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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    Missing adakitic granite and syn-subduction mafic dikes within Permian volcanic belts of the southern margin of the CAOB? Comment on "Permian oceanic slab subduction in the southernmost Central Asian Orogenic Belt: Evidence from adakite and high-Mg diorite in the southern Beishan"

    Songnan LiuLiyun ZhouYu Wang
    5页
    查看更多>>摘要：The petrogenesis of adakite and adakitic rocks and their tectonic settings are debated. Adakites may be derived by many processes which include: partial melting of subducted oceanic slab or lower crustal material; fractional crystallization of basaltic magma; magma mixing processes associated with arc-related and non-arc tectonic settings. It may be inaccurate to assume that adakitic rocks are solely derived from subducted oceanic slabs. The development of accretionary orogens, as it relates to adaldte formation, is a topic of significant interest. Geochemical data coupled with field observations are vitally important to address the interplay between tectonics and magma genesis. In this comment, we discuss the petrogenetic processes and possible tectonic setting of the "adakite" in the southern Beishan region of the Central Asian Orogenic Belt (CAOB), to provide insight on the use of geochemical data to discriminate the tectonic setting of their formation. Based on our field observations, we disagree with the model presented by Zheng et al. (2020) as it pertains to the existence of an "ophiolitic melange" and the evidence of a Permian arc system but rather, advocate that the adaldtic rocks at Beishan are generated at an exten-sional setting.
      原文链接:
    • NSTL

    Comment on Zafar et al., 2020: "Platinum-group element geochemistry and geodynamic evolution of Chilas complex gabbros, Kohistan Island Arc NE Pakistan"

    Amjad HussainUllah InayatMuhammad Saleem Mughal
    5页
    查看更多>>摘要：An article (Zafar et al., 2020,372-373,105,691) recently published in Lithos proposes new petrogenetic interpretations regarding the Chilas Complex (CC) gabbros, and collisional history of the Kohistan Island Arc (KIA). Geo-chemical modeling based on bulk analysis of a single rock type has led the authors to argue against the previously proposed interpretations about the (1) genesis of CC through diapirism or rift-related magma tism and (2) timing of the collision of the Indian continental plate with KIA. The authors have postulated that the CC gabbroic rocks originated as a result of the northward subduction of the Indian plate beneath the Eurasian plate at ca. 85 Ma; mature arc-type magmatism in the Neo-Tethys Ocean. The irrefutable evidence for the existence of the thousands of km thick Kohistan-Ladakh Island Arc (KIA) system and the Neo-Tethys Ocean between the Indian and Eurasian plates preclude the possibility for such subduction and direct collision in the northern territory of Paldstan. Furthermore, the geochemical signatures do not offer unequivocal evidence for arc-continent collisional setting; instead petrogenetic modeling based on a re-evaluation of Zafar et al. (2020) data favors a back-arc basin (BAB) origin for the CC gabbros and thus endorses the previously proposed BAB tectonic setting for CC Hence a thorough investigation of the data acquired and rationality of the interpretation by Zafar et al. (2020) is warranted since it contradicts the previously proposed and generally accepted concepts regarding the genesis of CC and collision of KIA with the Indian and Eurasian plates.
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    • NSTL

    "Calculating biotite formula from electron microprobe analysis data using a machine learning method based on principal components regression" by Li et al. (Lithos 356-357 (2020) 105371) - A discussion

    Eileena DasAbu Saeed Baidya
    6页
    查看更多>>摘要：Li et al. (2020) recently presented a new machine learning method for calculating biotite (sensu lato) structural formulae from major element oxide data obtained from electron microprobe analyses. They provided a user-friendly Microsoft Excel spreadsheet for biotite formula calculation and concluded that in comparison with previous normalization schemes, their model showed significant improvements in estimating OH and WO~(2-) at the 0(4) site and Fe~(3+)/ΣFe ratio of biotite.
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    • NSTL

