PTt evolution and textural evidence for decompression of Pan-African high-pressure granulites, Lurio Belt, north-eastern Mozambique: EVOLUTION OF PAN-AFRICAN HP GRANULITES (original) (raw)
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Gondwana Research, 2003
The geochemistry of the Leisure Bay Formation, Natal Metamorphic Province suggests that its protoliths were greywackes, pelites and arkoses that were deposited in an oceanic island arc environment. These rocks contain the mineral assemblage biotite + hypersthene + cordierite (with hercynite inclusions) + garnet + quartz + feldspar. Numerous generations of garnet genesis are evident from which a long history of metamorphism can be interpreted MI involved syn-D, high tempcrature/low pressure metamorphism (-4kb and > 8 5 0~) and dehydration melting to produce essentially anhydrous assemblages particularly in the vicinity of, and probably related to the intrusion of the Munster Suite sills. The inclusions of hercynite in cordierite and the garnet + quartz symplectites after hypersthene + plagioclase (-550°C and-5kb) suggests isobaric cooling after MI. This indicates an anticlockwise P-T loop related to the early intrusion of subduction related calc alkaline magmatic rocks. M, involved syn-D, dehydration melting of hydrous assemblages possibly related to the emplacement of many A-type rapakivi charnockite granitoids, which provided heat and loading. The D, tectonism postdated all lithologies in the region, except for synto late-D, granitoid plutons, and is interpreted as a transpressional tectonotherinal reworking of pre-existing (Proterozoic) crust at-1030Ma.
1990
The Namaqua Province of southwestern Africa is comprised of a number of distinct tectonostratigraphic subprovinces and terranes, which have in common a 1100-1200Ma structural and metamorphic imprint. In the western Bushmanland Subprovince, E-Wtrending belts of supracrustal gneisses are intruded by and infolded with granitic gneisses of varying ages. A central zone LIST OF PLATES Plate 1. Discordant contact of hornblende-biotite augen gneiss against homogeneous K-feldspar-quartz gneiss. Note the presence of recrystallized augen wrapped by the strong ferromagnesian mineral foliation. Hammer shaft is 29 em. Plate 2. The typical green, 'streaky' character of hornblendebiotite augen gneiss with refolded internal structure. The contact with homogeneous K-feldspar-quartz gneiss is commonly concordant. Hammer shaft is 26 em. Plate 3. Well-foliated, biotite-rich biotite-orthopyroxene granite gneiss at a concordant contact against biotite leucoparagneiss. Hammer shaft (27 em) is perpendicular to s 2 mineral foliation (s-surface), and pencil is parallel to the trace of the Nc-surface. Note the presence of a sheared, recrystallized parallel to the s2 mineral foliation. leucosome Plate 4. A stromatic migmatite root zone to a lenticular body of quartzofeldspathic anatectic granite (ca. 3 X 0.5 km) in biotite-orthopyroxene leucoparagneiss. Many of the dark spots in the leucosome assemblages are orthopyroxene. Hammer shaft is 26 em. Plate 5. Leucosome assemblages, containing garnet-quartz patches surrounded by anhydrous, quartzofeldspathic haloes, in a dyke of anatectic biotite granite that is intrusive into garnet-biotite quartzofeldspathic gneiss. The leucosomes comprise 50% of the rock volume. Hammer shaft is 26 em. Plate "6. Discordant leucosome, containing orthopyroxene (light green-brown), clinopyroxene (dark green-black) and plagioclase (white to pale blue), in hornblendepyroxene granulite gneiss. Hammer shaft is 83 em. Plate 7. An ovoid leucosome patch, containing rhombododecahedra of garnet intergrown with quartz, and surrounded by an anhydrous, quartzofeldspathic