Pb isotope geochemistry and reappraisal of Sr-Nd isotopes of the Cerro Morado basic magmatism ( Ischigualasto-Villa Union Triassic basin , NW Argentina ) : Implications for the mantle sources

Manuscript ID: 20170106. Received in: 09/08/2017. Approved in: 12/10/2017. ABSTRACT: Sr-Nd-Pb isotopic signatures were determined for silica-undersaturated hypabyssal alkaline basic rocks of the Los Baldecitos Formation in the Cerro Morado area, situated in the Ischigualasto — Vale de la Luna Provincial Park (northernmost San Juan Province, Argentina). The basic rocks show slight variations of the Sr-Nd-Pb isotopes and similar behavior of whole-rock geochemistry, which suggest a single source for the Cerro Morado volcanic rocks. The present-day Pb isotopic data show moderately radiogenic Pb compositions (206Pb/204Pb = 18.31 – 18.35), in addition to low values of 87Sr/86Sr initial ratios of 0.70314 – 0.70386 and highly radiogenic Nd initial isotopic compositions (+7.1 < εNd (228 Ma) < +9.3), which point to a PREMA mantle reservoir signature for the alkaline basalts with little evidence of crustal contamination. The involvement of a HIMU component in the source, previously suggested for the Cerro Morado volcanics, should be disregarded in the light of the Pb isotopic signature. The Sr-Nd signature together with Nd-TDM model ages not older than 295 Ma does not support the involvement of an ancient crust.


INTRODUCTION
The Ischigualasto-Villa Union Basin, northwestern Argentina, is part of a system of continental basins generated mainly by extensional tectonics related to the breakup of the Gondwana supercontinent.This basin corresponds to a hemigabren elongated in a NW-SE direction (Milana & Alcober 1994) and is filled by clastic continental sedimentationmainly fluvial-lacustrine -and volcanic and hypabyssal rocks of basic to intermediary composition.This volcanism is traditionally related to structures resulting from reactivation of Paleozoic lineaments during the Neotriassic.One of the main occurrences of silica-undersaturated alkaline basic hypabyssal rocks (Los Baldecitos Formation) is observed as a sill in the Cerro Morado area, overlying a basal unit formed by sedimentary rocks (Mesotriassic Chanãres Formation).Fieldwork, petrography and whole-rock geochemistry show that this magmatism was established in an intraplate environment in which mantle sources were responsible for the emplacement of the sill on the top of the Chañares Formation (Alexandre et al. 2009).
The aim of this paper was to discuss mantle sources of the basic rocks of Cerro Morado and to better understand the role of Triassic magmatism in the Villa Union Basin, based on a reappraisal of previously published geochemical and Sr-Nd isotope data, together with new Pb isotope data on the same rocks.A specific purpose was to test the hypothesis of a previously suggested HIMU mantle component as a source of these basic rocks (Alexandre et al. 2009).

GEOLOGICAL SETTING
The basement of the Ischigualasto-Villa Union Basin is composed of igneous and metamorphic rocks from the Precambrian Igneous Metamorphic Valle Fertil Complex (Bossi 1971) and Eopermian sedimentary rocks from the Patquia Formation of the Paganzo Group.The basin is bounded by Valle Fertil lineament to the west and the Sierra de los Tarjados to the east (Stipanicic & Bonaparte 1979) (Fig. 1).
The basal units of the basin are composed by clastic continental sequences of the Talampaya and Tarjados formations of lower Triassic age.The overlying Chañares Formation is composed of volcanogenic conglomerates with volcanic clasts immersed in a tuffaceous groundmass.These conglomerates, together with sandstones and mudstones, characterize a sequence of rocks related to debris flow (Milana & Alcober 1994).Intrusive in this unit, occur the the hypabissal rocks of Cerro Morado, which correspond to Los Baldecitos Formation.This volcanic sequence includes olivine-bearing diabases in the base, and porphyritic tephrites that predominate at the top (Alexandre et al. 2009).The Los Baldecitos Formation is stratigraphically succeeded by the Los Colorados Formation, which consists of reddish sandstone with tabular geometry and parallel stratification.Channeled conglomeratic bodies are observed in the central portions, often interleaved with Fe oxide-rich thin beds in the upper part of the formation (Kokogian et al. 1987, Lopez Gamundí et al. 1989, Limarino et al. 1990).
The magmatism of the Los Baldecitos Formation have basic to intermediate composition and occurs as lavas, sills, and associated laccolith.These sills and laccoliths are intrusive within Chañares deposits.Ar-Ar dating indicates an age of 228 ± 2 Ma (Chemale Jr. personal communication) and previous whole-rock geochemistry and Nd-Sr isotope led Alexandre et al. (2009) to consider this magmatism as typical intraplate alkaline basalts affected by metassomatism, whose source may be related to a HIMU reservoir.

