Alkaline magmatism

Top PDF Alkaline magmatism:

LATE ARCHAEAN - PALAEOPROTEROZOIC ALKALINE MAGMATISM  IN THE DHARWAR CRATON OF PENINSULAR INDIA

LATE ARCHAEAN - PALAEOPROTEROZOIC ALKALINE MAGMATISM IN THE DHARWAR CRATON OF PENINSULAR INDIA

Therefore, these these episodes (Koppal and Dhancharla) have been accounted as primary alkaline magmatic event in the Eastern DharwarCraton as well as the Indian sub-continent during the period of 2.52 Ga to 2.21 Ga.Mesoproterozoic to Neoproterozoic(̴ 1400 to 600) alkaline magmatism of the Prakasham Alkaline Province (PAP) have been taken place where the contact zone between Eastern DharwarCraton (EDC) and Eastern Ghat Mobile Belt (EGMB) represents intra-plate alkaline magmatic environment, considered as second event in the Eastern DharwarCraton. Thus, there is a huge gap identified between primary (late Archaean to Palaeoproterozoic) and secondary event (Mesoproterozoic to Neoproterozoic). In this gap there are no such alkaline magmatic occurrences found in the Eastern DharwarCraton (EDC).

12 Read more

Synkinematic ferro potassic cal alkaline magmatism from the yoro yangben massif, along the southern part of central domain of cameroon pan african fold belt

Synkinematic ferro potassic cal alkaline magmatism from the yoro yangben massif, along the southern part of central domain of cameroon pan african fold belt

The geochemical behaviors show that the Yoro-Yangben rocks are similar to those of the hyperpotassic calc-alkaline series of the orogenic domain. Orthogneiss show very significant negative Europium anomalies underlining the role of the plagioclase in the differentiation process of these rocks. They have high contents of Light Rare Earths Elements and low content in Heavy Rare earths elements. The very high contents of LREE is due to the abundance of pyroxenes, amphiboles and accessories minerals such as titanite, allanite and apatite; whereas the rare notable contents of HREE (10 to 50 times chondrites) are due to the significant presence of zircon. The linear correlations of the major elements with the SiO2 (Fig.12) and between the different Rare Earths Elements (Fig. 14) testify to an evolution by fractional crystallization as main process prevailing at the origin in the Yoro-Yangben rocks. The rock emplacement under stress due to the working of regional shear zone, produce variations in the magma composition, certainly after a magmatic mixture or, of a crustal contamination. This crustal contamination is underlined by (i) independence Ba-Rb, (ii) enrichment of LREE in rocks compared to HREE, (iii) the high contents of lithophile elements (K, Rb, Sr, Ba), (iv) the negative anomalies of Nb, Sr, TiO2 (Rollinson, 1993) and finally (v) Rb/Sr (0,1-2,3) ratios higher than those of the mantellic liquids (0.03; Wilson, 1989).

17 Read more

Tertiary Quaternary subduction processes and related magmatism in the Alpine Mediterranean region

Tertiary Quaternary subduction processes and related magmatism in the Alpine Mediterranean region

