云南鲁史三合洞组地层时代讨论

云南兰坪盆地三合洞组的时代存在争议,有早古生代和中生代两种观点。本文通过对云南鲁史上三叠统三合洞组和挖鲁八组岩石地层、古生物化石、微体化石及碎屑锆石U-Pb年代学研究,认为三合洞组时代为晚三叠世。本区三合洞组生物碎屑灰岩、泥晶灰岩及白云质灰岩与挖鲁八组炭质板岩呈整合接触关系,与歪古村组砂质板岩呈逆断层接触关系,整体表现为倒转地层。挖鲁八组炭质板岩中采获叉皱海燕蛤、虚海燕蛤等晚三叠世化石,而三合洞组灰岩中的小欣德牙形刺、奥泽克刺、滑梯螺、长卵形槽花介等微体化石时代也为晚三叠世。笔者从挖鲁八组粉砂岩中获得碎屑锆石U-Pb年龄,其最小年龄为218 Ma,指示挖鲁八组的最晚沉积时代为晚三叠世。综合生物化石和同位素年代学资料,认为兰坪盆地三合洞组的时代为晚三叠世。

An approach to the stratigraphic ages of the Upper Triassic Sanhedong Formation in Lushi, Yunnan

The Upper Triassic Sanhedong Formation in Lushi, Yunnan occurs as the main host horizons of the Jinding supergiant lead-zinc-silver deposit and Weishanzhacun gold deposit. However, there is still debate about the stratigraphic ages of the Upper Triassic Sanhedong Formation. The authors contend, according to lithostratigraphic division, organic fossils, microfossils and zircon U-Pb ages, that the bioclastic limestones, micritic limestones and dolomitic limestones in the Sanhedong Formation exhibit conformable contacts with the carbonaceous slates in the Waluba Formation, and thrust fault contacts with the sandy slates in the Waigucun Formation. The organic fossils including Halobia cf. Superbescens Kittl, Halobia fallax Mojsisovics, Posidonia sp., Halobia yandongensis chen, Halobi arugosoides Hsu, Halobi symmetrica Smith, Halobia cf. ganziensis che, Halobi moluccana J. Wann., Halobi alaskana Smith, and Mytilus sp. from the carbonaceous slates in the Waluba Formation and the microfossils including Hindeodus parvus, Ozarkodina sp., Eucycloscala sp., and Suicocythere vongovata Wei from the limestones in the Sanhedong Formation are both dated at the Late Triassic in age. In addition, the detrital zircons from the siltstones in the Waluba Formation give a minimum U-Pb age of 218 Ma, indicating that the deposition of the Waluba Formation may be traced back to the Late Triassic. It is concluded that the age of the Sanhedong Formation should be the Late Triassic in age.