长江水系沉积物碎屑石榴石化学组成及物源示踪

Detrital Garnet Chemistry of the Changjiang (Yangtze River) Sediments and Their Provenance Implication

摘要:

长江流域主要干流和支流碎屑沉积物中的单颗粒石榴石化学组成分析显示，石榴石元素组成以SiO2、FeO、Al2O3、MnO、CaO、MgO、TiO2为主，石榴石组合主要是镁铝榴石、铁铝榴石、钙铝榴石和锰铝榴石，其平均物质的量分数分别是65.4%、13.8%、9.4%和11.5%.利用石榴石GPAS物源分析方法，可以识别出长江流域的特征石榴石组合.长江水系沉积物中碎屑石榴石呈低Mg、高Mn特征，其源岩以中低级变质岩为主，长江流域出露的片岩、千枚岩和大理岩以及低温接触变质岩控制了沉积物石榴石的组成.金沙江是长江入海粗粒沉积物的主要来源，石榴石组合呈现为典型的低Mg、高Mn特征，与整个长江干流石榴石组成一致，但由于未受到中下游近源沉积物影响，其石榴石组合可以作为长江特征物源端元，用以中国东部海域沉积物物源示踪.

关键词:长江水系; 沉积物; 碎屑石榴石; 化学组成; 物源示踪

Abstract:

This study presents the compositions of major oxides including SiO2, FeO, Al2O3, MnO, CaO, MgO and TiO2 in the detrital garnet grains separated from the sediments of the Changjiang River and its major tributaries. The main garnet assemblage consists of almandine, grossularite, Grossular and spessartine, with the mean contents of 65.4%, 13.8%, 9.4% and 11.5%, respectively. The typical garnet assemblage of Changjiang sediments on the GPAS triangular diagram is characterized by low Mg and high Mn contents, which suggests they are mainly sourced from intermediate and lowgrade metasedimentary rocks. These garnet compositions are primarily determined by the widely distributed schist, phyllite, marble and the lowtemperature and lowpressure contact metamorphic rocks in the Changjiang catchment. Our data also suggests that the Jinshajiang River as the largest tributary in the upper drainage is the major supply of fine sands to the Changjiang mainstream. Similar to the average composition of the whole Changjiang sediments, the garnets of Jinshajiang are characterized by low Mg and high Mn contents, but much different to those of the middle and lower reaches. Therefore, the garnet assemblage of the Jinshajiang can indicate the contribution of Changjiang fine sand in the sediments provenance study of East China marginal seas.

Key words:Changjiang rivers; sediments; detrital garnet; geochemical composition; provenance identification