Retrogressive Microstructures of the Eclogites from the Northern Dabie Mountains, Central China: Evidence for Rapid Exhumation

The ultrahigh-pressure eclogites from the northern Dabie Mountains in central China occurred as tectonic lens or blocks within granitic gneisses or meta-peridotites. Petrologic studies suggest that the studied eclogites experienced strongly retrogressive metamorphism and produced a series of characteristic retrogressive microstructures. The retrograde structures mainly include: (1) oriented needle mineral exsolution, e.g., quartz needles in Na-clinopyroxene and rutile, clinopyroxene and apatite exsolution in garnet formed under decreasing pressure conditions during exhumation; (2) symplectite, especially, two generations of symplectites developed outside the garnet grains, which are called “double symplectite” here; (3) compositional zoning of minerals such as garnet and clinopyroxene; (4) minerals with a reaction rim or retrograde rim, e.g., omphacite with a diopside rim, diopside with an amphibole rim and rutile with a rim of ilmenite. These retrograde textures, especially mineral zoning and symplectite, provide important petrologic evidence for the exhumation process and uplift of high-grade metamorphic rocks such as eclogite in the northern Dabie Mountains, indicating a rapid exhumation process.

The Carboniferous at the Northern Foot of the Dabie Mountains and Its Tectonic Implications

In the Qinling orogenic belt. oceanic crust originated in the Early Palaeozoic. while the product of conti-nental collision appeared as late as after the Triassic. The Late Palaeozoic records there are of major impor-tance for understanding the tectonic regime at that time. The Carboniferous and even Permian sequences andthe distribution of sedimentary facies in northern Huaiyang indicate that the rocks were formed in a large basinopening towards the south. Regional stratigraphic correlation shows that the interior of the Qinling orogenicbelt was a sea trough lying between the Yangtze and North China plates in the middle part of the LatePalaeozoic. With subsequent northward migration of the South China Sea, the two seas were connected witheach other. Both the melanges and the Dabie block ia the eastern sector of the Qinling belt were formed in theMesozoic ?.

Mineral Composition and Metamorphic P-T Conditions of the Eclogitic Rocks in the Eastern Dabie Mountains and Their Genetic Implication

There are obvious differences in the mineral assemblage and metamorphic P-T conditions between the eclogites from the northern and southern parts of the eastern Dabie Mountains. Those from the northern part of the mountains are developed in Alpine peridotite and gneiss. They have a mineral assemblage of garnet+diopside with no quartz, and were formed at temperatures of 600℃-740℃. Those from the southern part are developed in gneiss and marble. They consist of garnet+omphacite+less quartz and were metamorphosed at temperatures in the range of 650°-800℃. These differences suggest that the former may be formed during the metamorphism of the deep subducted oceanic crust, whereas the latter may be genetically related to the subduction of the continental crust in this area.

Thrusting and Exhumation Processes of a Bounding Mountain Belt: Constraints from Sediment Provenance Analysis of the Hefei Basin

Lithic (or gravel) composition analyses of the Jurassic Sanjianpu Formation and Fenghuangtai Formation in the Hefei basin show that the sediment provenance consists mainly of four kinds of rock units: the basement metamorphic complex, granitic rocks, medium- and low-grade metamorphic rocks, and sandy and muddy sedimentary rocks, which are distributed along the bounding thrust belt. The whole stratigraphic section can be divided into 2 lithic sequences and 7 subsequences. The regular distribution and changes of lithic fragments and gravels in lithic (or gravel) sequences reflect that the bounding thrust belt of basin has undergone 2 thrusting cycles and 7 thrusting events. Lithic (or gravel) composition analyses of the basin fully reveal that the northern Dabie basement metamorphic complex was exhumed on the earth's surface in the Middle and Late Jurassic, and extensive intermediate and acid intrusive rocks were developed in the southern North Huaiyang or northern Dabie Mountains during the basin's syndepositional stage.