Proterozoic Orogenic Belts Research Articles

New insights into the lithosphere beneath the Superior Province from Rayleigh wave dispersion and receiver function analysis

SummaryWe present new models of shear wave velocity structure of the lithosphere and upper mantle beneath northern and eastern Ontario and surrounding regions. The study area is dominated by the Archean Superior Province, with Proterozoic orogenic belts to the south and southeast. Over the course of ∼3 Ga, the region has been shaped by accretionary and orogenic events, periods of rifting and the influence of a number of mantle hotspots.New data from the broad-band POLARIS/FedNor seismic network, along with permanent stations of the Canadian National Seismograph Network, are used for a seismic study of the crust and upper mantle beneath the region. This article concentrates primarily on a study of the upper mantle, carried out using two-station phase velocity analysis of teleseismic Rayleigh waves. Dispersion curves were measured for 100 two-station paths across the region, of which 30 were analysed to give 1-D path-averaged models of shear wave velocity structure. Dispersion measurements yielded phase velocity data for periods from ∼25 to ∼170 s, allowing us to resolve shear wave velocities to a depth of ∼300 km. The dispersion curves indicate an upper-mantle structure broadly characteristic of continental shield regions, but there are significant variations in the properties of the data across the province.In the central and western Superior Province, inferred lithospheric thicknesses vary between approximately 140 and 200 km. In general, the models are characterized by a well-developed ‘lid’ of high-velocity mantle underlain by a zone of reduced velocity. The highest lid velocities are modelled along a path whose azimuth corresponds to the fast direction of anisotropy resolved through SKS splitting analysis. The far northeast of Ontario has the thickest (∼220–240 km) lithospheric lid. Eastern Ontario has a complex structure that gives rise to a large variation in the structures modelled from the two-station paths. Apparent lithospheric thicknesses range from ∼100 to ∼220 km. This large variation is interpreted to arise from highly heterogeneous mantle structure and/or anisotropy across the Abitibi Greenstone Belt, the Southern Province, and the Proterozoic Grenville Province.Receiver function analysis shows variations in Moho depth and bulk Poisson's Ratio in the Superior Province crust. The crustal thickness varies from ∼34 to ∼44 km through most of the province, with a zone of anomalously thick crust (48 km) in the Kapuskasing Structural Zone region. Measurements of bulk Poisson's Ratio indicate that the crust is highly felsic in some parts of the province, but includes a significant mafic component in parts of the western Superior and eastern Ontario, regions that have been affected by uplift or rifting.The new results, together with information from other recent studies of shear wave splitting and body-wave tomography, show that the Superior craton is far from a homogeneous entity. Rather it is divisible, on the basis of thickness, anisotropy and velocity structure of the crust and lithosphere, into distinct subdomains that reflect the complex tectonic history of this region. The large range of inferred thicknesses of the crust (34–48 km) and lithosphere (100–240 km) within the Superior craton represents a significantly greater variability than has been previously observed for Archean lithosphere.

Geometry and evolution of the Mesoproterozoic Irumide Belt of Zambia

Abstract The Irumide Belt of eastern Zambia is one of a number of complex Proterozoic orogenic belts found in the African continent. The belt lies along the southeastern flank of the Tanzania-Bangweulu Craton and to the NW of the Mozambique Belt of East Africa. The dominant tectonic features of the belt are ductile shear zones, thrusts and folds related to a Mesoproterozoic, NW-SE crustal shortening event, and late-tectonic granite emplacement. Later tightening of Irumide structures and localized deformation of granites is also widespread. The timing of post-Irumide deformation is poorly constrained but is believed to be a mix of Pan-African and Late Palaeozoic reactivation and thermal reworking. This complex history is recently overprinted by significant Tertiary rifting. The Irumide deformation is evident in three tectonic zones stretching from the NW foreland of Mporokoso to the SE hinterland of Lundazi: (1) the NW-facing basement shear zones and associated cover deformation of the Luongo Fold Belt; (2) the Foreland Fold Belt of large upright folds defined by the kilometre-thick quartzite ridges of the Irumi, Shiwa Ngandu and Isoka hills; (3) an intensely deformed Internal Zone of basement and granite gneisses folded into major domes and associated tight synclines. Post-tectonic granites intrude the last of these zones and define an end to the Irumide deformation. The linking of these three zones is based on geometric, kinematic and geochronological evidence. The main Irumide deformation and metamorphism is believed to have occurred between 1100-950 Ma with a metamorphic peak between 1050 and 1000 Ma. Termination of Irumide deformation is dated by the emplacement of the post-tectonic granite and K/Ar biotite and hornblende closure dates of c. 950 Ma. Similar K/Ar dates from the Luongo shear zone indicate biotite closure and cooling during the period 1020-950 Ma. Together with parallel kinematic indicators, this supports a coeval development and tectonic link between the Irumide Belt and the Luongo Fold Belt. The intracontinental Irumide Belt deformation was driven by an accretionary collisional margin, to the SE of the present-day Irumide Belt, in southern Malawi and Mozambique. This accretionary margin is today represented by arc and island-arc magmatism and the complex terranes of the Mozambique Belt of East Central Africa.

