Timescales and mechanisms of batholith construction: Constraints from zircon oxygen isotopes and geochronology of the late Variscan Serre Batholith (Calabria, southern Italy)

    Timescales and mechanisms of batholith construction: constraints from zircon oxygen isotopes and geochronology of the late Variscan Serre Batholith (Calabria, southern Italy) Patrizia Fiannacca, Ian S. Williams, Rosolino Cirrincione PII: DOI: Reference:

S0024-4937(16)30124-4 doi: 10.1016/j.lithos.2016.06.011 LITHOS 3957

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LITHOS

Received date: Revised date: Accepted date:

16 January 2016 19 May 2016 16 June 2016

Please cite this article as: Fiannacca, Patrizia, Williams, Ian S., Cirrincione, Rosolino, Timescales and mechanisms of batholith construction: constraints from zircon oxygen isotopes and geochronology of the late Variscan Serre Batholith (Calabria, southern Italy), LITHOS (2016), doi: 10.1016/j.lithos.2016.06.011

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ACCEPTED MANUSCRIPT Timescales and mechanisms of batholith construction: constraints from zircon oxygen isotopes and geochronology of the late Variscan

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Serre Batholith (Calabria, southern Italy)

Patrizia Fiannacca1*, Ian S. Williams2 and Rosolino Cirrincione1 1. Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università degli Studi di

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Catania, C.so Italia 57, 95129 Catania, Italy

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2. Research School of Earth Sciences, Australian National University, Acton, ACT, 2601,

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Australia

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Abstract

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*Corresponding author. Tel.: +39 0957195738; E-mail address: [email protected]

The late Variscan Serre Batholith in central Calabria represents the middle portion, c. 13 km-thick,

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of a tilted crustal section continuously exposed from lower-crustal granulites to upper-crustal phyllites. The batholith is zoned, consisting of several granitoid types that were emplaced at depths ranging from c. 23 to c. 6 km. Deep strongly foliated quartz diorites and tonalites were emplaced into migmatitic metapelites, the intermediate level granitoids are weakly foliated to unfoliated porphyritic granodiorites and monzogranites, and the shallowest bodies are two-mica granodiorites and granites, grading upward to biotite ± amphibole granodiorites, emplaced into paragneisses and phyllites. Five samples, representative of the main granitoid types in terms of both composition and emplacement depth, have been dated by SHRIMP. Zircon from a lower-crustal quartz diorite gave an emplacement age of 297.3 ± 3.1 Ma; two middle-crustal strongly peraluminous K-feldspar megacrystic granites were emplaced at 296.1 ± 1.9 Ma and 294.9 ± 2.7 Ma; a middle-upper crustal 1

ACCEPTED MANUSCRIPT two-mica monzogranite was emplaced at 294.2 ± 2.6 Ma and finally, an upper crustal weakly peraluminous granodiorite from the batholith roof was emplaced at 292.2 ± 2.6 Ma. These results

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are consistent with development of the batholith by incremental multipulse overaccretion, each

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granitoid body being in a dominantly rigid state before intrusion of the next, with little or no

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possibility of magma mixing. The difference in age between the oldest and youngest granitoids, from the batholith floor and roof respectively, is 5.1 ± 4.0 Ma, providing an upper limit of about 9 Ma on the time taken for batholith construction. The presence in the c. 296–294 Ma granitoids of c.

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305–302 Ma anatectic zircon with varied Th/U and oxygen isotope compositions indicates a time

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interval of 8–9 Ma between incipient partial melting in the lower crust and magma emplacement in the middle crust. The emplacement of the first granitoid bodies into the top of the lower crust was controlled tectonically by the activation of a deep-seated shear zone. The shallowest granodiorites

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were emplaced during the waning stages of the shear zone activity, producing late- to post-tectonic

Keywords

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contact metamorphism in the upper crustal phyllites and mylonitic paragneisses.

Serre Massif

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Incremental pluton overaccretion; U-Th-Pb geochronology; oxygen isotopes; crustal recycling;

Highlights

Building the Serre Batholith started at c. 297 Ma and was completed in 5.1 ± 4.0 Ma.

From incipient crustal melting to granite emplacement in c. 9 Ma.

Batholith growth by incremental multipulse overaccretion. 2

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Distinct growth stages associated with melting of distinct magma sources.

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1. Introduction

A wealth of field, geochronological and geophysical studies over the last few decades has progressively changed common thinking about the dynamics and mechanisms of batholith

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construction (e.g., Coleman et al., 2004; De Saint Blanquat et al., 2011; Glazner et al., 2004; Miller et al., 2011). Although giant ignimbrite deposits with volumes greater than > 500 km3 and up to >

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5000 km3 demonstrate the prior existence of large reservoirs of felsic magmas in the continental crust (e.g., Chensner et al., 1991; Lipman and Bachmann, 2015; Willcock et al., 2015), the role of

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large magma chambers in the processes that produce batholiths has been considerably reduced. Many exposed batholiths and plutons are now considered to have formed by incremental

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emplacement of relatively small magma batches, rather than by crystallisation of large magma bodies. These alternative mechanisms have different implications for the way that magma pulses

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might interact with each other and, ultimately, for the processes responsible for the compositional diversity of granitoid magmas (e.g., fractional crystallisation and mixing versus near-source processes; Clemens and Stevens, 2012 and references therein; Fiannacca et al., 2015; Glazner et al., 2004). There also has been a strong focus on the timescales of batholith construction, with increasing amounts of data showing that batholiths commonly are formed by the assembly of granitoid bodies emplaced sequentially over time periods of the order of 105–106 years (e.g., Coleman et al., 2004, De Saint Blanquat et al., 2011; Díaz Alvarado et al., 2013, Glazner et al., 2004; Miller et al., 2007, 2011; Orejana et al., 2012). In situ zircon U-Pb dating in particular has provided new constraints on the duration of batholith-forming magmatic events, but doubt remains in some cases as to the significance of the ages measured. Also little is known about the time 3

ACCEPTED MANUSCRIPT elapsed between the start of partial melting of the crustal source and the final magma crystallisation at the site of batholith emplacement. This anatectic stage marks the beginning of large scale

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intracrustal differentiation and related information is essential to define the complete framework of

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batholith-forming magmatism and to quantify the rate of crustal recycling during the evolution of

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the continental crust.

The late Variscan Serre Batholith in central Calabria provides a particularly favourable opportunity for investigating the timescales and mechanisms of batholith construction. First, it

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represents the middle portion of a continuous tilted crustal section, c. 13 km thick, exposed in its

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entirety from lower-crustal granulites to upper-crustal phyllites (Schenk, 1980). Further, it is a zoned batholith consisting of several granitoid types, ranging from quartz diorites to leucogranites, that were emplaced at depths ranging from about 23 to 6 km (Caggianelli et al., 2000).

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The batholith is located in the southern Calabria-Peloritani Orogen, a Gondwana-derived terrane, the Variscan evolution of which was related to the closure of Palaeotethys and to the final

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amalgamation of Gondwana, peri-Gondwanan terranes and Laurussia leading to the formation of Pangea at c. 300 Ma (Fiannacca et al., 2015). Previous attempts to date the granitoids from the

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batholith have provided a range of late Carboniferous-early Permian ages (e.g., Fornelli et al., 1994; Langone et al., 2014; Schenk, 1980; 1990), but so far no systematic study has been carried out to determine the timescale of batholith construction and the emplacement sequence of the main plutonic bodies. We address these issues using SHRIMP zircon U-Pb dating of five samples that represent the main granitoid bodies from the different levels within the batholith. Further, we extract information from the Th/U and oxygen isotopic compositions of the dated zircon regarding the time interval between incipient partial melting of the crustal source and the final crystallisation of the magma at the emplacement site.

