Abstract
The eclogite-bearing Alag Khadny metamorphic complex in the Lake Zone, SW Mongolia occupies the central region of the Central Asian Orogenic Belt, the largest Phanerozoic orogenic belt in the world. The complex consists mainly of orthogneisses intercalated with eclogites and micaschists in a mélange zone. Most of eclogites are strongly amphibolitized. In this study, we examined petrography and mineral chemistry of eclogites and amphibolitized eclogites, respectively. The result of our research shows that Chandman eclogites experienced multiple events of metamorphism in throughout their subduction and subsequent collision history. We revealed that eclogites were subjected to blueschist facies metamorphism before the peak eclogite facies stage. In addition, we have studied amphibolitized eclogite, and revealed that another distinct progressive medium pressure (MP) epidote-amphibolite facies metamorphic event took place in the eclogite, consistent with collision process. The multiple events of metamorphism in eclogites have been revealed by zonation textures of HP amphiboles zoned with glaucophane→barroisite→Mg-hornblende and MP amphiboles zoned with actinolite/winchite→barroisite→Mg-hornblende/tschermakite/Fe-pargasite. These amphiboles with different zonation textures reflect their metamorphic history of subduction to collision events.
Highlights
Relics of subduction related high-pressure (HP) and relatively low-temperature (LT) rocks are common in orogenic belts
Petrographic textural relationships and mineral assemblages suggest multiple cycles of. This evolution is expressed by two types of zonation in HP and medium pressure (MP) amphiboles of eclogites—i.e., progressive metamorphism or events (HP and MP) proposed for the Chandman eclogites. This glaucophane→barroisite→Mg-hornblende (Amp2 coexisting with omphacite formed in HP event) evolution is expressed by two types of zonation in HP and MP amphiboles of eclogites—i.e., and actinolite/winchite→barroisite→Mg-hornblende/tschermakite/Fe-pargasite (Amp2 coexisting with glaucophane→barroisite→Mg-hornblende (Amp2 coexisting with omphacite formed in HP event) plagioclase and epidote formed in MP event)
The new insight of our results shows that zoning patterns of amphiboles in Chandman eclogite typically follow its multiple metamorphic events with subduction and collision histories
Summary
Relics of subduction related high-pressure (HP) and relatively low-temperature (LT) rocks are common in orogenic belts. Only one occurrence of a high-pressure eclogite-bearing complex has been identified in the Chandman district, SW Mongolia [15,16] This eclogite-bearing complex consist mainly of large bodies of mostly amphibolitized eclogite (up to 2 km long and 0.7 km width) with orthogneisess, micaschists, and serpentinite bodies, and is situated in the central part of the CAOB near the Zamtyn Nuruu and Dzavkhan-Baidrag basement blocks [17,18]. Our work presents the first account of blueschist to eclogite facies metamorphic events related to subduction processes followed by amphibolitization by collision Evidence for these processes have been revealed by textures and mineralogical studies of amphiboles. We propose that once subduction related metamorphism ended, the eclogites were juxtaposed and metamorphosed with surrounding country rocks during the collision
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