First partial melting inversion model for a rift-related origin of the Sierra de Chichinautzin volcanic field, central Mexican volcanic belt

Abstract

We present the geochemical and isotopic characteristics of Pleistocene to Holocene olivine-bearing mafic rocks from the Sierra de Chichinautzin (SCN) volcanic field, located in the central part of the Mexican Volcanic Belt (MVB). Some have geochemical characteristics of primary magmas, and their MORB-normalized patterns are similar to those displayed by extension-related mafic rocks. The SCN primary magmas show a limited range in Sr-87/Sr-86 (0.70348-0.70397

average((n = 7)) = 0.70370 +/- 0.00019), and Nd-143/Nd-144 (0.51279-0.51294

average((n = 6)) = 0.51288 +/- 0.00006). New mineral and whole-rock chemical data allow estimation of eruption temperatures and assure the near-primary nature of the studied samples. Equilibrium temperatures for the primary magmas were in the range 1070-1150degreesC. Trace-element concentration data for near-primary magmas from the SCN are used to develop a partial melting inversion model, the first for any area of the MVB. The source composition calculated by this inversion method shows an enrichment in highly incompatible elements (e.g., [C-La/C-Yh](n) similar to2.2), where [](n) represents mantle-normalized values), without a decoupling between LILE, REE, and HFSE. This relationship supports a rift-related origin for SCN volcanism. The calculated REE composition of the mantle source is comparable to that observed in mantle xenoliths of central Mexico. Concentration ratios of incompatible elements have also been used to infer 7-16% as the approximate range in degree of partial melting of the mantle. These results reinforce the hypothesis that SCN mafic magmatism reflects partial melting of an enriched lithospheric mantle in an extensional tectonic setting, precluding participation of the subducted Cocos plate in the genesis of these magmas.