Lateral displacement of a thermally weakened pluton overburden (Campiglia Mari<ma, Tuscany)

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Lateral displacement of a thermally weakened pluton overburden (Campiglia Mari<ma, Tuscany)

Abstract

Extensional tectonics commonly generates regional-‐scale structures that o4en hinders local varia7ons in the overall extensional regime that, if appropriately described and interpreted, could shed light on second-‐order processes leading to genera7on of anomalous structures, mass displacement, rock fracturing, hydrothermal mineraliza7ons. In the Campiglia MariBma area, a detailed field mapping, coupled with a detailed survey and mapping of 20 km of abandoned mining tunnels led to the reconstruc7on of a local deforma7on history that overlaps with regional extension. This local deforma7on was triggered at the Miocene-‐Pliocene transi7on by the intrusion of a monzograni7c pluton below a carbona7c sedimentary sequence. The carbonates were thermally weakened and did flow laterally, accumula7ng downslope of the pluton contact towards the east. As the thermal anomaly was decaying, the briLle-‐duc7le boundary was approaching the pluton, and the ongoing briLle deforma7on led to the genera7on of large tension gash-‐shaped volumes of fractured marble. These fractured volumes were exploited by rising fluids genera7ng skarn bodies that ul7mately took sigmoid shapes. Further fluids rising in the residual skarn porosity led to the genesis of Zn-‐Pb followed by small volumes of mafic magma, Fe-‐Cu ore and felsic melts emplaced as dykes marking the waning stage of the local, magma-‐induced deforma7on. All these processes occurred in a corridor bounded by SW-‐NE fault arrays, that can be defined as transfer zone accommoda7ng different extension rates, generated by concurrence of regional extension and local displacement of the pluton cover.

A complex magma7c-‐tectonic-‐hydrothermal history has thus been unraveled, with bearings on ore poten7als in seBngs where felsic and maﬁc magmas are associated and ac7vely interact with local tectonics.

IntroducBon

The mobility of magmas and hydrothermal fluids in the upper crust plays a key role during several geological processes as volcanic erup7ons, ore deposi7on, and establishment of geothermal fields. The necessary permeability in country rocks is commonly ac7vated by tectonic ac7vity, focussing magmas and fluids in structural traps. However, the ac7ve contribu7on to tectonic ac7vity by magma emplacement at local scale is yet to be assessed in full. This is also due to the general lack of evidence for the pathways followed by magmas and fluids remaining once emplacement processes are over, so that geometries and textures of igneous and ore bodies represent the only witness to those pathways.

Southern Tuscany is well suited to tackle with these issues, oﬀering the possibility to inves7gate the interplay between processes that, in the late Miocene-‐Quaternary ensialic back-‐arc extensional seBng, led to the genera7ons of the Tuscan Magma7c Province (Serri et al., 1993), diﬀerent types of ore deposits (mainly Fe-‐oxides, pyrite, base metals, and Sb-‐Hg ores; Tanelli, 1983), fossil hydrothermal systems (e.g., tourmaline veins; CavarreLa &

Puxeddu, 1990; Passerini & Marcucci, 1992; Gianelli & Ruggeri, 2002; Dini et al., 2008a;

Mazzarini et al., 2011), as well as ac7ve high-‐enthalpy geothermal ﬁelds (Larderello-‐Travale and Monte Amiata; Calamai et al., 1970; Duchi et al., 1992; Bellani, 2004). In detail, the Campiglia MariBma study area was aﬀected by igneous ac7vity during the late Miocene-‐

early Pliocene, linked with genera7on of metasoma7c rocks and ore bodies (Barberi et al., 1967). Here, the mapping of outcrops and 20 km of mining tunnels, integrated by deep

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boreholes and geophysical data, allowed us to reconstruct the tridimensional morphologies and textures of the magma7c rocks and ore bodies. The evolu7on of the magma7c-‐

hydrothermal system has thus been reconstructed, indica7ng that magmas and hydrothermal ﬂuids followed similar structural paLerns in the upper crust. But overall, the migra7on and emplacement of ﬂuids and magmas were ruled by the lateral displacement of the thermally weakened carbonate overburden of a pluton, a process which interacted with regional extension working on transfer zones.

