Tholeiitic Suite Research Articles

The tectonic significance of some basic dyke swarms in the Canadian Superior Province with special reference to the geochemistry and paleomagnetism of the Mistassini swarm, Quebec, Canada

The Mistassini dykes extend northwest from the Mistassini embayment and comprise both tholeiitic and komatiitic suites. They are probably > 2000 Ma old and yield two major paleomagnetic components. One of these, with a pole at 131°W, 13°S, is thought to be an overprint related to the Elsonian Disturbance 1400–1500 Ma ago. A very steeply down (and reversed) component may be primary and has a pole at 080°W, 50°N.These spacial, chemical, and age relationships between the Mistassini, Molson, Marathon, and Payne River dyke swarms and the Aphebian supracrustal fold belts on the perimeter of the Superior Province suggest a genetic relationship between the dyke swarms and the fold belts. The supracrustal belts are evidence of the opening and closing of oceans, and the dyke swarms are evidence of early-stage failed arms related to these openings. More rarely (for example, the Payne River dykes), early-stage dyke swarms are developed and preserved parallel to the edges of newly developed spreading plate boundaries. Presumably if a spreading episode stops, the dykes themselves may remain as the only evidence of that event. Probably all the world's great continental dyke swarms have the above-described tectonic setting, and the number and extent of dyke swarms during a geological epoch may be a measure of the number and vigour of spreading events.

The geochemistry, origin, and tectonic significance of rodingites from the Dun Mountain Ultramafics, D'Urville Island, New Zealand

Numerous rodingite dikes and rodingitised tectonic blocks occur within serpentinised harzburgite of the Dun Mountain Ultramafics, D’Urville Island, New Zealand. The rodingites formed during progressive metasomatic alteration of a wide range of mafic to silicic ophiolitic rocks, including metavolcanics and argillite, as well as gabbro and plagiogranite. Petrographic, mineralogical, and geochemical data distinguish between two types of rodingite which represent different degrees of metasomatic alteration. These are (1) coarse-grained rodingites that often preserve mineralogical features of their parent rocks, and (2) intensely altered, fine-grained rodingite pods which are bordered by distinctive chloritised serpentinite reaction zones. The two rodingite types differ in their degree of Ca and Al enrichment, and Si, total Fe and alkali depletion, as well as in their extent of oxidation, hydration or dehydration, and abundances of incompatible elements and transition metals. These differences are related to progressively changing mineralogies in the rodingite reaction zones that were imposed by chemically evolved fluids associated with serpentinisation of the enclosing ultramafic rocks. Despite alteration, the rodingites show geochemical affinities with light rare earth element (LREE) depleted lavas and intrusives comprising the Patuki Volcanics at D’Urville Island. No rodingitised samples of an earlier formed suite of LREE-enriched Patuki tholeiites have been found. Rodingitisation of the D’Urville rocks occurred during emplacement of the Dun Mountain Ultramafics in a continental crust environment, and the metasomatic processes are unrelated to early amphibolite and greenschist forming ocean-floor metamorphism of the Patuki Volcanics. In this respect, the D’Urville rodingites differ from other rodingites which have been dredged from the ocean floor.

Geodynamic setting of Early Devonian kuroko-type sulfide deposits in the eastern Klamath Mountains (Northern California) inferred by the petrological and geochemical characteristics of the associated island-arc volcanic rocks

In the West Shasta copper-zinc district, Kuroko-type massive sulfide deposits are present in Early Devonian volcanic rocks consisting of the Copley Greenstone and the overlying Balaklala Rhyolite. The Copley Greenstone is composed of pillow lava (mainly basalts with subordinate andesites) and pyroclastic deposits. High Mg andesites occur near the top of the 1,800-m-thick volcanic succession. The Balaklala Rhyolite, 1,000 m thick, consists of highly siliceous rhyolite flows, conglomerates, and tuffs. Both units were deposited in a submarine environment. The sulfide deposits are localized in the rhyolitic flows of the upper part of the middle unit of the Balaklala Rhyolite.Petrological and geochemical data show that these lavas belong to a tholeiitic suite formed in an immature island arc. At the close of Copley eruptions, the island arc underwent rifting and tectonic extension. This is suggested by the presence of boninites, a feeder dike system for the Balaklala Rhyolite, and bimodal volcanism. The extensional environment is similar to that classically observed in the domain of the Kuroko massive sulfide deposits. This rifting led subsequently to the development of a back-arc basin represented by the Trinity basalts associated with an ophiolitic sequence.

