Higgins, M.

Morata, D.

Varas, M. I.

Repositorio Digital Sernageomin

P – T conditions of crystallization of Mafic Microgranular Enclaves from the Illapel Plutonic Complex (IPC)  Varas, M.I.1,*, Morata, D. 1,2, Higgins, M. 3 1Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile. 2Centro de Excelencia en Geotermia de los Andes, CEGA (FONDAP-CONICYT). 3Sciences de la Terre, Université du Québec à Chicoutimi, 555 blvd de l'Université, Chicoutimi, Québec, G7H 2B1, Canada. *Current address: Instituto Andaluz de Ciencias de la Tierra – CSIC, Avda. Las Palmeras Nº 4, Armilla, 18100 Granada, España.   *E-mail: mvaras@iact.ugr-csic.es  Abstract.  The Illapel Plutonic Complex (IPC, 31°25’ – 32°30’S) exhibits a wide range of lithologies and a very good exposure, where mafic microgranular enclaves (MME) are particularly abundant. In this work we present new data about mineral chemistry (SEM-EDX) from MME, their respective host rock and ultramafic xenoliths. P-T calculated crystallization conditions for MME and their respective host–rocks are similar, suggesting that mixing and mingling processes must have occurred at pressures around 2 kbars and temperatures in the range 790-700ºC, which would correspond to the emplacement level of the plutonic complex.   Keywords: Geothermobarometry, mineral chemistry, emplacement, Coastal Cordillera, Chile.   1 Introduction   The Illapel Plutonic Complex (IPC, 31°25’ – 32°30’S), which is part of the Lower Cretaceous magmatic belt of the Coastal Range of central Chile, exhibit a wide range of lithologies and a very good exposure (Parada et al., 1999; Morata et al. 2009, 2010). Mafic microgranular enclaves (MME) are particularly abundant in the IPC (Varas et al., 2009), mainly located within the Main Tonalitic Unit (MTU), as isshown in Figure 1.  In this work we present new data about mineral chemistry (amphibole and plagioclase) from MME, their respective host rock and some ultramafic xenoliths. With this information we propose a P-T crystallization path for the magmatic system named above.   2 Sample selection and analytical procedure   18 samples were selected for mineral chemistry analyses. These samples belong to MME, granitoid host rocks and ultramafic xenoliths throughout the IPC, with special emphasis in the MTU. Samples were analyzed using a scanning electron microscope (SEM Hitachi, S-510) with backscattered electron images (BSE) and semi-quantitative measurements EDX (energy ray spectroscopy X) in order to identify textural and compositional relationships of minerals. This study was conducted at the Centro de Instrumentación Científica (CIC), University of Granada, Spain.    The chemical data collected on the minerals of the samples were used to calculate temperatures and pressures of the crystallization path of MME and their respective host rocks. For this purpose, structural formula for plagioclase was calculated based in 8 oxygens; and for amphiboles, in 23 cations following the procedures described in Holland & Blundy (1994).     Figure 1.  Simplified map of the Illapel Plutonic Complex (IPC; 31°50’ – 32°20’S)) showing areas where MME are found (solid circles) and sample location of MME (red circles). MU= mafic unit; TU= trondhjemitic unit; MTU= main tonalitic unit; GU= granodioritic unit. Modified and simplified from Rivano et al. (1993) y Rivano & Sepúlveda (1991).    3983 Petrography  A petrographic study allowed us to define the MTU as a composite body of monzogranite, granodiorites, quartz monzodiorites, and subordinately tonalities, all with amphiboles and biotites. For MME compositions are slightly different. They range in composition from diorites to monzodiorites, and subordinately quartz monzodiorites, all also with amphiboles and biotites. From a mineralogical point of view, the same minerals from the host rock are present in the MME, differing only in modal percentage and in grain size, with medium grain size host rocks, while MME present a fine grain size. MME show an increase in the percentage of mafic minerals with respect to their ost rock (Fig. 2). Also are found in the southern part of he IPC, ultramafic decimetric xenoliths, corresponding to hornblendites. Field relationships of these xenoliths contrast with MME, showing angular shapes and coarse to medium grain size.    