The role of radiogenic heat production in the thermal evolution of a Proterozoic granulite-facies orogenic belt: Eastern Ghats, Indian Shield

The Eastern Ghats Belt (EGB) is a deeply eroded Proterozoic orogenic belt juxtaposed against the Archaean Dharwar–Bastar–Singhbhum cratons in the Indian Shield. The tectono-metamorphic history of this belt is broadly similar to the Grenvillian orogenic belts. The EGB exposes dominantly the granulite-facies rocks such as charnockites, khondalites and migmatitic gneisses with less abundant intermediate to mafic granulites. The granulites are intruded by small plutons of granites, syenites and anorthosites. The northern and southern parts of the belt (NEGB and SEGB) have distinct crustal histories, but 1600–1000–550 Ma old tectono-metamorphic events are common to both the belts. In this research, we attempt to understand the present-day and past thermal state of this orogen on the basis of our radioelement (K, U and Th) measurements at 562 sites using in-situ gamma-ray spectrometry, with other geological and geophysical constraints. The dominant rock types of the NEGB and SEGB have similar radioelement abundances (>3% K, 3 ppm U and 30 ppm Th) and heat production. Heat production of charnockites, gneisses, khondalites and intermediate granulites of the NEGB is 2.9, 2.8, 2.9 and 1.1 μW m−3, respectively. In the SEGB, they are 2.7, 2.5, 2.8 and 0.6 μW m−3, respectively, and the mafic granulites are the lowest in heat production (0.3 μW m−3). On the basis of the heat production data and crustal petrologic models, we show that the crustal contribution of the NEGB and SEGB is 54 and 45 mW m−2, respectively, which are broadly in good agreement with the available surface heat flow data. Thermal models of the present-day EGB crust indicate that the temperature at the Moho is ∼550 °C. Radioelements and heat production of the Eastern Ghats granulites are higher than the other granulite belts in the Indian shield. The crustal radiogenic heat contribution and Moho temperatures of the EGB are higher than the adjoining Archaean cratons. This could also be responsible for the development of inverted metamorphic isograds in the tectonic boundary between the EGB and the adjoining cratons during a collisional orogenic event at about 550 Ma ago.

Hafnium Isotope and Trace Element Constraints on the Nature of Mantle Heterogeneity beneath the Central Southwest Indian Ridge (13°E to 47°E)

Hafnium isotope and incompatible trace element data are presented for a suite of mid-ocean ridge basalts (MORB) from 13 to 47 E on the Southwest Indian Ridge (SWIR), one of the slowest spreading and most isotopically heterogeneous mid-ocean ridges. Variations in Nd–Hf isotope compositions and Lu/Hf ratios clearly distinguish an Atlantic–Pacific-type MORB source, present west of 26 E, characterized by relatively low eHf values for a given eNd relative to the regression line through all Nd–Hf isotope data for oceanic basalts (termed the ‘Nd–Hf mantle array line’; the deviation from this line is termed DeHf) and low Lu/Hf ratios, from an Indian Ocean-type MORB signature, present east of 32 E, characterized by relatively high DeHf values and Lu/Hf ratios. Additionally, two localized, isotopically anomalous areas, at 13–15 E and 39–41 E, are characterized by distinctly low negative and high positive DeHf values, respectively. The low DeHf MORB from 13 to 15 E appear to reflect contamination by HIMU-type mantle from the nearby Bouvet mantle plume, whereas the trace element and isotopic compositions of MORB from 39 to 41 E are most consistent with contamination by metasomatized Archean continental lithospheric mantle. Relatively small source-melt fractionation of Lu/Hf relative to Sm/Nd, compared with MORB from faster-spreading ridges, argues against a significant role for garnet pyroxenite in the generation of most central SWIR MORB. Correlations between DeHf and Sr and Pb isotopic and trace element ratios clearly delineate a highDeHf ‘ Indian Ocean mantle component’ that can explain the isotope composition of most Indian Ocean MORB as mixtures between this component and a heterogeneous Atlantic–Pacific-type MORB source. The Hf, Nd and Sr isotope compositions of Indian Ocean MORB appear to be most consistent with the hypothesis that this component represents fragments of subduction-modified lithospheric mantle beneath Proterozoic orogenic belts that foundered into the nascent Indian Ocean upper mantle during the Mesozoic breakup of Gondwana.

