Joachim Jacobs

CONTINUATION OF THE MOZAMBIQUE BELT INTO EAST ANTARCTICA : GRENVILLE-AGE METAMORPHISM AND POLYPHASE PAN-AFRICAN HIGH-GRADE EVENTS IN CENTRAL DRONNING MAUD LAND

The about 500 km long coastal stretch of central Dronning Maud Land (DML), East Antarctica, is critical for understanding both Gondwana and Rodinia assembly. In common Gondwana reconstructions central DML lies at the potential southern extension of the Mozambique Belt. We report the first extensive geochronological study of magmatic and metamorphic rocks from the area. These new U‐Pb SHRIMP zircon and Sm‐Nd‐data of rocks sampled during the German international GeoMaud 1995/96 expedition indicate that the oldest rocks in central DML are Mesoproterozoic in age. The crystallization ages of metavolcanic rocks were determined at c. 1130 Ma. Syn‐tectonic granite sheets and plutons give ages of c. 1080 Ma, contemporaneous with metamorphic zircon growth at granulite facies conditions. An anorthosite intrusion and a charnockite are dated at c. 600 Ma. Subsequent metamorphism is recorded for at least two different episodes at c. 570–550 Ma and between 530 to 515 Ma. The latter metamorphic event reached granulite facies and is associated with the syn‐tectonic intrusion of a granodiorite body at Conradgebirge. Initial εNd,t‐values of the U‐Pb dated rocks with crystallization ages around 1.1 Ga range from c. +7 to –4. These values suggest that their magmatic precursors represent variable mixtures of a primitive mantle‐derived and continental crust component generated within a mature island arc. Initial Nd isotope data of Cambrian meta‐igneous rocks are indistinguishable from the Grenville‐age rocks, probably representing partial melts of the Grenville‐age basement. The occurrence of Pan‐African syn‐tectonic granitoids is unique in DML. The structure and shape of this body indicates that the main structural ENE‐WSW trend of the region is Pan‐African in age and not older, as previously assumed. Some major late ductile sinistral shear zones occuring in the study area fit well in the overall sinistral transpressional setting of the Mozambique Belt. Thus, central DML very probably represents the southern continuation of the Mozambique Belt into East Antarctica.

Himalayan-type indenter-escape tectonics model for the southern part of the late Neoproterozoic–early Paleozoic East African– Antarctic orogen

The East African–Antarctic orogen is one of the largest orogenic belts on the planet. It resulted from the collision of various parts of proto–East and West Gondwana during late Neoproterozoic–early Paleozoic time (between 650 and 500 Ma). We propose that the southern part of this Himalayan-type orogen can be interpreted in terms of a lateral-escape tectonic model. Modern Gondwana reconstructions show that the southern part of the East African– Antarctic orogen can best be reassembled when a number of microplates (the Falkland, Ellsworth-Haag, and Filchner blocks) are positioned between southern Africa and East Antarctica. This microplate assemblage is unusual. The microplates probably represent shear-zone–bounded blocks, produced by tectonic translation during lateral escape, similar to those currently evolving in Southeast Asia. One of the escape-related shear zones is exposed as the 20-km-wide Heimefront transpression zone in western Dronning Maud Land. Coats Land, a crustal block within the orogen, probably represents a block of older crust that was not subjected to tectonometamorphic reworking ca. 500 Ma by lateral tectonic escape. The southern part of the orogen is also typified by very large volumes of late-tectonic A2-type granitoids, intruded ca. 530–490 Ma, probably as a consequence of delamination of the orogenic root and the subsequent influx of hot asthenospheric mantle during tectonic escape. Erosional unroofing of the orogen is documented by the remnants of originally massive areas covered by Cambrian– Ordovician molasse-type sedimentary rocks throughout Africa, Arabia, and Antarctica, testifying to the past extent and size of this largest of orogens.

Accretion and indentation tectonics at the southern edge of the Kaapvaal craton during the Kibaran (Grenville) orogeny

A comparison of the lithologic, geochronologic, and kinematic features of the ∼1Ga orogens in southern Africa (Namaqua-Natal belt) and the Heimefrontfjella (East Antarctica) shows that the three areas originally constituted a contiguous belt that evolved within a prolonged, consistently northeast-oriented stress regime (African azimuths). An early northeast- or southwest-directed thrusting event has been identified along the entire belt. A later episode of transcurrent shearing can account for both the emplacement of extensive late-tectonic granitoid plutons in Natal and for the development of the Koras and Sinclair basins in Namaqualand and Namibia. The geometry and sense of movement of the later shear zones are functions of the varying orientation of the adjacent Archean cratonic margin and a prolonged period of northeast-directed plate convergence. Consequently, the craton is interpreted as a southwest-directed indenter during the Grenville orogeny.

