博格达造山带东段芨芨台子地区晚石炭世双峰式火山岩地球化学特征及其地质意义

Geochemical characteristics of the Late Carboniferous bimodal volcanic rocks in Jijitaizi area,eastern Bogda orogenic belt,and their geological significance

博格达造山带东段芨芨台子地区出露大量晚石炭世早期双峰式火山岩组合,岩石类型以亚碱性玄武岩为主,含有少量的流纹岩,属拉斑系列。其玄武岩富钠贫钾（Na2O/K2O=2.84~3.01）,Ti O2含量为1.32%~1.43%,略高于N型大洋中脊玄武岩,高Al（Al2O3含量为19.81%~20.60%）、低Mg（Mg O含量为2.95%~3.25%,Mg#为32.33~34.50）,表明其原始岩浆发生过明显的橄榄石和辉石的分离结晶作用。玄武岩具有近于平坦的稀土配分模式,轻微正Eu异常（δEu=1.02~1.10,平均为1.04）,富集Rb、Ba、P等大离子亲石元素,亏损Nb、Ta、Th、Sr、Ti等高场强元素的特征;其微量元素Zr/Nb比值41.13~41.38,Zr/Y比值4.26~4.52（均大于4.00）,Rb/Sr比值0.02~0.04（均小于0.05）,Dy/Yb比值1.92~2.00,La/Nb比值2.42~2.49,Ba/Nb比值113.58~126.05,Ba/La比值47.02~50.62以及火山岩Zr-Nb、La/Yb-Dy/Yb和La/Nb-La/Ba等判别图解,表明玄武岩可能是亏损尖晶石相地幔橄榄岩向石榴石相地幔橄榄岩过渡相较高程度部分熔融的产物,且在其上升过程中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,形成于陆内裂谷环境。流纹岩具有较高的Si O2（76.54%~77.74%）和全碱（Na2O＋K2O=6.70%~7.70%）含量,以及较低的Ti O2（0.15%~0.16%）、Al2O3（10.70%~10.90%）和Mg O（0.17%~0.18%,Mg#为13.93~14.52）含量;显示右倾负斜率稀土配分模式,铕负异常明显（δEu为0.32~0.33）,显著富集Rb、Th、U等大离子亲石元素,相对亏损Nb、Ta,强烈亏损Sr、Ti、P等高场强元素,指示流纹岩应来自于玄武岩浆上升过程中提供的热量对地壳物质重熔,源区有斜长石残留。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大陆裂谷环境,同时获得流纹岩锆石U-Pb年龄为（312±1）Ma,表明该套火山岩形成于晚石炭世早期。进一步的Rb-Sr地壳厚度网络图投影表明当时博格达地区地壳厚度为20.0~30.0 km,而且根据Zr-Zr/Y图解推断其地壳拉张速率介于2.0~5.0 cm/a,表明早石炭世早期开始的博格达初始裂谷在晚石炭世早期进一步快速拉张,发展到鼎盛阶段,也代表着博格达地区由碰撞挤压到伸展拉张这个地球动力学环境的重大转折,这一发现进一步证实了博格达造山带在石炭纪处于大陆裂谷演化的观点,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。

The genesis of the Late Carboniferous bimodal volcanic rocks in Jijitaizi area of eastern Bogda orogenic belt was revealed recently, which comprise mainly sub-alkaline basalts and subordinately rhyolite rocks, belonging mainly to the tholeiitic series. The basalts have high Al（Al2O3=19.81%-20.60%）, low Mg（Mg O=2.95%-3.25%, Mg#=32.33-34.50）, Na2O/K2 O ratios of 2.84-3.01 and Ti O2 values of 1.32%-1.43%, slightly higher than values of N-MORB, indicating obvious fractional crystallization of olivene and pyroxene during basaltic magmatism and an evolution product of original magma＇s fractional crystallization. They have the following characteristics： Zr/Nb=41.13-41.38, Zr/Y=4.26-4.52（all higher than 4.00）, Rb/Sr=0.02-0.04（all less than 0.05）, Dy/Yb=1.92-2.00, La/Nb=2.42-2.49, Ba/Nb=113.58-126.05, Ba/La=47.02-50.62, slight enrichment of trace elements Rb, Ba and P, slight positive Eu anomalies（δEu=1.02-1.10）, and relative depletion of Nb, Ta, Th, Sr and Ti. In addition, the Zr-Nb, La/Yb-Dy/Yb and La/Nb- La/Ba discrimination diagrams indicate that the basalts had geochemical features indicative of intraplate basalts formed within the continent, originated from high degree partial melting of the transition product from depleted spinel phase mantle peridotite to garnet phase mantle peridotite, and suffered a certain degree of crustal contamination during the rising process. The rhyolites of bimodal volcanic rocks have high Si O2（76.54%-77.74%） and ALK（Na2O＋K2O=6.70%-7.70%）, but low Ti O2（0.15%-0.16%）, Al2O3（10.70%- 10.90%） and Mg O（0.17%- 0.18%, Mg#=13.93- 14.52）. Their trace elements generally have features of enrichment of Rb, Th and U, obvious depletion of Sr, P and Ti, and slight depletion of Ta and Nb; the REE distribution patterns are characterized by LREE enrichment, right- oblique form and obvious depletion of Eu（δEu=0.32- 0.33）, suggesting that the rocks originated from the melting of crustal material with plagioclase as an important relic mineral in the source. Geochemical characteristics of the bimodal volcanic rocks in Jijitaizi area indicate that the volcanic rocks were formed in a typical tectonic setting of continental rift. The rhyolites with LA-ICP-MS zircon U-Pb age of（312±1） Ma suggest that this series of volcanic rocks were formed at the beginning of Late Carboniferous. According to these characteristics in combination with the Rb- Sr and Zr- Zr/Y diagrams, the crust thickness（20.0-30.0 kilometers） and extension speed（2.0 cm/a-5.0 cm/a） might suggest the fast development of Bogda rift, which stared in the Early Carboniferous and reached its culmination at the beginning of Late Carboniferous, and also represent the transitional period of geodynamic environment from collision and compression to stretching and extension. The results achieved by the authors further confirm the view that the Carboniferous Bogda orogenic belt belongs to continental rift and it could provide evidence for understanding the tectonic evolution and plate system of the Bogda orogenic belt of the Late Paleozoic.