THE PETROGENESIS OF 30-20 MA BASIC AND INTERMEDIATE VOLCANICS FROM THE MOGOLLON-DATIL VOLCANIC FIELD, NEW-MEXICO, USA

In the western USA calcalkaline magmas were generated hundreds of kilometres from the nearest destructive plate margin, and in some areas during regional extension several Ma after the cessation of subduction. The Mogollon-Datil Volcanic Field (MDVF) in southern New Mexico was a centre of active magmatism in the mid- to late-Tertiary, and a detailed field, petrographic and geochemical study has been undertaken to evaluate the relations between extensional tectonics and calcalkaline magmatism in the period 30-20 Ma. The rocks comprise alkalic to high-K calcalkaline lavas, ranging from basalt to high silica andesite. Most of the basaltic rocks have relatively low HFSE abundances, elevated Sr-87/Sr-86 and low Nd-143/Nd-144, similar to many Tertiary basalts across the western USA, and they are inferred to have been derived from the continental mantle lithosphere. Two differentiation trends are recognised, with the older magmas having evolved to more calcalkaline compositions by magma mixing between alkalic basaltic andesites and silicic crustal melts, and the younger rocks having undergone 30-40% fractional crystallisation to more alkalic derivatives. The younger basalts also exhibit a shift to relatively higher HSFE abundances, with lower Sr-87/Sr-86 and higher Nd-143/Nd-144, and these have been modelled as mixtures between an average post-5 Ma Basin and Range basalt and the older MDVF lithosphere-derived basalts. It is argued that the presence of subduction-related geochemical signatures and the development of calcalkaline andesites in the 30-20 Ma lavas from the MDVF are not related to the magmatic effects of Tertiary subduction. Rather, basic magmas were generated by partial melting of the lithospheric mantle which had been modified during a previous subduction event. Since these basalts were generated at the time of maximum extension in the,upper crust it is inferred that magma generation was in response to lithospheric extension. The association of the 30-20 Ma calcalkaline andesites with the apparently 'anorogenic' tectonism of late mid-Tertiary extension, is the result of crustal contamination, in that fractionated, mildly alkaline, basaltic andesite magmas were mixed with silicic crustal melts, generating hybrid andesite lavas with calcalkaline affinities.