collapse; (4) convective removal of the lower lithosphere. Torres-Roldán et al. (1986), Royden (1993), Lonergan & White (1997), Duggen et al. (2003; 2004) and Gill et al. (2004) assumed that contemporaneous subduction occurred with the calc-alkaline volcanism. Geophysical data indicate an east-dipping subducted slab (Gutscher et al. 2002) beneath the Alboran region. Duggen et al. (2004) and Gill et al. (2004) emphasized that the ‘subduction- related’ nature and particularly the strong depletion in the light REE and HFSE of the Alboran tholeiites (Fig. 7) could only be explained by formation in a metasomatized mantle wedge above a subducted slab. Furthermore, they assumed that all the other calc-alkaline volcanic rocks in the Betic-Rif province could be generated in the same geodynamic setting. Blanco & Spakman (1992) and Calvert et al. (2000) argued, however, that the seismic tomography models show a detached near-vertical lithospheric slab from about 180-200 km down to the 670 km discontinuity beneath the Alboran region. Zeck (1996) considered that slab break-off could have had a major role in melt generation. Influx of hot asthenospheric mantle into the widening gap above the sinking slab induced partial melting in the overlying lithosphere (particularly in the lower crust). The close relationship between the distribution of volcanism in the Alboran volcanic province and the surface projection of the sinking slab was used by Zeck (1996) to support this model. Fourcade et al. (2001) and Coulon et al. (2002) also invoked slab break-off to explain the calc-alkaline to alkaline magmatism in Northern Algeria. Other authors (Venturelli et al. 1984a; Platt & Vissers 1989; Zeck 1996; Benito et al. 1999; Turner et al. 1999) argued that the Betic-Alboran volcanism was post-collisional, following Late Cretaceous to Oligocene subduction and Late Oligocene to Early Miocene continental collision. Benito et al. (1999), Turner et al. (1999) and Coulon et al. (2002) suggested that the primary melts were generated in the lithospheric mantle, which had been metasomatised previously by fluids derived from subducted pelagic sediments. These mantle- derived magmas subsequently mixed with crustal melts. Zeck (1970; 1992; 1998) argued for a crustal anatectic origin for the calc-alkaline magmas of southern Spain. Platt & Vissers (1989), Benito et al. (1999) and Turner et al. (1999) emphasized that melt generation occurred by decompression melting due to extensional collapse of the overthickened orogenic wedge or convective removal of the lithospheric root. In North Africa, El Bakkali et al. (1998) also suggested an extension-related origin for the calc-alkaline to potassic magmas of the Eastern Rif (Morocco).

44 Read more

THE GEOTHERMAL SYSTEM NEAR PAISLEY, OREGON: A TECTONOMAGMATIC FRAMEWORK FOR UNDERSTANDING THE GEOTHERMAL RESOURCE POTENTIAL OF SOUTHEASTERN OREGON

THE GEOTHERMAL SYSTEM NEAR PAISLEY, OREGON: A TECTONOMAGMATIC FRAMEWORK FOR UNDERSTANDING THE GEOTHERMAL RESOURCE POTENTIAL OF SOUTHEASTERN OREGON

An important event in reconstructing the history of the Cascade arc is that of the accretion of the Siletzia terrain in early to mid-Eocene. The accretion of Siletzia allowed for the westward migration in arc-type magmatism from the Challis-Kamloops magmatic center to the Western Cascades, in which the Clarno Formation of central Oregon could represent a transitional phase of volcanism between these two entities. The Siletzia terrain has been characterized as an oceanic island seamount chain, probably created by intraplate hotspot volcanism within the Farallon plate (Duncan, 1982; Christiansen and Yeats, 1992; Dickinson, 2004). The accretion of Siletzia terrain most likely stalled subduction for a period of 5-10 million years. When subduction resumed, the trend of the ancestral Cascade arc was established farther west than the previous locus of volcanism (Schmandt and Humphreys, 2011). Tholeiitic magmatism dominated from about 45-18 Ma followed by Calc-alkaline magmatism until about 5 Ma (du Bray and John, 2011).

224 Read more

Geochemistry and tectonic development of Cenozoic magmatism in the Carpathian–Pannonian region

Geochemistry and tectonic development of Cenozoic magmatism in the Carpathian–Pannonian region

ratios), which is commonly suggested to reside within the mantle lithosphere. However genuine subduction-related calc-alkaline magmatic arcs show linear arrays of point magma sources, parallel to the mantle wedge above subduction zones (e.g. Stern, 2002; Kogiso et al., 2009 and references therein) with a melt zone resulting from the release of hydrous fluids from subducted materials lowering the melting temperature of the overlying asthenospheric mantle. Such a linear array cannot be found in the CPR. However, similar situations where lithospheric mantle enriched with subducted material was involved in late melt-production are not uncommon (e.g. McPherson and Hall, 1999; Kovács and Szabó, 2008; Karaoğlu et al., 2010). In the CPR the calc-alkaline magmatism is considered to be post-collisional and entirely of lithospheric origin. It is dominantly a result of extensional processes dependent on the rheological properties and specific lithosphere composition of the microplates and their boundaries (Fig. 2). In addition, the transition toward the asthenosphere-derived magmas, represented mainly by the Na-alkalic suite, is of crucial importance in understanding the tectonic processes.

