Geochemistry of peridotite xenoliths in alkali basalts from Jeju Island, Korea

Abstract   Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB–oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)_N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history.     

Hot-spots,mantle plumes and a model for the origin of ultramafic xenoliths in alkali basalts

The Wilson-Morgan hypothesis of hot-spots, characterized by high heat flow, positive gravity anomaly and alkalic volcanism, assumes that such hot-spots are surface expressions of mantle plumes rising by thermal convection. Possible evidence of this mantle upwelling is shown here from textural, structural and chemical aspects of ultramafic xenoliths in alkalic basalts. The xenolith-bearing basalts are constanly associated with Wilson-Morgan hot-spots in the ocean basins and with their continental counterparts in the rift valleys which show extensional tectonics. Most of the xenoliths are considered to be accidental fragments of the lithosphere in the host basalts. One remarkable aspect of xenoliths from all parts of the world is their ubiquitous tectonite fabric. The microstructures of these xenoliths are due to plastic deformation. Some of the xenoliths from Baja California show characteristic deformational features which are also found in the marginal parts of diapirically intruded high-temperature peridotite massifs. A model is proposed for the origin of xenoliths in alkalic basalts by mantle upwelling in which the plastic deformation of the xenoliths reflects this dynamic uprise.     

Petrogenesis of anhydrous clinopyroxenite xenoliths and clinopyroxene megacrysts in alkali basalts from the Ganseong area of South Korea

Late Cenozoic alkali basalts in the Ganseong area of South Korea contain abundant ultramafic xenoliths and clinopyroxene megacrysts. Anhydrous clinopyroxene‐rich wehrlite–clinopyroxenites make up the majority of the xenolith population and range from wehrlite through olivine clinopyroxenite to clinopyroxenite. This study investigates the petrogenesis of wehrlite–clinopyroxenite xenoliths and clinopyroxene megacrysts on the basis of petrography and mineral and whole‐rock chemistry. Observations such as an absence of carbonate or apatite, high Ti/Eu ratio, and clinopyroxene‐dominated mineralogy lead us to rule out peridotite–melt reactions as the origin of the Ganseong wehrlites– olivine clinopyroxenites. The whole‐rock compositions (e.g. high abundance of CaO at a given MgO content and low abundance of incompatible elements, such as U, K, P, and Ti compared with mafic melts) indicate that the pyroxenites do not represent crystallized magma itself, but are rather cumulates with a small amount of residual liquid. Anhydrous and orthopyroxene‐free mineral assemblages, crystallization sequence of olivine→clinopyroxene→plagioclase, and mineral chemistries (e.g. low Cr# and high TiO₂ abundances in spinels and high TiO₂ and Na₂O abundances in clinopyroxenes at a given Mg#) suggest that relatively anhydrous intraplate alkaline basalt is the most likely candidate for the parent magma. Texture and compositions of the clinopyroxene megacrysts preclude a cognate origin via high‐pressure crystallization of the host magma. The clinopyroxene megacrysts occupy the Fe‐rich end of the compositional trends defined by wehrlite–pyroxenite clinopyroxenes. Progressive decreases in Mg# and an absence of significant compositional gaps between pyroxenite xenoliths and clinopyroxene megacrysts indicate fractionation and differentiation of a similar parental magma. We suggest that the clinopyroxene megacrysts represent fragments of pegmatitic clinopyroxenites crystallized from more advanced fractionation stages of the evolution of a series of magmatic liquids formed Ganseong wehrlite–clinopyroxenites.     

A worldwide comparison of alkali olivine basalts and their differentiation trends

Worldwide alkali olivine basalts (AOB) and their differentiation series have been subdivided into continental, oceanic, or island-arc assemblages according to the inferred crustal environment at their time and place of eruption. No systematic differences have been found in major element composition of the AOB's from these three different environments. As plotted on (Na₂O + K₂O) vs. SiO₂ and AMF diagrams, AOB differentiation trends also show no differences between environments. Thus, AOB appears to be a primary magma generated at sufficient depth in the mantle that its major element content is unaffected by chemical or thermal differences between mantle regions underlying continents, ocean basins, or island arcs. The major element chemistry of AOB is also apparently unaffected by passage through different types of crust.     

