Implications for magmatic evolution of changes in plagioclase composition in the 26°S basalts, Southern Mid-Atlantic Ridge
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摘要: 南大西洋中脊26°S热液区玄武岩中斜长石的成分和结构特征为研究岩浆演化特征提供了重要依据。在对玄武岩岩相学观察、斜长石成分分析的基础上,对不同阶段斜长石的结构及成分进行对比,探讨岩浆演化的过程。结果表明:研究区以斑状玄武岩为主,斜长石斑晶常见连晶结构、韵律环带、溶解与再吸收、生长结构,基质中的斜长石主要呈针状和燕尾状;南大西洋中脊26°S(SMAR-26°S)热液区玄武岩主要为钠质拉斑玄武岩,岩浆演化早期以形成培长石为主,晚期以形成拉长石为主,整个演化过程中CaO和Al2O3含量下降,而SiO2、Na2O、FeOT和MgO含量上升;从斜长石斑晶的核部→边部→基质中的斜长石微晶,An值整体下降,斜长石斑晶边部的An值与基质中斜长石微晶An值部分重合,形成温度总体有逐渐下降趋势。由此反映出地幔岩浆形成后在上涌过程中可能经历了减压挥发分逸出、晶体在深部的熔蚀、熔体过饱和结晶、低Si同源岩浆的混合、同喷发脱气岩浆迅速减压的过程。Abstract: The composition and texture characteristics of plagioclase in basalt from the 26°S hydrothermal field of the Southern Mid-Atlantic Ridge provide an important basis for the study of magmatic evolution characteristics. On the basis of observing the petrography of basalt and analyzing the composition of plagioclase, the structure and composition of plagioclase at different stages are compared to explore the process of magma evolution. The results show that the study area is mainly composed of porphyritic basalt, and plagioclase porphyries are commonly characterized by glomerocryst structures, oscillatory-zoned, dissolved and resorbed rim, and growth structures. The plagioclase in the matrix is mainly acicular and swallow-tailed crystals; the basalts in the SMAR-26°S hydrothermal zone are mainly sodium tholeiitic basalts. In the early stage of magma evolution, bytownite was mainly formed, while in the late stage, labradorite was mainly formed. Throughout the entire evolution process, the content of CaO and Al2O3 decreases, while the content of SiO2, Na2O, FeO, and MgO increases; from the core to rim of plagioclase phenocrysts and then to plagioclase microcrystalline in the matrix, the An values decreases overall. The An values in the rim of plagioclase phenocryst partially overlaps with the An values of plagioclase microlites in the matrix, resulting in a gradual decrease in temperature. These represent the changes from the magma origin area to magma eruption. After the formation of mantle magma, it rapidly rises and erupts to form basalt. During the upwelling process, it may undergo the process of decompression accompanied by volatiles escaping, dissolving in the deep, supersaturated crystallizing of the melt, mixing of low-Si same sourced magma, and syn-eruptive fast decompression of degassed magma.
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图 2 SMAR-3岩石样品照片与镜下岩相学照片
a,c分别为样品26-3-1和样品26-3a,属于靠近外侧的玄武岩样品;b,d分别为样品26-3-2和样品26-3b,属于靠近内部的玄武岩样品。e. 橄榄石斑晶,表面可见裂理;f. 玄武岩中偶见的辉石斑晶;g. 具有双晶结构的斜长石斑晶;h. 具有光学环带的斜长石斑晶;i. 基质具有间隐结构,可见大量斜长石微晶。Ol. 橄榄石;Px. 辉石;Pl. 斜长石
Fig. 2 SMAR-3 rock sample photos and microscopic petrography photos
