Counterclockwise post-emplacement evolution of
metatroctolites from Aluminé Igneous-Metamorphic
Complex, Neuquén, Argentina.

Ivana Urraza1, 3, Sergio Delpino1, 2, Laura Grecco2

1 Departamento de Geología, Universidad Nacional del Sur, San Juan 670, Bahía Blanca, Argentina.
iurraza@uns.edu.ar; sdelpino@criba.edu.ar

2 Instituto Geológico del Sur, CONICET-Universidad Nacional del Sur, San Juan 670, Bahía Blanca, Argentina.
mlgrecco@criba.edu.ar

3 Departamento de Geología y Petróleo, Facultad de Ingeniería, Universidad Nacional del Comahue, Buenos Aires 1400, 830 Neuquén, Argentina.

Fig. 1. Location and geologic setting for the AIMC. A. Geologic map of the AIMC showing the locations of the metatroctolites (modified after Urraza et al., 2011); B. Geologic pre-Andean frame (modified after Vaughan and Pankhurst, 2008). Note the locations of the study area in a key zone of convergence of several terranes, and which counts up to the present with very scarce geologic information; C. Modified scheme of Gordon and Ort (1993), showing the locus, dimensions and orientation of the bodies that conform the SCB and the PB south of the 40ºS. The locations of the CPB and SB, are also showed.

 

There are many examples in literature of reaction textures, coronas and symplectites; some of them were given in: cumulates in NE Sardinia, Italia (Franceschelli et al., 2002); in metatroctolites from Buchcreek, North Carolina Blue Ridge (Lang et al., 2004); in Rymmen gabbro from Southern Sweden (Cleason, 1998); in gabbros from Black Hill, South Australia (Turner and Stüwe, 1992); in troctolitic gabbros in Norway (Ashworth, 1986), among others. In Argentina, there are some examples in mafic rocks from Sierras de San Luis (Cruciani et al., 2008, 2011, 2012; Brogioni et al., 2007) and in Sierra del Valle Fértil and La Huerta (Otamendi et al., 2010; Gallien et al., 2012). This contribution constitutes the first mention of mafic coronitic rocks outside the Sierras Pampeanas context.

This study was focused on metatroctolites that exhibit coronas between olivine and plagioclase, and involve the analysis of mineral assemblages, textures, microstructures, whole-rock/mineral geochemistry, geochronology and metamorphic reactions in order to define the P-T trajectory of the metatroctolites of Ñorquinco lake.

2. Geological Setting

The metatroctolites, previously denominated troctolitic metagabbros (Urraza et al., 2012, 2013; Urraza, 2014) form part of the Aluminé Igneous- Metamorphic Complex (AIMC), which has been defined as a set of intrusive-igneous pre-Andean and Andean rocks and its metamorphic country-rocks. The AIMC is located between the Aluminé, Moquehue, Ñorquinco and Pulmarí, lakes in the Neuquén province of Argentina (Urraza et al., 2008a, 2011). The AIMC forms the northern sector of the zone limited by the Pino Hachado-Picún Leufú (PH-PL) and Nahuel Huapi (NH) megalineaments, defined by Ramos (1978) as Aluminé Batholithic Belt (Fig. 1A).

The pre-Andean tectonic context for the AIMC, is represented in figure 1B. The AIMC is located within a complex tectonic framework. According to Vaughan and Pankhurst (2008), the sector between 38° and 40°S presents a high grade of complexity due to the scarcity of detailed information, from what they marked this area with a question mark. This zone limits to the north with the Chilean and Cuyanean terranes, whereas towards the south it is in contact with the North Patagonian Massif and Patagonia. The tectonic Andean frame also presents a high grade of complexity due to the successive emplacement of igneous bodies forming different batholiths. The Patagonian Batholith (PB), located between 38º30’S and 56ºS, records the Andean subducción initiation at around 150 Ma ago, in the Jurassic period (Hervé et al., 2007). The northern portion of the PB is named North Patagonian Batholith (NPB). Previously, Gordon and Ort (1993), had called Subcordilleran Patagonian Batholith (SCB) to the jurassic granitic rocks arranged in a NNE trend. Same authors named Cordilleran Patagonian Batholith (CPB) to the cretaceous granitic rocks distributed along the N-S direction (Fig. 1C). The locations of the Central Patagonian Batholith (CPB) and the Somuncurá Batholith (SB), are showed in figure 1C. In spite of in figure 1C outcrops of the PB and SCB are only showed to occur to the south of 40°S, rocks corresponding to these batholiths can be recognized up to the 38º30’S (Urraza et al., 2011).