Trace elements in alkali basalts and their inclusions in the Dariganga area,Mongolia

Alkali basalts of Dariganga contain inclusions of dunite, harzburgite, peridotite, and eclogite-like rock. The dunites and peridotites contain few trace elements, but these are somewhat more abundant in the eclogite-like rock, and the alkali basalts are rather rich in them. The trace elements show a more or less continuous change in abundance that can be related by recognized crystal-chemical laws to the major elements of the minerals that are present in the various rock types. Hypotheses for the origin of basaltic magma are discussed in relation to the trace-element distribution in the Dariganga rocks, but it is concluded that the available evidence does not permit any definite decision as to which is correct.     

Trace element and Sr and Nd isotope geochemistry of peridotite xenoliths from the Eifel (West Germany) and their bearing on the evolution of the subcontinental lithosphere

Peridotite xenoliths from the Eifel can be divided into incompatible element-depleted and -enriched members. The depleted group is restricted to dry lherzolites whereas the enriched group encompasses dry harzburgites, dry websterite and amphibole and/or phlogopite-bearing peridotites. Isotopically the depleted group is very diverse with¹⁴³Nd/¹⁴⁴Nd ranging from 0.51302 to 0.51355 and⁸⁷Sr/⁸⁶Sr from 0.7041 to 0.7019, thus occupying a field larger than expected for oceanic-type subcontinental mantle. These xenoliths are derived from a mantle which appears to have diverged from a bulk-earth Nd and Sr isotopic evolution path 2 Ga ago as a consequence of partial melting. The combination of high¹⁴³Nd/¹⁴⁴Nd with high⁸⁷Sr/⁸⁶Sr in some members of the depleted-xenoliths suite is likely to be the result of incipient reaction with incompatible element-enriched fluids in the mantle. In the enriched group such reactions have proceeded further and erased any pre-enrichment isotope memory resulting in a smaller isotopic diversity (¹⁴³Nd/¹⁴⁴Nd 0.51256–0.51273,⁸⁷Sr/⁸⁶Sr 0.7044–0.7032). An evaluation of SmHf and YbHf relationships suggests that the amphibole-bearing lherzolites and harzburgites acquired their high enrichment of light rare earth elements by fluid infiltration into previously depleted peridotite rather than by silicate melt-induced metasomatism. Upper mantle composed of such metasomatized peridotites does not represent a potential source for the basanites and nephelinites from the Eifel. The isotopic and chemical diversity of the subcontinental lithospheric part of the mantle may result from it having remained isolated from the convecting mantle for times > 1 Ga.     

Ladinian radiolarian fauna,siliceous rock from the Xianshuihe Belt,West Sichuan and their tectonic significance

Ladinian radiolarian fauna, including Muelleritortis, Baumgartneria, Oertlispongus, Paroertlispongus, Pseudoertlispongus, etc., was discovered from the siliceous rock of the Runiange Formation in the Xianshuihe belt, West Sichuan Province. Geochemical test on five samples from the siliceous rock indicates that SiO2 content varies in 71.16%-90.06% and Si/Al ratio, in 49-71, which shows that the siliceous rock contains more terrigenous mud sediments. The siliceous rock is characterized by the large ratios of Al2O3/(Al2O3+Fe2O3) (0.63-0.81) and Ti/V (>26), the low ratio of V/Y (<2.8), and low vanadium content (<23 μg/g), which are similar to the geochemical characteristics of continental margin siliceous rock. The Ce/Ce* ratios of the four samples vary in 1.02-1.47 and the LaN/CeN ratio, in 0.75-1.07, which imply that the siliceous rock was deposited in the continental margin basin. But only one sample is similar to the oceanic siliceous rock in REE. Turbidite-siliceous rock bearing radiolarian-basalt asse     

The palaeorotation of the troodos microplate,cyprus, in the late mesozoic-early cenozoic plate tectonic framework of the Eastern Mediterranean

