Search

The Earth is a unique planet, which has been highly evolved, diversified and complicated through geologic time, and underwent many key events, including giant impact, magma ocean, core formation, large-scale mantle differentiation and late heavy bombardment, especially in its dawn. But, our knowledge of early Earth is limited due to the lack of the Hadean supracrustal rocks. The supracrustal rocks with the Eoarchean ages provide key evidence for the Earth's early evolution, but few supracrustal rocks have been comprehensively investigated. Therefore, we mapped in seven areas of the Saglek Block, northern Labrador, where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses. Early studies suggested that some of them have the Mesoarchean ages because of the lack of the Mesoarchean Saglek dyke, but we found the Saglek dykes in the areas to recognize the Eoarchean Nulliak supracrustal rocks and Uivak Gneiss in all the areas. Recent reassessment of U–Pb dating and cathodoluminescence observation of zircons from the oldest suites of the Uivak Gneiss showed that the Uivak Gneiss has the Eoarchean age, > 3.95 Ga, and forms the Iqaluk–Uivak Gneiss series. Because our geological survey clearly showed that the Iqaluk–Uivak Gneisses were intruded into the Nulliak supracrustal belts, the Nulliak supracrustal rocks are the oldest supracrustal rock in the world. The supracrustal belts consist of piles of fault-bounded blocks, which are composed of the ultramafic rocks, mafic rocks and sedimentary rocks in ascending order, similar to modern ocean plate stratigraphy (OPS). In addition, small-scale duplex structures are found over the areas. The presence of duplex structure and OPS indicates that the > 3.95 Ga Nulliak supracrustal belts originate from an accretionary complex. The presence of the accretionary complex, ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early Earth.

We report on volcanic rocks of the Qinglong-Zhuzhangzi area in northeastern Eastern Hebei Province, North China Craton. The supracrustal rock associations in the study area consist of amphibole/biotite plagioclase gneiss, mica (plagioclase) gneiss/schist, biotite K-feldspar gneiss, (pyroxene) plagioclase amphibolite and amphibolite. Twenty-five new mafic, intermediate to felsic samples can be geochemically classified into four groups. Group 1 is similar to fore arc low Ti tholeiite and is characterized by low SiO 2 (49.7 wt%) and TiO 2 (0.43 wt%), slightly depleted LREE and MREE pattern with (La/Yb) N ratio of 0.74, (Gd/Yb) N ratio of 0.82. It most likely derived from depleted mantle in a subduction initiation environment. Group 2 is akin to primitive arc tholeiites with low SiO 2 (49.7–51.8 wt%), slight enrichments of LREE ((La/Sm) N : 1.06–1.53) and negative Nb anomalies ((Nb/La) PM : 0.50–0.76). Group 3 resembles evolved arc tholeiites and displays pronounced enrichments of LREE ((La/Yb) N of 2.38–11.3, (La/Sm) N of 1.70–2.86) and negative Nb, Ta anomalies ((Nb/La) PM : 0.14–0.34, except sample 11JD28-4 with that ratio of 1.00). Group 2 and 3 formed by partial melting of sub-arc depleted mantle wedge modified by differing quantities of slab flux addition. Group 4 comprises K-rich andesite-dacites, which are distinguished by the highest SiO 2 (54.0–71.8 wt%), K contents (up to 5.03 wt%), strong enrichments of LREE ((La/Yb) N ratios of 4.83–19) and negative Nb–Ta anomalies, analogous to shoshonitic rocks marking late stage of subduction. LA-ICPMS zircon U–Pb and Lu–Hf analyses of evolved arc basalts and K-rich andesite-dacites display crystallization ages of 2511–2604 Ma and single stage Hf model ages of 2534–2927 Ma (most focus on 2650–2800 Ma). In contrast to ∼2510–2560 Ma arc associations, ∼2604 Ma shoshonitic dacites most plausibly represent an older orogenic event. Collectively, supracrustal rocks in the Qinglong-Zhuzhangzi area constitute a series of volcanic arc associations that correspond to an entire subduction cycle ranging from orogen to primitive arc–evolved arc and back to an orogenic setting, which further demonstrate that, by the late Archean, plate tectonic and subduction processes already closely resembled modern counterparts.

