Petrogeochemistry Of Cocos Ridge Basement Basalt Offshore Western Costa Rica And Its Geodynamical Significance

The Cocos ridge,a typical aseismic ridge offshore Central America,has a complex tectonic evolution history due to long term interaction between Galapagos plume and the Cocos-Nazca spreading center.Rock samples for the Cocos ridge basement segment being subducting offshore Costa Rica along Central America trench were drilled at Sites U1381 and U1414 during the Integrated Ocean Drilling Program(IODP)Expeditions 334 and 344.Detailed petrological and geochemical study on these samples will provide some important clues for understanding the lithology of the mantle source,the magmatism process of the Cocos ridge,and the historical evolution of the Galapagos mantle plume.This study performed detailed petrographic,single mineral in situ major and trace elements chemical,and bulk-rock major and trace element geochemical studies on the Cocos ridge basement rocks from three holes(U1381A,U1381 C and U1414A)of Sites U1381 and U1414.(1)Petrographic characteristics show that the rocks are mainly basaltic rocks and the phenocryst minerals are plagioclase and clinopyroxene,microlites minerals are plagioclase and clinopyroxene and a small amount of opaque minerals,such as magnetite.(2)Plagioclases belong to bytownite and labradorite,with few andesine,and they are enriched in light rare earth elements(LREE)and large-ion lithophile elements(LILE),depleted in high field strength elements(HFSE),and have obvious positive Eu anomalies.Clinopyroxenes are augites,and depleted in highly incompatible elements such as LREE and LILE,with heavy rare earth(HREE)relatively flat in REE distribution patterns.According to plagioclase-liquid thermometers,the crystallization temperature of plagioclases is calculated to be 866?1269?.By using clinopyroxene-melt thermbarometer,this study obtained the crystallization temperature of clinopyroxenes is 1162?1268? and the equilibrium pressure is 0.12?0.97GPa(corresponding depth of 4.7?36.7km).Clinopyroxenes,plagioclases and host basaltic rocks came from the same magmatic system,and some plagioclase phenocrysts may be antecrysts.(3)Major element compositions of samples suggest that they are mainly basalt and basaltic andesite,and all rocks belong to tholeiitic basalts series.These samples are consistent with the characteristics of enriched mid-ocean ridge basalt(E-MORB).The mantle source lithology is inferred to be garnet pyroxenite by the major and trace elements of the basaltic rocks.The parent basalt magma may have been the melting product of garnet pyroxenite with a partial melting degree of 1%-5%.During its separation from the source and ascent to the surface,the magma underwent the effect of fractional crystallization of plagioclase,clinopyroxene and Ti-Fe oxide,suggested by the evolution trend of elements in clinopyroxenes and the co-variation diagrams among bulk-rock major oxides.Based on the comparison in the ratios of trace elements in the whole rocks and the tectonic evolution data,we suggested that from at least ? 14 Ma,the Cocos ridge basalts share a same mantle source with no obvious compositional variations,as those from the Galapagos archipelagos nowadays,and yet they may have experienced different magmatic processes.