Ocean Floor Basalts Research Articles

Geochemistry and stratigraphic correlations — Application to the investigation of geothermal and mineral resources of Tuscany, Italy: Contribution to the knowledge of the ore deposits of Tuscany, II

Major-element geochemistry (364 analyses) is used to obtain quantitative information on the initial mineralogical composition and the depositional environment of Paleozoic, more or less metamorphic, rocks from Tuscany and Elba. Furthermore, this method can be used to improve stratigraphic correlations by comparing the chemical compositions of the metamorphic rocks and their possible non-metamorphic counterparts of known stratigraphic position. The stratigraphic units were divided into two sets. The first includes the units of known stratigraphic or tectonic position, taken as reference groups: Permian Red Porphyries, Carboniferous rocks, Buti Group l.s., Porphyritic Schists, Porphyroids, Lower Phyllites and metabasites from the Apuan Alps and Elba. The second set comprises the units of uncertain position: metapelites and metapsammites from outcrops, mines and boreholes from the Boccheggiano—Niccioleta area, Calamita Schists and metabasites associated with these two units, Micaschists, Gneisses and associated amphibolites. The main results are: 1. (1) The Porphyroids and Porphyritic Schists differ strongly from the Permian Red Porphyries, thus confirming that they belong to two different volcanic episodes. 2. (2) All the reference units consist of shales, sandstones s.s. or graywackes, differing by their degree of maturity (increasing from the Lower Phyllites to the Buti Group and to the Carboniferous formations). 3. (3) The Carboniferous rocks, the Buti Group and the Lower Phyllites have distinct chemical compositions. 4. (4) The Boccheggiano Formation l.s. and the Calamita Schists are similar and include rocks chemically equivalent to the Lower Phyllites, the Buti Group and the Carboniferous formations. 5. (5) The Micaschists and Gneisses derive from shales and graywackes respectively, and chemically recall the Lower Phyllites. 6. (6) The metabasites from the Apuan Alps and from Niccioleta are “within-plate basalts”, whereas the amphibolites interlayered within the Micaschists and Gneisses seem “ocean-floor basalts”.

The Halifax County complex: Oceanic lithosphere in the eastern North Carolina Piedmont

Research Article| April 01, 1984 The Halifax County complex: Oceanic lithosphere in the eastern North Carolina Piedmont LUCILLE E. KITE; LUCILLE E. KITE 1Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695 Search for other works by this author on: GSW Google Scholar EDWARD F. STODDARD EDWARD F. STODDARD 1Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695 Search for other works by this author on: GSW Google Scholar Author and Article Information LUCILLE E. KITE 1Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695 EDWARD F. STODDARD 1Department of Marine, Earth, and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1984) 95 (4): 422–432. https://doi.org/10.1130/0016-7606(1984)95<422:THCCOL>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 LUCILLE E. KITE, EDWARD F. STODDARD; The Halifax County complex: Oceanic lithosphere in the eastern North Carolina Piedmont. GSA Bulletin 1984;; 95 (4): 422–432. doi: https://doi.org/10.1130/0016-7606(1984)95<422:THCCOL>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 The Halifax County complex, in the Eastern Slate Belt of North Carolina, consists of the low-grade metamorphic equivalents of the following lithologic groups: (1) an ultramafic group composed of peridotite, pyroxenite, and dunite, locally preserving cumulate textures; (2) gabbroids consisting of leucogabbro, anorthosite, and gabbro; (3) local quartz diorite and plagiogranite; and (4) a relatively large volume of porphyritic and massive basaltic rocks. Within the complex, the lithologies conform approximately, from west to east, to the sequence: peridotite and dunite-pyroxenite-anorthosite and leucogabbro-gabbro–diorite-basaltic rocks. On its west side, the complex is separated from metavolcanic rocks and metagraywackes of the Eastern Slate Belt by the late Paleozoic Hollister mylonite zone; to the east, the complex disappears beneath sediments of the Atlantic Coastal Plain. Relict primary silicate and oxide minerals include olivine of Fo79, clinopyroxene with compositional range of Wo44–49 En44–49 Fs5–9 in the ultramafic rocks and Wo34–49 En42–48 Fs8–21 in the gabbroids, plagioclase of as much as An93 in the leucogabbros, Cr-rich magnetite, and ilmenite. Major- and trace-element analyses of samples from each of the rock groups show an Fe-enrichment trend extending through the mafic sequence of gabbroids and basaltic rocks. The chemical characteristics of the basaltic rocks compare favorably with those of tholeiitic ocean-floor basalts and still more favorably with island-arc tholeiites. Field relationships, plus lithologic, geochemical, and mineralogical evidence, suggest that the complex originated as (early Paleozoic?) oceanic material. Either it represents the lower portions of a lithospheric section produced at a mid-ocean ridge or in a marginal basin, or, perhaps more likely, it was formed in the early stages of the development of an intraoceanic volcanic arc. We believe the Halifax complex is representative of the underpinnings of the Eastern Slate Belt volcanic arc. 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.

