Geochronological, geochemical and geothermal constraints on petrogenesis of the Indosinian peraluminous granites in the South China Block: A case study in the Hunan Province

The Indosinian granites in the South China Block (SCB) have important tectonic significance for the evolution of East Asia. Samples collected from Hunan Province can be geochemically classified into two groups. Group 1 is strongly peraluminous (A/CNK > 1.1), similar to S-type granites, and Group 2 has A/CNK = 1.0–1.1, with an affinity to I-type granites. Group 1 has lower FeOt, Al₂O₃, MgO, CaO, TiO₂ and ε_Nd(t) values but higher K₂O + Na₂O, Rb/Sr, Rb/Ba and ⁸⁷Sr/⁸⁶Sr(t) than those of Group 2. Samples of both groups have similar LREE enriched pattern, with (Eu/Eu) = 0.19–0.69, and strongly negative Ba, Sr, Nb, P and Ti anomalies. Geothermobarometry study indicates that the precursor magmas were emplaced at high-level depth with relatively low temperature (734–827 °C). Geochemical data suggest that Group 1 was originated from a source dominated by pelitic composition and Group 2 was from a mixing source of pelitic and basaltic rocks with insignificant addition of newly mantle-derived magma. Eight granitic samples in Hunan Province are dated at the cluster of 243–235 and 218–210 Ma by zircon U–Pb geochronology. Together with recent zircon U–Pb ages for other areas in the SCB, two age-clusters, including 243–228 Ma just after peak-metamorphism ( 246–252 Ma) and 220–206 Ma shortly after magma underplating event (224 Ma), are observed. It is proposed that in-situ radiogenic heating from the over-thickened crust induced dehydrated reaction of muscovite and epidote/zoisite to form the early Indosinian granites in response to the isostatic readjustments of tectonically thickened crust. Conductive heating from the underplating magma in the postcollisional setting triggered the formation of late Indosinian granites. Such a consideration is supported by the results from FLAC numerical simulation.     

Zircon U–Pb age and trace element evidence for Paleoproterozoic granulite-facies metamorphism and Archean crustal rocks in the Dabie Orogen

Zircon U–Pb ages and trace elements were determined for granulites and gneiss at Huangtuling, which are hosted by ultrahigh-pressure metamorphic rocks in the Dabie Orogen, east-central China. CL images reveal core–rim structure for most zircons in the granulites. The cores show oscillatory zoning, relatively high Th/U ratios, and HREE enriched patterns, consistent with a magmatic origin. They gave a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2766 ± 9 Ma, interpreted as dating magma emplacement of the protolith. The rims are characterized by sector or planar zoning, low Th/U ratios, negative Eu anomalies and flat HREE patterns, consistent with their formation under granulite-facies metamorphic conditions. Zircon U–Pb dating yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 2029 ± 13 Ma, which is interpreted to record a metamorphic event, possibly during assembly of the supercontinent Columbia. The gneiss has a protolith age of 1982 ± 14 Ma, which is younger than the zircon age of the granulite-facies metamorphism, suggesting a generally delay between HT metamorphism and the intrusion of post-collisional granites. A few inherited cores with igneous characteristics have ²⁰⁷Pb/²⁰⁶Pb ages of 2.90, 3.28 and 3.53 Ga, suggesting the presence of Mesoarchean to Paleoarchean crustal remnants in the Yangtze Craton. A few Cretaceous metamorphic ages were also obtained, suggesting the influence of post-collisional collapse in response to Cretaceous extension of the Dabie Orogen. It is inferred that the recently discovered Archean basement of the Yangtze Craton occurs as far north as the Dabie Orogen.     

High resolution (5 μm) U–Th–Pb isotope dating of monazite with excimer laser ablation (ELA)-ICPMS

Monazite [(LREE)PO₄], a common accessory mineral in magmatic and metamorphic rocks, is complementary to zircon in U–Th–Pb geochronology. Because the mineral can record successive growth phases it is useful for unravelling complex geological histories. A high spatial resolution is required to identify contrasted age domains that may occur at the crystal-scale. Bulk mineral techniques such as ID-TIMS, applied to single monazite grains recording multiple overgrowths or isotope resetting can result in partly scattered discordant analytical points that produce inaccurate intercept ages. Laser ablation (LA)-ICPMS has been demonstrated to be a useful technique for U–Th–Pb dating of zircons, and this study tests its analytical capabilities for dating monazite. A sector field high resolution ICPMS coupled with a 193 nm ArF excimer laser ablation microprobe is capable of achieving a high spatial resolution and producing stable and reliable isotope measurements.

