Post-collisional ultrapotassic rocks and mantle xenoliths in the Sailipu volcanic field of Lhasa terrane,south Tibet: Petrological and geochemical constraints on mantle source and geodynamic setting

Post-collisional ultrapotassic magmatic rocks (15.2–18.8 Ma), containing mantle xenoliths, are extensively distributed in the Sailipu volcanic field of the Lhasa terrane in south Tibet. They could be subdivided into high-MgO and low-MgO subgroups based on their petrological and geochemical characteristics. The high-MgO subgroup has olivine-I (Fo_87–92), phlogopite and clinopyroxene as phenocryst phases, while the low-MgO subgroup consists mainly of phlogopite, clinopyroxene and olivine-II (Fo_77–89). These ultrapotassic magmatic rocks have high MgO (4.6–14.5 wt%), Ni (145–346 ppm), Cr (289–610 ppm) contents, and display enrichment in light rare earth element (REE) over heavy REE and enriched large ion lithophile elements (LILE) relative to high field strength elements (HFSE) with strongly negative Nb-Ta-Ti anomalies in primitive mantle-normalized trace element diagrams. They have extremely radiogenic (⁸⁷Sr/⁸⁶Sr)_i (0.7167–0.7274) and unradiogenic (¹⁴³Nd/¹⁴⁴Nd)_i (0.5118–0.5120), high (²⁰⁷Pb/²⁰⁴Pb)_i (15.740–15.816) and (²⁰⁸Pb/²⁰⁴Pb)_i (39.661–39.827) at a given (²⁰⁶Pb/²⁰⁴Pb)_i (18.363–18.790) with high δ¹⁸O values (7.3–9.7‰). Strongly linear correlations between depleted mid-ocean ridge basalt-source mantle (DMM) and the Indian continental crust (HHCS) in Sr-Nd-Pb-O isotopic diagrams indicate that the geochemical features could result from reaction between mantle peridotite and enriched components (fluids and melts) released by the eclogitized Indian continental crust (HHCS) in the mantle wedge. The high-MgO (13.7–14.5 wt%) subgroup displays higher (¹⁴³Nd/¹⁴⁴Nd)_i, lower (⁸⁷Sr/⁸⁶Sr)_i and (²⁰⁶Pb/²⁰⁴Pb)_i ratios and lower δ¹⁸O values compared with the low-MgO (4.6–8.8 wt%) subgroup. High Ni (850–4862 ppm) contents of olivine phenocrysts and high whole-rock SiO₂, NiO, low CaO contents indicate that the low-MgO ultrapotassic magmatic rocks are derived from partial melting of olivine-poor mantle pyroxenite. However, lower Ni concentrations of olivine phenocryst and lower whole-rock SiO₂, NiO, higher CaO contents of the high-MgO ultrapotassic rocks may indicate their peridotite mantle source. This could be attributed to different amounts of silicate-rich components added into the mantle sources of the parental magmas in the mantle wedge caused by the northward subduction of the Indian continental lithosphere. The reaction-formed websterite xenoliths, reported for the first time in this study, are made up of anhedral and interlocking clinopyroxene (45–65 vol%) and orthopyroxene (30–50 vol%) with minor phlogopite (< 3 vol%) and quartz (< 2 vol%) and are suggested to be formed by silicate metasomatism of the mantle peridotite. The harzburgites, another major type of mantle xenolith in south Tibet, have a mineral assemblage of olivine (60–75 vol%), orthopyroxene (20–35 vol%), clinopyroxene (< 3 vol%), phlogopite (< 2 vol%) and spinel (< 2 vol%) and may have experienced subduction-related metasomatism. Combined with two types of ultrapotassic magmas, we propose that compositions of mantle wedge beneath south Tibet may gradually evolve from harzburgite through lherzolite to websterite with strong metasomatism of silicate-rich components in their mantle source region. Partial melting of the enriched mantle sources could be triggered by rollback of Indian continental slab during 25–8 Ma in south Tibet.     

Petrology and geochemistry of potassic and ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources

Major, trace element, Sr isotopic and mineral chemical data are reported for mafic volcanic rocks (Mg-value 65) from the northern-central sector of the potassic volcanic belt of Central Italy. The rocks investigated range from potassic series (KS) and high-K series (HKS) to lamproitic (LMP) and kamafugitic (KAM) through a transitional series (TRANS), thus covering the entire compositional spectrum of potassic and ultrapotassic magmas. KAM rocks are strongly silica undersaturated and, compared with the other rock series, have low SiO₂, Al₂O₃, Na₂O, Sc and V and high CaO, K/Na, (Na + K)/Al. KS and HKS have high Al₂O₃, CaO and variable enrichment in K₂O and incompatible elements. LMP rocks are saturated in silica and have high SiO₂, K₂O, K₂O/Na₂, MgO, Ni and Cr and low Al₂O₃, CaO, Na₂O, Sc and V. TRANS rocks display intermediate compositional characteristics between LMP and KS.

