Magnesium isotopic behaviors between metamorphic rocks and their associated leucogranites,and implications for Himalayan orogenesis

Magnesium isotopic compositions, along with new Sr–Nd–Pb isotopic data and elemental analyses, are reported for 12 Miocene tourmaline-bearing leucogranites, 15 Eocene two-mica granites and 40 metamorphic rocks to investigate magnesium isotopic behaviors during metamorphic processes and associated magmatism and constrain the tectonic-magmatic-metamorphic evolution of the Himalayan orogeny. The gneisses, granulites and amphibolites represent samples of the Indian lower crust and display large range in δ²⁶Mg from −0.44‰ to −0.09‰ in mafic granulites, −0.44‰ to −0.10‰ in amphibolites, and −0.70‰ to −0.03‰ in granitic gneisses. The average Mg isotopic compositions of the granitic gneisses (−0.19 ± 0.34‰), mafic granulites (−0.22 ± 0.17‰) and amphibolites (−0.25 ± 0.24‰) are similar, indicating the limited Mg isotope fractionation during prograde metamorphism from granitic gneisses to mafic granulites and retrograde metamorphism from mafic granulites to amphibolites. The Eocene two-mica granites and Miocene leucogranites are characterized by large variations in elemental and Sr–Nd–Pb isotopic compositions. The leucogranites and two-mica granites have their corresponding (⁸⁷Sr/⁸⁶Sr)_i varying from 0.7282 to 0.7860 and 0.7163 to 0.7191, (¹⁴³Nd/¹⁴⁴Nd)_i from 0.511888 to 0.512040 and 0.511953 to 0.512076, ²⁰⁷Pb/²⁰⁴Pb from 15.7215 to 15.7891 and 15.7031 to 15.7317, ²⁰⁸Pb/²⁰⁴Pb from 38.8521 to 39.5286 and 39.2710 to 39.4035, and ²⁰⁶Pb/²⁰⁴Pb from 18.4748 to 19.0139 and 18.7834 to 18.9339. However, they have similar Mg isotopic compositions (−0.21‰ to +0.06‰ versus −0.24‰ to +0.09‰), which did not originate from fractional crystallization nor source heterogeneity. Based on hornblende/biotite/muscovite dehydration melting reaction and Mg isotopic variations in two-mica granites and leucogranites with the proceeding metamorphism, along with elemental discrimination diagrams, Eocene two-mica granites and Miocene leucogranites could be related to hornblende dehydration melting and muscovite dehydration melting, respectively. Mg isotopic compositions of Eocene two-mica granites become heavier compared to the source because of residues of isotopically light garnet in the source; while those of Miocene leucogranites become lighter because of entrainment of isotopically light garnet from the source region. Thus, a new model for crustal anatexis and Himalayan orogenesis was proposed based on the Mg isotope fractionation in the leucogranites and metamorphic rocks. This model emphasizes a successive process from Indian continental subduction to rapid exhumation of the Higher Himalayan Crystalline Series (HHCS). The former underwent high-temperature (HT) and high-pressure (HP) granulite-facies prograde metamorphism, which resulted in the hornblende dehydration melting and the formation of Eocene two-mica granites; while the latter experienced amphibolite-facies retrogression and decompression, which resulted in the muscovite dehydration melting and the formation of Miocene leucogranites.     

Geochemical and isotopic constraints on the age and origin of mafic dikes from eastern Shandong Province,eastern North China Craton

Post-orogenic mafic dikes are widespread across eastern Shandong Province, North China Craton, eastern China. We here report new U–Pb zircon ages and bulk-rock geochemical and Sr–Nd–Pb isotopic data for representative samples of these rocks. LA-ICP-MS U–Pb zircon analysis of two mafic dike samples yields consistent ages of 118.7 ± 0.25 million years and 122.4 ± 0.21 million years. These Mesozoic mafic dikes are characterized by high (⁸⁷Sr/⁸⁶Sr) _i ranging from 0.7082 to 0.7087, low ?_Nd(t) values from??17.0 to??17.5, ²⁰⁶Pb/²⁰⁴Pb from 17.14 to 17.18, ²⁰⁷Pb/²⁰⁴Pb from 15.44 to 15.55, and ²⁰⁸Pb/²⁰⁴Pb from 37.47 to 38.20. Our results suggest that the parental magmas of these dikes were derived from an ancient, enriched lithospheric mantle source that was metasomatized by foundered lower crustal eclogitic materials prior to magma generation. The mafic dikes underwent minor fractionation during ascent and negligible crustal contamination. Combined with previous studies, these findings provide additional evidence that intense lithospheric thinning beneath eastern Shandong occurred at ～120 Ma, and that this condition was caused by the removal/foundering of the lithospheric mantle and lower crust.     

Zircon LA-ICP MS U-Pb Age, Sr-Nd-Pb Isotopic Compositions and Geochemistry of the Triassic Post-collisional Wulong Adakitic Granodiorite in the South Qinling, Central China, and Its Petrogenesis

