CAJ | 학술논문

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 (Al2 O3=14.06%~20.93%),富 Na 贫 K (Na2 O/K2 O =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(Na2 O+K2 O=8.56%~8.79%)含量,以及较低的 TiO2(0.12%~0.18%)、Al2 O3(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。
박격체조산대동단급급태자지구조이첩세현무암화중산성화산암재시공상구성쌍봉식화산암조합,기암석류형주요위아감성현무암화류문암,주체속개감성계렬。현무암 SiO2함량위47.70%~51.71%,TiO2함량(1.26%~1.52%)략고우 N 형대양중척현무암,고 Al (Al2 O3=14.06%~20.93%),부 Na 빈 K (Na2 O/K2 O =2.34~28.36),저 Mg(MgO =2.99%~8.62%, Mg#위27~52),표명기현무암장발생과명현적감람석화휘석적분리결정작용；현무암경희토원소략미부집,Eu 이상불명현(δEu=0.85~1.02),상대부집 Rb、Ba、P 등대리자친석원소,우손 Nb、Ta、Sr、Ti 등불상용원소。화산암지구화학특정표명,연구구현무질암장래자우우손암석권지만적부분용융,차수도일정정도적지각물질혼염,현시료판내현무암적지구화학특정형성우륙내신전배경。류문암구유교고적 SiO2(73.58%~75.45%)화전감(Na2 O+K2 O=8.56%~8.79%)함량,이급교저적 TiO2(0.12%~0.18%)、Al2 O3(12.77%~13.24%)화 MgO(평균위0.21%)함량；현시우경부사솔희토배분모식,Eu 부이상명현(δEu 위0.52~0.70),현저부집 Rb、Ba、Th 등대리자친석원소,강렬우손 Nb、Ta、Sr、P、Ti 등고장강원소,구유 A 형화강암적지구화학특정,위후팽당신전배경하저침현무암장결정분이적산물。박격체조산대동부급급태자지구쌍봉식화산암적지구화학특정표명,해투화산암응형성우팽당후신전배경,동시획득류문암고석 U-Pb 년령위(294.2±1.3)Ma,표명해투화산암형성우조이첩세조기。저일발현진일보증실료연구구재력경석탄기회취팽당사건후진입이첩기후팽당신전적연화시기,위진일보리해박격체지구만고생대구조격국급판괴구조체제제공료중요적지질의거。
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 (Al2 O3 =14.06%-20.93%),Na2 O/K2 O (2.34-28.36)and TiO2 (1.26%-1.52%),which are slightly higher than the ones of N – MORB,and they have low con-tents of Mg (MgO=2.99%-8.62%,Mg# =27-52). These major element characteristics indi-cate 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 val-ues of SiO2 (73.58%-75.45%)and ALK (Na2 O+K2 O=8.56%-8.79%),but low contents of TiO2 (0.12%-0.18%),Al2 O3 (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 ge-ochemical characteristic of the bimodal volcanic rocks in Jijitaizi area of the eastern section of Bog-da 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 bi-modal 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 evolu-tion and plate system of the Bogda orogenic belt of the late Paleozoic.