The structural petrology of portion of the eastern Mt. Lofty Ranges

The schists and gneisses of the Kanmantoo Group in the eastern Mt. Lofty Ranges show a well marked foliation and lineation. The foliation seen in the field is usually parallel to the bedding. The micas have a preferred orientation parallel to the lineation, resulting in girdles or partial girdles in the fabric diagrams. Quartz does not appear to have any preferred orientation. The lineations plunge to the S.S.E. or N.N.W., the mean plunge being about 20° to the S.S.E. This agrees with the plunge of the fold axes measured in the field and with the plunge of major structures deduced from field mapping. The area is thus one in which all the lineations are “b” lineations.     

Petrology of a portion of the Eastern Peninsular Ranges mylonite zone,Southern California

Peraluminous and metaluminous plutonic rocks of the Peninsular Ranges batholith near Borrego Springs in southern California were mylonitized in the large shear zone known as the eastern Peninsular Ranges mylonite zone (EPRMZ). Accompanying mylonitization in this portion of the EPRMZ was metamorphism at intermediate-low-pressure amphibolite-facies conditions. Deformation in the zone overlapped in time with Cretaceous intrusion of the batholith. In the San Ysidro Mountain — Pinyon Ridge area, four north-south trending zones of differing intensity of deformation have been defined; from east to west the degree and style of deformation gradually change from undeformed or weakly deformed rocks to strongly mylonitized rocks. Electron microprobe analysis shows that recrystallized hornblende, biotite, and plagioclase are variable in composition, probably reflecting a range of metamorphic conditions accompanying deformation. Comparison of mineral compositions with those from mafic schists of Vermont suggests conditions ranged from andalusite-staurolite through sillimanite-muscovite grades as defined for pelitic rocks. Stability of muscovite+quartz in mylonite assemblages and lack of remelting of granitic rocks indicate that temperature did not exceed about 650° C during mylonitization and lithostatic pressure did not exceed about 5 kbar. Over time, any given rock volume experienced a range of temperature, lithostatic pressure, and perhaps fluid pressure and differential stress. Mineral reactions in the zone involved hydration, requiring introduction of water. The possibility of large-scale migration of K and Fe is suggested by whole-rock chemical data. Brittle and ductile deformation features are closely associated in one part of the EPRMZ. The combined evidence suggests the presence of a pore fluid with fluid pressure close to lithostatic pressure. Short periods of low fluid pressure and possible high differential stress cannot be ruled out.     

Petrology of Mt Etinde Nephelinite Series

Mt Etinde is a volcano situated on the southwestern flank ofthe large Mt Cameroon. Its eruptions are dated at 065 Ma andtook place during the Mt Cameroon eruptive cycle (6 Ma withrecorded recent activity). The lava types, unrelated to theMt Cameroon alkali basalts, are melanephelinites, nephelinitessensu stricto, and numerous and varied nephelinites that containone or more of the following species: nosean, melilite, perovskite,garnet, aenigmatite, leucite, feldspar and haynophyres. Clinopyroxeneis the dominant mafic phase, with a variable composition betweenAl–Ti augite and aegirine. Zoning is also present in garnets,with conspicuous Ti enrichment in the border. Aenigmatite includesa fair proportion of Fe³⁺ Tschermak's component. Melilite issystematically Sr rich; its SrO contents increase continuouslywith MgO decrease, reaching 16 wt % in some facies. The chemicalcomposition of the lavas is extreme, with unusual concentrationsof volatiles (CO₂, H₂O, SO₃), most incompatible elements, suchas Ba, Sr and Zr, and the light rare earth elements (LREE).The Mt Etinde lavas define two lineages (MgO poor and MgO rich)that partly overlap. The chemical evolution of these two lineagescan be reproduced for major elements using a simple model ofcrystal fractionation. The major fractionating phase is an aluminousclinopyroxene, in accord with the petrographical observations.The scheme proposed can only be validated if the alkalis arenot taken into account, a hypothesis warranted by observationsof other nephelinite provinces or ijolite massifs and theirfenite aureoles. Nephelinite magmas were obviously generatedat great mantle depth, but their exotic composition can onlybe produced by partial melting of a metasomatic mantle. Comparisonwith other provinces would point to a source that has undergonecarbonatitic mtasomatism. KEY WORDS: nephelinites; Mt Etinde; Cameroon; petrogenesis; differentiation ^*Present address: Dpartement des Sciences de la Terre, Facult des Sciences, Universit de Yaound, BP 812, Yaound, Cameroon.Corresponding author.     

