Abstract
The Cocos Ridge, which is subducted beneath the Central American Volcanic Arc, has a complex tectonic evolution history due to plume-ridge interaction between the Galápagos plume and the Cocos—Nazca spreading center. This study presents major and trace element analyses of plagioclase and clinopyroxenes hosted by Cocos Ridge basaltic rocks that were drilled in three holes (U1381A, U1381C and U1414A) of Sites U1381 and U1414 on the Cocos Ridge close to the Middle America Trench during the Integrated Ocean Drilling Program (IODP) Expeditions 334 and 344. The results show that (1) plagioclases are mainly bytownite and labradorite with subordinate andesine, which are enriched in light rare earth elements (LREE) and some large-ion lithophile elements (LILE) and exhibit marked positive Eu anomalies; and (2) that clinopyroxenes are augites, which are depleted in highly incompatible elements such as LREE and LILE, have nearly flat heavy rare earth elements patterns (HREE) and lack Eu anomalies in chondrite-normalized rare earth element (REE) diagrams. During the ascent to the surface, the primary magmas experienced fractional crystallization of plagioclase, clinopyroxene, Ti-Fe oxides and possibly olivine (complete replacement of olivine by secondary minerals). The crystallization temperatures of plagioclase phenocrysts and microlites are 1050 to 1269 °C, and 866 to 1038 °C, respectively, and the pressures of plagioclase phenocrysts are 0.3–0.7 GPa. The crystallization temperatures of clinopyroxene phenocrysts/micro-phenocrysts is 1174–1268 °C, similar to those of plagioclase phenocrysts, suggesting some of clinopyroxene and plagioclase phenocrysts cotectic crystallized during early stage of magmatic evolution. In addition, the equilibrium pressures of clinopyroxene phenocrysts/micro phenocrysts are 0.02–0.97 GPa, implying that the clinopyroxene started to crystallize within the mantle, and magma evolution has undergone an early crystallization stage with clinopyroxene and no plagioclase.
Highlights
Seamount chains and/or aseismic ridges (e.g., Hawaii-Emperor chain) generally represent the surface expression of a long-lived hotspot activity as the tectonic plate moves over a relatively stationary mantle plume [1]
In situ major- and trace elements of plagioclases and clinopyroxenes hosted by ba ment basaltic rocks samples from Sites U1381 and U1414 were analyzed by electron pro
In situ major- and trace elements of plagioclases and clinopyroxenes hosted by basement basaltic rocks samples from Sites U1381 and U1414 were analyzed by electron probe micro-analyzer (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), respectively
Summary
Seamount chains and/or aseismic ridges (e.g., Hawaii-Emperor chain) generally represent the surface expression of a long-lived hotspot activity as the tectonic plate moves over a relatively stationary mantle plume [1]. Aseismic ridges near mid-oceanic spreading centers may have relatively complex magmatic histories due to the plume-ridge interactions. In the eastern Pacific region, the Cocos Ridge on the Cocos plate is generally interpreted as an aseismic ridge, which was formed as the stationary Galápagos hot spot affected the NE moving Cocos plate and interacted with the Cocos-Nazca spreading center [2,3,4]. Geist [10] suggested that the variations in lithospheric thickness may control the melting of a heterogeneous mantle and thereby cause regional geochemical variations in the composition of the erupted lavas in the Galápagos archipelago. Oorf)s[tu19d–ie2d1s].amOpf lsetsu, died samplfievs,e sfaivmeplseasm(3p34le-Us 1(338314A-U-141R38-21-AW-1436R/-123-W9, 33143-6U/1133891,A3-1364R-U-21-W3813A4-/1163R7,-23-3W4-U13841/1A3-178, R3-32-4W-U11/348,13A34--1U81R3-821-AW-201R/-42,- 334U1381WA-12105R/-121-8W, an1d153/3141-8U,1a3n8d1A3-3246-RU-11-3W818A0/-2836)Ra-r1e-Wfrom80h/8o3le) aUr1e3f8r1oAm, ahnodleonUe1s3a8m1pAle, a(3n4d4-oUn1e38s1aCm-1p3lXe-(13-W44-1U1/11368)1iCs -fr1o3mX-1-W 11/16)hioslefrUo1m38h1Col,eanUd1s3ix81saCm, panleds (s3i4x4-sUa1m41p4lAes-4(63R4-41--UW1140174/A11-40,63R4-41--UW141140A7/-1481R0,-13-4W4-1U171/411242A, 3-4448-RU-114-1W4A1-5117R/1-42-2W, 314246-/U13144,14A51R-4-3W44-1U2164/1143A4-,5364R4-1-U-W1411049A/1-5156,R3-414--WU1411049A/1-1595R, -324-4W-U821/48174Aan-d593R4-42-U-W141842A/8-670Ran-2d-W34545-/U615)4a1r4eAfr-o6m0Rh-2o-leWU15451/64A5). are from hole U1414A
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