Geochemistry of Pliocene/Pleistocene basalts along the Central Anatolian Fault Zone (CAFZ), Turkey

Abstract

Pliocene-Pleistocene volcanism accompanied strike-slip-related transtensional deformation along the K?z?l?rmak fault segment of the Central Anatolian fault zone (CAFZ) in the west of ?ark??la (Sivas-central Turkey). These volcanic rocks are represented by alkali olivine basalts. They can be divided into four different sub-groups on the basis of their Zr, Nb, TiO₂ contents. A primitive mantle-normalized incompatible trace element diagram for four subgroups shows close similarity to typical OIB pattern. Some of the incompatible trace element ratios (Ce/Y, Zr/Nb, La/Ba, La/Nb) are also akin to OIB values. Highly fractionated REE patterns (La/Yb_N=24.7–9.2) with no Eu anomaly are the main features of the alkali basalts and are comparable to alkaline volcanism in continental rift zones. On the basis of Al₂O₃/TiO₂, Nb/Y, Zr/Y Zr/Nb ratios, the geochemical differences among four sub-groups can be explained by variable degrees of partial melting of compositionally similar mantle source. Th/Nb, Th/Y, Nb/Y ratios and the primitive mantle-normalized trace element diagram suggests significant amount of crustal involvement for most of the alkali olivine basalts erupted along the CAFZ. Rupture of the continental lithosphere by strike-slip-related transtensional deformation might have caused decompressional partial melting of the asthenospheric mantle and generating alkali olivine basalts in this region. © 2001 Éditions scientifiques et médicales Elsevier SAS.     

Isotope geochemistry of Jeongok basalts,northernmost South Korea: Implications for the enriched mantle end-member component

South Korea separates two mantle source domains for Late Cenozoic intraplate volcanism in East Asia: depleted mid-ocean-ridge basalt (MORB) mantle-enriched mantle type 1 (DMM-EM1) in the north and DMM-EM2 in the south. We determined geochemical compositions, including Sr, Nd, Pb, and Hf isotopes for the Jeongok trachybasalts (∼0.51 to 0.15 Ma K–Ar ages) from northernmost South Korea, to better constrain the origin and distribution of the enriched mantle components. The Jeongok basalts exhibit light rare earth element (LREE)-enriched patterns ([La/Yb]_N = 9.2–11.6). The (La/Yb)_N ratios are lower than that of typical oceanic island basalt (OIB). On a primitive mantle-normalized incompatible element plot, the Jeongok samples show OIB-like enrichment in highly incompatible elements. However, they are depleted in moderately incompatible elements (e.g., La, Nd, Zr, Hf, etc.) compared with the OIB and exhibit positive anomalies in K and Pb. These anomalies are also prime characteristics of the Wudalianchi basalts, extreme EM1 end-member volcanics in northeast China. We have compared the geochemistry of the Jeongok basalts with those of available Late Cenozoic intraplate volcanic rocks from East Asia (from north to south, Wudalianchi, Mt. Baekdu and Baengnyeong for DMM-EM1, and Jeju for DMM-EM2). The mantle source for the Jeongok volcanics contains an EM1 component. The contribution of the EM1 component to East Asian volcanism increases toward the north, from Baengnyeong through Jeongok to Mt. Baekdu and finally to Wudalianchi. Modeling of trace element data suggests that the Jeongok basalts may have been generated by mixing of a Wudalianchi-like melt (EM1 end-member) and a melt that originated from a depleted mantle source, with some addition of the lithospheric mantle beneath the Jeongok area. In Nd–Hf isotope space, the most enriched EM1-component-bearing Jeongok sample shows elevation of ¹⁷⁶Hf/¹⁷⁷Hf at a given ¹⁴³Nd/¹⁴⁴Nd compared with OIB. Recycled pelagic sediments may explain the EM1-end-member component of northeastern Asian volcanism, possibly from the mantle transition zone.     

Geochemical characteristics of the La Réunion mantle plume source inferred from olivine-hosted melt inclusions from the adventive cones of Piton de la Fournaise volcano (La Réunion Island)

Major and trace element compositions were obtained for bulk rocks and melt inclusions hosted in olivine crystals (Fo > 85) from the adventive cones of the Piton de La Fournaise volcano (La Réunion Island). Ratios of highly incompatible trace elements for these magmas are used to identify the nature of the La Réunion mantle plume source. Although adventive cone lavas display unusual major element compositions compared to the historical lavas of the volcano (e.g., lower CaO/Al₂O₃), trace element data suggest that the magmas emitted by the adventive cones originate from a common chemical source. This source may correspond to either a homogeneous mixed source of different mantle components or a near-primitive less-differentiated mantle source. The melt inclusions display ratios of highly incompatible elements (e.g., Th/La, Nb/La) which are similar to primitive mantle values, and lower Nb/U ratios compared to most oceanic basalts. These results and previous isotopic and trace element data suggest that La Réunion plume samples a source which is intermediate between a primitive-like mantle domain and a slightly depleted one almost unaffected by the recycling processes. This source could have originated from early depletion of the primitive mantle. Assuming a depletion 4.45 Gyr ago, ~10% melting of this slightly depleted source could explain the enriched trace element concentrations of the melt inclusions.     