A groundwater salinity hotspot and its connection to an intermittent stream identified by environmental tracers (Mt Lofty Ranges,South Australia)

High and variable levels of salinity were investigated in an intermittent stream in a high-rainfall area (～800 mm/year) of the Mt. Lofty Ranges of South Australia. The groundwater system was found to have a local, upslope saline lens, referred to here as a groundwater salinity ‘hotspot’. Environmental tracer analyses (δ¹⁸O, δ²H, ^87/86Sr, and major elements) of water from the intermittent stream, a nearby permanent stream, shallow and deep groundwater, and soil-water/runoff demonstrate seasonal groundwater input of very saline composition into the intermittent stream. This input results in large salinity increases of the stream water because the winter wet-season stream flow decreases during spring in this Mediterranean climate. Furthermore, strontium and water isotope analyses demonstrate: (1) the upslope-saline-groundwater zone (hotspot) mixes with the dominant groundwater system, (2) the intermittent-stream water is a mixture of soil-water/runoff and the upslope saline groundwater, and (3) the upslope-saline-groundwater zone results from the flushing of unsaturated-zone salts from the thick clayey regolith and soil which overlie the metamorphosed shale bedrock. The preferred theory on the origin of the upslope-saline-groundwater hotspot is land clearing of native deep-rooted woodland, followed by flushing of accumulated salts from the unsaturated zone due to increased recharge. This cause of elevated groundwater and surface-water salinity, if correct, could be widespread in Mt. Lofty Ranges areas, as well as other climatically and geologically similar areas with comparable hydrogeologic conditions.     

Petrology and Petrogenesis of the Eclogite in Mt.Dabie Area,Central China

The occurrence,mineralogy and geochemistry of eclogites in the Mt.Dabie area show that they were subjected to a high-pressure metamorphism together with the country rocks,but their petrochemistry and REE geochemistry show some difference from those of the country rocks.The geochemical characteristics of the eclogites are similar to those of bot continental tholeiitic basalt and oceanic tholeiitic basalt.The rocks probably subducted to the upper mantle with the Dabie metamorphic complex.When elevated to the surface,they were subjected to different staes of retrogressive metamorphism.     

Traces from the past: the Cenozoic regolith and intraplate neotectonic history of the Gun Emplacement,a ferricreted bench on the western margin of the Mt Lofty Ranges,South Australia

The Gun Emplacement is a small but distinctive bench on the Eden–Burnside Fault Escarpment near Anstey Hill, in the northeastern suburbs of Adelaide, South Australia, occurring at an elevation of ～210–220 m asl. It is underlain by Middle Eocene North Maslin Sand and is capped by resistant, ferricreted colluvium. Paleomagnetic dating of hematitic mottles in the ferricreted colluvium, immediately underlying the emplacement, returned a Pliocene/Early Pleistocene age. This age is equivalent to that obtained for summit surface weathering. Fault scarps and exposures, including slickensides and fault gouge material, suggest that the Eden–Burnside Fault at this location has a strong en échelon pattern developed in response to reverse-sinistral oblique-slip faulting, reflecting continental stress fields. Remnants of ferricrete cappings forming stranded benches on the Eden–Burnside Fault Escarpment at elevations up to 25 m above the Gun Emplacement demonstrate recurrent tectonism of the South Mt Lofty Ranges related to intraplate deformation. There are at least four distinct ferricrete benches preserved on the eastern side of the active fault leading up from the Gun Emplacement surface. These benches demonstrate alternating periods of stability and tectonic activity disrupting and uplifting the ferricreted surfaces. A fresh surface rupture occurs and may be related to a recent seismic event.     

