Petrology and geochemistry of Tertiary volcanic rocks from the İkizce (Ordu) area, NE Turkey: Implications for the evolution of the eastern Pontide paleo-magmatic arc

Tertiary volcanism in the İkizce region at the western edge of the eastern Pontides paleo-magmatic arc is represented by basaltic and andesitic rocks associated with sediments deposited in a shallow basin environment. The basaltic rocks contain plagioclase (An_58–80), olivine (Fo_82–84), clinopyroxene (Wo_44–48En_35–42Fs_7–17), hornblende (Mg# = 0.68–0.76) phenocrysts, and magnetite microcrysts, whereas the andesitic rocks include plagioclase (An_25–61), clinopyroxene (Wo_46–49En_38–43Fs_11–13), hornblende (Mg# = 0.48–0.81), biotite (Mg# = 0.48–0.60) phenocrysts, titanomagnetite, apatite, and zircon microcrysts.Geochemical data indicate magmatic evolution from tholeiitic-alkaline transitional to calc-alkaline characteristics with medium-K contents. The geochemical variation in the rocks can be explained by fractionation of common mineral phases such as clinopyroxene, olivine, hornblende, plagioclase, magnetite, and apatite. The trace elements’ distributions of the volcanic rocks show similarities to those of E-Type MORB, have a shape that is typical of rocks from subduction-related tectonic setting with enrichment in LILE and to a lesser extent in LREE, but depletion in HFSE. The rocks evolved from a parental magma derived from an enriched source formed by subduction induced metasomatism of basaltic rocks, the latter formed through clinopyroxene ± olivine controlled fractionation in a high level magma chamber. The andesitic rocks developed through hornblende ± plagioclase controlled fractionation in shallow level magma chamber(s).     

Northwest Africa 773: lunar mare breccia with a shallow-formed olivine-cumulate component, inferred very-low-Ti (VLT) heritage, and a KREEP connection

Lunar meteorite Northwest Africa 773 (herein referred to as NWA773) is a breccia composed predominantly of mafic volcanic components, including a prominent igneous clast lithology. The clast lithology is an olivine-gabbro cumulate, which, on the basis of mineral and bulk compositions, is a hypabyssal igneous rock related compositionally to volcanic components in the meteorite. The olivine-gabbro lithology exhibits cumulus textures and, in our largest section of it, includes some 48% olivine (Fo₆₄ to Fo₇₀, average Fo₆₇), 27% pigeonite (En₆₀Fs₂₄Wo₁₆ to En₆₇Fs₂₇Wo₆), 11% augite (En₅₀Fs₁₇Wo₃₃ to En₄₇Fs₁₃Wo₄₀), 2% orthopyroxene (En₇₀Fs₂₆Wo₄), 11% plagioclase (An₈₀ to An₉₄), and trace barian K-feldspar, ilmenite, Cr-spinel, RE-merrillite, troilite, and Fe-Ni metal. The Mg/Fe ratios of the mafic silicates indicate equilibration of Fe and Mg; however, the silicates retain compositional variations in minor and trace elements that are consistent with intercumulus crystallization. Accessory mineralogy reflects crystallization of late-stage residual melt. Both lithologies (breccia and olivine cumulate) of the meteorite have very-low-Ti (VLT) major-element compositions, but with an unusual trace-element signature compared to most lunar VLT volcanic compositions, i.e., relative enrichment in light REE and large-ion-lithophile elements, and greater depletion in Eu than almost all other known lunar volcanic rocks. The calculated composition of the melt that was in equilibrium with pyroxene and plagioclase of the cumulate lithology exhibits a KREEP-like REE pattern, but at lower concentrations. Melt of a composition calculated to have been in equilibrium with the cumulate assemblage, plus excess olivine, yields a major-element composition that is similar to known green volcanic glasses. One volcanic glass type from Apollo 14 in particular, green glass B, type 1, has a very low Ti concentration and REE characteristics, including extremely low Eu concentration, that make it a candidate parent melt for the olivine-gabbro cumulate. We infer an origin for the parent melt of NWA773 volcanic components by assimilation of a trace-element-rich partial or residual melt by a magnesian, VLT magma deep in the lunar crust or in the mantle prior to transportation to the near-surface, accumulation of olivine and pyroxene in a shallow chamber, eruption onto a volcanic surface, and incorporation of components into local, predominantly volcanic regolith, prior to impact mixing of the volcanic terrain and related hypabyssal setting, and ejection from the surface of the Moon. Volcanic components such as these probably occur in the Oceanus Procellarum region near the site of origin of the green volcanic glasses found in the Apollo 14 regolith.     