The Troodos ophiolite formed during the Late Cretaceous (Turonian) in a spreading setting above a northerly-dipping subduction zone. Palaeomagnetic data establish that the ophiolite experienced a 90° anticlockwise palaeorotation that occurred during the Late Cretaceous-Early Eocene interval During this period, the ophiolite was stranded adjacent to a generally south-facing continental margin, the remnants of which are preserved in south-west Cyprus (Mamonia Complex), southern Cyprus (Moni melange) and in northern Cyprus (Kyrenia Range). A review of field evidence from these Mesozoic basin margin units shows that strike-slip played a critical role in their Late Cretaceous-Early Tertiary deformation while magnetic and gravity surveys support the existence of major lineaments preserved at depth beneath these areas. Thus, in agreement with available palaeomagnetic data from mainland Turkey and Africa, the rotated crustal unit is believed to be relatively small with its boundaries probably located in the vicinity of Cyprus. In the favoured palaeorotation model, a small supra-subduction zone oceanic crustal unit was rotated about a pole located close to the exposed ophiolite complex. Fragments of the northern continental margin became attached to the rotating microplate along strike-slip lineaments and were then carried southward to their present position. In the light of the regional tectonic setting, it was probably the oblique consumption of crust beneath the ophiolite and/or a collision outside the present area of Cyprus that provided the necessary driving force for the palaeorotation.     

The nature and distribution of carbon in submarine basalts and peridotite nodules

Primary carbonaceous material has been identified in submarine basaltic glasses and mantle-derived peridotite nodules from alkali basalts using electron microprobe techniques. In the submarine rocks carbon occurs (1) in quench-produced microcracks in glasses and phenocrysts, (2) in vesicles, where it is preferentially concentrated on the sulfide spherules attached to vesicle walls, and (3) in microcracks and CO₂-rich bubbles in inclusions of glass completely enclosed by phenocrysts. In peridotite nodules carbon exists in intergrain cracks, along grain boundaries, and on the walls of fluid inclusions disposed in two dimensional arrays. The carbonaceous material is believed to consist of a mixture of graphite, other forms of elemental carbon, and possibly small amounts of organic matter.It is suggested that carbon precipitates by disproportionation of CO according to the reaction 2 CO→C+CO₂ and that this reaction is catalyzed by sulfide-oxide surfaces in vesicles. Once deposition has begun, the reaction continues on carbon surfaces as well. Based on the large amounts of condensed carbon observed in some vapor inclusions and the apparent lack of oxidation features associated with them, it is proposed that carbon condensed from a magmatic vapor in which CO was a significant constituent. This implies that oxygen fugacities of undegassed basaltic melts under confining pressures of the shallow crust are typically lower than those of the QFM buffer at equivalent temperatures. This is in agreement with some intrinsic oxygen fugacity measurements on similar undegassed materials.Regardless of the mechanism of its formation, the presence of carbon in CO₂-rich vesicles and inclusions in basaltic glasses and mantle nodules adds uncertainty to estimates of minimum pressures of entrapment based on measurements of fluid densities. Condensed carbon also accounts for some of the carbon isotopic characteristics of these rocks.     

西昆仑库科西鲁克地区基性岩脉地球化学及构造意义

新疆库科西鲁克地区广泛发育基性岩脉,多呈岩墙、岩枝和小岩滴。基性岩脉岩石类型为辉长岩和辉绿岩。辉长岩属于碱性玄武岩,而辉绿岩属于过铝质碱性系列碱玄武岩与粗面玄武岩过渡型,其形成深度（浅成相）比辉长岩浅（中深成相）。区内基性岩脉形成于闭合边缘岛弧、活动陆缘造山带环境,是由幔源原生岩浆经过分异并同化混染地壳物质而形成,结晶分异是控制岩浆演化的主要因素。辉长岩中δEu具有弱的亏损。辉绿岩δEu为正异常,而C e均具弱亏损,成岩氧逸度较高,其成因与典型的I型花岗岩类相似,为壳幔混合型。辉长岩偏幔源,而辉绿岩偏壳源,可能为幔源岩浆上侵受围岩混染所致。辉长岩年龄（119 M a）要比辉绿岩年龄（46.1M a）老,辉长岩为冈底斯陆块向欧亚大陆板块碰撞拼贴过程中,逆冲挤压结束的标志,辉绿岩为大规模逆冲挤压剪切结束,青藏高原隆升初期拉张作用的产物。     

