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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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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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    Platinum-group elements in Late Quaternary high-Mg basalts of eastern Kamchatka: Evidence for minor cryptic sulfide fractionation in primitive arc magmas

    Nikolai NekrylovVadim S. KamenetskyDmitry P. Savelyev
    14页
    查看更多>>摘要：The geochemical variations of magmas across and along supra-subduction zones (SSZ) have been commonly attributed to profound changes in the phase and chemical compositions of the mantle source and subduction-derived melt and fluid fluxes, as well as the physical parameters (e.g. depth, temperature, oxygen fugacity etc) of slab dehydration, mineral breakdown and melting. Here we test the variability of the Late Quaternary primitive magmas in the southern and northern parts of the meridionally oriented Eastern Volcanic Belt (EVB) of Kamchatka, with a slab depth varying from 60 to 160 1cm. Eight high-Mg (Mg# > 60 mol%) basalts were characterized for major, trace and platinum-group element (PGE) abundances, as well as the compositions of olivine phenocrysts and olivine-hosted spinel inclusions. The basalts in our study are geochemically typical of SSZ magmas and contain similar liquidus assemblages of forsteritic olivine (Mg# 78-92 mol%), low-Ti Cr-spinel and clinopyroxene. Although the absolute abundances of major and trace elements, and their ratios, in the basalts fluctuate to some extent, the observed variability cannot be correlated with any of considered parameters in the geometry of the Kamchatka SSZ and conditions of melting. This unexpected result led to the evaluation of the platinum-group element (PGE) systematics against the lithophile and chalcophile trace element geochemistry and the compositions of phenocrysts. Total whole-rock PGE content varies from 2.3 to 11.7 ppb, whereas the normalized PGE concentration patterns are typical for supra-subduction zones magmas and broadly similar in all studied samples. They are enriched in Rh, Pd and Pt relative to mid-ocean ridge basalts (MORB) and have nearly identical concentrations of Ir-group PGE. The only parameter that correlates well with PGE contents is the average Mg# of olivine phenocrysts from 84 to 90.3 mol%. This is interpreted to result from minor cryptic fractionation of sulfide melt, together with primitive olivine, in low-to-mid crustal conditions. Negative Ru anomalies on chondrite-normalized diagrams correspond to the Fe~(2+)/Fe~(3+) ratios in spinel (a proxy for magma redox conditions), which reflects a replacement of monosulfide solid solution by laurite in the mantle wedge during oxidation.
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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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    Metals in Avachinsky peridotite xenoliths with implications for redox heterogeneity and metal enrichment in the Kamchatka mantle wedge

    Pavel KepezhinskasNikolai BerdnikovNikita Kepezhinskas
    17页
    查看更多>>摘要：Peridotite xenoliths from the Avachinsky volcano in southern Kamchatka contain diverse assemblages of native metals and intermetallic compounds associated with primary olivine and orthopyroxene and metasomatic amphibole. Mineral compositions in Avachinsky xenoliths (Mg-rich olivine and low-Al enstatite associated with Cr-spinel) coupled with common absence of primary clinopyroxene and low bulk Al2O3 and CaO contents, suggest a depleted mantle wedge beneath the southern segment of the Kamchatka volcanic arc.Metallic phases in Avachinsky xenoliths occur as euhedral to anhedral grains in fractures andirregularly shaped voids in Fe-Mg silicates as well as micro-particles localized along boundaries between oivine and orthopyroxene. Siderophile metal assemblage is composed of platinum, iron and gold along with Cr-Fe and W-Fe-Mn alloys. Chalcophile metals include native copper, bismuth, lead and zinc along with Cu-Ag, Cu-Sn, Cu-Sn-Au, Ag-Au, Cu-Ag-Au and Ni-Cu-Ag-Au intermetallic compounds. Metals in Avachinsky xenoliths are associated, both spatially and texturally, with magnetite, rutile, galena, cassiterite, Cu-Ag-bearing Sb-Bi-Pb sulfide phase, AgS and non-stoichiometric Cl-bearing silver compound. Siderophile metals were introduced by primitive metasomatizing melts into low fO2 domains in the mantle wedge below the Kamchatka arc. Phase relations in the Cu-Ag-Au system suggest high-temperature (>900 °C) origin for Cu-Ag-Au intermetallic compounds in Avachinsky xenoliths. While Cu-Ag-Au alloys precipitated from metal-rich, mantle-derived silicate melts, low-melting point chalcophile metals and their minerals (Ag, Bi, Sb, Zn, Pb, Sn) were deposited in the sub-Kamchatkan mantle from hydrous slab fluids under reduced conditions. Metal assemblages in Avachinsky xenoliths further attest to reducing conditions in some subduction systems and potentially indicate existence of extreme redox gradients at sufficiently small scales in sub-arc mantle wedge environments.
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    Petrogenesis of Mesozoic giant dike swarms and geodynamical insights about EMI-Gough flavors in the Equatorial Atlantic Magmatic Province

