ELSEVIER Earth and Planetary Science Letters 140 (1996) 227-241

EPSL

Age, origin and geodynamic significance of plagiogranites in lherzolites and gabbros of the Piedmont-Ligurian ocean basin

Laura Borsi a,b,*, Urs Schiirer a, Laura Gaggero c, Laura Crispini c a lx~boratoire de G~ochronologie, Unicersit[ Paris 7 and IPG-Paris, 2, place Jussieu, F-75251 Paris cedex 05. France

h lnstituto di Geocronologia e Geochimica lsotopica, CNR, Via C. Maffi, 36. 1-56100 Pisa, Italy L, Dipartimento di Scienze della Terra, Unicersith di Genoca, Corso Europa 26, 1-16132 Genova, Italy

Received 5 September 1995: accepted 14 February 1996

Abstract

U-Pb zircon dating, Sr-Nd isotope tracing and major/trace/RE element analyses were performed to constrain the age, origin and geodynamic significance of plagiogranites that intrude lherzolites and gabbros in the Ligurian Alps and the Northern Apennines. In addition, a host Fe-diorite was investigated. Samples from the Ligurian Alps were collected from the Voltri Group and the Sestri-Vohaggio Zone, whereas the plagiogranites from the Northern Apennines were taken in the Bracco unit. All these units have been affected by Alpine metamorphism reaching eclogite facies in the Voltri Group, blueschist degree in the Sestri Voltaggio samples, and prehnite-pumpellyite facies in the Bracco Unit, which has additionally been affected by rodingitization.

U-Pb zircon ages of 150 ± l, 153 _+ 1 and ~ 156 Ma were obtained, respectively, for two plagiogranites and the host Fe-diorite in the Ligurian Alps, and an age of 153 _+ 1 Ma was determined for the plagiogranite in Northern Apennines. Inherited components in zircon and initial Pb in plagioclase indicate mixing of variously differentiated basaltic magmas with small amounts of roughly 1.7-2.1 Ga old continental crust material. REE patterns in both the plagiogranites and the host diorite are characterized by high REE abundance, and moderate LREE enrichment. Nd isotopic compositions lie in the range of N-MORB sources, yielding initial epsilon Nd values between + 8.8 and + 9.7, whereas Sr is isotopically heterogeneous. The geochemical pattern of the plagiogranites and the host Fe-diorite requires melting of a MORB-type mantle source that experienced LREE enrichment shortly before melting. The most likely explanation for such enrichment is the injection of melts derived by small degrees of melting from an adjacent mantle region, The basaltic, LREE-enriched parent magmas generated from this enriched domain have probably undergone up to about 72% of low-pressure fractional crystallization prior to their emplacement into the gabbro-peridotite complex.

The 156-150 Ma magmatism occurred in close relation to normal faulting, sedimentation of breccias, and detachment of the mantle complex from its overlying continental crust, followed by exposure on the ocean floor. This tectono-magmatic event in the Ligurian Alps and the Northern Apennines reflects rifting of the Adriatic-Iberian continental plate segment, preceding wider opening of the Piedmont-Ligurian ocean basin and pillow basalt deposition.

Kevwords: Ligurian Alps; Northern Apennines: plagiogranites; U/Pb; Sm/Nd; geochemistry

* Corresponding author.

0012-821X/96/$12.00 © 1996 Elsevier Science B.V. All rights reserved PII S0012-821 X(96)00034-9

228 L. Borsi et al. / Earth and Planeta O' Science Letters 140 (1996) 227-241

1. I n t r o d u c t i o n

Serpentinite-gabbro complexes of the Ligurian Alps and the Northern Apennines are intruded by plagiogranites that cross-cut contacts between gab- bros, serpentinites and basaltic dikes (Fig. l) [1-7]. The plagiogranitic dikes also truncate a sedimentary breccia, which unconformably covers the serpenti- nite-gabbro complex. Fig. 2 schematically synthe- sizes the tectonic framework of the variously meta- morphosed Piedmont-Ligurian ocean basin rocks. Metamorphic overprint occurred either during rift-re- lated decompression or during the Alpine orogeny, including subduction and subsequent collision be- tween the Adriatic and European plates [7-16]. Al- though the plagiogranites have been identified in the field, their age, origin and geodynamic significance remains uncertain. In order to investigate these as- pects we have studied plagiogranitic dikes from three

different tectonic units in which mantle rocks of the Piedmont-Ligurian (Tethyan) oceanic basin are pre- served, These units are the Voltri Group, the Sestri- Voltaggio Zone, and the Ligurian nappes of the Northern Apennines (Fig. 1). Samples from the Lig- urian Alps are affected by metamorphism under eclogite to blueschist facies conditions, whereas the plagiogranites of the Apennines have experienced prehnite-pumpellyite degree metamorphic overprint.

2 . G e o l o g i c a l f r a m e w o r k

The region investigated has recorded all tectonic phases that characterize the evolution of the Alpine belt: opening and closure of the Piedmont-Ligurian ocean basin, and collision between the Adriatic and European plates. Closure of the basin probably started during the middle Cretaceous [17-24], in relation to

Structural sketch map of the Western Alps and Northern Apennines

Redrawn with modifications f • m St ia lc t twM Model of l l a l y , C N R

. P'U////~ ~g~q To,ino - ~ _ _

< > .,,:

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[ ] Quaternary and Pliocene deposits

] Tertiary Piemontese Basin and Ranzano Basin

~ ExtemM crystalline Massifs

[ ~ Provenqal - Dauphin6 units

] Brian~onnais units

] Piemontese and Valais Crystalline massifs

/

\ \

] Ligurian - Piemontese units

Main ophio|itic bodies of the ] Ligurian - Piemontese units

] Austroalpine units

] Helmintoides Flysch

] Main ophiolitic bodies of the Ligurian units

Limits of the Voltri Group and Sestri-Voltaggio Zone

- . - , .

, . , , o , o , IH

,~J. . ° •

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Tertiary Piemontese Basin

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V A I ~'-"~" ~ Voltri Group Iherzolites

10t: 1 @ sample location

Ligurian Sea GENOVA

0 i

0 lOkm L I

Bracco Unit

Fig, 1. Schematic map of the Ligurian Alps (Western Alps) and the Northern Apennines showing major lithotectonic units and sample localities.

L. Borsi et a l . / Earth and Planetary Science Letters 140 (1996) 227-241 229

opening of the North Atlantic and counter-clockwise rotation of Iberia. Collision is considered to be of latest Cretaceous to earliest Tertiary age (e.g. [17]).

