Reappraisal of the Sumé Complex: geochemistry and geochronology of metaigneous rocks and implications for Paleoproteorozoic subduction-accretion events in the Borborema Province, NE Brazil

Abstract

The Alto Moxotó Terrane represents the largest exposure of Paleoproterozoic rocks in the central Borborema Province. Several medium- to high-grade metamorphic units are described in the terrane, however geochronological data are still scarce, and the main periods of crust build-up and recycling are poorly known. In this study, we investigate the nature and age of metagranitic and metamafic-ultramafic rocks in the type area of the Sumé Complex. U-Pb zircon data from a metasyenogranite yields a concordant age of 1.97 Ga, which is in contrast with the previously published age of the complex (c. 640 Ma). In addition, Sm-Nd isotope signature indicates Paleoproterozoic to Mesoarchean sources that coupled with negative εNd(t) values suggest reworking of a preexisting crust. Whole-rock geochemical data indicate that metamafic rocks are similar to those already described in the terrane, corresponding to island-arc tholeiites, whereas the metagranitic rocks share similarities with magmas that were generated in an evolved stage of an orogenic setting. The obtained results are coherent with a long-lived Paleoproterozoic accretionary-collisional event (c. 2.1 - 1.9 Ga) described in the Borborema Province, which is correlative with Paleoproterozoic (c. 2.2 - 2.0 Ga) basement inliers/terranes from Pan-African Fold belts..

KEYWORDS:
Sumé Complex; Accretion tectonics; Alto Moxotó Terrane; Borborema Province; Western Gondwana

INTRODUCTION

The role of orogenic-collisional events in generation, preservation and reworking of continental crust in Precambrian provinces is one of the most debated topics in earth sciences (e.g., Hawkesworth & Kemp 2006, Hawkesworth et al. 2017). A critical point lies on how similar or not Early Earth and Phanerozoic thermal regimes are. For instance, geochemical and isotope record of granites, mafic rocks, supracrustal sequences and high-grade metamorphic assemblages, including granulites and eclogites, suggest that mid- to late Precambrian and Phanerozoic convergent tectonics could have been quite similar in most cases (Rudnick 1995, Cawood et al. 2009, 2013).

In contrast, early Paleoproterozoic and Archean regimes seem to be different in a number of aspects, including partial melting rates, speed of crustal growth and potential sources for melt generation (Martin et al. 2005, Brown 2006). Nevertheless, it is consensus that, in most Phanerozoic and Precambrian orogens, injection of high amounts of juvenile magmas via accretionary wedges generates new crust, whereas subduction-collisional orogens strongly rework the previous and ancient formed crust (Cawood et al. 2009, Hong & Kao 2015). In extremely deformed orogens, the nature of major crust building and reworking events needs careful and critical analysis, given that early crustal processes might have been hidden by younger subduction, collisional or rifting events. Another critical issue lies on the identification and description of basement inliers or exotic terranes that can be considerably older than the final tectonic stabilization. The recognition and detailed description of previously formed crust are hard tasks, but it can provide crucial information, once that they can represent travelling fragments that are missing in large cratonic blocks (e.g., Zhao et al. 2004, Meert 2012).

In Western Gondwana, several Neoproterozoic orogens are interpreted as the result of oceanic subduction followed by continent-continent collision of cratons and minor crustal blocks. Most of these orogens/provinces are characterized by Ediacaran to Cambrian arc-related lithotectonic units, that are widespread and preserved. Thus, the investigation of magmatic and metamorphic events provides reliable clues on processes related to the final assembly of Gondwana (Vaughan et al. 2005, De Wit et al. 2008, Brito Neves et al. 2014).

In the Neoproterozoic Borborema Province (NE Brazil, central portion of Western Gondwana), recent geochemical-isotopic investigations, focused on petrological and tectonic aspects of Archean and Paleoproterozoic basement inliers or exotic terranes, demonstrate the complexity and variety of crustal processes that took place prior to its present configuration (e.g., Brito Neves et al. 2014, Caxito et al. 2016, Ganade et al. 2017, Santos et al. 2017b, Costa et al. 2018). Within its central portion, the Alto Moxotó Terrane/Domain (AMT) is considered one of the most preserved Paleoproterozoic crustal segments of the entire province (Brito Neves et al. 2000). Among its numerous units, Medeiros and Torres (2000) described an important lithostratigraphic sequence that occupies a large area of the terrane, named as Sumé Complex. This unit is characterized by a large variety of rock types such as gneisses, banded iron formation (BIF), ultramafic rocks, granulites and calc-silicates rocks with innumerous tangential orthogneisses varying to granitic to leuco-tonalitic/anorthositic compositions. Their initial stratigraphic placement without using isotopic tools refer to Paleo- to Mesoproterozoic eras.

Silva et al. (2002) dated homogeneous zircon crystals (U-Pb sensitive high-resolution ion microprobe-SHRIMP) from a foliated hornblende granodiorite with porphyroclastic texture, interpreted as part of the Sumé Complex, yielding a date of 640 ± 6 Ma. Such age was interpreted as the crystallization age of the rock, whereas a zircon xenocryst of ~841 Ma delineates an early crustal inheritance (Silva 2006). This data led some authors to distinguish a number of Neoproterozoic rocks throughout the terrane. Recently, Santos et al. (2015) demonstrated that part of the Sumé Complex corresponds in fact to a major Paleoproterozoic association, including metamorphic, ultramafic, mafic and felsic rocks, marking two tectonically important contrasting events in the region:

1. Siderian and Rhyacian accretion-collision;

2. Statherian-Calymmian within-plate magmatism.

Such older associations are inconsistent with the previously obtained Ediacaran age. Therefore, the origin of the Sumé Complex is still an open question.

