Resumo

MERIGGI, Lorenzo et al. Heterogeneous magmas of the Quaternary Sierra Chichinautzin volcanic field (central Mexico): the role of an amphibole-bearing mantle and magmatic evolution processes. Rev. mex. cienc. geol [online]. 2008, vol.25, n.2, pp.197-216. ISSN 2007-2902.

The Quaternary Sierra Chichinautzin volcanic field (SCVF) is located at the volcanic front of the Trans-Mexican Volcanic Belt (TMVB), 350 km from the Middle American trench where the Cocos plate subducts beneath the North American plate. The SCVF is characterized by more than 200 monogenetic centers, ranging in composition from rare basalts to dacites. Less evolved terms have aphyric to porphyritic textures with phenocrysts of euhedral olivine (± Cr-spinels inclusions) + cpx, -whereas evolved terms have porphyritic textures with phenocrysts of pyroxene (cpx>opx), xenocrysts of corroded quartz, and sieve-textured plagioclase from the local basement. Regardless of the degree of magmatic evolution, plagioclase occurs as microphenocrysts or in the groundmass. New geochemical and isotopic data presented in this work are in good agreement with previous analyses; however, a new division for the SCVF mafic rocks (MgO >6.0 wt.%) is proposed. In fact, although concentrations of large ion lithophile elements (LILE) and light to medium rare earth elements (REE) are roughly constant in mafic rocks, TiO₂ and other high field strength elements (HFSE) such as Nb are scattered and vary from 0.8 to 1.8 wt.% and from 5 to 30ppm, respectively. These characteristics allow grouping the SCVF rocks in two main magmatic series with different TiO₂-HFSE enrichment: high(H)-TiO₂ and low(L)-TiO₂, with subordinate transitional samples. Additionally, a cinder cone with shoshonitic affinity and extremely high LILE/HFSE ratio is reported for the first time in central Mexico. Compositional variability is also observed among the mineral phases of these mafic rocks (e.g., the Cr# of spinel hosted in olivine phenocrysts), and the ⁸⁷Sr/⁸⁶Sr ratios that progressively increase from the H-TiO₂series (0.70307-0.70425) to the L-TiO₂ series (0.70365-0.70434), up to the shoshonitic scoria (0.70456). The calc-alkaline affinity, LILE, and Pb positive anomalies of the magmas confirm the occurrence of a subduction-related metasomatised mantle wedge beneath the studied area (particularly for the shoshonitic scoria), but enrichments in TiO₂ and other HFSE are controversial. These enrichments are not common in subduction-related magmas because these elements are not easily removed by aqueous fluids from the subducted slab. However, recent works have demonstrated that HFSE can be relatively soluble in high-temperature fluids/melts arising from the slab, and the presence of TiO₂-rich Cr-pargasite in Iherzolite xenoliths of the Valle de Bravo area prove the existence of a mantle phase with high-HFSE concentrations. Therefore, partial melting of such hydrated peridotite could explain the genesis of H-TiO₂ magmas as suggested by the proposed REE model. Moreover, isotopic ratios and variations in major and trace element concentrations of-western SCVF rocks indicate that fractional crystallization plus crustal assimilation are the main evolution processes in the studied area.

Palavras-chave : volcanic rocks; high-TiO₂; Sr and Nd isotopes; magmatic evolution; Sierra Chichinautzin; Trans-Mexican Volcanic Belt; Mexico.

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