新疆可可托海3号伟晶岩脉锆石U-Pb定年、Hf同位素特征及地质意义

Geochronology and Hf isotope of zircon for Koktokay No.3 granitic pegmatite in Xinjiang and its geological implications

研究了新疆可可托海3号伟晶岩脉多个结构带中锆石U-Pb定年及锆石Hf同位素组成,旨在厘定3号伟晶岩脉形成及岩浆-热液演化的时限,探讨岩浆形成的物源特征及大地构造背景。锆石的BSE、CL特征显示,可可托海3号脉岩浆结晶锆石普遍遭受了蜕晶化、流体交代和不同程度重结晶等作用。锆石的LA-ICP-MS U-Pb定年结果显示,3号伟晶岩脉岩钟体部分中II带、IV带和V带以及缓倾斜部分中II带分别形成于(211.9±3.2)、(214.9±2.1)、(212±4.1)和(212.0±1.8) Ma,指示岩浆阶段时限从～220 Ma至215 Ma,岩浆-热液过渡阶段时限从215 Ma至209 Ma,热液阶段时限从209 Ma至195 Ma。3号伟晶岩脉岩钟体部分中IV带和V带锆石Hf同位素组成变化于+1.25～+2.39之间,二阶段模式年龄变化于1103～1173 Ma之间,指示形成3号伟晶岩脉的岩浆很可能与中亚造山带在三叠纪为陆-陆碰撞后的伸展构造背景下导致加厚下地壳沉积物减压熔融有关。

This study presents zircon U-Pb dating and Hf isotope of the several textural zones of the Koktokay No.3 pegmatite,so as to declare the intrusion age and evolution time limitation of the pegmatite,and explore the geochemical characteristics of the source and tectonic setting of the pegmatite-forming magma.The results show that zircons from the studied pegmatite are commonly suffered from metamictization,fluid metasomatism and recrystallization based on back-scattered electron(BSE)and catholuminescence(CL)images.The result of zircon LA-ICP-MS U-Pb dating shows that the formation ages of II,IV and V zones from the“cupola”part and II zone from the plate part of the Koktokay No.3 pegmatite are(211.9±3.2),(214.9±2.1),(212.0±4.1)and(212.0±1.8)Ma,respectively,indicating that the time limitations for magmatic stage,magmatic-hydrothermal stage and hydrothermal stage last from about 220 Ma to 215 Ma,from 215 Ma to 209 Ma,and from 209 Ma to 195 Ma.The zircon Hf isotopic compositions of IV and V zones from the cupola part of the pegmatite are varied in a narrow range(in the range of+1.25^+2.39),with two-stage model ages of 1103-1173 Ma,suggesting that the petrogenesis of the pegmatite-forming magma was probably associated with decompression melting of thickened lower crust under continent-continent post-collisional extensional setting of the Central Asia Organic Belt(CAOB)in Triassic.