Neoproterozoic A-type granites of the Garevka massif, Yenisey Ridge: Age, sources, and geodynamic setting

This paper presents the results of geochemical, isotopic (Sm-Nd), and geochronological (U-Pb and Ar-Ar) investigations of leucogranites from the Garevka massif in the Transangara segment of the Yenisey Ridge. The most distinctive geochemical characteristics of these A-type granitoids are the enrichment in silica, potassium, iron, and fluorine and a considerable depletion in europium. Using U-Pb zircon geochronology, the age of the Garevka leucogranites was estimated as 752 ± 3 Ma, which allowed us to attribute them to a previously established Neoproterozoic tectonic event related to the collision of the Central Angara terrane and the Siberian craton. The parental melts of the granitoids were probably derived by melting of a mixed source composed of continental crustal rocks of Paleoproterozoic and Mesoproterozoic and (or) Neoproterozoic ages. Based on the obtained petrological, geochemical, and geochronological data, the leucogranites of the Garevka massif were assigned to the Neoproterozoic postcollisional Glushikha complex.     

Tectonometamorphic evolution of the Garevka polymetamorphic complex (Yenisei Ridge)

The Garevka metamorphic complex (GMC), located at the junction of the Central Angara and Isakovka terranes (western part of the Transangarian Yenisei Ridge), was studied in terms of its tectonometamorphic evolution and geodynamic processes in the Neoproterozoic history of the region. Geological, structural, geochronological, and petrological data permitted the recognition of two stages in the GMC evolution, which differ in thermodynamic regimes and metamorphic field gradients. These stages were related to crustal contraction and extension within the Yenisei regional shear zone, a large lineament structure in the region. Stage 1 was marked by the formation of metamorphic complexes in the middle to upper amphibolite facies moderate-pressure regional metamorphic settings at ～ 960 Ma, P = 7.7–8.6 kbar, and T = 582–631 °C. This suggests subsidence of the area to the middle continental crust with dT/dH = 20–25 °C/km. During stage 2, the rocks experienced Late Riphean (～ 880 Ma, SHRIMP II U–Pb and ⁴⁰Ar–³⁹Ar dating) dynamic metamorphism under epidote-amphibolite facies conditions (P = 3.9–4.9 kbar; T = 461–547 °C), indicating a metamorphic field gradient of dT/dH no greater than 10 °C/km, with the formation of blastomylonites in narrow zones of ductile and brittle deformations. In these zones, high-grade GMC blocks were exhumed to the upper continental crust and underwent low-temperature metamorphism. Comparison of the structural, geologic, and other evolutionary features (nearly identical age constraints in view of exhumation rate, similar PT-paths, and different types of metamorphism associated with different geodynamic settings, etc.) of the Garevka and Teya complexes suggests that they constitute a single polymetamorphic complex.     

Neoproterozoic collisional metamorphism in overthrust terranes of the Trans-Angarian Yenisey Ridge,Siberia

Four polymetamorphic complexes in the vicinity of regional faults in the Trans-Angarian region of the Yenisey Ridge were studied to determine their metamorphic evolution and to elucidate distinctive features of the regional geodynamic processes. Based on our geological and petrological studies using geothermobarometry and P–T path calculations, we show that a Neoproterozoic medium-pressure metamorphism of the kyanite-sillimanite type at c. 850 Ma overprinted regionally metamorphosed low-pressure andalusite-bearing rocks. A positive correlation between rock ages and P–T estimates for the kyanite-sillimanite metamorphism provides evidence for regional structural and tectonic heterogeneity. The medium-pressure recrystallization was characterized by (1) localized distribution of metamorphic zones in the area directly underlying thrust faults with a measured thickness of 2.5–8 km; (2) syntectonic formation of kyanite-bearing mineral assemblages related to thrusting; (3) gradual increase in metamorphic pressure towards the thrust faults associated with a low metamorphic field gradient (from 1–7 to 12°C/km); and (4) equally steep burial P–T paths recorded for the highest grade rocks. These specific features are typical of collisional metamorphism during overthrusting of continental blocks and are evidence of near-isothermal loading in accordance with the transient emplacement of thrust sheets. The proposed model for tectono-metamorphic evolution of the study areas due to crustal thickening at high thrusting rates and subsequent rapid exhumation explains these tectonic features. Data analysis allowed us to consider the medium-pressure kyanite-bearing metapelites as a product of collisional metamorphism, reflecting unidirectional thrusting of Siberian cratonal blocks onto Yenisey Ridge along regional deep faults (Angara, Mayakon, and Chapa areas) and by opposite movements in the zone of secondary splay faults (Garevka area).     