    On the improvement of calculating biotite formula from EPMA data: Reexamination of the methods of Dymek (1983), Yavuz and Oztas (1997), Li et al. (2020) and reply to the discussion of Baidya and Das

    Xiaoyan LiChao ZhangHarald Behrens
    6页
    查看更多>>摘要：Biotite (sensu lato) is a widespread silicate hydrous mineral in various geological conditions with potential petrological and geochemical implications. However, due to the chemical and structural complexities of biotite, calculating biotite formula from routine EPMA data has been a challenging task for decades. We thank Baidya and Das (2021) for their comments on our method presented in Li et al. (2020), which inspired us to further consider the issues about the improvement of calculating biotite formula from EPMA data.
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    • NSTL

    Episodic Proterozoic magmatism in Northwest Bangladesh: Implications for Columbia/Nuna and Rodinia reconstructions

    S.M. Mahbubul AmeenSimon A. WildeMd. Sakaouth Hossain
    22页
    查看更多>>摘要：A diorite from drillhole GDH 53 at Voktipur in the Dinajpur Block in NW Bangladesh records an age of 1686 ± 4 Ma, whereas a tonalite from drillhole GDH 54 at Barapaharpur has an age of 1641 ± 7 Ma. Conversely, char-nockite and monzogranite gneiss from drillhole GDH 31 at Gaibandha record considerably younger ages of 1144 ± 10 and 1133 ± 16 Ma, respectively. The diorite-tonalite-granodiorite suite is metaluminous, calc-alkaline, magnesian, with weak to moderate negative Eu anomalies and I-type characteristics. This suite is interpreted to have been derived from the melting of heterogeneous mafic crust, followed by a moderate degree of fractionation in an arc setting, identical to the source for similar rocks reported elsewhere in the basement of Bangladesh. The charnockite-monzogranite gneissic rocks from Gaibandha are ferroan, display weak negative Eu anomalies, belong to the high K calc-alkaline series, and plot as A-type granitoids (A2 subtype). It is considered that asthenospheric upwelling in an extensional/post-collisional setting provided the heat required to generate these rocks at the base of thinned crust. The geochemical data, lithostratigraphy and age of 1.73-1.64 Ga for the diorite-tonalite-granodiorite suite in NW Bangladesh do not match rocks of the Chotanagpur Granite Gneiss Complex and Central Indian Tectonic Zone to the west in India, negating their association with the collision that united the North and South Indian blocks. However, the Shillong-Meghalaya Gneissic Complex in NE India, with ~1.78-1.62 Ga granite gneiss and 1.28-1.08 Ga charnockite, provides a closer analogue. Events at these two times have also been recorded within the east Gondwana fragments in the Bunger and Obruchev Hills of East Antarctica. The similarities with both tbe Shillong-Meghalaya Gneissic Complex, lying to the east in India, and with East Antarctica, suggest that Proterozoic magmatic events in the Dinajpur Block of NW Bangladesh relate to events in both the Columbia/Nuna and Rodinia supercontinents and support a connection with these super-continents throughout the Proterozoic.
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    • NSTL

    Variscan granitic magmatism in the Western Carpathians with linkage to slab break-off