Silicate systems bearing on the crystallization courses in alkali basalts

Phase-equilibrium studies of the nepheline normative portion of the basalt tetrahedron nepheline-forsterite-silica-diopside ofYoder andTilley have shown that during the course of crystallization the composition of the liquid phase leaves this tetrahedron. When the tetrahedron is expanded to nepheline-forsterite-silica-Ca₂SiO₄ the courses of crystallization and composition of the liquid can be and have been followed by studying a series of joins within this expanded tetrahedron. These studies show that the ultimate goal of crystallization is the quaternary invariant point diopsidic pyroxene + nepheline solid solution + sodic plagioclase + wollastonite solid solution + liquid at 950 ± 5°"C. Attention is called to this low melting point in a dry system. With perfect equilibrium between the solid phases and liquid all melilite disappears by reaction with liquid below 1065 ± 5°C, the temperature of the quaternary reaction point with diopsidic pyroxene + melilite + nepheline solid solution + wollastonite solid solution + liquid. A flow sheet showing the reactions between all quaternary invariant points in the geologically interesting portion of the expanded tetrahedron is presented. This shows the interrelations (daughter-parent relations) between a large number of rock types found in intrusive and extrusive nepheline normative compositions as well as in the quartz normative tholeiitic basalts.     

The mineral chemistry of pyroxenite xenoliths in the volcanic rocks of Hoh Xil and their significance

The pyroxenite xenoliths in the volcanic rocks of Hoh Xil consist of clinopyroxenes and orthopyroxenes. The mineral composition of these pyroxenes is similar to that of mantle xenoliths including peridotite and pyroxenite from China and abroad, and different from that of granulites. The pyroxenes formed at 1101–1400°C (averaging 1250°C) and under 30–60 kb (averaging 46 kb). We deduced that the magma was derived from the mantle at a depth of more than 150 km, which fits in with the geophysical conclusion that the low-velocity layer existed in the mantle under 150 km.

     

Helium isotopes and mantle volatiles in Loihi Seamount and Hawaiian Island basalts and xenoliths

We examine in this paper the use of helium isotope ratios for the study of hotspot volcanism along age-progressive island volcanic chains. The Hawaiian Islands are the original “high ³He” hotspot, with ³He/⁴He ratios as high as 32 × the atmospheric ratio; in the Pacific they stand out against the surrounding sea of MORB (rather uniformly 8 × atmospheric) which fills the entire Pacific with the exception of the Macdonald-Mehetia-Samoa axis in the South Pacific. The recent availability of a variety of alkalic and tholeiitic glasses from the U.S. Geological Survey and our own dredge hauls has prompted us to look first at isotopic variability within a single fresh and new volcano which is probably sitting directly atop a mantle plume. Thus we have looked in some detail at the total helium in glass pillow rims, at He in the enclosed vesicles, and at He in the glass itself, in both tholeiitic and alkalic lavas, and also at helium in associated phenocrysts and xenoliths. The measured ³He/⁴He ratios range from atmospheric to 30 × atmospheric, but we see clear evidence that the highly vesiculated lavas suffer exchange of He between the thin glass walls of vesicles and ambient seawater, so that we observe a post-eruptive isotopic disequilibrium between glass and gas phases. The primary effect is the very large loss of initial He content during eruptive vesiculation, which results in quite large isotopic effects from small additions of ambient He (of the order of 0.02 μcc He per gram of basalt; corresponding to a “water/rock ratio” of 0.5). Phenocrystic He in olivines verifies that the gas-phase He is not affected by vesicularities up to about 5%. Alkali basalt He appears to be independent of vesicularity up to values as high as 35%; this He is somewhat lower in ³He/⁴He ratio, but matches precisely the associated xenolithic He. However, from the present data we cannot exclude the possibility that diffusive exchange with seawater has affected the He ratio in alkalic vesicles.On the large scale, along the 10% of the Hawaiian chain available for subaerial sampling, we find high ³He/⁴He ratios (24 × atmospheric) in 5.5 × 10⁶-year-old lavas on Kauai. Maximum values of the ratio so far observed are in the pre-erosional Kula basalts on Maui, confirming the previous results of Kaneoka and Takaoka. Hawaii, where these high values were first observed is now seen to range from MORB ratios at Mauna Loa to only 15 × R_A at Kilauea fumaroles. Most xenolithic He so far measured is MORB He, but Loihi xenoliths have high values and are quite different in this respect. Finally, we discuss also the hydrogen and carbon isotope results on Loihi lavas, and show that these elements resemble MORB and appear not to show a distinctive plume signature.     