Mineralogy and partial melt textures within an ultramafic-mafic body,Greece

Ultramafic-mafic rocks from Makrirrakhi, Central Greece exhibit features of an original ophiolite sequence which contains depleted mantle material, ultramafic containing partial melt textures and possibly the mafic pluton which resulted from the coalescing of these partial melt segregations. Considerable mineralogical variation exists: unzoned olivine crystals range in composition from Fo_78–84 (mafics) to Fo_88–92 (ultramafics), plagioclases An_64–79 (mafics) to An_80–90 (ultramafics) and spinel varies from a chromian spinel (ultramafics) to a more aluminous-titaniferous spinel (mafics). Pyroxenes from the ultramafics display a limited range: En_89–92 Fs_9–8 Wo_0–2 (orthopyroxene) and En_48–54 Fs_1–10 Wo_38–50 (clinopyroxene). Mafic rocks display a greater range being richer in ferrosilite En_36–65 Fs_3–20 Wo_33–51. Pyroxenes from within the partial melt segregations have chemical affinities with those from the gabbrotroctolite series. A model of partial melt within the upper mantle, and, a set of criteria to distinguish partial melt textures from cumulate textures, are developed from analytical data and textural evidence.     

Petrography and geochemistry of the chassignite Northwest Africa 2737 (NWA 2737)

We report on the petrology and geochemistry of the Northwest Africa 2737 (NWA 2737) meteorite that was recovered from the Morrocan Sahara in 2000. It is the second member of the chassignite subclass of the SNC (Shergotitte-Nakhlite-Chassignite) group of meteorites that are thought to have originated on Mars. It consists of black olivine- and spinel-cumulate crystals (89.7 and 4.6 wt%, respectively), with intercumulus pyroxenes (augite 3.1 wt% and pigeonite-orthopyroxene 1.0 wt%), analbite glass (1.6 wt%) and apatite (0.2 wt%). Unlike Chassigny, plagioclase has not been observed in NWA 2737. Olivine crystals are rich in Mg, and highly equilibrated (Fo = 78.7 ± 0.5 mol%). The black color of olivine grains may be related to the strong shock experienced by the meteorite as revealed by the deformation features observed on the macroscopic to the atomic scale. Chromite is zoned from core to rim from Cr_83.4Uv_3.6Sp_13.0 to Cr_72.0Uv_6.9Sp_21.1. Pyroxene compositional trends are similar to those described for Chassigny except that they are richer in Mg. Compositions range from En_78.5Wo_2.7Fs_18.8 to En_76.6Wo_3.2Fs_20.2 for the orthopyroxene, from En_73.5Wo_8.0Fs_18.5 to En_64.0Wo_22.1Fs_13.9for pigeonite, and from En_54.6Wo_32.8Fs_12.6 to En_46.7Wo_44.1Fs_9.2 for augite. Bulk rock oxygen isotope compositions confirm that NWA 2737 is a new member of the martian meteorite clan (Δ¹⁷O = 0.305 ± 0.02‰, n = 2). REE abundances measured in NWA 2737 mineral phases are similar to those in Chassigny and suggest a genetic relationship between these two rocks. However, the parent melt of NWA 2737 was less evolved and had a lower Al abundance.     

Crystallization, melt inclusion, and redox history of a Martian meteorite: Olivine-phyric shergottite Larkman Nunatak 06319

The Larkman Nunatak (LAR) 06319 olivine-phyric shergottite is composed of zoned megacrysts of olivine (Fo_76-55 from core to rim), pyroxene (from core to rim En₇₀Fs₂₅Wo₅, En₅₀Fs₂₅Wo₂₅, and En₄₅Fs₄₅Wo₁₀), and Cr-rich spinel in a matrix of maskelynite (An₅₂Ab₄₅), pyroxene (En_30-40Fs_40-55Wo_10-25,), olivine (Fo₅₀), Fe-Ti oxides, sulfides, phosphates, Si-rich glass, and baddeleyite. LAR 06319 experienced equilibration shock pressures of 30-35 GPa based on the presence of localized shock melts, mechanical deformation of olivine and pyroxene, and complete transformation of plagioclase to maskelynite with no relict birefringence. The various phases and textures of this picritic basalt can be explained by closed system differentiation of a shergottitic melt. Recalculated parent melt compositions obtained from melt inclusions located in the core of the olivine megacrysts (Fo_>72) resemble those of other shergottite parent melts and whole-rock compositions, albeit with a lower Ca content. These compositions were used in the MELTS software to reproduce the crystallization sequence. Four types of spinel and two types of ilmenite reflect changes in oxygen fugacity during igneous differentiation. Detailed oxybarometry using olivine-pyroxene-spinel and ilmenite-titanomagnetite assemblages indicates initial crystallization of the megacrysts at 2 log units below the Fayalite-Magnetite-Quartz buffer (FMQ - 2), followed by crystallization of the groundmass over a range of FMQ - 1 to FMQ + 0.3. Variation is nearly continuous throughout the differentiation sequence.LAR 06319 is the first member of the enriched shergottite subgroup whose bulk composition, and that of melt inclusions in its most primitive olivines, approximates that of the parental melt. The study of this picritic basalt indicates that oxidation of more than two log units of FMQ can occur during magmatic fractional crystallization and ascent. Some part of the wide range of oxygen fugacities recorded in shergottites may consequently be due to this process. The relatively reduced conditions at the beginning of the crystallization sequence of LAR 06319 may imply that the enriched shergottite mantle reservoir is slightly more reduced than previously thought. As a result, the total range of Martian mantle oxygen fugacities is probably limited to FMQ − 4 to − 2. This narrow range could have been generated during the slow crystallization of a magma ocean, a process favored to explain the origin of shergottite mantle reservoirs.     

