“intra-oceanic” volcanic arcs, Lesser Antilles and Central American volcanic arcs

Abstract

Silica-rich rocks characterize “oceanic” volcanic arcs. This gives rise to the “andesite problem” — they are not likely to derive from basalts of subducted oceanic crust. The interpretation is that continental rocks exist below the arcs, but these are new, formed in “subduction factories” by partial melting of mantle rocks, of mantle wedge rocks and sediments, or various combinations of these. This is where continental crust forms. Wisdom holds that “Intra-oceanic systems provide vital information on how subduction works. Formed upon oceanic crust, they do not overlie continental basement. However, in recent years ancient inherited zircons have been recorded from the Izu-Bonin, Luzon, Solomon, East Java and Vanuatu arcs and from the Laccadive-Chagos Ridge. These are now seen to include blocks of original continent, separated from continent margins by slab rollback. So, there is no “andesite problem” and continental crust does not form in “subduction factories”. The Lesser Antilles (LA) volcanic island arc is famous for having the most continental crust-like geochemical characteristics of all island arcs. The arc is 23-34 km thick, includes a 10 km mid-crust layer of typical continental velocities and has andesitic, high silica chemistry. Volcanoes on Grenada and Carriacou produce ancient, inherited zircons. This paper concludes that this arc, also, contains original continental material. The Central American arc is regarded a one of the best places to study volcanoes — there are many volcanic centres, easy to reach. Ignimbrites are present from Guatemala (continental basement) to Costa Rica/Panama (oceanic basement). They have chemical compositions within one order of magnitude of continental crust. Since those of Costa Rica and Panama do not lie above continental crust, those of Guatemala are not derived from continent. This circular reasoning concludes that the ignimbrites arise by fractionation of magmas derived from the mantle wedge. However, velocity data from Central America indicate continental Moho at around 40 km depth, gravity data indicate continental rocks below western Panama and Costa Rica’s Arenal volcano produces granulite xenoliths. This paper concludes that the arc includes original continental crust. No inherited zircon data are reported; it clearly should be sought.

AAPG Datapages/Search and Discovery Article #90330©2018 AAPG Hedberg: Geology of Middle America – the Gulf of Mexico, Yucatan, Caribbean, Grenada and Tobago Basins and Their Margins “Integration of the geology of the area from the southern onshore of North America to the northern onshore of South America”, Siguenza, Guadalajara, Spain, July 2-5, 2018