The U–Th–Pb systematic was applied to monazite grains from several samples: a lower Palaeozoic lens from high-grade terrains in Southern Madagascar, Neogene hydrothermal crystals from the Western Alps, a Palaeoproterozoic very high temperature granulite from central Madagascar and a Variscan leucogranite from Spain, directly on a polished thin section. The major aim was to compare and/or reproduce TIMS and EMP ages of monazite from a variety of settings and ages. The three independent ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U and ²⁰⁸Pb/²³²Th ratios and ages were calculated. Isotope fractionation effects (mass bias, laser induced fractionation) were corrected using a chemically homogeneous and U–Pb concordant monazite as external standard.

This study demonstrates that excimer laser ablation (ELA)-ICPMS allows U–Th–Pb dating of monazite with a high level of repeatability, accuracy and precision as well as rapidity of analysis. A spatial resolution almost comparable to that of EMP in terms of crater width (5 μm) produced precise ²⁰⁸Pb/²³²Th, ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ratios for dating Palaeozoic to Precambrian monazites. The advantages of (ELA)-ICPMS isotope dating are precision, accuracy and the ability to detect discordance. In the case of late Miocene hydrothermal monazites from the Alps, a larger spot size of 25 μm diameter is required, and precise and accurate ages were obtained only for ²⁰⁸Pb/²³²Th systematics. Results from the Variscan granite show that in situ U–Th–Pb dating of monazites with (ELA)-ICPMS is possible using a 5 μm spot directly on thin sections, so that age data can be placed in a textural context.     

Isotopic equilibrium/disequilibrium in granites, metasedimentary rocks and migmatites (Damara orogen, Namibia)—a consequence of polymetamorphism and melting

The overwhelming part of the continental crust in the high-grade part of the Damara orogen of Namibia consists of S-type granites, metasedimentary rocks and migmatites. At Oetmoed (central Damara orogen) two different S-type granites occur. Their negative ε_Nd values (− 3.3 to − 5.9), moderately high initial ⁸⁷Sr/⁸⁶Sr ratios (0.714–0.731), moderately high ²⁰⁶Pb/²⁰⁴Pb (18.21–18.70) and ²⁰⁸Pb/²⁰⁴Pb (37.74–37.89) isotope ratios suggest that they originated by melting of mainly mid-Proterozoic metasedimentary material. Metasedimentary country rocks have initial ε_Nd of − 4.2 to − 5.6, initial ⁸⁷Sr/⁸⁶Sr of 0.718–0.725, ²⁰⁶Pb/²⁰⁴Pb ratios of 18.32–18.69 and ²⁰⁸Pb/²⁰⁴Pb ratios of 37.91–38.45 compatible with their variation in Rb/Sr, U/Pb and Th/Pb ratios. Some migmatites and residual metasedimentary xenoliths tend to have more variable ε_Nd values (initial ε_Nd: − 4.2 to − 7.1), initial Sr isotope ratios (⁸⁷Sr/⁸⁶Sr: 0.708–0.735) and less radiogenic ²⁰⁶Pb/²⁰⁴Pb (18.22–18.53) and ²⁰⁸Pb/²⁰⁴Pb (37.78–38.10) isotope compositions than the metasedimentary rocks. On a Rb–Sr isochron plot the metasedimentary rocks and various migmatites plot on a straight line that corresponds to an age of c. 550 Ma which is interpreted to indicate major fractionation of the Rb–Sr system at that time. However, initial ⁸⁷Sr/⁸⁶Sr ratios of the melanosomes of the stromatic migmatites (calculated for their U–Pb monazite and Sm–Nd garnet ages of c. 510 Ma) are more radiogenic (⁸⁷Sr/⁸⁶Sr: 0.725) than those obtained on their corresponding leucosomes (⁸⁷Sr/⁸⁶Sr: 0.718) implying disequilibrium conditions during migmatization that have not lead to complete homogenization of the Rb–Sr system. However, the leucosomes have similar Nd isotope characteristics than the inferred residues (melanosomes) indicating the robustness of the Sm–Nd isotope system during high-grade metamorphism and melting. On a Rb–Sr isochron plot residual metasedimentary xenoliths show residual slopes of c. 66 Ma (calculated for an U–Pb monazite age of 470 Ma) again indicating major fractionation of Rb/Sr at c. 540 Ma. However, at 540 Ma, these xenoliths have unradiogenic Sr isotope compositions of c. 0.7052, indicating depleted metasedimentary sources at depth. Based on the distinct Pb isotope composition of the metasedimentary rocks and S-type granites, metasedimentary rocks similar to the country rocks are unlikely sources for the S-type granites. Moreover, a combination of Sr, Nd, Pb and O isotopes favours a three-component mixing model (metasedimentary rocks, altered volcanogenic material, meta-igneous crust) that may explain the isotopic variabilty of the granites. The mid-crustal origin of the different types of granite emphasises the importance of recycling and reprocessing of pre-existing differentiated material and precludes a direct mantle contribution during the petrogenesis of the orogenic granites in the central Damara orogen of Namibia.     