Basalt geochemistry as a diagnostic indicator of tectonic setting

Basalt geochemistry can be used as a diagnostic indicator for determining the tectonic setting of origin, because specific plate tectonic settings often impart distinctive geochemical characteristics. For example: (1) mid-ocean ridge basalts (MORB) and oceanic island basalts (OIB) have clearly distinguishable trace element and Sr-Nd isotope geochemical characteristics; (2) arc related basalts, including IOAB (intra-oceanic arc basalts), IAB (island arc basalts) and CAB (continental arc basalts), exhibit following distinguishing features: all are characterized by low Nb/La ratios (<0.85) and negative Nb, Ta and Ti anomalies; most exhibit low Nb concentrations (<8 ppm), high positive ɛ_Nd values and low enrichment of incompatible elements except the continental arc shoshonitic basalts that possess high concentrations of incompatible trace elements and lower to negative ɛ_Nd values; (3) although contamination by continental crust or lithosphere can impart subduction-like signature (e.g., low Nb, low Ta and low Ti) and lead to misidentification of contaminated continental intraplate basalts as arc related, there are still some essential differences between continental intraplate basalts and arc related ones; such as: uncontaminated continental intraplate basalts have high Nb concentrations, Nb/La > 1, “hump-shaped” OIB-like trace element patterns and moderate positive ɛ_Nd values that distinguish them from the arc related ones; whereas, the contaminated continental intraplate basalts are characterized by pronounced negative Nb, Ta and Ti anomalies, but their concentrations of incompatible trace elements are conspicuously higher than those of subduction-zone basalts that also distinguishes them from the arc related ones; (4) an important difference between back-arc basin basalts (BABB) and the MORB is that the former exhibit both MORB-like and arc-like geochemical characteristics; (5) most oceanic plateau basalts (OPB) show diagnostic geochemical characteristics of enriched MORB (E-MORB) to transitional MORB (T-MORB); only the Kerguelen Plateau is an exception; the early (pre 90 Ma) volcanism of the Kerguelen Plateau is associated with the Early Cretaceous break-up of Gondwana and displays features of continental flood basaltic volcanism; with time, the tectonic setting of the Kerguelen plume-derived volcanism changed from a rifted continental margin setting (133–118 Ma) through a young, widening ocean (118–40 Ma), finally to an oceanic intraplate setting (~40 Ma to the present).Tectonic discrimination diagrams should not be used in isolation, but can still be useful as part of holistic geochemical characterization. For example: (1) MORB and OIB are distinguishable from each other in the 3Tb-Th-2Ta diagram; (2) the arc related basalts, including IOAB, IAB and CAB, constantly plot in the arc-related basalts fields in the Th/Yb-Ta/Yb diagram; (3) the 3Tb-Th-2Ta diagram can be utilized to fully illustrate both MORB-like and arc-like characteristics of BABB; (4) some discriminant diagrams (such as Zr/Y-Zr, Th/Yb-Ta/Yb, 3Tb-Th-2Ta and Hf/3-Th-Nb/16 diagrams) can be used to distinguish continental intra plate basalts from arc related ones; (5) although there are not any discrimination diagrams published that delineate an OPB field, some trace element diagrams can still reveal diagnostic characteristics of the OPB.     

Geochronology and geochemistry of late Cenozoic basalts from the Leiqiong area,southern China

The Leiqiong area, which includes the Leizhou Peninsula and the northern part of the Hainan Island, is the largest province of exposed basalts in southern China. Ar–Ar and K–Ar dating indicates that incipient volcanism in the Leiqiong area may have taken place in late Oligocene time and gradually increased in tempo toward the Miocene and Pliocene Epoch. Volcanic activities were most extensive during Pleistocene, and declined and ended in Holocene. Based on radiometric age dating and geographic distribution, Pliocene and Quaternary volcanism in Hainan Island can be grouped into two stages and six eruptive regions. The early volcanism is dominated by flood type fissure eruption of quartz tholeiites and olivine tholeiites whereas the later phase is dominated by central type eruption of alkali olivine basalts and olivine tholeiites. The systematic decrease of MgO, ΣFeO and TiO₂ with increasing SiO₂ content for basalts from Hainan Island indicates that fractional crystallization of olivine, clinopyroxene and Ti-bearing opaques may have occurred during magmatic evolution. From coexisting Fe–Ti oxide minerals, it is estimated that the equilibrium temperatures range from 895–986°C and oxygen fugacities range from 10^−13.4 to 10^−10.7 atmospheres in the basaltic magmas. The incompatible element ratios and the chondrite-normalized REE patterns of basalts from the Leiqiong area are generally similar to OIB. The Nb/U ratios (less than 37) in most of the tholeiitic rocks and the negative Nb anomaly observed in the spidergram of some basalts indicated that the influence of a paleo-subduction zone derived component can not be excluded in considering the genesis of the basalts from the Leiqiong area. The tholeiites in the Leiqiong area may have mixed with a more enriched lithospheric mantle component as well as undergone relatively larger percentages of partial melting than the alkali basalts.     

