内蒙古渣尔泰地区中生代火山岩地球化学特征及构造环境分析

Geochemical characteristics and tectonic setting for Mesozoic volcanic rocks in the Zha'ertai area,Inner Mongolia

内蒙古渣尔泰地区处于华北地台西北部,其分布的中生代火山岩划分为两大类,形成于两个喷发旋回。早白垩世早期喷发旋回以中基性火山岩为主,沿近东西向的深大断裂呈北东东向裂隙式喷发;晚期喷发旋回以酸性火山岩为主,沿近南北向断裂带呈裂隙-中心式喷发。中基性火山岩主要为橄榄玄武粗安岩和安粗岩,其SiO_2介于49.65%~56.64%;酸性火山岩包括次火山岩、火山碎屑岩、火山熔岩,其Si O2介于73.18%~83.82%;两者组成典型的宽双峰火山岩。中基性火山岩富碱(Na_2O+K_2O=6.70%~7.47%)、高Ti(TiO_2=2.29%~3.55%)、Fe,相对贫Ca、Mg,富含大离子亲石元素(LILE)(Rb、Ba),轻稀土相对富集,轻、重稀土元素分馏明显,LREE/HREE=4.50~6.47,(La/Yb)N=4.17~8.85,具微弱的负Eu异常(δEu=0.67~0.96),具有与洋岛玄武岩(OIB)相似的地球化学特征,其岩浆来源于交代富集的软流圈地幔,岩浆上升过程中地壳组分混染较少。酸性火山岩富Si、高K,贫Ca、Mg、Fe、Ti,富含大离子亲石元素(LILE)(Rb、Th、U)和高场强元素(HFSE)(Nb、Ta、Zr、Hf),富集轻稀土,LREE/HREE=2.91~14.3,轻、重稀土分馏明显,(La/Yb)N=2.53~28.4,(La/Sm)N=1.16~4.57,(Gd/Yb)N=1.22~5.04,负Eu异常明显(δEu=0.02~0.25),具有A型流纹岩的特征,为幔源岩浆底侵,使上覆的先存地壳岩石发生部分熔融的产物。结合区域地质背景,认为研究区双峰式火山岩形成于西伯利亚板块与华北板块碰撞造山期后陆内拉张的构造环境。

The Mesozoic volcanic rocks in the mid-west area of Inner Mongolia are mainly distributed in the Zha＇ertai area, which is located in the northwest of North China plate. The Mesozoic volcanic rocks in the study area are divided into two types, which are formed in the two eruption cycles. The intermediate-to-basic volcanic rocks appear in early Early Cretaceous, and erupt as NEE trending eruption of fissure type along about EW deep fault ; the acid volcanic rocks appear in late Early Cretaceous, and erupt as eruption of fissure type and central eruption along about SN fault. The intermediate-to-basic volcanic rocks, whose SiOz ranges from 49. 65% to 56. 64% , are mostly shoshonite and atite, and the acid volcanic rocks, whose SiOz ranges from 73.18% to 83.82% , include subvolcanic rocks, volcanic lastic rocks and volcanic lava, which is a typical bimodal distribution. The intermediate-to-basic volcanic rocks are characterized by high total alkaline contents （ Na20 ＋ K20 = 6. 70% - 7. 47% ） , high-Ti （ TiO2 = 2.29% 3.55% ）, high-Fe, low-Ca, low-Mg, and the rocks are also enriched in large ion lithophile element （LILE） （Rb and Ba） and light rare earth elements （ LREE ） （ LREE/HREE = 4. 50 - 6. 47 ） with highly fractionated LREEs and HREEs （ （ La/Yb ） N = 4. 17 - 8.85 ） , with slightly negative Eu anomalies （ 6Eu = 0. 67 - 0. 96 ）. Their geochemical characteristics are similar to those of oceanic island basalt （OIB）, which implies that magma was mainly from asthenosphere, and the rocks did not experience obvious contamination of crustal materials. The acid volcanic rocks are characterized by high-Si, high-K, low-Ca, low-Mg, low-Fe, low-Ti, and the enrichment of large ion lithophile element （LILE） （ Rb, Th and U）, high field strength elements （HFSE） （ Nb, Ta, Zr and Hf） and light rare earth elements （LREE） （ LREE/HREE = 2. 91 - 14. 3 ）, with highly fractionated LREEs and HREEs （ （La/Yb） N =2. 53 -28.4, （La/Sm） N = 1.16 - 4. 57, （Gd/Yb） N = 1.22 - 5.04） , and with strong negative Eu anomalies （δEu = 0. 02 - O. 25 ）, which indicates that rhyolitic rock might be partial melting of crust-derived rock, stemming from partial melting of rocks in the pre-existing crust under plated by mantle-derived magma. On the basis of the regional geological background, it is concluded that the Mesozoic volcanic rocks in the study area are formed in the intracontinental extensional setting after the Siberian plate and the North China plate collision orogenetic period.