The 1590-1520 Ma Cachoeirinha magmatic arc and its tectonic implications for the Mesoproterozoic SW Amazonian craton crustal evolution

Isotopic and chemical data of rocks from the Cachoeirinha suite provide new insights on the Proterozoic evolution of the Rio Negro/Juruena Province in SW Amazonian craton. Six U-Pb and Sm-Nd analyses in granitoid rocks of the Cachoeirinha suite yielded ages of 1587-1522 Ma and TDM model ages of 1.88-1.75 Ga (EpsilonNd values of –0.8 to +1.0). In addition, three post-tectonic plutonic rocks yielded U-Pb ages from 1485-1389 Ma (TDM of 1.77-1.74 Ga and EpsilonNd values from –1.3 to +1.7). Variations in major and trace elements of the Cachoeirinha suite rocks indicate fractional crystallization process and magmatic arc geologic setting. These results suggest the following interpretations: (1) The interval of 1590-1520 Ma represents an important magmatic activity in SW Amazonian craton. (2) TDM and arc-related chemical affinity support the hypothesis that the rocks are genetically associated with an east-dipping subduction zone under the older (1.79-1.74 Ga) continental margin. (3) The 1590-1520 Ma age of intrusive rocks adjacent to an older crust represents similar geological framework along the southern margin of Baltica, corroborating the hypothesis of tectonic relationship at that time.


INTRODUCTION
The Amazonian craton represents a key region for reconstruction of Meso-and Paleoproterozoic continents. To understand its geological framework and crustal evolution is, therefore, essential for such re-Alternatively , Sadowsky and Bettencourt (1996) suggested that Amazonia was already joined to Laurentia-Baltica at 1.6 Ga. However, Amazonia became an isolated continental mass during the Mesoproterozoic rifting (probably correlated to the break-up of the Columbia supercontinent, according to Rogers (1996). This extensional event is probably coeval with the Pinwarian orogeny (1.51-1.45 Ga), reported by Gower and Tucker (1994) in the Grenville Province, followed by a complete Wilson cycle which led to further agglutination to Laurentia-Baltica and formation of Rodinia. There is a lack of paleomagnetic information in Amazonia between 1.6 Ga and 1.4 Ga. However, recent paleomagnetic data (Tohver et al. 2002) support juxtaposition of SW Amazonia with Laurentia-Baltica at ca. 1200 Ma and D´Agrella et al. (2000) reported paleomagnetic data indicating another collision at 1000 Ma.
The ages, structures, and compositions of the rock units and orogenic events within Amazonia are not well known, in spite of significant recent studies (Bettencourt et al. 1999, Tassinari et al. 2000, Geraldes et al. 2001. In order to improve the knowledge of the SW portion of the Amazonian craton, this paper presents new U-Pb ages, Sm-Nd isotopic results and chemical data on granitoids constraining the nature of the Mesoproterozoic plutonic rocks that crop out in SW Mato Grosso State (Brazil). Furthermore, the implications for the Mesoproterozoic crustal formation in SW Amazonian craton are discussed providing potential tectonic correlation between Amazonia and Laurentia-Baltica at 1.59-1.52 Ga.

THE AMAZONIAN CRATON
The Amazonian craton consists of five NW-SE trending Proterozoic provinces that developed adjacent to an Archean core (Teixeira et al. 1989, Tassinari and Macambira 1999, Santos et al. 2000. Recent U-Pb geochronology supported by Nd isotopic mapping defined the 2.25-2.00 Ga Maroni-Itacaiúnas Province (MIP) and the 2.00-1.80 Ga Ventuari-Tapajós Province (VTP) , Tassinari and Macambira 1999. The slightly younger Rio Negro-Juruena Province (RNJP) is dominated by granitic, granodioritic, tonalitic gneisses and migmatites with crystallization ages varying from 1.80 Ga to 1.63 Ga and Pb, Nd and Sr isotopic data also indicate that it is a juvenile accretionary province , Tassinari and Macambira 1999, Geraldes et al. 2001. VTP rocks are locally overlain by ca. 1.8-1.7 Ga felsic to intermediate volcanic rocks (Neder et al. 2002).
Alternatively, Santos et al. (2000) suggested a new configuration for the geochronological provinces of SWAmazonian craton. In this proposition, Santos et al. (2000) renamed the Rio Negro-Juruena, suggesting the name Rio Negro for the rocks above the Amazonas river, and Juruena-Rondonian for the Rio Negro-Juruena sensu Tassinari and Macambira (1999). In this paper we use the SW Amazonian craton division according to Tassinari and Macambira (1999).
The ca. 1.0 Ga Sunsás Province occurs west of the Rondonian/San Ignacio Province in Bolivia (Litherland et al. 1989) and includes the Sunsás and Aguapeí groups, comprised of low-grade to undeformed supracrustal sequences, and the high grade metamorphic rocks of the Nova Brasilândia Group (Rizzotto, personal communication (Geraldes et al. 1997) is apparently coeval with that identified in the Sunsás Province, and some authors (Tassinari and Macambira 1999) have interpreted the Aguapeí thrust belt as a branch of the Sunsás deformation front reported in Bolivia (Litherland et al. 1986).

