The behaviour of platinum-group elements in basalts from the East Greenland rifted margin

. The continental flood basalts of the East Greenland volcanic rifted margin were extruded during continental breakup above the ancestral Iceland mantle plume at 55 Ma. Three distinct magma types, the low-Ti, high-Ti and very high-Ti series (LTS, HTS and VHTS respectively), are found intercalated in the ~6-km-thick Plateau Lava sequence. Incompatible trace elements indicate that the LTS are derived from a more depleted mantle source compared to HTS and VHTS. The LTS is characterised by increasing Cu (105 to 248 ppm) and Pd (7 to 24 ppb), constant Cu/Pd ratio (~10,000), and decreasing Ir (1.1 to <0.05 ppb) and Ru (1.8 to <0.3 ppb) concentrations during magmatic differentiation (16 to 7 wt% MgO). The constant Cu/Pd ratio reflects silicate- and chromite-dominated fractionation without concurrent segregation of sulphide. S-undersaturated differentiation is also indicated in the HTS, which also displays increasing Pd (6-16 ppb) and decreasing Ir concentrations (1 to <0.05 ppb) during differentiation, and the Cu/Pd ratios for the entire series average 21,000. However, some HTS samples have elevated Cu/Pd ratios (up to 33,000). Cu/Pd ratios in the HTS do not correlate with MgO, and this is interpreted to reflect varying Cu/Pd ratios of HTS parental magmas rather than S-saturated differentiation. During S-undersaturated differentiation of the LTS and HTS, Pt/Pd ratios decrease from 1.3 to 0.11 and 1.1 to 0.2 respectively, which indicates that Pd is much more incompatible than Pt during S-undersaturated differentiation. The VHTS consists exclusively of highly evolved samples with low MgO (6.6-6.1 wt%) and Pd/Ir ratios 98-228. Here, Cu/Pd ratios increase from 17,500 to 35,000 with decreasing Cr concentrations which indicate that these magmas experienced silicate fractionation with concurrent segregation of sulphide. The LTS represent melting of a depleted source and show high PGE concentrations and constant Cu/Pd ratios during S-undersaturated differentiation. Melting of a normal depleted upper mantle source generates S-saturated melts (MORB), and the depleted LTS source is therefore considered an extraordinary S-poor component within the ancestral Iceland plume. Of the three series, the VHTS contain the largest contribution from enriched mantle portions. The VHTS have similar PGE but much higher Nb concentrations for instance compared to the most evolved LTS and HTS samples, indicating that the enriched source contributes Nb but not PGE.     

峨眉山大火成岩省：地幔柱活动的证据及其熔融条件

对苦橄岩中橄榄石斑晶及其中熔体包裹体的电子探针分析表明，峨眉山大火山岩省的原始岩浆具高镁（ MgO > 16％）特征。玄武岩的 REE反演计算揭示，参与峨眉山玄武岩岩浆作用的地幔具有异常高的潜能温度（ 1 550℃）。这些特征以及峨眉山玄武岩的大面积分布和一些熔岩所显示的类似于洋岛玄武岩 (OIB)的微量元素和 Sr- Nd同位素特征均为地幔热柱在能量和物质上参与峨眉山溢流玄武岩的形成提供了确凿证据。峨眉山两个主要岩类（高钛和低钛玄武岩）可能是不同地幔源区物质在不同条件下的熔融产物。低钛玄武岩形成于温度最高、岩石圈最薄的地幔柱轴部。地幔（ ISr≈ 0.705,ε Nd(t)≈＋ 2）熔融始于 140 km，并一直延续到较浅的深度（ 60 km,尖晶石稳定区 ),部分熔融程度为 16％，这类岩石可能代表了峨眉山玄武岩的主体。而高钛玄武岩的母岩浆的形成基本局限在石榴子石稳定区（ > 70 km），其源区特征为 : ISr≈ 0.704,ε Nd(t)≈＋ 5，可能代表了热柱边部或消亡期地幔小程度部分熔融（ 1.5％）的产物。     

河南信阳地区龟山岩组新元古代变质玄武岩的地球化学特征、Sr-Nd同位素组成及地质意义

对信阳地区商丹断裂带南侧龟山岩组新元古代变质玄武岩进行了岩石学、地球化学及Sr-Nd同位素研究,分析结果显示该套玄武岩为亚碱性拉斑玄武系列,分为低Ti及高Ti两种类型：低Ti型较富Mg,不相容元素富集程度及稀土分馏程度较低,具有E-MORB的微量元素地球化学特征,Sr-Nd同位素组成相对富集,可能来自地幔柱引发的岩石圈地幔的部分熔融,并受到一定程度的地壳混染;高Ti型较富Fe,强烈富集不相容元素,具有OIB的地球化学特征,Sr-Nd同位素组成较为亏损,可能来自地幔柱的部分熔融,并较少受到地壳物质的影响。综合构造判别显示该套玄武岩可能为地幔柱伸展背景下的岩浆活动产物,可能为区域上沿商丹断裂带分布的中—新元古代局部伸展背景岩浆活动产物的组成部分。     

