昌宁-孟连结合带东部泥盆系—石炭系南段组研究新进展及其对特提斯洋演化的启示

New progress in the study of the Devon-Carboniferous Nanduan Formation in the eastern Changning-Menglian Belt and its implications for the evolution of the Tethys Ocean

关键词:  碎屑锆石年代学  沉积环境  地球化学  特提斯  冈瓦纳大陆

Key words:  detrital zircon geochronology  sedimentary environment  geochemistry  Tethys  Gondwanaland

摘要:

原—古特提斯洋之间的构造转换方式一直是国际国内研究的热点与难点。南段组是昌宁-孟连结合带东侧地层重要组成部分，其沉积时代、沉积环境和与特提斯演化的关系均存在争议，制约了对昌宁-孟连结合带乃至东特提斯洋演化特征的认识和理解。本文对南段组变质砂岩样品开展了岩石学、岩相学、元素地球化学以及碎屑锆石U-Pb年代学研究工作，结果表明其为泥盆纪—石炭纪被动大陆边缘的浅海沉积，物源来自成熟的大陆。南段组碎屑锆石年龄谱可识别出560Ma、950Ma和1130Ma三个明显的年龄峰值，与来自澳大利亚北缘的拉萨地体上古生界碎屑锆石年龄谱可以对比。南段组物源特征具有亲冈瓦纳大陆属性，但不能据此限定所在地体泥盆—石炭纪古地理位置。保山地块东缘泥盆系曼信组碎屑锆石年龄谱可识别出440Ma和950Ma两个年龄峰值，与南段组存在显著区别。本文新获得昌宁-孟连特提斯洋两侧泥盆纪碎屑锆石年龄谱特征，为原特提斯洋与古特提斯洋之间为连续演化提供了新的沉积学证据。

Abstract:

The tectonic transformation from the Proto-to the Paleo-Tethys Ocean has always been a topic of heated debates amongst both domestic and international geological communities. The Nanduan Formation constitutes an important part of the sedimentary strata on the eastern side of the Changning-Menglian Belt. Its depositional age and environments as well as its relation to the evolution of the Tethys Ocean remain controversial, which inhibits the reconstruction the evolution of the Changning-Menglian Ocean and by extension of the East Tethys Ocean. In this contribution, petrological and petrographic characterizations, major and trace element geochemical analyses, and U-Pb dating of detrital zircons were carried out on the metamorphic sandstone samples of the Nanduan Formation in attempts to put the above-mentioned controversy to rest and to provide constraints on the evolution of the Tethys Ocean. The results show that the Nanduan Formation was deposited in a neritic setting on a passive continental margin during the Devonian-Carboniferous, and originated from a mature continental source. Three distinct age peaks, of 560 Ma, 950 Ma and 1130 Ma respectively, have been identified in the detrital zircon age spectrum of the Nanduan Formation, which can be correlated with the upper Paleozoic detrital zircon age spectrum of the Lhasa terrane from the northern margin of Australia. This in turn indicates an affinity of the Nanduan Formation to the Gondwanan continents. However, the paleo-geographical location of the terrane during the Devonian-Carboniferous transition remains to be constrained. The two age peaks of 440 Ma and 950 Ma identified from the Devonian Manxin Formation on the eastern margin of the Baoshan block differ significantly from those in the Nanduan Formation. The newly-obtained Devonian detrital zircon age spectrums on both sides of the Changning-Menglian Ocean in this study, therefore provide strong sedimentological evidences for the continuity of the Proto-and the Paleo-Tethys Oceans.