Volcanic sources and diagenetic alteration of Permian-Triassic boundary K-bentonites in Guizhou Province, South China
Volcanic sources and diagenetic alteration of Permian-Triassic boundary K-bentonites in Guizhou Province, South China
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贵州省二叠系—三叠系界线钾膨润土的火山来源及成岩蚀变
DOI：
10.1016/j.palaeo.2018.01.019
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发表时间：
2019-04-01
影响因子：
3
通讯作者：
Ji, Kaipeng
中科院分区：
文献类型：
--
作者：
Hong, Hanlie;Zhao, Lulu;Ji, Kaipeng
华南地区陆相二叠-三叠系界线（PTB）层序通常含有1 ~ 3层火山灰层（钾镁矾），这为陆相生物大灭绝的可能触发提供了证据，并可作为PTB界线从陆相到海相的对比标志。在这里，我们调查的粘土矿物学和地球化学的灰层的哲觉和韭菜冲部分，分别代表沼泽和湖泊环境，为了更好地了解这些灰层的来源（S）和保存。这两种钾质膨润土具有相似的组成，但与韭菜冲钾质膨润土有明显的不同。前者主要含粘土矿物，含少量石英、方解石和纤铁矿，这些层中的粘土矿物主要为R1和R3混合层伊利石/蒙皂石（VS），结晶良好至结晶不良的高岭石，而后者主要由R3 I/S组成，含少量石英、长石和方解石。前者有较高的CIA值（92.2和93.8）比后者（79.4），表明更强烈的化学风化，可能与沉积在沼泽环境中的孔隙流体中的有机酸浓度高。这些钾镁矾还表现出主元素化学的差异（由于风化作用强度较弱，可能是早期成岩作用从孔隙流体中吸收，故韭菜冲的MgO和K2 O含量较高），REE化学（韭菜冲Eu异常较大），Nd同位素组成浙珏钾镁矾的Nd-143/Nd-144比值分别为0.512436和0.512400，高于韭菜冲钾镁矾的Nd-143/Nd-144比值0.512190。虽然这些成分差异中的一些可能与成岩作用发生的不同化学环境有关（即，湖相与沼泽相），Eu异常和Nd同位素组成的差异反映了火山灰来源的不均一性。这些钾镁矾岩的源区可能是位于华南陆缘古南缘的一个大陆火山弧，该弧内可能同时存在多个火山喷口。
Terrestrial Permian-Triassic boundary (PTB) sequences in South China usually contain one to three ash beds (K-bentonites), which provide evidence of a possible trigger for the terrestrial mass extinction and are useful as a distinct marker for correlation of the PTB boundary interval from terrestrial to marine areas. Here, we investigate the clay mineralogy and geochemistry of ash beds in the Zhejue and Jiucaichong sections, representing paludal and lacustrine environments, respectively, in order to better understand the source(s) and preservation of these ash beds. The two Zhejue K-bentonites exhibit similar compositions that are distinctly different from that of the Jiucaichong K-bentonite. The former contain mainly clay minerals with minor quartz, anatase, and lepidocrocite, and the clay minerals in these layers are mainly R1 and R3 mixed-layer illite/smectite (VS) and well-crystallized to poorly-crystallized kaolinite, whereas the latter consists mainly of R3 I/S with minor quartz, feldspars, and anatase. The former have higher CIA values (92.2 and 93.8) than the latter (79.4), indicating more intense chemical weathering that may have been related to deposition in a paludal setting with high organic acid concentrations in pore fluids. These K-bentonites also exhibit differences in major-element chemistry (MgO and K2O contents are higher at Jiucaichong due to lesser weathering intensity and, possibly, early diagenetic uptake from pore fluids), REE chemistry (Jiucaichong exhibits a larger Eu anomaly), and Nd isotopic compositions (Zhejue K-bentonites have Nd-143/Nd-144 ratios of 0.512436 and 0.512400, higher than the 0.512190 ratio for Jiucaichong). Although some of these compositional differences may be related to the dissimilar chemical environments in which diagenesis occurred (i.e., paludal vs lacustrine), the differences in Eu anomalies and Nd isotopic compositions imply heterogeneous volcanic ash sources. The source of these K-bentonites was probably a continental volcanic arc located on the paleo-southern margin of the South China Craton, within which multiple volcanic vents may have been active concurrently.