Petrology and geochemistry of metamorphosed komatiites and basalts from the Sula Mountains greenstone belt, Sierra Leone

The Sula Mountains greenstone belt is the largest of the late-Archaean greenstone belts in the West African Craton. It comprises a thick (5 km) lower volcanic formation and a thinner (2 km) upper metasedimentary formation. Komatiites and basalts dominate the volcanic formation and komatiites form almost half of the succession. All the volcanic rocks are metamorphosed to amphibolite grade and have been significantly chemically altered. Two stages of alteration are recognised and are tentatively ascribed to hydrothermal alteration and later regional amphibolite facies metamorphism. Ratios of immobile trace elements and REE patterns preserve, for the most part, original igneous signatures and these are used to identify five magma types. These are: low-Ti komatiites – depleted in light REE; low-Ti komatiites – with flat REE patterns; high-Ti komatiitic basalts – with flat REE; low-Ti basalts – depleted in light REE; high-Ti basalts – with flat REE patterns. Much of the variation between the magma types can be explained in terms of different melt fractions of the mantle source, although there were two separate mantle sources one light REE depleted, the other not. The interleaving of the basalts and komatiites produced by this melting indicates that the two mantle sources were melted simultaneously. The simplest model with which to explain these complex melting processes is during melting within a rising mantle plume in which there were two different mantle compositions. The very high proportion of komatiites in the Sula Mountains relative to other greenstone belts suggests either extensive deep melting and/or the absence of a thick pre-existing crust which would have acted as a “filter” to komatiite eruption. Received: 10 February 1998 / Accepted: 28 July 1998     

A reappraisal of the high-Ti and low-Ti classification of basalts and petrogenetic linkage between basalts and mafic–ultramafic intrusions in the Emeishan Large Igneous Province, SW China

Based on published data, we reappraise the classification of high-Ti and low-Ti basalt from the Emeishan large igneous province (ELIP) and the correlations between basalts and mafic–ultramafic intrusions. Because of the lack of clear spatial and temporal variations of different types of basalts, we suggest that the basalts in the ELIP cannot be classified into high-Ti and low-Ti groups, by TiO₂ contents and/or Ti/Y ratios. The distinctive characteristics of these high-Ti and low-Ti lavas probably result largely from the different fractionating assemblages. Whether or not fractional crystallization of the Fe–Ti oxides occurred probably is the key factor that controls the Ti abundances and Ti/Y ratios in the residual melts, e.g., lavas, although the nature of the mantle sources, variable degrees of partial melting of mantle and crustal contamination also influence the geochemical signatures of the lavas. Therefore, neither Ti abundance nor Ti/Y ratios in basalts can reflect the nature of their mantle source. Moreover, the different types of mafic–ultramafic intrusions in the ELIP cannot simply be attributed to be genetically related special types of basalts, either high-Ti or low-Ti basalts. It is likely that they are merely the cumulus phases, i.e. chamber or conduit of the basaltic lavas. Hence, caution should be exercised in the use of high-Ti or low-Ti basalts as prospecting vectors for ore deposits in the region. Potential implications are proposed that both the Fe–V–Ti oxide and Cu–Ni–(PGE) sulfide mineralization in the ELIP intrusions is largely due to the variable differentiation and crustal contamination during magmatic processes.     

新疆托里蛇绿岩中两种不同玄武岩的地球化学特征及成因

托里蛇绿岩中分布着高Ti玄武岩与低Ti玄武岩,在地球化学特征上存在明显的差异;表明二者之间不存在过渡关系,有着不同的形成条件。它们分别代表岛弧拉张不同阶段之产物,本文讨论了玄武岩的地球化学特征,从而揭示了蛇绿岩形成于弧后盆地的微扩张环境。     

Oxygen and iron isotope constraints on near-surface fractionation effects and the composition of lunar mare basalt source regions

