Genesis and Characteristics of the Two Sub-belts in the Western Yarlung-Zangbo Suture Zone, Tibet, China

Abstract:

The Yarlung Zangbo Suture zone (YZSZ), extending more than 2000 km across southern Tibet, is thought to mark the tectonic boundary between the Indian and Asian plates. West of Saga county of the YZSZ is divided into two sub-belts (Dajiweng-Saga to the north and Daba-Xiugugabu to the south), which are separated by the Zhongba-Zhada microcontinent/terrane containing Sinian to Triassic metamorphic and sedimentary rocks.

Several large ophiolitic massifs that crop out in the southern sub-belt, e.g., the Dongbo (400 km²), Purang (650km²) and Xiugugabu (700km²), have a seamount-like stratigraphy, consisting from the base upward of peridotites, mafic dikes, massive basalt and basaltic hyaloclastites, mudstone and silty shale and radiolarian chert. The sedimentary sequence also locally contains discontinuous basalt flows and siliceous to massive limestones with minor oolites. The peridotites are highly depleted harzburgites, minor lherzolites and dunites, all of which were enriched by later fluids/melts. Mafic dikes (120-130 Ma) intruding the peridotites exhibit N-MORB-like chondrite-normalized REE patterns and negative Nb anomalies in spider diagrams. These geochemical features are similar to those of volcanic-rifted margin basalts. E-MORB-like basalts (137 Ma) within the Late Jurassic to Early Cretaceous radiolarian cherts and OIB-like basalts (140 Ma) between the silty shale and peridotites reflect limited seamount volcanism.

In the northern sub-belt ophiolitic peridotites, mafic dikes, cumulate gabbros, massive basalts, chert and limestone crop out discontinuously as lenses in mélange complexes. The peridotites are harzburgite and minor dunite, both of which were modified by later LREE-enriched melts/fluids. Mafic dikes (122-127 Ma) intruding the peridotites are geochemically similar to those in the southern sub-belt. A 500-meter-thick sequence of cumulate gabbro is tectonically sandwiched between the peridotites and basalts. A 1- 2 km sequence of OIB-type basalt unconformably overlies the gabbro in the Saga massif.

Ophiolitic rocks of the northern belt can be compared with the Xigaze ophiolite in the central YZSZ. If the later formed in Triassic (~170Ma) suggested in some publications, the southern belt should be represented a slightly younger Jurassic to Cretaceous oceanic basin.