All the rocks under study have fractionated hygromagmaphile element patterns with high LIL/HFS element values and negative anomalies of Ti, Ta, Nb and Ba. Negative Sr anomalies are observed in the LMP and TRANS rocks. LIL elements show overall positive correlations with K₂O, whereas different trends of Sr and HFSE vs. K₂O are defined by LMP-TRANS and KS-HKS-KAM. ⁸⁷Sr/⁸⁶Sr range from about 0.710 to 0.716. KS, HKS and KAM rocks have similar ⁸⁷Sr/⁸⁶Sr values clustering around 0.710. LMP and TRANS rocks have the highest ⁸⁷Sr/⁸⁶Sr values.

Geochemical and isotopic data reported for the most primitive Italian potassic and ultrapotassic rocks support the hypothesis that the interaction between crustal and mantle reservoirs was a main process in the genesis of Italian potassic magmatism. Simple mass balance calculations exclude, however, an important role of crustal assimilation during ascent of subcrustal magmas to the surface and indicate that the sources of Central Italy volcanics underwent contamination with fluids and/or melts released by upper crustal material previously brought into the mantle by subduction processes.

Different trends of incompatible elements vs. K₂O observed in the studied rocks suggest distinct metasomatic processes for the sources of the investigated magmas. Liquids derived by bulk melting of pelitic sediments are believed to be the most likely contaminants of the source of LMP rocks. Fluids or melts rich in Ca, Sr and with high LILE/HFSE value and Sr isotopic composition around 0.710 are the most likely contaminant of the source region of KS, HKS and KAM volcanics. Variations in CaO, Na₂O and ferromagnesian element abundances and ratios suggest that, in some zones, the mantle source of potassic magmas experienced partial melting with extraction of basaltic liquids prior to metasomatism.     

Post-collisional Plio-Pleistocene shoshonitic volcanism in the western Kunlun Mountains, NW China: Geochemical constraints on mantle source characteristics and petrogenesis

Major and trace element, Sr–Nd–Pb isotope and mineral chemical data are presented for post-collisional late Cenozoic shoshonitic volcanic rocks from the western Kunlun Mountains, NW China. They are distributed in two approximately E–W striking sub-belts, with the lavas in the southern sub-belt having been generated earlier than those in the northern sub-belt. The mineralogy of the rocks reflects crystallization from moderate temperature magmas (700–1000 °C) with high oxygen and water fugacities. They are geochemically characterized by relatively low TiO₂, Al₂O₃ and FeO and high alkalies coupled with very high contents of incompatible element concentrations. Remarkably negative Nb, Ta and Ti anomalies are displayed on primitive mantle-normalized incompatible element patterns. In addition, they show a relatively broad range of low ε_Nd (−1.8 to −8.7) at more restricted ⁸⁷Sr/⁸⁶Sr ratios (0.7081–0.7090). Pb isotopes are characterized by a range of ²⁰⁷Pb/²⁰⁴Pb (15.48–15.74) and ²⁰⁸Pb/²⁰⁴Pb (38.30–39.12) ratios at relatively invariant ²⁰⁶Pb/²⁰⁴Pb (18.60–18.83) values, except one sample with a ratio of 18.262, leading to near-vertical arrays. The lavas from the northern sub-belt have relatively high ⁸⁷Sr/⁸⁶Sr ratios. All lavas have extremely high La/Yb ratios, probably reflecting that the magmas were derived from a metasomatized lithospheric mantle source containing phlogopite–hornblende garnet peridotite affected by subducted sediments and hydrous fluids, rather than from a depleted asthenopheric mantle source or mantle plume source. However, the lavas from the southern sub-belt were derived from a lower degree of melting of more highly metasomatized sub-lithospheric mantle in comparison with those from the northern sub-belt. Processes responsible for partial melting of metasomatized lithospheric mantle and post-collision magmatism in the western Kunlun could be a consequence of continuously conductive heating of upwelling, hot asthenospheric mantle following the delamination subsequent to thickening, which is consistent with the spatial and temporal geochemical variations in shoshonitic rocks in Tibet.     

