Trace element and isotopic variations in a zoned pluton and associated volcanic rocks,Unalaska Island,Alaska: A model for fractionation in the Aleutian calcalkaline suite

Trace elements, including rare earth elements (REE), exhibit systematic variations in plutonic rocks from the Captains Bay pluton which is zoned from a narrow gabbroic rim to a core of quartz monzodiorite and granodiorite. The chemical variations parallel those in the associated Aleutian calcalkaline volcanic suite. Concentrations of Rb, Y, Zr and Ba increase as Sr and Ti decrease with progressive differentiation. Intermediate plutonic rocks are slightly enriched in light REE (La/Yb=3.45–9.22), and show increasing light REE fractionation and negative Eu anomalies (Eu/Eu^*=1.03–0.584). Two border-zone gabbros have similar REE patterns but are relatively depleted in total REE and have positive Eu anomalies; indicative of their cumulate nature. Initial ⁸⁷Sr/⁸⁶Sr ratios in 8 samples (0.70299 to 0.70377) are comparable to those of volcanic rocks throughout the arc and suggest a mantle source for the magmas. Oxygen isotopic ratios indicate that many of the intermediate plutonic rocks have undergone oxygen isotopic exchange with large volumes of meteoric water during the late stages of crystallization; however no trace element or Sr isotopic alteration is evident.Major and trace element variations are consistent with a model of inward fractional crystallization of a parental high-alumina basaltic magma at low pressures (6 kb). Least-squares approximations and trace element fractionation calculations suggest that differentiation in the plutonic suite was initially controlled by the removal of calcic plagioclase, lesser pyroxene, olivine and Fe-Ti oxides but that with increasing differentiation and water fugacity the removal of sub-equal amounts of sodic plagioclase and hornblende with lesser Fe-Ti oxides effectively drove residual liquids toward dacitic compositions. Major and trace element compositions of aplites which intrude the pluton are not adequately explained by fractional crystallization. They may represent partial melts derived from the island arc crust. Similarities in Sr isotopes, chemical compositions and differentiation trends between the plutonic series and some Aleutian volcanic suites indicates that shallow-level fractional crystallization is a viable mechanism for generating the Aleutian calcalkaline rock series.LDGO Contribution no. 2964     

Petrochemical Characteristics and Timing of Middle Eocene Granitic Magmatism in Kooh-Shah, Lut Block, Eastern Iran

The Kooh-Shah region located in a Tertiary volcanic-plutonic belt of the Lut Block in eastern Iran comprises several subvolcanic intermediate to acidic intrusive rocks, diorite to syenite in composition, which have intruded into volcanic rocks. The Kooh-Shah granitoid rocks are characterized by enrichment in large ion-lithophile elements (LILE: e.g. Sr, Ba, Rb) and depletion in high field-strength elements (HFSE: e.g. Nb, Ta, Ti). The chondrite-normalized REE patterns are characterized by moderate LREE enrichment (La/Yb)N=6.01-10.01, medium-heavy REE enrichment, and absence of Eu anomalies. The Kooh-Shah intrusive rocks are metaluminous, shoshonitic with calc-alkaline affinity and high values of magnetic susceptibility, and classified as the magnetite-series of oxidant I-type granitoids. The age of Kooh-Shah granitoid rocks based on zircon U-Pb age dating is 39.7±0.7 Ma (=Middle Eocene) and the ranges of their initial 87Sr/86Sr and 143Nd/144Nd ratios are from 0.704812 to 0.704920 and 0.512579 to 0.512644, respectively, when recalculated to an age of 39 Ma. The initial ?Nd isotope values for the Kooh-Shah intrusive rocks range from -0.18 to 1.09. This geochemical data indicates that the Kooh-Shah granitoid rocks formed from depleted mantle in an island arc setting. The geochemical signature of the studied granitoid rocks represents a characteristic guide for future exploration of copper-gold porphyry-type deposits in the Lut block.     

Volcanism in nascent back-arc basins behind the Shichito Ridge and adjacent areas in the Izu-Ogasawara arc,northwest Pacific: evidence for mixing between E-type MORB and island arc magmas at the initiation of back-arc rifting

