Sequence and tectonic deformation process of metamorphic complex in the Larsemann Hills, East Antarctica
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摘要: 东南极拉斯曼丘陵地区位于兰伯特裂谷东缘普里兹湾东岸，该地区主要出露一套麻粒岩相变质岩，前期对原岩时代、变质过程等进行了详细研究，但是对于变质杂岩的层序和变形过程研究相对薄弱。文章通过大比例尺地质填图，发现拉斯曼丘陵地区变质杂岩总体成层有序，在此基础上建立拉斯曼岩群，并将其划分成6个岩组，原岩形成时代为中元古代。拉斯曼岩群经历了格林维尔期和泛非期变质作用的叠加，变质程度均达到高角闪岩相-麻粒岩相。拉斯曼丘陵地区主体构造线方向为北东东—南西西方向，总体上构成往北东东方向翘起的复式向斜构造，几个岩组的分布也显示由东向西逐渐变新。东部米洛半岛一带明显叠加了北北西—南南东向的构造变形。研究表明，拉斯曼岩群经历了6次重要的构造变形，包括新元古代格林维尔期(D1)、新元古代—早古生代泛非期变质变形作用(D2，D3，D4，D5)以及中新生代伸展作用(D6)。目前岩石中保存的主变形面理是格林维尔期和泛非期两次构造热事件的复合型面理，主要是泛非事件形成，格林维尔期变形面理呈残留状。综合拉斯曼岩群变质年龄及早古生代进步花岗岩体形成时代，认为D2~D5变形时代为550~500 Ma左右。因此，拉斯曼丘陵地区变质变形特征显示，中元古代拉斯曼岩群经历了格林维尔期和泛非期两次重要的造山作用，以及冈瓦纳大陆的裂解。Abstract: The Larsemann Hills are located on the eastern coast of the Prydz Bay in East Antarctica. Based on large-scale geological mapping, metamorphic complex in the Larsemann Hills was found to be layered orderly in general, and therefore, the Larsemann Group is established. The Larsemann Group is subdivided into 6 rock formations, and the protolith formation age is the Mesoproterozoic. The group has experienced the superposition of the Grenvillian and Pan-African metamorphism, and the metamorphic grade reached upper amphibolite facies to granulite facies. The main structural line in the Larsemann Hills is in the NEE-SWW strain, which generally constitutes a synclinorum structure verging to NEE. The distribution of several rock formations also shows the gradually younging from east to west. The NNW-SSE deformation of the structural line is obviously superimposed on the eastern Mirror Peninsula. The study shows that the Larsemann Group has suffered 6 periods of deformation, including the early Neoproterozoic Grenvillian period (D1), the late Neoproterozoic to early Paleozoic Pan-African periods (D2, D3, D4 and D5) and the Meso-Cenozoic extension (D6). The foliations presented in the rocks are actually the composite foliations of both the Grenvillian and Pan-African events, and the Pan-African event is demonstrated stronger than the Grenvillian event, which is rarely preserved in the gneisses. Constrained with both the metamorphic age of the Larsemann Group and the intrusion time of the Progress granite, it is believed that the D2~D5 deformations occurred during the span of 550~500 Ma. Thus, both the metamorphism and deformation features of rocks from the Larsemann Hills show that the Mesoproterozoic Larsemann Group have witnessed two orogenies of Grenvillian and Pan-African periods, respectively, and the breakup of the Gondwana.
