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12 samples of basic intrusives within Triassic rocks &#194;&#171;ophites&#194;&#187;) 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. &#194;&#171;Ophite&#194;&#187; 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 &#194;&#171;ophites&#194;&#187; 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 &#194;&#171;ophites&#194;&#187; 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.  Doce muestras de cuerpos b&#225;sicos intrusivos en rocas tri&#225;sicas (&#194;&#171;ofitas&#194;&#187;) y 11 muestras de volcanitas y rocas intrusivas asociadas en secuencias jur&#225;sico-cret&#225;ceas de la zona Subb&#233;tica han sido objeto de dataci&#243;n radiom&#233;trica K/Ar (roca total) en combinaci&#243;n con an&#225;lisis qu&#237;mico-petrogr&#225;fico. Las edades anal&#237;ticas obtenidas son 's&#243;lo satisfactorias en relaci&#243;n con un cierto n&#250;mero de muestras de rocas volc&#225;nicas, no obstante, con una concordancia respecto de las edades estratigr&#225;ficas en el orden del 10% relativo; las del resto, fundamentalmente muestras de &#194;&#171;ofitas&#194;&#187;, presentan una fuerte dispersi&#243;n, con diferencias que pueden alcanzar valores superiores a los 30 Ma, incluso en conjuntos provenientes del mismo afloramiento o localidad. Se concluye que las p&#233;rdidas de Ar causantes de los rejuvenecimientos de las edades anal&#237;ticas observados son debidos a la existencia de transformaciones metam&#243;rficas alpinas, de muy bajo grado, que afectaron con mayor intensidad a las ofitas que a las rocas volc&#225;nicas presentes en niveles estratigr&#225;ficos m&#225;s altos. Otros cambios posteriores al emplazamiento magm&#225;tico, tales como el grado de oxidaci&#243;n secundaria, son as&#237; mismo distintivos en ambos grupos de muestras al tiempo que proveen de soporte adicional al concepto de que el medio de alteraci&#243;n de las &#194;&#171;ofitas&#194;&#187; debi&#243; haber producido condiciones favorables a una interacci&#243;n roca/fluidos m&#225;s penetrativa y, por tanto, una recristalizaci&#243;n m&#225;s homog&#233;nea. En conjunto, la actividad magm&#225;tica de la que derivaron las &#194;&#171;ofitas&#194;&#187; podr&#237;a haber comenzado en el Trias terminal y continuado en el Jur&#225;sico Inferior. Tanto las &#194;&#171;ofitas&#194;&#187; como las volcanitas se consideran el resultado de eventos magm&#225;ticos ligados a movimientos de distensivos a transtensivos que afectaron a las cuencas externas de las Cordilleras B&#233;ticas desde el Tr&#237;as terminal hasta el Cret&#225;ceo inferior

estudios geológicos

Attempts of whole-rock K/Ar dating of mesozoic volcanic and hypabissal igneous rocks from the Central Subbetic (Southern Spain): A case of differential Argon loss related to very low-grade metamorphism