In this paper, we present new U-Pb age, Sm-Nd isotope and whole-rock geochemistry data of metagranitoids and associated metamafic rocks from the Sumé Complex. Our main goals are:

1. to investigate the magma protoliths, including the associated tectonic setting;

2. to elucidate the proper stratigraphic position of the studied rocks and consequently the Sumé Complex (i.e., Neoproterozoic vs. Paleoproterozoic);

3. to demonstrate the importance of this crustal segment to the understanding of the Borborema Province Precambrian evolution.

GEOLOGICAL SETTING, FIELD RELATIONSHIPS AND PETROGRAPHY

The Sumé Complex is inserted in the AMT, a dominantly Paleoproterozoic domain that underlies a NE-SW segment of the central Borborema Province. The province itself comprises a major portion of NE South American platform, comprising several individual lithotectonic domains that record tectono-magmatic and metamorphic histories spanning Archean to Ediacaran-Cambrian times (Almeida et al. 1981, Brito Neves et al. 2000). As several other mobile belts of Western Gondwana, it has been built by tectonic convergence of major cratonic blocks (i.e., São Francisco-Congo and São Luís-West Africa cratons), during Neoproterozoic and Cambrian times (Brito Neves et al. 2014).

According to Van Schmus et al. (2011), the Borborema Province can be tectonically divided into the Northern, Transversal and Southern sub-provinces, which are limited by two major E-W shear zones: the Patos lineament, to the north; and Pernambuco lineament, to the south (Fig. 1). Up to now, the Neoproterozoic evolution of the Transversal sub-province is in the heart of a major debate, involving two main models:

1. accretion of crustal terranes during the Cariris Velhos (1000 - 920 Ma) and Brasiliano (650 - 540 Ma) orogenies (Brito Neves et al. 2000, Santos et al. 2010, 2017b, 2018);

2. intracontinental reworking of a larger Paleoproterozoic continent (Neves 2015, Neves et al. 2017).

Figure 1.
(A) Geodynamic context of the Borborema Province in pre-drift reconstruction for West Africa and northeastern South America; (B) tectonic framework of the Borborema Province; (B) simplified geological map of the Transversal sub-province and its domains with the study area highlighted.

In addition to the AMT, the Transversal sub-province is considered to consist of other four terranes/domains, namely São Pedro, Piancó-Alto Brígida or Cachoeirinha, Alto Pajeú and Rio Capibaribe (Santos & Medeiros 1999, Brito Neves et al. 2000, Basto et al. 2019). Within the AMT, two major units dominate, referred to as Floresta Complex or basement rocks (orthogneisses and migmatites with ages ranging from 2.2 to 2.1 Ga), and the Sertânia Complex (paragneisses, schists and migmatites dated at 2.1 - 2.0 Ga; Santos et al. 2004). However, Neves et al. (2017) reported Neoproterozoic detrital zircon ages (c. 700 Ma) from samples attributed to the Sertânia Complex.

Recently, Santos et al. (2015, 2017a) discussed the role of Archean and Paleoproterozoic rocks in the evolution of the AMT, which seems to be much more complex than the rest of the Transversal sub-province. It includes episodes of tonalite-trondhjemite-granodiorite (TTG) and sanukitoid generation, as well as several calc-alkaline metaluminous suites that record a long-lived Paleoproterozoic history of accretion and collision. Neves et al. (2015) achieved similar conclusions, studying different facies of orthogneisses and migmatitic rocks from the basement, as well as metamafic rocks.

The type area of the Sumé Complex comprises the central portion of the AMT and was defined in the work of the homonym geologic sheet (SB.24-Z-D-V; Medeiros & Torres 2000). This region is known by local and spaced occurrences of phosphate minerals, mainly apatite, hosted in skarn lenses (Nascimento et al. 2015). In the study area (Fig. 2), rocks of the Sumé Complex comprise interleaved granitic to metagranitic and metamafic-ultramafic rocks. The metagranitoids corresponds to monzogranites, syenogranites and minor alkali-feldspar granites (Fig. 3A). In the field, metamafic and metaultramafic rocks form pockets and rounded xenoliths within metagranitic members, suggesting magma mingling (Fig. 3B), whilst metagranitic rocks normally present a slightly to well developed foliation in response to regional stress, being whitish to pinkish-gray and pink, equi- to inequigranular, medium- to coarse-grained (Fig. 3C). Garnet-biotite paragneisses and leucossome-rich migmatites of the Sertânia Complex possibly host this set of metaplutonic rocks.

Figure 2.
Simplified geological map of the study area showing the location of the selected sample for U-Pb zircon dating.

Figure 3.
(A) Modal composition of the studied metagranitoids obtained by point counting; (B) rounded contact of alkali feldspar granite and pyroxenites, suggestive of preserved magma mingling; (C) foliated syenogranite of the outcrop selected for U-Pb dating; (D) metric layers of fine-grained massive metagabros; (E) deformed and elongated hornblende crystals following the regional foliation imposed on an amphibolite sample.

