Normative Diopside Research Articles

Petrography, Geochemistry and Petrogenesis of Maru Amphibolite, Northwestern Nigeria

Amphibolite occurrence in part of Maru schist belt, northwestern Nigeria was studied petrographically and geochemically inorder to establish its precursor and infer its geotectonic environment. Maru amphibolite comprises mainly of quartz + biotite + hornblende + epidote + plagioclase feldspar. Significant muscovite ± garnet ± magnetite ± pyrite ± microcline ± normative diopside and hypersthene are present. The grain size is fine to coarse, characterized by poikilitic texture, inequant and anhedral crystals. The protolith was metamorphosed to amphibolite facies conditions. Geochemically, these amphibolites are mainly intermediate to basic, subalkaline and tholeiitic in composition. Maru amphibolite is characterized with low compatible elements (Ni, Cr and Co) which implies the origin of its protolith from partial melting of depleted or metasomatised mantle or mafic crust. Its precursor is dominantly basaltic with some basaltic trachy-andesite. The paleo-tectonic environment is volcanic arc and plate alkaline basalts.Keywords: Amphibolite, Geochemistry, Protolith, Maru

Petrology and geochemistry of granitoids and their mafic micogranular enclaves (MME) in marginal part of the Małopolska Block (S Poland)

AbstractGranitic plutons (the Dolina Będkowska valley and Pilica area) were found in a few boreholes in the Małopolska Block (MB). These granitic rocks may represent apical parts (apophyses) of a great magmatic bodies (batholiths) located in deeper level of the Ediacaran/Paleozoic basement. They are described as ‘stitching intrusions’, generated during/after collision in Carboniferous/Permian period (~300 Ma) between the Upper Silesian Block (USB) and the Małopolska Block (MB).These rocks are fresh, unaltered granodiorites that are pale grey in colour. They have holocrystalline, medium- to coarse-grained structure and massive texture. For the first time, several mafic microgranular enclaves (MME), varying in size and colour, were found in the granodioritic host (HG). The occurrence of MME in the host granodioritic rocks is evidence of a mingling process between mafic and felsic magmas.The MME are pale/dark grey in colour, fine-grained rocks with ‘porphyritic’ textures. They consist of large megacrysts/xenocrysts of plagioclase, quartz, alkali feldspars and the fine-grained groundmass of pseudo-doleritic textures (lath-shaped plagioclases, blade-shaped amphiboles/biotites). According to their modal/mineral composition, they represent Q-diorites and tonalites.The MME, similar to the host granodiorites (HG), are I-type rocks, exhibit high Na2O content >3.2 wt%; normative diopside or normative corundum occurs (mainly <1%). They are metaluminous to slightly peraluminous (ASI <1.1) and have calc-alkaline, medium-K to high-K character. They generally belong to magnesian series (#Mg=0.20-0.40) and have low agpaitic index (<0.87). They are low evolved magmatic rocks. The rocks studied are enriched in LREEs (La, Ce, Sm) compared to HREEs. The Eu* negative anomaly and high Sr contents point to varying degrees of plagioclase fractionation connected to the mixing process rather than simple fractional crystallization. Both rocks studied (HG and MME) are characterized by a high content of LILEs (K, Ba, Rb) in normalized patterns and a low HFS/LIL elements ratio (Ta, Nb)/(K, Rb, La). The projection points of the rocks studied plot in different fields of various petrochemical diagrams: mainly in the arc granites that are rare in the pre-collisional granites as well as the syn-subductional to post-collisional granites fields.For the first time, inner textures in rock-forming minerals related to mixing processes are described both in the granodioritic host (HG) and in the MME. Mantled boxy cellular plagioclase megacrysts with ‘old cores’ of labradorite composition, and amphibole aggregates with titanite and opaque minerals, represent peritectic rather than primary residual minerals. The plagioclase, quartz and alkali feldspar megacrysts/xenocrysts were mechanically transferred from the granodioritic host (HG) to MME. The presence of lath-shaped plagioclases, blade-shaped amphiboles/biotites and acicular-shaped apatites in the groundmass of the MME is evidence of undercooling of hot mafic blobs in a relatively cold granodioritic magma chamber. The MME were hybridized by leucocratic melt squeezed from the granodioritic magma in a later stage of the mixing process (quartz and alkali crystals in the interstices in the MME groundmass). In the granodiorites (HG), the spike and spongy cellular zones as well as biotite/amphibole zones in plagioclase megacrysts are connected to the mixing process.Both of the rocks studied are characterized by different amounts of major elements (SiO2, Na2O and K2O), trace elements (Ni, Cr, V, Ti and P), #Mg and modified alkali-lime index (MALI) that is related to their origins from different sources. On the other hand, they have similar chondrite-normalized patterns (for trace elements and REE), LILEs contents (Sr, Ba, Rb), aluminum saturation index (ASI) and isotopic signatures (high 86Sr/87Sr (0.079-0.713) and low 143Nd/144Nd (0.512) values but lower than in continental crust), which are evidence of the strong hybridisation of mafic enclaves by the granodioritic host magma. The parental rocks of both rocks studied have a similar mafic signature but were generated in different sources: the host granodiorites (HG) magma in lower continental crust rocks, and the MME magma in enriched upper mantle. The MME crystallized from strongly hybridized magma of intermediate compositions (Q-diorite, tonalite) rather than from primary mafic magma. The host granodiorites (HG) originated from completely homogenized crustal granodioritic magma which inherited its geochemical signature from ancient arc-rocks in a subduction-related setting

