Clinopyroxenes from Somma-Vesuvius: Implications of Crystal Chemistry and Site Configuration Parameters for Studies of Magma Genesis

A clinopyroxene suite from Somma-Vesuvius clinopyroxenite cjectaand mcgacrysts has been investigated by single crystal X-raydiffraction combined with electron probe microanalysis. Theaim of the study was to characterize the intracrystalline configurationof the clinopyroxenes and delineate their possible parentalliquid(s). Vesuvius megacrysts and nodule clinopyroxenes show the initial‘signature’ of the leucitite (sensu lato) magmawhile Somma clinopyroxenes are characteristic of leucite phonolitemagma. The crystal chemical variations demonstrate that thespecimens investigated had originated by shallow-level crystallizationof leucitite and its derivatives, under reducing conditions.T-site (Si + Al) deficiency, typical of clinopyroxene from lamproitesis reported in some specimens from the Roman Volcanic Regionfor the first time. Intracrystalline Mg-Fe²⁺ ordering in M1-M2 appears to be configuration-dependent,yielding the largest K_D variation in the clinopyroxene fromphonolite. This is contrary to expectation.     

Crystal-chemistry of clinopyroxenes from potassic and ultrapotassic rocks in central Italy: implications on their genesis

The clinopyroxenes mentioned have been investigated by single crystal X-ray diffraction combined with electron microprobe analysis. The aim of this study was to characterize the crystal-chemical variations of clinopyroxenes in order to delineate the intracrystalline constraints which are characteristic of specific magmatic environments. Clinopyroxenes (cpx) crystallized from peralkaline ultrapotassic melt with kamafugitic and lamproitic affinities are characterized by high Si contents, which are insensitive to variations in silica abundance and silica saturation of the melt. The high Si occupancy in clinopyroxenes from kamafugitic magma is coupled to large M1 (i.e. Mg and Fe²⁺) and M2 (high Ca occupancy) sites, whereas in clinopyroxenes from magmas with lamproitic affinity, high Si content is combined with large M1 but small M2 sites. Clinopyroxenes from Romantype alkaline potassic and ultrapostassic rocks are characterized by an expanded tetrahedron (high ^IVA1 content) and small M1 site which is combined with small M2 polyhedron in clinopyroxenes from the potassic rocks and large M2 site in those from the ultrapotassic rocks.     

The Range of Spinel Compositions in Terrestrial Mafic and Ultramafic Rocks

Compositional fields for spinels from a wide variety of mafic–ultramaficigneous rock types and tectonic environments have been determinedfrom a global database of over 26 000 analyses. These fieldsare defined using contoured data density plots based on thespinel prism, and plots of T iO₂ vs ferric iron, for mantlexenoliths, ophiolitic rocks, continental layered intrusions,alkalic and lamprophyric rocks, tholeiitic basalts, Alaskanultramafic complexes and komatiites. Several trends appear regularlyin the various environments: a trend of widely variable Cr/(Cr+ Al) at low Fe²⁺/(Mg + Fe²⁺) (the Cr–Al trend); increasingFe³⁺, Fe²⁺/(Mg + Fe²⁺) and T iO₂ at constant Cr/(Cr + Al) (Fe–Ti trend); a trend found primarily in kimberlites, similar toFe–T i but at constant Fe²⁺/(Mg + Fe²⁺); and an unusualtrend of increasing Al found only in layered intrusions. TheCr–Al and Fe–T i trends are both found to varyingdegrees in tholeiitic basalts. The Cr–Al trend is prevalentin rocks that have equilibrated over a range of pressures, whereasthe Fe–T i trend is dominantly due to low-pressure fractionation.The most Cr-rich chromites found in nature occur in boninites,diamond-bearing kimberlites, some komatiites and ophioliticchromitites. Exceptionally reduced chromites are found in somekomatiites and in ophiolitic chromitites. Detrital chromitesfrom the Witwatersrand conglomerates are of komatiitic provenance. KEY WORDS: basalt; chromite; kimberlite; ophiolite; spinel     

Mineralogic Study of Eclogitic Rocks from Alpe Arami, Lepontine Alps, Southern Switzerland

Certain ultramafic-mafic lenses exposed in Ticino along thecontact zone between the underlying Simano and overlying Adulanappes display relatively high-pressure phase assemblages. AtAlpe Arami, metabasaltic layers associated with pyropic garnet-bearingIherzolite consist mainly of an early eclogitic assemblage characterizedby Alm₃₉Py₃₇Gross₂₃Spess₀₁+ Di₄₉Hd₀₈Jd₄₃+rutile±kyanite.Iron-magnesium fractionation between garnet+omphacite pairsyields a K_D, (Fe²⁺/Mg)_garnet/(Fe²⁺/Mg)_{cllnopyroxence}, of about6. This earlier assemblage has been replaced by a later, somewhatpargasitic hornblende+oligoclase+clinozoisite phase compatibility.Associated primary garnet peridotites contain Ca-rich clinopyroxeneand Al₂O₃-poor orthopyroxene. Both rock types have been affectedby a still later period of incipient chloritization. Available phase equilibrium and element partitioning data arecompatible with an inferred P-T condition of origin for theAlpe Arami mafic-ultramafic complex of 965–1000 °C,30–50 kilobars, indicating deep upper mantle generation.Amphibolites could have been produced during depressurizationaccompanying ascent of the mass through the upper mantle, butinasmuch as plagioclase accompanies the hornblende, the assemblageprobably crystallized after emplacement of the complex in theLepontine terrane prior to the termination of the Late Alpineregional metamorphism. Incipient production of high-rank greenschistphases certainly reflects a crustal event.     

