Fluid-mineral equilibria in prehnite-pumpellyite to greenschist facies metabasites near Flin Flon, Manitoba, Canada: implications for petrogenetic grids

A sequence of regional metamorphic isograds indicating a range from prehnite-pumpellyite to lower amphibolite facies was mapped in metabasites near Flin Flon, Manitoba. The lowest grade rocks contain prehnite + pumpellyite and are cut by younger brittle faults containing epidote + chlorite + calcite. Isobaric temperature- X _CO2 and pressure-temperature (constant X _CO2) diagrams were calculated to quantify the effects of CO₂ in the metamorphic fluid on the stability of prehnite-pumpellyite facies minerals in metabasites containing excess quartz and chlorite. Prehnite and, to a lesser extent, pumpellyite are stable only in fluids with X _co2 <0.002. For X _co2>0.002, epidote + chlorite + calcite assemblages are stable. Our calculated phase relations are consistent with regional metamorphism in the Flin Flon area in the presence of an H₂O-rich fluid and a more CO₂-rich fluid in the later fault zones. We believe that the potential effects of small amounts of CO₂ in the metamorphic fluid should be assessed when considering the pressure-temperature implications of mineral assemblages in low-grade metabasites.     

Petrologic Reconnaissance of Franciscan Metagraywackes from the Diablo Range, Central California Coast Ranges

The complexly folded and faulted Diablo antiform representsa core of pervasively metamorphosed Late Mesozoic Franciscanrocks surrounded by roughly contemporaneous, less deformed,only feebly recrystallized Great Valley strata. The contactbetween the two lithologic series is nearly everywhere a high-anglethrust, the Ortigalita fault. Thin sections of 679 metaclastics from the 3000 km² Franciscanarea of the Diablo Range have been studied. A very rough correlationseems to exist between the degree of textural reconstitutionand phase assemblage. Many feebly metamorphosed Franciscan rockscontain relict clastic biotite; chlorite, white mica, and minoramounts of pumpellyite appear to be newly generated from claysand detrital plagioclase which has been albitized. In more thoroughlyrecrystallized Franciscan rocks, lawsonite has grown by hydrationof the minor An component of plagioclase, but more commonlyby the inferred interaction of interstitial clays+calcium carbonate;some of these rocks carry metamorphic aragonite. Intensely recrystallizedFranciscan metagraywackes contain jadeitic pyroxene±glaucophane.The observed changes in mineral assemblages are thought to reflectprogressive pressure increment at nearly constant temperature. In the north-western portion of the Range, jadeitic metaclasticrocks are located along the faulted margin of the antiform.Elsewhere there appears to be no clear relationship betweensystematic Franciscan parageneses and the tectonic contact withrocks of the Great Valley sequence. Evidently Diablo Range metaclasticassemblages do not owe their present areal distribution to apostulated process involving tectonic overpressures accompanyingthrusting of the Great Valley strata over the Franciscan alongthe Ortigalita fault. Jadeitic pyroxenebearing metagraywackesalso appear to be unrelated to the emplacement of alpine-typeperidotites.     

Phase equilibria among pumpellyite,lawsonite, epidote and associated minerals in low grade metamorphic rocks

Phase relations of pumpellyite, epidote, lawsonite, CaCO₃, paragonite, actinolite, crossite and iron oxide are analysed on an Al-Ca-Fe³⁺ diagram in which all minerals are projected from quartz, albite or Jadeite, chlorite and fluid. Fe²⁺ and Mg are treated as a single component because variation in Fe²⁺/Mg has little effect on the stability of phases on the diagram. Comparison of assemblages in the Franciscan, Shuksan, Sanbagawa, New Caledonia, Southern Italian, and Otago metamorphic terranes reveals several reactions, useful for construction of a petrogenetic grid:

1. lawsonite+crossite + paragonite = epidote+chlorite + albite + quartz + H₂O
2. lawsonite + crossite = pumpellyite + epidote + chlorite + albite+ quartz + H₂O
3. crossite + pumpellyite + quartz = epidote + actinolite + albite + chlorite + H₂O
4. crossite + epidote + quartz = actinolite + hematite + albite + chlorite + H₂O
5. calcite + epidote + chlorite + quartz = pumpellyite + actinolite + H₂O + CO₂
6. pumpellyite + chlorite + quartz = epidote + actinolite + H₂O

     