Petrography, geochemistry and regional significance of crystalline klippen in the Garhwal Lesser Himalaya, India

Uphalda gneisses (UG) is a crystalline klippe located near Srinagar in Garhwal Himalaya. These gneisses are compared with Debguru porphyroids (DP) (≈Ramgarh group) of Garhwal–Kumaun Himalaya and Baragaon mylonitic gneisses (BMG) of Himachal Himalaya. Petrographic study reveals that the deformation of UG was initiated at higher temperature (above 350°C) and continued till lowering of temperature and deformation led to the mylonitization. Geochemically, these granitic gneisses (UG, DP and BMG) exhibit similar composition. Features such as high molecular A/CNK value (>1), presence of normative corundum and absence of normative diopside, enhanced Rb/Sr, Rb/Zr ratios, enrichment of Th and containing rounded zircons support their crustally-derived S-type granitic nature. The linear plot in major oxides is interpreted in terms of fractional crystallization processes. Mantle normalized multi-element spider diagram of UG illustrates depletion of Ba, Nb, Sr, P and Ti and enrichment of Th and show similarities with DP and BMG. Similarities were observed in lithology, petrographic characters and chemical composition of UG, DP, BMG and Ulleri augen gneisses (Nepal). Comparison with the rocks of Higher Himalayan crystallines (≈Vaikrita), suggests that these rocks (UG) are not transported from Higher Himalaya as understood earlier. This study however proposes that, these gneissic bodies represent an older basement occurring as a tectonic sliver which emplaced within the cover sequence as wedges at different structural levels. This is a regional phenomena observed throughout the Lesser Himalayan region.

Geochemistry and petrogenesis of granitoids of Lesser Himalayan crystallines, Western Arunachal Himalaya

Low to medium grade crystalline rocks locally known as Bomdila Group extensively covers the Lesser Himalayan region in Western Arunachal Himalaya. This Group consists dominantly of mylonitic gneisses of granitic composition of Palaeoproterozoic age, named as Bomdila mylonitic gneiss (BMG) and a small body of hornblende bearing granite of Mesoproterozoic age known as Salari granite (SG). The BMG is affinity to peraluminous (A/CNK > 1.1) with high content of SiO2, K2O/Na2O ratio, normative corundum, high ratio of FeOt/MgO in biotite (3.21–5.11) that shows characteristics of S-type granite whereas SG has granodiorite composition with high Na2O, low K2O, presence of hornblende, normative diopside, low A/CNK ratio (<1.1) and low FeOt/MgO ratio in biotite (1.58–1.60) indicates metaluminous I-type granite affinity. The SG has more fractionated nature of REE [(Ce/Yb)N = 9.06–18.53] and minor negative Eu anomalies [EuN/Eu* = 0.69–0.94] as compared to BMG which has less fractionation of REE [(Ce/Yb)N = 5.95–9.16] and strong negative Eu anomalies [EuN/Eu* = 0.37–0.43]. Geochemical and petrological studies suggest that the SG and BMG are not genetically related; SG appears to have derived from igneous source whereas the BMG have been derived from sedimentary source, however these granitoids might have produced during the same thermal event.