Fig. 2 schematically summarizes the paleotectonic setting of the Piedmont-Ligurian oceanic basin in- cluding: (1) serpentinized lherzolites containing gab- bro intrusions; (2) a first generation of basaltic dikes; (3) an overlying oceanic sedimentary breccia; (4) a second generation of basaltic dikes that truncate the breccia; and (5) the plagiogranites, which seem to be contemporaneous with the emplacement of the sec- ond generation of basaltic dikes. An important obser- vation is the presence of detrital plagiogranite com- ponents in the breccia, which shows that breccia formation and emplacement of both the plagiogran- ites and basaltic dikes are contemporaneous phenom- ena, corroborating a direct relationship between basaltic magmatism and extensional tectonics. On the other hand, deposition of the pillow lavas on top of both the basalts and breccias is clearly a younger event, post-dating magmatism in the lherzolite-gab-

bro complex. The pillow basalts are covered by radiolarian cherts, and locally by limestones and shales.

Concerning the mantle complexes preserved in the Ligurian Alps, petrological and structural data from the Voltri Group indicate subsolidus emplace- ment of the peridotites under progressively lower P and T conditions [15]. This pattern was interpreted as reflecting decompression of subcontinental litho- sphere during denudation, occurring in relation to passive extension [15]. For the same type of rocks in the Northern Apennines geochemical data suggest a similar scenario, with the continental lithospheric peridotites being emplaced during early stages of rifting of the Piedmont-Ligurian ocean basin [ 16].

In the Ligurian Alps, that is, the easternmost segment of the Western Alps (Fig. 1), first deforma- tions occurred under eclogite-blueschist facies, whereas the Northern Apennines show a low-grade degree of metamorphism. In consequence, the Alpine terrains have been interpreted as having undergone

- - - - "- . . . . . . : : ' " : : : : " : - ' : : : 1 ~ i ' ~.. :":" " ~ ' ~ l " ~ " e ' e ! - + , . d ~ l : ~ l r ~ + ~ ~ ~ - - ~ , • • L ~ * ~ t " ÷ ÷ ÷ ÷ @ ÷ • + ÷ + 4 - 4 - ÷ ÷ ÷ . . . . . . . . ~ 4''1" 4 - ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ ÷ - - - - • - - ~ * ~' ÷ 4- 4" 4 - ÷ ÷ 4 4 - ÷

, "¢ "~1 t-.r-l- ÷ ~. ~ ÷ ÷ ÷ ÷ 4. 4. ÷ 4. 4. 4. 4- ÷ 4- 4- 4- 4- 4,.Ig.

~ ~ r l~J~'a..--id~l* + + 41+ 4- + + + + +-~"---.--.._--'--"~ + ~'-'z ?|'41,1, + + 4 " ÷ ÷ 4 . + ÷ 4. 4. ÷ ÷ ~ I

>l'~+/it 4" .~. 4- + + + + + 4" 4" + 4" + + + +~..'~"~+ .~','," '/"~,!!','1 ",',', ', ', ', '," C7",'.~ ",',',

. - . - _ = - . - : ; . . ~ + .HS- 4- i1+ + + + + 4- 4- 4. + + + + 4- + ÷ ÷ ÷ + ~ 2 , ' 2 , , ~ 2 t 2 ~ ' , ~ ' ~ ' ~ , ' , ' , ' , T 7 " , ' , ' , ' I ¥ , ' , " ,

Serpentinized lherzolites ~ Dolerites

Gabbroic cumulates and dikes, with melatroctolite ~ Pillow basalts lenses

High temperature shear zones ~ Plagiogranites

Basaltic dikes ~ Sedimentary breccias with plagiogranite clasts

Hbl. + plag. veins Radiolarian cherts

Hydrothermalized tectonic breccia

Faults ~ Palombini shales

Fig. 2. Paleotectonic setting of the Piedmont-Ligurian ocean basin: synthetic cross-section through the serpentinite-gabbro complex.

230 L. Borsi et al. / Earth and Planetary Science Letters 140 (1996) 227-241

subduction and later exhumation, whereas the inter- nal Apennines are thought to represent relicts of the related accretionary wedge [12,22].

High-pressure metamorphism in the Ligurian Alps was followed by a retrograde evolution towards greenschist facies conditions [9,23]. Mafic-ultra- mafic rocks and the meta-sedimentary cover of the Voltri Group experienced eclogite to blueschist fa- cies peak metamorphism, whereas the adjacent Ses- tri-Voltaggio Zone is characterized by a more com- plex pattern including: (1) blueschist metamorphosed ultramafic-mafic rocks (Cravasco Voltaggio unit); (2) weakly metamorphosed (pumpellyite-actinolite) ultramafic-mafic lithologies that are covered by a continuous sedimentary sequence, containing Creta- ceous Flysch (Monte Figogna unit); and (3) a blueschist metamorphosed Triassic-Liassic calcare- ous to pelitic series, known as the Unith Triassico- Liassica [24]. Contacts between these three units are marked by mylonitic zones, often re-worked during much later subvertical faulting.

East, adjacent to the Sestri-Voltaggio zone lies the Northern Apennines, which consist of prehnite- pumpellyite metamorphic ophiolites, stratigraphically overlain by cherts and, locally, Calpionella limestone [25,26], as well as by a Lower Cretaceous shale- limestone sequence [3]. The Ligurian Alps and the Northern Apennines are separated by an important tectonic contact that corresponds to a zone of major movements in association with Alpine thrusting.

3. Previous radiometric ages

3.1. K - A r

Radiometric dating has been performed for the Apennine terrains only; no data are available for the Ligurian Alps. K-Ar ages on meta-basalts (118 and 56 Ma) of the Northern Apennines [27] are in con- flict with the biostratigraphically established Jurassic age of the overlying radiolarites [25,26] and, there- fore, they may reflect mixed ages due to partial Ar-loss during later metamorphism. For the Tuscan Apennines, about 150 km to the southeast, a whole rock K-Ar isochron age of 8 3 _ 5 Ma was deter- mined for metabasalts and gabbros from the ophio- lite sequence [20]. This age was ascribed to resetting

of the K - A t clock during initial stages of the Eoalpine metamorphism, because 4°Ar-39Ar ages of primary amphiboles from Fe-diorites of southern Tuscany are interpreted as dating magmatism at 152 _+ 4 Ma [20].

