Sergio Rocchi

The magmatic evolution of the late Miocene laccolith–pluton–dyke granitic complex of Elba Island, Italy

Since late Miocene time, post-collisional extension of the internal parts of the Apennine orogenic belt has led to the opening of the Tyrrhenian basin. Extensive, mainly acidic peraluminous magmatism affected the Tuscan Archipelago and the Italian mainland during this time, building up the Tuscan Magmatic Province as the fold belt was progressively thinned, heated and intruded by mafic magmas. An intrusive complex was progressively built on western Elba Island by emplacement, within a stack of nappes, of multiple, shallow-level porphyritic laccoliths, a major pluton, and a final dyke swarm, all within the span from about 8 to 6.8 Ma. New geochemical and Sr–Nd isotopic investigations constrain the compositions of materials involved in the genesis of the magmas of Elba Island compared to the whole Tuscan Magmatic Province. Several distinct magma sources, in both the crust and mantle, have been identified as contributing to the Elba magmatism as it evolved from crust-, to hybrid-, to mantle-dominated. However, a restricted number of components, geochemically similar to mafic K-andesites of the Island of Capraia and crustal melts like the Cotoncello dyke at Elba, are sufficient to account for the generation by melt hybridization of the most voluminous magmas ( c. e Nd (t) −8.5, 87 Sr/ 86 Sr 0.715). Unusual magmas were emplaced at the beginning and end of the igneous activity, without contributing to the generation of these hybrid magmas. These are represented by early peraluminous melts of a different crustal origin (e Nd (t) between −9.5 and −10.0, 87 Sr/ 86 Sr variable between 0.7115 and 0.7146), and late mantle-derived magma strongly enriched in incompatible elements (e Nd (t) = −7.0, 87 Sr/ 86 Sr = 0.7114) with geochemical–isotopic characteristics intermediate between contemporaneous Capraia K-andesites and later lamproites from the Tuscan Magmatic Province. Magmas not involved in the generation of the main hybrid products are not volumetrically significant, but their occurrence emphasizes the highly variable nature of crust and mantle sources that can be activated in a short time span during post-collisional magmatism.

GEOCHEMICAL AND ISOTOPIC STRUCTURE OF THE EARLY PALAEOZOIC ACTIVE MARGIN OF GONDWANA IN NORTHERN VICTORIA LAND, ANTARCTICA

Abstract The regional distribution of geochemical and isotopic compositions of granitoid rocks from a Gondwana continental margin is studied to highlight its structure and geodynamic evolution. The intrusive rocks emplaced during the early Palaeozoic Ross Orogeny in northern Victoria Land (Antarctica) constitute a high-K calc-alkaline association. The geographic patterns of isotope and geochemical data on granitoid rocks allow the distinction of two portions of the continental margin, separated by a sharp discontinuity. The portion towards the palaeo-Pacific Ocean (Oceanward Side) displays strongly regular inland increase of Sr- and decrease of Nd-isotope ratios, coupled with analogous variations in major and trace elements; on this basis we infer a NW-SE-trending margin affected by SW-directed subduction. The portion towards the East Antarctic Craton (Continentward Side) shows a similar regular variation only for Nd isotope compositions, consistent with a hypothesis of a N-S-striking margin with west-ward subduction. In the Oceanward Side, isotope and trace-element characteristics suggest that the granites were generated by extensive interaction of mantle-derived magmas with high-level crustal melts. The origin of Continentward Side intrusives is compatible with a process of interaction between mantle-derived melts and a mafic granulite lower crust. The granitoids of the two crustal sectors share the same Proterozoic Sm-Nd model ages, suggesting that they both belong to the same crustal province. We interpret this arrangement of crustal segments as due to the shift and rotation of a forearc sliver of the Gondwana margin. This movement was likely enhanced by oblique subduction under an irregular margin weakened by the presence of a magmatic arc.

Two-stage growth of laccoliths at Elba Island, Italy

At Elba Island, Italy, nine shallow-level late Miocene granite porphyry layers connected by feeder dikes built up three nested Christmas-tree laccoliths. Detailed mapping and reconstruction of tectonic history led to restoration of the original 5-km-thick sequence and determination of the dimensional parameters of each intrusive layer. The laccolith layers were emplaced at depths between 1.9 and 3.7 km, exploiting physical discontinuities that served as crustal magma traps inside a stack of nappes. The intrusive layers are 50–700 m thick, with diameters between 1.6 and 10 km. Length to thickness relationships for individual laccolith layers show a power-law correlation that does not fit the known dimensional distribution for laccoliths, but instead fits a line with a slope typical of the theoretical vertical-inflation stage of laccolith development. This is interpreted as the first reported natural example of the occurrence of a vertical-inflation stage during laccolith growth. The dimensional data for Elba intrusive layers also suggest that laccoliths and plutons commonly form by amalgamation of smaller sheet-like bodies, while multilayer laccoliths form when coalescence fails, possibly owing to the large availability of crustal magma traps.

