Geocronometría del magmatismo cretácico-terciario de la Sierra Madre del Sur

http://dx.doi.org/10.18268/BSGM1983v44n1a1

Jerjes Pantoja Alor*

* Consultor técnico de la Comisión Federal de Electricidad México. D. F .

 Abstract

Thirty six isotopic ages were analyzed, to which 15 were obtained by the Pb/Alfa method and 21 by the K/Ar method. The isotopic measures date from 100 to 26 m.y. (Early Cenomanian-Late Oligocene) even though the magmatic activity was initiated during the Late Jurassic. According to their mode of occurrence, 24 of the dated rocks belong to plutonic intrusives and volcanic rocks, 3 of them to metamorphic rocks and 2 to minerals of pegmatite. A plutonic event is well defined, which began at the end of the Albiane (100 m.y.) terminating in the Conacian (88 m.y.). After this plutonic event, there was a hiatus or period of time larger than 20 m.y. of none or few magrnatic activity. The magmatism was reinitiated in the Early Paleocene (66 m.y.) with an intrusive event that possibly continued through the Late Oligocene, joined by a comagmatic volcanism, of acid type in its final stage. According to chemical analysis, the cretaceous plutonism has a calcalkaline character with Ca and Mg deficiencies, as well as a light K enrichment. The tertiary plutonism and vulcanism has a calc-alkaline character with high deficiencies in Ca and Mg and a strong K enrichment, especially in their final volcanic stages. The few isotopic determinations of Sr⁸⁷/Sr⁸⁶ suggest the lack of precambrian earth crust below the actual mesozoic and possibly paleozoic cover.