Magmatic Evolution of Semisopochnoi Island, Alaska: Trace-Element and Isotopic Constraints

Semisopochnoi has erupted a tholeiitic suite of moderately evolved lavas (), whose bulk-rock chemistry and phenocryst mineralogy are generally compatible with an important role for fractional crystallization in controlling the overall evolution of the suite. Trace-element data, however, are not consistent with crystal fractionation operating alone. Instead, a variety of plots involving incompatible elements (or ratios among them) demonstrate that simple mixing between basaltic and silicic components can reproduce the essential aspects of the trace-element variations and must have been operative as well. We are unable to distinguish between a silicic end-member represented by residual dacitic liquid like one of the analyzed lavas (therefore implying magma mixing) and one consisting of pre-existing silicic crustal rocks (therefore implying assimilation). Variations in Sr and Nd isotopic compositions are very limited: and . With the exception of the Sr composition of one sample, the isotopic data are also compatible with a combined mixing-fractionation model. Coupled variations of and chondrite-normalized Ba/La ratios imply derivation of parental magmas from a source that has been enriched in incompatible elements, relative to MORB-sediment mixtures, perhaps by dehydration of the subducted slab and resulting metasomatism of the overlying mantle.

The platinum-group element and gold contents of the marginal rocks and sills of the Bushveld Complex

Previous work has defined three suites of sills and marginal rocks adjacent to and below the Rustenburg Layered Suite of the Bushveld Complex, namely an ultramafic (MgO approximately 30%), a magnesian basaltic (MgO approximately 12%), and a tholeiitic suite (MgO approximately 7%). These three suites are thought to represent magmas which have intruded and been incorporated into the Rustenburg Layered Suite at various times during its formation. The elements Ir, Ru, Rh, Pd, Pt, and Au have been analyzed in 17 samples representative of the three suites. The ultramafic sills have the highest total noble metal content (46 ppb) followed by the magnesian basaltic suite (37 ppb) and the tholeiitic suite (21 ppb). The ultramafic sills are characterized by a relatively fiat chondrite-normalized platinum-group element pattern with an average Pt/Ir ratio of 3.52 and a pronounced Rh anomaly. The magnesian basaltic and tholeiitic suites both have steep positive chondrite-normalized patterns with average Pt/Ir ratios of 16.14 and 40.5, respectively, and no Rh anomaly. Modeling done using incompatible trace elements and petrographic mixing suggests the ultramafic sills represent crystal mushes (of cumulate olivine and chromite) that were intruded into the floor during the crystallization of the lower portion of the Rustenburg Layered Suite. The enrichment of Ir, Ru, and Rh in the ultramafic sills appears to be associated with the cumulate enrichment of olivine or chromite in these sills.The chondrite-normalized patterns found for the Merensky Reef and the Platreef are similar to those of the magnesian basaltic magma. The pattern found for the UG-2 is much flatter and in some respects very similar to that found for the ultramafic sills.On a worldwide basis the platinum-group element contents of the magnesian basaltic and tholeiitic magmas generally fall in the range of basalts and chill zones of other layered suites. The magnesian basaltic magma does have one of the highest Pt contents measured for a magma.

