Miocene magmatic evolution in the Nefza district (Northern Tunisia) and its relationship with the genesis of polymetallic mineralizations
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Keywords:
Massif
Lile
Igneous differentiation
Asthenosphere
The Toprakkale (Osmaniye) region, located in the Yumurtalık fault zone in southern Turkey, contains Quaternary volcanic rocks, shown by their mineralogical and petrographical features to be alkali basaltic and basanitic. These alkaline rocks are enriched in the large ion lithophile elements (LILE) Ba, Th and U, and show light rare earth element (LREE) enrichment relative to heavy rare earth element (HREE) on primitive mantle trace and rare earth element patterns that indicate different partial melting of the same source. The isotopic 87Sr/86Sr ratio is relatively low (0.703534-0.703575 for the alkali basalts and 0.703120-0.703130 for the basanites) and the 143Nd/144Nd ratio is high (0.512868-0.512877 for the alkali basalts and 0.512885-0.512913 for the basanites), suggesting that both units originated from an isotopically depleted mantle source. The degree of partial melting of the Toprakkale volcanic unit was calculated using the dynamic melting method. The alkali basalts were formed by a high degree of partial melting (9.19%) whereas basanites were formed by a low degree of partial melting (4.58%) of the same mantle source. All the geochemical evidence suggests that the basic volcanism was generated by decompressional melting under a transtensional tectonic regime in the Yumurtalık fault zone, Southern Anatolia.
Lile
Incompatible element
Trace element
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In south Karakorum, the western prolongation of southern Tibet, three distinct types of magmatic rocks were emplaced during the Neogene: (1) 22–24 Myr old lamprophyres, characterized by strong enrichment in large ion lithophile (LILE) and light rare earth elements (LREE), 87Sr/86Sr(i) = 0·7096, εNd(i) = –7, and εHf = –9, interpreted to reflect partial melting of a previously metasomatized spinel-lherzolite mantle source; (2) the 21–26 Myr old Baltoro high Ba–Sr granitoids, likewise strongly enriched in LILE and LREE, with 87Sr/86Sr(i) = 0·7034–0·7183, εNd(i) = –6·5 to –11·0, and εHf = –1·8 to –8·0, produced by partial melting of amphibole-bearing rocks in the lower crust, possibly the root of south Karakorum Cretaceous magmatic arc; (3) the 8–9 Myr old Hemasil syenite and its associated lamprophyre, also both enriched in incompatible elements but with isotopic compositions closer to those of depleted mantle (87Sr/86Sr(i) = 0·7043–0·7055, εNd(i) = +3·5 – + 4·3, and εHf = + 10·4 – + 11·2). The Hemasil syenite is interpreted as the product of partial melting of a time-integrated depleted spinel-lherzolite source that was enriched in K and LREE during a recent metasomatic event. We propose that the lamprophyres were formed during partial melting of the South Asian mantle previously metasomatized by fluids derived from the subducted Indian continental crust. This melting episode is interpreted to be related to a break-off event that occurred within the subducting Indian continental lithosphere. Intrusion of the resulting lamprophyric melts into the previously thickened south Karakorum crust caused partial melting of calc-alkaline igneous protoliths and generation of the Baltoro granitoids. Late-stage syenitic magmas were produced by low-degree partial melting during upwelling and adiabatic decompression of depleted mantle along the Shigar strike-slip fault.
Lile
Metasomatism
Quartz monzonite
Adakite
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The role of partial melting in the uniform lithospheric stretching model of continental margin formation is explored. It is shown that the transition from continental lithosphere stretching to oceanic accretion is most probably controlled by the production of a significant amount of partial melting in the asthenosphere immediately below the lithosphere, which requires stretching factors larger than 3. It is also shown that, at stretching factors exceeding 2, the law of subsidence is significantly changed by the presence of partial melt in the underlying asthenosphere. The implications for the existence of deep continental margin basins on thinned continental crusts are examined. The Armorican deep continental margin basin is taken as an example.
Asthenosphere
Continental Margin
Passive margin
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Lile
Adakite
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Asthenosphere
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Geochemical characteristics and petrography of the acidic volcanic rocks from the South of Danesfahan indicate that these rocks are products of different petrogenetic processes. Magmatic differentiation through fractional crystallization has played the main role in the evolution of majority of acidic rocks. These rocks include the rhyolites with the following features: 1)porphyritic texture, 2)negative anomaly of Eu, and 3)decreasing LILE and increasing REE contents with increasing silica. A few of acidic rocks from the study area show characteristics implying their generation through partial melting of the upper crustal rocks. These include the rhyolites with the following features: 1)vitrophyric or perlitic texture, 2)lack of negative anomaly of Eu, and 3)the highest LILE/REE and LREE/HREE ratios amongst the acidic volcanic rocks.
Porphyritic
Lile
Fractional crystallization (geology)
Igneous differentiation
Petrogenesis
Texture (cosmology)
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Asthenosphere
Low-velocity zone
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Factors of importance in partial melting calculations are discussed. The thermal evolution of a geochemical and petrological model of the upwelling asthenosphere beneath a ridge crest is studied numerically. Partial melting, basalt eruption and differentiation of the upwelling asthenosphere is modelled. Melt distribution and density distribution in the top 100 km of the upper mantle are calculated. Partial melting takes place in a depth interval of 25—60 km below the ridge crest. The degree of partial melting is somewhat less than 20 %. About 2.5 times more liquid is produced by partial melting in the upwelling asthenosphere than is erupted at the ridge centre. This excess liquid solidifies in the lithosphere, off-ridge axis below the Moho. The calculated results are in agreement with the observations on the oceanic ridge basalt composition, its average eruption rate, and geochemical estimates of the degree of partial melting in the sub-ridge upper mantle.
Asthenosphere
Crest
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