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Nov 24, 2007 - Sonja Pabst & Gerhard Wörner & Lucia Civetta & ... S. Pabst . ...... Union within the ERUPT project; we thank Jon Davidson in particular for.
Bull Volcanol (2008) 70:961–976 DOI 10.1007/s00445-007-0180-z

RESEARCH ARTICLE

Magma chamber evolution prior to the Campanian Ignimbrite and Neapolitan Yellow Tuff eruptions (Campi Flegrei, Italy) Sonja Pabst & Gerhard Wörner & Lucia Civetta & Roberto Tesoro

Received: 17 July 2006 / Accepted: 15 October 2007 / Published online: 24 November 2007 # Springer-Verlag 2007

Abstract The Campi Flegrei (Campanian Region, Italy) experienced two cataclysmic caldera-forming eruptions which produced the Campanian Ignimbrite (39 ka, CI) and the Neapolitan Yellow Tuff (15 ka, NYT). We studied the minor eruptions before both these large events to understand magma chamber evolution leading towards such catastrophic eruptions. Major, trace element, and Sr and Nd isotope compositions of pre-Campanian Ignimbrite and preNeapolitan Yellow Tuff products define distinct geochemical groups, which are here interpreted as distinct magma batches. These batches do not show any transitional trend towards the CI and NYT eruptions. The CI and NYT systems are decoupled geochemically and isotopically. At least one of the pre-CI and one of the pre-NYT erupted magma batches qualifies as mixing endmembers for the large CI and NYT eruptions, and thus, must have been stored in reservoirs for some time to remain available for Editorial responsibility: D. Dingwell S. Pabst : G. Wörner Abteilung Geochemie, GZG, Georg-August-Universität Göttingen, Goldschmidtstr. 1, 37077 Göttingen, Germany L. Civetta : R. Tesoro Dept. of Scienze Fisiche, University of Napoli Federico II and Istituto Nazionale di Geofisica e Vulcanologia–Osservatorio Vesuviano, Via Diocleziano 328, 80124 Naples, Italy Present address: S. Pabst (*) Mineralogisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 236, 69120 Heidelberg, Germany e-mail: [email protected]

the CI and NYT eruptions. The least evolved, isotopically distinct magma compositions that are typical of the last phases of the NYT and CI eruptions did not occur before caldera-forming events. Based on the new data, we propose the following scenario: Multiple magma chambers with distinct compositions existed below the Campi Flegrei before the CI and NYT eruptions and remained generally separated for some time unless new magma was recharged. In each case, one of the residing magma reservoirs was recharged by a new large-volume magma input of intermediate composition from a deeper differentiating magma reservoir. This may have triggered the coalescence of the previously separated reservoirs into one large chamber which fed the cataclysmic caldera-forming eruption. Large magma chambers in the Campi Flegrei may therefore be ephemeral features, interrupted by periods of evolution in individual, separated magma reservoirs. Keywords Campi Flegrei . Phlegraean fields . Campanian Ignimbrite . Neapolitan Yellow Tuff . Geochemistry . Sr and Nd isotopes . Magma batches . Precursor activity

Introduction The study of volcanic rocks emplaced before large calderaforming eruptions is of great importance to understand the processes responsible for evolution, growth, and size of large magma chambers. These rocks should bear also essential clues on how magma systems evolve towards cataclysmic eruptions. As such, the products of the precursor activity of large caldera eruptions may provide information for assessing volcanic hazards that is of similar importance as the products of large caldera-forming events themselves.

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The Campi Flegrei (Campanian Region, Southern Italy) is a nested caldera structure formed during two climactic eruptions (Orsi et al. 1996): the Campanian Ignimbrite (CI, 39.28 ka; De Vivo et al. 2001) and the Neapolitan Yellow Tuff (NYT, 14.9 ka; Deino et al. 2004). We try to understand how these large and potentially dangerous magma chambers formed and developed by studying the products of many individual eruptive events before 39 ka and between 39 and 15 ka. In this paper, we present new geochemical and isotopical data of the pre-Campanian Ignimbrite (pre-CI) and preNeapolitan Yellow Tuff (pre-NYT) eruption products based on analyses of pumice clasts from two sections (Trefola quarry, TL and Ponti Rossi, PR).