In thin sections, metagranitic rocks show granoblastic texture. Its mineralogy consists of subhedric to anhedric quartz (45-40%), euhedric to subhedric K-feldspar ­(35-30%) and euhedric, subhedric and anhedric plagioclase crystals (25-20%). In the most deformed samples, hipidioblastic to xenoblastic plagioclase crystals may occur, exhibiting myrmekite intergrowth, besides the presence of millimetric light brown biotite inclusions that can be partially bent. The latter is the main mafic mineral of these rocks. Hornblende and muscovite also occur in minor amounts, in addition to apatite and titanite. Euhedral, subhedral and rounded zircon crystals are common inclusions in biotite lamellae. Lastly, rounded to elliptic mafic clots, up to 1.5 cm in diameter, can be locally found following a discrete magmatic foliation or even deformed by regional tectonics, mostly composed of Ca-rich amphibole.

Metamafic and metaultramafic rocks are similar of those ones described as Carmo Suite by Santos et al. (2015). In the study area, metamafic members dominate and form fine to medium grained massif aggregates or lenses of amphibolites and metagabbros (Fig. 3D). Metraultramafic rocks are punctual, corresponding to pyroxenites (websterites and clinopyroxenites). The presence of vein-like structures with amphibole + chlorite ± epidote suggests hydrothermal alteration, interpreted as skarnitization of metamafic rocks.

Amphibolites and metagabros are characterized by greenish to pale yellow prismatic hornblende (55-45%), subidioblastic plagioclase (35-25%) and hipidioblastic to subidioblastic clinopyroxene (15-10%). Hornblende crystals are usually characterized by a micro-planar fabric constrained by the regional foliation (Fig. 3E). Mg-rich biotite can be present, forming less than 5% of the rock mode. The main structure of these rocks varies from massive to foliate. Lastly, euhedral ilmenite is the main opaque phase. Metapyroxenites consists of augite-hyperstene assemblages with slight uralitization to hornblende. In some samples, garnet might be present and develop corona textures around clinopyroxenes (CPX) ­and/­or plagioclase crystals, suggesting retrograde metamorphism. Similar textures, including symplectites, are described in the Carmo Suite, which is the main Rhyacian mafic-ultramafic set of rocks of the AMT (Santos et al. 2015).

ANALYTICAL PROCEDURES

Laser ablation multi-collector induced coupled plasma mass spectrometry (LA-MC-ICP-MS) analyses were done from zircon grains separated from sample SC-05. This sample was collected just 2,5 km west of Sumé (see Fig. 2). Sample preparation and analysis were done in the facilities of the Geochronology Laboratory of Universidade de Brasília, Brazil. Standard procedures were followed, including crushing and grinding. The heavy minerals were separated using conventional gravimetric and magnetic methods. Zircon crystals were handpicked using binocular microscope and mounted on epoxy resin for isotope ratio acquisition. Data reduction was performed following Bühn et al. (2009) and Matteini et al. (2010). Back scattered electrons (BSE) and scanning electron microscopy (SEM) images were obtained prior to analysis to investigate the internal structure of the grains. Normalization was performed with internal GJ standard zircon (608 ± 1 Ma; Jackson et al. 2004), and the age calculations were performed using in-house developed Excel spreadsheets.

Four samples were selected for the Sm-Nd data acquisition. A thermo triton mass spectrometer using degassed double Re filaments (99.995 % H. Cross^®) at ~ 1,630°C were used in the Laboratory of the Center of Geochronological Researches (CPGeo), of the Universidade de São Paulo, and the applied methodology followed Magdaleno et al. (2017). The ¹⁴³Nd/¹⁴⁴Nd ratios were normalized to the value of ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219 [3], using an exponential law. Ion beam intensities of ¹⁴⁵Nd for JNdi-1 standard were always higher than 1V.

Thirteen representative samples were selected for whole-rock chemical analysis, including six from metagranitic and seven from metamafic rock samples. They were analyzed in the ACME Laboratories (Canada). Major elements were analyzed using borate flux and read by X-ray flourescence (XRF). Trace elements, including rare earths, were determined by Li-metaborate fusion and plasma spectrometry with inductively coupled plasma mass spectrometry (ICP-MS) and atomic emission (ICP-AES).

RESULTS

U-Pb zircon age

The analyzed sample (SC-05) corresponds to a discretely foliated biotite metasyenogranite. The obtained isotopic ratios and apparent ages of each zircon grain are displayed in Table 1. Zircon grains from this sample range between 100 and 150 µm long and are mostly euhedral to subhedral. They are translucent, showing oscillatory zoning and thin rims, representing discrete metamorphic overgrowths. They also present Th/U ratios always higher than 0.1, which supports their magmatic origin. The data plotted in the Concordia diagram yield a concordant age of 1976 ± 23 Ma (95% of concordance), with a mean square weighted deviation (MSWD) of 0.21. The result is interpreted as the crystallization age of the rock protolith (Fig. 4).

Table 1.
Summary of laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) data of zircon grains from sample SC-05.

