In addition to protolith age constraints on basement and supracrustal rocks, metamorphic ages from basement rocks and geochronological data from Brasiliano-age magmatic rocks provide key controls on the chronology and duration of all deformation phases in the MCD. The age of the D1 event in the basement rocks is inferred to be Transamazonian (2.0 – 2.2 Ga), as the ca. 1.79 Ga volcanic unit in the Ubajara Group records no evidence of D1 deformation. The timing and duration of D2 and D3 are constrained by U-Pb ages obtained from Brasiliano igneous bodies within and immediately southeast of the MCD. While convergence between the Borborema Province and the West African Craton may have begun as early as 775 Ma (Fetter et al, 2003), the timing and duration of tangential collision and crustal thickening (D2) has been determined from ages of syntectonic plutons, migmatitized paragneisses and late tectonic granitoids. The upper age limit for D2 is derived from the oldest metaplutonic rocks of the Santa Quitéria complex (located just SE of the Sobral-Pedro II lineament). This plutonic suite preserves a strong low-angle tectonic fabric related to thrust-nappe development with northwest vergence (Figure 15A), and yielded a crystallization age of 623.9 ± 1.3 Ma (U-Pb in zircons, Fetter et al. 2003). A migmatized paragneiss in the MCD with tangential deformation features yielded U-Pb monazite ages between 614 and 601 Ma (Fetter et al., 2000), indicating that D2 continued until this time. The Chaval granite, located in the northwest part of the MCD (Figure 4B) dates the late stages of D2 as it is only slightly affected by low-angle deformation, and its margins were affected by large NE-SW and E-W shear zones during its crystallization (Figure 15B,C). U-Pb data from igneous monazite indicates that the pluton crystallized around 591 ± 10 Ma (Fetter, 1999). Thus, D2 has lasted at least 30 myr between ca. 620 to 590 Ma.
Figure 15. tructures in granitic rocks in the the Médio Coreaú Domain
Structures in granitic rocks in the the Médio Coreaú Domain. a) Santa Quitéria granite with low-dipping S2 foliation and asymmetric folds and alkali feldspar porphyroclast indicating a NW sense of movement; b) Chaval granite with mylonitic foliation where feldspars porphyroclasts indicate dextral sense of shear during D3; c) Chaval granite showing prominent heterogeneous deformation, with alternate ultramylonite and protomylonite textures; d) Meruoca granite showing isotropic texture with no evidence for penetrative plastic deformation.
Many of the granitoid plutons that intruded during D3 intruded into transtensional zones both along and between shear zones. These plutons only record strike-slip shearing (Weinberg et al., 2004) and many such plutons throughout the Borborema Province have ages between 590 and 570 Ma (Fetter, 1999, Guimarães et al., 2000), suggesting that this was the peak interval of transcurrent (D3) tectonics. U-Pb SHRIMP zircon data from the Tucunduba granite, emplaced between two shear zones, suggests that the transcurrent D3 regime may have continued until 560 Ma (Fetter, 1999) alternatively, it may have been related to late-stage D4 transpression that resulted in rapid uplift and cooling of the orogen between ca. 565 and 555 Ma.
The rapid uplift of the granulites in the Granja massif is demonstrated by both a precise Sm-Nd mineral-whole rock isochron (plagioclase – whole rock –garnet) of 557 ± 1 Ma (Fetter, 1999), and by an Ar-Ar biotite plateau of 558 ± 3 Ma obtained by Monié et al. (1997). A U-Pb titanite cooling age of 554 ± 4 Ma from migmatitic gneiss the Granja massif is within error of the Sm-Nd and Ar-Ar ages, indicating rapid uplift of the basement complex during D4 transpression.
The undeformed Meruoca and Mucambo granites (Figure 4) represent the final magmatism in the MCD (Figure 15D). These granites are intimately linked to the development of the Jaibaras graben, hence are rift-related as opposed to post-tectonic. U-Pb zircon data from the Mucambo granite yield a crystallization age of 532 ± 6 Ma (Fetter, 1999).