Charnockitic rocks in the crystalline basement of Western Lithuania: implications on their origin and correlation with the Askersund suite in SE Sweden

Charnockitic rocks in the crys tal line base ment of West ern Lith u a nia:

im pli ca tions on their or i gin and cor re la tion with the Askersund suite in SE Swe den

Gediminas MOTUZA and Vykintas MOTUZA

Motuza G. and Motuza V. (2011) – Charnockitic rocks in the crys tal line base ment of West ern Lith u a nia: im pli ca tions on their or i gin and cor re la tion with the Askersund suite in SE Swe den. Geol. Quart., 55 (1): 63–70. Warszawa.

The polyphase Kuršiai batholith of charnockitic rocks, ex tend ing over some 10 000 km² with a few smaller plutons, has been re vealed in the West Lith u a nian Granulite Do main. Plutons in truded be tween 1850 and 1815 Ma, and are com posed of in ter me di ate and acid va ri et - ies of charnockitic rocks. They are ferroan, calc-alkalic to al kali-calcic, pre dom i nantly peraluminous, and re veal both S- and A-type gran ite char ac ter is tics. Based on pet ro chem i cal data charnockitic magma was gen er ated in the con ti nen tal crust more then 30 km-thick, that formed in a subduction-re lated collisional tec tonic en vi ron ment. Magmatism took place in the later oro gen esis pe riod, tran si tional from syn-ki ne matic collisional to post-ki ne matic extensional phases. Magma gen er a tion was pre sum ably trig gered by in tru sions of ba - salt magma in the crust. The rock com po si tion was de ter mined by the in ter ac tion of ba saltic mag mas with anatectic melt of Al-rich metasedimentary rocks, in volv ing mech a nisms of as sim i la tion and hy brid iza tion. The close geo chem i cal af fin ity of the Kuršiai suite in West ern Lith u a nia to the co eval Askersund plutonic suite in south east ern Swe den im plies a sim i lar tec tonic en vi ron ment and pro cesses of the for ma tion of con ti nen tal crust and sug gests pos si bil i ties of wider cor re la tion be tween these re gions.

Gediminas Motuza and Vykintas Motuza, De part ment of Ge ol ogy and Min er al ogy, Vilnius Uni ver sity, LT-03101 Vilnius, Lith u a nia, e-mails: gediminas.motuza@gf.vu.lt, vykintas.motuza@gmail.com (re ceived: Sep tem ber 24, 2010; ac cepted: March 23, 2011).

Key words: Charnockitic rocks, West Lith u a nian Granulite Do main, Kuršiai pluton, Askersund suite.

INTRODUCTION

Lith u a nia is sit u ated near the west ern mar gin of the East Eu ro pean Craton (EEC) within the Svecofennian Do main cov - ered by Phanerozoic platformal strata and sep a rated from the Fennoscandian Shield by the Bal tic Sea. The crys tal line crust has been in ves ti gated us ing drill ing and geophysical methods.

Plutons of charnockitic and co eval gra nitic rocks have been re vealed in the base ment of West ern Lith u a nia, mark ing an ex - ten sive mag matic event in the his tory of EEC for ma tion (Fig. 1). The Kuršiai batholith, which en com passes an area of ap prox i mately 140 ´ 80 km in West ern Lith u a nia and off shore un der the Bal tic Sea, is one of the larg est known charnockitic bod ies in the west ern part of the EEC. Charnockitic rocks of this pluton have been ob tained from 86 bore holes (Fig. 1). A few smaller charnockitic plutons – Sidabravas, Ariogala and Kybartai – are each char ac ter ized by cores from a sin gle bore - hole. To gether with the Kuršiai batholith they are re ferred to as the Kuršiai suite. U-Pb zir con dat ing places the ages of the charnockitic rocks at be tween 1850 and 1815 Ma (Claesson et al., 2001; Motuza et al., 2008). Along with the charnockitic

rocks, large gra nitic plutons have been re vealed. The gra nitic rocks have sim i lar geo chem i cal char ac ter is tics to the charnockites. The U-Pb zir con age of gar net- and cor di er - ite-bear ing gran ite from the Kužiai-65 bore hole is 1844 ±12 Ma and gneissic gran ite from the Graužai-105 bore - hole is 1832 ±4.9 Ma i.e. within the time span of the Kuršiai suite (Motuza et al., 2008).