    Igor BroskaMarian JanakMartin Svojtka
    15页
    查看更多>>摘要：The Variscan basement within the Western Carpathian Alpine architecture generally consists of metaluminous/ peraluminous tonalite/granodiorite massifs and high-grade metamorphic complexes of metapelites, metaul-tramafites, and metabasites with relics of eclogites. Unfortunately, the Variscan crystalline basement of the Western Carpathians is only fragmentally exposed. Therefore, the proposed geodynamic evolutionary model for the Variscan granites of the Western Carpathians is primarily based on granite data from the Mala Fatra Mts. with additional dating from the High Tatra Mts. The oldest magmatic age of 362 ± 4 Ma in the Mala Fatra horst was recorded in diatexites from a high-grade metamorphic complex, which is related to crustal anatexis during Variscan subduction. Subsequent collisional event and break-off of the subducted slab promoted exhumation of the diatexites within the high grade metamorphic complex and intrusion of 353 ± 3 Ma old Tournaisian tonalite. Intensive heat input after slab break-off from the rising asthenosphere generated melting of the lower crust and extensive calc-alkaline, Mg-rich granitic magmatism in a short time span from 347 ± 4 to 342 ± 3 Ma. These Visean granitic rocks caused thermal overprint on the roof metamorphic rocks, including diatexites and Tournaisian tonalites at ca. 348 ± 5.6 to 342 ± 3 Ma. The Visean granite formation was controlled by the mixing of hot magmas, which is indicated by the presence of composite oligoclase/andesine plagioclase with preserved labradorite cores, alkali feldspars with Na2O ≥ 2 wt%, zoned apatite, the presence of antiperthite, and quartz ocelli. Elevated contents of mantle-derived elements like V, Ni, Cr, Ba, high Sr/Y ratio of -44, steep LREE and flat HREE segments of chondrite-normalised patterns document adakite-like feature of the investigated granitic rocks which resulted from melting of a mixed lower-crustal and mantle sources and crystallisation in the presence of garnet. Unusual abundance of Fe-Ti oxides in granodiorites with magmatic cooling temperatures of 735-756 °C supports high-T input from mantle. In the High Tatra Mts., diorite xenolith shows the age of 359.2 ± 3 Ma, and its host granodiorite the age of 350.1 ± 2.6 Ma. The diorite contains acicular zircons, which points to rapid exhumation. Stubby zircon of the host granodiorite shows regular, oscillatory zoning controlled by a gradual temperature decrease. The non-comagmadc relationships between diorite and host granodiorite are indicated also by a difference in zircon Th/U ratio, which is 0.2 for the host granodiorite, but 1.0 for the diorite on average. The presented data show that slab break-off could have been a mechanism that promoted Variscan granitic magmatism in the Western Carpathians.
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    • NSTL

    Geochemistry and petrogenesis of alkaline rear-arc magmatism in NW Iran

    H. Shafaii MoghadamK. HoernleF. Hauff
    20页
    查看更多>>摘要：High Na- to K- alkaline magmatism is common in the rear-arc region of the Cenozoic Urumieh-Dokhtar Magmatic Belt of Iran, but their geochemical signatures, as well as their formation mechanisms, have been poorly studied thus far. In the Kleybar area of NW Iran, Middle Eocene magmatic activity comprises silica-undersaturated to -saturated gabbros, monzogabbros, monzosyenites to nepheline-monzosyenites, and nepheline syenite as well as tephritic to trachy-basaltic and trachy-andesitic dikes. New ~(40)Ar-~(39)Ar results show a restricted age range (39.2-43.6 Ma) for the intrusive rocks (43.16 ± 0.43 and 43.34 ± 0.43 Ma for gabbros, 43.56 ± 0.44 for monzogabbros, 39.22 ± 0.48 and 42.09 ± 0.42 Ma for nepheline syenites, 42.17 ± 0.42 Ma for nepheline-monzosyenites), whereas a cross-cutting trachy-andesitic dike yielded an ~(40)Ar-~(39)Ar age of 41.78 ± 0.42 Ma. The Kleybar rocks are enriched in alkalis (K2O and Na2O) with variable K2O/Na2O ratios (0.14 to 2.93), light rare earth elements and large ion lithophile elements such as Th, Rb, K, U and Pb. High field strength elements such as Nb-Ta are depleted in these rocks. The isotopic compositions of the different rock types are quite variable: gabbros and monzogabbros have ~(87)Sr/~(86)Sr_((t)) = 0.70419-0.70436, eNd(t) = +3.1 to +3.8 and eHf(t) = + 7.8 to +10.1; nepheline monzosyenites and nepheline syenites have ~(87)Sr/~(86)Sr_((t)) = 0.70359-0.70566 (except a nepheline syenite with a ratio of 0.74833 and very high Rb/Sr), eNd(t) = +1.5 to +4.2 and εHf(t) = +4.6 to +8.8; and trachy-basaltic to trachy-andesitic to tephritic dikes have ~(87)Sr/~(86)Sr_((t)) = 0.70437-0.70469, εNd(t) = +2.8 to +4.2 and εHf(t) = +8.3 to +10. In the thorogenic-Pb isotope diagram, the Kleyber igneous rocks define an array above the Northern Hemisphere Reference Line (NHRL), with A8/4 (deviation from the NHRL) of ～40-60. These samples also plot above the NHRL in uranogenic Pb space (A7/4-5-10). The positive A8/4 Pb and A7/4 Pb may reflect the involvement of subducted terrigenous sediments in their mantle source during the subduction of the Neotethyan oceanic lith-osphere. Two nepheline syenite samples have significantly higher thorogenic and uranogenic Pb isotopic compositions, that may reflect assimilation of surrounding clay-rich sedimentary rocks. Modelling of trace elements compositions using less fractionated Kleybar trachybasalt and fine-grained monzogabbro samples indicate that a 96:4 mixture of the depleted mantle and subducting (trench)-sediment melts with 6% aggregated fractional melting closely matches the trace-element abundances of the Kleybar trachybasalt and monzogabbro. Together with previous studies on high-K volcanic rocks from NW Iran, our results indicate that Neotethyan slab retreat and related extension of the Iranian continental lithosphere in the rear-arc region of the Urumieh-Dokhtar Magmatic Belt generated alkali-rich magmatic rocks throughout the NW Iran rear-arc during Middle-Late Eocene.
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    Geochemical variability among stratiform chromitites and ultramafic rocks from Western Makran, South Iran

