Geochronology and geochemistry of Cretaceous–Eocene granites,Tengchong Block (SW China): Petrogenesis and implications for Mesozoic-Cenozoic tectonic evolution of Eastern Tethys

The Early Cretaceous–Early Eocene granitoids in the Tengchong Block record the evolutionary history of the Mesozoic-Cenozoic tectono-magmatic evolution of Eastern Tethys. (a) The Early Cretaceous granitoids with relatively low (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7090–0.7169 and ε_Nd(t) values of ?9.8 to ?7.8 display metaluminous, calc-alkaline dominated by I-type granite affinity and hybrid mantle–crust geochemical signatures. They may have been derived from melting of the subducted Meso-Tethyan Bangong-Nujiang oceanic crust with terrigenous sediments in an arc-continent collisional setting. (b) The Late Cretaceous–Paleocene granitoids with relatively high (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7109–0.7627, and ε_Nd(t) values of ?12.1 to ?7.9 exhibit metaluminous to peraluminous, calc-alkaline dominated by S-type granite affinity and hybrid Lower–Upper crust geochemical signatures, which may be originated from partial melting of the Meso-Proterozoic continental crust in the collision setting between the Tengchong Block and Baoshan Block. (c) The Early Eocene granitoids have metaluminous, calc-alkaline I-type and S-type granites dual affinity, with relatively high (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.711–0.736, ε_Nd(t) values of ?9.4 to ?4.7, showing crust-mantle mixing geochemical signatures. They may have been originated from partial melting of the late Meso-Proterozoic upper crustal components mixed with some upper mantle material during the ascent process of mantle magma caused by the subduction of the Neo-Tethyan Putao–Myitkyian oceanic crust, and collision between the Western Burma Block and the Tengchong Block. It is these multi-stage subductions and collisions that caused the spatial and temporal distribution of the granitic rocks in the Tengchong Block.     

An early Paleozoic monzonorite–granite suite in the South China block: implications for the intracontinental felsic magmatism

Mineralogy and Petrology - Large granitoid complexes within the continental crust are believed to be closely linked to mantle-derived magmas based on field observations and isotopic studies....     

Petrogenesis and tectonic implications of early Devonian mafic dike–granite association in the northern West Junggar,NW China

Mafic dike–granite associations are common in extensional tectonic settings and important and pivotal in reconstructing crust–mantle geodynamic processes. We report results of zircon U–Pb and hornblende ⁴⁰Ar-³⁹Ar ages and major-element and trace-element data for mafic dike–granite association from the northern West Junggar, in order to constrain their ages, petrogenesis, and geodynamic process. The mafic dike–granite association was emplaced in the early Devonian. The Xiemisitai monzogranites have high SiO₂ contents and low MgO, Cr, and Ni concentrations, suggesting that they were mainly derived from crustal sources and were probably generated by partial melt of the juvenile mid-lower crust. The mafic dikes have low Mg^# and Cr and Ni abundances, suggesting that they have experienced significant fractional crystallization. The Xiemisitai mafic dikes contain hornblende and biotite and display negative Nb–Ta–Ti anomalies, enrichment of LREEs and LILEs, and depletion of HREEs and HFSEs, consistent with an origin from a lithospheric mantle metasomatized by subducted slab-derived fluids. In addition, the Xiemisitai mafic dikes are plotted within melting trends with little to no garnet (Cpx: Grt = 6:1) in their source. The La/Yb versus Tb/Yb plot also indicates the presence of less than 1% residual garnet in the source region for the Xiemisitai mafic dikes. Therefore, it can be inferred that the Xiemisitai mafic dikes were generated at a correspondingly shallow depth, mostly within the spinel stability field. The Xiemisitai mafic dikes were most probably generated by the partial melting of the metasomatized lithospheric mantle at relatively shallow depths (<80 km). The Xiemisitai mafic dike–granite association could have been triggered by asthenospheric upwelling as a result of the rollback of the subducted Irtysh–Zaysan oceanic lithosphere.     