拉萨地块西段中新世赛利普超钾质火山岩富集地幔源区和岩石成因:Li同位素制约

作为一种“非传统稳定同位素”,锂同位素地球化学研究已经成为近年来国际上研究的热点之一.文章成功应用锂同位素对青藏高原西南部赛利普超钾质火山岩进行了示范研究.研究表明,赛利普超钾质火出岩的w(Li)为11.2×10-6～22.9× 10-6,同位素组成δ7Li为1.2‰～+3.5‰,平均值为0 2‰,与平均上地壳的相当.超钾质火山岩的锂同位素组成与岩浆结晶分异程度参数之间不存在任何相关性,这表明在超钾质火山岩结晶分异过程中没有发生明显的锂同位素分馏,锂同位素组成特征反映了其形成时的源区特征.超钾质火山岩的锂同位素组成变化范围达4.7‰,并且与pb-Sr-Nd同位素和岩浆结晶分异参数之间亦无任何相关性,表明锂同位素异常可能反映了不均匀源区岩石特征.通过计算模拟以及与前人的类似研究成果进行对比,笔者认为俯冲印度地壳而不是特提斯洋壳(包括沉积物)的流体/熔体参与了超钾质火山岩的源区富集,并在此基础上提出了超钾质火山岩成因模式.     

冈底斯新近纪钾质火山作用:消减沉积物折返的地球化学与Sr-Nd-Pb同位素证据

在冈底斯岩浆岩带的羊八井地区,林子宗火山岩系上部出现小规模的粗面质火山岩.岩石学与地球化学研究表明粗面岩与时空密切相关的大体积安山质-英安质-流纹质火山岩属于不同的火山岩系:碱性钾质系列与钙碱性系列.详细的地球化学研究证明林子宗晚期小体积钾质岩具有独立的岩浆源区,而非早期钙碱性系统低压岩浆过程演化的产物.粗面质熔岩SiO2含量为62.91%～64.63%,具有高K2O(7.52%～8.05%)、Al2O3(16.64%-17.35%)、低TiO2(0.59%～0.68%)与MsO(0.15%～0.77%).钾质熔岩富集LILE与LREE,亏损HFSE,具有高Rb/Sr(1.1～2.3)、Th/La(0.59～0.80)、Th/U(6.3～10.9)和低Ce/Pb(4.1～6.2)与Ba/Th(4.3～14.5)比值,其同位素组成变化范围分别是87Sr/86Sr(I)=0.7068～0.7075、143Nd/144Nd(I)=0.51241～0.51252、206Pb/204Pb=18.87～18.95、207Pb/204Pb=15.63～15.70和208Pb/204Pb=39.24～39.68.粗面岩的地球化学与同位素特征表明冈底斯新近纪钾质火山岩来源于角闪岩相的消减沉积物熔融.因此,羊八井新近纪钾质火山熔岩提供了印度-亚洲大陆碰撞早期消减沉积物折返的证据.     

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.     

青藏高原西南部赛利普超钾质火山岩富集地幔源区和岩石成因:锆石U-Pb年代学和Hf同位素制约

西藏拉萨地块西南部赛利普钾质-超钾质火山岩为一套含地幔包体的粗面安山岩,高K2O,MgO、Cr、Ni含量,K2O/Na2O比值和Mg#,为地幔低度部分熔融的原始岩浆,或经橄榄石、单斜辉石或Fe-Ti氧化物分离结晶.岩石强烈富集大离子亲石元素和轻稀土元素、亏损高场强元素Nb、Ta、Ti,富集放射性成因Sr、Pb和Nd同位素,指示岩浆源区为富集地幔.采用LA-ICP-MS测定赛利普钾质-超钾质火山岩三件样品的18颗新生岩浆锆石U-Pb年龄为15.8～19.2Ma,其中钾质岩石样品SL0628中11个点的加权平均值为17.7±0.3Ma,与他人获得的40Ar/39Ar年龄一致.三件样品中新生岩浆锆石的εHf(t)变化范围为-7.6～3.9,平均地壳模式年龄(tDMC=0.86～1.59Ga)变化较大,除两个分析点显示亏损特征外,总体显示富集特征,表明岩石源于富集源区,但有少量亏损地幔物质加入.三件样品共获得49颗继承锆石的U-Pb年龄介于20～1907Ma,其Hf同位素组成(εHf(t)=-25.9～5.3)和平均地壳模式年龄(tDMC=0.79～4.08Ga)变化较大;其中的37颗年龄较小的继承锆石(20～110Ma)指示地幔源区可能受到四期明显的岩浆改造事件(62.2～64.0Ma,43.3～55.1Ma,29.5～37.7Ma和20.1～27.4:Ma)和两个岩浆活动间歇期(70～90Ma和37.7～43.3Ma).在拉萨地块首次发现29.5～37.7Ma的岩浆活动,并发现与林子宗火山岩同期的、Hf同位素富集的岩浆活动(62.2～64.0Ma,εHf(t)=-21.2～3.0).三件样品中49颗继承锆石的Hf同位素研究表明源区富集组分可能源自拉萨地块古老地壳基底和俯冲的印度大陆地壳.赛利普钾质-超钾质岩石形成可能是印度大陆地壳前缘撕裂和分段俯冲的结果.     

