Geochemistry of southern Pagan Island lavas,Mariana arc: the role of subduction zone processes

New major and trace element abundances, and Pb, Sr, and Nd isotopic ratios of Quaternary lavas from two adjacent volcanoes (South Pagan and the Central Volcanic Region, or CVR) located on Pagan Island allow us to investigate the mantle source (i.e., slab components) and melting dynamics within the Mariana intra-oceanic arc. Geologic mapping reveals a pre-caldera (780–9.4 ka) and post-caldera (<9.4 ka) eruptive stage for South Pagan, whereas the eruptive history of the older CVR is poorly constrained. Crystal fractionation and magma mixing were important crustal processes for lavas from both volcanoes. Geochemical and isotopic variations indicate that South Pagan and CVR lavas, and lavas from the northern volcano on the island, Mt. Pagan, originated from compositionally distinct parental magmas due to variations in slab contributions (sediment and aqueous fluid) to the mantle wedge and the extent of mantle partial melting. A mixing model based on Pb and Nd isotopic ratios suggests that the average amount of sediment in the source of CVR (~2.1%) and South Pagan (~1.8%) lavas is slightly higher than Mt. Pagan (~1.4%) lavas. These estimates span the range of sediment-poor Guguan (~1.3%) and sediment-rich Agrigan (~2.0%) lavas for the Mariana arc. Melt modeling demonstrates that the saucer-shaped normalized rare earth element (REE) patterns observed in Pagan lavas can arise from partial melting of a mixed source of depleted mantle and enriched sediment, and do not require amphibole interaction or fractionation to depress the middle REE abundances of the lavas. The modeled degree of mantle partial melting for Agrigan (2–5%), Pagan (3–7%), and Guguan (9–15%) lavas correlates with indicators of fluid addition (e.g., Ba/Th). This relationship suggests that the fluid flux to the mantle wedge is the dominant control on the extent of partial melting beneath Mariana arc volcanoes. A decrease in the amount of fluid addition (lower Ba/Th) and extent of melting (higher Sm/Yb), and an increase in the sediment contribution (higher Th/Nb, La/Sm, and Pb isotopic ratios) from Mt. Pagan to South Pagan could reflect systematic cross-arc or irregular along-arc melting variations. These observations indicate that the length scale of compositional heterogeneity in the mantle wedge beneath Mariana arc volcanoes is small (~10 km).     

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.     

Geochemistry and origin of volcanic rocks of the Andes (26°–28°S)

Mesozoic to Recent volcanic rocks from a transect of the Central Andes between latitudes 26 ° and 28 ° South in northern Chile and Argentina show chemical and temporal zonation with respect to the Peru-Chile trench. Jurassic to Eocene lavas occur closer to the trench and are comparable to calc-alkaline rocks of island arcs. Eastwards they are followed by Miocene to Quaternary sequences of typical continental margin calc-alkaline rocks which have higher contents of K, Rb, Sr, Ba, Zr, and REE and also higher K/Na and La/Yb ratios. The rocks occurring farthest from the trench have shoshonitic affinities. The distribution of major and trace elements is consistent with a model in which magmas were derived by anatexis of an upper mantle source already enriched in LILE and located above the descending oceanic slab. It is suggested that the chemical variations across the volcanic belt reflect systematic changes in the composition of the magmas due to a decreasing degree of partial melting with increasing depth, and probably also due to the heterogeneity of the source materials.     

Mineralogical and geochemical study of granular xenoliths from the Alban Hills volcano,Central Italy: bearing on evolutionary processes in potassic magma chambers