Deep chemical structure of the southern African mantle from kimberlite megacrysts

Research Article| June 01, 2004 Deep chemical structure of the southern African mantle from kimberlite megacrysts David R. Bell; David R. Bell Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, N.W., Washington DC 20015, U.S.A., Present address: Department of Chemistry and Biochemistry and Department of Geological Sciences, Arizona State University, Tempe AZ 85287-1604, U.S.A., Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa e-mail: David.R.Bell@asu.edu Search for other works by this author on: GSW Google Scholar Rory O. Moore Rory O. Moore Mineral Services Canada Inc., 409 Granville Street Ste 1300, Vancouver, B.C., V6C 1T2, Canada. Department of Geological Sciences, University of Cape Town, Rondebosch 7700, South Africa Email: Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2004) 107 (1-2): 59–80. https://doi.org/10.2113/107.1-2.59 Article history first online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David R. Bell, Rory O. Moore; Deep chemical structure of the southern African mantle from kimberlite megacrysts. South African Journal of Geology 2004;; 107 (1-2): 59–80. doi: https://doi.org/10.2113/107.1-2.59 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract Xenoliths of the Cr-poor megacryst suite were formed by the crystallization of magmas in the deeper regions of subcontinental lithospheric mantle of southern Africa, and may be used to probe its compositional structure. Some 2300 pyrope garnet megacrysts from 61 kimberlites in southern Africa define four broad arrays on the basis of Cr2O3 – Mg# systematics, referred to as H-, M-, L-and diffuse trends. M-trend megacrysts, representing typical Cr-poor megacryst compositions, occur over much of the southwestern Kaapvaal Craton, continuing into the Proterozoic Namaqua Province to the west. In contrast, all suites examined from the Zimbabwe Craton, as well as the Premier kimberlite cluster on the Kaapvaal craton, are dominated by H-trend megacrysts. The boundary between the two domains corresponds broadly to the locus of a region of slower seismic velocity in the uppermost mantle. L-trend megacrysts with very low Cr contents and diffuse, steeply inclined trends occur around the periphery of the continent and in zones of previous tectonic activity. Variations in Cr2O3 – Mg# trends are attributed to different megacryst magma sources and variations in the chemical character and depth of lithospheric mantle into which megacryst magmas are emplaced. In addition to the classic fractional crystallization processes, partial melting and magma-wall rock interaction are recognized as potentially important influences on megacryst composition. However, uncertainties in the quantitative contributions of each process render speculative the broader-scale geodynamic interpretations of the regional distribution patterns.Megacrysts in orangeites appear to reflect an origin and evolution in depleted lithospheric mantle. High-Cr megacrysts in kimberlites of the northern cratonic areas bear the influence of lithospheric mantle and their distribution its accessibility to infiltration. Commonly coherent M-trends of the southern Kaapvaal could be interpreted to indicate that the lithosphere in this region was locally less easily infiltrated, that complete conversion of depleted lithosphere to fertile mantle resembling the megacryst magma source by high magmatic flux occurred, or that shallow emplacement of megacryst-magma source mantle into the lower regions of the lithosphere preceded megacryst emplacement, effectively shielding the magmas from overlying lithosphere. The steeply trending, diffuse patterns are thought to result from some combination of changes in phase proportions in response to shallower intrusion of megacryst magmas, mixed source regions that include involvement of low-Cr, possibly oceanic mantle, and the enhanced degree of lithospheric interaction afforded by tectonized and more fertile lithosphere of Proterozoic orogenic belts and intra-continental rifts. Present megacryst sampling in space and time allows for the lateral differences to be long-lived features of the subcontinental mantle, and permit a possible origin for megacryst magmas with OIB affinities within the continental mantle root or boundary layer. These speculations may be resolved with an increased sampling density and understanding of megacryst petrogenesis. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Heat flow, deep temperature and thermal structure across the orogenic belts in Southeast China

The pattern of heat flow density in Southeast China is presented in the form of heat flow map based on 150 values of heat flow measured in recent years. A two-dimensional numerical solution of the heat conduction equation was used to calculate the temperature field to a depth of 60 km along two geotraverses: the 360 km long Wenzhou-Tunxi (WT) and the 1000 km long eastern segment of the Quanzhou-Heishui (QH). They cross all the major tectonic elements of orogenic belts in Southeast China. The steady-state model was solved using the finite element method. The distribution of heat production was derived from seismic refraction results applying the experimental relationship between seismic velocity ( v p ) and radiogenic heat production ( A) incorporated with the observed surface heat generation data in the near-surface crust. The thermal conductivity was assigned to be temperature dependent and related to crustal composition. In the upper crust, the temperature field is found to correspond to the regional variation of the surface heat flow, whereas the mantle heat flow is decisive for the temperature field in the lower crust. Low temperatures are typical of the Later Proterozoic orogenic belt and the Early Paleozoic Foreland Fold-Thrust Belt (FTB) (Moho temperature 415–575°C at a depth of 29–37 km). Temperatures increase toward the Triassic orogenic belt, and the maximum increase in Moho temperature and mantle heat flow occurs in the Triassic orogenic belt and its FTB (550–780°C). Low temperatures (400–500°C at Moho) are also calculated in the Coastal Min-Yue Belt supposed to be a Cretaceous orogenic belt. The “thermal” lithospheric thickness varies between 70 and 190 km, decreasing eastward from the Later Proterozoic Jiangnan in the west to the Triassic Zhe-Min orogenic belts. The mantle heat flow values across the orogenic belts in Southeast China range from 24 to 48 mW m −2 accounting for 51–63% of the surface heat flow.

