The Jequié Complex Revisited: a U-Pb Geochronological Reappraisal of the Geology and Stratigraphy of the Jequié-Itagi Area (Bahia, Brazil) Geocronológico

New geological and litogeochemical data plus La-ICP-MS U-Pb isotopic study on the Jequié Complex, or Jequié “Block”, a granulite facies terrain of the Northeastern São Francisco Craton, allowed to propose that a new lithodemic unit, the Volta do Rio Plutonic Suite, should be created as a lower rank unit. The Jequié Complex is deined here as an intrusive complex metamorphosed in the granulite facies, containing an assemblage of plutonic calc-alkalic maic to intermediate rocks, fractionated trondhjemites, tonalites and granodiorites, besides normal calc-alkaline tonalites/ granodiorites and rare metasedimentary remnants plus two diferent sets of leucogranites. The Volta do Rio Plutonic Suite is proposed as a lower range lithodemic unit belonging to the Jequié Complex. It is composed of granodiorites and monzogranites with porphyroclastic texture, even--grained granitoids and ine-grained granitoids, besides an association of amphibole-bearing leucogranites and maic to intermediate rocks. The metamorphosed maic, intermediate and felsic rocks of the Jequié Complex compose a Cordilleran-type magnesian calc-alkalic association, which age is 2.7 Ga. In contrast, all the metagranitoids of the Volta do Rio Plutonic Suite show a distinctive ferroan (“A-type”) geochemical signature and the maic and intermediate rocks associated to metaleucogranites show alkaline characteristics and host locally high-grade REE mineralizations contained in chevkinite group minerals. In the Volta do Rio Plutonic Suite, the porphyroclastic granites were dated at 2.6 Ga.The provisional age of alkaline maic rocks with magmatic-hydrothermal REE mineralizations and of possibly coeval leucogranites is 2.5 Ga. These mineralizations are inedit in the world and the obtained time frame indicates the need to re-evaluate the geology and metallogenic potential of the Jequié Complex by considering its primordial igneous nature and by screening out its “granulitic” or “charnockitic” nature. presents data from recent mapping in the 1:100,000 scale besides new geochemical and geochronological (U-Pb, LA-ICP-MS) It to review the potential for REE


Introduction
The Jequié -Itagi area is located in the Jequié Complex, which is part of the São Francisco Craton (Almeida, 1977; Figure 1), the most extensive Archean -Paleoproterozoic crustal segment in South America. The Jequié Complex was described by Cordani (1973) as an association of granulite facies rocks. It comprised an area of more than 100,000 km2 and was considered one of the largest areas of granulite facies rocks in the world (Barbosa, 1990). The complex has been later splitted by Barbosa & Sabaté (2002) in two diferent geotectonic entities: the so-called "Jequié Block" and the "Itabuna -Salvador -Curaçá Block" (Figure 1). The designation "Jequié Block" comes after the original proposal of Loureiro (1986) who studied part of the Jequié Complex and recognized a Jequié Nucleus, which was delimited based on gravimetric data and on the pre-dominance of Archean ages as compared to mostly Paleoproterozoic adjoining areas eastwards.
The original designation of Cordani (1973) has been left out in papers from 1986 to the present (e.g. Barbosa, 1986;, except for the Brazilian Geological Survey (CPRM) that still uses the "Complex" category to refer to the Jequié rocks (CPRM, 2009), following the Brazilian and international norms for the naming of lithodemic units (Petri et al., 1986;NACSN, 1983). Nowadays, the term "Jequié Complex" stands for the"Jequié Block" (Barbosa & Sabaté, 2002) which is the same as the former Archean Jequié Nucleus of Loureiro (1986). Thus, the Jequié Complex is bounded to the East by the "Itabuna-Salvador-Curaçá Block", which has been considered as a Paleoproterozoic continental margin represented by ~2.1 Ga calc-alkaline igneous rocks Peucat et al., 2011). This paper presents data from recent geological mapping in the 1:100,000 scale besides new geochemical and geochronological (U-Pb, LA-ICP-MS) data. It is intended to review the geology of the Jequié Complex in the Jequié-Itagi region and to propose a new lower range stratigraphic unit, the Volta do Rio Plutonic Suite, which is composed of high-K ferroan ("A-type") granitoids and subordinate maic to intermediate rocks. Although the Volta do Rio Suite was deined in the Jequié-Itagi region, there is evidence that it extends itself for more than 200 km northeastwards and to the South. It hosts high-grade REE mineralizations in maic to intermediate rocks and magmatic-hydrothermal segregations and veins associated to leucogranites which are inedit in the world. The recognition of such a sequence of geological processes in the Jequié Complex and their dating may contribute to change the geological approaches to study the Jequié Complex and incentivate the exploration of its metallogenic potential for REE deposits.

