low-K Tholeiites Research Articles

Magmatism in the South Sandwich arc

Abstract The South Sandwich Islands are one of the world’s classic examples of an intraoceanic arc. Formed on recently generated back-arc crust, they represent the earliest stages of formation of arc crust, and are an excellent laboratory for investigating variations in magma chemistry resulting from mantle processes, and generation of silicic magmas in a dominantly basaltic environment. Two volcanoes are examined. Southern Thule in the south of the arc is a complex volcanic edifice with three calderas and compositions that range from mafic to silicic and tholeiitic to calc-alkaline. It is compared to the Candlemas-Vindication edifice in the north of the arc, which is low-K tholeiitic and strongly bimodal from mafic to silicic. Critically, Southern Thule lies along a cross-arc, wide-angle seismic section that reveals the velocity structure of the underlying arc crust. Trace element variations are used to argue that the variations in both mantle depletion and input of a subducted sediment component produced the diverse low-K tholeiite, tholeiite and calc-alkaline series. Primitive, mantle-derived melts fractionally crystallized by c. 36% to produce the most Mg-rich erupted basalts and a high-velocity cumulitic crustal keel. Plagioclase cumulation produced abundant high-Al basalts (especially in the tholeiitic series), and strongly influenced Sr abundances in the magmas. However, examination of volumetric and geochemical arguments indicates that the silicic rocks do not result from fractional crystallization, and are melts of amphibolitic arc crust instead.

Geochemistry and tectonic environment of the Dagzhuka ophiolite in the Yarlung-Zangbo suture zone, Tibet.

The Cretaceous Dagzhuka ophiolite lies in the middle segment of the Yarlung-Zangbo suture zone, Tibet, and has a well-preserved ophiolitic section including a mantle sequence, cumulate complex, diabase dyke swarm and basaltic lavas. Compared to the primitive mantle, tectonized peridotites in the Dagzhuka ophiolite are depleted in Al, Ca, Ti and high field strength elements, but relatively enriched in large ion lithophile elements (Rb, Ba, Th). They have U-shaped, chondrite-normalized REE patterns suggesting an early stage of depletion by partial melting, followed by enrichment from slab-derived fluids. These features support a model in which the mantle peridotites at Dagzhuka formed in a forearc setting. The diabases and basalts are low-K tholeiites depleted in LREE, with compositions similar to N-MORB. However, these rocks are strongly depleted in Nb and Ta but enriched in Th and Rb, features characteristic of island arc lavas. Thus, the Dagzhuka ophiolite is believed to have formed in a supra-subduction zone environment, possibly a fore-arc basin.

Geochemistry and Geochronology of a Neoproterozoic Low-K Tholeiite-Boninite Association in Central Eritrea

The N-S trending Neoproterozoic volcano-sedimentary-plutonic associations in Eritrea are part of the Arabian-Nubian Shield. In Central Eritrea, the dominant rocks are low-grade supracrustal volcanic rocks with subordinate sedimentary rocks. They are intruded by pre- to syn-kinematic diorites and tonalites, as well as late- to post-kinematic granites. The low-K tholeiitic metavolcanic rocks range in composition from basalt to rhyolite. The low-K tholeiites (K 2O = 0.03-0.40 wt.%) represent parental magma (Mg# from 0.46 to 0.71, average 0.63) and are characterized by relatively high abundances of MgO (5-11 wt.%), Cr (174-874 ppm) and Ni (28-249 ppm), as well as low abundances of HFSE (TiO 2 = 0.43-0.69 wt.%, Zr = 15-36 ppm, and Y = 8-20 ppm). A chondrite-normalised REE pattern for a tholeiite is depleted (Ce/Yb N = 0.2). The low-K tholeiites have trace element characteristics and MORB-normalized geochemical patterns that are similar to those of modern arc lavas. Plotting of the intermediate to felsic metavolcanic rocks on various discriminant diagrams also indicates that the metavolcanic rocks are the result of plate-margin volcanism. The geochemical data, together with the low-K tholeiite-boninite association, indicate that the metavolcanic rocks were emplaced in an environment similar to a modern oceanic island-arc (fore-arc) setting. In comparison to the boninites from this region, the low-K tholeiites have low abundances of LILE and HFSE, lower ratios of the more to the less incompatible elements, and higher initial e Nd values. The low-K tholeiites are also characterized by higher total FeO contents and relatively lower contents of refractory elements than is observed for the boninites. This may suggest that the low-K tholeiitic rocks were derived by a relatively lower degree of melting from a more depleted mantle source than the boninites. SHRIMP U-Pb isotope analyses of zircons from a pre- or syn-kinematic tonalite yielded a concordant age of 813±9 Ma. This age is similar to the crystallization age of a pre-/syn-kinematic diorite from the same region and is interpreted to approximate the time of major magmatism in Central Eritrea. Magmatic zircons from a late to post-kinematic granite yielded a concordant U-Pb age of 585±6 Ma. These results indicate that the crust of Central Eritrea was formed between ca. 600 and 850 Ma. These ages are comparable to those of northern Eritrea and adjacent juvenile terranes in the Nubian Shield.

