Mutinaíte in zeolite assemblage in metabasalts of villegas, northern Patagonian Andes, Rio Negro Argentina María Elena Vattuone,a,b Pablo R Leal,a,b and Andrea Martíneza aBuenos Aires University, Buenos Aires, Argentina bConsejo Nacional de Investigaciones Cienctíficas y Técnicas, Bs As, Argentina

Abstract

In Northern Patagonian Andes, mutinaite in assemblage with clinoptilolite in amygdales of altered Jurassic/Cretacic metabasalts was found. The zeolites were studied by optic, SEM, EDS and XRD methods. The rocks have fresh pyroxene and andesine altered to pumpellyite, in a matrix with albite, interstratified chlorite/smectite, pumpellyite and epidote. There are veins with laumontite and amygdales infilled with heulandite/clinoptilolite and mutinaite. This last mineral is white and pinky with perfect cleavage parallel to {100} and imperfect cleavage parallel to {001}. It is orthorhombic, the elongation is negative, 2V� = 70°/80° and a0 = 19.166Å, b0 = 21.025Å, c0 = 14.558Å. Amygdales filled by mutinaite–clinoptilolite-quartz and veinlets with wairakite-laumontite suggest that their crystallization was related to low grade hydrothermal/geothermal metamorphism. The temperature of formation should be 200°C/250°C range. Keywords: mutinaite - zeolite assemblage - Jurassic/Cretacic metabasalts – Northern Patagonian Andes - República Argentina.

1. Introduction and geologic setting Mutinaite in assemblage with heulandite/clinoptilolite and offretite in amygdales of altered Jurassic/Cretacic basalts and basandesites was found in Villegas, Rio Negro Province in Northern Patagonian Andes and the country rocks extend from 41°30`LS to 42° LS, emerging between Jurassic/Cretacic plutonic rocks. In sectors volcanic rocks are intruded by Cretacic granitic rocks. Mutinaite is a rare silicic calcosodic zeolite founded in Mt. Adamson, Antarctica. [1] The outcrops of volcanic rocks that carry zeolites would belong to Lago la Plata Group that is a volcaniclastic sequence of Jurassic/Cretacic age and are affected previously by low grade metamorphism (LGM) in greenschists facies [2]; in this paper these are called metabasites.

2. Mineral Assemblage The secondary minerals of metabasites are found in microdomains as partial replacement of primary minerals phenocrysts and in the matrix of volcanic rocks and in breccias, amygdales, and infilling veins and veinlets. The zeolites were studied by conventional optic methods, SEM, EDS, XRD with cell reffinament. The smectites are calcic montmorillonites. The following associations were observed in thin sections as:

Zeolites and Related Materials: Trends, Targets and ChallengesProceedings of 4th International FEZA ConferenceA. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.

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h k l d(obs) I/Io h k l d(obs) I/Io3 2 1 5,235 15 5 1 4 2,548 103 1 2 4,601 33 3 0 5 2,475 81 0 3 4,413 7 1 5 5 2,255 64 3 0 4,019 95 8 3 3 2,091 61 3 3 3,699 13 8 0 4 2,024 92 3 3 3,455 9 3 5 6 1,885 66 0 0 3,366 30 4 2 7 1,776 71 2 4 3,158 18 8 0 6 1,664 73 5 7 3,076 8 4 12 1 1,584 135 0 3 3,015 100 8 7 5 1,550 85 2 3 2,872 16 0 1 9 1,492 56 3 2 2,755 30 8 5 7 1,437 7

6 0 3 2,690 8

1) replacement of phenocrysts and microphenocrysts : Plagioclase albite – S/C interestratified - pumpellyite Clinopyroxene Clinozoisite-epidote-pectolite-actinolite pumpellyite- smectite- hematite - magnetite smectite – S/C interestratified 2) replacement of matrix: albite - pumpellyite – prehnite - smectite- S/C interestratified - Mg-chlorite - pectolite - hematite – magnetite - titanite albite- tremolite/actinolite- Mg-chlorite palagonitic glass 3) amigdales: mutinaite-offretite-heulandite/clinoptilolite 4) cement of breccias: laumontite- wairakite - yugawaralite laumontite-pumpellyite smectite-mutinaìte-offretite-heulandite/clinoptilolite -quartz 5) infilling veins and cracks smectite - stilbite-stellerite calcite-quartz

3. Mineralogy of Mutinaite Mutinaite from Villegas has white and pale pinky colors. Occurs in laths and in a tabular cuadrangular sections; has a perfect cleavage parallel to {100} and an imperfect cleavage parallel to {001} (fig. 1); it is orthorhombic; the elongation is negative and has 2V� = 70°/80°; v> r dispersion; it shows abundant and very little inclusions of hematite The XRD spacing on a sample of pure mutinaite (powder method, CuK�1) are near coincident with mutinaite of Antarctica (table 1). The parameters of the unit cell [3] are: a0 = 19.166 Å �=0.00123, (20.223 Å); b0 = 21.025 Å �=0.00181 (20.052 Å); c0 = 14.558 Å �=0.00070 (13.491 Å) (the data between parenthesis are after Galli et al. [1]). Table 1: XRD pattern of one mutinaite crystal.