Geochemistry and magnetostratigraphy of deccan flows at Anjar, Kutch

Chemical analysis of nine Deccan flow basalts at Anjar, Kutch, western India, indicates that all, except the uppermost flow F-9, are alkaline. In their major and trace element composition, the alkali basalts resemble Ocean island basalts (OIB). Similarities of many diagnostic trace element ratios (e.g. Sm/Nd, Ba/Nb,Y/Nb and Zr/Nb) are similar to those found in the Réunion Island basalts. The uppermost basalt is tholeiitic and chemically resembles the least contaminated Deccan basalt (Ambenali type). The Anjar basalts have iridium concentration ranging between 2 and 178 pg/g. Some of these values are higher by about an order of magnitude compared to the Ir concentration in other basalts of the Deccan. A synthesis of chemical, palaeomagnetic and geochronologic data enables us to construct a chemical and magnetic stratigraphy for these flows. The three flows below the iridium enriched intertrappean bed (IT III) show normal magnetic polarity whereas all except one of the upper basalts show reversed magnetic polarity. The sequence seems to have started in polarity zones 31N and probably continued up to 28R or 27R. The results presented here support the view that Deccan volcanism in Kutch occurred on a time span of a few million years.     

Erratum to: Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex

Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO₂ ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP.     

Review of post-collisional volcanism in the Central Anatolian Volcanic Province (Turkey), with special reference to the Tepekoy Volcanic Complex

Neogene-Quaternary post-collisional volcanism in Central Anatolian Volcanic Province (CAVP) is mainly characterized by calc-alkaline andesites-dacites, with subordinate tholeiitic-transitional-mildly alkaline basaltic volcanism of the monogenetic cones. Tepekoy Volcanic Complex (TVC) in Nigde area consists of base surge deposits, and medium to high-K andesitic-dacitic lava flows and basaltic andesitic flows associated with monogenetic cones. Tepekoy lava flows petrographically exhibit disequilibrium textures indicative of magma mixing/mingling and a geochemisty characterized by high LILE and low HFSE abundances, negative Nb–Ta, Ba, P and Ti anomalies in mantle-normalized patterns. In this respect, they are similar to the other calc-alkaline volcanics of the CAVP. However, TVC lava flows have higher and variable Ba/Ta, Ba/Nb, Nb/Zr, Ba/TiO₂ ratios, indicating a heterogeneous, variably fluid-rich source. All the geochemical features of the TVC are comparable to orogenic andesites elsewhere and point to a sub-continental lithospheric mantle source enriched in incompatible elements due to previous subduction processes. Basaltic monogenetic volcanoes of CAVP display similar patterns, and HFS anomalies on mantle-normalized diagrams, and have incompatible element ratios intermediate between orogenic andesites and within-plate basalts (e.g. OIB). Accordingly, the calc-alkaline and transitional-mildly alkaline basaltic magmas may have a common source region. Variable degrees of partial melting of a heterogeneous source, enriched in incompatible elements due to previous subduction processes followed by fractionation, crustal contamination, and magma mixing in shallow magma chambers produced the calc-alkaline volcanism in the CAVP. Magma generation in the TVC, and CAVP in general is via decompression melting facilitated by a transtensional tectonic regime. Acceleration of the extensional regime, and transcurrent fault systems extending deep into the lithosphere favoured asthenospheric upwelling at the base of the lithosphere, and as a consequence, an increase in temperature. This created fluid-present melting of a fluid-enriched upper lithospheric mantle or lower crustal source, but also mixing with asthenosphere-derived melts. These magmas with hybrid source characteristics produced the tholeiitic-transitional-mildly alkaline basalts depending on the residence times within the crust. Hybrid magmas transported to the surface rapidly, favored by extensional post-collision regime, and produced mildly alkaline monogenetic volcanoes. Hybrid magmas interacted with the calc-alkaline magma chambers during the ascent to the surface suffered slight fractionation and crustal contamination due to relatively longer residence time compared to rapidly rising magmas. In this way they produced the mildly alkaline, transitional, and tholeiitic basaltic magmas. This model can explain the coexistence of a complete spectrum of q-normative, ol-hy-normative, and ne-normative monogenetic basalts with both subduction and within-plate signatures in the CAVP.     

