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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    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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    "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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    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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    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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    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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    Petrogenesis and tectonic implication of the Late Triassic A1-type alkaline volcanics from the Xiangride area, eastern segment of the East Kunlun Orogen (China)

    Yu-Xiang ZhuLian-Xun WangChang-Qian Ma
    23页
    查看更多>>摘要：A series of alkaline volcanic rocks with pronounced Nb contents, were identified in the Xiangride area of the eastern segment of the East Kunlun Orogen (EKO). An integrated investigation, involving mineral composition, whole-rock elemental and Sr - Nd isotopic geochemistry and zircon U-Pb dating and Lu-Hf isotopes was conducted on the alkaline volcanics as well as a spatially associated andesite. The Xiangride alkaline volcanics are mainly composed of trachyandesite, quartz trachyte, rhyolitic ignimbrite, and rhyolite. Zircon U-Pb dating reveals that the Xiangride alkaline volcanics were formed at 212-209 Ma, apparently postdating the associated andesite (231 Ma) and the volcanic rocks of the Elashan Formation (232-218 Ma) as well as the widespread I-type granitoids in the eastern EKO (265-217 Ma). The Xiangride alkaline volcanics display variably high total alkali, (Zr + Nb + Ce + Y) content, Ga/Al ratio, and zircon saturation temperature, typical of Ai-type granite. They are also characterized by high HFSEs and LREEs contents, along with strong depletion of Ba, Sr, P, Ti and Eu, similar to the OIB-type alkaline trachytic to rhyolitic rocks and obviously different from the coeval arc-related magmatic rocks in the eastern EKO. In contrast, the associated Xiangride andesite shows similar chemical signatures to the Elashan andesites, resembling the IAB-series rocks. Notably, the Xiangride alkaline volcanics all contain high Nb contents (up to 120 ppm) as compared to other Triassic granitoids and volcanic equivalents of the EKO (Nb = 5-60 ppm). The dominate Nb-hosts include primary titanite and rutile and secondary ilmenorutile and euxenite, indicating both contributions of magmatic and hydrothermal processes. The Xiangride alkaline volcanics yield slightly higher εNd(t) values (-3.7 to -2.4) and zircon εHf(t) values (-0.6 to 2.6) than the associated Xiangride andesites (-4.2 to -4.1 and - 4.7 to -3.5, respectively) and the widespread Elashan andesites (-6.4 to -3.4 and - 7.4 to -2.0, respectively). Based on our results, we suggest that the Xiangride A1-type alkaline volcanics were likely generated via partial melting of a metasomatized lithospheric mantle followed by prolonged fractional crystallization and minor crustal contamination, whereas the associated Xiangride andesite might have originated from partial melting of the juvenile mafic lower crust, similar to the Elashan andesites. We propose that the Xiangride A1-type alkaline volcanics may represent a within-plate anorogenic extensional magmatic pulse, whereas the associated andesite and the Elashan andesites recorded an earlier period of post-collisional volcanism. Therefore, the Triassic continental collision orogeny of the EKO is likely to have ended before ca. 212 Ma, followed by a transition from post-collisional to intracontinental extension.
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    Petrogenesis of the Mesoarchean (-3.05 Ga) mafic volcanics from the western Iron Ore Group volcano-sedimentary succession, Singhbhum craton, eastern India: Constraints from geochemical modelling and Sm-Nd geochronology

