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Gianluca Bianchini, Kingston University (United Kingdom)
Luigi Beccaluva, Università di Ferrara (Italy)
Costanza Bonadiman, Università di Ferrara (Italy)
Graham Pearson, University of Durham (United Kingdom)
Franca Siena, Università di Ferrara (Italy)
Marjorie Wilson, University of Leeds (United Kingdom)
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Mantle xenoliths from Calatrava (CLV), have an extremely wide compositional range, including lherzolites (prevalent) as well as rare harzburgites and dunites, generally displaying bulk-rock Mg#<89, i.e. lower than that of any primordial mantle estimate. Microprobe analyses of lherzolite minerals reveal the following compositions: olivine: Fo between 86.2 and 89.5; orthopyroxene: Wo 1.9-2.2, En 84.6-87.9, Fs 10-14.1, with Mg# 0.86-0.90; texturally equilibrated crystals of clinopyroxene (cpx): Wo 43.2-44.8, En 47.2-49.9, Fs 6.9-8.0, with Mg# 0.86-0.89, Na2O wt% 1.1-1.3 and TiO2 wt% 0.3-0.6; spinel typically shows brown cores with Mg# between 0.73 - 0.80 and Cr# 0.11-0.21, and black rims with lower Mg# and higher Cr# gradually transformed in a fine aggregate of vermicular Cr-Fe-Ti rich spinel and feldspar.
Intra-suite variation of the modal proportions is inconsistent with that expected by melting models whatever the starting composition, as also indicated by bulk-rock variation diagrams showing misfit between the CLV compositions (anomalously enriched in Fe-Ti) and those predicted by partial melting of primordial mantle material. The operation of other processes than pure melt extraction is confirmed by the whole-rock REE budget, typically characterized by LREE enrichments, with LaN/YbN (up to 6.7) probably related to the occurrence of pervasive metasomatic reactions. CLV mantle cpxs display fractionated REE patterns with upward convex shapes, characterized by low HREE (Tm-Lu) concentration, typically <6 x chondrite, and enrichments in Middle/Light REE (NdN/YbN up to 7, LaN/YbN up to 5) notable for their uniformity. These "enriched" compositions result from the re-equilibrium of pristine cpxs with an incoming metasomatic melt, and/or represent products crystallising directly from the metasomatic agent. The latter was plausibly generated at greater depths in the presence of residual garnet (from peridotite or eclogite starting materials).
Isotopic analyses (performed on separated cpxs) reveal a very homogeneous 87Sr/86Sr composition between 0.7031 and 0.7032; 143Nd/144Nd ranges from 0.51288 - 0.51295 (εNd 4.74-6.07) and 176Hf/177Hf 0.28302-0.28265 (εHf -3.6-9.0). Unlike mantle xenoliths and alpine-type peridotites from other Iberian occurrences, whose compositions range from Depleted Mantle (DM) to Enriched Mantle (EM), the CLV mantle cpxs approach the composition of the HIMU mantle end-member, the genesis of which is generally interpreted as the result of long-term recycling of oceanic basalts/gabbros (or their eclogitic equivalent) via ancient subduction.
This suggests that under the CLV volcanic district, sublithospheric convective instabilities - possibly triggered by the neighbouring subduction along the Betic collisional belt - could have remobilized deep mantle domains from the mantle Transition Zone (410-660 km) which possibly included eclogitic rocks, i.e. relicts of older subducted slabs.
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