LIETUVOS UGNINIS LANKAS
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1 33 Gediminas Motuza, Vilniaus universitetas LIETUVOS UGNINIS LANKAS Prologas Lietuvos kristalinio pamato kernà pirmà kartà pamaèiau prieð daugelá metø, kai tuometinës Lietuvos geologijos valdybos vyriausias geologas Vytautas Vonsavièius man, kà tik pradëjusiam dirbti Kompleksinës geologinës ekspedicijos giluminio geologinio kartografavimo bûryje, rodë bûsimø darbø plotus. Tuo metu ten vyko parametrinis græþimas. Pagal ðá projektà ávairiose Pietø Lietuvos vietose buvo numatyta iðgræþti 6 græþinius, ásigræþiant á kristaliná pamatà po 200 m. Ðie nauji duomenys turëjo padëti pasiruoðti tolesniam ilgalaikiam kristalinio pamato tyrimø etapui giluminiam geologiniam kartografavimui. Ðis projektas jau ëjo á pabaigà, buvo græþiamas prieðpaskutinis græþinys Jaskonys-269 ir liko tik græþ. Raigardas-302. Man ir teko uþbaigti parametriná græþimà ir pradëti giluminá geologiná kartografavimà, kuris truko beveik 20 metø. Kaip tik prie Jaskoniø græþinio netoli Ratnyèios mes su V.Vonsavièiumi ir nuvykome. Geologë Zubelytë kaip tik tvarkë eilinio reiso kernà, kuris atrodë kaip chaotiðkas margumynas iðskydusiø tamsiø dëmiø ir rausvø gysluèiø raizgalynë. Gerai prisimenu pirmà mintá kaip toje koðëje susigaudyti, kà èia galima suprasti ir apraðyti? Vëliau teko perþiûrëti ne vienà kilometrà kerno, ir tos senosios uolienos pasidarë aiðkesnës ir simpatiðkesnës. Bet ir tada, sëdëdamas ant eilinës dëþës, ne kartà pagalvodavau ar kada nors ateis toks laikas, kai þinosime, kaip ir kada susidarë tos margos uolienos ir visas mûsø kristalinis pamatas, kurá jos sudaro? Tuomet mes beveik nieko neþinojome apie litosferos plokðèiø tektonikos teorijà, mums buvo dar neprieinama ðiuolaikinë geocheminë analizë, kurios duomenys leidþia nustatyti, kokiomis tektoninëmis sàlygomis susidarë uolienos, ir radiologinis datavimas. O svarbiausia, nebuvo praktinës patirties, kuri padëtø bent ásivaizduoti, kaip ið tikrøjø keliø ðimtø metrø gylyje atrodo tas paslaptingas pavirðius, kurio tegalime pamatyti atskirus gabaliukus, iðkeltus ið græþiniø, nutolusiø vienas nuo kito per kilometrus. Praëjo daug metø, kol atsirado faktø, ið kuriø tarsi ið plyteliø galima sudëlioti pastatà-modelá, ir ðioks toks supratimas apie tai, kaip tai daroma. O kiek toks statinys pastovës, parodys laikas. Kiekviena hipotezë turi teisæ gyvuoti tol, kol ji paaiðkina esamus faktus, kol atsiranda naujø duomenø arba kol kas nors tuos paèius faktus-plyteles sudëlioja kitaip. Dvi Lietuvos dalys Jau pirmieji Lietuvos kristalinio pamato tyrëjai (Viktoras Vasiljevas, Rimantas Gailius ir kiti) pastebëjo, kad rytinë ir vakarinë Lietuvos kristalinës plutos dalys skiriasi savo sandara, uolienø rinkiniu kristalinës plutos pavirðiuje ir jø metamorfizmo laipsniu. Tai gerai atspindi gravitacinis ir magnetinis laukai. Todël ir buvo iðskirtos dvi sritys (domeinai) Vakarø Lietuvos sritis (VLS) ir Rytø Lietuvos sritis (RLS). Jos, tiesa, neapsiriboja vien Lietuva, tæsiasi ir kaimyniniuose kraðtuose, todël jas tiksliau bûtø vadinti atitinkamai Lenkijos-Lietuvos ir Lietuvos-Baltarusijos sritimis, arba