halo, in garnet-biotite quartzofeldspathic gneiss. Compass pouch is 8 em wide. Plate 8. Garnetiferous and entirely quartzofeldspathic leucosomes occurring as crenulated compositional bands in garnetbiotite metapelitic gneiss. At left, a coarse-grained garnetiferous. leucosome transgresses compositional banding and regional foliation. A K-feldspar-rich, pegmatitic vein occurs at the base of the outcrop, containing rootless stringers•of matrix gneiss. Hammer shaft is 29 em. Plate 9. Unzoned leucosomes occurring parallel to the regional, gneissic banding and foliation in garnet-biotite metapelitic gneiss. Note that many garnet grains intersect matrix-leucosome boundaries, but are always surrounded by, at least a narrow anhydrous halo. Hammer shaft is 29 em. Plate 10. Cross-section of a anatectic granite sill root zone in garnet-biotite metapeltic gneiss. Note the nebulitic internal structure of disjointed matrix stringers. Hammer shaft is 29 em. Plate 11. Coarse-grained orthopyroxene-K-feldspar leucosome patches in orthopyroxene-cordierite-phlogopite magnesian gneiss. Hammer shaft is 29 em. Plate 12. Transgressive, orthopyroxene-bearing quartzofeldspathic leucosome in biotite-orthopyroxene leucoparagneiss. Hammer head is 18 em. Plate 13. Vein and patch leucosomes, subparallel to regional foliation in biotite-orthopyroxene leucoparagneiss. The dark patches in both matrix and leucosome assemblages are orthopyroxene-biotite intergrowths. Hammer shaft is 29 em. Plate 14. Boudin-like, quartzofeldspathic sill in banded quartzofeldspathic gneiss. It is not clear whether these structures are igneous (i.e. pinch-and-swell) or truely tectonic (i.e. stretching boudins) in origin. Hammer head is 18 em.
Journal of Metamorphic Geology, 1998
To constrain the tectonic history of the Pan-African belt in Tanzania, we have studied the P-T evolution of granulites from northern and eastern Tanzania representative for a large part of the southern Pan-African belt of East Africa (e.g. Pare, Usambara, Ukaguru and Uluguru Mountains). Thermobarometry (conventional and multireaction equilibria) on enderbites and metapelites gives 9.5-11 kbar and 810±40°C during peak metamorphism at 650-620 Ma. This is consistent with the occurrence of both sillimanite and kyanite in metapelites and of the high-P granulite facies assemblage garnet-clinopyroxenequartz in mafic rocks. Peak metamorphic conditions are surprisingly similar over a very large area with N-S and E-W extents of about 700 and 200 km respectively. The prograde metamorphic evolution in the entire area started in the kyanite field but evolved mainly within the sillimanite stability field. The retrograde P-T evolution is characterized by late-stage kyanite in metapelites and garnet-clinopyroxene coronas around orthopyroxene in meta-igneous rocks. This is in agreement with thermobarometric results and isotopic dating, indicating a period of nearly isobaric and slow cooling prior to tectonic uplift. The anticlockwise P-T path could have resulted from magmatic underplating and loading of the lower continental crust which caused heating and thickening of the crust. Substantial postmetamorphic crustal thickening of yet unknown age (presumably after 550 Ma) led subsequently to the exhumation of high-P granulites over a large area. The results are consistent with formation of the Pan-African granulites at an active continental margin where tonalitic intrusions caused crustal growth and heating 70-100 Ma prior to continental collision. The P-T -t path contradicts recent geodynamic models which proposed tectonic crustal thickening due to continental collision between East and West Gondwana as the cause of granulite formation in the southern part of the Pan-African belt.