Quaternary Tholeiitic to Alkaline Volcanism in the Karasu Valley,Dead Sea Rift Zone,Southeast Turkey: Sr-Nd-Pb-O Isotopic and Trace-Element Approaches to Crust-Mantle Interaction

Rare-earth-element, radiogenic and oxygen isotope, and mineral chemical data are presented for tholeiitic and alkaline Quaternary volcanism from Karasu Valley (Hatay, southeastern Turkey). Karasu Valley is the northern segment of the Dead Sea transform fault and is filled with flood-basalt type volcanics of Quaternary age. This valley is an active fault zone that is known as “Karasu fault,” extending in a NE-SW direction. The Karasu Valley basaltic volcanics (KVBV) are subaphyric to porphyritic, with variable amounts of olivine, clinopyroxene, and plagioclase phenocrysts. Alkali basalts are generally characterized by high contents of olivine, clinopyroxene, and plagioclase phenocrysts. Their groundmass contains olivine, clinopyroxene, plagioclase, and Fe-Ti oxides. Tholeiitic basalts are subaphyric to porphyritic (high contents of olivine, clinopyroxene, and plagioclase). Their groundmass is similar to that of alkali basalts. The range of olivine phenocryst and microlite compositions for all analyzed samples is Fo₈₁ to Fo₄₃. Plagioclase compositions in both tholeiitic and alkali basalts range from andesine, An₃₈ to bytownite, An₇₂. Clinopyroxene compositions range from diopside to calcic augite. Most of the olivine, plagioclase, and clinopyroxene phenocrysts are normally zoned and/or unzoned. Fe-Ti oxides in both series are titanomagnetite and ilmenite.

Based on normative and geochemical data, the Karasu Valley basaltic volcanics are mostly olivine and quartz-tholeiites, and relatively lesser amount of alkali olivine-basalts. KVBV have low K₂O/Na₂O ratios, typically between 0.25 and 0.45. Olivine- and quartz-tholeiites are older than alkali olivine-basalts. Olivine tholeiites have Zr/Nb and Y/Nb ratios similar to alkaline rocks, but their Ba/Nb, Ba/La, and La/Nb ratios are slightly higher than alkali olivine-basalts. In contrast, quartz-tholeiites have the highest Ba/Nb, Ba/La, Zr/Nb, and Y/Nb and the lowest Nb/La ratios among the KVBV. Alkali basalts have ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios ranging from 0.703353 to 0.704410 and 0.512860 to 0.512910, respectively. In contrast, quartz-tholeiites have higher ⁸⁷Sr/⁸⁶Sr and lower ¹⁴³Nd/¹⁴⁴Nd ratios, which vary from 0.704410 to 0.705490 and 0.512628 to 0.512640, respectively. Olivine tholeiites have intermediate isotopic compositions ranging from 0.703490 to 0.704780 and 0.512699 to 0.512780, respectively. ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb isotopic ratios of KVBV range from 18.817 to 19.325, 15.640 to 15.718, and 39.054 to 39.223, respectively. The range of O isotope values is between +5.84 and +7.97‰. The higher O and Sr isotopes in olivine- and quartz-tholeiites relative to alkali olivine-basalts can be explained by contamination of magmas by crustal materials.

The KVBV have intraplate chemistry similar to that of other tholeiitic and alkaline basalts in other within-plate environments, and isotopes range from isotopically depleted mantle to enriched isotope compositions similar to some enriched ocean islands. Trace-element and isotope data indicate that the KVBV are derived from a common OIB-like asthenospheric mantle source, but they have experienced different degrees of crustal contamination during their ascent to the surface, contemporaneous with little fractional crystallization. Although quartz-tholeiites display significant effects of crustal contamination, alkali olivine-basalts appear to have negligible or no crustal contamination in their geesis.     

Seamount volcanism associated with the Xigaze ophiolite, Southern Tibet

Basaltic lavas at Renbu, Southern Tibet are associated with the Xigaze ophiolite in the Yarlung-Zangbo suture zone. They are alkaline lavas rich in large ion lithophile elements (LILE, Ba, Rb and Sr) and high field strength elements (HFSE, Nb, Ta, Zr and Hf), but poor in Cr, Co and Ni. All of the rocks have chondrite-normalized REE patterns enriched in light rare earth elements (LREE), comparable to modern basalts of the Society Islands, Kerguelen Plateau and Broken Ridge. Abundances of some immobile or moderately immobile elements (Nb, Ta, Zr, Hf, Y, Ti and REE) are also comparable to Kerguelen alkaline basalts. The Renbu basalts are geochemically similar to oceanic island basalts (OIB) and have some elemental ratios, such as Nb/Ta ratios = 15.7–18.1, Th/Nb =  0.06–0.10, La/Nb = 0.59–0.83 and Th/Ta = 1.03–1.52, similar to the primitive mantle. Their ⁸⁷Sr/⁸⁶Sr ratios (0.70453–0.70602) are relatively high, similar to OIB. In the ⁸⁷Sr/⁸⁶Sr vs. εNd(t) diagram, the Renbu basalts plot along a trend from N-MORB to EMII (enriched mantle II), suggesting the involvement of at least two mantle sources in their generation. The Renbu basalts represent seamount volcanism associated with the Xigaze ophiolite. They formed from an OIB-type mantle source within the Neo-Tethyan Ocean that had a composition similar to the modern Indian Ocean mantle.     