3. Country rocks

The metamorphic basement of the AIMC is represented by small isolated outcrops of quartzo-feldespatic metapelites and amphibolites. Most of the outcrops are constituted by gneisses intensely foliated, with injections of granitic material of the ‘lit par lit’ type and frequently showing isoclinal, intrafoliar and/or ptygmatic folding. With less representation, finely foliated amphibolites have been recognized (Fig. 2). These rocks also show thin injections of the ‘lit par lit’ type along the foliation planes and irregular granitic veins, usually of greater thicknesses, which cut the foliation with different angles (Urraza et al., 2009; Urraza, 2014). The varieties of basement described above correspond to in situ exposures, but they have been also observed as variable sized inclusions in gabbros and tonalites. Rarely and in thin strips, the gneissic basement loses partly its ptygmatic appearance and acquires a very well defined and very regularly orientated foliation. In this case, the foliation is marked by granitic thin injections, the stretching and boudinage of the thickest veins and by the distribution of abundant micas along the foliation planes. The loss of the intensely folded gneissic structure and the increase of phylosilicates in these bands, led to the rock to acquire a schistose appearance (Fig. 2B). The original metamorphic paragenesis of the injected gneisses is constituted by Qz+Pl+Ms+And±Bt±Grt+Zrn+Ap (Fig. 2A) (abbreviations after Withney and Evans, 2010). This association is stable at moderated to low temperatures (< 620 ºC at X_H2O=1, or still lower at lower X_H2O) and pressures below 3.8 Kb. This early association is overprinted by a ductile deformation event that led to the intense recrystallization of quartz, moderate recrystallization of plagioclase and the development of an anastomosed mylonitic foliation (Fig. 2A). The geothermobarometry of the association stable during this deformation event (Bt+Grt+Sil(Fib)+opaque minerals), gave pressures and temperatures in the ranges 6.2-6.7 Kb and 605-620 ºC, respectively. The presence of sillimanite in these rocks require of minimum temperatures of 615 ºC at 6.2 Kb and 640 ºC at 6.7 Kb (Urraza et al., 2009). The injected amphibolites are composed of Qz+Pl+Hbl±Bt, plus accessories and opaque minerals. Amphibolites also show ductile deformation evidenced by intense recrystallization of plagioclase and amphibole, which underwent significant reduction of the grain size and development of pseudo-polygonal new grains meeting at 120° triple junctions. These fine aggregates arrange in folia, which together with the preferential orientation of relict crystals define the mylonitic foliation (Fig. 2C).

 

Fig. 2. The field characteristics of the different lithological types conforming country-rocks, are showed on the left side of the figure, whereas a microphotography corresponding to each of them are presented on the right. A. Injected gneisses: are composed of the association Qz+Pl+Ms+And±Bt±Grt+Zrn+Ap. Garnet, biotite and the fibrolitic sillimanite (this last one not observed in the microphotography due to the scale), form the anastomosed mylonitic foliation corresponding to the ductile deformation event that led to the intense recrystallization of quartz and moderate recrystallization of plagioclase; B. Muscovitic schists: present similar deformation to gneisses, showing development of polycrystalline quartz aggregates forming 120° triple junctions, surrounded by folia composed mainly of muscovite; C. Injected amphibolites: composed by hornblende and intensely recrystallized plagioclase.

 

4. Field description of the metatroctolites

The studied metatroctolites show (at outcrop scale) a fine granular texture in the borders of the body that increases progressively in size towards the center, where it develops a coarse to very coarse granular texture (Fig. 3). This rocks show an anastomosed penetrative foliation with a general SSW trend (mean: 200º/80º DD/D) (Fig. 3D).

