| Citation: | WANG Bin, XIE Chaoming, DONG Yongsheng, SONG Yuhang, DUAN Menglong. Geochemical characteristics of ultramafic rocks in Sumdo area, Xizang and its enlightenment for the evolution of the Sumdo Paleo-Tethys Ocean[J]. Geological Bulletin of China, 2022, 41(7): 1144-1154. doi: 10.12097/j.issn.1671-2552.2022.07.002 |
The proposal of Tangjia-Sumdo Paleo-Tethys suture zone has changed the understanding of Paleo-Tethys evolution pattern in the Qinghai Xizang Plateau.The petrological and geochemistry of ultramafic rocks in Sumdo area are studied to manifest the genesis and type of the ophiolite in the Tangjia-Sumdo Paleo-Tethys suture zone.The results show the ultramafic rocks have higher MgO and TFe2O3 but lower Al2O3and TiO2 contents compared with those of the primitive mantle.The total rare-earth element(REE)contents of the samples are 4.04×10-6~9.31×10-6, which are lower than those of the primitive mantle.The chondrite-normalized REE patterns display a U-type, and they are enrichment in LREE.The primitive mantle-normalized spider diagrams show an abyssal peridotites affinity, there are obvious negative anomalies of Th and Nb.Most of the samples show slight negative Zr and Hf anomalies.Through quantitative modeling, we conclude that the ultramafic rocks are refractory residual formed by more than 25% batch melting of spinel lherzolite mantle, and the inheritance of some garnet signatures was observed in the samples to a certain extent.They are not modified by later rock-melt reaction, the protolith of the ultramafic rocks may be depleted abyssal peridotite.Combining with regional geology, we suggest that the ultramafic rocks in Sumdo area may have been formed in the mid ocean ridge(MOR)setting and are one of the typical end members of mid ocean ridge basalt(MORB)type ophiolites.
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Tectonic outline of the Xizang Plateau(a), geological map of the Sumdo area(b)and sampling location map of ultramafic rocks(c)
Geological section of Wenmulang ophiolite in Sumdo, Xizang
Discrimination diagrams of major elements in the ultramafic rocks
Chondrite-normalized REE patterns(a)and primitive mantle normalized trace elements patterns(b) for the ultramafic rocks in Sumdo
(MgO+TFeO)-(m+f)/Si diagram of the ultramafic rock in Sumdo
YbN-(Ce/Sm)N(a) and major and trace elements-MgO(b~f) diagrams of the ultramafic in Sumdo
Partial melting diagrams of the ultramafic rocks
Rock-melting reaction diagram of the ultramafic rocks