Geology of the Campi Flegrei area The Campi Flegrei caldera is located within the Campanian Plain along the western margin of the Southern Apennines mountain chain (Fig. 1). The Campanian Plain is composed of 2–3 km thick sequences of Plio-Quaternary continental, deltaic, and marine sediments intercalated with volcanic deposits. It is underlain by a graben formed during Fig. 1 Geological and structural sketch map of the Southern Campanian Plain (modified after Orsi et al. 2004)

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activation of NW–SE and NE–SW trending normal faults, which have downthrown the western Apennines during Quaternary times (Brancaccio et al. 1991). Seismic data show that the top of the Mesozoic limestone basement beneath the Campi Flegrei caldera occurs at about 4 km depth (Zollo et al. 2003) above which no evidence of magma reservoirs of significant size (>1 km3) has been found. Melt inclusion data of minerals in rocks younger than 39 ka indicate levels of crystallization at different depth: at about 10 and between 8 and 3 km, suggesting the occurrence of both deep and shallow magma reservoirs in the past (Cecchetti et al. 2001; Marianelli et al. 2006; Civetta and Rutherford, unpublished data). The Campi Flegrei caldera includes a subaerial and a submerged part, which cover a total area of about 230 km2. It is a resurgent nested structure (Fig. 2) formed during two major caldera collapses related to the eruptions of the Campanian Ignimbrite and the Neapolitan Yellow Tuff, respectively (Orsi et al. 1996). The geometry and dynamics of both large calderas, as well as of smaller volcanotectonic collapses that occurred later, were deeply influenced by both local and regional stress regimes. Each large collapse affected the structural conditions of the system and constrained the vent locations of later volcanism. The onset of

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1999). There have been about 70 minor eruptions, the last of which occurred in A.D. 1538. The volcanic system is therefore still active, as also demonstrated by fumarolic and seismic activity and by recurrent repetitive episodes of unrest in the past 30 years (Orsi et al. 1999b and references therein). The volcanic rocks younger than NYT range in composition from shoshonite to peralkaline phonolite with trachyte and alkalitrachyte as the most abundant rocks (D’Antonio et al. 1999).

Samples

Fig. 2 Topographic map of the Campi Flegrei Region (modified after Orsi et al. 2004) with the Campanian Ignimbrite and the Neapolitan Yellow Tuff calderas and the sampling locations (Trefola quarry and Ponti Rossi)

volcanism in the area is unknown. The oldest dated volcanic unit yielded an age of about 60 ka (Pappalardo et al. 1999) and is related to volcanism extending beyond the limits of the present caldera. Volcanic rocks older than the Campanian Ignimbrite are the product of both effusive and explosive eruptions. They are exposed only along the scarps that border the Campi Flegrei caldera and are mostly alkalitrachytic in composition (Pappalardo et al. 1999). The CI eruption and caldera collapse (Fig. 2) was the earliest event to profoundly influence the present geological setting of the area. During this eruption, at least 200 km3 of trachytic-to-phonotrachytic magma (DRE; Fedele et al. 2003) were emplaced as pyroclastic-fall and -flow deposits. An area of about 30,000 km2 was covered by these highly mobile ignimbrites (Fisher et al. 1993). Volcanism between the CI and the NYT was confined within the caldera and characterized by explosive, mainly phreatomagmatic eruptions. Rocks related to this volcanism vary in composition from trachyte to alkali-trachyte to phono-trachyte (Pappalardo et al. 1999). The NYT eruption and caldera collapse (Fig. 2) was the second more recent cataclysmic event in the history of the caldera and represents the largest known trachytic phreatoplinian eruption (Orsi et al. 1992). It emitted at least 40 km3 (DRE) of latitic-to-trachytic magma emplaced as pyroclasticfall and -flow deposits (Orsi et al. 1992, 1995). The latter deposits covered an area of more than 1,000 km2 (Orsi et al. 1992). The resulting caldera covered an area of about 90 km2 and was nested within the CI caldera (Orsi et al. 1996). After the NYT eruption, both volcanism and deformation have been very intense within the caldera (Di Vito et al.

