Lithogeochemistry of the meta ‐ igneous units from Arroio Grande Ophiolitic Complex , southernmost Brazil

Ophiolites are defined as slices of genetically-related upper mantle serpentinized peridotites and oceanic crustal rocks, tectonically displaced from its primary igneous origin of formation by plate convergence and associated (meta) sedimentary rocks of marine origin. From this premise, a meta-ultramafic-mafic-sedimentary complex (Cr-rich magnesian schists - upper mantle or crustal ultramafic cumulate candidates; epidote amphibolites, metadiorites and metagabbros - oceanic crust candidates; metasedimentary schists, quartzites and marbles - marine sedimentary rocks candidates), located in southeastern Dom Feliciano Belt (southernmost Brazil), started to be interpreted as possible slices of an ophiolitic complex related to the closure of a paleo-ocean during Brasiliano/Pan-African orogenic cycle and was called Arroio Grande Ophiolitic Complex. The present research fills the lack of geochemical data from previous studies and tests the hypothesis of an oceanic setting for the meta-igneous units of this complex from a lithogeochemistry point of view. The meta-ultramafics were interpreted as peridotites (mantle or crustal cumulates) that were subsequently serpentinized (probably in the ocean floor) and posteriorly metasomatized (probably in a continental setting). The meta-mafics were interpreted as oceanic gabbros/basalts formed in a back-arc basin. The results, together with field relationships, rock associations and petrographic evidences, support an oceanic origin for the protoliths of the meta-igneous units. The hypothesis that these rocks represent metamorphosed slices of an ophiolitic complex is still the most reasonable one. This work updates the geologic knowledge of the area and supports discussions about the evolution of Dom Feliciano Belt and Western Gondwana paleocontinent.

Lithogeochemistry of the meta-igneous units from Arroio Grande Ophiolitic Complex, southernmost Brazil

INTRODUCTION
Ophiolites are defined as slices of upper mantle and oceanic crust, tectonically displaced from their primary igneous origin of formation by plate convergence.They are frequently represented by a genetically-related rock association of partly to totally serpentinized upper mantle peridotites and crustal ultramafic cumulates, (meta) gabbros/basalts, volcanic units with or without sheeted dikes and (meta) sedimentary rocks of marine origin.Modern studies, like Dilek and Newcomb (2003), Dilek and Robinson (2003), Kusky et al. (2011), Dilek andFurnes (2011, 2014), have shown that the full Penrose layer-cake sequence defined in Anonymous (1972) represents only ≈ 10% of the Phanerozoic ophiolitic complexes and that the variations between individual ophiolites are as significant as their similarities, making it difficult to define a type succession.Furthermore, Precambrian ophiolites are frequently metamorphosed and dismembered, with some missing units (Kusky et al. 2011).
The present study aimed to fill the lack of geochemical data and to test the hypothesis of an oceanic crust and mantle origin for the Arroio Grande Ophiolitic Complex meta-ultramafic-mafic units from a lithogeochemical point of view, updating the geologic knowledge of the area and contributing to discussions about Dom Feliciano Belt and Western Gondwana paleocontinent evolution during Neoproterozoic.

Regional geology
The Arroio Grande Ophiolitic Complex is located at the southern portion of Dom Feliciano Belt (Rio Grande do Sul State, Brazil), a Neoproterozoic orogenic belt developed during the Brasiliano/Pan-African Orogenic Cycle.The belt extends for about 1,200 km from Punta del Este (Uruguay) to Santa Catarina State (Brazil).In Rio Grande do Sul State, it represents almost entirely the Sul-rio-grandense Shield and is divided into three geophysical domains: Eastern, Central and Western Domains (Fig. 1; Lenz et al. 2013).