GEOLOGY AND PETROGRAPHY OF THE CERRO MORADO ROCKS
The basic hypabyssal rocks of Los Baldecitos Formation show tabular subhorizontal geometry, which is conformable with the deposits of the Chañares Formation (Fig. 2A).
The basic rocks occur as a sill nearly 200 m thick and 1,200 m length (N-S), showing a massive aspect in the entire body, with sparse vesicles.Ophitic, subophitic and poikilitic textures are predominant in these intrusive bodies.Additionally, an irregular jointing with a columnar pattern, perpendicular to the walls of the sill, is common.
The basal part of the sill is approximately 30 m thick and consists of a porphyritic olivine-bearing diabase, where plagioclase phenocrysts and subordinate olivine are predominant (Figs.2B and 2C).These phenocrysts are encompassed by a matrix composed of plagioclase, clinopyroxene, opaque minerals and apatite (Fig. 2D), and predominant subophitic, intergranular, ophitic, and branching textures (Fig. 2D).In the intermediate and upper portions of the sill, tephrites predominate with porphyritic and glomeroporphyritic textures and show a diffuse contact with the olivine-bearing diabase.The tephrites exhibit imbrication of plagioclase crystals, suggesting cumulate processes occurring locally (Fig. 2E), as well as trachytic and branching textures.The matrix is finely to medium grained, composed by plagioclase, nepheline, analcime, clinopyroxene, opaque minerals, and apatite (Figs.2F and 2G).In the intermediate portion, subvertical fractures are filled by hydraulic breccias, consisted of moderately altered tephrite fragments surrounded by a matrix of banded chalcedony, carbonate, and bipyramidal quartz.Micro-vesicles are restricted to some samples, generally filled by quartz, chlorite, smectite and carbonate (Fig. 2H).

