Granulite Grade Research Articles

Archean Komatiitic Sill-hosted Chromite Deposits in the Zimbabwe Craton

Lenticular chromite deposits of widely varying age, metamorphic grade and strain rate, as well as size, shape, and ore quality, are widespread in greenschist to amphibolite grade greenstonegneiss terrain in the south-central part of the Zimbabwe craton and in granulite grade equivalents in the Northern Marginal zone of the Limpopo belt in the far south. In order of age and, very approximately, decreasing abundance, they form three groups: (1) early Archean (3.5–3.2 Ga) deposits associated with amphibolite grade greenstone remnants in the 3.6 to 3.35 Ga Tokwe gneisses (e.g., Hornet, Valley) and on the amphibolite grade eastern flank of the Shurugwi greenstone belt, and granulite grade greenstone remnants in the Northern Marginal zone (e.g., Inyala, Rhonda); (2) mid-Archean deposits within the ca. 3.0 Ga Shurugwi Group in the main greenschist part of the Shurugwi belt (e.g., Peak, Railway Block) and associated with the ca. 3.0 Ga Belingwean Supergroup in several other belts; and (3) late Archean (ca. 2.74 Ga) deposits mostly in the western segment of the Masvingo belt (e.g., Prince). With their host and country rocks, the deposits together preserve geologic and chromite compositional evidence of formation within poorly differentiated, komatiitic sills intruded into predominantly siliciclastic, BIF platformal sedimentary sequences associated with mafic and/or ultramafic lavas. Unlike the thin, laterally extensive chromitite layers associated with macrocyclic units of large, layered intrusions, the komatiitic sill-associated deposits are the remnants of laterally discontinuous (up to several tens of meters) thick chromitite layers or dunite-chromitite zones located at high stratigraphic levels of their host intrusions not far below the upper pyroxenite differentiates. In at least some deposits—for example, at Railway Block, where the chromitites form narrow, elongate bodies at least 1,000 m long—the lateral discontinuities are most likely primary; in many others, later deformation obscures the primary geometry. Chromitite formation may have been delayed by the late attainment of equilibrium crystallization and the reduced nature of the magma. The crystallization of thick, massive, cyclically layered, Cr-rich chromitites was most probably controlled by the stabilization of chromite relative to olivine by absorption of connate water derived from wet host sediments and by episodic, flow-through processes which, at Railway Block, may have been focused along narrow pathways across the magma chamber floor. The remarkable abundance of such chromitites and their formation in repeated episodes over 1,000 m.y. (cf. other cratons) can be attributed to generation of komatiitic magmas in a mantle source that was both Cr rich and strongly reduced. Archean komatiitic sill-hosted deposits constitute a third type of chromite deposit in addition to stratiform and podiform deposits in layered intrusions and ophiolites, respectively. The economically important variability of bulk ore compositions (especially Cr, Fe, Al, Mg) is due to several factors: magnetite introduction during serpentinization and talc-carbonate alteration at greenschist grade, subsolidus fluid-related and other metamorphic cation exchange processes (dependant on chromite body size), and, possibly, exsolution of Cr-Al and magnetite phase pairs, both at midamphibolite to granulite grade, as well as fluid-related retrogression and partial conversion of chromite to (mainly) chlorite during amphibolite metamorphism.

The contribution of geochronology to understanding the Paleozoic geological history of Australia