Vermicular orthopyroxene-magnetite symplectites from the Wateranga layered mafic intrusion,Queensland, Australia

Orthopyroxene-magnetite intergrowths (symplectites), partly or completely surrounding olivine, are described from the Wateranga layered mafic intrusion, Queensland, Australia. The texture occurs in unmetamorphosed plagioclase-rich norites, olivine gabbros and troctolites in which the primary minerals are olivine (Fo_63–69) orthopyroxene (En_66–72), clinopyroxene (Wo₄₂En₄₂Fs₁₆), plagioclase (An_49–65), hornblende, ilmenite, magnetite and sulphides. Symplectites range from incipient fine grained developments around corroded olivine grains to intricately formed pseudomorphs after olivine and slow a consistent orthopyroxene/magnetite ratio. Orthopyroxene in symplectites is commonly in optical continuity with surrounding magnetite-free orthopyroxene rims. Later intercumulus hornblended has replaced orthopyroxene. There is marked chemical similarity between primary and simplectite, orthopyroxenes and magnetites. Textures similar to those described here are considered elsewhere to have formed at a late magmatic stage or by solid state reactions involving subsolidus oxidation of olivine. In the Wateranga intrusion textural relations, the chemical similarity between primary and symplectite phases, and the consistent volume proportions of magnetite and orthopyroxene in the intergrowths suggest that they developed during late magmatic crystallization.     

Mineralogy and petrology of the diabasic rocks in a differentiated olivine diabase sill complex,Sierra Ancha,Arizona

The Precambrian Sierra Ancha sill complex, more than 700 feet thick, is a multiple intrusion with a central layer of feldspathic olivine-rich diabase, and upper and lower layers of olivine diabase derived from a high-alumina basalt magma. Minor rock types include albite diabase and albite-diabase pegmatite. Deuteric alteration was extensive. Principal primary minerals are plagioclase (An₇₂ to An₁₆), augite (Wo₄₃En₄₄Fs₁₃ to Wo₄₀En₃₈Fs₂₂), olivine (Fo₇₄ to Fo₅₄), orthopyroxene (En₇₇ to En₄₄), magnetite (Mgt₆₆Usp₃₄ to Mgt₈₉Usp₁₁), and ilmenite (Ilm₈₆Hem₁₄ to Ilm₉₆Hem₄). Ilmenite formed by reaction-exsolution from magnetite_ss is consistently different in compositon from primary ilmenite. Primary ilmenite became enriched in Mn and depleted in Mg as crystallization proceded. A systematic Fe-Mg partition between contacting olivine and orthopyroxene suggests that equilibrium prevailed on an extremely local scale during crystallization. Albite-diabase pegmatite contains a mineral assemblage including augite, ferrosalite (Wo₄₉En₂₈Fs₂₃ to Wo₄₉En₁₄Fs₃₇), albite (An₂ to An₀), and iron-rich chlorite. Altered diabase and albite diabase also have unusually calcium-rich pyroxenes. The calcium-rich pyroxenes, which occur in assemblages like those characterizing some spilites, are richer in calcium and lower in aluminum and titanium than basaltic augite.Contribution No. 1712 of the Division of Geological Sciences, California Institute of Technology, Pasadena, California.     

Mineralogical variations in the Delakhari sill,Deccan trap intrusion,central India

