Discovery of Jurassic ammonoids from the Shyok suture zone to the northeast of Chang La Pass, Ladakh, northwest India and its tectonic significance

Abstract   Callovian (late Middle Jurassic) ammonoids Macrocephalites and Jeanneticeras were recovered from the Shyok suture zone, northeast of Chang La Pass, Ladakh, northwest India. They are the first reliable Jurassic fossils and the oldest chronologic data from the Shyok suture zone. The ammonoid-bearing Jurassic strata, newly defined as the Tsoltak Formation, consist largely of terrigenous mudstone with thin sandstone beds and were probably a part of the continental basement to the Cretaceous Ladakh Arc.     

锆石He扩散模型与中国典型克拉通盆地古老层系热史重建

沉积盆地古老层系已成为我国油气勘探的重要领域,但由于缺乏有效古温标,其热史研究一直很薄弱.本文在系统综述锆石He扩散动力学模型及差异性基础上,探讨了锆石(U-Th)/He热定年技术在我国克拉通盆地古老层系热史恢复中的适用性.目前常用的锆石He扩散动力学模型包括简单模型、锆石辐射损伤积累和退火模型及扇形-线性模型.基于不同温度路径模拟发现,锆石辐射损伤积累和退火模型和扇形-线性模型在约束低温热史时具有较强的一致性,而在约束高温热史(＞200℃)时,则具有明显的差异性.塔里木盆地、四川盆地和华北克拉通盆地的前寒武系样品单颗粒锆石(U-Th)/He年龄均小于地层年龄,且与有效铀浓度呈负相关性,反映了辐射损伤是锆石He年龄分散的主因.基于正、反演热史模拟明确了三大盆地前寒武系样品自沉积以来的古温度史,并揭示出它们曾经历过160~185℃的高温热历史,这对深层、古老层系烃源岩成熟演化和资源评价具有重要的科学意义.