东昆仑南缘布青山构造混杂带得力斯坦南MOR型玄武岩地质、地球化学特征及岩石成因

东昆仑南缘布青山构造混杂带发育有较多OIB型玄武岩, 这类玄武岩成因与地幔柱密切相关.与灰岩密切伴生的具有MOR型特征的基性火山岩亦是东昆仑南缘古特提斯洋盆一类重要的海山玄武岩.为了查明布青山构造混杂带中不同类型洋岛或海山玄武岩的岩石成因, 对得力斯坦南玄武岩进行了详细的地质、地球化学和岩石成因研究.布青山地区得力斯坦南出露的玄武岩岩石类型复杂多样, 主要由枕状玄武岩、气孔-杏仁状玄武岩、角砾状玄武岩和块状玄武岩组成.主量元素地球化学特征表明, 该套玄武岩属于深海拉斑玄武岩和洋脊拉斑玄武岩系列.得力斯坦南玄武岩∑REE介于34.51×10-6~61.60×10-6, LREE/HREE介于0.89~1.37, (La/Yb)_N介于0.30~0.56, δEu介于0.90~1.18.球粒陨石标准化稀土元素配分图呈现轻稀土元素亏损的左倾型, 与NMORB型玄武岩稀土元素配分曲线基本相同.得力斯坦南玄武岩Zr、Hf、Nb和Ta含量均相当于NMORB的相应元素的丰度值.Zr/Nb值介于24.59~57.69, Nb/La值介于0.45~0.94, Hf/Ta值介于18.29~31.94.在原始地幔标准化微量元素蛛网图上, 曲线右侧高场强元素基本未分异(Nb、Ta、Zr、Hf等), 并贴近于NMORB标准线, 具有与NMORB玄武岩相似而明显不同于EMORB和OIB型玄武岩的特征.微量元素判别表明其形成于洋中脊或由于洋脊扩张向两侧后移的洋中脊构造环境, 结合其上覆盖有深水硅泥岩及浅水厚层状碳酸盐岩的地质事实, 认为其在地形地貌上属于古海山.岩石成因研究表明该套玄武岩起源于亏损地幔(DM), 并估算其为地幔二辉橄榄岩发生约10%部分熔融的产物.      

Trace Element Geochemistry of Tertiary Continental Alkali Basalts from the Liuhe—Yizheng Area,Jiangsu Province,China

Reported in this paper are the chemical compositions and trace element (REE,Ba,Rb,Sr,Nb,Zr,Ni,Cr,V,Ga,Y,Sc,Zn,Cu,etc)abundances of Tertiary continental alkali basalts from the Liube-yizheng area,Jiangsu Province,China.The olivine basalt,alkali olivine basalt and basanite are all derived from evolved melts which were once af-fected by different degrees of fractional crystallization of olivine and clinopyroxene(1:2)under high pres-sures.The initial melts were derived from the garnet lherzolite-type mantle source through low-degree par-tial melting.The mantle source has been affected by recent mantle-enrichment events(e.g.mantle metasomatism),resulting in incompatible trace element enrichment and long-term depletion of radiogenic isotopic compositions of Sr and Nd.     

Melting of metasomatized subcontinental lithosphere: undersaturated mafic lavas from Rungwe,Tanzania

 This paper uses the geochemistry of primitive mafic lavas from the Rungwe volcanic province (southwestern Tanzania) to infer the source mineralogy and melting history. Post-Miocene mafic lavas from Rungwe include alkali basalts, basanites, nephelinites and picrites with up to 18.9 wt% MgO; nephelinites (>13.5% normative nepheline) are restricted to Kiejo volcano in the southern portion of the province. Rungwe lavas differ from most Western Rift volcanics in that they are not unusually potassic (K₂O/Na₂O ca. 0.40). Sparsely phyric mafic lavas contain phenocrysts and xenocrysts of plagioclase (An_82–90), clinopyroxene (4.5–9.5 wt% Al₂O₃), and olivine (Fo_79–88); one basanite contains a 1 mm xenocryst of apatite included in magnesian clinopyroxene. All samples have high abundances of incompatible elements (e.g., 0.7–2.2 wt% P₂O₅) and are enriched in REE relative to HFSE (Hf, Zr, Ti, Y), Cs, Ba, and K. Some incompatible element ratios are constant throughout the Rungwe suite (e.g., Zr/Nb, Sr/Ce, K/Rb), but other ratios are extremely variable and exceed the range measured in global Ocean Island Basalts (OIB) (e.g., Ba/Nb, Sm/Zr, La/Nb, Pb/Ce, Nb/U). The range in degree of silica saturation, and its excellent correlation with P₂O₅/Al₂O₃, indicate that the Rungwe suite records variable degrees of melting. Variations of individual incompatible trace element abundances in nephelinite and basanite samples suggest that the source contains metasomatic amphibole, ilmenite, apatite, and zircon. The Rungwe suite is interpreted as a series of low-percentage melts of CO₂-rich peridotite at pressures that span the garnet-spinel transition. A geochemical comparison of Rungwe samples to lavas from other Western Rift volcanic centers requires that the source mineralogy varies along the rift axis, although each province is underlain by metasomatized peridotite. The incompatible trace element signatures of Western Rift lavas indicate that the source area is typically homogeneous on the scale of individual volcanoes, although lavas from each volcano reflect a range in degree of melting. Significantly, volcanoes with distinct geochemistry are always separated by major rift faults, suggesting that volcanic and tectonic surface features may correspond to metasomatic provinces within the subcontinental lithospheric mantle. Received: 30 May 1994 / Accepted: 5 April 1995     