Figure 2:  Petrographic characteristics of the contact (dotted red line) between host rock (left side) and a MME (right side) showing a clear difference in grain size. Sample ILL0859D, nic x (a), nic // (b), 4X. Anf: amphibole; Pl: plagioclase.   4 Results from mineral chemistry  4.1 Plagioclase  They are the most abundant mineral, both in MME and host rock. A total of 299 points for plagioclases were obtained from MME, host rock and ultramafic xenoliths. Plagioclases from the study samples cover almost the entire range of compositions; however th larger data rate is between An30-70. Plagioclase cores are usually more anorthitic than their rims.   4.2 Amphiboles  They are the most abundant ferromagnesian minerals, both in MME as in host rocks. A total of 234 points of analysis were obtained in MME, host rock and ultramafic xenoliths. SEM-EDX analyses show that amphibole classified mostly in two different calcic types: (1) magnesium hornblende and (2) actinolite (Fig. 3). Amphiboles composition from MME and host rock are almost indistinguishable, being amphiboles from the ultramafic xenolith the one that clearly differs incomposition (Fig. 3). It remarkable that actinolitic amphiboles are mainly located at the center of amphibole crystals, reason why they could correspond to pseudomorphs of pyroxenes.   Figure 3:  Classification diagram for calcic amphiboles from Leake et al. (1997). MME (filled triangles), host rock (squares), and xenolith amphiboles (filled circles).   5 Geothermobarometry  Pressures were obtained using the Schmidt (1992) calibration. The results show that the emplacement pressures for the host rocks, as well as for MME are very low, with values of 1.6 ±0.6 kbar and 1.8±0.6 kbar respectively. These results are consistent with the pressure range calculated for other Cretaceous plutonic rocks from the Coastal Batholith (1.5 – 3.0 kbar; Parada et al. 1999).   Using the geothermometer calibration of Blundy & Holland (1990) for the Hbl-Pl pairs, temperatures obtained in host rocks ranges between 790 – 640°C meanwhile figures ranging from 798 - 648°C were obtained for MME. Temperatures under 700ºC were interpreted as consequence of subsolidus re-equilibration rather than igneous crystallization.  For the ultramafic xenoliths, it was not possible to use the geothermobarometers exposed here, due to the mineral composition of the sample. However, Ernst & Liu (1998) proposed that the content of AlT in calcic amphiboles increases with pressure and temperature, while TiO2 content increases with temperature and is almost independent of pressure. Plotting the data of Al T v/s Ti (a.p.f.u) it is possible to have a better understanding of the path follow by the xenoliths and the MME. As can be seen in figure 4, the ultramafic xenolith shows a wide range of pressures, suggesting a complex history of crystallization and re-equilibration during plutonic emplacement and exhumation.     399 Figure 4:  Diagram of AlT v/s Ti (a.p.f.u.) from amphiboles of MME (filled triangles), host rock (squares), and xenolith (filled circles). Dotted and pointed lines are pressure estimations calculated based on Schimdt (1992) calibration.   6 Discussions and Conclusion  As already proposed, MME and their respective host rock could be the result of mixing and mingling processes (Varas et al. 2009, Morata et al. 2010) during magma generation and emplacement. Mineral chemistry from MME and host rock reported a similar compositional range, evidencing a partial re-equilibrium of minerals through chemical diffusion during the mingling and mixing process.  