Subduction of the Indian lower crust beneath southern Tibet revealed by the post-collisional potassic and ultrapotassic rocks in SW Tibet

New major and trace elemental, Sr–Nd–Pb isotope, and zircon U–Pb geochronological and Hf–O isotope data of post-collisional potassic and ultrapotassic volcanic rocks (PVRs and UPVs, respectively) along with geochemical data of PVRs, UPVs, and Mg-rich potassic rocks (MPRs) in the literature are used to constrain their mantle source and genesis. The PVRs, UPVs, and MPRs share similar geochemical features but with some discrepancies, suggesting that they were derived from subcontinental lithospheric mantle (SCLM) with isotopic heterogeneity resulting from the varying contributions of subducted Indian lower crust into the mantle source (ca. 6–20%, ca. 8–30%, and ca. 9–30%, respectively). The zircon Hf–O isotopic compositions of these rocks can be classified into two groups, including Group I rocks with high δ¹⁸O (6.7–11.3‰), low ε_Hf(t) (− 17.0 to − 12.0), and old Hf crustal model ages (1.87–2.19 Ga) that indicate an ancient SCLM source, and Group II rocks with δ¹⁸O values of 6.8–10.7‰, ε_Hf(t) values of − 11.8 to − 6.3, and younger Hf crustal model ages (1.50–1.86 Ga). The negative correlation defined by δ¹⁸O and ε_Hf(t) of Group II samples suggests a two-component mixing between mantle- and crust-derived melts, in which the latter would be the subducted Indian lower crust as indicated by the similar negative ε_Hf(t) values between Group II samples (− 11.8 to − 6.3) and the High Himalayan gneiss (− 14.2 to + 0.3). Thus we propose two enrichment events to account for the Hf–O isotopic compositions of the PVRs and UPVs/MPRs: the first involves the enrichment of the overlying SCLM that was metasomatized by fluids derived from dehydration of the subducted Indian lower crust, and the second invokes the enrichment of the overlying SCLM metasomatized by melts of the already dehydrated different proportions of the Indian lower crust. We argue that break-off of the northwards subducted Indian Plate in the early Miocene caused the asthenospheric upwelling under the Indian plate through slab window, resulting in varying degrees of partial melting of the overlying metasomatized heterogeneous SCLM to produce the primitive magmas of the PVRs, UPVs, and MPRs in an extensional setting. These observations and interpretations imply that the Indian lower crust was subducted beneath the Lhasa terrane in the Early–Middle Miocene.     

Subduction-related Late Cretaceous high-K volcanism in the Central Pontides orogenic belt: constraints on geodynamic implications

Mineral chemistry, major and trace elements, ⁴⁰Ar/³⁹Ar age and Sr–Nd–Pb isotopic data are presented for the Late Cretaceous Hamsilos volcanic rocks in the Central Pontides, Turkey. The Hamsilos volcanic rocks mainly consist of basalt, andesite and associated pyroclastics (volcanic breccia, vitric tuff and crystal tuff). They display shoshonitic and high-K calc-alkaline affinities. The shoshonitic rocks contain plagioclase, clinopyroxene, alkali feldspar, phlogopite, analcime, sanidine, olivine, apatite and titanomagnetite, whereas the high-K calc-alkaline rocks contain plagioclase, clinopyroxene, orthopyroxene, magnetite / titanomagnetite in microgranular porphyritic, hyalo-microlitic porphyritic and glomeroporphyritic matrix. Mineral chemistry data reveal that the pressure condition of the clinopyroxene crystallisation for the shoshonitic rocks are between 1.4 and 6.3 kbar corresponds to 6–18-km depth and the high-K calc-alkaline rocks are between 5 and 12 km. ⁴⁰Ar/³⁹Ar age data changing between 72 ± .5 Ma and 79.0 ± .3 Ma (Campanian) were determined for the Late Cretaceous Hamsilos volcanic rocks, contemporaneous with the subduction of the Neo-Tethyan Ocean beneath the Pontides. The studied volcanic rocks were enriched in the large-ion lithophile and light rare earth element contents, with pronounced depletion in the contents of high-field-strength elements. Chondrite-normalised rare earth element patterns (La_N/Lu_N = 6–17) show low to medium enrichment, indicating similar sources of the rock suite. Initial ⁸⁷Sr/⁸⁶Sr values vary between .70615 and .70796, whereas initial ¹⁴³Nd/¹⁴⁴Nd values change between .51228 and .51249. Initial ²⁰⁶Pb/²⁰⁴Pb values vary between 18.001 and 18.349, ²⁰⁷Pb/²⁰⁴Pb values between 15.611 and 15.629 and ²⁰⁸Pb/²⁰⁴Pb values between 37.839 and 38.427. The main solidification processes involved in the evolution of the volcanic rocks consist of fractional crystallisation, with minor amounts of crustal contamination ± magma mixing. According to geochemical evidence, the shoshonitic melts in the Hamsilos volcanic rocks were possibly derived from the low degree of partial melting of a subcontinental lithospheric mantle (SCLM), while the high-K calc-alkaline melts were derived from relatively high degree of partial melting of SCLM that was enriched by fluids and/or sediments from a subduction of oceanic crust.     

