W.R. Van Schmus

A chemical-petrologic classification for the chondritic meteorites

Abstract A two-dimensional classification grid based on chemical and petrologic subdivisions of the chondritic meteorites is proposed. This grid extends the current chemical subdivisions to account for varying petrologic (implied metamorphic) properties of chondrites within the primary chemical groups. Six petrologic types are recognized, which, together with the five major chemical groups, produce 30 possible chondritic types. Representatives of 20 of the types are known. An Appendix giving the extended classification for 460 chondrites is included.

A survey of the unequilibrated ordinary chondrites

Abstract The iron contents of olivines and low-calcium pyroxenes in thirty-one ordinary (H-, L- and LL-group) chondrites were determined by microprobe analysis. Twenty-five of these contain inhomogeneous silicates and are, in this respect, unequilibrated. Petrographic study of these chondrites reveals that those of each chemical group whose silicates are most heterogeneous possess many characteristics which suggest that they are nearly unmetamorphosed. These are textural (absence of chondrule-chondrule and chondrule-matrix intergrowth; presence of a fine-grained, dark matrix; presence in some chondrules of silicic glass), mineralogical (near-absence of visible plagioclase; predominance of monoclinic low calcium pyroxenes), and chemical (high abundances of water, carbon, argon, and other volatile constituents). A complete gradation of properties exists between these meteorites and the equilibrated ordinary chondrites. We believe the least-metamorphosed chondrites of each chemical group closely approximate the unmetamorphosed precursors of the equilibrated ordinary chondrites. That the three chemical groups were distinct at the earliest stages of metamorphism indicates that their different contents of the siderophile elements were established during or before agglomeration of the parent materials. Close similarities in the mean compositions and proportions of olivines and pyroxenes in the least metamorphosed members of the three groups suggest that the siderophile fractionation among these groups may have resulted from mechanical mixture of a common silicate fraction with different proportions of metal rich material. There appears to be no comparably simple chemical or genetic relationship between the ordinary chondrites and the carbonaceous or enstatite chondrites. The origin of chondrules and chondrites remains, in the light of this study, an open question, for our results are consistent with any disequilibrium process entailing the production of chondrules through rapid cooling of molten and semi-molten droplets.

NEW U–PB AND SM–ND DATA FROM NORTH-CENTRAL CAMEROON AND ITS BEARING ON THE PRE-PAN AFRICAN HISTORY OF CENTRAL AFRICA

Abstract North-central Cameroon is part of the Central African Fold Belt (CAFB) to the north of the Congo craton. In pre-drift reconstructions of Pangea, the CAFB matches with the Borborema province (BP) of NE Brazil. U–Pb analyses of zircons and Sm–Nd analyses of minerals and whole rock for samples from north-central Cameroon show a long and complex crustal evolution beginning in the late Archean and extending to the late Neoproterozoic. This crustal structure can be correlated with crustal boundaries in Brazil. The region north of the Congo craton in Cameroon was primarily the locus of two successive orogenies, Eburnian at 2100 Ma and Pan African at 600 Ma. The Eburnian event fused the Congo craton with the Sao Francisco craton of Brazil to form a larger, middle Paleoproterozoic Congo–Sao Francisco craton. Archean ages (2900–3000 Ma) north of the Congo craton have been obtained primarily as inheritance (xenocrystic zircons and Nd isotopic signatures) in high-grade metamorphic rocks of the Adamaoua region, although no exposed Archean units have been recognized. The Eburnian event is recorded as metamorphic U–Pb ages in both metasedimentary and metaplutonic rocks. Some of the Paleoproterozoic rocks were derived from juvenile crustal growth and others show Sm–Nd isotopic signatures indicating derivation from reworking or melting of Archean crust. Mesoproterozoic Sm–Nd model ages of 1600–1000 Ma occur in northern Cameroon, northwest of the Tchollire-Banyo fault system and correlate with similar terranes in NE Brazil. These model ages are probably the result of mixing of Neoproterozoic material with older crust, as found in Brazil. Early Neoproterozoic Sm–Nd model ages of 1000–700 Ma on metasedimentary and metavolcanic rocks may correspond to a period of Neoproterozoic crustal rifting and basin formation dominated by more juvenile sources. The Poli basin is characterized by important development of juvenile rocks while the Yaounde and Lom basins have substantial amounts of older crustal material. Late Neoproterozoic U–Pb ages (620–580 Ma) correspond to the period of West Gondwana assembly with calc-alkaline magmatism, convergent tectonism, metamorphism and anatexis, development of regional strike-slip faults, and finally the uplift of the belt associated with alkaline subcircular massifs. The results from north-central Cameroon are dominated by the effects of Eburnian and Pan African orogenesis; there is little evidence for the widespread ca. 1000 Ma Cariris Velhos activity found in NE Brazil. Some Sm–Nd data and limited zircon data suggest that central Africa may have had a limited middle to early Neoproterozoic tectonic history, similar to that recently documented for the BP of NE Brazil, but more detailed studies will be needed to explore this possibility.