Geochemistry and Petrogenesis of the Proterozoic Bandal Mafic Rocks,Himachal Pradesh,NW Himalaya

The Proterozoic Bandal mafic rocks, exposed in Kullu-Rampur window, Lesser Himalaya, Himachal Pradesh, indicate two distinct (high-Ti and low-Ti) magma types. The high-Ti basalts are characterised by high-TiO₂ (> 2 wt%), Ti/Y, Ti/Zr, TiO₂/K₂O and low Rb/Sr ratios. They are enriched in high field strength (HFS) elements (Nb, Zr, Ti) relative to low field strength (LFS) incompatible elements (K, Rb). The low-Ti basalts are charactersied by low TiO₂ (< 2 wt%), Ti/Y, Ti/Zr and high Rb/Sr and Rb/Ba ratios. Quartz-normative composition, continental tholeiite characteristics with Nb/La less than 1 are some of the common factors of the two groups of the Bandal mafic rocks. The trace element concentrations and their ratios of the two groups of the basalts indicate that they have been derived from the asthenosphere at different depths, low-Ti at shallow and high-Ti at deeper levels. Some of the chemical features like low Mg #, Cr, Ni, high incompatible element concentrations (especially Ba), light rare earth element (LREE) enriched patterns point towards assimilation and fractional crystallisation (AFC) process which may have played a significant role in the generation of these basalts.Furthermore, the Bandal mafic rocks, apart from field settings, are geochemically similar to other Proterozoic mafic bodies like the Rampur volcanics, Mandi-Darla volcanics, Garhwal volcanics and Bhimtal-Bhowlai volcanics of the Lesser Himalaya. This widespread Proterozoic continental tholeiitic magmatism over an area of 170,000 km² in the Lesser Himalaya provides an evidence of plume activity in the region.     

西秦岭甘肃夏河县腾布—日周早白垩世玄武岩年代学、地球化学及构造意义

甘肃省夏河县腾布—日周一带发育早白垩世多禾茂组(K1d)火山岩地层,该套地层属于典型的陆内裂谷沉积组合。岩石组成主要为玄武岩,底部偶见复成分砾岩。岩相学及岩石地球化学特征表明腾布—日周玄武岩为钠质碱性橄榄玄武岩,富集LREE及Nb、Ta、La、Zr等,亏损Rb和K等,稀土元素球粒陨石标准化配分曲线及微量元素原始地幔标准化蛛网图,均与世界典型的OIB型玄武岩相似。但是Th/Nb=0.055～0.060,平均为0.057(OIB为0.08),Zr/Nb=6.9～8,平均为7.25(OIB为5.83),Th/La=0.07(OIB为0.11),又与之不同,表明玄武岩岩浆来自软流圈地幔,同时受到地壳物质混染。LA-ICP-MS锆石U-Pb同位素年龄为(106.27±1.3) Ma,另外麦秀山一带多禾茂组中产以Classopollis-Osmundacidites为主的孢粉组合。认为该玄武岩是西秦岭晚中生代大陆裂谷岩浆作用的产物,但是未发育典型的双峰式火山岩,多禾茂组有陆相红层建造,上部万秀组发育类磨拉石建造。因此裂谷作用很可能夭折于岩石圈拉张的早期阶段,并未发展到陆间裂谷阶段。     

Metasomatism of the Pan-African lithospheric mantle beneath the Damara Belt,Namibia, by the Tristan mantle plume: geochemical evidence from mantle xenoliths

In situ trace element analyses of constituent minerals in mantle xenoliths occurring in an alnöite diatreme and in nephelinite plugs emplaced within the central zone of the Damara Belt have been determined by laser ablation ICP-MS. Primitive mantle-normalized trace element patterns of clinopyroxene and amphibole indicate the presence of both depleted MORB-like mantle and variably enriched mantle beneath this region. Clinopyroxenes showing geochemical depletion have low La/Sm_n ratios (0.02–0.2), whereas those showing variable enrichment have La/Sm_n ranging up to 3.8 and La/Yb_n to 9.1. The most enriched clinopyroxenes coexist with amphibole showing similar REE patterns (La/Sm_n = 1.3–4.1; La/Yb_n = 4.5–9). Primitive mantle-normalized trace element patterns allow further groups to be distinguished amongst the variably enriched clinopyroxenes: one having strong relative depletion in Rb–Ba, Ta–Nb and relative enrichment in Th–U; another with similar characteristics but with additional strong relative depletion in Zr–Hf; and one showing no significant anomalies. Amphiboles show similar normalized trace element patterns to co-existing clinopyroxene. Clinopyroxene and amphiboles showing LREE_N enrichment have high Sr and low Nd isotope ratios compared to clinopyroxene with LREE-depleted patterns. Numerical simulation of melt percolation through the mantle via reactive porous flow is used to show that the chromatographic affect associated with such a melt migration process is able to account for the fractionation seen in La–Ce–Nd in cryptically metasomatized clinopyroxenes in Type 1 xenoliths, where melt–matrix interactions occur near the percolation front, whereas REE patterns in clinopyroxenes proximal to the source of metasomatic melt/fluid match those found in modally metasomatized Type 2 xenoliths. The strong fractionation between Rb–Ba, Th–U and Ta–Nb shown by some cryptically metasomatized xenoliths can be also accounted for by reactive porous flow, provided amphibole crystallizes from the percolating melt/fluid close to its source. The presence of amphibole in vein-like structures in some xenoliths is consistent with this interpretation. The strong depletion in Zr–Hf in clinopyroxene and amphibole in some xenoliths cannot be accounted for by melt migration processes and requires metasomatism by a separate carbonate-rich melt/fluid. When taken together with published isotope data on these same xenoliths, the source of metasomatic enrichment of the previously depleted (MORB-like) sub-Damaran lithospheric mantle is attributed to the upwelling Tristan plume head at the time of continental breakup.     