Mantle Plumes and Their Geologic Manifestations

This paper interprets the conditions of plume formation with a focus on interpretations of origin at the core-mantle boundary. We also discuss important geological manifestations including formation of alkaline-basalt oceanic islands, kimberlite pipes, and plateau-basalt fields. The paper concentrates on the Siberian traps that formed between 249 and 245 Ma, one of the world's largest known plateau-basalt fields, and on the magmatic zonation and ore mineralization associated with the field and that are interpreted as forming from a Siberian mantle plume. One part of the field consists of undifferentiated low-K tholeiite basalts in the Tunguska syncline and contains only minor associated ore deposits. Major, unique Cu-Ni and PGE deposits are associated with differentiated alkaline basalt and with mafic-ultramafie intrusions in the northwestern Siberian platform. Angara-Ilim-type magnetite deposits occur in the southern part of the plateau-basalt province and are related to interaction of mafic magmas with overlying carbonate and evaporite sedimentary rocks. Permian and Triassic alkaline granitoid and bimodal magmatic rocks occur in fold-and-thrust belts that frame the southern, southwestern, and northwestern margins of the main trap area. The paper also presents new geochemical and geochronological data for A-granites for the Taymyr region that are synchronous with traps. For both the southern and northern framing areas, porphyry Cu deposits interpreted as derived from a mantle-crust system are associated with subalkaline and alkaline granitoids.

Nd, Pb and Sr isotopes in the Identidade Belt, an Archaean greenstone belt of the Rio Maria region (Carajás Province, Brazil): implications for the Archaean geodynamic evolution of the Amazonian Craton

The Rio Maria granite–greenstone terrain (RMGGT), located in the Amazonian craton at about 250 km south of the Carajás Ridge, is a well preserved Mesoarchaean domain composed of metavolcanic rocks and several suites of granitoids ranging in age from ca. 2.96 to 2.87 Ga. The metavolcanics comprise a basal unit of komatiites and low-K tholeiites overlain by sodic metadacites. A Sm–Nd whole-rock reference isochron for the metabasalts, metagabbros (tholeiites) and metadacites gives an age of 3046±32 Ma (MSWD of 1.6, 1 σ). Nd model ages ( T DM) of the metadacites range between 3.04 and 3.11 Ga. The ε Nd calculated at 3.0 Ga varies from +1.70 to +2.60 for the tholeiites and +0.30 to +3.22 for the metadacites. A Pb–Pb whole-rock linear array for the metabasalts yielded an anomalous older age of 3.41 Ga. The Sm–Nd age is coincident, within analytical error, with a Pb–Pb whole-rock errorchron age of 2944±88 Ma (1 σ) obtained for the metadacites. Rb–Sr systematic for the metadacites yielded an isochron age that is ca. 400 Ma youngest. The isotopic, geochemical and geological data suggest that oceanic lithosphere was the probable source of the ultramafic and mafic rocks in such a way that partial melting of the depleted mantle generated komatiites and low-K tholeiites. Partial melting of oceanic crust transformed into garnet amphibolite or eclogite generated the parental magmas of the metadacites. Isotopic and trace element characteristics of both metavolcanic rocks and associated plutonics suggest intra-oceanic island arcs as the most probable tectonic setting of the RMGGT. Two main events of crustal accretion are recognised at about 3.04–2.96 Ga and 2.87 Ga. Geological correlation indicates that the oceanic island arcs were completely amalgamated at around 2.76 Ga and acted as sources for the overlying sediments of the Rio Fresco Group.

Plate Tectonics at 3.8-3.7 Ga: Field Evidence from the Isua Accretionary Complex, Southern West Greenland.

