scholarly journals Les Roches Basiques Du Bas-Limousin (Massif Central Français) : Geochimie Et Implications Geodynamiques

2016 ◽  
Vol 12 (35) ◽  
pp. 382
Author(s):  
Jamil Ezzayani ◽  
Hassan El Hadi ◽  
Said Chakiri ◽  
Wafae Nouaim ◽  
Mohamed Allouza ◽  
...  
Keyword(s):  
Rare Earths ◽  
Regional Metamorphism ◽  
Igneous Rocks ◽  
Geochemical Data ◽  
Massif Central ◽  
Geochemical Study ◽  
Basic Rocks

The upper unit gneisses represent most of the Bas - Limousin. It is characterized by the intercalation of basic igneous rocks (metagabbro- and metadolerite). These are affected by regional metamorphism but primary textures are preserved. The aim of this work is to present new geochemical data, which allow characterizing the geochemical nature of these rocks and discussing their geodynamic meaning. Chemical analyzes of the major and traces elements were carried out at “Service commun des sciences de la terre de l’ Université de Nancy I”, rare earths elements were analyzed at CRPG of Nancy. The results of this geochemical study show that these basic rocks are comparable to continental tholeiitic rocks.

scholarly journals Regional geochemical variations in a metamorphosed black shale: a reconnaissance study of the Silurian Smalls Falls Formation, Maine, USA

2020 ◽  
Vol 56 ◽  
pp. 231-255
Author(s):  
John F. Slack ◽  
M.R. Van Baalen ◽  
Douglas N. Reusch
Keyword(s):  
Organic Matter ◽  
Black Shale ◽  
Nutrient Loading ◽  
Regional Metamorphism ◽  
Enrichment Factors ◽  
Geochemical Data ◽  
The Black Sea ◽  
Three Samples ◽  
Geochemical Study ◽  
Geochemical Variations

A reconnaissance geochemical study of 21 samples of sulphidic black phyllite and schist from the Silurian Smalls Falls Formation in Maine was undertaken in order to evaluate compositional changes during regional metamorphism. These samples represent variably metamorphosed black shale. Analyzed samples come from the chlorite zone in northern Maine and the biotite, garnet, and staurolite-andalusite zones in west-central Maine. Strata of the Smalls Falls Formation are distinctive in containing abundant pyrite and/or pyrrhotite (total S = 1.2–9.7 wt%), but only minor organic matter or graphite (TOC = 0.43–1.85 wt%); TOC/S ratios are uniformly low (average = 0.37 ± 0.22). Median enrichment factors were calculated for each element by normalizing the concentration to Ti in each sample to the Ti-normalized median composition of global black shale. In the chlorite zone, moderate to large decreases in enrichment factors (-23.1 to -49.8%) are evident for V, Cr, Cu, Ni, Zn, Pb, Sb, and U, attributed here to various factors during sedimentation plus variable element mobility during diagenesis. With increasing metamorphic grade (biotite through staurolite-andalusite zones), systematic small to extreme decreases (-14.5 to -99.0%) were found for Ba, Sb, Au, and U, together with less-systematic moderate to large decreases (-35.4 to -61.1%) for V and As. Molybdenum shows an extreme decrease (-94.7%) from the garnet to staurolite-andalusite zones. Excluding Ba, these results are interpreted to mainly reflect mobility of trace elements during pyrite recrystallization, and during the metamorphic transformations of organic matter to graphite and of pyrite to pyrrhotite. Moderate to large increases for Rb (+28.1 to +61.5%) and Th (+39.1 to +47.3%) from the biotite to staurolite-andalusite zones likely record the introduction of alkalis and mass loss, respectively, during metamorphism. Three samples from one site in the garnet zone differ in having anomalously high Fe/Al and low La/Yb ratios, attributed here to epigenetic formation of pyrite and related leaching of light rare earth elements during syn-metamorphic, channelized fluid flow.Geologic and geochemical data indicate that strata of the Smalls Falls Formation were deposited during an interval of anoxia on the northwestern flank of the Central Maine Basin, for which detrital sources included an evolved continental arc. Onset of anoxia coincided with deposition of the Mayflower Hill Formation of the Vassalboro Group, on the basin’s southeastern flank, related to emergence of the Brunswick subduction complex. We suggest that this emergence played a role in promoting both lateral and vertical circulation changes, nutrient loading, and deoxygenation through subsequent basin closure that culminated with Acadian deformation and metamorphism. Based on the relatively high contents of total sulphur present in our Smalls Falls samples, sediments in the Black Sea represent the only known plausible candidate among those in modern suboxic to euxinic basins.