Rise and fall of a nested Christmas-tree laccolith complex, Elba Island, Italy

Abstract In two separate areas of western and central Elba Island (Italy), Late Miocene granite porphyries are found as shallow-level intrusions inside a stack of nappes rich in physical discontinuities. Detailed mapping of intrusive rocks, along with their relations with country rocks, show that outcrops from western and central Elba Island expose the same rock types, with matching intrusive sequence, petrography and geochemical features. Structural and geological data indicate that these layers were originally part of a single sequence that was split by eastward-directed décollement and tilting. The two juxtaposed portions of the original sequence allow the restoration of a 5-km thick sequence, made up of nine main intrusive layers, building three Christmas-tree laccoliths nested into each other to support a structural dome. During their construction, the role of the neutral buoyancy level was of minor significance with respect to the role played by the relatively thin overburden and/or the large availability of magma traps inside the intruded crustal section. Emplacement of the Monte Capanne pluton into the base of the domal structure likely caused oversteepening and initiated decapitation of the complex, with gravity sliding of the upper half off the top.

Oligocene to Holocene erosion and glacial history in Marie Byrd Land, West Antarctica, inferred from exhumation of the Dorrel Rock intrusive complex and from volcano morphologies

The Dorrel Rock intrusive complex in Marie Byrd Land, West Antarctica, consists of a coarse-grained gabbro cut by fine-grained benmoreite and trachyte dikes, all exposed in a single nunatak. It is the only exposed plutonic body related to late Cenozoic volcanism in this part of the West Antarctic rift system. Our 40 Ar- 39 Ar age determinations indicate emplacement of the gabbro took place ca. 34 Ma, followed by dike injection at ca. 33.5 Ma. Marie Byrd Land volcanoes are all younger than ca. 27–29 Ma, and lie on a low-relief Late Cretaceous erosion surface that has been disrupted by block faulting and dome uplift since late Oligocene time. The erosion surface and overlying volcanoes are well preserved, but in contrast, we estimate that at least 3 km of overburden has been eroded away to expose the gabbro. This anomaly is most easily explained if most of the exhumation took place during a period of rapid erosion between ca. 34 Ma and 27–29 Ma and was followed by a pronounced decrease in erosion rate in the late Oligocene. Temporal anomalies in the degree of dissection of volcanic edifices, together with evidence from hydrovolcanic deposits, suggest there was an ice cap in Marie Byrd Land in the late Oligocene and that inland (200+ km) volcanoes were being actively eroded by glaciers until ca. 15 Ma. This is consistent with seismic and stratigraphic work in the Ross Sea, which documents at least two expansions of the West Antarctic Ice Sheet in the early mid-Miocene. We find that rates of glacial erosion in Marie Byrd Land increase significantly with nearness to the coast, and in nonresistant rock. Thus, the observation that inland volcanoes younger than ca. 15 Ma show no effects of glacial erosion, except for one with a basal section of weak tuffs, suggests that a transition from warm-based to cold-based glaciers took place around 15 Ma. These findings are similar to many of those reported from well-studied McMurdo Sound and Ross Sea localities, so they provide a wider regional picture of middle to late Cenozoic climatic and surficialgeologic events in Antarctica.

Intraplate strike-slip tectonics as an alternative to mantle plume activity for the Cenozoic rift magmatism in the Ross Sea region, Antarctica

Abstract The West Antarctic Rift System is one of the largest areas of crustal extension in the world. Current interpretations on its driving mechanisms mostly rely on the occurrence of one or more mantle plumes, active during the Cenozoic or the Mesozoic. Recent studies of structural-chronological relationships between emplacement of plutons, dyke swarms, and volcanic edifices since middle Eocene in northern Victoria Land imply that magma emplacement is guided by strike-slip fault systems that dissect the western rift shoulder in Victoria Land. These studies led to a critical re-examination of the arguments used to support plume models. In Victoria Land, the linear geometry of the uplift and the relative chronology of uplift and extension are inconsistent with the traditional concepts of lithospheric evolution above a mantle plume. The geochemical signature of the mafic rocks is equivocal, because both OIB and HIMU features cannot be exclusively interpreted in terms of plume activity. From a thermal point of view, magma production rates are low compared with the core part of plume-related provinces. Additionally, the hot mantle below the West Antarctic Rift System is not documented as deep as expected for mantle plumes and the shape of thermal anomaly is related to lithospheric geometry, being linear rather than having circular symmetry. The lack of any decisive evidence for plume activity is contrasted by evidence that large-scale tectonic features guide magma emplacement: the Cenozoic fault systems reactivated inherited Palaeozoic tectonic discontinuities and their activity is dynamically linked to the Southern Ocean Fracture Zones. As an alternative to both active, plume-driven rifting and passive rifting, we propose that lithospheric strike-slip deformation could have promoted transtension-related decompression melting of a subplate mantle already decompressed and veined during the late Cretaceous amagmatic extensional rift phase. Magma ascent and emplacement occurred along the main strike-slip fault systems and along the transtensional fault arrays departing from the master faults.