Pb, Nd, and Sr isotope mapping of Grenville‐age crustal provinces in Rodinia

New Pb, Nd, and Sr isotope data are presented for geochemically similar, ∼1.1–1.2 Ga, granitoids and tonalitic‐granitic orthogneisses from Antarctica, southern Africa, and the Falkland Islands and adjoining plateau, areas originally within the supercontinents of Rodinia and Gondwana. These data support contentions for the presence of a Mesoproterozoic (∼1.2 Ga) destructive plate margin running from Namaqua‐Natal (southern Africa), through the displaced microplates of the Falkland Plateau and Falkland Islands, the Haag Nunatak crustal block (West Antarctica) and into western Dronning Maud Land (East Antarctica). The bulk of these granitoids represent juvenile Mesoproterozoic additions to the crust, except for in parts of East Antarctica (i.e., the Sverdrupfjella) where older (Paleoproterozoic or Archean) crust was involved in granitoid generation. Our isotope data permit plate reconstructions in which southern Africa, East Antarctica, and the Falkland Islands and plateau were adjacent within Rodinia.

40Ar39Ar Thermochronological constraints on the structural evolution of the Mesoproterozoic Natal Metamorphic Province, SE Africa

Abstract Ten 40 Ar 39 Ar age spectra are presented for hornblende grains separated from mylonitized and weakly deformed amphibolites from the Mesoproterozoic (∼ 1.2-1.0 Ga) Natal Metamorphic Province (NMP). Since the proposal of the SWEAT hypothesis, the NMP has become a crucial area in which to study Grenville-aged (locally termed ‘Namaquan’) accretion in this part of Rodinia/Gondwana. Unlike many segments of the Worlds ∼ 1.1 Ga belts, the NMP is particularly suited to such studies since it is devoid of a high-temperature Pan-African (∼ 500 Ma) overprint. In this study we have dated hornblendes from within the earliest (D1) NE-directed thrust-dominated structural domains of the belt. The D1 domains are characterized by NE-directed recumbent fold and thrust nappes and a pervasive, generally S- to SW-dipping, metamorphic foliation with down-dip stretching lineations. The new data constrain the minimum age of this early event (which includes obduction of the northernmost Tugela Terrane onto the Kaapvaal Craton) to 1135 ± 9 Ma. In the D1 domains of the Mzumbe Terrane to the south, cooling to below ∼ 550°C had only been attained by ∼ 1005 Ma. We also dated amphibolites from the later (D2) structural domains which are characterized by sub-vertical sinistral mylonite belts with sub-horizontal to oblique stretching lineations. Our data show that D2 oblique shearing commenced at 1050-1035 Ma in the Mzumbe Terrane and only terminated at ∼ 980 Ma. The youngest movements also included reactivation of major D1 structures such as the Melville Thrust (Mzumbe-Margate Terrane boundary) at ∼ 990 Ma. Finally, indications of minor Pan-African resetting was detected during the first ∼ 5% of the ArAr spectra from mylonites of the Lilani-Matigulu shear zone (Tugela-Mzumbe Terrane boundary), indicating the polyphase nature of this major structure. It has been suggested that the D1 event records the early NE-SW directed arc-continent collision history of the belt whilst the D2 event represents a continuation of essentially the same convergence vectors after extensive crustal thickening. The new data place important time constraints on the major tectonic events in the NMP which appear comparable in many ways to other parts of the global ‘Grenvillian’ orogen.