    Shoji AraiWilliam L. GriffinHadi Shafaii Moghadam
    20页
    查看更多>>摘要：The geochemical compositions of minerals from the Moho transition zone of ophiolites potentially can help to understand the magmatic evolution of the ophiolites, and subsequent mantle-melt interactions. The Jurassic-Late Cretaceous Makran ophiolite of south Iran comprises one of the most extensive tracts of oceanic crust which were scraped off and preserved in an accretionary complex. The Makran ophiolite records traces of MORB-OIB-type magmatism during the Jurassic, but mostly supra-subduction zone magmatic activity during the Late Cretaceous. Despite a few geochemical studies on the crustal rocks, the nature and geochemical signatures of the mantle rocks from this ophiolite remain controversial. The Sorkhband mantle-crust transition zone underlying crustal cumulates in the western Makran consists of stratiform chromitites, harzburgites, chromite-rich dunites and dunites, with crosscutting dikes of olivine websterite and olivine clinopyroxenite. Major- and trace-element compositions of clinopyroxene grains in olivine websterite and clinopyroxenite dikes indicate crystallization from melts similar to boninites and low-Ti fore-arc basalts. Spinel compositions in olivine websterite and clinopyroxenite dikes suggest crystal fractionation from boninitic or high- Mg# magmas have played a major role in the genesis these rocks. We propose a two-stage model for the formation of the Sorkhband dunites including (1) supra-subduction zone-related melt infiltrates through harzburgites in the mantle-crust transition zone to dissolve peridotite orthopyroxene and leave dunites with high forsterite-NiO olivines, and (2) boninitic melts accumulate and react with surrounding harzburgites to crystallize cumulate dunites with low-Mg# olivine and high-Ti spinels. We conclude that there have been temporal changes in the composition of mantle melts in the fore-arc mantle section of the Makran ophiolite during the initial subduction of the Neotethyan ocean beneath the Lut block during the Late Cretaceous.
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    Elevation of zircon Hf isotope ratios during crustal anatexis: Evidence from migmatites close to the eastern Himalayan syntaxis in southeastern Tibet