The Mineralogy and Geochemistry of the Metamorphosed Basic and Ultrabasic Rocks of the Jijal Complex, Kohistan, NW Pakistan

The Jijal complex, covering more than 150 sq. km in the extremenorth of Pakistan, is a tectonic wedge of garnet granulitesintruded in the south by a 10 x 4 km slab of ultramafic rocks.The granulites are divisible into plagioclase-bearing (basicto intermediate) and plagioclase-free (ultrabasic to basic)types, the two types reflecting differences in bulk chemistry.Garnet + plagioclase + clinopyroxene + quartz + rutile ±hornblende ± epidote is the most common assemblage. Theplagioclase-free rocks are composed mainly of two or three ofthe minerals garnet, amphibole, clinopyroxene and epidote. Orthopyroxeneoccurs in websteritic rocks devoid of epidote. Much of the amphiboleand some epidote appear to be prograde products. Although variationdiagrams do not reveal a genetic link between the two typesof granulite, it is considered that they are comagmatic ratherthan the products of two or more unrelated magmas. The compositions of garnet (Py_28–46 Alm _27–43Gro_16–28),clinopyroxene (Mg_44–34Fe_5–17Ca_51–49, Al₂O₃3·0–9·9 per cent), orthopyroxene (with upto 5·5 per cent Al₂O₃), amphibole (with up to 16·3per cent Al₂O₃ and high Al^vi/Al^iv), and the abundance of garnetsuggest a high-pressure origin for the granulites. The rocksappear to have differentiated from a tholeiitic magma of oceanicaffinity or they may be genetically related to the pyroxenegranulites of Swat considered to have originally crystallizedfrom a calc-alkaline magma of island arc or continental marginaffinity. They probably crystallized in the ancient Tethyancrust/upper mantle (or less likely in a continental margin),later to be metamorphosed to granulites (670–790 °C,12–14 kb) during the collision of the Indian-Asian landmasses,and carried upwards during later Himalayan orogenic episodes. The ultramafic rocks are alpine-type in nature and devoid ofgarnet. They are dominated by diopsidites; dunites, peridotites,and harzburgites together form <50 per cent of the area ofoutcrop. The chemistry of the rocks, and their olivines (Fo_92–89)and clinopyroxenes (Mg_49.5–48Fe_2.8–5.2Ca_47.4–46.8)are similar to those of alpine complexes of the harzburgitesubtype. It is not clear whether they represent a faulted slabof suboceanic crust/upper mantle, mantle diapirs in deep orogenicroots, or dismembered ultramafic rocks differentiated from abasaltic magma. They seem to have a complex history; their presentmineralogy is suggestive of high grade metamorphism (800–850°C, 8–12 kb). They are magmatically unrelated to thegarnet granulites and were probably intruded into the latteras plastic crystalline material after both had been independentlymetamorphosed, but before the entire complex was carried tectonicallyinto its present surroundings. The abundances of the diopsiditesis in marked contrast to other alpine-type complexes and thepossibility of Ca and Si metasomatism during or before theirmetamorphism should not be totally ruled out.     

Paragenetic Types and Compositions of Metamorphic Pyroxenes

Paragenetic types of pyroxenes, selected according to mineralassociations, differ in their average contents of the majorcations. By comparing with the average composition it is seenthat both the Fe/(Mg+Fe) ratio and the Fe⁺², Mn, Mg, Fe⁺³, andCr contents are determined in the main by the composition ofthe host rocks, but the AI_z, Al_y, Ca, and Na contents in pyroxenesare influenced by the conditions under which the rock was formed.The dependence of the Al_z and Al_y contents of orthopyroxenesand clinopyroxenes on temperature and pressure is shown withthe help of a new P-T diagram and by comparison with chemicalanalyses of natural pyroxenes. The correlation between the compound cations in pyroxenes isused to test the hypothesis of real isomorphous substitutionsin each paragenetic type of pyroxenes, and to determine rationalmethods of calculating pyroxene analyses into components. The calculated discriminant functions are useful for referring(with 5–10 per cent error) analyses of pyroxenes to oneof a number of paragenetic types, and in particular for distinguishingpyroxenes from magmatic and metamorphosed gabbros, and fromhigh- and low-temperature two-pyroxene granulites.     

A clinopyroxene geobarometer for basaltic systems based on crystal-structure modeling

 The crystal chemical response of basalt clinopyroxene to increasing pressure was investigated by means of crystal-structure simulation (a procedure that enables modeling of the structural parameters of a clinopyroxene of known chemistry without requiring direct X-ray diffraction analysis) using available experimental chemical data. Pressure proved the main physical variable governing clinopyroxene behavior in a magmatic environment. The general internal consistency of the simulation data permitted construction of an empirical geobarometer based on the relationship of cell volume (Vc) vs M1-site volume (VM1). The straightforward geobarometric formulation in the absence of direct X-ray analysis is: P(kbar)=698.443+4.985⋅Al_T−26.826⋅Fe²⁺ _M1−3.764⋅Fe³⁺ +53.989⋅Al_M1+3.948⋅Ti+14.651⋅Cr −700.431⋅Ca−666.629⋅Na−682.848⋅Mg_M2−691.138⋅Fe²⁺ _M2−688.384⋅Mn−6.267⋅(Mg_M2)²−4.144⋅(Fe²⁺ _M2 where: (Fe²⁺ _M1⋅Mg_M2)/(Fe²⁺ _M2⋅Mg_M1)=e^**(0.238⋅R³⁺+0.289⋅CNM−2.315), CNM=Ca+Na+Mn, and R³⁺=Al_M1+Fe³⁺+Ti+Cr, with cations in atoms per formula unit. The geobarometer reproduces experimental pressures within ±2 kbar (=1σ; max. dev. ≤5 kbar; N=29) in the range 0–24 kbar and is applicable to near-liquidus C2/c clinopyroxenes crystallized from basaltic melts in the absence of garnet (excepting high-Al₂O₃ basalts). It is therefore suitable for many natural clinopyroxenes occurring as mega- or phenocrysts or forming well-preserved cumulate pyroxenites. If the above restrictions are not wholly satisfied, the Vc vs VM1 plot can also be used qualitatively to deduce the relative pressure conditions of clinopyroxenes forming from similar batches of magma. The structural simulation of experimental data also provided insight into the influence of minor chemical changes of the parental magma on the crystal chemistry of clinopyroxene at high pressure. Within the considered compositional space at given P-T, a _CaO and a _SiO2 in the melt have opposite effects on M2- and T-site cation populations. As a result, under similar physical conditions, clinopyroxenes from higher-CaO or more undersaturated basalts have higher VM2, VT and Vc and lower VM1. For basalts with normal contents of Al₂O₃ (<18 wt %), variations of major elements in the melt do not reduce the accuracy of the geobarometer. Received: 3 April 1994 / Accepted: 23 December 1995     

