Experimental Petrology of Melilite Nephelinites

Experimental study of natural melilite nephelinite lavas ofintermediate K/Na ratio at low pressure (fo₂ reveals the presenceof a peritectic ‘point’ of distributary type (1090?C)for liquids saturated with leucite, nepheline, and spinel. Withdecreasing temperature on the olivine + melilite cotectic, botholivine and melilite react with such liquids to produce high-calciumpyroxene at the peritectic. Both the olivine + high-calciumpyroxene and melilite + high-calcium pyroxene cotectics arestable at temperatures below the peritectic. Olivines coexistingwith such liquids are much more magnesian than those in comparabletholeiitic liquids. The olivine-liquid Fe-Mg distribution coefficient is a monotonically increasing function of silica activity over the composition range spannedby melilite nephelinite, ugandite, alkali basalt, and tholeiitebasalt liquids. The analogous Fe-Mg distribution coefficientfor melilite and liquid is effectively constant , while that for high-calcium pyroxene and liquidis highly dependent on the chemistry of high-calcium pyroxene(cf., Sack & Carmichael, 1984). Pseudoternary liquidus projectionsof multiply saturated liquids coexisting with nepheline, leucite,and spinel (?olivine?high-calcium pyroxene?melilite) have beenprepared to facilitate graphical analysis of the evolution oflava compositions during hypabyssal cooling. Major element chemicalanalyses and petrographic features of lavas from Mt. Nyiragongo,East Africa and Oahu, Hawaii (e.g., Denaeyer et al., 1965; Wilkinson& Stolz, 1983) confirm the validity of these diagrams andthe systematics established from the experimental data. ^*Reprint requests to R.O. Sack     

The Petrology of the Tholeiites through Melilite Nephelinites on Gran Canaria, Canary Islands: Crystal Fractionation, Accumulation, and Depths of Melting

We report major and trace element X-ray fluorescence (XRF) datafor mafic volcanics covering the 15-Ma evolution of Gran Canaria,Canary Islands. The Miocene (12–15 Ma) and Pliocene-Quaternary(0–6 Ma) mafic volcanics on Gran Canaria include picrites,tholeiites, alkali basalts, basanites, nephelinites, and melilitenephelinites. Olivineclinopyroxene are the major fractionatingor accumulating phases in the basalts. Plagioclase, Fe–Tioxide, and apatite fractionation or accumulation may play aminor role in the derivation of the most evolved mafic volcanics.The crystallization of clinopyroxene after olivine and the absenceof phenocrystic plagioclase in the Miocene tholeiites and inthe Pliocene and Quaternary alkali basalts and basanites withMgO>6 suggests that fractionation occurred at moderate pressure,probably within the upper mantle. The presence of plagioclasephenocrysts and chemical evidence for plagioclase fractionationin the Miocene basalts with MgO<6 and in the Pliocene tholeiitesis consistent with cooling and fractionation at shallow depth,probably during storage in lower-crustal reservoirs. Magma generationat pressures in excess of 3•0–3•5 GPa is suggestedby (a) the inferred presence of residual garnet and phlogopiteand (b) comparison of FeO¹ cation mole percentages and the CIPWnormative compositions of the mafic volcanics with results fromhigh-pressure melting experiments. The Gran Canaria mafic magmaswere probably formed by decompression melting in an upwellingcolumn of asthenospheric material, which encountered a mechanicalboundary layer at {small tilde}100-km depth.     

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.     