Granular xenoliths (ejecta) from pyroclastic deposits emplaced during the latest stages of activity of the Alban Hills volcano range from ultramafic to salic. Ultramafic types consist of various proportions of olivine, spinel, clinopyroxene and phlogopite. They show low SiO₂, alkalies and incompatible element abundances and very high MgO. However, Cr, Co and Sc are anomalously low, at a few ppm level. Olivine is highly magnesian (up to Fo%=96) and has rather high CaO (1% Ca) and very low Ni (around a few tens ppm) contents. These characteristics indicate a genesis of ultramafic ejecta by thermal metamorphism of a siliceous dolomitic limestone, probably with input of chemical components from potassic magma. The other xenoliths have textures and compositional characteristics which indicate that they represent either intrusive equivalents of lavas or cumulates crystallized from variably evolved ultrapotassic magmas. One sample of the former group has major element composition resembling ultrapotassic rocks with kamafugitic affinity. Some cumulitic rocks have exceedingly high abundances of Th (81–84 ppm) and light rare-earth elements (LREE) (La+Ce=421–498 ppm) and extreme REE fractionation (La/Yb=288–1393), not justified by their modal mineralogy which is dominated by sanidine, leucite and nepheline. Finegrained phases are dispersed through the fractures and within the interstices of the main minerals. Semiquantitative EDS analyses show that Th and LREE occur at concentration levels of several tens of percent in these phases, indicating that their presence is responsible for the high concentration of incompatible trace elements in the whole rocks. The interstitial position of these phases and their association with fluorite support a secondary origin by deposition from fluorine-rich fluids separated from a highly evolved potassic liquid. The Nd isotopic ratios of the cjecta range from 0.51182 to 0.51217. ⁸⁷Sr/⁸⁶Sr ratios range from 0.70900 to 0.71036. With the exception of one sample, these values are lower than those of the outcropping lavas, which cluster around 0.7105±3. This indicates either the occurrence of several isotopically distinct potassic magmas or a variable interaction between magmas and wall rocks. However, this latter hypothesis requires selective assimilation of host rocks in order to explain isotopic and geochemical characteristics of lavas and xenoliths. The new data indicate that the evolutionary processes in the potassic magmas of the Alban Hills were much more complex than envisaged by previous studies. Interaction of magmas with wall rocks may be an important process during magmatic evolution. Element migration by gaseous transfer, often invoked but rarely constrained by sound data, is shown to have occurred during the latest stages of magmatic evolution. Such a process was able to produce selective enrichment of Th, U, LREE and, to a minor degree, Ta and Hf in the wall rocks of potassic magma chamber. Finally, the occurrence of xenoliths with kamafugitic composition points to the existence of this type of ultrapotassic magma at the Alban Hills.     

Petrogenetic significance of rare-earth element behavior in the basement rocks of southern Obudu Plateau, Bamenda Massif, southeastern Nigeria

Rock samples representing various igneous and metamorphic rocks of southern Obudu Plateau were analyzed for rare-earth element ( REE ) behavior by ICP-MS. Results of the analyses indicate a range of REE abundances and distinctive patterns from highly fraetionated patterns with negative Eu anomalies in granitic rocks to relatively low abundances and less REE fractionated flat patterns with little Eu anomaly in some paragneisses, schists, enderbites and dolerites to unfractionated patterns with positive Eu anomalies in some paragneisses and charnockites. Over all, there are low to high ∑ REE contents with negative to positive Eu anomalies. The ratios of different parameters, especially La/Yb and Ce/Yb, show behaviors consistent with crustal to mantle derivation. The heterogeneity of REE abundances and REE patterns reflects mantle to crustal petrogenetic variations of different rock suites on the Plateau. The LREE content is higher than the HREE content in the highly differentiated rocks, as evidenced by their La/Yb,Ce/Yb and La/Sm ratios, which are normally higher in residual products than in primary melts. The dominantly intermediate nature of the source rock of the orthogneisses is suggested by the generally low ∑ REE. The granites enriched in LREE and depleted in HREE and some of the charnockites with negative Eu anomalies were probably formed by partial melting and crystallization.     

Geochemistry of tholeiites from Lanai,Hawaii

Lanai is the third smallest of the fifteen principal subaerial shield volcanoes of the Hawaiian hotspot. This volcano apparently became extinct during the shield-building stage of volcanism, as shown by the absence of both alkalic cap and post-erosional lavas. Major and trace element analyses of 22 new samples collected primarily from 3 stratigraphic sections show that Lanai tholeiites span a large range in composition. Some Lanai lavas are unique geochemically among Hawaiian tholeiites in having the lowest abundances of incompatible trace elements of any Hawaiian lavas and well-developed positive Eu anomalies. The geochemical characteristics of these low-abundance Lanai tholeiites are not the result of alteration, differences in mantle source modal mineralogy, the presence of residual accessory mantle phases or fractional crystallization of such phases, assimilation of depleted [MORB] wall-rock, or accumulation/resorption of phenocrysts or xenocrysts. Incompatible trace element ratios (e.g., Nb/La, Nb/Th, La/Th, La/Hf, Ce/Pb) in Lanai tholeiites span considerable ranges and form coherent trends with each other and with absolute abundances of these elements. Large variations in La/Sm, La/Yb, and absolute REE abundances at constant MgO suggest that Lanai tholeiites formed by variable amounts of partial melting. However, large ranges in incompatible element ratios cannot be explained solely by variations in partial melting of a geochemically homogeneous source, but must reflect geochemical heterogeneities in the Lanai source. Partial melting modeling indicates that the mixed Lanai source is probably LREE-enriched [i.e., (La/Yb)_CN>1]. One component in the Lanai source, exemplified by the low-abundance tholeiites, has markedly lower REE/HFSE, Th/HFSE, alkali/HFSE, and Ce/Pb ratios than other Lanai or Hawaiian tholeiites and may indicate the presence of recycled residual subduction zone materials in the Hawaiian plume source. The positive Eu anomalies that characterize the low-abundance Lanai tholeiites are not the result of plagioclase accumulation or assimilation but are a feature of this source component. Progressive temporal geochemical variations in Lanai tholeiites from 2 stratigraphic sections indicate that the source composition of these lavas probably evolved over time. This change could have resulted from a progressive decrease in the extent of partial melting of the Lanai source. The compositional variability of Lanai tholeiites suggests that geochemical heterogeneities in their source are larger than the scale of partial melting. Lanai tholeiites could not have formed by smaller degrees of partial melting of plume material than did the larger-volume Hawaiian shields. Therefore, volume differences between Hawaiian shields must be controlled primarily by differences in the volume of supplied plume material rather than by differences in the degree of partial melting. The premature cessation of eruptive activity at Lanai may be attributed to relatively large degrees of partial melting of a small plume.     