The Delakhari sill (maximum thickness cf. 200 m) is the most extensive Deccan Trap instrusion which occurs in central India, between longitutdes 78°3835 to 78°2240 and latitudes 22°26 and 22°2230. Based on petrographic examination, the sill is divided, from bottom to top, into (1) the Lower Chilled Zone (LCZ), up to 8 m thick, marked by abundant interstitial glass and an overall fine grain size, (2) the Olivine-Rich Zone (ORZ), 27 m thick, enriched in olivine (relative to the other zones in the sill), (3) the Central Zone (CZ), 70 m thick, marked by depletion in olivine and overall coarse grain size, (4) the Upper Zone (UZ), 55 m thick, marked by the presence of two chemically and morphologically distinct olivine types and abundant interstitial granophyre, and (5) the Upper Chilled Zone (UCZ), 10–25m thick, marked by abundant interstitial glass.Compositions of the pyroxenes and olivines show an overall increase in Fe/Mg with crystallization, but extensive interzonal and intrazonal variations and overlaps exist. Olivine ranges from Fa₂₄ (ORZ) to Fa₉₅ (UZ). In the UZ and inner UCZ, an equant (Fa_44–50, called type-A olivine) and interstitial skeletal olivine (Fa_70–95, called type-B olivine) occur together. Compositions of the Ca-rich and Ca-poor pyroxenes fall in the range Wo₃₈En₃₄Fs₂₈ to Wo₃₃En₈Fs₅₉ and Wo₁₄En₄₁Fs₄₅ to Wo₁₆En₁₉Fs₆₅, respectively. Overall, the two pyroxene trends converge with Fe-enrichment except for one anomalous sample from the UZ which contains a Ca-rich (Wo₃₄En₈Fs₅₈) and a Ca-poor (Wo₁₀En₁₈Fs₇₂) pyroxene well within the Forbidden Zone of Smith (1972).Compositions of coexisting oxide minerals indicate that the sill crystallized at oxygen fugacities from 10^–10 atm (ORZ) to 10^–13 (UZ). The magma prior to intrusion appears to have been derived from a more primitive melt from which a considerable amount of olivine and plagioclase have fractionated out. A model of open, interrupted fractional crystallization in the sill is proposed to explain the compositional variations exhibited by the major mineral phases.A previous study (Crookshank 1936) concluded that the sill is actually a multiple intrusion and has given rise to the lowermost (flow I) and the topmost (flow III) lava flows in the neighboring area around Tamia (78°4015, 22°2035). The olivines of flows I and III have compositions Fo₈₇ and Fo₈₈ respectively, and are much more Mg-rich than the maximum Mg-rich olivine (Fo₇₆) of the Delakhari sill, refuting the possibility of the sill being the feeder of the lava flows I and III.Geosciences Department, University of Texas at Dallas Contribution No. 338     

Al-rich calcic pyroxene in pseudotachylite: An indicator of high pressure and high temperature?

An aluminous pyroxene with an approximate formula (Ca_0.60Na_0.05□_0.20) (Mg_0.58Fe_0.11²⁺ Al_0.60^VI)[Si_1.68A1_0.32O₆] or {Jd_0.05Ts_0.32Es_0.20Wo_0.10En_0.27Fs_0.06} has been found in a pseudotachylite vein from gabbros in the Musgrave ranges, Central Australia. Textural evidence indicates that it formed by breakdown of calcic plagioclase, augite, and olivine which constitute the isochemical country rock. There is a large amount of M(2) vacancy compared to most pyroxenes. The vein mineral is most similar in composition to natural pyroxenes in kimberlites and eclogites and especially to synthetic pyroxenes which formed at pressures in excess of 3 GPa (= 30 kb). We suggest that these high pressures were not lithostatic but represent dynamic stresses released during catastrophic brittle rupture either at tips of propagating fractures or spallation of unsupported crackwalls. Such transient shear stresses may reach the shock range. We propose such a fracture mechanism as an alternative to frictional melting for the productoin of pseudotachylite veins.     

Phase relations in transitional and alkali basaltic glasses from Iceland

Basaltic glasses from the three alkalic areas of Iceland (Snaefellsnes Volcanic Zone, Sudurland Volcanic Zone and Vestmannaeyjar Volcanic Area) contain plagioclase, olivine, clinopyroxene, chromian spinel and titanomagnetite as phenocryst phases. The glasses are hypersthene to nepheline normative alkali basaltic with FeO/ MgO ratios between 1.4–4.7. Olivine ranges in composition from Fo₉₀ to Fo₅₅, plagioclase from An₉₀ to An₅₀ and clinopyroxene from En₄₅Fs₁₀Wo₄₅ to En₄₀Fs₁₇Wo₄₃. Clinopyroxene reveals systematic Ti:Al metastable crystallization trends related to the composition of the enclosing glass. Two types of phenocryst are present in most glasses and show a bimodality in size and composition. Microphenocryst phases are those most likely to have crystallized from the enclosing glass, while macrophenocrysts may have crystallized from a liquid of slightly less evolved composition. The glasses show complex phenocryst-glass relations which can be related to a polybaric effect. The normative glass compositions are related to 2-phase cotectic surfaces in the basalt tetrahedron and define the position of the 3-phase cotectic line. In general with increasing FeO/MgO in the glass the phenocryst assemblages vary from clinopyroxene, olivine and plagioclase along a clinopyroxene-olivine surface to olivine and plagioclase along an olivine-plagioclase surface. The normative glass compositions show a deflection from clinopyroxene-bearing to clinopyroxene-free glasses. The appearance of plagioclase together with clinopyroxene and olivine can be explained in the light of experimental investigations of the effect of pressure on phase relations. The major element variation of the glasses is interpreted as representing mantle derived magma batches of primary liquids, modified to some degree by high (6 kbar) and intermediate to low pressure (below 3 kbar) crystal fractionation towards equilibrium phase relations during ascent and residence in crustal magma chambers. The observed deflection in normative compositions of the glasses marks the position of the high pressure 3-phase cotectic line. The bimodality in size and composition of plagioclase and olivine phenocrysts can be related to high pressure crystal fractionation in the melt. The Fe-Ti basalt glasses from Sudurland are believed to be quenched high pressure compositions.     