北祁连山硬柱石蓝片岩p-T条件相平衡计算及其岩石学意义

北祁连硬柱石蓝片岩主要分布在甘肃省肃南县九个泉一带,是目前中国唯一报道的、确切地含有硬柱石的蓝片岩。文中在详细的岩石学和矿物学研究基础上,根据矿物共生组合的不同,将北祁连低温蓝片岩进一步划分为绿纤石蓝片岩、硬柱石蓝片岩和绿帘石蓝片岩。绿纤石蓝片岩的特征变质矿物组合为蓝闪石(>40%)+绿纤石(30%)+绿泥石(10%)+钠长石(8%)+石英(5%)+硬柱石(<3%)±方解石/文石(<1%)。硬柱石蓝片岩的矿物组合为蓝闪石(35%~40%)+硬柱石(35%~40%)+绿泥石(10%)+钠长石(10%)+石榴石(1%~2%)+黝帘石/斜黝帘石(<2%)+石英(<1%),副矿物有磷灰石和榍石,总含量小于2%。绿帘石蓝片岩的矿物组合为蓝闪石(30%~35%)+黝帘石/斜黝帘石/绿帘石(~30%)+绿泥石(15%)+钠长石(15%)+石榴石(2%)+石英(<2%),副矿物有金红石、磷灰石和磁铁矿,总含量小于2%。利用矿物内部一致性热力学数据和Domino/Theriak软件计算了这三种类型的蓝片岩形成的峰期温压条件,它们分别是绿纤石蓝片岩为320~350℃,0.75~0.85GPa;硬柱石蓝片岩为335~355℃,0.8~0.95GPa;绿帘石蓝片岩为345~375℃;0.75~0.85GPa。北祁连低温蓝片岩带由硬柱石蓝片岩相到绿帘石蓝片岩相的转化代表了俯冲变质过程中的递进变质过程。     

A second natural occurrence of yoderite

A second example of yoderite has been discovered in whiteschists from the Southern Chewore Hills of northern Zimbabwe. The mineral is pale green in colour and occurs in an equilibrium assemblage with talc+chlorite+kyanite+dravite+hematite. There is no quartz present. Recalculated microprobe analyses give a structural formula of Mg₂Al_5.7Fe_0.3Si₄O₁₈(OH)₂, similar to that obtained for the type locality at Mautia Hill, Tanzania, i.e. Mg₂Al_5.6Fe_0.4Si₄O₁₈(OH)₂. Textural relationships and relative proportions of minerals suggest that the yoderite was formed by reaction between talc, chlorite, kyanite and hematite. Experimental evidence suggests high-water-pressure metamorphic conditions at temperatures exceeding a reaction curve that extends between 13  kbar at 590  °C and 21  kbar at 650  °C. The yoderite-bearing whiteschist is associated with a 1.4  Ga dismembered ophiolite. It is proposed that this yoderite occurrence is associated with a relict subduction/suture zone.     

Reaction relationships during retrograde metamorphism at Olary, South Australia

Abstract In metapelitic schists of the north-eastern Weekeroo Inliers, Olary Block, Willyama Supergroup, South Australia, syn-S1 and syn-S2 assemblages involving staurolite, garnet, biotite and another mineral, most probably cordierite, were overgrown by large syn-S3 andalusite porphyroblasts, owing to isobaric heating from metamorphic conditions that existed during the development of S2. Conditions during the development of S3 probably just reached the andalusite—sillimanite transition. During the development of S4, at somewhat lower temperatures than those that accompanied the development of S3, the following reaction occurred:
staurolite + chlorite + muscovite ± biotite + andalusite + quartz + H₂O.
The amount of retrogression is controlled primarily by the amount of H₂O added by infiltration. As the syn-S3 matrix assemblage was stable during the development of S4, but the andalusite porphyroblasts were no longer stable with the matrix when H₂O was added, the retrogression is focused in and around the porphyroblasts. With enough H₂O available, and if quartz was consumed before biotite in a porphyroblast, then the following reaction occurred:
staurolite + chlorite + muscovite + corundum ± biotite + andalusite + H₂O.
This reaction allowed corundum inclusions in the andalusite to grow, regardless of the presence of quartz in the matrix assemblage.     