The Composition of Near-solidus Partial Melts of Fertile Peridotite at 1 and 1·5 GPa: Implications for the Petrogenesis of MORB

We have determined the near-solidus melt compositions for peridotite MM-3, a suitable composition for the production of mid-ocean ridge basalt (MORB) by decompression partial melting, at 1 and 1·5 GPa. At 1 GPa the MM-3 composition has a subsolidus plagioclase-bearing spinel lherzolite assemblage, and a solidus at ∼ 1270°C. At only ∼ 5°C above the solidus, 4% melt is present as a result of almost complete melting of plagioclase. This melting behaviour in plagioclase lherzolite is predicted from simple systems and previous experimental work. The persistence of plagioclase to > 0·8 GPa is strongly dependent on bulk-rock CaO/Na2O and normative plagioclase content in the peridotite. At 1·5 GPa the MM-3 composition has a subsolidus spinel lherzolite assemblage, and a solidus at ∼ 1350°C. We have determined a near-solidus melt composition at ∼ 2% melting within 10°C of the solidus. Near-solidus melts at both 1 and 1·5 GPa are nepheline normative, and have low normative diopside contents; also they have the highest TiO2, Al2O3 and Na2O, and the lowest FeO and Cr2O3 contents compared with higher degree partial melts. Comparison of these near-solidus melts with primitive MORB glasses, which lie in the olivine-only field of crystallization at low pressure, indicate that petrogenetic models involving aggregation of near-fractional melts formed during melting at pressures of 1·5 GPa or less are unlikely to be correct. In this study we use an experimental approach that utilizes sintered oxide mix starting materials and peridotite reaction experiments. We also examine some recent studies using an alternative approach of melt migration into, and entrapment within ‘melt traps’ (olivine, diamond, vitreous carbon) and discuss optimal procedures for this method.

The Effect of Cr2O3 on the Partial Melting of Spinel Lherzolite in the System CaO-MgO-Al2O3-SiO2-Cr2O3 at 1{middle dot}1 GPa

Chromium as Cr3þ substitutes for octahedrally coordinated Al in upper-mantle minerals, thereby reducing the activity of Al2O3 in the system and hence the concentration of Al2O3 in partial melts. The effect of Cr2O3 on melt compositions multiply saturated with the spinel lherzolite phase assemblage has been quantified in the system CaO–MgO–Al2O3–SiO2–Cr2O3 at 1 1 GPa as a function of 100 Cr/(Cr þ Al) in the spinel (Cr#sp). The decrease of Al2O3 in the melt with increasing Cr#sp is accompanied by increasing MgO and SiO2, whereas CaO remains almost constant. Consequently, the CaO/Al2O3 ratio of the melt increases with Cr#sp, and the melt becomes richer in normative diopside, hypersthene and quartz. The effect may explain certain mantle melts with unusually high CaO/Al2O3 ratios. The concentration of Cr2O3 in the melt remains low even at high Cr#sp, which means that the strong effect of Cr2O3 on partial melting equilibria is not readily apparent from its concentration in the melt itself. The existence of a highly refractory major component such as Cr2O3 nullifies simplified conclusions from the ‘inverse approach’ in the experimental study of basalt petrogenesis, as there is insufficient information in the composition of the partial melt to reconstruct the conditions of melting.

