Composition and age of Cenozoic volcanism in Libya

[摘要] Libya has five major Cenozoic volcanic provinces (Garian, Jabal Al Haruj al Aswad, Jabal Al Hasawinah, Jabal as Sawda and Jabal Nuqay) that have a surface area of approximately 66,000 km2. These volcanic provinces are dominated by alkali to mildly alkali basalts. The provinces are aligned NNW-SSE, typically occurring where NE-SW trending structural features intersect the main regional uplift structures. Small volumes of phonolites are associated with the basaltic volcanism at Garian and Jabal Al Hasawinah.Despite their size and relative accessibility the Cenozoic volcanic provinces of Libya have been rarely studied.In the first part of this thesis I report a new study of the petrology, geochemistry (major and trace elements, REE, Sr-Nd isotopes) and geochronology (40Ar/39Ar) of basalts and phonolites from the Garian volcanic province in north Libya. These analyses indicate that the plateau and late basalts are not distinct basalt types produced from melting of different mantle regions at different times as proposed by earlier studies. They are the product of fractional crystallisation of a common parent. There is little indication of crustal contamination. Trace element and REE data support an origin in 2 to 12 % melts of heterogeneous sub-lithosphere mantle. Trace elements and Nd and Sr isotopic composition of the Garian basalts overlap values measured in metasomatised peridotite xenoliths in the GVP. They are compositionally similar to Cenozoic volcanism of northern Libya (e.g. Jabal Al Haruj) and southern Italy (e.g. Etna and Pantelleria; European asthenosphere mantle reservoir), and they lack the influence of enriched mantle present in other North African Cenozoic basalt provinces. Compositional variation in the Garian province phonolitic magmas is dominated by extensive fractional crystallisation (50-83%) of plagioclase and alkali feldspars combined with an unusual style of assimilation (2-45%) of old upper crust typical of Pan-African shield. The new high precision age determinations of the phonolites suggest that they were produced over a very short period at ~8.1 Ma. This contrasts strongly with previous age determinations that were 40-50 Ma.In the second half of the thesis I report new Ar/Ar chronology measurements for extrusive and intrusive rocks all the major Cenozoic volcanic fields in Libya. The major pulse of basaltic volcanism in Garian in the north (from 6 to 2 Ma) overlaps in time in Jabal Al Haruj province in southern Libya. Jabal Al Hasawinah and Jabal as Sawda basalts were erupted significantly earlier (23-10 Ma). Dykes and plugs at all provinces imply basaltic volcanism started in Miocene, followed by periods of erosion. There is no systematic trend of time in Cenozoic basaltic volcanism of Libya. It appears to be related to reactivation of ancient structures during the passive rifting that has been produced in response to interaction of African and European plates since the late Mesozoic.

[发布机构] University:University of Glasgow;Department:School of Geographical and Earth Sciences

[关键词] QE Geology