Two-phase subduction and subsequent collision defines the Paleotethyan tectonics of the southeastern Tibetan Plateau: Evidence from zircon U-Pb dating, geochemistry, and structural geology of the Sanjiang orogenic belt, southwest China

Abstract

A curved belt consisting of terranes or blocks and associated late Paleozoic to early Mesozoic sutures are features of the south- eastern part of the Tibetan Plateau. The southeastern Tibetan Plateau Paleotethyan belt is divided into a northwestern segment (central Tibetan Plateau) and a southern seg- ment (the Sanjiang orogenic belt). The Paleo- tethyan tectonic history of this belt remains a subject of considerable debate. A major con- troversy concerns whether or not it evolved as a single genetic element. This paper pro- vides new geochronological, geochemical, and structural data to constrain the Paleotethyan tectonics of this belt. A detailed fi eld study across three sections of the Sanjiang orogenic belt has identifi ed two volcaniclastic succes- sions separated by a regional angular uncon- formity in the northwestern segment, and one continuous volcaniclastic succession in the southern segment. New zircon U-Pb ages reveal that: (1) the lower succession of the northwestern segment was produced by three volcanic phases at ca. 272 Ma, 253–247 Ma, and 240–235 Ma; (2) the upper succession of the northwestern segment formed during a shorter period along the Yushu–Yidun arc from ca. 227 to 211 Ma; and (3) the single volcaniclastic succession of the southern seg- ment was produced continuously from ca. 260 to 210 Ma. Whole-rock geochemical data including trace elements show that the late Paleozoic to early Mesozoic magmatic activ- ity has a continental arc–like affi nity.