Age and emplacement of late Sveconorwegian monzogabbroic dykes, SW Sweden

The monzogabbroic (jotunitic) Göteborg dykes in SW Sweden display igneous mineralogy and texture, with chilled margins that crosscut all ductile structures in the wall rock. Morphological features suggest emplacement into brittle crust along a set of pre-existing WNW- to W-trending fractures. U–Pb dating of baddeleyites from the largest of these dykes, the Tuve dyke, yields a late Sveconorwegian (Grenvillian) age of 935±3 Ma, which is considered as the crystallisation age of the dyke swarm. The Göteborg dykes and the Hakefjorden Norite–Anorthosite Complex on the Swedish west coast show many similarities with the jotunitic rocks in the Rogaland Anorthosite Complex of south-western Norway, in terms of age, major element composition and Fe–Ti oxide concentrations. The Blekinge–Dalarna dyke swarm east of and along the Protogine Zone in south-eastern Sweden, and the Hunnedalen dykes in south-western Norway, also record the characteristic high Ti–P–K, evolved “jotunitic” character. A distinct feature of all these late Sveconorwegian mafic rocks is the low content of normative diopside (low Ca), but in other aspects they resemble modern continental flood basalts. Ages around 930 Ma for many late Sveconorwegian intrusions correspond in time with the latest phase of extensional movements along crustal-scale shear zones, suggesting that this magmatism is related to extensional collapse and exhumation of the Sveconorwegian orogen.

Experimental tests of low degree peridotite partial melt compositions: implications for the nature of anhydrous near-solidus peridotite melts at 1 GPa

We present results of an experimental study to determine the nature of minimum to near-minimum melt compositions in equilibrium with upper mantle peridotite mineralogy at 1 GPa. We confirm earlier conclusions that anhydrous melts of lherzolite at 1 GPa are basaltic with ∼15–20% normative diopside, >10% normative olivine and at low degrees of melting are Na 2O and K 2O-rich and nepheline-normative in `fertile' mantle. The most extreme Na 2O-rich minimum melt composition is in equilibrium with an albite-bearing harzburgite residue at 1220°C. This melt composition is nepheline-normative with ∼64% SiO 2 and about ∼12% Na 2O. Our results disagree with recent reports [1, 2]that peridotitic minimum melt compositions have an `andesitic' character at 1 GPa. We present reversal experiments showing that these latter melts are not in equilibrium with a spinel or plagioclase lherzolite upper mantle assemblage. We use our new data and data from the literature to define minimum melts (i.e. melts in equilibrium with olivine+orthopyroxene+clinopyroxene+plagioclase±spinel) for fertile or enriched to refractory lherzolite at 1 GPa. The minimum melt compositions are nepheline+olivine-normative for sodium-rich sources and hypersthene+olivine-normative for refractory or depleted compositions with very calcic plagioclase or high Ca/Ca+Na ratios in spinel lherzolite. It is not possible to derive quartz-normative basaltic or `andesitic' melt compositions by partial melting of anhydrous lherzolite at 1 GPa.

The Bjerkreim-Sokndal Layered Intrusion, Rogaland, SW Norway: Evidence from marginal rocks for a jotunite parent magma

The Late-Proterozoic Bjerkreim-Sokndal Layered Intrusion (BKSK) consists of andesine anorthosite, leuconorite, troctolite, norite, gabbronorite, jotunite, mangerite, quartz mangerite and charnockite. The sequence of appearance of cumulus minerals and their compositions suggest a parent magma that was evolved, had plagioclase (±olivine) on the liquidus, was sufficiently TiO 2-rich for hemo-ilmenite to crystallise early, and low in CaO and CaO Al 2O 3 compared to basalts as reflected by the sodic plagioclases and the delayed appearance of cumulus augite. Fine- to medium-grained jotunites found along the northern contact of the BKSK consist of plagioclase (An 45–53), inverted pigeonite (Mg# = 55-50), sparse augite (Mg# = 69-59), Fe-Ti oxides, K-feldspar, quartz and apatite. They are basic to intermediate rocks with relatively high FeO total, high TiO 2, low MgO/MgO + FeO, moderate Al 2O 3 and low CaO and normative diopside. The jotunites have compositions that are consistent with the parental magma for the lower part of the BKSK Layered Series, and are interpreted as being marginal chills. Similar, but slightly more differentiated, jotunite magmas were subsequently emplaced into the BKSK and the surrounding region as broad dykes and small plutons. Jotunite is a minor rock type in most massif-type anorthosite provinces but may have an important petrological significance.

