风化壳^40Ar／^39Ar年代学研究意义：进展、问题与展望

^(40)Ar/^(39)Ar Geochronology of Weathering Crust:Significance,Problems,and Prospect

风化壳是不同地质历史时期风化作用的环境和物质记录,其组成、厚度、成熟度、保存程度等是古气候条件、区域构造活动性及地貌演化的直接反映。风化壳的形成年代是风化壳研究的关键内容,精确的风化壳年龄数据是查明一系列区域甚至全球事件的重要途径。风化壳中次生矿物(主要是钡硬锰矿族矿物和明矾石族矿物)的40Ar/39Ar年代学研究首次实现了对风化壳和风化作用的直接和精确定年,所获得的年龄数据为风化壳的形成演化、区域古气候古环境恢复、化学风化历史与矿床次生富集过程及新构造运动和山脉隆升历史等一系列重大地质问题的解决提供了重要途径。然而,风化壳中次生矿物定年存在的潜在问题及风化壳剖面取样的不完整性,有可能使风化壳的年龄分布变得相当复杂,导致对化学风化历史、风化壳形成过程和形成环境等的认识出现困难。为此,必须开展对风化壳的详细野外地质研究和精细取样,并综合采用多种先进测试手段,才能获得可靠的风化壳年龄数据。在对风化壳年龄数据进行解释时,还需要系统分析多种地质、环境资料(如古生物、盆地沉积物等),才可能获得有意义的地貌、构造和古气候信息。我国华南地区广泛分布的红色风化壳中含有丰富的钡硬锰矿族矿物,是研究该区新生代以来化学风化、大规模次生富集成矿及古气候古环境演变的理想对象。

Weathering crust is result and record of weathering processes and thus bears invaluable information on paleoclimate, regional tectonism, and landscape evolution of regional and even global significance. Geochronology of weathering crusts provides temporal constraints for establishing the relationships between a variety of global events and continental weathering. ^40 Ar/^39 Ar dating of supergene minerals is versatile to directly and precisely time the formation of weathering crust, yielding results required to determine the mechanisms and rates of propagation of weathering fronts, to unravel the climatic and geochemical history implicit in the evolution of continental regolith covers, and to correlate variation in the intensity of regional paleoclimate and mountain uplift. However, problems remain in sampling and isotopic dating of supergene minerals. Moreover, age distribution of weathering profile may be complicated due to episodes of weathering and preferential permeability of zones cutting through by faults. In this regard, systematic sampling and comprehensive geochronological study are imperative to unravel the history of continental weathering. Combination of geochronological results of weathering profiles with independent records such as paleontologic and sedimentary data is required while they are interpreted as paleoclimatic indicators. Laterites or red weathering crusts are widely distributed in South China, which contain appreciable amounts of hollandite-group minerals that can be dated by the ^40 Ar/^39 Ar Ar method. This will allow yielding geochronological framework of weathering and supergene ore enrichment, which is crucial in reconstruction of the Cenozoic paleoclimate and tectonism in South China.