The Petrology of Thingmuli, a Tertiary Volcano in Eastern Iceland

The Tertiary flood-basalt sequence of eastern Iceland is intermittentlydisturbed by central volcanic activity with the voluminous eruptionof acid magma. Associated with one of these central volcanoes,described in this paper, is an intense swarm of acid and basicdykes, a set of acid cone-sheets, and extensive superimposedhydrothermal alteration. The lavas and intrusions which makeup the volcano grade in composition from olivine-tholeiites,through olivine-free tholeiites, basaltic-andesites, and andesites(icelandites), to rhyolites. This series is unusually rich iniron, titanium, and manganese, and poor in magnesium; aluminaand total alkalis also tend to be low in the basic members.Magnetite (sensu lato) plays a varied role in the order of crystallization,and it is only in the intermediate stages of this fractionatedseries that magnetite is available for crystal fractionation.The otherwise progressive enrichment of iron relative to magnesiumthroughout the successive liquids of the series is halted duringan intermediate stage, as magnetite becomes an early-crystallizingphase.     

天山成矿带境内外地球化学特征对比——中国东天山和哈萨克斯坦楚伊犁地区

对天山成矿带中国境内的东天山地区和哈萨克斯坦境内的楚伊犁地区1∶100万比例尺的地球化学填图所揭示的信息进行了对比研究。造岩元素、铁族元素、稀有元素、稀土元素和分散元素在这2个地区的含量和富集程度都非常接近,说明天山地区具有相同的地质背景;贵金属元素Au在楚伊犁地区的含量和富集程度要显著高于东天山地区,楚伊犁地区的Au异常不仅与石英脉型金矿有关,还与黑色页岩微粒金矿化有关;有色金属元素在2个地区差别不大,不同的是Cu异常在东天山以独立或与其他有色金属元素伴生异常存在,而在楚伊犁主要以与贵金属元素伴生异常存在;铂族元素在2个地区含量比较接近,一类异常与基性和超基性岩有关,另一类与黑色岩系有关。尽管这2个地区都还未发现有经济价值的铂族元素矿床,但在楚伊犁地区已经发现与炭质硅质岩有关的微粒金、铂矿化。     

The Petrology of Moroto Mountain, Eastern Uganda, and the Origin of Nephelinites

Moroto Mountain is one of the Tertiary alkaline volcanoes ofeastern Uganda. Two differentiated series of alkaline undersaturatedrocks occur in the volcano: a nephelinite series which is commonto all the volcanoes of the province, and an alkaline olivinebasalt series which occurs only at Moroto. It is proposed that the nephelinite series derives from theincongruent melting of pargasite. Since pargasite occurs inhydrated ultrabasic rocks, it is also suggested that the twoseries are formed from the partial melting of hydrated peridotite:the nephelinites from the homblendic portions and the alkalineolivine basalts from the less volatile-rich portions. The two series of the Moroto volcano typify the two major associationsof East African alkaline vulcanicity and, spatially, the volcanolies between regions each characterized by one of these twomajor associations. It may be that the volcano lies above azone of transition in the upper mantle between a relativelyanhydrous, volatile-poor portion and a hydrated, volatile-richportion where hornblendites are developed.     

