Subduction and exhumation mechanisms of ultra‐high and high‐pressure oceanic and continental crust at Makbal (Tianshan, Kazakhstan and Kyrgyzstan)
Subduction and exhumation mechanisms of ultra‐high and high‐pressure oceanic and continental crust at Makbal (Tianshan, Kazakhstan and Kyrgyzstan)
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DOI：
10.1111/jmg.12097
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发表时间：
2014-10
影响因子：
3.4
通讯作者：
M. Meyer;R. Klemd;E. Hegner;D. Konopelko
中科院分区：
文献类型：
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作者：
M. Meyer;R. Klemd;E. Hegner;D. Konopelko
哈萨克斯坦和吉尔吉斯斯坦天山北部的Makbal杂岩由变质沉积岩组成，其中含有高P （HP）基性块体和超P （Grt - cold - Tlc）片岩（UHP由石榴石中的共生岩遗迹表明）。Grt -冷-薄层片岩的全岩主量元素和微量元素特征表明，该片岩可能来自弧后盆地的热液蚀变海洋地壳或与弧相关的火山碎屑。Grt - cold - Tlc片岩的峰值变质条件为~580℃，2.85 GPa，最大埋藏深度为~95 km。Sm - Nd石榴石年龄为475±4 Ma，被解释为石榴石在进峰变质过程中的平均生长年龄；- 11的低初始εΝd值表明原岩具有古地壳成分。洋壳深俯冲的岩石学证据对有效的掘出机制提出了疑问。在马巴尔杂岩中心嵌入陆源变质沉积岩中的其他高压基性海洋岩石的场关系和变质演化表明，在持续的俯冲过程中，海洋地壳和碎屑沉积岩的碎片在俯冲通道的不同深度被挖掘出来，现在作为一个构造模子暴露出来。此外，水道流不仅可以解释由具有不同俯冲历史的各种岩石类型组成的构造范围，而且还可以解释由低压推覆体（包括大陆地壳中的基性岩脉）和主要来自增生楔的大量变质沉积岩所包围的构造“圆顶”（核心（Makbal中央））的存在。
The Makbal Complex in the northern Tianshan of Kazakhstan and Kyrgyzstan consists of metasedimentary rocks, which host high‐P (HP) mafic blocks and ultra‐HP Grt‐Cld‐Tlc schists (UHP as indicated by coesite relicts in garnet). Whole rock major and trace element signatures of the Grt‐Cld‐Tlc schist suggest a metasomatized protolith from either hydrothermally altered oceanic crust in a back‐arc basin or arc‐related volcaniclastics. Peak metamorphic conditions of the Grt‐Cld‐Tlc schist reached ~580 °C and 2.85 GPa corresponding to a maximum burial depth of ~95 km. A Sm‐Nd garnet age of 475 ± 4 Ma is interpreted as an average growth age of garnet during prograde‐to‐peak metamorphism; the low initial εΝd value of −11 indicates a protolith with an ancient crustal component. The petrological evidence for deep subduction of oceanic crust poses questions with respect to an effective exhumation mechanism. Field relationships and the metamorphic evolution of other HP mafic oceanic rocks embedded in continentally derived metasedimentary rocks at the central Makbal Complex suggest that fragments of oceanic crust and clastic sedimentary rocks were exhumed from different depths in a subduction channel during ongoing subduction and are now exposed as a tectonic mélange. Furthermore, channel flow cannot only explain a tectonic mélange consisting of various rock types with different subduction histories as present at the central Makbal Complex, but also the presence of a structural ‘dome’ with UHP rocks in the core (central Makbal) surrounded by lower pressure nappes (including mafic dykes in continental crust) and voluminous metasedimentary rocks, mainly derived from the accretionary wedge.