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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    Chemical mass transport during deformation and metamorphism: Insights from the Main Central Thrust and its footwall of Western Arunachal Himalaya, NE India

    Tapos K. GoswamiPranjit KalitaPurbajyoti Phukon
    20页
    查看更多>>摘要：The Main Central Thrust (MCT) is one of the major tectonic discontinuities stretching about ～2500 km along the length of the Himalaya and it plays important role during the tectono-metamorphic evolution of Himalayan orogen. This contribution aims to find out the element mobility and loss or gain of chemical mass during mylonitization and migmatization of rocks and their relation with deformation and metamorphism within the Main Central Thrust Zone (MCTz) and its immediate footwall along the Bomdila-Dirang-Seppar section of the Western Arunachal Himalaya, India. Based on meso- and microstructural observations, we divided the Dirang Formation in the footwall of the MCT into 3 units: Lower Mylonite Zone (LMZ), Protomylonite Zone (PMZ) and Upper Mylonite Zone (UMZ), with increasing structural distance from Dirang thrust towards NW. Deformation of the Dirang Formation results in enrichment of Na, K and Si with significant gain in mass of ～55% in the UMZ relative to the PMZ. We inferred that shear strain localization during mylonitization in the UMZ induces textural transitions causing chemical alterations at peak metamorphism conditions of 0.62-0.66 GPa and 690°710°C. Migmatites of Lower Structural Position (LSP) show enrichment of Si, Na, K, Ca, Mn and P with depletion of Mg and Fe in the leucosome + mafic selvage (LM) relative to the paleosome with ～13% chemical mass gain. On the other hand, migmatites of Upper Structural Position (USP) are characterized by enrichment of Si, Mn and K and depletion of Ca, Mg, Na, P and Fe in the LM relative to the paleosome with ～17% chemical mass gain. Such chemical changes occur during near peak metamorphic conditions of 0.52-0.63 GPa and 680°-720 °C and it is accompanied by the appearance of sillimanite. This study improves our understanding of the chemical modification of rocks in the MCT and its footwall, which were subjected to deformation and metamorphism during its evolution in Himalayan orogen. The emplacement of the GHS along the MCT buried the Dirang Formation to a depth of ～23-25 km. Interaction of fluids, derived from dewatering of the underthrusted Dirang Formation, induced chemical alteration in the UMZ. We interpret that chemical mass transfer in migmatite zones directly influences the metamorphic reactions that occur during deformation and crustal anatexis in intracontinental shear zones, such as the MCT.
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    • NSTL

    Early Mesozoic magmatism records the tectonic evolution from syn- to post-collisional setting in the Central Lhasa subterrane, Tibet

    Xuhui WangXinghai LangYulin Deng
    15页
    查看更多>>摘要：The Early Mesozoic (ca. 240-185 Ma) magmatic rocks in the central Lhasa subterrane (CL) provide crucial insight into the geodynamics of the Early Mesozoic orogenic assembly in the Lhasa Terrane. Here we present an integrated geochemical and geochronological study combining new and published data of the Early Mesozoic magmatism in the CL to reveal the geodynamic processes and magmatic responses involved in this orogeny. The here studied A-type granodiorites from the Luoza batholith display zircon U-Pb ages of 199 Ma, and zircon saturation temperatures of 833-857 °C. In addition, zircons have highly enriched radiogenic Hf isotope compositions (e_(Hf(t)) = -20.1 to -4.3). The Luoza granodiorite samples have high (~(87)Sr/~(86)Sr)t ratios between 0.7174 and 0.7209, and negative e_(Nd(t)) values between -9.5 and - 9.1. We propose the granodiorites were generated via partial melting of ancient basement rocks with minor inputs of basaltic melts (～10%). The here studied monzogranites and syenogranites from the Menba batholith are normal calc-alkaline and highly fractionated I-type granites that yield zircon U-Pb ages from 197 to 184 Ma and 187 Ma, respectively. The zircon grains of both rock types have relatively higher e_(Hf(t)) values (-13.8 to +0.5) than those of the Luoza granodiorites. The Menba monzogranites have (~(87)Sr/~(86)Sr)_t ratios between 0.7062 and 0.7081, and e_(Nd(t)) values between -6.9 and - 3.4. The calculated e_(Nd(t)) values for the Menba syenogranites range from -4.5 to -4.0. The isotope data set indicates that the Menba granitoids were generated via mixing of ancient crustal melts and mantle-derived magmas (～50%). The high-temperature and low-pressure conditions indicated by the Early Jurassic Luoza A-type granitoids are geodynamically linked to a post-collisional setting. This evidence, in combination with Early Mesozoic magmatism identified within the CL, clearly displays a transitional conversion from ca. 240-210 Ma S-type-dominated to ca. 210-185 Ma I-type-dominated magmatism. This distinct change in magmatism reflects the geodynamic change from a syn-collisional to a post-collisional setting, involving the breakoff of the Sumdo Paleo-Tethys oceanic slab at ca. 210 Ma. The Early Mesozoic magmatic belt within the CL may record the orogenic assembly between the central and southern Lhasa subterranes, which represents the important evidence for closure of the Sumdo Paleo-Tethys Ocean. Consequentiy, the Early Mesozoic magmatic belt within the CL reveals that the Sumdo Paleo-Tethys Ocean extended at least from the Luoza area, central Lhasa Terrane, to the easternmost end of the Lhasa Terrane.
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    • NSTL