Variations in magnetization intensity of ocean floor basalts

Variations in magnetization intensity of ocean floor basalts

Imbricate structure in the Ettrick area, Southern Uplands

Synopsis Details of the imbricate structure of the Silurian greywackes across part of the Central Belt of the Southern Uplands are described. The evidence implies that the major structures are the result of the progressive shearing of early folds which formed penecontemporaneously with the deposition of the sediments. Local zones of intense imbrication are related to the presence, in these localities, of more ductile lithologies interbedded with the greywackes. Thrusts pre-dating the early folds are nowhere recognisable. This structural interpretation predicts that the imbricated greywackes extend to depths of some 3 km whereas the LISPB profile suggests that the basement is not present until a depth of about 15 km. If the grey-wackes form part of an accretionary wedge then the primary decollement between the wedge and the underlying basement cannot occur immediately below the greywackes, at the horizon of the Moffat Shales, but must occur within the underlying sequence of ocean floor basalts.

Variations in magnetization intensity of ocean floor basalts

Variations in magnetization intensity of ocean floor basalts

On dating the magmatism of Maio, Cape Verde Islands

Conventional K-Ar and 40Ar/ 39Ar studies of Mesozoic ocean floor basalts and Tertiary plutonic and volcanic rocks from Maio, Cape Verde Islands, have been determined to elucidate the magmatic evolution of this ocean island. Pillow lavas of the Basement Complex yield a minimum age of 113±8 Ma though thermal overprinting of their formation age by the younger Central Intrusive Complex (CIC) and subsequent sheet intrusions is in some cases almost total. Activity in the CIC began before 20 Ma and plutons continued to develop until about 8 Ma, the youngest ages possibly indicating a cooling history of more than 2 Ma for these bodies relative to their volcanic counterparts. Sheet intrusion occurred throughout the period 20 to 9 Ma though the peak of this activity probably occurred 11 Ma ago. Field relations allow the time of thrusting(s) on the Monte Branco Thrust to be bracketed between 9 and 7 Ma. Volcanic activity began in the Tertiary, probably before 12 Ma, and culminated in the development of a stratovolcano at 7 Ma.

The geochemical and oxygen-isotope affinities of Proterozoic mafic granulites from the Einasleigh Metamorphics, northern Queensland

1.6 Ga-old layered mafic rocks from the Einasleigh Metamorphics underwent two medium-high grade metamorphisms to produce a hornblende-stable granulite facies assemblage. The concentration of the immobile elements (Ti, P, Zr, Y, Nb) in 13 selected samples consistently indicates that these mafic granulites are indistinguishable from modern MORB. Furthermore, their K contents, K/Rb ratios, FeO ∗MgOTiO 2 signature and δ 18O values are similar to modern unaltered low-K tholeiites to ocean floor basalts which show some affinities to ‘plume’-type MORB. The only chemical features which separate the mafic rocks from pristine MORB are an increase in H 2O +, which predates the high grade event, a lower Fe 3+ ΣFe ratio, and a slight decrease in Na during the waning of the granulite facies metamorphism. These data are interpreted to indicate that the mafic rocks at Turpentine Hill underwent essentially isochemical granulite facies metamorphism, and their igneous precursors were derived from a mantle source similar to that which now underlies the mid-ocean ridges.