Geochemical and Sr–Nd–Pb isotopic compositions of the Eocene Dölek and Sariçiçek Plutons, Eastern Turkey: Implications for magma interaction in the genesis of high-K calc-alkaline granitoids in a post-collision extensional setting

The major and trace elements and Sr–Nd–Pb isotopes of the host rocks and the mafic microgranular enclaves (MME) gathered from the Dölek and Sariçiçek plutons, Eastern Turkey, were studied to understand the underlying petrogenesis and geodynamic setting. The plutons were emplaced at  43 Ma at shallow depths ( 5 to 9 km) as estimated from Al-in hornblende geobarometry. The host rocks consist of a variety of rock types ranging from diorite to granite (SiO₂ = 56.98–72.67 wt.%; Mg# = 36.8–50.0) populated by MMEs of gabbroic diorite to monzodiorite in composition (SiO₂ = 53.21–60.94 wt.%; Mg# = 44.4–53.5). All the rocks show a high-K calc-alkaline differentiation trend. Chondrite-normalized REE patterns are moderately fractionated and relatively flat [(La/Yb)_N = 5.11 to 8.51]. They display small negative Eu anomalies (Eu/Eu = 0.62 to 0.88), with enrichment of LILE and depletion of HFSE. Initial Nd–Sr isotopic compositions for the host rocks are ε_Nd(43 Ma) = − 0.6 to 0.8 and mostly I_Sr = 0.70482–0.70548. The Nd model ages (T_DM) vary from 0.84 to 0.99 Ga. The Pb isotopic ratios are (²⁰⁶Pb/²⁰⁴Pb) = 18.60–18.65, (²⁰⁷Pb/²⁰⁴Pb) = 15.61–15.66 and (²⁰⁸Pb/²⁰⁴Pb) = 38.69–38.85. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition, with I_Sr ranging from 0.70485 to 0.70517, ε_Nd(43 Ma) − 0.1 to 0.8 and with Pb isotopic ratios of (²⁰⁶Pb/²⁰⁴Pb) = 18.58–18.64, (²⁰⁷Pb/²⁰⁴Pb) = 15.60–15.66 and (²⁰⁸Pb/²⁰⁴Pb) = 38.64–38.77. The MMEs have T_DM ranging from 0.86 to 1.36 Ga. The geochemical and isotopic similarities between the MMEs and their host rocks indicate that the enclaves are of mixed origin and are most probably formed by the interaction between the lower crust- and mantle-derived magmas. All the geochemical data, in conjunction with the geodynamic evidence, suggest that a basic magma derived from an enriched subcontinental lithospheric mantle, probably triggered by the upwelling of the asthenophere, and interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on the Sr–Nd isotope data indicates that  77–83% of the subcontinental lithospheric mantle involved in the genesis. Consequently, the interaction process played an important role in the genesis of the hybrid granitoid bodies, which subsequently underwent a fractional crystallization process along with minor amounts of crustal assimilation, en route to the upper crustal levels generating a wide variety of rock types ranging from diorite to granite in an extensional regime.     

Sedimentary provenance in the central Ribeira belt based on laser-ablation ICPMSPb/Pb zircon ages

²⁰⁷Pb/²⁰⁶Pb ages from 420 zircons from 9 upper amphibolite and granulite facies quartzites of the post-1.8 Ga metasedimentary cover from the Occidental and Oriental terranes from the Neoproterozoic central Ribeira belt, in eastern Brazil, are discussed. Ages from the Occidental terrane show that Proterozoic ages predominate with a modal class at 2.1–2.2 Ga and that Archean ages are less common and lie within the 2.5–3.6 Ga range. A wider dispersion on Proterozoic ages down to 1.1 Ga and youngest Neoproterozoic ages (550–700 Ma) were also observed and could be related to the Brasiliano/PanAfrican metamorphic events. The data show that detrital sediments of the Occidental terrane were derived predominantly from erosion of a 1.8–2.2 Ga Paleoproterozoic terrane with a small proportion of a 2.6–3.6 Ga Archean crust. Ages from the Oriental terrane are greatly dispersed, but show a significant proportion of ages between 1.7 and 2.1 Ga, although the great majority are between 0.5 and 1.6 Ga. Only 3 out of 144 grains yield Archean ages, between 2.5 and 2.9 Ga. Based on the greatest errors observed on the data from the Oriental Terrane (100–300 Ma), and a continuum of obtained ages between 2072 Ma (Paleoproterozoic) and 517 Ma (Cambrian), the more plausible interpretation is that the dispersion of ages results from partial resetting of the U–Pb system by Brasiliano metamorphism in Proterozoic–Archean zircons, although a Mesoproterozoic source could not be discarded.     

