HR: 1340h
AN: V13B-0547    [Abstracts]
TI: The origin of high-Mg olivines and Cr-rich spinel inclusions from Mediterranean Tertiary lamproites: the case study from Vera, Spain
AU: * Prelevic, D M
EM: prelevic@uni-mainz.de
AF: Johannes Gutenberg - Universitaet Institut fuer Geowissenschaften, FB 09 Mineralogie, Becherweg 21, Mainz, 55099 Germany
AU: Foley, S F
EM: foley@uni-mainz.de
AF: Johannes Gutenberg - Universitaet Institut fuer Geowissenschaften, FB 09 Mineralogie, Becherweg 21, Mainz, 55099 Germany
AB: Si-rich Mediterranean type lamproites (ca. 55-56 wt. % SiO2) are olivine-phyric, having extremely Fo-rich millimetre-sized olivine macrocrysts that host high-Cr spinels. This is one of the most typical features of the whole lamproite group. The origin of olivine in Si-oversaturated lamproitic melts is an unresolved question related with lamproite genesis; previous studies have interpreted such high-Fo olivines as mantle xenocrysts. However, in most Mediterranean lamproites, the olivine is idiomorphic and tabular (magmatic) with consistent chemical compositions indicating its phenocrystal character, showing that the interpretation as xenocrysts is inappropriate. Si-rich lamproites from the Vera lava flow in Southeast Spain contain two types of olivine macrocrysts: the first are phenocrysts, and the second are mantle xenocrysts. In this contribution, we present major and trace element data for both types of olivine, and show that Vera may be a key lamproite locality where the origin of olivine may be resolved. (1) Tabular idiomorphic olivine phenocrysts up to 1.0 x0.3 mm show extremely high Fo (up to 96 %) and very high NiO contents up to 1 wt. %. In terms of measurable trace elements, the most noteworthy is the enrichment of Li and B up to 65 and 10 ppm, respectively. Cr# of tiny idiomorphic high Mg-chromite inclusions in the olivines reach up to 0.96. The olivine-spinel pairs plot in the most refractory part of the olivine-spinel mantle array diagram. (2) In contrast, xenomorphic mantle olivine xenocrysts show Fo components not exceeding 92 % and NiO contents less than 0.5 wt. %. Trace element analyses show considerably lower amounts of Li and B less than 10 and 5 ppm, respectively. They bear large spinel inclusions with considerably less refractory compositions with Cr# (86-88). We consider two different models which may explain the origin of the phenocrystal olivine: cognate phenocrysts crystallized from the host magma, and crystallization from parental melts of olivine lamproite composition. The extreme compositions of olivine-spinel pairs convey information about the extremely refractory character of the subcontinental lithosphere. It resembles harzburgitic suprasubduction-type orogenic peridotites, which have experienced twofold depletion in addition to the normal depleted upper mantle signature: first during the production of MORB, and secondly associated with intraoceanic subduction. We argue that the refractory component of the Mediterranean lamproites may be derived from an island-arc oceanic lithosphere accreted to the continent during the Alpine geotectonic and collisional processes. This best explains the extreme depletion in non-cratonic environments such as the Mediterranean area.
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 3640 Igneous petrology
DE: 8439 Physics and chemistry of magma bodies
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005