A 1&rcolon;5000 scale mapping was performed in the Isukasia area of the ca. 3.8-Ga Isua supracrustal belt, southern West Greenland. The mapped area is divided into three units bounded by low-angle thrusts: the Northern, Middle, and Southern Units. The Southern Unit, the best exposed, is composed of 14 subunits (horses) with similar lithostratigraphy, bound by layer-parallel thrusts. Duplex structures are widespread in the Isua belt and vary in scale from a few meters to kilometers. Duplexing proceeded from south to north and is well documented in the relationship between link- and roof-thrusts. The reconstructed lithostratigraphy of each horse reveals a simple pattern, in ascending order, of greenstone with low-K tholeiitic composition with or without pillow lava structures, chert/banded iron-formation, and turbidites. The cherts and underlying low-K tholeiites do not contain continent- or arc-derived material. The lithostratigraphy is quite similar to Phanerozoic "oceanic plate stratigraphy," except for the abundance of mafic material in the turbidites. The evidence of duplex structures and oceanic plate stratigraphy indicates that the Isua supracrustal belt is the oldest accretionary complex in the world. The dominantly mafic turbidite composition suggests that the accretionary complex was formed in an intraoceanic environment comparable to the present-day western Pacific Ocean. The duplex polarity suggests that an older accretionary complex should occur to the south of the Isua complex. Moreover, the presence of seawater (documented by a thick, pillow, lava unit at the bottom of oceanic plate stratigraphy) indicates that the surface temperature was less than ca. 100 degrees C in the Early Archean. The oceanic geotherm for the Early Archean lithosphere as a function of age was calculated based on a model of transient half-space cooling at given parameters of surface and mantle temperatures of 100 degrees and 1450 degrees C, respectively, suggesting that the Archean oceanic lithosphere was rigid. These conclusions-rigidity and lateral plate movement-support the idea that the modern style of plate tectonics was in operation only 0.7-0.8 G.yr. after the formation of the Earth.

On the relationship between subducted slab age and arc basalt petrogenesis, Cascadia subduction system, North America

Olivine-normalized ≤2.0 Ma magnesian basalts erupted close to the volcanic axis of the Cascadia subduction system exhibit arc-parallel compositional variations compatible with a northward decrease in slab-derived components in the underlying mantle wedge. Inferred decreases in slab input correlate strongly with a systematic decrease in age of the oceanic crust along the convergent margin. Geochemical trends are most pronounced in southwestern British Columbia and northern Washington basalts, where the Garibaldi belt strikes obliquely to the trend of isochrons on the subducted plate. From southern Washington to northern California, High Cascades arc segments trend nearly parallel to subducted plate isochrons, and basalts show more subdued arc-parallel variations. These observations support a model in which temperature and depth of melting in the mantle wedge beneath the arc are influenced by the age of the subducted plate. Where the arc is underlain by relatively young and hot oceanic crust, only a minor amount of heating during subduction is required before dehydration reactions begin in the subducted plate. The slab loses much of its volatiles trenchward of the arc, and the volatile budget in the mantle wedge beneath the arc is relatively low. In the Garibaldi belt, which overlies very young oceanic crust, this produces progressively more alkalic basalts to the north. The High Cascades overlies older oceanic lithosphere which was cooler at the time of subduction. As a result, dehydration reactions are delayed and a greater amount of volatiles are released beneath the arc. This results in lower melt temperatures and higher degrees of melting, producing predominantly low-K tholeiite and LILE-enriched HFSE-depleted calc-alkaline basalt magmas.

Melting experiments on hydrous low-K tholeiite: Implications for the genesis of tonalitic crust in the Izu-Bonin - Mariana arc

Melting experiments on hydrous low-K tholeiite: Implications for the genesis of tonalitic crust in the Izu-Bonin - Mariana arc

Late precambrian volcanism at Wadi Allaqi, south Eastern Desert, Egypt: evidence for transitional continental arc/margin environment

The Dokhan volcanics at Wadi Allaqi, situated in the south Eastern Desert of Egypt, range in composition from basaltic-andesite to dacite. Geochemically, they have a transitional character from low K-tholeiite to calc-alkaline with a relatively high Zr/Y ratio that characterises a continental arc/margin setting. The most basic sample has extremely low Mg# (40) and Ni (55 ppm) values, indicating significant fractionation of olivine and clinopyroxene and that it is not a direct partial melt of the mantle-peridotite. The examined andesites and dacites might have been formed, respectively, via low-pressure fractional crystallisation of basaltic-andesite and andesite melts with about 53% and 35% removal of mostly hornblende and plagioclase with little magnetite and biotite. The enrichment of LILE (supposed to be derived from the subducted slab) and the relative depletion of HFSE (Ta, P and Ti) seem to be inherited from the mantle source. Hence, the examined volcanics are affected by both slab-derived partial melts and aqueous fluids in their generation.

