江西井冈山地区早古生代沉积岩的地球化学特征及其对沉积环境的制约

Geochemical characteristics of Early Paleozoic sedimentary rocks in the Jinggangshan area, Jiangxi province and the constraining to the sedimentary environment.

本文研究的井冈山地区位于华夏地块中部,在早古生代寒武纪-奥陶纪时期沉积了巨厚的泥砂质韵律岩层夹碳酸盐岩和含碳岩层。它们具有较高的SiO_2(53.12%～86.37%,平均为70.16%)和Al_2O_3含量(5.43%～22.70%,平均为13.54%),高的Al_2O_3/(CaO+Na_2O)(1.36～174.6,平均为45.5)与K_2O/Na_2O比值(1.0～64.7,平均为20.4)以及低的TiO_2(0.30%～0.95%,平均为0.62%)和CaO含量(0.05%～6.05%,平均为0.63%)。它们具有与上地壳相似的微量元素和稀土元素组成,即富集大离子亲石元素和轻稀土元素,铕负异常明显。它们的主量元素、微量元素和稀土元素特征显示其沉积环境主要为被动大陆边缘浅海环境,物源区被剥蚀物质以上地壳成分为主。根据研究区内早古生代沉积物中没有蛇绿岩和火山岩、Ce异常较弱、有机碳的δ^(13)C_(org)值较低(-30.2‰～-25.4‰)、以及在沉积物中普遍发育的浅海沉积标志,如波痕、荷重模、泥砾、小型斜层理、含腕足类和腹足类等生物化石的灰岩透镜体等,认为研究区在早古生代时期不是洋盆,而是浅海-半深海或陆壳海槽,是由成冰纪时期的裂解事件,使原先的华南地块被肢解成许多次级块体(如武夷、赣中南、云开等)、各块体间成为浅海盆地的结果。

The Jinggangshan area is located the middle part of the Cathaysian block. As a lightly metamorphosed sedimentary cover, the Cambrian-Ordovician sequence consists of rhythmic slated sandstone and siltstone rocks with intercalated carbonate lens and carbonaceous layers, about total 1500 similar to 2000m thickness. The fragmentary rock samples more than 60 collected from the Jinggangshan region are used as the analysis of the Early Paleozoic sedimentary environment. Results suggest that on the major elements they have higher SiO(2)(53. 12% similar to 86.37%, average of 70.16%) and Al(2)O(3) contents (5.43% similar to 22.70%, average of 13.54%), high Al(2)O(3)/(CaO + Na(2)O) (1.36 similar to 174.6, average of 45.5) and K(2)O/Na(2)O ratios (1.0 similar to 64.7, average of 20.4), lower TiO(2) (0.30% similar to 0.95%, average of 0.62%) and CaO contents(0.05% similar to 6.05%, average of 0.63%). On the trace and rare earth elements, these samples are similar to the upper crust, which is characterized by rich in large ion radius elements and LREE, and obvious negative Eu anomaly. Geochemical characteristics indicate that their sedimentary environment during the Cambrian to Ordovician period belongs to a shallow sea in the passive marginal setting, and their source area was principally of upper crustal dominated by feisic rocks. Geochemical results and geological facts make us ensure that the study area was of a shallow sea-bathyal basin rather than oceanic basin during Early Paleozoic period because ophiolite and volcanic rock were never found in the Early Paleozoic outcrop areas, nor obvious Ce anomaly and carbonaceous rocks are characterized by lower delta(13)C(org). values (- 30.2 parts per thousand similar to -25.4 parts per thousand), implying a stable terrestrial environment. Moreover, plenty of shallow sea sedimentary structures are widely developed in the sedimentary rocks including ripple mark, load mark, foreset bedding, oblique bedding, olistostrome and lenticular limestone intercalations. This shallow sea basin was formed during Cryogenian by a splitting event, conducting that the Cathaysian old land was disrupted into several blocks such as the Wuyi, Central-Southern Jiangxi and Yunkai blocks. The shallow sea basins and troughs were yielded among these blocks.