Geochemistry of bimodal amphibolitic—felsic gneiss complexes from eastern Massif Central, France

1995 ◽  
Vol 132 (3) ◽  
pp. 321-337 ◽  
Author(s):  
Bernard Briand ◽  
Jean-Luc Bouchardon ◽  
Houssa Ouali ◽  
Michel Piboule ◽  
Paul Capiez
Keyword(s):  
Crustal Contamination ◽  
Igneous Rocks ◽  
Chemical Characteristics ◽  
Continental Margins ◽  
High Grade ◽  
Transitional Stage ◽  
Crust Formation ◽  
Massif Central ◽  
Mantle Sources ◽  
Two Populations

AbstractHigh-grade basic and acidic meta-igneous rocks are widespread in the bimodal amphibolitic—felsic gneiss complexes, which are characteristic formations of the ‘Middle Allochthonous Unit’ from eastern and southern French Massif Central. The metabasites from the Lyonnais and Doux complexes are chemically diverse and range from N-MORB type tholeiitic to transitional types. The two populations are not related by fractional crystallization or crustal contamination processes and their chemical characteristics reflect differences in their mantle sources. An ensialic setting is supported by the crustally-derived character of some of the associated felsic rocks, but the presence of N-MORB-type metabasites argues for an extensional environment. This bimodal association compares well with the magmatism of rifted continental margins and may reflect a transitional stage between continental rifting and oceanic crust formation during the Cambro-Ordovician spreading event.

Geochronology and geochemistry of igneous rocks in the southeastern Lesser Xing’an Range, northeastern China: petrogenesis and implications for the early Mesozoic tectonic evolution

2020 ◽  
Vol 57 (4) ◽  
pp. 506-523
Author(s):  
Jin-hua Qin ◽  
Cui Liu ◽  
Jin-fu Deng
Keyword(s):  
Field Strength ◽  
Inductively Coupled Plasma ◽  
High Field ◽  
Magma Mixing ◽  
Tectonic Setting ◽  
Alkali Feldspar ◽  
Igneous Rocks ◽  
Geochemical Data ◽  
High Field Strength ◽  
Alkaline Granite

We present systematic U–Pb age data collected by laser ablation multi-collector inductively coupled plasma mass spectrometry, precise geochemical data, and Nd isotope data for igneous rocks from the southeastern Lesser Xing’an Range (SE LXR). The results indicate that the formation ages as follows: Maojiatun alkaline granite, 207.2 ± 0.84 Ma and 204.6 ± 0.93 Ma; Diorite porphyrite, 164.5 ± 0.97 Ma; and Tieli syenogranite, 186.7 ± 1.50 Ma. The alkaline granite has high silicon, potassium, alkali, and FeOT contents; it is enriched in high field strength elements, Zr, Hf, Th, Rb, and U; is depleted in Ba, Sr, Nb, Ta, P, Ti, etc.; and has high ratios of 10000Ga/Al. It shows an A2-type granite affinity. The Tieli alkali-feldspar granite has high total alkali contents and is enriched in high field strength elements and rare earth elements and depleted in Sr, Ba, Ti, and P, and shows varying degrees of alkalinity. Rocks from SE LXR display similar εNd (t) values with corresponding to Nd model ages of 1095 to 813 Ma. The igneous rocks from the SE LXR are proposed to be derived from melting of the Neoproterozoic lower crust and potential magma mixing with ancient crystalline basement. The formation of the Maojiatun alkaline granite occurred in response to a postorogenic event following the closure of the Paleo-Asian Ocean. However, the SE LXR exhibited an extensional back-arc tectonic setting in the Early Jurassic. The Middle Jurassic diorite porphyrite could be related to the temporary stagnation of the westward subduction of the Paleo-Pacific plate.