The Indosinian post-collisional Wulong pluton intruded into the Mesoproterozoic Fuping Group, South Qinling, central China. In the southern part of the pluton, some mafic enclaves have sharp or gradational contact relationships with the host biotite granodiorite. Geochemistry, zircon LA-ICP MS （laser ablation inductively-coupled plasma mass spectrometry） U-Pb chronology and Sr- Nd-Pb isotope geochemistry of the pluton are reported in this paper. The biotite granodiorite shows close compositional similarities to high-silica adakite. Its chondrite-normalized REE patterns are characterized by strong HREE depletion （Yb = 0.33--0.96 10-6 and Y = 4.77-11.19 ×10^-6）, enrichment of Ba （775-1386 x 10-6） and Sr （643-1115 × 10^-6） and high Sr/Y （57.83-159.99） and Y/Yb （10.99-14.32） ratios, as well as insignificant Eu anomalies （6Eu = 0.70-0.83）, suggesting a feldspar-poor, garnet±amphibole-rich residual mineral assemblage. The mafic enclaves have higher MgO （4.15- 8.13%）, Cr （14.79-371.31 × 10-6）, Ni （20.00-224.24× 10^-6） and Nb/Ta （15.42-21.91） than the host granodiorite, implying that they are mantle-derived and might represent underplated mafic magma. Zircon LA-ICP MS dating of the granodiorite yields a ^206pb/^238U weighted mean age of 208±2 Ma （MSWD=0.50, 1σ）, which is the age of emplacement of the host biotite granodiorite. This age indicates that the Wulong pluton formed during the late-orogenic or post-collisional stage （〈242±21 Ma） of the South Qinling belt. The host biotite granodiorite displays ^87Sr/^86Sr = 0.7059-0.7062, Isr = 0.7044-- 0.7050,^143Nd/^144Nd = 0.51236-0.51238, εNd（t）= -2.26 to -2.66 to ^206Pb/^204pb = 18.099-18.209, ^207pb/^204pb = 15.873-15.979 and ^208pb/^204pb = 38.973-39.430. Those ratios are similar to those of the Mesoproterozoic Yaolinghe Group in the South Qinling. Furthermore, its Nd isotopic model age （-1.02 Ga） is consistent with the age （-1.1 Ga） of the Yaolinghe Group. Based on the integrated geological and ge     

U-Pb Zircon Age,Geochemical, and Sr-Nd-Pb Isotopic Constraints on the Age and Origin of Mafic Dykes from Eastern Shandong Province,Eastern China

U-Pb zircon age, geochemical, and Sr-Nd-Pb isotopic data of mafic dykes from eastern Shandong Province, eastern China is reported herein. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb zircon analyses of two samples from the investigated mafic dykes yield consistent ages ranging from 121.9 Ma ± 0.47 Ma to 122.9 Ma ± 0.61 Ma. The mafic dykes are characterized by high (87Sr/86Sr) i ranging from 0.7087 to 0.7089, low εNd(t) values ranging from -16.9 to -17.8, 206Pb/204Pb = 17.15 to 17.17, 207Pb/204Pb = 15.45 to 15.47, and 208Pb/204Pb = 37.59 to 37.68. Results from the current study suggest that the mafic dykes are derived from partial melting of ancient lithospheric mantle that was variably hybridized by melts derived from foundered lower crustal eclogite. The mafic dykes may have been generated through subsequent insignificant crystal fractionation and very minor crustal contamination during magma ascent. Combined with previous studies, the current findings provide new evidence that the intense lithospheric thinning beneath the eastern Shandong Province of eastern China occurred at ~120 Ma, and that this condition was caused by the removal of the lower lithosphere (mantle and lower crust).     

Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block,China

The Neoproterozoic Sugetbrak Formation in the Aksu area, which is located at the northwest margin of Tarim Block, comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent. However, the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear. In this study, the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some ...     

Laser-ICP-MS U–Pb zircon ages and geochemical and Sr–Nd–Pb isotopic compositions of the Niyasar plutonic complex,Iran: constraints on petrogenesis and tectonic evolution

We conducted geochemical and isotopic studies on the Oligocene–Miocene Niyasar plutonic suite in the central Urumieh–Dokhtar magmatic belt, in order better to understand the magma sources and tectonic implications. The Niyasar plutonic suite comprises early Eocene microdiorite, early Oligocene dioritic sills, and middle Miocene tonalite + quartzdiorite and minor diorite assemblages. All samples show a medium-K calc-alkaline, metaluminous affinity and have similar geochemical features, including strong enrichment of large-ion lithophile elements (LILEs, e.g. Rb, Ba, Sr), enrichment of light rare earth elements (LREEs), and depletion in high field strength elements (HFSEs, e.g. Nb, Ta, Ti, P). The chondrite-normalized rare earth element (REE) patterns of microdiorite and dioritic sills are slightly fractionated [(La/Yb)_n = 1.1–4] and display weak Eu anomalies (Eu/Eu* = 0.72–1.1). Isotopic data for these mafic mantle-derived rocks display I_Sr = 0.70604–0.70813, ?_Nd (microdiorite: 50 Ma and dioritic sills: 35 Ma, respectively) = +1.6 and ?0.4, TDM = 1.3 Ga, and lead isotopic ratios are (²⁰⁶Pb/²⁰⁴Pb) = 18.62–18.57, (²⁰⁷Pb/²⁰⁴Pb) = 15.61–15.66, and (²⁰⁸Pb/²⁰⁴Pb) = 38.65–38.69. The middle Miocene granitoids (18 Ma) are also characterized by relatively high REE and minor Eu anomalies (Eu/Eu* = 0.77–0.98) and have uniform initial ⁸⁷Sr/⁸⁶Sr (0.7065–0.7082), a range of initial Nd isotopic ratios [?Nd(T)] varying from ?2.3 to ?3.7, and Pb isotopic composition (²⁰⁶Pb/²⁰⁴Pb) = 18.67–18.94, (²⁰⁷Pb/²⁰⁴Pb) = 15.63–15.71, and (²⁰⁸Pb/²⁰⁴Pb) = 38.73–39.01. Geochemical and isotopic evidence for these Eocene–Ologocene mafic rocks suggests that the magmas originated from lithospheric mantle with a large involvement of EMII component during subduction of the Neotethyan ocean slab beneath the Central Iranian plate, and were significantly affected by crustal contamination. Geochemical and isotopic data of the middle Miocene granitoids rule out a purely crustal-derived magma genesis, and suggest a mixed mantle–crustal [MASH (melting, assimilation, storage, and homogenization)] origin in a post-collision extensional setting. Sr–Nd isotope modelling shows that the generation of these magmas involved ～60% to 70% of a lower crustal-derived melt and ～30% to 40% of subcontinental lithospheric mantle. All Niyasar plutons exhibit transitional geochemical features, indicating that involvement of an EMII component in the subcontinental mantle and also continental crust beneath the Urumieh–Dokhtar magmatic belt increased from early Eocene to middle Miocene time.     

