关于老虎洞白云岩及其上“粗晶灰岩”的形成机制

GENETIC MECHANISM OF THE CARBONIFEROUS LAOHUDONG DOLOMITE AND THE SECONDARY “MEGACRYSTALLINE LIMESTONE” IN THE LOWER YANGTZE AREA

下扬子区石炭系老虎洞白云岩的形成认为是被蒸发泵吸作用稍加改变的浅淡含水层与海水混合白云岩化的结果,并特别强调准同生期灰泥潮坪受蒸发泵吸作用形成的白云石籽晶在混合白云岩化中的动力学作用;其上“粗晶灰岩”的形成则是经历了白云化——溶解——菱面体孔隙形成及其相互连接——地下缝洞层的形成——粗晶方解石在缝洞空间自由生长——垮塌——再胶结的复杂过程。

Based on synthetical study of the geologic features, carbon and oxygen stable istopic composition and content of trace elements of the Carboniferous Laohudong dolomite and the secondary 'megacrystalline limestone' , their genesis is discussed in this paper. Laohudong dolomite consists of fine crystalline dolomite with relict bioclastic fabric, crystalline dolomite and few micritic dolomite with algal laminae , micritic dolomite ect., accompanied usually by some intercalated beds and lenticles of partial dolomitized bioclastic lime wackstone. The dolomite fabrics are complex. They are cquigranular micro-textured sutured mosaic ,peloidal and inequigranular porphyrotopic and mosaic fabriics. A continuous spectrum of dolomitic fabric ranging from early-sta e floating-rhomb porphyrotopic to later-stage contact-rhomb porphyrotopic fabrics is clearly revealed in the intercalated partial dolomitized limestone , and the mosaic fabrics of the fine crystalline dolomite appear to represent the completion stage of dolomitization. These fabric characteristics can be compare with the dolomitic fabrics of acquifer carbonate rocks (Tertiary) from Floridan , U.S.A.. The δ18O and δ13C isotope values of the dolomites (29 samples ) are variable, δ18O (PDB) from -0.48‰ to -20.15‰ and δ13C(PDB) from -3.05‰ to +3.69‰ . The content of trace elements of the dolomites compared with the limestone of the upper Carboniferous appear to be rich in Mn, Ba, poor in Sr. The rocks before dolomitization are interpreted to have been deposited in the extensive epeiric sea possessing many banks and islands. According to aforementioned, the authors suggest that the dolomites were the result of multiple dolomitization events. During the penecontemporancous diagenesis stage, many protodolomite microcrystals and crust were formed by evaporative pumping on the tidal flat of the islands and banks, and some protodolomite as a 'seed' crystal is introduced into the shallow sea deposits by storm tides. During the early diagenesis at shallow levels of burial the fresh groundwater of the aquifer beneath the islands or banks mixed with sea water so that the dorag dolomitization produced in the limestone. The pre-existing protodolomite 'seed' or 'nucleus' possessed significant kinetic effect of dolomitization and sped up the course of dolomitization. The secondary 'megacrystalline limestone' actually is a brecciola cemented by secondary megacrystal calcites. The composition of a gravels is complex, and they consist of the dolomite, limestone and a small amount of chert. Some megacrystalline calcites appear to be broken and recemented in the cathodluminescense photomicrograph. The carbon and oxygen stable isotope data of the 'megacrystalline limestone' show considerable lighter values. The average value of nine sample is that δ18O(PDB) value is -8.90‰ and δ13C (PDB) value is -1.94‰ . The content of its trace elements compared with overlying limestone appear also to be rich in Mn, Ba, and poorest Sr. The formation of the brecciola cemented by secondary megacrystal calcites, which usually is named 'Megacrystalline Limestone' , underwent complex course i.e. dedolomitization- dissolution -forming of rhombohcdral pores and connecting of the pores -rforming of the underground cave-channel bed -the free growth of calcite crystal and forming of megacrystalline calcite-collapse-collapse breccia and broken crystals-recemtation . The dedolomitized solution is the high Ca2+ / Mg2+ratio underground fresh water.