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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    Response of trace elements to partial melting of felsic crust at high to ultrahigh temperatures: Implications for granite geochemistry

    Qiang-Qiang ZhangXiao-Ying GaoYong-Fei Zheng
    22页
    查看更多>>摘要：It is known that partial melting and melt extraction of crustal rocks result in chemical differentiation of the continental crust. But it is unknown how these two processes have affected die composition of granites due to a limited knowledge of trace element behaviors during crustal anatexis. In order to quantify tbis issue, a combined study of whole-rock and mineral trace elements and mineral modal proportions was conducted for high-temperature (HT) to ultrahigh-temperature (UHT) felsic granulites from the Tongbai orogen in central China. Reconstruction of LREE budgets suggests that LREE mainly reside in monazite in both HT and UHT granulites. As monazite is predicted to be not stable at UHT conditions in the modelling of previous studies, the presence of monazite in the Tongbai UHT granulites suggest that it was not sufficiently dissolved into anatectic melts. Considering monazite mainly occurs as interstitial grains, fast melt-residue separation may be the dominant factor for impeding its dissolution. Calculation of mineral/mineral trace element ratios indicates that K-feldspar has a higher capacity to accommodate Ba and Sr than biotite and plagioclase, with D_(Ba)~(K-feldspar/biotite) of 2.5-3.0 and D_(Sr)~(K-feidspar/piagiociase) values of 1.4-1.9 and ilmenite can preserve more Nb and Ta than biotite with D_(Nb)~(biotite/ilmenite) values of 0.07-0.1 and D_(Ta)~(biotite/ilmenite) values of 0.04-0.05. In addition, partitioning of first row transition elements (FRTE) was constrained in the felsic granulites, with preference of Sc into garnet, and Co, V, Ni, and Cr into biotite. Modelling based on the presently constrained partition coefficients indicates that trace elements show consistent differentiation trends during anatexis of various felsic rocks, with enrichment of Sr, Ba, Nb, Ta and FRTE but depletion of Rb in anatectic restites. The high DNb/Ta values of biotite and ilmenite cause the residues produced by biotite dehydration melting to be commonly of higher Nb/Ta ratios than protoliths, indicating that the residues are potentially high Nb/Ta reservoirs. In contrast, the change of peritectic minerals due to various protolith compositions and melting conditions results in differential responses of Sr, Ba, and FRTE to partial melting, suggesting that these elements may be useful indicators to trace the petrogenetic process of granites. The correlation between Ba and Sc contents or between Sr/Ba and Sc/Co ratios is demonstrated to be a valid index in discriminating melts from metagreywackes and metapelites due to different modes of garnet, orthopyroxene, and K-feldspar in their anatectic residues. Moreover, as the proportions of garnet and ortbopyroxene in the residues are significantly influenced by melting pressures, the Sc/Co ratio and Sc content of granites can be used to trace their anatectic depth.
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    Modeling the chemical heterogeneity of tonalite-trondhjemite-granodiorite intrusive suites

    Lucie Mathieu
    19页
    查看更多>>摘要：Insights into the source and partial-melting conditions of tonalite-trondhjemite-granodiorite (TTG) and other tonalite-dominated intrusive suites provide essential constraints on die tectonic regimes prevailing before 2.5 Ga. Most petrogenetic interpretations rely on whole-rock chemical data and neglect the role of differentiation on primitive melt composition. Indeed, the processes of mineral separation from residual melt (fractionation) and of mineral sorting in a low-melt-fraction mush are rarely evaluated for tonalite-dominated magmatism. Here, numerical modeling provides quantitative constraints on fractional crystallization and mineral sorting for tonalite-dominated suites of the Abitibi greenstone belt, Superior craton, Canada. It is concluded that differentiation can be responsible for most within-suite chemical heterogeneity. The modeling shows that amphibole fractionation alone is sufficient to explain the range of HREE observed in the studied intrusive suites, indicating that the relationship between HREE content and partial-melting depth is not always straightforward. The modeling also shows that mineral sorting can explain chemical heterogeneities as well as primitive melts produced at distinct pressures. Also, the fractional crystallization of amphibole under various temperature conditions explains the contrasting Nb and Ti contents of the studied intrusive phases. The chemical heterogeneity of TTG suites can be explained using different modeling assumptions (e.g., fractional crystallization, mineral sorting, melts produced at distinct pressures) and Neoarchean tonalite-dominated magmatism likely results from a diversity of petrogenetic processes. The petrogenesis of individual intrusions should thus be interpreted with caution before drawing conclusions on source processes and, by extension, the geodynamic settings.
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    Role of inherited compositional and structural heterogeneity in shear zone development at mid-low levels of the continental crust (the Anzola shear zone; Ivrea-Verbano Zone, Southern Alps)

