THE EASTERN ALKALINE PROVINCE (MEXICO) REVISED: GEOLOGY, GEOCHRONOLOGY AND GEOCHEMISTRY OF NEOGENE VOLCANISM IN VERACRUZ STATE

The “Eastern Alkaline Province” (EAP) was defined by Robin (1981) as a roughly N-S belt of mafic volcanism paralleling the Gulf of Mexico from the U.S. Border to the southern Veracruz state. In this author’s model the EAP would consist of an intraplate-type volcanism resulting from Gulf-parallel extensional faulting that would have migrated from north to south from Oligoceno to Present. This simple view was questioned by Nelson et al. (1995), who found evidence of a subduction signature in the lavas of the Los Tuxtlas volcanic field. To clarify the tectonic setting of this peculiar volcanism we have undertook a geologic, geochronologic and geochemical study of the southern half of the EAP. Our new fieldwork, trace element and isotope geochemistry, 48 unspiked and 18 conventional K-Ar ages allow recognizing 5 volcanic fields from north to south: 1) Basanites to hawaiites that flowed for about 30 km from the Sierra Madre Oriental (SMOr) border (Tlanchinol-Huautla area) to the coastal plain with ages between 7.4 and 5.7 Ma. 2) Alkaline basalts of the Alamo volcanic field (about 50 volcanic necks) and the NE trending linear volcanic complex of Sierra de Tantima with ages between 7.5 and 6.6 Ma. 3) Alkaline basalts and hawaiites of the Poza Rica area that flowed from the SMOr to the coast for about 80 km, with ages of 1.6 to 1.3 Ma. 4) Alkaline basalts and hawaiites forming a plateau several hundreds m thick in the Chiconquiaco-Palma Sola area with ages ranging from 7.0 to 2.0 Ma. 5) The Los Tuxtlas volcanic field with a first mafic alkaline volcanism between 7.9 and 3.4 Ma and a younger, subduction related activity from late Pliocene to the Present. Our data define a contemporaneous but chemically heterogeneous Late Miocene to early Pliocene episode of volcanism along the entire southern half of the EAP. Rocks from areas 1 and 2 show an OIB affinity (Nb=33-90 ppm, Ba/Nb=6.7-8.1, Zr/Nb=3.7-6.5, and La/Yb=14.6-28.5) that can be modeled as low grade partial melting (~3%) of an enriched lherzolite with garnet and amphibol as residual phases. Areas 4 and 5 are characterized by both OIB and subduction related volcanism indicative of different grade of melting of a heterogeneous mantle. As expected area 3 shows a subduction signature (Nb=16-22 ppm, Ba/Nb=10.7-23.3, Zr/Nb=10.5-16.1, and La/Yb=6.5), since the vents of these lavas are located well inside the Trans-Mexican Vocanic Belt.