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Lithos
Elsevier Science B.V.
Lithos

Elsevier Science B.V.

0024-4937

Lithos/Journal LithosSCIISTP
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    Evidence of hydration of the peridotite mantle wedge recorded in low-CaO olivines from Los Tuxtlas Volcanic Field, Veracruz, Mexico

    Beatriz A. Diaz-BravoCarlos Ortega-ObregonPeter Schaaf
    20页
    查看更多>>摘要：Los Tuxtlas Volcanic Field (LTVF) is an isolated volcanic center located between the Trans-Mexican Volcanic Belt and the Central America Volcanic Arc. Its volcanic products include alkaline and sub-alkaline rocks with variable subduction signatures, some of them considered as high-MgO rocks and some also include ultramafic xenoliths. Most suites show MgO contents ranging from 11 to 16 wt%, reflecting peridotite-derived melts with a primitive character (Mg# = 66) that suffered crystal settling in magma batches during its ascent from the mantle to surface. Parental magma compositions were reconstructed using the significant forsterite compositions (Fo88 to Fo83) to estimate magmatic temperatures that range from 1095 to 1372 °C. Reconstructed parental magmas and low-CaO magmatic olivines allowed us to determine two main groups with water content between 4 and 6% and 8-9%. These magmatic parameters are consistent with those inferred by thermal models, supporting that magmatic products are derived from the melting of a mantle-wedge peridotite at relatively high temperature and wet conditions. Isotopic compositions point to the existence of a high-p signature (HIMU; μ = ~(238)U/~(204)Pb) in the mantle source similar to that identified at a regional scale from northern Mexico to South America. Low-CaO olivines and their use as a proxy for magmatic H2O content provide evidence of hydration of the mantle wedge by fluids derived from the slab. Hydrous slab-derived contribution induces a higher extent of melting and formation of wet magmas with high LILE/HFSE ratios. Relatively high-NiO olivine compositions record slight metasomatism of the mantle wedge by a small amount of silicic-rich slab components and the contribution of larger proportions of peridotite mantle to the melt over the slightly metasomatized mantle domains. In general, most of the magmatic products record stages of melting and metasomatism triggered and induced by fluids and melts of the slab into an enriched mantle peridotite wedge.
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    Late Neoproterozoic extended continental margin development recorded by the Seve Nappe Complex of the northern Scandinavian Caledonides

    Katarzyna WalczakGrzegorz ZiemniakChristopher Barnes
    15页
    查看更多>>摘要：The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record the opening and closure of the Iapetus Ocean, culminating with the collision of Baltica and Laurentia. The Seve Nappe Complex (SNC) of the Scandinavian Caledonides comprises relics of the outermost Baltoscandian passive margin that were subducted to mantle depths during ocean closure. Subduction of these rocks overprinted much of the Neoproterozoic record for Iapetus Ocean formation, and as a result, much of the work conducted in the SNC has focused solely on the Caledonian orogenic history. In this study, we combined petrological and geochronological work to expand the knowledge about the Neoproterozoic metamorphic history of the Baltoscandian margin in the understudied Vaivancohkka-Salmme-cohkat region of the northern Scandinavian Caledonides. The work focused on rocks that belong to the upper gneiss unit, which constitutes part of SNC in the region. The unit comprises migmatitic paragneisses and garnet-mica schist containing metamafic bodies. U-Pb zircon and Th-U-total Pb monazite dating of the migmatitic paragneiss yielded consistent age of metamorphism in 602 ±5 Ma and 599 ±3 Ma, respectively. A similar U-Pb age of 604 ±8 Ma was obtained for the zircon from the leucocratic vein transecting the amphibolite within the studied gneiss. Interestingly, no Caledonian ages were identified. Likewise, no evidence for high or ultra-high pressure conditions was found, neither in the gneisses/schists nor in the metamafic rocks. Petrographic observations and calculated metamorphic P-T conditions indicate that rocks belonging to upper gneiss unit underwent upper-amphibolite facies metamorphism in a melt stability field; 8.0-10.5 kbar at 750-790 °C. Since the studied rocks underwent high-grade metamorphism in the Ediacaran and lack obvious evidence for Caledonian high-pressure metamorphism, the studied part of the SNC offers an extraordinary insight into the Late Neoproterozoic history of the hot, extended Baltoscandian margin.
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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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    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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    CO2 storage in the Antarctica Sub-Continental Lithospheric Mantle as revealed by intra- and inter-granular fluids

