Volcanism at the Edge of the Hawaiian Plume: Petrogenesis of Submarine Alkalic Lavas from the North Arch Volcanic Field

Submarine lavas erupted onto the Hawaiian arch 200–400km north of Oahu show that the areal extent of Hawaiian volcanismis much larger than previously recognized. The North Arch volcanicfield comprises 25 000 km² of     

Geochemical Constraints on Possible Subduction Components in Lavas of Mayon and Taal Volcanoes, Southern Luzon, Philippines

Mayon is the most active volcano along the east margin of southernLuzon, Philippines. Petrographic and major element data indicatethat Mayon has produced a basaltic to andesitic lava seriesby fractional crystallization and magma mixing. Trace elementdata indicate that the parental basalts came from a heterogeneousmantle source. The unmodified composition of the mantle wedgeis similar to that beneath the Indian Ocean. To this mantlewas added a subduction component consisting of melt from subductedpelagic sediment and aqueous fluid dehydrated from the subductedbasaltic crust. Lavas from the highly active Taal Volcano onthe west margin of southern Luzon are compositionally more variablethan Mayon lavas. Taal lavas also originated from a mantle wedgemetasomatized by aqueous fluid dehydrated from the subductedbasaltic crust and melt plus fluid derived from the subductedterrigenous sediment. More sediment is involved in the generationof Taal lavas. Lead isotopes argue against crustal contamination.Some heterogeneity of the unmodified mantle wedge and differencesin whether the sediment signature is transferred into the lavasource through an aqueous fluid or melt phase are needed toexplain the regional compositional variation of Philippine arclavas. KEY WORDS: Mayon Volcano; Philippines; sediment melt; subduction component; Taal Volcano     

High-Field-Strength Elements in Mafic Volcanics from North China Craton: Implications for Archean-Proterozoic Boundary and Source Composition

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Petrology and Trace Element Geochemistry of the Honolulu Volcanics, Oahu: Implications for the Oceanic Mantle below Hawaii

The Honolulu Volcanics comprises small volume, late-stage (post-erosional)vents along rifts cutting the older massive Koolau tholeüticshield on Oahu, Hawaii. Most of these lavas and tuff of theHonolulu Volcanics have geochemical features expected of near-primarymagmas derived from a peridotite source containing Fo_87–89olivine; e. g. 100 Mg/(Mg + Fe²⁺) >65, >250 p. p. m. Ni,and presence of ultramafic mantle xenoliths at 18 of the 37vents. Consequently, the geochemistry of the alkali olivinebasalt, basanite, nephelinite and nepheline melilitite lavasand tuff of the Honolulu Volcanics have been used to deducethe composition of their mantle source and the conditions underwhich they were generated by partial melting in the mantle. Compositional trends in 30 samples establish that the magmaswere derived by partial melting of a garnet (<10 per cent)Iherzolite source, which we infer to have been carbon-bearing,from analogy with experimental results. This source was isotopicallyhomogeneous (Sr, Lanphere & Dalrymple, 1980; Pb, Sun, 1980;Nd, Roden et al., 1981), and we infer that the source was compositionallyuniform in all major-element oxides except TiO₂, in compatibletrace elements (Sc, V, Cr, Mn, Co and Ni), and in highly incompatibletrace elements (P, Th, La, Ce). However, the source appearsto have been heterogeneous in TiO₂, Zr, Hf, Nb, and Ta, elementsthat were not strongly incompatible during partial melting.Some nepheline melilitite samples may be derived from a sourcewith distinct Sc and heavy-rare-earth-elements (REE) abundances,or which had a phase or phases controlling the distributionof these elements. The relatively limited abundance range for several elements,such as Ti, Zr, Nb, is partly a consequence of the low degreesof melting inferred for the series (2 per cent for nephelinemelilitite, 11 per cent for alkali olivine basalt), which failedto exhaust the source in minor residual phases. We infer thatthese residual phases probably included phlogopite, amphibole,and another Ti-rich phase (an oxide?), but not apatite. In comparison with estimates of a primordial mantle compositionand the mantle source of mid-oceanic-ridge basalt the garnetperidotite source of the Honolulu Volcanics was increasinglyenriched in the sequence heavy REEs, Y, Tb, Ti, Sm, Zr, andHf all <P <Nd <Sr Ce <La <Nb Ta. A multi-stagehistory for the source of the Honolulu Volcanics is requiredbecause this enrichment was superimposed on a mantle that hadbeen previously depleted in incompatible elements, as indicatedby the relatively low ⁸⁷Sr/⁸⁶Sr ratio, high ¹⁴³Nd/¹⁴⁴Nd ratioand low contents of K, Rb, Ba, and Th. The composition of thesource of the Honolulu Volcanics differs from the source ofthe previously erupted tholeiitic shield. The modal mineralogyof the source of the Honolulu Volcanics is not represented inthe upper-mantle xenoliths, e. g. the garnet pyroxenite andolivine-poor garnet Iherzolite included within the lavas andtuff of the unit.     