Helium isotope studies of the mantle xenoliths and megacrysts from the Cenozoic basalts in the eastern China

Helium isotope compositions of the mantle xenoliths and megacrysts in the Cenozoic basalts in the eastern China were measured. The samples were collected from Ludao of Heilongjiang, Huinan and Jiaohe of Jilin, Kuandian of Liaoning, Hannuoba of Hebei, Nüshan of Anhui, Dingan of Hainan. The³He/⁴He ratios of the mantle xenoliths and megacrysts from the most areas were about 1 × 10^-5, and were similar to those of the MORB, thus reflecting the characteristics of the MORB-typed depleted mantle. The³He/⁴He ratios of the mantle xenoliths from Jiaohe were 4.8×10^-6 and the³He/⁴He ratios of xenoliths from Hannuoba vary from 0.15× 10^-6 to 7.4 ×10^-6, obviously lower than those of the MORB, and even lower than the atmospheric helium isotope ratios, indicating that the continental mantle was strongly replaced in Jiaohe and Hannuoba areas. The helium isotope compositions of the mantle xenoliths and megacrysts in the same region vary in a very wide range. It is inferred that the mantle xenoliths and megacrysts were from different parts of the continental mantle. There were not necessary origin relations between the mantle xenoliths, megacrysts and their host basalts. An extremely high³He/⁴He ratio of garnet megacryst from Hannuoba, Hebei was found.     

Comments on petrogeneses and the tectonic setting of Columbia River basalts

Data on major and a few trace element contents for specimens of five formational-scale units of the Columbia River Basalt Group are reviewed and compared with those for other flood basalts and the Chyulu basalts of Kenya. Flows of the Grande Ronde Formation, which constitutes more than two-thirds by volume of the Columbia River Basalt Group, in several respects resemble calc-alkaline rocks, and they plot in the field for magmas erupted at destructive plate margins on a ThHfTa diagram. These features of Grande Ronde flows may be related to the anomalous setting of the Columbia River Basalt Group as a whole. Calc-alkaline characteristics are exhibited by other flood basalts but rarely to the degree shown by Grande Ronde flows. Few if any of the Columbia River basalts represent primary magmas. Rather, they generally have been affected by multi-stage fractionation, and perhaps also by interaction with lower crustal rocks. Their compositions also may reflect partial melting of fragments of subducted lithospheric slabs.     

Geochemistry of the ophiolite and oceanic island basalt in the Kangxian-Pipasi-Nanping tectonic melange zone, South Qinling and their tectonic significance

Detailed studies indicate that Kangxian-Pipasi-Nanping tectonic zone is a complicated melange zone which includes many tectonic slabs of different origins. Ophiolite (MORB-type basalt), oceanic island tholeiite and alkaline basalt have been identified. Moreover, this tectonic melange zone is eastward connected with the Mianlue suture zone. The deformation characteristics, consisting components and volcanic rock geochemical features for the Kangxian-Pipasi-Nanping tectonic melange zone are much similar to those of the Mianlue suture zone and Deerni ophiolite. Therefore, the Kangxian-Pipasi-Nanping tectonic melange zone should be the westward extension part of the Mianlue suture zone. It indicates that the Mianlue suture zone had extended to the Nanping area.     