From the viewpoint of isotope geochronology, studies of the Paleozoic geological history of Australia can be divided into three main eras: (i) the era of stratigraphy, when the ages of igneous and metamorphic events were inferred primarily from stratigraphic relationships; (ii) the era of K–Ar and Rb–Sr, when the broad time frame of many of the main thermal events was mapped out; and (iii) the era of SHRIMP, when U–Pb microanalysis, in combination with techniques such as Ar–Ar, Sm–Nd and Lu–Hf, made it possible to study not only the most recent events in the thermal history but also cooling histories and the ages of precursors and protoliths. The work during the last era was the most ambitious and controversial. As age measurements became more precise, their accuracy and the accuracy of the calibration of the geologic time-scale were called into question. Isotopic analyses of eastern Australian granitic rocks, undertaken to help understand magma genesis and the age of the source rocks from which the magmas were derived (the basement of the Lachlan Orogen), were interpreted by different researchers in fundamentally different ways. Ages measured on detrital minerals from Paleozoic sedimentary rocks were interpreted as indicating erosion of quite different source regions. These debates did not detract from the main achievements, however. There is a large and expanding database of high-quality age measurements on thermal events throughout the Australian Paleozoic. In many cases, this provides directly determined ages for igneous, metamorphic and deformation events previously dated only approximately using stratigraphy. The distribution of coeval magmatism and the progressive migration of that activity have been mapped out. Isotopic compositions have been used to estimate the relative contributions of crust and mantle components to magmas and the ages of those components. In some cases, geochronology has changed the geological paradigm. For example, in central Australia, the Alice Springs Orogeny is now known to be a major intraplate tectonothermal event, and granulite grade metasediments in the east of the Paleoproterozoic Arunta Inlier have been shown to be deformed and metamorphosed equivalents of some of the lower units of the adjacent Centralian Superbasin, part of a previously unrecognised incipient Paleozoic transcontinental rift. In combination with field studies, dating of mineralisation has shown that ore genesis occurred not randomly, but in a series of distinct metallogenic episodes, many of the major Paleozoic mineral deposits in Australia having formed in response to far-field stresses associated with orogenic events, commonly late in a tectonic cycle. The principal episode of mineralisation (480–440 Ma) followed the end of the major tectonothermal event that coincided with the protracted collision between east and west Gondwana, the Pan-African/Ross/Delamerian Orogeny.

Understanding the 1720–1640 Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province, Southeastern Australia: Implications for tectonics, basin evolution and ore genesis

The ∼1720–1640 Ma Willyama Supergroup was deposited in an epi-continental rift basin accompanied by felsic and mafic magmatism, and hosts the largest known Pb–Zn–Ag accumulation at Broken Hill. The Supergroup occupies the southern portion of the Curnamona Province, a largely buried ∼50,000 km 2 crustal block, possibly the southernmost part of the Eastern Australian Pb–Zn belt (which includes Mt Isa). Additionally, the Province is a component of the early Mesoproterozoic IOCG ‘super-province’ that includes the Gawler Craton (Olympic Dam), Eastern Succession of the Mt Isa region (Ernest Henry) and perhaps parts of NW North America (Wernecke Mountains). The Curnamona Province, but a small part of the original basin, is divided into domains on depositional, tectonic and magmatic criteria with variably distributed stratigraphic units reflecting syn-depositional faulting and onlapping relationships resulting from rifting during the earlier extensional stage of basin evolution. Rifting controlled the variable distribution and thickness of the succession. In the Olary Domain to the west, the palaeo-flank exposes the ∼1720–1715 Ma Curnamona Group, but overlying sediment packages are but sporadically developed. To the east and closer to the rift axis, sedimentation in the Broken Hill Domain was more complete, comprising ∼1710–1705 Ma migmatitic-metasediments, and the overlying ∼1705–1695 Ma Thackaringa, ∼1695–1685 Ma Broken Hill and ∼1685–1670 Ma Sundown Groups. The ∼1670–1640 Ma Paragon and Strathearn Groups represent sag-phase deposition with a change in basin configuration and provenance. Earliest syn-depositional magmatism is recorded in the Olary Domain by ∼1720–1710 Ma A-type felsic volcanics and intrusives and restricted basaltic lavas. The locus of magmatism then moved to the Broken Hill Domain with partial melting of either the lower parts of the Willyama Supergroup, or underlying rocks of similar composition, producing S-type granitic sills of the ∼1705–1685 Ma Silver City Suite. Magmatism climaxed in the upper Broken Hill Group with emplacement of Fe-rich tholeiite sills and the ∼1685 Ma volcaniclastic felsic Hores Gneiss hosting the Broken Hill Pb–Zn–Ag orebody. Polyphase deformation and high T, low P metamorphism, ranging from greenschist to lower granulite grade, resulted from the ∼1620–1590 Ma Olarian Orogeny. Early layer-parallel schistosity was deformed progressively, first by large ductile recumbent and sheath-style folds, and later by upright folding. The mainly S-type granites of the ∼1595–1580 Ma Ninnerie Supersuite were intruded late in the deformation history. An extensive network of post-1580 Ma retrograde shear zones dismembered the Willyama Supergroup and was partially reactivated during the ∼500 Ma Delamerian Orogeny. Of the wide range of mineralisation styles in the Curnamona Province, Broken Hill-type Pb–Zn–Ag deposits and epigenetic Cu–Au and U are currently of greatest interest. Broken Hill-type mineralisation probably formed in a shallow depositional environment at or below the sea-floor, with maximum development approximating cessation of magmatic activity. Cu–Au (±Co, Mo) deposits (e.g. Kalkaroo, Portia) are located in chemical and structural traps near a regionally extensive redox boundary. By sharing geological aspects of the eastern Gawler Craton, the Curnamona Province has excellent prospectivity for Olympic Dam-style IOCG mineralisation. Uranium deposits, some economic, are located both in basement rocks and as roll-front concentrations in overlying Tertiary channel sands.

