长白山上新世以来玄武岩成分演变规律及其成因

Temporal variations and petrogenetic implications in Changbai basaltic rocks since the Pliocene

火山岩成分的多样性是岩浆物理和化学过程在其产生、运移、存储和喷发过程中的综合反映。长白山火山区自上新世以来喷发了大量的玄武质火山岩,其成分变化范围较大（Mg O 3.2%~7.8%）。以往研究认为其成分的变化主要受地幔不均一、部分熔融程度和分离结晶的影响,没有明显地壳混染。本研究发现这些玄武岩经历了不同程度的上、下地壳的混染。同时,结合火山岩的年龄发现玄武岩地球化学成分和同位素比值随时间呈现脉动式的变化。根据87Sr/86Sr和Mg O的突变点可以分为3段：5~2Ma,2~1Ma,1~0Ma。通过定性和定量的模拟发现地幔不均一性和部分熔融程度差异造成玄武岩成分的变化有限,而分离结晶、地壳混染和岩浆补给的岩浆作用是形成玄武岩成分随时间脉动变化的主要原因。并结合能量约束-补给-混染-分离结晶算法（ECRAFC）模拟得出以下结论：天池和望天鹅喷发中心的玄武质岩浆最初都存储于同一下地壳岩浆房,可能由于上地壳构造差异导致岩浆迁移路径和存储区不同;长白山岩浆房迁移有从5~2Ma阶段由下地壳向上地壳逐渐变浅,2~1Ma阶段由上地壳向下地壳快速变深的规律,而1~0Ma阶段的玄武岩由岩浆从下地壳直接快速喷出地表形成;长白山玄武质岩浆的活动与本区的构造断裂活动密切的关系,5Ma以来,火山岩成分随时间的周期性波动可能与本区构造应力的周期性的强拉张-弱拉张过程有关。

The compositional diversity of volcanic rocks reflects the complex interactions between physical and chemical processes during magma generation, ascent, storage and eruption. Since 5 million years, Changbai volcanoes （ including Mount Tianchi, Mount Wangtian＇ e and Mount Nampotae） erupt a lot of basaltic rocks. Their compositions vary significantly （3.2% -7. 8% MgO）. Previous studies suggested that the compositional variation is mainly controlled by mantle heterogeneity, differences in the degree of partial melting and fractional crystallization, with no significant crustal assimilation. This study found that these basalts experienced varied degrees of upper and lower crustal contamination. Meanwhile, the geochemical and isotopic composition show consistently temporal variations, which can be divided into three stages： 5 2Ma, 2 1Ma and 1 0Ma, base on STSr/Sr ratio and MgO concentration. By qualitative and quantitative geochemical examination and modeling, we found that the cyclic geochemical variation is mainly related to open-system magmatic processes, including fractional crystallization, crustal contamination and magma recharge, instead of mantle source heterogeneity and differences in degree of partial melting. To understand the detail magma stall and transportation information the cyclic variations record, we conducted energy-constrained recharge, assimilation and fractional crystallization （ECRAFC） modeling and found the best fit trajectories defined by the St-isotopic and Sr trace-element composition of 5 0Ma Changbai basalts. Our results indicate that： The parental basaltic magma of Tianchi and Wangtian＇ e eruption center initially store in a connected low-crustal magma reservoir, though they separated each other in upper-cruatal reservoir related to heterogeneity of regional tectonics; Magmatic storage systems of Changbai is progressively shallower from lower to upper-crust in 5 2Ma, and quickly deeper from upper to lower-crust in 2 -1Ma; During 1 -0Ma stage, the basaltic magma mainly restore in the lower-crust; The cyclic geochemical variation of basaltic magma during the last 5Ma is closely related the periodic stress-shift of regional faults, from fast to slow extending.