emplaced by the last sub-Plinian explosive eruption at Nisyros volcano (Greece). The deposit comprises a basal fallout, overlaid by pyroclastic density current units, followed by a lag-breccia level. White-yellow, porphyritic, rhyolitic pumices constitute the main juvenile component. Grey, crystal-rich juvenile clasts (CRCs) are less abundant (up to 10-15%), and are characterised by three different texture types (Type-A, -B and -C), with specific recurrence in the different depositional units and well correlated to the magma evolution. In the basal unit CRCs occur as andesitic to dacitic lapilli with Type-A and -B vesicular textures associated with highly variable trace element and isotopic compositions. In the lag-breccia deposit, the juvenile clasts occur as bombs with crenulated or bread-crust surfaces, displaying diktytaxitic Type-C textures and less evolved andesitic compositions, covering a larger Nd-isotope range at lower Sr-isotopes compared to the others. The CRCs are interpreted as the result of the rapid cooling of more mafic magma blobs sequentially intruded in the cooler rhyolitic host magma, in which they attained variable textures by different undercooling conditions, due to their variable compositions. We suggest that a two-stage AFC (Assimilation plus Fractional Crystallisation) process occurred at different pressures, before intrusion in the host magma, accounting for their heterogeneous chemical and isotopic characteristics. Firstly, the most primitive melts variably assimilated gneissic wallrock at depth, acquiring a variable Nd-isotope signature. On the way to the surface, they later experienced shallow AFC processes within different small magma reservoirs, involving heterogeneous carbonate-rocks such as pure limestone, metasomatised marble and skarn. Sequential dynamics of ascent and intrusion into the rhyolitic magma chamber lead the more evolved and skarn-contaminated Type-A and -B melts to firstly move in the upper part of the reservoir to be erupted in the early fallout deposits. Type-C more mafic melts later intruded the rhyolitic reservoir and were erupted in the lag-breccia deposit. The lowest Nd-isotopes recorded by CRCs, with respect to all the volcanic products of the Kos-Nisyros volcanic field, reveal the peculiar transient history for these magmas at relatively shallow levels in the crust. The CO2 release from the carbonate-rock assimilation has also possibly contributed to trigger the explosive eruption, discharging a large amount of CO2 into the atmosphere.
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    Highly siderophile element and Os isotope systematics of the Cenozoic volcanic rocks from the Eastern Pontides, NE Turkey: Constraints on the origin and evolution of subcontinental mantle-derived magmas

    Mehmet Arslanirfan TemizelLukas Ackerman
    19页
    查看更多>>摘要：The Cenozoic subalkaline to alkaline volcanic rocks from the Eastern Pontides Orogenic Belt (EPOB) in NE Turkey exhibit wide range of highly siderophile element (HSE) contents and Os isotope compositions that can be related to their mantle sources and parental melt evolution. Although some Eocene rocks show signs of late-stage modification, most of the studied volcanic rocks possess primary HSE and Os systematics. The Eocene volcanic rocks have largely variable Pd_N/Ir_N (~0.25-751), Pd_N/Pt_N (~0.25-87) and (~(187)os/~(188)Os)i ratios (~0.113 to 0.51). The Miocene volcanic rocks have high Pd_N/Ir_N (~2.6-441), low Pd_N/Pt_N (~0.11-1.7) and radiogenic (~(187)Os/~(188)Os)_i ratios ranging from -0.166 to 0.52 while the Mio-Pliocene volcanic rocks show high Pd_N/Ir_N of ~7.2-29, low Pd_N/Pt_N (~1.2-2.7), and only mildly radiogenic (~(187)Os/~(188)Os); values between ~0.132 and 0.177. Finally, the Quaternary volcanic rocks have low Pd_N/Ir_N (~2.3-3.6), Pd_N/Pt_N (~0.9-1.6) and radiogenic (~(187)Os/~(188)Os)i values ranging from -0.133 to 0.194. Overall, the primitive mantle-normalized HSE patterns of the studied volcanic rocks are characterized by PPGE (Pt, Pd) and Re enrichment with respect to IPGE (Os, Ir, Ru). The variations in HSE and radiogenic Os isotope compositions of the relatively differentiated volcanic rocks indicate that crustal assimilation played active role in their parental melt evolution, especially in the Eocene and Miocene period. Besides, the obtained HSE data also suggest tuat the parental magmas of all the studied volcanic rocks were generated mainly in the S-saturated condidons. The temporal and spatial HSE and Os isotope variations observed in the studied Cenozoic volcanic rocks reflect the heterogeneity of the mande source beneath the EPOB, which could be attributed to variations in response of different fluid/melt metasomatism of the underlying SCLM.
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    Multiple crustal melting pulses and Hf systematics in zircons

    Peng-Peng YuRoberto F. WeinbergYi Zheng
    14页
    查看更多>>摘要：This paper focuses on the U-Pb ages and Lu-Hf isotopes in zircons in a long-lived anatectic terrane, to understand both the nature of melting events and controls on Hf-isotope ratios in the zircon record. We focus on rocks from the Wuyi-Yunkai orogeny, South China, that mainly comprises Proterozoic basement rocks that generated numerous granitoids during Early Paleozoic granulite facies metamorphism. We investigate zircons from three anatectic granites and one migmatite. Zircon U-Pb dates for individual samples vary between -465 and -410 Ma, covering the duration of the orogeny. U-Pb dates for two samples define three normal distributions with populations spaced by ~15-20 Myr, indicating protracted and pulsed melting events. The inherited pre-orogenic zircon cores yield a wide spread of ε_(Hf)(435) values (-39.0 to -2.8), whilst all Early Paleozoic, magmatic syn-orogenic cores and rims have a narrower range shifted to higher values ranging between -14.3 and + 5.5 and no systematic variation with the date of the analytical spot. The data suggest that the magmatic syn-orogenic zircons grew from isotopically heterogeneous melts with -9 to 16 e units variation for individual samples. The narrower ranges compared to the inherited cores suggest that some magma homogenization must have occurred, and the upward shift of the ε_(Hf)(435) values is likely due to the influence of non-zircon Hf rather than due to mixing with an external less-evolved magma. Thus, the samples record a ~55 Myr, multi-pulsed melting event, during which heterogeneous crustal magmas had their Hf isotopic composition controlled by inherited zircon dissolution modified by contributions from non-zircon phase.
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