Permian volcanism in the Mongolian orogenic zone, northeast China: geochemistry, magma sources and petrogenesis
Permian volcanism in the Mongolian orogenic zone, northeast China: geochemistry, magma sources and petrogenesis
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DOI：
10.1017/s0016756801005210
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发表时间：
2001-03
影响因子：
2.3
通讯作者：
Yongfeng Zhu;Shihua Sun;Libing Gu;Y. Ogasawara;Neng Jiang;H. Honma
中科院分区：
文献类型：
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作者：
Yongfeng Zhu;Shihua Sun;Libing Gu;Y. Ogasawara;Neng Jiang;H. Honma
早二叠世火山活动是中国东部蒙古造山带碰撞事件后的第一次岩浆活动。因此，二叠纪火山岩是理解统一大陆岩石圈动力学的关键。火山岩由基性岩和中性岩组成。具有高初始87 Sr/86 Sr比值（0.7051至0.7052）和低εNd值（−0.73至−3.57）的中间岩通常覆盖在野外的基性岩上。基性岩具有相对较低的87 Sr/86 Sr比值（0.7034 ~ 0.7051）和较高的εNd值（2.72 ~ −0.10）。两条平行的Rb-Sr等时线给出了几乎相同的年龄，约为270 Ma。一个由基性岩组成，初始等时线87 Sr/86 Sr比值为0.7035。另一个由中间岩和一个玄武岩样品组成，其初始等时线87 Sr/86 Sr值为0.7051。SiO_2与其它主量元素之间的相关性表明，分离结晶作用在岩浆过程中起着重要作用。然而，分离结晶不能解释大多数不相容微量元素的地球化学和Sr-Nd同位素特征。Th/Nb与（La/Sm）N比值的正相关性表明，轻稀土元素的富集与陆相沉积物的污染有直接关系。大离子岩石圈元素（LILE）在二叠纪火山岩中的高含量可能暗示了一种额外的“地壳+流体”成分，特别是在相对于基性岩（> 200 ppm）而言Ba（> 400 ppm）高度富集的中间岩中。我们认为俯冲板片落入亏损地幔释放流体，引起地幔交代和大离子亲石元素富集。交代地幔部分熔融形成“原始”岩浆。这些原始岩浆在上升过程中与元古宙黑云母石英片岩同化，最终形成二叠纪火山岩。与元古代黑云母-石英片岩同化的少量岩浆可能产生基性岩，而大量岩浆的同化（由于同化时间较长）将产生中间岩。
Lower Permian volcanism was the first magmatic activity to occur after the collision events in the Mongolian orogenic zone, east China. The Permian volcanic rocks are therefore a key to understanding the dynamics of the unified continental lithosphere. The volcanic rocks consist of basic and intermediate rocks. The intermediate rocks with high initial 87Sr/86Sr ratios (0.7051 to 0.7052) and low εNd values (−0.73 to −3.57) generally overlie the basic rocks in the field. The basic rocks have relatively low initial 87Sr/86Sr ratios (0.7034 to 0.7051) and high εNd values (2.72 to −0.10). Two parallel Rb–Sr isochrons give almost the same age, about 270 Ma. One consists of the basic rocks giving an initial isochron 87Sr/86Sr ratio of 0.7035. The other consists of the intermediate rocks and one sample of basalt, which give an initial isochron 87Sr/86Sr value of 0.7051. The strong correlations between SiO2 and other major elements suggest that fractional crystallization played an important role in the magmatic processes. However, fractional crystallization cannot explain the geochemistry of most incompatible trace elements and Sr–Nd isotope characteristics. The positive correlation between Th/Nb and (La/Sm)N ratios demonstrates the direct relation between the enrichment of the light rare earth elements and the contamination of continental sediments. The high contents of large ion lithosphere elements (LILE) in the Permian volcanic rocks may suggest an additional ‘crust + fluid’ component, especially in the intermediate rocks, which are highly enriched in Ba (> 400 ppm) relative to the basic rocks (> 200 ppm). We propose that the subduction slab dropped into depleted mantle and released fluid, which induced the mantle metasomatism and LILE enrichment. The metasomatized mantle partially melted and formed the ‘primary’ magma. This primary magma assimilated with the Proterozoic biotite–quartz schist during its rise, and finally formed the Permian volcanic rocks. Magma assimilated with the Proterozoic biotite–quartz schist in small amounts could have produced the basic rocks, while assimilation of larger amounts of magma (because of longer assimilation time) would generate intermediate rocks.