五大连池钾质火山岩中辉石斑晶的反环带结构研究 及其对岩浆演化的约束

五大连池钾质火山岩中发现具有反环带结构的单斜辉石斑晶。反环带辉石的核部为次透辉石，与边部相比具有较低 的Mg#值（68~77）、TiO2 （0.23~0.50 wt.%）、Cr2O3 （<0.06 wt.%）和较高的Al2O3 （3.4~5.0 wt.%）、Na2O（0.43~0.78 wt.%）、 FeO（8.8~11.0 wt.%）、MnO含量。核部的稀土元素标准化曲线较为平坦，且具有非常显著的Eu和Sr的负异常（Eu/Eu*= 0.35~0.63，Sr/Sr* = 0.03~0.17），指示其为曾与斜长石平衡的变质岩辉石。在Mg#-TiO2相关图上，核部总体成分落入华北下 地壳低Mg麻粒岩中单斜辉石的成分范围，因此核部应该是来自下地壳麻粒岩的捕掳晶。反环带辉石边部与正常辉石斑晶成 分一致，具有较高的Mg#值（81~85）、TiO2（0.40~1.65 wt.%）、Cr2O3（0.03~0.25 wt.%）和较低的Al2O3（2.1~3.4 wt.%）、Na2O （0.34~0.63 wt.%）、FeO（4.6~6.6 wt.%）、MnO含量，轻重稀土分馏明显（La/Yb） N = 3.23~7.89，与玄武岩全岩的特征吻合。 利用主量和微量元素的分配系数进行的模拟计算均表明，反环带辉石边部与寄主岩浆已达到成分平衡，说明边部是在熔蚀 核部的基础上再生长而成的岩浆成因辉石。五大连池正常辉石斑晶的Mg#值与TiO2具有负相关性，指示岩浆在地壳深度经历 了一定程度的演化。反环带辉石斑晶边部由里到外Al2O3和Na2O含量逐渐增加的趋势支持岩浆经历演化的结论。核部辉石 的麻粒岩属性表明岩浆演化主要发生在下地壳。温压计计算结果也指示正常辉石斑晶和反环带辉石的边部都结晶于下地壳 深度的系列岩浆房。晚期结晶的辉石斑晶总是比早期结晶的辉石更富集不相容元素，说明分离结晶作用导致五大连池玄武 岩的不相容元素更富集。平衡计算表明，与辉石斑晶平衡的岩浆与玄武岩全岩在微量元素特征上高度相似。考虑到全岩高 度一致的微量元素特征，研究认为五大连池玄武岩的成分变化主要受下地壳岩浆房中的结晶分离作用控制，地壳混染（包 括壳源岩浆混合）可以忽略不计。

Investigation of Reverse Zoning in Pyroxene Phenocrysts from Wudalianchi Potassic Lavas and its Constraints on Magmatic Evolution

Clinopyroxene phenocrysts with reverse zoning have been observed in Wudalianchi potassic lavas. The cores are salites characterized by lower Mg# (68~77), TiO2 (0.23~0.50 wt.%), Cr2O3 (<0.06 wt.%) and higher Al2O3 (3.4~5.0 wt.%), Na2O (0.43~0.78 wt.%), FeO (8.8~11.0 wt.%) and MnO contents than those of the exteriors. Normalized REE patterns of the cores are relatively flat, and they also exhibit prominently negative anomaly of Eu and Sr (Eu/Eu* =0.35~0.63, Sr/Sr* =0.03~0.17), suggesting an origin from metamorphic rocks in equilibrium with plagioclase. In the plot of Mg#-TiO2, the overall composition of the cores falls within the compositional range of clinopyroxenes in lower crustal low-Mg granulites from North China Craton. As a result, the cores are inferred to be xenocrysts from granulites in the lower continental crust. The exteriors and normal pyroxene phenocrysts are similar in their compositions, which are higher in Mg# (81~85), TiO2 (0.40~1.65 wt.%), Cr2O3 (0.03~0.25 wt.%), and lower in Al2O3 (2.1~3.4 wt.%), Na2O (0.34~0.63 wt.%), FeO (4.6~6.6 wt.%), MnO. Their LREE and HREE are highly fractionated (La/Yb)N=3.23~7.89, which are consistent with the basaltic whole rocks. The exteriors were in equilibrium with the host magmas, as is confirmed by both major and trace element modeling based on their partition coefficients. Thus the exteriors are suggested to be magmatic pyroxenes formed by overgrowth on the eroded cores. The Mg# of Wudalianchi normal pyroxene phenocrysts are negatively correlated with TiO2 contents, pointing to a certain extent of magmatic evolution in crustal depth, consistent with the gradual increasement of Al2O3 and Na2O contents from the inside out of the exteriors. The granulite facies nature of the cores further constrains the evolution to have occurred in the lower continental crust. Thermobarometer estimations also suggest that the crystallization of normal pyroxenes and exteriors of reversely zoned pyroxenes were in serial magma chambers in the lower crust. Pyroxene phenocrysts crystallized in a later stage are more enriched in incompatible elements than those of earlier pyroxenes, indicating that fractional crystallization have enhanced the enrichment of incompatible elements of Wudalianchi basalts. Calculated melts in equilibrium with pyroxene phenocrysts are extremely similar with the ultimately erupted lavas in their trace element patterns. Considering the remarkably uniform whole-rock trace element patterns, we suggest that the evolution of Wudalianchi basalts in the crust were mainly controlled by the fractional crystallization in magma chambers, and crustal contamination (also mixing with crust-derived magmas) has had negligible effects on whole rock compositions.