However, these rocks show a widespread retrogression, the magmatic mineralogical association can be still recognized in some thin sections. The intrusive body contains inclusions of the surrounding metamorphic basement, whose mineral associations and textures have been strongly obliterated (Figs. 3B and C). Regarding mineralogy of these basement inclusions, only quartz and intensely altered plagioclase and biotite are preserved, and the primitive foliation is only marked by the preferred orientation of quartz ribbons. The body of metatroctolites is cut by aplitic dikes of up to 60 cm thickness oriented 130º/66° (dip direction/dip) and coarse pegmatitic dikes in which stand out perhitic K-feldspar crystals of up to 10 cm and tourmaline (Figs. 3A and E).

 

Fig. 3. A. Outcrop appearance of the metatroctolites. Note almost massive mafic body, cutting discordantly by an aplitic dike oriented 130º/66°; B. Metamorphic xenolith with irregular shape included in the metatroctolite; C. Detail of the xenolith. Note the good preservation of the metamorphic fabrica, similar to that observed in the in situ exposures of the same contiguous basement rocks; D. Rough disjuntive foliation affecting metatroctolites (S=200º/80º); E. Pegmatite composed of K-feldspar, quartz and tourmaline megacrystals, cutting the mafic body.

 

5. Geochemistry and classification

A detailed sampling traversing the whole gabbroic body including 11 samples were carried out, in order to asses the structural, textural and compositional variations. Geochemical analysis including major and trace elements, were performed in the Actlabs^® laboratory, Canada, using ICP-MS. For the classification of the original magmatic rock and the geothermobarometric analysis, samples of the center of the body which better preserve the magmatic (texture and mineralogical composition) and metamorphic characteristics (post-crystallization textures and minerals), were selected.

The representative geochemical composition of metatroctolites is presented in Table 1. According to its SiO₂ content (43.1%), this rock is located close to the limit between basic and ultrabasic rocks following the Le Maitre (1989) silica content classification. Taking into account that the mineral proportions of ferromagnesian minerals in the studied metatroctolites are significantly lower than 90%, these rocks are considered to be mafic. The alkalis sum (Na₂O+K₂O=0.71) and calcium content (11.62%), point to a sub-alkaline signature. In addition, this magnesian-rock presents an X_Mg=0.73 (in atomic proportions) and very low TiO₂ (0.068%). Considering its chemical and mineralogical composition, these metatroctolites can be compared with gabbronorites and gabbroic inclusions in granodiorites found in the same sector of the AIMC. The above mentioned rocks with SiO2 contents in the range 48.38-51.65% and total alkalis of up 4.44% would correspond to more evolved facies. Compared with the metatroctolites, these rocks are richer in iron (X_Mg=0.41-0.47) and TiO₂ (1.22-2.55%).

The REE plot in figure 4C, shows a relatively high concentration of LREE and depletion in HREE. The metatroctolites show a strong positive europium anomaly, suggesting the accumulation of plagioclase segregated from the melt. The enrichment of LREE can be explained as the result of contamination from the metasedimentary xenoliths incorporated by the mafic magma. In figure 4C, this rock pattern is compared with the REE diagrams corresponding to gabbronorites and gabbroic inclusions in cretaceous granodiorites (see section 8). The pattern is almost parallel to that of gabbronorites, although this last one shows a major enrichment in total REE. On the other hand, gabbroic xenoliths in granodiorites show total REE similar to gabbronorites, but differ from them by the presence of a slight negative europium anomaly (Fig. 4C).

On the base of the modal composition mafic rocks can be classified as troctolites (Fig. 4A), whereas according to its chemical composition as perido-gabbros (Fig. 4B).

 

Fig. 4. A. Modal classification based on the proportions of plagioclase, pyroxene and olivine; B. TAS Diagram (whole alkalis versus silica) of Cox et al. (1979), adapted by Wilson (1989) and Middlemost (1994) for plutonic rocks. The studied samples lie inside the subalkaline area, and within the perido-gabbros field; C. REE composition of metatroctolites, gabbronorites and gabbroic inclusions in granodiorites normalized to Boynton (1984).