Field occurrence, geochemistry and petrogenesis of the Archean Mid-Oceanic Ridge Basalts (AMORBs) of the Cleaverville area, Pilbara Craton, Western Australia

A 3.1–3.3 Ga Mid-Archean accretionary complex has been identified in the Cleaverville area, Pilbara granite-greenstone terrain, Western Australia, by using a well-defined duplex structure and by reconstructing the oceanic plate stratigraphy. Archean mid-oceanic ridge basalts (AMORBs) which are part of this sequence were selected for detailed petrochemical analysis to infer Archean divergent plate tectonic process. The Cleaverville AMORBs are low-K tholeiites which tend to be richer in FeO ∗ than modern MORBs. Abundance ratios of most incompatible elements such as REE, Ti, Y and Zr are chondritic. Neither Nb/Zr nor P/Zr suggest partitioning into the metallic core before the middle Archean. Estimated Mg ∗ values of the mantle source is about 85, and is lower than that of 89–92 modern MORBs. The Cleaverville AMORBs are considered to have been produced by partial melting at a pressure and temperature of 2.5 GPa and 1425 °C of a Fe-rich mantle peridotite (Mg ∗ value = 85.0). Potential mantle temperature (PMT) of the Earth of the mid-oceanic ridge at this time 3.1–3.3 Ga was estimated from the above constraints on the Cleaverville AMORBs to be 1400 °C. If the Cleaverville example is representative of Archean plate boundary processes at spreading ridges, the PMT was about 120 °C higher than today and extensive partial melting of adiabatically rising mantle material would have been initiated at 90 km depth. Net production of partial melt at the AMOR axis was equivalent to 15–20 km thickness of oceanic crust (roughly 2–3 times thicker than today).

Petrology of Mafic Plutons Associated with Calc-Alkaline Granitoids, Chilliwack Batholith, North Cascades, Washington

Small (<5 km), lithologically diverse gabbro and diorite stocks make up ~2% of the 34 to 2 Ma Chilliwack batholith, and overlap in age with associated calc-alkaline granitoids. These mafic plutons are similar to those in other I-type batholiths, and represent basaltic magmas present during batholith formation. Objectives of this study are: (I) to examine the origins of both interpluton and intrapluton petrologic diversity, and (2) to compare chemical and Sr-Nd isotopic traits of these gabbros with those of Cascade arc basalts. Mafic rocks in the Chilliwack are divided into a medium-K series (MKS) and a low-K series (LKS). The former contain 0-7-2-4 wt % K2O and are similar in composition to calc-alkaline basalts and basaltic andesites. Inverse REE modeling supports derivation of the MKS by 9-27% melting of a garnet free, LREE-enriched source (La/TbN ~ 2). Chilliwack LKS gabbros have chemical characteristics of low-K olivine tholeiites, including low K20 (0-3-0-5 wt%) and La/TbN (1-7-3-4), and high CaO (8-8-11-3 wt%) and Na20/K20 (6-22). These traits suggest a source with more clinopyroxene and lower La/Yb^ than the MKS source. Differences in Nd(0) between MKS and LKS gabbros suggest that lower Nd/Sm is a long-lived LKS source characteristic Lithologic variation within composite plutons of both series resulted primarily from multiple intrusion of related magmas, in some cases differentiates of a common parent. Two contrasting examples were studied in detail. At Mt Sefrit, MKS variation ( gabbronorite-quartz diorite) is modeled by low-pressure fractionation (ol+plag + cpx), accompanied by ~10% wallrock assimilation. In contrast, chemical and Sr-Nd isotopic variation among LKS gabbro—quartz diorite at Copper Lake points to crystallization dominated by clinopyroxene +plagioclase ± Cr-spinel, indicative of differentiation at pressures ~^10 kbar, although the assimilant in this case is poorly constrained. Chemical and isotopic similarities between these mafic plutons and Quaternary Cascade lavas indicate that mafic magmas present during the production of Chilliwack granitoids were lowand medium-K arc basalts.

East Mariana Basin tholeiites: Cretaceous intraplate basalts or rift basalts related to the Ontong Java plume?