    Antomat A. Macedo FilhoMaria Helena B.M. Hollanda
    18页
    查看更多>>摘要：The Rio Ceara-Mirim magmatism produced Mesozoic giant arcuate dike swarms during the initial stage of the West Gondwana breakup near the Equatorial Atlantic Ocean (NE Brazil), where they assemble tholeiites and subordinate intermediate rocks (SiO2 > 57 wt%). A multidata approach based on petrography, mineral chemistry, whole-rock geochemistry, and Sr-Nd-Pb isotopes enables the discrimination of four geochemical groups (two high-Ti and two low-Ti). High-Ti groups are composed of (1) basalts and basaltic andesites with TiO2 generally higher than 2.0 wt%, MgO > ~3 wt% (named the HT group) and rich in incompatible elements and (2) (trachy)andesites and trachytes with lower TiO2 (<2.5 wt%; Mg < 3 wt%) interpreted as fractionated magmas from the HT group, therefore named evolved HT. Both these groups have similar ~(87)Sr/~(86)Sr_((27)) (-0.706) and εNd_((127)) (-2.90 to -2.56) and moderately radiogenic ~(206)Pb/~(204)Pb_((m)) ratios between 18.66 and 18.21. Conversely, low-Ti tholeiites (TiO2 < 2 wt%) are (3) LT1 composed of evolved basaltic andesites, with ~Ti/Zr < 70, MgO <6 wt%, and Al2O3 > 15 wt%; and (4) LT2 composed of subalkaline (to transitional) basalts and basaltic andesites but ~Ti/Zr >70, and MgO > 6 wt%. These two low-Ti groups are, respectively, less radiogenic (LT1 εNd_((127))) -4.65 to -4.40) and slightly more radiogenic (LT2 εNd_((127))) -1.44 to -1.08) in Nd relative to the broad HT group. The geochemical groups present different degrees of enrichment in large ion lithophile and light rare-earth elements coupled with depletion in high-field-strength elements (Nb-Ta) combined with isotopic (Sr-Nd-Pb) signatures compatible with enriched mantle sources. We showed that the EMI (Gough-like) isotope compositions of Rio Ceara-Mirim magmatism may be explained by mixing DMM with enriched SCLM-derived melts plus minor crustal assimilation or by the involvement of OIBs derived from mantle plumes of the South Atlantic area mixed with lithospheric sources. In a geodynamical perspective, given the narrow similarity of HT Rio Ceara-Mirim isotope compositions with EMI-Gough type and high-Ti magmas of Transminas swarm, a genetic connection between Equatorial Atlantic and Parana-Etendeka provinces must be considered,
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    Titanite U-Pb dating and geochemical constraints on the Paleozoic magmatic-metamorphic events and Nb-Ta mineralization in the Yushishan deposit, South Qilian, NW China