Widely distributed on the southern margin of the North China Craton, the Taihua complex extends roughly in an east-west direction and the relatively complete successions are found in Lushan County, Henan Province. Like many other Archean terranes, the Taihua Complex can be divided into two major lithological units along the Dangze River, namely gneisses series and supracrustal rocks. The former is located on the north side of the river and chiefly composed of TTG gneisses and amphibolites; the latter on the south side of the river is mainly supracrustal rock. Coupled with the previous studies, the results obtained by this study show that the Taihua complex was formed in a large time span from Neoarchean to Palaeoproterozoic. The TTG gneisses and amphibolites are dated at Neoarchean (2794–2752 Ma). The 2.9 Ga and 3.1 Ga zircons in amphibolites could be xenocrysts. In the North China Craton, the 2.8–2.7 Ga old rocks crop out in several areas e.g. western Shandong Province, Jiaodong Peninsula and Lushan area of Henan Province. In addition, 2.8–2.7 Ga detrital zircons or xenocryst zircons have been recognized in Huai’an, Fuping, Wutai areas of North China Craton and also reported in the Early Paleozoic diamondiferous kimberlites in Mengyin and Fuxian. All these age results indicate that the 2.8–2.7 Ga rocks may have been developed much broader region than today’s outcrops. Zircon Hf and whole rock Nd isotopes show that the 2.8–2.7 Ga tectono-thermal event represents an important period of crustal growth with minor ancient crust reworked in the North China Craton. However the formation of supracrustal rocks is limited to 2.2–2.0 Ga in the Palaeoproterozoic time, not the Archaean, as previously believed. Combined with the chronological data of aluminium-rich metamorphic rocks (Khondalite series) on the southern margin of the North China Craton and adjacent areas, it is suggested that the above areas have widely developed Paleoproterozoic passive continental margin environment.

We report the first U–Th–Pb ion microprobe depth profiles of four Hadean zircons from the Jack Hills and Mount Narryer supracrustal belts of the Narryer Gneiss Complex (NGC), Western Australia. This ultra-high spatial resolution technique probes the age and origin of sub-micron features in individual crystals that can record episodes of zircon growth. Near-surface grain dates of 2700 Ma or older are coincident with post-depositional growth/modification. Some ages may coincide with documented pre-deposition metamorphic events for the NGC and igneous emplacement at ca. 3700 Ma. Separate events that do not correlate in time with known geologic episodes prior to the preserved rock record are also present on pre-4000 Ma zircons. We find evidence for a ∼3.9 Ga event, which is coterminous within age uncertainty with one or several large basin-forming impacts (e.g. Nectaris) on the Moon attributed to the late heavy bombardment of the inner solar system.

Chronological studies show that the Late Archean petrographic formation of the Tarim craton includes Tuogelakbulak complex in the northern margin and the Milan Group in the northern slope of the eastern Altun Mountains. The Tuogelakbulak complex is mainly plagioclase amphibolite, hornblende schist, gray gneiss, garnet biotite schist, actinolite mica quartz schist, biotite quartz schist, and migmatite, mixed with minor amount of marble lenticular bodies, and intruded by gneissic muscovite biotite granite. The Milan Group is composed of various types of felsic gneiss, granulite, marble, and plagioclase amphibolite, and belongs to hypometamorphic supracrustal rock series. The Neoarchean is an important episode of continental crust growth and basement cratonization of the Tarim craton, and a continental crust with the age of about 3.6 Ga might have existed in the Tarim craton basement. The Lower Proterozoic of the Tarim craton is extensively distributed and outcropped at Quruqtagh in the northern margin, Altun Mountains in the eastern margin and Yecheng area in the southwestern margin. Protoliths of the Paleoproterozoic Xingditag Group and Altun Mountains Group are mainly clastic sedimentary rock, carbonate rock, and volcanic formations, indicating a relatively stable and extensive littoral-neritic sedimentary environment. They are mainly composed of a suite of intermediate and high grade metamorphic rocks, including biotite plagioclase gneiss, muscovite biotite plagioclase gneiss, quartz schist, marble, and garnet sillimanite plagioclase quartz gneiss, resembling khondalite series. Magmatism was not significant during this episode, though the Paleoproterozoic magmatic rocks are observed in Altun Dunhuang, Tiekelike and Korla areas. During the Mesoproterozoic episode, the Tarim craton was relatively stable. The Jixianian Aierjigan Group is characterized by carbonate and clastic rock series formed in epicontinental sea. Magmatic formation consists of the Changchengian Sailajiazitage Group and the Jixianian Ailiankate Group formed in intracontinental extension setting.