tereinais (Bogdanova ir kt., 2006). Anksèiau, kol nebuvo radiologinio datavimo duomenø, buvo manoma, kad abi sritys yra skirtingo amþiaus: VLS archëjaus, RLS gal ir proterozojaus. Tiesa, tai buvo grindþiama tik tuo, kad VLS uolienos labiau metamorfizuotos, todël galbût ir senesnës. Vëliau, gavus pirmuosius datavimo duomenis U-Pb ir Sm-Nd metodais (Mansfeld, 2001) paaiðkëjo, kad abiejø srièiø kontinentinë pluta susidarë paleoproterozojuje, svekofeniðkosios kalnodaros metu kaip ir kaimyniø kraðtø Ðvedijos, Pietinës Suomijos, Rytø Lenkijos, Baltarusijos, Latvijos ir Estijos pluta. Archëjaus plutos visame ðiame plote nerasta. Tik metamorfizuotose pirminëse nuosëdinëse uolienose randama pavieniø archëjaus laikotarpio cirkono grûdeliø. Bet tai detritiniai grûdeliai kaþkada buvusios smiltelës, susidariusios ardant kaþkoká þemynà su archëjaus pluta, ir tai nerodo smëlio amþiaus. Juk ir dabartiniame kvartero smëlyje gali bûti ávairaus amþiaus smilteliø. Jie tik patvirtina, kad tai tikrai pirminës nuosëdinës uolienos m. atlikus giluminá seisminá zondavimà pagal projektus EUROBRIDGE ir Baltijskoje more paaiðkëjo, kad abiejø srièiø plutos ið esmës skiriasi ir gilumine sandara. VLS pluta
2 34 plona (apie km), palyginti vienalytë ir lengva, ypaè virðutinë, kur vyrauja uolienos, pagal sudëtá artimos granitui. RLS pluta gerokai storesnë (50-55 km), labai nevienalytës sandaros. Joje iðsiskiria keli skirtingo tankio ir atitinkamai sudëties sluoksniai, suskaidyti blokais, kuriuos skiria vertikalûs lûþiai (Motuza ir kt., 2002; Motuza, 2004). Paaiðkëjo, kad abiejø srièiø giluminë riba gana staigi plutos storis ir sandara pasikeièia 3040 km ruoþe. Ði riba yra kaip tik Lietuvos viduryje ir tæsiasi ðiaurës-pietø kryptimi. Tai reiðkia, kad 1 pav. Lietuvos kristalinio pamato geologinis þemëlapis (G.Motuza, 2004): 1 Grt-Qtz-Kfs-PI gneisai ±Sil, Crd, Mag, Hz pirminiai pelitai (a) ir psamitai (b); 2 Bt-Qtz-PI gneisai ±Kfs, Sil, Grt pirminiai psamitai; 3 amfibolitai pirminiai bazaltai ir diabazai; 4 baziniai granulitai (Hbl-CpxHyp-PI) pirminiai bazaltai ir diabazai; 5 Cpx-Hyp-BtQtz-PI gneisai ±Kfs, Hbl pirminiai felziniai vulkanitai; 6 smulkûs baziniø granulitø kûnai; 7 granulitinës facijos su metapelitø paleosoma; 8 granulitinës facijos su metapelitø ir metabazitø paleosoma; 9 granulitinës facijos su baziniø granulitø ir metapsamitø paleosoma; 10 granulitinës facijos su baziniø granulitø paleosoma; 11 amfibolitinës facijos su amfibolito ir metapsamito paleosoma; 12 amfibolitinës facijos su amfibolito paleosoma; 13 amfibolitinës facijos su amfibolito paleosoma ir tonalito GEOLOGIJOS PAÞANGA abiejø srièiø skiriasi ne tik uolienos, esanèios kristalinës plutos pavirðiuje, bet ir kad tai yra skirtingos giluminës sandaros blokai, kuriø ir amþius, ir geologinë istorija gali bûti skirtingi. Todël ði ribinë juosta buvo pavadinta Vidurio Lietuvos sandûros zona (VLSZ), suprantant jà kaip geologinæ ribà, skirianèià du skirtingos sandaros plutos blokus. Geriau iðtyrus græþiniø ið ðios zonos kernà paaiðkëjo, kad VLSZ sudaro ir uolienos, besiskirianèios tiek nuo Vakarø Lietuvos, tiek nuo Rytø Lietuvos srities (1 pav.). VLSZ rytinæ dalá daugiausia sudaro amfibo- Fig. 1. Geological map of the crystalline basement of Lithuania (G.Motuza, 2004): 1 Grt-Qtz-Kfs-Pl gneisses ±Sil, Crd, Mag, Hz primary pelites (a) and psammites (b); 2 Bt-Qtz-Pl gneisses ±Kfs, Sil, Grt primary psammites; 3 Amphibolite primary basalt and diabase; 4 Mafic granulites (Hbl-CpxHyp-Pl) primary basalt and diabase; 5 Cpx-Hyp-Bt-Qtz-Pl gneisses ±Kfs, Hbl primary felsic volcanics; 6 Minor bodies of mafic granulites; 7 Granulite facies with metapelitic palaeosome; 8 Granulite facies with metapelitc and metabasitic palaeosome; 9 Granulite facies with palaeosome of mafic granulite and metapsammite; 10 Granulite facies with palaeosome of mafic granulites; 11 Amphibolite facies with palaeosome of amphibolite and metapsammite; 12 Amphibolite facies with palaeosome of amphibolite; 13 Amphibolite facies with palaeosome of amphibolite and
3 neosoma; 14 anatektiniai granitoidai; 15 èarnokitoidai, granitoidai; 16 Cpx grandioritas; 17 gabroidai; 18 dioritoidai-grandioritai; 19 granitoidai; 20 granitas; 21 moncodioritas; 22 anortozitas; 23 gabras, anortozitas; 24 magnetitiniø uolienø (Fe rûdos) kûnai; 25 milonitø juostos; 26 lûþiai. litai pirminës vulkaninës uolienos bazaltas ar diabazas, tik aiðku metamorfizuotas. Tokias uolienas rodo ir geofiziniai duomenys. VLSZ rytinëje dalyje tæsiasi aukðtø gravitacinio lauko reikðmiø juosta. Uolienos gana tankios (apie 3000 kg/m 3 ), todël ir iðsiskiria didesniø svorio jëgos reikðmiø juosta. Tai patvirtina ir græþiniai. Tiesa, jø daugiau pietinëje dalyje, kur vadinamame Lazdijø plote buvo atliktas giluminis geologinis kartografavimas. Ðiauriau græþiniø maþiau ir jie daugiausia sutelkti atskiruose ploteliuose, nes buvo græþti dujø saugyklø paieðkoms Þieþmariø, Vepriø ir kituose plotuose. Amfibolito yra ir kai kuriose RLS vietose, bet ðioje juostoje amfibolito geocheminës ypatybës labiau bûdingos vadinamajai kalcio ðarminei serijai, be to, yra ir kitø poþymiø, kurie rodo, kad jos lydësi subdukcijos zonoje, vulkaniniø lankø tektoninëje aplinkoje. Ðiuose græþiniuose, be amfibolito, nustatyti nedideli gabro ir diorito kûnai, pirminiø smiltainiø sluoksniai, kuriuose iðliko net pirminës sandaros poþymiø (2 pav.). 35 neosome of tonalite; 14 Anatectic granites; 15 Charnockitic rocks and granites; 16 Cpx granodiorite; 17 Gabbro; 18 Dioritic-granodioritic rocks; 19 Granites; 20 Granite; 21 Monzodiorite; 22 Anorthosite; 23 Gabbro, anorthosite; 24 Bodies of magnetite rocks (Fe ores); 25 Milonite belts; 26 Faults. A B C 2 pav. Metamorfizuotas pirminis smiltainis ið græþ. Þieþmariai-5 (K.Èiûraitës nuotr.). Fig. 2. Metamorphosed primary sandstone from the well Þieþmariai-5 (Photo by K.Èiûraitë). Vakariniu Vidurio Lietuvos sandûros zonos pakraðèiu tæsiasi visai kitokios sudëties juosta. Beveik visuose èia iðgræþtuose græþiniuose (Akmenynai, Sasnava, Virbaliðkis-434, Kybartai-15; Pilviðkiai-140, 141, 142, Sutkai) surasta uolienø, pasiþyminèiø aiðkia porfyriðka struktûra (3 pav.). Jà sudaro keliø milimetrø dydþio taisyklingi plagioklazo kristalai smulkiagrûdëje pagrindinëje 3 pav. Metaporfyritas (hipersteno-amfibolo-biotito gneisas) ið græþ. Virbaliðkis-434 (A, B) ir græþ. Sasnava-7 (C), pietinë VLSZ dalis. Fig. 3. Metaporphyry (hyersthene-amphibole-biotite gneisse) from wells Virbaliðkis-343 (A, B) and Sanava-7 (C) in the Southern part of Middle Lithuanian Suture Zone. masëje. Ðie plagioklazo kristalai daþnai yra ryðkiai zoniðkos sandaros arba taisyklingos iðtæstos
