Abstract
Records of large crustal masses in the northwest of Borborema Province (BP) spread over more than 300 km are found as numerous granitic bodies amid a high-grade metamorphic gneissic-migmatitic terrain. The Tamboril-Santa Quitéria Complex (TSQC) is a Neoproterozoic example of these records; it is located in the north of the Ceará Central Domain (CECD) and its origin is related to a continental magmatic arc at different evolutionary stages of the arc. The Uruburetama Granite (UG), object of this study, fits into this context and constitutes one of the most representative batholiths occupying an area of 1,500 km2. Dozens of other similar plutons have been recorded in cartographic works that show similarities which suggests the grouping of these bodies in the Uruburetama Granitic Suite (UGS), previously included in the TSQC. The UG consists of a variety of plutonic rocks associated with mafic-dioritic dikes and its metamorphic products. Six petrographic facies were identified among the plutonic rocks: biotite-hornblende monzogranites and syenogranitic varieties, biotite-hornblende granodiorites, in addition to quartz syenite, leucomonzogranites, and diorites rocks. These rocks are affected by deformation, with greater intensity at the edges of the batholith, where thrust shear zones with transcurrent components were installed, generating tectonic fabrics (mylonitic foliations, stretch lineation, almond feldspar porphyroclasts, and ribbon quartz). Despite the superimposed deformational effects, magmatic relict textures are partially preserved, mainly toward the central portion of the pluton. The UG and the associated plutons are hosted by migmatized orthogneisses, paragneisses with garnet, sillimanite or kyanite, marbles, calcium-silicate gneisses, micaschists, sillimanite quartzites, and amphibolite lenses. The contact relations are diffuse with gradation for migmatites, and in rare cases, intrusive contacts with gneiss mega-xenolites are registered. The geochronological data obtained by Pb-evaporation zircon in two samples of the UG show average ages of 655 ± 2 and 656 ± 1 Ma. U-Pb zircon data for the same samples showed slightly younger age values of 559 ± 10 and 634 ± 10 Ma, respectively. The ages obtained are considered as representative of the magmatic phases of UG crystallization in the context of the evolution of the Santa Quitéria Magmatic Arc (SQMA), with the main magmatic phase of the UG in the Cryogenian. However, the age of 559 Ma would represent a younger magmatic event in the evolution of the arch. Overall, these ages correspond to the “Pre-Collisional I” or “Early-Sin-Orogenic” phase of a large collisional belt in the west of BP, and the younger ages must represent events related to the continuity of the orogen convergence of the Latest-Pre-collisional I (634 Ma) and Post-Collisional I (559 Ma) phases. Sm-Nd whole-rock isotopes data showed εNd(t) values predominantly negative (-25.6 and −0.9), and Nd-TDM model ages of 2.90 and 1.2 Ga. The results of the Lu–Hf isotopes analysis for the Bt-Hbl monzogranite sample showed negative εHf values (-26.75 to −35.48) and Hf-TDMC model ages of 3.12 to 3.65 Ga while the results for Bt-Hbl granodiorite showed εHf values of −2.07 to +1.08 and Hf-TDMC model ages of 1.46 to 1.66 Ga. These data point to the existence of two distinct and older crustal sources in the generation of these granitoids, one in the Mesoarchean, and the other in the Mesoproterozoic. The Archean ages correspond to the basement in the south of the CECD (Tróia Massif and Granjeiro Complex); on the other hand, the Mesoproterozoic ages are up for discussion, since terrains with this type of age do not occur adjacent to the BP. On the other hand, it can be interpreted as a mixture of sources, one of them probably juvenile, neoproterozoic, with contamination of the Archean crust. Thus, the UG is considered one of the most important records of the beginning of the evolution of the Santa Quitéria Magmatic Arc associated with a wide collisional belt in the “West Gondwana Orogen” in the west of Borborema Province.
Highlights
The present study focuses on this body, for the reasons reported above, and due to the few aggregated knowledge, large exposure area, difficult access and its importance as part of this arc magmatism in the context of the Borborema Province (BP) granitogenesis
Zircon U-Pb dating LA-MC-ICP-MS zircon U-Pb analyses were carried out using a high-resolution multi collector Neptune Thermo Finnigan mass spectrometer coupled with a Nd:YAG LSX-213 G2 CETAC laser microprobe at the Pará-Iso/Universidade Federal do Pará (UFPA), whose instrumental performance and analytical procedures were documented by Chemale Jr. et al (2012) and Milhomem Neto et al (2017b)
Considering that the Uruburetama Granite (UG) is not an isolated pluton, and that there are several other granitic plutons with similar correlated characteristics (Patos, Manoel Dias, Extrema, Mirindiba, Aracatiaçu, Tamboril, and Nova Russas granites) distributed from south to north for more than 300 km, and in the literature are informally defined as “Santa Quitéria granitoids” here we are proposing the formalization of these granitoids occurrence as a new lithostratigraphic unit, named “Uruburetama Granitic Suite”
Summary
Amidst high metamorphic gneissic terrain in northwestern Ceará State, whose main plutons are Uruburetama, Manoel Dias, Mirindiba, Extrema, Aracatiaçu, Tamboril, and Nova Russas granites (Figs. 1A and 1B). They show equigranular or porphyritic general texture, in which the lithotypes least affected by the deformation and the alignment of euhedral alkali-feldspar phenocrystals characterize a magmatic flow feature; lentiform enclaves rich in mafic minerals follow the flow direction Under the microscope, they are characterized by the granoblastic texture composed of plagioclase, microcline, and quartz, sometimes in polygonal aggregates, with foliation marked by the preferred orientation of the biotite and locally with porphyroclastic texture Biotite-hornblende granodiorite This petrographic type was found in the southeast portion of the batholith, they are dark gray, medium-grained, leucocratic (M = 21–24) rocks, which contained biotite and hornblende as main mafic phases (Tab. 2) They show porphyroclastic texture and well-defined tectonic foliation (Fig. 8A). These rocks represent segments of the lower crust tectonically exhumed in the region, which reveals the exposure of the roots of West Gondwana Orogen
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