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ACCEPTED MANUSCRIPT 2. Geological setting

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2.1. Regional overview

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The Serre Massif in southern Italy is part of the Calabria-Peloritani Orogen (CPO; Fig. 1), a poly-orogenic basement complex made up of remnants of Variscan and older mountain chains (e.g., Cirrincione et al., 2005; Fiannacca et al., 2012, 2013; Micheletti et al., 2008; Schenk, 1980, 1990;

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Williams et al., 2012) incorporated into the Alpine-Apennine orogenic system (e.g., Cirrincione et

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al., 2009, 2012, 2015).

The best preserved evidence for the evolution of continental crust in the CPO dates from the Variscan orogenic cycle, a period marked by voluminous felsic magmatism at about 300 Ma (e.g.,

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Appel et al., 2011; Fiannacca et al., 2005a, 2008; Graessner et al., 2000; Langone et al., 2014; Schenk, 1980). Apart from minor strongly peraluminous leucogranodiorite-leucogranite plutons,

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which are scattered all over the CPO, and small weakly peraluminous trondhjemite plutons which appear to occur only in the Aspromonte Unit of southern Calabria-northeastern Sicily, the bulk of

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the late Variscan plutonic rocks form granitoid batholiths in northern (Sila Massif) and central Calabria (Serre Massif – Capo Vaticano Promontory). The Serre Batholith is the middle portion of an exposed, continuous cross section of late Palaeozoic continental crust. The batholith is zoned from bottom to top; it consists of several granitoid types, ranging from quartz diorites to leucogranites, that were emplaced at different crustal levels (Caggianelli et al., 2000, Fiannacca et al., 2015, Rottura et al., 1990). . The contacts between the batholith and its underlying and overlying host rocks are also exposed, the whole providing an opportunity to investigate the architecture, processes and timing of batholith construction.

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2.2. Structure and composition of the Serre Massif

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The Serre Massif (Fig. 2) is a rootless nappe superimposed tectonically on low- to medium-

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grade mylonitic rocks. It consists of a continuous cross-section of late Palaeozoic continental crust, about 25 km thick (Schenk, 1980, 1990), exposed after regional tilting associated with Alpine tectonism and the opening of the southern Tyrrhenian basin (Festa et al., 2003). The crustal section

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consists of three lithologically distinct segments (lower crust, middle crust and upper crust) that are

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exposed in sequence from the NW to the SE (Fig. 2).

2.2.1. Lower crust

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The lower-crustal segment (7–8 km thick) is dominated by mafic granulites, including basal layered metagabbros, with intercalated felsic granulites and fine-grained metapelites (Granulite

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Complex; 2–3 km thick). These are overlain by a metapelitic complex (Migmatite Metapelite Complex; 5–6 km thick) consisting mostly of migmatitic paragneisses and minor felsic granulites,

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with interspersed metabasites and rare marbles and augen gneisses. The metapelitic complex is intruded by mafic dykes (quartz monzogabbro to quartz monzodiorite) up to 300 m thick (Fornelli et al., 2011; Schenk, 1980, 1984). The mafic granulites have been decompressed and dehydrated in several stages, passing from peak conditions of 1.1 GPa at 900 °C to 0.7–0.8 GPa at 650–700 °C (Acquafredda et al., 2008). The metapelites have been subject to amphibolite facies metamorphism (peak P-T 0.8 GPa at 700 °C; Acquafredda et al., 2006) related to crustal thickening, followed by multi-stage decompression with dehydration melting, and finally near-isobaric cooling associated with the final stages of exhumation.

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ACCEPTED MANUSCRIPT 2.2.2. Middle crust The mid-crustal segment (12–13 km thick) consists of the plutonic rocks of the Serre

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Batholith (Fiannacca et al., 2015 and references therein). These range in composition from quartz-

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diorites to leucogranites, with rare quartz-gabbros. The granitoids were emplaced at depths ranging

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from c. 23 to c. 6 km (Caggianelli et al., 2000) and variably deformed during and/or after their crystallisation (Angì et al., 2010; Caggianelli et al., 2007; Rottura et al., 1990). Strongly foliated quartz diorites and tonalites in the northern part of the batholith are

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considered to have been intruded relatively early at deep crustal levels, producing a migmatitic

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border zone in the lower-crustal host rocks (Rottura et al., 1990). Conversely, weakly foliated to unfoliated granodiorites and granites in the south were intruded later into higher crustal domains, producing contact aureoles in the low- to medium-grade host rocks of the upper crust (Angì et al.,

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2010; Festa et al., 2013). The intermediate-level granitoids in the central parts of the batholith are predominantly two-mica porphyritic granodiorites and monzogranites (BMPG) (Del Moro et al.,

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1994; Fiannacca et al., 2015) characterised by K-feldspar megacrysts up to 120 mm long. The uppermost granitoids in the south, representing the shallowest intrusions (Caggianelli et al., 2000;

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Fornelli et al., 1994), are two-mica granodiorites and granites (BMG) grading to biotite ± amphibole weakly peraluminous granodiorites (BAG). The various granitoids have a range of chemical and Sr-Nd isotopic features (87Sr/86SrI = 0.707–0.712), Nd = -0.2 to -10.5) indicating that the Serre Batholith was assembled by the emplacement of several magma batches produced by dehydration melting of different crustal sources (Fiannacca et al., 2015).

2.2.3. Upper crust The uppermost crustal segment, exposed in the southern part of the Serre Massif, consists of two metamorphic complexes that were brought into contact along a low-angle tectonic detachment before being intruded by the granitoids (Angì et al., 2010; Festa et al., 2013). The hanging wall complex (Stilo-Pazzano Phyllite Complex) consists of lower-greenschist facies phyllites with minor 7

ACCEPTED MANUSCRIPT marbles, quartzites, and metavolcanic layers. The footwall complex (Mammola Paragneiss Complex) consists mostly of amphibolite-facies paragneisses, with subordinate leucocratic gneisses

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and amphibolites. Both the Late Variscan granitoids and their host rocks are intruded locally by

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felsic to mafic dykes (Romano et al., 2011).

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According to Angì et al. (2010) the Mammola paragneisses experienced a similar history of crustal thickening to the lower-crustal Migmatite Metapelite Complex, reaching peak P-T conditions of 0.9 GPa at 530 °C. Subsequently they were rapidly exhumed along a major

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extensional shear zone that also promoted the emplacement of the granitoids. Those in turn were

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3. Previous dating of the Serre Massif

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responsible for the late- to post-tectonic thermal metamorphism of the upper-crustal rocks.