    Federico CasettaAndrea L. RizzoBarbara Faccini
    20页
    查看更多>>摘要：The investigation of the role played by CO2 circulating within the mantle during partial melting and meta-somatic/refertilization processes, together with a re-consideration of its storage capability and re-cycling in the lithospheric mantle, is crucial to unravel the Earth's main geodynamic processes. In this study, the combination of petrology, CO2 content trapped in bulk rock- and mineral-hosted fluid inclusions (FI), and 3D textural and volumetric characterization of intra- and inter-granular microstructures was used to investigate the extent and modality of CO2 storage in depleted and fertile (or refertilized) Sub-Continental Lithospheric Mantle (SCLM) beneath northern Victoria Land (NVL, Antarctica). Prior to xenoliths entrainment by the host basalt, the Antarctic SCLM may have stored 0.2 vol% melt and 1.1 vol% fluids, mostly as FI trails inside mineral phases but also as inter-granular fluids. The amount of CO2 stored in FI varies from 0.1 μg_((CO2))/g_((sample)) in olivine from the anhydrous mantle xenoliths at Greene Point and Handler Ridge, up to 187.3 μg/g in orthopyroxene from the highly metasomatized amphibole-bearing lherzolites at Baker Rocks, while the corresponding bulk CO2 contents range from 0.3 to 57.2 μg/g. Irrespective of the lithology, CO2 partitioning is favoured in orthopyroxene and clinopyroxene-hosted FI (olivine: orthopyroxene = 0.10 ±0.06 to 0.26 ±0.09; olivine: clinopyroxene = 0.10 ±0.05 to 0.27 ±0.14). The H2O/(H2O + CO2) molar ratios obtained by comparing the CO2 contents of FI to the H2O amount retained in pyroxene lattices vary between 0.72 ±.17 and 0.97 ±0.03, which is well comparable with the values measured in olivine-hosted melt inclusions from Antarctic primary lavas and assumed as representative of the partition of volatiles at the local mantle conditions. From the relationships between mineral chemistry, thermo-, oxy-barometric results and CO2 contents in mantie xenoliths, we speculate that relicts of CO2-depleted mantle are present at Greene Point, representing memory of a CO2-poor tholeiitic refertilization related to the development of the Jurassic Ferrar large magmatic event. On the other hand, a massive mobilization of CO2 took place before the (melt-related) formation of amphibole veins during the alkaline metasomatic event associated with the Cenozoic rift-related magmatism, in response to the storage and recycling of CO2-bearing materials into the Antarctica mantle likely induced by the prolonged Ross subduction.
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    Geochronology, geochemistry and petrology of the oligocene magmatism in SE segment of the UDMB, Iran

    Hadi Shafaii MoghadamWilliam L. GriffinJose F. Santos
    15页
    查看更多>>摘要：Despite diverse geochronological-geochemical studies on Cenozoic igneous rocks from the SE segment of the Urumieh-Dokhtar Magmatic Belt (UDMB) of Iran, the nature of the Oligocene magmatic rocks from the farthermost end of the SE segment- where it is linked to the Makran magmatic belt- has been ignored due to the difficulty of access. In this study, we focus on syn-collisional mafic to felsic igneous rocks of calc-alkaline and high-K calc-alkaline affinities from the SE segment of the Urumieh-Dokhtar Magmatic Belt (UDMB) near Nagisun, south of Bam. The Nagisun rocks have low Sr/Y and La(n)/Yb(n), similar to igneous rocks from typical arcs. Zircon U-Pb ages show comparable ages for plutonic (～ 34-25 Ma) and volcanic (～34-27 Ma) rocks. The eHf(t) values for zircons from plutonic rocks range from -0.3 to +12.8, whereas the eHf(t) values for the volcanic rocks vary from -2.6 to +13. Modelling of trace elements compositions using Nagisan basaltic samples indicate that an 87:2:11 mixture of the depleted MORB mantle, subducting (trench)-sediments and altered oceanic crust with 5% aggregated fractional melting closely matches the trace-element abundances of the Nagisun basaltic rocks. Indeed, the modelling of Sr and Nd isotopic data emphasizes that the Nagisun magmatic rocks could be products of bulk mixing between a depleted MORB mantle and/or a mixed, fertilized mantle with the Cadomian lower and upper continental crust. Furthermore, our compiled data display that the magmatism in the SE segment of the UDMB changed through time from normal calc-alkaline magmatism to adakitic magmatism at ~20 Ma, after the collision with Arabia began ca 27 Ma.
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    Petrology of Mt. Kira continental alkali lavas with arc-like signature, Batman, SE Anatolia, Turkey: Evidence for mafic juvenile lower crust assimilated intraplate basalts in the collision- and mantle flow-driven geodynamic setting of Arabian Foreland