Petrogenesis of isotopically unusual Pliocene olivine leucitites from Deep Springs Valley, California

High-K mafic alkalic lavas (5.4 to 3.2 wt% K₂O) from Deep Springs Valley, California define good correlations of increasing incompatible element (e.g., Sr, Zr, Ba, LREE) and compatible element contents (e.g., Ni, Cr) with increasing MgO. Strontium and Nd isotope compositions are also correlated with MgO; ⁸⁷Sr/⁸⁶Sr ratios decrease and ɛ_Nd values increase with decreasing MgO. The Sr and Nd isotope compositions of these lavas are extreme compared to most other continental and oceanic rocks; ⁸⁷Sr/⁸⁶Sr ratios range from 0.7121 to 0.7105 and ɛ_Nd values range from −16.9 to −15.4. Lead isotope ratios are relatively constant, ²⁰⁶Pb/²⁰⁴Pb ∼17.2, ²⁰⁷Pb/²⁰⁴Pb ∼15.5, and ²⁰⁸Pb/²⁰⁴Pb ∼38.6. Depleted mantle model ages calculated using Sr and Nd isotopes imply that the reservoir these lavas were derived from has been distinct from the depleted mantle reservoir since the early Proterozoic. The Sr-Nd-Pb isotope variations of the Deep Springs Valley lavas are unique because they do not plot along either the EM I or EM II arrays. For example, most basalts that have low ɛ_Nd values and unradiogenic ²⁰⁶Pb/²⁰⁴Pb ratios have relatively low ⁸⁷Sr/⁸⁶Sr ratios (the EM I array), whereas basalts with low ɛ_Nd values and high ⁸⁷Sr/⁸⁶Sr ratios have radiogenic ²⁰⁶Pb/²⁰⁴Pb ratios (the EM II array). High-K lavas from Deep Springs Valley have EM II-like Sr and Nd isotope compositions, but EM I-like Pb isotope compositions. A simple method for producing the range of isotopic and major- and trace-element variations in the Deep Springs Valley lavas is by two-component mixing between this unusual K-rich mantle source and a more typical depleted mantle basalt. We favor passage of MORB-like magmas that partially fused and were contaminated by potassic magmas derived from melting high-K mantle veins that were stored in the lithospheric mantle. The origin of the anomalously high ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios and low ɛ_Nd values and ²⁰⁶Pb/²⁰⁴Pb ratios requires addition of an old component with high Rb/Sr and Th/Pb ratios but low Sm/Nd and U/Pb ratios into the mantle source region from which these basalts were derived. This old component may be sediments that were introduced into the mantle, either during Proterozoic subduction, or by foundering of Proterozoic age crust into the mantle at some time prior to eruption of the lavas. Received: 28 February 1997 / Accepted: 9 July 1998     

塔里木盆地东南缘安南坝地区镁铁质麻粒岩的成因——来自地球化学及Sr-Nd-Pb同位素的制约

甘肃阿克塞县安南坝地区镁铁质麻粒岩呈脉状、透镜状赋存于新太古代米兰岩群和TTG片麻岩中。岩石主要由斜长石(Pl)+斜方辉石(Opx)+单斜辉石(Cpx)+角闪石(Amp)+磁铁矿(Mt)等组成。安南坝镁铁质麻粒岩中Ti、P、Nb、Ta、Th、Hf、Sr及REE等元素与Zr相关性较好,表明其在变质作用过程中保持基本稳定。地球化学数据显示其原岩属于拉斑玄武质岩系列,Si O_2、Ti O_2、Al_2O_3、P_2O_5含量相对较低,Ca O、Mg O含量相对较高。Mg~#值为41.52~43.09,低于原生玄武质岩石的Mg~#值,Fe_2O_3~T、Mg O、Ca O与Si O_2含量呈负相关性,指示原岩岩浆演化过程中可能发生了辉石、角闪石等镁铁质矿物的分异结晶作用。镁铁质麻粒岩∑REE较低,稀土元素配分模式为轻稀土元素弱富集、重稀土元素相对平坦的右倾型,Eu异常不明显(Eu/Eu~*=0.91~1.01)。岩石富集Rb、Ba、Sr等大离子亲石元素,亏损Nb、Ta、Zr、Ti等高场强元素,具有显生宙典型岛弧玄武质岩石的地球化学特征。Sr、Nd、Pb同位素组成显示镁铁质麻粒岩原岩源自富集地幔,并受到一定程度的地壳物质混染。构造环境分析表明安南坝镁铁质麻粒岩原岩形成于与俯冲有关的岛弧环境。在俯冲作用机制下,俯冲板片流体交代使地幔楔发生富集,形成富集地幔,随着(弧后)伸展作用的加强,进一步诱发富集地幔的部分熔融形成镁铁质岩浆,最终岩浆就位形成辉长岩或辉绿岩脉,后期在麻粒岩相变质作用条件下变质为镁铁质麻粒岩。     