The results of pressure and temperature calculations shows that MME and their respective host rock follow a similar crystallization path, where mixing and mingling processes must have occurred below 1.7±0.6 kbars that would correspond to the emplacement levelof the system. On the other hand, ultramafic xenolith shows a wide range of pressures, with maximum calculated values up to 6 kbar. These xenoliths could be the result of the first stages of crystal accumulation in the bottom f a deep magma chamber. The P-T range registered in the amphibole composition of these ultramafic xenoliths could report the different interaction processes P-T path between mafic and felsic magmas during the firstmagmatic stages at deep crustal levels up to the exhumation at upper crustal levels.   Acknowledgements  This research has been funded by the Chilean National Science Foundation (FONDECYT) Project 1080468. The first author carried out her Master of Science studies with a CONICYT grant.   References  Blundy, J. D. & Holland, T. J. B., 1990, Calcic amphibole equilibria and a new amphibole–plagioclase geothermometer. Contributions to Mineralogy and Petrology104, 208–224. Ernst, W. G. & Liu, J., 1998, Experimental phase equilibrium study of Al- and Ti-contents of calcic amphibole in MORB-A semiquantitative thermobarometer. American Mineralogist 83: 952-969. Holland, T.J.B.; Blundy, J.D. 1994. Non-ideal interactions in calcic amphiboles and their bearing on amphibole-plagioclase thermometry. Contributions to Mineralogy and Petrology, Vol. 116, p. 433-447 Leake, B.E.; Woolley, A.R.; Arps, C.E.; Birch, W.D.; Gilbert, M.C.; Grice, J.D.; Hawthorne, F.C.; Kato, A.; Kisch, H.J.; Krivovichev, V.G.; Linthout, K.; Laird, J.; Mandarino, J.A.; Maresch, W.V.; Nickel, E.H.; Rock, N.M.; Schuhmacher, J.C.; Smith,D.C.; Stephenson, N.C.; Ungaretti, L.; Whittaker, E.J.; Youzhi, G., 1997, Nomenclature of amphiboles: report of the subcommittee on amphiboles f the International Mineralogical Association, commission on new minerals and mineral names. – The Canadian Mineralogist, Vol. 35, pp. 219-246. Morata, D.; Higgins, M.; Varas, M.I.. 2009, Rhythmically layered gabbro in the Illapel Plutonic Complex (Coastal Cordillera of central Chile): petrography and field relationships. 12º Congreso Geológico Chileno, Noviembre 2009, Santiago (Chile). Morata, D.; Varas, M.I.; Higgins, M.D.; Valencia, V.; Verhoort, J.D. 2010, Episodic emplacement of the Illapel Plutonic Complex (Coastal Cordillera, central Chile): Sr and N  isotopic, and zircon UPb geochronological constrains. In VII SSAGI South American Symposium on Isotope Geology,Brasília. Parada, M.A.; Nyström, J.; Levi, B. 1999, Multiple sources for the Coastal Batholith of central Chile (31-34º S): geochemical and Sr-Nd isotopic evidence and tectoni implications. Lithos, vol. 46, 505-521. Rivano, S.; Sepúlveda, P.; Herve, M.; Puig, A. 1985, Geocronología K- Ar de las rocas intrusivas entre los 31°-32°S, latitud sur, Chile. Revista Geológica de Chile, vol. 17, .205 – 214.  Rivano, S.; Sepúlveda, P.; Boric, R.; Espiñeira, D., 1993, Hojas Quillota y Portillo. 1:250.000., Servicio Nacional de Geología y Minería, Carta Geológica de Chile, Santiago (1993) 73. Schmidt, M.W. 1992, Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer. Contributions to Mineralogy and Petrology, Vol. 110. p. 304-310. Varas, M.I.; Morata, D.; Arriagada, C.; Ferrando, R.; Higgins,M. 2009, Distribución y características morfológicas de los enclaves máficos del borde sur del Complejo Plutónico Illapel (CPI). 12º Congreso Geológico Chileno, Noviembre 2009, Santiago (Chile).  400

P-T conditions of crystallization of Mafic Microgranular enclaves from the Illapel Plutonic Complex [IPC]

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P-T conditions of crystallization of Mafic Microgranular enclaves from the Illapel Plutonic Complex [IPC]

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