Tectonophysics, 2003
U -Pb sensitive high resolution ion microprobe (SHRIMP) dating of zircons from charnockitic and garnet -biotite gneisses from the central portion of the Mozambique belt, central Tanzania indicate that the protolith granitoids were emplaced in a late Archaean, ca. 2.7 Ga, magmatic event. These ages are similar to other U -Pb and Pb -Pb ages obtained for other gneisses in this part of the belt. Zircon xenocrysts dated between 2.8 and 3.0 Ga indicate the presence of an older basement. Major and trace element geochemistry of these high-grade gneisses suggests that the granitoid protoliths may have formed in an active continental margin environment. Metamorphic zircon rims and multifaceted metamorphic zircons are dated at ca. 2.6 Ga indicating that these rocks were metamorphosed some 50 -100 my after their emplacement. Pressure and temperature estimates on the charnockitic and garnet -biotite gneisses were obscured by post-peak metamorphic compositional homogenisation; however, these estimates combined with mineral textures suggest that these rocks underwent isobaric cooling to 800 -850 jC at 12 -14 kbar. It is considered likely that the granulite facies mineral assemblage developed during the ca. 2.6 Ga event, but it must be considered that it might instead represent a pervasive Neoproterozoic, Pan African, granulite facies overprint, similar to the ubiquitous eastern granulites further to the east. D
Journal of Petrology, 2021
Gneiss domes cored by migmatites and granites represent the principal role of anatectic melts during the exhumation of high-grade metamorphic complexes. This study explores the exhumation history of a metapelitic granulite within the Ha-Tshanzi structure from the Central Zone of the Neoarchean-Paleoproterozoic Limpopo high-grade Complex, South Africa. Quartzofeldspathic garnet-bearing coarse-grained leucosomes in the rock alternate with attenuated shear bands consisting of biotite, cordierite, sillimanite and quartz that prominently modified the earlier garnet porphyroblasts. Cores of garnet porphyroblasts contain various polyphase inclusions that are interpreted as crystallized inclusions of melt. The phase equilibria modeling and regular zoning of garnet with respect to major (Mg, Fe, Ca) and some trace (P, Cr, Sc) elements reveals that a garnet + biotite + plagioclase + quartz ± sillimanite assemblage in the rock coexisted with the melt during the sub-isothermal (810-830 • C) exhumation from pressure of 10•0-10•2 kbar to 7•5-7•0 kbar during the Neoarchean event (2•68-2•62 Ga). The exhumation mediated by anatectic melt supports interpretation of the Ha-Tshanzi structure as a diapir-related granite-gneiss dome. During upwelling of the dome, the melt segregated into leucosome, while growth of cordierite sequestered water from the melt, assisting its crystallization at the end of the sub-isothermal decompression stage. As the rheology of the rock changed, melt-dominated deformation was transformed to solid-dominated ductile shear deformation. In contrast to the earlier sub-isothermal decompression P-T path, the gentler slope of ∼75 • C kbar-1 of the decompression-cooling path marks the exhumation from pressures of ∼7 kbar to pressures of 5-4•5 kbar and cooling to 600-550 • C. Dating of zircon, monazite and rutile shows that the Neoarchean evolution of the metapelite was strongly overprinted by the Paleoproterozoic event at c. 2•01 Ga. The results of the study highlight the significance of domed structures related to granitic diapirs in the exhumation of the Central Zone of the Limpopo Complex.
Journal of Metamorphic Geology, 2009
In this work, the factors controlling the formation and preservation of high-pressure mineral assemblages in the metamorphosed orthopyroxene-bearing metagranitoids of the Sandmata Complex, Aravalli-Delhi Mobile Belt (ADMB), northwestern India have been modelled. The rocks range in composition from farsundite through quartz mangerite to opdalite, and with varying K2O, Ca/(Ca + Na)rock and FeOtot + MgO contents. A two stage metamorphic evolution has been recorded in these rocks.An early hydration event stabilized biotite with or without epidote at the expense of magmatic orthopyroxene and plagioclase. Subsequent high-pressure granulite facies metamorphism (∼15 kbar, ∼800 °C) of these hydrated rocks produced two rock types with contrasting mineralogy and textures. In the non-migmatitic metagranitoids, spectacular garnet ± K-feldspar ± quartz corona was formed around reacting biotite, plagioclase, quartz and/or pyroxene. In contrast, biotite ± epidote melting produced migmatites, containing porphyroblastic garnet incongruent solids and leucosomes.Applying NCKFMASHTO T–M(H2O) and P–T pseudosection modelling techniques, it is demonstrated that the differential response of these magmatic rocks to high-pressure metamorphism is primarily controlled by the scale of initial hydration. Rocks, which were pervasively hydrated, produced garnetiferous migmatites, while for limited hydration, the same metamorphism formed sub-solidus garnet-bearing coronae. Based on the sequence of mineral assemblage evolution and the mineral compositional zoning features in the two metagranitoids, a clockwise metamorphic P–T path is constrained for the high-pressure metamorphic event. The finding has major implications in formulating geodynamic model of crustal amalgamation in the ADMB.