Some aspects of Tertiary acid magmatism in NE Ireland

1984 ◽  
Vol 48 (348) ◽  
pp. 351-363 ◽  
Author(s):  
I. G. Meighan ◽  
D. Gibson ◽  
D. N. Hood
Keyword(s):  
Northern Ireland ◽  
Central Complex ◽  
Geochemical Data ◽  
Igneous Province ◽  
Granitic Intrusions ◽  
Isotope Evidence ◽  
County Antrim ◽  
Geochemical Studies ◽  
Basic Rocks ◽  
Highly Evolved

Abstract Geochemical data (including REE determinations) are presented for all five Mourne Mountains granites and three Northern Ireland rhyolites. These confirm (1) the extremely fractionated nature of some of the rocks (Sr and Ba < 10 ppm, Rb > 400 ppm, Eu/Eu* < 0.1, and K/Rb < 100), and (2) a major revision to the outcrops of the E. Mourne granites G1 and G2 in which much of the former is reclassified as G2. Combined petrographic and geochemical studies have also indicated that magmatic pulses were involved in the emplacement of Mourne intrusions G2 (Revised)-G5 inclusive. The N. Ireland Tertiary acid rocks exhibit general geochemical similarities to their analogues elsewhere in the British Tertiary Igneous Province (in which Sr is generally < 100 ppm and CeN/YbN generally < 8 with Eu/Eu* often < 0.6), but as a suite the Mourne granites are enriched in Rb and some other LIL elements relative to their N. Arran counterparts. The more fractionated acid magmas of NE Ireland are believed to have evolved from primitive granitic parent liquids by crystal fractionation at depth which involved major and accessory phases (including zircon and allanite). In the Mourne (and County Antrim) areas the primitive acid compositions lie at the ends of basaltic (tholeiitic) differentiation series, and in the Mourne central complex there is a complete geochemical sequence from basic rocks through intermediate members to primitive and ultimately highly evolved, subalkaline, granitic intrusions. It is concluded that the data are consistent with the Mourne granites and Northern Ireland rhyolites being essentially basaltic differentiates, although Sr isotope evidence indicates some (probably minor) crustal involvement.

Origin of corundum within anorthite megacrysts from anorthositic amphibolites, Granulite Terrane, Southern India

2020 ◽  
Vol 105 (8) ◽  
pp. 1161-1174
Author(s):  
Shreya Karmakar ◽  
Subham Mukherjee ◽  
Upama Dutta
Keyword(s):  
High Pressure ◽  
Grain Boundaries ◽  
Chemical Potential ◽  
Regional Metamorphism ◽  
Phase Relations ◽  
Southern India ◽  
Triple Junctions ◽  
Medium Temperature ◽  
Hp Metamorphism ◽  
Basic Rocks

