Post-collisional melting of crustal sources constraints.pdf

Int J Earth Sci (Geol Rundsch) (2008) 97:705–723

DOI 10.1007/s00531-007-0185-z

ORIGINAL PAPER

Post-collisional melting of crustal sources: constraints

from geochronology, petrology and Sr, Nd isotope geochemistry

of the Variscan Sichevita and Poniasca granitoid plutons

(South Carpathians, Romania)

Jean-Clair Duchesne Æ Jean-Paul Li`geois Æ

Viorica Iancu Æ Tudor Berza Æ Dmitry I. Matukov Æ

Mihai Tatu Æ Sergei A. Sergeev

Received: 2 August 2006 / Accepted: 21 February 2007 / Published online: 21 March 2007

Springer-Verlag 2007

Abstract The Sichevita and Poniasca plutons belong to

an alignment of granites cutting across the metamorphic

basement of the Getic Nappe in the South Carpathians. The

present work provides SHRIMP age data for the zircon

population from a Poniasca biotite diorite and geochemical

analyses (major and trace elements, Sr–Nd isotopes) of

representative rock types from the two intrusions grading

from biotite diorite to biotite K-feldspar porphyritic

monzogranite. U–Pb zircon data yielded 311 ± 2 Ma for

the intrusion of the biotite diorite. Granites are mostly

high-K leucogranites, and biotite diorites are magnesian,

and calcic to calc-alkaline. Sr, and Nd isotope and trace

element data (REE, Th, Ta, Cr, Ba and Rb) permit distin-

guishing ﬁve different groups of rocks corresponding to

several magma batches: the Poniasca biotite diorite (P 1 )

shows a clear crustal character while the Poniasca granite

(P 2 ) is more juvenile. Conversely, Sichevita biotite diorite

(S 1 ), and a granite (S 2 *) are more juvenile than the other

Sichevita granites (S 2 ). Geochemical modelling of major

elements and REE suggests that fractional crystallization

can account for variations within P 1 and S 1 groups.

Dehydration melting of a number of protoliths may be the

source of these magma batches. The Variscan basement, a

subduction accretion wedge, could correspond to such a

heterogeneous source. The intrusion of the Sichevita–Po-

niasca plutons took place in the ﬁnal stages of the Variscan

orogeny, as is the case for a series of European granites

around 310 Ma ago, especially in Bulgaria and in Iberia, no

Alleghenian granitoids (late Carboniferous—early Permian

times) being known in the Getic nappe. The geodynamical

environment of Sichevita–Poniasca was typically post-

collisional of the Variscan orogenic phase.

) J.-P. Li`geois

Department of Geology, University of Li`ge,

Bat. B20, 4000 Sart Tilman, Belgium

e-mail: jc.duchesne@ulg.ac.be

J.-P. Li`geois

Department of Geology, Africa Museum, Tervuren, Belgium

e-mail: jean-paul.liegeois@africamuseum.be

V. Iancu T. Berza

Geological Institute of Romania, Bucharest, Romania

e-mail: viancu@igr.ro

T. Berza

e-mail: berza@igr.ro

Keywords Granite modelling Diorite Zircon dating

Getic nappe

D. I. Matukov S. A. Sergeev

Center of Isotopic Research,

All-Russian Geological Research Institute (VSEGEI),

74 Sredny prospect, 199106 St.-Petersburg, Russia

e-mail: Dmitry_Matukov@vsegei.ru

S. A. Sergeev

e-mail: Sergey_Sergeev@vsegei.ru

Introduction

Granitic rocks are a major constituent of the earth crust

and, in orogenic belts, are typical products of recycling

processes. Their study thus offers a most promising

opportunity to unravel the mechanism of magma formation

and evolution in the deep crust, together with giving insight

into the nature of the source rocks that are melted. The

M. Tatu

Geodynamical Institute of Romanian Academy,

Bucharest, Romania

e-mail: mtatu@geodin.ro

123

J.-C. Duchesne (

706

Int J Earth Sci (Geol Rundsch) (2008) 97:705–723

compositions of the primary melts, however, are often

blundered by fractionation processes and the occurrence of

crystals entrained from source rocks or from cumulates

formed in the early phases of differentiation. In regions

where granites occur together with maﬁc rocks, a major

role has been assigned to the basic magma either as a

source of heat to trigger the melting process, or as a mixing

or hybridization component. Moreover, several types of

material can potentially act as source rocks of granitic

magmas depending on their mineralogy, on the availability

of ﬂuids of various compositions, and on temperature. Fi-

nally, several sources can melt together, simultaneously

with different degrees of melting, or in sequence along a

PT path.