Textural, isotopic and REE variations in spinel peridotite xenoliths, Massif Central, France

Sr and Nd isotope analyses and REE patterns are presented for a suite of well-documented mantle-derived xenoliths from the French Massif Central. The xenoliths include spinel harzburgites, spinel lherzolites and some pyroxenites. They show a wide range of textures from undeformed protogranular material through porphyroclastic to equigranular and recrystallised secondary types. Textural differences are strongly linked to trace element geochemistry and variations in radiogenic isotope ratios. Many undeformed protogranular xenoliths are Type IA LREE-depleted with MORB-type ε_Sr values between − 30.7 and − 23.6, and ε_Nd values + 13.9 to + 9.4. A second group of undeformed xenoliths are Type IB LREE-enriched with higher ε_Sr values (− 22.7 to − 10.6) and lower ε_Nd values (+ 11.9 to + 5.6). Deformed xenoliths with porphyroclastic, equigranular and secondary recrystallised textures are all Type IB (LREE-enriched, ε_Nd < 6.4, ε_Sr > 11.8). It is proposed that two separate events have given rise to the observed mixing arrays: (1) MORB-source depleted mantle was enriched by a component derived from an enriched mantle. Deformation and recrystallisation accompanied this event. (2) Subsequently, unenriched MORB-source mantle interacted with magmas chemically akin to the host basalts, and enrichment occurred with little deformation. Hypotheses of Tertiary mantle diapirism resulting in isochemical deformation and refinement of protogranular mantle to equigranular mantle are untenable because of differences in REE patterns and isotopic ratios between different textural groups.     

Early volcanic rocks of réunion and their tectonic significance

A great sequence of hydrothermally altered and tectonically disturbed lavas and breccias is exposed in the erosional cirques of Piton des Neiges volcano, Réunion. These rocks are cut by intense swarms of minor intrusives and are overlain by a carapace of younger lavas which date back at least 2 m.y. The older lavas and breccias are believed to have been formed by submarine eruption of basalt magma during the pre-emergent growth stages of the volcano. It is suggested that their subsequent elevation to 2000 m. above sealevel is due to the thrusting of oceanic crust from the north-east, associated with movements along NE-SW fractures in the western Indian Ocean floor.     

Geochemistry,petrogenesis and tectonic significance of the late Triassic A-type granite in Fujian,South China

The late Permian–Triassic granites in southeastern China have important tectonic significance for the evolution of South China. Here, we present the detailed geochronological, geochemical and petrological analyses for the Jinlongyan(JLY) granite in northwest Fujian Province, southeast China. LA–ICP–MS zircon U–Pb dating yielded a weighted average ~(206)Pb/~(238)U age of 224.1 ±3.3 Ma. The granite is mainly comprised of K-feldspar,plagioclase, quartz, biotite and minor amphibole. It is characterized by enrichments in Rb, Th, REEs(total REE = 295.1–694.3 ppm), and HFSEs(e.g., Zr = 289–520 ppm, Hf = 9.3–15.0 ppm, Y = 36.2–68.2 ppm) but depletions in Ba, Sr, Eu and Ti. The granite is metaluminous to weakly peraluminous and show a clear A-type granite geochemical signature with high SiO_2(70.89 wt%–75.76 wt%), total alkalis(Na_2O + K_2O = 7.51 wt%–8.72 wt%), Ga/Al ratios(10000 Ga/Al = 2.72–3.43). Insitu zircon Hf isotope analysis shows their eHf(t) values ranging from-7.2 to-3.2, with Mesoproterozoic T2DM ages(1308–1525 Ma). Whole-rock Nd isotope data show their eNd(t) values in the range of-9.5 to-9.1 and yield paleoproterozoic TDMages(1606–1985 Ma). These characteristics indicate that the JLY A-type granite magma was formed by the partial melting of Meso-Paleoproterozoic crust rocks in the Cathaysia Block. Our study of the JLY A-type granite, together with other Triassic A-type granitesin South China, defines an extensional environment in the late Triassic which probably was caused by the collision of the South China Block with Indochina Block.