Grain spot	207 Pb/ 206 Pb ratios	± (1σ)	207 Pb/ 235 U ratios	± (1σ)	206 Pb/ 238 U ratios	± (1σ)	207 Pb/ 206 Pb ages	± (1σ)	207 Pb/ 235 U ages	± (1σ)	206 Pb/ 238 U ages	± (1σ)	Rho	Th/U ratio	Conc. (%)
Z07	0.12	1.96	5.57	3.08	0.34	2.37	1919.71	34.78	1910.91	26.51	1901.76	39.08	0.77	0.23	99.05
Z01	0.13	0.90	6.44	2.41	0.35	2.23	1957.53	16.00	2038.01	21.14	1928.00	37.17	0.93	0.29	96.31
Z30	0.12	1.10	5.65	2.21	0.34	1.92	1962.20	19.50	1924.35	19.07	1885.72	31.34	0.86	0.13	96.06
Z14	0.12	2.30	5.89	3.77	0.35	2.99	1925.52	40.59	1959.29	32.71	1939.67	50.05	0.79	0.13	100.13
Z05	0.13	0.85	5.92	1.39	0.34	1.10	1999.76	15.01	1963.52	12.09	1900.72	18.15	0.78	0.10	94.72
Z02	0.12	1.43	5.82	2.50	0.35	2.05	1971.14	25.30	1950.08	21.64	1924.87	34.05	0.82	0.59	97.59
Z06	0.13	1.37	5.99	2.19	0.35	1.70	1985.71	24.24	1973.78	19.04	1920.78	28.30	0.77	0.61	96.25
Z12	0.12	1.81	5.85	2.73	0.35	2.05	1953.32	32.02	1954.17	23.71	1951.20	34.45	0.76	0.12	99.85
Z04	0.12	2.64	5.86	4.08	0.36	3.11	1937.17	46.51	1955.61	35.37	1969.65	52.74	0.76	0.16	101.49
Z26	0.12	1.07	6.03	1.56	0.36	1.14	1964.25	19.01	1980.03	13.62	1994.45	19.52	0.71	0.10	101.50
Z11	0.12	1.94	6.06	3.22	0.36	2.57	1984.21	34.06	1984.47	28.05	1979.73	43.81	0.80	0.63	99.72
Z09	0.13	2.71	6.31	4.59	0.37	3.70	1974.23	47.59	2019.38	40.20	2010.02	63.85	0.81	17.98	101.22
Z25	0.12	0.80	6.10	1.53	0.36	1.31	1984.43	14.14	1989.95	13.35	1993.79	22.38	0.85	0.08	100.39
Z08	0.12	2.64	6.20	4.02	0.38	3.03	1952.29	46.48	2004.08	35.13	2053.59	53.22	0.76	0.11	105.13
Z27	0.12	1.80	6.20	2.50	0.36	1.75	1998.15	31.57	2004.93	21.90	2005.12	30.07	0.76	1.48	100.02
Z20	0.12	0.60	6.31	2.09	0.37	2.01	2005.95	10.57	2019.15	18.35	2030.50	34.96	0.96	0.24	101.14
Z15	0.12	1.92	6.54	3.10	0.39	2.44	1993.83	33.66	2050.78	27.30	2106.31	43.79	0.78	0.31	105.56
Z18	0.12	1.06	6.61	1.77	0.40	1.42	1958.01	18.79	2060.96	15.64	2164.66	26.16	0.80	0.30	110.51

• Conc.: concordance.

Figure 4.
U-Pb concordia diagram and back scattered electrons (BSE) images of selected zircon crystals with spot positioning of sample SC-05 (biotite metasyenogranite).

Sm-Nd isotopes

Sm-Nd isotope analyses were performed on four representative samples of felsic and mafic rocks, and the results are presented in Table 2. The calculated Nd T_DM model ages following DePaolo (1981), range from 3.2 to 2.2 Ga, indicating involvement of Rhyacian to Mesoarchean sources. Figure 5 shows the εNd (t) evolution of the studied samples based on the U-Pb age of 1.97 Ga. These data are in agreement with calculated model ages, with values ranging from slightly to highly negative (i.e., -0.08 to -10.44).

Table 2.
Summary of Nd isotope data for the metaplutonic rocks of the Sumé region. Samples SC-51, 14, and 05 stand for metagranitic members, whereas SC-78 corresponds to an amphibolite.

Sample	Sm (ppm)	Nd (ppm)	143 Nd/ 144 Nd	εNd (0)	εNd (t)	U-Pb age (Ga)	TDM (Ga)
SC-51	20.05	1,333.5	0.511437	-23.43	-0.08	1.997	2.97
SC-14	1.02	5.17	0.511099	-30.02	-10.44	1.997	2.97
SC-05	7.67	48.01	0.510916	-30.17	-4.89	1.997	2.97
SC-78	5.58	32.21	0.511120	-29.61	-6.40	1.997	2.97

Figure 5.
Nd evolution diagram for the studied rocks in the Sumé region.

Whole-rock geochemistry

The summary of results of whole-rock geochemical data is presented in Table 3. The studied metagranitic rocks present high SiO₂ values that vary from 61.31 to 75.50%. Na₂O values are low to intermediate, ranging from 1.01 to 3.05%, whereas high K₂O contents vary from moderate to high (4.86 to 10.87%), which are typical of evolved granitic and syenitic melts (Fig. 6A). This is in accordance with the obtained petrographic data. They plot in the calc-alkaline field on the AFM diagram and correspond to mainly metaluminous to slight peraluminous magmas (Figs. 6B and 6C). P₂O₅ and TiO₂ contents are low, ranging from 0.04 to 0.14% and 0.04 to 0.65%, respectively.