The 1998 edition of the National Geological Map of Botswana

A new National Geological Map of Botswana incorporates data acquired from a variety of sources; the map is produced as a 1:1 million hardcopy as well as in digital format. The new map shows the pre-Kalahari Group geology. The oldest rocks are exposed in eastern Botswana where three Archaean terranes are recognised: the western parts of the Kaapvaal and Zimbabwe Cratons and the western part of the Limpopo Mobile Belt. All three terranes are lithologically similar but differ in their structural styles and in the timing of major thermal events. The oldest (pre-3.0 Gal high-grade metamorphic rocks are found in the Kaapvaal Craton, and the youngest in the Limpopo Mobile Belt, which appears to record Palæoproterozoic ductile shearing. Proterozoic orogenic belts, mostly concealed beneath Karoo rocks, define the western limits of the Archaean terranes and pprogressively young westwards away from the Archaean rocks. The Palwoproterozoic Magondi and Kheis Belts are well-defined by regional magnetic maps, but both are very poorly exposed in Botswana. The Kheis Belt trends due north from South Africa into central Botswana to define the western edge of the Kaapvaal Craton. The western part of the Magondi Belt, as well as all of a Mesoproterozoic (Kibaran) belt and rift are overprinted by the Neoproterozoic Damara Belt; all have pronounced northeasterly trends. During the Palæoproterozoic, there was also significant intraplate magmatism, sedimentation and deformation within the Archæan terranes. Some of the magmatism (in southeastern Botswana) was contemporaneous with, and lithologically similar to, the Bushveld Igneous Complex of South Africa. The main feature of the Mesoproterozoic geology of Botswana is a northeast trending rift that extends right across the northwest of the country and which is partly infilled with ca 1 106 Ma volcanic rocks. Neoproterozoic sedimentary rocks overlie the volcanics within the rift. The various rocks are exposed along the Ghanzi Ridge and to the northeast in the Chobe District. New detailed airborne magnetic surveys in northwest Botswana (Ngamiland) show the detailed geology of the northeast trending inland branch of the Damara Belt and exactly define its northwestern and southeastern boundaries. The southeastern part of the Damara Belt comprises the Mesoproterozoic volcanics of the Kgwebe Formation and the Neoproterozoic Ghanzi Group sedimentary strata. The full extent of the volcanics, and of the three formations recognised in the Ghanzi Group, is shown on the new map. Deformation of these rocks increases to the northwest where they are bounded by the tectono-stratigraphical Roibok Group. To the northwest of the Roibok Group are poorly dated granitoid rocks separated into several units that are locally overlain by carbonate-dominated sequences. A cover sequence of metasedimentary rocks with northnorthwest trending folds lies northwest of the Damara Belt. These sediments may overlie the southernmost part of the Congo Craton in the extreme northwest of Botswana. Neoproterozoic/ Lower Palæozoic sediments of the Nama Group partly infill a foreland basin to the south of the Damara belt in western Botswana. Karoo strata deposited within the Kalahari Basin underlie central Botswana. The distribution of the four major sedimentary groups, as well as of the capping basalts, is shown. The total thickness of the sediments is < 2000 m and the basalts are up to 1000 m in thickness. The sediments comprise a lower sequence (Dwyka and Ecca Groups) related to regional sagging and an upper sequence (Beaufort and Lebung Groups) that succeeded regional uplift that created intra-Karoo unconformities. Karoo sedimentation commenced towards the end of the Carboniferous Period and the basalts were extruded at about 180 Ma before Present. Wherever there have been detailed studies undertaken on the Karoo rocks, they show intense faulting that may or may not mimic structures in the pre-Karoo bedrock. The faulting appears to be post-sedimentation. No evidence was found for growth faults producing abnormal thicknesses of Karoo sediments. It is always possible to correlate the internal stratigraphy, at least at the formational level across the faults. Abnormal thicknesses of the basalts are preserved on the downthrow sides of the major faults. A major dyke swarm coeval with the extrusive basalts trends east-southeast right across north-central Botswana to cut across older structural trends. Over 200 kimberlites are shown on the new map. The kimberlites are distributed throughout Botswana in a number of separate clusters. Most of the kimberlites are of Cretaceous age. Isopachs are shown of the Kalahari Group, which is generally < 180 m in total thickness.