华北克拉通孔兹岩带中段大青山-乌拉山变质杂岩立甲子基性麻粒岩年代学及地球化学研究

大青山-乌拉山变质杂岩立甲子基性麻粒岩主要由角闪二辉麻粒岩、含榴角闪二辉麻粒岩和黑云角闪二辉麻粒岩所组成,并以变形岩墙和不规则透镜体形式赋存于富铝片麻岩和花岗质片麻岩之中.立甲子基性麻粒岩中变质锆石含有单斜辉石(Cpx)+角闪石(Amp)+斜长石(Pl)+磷灰石(Ap)的包体矿物,与寄主岩石——基性麻粒岩矿物组合及其化学成分十分一致,相应的207 pb/206 Pb表面年龄变化于1933±39Ma ～ 1834±40Ma,加权平均年龄为1892±7Ma(MSWD =0.50,n=46),应代表立甲子基性麻粒岩原岩经历中低压麻粒岩相的变质时代.在变质过程中,以大离子亲石元素(K、Na、Sr、Rb)为代表的活动元素发生了显著的改变;而高场强元素(Nb、Zr、Ti)和稀土元素基本无变化,保持稳定.立甲子基性麻粒岩原岩属于拉斑玄武质岩石系列,其SiO2集中变化于45.58％ ～51.40％,Mg#值集中介于41 ～54之间;在球粒陨石标准化稀土配分图中,立甲子基性麻粒岩具有平坦型的稀土配分曲线特征((La/Yb)cN=1.30～1.51),Eu异常不明显(Eu/Eu*=0.93～1.04).与显生宙岛孤拉斑玄武岩类似,立甲子基性麻粒岩所有样品皆具有Nb、Zr、Ti负异常特征.综合分析认为,立甲子基性麻粒岩原岩形成于2450～1930Ma,并于～1900Ma经历中低压麻粒岩相变质作用的改造,其主量元素和微量元素组成具有岛弧拉斑玄武质岩石的地球化学特征,其原岩可能是板块汇聚动力学背景下,岛弧构造环境中形成的辉长岩或辉绿岩.     

Origin of anorogenic ‘lamproite-like’ potassic lavas from the Denizli region in Western Anatolia Extensional Province, Turkey

The Denizli region of the Western Anatolia Extensional Province (WAEP) includes a typical example of intra-plate potassic magmatism. Lamproite-like K-rich to shoshonitic alkaline rocks erupted in the Upper Miocene-Pliocene in a tensional tectonic setting. The absence of Nb and Ta depletion, low Th/Zr and high Nb/Zr ratios and distinct isotopic values (i.e. low ⁸⁷Sr/⁸⁶Sr, 0.703523–0.703757; high ¹⁴³Nd/¹⁴⁴Nd, 0.512708–0.512784; high ²⁰⁶Pb/²⁰⁴Pb, 19.079–19.227, ²⁰⁷Pb/²⁰⁴Pb, 15.635–15.682, ²⁰⁸Pb/²⁰⁴Pb, 39.144–39.302) mark an anorogenic geochemical signature of the Denizli volcanics. All of the lavas are strongly enriched in large-ion-lithophile elements (e.g. Ba 1,100–2,200 ppm; Sr 1,900–3,100 ppm; Rb 91–295 ppm) and light rare-earth elements (e.g. La_N?=?319–464), with a geochemical affinity to ocean-island basalts and lack of a recognizable subduction signature or any evidence for crustal contamination. The restricted range of isotopic (Sr, Nd, Pb) ratios in both near-primitive (Mg# 66.7–77.2) and more evolved (Mg# 64.6–68.7) members of the Denizli volcanics signify their evolution from an isotopically equilibrated parental mantle source. Their high Dy/Yb and Rb/Sr values also suggest that garnet and phlogopite were present in the mantle source. Their strong EM-II signature, very low Nd model ages (0.44–049 Ga) and isotopic (Sr-Nd-Pb) values analogous to those of the Nyiragongo potassic basanites and kimberlites from the African stable continental settings, suggest that the parental melts that produced the Denizli volcanics are associated with very young and enriched mantle sources, which include both sublithospheric and enriched subcontinental lithospheric mantle melts. Mantle-lithosphere delamination probably played a significant role in the generation of these melts, and could be related to roll-back of the Aegean arc, lithospheric extension and asthenospheric mantle upwelling.     

The genesis of felsic magmatism during the closure of the Northeastern Paleo-Tethys Ocean: Evidence from the Heri batholith in West Qinling,China

To better understand the origin of voluminous silicic rocks in a convergent continental margin, we conducted an integrated study in which we have obtained geochronological, mineralogical, and isotopic (including whole-rock Sr–Nd–Pb, in-situ zircon Hf) data of the Heri batholith in West Qinling on the NE Tibetan Plateau. The batholith is composed of metaluminous to weakly peraluminous granodiorites (235–233 Ma) and porphyritic granodiorites (230–223 Ma) with an I-type affinity. Both lithologies share similar major, trace elemental and SrNd isotopic compositions. Detailed elemental data demonstrate that these granodioritic rocks underwent fractional crystallization of hornblende and apatite, with plagioclase (i.e. sieve-textured plagioclase cores) accumulation to some extent. Except for porphyritic granodiorites, the Pb isotopes for other analyzed samples are characterized by high radiogenicity and uniformity ((²⁰⁶Pb/²⁰⁴Pb)_t: 17.263–18.472, (²⁰⁶Pb/²⁰⁴Pb)_t: 15.571–15.591, and (²⁰⁶Pb/²⁰⁴Pb)_t: 38.032–38.304), together with limited variations in initial Sr ((⁸⁷Sr/⁸⁶Sr)_t: 0.707251–0.708103) and Nd (ε_Nd(t) = −7.1 to −6.3) isotopes with two-stage model ages (T_DM2) of 1.58–1.52 Ga. These factors collectively point to a derivation from the Mesoproterozoic basement rocks at the lower crustal level, or a comprehensive mixing of different-age components that generated an average crustal residence age. The SrNd isotopic compositions of the porphyritic granodiorites are strikingly similar to those of granodiorites. Compared with the experimental melt compositions of amphibolites, the Heri granitoids are probably derived from an amphibolitic source under fluid-absent conditions due to the incongruent breakdown of amphibole and biotite. Based on the temporal–spatial distribution of granitic intrusions in West Qinling and the regional tectonic evolution, our interpretation is that the Heri batholith was formed during the initial collision between the North China Craton (NCC) and the South China Craton (SCC), which was accompanied by the closure of the Paleotethyan Ocean. Considering both previously published data and our new data, we propose that the Heri granitoids were mainly generated by the partial melting of lower crustal amphibolites, with minor mantle-derived melts.     

