Origin and tectonic significance of the graphite-bearing marble in the southern Dunhuang tectonic belt
The Dunhuang tectonic belt is located in the southern Central Asian Orogenic Belt(CAOB).Its tectonic attribute has been debated for a long time about whether Dunhuang tectonic belt is a Precambrian stable block or a Phanerozoic orogenic belt.This controversy restricts the understanding of the accretionary process of the southern CAOB and of the interaction between the Paleo-Asian Ocean and Tethyan Ocean.In this paper,structural geology,microscopic petrography,geochemistry and Raman spectroscopy have been conducted for the graphite-bearing marble exposed in Hongliuxia metamorphic complex,southern Dunhuang tectonic belt.In the Hongliuxia valley,the graphite-bearing marbles stack with amphibolites having the geochemistry affinity to seamount basalt or enclose these amphibolite blocks.Olivine,amphiboles and their serpentinized minerals can be seen under microscope observation.The whole-rock rare earth element(REE)contents of the samples are much less than that of post-Archean average Australian shale(PAAS).It shows that the protoliths of the marbles were not supplied by terrigenous debris but by large amount of basic igneous debris.It is presumed that their protoliths were a carbonate cap formed on a seamount or oceanic island.The graphite-bearing marbles in Ganhongou valley are outcropped as interlayers or lens in mica-quartz schists.They contain terrigenous debris such as quartz and white mica lacking basic igneous materials.Their PAAS normalized REE patterns show negative Ce anomalies,and their whole-rock Y/Ho ratios are similar to that of PAAS.These features indicate that the protoliths the marbles were deposited in a relatively oxidized coastal environment and influenced by terrigenous detrital materials.It is presumed that their protoliths were formed in the slope basin on accretionary wedge.The graphite Raman spectra indicate the metamorphic temperature of the two kinds of marble ranges from 552 ℃ to 577 ℃ and 431 ℃ to 476 ℃(T1,thermometer of Rahl),or from 534 ℃ to 539 ℃ and 472 ℃ to 503 ℃(T2,thermometer of Beyssac).This result indicates the two kinds of marble experienced different degrees of metamorphism.These data suggest that the protoliths of the marble in Hongliuxia metamorphic complex were formed in different tectonic environments including the seamount and slope basin on accretionary wedge.They experienced subduction-exhumation in the subduction zone and are considered as components of a subduction accretionary complex,and probably proves that the Dunhuang tectonic belt is an orogenic belt rather than a Precambrian block.
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