42 Read more

Geochemical and Geodynamic Implications of Mafic Dykes of the Iguerda Inlier (Central Anti Atlas, Morocco)

Geochemical and Geodynamic Implications of Mafic Dykes of the Iguerda Inlier (Central Anti Atlas, Morocco)

convergence back-arc setting (Gasquet et al., 2004, 2005, The Iguerda inlier is located at 190 km E-SE to city of 2008). According to the Palæoproterozoic in the Anti- Agadir (Fig. 2) and is part with the Zenaga, Bou Azzer-El Atlas is characterized by two distinguishable magmatic Grara inliers and Siroua massif of the Central Anti-Atlas. events both related to a subduction setting: the first at It belongs to the South West area and is located on 2110-2080 Ma (trondhjemitic magmatism) and the the northern edge of the West African Craton. It second at 2050-2030 Ma (calc-alkaline magmatism). Mafic consists by:

8 Read more

Generation of normal and adakite like calc alkaline magmas in a non subductional environment: An Sr O H isotopic study of the Apuseni Mountains neogene magmatic province, Romania

Generation of normal and adakite like calc alkaline magmas in a non subductional environment: An Sr O H isotopic study of the Apuseni Mountains neogene magmatic province, Romania

Figure 2 shows interpretations by Csontos et al. (2002) and Seghedi et al. (2004) of the geodynamic situation after the end of the differential rotation of the central and eastern parts of Tisia-Dacia, synchronous with graben opening in the western part of the Apuseni Mountains. The studied magmatism is closely connected with the major eastward rotations between 15.5 and 11 Ma and subsequent opening of narrow graben- type basins. During this interval, contemporaneous calc-alkaline magmatism also developed along the Carpathian arc in the front of Alcapa and Tisia-Dacia (Pécskay et al., 1995, 2006), in direct relationship with rollback subduction retreat and probable breakoff of the subducted slab (Nemčok et al., 1998; Seghedi et al., 1998, 2001, 2004; Wortel and Spackman, 2001). During Late Middle Miocene times (~11 Ma), collision of Tisia-Dacia with the East European platform took place, causing the retreating subduction processes to cease (Csontos, 1995; Maţenco, 1997; Zweigel, 1997).

46 Read more

SYNTHESIS AND CHARACTERIZATION OF CHITOSAN FROM CRAB SHELLS VS BACTERIOLOGICAL BIOMASS

SYNTHESIS AND CHARACTERIZATION OF CHITOSAN FROM CRAB SHELLS VS BACTERIOLOGICAL BIOMASS

The fermented broth from each flask was centrifuged at 12000 rpm for 15 minutes. The supernatant was discarded. The pellet contained mixture of bacteria, chitin and chitosan. To each of these pellets was added 10mL of 0.1N NaOH. The contents were mixed thoroughly and taken in separate clean test tubes that were autoclaved for 15 minutes. The tubes were then allowed to come to room temperature. Most of the cells were solubilized during the alkaline treatment. The tubes were again centrifuged at 12000 rpm for 15 minutes. The supernatants were carefully removed and pellets containing chitin, chitosan, and small amount of cell debris were mixed with 10mL of 2% acetic acid and mixtures were taken in clean test tubes that were left on a shaker overnight at room temperature to solubilize chitosan in 2% acetic acid. The contents of the above tubes were again centrifuged at 12000 rpm for 15 minutes. Pellet was discarded and 10mL supernatant was collected and the presence of chitosan in it was checked by the formation of white precipitate upon neutralization with 1N NaOH. [27]