P-T-t constraints on the metamorphic evolution of the Transangarian Yenisei Ridge: Geodynamic and petrological implications

Two metamorphic complexes of the Yenisei Ridge with contrasting composition are analyzed to unravel their tectonothermal evolution and geodynamic processes during the Riphean geologic history of the area. The structural, mineralogical, petrological, geochemical and geochronological data are used to distinguish two stages of the evolution with different ages, thermodynamic regimes, and metamorphic field gradients. Reaction textures, chemical zoning in minerals, shapes of the P-T paths, and isotope dates provide convincing evidence for a poly metamorphic history of the region. The first stage is marked by the formation of the ~ 970 Ma low-pressure zoned And-Sil rocks (P = 3.9-5.1 kbar, T = 510–640 °C) of the Teya aureole and a high metamorphic field gradient with dT/dH = 25–35 °C/km typical of many orogenic belts. At the second stage, these rocks experienced Late Riphean (853–849 Ma) collisional medium-pressure metamorphism of the kyanite-sillimanite type (P = 5.7-7.2 kbar, T = 660–700 °C) and a low metamorphic field gradient with dT/dH < 12 °C/km. This metamorphic event was almost coeval with the Late Riphean (862 Ma) contact metamorphism in the vicinity of the granitic plutons, which was accompanied by a high metamorphic field gradient with dT/dH > 100 °C/km. At the first stage, the deepest blocks of the Garevka complex in the vicinity of the Yenisei regional shear zone underwent high-pressure amphibolite-facies metamorphism within a narrow range of P = 7.1-8.7 kbar and T = 580–630 °C, suggesting the burial of rocks to mid-crustal depths at a metamorphic field gradient with dT/dH ~ 20–25 °C/km. At the second stage, these rocks experienced the Late Riphean (900–850 Ma) syn-exhumation dynamometamorphism under epidote-amphibolte facies conditions (P = 3.9-4.9 kbar, T = 460–550 °C) and a low gradient with dT/dH < 10 °C/km accompanied by the formation of blastomylonitic complexes in shear zones. All these deformation and metamorphic events identified on the western margin of the Siberian craton are correlated with the final episodes of the Late Grenville orogeny and provide supporting evidence for a close spatial connection between Siberia and Laurentia during early Neoproterozoic time, which is in good agreement with recent paleomagnetic reconstuctions.     

北秦岭造山带的早古生代多期变质作用

北秦岭造山带的秦岭岩群以高级变质岩石为特征,主要包括少量榴辉岩、高压麻粒岩和区域上广泛分布的麻粒岩-角闪岩相变质岩石。年代学研究显示秦岭岩群中不同岩石记录了多期变质作用。已有的定年资料给出北秦岭官坡地区的榴辉岩的年龄为500Ma左右,代表榴辉岩相的变质时代。结合岩相学资料,对两个高压麻粒岩样品的SHRIMP和LA-ICPMS U-Pb测定分别获得504±7Ma 和506±3Ma的年龄,应代表高压麻粒岩相变质时代。这表明高压麻粒岩和相邻的榴辉岩有相近的变质时代,但形成在造山带中不同的构热造环境中。西峡地区的角闪二辉麻粒岩的U-Pb定年给出两组早古生代年龄,一组为440±2Ma,可能代表了中低压麻粒岩相的变质时代,另一组为426±1Ma,应代表区域角闪岩相的变质时代。桐柏山北部的石榴二辉麻粒岩的U-Pb定年数据给出436±1Ma的年龄,为中压麻粒岩相的变质时代。这些资料表明北秦岭造山带经历了早奥陶世的俯冲和地壳增厚作用,并在晚志留世遭受了广泛的巴罗式区域变质作用。     