3.2. S m - N d

Plagioclase-clinopyroxene mineral separates from lherzolites of the Northern Apennines indicate sub- solidus re-equilibration of these minerals at about 165 ( _+ 20) Ma, and Nd and Sr model ages for whole rocks yield different Proterozoic values, which are interpreted as reflecting the minimum age of early mantle differentiation [ 16].

4. Analytical procedures

Major elements were analyzed with XRF, and trace elements with both INAA and ICP. Addition- ally, FeO was determined by wet-chemical analysis, and L.O.I. by gravimetric measurements. All REE were analyzed by ICP-MS and, for comparison, by INAA. Mineral separates were obtained by process- ing 10-15 kg samples by crushing, the disk-mill, the Wilfley table, the Frantz magnetic separator, and heavy liquids. For U-Pb analyses, all zircon grains were mechanically abraded [28] and selected grain by grain under the binocular microscope. Zircon was dissolved according to [29], and plagioclase was leached and dissolved as described in [30]. U-Pb analyses were performed using a 2°spb-235U-233U mixed isotope tracer, added prior to dissolution, and isotope ratios were measured on a Thomson 206 instrument, equipped with a single Faraday collector and a Secondary Electron Multiplier for U and Pb. Mass discrimination of U and Pb was determined by regular measurement of the NBS-981 common Pb standard, yielding values of 0.10 + 0.05%/ainu for both elements. Nd and Sr isotopic compositions were measured on a Finnigan MAT-262 instrument with multi-collection in dynamic mode. Rb concentrations were determined by isotope dilution using a STRb tracer, whereas Sr concentrations were measured by XRF. Decay constants were used as published in [31-33]. Linear regressions were calculated after [34] and modified for correlated errors after [35].

L. Borsi et a l , / Earth and Planetary Science Letters 140 (1996) 227-241 231

5. Sample description and analytical results

5.1. The Voltri Group plagiogranites

Major, trace, and RE element analytical results are listed in Table 1. The Voltri Group plagiogranites show high SiO 2 (69-76 wt%) and include quartz, albite and minor mafic minerals (< 5%), with pre-

dominantly amphibole as the rock-forming minerals. Their geochemical features satisfy the definition of plagiogranites [36]. Primary magmatic textures are often erased by blueschist metamorphism, producing albite, Na-amphibole, titanite and epidote. Relict pri- mary phases are euhedral plagioclase, quartz and amphibole. Blueschist metamorphism affected large parts of the Voltri Group including the plagiogran-

Table 1

M a j o r and t race e l emen t concen t r a t ions in p l ag iog ran i t e w h o l e - r o c k s a m p l e s

RIo RIo Rio B icc ia Bicc ia Bicc la Blcc la B H D - 1 Cane 1 Cane 3 Cane 4 1 2 3 4 hos t

diorite

BOC- B O C - Rio Rio 1 2 Gay 1 Gay 2

SiO2 68.5 75.0 76.1 65.6 65.6 75.5 75.7 56.3

TiO2 0.53 0.28 0.26 0.34 0.34 0.28 0.24 1.23

AI203 15.2 12.0 12.6 17.9 17.9 12.9 13.0 15.6

F e 2 0 3 0.55 0.49 0.73 0.68 0.74 1.0l 0.82 1.37

F e O 1.25 0.76 0.62 0.80 0.75 0.53 0.25 3.65

M n O 0.05 0.03 0.04 0.03 0.03 0.02 0.02 0.10

M g O 2.49 1,40 0.75 0.57 0.58 0.39 0.22 5.68

CaO 1,71 0.93 0.32 0,41 0.36 0.36 0.25 4.96

N a 2 0 7.46 6,78 7.81 11,7 11.6 8.78 8.92 6.75

K 2 0 0.55 0.14 0.10 0.30 0.04 0.05 0.05 0.08

P2Os 0.03 0.03 0.03 0.06 0,06 0.05 0.05 0.13

L O I 1.47 0.85 0,77 0.54 0.39 0.47 0.31 2.31

V 16 21 13 14 35 10 10 212

N i 23 18 13 13 101 31 11 95

Pb < 2 < 2 < 2 < 2 2 2 6 < 2

Th 3 3.2 2.2 2.2 2.1 1.5 1.1 0.6

U 1.2 1.7 1.3 1.6 0.6 0.5 0.6 0.6

Rb 5 0,078* 4.0 4.0 14 1.58* 6.0 0.5

S r 16 16 15 13 77 51 24 346

Y 223 199 169 156 198 137 128 40

Z r 552 549 443 407 480 416 411 133

N b 50 25 49 26 23 < 10 23 27

Ba 76 84 78 66 164 98 75 103

La 35.3 43.0 21.9 21.1 33.0 30.1 28.4 8.80

Ce 98.5 96.3 58.6 63.4 104 82.4 73.9 31.5

N d 53 50.7 33.5 33.5. 64.8 44,1 39.8 18.8

S m 11.8 11.5 8.0 7.8 13.9 9.1 9.0 4.1

Eu 2.2 1.6 1.7 1.2 2.0 1.6 1,6 1.9

Gd 17.6 16.4 12.1 11.4 18.8 11 9.1 6.7

Tb 3.7 3.7 2.2 2.3 3.3 2.0 2.5 1.0

Yb 17.9 18 12.3 11.9 14.6 12,l 11.8 5.2

Lu 2.84 2.81 2.03 1.88 2.28 2,03 1.84 0.84

Lan/Smn 2.99 3.74 2.74 2.71 2.37 3.31 3.16 2.15

Lan/Yb n 1.32 1.60 1,19 1.19 1.15 1.66 1.60 1.13

Eu /Eu* 0.47 0.36 0.53 0.39 0.38 0.49 0.54 1.11

REE 254.2 255.5 160.4 162.2 265.7 202.5 185.4 82.2

59.9 65.5 40.7 40.6

2.36 0.45 0.59 0.66

16.9 17.2 11.8 12.8

0.78 1.68 3.93 1.71

2.3l 0.9 1.78 1.07

0.07 0.04 0.12 0.07

3.38 0.96 8.91 9.48

3.67 0.89 29.0 29.2

8.77 11.4 <0.1 < 0 . I

0.06 0.06 < 0.1 < 0.1

0.14 0.10 0.02 0.23

1.54 0.47 3.93 4.16

211 31 134 58

66 21 104 34

< 2 < 2 < 2 < 2

1,0 0.8 < 0.5 1.1

0.7 0.7 < 0.5 0.5

5.0 0,376* 6.0 4.0

170 17 < 10 14

131 146 < 10 168

708 853 < 10 576

35 33 12 14

69 77 < 10 < 10

15.1 21.2 0.50 20.4

53.2 83.5 1.7 68.6

26,6 36.1 1.8 51.6

7.1 8.88 0.6 12.8

1.9 2.3 0.6 3.9

11.2 12.1 1.2 19.5

1.8 1.9 5.0 3.5

9.7 10.2 0.9 14.6

1.54 1.69 0.14 2.27

2.13 2.39 0.83 1,59

1.04 1.39 0.37 0.93

0.65 0.69 2.13 0.64

134.5 184.48 8.2 20.6

232 L. Borsi et al. / Earth and Planetao' Science Letters 140 (1996) 227-241

ites. Minor parts show higher degrees of overprint reaching eclogite facies conditions.