14 Read more

Early Proterozoic magmatism in Yukon, Canada: constraints on the evolution of northwestern Laurentia

Early Proterozoic magmatism in Yukon, Canada: constraints on the evolution of northwestern Laurentia

More than one quarter of Earth history passed between the time of cratonization in western Canada ( - 1850 Ma; Hoffman 1989; Villeneuve et al. 1991; Ross et al. 1991) and inception of the Neoproterozoic–Paleozoic Cordilleran miogeocline ( - 600 Ma; Gabrielse 1967; Young et al. 1979; Aitken and McMechan 1992; Gabrielse and Campbell 1992). During this interval, the Proterozoic landmass of ancestral North America, commonly referred to as Laurentia, underwent a series of extensional events along its western edge which ultimately led to its separation from another landmass, arguably Australia coupled with east Antarctica (Bell and Jefferson 1987; Moores 1991; Dalziel 1991, 1997; Ross et al. 1992), although Siberia (Sears and Price 1978, 2000) and south China (Li et al. 1995) have also been proposed. Prior to separation, Laurentia was part of one or more proposed supercontinents, namely Arctica, Nena, Kanatia, and Rodinia (Hoffman 1989; Young 1995; Rogers 1996). Con- figurations and nomenclature for these proposed continental regions are still evolving (e.g., Karlstrom et al. 1999; Burrett and Berry 2000; Sears and Price 2000). The divergence of Laurentia from the other parts of the proposed supercontinents occurred through discrete rift events that affected different parts of western Laurentia at different times, producing basins that filled with successions of clastic and carbonate rocks up to 20 km thick (Aitken and McMechan 1992; Gabrielse and Campbell 1992). Extension and sedimentation were punctuated by distinct events of magmatism, contractional deformation, and surges of hydro- thermal fluids (Eisbacher 1981; Roots and Thompson 1988; Höy 1989; Ross 1991; Thorkelson et al. 2001).

16 Read more

Petrology and Tectonic Setting of Volcanic Rocks in West and South West of Salafchegan, Qom, Iran

Petrology and Tectonic Setting of Volcanic Rocks in West and South West of Salafchegan, Qom, Iran

continent of Central Iran. The matching of Subduction zones and dealing with old hydrothermal blades which had created Mesozoic rift within the continent, could cause disturbance in classical magmatism in mentioned tectonic setting. A break of Neotethys oceanic crust in the upper Cretaceous allows that depressed and stopped part of the shell get uniformly warm (hot) and causes the creation of severe volcanoes in the Eocene period. These phenomena continued with less intensity in other parts of the Tertiary [1]. Some researchers [1] [21] believed that most intrusive and semi-volcanic masses of Iran’s tertiary are characterized by Oligo-Miocene ages which formed as result of pressure relief deep magmatic reservoirs of Eocene volcano in orogenic period of Pyrenees and astrains era [7].

24 Read more

Petrogenetic processes in the ultramafic, alkaline and carbonatitic magmatism in the Kola Alkaline Province: a review

Petrogenetic processes in the ultramafic, alkaline and carbonatitic magmatism in the Kola Alkaline Province: a review