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3.1 Lower crust

Schenk (1980) obtained an age of 300 ± 10 Ma, interpreted as the minimum age of the

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granulite facies metamorphism, from high-grade paragneisses and felsic granulites by ID-TIMS UPb dating of zircon and monazite. Intrusion of the large volumes of granitoid magma at about the same time has since been suggested as the possible source of heat leading to the thermal metamorphic peak recorded in the Calabrian crust (Graessner et al., 2000). ID-TIMS zircon U-Pb dating of a metamorphosed quartz-monzogabbro dyke intruded into the metapelitic unit gave an age of 298 ± 5 Ma, interpreted by Schenk (1980) as the age of magma intrusion. Subsequent LA-ICPMS U-Pb dating of zircon from a similar quartz-monzodiorite by Fornelli et al. (2011) gave a significantly older result, 323 ± 5 Ma. Fornelli et al. (2011) also reported LA-ICPMS zircon U-Pb dates between 357 ± 11 and 279 ± 10 Ma from three mafic granulites. Clusters of dates at about 347–340, 323–318, 300–294 and 279 Ma were interpreted as recording a sequence of crustal thickening, peak metamorphism and 8

ACCEPTED MANUSCRIPT multistage decompression. The age peak at c. 300 Ma was considered to record the end of partial melting in the lower-crustal metabasites, and the whole 60–70 Ma time interval was interpreted to

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be the duration of anatectic conditions in the Serre lower crust.

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A period of c. 40 Ma (from c. 325 Ma to 280–270 Ma) has been estimated for similar

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processes in the overlying metapelites. That crustal evolution would have involved: 1) crustal thickening at c. 325 Ma (Micheletti et al., 2008); 2) H2O-fluxed melting producing trondhjemitic leucosomes; 3) mica-dehydration melting during decompression between 300 and 280 Ma

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(Micheletti et al., 2008); and 4) the end of decompression at c. 270 Ma (Micheletti et al., 2008).

3.2. Intermediate level granitoids

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There have been several geochronological studies of the Serre Batholith granitoids, only a few of which used zircon-based techniques. Schenk (1980) measured an ID-TIMS zircon U-Pb age

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of 295 ± 2 Ma for the “Cardinale tonalite” from the basal levels of the batholith, and discordant zircon from a quartz diorite underlying the tonalite provided a similar intrusion age.

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Numerous muscovite and biotite Rb-Sr ages have been reported, including: a) 250–220 Ma and 150–130 Ma for the basal “Squillace tonalite” (Caggianelli et al., 2000; Del Moro et al., 2000); b) 180–130 Ma for the overlying “Cardinale tonalite” (biotite ages: Del Moro et al., 2000); c) 290– 250 Ma and 260–190 Ma for the intermediate-level BMPG (Del Moro et al., 1994, 2000); d) 288– 263 Ma and 280–240 Ma for the shallower BMG (Del Moro et al., 2000; Fornelli et al., 1994), and e) 290–270 Ma for the shallowest BAG (biotite ages: Del Moro et al., 2000; Fornelli et al., 1994). These mica ages, integrated with geobarometric data and microstructural observations (Caggianelli et al., 2000; Festa et al., 2013), mostly are considered to reflect diachronous cooling of the granitoids emplaced at different crustal levels, the deepest granitoids cooling later than the shallower ones. Recent zircon LA-ICP-MS dating of a tonalite from the bottom of the batholith (Cardinale 9

ACCEPTED MANUSCRIPT tonalite) and a weakly peraluminous granodiorite (Stilo granodiorite) from the top found two main peaks in the age spectra of both rocks (c. 306 and c. 295 Ma; Langone et al., 2014). The dominant c.

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306 Ma peak, from oscillatory-zoned domains in the tonalite zircon, was interpreted as reflecting

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the emplacement age of the tonalite, while the dominant peak at c. 295 Ma from the granodiorite

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was considered to record the emplacement age of the granodiorite. On the other hand, the minor peak at c. 295 Ma in the tonalite was considered to reflect disturbance of the tonalite zircon isotopic system by subsequent magma ascent; the minor peak at c. 306 Ma in the granodiorite was thought

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to reflect incorporation of tonalite zircon in the ascending granodiorite magma.

3.3. Upper crust

The upper-crustal rocks of the Stilo Unit are considered to have been derived from a

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volcano-sedimentary sequence ranging in age from Cambro-Ordovician to early Carboniferous (Bouillin et al., 1987), or from Silurian to early Carboniferous (Spalletta and Vai, 1990).

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Through LA-ICP-MS dating of monazite from the phyllite host rock of the Stilo granodiorite, Langone et al. (2014) obtained an age of 293 ± 4 Ma for the contact metamorphism

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caused by the intrusion of the granitoids. No other geochronological data have been produced until now to constrain the timing of the Variscan evolution of the uppermost crustal section.

4. SHRIMP zircon geochronology

Following a petrographic and geochemical study of 52 samples collected from different areas of the Serre Batholith (Fiannacca et al., 2015), five of the freshest samples, representative of the main granitoid types in terms of both composition and emplacement depth, were selected for the present study (Fig. 2). The selected samples are: 1) a lower-crustal quartz-diorite (SC42); 2) a middle-crustal two-mica porphyritic monzogranite (BMPG, PE115); 3) a middle-crustal two-mica syenogranite (BMG, NDP2); 4) a middle-upper crustal two-mica monzogranite (BMG, NDP11) and 10

ACCEPTED MANUSCRIPT 5) an upper-crustal weakly peraluminous granodiorite (BAG, SC29). Petrographic descriptions of the selected samples and details of their location within the

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batholith, as well as descriptions of analytical procedures and details of the zircon textural and CL

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features, can be found in Online Appendix A. Analytical procedures were based on those described

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by Williams and Claesson (1987), Williams (1998) and Ickert et al. (2008). Details can be found in Online Appendix A. Sensitive high resolution ion microprobe (SHRIMP) U-Th-Pb and oxygen data

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are reported in Online Table S1.

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4.1. Results

4.1.1. Lower-crustal quartz diorite (SC42)

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The zircon extracted from sample SC42 consisted mostly of sharply euhedral, prismatic crystals with fine, concentric oscillatory zoning throughout (Fig. 3). In rare cases (< 5% of the

core.

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grains) a contrasting zoning pattern in the centre of the grain suggested the presence of an older

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Of the 14 spots analysed, 10 targeted zircon with simple euhedral zoning from the outer parts of crystals, the other 4 targeted grain centres with contrasting zoning that might be inherited cores. All spots were subsequently analysed for O isotopes. There was a narrow range of U and Th contents in the simply zoned zircon (most 255–370 and 140–250 ppm respectively), and also a narrow range of Th/U (0.45–0.81) and 18O (7.3–8.3‰), consistent with all that zircon having precipitated from a single magma (Fig. 4, Table S1). In contrast, there was a large range of U (40– 1110 ppm) and Th (25–265 ppm) in the cores, and also a large range of Th/U (0.07–1.06) and 18O (7.9–9.9‰), consistent with at least some of the cores having crystallised before the magma in a range of crustal environments.

All the U-Pb isotopic analyses plotted in a concordant cluster at about 295 Ma (Fig. 5). 11

ACCEPTED MANUSCRIPT Contrary to expectations based on their textures, U and Th contents and 18O, there was no U-Pb evidence that the cores were older than the igneous zircon overgrowths or crystals. Considering just

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the igneous zircon, there was a small but significant range in radiogenic 206Pb/238U (MSWD = 2.4).

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If the scatter was interpreted as the result of minor radiogenic Pb loss, as seen in the cores, then 5

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low analyses had to be omitted before the scatter was eliminated. Conversely, if the scatter was interpreted as being due to the presence of a subtly older component, the analysis needing to be omitted (5.1) came from igneous zircon no different in Th/U or 18O from the rest. Given the 206

Pb/238U end of the range and the lack of evidence for an

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clustering of the analyses at the low

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inherited older component, neither of these explanations was acceptable. The best interpretation of the data was to include all analyses, giving a weighted mean 206Pb/238U age of 297.3 ± 3.1 Ma (95% c.l.) for the granitoid, the relatively large uncertainty reflecting the scatter in the data. The mean

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18O of the igneous zircon, 8.0 ± 0.2‰, was significantly higher than mantle values (5.3 ± 0.3‰:

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Valley, 2003) consistent with a crustal source for the magma.

4.1.2. Lower-middle-crustal two-mica porphyritic monzogranite (PE115)

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The zircon from sample PE115 consisted of subhedral to euhedral prismatic grains with a range of zoning patterns (Fig. 3). Most grains, about 10% of which contained a possible core, had simple concentric oscillatory igneous zoning, a further 10% consisted of a core surrounded by zircon with strong CL and very weak sector zoning, and the remaining grains consisted entirely of the sector zoned zircon. Thirty five spots were analysed for U-Th-Pb, all but one of which were analysed for 18O. Eleven of the spots were measured on the main igneous zircon population with simple oscillatory zoning. This zircon had mostly a narrow range of moderate U (110–335 ppm) and Th (70–260 ppm) contents, a narrow range of intermediate Th/U (0.43–0.77) and a narrow range of 18O (7.7–8.5), consistent with zircon precipitated from the melt phase of a single granitic magma (Fig. 4, Table 12

ACCEPTED MANUSCRIPT S1). The exceptions were two analyses (19.1, 35.1) with higher U and lower Th/U. The isotopic analyses of the igneous zircon plotted in a concordant cluster at c. 300 Ma (Fig. 6).

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There were 12 analyses with similar Th/U and 18O (9 edges and overgrowths, a centre and 206

Pb/238U

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2 'cores') that probably represented the igneous component. All had the same radiogenic

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within uncertainty (MSWD = 0.63), giving a weighted mean age of 294.9 ± 2.7 Ma (95% c.l.). Their mean 18O of 8.0 ± 0.4‰ was consistent with magma derived from a crustal source. There was a second group of analyses consisting of most of the grain centres, 3 areas that

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Pb/238U within analytical uncertainty (MSWD = 1.1),

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those 20 analyses had the same radiogenic

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looked like the igneous component in CL but had abnormal Th/U, and many of the cores. Sixteen of

giving a weighted mean age of 305.0 ± 2.5 Ma (95% c.l.). This component was distinguished from the main igneous component not only by its age, but also by having both higher and lower Th/U and

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18O. This range in composition made it unlikely to be an igneous component crystallised earlier in the magma history. More likely it was the product of crystallisation in disconnected melt pockets

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during an early episode of incipient partial melting. Five cores significantly older than the main zircon population gave a modest range of dates

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from c. 840 to 420 Ma, with a possible cluster at c. 430 Ma. These cores had a wide range in U (370–2165 ppm) and Th/U (0.04–0.6), but not so much in 18O (7.4–8.9‰), consistent with crystallisation in a range of igneous and metamorphic environments, perhaps containing a minor sedimentary component.

4.1.3. Middle crustal two-mica syenogranite (NDP2) The zircon from NDP2 had many features in common with that from PE115: both had a dominant population of zircon with simple oscillatory igneous zoning, and a subpopulation of CLbright sector zoned zircon, in both many of the zircon grains were surrounded by a CL-dark, trace element rich rim, and in both many of the grains contained a texturally discordant core (Fig. 3). 13

ACCEPTED MANUSCRIPT The zircon was analysed in two sessions, the first focused on a range of zircon types, the second on cores. Twenty six of the 'cores' dated proved to be significantly older than the igneous

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component. Only the grains dated in the first session were analysed for 18O. During the first

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session, 12 analyses were targeted near the edges of grains with simple igneous zoning, the aim

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being to date the emplacement of the granitoid. Those spots had a range of low to moderate U (60– 495 ppm) and Th (60–390 ppm) contents, and with two exceptions, a relatively narrow range of Th/U (0.40–0.95). With one exception, 18O was 8.1–9.1‰ (mean 8.6 ± 0.4‰), indicative of

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slightly more of a high 18O component in the magma than in the case of PE115 (Fig. 4, Table S1).

radiogenic

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All 12 analyses were concordant within analytical uncertainty (Fig. 7) but with a small range in Pb/238U (MSWD = 2.4). Rejecting the two lowest values as indicating Pb loss

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eliminated the scatter and gave a weighted mean age of 298.7 ± 3.0 Ma (95% c.l.). There were,

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however, several analyses of centres and 'cores' that fell within the same range of Th/U and O composition. When those were included in the age calculation no outliers were detected, giving a

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more precise weighted mean 206Pb/238U age of 297.5 ± 2.4 Ma (95% c.l.). The majority of the 'core' analyses from session 2 fell within the same range of Th, U and

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age as the analyses of the igneous zircon from session 1, providing the opportunity to refine the interpretation of the data. The combined analyses of grain centres showed a distinct bimodality in age, with clusters at c. 293 and 303 Ma. There was also a suggestion of bimodality in the ages measured on the young 'cores'. When the 68 analyses of the young component were combined and assessed purely on a statistical basis two age groups emerged, a younger group (n = 58) with a weighted mean

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Pb/238U age of 296.1 ± 1.9 Ma (95% c.l.), and a possible older group (n = 10)

with a weighted mean age of 305.3 ± 2.5 Ma. The former was the best estimate of the emplacement age of the syenogranite, and the latter probably the age of incipient partial melting. The dates obtained from the 26 inherited cores ranged from c. 320 to 2565 Ma. The 23 dates less than 1.0 Ga were distributed in 4 main groups: 320–365, 395–470, 515–675and c. 940–980 Ma. 14

ACCEPTED MANUSCRIPT As in PE115, there was a possible cluster (n = 5) at c. 420 Ma. The 3 analyses giving dates over 1.0 Ga were all ≥ 40% discordant, so the

207

Pb/206Pb dates for those grains (c. 1.65, 1.67 and 2.56 Ga)

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were minimum age estimates only. Two of the cores analysed in the first session, one

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Palaeoproterozic and the other Archaean in age, had low 18O (5.7 and 5.9‰, Fig 4., Tab. S1).

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These are the only indications in all the zircons analysed of the possible presence of a mantlederived component in the source rocks of the magmas. The dominant O isotopic signature in the

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NDP2 cores is indicative of derivation from rocks that have been altered or weathered.

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4.1.4. Middle-upper crustal two-mica monzogranite (NDP11) The zircon from sample NDP11 occurred mostly as sharply euhedral prismatic grains, most

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of which had fine concentric oscillatory zoning (Fig. 3). In about 10% of the grains there was a core

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marked by a contrasting zoning pattern.

Twenty six zircon grains were analysed for U-Th-Pb; none were analysed for O isotopes.