Mineralogy,petrology, and trace element geochemistry of the Johnstown meteorite: a brecciated orthopyroxenite with siderophile and REE-rich components

The Johnstown meteorite is a brecciated orthopyroxenite (diogenite) containing coarsegrained centimeter-sized clasts of cumulate origin that have undergone subsolidus recrystallization. The brecciated portion is dominated by subangular fragments of orthopyroxene (Wo_2–3En_72–74Fs_23–25) in a variably comminuted matrix of the same material. Minor and accessory phases include plagioclase (An_82–90Ab_10–18Or_0–1), diopside (Wo_44–45En_46–47Fs_9–10), olivine (Fo_71–72), tridymite, troilite, metallic Ni-Fe (~3% Ni), and chromite (Cm_71–80Hc_1–8Sp_11–19Mt_2–4Uv_1–3).The clastic component is parental to the brecciated matrix which contains no foreign lithic or mineralogic components. Siderophile trace element studies, however, reveal the presence of meteoritic (chondritic) contamination in the brecciated portion using unbrecciated clasts for indigenous values. Rare earth element abundances show a wide range of values for the light REE in different samples, although all samples exhibit a strong negative Eu anomaly, indicative of earlier plagioclase fractionation. Two pairs of adjacent brecciated and unbrecciated samples from different portions of the meteorite show, respectively, the most enriched and the most depleted light REE patterns. The variability in La content is over a factor of 100. However, in each case the REE pattern for the brecciated portion is very similar to that of the unbrecciated portion. These differences are attributed to sampling of variable amounts of residual, REE-enriched, trapped liquid. The most representative REE pattern for the bulk meteorite has an intermediate composition and was obtained from the largest sample. The data presented here indicate that Johnstown is a monomict breccia, in contrast to several other diogenites which may be considered to be polymict on the basis of their mineral compositions and/or clast populations.     

Petrology of the Kurancali Phlogopitic Metagabbro: An Island Arc-Type Ophiolitic Sliver in the Central Anatolian Crystalline Complex

The Kurancali metagabbro occurs as an isolated body in the central part of the Central Anatolian Crystalline Complex. It has been emplaced along a steep S-vergent thrust-plane onto the uppermost units of the Central Anatolian Metamorphics. The main body of the Kurancali metagabbro is characterized by distinct compositional layering. The layered gabbros comprise pyroxene and hornblende gabbros. Phlogopite-rich, plagioclase-hornblende gabbro occurs mainly as pegmatitic dikes intruding the layered gabbro sequence. The layered gabbros, in general, consist mainly of diopsidic augites, brown hornblendes, and plagioclase. Secondary phases are phlogopitic mica, brownish-green hornblende replacing clinopyroxenes, and fibrous, greenish actinolitic hornblende partially or completely replacing brown hornblende. The primary dark micas are phlogopitic in the range of phlogopite_(57-70) and annite_(30-43). The analyzed pyroxenes are diopsidic (En₃₂Fs₁₉Wo₄₉-En₃₅Fs₁₈Wo₄₈).

The whole-rock geochemistry of the gabbros indicates the presence of two distinct groups of rocks; a less pronounced group of phlogopite gabbro with island-arc calc-alkaline affinities, and a dominating layered gabbro sequence with island-arc tholeiite characteristics. They are extremely enriched in LILE, indicative of alkaline metasomatism in the source region, and display geochemical features of transitional backarc-basin basalts (BABB)/island-arc basalts (IAB)—and IAB-type oceanic crust. Based on their geochemical similarities to modern island-arc basements, we suggest that the Kurancali metagabbro may represent the basement of an initial island arc, generated in a suprasubduction zone setting within the Izmir-Ankara branch of Neotethys.     

