Silurian-Devonian Granites and Associated Intermediate-Mafic Rocks along the Eastern Kunlun Orogen, Western China: Evidence for a Prolonged Post-Collisional Lithospheric Extension
Jinyang Zhang Huanling Lei Changqian Ma Jianwei Li Yuanming Pan
Summarized by Makayla Palm
What data were used? The goal of this study was to gain insight into how the Kunlun mountain formation and surrounding area were initially formed. The Kunlun Mountain range primarily has an intermediate and mafic composition, with felsic granite intrusions, or dikes. Dikes are intrusions of magma that cut across previously formed layers and are an indicator of a secondary formation process. Depending on the mineral composition, or silica content, of these dikes (or intrusions) they will be either felsic, intermediate, or mafic, with felsic rocks containing the most silica. There are several kinds of secondary igneous rock formations found in the Kunlun called dikes. If the composition of the dike is different from the surrounding rock, this will provide insight into how the dikes formed in the Kunlun and how it can be explained using plate tectonic theory.
Granite is a commonly found felsic rock in the Kunlun and is formed intrusively (or underground). The mineral contents of the granite can tell the researchers how fast or slow the magma cooled, which will ultimately help answer the question of how the dikes formed. Within the granite, there were zircon crystals present with radioactive uranium decaying into lead. These ratios were recorded in order to estimate ages within the mountain range to determine when the different magma-cooling events took place. To summarize, this paper uses physical samples of the igneous rocks in the area to study mineral composition and isotope data from these rocks, too.
Methods: The samples that were collected from different rock types in this area were studied under a microscope in order to observe the composition and individual mineral grains. In plate tectonics, there are two kinds of plates: continental plates and oceanic plates. Granite (felsic) comprises less dense continental plates, while basalt (mafic) comprises a denser oceanic plate. In the Kunlun, the researchers observed several granite inclusions surrounded by mafic rock. The isotope ratios of uranium to lead were recorded and radiometrically dated. These data determine if the different intrusions formed at the same time, or if they formed during several events. This would help support or reject the hypothesis they posed that when the continental and oceanic plates collided, creating the Kunlun Mountains, the edge of the oceanic plate broke while bending under the continental plate (the oceanic plate always goes underneath a continental plate, due to higher density).
Results: The radiometric dating of the granite inside the intrusions (the magma formations added after the formation of the surrounding rock) indicated four different formation events, with the earliest taking place 427-414 million years ago (mya) and the latest from 373-357mya. (For more about how radiometric dating works visit Geologic Time.) The variation in the composition of the rocks (felsic, mafic, etc) indicates a complicated tectonic history; along with the multiple events of granitic intrusions, scientists also found ophiolites (oceanic crust that was pushed onto land during an oceanic- continental plate collision), which indicates that a piece of the oceanic plate was pushed up and broken off during the collision.
Why is this study important? This study looked to test the hypothesis of a broken oceanic plate’s impact on the formation of the Kunlun mountain range and gain more specific knowledge of its origin. By taking inventory of its intrusive rock formations, getting radiometric dating for these intrusions, and noting the differences in mineral compositions, they were able to confirm their hypothesized four magma events. These events represent different periods of magma formation, which confirms the researcher’s hypothesis about oceanic plate breakage during a collision.
The big picture: Clues from igneous geology, such as large crystal size, rock type, and mineral composition can give researchers details on how large formation events took place. Isotopes within radiometric dating were used to separate events from one another and place them in chronological order. This particular study answered questions about the origin of the Kunlun Orogen, or mountainous landscapes.
Citation: Zhang, Jinyang, Huanling Lei, Changqian Ma, Jianwei Li, Yuanming Pan. “Silurian-Devonian Granites and Associated Intermediate-Mafic Rocks along the Eastern Kunlun Orogen, Western China: Evidence for a Prolonged Post-Collisional Lithospheric Extension.” Gondwana Research, vol. 89, Oct. 2021, pp. 131–146., https://doi.org/10.1016/j.gr.2020.08.019.