Accreted fragments of the Late Cretaceous Caribbean–Colombian Plateau in Ecuador

The eastern part of the Western Cordillera of Ecuador includes fragments of an Early Cretaceous (≈123 Ma) oceanic plateau accreted around 85–80 Ma (San Juan–unit). West of this unit and in fault contact with it, another oceanic plateau sequence (Guaranda unit) is marked by the occurrence of picrites, ankaramites, basalts, dolerites and shallow level gabbros. A comparable unit is also exposed in northwestern coastal Ecuador (Pedernales unit).

Picrites have LREE-depleted patterns, high Nd_i and very low Pb isotopic ratios, suggesting that they were derived from an extremely depleted source. In contrast, the ankaramites and Mg-rich basalts are LREE-enriched and have radiogenic Pb isotopic compositions similar to the Galápagos HIMU component; their Nd_i are slightly lower than those of the picrites. Basalts, dolerites and gabbros differ from the picrites and ankaramites by flat rare earth element (REE) patterns and lower Nd; their Pb isotopic compositions are intermediate between those of the picrites and ankaramites. The ankaramites, Mg-rich basalts, and picrites differ from the lavas from the San Juan–Multitud Unit by higher Pb ratios and lower Nd_i.

The Ecuadorian and Gorgona 88–86 Ma picrites are geochemically similar. The Ecuadorian ankaramites and Mg-rich basalts share with the 92–86 Ma Mg-rich basalts of the Caribbean–Colombian Oceanic Plateau (CCOP) similar trace element and Nd and Pb isotopic chemistry. This suggests that the Pedernales and Guaranda units belong to the Late Cretaceous CCOP. The geochemical diversity of the Guaranda and Pedernales rocks illustrates the heterogeneity of the CCOP plume source and suggests a multi-stage model for the emplacement of these rocks. Stratigraphic and geological relations strongly suggest that the Guaranda unit was accreted in the late Maastrichtian (≈68–65 Ma).     

Erosion of lithospheric mantle beneath the East African Rift system: geochemical evidence from the Kivu volcanic province

This study presents new major and trace element and Sr–Nd isotopic results for a suite of Miocene–Recent mafic lavas from the Kivu volcanic province in the western branch of the East African Rift. These lavas exhibit a very wide range in chemical and isotopic characteristics, due to a lithospheric mantle source region that is heterogeneous on a small scale, probably <1 km. The chemical and isotopic variations are mostly geographically controlled: lavas from Tshibinda volcano, which lies on a rift border fault on the northwestern margin of the province, have higher values of ⁸⁷Sr/⁸⁶Sr, (La/Sm)_n, Ba/Nb, and Zr/Hf than the majority of Kivu (Bukavu) samples. The range of ⁸⁷Sr/⁸⁶Sr at Tshibinda (0.70511–0.70514) overlaps some compositions found in the neighboring Virunga province, while Bukavu group lavas include the lowest ⁸⁷Sr/⁸⁶Sr (0.70314) and highest _Nd (+7.6) yet measured in western rift lavas. The Tshibinda compositions trend towards a convergence for Sr–Nd–Pb isotopic values among western rift lavas. Among Kivu lavas, variations in ¹⁴³Nd/¹⁴⁴Nd correlate with those for certain incompatible trace element ratios (e.g., Th/Nb, Zr/Hf, La/Nb, Ba/Rb), with Tshibinda samples defining one compositional extreme. There are covariations of isotopic and trace element ratios in mafic lavas of the East African Rift system that vary systematically with geographic location. The lavas represent a magmatic sampling of variations in the underlying continental lithospheric mantle, and it appears that a common lithospheric mantle (CLM) source is present beneath much of the East African Rift system. This source contains minor amphibole and phlogopite, probably due to widespread metasomatic events between 500 and 1000 Ma. Lava suites which do not show a strong component of the CLM source, and for which the chemical constraints also suggest the shallowest magma formation depths, are the Bukavu group lavas from Kivu and basanites from Huri Hills, Kenya. The inferred extent of lithospheric erosion therefore appears to be significant only beneath these two areas, which is generally consistent with lithospheric thickness variations estimated from gravity and seismic studies.     

