Two types of garnets in the Cenozoic granites from the Himayalan Orogenic Belt: Geochemical characteristics and implications for crustal anatexis
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摘要:
在喜马拉雅碰撞造山带中, 石榴石是变泥质岩的主要造岩矿物, 也是花岗岩或淡色体的重要副矿物, 保存了有关地壳深熔作用的关键信息, 是揭示大型碰撞造山带中-下地壳物质的物理和化学行为的重要载体。在喜马拉雅造山带内, 新生代花岗质岩石(淡色花岗岩和混合岩中的淡色体)含两类石榴石, 大多数为岩浆型石榴石, 自形-半自形, 不含包裹体, 但淡色体中含有港湾状的混合型石榴石。岩浆型石榴石具有以下地球化学特征：(1)从核部到边部, 显示了典型的“振荡型”生长环带；(2)富集HREE, 亏损LREE, 从核部到边部, Hf、Y和HREE含量降低；(3)显著的Eu负异常；(4)相对于源岩中变质石榴石, Mn和Zn的含量显著增高。岩相学和地球化学特征都表明：变泥质岩熔融形成的熔体(淡色体)捕获了源岩的变质石榴石, 熔体与石榴石反应导致大部分元素的特征被改变, 只在核部保留了源岩的部分信息。同时, 在花岗质熔体结晶过程中, 形成少量的岩浆型石榴石。这些石榴石摄取了熔体中大量的Zn, 浓度显著升高, 在斜长石和锆石同步分离结晶作用的共同影响下, 石榴石中Eu为明显负异常，Hf、Y和HREE浓度从核部到边部逐渐降低。上述数据和结果表明, 花岗岩中石榴石的矿物化学特征记录了精细的有关花岗岩岩浆演化的重要信息。
Abstract:In the Himalayan Orogenic Belt, garnets occur as one of the key constituent phases in metapelites as well as one of the important accessory phases in leucogranites or leucosomes in migmatites. They have preserved critical information with regard to the geochemical nature of crustal anatexis and could yield important insights on the physical and chemical processes of middle to lower crustal rocks in large collisional orogenic belts. Along the Himalayan orogenic belt, two types of garnets, magmatic and metamorphic, occur in Cenozoic granitic rocks (leucogranite and leucosome). Magmatic garnets are euhedral to subhedral and commonly free of inclusions, whereas those derived from the source rocks show embayment texture and have been subjected to various degrees of melt-garnet reactions. Chemical analyses show that magmatic garnets are characterized by (1) typical oscillatory growth zonation; (2) enrichment in HREE, but highly depletion in LREE, and rim-ward decreases in Hf, Y and HREE; (3) pronounced negative Eu anomalies; (4) elevated Mn and Zn concentrations relative to those in metamorphic garnets derived from the source rocks. Petrographic and geochemical characteristics of garnets from leucogranites suggests that a large portion of Zn was incorporated in garnet and fractional crystallization of plagioclase and zircon is the major factor that regulate the magnitude of negative Eu anomalies and the abundance and rim-ward decreasing pattern of Hf, Y and HREE in magmatic garnet. Our data indicate that fine-scale chemical variations in magmatic garnet could serve as another important revenue to unravel the magmatic processes during the crystallization of leucogranitic melts.
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Key words:
- Leucogranite /
- Garnet /
- Trace element geochemistry /
- Crustal anatexis /
- Himalayan Orogenic Belt
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图 1
喜马拉雅造山带地质简图
Figure 1.
Simplified geologic map of the Himalayan Orogenic Belt, Southern Tibet
图 2
喜马拉雅造山带中不同地区含石榴石淡色花岗岩或淡色体的显微照片
Figure 2.
Photomicrographs showing the mineral assemblages and textures of granites and metapelite-hosted leucosome from different areas along the Himalayan Orogenic Belt
图 3
喜马拉雅造山带内淡色花岗岩和淡色体中石榴石的端元组分图解
Figure 3.
End-member discrimination diagrams for garnets in leucogranite and metapelite-hosted leucosome along the Himalayan Orogenic Belt
图 4
喜马拉雅造山带内不同地区淡色花岗岩中石榴石的端元成分剖面图
Figure 4.
Compositional profile for garnets in leucogranites and metapelite-hosted leucosome from different areas along the Himalayan Orogenic Belt
图 5
喜马拉雅造山带中不同地区淡色花岗岩类和变泥质岩中石榴石的Zn-ΣHREE (a)、Zn-Y (b)和Sc-Y (c)协变图
Figure 5.
Co-variation diagram Zn-ΣHREE (a), Zn-Y (b) and Sc-Y (c) for garnets in leucogranites and metapelite-hosted leucosome as well as metapelites from different areas along the Himalayan Orogenic Belt
图 6
喜马拉雅造山带中不同地区淡色花岗岩类中石榴石的稀土元素配分图(标准化值据Sun and McDonough, 1989)
Figure 6.
Chondrite-normallized REE distribution patterns for garnets in leucogranites and metapelite-hosted leucosome from different areas along the Himalayan Orogenic Belt (normalized values after Sun and McDonough, 1989)
图 7
喜马拉雅造山带中不同地区淡色花岗岩类中石榴石的(Gd/Yb)N-Y/Yb图解
Figure 7.
(Gd/Yb)N-Y/Yb diagram for garnets in leucogranites and metapelite-hosted leucosome from different areas along the Himalayan Orogenic Belt
图 8
喜马拉雅造山带中不同地区淡色花岗岩类的稀土元素配分图(标准化值据Sun and McDonough, 1989)
Figure 8.
Chondrite-normallized REE distribution patterns for leucogranites and metapelite-hosted leucosome from different areas along the Himalayan Orogenic Belt (normalized values after Sun and McDonough, 1989)
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