The Orano Porphyry dyke swarm (Elba Island, Italy): the last 'beat' of the Monte Capanne pluton system

Autor: Dini A. (1), Innocenti F. (2), Rocchi S (2), Westerman D.S. (3)
Jazyk: angličtina
Rok vydání: 2006
Zdroj: Lasi II: Physical geology of subvolcanic systems: Laccoliths, Sills, and Dykes, pp. 43–44, Isle of Skye, Scotland, April 1-3, 2006
info:cnr-pdr/source/autori:Dini A. (1), Innocenti F. (2), Rocchi S (2),. and Westerman D.S. (3),/congresso_nome:Lasi II: Physical geology of subvolcanic systems: Laccoliths, Sills, and Dykes/congresso_luogo:Isle of Skye, Scotland/congresso_data:April 1-3, 2006/anno:2006/pagina_da:43/pagina_a:44/intervallo_pagine:43–44
Popis: Recent advances in granite research established the many simi- larities in transfer and emplacement mechanisms between volca- nic and plutonic systems (magma transfer via dykes, high transfer rates, multiple injections, etc.) with the major difference defined by the role of magma traps in hampering the ascent of granitic magmas (Rocchi et al. 2002; Petford et al. 2000). In volcanic complexes, sequentially emplaced magma batches are quenched in distinct lithostratigraphic units at the surface. On the con- trary, the relatively long crystallization history of plutonic sys- tems (in the order of 104-105 years) may obliterate the record of such multiple events. Understanding the real history of a plu- tonic system cannot be addressed by a ''simple'' petrological approach aimed at identifying sources involved in the magma- tism since the time variable also has to be constrained. The reconstruction of the sequence of injection events is a pre- condition, however difficult, to build-up a realistic scenario in which the sequential arrival of distinct pulses of magmas at the emplacement level can be linked to significant changes occurring in both the source level as well as in tectonic setting controlling the magmatism (regional vs. local). The Miocene intrusive activity of Elba Island (Italy) covers a significant time span (from about 8.5 to 5.9 Ma), thus offering the opportunity to investigate temporal variations of deep-seated processes and materials during the post-collisional evolution of the Apennine orogenic belt. Ten intrusive units of various geometries, sizes and chemistries are exposed on Elba Island (Dini et al. 2002; Westerman et al. 2004) occupying almost half of its surface (two plutons, four laccoliths, and four dyke systems): seven of them build up the older central-western Elba complex (8.5-6.8 Ma), while the last three define the younger, and much smaller, eastern Elba complex (5.9 Ma). The magma formation processes recorded between ca. 8.5 and 6.8 Ma in central-western Elba, changed from crust-, to hybrid-, to mantle-dominated, as the Apennine fold belt was progressively thinned, heated, and intruded by mafic magmas during late Miocene time. Very unu- sual melts emplaced at the beginning and at the end of the igneous activity did not contribute to the generation of main hybrid magmas of Monte Capanne pluton. They do, however, emphasize the highly variable nature of crustal and mantle sources that can be involved, during a short time span, in post-collisional, exten- sion magmatism. After the reconstruction of this first-order history, new de- tailed mapping carried out on Monte Capanne pluton provided data to unravel a second-order history in which multiple injec- tions coalesced in a single, composite, and geochronologically indistinguishable pluton (?6.9 Ma). Several facies can be detected in the pluton, the most relevant being (i) the monzogranitic Sant'Andrea Facies, characterized by numerous large K-feldspar megacrysts and mafic enclaves, and (ii) the granodioritic-mon- zogranitic San Piero Facies, quarried for its homogeneous texture almost devoid of large megacrysts and mafic enclaves. The patchy distribution of the Sant'Andrea facies, dominantly around the margin of the pluton, suggests that its arrival is followed by stirring during emplacement of the San Piero facies. Both the facies are crust-mantle hybrid products. Finally, the last ''beat'' of the Monte Capanne magmatic system produced a relatively more mafic dyke swarm: the Orano Porphyries. The Orano dyke swarm was emplaced at 6.85 Ma, cutting the Monte Capanne pluton. Orano porphyry is typically dark and contains an assemblage of olivine, clinopyroxene, phlogopite, plagioclase. These petrographic features, coupled with geochemical and iso- topic data suggests a genesis from strongly modified mantle (Dini et al. 2002). This mantle-derived magma is distinctly different from that involved in the earlier main hybridization process (Monte Capanne pluton) and recorded by the mafic microgran- ular enclaves hosted in the monzogranite. Orano dykes occur primarily in the north-western portion of the Monte Capanne pluton and its contact aureole, but they also occur in the unmetamorphosed terrain of central Elba. According to the geological data and reconstruction proposed by Westerman et al. (2004), the Orano dyke swarm rose up into the pluton overburden before the tectonic unroofing of the pluton along a low angle detachment fault (Central Elba Fault) moved the upper part of them several km to the east. The Orano dykes have sub- vertical attitudes and range in size from few centimeter to 50 m thick and from a few meters to 6 km along strike; larger and continuous dykes can be followed from sea level up to the top of Monte Capanne at 1,000 m in elevation. Considering their intrusion up into the pluton overburden, the overall vertical extension of these dykes sum up to about 5 km. More than 70 dykes were mapped with a total surface area of approximately 2 km2, providing a rough estimate of the Orano magma volume at around 10 km3. Structural data on Orano dykes put in evidence a major trend ENE-WSE with few dykes oriented WNW-ESE, NW-SE, and N-S. In some instances, major ENE-WSW dykes have tips with WNW-ESE strike. Their attitude seems unrelated to plutonic structures of local origin (i.e., radial and tangential patterns), and suggest a link to regional tectonic patterns. Structural data, coupled with dyke distribution in the north- western portion of the pluton and with the presence of a magnetic anomaly localized at the base of the pluton itself (work in pro- gress), point out the interplay of local (emplacement of a batch of mafic magmas at the base of north-western portion of the pluton) and regional (large-scale stress field) factors during the emplace- ment of this last mafic ''beat'' in the life of the Monte Capanne magmatic system. References Dini A, Innocenti F, Rocchi S, Tonarini S, Westerman DS (2002) The magmatic evolution of the laccolith-pluton-dyke complex of the Elba Island, Italy. Geol Mag 139(3) Rocchi S, Westerman DS, Dini A, Innocenti F, Tonarini S (2002) Two-stage laccolith growth at Elba Island (Italy). Geology 30(11):983-986 Petford N, Cruden AR, McCaffrey KJW, Vigneresse JL (2000) Granite magma formation, transport and emplacement in the earth's crust. Nature 408:669-673 Westerman DS, Dini A, Innocenti F, Rocchi S (2004) Rise and fall of a nested Christmas-tree laccolith complex, Elba Island, Italy. In: Breitkreuz C, Petford N (eds) Physical geology of high-level magmatic systems, Geological Society Special Publication no 234. The Geological Society, London, pp 195-213
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