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摘要:
雅浦岛弧基本由变质岩组成, 与西太平洋的其他岛弧具有显著区别。利用在雅浦岛弧获得的变质岩样品, 研究了角闪岩的变质温压条件, 并进一步探讨了雅浦岛弧变质岩的构造环境, 以及加罗林洋底高原及海山的俯冲侵蚀对雅浦岛弧的影响。雅浦岛弧变质岩的矿物组合结果显示, 其变质相为角闪岩相和绿片岩相; 对雅浦岛弧变质岩进行了电子探针微区分析, 采用角闪石单矿物温度压力计、斜长石-角闪石Na-Ca交换温度计和角闪石-斜长石Al-Si压力计来计算变质P-T条件。矿物温压计的估算结果表明, 雅浦岛弧角闪岩变质P-T条件为493.6~630℃/3.8~6kbar, 为中等P/T型变质相系。结合矿物组合及温压计估算结果, 认为雅浦变质岩可能经历了岛弧区域变质作用。雅浦变质岩代表的构造环境为雅浦岛弧基底。雅浦角闪岩的埋藏深度大约为15~20km, 雅浦岛弧变质岩的出露是俯冲侵蚀的结果。加罗林洋底高原及海山促进了雅浦岛弧俯冲侵蚀的发育, 使得雅浦岛弧的弧前地区甚至岛弧都遭受侵蚀, 导致变质岩基底出露于海底。
Abstract:The Yap Island Arc consists largely of metamorphic rocks and thus differs significantly from the most of island arcs of the western Pacific.It is crucial to study the origin and process of the metamorphic rocks in the Yap arc for delineating the tectinic setting of the Yap arc-trench system.In this study, the metamorphic rocks in the Yap arc were analyzed using an electron microprobe (EMP) to reasearch temperature-pressure conditions of the amphibolite rocks.Results of mineral assemblages in metamorphic rocks of Yap arc show that metamorphic facies of Yap metamorphic rocks are greenschist and amphibolite.Temperature-pressure conditions of the Yap amphibolite rocks were calculated by the thermobarometry and barometer on the basis of compositions of mineral pairs.The temperature-pressure results are 493.6~630℃/3.8~6kbar, and the facies series of which belongs to medium P/T series.The medium P/T series is the characteristic of common orogenic belts.Considering the results of mineral assemblages and temperature-pressure conditions, we concluded that the metamorphic rocks in the Yap arc were formed in the setting of a volcanic arcs.Metamorphic rocks in the Yap arc represent the metamorphic basements of Yap arc.The Yap amphibolite rocks were buried in the depth of 15~20 km and the exposure of metamorphic rocks is resulted from subduction erosion of the Caroline plate.Caroline Ridges and seamounts enhanced subduction erosion.The Yap arc basements underwent regional metamorphism, then the uplifted section of volcanic arc were eroded bythe subduction erotion.Eventually the metamorphic basements of Yap arc croped out the earth surface.
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Key words:
- Yap island arc /
- amphibolite /
- regional metomorphism /
- subduction erosion
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表 1 雅浦岛弧变质岩样品的岩相学特征及其变质相
Table 1. Mineral assemblages of metamorphic facies at the Yap arc
样品 岩性 岩相学特征 变质相 Y3-9-6 角闪岩 变晶结构，致密块状构造，主要矿物组合：角闪石+斜长石（拉长石）；少量钠长石，辉石，褐铁矿 角闪岩相 Y3-9-8 角闪岩 变品结构，片状至弱片麻状构造，主要矿物组合：角闪石+斜长石（更长石）；少量钾长石、钠长石、黑云母、辉石、绿泥石、榍石，磁铁矿、钛铁矿 角闪岩相 Y3-9-10 角闪岩 变晶结构，致密块状构造，主要矿物组合：角闪石+斜长石（中长石、拉拉长石）；少量钾长石、钠长石.辉石、榍石、沸石、磁铁矿、钛铁矿 角闪岩相 Y3-9-12 绿片岩 变晶结构，片状构造，中等蚀变，主要矿物组合：绿帘石+绿泥石+阳起石+斜长石（更、中、拉、倍长石）；少量沸石、磁铁矿 绿片岩相 Y30059-1 角闪岩 变晶结构，致密块状构造，主要矿物组合：角闪石+斜长石（中长石）；少量钠长石、钛铁矿、榍石、绿泥石、沸石 角闪岩相 
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表 2 雅浦岛弧角闪岩样品代表性角闪石、单斜辉石和长石电子探针数据及相应离子数
Table 2. Representative mineral chemical composition of amphiboles, clinopyroxenes and feldsparsin amphibolites samples of Yap arc
