Petrological relationships between plutonic and volcanic rocks in a continental volcanic arc system: Evidence from plutonic xenoliths in the Sierra de Pachuca, Mexico

Abstract

Relatively abundant plutonic xenoliths found in some lava domes, lava flows, and scoria cone deposits were employed to study the features and processes between the magmatic plumbing systems and volcanic rocks in the Sierra de Pachuca, central-eastern Mexico. We present petrographic, geochemical, isotopic data and five U-Pb ages for some plutonic xenoliths and the Sierra de Pachuca host rocks to provide evidence of magmatic processes existing under this volcanic range. The phaneritic plutonic xenoliths were grouped into six lithological types based on petrography, chemical data, and isotopic ages. Four types (A to D) are hosted by the El Ventoso lava dome. Their compositions span from calc-alkaline gabbro to granite, with U-Pb zircon ages ranging from 23.1 ± 1.4 to 21.6 ± 0.6 Ma. These ages are contemporaneous with the emplacement of rhyolitic ignimbrites found at the base of the Sierra de Pachuca (22.8 ± 0.5 Ma U-Pb zircon age). The trace element patterns for these Late Oligocene-Early Miocene plutonic xenoliths display enrichment of LILE relative to HFSE and enrichment of LREE respecting to HREE typical of subduction-related rocks. Negative P, Ti, and Eu anomalies in these patterns also suggest fractional crystallization processes in magmas that produced the A to D types. Two more types of plutonic xenoliths (E and F) display gabbro and syenite compositions, respectively, and are hosted by the San Miguel-Santa María Regla columnar jointed basalts (gabbro samples) and by the El Prieto scoria cone (syenite samples). Based on one U-Pb zircon age (2.4 ± 0.2 Ma) and geological correlations, these plutonic xenoliths and their host rocks were emplaced in the Quaternary. They exhibit alkaline compositions, and their trace element patterns indicate an enriched mantle source with positive Ba and Pb anomalies, not typical of subduction-related magmas. The isotopic values (⁸⁷Sr/⁸⁶Sr = 0.70390 to 0.70502 and εNd = −2.99 to +0.76) for the Late Oligocene-Early Miocene A to D types and host rocks suggest the interaction of magmas with continental crust components. In contrast, the Quaternary xenoliths of E and F types and their host rocks display isotopic data (⁸⁷Sr/⁸⁶Sr = 0.70396 to 0.70602 and εNd = +0.13 to +3.66) that evidence of less interaction with continental crust components. In all studied xenoliths, the Pb isotopic ratios (²⁰⁶Pb/²⁰⁴Pb = 18.60–18.87; ²⁰⁷Pb/²⁰⁴Pb = 15.57–15.63 and ²⁰⁸Pb/²⁰⁴Pb = 38.39–38.68) lie on a mixing line between a mantle-like member and a radiogenic component such as the Paleozoic Acatlán complex that probably underlie the Sierra de Pachuca area. Based on the results obtained, it is proposed that the Late Oligocene-Early Miocene A to D type xenoliths represent the plumbing systems from which the volcanic rocks of the Sierra de Pachuca were extruded. On the other hand, the E and F types also represent the roots or magmatic reservoirs from which the Quaternary lava flows and scoria cones, located in the rear-arc part of the active front of the Trans-Mexican Volcanic Belt, were formed. Additionally, these two types of xenoliths and their Quaternary host rocks have hybrid geochemical characteristics of alkaline magmas influenced by crustal components produced during a slab rollback stage.