Local geology
The Arroio Grande Ophiolitic Complex is a meta-ultramafic-mafic-sedimentary rock association, cut in two by a large granitic body (Três Figueiras granite) and affected by ductile shear zones (Ayrosa Galvão and Arroio Grande shear zones) with N50-80 º E and secondary E-W directions.In the northern area, centimetric to kilometric xenoliths of the three main units (meta-ultramafic, meta-mafic and metasedimentary) occur within Pinheiro Machado Complex granitoids (Fig. 2A).In the southern area, the biggest meta-ultramafic and meta-mafic bodies are found (Fig. 2B).
The meta-ultramafic unit is represented by talc-serpentine schists, tremolitites and chloritites.Based on petrographic comparison with similar occurrences found in the literature (e.g.Strieder 1992;Hartmann & Remus 2000) and microscopic features, they were interpreted in previous works (Ramos 2011;Ramos & Koester 2014) as the product of contact metasomatism between serpentinites and chemically contrasting rocks, forming fronts of talcification, tremolitization and chloritization from a serpentinite core.
These rocks form NE-SW metric to decametric discontinuous elongated bodies (Fig. 3A) are fine to medium grained, show sub-vertical millimetric schistosity, sub-horizontal stretching lineation and frequently show folds.The less competent talc-rich schists locally show crenulation cleavages.The meta-ultramafics are commonly found together in the same outcrop in diffuse to gradational contacts (Fig. 3B).No relict igneous minerals (except probably the chromites) or textures are preserved.Intensive alteration makes it difficult to study this unit.The talc-serpentine schists are constituted by aggregates of talc and serpentine (both ≈ 50 -70%) and minor tremolite, clinochlore and disseminated chromite.The tremolitites are composed by ≈ 90% tremolite and subordinated clinochlore, talc and disseminated chromite.The chloritites are constituted by ≈ 95% clinochlore and minor ilmenite (1 -2%), zircon (1 -2%), serpentine and tremolite.One chloritite sample shows the presence of Cr-chlorite (kämmererite or kotschubeite), detected in X-ray diffraction analysis.The chromites found in talc-serpentine schists and tremolitites (Fig. 3C) are anhedral, fine to medium grained (< 1 -5 mm).Stretching lineations are common, marked by cataclastic (pull-apart) textures.Chromite accumulations in layers or pods are not found.
The meta-mafic unit is represented by epidote amphibolites, metadiorites and metagabbros.The epidote amphibolites form NE-SW centimetric to decametric discontinuous elongated bodies (Fig. 3D), are fine grained and show millimetric irregular and discontinuous banding marked by quartz and plagioclase.The metadiorites and metagabbros are centimetric to metric, found as xenoliths within granitoids (Pinheiro Machado and Três Figueiras; Fig. 3E).
Intense muscovitization and tourmalinization occur, related to the release of hydrothermal fluids by the cooling and emplacement of the Três Figueiras granite (Ramos & Koester 2014).These hydrothermalites (muscovitites and tourmalinites) cross-cut Arroio Grande Ophiolitic Complex mica schists.The muscovitites form metric to decametric veins of monomineralic rock.The tourmalinites are massive or layered and form centimetric to metric lenses and veins, constituted by fine to medium grained tourmaline (schorlite-dravite group; 50 -95%), quartz, muscovite and biotite.

MATERIALS AND METHODS
Seventeen representative and less altered samples were collected in the outcrops marked in Fig. 2. All the meta-ultramafic samples were collected in the southern area of the studied complex, were the best outcrops are found (Fig. 2B -locations 1, 3, 4 and 6).The meta-mafic samples were collected in outcrops located in the southern area (Fig. 2B -sample PU-2B in location 2, and PU-20 in location 5), in xenoliths within Três Figueiras granite (Fig. 2A -sample PF-41D in location 7, sample PF-43A in location 8 and sample PF-74C in location 9), and in the northern area (Fig. 2A -sample PF-47A).At the northeastern limit of the Ayrosa Galvão shear zone (Fig. 2 location 11), two metagabbro samples (PMB-2I and PMB-2F) were collected in xenoliths within Pinheiro Machado Complex granitoids in order to compare with the meta-mafics found further southwest in the main studied area.
Both meta-ultramafic and meta-mafic units were prepared in the Sample Preparation Laboratory at the Geosciences Institute, Rio Grande do Sul Federal University (Brazil).The samples were crushed and pulverized in a hydraulic press and in an agate grinding mill.After the preparation, they were analyzed in Acme Analytical Laboratories Ltd. (Canada) for whole rock geochemical data (packages 4A and 4B -whole rock major, minor and trace elements).The major and trace elements were analyzed by Inductively Coupled Plasma Optical Emission Spectrometry and the rare earth elements (REE), by Inductively Couple Plasma Mass Spectrometry.The loss on ignition (LOI) was determined by heating the powdered samples for 60 minutes at 1,000 º C.

