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.

Abstract

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 super 87 Sr/ super 86 Sr 0.70320-0.70344. The calc-alkaline series shows the lowest epsilon sub Nd and the highest super 87 Sr/ super 86 Sr 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 super 207 Pb/ super 204 Pb 15.58-15.61 and super 208 Pb/ super 204 Pb 38.41-38.65, but super 206 Pb/ super 204 Pb 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 super 206 Pb/ super 204 Pb and an enriched EM component characterized by high super 206 Pb/ super 204 Pb. 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.

19 samples from this reference