新疆卡拉麦里上-中泥盆统间角度不整合和346.8Ma后碰撞火山岩的意义

Geological Implications of Unconformity between Upper and Middle Devonian,and 346.8Ma Post-Collision Volcanic Rocks in Karamaili,Xinjiang

在东准噶尔卡拉麦里地区的五彩湾一带出露一套具磨拉石特征的火山沉积建造,下部为具磨拉石特征的砾岩、砂岩,上部为一套中基性的火山熔岩夹中酸性的火山凝灰岩.1∶20万卡拉麦里山幅将其归入下石炭统松喀尔苏组.通过1:5万地质调查研究发现,该火山沉积建造底部以一套粗砾岩高角度不整合于下-中泥盆统卡拉麦里组之上,含晚泥盆世植物化石Prelepidodendronsp.(先鳞木),中上部火山岩LA-ICP-MS锆石U-Pb年龄为346.8±3.3Ma,且被年龄为341.1±4.0Ma^340.9±5.1Ma后碰撞花岗岩侵入,表明其形成时代为晚泥盆世-早石炭世,时代上对应于北准噶尔地层分区的上泥盆统克安库都克组.该套地层中上部的火山岩的岩石组合为玄武岩、玄武安山岩、夹少量的流纹质凝灰岩,岩石化学特征上属钙碱性-高钾钙碱性系列,(La/Yb)N=2.97~6.66,Nb、Ta亏损,部分样品Zr、Ti弱显亏损,Nb/U、Ce/Pb比值分别为7.43~20.88、3.17~12.45.地球化学特征表明其兼具板内火山岩和弧火山岩的某些特点,形成于后碰撞伸展环境,是卡拉麦里洋盆于晚泥盆世之前闭合后后碰撞岩浆活动的产物.这一研究成果对广为关注的卡拉麦里洋盆的闭合时间进行了很好的限定.

A set of volcanic sedimentary formation outcrops in Wucaiwan, Karamaili area, eastern Junggar, composed of con- glomerate, sandstone characterized by molasse in its lower part, and intermediate-basic lava interbedded with intermediate- acidic tuff in its upper part. The volcanic sedimentary formation is categorized into the Lower Carboniferous Songkaersu Forma- tion in 1 ： 200 000 Karamaili Mountain report. The 1 ： 50 000 geological survey finds that a set of coarse conglomerate in the bottom high angle unconformably overlies the Early-Middle Devonian Karamaili Formation and contains plant fossil Prelepidodendron sp. of Late Devonian to Early Carboniferous. The volcanic rocks in the middle-upper part are intruded by post- collision granite of 341.1±4. 0 Ma to 340. 9±5.1 Ma, and its zircon U-Pb age is 346. 8±3. 3 Ma obtained by LA-ICP-MS. It indicates that the age of the volcanic sedimentary formation is Late Devonian-Early Carboniferous which might correspond to the Upper Devonian Keankuduke Formation in northern Junggar stratigraphic group. The volcanic rocks consist of basalt, basaltic andesitc, and a handful of rhyolitic tuff, belonging to calc-alkaline and high K calc-alkaline series;and they are enriched in LILE and LREE （ （La/Yb）N =2.97-6.66）, depleted slightly in Nb, Ta, and depleted weakly in Zr, Ti in some samples, with Nb/U=7.43--20. 88, Ce/Pb=3. 17-12. 45. These volcanic rocks show both intraplate and are volcanic rock geochemistry characteristics, formed in post-collision extensional period after the elosure of Karamaili Ocean before the Upper Devonian. This study determines the final amalgamation time of Karamali Ocean, an issue of wide concern.