Alkaline character of Eocene volcanism in the middle part of central Iran and its geodynamic situation

Alkaline volcanic rocks, sometimes feldspathoid bearing, appear in variable stratigraphic positions within the Eocene of the middle part of Central Iran. No magmatic zonation and no K₂O enrichment in both time and space can be found in this volcanic sequence. These facts together with other geochemical and geological features of the Iranian Eocene provinces do not agree with a volcanism related to a subduction zone model.Reversely, a rift model may be applied for the interpretation of the origin of Eocene volcanic rocks and their geodynamic situation. The emissions of alkaline magma are linked with rhythmic opening of a rift zone during Eocene.A mutual contamination between a basaltic magma and a bulky palingenetic acid magma is responsible for the few volcanic rocks with calc-alkaline trend.

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Zusammenfassung Alkali-Gesteine, die manchmal Feldspatoide enthalten, treten in verschiedenen stratigraphischen Stellungen innerhalb des Eozäns des Zentral-Irans auf.In diesen Vulkaniten ist keinerlei magmatische Zonierung und insbesondere keine systematische Variation der Kaligehalte festzustellen. Diese und andere geologische und geochemische Argumente sprechen gegen eine Deutung als Paläosubduktionszone.Als Alternative bietet sich das Modell der Öffnung eines Grabens an: der Alkali-Vulkanismus steht in Zusammenhang mit den verschiedenen Phasen der Entstehung und Entwicklung eines Rifts während des Eozäns.Die seltenen Vulkanite kalk-alkalischer Zusammensetzung erklären sich durch Mischung der Magmen von Alkali-Basalten mit den bedeutenden anatektischen Magmen.

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Résumé Des roches alcalines, éventuellement à feldspathoïde, existent à des niveaux et dans des secteurs variés de la séquence volcanique éocène de la partie médiane de la zone de l'Iran Central; par ailleurs, dans cette série, on n'a pu mettre en évidence ni zonation magmatique, ni enrichissement en K₂O. Ces faits, joints à d'autres caractères géochimiques et géologiques des provinces éocènes de l'Iran, sont en désaccord avec un volcanisme lié à une zone de subduction.En revanche, un modèle de rift peut être appliqué pour interpréter l'origine des volcanites éocènes et leur situation géodynamique. Les émissions de magma alcalin sont en effet en relation avec l'ouverture rhythmique d'une zone de rift pendant l'Eocène.Quant aux rares roches à tendance calco-alcaline, elles résultent d'une contamination mutuelle entre un magma basaltique et un important magma acide palingénétique.

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Investigation on mantle peridotites from Neyriz ophiolite, south of Iran: geodynamic signals

Neyriz ophiolite in Abadeh Tashk area appears as four major separated massifs in an area with 125 km², south of Iran. Peridotites including harzburgite, dunite, and lesser low-Cpx lherzolite are the major constituents of the ophiolite with very minor mafic rocks. Usual gabbros of ophiolite complexes are virtually absent from the study area. Mineral modality associated with bulk rock and mineral chemistry of the peridotites show a progression from fertile to ultra-refractory character, reflected by a progressive decrease in modal pyroxenes and in Al₂O₃, CaO, SiO₂, Sc, Ta, V, and Ga values of the studied rocks by approaching chromite deposits. The Neyriz peridotites vary from low-Cpx lherzolite (MgO, 41.97–43.1 wt.%; Al₂O₃, 0.8–1.3 wt.%) with low content of Cr# spinel (36.7–37.6) and Fo olivine (90.79–91.5) to harzburgite (MgO, 44.31–45.25 wt.%;Al₂O₃, 0.29–0.45 wt.%; Cr# spinel, 58.2–73.45; Fo olivine, 91.23–91.56), and then to dunite (MgO, 45.9–49.2 wt.%; Al₂O₃, 0.18–0.48 wt.%) with higher content of Cr# spinel (74.34–79.36) and Fo olivine (91.75–94.68). Compared to modern oceanic settings, mineral and rock composition of low-Cpx lherzolite plot within the field of mid-ocean-ridge environment, whereas those of harzburgite and dunite fall in the field of fore-arc peridotites. As a result of the studies on minerals and whole rock chemistry along with rock interrelationships, we contend that the peridotites were subsequently affected by percolating hydrous boninitic melt from which the high-Cr–Mg, low-Ti chromitites were formed within mantle wedge above the supra-subduction zone in a fore-arc setting.     