Table 3.
Major (wt. %) and trace element (ppm) concentrations of studied metaigneous rocks, Alto Moxotó Terrane, NE Brazil.

Sample	SC05*	SC14*	SC27*	SC51*	SC71*	SC135A*	SC05B**	SC14**	SC34**	SC37**	SC71A**	SC78**	LSM75**
Major elements (wt.%)
SiO2	72.11	69.51	61.31	75.49	72.37	68.54	50	46.83	48.07	47.43	51.03	47.63	49.9
Al2O3	15.29	14.17	15.9	13.03	12.91	14.84	6.76	10.26	5.64	7.08	8.87	6.12	3.18
CaO	1.22	1.51	3.3	1.34	4.9	2.95	22.11	18.53	22.7	22.08	19.78	22.82	20.7
Fe2O3	0.051	0.82	2.06	1.19	1.09	2.18	5.9	11.13	6.78	7.91	6.53	8.54	9.43
FeO	0.045	0.74	1.85	1.07	0.98	1.96	5.31	10.02	6.1	7.12	5.88	7.69	8.48
K2O	6.73	10.46	10.87	4.76	4.86	5.62	0.19	1	0.13	0.14	0.99	0.17	0.19
MgO	0.08	0.22	1.47	0.06	0.59	0.95	11.82	8.23	12.71	9.97	7.44	11.63	13.4
MnO	0.01	0.01	0.05	0.02	0.03	0.04	0.29	0.33	0.35	0.44	0.21	0.3	0.62
Na2O	2.84	1.01	1.85	3.05	1.42	2.81	0.67	1.14	0.38	0.87	1.25	0.35	0.58
P2O5	0.05	0.06	0.3	0.03	0.22	0.18	n.d.	0.13	0.02	0.02	1.77	n.d.	0.01
TiO2	0.04	0.65	0.07	0.06	0.06	0.14	0.25	0.53	0.38	0.24	0.3	0.54	0.26
Total	98	99	99	100	99	100	103	108	103	103	104	105	106
Trace elements (ppm)
Ba	835	1917	2534	91	2317	1749	33	267	343	120	458	270	121
Ce	75	50.8	130.8	17.1	44.2	145.5	34.5	74	57.1	34	166.1	24.5	202.4
Dy	0.9	1.7	1.7	0.7	1	2.3	2.2	4	2.8	1.1	3.2	2.7	4.8
Er	0.3	0.9	0.6	0.4	0.5	1.1	1.1	1.9	1.2	0.5	1.7	1.4	2.3
Eu	1	0.7	0.5	0.4	0.4	1.2	0.6	0.9	1.2	1.4	1.1	0.5	1.7
Ga	16.5	11.7	14.2	15.3	13	17.3	12.5	16.1	11.3	13.6	18.1	8.9	9.1
Gd	1.5	2.1	2.9	0.7	1.7	3.4	2.7	4.9	4.3	1.1	4.6	2.3	6.4
Hf	0.3	8.2	1	1.5	1.1	2.3	4.2	1.8	2.3	3.5	2.1	1.7	1.8
Ho	0.1	0.3	0.3	0.1	0.2	0.4	0.4	0.8	0.5	0.2	0.6	0.5	0.9
La	74.3	21.6	44.2	9.2	25.7	88	12.9	32.4	50.2	17	81.1	10.7	94.4
Lu	0.1	0.1	0.1	0.1	0.1	0.1	0.2	0.3	0.2	0.1	0.3	0.2	0.3
Nb	1.2	3.2	2.8	4.4	1.9	2.9	2.2	6.8	1.6	0.7	5	0.7	0.4
Nd	29.5	20.7	36.3	6.5	17	48.1	20.2	32.1	46.5	12.6	58.5	14.4	68.3
Pr	9.9	5.6	10.3	1.8	4.9	15.2	4.9	8.7	12.5	13.7	16.6	3.4	20.2
Rb	200.4	285.5	354	82.8	106.3	98.6	4.5	30.8	4	3.5	21.1	6.7	3.7
Sm	3.1	3.4	5.4	1.1	2.4	5.8	3.7	6.1	6.9	2.1	7.5	2.7	9.7
Sn	n.d.	6	1	n.d.	n.d.	1	5	3	3	3	3	7	4
Sr	182.4	267.8	322.8	112.9	734.6	400.3	94.2	233.9	122.2	222	866.3	173.5	71.6
Ta	0.1	0.8	0.3	0.3	0.1	0.1	0.6	0.8	0.3	0.1	0.5	0.2	0.1
Tb	0.2	0.3	0.4	0.1	0.2	0.5	0.4	0.7	0.6	0.2	0.6	0.4	0.9
Th	4	16.3	13	4	8.3	28.7	2.3	3	1.7	1.2	4.44	0.9	1.8
Tm	0.1	0.2	0.1	0.1	0.1	0.1	0.2	0.3	0.2	0.1	0.3	0.2	0.3
U	0.4	1.7	1	1.3	0.8	0.8	0.7	0.8	0.3	0.1	3.7	0.6	0.8
V	n.d.	17	9	n.d.	35	40	45	91	50	65	61	150	88
Y	3.9	9.4	7.1	3.4	5.4	10.5	11.2	22.1	12.8	5.4	16.5	12.6	24.8
Yb	0.2	1	0.4	0.5	0.5	0.1	1.1	1.9	1.1	0.6	1.7	1.4	2.1

• n.d.: not detected; *: metagranitic rocks; **: metamafic rocks.

