云南剑川龙门邑古钙华脊沉积学特征及发育机制

Sedimentary features and depositional mechanism of a fossil fissure-ridge travertine from Longmenyi,Jianchuan,Yunnan

构造活动地区常发育一类沿断层迹线呈线状分布的裂脊型钙华,目前对这类奇特的钙华沉积形态(体)发育机制的认识还不够明晰。本研究以云南剑川县龙门邑古钙华脊为例,通过对钙华脊的形态、内部结构、岩相特征以及碳氧同位素组成等进行系统分析,探讨裂脊型钙华的成因和发育演化机制。研究结果显示,龙门邑古钙华脊主要由轴部近直立的条带状钙华和两翼倾斜层状钙华构成,脊顶发育中央裂缝。脊轴部条带状钙华较为致密,主要由粗亮晶方解石厚层和泥晶方解石细层交替组成;而两翼钙华较为疏松多孔,其微岩相结构类型多样。钙华δ13C值为1.3‰~3.2‰,δ18O值为-26.9‰~-20.5‰,对其进一步分析表明,钙华脊由沿剑川盆地西缘的活动断裂出露的温泉水沉积而成。该古钙华脊的主要发育演化过程可能包括:1)热泉水沿断层迹线基岩裂缝出露,沉积钙华并连成脊;2)脊轴部发育中央扩张裂缝;3)泉水沿中央裂缝两壁沉积直立条带状钙华,并向脊两侧沉积倾斜层状钙华;4)中央裂缝被逐渐填充并闭合,且热泉活动减弱,钙华脊停止生长。对龙门邑古钙华脊的研究表明,裂脊型钙华的发育与当地断裂构造具有密切联系,加之这类钙华可以提供关键的地质年代约束,在新构造和古地震活动研究中具有巨大应用价值。

Fissure-ridge-type travertine is common in tectonically active areas and is often developed along the trace of active faults.However,the sedimentary features and depositional mechanism of this peculiar type of travertine have not been well-understood yet.In this study,a typical fissure-ridge travertine-Longmenyi travertine in Jianchuan County,Yunnan Province,Southwest China was taken as study site.The morphology,internal architecture,lithofacies,as well as isotopic geochemistry of the travertine were systematically investigated to elucidate the genesis and growth mechanism of fissure-ridge-type travertine.The results show that the fossil fissure-ridge travertine at Longmenyi is characterized by an apical fissure that follows the long axis of the ridge and consists of vertical to sub-vertical band-like travertine in the central portion of the ridge and bedded travertine that dips away from the apical fissure on the flanks of the ridge.The band-like travertines on the axis of the ridge are relatively dense and mainly composed of 2~3 mm-thick layers of coarse sparite that are alternating with<1 mm-thick layers of micrite;while the bedded travertines from the flanks of the ridge are somewhat loose and porous,with various types of micro-fabrics such as recrystallized sparites,crystalline dendrites,and biotic fabrics and porosity,etc.Theδ13C value of travertine ranges from 1.3‰to 3.2‰,and theδ18O value ranges from-26.9‰to-20.5‰.Based on the analysis of them,we infer that the travertine ridge was precipitated by hot spring water bound to an active fault located on the western margin of the Jianchuan Basin.The development of the fissure ridge was influenced by many factors,such as fault structure,hot spring activity,and the evolution of travertine body itself.The possible processes responsible for the formation of the fissure ridge include:(1)Hot spring water emerged from the fissure that had formed within bedrocks along the trace of an active fault,resulting in the accumulation of elongated ridge-shaped travertine;(2)A central expansion fissure developed along the long axis of the ridge;(3)Vertically banded travertine was deposited on both walls of the central fissure by upwelling thermal waters,while bedded travertine formed on both flanks of the ridge by waters that flowed out at the top of the ridge;and(4)Fracture sealing and reduced fluid discharge inhibited the growth of the ridge.The formation of Longmenyi fissure-ridge travertine was closely related to local tectonic structures,and the spatial distribution,morphology and internal architecture of such type of travertine could reveal much information about tectonic activities.Additionally,the absolute dating of these hydrothermal carbonates allows us to use them for neotectonic and paleoseismological studies.