马拉山穹窿的活动时限及其在藏南拆离系-北喜马拉雅片麻岩穹窿形成机制的应用

本文报道吉隆北喜马拉雅地区马拉山穹窿核部浅色花岗岩的锆石SHRIMP U-Pb和白云母激光40Ar/39 Ar年代学研究.花岗岩U-Pb年龄显示,穹窿核部浅色花岗岩岩浆活动(深熔及侵位)发生于～30Ma至～17Ma,其中最年轻的U-Pb年龄(17Ma)以及花岗岩白云母40Ar/39Ar年龄(17～15Ma)指示了马拉山穹窿的最后岩浆侵位时间及可能的穹窿冷却事件.已有研究表明,北喜马拉雅片麻岩穹窿带(NHGD)与藏南拆离系(STDS)中浅色花岗岩具有相似的最早侵位年龄,即～35 Ma,而STDS下盘U-Pb年龄老于35Ma的浅色花岗岩为增厚地壳重熔成因,表明北喜马拉雅在 ～35Ma地壳构造体制由挤压转为伸展,并暗示在始新世-渐新世转换期可能存在一更广泛意义的地质事件.～35 Ma以前增厚导致中下地壳部分熔融,形成中下地壳渠道流,渠道流活动触发增厚造山楔的垮塌,形成STDS.STDS的伸展减薄引发更大规模浅色花岗岩侵位,花岗岩底辟作用形成了NHGD,本文最年轻U-Pb年龄及40Ar/39Ar年龄(17～15Ma)即代表马拉山的底辟与穹窿作用,之后的构造体制由东西向伸展所取代(始于～13Ma).

Geochronology of the Malashan dome and its application in formation of the Southern Tibet detachment system (STDS) and Northern Himalayan gneiss domes (NHGD)

This paper presents the geochronology of zircon SHRIMP U-Pb and muscovite laser ⁴⁰Ar/³⁹Ar dating on the leucogranite from Malashan dome in Northern Himalaya, Gyirong. The U-Pb ages of the leucogranite show that the magmatism (anatexis and emplacement) of leucogranite in core of the dome took place since ~30Ma to ~17Ma. The youngest U-Pb age of ~17Ma and the muscovite ⁴⁰Ar/³⁹Ar ages of 17~15Ma indicate the latest magmatic emplacement and the possible event of doming and cooling. Recent studies have shown that the lecougranite both in the Northern Himalayan gneiss domes (NHGD) and the Southern Tibet detachment system (STDS) had the same initial intruding ages of ~35Ma, while the leucogranite with U-Pb ages older than 35Ma beneath the STDS had an origin of re-melting of thickened crust. This indicates that the crustal tectonic regime in Northern Himalaya had changed from compression to extension at ~35Ma, which might hint a much more significant geological event at the Eocene-Oligocene transform. The thickening before ~35Ma caused the partial melting of middle-lower crust to form the "channel-flow", which triggered the collapse of the thickened orogenic wedge and caused the onset of the STDS. The extension and thinning along the STDS resulted in the larger-scale emplacement of the later leucogranite, diapirism of which formed the NHGD. The youngest U-Pb age and the ⁴⁰Ar/³⁹Ar ages (17~15Ma) represents the diapirism and doming of the Malashan dome. The tectonic regime following that time may give the way to the east-west extension (beginning since ~13Ma).