Geological outline

Tuscany and the Northern Apennines

The geological seBng of Tuscany results from the evolu7on of the rela7ve movements between Adria (Africa) and Sardinia-‐Corsica (Europe) plates, whose convergence started in Late Cretaceous leading to Oligocene-‐Miocene con7nental collision with forma7on of the Apennine mobile belt (PlaL, 200; Molli, 2008). The Apennine tectonic units, stacked on a metamorphic Paleozoic-‐Triassic basement are, from boLom to top: (i) the Tuscan Nappe, formed onto the Tuscan con7nental margin, and consis7ng of late Triassic syn-‐ri4 evaporites and carbonates, early Jurassic to early Cretaceous deepening con7nental plahorm carbona7c-‐siliceous sequence, and a Cretaceous to late Oligocene/early Miocene foredeep detrital clayey-‐turbidi7c succession; (ii) the Sub-‐Ligurian units, deposited onto the transi7on zone between the oceanic and con7nental crust; (iii) the Ligurian units, consis7ng of Jurassic oceanic lithosphere and its Jurassic-‐Eocene sedimentary cover.

A4er the early Miocene collision, the Adria slab rollback coupled with the eastward retreat of the subduc7on zone drove the eastward migra7on of the compressional front, genera7ng an extensional ensialic back-‐arc basin with strongly thinned con7nental crust in southern Tuscany (20 to 25 km, Piana Agos7neB & Amato, 2009; Fig. 1). This crustal extension went through two main phases: (1) a late Oligocene-‐late Tortonian stage with extension exceeding 120 % on low-‐angle faults, and leading in southern Tuscany to elision of parts of the Tuscan Nappe stra7graphic sequence (Brogi, 2008a); (2) a late Miocene to Present stage, characterised by high-‐angle NNW-‐SSE and N-‐S normal faul7ng, producing horst-‐and-‐graben structures, with a total extension less than 10% (Carmignani et al., 1994;

Decandia et al., 2001). These structures are cut by strike-‐slip faults oriented SW-‐NE (e.g., Livorno-‐Sillaro Line). This overall extensiaìonal seBng is challenged by the view of a compressional tectonics ac7ve un7l Pliocene-‐Pleistocene 7mes (FineB et al., 2001; Bonini &

Sani, 2002; Musumeci et al., 2008), possibly only at a deep crustal level (BoccaleB et al., 2011).

The extensional phase is characterised by magma7c ac7vity from crustal and mantle sources (Innocen7 et al., 1992; Serri et al., 2001). The magma7c centres result distributed on SW-‐NE lineaments, on which magma7c ages decrease eastward. These structures have been interpreted as transfer zones triggering extrac7on, rising and emplacement of magmas (Dini et al., 2008b; Fig. 1). These magma7c centres drew the aLen7on of man thoughout twentyseven centuries for their associa7on with Fe, Cu, Pb, Zn, Ag, Au, Sb, Hg ores, pyrite and industrial minerals as well as super-‐heated steam (Dini, 2003).

The Campiglia MariBma area

The area of Campiglia MariBma (herea4er Campiglia) is characterized by a N-‐S trending horst mainly made of carbonate units of the Tuscan Nappe, bounded by high-‐angle extensional and strike-‐slip faults (Acocella et al., 2000; RosseB et al., 2000) (Fig. 2). During the Pliocene, the Campiglia area has been repeatedly aﬀected by intrusive and

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hydrothermal-‐metasoma7c events. The ﬁrst magma7c event led to the emplacement of the Botro ai Marmi monzogranite pluton (K/Ar age of 5.7±0.16 Ma; Borsi et al., 1967). Its primary paragenesis consists of quartz, K-‐feldspar, plagioclase and bio7te, along with accessory 7tanite, apa7te, zircon and tourmaline (Rodolico, 1945; Barberi et al., 1967). This assemblage is rarely preserved due to intense hydrothermal K-‐altera7on, making the pluton ideal for raw ceramic materials (LaLanzi et al., 2001). The granite crops out for as liLle as ca.

1 km² near the western border of the Campiglia horst, yet a larger, N-‐S elongated pluton is known from drilling logs (Stella, 1955; Grassi et al., 1990) and geophysical data (Aquater, 1994). The magma intruded below a Rhae7an grey carbonate unit at a depth corresponding to ca. 0.1 GPa (Barberi et al., 1967), producing an elongated thermal aureole in the carbona7c succession of the Tuscan Nappe (Giannini, 1955; Stella, 1938). A small-‐volume exoskarn is commonly found between the granite and the carbona7c host rock (Barberi et al., 1967) and endoskarn veins cuBng the granite are connected with the exoskarn.

