Bainang Terrane, Yarlung-Tsangpo suture, southern Tibet (Xizang, China): a record of intra-Neotethyan subduction-accretion processes preserved on the roof of the world

Journal of the Geological Society, May 2004 by Ziabrev, Sergey V, Aitchison, Jonathan C, Abrajevitch, Alexandra V, Et al

intrusions. Sills of high-Ti alkaline basalt up to tens of metres thick are abundant in the southern tract. They are particularly abundant in the Maniga unit, where they form up to 50% of the unit's volume. Baked contacts with host rocks indicate that metamorphism associated with intrusion predates shearing. The sills are structurally disrupted by the same tectonic features that imbricate other elements of the Bainang terrane stratigraphy. They intrude most lithologies, except the youngest red radiolarian cherts. Thus, basic magmatism probably predates deposition of this chert, and is inferred to be pre-Callovian. As the sills have intruded all other lithologies, including upper Aalenian-Bathonian tuffaceous chert, intrusion is constrained to a narrow interval in the late Mid-Jurassic. We note that coeval intraplate basic alkaline magmatism is known from potentially correlative rocks in the Western Ladakh Himalaya (Danelian & Robcrtson 1997; Robertson & Sharp 1998) where Middle jurassic lavas and volcaniclastic deposits occur in the Karamba Complex, and diabase sills are common in the adjacent Lamayuru Complex.

Synthesis: a model for Neotethys evolution

Detailed examination of the Bainang terrane elucidates two distinct aspects of Neotethyan evolution: (1) the history of the floor of this ocean is preserved in fragments of material that have been accreted into the terrane; (2) consumption of oceanic lithosphere and the nature of subduction-accretion processes at an intra-oceanic subduction system are recorded in the accretionary wedge.

Depositional setting and travel history of accreted material

The remnant stratigraphy of an oceanic plate fragment preserved in an accretionary wedge provides temporal constraints on the travel history of subducted oceanic material and its accretion (Isozaki et al. 1990; Matsuda & Isozaki 1991). Few remnants of any subduction complexes are well preserved along the length of the suture between India and Asia. Rocks within the Bainang terrane provide constraints on accreted Neotethyan oceanic material. They are interpreted in terms of depositional settings and compared with rocks from the modern ocean floor or exposed on land in accretionary wedges. Where the Bainang terrane record is incomplete, correlation with other units described from further west along the suture permits reconstruction of the history of sedimentation upon the now subducted floor of Ncotethys.

Northern tract: travel and approach towards a convergent margin. A similar mix of lithologies and stratigraphy occurs in the Bangga and Zongxia units. It therefore seems plausible that they accumulated in close proximity. The oldest rocks are red radiolarian chert, a distinctive oceanic pelagic lithology that is well known from many accretionary wedges preserved on-land (Isozaki et al. 1990; Matsuoka & Yao 1990; Zyabrev 1996; Kusky & Bradley 1999) as well as from drilling on the oceanic floor in the western Pacific (Matsuoka 1992). This lithology accumulated below the CCD far from the influence of any terrigenous sedimentary input.