Melting and Thermal Reconstitution of Pelitic Xenoliths, Wehr Volcano, East Eifel, West Germany

Pelitic xenoliths derived from amphibolite grade basement rocksoccur within a Pleistocene, trachytic, pyroclastic unit of theWehr volcano, East Eifel, West Germany: With increasing temperatureand/or prolonged heating at high temperature, quartz-plagioclaseand micaceous layers of the xenoliths have undergone meltingto form buchites and thermal reconstitution by dehydration reactions,melting and crystallization to form restites respectively. Thexenoliths provide detailed evidence of melting, high temperaturedecomposition of minerals, nucleation and growth of new phasesand P-T-f_o2 conditions of contact metamorphism of basement rocksby the Wehr magma. Melting begins at quartz-oligoclase (An_17·3Ab_82·3Or_0·4-An_20·0Ab_78·1Or_1·9)grain boundaries in quartz-plagioclase rich layers and the amountof melting is controlled by H₂O and alkalis released duringdehydroxylation/oxidation of associated micas. Initially, glasscompositions are heterogeneous, but with increasing degreesof melting they become more homogeneous and are similar to S-typegranitic minimum melts with SiO₂ between 71 and 77 wt. per cent;A/(CNK) ratios of 1·2–1·4; Na₂O < 2·95and normative corundum contents of 1·9–4·0per cent. Near micas plagioclase melts by preferential dissolutionof the NaAlSi₃O₈ component accompanied by a simultaneous increasein CaAl²Si²O⁸ (up to 20 mol. per cent An higher than the bulkplagioclase composition) at the melting edge. With increasingtemperature the end product of fractional melting is the formationand persistence of refractory bytownite (An_78–80) in thosexenoliths where extensive melting has taken place. Initial stage decomposition of muscovite involves dehydroxylation(H₂O and alkali loss). At higher temperatures muscovite breaksdown to mullite, sillimanite, corundum, sanidine and a peraluminousmelt. Mullite (40–43 mol. per cent SiO₂) and sillimanite(49 mol. per cent SiO₂) are Fe₂O₃ and TiO₂ rich (up to 6·1–0·84and 3·6–0·24 wt. per cent respectively).Al-rich mullite (up to 77 wt. per cent Al₂O₃) occurs with corundumwhich has high Fe₂O₃ and TiO₂ (up to 6·9 and 2·1wt. per cent respectively). Annealing at high temperatures andreducing conditions results in the exsolution of mullite fromsillimanite and ilmenite from corundum. Glass resulting fromthe melting of muscovite in the presence of quartz is peraluminous(A/(CNK) = 1·3) with SiO² contents of 66–69 percent and normative corundum of 4 per cent. Sanidine (An_1·9Ab_26·0Or_72·1-An_1·3Ab_15·9Or_82·9)crystallized from the melt. Dehydroxylation and oxidation of biotite results in a decreaseof K₂O from 8·6 to less than 1 wt. per cent and oxidetotals (less H₂O + contents) from 96·5 to 88·6,exsolution of Al-magnetite, and a decrease in the Fe/(Fe + Mg)ratio from 0·41 to 0·17. Partial melting of biotitein the presence of quartz/plagioclase to pleonaste, Al-Ti magnetite,sanidine(An_2·0Ab_34·9Or63·1) and melt takesplace at higher temperatures. Glass in the vicinity of meltedbiotite is pale brown and highly peraluminous (A/CNK = 2·1)with up to 6 wt. per cent MgO+FeO_{(total iroq)} and up to 10 percent normative corundum. Near liquidus biotite with higher Al₂O₃and TiO₂ than partially melted biotite crystallized from themelt. Ti-rich biotites (up to 6 wt. per cent TiO₂) occur withinthe restite layers of thermally reconstituted xenoliths. Meltingof Ti-rich biotite and sillimanite in contact with the siliceousmelt of the buchite parts of xenoliths resulted in the formationof cordierite (100 Mg/(Mg+Fe+Mn) = 76·5–69·4),Al-Ti magnetite and sanidine, and development of cordierite/quartzintergrowths into the buchite melt. Growth of sanidine enclosedrelic Ca-plagioclase to form patchy intergrowths in the restitelayers. Cordierite (100 Mg/(Mg+Fe+Mn) = 64–69), quartz,sillimanite, mullite, magnetite and ilmenite, crystallized fromthe peraluminous buchite melt. Green-brown spinels of the pleonaste-magnetite series have awide compositional variation of (mol. per cent) FeAl₂O₄—66·6–45·0;MgAl₂O₄—53·0–18·7; Fe₃O₄—6·9–28·1;MnAl₂O₄—1·2–1·5; Fe₂TiO₄—0·6–6·2.Rims are generally enriched in the Fe₃O₄ component as a resultof oxidation. Compositions of ilmenite and magnetite (single,homogeneous and composite grains) are highly variable and resultfrom varying degrees of high temperature oxidation that is associatedwith dehydroxylation of micas and melting. Oxidation mainlyresults in increasing Fe³⁺, Al and decreasing Ti⁴⁺, Fe²⁺ inilmenite, and increasing Fe²⁺, Ti⁴⁺ and decreasing Fe³⁺ in associatedmagnetite. A higher degree of oxidation is reached with exsolutionof rutile from ilmenite and formation of titanhematite and withexsolution of pleonaste from magnetite. Ti-Al rich magnetite(5·1–7·5 and 8·5–13·5wt. per cent respectively) and ilmenite crystallized from meltsin buchitic parts of the xenoliths. Chemical and mineralogic evidence indicates that even with extensivemelting the primary compositions of individual layers in thexenoliths remained unmodified. Apparently the xenoliths didnot remain long enough at high temperatures for desilicationand enrichment in Al₂O₃, TiO₂, FeO, Fe₂O₃, and MgO that resultsby removal of a ‘granitic’ melt, and/or by interactionwith the magma, to occur. T °C-f_o2 values calculated from unoxidized magnetite/ilmenitegive temperatures ranging from 615–710°C for contactmetamorphism and the beginning of melting, and between 873 and1054°C for the crystallization of oxides and mullite/sillimanitefrom high temperature peraluminous melts. f_o2 values of metamorphismand melting were between the Ni-NiO and Fe₂O₃-Fe₃O₄ buffer curves.The relative abundance of xenolith types, geophysical evidenceand contact metamorphic mineralogy indicates that the xenolithswere derived from depths corresponding to between 2–3kb P_load = P_fluid. The xenoliths were erupted during the latestphreatomagmatic eruption from the Wehr volcano which resultedin vesiculation of melts in partially molten xenoliths causingfragmentation and disorientation of solid restite layers.     