Figure 6.
Major element plots for the studied samples (data from Table 3). (A) TAS diagram (Na₂O + K₂O vs. SiO₂), from Le Bas et al. (1986); (B) AFM diagram from Irvine and Baragar (1971); (C) A/NK vs. ASI diagram, from Maniar and Picolli (1989), using Shand’s index. Blue circles are metagranitic rock samples, and green circles represent metamafic rock samples.

Metamafic samples are homogeneous and basic. SiO₂ content ranges from 46.8 to 51.0%. They also present low K₂O and Na₂O, which are 0.19 to 1% and 0.35 to 1.25%, respectively. These values are compatible with magmas that present gabbroic/basaltic composition (Fig. 6A). They are characterized by Al₂O₃ ranging from 3.18 to 10.26%, high CaO (18.5 - 22.8 %) and moderate to high contents of FeO, ranging from 5.3 to 10%, which is consistent with average values of magmas of the toleiitic series (Fig. 6B). TiO₂ and P₂O₅ ranges from 0.25 to 0.53 and 0.01 to 1.77%, respectively.

Trace elements of the metagranitic rocks show an enriched large ion litophile elements (LILE) pattern normalized to chondrite values of Nakamura (1974), exhibiting a discrete positive peak of Rb and depletion of high field strength elements (HFSE), such as Nb and Ta. Other characteristic negative anomalies are marked by P and Ti (Fig. 7A). Rare earth elements (REE) contents are moderate to high (ΣREE = 38.8 to 311.8), showing enrichment of light rare earth elements (LREE), indicated by the (La/Yb)_N ratio values that range from 12.27 to 247.67, and associated Eu anomalies (Eu/Eu* ratios ranging between 0.39 to 1.43; Fig. 7B).

Figure 7.
Chondrite-normalized spider diagrams (A) and (C) and rare earth elements (REE) patterns for the studied rocks (B) and (D). Standardization followed Nakamura (1974) normalizing values. Symbols are as in Figure 6.

The incompatible element spidergram normalized to chondrite values of Nakamura (1974) for the mafic rocks presents moderate to low values of LILE and prominent negative Nb, Ta, Sr and P anomalies in most of the analyzed samples. A minor negative Ti anomaly and positive peaks of La and Ce are observed (Fig. 7C).

Total REE contents ranges from 65.3 to 414.7 ppm, and the REE patterns normalized to chondrite values of Nakamura (1974) are steep, showing moderate to high concentrations of LREE in relation to the heavy rare earth elements (HREE), with (La/Yb)_N ratios ranging from 5.10 to 31.80. The samples are marked by a negative Eu anomaly (­Eu/­Eu* ~ 0.60 on average), except for sample SC37 (Fig. 7D).

In tectonic discriminant diagrams, metagranitic rocks share similarities with volcanic-arc related granites (Fig. 8A). The same is reflected on the classical discriminant diagram from Pearce et al. (1984), in which the studied samples plot both on volcanic arc granites (VAG) and syn-collisional (syn-COLG) fields (Fig. 8B). In addition, the distribution of trace elements for mafic members shows similar tectonic affinities. On the tectonic Hf/3-Th-Ta plot from Wood (1980), the samples share similarities with Arc-related basalts (Fig. 9A), also observed on the Nb/Th plot from Jenner et al. (1991) (Fig. 9B). Besides, on the DF1-DF2 diagrams proposed by Agrawal et al. (2008), samples correspond entirely to melts derived from island arc basalts (Figs. 9C and 9D).

Figure 8.
Discriminant diagrams of the studied rocks. (A) Hf-Rb/30-3Ta plot after Harris et al. (1986); (B) Y vs. Nb after Pearce et al. (1984). Fields indicate syn-collisional granites (syn-COLG), within-plate granites (WPG), volcanic arc-related granites (VAG) and ocean-ridge granites (ORG).

Figure 9.
Discriminant diagrams of the studied rocks of the Sumé Complex. (A) Hf/3-Th-Ta plot after Wood (1980), Nb/Th vs. Y plot after Jenner et al. (1991); (B) and (C) discriminant functions DF1 vs. DF2 plot from Agrawal et al. (2008). Fields indicate mid-ocean ridge basalts (MORB and N-MORB), enriched mid ocean ridge basalts (E-MORB), ocean island basalts (OIB), island arc basalts (IAB) and continental rift basalts (CRB). Symbols are as in Figure 6.

DISCUSSION

Magmatism and tectonic setting

New petrographic, isotope and geochemical data of metagranoids and metamafic rocks provide clues on the tectonic evolution of the AMT, Borborema Province and early crustal processes of Western Gondwana. The metagranitoids derived from calc-alkaline and metaluminous to peraluminous magmas that are mostly monzo- and syenogranitic in composition. On the other hand, associated metamafic rocks are mostly amphibolites and fine-grained metagabros with toleiitic affinity. Lesser occurrences of metaultramafic rocks in the area are mostly represented by clinopyroxenites and websterites.