The P-T-X(fluid) evolution of meta-anorthosites in the Eastern Granulites, Tanzania
Journal of Metamorphic Geology, 2011
Meta-anorthosite bodies are typical constituents of the Neoproterozoic Eastern Granulites in Tanzania. The mineral assemblage (and accessory components) is made up of clinopyroxene, garnet, amphibole; scapolite, epidote, biotite, rutile, titanite, ilmenite and quartz. Within the feldspar-rich matrix (70-90% plagioclase), mafic domains with metamorphic corona textures were used for P-T calculations. Central parts of these textures constitute high-Al clinopyroxene-which is a common magmatic mineral in anorthosites-and is therefore assumed to be a magmatic relict. The clinopyroxene rims have a diopsidic composition and are surrounded by a garnet corona. Locally the pyroxene is surrounded by amphibole and scapolite suggesting that a mixed CO 2-H 2 O fluid was present during their formation. Thermobarometric calculations give the following conditions for the metamorphic peak of the individual metaanorthosite bodies: Mwega: 11-13 kbar, 850-900°C; Pare Mountains: 12-14 kbar, 850-900°C; Uluguru Mountains: 12-14 kbar, 850-900°C. The P-T evolution of these bodies was modelled using pseudosections. The amount and composition of the metamorphic fluid and <0.5 mol.% fluid in the bulk composition is sufficient to produce fluid-saturated assemblages at 10 kbar and 800°C. Pseudosection analysis shows that the corona textures most likely formed under fluid undersaturated conditions or close to the boundary of fluid saturation. The stabilities of garnet and amphibole are dependent on the amount of fluid present during their formation. Mode isopleths of these minerals change their geometry drastically between fluid-saturated and fluid-undersaturated assemblages. The garnet coronae developed during isobaric cooling following the metamorphic peak. The cooling segment is followed by decompression as indicated by the growth of amphibole and plagioclase. The estimated X CO 2 of the metamorphic fluid is $0.3-0.5. Although the meta-anorthosites have different formation ages (Archean and Proterozoic) they experienced the same Pan-African metamorphic overprint with a retrograde isobaric cooling path. Similar P-T evolutionary paths are known from the hosting granulites. The presented data are best explained by a tectonic model of hot fold nappes that brought the different aged anorthosites and surrounding rocks together in the deep crust followed by an isobaric cooling history.