LITHOGEOCHEMISTRY
This section presents the results of the lithogeochemical analyses (Tables 1 and 2), as well as their discussion and interpretation.Major and minor elements are expressed as oxide weight percentages (wt%).The trace elements are expressed as parts per million (ppm) and total iron, as Fe 2 O 3 *.
The meta-ultramafics have SiO 2 contents between 25.51 and 59.68 wt%.The enrichment in silica is directly related to the increase of talc content, reflecting its mineral composition (≈ 63% SiO 2 ).The chlorite-rich samples show low SiO 2 values, reflecting the chemical composition of this mineral (≈ 30% SiO 2 ).The same relationship is seen for Al 2 O 3 , Fe 2 O 3 *, and MnO contents, which also reflect the chlorite chemistry.The MgO concentration does not show great variation (22.93 to 28.98 wt%) and the talc-rich samples show the highest values.The Rb (average 1.8 ppm), Co (average 68.5 ppm), V (average 67 ppm), U and Th (average 0.05 and 0.02 ppm, respectively, not including the chloritites, which have anomalous contents influenced by the presence of zircon), and the elevated Cr and Ni contents (average 1,666 and 1,387 ppm respectively) are consistent with peridotites (upper mantle or crustal cumulates) (Coleman 1977;Wilson 1989) and discard a possible sedimentary origin (e.g.dolomites and dolomitic marls).
In general, the meta-mafics have major element composition consistent with the average oceanic basalt composition of Metcalf and Shervais (2008).Average Cr, Ni, V, Co and Zr contents are consistent with gabbroic and basaltic compositions of Coleman (1977).

Element mobility
Due to mobility during metamorphic, hydrothermal and weathering processes, the relationship of the elements with the LOI (which reflects the volatile content of the samples) was investigated.The meta-ultramafics show high LOI values (5.1 -11.9 wt%), indicating large content of hydrated phases.The meta-mafics have lower LOI values (1.3 -4.3 wt%).
In major and minor elements versus LOI diagrams (Fig. 4), the meta-ultramafics show negative SiO 2 and positive Al 2 O 3 , Fe 2 O 3 * and MnO correlations.It represents chloritization and tremolitization processes, that increase especially water and Al 2 O 3 contents and decrease the silica content of the rocks.Sample PU-49 (talc-serpentine schist) has the higher LOI and CaO contents, possibly indicating the presence of carbonate.The meta-mafics show positive MgO and K 2 O and negative CaO, MnO and TiO 2 correlations with LOI (Fig. 4), possibly indicating the transformation of augite in hornblende, biotite and chlorite, which is consistent with thin section observations.
In trace elements versus LOI diagrams (Fig. 4), the meta-ultramafic samples show a positive V correlation that could represent the formation of magnetite and ilmenite during hydration.The meta-mafics show positive Cr, Ba and Rb correlations with LOI (Fig. 4) and may represent the formation of biotite.
In Figure 5A (REE + Y diagram), the talc-serpentine schists do not show relationship between LOI and the enrichment in these elements.They show an increase in light REE of 2 -30 times chondritic values, and in heavy REE of 1 -6 times (except 1 sample that is depleted in all REE, barring La).On the other hand, the tremolitites and chloritites show a clear relationship between LOI and the enrichment in REE + Y (Fig. 5B).The tremolitites have 6 -11 times chondritic values to light REE and 2 -7 times to heavy REE.The chloritites show higher enrichments: 100 -1,100 times to light REE and 20 -110 to heavy REE.These contents, much higher than the chondrite and also higher than the expected for serpentinites, suggest interaction with external fluid sources and an overlay of REE values caused by metasomatic processes subsequent to the protoliths serpentinization (Paulick et al. 2006).
Although varying in the contents, the REE distribution patterns of all meta-ultramafic samples show similar shapes, indicating a common source.The patterns ranging from slightly concave to flat in the middle and heavy REE are similar to the serpentinites derived from oceanic peridotites and those infiltrated by basaltic melts of Peltonen and Kontinen (2004).The negative Ce and Eu anomalies may indicate hydrothermal interactions, weathering and serpentinization, all of them caused by oceanic waters (D'Orazio et al. 2004;Spandler et al. 2008;De Hoog et al. 2009).
The meta-mafic samples do not show evident relationship between REE and LOI contents (Fig. 5C).The light REE vary from 20 to 100 times chondritic values.The heavy REE show a relatively flat pattern, without great variation (8 to 20 times chondritic values).