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Eleven of the analyses targeted areas with simple concentric oscillatory zoning in order to date the magmatism, 2 targeted the centres of grains without concentric zoning, and 13 targeted cores, with

300 Ma.

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the aim of characterising the inheritance. Only 3 of the cores gave ages significantly in excess of

The 11 analyses of zircon with oscillatory igneous zoning had a narrow range of U (170–640 ppm), Th (85–265 ppm) and Th/U (0.3–0.6), consistent with zircon grown from the melt phase of a single magma (Fig. 4, Table S1). In contrast, the two grain centres had high Th/U (0.8, 1.6) and the cores had a wide range of compositions (0.022–1.8), indicative of crystallisation in a range of different chemical environments. With the exception of the three inherited cores, the analyses plotted in a concordant cluster at about 300 Ma (Fig. 8). Omitting two analyses probably affected by Pb loss (1.1, 11.1) left 9 analyses with the same radiogenic 206Pb/238U within uncertainty (MSWD = 1.8), giving a weighted mean age of 294.6 ± 3.1 Ma (95% c.l.). When the grain centres and cores that gave dates in the same age range were included, the weighted mean age became 294.9 ± 2.5 Ma 15

ACCEPTED MANUSCRIPT (95% c.l.). There was a group of 6 cores, however, that appeared to be slightly but significantly older 206

Pb/238U within uncertainty

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than the igneous zircon group. Those 6 had the same radiogenic

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(MSWD = 2.0), giving a weighted mean age of 302.4 ± 4.4 Ma (95% c.l.). Removing those from

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the calculation of the igneous zircon age gave 294.2 ± 2.6 Ma (95% c.l.), based on 14 analyses, as the best estimate of the age of magmatism. As in the other samples, it is likely that the slightly older zircon, at c. 302 Ma, records the start of partial melting. The three old cores, at c. 700, 740 and 1965

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Ma, were inherited.

4.1.5. Upper crustal granodiorite (SC29)

The zircon from sample SC29 consisted mostly of prismatic grains with blunt pyramidal

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terminations and fine, simple concentric oscillatory igneous zoning throughout (Fig. 3). Fifteen spots were analysed for U-Th-Pb, 14 of those for 18O. Ten of the analyses targeted

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zircon with simple oscillatory igneous zoning. Four of the analyses targeted cores, all with bright CL, but with a variety of zoning textures ranging from banded zoning to sector zoning to unzoned.

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One analysis targeted the centre of a grain with banded zoning. The zircon with igneous zoning had a narrow range of moderately low U and Th contents (U: 140–290 ppm, Th: 75–180 ppm), very uniform, moderate Th/U (0.51–0.61) and a narrow range in 18O (7.5–8.4, mean 8.0 ± 0.3‰), consistent with all that zircon being precipitated from the same magma (Fig. 4, Table S1). The grain with banded zoning had the same 18O but slightly lower Th/U. In contrast, the U and Th contents of the CL-bright cores were lower and more diverse (U: 55–145 ppm, Th: 30–245 ppm), Th/U was not as uniform (0.35–1.68) and the 18O was much higher (9.3–10.0), implying zircon crystallisation in a range of different chemical environments. Except for the analysis of one core (5.2), the isotopic analyses formed a tight concordant cluster at c. 290 Ma (Fig. 9). Analysis 5.2 had lower radiogenic

206

Pb/238U than the other spots and 16

ACCEPTED MANUSCRIPT much higher common Pb, consistent with Pb isotopic exchange between the zircon and the host rock. The ten analyses of the igneous zircon grains and overgrowths all gave the same radiogenic Pb/238U within analytical uncertainty (MSWD = 0.55), equivalent to a weighted mean age of

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206

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292.2 ± 2.6 Ma (95% c.l.). The three analyses of low-U cores gave an age indistinguishable from

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this, 289 ± 10 Ma (2s), the large uncertainty reflecting the low radiogenic Pb content of two of those spots. There was no isotopic evidence that the cores were significantly older than the igneous zircon overgrowths around them. Either they lost all their radiogenic Pb during magma genesis, which was

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unlikely in such a high silica rock, or they represented a slightly earlier phase of zircon growth

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before the melt phase of the magma became chemically homogeneous.

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5. Discussion

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5.1. Timing and timescales of batholith construction

The primary goal of any geochronological investigation of a plutonic body is to define an

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accurate emplacement age, but this is commonly not a trivial task. Accurate age estimates are hampered by a number of complications ranging from analytical problems related to the technique used, to the issue of correctly interpreting the individual dates measured. Our results indicate that the construction of the Serre Batholith started at 297.3 ± 3.1 Ma, with the emplacement of the basal quartz diorites/tonalites (sample SC42). This age was obtained by dating zircon domains with oscillatory zoning assessed, on the basis of both textural features and chemical composition (Th/U ratios and δ18O), to be a single generation of zircon crystallised from a single magma. It is significantly younger than 306.4 ± 1.6 Ma obtained from LA ICP-MS zircon dating of the basal “Cardinale tonalite” (Langone et al., 2014; sample DH; Fig. 2), but consistent with the ID-TIMS U-Pb age of 295 ± 2 Ma measured by Schenk (1980) on two near-concordant zircon fractions from the same tonalite. 17

ACCEPTED MANUSCRIPT The range of radiogenic 206Pb/238U found by both LA ICP-MS and SHRIMP techniques was larger than expected from the analytical uncertainties. In both cases the analyses were bimodal.

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Langone et al. (2014) interpreted the older zircon (306.4 ± 1.6 Ma) to be the primary igneous

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component that dated the tonalite emplacement and the younger to be the product of

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recrystallisation. Conversely, we interpret the dominant, younger zircon population in SC42 (297.3 ± 3.1 Ma) to record the age of magmatism, and the minor older component to be pre-magmatic. An age of c. 297 Ma for the start of batholith construction is more consistent with the known

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late Variscan evolution of the southern CPO. Field and geochronological evidence suggest that

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crustal melting started at c. 347–325 Ma, producing trondhjemitic leucosomes (Fiannacca et al., 2005b; Fornelli et al., 2002, 2011) and, later, small trondhjemite plutons (c. 314 Ma; Fiannacca et al., 2005a, 2008). Larger magma volumes were produced at 304–300 Ma by decompression-

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dehydration melting to form leucogranodioritic-leucogranitic plutons. The great bulk of the granitoid magmatism that formed the major batholiths started after 300 Ma during the final stages

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of the Variscan Orogeny, as pervasive melting of the lower crust was triggered by basaltic underplating associated with lithospheric thinning (Fiannacca et al., 2015).

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The final stage of batholith construction was marked by the emplacement of the BAG into upper-crustal phyllites and paragneisses. The age of 292.2 ± 2.6 Ma that we obtained from sample SC29 is consistent with the LA ICP-MS age of 295 ± 1.4 Ma measured on the same granitoid (sample PD, Stilo granodiorite; Fig. 2) by Langone et al. (2014). The difference in age between the oldest and youngest granitoids that we dated is 5.1 ± 4.0 Ma (95% c.l.), placing an upper limit of about 9 Ma on the time taken to construct the Serre Batholith. This result provides a tighter constraint on the timing of batholith construction than previous work (upper limit of ~ 17 Ma, Langone et al., 2014). A time interval of less than 9 Ma for building the Serre Batholith is consistent with the timescales proposed for other late Variscan batholiths from south-western Europe (e.g., Casini et al., 2012; Diaz Alvarado et al., 2013; Orejana et al., 2012). This result supports the existence of a 18

ACCEPTED MANUSCRIPT strict relationship between the duration of pluton construction and pluton volume, as proposed by De Saint-Blanquat et al. (2011). Those authors have shown that, independently of the tectonic

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context, this duration ranges from less than 0.1 Ma for plutons of 100–200 km3 to about 8 Ma for

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batholiths of about 2500 km3. The latter has been obtained, for example, for the Tuolumne Batholith

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in the Sierra Nevada (Coleman et al., 2004), which has the same areal extent as the Serre Batholith and a comparable thickness.

