云南曲靖盆地蔡家冲粗面英安质凝灰岩——扬子克拉通内曲靖深断裂新生代强烈活动的证据

云南曲靖盆地内的蔡家冲火山岩作为扬子克拉通内部深断裂受青藏高原向东南逃逸而影响强度的标志之一,具有重要意义。在该区新发现的新生代火山岩的斑晶成分主要为正长石和少量黑云母,为钾质火山岩(粗面质火山岩);结合火山岩的全岩成分、微量元素分析,确定蔡家冲火山岩为粗面质火山岩(超钾质火山岩),经湖相水解后K、Na等碱质组分部分流失,导致该区火山岩在火山岩分类图解中落入碱质稍低的花岗闪长岩区域,其本质是水解的粗面质火山岩;全岩K/Ar同位素测年显示:火山岩活动时间范围在44.9±0.8~48.4±0.9Ma之间;火山岩下部至上部年龄差大致为3.5Ma,属于古近纪始新世路特阶的产物。该区的新生代火山岩反映出,曲靖深断裂的走滑拉分不仅形成了曲靖、陆良、弥勒、开远等一系列盆地,其活动强度还导致新生代始新世路特阶的蔡家冲组火山岩的喷发,曲靖深断裂同小江深断裂一样,新生代活动显示为超壳深断裂。火山岩显著富集Cd、Pb、Zn等元素,综合考虑东川播卡等地深部发现同时期的富碱侵入岩及金矿床,该区火山岩显示的金矿、铅锌矿的找矿潜力值得关注。曲靖深断裂还与地震活动、热水资源、农业生态环境、油气资源、固体矿产等密切相关,值得深入研究。

Trachytic dacitic tuffs in Caijiachong, Qujing basin, Yunnan Province: Evidence for a strong activity of the Qujing deep fault within the Yangtze Craton

As a symbol of the influence extent of the deep faults within the Yangtze Craton by the southeastward escape of the Tibetan Plateau, the volcanic rocks in the Qujing basin are of great research importance. By field observing and sampling, the Cenozoic volcanic rocks were found for the first time in this area. it is confirmed under the microscope that the volcanic rocks are trachytic (ultrapotassic) dacitic tuffs with phenocrystals of orthoclase and biotite, and K and Na of the volcanic rocks may be partly leached by hydrolysis, leading to the falling of volcanic rocks in the region of granodiorite on the volcanic rock classification diagram, as also shown by major and trace elements data. K/Ar dating result shows that the volcanic rock activity lasted for about 3.5Ma, i.e., from 44.93±0.8Ma to 48.39±0.9Ma, suggesting the products of the Cenozoic Middle Eocene (Lutetian, 48.6±0.2~40.4±0.2Ma) volcanic activities. The volcanic rocks in Qujing basin show that the pull-apart of the Qujing deep fault not only formed a series of basins such as Qujing, Luliang, Mile and Kaiyuan, but also brought about the extrusion of the Cenozoic Eocene Lutetian Caijiachong Formation (E2c) volcanic rocks, which implies a strong activity of the Qujing deep fault; like the Xiaojiang deep fault which was responsible for the intrusion in Boka, Dongchuan, the Qujing deep fault was an utracrustal fault during Cenozoic. The volcanic rocks are significantly rich in such elements as Cd, Pb and Zn, indicating that the potentials of Au, Pb-Zn deposits related to the volcanic rocks are worthy of attention. In combination with the finding of the homochromous alkaline intrusion and gold deposits in Boka of the Dongchuan deep fault, the Qujing deep fault, which is closely related to the seismic activities, hot spring resources, agricultural and ecological environment, oil-gas resources, and solid minerals, deserves thorough investigation.