Quests for low-degree mantle melts

Baker et al . report an experimental study at pressures of 1 GPa (1,000 million Pa) to determine the composition of a silicate melt produced at anhydrous, near-solidus conditions from a lherzolite composition. They obtained a composition at 1,250 °C with -57% Si0 2 and -5.7% Na 2 0, which is 'andesitic' in the sense of its low normative diopside (5% ), high normative hypersthene (> 15%) and high normative plagioclase (> 75%, andesine). These results conflict with earlier conclusions that anhydrous melts of fertile lherzolite at 1 GPa are basaltic, with ~15-20% normative diopside, > 10% normative olivine and, at low degrees of melting, nephelinenormative. From our own experience and experimental tests, we suggest the following possible explanations for the observations of Baker et al .

Experimental constraints on the low-pressure evolution of transitional and mildly alkalic basalts: the effect of Fe-Ti oxide minerals and the origin of basaltic andesites

One-atmosphere, melting experiments, controlled at the fayalite-magnetite-quartz oxygen buffer, on mildly alkalic and transitional basalts from Iceland show that these begin to crystallize Fe-Ti oxide minerals (magnetite and/or ilmenite) at 1105±5°C, apparently independently of bulk composition and the order of silicate and oxide mineral crystallization. Most samples crystalline plagioclase and olivine as the first two crystalline phases, augite as the third phase, and an Fe-Ti oxide mineral as the fourth phase. The main effects of Fe-Ti oxide crystallization are a marked decrease in FeO and TiO2 in the liquid, and a notable increase in SiO2 and Al2O3, and the minor oxides K2O and P2O5, with decreasing temperature. The most silicic glasses are compositionally mugearitic and shoshonitic basaltic andesites. Because the smallest amount of glass that could be analyzed with the microprobe represents 20–55 percent liquid remaining, it can be expected that more silicic liquids will occur at lower temperatures. On normative, pseudoternary projections, the general effect of Fe-Ti oxide crystallization for mildly alkalic and transitional basalts is a marked increase in normative quartz. This is caused by a strong systematic convergence, with the appearance of Fe-Ti oxides, of the bulk solid precipitates toward the liquid compositions, as projected on the triangle plagioclase-diopside-olivine. For alkalic basalts, the bulk solid precipitate shows an increase in normative diopside with falling temperature and Fe-Ti oxide crystallization. This causes the liquids to move toward decreasing normative diopside and relatively little variation in nepheline. The experimental observations imply that mildly alkalic and transitional magmas, without stabilizing a Fe-Ti oxide mineral, will not evolve toward early silica saturation.

Petrology and geochemistry of granitic intrusive rocks from the western part of the Oban Massif, southeastern Nigeria

The Oban Massif forms part of the Precambrian to Early Paleozoic Nigerian basement complex and predominantly consists of metasediments, granodiorites, granites with minor dolerites and microgranitic intrusions. The granitic rocks are perluminous and exhibit an apparent trend of decreasing normative diopside and increasing normative corundum with increasing SiO 2. Major and trace element compositions of the rocks show a typical calc-alkaline differentiation trend. The metasedimentary rocks have chemical compositions similar to those reported from other parts of Nigeria. High K 2O contents and numerous K-feldspar porphyroblasts relate the granitic rocks to widespread basement rejuvenation during the Pan-African thermotectonic event and imply an origin by partial melting of the lower crust at presumably different structural levels.

Fractional crystallization of the island arc tholeiitic magma in Hotaka volcano.