a, c. Samples 26-3-1 and 26-3a, both of which belong to the basalt samples near the outer side; b, d. samples 26-3-2 and 26-3b, both of which belong to the basalt samples near the interior. e. Olivine phenocrysts with visible parting on the surface; f. occasional pyroxene phenocrysts can be observed in basalt; g. plagioclase porphyry with twin crystal structure; h. plagioclase phenocrysts zonation under the perpendicular polarized light microscope; i. the matrix has aintersertal texture, with a large number of plagioclase microlites. Ol. Olivine; Px. pyroxene; Pl. plagioclase
图 6 环带斜长石显微照片、电子探针测点照片、Or-Ab-An成分图及An值变化曲线
a1、a2、a3、a4分别为样品26-3-2(4)中的环带斜长石显微照片、电子探针测点位置、斜长石三角图解和An值变化曲线,经历了岩浆演化I、II阶段;b1、b2、b3、b4分别为样品26-3b2(1)中的环带斜长石显微照片、电子探针测点位置、斜长石三角图解和An值变化曲线,经历了岩浆演化I、II、III阶段;c1、c2、c3、c4分别为样品26-3-1(4)中的环带斜长石显微照片、电子探针测点位置、斜长石三角图解和An值变化曲线,经历了岩浆演化III阶段;d1、d2、d3、d4分别为样品26-3b1(1)中的环带斜长石显微照片、电子探针测点位置、斜长石三角图解和An值变化曲线,经历了岩浆演化I、III阶段;a1、b1、c1、d1图中的红线是预设的沿该线打一排电子探针,两头的编号是电子探针的点号;a2、b2、c2、d2图中绿点是电子探针具体点位,黄色点及编号是其中个别点的位置和点号;a3、b3、c3、d3图中红、蓝、黑、橘色的圈所在位置代表的是对应斜长石的Or-Ab-An投点
Fig. 6 Microphotos of plagioclase zonation, electron probe images with microprobe point position, Or-Ab-An composition diagram of plagioclase phenocrysts, and An values variation curves
a1, a2, a3, and a4 are microphotos of plagioclase zonation, electron probe images with microprobe point position, Or-Ab-An composition diagram of plagioclase phenocrysts, and An values variation curve in samples 26-3-2(4), which have undergone stages I and II of magma evolution; b1, b2, b3, and b4 are microphotos of plagioclase zonation, electron probe images with microprobe point position, Or-Ab-An composition diagram of plagioclase phenocrysts, and An values variation curve in samples 26-3b2(1), which have undergone stages I, II and III of magma evolution; c1, c2, c3, and c4 are microphotos of plagioclase zonation, electron probe images with microprobe point position, Or-Ab-An composition diagram of plagioclase phenocrysts, and An values variation curve in samples 26-3-1(4), which have undergone Stage III of magma evolution; d1, d2, d3, and d4 are microphotos of plagioclase zonation, electron probe images with microprobe point position, Or-Ab-An composition diagram of plagioclase phenocrysts, and An values variation curve in samples 26-3b1(1), which have undergone stages I and III of magma evolution; the red line in figures a1, b1, c1, and d1 is a preset line for drawing a row of electron probes along this line, and the numbers on both ends are the point numbers of the electron probes; the green dots in figures a2, b2, c2, and d2 represent the specific positions of the electron probe, while the yellow dots and numbers represent the positions and numbers of individual points; the positions of the red, blue, black, and orange circles in figures a3, b3, c3, and d3 represent the projection point of corresponding Or-Ab-An composition of plagioclase