Post-collisional, K-rich mafic magmatism in south Tibet: constraints on Indian slab-to-wedge transport processes and plateau uplift

Post-collisional (23–8 Ma), potassium-rich (including ultrapotassic and potassic) mafic magmatic rocks occur within the north–south-trending Xuruco lake–Dangre Yongcuo lake (XDY) rift in the Lhasa terrane of the southern Tibetan Plateau, forming an approximately 130-km-long semi-continuous magmatic belt. They include both extrusive and intrusive facies. Major and trace element and Sr–Nd–Pb isotopic data are presented for all of the known exposures within the XDY rift. The potassium-rich, mafic igneous rocks are characterized by high MgO (5.9–10.8 wt.%), K₂O (4.81–10.68 wt.%), Ba (1,782–5,618 ppm) and Th (81.3–327.4 ppm) contents, and relatively high SiO₂ (52.76–58.32 wt.%) and Al₂O₃ (11.10–13.67 wt.%). Initial Sr isotopic compositions are extremely radiogenic (0.712600–0.736157), combined with low (²⁰⁶Pb/²⁰⁴Pb) _i (18.28–18.96) and (¹⁴³Nd/¹⁴⁴Nd) _i (0.511781–0.512046). Chondrite-normalized rare earth element patterns display relatively weak negative Eu anomalies. Primitive mantle-normalized incompatible trace element patterns exhibit strong enrichments in large ion lithophile elements relative to high-field-strength elements and display strongly negative Ta–Nb–Ti anomalies. The combined major and trace element and Sr–Nd–Pb isotopic characteristics of the K-rich igneous rocks suggest that the primitive magmas were produced by 1–10 % partial melting of an asthenospheric mantle source enriched by both fluids and partial melts derived from Indian passive continental margin sediments subducted into the shallow mantle as a consequence of the northward underthrusting of the Indian continental lithosphere beneath Tibet since the India–Asia collision at ~55 Ma. The best-fit model results indicate that a melt with trace element characteristics similar to those of the K-rich rocks could be generated by 8–10 % partial melting of a metasomatized mantle source in the south and 1–2 % melting in the north of the XDY rift. Trace element and Sr–Nd–Pb isotopic modeling indicate that the proportion of fluid derived from the subducted sediments, for which we use as a proxy the Higher Himalayan Crystalline Sequence (HHCS), in the mantle source region increases from north (rear-arc) to south (front-arc), ranging from 0 to 5 %, respectively. Correspondingly, the proportion of the melt derived from the subducted HHCS in the source increases from north (2 %) to south (15 %). The increasing proportion of the fluid and melt component in the mantle source from north to south, together with a southward decreasing trend in the age of the K-rich magmatism within the XDY rift, is inferred to reflect rollback of the subducted Indian lithospheric mantle slab during the period 25–8 Ma. Slab rollback may be linked to a decreasing convergence rate between India and Asia. As a consequence of slab rollback at 25 Ma beneath the Lhasa terrane, its geodynamic setting was transformed from a convergent (55–25 Ma) to an extensional (25–8 Ma) regime. The occurrence of K-rich magmatism during the period 25–8 Ma is a consequence of the decompression melting of an enriched mantle source, which may signal the onset of extension in the southern Tibetan Plateau and provide a petrological record of the extension process.     

Geochemistry and petrogenesis of carbonatites from South Nam Xe,Lai Chau area,northwest Vietnam

This paper presents a study of the petrography, mineral chemistry, geochemistry, and Sr–Nd–Pb–C–O isotope systematics of carbonatite dykes and associated rocks from the northeastern part of the Song Da intracontinental rift in South Nam Xe (northwest Vietnam) aimed at constraining the origin of the carbonatite magmas. The carbonatites are characterized by SiO₂ < 12.18 wt.% and by wide ranges in FeO, MgO and CaO content that define them as calciocarbonatite and ferrocarbonatite. On U–Th–Pb isochron diagrams, whole rocks and mineral separates from the ferrocarbonatites form linear arrays corresponding to ages of 30.2–31.6 Ma (Rupelian, Oligocene). The South Nam Xe carbonatites are extremely enriched in Sr, Ba, and light rare earth elements (LREE), and depleted in high field strength elements (HFSE) (e.g. Ti, Nb, Ta, Zr and Hf). The age–corrected Sr–Nd–Pb isotope ratios and C isotope data are relatively uniform (⁸⁷Sr/⁸⁶Sr_(t) = 0.708193–0.708349; ¹⁴³Nd/¹⁴⁴Nd_(t) = 0.512250–0.512267; ε_Nd(t) = ?6.46 to ?6.80; ²⁰⁶Pb/²⁰⁴Pb_(t) = 18.26–18.79; ²⁰⁷Pb/²⁰⁴Pb_(t) = 15.62–15.64; ²⁰⁸Pb/²⁰⁴Pb_(t) = 38.80–39.38; δ¹³C_V-PDB = –2.7?‰ to ?4.1?‰). These isotopic compositions indicate source contamination that occurred before the production of the carbonatite magmas, and did not change noticeably during or after emplacement. The variation in oxygen isotopes is consistent with the change in mineral compositions and trace element abundances: the lower δ¹⁸O values (9.1–11.0?‰) coupled with Sr-rich, Mn-poor calcite, and igneous textures such as triple junctions among calcite grain boundaries, define a magmatic origin. However, the elevated δ¹⁸O values of the ferrocarbonatites (12.0–13.3?‰) coupled with a volatile-bearing mineral assemblages (including REE fluorcarbonates, sulfates, sulfides and fluorite) may be due to interaction with meteoric water during low-temperature alteration. High δ¹³C values and Sr–Pb ratios, and low Rb/Sr (0.00014–0.00301), Sm/Nd (0.089–0.141) and ¹⁴³Nd/¹⁴⁴Nd ratios, coupled with very high Sr-Nd concentrations, suggest the involvement of an enriched mantle component, which probably resulted from metasomatism due to the migration of subducted material. Because of the lack of tectonic data and the limited number of samples studied, this conclusion is still ambiguous and requires further study.     