Fig. 3a shows the chondrite-normalized REE pat- terns for three of the plagiogranites. REE abundances range between 50 and 100 × chondrite and there is a significant negative Eu anomaly with Eu/Eu* be- tween 0.38 and 0.54 (Table 1). All patterns are weakly enriched in LREE yielding LaN/YbN be- tween 1.15 and 1.66.

For U-Pb dating, all zircons were selected ac- cording to criteria such as habit, size, transparency, colour (state of metamictization), inclusions and cracks. Grain characteristics are illustrated in Fig. 4,

and a short description of the fractions analyzed is given in the first column of Table 2, which lists the U-Pb dating results. To correct the zircon U-Pb data for initial common Pb, leached feldspar from each rock was analyzed for U and Pb, and initial ratios were calculated for the age of the rock. U and Pb abundances, and corresponding Pb isotopic com- positions are listed in Table 3.

Zircons from sample RC-3 are euhedral, showing two different types of grains, with short to medium prismatic, transparent, coiourless, medium-size grains as one end member, and bi-pyramidal, milky yellow- ish small crystals as the other. These two types of

IOO0-

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]OO-

8

e¢

V o l t r i G r o u p

10"

a I i i [ i i i i i i i L i , i 'J

L a C e N d S m E u G d Y b L u

I - - c - - RC-I ----0-- RC-3

---0-- RC-4

1000 1 Sestri-Voltaggio Zone

lOO'

1 0

i i

La Ce

10OO. S e s t r i - V o l t a g g i o Z o n e

tOO.

~ ~ ~ _ 1 . ~ - I 1 ~ 1 1

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1 , i i i i i i i J i I i i i i

L a C e N d S i n E u G d Y b L u

l -" Fe-diorite host-rock + BOC-1

"---V--- BOC-2

10OO,

]OO~

10":

Bracco unit ( N o r t h e r n A p e n n i n e )

• " - ' - - ~ . ~ _ V - - T

. . . . . . . . . . . . ~ d N d S m E u G d Y b L u I , , , , , ~ t ~ , ~ ~ I

L a C e N d S m E u G d Y b L u

Bieeia3 ~ Biccia4 ~ RG-1 V RG-2

Fig. 3. REE patterns of the plagiogranites from (a) the Voltri Group, (b and c) the Sestri Voltaggio Zone, and (d) the Northern Apennines, as well as for a host Fe-diorite (b). The RG-I pattern (d) is due to strong rodingitization of the plagiogranite.

L. Borsi et al. / Earth and Planetary Science Letters 140 (1996) 227-241 233

zircon grains are shown in Fig. 4a-c. In both types of grains, dark domains are observed in the centre of the crystals, representing relicts of older, inherited zircon components. The presence of such relict ra- diogenic Pb components, occurring in different pro- portions, is confirmed by the U-Pb analytical data, which reveal reverse age patterns, as discussed be- low.

From the milky type of zircons we analyzed a single fraction, and from the transparent grains, 3 different fractions. In the concordia plot (Fig. 5a) a transparent and the milky fraction yield concordant ages, whereas the other two transparent fractions are about 3% and 16% discordant. These four analyses plot within individual analytical uncertainties on a linear regression line that intercepts the concordia curve at 150.1 + 0 . 6 / - 0 . 7 (20-) Ma and 1704 + 1 7 / - 16 Ma.

As shown in Table 4, Rb-Sr and Sm-Nd analy- ses on whole rock powders of RC-3 yield a 87 Sr/86 Sr of 0.70373 and a ~43Nd/144Nd of 0.513063 (Table 3). Using the zircon age of 150 Ma, the correspond- ing initial values are 0.70354 and 0.512938. Ex- pressed in the epsilon notation, initial values lie at - 11.2 for Sr, and at + 9.66 for Nd.

5.2. The Sestri-Voltaggio Zone plagiogranites

Plagiogranites from the Sestri-Voltaggio zone show a range of SiO 2 contents (60-75 wt%), identi- cal to those of the Voltri Group. Their mineralogical and chemical composition is also in agreement with the definition of plagiogranites [36], with albite, Na-amphibole, Na-Ca clinopyroxene, titanite and epidote reflecting Alpine metamorphic overprint un- der blueschist facies conditions. Magmatic relict grains are euhedral plagioclase, quartz, and amphi- bole (hornblende). As shown in Fig. 3b, plagiogran- ites (BOC-1 and BOC-2) are again characterized by weakly LREE-enriched patterns and pronounced Eu anomalies. Also given in this figure is the REE pattern of the host Fe-diorite (BHD-1), which shows an almost flat pattern having a very weak positive Eu anomaly (Eu/Eu* of !.11). Four other plagiogran- ites from the Sestri-Voltaggio zone (samples Biccia 1-4) yield similar REE patterns (Fig. 3c), also show- ing weak LREE enrichment (LaN/Yb N between 1.17 and 1.32), and Eu anomalies ranging between 0.36 and 0.53 for Eu/Eu*.

Zircons from the BOC-2 plagiogranite are devel- oped as short prismatic, transparent, colourless grains

Fig. 4. Plagiogranite zircons. (a) = 100 /zm long prismatic zircon with an old core (RC-3 plagiogranite of the Voltri Group, Ligurian Alps. (b) and (c) = 50 /*m large bi-pyramidal grains, some also with old cores, from the same rock. (d) ~ 200 /xm long, abraded, transparent and prismatic grains from the RG-I plagiogranite (Bracco Unit, Northern Apennines).

234 L. Borsi et al. / Earth and Planetary Science Letters 140 (1996) 227-241

with few inclusions or cracks. In the concordia dia- gram (Fig. 5b), six of the seven fractions analyzed yield identically concordant ages, whereas one frac- tion has a slightly older age. From the six identical fractions a crystallization age of 153.3 + 1.0 Ma is obtained.