Geophysical constraints on geodynamic processes at convergent margins: A global perspective

Convergent margins, being the boundaries between colliding lithospheric plates, form the most disastrous areas in the world due to intensive, strong seismicity and volcanism. We review global geophysical data in order to illustrate the effects of the plate tectonic processes at convergent margins on the crustal and upper mantle structure, seismicity, and geometry of subducting slab. We present global maps of free-air and Bouguer gravity anomalies, heat flow, seismicity, seismic Vs anomalies in the upper mantle, and plate convergence rate, as well as 20 profiles across different convergent margins. A global analysis of these data for three types of convergent margins, formed by ocean–ocean, ocean–continent, and continent–continent collisions, allows us to recognize the following patterns. (1) Plate convergence rate depends on the type of convergent margins and it is significantly larger when, at least, one of the plates is oceanic. However, the oldest oceanic plate in the Pacific ocean has the smallest convergence rate. (2) The presence of an oceanic plate is, in general, required for generation of high-magnitude (M > 8.0) earthquakes and for generating intermediate and deep seismicity along the convergent margins. When oceanic slabs subduct beneath a continent, a gap in the seismogenic zone exists at depths between ca. 250 km and 500 km. Given that the seismogenic zone terminates at ca. 200 km depth in case of continent–continent collision, we propose oceanic origin of subducting slabs beneath the Zagros, the Pamir, and the Vrancea zone. (3) Dip angle of the subducting slab in continent–ocean collision does not correlate neither with the age of subducting oceanic slab, nor with the convergence rate. For ocean–ocean subduction, clear trends are recognized: steeply dipping slabs are characteristic of young subducting plates and of oceanic plates with high convergence rate, with slab rotation towards a near-vertical dip angle at depths below ca. 500 km at very high convergence rate. (4) Local isostasy is not satisfied at the convergent margins as evidenced by strong free air gravity anomalies of positive and negative signs. However, near-isostatic equilibrium may exist in broad zones of distributed deformation such as Tibet. (5) No systematic patterns are recognized in heat flow data due to strong heterogeneity of measured values which are strongly affected by hydrothermal circulation, magmatic activity, crustal faulting, horizontal heat transfer, and also due to low number of heat flow measurements across many margins. (6) Low upper mantle Vs seismic velocities beneath the convergent margins are restricted to the upper 150 km and may be related to mantle wedge melting which is confined to shallow mantle levels.     

Central European Cenozoic plume volcanism with OIB characteristics and indications of a lower mantle source

Average concentrations of incompatible elements and isotopes of radiogenic Sr, Nd and Pb in a 350 km long belt of Central European volcanics (CECV) resemble those in OIB in general and in OIB from the N Atlantic in particular. This similarity allows to infer comparable sources for CECV and OIB which might have been located in the lower mantle according to seismic tomography and chemistry, with the latter unlike a MORB source. The incompatible element contribution of lower mantle origin can be modelled from primitive mantle minus continental crust and upper mantle inventories. Alkali basaltic magmas from the modelled source are close in composition to CECV and OIB. The continental crust contains almost half of the silicate Earth's content of Rb, K, Pb, Ba, Th and U, which were mobilized to a major extent through dehydration of subducted ocean crust. Related losses from the lower mantle had to be replaced by deep subductions of oceanic lithosphere recognized from their isotopic imprint. From a balance based on Nd isotopes it can be concluded that average CECV contains 60% matter from residual primitive mantle and 40% from deeply subducted lithosphere (including some young upper mantle materials). Plume products from separate CECV regions developed, within 45 Ma, from rather depleted to more primitive isotopic signatures. Four periods of volcanism from Eocene to late Quaternary time are explained as four pulses of an almost stationary ultrafast plume uprise as modelled by Larsen and Yuen (1997). Magma production has increased from the first to the third pulse with the peak during Miocene time in the Vogelsberg region. The final pulse produced the Quaternary Eifel volcanoes. Tectonism from the Alpine orogen has probably triggered the synchronous volcanism of CECV, Massif Central etc. The European lithospheric plate has moved under the control of the opening Atlantic almost in an eastern direction with a velocity of 1 cm per year and has shifted extinct volcanoes off their source channels. Received: 10 July 1998 / Accepted: 10 February 1999     

Depleted arc volcanism in the Alboran Sea and shoshonitic volcanism in Morocco: geochemical and isotopic constraints on Neogene tectonic processes