ANALYTICAL PROCEDURES FOR WHOLE-ROCK GEOCHEMISTRY AND Sm-Nd, Rb-Sr AND Pb-Pb ISOTOPE
The whole-rock chemical analysis were performed at ACME Laboratories Ltd (Canada) in which the 4A+4B package was used to analyze major oxides by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES).Trace elements and rare earth elements (REE) were analyzed by Inductively Coupled Plasma Atomic Mass Spectrometry (ICP-MS).
Sr, Nd, Pb isotope analyses were carried out at the Isotope Geology Laboratory of the Universidade Federal do Rio Grande do Sul with a VG Sector 54 multicollector thermal ionization mass spectrometer.For the sample digestion procedure, rock samples were powered in agate mortar at a grain size lower than 200 mesh, where, ~100 mg of the rock sample were mixed with a 149 Sm/ 150 Nd and 87 Rb/ 84 Sr spike and dissolved with HF-HNO 3 and HCl in 7 mL Teflon vials on a hot plate for seven days.Rb, Sr and REE were purified by using cation exchange chromatography with a DOWEX AG 50X8 resin (200-400 mesh).Nd and Sm were separated from the other REE by using exchange columns with Di-(2-ethylhexyl) orthophosphoric acid (HDEHP) resin (50-100 nm).Pb was purified using anion exchange DOWEX AG-1X8 resin (200-400 mesh).
For the isotopic analyses, Rb, Sr, Sm and Pb were run on Re single filaments and Nd on Ta-Re triple filaments.The Pb concentrates were deposited with silica gel.The Sr and Nd isotopic compositions were normalized to 86 Sr/ 88 Sr = 0.1194 and 146 Nd/ 144 Nd = 0.7219, respectively.Pb was corrected from mass discrimination by using a factor of 0.1% amu -1 based on 38 analyses of NBS-981 reference material.Repeated analyses of NBS 987 and La Jolla reference material furnished 87 Sr/ 86 Sr and 143 Nd/ 144 Nd values of 0.71026 ± 0.000011 (1 σ; n = 100) and 0.511848 ± 0.000021 (1 σ n = 100), respectively.The blanks used for the procedure were lower than 500 pg for Rb and Sm, and than 60 pg, 150 pg and 100 pg for Sr, Nd and Pb, respectively.Typical analytical errors for 87 Rb/ 86 Sr, 147 Sm/ 144 Nd and 208,207,206 Pb/ 204 Pb ratios did not exceed 0.1%.Nd-T DM model ages were calculated according to the Depleted Mantle model of De Paolo (1981).The decay constants were those recommended by Steiger and Jäger (1977) for 87 Rb and by Lugmair and Marti (1978) for 147 Sm.

WHOLE-ROCK GEOCHEMISTRY
The high values of Loss on Ignition (LOI) obtained in the tephrite samples (Tab. 1) due to carbonation of feldspathoids, and high values in the olivine-bearing diabase sample do not allow the classification of the rocks in the Total-Alkali (Na 2 O+K 2 O) versus silica (SiO 2 ) (TAS) diagram, as recommended by International Union of Geological Sciences (IUGS).We used the Nb versus Zr/TiO 2 * 0.0001 diagram (Winchester & Floyd 1977), which establishes the relationship between major and trace elements with low degree of mobility.In this diagram, the Cerro Morado rocks occupy the alkali basalts field (Fig. 3).The behavior of major and trace elements, when correlated to the differentiation index (DI = Ab+Or+Q+Ne, normative -Thornton and Tuttle 1960), suggests a differentiation process by mineral fractionation (Figs. 4 and 5).The samples show inverse correlations of Al 2 O 3 , CaO and Sr and positive correlation of K 2 O with DI, which suggest a major participation of plagioclase toward the olivine-bearing diabase.The dispersion of Fe 2 O 3 (FeO+ Fe 2 O 3 ) and TiO 2 contents may reflect fractionation and accumulation of mafic phases.The MgO, Ni, Cr and Co contents are very low.Ba, Nb, Y and Zr show an incompatible behavior in relation to DI.The Rb contents also display a positive correlation with DI, which is coherent with K 2 O contents (Fig. 5).
When normalized to oceanic island basalts (OIB) (Sun & McDonough 1989), the samples show higher values of Rb, Ba and K in relation to U and Th (Fig. 6A), which suggests some remobilization of low ionic potential elements by aqueous fluids.