 

6. Petrography

The Ñorquinco metatroctolites show a coarse granular texture (Figs. 5A and B), and variable proportions of preserved igneous and late magmatic/subsolidus metamorphic minerals. Relict magmatic olivine crystals are usually fractured and normally exceed 500 µm in size. Magnetite appears filling internal microfractures and in the crystals rims. The most conspicuous characteristic showed by relict olivine crystals is the formation of reaction coronas around it, which are composed mainly of pyroxene, amphibole and spinel.

Plagioclase appears either as relict magmatic crystals of the original mineralogy (Pl₁), or filling fractures (Pl₂).Its presents deformation twins and rounded rims when it is in contact with amphibole. The orthopyroxene is mainly present in the reaction coronas, although a few magmatic big interstitial crystals were recognized in some thin sections. Magmatic clinopyroxene (Cpx₁) appears as small relict crystals surrounded by other phases (Fig. 6B and D) Cpx₂ forms part of the coronas around olivine and plagioclase crystals, sometimes forming symplectitic intergrowths with spinel. The presence of orthopyroxene and Cpx₂+Spl symplectites developed around olivine crystals can be attributed to the typical reaction: Pl+OlCpx+Opx+Spl. These coronas and the presence of different types of amphiboles, allow tracking the succession of events that affected to the mafic rock following its emplacement. Hbl₁ name is assigned to the oldest recognized crystals of amphibole, which was initially in contact with relict Pl₁ and Ol (Figs. 5C and D). Hbl₂ is characterized by big interstitial crystals, which replace Hbl₁ as well as all of the other constituents of the rock. Green spinel is disseminated everywhere inside the rock and forms part of the previously mentioned symplectites (Fig. 6A, B and C). The formation of coronas and retrograde interstitial mineral association can be represented by the general reaction: Pl₁+Ol+Cpx₁+Hbl₁+H₂O Opx+Cpx₂+Hbl₂+Pl₂+Spl. Excess of H₂O needed for the reaction, can be either attributed to a late magmatic origin, or to an external incorporation. The metatroctolites are carriers of an important sulfide mineralization, represented principally by pyrrotine, pentlandite and chalcopyrite. These ore minerals were also found filling fractures in the adjacent country rocks.

 

Fig. 5. Parallel light (left) and crossed nicols (right) microphotographs of the metatroctolites. A. Granular texture of the metatroctolites formed by big-sized olivine crystals, with development of coronas composed of Opx+Cpx+Hbl2. Note also the presence of irregular and corroded relict Hbl1 crystals, surrounded by Hbl2 and small Spl crystals; B. Microphotograph showing replacement of Hbl1 (initially in ‘normal’ contact with magmatic plagioclase and olivine), by widespread interstitially distributed Hbl2.

 

 

Fig. 6. BSE images of coronas and replacement textures observed in the metatroctolites of Ñorquinco lake. A. Amphibole-spinel corona developed around a relict-magmatic olivine crystal. Note symplectitic intergrowth of both minerals towards the external edge of the corona, against plagioclase. Note also, late chlorite and magnetite along olivine-corona contact and filling microcracks; B. Interstitial orthopyroxene replacing olivine, plagioclase and clinopyroxene. Observe also, olivine, plagioclase, clinopyroxene and orthopyroxene replaced in a first stage by Hbl1 and in a second stage by Hbl2; C. Enlargement of the sector delimited by the rectangle in b, showing a Spl-Hbl2 symplectitic corona developed around a relict-magmatic plagioclase. Note that symplectites develop towards orthopyroxene crystal, and not to the contact with the other phases; D. Olivine, plagioclase and clinopyroxene first replaced by Hbl1 and then by Hbl2; E. Interstitial spinel between plagioclase and olivine. The presence of spinel is attributed to reactions in which these two later phases are the reactants (details in the text). Note also the presence of retrograde chlorite, either interstitial or filling-cracks.

