甘肃敦煌红柳峡地区石榴石斜长角闪岩的变质特征、锆石U-Pb年龄及地质意义  

Metamorphism,zircon U-Pb dating and tectonic implications of garnet amphibolites from Hongliuxia,Dunhuang,Gansu Province

作　　者：王忠梅[1,2,3] 肖文交[1,2] 韩春明[1,2] 敖松坚[1,2] 王洛娟[1,3] 

机构地区：[1]中国科学院地质与地球物理研究所,北京100029 [2]新疆矿产资源研究中心,乌鲁木齐830011 [3]中国科学院大学,北京100049

出　　处：《岩石学报》2013年第5期1685-1697,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41272107;41230207;41190072;41202150;41102132);国家科技支撑计划项目(2011BAB06B04-1)联合资助

摘　　要：甘肃敦煌红柳峡石榴石斜长角闪岩原岩为岛弧拉斑玄武岩,形成于不成熟岛弧。其典型矿物组合为:石榴石(Grt)-角闪石(Amp)-透辉石(Di)-斜长石(Pl),其中较粗大的变斑晶石榴石中通常保存进变质生长环带。根据岩相学和矿物化学研究得出,该区石榴石斜长角闪岩经历了四个变质演化阶段:早期进变质阶段(M1)、变质高峰期阶段(M2)、近等温减压阶段(M3)和晚期降温退变质阶段(M4)。M1阶段以石榴石核部成分及其核部包体矿物组合石榴石(GrtⅠ)+角闪石(AmpⅠ)+斜长石(PlⅠ)+石英(Qtz)为特征,P-T估算结果为T=550～600℃,P=0.3～0.5GPa;M2阶段以变斑晶石榴石幔部成分(GrtⅡ)及基质中的矿物组合角闪石(AmpⅡ)+透辉石(Di)+斜长石(+PlⅡ)+石英(Qtz)为特征,P-T估算结果为T=650～780℃,P=0.8～0.9GPa;M3阶段的代表性结构为石榴石边缘(GrtⅢ)形成由绿色角闪石(AmpⅢ)和斜长石(PlⅢ)组成的"白眼圈",该阶段温度估算结果为630～700℃,压力大幅降低;M4阶段透辉石(Di)开始向角闪石(AmpⅣ)和斜长石(PlⅣ)转化,该阶段压力变化不大,主要是温度的降低。温压估算结果表明,红柳峡石榴石斜长角闪岩完整记录了从早期升温升压的进变质到后来的近等温减压再到最后的降温退变质的一个顺时针的PTt演化轨迹,反映的构造演化历史为:板块俯冲到地壳深部遭受高温变质作用,然后该区出现拉张的构造体制,随之地壳减薄,但是原来处于地壳深部的岩石并未出现大幅度折返和抬升,而是仍然处于原来的深度,慢慢的冷却直至正常的地温梯度。锆石U-Pb定年结果表明,红柳峡石榴石斜长角闪岩原岩的结晶年龄为1611±6Ma,该期年龄代表的构造事件与哥伦比亚超大陆的裂解事件一致,结合前人的研究成果我们推断,塔里木板块可能与哥伦比亚超大陆的裂解有联系,且当时位于塔里木东北缘的敦煌地块还是岛弧的构造环境,后来在中元�The protolith of the Hongliuxia amphibolites is tholeiite formed in immature arc, and the typical mineral assemblages are garnet （Grt）, amphibole （Amp）, diopside （Di） and plagioclase （P1）. The coarse-grain garnet porphyroblasts usually display obvious zonings. Petrographic and mineral chemistry indicate that the amphibolites have undergone four successive stages： prograde, peak, isothermal decompression and isobaric cooling. The prograde metamorphic assemblages （ M1 ） are the minerals preserved in garnet porphyroblasts （Grt I ＋ Amp I ＋ P1 I ＋ Qtz） , and the P-T metamorphic conditions are estimated to be 550℃ to 600℃ and 0. 3 Gpa to 0. 5Gpa; the peak metamorphic assemblages （M2） are the mantle part of garnet and the minerals in the matrix （Grt II ＋ Amp 11 ＋ Di ＋P1 II ＋ Qtz）, P-T estimates give peak metamorphic conditions of 650℃ to 780℃ and 0. 8 Gpa to 0. 9Gpa; the M3 stage is characterized by the P11I ＋ Amp m corona around garnet porphyroblasts, which infer an obvious decompression process, and the temperature of metamorphism is estimated to be 630℃ to 700℃ ; the temperature of the M4 stage drop a lot, which is inferred from the breakdown of diopside to produce AmpIV ＋ PIIV. The estimated P-T conditions indicate a clockwise P-T-t path. The tectonic evolution history is： plate subduction cause high-temperature metamorphism, after that the ernst tends to be thin, then the deep rock cool down to the normal geothermal gradient. The crystallization age of garnet-bearing amphibolite is 1611 ± 6Ma, which is contemporary to the breakup Columbia Supercontinent, so the Tarim Craton may connect with the breakup of Columbia Supercontinent.

关 键 词：石榴石环带 矿物反应 石榴石斜长角闪岩 锆石U-PB定年 变质演化 塔里木克拉通 

分 类 号：P588.346[天文地球—岩石学] P597.3[天文地球—地质学]