GEOLOGY OF SW MATO GROSSO STATE
Three major geochronological and tectonic provinces (Teixeira et. al. 1989, Tassinari andMacambira 1999) are traditionally considered and represented in the western part of the Amazonian craton in Mato Grosso state: Rio Negro-Juruena Province, Rondonian-San Ignacio Province, and Sunsás Province.
Recent advances in understanding the evolution of these provinces, based on newly achieved geochronological and geological data , Sato and Tassinari 1997, Pinho et al. 1997, Bettencourt et al. 1999) among others, provide the basis to the better understanding of these provinces. In this way the geochronological provinces can be subdivided in various orogenies and terranes, which have evolved within the timeperiod established for each province.
The Proterozoic basement in SW Mato Grosso ( Figure 1) consists of igneous and metamorphic rocks interpreted as belonging to the RNJP. This province includes distinct rock associations, such as the oldest Alto Jauru orogenic rocks (Pinho et al. 1997) encompassing acid metavolcanics associated to BIF's and metasedimentary rocks and tonalitic to granitic gneisses. U-Pb ages (volcanic and plutonic rocks) vary from 1790 to 1740 Ma (ε Nd from +2.6 to +2.2 and T DM from 2.00 to 1.80 Ga) interpreted as originated in a volcanic arc due to their juvenile isotopic signatures and chemical characteristics (Pinho et al. 1997, Geraldes et al. 2001. The geological map presented in Figure 1 is compiled from (unpublished data) and Carneiro et al. (1992) and shows intrusive rocks in the Alto Jauru greenstone belt rocks. These intrusive rocks are here named Cachoeirinha suite and petrologic, field evidences, U-Pb and Sm-Nd geochronology of these rocks are the objectives of this work.
In the eastern sector of the studied area occurs the Rio Branco suite, composed of bimodal intrusive rocks (rapakivi granites and alkaline basalts) dated about 1.47-1.42 Ga (unpublished data). The youngest unit in the studied area ( Figure 1) is represented by the Aguapeí Group, which is composed by slightly metamorphosed claystones, sandstones and conglomerates. These sedimentary rocks were probably deposited between 1.3 Ga to 1.0 Ga, and cover all the units previously described. In addition, the rocks in the western boundary of the RNJP are ascribed to the Rondonian-San Ignácio Province, where three orogenic belts have been described. These are the Santa Helena, Rio Alegre (in Brazil) and San Ignácio (in Bolivia). Several granites occur in the west of the studied area such as the large batholith defined as 1.45-1.42 Ga Santa Helena orogen (Geraldes et al. 2001). The 1.42-1.36 Ga San Ignacio orogen (Geraldes et al. 2002) and the 1.50-1.48 Ga Rio Alegre orogen (Matos et al. 2004) may be interpreted as accretionary arcs related of the SW Amazonian craton crust formation during the Mesoproterozoic.