    Shao-Yong JiangRuo-Hong ZhengTao Liu
    13页
    查看更多>>摘要：Titanite, as one significant petrogenetic tracer, occurs in the ore-bearing alkaline unit (syenite and aegirine-augite syenite dikes) and in the barren calc-alkaline unit (monzodiorite) and metamorphosed strata (called as leptynite) in the recently found large Yushishan Nb-Ta deposit from South Qilian, NW China. Most titanite grains in the alkaline unit display obvious shape-preferred orientation and zonation, coexisting with allanite, zircon and alkali feldspar. Some titanite grains from the alkaline unit have been partially or completely altered into finegrained euxenite and/or Nb-rich rutile, but retained their original crystal shapes. Although some titanite grains in the igneous rocks show obvious core-rim textures, both of them yield almost the same U-Pb ages. Combined with U-Pb dating of zircon from these different rock units, the complex magmatic-metamorphic events in the Yushishan district can be subdivided into two different periods: the first one occurring in the Neo-proterozoic (ca. 834-790 Ma, zircon U-Pb ages) that are related to the regional volcanism and metamorphic activities and the formation of leptynite during the metamorphism, while the second one occurring in the early Paleozoic (ca. 490-470 Ma, titanite and zircon U-Pb ages) that are related to the formation of granitic intrusions and subsequent metamorphic process and Nb-Ta mineralization. Geochemically, titanite from these igneous rocks is consistent with the typical features of igneous titanite, such as high Fe and REE contents, and negative Eu anomalies. The zoned titanite in both alkaline (mainly syenite) and calc-alkaline units uniformly displays a core-to-rim decreasing trend for the Fe, REE, Nb and Ta contents and Th/U ratios, corresponding to a secular variation from the magmatic to metamorphic evolution. Titanite from the leptynite is interpreted to be of metamorphic origin, with low Fe and REE contents, weakly positively sloping REE patterns, along with positive Eu anomalies. It is suggested that the titanite can be used as an indicator of metal fertility and ore potential for the host rocks/ fluids. The high Nb and Ta features in titanite from the alkaline unit suggest a genetic relationship between the alkaline granitic magmatism and Nb-Ta mineralization in the Yushishan deposit. In addition, the high and variable Nb and Ta contents in titanite from the leptynites reveal an inherited signature of the metamorphic fluids. Considering the absence of titanite in the ore-bearing leptynites, we speculate that titanites with high and variable Nb and Ta contents of titanite in the barren leptynites have the potential to provide abundant metals for the precipitation of Nb-Ta-bearing minerals to form economic orebodies.
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    Differences in composition of chromites from low-Ti and high-Ti picrites of the Emeishan Large Igneous Province and comparison with chromites of the UG-2 platinum-deposit of the Bushveld complex

    Sarah-Jane BarnesEduardo T. MansurPhilippe Page
    22页
    查看更多>>摘要：Chromite is among the first minerals to crystallize from mantle derived magmas and one of the last to be consumed during partial melting of the mantle. Chromite is also an important mineral in major ore deposits of Cr and Pt. The composition of chromite could, therefore be of use in interpreting the petrogenetic conditions during partial melting of the mantle, crystallization of primitive magmas and formation of Pt and Cr ore deposits. However, most mafic rocks contain very little chromite and post-crystallization processes such as re-equilibration during cooling, metamorphism and weathering could change the composition. The composition of chromites from high- and low-Ti picrites from the Emeishan large igneous province have been determined to assess the degree to which the chromite compositions reflect the melt compositions. Aluminium, Sc, Ti, Ga, Nb, Sn, Hf and Ta concentrations in the chromites do appear to reflect the melt compositions in that they correlate with the whole rock compositions and have empirical partition coefficients similar to those determined in experiments. The V contents of both types of picrite are similar, but concentrations of chromites from high-Ti picrites are higher than those of low-Ti picrites. This can be explained if, in the high-Ti picrite more of the V was in the V3+ state (which can more readily substitute into chromite than V4+ or V5+) than in the low-Ti picrite. This implies that fO2 was lower for high-Ti picrites than low-Ti picrites. Concentrations of elements with a 2+ charge, Mg, Mn, Fe, Co, Ni and Zn are different in chromites included in olivine and chromites in the matrix and appear to have re-equilbrated. The compositions of the UG-2 chromites from the World's largest Pt deposit show some similarities with the chromites from the high-Ti picrite, but are depleted in Sc and Ti and enriched in Ga and Al.
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