Eocene initiation of Ross Sea dextral faulting and implications for East Antarctic neotectonics

The Ross Sea region of the East Antarctic plate provides evidence for intraplate tectonic activity in Cenozoic times. Still unresolved are the cause, timing and kinematics of this intraplate tectonism. By integrating and discussing the different (kinematic and temporal) signals of Cenozoic tectonism, intraplate dextral shearing is recognized as the main tectonic regime controlling the structural architecture of the Ross Sea region from the Mid-Eocene (c. 40–50 Ma) onward. We speculate that propagation and persistence of this tectonic regime through time constitutes a feasible seismogenetic framework to explain past and current tectonism in the Ross Sea region.

Rapid incremental assembly of the Monte Capanne pluton (Elba Island, Tuscany) by downward stacking of magma sheets

The late Miocene Monte Capanne pluton (Elba Island, Italy) is characterized by the widespread occurrence of euhedral K-feldspar megacrysts, for which variations in size and abundance have been determined at 392 stations. The variability of megacryst distribution defines three main facies characterized by low (San Piero facies), high (Sant9Andrea facies), and intermediate (San Francesco facies) megacryst abundance. The three facies show minor yet systematic differences in whole-rock major- and trace-element contents, isotopic composition, and biotite mineral chemistry, with no detectable link between chemical variabilities and megacryst abundance. These results, along with the reconstruction of the crystallization sequence, suggest that the facies formed at depth as distinct magma batches with their own peculiar geochemical features, which were preserved after ascent and emplacement. The new geological map based on K-feldspar megacryst distribution thus reveals the composite structure of the pluton, which was built up incrementally by downward stacking of three slightly different magma batches, resulting in a sheeted pluton in the intermediate-shallow crust. The three magma batches were emplaced in a short time sequence in order to have internal magmatic contacts preserved, thus giving a new perspective to the ongoing geochronological efforts to unravel age differences between internal plutonic facies.

Intrusive sheets and sheeted intrusions at Elba Island, Italy

The 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 ∼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.

Granite-lamprophyre connection in the latest stages of the Early Paleozoic Ross Orogeny (Victoria Land, Antarctica)

The Ross Orogen of the Transantarctic Mountains developed in response to the early Paleozoic convergence between the paleo–Pacific plate and the Antarctic margin of Gondwana. The central Victoria Land sector of the orogen is characterized by the widespread occurrence of pink Irizar granite plutons and dikes and Vegetation lamprophyric dikes and sills, which were emplaced in a tensional regime during a restricted time interval of the latest Ross Orogeny, ca. 490 Ma, as documented by new geochronological zircon U-Pb and mineral 40 Ar- 39 Ar data. The syenomonzogranitic Irizar granites-dikes and the Vegetation lamprophyres are all potassic and, despite the chemical gap between them, have overlapping 87 Sr/ 86 Sr (490 Ma) and ϵ Nd (490 Ma) values, within 0.7074–0.7092 and −4.4 to −7.5, respectively. The genesis of Vegetation lamprophyres can be ascribed to the melting of previously enriched subcontinental lithospheric mantle further metasomatized by a subduction component during the Ross convergence. Melting was probably linked to asthenospheric upwelling during postcollisional slab rollback and convective thinning and/or delamination of overthickened lithosphere. On the other hand, the overlap of age, geochemical, and Sr-Nd isotope data between Vegetation and Irizar products, supported by geochemical modeling, suggests that the Irizar felsic magmas were derived by partial remelting of underplated material similar in composition to the Vegetation lamprophyres. This scenario provides new insights into the genesis of widespread postcollisional granites in orogens worldwide, implying significant net crustal growth by magma underplating in the very latest orogenic stages. In the regional geodynamic framework, the NE strike shared by both mafic and felsic dikes along 300 km of the convergent margin points to NW-SE extension, which in turn suggests oblique convergence of paleo–Pacific and Antarctic plates during the latest orogenic stages. A comparison of latest igneous activity in the Ross Orogen with contemporary magmatism in southeastern Australia-Tasmania implies that different mechanisms triggered the magmatic activity: slab rollback in the Antarctic sector of the margin versus slab tear in the Australia-Tasmania sector.