U–Pb dating of zircon and cassiterite from the Early Cretaceous Jiaojiguan iron-tin polymetallic deposit,implications for magmatism and metallogeny of the Tengchong area,western Yunnan,China

The newly discovered Jiaojiguan deposit, a medium-scale skarn iron-tin polymetallic deposit on the Sino-Burma boundary of Yunnan Province (SW China), is spatially associated with the biotite monzonitic granite. Here, we report new in situ zircon LA-MC-ICP-MS U–Pb ages, trace element and Hf isotope data from the granite, and U–Pb dating ages of cassiterite from the ore bodies. In this study, we obtain a weighted mean ²⁰⁶Pb/²³⁸U age of 124.1 ± 1.4 Ma for the zircon and a ²⁰⁷Pb/²⁰⁶Pb-²³⁸U/²⁰⁶Pb intercept age of 123.8 ± 2.2 Ma for the cassiterite. The granite crystallized during the Early Cretaceous, with zircons exhibiting εHf(t) values from ?5.8 to ?0.6 and two-stage Hf model ages (T_DM2) of 1.21–1.54 Ga. The close temporal and spatial links between pluton emplacement and ore-forming events suggest that magmatic-hydrothermal events were the key factors that triggered the genesis of the iron-tin polymetallic deposits in the area. Regional geochronological data show that tin mineralization took place three times during the Cretaceous–Palaeogene in the Tengchong block due to re-melting of the underlying supposed Proterozoic (1.5 ± 0.5 Ga) Sn-rich strata/materials. Compared with those in the Bangong–Nujiang metallogenic belt (BNMB), we propose that the Cretaceous iron-tin polymetallic mineralization events in Tengchong–Baoshan closely resemble those of the Bangong–Nujiang belt in northern Tibet, both of which have experienced similar tectono-magmatic-metallogenic histories since the Mesozoic.     

Geochemistry,zircon U–Pb geochronology and Hf isotopes of Jurassic-Cretaceous granites in the Tengchong terrane,SW China: implications for the Mesozoic tectono-magmatic evolution of the Eastern Tethyan Tectonic Domain

ABSTRACT

Recently identified Early Jurassic, Early Cretaceous, and Late Cretaceous granites of the Tengchong terrane, SW China, help to refine our understanding of the Mesozoic tectonic-magmatic evolutionary history of the region. We present new zircon U–Pb geochronological, Lu–Hf isotopic and geochemical data on these rocks. The zircon LA-ICP-MS U–Pb ages of the Mangzhangxiang, Laochangpo, and Guyong granites, and Guyong granodioritic microgranular enclaves are 185.6, 120.7, 72.9, and 72.7 Ma, respectively. Geochemical and Hf isotopic characteristics suggest the Mangzhangxiang and Laochangpo S-type granites were derived from partial melting of felsic crust and that the Guyong I-type granite and associated MMEs were generated through magma mixing/mingling. Mesozoic magmatism in the Tengchong terrane can be divided into three episodes: (1) the Triassic syn- and post-collisional magmatic event was related to the closure of the Palaeo-Tethyan Ocean, as represented by the Changning-Menglian suture zone; (2) the Jurassic to Early Cretaceous magmatism was related to the subduction of the Meso-Tethyan oceanic crust, as represented by the Myitkyina ophiolite belt; and (3) the Late Cretaceous magmatism was related to the subduction of the Neo-Tethyan oceanic crust, as represented by the Kalaymyo ophiolite belt.     

The first dating results for gabbro of the dunite–clinopyroxenite–gabbro complex of the Chistop massif (North Urals)

The first data on the Late Riphean age by U–Pb and Sm–Nd analysis (≥922 ± 14 and 686 ± 19 Ma, respectively) were obtained for rocks of the dunite–clinopyroxenite–gabbro complex of the Chistop massif in the Patinum-bearing Belt of the Urals. These data allow one to assume that the formation of the Ural paleoocean probably started immediately after the break-up of Rodinia.     