506 M.E. Vattuone et al.

Table 2 shows the chemical composition that evidences a high R=Si/(Si + Al) = 0.90/0.91, characteristic of this specie. The amount of Ca and Na indicate that this mutinaite is in the range proposed by Passaglia and Sheppard [4]. Table 2: Chemical analyses of mutinaite. .

Sample 1

(wt%)

Sample 2

(wt%)SiO2 74.67 75.01

Al2O3 6.51 6.23

CaO 2.73 2.68

Na2O 1.09 1.09 Number of ions on the basis of 192 O

Si 86.98 87.31

Al 8.94 8.55

Ca 3.41 3.34

Na 2.45 2.45

4. Discussion According to the mineralogy and textural relationships, the volcanic sequences were affected by several metamorphic events: one generalized in the area in greenschist facies of LGM (evidenced by albite, actinolite, Mg-chlorite, epidote, clinozoisite and pectolite associations) reached 2kb of pressure at 350°C of temperature [2] similar to LGM in Northern Patagonian Andes in Chubut Province south 42°LS [5]. The next events are geothermal and hydrothermal processes of LGM in zeolite facies. 4.1. Geothermal LGM The characteristic assemblage is given by laumontite, scarces wairakite and yugawaralite, albite and pumpellyite; in a few sectors there is prehnite and pumpellyite is absent which would indicate differences in the compositions of the country rocks. This associations correspond to zeolite facies and according to Deer et al. [6], the presence of yugawaralite and laumontite both in veins and in cement of the breccias, confirms the precipitation from hydrothermal fluids and the presence of wairakite, indicate that the temperatures would range 250°C at pressures of 0,5kb [7]. 4.2. Hydrothermal LGM The sequence in the formation of wairakite, yugawaralite, laumontite, mutinaite and offretite shows that when the temperature goes down, the sodic fluids increased. Mutinaite and offretite with dioctaedric smectites in veins and amygdales would have formed at lower temperatures than 200°C.

Figure 1: SEM photo of Mutinaite

507Mutinaíte in zeolite assemblage in metabasalts of Patagonian Andes

According to this, cold fluids that deposited these assemblages would have had little interaction with the mesostasis of the country rocks and would be indicative in agreement to Schiffman and Staudigel [8] of a low interaction fluid/rock. Later, at low temperatures, a restrictive process due to a calcic event formed Ca-heulandite/clinoptilolite at the core of amygdales, and infilling cracks, were deposited Ca-stilbite and stellerite.

5. Conclusion The LGM in a geothermal environment in a zeolite facies is in accordance to the thermodinamic variables necessary for the formation of laumontite, wairakite and yugawaralite at temperatures lower than 250°C and 0.5kbars. The hydrothermal metamorphism in low temperature (zeolite facies), whose deposited mutinaite and offretite with smectites, was developed in an environment with strong presence of alkaline rich fluids at probably temperatures lower than 200°C. This process was followed by another one with calcic zeolites heulandite and then, stilbite-stellerite. Acknowledgments The authors wish to acknowledge the support provided by PIP CONICET 5064 and UBACyT X840.

References [1] E. Galli, G. Vezzalini, S. Quartieri, A. Alberti and M. Franzini, Zeolites, 19 (1997) 318. [2] Vattuone at al., in preparation [3] T. Holland and S. Redfern, Mineralogical Magazine, 61 (1997) 65. [4] E. Passaglia and R. Sheppard, Reviews in Mineralogy and Geochemistry, 45 (2001) 69. [5] M. Vattuone, C. Latorre and P. Leal, Revista Mexicana de Ciencias Geológicas, 23 (2005)

315. [6] W. Deer, R. Howie, J. Zussman and W. Wise, Rock Forming Minerals. Framework Silicates.

4B. The Geological Society. 2nd Edition (2004) London, 982. [7] Y. Zeng and J. Liou, American Mineralogist, 67 (1982) 937. [8] P. Shiffman and H. Staudigel, Journal of Metamorphic Geology, 13 (1995) 487.

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