Petrology and Geochemistry of MORB from 25?E to 46?E along the Southwest Indian Ridge: Evidence for Contrasting Styles of Mantle Enrichment

The 1984 PROTEA expedition, leg 5, to the central SouthwestIndian Ridge recovered basaltic lavas from fracture zones andridge segments between 25?E and 48?E. In terms of petrographyand major element variations the samples are unremarkable forocean ridge basalts and range from aphyric to highly plagioclasephyric and from primitive (mg-number = 70) to moderately evolved(mg-number = 40) in composition. Multiply saturated (i.e., olivine,plagioclase, and clinopyroxene) basalts are common within thisregion. There is no systematic difference in compositional characteristicsbetween basalts dredged from fracture zone walls and those dredgedfrom ridge segments, and fractional crystallization has playedan important role in controlling the overall range in lava compositionin both tectonic environments. Incompatible element abundance ratios in the basalts are morenotable and distinguish between geochemically depleted (N-type)MORB with high Zr/Nb (1668) and Y/Nb (4?723) ratios and low(La/Sm)^m, ratios (0-?76–1?00), and geochemically enriched(E-type) MORB with low Zr/Nb (3?4–15?8) and Y/Nb (0?5–8?8)and high (La/Sm). ratios (1?07–3?8). N-type MORB appearsto be absent in the immediate vicinity of Marion Island, butoccurs further along the ridge to the northeast and southwest.Geochemically enriched MORB occurs at scattered localities alongthe ridge but is particularly abundant along the section ofthe ridge closest to the Marion hotspot. In detail, two distinct varieties of E-type MORB can be recognized.The one type has incompatible element and isotopic ratios similarto, although slightly less enriched than, those characteristicof the Marion hotspot (Zr/Nb=5?8–8?6; Y/Nb=0?5–0?8;Ba/Nb=5?1–9?0). The second type can be distinguished byhaving high Ba/Nb ratios (9–22), unlike any lavas directlyassociated with the Marion hotspot, but similar to those characteristicof DUPAL ocean island basalts (OIB). A single sample from thisgroup for which there are isotopic data indicates derivationfrom an isotopically anomalous source region. A model is proposed whereby the sub-oceanic mantle below thisportion of the southwest Indian Ocean has experienced at leasttwo distinct enrichment events. The one is associated with theupwelling of the Marion mantle plume (geochemically characterizedby having low Ba/Nb ratios and normal OIB isotopic ratios).The other is associated with upwelling from a DUPAL source (characterizedby having high Ba/Nb ratio and unusual isotopic ratios) whichhas been proposed to exist beneath this portion of the southwestIndian Ocean (Hart, 1984). On the basis of Ba/Nb and Nb/U ratios,recycled oceanic lithosphere is favoured as a source for theMarion hotspot, while recycled oceanic lithosphere plus ancientpelagic sediment appears to be the most likely source for theDUPAL anomaly and the DUPAL E-type MORB in this region.     

Tholeiitic and alkalic basalts of the oldest Pacific Ocean crust

Approximately 160 Ma old basaltic lavas obtained from ODP Site 801 in the Pigafetta Basin represent the first Jurassic oceanic crust recovered in the Pacific Ocean and the oldest in situ oceanic crust discovered anywhere. The basement consists of an upper alkali olivine basalt sequence and a lower tholeiitic sequence separated by a yellow Fe-rich hydrothermal sedimentary deposit. The aphyric and sparsely plagiodase-olivine±spinel phyric tholeiites exhibit depleted, open–system fractionated characteristics with trace element abundances and Pb–Nd isotopic compositions similar to normal mid-ocean ridge basalts (N-MORB). The aphyric alkali basalts, although showing some overlap in isotopic composition with MORB, exhibit strong similarities in terms of incompatible element abundances to ocean island basalts (OIB). They could represent either OIB-type off-axis volcanism or an alkalic event possibly associated with the waning stages of spreading axis volcanism in the Pigafetta Basin. All lavas have undergone low-grade anoxic smectite–carbonate alteration, although flows underlying the Fe-rich sediments have suffered hydrothermal alteration and fracturing.     