Sequence of magma emplacement and sulfide saturation in the Gaojiacun–Lengshuiqing intrusive complex (SW China)

The Lengshuiqing area contains several small intrusions made up of peridotite ± quartz diorite ± granite spatially associated with the Gaojiacun pluton (gabbroids + peridotite + diorite). Ni–Cu sulfide ore occur at Lengshuiqing, hosted in peridotite. SHRIMP U–Pb zircon dating produced the ages of 803 ± 4.2 Ma (peridotite), 807 ± 2.6 Ma (oikocrystic hornblende gabbro), 809 ± 4.3 Ma (hornblende gabbronorites) for the Gaojiacun pluton and 807 ± 3.8 Ma (diorite, intrusion I), 817 ± 6.3 Ma (quartz diorite, intrusion II) and 817 ± 5 Ma (peridotite, intrusion 101) for Lengshuiqing. These ages suggest the emplacement of the Gaojiacun pluton later than the intrusions from Lengshuiqing. The olivine from Lengshuiqing does not contain sulfide inclusions and is relatively Ni-rich (1,150–1,550 ppm Ni), suggesting its crystallisation before the sulfide saturation that generated the Ni–Cu deposits. The olivine of the gabbros in the Gaojiacun pluton is Ni-poor (250–800 ppm), which indicates crystallisation from a severely metal-depleted magma after a sulfide saturation event. The olivine in the peridotites from the Gaojiacun pluton has 800–1,150 ppm Ni and contains sulfide inclusions. Moreover, geological evidence suggests the genesis of the peridotites from Gaojiacun in conduits that were ascending through the gabbroids. A sequence of at least three stages of magma emplacement is proposed: (1) Lengshuiqing; (2) gabbroids from Gaojiacun; (3) peridotites from Gaojiacun. Given the age differences, the intrusions at Lengshuiqing and the Gaojiacun pluton might have been produced by different magmatic events.     

Paleoproterozoic gabbro–diorite–granite magmatism of the Batomga Rise (NE Aldan Shield): Sm–Nd isotope geochemical evidence

The geochemical similarity and almost simultaneous (2055–2060 Ma) formation of Utakachan gabbro-amphibolite, Jagdakin granodiorite-diorite, Khoyunda granitoid, and Tygymyt leucogranite complexes, which inruded metamorphic formations of the Batomga Group are evidence of their formaton from unified magmatic source. All this makes it possibble to combine aforementioned complexes into the unified Early Proterozoic diferentiated gabbro-diorite-granite complex.     

Geochemistry and Petrogenesis of Late Cretaceous–Paleocene Granites from the Tengchong Block, Western Yunnan: Implications for Angle-switching of Subducting Slab

The Cenozoic geological hallmark of Western Yunnan is the characteristic voluminous Late Cretaceous-Eocene granites; however, their geological background and petrogenesis have not been well constrained and elucidated. In this study, we present new zircon U-Pb dating, along with geochemical and Sr-Nd-Hf isotopic data for granites from the Tengchong–Lianghe granitoid belt (as abbreviated to Tengliang belt) and West Yingjiang batholiths from the Tengchong block. The mineralogical and geochemical features of the Tengliang granites and the West Yingjiang batholiths are ascribed to aluminous S-type granites and weak peraluminous I-type, respectively. Zircon U-Pb analyses yielded consistent ages ranging from 67.5 Ma, 68.4 Ma and 66.2 Ma from the Tengliang granitoid belt and 50.4 Ma to 60.8 Ma for three samples from the west Yingjiang batholiths. The Tengliang granites were emplaced during the Late Cretaceous (68?66 Ma) and demonstrate negative εHf(t) values (?24 to ?4) and initial?87Sr/86Sr ratios of 0.7101–0.7139 and significant negative εNd(t) values from ?8.91 to ?13.2, indicating a Proterozoic sedimentary source or enriched components. The hornblende-bearing I-type granites from West Yingjiang are characterized by lower initial?87Sr/86Sr ratios of 0.7076–0.7106, compared to Tengliang granite and negative whole-rock εNd(t) values from ?4.0 to ?11.9. The early Eocene west Yingjiang gneissic granites show wide ranges of εHf(t) values from +7.4 to ?8.5 and T2DM of 1.30–0.65 Ga, indicating partial melting of ancient crust with contributions of depleted mantle materials. In combination with the regional background and previous studies, we propose that such a spatio-temporal distribution of the Tengchong granitoid belt might be related to the rollback or angle-switching of the Neo-Tethyan subducting slab. This study sheds new light on the evolutionary history of the Tengchong block.     