Luminescence dating of Quaternary alluvial successions,Sellicks Creek,South Mount Lofty Ranges,southern Australia

Abstract

Quaternary alluvial and colluvial sediments infill major river valleys and form alluvial fans and colluvium-filled bedrock depressions on the range fronts and within the Mount Lofty Ranges of southern Australia. A complex association of alluvial successions occurs in the Sellicks Creek drainage basin, as revealed from lithostratigraphy, physical landscape setting and optically stimulated luminescence (OSL) ages. Correlation of OSL ages with the Marine Oxygen Isotope record reveals that the alluvial successions represent multiple episodes of alluvial sedimentation since the penultimate glaciation (Marine Isotope Stage 6; MIS 6). The successions include a penultimate glacial maximum alluvium (Taringa Formation; 160?±?15?ka; MIS 6), an unnamed alluvial succession (42?±?3.2?ka; MIS 3), a late last glacial colluvial succession within bedrock depressions (ca 15?ka; MIS 2) and a late last glacial alluvium (ca 15?ka; MIS 2) in the lowest, distal portion of Sellicks Creek. In addition, the Waldeila Formation, a Holocene alluvium (3.5?±?0.3?ka; MIS 1), and sediments deposited during a phase of Post-European Settlement Aggradation (PESA) are also identified. The age and spatial distribution of the red/brown successions, mapped as the Upper Pleistocene Pooraka Formation, directly relate to different topographic and tectonic settings. Neotectonic uplift locally enhanced erosion and sedimentation, while differences in drainage basin sizes along the margin of the ranges have influenced the timing and delivery of sediment in downstream locations. Close to the Willunga Fault Scarp at Sellicks Creek, sediments resembling the Pooraka Formation have yielded a pooled mean OSL age of 83.9?±?7?ka (MIS 5a) corroborating the previously identified extended time range for deposition of the formation. Elsewhere, within major river valleys, the Pooraka Formation was deposited during the last interglacial maximum (128–118?ka; MIS 5e). In general, alluviation occurred during interglacial and interstadial pluvial events, while erosion predominated during drier glacial episodes. In both cases, contemporaneous erosion and sedimentation continued to affect the landscape. For example, in the Sellicks Creek drainage basin, which lies across an actively uplifting fault zone, late glacial age sediments (MIS 2) occur within the ranges and near the distal margin of the alluvial fan complex. OSL dating of the alluvial successions reported in this paper highlights linkages between the terrestrial and marine environments in association with sea-level (base-level) and climatic perturbations. While the alluvial successions relate largely to climatically driven changes, especially in major river valleys, tectonics, eustasy, geomorphic setting and topography have influenced erosion and sedimentation, especially on steep-sloped alluvial fan environments.

1. KEY POINTS

2. Luminescence dating of the Sellicks Creek alluvial fan complex reveals that sedimentation occurred predominantly during the later stages of glacial cycles accompanying lower sea-levels than present.

3. Luminescence dating confirms that the stratigraphically lower portions of the Pooraka Formation are beyond the range of radiocarbon dating.

4. Upper Pleistocene alluvial fan sedimentation at Sellicks Creek correlates with pluvial events in southeastern Australia.

     