The main hole (MH) of the Chinese Continental Scientific Drilling Project (CCSD) in southern Sulu has penetrated into an ultrahigh-pressure (UHP) metamorphic rock slice which consists of orthogneiss, paragneiss, eclogite, ultramafic rock and minor schist. Recovered eclogites have a UHP metamorphic mineral assemblage of garnet + omphacite + rutile ± phengite ± kyanite ± coesite ± epidote. Ultramafic rocks contain garnet + olivine + clinopyroxene + orthopyroxene ± Ti-clinohumite ± phlogopite. Gneisses and schists contain an amphibolite-facies paragenesis, but their zircons have coesite, garnet, omphacite (or jadeite) and phengite inclusions, indicating that eclogites and gneisses have been subjected to in situ UHP metamorphism. Using available geothermobarometers, P–T estimates of 3.1–4.4 GPa and 678–816°C for eclogites were obtained. If surface outcrops and neighboring shallow drill holes are considered together, we suggest that a huge supracrustal rock slab (> 50 km long × 100 km wide × 5 km deep) was subducted to a depth > 100 km and then exhumed to the surface. The depth interval (0–2,050 m) of the CCSD-MH can be divided into six lithological units. Unit 1 consists of alternating layers of quartz-rich and rutile-rich eclogites, with thin interlayers of gneiss and schist. Eclogites of unit 1 are characterized by Nb, Ta, Sr and Ti depletions, low Mg number and general LREE enrichment. Unit 2 comprises rutile- and ilmenite-rich eclogite and minor “normal” eclogite and is characterized by high TiO2, total Fe, V, Co and Sr, and very low SiO2, alkali, Zr, Ba, Nb, Ta and total REE contents, and LREE-depleted REE patterns with slightly positive Eu anomalies. Unit 3 contains ultramafic rock and minor MgO-rich eclogite. Protoliths of UHP rocks from units 1, 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth. Units 4 and 6 consist of interlayered eclogite and paragneiss; the eclogites are characterized by Th, U, Nb, Ta and Ti depletion and K enrichment and LREE-enriched REE patterns. Paragneisses show Nb, Ta, Sr and Ti depletions and LREE-enriched REE patterns occasionally with slightly negative Eu anomalies, indicating that their protoliths represent metamorphic supracrustal series. Unit 5 consists mainly of orthogneisses, showing distinct Nb, Ta, Sr and Ti depletions, and LREE-enriched REE patterns with pronounced negative Eu anomalies, suggesting granitic protoliths. In conclusion it is proposed that the southern Sulu UHP belt consists of a series of meta-supracrustal rocks, a layered mafic–ultramafic complex and granites.

Best known for its Neoarchean greenstone-granite terranes in the east and west, the Wabigoon subprovince of the Superior Province features a central transverse corridor of Meso- and Neoarchean gneissic, granitoid, and minor supracrustal rocks resembling an eroded intermontane plateau. Structural analysis and U–Pb geochronology in the north-central Wabigoon subprovince reveal a complex history of plutonism, supracrustal rock deposition, polyphase deformation and amphibolite-facies metamorphism between 3075 and 2670 Ma. The geometric and kinematic character of D 1 and D 2 deformational events recorded in 2890–2718 Ma tonalitic gneisses remain cryptic as a result of magmatic and structural (D 3–5 ) overprinting, but the early structures are bracketed in age between 2725 and 2713 Ma by a polydeformed gneiss and tonalite dyke. These events occurred deep within an active continental magmatic arc and correspond in age to a collision between the juvenile Wabigoon and continental Winnipeg River subprovinces recognized to the west. Clastic sediments deposited after 2701 Ma are the youngest rocks involved in subsequent regional deformation. A D 3 event is responsible for rapid burial of supracrustal units to 15 km within warm arc crust, and imposition of a regional, penetrative, S 3 foliation that accommodated some relative uplift prior to 2697 Ma, the age of a discordant granodiorite dyke. This event corresponds to variably preserved, steep, north-trending D 1 structures recognized in adjacent greenstone belts. Subsequent (D 4 ) ductile deformation defines the map pattern in the deep crustal rocks of the central Wabigoon subprovince. Upright, east-trending F 4 folds developed synchronously with D 2 dextral transpressive structures that formed throughout the Wabigoon subprovince prior to 2698 Ma. The final (D 5 ) sinistral shearing event occurred during emplacement of mantle-derived granodiorites of sanukitoid affinity (2688 Ma) and crust-derived granites (2685 Ma). This event appears to have triggered rapid cooling, reflected in 2680–2670 Ma titanite dates and late brittle increments of displacement.

