Home

Search Abstracts

Author Index

Symposia Programmes

Sponsors

Help

UHP-02 Collisional orogeny, ultrahigh-pressure metamorphism and crustal melting

The geochemical characteristics of the metabasites from Ganghe, Dabie mountains, China

Zhi Xie, CAS Key Laboratory of Crustal-Mantle Materials and Environments (China)
Jiangfeng Chen, CAS Key Laboratory of Crustal-Mantle Materials and Environments (China)
 

The Ganghe metabasites, which preserve pillow structure, belong to the Yangtze unit within ultrahigh-pressure (UHP) metamorphic terrain in Dabie Mountains, China. Like the adjacent Bixiling eclogite body, these metabasites were initially emplaced in the crust and then subducted to mantle depth, and underwent UHP metamorphism during the subduction of the Yangtze craton under the North China Block. Zircon U-Pb SHRIMP data give a discordia, yielding an upper intercept age of 770 Ma, which is inerpreted as the time of protolith emplacement, consisting with the age of the volcanoclastic rocks in the same unit. Data points around the lower intercept give a weighted average ²⁰⁶Pb/²³⁸U age of 227±9 Ma, reflecting the Triassic UHP metamorphism. Thus, the metabasites underwent open-system chemical and isotopic processes three times. Early crustal contamination during magmatic differentiation is indicated by the negative Nb anormaly, high initial ⁸⁷Sr/⁸⁶Sr ratios (0.705 ∼ 0.708) and negative ε_Nd(T) values of -6 to -12 at 770 Ma (the protolith age). Post-eruption thermal alteration is suggested by the lack of whole-rock Sm-Nd isochron relationship, and an extreme enrichment of LREE. Finally, highly mobile elements, such as K and Sr, are heterogeneous, but P, Fe, Mg and Ti are homogeneous during UHP metamorphism.
Furthermore, the metabasites can be divided into two groups. Three closely located pillows surrounded by muscovite schist are similar, with higher ΣREE, ⁸⁷Sr/⁸⁶Sr initial ratios and lower negative ε_Nd(T) values than that of the forth pillow about 50 m away. The geochemical and isotopic compositions of the former pillows suggested that their source was contaminated by crustal materials, and that of the forth pillow was the lest contaminated. Unlike the Maowu layered intrusion, which was contaminated by the upper crust, the magmas of the Ganghe metabasite were contaminated by the lower crust in the magma chamber and during magma acsending. The metabasites may represent magmatic rocks erupted in a rift setting during the Neoproterozoic, and be distinct from the Maowu and Bixiling intrusive bodies, which were intruded in an intracontinent envoirment.
This work is supported by the CAS Key Program (KZCX2-yw-131).