HIMU型洋岛玄武岩的地球化学特征

    洋岛玄武岩的元素和同位素地球化学特征可以示踪深部地幔的化学结构和化学演化过程。HIMU(Highμ,μ=238U/204Pbt=0) 型玄武岩是一类元素和同位素组成特殊的洋岛玄武岩,被认为与地幔柱中再循环的洋壳物质直接相关,因此,HIMU型玄 武岩的成因是地幔柱（热点） 研究中长期关注的话题。本文概述了HIMU型玄武岩的地球化学定义,对HIMU洋岛的分布、 火山演化阶段以及岩性变化做了综合阐述,并在对比经典HIMU型玄武岩与其他板内玄武岩元素地球化学特征、放射成因 同位素组成以及惰性气体同位素组成特征的基础上,简要探讨其源区组成和成因上的不同。     

The contribution of the young Cretaceous Caribbean Oceanic Plateau to the genesis of late Cretaceous arc magmatism in the Cordillera Occidental of Ecuador

The eastern part of the Cordillera Occidental of Ecuador comprises thick buoyant oceanic plateaus associated with island-arc tholeiites and subduction-related calc-alkaline series, accreted to the Ecuadorian Continental Margin from Late Cretaceous to Eocene times. One of these plateau sequences, the Guaranda Oceanic Plateau is considered as remnant of the Caribbean–Colombian Oceanic Province (CCOP) accreted to the Ecuadorian Margin in the Maastrichtien.Samples studied in this paper were taken from four cross-sections through two arc-sequences in the northern part of the Cordillera Occidental of Ecuador, dated as (Río Cala) or ascribed to (Macuchi) the Late Cretaceous and one arc-like sequence in the Chogòn-Colonche Cordillera (Las Orquídeas). These three island-arcs can clearly be identified and rest conformably on the CCOP.In all four localities, basalts with abundant large clinopyroxene phenocrysts can be found, mimicking a picritic or ankaramitic facies. This mineralogical particularity, although not uncommon in island arc lavas, hints at a contribution of the CCOP in the genesis of these island arc rocks.The complete petrological and geochemical study of these rocks reveals that some have a primitive island-arc nature (MgO values range from 6 to 11 wt.%). Studied samples display marked Nb, Ta and Ti negative anomalies relative to the adjacent elements in the spidergrams characteristic of subduction-related magmatism. These rocks are LREE-enriched and their clinopyroxenes show a tholeiitic affinity (FeO_T–TiO₂ enrichment and CaO depletion from core to rim within a single crystal).The four sampled cross-sections through the island-arc sequences display homogeneous initial Nd, and Pb isotope ratios that suggest a unique mantellic source for these rocks resulting from the mixing of three components: an East-Pacific MORB end-member, an enriched pelagic sediment component, and a HIMU component carried by the CCOP. Indeed, the ankaramite and Mg-basalt sequences that form part of the Caribbean-Colombian Oceanic Plateau are radiogenically enriched in ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb and contain a HIMU component similar to that observed in the Gorgona basalts and Galápagos lavas. The subduction zone that generated the Late Cretaceous arcs occurred far from the continental margin, in an oceanic environment. This implies that no terrigenous detrital sediments interacted with the source at this period. Thus, the enriched component can only result from the melting of subducted pelagic sediments.We have thus defined the East-Pacific MORB, enriched (cherts, pelagic sediments) and HIMU components in an attempt to constrain and model the genesis of the studied island-arc magmatism, using a compilation of carefully selected isotopic data from literature according to rock age and paleogeographic location at the time of arc edification.Tripolar mixing models reveal that proportions of 12–15 wt.% of the HIMU component, 7–15 wt.% of the pelagic sediment end-member and 70–75 wt.% of an East-pacific MORB end-member are needed to explain the measured isotope ratios. These surprisingly high proportions of the HIMU/CCOP component could be explained by the young age of the oceanic plateau (5–15 Ma) during the Late Cretaceous arc emplacement. The CCOP, basement of these arc sequences, was probably still hot and easily assimilated at the island-arc lava source.     

The contribution of the young Cretaceous Caribbean Oceanic Plateau to the genesis of late Cretaceous arc magmatism in the Cordillera Occidental of Ecuador