Petrology of the Caribou Mountain Pluton, Klamath Mountains, California

The Caribou Mountain pluton is a small trondhjemitic body thatintruded semipelitic schist of the Stuart Fork terrane in lateMiddle Jurassic to Early Cretaceous time. Its emplacement followedthe intrusion of an adjoining body of hornblende quartz dioritecalled the Middle Fork pluton and the mode of its emplacementwas as an asymmetric ballooning diapir (Davis, 1963), as shownby concentric foliation, radial late-stage dikes, foliated enclaves,and folded blocks of schlieren-banded tonalite. Coarse-grainedhornblende-bearing trondhjemite is the dominant rock type inthe Caribou Mountain pluton, and it is called the ‘maintrondhjemite’. It was followed by medium-grained ‘latetrondhjemite’ and by late-stage trondhjemitic and granodioriticdikes. All the trondhjemitic rock types are characterized bylow alkali contents, high light rare earth elements, low initial⁸⁷Sr/⁸⁶Sr, and low ¹⁸O. However, the late trondhjemite has higherNa₂O and a higher initial ⁸⁷Sr/⁸⁶Sr value than the main trondhjemite,and the two units cannot be related by fractional crystallization.The late granodioritic dikes are richer in Ba, Rb, Y, and Scthan the late trondhjemite and probably reflect assimilationof Stuart Fork metasedimentary rocks by late-stage trondhjemiticmagma. Mafic enclaves in the main trondhjemite contain xenocrysts ofquartz and plagioclase derived from the host by magma mixing.The enclaves have K₂O, Ba, and Rb contents similar to, or higherthan those of the host rocks. Their rare earth element (REE)patterns display strong middle REE enrichment caused by accumulationof hornblende, probably as the result of filter pressing. The main trondhjemite cannot be derived from Middle Fork magmabecause the initial ⁸⁷Sr/⁸⁶Sr of the Middle Fork pluton is lowerthan that of the trondhjemite. The absence of parental maficmagmas of appropriate composition suggests that the CaribouMountain trondhjemitic magmas formed by partial melting of anamphibolitic source rock compositionally similar to low-K tholeiite.     

Petrology of the Blue Mountain Complex, Marlborough, New Zealand

Blue Mountain is a central-type alkali ultrabasic-gabbro ringcomplex (1?1?5 km) introducing Upper Jurassic sediments, Marlborough,New Zealand. The ultrabasic-gabbroic rocks contain lenses ofkaersutite pegmatite and sodic syenite pegmatite and are intrudedby ring dykes of titanaugite-ilmenite gabbro and lamprophyre.The margin of the intrusion is defined by a ring dyke of alkaligabbro. The plutonic rocks are cut by a swarm of hornblende-biotite-richlamprophyre dykes. Thermal metamorphism has converted the sedimentsto a hornfels ranging in grade from the albite-epidote hornfelsfacies to the upper limit of the hornblende hornfels facies. The rocks are nepheline normative and consist of olivine (Fo_82-74),endiopside (Ca₄₅Mg₄₈Fe₇-Ca₃₆Mg₅₅Fe₉), titanaugite (Ca₄₀Mg₅₀Fe₁₀-Ca₄₄Mg₃₉Fe₁₇),plagioclase (An_73-18), and ilmenitetitaniferous magnetite, withvarious amounts of titaniferous hornblende and titanbiotite.There is a complete gradation between end-iopside and titanaugitewith the coupled substitution R_y^+z+Si(Ti⁺⁴+Fe⁺³)+Al⁺³ and asympathetic increase in CaAl₂SiO₆ (0?2-10?2 percent) and CaTiAl₂O₆(2?1-8?1 per cent) with fractionation. Endiopside shows a small,progressive Mg enrichment along a trend subparallel to the CaMgSi₂O₆-Mg₂Si₂O₆boundary, and titanaugite is enriched in Ca and Fe⁺²+Fe⁺³ withdifferentiation. Oscillatory zoning between endiopside and titanaugiteis common. Exsolved ilmenite needles occur in the most Fe-richtitanaugites. The amphiboles show the trend: titaniferous hornblende(1?0–5?7 per cent TiO₂)kaersutite (6?4 per cent TiO₂)Fe-richhastingsite (18?0–19?1 per cent FeO as total Fe). Biotiteis high in TiO₂ (6?6–7?8 per cent). Ilmenite and titaniferousmagnetite (3?5–10?6 per cent TiO₂) are typically homogeneousgrains; their composition can be expressed in terms of R⁺²RO₃:R⁺²O:R₂⁺³O₄. The intrusion of igneous rocks was probably controlled by subterraneanring fracturing. Subsidence of the country rock within the ringfracture provided space for periodic injections of magma froma lower reservoir up the initial ring fracture to form the BlueMountain rocks at a higher level. Downward movement of the floorof the intrusion during crystallization caused inward slumpingof the cumulates which affected the textural, mineralogical,and chemical evolution of the rocks in different parts of theintrusion. The order of mineral fractionation is reflected by the chemicalvariation in the in situ ultrabasic-gabbroic rocks and the successiveintrusions of titanaugite-ilmenite gabbro and lamprophyre ringdykes, marginal alkali gabbro and lamprophyre dyke swarm. Aninitial decrease, then increase in SiO₂; a steady decrease inMgO, CaO, Ni, and Cr: an initial increase, then decrease inFeO+Fe₂O₃, TiO₂, MnO, and V; almost linear increase in Al₂O₃and late stage increase in alkalis and P₂O₃, implies fractionationof olivine and endiopside, followed by titanaugite and Fe-Tioxides, followed by plagioclase, hornblende, biotite, and apatite.Reversals in the composition of cumulus olivine and endiopsideand Solidification Index, indicate that the ultrabasic-gabbroicsequence is composed of four main injections of magma. The ultrabasic rocks crystallized under conditions of high PH₂Oand fairly high, constant P_O2; P_H2 and P_O2 increased duringthe formation of the gabbroic rocks until fracturing of thechamber roof occurred. The abundance of euhedral amphibole inthe latter injection phases suggests that amphibole accumulatedfrom a hydrous SiO₂ undersaturated magma when an increase inP_O2, stabilized its crystallization. Plutonic complexes similar to Blue Mountain are found withinand beneath the volcanic piles of many oceanic islands, e.g.Canaries, Reunion, and Tahiti, and those intruding thick sedimentarysequences, as at Blue Mountain, e.g. the pipe-like intrusionsof the Monteregian Hills, Quebec.     