Oxygen and iron isotope analyses of low-Ti and high-Ti mare basalts are presented to constrain their petrogenesis and to assess stable isotope variations within lunar mantle sources. An internally-consistent dataset of oxygen isotope compositions of mare basalts encompasses five types of low-Ti basalts from the Apollo 12 and 15 missions and eight types of high-Ti basalts from the Apollo 11 and 17 missions. High-precision whole-rock δ¹⁸O values (referenced to VSMOW) of low-Ti and high-Ti basalts correlate with major-element compositions (Mg#, TiO₂, Al₂O₃). The observed oxygen isotope variations within low-Ti and high-Ti basalts are consistent with crystal fractionation and match the results of mass-balance models assuming equilibrium crystallization. Whole-rock δ⁵⁶Fe values (referenced to IRMM-014) of high-Ti and low-Ti basalts range from 0.134‰ to 0.217‰ and 0.038‰ to 0.104‰, respectively. Iron isotope compositions of both low-Ti and high-Ti basalts do not correlate with indices of crystal fractionation, possibly owing to small mineral-melt iron fractionation factors anticipated under lunar reducing conditions.The δ¹⁸O and δ⁵⁶Fe values of low-Ti and the least differentiated high-Ti mare basalts are negatively correlated, which reflects their different mantle source characteristics (e.g., the presence or absence of ilmenite). The average δ⁵⁶Fe values of low-Ti basalts (0.073 ± 0.018‰, n = 8) and high-Ti basalts (0.191 ± 0.020‰, n = 7) may directly record that of their parent mantle sources. Oxygen isotope compositions of mantle sources of low-Ti and high-Ti basalts are calculated using existing models of lunar magma ocean crystallization and mixing, the estimated equilibrium mantle olivine δ¹⁸O value, and equilibrium oxygen-fractionation between olivine and other mineral phases. The differences between the calculated whole-rock δ¹⁸O values for source regions, 5.57‰ for low-Ti and 5.30‰ for high-Ti mare basalt mantle source regions, are solely a function of the assumed source mineralogy. The oxygen and iron isotope compositions of lunar upper mantle can be approximated using these mantle source values. The δ¹⁸O and δ⁵⁶Fe values of the lunar upper mantle are estimated to be 5.5 ± 0.2‰ (2σ) and 0.085 ± 0.040‰ (2σ), respectively. The oxygen isotope composition of lunar upper mantle is identical to the current estimate of Earth’s upper mantle (5.5 ± 0.2‰), and the iron isotope composition of the lunar upper mantle overlaps within uncertainty of estimates for the terrestrial upper mantle (0.044 ± 0.030‰).     

Zircon U-Pb geochronology and elemental and Sr–Nd isotope geochemistry of Permian mafic rocks in the Funing area, SW China

Mafic-layered intrusions and sills and spatially associated andesitic basalts are well preserved in the Funing area, SW China. The 258±3 Ma-layered intrusions are composed of fine-grained gabbro, gabbro and diorite. The 260±3 Ma sills consist of undifferentiated diabases. Both the layered intrusions and volcanic rocks belong to a low-Ti group, whereas the diabases belong to a high-Ti group. Rocks of the high-Ti group have FeO, TiO₂ and P₂O₅ higher but MgO and Th/Nb ratios lower than those of the low-Ti group. They have initial ⁸⁷Sr/⁸⁶Sr ratios (0.706–0.707) lower and ɛNd (−1.5 to −0.6) higher than the low-Ti equivalents (0.710–0.715 and −9.6 to −4.0, respectively). The high-Ti group was formed from relatively primitive, high-Ti magmas generated by low degrees (7.3 –9.5%) of partial melting of an enriched, OIB-type asthenospheric mantle source. The low-Ti group may have formed from melts derived from an EM2-like, lithospheric mantle source. The mafic rocks at Funing are part of the Emeishan large igneous province formed by a mantle plume at ∼260 Ma.     

Petrology,isotope characteristics,and K-Ar ages of the Maranhão,northern Brazil,Mesozoic basalt province

