A cross-section of the seafloor
The rocky outcrops of the Liaodong Peninsula contain a classic ophiolite sequence. An ophiolite is a segment of oceanic crust and the underlying upper mantle that has been violently thrust onto a continental landmass. These formations are geological rarities, providing direct access to rocks that are normally thousands of meters below the ocean surface.
A complete ophiolite presents a distinct layering of rocks that mirrors the structure of oceanic lithosphere. At the base are peridotites, ultramafic rocks from the Earth's upper mantle, often altered by seawater into a greenish rock called serpentinite. Above the mantle rocks lie gabbros, which are coarse-grained intrusive rocks that crystallized slowly in magma chambers beneath the seafloor. This is followed by a sheeted dike complex—a series of vertical, parallel intrusions of basalt that acted as conduits for magma. The sequence is topped by pillow lavas, bulbous structures formed when basaltic lava erupts underwater and is rapidly cooled. Capping the entire sequence are deep-sea sediments, like chert, that slowly accumulated on the ocean floor.
Closing an ancient ocean
The Liaodong Peninsula Ophiolite is a remnant of the Paleo-Tethys Ocean, which once separated the North China and South China continental blocks. The closure of this ocean was an event during the assembly of the supercontinent Pangea. Beginning around 290 million years ago, in the Early Permian, the oceanic crust of the Paleo-Tethys began to subduct, or slide beneath, the Eurasian plate.
This process continued for millions of years. By the Middle to Late Triassic, between 250 and 220 million years ago, the North and South China blocks finally collided. During this collision, a fragment of the Paleo-Tethys ocean floor was not subducted. Instead, it was scraped off and pushed (a process called obduction) onto the edge of the North China Craton, where it remains today. Geologists use radiometric dating techniques, such as Uranium-Lead dating on zircon crystals found within the igneous rocks, to pinpoint the timing of these events with precision.
The geochemistry of the basalts within the Liaodong ophiolite suggests they may have formed in a back-arc basin, a type of seafloor spreading center that opens behind a volcanic island arc at a subduction zone. This shows the complex tectonic environment that existed as the ancient ocean was closing.