METAMORPHISM OF THE CANADIAN SHIELD, ONTARIO, CANADA. II. PROTEROZOIC METAMORPHIC HISTORY

This paper is a complement to the Tectonometamorphic Map of the Canadian Shield, and contains a brief description of the metamorphic history of Proterozoic orogenic belts and rock sequences in the Canadian Shield in Ontario, with a focus on rocks of the Southern and Grenville provinces. Metamorphism in the Southern Province has generally been attributed to the Penokean orogeny (~1879–1820 Ma), although the timing of this metamorphism is poorly constrained by either relative or absolute ages. In the Sault Ste. Marie – Sudbury area, previously described regional Na- and K-metasomatism has likely altered the original assemblages of metamorphic minerals, making unravelling of the metamorphic history of these rocks problematic. Metamorphism appears to postdate the emplacement of the Sudbury Igneous Complex at 1850 Ma; however, it is unclear if regional metamorphism in the Sault Ste. Marie – Sudbury area is related to the early (1870–1820 Ma) or late-stage (1740–1700 Ma) events of the Penokean orogeny. Three main metamorphic events are associated with the Grenville (1300–950 Ma) orogeny. These events are not necessarily orogen-wide in their effects. The first, at 1250–1240 Ma, affected rocks of the Composite Arc Belt, and is similar in style to that found in Superior Province granite–greenstone belts. It consists of a dominant subgreenschist- to greenschist-facies event, with amphibolite-facies assemblages developed near larger plutonic bodies. The next event (1190–1170 Ma) is restricted mainly to the southern part of the orogen (southern Laurentian margin, Composite Arc and Frontenac–Adirondack belts), and is responsible for regional upper-amphibolite- to granulite-facies metamorphism. In parts of the orogen, metamorphic rocks formed at this time are well preserved ( e.g., Parry Sound and Frontenac domains), whereas in other areas ( e.g., Adirondack Highlands), the rocks are overprinted by younger, high-grade metamorphism. Pan-Grenville greenschist- to granulite-facies metamorphism in Ontario in the period 1070–1050 Ma was associated with major thrusting. In deeper structural levels, this event persists to 990 Ma. Tectonic unroofing after 1070 Ma likely played a major role in the current distribution of metamorphic rocks within the Grenville Province in Ontario.

Shear wave splitting observations in the Archean Craton of western Superior

Shear wave splitting observations in the Archean Superior Province of the Canadian Shield show moderate to large delay times (1.1–2.1 s) with azimuths suggesting upper‐mantle anisotropy is subparallel to the structural grain of the craton. Two regions with uniform anisotropy azimuth (62°±7° and 87°±3°) are separated by a transitional zone showing strong dependence of observed azimuth on source direction, suggesting that lateral structure is being observed. No splitting was observed at stations near the coast of Hudson Bay, continuing a pattern of weak splitting in surrounding Proterozoic orogenic belts. Current absolute plate motion is consistent with the direction of anisotropy, but would not explain the regional contrasts. Instead, the anisotropy appears to be related to Archean structure and tectonic history.

Rifting Archaean lithosphere: the Eyasi-Manyara-Natron rifts, East Africa

Long (&gt;50km) normal faults bound one or both sides of narrow basins within the East African Rift System, but the dimensions and internal geometry of individual basins vary along its length. We examine basins in N Tanzania that developed in Archaean and Late Proterozoic (Pan-African) crust, and the relationship of Neogene-Recent faulting and volcanism to pre-existing lithospheric structure. In northern Tanzania the c . 50 km-wide Eastern (Gregory) Rift splays into three seismically active arms, with active extension distributed across a 200 km wide zone. We use new gravity and aeromagnetic data, and existing seismic, gravity, heat flow, and geochemical data to model lithospheric structure beneath the Archaean craton and Proterozoic orogenic belts, and its influence on rift development in N Tanzania. Depths to source bodies determined from Euler deconvolution of aeromagnetic data (1 km grid) indicate that c . 40 km wide basins are less than 3.5 km deep, with basin depths decreasing to the south, consistent with depths estimated from gravity anomalies. Asymmetric basins in the Archaean and zone of reworked Archaean and Proterozoic nappes are bounded by unusually long (100 km) border faults associated with seismicity to depths &gt;25 km. Estimates of flexural rigidity, or effective elastic thickness (TJ, suggest that the lithosphere beneath the Tanzania craton, including the western rift arm, is stronger (64 ± 5 km) than that beneath the Proterozoic belt and the transition zone (30 ± 4 km), with the lowest T e values found beneath the central rift arm (23 +2 4 km). Heat flow, seismicity, and mantle xenolith data also indicate that the lithosphere beneath the Archaean craton was and is colder and stronger than the post-Archaean lithosphere. Geophysical and geochemical data suggest that (a) pre-existing heterogeneities in the Archaean crust influenced the orientation of border faults bounding basins, and (b) that topography at the base of the lithosphere guided the location of rifting in Tanzania, producing a broader rift zone. These results indicate the persistence of a deep cratonic root despite the impingement of a mantle plume in Cenozoic time.