The greenschist metamorphism of Archaean synvolcanic stocks near Wawa, Ontario, Canada

Abstract Two Archaean synvolcanic stocks with contact aureoles occur in the Wawa greenstone belt near Wawa, Ontario, Canada. The Gutcher Lake and Jubilee stocks consist mainly of granitoid trondhjemite with feldspar laths mottled by white mica + calcite + epidote and rimmed by clear albite. Biotite is partly or wholly pseudomorphosed by chlorite + sphene; some epidote is partly altered to calcite + chlorite. The granitoid phase grades into a foliated phase of quartz + albite + white mica + calcite + chlorite near fracture zones traversing the stocks.
The alteration of the Gutcher Lake stock along its foliated margin involved addition of K₂O, H₂O + CO₂, MnO, plus Rb; loss of CaO plus Sr; and a shift in Fe⁺²/Fe_t from 0.66 to 0.81. The alteration of the Jubilee stock along the Darwin Shear involved addition of H₂O + CO₂; loss of Sr; and no significant shift in Fe⁺²/Fe_t. The greenschist alteration also modified the contact aureoles bordering both stocks.
One interpretation is that regional metamorphism in the Archaean overprinted a greenschist assemblage on both stocks. The alteration was intense near fracture zones and sporadic remote from fractures. Lower integrated water to rock ratios along the Darwin Shear compared to the margin of the Gutcher Lake stock may explain the comparatively lower perturbation of the element abundances and redox state of iron.     

Lawsonite zone blueschists and a sodic amphibole producing reaction in the Tavşanli Region,Northwest Turkey

The petrology and mineralogy of lawsonite zone metabasites have been studied northeast of town of Tav?anli, NW Turkey. In the field the metabasites are characteristically green and lack foliation; the essential mineral assemblage being sodic pyroxene+ lawsonite+chlorite+quartz±sodic amphibole. Sodic pyroxene of aegirine-jadeite composition occurs as pseudomorphs after magmatic augite. Lawsonite and chlorite are the other two dominant minerals. Sodic amphibole forms progressively from a reaction between sodic pyroxene, chlorite and quartz, and an isograd representing the first abundant occurrence of sodic amphibole in basic rocks has been mapped. The widespread occurrence of sodic pyroxene pseudomorphs in other blueschist terrains indicates that the inferred sodic amphibole producing reaction is of general significance for blueschist metabasites.The conversion of greenstones with the assemblage albite+chlorite+actinolite directly into glaucophane-lawsonite blueschists without any intervening lawsonite zone illustrates the influence of the initial mineral assemblage on the reaction path.     

Lawsonite zone blueschists and a sodic amphibole producing reaction in the Tavşanli Region,Northwest Turkey

The petrology and mineralogy of lawsonite zone metabasites have been studied northeast of town of Tavanli, NW Turkey. In the field the metabasites are characteristically green and lack foliation; the essential mineral assemblage being sodic pyroxene+ lawsonite+chlorite+quartz±sodic amphibole. Sodic pyroxene of aegirine-jadeite composition occurs as pseudomorphs after magmatic augite. Lawsonite and chlorite are the other two dominant minerals. Sodic amphibole forms progressively from a reaction between sodic pyroxene, chlorite and quartz, and an isograd representing the first abundant occurrence of sodic amphibole in basic rocks has been mapped. The widespread occurrence of sodic pyroxene pseudomorphs in other blueschist terrains indicates that the inferred sodic amphibole producing reaction is of general significance for blueschist metabasites.The conversion of greenstones with the assemblage albite+chlorite+actinolite directly into glaucophane-lawsonite blueschists without any intervening lawsonite zone illustrates the influence of the initial mineral assemblage on the reaction path.     

A new petrogenetic grid for low-grade metabasites

Abstract We have used internally-consistent thermodynamic data to present calculated phase equilibria for the system Na₂O-CaO-MgO-Al₂O₃-SiO₂-H₂O (NCMASH), in the range 0–500° C and 0.1–10 kbar, involving the phases anorthite, glaucophane, grossular, heulandite, jadeite, laumontite, lawsonite, paragonite, prehnite, pumpellyite, stilbite, tremolite, wairakite, zoisite with excess albite, clinochlore, quartz and pure water. Average activity terms derived from published mineral chemical data were included for clinochlore, glaucophane, prehnite, pumpellyite, tremolite, and zoisite. The new petrogenetic grid delineates stability fields and parageneses of common index minerals in zeolite, prehniteactinolite, prehnite-pumpellyite, pumpellyite-actinolite, blueschist and greenschist facies metabasites. The stability fields of mineral assemblages containing prehnite, pumpellyite, epidote, actinolite (+ albite + chlorite + quartz) were analysed in some detail, using activity data calculated from five specific samples. For example, the prehnite-actinolite facies covers a P-T field ranging from about 220 to 320° C at pressures below 4.5 kbar. The transition from the prehnite-actinolite and pumpellyite-actinolite to greenschist facies occurs at about 250–300° C at 1–3 kbar and at about 250–350° C at 3–8 kbar. P-T fields of individual facies overlap considerably due to variations in chemical composition.     