Spatial variations in the geochemistry of quaternary lavas across the Sunda arc in Java and Bali

Since Mesozoic time, Java and Bali have formed part of an evolving system of island arcs comprising the Sunda arc of Indonesia. The present tectonic setting is relatively simple with subduction occurring at the Java Trench to the south. A north-dipping Benioff seismic zone delineates an underthrust lithospheric slab to depths of approximately 600 km beneath the Java Sea. Quaternary lavas of the normal island arc association range from tholeiites to high-K calc-alkaline lavas over Benioff zone depths from 120–250 km, respectively. More abundant calc-alkaline lavas lie between these extremes. High-K alkaline lavas are found over Benioff zone depths in excess of 300 km.Both within and between these groups of rocks there are consistent spatial variations in the observed geochemistry. For approximately 200 rocks, incompatible elements such as K, Rb, Cs, Sr, Ba, light REE, U and Th show an increase in abundance of almost an order of magnitude with increasing depth to the seismic zone. Abundances of compatible elements show little consistent variation and trace elements such as Ni, Co, Cr, and Sc are characteristically depleted except in some of the alkaline lavas. Major element abundances in rocks of the normal island arc association show little variation, except for K and P, which both increase in abundance across the arc and Al, which shows a relative decrease.The major and trace element data are inconsistent with the derivation of the analyzed rocks by partial melting of the crustal component of the subducted lithosphere. On the other hand, low Ni abundances (20 ppm) in the basalts suggest that most of the lavas are fractionated and few if any represent primary mantle-derived melts. The spatial variations in the geochemistry of erupted lavas across Java and Bali are best explained by a combination of two processes: melting of a geochemically zoned mantle source and smaller degrees of partial melting of that material at progressively greater depths. Primary tholeiitic magmas could be formed by 20–25% melting at depths of 30–40 km, primary high-K calc-alkaline magmas by 5–15% melting at 40–60 km depth, and primary alkaline magmas by 5% melting at depths of 80–90 km. The geochemical zoning in the mantle, which is also manifested by increasing ⁸⁷Sr/⁸⁶Sr ratios in lavas across the arc, is interpreted to result from the addition of a small melt fraction derived from the crustal component of the subducted lithosphere.     

沂沭断裂带高桥盆地早白垩世火山岩的地球化学及岩石成因

沂沭断裂带内高桥盆地早白垩世火山岩SiO₂含量为51.97%～68.94%;由玄武粗安岩、 粗面岩和流纹岩组成;都属于碱性岩。岩石富集Rb、 Ba、 K等大离子亲石元素和轻稀土元素;相对亏损Nb、 Ta、 Ti等高场强元素和重稀土元素;并具有富集的Sr-Nd-Pb同位素组成。钾质粗面岩具有高稀土Cr、 Ni含量、 La/Yb、Sr/Y和Th/U高比值;这类似于华北克拉通东南缘早白垩世富集岩石圈来源的基性岩（如方城玄武岩、 沂南辉长岩）;其可能主要来源于富集岩石圈地幔部分熔融。与之相比;钠质玄武粗安岩具有低Cr、 Ni含量 、 ⁸⁷Sr/⁸⁶Sr（t）、 Th/U和高ε_Nd（t）值;表明它可能由岩石圈地幔熔体与软流圈物质混合而成。沂沭断裂带高桥盆地火山岩形成于岩石圈伸展背景下;沂沭断裂带的活动可能诱使软流圈物质的上涌;导致岩石圈地幔升温发生部分熔融;并为软流圈物质的上升提供了通道。     

