Current state of geochemical research in monogenetic field of the Sierra de Chichinautzin; information analysis and perspectives

Velasco, T.F.; Verma, S.P.

Revista Mexicana de Ciencias Geologicas 18(1): 1-36

2001


ISSN/ISBN: 1026-8774
Accession: 018664981

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Abstract
In the present paper are reported the results of an exhaustive literature survey and a critical analysis of information related to geological, geochronological and geochemical studies in the Sierra de Chichinautzin volcanic field (SCN), located in the central part of the Mexican Volcanic Belt (MVB). The SCN is composed by 221 monogenetic-type volcanoes that can be classified as cinder cones with associated lava flows, shield volcanoes and lava domes. Geochronological (super 14) C data suggest ages younger than 40,000 years for volcanic activity in the SCN. The average magma output rate has been estimated as approximately 11.75 km (super 3) /1,000 years, being greater than that for the Michoacan-Guanajuato monogenetic field. The volcanic cones in the SCN are preferentially aligned along an east-west trend, which has been related with a north-south extensional environment. In general, SCN rocks show a porphyritic texture with <26% of phenocrysts and a fine-grained groundmass, constituted by plagioclase together with orthopyroxene, clinopyroxene, and titanomagnetite. Ol+ or -Plg and Opx+ or -Ol+ or -Cpx+ or -Plg have been reported as mineralogical assemblages, although in some lavas were identified abundant mineralogical disequilibrium textures (e.g., olivine or pyroxene with reaction rims, plagioclase with oscillatory zoning) and hydrated minerals (biotite or amphibole), suggesting magma mixing phenomena. In the surveyed literature, the geochemical information was used to classify the magmas and, in some cases, develop quantitative models. However, in many cases the applied analytical methodology or geochemical data handling was inadequate. According to TAS diagram, the mafic rocks are classified as basalts, trachybasalts and basaltic trachyandesites, with normative hy or ne. Evolved magmatism is represented by basaltic trachyandesites and basaltic andesites with high-MgO (SiO (sub 2) = 53-55%, MgO = 8.8-10.1%), and by basaltic andesites, andesites and dacites. Disequilibrium magmas cover a wide range of whole-rock compositions (SiO (sub 2) = 55.5-67.0%) and are classified as basaltic trachyandesite, andesite and dacite. In MORB-normalized multielement plots, mafic magmas are characterized by LILE enrichments, but do not show significant negative anomalies for HFSE. Primitive mantle-normalized REE plots for these magmas are characterized by enrichments in light REE, a nearly horizontal pattern for heavy REE and an absence of Eu and Ce anomalies. Evolved magmas showed multielement patterns with enriched LILE and negative anomalies for HFSE, whereas the REE plots display enrichment in light REE and, in some cases, a small negative Eu anomaly. REE contents in some evolved magmas were lower than in other magmas with a minor SiO (sub 2) concentration, which precludes simple fractional crystallization as a viable process for magma evolution. In discrimination diagrams, mafic magmas plotted in the rift or oceanic island field. These magmas are also characterized by (super 87) Sr/ (super 86) Sr and (super 143) Nd/ (super 144) Nd isotopic ratios that plot on the "mantle array". In comparison, the evolved magmas showed slightly lower isotopic ratios in Nd and significantly higher in Sr. Quantitative modeling was used to decide between the different hypotheses proposed to explain the origin of the SCN mafic magmatism: subduction and rifting. However, quantitative models reported in favor of subduction are strongly dependent on assumed chemical and mineralogical source composition and not supported by geochemical and geological data. Sr, Nd and Pb isotopic ratio mixing models and trace-element geochemistry, suggest that the mafic magmas in the SCN were generated by partial melting of a heterogeneous peridotitic source. The eruption from monogenetic volcanoes was facilitated by crustal weakening, caused by tensional stress in this area. The origin of the most evolved magmas was related to partial melting of the lower crust. Intermediate magmas may reflect magma mixing processes between the most evolved magmas and the mantle-derived mafic magmas. As a complement of this review on the SCN, a preliminary partial melting model (inversion methods) in equilibrium conditions was developed for mafic magmas. The results of this exercise suggest that the mafic magmas were formed by approximately 2-10% partial melting of a peridotitic mantle.