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Sm-Nd and zircon SHRIMP U-Pb dating of Huilanshan mafic granulite in the Dabie Mountains and its zircon trace element geochemistry [查看] HOUZhenhui、LIShuguang、CHENNengsong、LIQiuli、LIUXiaoming

Abstract The mafic granulites from Huilanshan are outcropped on the center of the Luotian dome in the northern Dabie Mountains. The Sm-Nd isochron defined by granulite-facies metamorphic minerals (garnet + clinopyroxene + hypersthene) yields an age of 136 ± 18 Ma indicating the early Cretaceous granulite-facies metamorphism. The cathodoluminescence (CL) images of zircons from the granulite show clearly core-mantle-rim structures. The zircon cores are characterized by typical oscillatory zoning and highly HREE enriched patterns, which suggests their magma origin. Some zircon cores among them with little Pb loss give SHRIMP U-Pb ages ranging from 753 to 780 Ma, which suggests that the protolith of Huilanshan granulite is Neoproterozoic mafic rocks. The zircon mantles usually cut across the oscillatory zone of the zircon cores have 3―10 times lower REE, Th, U, Y, Nb and Ta contents than the igneous zircon cores but have high common Pb contents. These characteristics suggest that they were formed by hydrothermal alteration of the igneous zircons. The part of zircon mantles with little Pb loss give a similar SHRIMP U-Pb age (716―780 Ma) to the igneous zircon cores, which implies that the hydrothermal events occurred closely to the magmatic emplacement. In view of the strong early Cretaceous magmatism in the Luotian dome, consequently, the Huilanshan mafic granulite was formed by heating of the Neoproterozoic mafic rocks in mid-low crust, which caused the granulite-facies metamorphism underneath the Dabie Mountains. The similarity between the granulite metamorphic age (136 ± 18 Ma defined by Sm-Nd isochron and K-Ar age of 123―127 Ma given by amphible from the gneiss in Luotian dome suggests a rapid uplifting of the Luotian dome,which may result in further exhumation of the ultrahigh pressure metamorphic rocks in the Dabie Mountains.

A combined study of SHRIMP U-Pb dating ,trace elemnt and mineral inclusions on high-pressure metamorphic overgrowth zircon in eclogite from Qinglongshan in the Sulu terrane [查看] LIQiuli、LIShuguang、HOUZhenhui、HONGJi'an、YANGWei

Abstract Methods recently advanced for discrimination on the genesis of metamorphic zircon,such as annalysis of mineral inclusions and trace elements,provide us powerful means to distinguish zircon overgrowth during high-pressure metamorphism.Zircons in ultrahigh-pressure eclogite from Qinglongshan in the Sulu terrane were studied by the SHRIMP U-Pb method in combining with trace element and mineral inclusion analyses. No inherited core was identified in the annalyzed zircons by means of cathodoluminescence image.The occurrence of high-pressure metamorphic mineral inclusions in zircon,such as garnet,omphacite,rutile,and the flat HREE pattern in zircon indicate that the zircon formed at high-pressure metamorphic conditions.Therefore,a weighted average U-Pb age of 227.4 ±3.5Ma obtained from such a kind of zircon is interpreted to represent the timing of peak metamorphism for the Qinglongshan eclogite

Occurrence of Neoproterozoic low-grade metagranite in the western Beihuaiyang zone, the Dabie orogen [查看] LIUYiCan、LIULiXiang、GUXiaoFeng、LIShuGuang、LIUJia1、SONGBiao

Based on detailed field investigations and petrographic observations, we discovered Neoproterozoic-emplaced granite from the metavolcanics of the Dingyuan Formation in the western Beihuaiyang zone, the Dabie orogen. This study reports the results of zircon U-Pb dating and preliminary petrographic observations on two metagranite samples. The studied rocks experienced epidote-amphibolite facies metamorphism and strong structural deformation. Their U-Pb ages are 726 ± 6 and 758 ± 12 Ma, respectively, similar to those for the Luzhenguan complex in the eastern segment of the Beihuaiyang zone. In combination with previously determined 635 ± 5 Ma low-grade metagabbro, this study suggests the occurrence of at least two types of Neoproterozoic intrusive rocks in the Beihuaiyang zone, the northern margin of the South China Block (SCB): 726−758 Ma metagranite and 635 Ma metagabbro. These rocks occur within the metamorphosed Ordovician volcanic zone (originally named the Dingyuan Formation) and are in tectonic contact to each other, but they formed in different tectonic settings. The protolith ages for the Neoproterozoic low-grade metaigneous rocks are in good agreement not only with ages for two episodes of mid- and late-Neoproterozoic mafic and felsic magmatism in the Suizhou and Zaoyang area, Hubei Province, but also agree with protolith ages of ultrahigh-pressure metaigneous rocks in the Dabie-Sulu orogenic belt. In view of their tectonic relationships to country rocks, it appears that these Neoproterozoic low-grade rocks are exotic and they may have been detached and scraped from subducting SCB crust in the early Triassic during the initial continental subduction, and thrusted over Paleozoic metamorphosed rocks in the southern margin of the North China Block during continental collision.

Zircon SHRIMP U-Pb dating for olivine gabbro at Wangmuguan in the Beihuaiyang zone and its geological significanc [查看] LIUYican、LIShuguang、GUXiaofeng、HOUZhenhui

Zircon SHRIMP U-Pb dating was per-formed for olivine gabbro at Wangmuguan in the Beihuaiyang zone of the Dabie orogen and its coun-try rock (garnet-bearing epidote-mica-quartz schist). The results show that the gabbro was crystallized at 635 ± 5 Ma, in the late Neoproterozoic rather than in the late Paleozoic as previously suggested; its coun-try rocks formed at 464±7 Ma, younger than the en-closed gabbro. The U-Pb age for the gabbro is in good agreement with ages for tuff interbedded with sediments from the Doushantuo Formation in the South China Block and late-Neoproterozoic basic dyke swarms distributed on a large scale over areas of Suizhou to Zaoyang of Hubei Province in the northern margin of the South China Block. This sug-gests a large-scale magmatic activity occurred at the late Neoproterozoic in the South China Block, so that the gabbro at Wangmuguan in the western segment of the Beihuaiyang zone is geotectonic affinity to the northern margin of the South China Block. Since the olivine gabbro occurs within the schist of Ordovician protolith with tectonic contact between them but forming in different tectonic settings, it is concluded that the late-Neoproterozoic gabbro was detached from the Precambrian basement of the South China Block during the Triassic subduction of the South China Block, and tectonically thrusted over the metamorphosed rocks in the southern margin of the North China Block.

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