Ultrahigh-pressure pyrope-kyanite granulites and associated eclogites in Neoproterozoic nappes of southeast Brazil
(2) Departamento de Mineralogia e Petrologia Ígnea, Universidade do Estado do Rio de Janeiro (DMPI/FGEL/CTC/UERJ). Rua São Francisco Xavier 524, Sala 4005, Bloco A, Maraca, Rio de Janeiro, CEP 20559-900, Brasil.
(3) Geoscience Research Group, Japan Nuclear Cycle Development Institute (JNC), Mizunamishi, Gifu, 509-6132 Japan.
(4) Department of Earth & Planetary Sciences, Tokyo Institute of Technology, Meguro, Tokyo, 152-8551 Japan.
Neoproterozoic nappes in Minas Gerais State, SE Brazil contain some of the few recognized eclogites in the South American continent. The Tres Pontas, Varginha and Andrelandia nappes constitute a 4-5 km thick sequence of shallow-dipping high-pressure and high-grade rocks of continental parentage disposed between overlying high-grade granulites, migmatites and granites of the Socorro-Guaxupe terrane (Rio de la Plata plate) and underthrust, feebly metamorphosed metasediments atop the Sao Francisco craton (Fig. 1). They were recrystallized in a major Pan-African continental collision zone at 640-630 Ma. The nappes comprise coarse-grained kyanite-garnet granulite with intercalations of impure quartzite, calc-silicate rocks, and minor lenses and sills of mafic-ultramafic rocks, including eclogite. Spatial dimensions, thermobaric structure, orogen-scale architecture and tectonic development of these nappes (e.g. Campos Neto, 2000) display gross, but striking similarities with the Nepalese Himalaya.
Previous petrological research applying KFMASH and CMFASH systems and conventional cation exchange geothermobarometry on these rocks has established that amphibolite/granulite re-equilibration occurred at around 800-900C at 12-13 kbar, 600- 750C at 10-15 kbar and and 600-650C at 12-14 kbar for the Varginha, Tres-Pontas and Andrelandia nappes, respectively (Campos Neto, 2000; Campos Neto and Caby, 2000). However, all previous researchers have noted that some constituents, especially the metabasites, contain evidence of an earlier, higher pressure stage of eclogite facies metamorphism. Kyanite-garnet metapelites (possibly whiteschists) are interpreted to have originally recrystallized under eclogite facies conditions and contain pseudomorphs after CaNa pyroxene. Strongly retrogressed eclogite boudins within paragneiss occur within the Andrelandia nappe in four locations (Passos, Pouso Alegre, Andrelandia and Baependi), and comprise pyrope-almandine, rutile, retrograde hornblende and plagioclase, and patchy diopside-plagioclase symplectites. Through integration of precursor compositions, the latter are interpreted to have recrystallized from primary omphacite during granulite facies metamorphism and minimum pressures of 15 kbar for the eclogite stage have been suggested (Choudhuri et al., 1978). Furthermore, Campos Neto & Caby (1999) reported inclusions of omphacite and jadeitic pyroxene (up to 76 mol% jadeite component) in garnets in eclogite of the same nappe, indicating minimum pressures of 17.5 kbar (for a T fixed at 660C).
On this basis we speculated that, like the correlative coesite-bearing eclogites of the PanAfrican nappes of northern Mali, at least some components of the Brazilian nappes had experienced ultrahigh-pressure metamorphism, that had been subsequently overprinted by a high-grade granulite recrystallization at more shallow levels during exhumation in the collision zone. Therefore, we collected around 300 representative samples from all three nappes in November 2000, in order to study mineral zonation, symplectites, exsolution and mineral inclusion compositions and textures, as well as structural and microstructural characteristics of the nappes.
Rocks of the Tres Pontas nappe, from the Tres Pontas quarry, where it is best exposed, consist predominantly of lensoidal quartzo-feldspathic leucosomes within kyaniterich aluminous granulite. Rare ultramafic lenses consist of phlogopite- orthopyroxene rocks and garnet clinopyroxenite. Cores of boudins of the granulite retain an early assemblage of almandine-pyrope + quartz + K-feldspar + plagioclase + kyanite + rutile +- muscovite. Remarkably, some of the larger garnets (> 5 mm) are strongly compositionally zoned with up to 15 mol% spessartine component in the cores (c. alm38.prp40.grs07.sps15) and up to 60 mol% pyrope in the mantles (c. alm3l.prp60.grs08.sps01). Thin rims generally display a decrease in the almandine component to (c. alm50.prp45.grs04.sps01). Spessartine-rich cores preserve a "fossilized" low-grade inclusion micro-assemblage of graphite, quartz, pyrophyllite calcite, ilmenite and abundant fluid inclusions. Garnet mantles contain inclusions of quartz, rutile, apatite, phengite, kyanite, monazite and a sodic clinopyroxene. Quartz inclusions (20-100 micro meter) within the mantle regions of the garnets are invariably surrounded by intense radial fractures (Fig. 2A), suggestive of the former presence of coesite.
