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Geochemistry of basaltic rocks from the Sasso di Castro ophiolite and comparaison with other mediterranean ophiolites
No full textPetrographic and geochemical characters of magmatic enclaves occurring in San Vincenzo rhyolites (Tuscany, Italy)
No full textPhase disequilibria and crustal contributions in the magmatic mafic enclaves of San Vincenzo volcanic district
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Geology and petrology of the Sasso di Castro ophiolites and associated plagiogranites.
No full textThe Sasso di Castro ophiolite is a huge olistolith within the chaotic complex (late Cretaceous) of the external Liguride unit at Futa Pass, Tuscan Apennines. The ophiolite consists of massive basalts cut by few dykes and overlain by pillow basalts which grade up into pillow basalt breccia covered by red cherts. The whole sequence was affected by ocean-floor metamorphism under static conditions from lower amphibolite to zeolite facies. As opposed to other occurrences in the Northern Apennines, the Sasso di Castro plagiogranites (trondhjemites and very scarce diorites) occur as two small stock-like bodies intruded in the lower part (massive basalt) of the effusive complex. The largest intrusion produced a small thermometamorphic aureole whose hornfelses reached lower amphibolite facies. The thermal effect was associated with fragmentation of the wall-rock and locally also with plastic deformation. Chemical exchange between the host massive basalts and the intruded plagiogranite body has been recognized. The plagiogranites, particularly the trondhjemites, have extremely low K2O (50) ratio. However the presence of a small amount of modal biotite indicates crystallization from a relatively K2O-rich 'granitic' liquid. Low K2O and CaO contents and absence of K-feldspar in the Sasso di Castro plagiogranite probably result from K/Na and Ca/Na exchange in feldspars related to hydrothermal ocean-ridge metamorphism. All the analysed basalts were sampled closely to one another in a quarry. The ratios between refractory elements such Ti, Zr, Nb, Y allow the subdivision of the Sasso di Castro basalts into two groups with high and low Zr/Y; they can be interpreted as two series fractionated from two different parental magmas, which are more enriched in incompatible element than normal MORB. Melting models of the Sasso di Castro basalts as well as of basaltic rocks from Northern Apennines, Western Alps and Corsica ophiolites point to at least two significantly different types of mantle sources: one LREE enriched (Maggiorasca, external Ligurides, Montgenevre, Balagne), the other LREE depleted (Inzecca, Eastern Liguria). It is shown that the Western Mediterranean ophiolite basalts (which are more enriched in incompatible elements than normal MORB) are clearly distinguishable from both transitional and enriched MORB
Petrogenesis of orenditic and kamafugitic rocks from Central Italy
No full textIn central Italy, ultra potassic rocks having an orenditic or lamproitic and kamafugitic affinity are found associated with potassic (KS) and highly potassic (HKS) volcanic rocks of the Roman Province. Orenditic rocks (OREN) are silica oversaturated, intermediate in composition and have a high (71-80) value of Mg# [100 Mg/(Mg + FeZ+)], high KINa, and high abundances of incompatible elements, Cr and Ni. Kamafugitic rocks (KAM) are ultrabasic, strongly silica undersaturated, and show the highest Ca(%), Mg# (75-81) and K/Na in the Roman province; incompatible elements are highly enriched; Ni and Cr range from 79 to 153 and from 40 to 830 ppm, respectively. Orenditic rocks have higher Sr-isotope ratio (0.71256-0.71715) and lower abundance of Sr (402-847 ppm) with respect to KAM (87Sr/86Sr 0.71037-0.71120; 1724-3704 ppm Sr). Derivation of OREN from KAM or HKS magmas by assimilation of crustal material, although consistent with Sr isotopic variations, is ruled out by the high Mg#, Ni and Cr of OREN, as well as by a large number of other geochemical and petrological data. The genetic model proposed suggests that KAM and OREN were generated by melting at different depths of a residual, phlogopite-bearing upper mantle enriched in LILE and radiogenic Sr. Enrichment was provided by addition of liquids derived by melting of sediments carried down by subduction processes that were active under the Appennines during Tertiary times. The strong degree of silica undersaturation and the very high Mg# exclude significant interaction of KAM magmas with crustal rocks. Contamination of OREN magma with significant amounts of crustal material is possible only if parental magma had extremely high Mg, Ni, and Cr, as found in kimberlites and some highMg lamproites which, however, are not observed in the Italian peninsula
Coexistence of IAB-Type and OIB-type magmas in the southern Tyrrhenian back-arc basin : evidence from recent seafloor sampling and geodynamic implications
No full textNeogene-Quaternary magmatic activity and its geodynamic implications in the Central Mediterranean region
No full textThe petrogenesis and time/space distribution of the magmatism associated with the formation of the Northern and Southern Tyrrhenian basins, together with the directions and ages of lithospheric extension and/or spreading north and south of the 410N discontinuity, show that the two arc/back-arc systems have undergone a different structural evolution at least since the middle Miocene (Langhian). The geochemical components involved in the genesis of the heterogeneities of the mantle sources of this magmatism require two separate, compositionally different slabs: 1) an old oceanic (Ionian) lithosphere still seismically active below the Calabrian arc and the Southern Tyrrhenian region; 2) an almost seismically inactive continental (Adriatic) lithosphere which carried large amounts of upper crustal materials within the upper mantle under the NW Roman Province/Tuscan/Northern Tyrrhenian region. The proposed geodynamic models require: 1) for the Northern Tyrrhenian/Northern Apenninic arc/back-arc system, the delamination and foundering of the Adriatic continental lithosphere as a consequence of the continental collision between the Corsica block and the Adriatic continental margin. This delamination process, which is still ongoing, probably started in the early-middle Miocene, but earlier than 15-14 Ma, as indicated by the age and petrogenesis of the first documented magmatic episode (the Sisco lamproite) of the Northern Apennine orogenesis; 2) for the Southern Tyrrhenian/Southern Apenninic-Calabrian arc/back-arc system, the roll-back subduction and back-arc extension driven by gravitational sinking of the Ionian oceanic subducted lithosphere. This process started after the end of the arc volcanism of Sardinia (about 13 Ma) but earlier than the first recorded episode of major rifting (about 9 Ma) in the Southern Tyrrhenian back-arc basin.JCR Journalope
OIB-type magmas within the Tyrrhenian back-arc basin: a slab tear-induced lateral African mantle flow product.
No full textMagmatism from Mesozoic to Present: petrogenesis, time-space distribution and geodynamic implications
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