Discovery of a Neoproterozoic basin in the Prydz belt in East Antarctica and its implications for Gondwana assembly and ultrahigh temperature metamorphism

Detrital zircon U–Pb age data from ultrahigh-temperature metasedimentary rocks (950 °C and 10–12 kbar) in the Rauer Group and a granulite grade metasedimentary rock (850 °C and 7–8 kbar) in the Bolingen Islands in Prydz Bay, East Antarctica, indicate the existence of a Neoproterozoic (Ediacaran) cover sequence that was deposited on basement ranging in age between approximately 2800 Ma and 940 Ma. The Neoproterozoic succession has a maximum depositional age of approximately 600 Ma with derivation from igneous and metamorphic sources of Archaean, Palaeoproterozoic and Neoproterozoic age. The minimum depositional age is constrained to be approximately 575 Ma based on existing (Th + U)–Pb monazite metamorphic ages from the UHT metasedimentary rocks. The identification of Late Neoproterozoic protoliths to the UHT granulites in the Rauer Group demonstrate that ultrahigh-temperature metamorphism in Prydz Bay must be Ediacaran to Cambrian in age. The timing of basin development ( ca. 600–575 Ma) is coincident with the amalgamation of eastern Gondwana. The palaeogeography of the Prydz Bay region during the amalgamation of Gondwana is uncertain, but in our proposed reconstruction we suggest that inherited zircons were derived from Indo-Antarctic crust. Ocean closure is inferred to have occurred along a suture zone to the southeast of Prydz Bay and the Prince Charles Mountains. Our palaeogeographic model invites further work on the possibility of a shared history between India and a large portion of East Antarctica prior to the final amalgamation of eastern Gondwana during the Ediacaran–Cambrian.

Provenance and tectonic development of the late Archaean Gawler Craton, Australia; U–Pb zircon, geochemical and Sm–Nd isotopic implications

Integrated Sensitive High Mass Resolution Ion Microprobe (SHRIMP) U–Pb zircon, geochemical and Sm–Nd isotopic data from the late Archaean Gawler Craton, southern Australia, constrain major basin-forming and magmatic processes to the interval 2560–2500 Ma. This terrane represents a convergent margin associated with the second half of a full global Wilson cycle, during which continental fragments formed between 2780 and 2590 Ma amalgamated with younger cratons, perhaps forming the Earth's first supercontinent. Metasedimentary rocks in the central and southern Gawler Craton deposited between 2535 and 2500 Ma, have dominant detrital SHRIMP U–Pb zircon ages, geochemical and ɛ Nd signatures which reflect input from predominantly felsic, juvenile to moderately evolved late Archaean crust. Geochemical patterns in ca. 2560–2500 Ma felsic to intermediate igneous lithologies that in part were produced during on-going basin development, suggest affinities with an arc-like environment, and have a range of ɛ Nd values that indicate variable crustal contamination of primitive arc components. However, approximately coeval komatiites have geochemical and ɛ Nd signatures consistent with a mantle–plume origin. We envisage a tectonothermal regime for development of the late Archaean Gawler Craton that reflects interaction between a convergent margin and mantle–plume, in which sedimentary deposition occurred during active basin development in a back-arc or arc-rift setting. Basin formation and associated magmatism was terminated by regional granulite grade metamorphism and crustal thickening during the ca. 2500–2400 Ma Sleafordian Orogeny, which was proceed by ∼400 Ma of tectonic quiescence. This suggests that the Sleafordian Orogeny was driven by collision between continental components, leading to the formation of a continental interior. While the late Archaean Gawler Craton is younger than prodigious late Archaean metallogenic provinces associated with the first half of a full global Wilson cycle (ca. 2780–2590 Ma), there are important lithological and tectonic similarities to those mineralized systems.