Mineral chemistry, major and trace elements, and ⁸⁷Sr/⁸⁶Sr ratios are presented for 29 igneous rocks dredged from the northern portion of the Izu-Ogasawara arc. These rocks are compositionally bimodal. Basement gabbro and trondhjemite from the arc are extremely poor in K₂O (0.05–0.19%) and Rb (0.48–0.62 ppm), and their REE patterns and Sr isotope ratios indicate that there are island arc tholeiites. Quaternary volcanic rocks from the present volcanic front (Shichito Ridge; active arc), back-arc seamounts (east side; inactive arc) and Torishima knoll between the two back-arc depressions (incipient back-arc basins) behind the active arc have the same geochemical characteristics as the above plutonic rocks though they are not as depleted in K and Rb. Rhyolite pumice from the backarc depression is also the depleted island arc tholeiite, whereas basalts from the depression have compositions that are transitional between MORB and island arc tholeiites in trace element (Ti, Ni, Cr, V, Y and Zr) and mineral chemistries. The back-arc depression basalts have relatively high Ba_N/Ce_N(0.66–1.24), Ce_n/Yb_N(1.1–1.9) and K/Ba(45–105) and low ⁸⁷Sr/⁸⁶Sr (0.70302–0.70332) and Ba/Sr (0.1–0.2), which are similar to other back-arc basin basalts and E-type MORB, but are quite unlike the depleted island arc tholeiites. The diverse trace element and Sr isotope compositions of basalt-andesite from the back-arc depressions imply the interplay between E-type MORB and island arc tholeiite. These chemical characteristics and the relationships of (Ce/Yb)_N vs (Ba/Ce)_N and (Ce/Yb)_N vs ⁸⁷Sr/⁸⁶Sr suggest that the back-arc depression magmas are generated by mixing of E-type MORB and depleted island arc tholeiite magmas. Geochemical characters of the associated rhyolite from the depression are compatible with partial melting of lower crust.     

西秦岭天水地区李子园群变质火山岩的地球化学特征及其地质意义

西秦岭天水地区的早古生代李子园群为一套中浅变质的沉积-火山岩系。沉积岩系主要由变质碎屑岩和碳酸盐岩组成,火山岩系主要由变玄武岩、变玄武安山岩和变安山岩组成,包括岛弧型火山岩和玻安岩。岛弧型火山岩SiO2含量介于48.79%~54.64%之间,TiO2含量较低(0.29%~0.88%);稀土元素分布型式呈LREE略富集型,富集大离子亲石元素(LILE)Cs、Sr、Th、U,相对亏损Rb、K和高场强元素(HFSE)Nb、P、Sm、Ti和Y,具Nb负异常,类似于低钾岛弧拉斑玄武岩和钙碱性玄武岩特征。玻安岩具有中等的SiO2含量(53.59%~59.28%),低的TiO2含量(0.24%~0.48%);相对中等的MgO含量(4.90%~4.96%)、较低的CaO/Al2O3比值(0.39~0.54)和较高的Mg#值(0.54~0.58)、Al2O3/TiO2比值(33.88~64.29);同时具有较低的Ti/Zr比值(15~83)、V/Zr比值(2.18~8.35)和较高的Zr/Y比值(3.82~12.08),相对富集大离子亲石元素(LILE),特别是Rb、Ba、Th,而亏损高场强元素(HFSE),如Nb、Ta、P、Ti、Y、Yb,显示为亏损MREE的U型稀土元素分布型式。岛弧型火山岩和玻安岩的存在表明,李子园群及其中的中基性火山岩系形成于俯冲带之上的岛弧或弧前环境。     

Ultramafic Xenoliths in Plio-Pleistocene Alkali Basalts from the Eastern Transylvanian Basin: Depleted Mantle Enriched by Vein Metasomatism