ANALYTICAL PROCEDURES
Major element analyses were carried out at the Geochemistry Laboratory, Department of Mineralogy and Geotectonics of the University of São Paulo, using an ICO-ES according to the procedures described in Janasi et al. (1996). Trace element including REE were analyzed at the ACTLAB (Toronto, Canada) using ICP-MS routine.
For the U-Pb analyses 20 to 30 kg of sample were crushed, milled and heavy minerals were concentrated in wiffley table at University of São Paulo (Brazil). Heavy liquids were used for separation of zircon. U-Pb zircon analyses were carried out in the Isotope Geochemistry Laboratory (IGL), Department of Geology, University of Kansas (USA). The less magnetic fraction was abraded and handpicked single grains were spiked with 205 Pb-235 U mixed tracer. Zircons were dissolved and Pb and U were separated using procedures modified after Krogh (1973Krogh ( , 1982 and Parrish (1987). Zircon weight varied from 0.001 to 0.005 mg. Isotopic ratios were measured using a VG Sector multicollector mass spectrometer in a single collector mode using Daly detector.
Pb isotope compositions were analyzed on single Re filaments using silica gel and phosphoric acid. Uranium was loaded with Pb on the same filament and analyzed as UO + 2 . Radiogenic 208 Pb, 207 Pb, and 206 Pb were calculated by correcting for laboratory Pb blank (from 55 to 17 pg of total Pb during the analyses) and for nonradiogenic common Pb corresponding to Stacey and Kramers (1975)    stants used were 0.155125 × 10 -9 year -1 for 238 U and 0.98485 × 10 -9 year -1 for 235 U (Steiger and Jäger 1977). Zircon data were regressed using the ISO-PLOT program of Ludwig (1998). Uncertainties on concordia intercept ages are given at the 2 sigma (σ ) level.
For the Sm-Nd analyses, rock powders were dissolved in bombs at ca. 180 • C and spiked with 145 Nd and 144 Sm. REE were extracted using the methodology of Patchett and Ruiz (1987). Isotopic compositions were measured in a VG Sector 5collectors mass spectrometer. Sm was loaded with H 3 PO 4 on a single Ta filament and typically analyzed as Sm + in a static multicolector mode. Nd was loaded with phosphoric acid on a single Re filament having a thin layer of AGW-50 resin beads and analyzed as Nd + using static mode. Analyses of BCR-1 during the period when unknown samples were analyzed yielded Nd = 29.44 ± 0.70 ppm, Sm = 6.77 ± 0.21 ppm, 147 Sm/ 144 Nd = 0.13931 ± 0.00071, and 143 Nd/ 144 Nd = 0.512641 ± 0.000007, yielding ε Nd = 0.07 ± 0.12 (all at 1σ ). Sm-Nd model ages (T DM ) were calculated according to DePaolo (1988). During the course of these analyses Nd blanks ranged from 500 to 150 pg, with corresponding Sm blanks of 100 to 50 pg. Correction for blanks was insignificant for Nd isotopic composition and insignificant for Sm-Nd concentrations and ratios. Sm-Nd ratios are corrected to within ±0.5 percent based on analytical uncertainties.

U-Pb AND Sm-Nd RESULTS
Nine rocks of the studied area ( Figure 1) were analyzed for U-Pb and Sm-Nd (summarized in Table II). Complete U-Pb results are presented in Table III and Sm-Nd results in Table IV. Chemical results from 9 samples of studied area are presented in Table V.
The Cachoeirinha suite rocks show compositional variations from tonalites, granodiorites to granites. These rocks are leucocratic, gray to green in color and medium-grained size (0.1 to 1 cm), presenting isotropic, slightly foliated and banded fabrics. The essential mineralogy includes plagioclase, amphibole, biotite, quartz and K-feldspar. Zircon, apatite, allanite and oxides are the accessory minerals.
Several plutonic bodies were individualized from the gneissic basement in the Cachoeirinha region according to the geologic mapping published by Saes et al. (personal communication), (unpublished data) and Carneiro et al. (1992). The authors above cited worked in different locations, what hampered the temporal relationship between the granitoid intrusions observed in the respective studied areas. However, the nomenclature described by these authors was used during the sampling work and include the following granitoids: Quatro Marcos, Cachoeirinha, Santa Cruz, Alvorada and Água Clara. The new units sampled and analyzed in this paper without previous studies were described using new denominations as Santa Fé, São Domingos and Araputanga.

Quatro Marcos Tonalite
Two tonalitic rocks were sampled for U-Pb dating. The first one (sample 97-134) was collected in the southeastern region of the Santa Fé town. In this region Carneiro et al. (1992) identified Paleoproterozoic (1.9-1.7 Ga) Rb-Sr ages on a gray gneiss. Four zircons were analyzed for U-Pb from a gray tonalitic gneiss and yielded an upper intercept age of 1536 ± 11 Ma with T DM = 1.77 and ε Nd = +0.5 ( Figure 2A; Table II).