Due to a very small zircon abundance, only three zircon fractions could be analyzed from the Fe-di- orite host rock (BHD-1). As shown in the inset of Fig. 5b, one fraction yields a concordant age, whereas the other two analyses are discordant. The concor- dant data with an age of about 156 Ma lies in the

Table 2

U - P b analytical results for zircons from three plagiogranites and a Fe-diorite host rock

Sample description Wt. Concentra- Radiogenic Pb Atomic ratios Apparent ages in Ma (a) tions 206pb in atomic % (c) (C) (C)

(rag) (ppm) 2O4p b

U Pb mea- rad. sured

(b)

206pb 207pb 208pb 206pb 2 0 7 p b 2 0 7 p b 206pb 207pb 207p b

238 U 235 U 206pb 238 U 235 U 206pb

RC-3: plagiogranite (Voltri Group, Ligurian Alps)

1)smaU, bipyramidal, milky .1336 332.1 8.74 265.8 79.7 4.04 16.3 .02438 .1703 .05067 155.3 159.7 225.7 2) small, prismatic, transparent .0671 647.6 16.89 483.3 78.4 3.89 17.7 .023749 .1627 .04970 151.3 153.1 180.9 3) small, prismatic, transparent .2402 413.1 10.53 173.2 79.7 3.93 16.4 .02361 .1605 .04929 150.4 151.1 161.6 4) small, prismatic, transparent .1574 197.9 5.76 195.2 80.2 4.58 15.2 .02710 .2134 .05712 172.4 196.4 495.4

BOC-2: plagiogranite (Sestri-Voltaggio Zone, Ligurian Alps)

5) small,prismatic, transparent .3944 19.2 .49 203.0 81.2 4.06 14.8 .02423 .1671 .05001 154.3 156.9 195.7 6) small, prismatic, transparent .2774 10.7 .28 86.2 78.2 3.86 17.9 .02372 .1612 .04930 151.1 151.8 162.3 7) small, prismatic, transparent .1761 44.7 1.21 108.2 80.4 3.95 15.7 .02530 .1713 .04910 161.1 160.6 152.7 8) medium, prismatic, transparent .1908 77.3 2.03 219.6 77.7 3.83 18.5 .02369 .1610 .04927 151.0 151.6 160.8 9) small, prismatic, transparent .0466 56.9 1.52 112,0 77.4 3.83 18.7 .02409 .1643 .04945 153.6 154.9 173.5 lO) small, prismatic, transparent .0769 91.9 2.48 213.5 76.1 3.79 20.1 .02382 .1637 .04986 153.5 154.4 169.3 l l )small , prismatic, transparent .1254 32.4 .89 182.3 75.8 3.75 20.5 .02412 .1648 .04954 151.7 154.0 188.3

B H D - I : host Fe-diorite (Sestri-Voltaggio Zone, Ligurian Alps)

12) very small, prismatic, transp. .2317 94.2 2.50 140.8 78.8 3.91 17.3 .02436 .1664 .04956 155.I 156.3 174.2 13) very small, prismatic, transp. .2594 148.8 3.52 323.8 77.2 3.88 18.9 .02121 .1470 .05025 135.3 139.2 206.5 14) verysmall, prismatic, transp. .2514 106.5 3.02 556.1 74.5 3.97 17.5 .02588 .1806 .05062 164.7 168.6 223.5

RG-I: rhodingized plagiogranite (Bracco Unit, Northern Apennine)

15) medium tolarge, transparent .3588 12.0 .32 215.9 80.0 4.02 16.0 .02485 .1720 .05018 158.2 161.1 203.7 16) small, prismatic, transparent .2002 9.7 .26 118.0 80.7 4.28 15.0 .02521 .1842 .05298 160.5 171.6 328.1 17) large, prismatic, transparent .1647 24.0 .65 137.2 81.2 4.38 14.4 .02544 .1891 .05390 161.9 175.8 367.0 18) medium, prismatic, transp. .2703 26.2 .93 186.7 83.1 5.67 11.2 .03452 .3249 .06827 218.7 285.6 876.7 19) large, prismatic, transparent .1512 30.1 .26 169.5 75.9 10.18 14.0 07720 1.429 .13421 480.2 908.1 2171.3

a Individual analyses were performed on euhedral, unbroken, crack- and inclusion-free grains of highest transparency possible. Very small

grains: < 50 /.tm; small grains: 50 -100 /xm long; medium size: 100-150 /xm; and large grains > 150 mm. Zircons were dissolved in HF

> 50% at 220°C for 4 days in Teflon bombs [29], and abrasion was performed according to [28]. U decay constants used are those given in [31,33]. For isotopic measurements, Pb and U were loaded together on the same single Re filaments with Si gel and H3P Q and run at

1350-1450°C and 1450-1550°C, respectively, on a Thomson 206 mass spectrometer. Mass-discrimination is 0.10 _+ 0 . 0 5 % / a m u for Pb and

U. b Ratios corrected for mass-discrimination and isotope tracer contribution, c Corrected for mass-discrimination, isotopic tracer contribution, 15 pg of Pb blank. 1 pg of U blank, and initial common Pb having the composition of initial Pb determined in plagiclase from

the same rocks (Table 4). Together with mass-spectrometric precisions and uncertainties from spike calibration, such correction for initial Pb yields analytical errors of 0 .5-1 .0% for 2°6pb/238U and 0 .6-2 .0% for 2117pb/235U, having correlation coefficients between 0.5 and 0.9, dependent upon in-run precision and -'~)6pb/2~4Pb measured.

