Sources of granite magmatism in the Embu Terrane (Ribeira Belt,Brazil): Neoproterozoic crust recycling constrained by elemental and isotope (Sr-Nd-Pb) geochemistry

Whole rock elemental and Sr–Nd isotope geochemistry and in situ K-feldspar Pb isotope geochemistry were used to identify the sources involved in the genesis of Neoproterozoic granites from the Embu Terrane, Ribeira Belt, SE Brazil. Granite magmatism spanned over 200 Ma (810–580 Ma), and is dominated by crust-derived relatively low-T (850–750 °C, zircon saturation) biotite granites to biotite-muscovite granites. Two Cryogenian plutons show the least negative εNd_t (−8 to −10) and highest mg# (30–40) of the whole set. Their compositions are strongly contrasted, implying distinct sources for the peraluminous (ASI ∼ 1.2) ∼660 Ma Serra do Quebra-Cangalha batholith (metasedimentary rocks from relatively young upper crust with high Rb/Sr and low Th/U) and the metaluminous (ASI = 0.96–1.00) ∼ 630 Ma Santa Catarina Granite. Although not typical, the geochemical signature of these granites may reflect a continental margin arc environment, and they could be products of a prolonged period of oceanic plate consumption started at ∼810 Ma. The predominant Ediacaran (595–580 Ma) plutons have a spread of compositions from biotite granites with SiO₂ as low as ∼65% (e.g., Itapeti, Mauá, Sabaúna and Lagoinha granites) to fractionated muscovite granites (Mogi das Cruzes, Santa Branca and Guacuri granites; up to ∼75% SiO₂). εNd_T are characteristically negative (−12 to −18), with corresponding Nd T_DM indicating sources with Paleoproterozoic mean crustal ages (2.0–2.5 Ga). The Guacuri and Santa Branca muscovite granites have the more negative εNd_t, highest ⁸⁷Sr/⁸⁶Sr_t (0.714–0.717) and lowest ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb, consistent with an old metasedimentary source with low time-integrated Rb/Sr. However, a positive Nd–Sr isotope correlation is suggested by data from the other granites, and would be consistent with mixing between an older source predominant in the Mauá granite and a younger, high Rb/Sr source that is more abundant in the Lagoinha granite sample. The Ediacaran granites are coeval with profuse granite magmatism attributed to continental arc magmatism in northern Ribeira and Araçuaí belts. However, their evolved compositions with low mg# and dominantly peraluminous character are unlike those of magmatic arc granites, and they are more likely products of post-collisional magmatism or correspond to an inner belt of crust-derived granites.     

胶北地区粉子山群碎屑锆石U-Pb年龄、Hf同位素及其对胶-辽-吉带构造演化的制约

胶北地区粉子山群和荆山群是胶-辽-吉带重要的组成部分, 对粉子山群和荆山群中碎屑锆石的年龄分布和Hf同位素特征的研究, 有助于深入认识胶-辽-吉带古元古代构造演化过程。对粉子山群小宋组、张格庄组和巨屯组中碎屑锆石U-Pb定年结果表明, 其锆石谐和年龄分布在1842~3663Ma之间, 其中13颗锆石的变质边给出了1842~1877Ma的变质年龄, 指示粉子山群经历了古元古代晚期变质作用, 与荆山群变质作用时代一致。本次研究的三个组中都存在2.2~2.1Ga的碎屑锆石, 结合前人研究成果, 粉子山群主体应沉积于2.1~1.9Ga。粉子山群碎屑锆石的年龄峰和ε_Hf(t)值与胶北地区早前寒武纪岩浆岩基本一致, 表明其碎屑物质很有可能来自胶北地体。另外在巨屯组样品中发现约17%的新太古代晚期碎屑锆石具有明显偏低的ε_Hf(t)值(-5.79~-2.44), 可能由早期岩浆岩部分熔融而成, 这也暗示胶北~2.5Ga不仅发生大规模的初生地壳生长, 还存在早期地壳的重熔。粉子山群和荆山群碎屑锆石年龄分布特征基本一致, 均含有胶北特征的年龄峰(~2.7Ga和~2.9Ga), 结合二者空间分布特征、一致的变质作用时代以及顺时针的P-T-t轨迹, 本文认为粉子山群和荆山群可能均沉积于胶北地体一侧, 并随胶北地体向东南俯冲。

     

Petrogenesis of the Orthogneisses of the Mantiqueira Complex, Central Ribeira Belt, SE Brazil: An Archaean to Palaeoproterozoic Basement Unit Reworked During the Pan-African Orogeny