Two volcanic successions at Trefola quarry (TL) and Ponti Rossi (PR; Fig. 2), have been chosen and sampled, as they represent the most complete pre-CI and pre-NYT succession exposed in the area. Trefola quarry is situated in the northern Campi Flegrei area close to the northern border of the CI caldera. Ponti Rossi is situated north of Naples near the outer margin of the CI caldera. Both stratigraphic sections were described in detail by Orsi et al. (1996), and we follow their stratigraphic nomenclature. All collected samples are fresh, unweathered, and unaltered pumice and obsidian clasts. Trefola quarry exposes a thick sequence of pyroclastic deposits including twelve units below the CI (units TLa to TLn), five units between the CI and NYT deposits (units TLo to TLs), and five units above the NYT. The units are intercalated by reworked layers or palaeosols. The samples collected from units TLc and TLf are large pumice fragments (and in one case obsidian). Due to quarry conditions, the pre-CI units TLa, TLb, TLd, and TLe, as well as units TLg to TLn, were not sampled. Available geochemical data of these units will be taken from literature for the later discussions. The preNYT units TLo to TLs have been sampled completely. Orsi et al. (1996) described unit TLa as a pyroclastic-flow deposit, TLn is a succession of ashy pyroclastic-flow deposits, and units TLb and TLm are cross-laminated surge beds with minor fallout layers. The units TLc and TLf are complex sequences of pyroclastic deposits; units TLd, TLe, TLg, TLh, and TLi are distal fallout deposits (Orsi et al. 1996). The pre-NYT units TLo and TLs are pumice fallout deposits with minor surge beds, TLp is a distal ash fallout, TLr an ashy surge deposit, and unit TLq is an almost completely reworked ash deposit (Orsi et al. 1996). Ponti Rossi exposes a complete sequence between CI and NYT of nine units, which are intercalated by palaeosols. Units PRa, PRc, PRd, PRf, and PRh have been sampled (but not units PRb, PRe, PRg, and PRi). Available data of the missing units will be taken from literature for later discussion. Orsi et al. (1996) described unit PRa as a succession of distal surge beds with minor fallout layers. Units PRb, PRh, and PRi are described as surge beds, PRi also contains angular pumice clasts. The units PRc, PRd,

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Table 1 XRF and ICP-MS whole rock data of pre-CI and pre-NYT eruptions Sample

TLc-1

TLc-2

TLc-3

TLc-4

TLc-5

TLc-6

TLc-7

TLc-8

TLf-9

TLf-10

SiO2 TiO2 Al2O3 Fe2O3tot MnO MgO CaO Na2O K2O P2O5 L.O.I. Sum Sc V Ni Zn Ga Rb Sr Y Zr Nb Mo Sn Sb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W Pb Th U

56.4 0.40 18.7 4.07 0.26 0.33 2.03 6.61 7.17 0.06 2.70 96.03 4 11 4 141 22 420 20 71 733 131 6.60 8.66 2.10 36.5 23 114 225 22.6 77.0 13.7 1.30 13.9 1.85 10.20 1.98 5.78 0.855 5.81 0.899 17.0 5.93 8.59 65.1 57.2 18.9

56.7 0.41 18.9 4.09 0.26 0.33 2.04 6.61 7.2 0.06 2.62 96.60 3 13 4 142 24 418 23 68 735 134 6.97 8.72 1.73 35.1 33 95 204 19.1 65.1 11.6 1.14 12.0 1.63 9.14 1.76 5.25 0.783 5.40 0.820 16.9 4.02 9.12 63.3 49.4 15.4

56.8 0.41 18.8 4.11 0.26 0.32 2.01 6.67 7.25 0.06 2.39 96.68 1 12 3 144 24 423 20 71 751 135 7.31 8.91 1.98 37.2 32 107 209 21.9 74.0 12.8 1.24 13.2 1.74 9.90 1.82 5.45 0.805 5.50 0.821 17.0 6.49 9.36 63.6 51.5 14.9

57.5 0.41 19.2 4.15 0.26 0.35 2.07 6.79 7.32 0.06 1.17 98.12 1 15 5 142 24 424 21 72 745 131 6.39 8.90 2.27 36.6 8 61 171 12.8 44.9 8.7 0.85 9.0 1.29 7.49 1.47 4.45 0.689 4.79 0.744 17.4 6.72 8.07 68.1 37.8 11.1

56.6 0.41 18.9 4.1 0.26 0.33 2.02 6.56 7.29 0.06 1.96 96.53 1 15 3 143 23 420 20 73 740 130 7.45 9.03 2.20 37.9 20 114 223 22.4 77.7 13.5 1.29 14.0 1.80 10.20 1.95 5.66 0.872 5.71 0.885 17.3 7.19 10.00 62.2 53.0 16.1

56.3 0.40 18.8 4.08 0.26 0.34 1.98 6.52 7.19 0.06 2.96 95.94 4 17 3 140 22 418 24 73 741 130 7.27 8.76 2.11 36.3 13 111 198 21.9 75.3 13.2 1.25 13.5 1.75 9.77 1.87 5.37 0.818 5.36 0.835 16.7 6.65 9.74 62.5 52.3 15.7

56.5 0.41 18.8 4.11 0.26 0.35 2.09 6.54 7.15 0.06 2.97 96.27 6 17 6 142 23 418 33 72 738 130 7.20 8.89 2.26 37.2 41 123 234 23.9 82.1 14.4 1.37 14.6 1.94 10.60 2.04 5.91 0.883 5.91 0.904 17.1 7.44 10.00 63.6 56.7 18.2

56.1 0.41 18.6 4.11 0.26 0.37 2.12 6.31 7.05 0.07 3.46 95.39