Geothermometry of exsolved augites from the Laramie Anorthosite Complex,Wyoming

Exsolved augite pyroxenes from the ferromonzonite border facies of the ferrosyenite in the Laramie Anorthosite Complex have been studied with the transmission electron microscope and the electron microprobe to determine their exsolution histories. The Lindsley and Andersen (1983) geothermometer gives initial crystallization temperatures of 1000° C for the bulk augite crystal (Wo₃₂ En₂₂ Fs₄₆). Exsolved lamellae are predominantly pigeonites with very low calcium contents (Wo_1–3 En_23–24 Fs_71–74) and have formation temperatures estimated to be in the range of 600 to 975° C. The uniform compositions of lamellae and hosts, despite the range in lamellar size and orientation, suggest that either 1) the ferromonzonite experienced an extended plateau in cooling or a reheating event at 600 to 650° C or 2) the pyroxenes recorded a blocking temperature. Two-feldspar geothermometry on exsolved feldspars also records 600° C and suggests that these low temperatures are not blocking temperatures.     

Textural evolution and metamorphism of pillow basalts from the Franciscan Complex,western Marin County,California

Petrogenesis of Franciscan pillow basalts from the Franciscan Complex of western Marin County California entails both dynamic crystallization of tholeiitic magma and subsequent low-temperature metamorphism. Brittle deformation during tectonic emplacement of pillow basalts into a chert greywacke terrain is manifested by the shearing of interpillow matrix and polishing of pillow rims, but the igneous textures within pillows are well preserved.The cooling history of pillow basalts can be understood through analysis of morphologic variations of primary olivine and plagioclase from rim to core of the pillow. Crystal sizes and plagioclase dendrite spacings are consisted with a cooling rate which generally decreases inward. Some pillows show a marked asymmetry in plagioclase and olivine morphology suggesting lower cooling rates caused by asymmetric cooling of the pillows. Olivine morphologies, primarily hopper and chain forms, are consistent with cooling rates of 2–10 °C/h for pillow cores and 50–75 °C/h for pillow rims.Low temperature hydrothermal alteration has produced secondary minerals indicative of zeolite facies conditions. Pillow matrix is either chloritic or zeolitic (in part laumontized). Pillow rims display incomplete replacement of calcic palagonite by pumpellyite (Fe₂O₃=9–21 wt%), prehnite (Fe₂O₃=5–7 wt%), sphene and quartz. Metamorphism of pillow interiors, manifested by: (1) veins of quartz, pumpellyite, calcite, or harmotome (BaO=15 wt%); (2) amygdules containing analcime, chlorite or quartz; and (3) replacement of olivine by pumpellyite or smectite/illite, of plagioclase by albite (An3)+sericite, and of glassy groundmass by fine-grained chlorite. Primary augite (Wo3₃₉En₁₃Fs₄₈) was not altered. The described paragenesis may be attributed to oceanfloor and/or Franciscan-type metamorphism.     

Peridotites from the Mid-Atlantic Ridge at 43° N and their petrogenetic relation to abyssal tholeiites

Whole-rock, major and trace element analyses and microprobe mineral analyses were conducted on serpentinized peridotites recovered from the walls of a MAR (Mid-Atlantic Ridge) 43° N fracture zone. These peridotites are extensively serpentinized; serpentine usually makes up 30–100 vol. percent of the bulk rocks. The relict minerals observed consist mainly of olivine and orthopyroxene with subordinate amounts of clinopyroxene and brown spinel. The range in olivine composition is very limited (Fo_91–92). Orthopyroxene forms large, anhedral crystals with clinopyroxene exsolution lamellae and shows undulose extinction with bent cleavages and lamellae. Broad beam microprobe analyses indicate that the composition range of orthopyroxene is also limited (En_89.1–87.6Fs_8.2-8.0Wo_2.7–4.4; Al₂O₃=1.82–2.64 wt%; Cr₂O₃=0.63–0.88 wt%). Clinopyroxene tends to fringe large orthopyroxene crystals or fills the interstices between them. The Mg/Fe ratios of clinopyroxene are practically constant; however, the Ca/(Ca + Mg + Fe) ratios range from 0.48 to 0.45. The Cr/(Cr+Al) and Mg/(Mg+ Fe²⁺) ratios of brown spinel range from 0.57 to 0.36 and 0.69 to 0.56, respectively. The geothermometers utilizing coexisting spinel lherzolite mineral assemblages suggest that the MAR 43° N peridotites attained equilibrium at temperatures from 1100° to 1250° C.Peridotites recovered from the ocean floor are generally considered to have been subjected to partial melting processes and are regarded as residues left after primary magma was removed. Major element chemistry of the MAR 43° N peridotites are compared with those of the ocean-floor ultramafic tectonites reported previously and used together with those published data to demonstrate that the major element abundances of the oceanfloor peridotites define an average trend which is compatible with removal of primary magma from these peridotites at moderate pressures (10–15 kb). Then, the most primitive abyssal tholeiite glasses could be produced by ca. 10% olivine fractionation of such primary magma. Extensive fractionation of olivine and/or orthopyroxene from picritic liquids which are in equilibrium with the lherzolitic or harzburgitic mantle sources at higher pressures (>20 kb) could not yield the majority of the most primitive abyssal tholeiite glasses.     