地中海东部地幔演化过程:来自希腊Lesvos岛中新世钾质岩浆作用的启示

希腊Lesvos岛广泛出露的中新世钾质岩浆岩为深入探讨地中海东部地区新生代构造演化和深部过程提供了宝贵机会。本文对Lesvos岛碰撞后钾质岩石开展了系统的矿物电子探针分析、锆石U-Pb定年和Hf同位素分析及全岩地球化学分析。研究结果表明：Lesvos岛出露的碰撞后火山岩主要由玄武粗安岩、玄武安山岩和粗面岩组成,锆石U-Pb年龄为17.2Ma。三类岩石均表现出相对富集大离子亲石元素以及相对亏损高场强元素的地球化学特征。玄武粗安岩和玄武安山岩样品具有相似且富集的Sr-Nd-Pb同位素组成,其较低的Dy/Yb和整体较高且变化范围较大的Ba/La、Zr/Hf比值表明这两类钾质基性岩石起源于受到俯冲陆源沉积物起源流体/熔体交代的尖晶石相岩石圈地幔的低度部分熔融。相比之下,同时代粗面岩具有更亏损的Sr-Nd同位素组成,其均匀的同岩浆锆石Hf同位素组成排除了在岩浆演化过程中发生显著地壳混染或者岩浆混合作用的可能。而较高的Y和Yb含量、相对较低的Sr/Y和（La/Yb）_N比值则表明这种粗面质岩浆起源于中-上地壳的部分熔融。考虑到地震层析结果和Pelagonia陆块与Apulian陆块初始碰撞的时间,研究区各类碰撞后钾质岩浆活动可能反映了深俯冲的Pindos洋壳在中新世发生了板片断离过程。     

Partial melting of a phlogopite-clinopyroxenite nodule from south-west Uganda: an experimental study bearing on the origin of highly potassic continental rift volcanics

Melting experiments on a mantle-derived nodule assemblage consisting of clinopyroxene, phlogopite and minor titanomagnetite, sphene and apatite have been done at 20 and 30 kbar between 1,175 and 1,300° C. The nodule composition was selected on the basis of modal and chemical analyses of 84 mantle derived nodules with metasomatic textures from the Katwe-Kikorongo and Bunyaruguru volcanic fields of south-west Uganda. At 30 kbar, 1,225 and 1,250° C, representing 20–30% partial melting, the compositions of glasses compare favourably to those of the average composition of 26 high potassic mafic lavas from the same region. Glasses produced by sufficiently low degrees of partial melting at 20 kbar could not be analysed. Glass compositions obtained for 20–30% melting at 30 kbar have high K₂O (3.07–5.05 wt.%), low SiO₂ (35.0–39.2 wt.%), high K/K + Na (0.54–0.71), K + Na/Al (0.99–1.08) and Mg/ Mg + Fe_T of 0.59–0.62. These results support the suggestion of Lloyd and Bailey (1975) that the nodules represent the source material for the high K-rich lavas of south-west Uganda. If this conclusion is correct it implies that anomalous mantle source of phlogopite clinopyroxenite composition could produced the Ugandan lavas by relatively higher degrees of partial melting than that normally considered for highly alkaline mafic magmas derived from a pyrolitic mantle source. Higher degrees of melting are considered likely from such a different source region, rich in alkalis, water and radioactive elements. Steeper geotherms and increased fluxing of sub-rift mantle by degassing would also produce higher degrees of partial melting.     

Geochemistry of an ophiolitic complex from New Caledonia

The ophiolites of New Caledonia are composed of ultramafics overlain by mafic rocks, all of which were affected by low P metamorphism. The mafic rocks studied (gabbroic cumulates, and basaltic flows and dikes) from Montagne des Sources are similar to recent mid-ocean ridge rocks. They are olivine-normative with Mg/Mg+Fe²⁺ ratios ranging from 0.69 in lavas to 0.90 in gabbroic cumulates and show tholeiitic fractionation trends such as a negative correlation of Ti and V with the Mg/Fe ratio. The lavas have a flat REE pattern with a slight depletion of light REE and a La/Yb ratio <2. The dikes have three different types of REE patterns. The first type is nearly parallel to that of lavas, the second one is enriched in LREE (La/Yb4) and the third type with the lowest REE contents and a distinct LREE depletion is similar to that of cumulitic pyroxene gabbro. The variations in chemical compositions of the mafic rocks can be accounted for by the dynamic partial melting process of Langmuir et al. (1977). In agreement with structural and tectonic observations, the geochemical data suggests that the ophiolites were formed during the spreading of a mid-ocean ridge with a spreading half-rate of about 1 cm/ year.     