These processes were followed by the forma7on of a voluminous skarn consis7ng essen7ally of clinopyroxene (hedenbergite along with minor johannsenite) and ilvaite, along with very minor garnet in associa7on with rhodonite-‐pyroxmangite, thaumasite, quartz, calcite and epidote (Vezzoni et al., in prep-‐b; Capitani & Mellini, 2000; Corsini et al., 1980).

The skarn host rock is a white marble derived from thermal metamorphism of pure, homogeneous, massive HeLangian reef limestone of the Tuscan Nappe. Sulphides and Fe-‐

oxides (chalcopyrite, pyrite, pyrrothite, sphalerite, galena, magne7te, hema7te) are associated with Ca-‐silicates and were ac7vely exploited for Cu, Pb, Zn, and Ag un7l 1979, mainly in the Temperino and Lanzi mines (Corsini et al., 1980). A low-‐grade Sn deposit (∼0.4 wt%) with cassiterite, pyrite, scheelite, arsenopyrite and bismuthinite is part of a Sn-‐W-‐As-‐Bi belt (Monte Valerio-‐Santa Caterina-‐Campo alle Buche) on the southern side of the buried Botro ai Marmi pluton (Dini & Senesi, 2013), and was ac7vely exploited un7l 1946 near Monte Valerio (Venerandi-‐Pirri & Zuﬀardi, 1981; Stella, 1955).

A4er the skarn forma7on, a mafic magma intruded as small dykes or as a skarn pockets infill (Vezzoni et al., in prep-‐a). This mafic magma solidified as the Temperino porphyry, with phenocrysts of plagioclase, clinopyroxene and bio7te, as well as abundant xenocrysts of sanidine (up to 5 cm) and quartz. This porphyry is strongly altered, with par7al oblitera7on of the primary mineralogy and texture (Barberi et al., 1967).

A4er the maﬁc porphyry, two types of felsic dykes emplaced with a primary paragenesis consis7ng of phenocrysts of quartz, sanidine, plagioclase, bio7te, and pini7zed cordierite, overprinted by potassic altera7on (Barberi et al., 1967; Vezzoni et al., in prep-‐b). The felsic Coquand dykes are spa7ally associated to the main skarn bodies (Vezzoni et al., in prep-‐b).

The single felsic Ortaccio dyke, readily iden7ﬁed by the occurrence of abundant cm-‐sized sanidine phenocrysts and rare maﬁc enclaves, crosscuts all the other metasoma7c and magma7c rocks (e.g., Bodechtel, 1967; Vezzoni et al., in prep-‐b) cropping out almost con7nuously for about 8 km from the Temperino Valley to the north of Santa Maria Valley (Fig. 3; Giannini, 1955; Vezzoni et al., in prep b). A K-‐Ar date of 4.3±0.13 Ma on an Ortaccio sample (Borsi et al., 1967) was interpreted as the age of the potassic altera7on (Barberi et al., 1967).

The ﬁnal igneous event in the area is the extrusion of rhyoli7c lavas characterized by phenocrysts of quartz, alkali feldspar, plagioclase, bio7te and cordierite along with variable amounts of small maﬁc enclaves (Giraud et al., 1986; Ferrara et al., 1989; Pinarelli et al., 1989; Feldstein et al., 1994). The ⁴⁰Ar/³⁹Ar emplacement age (4.38±0.04 Ma; Feldstein et al., 1994) is similar to the altara7on age of the felsic porphyry dyke, although direct geological rela7onships are not found.

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DeformaBon styles and geometries

Roof morphology of the Botro ai Marmi pluton

In the Campiglia area, the geometric characteris7cs of the rock bodies and their deforma7on styles are clearly spa7ally related to the loca7on and shape of the Botro ai Marmi pluton and its thermal metamorphic aureole. Intensity of deforma7on decreases with distance from the pluton, and deforma7on type changes accordingly, with records of both duc7le and briLle styles. Therefore, reconstruc7on of the 3D morphology of the pluton-‐host rock surface is crucial to the understanding of local stress evolu7on during the development of the magma7c-‐hydrothermal system.