Ultramafic xenoliths from alkali basalts in the Perjani Mountainsin the Eastern Transylvanian Basin (ETB) of Romania are mainlyspinel Iherzolites, although spinel harzburgites, websterites,clinopyroxenites and amphibole pyroxenites are also present.Amphibole veins cut some spinel peridotite samples. All arederived from the shallow lithospheric upper mantle. In general,textural variations are restricted to protogranular and porphyroclastictypes, compared with the more varied textures found in mantlexenoliths from the alkali basalts of the neighbouring PannonianBasin. Also, ETB peridotites are richer in amphibole. Thus,the mantle beneath the edge of the ETB is less deformed butmore strongly metasomatized than the mantle closer to the centreof the Pannonian Basin.Mineralogical and bulk-rock geochemicalvariations resemble those of spinel Iherzolites from other sub-continentalshallow mantle xenolith suites. There is no apparent correlationbetween deformation and geochemistry, and much of the majorand trace element variation is due to variable extraction ofpicritic melts. The REE patterns of separated clinopyroxenesfrom the peridotite xenoliths are mostly LREE depleted, althoughclinopyroxenes from regions adjacent to amphibole veins haveexperienced an enrichment in La and Ce and a change in theirSr and Nd isotopic values towards those of the vein, while stillretaining an overall LREE depletion. Clinopyroxenes from thewebsterites and clinopyroxenites are more variable. Amphibolein the hydrous pyroxenites and amphibole veins is strongly LREEenriched and is considered to be metasomatic in origin. ⁸⁷Sr/⁸⁶Srand ¹⁴³Nd/^l44Nd isotopic ratios of the xenoliths vary between07018 and 07044, and 051355 and 0 51275, respectively. Thesevalue are more depleted than those obtained for xenoliths fromthe Pannonian Basin. The lower ^l43Nd/^l44Nd and higher ⁸⁷Sr/Sr⁸⁶values are found in anhydrous pyroxenites, metasomatic amphibolesin veins and amphibole pyroxenites, and in the only exampleof an equigranular spinel Iherzolite in the suite.The ETB xenolithswere brought to the surface in alkaline vokanism which post-dateda period of Miocene to Pliocene subduction-related cak-alkalinevolcanism. However, the effects of the passage of either slab-derivedfluids or cak-alkaline magmas through the ETB lithospheric mantlecannot be discerned in the chemistry of the xenoliths. The metasomaticamphibole has ⁸⁷Sr/Sr⁸⁶ and ¹⁴³Sr/Sr¹⁴⁴ ratios similar to thehost alkali basalts, but the least evoked cak-alkaline magmasalso have similar Sr and Nd isotope compositions. The REE patternsof the amphibole resembk those of amphiboles considered to havecrystallized from alkaline melts. No preferential enrichmentin elements typically associated with slab-derivedfluids (K,Rb and Sr) relative to elements typically depleted in cak-alkalinemagmas (Ti, 2jr and Nb) has been observed in the vein amphiboles,although some interstitial amphibole is depleted in all incompatibletrace elements, including LREE. Thus, despite its position closeto the calc-alkaline volcanic arc of the Eastern Carpathians,we cannot readily detect any interaction between the lithosphericupper mantle beneath the ETB and subduction-related magmas orfluids. Metasomatism in the lithospheric mantle is instead largelyrelated to the passage of a primitive alkaline magma similarto the host alkali basal ^*corresponding author     

Petrogenesis of Mafic Garnet Granulite in the Lower Crust of the Kohistan Paleo-arc Complex (Northern Pakistan): Implications for Intra-crustal Differentiation of Island Arcs and Generation of Continental Crust

We report the results of a geochemical study of the Jijal andSarangar complexes, which constitute the lower crust of theMesozoic Kohistan paleo-island arc (Northern Pakistan). TheJijal complex is composed of basal peridotites topped by a gabbroicsection made up of mafic garnet granulite with minor lensesof garnet hornblendite and granite, grading up-section to hornblendegabbronorite. The Sarangar complex is composed of metagabbro.The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like,light rare earth element (LREE)-enriched REE patterns similarto those of island arc basalts. Together with the Jijal garnetgranulite, they define negative covariations of La_N, Yb_N and(La/Sm)_N with Eu^* [Eu^* = 2 x Eu_N/(Sm_N + Gd_N), where N indicateschondrite normalized], and positive covariations of (Yb/Gd)_Nwith Eu^*. REE modeling indicates that these covariations cannotbe accounted for by high-pressure crystal fractionation of hydrousprimitive or derivative andesites. They are consistent withformation of the garnet granulites as plagioclase–garnetassemblages with variable trapped melt fractions via eitherhigh-pressure crystallization of primitive island arc basaltsor dehydration-melting of hornblende gabbronorite, providedthat the amount of segregated or restitic garnet was low (<5wt %). Field, petrographic, geochemical and experimental evidenceis more consistent with formation of the Jijal garnet granuliteby dehydration-melting of Jijal hornblende gabbronorite. Similarly,the Jijal garnet-bearing hornblendite lenses were probably generatedby coeval dehydration-melting of hornblendites. Melting modelsand geochronological data point to intrusive leucogranites inthe overlying metaplutonic complex as the melts generated bydehydration-melting of the plutonic protoliths of the Jijalgarnet-bearing restites. Consistent with the metamorphic evolutionof the Kohistan lower arc crust, dehydration-melting occurredat the mature stage of this island arc when shallower hornblende-bearingplutonic rocks were buried to depths exceeding 25–30 kmand heated to temperatures above c. 900°C. Available experimentaldata on dehydration-melting of amphibolitic sources imply thatthickening of oceanic arcs to depths >30 km (equivalent toc. 1·0 GPa), together with the hot geotherms now postulatedfor lower island arc crust, should cause dehydration-meltingof amphibole-bearing plutonic rocks generating dense garnetgranulitic roots in island arcs. Dehydration-melting of hornblende-bearingplutonic rocks may, hence, be a common intracrustal chemicaland physical differentiation process in island arcs and a naturalconsequence of their maturation, leading to the addition ofgranitic partial melts to the middle–upper arc crust andformation of dense, unstable garnet granulite roots in the lowerarc crust. Addition of LREE-enriched granitic melts producedby this process to the middle–upper island arc crust maydrive its basaltic composition toward that of andesite, affordinga plausible solution to the ‘arc paradox’ of formationof andesitic continental-like crust in island arc settings. KEY WORDS: island arc crust; Kohistan complex; Jijal complex; amphibole dehydration-melting; garnet granulite; continental crustal growth     

