博格达造山带东段芨芨台子地区早二叠世双峰式火山岩地球化学特征及其地质意义

博格达造山带东段芨芨台子地区早二叠世玄武岩和中酸性火山岩在时空上构成双峰式火山岩组合,其岩石类型主要为亚碱性玄武岩和流纹岩,主体属钙碱性系列。玄武岩SiO2含量为47.70%~51.71%,TiO2含量(1.26%~1.52%)略高于N型大洋中脊玄武岩,高Al(Al2O3=14.06%~20.93%),富Na贫K(Na2O/K2O=2.34~28.36),低Mg(MgO=2.99%~8.62%,Mg#为27~52),表明其玄武岩浆发生过明显的橄榄石和辉石的分离结晶作用;玄武岩轻稀土元素略微富集,Eu异常不明显(δEu=0.85~1.02),相对富集Rb、Ba、P等大离子亲石元素,亏损Nb、Ta、Sr、Ti等不相容元素。火山岩地球化学特征表明,研究区玄武质岩浆来自于亏损岩石圈地幔的部分熔融,且受到一定程度的地壳物质混染,显示了板内玄武岩的地球化学特征形成于陆内伸展环境。流纹岩具有较高的SiO2(73.58%~75.45%)和全碱(Na2O+K2O=8.56%~8.79%)含量,以及较低的TiO2(0.12%~0.18%)、Al2O3(12.77%~13.24%)和MgO(平均为0.21%)含量;显示右倾负斜率稀土配分模式,Eu负异常明显(δEu为0.52~0.70),显著富集Rb、Ba、Th等大离子亲石元素,强烈亏损Nb、Ta、Sr、P、Ti等高场强元素,具有A型花岗岩的地球化学特征,为后碰撞伸展环境下底侵玄武岩浆结晶分异的产物。博格达造山带东部芨芨台子地区双峰式火山岩的地球化学特征表明,该套火山岩应形成于碰撞后伸展环境,同时获得流纹岩锆石U-Pb年龄为(294.2±1.3)Ma,表明该套火山岩形成于早二叠世早期。这一发现进一步证实了研究区在历经石炭纪汇聚碰撞事件后进入二叠纪后碰撞伸展的演化时期,为进一步理解博格达地区晚古生代构造格局及板块构造体制提供了重要的地质依据。

Geochemical Characteristics and its Significance of the Early Permian Bimodal Volcanic Rocks in Jijitaizi Area,East of the Bogda Orogenic Belt

The early Permian Bogda orogenic basalts and acidic volcanic rocks, developed in the Jijitaizi area of the eastern section of Bogda orogenic belt, are bimodal volcanic rocks, which are composed by sub-alkaline basalt and rhyolite, mainly belonging to the calc-alkaline series. The basalts have high values of Al(Al2O3=14.06%-20.93%), Na2O/K2O(2.34-28.36) and TiO2(1.26%-1.52%), which are slightly higher than the ones of N-MORB, and they have low contents of Mg(MgO=2.99%-8.62%, Mg#=27-52). These major element characteristics indicate that these basalts had experienced an obvious fractional crystallization of olive and pyroxene during basaltic magmatism, which were the evolution products of original magma's fractional crystallization. The basalts are slightly enriched in trace elements(such as Rb, Ba and P), with no Eu anormalies(δEu=0.85-1.02), but are relatively depleted in Nb, Ta, Sr and Ti. These trace element characteristics of volcanic rocks suggest that,these basalts were originated from the partial melting of the depleted lithospheric peridotite mantle and had suffered a certain degree of crustal contamination, showing the geochemical features of intraplate basalts formed within the intracontinental extensional environment. The rhyolites of bimodal volcanic rocks have high values of SiO2(73.58%-75.45%) and ALK(Na2O+K2O=8.56%-8.79%), but low contents of TiO2(0.12%-0.18%), Al2O3(12.77%-13.24%) and MgO(average about 0.21%).Their trace elements generally have the enrichment features of Rb, Th and Ba, obvious depletion of Nb, Ta, Sr, P and Ti, and the REE distribution patterns are characterized by LREE enrichment, right-deviation type and obvious depletion of Eu(δEu=0.52-0.70). The REE characteristics indicate that, the rhyolites have similar geochemical characteristics of A-type granite, suggesting that the rhyolites were originated from the fractionation of underplating basaltic magma. The geochemical characteristic of the bimodal volcanic rocks in Jijitaizi area of the eastern section of Bogda orogenic belt indicate that,these volcanic rocks were formed in a post-collisional extensional environment.The LA-ICP-MS zircon U-Pb ages of rhyolites are(294.2±1.3) Ma, suggesting that this set of volcanic rocks was formed in the beginning of early Permian. Our study of the bimodal volcanic rocks in Jijitaizi area indicate that, the research area had witnessed the transitional period of geodynamic environment from Carboniferous collision and compression to Permian stretching and extension, and it could provide an evidence for understanding the tectonic evolution and plate system of the Bogda orogenic belt of the late Paleozoic.