Unusual coexistence of subduction-related and intraplate-type magmatism: Sr, Nd and Pb isotope and trace element data from the magmatism of the San-Pedro-Ceboruco graben (Nayarit, Mexico)

摘要

Subduction-related and continental intraplate type magmatism coexisted in the San Pedro-Ceboruco (SPC) graben in western Mexico in a rather unusual close association. The magmatic systems belong to the Trans-Mexican Volcanic Belt (TMVB), the active volcanic arc linked to subduction of the Rivera and Cocos plates beneath the North American plate. Three different magmatic series are recognized in the San Pedro-Ceboruco graben: calc-alkaline, Na-alkaline, and transitional. The transitional series can be further subdivided into: Low-Ti; High-Ti and Amado Nervo groups. The different magmatic series are characterized by variable LILE/HFSE and LREE/HFSE ratios. The Na-alkaline series has the most radiogenic Nd compositions and the lowest Sr-87/Sr-86 (0.70320-0.70344). The calc-alkaline series shows the lowest epsilon(Nd) and the highest Sr-87/Sr-86 (0.70395-0.70402) overlapping the values for the transitional High-Ti group (0.70390-0.70404). Sr isotopic compositions for transitional Low-Ti (0.70367-0.70394) and transitional Amado Nervo (0.70351-0.70389) groups are intermediate between those of the Na-alkaline and calc-alkaline rocks. All the studied rocks show similar Pb-207/Pb-204 (15.58-15.61) and Pb-208/Pb-204 (38.41-38.65), but Pb-206/Pb-204 discriminates well the Na-alkaline series (18.90-19.03) from all the others (18.68-18.75). Compositional and isotopic data suggest that the different series derive from distinct parental magmas, which were generated by partial melting of a heterogeneous mantle source characterized by two different components. A depleted mantle (DM) component with low Pb-206/Pb-204 and an enriched (EM) component characterized by high Pb-206/Pb-204. These two components mix together in various degrees and are both overprinted, although in different proportions, by metasomatic processes. Metasomatism of the mantle wedge was induced by subduction-related agents resulting from the dehydration/melting of subducted sediments and characterized by variable fluid/melt proportions. (C) 2003 Elsevier Science B.V. All rights reserved. [References: 52]