PALEOZOIC OPHIOLITIC MELANGE IN CENTRAL QIANGTANG, NORTHERN TIBET: GEOCHEMISTRY AND SHRIMP U-PB DATING

E-W trending mafic-ultramafic rocks occur widely in the middle part of the Qiangtang Block in northern Tibet. Due to the very limited accessibility, these rocks have only been discovered recently. They were interpreted by some workers as an ophiolite sequence, but such interpretation has not been universally accepted. In order to resolve this controversy, we undertook a detailed zircon geochronology and geochemical-isotopic study of these rocks. Our results confirm the ophiolite interpretation.

In the central Qiangtang block, two ophiolitic mélange suites have been identified: an Early Paleozoic (467-345 Ma) and a Late Paleozoic (Permian). The Early Paleozoic ophiolitic mélange occurs in the Gangma Co and Guoganjianian area, and it is composed of cumulate gabbro, basalt, hornblendite and plagiogranite. All the rock types have been metamorphosed to the greenschist facies. Geochemical data indicate that these rocks show typical N-MORB characteristics, with depleted or flat REE patterns, and positive e_Nd(T) values (4.9-8.8). SHRIMP Zircon U-Pb dating on the cumulate gabbros yielded three weighted mean ages of 467 ± 4 Ma, 438 ± 11 Ma and 345 ± 5 Ma. e_Hf(T) values of zircons range from 4.5 to 5.9 for cumulate gabbro from the Gangma Co area, and 5.1 to 13.1 for those from the Guoganjianian area. We conclude that this suite of rocks represent a Paleozoic ophiolitic mélange.

The Late Paleozoic or Permian ophiolitic mélange lies in the Jiaomuri area, and consists of ultramafic rocks (harzburgite), mafic dyke rocks (gabbro and diabase), pillow lavas and minor amount of chert. Geochemical analyses indicate that the basaltic rocks belong to two distinct groups, namely, alkaline and tholeiitic. The alkaline basalt samples are similar to OIB, whereas the tholeiitic basalt samples exhibit E-MORB characteristics. So far, no zircon ages have been determined for this ophiolitic mélange suite, but its age can be inferred as Permian from the index fossils (fusulinid) in the intercalated or overlying limestone. In addition, Permian oceanic island basalt with interbedded limestone and chert of the Lugu Formation are widespread in central Qiangtang.

The newly discovered ophiolitic mélange suites suggest that a Paleozoic Ocean basin (Paleo-Tethys) existed in central Qiangtang block during Middle Ordovician to Permian. The ocean was closed in the Middle to Late Triassic as inferred from the metamorphic ages of eclogite and blueschist that occur nearby and seem to have formed from the protolith of the Permian ophiolitic melange. The ophiolitic mélange in central Qiangtang block could be the western extension of the Sanjiang Paleo-Tethys ophiolite. A tectonic framework of the Qiangtang block and its implications will be presented.