Geochemical and Sr-Nd-Pb isotopic evidence for a combined assimilation andfractional crystallisation process for volcanic rocks from the Huichapan caldera, Hidalgo, Mexico

This study reports new geochemical and Sr-Nd-Pb isotopic data for Miocene t o Quaternary basaltic to andesitic, dacitic, and rhyolitic Volcanic rocks f rom the Huichapan caldera, located in the central part of the Mexican Volca nic Belt (MVB). The initial Sr and Nd isotopic ratios, except for one rhyol ite, range as follows: Sr-87/Sr-86 0.70357-0.70498 and Nd-143/Nd-144 0.5126 5-0.51282. The Sr-Nd-Pb isotopic ratios are generally similar to those for volcanic rocks from other areas of the central and eastern parts of the MVB . The isotopic ratios of one older pre-caldera rhyolite (HP30) from the Hui chapan area, particularly its high Sr-87/Sr-86, are significantly different from rhyolitic rocks from this and other areas of the MVB, but are isotopi cally similar to some felsic rocks from the neighbouring geological provinc e of Sierra Madre Occidental (SMO), implying an origin as a partial melt of the underlying crust. The evolved andesitic to rhyolitic magmas could have originated from a basaltic magma through a combined assimilation and fract ional crystallisation (AFC) process. Different compositions, representing l ower crust (LC) and upper crust (UC) as well as a hypothetical crust simila r to the source of high Sr-87/Sr-86 rhyolite HP30, were tested as plausible assimilants for the AFC process. The results show that the UC represented by granitic rocks from a nearby Los Humeros area or by Cretaceous limestone (L) rocks outcropping in the northern part of the study area, and the LC r epresented by granulitic xenoliths from a nearby San Luis Potosi (SLP) area are not possible assimilants for Huichapan magmas, whereas a hypothetical crust (HA) similar in isotopic compositions to rhyolite HP30 could be consi dered a possible assimilant for the AFC process. Chemical composition of as similant HA, although not well constrained at present, was inferred under t he assumption that HP30 type partial melts could be generated from its part ial melting. These data were then used to evaluate the proposed AFC process to explain the evolution of the Huichapan magmas. (C) 2001 Elsevier Scienc e B.V. All rights reserved.