 

7. Mineral Chemistry

The chemical compositions of the metagabbro mineral phases present in the metatroctolite were analyzed using a microprobe JEOL 8200 from the Earth and Atmospheric Sciences laboratory of the Alberta University, Canada. The analytical conditions were 15kV accelerating voltage, beam current of 15nA and 3-5 µm spot size. Analyses representative of the different mineral phases and their structural formulas, are presented in Table 2.

7.1. Olivine

It appears as a primary magmatic mineral bordered by reaction coronas composed of pyroxenes and amphiboles. The composition of olivine varies from Fo₆₉ to Fo₇₃. Olivine shows only negligible compositional variations between crystals cores and rims.

7.2. Plagioclase

Plagioclase forms part of the original magmatic association of the metatroctolites. It is characterized by high An contents, which varies between 89.2% and 95.6% (Table 2) and lies in the Anorthite when plotted on the ternary diagram Ab-An-Or (Fig. 7B). Both petrographycally and geochemically is possible to recognize two generation of plagioclase: as phenocrystals (Pl₁) and interstitially (Pl₂). The composition of Pl₁ is fairly uniform, varies between An₈₉-An₉₀, while Pl₂ is substantially more calcium -rich (An₉₆).

 

Fig. 7. A. Olivine classification diagram (isomorphic binary series forsterite-fayalite, based on the Mg/Mg+Fe2+ and Fe2+/Fe2++Mg proportions; B. Ab-An-Or ternary diagram; C. Ternary diagram for orthopyroxene classification. Observe the intermediate compositions of the isomorphic series between the end-members enstatite (En) and ferrosilite (Fs); D. Ternary diagram for classification of clinopyroxene. Note the intermediate compositions of the isomorphic series between the extremes clinoenstatite (Cen) and clinoferrosilite (Cfs); E. Prism of Jhonston for classification of spinel group minerals; F. Triangular diagram of trivalent cations (Fe3+-Cr3+-Al3+); G. Diagram of spinel classification according to the relative proportions of bivalent and trivalent ions; H. Classification of calcic amphiboles [(Na+K)< 0.5, Ti< 0.5] according to Leake et al. (1997); I. Classification of calcic amphiboles [(Na+K)>0.5, Ti< 0.5] according to Leake et al. (1997); J. Chlorite classification diagram (taken from Deer et al., 1992).

 

7.3. Orthopyroxene

This phase appears in the reaction coronas developed between olivine and plagioclase. Classification of orthopyroxene was carried out using the triangular diagram En-Fs-Wo (Fig. 7C). The proportion of enstatite (En) and ferrosilite (Fs) varies between 69% and 73% (X_{Mg 77}), whereas the wollastonite component does not overcome 5% for any crystal (Table 2). Orthopyroxene crystals in the metatroctolites have predominantly magnesian compositions, and lie in field of broncite.

7.4. Clinopyroxene

Chemical composition of clinopyroxene crystals (Fig. 7D), can be expressed in terms of the relative proportion of the clinoenstatite-clinoferrosilite-wollastonite components. The analyzed grains indicate that clinopyroxenes are calcium-magnesium diopsides, with a wollastonitic component varying between is 46.16%-48.19%, and X_{Mg 83-85} (Table 2). Cpx₁ is the relict phases, although scarce Cpx₂ is found only in coronas.

7.5. Spinel

Pure end-members of the spinel group are rarely found in nature. They can be subdivided on the base of their dominant divalent and trivalent cations. The compositions of the multicomponent system of the spinel group, can be plotted on the prism of Johnston (Fig. 7E), whose extremes are given by magnetite, magnesioferrite, spinel, magnesiochromite and chromite (Deer et al., 1992). Gargiulo et al. (2013), propose classification diagrams based on the faces of the Johnston’s prism. Figure 5F, shows a classification of the spinel group based on the exchange of the trivalent cations on the Y position. This classification allows separating the fields of Fe³⁺-rich spinels representing the magnetite end-member, from the Cr³⁺-rich and Al³⁺-rich fields corresponding to chromite and hercynite end-members, respectively. Figure 5G, shows that the analyses individuals have intermediate compositions between spinel and hercynite, with most of them being located within the pleonaste field. Only one analysis lies on the spinel field, which corresponds to aSpl-Hbl symplectite developed in the contact between olivine and plagioclase crystals (Fig. 6 A-C).

