扬子陆核区白竹坪火山岩建造形成时代的重新厘定——来自LA-ICP-MS锆石U-Pb年代学的证据

ZIRCON U-PB AGE CONSTRAINTS ON THE CHRONOSTRATIGRAPHY OF THE BAIZHUPING FORMATION,NORTHERN YANGTZE BLOCK

分布于扬子陆核区北缘的白竹坪火山岩—次火山岩建造为一套变酸性凝灰岩、酸性晶屑岩屑凝灰岩、流纹岩(或安山流纹岩)、次流纹斑岩的岩石组合,部分层位变质为含黄铁矿绢云板岩、含黄铁矿纳长浅粒岩。前人依据地层接触关系,将其归于青白口系,作为扬子陆核区新元古代早期的沉积建造。于北崆岭大树垭地区该火山岩建造层型剖面上分别采集了含石榴变质流纹斑岩(次流纹斑岩)和黑云二长变粒岩(变酸性晶屑凝灰岩)两个样品,并进行了LA-ICP-MS锆石U-Pb同位素年代学测定。结果表明,次流纹斑岩加权平均年龄为1852±16 Ma,而黑云二长变粒岩的年龄一组为代表其成岩年龄的1856±24 Ma,另一组为加权平均年龄为1959±11Ma,属带入的围岩锆石年龄。因此,白竹坪火山岩建造的形成时代应归为古元古代,与华山观岩体、圈椅埫岩体、殷家坪基性岩脉等Columbia超大陆裂解事件记录基本同期。黑云二长变粒岩中带入围岩的锆石可能与黄陵地区巴山寺片麻岩为同一期次岩浆活动的产物。

The volcanic-subvolcanic Baizhuping Formation in the northern margin of the Yangtze block is mainly composed of metamorphosed felsic tufts, intermediate-felsic lavas, and felsic hypabyssal intrusive rocks. Due to regional metamorphism, some rocks have been transformed into sericite-bearing slate or al- bite-bearing leptynite with minor pyrite. On the basis of stratigraphic contacts, previous researchers regar- ded this suite of rocks as Neoproterozoic sedimentary rocks of the Qingbaikouan System. In this contribu- tion, we present LA-ICP-MS zircon U-Pb ages for the garnet-bearing rhyolite porphyry and biotite granu- lite from the Dashuya area. Zircon U-Pb dating of the garnet-bearing rhyolite porphyry yielded an age of 1852 -＋- 16 Ma （MSWD z 0.65）, whereas dates for the biotite granulite can be divided into two groups at 1856 ~ 24 Ma and 1959 -＋- 11 Ma, the older age probably coming from xenoliths. Geochronologic data suggest that the volcanic-subvolcanic Baizhuping Formation was formed in the Paleoproterozoic. We pro- pose that these rocks are related to break-up of the Columbia super-continent. In addition, we infer that the xenoliths in the biotite granulite have a geodynamic setting similar to the Bashansi gneiss.