REE Variations Across the Peninsular Ranges Batholith: Implications for Batholithic Petrogenesis and Crustal Growth in Magmatic Arcs

Rare earth element (REE) patterns of plutonic rocks across theCretaceous Peninsular Ranges batholith vary systematically westto east, transverse to its long axis and structural trends andgenerally parallel to asymmetries in petrologic, geochronologicand isotopic properties. The batholith can be ivided into threedistinct parallel longitudinal regions, each defined by distinctREE pattern types. An abrupt transition occurs between rockswith slightly fractionated REE patterns in the western (coastal)region and rocks with middle to heavy REE fractionated and depletedpatterns in the central region. Further to the east a secondtransition to strongly light REE enriched rocks occurs. Theslopes of the REE patterns within each of these regions arelargely independent of rock type. The first REE transition isclosely coupled to regional discontinuities in other parameters:elimination of negative Eu anomalies, an increase in Sr content,and a marked restriction in petrologic diversity. This transitionoccurs over a range of initial ⁸⁷Sr/⁸⁶Sr ratios and ¹⁸O values,but approximately correlates to a major shift in the emplacementstyle of the batholith from a stationary arc to a rapidly eastward-migrating(cratonward) arc. The sense of the regionally consistent REE trends cannot beexplained by crystallization, assimilation, combined crystallization-assimilation,or mixing processes. The consequences of assimilation and high-leveldifferentiation are not observed generally, despite the sensitivityof the REE to these processes. Geochemical and petrologicalfeatures argue that the partial melting of mafic source rocksin which plagioclase-rich (gabbroic) residual assemblages abruptlygave way laterally and downward to garnet-bearing (eclogitic)residual assemblages produced all the changes associated withthe first REE transition. The change in residual assemblagesfrom gabbroic to eclogitic was superimposed on source regionsalready zoned in light REE abundances, ⁸⁷Sr/⁸⁶Sr and ¹⁸O. Temperatureand pressure constraints on the source regions place them ina subcrustal location. The calcic nature of the batholith andthe dominance of tonalite and low-K²O granodiorite in all itsregions argue that the source materials are broadly basalticin composition. Experimental studies are consistent with thegeneration of the abundant tonalitic magmas by the partial meltingof basalt under both low and high pressure conditions. Arc basaltssuch as those commonly erupted in modern island arcs and continentalmargins are inferred to have provided much of the source materialand the heat. Additional high-¹⁸O components are needed in themore easterly source regions. These materials must be distributedso as to contribute equally to the range of magmas that occurin a given local region, and must preserve the calcic natureof batholithic sources. Altered basalts of ancient oceanic crustand possibly their associated metasediments, previously incorporatedinto the lithosphere beneath the continental margin during earliercycles of subduction, most readily satisfy these constraints. The REE geochemistry of the central and eastern regions of thebatholith differs from that of oceanic island arcs in the presenceof strongly heavy REE depleted and fractionated magmas. A modelis proposed in which arc basalts accumulate beneath a crustallayer. Melting of accumulated material at low pressure producesmagmas of the western region. Where thickening of the basalticunderplate is sufficient to form eclogitic assemblages, eclogite-derivedmagmas of the central and eastern region are produced. The abrupttransition to eclogite-derived magmas that suggests a processdriven by a density instability is responsible for their origin. The Peninsular Ranges batholith appears to be representativeof a major differentiation process in which mantle-derived basaltis remelted, contributing its more sialic fractions to the continentalcrust and leaving its mafic to ultramafic residues in the mantle.This process preserves the sialic character of the continentalcrust and may play a significant role in its growth and evolution.The batholith and the processes that produced it may be a moreappropriate basis than immature oceanic island arcs on whichto construct models of continental growth and evolution.     