The petrochemistry of the basic volcanic rocks of the South Connemara Group (Ordovician), western Ireland

Summary16 samples of Ordovician basic volcanic rocks of the South Connemara Group, which abut the southern side of the metamorphic rocks of the Connemara massif in western Ireland, have been analysed for both major and trace elements. Although subject to low grade regional metamorphism and subsequently hornfelsed by the Galway Granite (400 Ma), their immobile element contents do not appear to be significantly disturbed. These elements characterise the metabasites of the South Connemara Group as ocean floor basalts having their origins in a marginal basin. The Skird Rocks Fault, separating the South Connemara Group from high grade metamorphic rocks of the Connemara massif, is consequently regarded as the northern margin of the vestiges of the lapetus Ocean which can be traced into, and along, the Southern Uplands Fault.

Variations in magnetization intensity and low-temperature titanomagnetite oxidation of ocean floor basalts

The remanent magnetization of the oceanic crust exhibits a systematic long-term variation which correlates with the amplitudes of marine magnetic anomalies. After a sharp initial decrease of natural remanent magnetization intensity, a minimum is reached at ∼20Myr, followed by a gradual increase up to ages of 120 Myr. The progressive sea floor alteration of the magnetic minerals carrying the crustal magnetism is proposed as a cause for this behaviour.

Inert gas abundances in basalts of the Lesser Antilles island arcs: possible implications for their volcanic evolution

Measurements of noble gas element abundances in igneous rock samples from the Lesser Antilles island arc (LAIA) show an enrichment of Ne, Kr and Xe relative to Ar, when compared to the atmospheric inert gas composition. The element concentrations vary as a function of time and a cyclic variation appears to occur in the pre-Pliocene stage of arc development. The pattern of elemental abundances closely resembles that of ocean floor basalts, suggesting that, (1) the origin of the inert gases in these samples is related to the subduction of oceanic lithosphere responsible for the formation of the LAIA, and (2) the subduction process does not modify the noble gas patterns of the materials being subducted. There is no evidence of a change in concentration or composition of noble gases occurring at ca. 9 Ma, the time when it has been suggested the LAIA shifted to form the western, younger, volcanic Caribbees. However, a major hiatus in the noble gas evolution is recognised at ca. 16 Ma.

Dredged volcanic rocks from the southern oceans: theEltanincollection

Abstract Here we synthesise available data and present 22 new chemical analyses of dredged volcanic rocks, recovered by the USNS Eltanin, from a wide variety of morphotectonic settings in the southern oceans. Generally, these samples have undergone the variable affects and styles of postmagmatic alteration. The products of alkali volcanism were recovered from scattered localities in the back-arc environment of the Scotia Sea. Volcanic rocks from the Pacific-Antarctic and Chile Rises mainly consist of quartz and olivine normative ocean-floor basalts similar in composition to those from the East Pacific Rise and Galapagos Spreading Centre. To the north of the Eltanin Fracture Zone, a wider compositional diversity seems to occur, including FeTi-basalts and trachyandesite, than that observed from within and to the south of this fracture zone and on the Chile Rise. Reported values for the ratios K/Rb Rb/Sr La/Yb and Sr87/Sr86 for fresh material, gen'era"lly hint at a derivation of these basalts from a typically depleted mid-ocean ridge basalt source region. Both geophysical and petrological interpretations point to a possible analogy between segments of the PacificAntarctic Rise and youthful migrating-propagating accretion axes, in areas of intense axial primordial gas emission and hydrothermal activity, such as the East Pacific Rise between 4° and 17°S and the Galapagos Spreading Centre. From the Ross Sea, phonolites, hawaiites, nepheline-mugearite, and alkali basalt were recovered. With the exception of the alkali basalt from the Scott Seamounts, these samples show chemical affinities to both the Erebus and Hallett volcanic provinces. The phonolites from Scott Island and platform are similar in composition to those of Cape Crozier (Ross Island), all of which probably have evolved from a common magma type by fractional crystallisation of a basanite or alkali basalt of similar composition to those of the Balleny Islands. Though altered, basalts from the Broken Ridge and Hjort Trench share some compositional similarities and might be termed ferrobasalts. The data support the inference that the Broken Ridge is at least m part volcanic in nature, while a relationship between the Hjort Trench and the West Pacific island-arc system remains inconclusive. The rough topography between the southern end of Ninetyeast Ridge and the Indian-Antarctic Spreading. Centre may represent intraplate volcanic activity.