Abstract Growth of corundum in metamorphosed anorthosites and related basic-ultra-basic rocks is an exceptional feature, and its origin remains elusive. We describe the occurrence of and offer an explanation for the genesis of corundum in anorthositic amphibolites from ~2.5 Ga old basement of the Granulite Terrane of Southern India (GTSI). The studied amphibolites from two localities, Manavadi (MvAm) and Ayyarmalai (AyAm), contain anorthite lenses (An90–99) with euhedral to elliptical outline set in a finer-grained matrix of calcic plagioclase (An85–90) and aluminous amphibole (pargasite-magnesiohastingsite). The lenses, interpreted as primary magmatic megacrysts, and the matrix are both recrystallized under static condition presumably during the regional high pressure (HP) metamorphism (~800 °C, 8–11 kbar) at ~2.45 Ga. Corundum occurs in the core of some of the recrystallized anorthite lenses (An95–99) in two modes: (1) Dominantly, it forms aggregates with magnetite (with rare inclusion of hercynite; in MvAm) or spinel (and occasionally hematite-ilmenite; in AyAm). The aggregates cut across the polygonal grain boundaries of the anorthite and contain inclusions of anorthite. (2) Corundum also occurs along the grain boundaries or at the triple junctions of the polygonal anorthite grains, where it forms euhedral tabular grains, sieved with inclusions of anorthite or forms skeletal rims around the recrystallized anorthite, such that it seems to be intergrown with anorthite. Combined petrological data and computed phase relations are consistent with growth of corundum in an open system during regional metamorphism in the presence of intergranular fluids. Two mechanisms are proposed to explain the formation of the corundum in the amphibolites: (1) corundum + magnetite/spinel aggregates formed dominantly by oxy-exsolution of pre-existing Al-Fe-Mg-(Ti)-spinel. This pre-existing spinel may be primary magmatic inclusions within the anorthite phenocrysts or could have formed due to reaction of primary magmatic inclusions of olivine with the host anorthite. Pseudosections of fO2-nH2O-T-P in the CaO–FeO–MgO–Al2O3–SiO2–H2O (CFMASH) system indicate that fO2 and H2O strongly influence the formation of corundum + amphibole from the initial magmatic assemblage of anorthite (phenocrysts) + spinel ± olivine (inclusions). (2) The corundum with anorthite presumably formed through desilification and decalcification of anorthite, as is indicated by computed phase relations in isobaric-isothermal chemical potential diagrams (µSiO2-µCaO) in parts of the CASH system. Growth of corundum in this mode is augmented by high activity of anorthite in plagioclase, high pressure, and low-to-medium temperature of metamorphism. This study thus presents a new viable mechanism for the origin of corundum in anorthositic amphibolites, and basic-ultra-basic rocks in general, which should provide new insight into lower crustal processes like high-pressure metamorphism.

Les eclogites et roches associees des massifs basiques de Saint-Joseph (monts du Lyonnais, Massif central francais)

1971 ◽  
Vol S7-XIII (3-4) ◽  
pp. 283-291 ◽  
Author(s):  
Denis Coffrant ◽  
Michel Piboule
Keyword(s):  
Regional Metamorphism ◽  
Massif Central

Abstract Laccolithic massifs differentiated to gabbros and serpentinites, regional metamorphism (P-T conditions), plagioclase pyroxenite with omphacite, no garnet coronite

The Genesis of Oligoclase in Certain Schists

1933 ◽  
Vol 70 (12) ◽  
pp. 529-541 ◽  
Author(s):  
F. J. Turner
Keyword(s):  
Chemical Composition ◽  
Bulk Composition ◽  
Regional Metamorphism ◽  
Slight Modification ◽  
Igneous Rocks ◽  
Calcic Plagioclase ◽  
Irregular Distribution ◽  
Thermal Metamorphism