Petrochemical and Petrogenetic characteristics of precambrian amphibolites of the Alawa district, northwest Nigeria

The Alawa amphibolites are well exposed as lenticular and extensive ovoid-shaped bodies closely associated with basement gneisses and migmatites, semi-pelitic schists and intrusive granites. In terms of textural character, the series comprises slightly massive, folded, banded, laminated and schistose units.Chemical data show that these assemblages are in some respects — such as Ga and Y contents and Ni/Co ratios — similar to basaltic rocks. However, the petrographic subgroups are distinctive in certain petrochemical trends and features. The cataclastic and the schistose types are readily discriminated from the others, and seemingly show strong evidence of the mobility of elements. Changes in the foliated rocks are believed to reflect mainly the effect of contamination by the adjacent metaclastics, whereas the modifications in the laminated ones are possibly due to dynamic metamorphism.On the basis of field relationships and overall petrochemical nature, it is surmised that the Alawa meta-basites represent ultimately a tholeiitic suite emplaced through rifted crustal segment, into an environment of active deposition. It is considered that subsequent metasomatism, metamorphism and tectonization have contributed to varying in different extent, the original composition and fabric of the parent rocks.

Early Devonian volcanism in the eastern Klamath Mountains, California: evidence for an immature island arc

The Early Devonian volcanic sequence of the eastern Klamath Mountains (northern California) consists of the Copley Greenstone, a basalt–andesite series, overlain by the Balaklala Rhyolite. All the rocks are metamorphosed to the greenschist facies.The Copley Greenstone consists of massive flows and pyroclastic deposits in the lower part that are overlain by pillow lavas. No sediments are interlayered in the volcanic pile except in the easternmost outcrop area, where sandstones with granitic debris and shaly tuffs are interbedded in the uppermost flows. High-Mg andesites occur only near the top of the basic volcanic succession. The Balaklala Rhyolite is formed of massive flows, breccias, and tuffs.The Copley volcanic rocks, poor in titanium, belong to a low-K tholeiitic suite formed in an island-arc geodynamic environment. The occurrence of olivine with chromium spinels in almost all the lava types, the enrichment in magnesium, chromium, and nickel, and the depletion in hygromagmaphile elements suggest that magmatic differentiation was a marginal process. The Balaklala Rhyolite shows very homogeneous petrographic and geochemical features, characteristic of tholeittic products.From its characteristics here described (pillowed flows, lack of sedimentation, presence of high-Mg andesites and low-K rhyolites, bimodality of the volcanism) the Early Devonian volcanic sequence represents an immature island arc related to a back-arc basin, similar to the present-day Mariana island arc.

Chemical and isotopic evidence for mixing between depleted and enriched mantle, northwestern U.S.A.

Combined elemental and Sr, Nd, Pb and O isotopic data for late Cenozoic olivine tholeiite lavas from the northwestern Great Basin indicate derivation from at least two chemically and isotopically distinct mantle source regions with no significant modification by interaction with continental crust. The lack of crustal involvement is a direct reflection of the extensional tectonic environment which favors rapid ascent of magmas, minimal residence time in crustal magma chambers and scattered fissure eruptions. The observed chemical and isotopic variations in the tholeiite suite are attributed to mixing between depleted oceanic type mantle ( 87Sr 86Sr ~ 0.7030 and 143Nd 144Nd ~ 0.51305 ) and old, chemically heterogeneous, isotopically enriched subcontinental mantle ( 87Sr 86Sr ~ 0.7078 and 143Nd 144Nd ~ 0.51233 ). Model incompatible element concentrations suggest strong similarities between the depleted mantle and the mantles beneath normal oceanic ridge segments and back-arc basins and between the enriched mantle and the mantle beneath enriched oceanic ridge segments such as the Azores. Superimposed upon the characteristics derived from the two component mixing model may be the effects of a third mantle source which is identifiable only by its apparent radiogenic 206Pb 204Pb ratios. If present, this third source may reflect a component derived from the downgoing slab of an ancient subduction zone.

