Trace element geochemistry of zircons from the Kahnouj ophiolite complex: implications for petrogenesis and geodynamic setting

Dikes of plagiogranites and aplitic granites from the Kahnouj ophiolitic complex are considered as the latest products of an ophiolitic magma and subsequent remelting of previously subducted sediments. These felsic intrusions bear an important role on understanding of the petrogenesis of the ophiolite, and zircons from these acidic rocks reflect the magmatic composition and record processes of felsic melt formation and its evolution. In this study, we analyzed the trace element composition of zircons from six samples of plagiogranite intrusions, plagiogranite, and granitic dikes. Zircon grains are subdivided in two groups, namely euhedral (Type 1) and subhedral to anhedral (Type 2) grains, and are characterized by variable Th/U ratios, rare earth element (REE) patterns enriched in heavy REEs, pronounced positive Ce, and negative Eu-anomalies. U/Yb versus Hf or Y discrimination diagrams are consistent with the magmatic oceanic origin of zircons. Trends between Ti and U, Y, REE, and Eu/Eu* of zircons from plagiogranite intrusion indicate that these rocks formed by hydrous partial melting of gabbro cumulates. On the other hand, zircons from plagiogranite dikes are characterized by more fractionation of a felsic melt produced by partial melting. However, different ages of plagiogranite intrusions (130.8 ± 1.8 Ma and 128.5 ± 1.3 Ma) and plagiogranite dikes (126.4 ± 1.7 Ma and 126 ± 1.3 Ma) reported in previous study can be interpreted as formation of plagiogranite by partial melting of gabbro cumulated followed by fractionation in a shear zone at the back-arc spreading center. Aplitic granite dikes (123.0 ± 1.7 Ma and 123.3 ± 1.9 Ma) formed by partial melting of sedimentary material during ophiolite obduction.