三江盆地绥滨断陷基底花岗岩的锆石U-Pb年代学及其构造意义

根据三江盆地绥滨断陷滨参1井基底花岗岩锆石LA-ICP-MS U-Pb定年结果,探讨了盆地基底的性质。滨参1井基底花岗岩中的锆石呈自形-半自形晶,并具有典型的岩浆生长环带;锆石的Th/U值为0.22~0.82,表明其岩浆成因。花岗岩中锆石22个测点的²⁰⁶Pb/²³⁸U年龄值为427~502 Ma,其加权平均年龄分为2组,分别为（430±8） Ma和（496±6） Ma。（430±8） Ma的年龄代表了三江盆地绥滨断陷基底花岗岩的形成时代为早志留世晚期;而（496±6） Ma可能是花岗岩浆形成或侵位过程中的捕获锆石年龄,并且它与佳木斯地块麻山群和泛非期片麻状花岗岩的形成时间一致。上述结果表明：三江盆地绥滨断陷基底中存在加里东期花岗岩,三江盆地西部基底与佳木斯地块的属性相同。     

Sveconorwegian crustal underplating in southwestern Fennoscandia: LAM-ICPMS U–Pb and Lu–Hf isotope evidence from granites and gneisses in Telemark, southern Norway

Laser ablation ICPMS U–Pb and Lu–Hf isotope data on granitic-granodioritic gneisses of the Precambrian Vråvatn complex in central Telemark, southern Norway, indicate that the magmatic protoliths crystallized at 1201 ± 9 Ma to 1219 ± 8 Ma, from magmas with juvenile or near-juvenile Hf isotopic composition (¹⁷⁶Hf/¹⁷⁷Hf = 0.2823 ± 11, epsilon-Hf > + 6). These data provide supporting evidence for the depleted mantle Hf-isotope evolution curve in a time period where juvenile igneous rocks are scarce on a global scale. They also identify a hitherto unknown event of mafic underplating in the region, and provide new and important limits on the crustal evolution of the SW part of the Fennoscandian Shield. This juvenile geochemical component in the deep crust may have contributed to the 1.0–0.92 Ga anorogenic magmatism in the region, which includes both A-type granite and a large anorthosite–mangerite–charnockite–granite intrusive complex. The gneisses of the Vråvatn complex were intruded by a granitic pluton with mafic enclaves and hybrid facies (the Vrådal granite) in that period. LAM-ICPMS U–Pb data from zircons from granitic and hybrid facies of the pluton indicates an intrusive age of 966 ± 4 Ma, and give a hint of ca. 1.46 Ga inheritance. The initial Hf isotopic composition of this granite (¹⁷⁶Hf/¹⁷⁷Hf = 0.28219 ± 13, epsilon-Hf = − 5 to + 6) overlaps with mixtures of pre-1.7 Ga crustal rocks and juvenile Sveconorwegian crust, lithospheric mantle and/or global depleted mantle. Contributions from ca. 1.2 Ga crustal underplate must be considered when modelling the petrogenesis of late Sveconorwegian anorogenic magmatism in the region.     

湖南白马山龙潭超单元、瓦屋塘花岗岩锆石SHRIMP U-Pb年龄及其地质意义

运用阴极发光技术,对湖南白马山龙潭超单元2个样品和瓦屋塘花岗岩1个样品的锆石进行了内部结构分析,在此基础上利用锆石SHRIMP U-Pb定年方法进行了同位素年代学测定。其中,白马山龙潭超单元2个黑云母二长花岗岩样品分别给出了(215.9±1.9) Ma和(212.2±2.1) Ma的主体谐和年龄。同时还测得了一组较年轻的谐和年龄((201.0±2.8) Ma)和一组较老的锆石核部年龄(230.3~227.0 Ma),表明研究区印支晚期存在多期花岗质岩浆的侵入活动。瓦屋塘岩体黑云母二长花岗岩1个样品给出了(217.7±1.8) Ma的谐和年龄。这2个岩体的形成进一步佐证了华南大陆印支晚期岩浆活动于210~225 Ma,达到岩浆活动的峰期。地球化学测试结果显示,白马山和瓦屋塘岩体均为弱过铝-强过铝质花岗岩,具壳源型花岗岩的特征,形成于后碰撞期或碰撞晚期的构造环境,源于早元古代变质杂砂岩的部分熔融。结合区域大地构造背景认为,这2个岩体形成于秦岭-大别和松马2条印支期缝合带碰撞结束后的印支晚期伸展构造背景下,为热-应力松弛阶段,板内挤压加厚的地壳减压熔融作用的产物。     