图 7 斜长石斑晶和斜长石微晶的Or-Ab-An成分图和An值
a、b. 斜长石斑晶和斜长石微晶的三角图解及An值(未包括环带斜长石);c、d. 环带斜长石的三角图解及An值
Fig. 7 Or-Ab-An composition diagram and An values of plagioclase phenocrysts and plagioclase microlites
a and b are Or-Ab-An composition diagram and An values scatter plot of plagioclase phenocrysts and plagioclase microlites; c and d are Or-Ab-An composition diagram and An values scatter plot of plagioclase zonation
图 8 斜长石An值及温度变化箱线图
a. 从斜长石阶段I→阶段II→阶段III→基质中的斜长石微晶的An值变化箱线图;b. 从斜长石阶段I→阶段II→阶段III→基质中的斜长石微晶的形成温度变化箱线图
Fig. 8 Box plot of plagioclase An value and temperature change
a. Box plot of An value changes during the formation of plagioclase Stage I → Stage II → Stage III→ plagioclase microlites in matrix; b. box plot of temperature changes during the formation of plagioclase from Stage I → Stage II → Stage III→ plagioclase microlites in matrix
表 1 研究区玄武岩中斜长石斑晶的电子探针分析组分(部分结果)(%)
Tab. 1 EPMA compositions of plagioclase phenocrysts in basalts of the study area (partial results) (%)
打点编号 组分 总量 SiO2 TiO2 Al2O3 FeOT MnO MgO CaO Na2O K2O P2O5 Cr2O3 SrO 3b3(1)#4 48.09 0.07 32.10 0.43 0.02 0.25 15.53 2.61 0.01 bdl. bdl. 0.06 99.18 3b3(1)#5 49.31 0.07 31.73 0.51 bdl. 0.27 15.00 2.82 0.01 bdl. 0.01 0.08 99.83 3b3(2)#3 47.71 0.06 32.93 0.53 bdl. 0.22 16.35 2.18 0.03 0.04 bdl. 0.01 100.10 3b3(2)#4 46.61 bdl. 32.92 0.52 bdl. 0.23 16.23 2.15 bdl. 0.04 bdl. 0.03 98.77 3b3(3)#1 50.35 bdl. 31.05 0.50 0.02 0.27 14.53 3.19 0.02 0.02 0.03 bdl. 100.03 3b3(3)#2 49.83 0.06 31.10 0.61 bdl. 0.24 14.30 3.07 0.01 bdl. bdl. 0.04 99.26 3b3(4)#3 50.50 0.05 30.88 0.63 bdl. 0.21 14.23 3.30 0.02 bdl. 0.02 0.01 99.90 3b3(4)#4 50.01 0.03 31.04 0.50 bdl. 0.25 14.31 3.35 0.02 bdl. 0.05 0.06 99.61 3b3(5)#2 45.99 0.02 32.89 0.43 0.02 0.20 16.78 1.76 0.03 0.03 0.01 0.02 98.21 3-1(2)#1 50.91 0.11 30.63 0.56 0.06 0.21 13.95 3.41 0.01 bdl. 0.01 0.02 99.94 3-1(2)#2 50.88 0.10 31.14 0.49 0.04 0.22 14.09 3.36 0.02 bdl. bdl. 0.05 100.39 3-1(3)#1 49.75 0.05 30.87 0.51 0.06 0.19 14.38 3.12 0.03 bdl. 0.04 0.01 99.01 3-1(3)#2 48.79 0.01 32.00 0.59 0.02 0.21 15.23 2.69 0.02 0.06 bdl. bdl. 99.67 3b2(2)#1 49.37 0.08 30.64 0.53 bdl. 0.23 13.93 3.11 bdl. bdl. bdl. 0.11 98.04 3b2(2)#2 49.33 0.05 31.72 0.47 0.02 0.23 15.24 2.65 bdl. 0.02 bdl. bdl. 99.74 注:该数据由电子探针测试分析得出;bdl.表示数据低于检测限。 表 2 研究区玄武岩中环带斜长石的电子探针分析组分(部分结果)(%)
Tab. 2 EPMA compositions of plagioclases zonation in basalts of the study area (partial results) (%)
打点编号 组分 总量 SiO2 TiO2 Al2O3 FeOT MnO MgO CaO Na2O K2O P2O5 Cr2O3 SrO 3-2(4)#3 46.33 0.02 32.67 0.49 0.01 0.13 16.45 1.83 0.01 0.02 bdl. 0.06 98.02 3-2(4)#4 47.12 0.04 32.66 0.38 bdl. 0.17 16.12 2.16 bdl. 0.03 bdl. 0.01 98.68 3-2(4)#5 48.26 0.01 32.21 0.43 bdl. 0.19 15.68 2.29 bdl. 0.02 0.04 0.05 99.17 3-2(4)#6 47.40 0.01 32.35 0.37 bdl. 0.19 16.23 2.06 0.02 0.02 0.11 0.04 98.81 3-2(4)#7 47.21 0.03 32.66 0.43 0.02 0.19 16.16 2.07 0.02 bdl. 0.06 bdl. 