Isotope Compositions of Submarine Hana Ridge Lavas, Haleakala Volcano, Hawaii: Implications for Source Compositions, Melting Process and the Structure of the Hawaiian Plume

We report Sr, Nd, and Pb isotope compositions for 17 bulk-rocksamples from the submarine Hana Ridge, Haleakala volcano, Hawaii,collected by three dives by ROV Kaiko during a joint Japan–USHawaiian cruise in 2001. The Sr, Nd, and Pb isotope ratios forthe submarine Hana Ridge lavas are similar to those of Kilauealavas. This contrasts with the isotope ratios from the subaerialHonomanu lavas of the Haleakala shield, which are similar toMauna Loa lavas or intermediate between the Kilauea and MaunaLoa fields. The observation that both the Kea and Loa componentscoexist in individual shields is inconsistent with the interpretationthat the location of volcanoes within the Hawaiian chain controlsthe geographical distribution of the Loa and Kea trend geochemicalcharacteristics. Isotopic and trace element ratios in Haleakalashield lavas suggest that a recycled oceanic crustal gabbroiccomponent is present in the mantle source. The geochemical characteristicsof the lavas combined with petrological modeling calculationsusing trace element inversion and pMELTS suggest that the meltingdepth progressively decreases in the mantle source during shieldgrowth, and that the proportion of the recycled oceanic gabbroiccomponent sampled by the melt is higher in the later stagesof Hawaiian shields as the volcanoes migrate away from the centralaxis of the plume. KEY WORDS: submarine Hana Ridge; isotope composition; melting depth; Hawaiian mantle plume     

Geochronological Constraints on Evolution of Singhbhum Mobile Belt and Associated Basic Volcanics of Eastern Indian Shield

The Singhbhum Mobile Belt (SMB) of the eastern Indian shield represents a roughly east-west-trending arcuate belt of folded supracrustals overlying the granite-greenstone basement of the Singhbhum-Orissa Craton along its northern, eastern and western margins and is bounded by the Chotanagpur Gneissic Complex to further north. The radiometric ages of the basement Singhbhum and equivalent granites and the intrusive anorogenic Mayurbhanj granite pluton constrain the time of evolution of this mobile belt between 3.12 and 3.09 Ga. Hence, the SMB supracrustals also known as Singhbhum Group, is late Mesoarchaean in age and not Proterozoic as thought earlier. The evolution of the SMB was followed by emplacement of some major basic igneous rocks within or adjacent to the supracrustals. These include Simlipal volcanics at >3.09 Ga on the SMB, Mayurbhanj gabbro along with Mayurbhanj granite at 3.09 Ga along the marginal part of the craton near the SMB, and the Dalma volcanics on the SMB along with the Dhanjori volcanics adjacent to SMB at 2.80 Ga. The 2.80 Ga old basic volcanics is also associated with emplacement of some small granite plutons occurring along the marginal part of the craton, one of them, the Tamperkola granite intrudes the SMB. The >3.09 Ga onward igneous activities along the marginal part of Singhbhum-Orissa Craton took place essentially under anorogenic tectonic setting before being affected by a major metamorphism at 2.50 Ga, which is recorded on the Dalma volcanics and on some small granite pluton occurs along the marginal part of the craton. The Jagannathpur and stratigraphically equivalent Malangtoli volcanics, occurring within the Singhbhum-Orissa Craton at the west, were erupted at 2.25 Ga. The boundary between the SMB supracrustals and the Singhbhum-Orissa Craton is demarked by a prominent shear zone known as the Singhbhum Shear Zone, which shows multiple reactivation, the oldest being at 3.09 Ga, followed by subsequent reactivation during Palaeo- and Mesoproterozoic periods at 2.2, 1.8, 1.6-1.5, 1.4 and 1.0 Ga respectively. The Singhbhum Group and the adjacent Chotanagpur Gneissic Complex appear to have evolved from a near shore syn-rift and a distal post-rift stable shelf sedimentary assemblages respectively, which were deposited without any stratigraphic break in a marine basin existed in the present north of the Singhbhum-Orissa Craton. Both of these assemblages were deformed and metamorphosed together during Proterozoic at 2.5 to >2.3 Ga, 1.6 Ga and 1.0 Ga.     