7.6. Amphiboles

Three generations of amphiboles were recognized in the metatroctolites. The first two classify as hornblendes (Hbl₁, Hbl₂) and the third as tremolite (Tr). According to Leake et al. (1997), Hbl₁ classifies as magnesium-hornblende, although most of them lie on the limit with the tchermarkitic-hornblende field as shown in figure 4G. The petrographic analysis shows that Hbl₁ is replaced by Hbl₂, whose composition corresponds to an edenite-hornblende (Fig. 7H). The tremolitic amphibole, replaces the olivine crystals and the minerals forming the reactions coronas, and clearly corresponds to a very late-low temperature retrogression.

7.7. Chlorite

Chlorites are magnesian and classify as chlinoclores in the Si versus (Fe³++Fe²⁺) diagram (Deer, 1992) (Fig. 7I). Like tremolites, chlorites correspond to a late-low temperature alteration event.

8. Mineral re-equilibration

The sequence of mineral re-equilibrations in metatroctolites from Ñorquinco lake is shown in Table 3. The original igneous association is constituted by Ol+Pl₁+Cpx₁. First replacement textures are represented by the growth of Opx on Cpx₁ crystals (stage A in Table 3). The appearance of Hbl1 due to hydration of the previous phases form the association corresponding to stage B. Spl, Hbl₂ and Pl₂ formed during stage C. Stage D is characterized by the late formation of Tr and Chl.

8.1. Stage A formation of Orthopyroxene

Orthopyroxene is the first mineral representing replacement textures in the metatroctolites. It clearly replace the igneous minerals Ol-Cpx₁. Convex grain boundaries towards olivine in Ol-Opx contacts, indicate growth of orthopyroxene at the expense of olivine crystals.

8.2. Stage B formation of Mg-Hornblende (Hbl1)

The development of amphibole (Hbl₁) is the result of the reaction between Ol-Cpx_{1/ and Pl1 with the progressive increase in H2O concentration. The widespread occurrence of Hbl1 generates lobed edges around Pl, and sometimes full replacement of Ol, although some small remnants of Ol with irregular borders were found. It is not possible to recognize close contacts between Ol and Pl because Hbl1 is always present between these original phases.}

8.3. Stage C: formation of Mg-Hornblende (Hbl2), Spinel and Cpx2

As a result of the progressive changes in the physical conditions, a new amphibole (Hbl₂) replacing the pre-existing phases is formed. Spl generates symplectites with Hbl₂ around Ol and Pl. Hbl₂ replaces Hbl₁ and forms reaction rims around Cpx-Hbl₁ and Opx-Hbl₁. Cpx₂ was recognized in some coronas associated with Spl. Hbl₂ and Spl form symplectites around relict phases.

8.4. Stage D: formation of Chl-Tr

The formation of Chl and Tr in very low proportions is attributed to the last stage of retrogression. Chl is mainly replacing the different phases of the corona and surrounding olivine. Magnetite filling fractures inside olivine crystals, is also assigned to this stage.

9. Phase relations, geothermobarometry and P-T path

In order to evaluate the stability of the observed mineral associations, a suitable P-T pseudosection was constructed with the program Perplex_X (Connolly, 1990, version 2013). The diagram was calculated using the whole-rock chemistry of the metatroctolites (Fig. 8), considering the chemical system Na₂O-CaO-FeO-MgO-Al₂O₃-SiO₂-H₂O and including the following phases: plagioclase, olivine, orthopyroxene, clinopyroxene, spinel, amphibole, garnet, chlorite and water. The solid solution models for olivine, clinopyroxene, orthopyroxene garnet, spinel and chlorite correspond to Holland and Powell (1998, and updated versions); for plagioclase and amphibole were used the formulations of Newton et al. (1980) and Dale et al. (2005), respectively.

 

Fig. 8. P-T Pseudosection for the metatroctolites calculated for the system NCFMASH and a whole-rock chemical composition (wt% oxides): SiO2=43.1; TiO2=0.068; Al2O3=19.7; FeO=9.57; MgO=14.67; CaO=11.62; K2O=0.1; Na2O=0.61; H2O=1.5. The points A, B, C and D have been obtained through classical geothermobarometry applied to different mineral pairs.