Methodological Procedures
Geological mapping in an area of approximately 900 km2 ( Figure 2) was performed in the 1:100,000 scale in parts of the SUDENE 1:100,000 quadrangles of Jequié (SD-24-V-D-IV), Jaguaquara (SD-24-V-D-V), Manoel Vitorino (SD-24-Y-B-I) and Ipiaú (SD-24-Y-B-II). Around 200 samples have been studied in thin section. Sixty-eight samples of granitoids, maic rocks and a few enclaves of the undivided Jequié Complex and of the Volta do Rio Plutonic Suite were destined to chemical analysis. These were crushed into small fragments (2-3 cm) in the ield and examined to avoid any contamination from veinlets of leucogranites.
After crushing to ~ 4 mesh, samples were reexamined to sort out possible granitic veinlets and then pulverized to -200 mesh in a tungsten carbide ring mill. Pulps have been sent to ACTLABS (Vancouver, Canada) where they were fused in a Lithium Tetraborate/ Metaborate lux and digested with nitric acid for major element and Sc, Be, V, Sr, Y, Zr analysis by ICP-OES. Detailed descriptions of analytical procedures for major and trace elements may be found at ACTLABS (2012).
Heavy minerals have been concentrated in a shaking table and thirty zircon grains were picked from each sample under a stereomicroscope after preconcentration with bromoform. Grains were examined by BSE imaging and cathodoluminescence. Isotopes were measured on individual zircon crystals by NewWave 213 nm laser workstations (New Wave UP213) inductively coupled to plasma mass spectrometry (LA-ICPMS). Geochronological analyses were performed at the Jack Satterly Geochronology Laboratory, University of Toronto, Canada (VG PQ Excell ICP-MS; samples 369B and 47C), and at LGI -Laboratório de Geologia Isotópica, Universidade Federal do Rio Grande do Sul, Brazil (MC-ICP-MS Neptune; samples 94 and 210-04).
Zircon crystals from poriroclastic granitoids samples 94 and 210-04 have been analysed using static mode with spot sizes of 15 and 25 µm. Laser-induced elemental fractional and instrumental mass discrimination were corrected by the analyses of the reference zircon GJ-1 (Jackson et al., 2004) after every ten zircon spots. The external error was calculated after propagation error of the GJ-1 mean and the individual zircon (or spot).
At irst, the more clear and transparent crystals found on samples 369B (leucodiorite) and 47C (gabbroic enclave) were mounted onto double sided tape and targeted on natural surfaces. In a second round of analyses pre-analyzed grains were picked and mounted in resin, have their surfaces polished, imaged by cathodoluminescence and then re-analysed. Selected sample spots were ablated at 5 Hz and about 5 J/cm2 with beam diameter of 40 microns. Mass spectrometry was carried out on a Plasmaquad quadrupole ICP-MS equipped an S-option 75 l/sec rotary pump to increase sensitivity. Data were collected on 88Sr (10 ms), 206Pb (30 ms), 207Pb (70 ms), 232Th (10 ms) and 238U (20 ms). Prior to analyses spots were pre-ablated using a raster pattern to clean the surface. Data were edited and reduced using custom VBA software written by the author (D.W. Davis). 