Anatexis and enrichment mechanism of the Fe-Ti oxide minerals in the quartzofeldspathic gneisses from the Larsemann Hills, East Antarctica
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摘要: 东南极拉斯曼丘陵高级变质长英质岩石中铁钛氧化物的局部聚集与高级变质作用过程中的深熔作用有关，并非原岩富集这些组分。深熔作用造成惰性组分如铁钛氧化物滞留原地或略有聚集及活动性组分的迁移，而流体挥发组分优先聚集于熔体之中。当体系中水含量较低、处于不饱和状态时，深熔作用过程中形成局部"熔体"，其结晶所成的浅色体不具低共结组分，没有熔体结晶结构，不是真正的熔体，可能是(准)熔体。较粗粒的浅色体或伟晶岩也是与深熔作用有关的产物，其形成早于花岗岩脉或岩体，而与花岗质岩浆分异无关。伴随(准)熔体的出现，体系中组分的萃取、分异效果较为明显，即可造成组分分异，形成截然不同的异地、二相分异结构，分别形成固相残留物(组成可以不固定)和(准)熔体相。固相残留体中富铝、铁组分，形成矽线石和铁钛氧化物团块，其中少或无挥发分；与此对应，短距离迁移浅色体中往往贫铁钛组分，可见石榴子石、偶见铁钛氧化物矿物。这种挥发分不饱和状态下的深熔作用基本属于封闭体系，整体失水不显著，高级变质岩中的一些特征矿物如矽线石、石榴子石、堇青石、尖晶石的形成也与这种分异作用有关，但组分迁移范围有限，并可保存组分分异各阶段的产物。拉斯曼丘陵长英质岩系中大量铁钛氧化物和矽线石类矿物组合的形成，反映了临界状态下的局部或差异抬升，变形作用的非均匀性及相伴随的组分分异作用，很可能相当于早期格林维尔期构造的泛非期再活动。Abstract: Fe-Ti oxide minerals can be locally aggregated or enriched in the quartzofeldspathic gneisses from the Larsemann Hills, East Antarctica. The enrichment is related to anatexis and subsequent high-grade metamorphism, not inherited from the protolith. The partial melting process was responsible for the residues or local enrichment of the inert Fe-Ti-Al elements and the migration of mobile components, and the volatiles were preferentially incorporated in the melt. In the water-deficient system, the local "melt" in anatexis crystallized to form the leucosomes without melt texture or minimum eutectic composition, suggesting the meta-melt feature, not real melt. The coarse leucosome or pegmatite occurred in anataxis as pre-granitic vein or body and had nothing to do with the late stage differentiation of the granitic magma. Together with the presence of the meta-melt, the substantial differentiation of the components resulted in solid residues and corresponding meta-melt phases, with the former enriched in Al, Fe elements and sillimanite, Fe-Ti oxide minerals occurred. On the contrary, the leucosomes with possible short distance migration of components were poor in Fe, Ti compositions, and seldom formed garnet and Fe-Ti oxides. The volatile-unsaturated anatexis was an essentially closed system and the total dehydration effect was not obvious. The presence of some typical minerals in high-grade metamorphism of the quartzofeldspathic gneisses, such as sillimanite, garnet, cordierite and spinel, was derived from component differentiation in partial melting of the rocks. However, the components migration was limited, and the evolving stages of metamorphism and differentiation could be preserved. Voluminous Fe-Ti oxides and sillimanite in the quartzofeldspathic gneisses suggest the possible local or differential uplifting of the area, the heterogeneity of deformation and accompanying components differentiation, corresponding to the reactivation of the earlier structures in the Pan-African event.
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Key words:
- Fe-Ti oxides /
- anatexis /
- component differentiation /
- Pan-African event /
- Larsemann Hills
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图 1 拉斯曼丘陵铁钛氧化物的野外宏观产状
Op-不透明矿物
a-拉斯曼丘陵米洛半岛铁钛氧化物富集区；b-淡色花岗岩中铁钛氧化物不规则团块；c-雁列状排列铁钛氧化物和浅色体；d-浅色体中夹有铁钛氧化物Figure 1. Mesoscopic occurrence of the Fe-Ti oxides in the Larsemann Hills
(a) Fe-Ti oxide minerals abundant in Mirror Peninsula, Larsemann Hills; (b) Irregular patches of Fe-Ti oxides within the leucocratic granite; (c) En echelon Fe-Ti oxides and leucosomes; (d) Fe-Ti oxide minerals interlayered with leucosomes