North and South of the Danube Gorges that cross the

South Carpathians and separate Romania from Serbia, four

major granitoid bodies cut across the basement of the Getic

nappe, which is the most important Alpine nappe of the

South Carpathians (Fig. 1 ). These bodies form a discon-

tinuous alignment 100 km long and up to 10 km wide

(Sandulescu et al. 1978 ), suggesting a continuous batholith

buried beneath the Mesozoic and Cenozoic cover se-

quences. In Serbia (from south to north), the main plutons

are known as the Neresnica and Brnjica plutons (Vaskovic

and Matovic 1997 ; Vaskovic et al. 2004 ), the latter in di-

rect continuation across the Danube river with the Sich-

evita pluton in Romania. A fourth granitoid pluton, 15 km

north of the latter and separated by Mesozoic sediments,

was named the Poniasca pluton by Savu and Vasiliu

( 1969 ).

The similarity in petrography, mineralogy and major

element geochemistry of the Sichevita and Poniasca plu-

tons supports the hypothesis of a geometrical continuity

between the two Romanian plutons. Comparison with

available data on the Serbian plutons is further pointing to

the occurrence of a regional batholith. More detailed geo-

chemical studies on the Romanian occurrences, including

trace elements and Sr and Nd isotopes which are presented

here, show, however, a more complex image. Both intru-

sions result from different crystallization processes and

imply several magma types. Moreover, it is inferred that

both plutons originated by partial melting of several dis-

tinct sources.

many syntheses based on hundreds of studies have pro-

posed complex and partly conﬂicting models (see the re-

views of Berza 1997 and Iancu et al. 2005a ).

The South Carpathians are viewed as a Cretaceous

nappe pile (Iancu et al. 2005a and references therein), in

tectonic contact with the Moesian Platform (Stefanescu

1988 ; Seghedi and Berza 1994 ). The uppermost Cretaceous

nappes of the South Carpathians are the Getic and Supra-

getic nappes. They include both pre-Alpine metamorphic

basement and Upper Paleozoic–Mesozoic sedimentary

cover (Iancu et al. 2005b ). The Sichevita, Poniasca, Ne-

resnica and Brnjica plutons have intruded into the meta-

morphic basement of the Getic nappe. According to Iancu

et al. ( 1988 ), Iancu and Maruntiu ( 1989 ) and Iancu ( 1998 ),

the pre-Alpine basement of the Getic nappe in the Roma-

nian Banat is made up of several lithotectonic units

(Fig. 1 ), assembled in Variscan times as thrust sheets and

composed of various sedimentary, volcanic and (ultra)

maﬁc protoliths, metamorphosed in several low to med-

ium-high grade episodes.

Late Variscan post-thrust folding of the getic nappe

basement is well expressed by regional dome-shaped

structures. The alignment of the granitic plutons, though

conspicuous on a large scale (Fig. 1 ), is considered by

Savu et al. ( 1997 ) and Iancu ( 1998 ) to be tectonically

controlled either by a host anticline or by a transcurrent

fault. Late Variscan, extension-related movements follow-

ing the nappe stacking and folding could also be envisaged.

The Sichevita and Poniasca granitoids

Former studies on Sichevita were made by Birlea ( 1977 ),

Stan et al. ( 1992 ), Stan and Tiepac ( 1994 ) and Iancu

( 1998 ), while Poniasca granitoids have received less

attention (see references in Savu et al. 1997 ).

Both granitoid plutons were re-interpreted as composite

intrusions crosscutting the Variscan nappe pile of the Getic

basement, north of Danube (Iancu et al. 1996 ; Iancu 1998 ).