Petrology of the Blue Mountain Complex, Marlborough, New Zealand

Blue Mountain is a central-type alkali ultrabasic-gabbro ringcomplex (lxl7middot;5 km) introducing Upper Jurassic sediments,Marlborough, New Zealand. The ultrabasic-gabbroic rocks containlenses of kaersutite pegmatite and sodic syenite pegmatite andare intruded by ring dykes of titanaugite-ilmenite gabbro andlamprophyre. The margin of the intrusion is defined by a ringdyke of alkali gabbro. The plutonic rocks are cut by a swarmof hornblendebiotite-rich lamprophyre dykes. Thermal metamorphismhas converted the sediments to a hornfels ranging in grade fromthe albite-epidote hornfels facies to the upper limit of thehornblende hornfels facies. The rocks are nepheline normative and consist of olivine (Fo_82–74),endiopside (Ca₄₅Mg₄₈Fe₇–Ca₃₆Mg₅₅Fe₉), titanaugite (Ca₄₀Mg₅₀Fe₁₀–Ca₄₄Mg₃₉Fe₁₇),plagioclase (An_73–18), and ilmenitetitaniferous magnetite,with various amounts of titaniferous hornblende and titanbiotite.There is a complete gradation between endiopside and titanaugitewith the coupled substitution R_y⁺²+Si;;(Ti⁺⁴+Fe⁺³+Al⁺³ and asympathetic increase in CaAl₂SiO₆ (0·2–10·2percent) and CaTiAl₂O₆ (2·1–8·1 per cent)with fractionation. Endiopside shows a small, progressive Mgenrichment along a trend subparallel to the CaMgSi₂O₆–Mg₂Si₂O₆boundary, and titanaugite is enriched in Ca and Fe⁺²+Fe⁺³ withdifferentiation. Oscillatory zoning between endiopside and titanaugiteis common. Exsolved ilmenite needles occur in the most Fe-richtitanaugites. The amphiboles show the trend: titaniferous hornblende(1·0–57middot;7 per cent TiO₂) kaersutite (6·4per cent TiO2) Fe-rich hastingsite (18·0–19·1per cent FeO as total Fe). Biotite is high in TiO₂ (6·6–7·8per cent). Ilmenite and titaniferous magnetite (3·5–10·6per cent TiO₂) are typically homogeneous grains; their compositioncan be expressed in terms of R⁺²RO₃:R⁺²O:R₂⁺³O₄. The intrusion of igneous rocks was probably controlled by subterraneanring fracturing. Subsidence of the country rock within the ringfracture provided space for periodic injections of magma froma lower reservoir up the initial ring fracture to form the BlueMountain rocks at a higher level. Downward movement of the floorof the intrusion during crystallization caused inward slumpingof the cumulates which affected the textural, mineralogical,and chemical evolution of the rocks in different parts of theintrusion. The order of mineral fractionation is reflected by the chemicalvariation in the in situ ultrabasic-gabbroic rocks and the successiveintrusions of titanaugite-ilmenite gabbro and lamprophyre ringdykes, marginal alkali gabbro and lamprophyre dyke swarm. Aninitial decrease, then increase in SiO2; a steady decrease inMgO, CaO, Ni, and Cr: an initial increase, then decrease inFeO+Fe₂O₃, TiO₂, MnO, and V; almost linear increase in A1₂O₃and late stage increase in alkalis and P₂O₃, implies fractionationof olivine and endiopside, followed by titanaugite and Fe-Tioxides, followed by plagioclase, hornblende, biotite, and apatite.Reversals in the composition of cumulus olivine and endiopsideand Solidification Index, indicate that the ultrabasic-gabbroicsequence is composed of four main injections of magma. The ultrabasic rocks crystallized under conditions of high P_H2Oand fairly high, constant     