Litholog and structure of the Grenville-aged (≈1.1 Ga) basement of heimefrontfjella (East Antarctica)

The Heimefrontfjella mountains, Western Dronning Maud Land (East Antarctica), are dominantly composed of Grenville-aged (≈ 1.1 Ga) rocks, which were reworked during the Pan -African orogeny at ≈500 Ma. Three discontinuity-bounded Grenville-aged terranes have been recognized namely (from north to south) the Kottas, Sivorg and Vardeklettane terranes. The terranes contain their own characteristic lithological assemblages, although each is made up of an early supracrustal sequence of metavolcanic and/or metasedimentary gneisses, intruded by various (predominantly granitoid) suites. No older basement upon which the protoliths of these older gneisses were deposited has been recognized. In each terrane the older layered gneisses were intruded by various plutonic suites ranging in age from ≈ 1150 to ≈1000 Ma. The Vardeklettane terrane is characterized by abundant charnockites and two-pyroxene granulite facies parageneses in metabasites, whereas the Sivorg and Kottas terranes were metamorphosed to amphibolite facies grade. P-T estimates show that peak metamorphic conditions changed from ≈600°C at 8 kbar in the south, to ≈700 °C at 4 kbar in the northern Sivorg terrane. Regional greenschist retrogression of high-grade assemblages may be of Pan-African age. The Heimefrontfjella terranes were juxtaposed and pervasively deformed during a complex and protracted period of E-W collision orogenesis in a transpressive regime at ≈ 1.1 Ga. This is manifest as early, gently dipping thrust-related shear fabrics (D1), succeeded by the initiation of an important (D2) steep dextral shear zone (Heimefront shear zone, HSZ), during which the early fabrics and structures were steepened and rotated in an anticlockwise sense. The HSZ is a curvilinear structure which changes from a dextral oblique strike-slip lateral ramp in the north to a steep dip-slip frontal ramp in the south, where it forms the boundary between the Sivorg and Vardeklettane terranes. The Pan-African event is manifested as discrete, low- to medium-temperature ductile to brittle shears (D3) and numerous K/Ar cooling ages.

Grenville‐Age versus Pan‐African Magnetic Anomaly Imprints in Western Dronning Maud Land, East Antarctica

In this article, we examine aeromagnetic data from a part of the western margin of the Pan‐African East Antarctic Orogen. The East Antarctic Orogen represents the southern continuation of the East African Orogen that together formed during the collision of East and West Gondwana during late Neoproterozoic/Early Paleozoic times (at ca. 580–515 Ma). The western margin of the East Antarctic Orogen is exposed in Heimefrontfjella, western Dronning Maud Land, where the western front of this orogen crops out as the Heimefront Shear Zone. Crust west of the Heimefront Shear Zone has typical Mesoproterozoic to early Neoproterozoic (Grenville‐age) K‐Ar and Ar‐Ar mineral cooling ages, and magnetic anomalies are broad, of high amplitude, elongate, and craton parallel with long wavelengths. East of the Heimefront Shear Zone, K‐Ar and Ar‐Ar mineral cooling ages range at between ca. 570 and 470 Ma, and the magnetic anomaly pattern is entirely different. Here, a large magnetic low persists, which is overprinted by small‐scale anomalies that are oriented parallel to the regional Pan‐African structural trends at a high angle to the Mesoproterozoic anomalies. Thus, the Pan‐African tectono‐thermal overprint has caused a fundamental redistribution of magnetic minerals. The data show that the combination of aeromagnetic mapping along with detailed fieldwork is a powerful method to delineate the extent of the East Antarctic Orogen in poorly exposed Antarctica.

Early Palaeozoic orogenic collapse and voluminous late-tectonic magmatism in Dronning Maud Land and Mozambique : insights into the partially delaminated orogenic root of the East African-Antarctic Orogen?

Abstract The late tectonic history of the southern part of the Late Neoproterozoic–Early Palaeozoic East African–Antarctic Orogen (EAAO) is characterized by lateral extrusion, extensional collapse and large volumes of high-temperature A2-type granitoids. This late-tectonic igneous province covers an area more than 15 000 km2 of the EAAO in Dronning Maud Land (East Antarctica) and its northerly continuation as the Nampula Complex of NE Mozambique. The magmatic province is bounded in the north by the Lurio Belt. New secondary ionization mass spectrometry (SIMS) U–Pb analyses of zircons from two major late-tectonic granitoid intrusions from Dronning Maud Land indicate crystallization ages of 501±7 and 499±4 Ma, whereas a major extensional shear zone was dated at 507±9 Ma. New SIMS zircon U–Pb analyses of late-tectonic granitoid sheets and plutons from the Nampula Province indicate ages of 512±4, 508±4, 508±2 and 507±3 Ma. Consequently, the late-tectonic magmatism can be bracketed between c. 530 and 485 Ma. It started with small gabbro bodies emplaced at c. 530–520 Ma, culminated with the intrusion of major granite–charnockite plutons at c. 510–500 Ma and terminated with the introduction of small volumes of sheet-like granite at c. 485 Ma. The new dates demonstrate that extensional shearing and granitoid intrusion are synchronous, and that orogenic collapse and the magmatism are related. We ascribe the distribution, structural style, geochemical composition and age of the late magmatic province to a process of partial delamination of the orogenic root in the southern third of the EAAO. It remains to be tested whether there is a relationship between orogenic collapse–granitoid magmatism and south-directed escape tectonics in the southernmost EAAO.

