Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Surendra P. Verma ¹, Mahendra P. Verma ¹

Affiliations
1 Departamento de Geotermia, Division de Fuentes de Energia Instituto de Investigaciones, Electricas Apdo. Postal 475, Cuernavaca, Mor. 62008, MX

Abstract

A compilation is given of the available Sr and Nd isotope data on Mexican volcanic-plutonic terranes which cover about one-third of Mexico's territory. The available data are arranged according to a subdivision of the Mexican territory in terms of geological provinces. Furthermore, site and province averages and standard deviations are calculated and their petrogenetic implications are pointed out.

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K. Pandarinath ¹, K. R. Subrahmanya ², M. G. Yadava ³, Surendra P. Verma ¹

Affiliations
1 Centro de Investigacion en Energia, UNAM, Priv. Xochicalco s/no., Col Centro, Apartado Postal 34, Temixco, Mor. 62580, MX
2 0cean Science and Technology Cell, Mangalore University, Mangalagangotri 574199, IN
3 Earth Science Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, IN

Abstract

Organic matter in sediment layers of two sediment cores from the continental slope off southwest coast of India was dated by radiocarbon method and sedimentation rates were estimated. The age data show lateral and temporal variations in sediment accumulation rates. Sedimentation rates decrease gradually from the inner shelf to the deep-sea in the study area, with an exception at 332 m water depth. The sediment accumulation rates on the shallower continental slope (332 m water depth; 0.003-0.013 mm/yr) are lower than on the deeper slope (776 m water depth; 0.071-0.190 mm/ yr) during the entire period of core deposition (32.8 ka BP to present). The unusually lower sedimentation rates on the shallower slope region may be the result of prevailing adverse depositional conditions. The sedimentation rates were lower during 32.80-18.98 ka BP and 16.04-8.69 ka BP and higher during 18.98- 16.04 ka BP and 8.69 ka BP to present day. The temporal variations in sedimentation rates reflect the changes in detrital input to the sea as a result of variations in intensity of paleomonsoonal conditions.

Keywords

Sedimentation record, Paleomonsoon, Continental slope, Southwest coast.

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Surendra P. Verma ¹

Affiliations
1 Centra de Investigacron en Energia, UNAM, Pnv Xochicalco S/No Apartado Postal 34, Temixco, Mor 62580, MX

Abstract

New geochemical and Sr and Nd isotope data are presented on six selected rock samples of hy-normative late Quaternary basalt, basaltic trachyandesite, basaltic andesite, andesite, and dacite from Tizayuca Volcanic Field (TVF), located in the State of Hidalgo, Mexico, at the central part of the Mexican Volcanic Belt (MVB). This province (MVB) is a modern volcanic zone (Miocene to Recent), which in spite of ongoing subduction, presents widespread magmas whose origin is shown to be unrelated to subduction. The initial isotopic ratios of the sampled rocks, practically identical to their measured ratios, range as follows ⁸⁷Sr/⁸⁶Sr 0 70390-0 70540 and ¹⁴³Nd/¹⁴⁴Nd 0 512598-0 512907. There is a genera] increase in ⁸⁷Sr/⁸⁶Sr and a decrease m ¹⁴³Nd/¹⁴⁴Nd with increasing Si0₂ and decreasing MgO contents. The combined geochemical and isotopic evidence supports the generation of these magmas from the mantle or even in the lower crust, and subsequent modification through crustal assimilation processes. In spite of ongoing subduction of the Cocos plate, the TVF seems to owe its origin to rift-related processes as is the case of many other areas of the central and eastern MVB. A broader implication of the isotopic data from the MVB is that the basic assumption of geochronology of the same initial isotopic ratios seems to fall apart because none of the so far studied areas of the MVB shows a uniform Sr or Nd isotopic ratio Indeed, the ⁸⁷Sr/⁸⁶Sr and ¹⁴³Nd/¹⁴⁴Nd ratios vary according to the rock-type for rocks of practically zero age.

Keywords

Geochemistry, Isotopes, Subduction, Rift, Quaternary, Central American Volcanic Arc, Cocos Plate, Mexican Volcanic Belt, Mexico.

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