Studies of seafloor magnetic anomaly patterns suggest the presence of Jurassic oceanic crust in a large area in the western Pacific that includes the East Mariana, Nauru and Pigafetta Basins. Sampling of the igneous crust in this area by the Deep Sea Drilling Program (DSDP) and the Ocean Drilling Program (ODP) allows direct evaluation of the age and petrogenesis of this crust. ODP Leg 129 drilled a 51 m sequence of basalt pillows and massive flows in the central East Mariana Basin. 40 Ar 39 Ar ages determined in this study for two Leg 129 basalts average 114.6 ± 3.2 Ma. This age is in agreement with the Albian-late Aptian paleontologic age of the overlying sediments, but is distinctively younger than the Jurassic age predicted by magnetic anomaly patterns in the basin. Compositionally, the East Mariana Basin basalts are uniformly low-K tholeiites that are depleted in highly incompatible elements compared to moderately incompatible ones, which is typical of mid-ocean ridge basalts (MORB) erupted near hotspots. The Sr, Nd and Pb isotopic compositions of the tholeiites ( 87 Sr 86 Sr init = 0.70360–0.70374 ; 143 Nd 144 Nd init = 0.512769–0.512790 ; 206 Pb 204 Pb meas = 18.355–18.386 ) also overlap with some Indian Ocean Ridge MORB, although they are distinct from the isotopic compositions of Jurassic basalts drilled in the Pigafetta Basin, the oldest Pacific MORB. The isotopic compositions of the East Mariana Basin tholeiites are also similar to those of intraplate basalts, and in particular, to the isotopic signature of basalts from the nearby Ontong Java and Manihiki Plateaus. The East Mariana Basin tholeiites also share many petrologic and isotopic characteristics with the oceanic basement drilled in the Nauru Basin at DSDP Site 462. In addition, the new 110.8 ± 1.0 Ma 40 Ar 39 Ar age for two flows from the bottom of Site 462 in the Nauru Basin is indistinguishable from the age of the East Mariana Basin flows. Thus, while magnetic anomaly patterns predict that the igneous basement in the Nauru and East Mariana Basins is Jurassic in age, the geochemical and chronological results discussed here suggest that the basement formed during a Cretaceous rifting event within the Jurassic crust. This magmatic and tectonic event was created by the widespread volcanism responsible for the genesis of the large oceanic plateaus of the western Pacific.

Experimental melting of hydrous low-K tholeiite: evidence on. the origin of Archaean era tons

Experimental melting studies were performed on a natural high-Al basalt and a synthetic average Archaean tholeiite (AAT) composition (0.3 wt.% K20) with variable amounts of H20. Microprobe analyses of quenched melts (glass) from runs at 5-30 kbar and 750°-l100°C showed that typical Archaean tonalitic and trondhjemitic "grey gneiss" compositions were produced from the average Archaean tholei-ite over the entire experimental range, with 15% to less than 1 % H20. The high-Al basalt produced liquids too high in Al203 (18--23%) for realistic grey gneiss compositions. The persistent generation in our experiments of low-K calc-alkaline magmas directly by vapor-undersaturated partial melting of low-K Archaean tholeiite strongly suggests this mechanism for the origin of early continents. Temperatures of 850°-l000°C and pressures around 15 kbar are appropriate melting conditions. Ton­alitic magmas are favored by higher temperatures, lower pressures, and higher H20 contents in the source. Trondhjemitic magmas are favored by lower temperatures, higher pressures, and lower H20 contents. Heavy REE depletion of magmas would be possible for partial melting above 15 kbar because of the stability at higher pressures of residual garnet. Unfractionated REE patterns of magmas could result from melting at lower pressures, where garnet does not coexist with liquid. The low-K trends of melts are maintained by very refractory amphibole (up to 0.7 wt. % KzO) which coexists with liquid for bulk H20 contents of 2 wt. % or more. Shallow subduction-zone melting of amphibolite with magma extraction, and partial melting of amphibolite under deep-crustal metamorphic conditions are models for early crustal evolution which appear to satisfy the experimental constraints.