The morphology of the roof of the Botro ai Marmi pluton (Fig. 3) has been reconstructed on the basis of geological surveys (this work; Giannini, 1955; Acocella et al., 2000; RosseB et al., 2000; Cerrina Feroni, 2007b), exploratory boreholes (Axerio, A.M.M.I. and RIMIN internal reports; Stella, 1938; Stella, 1955; Grassi et al., 1990), and reﬂec7on seismics-‐gravimetric data (Aquater, 1994), in detail: (i) ﬁeld, boreholes, and geophysical data has been used in the central area, around the pluton’s outcrop, (ii) geophysical data in the northern area, and (iii) borehole data in the southern area.

The pluton’roof is N-‐S elongated, with length/width ra7o of 3 to 6. In transversal sec7on, the pluton has an asymmetric proﬁle, with the western side dipping > 70° (this contact is interpreted as a fault by Acocella et al., 2000: see further on), opposed to an mean slope of 25-‐30° on the eastern and southern ﬂanks. The outcropping por7on of the pluton is at the top of a bulge that was mostly uncovered by mining ac7vity for raw ceramic materials (Fig.

3).

Duc7le deforma7on

The early Jurassic carbona7c host-‐rocks of the Botro ai Marmi pluton were thermally metamorphosed (Rodolico, 1931; RosseB et al., 2000) in an aureole about 5 km long in N-‐S direc7on and 2 km wide in E-‐W (Giannini, 1955). Its thickness, which is approximately 300 m to the south (Monte Valerio; Stella, 1938), in the eastern Temperino mining area, reaches at least 900 meters, as aLested by borehole and geophysical data.

The thermally metamorphosed units, even those that were originally massive, are pervasively foliated, deﬁning a broad an7form with a NE-‐SW to N-‐S axial plane, accompanied by minor an7forms and synforms (Acocella et al., 2000; RosseB et al., 2000). The aBtudes of bedded units outside the contact aureole are parallel to the folia7on planes (Giannini, 1955; Acocella et al., 2000; RosseB et al., 2000; Cerrina Feroni, 2007b). As for thermal aureole, also the thickness of the Rhae7an grey carbonate unit in direct contact with the pluton increases outward with respect to the pluton outcrop (Fig. 3): in the south it is 450 m thick, while to the east is about half that thickness (Fig. 3, cross-‐sec7ons A and B). Similar variabili7es are shown by the overlying carbonate units of the Tuscan Nappe, with the reef limestone showing an impressive diﬀerence in thickness from about 150 m on top of the pluton’s center to 500 m to the south (Monte Valerio) to about 1000 m in the eastern side (Lanzi mine; Fig. 3, sec7ons A and B).

The pluton’s thermal aureole is characterized by several folds, whose geometry varies with distance from the pluton. A narrow volume close to the contact with the pluton (tens of m thick) is characterized by decametric folds with non-‐cylindrical geometry, small inter-‐limb angle (7ght to isoclinal) and disharmonic folds with variably oriented axes (Fig. 4A, B). The axial planes are generally slightly dipping and sub-‐parallel to the contact with the pluton. The limbs of the main folds are characterized by minor cm-‐sized isoclinal folds. These features are highlighted by the diﬀerent alternate colors of the beds in the original Rhae7an

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carbonate unit. Minor isoclinal folds in the overlying metamorphosed reef limestone have been also described (Acocella et al., 2000).

Further away from the contact, the overlying carbona7c forma7ons of the Tuscan Nappe show a diﬀerent style of east-‐verging folding, characterized by asymmetric shape and close to open inter-‐limb angle. These features are evident in the Temperino and Lanzi mines, where metric-‐sized lenses of red nodular limestone, with limbs parallel to the marble folia7on, are embedded in the older HeLangian reef limestone (Fig. 4C). The geological map and sec7ons 1:10,000 of the Regione Toscana (Cerrina Feroni, 2007a, b) report similar fold structures in the eastern side of the pluton aureole. The eastern side of the Campiglia MariBma horst show diﬀerent features characterized by chevron folds with sub-‐ver7cal axial planes and sub-‐horizontal NW-‐SE hinges.

BriLle deforma7on and hydrothermal bodies

BriLle deforma7on overprint duc7le deforma7on features. The briLle structures are less spectacular than duc7le ones, and have to be carefully reconstructed in minor skarn facies at the pluton contact, large isolated skarn bodies, maﬁc and felsic dykes, and in areas distal from the pluton.