Effects of basement composition and age on silicic magmas across an accreted terrane-Precambrian crust boundary,Sierra Madre Occidental,Mexico

Trace element and isotopic compositions of mid-Tertiary siliceous magma sequences from two localities of the Sierra Madre Occidental, northern Mexico, display differences that reflect the composition and age of the basement through which they erupted. The crust beneath the section at San Buenaventura is thicker and more evolved and forms part of the North American basement, while that under El Divisadero consists of allochthonous terranes of island arc/oceanic? crust accreted during the Mesozoic.The volcanics are highly differentiated and range in composition from basalt to rhyolite (SiO₂=50–76%). Those erupted through the accreted terranes display a small range of isotope ratios and have lowest initial (age-corrected) Sr isotope ratios (>0.7044) and the highest Nd (<0.5126) and Pb isotope ratios (²⁰⁶Pb/²⁰⁴Pb ∼18.9). Isotope ratios of the continental suite are more variable and form an array which trends away from that of the accreted terrane suite toward compositions more typical of old crust (to ⁸⁷Sr/⁸⁶Sr ∼0.710 and ¹⁴³Nd/¹⁴⁴Nd ∼0.5123). The volcanics in the continental zone are relatively more enriched in moderately incompatible elements compared with those within the accreted terranes (Ce/Yb=25–45 vs. 13–33, respectively), but are depleted in some highly incompatible elements such as U and Rb (e.g., Th/U=3.8–7.5 vs. 2.5–4.0, respectively). Those higher in the stratigraphic sections have higher ⁸⁷Sr/⁸⁶Sr, ²⁰⁸Pb/²⁰⁴Pb, and Th/U ratios, and lower ¹⁴³Nd/¹⁴⁴Nd ratios than those lower in the sections.The data have implications for the nature of the sources and the petrogenesis of these volcanics. The isotope ratios of both suites fall between those of mafic magma compositions from the Sierra Madre Occidental, and intermediate and felsic lower crustal xenoliths in northern Mexico and the southwestern USA. The relationship between the isotope ratios of the sequences and the age of the basement, combined with the fact that the overall data set forms well-defined isotopic arrays, demonstrates the strong effects of the crust on the chemistry of the silicic magmas. In the continental suite, isotope ratios covary with Th/Pb and U/Pb ratios, approaching the compositions found in the intermediate and felsic granulite facies xenoliths, strongly indicating that they are not anatectic melts of the lower crust but rather reflect interaction between mantle-derived basaltic parental magmas and the crust. Crustal contributions appear to be large, on the order of 20–70%. The small range of isotope ratios in the accreted terrane suite appears to reflect interaction of the basaltic parent with relatively juvenile crust whose isotopic composition is similar to the mantle-derived magmas. High Th/U and Th/Rb ratios indicate that the crustal contamination occurs in the lower crust. Moreover, the less radiogenic ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios in the continental suite indicate that the depletion in highly incompatible elements in the continental lower crust is an old feature. The secular changes in the isotope ratios within the stratigraphic sections indicate increasingly shallow crustal contributions with time, initially by predominantly mafic deep lower crust and later by more felsic middle crust. Using lavas from outside of the two heavily sampled stratigraphic sections, the differences in the isotopic compositions between volcanics erupted through the accreted terranes and the continental basement help to delineate the location of the boundary.     

Quaternary bimodal volcanism in the Niğde Volcanic Complex (Cappadocia,central Anatolia,Turkey): age,petrogenesis and geodynamic implications

The late Neogene to Quaternary Cappadocian Volcanic Province (CVP) in central Anatolia is one of the most impressive volcanic fields of Turkey because of its extent and spectacular erosionally sculptured landscape. The late Neogene evolution of the CVP started with the eruption of extensive andesitic-dacitic lavas and ignimbrites with minor basaltic lavas. This stage was followed by Quaternary bimodal volcanism. Here, we present geochemical, isotopic (Sr–Nd–Pb and δ¹⁸O isotopes) and geochronological (U–Pb zircon and Ar–Ar amphibole and whole-rock ages) data for bimodal volcanic rocks of the Ni?de Volcanic Complex (NVC) in the western part of the CVP to determine mantle melting dynamics and magmatic processes within the overlying continental crust during the Quaternary. Geochronological data suggest that the bimodal volcanic activity in the study area occurred between ca. 1.1 and ca. 0.2 Ma (Pleistocene) and comprises (1) mafic lavas consisting of basalts, trachybasalts, basaltic andesites and scoria lapilli fallout deposits with mainly basaltic composition, (2) felsic lavas consisting of mostly rhyolites and pumice lapilli fall-out and surge deposits with dacitic to rhyolitic composition. The most mafic sample is basalt from a monogenetic cone, which is characterized by ⁸⁷Sr/⁸⁶Sr = 0.7038, ¹⁴³Nd/¹⁴⁴Nd = 0.5128, ²⁰⁶Pb/²⁰⁴Pb = 18.80, ²⁰⁷Pb/²⁰⁴Pb = 15.60 and ²⁰⁸Pb/²⁰⁴Pb = 38.68, suggesting a moderately depleted signature of the mantle source. Felsic volcanic rocks define a narrow range of ¹⁴³Nd/¹⁴⁴Nd isotope ratios (0.5126–0.5128) and are homogeneous in Pb isotope composition (²⁰⁶Pb/²⁰⁴Pb = 18.84–18.87, ²⁰⁷Pb/²⁰⁴Pb = 15.64–15.67 and ²⁰⁸Pb/²⁰⁴Pb = 38.93–38.99). ⁸⁷Sr/⁸⁶Sr isotopic compositions of mafic (0.7038–0.7053) and felsic (0.7040–0.7052) samples are similar, reflecting a common mantle source. The felsic rocks have relatively low zircon δ¹⁸O values (5.6 ± 0.6 ‰) overlapping mantle values (5.3 ± 0.3 %), consistent with an origin by fractional crystallization from a mafic melt with very minor continental crustal contamination. The geochronological and geochemical data suggest that mafic and felsic volcanic rocks of the NVC are genetically closely related to each other. Mafic rocks show a positive trend between ⁸⁷Sr/⁸⁶Sr and Th, suggesting simultaneous assimilation and fractional crystallization, whereas the felsic rocks are characterized by a flat or slightly negative variation. High ⁸⁷Sr/⁸⁶Sr gneisses are a potential crustal contaminant of the mafic magmas, but the comparatively low and invariant ⁸⁷Sr/⁸⁶Sr in the felsic volcanics suggests that these evolved dominantly by fractional crystallization. Mantle-derived basaltic melts, which experienced low degree of crustal assimilation, are proposed to be the parent melt of the felsic volcanics. Geochronological and geochemical results combined with regional geological and geophysical data suggest that bimodal volcanism of the NVC and the CVP, in general, developed in a post-collisional extensional tectonic regime that is caused by ascending asthenosphere, which played a key role during magma genesis.     

