[SCG54-P10] オマーンオフィオライト北部のV2期岩脈群の岩石学
キーワード：V2期岩脈群、無人岩系列岩、ソレアイト系列岩
The Oman Ophiolite is the world largest and best preserved fragments of oceanic lithosphere obducted on land, that provides a geological record of formation of juvenile arc (V2 stage) on oceanic crust produced at a spreading ridge (V1 stage) [1,2]. Although volcanic stratigraphy and geochemical evolution of the V2 arc magmatism are well constrained, the V2 magma plumbing system is poorly understood. In the Fizh block of northern Oman, V2-stage extrusive rocks are underlain by intense swarms of dikes emanated from plutonic rocks beneath. This study focuses on the petrological and geochemical characteristics of the V2-stage dike swarms that supplied arc tholeiitic and boninitic series extrusive rocks.
The Fizh area in northern Oman is underlain by 96-94 Ma V2 arc volcanic rocks, which is in turn underlain by V1 volcanic rocks generated at a spreading axis at 98-96 Ma. The majority of V2 volcanics are tholeiitic flows and pyroclastic rocks, which are sporadically overlain by boninite-series rocks. The V1-stage sheeted dikes strike N-S, whereas the V2 dike swarms strike dominantly E-W and rarely N-S, and were emanated from lower crustal plutonic rocks ranging from ultramafic cumulates through gabbronorite to diorite and plagiogranite.
A total of 57 samples randomly collected from the V2 stage dikes in the Fizh area were examined by polarizing microscope on thin sections. 28 samples were tholeiite series rocks and 29 samples were boninite series. Tholeiite-series rocks have phenocrysts of olivine, clinopyroxene, plagioclase and magnetite set in an intergranular to intersertal groundmass, while boninite-series rocks have phenocrysts of olivine, clinopyroxene, orthopyroxene, plagioclase and Cr-spinel set in a hyalo-ophitic groundmass. Tholeiite-series rocks pervasively suffered actinolite-prehnite to greenschist facies metamorphism that replaced almost all igneous minerals by secondary minerals. Boninite-series rocks are less affected by hydrothermal alteration and pyroxenes and Cr-spinel are generally preserved.
Whole rock compositions determined by XRF show that boninite-series rocks are significantly lower in TiO2 and P2O5, which are positively correlated with SiO2. Tholeiite-series rocks are discriminated from boninite by their higher Na2O, TiO2 and P2O5, which are highly varied. A negative correlation of Na2O and CaO is consistent with the low-T hydrothermal alteration. Most of the latter plot within the range of V1 volcanic rocks by [1,2]. Some tholeiite-series samples plot outside of the V1 volcanics but are more akin to arc tholeiitic andesite. Higher degrees of alteration for the tholeiite-series rocks than for the boninite-series are consistent with their older age of intrusion. Most tholeiite-series rocks in Wadi Baqara strike E-W, which are intruded by rare N-S-striking tholeiite dikes. This indicates simultaneous activity of the MORB-like V1 and arc V2 magmas during the transitional stage from V1 to V1.
[1] Kusano et al. 2014. Geol. Soc. London Spec. Pub., 392, 177-193 [2] Kusano et al. 2017. Chemical Geol., 449, 206 – 225 [3] Weiler 2000. Marine Geophysical Res., 21, 195 – 210 [4] Umino et al. 2018. JpGU S-MP36.
The Fizh area in northern Oman is underlain by 96-94 Ma V2 arc volcanic rocks, which is in turn underlain by V1 volcanic rocks generated at a spreading axis at 98-96 Ma. The majority of V2 volcanics are tholeiitic flows and pyroclastic rocks, which are sporadically overlain by boninite-series rocks. The V1-stage sheeted dikes strike N-S, whereas the V2 dike swarms strike dominantly E-W and rarely N-S, and were emanated from lower crustal plutonic rocks ranging from ultramafic cumulates through gabbronorite to diorite and plagiogranite.
A total of 57 samples randomly collected from the V2 stage dikes in the Fizh area were examined by polarizing microscope on thin sections. 28 samples were tholeiite series rocks and 29 samples were boninite series. Tholeiite-series rocks have phenocrysts of olivine, clinopyroxene, plagioclase and magnetite set in an intergranular to intersertal groundmass, while boninite-series rocks have phenocrysts of olivine, clinopyroxene, orthopyroxene, plagioclase and Cr-spinel set in a hyalo-ophitic groundmass. Tholeiite-series rocks pervasively suffered actinolite-prehnite to greenschist facies metamorphism that replaced almost all igneous minerals by secondary minerals. Boninite-series rocks are less affected by hydrothermal alteration and pyroxenes and Cr-spinel are generally preserved.
Whole rock compositions determined by XRF show that boninite-series rocks are significantly lower in TiO2 and P2O5, which are positively correlated with SiO2. Tholeiite-series rocks are discriminated from boninite by their higher Na2O, TiO2 and P2O5, which are highly varied. A negative correlation of Na2O and CaO is consistent with the low-T hydrothermal alteration. Most of the latter plot within the range of V1 volcanic rocks by [1,2]. Some tholeiite-series samples plot outside of the V1 volcanics but are more akin to arc tholeiitic andesite. Higher degrees of alteration for the tholeiite-series rocks than for the boninite-series are consistent with their older age of intrusion. Most tholeiite-series rocks in Wadi Baqara strike E-W, which are intruded by rare N-S-striking tholeiite dikes. This indicates simultaneous activity of the MORB-like V1 and arc V2 magmas during the transitional stage from V1 to V1.
[1] Kusano et al. 2014. Geol. Soc. London Spec. Pub., 392, 177-193 [2] Kusano et al. 2017. Chemical Geol., 449, 206 – 225 [3] Weiler 2000. Marine Geophysical Res., 21, 195 – 210 [4] Umino et al. 2018. JpGU S-MP36.