云开地块印支期变质-深熔作用:混合岩、片麻岩锆石U-Pb年代学和Hf同位素证据

云开地块中生代构造演化是华南地区的研究热点之一.通过对云开地块变质基底中的混合岩、片麻岩（5个样品）和花岗岩（1个样品）开展锆石LA-ICP-MS U-Pb定年,获得440.3±3.3 Ma、230.2±2.9 Ma、230.7±1.3 Ma、459.5±2.7 Ma、431.5±4.3 Ma、229.2±5.4 Ma、229.7±2.7 Ma 7组变质（深熔）或岩浆年龄和2组（样品1432-1和ID7-3）碎屑锆石年龄,碎屑锆石年龄范围均为太古代-新元古代,且具有~1.0 Ga年龄主峰,与天堂山岩群和云开群碎屑锆石年龄谱相似.区域资料表明云开地块天堂山岩群和云开群具有相似的物质组成,均形成于早古生代-新元古代,存在变质程度和物质面貌的差异;在加里东期构造-热事件的基础上,广泛叠加了印支期区域变质（深熔）-构造-流体作用影响.4个样品中（1431-1、1432-1、D116-3和ID7-3）锆石原位Lu-Hf同位素组成显示,加里东期变质和深熔锆石Lu/Hf同位素组成基本一致,应继承了原岩的同位素组成特征.印支期变质和深熔锆石Lu/Hf同位素组成不同,可能主要由变质作用和深熔作用的差异所致.以古-中元古代为主的地壳物质参与了加里东期和印支期变质-深熔作用,在加里东期和印支期深熔作用过程中,均有少量幔源物质的加入,印支期幔源物质的贡献相对显著.      

Age and emplacement of late-Variscan granites of the western Bohemian Massif with main focus on the Hauzenberg granitoids (European Variscides, Germany)

The Variscan Hauzenberg pluton consists of granite and granodiorite that intruded late- to postkinematically into HT-metamorphic rocks of the Moldanubian unit at the southwestern margin of the Bohemian Massif (Passauer Wald). U–Pb dating of zircon single-grains and monazite fractions, separated from medium- to coarse-grained biotite-muscovite granite (Hauzenberg granite II), yielded concordant ages of 320 ± 3 and 329 ± 7 Ma, interpreted as emplacement age. Zircons extracted from the younger Hauzenberg granodiorite yielded a ²⁰⁷Pb–²⁰⁶Pb mean age of 318.6 ± 4.1 Ma. The Hauzenberg granite I has not been dated. The pressure during solidification of the Hauzenberg granite II was estimated at 4.6 ± 0.6 kbar using phengite barometry on magmatic muscovite, corresponding to an emplacement depth of 16-18 km. The new data are compatible with pre-existing cooling ages of biotite and muscovite which indicate the Hauzenberg pluton to have cooled below T = 250–400 °C in Upper Carboniferous times. A compilation of age data from magmatic and metamorphic rocks of the western margin of the Bohemian Massif suggests a west- to northwestward shift of magmatism and HT/LP metamorphism with time. Both processes started at > 325 Ma within the South Bohemian Pluton and magmatism ceased at ca. 310 Ma in the Bavarian Oberpfalz. The slight different timing of HT metamorphism in northern Austria and the Bavarian Forest is interpreted as being the result of partial delamination of mantle lithosphere or removal of the thermal boundary layer.     

The origin of basic rocks of the Korosten AMCG complex, Ukrainian shield: Implication of Nd and Sr isotope data

The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km² in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A₁ (1800–1780 Ma), main anorthositic A₂ (1760 Ma), early gabbroic G₃ (between 1760 and 1758 Ma), late gabbroic G₄ (1758 Ma), and a suite of dykes D₅ (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A₁ rocks have lower Sr (⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70233–0.70288) and higher Nd (εNd₍₁₇₆₀₎ = 1.6–0.9) isotopic composition. The most widespread A₂ anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ = 0.70362, εNd₍₁₇₆₀₎ varies from 0.2 to − 0.7. The G₃ gabbro–norite has slightly lower εNd₍₁₇₆₀₎ varying from − 0.7 to − 0.9. Finally, G₄ gabbroic rocks show relatively high initial ⁸⁷Sr/⁸⁶Sr (0.70334–0.70336) and the lowest Nd isotopic composition (εNd₍₁₇₆₀₎ varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd₍₁₇₆₀₎ values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. ⁸⁷Sr/⁸⁶Sr₍₁₇₆₀₎ values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia.     