A liquid line of descent is well demonstrated in the Hotaka volcano. Mineral assemblage and chemical composition of phenocrysts change systematically with the change of the chemical composition of the groundmass (magmatic liquid). Crystallization sequence with increasing SiO2 content of the groundmass is as follows. Olivine, hypersthene, augite, plagioclase and magnetite coexisting in the liquid whose SiO2 content ranges from 53.5 to 57 wt%. In liquids with more than 57 wt% SiO2 content, olivine disappears. Augite, hypersthene, magnetite and plagioclase continuously crystallize until the SiO2 content of liquid attains at least 70 wt%. The amount of the crystallized minerals during fractional crystallization is estimated to be 46 wt% (plagioclase, 22; hypersthene, 6; augite, 11; magnetite, 7), while the SiO2 content of liquid changes from 57 to 70 wt%. Plagioclase accumulation during fractional crystallization is inferred from the difference between the estimated ratio of plagioclase to other phenocrysts and the observed ratios in the modal compositions of phenocrysts. The assemblage of groundmass pyroxenes changes with increasing SiO2 content of the groundmass in the following order: augite+pigeonite→pigeonite→pigeonite+hypersthene→hypersthene. This change reflects the decrease of the normative diopside content of the liquid. The chemical and mineralogical features of fractional crystallization is close to that of other island arc tholeiites in Japan, though the normative diopside content of the rocks of the Hotaka volcano in slightly lower than that of the others and groundmass hypersthene occurs in the rocks with more than 65 wt% SiO2.

Mineral chemistry and genetic relations among H-group chondrites

The chondrites Bremervorde and Tieschitz (both H3), Beaver Creek, Menow, Monroe and Quenggouk (all H4), Allegan and Ambapur Nagla (both H5), and Butsura and Kernouve (both H6) were studied. Ca-poor pyroxenes almost invariably show enrichment in Mg relative to coexisting olivines; this was established in Tieschitz chondrules and in Bremervorde by crystal-liquid equilibria. In each chondrite, Ca-poor pyroxenes typically have less than 2 mol % wollastonite. In petrologic types 3 and 4, chondrule mesostases frequently show enrichment in normative diopside, which occurs as a resolvable modal phase in the type 6 chondrites. Partition of Ca between the two pyroxenes was initiated by crystal-liquid fractionation. Various chemical inhomogeneities may be traced from type 3 meteorites to those of type 6. Polycrystalline taenite occurs in Tieschitz, Bremervörde, Menow, Quenggouk and Butsura; it is rarely present in Kernouve. This textural form of taenite is interpreted as a relict solidification structure produced from a quenched liquid, which must have cooled rapidly to below about 700 °C. We argue that equilibration of the H-group chondrites took place during cooling, mainly from about 700 °C, which is compatible with several lines of evidence. If metamorphism is defined as change induced by an increase in temperature, then in the H-group chondrites studied we recognize it only in transient reheating probably produced by shock.

The solubility of olivine in basaltic liquids: an ionic model

A simple ionic model which describes the solution of the forsterite component of olivine in silicate liquids is reported. The melting relation is represented: ( Mg 2 SiO 4) ol = 2( Mg 2+) L + ( SiO 4 4− L and is extended to all silicate liquids by normalizing their compositions to 4 oxygens. At 1 bar, the temperature at which olivine is in equilibrium with any alkali-depleted basaltic composition can be calculated to within ±30° C. This error is increased considerably when applied to terrestrial basalts which contain several weight percent alkalis. Alkalis interfere with the equilibrium by generating strongly repulsive interionic forces which can be crudely modelled in a manner consistent with constraints imposed by regular solution theory. The model quantifies the reduced activity of SiO 4 4− monomers due to increasing SiO 2 concentrations in the melt. This is a consequence of polymerization which does not appear to operate gradually over the entire spectrum of mafic and ultramafic compositions. The coordination of alumina in melts which precipitate olivine only appears to be dominantly octahedral. Titanium acts as a polymerizing agent by interconnecting previously isolated SiO 4 4− monomers. Calcium associated with normative diopside tends to exhibit small but perceptible repulsive forces involving Mg 2+.

