A MARINE BACK-ARC ORIGIN FOR THE UPPER CARBONIFEROUS BASEMENT OF INTRACONTINENTAL GREATER TURPAN-JUNGGAR BASIN – VOLCANIC, SEDIMENTARY, AND GEOCHRONOLOGIC EVIDENCE FROM SOUTHERN BOGDA MOUNTAINS, NW CHINA

The Turpan-Junggar microplate in the SE Central Asia Tectonic Belt may have been formed by amalgamation of arcs, microcontinents, and/or exotic terranes, followed by intracontinental rifting in late Paleozoic. The Tarlong-Taodonggou section in southern Bogda Mountains in the plate interior provides data for the timing and nature of these processes. Pre-rift sedimentary and volcanic strata are 571 m thick, of which 295 m are theoleiitic pillow basalt flows. Thin felsic tuffs occur in siliciclastics; an 11-m ignimbrite caps pre-rift deposits, indicating mafic volcanism in a back-arc basin with nearby felsic volcanism that peaked at the end of amalgamation. A 70-m interval of graded lithic arenite, wacke, siltstone, and shale in the lower section contains 10s of cm-thick laterally-persistent upward-fining successions with erosional bases, interpreted as coarse- and fine-grained turbidites comprising two upward-fining and thinning retrogradational and one upward-coarsening and thickening progradational submarine fan complexes. Massive matrix-supported conglomerates in the upper section are volcaniclastic debris flow deposits. A 44-m interval of hummocky cross-stratified arenite, algal-laminated wacke with desiccation-cracked mud drapes, and clast-supported sheet conglomerate in the uppermost section contains plagioclase, K-feldspar, quartz, and basaltic and andesitic lithics, interpreted as shorezone deposits. A similar sequence occurs in Zaobishan area ~110 km to the east. The Bogda records indicate shallowing from basin-slope to shoreline environments and mixed volcanic and continental sediment sources in a back-arc basin. A regional unconformity separates pre-rift from overlying alluvial deposits. Timing of arc-continent amalgamation, marine-terrestrial transition, and rifting is constrained by U-Pb zircon TIMS dates. Back-arc theoleiitic and minor felsic volcanism occurred before 306.6 Ma and persisted to 301.1 Ma when amalgamation completed and the thick ignimbrite accumulated. Submarine fans developed at 306.6 Ma, progressed at 305.5 Ma, and stopped at an extrapolated age of 303.9 Ma. After a regional uplift, rifting started before 301.3 Ma, indicating a rapid transition from amalgamation to rifting in an ~0.1-m.y. span.