柴北缘锡铁山花岗斑岩锆石U—Pb年代学、地球化学及Hf同位素

对锡铁山北东侧的花岗斑岩体进行锆石U—Pb年代学、岩石地球化学及Hf同位素研究。通过LA—ICP—MS锆石U—Pb同位素测年,获得花岗斑岩的成岩年龄为373. 9±2. 2 Ma (MSWD=0. 069),属于晚泥盆世岩浆活动的产物。岩石地球化学特征显示岩石富硅(SiO_2=76. 33%～76. 99%)和碱(K_2O+Na_2O=7. 28%～8. 19%),低钙(CaO=0. 58%～0. 75%)、镁(MgO=0. 20%～0. 31%)和Mg~#值(Mg~#=23. 29～30. 17),A/CNK介于0. 97～0. 98,属于准铝质钙碱性岩石系列。岩石相对富集Rb、Th、U等大离子亲石元素,亏损Nb、Ta、Ti、P等高场强元素及Ba、Sr等部分大离子亲石元素。稀土元素配分曲线呈右倾型,轻稀土元素分馏明显,重稀土元素分馏较弱,且相对富集轻稀土元素,亏损重稀土元素,显示明显的负Eu异常(δEu=0. 36～0. 43)。锆石Hf同位素ε_(Hf)(t)比值为+6. 6～+9. 6,二阶段模式年龄T_(DM2)为760～950 Ma。综合岩石地球化学及同位素的研究表明,锡铁山花岗斑岩为高分异的I型花岗岩,岩浆源区主要为起源亏损地幔的新元古代新生地壳物质的部分熔融。结合区域构造背景和前人研究成果,认为本文锡铁山花岗斑岩形成于柴达木地块与祁连地块碰撞后伸展的大地构造环境。

Zircon U-Pb geochronology,geochemistry and Hf isotopic of granite porphyry in Xitieshan area of north margin of Qaidam Basin

The U-Pb chronology, geochemistry and Hf isotopic data of granite porphyry in the northeast Xitieshan area were studied. The results of zircon U-Pb geochronology and geochemical data for the granite porphyry show that the weighted mean age of the rock is 373.9±2.2 Ma (MSWD=0.069), suggesting that the intrusion was emplaced in Late Devonian. Petrogeochemically, the granites are characterized by high content of SiO2 (76.33%~76.99%) and (K2O+Na2O) (7.28%~8.19%), low content of CaO (0.58%~0.75%), MgO (0.20%~0.31%) and Mg# (23.29~30.17). The aluminum saturation index (A/CNK) is 0.97~0.98, belonging to metaluminous calc-alkaline rock series. The samples are enriched in large ion elements (Rb, Th, U), depleted in high field strength elements (Nb, Ta, Ti, P) and partial large ion elements (Ba, Sr). In the REE distribution patterns, the samples show a right inclined LREE/HREE pattern. The fractionation of LREE is obvious, while the fractionation of HREE is weak. And the granite porphyry is rich in LREE and poor in HREE, with obviously negative Eu anomalies (δEu=0.36~0.43). The εHf(t) values of the granite porphyry range from +6.6 to +9.6, and two-stage Hf model ages (TDM2) range from 760 to 950 Ma. Integrating the study of geochemistry and isotopes, the granitic rocks belong to highly fractionated I-type granites, and the magma was originated from the partial melting of Neoproterozoic juvenile accreted lower crust that was derived from the depleted mantle. Combining with the regional tectonic background and previous research, it is concluded that the granite porphyry from Xitieshan were formed in extensional tectonic setting after the collision between Qaidam block and Qilian block.