Neoproterozoic accretionary and collisional events on the western margin of the Siberian craton: new geological and geochronological evidence from the Yenisey Ridge

The geological, structural and tectonic evolutions of the Yenisey Ridge fold-and-thrust belt are discussed in the context of the western margin of the Siberian craton during the Neoproterozoic. Previous work in the Yenisey Ridge had led to the interpretation that the fold belt is composed of high-grade metamorphic and igneous rocks comprising an Archean and Paleoproterozoic basement with an unconformably overlying Mesoproterozoic–Neoproterozoic cover, which was mainly metamorphosed under greenschist-facies conditions. Based on the existing data and new geological and zircon U–Pb data, we recognize several terranes of different age and composition that were assembled during Neoproterozoic collisional–accretional processes on the western margin of the Siberian craton. We suggest that there were three main Neoproterozoic tectonic events involved in the formation of the Yenisey Ridge fold-and-thrust belt at 880–860 Ma, 760–720 Ma and 700–630 Ma. On the basis of new geochronological and petrological data, we propose that the Yeruda and Teya granites (880–860 Ma) were formed as a result of the first event, which could have occurred in the Central Angara terrane before it collided with Siberia. We also propose that the Cherimba, Ayakhta, Garevka and Glushikha granites (760–720 Ma) were formed as a result of this collision. The third event (700–630 Ma) is fixed by the age of island-arc and ophiolite complexes and their obduction onto the Siberian craton margin. We conclude by discussing correlation of these complexes with those in other belts on the margin of the Siberian craton.     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

Three metamorphic events in the precambrian P-T-t history of the Transangarian Yenisey ridge recorded in garnet grains in metapelites

A study of gneisses and schists from the Yenisey regional shear zone (Garevka complex) at the western margin of the Siberian Craton has provided important constraints on the tectonothermal events and geodynamic processes in the Yenisey Ridge during the Riphean. In situ U-Th-Pb geochronology of monazite and xenotime from different garnet growth zones and the calculation of P-T path derived from chemical zoning pattern in garnet were used to distinguish three metamorphic events with different ages, thermodynamic regimes and metamorphic field gradients. The first stage occurred as a result of the Grenville orogeny during late Meso-early Neoproterozoic (1050–850 Ma) and was marked by low-pressure zoned metamorphism at ～4.8–5.0 kbar and 565–580°C and a metamorphic field gradient with dT/dH = 20–30°C/km typical of orogenic belts. At the second stage, the rocks experienced Late Riphean (801–793 Ma) collision-related medium-pressure metamorphism at ～7.7–7.9 kbar and 630°C with dT/dH ≤ 10°C/km. The final stage evolved as a syn-exhumation retrograde metamorphism (785–776 Ma) at ～4.8–5.4 kbar and 500°C with dT/dH ≤ 12°C/km and recorded a relatively fast uplift of the rocks to upper crustal levels in shear zones. The range of exhumation rates at the post-collisional stage (500–700 m/Ma) correlates with the duration of exhumation and the results of thermophysical numerical modeling of metamorphic rocks within orogenic belts. The final stages of collisional orogeny are marked by the development of rift-related bimodal dyke swarms associated with Neoproterozoic extension (797 ± 11 and 7.91 ± 6 Ma; U-Pb SHRIMP II zircon data) along the western margin of the Siberian craton and the beginning of the breakup of Rodinia. Post-Grenville metamorphic episodes of regional evolution are correlated with the synchronous succession and similar style of the later tectono-metamorphic events within the Valhalla orogen along the Arctic margin of Rodinia and support the spatial proximity of Siberia and North Atlantic cratons at about 800 Ma, as indicated by the latest paleomagnetic reconstructions.     