Paired uraninite and molybdenite dating of the K?nigshain granite: implications for the onset of late-Variscan magmatism in the Lausitz Block

We present geochronological data for late-Variscan magmatism in the Lausitz Block of the Saxo-Thuringian Zone, Germany. The Th–U–total Pb age of uraninite and the Re–Os age of molybdenite from the composite biotite–monzogranite pluton of K?nigshain overlap at the 2σ confidence limit: 328.6 ± 1.9 Ma (uraninite), and 327.0 ± 1.3 Ma and 327.6 ± 1.3 Ma (molybdenite), indicating that crystallization of magmatic uraninite and deposition of molybdenite were nearly contemporaneous. These data imply that magmatic processes in this part of the Variscan orogen already started in latest Visean time, about 10 Ma earlier than previously assumed (315–320 Ma). The new ages correspond to ages for plutonic rocks in the Elbe Zone immediately west of the Lausitz (around 335–325 Ma) and the bulk of late-Variscan igneous rocks in the Saxo-Thuringian Zone (335–320 Ma).     

Characteristics of the Reservoir of the Rehai Geothermal Field in Tengchong, Yunnan Province, China

The Rehai geothermal field in Tengchong County, Yunnan Province is a significant high-temperaturehydrothermal convective system. The geothermal reservoir is composed of granite. Various geothermometersare used to evaluate the reservoir temperature. The most likely temperature of the reservoir as representedby T_(Na-K-Ca) is about 230℃. The chemical and isotopic compositions of fluids before boiling within the reser-voir are estimated. The mixing and dilution of cold and warm waters are discussed. The Rehai geothermal fieldis a high-temperature (hot) water system with the subsurface boiling zone close to the surface. The reservoirpressure at different depths is calculated. And finally the water-rock equilibration is inferred.     

Paleocene–early Eocene post-subduction magmatism in Sikhote-Alin(Far East Russia):New constraints for the tectonic history of the Izanagi-Pacific ridge and the East Asian continental margin

New isotopic,geochemical and geochronological data justify the widespread occurrence of middle Paleocene to early Eocene(60–53 Ma) post-subduction felsic magmatism across the entire Sikhote-Alin territory(southeastern Russia),conform with previous observations in Northeast China,the southern Korean Peninsula,and the Inner Zone of Japan.This igneous activity in East Asia coincided with the reactivation(after tectonic quiescence between ～93–60 Ma) of left-lateral strike-slip displacements along the Tan-Lu and Central Sikhote-Alin faults and with the post-60 Ma cessation of subduction/accretion recorded in the Shimanto belt of SW Japan.The SikhoteAlin post-subduction igneous A-type rocks present diverse mineralogical and geochemical features that suggest interactions of the subducting plate with anhydrous mantle upwelling through slab tears in the continental margin.The middle Paleocene–early Eocene magmatism is not related to subduction but is synchronous with strikeslip tectonics and the termination of accretionary prism development,suggesting a shift in tectonic regime from oceanic plate subduction at a convergent margin to parallel sliding and initiation of a transform continental margin.These new observations are inconsistent with the current tectonic model of 60–50 Ma Izanagi-Pacific ridge subduction beneath East Asian continental margin.     