U Pb and Sm Nd geochronologic studies of the eastern Borborema Province, Northeastern Brazil: initial conclusions

The Borborema Province of NE Brasil comprises the central part of a wide Pan-African-Brasiliano orogenetic belt that formed as a consequence of late Neoproterozoic convergence and collision of the Sao Luis-West Africa craton and the Sao Francisco-Congo-Kasai cratons. New SmNd and UPb results from the eastern part of this province help to define the basic internal architecture and pre-collisional history of this province, with particular emphasis on delineating older cratonic terranes, their fragmentation during the Mesoproterozoic, and their assembly into West Gondwana during the Pan African-Brasiliano orogeny at ca. 600 Ma. The region can be divided into three major geotectonic domains: a) Rio Piranhas-Caldas Brandao massif, with overlying Paleoproterozoic to Neoproterozoic supracrustal rocks, north of the Patos Lineament; b) the Archean to Paleoproterozoic Sao Francisco craton (SFC) to the south; and c) a complex domain of Paleoproterozoic to Archean basement blocks with several intervening Mesoproterozoic to Neoproterozoic fold belts in the center (south of Patos Lineament and north of SFC). The northern and central domains comprise the Borborema Province. Archean basement gneiss and Transamazonian granulite of northern SFC are exposed in the southern part of the central domain, underlying southern parts of the Sergipano fold belt. Basement in the Rio Piranhas massif appears to consist mostly of Transamazonian (2.1 to 2.2 Ga) gneissic rocks; Nd model ages (TDM) of ca. 2.6 Ga for 2.15 Ga gneisses indicate a substantial Archean component in the protoliths to these gneisses. The Caldas Brandao massif to the east yields both Transamazonian and Archean UPb zircon and Nd (TDM) ages, indicating a complex architecture. Metasedimentary rocks of the Jucurutu Formation yield detrital zircons with original crystallization ages as young as 1.8 Ga, indicating that these rocks may be late Paleoproterozoic and correlate with other ca. 1.8 Ga cratonic supracrustal rocks in Brazil such as the Roraima Group and Espinhaco Group. Most metavolcanic and pre-Brasiliano granitic units of the Sergipano (SDS), Pajeu-Paraiba (SPP), Riacho Pontal (SRP), and Pianco-Alto Brigida (SPAB) fold belts in the central domain formed ~ 1.0 ± 0.1 Ga, based on UPb ages of zircons. Nd model ages (TDM) for these same rocks, as well as Brasiliano granites intruded into them and large parts of the Pernambuco-Alagoas massif, are commonly 1.3–1.7 Ga, indicating that rocks of the fold belts were not wholly derived from either older (> 2.1 Ga) or juvenile (ca. 1.0 Ga) crust, but include mixtures of both components. A simple interpretation of Brasiliano granite genesis and the Nd data implies that there is no Transamazonian or Archean basement underlying large parts of these fold belts or of the Pernambuco-Alagoas massif. An exception is a belt of syenitic Brasiliano plutons (Syenitoid Line) and host gneisses between SPAB and SPP that clearly has a Transamazonian (or older) source. In addition, there are several smaller blocks of Archean to Transamazonian gneiss that can be defined within and among these fold belts. These blocks do not appear to constitute a continuous basement complex, but appear to be isolated older crustal fragments. Our data support a model in which ca. 1.0 Ga rifting was an important tectonic and crust-forming event along the northern edge of the Sao Francisco craton. Our data also show that significant parts of the Borborema Province are not remobilized Transamazonian to Archean crust, but that Mesoproterozoic crust is a major feature of the Province. There are several small remnants of older crust within the area dominated by Mesoproterozoic crust, suggesting that the rifting event created several small continental fragments that were later incorporated into the Brasiliano collisional orogen. We cannot at present determine if the Rio Piranhas-Caldas Brandao massifs and the older crustal blocks of the central domain were originally part of the Sao Francisco craton or whether some (or all) of them came from more exotic parts of the Proterozoic Earth. Finally, our data have not yet revealed any juvenile terranes of either Transamazonian or Brasiliano age.