On the origin of CO2-rich fluid inclusions in migmatites

Abstract Nearly pure CO₂ fluid inclusions are abundant in migmatites although H₂O-rich fluids are predicted from the phase equilibria. Processes which may play a role in this observation include (1) the effects of decompression on melt, (2) generation of a CO₂-bearing volatile phase by the reaction graphite + quartz + biotite + plagioclase = melt + orthopyroxene + CO₂-rich vapour, (3) selective leakage of H₂O from CO₂+ H₂O inclusions when the pressure in the inclusion exceeds the confining pressure during decompression, and (4) enrichment of grain-boundary vapour in CO₂ by subsolidus retrograde hydration reactions.     

Metamorphic evolution of the Susunai metabasites in southern Sakhalin, Russian Republic

The Susunai Complex of southeast Sakhalin represents a subduction-related accretionary complex of pelitic and basic rocks. Two stages of metamorphism are recognized: (1) a local, low- P / T  event characterized by Si-poor calcic amphiboles; (2) a regional, high- P / T  event characterized by pumpellyite, actinolite, epidote, sodic amphibole, sodic pyroxene, stilpnomelane and aragonite. The major mineral assemblages of the high- P / T  Susunai metabasites contain pumpellyite+epidote+actinolite+chlorite, epidote+actinolite+chlorite, epidote+Na-amphibole+Na-pyroxene+chlorite+haematite. The Na- amphibole is commonly magnesioriebeckite. The Na-pyroxene is jadeite-poor aegirine to aegirine-augite. Application of empirically and experimentally based thermobarometers suggests peak conditions of T  =250–300 °C, P= 4.7–6 kbar. Textural relationships in Susunai metabasite samples and a petrogenetic grid calculated for the Fe³⁺-rich basaltic system suggest that pressure and temperature increased during prograde metamorphism.     

Reaction textures in calc-silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica: implications for the retrograde P–T path

Calc-silicate granulites from the Bolingen Islands, Prydz Bay, East Antarctica, exhibit a sequence of reaction textures that have been used to elucidate their retrograde P–T path. The highest temperature recorded in the calc-silicates is represented by the wollastonite- and scapolite-bearing assemblages which yield at least 760°C at 6 kbar based on experimental results. The calc-silicates have partially re-equilibrated at lower temperatures (down to 450°C) as evidenced by the successive reactions: (1) wollastonite + scapolite + calcite = garnet + CO₂, (2) wollastonite + CO₂= calcite + quartz, (3) wollastonite + plagioclase = garnet + quartz, (4) scapolite = plagioclase + calcite + quartz, (5) garnet + CO₂+ H₂O = epidote + calcite + quartz, and (6) clinopyroxene + CO₂+ H₂O = tremolite + calcite + quartz.
The reaction sequence observed indicates that a CO₂ was relatively low in the wollastonite-bearing rocks during peak metamorphic conditions, and may have been further lowered by local infiltration of H₂O from the surrounding migmatitic gneisses on cooling. Fluid activities in the Bolingen calc-silicates were probably locally variable during the granulite facies metamorphism, and large-scale CO₂ advection did not occur.
A retrograde P–T path, from the sillimanite stability field ( c. 760°C at 6 kbar) into the andalusite stability field ( c. 450°C at <3 kbar), is suggested by the occurrence of secondary andalusite in an adjacent cordierite–sillimanite gneiss in which sillimanite occurs as inclusions in cordierite.     

