澎湖群島二輝粒變岩及含斜長石輝石岩包體之岩石學與地球化學研究

Petrography and geochemistry of two-pyroxene granulite and plagioclase-bearing pyroxenite xenoliths of Penghu Islands

何恭算(Kung-Suan Ho)

《國立臺灣博物館學刊》 74卷1期 (2021/03) Pp. 1-37

https://doi.org/10.6532/JNTM.202103_74(1).01

摘要

本文針對澎湖鹼性玄武岩及玄武質集塊岩內31件包體進行分析，這些包體可分為二輝粒變岩和含斜長石輝石岩兩大類，並具有層狀堆晶或粒狀至粒狀變晶的岩石組織。二輝粒變岩包體是由斜輝石、直輝石和斜長石，以及少量的橄欖石、鉀長石和鈦鐵氧化物等礦物所組成，它們具有基性（SiO_2介於47 wt.%至52 wt.%之間）但全岩化學成分並不均質，且輕稀土元素中度富集（(La/Yb)_N = 4.00-10.86）的特徵。此類包體之主要及微量元素含量和鍶—釹同位素比值（^(87)Sr/^(86)Sr = 0.703678-0.704251;^(143)Nd/^(144)Nd = 0.512858-0.512948）與晚新生代華南玄武岩者類似，因此，筆者認為它是由底附玄武質岩漿經結晶、變質作用所形成。含斜長石輝石岩包體是由斜輝石、斜長石、填隙型角閃石、石榴子石和（或）不透明礦物所組成，它比二輝粒變岩包體有較低的SiO_2（40-44 wt.%）、輕稀土元素（(La/Yb)_N = 0.49-1.57）和較高的Al_2O_3含量，成分上與矽質苦橄岩相近。含斜長石輝石岩因組成礦物之間沒有達到同位素平衡，研判這些礦物是從不同熔體相結晶而來，它們可能是在正發生結晶分化和交換填補作用下的岩漿庫裡晶出，或是在岩脈系統中逐次結晶的產物，隨後再與富含揮發性熔體晶出之填隙型角閃石一起膠結的結果。由於兩類包體之鍶—釹同位素比值（含斜長石輝石岩之^(87)Sr/^(86)Sr = 0.704438-0.704655;^(143)Nd/^(144)Nd = 0.512914-0.513182）有明顯的差異，顯示兩者並無成因關係。二輝粒變岩包體以二輝地溫計算出平衡溫度為802-973℃，而含斜長石輝石岩用斜輝石－石榴子石地溫計求得平衡溫度為1006-1158℃。根據鍾孫霖（1990）推演之澎湖古地溫研判，前者係源自下部地殼（相當深度約20-32公里），後者依照斜輝石－石榴子石－斜長石地壓計算出平衡壓力為8.44-11.72 Kb，是來自30-42公里的上部地函。

關鍵字

岩石學 ； 地球化學 ； 二輝粒變岩 ； 含斜長石輝石岩 ； 澎湖群島

並列摘要

31 xenoliths collected from Penghu alkali basalts and basaltic pyroclastic rocks were analyzed. These xenoliths include two-pyroxene granulites and plagioclase-bearing pyroxenites, with textures ranging from meta-igneous cumulates to metamorphic granular. Granulites have mineral assemblages of clinopyroxene, orthopyroxene and plagioclase, with accessory olivine, alkali feldspar and Ti-Fe oxide minerals. These xenoliths are basic (SiO_2 = 47-52 wt.%) but heterogeneous in bulk chemical compositions, with mild LREE ((La/Yb)_N = 4.00-10.86) enrichment. Major- and trace elements and Sr-Nd isotopic ratios (^(87)Sr/^(86)Sr = 0.703678-0.704251; ^(143)Nd/^(144)Nd = 0.512858-0.512948) of the granulite resemble those of late Cenozoic SE China basalts and it is suggested that this xenolith was formed by the crystallization and metamorphism of the underplated basaltic melt. Plagioclase-bearing pyroxenites xenoliths contain clinopyroxene, plagioclase, interstitial amphibole, garnet and/or opaque minerals. They have low SiO_2 (40-44 wt.%), LREE ((La/Yb)_N = 0.49-1.57) and high Al_2O_3 relative to granulite xenoliths, and compositionally are similar to tholeiitic picrites. Because there is no isotopic equilibrium among the constituent minerals of plagioclase-bearing pyroxenite, it is suggested that the minerals crystallised from different melt phases, possibly in a magma chamber undergoing fractionation and recharge or by progressive crystallisation in a vein system. The plagioclase-bearing pyroxenite thus formed was then subsequently fragmented and/or re-cemented by a subsequent volatile bearing-melt which crystallised the interstitial amphibole. Isotopic differences between two-pyroxene granulite and plagioclase-bearing pyroxenite (^(87)Sr/^(86)Sr = 0.704438-0.704655; ^(143)Nd/^(144)Nd = 0.512914-0.513182) indicate these two types of xenoliths are not cogenetic. Thermobarometric studies suggested that the two-pyroxene granulites yielded 2-pyroxene temperatures in the range of 802-973℃, whereas the plagioclase-bearing pyroxenites gave clinopyroxene-garnet temperatures of 1006-1158℃. These temperatures indicate that the studied two-pyroxene granulite xenoliths were derived from the lower crust (depths of ca. 20-32 km) on the basis of the Penghu geotherm suggested by Chung (1990). Based on garnet-clinopyroxene-plagioclase barometer the plagioclase-bearing pyroxenites equilibration pressures were estimated around 8.44-11.72 kbar (depths of ca. 30-42 km), well within the upper mantle.

並列關鍵字

Petrography ； Geochemistry ； Two-pyroxene granulite ； Plagioclase-bearing pyroxenite ； Penghu Islands

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