Accessory phases and the generation of LREE-Enriched basalts — A test for disequilibrium melting

The REE contents of tholeiitic rocks can be derived by simple mantle melting models. However this type of model has been less successful in accounting for rocks of the alkali basalt-kimberlite suite with strong LREE enrichment. Rocks from these associations have similar La/Sm ratios over a range of total REE concentrations and their generation by simple melting models requires an enriched source. These requirements conflict with the conclusions reached from Nd-Sm studies.An alternative hypothesis involves the disequilibrium melting of a LREE-rich accessory phase in the mantle. Such a phase will be among the first to melt and thus will dominate the REE patterns of early-formed liquids, even if subsequent partial re-equilibration occurs. This model could account for liquids with strong LREE enrichment and similar La/Sm ratios at different levels of total REE enrichment by partial melting of a simple chondritic mantle. Since the model predicts the existence of an accessory phase in the mantle with a REE pattern parallel to that found in strongly LREE-enriched magma, it provides a sensitive test for disequilibrium melting.     

Variations in andean andesite compositions and their petrogenetic significance

Three linear zones of active andesite volcanism are present in the Andes — a northern zone (5°N–2°S) in Colombia and Ecuador, a central zone (16°S–28°S) largely in south Peru and north Chile and a southern zone (33°S–52°S) largely in south Chile. The northern zone is characterized by basaltic andesites, the central zone by andesite—dacite lavas and ignimbrites and the southern zone by high-alumina basalts, basaltic andesites and andesites. Shoshonites and volcanic rocks of the alkali basalt—trachyte association occur at scattered localities east of the active volcanic chain,The northern and central volcanic zones are 140 km above an eastward-dipping Benioff zone, while the southern zone lies only 90 km above a Benioff zone. Continental crust is ca. 70 km in thickness below the central zone, but is 30–45 km thick below northern and southern volcanic zones. The correlation between volcanic products and their structural setting is supported by trace element and isotope data. The central zone andesite lavas have higher Si, K, Rb, Sr and Ba, and higher initial Sr isotope ratios than the northern or southern zone lavas. The southern zone high-alumina basalts have lower Ce/Yb ratios than volcanics from the other zones. In addition, the central zone andesite lavas show a well-defined eastward increase in K, Rb and Ba and a decrease in Sr.Andean andesite magmas are a result of a complex interplay of partial melting, fractional crystallization and “contamination” processes at mantle depths, and contamination and fractional crystallization in the crust. Variations in andesite composition across the central Andean chain reflect a diminishing degree of partial melting or an increase in fractional crystallization or an increase in “contamination” passing eastwards. Variations along the Andean chain indicate a significant crustal contribution for andesites in the central zone, and indicate that the high-alumina basalts and basaltic andesites of the southern zone are from a shallower mantle source region than other volcanic rocks. The dacite-rhyolite ignimbrites of the central zone share a common source with the andesites and might result from fractional crystallization of andesite magma during uprise through thick continental crust. The occurrence of shoshonites and alkali basalts eat of the active volcanic chain is attributed to partial melting of mantle peridotite distant from the subduction zone.     

云南宾川地区峨眉山玄武岩地球化学特征:岩石类型及随时间演化规律

分布于云南宾川地区、厚逾5000m的晚二叠世峨眉山玄武岩以拉斑玄武岩为主,少量碱性玄武岩。根据它们的岩相学和主量元素、微量元素特征,将其划分为两个地球化学类型:低钛玄武岩(LT)和高钛玄武岩(HT)。低钛玄武岩主要分布于岩石剖面的中下部,其主要地球化学标志为低Ti/Y(<500),高Mg^#(048～067),低的∑REE(<120μg/g)、(La/Yb)_N(<6)和(Sm/Yb)_N(<25)。高钛玄武岩分布于剖面的顶部,以高Ti/Y(>500),低Mg^#(039～053)、高的∑REE(>150)、(La/Yb)_N(>9)和(Sm/Yb)_N(>3)为特征。根据LT的分异指数和Th、U异常特征等,将其分为LT1和LT2两个亚类。其中LT1位于岩石剖面下部,表现为明显的Th和U正异常,高Mg^#(061～067)和低Nb/U比值等。LT2位于岩石剖面中部,具Rb、Ba正异常,无Th和U异常和较低的分异指数(Mg^#=048～054)。LT玄武岩可能是峨眉地幔热柱主活动期地幔柱头部熔融的产物,后经较弱的橄榄石+单斜辉石±斜长石结晶分异形成。剖面最底部的LT1玄武岩显示较强的“壳源”印记,可能与岩石圈地幔中富集组份的活化有关。这些壳源物质的参与程度自底部向上有降低的趋势。晚期HT玄武岩为地幔热柱消亡期的产物,其在地壳浅部经历强烈的以斜长石为主的结晶分异,壳源物质的混染不明显。     