South-to-north pyroxenite–peridotite source variation correlated with an OIB-type to arc-type enrichment of magmas from the Payenia backarc of the Andean Southern Volcanic Zone (SVZ)

New high-precision minor element analysis of the most magnesian olivine cores (Fo_85–88) in fifteen high-MgO (Mg#_66–74) alkali basalts or trachybasalts from the Quaternary backarc volcanic province, Payenia, of the Andean Southern Volcanic Zone in Argentina displays a clear north-to-south decrease in Mn/Fe_ol. This is interpreted as the transition from mainly peridotite-derived melts in the north to mainly pyroxenite-derived melts in the south. The peridotite–pyroxenite source variation correlates with a transition of rock compositions from arc-type to OIB-type trace element signatures, where samples from the central part of the province are intermediate. The southernmost rocks have, e.g., relatively low La/Nb, Th/Nb and Th/La ratios as well as high Nb/U, Ce/Pb, Ba/Th and Eu/Eu* = 1.08. The northern samples are characterized by the opposite and have Eu/Eu* down to 0.86. Several incompatible trace element ratios in the rocks correlate with Mn/Fe_ol and also reflect mixing of two geochemically distinct mantle sources. The peridotite melt end-member carries an arc signature that cannot solely be explained by fluid enrichment since these melts have relatively low Eu/Eu*, Ba/Th and high Th/La ratios, which suggest a component of upper continental crust (UCC) in the metasomatizing agent of the northern mantle. However, the addition to the mantle source of crustal materials or varying oxidation state cannot explain the variation in Mn and Mn/Fe of the melts and olivines along Payenia. Instead, the correlation between Mn/Fe_ol and whole-rock (wr) trace element compositions is evidence of two-component mixing of melts derived from peridotite mantle source enriched by slab fluids and UCC melts and a pyroxenite mantle source with an EM1-type trace element signature. Very low Ca/Fe ratios (~1.1) in the olivines of the peridotite melt component and lower calculated partition coefficients for Ca in olivine for these samples are suggested to be caused by higher H₂O contents in the magmas derived from subduction zone enriched mantle. Well-correlated Mn/Fe ratios in the wr and primitive olivines demonstrate that the Mn/Fe_wr of these basalts that only fractionated olivine and chromite reflects the Mn/Fe of the primitive melts and can be used as a proxy for the amount of pyroxenite melt in the magmas. Using Mn/Fe_wr for a large dataset of primitive Payenia rocks, we show that decreasing Mn/Fe_wr is correlated with decreasing Mn and increasing Zn/Mn as expected for pyroxenite melts.     

Crystallization processes beneath the southern Mid-Atlantic Ridge (40–55°S), evidence for high-pressure initiation of crystallization

New major and trace element data are reported for a suite of basalts dredge sampled from the southern MAR (40.6-52.5°S) during cruise 9309 of the R/V Maurice Ewing and cruise 32 of the S.A. Agulhas (51-54.5°S). Samples range from aphyric to moderately porphyritic with plagioclase and olivine the dominant phenocryst and microphenocryst phases. Clinopyroxene occurs rarely as a phenocryst phase. Bulk rock and quench glass samples have major element compositions ranging from moderately primitive (Mg#=68.5) to evolved (Mg#=41.1), with a comparable range in incompatible (Zr=54-232 ppm; Nb=0.8-21 ppm) and highly compatible trace element (Ni=51-289 ppm; Cr=39-949 ppm) abundances. Incompatible trace element ratios (e.g., Zr/Nb=5.9-69; Y/Nb=0.88-29; (La/Sm)_N=0.48-2.9) indicate both enriched and depleted MORB occurring along this section of the MAR, agreeing with along-ridge radiogenic isotope ratio variations delineating compositional influences of the off-axis Discovery, and a so-called LOMU-component, and ridge-centered Shona mantle plumes (Douglass et al. 1999; Douglass and Schilling 2000). Major- and trace-element variations, particularly decreasing CaO/Al₂O₃ ratios and relatively constant Sc abundances with decreasing Mg# suggest crystallization of olivine, plagioclase, and clinopyroxene in most of these basalts prior to eruption. Liquid-lines-of-descent, derived from forward modeling of various reasonable parental magma compositions at pressures between 1 atm-8 kbar, confirm clinopyroxene as an early crystallizing phase, along with olivine and plagioclase, and its pivotal role in the compositional evolution of these basalts. Pressures deduced using algorithms of Grove et al. 1992 and Michael and Cornell 1998 indicate crystallization beneath ridge segments unaffected by mantle plumes occurred predominately at depths of 3-6 kbar (10-20 km). Crystallization beneath ridge segments affected by the Discovery and Shona mantle plumes occurred over a larger pressure range (1 atm-~7 kbar), but predominantly at crustal depths (1 atm-3 kbar), possibly due to more constant magma fluxes and higher temperatures in the crust and lithosphere in these regions favoring the existence and longevity of subaxial magma chambers.