    Early Mesozoic magmatism records the tectonic evolution from syn- to post-collisional setting in the Central Lhasa subterrane, Tibet

    Xuhui WangXinghai LangYulin Deng
    20页
    查看更多>>摘要：The Early Mesozoic (ca. 240-185 Ma) magmatic rocks in the central Lhasa subterrane (CL) provide crucial insight into the geodynamics of the Early Mesozoic orogenic assembly in the Lhasa Terrane. Here we present an integrated geochemical and geochronological study combining new and published data of the Early Mesozoic magmatism in the CL to reveal the geodynamic processes and magmatic responses involved in this orogeny. The here studied A-type granodiorites from the Luoza batholith display zircon U-Pb ages of 199 Ma, and zircon saturation temperatures of 833-857 °C. In addition, zircons have highly enriched radiogenic Hf isotope compositions (e_(Hf)(t) = -20.1 to -4.3). The Luoza granodiorite samples have high (~(87)Sr/~(86)Sr)_t ratios between 0.7174 and 0.7209, and negative emit) values between -9.5 and - 9.1. We propose the granodiorites were generated via partial melting of ancient basement rocks with minor inputs of basaltic melts (～10%). The here studied monzogranites and syenogranites from the Menba batholith are normal calc-alkaline and highly fractionated I-type granites that yield zircon U-Pb ages from 197 to 184 Ma and 187 Ma, respectively. The zircon grains of both rock types have relatively higher e_(Hf)(t) values (-13.8 to +0.5) than those of the Luoza granodiorites. The Menba monzogranites have (~(87)Sr/~(86)Sr)_t ratios between 0.7062 and 0.7081, and e_(Nd)(t) values between -6.9 and - 3.4. The calculated e_(Nd)(t) values for the Menba syenogranites range from -4.5 to -4.0. The isotope data set indicates that the Menba granitoids were generated via mixing of ancient crustal melts and mantle-derived magmas (～50%). The high-temperature and low-pressure conditions indicated by the Early Jurassic Luoza A-type granitoids are geodynamically linked to a post-collisional setting. This evidence, in combination with Early Mesozoic magmatism identified within the CL, clearly displays a transitional conversion from ca. 240-210 Ma S-type-dominated to ca. 210-185 Ma I-type-dominated magmatism. This distinct change in magmatism reflects the geodynamic change from a svn-collisional to a post-collisional setting, involving the breakoff of the Sumdo Paleo-Tethys oceanic slab at ca. 210 Ma. The Early Mesozoic magmatic belt within the CL may record the orogenic assembly between the central and southern Lhasa subterranes, which represents the important evidence for closure of the Sumdo Paleo-Tethys Ocean. Consequently, the Early Mesozoic magmatic belt within the CL reveals that the Sumdo Paleo-Tethys Ocean extended at least from the Luoza area, central Lhasa Terrane, to the easternmost end of the Lhasa Terrane.
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    • NSTL