The gen eral struc tural, min er al og i cal and geo chem i cal pa - ram e ters of the charnockitic and gra nitic rocks, and the orig i nal chem i cal anal y ses and dat ing re sults, are pub lished and avail - able on the web (Motuza et al., 2008).

This pa per is de voted to a more de tailed pet ro log i cal char - ac ter iza tion of the charnockitic rocks, the im pli ca tions for their for ma tion, and a pro posed cor re la tion with the Askersund suite in south east ern Swe den.

The Askersund suite in SE Swe den is lo cated geo graph i - cally close to West ern Lith u a nia (Fig. 1). That suite em braces a num ber of plutons com posed of gra nitic rocks, sub or di nated monzodiorite, quartz monzonite and gab bro (e.g., Wikström and Andersson, 2004). In the gran ite of the Graversfors in tru - sion and the quartz monzonite of the Tiveden area both clino- and orthopyroxenes are pres ent, thus rep re sent ing

acharnockitic as so ci a tion. Plutons of the Askersund suite are lo cated in the south ern part of the Transscandinavian Ig ne ous Belt (TIB), along the bor der with the Svecofennian prov ince, but are older com pared to the ma jor part of the TIB in this area (1.81–1.76 Ga), partly de formed and re ferred to as “TIB-0”

(Ahl et al., 2001; Wikström and Andersson, 2004). The U-Pb in zir con ages of the Askersund suite fall in the age range 1.86–1.83 Ga (e.g., Persson and Wikström, 1993; Wikström, 1996; Andersson, 1997a; Andersson et al., 2006; Andersen et al., 2009).

GEOLOGICAL SETTING OF THE CHARNOCKITIC ROCKS IN WESTERN LITHUANIA

The ex tent of the charnockitic and co eval gra nitic plutons stud ied is lim ited to the West Lith u a nian Granulite Do main (WLGD), which is a par tic u lar lithospheric block with a com - po si tion and struc ture dis tinct from the ad ja cent do mains (Fig. 1). The crust in the WLGD is 40–45 km-thick while its lower crust is thin (~10 km). In the ad ja cent East Lith u a nian

Fig. 1A – geo log i cal sketch of the crys tal line base ment of West ern Lith u a nia; B – geo graph ical po si tion of the re search area, Askersund suite plutons marked in black; C – lo ca tion of bore holes and their groups in the Kuršiai batholith

ELD – East Lith u a nian Do main, MLSZ – Mid-Lith u a nian Su ture Zone, WLGD – West Lith u a nian Granulite Do main; plutons of charnockitic rocks: A – Ariogala; Kb – Kybartai; Kr – Kuršiai; S – Sidabravas; bore holes: Abl – Ablinga, Akm – AkmenÅ, Bbl – Baubliai, D – Dar ius, Et – Eituèiai, Grk – Girkaliai, Grn – Gorainiai, Krt – Kretinga, Kul – Kuliai, Lz – Laužai, Mcc – Macuièiai, Mkl – Mykoliškis, Mm – Mamiai, Nsd – Nausodis, Pjr – Pajñris, Plk – PaluknÅ, Plng – PlungÅ, Sdr – Syderiai, Slp – žlapgiriai, Slt – Salantai, Slgl – žilgaliai, Stmb – StumbrÅs, Tbs – Tñbausiai, Tl – Toliai, Trb – Traubai, Trsk – Tryškiai, Vd – Vydmantai, Vlc – VÅlaièiai, Vvr - VeivirÅûnai, Vzc – VÅûaièiai, Zmt – ŽemytÅ, Žtt – Žutautai

Do main the crust is 50–55 km-thick and more vari able both in terms of thick ness and com po si tion (Giese, 1998; Eurobridge Seis mic Work ing Group, 2001; Bogdanova et al., 2006). Both do mains are sep a rated by the Mid-Lith u a nian Su ture Zone, re - garded as a subduction-re lated collisional zone formed at 1845–1830 Ma (Motuza, 2005).