    S. CorvoM. MainoS. Piazolo
    21页
    查看更多>>摘要：The formation of shear zones is crucial to understand the deformation of the crust and the evolution of rifted margins. However, despite their intrinsic importance, a detailed characterization of the compositional and structural patterns of shear zones is often lacking, resulting in poorly constrained models of shear initiation. In this contribution, we reconstruct the pre-shearing lithological, structural and metamorphic proprieties of rocks forming a major, rift-related shear zone with the aim to assess the conditions promoting the strain localization. We focus on the Anzola shear zone, a major extensional structure from one of the best-preserved cross-sections through the middle to lower continental crust of a fossil passive margin, the Ivrea-Verbano Zone (Southern Alps, Italy). Until now, the Anzola shear zone is believed to have developed within a Theologically hard and isotropic gabbro rather than in the surrounding weaker and anisotropic volcano-sedimentary metamorphic sequence. New geological mapping shows that significant pre-existing heterogeneities related to the composition and deformation structures, characterize the Anzola shear zone. Field relationships and geochemistry reveal that the (ultra-)mylonitic rocks overprinted a multi-lithological sequence that have already experienced Variscan deformation and late Variscan High-Temperature metamorphism, at the boundary between amphibolite and granulite fades. Our in-depth trace elements study is shown to be a powerful tool in reconstructing the pre-shearing relationships between wall rocks and mylonites and determining the protoliths of tectonites. Estimated P-T conditions indicate that mylonitic deformation started at high temperature (—820 ° C) with presence of melt and continued as solid-state deformation down to amphibolite fades (—650 °C), following a retrograde path. We argue that strain localization was promoted by the combination of rheological boundaries derived from pre-existing conditions, including: i) compositional and structural anisotropics of the volcano-sedimentary metamorphic sequence contrasted by ii) the close intrusion of a nearly isotropic gabbro and iii) the presence of melt within the metamorphic boundary depicted by the transition between granulites (dominated by anhydrous minerals) and amphibolite fades (dominated by hydrous minerals). Our findings finally suggest that preexisting significant heterogeneities relate to rock composition, deformation and metamorphism represent the preferential loci for strain localization controlling the initiation and development of rift-related structures in the mid to lower crust of passive margins.
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    Subduction initiation in the Neo-Tethys and formation of the Bursa ophiolite in NW Turkey

    Guang-Yao XinYang ChuBen-Xun Su
    17页
    查看更多>>摘要：Subduction of oceanic lithosphere is the primary driver for plate tectonics on earth, but a paucity of geological records hinders our understanding of how subduction is initiated. Two general mechanisms have been proposed-induced and spontaneous nucleation of subduction zones. For both mechanisms, subduction inception begins with initial lower plate burial, but whether upper plate extension and magmatic activity coincide with, or are later than, lower plate underthrusting is the key to identifying spontaneous or induced subduction initiation. Here we present geological, geochemical and geochronological characteristics of mafic dikes and amphibolites from the Bursa ophiolite in NW Turkey, to identify the mechanism of the subduction initiation of the Neo-Tethys Ocean. The mafic dikes are depleted in light rare earth elements (LREE) and slightly enriched in large-ion lithophile elements (LILE), which together with high ε_(Nd)(t) values and low ~(207)Pb/~(204)Pb ratios, indicate a MORB-like mantle source similar to the earliest forearc basalts (FAB) of the Izu-Bonin-Mariana subduction initiation system. Zircon U-Pb geochronology shows a crystallization age of 89.8 ± 0.9 Ma. In contrast, the amphibolites are enriched in LREE and LILE, and have low ε_(Nd)(t) values and high ~(207)Pb/~(204)Pb ratios, suggesting an E-MORB-like geochemical affinity. Zircon U-Pb ages of the amphibolites reveal that the metamorphism occurred at 96.4 ±1.9 Ma, representing the initial burial of the lower plate. Thus, subduction predated upper plate extension and formation of the Bursa oceanic crust by 9.4-3.8 Myr, arguing for induced subduction of Neo-Tethyan lithosphere that formed the Bursa ophiolite.
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    Cretaceous basalt-andesite sequence in the Southern Pamir: arc—back-arc architecture at the Pamir Plateau genetically related to the northward flat subductions of the Neo-Tethys Ocean

