Petrološki i geohemijski efekti procesa parcijalnog stapanja metasomatizma u subkontinentalnom omotaču ispod južne Libije

Abstract

U disertaciji su prikazani rezultati proučavanja kenozojskih bazalta i u njima pronaĎenih ksenolita iz oblasti Jabal Eghei (juţna Libija). UtvrĎeno je da su bazalti obrazovani u dvema vulkanskim epizodama; za vreme prve epizode (16-4 Ma) nastali su tranzicioni bazalti (TB), a za vreme druge (4–≤1 Ma) formirani su alkalni bazalti/bazaniti (AB/B). Primarne magme i TB i AB/B nastale su stapanjem sličnog metasomatizovanog granatskog peridotita uz najvaţniju razliku u stepenu stapanja (TB: 3–5 %, AB/B: ≤1%). TB pokazuju korelaciju izmeĎu K/Ar starosti i nekih geohemijskih parametara (npr. La/Ce i K/Nb), što ukazuje da se za vreme prve epizode odvijala postepena promena petrogenetsko-geodinamičkih uslova. Ksenoliti omotača su pronaĎeni u mlaĎim AB/B lavama, dominantno su predstavljeni fertilnim lerzolitom i dele se u dve strukturne grupe: 1) nedeformisani (protogranularni) i 2) deformisani (porfiroklastični ±ekvigranularni) ksenoliti. Nedeformisani ksenoliti su postigli ravnoteţu na višim temperaturama (>1000 oC) i osiromašeniji su bazaltnom komponentom (do 10-13 %) od deformisanih (<1000 oC, odnosno 2-7 %), pri čemu obe strukturne grupe sadrţe tragove tzv. ‘skrivenog‘ metasomatizma, dok neki deformisani ksenoliti pokazuju tragove i tzv. ‘nevidiljvog‘ metasomatizma. Na osnovu sinteze podataka o bazaltima i ksenolitima predloţen je geohemijsko-geodinamički model evolucije gornjeg omotača ispod područja Jabal Eghei u juţnoj Libiji, koji obuhvata četiri perioda: 1) predvulkanski period (>20 Ma), 2) vreme metasomatizma (20-16 Ma), 3) period obrazovanja TB (~16 – ~4 Ma), i 4) period zahvatanja ksenolita i erupcije AB/B (~4 – ~1 Ma).

The Thesis reports and discusses the results of investigations of late Cenozoic basalts and basaltborne mantle xenoliths of the Jabal Eghei area (south Libya). The basalts formed during two volcanic episodes, first at 16-4 Ma and second at 4–≤1 Ma, during which transitional basalts (TB) and alkali basalt/basanite (AB/B) originated, respectively. Both TB and AB/B primary melts formed via partial melting of a similar metasomatized garnet-bearing peridotite mantle, with the only difference in the amount of melting, ranging 3–5 %, and ≤1% for TB and AB/B, respectively. TB shows a correlation between the K/Ar ages and some geochemical parameters (e.g., La/Ce and K/Nb), which indicates that the first volcanic episode was associated with gradual changes in petrogenetic-geodynamic conditions. Mantle xenoliths (found in younger AB/B lavas, only) are represented by fertile lherzolite and are distinguished into: 1) non-deformed (protogranular) and 2) deformed (porphyroclastic ±equigranular) xenoliths. The non-deformed xenoliths equilibrated at higher temperature (>1000 oC) and are more depleted (10-13 %) than the deformed ones (<1000 oC and 2-7 %, respectively). Both groups of xenoliths display evidence of cryptic metasomatism, whereas some non-deformed samples have preserved traces of so-called ‘stealth‘ metasomatism. The synthesis of all the available data on the basalts and the xenoliths allowed us to propose a simplified model of geochemical-geodynamic evolution of the lithospheric mantle underneath southern Libya; the model involves four different steps: 1) the pre-volcanic period (>20 Ma), 2) the time of metasomatism (20–16 Ma), 3) the formation of TB (16–4 Ma), and 4) the capture of xenoliths and the euption of AB/B (4–1 Ma).