New PGE and Re/Os-isotope data from lower crustal sections of the Vredefort Dome and a reinterpretation of its "crust on edge" profile

Granulites exposed in the core of the Vredefort Dome are generally thought to represent components of the Archean middle crust of the central Kaapvaal Craton. These rocks were exhumed from depths of at least 20 km following an ~2.02 Ga meteorite impact event, exposing a “crust on edge” section believed to represent a continuous magmatically-differentiated sequence from upper crust possibly down to Moho. Platinum group element concentrations suggest that mafic and ultramafic enclaves in the lower most sections are fragments of greenstone belt material metamorphosed at granulite grade. Re/Os isotopic data are consistent with previously published U-Pb zircon data, which suggest that the mafic and ultramafic rocks have a primary age of ~3.5 Ga and were metamorphosed to granulite grade at ~3.1 Ga. However, near the core of the Vredefort Dome across a major shear zone, other ~3.5 Ga greenstone belt remnants are at distinctly lower metamorphic grade. The juxtaposition of mafic-ultramafic rocks with a different metamorphic history in the core of the Vredefort structure requires a modified “crust on edge” model that incorporates tectonic stacking of different crustal segments at ~3.1 Ga.

The petrology of two distinct granulite types in the Hengshan Mts, China, and tectonic implications

The Archean to Proterozoic Hengshan Complex (North China Craton), comprises tonalitic and granodioritic gneisses with subordinate mafic lenses, pegmatites and granites. Amphibolite facies assemblages predominate, although granulite-facies relics are widespread, and greenschist-facies retrogression occurs in km-wide shear zones. Mafic lenses, locally abundant, occur as strongly deformed amphibolite (hornblende+plagioclase) boudins or sheets. In contrast to previously published models we find two series of mafic rocks with distinctly different granulite-facies evolutions. In the north of the Complex, relict high-pressure mafic granulites are garnet+clinopyroxene-bearing rocks with a secondary development of orthopyroxene around both garnet (kelyphites) and clinopyroxene (coronas). South of the newly defined central, E–W-trending, Zhujiafang shear zone, numerous mafic boudins and less-deformed dykes exhibit a macroscopically visible magmatic texture with coronitic growth of metamorphic garnet (full of quartz inclusions) between the magmatic plagioclase and pyroxene domains. Additional orthopyroxene (after magmatic augite) and sodic rims to magmatic plagioclase clearly indicate medium-pressure granulite-facies metamorphism. These findings suggest tectonic juxtaposition in this area of three different structural levels of the same Proterozoic-imprinted crust: high-pressure granulite grade in the northern Hengshan, medium-pressure granulite grade in the southern Hengshan and amphibolite- to greenschist-facies grade in the Wutaishan to the SE.

Rock magnetism and palaeomagnetism of the Oddanchatram anorthosite, Tamil Nadu, South India

Rock magnetic and palaeomagnetic data are reported on the Proterozoic massif-type anorthosite body from Oddanchatram, on the northern slopes of the Kodaikanal (Palani) Ranges, Tamil Nadu. Alternating field demagnetization treatment indicated a very stable direction of magnetization. Progressive heating revealed a remanence that became unblocked at 580°C. The demagnetization analysis yields a characteristic component with a mean direction: Dm= 97.3° Im =− 28.8° (k= 51, α95= 7.2° and N= 9 sites). In addition to this characteristic component, specimens from a few sites show the presence of a low coercivity component with a mean palaeomagnetic direction of D= 36.7°; I=−29.4° (k= 63, α95= 4.4° and N= 18). The isothermal remanent magnetization acquisition curves indicate magnetite as the main carrier of the stable remanence. The direction of the characteristic component corresponds to a pole position at 9.7°N, 1.7°E (dp= 4.4°, dm= 7.9°). The location of this pole is comparable to palaeomagnetic poles reported for the period of 1100–1000 Ma for a range of formations from the Indian shield. The pole for the secondary component (45°S, 23°E) fits well with poles for 550 Ma, associated with the Pan-African thermal event that extensively caused granulite grade metamorphism in the Southern Granulite Terrain. Despite the high temperature (>700 °C) that prevailed during the Pan-African event, the remanence magnetization of 1100–1000 Ma is preserved in the Oddanchatram anorthosite. It is inferred that the northern boundary of the terrain affected by the dominant 550 Ma granulitic metamorphism lies close to the southern margin of the Oddanchatram anorthosite body. Thus Oddanchatram anorthosite escaped the high temperatures metamorphism. The weak secondary magnetization found in a few specimens is attributed to hydrothermal activity associated with the intrusion of pink granites, immediately south of the Oddanchatram anorthosite body. A mid-Proterozoic magnetization age of the Oddanchatram anorthosite supports the view that this body was emplaced/remobilized during the Eastern Ghat Orogeny (∼1000 Ma) and this body can be seen as a western extension of the string of massif-type anorthosites in the Eastern Ghat Mobile Belt.

