Ancient Granites in China Reveal the Final Death of a Lost Ocean

More than 400 million years ago, a vast ocean that once separated the great landmasses of early Earth quietly vanished, its seafloor swallowed whole by colliding continents. The exact timing of that disappearance has long divided geologists, but a new study of two granite bodies buried in the mountains of central China now offers some of the sharpest constraints yet. By dissecting the chemistry and crystal clocks of the Liqiao and Xianping plutons in the western North Qinling Orogen, researchers have pinned down when an ancient ocean closed and when the mountains born from that collision finally began to relax.

The Wushan-Shangdan Ocean was a northern arm of the Proto-Tethys Ocean, the immense water body that separated the northern continent of Laurasia from the southern supercontinent Gondwana after the breakup of Rodinia. Its remnants survive today as the Wushan-Shangdan Suture, the great tectonic seam that separates the North China Block from the Yangtze Block. For decades, scientists have argued over how long the subduction-collision cycle lasted: some place the main orogeny from the late Neoproterozoic through the Middle-Late Triassic, while others argue the ocean opened in the Late Cambrian and that collision did not arrive until the Silurian to Late Devonian. The new work, led by Hao Lin and Zuochen Li of Chang’an University and published in Solid Earth, cuts through that uncertainty with high-precision dating of granites that straddle the critical transition.

The team focused on two plutons, the Liqiao and Xianping, exposed in the western section of the North Qinling Orogen near Tianshui. Both are high-potassium, calc-alkaline granites, metaluminous to weakly peraluminous in composition, dominated by K-feldspar, plagioclase, quartz and biotite. But their ages and isotopic fingerprints tell strikingly different stories. Zircon U-Pb dating, performed by laser ablation ICP-MS on magmatic zircons with characteristic oscillatory zoning, yielded a weighted mean age of 429.9 ± 2.6 million years for the Liqiao monzogranite and 429.4 ± 2.4 million years for its syenogranite phase. The Xianping syenogranite, by contrast, crystallized at 421.8 ± 2.2 and 421.5 ± 2.5 million years across two samples, a gap of roughly eight million years between the two intrusions.

That age gap matters because it captures the moment the tectonic regime flipped. Geochemically, the Liqiao pluton is a classic I-type granite: silica contents of 71 to 76 weight percent, high strontium and barium, strongly fractionated rare earth element patterns with lanthanum-to-ytterbium ratios between 27 and 52, and only weak negative europium anomalies. Its zircons carry positive epsilon hafnium values from −0.1 to +3.4, with two-stage model ages of roughly 1.2 to 1.4 billion years, pointing to a source in juvenile felsic lower crust, crust that had been newly extracted from the mantle not long before. Notably, the magnesium numbers of Liqiao samples range from about 38 to 48, and values above 40 are widely taken as evidence that mantle-derived material contributed to the melt.

The Xianping pluton reads like a different planet. Its zircons carry strongly negative epsilon hafnium values between −18.5 and −13.6, with two-stage model ages of 2.3 to 2.6 billion years, indicating melting of ancient, mature continental crust. The rocks are highly fractionated I-type granites, with silica up to 78 weight percent, extreme depletion in strontium and barium, pronounced negative europium anomalies of 0.18 to 0.33, and magnesium numbers as low as 20, ruling out any meaningful mantle input. The team interprets the Xianping magma as the product of partial melting of old felsic crust followed by extensive fractional crystallization, with plagioclase and K-feldspar removal driving the strontium and barium depletion and apatite and titanite crystallization accounting for the phosphorus and titanium anomalies.

Placed in tectonic context, the two plutons become a paired record of an orogeny in motion. Tectonic discrimination diagrams place the Liqiao granite firmly in the syn-collision field, while the Xianping granite plots in the post-collision region. The Liqiao pluton’s age of 429 million years coincides with a well-documented pulse of magmatism attributed to slab break-off, the dramatic moment when the dense oceanic slab, no longer able to drag the buoyant continental crust downward, snaps and allows hot asthenosphere to surge upward. That upwelling supplied both heat and mantle material, triggering partial melting of the juvenile lower crust that had accreted from Wushan-Shangdan oceanic crust and sediments. The Xianping pluton, at 421 million years, falls squarely within the post-collisional window of roughly 420 to 409 million years, when delamination of a thickened, eclogite-facies lithospheric root caused the orogen to extend and collapse.

Synthesizing these results with the regional record, the authors propose a three-stage evolution for the Wushan-Shangdan Ocean. From about 472 to 438 million years ago, the ocean floor subducted northward beneath the North Qinling Orogen, building island arcs recorded in the Liziyuan and Caotangou groups, gabbroic complexes, and subduction-type quartz diorites and tonalites. By 438 million years ago, the ocean had closed, and from 438 to 423 million years the northern margin of the Yangtze Block collided with the North Qinling Orogen, thickening the crust to the point of granulite-facies metamorphism between 433 and 424 million years ago. Finally, from about 421 to 409 million years ago, the orogen entered a post-collisional extensional phase, with delamination and asthenospheric upwelling melting the ancient lower crust to produce granites like Xianping.

The implications ripple outward across the globe. The Proto-Tethys Ocean was one of the defining features of the early Paleozoic world, and its closure stitched together fragments that would eventually join eastern Gondwana. The new dates indicate that the Wushan-Shangdan Ocean, the northernmost branch of that system in eastern Asia, had already disappeared before 438 million years ago, and that the pivotal transition from active collision to post-collisional relaxation occurred between 429 and 421 million years ago, in the late Silurian. That timing aligns remarkably well with evidence from the Iapetus Ocean in the North Atlantic realm, where Proto-Tethyan waters are thought to have vanished around 435 million years ago, suggesting a broadly synchronized global reorganization of plates during the Caledonian orogeny.

What makes the study compelling is how two modest granite bodies, separated by just a few kilometers and a few million years, encode an entire tectonic drama. The Liqiao granite, born of a snapping slab and mingled mantle heat, marks the violent crescendo of continental collision. The Xianping granite, born of sinking lithosphere and stretching crust, marks the mountain belt’s long exhale. Together they demonstrate that the death of an ocean is not a single event but a sequence, and that the granites it leaves behind, read carefully enough, can replay that sequence with a precision measured in millions of years.

Subject of Research: Silurian syn- and post-collisional granitic magmatism and the closure of the Wushan-Shangdan Ocean in the North Qinling Orogen

Article Title: Silurian syn- and post-collision granitic magmatism in the western section of the North Qinling Orogen: implications for collisional orogenic processes

Article References: Silurian syn- and post-collision granitic magmatism in the western section of the North Qinling Orogen: implications for collisional orogenic processes. (n.d.). https://doi.org/10.5194/se-17-1035-2026

Image Credits: AI Generated

DOI: 10.5194/se-17-1035-2026

Keywords: North Qinling Orogen, Wushan-Shangdan Ocean, Proto-Tethys, Silurian granites, zircon U-Pb geochronology, Lu-Hf isotopes, slab break-off, post-collisional extension, continental collision, I-type granite, tectonics, Solid Earth