    Olivines as probes into assimilation of silicate rocks by carbonatite magmas: Unraveling the genesis of reaction rocks from the Jacupiranga alkaline-carbonatite complex, southern Brazil

    Luanna ChmyzRogerio Guitarrari AzzoneExcelso Ruberti
    18页
    查看更多>>摘要：Assimilation of clinopyroxenite xenoliths by carbonatite magma plays an important role in the Jacupiranga complex, a classical mafic-ultramafic alkaline-carbonatite occurrence outcropping in Southeastern Brazil. In this complex, different batches of dolomite-bearing carbonatites and calcite carbonatites are found, all of them intrusive in the main clinopyroxenite body. Particularly for the calcite-rich carbonatites (with accessory olivine), the contacts with wall-rock clinopyroxenite are marked by xenolith-rich zones, formed by clinopyroxenite blocks embedded in the carbonatite. Banded reaction rocks occur enveloping such xenoliths, presenting conspicuous textures and compositions, with mineral assemblages changing from clinopyroxenite towards the carbonatite from (i) amphibole + phlogopite ±18titanite (amphibole-domain) via (ii) phlogopite (phlogopite domain) to (iii) olivine + phlogopite (olivine domain). The contents of Ni, Co and Sc in olivines from reaction rocks suggest that their trace element budget was strongly influenced by clinopyroxenite assimilation. Schematic reactions between clinopyroxenite and carbonatite melt are presented and these suggest that the observed features can be explained by a single magmatic high-T event. The required carbonatite component increases from amphibole- through phlogopite- to olivine-domains, mirroring their distance to the carbonatite front, reinforcing that carbonatite melt activity is a major control for the formation of the reaction rocks. The proposed reactions also imply that initially Mg-rich carbonatite melts consume diopside in the clinopyroxenite to produce the silicate phases (olivine, phlogopite, amphibole, and titanite), while shifting to Ca-rich melt compositions. The lack of xenoliths zones related to the olivine-free dolomite-bearing carbonatites, the absence of dolomite in the carbonatites proximal to the reaction rocks, and the presence of olivine seams in the carbonatite-reaction rock interface reinforce the link between olivine crystallization in the carbonatite and the consumption of dolomite component of the melt. Unlike previous models, we propose that the investigated reaction rocks formed in a magmatic environment, where carbonatite magmas have both assimilated and modified clinopyroxenite xenoliths. These observations may have general implications and further underline the capability of carbonatitic magmas for reactive bulk assimilation of diverse silicate rocks.
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    Late Early Cretaceous magmatic constraints on the timing of closure of the Bangong-Nujiang Tethyan Ocean, Central Tibet