    Qiong-Xia XiaYi-Xiang ChenRen-Xu Chen
    18页
    查看更多>>摘要：The initial Hf isotope composition of zircon in high-grade metamorphic rocks is an important tracer for protolith origin as well as for the growth and evolution of continental crust. However, less attention has been paid to the behavior of Lu-Hf isotopes in peritectic reactions during partial melting of the continental crust. In this paper, we present a combined study of zircon U-Pb ages, Lu-Hf isotopes and trace elements as well as petrology and whole-rock geochemistry for migmatites close to the eastern Himalayan syntaxis in southeastern Tibet. The results show that peritectic zircons in the migmatites have significantly higher ~(176)Hf/~(177)Hf ratios than their protolith zircons. More interestingly, the ~(176)Lu/~(177)Hf ratios and the contents of high field strength elements (such as Nb, Ta, and Hf) and heavy rare earth elements are significantly elevated in the peritectic zircons compared to the protolith zircons. This suggests that garnets with high Lu/Hf ratios and other minerals (such as titanite, ilmenite, amphibole, and biotite) with moderate Lu/Hf ratios were decomposed during crustal anatexis, contributing to the Hf isotope composition of peritectic zircons. This is confirmed by petrographic observations that garnet and biotite occur as residues or remnants in mesosomes (biotite gneiss) and some leucosomes (felsic veins), but are absent in the melanosomes of the target migmatites. Therefore, the peritectic zircons were produced together with anatectic melts through peritectic reactions and acquired the elevated ~(176)Lu/~(177)Hf ratios compared to the inherited zircons from the protoliths if no peritectic garnet was coevally produced during the crustal anatexis. In this regard, the greatly elevated Hf isotope compositions of peritectic zircons in anatectic granites cannot faithfully reflect the Hf isotope composition of parental rocks. As such, great care must be taken when using Hf isotope ratios to trace the nature of parental rocks with respect to magmatic processes and crustal evolution.
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    Biotite compositions and geochemistry of porphyry-related systems from the central Urumieh Dokhtar Magmatic Belt, western Yazd, Iran: Insights into mineralization potential

    Amir TaghaviMohammad MaanijouDavid R. Lentz
    19页
    查看更多>>摘要：Commonly biotite occurs as a minor rock-forming mineral in a wide range of felsic to intermediate intrusive rocks. It can record the physicochemical conditions of the parent magma due to its complex crystal structure that allows multiple site substitutions of a wide range of elements. Geochemical studies of whole rocks and biotite (using EPMA and LA-ICP-MS methods) with petrographic analysis of biotite provide distinction between barren and fertile granitoids in western Yazd. Fertile suites are characterized by Si- and Mg-rich (3.47-4.35, ave. 3.75 a. p.f.u.) and total REE-poor biotite. In contrast, Fe (2.66-3.15, ave. 3.08 a.p.f.u.), Al, Ti, and REEs (in particular HREE+Y) increase in barren rocks. On the basis of biotite geobarometry, the fertile rocks (Darezereshk Porphyry Copper Deposit (PCD)) solidified at low pressure (ca. 0.6 kb, i.e., subvolcanic); in contrast, the barren rocks (Galvic and Shirkouh granodiorites) crystallized at higher pressures (1.1 to 3.6 kb). The temperature of the fertile granitoids is inferred to be about 750 °C, whereas T's of 625-675 °C for barren granitoids. The fertile suites have redox characteristic near the Ni-NiO buffer, whereas the barren suites are more reduced, near the quartz-fayalite-magnetite (QFM) buffer. Therefore, biotite major-element compositions, fertile magma bodies are emplaced at higher temperature and redox to lower pressures (subvolcanically) than the barren intrusive bodies. The whole-rock geochemical signatures show that both barren and fertile suites are represented by peraluminous to met-aluminous granite to diorite that have calc-alkaline characteristics with an I-type volcanic arc (VAG) tectonic setting. However, fertile suites originated from the mantle-crust mixed magma exhibiting calc-alkaline affinity, whereas the barren suites are derived predominandy from recycling of continental crust.
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    • NSTL