Geochemistry of Lavas from the Emperor Seamounts, and the Geochemical Evolution of Hawaiian Magmatism from 85 to 42 Ma

The Hawaiian–Emperor Seamount Chain (ESC), in the northernPacific Ocean, was produced during the passage of the PacificPlate over the Hawaiian hotspot. Major and trace element concentrationsand Sr–Nd–Pb isotopic compositions of shield andpost-shield lavas from nine of the Emperor Seamounts providea 43 Myr record of the chemistry of the oldest preserved Hawaiianmagmatism during the Late Mesozoic and Early Cenozoic (from85 to 42 Ma). These data demonstrate that there were large variationsin the composition of Hawaiian magmatism over this period. Tholeiiticbasalts from Meiji Seamount (85 Ma), at the northernmost endof the ESC, have low concentrations of incompatible trace elements,and unradiogenic Sr isotopic compositions, compared with youngerlavas from the volcanoes of the Hawaiian Chain (<43 Ma).Lavas from Detroit Seamount (81 Ma) have highly depleted incompatibletrace element and Sr–Nd isotopic compositions, which aresimilar to those of Pacific mid-ocean ridge basalts. Lavas fromthe younger Emperor Seamounts (62–42 Ma) have trace elementcompositions similar to those of lavas from the Hawaiian Islands,but initial ⁸⁷Sr/⁸⁶Sr ratios extend to lower values. From 81to 42 Ma there was a systematic increase in ⁸⁷Sr/⁸⁶Sr of boththoleiitic and alkalic lavas. The age of the oceanic lithosphereat the time of seamount formation decreases northwards alongthe Emperor Seamount Chain, and the oldest Emperor Seamountswere built upon young, thin lithosphere close to a former spreadingcentre. However, the inferred distance of the Hawaiian plumefrom a former spreading centre, and the isotopic compositionsof the oldest Emperor lavas appear to rule out plume–ridgeinteraction as an explanation for their depleted compositions.We suggest that the observed temporal chemical and isotopicvariations may instead be due to variations in the degree ofmelting of a heterogeneous mantle, resulting from differencesin the thickness of the oceanic lithosphere upon which the EmperorSeamounts were constructed. During the Cretaceous, when theHawaiian plume was situated beneath young, thin lithosphere,the degree of melting within the plume was greater, and incompatibletrace element depleted, refractory mantle components contributedmore to melting. KEY WORDS: Emperor Seamounts; Hawaiian plume; lava geochemistry; lithosphere thickness; mantle heterogeneity     