Low-temperature oxidation of synthetic Al- and Mg-doped titanomagnetites

Al- and Mg-doped titanomagnetites were synthesized at 1300°C using the gas-mixing technique. A composition, representative of average natural titanomagnetites in ocean floor basalts, was sought. The samples were ball-milled in acetone to average grain sizes of 0.5 μm and 5 μm and the material was then oxidised, in air, at temperatures below 300°C. This procedure formed titanomaghemite, a cation-deficient titanomagnetite. Low-temperature oxidation is described as the diffusion of Fe-ions out of the spinel lattice and the process is observed to be distinctly dependent upon grain size.

Diffusion of dissolved carbonate in magmas: Experimental results and applications

The diffusivity of dissolved carbonate in sodium aluminosilicate (60% SiO2, 30% Na2O, 10% Al2O3) and iron-free “basalt” melts was measured by a 14C thin-source tracer technique at conditions of combined high temperature and high pressure, ranging from 800°C to 1350°C and 0.5 to 18 kbar for the simple melt and 1350° to 1500°C at 15 kbar for the haplobasalt melt. The two melts have similar dissolved carbonate transport properties, expressed for the simple system at 0.5 kbar by: Dcarbonate=3.559exp(−46,600/RT) The pressure dependence at 1200°C is given by: Dcarbonate=4.13×10−7exp(−11.02P/RT) The activation energy of 46.6 kcal/mole is low relative to expected values for small, highly-charged cations, suggesting that the actual diffusing species is a larger carbonate anion. This interpretation is supported by the 11 cm3/mole activation volume, which corresponds to twice the molar volume of oxygen anions. Because the 14C diffusion gradients conform quite well to a constant-diffusivity, thin-source model, there can be no strong dependence of carbonate diffusivity upon dissolved carbonate content, at least in the 0–0.2 wt.% range represented by the experimental profiles. Carbonate diffusion is markedly different from previously determined systematics for water, being 1–2 orders of magnitude slower in granitic magmas and considerably faster in basalts. This difference can result in diffusional fractionation of CO2 and H2O in magmas, which may be most significant in small-scale processes such as growth of vapor bubbles and formation of some types of glass inclusions in phenocrysts. The new data also place constraints on the rate at which CO2-dominated fluid bubbles can actually grow. Under more or less isothermal conditions at ∼ 10 kbar, the growth rate is on the order of 1 mm/day, depending upon the degree of supersaturation in CO2. When combined with knowledge of initial CO2 contents of magmas and observed characteristics of dispersed bubbles (e.g., in ocean-floor basalt glasses), the diffusivity data may expedite the reconstruction of magma decompression paths.

Rare earth element geochemistry of the Betts Cove ophiolite, Newfoundland: complexities in ophiolite formation

The Betts Cove ophiolite includes the components of typical ocean crust: pillow lavas, sheeted dikes, gabbros and ultramafics. However, the trace element geochemistry of basaltic rocks is unusual. Three geochemical units are recognized within the lava and dike members. Within the pillow lavas, the geochemical units correspond to stratigraphic units. Upper lavas have ‘normal’ ( i.e., typical for ocean floor basalts) TiO 2 contents (0.75 to 2.0 wt%), heavy rare earth elements (HREE) values in the range 6–20× chondrites and chondrite-normalized REE patterns with relative LREE depletion. Intermediate lavas have TiO 2 contents between 0.30 and 0.50 wt%, HREE contents from 4–7× chondrites and extreme relative LREE depletion. Lower lavas have anomalously low TiO 2 contents (<0.30 wt%) and unusual convex-downwards REE patterns with REE abundances around 2–5 × chondrite. These geochemical differences can be explained if the three groups were derived from different mantle sources. Independent mantle sources for the three units are consistent with their different 143Nd 144Nd ratios varying at 480 m.y.B.P. from 0.51222 in a lower lava to 0.51238 in an upper lava. The upper lavas may be partial melts of a source similar in composition to that of modern MORB, the intermediate lavas may be from a very depleted oceanic mantle (second stage melt), and the lower lavas may have formed by melting an extremely depleted mantle that had been invaded by a LREE-enriched fluid. A possible tectonic environment where these different sources could be juxtaposed is a back-arc or inter-arc basin.

