Genesis and paleoenvironment of the iron ooids from the Ganxi prolife of the Devonian Yangmaba Formation in Beichuan County, Longmenshan Area
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摘要:
龙门山区中泥盆统养马坝组底部发育丰富的铁质鲕粒。为分析该铁质鲕粒成因，本文以甘溪石沟里剖面精细实测为基础，通过详细的薄片和扫描电镜等分析，对铁质鲕粒的特征进行了研究，分析结果表明：龙门山区北川甘溪泥盆系养马坝组鲕粒类型多样，鲕粒核心包括石英碎屑颗粒、海百合碎片、黑色赤铁矿和方解石等4种类型，鲕粒圈层可分为明暗相间圈层、颜色均匀圈层和绿泥石圈层等3种类型。根据鲕粒核心和外部圈层的组合，可将甘溪泥盆系养马坝组的铁质鲕粒划分为粉砂质鲕粒、铁化鲕粒和绿泥石薄皮鲕粒等3种类型。龙门山区北川甘溪泥盆系养马坝组铁质鲕粒对古环境具有重要的指示意义，粉砂质鲕粒形成于开放近岸浅海氧化环境，而铁化鲕粒和绿泥石薄皮鲕粒则指示浅海滨岸中相对封闭的泻湖与残积鲕粒滩环境。
Abstract:There exist abundant iron ooids at the bottom of the Middle Devonian Yangmaba Formation in the Longmenshan area. In order to analyze the genesis of the iron ooids, their characteristics were studied by means of thin section and scanning electron microscope based on the detailed measurement of the Shiligou Profile in Ganxi village. The analysis results show that there are various types of ooids in Middle Devonian Yangmaba Formation of Longmenshan area. The ooidal cores include four types:quartz clastic particles, crinoid fragments, black hematite and calcite. The outer laminas of ooids can be divided into three types:alternate dark and bright one, uniform-colored one and chlorite-fragmented one. The iron ooids of Middle Devonian Yangmaba Formation in the Longmenshan area have an important significance to the ancient environment. The silty ooids were formed in the open environment nearshore shallow water under oxidation condition, and iron ones and chlorite ones with thin layers were deposited in the relatively closed lagoon and residual ooids beach of shallow seashore.
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图 1 石沟里养马坝组剖面位置图
Figure 1. Location of the Ganxi profile from the Yangmaba Formation in Shigouli
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图 2 龙门山区石沟里养马坝组沉积综合柱状图（据侯鸿飞等，1988；郑荣才等，2016等文献修编）
Figure 2. Stratigraphic column of the Yangmaba Formation in the Shigouli profile, Longmenshan area, Sichuan Basin (modified from Hou et al., 1988; Zheng et al., 2016)
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图 3 龙门山区石沟里养马坝组铁质鲕粒富集层综合柱状图
Figure 3. Column of iron ooids-enriched layer within the Yangmaba Formation of the Shigouli profile, Longmenshan area
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图 4 龙门山区石沟里养马坝组铁质鲕粒岩石特征照片
a—铁质鲕粒灰岩，野外宏观照片，可见红色鲕粒赤铁矿呈透镜状分布于生屑灰岩，第7层；b—鲕粒生屑灰岩，鲕粒呈球状、椭球状，SEM，N-2；c—鲕粒生屑灰岩，真鲕，核心为单晶石英颗粒，少量铁质，沿鲕粒纹层分布，N-2；圆形鲕粒，N-1；鲕粒形态，SEM，N-2；d—粉砂质鲕粒生屑灰岩，真鲕，核心为多晶石英颗粒，基质中分布石英颗粒，正交偏光下鲕粒呈十字消光，N-1；e—鲕粒生屑灰岩，海百合核心，绿泥石纹层，薄皮鲕，B1-2；f—生屑鲕粒灰岩，鲕粒核心为海百合碎片，薄皮鲕，纹层成分绿泥石质，基质为黑褐色赤铁矿，N-4；g—生屑鲕粒灰岩，鲕粒核心为海百合碎片和赤铁矿，薄皮鲕，纹层成分绿泥石质，基质为黑褐色赤铁矿，N-4；h—生屑鲕粒灰岩，真鲕，鲕粒核心不明显，B1-3；i—生屑鲕粒灰岩，铁质含量丰富，沿鲕粒纹层分布，B1-3；j—生屑鲕粒灰岩，铁质鲕粒，海百合内充填铁质，B1-3；k—生屑鲕粒灰岩，铁质鲕粒，海百合内充填铁质，B1-3；l—生屑鲕粒灰岩，鲕粒核心为海百合碎片和赤铁矿，薄皮鲕，纹层成分绿泥石质，基质为黑褐色赤铁矿，B2-2-1
Figure 4. Photos of rocks with iron ooids from the Shigouli profile in the Yangma Formation, Longmenshan area
a- Ferric oolitic limestone. In the field macroscopic photos, it can be seen that red oolitic hematite is distributed lenticular in bioclastic limestone, the 7th layer; b-Bioclastic oolitic limestone, globular and ellipsoidal oolitic, SEM, N-2; c-Bioclastic oolitic limestone, true oolite, core is single crystal quartz grain, a small amount of iron distributed along oolitic striatum, N-2; Round oolite, N -1; Oolite morphology, SEM, N-2; d-Silt-bioclastic oolitic limestone, true oolite, core is polycrystalline quartz grain, quartz grain distribution in the matrix, cross extinction under orthogonal polarization, N-1; e- o Bioclastic oolitic limestone, core of crinoids, chloritic stromatolite, thin-skinned oolite, b1-2; f-Bioclastic oolitic limestone, oolitic core is fragments of crinoids, thin-skinned oolitic, striatum composition is chloritic, matrix is black brown hematite, n-4; g- Bioclastic oolitic limestone, oolitic core is crinoids fragments and hematite, thin crust oolitic, striatum composition is chloritic, matrix is black brown hematite, N-4; hBioclastic oolitic limestone, true oolite, oolitic core is not obvious, B1-3; i-Bioclastic oolitic limestone, rich in iron, distributed along oolitic striatum, B1-3; j- Bioclastic oolitic limestone, ferric oolitic, ferric fill in crinoids, B1-3; k-Bioclastic oolitic limestone, ferruginous oolitic, ferric fill in crinoids, B1-3; l- Bioclastic oolitic limestone, oolitic core consists of crinoids fragments and hematite, thin-skinned oolitic, striatum composition is chloritic, matrix is black brown hematite, B2-2-1
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图 5 龙门山区石沟里养马坝组铁质鲕粒能谱分析
a—鲕粒能谱分析位置图，在鲕粒核心和外部圈层中铁的赋存形式是Fe2O3（赤铁矿），N-1；b—鲕粒能谱分析位置图，在鲕粒核心和外部圈层中铁的赋存形式是FeCO3（菱铁矿），B1-3
Figure 5. Energy spectrum analysis of iron ooids from the Shigouli profile of the Yangma Formation, Longmenshan area
a-Energy spectrum analysis location diagram, in the oolite core and the outer ring, the occurrence of iron is Fe2O3 (hematite), N-1; b-Energy spectrum analysis location diagram, the occurrence form of iron in the oolite core and outer ring is FeCO3 (siderite), B1-3
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