Samples of volcanic rocks from Alborán Island, the Alboran Sea floor and from the Gourougou volcanic centre in northern Morocco have been analyzed for major and trace elements and Sr–Nd isotopes to test current theories on the tectonic geodynamic evolution of the Alboran Sea. The Alborán Island samples are low-K tholeiitic basaltic andesites whose depleted contents of HFS elements (0.5×N-MORB), especially Nb (0.2×N-MORB), show marked geochemical parallels with volcanics from immature intra-oceanic arcs and back-arc basins. Several of the submarine samples have similar compositions, one showing low-Ca boninite affinity. ¹⁴³Nd/¹⁴⁴Nd ratios fall in the same range as many island-arc and back-arc basin samples, whereas ⁸⁷Sr/⁸⁶Sr ratios (on leached samples) are somewhat more radiogenic. Our data point to active subduction taking place beneath the Alboran region in Miocene times, and imply the presence of an associated back-arc spreading centre. Our sea floor suite includes a few more evolved dacite and rhyolite samples with (⁸⁷Sr/⁸⁶Sr)₀ up to 0.717 that probably represent varying degrees of crustal melting. The shoshonite and high-K basaltic andesite lavas from Gourougou have comparable normalized incompatible-element enrichment diagrams and Ce/Y ratios to shoshonitic volcanics from oceanic island arcs, though they have less pronounced Nb deficits. They are much less LIL- and LREE-enriched than continental arc analogues and post-collisional shoshonites from Tibet. The magmas probably originated by melting in subcontinental lithospheric mantle that had experienced negligible subduction input. Sr–Nd isotope compositions point to significant crustal contamination which appears to account for the small Nb anomalies.

The unmistakable supra-subduction zone (SSZ) signature shown by our Alboran basalts and basaltic andesite samples refutes geodynamic models that attribute all Neogene volcanism in the Alboran domain to decompression melting of upwelling asthenosphere arising from convective thinning of over-thickened lithosphere. Our data support recent models in which subsidence is caused by westward rollback of an eastward-dipping subduction zone beneath the westernmost Mediterranean. Moreover, severance of the lithosphere at the edges of the rolling-back slab provides opportunities for locally melting lithospheric mantle, providing a possible explanation for the shoshonitic volcanism seen in northern Morocco and more sporadically in SE Spain.     

Eocene mafic volcanism in northern Anatolia: its causes and mantle sources in the absence of active subduction

Eocene mafic volcanic rocks occurring in an E–W-trending, curvilinear belt along and north of the Izmir–Ankara–Erzincan suture zone (IAESZ) in northern Anatolia, Turkey, represent a discrete episode of magmatism following a series of early Cenozoic collisions between Eurasia and the Gondwana-derived microcontinents. Based on our new geochronological, geochemical, and isotope data from the Kartepe volcanic units in northwest Anatolia and the extant data in the literature, we evaluate the petrogenetic evolution, mantle melt sources, and possible causes of this Eocene volcanism. The Kartepe volcanic rocks and spatially associated dikes range from basalt and basaltic andesite to trachybasalt and basaltic trachyandesite in composition, and display calc-alkaline and transitional calc-alkaline to tholeiitic geochemical affinities. They are slightly to moderately enriched in large ion lithophile (LILE) and light rare earth elements (LREE) with respect to high-field strength elements (HFSE) and show negative Nb, Ta, and Ti anomalies reminiscent of subduction-influenced magmatic rocks. The analysed rocks have ⁸⁷Sr/⁸⁶Sr_(i) values between 0.70570 and 0.70399, positive ?_Nd values between 2.7 and 6.6, and Pb isotope ratios of ²⁰⁶Pb/²⁰⁴Pb_(i) = 18.6–18.7, ²⁰⁷Pb/²⁰⁴Pb_(i) = 15.6–15.7, and ²⁰⁸Pb/²⁰⁴Pb_(i) = 38.7–39.1. The ⁴⁰Ar/³⁹Ar cooling ages of 52.7 ± 0.5 and 41.7 ± 0.3 Ma obtained from basaltic andesite and basalt samples indicate middle to late Eocene timing of this volcanic episode in northwest Anatolia. Calculated two-stage Nd depleted mantle model (T_DM) ages of the Eocene mafic lavas range from 0.6 to 0.3 Ga, falling between the T_DM ages of the K-enriched subcontinental lithospheric mantle of the Sakarya Continent (1.0–0.9 Ga) to the north, and the young depleted mantle beneath central Western Anatolia (0.4–0.25 Ga) to the south. These geochemical and isotopic features collectively point to the interaction of melts derived from a sublithospheric, MORB-like mantle and a subduction-metasomatized, subcontinental lithospheric mantle during the evolution of the Eocene mafic volcanism. We infer triggering of partial melting by asthenospheric upwelling beneath the suture zone in the absence of active subduction in the Northern Neotethys. The geochemical signature of the volcanic rocks changed from subduction- and collision-related to intra-plate affinities through time, indicating an increased asthenospheric melt input in the later stages of Eocene volcanism, accompanied by extensional deformation and rifting.     