 

Pseudosection analysis was complemented with the application of classical geothermobarometry to selected mineral pairs. Using the Opx-Cpx₁ geothermometer of Wood and Banno (1973) and the Cpx₁-Pl geobarometer of Kretz and Jen (1978), a temperature of 936 ºC and a pressure of 2.7 Kb were obtained (Table 4). This point, when plotted on the pseudosection (A) of figure 8, locates within the stability field of the association Ol+Pl₁+Cpx₁+Opx+H₂O. Possibly, there was not available free water during this first stage, but the incorporation of this fluid component is necessary in order to generate the hydrated phases observed. This free water might have been originated during late magmatic crystallization, supplied by an external source, or by a combination of both provenances. Point A is considered to be the nearest condition to the reached metamorphic peak by the metatroctolites, and it is established as the starting point for the post-emplacement trajectory.

The conditions of the point B in figure 8, were calculated through the geothermometer TB of Holland and Blundy (1994), using the mineral pair Pl₂-Hbl₁ and the Al-in-Hbl geobarometer of Johnson and Rutherford (1989) for Hbl₁, obtaining a temperature of 851 ºC and a pressure of 4.26 Kb. Point B lies in the stability field of the association Ol+Pl+Cpx+Opx+Hbl₁+H₂O, and differs from the previous one for the first appearance of the amphibole (Hbl₁). The trajectory between points A and B implies increasing pressures and decreasing temperatures, and requires of the availability of free water to form amphibole. The P-T condition of stage B is well constrained by the compositional isopleths of X_Mg in Opx (0.77), as well as in Cpx X_Mg=0.85 (Fig. 9A, B). Anorthite proportions in Pl for the stage B is around (X_Ca=0.96), whose composition is the same as that obteined for Pl₂ with microprobe analysis. X_Si in amphibole varies between 6.5 and 6.6 (a.p.f.u) (Fig. 9 C and D). The estimated isopleths are fully coincident with the P-T conditions obtained for stage B using classical geothermobarometry.

 

Fig. 9. Compositional isopleths. A. XMg in Orthopyroxene; B. XMg in Clynopyroxene; C. An in plagioclase; D. XSi in amphiboles. The post-emplacement stages B, C and D (showed with stars) are the P-T values calculated with conventional geothermobarometry.

 

Textural relationships indicate that Hbl₁ is replaced by the Hbl₂, which indicates a new stage of mineralogical re-equilibrium. Applying the same thermometer and geobarometer to Pl₂-Hbl₂ pair, a temperature of 780 ºC and a pressure of 5.69 Kb were obtained. This point represented in figure 8 with the letter C, corresponds to the stability field of the mineral association Pl+Cpx+Opx+Amph+Spl+H₂O. The transition from the previous to this stability field carries with it the disappearance of Ol and the appearance of the Hbl₂+Spl pair. Petrography shows that this later phases are always spatially and texturally associated. The appearance of spinel is consistent with petrographic observations, which indicates the development of the reaction Ol+PlOpx+Cpx+Spl, typical of these rock types. Given that reaction Spl+PxGrt+Ol, which fixes the upper pressures limit for the stability of the association listed above, did not take place considering the total absence of garnet in the studied rocks, point C establishes an approximated maximum for pressures reached by these rocks. The X_Si composition in Hbl2 is around 6.6-6.7 (a.p.f.u), though the stage C calculated using conventional geothermobarometry corresponds to a little high composition X_Si 6.9-7.0 for the isopleths (Fig. 9D).

Finally, the presence of chlorite allows estimation of a temperature of 694 ºC, calculated with the formulation Al-in-Chl of Cathelineau (1988). According to pseudosection of figure 8, the formation of chlorite is associated to the total consumption of water and clinopyroxene present in the previous paragenesis, giving place to the new stable association Pl+Opx+Amp+Spl+Chl. Considering the calculated temperature for chlorite, a range of pressures can be estimated considering the absence of olivine in the above listed association which fix a minimum pressure at ~4.5 Kb, and the entire absence of garnet in these rocks establishing a maximum pressure of ~5.2 Kb (Fig. 8). Letter D in figure 8, is located in the middle of these values and at the calculated temperature. Taking into account that the continuity of the trajectory is limited by the last stability field, the trajectory must continue along a path characterized by the decrease of temperatures and pressures (Fig 8).