The Bangong Lake ophiolite (NW Tibet) and its bearing on the tectonic evolution of the Bangong–Nujiang suture zone

The Jurassic Bangong Lake ophiolite, NW Tibet, is a key element within the western part of the Bangong–Nujiang suture zone, which marks the boundary between the Lhasa and Qiangtang blocks. It is a tectonic mélange consisting of numerous blocks of peridotite, mafic lavas and dikes. The mantle peridotites include both clinopyroxene-bearing and clinopyroxene-free harzburgites. The Cpx-bearing harzburgite contains Al-rich spinel with low Cr#s (20–25), resembling peridotites formed in mid-ocean ridge settings. On the other hand, the Cpx-free harzburgite is highly depleted with Cr-rich spinel (Cr# = 69–73), typical of peridotites formed in subduction zone environments. Mafic rocks include lavas of N-MORB and E-MORB affinity and boninites. The N-MORB rocks consist of pillow lavas and mafic dikes, whereas the E-MORB rocks are brecciated basalts. The boninites have high SiO₂ (53.2–57.9 wt%), MgO (6.5–12.5 wt%), Cr (166–752 ppm) and Ni (63–213 ppm) and low TiO₂ (0.22–0.37 wt%) and Y (5.34–8.10 ppm), and are characterized by having U-shaped, chondrite-normalized REE patterns. The N-MORB and E-MORB lavas probably formed by different degrees of partial melting of primitive mantle, whereas the boninites reflect partial melting of depleted peridotite in a suprasubduction zone environment. The geochemistry of the ophiolite suggests that it is a fragment of oceanic lithosphere formed originally at a mid-ocean ridge (MOR) and then trapped above an intraoceanic subduction zone (SSZ), where the mantle peridotites were modified by boninitic melts. The Bangong–Nujiang suture zone is believed to mark the boundary between two blocks within Gondwanaland rather than to separate Gondwanaland from Eurasia.     

Chemical discontinuities in Archean metavolcanic terrains and the development of Archean crust

Most large Archean greenstone belts ( 2.7 Ga), comprise thick (12–15 km) mafic to felsic metavolcanics sequences which exhibit consistent but discontinuous geochemical patterns resulting from mantle-crust processes. In a typical Archean metavolcanic sequence, thick (5–8 km) uniform tholeiitic basalt is followed by geochemically evolved rock units (4–7 km thick) containing intermediate and felsic calc-alkaline rocks. This major geochemical discontinuity is marked by a change from LIL-element depleted basalts which show unfractionated REE abundance patterns, to overlying andesites with higher LIL-element contents, fractionated REE patterns and relatively depleted HREE. A less well marked discontinuity separates andesitic rocks from still later more felsic dacite-rhyolite extrusive assemblages and their intrusive equivalents, and is identified by a further increase in LIL element content and REE fractionation. The major geochemical discontinuity apparently separates rocks derived by partial melting of mantle (either directly or through shallow fractionation processes) from those which originated either by partial melting of mantle material modified by crustal interactions or by partial melting of crustal material.We suggest that accumulation of a great thickness of mantle derived volcanic rocks can lead to sagging and interaction of the lower parts of the volcanic piles with upper mantle material. The resulting modified mantle acts as a source for some of the geochemically evolved rocks observed in volcanic successions. Subsequent direct melting of the volcanic pile produces the felsic magmas observed in the upper parts of Archean volcanic successions. This process, termed sag-subduction, is the inferred tectonic process operating in the comparatively thin, hot Archean crustal regime. By this process, large masses of ultimately mantle-derived material were added to the crust.     

