Aeromagnetometry and aerogammaspectrometry integrated with U-Pb zircon geochronology of northern Bossoroca ophiolite, Brasiliano Orogen

: Age delimitation integrated with aeromagnetometric and aerogammaspectrometric survey advances the understanding of ophiolite evolution in the Brasiliano Orogen. We focused on the Bossoroca ophiolite, because oceanic crustal and mantle rocks contain zircon in metasomatic chloritite. A metadiorite and a metavolcanoclastic rock were also studied to delimit relationship between ophiolite and island-arc infrastructure and superstructure. Zircon crystals were dated by laser ablation inductively coupled plasma emission spectroscopy. Ages of zircon from Campestre metavolcanoclastic rock are 920-840 (peak 842) Ma, Bossoroca chloritite 900-800 (peak 868 Ma) and Capivaras metadiorite 698 Ma. Ages 920-800 Ma correspond to processes in the oceanic crust, whereas 698 Ma was a late magmatic intrusion (Capivaras metadiorite) into the island-arc infrastructure. Aeromagnetometric and aerogammaspectrometric data delimit the occurrence and structure of the ophiolite. These are major multiproxy markers of geotectonic processes early in the Brasiliano


INTRODUCTION
Integrated retrieval of U-Pb ages of z i rco n a n d a e ro m a g n e t o m e t r i c a n d aerogammaspectrometric description of ophiolites establish fundamental parameters in the evolution of oceanic crust and mantle within host infrastructure of island arcs (Blakely 1995, Dickson & Scott 1997. Uncoding the time capsule is commonly made from associated rocks that contain zircon, such as plagiogranite and gabbro (e.g. Samson et al. 2004, Queiroga et al. 2007, Dilek & Thy 2006, Karaoglan et al. 2013. Host granitic rocks are regularly dated with use of zircon. But direct dating of zircon formation and alteration in the oceanic crust and mantle is a large step toward elucidation of processes in the oceanic realm and later accretion to the island arc (e.g. Arena et al. 2016Arena et al. , 2017Arena et al. , 2018. This dating can be done on zircon formed during serpentinization of harzburgite either in the mid-ocean ridge or above subduction zone. Geophysical survey (aeromagnetometry and aerogammaspectrometry) are proxies for description and delimitation of ophiolite structure and localization of subsurface geological targets. Use is made of data density and physical contrast between target and host rocks (Blakely 1995, Dickson & Scott 1997, Rosa & Fuck 2014. Mesozoic ophiolites are abundant in large collisional orogens such as Himalayas-Alps (Liu et al. 2016). Less ubiquitous in the Tonian, e.g. Brasiliano Orogen, ophiolites are nevertheless abundant both in juvenile crust and fold and thrust belts, including the Araguaia, Brasília, Araçuaí belt and Dom Feliciano belts (e.g., Szubert 1980, Hartmann et al. 2019, Strieder & Nilson 1992, Queiroga et al. 2007, Suita et al. 2004, Hodel et al. 2019. Pioneering age delimitation of oceanic alteration processes by Arena et al. (2016Arena et al. ( , 2017Arena et al. ( , 2018 and Hartmann et al. (2019) allows deeper understanding of ophiolites and sutures in the largest orogen of South America.
We selected the Bossoroca ophiolite because of presence of key rock associations and structures, including metasomatic rocks, and because the ophiolite is positioned at the base of the suprastructure and delimited by the infrastructure. Chloritite related to serpentinite, in addition to amphibolite, allows the study of oceanic processes; age of a diorite constrains the formation of the host island arc infrastructure. Oceanic processes occurred in the Tonian (920-800 Ma), ophiolite obducted into a 698 Ma granitic-volcanic island arc. The arcuate shape of the ophiolite seen in aerogeophysical images was caused by obduction onto an island arc.

MATERIALS AND METHODS
W e i n t e g r a t e d f i e l d g e o l o g y w i t h a e r o m a g n e t o m e t r i c a n d aerogammaspectrometric data and zircon U-Pb geocronology. Geological data were obtained over several decades on the shield and on the ophiolite (Jost & Hartmann 1984, Szubert 1980, Laux 2017, Gubert et al. 2016, Hartmann et al. 2019; detailed work led to discovery of metasomatic rocks, including chloritite and tourmalinite. Techniques here reported for the airborne geophysical survey of the shield by the Geological Survey of Brazil (CPRM 2010a), including aerogammaspectrometry and aeromagnetometry, follow Hartmann et al. (2016). Data acquisition was made by LASA PROSPECÇÕES S.A. (CPRM 2010a). The flight was at an elevation of 100 m above the terrain, line spacing at 500 m and control lines spaced 10,000 m oriented NS and EW. The survey covered 159,789.21 km of flights. Border regions of the shield were also covered, including strips of the Paleozoic-Mesozoic Paraná Basin to the north, west and south and the Quaternary coastal plain in the east. A Scintrex CS-2 equipment was used for the acquisition of magnetic data. Two equipment were used in two different airplanes for the acquisition of gammaspectrometric data, the Exploranium GR-820 and the Radiation Solutions Inc./RS500 spectrometers. Radar altimeters King 405 and Collins ALT-50 and barometers Fugro/Enviro were used in different airplanes to obtain the digital terrain model of the shield.
The geophysical magnetic (total magnetic field) and gamma spectrometry (potassium, thorium and uranium channels) data processing was done at LASA PROSPECÇÕES S.A., Rio de Janeiro, involving the application of Oasis Montaj GEOSOFT routines, version 7.1.1. Maps were generated in several scales, and also a data bank. This data bank was deposited at UFRGS by the Geological Survey of Brazil office in Porto Alegre; maps were produced by the authors of the digital elevation model, anomalous magnetic field (AMF), analytical signal, and K%. Selected maps are here presented in two scales and interpreted. This remote sensing of rock types allowed the contouring of the geology and interpretation of structures.
Three selected rock samples (Supplementary Material - Table SI) were studied with optical petrography, whereas backscattered electron images of zircon crystals were done at Universidade Federal do Rio Grande do Sul.
Zircon is present in metasomatic chloritite, also in amphibolite and metadiorite. Zircon was separated mechanically from rock samples at UFRGS by standard crushing and milling followed by heavy liquids. The crystals were mounted in epoxy and polished to half their thickness. Detailed methodology of U-Pb isotopic measurements is in Supplementary Material.

Geological setting
The Bossoroca ophiolite is part of the early, juvenile segment of the Brasiliano Orogen. The São Gabriel terrane is in the western portion of the Sul-Riograndense Shield ( Figure  1), southern Brasiliano Orogen (Hartmann et al. 2000), and is composed of two main rock associations (Table II) We adopt a lithostratigraphic classification equivalent to Kozdrój et al. (2018) in the Arabian Shield, because the overall geological controls are similar and clarify the organization of the terrane. Previous subdivisions of the terrane included several formations and complexes (e.g., Robertson 1966, Ribeiro & Fantinel 1978, Naumann et al. 1984, Koppe et al. 1985, Babinski et al. 1996, Hartmann et al. 1999, 2007, 2011, Garavaglia et al. 2002, Lena et al. 2014, Gubert et al. 2016, Vedana et al. 2018, including metavolcano-sedimentary sequences and juvenile calk-alkaline gneisses (e.g., Saalmann et al. 2005, Hoerlle et al. 2019). The stratigraphic nomenclature here simplified includes the historical names 'Vacacaí, Cambaí' and extracts ophiolites into a new stratigraphic class, thus solving the long-lived, apparently contradictory stratigraphic organization of the terrane. The ophiolites are named for each occurrence; integration can be made with Cerro do Ouro ophiolite, following Goñi (1962). Main rocks in the ophiolites are metaserpentinite, amphibolite, quartzplagioclase granofels, banded iron formation, albitite, and chert. Metasomatic rocks are minor in volume but significant and include rodingite, chloritite and tourmalinite. Grade of metamorphism reached greenschist facies, e.g., Ibaré ophiolite, middle amphibolite facies (Cerro Mantiqueiras ophiolite) and dominantly low-amphibolite facies (Cambaizinho and Bossoroca ophiolites; e.g., Massuda et al. 2020). Contact metamorphism by TTG on the ophiolites is not described, but postectonic granites (e.g., Cerro da Cria, São Sepé Granites) caused intense The 2-km wide Bossoroca ophiolite extends for 20 km NE and dips 60-80⁰ to NNW. The ophiolite was obducted during the Neoproterozoic into the base of the Campestre Formation volcanoclastic rocks which occur to the east. Dominant foliation contains lowamphibolite facies mineral assemblage and corresponds to D2 of Saalmann et al. (2006,2007). The foliation marks the obduction of the ophiolite into an oceanic volcanic-sedimentary arc. Transcurrent faulting marked D3, whereas D4 corresponds to local thrusting. Rio de La Plata Craton rocks occur below the juvenile terrane, as described from isotopic composition of young granites and base metal ore, corresponding to a metacraton in the region ( 2b, 3, 4) and is mapped more accurately with support from aerogeophysical images. Anomalous magnetic field (Figure 3a) shows magnetic dipoles corresponding to contrasting magnetic rock bodies. Analytic signal applied to AMF (Figure 3b) allowed delineation of the anomalies with greater magnetic susceptibility, peaks of the sources centered on the edges of the anomalous body. The ophiolite is mostly intensely magnetic, but non-magnetic portions, e.g. chromite-talc-magnesite granofels, increase the width of the body as seen on the analytical signal image. The signal is strong (0.1-0.2 nT/m) in magmatic Cambaí Complex, weak (0.02-0.04 nT/m) in metasedimentary Campestre Formation (Figure 4). Gamma-ray emission is low over most of the ophiolite (Figure 4), which shows low K concentration (0.1 -0.3%), but K concentration is high in Cambaí Complex and Campestre Formation (0.5-1.0%). Emission rates from eTh and eU follow distribution of K (not shown).

Structure of the ophiolite is arcuate (Figures
The ophiolite has many serpentinite bodies with sizes ranging from 10-1000 m. Intense serpentinization partly obliterates the metamorphic 'jackstraw' texture of olivine + talc observed in the serpentinite bodies. Metasomatic rocks include chromitetalc-magnesite granofels, chloritite and massive tourmalinite, all in direct contact with serpentinite. These rocks and studied amphibolite and metadiorite are in amphibolite facies of regional metamorphism (Massuda et al. 2020). Studied sample distribution inside, east and west of ophiolite body is shown in Figure 5.
Studied zircon crystals have varied internal structures. In sample BO17 (chloritite), observation of 140 crystals in BSE images (20 shown in Figure 6) displays mostly anhedral (some euhedral) external faces. Internal structure shows large homogeneous cores and complex narrow rims. No euhedral zoning is observed. Size of crystals is ~120 mm with aspect ratio 2:1. Crystals have few inclusions and are little fractured but a fracture may be present around the core. Baddeleyite overgrowth on zircon is seen in Figure 4g.
Zircon from sample C3P4 (Campestre volcanoclastic rock) was observed in 30 BSE images (16 shown in Figure 7). Crystals are homogeneous and similar, with marked internal euhedral zoning, anhedral to euhedral. Size of crystals is 100-120 mm, aspect ratio 1.2:1. Very few mineral inclusions and fractures are observed. Sector zoning is present in Figures 5a, o. In sample C3P17 (Capivaras metadiorite), observation of 41 BSE images (24 shown in Figure  8) reveals anhedral to subhedral (rounded) zircon crystals. Internal structure is similar in the crystals and complex. Diffuse geometry corresponds to medium gray tones (BSE) in cores, surrounded or crossed by lighter grey tones in zircon portions. Opposite light and dark tones are seen in CL images. Size of crystals is 200-300 mm and reaching 400 mm, aspect ratio 2:1. Very few mineral inclusions are present; a few fractures cross the crystal, concentrated in the darker in BSE portions.
Zircon U-Pb isotopic data (Table SI) are displayed in concordia diagrams (Figure 9). Sample BO17 (chloritite) had 70 analyses made on zircon; 62 were used for age calculation and 8 were discarded. Concordia diagram shows a spread of concordant ages from 960-700 Ma but mostly from 920-820 Ma. Bell-shaped distribution shows maximum at 869 Ma.
Out of a total of 42 analyses on zircon from sample C3P17 (Capivaras metadiorite), 31 were used for calculations and 11 discarded. Concordia age is 698 ±4 Ma, interpreted as the magmatic age of the diorite. Analyses on a few light grey (BSE) portions of zircon indicate similar age to the magmatic portions.
I n s a m p l e C 3 P 4 ( C a m p e s t r e metavolcanoclastic rock), 42 spots were analyzed on zircon, 32 were used in calculations and 10 discarded. Concordia age is 842 ±5 Ma, corresponding to the magmatic age of detrital inheritance.

DISCUSSION
Bossoroca ophiolite is a key oceanic crustal and mantle fragment in the Brasiliano Orogen. Study by Hartmann et al. (2019) ascertained the depleted mantle affinity of zircon from a tourmalinite and oceanic origin of the tourmaline. We presently establish the structure of the ophiolite from field and aerogeophysics. Ages of geological processes with U-Pb isotopes are constrained on zircon from a metasomatic chloritite (960-700 Ma but mostly from 920-820 Ma, peak 869 Ma) within the ophiolite, metadiorite (698 ±4 Ma) from the infrastructure of the juvenile island arc, and detrital zircon (842 ±5 Ma) from a Campestre metavolcanoclastic rock from the superstructure. Volcanism in the superstructure is considered 757 ± 17 Ma old from a dacitic metatuff -Zrn U/Pb SHRIMP II (e.g., Remus et al. 1999).
Capivaras metadiorite is part of the Cambaí Complex, and is the youngest intrusion identified in the infrastructure along the western border of the studied ophiolite. Most of the granitic and gneissis rocks in the complex have ages 750-700 Ma (Hartmann et al. 2011). Age of zircon from the metavolcanoclastic rock is older than previous studies, which placed the interpreted magmatic age near 750 Ma. We interpret the age 842 ±5 Ma as corresponding to the source area of the detrital zircon. This is supported by the sedimentary characteristics of the rock and the structure of zircon crystals, which is similar in size and geometry to the core of the zircon crystals from the chloritite within the ophiolite. Ages from the chloritite place the ophiolite within the time span of formation of the Cerro do Ouro Ophiolite in the terrane (Arena et al. 2016, 2017, Hartmann et al. 2019 Two types of ophiolites are present in the terrane, one contained in the infrastructure -Cambaizinho and Cerro Mantiqueiras ophiolite, and the other at the base of the superstructure -studied Bossoroca, and Palma, Ibaré ophiolites. In the Arab-Nubian Shield, this classification is significant, because gold deposits are concentrated in ophiolites at the base of the superstructure. All ophiolites in the terrane are mélanges, because serpentinite is interspersed with rocks formed in the oceanic crust, e.g. amphibolite (extensive in Cerro Mantiqueiras ophiolite), banded iron formation and metachert (all ophiolites), metapelite and para-amphibolite (Cambaizinho ophiolite), metavolcanoclastic rocks (Bossoroca, Palma, Ibaré ophiolites). These are tectonic mélanges in the sense of Raymond (2019), commonly interpreted as formed in the accretionary prism above a subduction zone. Because main trusting of the São Gabriel terrane occurred towards ESE, the presence of a west-dipping subduction zone is suggested to the east of the region in the Tonian (Saalmann et al. 2006).
Integrated investigation of an ophiolite, infrastructure and superstructure, with use of field survey, aeromagnetometry and aerogammaspectrometry and zircon U-Pb geochronology allows the perception of several key points in evolution. Improved cartography of the Bossoroca ophiolite leads to dating of the three main units, namely a chloritite within the ophiolite formed by metasomatism in oceanic floor, a metadiorite formed late in the Cambaí Complex and a metavolcanoclastic rock from the Porongos Group (Campestre Formation).

CONCLUSIONS
The Bossoroca ophiolite is an independent stratigraphic unit obducted in the Tonian at the base of island-arc superstructure (Porongos Group, Campestre Formation) and on top of the infrastructure (Cambaí Complex). Zircon formation in the chloritite from the ophiolite started at 920 Ma, peaked at 869 Ma and continued mostly until 800 Ma and even 700 Ma, therefore mostly during the Tonian. Host island-arc concluded formation at 698 Ma as observed in syntectonic metadiorite from Cambaí Complex. Metavolcanoclastic rock from superstructure has detrital zircon with age of 841 Ma; Campestre Formation is considered 757 Ma old from previous studies. Ophiolite obduction is estimated as syntectonic with metadiorite intrusion and deformation at 698 Ma.