Genesis of fibrous veins in the shales of Longmaxi Formation in Shizhu area, Eastern Chongqing
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摘要: 渝东石柱地区志留系龙马溪组页岩地层发育特殊的纤维状方解石脉体。为揭示其流体来源及形成机理，结合岩石学及地球化学特征等开展系统分析。研究表明，纤维状方解石脉（FCV）以顺层或近平行层面充填于黑色粉砂质页岩和泥质粉砂岩微裂缝中，脉体宽0.1~4.0 cm、横向延伸一般为0.5~8.0 m。纤维状方解石晶体以柱状、纤维状垂直裂缝壁生长为主，多与石英共生产出。晶体表面洁净，局部见方解石压力影、机械双晶及晶体间锯齿状晶界。阴极发光下纤维状方解石呈暗红色-橘红色；碳-氧同位素分析表明δ13CPDB略偏负、δ18OPDB负异常明显（δ13CPDB、δ18OPDB平均值分别为-1.549‰及-12.654‰），指示其形成过程中受温度升高影响明显；流体包裹体均一温度平均值为159.5℃。结合区域构造-热演化史分析，结果表明石柱地区龙马溪组FCV形成流体具造山带同构造期超高压有机流体性质特征，为中-晚三叠世印支期同构造作用驱动深部流体叠加生烃超高压释放导致水力压裂所致，裂缝的开启导致孔隙流体的过饱和及纤维状方解石的快速沉淀。Abstract: Fibrous calcite veins (FCV) filled in microfractures in Lower Silurian Longmaxi Formation are widely found in the Eastern Chongqing of Southwestern China. The mechanism and time of fibrous calcite veins is elucidated by the combination of petrographic descriptions integrated with fluid inclusion, stable carbon and oxygen isotopic data and burial history with thermal history evidence. Field observations indicate that the syntaxial fibrous calcite veins are filled in black silty shale/argillaceous siltstone microfractures with bedding-parallel or subhorizon, with width of 0.1-4.0 cm and lateral extended distance of 0.5-8.0 m. Fibrous calcite crystals grow mainly in columnar and fibrous vertical wall rock and are mostly co-precipitated with quartz fibers. The surface of the crystal is clean and the pressure shadow, mechanical twin of calcite and serrated grain boundary between crystals can be seen locally. The fibrous calcites are dark red-orange under cathodoluminescence, and also exhibit low negative values of δ13C and extreme negative values of δ18O, respectively with an average of -1.549‰ and -12.654‰, meaning that calcites may have been obviously affected by the increase of pore-water temperature. Fluid inclusion analyses of two-phase aqueous inclusions in fibrous calcites suggest that calcites occurre at temperatures with an average of 159.5℃. It is inferred that the FCV may have formed during uplift in the Middle-Late Triassic tectonic movement due to fracture-controlled fluid migration and precipitation by syntectonic superhigh pressure and excretion of geofluids in second paleohydrologic system.
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Key words:
- Eastern Chongqing /
- Silurian /
- Longmaxi Formation /
- fibrous calcite vein /
- shale /
- syntectonic superhigh pressure
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图 1 渝东石柱地区构造示意图(A)、取样位置地质简图(B)及龙马溪组地层柱状图(C)
Figure 1. Structural schematic diagram (A), simplified geological map showing the position of sampling section (B) and simplified stratum column of the Lower Silurian Longmaxi Formation in Shizhu area, Eastern Chongqing (C)
