Provenance derived from the geochemistry of late Paleozoic–early Mesozoic mudrocks of the Pyeongan Supergroup, Korea

Abstract

Late Carboniferous–Early Triassic Pyeongan Supergroup, Korea, was deposited in paralic to nonmarine environments developed disconformably on a lower Paleozoic platformal sequence. Pyeongan Supergroup mudrocks show progressive upsection changes in major, trace, and rare earth element compositions. Mudrocks in the lower and middle parts of the Pyeongan Supergroup are strongly depleted in CaO and Na2O, while those in the upper part are enriched in these elements. In parallel with CaO and Na2O, the ratio of K2O/Al2O3 in the lower and middle sections is less than 0.3, while uppermost strata are enriched in K2O having K2O/Al2O3 ratios greater than 0.3. The index of compositional variability, a measure of compositional maturity, is less than 0.6 in the lower to middle section, whereas in the uppermost strata it is greater than 0.7. Similar trends are reflected in A–CN–K space. Compared to PAAS mudrocks of the Pyeongan Supergroup (except for those in uppermost units), they are depleted in Mn, Ca, and Na and are slightly depleted in Mg and P. Contrasting major oxide trends between the lower and middle parts of the Pyeongan and the uppermost units are due to decreasing proportions of clay minerals and a concomitant increase in the proportion of nonclay silicate minerals due to increasing input of first-cycle detritus. Excursions marked by increases in MgO, Na2O, and CaO reflect large-scale input of nonclay first-cycle minerals from crystalline source rocks due to large-scale basement uplift. Incompatible elements are generally enriched in mudrocks of the Pyeongan Supergroup relative to PAAS, while transition metals (Co, Ni, Cr, and V) are progressively depleted in younger parts of the section. Total rare earth element concentrations are higher than PAAS, but chondrite-normalized patterns display the typical LREE-enriched pattern of PAAS. Eu anomalies for most mudrocks are close to that of PAAS. The youngest mudrocks in the Pyeongan Supergroup have the most negative values. These trends can be explained by a sedimentary system dominated by detritus derived from recycled highly weathered granitic materials in the lower to middle Pyeongan section, with an increased proportion of less weathered detritus derived from fractionated granitic materials in uppermost strata. Such an interpretation is consistent with the results of studies on coarse-grained sediments of the Pyeongan Supergroup.