88 Sr was monitored from zircon in order to detect intersection of the beam with zones of alteration or inclusions and data showing high Sr or irregular time resolved proiles were either averaged over restricted time windows or rejected. Laser-The Jequié Complex Revisited: a U-Pb Geochronological Reappraisal of the Geology and Stratigraphy of the Jequié-Itagi Area (Bahia, Brazil) Paulo César Dávila Fernandes; José Carlos Frantz; Débora Correia Rios; Donald Wayne Davis; Carla Cristine Porcher; Rommulo Vieira Conceição & Rodrigo Estevam Coelho -induced elemental fractional and instrumental mass discrimination were corrected by the analyses of the reference zircons DD-91-1, from Lac Fourniere Batholith (Jackson et al., 2004) and DD85-17, from the Marmion batholith (Tomlinson et al., 2003), both of Precambrian ages. Sets of 4 sample measurements are bracketed by measurements on standards. Differences between standards are time interpolated to correct sample measurements. A "standard-sample--standard" method was used to correct instrumental drift during a single laser-ablation session and common Pb correction was applied using an initial Pb composition taken from Stacey& Kramers (1975).
The Jequié Complex and nearby areas have been afected by Paleoproterozoic granulite facies metamorphism . However, in this text rock types have been also refered to by their igneous classiication, it being implicit that all of them have been metamorphosed in the granulite facies. The nomenclature of Streckeisen (1974) will be avoided since "charnockite" and related terms should be used for igneous rocks (Robertson, 1999) and im most of these rocks there is no indication that these rocks have igneous orthopyroxene.
Besides these rocks, the Jequié Complex contains a great diversity of younger felsic metaplutonic rocks, which constitute the main aim of this paper: (i) the Volta do Rio Plutonic Suite, (ii) the Aiquara Massif, (iii) orthopyroxene-metaleucogranites and garnet-metaleucogranites ( Figure 2; Figures 3C, 3D).
The high density of veinlets, veins, sheets and dykes of these orthopyroxene-and garnet-leucogra-nites cutting the metaigneous rocks (tonalites-trondhjemites-granodiorites and maic -intermediate rocks, Figure 3B, 3C) are probably the reason why part of the Jequié Complex was originally classiied as "migmatites" (Barbosa, 1986). However, the clear intrusive character of the granitoids suggests that they correspond to a deformed and recrystallized intrusive complex (Sawyer, 2008), that we propose here.
These high K ferroan calk-alkalic to alkali--calcic felsics and subordinate maic to intermediate rocks constitute a large NNE-SSW elongated batholith which outcrops East of Jequié City (Figure 2). Besides, there are a few small enclaves of paragranofels (Robertson, 1999). Most of the high-M granitoids of VRPS are even-grained, but a facies with mesoperthite porphyroclasts ( Figure 5A) is present. This shows coarse-grained mesoperthite, ortho-and clinopyroxene and amphibole (± biotite) grains in a granoblastic-polygonal matrix composed of the same minerals. The ine-grained rocks with an orthopyroxene-bearing polygonal granoblastic texture seem to result from granulite facies comminution and recrystallization of the coarser-grained rocks.  (Figure 4) due to high alkali contents in mesoperthite and biotites. The granodiorites and monzogranites difer from the leucogranites in having high M and Q numbers numbers (Q ≈ 24 and M ≈ 12) (Streckeisen, 1976) and low silica contents (less than ~75% SiO 2 , Figure 4). With no exception, all the granitoids of the VRPS difer from the other granitoid rocks of the Jequié Complex in being ferroan ("A-type") ( Figure 6). Trace elements also conirm that all of the granitoid rocks of the VRPS are "A-type" (Fernandes et al., 2017).