Petrology of Ultramafic Xenoliths from Loihi Seamount, Hawaii

Ultramafic xenoliths were recovered in four alkalic lava flowsfrom Loihi Seamount at depths between 2200 and 1400m. No xenolithbearing flows were sampled near the summit despite a concentrateddredge program. The flows, three of alkalic basalt and one ofbasanite, contain common olivine megacrysts and small xenolithsof dunite, rarer harzburgite, and a single wehrlite. Olivinemegacrysts as large as 8 mm are Fo_{84–88 6} and containmagnesiochromite inclusions with 1?1–3?5 wt.% TiO₂ Dunitecontains Fo_{83 5–88?5} olivine, magnesiochromite with l?5–6?9wt.% TiO₂ (avg. 3?2 wt.%), and extremely rare chrome-rich diopside.The wehrlite contains euhedral Fo_{85 9} olivine and magnesiochromitewith 1?9–4?7 wt.% TiO₂ poikilitically enclosed in chrome-richdiopside (Wo_{45 4}En_{48 0}Fs_6?6).Most of the olivine megacrysts,dunite, and the wehrlite are cumulates of Loihi alkalic lavasthat accumulated in a magma storage zone located at least 16kmbelow sea level. The rarity of dunite related to tholeiiticmagmas supports the interpretation that the alkalic lavas atLoihi generally predate the tholeiitic lavas. The harzburgitexenoliths have cataclastic textures and contain Fo_{89 5–926} olivine, enstatite (Wo_{2 0–2?7}En_{90?0–88 7}Fe_8?0–8?6),Cr-rich endiopside (Wo_{43 4–44 5}En_{52 0–50 0}Fs_{4 6–45}), and translucent red-brown magnesiochromite. The harzburgitexenoliths, which have 2-pyroxene temperatures of 1066 ? 35?C,originated in the uppermost mantle in a region of high strainrate, probably near the boundary between the mantle and theoverlying ocean crust. The presence of upper mantle xenolithsindicates that the magma storage zone is located below the baseof the ocean crust within the uppermost mantle.     

The formation of garnet in olivine-bearing metagabbros from the Adirondacks

A regional study of olivine-bearing metagabbros in the Adirondacks has permitted testing of the P(pressure)-T(temperature)-X(composition) dependence of garnet-forming reactions as well as providing additional regional metamorphic pressure data. Six phases, olivine, orthopyroxene, clinopyroxene, garnet, plagioclase and spinel, which can be related by the reactions: orthopyroxene+clinopyroxene+spinel +anorthite=garnet, and forsterite+anorthite=garnet occur together both in coronal and in equant textures indicative of equilibrium. Compositions of the respective minerals are typically Fo_25–72, En_44–75, En_30–44Fs_9–23Wo_47–49, Pp_13–42Alm_39–63Gr_16–20, An_29–49 and Sp_16–58. When they occur in the same rock, equant and coronal garnets are homogeneous and compositionally identical suggesting that chemical equilibrium may have been attained despite coronal textures. Extrapolating reactions in the simple CMAS system to granulite temperatures and making thermodynamic corrections for solid solutions gives equilibration pressures (using the thermometry of Bohlen et al. 1980b) ranging from about 6.5 kb in the Lowlands and southern Adirondacks to 7.0–8.0 kb in the Highlands for the assemblage olivine-plagioclase-garnet. These results are consistent with inferred peak metamorphic conditions in the Adirondacks (Valley and Bohlen 1979; Bohlen and Boettcher 1981). Thus the isobaric retrograde path suggested by Whitney and McLelland (1973) and Whitney (1978) for the formation of coronal garnet in olivine metagabbros may not be required. Application of the same equilibria gives >8.7 kb for South Harris, Scotland and 0.9 kb for the Nain Complex. Disagreement of the latter value with orthopyroxeneolivine-quartz barometry (Bohlen and Boettcher 1981) suggests that the use of iron-rich rocks (olivines Fa₅₀) results in errors in calculated pressures.Contribution No. 385 from the Mineralogical Laboratory, Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109, USA     