五莲七宝山地区早白垩世碱性侵入岩成因及其地质意义

山东五莲七宝山地区早白垩世的碱性侵入岩位于火山机构的中央部位,该岩体具有高Ba-Sr含量、高Nb/Ta和Zr/Hf比、低Ti/Eu比等特征,前人的研究指出其起源于岩石圈地幔。然而,该侵入体中的岩性与成分变化所反映的深部动力学过程尚未理清。本文对七宝山二长辉长岩和两类辉石二长岩开展了详细的矿物学和岩石地球化学研究,识别出钠质和钾质两类钾玄质岩石系列。该套碱性中基性侵入岩具有富碱、富轻稀土和富大离子亲石元素的特征,同时具有高的(La/Yb)_N和(Gd/Yb)_N值。碱性侵入岩中两类单斜辉石和两类斜长石作为再循环晶,记录了不同批次岩浆/熔体的混合,这些矿物组分和全岩成分共同约束了岩浆的起源与演化过程。结合前人的地球化学资料,本文指出七宝山碱性侵入岩的源区是曾受到沉积物交代的富集地幔,源区存在金云母脉体和角闪石脉体。上述脉体连同周围的地幔橄榄岩共同发生部分熔融,形成原生的碱性熔体。七宝山碱性侵入岩显示高的Nb/Ta和Zr/Hf比、低的Ti/Eu比,同时在微量元素蜘蛛图上呈现Ti^*和Hf^*的负异常,结合高稀土单斜辉石平衡熔体的属性,共同指示了碳酸盐熔体组分对该套碱性侵入岩的形成发挥了重要作用。钠质系列与钾质系列岩石反映了源区富碱矿物相类型相对贡献量的差异,即钠质为主的碱性岩反映源区角闪石的贡献更大,而钾质为主的碱性岩反映源区金云母的贡献占优势。此外,碱性侵入岩中的钾质系列具有异常高的Rb-Zr-Hf-U含量,很可能反映了源区在部分熔融过程中热液锆石熔解后形成的熔体加入到了钾质岩浆房内。本研究强调了碳酸盐熔体组分对高Nb/Ta碱性中基性的形成发挥着重要作用,亦强调了热液锆石的熔解加入导致岩浆具有高Zr-Hf-U含量的特征。     

Geochemistry and origin of massif-type anorthosites

Samples of Proterozoic anorthosite complexes from the Adirondack Mountains of New York, Burwash Area of Ontario, and the Nain Complex of Labrador, ranging in composition from anorthosite to anorthositic gabbro, have been analyzed for major elements, Rb, Sr, Ba and nine rare-earth elements (REE), in order to set limits on the compositions and origins of their parent magmas. Similar rock types from the different areas have similar major and trace element compositions. The anorthosites have high Sr/Ba ratios, low REE abundances (Ce about 10, Yb about 0.5–1.5 times chondrites) and large positive Eu anomalies. The associated anorthositic gabbros have lower Sr/Ba ratios, REE abundances nearly an order of magnitude higher than the anorthosites, and small to negligible positive Eu anomalies.Model calculations using the adcumulate rocks with the lowest REE abundances and published distribution coefficients yield parent liquids having REE abundances and patterns similar to those of the associated anorthositic gabbros with the highest REE abundances. Rocks with intermediate REE abundances are the result of incorporation of a liquid component by a plagioclase-rich cumulate similar to the adcumulate samples. The analytical data and model calculations both suggest parent liquids having compositions of 50–54% SiO₂, greater than 20% Al₂O₃, about 1% K₂O, atomic Mg/(Mg+Fe²⁺) ratios (Mg No.'s) of less than 0.4, 15–30 ppm Rb, 400–600 ppm Sr and 400–600 ppm Ba, 40–50 times chondrites for Ce and 8–10 times chondrites for Yb.The low atomic Mg/(Mg+Fe²⁺) values for these rocks combined with geophysical evidence suggesting there are not large quantities of ferromagnesian material at depth, indicate that the anorthositic masses are not products of fractional crystallization of mafic melt derived from melting of the mantle. Rather, it is suggested that they are a result of partial melting of tholeiitic compositions at depths shallower than the basalt-eclogite transformation, leaving a pyroxene-dominated residue.     