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5.2. The time interval between initial crustal melting and granite emplacement

In addition to the inherited/xenocrystic zircon cores older than c. 320 Ma, which are a minor component in the two-mica granites (samples PE115, NDP2, NDP11), most of the samples contain

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texturally discordant < 320 Ma zircon cores with varied Th/U and 18O, indicating that they did not crystallise in the same chemical environment as the zircon crystallising from the melt phase of the

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mobilised magma. The significance of these zircon cores in the more mafic samples (quartz diorite SC42 and granodiorite SC29) is not immediately obvious because, despite their compositions

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contrasting strongly with those of the magmatic zircon, they are not detectably older. In contrast, the three granites contain a core population that is c. 10 Ma older than the magmatic zircon (Fig. 10). These cores form a single component together with coeval grain centers and a few edges having igneous-like appearance in CL, but anomalous compositions. This component is distinguishable from the main igneous population either by being slightly older or by having both higher and lower Th/U and 18O. The presence of a zircon component a few million years older than the main igneous population could indicate that zircon which crystallised from an early magma pulse

was

incorporated into a subsequent magma ("antecrystic" zircon; e.g. Charlier et al., 2005). Nevertheless, the wide range in Th/U and O composition makes that zircon unlikely to be an early igneous component crystallised from a chemically and isotopically homogeneous granite magma. It 19

ACCEPTED MANUSCRIPT appears even more unlikely that it represents zircon from the more mafic granitoids incorporated by the granite magma during its ascent, considering both the different age and the narrow

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compositional range of the magmatic zircon from the quartz diorite.

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The varied compositions of the zircon, together with its crystallisation age, suggest that this

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component is the product of zircon crystallisation within disconnected melt pockets produced during incipient partial melting of the magma source, similar to that identified in the Permian S-type granites of the New England Orogen, eastern Australia (Jeon et al., 2012). It would therefore

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correspond to anatectic zircon produced below the “liquid percolation threshold” (Vigneresse et al.,

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1996), or “melt connectivity transition” (Rosenberg and Handy, 2005), proposed as the minimum of 7-8 vol% melt above which melt pockets connect. After crossing this transition the melt would start approaching chemical-isotopic homogeneity (e.g., Yakymchuk and Brown, 2014). This zircon

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component is best characterised in the porphyritic granite (PE115), due to the narrower range in Th/U and O composition of the main igneous zircon in that sample, but similar evidence can also be

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found in the other two granites. The identification of early-anatectic zircon offers the opportunity of gaining information on the time interval between the beginning of partial melting in the crustal

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source and the crystallisation of the granitoid magma at the emplacement level. In the three granites with emplacement ages of 294.2 ± 2.6, 294.9 ± 2.7 and 296.1 ± 1.9 Ma we have found evidence for zircon crystallisation during pre-magmatic crustal melting at 302.4 ± 4.4, 305.0 ± 2.5 and 305.3 ± 2.5 Ma, respectively. This gives a mean time interval of 9.3 ± 2.2 Ma (95% c.l.) between incipient partial melting of the magma source and granitoid emplacement. This interval is similar in magnitude to the 4.7 ± 2.6 Ma estimated by Jeon et al. (2012) as the mean time between the beginning of crustal melting and the emplacement of granites from the New England Orogen. Short time intervals between high-grade metamorphism/anatexis and granite emplacement have also been found in other areas (e.g., Cesare et al., 2009; Solar, 1998; Zeck and Williams, 2002). Further, measurements of the maximum time elapsed between sediment deposition and partial melting of those sediments to produce granite magma indicate that crustal recycling in different geodynamic 20

ACCEPTED MANUSCRIPT contexts can occur in less than 15 Ma (e.g., Fiannacca et al., 2013; Jeon et al., 2012; Matzel et al., 2006; Schulmann et al., 2002).

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On the other hand, several studies suggest that melts reside in the deep anatectic crust for

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tens of millions of years (e.g., Brown, 2013) and geochronological studies of the Serre lower crust

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(Fornelli et al., 2011) have suggested that anatectic conditions endured for c. 40 Ma in the Migmatite Metapelite Complex and for c. 60–70 Ma in the underlying Granulite Complex (Fig. 10). Regardless of whether melt was present in the Serre lower crust continuously over those

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periods or if melt production was episodic, our results do not preclude anatectic conditions starting

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tens of millions years before the crystallisation of the c. 305 Ma anatectic zircon. Indeed earlier anatectic events related to the near-peak stages of the tectono-metamorphic evolution produced only small melt volumes that crystallised in-situ as leucosomes. As proposed for other sectors of the

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Variscan Belt (e.g., Spalla et al., 2014; Von Raumer et al., 2009; Wilson et al., 2004), the latest post-collisional stages were probably associated with lithospheric thinning and massive basalt

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underplating. Crystallisation of the mantle magmas provided an extra heat source, promoting extensive crustal melting and the generation of melt volumes greater than the “rheological critical

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melt percentage” (c. 20–35 vol%; van der Molen and Paterson, 1979), enabling magma to leave the source region and migrate toward the emplacement site. In the case of the post-collisional Serre Batholith the c. 305 Ma anatectic zircon, carried away from the source by the granitoid magmas, would therefore mark the beginning of this final stage of the Variscan orogenic cycle in southern Italy.

5.3. Timing and mechanisms of sequential granitoid emplacement

A pluton overaccretion model has been proposed for the Serre Batholith by Langone et al. (2014), based on dating of the “Cardinale tonalite” from the bottom of the batholith and of the 21

ACCEPTED MANUSCRIPT “Stilo granodiorite” from the top, suggesting that emplacement of the granodiorite postdated emplacement of the tonalite by about 10 Ma.

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Our dating of a larger number of granitoid types, collected not only from the batholith floor

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and roof, but also from intermediate levels, has clarified the granitoid emplacement sequence and

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placed firmer constraints on the mechanisms of batholith accretion.

In addition to the ages of 297.3 ± 3.1 and 292.2 ± 2.6 Ma determined for the intrusion of the batholith floor and roof granitoids respectively, we obtained ages of 294.9 ± 2.7, 296.1 ± 1.9 and

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294.2 ± 2.6 Ma for intermediate-level granitoids from progressively higher crustal levels. These

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ages are too closely spaced to allow us to construct a detailed step by step chronology of batholith growth, but together support a process of pluton overaccretion. Significant differences from the model proposed by Langone et al. (2014) emerge, however. They envisaged emplacement of the

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batholith in two main pulses separated by c. 10 Ma, deeper tonalites at c. 306 Ma, and shallower granodiorites at c. 295 Ma. Their samples contained the same two zircon age groups (c. 306 and c.

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295 Ma), which they interpreted to indicate that the tonalite was emplaced at c. 306 Ma, but underwent zircon recrystallisation at c. 295 Ma during the ascent of the granodiorite magma. That

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magma, emplaced at c. 295 Ma, was considered to have incorporated c. 306 Ma zircon from the tonalite during its ascent.