The northern Baltic Shield is composed predominantly of tonalite-trondhjemite-granodiorite complexes formed around 2.95-2.90 Ga ago, with subsidiary supracrustal Late Archaean and Early Proterozoic formations (Balagansky et al., 1998). The crust of the region is approximately 40km thick and the present-day lithosphere thickness is estimated at >200km (see review by Artemieva (2003)). During Palaeozoic times, the area was part of Baltica, a continent consisting of present-day Scandinavia, Spitsbergen, Russia and Ukraine. Paleogeographic reconstructions show that Baltica was situated in the southern hemisphere during Devonian times and was moving northward (Torsvik et al., 1996). Several authors have linked the KACP with either a 'North Atlantic Alkaline Province' that includes complexes in Greenland and the Canadian Shield (Vartiainen and Woolley, 1974) or with the extensive Devonian rifting and magmatism on the East European Platform (Kramm et al., 1993). The age of the KACP magmatism is much more restricted than that of the so- called 'North Atlantic Alkaline Province' and is much closer to the ages of the East European Platform Devonian magmatism. The existence of a ENE-WSW trending rift system on the Kola Peninsula (sometimes called the Kontozero rift) was first suggested by Kukharenko (1967) from the general alignment of several KACP massifs. This trend also encompasses the outcrops of Devonian volcano-sedimentary units (Arzamastsev et al., 1998). However, the generally N-S to NE-SW orientation of the KACP dyke swarms on the south coast of the Kola Peninsula argues for an approximately E-W extensional stress field at the time of their emplacement. Kukharenko et al. (1971) and Vartiainen and Paarma (1979) suggested that the presence of several carbonatite complexes and dyke swarms on the south coast of Kola was related to NNW-SSE trending 'Kandalaksha deep fracture system', which may be associated with the Mezen rifts beneath Arkhangelsk.

22 Read more

Ar Ar age constraints on the timing of Havre Trough opening and magmatism

Ar Ar age constraints on the timing of Havre Trough opening and magmatism

211 Our results show that there has been arc and rift-related magmatism across the entire 212 southern Havre Trough within the last c. 1 Ma, both within rifts (e.g., Ngatoro Rift) and 213 constructing large stratovolcano cones such as Gill and seamounts of Rumble V Ridge 214 (Wright e al., 1996; Todd et al., 2010). This, together with the >4 km water depth in the 215 deepest parts of the basin, is more consistent with distributed rifting across the basin than 216 ocean spreading. Whether there are differences in age between rift-related magmas erupted at 217 different depths, or distance across the basin, or distance northward from New Zealand, is 218 important for understanding the tectonic evolution of the basin but remains to be discovered. 219 Our experience shows that 40 Ar/ 39 Ar ages can be obtained for the challenging Havre Trough

20 Read more

A Paleoproterozoic intra arc basin associated with a juvenile source in the Southern Brasilia Orogen : application of U Pb and Hf Nd isotopic analyses to provenance studies of complex areas

A Paleoproterozoic intra arc basin associated with a juvenile source in the Southern Brasilia Orogen : application of U Pb and Hf Nd isotopic analyses to provenance studies of complex areas

The Juiz de Fora Complex orthogranulites represent a calc-alkaline suite associated with convergent tectonic settings (Heilbron et al., 1998) and the basic rocks are tholeiites similar to island-arc and back-arc basalts (Costa, 1998; Noce et al., 2007). Inspite of its Ryacian age (2195- 2084 Ma) and arc-related signature, it is interpreted as the basement of the Araçuaí orogen magmatic arc (item 2.2.2 - Noce et al., 2007; Pedrosa-Soares et al., 2008, 2001). Therefore, even though this complex satisfies the necessary age and isotope requirements to be a major source for the São Vicente Complex sedimentary basin, its paleogeographic position during the

46 Read more

The metasedimentary Macururé Complex (Sergipano Belt, Borborema Province, Brazil) in southern Alagoas state: geology and geochronology

The metasedimentary Macururé Complex (Sergipano Belt, Borborema Province, Brazil) in southern Alagoas state: geology and geochronology

of metagranitoids with A-type signature favors an intraplate origin for the magmatism (Neves 2003; Guimarães et al. 2011, 2012, 2016). Given the wide distribution of the Cariris Velhos-related rocks, it is not possible to ascertain if zircon grains of the dominant age group were sourced from proximal rocks (Marancó-Poço Redondo Domain) or had more distal provenance. The absence of zircon grains younger than c. 900 Ma in our and in previously analyzed samples could indicate that deposition of the Macururé Complex occurred at the end or shortly afterward the Cariris Velhos event. Alternatively, it may simply be that younger sources were either not available for erosion at the time of deposition or were not present along the drainage system.