Origin of lamprophyres by the mixing of basic and alkaline melts in magma chamber in Beiya area, western Yunnan, China

Lamprophyres consisting mainly of diopside, phlogopite and K-feldspar formed in the early Tertiary around 60 Ma in the Beiya area and are characterized by low SiO₂ ± 46–50 wt.%), Rb (31–45 ppm) and Sr (225–262 ppm), high Al₂O₃, (11.2–13.1 wt.%), CaO (8.0–8.7 wt.%), MgO (11.5–12.1 wt.%), K₂O(4.9–5.5 wt.%), TiO₂ (2.9–3.3 wt.%) and REE (174–177 ppm), and compatible elements (e.g. Sc, Cr and Ni) and HSF elements (e.g. Th, U, Zr, Nb, Ta, Ti and Y), and low ¹⁴³Nd/¹⁴⁴Nd 0.512372–0.512536, middle ⁸⁷Sr/⁸⁶Sr 0.707322–0.707395, middle ²⁰⁶Pb/²⁰⁴Pb 18.50–18.59, ²⁰⁷Pb/²⁰⁴Pb 15.60–15.65 and ²⁰⁸Pb/²⁰⁴Pb 38.75–38.8. These rocks developed peculiar quartz megacrysts with poly-layer reaction zones, melt inclusions, and partial melted K-feldspar and plagioclase inclusions, and plastic shapes. Important features of these rocks include: (1) hybrid composition of elements, (2) abrupt increase of SiO₂ content of the melt, recorded by zoned diopside, (3) development of sanidine and aegirine-augite reaction zones, (4) alkaline melt and partial melted K-feldspar and plagioclase inclusions, (5) deformed quartz inclusions associated with quartz megacrysts, (6) the presence of quartz megacrysts in plastic shape with their parent melts, (7) the occurrence of olivine, high-MgO ilmenite and spinel inclusions within earlier formed diopside, phlogopite and magnetite. Median ⁸⁷Sr/⁸⁶Sr values between Tertiary alkaline porphyries in the Beiya area and the western Yunnan and Tertiary basalt in the western Yunnan indicate that the Beiya lamprophyre melts were derivative and resulted from the mixing between basic melts that were related to the partial melting of phenocrysts of spinel iherzolite from a mantle source. The alkaline melts originated from partial melting along the Jinshajiang subduction ductile shear zone at the contact between the buried Palaeo-Tethyan oceanic lithosphere and the upper mantle lithosphere. The alkaline melts are composed of 65% sanidine (Or₇₀Ab₂₈An₂) and 35% SiO₂. The melt mixing occurred in magma chambers in the middle-shallow crust at 8–10 km before the derivative lamprophyre melts intruded into the shallow cover in Beiya area. This mixing of basic and alkaline melts might represent a general process for the formation of lamprophyre in the western Yunnan.     

Tonian rift-related, A-type continental plutonism in the Araçuaí Orogen, eastern Brazil: New evidence for the breakup stage of the São Francisco–Congo Paleocontinent

Zircon U–Pb SHRIMP, petrographical and geochemical data lead to the first characterization of the Tonian plutonism (Salto da Divisa Granite Suite), ascribed to the continental rift stage of the precursor basin of the Araçuaí Orogen (Eastern Brazil). The suite includes batholitic plutons and comprises mainly fluorite-bearing, dominantly mesoperthitic hornblende–biotite leucogranites. The presence of mafic (tholeiitic) gabbroic enclaves and syn-plutonic dykes confers to the suite a bimodal character. The plutons were locally deformed and foliated under amphibolite facies conditions, in response to the Neoproterozoic collage of the Araçuaí Orogen against the São Francisco Cratonic margin. However, undeformed magmatic facies are well preserved at inner portions of the plutons. The granitoids are metaluminous, with high SiO₂ and HFSE: Nb, Zr, Y, Ta and REE (except Eu); low CaO, Al₂O₃, Sc, Ba, Sr; high FeOt/MgO ratios, characterizing a chemical signature akin to the subalkaline, A-2 type granites. U–Pb SHRIMP data obtained on zircons from the main pluton yielded a magmatic crystallization age of 875 ± 9 Ma. Some inherited xenocrysts revealed ages of ca. 2080 Ma, corresponding to ages of the host rocks, a Paleoproterozoic basement. Nd isotopic evolution studies confirm the Paleoproterozoic influence on magma genesis with a T_DM model age of ca. 1.6 Ga and ε_Nd of − 5.58 at 880 Ma. The African counterpart, the West Congo Belt, encompasses thick rift-related alkaline volcanic-sedimentary basin (Zadinian and Mayumbian groups, and associated anorogenic granites), dated in the interval of ca. 1000–900 Ma. The age differences between the Salto da Divisa Suite intrusion and the anorogenic magmatic episode at the West Congo Belt suggests a westward migration (i.e. to the Brazilian side) of the thermal axis of the rift, ca. 30 Ma after the ending of the extensional process in Africa.     