10A. Trace element geochemistry of the Deccan tholeiitic suite(Abstracts of Papers Presented at the Spring Meeting of the Society)

10A. Trace element geochemistry of the Deccan tholeiitic suite(Abstracts of Papers Presented at the Spring Meeting of the Society)

Hydrogen isotope systematics of submarine basalts

The D/H ratios and water contents in fresh submarine basalts from the Mid-Atlantic Ridge, the East Pacific Rise, and Hawaii indicate that the primary D/H ratios of many submarine lavas have been altered by processes including (1) outgassing, (2) addition of seawater at magmatic temperature, and (3) low-temperature hydration of glass. Decreases in δD and H 2O + from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between δD and H 2O + in basalts from the Galapagos Rise and the FAMOUS Area are attributed to outgassing of CH 4 and H 2. A good correlation between δD values and H 2O is observed in a suite of submarine tholeiites dredged from the Kilauea East Rift Zone where seawater (added directly to the magma), affected only the isotopic compositions of hydrogen and argon. Analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having δD values as low as −100. δD values vary with H 2O contents of subaerial transitional basalts from Molokai, Hawaii, and subaerial alkali basalts from the Society Islands, indicating that the primary δD values were similar to those of submarine lavas. Extrapolations to possible unaltered δD values and H 2O contents indicate that the primary δD values of most thoteiite and alkali basalts are near −80 ± 5: the weight percentages of water are variable, 0.15–0.35 for MOR tholeiites, about 0.25 for Hawaiian tholeiites, and up to 1.1 for alkali basalts. The primary δD values of −80 for most basalts are comparable to those measured for deep-seated phlogopites. These results indicate that hydrogen, in marked contrast to other elements such as Sr, Nd, Pb, and O, has a uniform isotopic composition in the mantle. This uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the Earth.

Geochemistry of Precambrian ophiolites from Bou Azzer, Morocco

The Upper Proterozoic ophiolite complex of Bou Azzer, Morocco, includes ultramafic rocks, cumulate gabbros, sheeted dykes, pillow lavas and diorite-quartz diorite intrusions and an overlying volcano-sedimentary sequence. The gabbroic cumulates, basaltic flows and dykes have compositions similar to recent ocean-floor rocks (N- and/or T-type). Among other features, they have comparable light REE-depleted patterns and relations of Ti-Zr and La-Nb. Although fractional crystallization played an important role in the evolution of these rocks, the large variations in their chemical compositions require generation from a heterogeneous upper mantle source and/or by a dynamic partial melting process. Diorites, quartz diorites and the volcanic rocks of the overlying sequence are calc-alkaline, genetically unrelated to the tholeiitic suite and indicative of an island arc setting. A possible tectonic model for the ophiolite complex is a marginal basin just behind a still active island arc.

Geochemistry of tholeiitic and alkalic lavas from the Koolau Range, Oahu, Hawaii: Implications for Hawaiian volcanism

Lavas of the post-erosional, alkalic Honolulu Volcanics have significantly lower 87Sr/ 86Sr and higher 143Nd/ 144Nd than the older and underlying Koolau tholeiites which form the Koolau shield of eastern Oahu, Hawaii. Despite significant compositional variation within lavas forming the Honolulu Volcanics, these lavas are isotopically (Sr, Nd, Pb) very similar which contrasts with the isotopic heterogeneity of the Koolau tholeiites. Among Hawaiian tholeiitic suites, the Koolau lavas are geochemically distinct because of their lower iron contents and Sr and Nd isotopic ratios which range to bulk earth values. These geochemical data preclude simple models such as derivation of the Honolulu Volcanics and Koolau tholeiites from a common source by different degrees of melting or by mixing of two geochemically distinct sources. There may be no genetic relationship between the origin and evolution of these two lava suites; however, the trend shown by Koolau Range lavas of increasing 143Nd/ 144Nd and decreasing 87Sr/ 86Sr with decreasing eruption age and increasing alkalinity also occurs at Haleakala, East Molokai and Kauai volcanoes. A complex mixing model proposed for Haleakala lavas can account for the variations in Sr and Nd isotopic ratios and incompatible element abundances found in lavas from the Koolau Range. This model may reflect mixing and melting processes occurring during ascent of relatively enriched mantle through relatively depleted MORB-related lithosphere. Although two isotopically distinct components may be sufficient to explain Sr and Nd isotopic variations at individual Hawaiian volcanoes, more than two isotopically distinct materials are required to explain variations of Sr, Nd and Pb isotopic ratios in all Hawaiian lavas.

