Rhyacian and Neoproterozoic magmatic associations of the Gurupi Belt,Brazil: Implications for the tectonic evolution, and regional correlations

The Gurupi Belt,in north-northeastern Brazil,is a mobile belt developed in the south-southwestern margin of the Sao Luis cratonic fragment and crops out as a tectonic and erosional window within the Phanerozoic cover.Field,petrographic,geochemical,geochronological,and Nd isotopic information(new and published)constrain the timing and types of magmatic associations present in the belt and the tectonic settings in which they formed.The Rhyacian was the main period of magmatic activity,which can be grouped into two main stages.(1)~2185-2130 Ma:pre-collisional,juvenile,calc-alkaline magnesian and calcic ferroan granitoid suites,and minor calc-alkaline and tholeiitic mafic plutonism(now amphibolites),formed in intra-oceanic to transitional/continental arcs;and intra-or back-arc volcano-sedimentary basin.(2)~2125-2070 Ma:syn-(two-mica granites)to late-collisional(potassic to shoshonitic granites and quartz-syenite)plutonic suites produced after crustal thickening and melting,with localized migmatization,that intruded during the compressive D1 deformational phase and concomitantly with greenschist to amphibolite metamorphism.There is a zonation of the Rhyacian episodes,with intra-oceanic stages occurring to the northeast,and the continental arc and collisional phases occurring to the southwest,indicating the presence of an active continental margin to the southwest,and subduction from NE to SW(present-day configuration).This magmatic framework is a continuation to the south of what is described for the Sao Luis cratonic fragment to the north,and the orogenic scenario is identical to what is observed for the same period in the West African Craton(Eburnean/Birrimian orogen),which additionally supports previous geological correlations.In the Neoproterozoic,a few magmatic occurrences are recognized.An extensional event allowed the intrusion of an anorogenic,nepheline syenite at ca.730 Ma,which was followed by the intrusion of a crustal,calc-alkaline microtonalite,of uncertain tectonic setting,at 624 Ma.Both intrusions underwent greenschist to amphibolite facies metamorphism between 580 Ma and 529 Ma.This metamorphic event is probably related to crustal thickening,which produced crustal melting and intrusion of two-mica granites between 595 Ma and 549 Ma.The absence of oceanic and arc-related assemblages,along with geophysical information about the basement of the Phanerozoic cover indicates an intracontinental setting for the Neoproterozoic-Early Cambrian evolution of the Gurupi Belt,with rifting and posterior closure of the basin,without oceanization.Rifting and closure correlate in time with the onset of Rodinia breakup and West Gondwana assembly,respectively,but we interpret the events in the Gurupi Belt as having no direct role in these two global supercontinent-related events,but,instead,as being related to orogenic events occurring in the periphery of the West African and Amazonian cratons at that time.

Lu–Hf constraints on pre-,syn,and post-collision associations of the Gurupi Belt,Brazil:Insights on the Rhyacian crustal evolution

The Gurupi Belt(together with the São Luís cratonic fragment),in north-northeastern Brazil,has been described in previous studies that used extensive field geology,structural analysis,airborne geophysics,zircon U–Pb dating,and whole-rock Sm–Nd isotope and geochemical data as a polyphase orogenic belt,with the Rhyacian being the main period of crust formation.This was related to a 2240 Ma to 2140 Ma accretionary processes that produced juvenile crust,which has subsequently been reworked during a collisional event at 2100±20 Ma,with little evidence of Archean crust.In this study,we use Lu–Hf isotopic data in zircon from granitoids(including gneiss)of variable magmatic series,and amphibolite to improve the knowledge of this scenario,and investigate additional evidence of recycling of Archean basement.Pre-collisional high Ba-Sr and ferroan granitoids and amphibolite formed in island arc(2180–2145 Ma),show only zircons with suprachondriticεHf values(ca.+1 to+8)indicating the large predominance of juvenile magmas.Only 10%of the data show slightly negativeεHf values(0 to-4),which have been observed in granodiorite-gneiss formed in continental arc(2170–2140 Ma),and in strongly peraluminous collisional granites(2125–2070 Ma),indicating the rework of older Paleoproterozoic to Archean components(Hf_(TDM)=2.11–3.69 Ga).A two-component mixing model using both Hf and published Nd isotope data are in line with this interpretation and indicate more than 90%of juvenile material,and less influence of Archean materials.Comparing with other Rhyacian terranes that are interpreted to have been close to Gurupi in a pre-Columbia configuration(ca.2.0 Ga),our results differ from those of SEGuiana Shield,which show strong influence of Archean protoliths,and are very similar to those of the central-eastern portion of the Baoulé-Mossi Domain of the West African Craton,which has also been formed largely by juvenile magmas in an accretionary-collisional orogen.