Petrogenesis and tectonic significance of the Early Devonian lamprophyres and diorites in the Alxa Block,NW China

The North Qilian orogenic belt (NQOB) has been defined as a subduction-collision zone between the Alxa Block and the Qilian Block during the Early Paleozoic. To constrain the post-collisional tectonic evolution of the NQOB, analyses of zircon U-Pb-Hf isotopes, whole-rock major, trace element and Sr-Nd-Pb isotope compositions of the newly discovered Early Devonian lamprophyres and diorites dikes from the Longshoushan area in southwestern margin of the Alxa Block were conducted. Zircon U-Pb dating yields emplacement ages of 400 ± 4 Ma and 403 ± 6 Ma for two lamprophyre dikes and 391 ± 3 Ma for two diorite dikes. The lamprophyres dikes are shoshonitic-high-K (calc-alkaline) in nature, and are characterized by SiO₂ contents of 53.6–56.3 wt %, (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7064 to 0.7072, εNd(t) values of 0.1–1.0, and zircon εHf(t) values of −8.0 to −2.9. The diorite dikes are high-K (calc-alkaline), and are characterized by MgO contents of 6.32–6.98 wt %, (⁸⁷Sr/⁸⁶Sr)_i ratios of 0.7089–0.7137, εNd(t) values of −3.8 to −3.5, and zircon εHf(t) values of −9.9–0.4. Both the lamprophyre and diorite dikes show parallel enrichments in LREEs and LILEs and depletions in HREEs and HFSEs and have similar ratios of (²⁰⁶Pb/²⁰⁴Pb)_i (17.587–18.133), (²⁰⁷Pb/²⁰⁴Pb)_i (15.518–15.584) and (²⁰⁸Pb/²⁰⁴Pb)_i (37.676–38.058). Geochemical and isotopic data suggest that the lamprophyre and diorite dikes were derived from low-degrees melts of amphibole- and phlogopite-bearing lherzolite and phlogopite-bearing lherzolite, respectively, in the spinel-garnet transition facies. Their parental magmas both experienced extensive fractional crystallization in a deep magma chamber and negligible crustal contamination during their ascent. Regarding the Palaeozoic tectonic development of the North Qilian orogenic belt, we propose that the Early Devonian lamprophyres and diorites possibly are related to North Qilian orogen unrooting and collapse and marking the end of the North Qilian orogenic events.     

Petrogenesis of the Late Jurassic Laomengshan rhyodacite (Southeast China): constraints from zircon U–Pb dating,geochemistry and Sr–Nd–Pb–Hf isotopes

Extensive magmatism occurred in southeast China during Late Jurassic time, forming large-scale granitic and volcanic rocks associated with non-ferrous, rare earth and rare, radioactive metal deposits. The Shuikoushan Pb–Zn–Au orefield is a typical example located in Hunan Province. This study reports LA-ICP-MS zircon U–Pb ages, whole-rock chemistry, and Sr–Nd–Pb isotopic compositions, and in situ Hf isotopic geochemistry of zircons from the Laomengshan rhyodacite in the Shuikoushan Pb–Zn–Au orefield. Zircon U–Pb dating yields a weighted average age of 156.7 ± 1.6 Ma for the intrusion of the rhyodacite. The rhyodacite samples are mainly shoshonitic series, having metaluminous to weakly peraluminous A/CNK values ranging from 0.96 to 1.09, with moderately high magnesium content (Mg# = 42.4–47.5). Samples display high (⁸⁷Sr/⁸⁶Sr)_i values (0.71165–0.71176), low ε_Nd(T) values (?10.7 to ?10.3), old Nd model ages (T_DM = 1.73–1.86 Ga), and relatively homogeneous Pb isotopic compositions [(²⁰⁶Pb/²⁰⁴Pb)_i = 18.365–18.412, (²⁰⁷Pb/²⁰⁴Pb)_i = 15.663–15.680, and (²⁰⁸Pb/²⁰⁴Pb)_i = 38.625–38.666]. The zircons exhibit enriched ε_Hf(T) values (?16.22 to ?9.86) and old two-stage Hf model ages (T_DM2 = 1.82–2.22 Ga). All the above data indicate that the Laomengshan rhyodacite originated from melting Palaeoproterozoic basement, perhaps contaminated by subordinate mantle melts. Intense extension and thinning of the continental lithosphere during Late Jurassic time resulted in melting of upwelling asthenosphere, and mafic mantle melts interacted with and melted Palaeoproterozoic lower crust, thus forming the Laomengshan rhyodacite.     