五大连池钾质火山岩中辉石斑晶的反环带结构研究 及其对岩浆演化的约束

五大连池钾质火山岩中发现具有反环带结构的单斜辉石斑晶。反环带辉石的核部为次透辉石,与边部相比具有较低 的Mg#值（68~77）、TiO2 （0.23~0.50 wt.%）、Cr2O3 （<0.06 wt.%）和较高的Al2O3 （3.4~5.0 wt.%）、Na2O（0.43~0.78 wt.%）、 FeO（8.8~11.0 wt.%）、MnO含量。核部的稀土元素标准化曲线较为平坦,且具有非常显著的Eu和Sr的负异常（Eu/Eu*= 0.35~0.63,Sr/Sr* = 0.03~0.17）,指示其为曾与斜长石平衡的变质岩辉石。在Mg#-TiO2相关图上,核部总体成分落入华北下 地壳低Mg麻粒岩中单斜辉石的成分范围,因此核部应该是来自下地壳麻粒岩的捕掳晶。反环带辉石边部与正常辉石斑晶成 分一致,具有较高的Mg#值（81~85）、TiO2（0.40~1.65 wt.%）、Cr2O3（0.03~0.25 wt.%）和较低的Al2O3（2.1~3.4 wt.%）、Na2O （0.34~0.63 wt.%）、FeO（4.6~6.6 wt.%）、MnO含量,轻重稀土分馏明显（La/Yb） N = 3.23~7.89,与玄武岩全岩的特征吻合。 利用主量和微量元素的分配系数进行的模拟计算均表明,反环带辉石边部与寄主岩浆已达到成分平衡,说明边部是在熔蚀 核部的基础上再生长而成的岩浆成因辉石。五大连池正常辉石斑晶的Mg#值与TiO2具有负相关性,指示岩浆在地壳深度经历 了一定程度的演化。反环带辉石斑晶边部由里到外Al2O3和Na2O含量逐渐增加的趋势支持岩浆经历演化的结论。核部辉石 的麻粒岩属性表明岩浆演化主要发生在下地壳。温压计计算结果也指示正常辉石斑晶和反环带辉石的边部都结晶于下地壳 深度的系列岩浆房。晚期结晶的辉石斑晶总是比早期结晶的辉石更富集不相容元素,说明分离结晶作用导致五大连池玄武 岩的不相容元素更富集。平衡计算表明,与辉石斑晶平衡的岩浆与玄武岩全岩在微量元素特征上高度相似。考虑到全岩高 度一致的微量元素特征,研究认为五大连池玄武岩的成分变化主要受下地壳岩浆房中的结晶分离作用控制,地壳混染（包 括壳源岩浆混合）可以忽略不计。     

铅同位素特征对成矿物质来源的制约--以华北地块北缘中段铅锌银矿床为例

华北地块北缘中段是我国铅锌银矿床的重要富集区之一.对这些矿床的成因研究长期以来存在诸多争论.笔者依据成矿地质、地球化学和成矿年代学等特征,将本区铅锌银矿床划分为四种类型.通过对铅同位素地质特征的研究,探讨了成矿物质来源,指出区域古元古代变质基底和中新元古代地层是成矿物质的重要来源之一.     

庐枞早白垩世火山岩的地球化学特征及其源区意义

从中生代到新生代,华北东部岩石圈地幔发生了减薄以及地球化学性质置换, 而扬子地块东部中生代岩石圈地幔也表现出类似的过程,对中生代火山岩的地球化学研究有助于了解这一变化过程以及发生置换时的时空关系。庐枞火山岩出露于扬子地块东部,为一套包括粗玄岩–玄武粗安岩–粗面岩的富碱橄榄安粗岩系。研究了双庙组基性火山岩,这些岩石富集Rb,K,Sr,Th和轻稀土元素,亏损高场强元素。(87Sr/86Sr)i = 0.7060~0.7063,εNd(t )=-3.9~-6.2,(206Pb/204Pb)i=17.788~18.125,(207Pb/204Pb)i= 15.511~15.546,(208Pb/204Pb)i =37.735~38.184。在喷出地表过程中,火山岩没有受到明显的地壳物质混染,因此元素和同位素组成反映了地幔源区的地球化学特征。 其地幔源区具有同位素富集特征,表明火山岩源区曾受到地壳物质的影响,是富集地幔部分熔融的产物,并经历明显的结晶分异作用。庐枞火山岩的岩浆成分和源区特征反映该地区在晚中生代岩石圈地幔的伸展和软流圈地幔上涌的演化过程。     

中拉萨地块南缘孔隆晚白垩世火山岩成因及对地壳演化的约束

拉萨地块晚白垩世岩浆岩的研究集中于南北两缘,中部的岩浆-构造演化对班公湖—怒江洋或新特提斯洋演化过程的响应及地幔贡献度仍然缺乏精确约束.本次研究报道了中拉萨地块南缘孔隆地区发现的一套流纹质火山岩.锆石U-Pb年代学、全岩地球化学及Hf同位素分析结果显示,孔隆火山岩LA-ICP-MS锆石U-Pb年龄为(88.0±1.7)...     