The eastern part of the Cordillera Occidental of Ecuador comprises thick buoyant oceanic plateaus associated with island-arc tholeiites and subduction-related calc-alkaline series, accreted to the Ecuadorian Continental Margin from Late Cretaceous to Eocene times. One of these plateau sequences, the Guaranda Oceanic Plateau is considered as remnant of the Caribbean–Colombian Oceanic Province (CCOP) accreted to the Ecuadorian Margin in the Maastrichtien.Samples studied in this paper were taken from four cross-sections through two arc-sequences in the northern part of the Cordillera Occidental of Ecuador, dated as (Río Cala) or ascribed to (Macuchi) the Late Cretaceous and one arc-like sequence in the Chogòn-Colonche Cordillera (Las Orquídeas). These three island-arcs can clearly be identified and rest conformably on the CCOP.In all four localities, basalts with abundant large clinopyroxene phenocrysts can be found, mimicking a picritic or ankaramitic facies. This mineralogical particularity, although not uncommon in island arc lavas, hints at a contribution of the CCOP in the genesis of these island arc rocks.The complete petrological and geochemical study of these rocks reveals that some have a primitive island-arc nature (MgO values range from 6 to 11 wt.%). Studied samples display marked Nb, Ta and Ti negative anomalies relative to the adjacent elements in the spidergrams characteristic of subduction-related magmatism. These rocks are LREE-enriched and their clinopyroxenes show a tholeiitic affinity (FeO_T–TiO₂ enrichment and CaO depletion from core to rim within a single crystal).The four sampled cross-sections through the island-arc sequences display homogeneous initial Nd, and Pb isotope ratios that suggest a unique mantellic source for these rocks resulting from the mixing of three components: an East-Pacific MORB end-member, an enriched pelagic sediment component, and a HIMU component carried by the CCOP. Indeed, the ankaramite and Mg-basalt sequences that form part of the Caribbean-Colombian Oceanic Plateau are radiogenically enriched in ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb and contain a HIMU component similar to that observed in the Gorgona basalts and Galápagos lavas. The subduction zone that generated the Late Cretaceous arcs occurred far from the continental margin, in an oceanic environment. This implies that no terrigenous detrital sediments interacted with the source at this period. Thus, the enriched component can only result from the melting of subducted pelagic sediments.We have thus defined the East-Pacific MORB, enriched (cherts, pelagic sediments) and HIMU components in an attempt to constrain and model the genesis of the studied island-arc magmatism, using a compilation of carefully selected isotopic data from literature according to rock age and paleogeographic location at the time of arc edification.Tripolar mixing models reveal that proportions of 12–15 wt.% of the HIMU component, 7–15 wt.% of the pelagic sediment end-member and 70–75 wt.% of an East-pacific MORB end-member are needed to explain the measured isotope ratios. These surprisingly high proportions of the HIMU/CCOP component could be explained by the young age of the oceanic plateau (5–15 Ma) during the Late Cretaceous arc emplacement. The CCOP, basement of these arc sequences, was probably still hot and easily assimilated at the island-arc lava source.     

Trace element evidence for the origin of ocean island basalts: an example from the Austral Islands (French Polynesia)

The Austral Islands, a volcanic chain in the South-Central Pacific Ocean (French Polynesia) are composed mainly of alkali basalts and basanites with subordinate amounts of olivine tholeiites and strongly undersaturated rocks (phonolite foidites and phonolite tephrites). The basaltic rocks have geochemical features typical of oceanic island suites. The distribution of incompatible trace elements indicate that the lavas were derived from a heterogeneous mantle source. The chondrite-normalized patterns of the incompatible elements in basaltic rocks of the Austral Islands are complementary to those of island arc tholeiites. As supported by isotope data, the observed trace element heterogeneities of the source are probably due to mixing of the upper mantle with subducted oceanic crust from which island arc tholeiitic magma was previously extracted.     

A Mid Cretaceous origin for the Galápagos hotspot: volcanological, petrological and geochemical evidence from Costa Rican oceanic crustal segments

The Quepos, Nicoya and Herradura oceanic igneous terranes in Costa Rica are conspicuous features of a Mid to Late Cretaceous regional magmatic event that encompasses similar terranes in Central America, Colombia, Ecuador and the Caribbean. The Quepos terrane (66?Ma), which consists of ol-cpx phyric, tholeiitic pillow lavas overlain by highly vesicular hyaloclastites, breccias and conglomerates, is interpreted as an uplifted seamount/ocean island complex. The Nicoya (～90?Ma) and Herradura terranes consist of fault-bounded sequences of sediments, tholeiitic volcanics (pillow lavas and massive sheet flows) and plutonic rocks. The volcanic rocks were emplaced at relatively high eruption rates in moderate to deep water, possibly forming part of an oceanic plateau. Major and trace element data from Nicoya/Herradura tholeiites indicate higher melting temperatures than inferred for normal mid-ocean-ridge basalts (MORB) and/or a different source composition. Sr–Nd–Pb isotopic ratios from all three terranes are distinct from MORB but resemble those from the Galápagos hotspot. The volcanological, petrological and geochemical data from Costa Rican volcanic terranes, combined with published age data, paleomagnetic results and plate tectonic reconstructions of this region, provide strong evidence for a Mid Cretaceous (～90Ma) age for the Galápagos hotspot, making it one of the oldest known, active hotspots on Earth. Our results also support an origin of the Caribbean Plate through melting of the head of the Galápagos starting plume.     