Chachoeirinha Tonalite
The second tonalitic sample (97-150) was collected 2 km to the west of Cachoeirinha town (Figure 1). Zircons obtained from this rock yielded upper intercept U-Pb age ( Figure 2B; Table II) of 1549 ± 10 Ma and whole rock Sm-Nd analysis yielded T DM = 1.83 and ε Nd = +1.0.

São Domingos Gneiss
This sample (97-149) presents a compositional banding of quartz and feldspar, in felsic layers, and biotite and amphibole-enriched in mafic layers. The São Domingos gneiss yielded an U-Pb zircon

Santa Cruz Gneiss
This granodioritic rock (sample 97-145) is surrounded by the São Domingos gneiss and may comprise lateral variations (unpublished data) of the last unit. The Santa Cruz sample yielded U-Pb zircon age ( Figure 2D; Table II) of 1562 ± 36, and T DM age of 1.79 Ga and ε Nd(1562) = +0.9.
In addition, three post-orogenic plutons of granitic and granodioritic compositions reported in previous studies (Saes, Monteiro and Matos personal communication) were analyzed. The Água Clara is described as a large anorogenic batholith in the Jauru region (600 km 2 ) and comprises mainly granodiorites with subordinate granites. The Alvorada granite comprises several isotropic granitic bodies widespread within the Jauru-Araptanga region. Araputanga granite is a small body intrusive in banded gneisses in Araputanga region.

Araputanga Granite
These granitic bodies are interpreted (Pinho et al. 1997 among others) as the youngest magmatic event in the region. The 97-139 sample yielded U-Pb zircon age ( Figure 3C; Table II)

GEOCHEMISTRY
The major and trace elements analyses (n = 9, Table V) of tonalites, granodiorites and granites of Cachoeirinha suite show variation content in compatible elements related to a calc-alkaline fractional differentiation. The SiO 2 amount ranges from 44% to 55% in the more primitive rocks, 67% to 68% in the intermediate rocks, and reaches 70 to 73% in the more fractionated rocks. CaO ranges from 7% to 8%, 4% to 5%, and 2% to 3%, and MgO ranges from 4.7% to 7.2%, 1% to 1.2%, and 0.3% to 0.8%, respectively in the tonalitic (more primitive), granodioritic (intermediated) and granitic (evolved) rocks. Trace element results plot within the field of volcanic arc granites (VAG), following the Pearce et al. (1984) tectonic discrimination diagram ( Figure  4A) for both orogenic and post-orogenic rocks. In  Steiger and Jäger (1977). the Figure 4B (alumina index; Maniar and Piccoli 1989), the studied samples vary from peraluminous (granitic and granodioritic rocks) to metaluminous (tonalitic rocks). The hypothesis of fractionation is confirmed by Rb/Sr versus Sr diagram ( Figure 5) which analytical data plot indicates an increasing Rb/Sr ratio and decreasing Sr content. The post-tectonic plutonic rocks (U-Pb ages from 1.48 to 1.39 Ga) show SiO 2 variation from 73% to 70%, with low Ca 2 O content (from 3% to 1.37%) and high Na 2 O and K 2 O val-ues (4.3% to 3.7% and 4.6% to 2.6%), indicating that this group of rocks present highly fractionated signature.
The REE patterns indicate a higher fractionation between LREE and HREE for the felsic rocks (97-138, 97-132, 97-129 and 97-139), compared with the intermediate (97-147, 97-145 and 97-136) and primitive rocks (97-134 and 97-150). The REE distribution between the rocks is also characterized by slightly positive Eu anomaly in the most primitive rocks, a slightly negative Eu anomaly in the inter-  mediate ones, and moderate Eu negative anomaly in the felsic rocks ( Figure 6). The post-orogenic granites show two distinctive REE patterns ( Figure 6). The Agua Clara granodiorite (sample 97-136) presents REE distribution similar to the primitive rocks of the Cachoeirinha suite rocks, and the other two samples (97-129 and 97139) show REE patterns similar to the felsic rocks of the Cachoeirinha suite rocks.