北祁连西段石鸡河地区火山岩地球化学特征及其动力学意义

石鸡河一带是近年新发现的北祁连山多金属成矿带, 气侯和自然条件恶劣, 研究程度很低.该区地层岩性为阳起石岩、角岩和细碧岩; 岩石地球化学和Nd同位素数据显示岩石来源于亏损地幔, 具有N-MORB洋脊玄武岩特征; Sr同位素特征显示地层Sr同位素组成的改变是蚀变引起, 而不是由地壳物质加入引起的.Sm-Nd等时线年龄为481±20 Ma.提出了石鸡河地区地层形成于早奥陶世, 成岩环境为北祁连洋扩张环境, 而非区域资料上显示的残留海盆封闭、大陆碰撞造山环境.      

Early Triassic Silicic Volcanics of South China: Petrogenesis and Constraints on the Geodynamic Evolution of the Paleo-Tethys Ocean Region

The only occurrence of Lower Triassic silicic volcanic rocks within the South China Block is in the Qinzhou Bay area of Guangxi Province. LA-ICP-MS zircon U-Pb dating reveals that volcanic rocks of the Beisi and Banba formations formed between 248.8 ± 1.6 and 246.5 ± 1.3 Ma, coeval with peraluminous granites of the Qinzhou Bay Granitic Complex. The studied rhyolites and dacites are characterized by high SiO2, K2O, and Al2O3, and low MgO, CaO, and P2O5 contents and are classified as high-K calc-alkaline S-type rocks, with A/CNK = 0.98–1.19. The volcanic rocks are depleted in high ?eld strength elements, e.g., Nb, Ta, Ti, and P, and enriched in large ion lithophile elements, e.g., Rb, K, Sr, and Ba. Although the analyzed volcanic rocks have extremely enriched zircon Hf isotopic compositions (εHf(t) = ?29.1 to ?6.9), source discrimination indicators and high calculated Ti-in-zircon temperatures (798–835°C) reveal that magma derived from enriched lithospheric mantle not only provided a heat source for anatectic melting of the metasedimentary protoliths but was also an endmember component of the S-type silicic magma. The studied early Triassic volcanics are inferred to have formed immediately before closure of the Paleo-Tethys Ocean in this region, as the associated subduction would have generated an extensional setting in which the mantle-derived upwelling and volcanic activity occurred.     

The Role of Continental Crust and Lithospheric Mantle in the Genesis of Cameroon Volcanic Line Lavas: Constraints from Isotopic Variations in Lavas and Megacrysts from the Biu and Jos Plateaux

We present a combined Sr, Nd, Pb and Os isotope study of lavasand associated genetically related megacrysts from the Biu andJos Plateaux, northern Cameroon Volcanic Line (CVL). Comparisonof lavas and megacrysts allows us to distinguish between twocontamination paths of the primary magmas. The first is characterizedby both increasing ²⁰⁶Pb/²⁰⁴Pb (19·82–20·33)and ⁸⁷Sr/⁸⁶Sr (0·70290–0·70310), and decreasing_Nd (7·0–6·0), and involves addition of anenriched sub-continental lithospheric mantle-derived melt. Thesecond contamination path is characterized by decreasing ²⁰⁶Pb/²⁰⁴Pb(19·82–19·03), but also increasing ⁸⁷Sr/⁸⁶Sr(0·70290–0·70359), increasing ¹⁸⁷Os/¹⁸⁸Os(0·130–0·245) and decreasing _Nd (7·0–4·6),and involves addition of up to 8% bulk continental crust. Isotopicsystematics of some lavas from the oceanic sector of the CVLalso imply the involvement of a continental crustal component.Assuming that the line as a whole shares a common source, wepropose that the continental signature seen in the oceanic sectorof the CVL is caused by shallow contamination, either by continent-derivedsediments or by rafted crustal blocks that became trapped inthe oceanic lithosphere during continental breakup in the Mesozoic. KEY WORDS: crustal contamination; CVL; megacrysts; ocean floor; osmium isotopes     