Petrogenesis of Ultramafic Xenoliths from the 1800 Kaupulehu Flow, Hualalai Volcano, Hawaii

The 1800 Kaupulehu flow on Hualalai Volcano, Hawaii, containsabundant xcnoliths of dunitc, wehrlite, and olivine clinopyroxenitewith minor gabbro, troctolite, anorthosite, and wcbstcrite.The petrography and mineral compositions of 41 dunite, wehrlite,and olivine clinopyroxenite xenoliths have been studied, andclinopyroxene separates from eight of these have been analyzedfor Ba, K, Rb, Sr, rare earth elements, ⁸⁷Sr/⁸⁶Sr, and ¹⁴³Nd/¹⁴⁴Nd.Temperatures of equilibration obtained by olivine-spinel andpyroxene geothermometry range from 1000 to 1200 C. Mineralogicaldata combined with published fluid inclusion data indicate depthsof origin in the range of 8–30 km. The rarity of orthopyroxene, the presence of Fe-rich olivine(Fo⁸¹–⁸⁹) and clinopyroxene (Fs⁵–¹²), and the occurrenceof high TiO² in spinel (0.9–2.8 wt.%) and clinopyroxene(035–1.33 wt%) all indicate that the xenoliths are cumulates,not residues from partial fusion. The separated clinopyrox-eneshave ⁸⁷Sr/⁸⁶Sr (0-70348.0-70367) and ¹⁴³Nd/¹⁴⁴Nd (0.51293–0.51299)values that are different from Sr and Nd isotope ratios of Pacificabyssal basalts (>0.7032 and >0-5130, respectively). Also,clinopyroxenes and spinels in the xenoliths have generally higherTiO₂ contents (>O.35 and >0.91 wt.%, respectively) thantheir counterparts in abyssal cumulates (<0.40 and <0.70wt%,respectively). These differences indicate that the xenolithsare not a normal component of oceanic crust. Because the xenoliths and alkalic to transitional Hualalai lavashave similar values for Cr/(Cr + Al) and Cr/(Cr + Al + Fe³⁺)of spinels, ⁸⁷Sr/⁸⁶Sr of clinopyroxenes, and whole-rock ³He/⁴He,we conclude that the xenoliths are cumulates from such magmas.Multiple parental magmas for the xenoliths are indicated byslightly heterogeneous ⁸⁷Sr/⁸⁶Sr of clinopyroxene separates.Depths of formation of the xenoliths are estimated to be {smalltilde}8–30 km. Extensive crystallization of olivine in the absence of pyroxenesand plagioclase is a characteristic and prominent feature ofHawaiian tholeiitic magmatism. Dunite xenoliths crystallizedfrom alkalic magmas have previously been reported from MaunaKea Volcano (Atwill & Garcia, 1985) and Loihi Seamount (Clague,1988). Our finding of an alkalic signature for dunite xenolithsfrom a third Hawaiian volcano, Hualalai, shows that early olivinecrystallization should be considered a characteristic not justof Hawaiian tholeiitic magmatism but also of Hawaiian alkalicmagmatism.     

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     

Structure and Petrology of the Alpine-type Peridotite at Burro Mountain, California, U.S.A.