9 Read more

Seismic tomography of the uppermost mantle beneath southwestern Japan: Seismological constraints on modelling subduction and magmatism for the Philippine Sea slab

Seismic tomography of the uppermost mantle beneath southwestern Japan: Seismological constraints on modelling subduction and magmatism for the Philippine Sea slab

Many studies have been made on the subduction of the Pacific slab and the magmatism in northeastern Japan, but not on the subduction of the Philippine Sea slab and the magmatism in southwestern Japan. Primary reasons may be that seismological networks in southwestern Japan were sparse as compared with those in northeastern Japan and that geology including volcanism of southwestern Japan is more complicated than that of northeastern Japan. However, recent instrumental development of dense seismological networks in the Japanese Islands has provided us with high quality data not only for northeastern Japan but for southwestern Japan. One of the outcomes from the development is the increase of accuracy of arrival time readings of P- and S-waves and resultant hypocenter determination. We attempt to elucidate fine image of the uppermost mantle structure beneath the Japanese Islands and to find evidence for the relation between the magmatism and subduction process. We apply travel time tomography to 216,247 P- and 98,207 S-wave arrival times observed at 1,328 seismic stations from 5,242 earthquakes in and around the Japanese Islands, and obtain three-dimensional variations of P- and S-wave velocity structure. In Chubu and Kyushu, the subducting Philippine Sea slab bends downward in the depth range of 50 to 70 km. In some nonvolcanic regions, remarkable anomalies of high Poisson’s ratio (and low S-wave velocity) are seen in the depth range of 25 to 40 km near the upper boundary of the Philippine Sea slab or the Moho discontinuity, and approximately coincide with the hypocenter distribution of deep low-frequency earthquakes. The anomalies of high Poisson’s ratio are also seen near the upper boundary of the Philippine Sea slab or the overlying mantle wedge down to a depth of about 60 km, but are not seen after the downward bending of the slab, in the forearc region. The anomalies are probably caused by separated fluid or hydrous minerals. These characteristics should be taken into account in numerical modelling of the subduction of young slabs (e.g., Philippine Sea slab) and associated phenomena (e.g., magmatism).

20 Read more

Production and Performance of Bio Diesel from Cotton Seed and Rice Bran Oil

Production and Performance of Bio Diesel from Cotton Seed and Rice Bran Oil

The alkaline catalyzed transesterification process is the reaction of a triglyceride (fat/oil) with an alcohol in the presence of alkaline catalyst such as alkaline meta l alko xides and hydroxides as well as sodium or potassium carbonates to form esters (biodiesel) and glycerol. The alka line cataly zed transesterification of vegeta ble oils proceeds faster than the acid catalyzed reaction. Due to this reason, together with the fact that the alkaline catalysts are less corrosive than acidic compounds, industrial processes usually favour alka line catalysts, such as alkaline metal a lko xides and hydroxides as well as sodium or potassium carbonates.

9 Read more

Zircon geochronology reveals polyphase magmatism and crustal anatexis in the Buchan Block, NE Scotland : implications for the Grampian Orogeny

Zircon geochronology reveals polyphase magmatism and crustal anatexis in the Buchan Block, NE Scotland : implications for the Grampian Orogeny

