西藏林周盆地林子宗群火山岩的精细年代框架

U-Pb AGE FRAMEWORK OF THE LINZIZONG VOLCANIC ROCKS FROM THE LINZHOU BASIN, TIBET

林子宗群火山岩是一套出露于拉萨地块南缘的冈底斯弧之上的未变形的新生代火山岩，是研究拉萨地块在新特提洋北向俯冲到印度一欧亚大陆碰撞的转换过程中古地磁位置、古高程变化及岩浆演化的窗口。位于拉萨市北部约30km的林周盆地是一个火山沉积盆地，因其火山岩剖面完整、岩组之间界限清晰而被视为研究林子宗群火山岩的理想地区。本文选取林周盆地林周县典中村北和强噶乡北两条经典的林子宗群火山岩剖面进行详细的地层学及岩石学研究，并在剖面关键位置挑选18件样品进行了详细的岩石薄片镜下观察、锆石阴极发光及U-Pb年代学研究（LA—ICP-MS）。结果表明：厚约3500m的林子宗群火山岩是一套火山-沉积岩相的地层，自下而上可分为典中组（E1d）、年波组（E2n）和帕那组（E2P）3个组，其中帕那组又可以分为帕那组一段（E2p^2）和帕那组二段（E2p^2）两个岩性段。典中组主要岩性为玄武安山质熔岩，年龄范围为66～59Ma；年波组主要为一套河湖相泥岩、砂岩及砾岩等碎屑沉积岩，夹有少量的湖相灰岩及薄层凝灰岩，并被后期基性岩墙切割，年龄范围为56～54Ma；帕那组一段由具柱状节理的流纹质安山岩及安山质熔结凝灰岩组成，年龄范围为54～50Ma；帕那组二段则主要由河流相沉积夹数层熔结凝灰岩组成，年龄范围为为49～47Ma。结合本区已有的年龄数据和地质资料，根据上覆典中组底部火山岩的年龄，进一步限定林周地区林子宗群火山岩与下伏白垩纪设兴组地层之间区域不整合的上限时代为66Ma。典中组与年波组之间存在一个平行不整合，为3Ma的火山活动休眠期，而年波组和上覆帕那组之间为连续过渡。

The Linzizong Group is mainly a suit of undeformed Cenozoic volcanic rocks that crop out along the Gangdese arc in southern Tibet. It has been used by many researchers to study the paleolatitude, paleoelevation, and magmatic evolution of the Lhasa terrane during the transition from the northward subduction of the Neo-Tethyan oceanic lithosphere to the India continent. Therefore, establishing the precise age framework of the Linzizong volcanic rocks is valuable for constraining the timing of the geological events above. The Linzhou Basin, located ca.3Okm north of Lhasa in Linzhou County, is a volcanic-sedimentary basin which is deemed to be an ideal site of the Linzizong Group with well-exposed interlayered volcanic-sedimentary sections and a clear unconformity atop the underlying folded Cretaceous and older rocks. In this study, we present detailed descriptions and age constraints of interlayered volcanic-sedimentary. In the Linzhou Basin, the Linzizong volcanic rocks consist of ca. 3500m of volcanic-dominated strata that were separated into four units from the base to the top. The Paleocene Dianzhong Formation （Eld） is chiefly composed of basaltic and andesitic lava flows; the Eocene Nianbo Formation （E2n） consists of fluvial sandstones interbedded with minor lacustrine limestones and tufts, cut by mafic dyke swarms; the lower Eocene Pana Formation （E2p^1 ） is mostly comprised of rhyolitic andesites and andesitic ignimbrites with columnar joints, and the upper Eocene Pana Formation （E2p^2） is mostly composed of fluvial deposits and several interbedded andesitic ignimbrites. After the stratigraphic and petrological study, we conducted systematic U-Pb zircon dating on 18 selected samples （one sandstone and the rest volcanic rocks） taken from the Dianzhong and Qiangga sections crossing the Linzizong volcanic rocks exposed in the Linzhou Basin. The U-Pb dating yielded an age range of ca.66 to 47Ma for the Linzizong volcanic rocks. The Dianzhong Formation （E^d） , Nianbo Formation （E2n）, lower Pana Formation （E2p^1） and upper Pana Formation （E2p2） were formed during ca.66-59Ma, 56-54Ma, 54-50Ma and 49-47Ma, respectively. The oldest age （ca.66Ma） of the Dianzhong Formation marks the upper limit for the regional angular unconformity against the underlying Upper Cretaceous Shexing Formation. This indicates that the strong deformation ended before ca.66Ma ago. There is a ca.3Ma magmatic gap between the Dianzhong Formation and the Nianbo Formation and an uninterrupted volcanic-sedimentary process from the Nianbo Formation to the upper Pana Formation.