甘肃敦煌党河水库TTG地球化学特征、锆石SHRIMP U-Pb定年及其构造意义
Geochemical characteristics and SHRIMP U-Pb age of zircons from the Danghe reservoir TTG in Dunhuang, Gansu Province, and its significations
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摘要: 甘肃敦煌党河水库岩体以花岗闪长岩为主,并构成英云闪长岩-花岗闪长岩-奥长花岗岩的TTG组合。锆石SHRIMP U-Pb年为440±12Ma (MSWD=2.5),代表TTG系列岩石的侵位时代。岩石化学成分中SiO2含量约为57.02%~72.75%,随着SiO2含量的增加,TiO2、Al2O3、MgO、CaO和FeO等的含量相应下降,这表明该套岩石具岩浆成因,是角闪石和斜长石的分离作用控制下岩浆分异的产物。岩石的稀土元素总量较低,轻重稀土分异明显,(La/Yb) N＝4.70~58.88,无或略具微弱的Eu异常,近于平行的稀土元素配分曲线表明所有岩石具有同成因的亲缘关系。岩石富集大离子亲石元素Rb、K、Th、Sr等,亏损Nb和Ta等高场强元素以及P和Ti。在Rb/30-Hf-3×Ta和Rb/30-Hf-0.25×Nb图解上,所有岩石的投影点落到岛弧到后碰撞区。但从区域地质背景、TTG的产出环境、形成时代及与敦煌地块南北缘火成岩对比分析,认为该TTG系列岩石并非形成于岛孤构造环境,而是加里东造山作用晚期(约440Ma)红柳河缝合带和(或者)阿尔金北缘红柳沟拉配泉俯冲碰撞带,引发富含角闪石的下地壳源岩部分熔融形成岩浆,上侵过程中同化部分陆壳物质形成具有岛弧性质的岩浆岩。Abstract: The Danghe reservoir intrusive rocks are compositionally comparable to the tonalite – trondjemite – granodiorite association (TTG), which of the principle component is the granodiorite, and have the SiO2 contents ranging from 57.02 to 72.75%. The SHRIMP U-Pb age of zircons from TTG is 440±12Ma (MSWD=2.5), which is the intrusive age of TTG magma. The concentrations of TiO2, Al2O3, MgO, CaO, FeO and P2O5 decrease with the increase in the SiO2 contents, showing negative correlations, and implying a magmatic differentiation model for the origin of the TTG controlled by hornblende and plagioclase fractionation. The TTG have lower contents of ∑REE with strong fractionation of LREE/HREE ((La/Yb) N＝4.70~58.88) and no or slightly Eu anomaly. On C1 chondrite normalized REE fractional diagram, the subparallel patterns indicate a cogenetic relation for all samples. The concentrations of large ion lithophile elements (LILE) such as Rb, K, Th, Sr are enriched, whereas the high field strength elements (HFSE) (e. g. Nb and Ta) as well as P and Ti contents are depleted. On Rb/30-Hf-3×Ta and Rb/30-Hf-0.25×Nb diagrams, all plots set in the area from island arc type to post collision type. All of these imply that the TTG is of the calc-alkaline association forming in a volcanic arc. The regional geological setting, tectonic setting and formation times of the TTG, and contrasting analysis of igneous rocks from the south and north fringe of the Dunhuang terrain indicate that TTG rocks didn’t form in island arc tectonic environment, but was produced by partial melting of the hornblende-rich lower crust under the condition of plate collisions along the northern and (or) southern margins of Dunhuang block in Late Caledonian (440Ma).
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Key words:
- Dunhuang /
- Danghe reservoir /
- TTG /
- SHRIMP U-Pb data of zircon /
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