拉萨地体的起源和古生代构造演化

摘要

The history of breakup, drift, subduction, and collision of the Lhasa Terrane during the Paleozoic remains poorly understood because of the scarcity of data. This paper reviews the Mesozoic lithospheric architecture and explores the origin and Paleozoic evolution of the Lhasa Terrane on the basis of geological and geochemical data currently available. Abundant zircon Hf isotope and whole-rock Nd isotope data of silicic rocks indicate that the southern and northern Lhasa subterranes aredominated by juvenile crust with possible Precambrian crystalline basement locally preserved, while the central Lhasa subterrane is a microcontinental ribbon with ancient basement rocks as old as Archean. Over 3000 U-Pb ages of detrital zircons from Late Neoproterozoic-Paleozoic strata define an age peak of ca. 1170 Ma for the Lhasa Terrane and identical age peaks of ca. 950 Ma for the Amdo, Southern Qiangtang, and Tethyan Himalaya. Thus the most plausible explanation for the origin of the Lhasa Terrane is that it came from the northern Australia. The ca. 492 Ma bimodal metavolcanic rocks identified in the central Lhasa subterrane were likely developed in an active continental margin representing a part of a magmatic arc paleogeographically located in the northern Australian continent and may be a result of slab break-off of the subducting proto-Tethyan oceanic lithosphere. The Late Devonian to Early Carboniferous gneissic granitoids in both the southern margin of the Lhasa Terrane and Southern Qiangtang are S-type granitoids with significant contributions from mantle-derived materials. These rocks may have been emplaced in a back-arc setting that ultimately evolved to what is known as the Songdo Tethyan Ocean. The Lhasa-northern Australia collision at the end of the Middle Permian may have triggered the southward subduction of the Bangong-Nujiang oceanic lithosphere, imposing a critical role on the subsequent tectonomagmatic evolution of the Lhasa Terrane during the Mesozoic.%早期由于资料有限,对拉萨地体古生代时期的裂解、漂移、俯冲和碰撞历史的认知程度还很低.本文利用目前已有地质和地球化学资料,分析了拉萨地体的中生代岩石圈结构,探讨了拉萨地体的起源和古生代演化历史.大量长英质岩石的锆石Hf同位素和全岩Nd同位素表明,南部和北部拉萨地体以新生地壳为主,部分地区可能存在前寒武纪结晶基底,而中部拉萨地体是具有古元古代甚至太古代结晶基底的条带状微陆块.大量古生代沉积岩的碎屑锆石U-Pb年龄数据表明,拉萨地体约1170 Ma的碎屑锆石年龄指标,明显不同于以约950 Ma为碎屑锆石年龄指标的安多、羌塘和特提斯喜马拉雅.拉萨地体起源于澳大利亚大陆北缘是目前资料情况下的最合理解释.中部拉萨地体约492 Ma的双峰式火山岩形成于活动大陆边缘背景,代表了古地理上位于澳大利亚大陆北缘的岩浆弧的一部分,可能与原特提斯洋岩石圈板片的断离有关.拉萨地体南缘和南羌塘的泥盆纪末期一石炭纪早期片麻状花岗岩类为存在明显幔源物质输入的S型花岗岩,可能形成于最终演化为松多特提斯洋的弧后盆地背景.中二叠世末期发生的拉萨地体与澳大利亚大陆北缘的碰撞造山事件可能触发了班公湖一怒江特提斯洋岩石圈的南向俯冲,并随后对拉萨地体的中生代构造岩浆演化发挥关键性作用.