A Cambrian island arc in Iapetus: geochronology and geochemistry of the Lake Ambrose volcanic belt, Newfoundland Appalachians

AbstractThe Lake Ambrose volcanic belt (LAVB) outcrops as a 45 km long northeast-trending belt of mafic and felsic volcanic rocks along the eastern side of the Victoria Lake Group in south-central Newfoundland. It comprises roughly equal proportions of mafic pillow basalt and high silica rhyolite, locally interbedded with epiclastic turbidites. Volcanic rocks have been metamorphosed in the greenschist facies and are extensively carbonatized.U-Pb (zircon) dates from rhyolite at two, widely separated localities give identical ages of 513 ± 2 Ma (Upper Cambrian), and this is interpreted as the eruptive age of the volcanic sequence. Primitive arc and low-K tholeiites can be recognized on the basis of major and trace element geochemistry, ranging from LREE-depleted to LREE-enriched. Geochemical variation between mafic volcanic types is interpreted predominantly to reflect contrasts in source characteristics and degree of partial melting; some variation within each geochemical type attributable to fractional crystallization can be recognized. Detailed examination of some samples indicates that the heavy REE and related elements have locally been mobile, probably as a result of carbonate complexing.The LAVB is the oldest well-dated island arc sequence in Newfoundland, and perhaps in the Appalachian–Caledonian Orogen. Its age requires modification of widely held models for the tectonic history of central Newfoundland. It is older than the oldest known ophiolite, demonstrating that arc volcanism was extant before the generation of the oldest known oceanic crust in this part of Iapetus. It further demonstrates that there was a maximum of approximately 30 Ma between the rift-drift transition which initiated Iapetus, and the initiation of subduction. This suggests that the oceanic sequences preserved in Newfoundland represent a series of arcs and back arc basins marginal to the main Iapetus Ocean, and brings into question whether the Appalachian accreted terranes contain any remnants of normal mid-ocean ridge type Iapetan crust.

Nature and record of igneous activity in the Tonga arc, SW Pacific, deduced from the phase chemistry of derived detrital grains

Abstract Sedimentary rocks in dredge samples from the Tonga arc, SW Pacific, are simple two-component admixtures of volcanogenic detritus and bioclastic material. The volcanic detritus consists largely of lithic and plagioclase grains, but also includes, in decreasing order of abundance, significant minor amounts of clinopyroxene, orthopyroxene and pigeonite. In addition, amphibole, olivine and opaque oxide minerals are accessory phases and trace amounts of monocrystalline quartz occur in a few samples. Plagioclase grains have high An and low Or contents and fall largely in the bytownite field. Pyroxene grains show progressive Fe enrichment and low Ti contents. Volcanic glasses also show Fe enrichment and have low K 2 O contents. These features indicate derivation of the volcanic detritus from a low-K tholeiite source. The uniform compositional range and Oligocene to Holocene age range of the detritus is consistent with their derivation from the adjacent Tongan arc, and indicate that Pliocene rifting of the arc and associated back-arc basin formation did not affect the composition and affinities of the magma source.

The geochemistry and tectonic setting of the northern section of the Luzon arc (The Philippines and Taiwan)

The Luzon arc consists of a 1200 km chain of stratovolcanoes and volcanic necks stretching from Mindoro (13° N) to the Coastal Range of Taiwan (24° N). This study is concerned with three of the five major segments along the arc: the Northern Luzon, Babuyan, and Taiwan segments. The late Tertiary to Quaternary volcanics of these segments are primarily andesitic but range in composition from basalt to rhyolite and are typical arc volcanics: porphyritic plagioclase textures, primarily calc-alkaline with a few tholeiitic volcanic centers, low TiO 2 concentrations, and low high-field strength element (HFSE) to large-ion lithophile element (LILE) ratios. There is a large range in K 2O and other LILEs from low-K tholeiites to high-K calc-alkaline suites. Calayan island and Mt. Tabungon (volcanic substratum of Mt. Cagua) volcanics in the Babuyan segment and most rocks from the Taiwan Coastal Range are low-K tholeiites (e.g., nearly flat REE patterns, low LREE and LILE concentrations, and low Th/U ratios). The dominant calc-alkaline series ranges from medium-K to high-K rocks. The high-K calc-alkaline rocks are mainly young and are found on Batan and Lutao islands. When the low-K tholeiitic and medium- and high-K calc-alkaline rocks are associated in the same region, the low-K rocks are usually older. There is a general relationship between K 2O (and other LILE concentrations) and K-Ar radiometric dates. To some extent, 87Sr 86Sr ratios also increase with time throughout the arc. There is a correlation between latitude and 87Sr 86Sr , and a positive correlation between 87Sr 86Sr and [La Sm] CN . An increase in both these geochemical parameters has been associated with an increase in the input of continental crustal material in other arc regions. The lowest 87Sr 86Sr ratios found along the entire Luzon arc (the samples from Baguio) are equivalent to those found in samples from mid-Tertiary plutons from Northern Luzon and the Bicol arc. Both groups of rocks are associated with westward subduction along the Philippine Trench where presumably deep oceanic sediments with little or no continental crustal component have been subducted. The absence of continental crust below the arc in the Northern Luzon and Babuyan segments has been suggested by several groups of researchers. This, together with the fact that metasomatized ultramafic nodules (probably of mantle origin) in disequilibrium with their Batan host lavas (but with similar 87Sr 86Sr and [La Sm] CN ratios to the host rocks), suggests that upper level assimilation (AFC processes) does not appear to be the major influence on the geochemical signatures. A collision zone between the upper crustal block of Eastern China and Taiwan and the Manila Trench has been recorded in the northern section of the arc (Taiwan). Sediments have also been shown to decrease in thickness along the South China Sea basin nearly parallel to the Manila Trench from north to south. The sediment source region is most probably Taiwan, and perhaps Eastern China. This latitudinal in variation sediment thickness may explain the crustal signature related to both geochemical and age parameters. We suggest that this crustal input has taken place via subduction of sediment rich in a crustal component.

