黔北普宜地区晚三叠世二桥组砂岩碎屑锆石U- Pb年龄、重矿物分析及地质意义

普宜地区位于上扬子腹地——四川盆地东南缘，区内晚三叠世碎屑岩记录了上扬子前陆盆地演化信息，是研究盆地东缘盆山耦合的理想对象。二桥组主要由三角洲平原- 三角洲前缘相砂岩组成，交错层理恢复的物源主要来自东侧，砂岩岩屑主要由岩浆岩岩屑和变质岩岩屑组成，碎屑重矿物主要由锆石、电气石、金红石、白钛石、黄铁矿、锐钛矿、石榴子石、铬铁矿等组成。本文作者使用电子探针和LA- ICP- MS测定技术，测定了二桥组砂岩中电气石的化学成分和锆石U- Pb同位素组成，进而研究二桥组物源体系，探讨晚三叠世上扬子盆地演化。电子探针测得的化学成分显示，电气石主要为镁电气石和黑电气石，来自贫锂花岗岩、变质砂岩、板岩等。测得的锆石U- Pb年龄显示，二桥组碎屑锆石主要存在2662~2380 Ma，2077~1688 Ma，840~722 Ma，475~412 Ma，293~246 Ma五组年龄峰值，前两组年龄峰值所占比例最大，表现为多物源特征，结合砂岩碎屑组成和电气石成分推测物源主要来自江南造山带梵净山地区新元古代再旋回沉积区，扬子北部崆岭地区可能提供部分物源，区内加里东期火山碎屑岩和印支- 海西期峨眉山玄武岩及同期侵入岩、凝灰岩提供少量物源。综合研究表明晚三叠世上扬子前陆盆地前缘隆起带持续向南东迁移，中晚期江南造山带成为前缘隆起带，为盆地东南部提供主要物源。

Implications of detrital zircon U- Pb ages and analysis of heavy minerals from sandstone of the Late Triassic Erqiao Formation in Puyi area, North Guizhou

The Puyi area is located in the Middle of upper Yangtze, southeast margin of Sichuan basin. The Late Triassic clasolite record the evolution information of the upper Yangtze foreland basin, which is an ideal object for studying the coupling relationships between sedimentary basins and orogenic belts in the eastern margin of the basin. The Erqiao Formation is mainly composed of delta sandstone. The provenance of cross bedding restoration is mainly from the East. Sandstone detrital are mainly composed of magmatic detrital and metamorphic detrital. Detrital heavy minerals are mainly composed of zircon, tourmaline, rutile, albite, pyrite, garnet, chromite and anatase. In this paper, we determined the zircon tourmaline chemical composition and U- Pb isotope from sandstone of the Erqiao Formation by means of electron microprobe and LA- ICP- MS, further studied the provenance of the Erqiao Formation, and discussed the evolution of the upper Yangtze basin in Late Triassic. The chemical composition of tourmaline shows that it is mainly composed of magnesium tourmaline and black tourmaline, which comes from granite, meta- sandstone, slate etc. The zircon U- Pb ages show detrital zircons of the Erqiao Formation mainly have five peaks at 2662~2380 Ma, 2077~1688 Ma, 840~722 Ma, 475~412 Ma and 293~246 Ma. The first two age peaks account for the largest proportion, showing the characteristics of multiple provenances. Combined with sandstone detrital composition and tourmaline chemical composition, it is inferred that the provenance is mainly from the Neoproterozoic recycle sedimentary area in Fanjingshan area of the Jiangnan orogenic belt. Some provenances may be provided by Kongling area in the north of Yangtze, and a small amount by the Caledonian pyroclastic rocks, the Indosinian- Hercynian Emeishan basalt and contemporaneous intrusive rocks and tuffs. It shows that the front uplift belt of the upper Yangtze foreland basin continued to move southeast in the Late Triassic, and the Jiangnan orogenic belt became the front uplift belt in the middle and late stage, providing the main provenances for the southeast of the basin.