Two stages of granulite facies metamorphism in the eastern Himalayan syntaxis,south Tibet: petrology,zircon geochronology and implications for the subduction of Neo‐Tethys and the Indian continent beneath Asia

The eastern Himalayan syntaxis in southeastern Tibet consists of the Lhasa terrane, High Himalayan rocks and Indus‐Tsangpo suture zone. The Lhasa terrane constitutes the hangingwall of a subduction zone, whereas the High Himalayan rocks represent the subducted Indian continent. Our petrological and geochronological data reveal that the Lhasa terrane has undergone two stages of medium‐P metamorphism: an early granulite facies event at c. 90 Ma and a late amphibolite facies event at 36–33 Ma. However, the High Himalayan rocks experienced only a single high‐P granulite facies metamorphic event at 37–32 Ma. It is inferred that the Late Cretaceous (c. 90 Ma) medium‐P metamorphism of the southern Lhasa terrane resulted from a northward subduction of the Neo‐Tethyan ocean, and that the Oligocene (37–32 Ma) high‐P (1.8–1.4 GPa) rocks of the High Himalayan and coeval medium‐P (0.8–1.1 GPa) rocks of the Lhasa terrane represent paired metamorphic belts that resulted from the northward subduction of the Indian continent beneath Asia. Our results provide robust constraints on the Mesozoic and Cenozoic tectonic evolution of south Tibet.     

SHRIMP II dating of zircons and monazites: reassessing the timing of high-grade metamorphism and fluid flow in the Reynolds Range, northern Arunta Block, Australia

ABSTRACT Key insights into the timing of tectonometamorphic events in a complex high-grade metamorphic terrane can be obtained by combining results from SHRIMP II ion microprobe studies of individual monazite grains with SHRIMP II studies and scanning electron microscope (SEM)-based cathodoluminescence (CL) imaging of zircons. Results from the Reynolds Range region, Arunta Block, Northern Territory, Australia, show that the final episode of regional metamorphism to high-T and low-P granulite facies conditions is most likely to have occurred at c. 1580 Ma, not at 1785–1775 Ma, as previously accepted. The previous interpretation was based on zircon studies of structurally controlled granitoids, without SEM-based CL imaging. Monazites in a 1806± 6 Ma megacrystic granitoid preserve rare cores that are interpreted to be inherited magmatic monazite, but record no evidence of another high-T event prior to 1580 Ma. Most monazites from the region record only a single high-T metamorphic event at c. 1580 Ma. Zircon inheritance is very common. Zircons or narrow overgrowths of zircon dated at c. 1580 Ma have only been found in two types of rocks: rocks produced by metasomatic fluid flow at high temperatures (≤750°C), and rocks that have undergone local partial melting. Previous explanations that attributed these 1580 Ma zircon ages to widespread hydrothermal fluid fluxing associated with post-tectonic pegmatite emplacement at amphibolite facies conditions are not supported by the available evidence including oxygen isotope data. The observed high regional metamorphic temperatures require the involvement of advective heating. However, contrary to a previous tectonic model for the formation of this and other low-P, high-T metamorphic belts, the granites that are exposed at the present structural level do not appear to be the source of that heat, unless some of the granites were emplaced at c. 1580 Ma.     