UPb and SmN edvidence for Eburnian and Pan-African high-grade metamorphism in cratonic rocks of southern Cameroon

Abstract UPb analyses of zircons and SmNd mineral and whole-rock analyses of samples from southern Cameroon document three distinct rock-forming or high-grade metamorphic events: ∼2900 Ma, ∼2050 Ma, and ∼600 Ma. Zircons from the Ebolowa charnockite in southern Cameroon yield an age of 2896±7 Ma, consistent with published RbSr total rock data for the region. SmNd crustal formation ages for these rocks are ∼3000 Ma, indicating that the granitoids and Ntem Series gneisses in the north end of the Congo Craton formed ∼2900 Ma ago and do not contain major contributions from older Archean crust. A second metamorphic event of Eburnian age is identified from re-equilibration of Sm and Nd in gneisses and metadolerites of the Ntem Complex at ∼2050 Ma. Zircons from the younger Nyong Series gneisses are a complex mixture of cores and overgrowths. The overgrowths in most cases formed during a high-grade metamorphic event between 2100 and 2000 Ma and clearly postdate the Ebolowa charnockite. The ages of the cores are not precisely known, but we believe most represent detrital zircons from 2900 Ma protoliths. SmNd crustal formation ages for samples of the Nyong Series gneisses are generally 2500 to 2900 Ma, indicating that their precursors were apparently derived mainly from the Archean craton with variable amounts of younger component. SmNd garnet-total rock isochrons for metamorphosed mafic rocks in the Nyong Series yield ages of ∼2050 Ma, and zircons from one of them yield an upper intercept age of 2037±10 Ma, further documenting the existence of a major event with magmatism and charnockitization at 2000 to 2100 Ma. We interpret the Nyong Series metasedimentary gneisses as post-2900 Ma cratonic cover that was deformed and metamorphosed during the 2000 to 2100 Ma Eburnian orogeny; we cannot define precise limits on the ages of sedimentation. Preliminary data on metamorphosed intrusive rocks associated with the Yaounde Series yield UPb concordant ages around 620 Ma, consistent with SmNd garnet—whole-rock ages from the same series. This dates the Pan African tectono-metamorphic activity in the southern end mobile belt. The Pan African lower intercepts of zircons and SmNd garnet-whole-rock ages from thNyong Series are the consequences of this event, which is further represented by a ∼590 Ma zircon age from a late syenitic pluton. Results from Cameroon are consistent with recent results from the western part of the Congo Craton farther south and from the northeastern part of the Sao Francisco Craton in Brazil. One interpretation of these results is that the Congo and Sao Francisco cratons were joined during the Ebumian-Trans-Amazonian orogeny ≈2100 Ma and are bordered to the north by a late Mesoproterozoic rift basin that was closed during the Pan-African-Brasiliano orogeny ∼600 Ma.

Juvenile accretion at 750–700 Ma in southern Brazil

A revista Economic Geology nao autoriza a publicacao de seus artigos em repositorios institucionais.

The composition and structural state of feldspar from chondritic meteorites

Abstract Electron microprobe analysis and X-ray diffraction techniques were used to determine the chemical compositions and structural states of feldspar in recrystallized (Type 6) chondritic meteorites from the L, LL, H, and E Groups. The feldspar in these meteorites is a secondary reerystallization product and is commonly partially or wholly converted to maskelynite. The feldspars are rich in the albite (Ab) end-member, containing small amounts of anorthite (An) and orthoclase (Or). Their compositions cluster near Ab82An12Or6 for H Group chondrites, Ab84An10Or6 for L Group chondrites, and Ab81An15Or4 for Enstatite chondrites. The compositional clusters for these three groups are essentially discrete and non-overlapping; ±1 mole per cent ranges on the An and Or contents will include most chondrites belonging to any given group. This compositional uniformity of feldspar within a single group reflects the bulk chemical uniformity of chondrites within a group, and the differences in feldspar composition from group to group are probably due to primary elemental fractionations. The feldspar from LL Group chondrites also has a narrow range of An content (10.0–10.5 mole per cent), but has a range in Or content from 0.9 to 5.2 mole per cent that is a consequence of varying bulk potassium contents within this group. X-ray diffraction patterns of crystalline feldspar from chondrites yield values of [2θ(131) –2θ(1 3 1)] = 1.75 ± 0.10° indicating a high-intermediate structural state. Unit cell refinements were carried out for some samples, and consideration of the cell angle γ puts the structural state in the high to high-intermediate range. Although such structural states are indicative of high equilibration temperatures (650–850°C), the fact that chondritic feldspars are still in the high-temperature state does not preclude cooling of these meteorites over 106-108 yr within asteroidal sized bodies. Finally, all of our observations and data are consistent with earlier suggestions that maskelynite is produced by shock.