Tectonic environments
Chemical modifications caused by serpentinization and subsequent metasomatism, together with the absence of relict igneous minerals or textures, make it difficult to define clearly the protoliths of the meta-ultramafics and their tectonic environment of formation on the basis of the available data.However, some considerations can be made.As already discussed in Lithogeochemistry section, the high Cr and Ni contents of these rocks point to a peridotite protolith (upper mantle or crustal ultramafic cumulate).In the Cr versus Ni diagram (Fig. 5D), the samples plot in the ophiolitic cumulates and associated mantle-derived ultramafic rocks field, except for one talc schist sample, which has lower Ni content.All meta-ultramafic samples (except the chloritites, which have anomalous Zr contents) plot in the ocean floor peridotite field of Fig. 5E.
The less altered and less modified meta-mafic samples provided a large amount of clues about the tectonic environments in which their protoliths may have been formed.The Zr, P 2 O 5 , Nb and Y contents indicate a tholeiitic (oceanic) chemical affinity (Fig. 5F).In Figs.5G and 5H diagrams, the samples plot in the mid-ocean ridge basalts (MORB) fields (except sample PF-47A, which has lower Y content; Fig. 5H).The MORB field of Fig. 5H includes back-arc basin basalts (Pearce et al. 1984).Sample PF-74C plots in the volcanic arc (Fig. 5G) and arc tholeiites (Fig. 5H).
In the MgO/TiO 2 versus Zr diagram (Fig. 5I), samples PF-74C and PU-20 plot in the primitive or evolved island arc gabbro field.All other samples plot in the normal mid-ocean ridge (MOR) (cumulate) gabbro field.
In the Th-Ta-Hf /3 diagram (Fig. 5J), most of the samples plot in the supra-subduction zone (SSZ) (i.e.The region above an active subduction zone like forearcs, back-arcs and volcanic arcs) basalts field, with exception of one epidote amphibolite sample (PU-2B), which plots in the enriched MORB (E-MORB) field (this field also includes back-arc basin basalts with no detectable subduction component; Pearce et al. 1984).According to Hawkins (2003), the adittion of sediments in the magma source, expected in a subduction zone setting, shifts the composition toward the Th apex of the diagram.In the normal MORB (N-MORB) normalized spidergram (Fig. 5K), the zigzag REE patterns are typical of supra-subduction zone, where subducting slab-derived elements (Cs, Rb, Ba, Th, U, K, La, Ce, Pb, Sr) superimpose that of the mantle wedge (Nb, Zr, Sm, Eu, Ti, Dy, Y, Yb, Lu).The sample PU-2B is an exception, showing a pattern similar to the E-MORB.The same occurs with the Pb anomalies: the PU-2B sample shows Pb negative anomaly, typical of MORB, while all the other samples show positive Pb anomalies (and also Ce negative anomalies), which suggests influence of sediments in the magma composition in a subduction zone setting (Hawkesworth et al. 1993;Marini et al. 2005;Godard et al. 2006;Metcalf & Shervais 2008).
The high Ce/Pb (> 10) ratios of all meta-mafics indicate a MORB affinity (Plank 2005).To Pb/Ce ratios, the typical MORB value is ≈ 0.04 and all the samples show this feature, except PF-47A and PMB-02F samples, which have higher values suggesting contamination of the mantle source by crustal material and fluid flux in a subduction zone setting (Porter & White 2003;Rollinson 2007).The La/Nb and Th/La ratios (ranging from MORB to sediment-contaminated basaltic magmas) together with Sr/Nd ratios (indicating enrichment of the magma source caused by oceanic waters and subduction zone-derived fluids) seem to reinforce the above assumption (Thompson et al. 1984;McDonough & McCulloch 1987;Metcalf et al. 2000;Plank 2005;Koglin et al. 2009).
Both MORB and back-arc basin basalts share geochemical features mainly when the spreading center of the basin is distal from the subduction zone (Pearce et al. 1984;Hawkins 2003;Pearce & Stern 2006;Metcalf & Shervais 2008;Dilek & Furnes 2011).The meta-mafic samples concomitantly show a MORB signature and the influence of subduction components, which suggests, in the proposed suprasubduction zone model, that the back-arc basin region is the most suitable tectonic environment of formation of the epidote amphibolite, metadiorite and metagabbro protoliths.According to Hawkins (2003), the vast majority of MORB geochemical signatures found in ophiolites represents oceanic crust formed in a back-arc basin setting.