Isotopic characteristics of river sediments on the Tibetan Plateau

We systematically collected 40 modern clastic sediment samples from rivers in different tectonic units of the Tibetan Plateau and measured their Sr–Nd isotopic compositions. The isotopic characteristics provide insight into the controversial paleo-tectonic affinity of terranes of the Tibetan Plateau and the provenance of Songpan–Ganzi flysch complex. The Qilian Terrane and Himalaya Terrane have more negative ε_Nd(0) values (from ? 14.3 to ? 11.8 and from ? 20.64 to ? 13.26, respectively) and high ⁸⁷Sr/⁸⁶Sr values (from 0.719674 to 0.738818 and from 0.721020 to 0.824959, respectively), reflecting old and mature continental crust origin of these two terranes. The southern Lhasa Terrane is more radiogenic in ε_Nd(0) values (from ? 8.82 to ? 3.8) and low in ⁸⁷Sr/⁸⁶Sr values (from 0.711504 to 0.719489), implying the combined impact of the Neo-Tethys mantle and Himalaya old continental crust. Sr–Nd isotopic compositions of the Qilian Terrane are similar to those in the Yangtze Craton, indicating that the Qilian Terrane was probably separated from the Yangtze Craton. Sr–Nd isotopic characteristics of the Songpan–Ganzi Terrane are similar to the Yangtze Craton and are remarkably different to those in the North China Craton, eastern Kunlun–Qaidam and the central Qiangtang metamorphic belt, implying that the widely distributed flysch complex of the Songpan–Ganzi Terrane was sourced from the Yangtze Craton.     

Sr–Nd–Pb isotopic constraints on the nature of the mantle sources involved in the genesis of the high-Ti tholeiites from northern Paraná Continental Flood Basalts (Brazil)

There has been little research on geochemistry and isotopic compositions in tholeiites of the Northern region from the Paraná Continental Flood Basalts (PCFB), one of the largest continental provinces of the world. In order to examine the mantle sources involved in the high-Ti (Pitanga and Paranapanema) basalt genesis, we studied Sr, Nd, and Pb isotopic systematics, and major, minor and incompatible trace element abundances. The REE patterns of the investigated samples (Pitanga and Paranapanema magma type) are similar (parallel to) to those of Island Arc Basalts' REE patterns. The high-Ti basalts investigated in this study have initial (133 Ma) ⁸⁷Sr/⁸⁶Sr ratios of 0.70538–0.70642, ¹⁴³Nd/¹⁴⁴Nd of 0.51233–0.51218, ²⁰⁶Pb/²⁰⁴Pb of 17.74–18.25, ²⁰⁷Pb/²⁰⁴Pb of 15.51–15.57, and ²⁰⁸Pb/²⁰⁴Pb of 38.18–38.45. These isotopic compositions do not display any correlation with Nb/Th, Nb/La or P₂O₅/K₂O ratios, which also reflect that these rocks were not significantly affected by low-pressure crustal contamination. The incompatible trace element ratios and Sr–Nd–Pb isotopic compositions of the PCFB tholeiites are different to those found in Tristan da Cunha ocean island rocks, showing that this plume did not play a substantial role in the PCFB genesis. This interpretation is corroborated by previously published osmium isotopic data (initial γ_Os values range from +1.0 to +2.0 for high-Ti basalts), which also preclude basalt generation by melting of ancient subcontinental lithospheric mantle. The geochemical composition of the northern PCFB may be explained through the involvement of fluids and/or small volume melts related to metasomatic processes. In this context, we propose that the source of these magmas is a mixture of sublithospheric peridotite veined and/or interlayered with mafic components (e.g., pyroxenites or eclogites). The sublithospheric mantle (dominating the osmium isotopic compositions) was very probably enriched by fluids and/or magmas related to the Neoproterozoic subduction processes. This sublithospheric mantle region may have been frozen and coupled to the base of the Parana basin lithospheric plate above which the Paleozoic subsidence and subsequent Early Cretaceous magmatism occurred.     

Petrogenesis of Jurassic granitoids in the west central Lhasa subterrane,Tibet, China: the Geji example

The Jurassic magmatic and volcanic rocks are widespread along the west central Lhasa subterrane. However, the petrogenesis of these rocks is poorly understood because of lacking high-quality geochronology and geochemical data. Here, we present new zircon U–Pb age and Hf isotopic data, whole-rock geochemical and Sr–Nd–Pb isotopic data for the Songduole and Qiangnong plutons in Geji area. LA-ICP-MS dating of zircon yield crystallization ages of 172.1 ± 1.9 and 155.9 ± 1.2 Ma for the Songduole and Qiangnong plutons, respectively. Geochemically, Songduole and Qiangnong granodiorite are characterized by high MgO (2.63–3.49 wt%), high Mg# (49–50), and low TiO₂ (0.48–0.57 wt%). Besides, all rocks show metaluminous, calc-alkaline signatures, with strong depletion of Nb, Ta, and Ti, enrichment of large-ion lithophile (e.g. Rb, Th, K), and a negative correlation between SiO₂ and P₂O₅. All these features are indicative of arc-related I-type magmatism. Five samples from the Songduole granodiorite have whole rock (⁸⁷Sr/⁸⁶Sr)_i of 0.71207–0.71257, ε_Nd(t) values of ?15.1 to ?13.9, zircon ε_Hf(t) values of ?17.4 to ?10.5, (²⁰⁶Pb/²⁰⁴Pb)_t ratios of 18.402–18.854, (²⁰⁷Pb/²⁰⁴Pb)_t ratios of 15.660–15.736, and (²⁰⁸Pb/²⁰⁴Pb)_t ratios of 38.436–39.208. Samples from the Qiangnong granodiorite have (⁸⁷Sr/⁸⁶Sr)_i of 0.71230–0.71252, ε_Nd(t) values of ?15.1 to ?14.2, zircon ε_Hf(t) values of ?12.6 to ?6.4, (²⁰⁶Pb/²⁰⁴Pb)_t ratios of 18.688–18.766, (²⁰⁷Pb/²⁰⁴Pb)_t ratios of 15.696–15.717, and (²⁰⁸Pb/²⁰⁴Pb)_t ratios of 38.546–39.083. These geochemical signatures indicate that the two plutons most likely originated from partial melting of the ancient Lhasa lower crust with obvious inputs of mantle-derived melts. Combined with regional geology, our results indicate that the Jurassic magmatism in the west central Lhasa subterrane most likely resulted from the southward subduction of the Bangong Ocean lithosphere beneath the central Lhasa terrane.