Late Neoarchean volcanic rocks in the southern Liaoning Terrane and their tectonic implications for the formation of the eastern North China Craton

The late Neoarchean metamorphosed volcanic rocks in the southern Liaoning Terrane (SLT) of the eastern North China Craton (NCC) are mainly composed of amphibolites and felsic gneisses and can be chemically classified as basalt (Group#1), basaltic andesite (Group#2), dacite (Group#3) and rhyodacite (Group#4). LA-ICP-MS zircon U–Th–Pb dating reveals that they formed at ~2.53–2.51 ​Ga. Group#1 samples are characterized by approximately flat chondrite-normalized rare earth element (REE) patterns with low (La/Yb)_N ratios and a narrow range of (Hf/Sm)_N ratios, and their magmatic precursors were generated by partial melting of a depleted mantle wedge weakly metasomatized by subducted slab fluids. Compared to Group#1 samples, Group#2 samples display strongly fractionated REE patterns with higher (La/Yb)_N ratios and more scattered (Hf/Sm)_N ratios, indicative of a depleted mantle wedge that had been intensely metasomatized by slab-derived melts and fluids. Group#3 samples are characterized by high MgO and transition trace element concentrations and fractionated REE patterns, which resemble typical high-Si adakites, and the magmatic precursors were derived from partial melting of a subducted oceanic slab. Group#4 samples have the highest SiO₂ and the lowest MgO and transition trace element contents, and were derived from partial melting of basaltic rocks at lower crust levels. Integrating these tholeiitic to calc-alkaline volcanic rocks with the mass of contemporaneous dioritic-tonalitic-trondhjemitic-granodioritic gneisses, the late Neoarchean volcanic rocks in the SLT were most likely produced in an active continental margin. Furthermore, the affinities in lithological assemblages, metamorphism and tectonic regime among SLT, eastern Hebei to western Liaoning Terrane (EH–WLT), northern Liaoning to southern Jilin Terrane (NL–SJT), Anshan-Benxi continental nucleus (ABN) and Yishui complex (YSC) collectively indicate that an integral and much larger continental block had been formed in the late Neoarchean and the craton-scale lateral accretion was a dominantly geodynamic mechanism in the eastern NCC.     

Platinum-group elemental geochemistry of mafic and ultramafic rocks from the Xigaze ophiolite, southern Tibet

The Xigaze ophiolite in the central part of the Yarlung–Zangbo suture zone, southern Tibet, has a well-preserved sequence of sheeted dykes, basalts, cumulates and mantle peridotites at Jiding and Luqu. Both the basalts and diabases at Jiding have similar compositions with SiO₂ ranging from 45.9 to 53.5 wt%, MgO from 3.1 to 6.8 wt% and TiO₂ from 0.87 to 1.21 wt%. Their Mg^#s [100Mg/(Mg + Fe)] range from 40 to 60, indicating crystallization from relatively evolved magmas. They have LREE-depleted, chondrite-normalized REE diagrams, suggesting a depleted mantle source. These basaltic rocks have slightly negative Nb- and Ti-anomalies, suggesting that the Xigaze ophiolite represents a fragment of mature MORB lithosphere modified in a suprasubduction zone environment. The mantle peridotites at Luqu are high depleted with low CaO (0.3–1.2 wt%) and Al₂O₃ (0.04–0.42 wt%). They display V-shaped, chondrite-normalized REE patterns with (La/Gd)_N ratios ranging from 3.17 to 64.6 and (Gd/Yb)_N from 0.02 to 0.20, features reflecting secondary metasomatism by melts derived from the underlying subducted slab. Thus, the geochemistry of both the basaltic rocks and mantle peridotites suggests that the Xigaze ophiolite formed in a suprasubduction zone.Both the diabases and basalts have Pd/Ir ratios ranging from 7 to 77, similar to MORB. However, they have very low PGE abundances, closely approximating the predicted concentration in a silicate melt that has fully equilibrated with a fractionated immiscible sulfide melt, indicating that the rocks originated from magmas that were S-saturated before eruption. Moderate degrees of partial melting and early precipitation of PGE alloys explain their high Pd/Ir ratios and negative Pt-anomalies. The mantle peridotites contain variable amounts of Pd (5.99–13.5 ppb) and Pt (7.92–20.5 ppb), and have a relatively narrow range of Ir (3.47–5.01 ppb). In the mantle-normalized Ni, PGE, Au and Cu diagram, they are relatively rich in Pd and depleted in Cu. There is a positive correlation between CaO and Pd. The Pd enrichment is possibly due to secondary enrichment by metasomatism. Al₂O₃ and Hf do not correlate with Ir, but show positive variations with Pt, Pd and Au, indicating that some noble metals can be enriched by metasomatic fluids or melts carrying a little Al and Hf. We propose a model in which the low PGE contents and high Pd/Ir ratios of the basaltic rocks reflect precipitation of sulfides and moderate degrees of partial melting. The high Pd mantle peridotites of Xigaze ophiolites were formed by secondary metasomatism by a boninitic melt above a subduction zone.