Merineroetal SEM2014

macla nº 19. ju lio ‘ 14 revista de la sociedad española de mineralogía Inverse Zoning in Poly-Metamorphosed Chromite from the Calzadilla de los Barros Ultramafic Bodies (SW Iberian Peninsula) / RAUL MERINERO (1*), ROSARIO LUNAR (1,2), LORENA ORTEGA (1), RUBÉN PIÑA (1), SERAFÍN MONTERRUBIO (3), FERNANDO GERVILLA (4) (1) Departamento de Cristalografía y Mineralogía. Universidad Complutense de Madrid. C/José Antonio Novais 2. 28040, Madrid ( Spain ) (2) Instituto de Geociencias, departamento de Cristalografía y Mineralogía, UCM, C/José Antonio Novais 2. 28040, Madrid (Spain) (3) Escuela Politécnica Superior de Zamora. Universidad de Salamanca, Avda. Príncipe de Asturias 51, 49022, Zamora (Spain) (4) Departamento de Mineralogía y Petrología , Facultad de Ciencias, Universidad de Granada , Avda. Fuentenuev a s/n, 18002, Granada ( Spain ) INTRODUCTION episodes of regional metamorphism: the composition: Cr# [(Cr/(Cr+Al)] varies Cadomian (Neoproterozoic) and Variscan from 0.47 to 0.51 and Mg# Chromites in chromitites and ultramafic (Late Palaeozoic) orogenies, and the [(Mg/(Mg+Fe 2+ )] varies from 0.64 to rocks are commonly zoned, reflecting intrusion of abundant plutons during 0.77. Two types of zoned chromite are mineralogical, textural and chemical Cambrian and Variscan times. observed: (1) chromite with zoned cores changes induced by post-magmatic characterised by Mg# decrease from processes (e.g. González-Jiménez et al., There are two main ultramafic bodies inner to outer core with constant Cr#, 2009). Despite the large number of cropping out in the Calzadilla de los surrounded by Fe 3+ -poor, porous rims studies published on zoned chromite, Barros area: Cerro Cabrera and Sierra and (2) grains with the opposite little is known on how multiple Cabeza Gorda. The rocks that constitute chemical trend (Mg# increase from inner metamorphic episodes or high- both massifs are highly serpentinised to outer core) surrounded by Fe 3+ -rich temperature contact metamorphism harzburgite and dunite hosting several porous rims (Fig. 2). The second type of impact on the formation of zoned chromitite bodies (Merinero et al., zoned chromite is only observed in chromite. 2013). Strongly mylonitised meta- chromitites sampled in a pod located in gabbros crop-out in contact with the the western part of the Cerro Cabrera Here, we report a detailed study of western part of the Cerro Cabrera body body (RO in Fig. 1). Clinochlore is the zoned chromite in chromitites and (Jiménez-Díaz et al., 2009). only silicate mineral surrounding dunites from the ultramafic bodies of chromite grains and filling rims pores in Calzadilla de los Barros (Ossa-Morena CHROMITE IN CHROMITITES all chromitites. zone, SW Iberian Peninsula). This zone experienced a complex tectono- ACCESSORY CHROMITE IN DUNITES metamorphic evolution from the Neoproterozoic to late Carboniferous (Quesada 2006). GEOLOGICAL SETTING fig 2. Electron microprobe element maps (Mg 2+ and fig 3. Backscattered-electron image of accessory Fe 2+ ) showing the compositional zoning of the chromite in dunite showing complex textural and second type of zoned chromite: Mg 2+ increase compositional zoning. IC = inner core, OC = outer whereas Fe 2+ decrease from inner to outer core. fig 1. Simplified geologic map of the Calzadilla de core, IR = inner rim, OR = outer rim. los Barros ultramafic bodies (modified from Merinero et al., 2013) with the locations of studied Chromite grains in massive and semi- The grains of accessory chromite in chromitites (RO, CAB, CUCO and CM sites) and massive chromitites of the Calzadilla de dunites (CZ5 and CZ10 drill cores). dunites of the Calzadilla de los Barros los Barros ultramafic bodies are ultramafic bodies are subhedral to The ultramafic bodies of Calzadilla de commonly fractured and show complex anhedral with thick corroded margins; los Barros are located on the NE flank of patterns of zoning. These patterns are they are enveloped by clinochlore. Most the Olivenza-Monesterio antiform, in the characterised by composite cores of them are of very small size (< 1 mm) Ossa-Morena zone (Fig. 1). This zone of surrounded by thin rims of porous and show high degree of alteration and the Iberian Massif records a complex chromite. Inner cores of chromite almost complete transformation to Cr- tectonic evolution with two major present homogeneous high-Al magnetite. However, some larger grains palabras clave: Zonado Inverso en Cromita, Metamorfismo de key words: Inversely Z oned Chromite, High -Temperature Contact Contacto de Alta Temperatura, Poli-Metamorfismo Metamorphism, Poly-Metamorphism resumen SEM 2014 * corresponding author: rmeriner @ucm .es macla nº 19. ju lio ‘ 14 revista de la sociedad española de mineralogía (> 1 mm) exhibit complex mineralogical, dunites sampl ed in the CZ10 drill