全球幔源岩Pb-Sr-Nd同位素体系

根据各种同位素数据库得到的3万多个晚古生代以来的幔源岩(包括洋中脊玄武岩、洋岛玄武岩、岛弧火山岩、大陆与大洋溢流玄武岩以及大陆板内玄武岩)Pb-Sr-Nd同位素资料和图解分析,对各类火山岩的源区以及地幔的垂向与横向不均一性问题作了进一步讨论。笔者认为不存在具有公共性质的EM1、EM2和HIMU地幔端员,它们的源区可能来自上、下地幔过渡带,只在局部地区出现,独一无二。PREMA(FOZO)则是洋岛玄武岩和溢流玄武岩公共端员。DUAPAL异常现象不只是在洋中脊玄武岩中出现,在洋岛玄武岩、岛弧火山岩和大洋溢流玄武岩中也存在同步的地球化学分区现象。溢流玄武岩的同位素体系特征表明它们的源区涉及再循环地幔的壳幔混合、岩石圈减压熔融、上—下地幔过渡带和似原始-略亏损的下地幔。Pb同位素体系为鉴别俯冲带的存在提供了更严格的证据,这种鉴别表明,安第斯弧火山作用不是洋陆俯冲带产生的。     

Geochemical diversity in submarine HIMU basalts from Austral Islands, French Polynesia

We present the first report of geochemical data for submarine basalts collected by a manned submersible from Rurutu, Tubuai, and Raivavae in the Austral Islands in the South Pacific, where subaerial basalts exhibit HIMU isotopic signatures with highly radiogenic Pb isotopic compositions. With the exception of one sample from Tubuai, the ⁴⁰Ar/³⁹Ar ages of the submarine basalts show no significant age gaps between the submarine and subaerial basalts, and the major element compositions are indistinguishable at each island. However, the variations in Pb, Sr, Nd, and Hf isotopic compositions in the submarine basalts are much larger than those previously reported in subaerial basalts. The submarine basalts with less-radiogenic Pb and radiogenic Nd and Hf isotopic compositions show systematically lower concentrations in highly incompatible elements than the typical HIMU basalts. These geochemical variations are best explained by a two-component mixing process in which the depleted asthenospheric mantle was entrained by the mantle plume from the HIMU reservoir during its upwelling, and the melts from the HIMU reservoir and depleted asthenospheric mantle were then mixed in various proportions. The present and compiled data demonstrate that the HIMU reservoir has a uniquely low ¹⁷⁶Hf/¹⁷⁷Hf decoupled from ¹⁴³Nd/¹⁴⁴Nd, suggesting that it was derived from an ancient subducted slab. Moreover, the Nd/Hf ratios of the HIMU basalts and curvilinear Nd–Hf isotopic mixing trend require higher Nd/Hf ratios for the melt from the HIMU reservoir than that from the depleted mantle component. Such elevated Nd/Hf ratios could reflect source enrichment by a subducted slab during reservoir formation.     

Geochemical and isotopic investigation of the Laiwu–Zibo carbonatites from western Shandong Province, China, and implications for their petrogenesis and enriched mantle source

Major and trace element and Nd–Sr isotope data of the Mesozoic Laiwu–Zibo carbonatites (LZCs) from western Shandong Province, China, provide clues to the petrogenesis and the nature of their mantle source. The Laiwu–Zibo carbonatites can be petrologically classified as calcio-, magnesio- and ferro-carbonatites. All these carbonatites show a similarity in geochemistry. On the one hand, they are extremely enriched in Ba, Sr and LREE and markedly low in K, Rb and Ti, which are similar to those global carbonatites, on the other hand, they have extremely high initial ⁸⁷Sr/⁸⁶Sr (0.7095–0.7106) and very low _Nd (−18.2 to −14.3), a character completely different from those global carbonatites. The small variations in Sr and Nd isotopic ratios suggest that crustal contamination can not modify the primary isotopic compositions of LZC magmas and those values are representatives of their mantle source. The Nd–Sr isotopic compositions of LZCs and their similarity to those of Mesozoic Fangcheng basalts imply that they derived from an enriched lithospheric mantle. The formation of such enriched lithospheric mantle is connected with the major collision between the North China Craton (NCC) and the Yangtze Craton. Crustal materials from the Yangtze Craton were subducted beneath the NCC and melts derived from the subducted crust of the Yangtze Craton produced an enriched Mesozoic mantle, which is the source for the LZCs and Fangcheng basalts. The absence of alkaline silicate rocks, which are usually associated with carbonatites suggest that the LZCs originated from the mantle by directly partial melting.     

Hf isotope compositions of northern Luzon arc lavas suggest involvement of pelagic sediments in their source

New Hf isotopic compositions for island arc basalts from the Luzon arc (Philippines) define a remarkable sub-horizontal trend in Hf–Nd isotopic space with a small range of _Hf (+5 to +17) associated with a large variation in _Nd (–7 to +8). The data plot above and barely overlap the terrestrial array defined by oceanic basalts and continental crust. Mixing hyperbolas passing through the data intersect fields for depleted mantle and pelagic sediments suggesting that these two components formed the source of the Luzon arc lavas. An exception is the Batan Island where the low _Nd ratios are associated with low _Hf values. A mixing hyperbola fitting the Batan samples suggests that their mantle source was modified by subducted material prior to contamination by terrigenous clays. More generally, the geochemical relationships in Luzon lavas show that the mixing endmembers are source components rather than melts. The relationship between Nd and Hf isotopic compositions in the Luzon volcanics show that the type of sediment subducted under an island arc is a determining factor in the control of the two isotopic systems in island arc environments.Electronic Supplementary Material Supplementary material is available for this article at     

