Plagiogranite is a felsic intrusive rock commonly found in ophiolite complexes, which are fragments of oceanic crust and upper mantle that have been uplifted and exposed on land. It is characterized by its high quartz content and dominance of plagioclase feldspar, with minimal mafic minerals such as amphibole or pyroxene. Plagiogranite is considered an analog to continental granite but forms in oceanic rather than continental settings.
Geological Formation[]
Plagiogranite primarily forms through two processes:
- Partial melting of oceanic crust – This occurs when basaltic or gabbroic rocks in the lower oceanic crust undergo partial melting, producing a silica-rich melt that crystallizes into plagiogranite.[1]
- Magmatic differentiation – This involves fractional crystallization of a mafic parent magma, where early-crystallizing mafic minerals (e.g., olivine, pyroxene) are removed, leading to the concentration of more felsic components such as plagioclase and quartz.[2]
It is typically associated with sheeted dike complexes, gabbroic intrusions, and pillow basalts within ophiolite sequences, reflecting its origin in the mid-ocean ridge and supra-subduction zone environments.[3]
Mineralogy and Petrology[]
Plagiogranite is a leucocratic (light-colored) rock composed mainly of:
- Plagioclase feldspar (dominant)
- Quartz (abundant)
- Minor amounts of amphibole, biotite, and pyroxene
Unlike continental granite, which commonly contains potassium feldspar, plagiogranite is distinguished by its complete absence or minimal presence of potassium-rich minerals.[4]
Tectonic Settings[]
Plagiogranites are most commonly found in:
- Ophiolite complexes – Examples include the Troodos Ophiolite (Cyprus), Oman Ophiolite, and Bay of Islands Ophiolite (Canada).[5]
- Mid-ocean ridges – Forming as minor felsic intrusions within basaltic oceanic crust.[6]
- Back-arc basins and island arcs – Occurring in subduction-related magmatic systems where differentiation of mafic magmas produces felsic melts.[7]
Geological Importance[]
Plagiogranite is significant in understanding:
- The evolution of the oceanic crust – It represents late-stage differentiation of mid-ocean ridge magmatism.[8]
- Tectonic reconstructions – As an indicator of past spreading center environments, its presence in ophiolites provides evidence of former ocean basins and plate tectonic processes.[9]
- Geochemical evolution of the Earth's mantle and crust – Plagiogranites help geologists study the differentiation of mantle-derived magmas into continental-like compositions.[10]
See Also[]
- Ophiolite
- Gabbro
- Mid-ocean ridge
- Granite
References[]
- ↑ Coleman, R. G. (1977). Ophiolites: Ancient Oceanic Lithosphere? Springer-Verlag.
- ↑ Floyd, P. A., Exley, R. A., & Styles, M. T. (1991). Igneous Rocks of Ophiolites and Oceanic Crust. Geological Society of London.
- ↑ Pearce, J. A. (2003). "Supra-subduction zone ophiolites: The Earth's natural laboratories for crustal-accretion processes." Geological Society Special Publications, 218, 417–438.
- ↑ Dilek, Y. & Furnes, H. (2014). Ophiolites and Their Origins. Geological Society of America.
- ↑ Robertson, A. H. F. (2002). "The Troodos Ophiolite (Cyprus): Formation and Emplacement in an Evolutionary Tethyan Context." Geological Society, London, Special Publications, 195, 45–72.
- ↑ Taylor, B., & Martinez, F. (2003). "Back-arc Basin Basalt Systematics." Earth and Planetary Science Letters, 210, 481–497.
- ↑ Stern, R. J. (2004). "Subduction Initiation: Spontaneous and Induced." Earth and Planetary Science Letters, 226, 275–292.
- ↑ Casey, J. F. (1997). "The Composition of the Lower Oceanic Crust and the Genesis of Plagiogranites in Ophiolites." Reviews in Mineralogy and Geochemistry, 35, 85–120.
- ↑ Moores, E. M. (2002). "Ophiolites, the Sierra Nevada, and Plate Tectonics." GSA Bulletin, 114, 993–1009.
- ↑ Rollinson, H. (2009). Early Earth Differentiation and the Role of Plagiogranites. Cambridge University Press.