The WLGD is a high-grade ter rain char ac ter ized by tem per - a tures of peak meta mor phism of up to T = 850^oC, and cor re - spond ing pres sures of up to 8 kbar (SkridlaitÅ and Motuza, 2001). Pre sum ing an av er age crustal den sity of 2800 kg/m³, a lithostatic pres sure of 8 kbar cor re sponds to a depth of ca. 29 km.

Supracrustal rocks are pre served as highly migmatized rel - ics. They are rep re sented by metapelitic paragneisses with gar - net and sillimanite, pyroxene-bear ing cal cium-rich paragneisses and fel sic bi o tite paragneisses grad ing into quartzites. The supracrustals are in ter preted as pri mary greywackes, in places with an ad mix ture of pyroclastic ma te - rial, and arkosic sand stones (Motuza and Staškus, 2009). Mafic meta vol can ics are not known on the subsurface of the crys tal - line crust in the WLGD. The pos si ble age of sed i men ta tion is lim ited by an in ter val 2.1–1.85 Ga set by the youn gest age of the de tri tal zir con (2.1–2.145 Ga) and the old est crys tal li za tion age of charnockitic rocks (1.846 ±12 Ga; Claesson et al., 2001;

SkridlaitÅ et al., 2007; Motuza et al., 2008).

PETROGRAPHIC FEATURES OF THE WLGD CHARNOCKITIC ROCKS

The Kuršiai batholith is al most en tirely com posed of charnockitic rocks ex cept for a few mi nor bod ies of gran ite and lo cal en claves of migmatized supracrustals. The clas si fi ca tion of charnockitic rocks fol lows that of Le Maitre (2002). The principal va ri et ies of charnockitic rocks in Kuršiai pluton are:

opdalite, mangerite, charnockite, enderbite. The Sidabravas pluton is rep re sented by mangerite, and the Ariogala and Kybartai plutons by charnockite.

Typ i cal rock-form ing min er als are plagioclase (An 40-55), K-feld spar, quartz, orthopyroxene (ferroenstatite to magnesian ferrosalite), bi o tite, clinopyroxene (ferrodiopside), and gar net (almandine). Com mon ac ces sory min er als are mag ne tite, il - men ite, hercynite, zir con, monazite, xeno time and fluorapatite.

Hornblende ap pears lo cally as a sec ond ary min eral, form ing re - place ment rims on clinopyroxene. The bulk con tent of mafic min er als is up to 20–25%.

The ig ne ous tex ture is well-pre served in the charnockitic rocks. They are me dium-grained (1–5 mm) to coarse-grained (5–10 mm), of ten por phy ritic, formed by K-feld spar and plagioclase pheno crysts up to 20–30 mm in di am e ter.

Plagioclase is of ten euhedral or hypidiomorphic with sim ple twins and os cil la tory zon ing. Orthopyroxene ap pears mainly in sep a rate grains, in places over grown by clinopyroxene, in di cat - ing an ear lier crys tal li za tion of the orthopyroxene (Motuza et al., 2008). The rock struc ture is pre dom i nantly mas sive, but in shear zones the rocks are strongly de formed up to mylonites.