Late Mesozoic Huangbeiling S-type granite in the East Qinling Orogen,China: Geochronology,petrogenesis and implications for tectonic evolution

S-type granites are typical features of collisional orogenic belts and could provide insights into the tectonic process associated with the final phase of orogeny. The East Qinling Orogen, one of significant segments in the Central China Orogen, witnessed complex tectonic evolution during the Late Mesozoic. The rare S-type granites in this orogen can be used as important proxies to understand the Late Mesozoic tectonic processes. Although a few previous studies suggested that the Huangbeiling pluton in the East Qinling Orogen might be S-type granite, detailed studies are lacking. Thus, we report the results from a systematic petrological, whole-rock geochemical and zircon U-Pb-Lu-Hf isotopic studies on the Huangbeiling pluton, with a view to constrain the timing of magmatism, petrogenetic evolution and genetic type, and to evaluate the implications for Late Mesozoic tectonic evolution of the East Qinling Orogen. Zircon U-Pb analysis yield ²⁰⁶Pb/²³⁸U spot ages in the range of 156.7–132.2 Ma, with weighted ²⁰⁶Pb/²³⁸U mean ages varying from 146.8 to 141.9 Ma, suggesting the Huangbeiling pluton formed during the Late Jurassic to Early Cretaceous. Zircon Lu-Hf isotopic data show negative εHf(t) values of ?21.5 to ?14.9 and two-stage Hf model ages of 2546–2131 Ma, which are correlated with the Neoarchean to Paleoproterozoic (3.0–2.1 Ga) meta-sedimentary rocks from nearby Taihua Group, indicating that the magma was sourced from reworked ancient crustal components involving meta-sedimentary rocks. Whole-rock geochemical data display enrichment of LREEs, Pb, Hf and Y as well as depletion in HREEs, Ba and HFSEs (e.g., Ta, P, Ti), with weakly negative Eu anomalies. The Huangbeiling granitoids are identified as S-type granites, which generated through partial melting of lower-middle crust and upper crustal fractional crystallization in syn-collisional settings. In conjunction with published information related to the tectonic evolution of the East Qinling Orogen, we propose that the Late Mesozoic Huangbeiling S-type granites might response to the complex tectonic evolution related to extensional tectonics induced by multi-directional (intra-continental) subductions from the Yangtze and North China Cratons as well as the Paleo-Pacific Plate during the Late Mesozoic.     

华北东部早白垩世A型花岗岩与克拉通破坏

A型花岗岩是非造山的、无水的碱性花岗岩, 具有独特的矿物学、岩石学和地球化学特征, 形成于造山后、板内非造山或者地幔热柱等伸展动力学背景, 蕴涵着大陆地壳生长、岩石圈演化及区域构造发展等大陆动力学的重要信息.在华北东部广泛分布着一期中生代A型花岗岩及伴生的碱性岩, 结合前人的研究成果以及笔者近年来的研究发现: 它们主要侵位于早白垩世(130~110Ma), 来源于上地壳、下地壳和地幔等多元源区, 是不同成分的岩浆经过分离结晶、岩浆混合作用形成; 综合区域地球动力学研究和A型花岗岩本身的地球化学特征, 认为华北东部早白垩世A型花岗岩形成于地壳伸展背景之下, 是岩石圈减薄和克拉通破坏的浅部地质响应, 标志着华北东部岩石圈减薄和克拉通破坏作用的峰期.      

Neoproterozoic metamorphic evolution in the Transangarian Yenisei Ridge: Evidence from monazite and xenotime geochronology

Chemical mapping and in situ dating of U-Th-rich minerals in zoned garnets from gneisses of the Garevka metamorphic complex were used to constrain multiple metamorphic events in the Transangarian Yenisei Ridge. The data provide supporting evidence for three distinct metamorphic stages. The first episode occurred as a result of the Grenville orogeny during the Late Mesozoic and Early Neoproterozoic (1050–850 Ma) and was marked by low-pressure zoned metamorphism and a metamorphic field gradient with dT/dH = 20?30°C/km typical of orogenic belts. At the second stage, the rocks experienced Late Riphean (801–793 Ma) syn-collisional medium-pressure metamorphism with a low metamorphic field gradient (dT/dH ≤ 10°C/km). The final stage evolved as a synexhumation dynamic metamorphism (785–776 Ma) with dT/dH ≤ 12°C/km and reflected rapid exhumation of rocks in shear zones. The sequence of collisional events within the western margin of the Siberian craton affected by the Valhalla orogen suggests that Siberia and cratons of the North Atlantic region were in close proximity to one another at about 800 Ma, which is supported by recent paleomagnetic reconstructions.     