The alpine-type peridotite at Burro Mountain is a partiallyserpentinized harzburgite-dunite body approximately 2 km indiameter. It lies in a chaotic mélange derived from theFranciscan Formation (Upper Jurassic to Upper Cretaceous) ofthe southern Coast Ranges of California. The peridotite is boundedon the east by a vertical fault in the Nacimiento fault zonethat brings sedimentary rocks of Taliaferro's (1943b) AsuncionGroup (Upper Cretaceous) into contact with the peridotite. Theperidotite appears to be one of a number of tectonic lenses,having a wide range in size, that make up the mélange.These lenses include metagraywacke, metachert, greenstone, amphibolite,and blueschist, as well as ultramafic rocks, and represent awide range of pressure-temperature environments. The outer shell of the peridotite is a sheared serpentinitezone 10–15 m thick. The peridotite was tectonically emplacedat its present level as a cold solid mass and had little effecton the mineral assemblages of the Franciscan Formation. Localdevelopment of lawsonite and aragonite in shear zones may berelated to the peridotite emplacement. Foliated harzburgite forms approximately 60 per cent of theperidotite. It is a lithologically uniform rock that has anolivine: orthopyroxene ratio of approximately 75:25. Accessoryclinopyroxene and chromian spinel generally make up less than5 per cent of the harzburgite. Dunite, composed of olivine,accessory chromian spinel (< 5 per cent), and trace amountsof pyroxene, makes up approximately 40 per cent of the peridotiteand occurs as dikes, sills, and irregular bodies in the harzburgite. Olivine and pyroxene show small but significant compositionalvariations and chromian spinel shows a large range in the cationratio Cr/(Cr+Al+ Fe³⁺). The compositional variations in theseminerals are related to original differences in bulk chemicalcomposition. The following compositional ranges were determinedfor minerals in the harzburgite: olivine, Fo_91.1–Fo_91.4;orthopyroxene, En_89.8–En_91.1; clinopyroxene, Ca_47.0Mg_50.0Fe_3.0–Ca_48.7Mg_48.2Fe_3.1;chromian spinel, Cr/(Cr+Al+Fe³⁺) 0.37–0.55. The pyroxeneshave a range in A1₂O₃ content of 1.3–3.0 wt per cent.Olivine from dunite ranges from Fo₉₁ to Fo_{92 7} and the chromianspinel has a range in the Cr/(Cr+Al+Fe³⁺) ratio of 0.30–0.75.Although all the dunites are lithologically similar, three distincttypes are recognized on the basis of composition of coexistingolivine and chromian spinel. Structural relations between thethree types of dunite suggest three periods of emplacement (possiblyoverlapping) of dunite into harzburgite. The evidence indicatesthat the dunite, and probably also the harzburgite crystallizedfrom an ultramafic magma, probably in the upper mantle. After the magmatic episode and crystallization, the peridotitewas subjected to a deep-seated plastic deformation and recrystallization.The first phase of the deformation produced a pervasive, planarstructural element (S₁) that crosscuts many harzburgite-dunitecontacts. It is probable that some of the dunite sills wereemplaced during this deformation. The foliation, S₁, is definedby layers of different orthopyroxene content in harzburgite,and by discontinuous layers of chromian spinel in dunite. Flowor slip along S₁ produced slip folds in harzburgite—dunitecontacts with axial planes parallel to S₁. At a later stage,isoclinal folds developed in S₁, and the present olivine microfabricwas probably formed by recrystallization in the stress fieldthat produced the isoclinal folding. In the olivine microfabric,X tends to be perpendicular to the axial planes (S₂) of theisoclinal folds and Y and Z tend to form double maxima in S₂approximately 90° apart. Mg–Fe²⁺ distribution betweencoexisting mineral pairs yields a calculated temperature offormation of approximately 1200 °C. Although this temperatureis only a nominal value, it indicates that the mineral pairsequilibrated at a significantly high temperature. In view ofthe deformation and recrystallization, the calculated temperaturepossibly represents subsolidus re-equilibration of the mineralsduring this event. The deformation and recrystallization probablyoccurred shortly after crystallization while the peridotitewas still at a high temperature. A later deep-seated deformation produced small scattered kinkfolds in S₁ that tend to disrupt the major olivine microfabric.The kink folding was accompanied or followed by the developmentof kink bands in olivine that reflect intragranular glidingon the system T = [Okl], t = [100]. The kink bands probablyformed at a minimum temperature of 1000 °C. Following the deep-seated deformation, which probably took placein the mantle, the peridotite mass was tectonically detachedand moved upward to its present level in the crust. Cleavages,joints, and faults provided channels for water to pervade theperidotite and allow alteration of the primary minerals.     

Open-System, Sub-Volcanic Magmatic Evolution: Constraints on the Petrogenesis of the Mount Brome Alkaline Complex, Canada

The Mount Brome alkaline complex of southern Quebec (Canada)comprises gabbroic to silica-oversaturated and -undersaturatedfelsic rocks which have variable initial ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Ndratios: 0.70345–0.70431 and 0.51270–0.51258, respectively,in mafic lithologies (gabbro, nepheline diorite, and biotitemonzodiorite); 0.70353–0.70403 and 0.51270–0.51263in silica-undersaturated felsic units (pulaskite and nephelinesyenite); and 0.7051–0.7115 and 0.51262–0.51259in silica-oversaturated nordmarkite. Negatively correlated ⁸⁷Sr/⁸⁶Srvs. ¹⁴³Nd/¹⁴⁴Nd trends for the various rock types appear toconverge at the composition ⁸⁷Sr/⁸⁶Sr = 0.70345 and ¹⁴³Nd/¹⁴⁴Nd= 0.51270 (_Nd = 4.3). This suggests that all rock types sharea common parental magma composition, either through derivationfrom a single batch of liquid, or (more probably) through twoepisodes of melting of the same depleted mantle source region.Delta ¹⁸O ranges from + 5.5 to + 8% and is broadly correlatedwith radiogenic isotopes and bulk composition. Isotopic, and major and trace element compositions suggest thatfractional crystallization (FC) of the parental magma at deeplevels produced evolved magmas, apparently without significantcrustal contamination [FC without assimilation (A)]. Duringascent and emplacement into the upper crust, these magmas thenevolved by simultaneous crystal fractionation and country-rockassimilation (AFC). Within both basic and felsic rocks thereis a clear relationship between silica saturation and degreeof contamination. Indeed, the oversaturated nordmarkites owetheir origin to assimilation of siliceous country rocks by acritically undersaturated magma. More generally, it seems likelythat this type of process is the normal mode of origin for coexistingquartz and nepheline syenites in many sub-volcanic alkalineigneous complexes. Additionally, such complexes would be likelyto develop by punctuated FC and AFC processes throughout theirpetrogenetic history.     

