Schirnick, Carsten (1996) Formation of an intracaldera cone sheet dike swarm (Tejeda Caldera, Gran Canaria). (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, IX, 204, 133 pp.

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Abstract

A new model of cone sheet dike formation is presented and constrained based on the results concluded from an integrated study of the structure, petrology, stratigraphy, and Ar-isotopic compositions of a cone sheet dike swarm and its environment, the fill of the Miocene Tejeda caldera on Gran Canaria, Spain. The main results of this study are as follows: The Tejeda caldera (-20 km diameter) formed synchronous with eruption of ignimbrite Pl at 14.1 Ma and was subsequently filled with minor epiclastic sediments and with predominating widespread, up to 100 m thick cooling units of extremely welded, dense ignimbrites and lava flows (comenditic trachytes, comendites and pantellerites) of the Montafia Horno and the overlying Montafia Tirma Formation (comenditic trachytes, peralkaline trachytes, phonolites). Laser-40 A1ß9 Ar single feldspar grain dating indicates eruption of 5 of the oldest exposed intracaldera ignimbrites between 13.61 ±0.07 and 12.97 ±0.24 Ma and concurrent intrusion of 2 dikes into the caldera fill at 13.45 ±0.12 and 12.86 ±0.08 Ma. Eruption of these intracaldera ignimbrites and intrusion of the dikes was contemporary with eruptions of extracaldera ignimbrites of the Upper Mogan Formation, but prior to intrusion of the cone sheet dike swarm. The absence of major faults indicates that the caldera fill behaved as a rigid plate when intrusion of cone sheet magmas started at the base of the caldera fill and caused central uplift, respectively doming or bending over the central area. Bending of a plate, respectively the caldera fill, causes a differential stress field, which is tensional in the upper half and directed radial and tangential, whereas the forces in the lower half of the plate are compressive in radial and tangential direction. Failure of the plate, respectively the caldera filling rocks, results in formation of a conical crack, which begins at the surface predominantly as a tensional crack and propagates downwards with increasing shear component to the circumference of the source of central uplift, respectively the intrusion at the base of the caldera fill. Intrusion of the magma into the fracture resulted in the formation of a cone sheet dike. Detailed structural analysis of about 2000 cone sheet dikes reveal an average dip of 43° (<1000 m a.s.l.) decreasing to 37° (> 1000 m a.s.l.), which compares weil with experimentally observed cracks developing in rigid concrete plates punched from the bottom. Radial cracks occur as a consequence of the tensional forces in the upper half of the plate, and may result in radial dikes when the cracks propagate downward and intersect the intrusion. The model reveals that about 246 km3 of trachytic and 77 km 3 of younger, peralkaline trachytic and late phonolitic magma intruded as cone sheet dikes into the caldera fill. Radiometrie ages of feldspars indicate intrusion of trachytic cone sheet dikes before 9.65 ±0.03 Ma, whereas intrusion of peralkaline trachytic and phonolitic cone sheet dikes occurred during a period that lasted at least from >9.65 ±0.03 Ma until 7.40 ±0.22 Ma. Intrusion of cone sheet dikes was contemporary with extrusive activity of the Fataga Group. Cone sheet dike rocks reflect chiefly two distinct compositional patterns: (1) a first order temporal evolution during a roughly 4.8 Ma long period from trachytic to peralkaline trachytic and phonolitic cone sheet magmas and (2) a second order variation reflected in up to 3- to 6-fold enrichment of incompatible elements (i.e. Zr, Nb, LREE, Th, Y, Rb) in the most fractionated magmas. This second order variation is due to variable degrees of fractionation, chiefly of alkali feldspar and minor biotite and Fe-Ti oxides, during residence in a small (<l km 3 ), sill-like magma chamber at the base of the caldera fill, which causes the uplift of the fill and eventually the generation of the cone fracture. Fractionation proceeded until the cone fracture intersected the reservoir and magma was extracted into the new cone sheet dike. Replenishment of magma from a major magma reservoir located at greater depth, resulted in resurgent uplift brought about by the small magma chamber and formation of a new cone sheet dike. Comparison of the number of cone sheet dikes with time it took to form the cone sheet dike swarm indicates a cycle (intrusion, uplift, fractionation, cone shaped fracture and formation of a new cone sheet dike) of approximately 5000 years.

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