INITIATION AND DEVELOPMENT OF THE CENTRAL ASIAN OROGENIC BELT: THE VIEW FROM NORTH CHINA

The Central Asian Orogenic Belt (CAOB) was initiated by late Paleozoic or early Mesozoic closure of the Paleoasian Ocean, which led to amalgamation of the North China block (NCB) with oceanic arc terrane(s) that now constitute much of Mongolia. We present a new detrital zircon (DZ) dataset from late Paleozoic-early Mesozoic strata of the NCB that contains more than 4000 U-Pb ages and 500 εHf(t) values. This dataset provides new constraints on the timing of closure of the Paleoasian Ocean, and on the tectonic setting of the NCB before, during, and after this closure.

In the eastern NCB, numerous fault-bounded exposures of a conformable, upward-coarsening sequence of Carboniferous-Triassic strata with recycled orogen and transitional arc provenance are strongly suggestive of deposition in an expansive retroarc foreland basin south of an active continental arc. Phanerozoic U-Pb DZ ages in these strata define a periodicity of ~60 Ma in this North China arc, with major peaks of magmatism at ~380 Ma, ~320 Ma, and ~260 Ma. The εHf(t) values for these arc-derived zircons range from +2 to -25, tend to become more negative as each magmatic phase progresses, and abruptly become more positive with each successive magmatic phase. Major Precambrian peaks in the DZ spectra are consistent with NCB continental crust.

Zircons with Phanerozoic U-Pb ages and εHf(t) values that do not fit this pattern are likely derived from elsewhere. The first appearance of such "foreign" zircons occurs in sandstone and conglomerate with a maximum depositional age of 245 Ma. Strata of Middle Triassic age and younger in the NCB tend to contain Phanerozoic zircons with both strongly positive (>+5) and strongly negative (<-30) εHf(t) values. They also tend to contain ages that do not occur on the NCB. U-Pb ages younger than 245 Ma comprise a peak at ~230 Ma, followed by a distinct magmatic gap until ~190 Ma.

We interpret that a Carboniferous-Lower Triassic retroarc foreland basin spanning the NCB was disrupted by orogeny associated with closure of the Paleoasian ocean at ~245 Ma. Post-orogenic magmatism followed ocean basin closure at ~230 Ma, succeeded by a documented episode of post-orogenic extension in Late Triassic-Early Jurassic time. Magmatism then ceased until the onset of paleo-Pacific arc volcanism at ~190 Ma.