    Wan-Long HuQiang WangGong-Jian Tang
    16页
    查看更多>>摘要：As one of the most important parts of the eastern Tethys, the evolution of the Bangong-Nujiang Tethyan Ocean (BNTO) has important geological implications for paleogeographic reconstructions of the Tethyan Realm. However, the closure time of the BNTO remains highly debated. Here, we report geochronological, petrological and geochemical data from three newly identified undeformed stitching plutons that intrude a north-south-oriented section of ophiolitic melanges in the middle-eastern parts of the Bangong-Nujiang suture zone (BNSZ), central Tibet. The stitching plutons comprise the Chaqu diorite-granite association, the Jiang Co granite, and the Konglong diorite porphyry. Secondary ion mass spectroscopy (SMIS) zircon U-Pb dating suggests that these three plutons were emplaced at ca. 116-112 Ma. The stitching plutons show heterogeneous Sr-Nd-Hf-O isotopic compositions. The Chaqu diorites have initial ~(87)Sr/~(86)Sr ratios varying from 0.7062 to 0.7064 and e_(Nd)(t) values from -2.0 to -1.6, and slightly high zircon d~(18)O (6.2-6.8± and positive zircon e_(Hf)(t) (+2.6 to +7.4) values. The Chaqu granites show high initial ~(87)Sr/~(86)Sr (0.7064-0.7065) ratios and low e_(Nd)(t) (-2.3 to -2.1) values, and elevated zircon d~(18)O (6.2-7.3± values and variable zircon e_(Hf)(t) (-1.7 to +4.9) values, similar to those of the associated Chaqu diorites. The Jiang Co granites exhibit higher initial ~(87)Sr/~(86)Sr (0.7085-0.7094) ratios and negative e_(Nd)(t) (-8.33 to -8.27) values, and variable zircon d~(18)O (4.8±to 6.0± and e_(Hf)(t) (-6.9 to -2.0) values. The Konglong diorite porphyries have initial ~(87)Sr/~(86)Sr isotopic ratios ranging from 0.7069 to 0.7075 and negative e_(Nd)(t) values ranging from -4.2 to -4.1, and high zircon d~(18)O (9.0±to 9.8± and variable zircon e_(Hf)(t) (-7.3 to +0.7) values. Geochemical evidence indicates that they were derived from different magma sources that included varying proportions of mantle and crustal components. The Chaqu diorites were derived by partial melting of an ancient metasomatized mantle source, and the Chaqu granites were formed by fractional crystallization of the Chaqu dioritic magmas. The Jiang Co granites were generated by partial melting of low d~(18)O high-temperature hydrothermally altered ancient continental crust. The Konglong diorite porphyries were likely generated by partial melting of high-d~(18)O enriched lithospheric mantle. In combination with data from adjacent ophiolites, magmatic rocks, and Cretaceous sedimentary rocks, we suggest that the stitching plutons were generated in a post-collisional setting induced by upwelling of asthenospheric mantle due to slab breakoff beneath the BNSZ. Our data provide an important new upper age limit for the time of collision between the Lhasa and Qiangtang terranes, i.e., the latest closure time for the BNTO.
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    The Calatrava paradox to decipher the origin of carbonatites: A petrological insight on Finca La Nava, Calatrava Province (central Spain)

    G. RosatelliE. Humphreys-WilliamsF. Wall
    22页
    查看更多>>摘要：The paper presents a detailed petrographic and geochemical study of the pyroclastites outcropping at La Nava maar (Calatrava Volcanic Province-CVP). Here, extrusive carbonatites mix with melilite nephelinite lapilli and bombs, crustal and mantle debris. Nephelinite and carbonatite are not characterised by a HFSE distribution typical of immiscible pairs. IREE and La/Lu ratios reflect primitive mantle carbonates and kimberlites. Igneous carbonates have 613C and 6180 isotopic ratios in the range of extrusive carbonatites and differ from local sedimentary carbonates. Carbonatites have 87Sr/86Sr and 144Nd/143Nd isotopic ratios similar to Calatrava leucitites but different to melilite nephelinite. The difference indicates that melilite nephelinites and carbonatites are co-eruptive but not comagmatic. The mingled magmatic convoy, underwent decarbonation processes, releasing Ca and Mg that reacted with parental silicate melt. Consequently, the nephelinitic melt evolved towards a melilititic composition producing Ca-rich overgrowths on mafic phenocrysts and reacted to produce high Mg#, low Cr + Ni forsterite. Mass balance calculations indicats that the nephelinite magma assimilated up to 30% of the carbonatite magma. Mixing modelling based on the Sr and Nd isotopic composition of mantle xenoliths and carbonatites indicate that La Nava carbonatite may represent the mantle metasomatic agent.
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    HT overprint of HP granulites in the Oisans-Pelvoux massif: Implications for the dynamics of the Variscan collision in the external western Alps