Geochronology of the Sikombe Granite, Transkei, Natal Metamorphic Province, South Africa

A new U-Pb SHRIMP zircon date of 1181 ± 15 Ma has been obtained from a sample of the syntectonic, gneissose Sikombe Granite from northeastern Transkei (Eatern Cape Province). The outcrops form the most southerly exposures of the ~1.1 Ga Natal Metamorphic Province (NMP). Previously, these isolated outcrops have been regarded as possibly forming a separate southerly, amphibolite facies terrane of the NMP, as the lithological assemblage is somewhat different from those of the granulite grade Margate terrane to the north. The new data do not conclusively demonstrate this, but they do show that the accretion age of the southerly terrane of the NMP is not significantly younger than the more northerly terranes. Sm-Nd analyses of three samples of the Sikombe Granite give consistent, positive End values (+4 at t = 1180 Ma) showing that, like the rest of the NMP, the granite was derived from juvenile Mesoproterozoic crust, with little or no involvement of older material in its genesis

Origin and significance of Archean quartzose rocks at Akilia, Greenland.

Fedo and Whitehouse ([1][1]) presented data to support their hypothesis that granulite grade quartz+magnetite rocks on the island of Akilia, southwest Greenland—which are conformably interposed with pyroxenite and hornblendite units that date from before 3830 million years ago (Ma) ([2–4][2])—

Archean and Early Proterozoic Events Along the Snowbird Tectonic Zone in Northern Saskatchewan, Canada

The Snowbird tectonic zone is a major boundary structure in the Canadian shield, separating rocks of the Rae and Hearne lithostructural domains. Field investigations along portions of the Snowbird tectonic zone exposed in Northern Saskatchewan have revealed a major granulite facies mylonite belt that records deformation and metamorphism at several intervals in the Archean and early Proterozoic. Early assemblages are dominated by granulite grade gneisses that were deformed in a major left-lateral strike-slip shear zone at ca. 3200 Ma. The major episode of deformation along this portion of the Snowbird tectonic zone occurred at ca. 2600 Ma, again at granulite facies in a dominantly right-lateral strike-slip shear zone. Little evidence of Proterozoic deformation is recorded within this region of the Snowbird tectonic zone itself, but neighboring rocks in the Rae province to the west preserve granulite grade right-lateral fabrics dated at ca. 1850 Ma. These constraints suggest that the Snowbird tectonic zone was a focus of major regional deformation throughout the middle and late Archean, but that regional deformation was partitioned outward to the surrounding Rae (and Hearne?) province rocks during the early Proterozoic. These data suggest that the Snowbird tectonic zone was not a major continental suture zone in the early Proterozoic.

Tectonic evolution of the Reynolds-Anmatjira Ranges: a case study in terrain reworking from the Arunta Inlier, central Australia

Abstract The Reynolds-Anmatjira Range region forms part of the Arunta Inlier in central Australia and has undergone four tectonothermal cycles that span an interval of c. 1450 Ma. The first two cycles were the Stafford Tectonic Event c. 1820 Ma, and the Strangways Orogeny c. 1770–1780 Ma, both of which were associated with regional low-pressure high-temperature metamorphism up to granulite grade that was coeval with the emplacement of voluminous sheet-like granites. The subsequent Chewings Orogeny occurred at around 1590–1570 Ma and was a long-lived event that produced regional low-pressure greenschist to granulite facies metamorphism without obvious associated magmatism. During the mid-Palaeozoic Alice Springs Orogeny (400–300 Ma), the terrain was dissected by a system of sub-greenschist to mid-amphibolite facies shear zones. In the Reynolds Range, the Proterozoic events produced a single regional foliation that is axial planar to simple large-scale folds. The composite regional Proterozoic foliation increases in grade smoothly from northwest to southeast, producing a pattern of isograds that is remarkably similar to those that formed during the mid-Palaeozoic Alice Springs Orogeny. Despite this simple pattern, the isograds reflect the superimposed metamorphic effects of four unrelated tectonothermal cycles. Without geochronological and stratigraphic information, the degree of terrain reworking in the Reynolds-Anmatjira Range region could have been largely obscured by the apparent simplicity of many of the structural and metamorphic relationships.