Pseudostratigraphy and origin of the Early Proterozoic Payson ophiolite, central Arizona

The Payson ophiolite (ca. 1.73 Ga), exposed within the Early Proterozoic orogenic belt of central Arizona, is a shallow-dipping, pseudostratigraphic sequence of gabbro, sheeted dikes, and submarine volcanic rocks, partly disjointed by later intrusion and deformation. An older basement complex occurs as roof pendants in the gabbro and screens in the sheeted dike complex and was tilted and eroded prior to development of the ophiolite. Gabbro-dike mingling and mutual intrusion in the transition from gabbro to sheeted dikes indicates that the dikes are rooted in the gabbro. Analysis of detailed measured sections of well-exposed sheeted dikes (600 m) reveals 90%–100% dikes with lateral variations in the proportions of tonalitic or dacitic dikes and basement screens. The transition from sheeted dikes to submarine basalts is abrupt and marked by a stratigraphically continuous zone of intensely altered rocks. The sheeted dike complex, pseudostratigraphic structure, and synmagmatic hydrothermal alteration of the Payson ophiolite are diagnostic of a crustal section formed by sea-floor spreading. The Payson ophiolite intrudes, and is intruded by, arc granitoids and is overlain by arc-derived volcanic and sedimentary rocks. A model of an intra-arc basin formed along an arc parallel strike-slip fault is proposed to account for (1) screens of older arc crust, (2) inferred arc-parallel extension, and (3) juxtaposition of the ophiolite with a distinct arc terrane prior to regional convergent deformation and accretion of the arc to North America during the Yavapai orogeny (ca. 1.70 Ga).

The main uplift processes of deep crustal rocks of the early proterozoic orogenic belt in Daqingshan area, Inner Mongolia

Deep crustal rocks of the Early Proterozoic orogenic belt (DCR) in Daqingshan area mainly included the synorogenic-metamorphic khondalite series and reworked Archean-basement granulites. During the early stage about 1900–1800 Ma, the DCR consisted of five huge slices which obducted over and stacked up along a series of high-temperature shear zone (HTSZ). and were subsequently uplifted from the deep crustal level to the middle one accompanied with granulite-amphibolite facies retrometamorphic reworking in HTSZ. From 1800 – 1700 Ma, some slices and HTSZ mentioned above were oveprinted by several low-temperature shear zone (LTSZ), of which the low-angle oblique thrusts caused these DCR to uplift again from the middle crustal level to the shallow one, and experienced retrometamorphic reworking of greenschist facies. These two stages of uplifting processes should be included in an entire Early Proterozoic orogenic cycle.

Tectonic Units and Their Fundamental Characteristics on the Northern Margin of the Alxa Block1

Abstract The northern margin of the Alxa block is the junction of a tectonic units. Four first—order tectonic units are distinguished: 1. the Yagan structural zone characteristic of an immature island arc; 2. the Zhusileng—Hangwula structural zone, which was a passive continental margin in the Early Palaeozoic and was transformed into an active continental margin in the Late Palaeozoic; 3. the Shalazha structural zone characteristic of a mature island arc; 4. the Nuru—Langshan structural zone, which was a Proterozoic orogenic belt and later evolved into an extensional transitional crust in the Palaeozoic. The above—mentioned tectonic units differ remarkably in sedimentary formations, magmatic rock associations, metamorphism and geochemistry and are bounded by faults between one another.

Zircon ages and the distribution of Archaean and Proterozoic rocks in the Rauer Islands

The application of zircon U-Pb geochronology using the SHRIMP ion microprobe to the Precambrian high-grade metamorphic rocks of the Rauer Islands on the Prydz Bay coast of East Antarctica, has resulted in major revisions to the interpreted geological history. Large tracts of granitic orthogneisses, previously considered to be mostly Proterozoic in age, are shown here to be Archaean, with crystallization ages of 3270 Ma and 2800 Ma. These rocks and associated granulite-facies mafic rocks and paragneisses account for up to 50% of exposures in the Rauer Islands. Unlike the 2500 Ma rocks in the nearby Vestfold Hills which were cratonized soon after formation, the Rauer Islands rocks were reworked at about 1000 Ma under granulite to amphibolite facies conditions, and mixed with newly generated felsic crust. Dating of components of this felsic intrusive suite indicates that this Proterozoic reworking was accomplished in about 30–40 million years. Low-grade retrogression at 500 Ma was accompanied by brittle shearing, pegmatite injection, partial resetting of U-Pb geochronometers and growth of new zircons. Minor underformed lamprophyre dykes intruded Hop and nearby islands later in the Phanerozoic. Thus, the geology of the Rauer Islands reflects reworking and juxtaposition of unrelated rocks in a Proterozoic orogenic belt, and illustrates the important influence of relatively low-grade fluid-rock interaction on zircon U-Pb systematics in high-grade terranes.