Evolution of basaltic and differentiated lavas from Anjouan, Comores Archipelago

Anjouan is one of four volcanic islands comprising the Comores Archipelago. Three (arbitrarily defined) categories of basic magma are recognised on Anjouan: ‘hypersthenenormative’, ‘alkalic’ and ‘basanitic’, which appear in that order with an eruptive sequence involving 1) shield construction, 2) peripheral fissure-controlled activity, and 3) rejuvenescent (posterosional) eruptions. Differentiated magmas have evolved within the three chemical groupings, and trend mainly towards undersaturated trachyte and phonolite. These trends are considered to have developed by initial removal of olivine and clinopyroxene, followed by Fe-Ti oxides, apatite and amphibole from arrested liquid pools within and beneath the volcano. The appearance of feldspar on the liquidus was clearly inhibited by the high contents of normative diopside in most Anjouan magmas, although late stage plagioclase fractionation is probably responsible for development of peralkaline phonolites at shallow depths, assisted eventually by alkali feldspar. Lherzolite-xenolith-bearing lavas are likely to be directly mantle-derived liquids. Three analysed representatives with 100 · Mg/Mg + Fe2+ (atomic) ratios approaching 70 are characteristically rich in normative diopside and may confirm the suggestion of primitive ankaramitic melts in mantle regions. High pressure fractional crystallisation may involve fractionation of ‘eclogite’, orthopyroxene or clinopyroxene, with or without olivine. It is believed however that such processes do not adequately explain the compositional change from hypersthene-normative basalt towards basanite, as they imply unacceptable degrees of Fe-enrichment. Problems of large ion element enrichment and fractionation would also remain, while the high contents of Cr and Ni place limits on the extent of crystal fractionation. A partial melting model of magma genesis is able to explain the main features of basalt geochemistry, especially if the upper mantle low velocity zone is regarded as having been severely fractionated due to upward migration of large ion elements. An ‘open’ system of magma production in the context of regional plate movement, and the implied decoupling between lithosphere and asthenosphere, accomodates realistic degrees of partial melting and allows a greater potential volume of mantle available for melting than possible ‘closed’ systems. The model also accounts for the migratory pattern of Comores volcanism.

Eclogites from Southwestern Oregon

Research Article| August 01, 1973 Eclogites from Southwestern Oregon EDWARD D. GHENT; EDWARD D. GHENT 1Department of Geology, University of Calgary, Calgary 44, Alberta, Canada Search for other works by this author on: GSW Google Scholar ROBERT G. COLEMAN ROBERT G. COLEMAN 2U.S. Geological Survey, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information EDWARD D. GHENT 1Department of Geology, University of Calgary, Calgary 44, Alberta, Canada ROBERT G. COLEMAN 2U.S. Geological Survey, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1973) 84 (8): 2471–2488. https://doi.org/10.1130/0016-7606(1973)84<2471:EFSO>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation EDWARD D. GHENT, ROBERT G. COLEMAN; Eclogites from Southwestern Oregon. GSA Bulletin 1973;; 84 (8): 2471–2488. doi: https://doi.org/10.1130/0016-7606(1973)84<2471:EFSO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Eclogite, high-grade blueschist, and amphibolite blocks occur within the Mesozoic Otter Point Formation of southwestern Oregon and are inferred to have been tectonically emplaced by eastward-directed overthrusting involving Colebrooke Schist and serpentinite.Eclogite from southwestern Oregon is very similar in bulk chemistry and mineralogy to the well-studied eclogite of California.Calculations of phase equilibria at load pressures of 7 and 10 kb and T = 400°C to 550°C suggest that many of the hydrates found in eclogite could have been stable at very low H2O fugacities. The lack of lawsonite and the presence of almandine-grossular garnet set a maximum limit on H2O fugacity for a given Ps-T.Chemically, Group C eclogite from Oregon and California characteristically is nepheline normative and is enriched in normative diopside relative to basaltic compositions. The present chemistry of this eclogite may be the result of metasomatism in an ultramafic environment with low a SiO2 and high a Ca, but outside the stability field of serpentine.The generally high jadeite content of clinopyroxene from Group C eclogite compared with Group A and Group B eclogite is largely a function of bulk rock chemistry. Crystallization under low a SiO2 conditions stabilizes jadeite in clinopyroxene at lower Ps for a given T. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.