Both granitoids and their metamorphic country rocks are

sealed to the west by unconformable Upper Carboniferous-

Permian continental deposits and Mesozoic covers (Fig. 1 ).

They are crosscutting the Variscan nappe pile of the Getic

metamorphic basement, which is made up of four units

(Iancu 1998 ). The Nera unit is mainly composed of me-

tasedimentary micaschists and gneisses. The Ravensca unit

is made up of gneisses with maﬁc and ultramaﬁc protoliths,

metamorphosed in amphibolite and eclogite facies condi-

tions and retrogressed in greenschist facies conditions.

Different from these, the low-grade Paleozoic formations

are mainly represented by metabasalts and metadolerites of

ensialic, back-arc related origin (Maruntiu et al. 1996 ),

with associated carbonate rocks and black shales (Buceava

unit) or metapelites (Minis unit).

Geological framework of the granite intrusions

The South Carpathians represent a segment of the Alpine-

Carpathian-Balkan fold-thrust belt, moulded against the

Moesian Platform as a horse shoe, with an eastern E–W

oriented part and a western N–S oriented part, in the

Romanian Banat and Eastern Serbia province (Fig. 1 b).

Since Murgoci ( 1905 ) discovered the main nappe structure,

123

Int J Earth Sci (Geol Rundsch) (2008) 97:705–723

707

Fig. 1 a Generalised geological

map of the Romanian Poniasca

and Sichevita granites and

related Serbian intrusions

(modiﬁed after Sandulescu et al.

1978 and Iancu et al. 2005b ).

b Sketch map of the various

geological units in the

Carpathians belt; c Tentative

cross-section through the

geological units, parallel to the

Danube river

Contact metamorphism of the studied granitoid plutons

is marked by neoformation of biotite and andalusite (Savu

et al. 1997 ) as well as of garnet and muscovite (Iancu

1998 ). Detailed mapping of the Poniasca pluton shows that

the contacts are grossly parallel to a foliation in the sur-

rounding gneisses (Savu et al. 1997 ). Locally, clear

crosscutting relationships are observed with the foliation in

the Ravesca unit. The round northern end of the pluton

(Fig. 1 ) ﬁts an antiform structure in the country rocks

(Savu et al. 1997 ). The granitoids show a foliation parallel

to the border, with microgranular dark enclaves and crys-

talline schist xenoliths elongated in the same plane. This

foliation itself is crosscut by undeformed late pegmatitic

and aplitic veins (see Fig. 4 in Savu et al. 1997 ). The

observed magmatic planar ﬂow structures inside both

Sichevita and Poniasca plutons (Savu et al. 1997 ; Iancu

1998 ) could result from a syn-emplacement ballooning

deformation of the intrusion.

North-East/South-West sub-vertical faults follow part of

the eastern border of the Sichevita pluton and both the

eastern and western borders of the Poniasca body. They

sometimes contain thin concordant granitoid dykes and are

marked by local low-temperature mylonites (actinolite-

chlorite-albite schists). These faults and the elongation of

the massifs suggest some kind of tectonic control (Iancu

et al. 1996 ) on the emplacement in zones of apparent

weakness parallel to the regional foliation and shear zones.

Considering the structural features mentioned above and

the Late-Variscan age of the plutons, the tectonic setting

can be deﬁned as post-collisional (Li´geois 1998 ).

123

708

Int J Earth Sci (Geol Rundsch) (2008) 97:705–723

Petrography of the Sichevita and Poniasca granitoids

granitoids, Birlea ( 1977 ) quotes 328–350 Ma U–Pb mon-

azite ages (determined by Gr ¨nenfelder at ETH Z ¨rich)

and 250–310 Ma K–Ar microcline and biotite ages

(determined by Tiepac at Nancy). In the Serbian Brnjica

pluton, Rb–Sr ages of 259–272 Ma are reported by Va-

skovic et al. ( 2004 ). A Carboniferous emplacement mini-

mum age is in agreement with the presence of pebbles from

the granitoid plutons in the Upper Carboniferous con-

glomerates exposed at the base of the unconformable

sedimentary cover.