The dated sample of a metagranitic member yields an age of 1.97 Ga, which is interpreted as the crystallization age of its protolith. The Nd isotopic composition is variable for felsic and mafic members, being characterized by T_DM model ages, suggestive of the involvement of Paleoproterozoic and Archean components in the magma genesis, whilst the calculated εNd_(1.97) is dominantly negative. These data indicate that the generation of the studied precursor magmas were coeval to crust reworking of the AMT. Indeed, rocks with slight similar age and isotope characteristics described within the terrane are mostly associated with crustal recycling with minor juvenile inputs (Santos et al. 2015 and references therein).

In both groups of rocks, trace elements signature share similarities with classical Phanerozoic “Arc-related magmas.” Hence, we suggest that they were emplaced in a subduction-related setting. Our statement is mostly based on:

1. enrichment of LILE and depletion of HFSE;

2. prominent negative anomalies, such as Nb, Ta, besides low P and Ti;

3. the “arc-like to syn-collisional signature” recorded on discriminant diagrams (e.g., Pearce 1982, Pearce et al. 1984, Harris et al. 1986, Agrawal et al. 2008).

Such pattern clearly characterizes depletion of HFSE in the source region, which can be explained by their low mobility compared to lithophile elements (Tatsumi et al. 1986). This geochemical behavior of trace elements is usually attributed to fluids released during dehydration of the subducting oceanic slab that migrates into the mantle wedge promoting partial melting by depressing solidus temperature (Tatsumi & Eggins 1995, Tatsumi 2005).

On the other hand, the depletion on Nb-Ta-Ti can also reflects the presence of rutile, sphene or Ti-bearing amphibole in the dehydrating slab during subduction, whereas significant depletion of HREE might also reflect certain amount of garnet as a residual phase (Foley et al. 2000, Klemme et al. 2006). Also, these geochemical features support fluid transport of trace elements under low pressure conditions (Baier et al. 2008). In addition, negative and positive Eu anomalies are consistent with residual plagioclase in the source and anomalous accumulation of this mineral in the melt, that can be ascribed to fractional crystallization.

Similar petrographical and geochemical aspects are also observed in several metaplutonic rocks attributed to a major Rhyacian-Orosirian orogeny described for the AMT (e.g., Brito Neves 2011, Brito Neves et al. 2014, Santos et al. 2017b). Indeed, the geochemical and isotopic signatures of the studied metamafic rocks are very similar to those from Carmo mafic-ultramafic suite, investigated just a few kilometers from the study area (Santos et al. 2015). On the other hand, high SiO₂, Al₂O₃ and K₂O contents of the studied granitic rocks suggest a more evolved stage of the magmatic arc. Such parameters are adequate for such interpretations, once that their protoliths were already potassic in nature (K-feldspar-rich members) and might explain the obtained younger age (c. 1.97 Ga). In addition, some samples plot in the syn-collisional field from Pearce et al. (1984) and are slightly peraluminous, indicating that they also might be associated with a final stage of continent-continent collision. Such hypothesis is in agreement with the late Paleoproterozoic obtained U-Pb age and the evidence of crust reworking provided by Nd data (Floresta Suite, Pedra d’Água Suite, Cabaceiras Complex, among others; Santos et al. 2017a and references therein). However, nature and conditions of continent-continent collision in the AMT still needs further investigations and must be expanded to other portions of the domain, mainly due to the scarcity of geochronological and isotope data.

Lastly, based on the combination of our data and those from the literature, we conclude that, after a major Neoarchean crustal growth event (Neves et al. 2015, Santos et al. 2017b and references therein), the AMT was strongly affected by a Paleoproterozoic accretion-collisional event that reworked previous crust. This event is characterized by the development of a long-lived continental magmatic arc, in which the studied potassic rocks record an evolved stage of it. Finally, we suggest that such magmatism spanned the Rhyacian and the Orosirian periods (i.e., 2.15 - 1.97 Ga).

Stratigraphic implications and regional correlations

The studied rocks crop out in the type area of the Sumé Complex, described by Medeiros and Torres (2000) as a Neoproterozoic unit based on the available U-Pb from Silva et al. (2002). Rocks attributed to this sequence cover a large petrographic range (orthogneisses, migmatites, mafic rocks, paragneisses, schists and calc-silicate rocks) and are widespread throughout the AMT, being dubiously placed in the Neoproterozoic. However, in this study, as mentioned, a metagranitic rock that is attributed to this complex yielded a Paleoproterozoic age (ca. 1.97 Ga), which is coherent with several recently published geochronological/isotopic data available for the region that stands for the predominance of Paleoproterozoic crust (Neves et al. 2015, Santos et al. 2015, 2017b).

Due to the abundance of Rhyacian/Orosirian ages for the major units of the AMT in the recently published studies, besides the new U-Pb age obtained in this study, we suggest that the use of the term Sumé Complex as a Neoproterozoic unit should be avoided. The duality of crustal ages within the AMT is not only found in these rocks. For instance, the Sertânia Complex (garnet-bearing paragneisses, garnet-sillimanite-biotite schists and migmatites) is another important unit exposed in the terrane, considered Paleoproterozoic in age by many authors, including Santos et al. (2004). Recently, Neves et al. (2017) documented Neoproterozoic zircon grains in metasedimentary rocks attributed to this unit, interpreting them as the maximum deposition age of the sedimentary pile. Hence, one must consider that it is possible that Neoproterozoic sequences can be distributed and deposited throughout major Paleoproterozoic units that formed the basement of the Borborema Province, being later deformed and metamorphosed together during the Brasiliano Orogeny, further complicating the original stratigraphic relations.