The Jiaodong Terrane of the Eastern Shandong Complex in the Eastern Block of the North China Craton (NCC) consists predominantly of Archean granitoid gneisses with minor supracrustal rock enclaves or lenses. This study presents new zircon LA-ICP-MS U–Pb and Lu–Hf isotopic data for these lithologies, which help to better understand the Archean crustal evolution of the Eastern Block of the NCC. Magmatic zircon U–Pb data reveal that zircons in the supracrustal rocks and granitoid gneisses were generated by multistage events at ~2.9, ~2.7 and ~2.5 Ga. Metamorphic zircon U–Pb data obtained for these rocks show distinct metamorphic ages at ~2.50 and ~1.9–1.8 Ga, suggesting that the Jiaodong Terrane experienced a regional metamorphic event at the end of the Neoarchean and encountered reworking by a tectonothermal event that was associated with the formation of the Paleoproterozoic Jiao-Liao-Ji Belt. Magmatic zircons have variable eHf(t) values from −5.5 to +7.7 with model ages of 3.92–2.57 Ga, of which most eHf(t) values are positive with a predominant peak of model ages at 3.4–3.1 Ga and a subordinate peak at 2.8–2.7 Ga. These Hf features reveal major juvenile crustal growth stages with significant additions of older crustal materials at 3.4–3.1 and 2.8–2.7 Ga, and a crustal reworking event with minor juvenile additions at ~2.5 Ga in the Jiaodong Terrane.

A layered quartzo‐feldspathic amphibolite facies gneiss from the Willyama Supergroup in the southern part of the Broken Hill block (New South Wales) contains predominantly 2670–2550 Ma old zircons and a lesser number of ca 2400 Ma, 2100 Ma and rare ca 1950 Ma, ca 1820 Ma grains. The gneiss could be an Archaean basement rock, affected by several Proterozoic tectonothermal events. An alternative interpretation is that all its zircons are inherited and it is a Palaeoproterozoic supracrustal rock. If the latter, this sample is anomalous, because it is missing 1800–1730 Ma zircons, the most important age component in the zircon populations of all other Willyama Supergroup supracrusfal rocks and the commonest age for inherited zircons in Broken Hill area felsic igneous rocks. Whichever interpretation of the rock is chosen, the predominance of Archaean zircons strongly suggests that Archaean crust in some form is present in the area. As the Broken Hill Block is near the eastern margin of Australian Rodinia, this...

Scapolite is an important mineral in the metamorphism of calc-silicates, marbles, amphibolites, and metagabbros in the Lufilian–Zambezi belt of Zambia. Both field and petrographic studies on granite gneisses, amphibolites, and metagabbros from the Munali hills area, in the southern part of the Lufilian–Zambezi belt indicated that scapolitization was due to metasomatism. The scapolite occurs as a pervasive replacement of plagioclase in the Munali hills granite gneiss, amphibolites, and metagabbros, and is associated with mineral assemblages that are indicative of amphibolite-facies metamorphism. Results of mineral analyses show that all the scapolites have calcian–marialite compositions, which range from 27 to 47 Me % and XCl contents of 0.37–0.50 a.p.f.u. The anorthite equivalent (31–46 EqAn %) of the scapolites overlaps with that of coexisting plagioclase (21–48 XAn %). The composition of scapolite is similar to that of the Copperbelt region of Zambia, where there is documented evidence of evaporite horizons. Moderate to high NaCl salinities, which range from 0.2–0.5 mol, and high contents of Cl in scapolite indicate that metamorphism in the belt was accompanied by metasomatic introduction of NaCl-rich fluids, which were derived from evaporite horizons that existed in the metasedimentary succession in the belt. This study shows that evaporites, from which NaCl-rich fluids were derived, were widespread in the Lufilian–Zambezi belt and played an important role in the metamorphic history of the belt.