The Petrology and Origin of the Tertiary Lundy Granite (Bristol Channel, UK)

The British Tertiary Volcanic Province (BTVP) comprises within-platecentral igneous complexes associated with plateau lavas andregional dyke swarms. Lundy is the southernmost complex of theBTVP and comprises granite ({small tilde}90%) emplaced intodeformed Devonian sedimentary rocks within the Hercynian Cornubiangranite province of southwest England. The complex is intrudedby a northwest-southeast trending dyke swarm. In common withother BTVP igneous complexes, Lundy is associated with positivegravity and magnetic anomalies which are interpreted in termsof the presena of an underlying basic intrusion at shallow depth,with a volume exceeding that of the overlying granite. The Lundy intrusion is a coarse-grained megacrystic granitecontaining up to 20% alkali feldspar megacrysts in a coarse-grainedgroundmass composed of alkali feldspar, quartz, lithium-bearingmuscovite, and ‘biotite’ (lithian siderophyllite),with a range of aaxssory minerals. The main granite has a coarse-grained(locally miarolitic) pegmatitic facies and is intruded by thinsheets and veins of fine-grained aplite and microgranite. Themineralogy indicates crystallization of the Lundy granite froma highly fractionated H₂O- and halogen-rich magma at a relativelyshallow crustal level. The main Lundy granite is a peraluminous leucogranite with Na₂O=3–4%,K₂O{small tilde}5%, low TiO₂, MeO, CaO, Zr, and Sr, and highRb and Rb/Sr in comparison with many other peralurninous granites,including those from the Cornubian batholith and the BTVP. Anew Rb-Sr whole-rock isochron for the granite yields an ageof 58?7?1?6 Ma with an initial ⁸⁷Sr/86Sr of 0?715?0?006. _Ndvalues for the granite (–0?9 to –1?9) plot betweencontemporaneous mantle (positive _Nd and Cornubian granites (_Nd=ca.–11). The trace element data (Rb, Y, Nb) show affinities with syn-collisionand within-plate granites. As the Sr isotope data indicate amajor crustal component, and the Nd isotope data suggest bothmantle and crustal components, we propose that the Lundy graniteis derived from a parental magma comprising crustal components(derived from a similar source to that of the Cornubian granitebatholith) and a mantle-derived component (derived from a differentiateof contemporaneous basaltic magma This magma experienced fractionalcrystallization of plagioclase, alkai feldspar, Fe-Mg minerals,and REE-bearing accessory minerals before emplacement, and theLundy granite experienced further in situ fractional crystallization,associateded with crustal contamination by the Devonian shaleafter emplacement.     

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.     