This model is at odds with the presence of various genetically unrelated granitoid bodies intruded at different crustal levels, for which SHRIMP zircon dating indicates very closely-spaced emplacement ages, with little or no interaction beween successive batches of magma. It also appears inconsistent that no evidence of the ascent of the strongly peraluminous magmas that form large bodies at intermediate levels of the batholith has been recorded in the samples that Langone et al. (2014) studied. If the c. 295 Ma granodiorite magma did incorporate zircon from older granitoids, it is difficult to understand how it selectively picked up the c. 306 Ma population from the tonalite, ignoring all the igneous and inherited zircon in the intervening BMPG-BMG. There is virtually no trace of such zircon in the late BAG, either in their sample PD or our sample SC29. 22

ACCEPTED MANUSCRIPT Furthermore, if the passage of younger magmas caused zircon recrystallisation in the older granitoids as Langone et al. (2014) proposed, why has only the youngest granodiorite magma had

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this effect? There should also have been an influence from the voluminous strongly peraluminous

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magmas of intermediate age. We find no microstructural or isotopic evidence for post-magmatic

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zircon recrystallisation in the early-emplaced quartz diorite, neither do we observe the degree of dispersion in Pb/U ratios that Langone et al. (2014) reported in the tonalite sample that they studied. Our late granodiorite is also characteristically simple. Like SC42, the granodiorite does not have

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CL-bright or dark grains or overgrowths, and there is no evidence for a component in the zircon as

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old as the igneous component in the three other samples, let alone any component as old as 306 Ma. These considerations carry the general implication that zircon can be a potent tracer of magma mixing processes, because hybrid rocks should contain mixed zircon populations. The Serre

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upper-crustal granodiorites are characterised by a highly homogeneous zircon population. The virtual absence of zircon from the older granitoids indicates that the BAG magmas, although

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traversing large volumes of granitoid, did not interact with them in any significant way. Summing up all the evidence, our results are consistent with a model of incremental

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multipulse overaccretion, a conclusion also supported by geochemical data suggesting assembly of the batholith from numerous magma batches derived from different crustal sources (Fiannacca et al., 2015).

A noteworthy aspect emerging from this study is that the composition of each granitoid magma appears to have been a primary factor in controlling the depth of magma emplacement. The main batholith bodies are arranged in a lithological sequence, with quartz diorites being the deepest granitoids, passing progressively upward into tonalites, BMPG, BMG and BAG. These main granitoid units have a substantial thickness, each at least 2 km. This leads us to the following concept for the process of batholith construction. Once partial melting conditions for a specific lower-crustal source rock are achieved, magmas are produced in an amount, and over a time period, determined by the fertility of that 23

ACCEPTED MANUSCRIPT source under those specific melting conditions. Magmas then ascend to a specific emplacement level where they finally spread laterally to form a sheeted intrusive unit. When the conditions for

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partial melting are reached in another source the process starts again, but the level that the

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ascending magmas reach is influenced by the presence of the preexisting granitoid bodies, so the

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incoming magma either pools beneath the accreting batholith (e.g., Coleman et al., 2004; De Saint Blanquat et al., 2001; Michel et al., 2008) or cuts through all the older units to pool at the batholith roof (e.g., this study; Zak et al., 2013; Zibra et al., 2014). It does not pool within older granite units.

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The dearth of zircon and geochemical evidence for magma mixing in the Serre Batholith

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would suggest that the intrusive bodies rapidly achieved a dominantly rigid state (e.g., rigid sponge of Miller et al., 2011) with more than 60% crystallised volume. Under such conditions magmas could rise through dykes along possible dilatant shear zones and not mix with preexisting magmas

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or spread horizontally, because they are confined laterally by the rigid granitoid framework. Thermal models indicate that even large volumes of magma complete their solidification in the

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middle-upper crust in less than 1–2 Ma, usually before the intrusion of subsequent magmas, and are therefore unlikely to interact with each other (e.g., Coleman et al., 2004; Glazner et al., 2004). The

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opportunity for the interaction between different granitoid bodies would be higher at their horizontal boundary, possibly involving some remelting of the older magma by the younger. Indeed, the only evidence in the Serre Massif for magma interaction is local transitional petrographic and geochemical features at the BMPG-BMG and BMG-BAG boundaries (Fig. 2; Fiannacca et al., 2015, 2016). As first pointed out by Rottura et al. (1990), structural evidence suggests that the emplacement of the early granitoid bodies was controlled tectonically by the activation of deepseated shear zones. Indeed, the quartz dioritic/tonalitic bodies are strongly foliated and the porphyritic granitoids locally have aligned feldspar megacrysts, consistent with syn-tectonic crystallisation. Shear zone control on the ascent and emplacement of granitoid magmas has been proposed to be a common mechanism by several authors (e.g., Brown and Solar, 1998; Florisbal et 24

ACCEPTED MANUSCRIPT al., 2012a, b; Vigneresse, 1995; Weinberg et al., 2004, 2009). Weinberg et al. (2009) have also suggested that periodic changes in the nature of the regional state of strain (e.g., transtensional to

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transpressional) might trigger the transition from periods of magma accumulation in the anatectic

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region to periods of effective magma expulsion, with the magma batches pumped upward by the

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shear zones. According to those authors, this mechanism could account for the incremental growth of composite plutons over millions of years by sequential emplacement of granitoid bodies with distinct chemical signatures.

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The link between shear zone activity and intrusion of the Serre Batholith granitoids has been

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demonstrated by Angì et al. (2010). They have shown that the Mammola paragneisses (Fig. 2), after reaching peak lower-crustal metamorphic conditions (c. 0.9 GPa at 530 °C) during collision-related thickening, were detached from the lower-crustal metapelites before achieving thermal equilibrium

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and were rapidly uplifted to upper-crustal levels (up to c. 0.3 GPa at 470 °C) along a shear zone operating in a dominantly extensional regime. The ductile shear zone became the preferential

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pathway for ascent of the granitoid magmas, and it now appears likely that the Mammola paragneisses acted as the continuously uplifted lid of the batholith, accommodating the

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emplacement of the newly arriving magmas for most of its accretion history. During the last stages of batholith construction and shear zone activity the rising Mammola Paragneiss Complex came into tectonic juxtaposition with the upper-crustal Stilo-Pazzano Phyllite Complex (Angì et al., 2010). The weakly foliated to unfoliated BAG were emplaced during these waning stages of the shear zone activity, suturing the contact between the two metamorphic complexes at the batholith roof and producing late- to post-tectonic contact metamorphism in the host rocks (Festa et al., 2013). This also involved annealing of the mylonitic foliation in the paragneisses closest to the contact with the magmatic rocks (Angì et al., 2010).

25

ACCEPTED MANUSCRIPT 6. Concluding remarks

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The crustal section exposing the multiple levels of the c. 13 km-thick late Variscan Serre

IP

Batholith provides a rare opportunity to investigate the timescales and mechanisms of construction

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of granite batholiths.