The Barberton granitoid-greenstone belt is a classic dome-and-keel province, characterized by kilometer-scale gneiss domes and elongate keels of largely folded supracrustal rocks. Combined U–Pb SHRIMP data and structural mapping demonstrate that the geometry of the Barberton belt reflects events that occurred over ∼30 million year interval, from ca. 3230 and 3203 Ma. Early deformation with NW–SE shortening in the upper crust was accompanied by emplacement of tonalite-trondhjemite-granodiorite TTG magmas at 3234 ± 12 and 3226 ± 9 Ma. Much of the structural grain of the greenstone belt relates to a long episode of post-orogenic extension, with NE-directed extension in the lower crust leading to exhumation of high-grade gneisses in the southern Barberton terrane. Advective heat transfer during emplacement of kilometer-scale (TTG) plutons around the margins of the greenstone belt facilitated the infolding of the relatively denser and colder greenstone sequence. The end of this tectonic cycle is punctuated by the emplacement of the undeformed, 3203 ± 7 Ma Dalmein pluton, which sharply truncates not only anticlines and synclines in the greenstone belt but also the dominant fabric in the high-grade gneisses.

The 10–14 km thick Archean volcano-sedimentary sequence in the Chibougamau region is composed of two mafic to felsic cycles, and of two thick, predominantly epiclastic, sedimentary sequences. The supracrustal suite is isoclinally folded along east-west axes, and punctuated by syn-volcanic, syn-kinematic and post-kinematic plutons. Tonalitic basement occurs along the northern margin, in a central horst, and as large rafts in later batholiths. Events affecting the supracrustal rocks are recorded in the tectono-magnetic history of the basement gneiss superimposed on the events related to basement formation.

In the previous chapters we have given an impression of the way in which the analysis of high-grade rocks can be approached in the field. Here we conclude with some current larger-scale ideas on the origin and evolution of such rock assemblages.

The early Precambrian (Archaean) Dharwar sequence in south India provides evidence for a distinct trend of evolution which differs from trends exhibited in many other early Precambrian regions of the world. The supracrustal rock associations preserved in the Dharwar schist belts suggest the evolution of the depositional environment from a stable to a mobile regime. Layered igneous complexes, mature to supermature quartz-pebble conglomerates and quartzites deposited on platforms, ortho-quartzite-carbonate shelf assemblages and deposition of persistent beds of iron and manganese formations which serve as stratigraphic markers — all these developed in the lower sections of the Dharwar supracrustal sequence may be cited as evidence for early stability. The mobile regime which can be shown on sedimentological grounds to have succeeded the stable regime witnessed accumulation of turbidite greywackes in a volcanically active environment. The tectonic environment of sedimentation and magmatism in the supracrustal belts seems to have evolved from a rifted continental margin, through a stable shelf to a geosynclinal type of setting. Sialic crustal differentiation about 3400 Ma. ago may have been responsible for the early stable conditions. The supracrustal rocks show evidence of three phases of deformation (DF 1 to DF 3 ). A structural unity is evident amongst the supracrustal sequence, the Peninsular Gneiss and granulites of the Dharwar tectonic province. This structural unity suggests the evolution of the Peninsular Gneiss and granulites synkinematically with the deformation of the Dharwar supracrustal sequence. There is a wide spread of ages from 3400 Ma. to 2600 Ma. in the Dharwar craton. The structural unity amongst rocks developed over such a wide range of ages implies long-term stability of the stress regimes in the Archaean lithosphere. Dharwar schist belts are a rich repository of iron, manganese, and limestone deposits, indicating substantial biological activity during the early Precambrian times. Stromatolites and o 13 C values for carbon in graphitic schists and preservation of syngenetic cyanobacterial microfossils have been recognised in widely separated regions in the Dharwar schist belts. The morphological diversity as exhibited by Dharwar stromatolites are rare in pre-Riphean sequences. The foregoing observations indicate that, as compared to other Archaean terranes, the Archaean Dharwar craton appears to have more evolved characteristics.