4 36 formos su paprastais dvynukais. Dabartiniame metamorfizuotame pavidale tai amfibolo-biotito feldðpatiniai-kvarciniai gneisai, daþnai su hiperstenu. Uolienos gerokai pakitusios, kai kur net pradëjusios lydytis (migmatizuotis), bet iðlikusios pirminës sandaros ypatybës rodo, kad tai galëjo bûti vulkaninës uolienos, kurioms labai bûdinga porfyrinë sandara ir taisyklingi zoniðki plagioklazo fenokristai. Jø sudëtis atitinka bazaltiná andezità, andezità ir dacità vidutinës ir rûgðèios sudëties vulkanitus. Beje, tokiø uolienø yra ir rytinëje metabazaltø juostoje. Jø rasta Vepriø ir Vaðkø græþiniuose, kur taip pat iðliko pirminës vulkaninës struktûros liekanø (4 pav.). Matyt ðios uolienos sudaro tik atskirus sluoksnius tarp vyraujanèiø bazaltø. 4 pav. Mataporphyras (muskovito-biotito plagiogneisas) ið græþ. Vaðkai-1, ðiaurinë Vidurio Lietuvos sandûros zonos dalis. Fig. 4. Metaporphyry (moscovite-biotite plagiogneisse) from the well Vaðkai-1 in the Northern part of the Middle Lithuanian Suture Zone. Taigi Vidurio Lietuvos sandûros zonoje turime dvi skirtingos sudëties vulkaniniø uolienø juostas: rytuose tæsiasi pirminio bazalto ar diabazo su smiltainiø ir rûgðèiø vulkanitø sluoksniais juosta, vakariau andezito ir dacito sudëties vulkaniniø uolienø juosta. VLSZ vulkaniniø arba efuziniø uolienø rinkiná papildo ir intruzinës uolienos gabras, dioritas, granodioritas, kuriø kûnai iðskirti pagal geofizinius duomenis, o kai kurie patvirtinti ir græþimu. Èia yra nemaþø kûnø, pavyzdþiui, Randamoniø plutonas, kuris tæsiasi nuo Druskininkø beveik iki Puvoèiø kaimo. Pagal ðià intruzijà, kuri yra ne tik viena didþiausiø, bet ir neblogai iðtirta keliais græþiniais, visas panaðios sudëties intruzijø kompleksas pavadintas Randamoniø vardu. Dioritas ir granodioritas yra atitinkamai andezito ir dacito giluminiai analogai, taigi vulkanai ir intruzijos gali bûti giminingi ne tik pagal sudëtá, bet ir pagal susidarymo tektoninæ aplinkà, kuri veikia uolienø sudëties ypatybes. Vidurio Lietuva tektoniniø plokðèiø sandûroje GEOLOGIJOS PAÞANGA Tiek vulkaniniø, tiek giluminiø uolienø cheminë sudëtis, ypaè mikroelementø kiekiai rodo, kad tos uolienos greièiausiai susidarë lydantis plutai subdukcijos zonoje, t.y. ten, kur susiduria dvi litosferos plokðtës ir viena jø nyra po kita. Andezitas apskritai yra bûdinga ugnikalniø, iðaugusiø virð nyranèios plokðtës, uoliena. Galima sakyti, kad kitokioje tektoninëje padëtyje jis ir nesusidaro. Dabar andezito ir dacito lavos iðplitæ vadinamame Ugnies þiede, kuris juosia Ramøjá vandenynà ir apima Andus, Ðiaurës Amerikos Kordiljerus, Kamèiatkà, Japonijà, Aleutø, Kurilø ir kitus vulkaninius lankus. Beje, nuo Andø kalnø kilæs ir andezito pavadinimas. Visi tie ugnikalniø lankai susidaro virð nyranèiø litosferos plokðèiø. Ávairaus