Although Rhyacian rocks are common and widespread in the Atlantic shield (in effect composing the majority of it), Orosirian rocks, on the other hand, are quite scarce, and might represent a magmatic event that was not previously considered in its full importance until recent times. In effect, in the southern Borborema Province, in the basement of the Rio Preto Fold belt, Caxito et al. (2015) recognized the first well described Orosirian rocks of the region, amphibolites which were then interpreted as developed in a supra-subduction zone setting, much like our interpretation for the Sumé Complex. The recognition of Orosirian fragments in both the southern and central Borborema Province might provide clues to a previously undetected important tectonic scenario, which deserves further studies and investigation.

On the other hand, older correlatives of the studied rocks are widespread in the province, Pan African Mobile belts and adjoining cratons. In central Borborema Province, they can be related to other subduction-related suites and coeval metamorphism. For instance, in other areas of the Transversal sub-province, Neves et al. (2015) described juvenile inputs of ca. 2.1 Ga, followed by 2.0 to 1.98 Ga continent-continent collision, crustal reworking and coeval metamorphism. A similar scenario is also observed in the northern sub-province: Rhyacian to Orosirian metagranitic rocks of the Troia Massif are related to accretionary and collisional tectonics at 2.1 and 2.0 Ga, respectively (Costa et al. 2015), whereas Paleoproterozoic reworking is also present in preserved Archean domains, such as the São José do Campestre (Dantas et al. 2013) and Granjeiro terrane (Silva et al. 2014).

Reworking of Archean basement resulted from collisional magmatism and coeval metamorphism is also recorded on metagranitic and gneissic-migmatitic suites of the São Francisco-Congo and São Luis-West Africa cratons (Barbosa & Sabaté 2004, Klein et al. 2012, among others). In the Pan-African mobile belts, preexisting crust, including TTG-like rocks, calc-alkaline and potassic rock suites are also widespread, mostly interpreted as originated via subduction-related tectonics. In most cases, such associations have been referred to be the result of long-lived Paleoproterozoic magmatism attributed to the Eburnean orogeny, similar to that observed within the AMT (e.g., Penaye et al. 2004, Dada 2008, Baratoux et al. 2011, Block et al. 2016).

CONCLUSIONS

The main conclusions of this paper are:

1. Metaigneous rocks that occur in the Sumé region comprise monzogranites, syenogranites, alkali-feldspar granites, amphibolites, fine-grained metagabros and minor pyroxenites. These rocks have been recently attributed to the Neoproterozoic Sumé Complex. However, our obtained U-Pb age of 1.97 Ga of a metasyenogranitic rock, as well as similar geochemical and isotopic characteristics of the studied mafic rocks and other Paleoproterozoic suites within the terrane, indicates that this set of rocks is in fact Rhyacian to Orosirian in age;

2. Based on geochemical and Sm-Nd isotope characteristics, we suggest that such rocks are associated with a major Paleoproterozoic orogeny (ca. 2.1 - 1.9 Ga), that is described in the central Borborema Province. The magmagenesis involved reworking of previously formed crust, with sources that range from to Mesoarchean to early Paleoproterozoic;

3. The obtained results are compatible with other Paleoproterozoic belts of the Borborema Province, adjoining cratons and West African mobile belts underlying Nigeria and Cameroon. Such correlations demonstrate the importance in investigating Paleoproterozoic crustal segments within Neoproterozoic belts of Western Gondwana, providing clues for paleogeographic reconstructions. However, Orosirian magma genesis is quite scarce, and its significance must be considered in regional correlations/interpretations.

ACKNOWLEDGEMENTS

One of the major retirement plans of Edilton José dos Santos was to write a manuscript concerning the nature of the Sumé Complex and its role in the AMT evolution. Unfortunately, he did not live enough to it, and Lauro Cézar Montefalco de Lira Santos felt that he had to move on with this topic. The final result is this paper, which is Lira Santos’ personal tribute to him. Christian Carmona, Cesar Verissimo and Adauto Neto are thanked for the geochemical data concession. Elton Dantas and Benjamin Bley are equally thanked for providing facilities on isotope data acquisition, and Reinhardt Fuck and Geysson Lages are thanked for early discussion. The authors also would like to express gratitude to Editor Claudio Riccomini, as well as Fabricio Caxito and the anonymous reviewers for their criticism and suggestions that helped improve the original manuscript. This is a late contribution to the Project O Magmatismo Anorogênico Pré-Cariris Velhos na Região de Sumé e Camalaú (PB) e seu significado na Evolução da Província Borborema, granted to Edilton José dos Santos in 2008 by the Brazilian Council for Scientific and Technological Development (CNPq).

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ARTICLE INFORMATION

• 2
  Manuscript ID: 20180083.

• 3
  E. J. S. wrote a preliminary draft of the manuscript, collected the data and formulated the former ideas regarding the geology of the Sumé area and the AMT as well. L. C. M. L. S. contributed with the figures, tables, data interpreting, final geological model, reshaping of the text, as well as took care of all the submitting phases, once that, unfortunately, the first author passed away early in the submission year.

Publication Dates

• Publication in this collection
  08 Apr 2019
• Date of issue
  2019

History

• Received
  26 July 2018
• Accepted
  07 Dec 2018