A biotite diorite from the Poniasca pluton is dated here

using the U–Pb on zircon chronometer. Eleven zircon

grains from sample#1 (R4710) were analysed (Table 1 ).

The measurements were carried out on a SHRIMP-II ion

microprobe at the Centre for Isotopic Research (VSEGEI,

St. Petersburg, Russia; for methodology see Appendix).

The zircon crystals are zoned and may have relic cores

(Fig. 2 a–d). Two cores were analysed and give older ages

than the outer rims. One core (6.1, Fig. 2 c) is nearly con-

cordant (3% discordant) and its 207 Pb/ 206 Pb age is 891 ±

20 Ma; the other core (5.2, Fig. 2 c) is more strongly dis-

cordant and no meaningful age can be calculated on this

single grain. Within the other nine zircon crystals, seven

measurements determine a Concordia age of 311 ± 2 Ma

(2r; MSWD= 0.06; Fig. 2 e). The two remaining zoned

zircon crystals (4.1 and 5.1; Fig. 2 b and 2 c) are also con-

cordant but at a slightly older age of 324 ± 4 Ma (2 r;

MSWD= 0.84). Taken together, the 11 zircon analyses

deﬁne a discordia with an upper intercept at 895 ± 56 Ma

and a lower intercept at 319 ± 14 Ma.

The emplacement of the Poniasca pluton is precisely

dated at 311 ± 2 Ma by magmatic zircons or magmatic

overgrowths on inherited zircons. Although based only on

a few core analyses, the 207 Pb/ 206 Pb age of 891± 20 Ma

(Fig. 2 e, inset) can be considered either as the age of the

source of the magma (inherited zircon grain), or at least of

a major contaminant of the diorite magma. The position of

the core 5.2 in the Concordia diagram (Fig. 2 e) is consid-

ered as the result of Pb loss of a ca. 890 Ma old zircon

during the Poniasca magmatic event. The concordant

fractions giving the 324 ± 4 Ma age are interpreted as

being inherited from an early partial melting event. The

main conclusions are that the intrusion of the Poniasca

pluton (and by correlation also of the Sichevita pluton)

occurred at 311± 2 Ma. These granitoids are contempora-

neous to the Variscan granites on the other side of the

Moesian platform, i.e. the San Nikola calc-alkaline granite

at 312 ± 4 Ma and the Koprivshtitsa two-mica leucocratic

granite at 312 ± 5 Ma (Carrigan et al. 2005 ). As is the case

in Bulgaria, this puts the Poniasca-Sichevita plutons on the

young side of the European post-collisional magmatism

that are predominantly 340–320 Ma old (see review in

Carrigan et al. 2005 ). In addition, at least the dated biotite

The Sichevita and Poniasca granitoid plutons consist of a

series of rocks intermediate between two major petro-

graphic types: (1) hornblende biotite diorite, and (2) biotite

K-feldspar porphyritic granite. The contacts between the

two rock types are commonly sharp and lobated, giving

evidence that both were intruded at the same time. Both

rock types contain maﬁc magmatic enclaves (=MME; Di-

dier and Barbarin 1991 ) or schlieren of dioritic composi-

tion, suggesting that mixing processes may have played an

important role in the formation of intermediate composi-

tions. In the present study, the biotite diorite will be named

S 1 (for Sichevita) and P 1 (for Poniasca), and the biotite K-

feldspar porphyritic granites will be deﬁned S 2 and P 2 ,

respectively.

(1) Biotite diorite

Biotite diorite (S 1 ,P 1 ) is a medium-to coarse-grained

inequigranular massive rock, composed of plagioclase,

biotite, hornblende, quartz, zircon, apatite, titanite, allanite

and Fe–Ti oxides. K-feldspar is rarely present. Plagioclase

(An 20–30 ) shows albite twinning and wavy oscillatory

zoning, is anhedral and partially albitised or rimmed with

albite. Dark brown biotite is present as inclusions in pla-

gioclase, or as large interstitial crystals, containing zircon,

apatite, zoned allanite and skeletal titanite. Green horn-

blende is common and always replaced by biotite. Epidote,

chlorite, sericite and albite occur in deformed and altered

samples.