    Emilie JanotsStephane GuillotJean-Baptiste Jacob
    23页
    查看更多>>摘要：The Oisans-Pelvoux massif belongs to the Paleozoic basement of the External western Alps and records high temperature (HT) metamorphism associated with intense migmatization during the syn to post-collisional stages of the Variscan orogeny. Metamorphic assemblages related to the early collision stages have been obliterated making it difficult to constrain the earlier evolution of the unit. In this study, we report the finding of new bodies of high pressure (HP) mafic granulites, which preserve relics of the prograde evolution. Relics of Mn-rich garnet cores containing inclusions of epidote, titanite, chlorite and rare white micas constrain an early prograde stage at 460-550 °C and 0.4-1.0 GPa. The HP assemblage consists in garnet + clinopyroxene+ quartz + rutile ±plagioclase ±amphibole ±biotite and yield peak-P conditions at 650-730 °C and 1.5-1.7 GPa. Decompression was associated with heating to HT granulite-facies conditions of 800-870 °C and 0.6-0.9 GPa, which led to the development of granoblastic polycrystalline mosaics over the resorbing HP assemblage. The U-Pb dating of magmatic zircon cores constrains the emplacement age of the mafic protoliths at 479 ±5 Ma. Timing of the prograde to HP evolution is estimated around 345-330 Ma based on rutile U-Pb dating, which is contemporaneous with HP recorded in the other crystalline massifs of the western Alps. Zircon metamorphic rims yields U-Pb dates scattering between 337 and 294 Ma, which are interpreted to record crystallization during decompression and heating to granulite facies. Metamorphic conditions during the early prograde stage precludes scenarios involving subduction of a cold passive margin and are better reconciled with thickening of an orogenic wedge during collision. In contrast with other Variscan HP relics of the external Alps, the HP rocks of the Oisans-Pelvoux massif record heating to HT granulitic conditions during decompression, which may result from longer residence time in the lower crust before exhumation.
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    Unmixing of REE-Nb enriched carbonatites after incremental fractionation of alkaline magmas in the Shaxiongdong complex, Central China

    Jian-Hui SuXin-Fu ZhaoXiao-Chun Li
    18页
    查看更多>>摘要：Carbonatites are often temporally and spatially associated with alkaline silicate rocks. However, petrogenetic models of formation of alkaline-carbonatite complex and the related rare metal element enrichment processes are sdll in a matter of debate. In this study, we present detailed petrographic observations and geochemical data for the Shaxiongdong complex, consisting of meta-pyroxenite, syenite, and carbonatite in the South Qinling Belt, Central China. Meta-pyroxenite crops out as a marginal face of the complex and has higher Mg, Fe, Ti, Ca and P contents relative to syenite, consistent with a cumulate of pyroxene, magnetite, ilmenite, titanite and apatite. Syenite is the main central phase of the complex and can cut the meta-pyroxenite as dykes or veins. Both meta-pyroxenite and syenite show various degrees of fenidzation, in which the residual syenite looks like breccia cemented by fenite. Carbonatite locally occurs as dykes of dozens of centimeters in width within syenite and meta-pyroxenite and hosts the most Nb and REE of the complex. Nb-rich minerals, e.g., pyrochlore, crystallized coeval with aegirine and apatite in carbonatite, implying a magmatic origin. In contrast, most REE minerals are spatially associated with barite and are interstitial to calcite, implying they were mainly crystallized from residual carbonatite magmas or magmatic-hydrothermal fluids. Carbonatite samples with REE mineralization have oxygen isotopes slightly higher than those of unmineralized and Nb mineralized samples, supporting the involvement of hydrothermal fluids in REE mineralization. U-Pb dating of titanite and zircon from meta-pyroxenite, syenite and the associated fenite yielded identical ages of ~440 Ma. Carbonatite, syenite, and meta-pyroxenite all have nearly identical Sr-Nd isotopes ((~(87)Sr/~(86)Sr)_i: 0.70298 to 0.70367; eNd(t):+3.6 to +4.6), suggesting that all of these rocks were derived from the same mantle source. Fractionation modeling suggests the syenites evolve from a parental alkali basalt magma by separation of cumulate pyroxenite in the complex and finally lead to the unmixing of Nb-REE enriched carbonatites. The subsequent fractionation of carbonatite melts led to Nb and REE mineralization in the Shaxiongdong carbonatite. Niobium minerals formed at early magmatic stage, while most REE minerals likely crystallized from the late magma residuals or precipitated from magmatic-hydrothermal fluids. This genetic model can be widely applicable to other alkaline-carbonatite complexes and provides a reasonable explanation for extreme enrichment of rare metals in such rocks.
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    The delamination of lower crust in continental back-arc basin: Evidence from Sr isotope and elemental compositions of plagioclase and clinopyroxene in andesites from Kueishantao, north of Taiwan, China

