内蒙古狼山地区早石炭世角闪辉长岩、花岗闪长岩的岩石成因及构造意义

内蒙古狼山山脉西侧分布有大面积的晚古生代岩浆岩,时代集中在早石炭世—晚二叠世,早石炭世角闪辉长岩、花岗闪长岩体出露于潮格温都尔镇西侧。角闪辉长岩体呈岩滴状产出,被花岗闪长岩体侵入,LA-ICP-MS锆石U-Pb年龄显示,角闪辉长岩的~(206)Pb/~(238)U加权平均年龄为329.0±2.3 Ma,花岗闪长岩的~(206)Pb/~(238)U加权平均年龄为331.1±0.9 Ma~330.0±4.2 Ma。花岗闪长岩暗色矿物以角闪石为主,富钠(Na2O=3.48%~4.46%),高钠钾比值(Na2O/K2O=1.03~2.39),钙碱性系列,P2O5-SiO_2之间存在较好的负相关性,岩石地球化学特征具Ⅰ型花岗岩的特点。Hf同位素及元素地球化学特征指示了角闪辉长岩及花岗闪长岩均来自于受地壳混染的亏损地幔,为同源岩浆演化的产物。角闪辉长岩及花岗闪长岩稀土元素配分型式一致,均为轻稀土元素富集,重稀土元素亏损,具弱的负Eu异常;角闪辉长岩富集Ba、Sr,亏损Nb、Ta、Zr、Hf;花岗闪长岩富集大离子亲石元素Rb、K、Pb、Sr,不同程度地亏损高场强元素Nb、Ta、P、Ti,总体反映了岩浆弧的地球化学特征。结合区域地质背景,早石炭世狼山地区侵入岩岩石组合为角闪辉长岩(闪长岩)+石英闪长岩+花岗闪长岩,认为狼山地区早石炭世处于大陆边缘弧构造背景。

Petrogenesis and tectonic significance of the Early Carboniferous hornblende-gabbro and granodiorite in Langshan area, Inner Mongolia

Large quantities of Early Carboniferous-Early Permian magmatic rocks are exposed on the western side of the Langshan Mountain, Inner Mongolia. The Early Carboniferous rocks mainly consist of hornblende gabbro and granodiorite outcropped on the west side of Chaogewenduer Town. The hornblende gabbros assume the form of individual guttate bodies and were cut by the granodiorite. LA-ICP-MS U-Pb dating of hornblende gabbro yielded weighted mean ²⁰⁶Pb/²³⁸U age of 329.0±2.3 Ma, while dating of two granodiorite samples yielded 331.1±0.9 Ma and 330.0±4.2 Ma respectively. The mafic mineral in granodiortie is dominated by hornblende. Whole rock geochemical data reveal that the granodiorite is calc-alkaline nature characterized by enrichment of Na₂O (3.48%~4.46%), high Na₂O/K₂O ratios (1.03~2.39) and shows good negative correlation between P₂O₅ and SiO₂. The similar Hf isotopic signature between the hornblende gabbro and the granodiorite and their behavior in elemen-tal geochemical characteristics together indicate their co-magmatic origin derived from depleted mantle which suffered from crustal contamination. The two types of rocks show similar patterns on the chondrite-normalized REE patterns, and display relatively high concentration of light rare earth elements but low content of heavy rare earth elements with minor negative Eu anomalies. The overall chemical similarities of these rocks on the primitive mantle-normalized variation diagrams suggest affinity to arc signature. In combination with the authors''work and the large scale regional exposure of hornblende gabbro+quartz diorite+ granodiorite rock assemblage, the authors hold that the Langshan area was under the tectonic setting of continental margin arc during Early Carboniferous.