Interpretation of the relations between massive, pillowed and brecciated facies in an archean submarine andesite volcano — amulet andesite, rouyn-noranda, Canada

The Amulet andesite formation in the Archean terrain of Rouyn-Noranda, P.Q., consists of 19 flows, distinguished by variations in phenocryst content and vesicularity and by the presence of concentric contraction fractures. Detailed mapping of flows revealed the presence of three main facies: (1) massive facies; (2) pillowed facies; and (3) foreset-bedded (brecciated) facies. The massive facies consists of > 50% massive lava overlain by pillow lava and/or pillow breccia. The pillowed facies consists of > 50% pillows. In some flows, the pillowed facies contains a thin sheet of massive lava at the base of the flow (facies 2a). Generally, massive lava fills braided channels 10–50 m wide (facies 2b). Facies 2c consists of pillows and large, irregular, megapillows. Determination of the flow direction shows that facies 1 is proximal, facies 2 distal. The foreset-bedded facies 3 consists of alternating thin (1–2 m) lobes of massive lava, pillow lava and broken-pillow breccia. It makes up flows 16–19 at the top of the sequence. Flow M, a unit entirely composed of massive lava, is ponded against flows 9 and 10. We interpret the growth of the Amulet andesite volcano in the light of new work on ocean-floor basalt and on the 1969–1973 Mauna Ulu eruption. The feeding fissure was located in the area of massive facies. At the beginning of eruptions lava spread laterally as a sheet-flood flow (massive base of facies 2a) but rapidly became channelized. The pillow lava and in particular the facies 2b, 2c and 3 are considered as the subaqueous equivalents of tube-fed pahoehoe. Flow M probably represents a lava lake. Shallowing-upward of the sequence during the built-up of the volcano is indicated by increasing vesicularity of the flows and by an upward increase of the proportion of broken pillow breccia. This increase is gradual from flow 1 to flow 15 but abrupt from flow 15 to 16–19.

Discrimination between ophiolitic metabasalts, north D'Urville Island, New Zealand

Whole-rock minor and trace element abundances, relict clinopyroxene compositions and petrographic features discriminate between 2 metabasalt units within the Patuki Volcanic complex, north D'Urville Island, New Zealand. The majority of the Patuki metabasalts show P, Zr, Nb, and light rare earth element (LREE) depletion and exhibit tholeiitic differentiation characteristics. They show close similarities to many abyssal tholeiites produced at modern spreading ridges (Ntype ocean ridge basalts). Pristine relict clinopyroxene preserves Ti-poor endiopside and endiopsidic augite compositions somewhat enriched in Cr, and shows systematic iron enrichment. A smaller proportion of the metabasalts are enriched in Nb, LREE, and related hygromagmatophile elements, but retain phenocryst assemblages and some trace element abundances similar to ocean-floor basalts. They contain Ca- and AI-rich titaniferous salites. These metabasalts are atypical of ophiolitic metabasalts and of metabasalts formed at normal ridge segments and show closest similarity to largeion- lithophile element enriched, ocean-floor tholeiites (E-type mid-ocean ridge basalts). At north D’Urville Island, the N-type metabasalts form an upper unit composed of massive and more rarely pillowed flows, and the group of E-type metabasalts form a lower unit composed of pillow lava and rare tabular bodies of ophitic spilite, associated with volcanic breccia. Interbedded marine sediments include sandstone, argillite, mudstone and minor tuffs, and ophiolitic breccia The trace element data indicate that the 2 suites of Patuki metabasalts at north D’Urville Island are derived from geochemically distinct mantle sources, similar to sources from which N- and E-type midocean ridge basalts originate. The data imply mantle heterogeneity beneath a poorly evolved Middle Permian seafloor.

Sr, Y, Zr, Nb, Ti, and REE in Grenville amphibolites at Montauban-les-Mines, Quebec

Distribution of Ti, Sr, Y, Zr, Nb, and REE in Proterozoic amphibolites and a rarely preserved pillowed metabasalt in the vicinity of Pb–Zn mineralization at Montauban-les-Mines in the Grenville Province of Quebec are used to assess the tectonic setting of the volcanism and, hence, the environment of ore deposition. Absolute abundances of Ti, Sr, Y, Zr, and Nb are close to those reported for modem island-arc and back-arc basalts. The flat patterns of the chondrite-normalized REE distribution and the discrimination plots of the other elements indicate a tholeiitic affinity for the volcanism. Specifically, plots of Ti against Zr and Ti–Zr–Y are mostly in the fields reported for ocean-floor (MOR and back-arc) basalts, but overlap with arc tholeiites. Cr and Ni are, however, higher than for most arc tholeiites. Overall, the data are most compatible with tholeiitic volcanism in an island-arc (including back-arc) environment.