    Chuan-Lin ZhangHaibo ZouXiao-Qiang Liu
    13页
    查看更多>>摘要：The Pamir-Karakorum is one of the most important regions for our understanding the Tethys evolution. Although it is commonly known that the northward subduction of the Neo-Tethys Ocean induced the formation of the magmatic arc in Southern Pamir, the temporal-spatial configuration of the arc—back-arc during the Neo-Tethys evolution remains unclear. In this study, we report sedimentary feature, petrography, zircon U—Pb ages and Hf—O isotope compositions as well as bulk-rock and mineral geochemistry of the well-preserved Cretaceous volcanic sequence in Chinese Wahan Corridor at South Pamir. The volcanic sequence is mainly composed of phenocrysts-bearing basalt-andesites with minor rhyolite and volcaniclastic rock. Zircon U—Pb ages reveal the basalt-andesite sequence erupted at 100-98 Ma. Bulk-rock geochemistry defines their potassic calc-alkaline signature and both their rock association and elemental geochemistry share most features of the continental arc volcanic sequences. On the other hand, they have variably enriched Nd isotope compositions with ewd(t) varying from-5.9 to-9.6, coupled with their enriched zircon Hf isotope signature with ε_(Hf)(t) between-13 to-8. Whole-rock geochemistry and zircon oxygen isotope compositions (average δ~(18)O = 7.74 ± 0.14) demonstrate that the primitive magma of the basalt-andesites were derived from partial melting of a metasomatized sub-continental lithospheric mantle and underwent variable assimilation fractional crystallization. Late Jurassic to Cretaceous volcanic-sedimentary sequences and coeval voluminous arc-like granodiorite and granites are widely distributed along the Southern Pamir. In the Central Pamir and further north, late Jurassic to Cretaceous sequences resemble features of back-arc basins. Taken together, we suggest that a northward low-angle or flat subduction of the Neo-Tethys Ocean along the Shyok zone was geodynamically related to the formation of the broad magmatic arc in South Pamir-Karakorum and the back-arc basins north of the South Pamir.
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    High-and low-Mg adakitic rocks in southern Tibet: Implication for the crustal thickening and geodynamic process in the late Cretaceous

    Zhenzhen WangMingchun DongZhidan Zhao
    14页
    查看更多>>摘要：The history of crustal thickening is critical for comprehending the evolution process of the Tibetan plateau. The genetic relationship and spatiotemporal distribution of adakitic rocks with high Sr/Y, (La/Yb)N ratios may impose fundamental constraints on specific geodynamic and crustal thickening processes. The Songka intrusive suite consists of adakitic rocks with high-Mg adakite (monzonites), low-Mg adakitic rocks (quartz monzonites), and gabbroic-diorites. Their geochemical characteristics, pedogenesis, and geodynamic implication is discussed in this paper. Songka high-Mg adakites formed when subducting Neo-Tethyan oceanic slab melts were assimilated by mantle peridotite at elevated temperatures (average Ti-in-zircon temperatures of sample SK1319 are 830 °C). Low-Mg adakitic rocks exhibit markedly different geochemical characteristics than those formed in the juvenile thickened lower crust. The least-squares mass balance and trace element simulations indicate that Songka low-Mg adakitic samples were formed at a degree of 60-65% fractional crystallization of melts resembling Songka high-Mg adakites. Songka gabbroic diorites are formed in the metasomatic mande wedge. The genetic relationship between these rocks indicates that the crust of die southern Tibet did not appear to thicken obviously during the early stage of late Cretaceous (100-90 Ma). The intensive emplacement of late Cretaceous high-Mg adakites (Mg# >50) and mafic rocks (SiO2 < 52%) in the southern Lhasa terrane around 95-90 Ma, together with the slow convergence velocity of the Indian-Asian continent, can be explained by die response of rapid crustal thickening and Neo-Tethyan slab rollback at 95-90 Ma.
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    Apatite as a magma redox indicator and its application in metallogenic research