角闪石 Y3-9-6 Y3-9-8 Y3-9-10 Y30059-1 单斜辉石 Y3-9-6 Y3-9-8 Y3-9-10 N=25 N=14 N=16 N=12 N=1 N=2 N=9 SiO2 45.24 44.92 45.02 44.59 SiO2 51.70 45.50 52.33 TiO2 1.20 0.82 0.88 1.74 TiO2 0.25 0.78 0.12 Al2O3 10.45 10.62 9.53 9.95 Al2O3 2.75 9.53 1.45 Cr2O3 0.13 0.08 0.07 0.05 Cr2O3 0.03 0.12 0.02 FeO 15.93 13.35 15.74 13.70 FeO 10.19 12.92 8.48 MnO 0.29 0.26 0.34 0.31 MnO 0.35 0.24 0.42 MgO 12.11 13.14 12.34 18.23 MgO 12.64 13.65 13.22 CaO 10.97 11.79 11.85 11.42 CaO 21.37 11.91 23.25 Na2O 1.86 1.68 1.30 1.66 Na2O 0.58 1.65 0.44 K2O 0.08 0.81 0.69 0.86 K2O 0.03 0.71 0.02 Total 98.26 97.47 97.76 97.50 Total 100.40 97.00 99.75 T位 Si 6.52 6.55 6.59 6.51 Si 1.93 1.76 1.96 AlⅣ 1.48 1.45 1.41 1.49 AlⅣ 0.07 0.24 0.04 Ti(Ⅳ) 0.00 0.00 0.00 0.00 AlⅥ 0.06 0.19 0.03 C位 AlⅥ 0.30 0.38 0.23 0.22 Ti 0.01 0.02 0.02 Ti 0.13 0.09 0.10 0.19 Cr 0.00 0.00 0.00 Cr 0.01 0.01 0.01 0.01 Fe3+ 0.06 0.24 0.05 Fe3+ 0.98 0.56 0.76 0.69 Fe2+ 0.27 0.17 0.21 Fe2+ 0.94 1.07 1.17 0.98 Mn 0.01 0.01 0.01 Mg 2.60 2.86 2.69 2.88 Mg 0.72 0.79 0.74 Mn 0.04 0.03 0.04 0.04 Ca 0.86 0.49 0.93 B位 Fe2+ 0.00 0.00 0.00 0.00 Na 0.04 0.12 0.03 Ca 1.70 1.84 1.86 1.79 K 0.00 0.04 0.00 Na 0.30 0.16 0.11 0.21 A位 Na 0.21 0.32 0.23 0.26 Wo 44.82 29.06 47.86 K 0.01 0.15 0.13 0.16 En 37.49 46.36 37.89 Total 0.23 0.47 0.36 0.42 Fs 17.69 24.58 14.25 长石 Y3-9-6 Y3-9-8 Y3-9-10 Y3-9-12 Y30059-1 钠长石 拉长石 钾长石 钠长石 更长石 钾长石 钠长石 中长石 拉长石 更长石 中长石 拉长石 倍长石 中长石 N=7 N=7 N=5 N=ll N=6 N=15 N=2 N=ll N=7 N=7 N=2 N=2 N=4 N=23 SiO2 59.78 52.51 63.54 65.58 62.56 64.27 64.25 56.72 54.32 64.03 61.41 53.21 48.63 57.89 Al2O3 32.04 29.94 17.93 21.71 23.25 18.31 25.24 27.17 28.21 23.48 26.16 29.06 32.40 26.15 FeO 0.07 0.29 0.16 0.18 0.24 0.06 0.16 0.24 0.27 0.22 0.16 0.20 0.31 0.28 CaO 0.81 12.83 0.04 1.32 3.96 0.04 0.77 9.07 11.01 4.24 7.62 11.90 15.84 8.38 Na2O 6.94 4.30 0.16 9.52 8.92 0.30 8.58 6.16 5.17 8.88 4.89 4.74 2.54 6.78 K2O 0.02 0.03 17.05 1.45 0.74 17.25 0.23 0.23 0.21 0.25 0.08 0.03 0.03 0.21 Total 99.67 99.90 98.88 99.75 99.66 100.22 99.22 99.60 99.18 101.09 100.32 99.13 99.75 99.68 Si 2.60 2.39 2.99 2.89 2.78 2.98 2.82 2.56 2.47 2.80 2.70 2.43 2.23 2.60 AI 1.64 1.60 0.99 1.13 1.22 1.00 1.30 1.44 1.51 1.21 1.35 1.56 1.75 1.39 Fe 0.00 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 Ca 0.04 0.62 0.00 0.06 0.19 0.00 0.04 0.44 0.54 0.20 0.36 0.58 0.78 0.40 Na 0.59 0.38 0.01 0.81 0.77 0.03 0.73 0.54 0.46 0.75 0.42 0.42 0.23 0.59 K 0.00 0.00 1.02 0.08 0.04 1.02 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.01 Total 4.87 5.00 5.03 4.99 5.01 5.04 4.90 5.00 5.00 4.98 4.84 5.00 5.00 5.01 An 6.07 62.14 0.18 6.51 18.86 0.20 4.62 44.28 53.42 20.57 46.02 58.03 77.37 40.09 Ab 93.71 37.68 1.37 84.99 76.94 2.57 93.75 54.37 45.37 77.97 53.41 41.82 22.45 58.73 注，角闪石中的Fe3+由Holland等[27]方法计算：单斜辉石中的Fe3+通过化学计量平衡和电价平衡规则[31, 32]估算。
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表 3 雅浦岛弧角闪岩变质温压条件计算结果
Table 3. P-T estimates for the metamorphism of amphibolite samples in Yap arc
P-T 角闪石单矿物温度压力计 斜长石-角闪石 Na-Ca温度计/℃ 角闪石-斜长石 Al-Si压力计/kbar 温度/℃ 压力/kbar Y3-9-6 566.6~611, 6 4.7~5.8 — — Y3-9-8 572.2~630.1 5.0~6.0 — — Y3-9-10 493.6~620.6 3.8~5.7 721~851 1.4~5.9 Y3005-1 582.4~625.6 4.6~5.5 742~822 2.3~4.8
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