The Stream Sediment Geochemistry of the Walawe Ganga Basin of Sri Lanka - Implications for Gondwana Mineralization

A regional geochemical and mineralogical study aimed at investigating the mineralization in the western-part of the Walawe Ganga (river) Basin in Sri Lanka is represented in this paper. The river basin is the 3 rd largest in the country and has within it a boundary zone between two geologically different crustal blocks, which are marked by granulitic grade rocks and amphibolite grade rocks. Size fractions of stream sediments (< 63 μm; 63–125 μm; 125–177 μm and 177–250 μm) developed on the granulite-grade metamorphic terrain have been analysed at their source for their mineralogical and selected element compositions. Thirty-eight (38) sediment samples and 15 representative probable parent rock samples were chemically analysed giving special emphasis to the High Field Strength trace Elements (HFSE) including the Rare Earth Elements (REE). The granulite grade rocks in the study area is geochemically similar to that of post Archean upper crust. However the stream sediments developed from the high-grade rocks during the intense weathering, are markedly enriched with HFSE and REE. The enrichment of HFSE and LREE is accounted for by the presence of HFSE- and REE- rich accessory mineral phases such as zircon, monazite, apatite, garnet and rutile in the sediments. In some samples, the content of heavy minerals contributes as much as 50 wt. %. These minerals act as a source of elements in the sediments. However, extreme hydraulic sorting of HFSE- and REE-bearing minerals during the sediment deposition cannot be expected within a short distance from near the sources except from a mineralized occurrence. Therefore, the higher enrichment of these elements presumably indicates occurrences of scattered mineral sources such as highly differentiated granites and associated pegmatites within the Walawe Ganga drainage basin. These granitic pegmatites are probably intruded during or soon after the main granulite-facies metamorphic event and similar events are seen in other terrains of East-Gondwana.

The Nilgiri enderbites, South India: nature and age constraints on protolith formation, high-grade metamorphism and cooling history

The trident-shaped Nilgiri highland massif represents the deepest level exposure (9–10 kbar; c. 35 km palaeo-depth) of granulite grade lower crust in southern Peninsular India. The crustal section is essentially composed of garnetiferous enderbitic granulites with minor kyanite-bearing gneisses, quartzites and banded magnetite quartzites. Its deeper, northern part is intruded by intermediate and basic magmatic rocks now represented by non-garnetiferous enderbites, garnetiferous basic granulites, gabbroic to anorthositic two-pyroxene granulites and pyroxenites. The garnetiferous enderbitic granulites are tonalitic in composition, displaying a typical calc–alkaline trend, and exhibit chemical features of both I-type as well as S-type granitoids. They show moderate enrichment of LREE (La N 35–70), no to weakly negative Eu anomalies, and fairly constant levels of HREE (Yb N 6.5–13.0), and resemble the REE spectra of tonalitic–trondhjemitic granitoids and greywackes of late Archaean to Palaeoproterozoic age. Sedimentary protoliths are indicated by negative DF values ranging from −1 to −4, Al 2O 3/(CaO+Na 2O+K 2O) molar ratios >1.1, high δ 18O whole-rock values (10–12‰) which match the values reported for greywackes. Whole-rock isotope data for garnetiferous enderbites define linear arrays in the Rb–Sr and Sm–Nd evolution diagrams (Rb–Sr system: 2460±81 Ma with Sr i 0.7030±5; Sm–Nd system: 2506±70 Ma with Nd i 0.509363±60), implying a provenance of the sedimentary precursors that was dominated by felsic volcanic rocks or tonalitic–trondhjemitic granitoids of uniform isotopic composition and early Proterozoic age. The U–Pb systematics of composite zircon grains with detrital cores and metamorphic overgrowths, in accord with the Sm–Nd and Rb–Sr errorchron ages and Sm–Nd garnet–whole-rock data, suggest that formation of the provenance, deposition of the sedimentary precursors and subsequent high-grade tectono-metamorphic processes must have occurred in the short time span between 2600 and 2500 Ma. Based on the regional geological framework, the petrological, geochemical and isotopic data, the Nilgiri granulite terrain is interpreted as a metasedimentary allochthonous unit that was thrust onto the Dharwar Craton during Palaeoproterozoic collisional tectonics.