    Hairuo WangKeda CaiMin Sun
    16页
    查看更多>>摘要：As an early crystallizing phase in the magma, apatite can accommodate a wide range of multivalent cations and has a capacity resistant to weathering and hydrothermal alteration. These features enable apatite to record the primary redox state of precursor magma. Here, we present new electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) geochemical data on apatite and zircon grains from the epithermal gold ore-hosting Dahalajunshan Formation volcanics in the Tulasu basin (Western Tianshan Orogen, Central Asia). Calculated zircon Ce-/Ti-based oxybarometric results and sulfur content of apatite suggest that the parental magma of the volcanics was relatively oxidized. Furthermore, we compiled a regional apatite chemical dataset to assess whether other elements (Mn, V, Eu, Ce, As and Ga) in apatite can serve as a magma redox proxy. Negative correlation between apatite Mn and whole-rock MgO is evident, implying the control of magma fractionation on apatite Mn contents. Also, oxygen fugacity (fO2) calculated by Mn-in-apatite oxybarometer correlates strongly with magma fractionation, which indicates that fractional crystallization (instead of fO2) controlled the Mn-uptake in apatite. The apatite As budget is sensitive to_fO2 variation but does not show any correlation with the ratio of nonbridging oxygens to tetrahedral cations (NBO/T), whole-rock SiO2, or aluminum saturation index. In contrast, apatite V and Ga contents correlate obviously with whole-rock SiO2 and NBO/T but not fO2, and apatite Eu and Ce anomalies show no correlation with fO2. Therefore, we conclude that only the apatite As and S contents can reflect the magma redox state. The calculated fO2 of the Dahalajunshan Formation volcanic rocks is AFMQ+0.53 ± 0.52 (based on the fayalite+magnetite+quartz buffer), which is markedly lower than those of typical porphyry Cu ore-forming magmas worldwide. Thus, from the perspective of magma fO2, regional magma is inferred to have little potential to form the porphyry Cu deposit.
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    The retrograde evolution of F-rich skarns: Clues from major and trace element chemistry of garnet, scheelite, and vesuvianite from the Belka Pahar wollastonite deposit, India

    Urmi GhoshDewashish Upadhyay
    20页
    查看更多>>摘要：Skarn settings host many ore deposits and are ideal for studying the mobility of elements in hydrothermal fluids. Belka Pahar, in the Sirohi district of Rajasthan (NW India), is the largest wollastonite deposit in India (estimated wollastonite reserve of 56 million tonnes). No detailed textural and geochemical study has been conducted on these skarn rocks. In this study, we use petrographic studies, and major-trace element chemistry of skarn minerals from Belka Pahar to deduce the sequence of mineral transformations and to characterize the changes in the composition of the fluid with the progress of fluid-rock interaction during skarn formation. Two major rock types from the exoskarn have been considered for detailed study: prograde skarn rocks comprising pyroxene, garnet, wollastonite, quartz, and calcite, and retrograde skarn rocks containing garnet, scheelite, vesuvianite, fluorite, and sphalerite. Two types of garnet are recognized in the rocks-texturally earlier prograde garnet (Grt1) and texturally late retrograde oscillatory-zoned Grt2, which is further sub-divided into three generations Grt2a, Grt2b and Grt2c. The Grt1 (And27_47) and Grt2a (And_(50-60)) have high Y-concentrations, and high HREE/LREE ratio, which is in stark contrast to the late Grt2b (And_(40-50)) and Grt_(2c) (And_(27-40))) which is characterized by high F, low Y-concentrations, and HREE-depleted REE patterns. The chemistry of different generations of garnet suggests a shift from diffusive to more advective metasomatic conditions and increasing water/rock ratio, accompanied by a shift in pH of the fluid from neutral to acidic to weakly acidic. The crystallization of F-rich garnet, vesuvianite, and fluorite indicates an increase in F-concentration in the later-stage fluid. An increase of W concentration in Grt2b and co-precipitation of scheelite and fluorite also suggests its possible link with scheelite crystallization. The high grossular component of Grt2b and Grt2c suggests transport of Al in the hydrothermal fluid as F-com-plexes. The hydrothermal fluid composition extrapolated from scheelite chemistry agrees with the LREE enriched patterns exhibited by all skarn minerals. Principal Component Analysis done on a compiled dataset of skarn garnet shows that its trace element composition can be a useful proxy of fluid composition and can be used as an indicator of associated ore type.
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    Thermodynamic constraints on the petrogenesis of massif-type anorthosites and their parental magmas

