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Magma emplacement in a transfer zone: the Miocene mafic Orano dyke swarm of Elba Island, Tuscany, Italy
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Reaction microtextures of REE-Y-Th-U accessory minerals in the Monte Capanne pluton (Elba Island, Italy): a possible indicator of hybridization processes
No full textThe study of accessory minerals in granitoid rocks can provide clues to the history of magmatic processes. In particular, the
textural–chemical characteristics of accessories could represent effective markers of hybridization processes. The concomitant
occurrence of contrasting reaction microtextures of REE–Y–Th–U accessory minerals in the Monte Capanne anatectic–hybrid
pluton suggests the occurrence of transient chemical conditions (alumina saturation up and down in the same system) in the
early stages of crystallization. Incongruent dissolution of apatite produced microcrystal clusters of huttonitic monazite, while
monazite-(Ce) crystals were replaced by allanite-(Ce)Fapatite assemblage at the same location. A magma mingling process,
involving acidic peraluminous and mafic metaluminous end-members, can provide the expected initial strong differences in
alumina saturation that are able to induce such contrasting reactions. The proposed double exchange of accessory minerals
between the two magmas strongly suggests a dynamic setting (stirring and straining of crystal-rich melts) in which anatectic and
hybridization processes evolved
Introduction: LASI III–Magma pulses and sheets in tabular intrusions: Geosphere, 6: 161-162; doi: 10.1130/GES00581.1
No full textThe origins of granites and intrusive rocks
have been widely discussed for a couple of
centuries, and the way volcanoes work and
their magma forms have attracted scientists,
naturalists, and laymen since the dawn of
humankind. However, shallow igneous intrusions,
representing the obvious link between
the hidden kingdom of Pluto and the fi ery
realm of Vulcanus, have been partly overlooked,
leading to some lack of communication
between “plutonic” and “volcanic”
researchers. An effort devoted to heal this
breach has been contributed to by the establishment
of the LASI conferences (named
after laccolith and sill, the main types of shallow
igneous intrusions)
): The Montecristo Monzogranite (Northern Tyrrhenian Sea Italy): a collisional pluton in an extensional setting
No full textThe Montecristo monzogranite (MM) is a near-circular peraluminous monzogranite pluton occupying the entire 10 km(2) of Montecristo Island. Outcrops of country rock are scarce, and are mainly roof pendants of metagabbros and calcsilicate hornfels of the Apenninic ophiolite sequence. Emplacement of the pluton (Rb-Sr age = 7.1 +/- 0.2 Ma), following the early Miocene onset of continental collision, occurred during an extensional phase which migrated eastward via a combined process of subduction-delamination. The MM rocks are strongly porphyritic, the assemblage being composed of alkali-feldspar, quartz, plagioclase (all occurring as mega- or phenocrysts), biotite and minor cordierite. Accessory minerals include tourmaline, apatite, zircon, ilmenite, allanite, monazite, rutile and hellandite. Reconstructed crystallization histories for the mineral phases reveal a polybaric crystallization starting at about 5 kb. Textural variations of MM occur in sharp contact with each other; darker types often form globular masses containing fewer megacrysts and more abundant mafic microgranular enclaves. Geochemical, isotopic and petrographic data indicate that the MM magma was produced by anatectic melting of an intermediate to deep pelitic crustal source. On the basis of the geochemical and mineralogical characteristics of the enclaves, modification of their parent magma occurred by crystal fractionation coupled with mixing and mingling of components from the MM magma. The limited geochemical variation in MM is interpreted as due to crystal fractionation processes during the magma's ascent. Younger porphyritic dykes with more potassic and alkaline affinities cut the pluton; these dykes are concentrated in a major fracture zone and are associated with contemporaneous pseudotachylites
The late Miocene intrusive complex of Elba Island: two centuries of studies from Savi to Innocenti. Acta Vulcanologica, 20/21: 11-32
No full textElba Island represents a key site for the understanding of the tectono-magmatic evolution of the northern Tyrrhenian-Apennine system.
Since xviii century the presence on the island of granites showing similarities with volcanic rocks of inland Tuscany attracted
the interest of naturalists (G. Targioni Tozzetti). Textural and mineralogical analogies between Elba Late Miocene granites and Tuscan
acidic Pliocene-Quaternary volcanites, coupled with the clear evidence of intrusion into ‘young’ sedimentary-ophiolitic rocks
helped the earlier geologists (e.g., P. Savi and B. Lotti, xix century) to firmly discard the idea that all the granites are ‘old’. Lotti’s geological
mapping at Elba Island defined the standard for geological surveys in Italy and provided the necessary geometrical detail of intrusions
for the earlier interpretative model of the acidic magmatism. It is astonishing that at the dawn of granite geology in Tuscany
those early pioneers correctly described the multilayer laccoliths of central Elba Island as well as the bannock-like shape of the Monte
Capanne pluton. After this period of intensive investigations, about 40 years must be waited for new and innovative studies on Elba
magmatism. The multidisciplinary project of L. Trevisan and G. Marinelli (1950-1960s) produced a new geological map of the magmatic
complex, supported by accurate petrographic and geochemical data, and first isotope dating of intrusions. During the 1970-
2000s period, well-constrained petrological models were formulated, allowing a characterisation of the magmatic end members involved
in the petrogenesis of magmatic rocks from Elba Island (mixing-mingling, mixing plus crystal fractionation). Although quite
sophisticated, these models lack of any detailed field study devoted to define the geometry, internal variations and emplacement mechanisms
of the intrusive bodies and their relationships with local tectonic evolution. Starting from the 1990’s, Fabrizio Innocenti and
his research group did overcome the limits of standard granite petrology, understanding the importance of fieldwork, and stimulating
an integrated investigation of the Elba Island granites. Such a multidisciplinary approach led to the definition of an integrated
model for Elba Island magmatism that include understanding of melts generation at depth, transfer/emplacement mechanisms, magma
modifications at emplacement level and interplay of magmatism with local/regional tectonics. The studies developed at Elba Island
have potential to elucidate the geological evolution of granite intrusions throughout the Tuscan Magmatic Province
Intrusive sheets and sheeted intrusions at Elba Island (Italy)
No full textThe processes leading to successful versus failed coalescence of similar magma batches upon their emplacement are investigated at Elba Island (Tuscany), where several magma bodies were generated at a single magmatic center over a time span of similar to 1 Ma during the Late Miocene. Three nested Christmas-tree laccoliths made up of separated, shallow-level felsic sheets were emplaced at 2-3 km depth with associated roof uplift. Then, at a deeper level, a granite pluton was constructed over a short time span by three magma pulses stacked downward as subhorizontal intrusive sheets, with space for magma generated mostly by roof uplift and tectonic-gravitational displacement of the overburden. Length-to-thickness relationships for individual laccolith layers, as well as for pluton sheets, show a power-law correlation interpreted as the frozen evidence for the occurrence of a vertical inflation stage during intrusion growth. We infer that laccolith sheets failed to coalesce and form a larger pluton because their magma driving pressure exceeded the lithostatic load in a crustal section rich in subhorizontal magma traps (a thrust stack of bedded rocks). However, the driving pressure of the first magma batch of the Monte Capanne pluton was presumably enhanced by an increased magma supply rate, so that the driving pressure exceeded the load at the level of a deeper magma trap represented by a major thrust fault. The following magma batches arrived in rapid succession and were not able to penetrate the still mushy tabular mass. Thus the laccolith sheets and the sheeted pluton represent different outcomes of similar processes occurring under slightly different conditions
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