10. Estimation of the metatroctolites relative age

The presence of abundant metasedimentary xenoliths from the country-rocks fixes a maximum Late Devonian age (360 Ma, Urraza et al., 2009). On the other hand, xenoliths of strongly deformed metagabbros composed of Opx+Pl+Hbl+Bt+Ilm, were found included in granodiorites. In spite of these xenoliths are strongly affected by deformation, they preserve a mineralogical and geochemical composition similar to the studied metatroctolites. In the present contribution, a U-Pb zircon age of 309±31 Ma has been determined for the metagabbroic xenoliths, whereas for the host granodioritic rocks a U-Pb zircon age of 82±1.6 Ma was established (Fig. 10 A and B). Isotopic data were acquired using a Nu Plasma MC-ICP-MS coupled to a laser ablation system. The well-calibrated zircon standard LH-9415 was used as the standard for the normalization of the Pb/U values in the University of Alberta. Simonetti et al. (2006) show the advantages of using U-Pb laser ablation for zircon, monazite and titanite grains in standard petrographic thin section and demonstrate the effectiveness of the in situ technique in providing accurate and relatively precise age information. A laser beam spot of 40 µm was used; considering it is the appropriate size for Paleozoic rocks. Simonetti et al. (2006) mention that it should prove particularly valuable for obtaining U-Pb ages in reconnaissance-type studies, such as preliminary geological studies of regions that contain little or no prior geochronological age information. Therefore, it is an appropriate technique for preliminary information in the AIMC.

The ages found for the country-rock xenoliths and gabbroic inclusions in granodiorites, constrain the metatroctolites formation to Upper Paleozoic times, which allows linking its intrusion to the early stages of gondwanian magmatism.

 

Fig. 10. A. Tera-Wasserburg concordia representation of the data obtained from analyzed zircons, using thin section in situ LA-ICP-MS with a 40 µm bundle. U-Pb age for gabbroic inclusions in granodiorites; B. Wetherill concordia U-Pb age for host granodiorites.

 

11. Conclusions

• Mafic rocks from the north of Ñorquinco lake were classified based on the primary modal composition as metatroctolites, and on the base of the alkalis content as perido-gabbros.
  
  
• The primary-magmatic association composed mainly of Pl₁+Ol+Cpx₁+Hbl₁, was re-equilibrated during the post-emplacement thermal descent giving place to the formation of coronas around olivine crystals, with successive formation of orthopyroxene, clinopyroxene (Cpx₂), amphibole (Hbl₂) and spinel.
  
  
• The presence of water, either late-magmatic or externally incorporated, gave place to the appearance of high temperature amphiboles (Hbl₁, Hbl₂) and the late lower-temperature phases, chlorite, magnetite and tremolite. These phases formed by the hydration of the primitive anhydrous phases, and crystallized at different stages of the evolution of these rocks.
  
  
• Considered together, textural relationships established via petrographic studies, pseudosection analysis and geothermobarometry, allow us to reconstruct a counterclockwise P-T path for the post-emplacement evolution of the metatroctolite. This trajectory is consistent with the evolution determined for the adjacent country-rocks previously studied by the authors (Urraza et al., 2009).
  
  
• Based on the established ages for basement xenoliths in metatroctolites and gabbroic inclusions in granodiorites, the emplacement of the Ñorquinco lake metatroctolites can be constrained to the Upper Paleozoic.

 

Acknowledgements
This work was financed by funds of the PICT2008 No. 1379 granted to Dra. L. Grecco. We thank Dr. M. Zentilli of Dalhousie University and Dr. L. Heamann of University of Alberta, Canada, where the laboratory works were carried out. We are grateful to Dr. Indares, Dr. Steenken and Dr. Cruciani for the constructive reviews.

 

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