Sr-Nd-Pb ISOTOPE GEOCHEMISTRY
The Sr, Nd, and Pb isotope analyses obtained in one sample of olivine-bearing diabase and seven from tephrites are shown in Tab. 2 and Figs. 8, 9, 10 and 11.The initial 87 Sr/ 86 Sr and 143 Nd/ 144 Nd ratios were re-calculated to an age of 228 Ma.As Pb concentrations were not analyzed in the whole-rock geochemistry analyses, the corrections of the initial Pb ratios were not possible, thus using present-day Pb isotope compositions.The Cerro Morado rocks show initial 87 Sr/ 86 Sr ratios ranging from 0.70314 to 0.70386, and ε Nd(228 Ma) values ranging from +7.1 to +9.3 with Nd-T DM ages between 93 and 295 Ma.Present-day Pb isotope compositions range from 18.319 to 18.357 for 206 Pb/ 204 Pb; 15.509 to 15.518 for 207 Pb/ 204 Pb; and 37.834 to 37.880 for 208 Pb/ 204 Pb.
In the 143 Nd/ 144 Nd i versus 87 Sr/ 86 Sr i diagram (Fig. 8), the samples exhibit high 143 Nd/ 144 Nd i and low 87 Sr/ 86 Sr i ratios, within the mantle array, close to the PREMA reservoir.In the Pb isotopic diagrams (Fig. 9), the samples plot below of the Northern Hemisphere Reference Line (NHRL) reference line and lying close to the PREvalent (PREMA) mantle reservoir field, similarly to the Nd-Sr isotopic signature.
In both 206  Nd/ 144 Nd diagrams (Fig. 10), the studied rocks show a similar behavior and plot together overlapping the PREMA mantle reservoir field, besides plotting away from the HIMU field.Triangles: olivine-bearing diabase; circles: tephrites.

DISCUSSION AND CONCLUSIONS
The diabase and tephrite samples of Cerro Morado show slight variations of their Sr-Nd-Pb isotopic composition, although all of the samples occupy the same field in the isotopic diagrams (Figs. 8,9,10 and 11).This suggests a same source for all volcanic rocks.The similar wholerock geochemical behavior of all studied rocks reinforces this assumption.
The Sr-Nd isotopic data previously obtained for the Cerro Morado rocks suggested that their source was related to HIMU type mantle reservoir (Alexandre et al. 2009).However, in this previous study, the mantle reservoir fields were not recalculated to crystallization age, which would shift their positions.Additionally, in the geochemical comparison by Chart 1 from Alexandre et al. (2009), the samples move away from HIMU basalt compositions.The new set of Pb isotopic data obtained in this paper definitively discard the HIMU reservoir as a possible source for the Nb/Y vs Zr/TiO 2 *0.0001 diagram (Winchester & Floyd 1977) where the samples occupy the alkali basalt field.

Figure 4 .
Figure 4. Major and minor element variation diagrams versus differentiation index (DI) for the Cerro Morado rocks (cf.Alexandre et al. 2009).

Figure 9 .
Figure 9. Lead isotope compositions for the studied rocks in comparison to mantle components PREMA, EM-I, EM-II, and DM(Zindler & Hart 1986).NHRL, Northern Hemisphere Reference Line(Hart 1984).The reservoirs fields are present-day Pb ratios.

Figure 10 .
Figure10.87Sr/ 86 Sr i and 143 Nd/ 144 Nd i against present-day Pb isotope compositions for the studied rocks compared to the fields of the mantle components HIMU, PREMA, EM-I, EM-II and DM(Zindler & Hart 1986).

Figure 8 .
Figure 8. 143 Nd/ 144 Nd i versus 87 Sr/ 86 Sr i variation diagram for the studied hypabissais rocks.The mantle components DM, PREMA, HIMU, EM-I, and EM-II are taken from Zindler and Hart (1986).The mantle reservoir fields are those recalculated at 200 Ma by Deckart et al. (2005), close to the approximate crystallization age estimated for the Cerro Morado around 228 Ma.The mantle array position was recalculated at 285 Ma by Vozár et al. (2015), also close to the crystallization age of the Cerro Morado.

Table 1 .
Major and trace element geochemical data of the Cerro Morado rocks.Sr-Nd-Pb isotope geochemistry of the Cerro Morado basic magmatism (NW Argentina): Implications for the mantle sources All Fe expressed as Fe 2 O 3 120 Brazilian Journal of Geology, 48(1): 115-126, March 2018

Table 2 .
Isotopic compositions of Sr, Nd, and Pb of the Cerro Morado rocks in the Ischigualasto-Villa Union Basin.