碎屑石榴石地球化学物源分析与解释:粒度的影响

单矿物地球化学分析是沉积物源分析研究的重要手段,运用广泛。沉积物中碎屑矿物的粒度组成,即水动力分选作用,是否影响到基于这种方法的物源解释,目前尚不清楚且受到的关注不多。通过研究柴达木盆地北缘地区第三系沉积物中的680颗碎屑石榴石(0.068~0.557 mm)的主量元素地球化学组成数据,解释了不同粒度的碎屑石榴石物源。结果显示,0.063~ 0.125 mm的碎屑石榴石的地球化学结果中Fe^2+与Mn^2+含量更高,表明其母岩的变质结晶程度较弱,故物源解释结果更可能为中—低级角闪岩相变沉积岩,而其他粒径的碎屑石榴石Ca^2+与Mg^2+含量更高,这说明其物源区的温压条件较前者高,因而物源解释结果为低级变质相,中酸性火成岩,榴辉岩等宽泛结果。这种碎屑石榴石粒度差异导致的地球化学组成的不同,说明碎屑石榴石的颗粒大小在一定程度上会影响物源解释结果。因此,在进行单矿物地球化学物源研究时,选取特定粒径(0.063~ 0.125 mm或0.125~ 0.25 mm)组成的碎屑矿物进行分析,可以消除水动力分选作用的影响,有助于获得更为准确的物源解释结果。

Detrital Garnet Geochemistry-based Provenance Analysis and Interpretation: The effect of grain size

Single mineral geochemistry is an important approach for sedimentary provenance analysis and is widely used. Whether grain size compositions of clastic minerals (i.e., hydrodynamic sorting process) affect sedimentary provenance interpretation is not yet clear, and not enough attention has been given to this issue. In this paper, 680 detrital garnets (0.068-0.557 mm) from the Tertiary sedimentary rocks in the northern Qaidam basin were studied, and the provenance of the detrital garnets with different grain sizes was interpreted based on major element geochemical data. Fe2+ and Mn2+ contents are higher when the grain size of the garnet is between 0.063 mm and 0.125 mm, so the result of sedimentary provenance interpretation is more likely to be from the metamorphism of lower-level amphibolite facies. In the other grains, Ca2+ and Mg2+ contents are higher, and the resulting explanation of provenance is the broad result of low-grade metamorphic facies, intermediate-acidic igneous rocks and eclogite, etc. Different geochemical compositions caused by the difference of grain size of the clastic garnet show the grain size of the detrital garnets, which will affect the geochemistry-based provenance interpretation. Therefore, in the case of single mineral geochemistry-based provenance analysis, concentrating on detrital minerals with certain grain size compositions (0.063-0.125 mm or 0.125-0.25 mm) can eliminate the influence of the hydrodynamic sorting process and help obtain more accurate provenance interpretation results.e