Endo-‐ and exo-‐skarn occur near the contact between Botro ai Marmi monzogranite and the metamorphosed Rhae7an carbona7c rock. Endoskarn veins (diopside and scapolite) cut the monzogranite and are connected with the exoskarn (diopside, phlogopite, scapolite, vesuvianite, and wollastonite; Fig. 4). The exoskarn occurs as a massive metric zone at the contact with the pluton or as a selec7ve replacement of folded beds of the Rhe7an carbonate, thus mimicking the geometries of the isoclinal folds (Fig. 4A, B).

The tridimensional characteris7cs of the ore bodies (Fig. 5) allow dis7nguishing two diﬀerent groups. The ﬁrst group includes skarn bodies with a sigmoid-‐tabular shape (akin to a mega-‐tension gash), and maximum thickness in their central part (> 40 m; Earle body), which plunge steeply to the NE and is elongated in SE-‐NW direc7on. These bodies taper out at the upper and lower termina7ons toward SW and NE, respec7vely (Fig. 5) and are arranged in en-‐echelon paLern in plan view. Lateral tongues, gently dipping to the NE, are aLached to the NE side of the main sigmoidal bodies, and follow the folia7on of the host marble (further details in Vezzoni et al., in prep-‐b).

The second group includes two skarn bodies, exploited by the Lanzi mine, that share an overall SSW-‐NNE elonga7on, but diﬀers in their ver7cal development. The main Lanzi body is made of a partly coalescing cluster of small sigmoid-‐tabular bodies, collec7vely building up a tabular body striking 040N and steeply (70-‐80°) dipping to the SE (Vezzoni et al., in prep-‐b).

The northeastern side of the Lanzi main skarn body is characterized by several sub-‐

horizontal small tongues. At Lanzi mine, numerous and well-‐developed skarn veins branch oﬀ the skarn mass, tapering out in some meters. The marble host-‐rock shows a well-‐

developed and closely spaced (few cm) array of subver7cal, parallel fractures with preferen7al 025N strike. Fractures and skarn veins share the same aBtude, yet only fractures intersec7ng skarn bodies contain skarn veins. The minor Lanzi skarn body is made of several bodies with decimetric to metric thickness, striking around N030 gently dipping to the NW, and interconnected by SW-‐NE sub-‐ver7cal veins. Also sub-‐horizontal tongues have been observed at Lanzi mine following the folia7on in the marble host-‐rock (Vezzoni et al., in prep-‐b). In summary, all the skarn bodies were formed by ﬂuids exploi7ng a sigmoid-‐shaped volume of briLle subver7cal fractures.

Three intrusive events followed the forma7on of skarn. First, the maﬁc Temperino porphyry magma formed dykes and ﬁlled pockets in both sigmoid-‐shaped and sub-‐horizontal skarn bodies at Temperino mine. In the second event, the felsic Coquand porphyry formed

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two dykes cropping out in the middle of sub-‐ver7cal skarn bodies. The main dyke can be followed discon7nuously for 2 km, cuBng three diﬀerent sigmoid skarn bodies. The minor dyke is partly intruded in the southern part of the Le Marchand skarn body (Figs. 3 and 5).

The third event led to the emplacement of the felsic Ortaccio porphyry dyke, which crops out for 8 km (Fig. 3), and is characterized by steps and bridges. It is worth no7ng that the steps and the bridges are systema7cally arranged in a NNW-‐SSE right-‐lateral en-‐echelon paLern in the southern half of the dyke, while in the northern half they arranged in a N-‐S le4-‐lateral en-‐echelon (Fig. 3).

BriLle structures also occur at the eastern border of Campiglia MariBma horst as (i) small thrust-‐ramp structure in pelagic limestones of the Tuscan Nappe and (ii) normal faults displacing reef and nodular early Jurassic limestones.

Discussion

Kinema7cs of duc7le deforma7on

The morphology of the pluton’s roof is N-‐S elongated with strong asymmetry between the western, steeply dipping contact, and the eastern-‐southern, gently dipping contact.

Carbonate rocks in direct contact with the pluton are characterized by isoclinal non-‐

cylindrical, disharmonic folds with gently dipping axial planes, sub-‐parallel to the pluton roof.