Evolution of Calc-alkaline Magmas in Continental Arc Volcanoes: Evidence from Alicudi, Aeolian Arc (Southern Tyrrhenian Sea, Italy)

Major, trace element, and Sr isotopic data are reported forvolcanic rocks from the island of Alicudi, Aeolian Arc, SouthernTyrrhenian Sea. The island is constructed of basalt, basalticandesite to high-K andesite lavas, and pyroclastites, whichshow a continuum in the variation of many major and trace elements.Total iron, MgO, CaO, Ni, Co, Sc, and Cr decrease with increasingsilica, whereas incompatible elements Rb, Ba, Th, and LREE displaythe opposite tendency. Very significant positive correlationsare defined by incompatible elements on interelemental variationdiagrams. Sr isotopic ratios vary from 0–70352 to 0–70410.Overall, basalts (0–70352–O-70410) and basalticandesltes (0–70356–0–70409) are enriched in⁸⁷Sr compared with high-K andesites (O–70352–O–70367),which display the lowest Sr isotopic ratios within the entireAeolian archipelago. Overall negative relationships exist between⁸⁷Sr/⁸⁶Sr and several incompatible trace element abundancesand ratios, such as Th, U, LREE, Zr, La/Yb, and Th/Hf. Otherelemental ratios such as La/Rb, Ba/Rb, and Sr/Rb show more complexbehaviour, even though negative correlations with Sr isotopicratios are observed in the basalts. The observed compositional variations are best explained interms of a model in which primitive calc-alkaline magmas evolvedby crystal-liquid fractionation to give a series of variouslydifferentiated liquids, which underwent different degrees ofinteraction with crustal material. The more mafic and hotterbasaltic liquids appear to have assimilated higher amounts ofmetamorphic wall rocks than did the cooler late erupted andesiticmagmas. This process produced significant variations of Sr isotopicratios, Rb, Cs, Rb/Sr ratios, and LILE/Rb ratios in mafic magmas,but had only minor effects on the abundances and ratios of otherincompatible elements such as Th, LREE, La/Yb, and Th/Hf. When compared with mafic rocks from other Aeolian islands, theAlicudi basalts are more primitive geochemically and isotopically.Going eastward, there is a decrease in Ni and Cr abundances,mg-number and Nd isotopic ratios which parallels an increaseof Sr isotopic ratios in basaltic rocks along the arc. Thesecompositional variations are typical of volcanic series whichhave undergone interaction with upper-crustal material, andsuggest that this process may have contributed significantlyto the regional geochemical and isotopic trends observed inthe Aeolian arc.     

Geodynamics and sources of preaccretionary magmatism in western Mongolia

The paper reports original isotopic and geochemical data on Early Precambrian lavas in the Ozernaya Zone in Mongolia. According to their normalized trace-element patterns, the rocks are classified into the following groups: (1) rocks similar to N-MORB; (2) rocks similar to E-MORB; (3) basalts enriched in trace elements, with HFSE minima; and (4) basalts depleted in trace elements, with HFSE minima. All of the lava types could be produced in an island arc—backarc basin system. The magmatic rocks of group (1) were likely formed in a spreading backarc basin, and those of group (2) were likely generated within the influ- ence zone of a hotspot or were derived from heterogeneous upper mantle domains. The lavas of group (3) seem to be fragments of an ensimatic, relatively primitive island arc. The basalts and basaltic andesites of group (4) were likely produced by mixing melts of groups (1) and (3). The fact that lavas of groups (1) and (4) sometimes intercalate within a single stratigraphic section suggests that the extension and subduction zones were closely spaced and operated simultaneously. The magmas of groups (1), (2), and (3) were derived from different mantle sources, which possessed different ratios of trace elements and were different in isotopic composition.     

Petrology, geochemistry and geodynamic implications of Jurassic island arc magmatism as revealed by mafic volcanic rocks in the Mesozoic low-grade sequence, eastern Rhodope, Bulgaria