    Avishek AdhikariRavikant Vadlamani
    19页
    查看更多>>摘要：Cratonic Mesoarchean volcano-sedimentary successions preserve compositionally diverse volcanic rocks which give insight into Archean mantle characteristics and crustal evolution. The late Archean western Iron Ore Group (W-IOG) succession in the Singhbhum craton represents deposition during this time period. As the W-IOG depositional age has remained poorly constrained, to between Paleo- and Mesoarchean, petogenesis and chronology of the interbedded volcanics would be an important geological constraint. Mafic volcanics, interlayered with BIF and Fe-Mn-rich phyllite/shale, were sampled from Kalta and Ongarbira, and petrogenetic modelling of their mantle source and Sm-Nd geochronology were conducted. The Ongarbira basalts show tholeiitic affinity, are depleted in LILE, LREE, Zr and Th and an absence of Nb-Ta-Ti anomalies that imply a depleted MORB-type mantle source. Tholeiitic basalts yield an isochron age of 3050 ± 71 Ma (2SD) with Ndi = 0.50885 ± 0.00010, MSWD = 0.17 (n = 10) and ε_(Nd(T)) = +3.3 ± 1.6. Geochemical modelling indicates the most primitive tholeiitic basalt (Mg# = 61) formed by ～14% partial melting of depleted mantle whereas the most enriched variant formed by its subsequent assimilation-fractional crystallization (AFC) (-15% with r = 0.2 being the ratio between assimilation rate and fractional crystallization rate). In contrast, the Kalta basaltic andesite have calc-alkaline affinity, are enriched in LILE, LREE, Zr and Th with pronounced negative Nb-Ta-Ti anomalies indicating incompatible trace element enrichment in their petrogenesis. The basaltic andesite yield an isochron age of 3041 ± 94 Ma (2SD) with Ndi = 0.50875 ± 0.00009, MSWD = 0.62 (n = 10) and ε_(Nd(T)) = +1.1 ± 1.6. Geochemical modelling indicates that their compositions were generated by 20-40% AFC (r = 0.2) of -14% partial melt of depleted mantle. The source magma to the Ongarbira basalt is interpreted to have formed during lithospheric extension of the Singhbhum craton, by partial melting of depleted mantl followed by minor AFC, whereas the Kalta basaltic andesite represents higher extents of AFC of the Ongarbira-type tholeiitic magma.
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    Petrogenesis of high-maficity S-type granites: Insight from the early Paleozoic Jinxi granite, South China

    Zhen-Tao SongXi-Sheng Xu
    17页
    查看更多>>摘要：Experimental S-type melts are characterized by high silica (SiO2 > 70 wt%) and low-maficity (FeO~T + MgO < 4 wt%) natures. However, more and more studies revealed that many natural S-type granites exhibited chemical compositions differing from the experimental melts, these S-type granites have relatively low SiO2 but high FeO~T + MgO contents. Several models have been proposed for their petrogenesis: magma mixing, source heterogeneity, entrainment of metamorphic garnets, crystal accumulation, etc. In this study, we carried out integrated petrological and mineralogical researches on the early Paleozoic Jinxi granites (ca. 442 Ma) in the Cathaysia Block, South China. They are strongly peraluminous (average A/CNK = 1.31), K-rich (average K2O/Na2O = 1.50) S-type granites, and can be divided into high-maficity (FeO~T + MgO > 4%) and low-maficity types (FeO~T + MgO < 4%). A combing of field, petrological, and lithochemical evidence indicates that the Jinxi granites did not undergo magma mixing process, and they were derived from ancient crustal rocks without the input of mantle-derived ferromagnesian components, ruling out the mixing model. Both the high-maficity and low-maficity samples of the Jinxi granites show whole-rock Sr-Nd and zircon Hf-0 isotopic characteristics indicating a met-asedimentary origin, precluding the mafic meta-igneous source. The metasedimentary origin is also supported by the evidence of inherited zircons, whole-rock geochemistry and field observation. Melt extraction and crystal accumulation seem unlikely due to the absences of a prominent silica gap and the complementary geochemical relationship between the high-maficity and low-maficity granites. Furthermore, the absences of metamorphic garnets and significant positive correlations of FeO~T + MgO vs. Y, Yb, and A/CNK, argue against the metamorphic garnet entrainment model. The biotites with a very high proportion (up to -25%) are observed in the high-maficity granites, indicating that these biotites are not all magmatic biotites. It is supported by their occurrences and compositions. Given that the Jinxi granites formed via biotite fluid-absent melting at ~828 °C and ~ 5 kbar, remnant reactant biotites and potential peritectic biotites should be high-temperature and Ti-rich, which differs from the Ti-poor characteristic of the biotites in the Jinxi high-maficity granites. Moreover, highly variable whole-rock chemistry, particularly Na:K ratios, and petrographic observations suggest that the Jinxi granites have been modified by late- to post-magmatic processes involving introduction of alkalis and H2O during cooling. These features suggest the partial replacement of early magmatic garnet by biotite, and plenty of secondary biotites formed in post-magmatic metamorphism and alteration, which leads to a high proportion of biotite, and thus high FeO~T + MgO contents in the Jinxi high-maficity granites. In this case, the effect of late- to post-magmatic processes should be noticed in the study on high-maficity S-type granites.
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    Geochronological, geochemical and isotopic characterisation of the basement of the Choco-Panama Block in Colombia