Geochemistry of the Lesser Antilles volcanic island arc

New analyses of 1518 rocks for major and certain trace elements are used to examine chemical variations between the 15 larger volcanic islands of the Lesser Antilles island arc. The depth to the top of the subduction zone dipping westward at about 40° lies about 100km below all the volcanoes of the arc. Most of the sampled eruptions are post-Miocene (5-1 m.y.) although south of Martinique, the Oligocene-Miocene and the younger arc are superimposed.There is a chemical variation along the arc axis, from alkalic (southern) through calc-alkalic (central) to tholeiitic (northern) volcanic suites. Three islands are examined in detail as type examples of this variation, i.e. Grenada (south), Dominica (centre), and St. Kitts (north). The Grenada suite includes basanites, alkalic basalts, and subalkalic basalts, andesites and dacites. The subalkalic basalts, andesites and dacites each fall into three chemical groupings along the axis of the arc, distinguished especially by K, Zr, Ni and Cr abundances. The whole Lesser Antilles assemblage is characterised by low K abundances and low K/Rb ratios, compared with other island arcs.The magmas are believed to have evolved through processes of partial melting and crystal fractionation. Partial melting of garnet Iherzolite at about 100km depth in a relatively ‘fertile’ zone of upper mantle in the southern sector, above the subducted slab of basaltic ocean crust, could have produced the undersaturated alkalic magmas. In the central and northern sectors, where the crustal structures are more complex, partial melting may have occurred within more ‘barren’ upper mantle, to produce tholeiitic and calc-alkalic magmas depleted in certain trace elements. In either case, water was probably added to the melted zone from the subducted and hydrated oceanic crust, since the whole arc assemblage was erupted explosively and the rocks are rich in A1₂O₃, plagioclase is very calcic, and amphibole is an important phase. The second process was crystal fractionation at low pressure, as evidenced by the abundance of cumulate xenoliths. Separating phases for the southern volcanoes were olivine, calcic augite and Cr-spinel, followed by hornblende, anorthite and Ti-magnetite at lower temperatures. There is little evidence of the higher-temperature fractionation controls for the central and northern volcanoes.     

Magma genesis in the New Britain island-arc: Constraints from Nd and Sr isotopes and trace-element patterns

A selected suite of fresh volcanic rocks from the New Britain island arc has been analyzed for ¹⁴³Nd/¹⁴⁴Nd, ⁸⁷Sr/⁸⁶Sr, major and trace elements to investigate relationships between isotopes, trace elements and petrology, and depth to the underlying Benioff zone. From these relationships inferences about magma generation are made utilizing Nd and Sr isotope systematics in possible source materials. Lavas ranging in composition from basalt to rhyolite show minimal variation of ¹⁴³Nd/¹⁴⁴Nd. Small variations in ⁸⁷Sr/⁸⁶Sr do not correlate with depth to the Benioff zone, but are related to magma type. Nd-Sr isotopes suggest that island arc lavas in general are derived from a mixture of suboceanic mantle and hydrothermally altered mid-ocean ridge-type basalt, but the New Britain magma source appears homogeneous with little indication of either the involvement of oceanic crust or mantle inhomogeneity. Trace element patterns in New Britain lavas are not consistent with Nd isotope data for currently accepted petrologic and trace element models of magma genesis. Mafic lavas from New Britain and other island arcs have anomalously high Sr/Nd, possibly due to components derived from subducted oceanic crust.     

Geochronologic and isotopic study of the La Désirade island basement complex: Jurassic oceanic crust in the Lesser Antilles?

La Désirade, a small island east of Guadeloupe, is underlain by the only exposed pre-Tertiary basement rocks in the Lesser Antilles. The basement complex comprises spilitic and keratophyric flows and pillow lavas (with interbedded and overlying radiolarian cherts), swarms of mafic to silicic dikes, and subjacent plagiogranite. These features, and the absence of carbonates, terrigenous clastic sediments, or tuffaceous sediments from the complex indicate that it developed in a deep marine environment beyond the reach of terrigenous sedimentation or emergent island arc pyroclastic deposition. Previous workers have suggested that the Désirade basement complex originated either as oceanic crust or during an early (tholeiitic) stage of island arc growth. The isotopic compositions of Sr and Pb from the complex, and previously reported rare earth data (Johnston and Schilling, 1974) do not provide a clear distinction between these two possibilities. Nor does the presence of siliceous keratophyre in the complex rule out an oceanic crustal origin-such rocks are common in well studied ophiolites that originated as oceanic crust. Hence we turn to the age relationships of the complex, the surrounding ocean floor, and adjacent island arcs in an attempt to resolve this problem. The age of the complex strongly supports an oceanic crustal (ophiolitic) origin. The ages of zircons and a previously reported K-Ar age indicate that the complex is 145±5 m.y. old. The complex predates the next oldest volcanic rocks of the Lesser Antilles arc by ca. 110 m.y., and the oldest known rocks of the Aves Ridge, a possible Mesozoic precursor of the Lesser Antilles arc, by 50–60 m.y. This makes it unlikely that the Désirade complex is related to an early phase of either of these arcs. Instead, the age of the complex falls in the range of ages expected for oceanic crust in the vicinity of the Lesser Antilles prior to the development of any subduction zone and resulting arc. Thus we interpret the Désirade complex to be an uplifted segment of oceanic crust that represents the basement on which the later island arcs grew: first the Aves Ridge, an arc that was active in middle to late Cretaceous time (but whose exact mode of origin is enigmatic, and is considered in four alternate tectonic models), then the Eocene to Recent Lesser Antilles arc.     