In the eastern Bulgarian Rhodope, mafic extrusive rocks and underlying greenschists are found in the Mesozoic low-grade unit, which represents the northern extension of similar sequences including the Evros ophiolites in Thrace (Greece). Both rock types define a suite of low-Ti tholeiitic basalts to transitional boninitic basaltic andesites and andesites and associated metapyroclastites (greenschists), intruded at its base by diorite dikes of a boninitic affinity. Mafic lavas and greenschists display large ion lithophile element (LILE) enrichment relative to high-field strength elements (HFSE), flat REE patterns of a slight light REE depletion, a strong island arc tholeiite (IAT) and weak MORB-like signature. All these rocks are characterized by negative Nb anomalies ascribed to arc lavas. They have positive Nd_i values in the range of + 4.87 to + 6.09, approaching the lower limit of MORB-like source, and relatively high (²⁰⁷Pb/²⁰⁴Pb)_i (15.57–15.663) at low (²⁰⁶Pb/²⁰⁴Pb)_i (18.13–18.54) ratios. The Nd isotopic compositions coupled with trace element data imply a dominantly depleted MORB-like mantle source and a contribution of subduction modified LILE-enriched component derived from the mantle wedge. The diorite dike has a low Nd_i value of − 2.61 and is slightly more Pb radiogenic (²⁰⁷Pb/²⁰⁴Pb)_i (15.64) and (²⁰⁶Pb/²⁰⁴Pb)_i (18.56), respectively, reflecting crustal contamination. Petrologic and geochemical data indicate that the greenschists and mafic extrusive rocks represent a magmatic assemblage formed in an island arc setting. The magmatic suite is interpreted as representing an island arc–accretionary complex related to the southward subduction of the Meliata–Maliac ocean under the supra-subduction back-arc Vardar ocean/island arc system. Magmatic activity appears to have initiated in the north during the inception of the island arc system by the Early–Middle Jurassic time in the eastern Rhodope that most likely graded to back-arc spreading southwards as represented by the Late Jurassic MORB-type Samothraki Island ophiolites. This tectonic scenario is further constrained by paleotectonic reconstructions. The arc–trench system collided with the Rhodope in the Late Jurassic times.     

Geochemical Constrains on Petrogenesis and Tectonic Setting of Volcanic Rocks from the Baimianxia and Sanwan Formations in the Northern Margin of the Yangtze Plate

On the basis of petrogeochemical data, the volcanic lavas of the Baimianxia Formation can be classified into two units: high TiO2 and low TiO2. The TiO2 concentration of the former is generally higher than 1%, which occurs in the lower part with high-grade metamorphism, but the latter is less than 1% and crops out in the upper part with low-grade metamorphism. The high-TiO2 unit is dominated by tholeiitic lavas showing high rare earth element (REE) contents (ΣREE?=?83.4–180.8?μg/g), high light/heavy REE (LREE/HREE) ratios (LREE/HREE=2.17–5.85) and weak negative Eu anomaly (Eu=0.79–1.01). Its trace element patterns display weak Nb-Ta anomalies with respect to Th, K, La, Ce, showing within-plate basalt affinities. In contrast, the low-TiO2 unit is characterized by low REE contents, low LREE/HREE ratios, and pronounced Nb-Ta anomalies, indicating typical arc or continental arc signature. Chondrite-normalized REE patterns of basalts and andesites from the Sanwan Formation are flat or LREE depletion, which is very similar to normal mid-oceanic basalt. Therefore, we suggest that these lavas should be formed in a back-arc basin setting. Sr-Nd isotopic data of the basalt in the lower part suggest that the rocks would have been formed in ~1144?Ma. Based on the geochemical and isotopic features of the basalts, we suggest that these rocks in the low part of the Baimianxia Formation should originate from an asthenospheric oceanic-island basalt-like mantle source, which may be produced by partial melting of garnet lherzolite, and significantly underwent fractional crystallization and crustal or lithospheric mantle contamination en route to the surface. However, laser ablation inductively coupled plasma mass spectrometry zircon U-Pb dating of the basalt sample from the upper part of the Baimianxia Formation gives a 437 Ma, indicating a Early Paleozoic age. The geochemical analysis in this paper suggests that they may originate from an arc or continental arc in response to aqueous fluids or melt expelled from a subducting slab, and the partial melting occurred in the garnet stability field. The samples of basalts and andesites in the Sanwan Formation show they are derived from depleted mantle source similar to normal mid-oceanic basalt. Finally, we can conclude that the lavas in the lower part of the Baimianxia Formation represent the geological records of rift-related volcanism in the middle Proterozoic, which is commonly considered to be the precursor of continental breakup and followed by oceanic basin forming from Neoproterozoic to early Paleozoic. Whereas, the lavas in upper part of the Baimianxia Formation and Sanwan Formations may have been generated by the oceanic and continental conversion that occurred in the early Paleozoic.     