The Kapuskasing uplift provides a window on the deep crust of the Superior Province. Evidence in support of an exposed oblique crustal cross section includes: a systematic easterly increase in metamorphic grade from 3 kbar greenstones at Wawa to 8 kbar Kapuskasing granulites; structural and paleomagnetic data on gneisses and dykes indicating 10–15° of east-side-up block rotation; seismic refraction and reflection data tracing the high-velocity Kapuskasing zone from surface down dip at 15 °W to about 20 km; and progressively slower cooling rates with paleodepth. The composite cross-section derived from the oblique exposure comprises an upper, 2–3 kbar greenstone- granite megalayer (2.89–2.67 Ga), an intermediate, 5–6 kbar tonalite gneiss megalayer (2.72–2.67 Ga) and a lower, 7–9 kbar, heterogeneous granulite gneiss megalayer (2.76–2.60 Ga). Regional cooling and uplift preceded thrust uplift along the Ivanhoe Lake fault 2.2–1.95 Ga ago. Steep normal faults transecting the uplift possibly reflect topographic collapse and are cut by 1.88 Ga carbonatites. Regional NW-SE shortening of about 70 km during the 2.2–1.88 Ga Kapuskasing event was accomodated above a mid-crustal decollement by 15 km of brittle uplift of granulites along a 15°-dipping fault and below it by 10–15 km of ductile thickening of the present lower crust over a 200-km-wide zone.

The author, S. Lin, has made major contributions to the understanding of the complex geology in the Cape Breton Highlands through his detailed mapping in the vicinity of the Eastern Highlands shear zone (see also Lin 1992). However, some generalizations made in his paper do not accurately portray the geology, and imply an unrealistic degree of certainty about some interpretations. The purpose of this discussion is to emphasize that many problems remain unresolved in the vicinity of the Eastern Highlands shear zone, contrary to the impression given by him. The author asserted that the late Precambrian rocks of the Bras d'Or terrane are overlain by the Clyburn Brook formation, a volcanic sedimentary sequence of Silurian age. However, the age of the Clyburn Brook formation is uncertain because it is based on a Rb-Sr isochron; such ages have been shown to be unreliable in the Cape Breton Highlands and elsewhere (e.g., Dunning et al. 1990). In addition, the reported isochron (Keppie and Halliday 1986) is based (with one exception) on Rb Sr data for samples from rhyolite on Ingonish Island, not on samples from the Clyburn Brook formation. Unlike rhyolite in the Clyburn Brook formation, the Ingonish Island rhyolite is unmetamorphosed and lacks associated basaltic, volcaniclastic, and sedimentary units (Raeside and Barr 1992); hence, correlation between the Ingonish Island rhyolite and the Clybum Brook formation is speculative. The statement in the abstract of the paper that the Clyburn Brook formation is Silurian is premature. It may be, but that has yet to be demonstrated. The author (p. 1773) implied that amphibolite-facies rocks occur only in the part of the McMillan Flowage Formation within the Eastern Highlands shear zone, with the further implication that there is a connection between the amphibolitefacies metamorphism and the shear zone. Amphibolite-facies metamorphism in the McMiiian Flowage Formation persists for 16 km southeast of (away from) the shear zone. Furthermore, titanite in the McMillan Flowage Formation has yielded ages of Ca. 540 Ma (Raeside and Barr 1992; G.R. Dunning, personal communication, 1990), showing that peak metamorphism was late Precambrian to Early Cambrian, much older than the deformation associated with the Eastern Highlands shear zone as documented by the author. In describing the Aspy terrane, the author (p. 1774) implied that the Cheticamp Lake Gneiss is the only supracrustal rock unit in the Aspy terrane. As documented by Raeside and Barr (1992), as well as numerous other workers (e.g., Currie 1982; Jarnieson et al. 1987, 1990), the Aspy terrane includes various