Geochemistry and Geotectonic Setting of Neoproterozoic Rocks from Northern Ethiopia (Arabian-Nubian Shield)

Whole-rock geochemical analysis of metavolcanic and plutonic rocks from Mai Kenetal-Negash area, Tigrai, northern Ethiopia was carried out to evaluate their magma type and original tectonic environment. Their major element content has been modified by secondary alteration, and trace and rare-earth elements have been used to investigate their petrogenesis. The low content of compatible elements of the metavolcanic rocks and their relatively high content of incompatible elements and light REE point to a depleted source region, whereas the plutonic rocks show a more pronounced REE fractionation (La_N/Lu_N >4). Metavolcanic and plutonic rocks seem to be cogenetic. Discriminant diagrams suggest that the majority of the metavolcanic and all the plutonic rocks are members of a calc-alkaline suite developed in a volcanic arc setting. The overall geochemical characteristics of both sets of rocks are consistent with the arc accretion models postulated in Sudan, Egypt and Saudi Arabia for the Neoproterozoic evolution of the Arabian-Nubian Shield.     

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

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

Ce and Nd isotope geochemistry on island arc volcanic rocks with negative Ce anomaly: existence of sources with concave REE patterns in the mantle beneath the Solomon and Bonin island arcs

¹³⁸Ce/¹⁴²Ce isotope ratios in Cenozoic island arc volcanic rocks are reported for the first time, together with isotope ratios of Nd and Sr and abundances of REE, Ba and Sr. The island arc volcanics studies here are boninites from Chichijima, the Bonin Islands, and basalts and andesites from the Solomon Islands. REE patterns of the island arc volcanic rocks from the Solmon Islands and the Bonin Islands are confirmed to have negative Ce anomalies. It is also disclosed that the majority of these island arc volcanic rocks show mainly positive values for both _Ce and _Nd. It is shown that these _Ce and _Ce values can hardly be interpreted by simple mixing between MORB and oceanic or continental crustal rocks; the former have positive _Nd and negative _Ce and the latter have negative _Ce and positive or negative _Nd. Existence of sources having positive _Ce and _Nd values is strongly suggested. If the sources are assumed to have been fractionated from CHUR (chondritic uniform reservoir) at the early or middle Precambrian era, the sources from which the volcanics were derived are concluded to have kept concave REE patterns with larger (La/Ce)_N and smaller (Nd/Sm)_N ratios than chondritic values over a substantial period of time, until the time of Cenozoic magmatism forming island arc volcanic rocks in question. During the periods of the Cenozoic magmatic activities and their related events, Ce anomalies are considered to have been created. From Ce and Nd isotope ratios, however, it is difficult to determine which of the following processes was responsible for the Ce anomaly; the incorporation process of subducted oceanic crust into magma at the mantle or the slab dehydration and metasomatism process. Nevertheless, so far as Ce and Nd isotopic ratios are concerned, incorporation of oceanic sediments did not take place to any clearly detectable degree.     

安徽伏川蛇绿岩套的Nd-Sr-O同位素研究

安徽歙县伏川的蛇绿岩套形成于中-晚元古宙,其Nd、Sr和O同位素组成是:ε_Nd(T)=+0.7-+3.8,ε_Sr(T)=+30.7-+53.9,δ¹⁸O=3.2-11.0‰。据地质学和同位素地球化学特征,该岩套位于杨子板块南缘、江南古岛弧的弧后小洋盆地轴部。ε_Nd(T)值的变化是由于蛇绿岩形成过程中受到下伏不成熟硅铝质基底地壳的混染引起的;ε_Sr(T)和δ¹⁸O的变化,是在蛇绿岩形成时或形成后不久遭受海水热液蚀变的结果。     