The asymmetry of fold limbs indicate a vergence of the displacement outward from the highest zone of the pluton roof. Moving away from the contact, open folds and sigmoidal lenses of red limestone embedded in the older metamorphosed white limestone (observed in mining tunnels and reported in geological maps as well: Cerrina Feroni, 2007b) invariably have top-‐to-‐the-‐east vergence: overall, this is inferred to be the main direc7on of transport during fold development (Figs. 3 and 4).

These duc7le structures tes7fy for decreasing deforma7on with distance from the pluton, as well as an outward vergence of the structures, thus poin7ng out a main role of the pluton in ruling the duc7le deforma7on. The anomalous thickness of thermal aureole and the reef limestone (now marble) in the eastern side of the pluton (Fig. 3), coupled with the occurrence of lenses of red limestone within the older reef limestone, suggests that the carbonate sequence in the east has been thickened by duc7le sliding/accumula7on of thermally weakened carbona7c material. These deforma7on eﬀects are focussed in a ca. 1.5 km-‐wide, SW-‐NE belt.

Kinema7cs of briLle deforma7on

Overprin7ng of duc7le structures by briLle deforma7on is common occurrence in the Botro ai Marmi thermal aureole. During the briLle phase, the metasoma7c and magma7c rocks did emplace recording the local stress ﬁeld. In fact, exoskarn bodies cut the marble folia7on and replaced the duc7lely folded carbonate host-‐rocks (Fig. 6A, B, C). Also, the endoskarn veins that fed the exoskarn are observed to follow briLle fractures in the Botro ai Marmi pluton.

The sigmoidal Temperino skarn bodies were generated by drawing hydrothermal ﬂuids into tension gash-‐shaped volumes of marble fractured in briLle regime (Vezzoni et al., in prep-‐b). The geometries of these skarn bodies indicate a top-‐to-‐NE sense of shear. Also the skarn veins developed in a briLle regime: they indeed cut the marble folia7on, ﬁlling the pre-‐

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exis7ng briLle fractures in the host marble. These skarns are located within the main SW-‐NE deforma7on belt and are arranged in a right-‐lateral en-‐echelon paLern that, in 3D, has a top-‐to-‐the-‐NE sense of shear.

The Lanzi skarn bodies also have sigmoidal shapes, yet with diﬀerent orienta7ons. The main Lanzi body is made of coalescing minor sigmoid-‐shaped bodies with a SSW-‐NNE elonga7on and steeply plunging to the SE (Vezzoni et al., in prep-‐b). The minor Lanzi body consists of small sub-‐horizontal bodies slightly plunging (<30°) toward SE, interconnected by sub-‐ver7cal SW-‐NE skarn veins.

Overall, these geometries are located along the northern boundary of the main deforma7on belt, arranged in a le4-‐lateral en-‐echelon paLern that, in 3D, indicate a transtensional top-‐to-‐the-‐E sense of shear.

A4er skarn development, different magmas were emplaced in sequence (Vezzoni et al., in prep-‐a). First, a mafic magma formed dykelets and filled pockets in the sigmoidal skarn bodies (Temperino mine; Vezzoni et al., in prep-‐a). The second magma batch emplaced as the Coquand dykes, intruding in the middle of sub-‐ver7cal sigmoidal skarn bodies (Fig. 5).

The 7ght spa7al/geometric rela7onships between magma7c and metasoma7c rocks suggest similar ascent mechanisms for metasoma7c ﬂuids and magmas. The latest magma7c event was the emplacement of the Ortaccio felsic dyke, with completely diﬀerent geometry (Fig.

3), marking the last change in the local tectonics.

Local duc7le-‐briLle transi7on

A transi7on from duc7le to briLle rheological regime is thus clearly recorded by structures in the host-‐rocks of the Botro ai Marmi pluton. The deforma7on regime reversed back to regionally-‐controlled before the emplacement of the Ortaccio felsic dyke, constraining the duc7le regime between pluton emplacement (5.7 Ma) and an age older than the ﬁnal potassic metasoma7sm aﬀec7ng the dyke (4.3 Ma). To further constrain this 7me interval, a comparison can be made with the thermo-‐rheological evolu7on of the host-‐

rocks modelled for the nearby Monte Capanne pluton, Elba Island (Caggianelli et al., 2014), sugges7ng that the duc7le-‐briLle transi7on could have occurred in less than 500 ka.