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.     

Lu–Hf and U–Pb isotope systematics of zircons from the Storgangen intrusion, Rogaland Intrusive Complex, SW Norway: implications for the composition and evolution of Precambrian lower crust in the Baltic Shield

The Storgangen orebody is a concordantly layered, sill-like body of ilmenite-rich norite, intruding anorthosites of the Rogaland Intrusive Complex (RIC), SW Norway. 17 zircon grains were separated from ca. 5 kg of sand-size flotation waste collected from the on-site repository from ilmenite mining. These zircons were analysed for major and trace elements by electron microprobe, and for U–Pb and Lu–Hf isotopes by laser ablation microprobe plasma source mass spectrometry. Eight of the zircons define a well-constrained (MSWD=0.37) concordant population with an age of 949±7 Ma, which is significantly older than the 920–930 Ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the RIC. The remaining zircons, interpreted as inherited grains, show a range of ²⁰⁷Pb/²⁰⁶Pb ages up to 1407±14 Ma, with an upper intercept age at ca. 1520 Ma. The concordant zircons have similar trace element patterns, and a mean initial Hf isotope composition of ¹⁷⁶Hf/¹⁷⁷Hf_{949 Ma}=0.28223±5 (_Hf=+2±2). This is similar to the Hf-isotope composition of zircons in a range of post-tectonic Sveconorwegian granites from South Norway, and slightly more radiogenic than expected for mid-Proterozoic juvenile crust. The older, inherited zircons show Lu–Hf crustal residence ages in the range 1.85–2.04 Ga. One (undated) zircon plots well within the field of Hf isotope evolution of Paleoproterozoic rocks of the Baltic Shield. These findings indicate the presence of Paleoproterozoic components in the deep crust of the Rogaland area, but do not demonstrate that such rocks, or a Sveconorwegian mantle-derived component, contributed significantly to the petrogenesis of the RIC. If the parent magma was derived from a homogeneous, lower crustal mafic granulite source, the lower crustal protolith must be at least 1.5 Ga old, and it must have an elevated Rb/Sr ratio. This component would be indistinguishable in Sr, Nd and Hf isotopes from some intermediate mixtures between Sveconorwegian mantle and Paleoprotoerzoic felsic crust, but it cannot account for the initial ¹⁴³Nd/¹⁴⁴Nd of the most primitive, late Sveconorwegian granite in the region, without the addition of mantle-derived material.     

Late Triassic granitoids of the eastern margin of the Tibetan Plateau: Geochronology, petrogenesis and implications for tectonic evolution

Late Triassic granitoids in the Songpan-Garzê Fold Belt (SGFB), on the eastern margin of the Tibetan Plateau, formed at 230 to 220 Ma and can be divided into two groups. Group 1 are high-K calc-alkaline rocks with adakitic affinities (K-adakites), with Sr > 400 ppm, Y < 11 ppm, strongly fractionated REE patterns ((La/Yb)_N = 32–105) and high K₂O/Na₂O (≈ 1). Group 2 are ordinary high-K calc-alkaline I-types with lower Sr (< 400 ppm), higher Y (> 18 ppm) and weakly fractionated REE patterns ((La/Yb)_N < 20). Rocks of both groups have similar negative Eu anomalies (Eu/Eu = 0.50 to 0.94) and initial ⁸⁷Sr/⁸⁶Sr (0.70528 to 0.71086), but group 1 rocks have higher ε_Nd(t) (− 1.01 to − 4.84) than group 2 (− 3.11 to − 6.71). Calculated initial Pb isotope ratios for both groups are: ²⁰⁶Pb/²⁰⁴Pb = 18.343 to 18.627, ²⁰⁷Pb/²⁰⁴Pb = 15.610 to 15.705 and ²⁰⁸Pb/²⁰⁴Pb = 38.269 to 3759. Group 1 magmas were derived through partial melting of thickened and then delaminated TTG-type, eclogitic lower crust, with some contribution from juvenile enriched mantle melts. Group 2 magmas were generated by partial melting of shallower lower crustal rocks. The inferred magma sources of both groups suggest that the basement of the SGFB was similar to the exposed Kangding Complex, and that the SGFB was formed in a similar manner to the South China basement. Here, passive margin crust was greatly thickened and then delaminated, all within a very short time interval ( 20 Myr). Such post-collisional crustal thickening could be the tectonic setting for the generation of many adakitic magmas, especially where there is no spatial and temporal association with subduction.     