Some maic to intermediate rocks and cumu-
lates of the VRPS contain Chevkinite Group Minerals (CGM) and some individual samples with high contents of REE reach ore grade. Moreover, some quartz segregations linked to pegmatites and with CGM also reach ore grade in individual samples (Fernandes et al., 2018).

Rocks of the Jequié Complex
Zircon grains from a calc-alkaline gabbronorite (a hypersthene diorite), sample 369B, located at the Cidade Nova Quarry, outskirts of Jequié ( Figure  2), were dated by U-Pb-Th geochronology. These rocks are the basement for the VRPS. Sample 369B is associated to Mg-rich maic rocks and to tonalites/ trondhjemites and was intruded by hypersthene-bearing leucogranitoid veins and sheets. At 369B most zircon crystals are white to brown coloured, subhedral, prismatic and few of them display overgrowths. Some grains had fractures and/or are twinned.
Only three out of twelve grains directly mounted onto double-sided tape were targeted on natural surfaces. The results range from 2721±6 Ma (Zr1 core) to 2718±7Ma (Zr2 core). The most concordant result was yielded by Zr1 border, resulting in 2493±16Ma. Zr3 presented a low Th/U ratio (0.042) associated to a Neoproterozoic age (951±14Ma) and was interpreted as a metamorphic grain. As all of them presented a high level of discordance (Table  1) it was decided not to continue analyses on natural surfaces, transfering all grains to a resin polished mount in order to expose their interior The less damaged grains provide a provisional four-point Pb-loss line with an age of 2681±48 Ma (MSWD = 6.3) (Figure 7, Table 1), been interpreted as the minimum crystallization age for the maic to intermediate rocks of Jequié Complex.

The Ages of the Volta do Rio Plutonic Suite
U-Pb dating has been performed in zircon grains from two samples of porphyroclastic grani-  Figure 4. Samples from  were added for comparison: grey squares = "CH2 High-Ti charnockites"; grey triangles= "CH1 Low-Ti Charnockites" In addition to these U-Pb ages in granitoids a sample (47C) of a hypersthene monzodiorite, which shows evidence of being coeval with the leucogranites, has been dated.

The Granodiorites and Monzogranites
Samples 210-04 (a granodiorite) and 94 (a monzogranite) are porphyroclastic granitoids with amphibole and orthopyroxene and common accessory minerals as zircon, apatite and opaque minerals. Most of the coarser assemblage, including hypersthene, has been recrystallized in the granulite facies as a polygonal ine-grained matrix, which has a granulite facies paragenesis containing hypersthene+clinopyroxene + amphibole. They show a ferroan ("A-type") high-K chemical signature ( Figure 6).
In sample 94, collected in a dimension stone quarry, North from the road Jequié -Jitaúna, a crystallization age of 2.6 Ga can be constrained by analytical spots in zircon cores, which have been grouped in a 12-point Concordia of 2621±16 Ma, with MSWD of concordance of 0%, and probability of concordance = 0.995 (Table 1, Figures 8A to 8C).
Sample 210-04 was collected on a unpaved road, which links the town of Itagi to the village of Oriente Novo, in a small quarry where boulders with diameter up to 3 metres had been exploited for street paving. The analysis of the cores of igneous zircon grains (Table 1, Figures 8D, 8E) have deined a concordia with eleven points at 2576±30 Ma and MSWD = 3.1.

Magmatism and REE Mineralization
Sample 47C is a coarse-grained gabbronorite (which is a monzodiorite in the TAS chemical classiication, Figure 5) of the maic-ultramaic intermediate group of rocks of VRPS. It is composed of biotite, amphibole, chevkinite group minerals, orthoclase mesoperthite plagioclase, zircon and opaque minerals. It was collected on a depression in a hilly area from metre-sized boulders.