    Riikka FredJussi S. HeinonenAku Heinonen
    16页
    查看更多>>摘要：Development of computational modeling tools has revolutionized studies of magmatic processes over the last four decades. Their refinement from binary mixing equations to tiiermodynamically controlled geochemical assimilation models has provided more comprehensive and detailed modeling constraints of an array of magmatic systems. One of the questions that has not yet been vigorously studied using thermodynamic constraints is die origin of massif-type anordiosites. The parental melts to these intrusions are hypodiesized to be either mantle-derived high-Al basaltic melts that undergo crustal contamination or monzodioritic melts derived directly from lower crust. On the otiier hand, many studies suggest tiiat die monzodioritic rocks do not represent parental melts but instead represent crystal remnants of residual liquids left after crystal fractionation of parental melts. Regardless of the source or composition, magmas tliat produce massif-type anordiosites have been suggested to have undergone polybaric (—1000-100 MPa) fractional crystallization while ascending through die lithosphere. We conducted lower crustal melting, assimilation-fractional crystallization, and isobaric and polybaric fractional crystallization major element modeling using two thermodynamically constrained modeling tools, die Magma Chamber Simulator (MCS) and rhyolite-MELTS, to test the suitability of tiiese tools and to study the petrogenesis of massif-type anordiosites. Comparison of our models with a large suite of whole-rock data suggests that the massif-type anorthosite parental melts were high-Al basalts that were produced when hot mantle-derived partial melts assimilated lower crustal material at Moho levels. These contaminated basaltic parental magmas tiien experienced polybaric fractional crystallization at different crustal levels (—40 to 5 km) producing residual melts tiiat crystallized as monzodioritic rocks. Model outcomes also support die suggestion that the cumulates produced during polybaric fractional crystallization likely underwent density separation, thus producing die plagioclase-rich anortiiositic rocks. The modeled processes are linked to a four-stage model that describes the key petrogenetic processes that generate massif-type anorthosites. The presented framework enables further detailed thermodynamic and geochemical modeling of individual anorthosite intrusions using MCS and involving trace element and isotope constrains.
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    Protracted extraction of high-silica melts from an upper-crustal magma reservoir recorded by the Wuchagou volcanic rocks in central Great Xing'an Range, NE China

    Min LiuDa ZhangYongjun Di
    13页
    查看更多>>摘要：High-silica volcanic rocks and their plutonic counterparts carry critical information concerning the evolution of shallow magmatic systems and construction of silicic upper continental crust, yet their origins remain debatable. Here, we examine high-silica volcanic rocks from the Lower Cretaceous Manitu and Baiyin'gaolao formations of the Wuchagou volcanic field, central Great Xing'an Range (GXR), to constrain the genesis of these high-silica volcanic rocks and the evolution of the Wuchagou magmatic system. Geochronological data suggest that the Manitu and Baiyin'gaolao volcanic rocks represent a prolonged eruption from the Wuchagou magmatic system, spanning Period I (135-129 Ma) and Period II (127-122 Ma) magmatism in the central GXR. Their slightly depleted ε_(Nd)(t) values and relatively young model ages coincide with a juvenile crustal origin. We suggest that the Manitu and Baiyin'gaolao high-silica volcanic rocks are the evolved compositions of their coexisting intermediate rocks given the similarities in both formation age and Sr-Nd isotopic composition and systematic geochemical variations. Clinopyroxene-and amphibole-based thermobarometers respectively yield low average pressures of —3.3 kbar and — 1.8 kbar with low temperatures of —1013 °C and 875 °C. Also, Clinopyroxene-and amphibole-based hygrometry yield relatively high H2O contents of —2.7-4.1 wt% and — 3.1-4.6 wt%. Together, they indicate the presence of a hydrous upper-crustal magma reservoir feeding the eruptions of the Manitu and Baiyin'gaolao high-silica volcanic rocks. Combining the behavior of highly incompatible elements (Rb and Th) with trace element modeling results, we show that the Wuchagou high-silica volcanic rocks formed by protracted extraction of high-silica melts in the upper crust, with complementary residual cumulates partially disguised in the Early Cretaceous felsic plutons in the central GXR. Our results highlight the significance of prolonged upper-crustal differentiation of magmas derived from juvenile crust in the development of the high-silica upper continental crust.
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