新疆准噶尔与华南湘中法门期碳同位素特征的对比研究
Comparative Study of Famennian Carbon Isotope Characteristics of Junggar, Xinjiang and Central Hunan, South China

摘要 针对新疆准噶尔盆地西北缘洪古勒楞组下段是否存在弗拉？法门阶界线的问题, 对比研究布龙果尔剖面该段地层以及湖南中部锡矿山剖面上泥盆统法门阶中下部碳酸盐岩的碳同位素特征, 并结合已有的生物地层学证据, 重新讨论洪古勒楞组下段的时代。结果表明, 布龙果尔剖面洪古勒楞组下段碳同位素的变化趋势与国内外诸多典型剖面弗拉？法门期之交的δ13C偏移明显不同: 未见到显著正偏移; 布龙果尔剖面δ13C在0~1‰范围内变化, 小于国际上目前已知的弗拉？法门期之交δ13C正偏移的幅度(2‰~3‰)。布龙果尔剖面洪古勒楞组下段可能不存在弗拉期地层, 甚至可能缺失法门阶底部(牙形石Pa. triangularis带)。法门早中期, 布龙果尔剖面碳同位素曲线可与湖南锡矿山剖面同时期地层进行较好的对比, 分为3个阶段: 2个完整的正偏—负偏旋回以及之上的小幅正偏。主量元素特征表明, 布龙果尔剖面第3-1层的下半部受陆源物质影响较大, 推测可能与区域构造活动变强烈有关。
Abstract In order to clarify the position of the Frasnian/Famennian (F/F) boundary in the Lower Member of the Hongguleleng Formation of northwestern Junggar Basin, Xinjiang, carbon isotope characters were comparatively studied for the carbonates from the Lower Member of this formation in the Bulongguoer section of Junggar and the interval (lower-middle Famennian of the Upper Devonian) at the Xikuangshan section of central Hunan. The age of the Lower Member of the Hongguleleng Formation is re-investigated, on the basis of its integrative biostratigraphical evidence, carbon isotope features, and a comparison with carbon isotope data from the Xikuangshan section. The carbon isotopic trend from the Lower Member of the Hongguleleng Formation does not show any prominent positive excursion and is distinctly different from the δ13C oscillation pattern across the F/F boundary recorded in several typical sections around the world; the δ13C values at Bulongguoer vary from 0 to 1‰, which is lower in amplitude than the currently known positive shift (2‰？3‰) across the F/F boundary in the world. The Lower Member of the Hongguleleng Formation probably may not contain Frasnian deposits, or even lacks the basal part of the Famennian (Pa. triangularis conodont Zone). For the early-middle Famennian interval of the studied sections, the carbon isotope trend seems to be well comparable; and this trend can be divided into three phases: two positive-negative oscillation cycles, followed by a weak positive excursion. The characteristics of major elements indicate that the lower part of Unit 3-1 in the Bulongguoer section was affected by terrigenous supply, which is inferred to probably be related to strong regional tectonic activities.