图 2 渝东石柱漆辽地区龙马溪组纤维状方解石脉宏观照片
A.粉砂质页岩中重晶石结核(Bc)及纤维状方解石脉(FCV)发育；B.条带状方解石脉轻微变形，宽2~3 cm；C.低角度方解石脉，宽0.1~4 cm均可见；D.顺层方解石脉，脉体遭受挤压变形；E.页岩中纤维状方解石脉发育，富含各类笔石化石(箭头所指)；F.页岩中纤维状方解石脉及黄铁矿(Py)近平行层面发育，被黄铁矿分割成透镜状
Figure 2. Macro-photographs for fibrous calcite veins in the shales of Longmaxi Formation in Qiliao, Shizhu area, Eastern Chongqing
图 3 石柱漆辽地区龙马溪组纤维状方解石脉显微特征
A.围岩及纤维状方解石脉体，脉体以垂直围岩(WR)生长方式为主，单偏光；B.围岩(砂质页岩)及纤维状方解石脉体充填物，脉体被石英(Qz)、方解石(Cal)及黄铁矿(Py)全充填；C.沿围岩黄铁矿底质向上依次发育微晶方解石、白云石、粒状方解石及纤维状方解石-石英，单偏光；D.沿围岩底质向上发育纤维状方解石与石英共生矿物，正交偏光；E.纤维状方解石脉体中矿物充填序列：黄铁矿-纤维状石英+方解石，SEM；F.纤维状方解石及自形立方体状黄铁矿，SEM；G.纤维状方解石中石英及方解石压力影、机械双晶及晶体间的锯齿状晶界，正交偏光；H.纤维状方解石全貌，单偏光；J.H对应的阴极发光照片，围岩及裂缝中的石英不发光，纤维状方解石发橘红-暗红色光
Figure 3. Micro-photographs for fibrous calcite veins in the shales of Longmaxi Formation in Qiliao, Shizhu area
图 4 研究区龙马溪组纤维状方解石脉中充填物包裹体特征(A, B)及均一温度直方图(C)
Figure 4. Micro- and fluorescent photograph (A, B) and corresponding homo-temperature histograms (C) of fluid inclusion for FCV in Longmaxi Formation in the study area
图 5 渝东石柱漆辽地区纤维状方解石脉简图(a)及碳-氧同位素(b，c)变化关系图
FCV.纤维状方解石脉; WR.围岩; C1.微晶方解石; C2.中-粗晶粒状方解石
Figure 5. Diagrammatic sketch (a) and corresponding C-O stable isotope values (b, c) for samples of fibrous calcite veins in Longmaxi Formation in Qiliao, Shizhu area, Eastern Chongqing
图 6 渝东石柱地区志留世龙马溪组埋藏史-热史及裂缝充填对应时期示意图(据文献[31]修改)
Figure 6. Burial-thermal history plots and schematic for correspondingly formation time of FCV in Silurian Longmaxi Formation in Shizhu area, Eastern Chongqing
表 1 研究区龙马溪组纤维状方解石脉及围岩部分矿物成分特征、碳氧同位素特征及其成脉温度
Table 1. Mineral composition characteristics, carbon and oxygen isotope characteristics and vein forming temperature of fibrous calcite veins and surrounding rocks in Longmaxi Formation in the study area
取样位置 样品号 矿物成分wB/% C-O同位素/‰ 包裹体特征 FeO MnO SrO δ13CPDB δ18OPDB Th/℃ C1 SP-1 0.934 0.148 0.051 0.814 -8.345 － SP-2 0.668 0.045 0.061 0.868 -8.672 － 0.630 0.070 0.027 C2 SP-3 0.595 0.079 0.053 -1.356 -11.873 106.4, 120.1 SP-4 0.530 0.202 0.104 -1.228 -12.000 107.4, 106.1, 118.0 0.475 0.175 0.265 FCV SP-5 0.370 0.097 0.484 -1.652 -13.106 141.5, 186.7 0.549 0.383 0.335 0.640 0.298 0.250 SP-6 0.683 0.381 0.318 -1.526 -12.765 143.8, 165.8, 166.4 0.841 0.394 0.364 0.651 0.386 0.416 SP-7 0.752 0.354 0.333 -1.534 -12.218 140.2, 145.2, 167.4, 161.5, 156.6 0.728 0.195 0.448 0.591 0.231 0.509 SP-8 0.582 0.221 0.426 -1.485 -12.527 154.6, 184.6 0.500 0.184 0.567 0.514 0.193 0.254 取样位置 样品号 矿物成分wB/% C-O同位素/‰ 包裹体特征 TFe2O3 MnO SrO δ13CPDB δ18OPDB Th/℃ WR SP-9 1.730 0.018 0.023 0.684 -7.468 － SP-10 5.580 0.032 0.016 0.712 -7.325 － 注：－表示未检测；FCV.纤维状方解石脉; WR.围岩; C1.微晶方解石; C2.中-粗晶粒状方解石 
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