GEOCHEMICAL CHARACTERISTICS OF THE WLGD CHARNOCKITIC ROCKS

AND THE ASKERSUND SUITE

47 anal y ses of the Kuršiai suite rocks were com piled from Motuza et al. (2008) and com pared with TIB rocks of the Askersund suite com piled from Andersson (1997b). The SiO2

con tent in the WLGD charnockitic rocks var ies from 53 to 73%, while the av er age is 63%, in di cat ing a pre dom i nance of in ter me di ate va ri et ies.

All charnockitic rocks of the Kuršiai pluton have fea tures of S-type gran ite based on pa ram e ters pro posed by Chappell and White (2001). In par tic u lar, the Alu mi num Sat u ra tion In dex (Al2O3/K2O+Na2O+CaO) var ies be tween 1.2 and 2.1, thus most of rocks are peraluminous, but on the Shand ASI di a gram (Maniar and Piccoli, 1989) al most 20% of the sam ples fall within the metaluminous field (Fig. 2). At the same time, most of Kuršiai charnockitic rocks re veal an A-type af fin ity (Motuza et al., 2008).

Fol low ing the geo chem i cal clas si fi ca tion of gra nitic rocks pro posed by Frost et al. (2001), the Kuršiai charnockitic rocks are ferroan in terms of the Fe-num ber, al kali-calcic or calc-alkalic in terms of their mod i fied al kali-lime in dex, a high-K or shoshonitic, ex cept for the enderbites which are me - dium-potassic (Fig. 3).

Gen er ally most of the S-type granitoids are magnesian and in vari ably peraluminous, while the A-type granitoids typ i cally are ferroan, metaluminous, alkalic or al kali-calcic (Frost et al., 2001). The WLGD charnockitic rocks dem on strate an af fin ity to both S- and A-type granitoids be ing at the same time ferroan, peraluminous and calc-alkalic.

Fig. 2. Aluminum Sat u ra tion In dex di a gram (Maniar and Piccoli, 1989) for the charnockitic rocks of the Kuršiai suite (full cir cles;

Motuza et al., 2008) and the Askersund suite (open squares;

Andersson, 1997b)

CONSIDERATION OF MAGMA SOURCE AND TECTONIC SETTING

Mod els of magma gen er a tions pro duce the charnockitic rocks sug gest var i ous sources, but mainly a com bined man tle and crustal source. Ac cord ing to Frost and Frost (2008) and Rajesh (2007) in tru sion of ba sic magma from the man tle into the lower crust and its sub se quent in ter ac tion with the crustal ma te rial (in terms of as sim i la tion-frac tional crys tal li za tion mod els) are the prin ci pal mech a nisms of magma gen er a tion.

PatiÔo Douce (1999), Frost et al. (2001), Rajesh and Santosh (2004) em pha size the con tri bu tion of re duced tholeitic or mildly alkalic ba saltic magma to the gen er a tion of iron-en - riched melts, pro duc ing ferroan gra nitic and charnockitic rocks while peraluminous potassic mag mas are orig i nated by melt ing and as sim i la tion of pelitic or semipelitic rocks. PatiÔo Douce (1999) ex per i men tally dem on strated that the best ex pla na tion for the or i gin of charnockitic rocks is an in ter ac tion be tween ba saltic mag mas and Al-rich metasedimentary rocks at rel a - tively shal low depths (15–20 km), where the for mer acts as the source of both mat ter and heat. The geo chem i cal and min er al - og i cal com po si tion of the Kuršiai rocks fit these petro gen etic sce nar ios (Fig. 4). The af fin ity of the Kuršiai rocks to the S-type gran ites (sensu Chappell and White, 2001) sug gests that both a tholeiitic magma and supracrustals (metagreywackes and metapelites) were in volved in the melt gen er a tion.