Proterozoic links between the Borborema Province, NE Brazil, and the Central African Fold Belt

Abstract The Congo (CC) and the São Francisco (SFC) cratons were joined at about 2.05 Ga; northern parts of Palaeoproterozoic basement subsequently underwent extension at about 1 Ga, forming intracratonic basins. Neoproterozoic metasedimentary rocks in these basins yield detrital zircons as young as 630 Ma. The Brasiliano and Pan-African (c. 620–580 Ma) assembly of West Gondwana extensively altered this system. The Sergipano domain occurs north of the SFC, and the comparable Yaoundé domain occurs north of the CC. Crust north of the Sergipano domain comprises the Pernambuco–Alagoas (PEAL) domain. The NE–SW-striking Tcholliré–Banyo fault in Cameroon may extend southwestwards between the PEAL and Sergipano domains, defining northern limits of abundant SFC/CC basement. The Adamawa–Yadé domain in Africa does not appear to extend into Brazil. The Transverse domain of Brazil is a collage of Palaeoproterozoic crustal blocks, the 1.0 Ga Cariris Velhos orogen (CVO), late Neoproterozoic basins, and Brasiliano granites. The CVO extends ENE for more than 700 km in Brazil, but eastern continuation into Africa has not been identified. North of the Transverse domain contiguous c. 2.15 Ga gneisses comprise basement of Rio Grande do Norte and Ceará domains, which continue eastwards into western Nigeria and western Sahara.

U-Pb and Sm-Nd geochronology of the neoproterozoic granitic-gneissic Dom Feliciano belt, Southern Brazil

The Brasiliano Cycle in southern Brazil and Uruguay is represented by three major NE-SW trending geotectonic units: the Vila Nova belt, Tijucas belt and Dom Feliciano belt. The Vila Nova belt is located in western part of Rio Grande do Sul State; its evolution took place between 900 and 700 Ma and it corresponds to one of the few areas with juvenile accretion during the Neoproterozoic in Brazil. The Tijucas belt, situated between the Vila Nova and Dom Feliciano belts, consists of a rift-related Mesoproterozoic (?) volcano-sedimentary sequence which was strongly deformed during the Brasiliano cycle. The Dom Feliciano belt is located along the eastern coast of southern Brazil and Uruguay and is a typical granite-gneiss-migmatite terrane. This belt is a key area for understanding West Gondwana assembly during Neoproterozoic and Early Paleozoic times, because of its direct connection to the Gariep and Damara belts in southern Africa. The present study defines the main tectonic phases of the ca. 600 Ma Dom Feliciano event in the Sao Feliciano belt. U-Pb zircon data for flat-lying gneisses yield ages between 610 ± 5 Ma and 616 ± 2 Ma, which we believe correspond to the approximate age of thrusting. The strike-slip deformation (main transcurrent phase) is well dated by U-Pb zircon ages for the syn-transcurrent granites (Arroio Moinho Granite, 595 ± 1 Ma; Encruzilhada do Sul Granite, 594 ± 5 Ma). These results indicate a relatively rapid evolution, from about 620 Ma (upper limit for the age of the gneiss) to 594 Ma (syn-trancurrent granites), for the known thrust related and strike-slip related tectonic phases of the Dom Feliciano belt. Sm-Nd results can be considered in three major groups. The first group (I) includes Brasiliano gneisses, granitoids, and one anorthosite with TDM ages of ca. 2.0 Ga and very negative eeNd(600) values. They may represent either direct melting of Transamazonian (Paleoproterozoic) basement or extensive contamination with older material of Paleoproterozoic to Archean age. The second group (II) includes granitoids and gneisses with TDM model ages from 1.31 to 1.41 Ga. The third group (III) comprises samples with TDM ages between 1.58 to 1.75 Ga. For groups II and III it is clear these rocks or their protoliths represent pre-Brasiliano continental crust. Unlike Group I rocks, groups II and III granites and gneisses may also contain a small fraction of a juvenile Brasiliano material. However, we have not yet found any sample from the Dom Feliciano belt with a Neoproterozoic TDM age and positive eNd value at 600 Ma that could be considered largely juvenile. Based on results from the Vila Nova belt, in which the main orogenic process developed between 753 and 704 Ma, we conclude that the Vila Nova belt was stable for over 100 Ma before the Dom Feliciano event reached its peak. It is probable that the collage of terranes in the Dom Feliciano belt and the region comprised by the Tijucas and Vila Nova belts were assembled during the Dom Feliciano event (ca. 600 Ma).

Proterozoic collisional tectonism in the Trans-Hudson orogen, Saskatchewan

Isotopic and structural data from the juvenile Reindeer zone of the Trans-Hudson orogen, northern Saskatchewan, indicate a pre-1.85 Ga thermotectonic event, possibly reflecting arc-continent collision, followed by a more extensive, nappe-forming, ca. 1.83-1.80 Ga thermotectonism during terminal continent-continent collision. Preliminary data from the adjacent, ensialic Cree Lake zone suggest high-grade reworking of Archean crust by the pre-1.85 Ga event. In the Rae province to the west, high-grade metamorphism and reworking of Archean crust occurred about 2.0 Ga and may be related to the formation of the coeval Taltson magmatic zone.