A unifying model -‐ Interac7on between extension and hydrothermal-‐magma7c system At Campiglia, the ac7ve regional extensional tectonic regime interplayed with the local magma-‐induced tectonics and fluid transfer. A unifying model for all these intertwined events is therefore needed to shed light on a series of significant geological processes, that could be difficult to be understood if tackled as single, isolated phenomena. As a whole, the evidence reported can be interpreted in a 7me-‐space sequence (Fig. 7).

1. The triggering event of all is the emplacement of the Botro ai Marmi monzogranite pluton at ca. 5.7 Ma. This crustal melt was generated in the extending ensialic back-‐arc of the Northern Apennines and emplaced exploi7ng the tectonic discon7nuity at the base of the Tuscan Nappe, one of the main tectonic units of the Apennine belt.

2. The host rocks of the pluton were a Rhae7an grey plahorm carbonate, overlain by a HeLangian white reef limestone and a Sinemurian red nodular limestone. These carbonate units were thermally metamorphosed to marbles, with temperatures reaching 500 °C (P< 0.1 GPa, Barberi et al., 1967) in the inner part of the aureole. The eﬀec7ve viscosity of a marble at these temperatures, if coupled with high strain rates, could be comparable to a crystal-‐loaded viscous felsic magma (Pehord et al., 2003, Zulauf & Zulauf, 2004).

3. The pushing-‐up of the emplaced magma, coupled with the asymmetric shape of the intrusion roof, forced the rheologically weak marbles to “squeeze out” mainly to the east,

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helped by gravity-‐assisted sliding on the eastward dipping slope of the pluton-‐marble contact. Disharmonic, east verging folds were formed in the marble at the pluton contact.

The marbles decreased their original stra7graphic thickness above the pluton by transfer of material to the east, where anomalous thickness of carbonate rocks was accumulated.

Disharmonic transfer of material drove sigmoid lenses of metamorphosed red limestone to be included within older white HeLangian limestone, as well as sigmoids of white limestone to be included within younger red Sinemurian limestone. Such a deforma7on is focussed in a SW-‐NE belt and decreases and becomes less duc7le with distance from the heat source, to the forma7on of small thrust ramps in the easternmost, frontal accre7on zone. Here, chevron folds with sub-‐horizontal, NW-‐SE hinge lines (Acocella et al., 2000) and a thrust structure (N139, 67SW) are observed, orthogonal to the displacement direc7on. This scenario has been well modelled in analogic studies (Merle and Vendeville, 1995), thus accoun7ng for magma-‐induced local compressional structures in a regional extensional regime. The extensional strain in this belt is higher than in the adjacent boundary zones (Fig. 7), making this corridor a transfer zone.

4. The eastward displacement of carbonate material was waning as the thermal anomaly was decaying, so briLle deforma7on progressively overprinted towards the pluton the previous duc7le structures. The main effect of the eastward, gravity-‐assisted, disharmonic displacement of carbonate material was the genera7on of sigmoid-‐shaped large volumes of fractured marble in an en-‐echelon paLern, similarly to the dyke emplacement described in volcanic seBngs (Klugel et al. (2005). These porous volumes acted as structural traps drawing in hydrothermal fluids, that replaced the carbonate host to generate the sigmoidal calc-‐silicate Temperino skarn bodies. These skarns were thus formed in en-‐echelon paLern within a transfer zone. The northern boundary of the transfer zone is defined by the alignement of the minor Lanzi skarn bodies, generated in a set of minor tension gashes striking N040 and arranged to indicate a le4-‐lateral displacement on that boundary. The southern boundary of the transfer corridor is defined by the limit of the eastward (dextral) displaced carbonate units (SAMIM, 1980; Cerrina Feroni, 2007), that explain the asymmetric shape of the carbonate horst; hints to the possible occurrence of such a lineament are also found in Giannini, (1955); Barberi et al., (1967); Acocella et al., (2000); RosseB et al., (2000); Tanelli, (1977).

5. Further fluid ac7vity deposited the Zn-‐Pb, following by mafic magma that emanate the Cu-‐Fe sulfide ores (Vezzoni et al., in prep-‐b). A felsic melt emplaced genera7ng a porphyry dyke (Coquand) in the middle zone of the main skarn bodies, aLes7ng a prolonged ac7vity of the briLle, top-‐to-‐the-‐east displacement process.