Role of Pan-African events in the Circum-East Antarctic Orogen of East Gondwana: a critical overview

Abstract Recent studies of Pan-African events in East Gondwana are critically reviewed, particularly recent models of amalgamation of East Gondwana during the Pan-African period. It is pointed out that critical data are insufficient to constrain the newly proposed models and so the classical model of the Grenvillian Circum-East Antarctic Orogen cannot yet be replaced. Grenvillian tectonothermal events with a peak between 1.0 and 1.2Ga assembled different crustal blocks of the East Antarctic Shield with different geohistories. Pan-African tectonothermal reworking took place over wide but selected areas of the orogen. Careful geochronological studies, including SHRIMP dating associated with structural and petrological investigations to correlate ages with those events, are shown to be important, since fluid-rich and/or deformational conditions are equally effective as temperature conditions for mineral recrystallization and resetting of isotopic systematics. Pan-African suture zones, one extending from the Mozambique Belt to the Shackleton Range and another connecting the Mozambique Belt to the Zambezi Belt, are equally possible, although the width of the southern Mozambique Ocean is poorly understood. The extent of Pan-African sutures in the Prydz Bay area is enigmatic, although they represent definite orogens. Palaeomagnetic studies may provide critical constraints in evaluating the sutures, provided that the age of magnetization is well established.

Age and thermal evolution of the Mesoproterozoic Cape Meredith Complex, West Falkland

U-Pb SHRIMP zircon and 40Ar/39Ar mineral ages are reported for rocks of the Cape Meredith Complex, West Falkland. Felsic gneisses of the oldest Big Cape Formation give a zircon date of 1118 ± 8 Ma, interpreted as the time of extrusion of the rhyolitic protoliths. The three phases of granitoid which intrude the Big Cape Formation, namely granodiorite orthogneiss (Gl), syntectonic granite gneiss (G2) and post-tectonic granite (G3), gave U–Pb dates of c. 1090, 1067 ± 9 and 1003 ± 16 Ma respectively. Metamorphic overgrowths on zircons from the felsic gneisses were dated at c. 1000 Ma, coeval with the G3 granite, whereas a date of 1135 ± 11 Ma from inherited cores in G2 zircons were probably derived from a slightly older component of the Big Cape Formation. 40Ar/39Ar hornblende ages from amphibo-lites of the Big Cape Formation (1009 ± 14 and 1015 ± 6 Ma), along with muscovite (989 ± 3 Ma) and biotite (989 ± 7 Ma) from G3 pegmatites show that the complex cooled relatively rapidly to below 350°C, with no evidence of any Pan-African (c. 500 Ma) thermal overprinting. These data show that the older rocks of the Cape Meredith Complex are significantly younger than the equivalent rocks of the Natal Metamorphic Province, SE Africa (c. 1200 Ma), though they are comparable in age to those of Western Dronning Maud Land (East Antarctica), areas which Gondwana reconstructions place west and east of the Falkland Microplate respectively. The older rocks of the Cape Meredith Complex may therefore represent younger outboard arc terranes relative to those exposed in Natal. However, the syn- to post-tectonic granites of the Cape Meredith Complex are comparable in age in all three areas (1070–1000 Ma), suggesting similar collisional and post-collisional histories throughout the entire region. The lower temperature history of the Cape Meredith Complex is comparable to data from Natal, but does not show the Pan-African (c. 500 Ma) overprint which is characteristic of much of western Dronning Maud Land. This situation is consistent with the proposed position of the Falkland Microplate between SE Africa and East Antarctica in Gondwana, where a general eastward increase in the intensity of the Pan-African thermal effects has been recorded.