A Complex Petrogenesis for an Arc Magmatic Suite, St Kitts, Lesser Antilles

St Kitts forms one of the northern group of volcanic islandsin the Lesser Antilles arc. Eruptive products from the Mt Liamuigacentre are predominantly olivine + hypersthene-normative, low-Kbasalts through basaltic andesites to quartz-normative, low-Kandesites. Higher-Al and lower-Al groups can be distinguishedin the suite. Mineral assemblages include olivine, clinopyroxene,orthopyroxene, plagioclase and titanomagnetite with rarer amphibole,ilmenite and apatite. Eruptive temperatures of the andesitesare estimated as 963–950°C at fO₂ NNO + 1 (whereNNO is the nickel–nickel oxide buffer). Field and mineralchemical data provide evidence for magma mixing. Glass (melt)inclusions in the phenocrysts range in composition from andesiteto high-silica rhyolite. Compositional variations are broadlyconsistent with the evolution of more evolved magmas by crystalfractionation of basaltic parental magmas. The absence of anycovariation between ⁸⁷Sr/⁸⁶Sr or ¹⁴³Nd/¹⁴⁴Nd and SiO₂ rulesout assimilation of older silicic crust. However, positive correlationsbetween Ba/La, La/Sm and ²⁰⁸Pb/²⁰⁴Pb and between ²⁰⁸Pb/²⁰⁴Pband SiO₂ are consistent with assimilation of small amounts (<10%)of biogenic sediments. Trace element and Sr–Nd–Pbisotope data suggest derivation from a normal mid-ocean ridgebasalt (N-MORB)-type mantle source metasomatized by subductedsediment or sediment melt and fluid. The eruptive rocks arecharacterized by ²³⁸U excesses that indicate that fluid additionof U occurred <350 kyr ago; U–Th isotope data for mineralseparates are dominated by melt inclusions but would allow crystallizationages of 13–68 ka. However, plagioclase is consistentlydisplaced above these ‘isochrons’, with apparentages of 39–236 ka, and plagioclase crystal size distributionsare concave-upwards. These observations suggest that mixingprocesses are important. The presence of ²²⁶Ra excesses in twosamples indicates some fluid addition <8 kyr ago and thatthe magma residence times must also have been less than 8 kyr. KEY WORDS: Sr–Nd–Pb isotopes; U-series isotopes; crystal size distribution; petrogenesis     

钾玄岩系列:藏东玉龙铜矿带含矿斑岩Sr、Nd、Pb同位素组成

西藏东部玉龙铜矿带,包括玉龙、扎拉尕、莽总、多霞松多和马拉松多含矿斑岩,马牧普钾质碱性斑岩和总郭碱性火山岩等Sr、Nd、Pb同位素组成比较一致,其数据点均分布在地幔演化区,接近EMI地幔端元,暗示其物质来源于交代地幔源区。     

The Pleistocene-Recent Tonga-Kermadec Arc Lavas: Interpretation of New Isotopic and Rare Earth Data in Terms of a Depleted Mantle Source Model

New REE data, and new Nd, O, Sr, and Pb isotopic data are presentedand integrated with previous data for this low-K intra-oceanicarc suite. Geochemically, the arc tholeiites and basaltic andesitesrange from extremely HFS element depleted (northern Tonga) tonear N-MORB-like HFS element abundances in L'Esperance (southernKermadecs). LIL elements (Sr, Rb, K, Rb, Ba, Th) show the characteristicselective enrichment generally recognized in arc magmas, andthus indicate decoupling of the HFS and LIL elements. Modellingsuggests a compositionally variable source (mantle wedge) alongthe arc, ranging from restite after remelting an N-type MORBsource (northern end), to progressively less depleted, MORB-likesources southwards. Thus, the low HFS/LIL element ratios areinterpreted in terms of HFS depletion followed by LIL elementenrichment associated with subduction; broad correlations occurbetween Zr/Ba and Sr/Nd ratios (fractionation corrected) and87Sr/86Sr and ¹⁴³Nd/¹⁴⁴Nd ratios. Derivation of the arc magmas from depleted peridotote requiressuperimposed fractional crystallization, which has been modelledthermodynamically using SILMIN (Ghiorso, 1985), utilizing experimentallyproduced partial melts from depleted lherzolite (Jaques &Green, 1980). It is shown that the arc tholeiites and basalticandesites (and also high Mg-andesites) are potentially developedat low pressures 5 kb), from parental magmas also generatedat relatively low pressure ( 10 kb). These data further suggestthat a southward increasing depth of magma segregation (correlatingwith Benioff Zone geometry) could account for differences inchemistry between the Tonga and Kermadec arc segments. The mechanism of LILE enrichment is still highly problematic,but it is suggested that the model of Tatsumi et al. (1986)may account for much of the geochemical data; this involvesrelatively shallow release, via fluids, of LIL elements intooverlying peridotite beneath the fore-arc region. Induced convectionin the mantle wedge moves the metasomatized mantle into thezones of magma generation. The development of the inferred, variably depleted mantle wedgesource is here related to active back-arc spreading which isslightly older and more rapid behind the northern region ofthe arc. The lherzolite restite from this spreading processis interpreted to undergo further partial melting in the metasomatizedmantle wedge overlying the subduction zone, involving inducedconvection. The back-arc island of Niua fo'ou is geochemically quite distinctfrom the arc magmas, being similar to N-MORB in its trace elements,but to OIB in its isotope ratios.     