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Key words:
- East Antarctica /
- Larsemann Hills /
- Larsemann Group /
- tectonic deformation /
- tectonic framework
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图 2 东南极拉斯曼丘陵地区构造纲要图
Figure 2. Structural geological map of the Larsemann Hills in East Antarctica
图 3 拉斯曼岩群各岩组代表性岩石
a—深灰色薄层状含辉石长英质片麻岩；b—混合岩化石榴黑云长英质片麻岩；c—含磁铁矿泥质长英质片麻岩；d—混合岩化长英质片麻岩；e—基性麻粒岩；f—细粒块状片麻岩
Figure 3. Typical rocks of the Larsemann Group in the Larsemann Hills
(a) Dark gray thin bedded pyroxene-bearing quartzofeldspathic gneiss; (b) Migmatized garnet-biotite quartzofeldspathic gneiss; (c) Magnetite-bearing pelitic-quartzofeldspathic gneiss; (d) Migmatized quartzofeldspathic gneiss; (e) Mafic granulite; (f) Fine-grained massive gneiss
图 4 条带状磁铁矿化泥质长英质片麻岩
a—磁铁矿化泥质长英质片麻岩露头特征；b—富磁铁矿条带
Figure 4. Banded magnetically mineralized pelitic-quartzofeldspathic gneiss
(a)Outcrop of argillaceous felsic gneiss formation containing magnetite; (b) Magnetite-rich bands
图 5 拉斯曼丘陵米洛半岛中山站地区地质构造图
Figure 5. Structural geological map of the Mirror Peninsula in the Larsemann Hills
图 6 高温变质泥质片麻岩中发育的不同期次变形面理(S1、S1-2及S3)
a—S3、S1-2及残留早期面理S1；b—S3平行于S1和S2叠加形成的复合变形面理
Figure 6. Deformation foliations (S1, S1-2, and S3) developed in the high-temperature metamorphic pelitic gneiss
(a) S3, S1-2 and residual early foliation S1; (b) S3 is parallel to the composite foliation formed by the superimposition of S1 and S2
图 7 条带状混合片麻岩和泥质长英质片麻岩中发育的顺层褶皱(进步站)
a—拉伸线理；b—剪切褶皱；c—变形面理、线理赤平投影
Figure 7. Bedding folds developed in banded migmatitic gneiss and pelitic-quartzofeldspathic gneiss in the Progress Station
(a)Straching lineation; (b) Shearing folds; (c) Stereographic projection of deformation foliation and lineation
图 8 中山站附近低角度韧性剪切带内发育的A型褶皱群
a、b—平行于低角度韧性剪切带的A型褶皱轴，深色岩石为基性麻粒岩，浅色岩石为泥质长英质片麻岩；c—变形岩石中发育的拉伸线理平行于褶皱轴；d—剪切带中面理与线理投影
Figure 8. A-type fold group developed in a low angle ductile shear zone near the Zhongshan Station
(a and b) The A-type fold parallel to the extensional lineation in the low angle ductile shear zone (The dark rock is mafic granulite, and the light rock is pelitic-quartzofeldspathic gneiss); (c) The extensional lineation developed in the deformed rock is parallel to the fold axis; (d) Polar stereographic projection of foliation and lineation in the shear zone
图 9 剪切变形带中泥质长英质片麻岩矿物定向组构
Qtz—石英；Pl—斜长石；Bt—黑云母；Sil—矽线石
a—富矽线石黑云斜长片麻岩中片柱状矿物强烈定向；b—韧性变形的长英质浅色脉体，石英与斜长石塑形变形呈长条状Figure 9. Mineral oriented fabric of pelitic-quartzofeldspathic gneiss in ductile shear zone
(a) Schistose and columnar minerals are strongly oriented in biotite plagioclase gneiss rich in sillimanite; (b) Ductile deformed granitic leucosome veins, in which quartz and plagioclase are plastically deformed into strips
Qtz—quartz; Pl—plagioclase; Bt—biotite; Sil—sillimanite
图 10 拉斯曼丘陵条带状混合片麻岩和泥质长英质片麻岩中顺层褶皱(进步站)
a—条带状混合片麻岩；b—泥质长英质片麻岩
Figure 10. Bedding folds in the banded migmatitic gneiss and pelitic-quartzofeldspathic gneiss near the Progress Station in the Larsemann Hills
(a) Banded migmatitic gneiss; (b) Pelitic-quartzofeldspathic gneiss
图 11 浅色混合花岗质脉体沿泥质长英质片麻岩的脆性破裂贯入
a—沿脆性破裂贯入的浅色脉体；b—剪切变形的深熔浅色脉体
Figure 11. Leucosome veins injected along the brittle fractures of pelitic-quartzofeldspathic gneiss
(a) Leucosome veins injected along the brittle fractures; (b) Shear deformed lecosome veins
图 12 拉斯曼岩群晚期变形面理(S6)
a、b—晚期北东—北北东向密集破劈理；c、d—晚期变形面理的赤平投影图(下半球投影)
Figure 12. Late deformation foliation (S6) of the Larsemann Group
(a and b) Late dense cleavage in NE-NNE trending; (c and d) Stereographic projection of late deformation foliation(Lower hemisphere projection)
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