The Jequié Complex Revisited: a U-Pb Geochronological Reappraisal of the Geology and Stratigraphy of the Jequié-Itagi Area (Bahia, Brazil)
Paulo César Dávila Fernandes; José Carlos Frantz; Débora Correia Rios; Donald Wayne Davis; Carla Cristine Porcher; Rommulo Vieira Conceição & Rodrigo Estevam Coelho Zircon crystals from sample 47C are euhedral, prismatic, dark brown in color, many with a transparent preserved core and a white cloudy border, also showing metamict, translucid and cloudy areas, and some with overgrowths, inclusions and zonation. Sample 47C seems to have a second population of colorless to pale brown, long prisms zircon crystals.
Eight zircon grains were dated by LA-ICP--MS. Selected crystals are euhedral, with a brown dusty core indicating severe radioactive damage and colourless transparent overgrowth rims. The results are not quite satisfactory in view of severe radioactive damage but yielded a provisional age of 2498 ± 78 Ma (Table 1, Figure 8E).

There is ield evidence suggesting coeval emplacement of both maic and intermediate rocks and
their cumulates and the amphibole-bearing leucogranites (Fernandes et al., 2017) and therefore the 2.5 Ga age may be considered as an approximation to the age of leucogranitic magmatism of the Volta do Rio Suite. Besides radiation damage, the high MSWD may be a consequence of granulite facies metamorphism and Pb loss in the Paleoproterozoic, around 2.05 Ga . The dated sample is REE-rich, so 2.5 Ga is also a provisional minnimum age also for REE mineralization in maic magmas and cumulates (Fernandes et al., 2018). U-Pb dating performed in zircon grains from the porphyroclastic granitoids belonging to the population of "granodiorites and monzogranites" of the VRPS allowed the identiication of a Neoarchean granitogenesis. Besides, it was possible to conim previous data on the age of Paleoproterozoic granulite facies metamorphism. In addition to these ages in granitoids, the hypersthene monzodiorite shows evidence of being coeval with the leucogranites and resulted in a younger crystallization age of ~2.5 Ga.

The age of the Paleoproterozoic Granulite Facies Metamorphism
At the granodiorites and monzogranites of the VRPS all the selected zircon grains show clear colourless rims (Figure 8). These areas have been dated in sample 94 where an age of 2050 ± 9.2 Ma (MSWD of concordance = 0.57; probability of concordance = 0.46) was obtained. In sample 210-04 the zircon grain rims deined an age of 2013 ± 47 Ma (MSWD = 4.6; n= 6, Table 1). Zircon grains of sample TR47B also record this Paleoproterozoic metamorphism on their borders as an ill-deined Pb--loss curve at 1915 ± 93 Ga. Silva et al. (2002) dated granulite facies metamorphism in the Jequié Block at 2.05 Ga in a nearby region, and as such we consider that the most acceptable age for granulite facies metamorphism is the age obtained in sample 210-04 ( Figure 8E). A 2.4 Ga age in a zircon grain from sample 94 seems to be an artefact, resulting from dating a spot that represents the edge between an Archean core and a Paleoproterozoic rim ( Figure 8B).

Discussion
In the following paragraphs, we will discuss separately genetic aspects of the Jequié Complex and the Volta do Rio Plutonic Suite.
The maic rocks of the Jequié Complex may be described as three diferent groups: (i) "normal" calc-alkaline maic rocks; (ii) high-Mg cumulates and (iii) Fe-rich maic rocks. The maic to intermediate rocks plus tonalites, trondhjemites and granodiorites compose a calcic to calc-alkalic magnesian association in the sense of Frost et al. (2001, Figure  6). Major element data and trace element signatures of the calc-alkaline rocks are similar to those of contaminated continental arc calc-alkali plutonic and volcanic rocks with equivalent silica contents as the ones studied by Franchini et al. (2003); Hervé et al. (2007) in Andean batholiths.
Magnesian calcic to calc-alkalic associations with compositional ranges equivalent to the maic rocks plus tonalites, trondhjemites and granodiorites as those of the Jequié Complex (Figures 4, 6) are found in the Cordilleran batholiths of North America and in the Andes, (Frost et al., 2001). Most rocks of these associations are melts of orthometamorphic rocks and they hold a position immediately next to the oceanwards portion of the continental batholiths (Frost et al., 2001). However, this resemblance to Cordilleran magmatic rocks does not necessarily imply that subducction processes generated the Jequié Cordilleran assemblage in the Archean, since according to Bedard (2018) and Van Kranendonk (2011), subduction processes would not have been operative before 2.5 Ga.
Although most of the Jequié Complex maic to intermediate rocks show geochemical signatures traditionally attributed to arc processes (e.g. Ta--Nb-Ti negative anomalies) in Archean rocks, these features that can be mimicked by crustal contamination (Pearce, 2008;Condie, 2015). Bedard (2018) advocates that Archean calc-alkaline analogues of Cordilleran-type magmatism do not necessarily imply subduction processes and that Archaean basalts were derived from melting in overturn upwelling zones of fertile mantle. He also defends the notion that modern-style subduction only started operating at 2.5 Ga.
The hypersthene-bearing leucogranites, which intrude the Cordilleran assemblage of maic to intermediate rocks and tonalites, trondhjemites and granodiorites of the Jequié Complex are ferroan and potassic, whilst the garnet-bearing leucogranites are magnesian (Figure 4). Similar garnet-bearing leucogranites were dated at 2.05 Ga in the Jequié Complex (Barbosa et al., 2004). They are certainly partial melts from Al-rich metasediments, as demonstrated by their metric enclaves of peraluminous gneisses with biotite + garnet + cordierite + sillimanite. The 2.05 Ga age links them to a continental collision between the Jequié "Block" and the Itabuna-Salvador--Curaçá "blocks" (Figure 1) which was proposed by Silva et al. (2002) and Peucat et al. (2011). On the other hand, the hypersthene-bearing ferroan leucogranites intruding the Jequié Complex are metaluminous to slightly peraluminous and their age and sources are unknown.
In contrast to the magnesian Cordilleran rocks of the Jequié Complex, all the Volta do Rio granitoids are high-K calc-alkaline and show ferroan (A-type) characteristics ( Figure 6). This type of magmatism is considered to result from melting in an oxygen-poor environment, that is, in a source region with low fO 2 , in a number of speciic geotectonic settings and timings in the the Wilson Cycle (Frost et al., 2001).

Final Remarks
In summary, geochronological and geological data presented in this paper allowed recognizing: (i) A 2.7 Ga association of Cordilleran-type igneous rocks which is similar to all "low--Ti charnockites" Macedo, 2004) described elsewhere in the Jequié Complex.
The use of terms like "Cordilleran" or "post--orogenic" however does not imply that these rocks have been generated in a continental margin, since the idea of Archean plate ubduction is controversial (Bedard, 2018;Hahn et al., 2017;Barros et al., 2009) and more evidence to advocate a Neoarchean Jequié continental margin, such as eclogites, blueschists and ophiolites would be necessary. However, even if this evidence were present in Archean rocks, probably the pervasive high-strain and high-T granulite facies recrystallization at 2.05 Ga would have erased it.
Whatever the tectonic setting of the Jequié Complex was in the Neoarchean, these new geological and geochronological data show a sucession of geological events and constrain the REE mineralization to a speciic geological association -that is, the maic to intermediate rocks and ferroan ("A-type") amphibole-bearing leucogranites of the Volta do Rio Plutonic Suite -and propose a time reference (~2.5 Ga) for REE mineralization which however must be more precisely deined. The recognition of this sequence of geological events indicates an urge to investigate the Jequié rocks considering their primordial igneous pre-granulite facies metamorphism.

Acknowledgments
Thanks to Companhia de Pesquisa de Recursos Minerais -CPRM for support during ieldwork and thin section preparation, for which Pedro Cleones was responsible, and to Fundação de Amparo à Pesquisa do Estado da Bahia (FAPESB) for inancing part of the chemical analysis. Thanks also to Dr Ivana de Araujo Pinho for igure 1. This study was inanced in part by the CNPq -Conselho Nacional de Pesquisas -during the doctorate degree of PCDF at Universidade Federal do Rio Grande do Sul. DCR thanks CNPq for her research grant (process 307554/2015-5).