L. Borsi et a l . / E a rth and Planetary Scienee Letters 140 (1996) 227-241

Table 3 U-Pb analytical data for leached plagioclase

235

S a m p l e concentrations rat ios m e a s u r e d (a) in i t ia l ra t ios (b)

U Pb 206pb/204p b 207pb/204pb 208pb/204pb 238U/204pb 206pb/204pb 207pb/204pb bt = 23Sut204pb (c)

lppb] [ppb] (g) of feldspar [Th/U] (d)

of the source

RC-3 6 169 18.128 15.610 38.183 2.35 18.073 15.610 8.75 [4.06]

BOC-2 < 1 46 18.372 15.409 38.429 0.276 18.366 15.409 9.04 [4.041

BIC-2 21 66 18.787 15.711 39.067 20.5 18.307 15.687 8.98 [4.36]

RG-1 1 97 17.954 15.456 37.742 0.732 17.937 15.455 8.61 [3.92]

" Corrected for 0.10c# + 0.05/ainu mass-discrimination (blanks for U and Pb are negligible), b Initial isotope ratios, calculated for 150 Ma

in situ decay of U, using the /x values measured. ~ /x value of the source region at 150 Ma. calculated for a single-stage evolution of a 4.55 Ga old Earth. ,t Maximum T h / U of the source region at 150 Ma, calculated from the measured 2°spb/2°4pb ratios, for single-stage

evolution of a 4.550 Ga old Earth. Th was not mesured in plagioclase and, therefore, these values reflect maximal ratios; however, these

values must lie very close to the real ratios, given the very low U (Th) concentrations in plagioclase.

range of concordant data from the BOC-2 pla- giogranite. Discordancy of the older fraction has to be ascribed to the presence of inherited components, whereas the position of the younger discordant frac- tion can be explained by a combination of later Pb loss (Alpine metamorphism) and the presence of inherited components.

Three plagiogranites and the Fe host diorite have S7Sr/86Sr between 0.70421 and 0.70600, and 143Nd/144Nd lie between 0.513054 and 0.513058 (Table 4). Their initial epsi[on values for 150 Ma are - 2.1 to + 23 for Sr, and + 8.5 to + 9.9 for Nd. The initial epsilon values for the Fe-diorite lie at + 22 for Sr and at + 9.5 for Nd.

5.3. The Northern Apennines plagiogranites

The two dikes from the Bracco Unit (RG-1 and RG-2) have much lower SiO~ (41 wt%) than the plagiogranites from the Ligurian Alps, and alkali elements are absent ( < 0.1 wt%). This peculiar com- position is the result of strong chemical exchange between the plagiogranite dike and the serpentinite host rock, yielding a typical rodingite mineral para- genesis with prehnite, vesuvianite and hydro-grossu- lar. Despite its strong metamorphic modification, the sample can still be identified as one of the pla-

t ~ ] 1 2o~/t 0~ plagiogranite RC-3 ~ 9 ° ~ ~

Voltri Group (Ligurian Alps) /

+0.6 .... ~ 150.1_ 0.7 Ma

/ / ~ ,q - " J ~

plagiogranitc BOC-2 1 33~

Sestri-Voltaggio Zone (Ligurian Alps)

1 5 3 . 3 + ].0 Ma 206Pb 238 U

= 156 Ma

1

.22 207Pb / 235U

Fig. 5. Concordia diagrams for zircons from two plagiogranites. (a) RC-3 sample from Voltri Group. (b) BOC-2 intrusion from the Sestri Voltaggio Zone. The inset in (b) shows data for the host Fe-diorite of the BOC-2 plagiogranite.

236 L. Borsi et al. / Earth and Planetao" Science Letters 140 (1996) 227-241

Table 4 Rb-Sr and Sm-Nd analytical data

sample concentrations [in ppm] 87Rb/86Sr 87Sr/86Srnorm. 87Sr/86SrlSOMa 147Sm/144Nd 143Nd/144Ndnorm. 143Nd/144NdlSOMa [~NdlsOMa

Rb Sr Sm Nd

Voltri Group

RC-3 1.6 51 9.1 44 0.09 0.70373:t:2 0.70354 0.128 0.513063+9 0.512938 + 9.7

Sestri Voltaggio Zone

BIC-2 0.08 16 11.5 50.7 0.0l 0.70421 + 1 0.70418 0.141 0.513045 + 13 0.512907 + 9.1

BOC-2 0.38 17 8.8 36.1 0.06 0.713437 + 1 0.70423 0.151 0.513051 + 10 0.512903 + 9.0

BOC-1 5.0 170 1.9 7.1 0.08 0.70600+ 1 0.70583 0.166 0.513058:t: II 0.512895 + 8.8

BHD-1 0.50 350 4.1 18.8 <0.01 0.70593+1 0.70591 0.135 0.513061+10 0.512929 + 9.5

Bracco Unit

RG-1 6.2 10 0.6 1.8 1.79 0.70587 -+ 1 0.70206 0.207 0.513140 + 13 0.512937 + 9.7

146 144 (87Sr /s6Sr) . . . . is normalised to S6Sr/SSSr = 0.1194, and (143Nd/144Nd) ....... to N d / Nd =0 .7219 . Analytical uncertainties are _+2% for STRb/s6Sr, and _+0.5% for 147Sm/144Nd, For (SVSr/S6Sr),or,~ and (143Nd/144Nd),or, , uncertainties are given in the table,

relative to the last digits of the values measured. Decay constant used are 1.42 - 10- i t y - i for srRb [33], and 6.54 • 10-12 y i for 147Sm

[32]. For the initial E TM calculation we used: (147Sm/144Nd'J, CHUk = 0.1967, and a present day value of ( t43Nd/144Nd)cHvR of 0.512638.

For initial E sr (given in Table 5 only) we used: (87Rb/86Sr)uR = 0.0816, and a present day (87Sr/86Sr)uR of 0.7045.

giogranites of the Piedmont-Ligurian basin [5,6]. Fig. 3d displays the chondrite-normalized REE pat- terns, documenting a large difference between the two dikes. RG-2 has a flat pattern (LaN/YbN: 0.93) at 50-90 × chondrite abundance, whereas the other rock (RG-1) is depleted in LREE (LaN/Ybs: 0.37), and its REE abundance is less than 10 × chondrite. This latter sample has a positive Eu anomaly (Eu/Eu*: 2.13). Such contrasting patterns reflect different degrees of interaction between the dikes and their ultramafic host rock, as also suggested by the high Ca contents of the dikes (Table 1). This chemical exchange produces patterns that lie be- tween those of plagiogranites (Fig. 3a-c) and LREE-depleted lherzolites.

The zircons from the RG-1 Apennines sample are euhedral, equant to short prismatic, transparent and colourless grains. Some of them reach exceptionally large sizes, of the order of 1 mm. A photograph of a series of abraded grains is shown in Fig. 4d. In the concordia diagram (Fig. 6), three zircon fractions lie close to the concordia curve, and a fourth fraction is about 25% discordant. These four analyses yield a regression line with intercept ages of 153.4 + 0 . 7 / -

0.8 Ma (2o-) and 1777 + 7 5 / - 7 3 Ma. A fifth fraction (not shown in Fig. 6) is characterized by much older ages, having a 2°Tpb/2°6pb age of 2171 Ma (Table 2). This age is an average minimum estimate for inherited components in this fraction, which is significantly older than the 1777 Ma upper intercept age of the other fractions. The RG-1 pla- giogranite has a 87Sr/86Sr of 0.70587 and a

I i - I I I - - 7 I plagiogranite RG-1 2s0, J

~ I Bracc° Unit (N°rthern AP ermine) 2 ~ / ~ /

+ 0.7 ~,~

. . . . .

zircons

207Pb / 235U

Fig. 6. Concordia diagram for zircons of the RG-I plagiogranite

sample from the Bracco Unit of the Northern Apennines.

