摘要
塔克什肯口岸富碱侵入岩体是阿尔泰造山带典型的后造山岩体。本次锆石U-Pb定年给出^(206)Pb/^(238)U年龄286±1Ma(MSWD=0．05),代表其形成年龄。这为阿尔泰后造山岩浆作用提供了一个可靠的年代学证据。该岩体主要岩石类型为正长岩、石英二长岩、石英碱长正长岩、正长花岗岩,富钾、富钠、准铝,富集轻稀土、大离子亲石元素,亏损高场强元素,与区内相邻的碱性花岗岩(A型花岗岩)在岩石学、地球化学方面不同。该岩体Sr初始值变化于0．7038～0．7040,ε_(Nd)(t)值为正值(+6．2～+6．3),Nd模式年龄T_(DM)为542～546Ma,与中亚造山带典型的高(正)ε_(Nd)(t)值花岗岩类似。而且,其ε_(Nd)(t)值远高于区内同造山花岗岩,也高于造山带内部同时期的Ⅰ-A型后造山花岗岩。依据岩体构造特征、年代学、地球化学和区域地质背景综合分析,该岩体应为后造山岩体,在时空上可与蒙古南部碱性岩带对比,其形成可能与富集高场强元素的亏损地幔岩浆底侵,导致下地壳重熔,并发生岩浆混合有关。这说明,在阿尔泰造山带后造山阶段,除了可能的俯冲下埋的年轻洋壳或岛弧物质外,还有新的幔源物质加入到地壳。这为中亚造山带后造山阶段陆壳垂向生长提供了一个新证据。同时,也为东北北部-蒙古南部碱性岩带向西沿入阿尔泰造山带提供了证据。
The Takeshiken alkali-rich intrusion is a typical postorogenic pluton in Chinese Ahay orogen. Zircon U-Pb （TIMS） dating yield a ^206pb/^238U age of 286 ＋ 1Ma（ MSWD = 0. 05 ）, which is interpreted as its formation age. This pluton is composed of syenite with few quartz monzonite, quartz alkali-feldspar syenite and syenogranite. The rocks are enriched in K, Na, Al, REE and LILE （Rb, Th, Ba） with negative anomaly in Nb, Sr, Ti and P, different from the alkaline （A-type） granites in the same area. Their Isr values are between 0. 7038 - 0.7040, εNd （t） values between ＋ 6.2 - ＋ 6.3, and TDM between 542 - 546 Ma, which are similar to those of high （positive） εNd （t） value granites from the Central Asian Orogenic Belt. Importantly, their εNd （t） values are higher than those of synorogenic granites, and also slightly higher than those of coeval Ⅰ-A type post-orogenic granites in the central Altay. Integration analysis of field occurrence, geochronology, geochemistry and regional geology suggest that this pluton is post orogenic, and was probably related to the melting of lower crust due to the underplating of mafic magma derived from DM with enriched in LILE. This suggests that new juvenile mantle-derived materials, besides subducted juvenile ocean crust or arc rocks, probably entered into continental crust. Accordingly, this study provides evidences for vertical continental crustal growth in the Central Asian Orogenic Belt. In addition, our new data indicate that the alkali rock zone in the southern Mongolia and NE China extends to Ahay orogen, constituting a large scale alkali rock zone in the Central Asia .
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
2006年第5期1267-1278,共12页
Acta Petrologica Sinica
基金
国家重大基础研究规划项目(2001CB409802)
国家自然科学基金项目(40472101
49772105)
国际合作基金项目(40210647)资助.
关键词
锆石U-PB年龄
ND同位素
后造山
富碱侵入岩
中亚造山带
地壳生长
Zircon U-Pb age, Nd isotope, Postorogenic, Alkali-rich intrusive rock, Central Asian Orogenic Belt, Crustal growth