Magma source components in an arc-continent collision zone: the Flores-Lembata sector, Sunda arc, Indonesia

Major, trace-element, and Sr-, Nd-and Pbisotope data are presented for volcanics from 12 active or recently active volcanoes from the islands of Flores, Adonara, Lembata and Batu Tara in the eastern Sunda are. The volcanics vary in composition from low-K tholeiite, through medium-and high-K calcalkaline types to the K-rich leucite basanites of Batu Tara. From the tholeiites to the leucite basanites there are marked increases in the concentrations of LILE (K, Rb, Ba, Sr), LREE and La/Yb, and all the volcanics have high Ba/ Nb, La/Nb and Ba/La compared with mid-ocean ridge and intraplate eruptives. K/Cs values are generally lower than OIB values, and overlap those of other arc volcanics and northeast Indian Ocean sediments. The volcanics exhibit a broad range of 87Sr/86Sr (0.70468–0.70706), 143Nd/144Nd (0.512946–0.512447), and a moderate range in 206Pb/204Pb (18.825–19.143), 207Pb/ 204Pb (15.643–15.760) and 208Pb/204Pb (38.97–39.51). Trace-element and isotopic data suggest that the mantle beneath the eastern Sunda arc is a complex heterogeneous mixture of 3 or 4 major source components: MORB-source or depleted MORB-source, OIB-source and subducted Indian Ocean sediment. The low-K tholeiites were probably formed by relatively large degrees of melting of depleted MORB-source mantle, modified by subduction-related fluids, whereas the trace-element and isotopic characteristics of the K-rich volcanics suggest that they were derived from an OIB source which and been modified by a subduction-related melt component. The source components of the medium-to high-K calcalkaline rocks are more difficult to determine, and probably include mixtures of MORB-source or OIB-source, and melt/fluid derived from subducted oceanic sediment. Minor-and trace-element modelling calculations indicate substantial difficulties in producing the relatively low Ti-contents of arc volcanics by melting OIB-source mantle. Where OIB mantle is considered to be an important component of arc magmas it is suggested that the HFSE are buffered to relatively low concentration by a residual Ti-rich accessory phase.

Petrology of the Rainy Lake area, Minnesota, USA-implications for petrotectonic setting of the archean southern Wabigoon subprovince of the Canadian Shield

