PLEISTOCENE PILLOW LAVA FLOWS AND THEIR RELATION TO THE CLOSURE OF THE AMAJAC LAKE IN CENTRAL HIDALGO, MEXICO

The eastern part of the Trans-Mexican Volcanic Belt, TMVB, occupies the south central portion of the state of Hidalgo, Mexico. In this area, crops out the Atotonilco El Grande Formation, that consists of a clastic and volcanic sequence deposited in the Amajac paleolake during the Late Pliocene-Pleistocene. Recent findings indicate that the top of this sequence consists of fissure basaltic flows that, at present, constitute topographic plateaus of large extent and columnar structure. The presence of degassing tubes and abundant pillow lavas to the base of these flows implies that the flows were deposited in shallow water.

Diameter of the pillow structures ranges from 5 to 40 cm, with radial exfoliation and abundant vesicles of 2 to 4 cm in diameter. In general, the smaller pillow structures are embedded in muddy lacustrine sediments and the bigger are associated with altered basaltic tuff. Pillow structures of 4 to 5 m in diameter and concentric exfoliation are also present, and probably were formed due to sudden vaporization of water in the lake.

The microtexture of these rocks is microlithic-porfidic with hyalo-ophitic matrix of euhedral to subhedral zoned microphenocrystals of labradortite, included in a mesostasis of plagioclase microliths with intersticial volcanic glass and illmenite inclusions. Disseminated in the mesostasis occur subhedral phenocrystals of augite and iron oxides.

The lava flows are constituted by microlithic porfidic augite basalt and, most likely, are associated with the fate of the Amajac Lake during the Pleistocene. The origin of the flows is attributed to the younger phase of fissure volcanic activity, at present, recognized in TMVB in Hidalgo.