Contributions To Mineralogy and Petrology, 2004
The present study from the Sausar Mobile Belt (SMB) in the southern part of the Central Indian Tectonic zone (CITZ) demonstrates how microdomainal compositional variation of a single garnet porphyroblast in a metapelite granulite sample records the different segments of a near complete P-T path of metamorphic evolution. The microdomainal variation is ascribed to the preservation of growth zoning and heterogeneous distribution of diverse inclusion mineral assemblages. Subsequent mineral reactions under changing P/T conditions were controlled by this compositional heterogeneity. Four stages of metamorphic evolution have been deciphered. An early prograde stage (Mo) is implied by the rare presence of staurolite-biotite-quartz and in places of kyanite inclusion assemblages in other metapelite samples, together with the growth zoning preserved in garnet. The peak metamorphism (M1) at ~9.5 kbar, ~850 °C is consistent with the biotite dehydration melting that produced garnet-K-feldspar and granitic leucosomes. This was followed by near isothermal decompression (M2) at ~6 kbar, ~825 °C, during which different garnet segments behaved as separate microscale bulk compositions and decomposed both internally and externally to produce different retrograde mineral assemblages. In the quartz-bearing domain of almandine-rich and grossular-rich garnet core, grossular components in garnet reacted with included sillimanite and quartz to produce coronal plagioclase (XAn=0.90). By contrast, grossular-rich garnet in quartz-absent domain reacted with included sillimanite to produce layered spinelss {XMg (Mg/Mg+Fe2+) = 0.23–0.26}, XAl (Al/Al+Fe3+)=0.71–0.81}-plagioclase (XAn=0.91)-cordierite {XMg (Mg/Mg+Fe2+) = 0.80–0.83} coronas both in the core and inner rim region of garnet. During post-decompression cooling, reactions occurred at about 600 °C (M3), whereby quartz-bearing, sillimanite-absent microdomains of pyrope-rich, grossular-poor garnet outer rim decomposed to form relatively magnesian assemblages of cordierite-anthophyllite and cordierite-biotite-quartz. M2 spinelss decomposed to polyphase domains of spinel-magnetite±högbomite at this stage. Collating the textural and geothermobarometric results, a clockwise P-T path has been deduced. The deduced P-T loop is consistent with a model of crustal thickening due to continental collision, followed by rapid vertical thinning, which appears to be the general feature of the Sausar Mobile Belt. Using model calculations of the preserved growth and diffusion zoning in garnet, we demonstrate rather short-lived nature of this collision orogeny (in the order of 40–60 Ma).
Journal of Petrology, 2022
In collision-type orogens, where high-pressure and ultrahigh-pressure (HP-UHP) metamorphism usually occurs, deeply subducted continental slabs with eclogitized mafic rocks often undergo recrystallization/overprinting with various geothermal gradients after the peak conditions at lowerto-middle-crustal levels. During the crustal stabilization, the transition from eclogite-to granulitefacies is common. We conducted metamorphic petrology and zircon geochronology on (1) bimineralic and (2) partially granulitized eclogites from the Neoproterozoic Ufipa Terrane (Southwestern Tanzania). Microtextural relationships and mineral chemistry define three metamorphic stages: eclogite metamorphism (M1), HP granulite-facies overprinting (M2), and amphibolite-facies retrogression (M3). The bimineralic eclogite has a basaltic composition and lacks M2 minerals. In contrast, the kyanite eclogite is characterized by a gabbro-dioritic whole-rock composition and contains inherited magmatic zircon. Although the matrix is highly granulitized, garnet and kyanite contain eclogite-facies mineral inclusions. Phase equilibria modeling revealed P-T conditions of 2.1-2.6 GPa and 650-860 • C for the M1 stage and 1.4-1.6 GPa and 750-940 • C for the M2 stage. Zircon with eclogite-facies mineral inclusions from the bimineralic eclogite lacks Eu anomaly in the REE patterns and yielded the M1 eclogite metamorphic age of 588 ± 3 Ma. Zircon overgrowths surrounding the inherited Paleoproterozoic magmatic cores in kyanite eclogite yielded 562 ± 3 Ma. A weak negative Eu anomaly in the REE patterns and the absence of eclogitic mineral inclusions suggest the zircon growths at the M2 HP granulite-facies metamorphic stage. These new data indicate an eclogite-to granulite-facies transition time of 26 ± 4 million years (Myr), suggesting a rate of HP rock exhumation toward a lower crustal level of 0.7-1.5 mm/year. Furthermore, the density evolution model indicates that buoyant host orthogneiss with low-density gabbro-dioritic eclogite plays an important role in carrying high-density basaltic eclogite. Our 2D thermomechanical modeling also suggests that a slab break-off with a lower angle subduction of <20 • triggers the exhumation of the HP slab sliver with 20-30 Myr eclogite-to granulite transition time of large HP-UHP terranes in major collision zones.