Although most inclusions are monocrystalline, some have quasi-palisade textures. Coesite has not yet been detected as inclusions in Tres Pontas granulites. Some smaller almandinerich (60-80 mol%) garnets are relatively compositionally homogeneous and display dense distribution of rutile rods (5-100 pm in length) in core and mantle regions in 3 distinct orientations. The rutile has a clear topotaxial relationship with the host and is interpreted to have exsolved from the garnet. In addition, shorter prismatic rods of clinopyroxene (of indeterminant composition) and apatite (~ 10-20 micro meter in length) are dispersed throughout the garnets, reminiscent of the exsolved rutile + apatite + clinopyroxene described by Ye at al. (2000) in UHP eclogite from the Dabie terrane. Those authors suggested phosphorus may enter the garnet structure through the coupled substitution PIV + NaVIII = SiIV + CaVIII and that (CaNa2)Ti2Si3O2 and (Ca2Na)(AITi)Si3O2 are probably significant Ti and Na bearing components in garnet under UHP conditions. However, from the textural evidence in the Tres Pontas rocks it is, as yet, unclear whether the apatite and clinopyroxene rods are also exsolved phases, or merely inclusions. Similar garnet granulites and other orthogneisses also occur in the Varginha nappe. Granulites from near Varginha city contain the assemblage: quartz + plagioclase + kyanite + hypersthene + diopside + K-feldspar +- biotite +- garnet, and some remarkable exsolution textures and inclusions. Some rocks contain appreciable quantities of REE-bearing accessory minerals (particularly zircon, apatite and monazite), with apatite constituting up to 1-2 vol%. Zircon, which may attain a size of 500 pm, generally contains numerous micro-inclusions. One large composite inclusion consists of K-feldspar + kyanite + coesite.
Spherules of coesite were also detected in several other zircon grains of the same rock (VCP33; Fig. 2B) The coesite was confirmed by its characteristic Raman spectrum with a strong peak at 522 cm-1. The cores of many apatite grains display abundant oriented (parallel to c-axis of the apatite) rods of monazite, and occasionally, composite rods of monazite + thorite (ThSi04) and monazite + quartz. Some apatites also have thin overgrowths of monazite at their rims.
Monazite rods are 5-100 micro meter in length and up to 5 micro meter in width. Since there is little doubt that these are exsolved phases, the precursor Varginha apatites clearly contained appreciable LREEs, MREES, Th and Si. It is well known that REEs can substitute for Ca in the apatite structure through the coupled substitution Ca 2+ + P5+ = REE3+ + S14+, but there are very few experimental constraints on the pressure- and/or temperature-dependent solubility of these elements. It is likely that REEs and Si may be present in apatite as the lessingite component (Ca2REE3(SiO4)3OH). Exsolved monazite rods in apatite have previously been described from UHPM rocks of the Kokchetav massif of Kazakhstan and the Dabie Terrane of China (e.g. Zhang & Liou, 1999).
Commonplace mymerkitic and antiperthitic textures are exhibited by feldspars in the Varginha rocks. The cores of large (> 5 mm) K-feldspar porphyroclasts also contain unusual exsolution textures. These comprise rods of rutile and platelets of ilmenite. Although substitutions of a considerable number of elements into the K-feldpar structure and their subsequent exsolution during cooling and/or decompression are relatively well known, exsolution of rutile + ilmenite has not, as far as we are aware, been previously reported. At present, the significance of and factors for enhanced Ti solubility in K-feldspar are unknown. Eclogitic rocks and garnet pyroxenites occur within the cores of mafic lenses in the Andrelandia nappe, and as float. Large patchy symplectites (several mm) interstitial to relatively homogenous almandine-rich garnets consist of diopside + Na-plagioclase + quartz.
Precursor pyroxene compositions integrated from areal analyses are omphacite. Omphacite, as well as rare jadeite inclusions (up to 200 pm) occur within mantle regions of the garnets. We have, so far, not detected any UHP indicator phases in the eclogite, but as noted by Campos Neto and Caby (1999), jadeite inclusion compositions (up to 75 mol%) indicate minimum pressures of c. 18 kbar. As far as we are aware, this is the first report of coesite-grade ultrahigh-pressure rocks in the Americas, and the second from a Pan-African orogenic belt (following the report of coesite in eclogite from northern Mali by Caby, 1994). Ultradeep subduction and exhumation of supracrustal components has important implications for the tectonic development of Neoproterozoic nappes in SE Brasil. We speculate that eclogtic assemblages and probable UHPM components may be disposed in other, correlative nappe systems at the margins of the Sao Francisco craton in Brazil.Acknowledgments
Fieldwork in the Varginha region was funded by a grant on Whole Earth Tectonics to Shige Maruyama from the Science and Technology Agency of Japan. Analytical studies at USC were supported by NSF-EAR0073803 to Matt Kohn.References
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