Geochemical constraints on the depositional environment of Upper Devonian radiolarian cherts from Loei, north-eastern Thailand

摘要

Late Devonian radiolarian chert sequences in the Indochina block of north-eastern Thailand are exposed in a narrow belt located to the east of Loei province. The analyzed radiolarian cherts were collected from Chiang Klom, Sumnoi and Pha Samyod localities. They are characterized by high silica content (90 wt.%), high aluminum content and low iron content (Al2O3/(Al2O3 + Fe2O3): geomeans 0.91, 0.88 and 0.92). The Al/(Al + Fe + Mn) values are high in both cherts and interbedded shales (geomean 0.89). High Si/(Si + Al + Fe) ratios are observed in cherts (geomean 0.97) and slightly low in shales (geomean 0.69), whereas Fe2O3/TiO2 values are low (geomean 5.91). For rare earth element (REE) analysis, the cherts exhibit low La abundances (geomean 4.31, 3.59 and 4.22), slightly negative Ce anomalies (Ce/Ce*: geomean 0.81, 0.76 and 0.93), intermediate ratios of North American Shale Composite (NASC) normalized Lan/Cen (geomean 1.33, 1.37 and 1.12) and intermediate Lan/Ybn values (0.79, 0.94 and 1.22). In the interbedded shales, REE characteristics are more or less equal to the cherts. The results indicate that these late Devonian radiolarian cherts were deposited in a continental margin environment. High Si/(Si + Al + Fe) values indicate a biogenic origin of the cherts, however, additional silica content in the cherts could be the result of diagenetic alteration. Intermediate positive Eu anomalies (Eu/Eu*: geomean 1.32, 1.25 and 1.44) are interpreted as the result of detrital feldspar contribution corresponding to the distinctive low content of Fe. Geological evidences from the field support volcanic activities during the Late Devonian-Early Carboniferous. Weathered materials and fragments of basalts and andesites would possibly be supplied to the basin during chert sedimentation. These geochemical constraints indicate that, in western portion of Indochina, deep marine basin was closed before Late Devonian which was followed by the incursion of Early Carboniferous orogeny.