河北矾山杂岩体中单斜辉石的研究

河北矾山杂岩体属于二氧化硅不饱和的超钾质碱性-过碱性岩浆系列,由单斜辉石岩、辉石正长岩和碱长正长岩等不同类型岩石组成。各类型岩石主要组成矿物为单斜辉石、黑云母、石榴石和正长石。本文利用电子探针对单斜辉石进行了详细研究,发现所有单斜辉石属于高钙透辉石,随着岩浆的演化,主要表现为Fe²⁺对Mg²⁺的替代关系,结晶趋势为透辉石→钙铁辉石,这揭示矾山杂岩体岩浆体系的特点是高温、中等大小的氧逸度、贫硅、富碱（尤其是钾）。辉石中的Al^IV含量取决于岩浆的硅饱和度,硅越不饱和,Al^IV含量越高;Al^VI含量则与体系中的Al含量呈正相关关系。由Al对Si的替代引起的电荷不平衡主要由八面体位置的Fe³⁺来补偿,其次为少量的Ti⁴⁺和更少量的Al³⁺。体系中的Ti含量与体系的温度呈正相关关系,而Na含量则与辉石中的Ti和Fe³⁺含量成正相关关系。不同类型岩石中单斜辉石从核部到边部随着Mg^#的降低,Al、Ti含量呈现不同的演化趋势,这是因为它们于岩浆演化的不同阶段开始结晶,经历了不同的岩浆演化史。碱性岩中高钙辉石的出现和成分环带的普遍发育是由岩浆的贫硅富钾特征决定的。单斜辉石的成分不仅受控于结晶时的温度、压力条件,也受控于岩浆的总成分及其变异。     

A Metamorphosed, Early Archaean Chromitite from West Greenland: Implications for the Genesis of Archaean Anorthositic Chromitites

An early Archaean (>3·81 Ga) chromitite–ultramaficlayered body from the Ujaragssuit nunât area, west Greenland,may represent the Earth’s oldest chromitite. The layeredbody occurs as a large xenolith (800 m x 100 m) entrained withintonalitic gneisses and preserves primary igneous layering andtextures. New Re–Os and Pb–Pb isotope results supportthe view that it has been metamorphosed twice, in the earlyand late Archaean at 3·75 Ga and 2·8 Ga. Mineralchemistry and textures indicate that the chromite compositionspreserve two different evolutionary trends. There is a mainmagmatic trend in which Cr/(Cr + Al) ratios remain relativelyconstant but in which there is strong enrichment in Fe³⁺, Fe²⁺and Ti with progressive differentiation. This trend is a compositeof magmatic-liquidus, magmatic-cooling and subsolidus re-equilibrationprocesses. A second trend is defined by chromites from harzburgitesin the upper part of the layered body. These chromites showmagmatic replacement textures in which Fe-rich chromites arealtered to aluminous chromites. Chromites showing magmatic replacementtextures are thought to have formed by reaction with a late,interstitial melt during the solidification of the layered body.The close association between the Fe³⁺–Cr-chromites ofthe main trend and Al-rich chromites of the type found in otherArchaean megacrystic anorthosites suggest a magmatic-geneticrelationship between the two types of chromite. We propose thatanorthositic chromites form in an Fe-rich basaltic melt derivedfrom a komatiitic, boninitic or basaltic parent magma throughreaction between the melt and early-formed Fe-rich chromite. KEY WORDS: chromite; anorthosite; Archaean; Greenland; Re–Os     

Distinctive crystal chemistry and site configuration of the clinopyroxene from alkali basaltic rocks

A crystal chemical investigation of clinopyroxenes from a suite of nepheline-bearing lavas located in the Nyambeni Range of Kenya has delineated the polyhedral site configurations and related intracrystalline relationships. These are distinct from those determined for the clinopyroxene in an analogous suite of leucite-bearing lavas from the Sabatini volcanoes in the Roman Region of Italy (Dal Negro et al. 1985).The Nyambeni clinopyroxene, varying from salite to hedenbergite, preferentially accepts Na in the M2 site to balance increasing Fe²⁺ and Si, respectively, whereas the Sabatini clinopyroxene is confined within the salite field and preferentially accepts Al^iv to balance the effect of increasing (Fe³⁺+Ti⁴⁺+Al^vi+Cr³⁺)_M1.The Fe²⁺/Fe³⁺ and K/Na ratios of the host rocks emerge as significant factors in determining the different polyhedral configurations and evolutions of the clinopyroxene from the two lava suites, respectively. The resulting Mg-Fe²⁺ order-disorder relationships in M1–M2 are also distinct in the two clinopyroxenes. A high degree of MgFe²⁺ order in M1–M2 corresponds to the largest configurational, hence energetic, difference between M1 and M2 in the Nyambeni clinopyroxene, whereas the converse applies to the Sabatini clinopyroxene.In view of the significant crystal chemical differences and distinct evolution trends, it is proposed that salites from alkali volcanic rocks may be referred to as Nyambeni-type or Sabatini-type, respectively.     

