Genesis and geological significance of Early Jurassic metamorphic gabbro in the Sumdo area, Tibet

The metamorphic gabbro newly discovered in the Sumdo area, the eastern part of the Gangdese magmatic belt, is of importance for discussing the evolution history of the Sumdo Paleo-Tethyan Ocean. In this paper, on the basis of detailed field geological investigation, we report LA-ICP-MS zircon U-Pb ages, zircon Hf isotopic and whole-rock major and trace element compositions from the metamorphic gabbro, with the aims of understanding their petrogenesis and the implications for Sumdo Paleo-Tethyan Ocean evolution. LA-ICP-MS zircon U-Pb isotopic dating indicates that the gabbros emplaced at 195 +/- 6Ma similar to 202 +/- 2Ma. It is characterized by its contents of variable SiO2 (45.56% similar to 53.69%), and high MgO (6.04% similar to 11.75%), Mg-# (66 similar to 77), Cr (209x10(-6) similar to 746x10(-6)) and Ni (80x10(-6) similar to 234x10(-6)). The REE distribution patterns are between E-MORB and OIB. They are characterized by smooth REE patterns curves inclining gently to right with LREE slightly enriched. The large ion lithophile elements (LILEs) and the high field strength element (HFSEs) (such as Nb, Ta) are riched. They have low (La/Yb)(N)=2.06 similar to 4.81, (La/Sm)(N)=1.50 similar to 2.48, (Gd/Yb)(N)=1.21 similar to 1.67 and (Ce/Y)(N)<2. Lu-Hf isotope study demonstrates that its epsilon(Hf)(t) is between -1.9 and 3.9,and the value of ancient zircon Hf crustal modal ages changes greatly from 988Ma to 1361Ma. Combining with regional geology, geochronolagical, geochemical and zircon Hf isotopic data, we propose that the primary of Early Jurassic Sumdo Metamorphic gabbro was derived by partial melting of a depleted lithospheric mantle modified by ancient lower crustal material. They were formed in an extensional tectonic environment related to the lithospheric thinning by slab breakoff of Sumdo Paleo-Tethyan Ocean.