The mineralogical changes in green schists and related quartzofelspathic schists of sedimentary origin are discussed, and the following conclusions are reached as to the conditions of formation of oligoclase in these rocks:—(1) Oligoclase normally appears as a product of dynamothermal metamorphism at relatively high grades such as prevail in the zones of almandine and perhaps kyanite. It is accompanied either by deeply-coloured hornblende, hornblende and biotite, or biotite and muscovite, according to the chemical composition of the rocks in which it occurs.(2) Sodic oligoclase containing 10 per cent to 15 per cent of anorthite may occur with pale aluminous hornblende in green schists lying within the more strongly metamorphosed portion of the chlorite zone. The rocks in question are low in potash and have been formed by reconstitution, at a higher grade, of chlorite-epidotealbite-schists containing calcite. This oligoclase-hornblende association is not to be confused with the actinolite-epidote-albite-chlorite assemblage which is formed at any grade within the zone of chlorite, by direct reconstitution of basic igneous rocks without change in bulk composition and in the absence of CO2. A slight modification of Tilley’s subdivision of the green schist facies of Eskola is therefore introduced.(3) A zone of oligoclase representing a grade of metamorphism higher than that attained in the biotite zone, may be recognized for quartzo-felspathic schists of appropriate composition and for many green schists, in areas of progressive regional metamorphism. In the latter case, blue-green hornblende often accompanied by biotite is also present.(4) Oligoclase or more calcic plagioclase and deeply-coloured hornblende form readily during purely thermal metamorphism of only medium grade in the absence of stress. This accounts for the irregular distribution of both these minerals in districts where purely thermal and regional metamorphism have both occurred during the same period of orogeny.

Varying Trends in the Metamorphism of Dolerites

1951 ◽  
Vol 88 (1) ◽  
pp. 25-35 ◽  
Author(s):  
John Sutton ◽  
Janet Watson
Keyword(s):  
Regional Metamorphism ◽  
Metamorphic Grade ◽  
Low Grade ◽  
Basic Rocks ◽  
High Metamorphic Grade

AbstractThe metamorphism of Pre-Cambrian dolerites in the Northwest Highlands is described and compared with that of basic rocks in the Southwest Highlands, Banffshire and other regions. The first products of metamorphism are not the same in every area. It is suggested that the trend of regional metamorphism differs according to the environment in which it took place, and that a sequence of changes established in one area cannot be used as a standard by which to judge the changes in other areas. In particular, rocks of high metamorphic grade have not invariably passed through stages characterized by low-grade minerals.

The tectonic significance of Ordovician basic igneous rocks in the Southern Uplands, southwest Scotland

1995 ◽  
Vol 132 (5) ◽  
pp. 549-556 ◽  
Author(s):  
E. R. Phillips ◽  
R. P. Barnes ◽  
R. J. Merriman ◽  
J. D. Floyd
Keyword(s):  
Volcanic Rocks ◽  
Accretionary Prism ◽  
Igneous Rocks ◽  
Geochemical Data ◽  
Published Data ◽  
Volcanic Material ◽  
Tectonic Significance ◽  
Arc Setting ◽  
Back Arc ◽  
Midocean Ridge

AbstractIn the northern part of the Southern Uplands, restricted volumes of basic igneous rocks occur at or near the base of the Ordovician sedimentary strata. These rocks have previously been interpreted as ocean-floor tholeiites representative of the subducted Iapetus oceanic plate, preserved as tectonic slivers in a fore-arc accretionary prism. The alternative, back-arc basin model proposed for the Southern Uplands on sedimentological evidence raises questions over the origin of these rocks. New geochemical data and previously published data clearly indicate that the volcanic material does not have a simple single source. The oldest (Arenig) volcanic rocks from the Moffat Shale Group associated with the Leadhills Fault include alkaline within-plate basalts and tholeiitic lavas which possibly display geochemical characteristics of midocean ridge basalts. In the northernmost occurrence, alkaline and tholeiitic basalts contained within the Caradoc Marchburn Formation are both of within-plate ocean island affinity. To the south, in the Gabsnout Burn area, the Moffat Shale Group contains lenticular bodies of dolerite and basalt which have characteristics of island-arc to transitional basalts. This complex association of basaltic volcanic rocks is, at the present time, difficult to reconcile with either a simple fore-arc or back-arc setting for the Southern Uplands. However, the increasing arc-related chemical influence on basic rock geochemistry towards the southeast may tentatively be used in support of a southern arc-terrane, and as a result, a back-arc situation for the Southern Uplands basin. An alternative is that these volcanic rocks may represent the local basement to the basin and include remnants of an arc precursor to the Southern Uplands basin.

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