core Kerestedjian, T., Sergeeva, I., González - textural and chemical zoning with three reveals the superimposition of various Jiménez, J.M., Fanlo, I. (2012): Formation concentric zones (from inner core to alteration episodes. The slightly porous of ferrian chromite in podiform chromitites outer core and to inner rim) texture of the inner core as well as its Cr- from the Golyamo Kamenyane serpentinite, Eastern Rhodopes, SE characterised by progressive Mg# magnetite composition suggests that it Bulgaria: a two-stage process. Contrib. increase and Cr# decrease, surrounded form by magnetite precipitation within Mineral. Petr, 162 , 643-657. by an outer rim with almost the same Cr- highly porous chromite (as occurred in González-Jiménez, J.M., Kerestedjian, T., magnetite composition as the inner core the rims of RO chromite) followed by Proenza, J.A., Gervilla, F. (2009): (Fig. 3). This type of zoned chromite only extensive re-equilibrium between Metamorphism on chromite ores from the occurs in dunites sampled in the drill magnetite and host chromite. The low Dobromirtsi ultramafic massif, Rhodope core CZ10 (see location in Fig. 1). chromite/silicate ratio in dunites favours mountains (SE Bulgaria). Geol. Acta, 7, extensive percolation of metamorphic 413-429. Jiménez-Díaz, A., Capote, R., Tejero, R., Lunar, DISCUSSION fluids promoting almost complete R., Ortega, L., Monterrubio, S., Maldonado , transformation of primary chromite into C., Rodríguez , D. (2009): La fábrica de las The decrease of Mg# at constant Cr# porous chromite and later, the rocas miloníticas de la zona de cizalla de from inner to outer core observed in precipitation of magnetite. The Mg# Los Llanos (Calzadilla de los Barros, most studied zoned chromite from increase and the Cr# decrease from Badajoz). Geogaceta, 46 , 27-30. chromitites can be interpreted as a inner core to outer core and to inner rim Merinero, R., Lunar, R., Ortega, L., Piña, R., consequence of the sub-solidus re- involve the existence of a new alteration Monterrubio, S., Gervilla, F. (2013): equilibrium commonly described in event. This trend can be only explained Hydrothermal palladium enrichment in podiform chromitites of Calzadilla de los unaltered chromitite of ophiolites by assuming a heat ing event that promoted Barros (SW Iberian Peninsula). Can. cooling (e.g. Barnes, 1998). However, a reaction between Cr-magnetite and Mineral., 51 , 387-404. the chemical trend recorded in cores of the chlorite envelope, thus giving rise to Quesada, C. (2006): The Ossa-Morena zone of zoned chromite from RO site requires a slightly Al-richer chromite and minute the Iberian Massif: a tectonostratigraphic different explanation. According to Sack olivine grains. The proximity of the CZ10 approach to its evolution. Z. Deut. G. & Ghiorso (1991), the observed increase drill core and RO chromitites (Fig. 1) Geowiss., 157 , 585-595. of Mg# from inner to outer core at allows to correlate this heating event Sack, R.O. & Ghiorso, M.S. (1991): Chromian constant Cr# requires strong increase in with that invoked to explain the zoning spinels as petrogenetic indicators: thermodynamic and petrological temperature (> 1200 ˚C). Fig. 2 shows of the second type of zoned chromite in applications. Am. Mineral., 76 , 827-847. that Mg# increase from inner to outer chromitites. The development of the Sánchez-García, T., Quesada, C., Bellido, F., core takes place toward fracture edges outermost rim requires further reaction Dunning, G., González de Tanago, J. (2008): (zoning post-date fracturing) discarding between the newly formed Al-rich inner Two-step magma flooding of the upper a magmatic origin for this zoning and rim and olivine to produce chromite with crust during rifting: the Early Paleozoic of suggesting that ultra-heated fluids the same composition as that of the the Ossa-Morena zone (SW Iberia). migrating along fractures promoted inner core and clinochlore. This reaction Tectonophysics, 461 , 72-90. Mg 2+ partitioning towards chromite and must take place during the retrograde Tornos, F., CasQuet, C., Relvas, J. (2005): Transpressional tectonics, lower crust 2+ Fe towards associated silicates. The evolution postdating the heating event. decoupling and intrusion of deep mafic emplacement of small Mg-rich gabbroic sills: a model for the unusual plutons in the Ossa-Morena zone during CONCLUSIONS metallogenesis of SW Iberia. Ore Geol. Cambrian or Early Carboniferous times Rev., 27 , 133 163. (Tornos et al., 2005, Sánchez-García et The chemical and textural variations in al., 2008) could be the source of the zoned chromite from chromitites and high-temperature required for this event. dunites from Calzadilla de los Barros ultramafic bodies can be interpreted in The outer core to rim chemical

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