滇西腾冲-梁河地区花岗岩锆石稀土元素组成和U-Pb同位素年龄

在详细的区域地质调查基础上,对滇西腾冲-梁河地区花岗岩的锆石进行了阴极发光观察,并通过LA-ICP-MS分析了其稀土元素组成和U-Pb同位素年龄。花岗岩中大部分锆石具有振荡环带,锆石稀土元素表现为亏损轻稀土、阶梯式富集重稀土、强烈正Ce异常及强烈负Eu异常,表明锆石为典型的岩浆锆石。花岗岩（D0053）锆石的²⁰⁶Pb/²³⁸U加权平均年龄为（127.9±1.0） Ma (MSWD=2.9,27个点),花岗岩（D4310）锆石的²⁰⁶Pb/²³⁸U加权平均年龄为（58.3±0.5） Ma (MSWD=1.8,21个点),表明腾冲-梁河地区存在大量早白垩世和早第三纪花岗岩浆活动,它们是特提斯演化过程的岩浆响应。     

The timing of sub-solidus hydrothermal alteration in the Central Zone, Limpopo Belt (South Africa): Constraints from titanite U–Pb geochronology and REE partitioning

In the Central Zone of the Limpopo Belt (South Africa), Palaeoproterozoic granulite-facies metamorphism was superimposed on an earlier Archaean orogenic history. Previously determined ages of  2030–2020 Ma obtained from high-temperature chronometers (zircon, garnet, monazite) are generally thought to provide the best estimate of the peak of Palaeoproterozoic granulite-facies metamorphism in the Central Zone, whereas ages as young as  2006 Ma from late melt patches suggest that temperatures remained above the wet solidus for an extended period. We present a new MC-ICP-MS ²⁰⁷Pb–²⁰⁶Pb age of 2030.9 ± 1.5 Ma for titanite found in amphibolite- to greenschist-facies alteration zones developed adjacent to quartz vein systems and related pegmatites that cut a strongly deformed Central Zone metabasite. This age could potentially date cooling of rocks at this locality to temperatures below the wet solidus. Alternatively, the titanite could be inherited from the metabasite host, and the age determined from it date the peak of metamorphism. Integration of the geochronology with LA-ICP-MS trace element data for minerals from the metabasite, the hydrothermal vein systems and comparable rocks elsewhere shows that the titanite formed during the amphibolite-facies hydrothermal alteration, not at the metamorphic peak or during the greenschist-facies phase of veining. This suggests that high-grade rocks in the Central Zone have cooled differentially through the wet solidus, and provides timing constraints on when Palaeoproterozoic reworking in the Central Zone began. This study illustrates the potential of combined geochronological and high-resolution geochemical studies to accurately match mineral ages to distinct crustal processes.     

云开地块印支期构造热事件叠加改造:来自片麻岩中多矿物U-Pb年代学的证据

云开地块显生宙的构造演化一直是华南板块的研究热点之一.通过对云开地块变质基底中片麻岩进行独居石、磷灰石和锆石LA-ICP-MS U-Pb定年,获得了434±6 Ma（MSWD=3.0）和437±12 Ma（MSWD=6.0）的岩浆锆石年龄,结合镜下观察,认为其代表的是片麻岩原岩（花岗质岩石）的形成时代,与区域上加里东期构造-岩浆-变质-深熔事件的时限基本一致,为武夷-云开加里东造山作用的产物;磷灰石经过普通铅校正后分别给出了241±13 Ma（MSWD=2.2）和242±6 Ma（MSWD=1.4）的变质年龄,独居石则分别获得了236.2±0.8 Ma（MSWD=0.66）和229.4±1.2 Ma（MSWD=2.4）的变质年龄,这些年龄结果与云开地块构造岩云母Ar-Ar年龄相一致,暗示云井地块经历了印支期（230~240 Ma）构造热事件的叠加改造.对片麻岩中黑云母电子探针数据分析显示其具有相对高的Al、Ti和Mg含量,X_Mg值为0.37~0.45,属于铁质黑云母,其Ti饱和温度计给出温度范围为650~700℃（±50℃）,与独居石估算温度580~640℃（±25℃）在误差范围内基本一致,暗示印支期构造热事件对云开地块的叠加改造温度达到了700℃.综上所述,认为云开地块现今的构造格局是在加里东期造山作用基础上叠加了印支期构造塑造而成,叠加温度可能达到了700℃;此外,在利用矿物对温压计恢复加里东期变质作用P-T轨迹时应着重考虑印支期构造热事件对矿物成分的再造作用.