The petrology and geochemistry of Cretaceous to Recent volcanism in Colombia: the magmatic history of an accretionary plate margin

The Pacific margin of Colombia records a complex history of subduction-related magmatism from at least Cretaceous times. Cretaceous lavas from the Western Cordillera and part of the Central Cordillera of the Colombian Andes exhibit some chemical characteristics typical of island-arc tholeiites, including low LIL-element abundances and flat to light depleted REE patterns, but also have chemical and geological affinities with deposits from marginal basins, and with transitional type ocean floor basalts. An appraisal of the geological and chemical evidence leads to the conclusion that these rocks were erupted in an oceanic environment in close proximity to the continent. A suite of basic rocks from the Coastal Cordillera of Upper Cretaceous to Tertiary age has been interpreted elsewhere as abyssal tholeiites. Following accretion of the Cretaceous sequences on to cratonic S America, volcanism continued in the upper Miocene with the eruption of basalts and basaltic andesites on the eastern flanks of the Western Cordillera. This tholeiitic suite is LREE and LIL-element enriched compared with the Cretaceous lavas, and represents the trenchward portion of an ensialic island-arc sequence. Subduction-related magmatism continued from the Miocene with the formation of composite calc-alkaline strato-volcanoes, mostly along the axis of the Central Cordillera. These Miocene to Recent lavas are dominantly andesitic, with strong LIL and LREE enrichment and 87 Sr/ 86 Sr ratios of 0.7042:0.7047.

Geochemical evolution of St Kitts and Montserrat, Lesser Antilles

The volcanic rocks of St Kitts have uniform 87 Sr/ 86 Sr ratios (c. 0.7037) and high 143 Nd/ 144 Nd ratios (c. 0.5130) in a suite which ranges from basalt to rhyolite. Trace element abundances and the flat chondrite-normalized REE patterns resemble those of island-arc tholeiitic suites. In comparison, the Montserrat lavas have higher concentrations of alkaline elements and are LREE enriched. Geochemical differences may be related to partial displacement of the axis of the volcanic arc about 9 Ma. This would have led to the development of the St Kitts’ suite in an essentially new and immature island-arc environment. Being closer to the junction with the former arc axis, the composition of the Montserrat lavas may reflect previous subduction-related mantle enrichment and/or the influence of a sedimentary component. Isotopic ratios and most trace elements in the Redonda basalt indicate island-arc tholeiite or in some respects MORB affinities, but it has a high Sr content and is LREE-enriched. Despite its distinctive composition, Redonda is situated on the main axis of the volcanic arc and its lavas were evidently contemporaneous with the younger eruptions of St Kitts and Montserrat.

Field setting, mineralogy, chemistry, and genesis of arc picrites, New Georgia, Solomon Islands

The field setting, petrography, mineralogy, and geochemistry of a suite of picrite basalts and related magnesian olivine tholeiites (New Georgia arc picrites) from the New Georgia Volcanics, Kolo caldera in the active ensimatic Solomon Islands arc are presented. These lavas, with an areal extent in the order of 1002 km and almost 1 km thick in places, are located close to the intersection of the Woodlark spreading zone with the Pacific plate margin. They contain abundant olivine (Fo94-75) and diopside (Cr2O3 1.1-0.4%, Al2O3 1–3%), and spinels characterised by a large range in Cr/(Cr+Al) (0.85–0.46) and Mg/(Mg+ Fe++) (0.65−0.1). The spinels are Fe+++ rich, with Fe+++/ (Fe++++Cr+Al) varying from 0.06 to 1.0. A discrete group of spinels with the highest Cr/(Cr+Al) (0.83–0.86) and lowest Fe+++ contents are included in the most Mg-rich olivine (Fo91–94) and both may be xenocrystal in origin. The lavas, which range between 10–28% MgO, define linear trends on oxide (element) — MgO diagrams and these trends are interpreted as olivine (∼0.9) clinopyroxene (∼0.1) control lines. For the reconstructed parent magma composition of these arc picrites, ratios involving CaO, Al2O3, TiO2, Zr, V and Sc are very close to chondritic. REE patterns are slightly LREE — enriched ((La/Sm)N 1.3–1.43) and HREE are flat. All lavas show marked enrichments in K, Rb, Sr, Ba, and LREE relative to MORB with similar MgO contents, but the TiO2 content of the proposed parent magma is close to those of postulated primary MORB liquids. It is proposed that the arc parent magma was produced by partial melting of sub-oceanic upper mantle induced by the introduction of LILE — enriched hydrous fluids derived by dehydration and/or partial melting of subducted ocean crust and possibly minor sediments.