Amphibolite associated with the Thetford Mines Ophiolite Complex at Belmina Ridge, Quebec

Amphibolite in the Quebec Appalachians at Belmina Ridge (45°58′N; 71°28′W) constitutes a steeply dipping, allochthonous sheet as much as 800 m thick, between the obducted Thetford Mines Ophiolite Complex to the east and rocks of the allochthonous Caldwell Group to the west. The amphibolite, which structurally and chemically is unlike amphibolite in the adjacent Caldwell, is fresh, coarse to fine grained, and massive to layered. It is composed chiefly of edenitic to pargasitic amphibole, pyrope- and grossular-rich almandine, salite, and epidote, with small amounts of quartz, plagioclase, and accessory minerals. Metamorphism ranged from [Formula: see text] at the base of the sheet to T ~ 780 °C at the top, near the tectonic contact with rocks of the superjacent ophiolite complex, under a Barrovian facies series with Ptotal between 5 and 7 kbars (500–700 MPa).The amphibolite is interpreted to have been derived from ocean-floor basalt that was metamorphosed under a 20 km thick, westward-directed, obducted allochthon of oceanic lithosphere. With a thermal gradient of 40 °C∙km−1, the temperature at the base of the allochthon would have been 800 °C, whereas the temperature at the base of the Thetford Mines Ophiolite Complex (the uppermost third of the allochthon) would have been 312 °C or less, in agreement with the serpentinization history of the complex. The dynamic environment during obduction could continually furnish "fresh" (hot) lithosphere to override the site of metamorphism of the amphibolite. At the site, the temperature of the base of the allochthon rose progressively, and a relatively high temperature was maintained for ample time, of the order of 1 Ma, to have continuously heated the underlying rocks to raise them to a high temperature without the allochthon ever having been unrealistically hot. The allochthon was dismembered tectonically toward the close of obduction. The amphibolite was thrust westward and upward, to be emplaced at relatively little depth on rocks of the Caldwell Group, and under the ophiolite complex. The petrogenesis proposed for the amphibolite at Belmina Ridge is consistent with the apparently simple metamorphic history of the rock.

Geochemical clues to elucidate the tectonic environment of the Chamoli Volcanics, Lesser Himalayas, Uttar Pradesh, India

Geochemical data on the Chamoli Volcanics of the Garhwal Group suggest their strong affinity with oceanic tholeiites. However, the field relations and other geological information do not support this conclusion and indicate an epicontinental rather than eugeosynclinal environment of eruption. The lack of correlation of chemical data with geological setting precludes the possibility that these basalts are true ocean floor basalts. It is inferred that the Chamoli Volcanics with an oceanic tholeiite affinity were probably erupted as a result of initial rifting in the Proto-Tethys, which at that time was an intercontinental sea. The rifting was started during the depositional regimes of this intercontinental sea in which shallow water sediments were being deposited. Ocean type tholeiitic magma, guided by the rift zone, disrupted the cycle of sedimentation and gave rise to the development of the quartzite-lava sequence of the Chamoli Formation.

Alkalibasalts from the Tyrrhenian sea Basin: Magmatic and geodynamic significance

Petrological characteristics of basaltic rocks from the Tyrrhenian deep-sea are discussed and related to the geotectonic situation. For the first time, distinctly alkaline basalts (hawaiites) have been found in the Tyrrhenian deep-sea. These are typical within-plate basalts related to the tensional fracturing of the Tyrrhenian area. A suggested age of 100,000 years is among the youngest indications for the Tyrrhenian Sea volcanism. Since the Miocene, magmatic activity in the inner Tyrrhenian sea basin evolved from ocean-floor basalts to ocean island tholeiites and transitional basalts, with alkaline basalts as the most recent products. Seamounts in the southern part of the Tyrrhenian deep-sea (Palinuro, Marsili) add shoshonitic and calcalkaline lavas (some with high Mgvalue) to the complexity of the Tyrrhenian magmatic evolution.