贵州省梵净山区新元古代岩浆活动的年代学格架及其大地构造意义

江南造山带位于华夏与扬子地块之间,发育大量新元古代火成岩,是研究地球早期壳幔相互作用、板块构造演化的理想对象。江南造山带东段新元古代板块演化研究已初具雏形,但对其西南段构造演化还争议不断,主要问题在于没有构建与区域地质特点吻合的岩浆活动年代学格架。本文以江南造山带西南段梵净山为研究区,基于详细的区域地质填图结果,针对侵入于梵净山群(沉积时代850~830 Ma)中的基性-超基性岩样品开展原位锆石年代学研究。新的年代学数据表明,侵入于梵净山群的3个辉长岩体结晶年龄分别为813 Ma、804 Ma、748 Ma。结合前人发表数据,提出梵净山地区岩浆活动可划分为两个阶段:晋宁Ⅰ期(850~825 Ma)、晋宁Ⅱ期(820~750 Ma),分别对应于梵净山期、下江期盆地演化。该岩浆岩年代学格架与区域地层年代格架吻合,形成了完整的新元古代沉积-岩浆活动记录。这套沉积-岩浆组合的大地构造背景尚不明确,需要通过大地构造相分析予以确定。

Neoproterozoic geochronologic framework of magmatism in Fanjingshan area and its tectonic implications

The Jiangnan orogen is located between the Cathaysian and Yangtze blocks, where numerous Neoproterozoic igneous rocks have been identified, which provide a chance to study the mantle-crust interaction and plate tectonics of the earth''s earlier history. The tectonic evolution of the eastern segment of the Jiangnan orogen seems to be clear, but the tectonic evolution of its southwestern segment remains controversial mainly due to absence of field relationship-consistent geochronologic framework. The Fanjingshan area is located in the northern portion of the southwestern segment of the Jiangnan orogen. 1:50000 geological mapping of that area revealed detail field relationship between different geological units, thus it is an ideal area to construct a reasonable geochronologic framework. This paper reports four in- situ zircon U/Pb dating results of mafic to ultramafic plutons which intruded the Neoproterozoic Fanjingshan Group. (deposition time 850- 830 Ma). The new data obtained by the authors demonstrate that the three gabbroic plutons crystallized at 813 Ma, 804 Ma, and 748 Ma, respectively. Combined with previously published geochronologic data, the authors hold that the magmatism that occurred in the southwestern segment of the Jiangnan orogen consisted of two stages: the Jinning-I stage (850 to 825 Ma) and the Jinning-II stage (820 to 750 Ma), which correspond to the sedimentation time of Fanjingshan and Xiajiang Groups, respectively. Between them is an angular unconformity. Such a magmatic geochronological framework is consistent with that of the sedimentary rocks. The tectonic setting of the Neoproterozoic tectonostratigraphy is not very clear due to the fact that regional tectonic setting remains unclear.