The Occidental terrane of the central segment of the Brasiliano-Pan-African Ribeira belt comprises two crustal scale thrust sheets (Andrelândia and Juiz de Fora domains) taken as reworked Neoproterozoic products of the São Francisco cratonic margins. Pre-1.8 Ga orthogneisses and associated rocks of the Mantiqueira Complex comprise the basement for rocks of the Andrelândia Depositional Cycle within the Andrelândia tectonic domain. Geochemical data indicate that the Mantiqueira Complex comprises rocks that can be grouped as follows: intermediate to acid calc-alkaline rocks and a transitional basaltic series. On the basis of quantitative analysis of the lithogeochemical data, these lithotypes cannot be related. Statistical and/or petrological criteria made it possible to define suites and/or groups within each one of those units and to constrain petrogenetic models based mostly on their REE data. Simple least-square regression analysis indicates that the basic rocks are unlikely to constitute a single suite themselves. The results of the geochemical modelling presented in this work suggest that crustal partial melting rather than fractional crystallisation is the most likely petrogenetic process associated with the rocks of the Mantiqueira Complex. The partial melting processes might have taken place under oxidising conditions, typical of tectonic settings associated with the generation of calc-alkaline rocks.     

The Juiz De Fora Granulite Complex of the Central Ribeira Belt, SE Brazil: A Paleoproterozoic Crustal Segment Thrust During the Pan-African Orogeny

The granulites of the Juiz de Fora complex occur within thick basement thrust slices associated with the Pan-African shortening process in the central segment of the Ribeira belt. Five lithological units of the Intermediate tectonic domain of the belt can be identified on the basis of detailed geological mapping: a) orthogranulites, b) orthogneisses; c) kinzigite; d) intrusive garnet charnockite and e) amphibolite facies metasediments of probable Meso to Neoproterozoic age, correlated to the cover of the belt. Petrological data indicate high temperatures and intermediate to low lithostatic pressure conditions for the Paleoproterozoic granulite facies metamorphism. Textures and CO₂-rich fluid inclusions are probably related to an IBC path. Geochemical data do not show relevant compositional change as a result of the granulite metamorphism. Two calc-alkaline suites and tholeiitic to alkaline basic rocks can be related to compressional and extensional settings, respectively. The overall composition of the granulites, the lack of substantial LILE depletion as well as the composition of the fluid inclusion points to granulitization process driven by CO₂-rich fluids. Orthogranulites gave rise to banded gneisses as a result of the Pan-African retrograde metamorphism and intense deformation. The U and Th depletion detected in few rocks is possibly related with the hydrated conditions of the retrograde reactions.     

Petrology,geochemistry, and zircon U-Pb geochronology of the Zambezi Belt in Zimbabwe: Implications for terrane assembly in southern Africa

The Zambezi Belt in southern Africa has been regarded as a part of the 570-530 Ma Kuunga Orogen formed by a series of collision of Archean cratons and Proterozoic orogenic belts.Here,we report new petrological,geochemical,and zircon U-Pb geochronological data of various metamorphic rocks(felsic to mafic orthogneiss,pelitic schist,and felsic paragneiss) from the Zambezi Belt in northeastern Zimbabwe,and evaluate the timing and P-T conditions of the collisional event as well as protolith formation.Geochemical data of felsic orthogneiss indicate within-plate granite signature,whereas those of mafic orthogneiss suggest MORB,ocean-island,or within-plate affinities.Metamorphic P-Testimates for orthogneisses indicate significant P-T variation within the study area(700-780 C/6.7-7.2 kbar to 800-875 C/10-11 kbar) suggesting that the Zambezi Belt might correspond to a suture zone with several discrete crustal blocks.Zircon cores from felsic orthogneisses yielded two magmatic ages:2655±21 Ma and 813士5 Ma,which suggests Neoarchean and Early Neoproterozoic crustal growth related to within-plate magmatism.Detrital zircons from metasediments display various ages from Neoarchean to Neoproterozoic(ca.2700-750 Ma).The Neoarchean(ca.2700-2630 Ma) and Paleoproterozoic(ca.2200-1700 Ma) zircons could have been derived from the adjacent Kalahari Craton and the Magondi Belt in Zimbabwe,respectively.The Choma-Kalomo Block and the Lufilian Belt in Zambia might be proximal sources of the Meso-to Neoproterozoic(ca.1500-950 Ma) and early Neoproterozoic(ca.900-750 Ma) detrital zircons,respectively.Such detrital zircons from adjacent terranes possibly deposited during late Neoproterozoic(744-670 Ma),and subsequently underwent highgrade metamorphism at 557-555 Ma possibly related to the collision of the Congo and Kalahari Cratons during the latest Neoproterozoic to Cambrian.In contrast,670-627 Ma metamorphic ages obtained from metasediments are slightly older than previous reports,but consistent with~680-650 Ma metamorphic ages reported from different parts of the Kuunga Orogen,suggesting Cryogenian thermal events before the final collision.