Petrological characterization of the source components of potassic magmas: geochemical and experimental constraints

The source mineralogy and conditions of origin of the three main groups of ultrapotassic rocks are outlined by combining experimental constraints and an abstraction of evidence from whole-rock chemistry (including volatiles), tectonic setting and xenolith contents. Lamproites originate from a depleted source rock which was strongly re-enriched at a later stage, thus producing mica-harzburgite. Melting conditions are H₂O-rich and in most cases strongly reducing. Kamafugites originate from a clinopyroxene-reich source, also with abundant mica, in more oxidizing, CO₂-rich conditions. Members of the third group form in a relatively fertile spinel-peridotite also containing abundant clinopyroxene and mica. Contrasting effects of variation in (i) pressure of melting and (ii) oxygen fugacity, emphasize the importance of these parameters in the sources of ultrapotassic rocks.

Currently popular models for the origin of ultrapotassic melts by partial melting of phlogopite-bearing lherzolite are inconsistent with the now extensive array of liquidus experimental results on ultapotassic rock compositions. The discrepancy between partial melting models and liquidus results is attributed to the implicit, invalid assumption in the partial melting models that incompatible elements are homegeneously distributed on a large scale. Non-peridotitic assemblages rich in mica and pyroxenes which may be completely free of olivine must have an important role in the genesis of potassic rocks as spatially restricted components of inhomogeneous source regions.     

青藏高原东缘地壳上地幔结构及其动力学意义

本文综述了我们在青藏高原东缘实施的垂直切过龙门山断裂带宽频带地震探测的研究成果,揭示了研究区复杂的地壳上地幔结构,结果表明松潘-甘孜地块与四川盆地西缘莫霍面深度为58 km与40 km±,在龙门山断裂带下方存在约15 km的莫霍面错断; 松潘-甘孜与龙门山断裂带域地壳纵横波速度比Vp/Vs比值远大于173,预示着粘性下地壳流或基性/超基性物质的存在。探讨了研究区强烈的盆山之间以及深部不同层圈之间的相互作用,推断四川盆地对青藏高原东缘软流圈驱动的物质东向逃逸阻挡作用可能深达整个上地幔。     

拉萨地块麻江地区具有“超钾质”成分的钾质火山岩的识别及成因

一般认为青藏高原拉萨地块后碰撞钾质-超钾质岩浆活动由西向东逐渐喷发,然而本文在拉萨地块中部麻江地区识别出一套钾质火山岩,利用单矿物金云母的40Ar-39Ar方法确定其形成于21.3Ma.这套火山岩具有高镁(＞3％)和高钾( K2O/Na2O ＞2)等的超钾质火山岩成分特征,但其高的MgO含量是因岩石中含有后期蚀变矿物白...     

Crystal-chemistry of clinopyroxenes from potassic and ultrapotassic rocks in central Italy: implications on their genesis

The clinopyroxenes mentioned have been investigated by single crystal X-ray diffraction combined with electron microprobe analysis. The aim of this study was to characterize the crystal-chemical variations of clinopyroxenes in order to delineate the intracrystalline constraints which are characteristic of specific magmatic environments. Clinopyroxenes (cpx) crystallized from peralkaline ultrapotassic melt with kamafugitic and lamproitic affinities are characterized by high Si contents, which are insensitive to variations in silica abundance and silica saturation of the melt. The high Si occupancy in clinopyroxenes from kamafugitic magma is coupled to large M1 (i.e. Mg and Fe²⁺) and M2 (high Ca occupancy) sites, whereas in clinopyroxenes from magmas with lamproitic affinity, high Si content is combined with large M1 but small M2 sites. Clinopyroxenes from Romantype alkaline potassic and ultrapostassic rocks are characterized by an expanded tetrahedron (high ^IVA1 content) and small M1 site which is combined with small M2 polyhedron in clinopyroxenes from the potassic rocks and large M2 site in those from the ultrapotassic rocks.     

大型花岗岩基形成和演化的深部动力学过程：滴水构造、钾质火山作用与地表地质过程

以美国内华达山脉复合岩基为例,系统评述了与大型花岗岩基的形成、演化相关的深部地球动力学过程及构造地貌学响应。在大陆岛弧环境下,基性岩浆的底侵作用促使下地壳发生角闪岩脱水部分熔融,在岩基根部形成高密度的石榴辉石岩,岩基根部最终发生重力失稳,形成滴水构造;在地貌上反映为滴水构造对应区域的沉降和相应的张性构造,在岩浆作用上则表现为软流圈地幔上涌和残余富集岩石圈地幔的低程度部分熔融,形成钾质火山岩。这种高度动态的深部动力学过程是维持大型花岗岩基地区较高高程或促使这些区域高程骤然增加的重要因素。