Die Glaukophangesteine,ihre stofflichen Äquivalente und Uniwandlungsprodukte in Nordcalabrien (Süditalien)

In the southern Apennin (= northern part of the region dealt with) and the Coasta Chain (= southern part) there are metabasalts wich are classified in the northern part as:

1. Glaucophane rocks of the albite-lawsonite-glaucophane-subfacies with the assemblage glaucophane + pumpellyite + lawsonite ±albite ±aragonite ±muscovite (7 rock analyses, 8 mineral analyses). These rocks are conceived as relics of an older burial metamorphism.
2. Rocks with pumpellyite and chlorite or also chlorite alone, that are interpreted as reaction rims between the metastable glaucophane rocks and the country rock (phyllites, quartzites). The assemblages pumpellyite + chlorite and chlorite alone are to be found (2 rock analyses and 2 mineral analyses).
3. Rocks with lawsonite and/or epidote belong to the same mineral facies as the country rock: a facies similar to the greenschist facies (called “lawsonite-albite-chlorite-subfacies”) which is characterized by the assemblages lawsonite + albite + chlorite ±calcite and also epidote ±lawsonite + albite + chlorite ± muscovite. These types are attributed to a younger dynamo-metamorphism (2 rock analyses).

In the southern part, the metabasalts can be found only as rocks with epidote and/or lawsonite, a metamorphism with more than one event cannot be proved petrologically (3 rock analyses). Equations of the observed mineral reactions are given. The transitions of one facies into another are represented in the pseudo-quaternary system Al₂O₃-CaO-Na₂O · Al₂O₃-2 Fe₂O₃ + FeO + MnO + MgO-(H₂O). The pressure-temperature conditions are estimated on the basis of published experimental data (300° C and 6–7 kb for the glaucophane rocks; 400° C and about 6 kb for the rocks with lawsonite and/or epidote) and are compared with geologic facts.     

Glaucophane and pyroxene breakdown reactions in the Pennine units of the Eastern Alps

Abstract New occurrences of crossite and jadeitic pyroxene are described from a thick metabasite unit within the upper levels of the Peripheral Schieferhülle in the Tauern Window, Austria. Unusual textures are preserved which provide evidence for the reactions and mechanisms involved in the breakdown of crossite and jadeitic pyroxene. Zones of albite and chlorite, produced by reaction between crossite and paragonite, have been preserved due to sluggish reaction kinetics during decompression from the blueschist to the greenschist facies. The zonal sequence is interpreted in terms of chemical potential gradients in Na, Mg and Al, which have been established by overstepping the equilibrium boundary. Breakdown textures of jadeite-acmite pyroxene to a symplectite of albite + hematite + actinolite, and of crossite to talc and actinolite are also described.
The occurrence of crossite and jadeitic pyroxene at high levels within the Peripheral Schieferhülle implies that even upper levels of the structural sequence have undergone blueschist facies metamorphism with pressures in excess of 8 kbar during the Alpine collisional event.     

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.     

Low-grade metamorphism of the Mikabu and northern Chichibu belts in central Shikoku, SW Japan: implications for the areal extent of the Sanbagawa low-grade metamorphism

In central Shikoku, SW Japan, the Mikabu belt is bounded to the north by the Sanbagawa belt, and to the south by the northern (N) Chichibu belt. The N-Chichibu belt can be further subdivided into northern and southern parts. There is no apparent difference in the overall geology, structure, or fossil and radiometric ages between the Mikabu belt and the northern part of the N-Chichibu belt. Greenstones from the Mikabu belt and the northern part of the N-Chichibu belt show evidence for similar low-grade metamorphism, and include the following mineral assemblages with albite+chlorite in excess: metamorphic aragonite, sodic pyroxene+quartz, epidote+actinolite+pumpellyite, glaucophane+ pumpellyite+quartz, and lawsonite (not with actinolite or glaucophane). These similarities suggest that the Mikabu belt and the northern part of the N-Chichibu belt belong to the same geological unit (the MB-NNC complex). The mineral assemblages also indicate that the MB-NNC complex belongs to a different metamorphic facies from the low-grade part of the Sanbagawa belt, that is, the former represents lower temperature/higher pressure conditions than the latter. Structural and petrological continuity between the MB-NNC complex and Sanbagawa belt has not yet been confirmed, but both have similar radiometric ages. It is therefore most likely that the MB-NNC complex and Sanbagawa belt belong to the same subduction complex, and were metamorphosed under similar but distinct conditions. These two units were juxtaposed during exhumation. In contrast, the southern part of the N-Chichibu belt is distinct in lithology and structure, and includes no mineral assemblages diagnostic of the MB-NNC complex and the Sanbagawa belt. Thus, the southern part of the N-Chichibu belt may represent a different geological unit from the MB-NNC complex and Sanbagawa belt.     