amþiaus ðios uolienos daþnos ir þemynuose, kuriø pluta susidarë susidûrus litosferos plokðtëms. Kartografuojant Ringvasojos þaliøjø skalûnø juostà Norvegijoje, pavyko iðskirti beveik 3 mlrd. metø senumo andezito-dacito storymæ, kuri rodo, kad litosferos plokðèiø tektonika veikë jau tada. Vidurio Lietuvos sandûros zonoje tektoninë aplinka buvo panaði èia taip pat vyko litosferos plokðèiø susidûrimas, viena niro po kita, lydësi, o magma, prasimuðusi á virðø, sudarë ugnikalniø grandines ir lavø bei piroklastø klodus. Taèiau kyla klausimas, kada visa tai vyko, kaip judëjo tos plokðtës ir kuri plokðtë po kuria niro? Duomenø apie VLSZ uolienø amþiø nëra daug, bet keli datavimai U-Pb metodu padaryti. Bendradarbiaujant su Rusijos MA Prekambro geologijos ir geochronologijos instituto mokslininkais J.Salnikova ir A.Kotovu buvo nustatytas dacitinio metaporfyro ið græþ. Virbaliðkis-434 amþius 1842±6 mln. metø. A. Rimða nustatë panaðø granodiorito ið Randamoniø plutono amþiø apie 1837 mln. metø (Rimsa ir kt., 2003). Lenkijoje nustatytas panaðus metavulkanitø ið græþinio Monki amþius (Wiszniewska ir kt., 2005). Ðis græþinys, atrodo, yra Vidurio Lietuvos sandûros zonos tæsinyje Lenkijoje. Ðie duomenys leidþia manyti, kad subdukcija ir su ja susijæs magmatizmas VLSZ vyko prieð 1,84 mlrd. metø. Tiesa, tokie procesai trunka bent deðimtis milijonø metø, todël gautos datos greièiausiai rodo ðio proceso pabaigà arba intensyvià fazæ. Kitas klausimas kuri plokðtë niro po kuria
5 ir kokia kryptimi vyko subdukcija? Jau minëjome, kad Vakarø Lietuvos srityje palyginti daug intruziniø uolienø. Tai èarnokitoidai ir granitoidai, sudarantys stambius kûnus. Didþiausio Kurðiø èarnokitoidø plutono plotas apie 15 tûkst. km 2. Jo amþius yra apibûdintas dviem datom: èarnokito ið græþ. Macuièiai-1 amþius, nustatytas U-Pb metodu, 1,846±12 mln. metø (G.Motuza, 2005), o èarnokito ið græþ. Vydmantai-1 1,815 mln. metø (Claesson, 2001). Datø skirtumas tokio dydþio kûnams nëra didelis. Stambûs batolitai susidaro pamaþu ásiskverbiant magmos porcijoms per kelias deðimtis milijonø metø. Mums svarbesnë senesnë data, rodanti magmatizmo pradþià, nes panaðaus amþiaus yra ir granitoidai ið græþ. Kuþiai-65 ir Grauþai-105 atitinkamai 1840±2 ir 1837±6 mln. metø (Motuza, 2004). Visos trys datos labai artimos ir rodo intensyvaus giluminio magmatizmo Vakarø Lietuvos srityje laikà, kuris paklaidos ribose yra toks pat, kaip ir VLSZ subdukcijos zonos andezito-dacito lavø bei Randamoniø komplekso intruzijø. Èarnokitai yra savitos uolienos, kuriø ypatybë kvarco ar/ir kalio feldðpato derinys su hiperstenu. Jos susidaro aukðtoje temperatûroje, bevandenëje aplinkoje ir dideliame gylyje. Tokios sàlygos galimos tik kontinentinëje plutoje, nes okeaninë yra pernelyg plona ir ðalta. Èarnokitoidø ir to paties amþiaus granitoidø mineraloginës ir geocheminës ypatybës rodo, kad magma, ið kurios jie kristalizavosi, iðsilydë ið metanuosëdiniø uolienø. Tai patvirtino jø bendras aliuminingumas ir daug kitø geocheminiø rodikliø, pagal kuriuos ðios uolienos priskiriamos vadinamajam S tipui (angl. sedimentary, reiðkianèio magmos ðaltiná). Èia daþnai randama