Sr–Nd isotope data of basement rocks from the northernmost argentine Precordillera and its implications for the early Paleozoic evolution of SW Gondwana margin

The Precordillera terrane (Cuyania) in western Argentina is commonly accepted as an exotic fragment derived from Laurentia in the Early Paleozoic. Evidence supporting such an interpretation is manly based on similarities in the sedimentary cover successions and their paleontological content. Little is known about the basement of the Precordillera terrane. Its isotopic characterization is essential to better constrain the present areal distribution of the terrane and it may provide more insight into the pre-rifting evolution of the Precordillera terrane along the Iapetan margin of Laurentia. We present new Sr and Nd isotope data of pre-Late Ordovician meta-igneous rocks from the Río Bonete region in NW Argentina, interpreted as the northernmost extent of the Precordillera. The Nd systematics of the Río Bonete basement rocks including greenschists and metagabbros (εNd₍₄₇₀₎ = +2.14–−0.19; T_DM = 0.99–1.2 Ga), a garnet-amphibolite (εNd₍₄₇₀₎ = −0.53; T_DM = 1.32 Ga) and a quartz-phyllite (εNd₍₄₇₀₎ = −3.83; T_DM = 1.55 Ga), are similar to other pre-Ordovician meta-igneous rocks from Sierra de Umango, Pie de Palo and the Ullum xenoliths, usually interpreted as the basement of the Precordillera terrane. Nd model ages around 1.2 Ga are also typical from the Mesoproterozoic Grenvillian basement of southern North America, currently exposed in the Llano region. In addition, the greenschists and metagabbros show a robust correlation with the Late Neoproterozoic Catoctin volcanics in the central Appalachians. The Sr isotope data (when not disturbed) also supports this novel interpretation and suggests the presence of the Blue Ridge rifting event in Precordillera. According to our interpretation, some lithotypes included within the basement complex of the Río Bonete area belonged to the basement of the Precordillera terrane supporting previous correlation between both regions.     

Evolution of Jurassic–Cretaceous magmatism in the Khambin volcanotectonic complex (western Transbaikalia)

The Khambin volcanotectonic complex is a horst framing the Late Cretaceous Lake Gusinoe basin in the northwest. This complex is due to the intracontinental rift conditions which existed in western Transbaikalia in the Late Mesozoic. They gave rise to a system of subparallel grabens and horsts in present-day topography. The magmatic evolution of this complex spans from 159 to 117 Ma and is divided into three stages. The first stage (159–156 Ma) witnessed the formation of thick (up to 1500 m) volcanic masses of trachybasalts, basaltic trachyandesites, trachytes, trachydacites, trachyrhyolites, and pantellerites. The next two stages were the formation of isolated ancient volcanoes (127–124 Ma) composed of trachybasalts, basaltic trachyandesites, phonotephrites, tephriphonolites, and alkali trachytes and the formation of the Murtoi (Lake Gusinoe) essexite dike (122–117 Ma). The main trends for igneous associations from early to late stages are reduced magmatism and reduced rock diversity because of the decreasing portion of felsic volcanic rocks. Mafic rocks show an increase in total alkalinity, the content of incompatible elements (Th, U, K, Rb, Pb, Nb, Ta, Zr, Hf), total REE content, and the LREE/HREE ratio. The Sr–Nd isotopic composition of these rocks remained nearly constant and corresponds to that of OIB-EMII mantle sources. Compositional variations are attributed to a time-dependent decrease in the degree of partial melting of a similar magma source.     