Northern Brazil contains remnants of Mesozoic flood basalts and hypabyssal rocks that were apparently emplaced during tectonism related to opening of the Atlantic Ocean. Analyses and new K-Ar ages reveal that this 700x250 km Maranhão province (5°–8°S) has low-Ti basalts (1.1 wt% TiO₂) in the western part that range about 160 to 190 Ma, and high-Ti basalts (3.4–4.4 wt% TiO₂) in the eastern part about 115–122 Ma. Low-Ti basalt compositions are less evolved and have a smaller range, Mg# 62-56, than the high-Ti basalts, Mg# 44–33. General characteristics of the least evolved members of low- and high-Ti groups include, respectively, Zr 100 and 250 ppm, Sr 225 and 475 ppm, Ba 200 and 500 ppm, Nb 10 and 26 ppm, Y 29 and 36 ppm, La/Yb_(n) 4.2 and 8.8, where La_(n) is 30 and 90. Overall compositions resemble the low- and high-Ti basaltic rocks of the Mesozoic Serra Geral (Paraná) province in southern Brazil. The Maranhão low-Ti basalts have more radiogenic Sr and Pb and higher ¹⁸O than the high-Ti basalts. Respectively, low- vs high-Ti: _Sr26–54 vs 15–18; ²⁰⁶Pb/²⁰⁴Pb=18.25–.78 vs 18.22–.24; and ¹⁸O 8.9–12.6 vs 6.5–8.6. Nd isotopes overlap: _Nd–1.6 to –3.8 vs –2.1 to –3. Ages, compositions, and isotopes indicate that the low- and high-Ti groups had independent parentages from enriched subcontinental mantle. However, both groups can be modeled from one source composition if low-Ti basalt isotopes reflect crustal contamination, and if the parentages for each group were picritic liquids that represent either higher (for low-Ti) or lower (for high-Ti) percentages of melting of that single source. When comparing Pb isotopes of Maranhão and Serra Geral high-Ti basalts (uncontaminated) to evaluate the DUPAL anomaly, Maranhão has Pb 7/4=4.6–11, and Pb 8/4=72–87; Serra Geral has Pb 7/4=10–13, and Pb 8/4=95–125. The small difference is not enough to conform to DUPAL contours, and is inconsistent with large-scale isotopic heterogeneity of mantle beneath Brazil prior to rifting of South America from Africa. Maranhão low-Ti magmas probably relate to the opening of central North Atlantic, and high-Ti magmas to the opening of equatorial Atlantic. The proposed greater percentage of source melting for low-Ti basalts may reflect a Triassic-Jurassic hotspot, while lesser melting for high-Ti magmas may relate to Cretaceous decompressional (rifting) melting.     

Os, Pb, and Nd isotope geochemistry of the Permian Emeishan continental flood basalts: Insights into the source of a large igneous province

The nature of the source of continental flood basalts (CFB) is a highly debated topic. Proposed mantle sources for CFBs, including both high- and low-Ti basalts, include subcontinental lithospheric mantle (SCLM), asthenospheric mantle, and deep, plume-related mantle. Re-Os isotope systematics can offer important constraints on the sources of both ocean island basalts (OIB) and CFB, and may be applied to distinguish different possible melt sources. This paper reports the first Re-Os isotope data for the Late Permian Emeishan large igneous province (LIP) in Southwest China. Twenty one CFB samples including both low- and high-Ti basalts from five representative sites within the Emeishan LIP have been analyzed for Os, Nd, and Pb isotopic compositions. The obtained Os data demonstrate that crustal assimilation affected Os isotopic compositions of some Emeishan basalt samples with low Os concentrations but not all of the samples, and the Emeishan basalts with high Os contents likely experienced the least crustal contamination. The low and high-Ti basalts yield distinct Os signatures in terms of ¹⁸⁷Os/¹⁸⁸Os and Os content. The low-Ti basalt with the highest Os concentration (400 ppt) has a radiogenic Os isotopic composition (γOs(t), +6.5), similar to that of plume-derived OIB. Because the Os isotopic composition of basalts with relatively high Os concentrations (typically >50 ppt) likely represents that of their mantle source, this result implies a plume-derived origin for the low-Ti basalts. On the other hand, the high-Ti basalts with high Os concentration (over 50 ppt) have unradiogenic Os isotopic signatures (γOs(t) values range from −0.8 to −1.4), suggesting that a subcontinental lithosphere mantle (SCLM) component most likely contributed to the generation of these magmas. Combining Pb and Nd isotopic tracers with the Os data, we demonstrate that the low-Ti basaltic magmas in the Emeishan CFB were mainly sourced from a mantle plume reservoir, whereas the high-Ti basaltic magmas were most likely derived from a SCLM reservoir or were contaminated by a significant amount of lithospheric mantle material during plume-related magma ascent through the SCLM.     