L. Borsi et al. / Earth and Planetao' Science Letters 140 (1996) 227-241 237

143Nd/144Nd of 0.513140, and corresponding initial epsilon values lie at -32 .2 for Sr and + 9.7 for Nd (Table 4).

6. Interpretations and implications

6.1. Age and origin

Table 5 summarizes the new U-Pb zircon ages of the Piedmont-Ligurian plagiogranites and host Fe- diorite, as well as their initial Pb-Sr -Nd isotope signatures. Intrusion ages of the Ligurian Alps pla- giogranites lie at 150 and 153 Ma, with the Voltri Group samples being slightly younger than the Sestri Voltaggio dike (150.1 + 0 . 6 / - 0.7 Ma vs. 153.0 _+ 1.0 Ma). A concordant zircon from the host diorite in the Sestri Voltaggio Zone indicates an emplacement age of about 156 Ma. The 153.0 + 0 . 7 / - 0 . 8 Ma age for the Northern Apennines plagiogranite is iden- tical with that from the Sestri-Voltaggio dike, cor- roborating that the Piedmont-Ligurian plagiogranites and host diorites were formed during a short period of time between 156 and 150 Ma.

Initial Nd isotope signatures of all plagiogranites

and the host diorite yield identical values lying be- tween +8.8 and +9.5. This signature substantiates magma formation from a mantle reservoir strongly depleted in LREE, corresponding to a N-MORB type source (e.g., [37]). Homogeneity of initial Nd shows that crustal contamination was very small; only in- herited zircon and initial Pb in plagioclase are suffi- ciently sensitive to reveal the presence of old conti- nental material in the magma. Plagioclase of two of the plagiogranites (RC-3 and BIC-2) have upper crustal 2°Tpb/2°4pb (/3) ratios at about 15.6 (Table 3) whereas two other dikes (BOC-2 and RG-1) yield mantle source values at about 15.4 (e.g., [37,38]). In contrast to /3, all initial 2°6pb/2°4Pb ratios (or) lie in the range of mantle values, yielding corresponding 238U/2°4pb ratios ( ~ ) for the magma source areas between 8.61 and 9.04. The high /3, low c~ values of RC-3 and BIC-2 confirm that small portions of old continental crust material was incorporated in the magmas, as already documented by the age of the inherited zircon components.

In contrast to Nd, Sr signatures are heteroge- neous, yielding initial epsilon values between - 1 1 and + 22. A particular case is the strongly rhodingi- tized dike from the Northern Apennines, which has

Table 5

Summary of U - P b zircon ages and Pb, Sr and Nd isotope characteristics of the Piedmont-Ligur ian plagiogranites and host diorite

sample age [in Ma] 238UI204pb 87Sr/86SrlsOMa ~SrlsOM a ~NdlsoM a

[Th /Umax . ] of the source area

Voltri Group

RC-3 150.1 +0.6t-0.7

Sestri Voltaeeio Zone

BIC-2

BOC-2 153.3 ± 1.0

BOC-I

BHD-I = 156 host diorite

Bracco Unit

RG-1 153.0 +0.7/-0.8

8.75 0.70354 - 11 + 9.7 [4,06]

8.98 0.70418 ~ 2.1 + 9.1 [4.04]

9.04 0."/0423 + 1.4 + 9.0 [4,36]

0,70583 + 21 + 8.8

0.70591 + 22 + 9.5

8.61 0.70206 - 32 + 9.7 [3.92]

238 L. Borsi et a l . / Earth and Phmeta O' Science Letters 140 (1996) 227-241

an initial epsilon of - 32 . This value is basically consistent with a N-MORB type source; however, in all cases Rb-Sr systematics are potentially influ- enced by: (1) the very small continental crust com- ponents in the melts; (2) chemical exchange with the host rocks during subduction-related metamorphism; and (3) sea water alteration during ocean floor meta- morphism. We therefore consider the Sr signatures not to be representative for the primary mantle source magma. As substantiated by their homogeneity, REE patterns of the plagiogranites have not been or were only very weakly affected by these processes, except for RG-I and RG-2.

Negative Eu anomalies are observed in all pla- giogranites, corroborating fractional crystallization in the parent magmas prior to dike formation. This agrees with the generally accepted model for pla- giogranite genesis (e.g., [37,39]). An exception is the highly rhodingitized Apennines sample (RG-t). For the host rock of the plagiogranites, fractional crystal- lization was significantly lower, as indicated by its dioritic composition, its absence of an Eu anomaly, and its much lower REE abundance. In certain cases, where plagiogranites are LREE depleted (e.g., [36,39]), geochemical patterns can entirely be ex- plained by such fractionation, following 15-20% of lherzolite melting (MORB-type source). In our case, where all plagiogranites are LREE enriched, frac- tional crystallization alone does not explain the pat- tern, and another process must have been involved. Candidates to explain this difference may be either very small degrees of lherzolite melting or magma formation from an enriched mantle source. Experi- mental data [40-42] indicate that even liquids pro- duced by only 5% of partial lherzolite melting at 1-3 GPa are not sufficiently enriched in incompatible elements to explain the plagiogranite pattern. In con- sequence, tapping of a LREE-enriched mantle source appears more likely. Given the homogeneously high initial epsilon Nd values (+8.8 to +9.5), such enrichment must have occurred shortly before melt- ing. It seems plausible that the N-MORB mantle domain was enriched by injection of melts, derived by very low degrees of partial melts from another, probably adjacent MORB-type mantle segment, prior to 15-20% melting of the thus enriched domain. This mechanism would explain the chemical pattern of the plagiogranite and Fe-diorite parent magmas.

100 ":

10"

] I i i i i i i i i i i i i i i

La Ce Nd Sm Eu Gd Tb Yb Lu

Initial ma~mna ~ Boe-1

• 72% fractiotml .i. 72% f.c. + crystallization (f.c.) 2% zircons

Boc-2

Fig. 7. Model l ing o f REE patterns in plagiograni tes in assuming

15% melt ing of a LREE-enr iched N - M O R B mantle source, fol-

lowed by up to 72% fractional crystal l izat ion of the basaltic parent

magma . An incorporat ion of 2% inherited zircon is also consid-

ered, to explain the slightly too low HREE concentra t ion of the

plagiograni te model patterns.

