High-pressure mafic oceanic rocks from the Makbal Complex, Tianshan Mountains (Kazakhstan & Kyrgyzstan): Implications for the metamorphic evolution of a fossil subduction zone
High-pressure mafic oceanic rocks from the Makbal Complex, Tianshan Mountains (Kazakhstan & Kyrgyzstan): Implications for the metamorphic evolution of a fossil subduction zone
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DOI：
10.1016/j.lithos.2013.06.015
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发表时间：
2013-09
期刊：
影响因子：
3.5
通讯作者：
M. Meyer;R. Klemd;D. Konopelko
中科院分区：
文献类型：
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作者：
M. Meyer;R. Klemd;D. Konopelko
哈萨克斯坦和吉尔吉斯斯坦西天山的马克巴尔杂岩由高压/超高压变质沉积主岩组成，包裹着各种高压镁铁质地块或包裹体。这些镁铁质岩石包括罕见的榴辉岩(严格意义上的榴辉岩和意义上的拉托榴辉岩)、石榴石角闪岩(退变质的榴辉岩)和新发现的蓝闪石(蓝闪石-石榴石-绿辉石)。到目前为止，马克巴尔杂岩被解释为主要由大陆岩性组成，镁铁质岩被认为是侵入大陆变质沉积的解体岩墙。这一解释主要基于不同岩石类型的地质关系和块体岩石化学。为了揭示马克巴尔杂岩中镁铁质高压岩的变质演化和原岩特征，将常规地质测温、P-T假剖面模拟和常量元素全岩地球化学研究相结合，揭示了马克巴尔杂岩中镁铁质高压岩的变质演化和原岩特征。结果表明，蓝闪石和榴辉岩(Sensu Lato)样品经历了相似的P-T进动轨迹和略有不同的峰期变质条件，前者在2.4 GPa时的P-T进动温度约为560°C，后者在2.2 GPa时的约520℃和~22.5 GPa时的约555°C之间，对应的埋藏深度在70~85 Km之间。榴辉岩(Sensu Lato)被认为是强退退的以前的榴辉岩相岩石，由于不适宜的贫碱主体成分(Na2O<；)，从未成为榴辉岩(Sensu Strecto，即70%的石榴石和绿辉石)。本文研究的4个高压镁铁质岩石样品来自洋壳(Zr/Hf比值为33~35)，这与以往认为马克巴勒地区所有镁铁质高压/超高压岩石均为大陆原岩的研究结果相矛盾。不同基性大洋(本研究)和大陆岩石的并置可能是由于浮力驱动俯冲通道中的变质沉积主岩的折返，在不同的深度捕获了俯冲洋壳的解体碎片。
The Makbal Complex in the western Tianshan Mountains of Kazakhstan and Kyrgyzstan consists of HP/UHP metasedimentary host rocks which enclose various HP mafic blocks or boudins. These mafic rocks comprise rare eclogites (sensu stricto and sensu lato), garnet amphibolites (retrograded eclogites) and a newly discovered glaucophanite (glaucophane–garnet–omphacite bearing rock).So far the Makbal Complex has been interpreted to predominantly consist of continental lithologies and the mafic rocks were considered as dismembered dikes intruding continental metasediments. This interpretation is mainly based on the geological relationship and bulk rock chemistry of the different rock types. It was further suggested that the continental lithologies of the Makbal Complex underwent eclogite-facies metamorphism in a former subduction zone.In the present study we combined conventional geothermometry, P–T pseudosection modeling and major and trace element whole rock geochemistry for different mafic samples (glaucophanite and eclogites (sensu lato)) in order to shed light on both the metamorphic evolution and the protoliths of the mafic HP rocks in the Makbal Complex.Prograde to peak-pressure clockwise P–T paths of glaucophanite and eclogites (sensu lato) were modeled using garnet isopleth thermobarometry. The results show that the glaucophanite and eclogite (sensu lato) samples experienced similar prograde P–T paths and slightly different peak metamorphic conditions at ~ 560 °C at 2.4 GPa for the former and between ~ 520 °C at 2.2 GPa and ~ 555 °C at ~ 2.5 GPa for the latter, corresponding to burial depths between 70 and 85 km.Whole rock major and trace element analyses and petrological evidence imply that the various rock types at the Makbal Complex most likely originated from different precursor rocks. Eclogites (sensu lato) are believed to represent strongly retrogressed former eclogite-facies rocks that had never been eclogites (sensu stricto, i.e. > 70 vol.% garnet and omphacite) due to an unfavorable alkali-poor bulk composition (Na2O < 1 wt.%). The four high-pressure mafic samples investigated in this study originated from oceanic crust (Zr/Hf ratio of 33 to 35) which contradicts all previous studies suggesting a continental protolith for all mafic HP/UHP rocks at Makbal.The present study indicates that the mafic high-pressure rocks represent incoherent segments of exhumed oceanic crust. Juxtaposition of different mafic oceanic (this study) and continental rocks is suggested to be due to buoyancy-driven exhumation of the metasedimentary host rock in the subduction channel where dismembered fragments of the subducted oceanic crust were captured in different depths.