Co-development of Jurassic I-type and A-type granites in southern Hunan,South China: Dual control by plate subduction and intraplate mantle upwelling

Two types of spatially and temporally associated Jurassic granitic rocks, I-type and A-type, occur as pluton pairs in several locations in southern Hunan Province, South China. This paper aims to investigate the genetic relationships and tectonic mechanisms of the co-development of distinct granitic rocks through petrological, geochemical and geochronological studies. Zircon LA-ICPMS dating results yielded concordant U–Pb ages ranging from 180 to 148 Ma for the Baoshan and Tongshanling I-type granodiorites, and from 180 to 158 Ma for the counterpart Huangshaping and Tuling A-type granites. Petrologically, the I-type granodiorites consist of mafic minerals such as hornblende whereas the A-type granites are dominated by felsic minerals (e.g., quartz, K-feldspar and plagioclase). Major and trace element analyses indicate that the I-type granodiorites have relatively low SiO₂ (64.5–71.0%) and relatively high TiO₂ (0.28–0.51%), Al₂O₃ (13.8–15.5%), total FeO (2.3–4.7%), MgO (1.3–2.6%) and P₂O₅ (0.10–0.23%) contents, and the A-type granites are characterized by high concentrations of Rb (212–1499?ppm), Th (18.3–52.6?ppm), U (11.8–33.6?ppm), Ga (20.0–36.6?ppm), Y (27.1–134.0?ppm) and HREE (20.3–70.0?ppm), with pronounced negative Eu anomalies (Eu/Eu*?=?0.01–0.15). Moreover, the I-type granodiorites are classified as collision-related granites emplaced under a compressional environment, whereas the A-type granites are within-plate granites generated in an extensional setting. Zircon Hf isotopic compositions vary substantially for these granitic rocks. The I-type granodiorites are characterized by relatively young Hf model ages (T_DM1?=?1065–1302 Ma, T_DM^C =1589–2061 Ma) and moderately negative εHf(t) values (–5.9 to –11.5), whereas the A-type granites have very old model ages (T_DM1?=?1454–2215 Ma, T_DM^C?=?2211–2974 Ma) and pronounced negative εHf(t) values (–15.8 to –28.3). These petrochemical and isotopic characteristics indicate that the I-type granodiorites may have been derived from a deep source involving mantle-derived juvenile (basaltic) and crustal (pelitic) components, whereas the A-type granites may have been sourced from melting of meta-greywacke in the crust. This study proposes that the pressure and temperature differences in the source regions caused by combined effects of intra-plate mantle upwelling and plate subduction are the major controlling factors of the co-development of the two different types of magmas. Crustal anatexis related to lithospheric delamination and upwelling of hot asthenosphere under a high pressure and temperature environment led to the formation of the I-type magmas. On the other hand, the A-type magmas were formed from melting of the shallower part of the crust, where extensional stress was dominant and mantle-crust interaction was relatively weak. Rifts and faults caused by mantle upwelling developed from surface to depth and successively became channels for the ascending I- and A-type magmas, resulting in the emplacement of magmas in adjacent areas from sources at different depths.     