Caribbean island-arc rifting and back-arc basin development in the Late Cretaceous: Geochemical, isotopic and geochronological evidence from Central Hispaniola

We present new regional petrologic, geochemical, Sr–Nd isotopic, and U–Pb geochronological data on the Turonian–Campanian mafic igneous rocks of Central Hispaniola that provide important clues on the development of the Caribbean island-arc. Central Hispaniola is made up of three main tectonic blocks—Jicomé, Jarabacoa and Bonao—that include four broad geochemical groups of Late Cretaceous mafic igneous rocks: group I, tholeiitic to calc-alkaline basalts and andesites; group II, low-Ti high-Mg andesites and basalts; group III, tholeiitic basalts and gabbros/dolerites; and group IV, tholeiitic to transitional and alkalic basalts. These igneous rocks show significant differences in time and space, from arc-like to non-arc-like characteristics, suggesting that they were derived from different mantle sources. We interpret these groups as the record of Caribbean arc-rifting and back-arc basin development in the Late Cretaceous. The> 90 Ma group I volcanic rocks and associated cumulate complexes preserved in the Jicomé and Jarabacoa blocks represent the Albian to Cenomanian Caribbean island-arc material. The arc rift stage magmatism in these blocks took place during the deposition of the Restauración Formation from the Turonian–Coniacian transition (~ 90 Ma) to Santonian/Lower Campanian, particularly in its lower part with extrusion at 90–88 Ma of group II low-Ti, high-Mg andesites/basalts. During this time or slightly afterwards adakitic rhyolites erupted in the Jarabacoa block. Group III tholeiitic lavas represent the initiation of Coniacian–Lower Campanian back-arc spreading. In the Bonao block, this stage is represented by back-arc basin-like basalts, gabbros and dolerite/diorite dykes intruded into the Loma Caribe peridotite, as well as the Peralvillo Sur Formation basalts, capped by tuffs, shales and Campanian cherts. This dismembered ophiolitic stratigraphy indicates that the Bonao block is a fragment of an ensimatic back-arc basin. In the Jicomé and Jarabacoa blocks, the mainly Campanian group IV basalts of the Peña Blanca, Siete Cabezas and Pelona–Pico Duarte Formation, represent the subsequent stage of back-arc spreading and off-axis non-arc-like magmatism, caused by migration of the arc toward the northeast. These basalts have geochemical affinities with the mantle domain influenced by the Caribbean plume, suggesting that mantle was flowing toward the NE, beneath the extended Caribbean island-arc, in response to rollback of the subducting proto-Caribbean slab.     

A geochemical and Nd isotopic study of Barberton komatiites (South Africa): implication for the Archean mantle

The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al₂O₃/TiO₂ and CaO/Al₂O₃ ratios of 15–18 and 1.1–1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb)_N ratios. In contrast, the Al-depleted komatiites show significantly lower Al₂O₃/TiO₂ ratios of 8–12, highly variable CaO/Al₂O₃ (0.19–2.81) ratios combined with (Gd/Yb)_N ratios varying from 1.08 to 1.56. A Sm–Nd whole rock isochron for komatiites of the Komati Formation gives an age of 3657±170 Ma. ¹⁴⁷Sm/¹⁴⁴Nd ratios (0.1704 and 0.1964) are all lower than the chondritic value of 0.1967. The komatiite _i,Nd(3.45) values cluster at +1.9±0.7.

Trace element distribution indicates that most of the primary geochemical and isotopic features of the komatiites were preserved in line with the conservation of the primary chemical composition of clinopyroxene. High field strength element and rare earth element abundances indicate that crustal contamination and post-crystallisation processes did not disturb the primary features of komatiites.

The Sm/Nd and Nb/U ratios of komatiites indicate that the Barberton greenstone belt mantle source has undergone melt extraction prior to komatiite formation. Variations of Al₂O₃/TiO₂, (Gd/Yb)_N, Zr/Sm and Sm/Nd ratios of komatiites indicate that a batch melting of slightly depleted mantle source during with garnet and/or clinopyroxene remained in the residue can produce the geochemical isotopic feature of the Barberton greenstone belt komatiites. Typical geochemical fingerprints of subduction-related processes (LILE enrichment, HFSE depletion compared to REE), as known from modern subduction zones, are not observed. Komatiites exhibit Ti/Zr, La/Nb, Nb/U, Sr/Nd and Ba/La ratios comparable to those of oceanic island basalt and mid-ocean ridge basalt. (La/Nb)_PMN, (Sm/Yb)_PMN, positive δNb values and flat or slightly enriched REE patterns suggest that BGB komatiites are part of an oceanic plateau rather than an oceanic island such as Iceland. Therefore, an oceanic plateau or mid-ocean ridge, in connection with an oceanic plateau, such as Ontong Java plateau or Caribbean–Colombian oceanic plateau, is a suitable tectonic setting for the formation of the BGB komatiites.     