Pumpellyite-Actinolite Facies Schists of the Taveyanne Formation near Lo?che, Valais, Switzerland

Electron microprobe analyses are presented for new-formed mineralsfrom a small exposure of semi-schistose Taveyanne Formationof the pumpellyite-actinolite facies near Lo?che, Valais. Comparisonsare drawn with minerals of other low-grade metamorphic areas,especially in southern New Zealand. Sphene shows considerablesubstitution of Ca(Al,Fe)SiO₄(OH) for CaTiSiO₅. Epidotes aresharply divided into early pistacitic (Ps = 0.28–0.37)and later clinozoisitic varieties (Ps = 0.11–0.19). Pumpellyitesrange from pumpellyite-(Fe) to pumpellyite-(Al) and are generallyless Fe-rich than those of zeolite and prehnite-pumpellyitefacies. Pumpellyite inclusions in albitized plagioclase areparticularly low in Mg. Actinolites are low in A1₂O₃, TiO₂,and Na₂O, essentially identical compositions being nucleatedon detrital augite, hornblende, and in the matrix. Phengitesare also extremely low in Na₂O and TiO₂. Chlorites are ripidolites.Albitized clastic plagioclase has the composition An0.7–1.6and albite in clinozoisite-calcite-albite-phengite-chloriteveins An2.1–2.3. Calcites carry minor Mn > Fe ? Mg.New-formed iron oxides are absent, whereas pyrrhotite and minorpyrite occur in one rock, buffering fs₂ and indicating low fo₂. Ratios Mg: Fe^* (Fe^* = total Fe) in coexisting chlorites andA1, Na-poor actinolites vary sympathetically both in the Lo?cheand southern New Zealand rocks here considered, giving K_D =(Mg/Fe^*) _actlnolIte/(Mg/Fe^*)_chlorle = 1.72. Mg/Fe^* ratios inpumpellyites tend to vary sympathetically with those of coexistingchlorites and actinolites but are more variable. Substitutionof (Fe, Mg)Si for A1₂ in phengitic micas and chlorites variessympathetically in the same suites between mafic volcanic andmore pelitic extremes. Various minor elements also behave ina consistent fashion, indicating an encouraging tendency towardsequilibrium. Variable (though small) A1₂O₃ contents of actinolite,Fe: Al ratios in epidotes and pumpellyites, and Mg: Fe^* ratiosin phengites, even within a single grain, are evidence of short-rangedisequilibrium; metamorphic equilibration is evidently easierbetween some crystal structures and structural sites than betweenothers. In phase rule analysis of assemblages in such rocks it is commonlynecessary to treat Fe²O₃, FeO, and MgO as separate componentsand it may also be necessary to regard CO² as an inert componentand/or to interpret observed assemblages as of low variance.The presence of the Ca-Al silicates and sphene indicates verylow X_co2 in the metamorphic fluids in all rocks examined exceptan albite-chlorite-calcite-quartz-anatase assemblage. But higherAn in albites than in isofacial and in greenschist facies rocksof southern New Zealand can be ascribed to significantly higherXco₂ at Lo?che, especially in the veins, than in New Zealand. Pumpellyite and epidotes of the pumpellyite-actinolite faciestend to be lower in Fe and richer in Al than those of lowergrade facies. Important reactions include those of the formpumpellyite-(Fe³⁺)+chlorite+quartz+H₂=pumpellyite-(Al)+actinolite,and pumpellyite+chlorite+quartz- ‘epidote’+actinolite+water.Careful selection of pumpellyite and chlorite compositions isrequired for experimental and chemographic analysis of pumpellyitestability. In the absence of critical data, temperatures ofabout 250–350? and pressures of several kilobars are provisionallysuggested for the Lo?che metamorphism.     

Garnetite Xenoliths and Mantle-Water Interactions Below the Colorado Plateau, Southwestern United States

Garnetite xenoliths from ultramafic diatremes in northeasternArizona provide insights into hydration and metasomatism inthe mantle. The garnetites contain more than 95% garnet, someof which has complex compositional zonation related to growthin fractures within grains. Accessory minerals include rutile,ilmenite, chlorite, clinopyroxene, and zircon. Zircon grainsin one rock were analyzed in situ to determine U–Pb agesand Hf isotopic compositions. Most U–Pb analyses ploton or near concordia in the range 60–85 Ma but a few arediscordant. The range in ¹⁷⁶Hf/¹⁷⁷Hf is about 0·2818–0·2828,with grains zoned to more radiogenic Hf from interiors to rims.The garnetite protolith contained zircons at least 1·8Ga in age, and garnet and additional zircon crystallized episodicallyduring the interval 85–60 Ma. The garnetites are interpretedas mantle analogues of rodingites, formed in metasomatic reactionzones caused by water–rock interactions in Proterozoicmantle during late Cretaceous and Cenozoic subduction of theFarallon plate. Associated eclogite xenoliths may have beenparts of these same reaction zones. The rodingite hypothesisrequires serpentinization in the mantle wedge 700 km from thetrench, beginning 5–10 Myr before tectonism related tolow-angle subduction. KEY WORDS: garnetite; Lu–Hf, mantle; rodingite; metasomatism     

Chemistry of Kornerupine and Associated Minerals, a Wet Chemical, Ion Microprobe, and X-Ray Study Emphasizing Li, Be, B and F Contents