Geochemistry of mafic igneous rocks of the northern Prince Charles Mountains, Antarctica

Abstract Rare mafic dykes, which intrude 1000 Ma high‐grade metamorphic rocks of the northern Prince Charles Mountains‐Mawson Coast area, are compositionally distinct from abundant early to middle Proterozoic tholeiite dykes, which are confined to Archaean or early Proterozoic terrains in the southern Prince Charles Mountains and elsewhere in East Antarctica, and which have therefore proved useful as stratigraphic markers. The younger dykes (and extrusive rocks) are a composition‐ally heterogeneous group with a wide range of ages (at least Cambrian to Eocene), although most are of K‐rich alkaline composition or have alkaline affinites. Their strong enrichment in highly incompatible elements (Rb, Ba, Th, Nb, K, Pb, Th and U) relative to less incompatible elements (La, Ce and P) suggests derivation by partial melting of more enriched mantle source regions than those of most of the Proterozoic tholeiite suites. However, unlike the latter, many incompatible element ratios have been significantly affected by f...

An introduction to the Sangihe arc: Volcanism accompanying arc—Arc collision in the Molucca Sea, Indonesia

In the Molucca Sea region of northeastern Indonesia, the Sangihe and Halmahera arcs are presently in the process of colliding, the earth's only example of a collision between facing volcanic arcs. The collision event is more advanced in the northern Molucca Sea where back-arc thrusting occurs along the Cotobato and Philippine trenches and volcanic centers are inactive and dissected. This paper describes the Sangihe arc, the western arc of the collision. The Sangihe are is approximately 500 km in length extending from the NE tip of Sulawesi to Mindanao, Philippines. The arc comprises over 25 Quaternary volcanic centers, the eight presently active volcanoes being confined to the southern half of the arc. The active volcanic front lies 100–110 km above the top of the west-dipping Benioff zone and volcanoes extend up to 70 km behind the front, thereby defining a volcanic arc overlying earthquakes 100 to 180 km deep. Rock types range from basalt to rhyolite, andesites are dominant. Pyroxene-andesites are common on Sulawesi and at or near the volcanic front from Ruang to Buhias-Pahepa and Buang to Mamanuk. Hornblende-andesites are common at the volcanic front between Mahengetang and Sangihe Island and in centers located well behind the volcanic front. Tholeiitic (TH) suites, defined on the basis of relative degrees of iron enrichment, are confined to the southern volcanic front, lack hornblende, are low-K (K55 < 1.05), have low Rb/Zr ratios (0.10–0.17), low Ba/Y ratios (3.4–4.9), low Rb/Cs ratios (8–14) and have the highest 87Sr/86Sr ratios (0.7039–0.7042). Calcalkaline (CA) suites occur throughout the arc, are low- to high-K (K55 ≅ 0.6–2.0), have variable Rb/Zr ratios (0.15–0.65) and Ba/Y ratios (4.2–12.4), high Rb/Cs ratios (14–82) and low 87Sr/86Sr ratios (0.7035–0.7040). Zr and Y distribution coefficients are lower in TH suites (DZr = 0.0, DY = 0.2–0.5) than in CA suites (DZr = 0.0–0.8, DY = 0.3–1.2). Ti initially increases with increasing SiO2 in some TH suites while decreasing with SiO2 in all CA suites. With the exception of Bukide (Ti/Zr = 138), Ti/Zr ratios of basalts exceed 100 only in TH suites (76–130). LIL-element contents and LIL/HFS-element ratios increase 2 to 3 times across the are away from the volcanic front and 2 to 3 times from south to north along the arc. 87Sr/86Sr ratios decrease away from the volcanic front. The south to north increase in LIL-element contents and LIL/HFS-element ratios, without corresponding changes in Sr-isotopic compositions are interpreted as resulting from decreasing degrees of partial melting northward. This reflects the gradual cessation of volcanism in the north due to collision.