    Semsettin CaranSuleyman Polat
    22页
    查看更多>>摘要：Mineral phase variations and compositions of Early Pliocene (~(40)Ar-~(39)Ar ages ranges 5.092 ±0.030-4.892 ±0.024 Ma) continental alkali basalts cropped out on Mt. Kira, NE of Batman Province, SE Anatolia, and located approximately 75 km east of well-known Karacadag Volcanic Complex (KVC) in Diyarbakir Province, record indications of pre-eruptive assimilation of the lower crust by OIB-like basaltic magmas from Arabian Foreland, which are proved by available data of juvenile lower crustal xenoliths in northern Syria Cenozoic basalts. Diopsitic augites with low Mg#'s (0.65-0.73) in Mt. Kira basalts depict a compositional prone to clinopyroxenes (Mg#, 0.60-0.76) in Syrian lower crustal xenoliths and coexist with Ca- and Mn-rich fayalitic olivines (Fo_(63-36)), Ca-poor (An_(72-13)) plagioclases, and Fe-Ti oxides. Mt. Kira lavas, classified as hawaiites, are characterized by the narrow ranges of low SiO2 (46.30-47.68 wt%), Al2O3 (13.74-14.41 wt%), MgO (3.48-4.16 wt%), and high total alkali (Na2O + K2O, 5.26-5.89 wt%), Fe2O3 (13.97-15.38 wt%), TiO2 (3.44-3.61 wt%), CaO (7.20-8.99 wt%), and P2O5 (1.54-1.71 wt%) contents, and with low Cr (< 14 ppm) and Ni (< 20 ppm) values. Except for unusual Zr-Hf depletion, they exhibit typical arc-like (or crust-like) sharp negative anomalies of Nb-Ta and Ti and possess high concentrations of Y (45-51 ppm), Sc (28-33 ppm), and Heavy REE (e.g., Dy, 9.25-10.73 ppm; Er, 4.74-5.27 ppm; Yb, 3.79-4.44 ppm). Two-component mixing model, and radiogenic ~(87)Sr/~(86)Sr (0.7,044,010-0.705396) and ~(143)Nd/~(144)Nd (0.512676-0.512812) isotopic compositions, which overlap with bulk-silicate earth, for Mt. Kira basalts, provide evidence for the mafic juvenile lower crustal assimilation by OIB-like basaltic magmas derived from a common enriched mantle source. High Ba/La, Ba/Ta, Ba/Nb, Zr/Nb, and low Nb/La, La/Sm, Nb/Th, La/Yb, and Dy/Yb ratios, which coincide with those of Syrian lower crustal xenoliths, appears as geochemical hallmarks for decoupling of juvenile lower crust assimilated Mt. Kira, NW Syria and some of KVC basalts from least contaminated ones in Arabian Foreland. The enrichments in Y, Fe, and Ti and depletions in Al and Sr in Mt. Kira lavas, however, are attributed to post-eruptive chemical weathering processes. Based on petrological and tectonic data, it is proposed that the northern Arabian Foreland Cenozoic volcanism was triggered by the combined effect of both collision- and mantle flow-driven forces with a kinematic linkage. Asthenospheric mantle flow and upwelling, which supply a thermal source, caused the deep mantle processes and interaction between asthenosphere and lithosphere, which was formed the amphibole-bearing garnet Iher-zolitic (asthenospherized) metasomes (mantle flow-induced metasomatism) at the base of the SCLM accreted from the ductile asthenosphere, prior to partial melting. Partial melting of such re-fertilized metasomes and surrounding wall rock was most likely produced hybrid parental magmas via mixing variable proportions of melt fractions from a garnet lherzolite source with melt fractions from an amphibole-bearing garnet lherzolite source. On the other hand, concurrently running of collision-induced temporarily changing regional stress -compressional or extensional- regimes caused i) the development of surface tectonic structures in a dynamic topography that reactivated the pre-Neogene deep tectonic structures, ii) multiphase rapid uplifts, and iii) prominent deep channeling for the upwelling enriched mantle-derived hybrid magmas to penetrate. The shallow level lower crustal assimilation of this OIB-like hybrid mantle melts, generated the Mt. Kara, NW Syria, and some of KVC basalts in the Arabian Foreland, during magmas en route to die surface.
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    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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