Pan-African pressure-temperature evolution of the Merelani area in the Mozambique Belt in northeast Tanzania

The bedrock of the Merelani area in the Mozambique Belt in northeast Tanzania was multiply deformed and metamorphosed in Pan-African (Neoproterozoic) times. The first metamorphic event took place at the granulite grade and later the rocks suffered retrogression during isobaric cooling (IBC) at the amphibolite grade. Thermobarometric data from the Merelani gneisses indicate temperatures in the range 670–610°C and pressures of 6.5–6.0 kbar. Isobaric cooling is exemplified by late, post-tectonic growth of kyanite porphyroblasts at the expense of sillimanite in graphitic paragneisses as well as by garnet zoning in the same rock. Further retrogression was due to hydration along shear zones and faults. This led to the alteration of forsterite, enstatite and pargasite into serpentine minerals. Crystallisation of vanadiferous zoisite (tanzanite) in hydrothermally altered rocks took place at this late stage when X H 2O = 0.5 in calcsilicate rocks. These findinds, combined with the textural and thermobarometric data from the Uluguru Mountains and the Furua area, suggest that the IBC P- T path was prominent in the metamorphic evolution(s) of the Pan-African Mozambique Belt rocks in Tanzania. This was most probably caused by addition of igneous rocks (underplating) at the initial stage of the formation of the granulite-facies rocks in the belt: followed by their residence and coolingin the middle and upper crustal levels. The slight decompression during this stage could be explained by either extensional or transtensional deformation. Finally, the Merelani rocks, like those in the Uluguru Mountains, were rapidly exhumed.

Geology of the High Grade Proterozoic Terrains of Sri Lanka, and the Assembly of Gondwana: an Update on Recent Developments

Nd model ages determined for the high-grade rocks of Sri Lanka delineate three crustal units, viz., the Highland Complex (HC), the Wanni Complex WC), and the Vijayan Complex (VC). The distribution of these three units differs considerably from the three geological divisions demarcated previously on the basis of geological mapping. The centrally located HC comprises mainly granulite grade charnockitic rocks, and metasediments characterized by older Nd model ages (2.0–3.4 Ga). The Highland sedimentary pile was thickened by intermittent granitoid intrusions, most of which are now charnockitic gneiss, and granulites, and basaltic sills, and dikes. All these metaigneous rocks now occur as conformable bands or layers due to intense polyphase deformation. The HC is bounded on the east by the amphibolite grade VC, composed mainly of granitic gneisses, basic gneisses, and migmatites, and they have ‘younger’ Nd model ages (1.1–1.8 Ga). The isotopic, and geochemical characteristics identify the precursors to the Vijayan rocks as I-type calc-alkaline granitoids originated at an ‘arc’-related tectonic environment. Thus, the earlier interpretation that the Vijayan rocks represent reworked HC was rejected. The granulite inliers within the VC, earlier considered as “resisters” to re-working, are now shown as overthrust klippen or rotated rafts of the HC. The WC, demarcated on the basis of Nd model ages (1.1–1.8 Ga) similar to those of the VC, lies west of the HC. It consists mainly of granitic gneisses, charnockitic gneisses, and migmatites, and the metamorphic grade ranges from amphibolite to granulite. Comprehensive geothermobarometric surveys constrain the P-T evolution of the three crustal units, and indicate that both the HC, and WC underwent near isobaric cooling, followed by a decompression with decreasing temperature. Extensive isotopic studies (U-Pb, Pb-Pb, Sm-Nd, Rb-Sr) have established a new geochronological framework for these high-grade rocks of Sri Lanka. The new framework has bracketed the age of high grade metamorphism in the three crustal units at 550–600 Ma. The recent advances in knowledge of the geology of Sri Lanka favour a strong geological correlation of the HC, and the VC of Sri Lanka, respectively, with the Lutzöw-Holm Complex, and the Yatmato-Belgica Complex in the East Antarctica. The geology of the WC suggests a possible correlation with Madagascar, and East Africa. The amalgamation of the three crustal units of Sri Lanka, is apparently related to the two distinct orogenic events that resulted in the assembly of the Gondwana supercontinent.

