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.

Permian-Triassic magmatic evolution of granitoids from the southeastern Central Asian Orogenic Belt:Implications for accretion leading to collision

Many orogenic belts in the world exhibit accretionary and collisional orogenic phases to varying extents.How accretion evolves into collision of the Central Asian Orogenic Belt(CAOB),the largest Phanerozoic accretionary orogenic belt,is an intriguing question.In this paper,we present new U-Pb age,geochemical and isotopic data for Permian-Triassic granitoids from middle Inner Mongolia,Northern China in the southeastern CAOB,and delineate the magmatic transition from subduction to(soft)collision.The magmatic record of soft collision is identified and characterized by thickened lower crust-derived high Sr/Y granitoids with a sub-linear distribution along the Solonker suture zone.Granitoids from Early Permian to Late Permian became more enriched in whole-rock Nd and zircon Hf isotopic compositions(εNd(t)values from 2.4 to-19.5,εHf(t)values from 11.6 to-33.7),indicating increasing incorporation of old crust.The change in peak timing of magmatism from west(ca.264 Ma)to east(ca.251 Ma)along the Solonker suture zone implies"scissor-like"closure of the Paleo-Asian Ocean.Integrated with previous studies,a three-stage tectonic model from the Permian to Triassic by accretion leading to collision on the south-eastern margin of CAOB is proposed.(1)Early Permian(>ca.285 Ma):Juvenile magmatism on an active continental margin with double-sided subduction of the Paleo-Asian Ocean;(2)Middle Permian to Middle Triassic(ca.285–235 Ma):Magma source transition from juvenile to old crust induced by a tectonic switch from arc to"scissor-like"closure and subsequent intracontinental orogenic contraction;(3)Late Triassic(<ca.235 Ma):A-type and alkaline magmatism in response to post-collisional extension.