The detailed mineralogy and significance of an Olivine-two pyroxene gabbro nodule from lanzarote,Canary Islands |
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| Authors: | Thomas Frisch |
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| Institution: | (1) Mineralogisch-Petrographisches Institut der Universität München, Deutschland;(2) Present address: Department of Geology, University of Alberta, Edmonton 7, Alta, Canada |
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| Abstract: | A gabbro inclusion typical of the gabbroic xenolith suite in alkalic basalts on Lanzarote is composed of essentially four minerals. Olivine (5 vol.%) and plagioclase An81 (60%) were first to crystallize. The olivine, Fo76 with 0.02% CaO, was subsequently subjected to high-temperature oxidation followed by hydrolysis, resulting in its partial alteration to Fo92 and Fe-oxide-hydroxide plus (free?) silica. Clinopyroxene (30%) Ca43Mg46Fe11 has exsolved orthopyroxene Ca1.6Mg76.0Fe22.4 with 2.10% A12O3 in lamellae ∥ {100}. Orthopyroxene (5%) of the same composition also occurs as discrete crystals and petrographic and chemical criteria suggest that it formed by reaction of olivine with melt. The xenolith probably crystallized from a tholeiitic melt at a depth less than 9 km, i.e. above the Moho discontinuity under Lanzarote. The existence of basalts of olivine-tholeiitic chemistry on Lanzarote suggests a source for the gabbro nodules. Separation of calcic plagioclase from a tholeiitic melt may explain the relatively high alkali contents (Na2O+ K2O= 3.2 to 3.7%) of the Lanzarote olivine-tholeiitic basalts. |
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