How well established is isobaric cooling in Proterozoic orogenic belts? An example from the Arunta inlier, central Australia: Comment and Reply

How well established is isobaric cooling in Proterozoic orogenic belts? An example from the Arunta inlier, central Australia: Comment and Reply

Episodic Early Proterozoic granitoid plutonism in the Makkovik Province, Labrador: U–Pb geochronological data and geological implications

Episodic granitoid plutonism in the Early Proterozoic Makkovik Province of the easternmost Canadian Shield is revealed by high-precision U–Pb zircon studies of a range of intrusive rock types. The oldest granites yield ages of 1893 ± 2 and 1891 ± 5 Ma and document a previously unrecognized event that may correlate with an early migmatization of reworked Archean basement. These dates also constrain early structural reworking of the basement, the earliest deformational event grouped as part of the Makkovikian orogeny. Four samples have essentially identical zircon ages of 1801 ± 2, 1802 ± 2, [Formula: see text], and 1825–1799 Ma, and a fifth is slightly older, at [Formula: see text]. These ages suggest correlation with local volcanic sequences, dated in part at 1807 ± 4 Ma. The plutonic suites include both syn- and posttectonic granitoid assemblages and define the main magmatic pulse associated with the Makkovikian orogeny and constrain its final deformational episode. Distinctive, fluorine-enriched "A-type" granites yield an age of 1719 ± 3 Ma and represent a previously unrecognized late postorogenic to anorogenic magmatism of regional significance. Two layered, gabbro–diorite–monzonite–syenite suites yield identical zircon ages of 1649 ± 1 and 1649 ± 3 Ma. A regionally extensive granodioritic unit gives an age of 1647 ± 2 Ma, and a high-level alaskitic granite is dated imprecisely at 1640–1650 Ma. These plutonic suites correlate with volcanic rocks previously dated at 1649 ± 1 Ma.These data show that the plutonic evolution of the Makkovik Province is significantly more complex than previously supposed. The clustering of ages suggests episodic, rather than continuous, magmatism. The different age groupings can, to some extent, be correlated with compositional associations defined by major- and trace-element geochemistry. The new data also raise questions about the regional configuration of Early and Middle Proterozoic orogenic belts in Labrador. Previous correlations between the Makkovik Province, the Ketilidian Mobile Belt of Greenland, and the Svecofennian and Trans-Scandinavian belts of Sweden are supported and expanded by these new results.

The Chaparral shear zone: Deformation partitioning and heterogeneous bulk crustal shortening during Proterozoic orogeny in central Arizona

The Proterozoic Chaparral shear zone of central Arizona is one of a network of subvertical, northeast-striking shear zones that divide the Proterozoic orogenic belt of Arizona into tectonic blocks. The zone is several kilometers wide and contains variably developed mylonitic foliation that is subparallel to the regional subvertical foliation. Stretching lineations plunge shallowly northeast, and displacement across the zone was dominantly right-lateral. Several lines of evidence constrain displacement across the zone to be greater than 5 km, but probably less than tens of kilometers: (1) stratigraphic and plutonic rocks can be correlated across the zone, (2) structural and metamorphic histories of tectonic blocks on opposite sides of the zone are similar, and (3) integration of shear strain (estimated by deflection of earlier fabric) suggests greater than 5 km of strike slip. More important than scale of movement across the zone, structural studies have clarified several aspects of the assembly history of the orogen. Early north- to northwest-striking S 1 foliation, on both sides of the Chaparral shear zone, records one or more deformational events that took place between 1.75 and 1.7 Ga. Early fabrics record northeast-southwest or east-west shortening and west-verging thrusting that we interpret to have formed during development of a primitive arc complex. In contrast, intense deformation in the Chaparral shear zone took place during regional northwest-southeast shortening, D 2 , that produced the dominant subvertical northeast-striking foliation and the present block architecture of the orogen. D 2 is interpreted to have been in part synchronous with emplacement of 1.70 Ga granitoids and to record assembly of volcanic belts to North America. Of general interest is the character of D 2 deformation partitioning in and across the shear zone. Two types of high-strain domains were generated during shortening. One type, represented by rocks southeast of the shear zone, involved extreme shortening and transposition by folding. The other type, represented by the shear zone, involved simple shear deformation in a complicated array of anastomosing and conjugate shear zones. Overprinting relationships suggest that the second type may have nucleated on the first, implying an important component of deformation partitioning in time, as well as space. Mylonitic fabric in the shear zone developed by progressive heterogeneous simple shear followed by brittle fracturing. Conjugate shear bands suggest a shallow southeast-plunging σ 1 late during strike-slip deformation. This is consistent with regional D 2 shortening but not with the incremental shortening direction expected during right-lateral simple shear. This supports a regional kinematic model in which the Chaparral shear zone (right-lateral) and a temporally related left-lateral shear zone were regional-scale conjugate shear zones that accommodated heterogeneous D 2 northwest-southeast orogenic shortening via escape-block tectonics. Thus, orogen-parallel strike-slip displacements in the Chaparral shear zone were a response to partitioned shortening in the late stages of accretion rather than transpressional accretion during oblique subduction.