98.85 3-2(4)#8 47.80 0.06 32.68 0.39 0.04 0.17 16.24 2.09 0.02 0.03 bdl. bdl. 99.50 3-2(4)#9 46.89 0.04 32.32 0.37 bdl. 0.21 16.13 1.98 bdl. 0.02 bdl. 0.04 97.99 3-2(4)#10 46.79 0.02 32.48 0.41 0.05 0.22 16.16 1.96 0.03 bdl. bdl. 0.02 98.13 3-2(4)#12 46.83 0.03 32.71 0.33 0.01 0.21 16.34 1.97 0.01 0.02 bdl. 0.04 98.49 3-2(4)#13 46.37 0.13 33.11 0.36 0.02 0.18 16.55 1.95 0.02 bdl. 0.01 0.03 98.73 3-2(4)#14 46.65 0.09 33.28 0.36 0.03 0.17 16.53 1.85 0.02 0.04 bdl. 0.09 99.11 3-2(4)#15 45.51 0.05 33.83 0.34 0.01 0.14 17.61 1.40 0.01 bdl. bdl. 0.05 98.95 3-2(4)#16 46.25 bdl. 33.77 0.36 bdl. 0.14 17.57 1.37 0.01 0.04 0.04 bdl. 99.53 3-2(4)#17 45.97 0.02 33.73 0.35 bdl. 0.17 17.57 1.44 0.01 bdl. bdl. 0.06 99.32 3-2(4)#18 46.01 0.02 33.66 0.25 bdl. 0.14 17.47 1.47 0.01 bdl. bdl. bdl. 99.03 3-2(4)#19 45.10 bdl. 33.93 0.34 bdl. 0.15 17.56 1.38 bdl. bdl. bdl. 0.09 98.54 3-2(4)#20 45.79 0.01 33.95 0.38 0.01 0.15 17.53 1.36 0.02 0.02 0.01 bdl. 99.23 3-2(4)#21 45.26 bdl. 34.00 0.29 0.03 0.11 17.61 1.38 bdl. bdl. bdl. 0.12 98.79 3-2(4)#22 45.61 0.01 33.95 0.38 bdl. 0.14 17.74 1.32 0.01 0.04 0.06 bdl. 99.25 3-2(4)#23 45.69 bdl. 34.08 0.30 0.04 0.14 17.60 1.22 0.01 0.02 0.05 0.03 99.18 3-2(4)#24 45.38 bdl. 33.60 0.32 bdl. 0.14 17.50 1.26 bdl. bdl. 0.06 0.01 98.27 3-2(4)#26 45.35 bdl. 34.11 0.29 bdl. 0.12 17.43 1.32 0.01 0.03 0.06 0.07 98.79 注:该数据由电子探针测试分析得出;bdl.表示数据低于检测限。 表 3 研究区玄武岩中斜长石微晶的电子探针分析组分(部分结果)(%)
Tab. 3 EPMA compositions of plagioclase microlites in basalts of the study area (partial results) (%)
打点编号 组分 总量 SiO2 TiO2 Al2O3 FeOT MnO MgO CaO Na2O K2O P2O5 Cr2O3 SrO 3b3(1)#6 49.33 0.11 30.68 0.61 0.02 0.27 14.23 3.25 0.02 0.01 0.02 0.02 98.56 3b3(1)#7 52.44 0.15 28.21 0.99 0.02 0.45 12.14 4.22 0.02 bdl. 0.04 0.05 98.73 3b3(2)#5 50.24 0.09 30.40 0.61 bdl. 0.26 14.22 3.32 0.03 0.01 bdl. 0.04 99.21 3b3(2)#6 50.64 0.10 30.16 0.73 bdl. 0.34 13.82 3.46 0.02 0.03 bdl. bdl. 99.29 3b3(2)#8 54.99 0.17 26.63 1.27 0.01 0.35 10.20 5.55 0.09 bdl. 0.04 bdl. 99.30 3b3(3)#5 51.42 0.11 28.26 0.84 0.02 0.37 12.47 4.37 0.02 bdl. 0.01 0.11 98.00 3b3(4)#5 52.08 0.17 28.43 1.01 0.04 0.52 13.06 3.91 0.02 bdl. bdl. 0.01 99.24 3b3(4)#6 53.13 0.14 28.79 0.84 bdl. 0.37 12.49 4.21 0.03 0.04 bdl. bdl. 100.03 3b3(5)#5 51.32 0.05 29.75 0.89 0.02 0.35 13.90 3.50 0.04 0.01 0.02 bdl. 99.85 3b3(5)#6 52.88 0.03 28.88 1.15 bdl. 0.47 13.05 3.79 0.03 0.05 bdl. bdl. 100.33 3-1(2)#4 52.08 0.13 28.95 1.08 bdl. 0.51 13.09 3.67 0.03 bdl. 0.04 bdl. 99.56 3b2(2)#3 52.47 0.12 28.05 1.27 0.05 0.53 12.45 4.23 0.03 bdl. bdl. 0.04 99.25 3b2(2)#4 51.39 0.27 27.60 1.69 0.05 0.56 12.63 4.02 0.04 0.05 0.01 0.07 98.36 3b2(3)#5 52.29 0.10 28.88 1.08 0.03 0.51 13.11 3.85 0.03 bdl. 0.01 0.01 99.91 3b2(3)#6 50.04 0.07 30.98 0.55 bdl. 0.25 14.57 3.00 0.02 0.05 bdl. 0.03 99.55 注:该数据由电子探针测试分析得出;bdl.表示数据低于检测限。 -
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