granatø, pasitaiko kordierito. Ðie mineralai rodo aliuminio pertekliø uolienose. Be to, magminiuose cirkono grûdeliuose, pagal kuriuos ir buvo nustatytas uolienø amþius, surasti gerokai senesni branduoliai, kurie greièiausiai yra liekanos smilteliø, buvusiø nuosëdinëje uolienoje. Toks detritinis cirkonas daþnai randamas magminëse uolienose, nes jo lydimosi temperatûra labai aukðta. Taigi visos ðios plutoninës magminës uolienos susidarë kontinentinëje plutoje, o tai reiðkia, kad tokia pluta jau buvo Vakarø Lietuvoje, ir Vidurio Lietuvos sandûros zonoje vyko subdukcija. Litosfera, kurià sudaro kontinentinë pluta, yra lengva, todël negali subdukuoti. Reiðkia niro kita, Rytø Lietuvos plokðtë. Tai patvirtina ir dar keli faktai. Kaip minëta, metabazaltø juosta yra rytinëje VLSZ dalyje, o andezito ir dacito 37 sudëties metavulkanitø vakarinëje. Paprastai vulkaniniuose lankuose bazaltai sudaro juostas iðorinëje, o felziniai vulkanitai vidinëse lankø dalyse. Taigi vulkaninio lanko iðorë buvo dabartiniuose rytuose. Be to, susidûrusiø plokðèiø pakraðèiai deformuojasi sueiþëja lûþiais, kurie atskiria pleiðto pavidalo blokus, o ðie spaudþiami slenka aukðtyn. Tuo pat metu blokai yra ardomi, o jø uolienos iðsidësto juostomis lygiagreèiai sandûros zonai. Toks darinys vadinamas akrecine prizme. Rytø Lietuvoje yra ryðkus juostinis uolienø iðsidëstymas. Juostos, kuriose vyrauja amfibolitai, kaitaliojasi su juostomis, kuriose yra daugiau nuosëdiniø uolienø, o jø tása yra ðiaurësðiaurës rytø krypties, t.y. kaip tik lygiagreti VLSZ. Tai labai gerai atsispindi ir geofiziniuose þemëlapiuose, ypaè magnetinio lauko, ir primena akrecinës prizmës sandarà. Be to, subdukcija po kontinentine Vakarø Lietuvos pluta gali paaiðkinti ir èia nustatytà intensyvø plutoniná magmatizmà, vienalaiká su subdukcija. Virð nyranèios plokðtës mantijoje gali susidaryti vietinë konvekcija, kai karðta mantijos medþiaga kyla aukðtyn ir sukelia lydimàsi aukðtesnëse mantijos dalyse bei plutoje (Kearey, Vine, 1996). Tiesa, kol kas tai tik prielaida. Jà paremia tai, kad intruzijø susidarymas ir subdukcija vyko vienu metu. Bet gali bûti ir kitø Vakarø Lietuvos giluminio magmatizmo prieþasèiø. Tai gali bûti susijæ ir su pirminës VLS kontinentinës plutos susidarymu, apie kurá maþai þinoma. Stiprus metamorfizmas, lydymasis ir plataus masto giluminis magmatizmas apnaikino pirmines nuosëdines ir vulkanines uolienas, kurios geriausiai prisimena VLS susidarymo istorijà. Greièiausiai VLS susidarë litosferos plokðtëms slenkant ir akrituojant ðiaurës-ðiaurës rytø kryptimi, ta paèia kaip ir susidarant Ðvedijos bei Suomijos plutai (5 pav.). Tai rodo ir geofiziniø laukø anomalijos, kurios vakarinëje Lietuvos dalyje ir ypaè Baltijos jûroje tæsiasi á ðiaurës vakarus, kaip ir kaimyninëje Ðvedijoje, ir atspindi kristalinës plutos uolienø kompleksø padëtá. Ðiø geofiziniø lineamentø tæsinyje, jau Ðvedijos pusëje, kontinentinë pluta susidarë prieð 1,88-1,86 mlrd. metø (Sultan, Claesson ir kt., 2005). Greièiausiai ir VLS kontinentinë plutos amþius yra panaðus. Truputis fantazijos Apibendrinant turimus duomenis susidaro vaizdas, kad Lietuvos kristalinë pluta susidarë
