| Popis: |
Subduction initiation processes, such as those evidenced in the Izu-Bonin-Mariana (IBM) forearc crust are a key feature of modern-style plate tectonics. However, the evidence for this process in the Precambrian is limited, due to the scarcity of ophiolite suites and in particular a lack of typical complete ophiolitic sequences similar to those observed in Phanerozoic ophiolites. Mineral analyses of olivine, orthopyroxene, and chrome spinel from the harzburgite and two types of clinopyroxene from the cumulate gabbro of the Fuchuan ophiolite, South China suggest formation in a Neoproterozoic subduction system similar to the IBM. High temperature (1,000–1,200°C), low pressure along with a high degree of melt extraction (>18%) have been recorded in the harzburgite. This evidence, in conjunction with magma evolution from forearc basalt to boninite to hydrous calc-alkaline magmatism, recorded in the crustal section of the Fuchuan ophiolite indicates a transition from initial to mature subduction in a forearc setting. Statistical analysis of global ophiolites through time suggests that IBM-like subduction systems may have successfully operated since at least the latest Neoproterozoic (ca. 1,000 Ma) and be pervasive on Earth from ca. 850 Ma. Additional evidence for Neoproterozoic ophiolites from India, Siberia, and Arabian-Nubian Shield, which are also similar to the IBM forearc ophiolite, suggests that cold and deep subduction became much more common in the Neoproterozoic (ca. 850 Ma) around the periphery of the Rodinia supercontinent. This IBM-like subduction system may have acted as the geodynamic trigger for the break-up of Rodinia. |
