摘要:
提要: 野马山岩基位于中祁连地块西段, 由早期岩体(花岗闪长岩、斑状二长花岗岩)和晚期岩体(二长花岗岩)组成, 二者呈侵入接触。LA-ICPMS 锆石U-Pb定年表明, 早期岩体侵位时代为中奥陶世((469.0±1.3) Ma), 晚期岩体侵位时代为晚奥陶世((450.0±1.0) Ma)。早期岩体SiO2=59.8%~64.2%, K2O/Na2O>1, 且A/NKC=0.8~1.0, 为准铝质岩石; 微量元素相对富集Rb、U、Th和亏损Ba、Nb、Ta、Sr、P、Ti; 稀土总量为97.7×10-6~185×10-6, 且(La/Yb)N=5.57~12.47, LREE/HREE=7.7~11.3, 具轻重稀土分馏明显, 轻稀土富集, 弱Eu负异常(δEu=0.66~0.89)特征。晚期岩体SiO2=69.8%~76.5%、K2O/Na2O=1.2~1.7、A/NKC=1.0~1.1, 属弱过铝质花岗岩; 稀土总量为78.97×10-6~244.92×10-6, 轻重稀土分馏不明显((La/Yb)N=1.90~5.72), 强Eu负异常(δEu=0.11~0.24)。岩石地球化学特征表明, 野马山岩基早期岩体为I型花岗岩, 形成于俯冲环境, 晚期岩体为高分异的I型花岗岩, 形成于后碰撞环境。结合岩体产出的区域构造位置及区域地质演化, 认为早古生代北祁连洋发生了双向俯冲, 野马山岩基为其向南俯冲碰撞的产物。
Abstract:
Abstract: Located in western Central Qilian Mountain, the Yemashan batholiths are composed of early pluton (granodiorite and porphyritic monzogranite) and late pluton (monzogranite). U-Pb age determination of zircon by LA-ICPMS shows that the early pluton of Yemashan batholiths was intruded at (469.0±1.3) Ma, and the late pluton was intruded at (450.0±1.0) Ma. Compositionally, the early pluton of Yemashan batholiths has SiO2 values ranging from 59.8% to 64.2%, K2O/Na2O ratios higher than 1, and molar A/NKC ratios from 0.8 to 1.0, indicating typical metaluminous granitoid. The pluton is depleted in Ba, Nb, Ta, Sr, P and Ti, and enriched in Rb, U and Th, and has total REE content ranging from 97.73×10-6 to 185.76×10-6, (La/Yb)N from 5.57 to 12.47, and LREE/HREE from 7.7 to 11.3. The chondrite-normalized REE patterns of the pluton exhibit enrichment of LREE and depletion of HREE, with moderate negative Eu anomalies (δEu=0.66-0.89). The late pluton of Yemashan batholiths has SiO2 ranging from 69.8% to 76.5%, K2O/Na2O from 1.2 to 1.7, and molar A/NKC from 0.8 to 1.0, implying weakly peraluminous granitoid. It also shows enrichment of Rb, U and Th, and depletion of Ba, Nb, Ta, Sr, P and Ti, with ∑REE from 78.97×10-6 to 244.92×10-6, and (La/Yb)N from 5.57 to 12.47. The chondrite-normalized REE patterns of the pluton exhibit enrichment of LREE and depletion of HREE, with pronounced negative Eu anomalies (δEu=0.11-0.24). The petrological and geochemical characteristics indicate that the early pluton of Yemashan batholiths could represent typical I-type granite formed in a subduction orogenic setting. The late pluton should be assigned to highly fractionated I-type granite formed in a post-collisional orogenic setting. On the basis of these features in combination with the tectonic locations of granite occurrence and field observation data, the authors hold that southward and northward subductions of the North Qilian oceanic plate took place in the Early Paleozoic, during which the southward subduction and collision resulted in the formation of Yemashan batholiths.
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