Origin and differentiation of picritic arc magmas,Ambae (Aoba), Vanuatu

Key aspects of magma generation and magma evolution in subduction zones are addressed in a study of Ambae (Aoba) volcano, Vanuatu. Two major lava suites (a low-Ti suite and high-Ti suite) are recognised on the basis of phenocryst mineralogy, geochemistry, and stratigraphy. Phenocryst assemblages in the more primitive low-Ti suite are dominated by magnesian olivine (mg 80 to 93.4) and clinopyroxene (mg 80 to 92), and include accessory Cr-rich spinel (cr 50 to 84). Calcic plagioclase and titanomagnetite are important additional phenocryst phases in the high-Ti suite lavas and the most evolved low-Ti suite lavas. The low-Ti suite lavas span a continuous compositional range, from picritic (up to 20 wt% MgO) to high-alumina basalts (<5 wt% MgO), and are consistent with differentiation involving observed phenocrysts. Melt compositions (aphyric lavas and groundmasses) in the low-Ti suite form a liquid-line of descent which corresponds with the petrographically-determined order of crystallisation: olivine + Cr-spinel, followed by clinopyroxene + olivine + titanomagnetite, and then plagioclase + clinopyroxene + olivine + titanomagnetite. A primary melt for the low-Ti suite has been estimated by correcting the most magnesian melt composition (an aphyric lava with 10.5 wt% MgO) for crystal fractionation, at the oxidising conditions determined from olivine-spinel pairs (fo₂ FMQ + 2.5 log units), until in equilibrium with the most magnesian olivine phenocrysts. The resultant composition has 15 wt% MgO and an mg ^Fe2 value of 81. It requires deep (3 GPa) melting of the peridotitic mantle wedge at a potential temperature consistent with current estimates for the convecting upper mantle (T _p 1300°C). At least three geochemically-distinct source components are necessary to account for geochemical differences between, and geochemical heterogeneity within, the major lava suites. Two components, one LILE-rich and the other LILE- and LREE-rich, may both derive from the subducting ocean crust, possibly as an aqueous fluid and a silicate melt respeetively. A third component is attributed to either differnt degrees of melting, or extents of incompatible-element depletion, of the peridotitic mantle wedge.     

桂西那坡基性岩地球化学:峨眉山地幔柱与古特提斯俯冲相互作用的证据

华南板块西南缘、越北地块以北桂西那坡县城以西及西南一带发育一套晚二叠世基性岩,由层状、似层状次火山岩相辉绿岩、辉绿玢岩及球状岩组成。根据岩石地球化学特征,那坡基性岩可划分为高Ti(TiO_22.8%和Ti/Y500)和低Ti两部分。高Ti基性岩为碱性玄武岩,而低Ti基性岩为拉斑玄武岩。与低Ti基性岩相比,高Ti基性岩整体具有相对较低的SiO_2、MgO和较高的FeO_t、P_2O_5,轻、重稀土分馏明显,富集大离子亲石元素(LILE)和高场强元素(HFSE),显示出似OIB地球化学特征,与峨眉山高Ti玄武岩具高度亲缘性;低Ti基性岩具有相对较高的SiO_2、MgO和较低的FeO_t、P_2O_5,稀土配分曲线较平坦,富集LILE,严重亏损HFSE(Nb、Ta),与岛弧玄武岩地球化学特征类似。从微量元素比值及相关图解对岩浆源区和构造环境判别,那坡高Ti基性岩来自富集OIB地幔源区,而低Ti基性岩兼具OIB和岛弧岩浆源区的过渡特征。结合岩石地球化学特征及区域地质背景,认为那坡高Ti基性岩可能为峨眉山地幔柱岩浆作用的产物,低Ti基性岩为古特提斯俯冲与峨眉山地幔柱共同作用的产物,揭示了那坡地区晚二叠世同时受到峨眉山地幔柱和古特提斯俯冲相互作用的影响。     

Origin of titaniferous lunar basalts

Delineation of low pressure phase equilibria in the composition space relevant to titaniferous lunar basalts demonstrates a significant degree of control by those equilibria on the compositions of the basalts. The existence of two distinct chemical groups of basalts (high and low-K) which cannot be related one to the other by fractional crystallization at any pressure, suggests that melting is responsible for the two groups. Consideration of the pressure shift required to produce the differences between groups constrains magma segregation to have occurred in the outer 150 km of the Moon. It is difficult to relate low-Ti and high-Ti basalts to the same source region. The preferred source region of high-Ti basalts, based on phase equilibrium considerations, is a late ilmenite-rich cumulate produced from the residual liquid of the primordial differentiation of the outer portions of the Moon. This ilmenite-rich layer is sandwiched between the lunar feldspathic crust and a complementary mafic cumulate.     

北祁连乌鞘岭蛇绿混杂岩地球化学特征及其构造环境

乌鞘岭蛇绿混杂岩位于北祁连造山带东段,具有相对完整的蛇绿岩序列,包括：变质地幔橄榄岩单元（蛇纹岩＋辉橄岩）,镁铁质一超镁铁质堆晶岩单元（橄辉岩＋辉石岩）,镁铁质侵入岩单元（辉长岩）,及基性火山岩单元（玄武岩）。依照TiO,的含量,本文从乌鞘岭蛇绿混杂岩中分出两类玄武岩,即：低n玄武岩（Ti02=0．55％～0．76％）和高Ti玄武岩（Ti02=1．35％-1．99％）。低Ti玄武岩大离子亲石元素含量波动较大,具明显n负异常,LREE呈略富集的配分模式,整体上具有弧火山岩的特征;高Ti玄武岩大离子亲石元素含量在小范围内变化,未见Nb、Ti负异常,LPtEE呈略亏损的配分模式,具有典型N—MORB的性质。在构造环境判别图上,低n玄武岩和高Ti玄武岩分别落入陆缘弧和大洋中脊环境。高Ti玄武岩是鸟鞘岭蛇绿混杂岩的一部分,源于亏损地幔的部分熔融,与陆缘弧型低rri玄武岩构造混杂在一起。乌鞘岭蛇绿混杂岩大概于中一晚奥陶世形成于北祁连造山带老虎山一毛毛山弧后盆地。     