The Ghingee Granite, Tamil Nadu, South India: Geochemistry and Petrogenesis

The Palaeo-Proterozoic Ghingee granite is an anatectic granite formed in high grade granulite terrain by ultrametamorphism. The compositional variations both in major and trace elements observed in this granite (SiO 2 : 64.16-73.81; Fe 2O 3 : 0.12-2.19; FeO : 0.12-2.80; MgO : 0.10-2.19; CaO : 1.66-4.71; K 2O : 1.09-5.09; Ba: 223-1883 ppm; Cr : 4-60 ppm) are attributed to a) source rock heterogeneity and b) the tectonic disturbances that might have abruptly ended the anatectic melting process. The granite is compositionally similar to Perur, Closepet and Hyderabad granites and is formed during Archaean-Proterozoic transition by anatectic and crustal remelting processes.

A structural transect across the coastal mobile belt in the Brazilian Highlands (latitude 20°S): the roots of a Precambrian transpressional orogen

We present results of a detailed structural analysis from a 250 km-long, east–west-trending transect crossing the Coastal Mobile Belt, a part of the Precambrian orogen which lies between the eastern edge of the São Francisco craton and the Atlantic coast. The region exposes amphibolite–granulite grade metamorphic rocks and migmatites which formed at mid-lower crustal depths during the Brasiliano orogeny (0.63–0.52 Ga). This event marked the closure of the northernmost Adamastor Ocean and collision between the São Francisco and Congo Cratons during West Gondwana assembly. Brasiliano deformation resulted in W-vergent structures including thrust faults which accommodated kilometer-scale transport of crystalline basement, overturned kilometer-scale folds, sheath folds and penetratively developed gneissosity and schistosity. Isolated relics of an older folded fabric occur locally and may represent Transamazonian (2.2–2.0 Ga) deformation. The orogen is kinematically partitioned with the eastern 175 km dominated by moderately- to steeply dipping north-trending dextral strike-slip and oblique-slip faults and associated flower structures, whereas the western 75 km is dominated by W-vergent shallowly- to moderately east-dipping thrust faults. The boundary between these two provinces may mark a Brasiliano suture. Throughout the transect, quartzite and metasedimentary belts form strongly deformed zones between massive crystalline basement thrust sheets. The granulite-cored Serra do Caparaù massif, the highest mountains in South America outside of the Andes and Guyana shield, occupies a restraining bend between two Brasiliano dextral shear zones. The W-vergent Coastal Mobile Belt formed contiguously with the E-vergent Pan-African West Congo orogen now exposed along the conjugate margin of Africa. Thus an important late Precambrian boundary between structurally linked but kinematically opposed structural provinces must lie hidden in the extended offshore continental margins of either continent. Cretaceous opening of the South Atlantic and separation of the West Congo belt from the Coastal Mobile Belt may have been structurally influenced by this boundary.

Fluorine-rich Clinohumite from Ambasamudram Marbles, Southern India: Mineralogical and Preliminary FTIR Spectroscopic Characterization

AbstractHumite group minerals occur in the marbles of granulite grade at Ambasamudram, southern India. Detailed mineralogical and mineral chemical characterization indicate the mineral is a flourine-rich titanian-poor variety of clinohumite. Average MTi/Si values of 2.22 with typical XRD pattern indicate the mineral to be clinohumite. Petrological constraints on the clinohumite formation shows a high temperature (&gt;700°C) and low aco2 during its formation. The fluorine content of these clinohumites is the highest reported in any environment, with F/(F+OH) ratio reaching a value of 0.70. The high fluorine content reflect the high P-T condition of formation. The OH content of the clinohumites is around 0.59 mole fraction. Preliminary FTIR spectra of the clinohumites show eight sharp absorption peaks between wave numbers 3700 and 3400 cm−1 and a broad absorption band with a peak at 3840 cm−1. The sharp peaks are due to OH in the clinohumite. The high fluorine content of the Ambasamudram clinohumites possibly resulted from the isochemical reactions involving (OH-F) silicates such as amphiboles or phlogopites. The internal fluid buffering is also supported by the stable isotope as well as the petrological studies of the marble assemblages.

Geochemical Evolution of the Massif-type Anorthosite Complex at Bolangir in the Eastern Ghats Belt of India

granulite facies recrystallization [750–800°C, 7–8 kbar, a(H2O) The Bolangir massif-type anorthosite in the Eastern Ghats Belt of <0·25] during cooling from the igneous to the regional thermal India is composed of homogeneous anorthosite and subordinate regime. volumes of schlieric leuconorite in its marginal parts. The massif is separated from granulite grade country gneisses by coarse-grained garnetiferous granitoids. Extremely iron-rich ferrodiorites (FeO∗ 39–30 wt %, SiO2 36–43 wt %, dO = 7·7–8·0‰) occur