Arc-tholeiite and ultramafic cumulate, Brook Street Volcanics, west O'Urvilie Island, New Zealand

Abstract Typically porphyritic basaltic to basaltic andesitic pyroclastic rocks (mainly tuff, tuffbreccia and agglomerate), together with subordinate massive and pillowed lava flows, dikes, and interbedded volcaniclastic sediments, comprise the 2400 m thick sequence of Brook Street Volcanics at west D'Urville Island, New Zealand. The Brook Street rocks show a trend of moderate iron enrichment and possess anomalously low Ti, Zr, and Nb abundances, as well as low Zr/Y, Ti/Y, and Nb/Zr ratios, features typical of island arc tholeiites. Rare earth element (REE) abundance patterns are relatively unfractionated and show only slight light rare earth element (LREE) enrichment. The lack of Eu anomalies, as well as the increase of Al2O3 contents with decreasing MgO and the large range of Ni and Cr contents, indicate that fractionation was dominated by ferromagnesian phases over most of the observed range of compositions, typical of island-arc tholeiitic suites. Some of the basalts possess ankaramitic affinities. Ve...

Origin of calc-alkalic andesites, Nasu zone, northeastern Japan: Kuno revisited.

KUNO (1950, 1959) first identified the existence of two rock series in the Japanese volcanic arc and proposed the origin of one from the other by assimilation of granitic material. In the Nasu zone in northeastern Japan, both the tholeiitic suite with pigeonite in the groundmass and the calc-alkalic suite with hypersthene as the Ca-poor pyroxene in the groundmass have been shown to have erupted from one volcano and to have differences in major element chemistries (KAWANO and AOKI, 1960; KAWANO et al., 1961). MASUDA and AOKI (1979), using mainly trace elements, showed that the calc-alkalic suite has higher Mg, K, Co, Cr, Ni, Th, and U than the tholeiitic suite, and also that each suite had a separate fractional crystallization. A model is presented here which is intended to explain both the mineralogical and chemical differences. KUNO's idea of assimilation is considered from a different point of view; that is, calc-alkalic basaltic andesite is formed by the partial assimilation of mafic rather than granitic material by a silicic tholeiitic andesite. Changes in the chemistry, mainly in the Mg/Fe ratio in the contaminated magma, prevents the crystallization of pigeonite; a drop in temperature as KUNO suggested is not as important although lower temperatures will help. It is demonstrated how such elements as K, Th, and U, which tend to be concentrated in granitic material, can be increased in the calc-alkalic rocks by addition of mafic xenoliths. This process is offered as an alternative to magma mixing, since it is believed that the mineralogical features used to support magma mixing could be explained equally well by assimilation.

The origin of the Naturaliste Plateau, SE Indian Ocean: Implications from dredged basalts

Abstract Although a 1972 dredging by USNS Eltanin from the submarine Naturaliste Plateau was reported to yield rocks of continental origin, a re‐examination of the dredge haul shows that the rock clasts are in fact altered tholeiitic basalts. They have affinities both with MOR basalts and, especially, within‐plate basalts. Petrographically they correlate most closely with the Bunbury Tholeiitic Suite on the Australian mainland to the east. The basalts are reworked cobbles in a manganiferous Quaternary slump mass, which contains a quartz‐rich, felsic, detrital mineral suite with a granite‐gneiss provenance. The basalt cobbles were part of a basal conglomerate, which covered large areas of the Plateau. It is suggested that this was laid down from nearby elevated volcanic structures formed during the inception of seafloor spreading and the separation of Greater India from Australia at about 122 Ma BP.