The Petrogenetic Significance of Interstratified High- and Low-Ti Basalts in Central Nicaragua

Basalts erupted from recent volcanoes in central Nicaragua canbe divided into distinct high-and low-Ti suites. Low-Ti basaltshave higher concentrations of LILE and LREE than high-Ti basalts.In addition, low-Ti basalts have obviously higher Ba/La, La/Sm,and ⁸⁷Sr/⁸⁶Sr, and lower Ti/Zr, than high-Ti basalts. In contrast,there are no mineralogical or petrographic differences betweenthe two suites. The differences between the high-and low-Ti basalts of centralNicaragua are inherited from their source regions. The primarymagmas of both are generated in the mantle wedge. However, low-Tiprimary magmas come from parts of the wedge which bear a strongsubduction zone signature, including that of subducted pelagicsediment. On the other hand, the primary magmas of the high-Tibasalts are generated in parts of the wedge relatively freeof subduction zone influence. Subducted pelagic sediment can therefore be a key source componentat active continental margins as well as at island arcs. Pelagicsediment could also be responsible for subtle high-field-strengthelement fractionations within subduction zone magmas. The mantlewedge beneath Nicaragua, which is variably modified by the subductingplate, is relatively enriched suboceanic mantle.     

Volcanic Rocks from the Nejapa and Granada Cinder Cone Alignments, Nicaragua, Central America

Mafic tholeiitic basalts from the Nejapa and Granada (NG) cindercone alignments provide new insights into the origin and evolutionof magmas at convergent plate margins. In comparison to otherbasalts from the Central American volcanic front, these marietholeiitic basalts are high in MgO and CaO and low in Al₂O_p,K₂O₁, Ba and Sr. They also differ from other Central Americanbasalts, in having clinopyroxene phenocrysts with higher MgO,CaO and Cr₂O₃ concentrations and olivine phenocrysts with higherMgO contents. Except for significantly higher concentrationsof Ba, Sr and ⁸⁷Sr/₈₆Sr, most of the tholeiites are indistinguishable in compositionfrom mid-ocean ridge basalts. In general, phenocryst mineralcompositions are also very similar between NG tholeiites andmid-ocean ridge basalts. The basalts as a whole can be dividedinto two groups based on relative TiO₂-K₂O concentrations. Thehigh-Ti basalts always have the lowest K₂O and Ba and usuallyhave the highest Ni and Cr. All of the basalts have experienced some fractional crystallizationof olivine, plagioclase and clinopyroxene. Relative to otherCentral American basalts, the Nejapa-Granada basalts appearto have fractionated at low P_T and P_H2O. The source of primarymagmas for these basalts is the mantle wedge. Fluids and/ormelts may have been added to the mantle wedge from hydrothermally-altered,subducting oceanic crust in order to enrich the mantle in Sr,Ba and ⁸⁷Sr/⁸⁶Sr, but not in K and Rb. The role of lower crustaicontamination in causing the observed enrichments in Sr, Baand ⁸⁷Sr/⁸⁶Sr of NG basalts in comparison to mid-ocean ridgebasalts, however, is unclear. Rutile or a similar high-Ti accessoryphase may have been stable in the mantle source of the low-TiNG basalts, but not in that of the high-Ti basalts. Mafic tholeiiticbasalts, similar to those from Nejapa and Granada, may representmagmatic compositions parental to high-Al basalts, the mostmafic basalts at most Central American volcanoes. The characterof the residual high-Al basalts after this fractionation stepdepends critically on P_H2O Both high and low-Ti andesites are also present at Nejapa. Likethe high-Ti basalts, the high-Ti andesites have lower K₂O andBa and higher Ni and Cr in comparison to the low-Ti group. Thehigh-Ti andesites appear to be unrelated to any of the otherrocks and their exact origin is unknown. The low-Ti andesitesare the products of fractional crystallization of plagioclase,clinopyroxene, olivine (or orthopyroxene) and magnetite fromthe low-Ti basalts. The eruption that deposited a lapilli sectionat Cuesta del Plomo involved the explosive mixing of 3 components:high-Ti basaltic magma, low-Ti andesitic magma and high-Ti andesiticlava.     