Age and geochemistry of the Neoproterozoic granitoids in the the Songnen–Zhangguangcai Range Massif, NE China: Petrogenesis and tectonic implications

<正>1 Introduction The Songnen–Zhangguangcai Range Massif(SZRM)crops out over an extensive part of NE China and was thought to contain Precambrian crystalline basement material,as evidenced by the presence of what appears to bePaleoproterozoicbasementmaterialwithin exploration drillholes(Pei et al.,2007).An alternative view is that the basement within the SZRM is     

Be-daughter minerals in fluid and melt inclusions: implications for the enrichment of Be in granite–pegmatite systems

The study of re-homogenized melt inclusions in the same growth planes of quartz of pegmatites genetically linked to the Variscan granite of the Ehrenfriedersdorf complex, Erzgebirge, Germany, by ion microprobe analyses has determined high concentrations of Be, up to 10,000 ppm, in one type of melt inclusion, as well as moderate concentrations in the 100 ppm range in a second type of melt inclusion. Generally, the high Be concentrations are associated with the H₂O- and other volatile-rich type-B melt inclusions, and the lower Be concentration levels are connected to H₂O-poor type-A melt inclusions. Both inclusion types, representing conjugate melt pairs, are formed by a liquid–liquid immiscibility separation process. This extremely strong and very systematic scattering in Be provides insights into the origin of Be concentration and transport mechanisms in pegmatite-forming melts. In this contribution, we present more than 250 new analytical data and show with ion microprobe and fs-LA-ICPMS studies on quenched glasses, as well as with confocal Raman spectroscopy of daughter minerals in unheated melt inclusions, that the concentrations of Be may achieve such extreme levels during melt–melt immiscibility of H₂O-, B-, F-, P-, ± Li-enriched pegmatite-forming magmas. Starting from host granite with about 10 ppm Be, melt inclusions with 10,000 ppm Be correspond to enrichment by a factor of over 1,000. This strong enrichment of Be is the result of processes of fractional crystallization and further enrichment in melt patches of pegmatite bodies due to melt–melt immiscibility at fluid saturation. We also draw additional conclusions regarding the speciation of Be in pegmatite-forming melt systems from investigation of the Be-bearing daughter mineral phases in the most H₂O-rich melt inclusions. In the case of evolved volatile and H₂O-rich pegmatite systems, B, P, and carbonates are important for the enrichment and formation of stable Be complexes.     

Zircon U–Pb dating and the petrological and geochemical constraints on Lincang granite in Western Yunnan,China: Implications for the closure of the Paleo-Tethys Ocean

Lincang granite is a batholith located in the Sanjiang region and is an important research subject for understanding subduction and collision during the Paleo-Tethyan period. It is widely exposed in the Lincang Terrane and extends south into Burma. Based on various petrological and geochemical investigations performed from south to north across the Lincang granite, a new set of data, which includes zircon chronological and Hf isotopic data, is presented to discuss the origin of the Lincang granite and its tectonic significance. The Lincang granite is a peraluminous, high-K calc-alkaline body with sub-parallel REE patterns and a strong negative Eu anomaly. This anomaly is characteristic of a post-collision peraluminous S-type granitic batholith. The 200–230 Ma formation age of the Lincang granite was determined using LA-ICP-MS zircon U–Pb dating. Thus, it has been confirmed that the granite formed during the late Triassic period, and the formation process lasted for approximately 30 Ma. Geochemical and isotopic compositions indicate that the primary magma of Lincang granite most likely originated from a crustal source, and possibly underwent an assimilation–fractionation crystallization (AFC) process during its emplacement. The Lincang granite formed during the continental collision between the Baoshan–Gengma Terrane and the Lanping–Simao Terrane after the northeast subduction of the Paleo-Tethyan Oceanic Plate. Therefore, the late Triassic Lincang granite is important evidence for the closure of the Paleo-Tethyan Ocean.     