柴达木山花岗岩的岩石学、地球化学及锆石U-Pb年代学研究

正柴北缘高压—超高压变质带内发育的榴辉岩、蛇绿岩、麻粒岩及片麻岩等前人进行了大量的研究。大柴旦地区是柴北缘古生代超高压带的重要组成部分,与超高压岩石相伴的花岗岩十分发育,查明该超高压带上花岗质岩浆活动期次及其与超高压变质作用、造山带构造演化之间的关系,对探讨中国西部大地构造演化具有十分重要的意义(吴才来等,2007)。     

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 plagiogranite in the Canyon Mountain ophiolite,Oregon

Plagiogranites in the Canyon Mountain ophiolite, Oregon, include a wide range of rock types ranging from diorite to trondhjemite. The plagiogranites are mostly concentrated as an intrusive sill swarm at the top of a section of gabbroic cumulates. The plagiogranites are typically low in K₂O and high in Na₂O, and are enriched 10–20 times chondrites in REE, and overlap with abundances in basic rocks from Canyon Mountain. All samples of plagiogranite are relatively depleted in LREE, with more silicic samples characterized by a slightly lesser degree of LREE depletion. Total REE content is not consistently correlated with contents of major and other trace elements. Fractional crystallization of basaltic magma may give rise to plagiogranites; however this model applied to Canyon Mountain plagiogranites is discounted because of the significant volume of plagiogranites relative to basic rocks, and the complete overlap of REE abundances of the basic rocks and the plagiogranites. The latter is also a major reason for rejecting the hypothesis of silicate liquid immiscibility in the generation of the plagiogranites. Field observations coupled with major-element and trace element chemistry lend support to a model by which the plagiogranites were produced by partial melting of basic rocks under hydrous conditions. REE data for the plagiogranites were used in calculations to delimit source REE contents. Relevant parameters in the calculations were estimated from experimentally determined phase relations of basalt under hydrous conditions. The resulting calculated source patterns are similar to those of basic rocks in ophiolites and oceanic settings, and suggest boundary conditions for the model. Partial melting as suggested for the Canyon Mountain plagiogranites probably occurred at relatively shallow depths (i.e., total pressures less than 5 kb).     

The Structural Petrology of Portion of the Eastern Mt. Lofty Ranges

Abstract

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.     

The Petrology of the Lower Old Red Sandstone Lavas of the Eastern Sidlaw Hills, Perthshire, Scotland

The calc-alkaline lava sequence of the eastern Sidlaw Hillsforms a small part of an extensive volcanic province of LowerOld Red Sandstone (Devonian) age in Scotland and N. England.The Sidlaw lavas ranging from olivine basalt to dacite are allporphyritic with combinations of olivine, plagioclase, clinopyroxene,orthopyroxene, and opaque oxide pheno-crysts. Chemically, thelavas are slightly more alkalic than modern calc-alkaline lavas.There is considerable variation in the ‘incompatible elements’.The differentiation of the lavas can be accounted for by fractionationof olivine+plagioclase+minor ore from a chemically variable,immediately parental magma at low pressure (c. 1 kb P_H2O). Itis suggested that fractionation of variable amounts of olivineand clinopyroxene from an olivine tholeiite at moderate P_H2Ocould give rise to this chemically variable, high alumina, immediatelyparental magma.     

The Petrology and Geochemistry of Vesteris Seamount, Greenland Basin--an Intraplate Alkaline Volcano of Non-Plume Origin

Vesteris Seamount is a solitary alkaline volcano in the GreenlandBasin some 280 km NW of Jan Mayen. Topographic and geophysicalstudies have shown no sign of an associated plume trace. Evidencefrom ash layers in sediment cores around the volcano and datingof dredged samples show that it has been active in Quaternarytimes. The lavas from Vesteris studied here consist of basanites,tephrites, mugearite, and alkali basalts. Crystal fractionationmodels are consistent with the generation of the tephrites andmugearite from a basanitic parent. Extensive kaersutite fractionationis required late in the fractionation sequence to produce theextreme mugearite composition. Na-Al-Fe-rich green cores tomany clinopyroxene phenocrysts at Vesteris suggest a fractionationhistory beginning at high pressure in the mantle. Differencesbetween Vesteris and Jan Mayen in the ratios of highly incompatibletrace elements such as Ce/Pb and Rb/Cs, which will not normallybe fractionated from one another during mantle melting, suggestthat the two are not derived from the same source. Relativelyunradiogenic Sr isotope ratios (compared with Bulk Earth), andhighly incompatible trace element patterns similar to thosefor St. Helena, suggest that Vesteris magmas are derived froma depleted, asthenospheric source. We propose that the Vesterisbasanites are very low degree partial melts ({small tilde}1%)of this source, most probably those which give rise to the seismiclow-velocity zone (LVZ). Such small-degree melts may preferentiallytap small-scale heterogeneities in the asthenosphere. Vesterislies at the intersection of two major structural trends in theGreenland Basin—(1) a zone of major reorientation of spreadingdirection on the Mohns Ridge north of Jan Mayen and (2) theextension of the Kolbeinsey Ridge axis. We propose that a combinationof the extensional stress fields related to these two lineamentsproduces sufficient dilation of the lithosphere at Vesteristo allow magmas from the LVZ to reach the surface.     

