The Upper Neoproterozoic-Lower Cambrian of the Central Iberian Zone, Spain: chemical and isotopic (Sm-Nd) evidence that the sedimentary succession records an inverted stratigraphy of its source |
| |
| Authors: | JM Ugidos MI Valladares RM Ellam |
| |
| Institution: | 1 Departamento de Geología, Facultad de Ciencias, 37008 Salamanca, Spain
2 Scottish Universities Environmental Research Centre, East Kilbride G75 OQF, UK |
| |
| Abstract: | The Upper Neoproterozoic-Lower Cambrian sedimentary succession in the Central Iberian Zone has recently been divided into 12 sedimentary units (I to XII). Units I to IV are of Late Neoproterozoic age, and units V to XII are Early Cambrian.Throughout the stratigraphic section, shales with similar ranges of SiO2 and Al2O3 have TiO2 and Zr contents that are coupled and vary gradually from the bottom (unit I, TiO2 = 1.0-1.1%, Zr = 245-287 ppm) to the top (unit XII, TiO2 = 0.74-0.86%, Zr = 141-192 ppm). These two parameters clearly distinguish Upper Neoproterozoic from Lower Cambrian compositions, as do certain elemental ratios (Al2O3/TiO2, Rb/Zr, Nb/Ti), which vary from low values at the bottom of the section to high at the top. However, other elements (the rare earth elements REEs], Y) do not fit this general trend, with heavy REEs and Y showing extreme concentrations in some Lower Cambrian units, which also have negative Ce anomalies.Units I to III and units XI to XII have uniform TDM neodymium model ages (∼1.1 and 1.7 Ga, respectively), but the units between these two groups have erratic values between 1.4 Ga (unit VII) and 3.4 Ga (unit VIII). Thus, the bottom and top units have neodymium isotope ratios that indicate the presence of two source compositions: a composition that records a juvenile contribution younger than 1.1 Ga and a composition mainly derived from an old basement. Some samples from Lower Cambrian units have disturbed REE abundances and neodymium isotope ratios that are attributed to extensive chemical alteration that resulted in REE and Y redistribution. This alteration probably affected the material now preserved as unit IV during a period of relative sea level fall at the end of the Late Neoproterozoic. Both elemental and isotopic results support the suggestion that the Upper Neoproterozoic-Lower Cambrian stratigraphic section records an inverse stratigraphy of a source hinterland composed of a cover sequence of relatively juvenile crustal materials underlain by an older basement. Through time, the deeper crust gradually increased its contribution to the sediments in the basin from Late Neoproterozoic to Lower Cambrian times. These results are compatible with data from many other European zones. |
| |
| Keywords: | |
| 本文献已被 ScienceDirect 等数据库收录！ |
|