Op-opaque mineral
图 2 拉斯曼丘陵与铁钛氧化物有关的岩石结构和矿物
Crd-堇青石；Qtz-石英
a-铁钛氧化物与伟晶岩相邻；b-铁钛氧化物-富黑云母条带与伟晶岩相邻；c-原地深熔形成含石榴子石的浅色体团块；d-深熔形成不规则含石榴子石的浅色体团块；e-透镜状伟晶岩一侧有铁钛氧化物；f-布丁状张裂处充填堇青石-石英，附近为不规则浅色体Figure 2. Minerals related to the Fe-Ti oxides in the Larsemann Hills
(a) Fe-Ti oxides with adjacent pegmatite; (b) Fe-Ti oxide and biotite-rich band with pegmatitic leucosome; (c) In situ anatexis with garnet-bearing leucosome patch; (d) Irregular garnet-bearing leucosome; (e) Fe-Ti oxide band with granitic lens; (f) Crd-Qtz emplaced between boudin fissure and nearby leucosome patch
Crd-cordierite; Qtz-quartz
图 3 拉斯曼丘陵铁钛氧化物的露头产状
Bt-黑云母；Grt-石榴子石；Sil-矽线石；其他矿物缩写同图 2
a-铁钛氧化物及矽线石、石英；b-铁钛氧化物及矽线石；c-与浅色体相伴的堇青石；d-浅色体中的铁钛氧化物、石榴子石和黑云母条带；e-铁钛氧化物及石榴子石；f-伟晶质铁钛氧化物及长石、石英Figure 3. Outcrops of the Fe-Ti oxides in the Larsemann Hills
(a) Fe-Ti oxides with sillimanite and quartz; (b) Fe-Ti oxides and sillimanite; (c) Leucosome and accompanying cordierite; (d) Fe-Ti oxides, garnet and biotite bands within leucosome; (e) Fe-Ti oxides and garnet; (f) Pegmatitic Fe-Ti oxides and feldspar, quartz
Bt-biotite; Grt-garnet; Sil-sillimanite; Abbreviations for other minerals are shown in Fig. 2
图 4 与铁钛氧化物有关的显微结构
Spl-尖晶石；Crn-刚玉；Hem-赤铁矿；Ilm-钛铁矿；Mgt-磁铁矿；Sulphide-硫化物；其他矿物缩写同图 2，图 3
a-部分的矽线石转化为堇青石、尖晶石；b-矽线片麻岩中的尖晶石、堇青石；c-铁钛氧化物和刚玉；d-磁铁矿、尖晶石和黑云母的先后关系；e-与铁钛氧化物相伴的硫化物；f-石榴子石边缘转化为尖晶石、石英Figure 4. Microscopic textures with Fe-Ti oxides
(a) Sillimanite partially transformed into cordierite and spinel; (b) Spinel and cordierite in the sillimanite gneiss; (c) Fe-Ti oxides and corundum; (d) The successive magnetite, spinel and biotite; (e) Sulphides accompanying the Fe-Ti oxides and sillimanite; (f) Garnet rim retrograded to spinel and quartz. a, b, d, f-plane polarized in transmitted light; c, e-plane polarized in reflection light.
Spl-spinel; Crn-corundum; Hem-hematite; Ilm-ilmenite; Mgt-magnetite; Sulphide-sulfide; Abbreviations for other minerals are shown in Fig. 2 and Fig. 3表 1 拉斯曼丘陵赤铁-钛铁矿及相关矿物的电子探针成分分析
Table 1. Microprobe composition of Fe-Ti oxides and related minerals in the Larsemann Hills
矿物成分/% Sil Mgt Hem-Ilm Ilm-Hem Spr Spl Bt1 Bt2 SiO2 35.81 0.02 0.02 0.02 10.84 0.03 37.91 38.27 TiO2 0.01 0.03 45.63 18.48 0.03 0.01 4.36 4.40 Al2O3 63.11 0.10 0.00 0.02 64.41 62.61 14.75 14.94 Cr2O3 0.04 0.28 0.03 0.15 0.11 0.31 0.05 0.04 MgO 0.12 0.08 0.87 0.26 14.30 12.07 17.57 17.94 CaO 0 0 0 0 0 0 0 0 MnO 0 0 0.02 0 0.01 0 0.01 0 FeO 0.49 92.63 53.05 75.18 10.19 25.71 10.37 9.80 ZnO 0.005 0 0 0.027 0.068 0 0.017 Na2O 0 0.04 0 0 0 0.01 0.12 0.11 K2O 0 0 0 0 0 0 10.57 10.45 Total 100.54 93.18 99.62 94.09 99.92 100.83 95.70 95.96 O 5 4 3 3 20 4 11 11 Si 0.9741 0.0009 0.0005 0.0006 1.3092 0.0008 2.7754 2.7814 Ti 0.0002 0.0012 0.9001 0.4567 0.0029 0.0002 0.2402 0.2406 Al 2.0238 0.0060 0.0008 9.1689 1.9590 1.2729 1.2802 Cr 0.0043 0.0111 0.0004 0.0039 0.0102 0.0064 0.0031 0.0022 Fe3+ 0.0100 Mg 0.0049 0.0061 0.0006 0.0128 2.5749 0.4777 1.9179 1.9441 Ca 0 Mn 0.0005 0.0010 0.0004 Fe2+ 3.9618 1.1635 2.0655 1.0291 0.5709 0.6353 0.5958 Zn 0.0002 0.0022 0.0013 0.0009 Na 0.0040 0 0.0005 0.0172 0.0157 K 0.9860 0.9671 注：电子探针WDS(波谱仪)分析在中国地质科学院地质研究所电子探针室完成，仪器型号JXA-8230，电压15KV，电流20nA，束斑直径5 μm，标样为天然或合成的矿物和氧化物，主要氧化物的分析误差约为1%。其中Hem-Ilm、Ilm-Hem为铁、钛氧化物混溶体，前者以赤铁矿(Hem)为主，后者以钛铁矿(Ilm)为主。矿物缩写：Sil-矽线石；Mgt-磁铁矿；Hem-赤铁矿；Ilm-钛铁矿；Spl-尖晶石；Bt-黑云母(Bt1和Bt2分别为早期和晚期黑云母)；Spr-假蓝宝石 
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