In spite of the fact that ba sic rocks are rare on the subsurface of the WLGD, ev i dence for the pos si ble in volve ment of ba - saltic magma is pro vided by ba sic en claves rarely ap pear ing within the charnockitic rocks as a few me tres-thick in ter vals within drillcores. These rocks con sist of clino- and orthopyroxene and plagioclase, are fine-grained and granoblastic, lo cally with a relic por phy ritic tex ture de fined by the pres ence of 1–2 mm plagioclase pheno crysts. Some ba sic en claves (bore hole Grk-4) con tain pheno crysts of feld spar up to 10–15 mm in size, mac ro scop i cally sim i lar to those in the

sur round ing rocks, a fea ture in di cat ing min gling of mag mas.

The in volve ment of Al-rich metasedimentary rocks in the magma gen er a tion of the mostly peraluminous Kuršiai suite is sup ported by the fact that the suite is hosted by migmatized metasedimentary se quences and con tains such en claves. More - over, in her ited de tri tal cores of Paleoproterozoic age (2.15–2.45 Ga) have been re vealed in zir cons from the charnockitic rocks, in di cat ing in put from meta sedi ments (Claesson et al., 2001).

The crys tal li za tion tem per a ture of the Kuršiai and Askersund rocks was roughly eval u ated us ing ex per i men tal di - a grams by Green and Pearson (1986) and Har ri son and Wat son (1984) re spec tively (Fig. 5). The iso therms show Fe-Ti ox ide and ap a tite sat u ra tion at a pres sure of 7.5 kbar. On the TiO2 vs.

SiO2 plot the ma jor ity of points are grouped along the 900–950^oC iso therms, while on the P2O5 vs. SiO2 plot they are grouped pre dom i nantly be tween the 800–900^oC iso therms, which is close to the typ i cal em place ment tem per a ture (900–1100^oC) of charnockitic plutons (Frost and Frost, 2008).

De tri tal cores in zir cons sug gest that the as sim i la tion took place at a magma tem per a tures not ex ceed ing 850^oC, be cause at higher tem per a ture zir cons might have dis solved (Wat son, 1996). This pres sure (7.5 kbar) and es ti mated tem per a ture cor - re spond to pa ram e ters for the peak meta mor phism in the WLGD (8 kbar and 850^oC), in di cat ing the gen er a tion of the magma at a crustal level of around 30 km or slightly deeper. At the same time it pro vides ev i dence for the ex is tence of a thick (>30 km) crust be fore 1.85 Ga – the time in di cated by the old est age of the charnockitic rocks.

On the Y-Nb-Ga and Rb/Nb vs. Y/Nb di a grams (Fig. 6) the charnockitic rocks of the WLGD re veal an af fin ity to A2-type gran ites. Rocks of the A2 group may be emplaced in a va ri ety of tec tonic set tings, in clud ing postcollisional, but it is es sen tial that their “mag mas were gen er ated from crust that had been through a cy cle of subduction zone or con ti nent-con ti nent col - li sion magmatism” (Eby, 1992, p. 643).

It has been noted that the geo chem i cal com po si tion of gra - nitic mag mas pri mar ily re flect the com po si tion and tec tonic set - ting of their source rocks (Eby, 1992; Frost et al., 2001). Nev er - the less cer tain tec tonic im pli ca tions are pos si ble. Thus con di - tions for the gen er a tion of ferroan, rel a tively an hy drous mag mas are com mon in extensional en vi ron ment (Frost et al., 2001).

CORRELATION OF THE KURŠIAI AND ASKERSUND SUITES

Look ing for pos si ble cor re la tives, the Kuršiai suite has been com pared with the Askersund suite in south east ern Swe den. The di a grams (Figs. 2–6) dem on strate cer tain sim i lar i ties of both suites in com po si tion, and pos si bly also in or i gin. Con tent and pro por tions of both ma jor and trace el e ments vary within the same lim its and form over lap ping ar eas on the di a grams. One dif fer ence is that the Kuršiai suite has a higher abun dance of peraluminous rocks and a sys tem at i cally lower con tent of Na2O.

On the Y-Nb-3Ga ter nary plot and Y/Nb vs. Rb/Nb plot both the Kuršiai and Askersund suites fall in the same A2 field, im ply ing the gen er a tion of mag mas by melt ing of the crustal ma te rial, which had un der gone subduction (Eby, 1992).