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.     

Petrology of the mafic sill of Narshingpur-Lakhnadon section,Eastern Deccan volcanic province

Deccan volcanism with a tremendous burst of volcanic activity marks a unique episode in Indian geological history and covers nearly two third of Peninsular India. Occurrences of mafic sill in the continental basalts are rather rare throughout the flood basalt provinces and only few sporadic reports have been described from different Continental Flood Basalts of the world. In the present article, petrology of mafic sill from the Narshingpur-Lakhnadon section of Eastern Deccan province of India has been presented. The mafic sill in the field is found to occur in a relatively deep valley amidst Gondwana rocks, which occur as the basement of the extrusion. The sill is spatially associated with three initial flows viz. flow I, II and III of adjacent Narshingpur-Harrai-Amarwara section. The sill in its central part is a medium grained rock and petrographically corresponds to dolerite containing augite, plagioclase and rare olivine grains; the chilled facies of the sill is characterized by phenocrysts of olivine, plagioclase and augite that are set in groundmass consisting predominantly of plagioclase, olivine and glass. Mineral chemistry indicates that olivine phenocrystal phase is magnesian (Fo61). Plagioclase phenocrystal composition ranges from An 51 to An 71 whereas the same variation of the groundmass plagioclase composition corresponds to An 31 to An 62. The overlap in the compositions for groundmass and phenocrystal plagioclase may be explained due to fluctuating PH2O condition. The pyroxene compositions (both groundmass and phenocryst) in majority of the cases are clubbed well within the augite field, however, in a few cases, groundmass compositions are found to fall in the sub-calcic augite and pigeonite field. Some zoned pyroxene phenocrysts, characteristically display different types of zoning patterns. Opaque minerals in the mafic sill are found to be magnetite and ilmenite and this coexisting iron-oxide composition helps to constrain the prevalent fO2 condition in the parent magma. The geochemistry of the mafic sill and associated basaltic lava flows indicates close genetic link amongst them. Critical consideration of trace elements indicates a distinct enriched mantle source (EM-I/EM-II/HIMU) for the parental magma. Trace element modeling indicates that equilibrium batch-melting of plume source followed by fractionation of olivine, clinopyroxene and plagioclase and subsequent heterogeneous mixing of melt and settled crystals can very well explain the genesis of the mafic sill and the associated basaltic flows.     

Petrology of Cr-rich Ophiolitic Chromitites of Bulqiza, Eastern Ophiolitic Belt, Albania

<正>The ultramafic massif of Bulqiza,which belongs to the eastern ophiolitic belt of Albania,is the most important area for metallurgical chromitite ores.The massif consists of a thick(4 km)rock sequence,with a generalized profile from the bottom to the top as follows.The tectonite     

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.     

On the Origin of One Basin-Multiple Mountain Couplings in the Mesozoic-Cenozoic Basin-Range Area in Eastern North China

The basin-range coupling relation is a leading subject of the modern geology. In geometry, relations of this type include couplings between stretched orogenic belt and down-faulted basin, compressional orogenic belt and foreland basin, strike-slip orogenic belt and strike-slip basin and so on. Fault chains are the key for these couplings and there are typical examples for all these cases. The North China down-faulted basin is coupled west with the Taihang uplift, east with the Jiao-Liao Mountains, north with the Yanshan orogenic belt and south with the Dabie orogenic belt, that is to say, the central down-faulted basin and the surrounding orogenic belts bear a coupling relation within a uniform dynamistic system. Study shows that the central down-faulted basin and the North China mantle sub-plume structure have a close relation during their formation. Owing to intensive mantle sub-plume uplifting, the bottom of the lithosphere suffered from resistance, which caused the lithosphere of the eastern North Ch