He–Ar and Nd–Sr isotopic compositions of late Pleistocene felsic plutonic back arc basin rocks from Ulleungdo volcanic island, South Korea: Implications for the genesis of young plutonic rocks in a back arc basin

We report analyses of noble gases and Nd–Sr isotopes in mineral separates and whole rocks of late Pleistocene (< 0.2 Ma) monzonites from Ulleungdo, South Korea, a volcanic island within the back arc basin of the Japan island arc. A Rb–Sr mineral isochron age for the monzonites is 0.12 ± 0.01 Ma. K–Ar biotite ages from the same samples gave relatively concordant ages of 0.19 ± 0.01and 0.22 ± 0.01 Ma. ⁴⁰Ar/³⁹Ar yields a similar age of 0.29 ± 0.09 Ma. Geochemical characteristics of the felsic plutonic rocks, which are silica oversaturated alkali felsic rocks (av., 12.5 wt% in K₂O + Na₂O), are similar to those of 30 alkali volcanics from Ulleungdo in terms of concentrations of major, trace and REE elements. The initial Nd–Sr isotopic ratios of the monzonites (⁸⁷Sr/⁸⁶Sr = 0.70454–0.71264, ¹⁴³Nd/¹⁴⁴Nd = 0.512528–0.512577) are comparable with those of the alkali volcanics (⁸⁷Sr/⁸⁶Sr = 0.70466–0.70892, ¹⁴³Nd/¹⁴⁴Nd = 0.512521–0.512615) erupted in Stage 3 of Ulleungdo volcanism (0.24–0.47 Ma). The high initial ⁸⁷Sr/⁸⁶Sr values of the monzonites imply that seawater and crustally contaminated pre-existing trachytes may have been melted or assimilated during differentiation of the alkali basaltic magma.A mantle helium component (³He/⁴He ratio of up to 6.5 RA) associated with excess argon was found in the monzonites. Feldspar and biotite have preferentially lost helium during slow cooling at depth and/or during their transportation to the surface in a hot host magma. The source magma noble gas isotopic features are well preserved in fluid inclusions in hornblende, and indicate that the magma may be directly derived from subcontinental lithospheric mantle metasomatized by an ancient subduction process, or may have formed as a mixture of MORB-like mantle and crustal components. The radiometric ages, geochemical and Nd–Sr isotopic signatures of the Ulleungdo monzonites as well as the presence of mantle-derived helium and argon, suggests that these felsic plutonic rocks evolved from alkali basaltic magma that formed by partial melting of subcontinental lithospheric mantle beneath the back arc basin located along the active continental margin of the southeastern part of the Eurasian plate.     

Resolving Sediment Subduction and Crustal Contamination in the Lesser Antilles Island Arc: a Combined He-O-Sr Isotope Approach

We report an extensive helium isotope survey of basaltic toandesitic lavas from the Lesser Antilles island arc—anarc system with well-documented evidence of crustal contamination.Given the sensitivity of helium isotopes as a tracer of theeffects of crustal additions, our aim is to evaluate the relationshipof ³He/⁴He ratios to other indices of contamination processessuch as oxygen and strontium isotopes. To this end, we havecarried out 53 ³He/⁴He analyses on separated minerals (olivinesand pyroxenes) from throughout the arc, which we compare withwhole-rock strontium and phenocryst oxygen isotope measurements.We show that the three isotopic tracers show coherent patternsthroughout the Lesser Antilles, indicating a regional controlon crustal contamination. The southern section of the arc (Grenadato Martinique) shows clear evidence for major crustal contaminationin all three isotopic systems with results for our samples inthe range ³He/⁴He_(olivine) 3·6–7·6R_A,     

The Koloa Volcanic Suite of Kauai, Hawaii

The Koloa lavas of the post-erosional volcanic suite of Kauaivary in composition from melilitite to alkali olivine basalt.⁸⁷Sr/⁸⁶Sr varies from {small tilde}0.7030 to 0.7033, the rangebeing similar to that of other Hawaiian post-erosional suites.The chemical composition within single lava flows is variableand forms major-element trends that are different from the mainKoloa trend. The main trend can be related to increased degreesof partial melting during decreasing pressure. The combinedevidence from Sr and Nd isotopes and rare earth elements (REEs)could suggest that an ocean island basalt (OIB) plume was infiltratedby a carbonate-rich melt with a high concentration of Sr andREEs derived from the mid-ocean ridge basalt (MORB) source.However, the Pb-isotope systematics of the Hawaiian islandsdo not suggest that a MORB source was involved during the generationof the posterosional lavas. Instead, we suggest that the Hawaiiantholeiitic and nephelinitic magma suites are derived from azoned plume with a low ⁸⁷Sr/⁸⁶Sr core and a high ⁸⁷Sr/⁸⁶Sr concentricmargin. The trends of the single flows and their variation inisotope ratios suggest that they formed from magmas accumulatedmainly from the central core of the zoned plume.