热力学计算模拟对月球镁质岩套源区的约束

镁质岩套是月球上最重要的岩石之一.通常认为该类岩石的母岩浆来自月幔翻转引起的深部物质的减压熔融或者月壳底部的混合杂化.但是,这两类模型分别面对无法重现样品中橄榄石的Cr203含量过低等地球化学特征,也无法解释镁质岩套与其他月球样品年龄重合的问题.本文采用热力学计算方法,通过改变源区物质组成、熔融程度、熔融方式及结晶方式等参数,对两种模型进行了一系列的正演研究.结果 表明,月球的镁质岩套有多种成因:月幔物质减压熔融形成的熔体通过平衡结晶可以形成镁质岩套,并满足样品中橄榄石Cr2O3含量的制约;减压熔融模型可以解释某些月球样品年龄的重合,但是成分较为单一,而混合杂化模型形成的镁质岩套成分范围较广.     

Source and Differentiation of Deccan Trap Lavas: Implications of Geochemical and Mineral Chemical Variations

The basalt stratigraphy of the Deccan Trap between MahabaleshwarGhat and Belgaum over-steps the basement from north to south.Sr-isotope and Zr/Nb ratios, and Sr, Rb, and Ba concentrationscorrelate portions of the post-Poladpur stratigraphy over 250km along the Western Ghats, thereby confirming a southerly componentof dip of 0?06?. At the southwestern margin, the stratigraphyextends upwards from the compositionally uniform Ambenali Formation(Cox & Hawkesworth, 1984) into a sequence of grossly heterogeneousflow units which have been allocated to the Mahabaleshwar andPanhala Formations (Lightfoot & Hawkesworth, 1988). TheMahabaleshwar Formation is represented only by a sequence ofhighly fractionated flows (termed the Kolhapur unit) with similar⁸⁷Sr/⁸⁶Sr₀ to the Mahabaleshwar (0?7045), but with Sr<240ppm and TiO₂>2?25%. Succeeding the Kolhapur unit are a seriesof flows with high ⁸⁷Sr/⁸⁶Sr₀ (0?7045-0?705), Zr/Nb > 13,and low Sr (< 200 ppm), which have been allocated to thePanhala Formation, and a group of flows with high ⁸⁷Sr/⁸⁶Sr₀(0?707–0?708) and Sr (>230), but trace element concentrationssimilar to the Mahabaleshwar Formation; these have been allocatedto the Desur unit of the Panhala. Geochemical variations in flows overlying the Ambenali definetwo distinct trends: one is attributed to gabbro fractionation,and the other to variations in the compositions of the parentalmagmas, and arguably their source regions. There is little evidencefor significant crustal contamination in these flows, and thedegree of fractionation and the composition of the phase extractare shown to vary along strike within the Mahabaleshwar Formation.The high TiO₂ content of Kolhapur unit flows is shown to bethe result of shallow-level gabbro fractionation, rather thanthe presence of a primitive high-Ti magma. Mahabaleshwar Formationbasalts exhibit a broad negative correlation between the degreeof fractionation and Sr-isotopic composition. The endmemberwith lower ⁸⁷Sr/⁸⁶Sr₀ has different Zr/Y from the Ambenali basalts,and would appear to have been generated by lower degrees ofmelting of a similar source. The other endmember has more radiogenicSr, lower Zr/Nb, similar Zr/Y, but higher mg-number. The simplestinterpretation is that these magmas were more primitive andhence hotter and more able to interact with the lithosphereen route to the surface, and that they then mixed to producethe Mahabaleshwar array. The Panhala Formation basalts ploton the Sr-Nd array defined by the Mahabaleshwar Formation, andthe Desur unit basalts plot on an extension of this array; thissuggests that the source characteristics are also lithospheric.The absolute elemental abundances may then be a function ofmelting and fractionation. We are impressed by the apparentswitch from crustal lithospheric contributions to mantle lithosphericcontributions through the stratigraphy, and suggest that this,together with the more protracted fractionation of the magma,reflects a change in the availability of the lithospheric componentsaccompanying the southerly migration of the volcanic edifice. ^* Present address: Geoscience Laboratories, Ontario Geological Survey, 11th Floor, 77, Grenville Street, Toronto, Ontario, M7A 1W4, Canada     

Secular Geochemistry of Central Puerto Rican Island Arc Lavas: Constraints on Mesozoic Tectonism in the Eastern Greater Antilles