The Cenozoic Basaltic Rocks of Eastern China: Petrology and Chemical Composition

The Cenozoic volcanicity of eastern China is entirely basalticand occurred as relatively small eruptions widely dispersedin space and time, closely associated with graben basins andtheir regional bounding faults. Samples (157) from over 30 sitesin eastern China have been studied. They are predominantly alkalinebasalts, but vary in composition from olivine nephelinites andleucitites to quartz tholeiites. The majority are aphyric butsome contain olivine and clinopyroxene phenocrysts. Whole-rockanalyses (X-ray fluorescence) of all samples for the major and13 trace elements are used, as are the compositions of all themajor mineral phases determined by electron microprobe. It is argued that the most primitive basanites, alkali olivinebasalts, and olivine tholeiites represent primary or near-primarymagmas which were formed by different degrees of partial meltingof the upper mantle at different depths. The olivine tholeiitesrepresent larger degrees of partial melting (8–9%) ofa spinel peridotite at depths of <66 km. The alkalic basaltscarry xenoliths of spinel and garnet peridotite and appear tohave been derived by 1–7% partial melting of a garnetlherzolite (50% ol, 25% opx, 15% cpx, 10% garnet) at depths> 79 km. The olivine nephelinite may have formed by evensmaller degrees of partial melting. Most flows are not primary; the variations in their compositionsare consistent with fractional crystallization from the spectrumof primary parents created by varying degrees of partial meltingof a mineralogically heterogeneous source. The tholeiites havefractionated by the removal of clinopyroxene and some olivine;the alkali basalts by the removal of clinopyroxene with a smallerproportion of olivine. The incompatible behavior of Sr impliesthe absence of plagioclase from any of the fractionating assemblagesand, together with the high Al content of the pyroxene phenocrysts,suggests that much of the fractionation occurred at mantle depthsand pressures. The Cenozoic magmatism of eastern China is seen as a typicalexample of volcanism associated with continental extension.That is, small volumes of predominantly alkalic basalts andolivine tholeiites erupted over a prolonged period and associatedwith extensional basins and their bounding faults. As such,the province is distinct from continental flood basalt provinces.     

The Sulu UHP Terrane: A Review of the Petrology and Structural Geology

Petrological and isotopic evidence suggests that the protolith of the Sulu ultrahigh-pressure (UHP) terrane was Precambrian continental crust consisting of granite, granodiorite, gabbro, marble, and basic dikes, with local granulite-facies assemblages. Around 220 Ma this unit of continental rocks was buried to depths up of ～120 km within the mantle. Structures formed during exhumation suggest highly mobile behavior of acidic rocks, even under conditions of very low water activity. Petrological studies show that the Sulu terrane underwent isothermal decompression, which implies relatively rapid exhumation, and suggests that the role of melting during exhumation may have been underestimated. The later stages of exhumation are associated with NW-SE-directed tectonic transport and the formation of at least one major normal detachment.     

Kalaymyo Peridotite Massif in the Indo-Myanmar Ranges (Western Myanmar): Its Mineralogy and Petrology

Mesozoic ophiolites crop out discontinuously in the Indo-Myanmar Ranges in NE India and Myanmar,and represent the remnants of the Neotethyan oceanic lithosphere(Sengupta et al.,1990;Mitchell,1993).These ophiolites in the Indo-Myanmar Ranges are the southern continuation of the Neotethyan ophiolites occurring along the Yarlung Zangbo Suture Zone(YZSZ)in southern Tibet farther northwest(Mitchell,1993;Fareeduddin and Dilek,2015),as indicated by their coeval crystallization ages and geochemical compositions(Yang et al.,2012;Liu et al.,2016).The Kalaymyo ophiolite is located in the central part of the eastern Indo-Myanmar Ranges(Fig.1).composition of these ophiolites from the central Tibetan Plateau(CTP)is dominated by MORBs and minor OIBs and a distinct lack of IATs and BONs,which is inconsistent with most ophiolites worldwide(Robinson and Zhou,2008;Zhang et al.,2008).But the generation and tectonic nature of these ophiolites are still controversial.*The Kalaymyo peridotites consist mainly of harzburgites,which show typical porphyroclastic or coarse-grained equigranular textures.They are composed ofolivine(Fo=89.8–90.5),orthopyroxene(En86-91Wo1-4Fs8-10;Mg#=89.6–91.9),clinopyroxene(En46-49Wo47-50Fs3-5;Mg#=90.9–93.6)and spinel(Mg#=67.1–78.9;Cr#=13.5–31.5),and have relatively homogeneous whole-rock compositions with Mg#s of90.1–90.8 and Si O2(41.5–43.65 wt.%),Al2O3(1.66–2.66wt.%)and Ca O(1.45–2.67 wt.%)contents.TheydisplayLightRareEarthElement(LREE)-depleted chondrite-normalized REE patterns with(La/Yb)CN=0.04–0.21 and(Gd/Yb)CN=0.40–0.84,and show a slight enrichment from Pr to La with(La/Pr)CN in the range of 0.98–2.36.The Kalaymyo peridotites are characterized by Pd-enriched chondrite-normalized PGE patterns with superchondritic(Pd/Ir)CN ratios(1.15–2.36).Their calculated oxygen fugacities range between QFM–0.57 and QFM+0.90.These mineralogical and geochemical features collectively suggest that the Kalaymyo peridotites represent residual upper mantle rocks after low to moderate degrees(5–15%)of partial melting at a mid-ocean-ridge(MOR)environment.The observed enrichment in LREE and Pd was a result of their reactions with enriched MORB-like melts,percolating through these already depleted,residual peridotites.The Kalaymyo and other ophiolites in the Indo-Myanmar Ranges hence represent mid-ocean ridge(MOR)–type Tethyan oceanic lithosphere derived from a downgoing plate and accreted into a westward migrating subduction–accretion system along the eastern margin of India.     