Constraints from melt inclusions and their host olivines on the petrogenesis of Oligocene-Early Miocene Xindian basalts, Chifeng area, North China Craton

The geochemical characteristics of melt inclusions and their host olivines provide important information on the processes that create magmas and the nature of their mantle and crustal source regions. We report chemical compositions of melt inclusions, their host olivines and bulk rocks of Xindian basalts in Chifeng area, North China Craton. Compositions of both bulk rocks and melt inclusions are tholeiitic. Based on petrographic observations and compositional variation of melt inclusions, the crystallizing sequence of Xindian basalts is as follows: olivine (at MgO > ~5.5 wt%), plagioclase (beginning at MgO = ~5.5 wt%), clinopyroxene and ilmenite (at MgO < 5.0 wt%). High Ni contents and Fe/Mn ratios, and low Ca and Mn contents in olivine phenocrysts, combining with low CaO contents of relatively high MgO melt inclusions (MgO > 6 wt%), indicate that Xindian basalts are possibly derived from a pyroxenite source rather than a peridotite source. In the CS-MS-A diagram, all the high MgO melt inclusions (MgO > 6.0 wt%) project in the field between garnet + clinopyroxene + liquid and garnet + clinopyroxene + orthopyroxene + liquid near 3.0 GPa, further suggesting that residual minerals are mainly garnet and clinopyroxene, with possible presence of orthopyroxene, but without olivine. Modeling calculations using MELTS show that the water content of Xindian basalts is 0.3–0.7 wt% at MgO = 8.13 wt%. Using 20–25 % of partial melting estimated by moderately incompatible element ratios, the water content in the source of Xindian basalts is inferred to be ≥450 ppm, much higher than 6–85 ppm in dry lithospheric mantle. The melting depth is inferred to be ~3.0 GPa, much deeper than that of tholeiitic lavas (<2.0 GPa), assuming a peridotite source with a normal mantle potential temperature. Such melting depth is virtually equal to the thickness of lithosphere beneath Chifeng area (~100 km), suggesting that Xindian basalts are derived from the asthenospheric mantle, if the lithospheric lid effect model is assumed.     

Composition and age of tertiary sills and dykes, Jameson Land Basin, East Greenland: relation to regional flood volcanism

The Paleozoic–Mesozoic Jameson Land Basin (East Greenland) is intruded by a sill complex and by a swarm of ESE trending dykes. Together with dykes of the inner Scoresby Sund fjord, they form a regional Early Tertiary intrusive complex located 200–400 km inland of the East Greenland rifted continental margin. Most of the intrusive rocks in the Jameson Land Basin are geochemically coherent and consist of evolved plagioclase–augite–olivine saturated, uncontaminated high-Ti basalt with 48.5–50.2 wt.% SiO₂, 2.2–3.2 wt.% TiO₂, 5.1–7.4 wt.% MgO, 9–17 ppm Nb and La/Yb_N=2.8–3.6. Minor tholeiitic rock types are: (a) low-Ti basalt (49.7 wt.% SiO₂, 1.7 wt.% TiO₂, 6.8 wt.% MgO, 2.6 ppm Nb and La/Yb_N=0.5) akin to oceanic basalts; (b) very-high-Ti basalt (48.6 wt.% SiO₂, 4.1 wt.% TiO₂, 5.1 wt.% MgO and 21 ppm Nb); and (c) plagioclase ultraphyric basalt. The tholeiitic dolerites are cut by alkali basalt (43.7–47.3 wt.% SiO₂, 4.1–5.1 wt.% TiO₂, 4.9–6.2 wt.% MgO, 29–46 ppm Nb and La/Yb_N=16–17) sills and dykes.Modelling of high-field-strength and rare-earth elements indicate that the high-Ti basalts formed from 6–10% melting of approximately equal proportions of garnet- and spinel-bearing mantle of slightly depleted composition beneath thick continental lithosphere. Conversely, dolerite intrusions and flood basalts of similar compositional kindred from adjacent but more rift-proximal occurrences in Northeast Greenland formed from shallower melting of dominantly spinel-bearing mantle beneath extended and thinned continental lithosphere. These variations in lithospheric thickness suggest the continent–ocean transition of the East Greenland rifted volcanic margin is sharp and narrow.⁴⁰Ar–³⁹Ar dating and paleomagnetism show that the high-Ti dolerites were emplaced at 53–52 Ma (most likely during C23r) and hence surprisingly postdate the main flood volcanism by 2–5 Ma and the inception of seafloor spreading between Greenland and Europe by 1–2 Ma. The formation of tholeiitic and alkaline magmas emplaced into the Jameson Land Basin corroborates to the importance of post-breakup magmatism along the East Greenland volcanic rifted margin. Upwelling of the ancestral Iceland mantle plume under central Greenland at 53–52 Ma (rather than under the active rift), perhaps accompanied by a failed attempt to shift the rift zone westward towards the plume axis, may have triggered post-breakup continental magmatism of the Jameson Land Basin and the inner Scoresby Sund region, along preexisting structural lineaments.     