Kornerupine and associated minerals in 31 samples of high-graderocks relatively rich in Al and Mg were analysed by wet chemistry,ion microprobe mass analyser, electron microprobe and X-raypowder diffraction. For 11 samples of kornerupine and threesamples of biotite (F only) analysed by both wet chemical andion microprobe methods, the best agreement was obtained forB₂O₃, whereas the ion microprobe Li₂O values were systematicallysomewhat higher than the wet chemical values. The wet chemicalmethods give Li₂O=0–0?19 wt.%; BeO=0–0?032 wt.%;B₂O₃=0–4?01 wt.%; and F=0?07–0?77 wt.% in kornerupine,whereas ion microprobe analyses on other kornerupines give valuesup to 0?35 wt.% Li₂O, O066 wt.% BeO, and 4?72 wt.% B₂O₃. Thesum B+Al+Fe³⁺+Cr is close to 6?9 atoms per 22 (O, OH, F) or21?5 (O) in kornerupine. In general, Li/Fe ratios decrease as follows: kornerupine ?sapphirinebiotite> Crd (Na<0?03 per 18 oxygens)>tourmaline, garnet,orthopyroxene. However, for cordierite with Na>004, Li/Fedecreases as follows: cordierite>kornerupine. Sapphirineand sillimanite are the only associated minerals to incorporatesignificant boron (0?1–0?85 wt.% B₂O₃) and then only whenthe single site for B in kornerupine is approaching capacity.Sillimanite B₂O₃ contents increase regularly with kornerupineF. Fractionation of fluorine increases as follows: kornerupine<biotite<tourmaline,and K^krn-BtD=(F/OH)^Krn/(F/(OH)^Bt (assuming ideal anion composition)increases with biotite Ti. Kornerupine B₂O₃ content is a measureof B₂O₃ activity in associated metamorphic fluid, whereas sillimaniteB₂O₃ content increases with temperature, exceeding 0?4 wt.%whenT=900?C at very low water activities. New data on 11 kornerupines and literature data indicate thatthe unit cell parameters a, c, and V decrease with increasingB content and b, c, and V increase with increasing Fe³⁺ content.In Fe³⁺-poor kornerupines, b increases with Mg and with (Mg+ Fe²⁺) but the effect of Mg on b via the substitution ^VIMg+^IVSi=^VIAl+^IVAloverwhelms the effect of Fe²⁺=Mg substitution.     

Genesis of Pyroxenite-rich Peridotite at Cabo Ortegal (NW Spain): Geochemical and Pb-Sr-Nd Isotope Data

Petrographic and field data indicate the existence of four mainrock types within the allochthonous Cabo Ortegal ultramaficunits: (1) harzburgites; (2) dunites; (3) massive, occasionallygarnet-bearing, pyroxenites; (4) less abundant mafic rocks withvariable amounts of garnet-rich pyroxenite. The major and traceelement compositions of the analysed ultramafic rocks definewell-delimited fields in binary variation diagrams. Normalizedtrace element patterns, however, exhibit large ion lithophileelement (LILE) and light rare earth element (LREE) enrichmentthat do not correlate with the main rock types distinguished.NiO contents and fo-number of olivine in the harzburgites matchthose of the mantle olivine array, whereas a fractional crystallizationtrend is observed from dunites to pyroxenites. Spinel and olivinein the harzburgites have residual characteristics comparablewith those of abyssal peridotites or peridotites from arc settings,whereas in most of the dunites and pyroxenites the range offo-number and Cr/(Cr + Al) ratio suggests crystallization fromprimitive subduction-related magmas. Whole-rock major and traceelement and Pb–Sr–Nd isotope data suggest that regional-scalemassive pyroxenites from Cabo Ortegal originated from relativelyhomogeneous parental melts. Fractional crystallization processes,coeval with intense deformation, might result in the formationof cumulate layers (clinopyroxene, orthopyroxene, olivine, chromite,etc.). Some less abundant mafic rocks and associated pyroxenitesare also homogeneous but have different chemical and isotopicsignatures suggesting a different parental melt from that ofthe massive pyroxenites. Although some differences exist inthe major element and isotopic composition of the clinopyroxenes,their initial isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 17·845–18·305,²⁰⁷Pb/²⁰⁶Pb = 15·433–15·634; ⁸⁷Sr/⁸⁶Sr =0·70330–0·70476; ¹⁴³Nd/¹⁴⁴Nd = 0·512539–0·512916)suggest involvement of an enriched component in their mantlesource, which may be related to the subduction of terrigenoussediments (i.e. EMI). The new data obtained confirm that ultramaficunits of Cabo Ortegal experienced a complex tectonothermal historysimilar to that of other units of the same area and allow usto distinguish at least two different events. Sm–Nd whole-rock–clinopyroxeneages suggest formation of the ultramafic units at     

Melting and Melt Segregation in the Mantle Wedge above a Subduction Zone: Evidence from the Chromite-bearing Peridotites of the Miyamori Ophiolite Complex, Northeastern Japan

Chromite-bearing peridotites of the Ordovician Miyamori ophiolitecomplex exhibit spatial mineralogical variations on scales rangingfrom several centimeters to a few kilometers. The largest variationscorrespond to the entire structure of the complex, which featuresa layered zone of interstratified harzburgite, wehrlite, andvarious pyroxenites sandwiched between zones of unlayered harzburgiteand dunite containing only minor pyroxenite bands. All zonesexhibit the same deformation microstructures, tabular equigranularto porphyroclastic textures, and strong mineral aggregate lineation.Harzburgite from the unlayered zones is characterized by olivinevalues of 100Mg/(Mg+Fe)=91–93.5 and chromite values of100Cr/(Cr+Al+Fe³⁺)=40–75. These variables exhibit a positivecorrelation, which is typical of harzburgites and lherzolitesfrom the basal units of ophiolites and from xenoliths in alkalibasalts and kimberlites. The harzburgite is therefore interpretedas a residue from partial melting in the mantle. By contrast,harzburgite in the interlayered zone features olivine valuesof 100Mg/(Mg+Fe)=88–92 and chromite values of 100Cr/(Cr+Al+Fe3+)=40–60,and in this case the variables tend to show a negative correlationin any given locality and they partly overlap data from theintercalated wehrlite and dunite. The harzburgite of the layeredzone is interpreted as residual mantle that reacted with evolvedmelts that then crystallized as wehrlite and dunite. The harzburgitein the unlayered zones is more refractory than that in the layeredzone, even after removing effects of reaction. This differencecan be explained either by enhanced partial melting and meltextraction in the unlayered zones, possibly owing to the preferentialintroduction of a waterrich fluid, or by melt segregation fromthe unlayered zones and transfer to the layered zone in responseto a piezometric pressure gradient and compaction of a solidresidual matrix. Mineralogical evidence suggests that evolvedmelts migrated through conduits formed in the layered zone byfracturing or diapirism.