(2) Biotite K-feldspar porphyritic granite

Biotite K-feldspar granite (S 2 ,P 2 ) is porphyritic, with

variably rounded phenocrysts of poikilitic microcline

perthite. The latter is rimmed by albite and may contain

inclusions of plagioclase and small quartz grains, as well as

ﬁne-grained biotite and muscovite. Anhedral plagioclase

(An 20–30 ), with strong oscillatory zoning, typically shows

corroded rims of albite or microcline. Quartz is interstitial.

Rare myrmekites develop at the contact between plagio-

clase and K-feldspar. Biotite is dark brown, mainly inter-

stitial, and contains accessory phases (zircon, apatite,

titanite and Fe–Ti oxides). It is replaced by white mica.

Primary muscovite locally occurs in P 2 samples. Horn-

blende is rare in this petrographic type, but garnet is

common. In rare deformed and altered samples, epidote,

albite, chlorite, sericite and hematite are also present.

Geochronology

Published isotopic ages from the various outcrops deﬁ-

nitely differ, but all point to Variscan events. For Sichevita

123

Int J Earth Sci (Geol Rundsch) (2008) 97:705–723

709

diorite contains old material whose age is 891 ± 20 Ma, an

age also known from zircon cores in Bulgarian orthog-

neisses of Variscan age (Carrigan et al. 2006 ).

Geochemistry

Major element geochemistry

The Ponisaca and Sichevita plutons were extensively

studied by Romanian petrologists and a large amount of

analyses are available (Savu and Vasiliu 1969 ; Savu et al.

1997 ; Birlea 1976 ; Stan et al. 1992 ; Iancu et al. 1996 ). In

this study, representative samples of the main petrographic

types were collected in the ﬁeld (Table 2 ), and analysed for

major elements (XRF), trace elements (ICP-MS) and iso-

topes (TIMS). The methods are brieﬂy described in the

appendix. The new major element data (Table 3 ) are

compared to the former analyses in Figs. 3 , 4 . The various

element contents form continuous trends from ca. 60 to

75% SiO 2 , i.e. from biotite diorite to granite. The trends are

roughly linear and point to a second group of rocks, made

up by maﬁc microgranular inclusions (sensu Didier and

Barbarin 1991 ). It is shown by Figs. 3 , 4 that our sample

selection covers reasonably well the interval of composi-

tion of the Romanian samples from biotite diorite to

granite. The maﬁc microgranular inclusions were not re-

studied because of exceptional occurrence. The calc-alka-

line character already noted by Iancu et al. ( 1996 ) of the

composite series is conﬁrmed in the AFM diagram (Fig. 4 )

in which the samples show a linear trend with little vari-

ation in the Fe/Mg ratio. The granites (Table 3 ) have high

silica contents and are slightly peraluminous (ASI: 0.96–

1.13). Their normative quartz and feldspar contents are

above 95% (except#8 at 92%), which indicates leuco-

granitic compositions. In the classiﬁcation of Frost et al.

( 2001 ), they show calcic to alkali-calcic compositions in

the modiﬁed alkali-lime index (MALI) and are magnesian

to ferroan. In the K 2 O versus SiO 2 diagram (Peccerillo and

Taylor 1976 ) the samples have a medium- to high-K

composition.

In the Harker diagram for Na 2 O (Fig. 3 ) the large dis-

persion of the data points may result from the late albiti-

sation process revealed by the petrographical study. The

dispersion of some K 2 O values (Fig. 3 ) can also be ex-

plained by a late metasomatic alteration. In particular, the

high K 2 O content of the maﬁc enclaves suggests interdif-

fusion of K between the granitic and the basic melts.

Crystallization of biotite in the basic melt could have

maintained the K content in the melt at a low value, thus

promoting exchange of K with the granitic melt (see the

review by Debon 1991 ). If the two mobile elements Na and

K are excluded, the linear trends observed for the immobile

123

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: