Decratonization by Rifting followed by Orogeny and Transcurrent Dispersal of Old Terranes in NE Brazil: A Common Feature of Gondwana Amalgamation?

Dispersion and deformation of cratonic fragments within orogens in the periphery of cratons require weakening of the craton margins in a process of decratonization. The Borborema orogenic province, in NE Brazil, is one of several Brasiliano/Panafrican late Neoproterozoic orogens that led to the amalgamation of Gondwana. A common feature of these orogens is that a period of extension and opening of narrow oceans preceded inversion and collision. For the case of the Borborema Province, the São Francisco Craton was pulled away from its other half, the Benino-Nigerian Shield, during an extension event lasting between 1 Ga and 0.65 Ma. This was followed by inversion and a transpressional orogeny from c. 0.60 Ga onwards. Here we investigate the boundary region between the north São Francisco Craton and the Borborema Province and demonstrate how cratonic blocks became physically involved in the orogeny. We combine these results with a wide compilation of U-Pb and Nd-isotopic model ages to show that the BP consists of up to 65% of strongly sheared ancient rocks aliated with the Sao Francisco/Benino-Nigerian Craton, separated by major transcurrent shear zones, with only ~ 15 % addition of juvenile material during the orogeny. This evolution is repeated across a number of Brasiliano/Panafrican orogens, with signicant local variations, and indicate that extension weakened entire cratonic regions in a process of decratonization that prepared them for involvement in the orogenies that led to the amalgamation of Gondwana.


Background
Cratons are old, stable continental regions with thick buoyant keels that resist deformation, protecting them from tectonic reworking. Their thick and cold refractory lithospheric keel is taken to be responsible for their integrity 1 . However, during continental collisions, colliding cratonic blocks can be partially reworked to generate continental tracts that are no longer cratons but that are not typical orogens either 2 .
Thus, understanding the origin of these old continental crust within orogenic areas must address the speci c mechanisms for both the weakening of the craton lithosphere (c.f. decratonization) and their subsequent reworking. First order controls in the reworking of these strong lithospheres are temporal and spatial variations of their thermal state and pre-existing mechanical anisotropies or major compositional boundaries [3][4][5][6][7][8] . The North China Craton is a classic example where lithospheric thinning, asthenospheric decompression and magmatism changed the thermal and chemical state of the cratonic lithosphere resulting in wholesale decratonization [9][10][11][12] .
Here we describe the process of decratonization of the São Francisco Craton (SFC) during the Neoproterozoic in northeast Brazil. This craton is bound to the north by the late Neoproterozoic orogenic Borborema Province. In the province, pre-orogenic extension started as early as 1000-900 Ma and resulted in the development of intracontinental extensional basins 13 , passive margin basins anking the SFC [14][15][16] , and separation of the SFC from the African Benino-Nigerian Shield 17,18 . Remarkably, this orogenic province includes a number of reworked and deformed Archean-Paleoproterozoic terranes among the Neoproterozoic sedimentary basins and granitic intrusions 19 . These old terranes are relatively small (10,000 to 40,000 km 2 ) and bounded by large-scale continental shear zones 19,20 (Fig. 1A and B).
Although no quantitative crustal growth curves of the orogenic Borborema Province have been reported, present models range from purely intracontinental reworking 21 , to juvenile arc accretion 22,23 and docking followed by reworking of unrelated terranes 24 , or even to questionable autochthonous accretion surrounding old small Archean crust 25 .
The Borborema Orogen comprises two separate and interacting collisions: one on the west side as part of the vast West Gondwana Orogen (WGO) 26 and one in the south against the SFC and termed here Southern Borborema Orogen (SBO). These two collisions were partly contemporaneous and interacted involving the entire Borborema Province (BP). In this paper we will use the term BP to refer to the entire orogenic area. The current con guration of the Province seems to be controlled by these collisions as indicated by the record of UHP metamorphism and eclogites 26,27 , marginal ophiolites 28,29 and arc sequences 16,23,30 that together suggest two separate but related collisions: an oceanic subduction followed by collision along the WGO 19,26 , and a later but in part contemporaneous collision with the SFC. The two different orogens interacted in a complex collisional zone, giving rise to an intricate network of continental shear zones that controlled deformation 19 . In this context, the large Transbrasiliano-Kandi shear zone, that developed as a result of the WGO, acted as a dextrally transfer zone leading to collision and development of the Southern Borborema Orogen 17 .
In this paper, we compile large volumes of whole-rock Nd isotope data, zircon U-Pb geochronology data, as well as geological and geophysical data of the Borborema Province and northern part of the SFC and the Benino-Nigerian Shield. We use the data to rst quantify the Neoproterozoic continental growth of the Borborema Province and establish the dominance of recycled cratonic material, and then to investigate how the decratonization allowed former cratonic Archean-Paleoproterozoic terranes to be entrained within the BP during the Neoproterozoic Orogeny.

Geological Setting Of The Borborema Province
The distribution of lithologies in the triangular wedge-shaped orogenic BP ( Fig. 1) was ultimately controlled by a set of Neoproterozoic strike-slip shear zones 19,20 . These shear zones bound the north, central and south sub-provinces and within them several other informal domains 31 . Shearing was accompanied by granitoids with intrusions concentrated at 585-565 Ma 19 . Archean and Paleoproterozoic lithologies are found as blocks all over the BP, always limited by Neoproterozoic shear zones (Fig. 1).
Early Neoproterozoic extensional events starting as early as 1000-900 Ma are marked by granitoid intrusions, bimodal volcanism and deposition of immature terrigenous and minor carbonatic sediments in the so-called Cariris Velhos event 13 . In the southern BP this event culminated in the development of a passive margin sequence along the edge of the SFC associated to 900-800 Ma ma c-ultrama c intrusions, proximal margin-type ophiolites and A-type granites 14,16,28,32 . In this region, extensional tectonics may have lasted most of the Neoproterozoic, with the last pulses of rifting recorded by the 720- Early Neoproterozoic passive margin and intracontinental sedimentation was overlain by orogenic sedimentary successions as young as 580 Ma 16 contemporaneous with other basins within the central and north BP 33 . The lithological boundary of the SFC and BP is de ned in many geological maps (Fig.  1A), however seismic data indicate that the lithospheric cratonic signature at 100 km depth extend further north ending within the central portion of BP 34 (Fig. 1C).
During the Late Neoproterozoic, from 650 to 610 Ma, predating continental collisional events, continental magmatic arcs developed in the west 23 and south 35 parts of the province, closing intervening oceanic basins. During the collisions that ensued the province was squeezed between two impinging continents, one continent coming in from the west and the other, the São Francisco Craton, coming in from the south 19 . As a consequence, the BP "escaped" obliquely with the development of a throughgoing network of transcurrent shear zones 19,20 that shear and limits Archean-Paleoproterozoic terranes of unknown origin [36][37][38] , such as the Alto Moxotó Terrane 24 (Fig. 1). Just how these old terranes became involved in the orogen remains unclear. The results based on the exposed surface area, incorporating both zircon U-Pb and Nd isotopes, indicate that 65-60% of the province was already formed by the end of the Paleoproterozoic, with a rapid growth rate between 2.3 to 2.0 Ga, when 55-50% of the continental crust of the Borborema Province formed ( The combination of our eld data and available geological maps, isotopic data and geophysical images

U-Pb ages, Nd isotopic spatial distribution and crustal growth
show that the Entremontes Block is bound by the Pernambuco shear zone in the north and the Boa Vista shear zone in the south (Fig. 3A). Movement on these shear zones forced block rotation and internal deformation of the block under a transpressive regime (Fig. 3B) leading to the folds documented in the aeromagnetic images at wavelengths of 5 to 20 km with 2D axial planes subparallel to the shear zones (block diagram in Fig. 3B). In outcrop, folds are tight and fold a previous foliation (Sn), generating a steep axial plane (Sn+1) permeated by leucosomes indicative of syn-tectonic partial melting (Fig. 3B). Stretching lineation has low rake and is commonly parallel to the fold axes plunging at low angles to WSW or ENE. Kinematic indicators on Sn+1 planes indicate consistent top-to-NE or E transport in the Entremontes block (Fig. 3A). This direction could either be a result of rotation of early-formed shear zones, or formed contemporaneously with transcurrent movement, de ning a constrictional  (Fig. 3A).  [40][41][42] .
In summary, much of the Borborema province has old Nd model ages and zircon population signatures similar to the São Francisco Craton/Benino-Nigerian Shield ( Fig. 1 and 2A), suggesting direct or indirect derivation from these areas. The stepwise process of breakdown and involvement of cratonic blocks is Transbrasiliano-Kandi strike-slip belt ( Fig. 4B and C). The second step was a result of dextral movement on the Transbrasiliano-Kandi strike-slip belt. This movement brought the Benino-Nigerian Shield closer to the São Francisco Craton deforming the weakened decratonized area in between these two stiffer lithospheric domains leading to the transpressional Borborema Province Orogeny (see cross section in Fig. 4D).
The transcurrent shear zones that characterize the Borborema Province splayed out of the Transbrasiliano-Kandi shear zone 23 and deformed the weakened cratonic margin. Deformation was a result of these two quasi-contemporaneous collisions 23,26 (Fig. 4D). In the north part of the Borborema Province, old Paleoproterozoic and minor volumes of Archean rocks dominate and represent the southern continuation of the Benino-Nigerian Shield 17 (north of the Patos shear zone in Figure 1B). To the south, the Borborema Province record the interaction of the decratonized lithosphere and the São Francisco Craton. In this region, the transcurrent shear zones wrenched the blocks of the northern margin of the São Francisco Craton that had been previously weakened by the Cariris Velho extension. Deformation included a number of continental ribbons (e.g. PEAL terrane) pulled away from the craton and the intervening Tonian sedimentary basins ( Fig. 4C and D).
The exact geometry of the decratonized terrane at the start of wrenching, and the source region of individual blocks now embedded in the BP are generally unknown. However, using rock ages, isotopic and geophysical signatures, we have linked the Tonian-aged Afeicão domain and the ancient Entremontes block south of the Pernambuco shear zone, with the Tonian-age Alto Pajeú and the ancient Alto Moxotó terranes, north of the shear zone. This implies a ≈ 200 km dextral wrenching across the Pernambuco shear zone (Fig. 3). The rocks south of the shear zone record only incipient deformation, whereas those to the north are intensely strained into sigmoidal terranes, in harmony with the regional transcurrent deformation and geological-geophysical structure of this region.  Figure  4B). Finally, Ar-Ar colling ages and emplacement of poorly deformed to isotropic granitoids indicate slow cooling rates with continuous heat supply by the delamination process until the Cambrian (530-500 Ma) 19 .
We conclude that the cratonic roots of the São Francisco Craton/Benino-Nigerian Shield, responsible for craton integrity, were weakened by Tonian-age extension, similar to the evolution of the North China Craton where extension and alkaline magmatism provided a mechanism for decratonization. This created the conditions required for the involvement and dispersal of decratonized inliers within the Brasiliano orogen. We suggest that this may have been the general sequence of events for many of the Brasiliano/Panafrican orogens, where extension related to the break-up of cratonic masses and opening of oceanic realms with varying degrees of maturity were followed by convergence and wrench tectonics during Gondwana amalgamation.

Methods
Nd isotopic maps.
In order to discriminate terranes with similar signatures, a large compilation comprising 360 zircon U-Pb ages and 1331 Sm-Nd whole-rock isotope analyses of samples from the BP and the northern SFC were used (Fig. 1B). Sm-Nd isotope distribution of a large number of samples is suitable to identify correlated terranes due to resistance of the Sm-Nd isotope system to post-crystallization thermal disturbance 71 .  [73]. The compilation was augmented with data from the literature. Most of the data is from (meta)igneous rocks with subordinated input from metasedimentary rocks.
In general, Nd model ages (TDM) do not correspond to a speci c crust-formation event but instead re ect mixing of material derived from the mantle at different times and are determined by calculating the time when a sample had an isotopic composition identical to that of its source 74 , so they can be understood as minimum ages of crust formation. For gure 1B we used Sm-Nd TDM ages as originally reported by different authors (Table 1 of supplementary material). The Sm-Nd TDM ages were gridded in the ArcGIS software using the Inverse-Distance-Weighted Interpolation (IDW). Since signi cant discontinuities modify the surrounding geological environment, we de ne the major shear zones as interpolation barriers. We also applied the Gaussian low-pass lter to attenuate high frequencies due to variable spacing between samples. We compared compiled U-Pb zircon crystallization ages and freely available geological maps (http://geosgb.cprm.gov.br) of the Northern SFC and BP to cross-check terrane a nity based on the Sm-Nd TDM map, to identify old terranes within the Borborema orogenic province. From the compiled Sm-Nd dataset only data with reported ppm content of Nd, Sm and 143 Nd/ 144 Nd ratio associated with reliable reported U-Pb ages for the magmatic crystallization were considered for building the eNd (t) maps and further crustal growth curve for the Borborema Province. This resulted in 889 data points with assigned geographical position and recalculated 147 Sm/ 144 Nd ratio. A new screening was applied to eliminate unreliable 147 Sm/ 144 Nd resulting in the 837 data points that were used for further gridding of eNd (t) values, as described above (Fig. 3). The pixels with eNd > 0 were binned to 100 m.yr. and the cumulative percentage area covered by juvenile rocks was used to infer the crustal growth of Borborema Province from 3.4 to 0.5 Ga. Figure S1 (supplementary material) show the data distribution used to grid the Sm-Nd TDM ages.
Magnetic maps.
The airborne magnetic database comprises data from seven surveys between 2001 and 2010, with 500 m ight-line spacing in the N-S direction and ight height of 100 m (http://geosgb.cprm.gov.br). The Total Magnetic Intensity map (TMI) was created by interpolating the magnetic data into a 125 m grid cell size using the bi-directional method and subsequently ltered by a Gaussian low-pass lter. To highlight the regional tectonic framework, we calculated the rst vertical derivative of TMI (1VD).