SHRIMP U-Pb dating and oxygen isotopic analysis of zircon from granitoid samples representing the main magma bodies emplaced at a range of crustal levels shows that the batholith

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developed over a relatively short time period of 5.1 ± 4.0 Ma, supporting the existence of a strict

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relationship between the duration of magmatism and volume of magmatism, as proposed by De Saint Blanquat et al. (2011). The oldest dated unit is a quartz diorite (297.3 ± 3.1 Ma) from the deepest levels of the batholith. The youngest is a weakly peraluminous granodiorite (292.2 ± 2.6

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Ma) from the batholith roof. Three strongly peraluminous granites from progressively higher intermediate levels, gave mutually indistinguishable intermediate ages of 294.9 ± 2.7, 296.1 ± 1.9

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and 294.2 ± 2.6 Ma respectively.

These results are consistent with development of the batholith by incremental multipulse

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overaccretion. A model of overaccretion implies that any magma traversing earlier magma bodies could easily interact with them through mixing processes. In this regard, the absence of mixed zircon populations showing contributions from the older granitoids in the younger ones, together with geochemical and petrographic evidence indicating only local magma interaction at the boundaries between the most felsic granitoid units (Fiannacca et al., 2015), suggests a limited involvement of magma mixing and appears consistent with a rigid sponge state (Miller et al., 2011) of the preexisting magmas at the time of the subsequent magma intrusion. This dominantly solid state of the granitoid bodies is reflected in the arrangement of the main granitoids in a quite well ordered lithological sequence; there is an upward progression from quartz diorites to tonalites, to two-mica porphyritic granodiorites-granites, to two-mica granodiorites-granites and finally to weakly peraluminous granodiorites. This arrangement suggests 26

ACCEPTED MANUSCRIPT that melting conditions in specific lower crustal magma sources were reached episodically, producing magmas in amounts and over periods determined by the source fertility under those

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specific conditions. When partial melting conditions were reached in another source the whole

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process restarted, but with the emplacement level of the new magmas strongly influenced by the

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presence of the preexisting granitoid bodies. In the Serre Batholith this resulted in a process of overaccretion, although underaccretion is considered to be a more common mechanism in granite batholiths from a range of other tectonic settings (e.g., Annen 2011).

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Emplacement of the strongly foliated quartz diorites-tonalites in the top of the lower crust

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was controlled by the activation of a deep-seated shear zone; the shallowest weakly foliated to unfoliated granodiorites were emplaced during the waning stages of the shear zone activity, producing late- to post-tectonic contact metamorphism in upper crustal phyllites and mylonitic

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paragneisses.

Finally, the presence in the c. 296–294 Ma granites of c. 305–302 Ma anatectic zircon places

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a limit of 8–9 Ma on the time required for effective crustal differentiation, involving recycling of lower crustal components into granites to make up plutonic complexes in the middle crust.

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Although anatectic processes might have started at c. 325 Ma in the Serre migmatitic metapelites and as early as c. 347 Ma in the underlying granulites, these early processes were only able to produce small melt volumes that crystallised as leucosomes. In contrast, the latest post-collisional stages were probably associated with lithospheric thinning and basalt underplating promoting extensive crustal melting that resulted in the generation of magma volumes large enough to leave the source region and accrete the batholith. Although partial melting of mafic sources to produce the early-emplaced quartz dioritestonalites is likely to have started slightly earlier, the dated c. 305–302 Ma anatectic zircon, carried away from the source by the rising granite magmas, is at present the only constraint on the beginning of this final stage of the Variscan orogenic cycle in southern Italy.

27

ACCEPTED MANUSCRIPT Acknowledgments This work was supported by PRIN 2009, PRA 2012 and FIR 2014 grants, and carried out in large

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part while PF was visiting RSES, ANU, in 2012. We thank Luana Moreira Florisbal and Sven

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Morgan for thoughtful and constructive reviews and the guest editor Maria de Fatima Bitencourt for

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editorial handling of the manuscript.

References

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Acquafredda, P., Fornelli, A., Paglionico, A., Piccarreta, G., 2006. Petrological evidence for crustal

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thickening and extension in the Serre granulite terrane (Calabria, southern Italy). Geological Magazine 143, 145–163.

Acquafredda, P., Fornelli, A., Piccarreta, G., Pascazio, A., 2008. Multi-stage dehydration–

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decompression in the metagabbros from the lower crustal rocks of the Serre (southern Calabria, Italy). Geological Magazine 145, 397–411.

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Angì, G., Cirrincione, R., Fazio, E., Fiannacca, P., Ortolano, G., Pezzino, A., 2010. Metamorphic evolution of preserved Hercynian crustal section in the Serre Massif (Calabria– Peloritani

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Orogen, southern Italy). Lithos 115, 237–262. Annen, C., 2011. Implications of incremental emplacement of magma bodies for magma differentiation,

thermal

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dimensions

and

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Fig. 1. (a) Distribution of pre-Alpine basement in Europe (after von Raumer et al. 2009). (b)

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Distribution of Alpine and pre-Alpine (Variscan and/or pre-Variscan) basement rocks in the

Fig. 2.

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Calabria-Peloritani Orogen and main tectonic lineaments (modified after Angì et al., 2010).

Geological sketch map and simplified lithological cross-section of the Serre Massif

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(modified after Caggianelli and Prosser, 2002; Caggianelli et al., 2013; Fiannacca et al., 2015;

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Fornelli et al., 1994; Schenk, 1980) with the locations of dated granitoid samples (stars). Locations of samples dated by Langone et al. (2014; circles) are also shown.

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Fig. 3. Cathodoluminescence images of sectioned zircon grains from the dated granitoids from the Serre Batholith, selected to show the range of textural features present. Grain numbers correspond

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to analysis numbers in Table S1. Uncertainties in the spot dates 1.

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Fig. 4. Dates, oxygen isotopic compositions and Th-U contents of zircon from granitoids SC42, PE115, NDP2, NDP11 and SC29, representing the succession from bottom to top of the Serre Batholith. Analyses plotting beyond the range of the diagrams are indicated where possible. Lines on the Th-U plots indicate Th/U = 0.5, a common ratio for zircon from intermediate composition granitoids. The locations of the analyses within individual zircon crystals are designated as in Table S1.

Fig. 5. U-Pb analyses of zircon from lower-crustal quartz diorite sample SC42, plotted before correction for the very low levels of common Pb. Grey (green): cores, Black (red): igneous centres and edges. Analytical uncertainties 1. 37

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large uncertainties omitted for clarity. Grey (green): cores, black (red): igneous centres and edges,

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shaded (blue): probable anatectic component. Analytical uncertainties 1.

Fig. 7. U-Pb analyses of zircon from middle crustal two-mica syenogranite sample NDP2, plotted before correction for the very low levels of common Pb. Two analyses with large uncertainties

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omitted for clarity. Grey (green): cores, black (red): igneous centres and edges, shaded (blue):

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probable anatectic component. Analytical uncertainties 1.

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Fig. 8. U-Pb analyses of zircon from middle-upper crustal two-mica monzogranite sample NDP11,

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igneous centres and edges, shaded (blue): probable anatectic component. Analytical uncertainties

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Fig. 9. U-Pb analyses of zircon from upper crustal granodiorite sample SC29, plotted before correction for the very low levels of common Pb. Grey (green): cores, black (red): igneous centres and edges. Analytical uncertainties 1.

Fig. 10.

Summary of geochronological data from the Serre Batholith. Squares: granitoid

emplacement ages (white: Langone et al., 2014); triangles: anatexis ages. Gray circles: age of contact metamorphism in the upper crust (Langone et al., 2014). Gray fields: proposed duration of anatectic conditions in the lower crust (Fornelli et al., 2011). See text for more details.

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