6 38 GEOLOGIJOS PAÞANGA 5 pav. Svekofeniðkojo orogeno raida prieð 1,9 mlrd. metø. Brûkðniuotas plotas archëjaus kratonas; baltas plotas okeaninë pluta ir nuosëdinës uolienos; uþtaðkuotas plotas vulkaniniø lankø kompleksas; stora dantyta linija subdukcijos ruoþas; didelës rodyklës rodo plokðèiø judëjimo kryptá (Nironen, 1997). Fig. 5. A model of the development of Svecofennian orogen 1.9 Ga ago. Broken lines Archaean craton; the area without overprint oceanic crust and sedimentary rocks; the dotted area is the arc complex; thick black line the subduction zones; big arrows show the direction of plate motion (Nironen, 1997). dviejø litosferos plokðèiø, slenkanèiø skirtingomis kryptimis, sandûroje. Vakarø Lietuvos sritis susidarë slenkant ir akrituojant plokðtei ðiaurës rytø link, kaip ir kitø svekofenidø. Pagrindinio jos kratonizacijos etapo pabaigà þymi èarnokitoidøgranitoidø intruzijos, susidariusios prieð 1,846-1,815 mlrd. metø. Tuo tarpu Rytø Lietuvos srities pluta susidarë kitos plokðtës pakraðtyje, kuri judëjo dabartiniø ðiaurës vakarø link ir susidûrë su VLS plokðte, kai ði jau turëjo kontinentinæ plutà. Kolizijos metu Rytø Lietuvos plokðtë niro po Vakarø Lietuvos plokðte, lydësi pati ir sukëlë lydymàsi dengianèioje plokðtëje. Èia á plutà ásiskverbë gabro, diorito, granodiorito, tonalito intruzijos, o ten, kur lûþiais magma prasimuðë á pavirðiø, iðkilo ugnikalniø grandinë Vidurio Lietuvos vulkaninis lankas. Jis turëjo bûti panaðus á dabartinius Kamèiatkos, Japonijos ar Kurilø vulkaninius lankus (6 pav.). Tokioje tektoninëje aplinkoje ið ugnikalniø daugiausia liejasi andezito, dacito (reèiau bazalto) sudëties lava, kuri, bûdama tirðta, nuo iðsiliejimo vietos tenuteka 6 pav. Kurilø salos dabartinis vulkaniniø salø lankas. Geltona rodyklë Ramiojo vandenyno plokðtës judëjimo kryptis. Apaèioje deðinëje schematiðkas subdukcijos zonos vaizdas; kairëje Vidurio Lietuvos sandûros zonos padëtis, kuri, galbût susidarë panaðioje tektoninëje aplinkoje. Fig. 6. Kurily islands modern volcanic arc. Arrow direction of motion of the Pacific plate. Below right, sketch of subduction zone; left position of the Mid-Lithuanian suture zone presumably formed in similar tectonic setting. netoli. Jos klodai kaitaliojasi su tufais ir kitais piroklastais ir sudaro aukðtus taisyklingo kûgio pavidalo ugnikalnius stratovulkanus. Taigi ásivaizduokime per visà Lietuvà nutásusià virtinæ staèiaðlaièiø ugnikalniø, iðkëlusiø smailas rûkstanèias virðûnes á keliø kilometrø aukðtá. Ið dabartiniø rytø ðá ugnies lankà skalavo vandenynas, o vakarinëje pusëje greièiausiai buvo þemynas arba sekli uþulankio jûra, panaði á dabartinæ Ochotsko ar Japonø (6 pav.). Ir vëlesnëje mûsø kraðto geologinëje istorijoje Vidurio Lietuvos sandûros zona buvo reikðminga geologinë riba. Kaip tik ja vedama (gal kiek sàlyginë) Baltijos sedimentacinio baseino riba (Stirpeika, 1999), iðilgai jos iðsidësèiusios daugelio sistemø litofacijø juostos, èia baigiasi Vakarø Lietuvos geoterminë anomalija. Gal ir daugiau svarbiø geologinës sandaros bei raidos ypatybiø yra susijæ su ðia bene reikðmingiausia mûsø kraðto tektonine siûle?