Fig. 3. FeO/FeO+MgO vs. SiO2 di a gram dis tin guish ing ferroan and magnesian plutons by Frost et al. (2001)

Ex pla na tions and ref er ences as in Fig ure 2

The for ma tion of the Askersund suite has been ex plained as due to melt ing of the pre-ex ist ing calc-al ka line crust, pro voked by mafic underplating in a con ti nen tal arc in volv ing a mix ing of gra nitic and gabbroic mag mas to pro duce the in ter me di ate rocks, and marks the shift from a collisional to the post-collisional extensional re gime (Andersson, 1991, 1997b;

Andersson and Wikström, 2004; Andersen et al., 2009).

Based on the geo chem i cal data pre sented in the pre vi ous sec tions a sim i lar model might be ap plied for the for ma tion of the Kuršiai suite. The higher con tent of alu mina and lower con - tent of so dium in the Kuršiai suite might be caused by a dif fer - ent com po si tion of the supracrustals, in par tic u lar a higher abun dance of metapelitic rocks which in flu enced the com po si - tion of the Kuršiai magmas.

CONCLUSIONS

The Kuršiai plutonic suite, em brac ing the polyphase Kuršiai batholith and smaller plutons in truded be tween 1.850 and 1.815 Ga, has been re vealed in the WLGD, West ern Lith u - a nia. It is com posed of in ter me di ate and acid va ri et ies of charnockitic rocks. They are ferroan, calc-alkalic to al - kali-calcic, pre dom i nantly peraluminous, and have both S- and A-type gran ite char ac ter is tics.

Charnockitic magma in the WLGD was gen er ated in con ti - nen tal crust, more than 30 km-thick, formed in a subduction-re - lated tec tonic en vi ron ment be fore 1850 Ma.

Magma gen er a tion pre sum ably was trig gered by in tru sions of ba saltic magma into the crust pro vok ing its melt ing. The

Fig. 4. The po si tion of the Kuršiai and Askersund suites on ex per i men tal di a grams of magma com po si tion, gen er ated by the in ter ac tion be tween ba salt melt and melt from var i ous supracrustals (PatiÔo Douce, 1999)

HP – at high pres sure (>10 kbar); LP – at low pres sure (5–6 kbar); PSGS – Peraluminous S-type Gran ite Suite; R – magma com po si tion that would re sult from melt-restite mix ing in a pelitic sys tem with ad di tion of ba saltic com po nents; ex pla na tions and ref er ences as in Fig ure 2

com po si tion of the charnockitic rocks was de ter mined by the in ter ac tion of ba saltic mag mas with anatectic melt of Al-rich metasedimentary rocks, in volv ing mech a nisms of as sim i la tion and hybridization.

Magmatism took place in late oro gen esis, tran si tional from syn-ki ne matic collisional, to post-ki ne matic extensional phases.

The close geo chem i cal af fin ity be tween the Kuršiai suite in West ern Lith u a nia and the co eval Askersund plutonic suite in south east ern Swe den sug gests a sim i lar tec tonic en vi ron ment and pro cesses of the con ti nen tal crust for ma tion, and a wider cor re la tion be tween these re gions.

Ac knowl edg ments. This work was sup ported by the Lith - u a nian Sci ence and Stud ies Foun da tion, the Swed ish in sti tute’s VISBY Programme and the Foun da tion of Vilnius Uni ver sity.

The au thors wish to ac knowl edge all of these in sti tu tions for their con tri bu tion. The au thors are par tic u larly thank ful to Dr.

U. B. Andersson for anal y sis of the Askersund suite, con struc - tive com ments and dis cus sion, Dr. S. Bogdanova and Dr.

E. Krzemiñska for valu able re marks and cor rec tions, and Dr.

S. Keen for im prov ing the lan guage of the manu script.

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