Comparison of petrology and isotope geochemistry of the Bulka peridotite–gabbro massif and granitoids of the Kyzykchadra complex (West Sayan)

The study provides new petrologic and isotope geochemical data for rocks of the 465 ± 5 Ma Bulka massif (Borodina et al., 2011). The primary amphibole from granitoid stocks cutting across the layered series of the massif yielded an Ar–Ar age of 415.9 ± 3.7 Ma. The rocks of the Bulka massif have ¹⁴³Nd/¹⁴⁴Nd ratio of 0.513243 and εNd (Т) values of +12.00. The granitoids have ¹⁴³Nd/¹⁴⁴Nd ratios between 0.512919 and 0.512961 and εNd (Т) values between +8.03 and +9.25. The Nd isotope composition indicates that the rocks of the Bulka massif and granitoids were derived from a depleted mantle source. Depletion of the rocks of the massif in LILE, LREE, and HFSE over LILE is inherited from the mantle source, which has geochemical signatures of N-MORB and subduction-related components. Granitoids are metaluminous I-type granites, which were probably generated either by differentiation of intermediate to mafic mantle-derived magmas or by melting of metabasites. The rocks of the granitoid stocks are characterized by enrichment in LILE and LREE and depletion in HFSE over LILE, which suggests derivation from arc-related parental magmas.     

Petrogenesis of the Xuanwoling mafic–ultramafic intrusion in the northeastern Tarim Block(Northwest China)

Beishan Terrane, located in the northeast of the Tarim Block, in northwest China, has developed a 500-km long and 100-km wide belt of Permian mafic–ultramafic intrusions One of these mafic–ultramafic intrusions, the Xuanwoling Intrusion, is composed of dunite, troctolite, olivine gabbros and gabbros, with cumulate texture and rhythmic layering The crystallization sequence is olivine ? spinel ? plagio clase ? pyroxene, indicating that the crystallization pressure is lower than 0.5–0.8 GPa and that the intrusion has undergone variable degrees of crustal contamination, increasing from dunite to gabbros. The olivines found in the Xuanwoling Intrusion have high Fo values(up to 90), suggesting a primary magma with a high composition of mg. It is likely that this high-mg magma was produced at extremely high temperatures(1,330–1,350 °C), and as a result, Nd–Sr isotopic compositions similar to oceanic island basalts are found in the Xuanwoling Intrusion, which we propose arose from the mantle plume.     

Petrogenesis of the Wudaogou intermediate–mafic complex,NW China: zircon U–Pb dating,whole-rock and isotope geochemistry,and geological implications

The Wudaogou plutonic complex is located in the eastern Yanbian area of Jilin Province and consists of hornblende gabbros, gabbroic diorites, and quartz diorites that contain abundant dioritic microgranular xenoliths. Zircon U–Pb dating of gabbroic and quartz diorites yielded weighted mean ²⁰⁶Pb/²³⁸Pb ages of 263.5 ± 5.1 Ma (N = 12, mean squared weighted deviation (MSWD) = 0.78, probability = 0.66) and 262.0 ± 5.6 Ma (N = 10, MSWD = 0.50, probability = 0.87), respectively. These units are characterized by high Na₂O/K₂O (0.33–0.77) ratios and Al₂O₃ (15.05–18.91 wt%) concentrations and are large ion lithophile element (LILE) (light rare earth element (LREE), Rb, Ba, K, etc.) enriched and high field strength element (HFSE) (Nb, Ta, P, Ti) depleted. They also have initial ⁸⁷Sr/⁸⁶Sr values of 0.70192–0.70420 and ?_Nd(t) values of +1.9 to +4.7 with two-stage model ages (T_DM2) of 653–878 Ma. These characteristics indicate that these rocks formed from calc–alkaline magmas derived from partial melting of a mixture of juvenile crust formed attending the Neoproterozoic subduction of the Palaeo-Asian oceanic crust and lower crustal material. The dioritic xenoliths have whole-rock compositions that are similar to their host rocks, but with negative ?_Nd(t) values (?1.6 to ?4.3) and older T_DM2 ages (1166–1382 Ma), further indicating that this magmatic event involved older crustal material. Combining these data with existing knowledge of the crustal evolution of this area, we conclude that this complex formed in a post-collisional extensional setting during closure of the Palaeo-Asian Ocean.