Freundt-Malecha, Birgit (1997) The crystallized portions of magmatic systems exposed by mafic through felsic plutonic fragments in ignimbrites on Gran Canaria: Keys to felsic magma formation and development of a compositional gap . (PhD/ Doctoral thesis), Christian-Albrechts-Universität, Kiel, 208 pp.

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Abstract

The Miocene phase (14-9 Ma) of volcanic activity on Gran Canaria produced the largest known volume (>800 km3) of felsic igneous rocks on an ocean island and developed a pronounced compositional gap across rocks of intermediale compositions. Felsic-rock compositions changed from rhyolitic-trachytic (Lower Mogan Formation, 14-13.9 Ma) through peralkaline rhyolitic­trachytic (Middle and Upper Mogan Formations, 13.9-13.3 Ma) to undersaturaled trachytic­trachyphonolitic (Fataga Group, 13-9 Ma). Plutonic fragments erupted with lhree ignimbrites, PI (13.95 Ma), P2 (13.89 Ma), and FBl-4 (11.89 Ma), are analyzed to derive petrogenetic processes leading to production of voluminous felsic magmas and to development of a compositional gap in the volcanic deposits.
Fine-to coarse-grained syenite fragmenls from undersaturated trachytic, anorthoclase and biotite bearing Fataga Group ignimbrite FB 1-4 are composed of exsolved alkali feldspar forrning an interlocking framework, amphibole, and interstitial quencbed aegirine-augite. Preservation ofup to 30% miarolitic cavities suggesls that tbe syenites resided at shallow crustal levels. Bulk chemical compositions of syenites and ignimbrite overlap but fine-grained syenites are more evolved (highest contents of incompatible trace elemenls) and represent crystallized bulk magmacompositions wbile coarse-grained syenites are partially fractionated cumulates, having incompatible trace element concentrations diluted by 10-50% feldspar accumulation. The Fataga FBI-4 magma reservoir evolved as a closed system until penetrating hydro thermal fluids increased the alkali-content of some syenites. ...

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