Low-grade blueschist facies metamorphism of metagreywackes, Franciscan Complex, northern California

Abstract Metagreywackes in the Eastern Belt of the Franciscan Complex contain the assemblage: Qtz + Ab + Lws + Chl + Ph + Pmp + Fgl + Hem ° Cal/Arg or compatible subassemblages. Blue amphibole first appears in the westernmost part of the belt and pumpellyite is absent in the eastern part. The compositions of the coexisting minerals and the nature of the continuous reactions in these low-grade blueschists suggest that the distribution of blue amphibole and pumpellyite in the Eastern Belt of the Franciscan Complex reflects differences of effective bulk composition rather than differences in physical conditions of metamorphism. In rocks lacking pumpellyite, white mica may be essential to the growth of blue amphibole, but carbonate plays only a limited role. The continuous reaction that limits the appearance of blue amphibole and the disappearance of coexisting pumpellyite has the general form: Pmp + Chl + Ab + Qtz + Hem + H₂O + FeMg_-1= Fgl + Lws. This reaction requires significant hydration as pressure increases in order to produce blue amphibole. Most of the Eastern Belt of the Franciscan Complex formed in limited ranges of temperature and pressure, which are estimated to be 240—280° C, 6.5-7.5 kbar. Pressures in the westernmost part of the area were about 1 kbar lower than in the east. Pressures of about 8.5-10 kbar are estimated for tectonic blocks that contain sodic clinopyroxene.     

Magmatic and hydrothermal processes in the Bouvet Triple Junction Region (South Atlantic)

Results of electron microprobe and microthermometric studies of samples collected from the Bouvet Triple Junction Region (BTJR) during a joint Russian-Italian geological expedition on the R/V Academician Nikolaj Strakhov (1994) have revealed new data on the composition of basaltic magmas and oceanic hydrothermal fluids connected with magmatic processes. Detailed analysis of basaltic glasses shows that the modem Mid-Atlantic Ridge (MAR) rift valley is composed of normal mid-ocean ridge basalts with low concentrations of K₂ O and TiO_z (N-MORB), while its flanks are more enriched with these components approaching E-MORB. A marked influence of the Bouvet hot spot volcanism on magma generation on the South-West Indian Ridge (SWIR) near Bouvet Island is observed. Basaltic melts in this area belong to alkalic and transitional series and have maximum contents of K₂O, TiO₂, H₂O.
Microthermometric analyses of fluid inclusions in the samples from the BTJR have revealed major differences in the oceanic hydrothermal fluid systems on the MAR and near SWIR, which depends on the peculiarities of magma. In the area of the MAR (with dry melts) only H₂O solution inclusions in quartz were found; thus, seawater is probably the only primary source of hydrothermal fluids (NaCl + MgCl₂+ H₂O; T = 170–200°C). In the SWIR area (with the high content of water in melts) syngenetic liquid CO₂ and H₂O solution inclusions in quartz indicate the influence of the magmatic fluid component on the ore-forming water/carbon dioxide solutions (NaCl + CaC1₂+ H₂O + CO₂; T = 200–310 °C; P = 900–1700 bar).     

A ternary solid solution model for omphacite and its application to geothermobarometry of eclogites from the Middle Adula nappe (Central Alps, Switzerland)

A ternary solid solution model for omphacite with the end-members jadeite (NaAlSi₂O₆), diopside (CaMgSi₂O₆) and hedenbergite (CaFeSi₂O₆) was derived from experimental data from the literature. The subregular solution model, fitted by linear programming, is best suited to omphacites with very little aegirine component in common eclogites. Applying this solution model to the calculation of equilibrium phase diagrams of eclogites from the Adula nappe (Central Alps, Switzerland) results in large stability fields for common eclogite assemblages (garnet+omphacite+quartz+H₂O±kyanite). Within this field the compositions of garnet and omphacite show very little variation. A precise determination of the peak-pressure and temperature is not possible. The occurrence of amphibole, overgrowing the peak-pressure assemblage in fresh eclogite, suggests retrograde re-equilibration, still under eclogite facies conditions. The computation of isopleths for garnet and pyroxene end-members allows the estimation of the pressure and temperature conditions of this re-equilibration event (19–21  kbar, c .  700 °C).