Geochemistry and origin of massif-type anorthosites

Samples of Proterozoic anorthosite complexes from the Adirondack Mountains of New York, Burwash Area of Ontario, and the Nain Complex of Labrador, ranging in composition from anorthosite to anorthositic gabbro, have been analyzed for major elements, Rb, Sr, Ba and nine rare-earth elements (REE), in order to set limits on the compositions and origins of their parent magmas. Similar rock types from the different areas have similar major and trace element compositions. The anorthosites have high Sr/Ba ratios, low REE abundances (Ce about 10, Yb about 0.5–1.5 times chondrites) and large positive Eu anomalies. The associated anorthositic gabbros have lower Sr/Ba ratios, REE abundances nearly an order of magnitude higher than the anorthosites, and small to negligible positive Eu anomalies.Model calculations using the adcumulate rocks with the lowest REE abundances and published distribution coefficients yield parent liquids having REE abundances and patterns similar to those of the associated anorthositic gabbros with the highest REE abundances. Rocks with intermediate REE abundances are the result of incorporation of a liquid component by a plagioclase-rich cumulate similar to the adcumulate samples. The analytical data and model calculations both suggest parent liquids having compositions of 50–54% SiO₂, greater than 20% Al₂O₃, about 1% K₂O, atomic Mg/(Mg+Fe²⁺) ratios (Mg No.'s) of less than 0.4, 15–30 ppm Rb, 400–600 ppm Sr and 400–600 ppm Ba, 40–50 times chondrites for Ce and 8–10 times chondrites for Yb.The low atomic Mg/(Mg+Fe²⁺) values for these rocks combined with geophysical evidence suggesting there are not large quantities of ferromagnesian material at depth, indicate that the anorthositic masses are not products of fractional crystallization of mafic melt derived from melting of the mantle. Rather, it is suggested that they are a result of partial melting of tholeiitic compositions at depths shallower than the basalt-eclogite transformation, leaving a pyroxene-dominated residue.     

The petrology of ultramafic xenoliths from Summit Lake,near Prince George,British Columbia

Approximately 200 upper mantle xenoliths from Summit Lake, near Prince George, British Columbia, were collected from a basanitoid flow of Late Cenozoic (possibly post-glacial) age. The most abundant xenolith is spinel lherzolite (55%), with subordinate wehrlite (22%), clinopyroxenite (10%), olivine websterite (10%), websterite (2%) and dunite (1%). Xenoliths have granular textures and both green chrome diopside-bearing and black aluminous augitebearing xenoliths are present. About 5% of the xenoliths are banded on a cm scale, suggesting that the upper mantle beneath north-central British Columbia is heterogeneous on a scale of cm to meters.Microprobe data on the mineral phases indicate that the xenoliths are generally well equilibrated. Typically in spinel lherzolite, olivines are Fo₈₉, orthopyroxenes are En₉₀ and chrome diopside is Wo₄₅En₅₀Fs₅. Spinels vary in composition from xenolith to xenolith. The evidence for partial melting observed in five xenoliths, may be due to heating during incorporation of the xenoliths within the host magma or to instability caused by decompression as the xenoliths are transported to the surface.Using element partition geothermometers, equilibration temperatures are calculated to be between 1080–1100° C. Pressures, estimated from a Cordilleran geotherm, are between 18–20 kbar. These temperatures are somewhat higher than estimates from xenoliths from other localities in Late Cenozoic alkali basalts in south and central British Columbia. It is concluded, therefore, that either the Summit Lake suite represents samples from a deeper source region in the upper mantle or the Late Cenozoic geotherm varied in time and space.On leave from the Geological Institute, University of Tokyo     

Fertile Lithospheric Mantle Beneath the North African Plate: Evidence from Gharyan Lherzolite Xenoliths (Northern West Libya)

The late-stage basanite (～12–1 Ma) of the NNW-SSE extending Gharyan Cenozoic volcanics (Northern West Libya) contains numerous fresh lherzolite xenoliths. These xenoliths display magmatic protogranular and porphyroclastic textures. Chemistry of olivine (forsterite content –90–91, NiO = 0.26–0.39 wt %), orthopyroxene (Mg# = 0.91–0.92, Cr# = 0.03–0.07, Al₂O₃ = 3.64–4.43 wt %), clinopyroxene ((Wo_{45.59–48.61}, En_{45.89–48.80}, Fs_4.47–5.81), Mg# = 0.82–0.92, Al₂O₃ = 5.14–6.58 wt % and Cr₂O₃ = 0.5–0.95 wt %) and spinel (hercynite–picotite–Al-rich chromite spinels with Cr# = 0.08–0.11) refer to the fertile nature of Gharyan peridotites. Mantle source region was close to the primitive composition with low degrees of melting and depletion in melt (1.5% in average). However, it underwent metasomatism as illustrated by formation of secondary clinopyroxene (Wo_{37.96–44.77}, En_{47.44–54.18}, Fs_7.59–8.03) with high contents of Na₂O, and enrichment of the Al-spinel in TiO₂. The estimated pre-eruptive temperature ranges from 1066 to 837°C. Despite the Gharyan peridotite is similar to mantle components in many districts in Saharan belt of North Africa, it represents juvenile mantle source with minor refractory residues. This compositional heterogeneity is mainly attributed to the local effect of the interaction of the Gharyan mantle with the host basanite magma that may be related to the Cenozoic rifting of the Pan-African basement.