Inliers of a Proterozoic gneiss terrane, known as the Natal Province, are exposed in eastern Natal and constitute an elongated cross section some 280 km long subparallel to the present coastline (Fig. 8-1). Phanerozoic cratonic sediments obscure the western, southern, and most of the eastern margins of the Natal Province. The gneiss terrane may extend westward beneath the cratonic cover skirting the southern margin of the Archean Kaapvaal Province to link up with the high-grade Namaqua Province (Fig. 1-4), which has yielded comparable Mid Proterozoic radiometric ages (Nicolaysen and Burger, 1965; Burger and Coertze, 1973). Based on reconstructions of pre-Mesozoic Gondwana (Smith and Hallam, 1970; Piper, 1974; Norton and Sclater, 1979), the eastern extension of the Natal Province is most likely to be the high-grade terrane of Dronning Maud Land, Antarctica.

The elements Rb, Sr, Sm, Nd, U and Pb are all redistributed during the movement of fluids associated with regional metamorphism. The effect of this redistribution on the isotopic systems commonly used for age determination depends on several factors. Rb, U and Pb are lost on a regional scale during prograde metamorphism to granulite facies assemblages and are regained during retrogression. Sr is redistributed on a cm scale during both prograde and retrograde metamorphism and is mobile on a scale of tens of meters during retrogression associated with shearing. As Rb is markedly more mobile than Sr, Rb/Sr ratios of individual samples can be modified without homogenisation of Sr on outcrop scale. Addition of Rb to a sample suite will displace an original Rb-Sr isochron to the right, giving rise to an anomalously low intercept 87Sr/86Sr ratio. Loss of Rb has the opposite effect. A high amount of Rb movement is required before isochron relationships are totally destroyed. Redistribution of Sm and Nd takes place on outcrop scale in response to the formation of partial melts and new mineral assemblages. This can lead to local resetting of the Sm-Nd whole rock isotopic system at the time of high grade metamorphism. The effect of regional metamorphism on the redistribution of the REE on a larger scale is not well constrained by bulk rock analyses. Addition of LREE is recorded in tonalitic gneisses up to tens of meters from adjacent granite sheets and pegmatites. This results in both changes in bulk REE patterns and in changes in the Nd isotopic signatures depending on the isotopic composition of the Nd introduced. Basic rocks and ironstones with low primary REE contents from the 3.8 Ga Isua supracrustal belt show variable amounts of LREE addition during late Archaean and Proterozoic metamorphism. Estimates of crustal residence times or early Archaean mantle compositions based on initial end values from individual samples of these polymetamorphic rocks must be evaluated using field constraints. U loss during granulite facies metamorphism will arrest the radiogenic evolution of Pb. Straightforward interpretation of the resulting “palaeoisochrons” without allowing for secondary U loss leads to spuriously high ages and µ1 values. U loss during granulite facies metamorphism is commonly accompanied by Pb loss, and in some examples by partial isotopic homogenisation of the remaining Pb. Addition of U to rocks with approximately isotopically homogenous Pb (for example during retrogression from granulite facies) will yield ages close to that of U addition. The effect of Pb addition is more complex as it depends on the isotopic composition of the introduced Pb itself depending on the source of the Pb. Under certain circumstances mixing of Pb from different sources can give rise to Pb-Pb pseudoisochrons.

Geophysical modelling of Archean tectonics during times of higher heat production and heat loss than today predicts that plate-tectonic motions should have operated more rapidly in the Archean. The Archean geological record is still too poorly understood to accept or reject this prediction with confidence. Large volumes of calc-alkaline rocks and evidence of horizontal tectonics are the features of Archean terrains most suggestive of a plate-tectonic regime. However, it is puzzling that there are no obvious major sedimentary, volcanic, metamorphic, and structural facies belts in granite-greenstone terrains related to collision orogeny. Features that distinguish Archean from modern terrains include the basalt—dominated greenstone sequences disposed around gregarious composite granitoid batholiths and the prevalence of granulite facies gneiss belts. These features are difficult to interpret since there is still considerable controversy and uncertainty concerning their basic geological relationships.