Abstract
12 samples of basic intrusives within Triassic rocks «ophites») and 11 samples of volcanic and associated intrusives within Jurassic to Early Cretaceous sequences of the Subbetic Zone were subjected to whole-rock K/Ar dating in combination with chemical/petrological analysis. Satisfactory results were obtained only from a number of samples of volcanic rocks, however, analytical ages commonly agree, within about 10 relative percent, with those deduced from stratigraphic location. «Ophite» samples, on the other hand, may reveal considerably lower analytic ages than the volcanics and show much stronger scattering, even among samples collected within a small area. It is argued that the inferred loss of Ar results from very-low-grade alpine metamorphic alteration, which affected the «ophites» more intensely than the higher volcanic rocks. Other post-emplacement chemical changes, such as the degree of secondary oxidation of Fe, are also distintive among the two groups of samples, and are to some extent consistent with the above view in that the alteration environment of the ophites should have produced conditions for more penetrative fluid-rock interactions and homogeneous recrystallization. Overall, the magmatic activity from which the ophitic rocks originated might have started in the Late Triassic and continued in the Lower Jurassic. 80th, the «ophites» and the volcanics are though to be the result of magmatic events Collowing tensional to transtensive crustal movements affecting the external basins of the Betic Cordilleras Crom Late Triassic to Early Cretaceous times.
Highlights
The external zones of the Betic Cordilleras represent various sOOimentary realros of rocks that accumulatOO in basins adjacent to the Iberian subplate from Triassic to Miocene times
On tbe present paper we report tbe results of attempts oC K/Ar dating oC an additional set oC samples oC volcanic and intrusive rocks Crom tbe Subbetic series oC tbe western and central segments oC tbe Subbetic Belt, and discuss tbeir significance as regards true ages oC emplacement and possible later dJangcs, in oonnectioo with sorne aspeds of the evolution oC the Subbetic realm and of tbe External Zone oC tbe Betic Cordillera in general
61.6 over a mucb wider area witbin tbe Subbetic Zone, as bere reported, witb mucb greater discrepancies between tbe analytical ages given by tbe two groups of basic rocks, as well as the description of unmistakebly metamorpbosed «opbites» in tbe Archidona region (Puga et al, 1983), suggest that the occurrence of a postemplacement very-low-grade metamorphism should instead be taken into consideration
Summary
Tanto las «ofitas» como las volcanitas se consideran el resultado de eventos magmáticos ligados a movimientos de distensivos a transtensivos que afectaron a las cuencas externas de las Cordilleras Béticas desde el Trías terminal hasta el Cretáceo inferior. 12 samples of basic intrusives within Triassic rocks «
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