Metasomatism of former serpentinite bodies
On the basis of the available data, it is not possible to define precisely yet if the metasomatism of former serpentinite bodies, responsible for the formation of talc-, tremolite-and chlorite-rich zones, occurred after their incorporation into the orogenic belt or in the ocean floor.In orogenic belts, metasomatism is common in the contact of ultramafic bodies and the country rocks (usually quartz-feldspathic) in regional metamorphic terrains, a set where silica-rich fluids migrate from the country rocks to the ultramafics, often generating virtually monomineralic metasomatic zones around the peridotitic/ serpentinitic core (Brady 1977;Coleman 1977;Evans 1977;Bucher & Grapes 2011;Bach et al. 2013).In the ocean floor setting metasomatic reaction zones mark contacts between ultramafic rocks and gabbroic bodies.Reactions like talcification (partial or total) of serpentinites may occur by Mg remotion or Si addition by diffusion, with the chemical changes occurring in the ultramafic/gabbro contact, or by infiltration, when the metasomatic fluids are formed by the gabbro/ocean water interaction and posteriorly infiltrate the ultramafic body through faults and fractures (Bach et al. 2013;Klemd 2013).Some factors, like the presence of zircon and ilmenite (1 -2%) in the chloritites and their enrichment in REE, suggest that these rocks represent the contact zone between a former serpentinite body and a granitic intrusion, similar to what can be found, in a continental setting, in the Ronda Peridotites, Betic Cordilleras, Spain (Esteban et al. 2007).

CONCLUSIONS
Based on major, minor and trace composition, a peridotite protolith is suggested for the meta-ultramafic rocks.The serpentinization of these protoliths may have happened in the ocean floor by rock/ocean water interaction.The serpentinites were subsequently metasomatized probably in a continental crust setting, forming zones of talcification, chloritization and tremolitization.
The major, minor and trace elements suggest oceanic gabbroic/basaltic protoliths to the meta-mafic unit rocks.These protoliths may have been formed in a supra-subduction zone setting, with magma source contamination by sediments and subduction zone-derived fluids by the closure of a back-arc basin.
The two studied metagabbro xenoliths within Pinheiro Machado Complex granitoids, collected near the northeastern limit of Ayrosa Galvão Shear Zone, have similar geochemical features to the meta-mafics located further southwest.It confirms the presence of Arroio Grande Ophiolitic Complex rocks further northeast.
The results of this lithogeochemistry study, along with field relationships, rock associations and petrographic evidences, support an oceanic setting to the protoliths of the meta-ultramafic and meta-mafic units.The hypothesis that these rocks represent metamorphosed slices of an ophiolitic complex still seems to be the most reasonable one.

Figure 1 .Figure 2 .
Figure 1.Lithotectonic map of Sul-rio-grandense and Uruguayan shields.(A) Gondwana configuration with main cratonic areas and belts related to Dom Feliciano Belt.(B) Localization of Dom Feliciano Belt and adjacent African belts in the Gondwana configuration.(C) Geological map of Dom Feliciano Belt and Río de La Plata Craton in South Brazil and Uruguay (modified from Lenz et al. 2013).

Figure 4 .
Figure 4. Bivariate diagrams of major, minor and trace elements versus loss on ignition.

Table 1 .
Whole rock composition of representative samples from the meta-ultramafic unit.

Table 2 .
Whole rock composition of representative samples from the meta-mafic unit.