Boninitic and tholeiitic basaltic lavas and dikes from dispersed Jurassic East Othris ophiolitic units,Greece: petrogenesis and geodynamic implications

ABSTRACT

Pillow lavas, massive lava flows, and sub-volcanic dikes of tholeiitic basaltic composition are found to be members of the Vrinena, Aerino, Eretria, and Velestino dispersed Middle–Upper Jurassic ophiolitic units in East Othris. The Vrinena and Eretria ophiolitic units appear to have been emplaced onto the Pelagonian continental margin during the Upper Jurassic–Lower Cretaceous, whereas the Aerino and Velestino units seem to have been finally emplaced during post-Palaeocene times. Geochemically these are divided into two groups: Group I includes subduction-related boninites and low-Ti basalts from the Vrinena and Aerino units, and Group II high-Ti basalts show spreading-type characteristics occurring in the Eretria and Velestino units. Primary magma of the Group I volcanics appears to have been formed after high partial melting degrees (~18%) of a highly depleted harzburgitic mantle source, under relatively high temperatures (mantle potential temperature ~1372°C). Petrogenetic modelling also suggests that the primary magma of the Group II volcanics were formed after lower partial melting degrees (~7%) of a moderately depleted mantle source. The petrological and geochemical data from the East Othris dispersed and diversely emplaced ophiolitic units provide evidence of a common intra-oceanic supra-subduction zone (SSZ) origin within the Pindos oceanic strand of the Western Tethys. Specifically, Group I lavas and dikes from Vrinena seem to represent the extrusive part of an almost complete fore- to island-arc ophiolitic sequence. Dikes of Aerino most likely correspond to fore-arc magmatic material that intruded within exhumed serpentinized ultramafic rocks through a subduction channel that developed close to the slab and towards the fore-arc and the accretionary prism. The Group II volcanics either corresponded to a fore-arc magmatic expression, which extruded earlier than Group I volcanics and prior to the establishment of a mature subduction zone, or represent back-arc to island-arc magmatism that was contemporaneous to the fore-arc magmatic activity during rollback subduction.     

The Stonyford Volcanic Complex: a Forearc Seamount in the Northern California Coast Ranges

Jurassic age volcanic rocks of the Stonyford volcanic complex(SFVC) comprise three distinct petrological groups based ontheir whole-rock geochemistry: (1) oceanic tholeiites; (2) transitionalalkali basalts and glasses; (3) high-Al, low-Ti tholeiites.Major and trace element, and Sr–Nd–Pb isotopic dataindicate that the oceanic tholeiites formed as low-degree partialmelts of normal mid-ocean ridge basalt (N-MORB)-source asthenospheresimilar in isotope composition to the East Pacific Rise today;the alkalic lavas were derived from an enriched source similarto that of E-MORB. The high-Al, low-Ti lavas resemble second-stagemelts of a depleted MORB-source asthenosphere that formed bymelting spinel lherzolite at low pressures. Trace element systematicsof the high-Al, low-Ti basalts show the influence of an enrichedcomponent, which overprints generally depleted trace elementcharacteristics. Tectonic discrimination diagrams show thatthe oceanic tholeiite and alkali suites are similar to present-daybasalts generated at mid-oceanic ridges. The high-Al, low-Tisuite resembles primitive arc basalts with an enriched, alkalibasalt-like overprint. Isotopic data show the influence of recycledcomponents in all three suites. The SFVC was constructed ona substrate of normal Coast Range ophiolite in an extensionalforearc setting. The close juxtaposition of the MORB-like olivinetholeiites with alkali and high-Al, low-Ti basalts suggestsderivation from a hybrid mantle source region that includedMORB-source asthenosphere, enriched oceanic asthenosphere, andthe depleted supra-subduction zone mantle wedge. We proposethat the SFVC formed in response to collision of a mid-oceanridge spreading center with the Coast Range ophiolite subductionzone. Formation of a slab window beneath the forearc duringcollision allowed the influx of ridge-derived magmas or themantle source of these magmas. Continued melting of the previouslydepleted mantle wedge above the now defunct subduction zoneproduced strongly depleted high-Al, low-Ti basalts that werepartially fertilized with enriched, alkali basalt-type meltsand slab-derived fluids. KEY WORDS: CRO; oceanic basalts; California