Makran ophiolitic basalts (SE Iran) record Late Cretaceous Neotethys plume-ridge interaction

ABSTRACT

The Makran complex in southeast Iran provides a spectacular subduction-related accretionary complex to understand the mechanism of oceanic accretion and the evolution of subduction zones. In this paper, we present new major and trace element data as well as isotopic compositions of mafic volcanic blocks from the Makran ophiolitic mélange complex (OMC). Our aim is to assess the genesis of these rocks and discuss their implications on the evolution of Neotethys Ocean. These volcanic blocks are composed mainly of basalts with minor trachytes. The Makran lavas are occasionally interlayered with tuff layers. Zircons from these tuffs give U-Pb ages of 95 Ma, which is well in accordance with the reposted microfossil data for the interlayered pelagic limestones with pillow lavas. Makran basalts can be geochemically subdivided into four groups; normal to transitional MORB, enriched-MORB, Plume-type MORB and alkaline (-OIB-like) basalts. The OIB-like pillow lavas are represented by high values of Th/Tb (6.3–7.4) which are higher than other basalts (group 1 = 0.3–0.8; groups 2 = 0.7–1.6; group 3 = 1.58–1.36).¹⁴³Nd/¹⁴⁴Nd_(t) ratios for basalts ranges from 0.51247 to 0.51292, whereas ⁸⁷Sr/⁸⁶Sr_(t) isotopic composition of the OMC lavas varies from 0.704433 to 0.709466. The Pb isotopic composition of the lavas are quite high, ranging from 15.49–15.66 for ²⁰⁷Pb/²⁰⁴Pb_(t), 18.09–19.12 for ²⁰⁶Pb/²⁰⁴Pb_(t) and 37.80–39.23 for ²⁰⁸Pb/²⁰⁴Pb_(t). The chemistry of these rocks suggests that they were formed most likely in an oceanic setting with clear plume-ridge interaction. These rocks can form from partial melting of a highly heterogeneous mantle source, which is extensively metasomatized with deep mantle OIB-type components. We suggest these rocks have been generated in an oceanic ridge with plume-ridge interaction, similar to the Iceland-Reykjanes Ridge, before being fragmented and accreted into the Makran accretionary complex.     

The geochemistry and petrogenesis of the late-Cretaceous picrites and basalts of Curaçao,Netherlands Antilles: a remnant of an oceanic plateau

The island of Curaçao in the southern Caribbean Sea is composed mainly of a thick sequence (>5?km) of pillow lavas, grading upwards from picrites at the base of the exposed section, to basalts nearer the top. Modelling suggests that picrites are related to the basalts by fractional crystallisation. Initial radiogenic isotope ratios of the picrites have a restricted compositional range: ?_Nd=+6.1 to +6.6, ⁸⁷Sr/⁸⁶Sr=0.70296–0.70319; whereas the basalts display a wider range of compositions: ?_Nd=+6.6 to +7.6, ⁸⁷Sr/⁸⁶Sr=0.70321–0.70671. This variation in isotope ratios between basalts and picrites may be due to the assimilation of altered oceanic crust (or possibly partial melts of such crust) by a picritic magma along with fractional crystallisation. The relatively narrow range of Nd and Pb isotopic compositions in the Curaçao lavas suggests either that the source region was homogeneous, or that melts from a heterogeneous mantle source were well mixed before eruption. Chondritic to slightly light rare earth element enriched patterns, combined with long-term light rare earth element depletion (positive ?_Nd), suggest that the lavas were formed by polybaric melting of spinel lherzolite, with small a contribution from garnet lherzolite melts. High-MgO lavas, the absence of a subduction related chemistry, and the chemical similarity to other oceanic plateaux, suggest a mantle plume origin for the Curaçao lava succession. The Curaçao volcanic sequence is part of an oceanic plateau formed at about 88–90?Ma, fragments of which are dispersed around the Caribbean as well as being obducted onto the western margin of Colombia and Ecuador. The occurrence of high-Mg lavas throughout this Cretaceous Caribbean–Colombian igneous province requires anomalously hot mantle (>200^°?C hotter than ambient upper mantle) over a large part of a putative plume head, which is inconsistent with some mantle plume models.     

中国东南沿海中-新生代玄武岩微量元素和Nd-Sr-Pb同位素研究

本文对中国东南沿海不含幔源包体的中生代玄武岩和含幔源包体的新生代玄武岩进行了微量元素和Nd-Sr-Pb同位素对比研究。中生代玄武岩呈Ta、Nb和Hf负异常,低Ce/Pb、Nb/U比值和高La/Nb比值,与岛弧火山岩和陆壳岩石的微量元素特征相类似,说明在岩浆生成和上升过程中,幔源组分受到了陆壳组分的混染。新生代玄武岩呈Ta、Nb正异常和Pb负异常,高Ce/Pb、Nb/U比值和低La/Nb比值,与海岛玄武岩(OIB)相类似,Nd-Sr同位素成分与夏威夷玄武岩类似,因而它们未受明显的陆壳混染。143Nd/144Nd与206Pb/204Pb之间的负相关关系和87Sr/86Sr与206Pb/204Pb之间的正相关关系说明本区新生代玄武岩起源于中等亏损程度的软流圈地幔,并与EMII富集地幔组分发生了混合。