7 39 Literatûra Bogdanova S., Gorbatchev R., Grad M., Guterch A., Janik T., Kozlovskaja E., Motuza G., Skridlaite G., Starostenko V., Taran L & EUROBRIDGE and POLONAISE Working Groups. EUROBRIDGE: New insight into the geodynamic evolution of the East European Craton // European Lithosphere Geodynamics / eds. D.G. Gee and R. A. Stephenson. Geological Society London Memoirs, Claesson S., Bogdanova S., Bibikova E., Gorbatschev R. Isotopic evidence for Palaeoproterozoic accreation in the basement of the East European Craton // Tectonophysics P Kearey P., Vine F.J. Global tectonics Mansfeld J. Age and end constraints on the Paleoproterozoic tectonic evolution in the Baltic-Sea region // Tectonophysics P Motuza G., Korabliova L., Nasedkin V. Giluminis seisminis zondavimas þvilgsnis á litosferos gelmes // Geologijos akiraèiai, P Motuza G. Litosferos sandara. Lietuvos Þemës gelmiø raida ir iðtekliai // Litosfera P Nironen M. The Svekofennian Orogen: a tectonic model // Precambrian Research, P Ostrovsky A.A., Flueh E.R., Luosto U. Deep seismic structure of the Earth s crust along the Baltic Sea profile // Tectonophysics, P Rimsa A., Bogdanova S., Skridlaite G., Bibikova E. The Randamonys TTG-intrusion in southern Lithuania: Evidence of an 1.84 Ga island arc // EUG XI abstract volume. Strasbourg, Stirpeika A. Tectonic evolution of the Baltic Syneclise and Local Structures in the South Baltic Region with Respect to ther Petroleum Potential Wiszniewska J., Krzeminska E., Skridlaitë G., Motuza G., Williams I., Whitehouse M. Metasedimentary and metavolcanic rocks from NE Poland and Lithuania: implications for Precambrian crustal evolution // 12th Meeting of the Petrology Group of the Mineralogical Society of Poland, Stary Folwark, October 13-16, 2005: Crystalline Rocks of the East-European Craton [Scientific Communications]. Kraków, P Summary Volcanic Arc of Lithuania Two main domains have been recognized in the crystalline basement of Lithuania West Lithuanian (WLD) and East Lithuanian (ELD), which differ essentially in the deep structure and lithological composition of the crust (Fig. 1). The contact zone distinguished as Mid-Lithuanian Suture Zone (MLSZ) is characterized by transitional structure of crust and specific assemblage of rocks. In the eastern part of MLSZ continues the belt of amphibolites primary metabasalts and diabases, interbedding with metapsammitic gneisses (Fig. 2). They reveal geochemical patterns characteristic of calc-alkaline series, and geochemical signatures of volcanic arc tectonic setting. In the western part of MLSZ the pyroxene-hornblende-biotitic gneisses are predominant. They are characterized by andesiticdacitic composition and remarkable porphyry texture (Fig. 3 and 4). Such rocks typically occur in the volcanic arc tectonic setting. Based on these data MLSZ is regarded as former subduction related volcanic arc, formed in course of the collision between two different lithospheric plates. Few U-Pb (zircon) dating suggest formation of the arc around 1.84 Ga. Roughly same ( ) age have large plutonic bodies of charnockitic and granitic rocks in WLD. The mineralogical (presence of garnet and cordierite) and geochemical patterns of these rocks corresponds same time to S- and A-type granite and suggest formation of intrusions in the thick continental crust, presumably on the late stages of orogenic period. These facts implies the following tectonic model. The WLD has been formed in course of plate motion and accretion to recent N-NE along main Svcofennian trend before 1.85 Ga (Fig. 5). East Lithuanian oceanic plate was moving towards recent NW and subducting beneath WLD, which at that time (around 1.84 Ga) was already cratonised. Chain of stratovolcanoes emerged along the edge of WLD above the subduction zone, comparable to recent volcanic arcs as Kamchatka-Kurily (Fig. 6). The Central Lithuanian joint zone was an important geological boundary in further geological history of our land as well. Along it, the Baltic sedimentation basin boundary (Stirpeika, 1999) (slightly relative) is drawn, and there are numerous belts of different lithofacies, here is also the margin of the West Lithuanian geothermal anomaly. Maybe, there are more important peculiarities in geological setup and development related to the most significant tectonic line of Lithuania?
Due to the especially interesting geological structure ~280 km 2 territory of Druskininkai area is covered by detail complex geological and
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