How well established is isobaric cooling in Proterozoic orogenic belts? An example from the Arunta inlier, central Australia

Orogenic pressure-temperature paths can only be deduced from metamorphic textures if they formed during a single metamorphic event, rather than recording the effects of two or more separate events. Erroneous tectonic models are likely to result if textures formed in several events are ascribed to one event. We illustrate this problem for the Arunta inlier in central Australia, which has been considered to be a good example of an isobarically cooled terrain. Rocks of the Anmatjira Range in the central Arunta inlier, which has been described in terms of isobaric cooling, actually record the effects of two low-pressure, granulite facies, metamorphic events of similar peak metamorphic grade, M 1 and M 2 , that are separated in time by a period of basin development and deposition of a succession of sedimentary rocks. Correlations of stratigraphic, structural fabric, and metamorphic texture data allow the effects of M 1 and M 2 to be distinguished. Without this fine control, the textures formed by superposition of M 1 upon M 2 , considered as a manifestation of one event, could easily be interpreted as indicating isobaric cooling, because the overprinting assemblages are of similar pressure but slightly lower temperature than the original assemblages. We call this apparent isobaric cooling; in fact, we now recognize textures reflecting decompression that formed immediately following the peaks of both M 1 and M 2 . It is now a matter for conjecture whether isobaric cooling is more apparent than real in other terrains for which it has been recorded.

Late Proterozoic paleomagnetism and tectonic models: a critical appraisal

Interpretations of Proterozoic orogenic belts in terms of plate tectonic processes have been widely divergent. Published models range between the extremes of no relative motions between continental nuclei (implying ensialic orogenic processes) on the one hand, to large-scale relative motions with oceans opening and closing (resulting in continent-continent collisions) on the other hand. Paleomagnetic data can, in theory, contribute significantly to this debate; however, as shown in this paper, several tectonic interpretations on the basis of paleomagnetic data have been premature. A critical continent in many of the previous models is Africa. In order to test hypotheses, for instance, for the late Proterozoic-Cambrian Pan African orogeny, a compilation of paleopoles has been made for Africa, with age ranges falling fully or partially within the interval of 1150 to 500 Ma. A quantitative comparison of the quality of this African dataset with the Phanerozoic poles for North American and Europe shows that the late Proterozoic paleopoles of Africa generally have very low reliability. It appears that the data from other Gondwana continents are equally unreliable and even less abundant. This means that currently the dataset of Gondwanaland cannot be used with confidence for the testing of tectonic models such as the Precambrian supercontinent, at least for the time after 1150 Ma. Well-dated late Proterozoic paleopoles from the three cratonic nuclei within Africa (Congo, Kalahari, West Africa) define relatively short apparent polar wander path segments, but each with different age ranges. This implies that they cannot be compared with each other to test relative motions between the cratonic nuclei and that a choice between ensialic and ensimatic models for the Pan African orogenic belts cannot yet be based on paleomagnetic data. While this does not imply that the tectonic models (e.g. those of Piper and McWilliams) are wrong, it does mean that substantial paleomagnetic support for them will have to wait more and higher-quality paleopole determinations with better dating precision.

Granitoids of rapakivi granite-anorthosite and related associations

Mid-Proterozoic anorthosite-mangerite-charnockite-rapakivi granite (AMCG) magmatic suites have commonly intruded Proterozoic orogenic belts within 100 Ma or less to as much as 600 Ma after cessation of calc-alkalic crustal generation processes. High FeO (FeO+MgO) , high Na 2 O+K 2 O, low Al 2 O 3 , and reduced redox condition of the granitoids coupled with the bimodal character of AMCG magmatism are dissimilar to calc-alkalic associations. Although included in the broad classification of A-type granites, the granitoid rocks of AMCG suites have mineral and chemical characteristics that permit them to be more precisely defined as a distinctive subset of that category. Comparison with other mid-Proterozoic late orogenic, post-orogenic, and anorogenic (LPA) granites illustrates the extended compositional range and evidence for higher temperature earlier magmas exhibited by AMCG granitoid suites. Many AMCG granitoid assemblages contain clear evidence of extensive fractionation that led in some cases to extremely evolved products. Common abundance of the earlier crystallized fractions supports the view that AMCG granitoid suites developed by way of extended crystal-liquid fractionation from voluminous parent melts and not by small degrees of partial melting of relatively depleted granulitic sources.