    Kun GuoXiaoyuan WangShuai Chen
    19页
    查看更多>>摘要：Subduction zones (arcs and back-arcs) are major sites for elemental cycling via slab dehydration and subsequent mande metasomatism and melting; many models have been suggested by researchers to explain these processes. However, the influence of the overriding lithosphere, especially the lower crust, on the generation of back-arc magmas during the early spreading stage remains largely unknown. Kueishantao (KST), located at the tectonic junction of the southernmost part of the Okinawa Trough (SPOT) and the Ryukyu arc, is an ideal setting to investigate these issues because of the obvious differences between the subducted slab and the overriding continental lithosphere. To better address the contribution of the overriding lithosphere to a nascent back-arc basin, we examined the in situ major and trace element compositions of plagioclase (PI) and clinopyroxene (Cpx) phenocrysts and the in situ ~(87)Sr/~(86)Sr of PI phenocrysts in andesites from KST. The Pl and Cpx in KST andesites show obvious evolution (An contents of Pl, 39-91; Mg~# of Cpx, 73-92). The valid thermobarometers of Pl and Cpx imply crystallization temperatures and depths of 1059-1117 °C and 1-14 km, respectively, similar to those of the SPOT. The compositions of Cpx suggest that the KST andesitic magma belonged to the subalkaline, tholeiitic and calc-alkaline magma series. The dEu values of Pl are 9.27-25.73, and those of Cpx are 0.50-0.92, indicating that the magma experienced Pl crystallization and separation. The ~(87)Sr/~(86)Sr values of PI range from 0.705630 to 0.706985 and remain constant with varying An contents (52-85), indicating an enriched magma source not resulting from assimilation of continental crust. Taking into consideration all geochemical, trace elemental, ~(87)Sr/~(86)Sr isotope and geophysical constraints, the principal magma source of KST andesites was probably the lower continental crust, which foundered into the convecting mantle and subsequently melted and interacted with asthenospheric mantle materials. The over-thickened crust, mantle corner flow and addition of subducted fluids in the KST area led to delamination. The structure and geochemistry of Pl and Cpx imply a simple two- or multilayer magma chamber system in the continental crust and a mixing magma process between a relatively silicic magma (H2O-saturated and high fO2) and a primitive mafic magma at shallow depths.
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    Monazite record for the Paleoproterozoic Svecofennian orogeny, SE Finland: An over 150-Ma spread of monazite dates

    Paula E. SalminenPentti HolttaRaimo Lahtinen
    23页
    查看更多>>摘要：The Svecofennian orogeny in Fennoscandia is an example of a long-lived orogeny characterised by low pressure, high temperature (LP-HT) metamorphism. In this paper, we present new LA-ICP-MS age data on monazite from fifteen paragneiss samples and two granitoid samples from the southeastern part of the Svecofennian orogen, including the Sulkava granulite complex. The high-grade rocks in the study area were metamorphosed at ca. 750-800 °C and 5-6 kbar, followed by near-isothermal decompression down to 3-4 kbar. The analysed monazites yielded ~(207)Pb/~(206)Pb dates from 1953 to 1737 Ma. The youngest dates are typically yielded by rims and overgrowths of monazites and by younger domains in patchy monazite grains. Psammitic and pelitic layers from the same outcrop display differences in monazite date distributions, with younger dates being more common in pelitic layers. Both detrital and metamorphic origins are possible for the =1.91 Ga monazite grains found in some samples. For the younger (<1.91 Ga) monazite grains, we found three main age peaks. The peak at 1.89-1.86 Ga fits well with the high-grade metamorphic event at 1.89-1.88 Ga observed elsewhere in southern and central Finland. The 1.87-1.86 Ga interval dates a near-isothermal decompression stage. The age peak at 1.83-1.82 Ga records the age of younger high-grade metamorphism in the study area. The age peak at 1.80-1.77 Ga is interpreted to represent isotopic resetting due to fluid-induced alteration during a shield-wide exhumation stage, when waning magmatism, leucosome crystallisation and a change towards brittle-ductile deformation led to localised fluid flow before cratonisation. The dates between the age peaks could be explained in part by mixed isotopic ages in patchy grains without clear growth zones.
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