Geofísica Internacional (Jan 2010)
Rare earth element geochemistry of the Late Quaternary tephra and volcano-clastic sediments from the Pachuca sub-basin, north-eastern Basin of Mexico
Abstract
Chondrite normalized REE patterns, fractionations of light (La/Sm), heavy (Gd/Yb) and total (La/Yb) REE and Eu anomalies of the Late Quaternary multi-layer tephra fall deposits and overlying volcano-clastic sediments deposited in the Pachuca sub-basin (north-eastern Basin of Mexico) are studied to characterize their geochemical signatures. The REE geochemistry is compared with mineralogical abundances and multi-element concentra- tions to identify the possible source terrains contributing sediments into the basin. The basalt to basaltic-andesite tephra deposits have chemical compositions comparable to the rocks from Apan-Tezontepec volcanic field and consist of plagioclase, pyroxene and olivine. They are characterized by lower total REE abundances, lower light and heavy REE fractionations and a positive Eu anomaly. The overlying sediments show significantly different REE characteristics and can be divided into two different groups (i.e. I and II). The geochemical composition and REE patterns of group I (15-17 m, 4.6-9.6 m and 2-4.6 m depths) are comparable to the tephra deposits and suggest a mafic provenance. They are characterized by the absence of the Eu anomaly and lower total REE concentrations compared to group II. In group II (9.6-15 m and 0-2 m depths), higher total REE, a negative Eu anomaly, abundant K-feldspar and enriched Zr, Y and Th indicate felsic source terrains containing heavy minerals such as allanite, monazite and zircon.