Metamorphic Petrology of the Northern Tokoro Metabasites, Eastern Hokkaido, Japan

The metabasites within the Tokoro belt of eastern Hokkaido,Japan, suffered pervasive high–P/ Tetamorphism. Mineralassemblages and compositions of more than 400 metabasites fromthe Saroma–Tokoro district were investigated. The metabasites are divided into six metamorphic zones basedon mineral assemblages. The laumontite (Lm) zone is definedby the presence of laumontite. The prehnite–pumpellyite(Pr–Pp) zone is characterized by the association of prehnite+ pumpellyite. The lawsonite–sodic. pyroxene (Lw–Napx)zone is defined by the assemblage lawsonite + pumpellyite +sodic pyroxene + chlorite. The epidote–sodic pyroxene(Ep–Napx)(1) and (2) zones are charecterized by the assemblage epidote+ pumpellyite + sodic pyroxene + chlorite. The former is characterizedby the absence of aragonite, sodic amphibole, and winchite,as well as the presence of jadeite–poor sodic pyroxene(maxJd mol% = 13), whereas these minerals occur in the Ep–Napx(2)zone, together with jadeite–rich sodic pyroxene (max.Jd mol % = 34). In the epidote–actinolite (Ep–Act)zone, the most common assemblages contain epidote+ actionolite+ pumpellyite + chlorite. The Lm zone corresponds to the zeolite facies (150–200?Cand 1–2 kb) and the Pr–Pp zone is equivalent tothe prehnite–pumpellyite facies (200–250?C and 2–2–5kb). The Ep–Napx(I) zone appears to be stable at 200–250?C and 2? 5?3?5 kb. The pressure conditions in the Lw–Napx,Ep-Napx(2), and Ep–Act zones appear to range from 5 to6 kb, and the temperatures are estimated to be 200–230,230–270, and 270–300? C, respectively. The sequenceof the metamorphic zones is charaterized by the curved P–Tpath. The stability field of pumpellyite+ sodic+ pyroxene+ chloritein Fe₃+ bearing metabasites is located in the lower–temperatureand higher–pressure part of the pumpellyite–actionolitefacies. On the basis of Schreinmaker's method, the stabilityfield of the assemblage is bounded by a high–pressurereaction Pp+ Napx+ Chl+ Ab+ Qz+ H₂O= Lw+ Gl, and by a high-temperaturereaction Pp Napx+ Chl+ Ab+ Qz = Ep + Gl + H₂O.     