Island arc volcanism in the Greater Antilles persisted for >70m.y. from Middle Cretaceous to Late Eocene time. During theinitial 50 m.y., lavas in central Puerto Rico shifted from predominantlyisland arc tholeiites (volcanic phase I, Aptian to Early Albian,120–105 Ma), to calc-alkaline basalts (phase II, LateAlbian, 105–97 Ma), and finally to high-K, incompatible-element-enrichedbasalts (phases III and IV, Cenomanian–Maastrichtian,97–70 Ma). Following an island-wide eruptive hiatus, geochemicaltrends were reversed in the Eocene with renewed eruption ofcalc-alkaline basalts (phase V, 60–45 Ma). Progressiveincreases in large-ion lithophile elements (LILE)/light rareearth elements (LREE), LILE/high field strength elements (HFSE),LREE/HFSE, and HFSE/heavy rare earth elements (HREE) characterizethe compositional evolution of the first four volcanic phases.The shift in trace element compositions is mirrored by increasingradiogenic content of the lavas. Pb     

Petrology and Geochemistry of Early Cretaceous Bimodal Continental Flood Volcanism of the NW Etendeka, Namibia. Part 1: Introduction, Mafic Lavas and Re-evaluation of Mantle Source Components

The bimodal NW Etendeka province is located at the continentalend of the Tristan plume trace in coastal Namibia. It comprisesa high-Ti (Khumib type) and three low-Ti basalt (Tafelberg,Kuidas and Esmeralda types) suites, with, at stratigraphicallyhigher level, interstratified high-Ti latites (three units)and quartz latites (five units), and one low-Ti quartz latite.Khumib basalts are enriched in high field strength elementsand light rare earth elements relative to low-Ti types and exhibittrace element affinities with Tristan da Cunha lavas. The unradiogenic²⁰⁶Pb/²⁰⁴Pb ratios of Khumib basalts are distinctive, most plottingto the left of the 132 Ma Geochron, together with elevated ²⁰⁷Pb/²⁰⁴Pbratios, and Sr–Nd isotopic compositions plotting in thelower ¹⁴³Nd/¹⁴⁴Nd part of mantle array (EM1-like). The low-Tibasalts have less coherent trace element patterns and variable,radiogenic initial Sr (     

Petrological and trace element geochemical features of the Okete Volcanics,western North Island,New Zealand

The Okete Volcanics form small volume monogenetic volcanoes situated around the flanks of larger tholeiitic cones of the Plio-Pleistocene Alexandra Volcanics, in the back-arc tectonic environment of western North Island, New Zealand. The lavas and tuffs of the Okete Volcanics have compositions which include basanites, alkali olivine basalts, olivine tholeiites, and hawaiites. Most rocks have Mg numbers >66, >250 p.p.m. Ni, >500 p.p.m. Cr, and often contain ultramafic xenoliths, which indicate that they are very close to being primary magmas. The Okete Volcanics show geochemical trends, from basanite to hawaiite, of progressive depletion of both compatible and incompatible trace elements, progressive increase in Al₂O₃, and heavy REE and Y enrichment with crossingover REE patterns in the hawaiites. These geochemical trends can be accounted for by varying degrees of partial melting of a light REE enriched garnet peridotite with subsequent modification of the melts near source or during ascent by fractional crystallization of olivine and minor clinopyroxene. Mass balance calculations cannot quantitatively constrain the degree of partial melting or fractional crystallization, but nevertheless indicate that the Okete alkali olivine basalts, olivine tholeiites, and hawaiites have been derived by successively larger degrees of partial melting relative to basanites, and have also been progressively more modified by fractional crystallization than have the basanites. Sources of the alkalic melts lay at depths corresponding to >20 kb, and most of the ultramafic xenoliths, apart from some which may be cognate cumulates, are unrelated to the magmas that brought them to the surface. Magmas have changed in composition with time from older smaller-volume volcanoes of basanite or alkali olivine basalt compositions, to younger and more voluminous volcanoes which contain hawaiites. The geochemical trends shown by the Okete Volcanics and their spatial association with voluminous tholeiitic volcanism, are features which are different from those observed elsewhere in the Pliocene to Recent basaltic fields of northern North Island, and may be related to their unique tectonic setting, situated in a distinct structural domain.