6. Finally, with the end of gravity sliding, the Ortaccio felsic dyke was emplaced parallel to the western horst-‐bounding fault. Its apparently contradictory paLern, with right-‐lateral en-‐echelon paLern in the southern half and le4-‐lateral en-‐echelon paLern in the northern half, can be reconciled in a normal, east-‐dipping extensional faul7ng system where the segment arrays are connected by relay ramps (Walsh et al., 2003). Outwards from the igneous-‐hydrothermal system, this ﬁnal extensional event is recorded by large-‐throw, NW-‐SE normal faults to the east and N-‐S faults to the west (Giannini, 1955; Acocella et al., 2000).

Conclusions and implicaBons

Transfer zones in the Apennines

Transversal structures in the Northern Apennines are known since Signorini (1935) and Sacco (1935). However, their interpreta7on as transfer zones is accumula7ng only in the latest years (e.g., Bartolini et al., 1983, Costan7ni et al., 1993, Acocella & Funiciello, 2006,

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Dini et al., 2008b; Brogi & Fabbrini, 2009; Brogi et al., 2010; Brogi, 2011; Brogi et al., 2011;

Brogi & Fuligna7, 2012). All these transfer structures are associated with igneous and/or hydrothermal ac7vity, and transfer movements are interpreted to drive magma emplacement in both intrusive (Dini et al., 2008b) and volcanic environments (e.g., Acocella et al., 2006; Brogi et al., 2010).

Nevertheless the way these magmas ascended from their sources is s7ll enigma7c because extensional structures are either (i) older than magma7sm and with low-‐angle geometry, or (ii) coeval with magma7sm, but with listric geometries in seismic proﬁles (Brogi et al., 2005; FineB et al., 2001) and therefore not penetra7ng the crust at depth. It is thus temp7ng to speculate that the way magma rose from source to shallow crust was via these transfer structures, that should be deep-‐reaching enough to tap magmas from their sources, even in the mantle.

Gravity-‐assisted sliding

The Campiglia transfer zone, when compared to other Apennine transfer zones, behaved in a somewhat different way: not only it did draw magmas and fluids towards the surface, it also defines a corridor where lateral displacement of thermally weakened marble occurred at higher-‐than-‐regional extensional rates, thus determining a southern boundary characterized by right-‐lateral movement, and a northern boundary with le4-‐lateral movement.

Another, nearby, prominent example of eastward lateral displacement of pluton overburden occurred above the 7 Ma Monte Capanne intrusion (Elba Island, Westerman et al., 2004). However, in Elba the crustal slice was displaced by ca. 8 km as a thick, coherent, briLle body, whereas in Campiglia the pluton intrusion in carbonate units generated a lateral diplacement in duc7le regime for the deeper parts, gradually changing to briLle movements more distally. Thus, the resul7ng displaced material in Elba preserved its tectono-‐stra7graﬁc and intrusive layout, whereas in Campiglia the deforma7on structures formed in the thermally weakened, outward squeezing aureole.

Addi7onally, the proposed model simply explain the two types of compressional features in the Campiglia area, that are clearly related to the emplacement of the Botro ai Marmi pluton: (i) the isoclinal folds in the marble at the pluton contact, which would otherwise have an anomalous style in the frame of the Apennine compressional features, and (ii) the small compressional thrust ramps at the easternmost reach of the tranfer zone. This scenario thus points out two of the mul7ple ways how local compressional structures can form in presence of ac7ve magma7sm in an overall extensional seBng.

Tectonic traps for magmas and hydrothermal ﬂuids

In southern Tuscany, ore bodies and magma7c rocks did generate exploi7ng similar tectonic traps (e.g., Amiata: Brogi et al., 2010; Brogi et al., 2011, Elba Island; Dini et al., 2008b; Gavorrano; RosseB et al., 2001; Roccastrada; Brogi & Fuligna7, 2012). At Campiglia, a more speciﬁc inves7ga7on reﬁnes this scenario, poin7ng out that the forma7on of mega-‐

tension gash-‐like fractured volumes was able to enhance permeability in the shallow crust and draw-‐in hydrothermal ﬂuids and magmas from deeper sources. These anomalous tectonic structures should be taken into account in ore and geothermal explora7on.

Acknowledgements

This work has been carried out as part of the PhD of SV, in the framework, of the PhD program of the University of Pisa. Thanks are due to Marco Pistolesi and Luca Tinagli for their help during ﬁeld survey.

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