    M.M. Ariza-AceroR. SpikingsA. Beltran-Trivino
    27页
    查看更多>>摘要：The Choco-Panama block is the land bridge that connects Central and South America and forms the trailing edge of the Caribbean Large Igneous Province (CLIP). New field observations are combined with new geochemical, isotopic and ~(40)Ar/~(39)Ar analyses of the mafic basement of the Choco block within Colombia. These are integrated with new U/Pb ages of detrital zircons extracted from siliciclastic and carbonate rocks that unconformably overlie the mafic basement. The data are used to constrain the age, tectonic origin and evolution of the Choco Block, which are used to improve tectonic models of the Caribbean Plate. The whole-rock chemical and isotopic compositions of the mafic lavas suggests they formed within an oceanic plateau, and are extremely similar to plateau units in Colombia, Ecuador, Panama and Costa Rica. ~(40)Ar/~(39)Ar (groundmass) dates of the mafic basement are affected by early calcic and late potassic alteration, and none are accurate measurements of the time of crystallisation. A comparison of ~(40)Ar(radiogenic), ~(39)Ar(K) and ~(37)Ar(Ca) reveals at least three argon reservoirs within the mafic groundmass, where at least two are a consequence of alteration. Our most accurate albeit imprecise estimate of the crystallisation age of the mafic basement is >86.0 ± 6.3 Ma, which is a plateau ~(40)Ar/~(39)Ar date from a groundmass aliquot which contains the lowest proportion of secondary Ar reservoirs. This age estimate is consistent with the few reliable crystallisation ages obtained for oceanic plateau rocks of the CLIP. U/Pb concordia dates of detrital zircons constrain the maximum depositional age of the overlying sedimentary rocks to -28 Ma, supporting previous conclusions that the Choco-Panama block collided with South America during the Oligocene to Early Miocene. Deuital Precambrian - Triassic zircons in the older cover sequences are not present in younger sedimentary rocks due to surface uplift of the Serrania del Baudo, presumably due to collisional tectonics.
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    Magmatic-hydrothermal processes of the Laojunshan metamorphic massif in Southeastern Asia: Evidence from chemical and B-isotopic variations of deformed tourmalines

    Wei LiShuyun CaoEizo Nakamura
    22页
    查看更多>>摘要：Multi-stage fluid influx has led to the volumetrically important occurrence of granite-related tourmalines at the Laojunshan metamorphic massif in Southeast Asia, which exhibits variable deformed structures and decomposition textures. These include disseminated tourmaline porphyroclasts (type-I), crosscutting tourmaline-quartz veins (type-II) and tourmaline veinlets (type-Ill). The chemical variations of these three types of tourmalines reveal a source transition from early boron-rich melt to late hydrothermal fluid, accompanying with variable fluid-rock interaction. Hydrothermal tourmalines are characterized by higher Mg/(Mg + Fe) ratios, lower Na/ (Na + Ca) ratios, more pronounced positive Eu anomalies, higher Sr and heavy rare earth element (HREE) concentrations, and lower Li, Nb, Zr, Hf, and light rare earth element (LREE) concentrations compared to magmatic tourmalines. The tourmalines exhibit δ~(II)B values ranging from -13 to -7.9%o for type-I, from -15.5 to -7.5%o for type-II, and from -18.6 to -11.6%o for type-Ill. This variation is mainly due to multiple fluid exsolution, Rayleigh fractionation and the mixing of two isotopically distinct sources. U-Pb dating results of magmatic zircons from two tourmaline-bearing gneisses reveal the ages of magma emplacement and crystallization from 445 to 420 Ma. Magmatic-hydrothermal fluids released from Silurian peraluminous granites are responsible for the formation of the disseminated type-I tourmalines. Further hydraulic fracturing during exhumation led to the formation of the tourmaline-quartz veins (type-II) and the tourmaline veinlets (type-Ill), possibly from Cretaceous granitic magma-derived fluids that interacted with metamorphic rocks. The decreasing grain size resulted from decomposition and cataclasis coupled with the overpressured fluids weakening the host rocks along fractures and eventually promoting the deformation during exhumation of the metamorphic massif. The chemical and isotopic compositions of tourmaline suggest that the oxidized conditions possibly through fluid boiling and fluid-rock interaction may promote the regional Sn-mineralization.
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