Geochemistry of the subducting Cocos Plate and the origin of subduction-unrelated mafic volcanism at the volcanic front of the central Mexican volcanic belt.
Verma, Surendra P (2000) Geochemistry of the subducting Cocos Plate and the origin of subduction-unrelated mafic volcanism at the volcanic front of the central Mexican volcanic belt. Special Paper - Geological Society of America. 334; Pages 195-222. 2000.
| Authors | Verma, Surendra P |
|---|---|
| Year | 2000 |
| Title | Geochemistry of the subducting Cocos Plate and the origin of subduction-unrelated mafic volcanism at the volcanic front of the central Mexican volcanic belt. |
| Source | Special Paper - Geological Society of America. 334; Pages 195-222. 2000. |
| Publisher | Geological Society of America GSA. Boulder, CO, United States. 2000. |
| Document type | Serial; Conference-Document |
| ISBN | 1-81372-334-5 |
| ISSN | 0072-1077 |
| GeoRef | 2001-040169 |
| Keywords | alkaline-earth-metals; basalts-; Cenozoic-; Cocos-Plate; Deep-Sea-Drilling-Project; DSDP-Site-487; DSDP-Site-488; East-Pacific; enrichment-; experimental-studies; Federal-District-Mexico; geochemical-anomalies; geochemistry-; igneous-rocks; IPOD-; isotope-ratios; isotopes-; lead-; Leg-66; lithogeochemistry-; mafic-composition; major-elements; marine-sediments; metals-; Mexican-volcanic-belt; Mexico-; Mexico-state; mid-ocean-ridge-basalts; Nd-144-Nd-143; neodymium-; North-Pacific; Northeast-Pacific; Pacific-Ocean; partial-melting; Pb-206-Pb-204; Pb-207-Pb-204; Pb-208-Pb-204; plate-tectonics; Quaternary-; radioactive-isotopes; rare-earths; rifting-; sediments-; Sierra-de-Chichinautzin; spectra-; Sr-87-Sr-86; stable-isotopes; strontium-; subduction-; trace-elements; volcanic-rocks; volcanism-; X-ray-fluorescence-spectra |
Abstract
I report new geochemical and radiogenic isotope data on near-trench basalts and sediments from the Cocos plate and hypersthene hy-normative Pleistocene to Holocene mafic volcanic rocks from Sierra de Chichinautzin, located south of Mexico City, at the volcanic front of the central part of the Mexican Volcanic Belt. The altered basalts from the Cocos plate are mostly hy-normative and have light rare earth element REE depleted patterns, with a small negative Ce anomaly. Their Sr, Nd, and Pb isotopic ratio ranges are: super 87 Sr/ super 86 Sr 0.70317-0.70340, super 143 Nd/ super 144 Nd 0.51319-0.51326, super 206 Pb/ super 204 Pb 18.05-18.40, super 207 Pb/ super 204 Pb 15.45-15.55, and super 208 Pb/ super 204 Pb 37.45-38.04. All sediments are light REE enriched with negative Eu anomalies. Some of them also show large negative Ce anomalies, characteristic of seawater. Their isotope ratios are super 87 Sr/ super 86 Sr 0.70799-0.71034, super 143 Nd/ super 144 Nd 0.51244-0.51256, super 206 Pb/ super 204 Pb 18.59-18.94, super 207 Pb/ super 204 Pb 15.58-15.67, and super 208 Pb/ super 204 Pb 38.31-38.88. In multielement mid-oceanic ridge basalt MORB-normalized plots, both basalts and sediments show a significant negative Nb anomaly, as compared to K and La. The basalts and basaltic trachyandesites from the Sierra de Chichinautzin are hy-normative, high Mg light REE-enriched, and relatively high-Nb rocks. Their major and trace element characteristics are similar to the hy-normative mafic rocks from continental rift and oceanic island basalt OIB tectonic settings. Their isotope ratios show the following ranges: super 87 Sr/ super 86 Sr 0.70348-0.70433, super 143 Nd/ super 144 Nd 0.51275-0.51293, super 206 Pb/ super 204 Pb 18.65-18.76, super 207 Pb/ super 204 Pb 15.57-15.61, and super 208 Pb/ super 204 Pb 38.34-38.53. On the basis of combined geochemical and isotopic evidence, it appears that the Sierra de Chichinautzin mafic rocks cannot be generated by any direct slab melting or indirect fluid transport to the mantle participation of the subducted Cocos plate. Instead, they were probably generated in a rifting environment from partial melting of the underlying mantle and their eruption from monogenetic volcanoes was facilitated by crustal weakening caused by tensional stress in this area. The results have important bearing on the fate of the subducted slab below the central Mexican Volcanic Belt, perhaps implying that for some yet-unknown reason, it does not contribute to the generation of these mafic rocks from the mantle.