DISCUSSION
Geochronological, isotopic and chemical data provide informations about the nature and tectonic set- Chondrite normalized according to Taylor and McLennan (1985).
ting of the Cachoeirinha suite, which have important bearing for the understanding of the crustal evolution of the SW Amazonian craton at Mesoproterozoic time. U-Pb zircon ages indicate that the Cachoeirinha granitoids comprise a voluminous rock suite formed during a short period of time (1590 Ma to 1520 Ma), implying that a regional tectonic magmatic event took place succeeding the generation of the Alto Jauru volcanic-plutonic accretionary complex (Geraldes et al. 2001). The T DM between 1.88 Ga and 1.75 Ga and the ε Nd(t) values varying from slightly negative (-0.8) to positive (+1.0) for the investigated Cachoeirinha rocks together with their calc-alkaline affinity is consistent with an arc-   Figure 7) suggests crustal contribution in the genesis of the Cachoeirinha suite. This is corroborated by the U-Pb results of sample 97-149, with several inherited zircon populations, probably due to contribution of Alto Jauru crust in the source for this rock. If this interpretation is correct, the sample 97-149 grains of zircon may have retained isotopic signature of ancient rocks, and the magmatic event responsible for the Cachoeirinha suite formation kept part of these zircon within the analyzed sample. The alignment of four U-Pb analytical results of this sample yielded an age of about 1.7 Ga ( Figure 2C), suggesting that the older protholith may be the Alto Jauru country rocks. Post-orogenic magmatism represented by samples 97-129, 97-136 and 97-139 shows crystallization U-Pb zircon ages ( Figures 3A, 3B and 3C) varying from 1485 Ma to 1389 Ma, with correspondent Sm-Nd results (ε Nd(t) values between -1.3 and +1.7) indicating a probable participation of the Alto Jauru rocks in their sources. The Cachoeirinha suite rocks SiO 2 contents range from about 68% to 73%, contains primitive (high-Ca), and highly evolved (high-K, Rb) phases. The post-tectonic rocks (samples 97-136, 97-139 and 97-129) are the highest evolved rocks and they show SiO 2 contents from 70% to 73%. The Cachoeirinha granitoids show I-type affinities for the primitive and intermediate rocks and A-type affinities for the pos-tectonic more evolved rocks ( Figure  4A) and chemical results plot near the peraluminousmetaluminous boundary on the A/CNK versus ZNCY diagram ( Figure 4B). REE data for the Cachoeirinha suite have steep LREE patterns and relatively flat HREE patterns with moderately negative to absent Eu anomalies. Both major and trace elements in the analyzed samples show distributions characteristic of fractional crystallization. As example, the increasing Rb/Sr ratio with decreasing Sr content ( Figure 5) can be explained by removal of plagioclase, K-feldspar, and biotite in the approximate proportions that they occur in the rocks.
The Cachoeirinha suite presents isotopic and chemical signatures that define their juvenile character. Consequently we propose that the rock association here reported was formed in a magmatic arc setting, (e.g., Cachoeirinha magmatic arc) close to an older continental crust (as speculated in Figure 8) during an orogenic event that occurred in the SW (actual) margin of the Amazonian craton which magmatism source had an important contribution of the older crust comprised by the 1790-1740 Ma Alto Jauru volcano-plutonic rocks.
Cachoeirinha rocks may be correlated to rocks formed in coeval magmatic arcs described in Baltica, where continent-continent collision occurred at ca. 1.58 Ga and eastward subduction was renewed (Åhäll and Gower 1997). Evidence is given by calc-alkaline ca. 1.55 Ga mafic-ultramafic tonalitic intrusions (northern Telemark) and ca. 1.53-1.50 Ga rapakivi magmatism in central Sweden. These events are coeval and consistent with the onset of the Cachoeirinha orogen in SW Mato Grosso, and in Finland, by the Aland Riga Group (1.58-1.54 Ga) and ca. 1.56-1.54 Ga Salmi Group rapakivi plutons (Rämö and Haapala 1995). Similarly, the 1.59-1.52 Ga age pattern of the Cachoeirinha suite is partially coeval with the 1.61-1.53 Ga Serra da Providência suite, described as a bimodal (rapakivi granites and gabbros) intrusive suite, according to Bettencourt et al. (1999).