Our proposed model for punctuated tectonothermal activity during the Grampian Orogeny is illustrated in Fig. 9. Subduction- related tectonism may have started with rollback prior to ca. 490 Ma, producing a back-arc environment with associated arc magmatism and high dT/dP metamorphism in the Buchan Block (Fig. 9a). Arrival of an outboard arc then produced a phase of shortening (the initial phase of the Grampian Orogeny) starting at ca. 488 Ma (see Fig. 9b, at ca. 480 Ma), before rollback of a sub- duction zone located further to the SE began at ca. 473 Ma. Asso- ciated lithospheric-scale extension led to decompression melting, magmatism and advective heating of the middle crust, producing the widespread ca. 470 Ma Grampian (classic Barrovian and Buchan) regional metamorphism (Fig. 9c). Resumed hinge advance by ca. 465 Ma cut off the heat supply for Grampian metamorphism and produced the fi nal shortening phase of the Grampian Orogeny (see Fig. 9d, at ca. 461 Ma). The Grampian (488e461 Ma) phases of the model (Fig. 9bed) follow Viete et al. (2010, 2013), who proposed tectonic mode switches and a shortening e extensioneshortening history during the Grampian Orogeny to explain: (1) a top-Wetop-Eetop-W sequence of shear kinematics in the Portsoy Shear Zone; (2) ‘ syn-orogenic ’ decompression melting at asthenospheric depths < 70 km to produce the Grampian Gabbro Suite; (3) a discrete phase of widespread Buchan and Bar- rovian metamorphism at 473 e 465 Ma.

10 Read more

U Pb Geochronology of the Jebba Granitic Gneiss and Its Implications for the Paleoproterozoic Evolution of Jebba Area, Southwestern Nigeria

U Pb Geochronology of the Jebba Granitic Gneiss and Its Implications for the Paleoproterozoic Evolution of Jebba Area, Southwestern Nigeria

forming period of time marked by long-lived granitic magmatism in Africa and South America [29,30] as si- milar ages on orthogneisses and granitoids have been do- cumented from the Nigerian basement complex, parts of the West African craton, the Zenaga inlier, Morrocco [31], the Borborema Province [32], Gurupi Belt, Brazil [33], Sao Luis craton, Brazil [34], and the Sao Francisco craton, Brazil [35], among others.

9 Read more

Magmatism of the Kenya Rift Valley : a review

Magmatism of the Kenya Rift Valley : a review

ABSTRACT: Tertiary±Recent magmatism in the Kenya Rift Valley was initiated c. 35 Ma, in the northern part of Kenya. Initiation of magmatism then migrated southwards, reaching northern Tanzania by 5±8 Ma. This progression was accompanied by a change in the nature of the lithosphere, from rocks of the Panafrican Mozambique mobile belt through reworked craton margin to rigid, Archaean craton. Magma volumes and the geochemistry of ma®c volcanic rocks indicate that magmatism has resulted from the interaction with the lithosphere of melts and/or ¯uids from one or more mantle plumes. Whilst the plume(s) may have been characterised by an ocean island basalt- type component, the chemical signature of this component has everywhere been heavily overprinted by heterogeneous lithospheric mantle. Primary ma®c melts have fractionated over a wide range of crustal pressures to generate suites resulting in trachytic (silica-saturated and -undersaturated) and phonolitic residua. Various Neogene trachytic and phonolitic ¯ood sequences may alternatively have resulted from volatile-induced partial melting of underplated ma®c rocks. High-level partial melting has generated peralkaline rhyolites in the south±central rift. Kenyan magmatism may, at some future stage, show an increasing plume signature, perhaps associated ultimately with continental break-up.

15 Read more

Thermal impact of magmatism in subduction zones

Thermal impact of magmatism in subduction zones

mal structure and solid flow, which are coupled through the temperature-dependence of mantle viscosity, until a steady-state is achieved. The thermal impact of magma- tism is then defined as the difference between the cal- culated and reference temperature fields. Note that we do not consider any latent heat effects in this exercise because we previously observed that these are negligible. The two-dimensional calculations predict that fluid flow substantially alters the thermal structure in sub- duction zones, as shown in Fig. 2. The main effect is to raise temperatures near the base of the lithosphere, where warm material is transported from the mantle up- ward. These 2D results are qualitatively similar to the 1D column models (cooling above the slab, warming near the surface), indicating that the physical mechanisms dis- cussed in the previous section remain pertinent. Some features only occur in two dimensions, such as the along- slab cooling observed deeper than the fluid source that is caused by advection by the mantle flow. Thus the thermal impact of magmatism is felt beyond where the magma itself flows.

15 Read more

Show all 1837 documents...

Related subjects