摘要
下扬子区早三叠世孤峰组中产出的层状硅质岩厚度为近20—80m,含有数量不等的放射虫和海绵骨针。孤峰组剖面有硅质岩型、碳酸盐岩型和硅质岩-碳酸盐岩混合型3种类型。层状硅质岩主要产在第一、第三2种类型剖面中。硅质沉积中心是一些菱形或椭圆形小型(面积在数千平方公里以内)凹地,其形成受断裂控制。硅质岩中产出泥化火山岩,层薄,且层数很多。硅质岩富Fe贫Al,富集As,Sb,Bi,Ga,Au,Ag,Cr等微量元素。稀土元素总量低,Ce负异常,重稀土有不同程度的富集,具有热水硅质岩的特征。硅质岩中混有少量非热水成因的物质。硅质沉积时海水温度约为几十度至160余度。
Cherts of the Early Permian Gufeng Formation in the lower Yangtze area are marked by bedded black rocks, containing more or less clayey and organic materials, and there is minor calcite in some samples. The thickness of the chert sequence ranges between 20 and 80m, and the thickness of a single bed of chert generally varies from several to 30 cm. The chert beds usually alternate with very thin beds of clayey rocks, each only a few mm thick.Remains of radiolarians and spongy spicules, perfectly preserved in many samples, have been discovered in the cherts for the first time.Gufeng cherts can be identified as hydrothermal ones by the following evidence: First, the heat source responsible for hydrothermal convection is present in this area. The present authors have discovered alternating beds of volcanic rocks and cherts; for example, there are at least 15 beds of volcanic rocks in the Gufeng Formation at Taochong, Fanchang County, Anhui Province. They are mainly intermediate-acid tuffs with a bed of basic lava; their thickness varies from several to 104mm. All the volcanic rocks have been argillized, which indicates that they underwent low-temperature hydrothermal alteration immediately after the accumulation of each volcanic bed.Second, there were several syngenetic faults transecting the crust of this area, which provided conduits for the hydrothermal activity. Crustal extension along these faults resulted in the formation of a series of basins and uplifts inserted among the basins. Cherts were mainly deposited in the basins, whereas shallow-water carbonates up to 300 m thick were accumulated on the uplifts.Third, geochemically the cherts are characterized by higher contents of Fe and Mn, slightly lower contents of Al and Ti, higher abundances of As, Sb, Bi, Au, Ag, Ga and Cr, lower total REE, negative Ce anomaly and slight enrichment in HREE. All these characteristics are important evidence for hydrothermal cherts.The δy18O of the cherts shows that the temperatures for chert deposition changed from several tens of degrees to about 160℃. Data of the highest temperature were obtained from the sample collected at Taochong, Fanchang County. Thus, an emanation orifice of a hot spring might exist probably around this place.
出处
《地质学报》
EI
CAS
CSCD
北大核心
1995年第2期125-137,T001,共14页
Acta Geologica Sinica
基金
国家自然科学基金(编号 4880131)
关键词
硅质岩
成因
孤峰组
早二叠世
放射虫
hydrothermal chert, argillized volcanic rocks, radiolarian