大别岳西地区花岗岩类岩石学及其成因

岳西地区花岗岩类主要由主簿原和白马尖二长花岗岩－正长花岗岩岩基、花岗闪长岩－石英二长闪长岩小岩体和晚期的淡色碱长花岗岩岩体组成。主要岩体形成于燕山晚期,为造山后花岗岩。岩基中花岗岩的暗色矿物为黑云母,主簿原花岗岩含有褐帘石,岩石化学显示为过铝质（Ａ／ＣＮＫ约为１．１）,稀土配分为右倾的、中等负铕异常（Ｓｍ／Ｅｕ为０．１６～０．２５）曲线。花岗闪长质的、没有变形的小岩体中普遍含有角闪石和榍石,岩石化学表现为准铝质（Ａ／ＣＮＫ为０．８～０．９,Ａ／ＮＫ为１．５～１．８）,稀土配分为右倾的无铕异常（或略有正铕异常）的曲线,但其形成时代和εＮｄ（０）值与主要岩基相同。花岗岩类的εＮｄ（０）值均为较大的负值（－１７～－２６）,在εＮｄ（０）－εＳｒ（０）图上,表现为一条水平带状分布,显示其源岩为存留时间很长的古老地壳。     

Petrology of the Udayagiri anorthosite complex, Eastern Ghats Belt, India

The Eastern Ghats Belt (EGB), characterised by pervasive Grenvillian granulite facies metamorphism, is the host to several 950–1000 Ma old massif-type anorthosite complexes. The present work describes one such complex near Udayagiri from the northern margin of the EGB, reported for the first time as “Udayagiri anorthosite complex” (UAC). The ‘massif type’ UAC comprises mainly of anorthosite, leuconorite-olivine leuconorite and norite in the decreasing order of areal extent. Mineralogically, these rocks dominantly consist of cumulates of moderately calcic plagioclase (～An_50–60), moderately magnesian intercumulus olivine (X_Mg: ～0.6) and orthopyroxene (X_Mg: 0.47 to 0.70). Metamorphic garnet (Alm: ～50 mol%) is also common in these rocks. Anorthosite and leuconorite of the UAC exhibit a moderate ‘+ve’ Eu anomaly. Norite occurs locally as schlierens and is relatively rich in Fe, P, Rb, Sr, Th, Nb, Ta, Y and REE which could be a residual melt product. These rocks exhibit both relict magmatic mineralogy and textures with a metamorphic impress manifested by the development of multilayered corona involving olivine, orthopyroxene, garnet, phlogopite, ilmenite and plagioclase during cooling of the pluton. The corona development is a result of combination of significant magmatic and metamorphic reactions which have the potential to provide important clues for deciphering the magmatic and metamorphic evolution of such plutons in ambient granulite facies conditions.     

Petrology and Geochemistry of Cretaceous Ultramafic Volcanics from Eastern Kamchatka

The origin, evolution and primary melt compositions of lateCretaceous high-K ultramafic volcanics and associated basaltsof Eastern Kamchatka are discussed on the basis of a study ofthe mineralogy and geochemistry of the rocks and magmatic inclusionsin phenocrysts. The exceptionally primitive composition of thephenocryst assemblage [olivine—Fo;_88–95, Cr-spinel—Cr/(Cr + Al) up to 85] provides direct evidence of the mantleorigin of primary melts, which were highly magnesian compositions(MgO 19–24 wt%). The rocks and meltsare characterizedby strong high field strength element (HFSE) depletion in comparisonwith rare earth elements, and high and variable levels of enrichmentin large ion lithophile elements (LILE), P, K and H₂O (0.6–12wt % in picritic to basaltic melts). _Nd values lie in a narrowrange (+107 to +91), typical of N-MORB (mid-ocean ridge basalt),but ⁸⁷Sr/⁸⁶Sr (0.70316–0.70358) is slightly displacedfrom the mantle array. High-K ultramafic melts from Kamchatkaare considered as a new magma type within the island-arc magmaticspectrum; basaltic members of the suite resemble arc shoshonites.The primary melts were produced under high-pressure (30–50kbar) and high-temperature(1500–1700C) conditions bypartial melting of a refractory peridotitic mantle. KEY WORDS: Kamchatka; Late Cretaceous magmatism; ultramafic volcanics; shoshonites ^*Corresponding author. Present address: Department of Geology, University of Tasmania, GPO Box 252C, Hobart, Tas., Australia