Small volumes (1-5%) of basic melts, enriched in incompatible elements and REE are able to infiltrate and metasomatise mantle peridotites [43]. Under the condition that this process is cryptic in nature [44], percolation by such melts does not significantly change modal or major-minor element compositions of the peridotites [45]. In consequence, our dike parent magmas may have been derived by melting of metasomatized mantle, followed by strong fractional crystallization, to produce the plagiogranitic liquids.

To quantify this concept schematically, and to constrain compositions of the plagiogranite parent magmas, we have modelled partial melting of the metasomatized peridotites using the inversion method from [46]. Fig. 7 shows different model REE patterns in assuming: (1) 1% melt production from a N- MORB mantle source in the spinel stability field; (2) injection of these melts to 2% in the overlying lherzolites of the plagioclase stability field; (3) melt- ing of this enriched mantle to 15%; and (4) fractional crystallization up to 72% to produce the plagiogran- ites. Moreover, 2% inherited zircon was also in-

L, Borsi et al. / Earth and Planetary Science Letters 140 (1996) 227-241 239

cluded in the model to compensate for the slightly too low model HREE abundances. These model patterns are identical to those actually measured in the plagiogranites and, therefore, the idea of magma extraction from a secondarily LREE-enriched N- MORB type mantle region seems to be the most likely explanation to produce the plagiogranites,

An important observation is the presence of inher- ited Precambrian components in the dikes, indicating ages of 1.70, 1.78, and > 2.1 Ga for continental material involved in magma genesis of both the plagiogranites and the host Fe-diorites. These ages are in good agreement with the 1.2-2.5 Ga Sr and Nd model ages determined for the Northern Apen- nines lherzolites [16]. Such age coincidence may be an indication that Proterozoic mantle differentiation occurred in relation to a subduction event that brought relicts of continental crust material (the inherited zircon components and crustal Pb) into the subconti- nental mantle, from where they were extracted by injection of the Jurassic melts. Since initial epsilon Nd values of our plagiogranites sources (+ 8.8 to + 9.7) are identical to those of most clinopyroxenes from the lithospheric Voltri peridotites ( + 7.7 and +9.9) [16], assimilation of continental lithospheric mantle (i.e., the zircon host) would not measurably modify Nd-isotope signatures of the asthenospheric magmas. Another mechanism for introducing crustal contamination could be immediate contact of the Jurassic mantle melts with the lowermost crust; how- ever, there are no field observations available to substantiate such a model and the real mechanism of crustal contamination remains to be clarified.

6.2. Geodynamic significance

Our period of melting at 156-150 Ma is roughly coeval with about 163-165 (_+20) Ma subsolidus re-equilibration of the Sm-Nd system observed for plagioclase and clinopyroxene in lherzolites of the Northern Apennines [16]. Since such Sm-Nd re- equilibration most likely occurred during decompres- sion of lithospheric mantle [16], both events may reflect the same process, including unroofing of man- tle complexes in relation to extension, as well as melt injection from the decompressed and melted asthenospheric mantle underneath. Limited partial melting was actually also described to have occurred

in the Voltri Group continental lithosphere peri- dotites [ 15].

As depicted in Fig. 2, plagiogranitic and doleritic intrusions roughly follow fault zones that truncate the gabbros and the serpentinites. Moreover, the plagiogranites truncate the sedimentary breccia which contains plagiogranite and basaltic dike fragments. This feature suggests that mantle extension, basaltic dike injection, plagiogranite intrusion, and breccia formation are strongly related processes. The ages determined here document that this period of tec- tonic and magmatic activity occurred from 156 to 150 Ma. Another, 152 + 4 Ma old Fe-diorite intru- sion [20] in southern Tuscany probably reflects the same magmatic event. In our case, the intruded mantle complex most likely was part of the Adriatic plate segment (e.g. [24]).

The 156-150 Ma tectono-magmatism clearly pre-dates the deposition of pillow basalts on top of the breccias (Fig. 2). Neither basaltic or plagiogran- ite dikes intrude the pillow lavas in the Ligurian Alps and the Northern Apennines. This observation is in agreement with the age of the Callovian to Tithonian radiolarian cherts [24,25], and the locally occurring Tithonian-Neocomian Calpionella lime- stones [3] that cover these basalts in the Northern Apennines. Denudation of the mantle complex, and its direct exposure to sea water, were probably caused by movements along large-scale detachment faults [47]. Such a situation is observed in the North At- lantic Galicia margin, where mantle rocks were sepa- rated from their overlying continental crust and ex- posed on the ocean floor, in relation to rifting prior to oceanic crust formation (e.g. [48]). There the latest stages of rifting were also accompanied by the intru- sion (underplating) of dolerites and Fe-gabbroic dikes into peridotites (e.g. [49]).

,

1.

2.

General conclusions

Diorites and plagiogranites were emplaced at 156-150 Ma into the subcontinental mantle of the Adriatic-European plate; that is, into the margins of the subsequently forming Piedmont-Ligurian basin. This magmatism occurred during the late stages of rifting, in association with normal faulting,

240 L. Borsi et a l . / Earth and Planetar)' Science Letters 140 (1996) 227-241

low-angle detachment of the mantle complex from its overlying continental crust, and direct mantle- rock exposure on the ocean floor.

3. Small amounts of Precambrian continental crust material was incorporated into the variously dif- ferentiated basaltic liquids.

4. The parent magmas were generated by 15-20% melting of a N-MORB type mantle in the plagio- clase stability field, which had been enriched in LREE shortly before melting. This enrichment is most probably due to the injection of melts pro- duced by small degrees of melting in an deeper, adjacent N-MORB mantle domain.

Acknowledgements

We thank L. Cortesogno, G. Ferrara and G. Giglia for inspiring discussions, field guidance and impor- tant financial support of this work. We also acknowl- edge CNRS France for financing the U-Pb analyses and we thank L. Cortesogno and R. Petrini for critical reading of the manuscript. We also acknowl- edge M. D'Orazio for the inverse method computer program, and we are very much indebted to Daniel Bernoulli, Jacques Kornprobst and an anonymous reviewer for their competent comments, which helped to improve the manuscript. This is IPG Paris contri- bution No. 1414. [TA]

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