大别造山带在大陆裂解、地壳的俯冲-折返及山根垮塌期间的多期部分熔融作用

大别造山带发育了与大陆俯冲-折返和碰撞造山等相关的不同构造岩石单位.针对存在的问题,本项研究开展了宿松变质带、中大别超高压带和北大别杂岩带等不同俯冲岩片花岗质岩石的野外地质调查以及岩石学、元素-同位素地球化学和锆石年代学等方面系统研究.研究结果表明:（1）宿松变质带花岗片麻岩的原岩时代包括晚太古代（2.5~2.7 Ga）和新元古代（770~830 Ma）两大类,其中新元古代花岗片麻岩的原岩是由经历了~2.0 Ga变质作用的晚太古代岩石在新元古代大陆裂解过程中发生重熔作用形成的;（2）首次揭示中大别花岗片麻岩至少包含两种不同的原岩时代（~750 Ma和780~800 Ma）与岩石成因,并在三叠纪俯冲-折返期间经历了~230 Ma和~220 Ma两期部分熔融作用;（3）北大别混合岩中发育折返早期（209±2 Ma）因高温减压而引起的黑云母脱水熔融以及山根垮塌期间（110~145 Ma）有水加入的加热熔融（水致熔融）形成的多种浅色体;（4）发现并限定了北大别变质闪长岩是在燕山期山根垮塌期间,由三叠纪深俯冲的新元古代镁铁质下地壳岩石发生部分熔融作用而形成的.因此,这为大别造山带在新元古代大陆裂解、印支期地壳的俯冲-折返及燕山期山根垮塌期间发生的多种部分熔融作用提供了新的制约.      

黑龙江东南部穆棱地区“麻山群”的特征及花岗岩锆石SHRIMP U-Pb定年——对佳木斯地块最南缘地壳演化的制约

通过对佳木斯地块南缘穆棱地区常兴村-新兴村剖面的研究,指出这里是“麻山群”和“黑龙江群”的结合部位,具有古大陆边缘性质。穆棱地区的“麻山群”为佳木斯地块南缘的陆壳基底,其南侧的“黑龙江群”为包括洋壳残片在内的增生-碰撞杂岩。对“麻山群”混合岩的SHRIMP锆石U-Pb定年结果表明：佳木斯地块存在中-新元古代的结晶基底,并遭受到~500Ma变质作用的影响。侵入“麻山群”杂岩的花岗岩的岩石学、地球化学研究表明,这些花岗岩具有S型花岗岩的特征;其SHRIMP锆石U-Pb分析表明其形成年龄为486Ma±3Ma,略晚于前人确定的“麻山群”杂岩约500Ma的麻粒岩相变质作用,为同碰撞或碰撞后花岗岩。这些资料进一步证明,该地区可能经历了晚泛非-早加里东期的碰撞造山作用。     

Magmatism evolution and carbonatite-granite association in the neoproterozoic active continental margin of the Siberian craton: Thermochronological reconstructions

Neoproterozoic carbonatites and related igneous rocks, including A-type granites in the Tatarka-Ishimba suture zone of the Yenisey Ridge are confined to a horst-anticlinal structure that was formed in a transpression setting during the oblique collision between the Central Angara terrane and the Siberian craton. The carbonatites, associating mafic (including alkaline) dikes as well as the Srednetatarka nepheline syenites are the oldest igneous formations of the Tatarka active continental margin complex. Geochronological data indicate that magmatic evolution continued in the studied anticline for nearly 100 m.y. On the earliest stage carbonatites were formed and on the last stage — the emplacement of mantle-crustal A-type Tatarka granites took place. According to new U/Pb zircon studies, the earliest rocks in the Tatarka pluton are A-type leucogranites aged 646 ± 8 Ma. The younger ⁴⁰Ar/³⁹Ar ages of carbonatites obtained for phlogopites (647 ± 7 and 629 ± 6 Ma) are related to the last tectonic events in the studied region of the Tatarka-Ishimba suture zone, which are coeval with the formation of the A-type granitoids (646–629 Ma).     