The Rainy Lake area in northern Minnesota and southwestern, Ontario is a Late Archean (2.7 Ga) granite-greenstone belt within the Wabigoon subprovince of the Canadian Shield. In Minnesota the rocks include mafic and felsic volcanic rocks, volcaniclastic, chemical sedimentary rocks, and graywacke that are intrucded by coeval gabbro, tonalite, and granodiorite. New data presented here focus on the geochemistry and petrology of the Minnesota part of the Rainy Lake area. Igneous rocks in the area are bimodal. The mafic rocks are made up of three distinct suites: (1) low-TiO2 tholeiite and gabbro that have slightly evolved Mg-numbers (63–49) and relatively flat rare-earth element (REE) patterns that range from 20–8 x chondrites (Ce/YbN=0.8–1.5); (2) high-TiO2 tholeiite with evolved Mg-numbers (46–29) and high total REE abundances that range from 70–40 x chondrites (Ce/YbN=1.8–3.3), and (3) calc-alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive Mg-numbers (79–63), enriched light rare-earth elements (LREE) and depleted heavy rare-earth elements (HREE). These three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma; they resulted from melting of heterogeneous Archean mantle. The felsic rocks are made up of two distinct suites: (1)low-Al2O3 tholeiitic rhyolite, and (2) high-Al2O3 calc-alkaline dacite and rhyolite and consanguineous tonalite. The tholeiitic felsic rocks are high in Y, Zr, Nb, and total REE that are unfractionated and have pronounced negative Eu anomalies. The calcalkaline felsic rocks are depleted in Y, Zr, and Nb, and the REE that are highly fractionated with high LREE and depleted HREE, and display moderate negative Eu anomalies. Both suites of felsic rocks were generated by partial melting of crustal material. The most reasonable modern analog for the paleotectonic setting is an immature island arc. The bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre- and syntectonic granitoid rocks. However, the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments. The low-TiO2 tholeiite is similar to both N-type mid-ocean-ridge basalt (MORB) and low-K tholeiite from immature marginal basins. The calc-alkaline basaltic andesite is like that of low-K calc-alkaline mafic volcanic rocks from oceanic volcanic arcs; however, the high-TiO2 tholeiite is most similar to modern E-type MORB, which occurs in oceanic rifts. The conundrum may be explained by: (1) rifting of a pre-existing immature arc system to produce the bimodal volcanic rocks and high-TiO2 tholeiite; (2) variable enrichment of a previously depleted Archean mantle, to produce both the low- and high-TiO2 tholeiite and the calc-alkaline basaltic andesite, and/or (3) enrichment of the parental rocks of the high-TiO2 tholeiite by crustal contamination.

Comparison between two Palaeozoic island-arc terranes in northern California (eastern Klamath and northern Sierra Nevada): geodynamic constraints

The Palaeozoic geodynamic environment of northern California is considered to be one of offshore island arcs bounded by marginal seas. The petrological and geochemical features of the eastern Klamath and northern Sierra Nevada island-arc sequences are important indicators of the geodynamic evolution of the Cordillera. The eastern Klamath pre-Early Devonian island-arc sequence consists of pillowed basalts andesites and boninites, overlain by low-K rhyolites, both suites erupted in a submarine environment. These volcanics, characterized by almost flat REE patterns and high ϵ Nd represent a primitive intraoceanic island arc. The mid-Palaeozoic volcanic succession of northern Sierra Nevada is formed of calc-alkaline rhyolites conformably overlain by either calc-alkaline (in the north) or low-K tholeiite (in the south) basalts and andesites. The volcanic episode ended in the Early Carboniferous with a differentiated shoshonitic suite. All the massive or pillowed flows are interbedded with volcaniclastic sediments, suggesting that they were erupted partly in a shallow-water environment. The geochemical features of the volcanics, moderate to high LREE enrichment and low ϵ Nd values, are evidence for the involvement of a crustal component in the magma genesis. The mid-Palaeozoic volcanic sequence therefore represents a continent-based island arc. This microcontinent could be the Sonomia microplate (excluding the eastern Klamath plate). Thus, these two northern Californian Palaeozoic sequences cannot be considered as lateral counterparts but could represent the remnants of oceanic and continental island arcs as observed in the present-day East Pacific areas along the Eurasiatic margin.

Diversity of magma types in a lower Paleozoic island arc-marginal basin system (Eastern Klamath Mountains, California, U.S.A.)

A trace-element investigation has been conducted on Ordovician to Devonian volcanics of the eastern Klamath Mountains spatially associated to the Siluro-Ordovician Trinity peridotite and gabbros. All the studied volcanics (low-K tholeiites and calc-alkaline volcanics) present strong island-arc geochemical features (low TiO 2 contents, negative Nb, Zr and Ti anomalies, mild LREE enrichment and depletion). Some volcanics from Lovers Leap and Copley exhibit strong immature island-arc geochemical affinities (very depleted LREE patterns and very low Nb contents), suggesting that they were formed by high degrees of partial melting (20%) of a residual lithosphere. Diminution of the partial melting degree and addition of sediments to the source will explain the occurrence of less depleted or slightly LREE-enriched volcanics. The Grey Rocks basaltic pillow-lavas thrust onto the Trinity peridotitic sheet probably represent the extrusive member of the ophiolitic sequence. Their strong island-arc geochemical affinities suggest that the ophiolite was formed during the first stages of a back-arc basin development. Differentiation took place in the arc by melting of the deeper basaltic layers, leading to the formation of abundant rhyolitic and trondhjemitic magmatism representing the first stages of continental accretion.