Palinspastic reconstruction of the Grenville terrane in the Blue Ridge Geologic Province,southern and central Appalachians,U.S.A

Examinations of Grenville massifs in the Blue Ridge Geologic Province of Virginia and North Carolina indicate that the country rocks (∼ 1100–1450 Ma) are layered gneisses that were metamorphosed during Grenville orogenesis (∼ 1000–1100 Ma) to amphibolite to granulite facies and intruded by plutonic suites. Subsequently, the Grenville terrane was intruded by a suite of peralkaline granitic plutons (∼ 700 Ma) and progressively overlapped westward by Upper Precambrian to Cambrian sedimentary and volcanic rocks. Following deposition of Upper Precambrian and Palaeozoic rocks, the Blue Ridge Geologic Province was subjected to Taconic metamorphism (∼ 450–480 Ma) which generally increased in intensity southeastward from greenschist (chlorite grade) to upper amphibolite (sillimanite grade) facies. Large-scale late Devonian thrusting (∼ 350 Ma) along the Fries fault system and the Brevard zone-Yadkin fault system produced the present day distribution of juxtaposed Grenville massifs and Palaeozoic metamorphic zones in the Blue Ridge Geologic Province. Palinspastic restoration of the Taconic metamorphic zones to their pre-late Devonian relative positions yields an ∼ 50 km displacement on the Fries fault system near the Grandfather Mountain window and and an ∼ 80 km displacement on the Smith River allochthon farther east. Restoration of the Grenville massifs to this same palinspastic base shows that Grenville metamorphic grade decreased southeastward from the deeper granulite facies (opx + gar) to the shallower granulite facies (opx ± amp) to amphibolite facies.     

江苏省东海县片麻状碱性花岗岩的成因及年代学研究

江苏省东海地区位于苏鲁造山带的西南缘,区内广泛出露片麻状碱性花岗岩.本文对东海地区磨山、虎山和房山片麻状碱性花岗岩的产状、岩石结构构造、碱性铁镁矿物、元素地球化学和锆石成因及SHRIMP定年等综合研究结果表明：该类岩石具有高硅、富碱、富铁贫镁和含碱性铁镁矿物等特征,说明其为碱性花岗岩;岩体还保留有沉积岩特有的层理（走向北东、倾向南东等一致的产状）,长石等造岩矿物显示在固相条件下结晶的特征,以及继承锫石大部分是不同时代的岩浆或变质锆石碎屑等,均证实其原岩是沉积岩.SHRIMP定年对变质锆石的边部获得两组₂₀₆Pb-₂₃₈U加权平均年龄,分别为224.7±9.4 Ma和209.4±2.5～212±3.5Ma,反映了东海片麻状碱性花岗岩成岩时间属于印支期,相当于晚三叠世,变质锆石的核部₂₀₆Pb-₂₃₈U年龄变化于866～248 Ma,反映原岩时代不会早于晚古生代.     

北祁连榴辉岩相变沉积岩的特征及其构造意义

在北祁连造山带中,出露典型的高压/低温变质岩石,前人对其中的低温榴辉岩已做过较多的研究,但对其中的变沉积岩研究涉及很少.本文展示了榴辉岩相变质沉积岩的岩石学、地球化学、锆石U-Pb年代学和Hf同位素方面的一些新的研究结果.变沉积岩含有榴辉岩相的矿物组合,峰期温压条件为t= 450～520℃,p=1.9～2.3 GPa,与相邻榴辉岩的温压条件一致.地球化学显示这些岩石的原岩为不成熟的沉积岩,可能形成于大陆边缘或大陆岛弧环境.变沉积岩中的碎屑锆石U-Pb年龄主要集中在1800 Ma左右和540～600 Ma之间,结合锆石Hf同位素特征,表明其原岩的碎屑来源既有周缘陆块的前寒武纪变质基底物质,又有新元古代-早古生代新生洋壳或增生物质.同时,这些数据也表明北祁连早古生代洋壳俯冲过程中发生了活动大陆边缘的构造剥蚀作用,即形成于上盘的沉积物(弧前盆地或增生楔)被构造作用运移到俯冲带中,并俯冲到60～70km深处,遭受榴辉岩相变质作用,然后折返到地表.