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A Model for the Origin of Early Crust of East Asia
RODINIA, GONDWANA AND ASIA
703
Middle to Late Devonian Paleomagnetic Pole for Gondwanaland: Results from Dulladerry Volcanics, Eastern Australia Masako Mikil and Chris Klootwijk2 Graduate School of Science and Technology, Kobe University, Kobe 6571-8501,Japan, E-mail: [email protected] Department of Geolopj, The Australian National University, Canberra, ACT 0200,Australia, E-mail:[email protected] Paleomagnetic results are reported for the late Middle Devonian Dulladerry Volcanics of the eastern Lachlan Orogen, Tasman Orogenic System. The volcanics had yielded SHRIMP U-Pb zircon age of 376f 4 Ma (Pogson and Watkins, 1998) and Givetian-Frasnian biostratigraphic age (Sherwin, 1996). The characteristic high temperature components were obtained from 11 sites of rhyolitic ignimbrites and rhyolitic lavas. Normal and reverse polarities and a positive fold test indicate a pre-folding (Early-middle Carboniferous Kanimblan Orogeny: terminal deformation event of the Lachlan Orogen), presumably primary, origin. The magnetic carrier of magnetite and hematite were identified from progressive thermal demagnetizations, three component Lowrie-tests and high and low temperature susceptibility cycling. The mean direction for 11sites is D= 177.8 degrees, I = 49.1 degrees with k= 9.7 and the radius of 9 5 % confidence circle of 15.4 degrees. The pole position (86.4 S and 296.5 E) calculated from the mean direction is close to the pole position for the contemporaneous Comerong Volcanics of the eastern Lachlan Orogen (Schmidt et al., 1986).The Dulladerry Volcanics postdate the early Middle Devonian Tabberabberan Orogeny, generally
regarded as a major cratonizing event of the Lachlan Orogen. The preliminary pole for the Dulladerry Volcanics could therefore be indicative of Middle to Late Devonian pole position for the Australian carton and for Gondwanaland. Middle temperature components with unblocking temperature of about 400 degrees are obtained from several sites. The scatter of these directions increases after tilt corrections, although the scatter is still large. The component is characterized by steep inclination of normal polarity. It might be correlated to secondary magnetization at the Carboniferous orogeny.
References Pogson, D.J. and Watkins J.J. (1998) Bathurst 1250 000 geological sheet SI/55-8: explanatory notes. Geol. Surv. New South Wales, Sydney. Schmidt, P.W., Embleton, B.J.J., Cudahy, T.J. and Powell, C.McA. (1986) Prefolding and premegaking magnetization from the Devonian Comerong volcanics, New South Wales, Australia, and their bearing on the Gondwana pole path. Tectonics, v. 5, pp. 135-150. Sherwin, L. (1996) Narromine 1250 000 geological sheet SI/55-3: explanatory notes. Geol. Surv. New South Wales, Sydney.
A Model for the Origin of Early Crust of East Asia M.A. Mishkin Far East Geological Institute, FEB R A S , Vladivostok, Russia The early crustal formations of granulite - gneissic terrains in ancient shield areas comprise a variety of enderbites that are broadly analogous with grey gneisses of the granite - greenstone areas. Ourcrops of enderbitic complexes are found in the Aldan and Anabar shields of the Siberian craton. During the past decades, petrological studies have demonstrated that the most deepest of them are widespread towards the south of Aldan shield, where outcrops are found in different tectonic blocks - Sutamsky, Zverevsky, Kurultinsky and Djugdjursky. Investigations in these blocks have contributed significantly to our understanding of the origin of early crust. The present work investigates the nature of protoliths and time scale of formation of the Sutamsky block using geochemical and isotope geochronological techniques. The Sutamsky granulitic complex comprises hypersthene plagiogneisses containing interlayers of two-pyroxene rocks, metaultrabasites and, rarely quartzites. We suggest this strata to be metabasite enderbitic formation. Based on geochemical criteria, the Gondwana Research, I? 4, No. 4,2001
protoliths of the metabasite - enderbitic formation are inferred to be volcanics of calc-alkali and komatiitic-tholeiitic series. The calc-alkali series (comprising 90%) consist of basalt-andesitedacite-rhyodacite, with essential prevalence of andesites and dacites. (Mishkin et al., 1999). The calc-alkaline volcanic series are thought to be derived through the partial melting of a metamorphosed mafic primary crust, such as garnet amphibolite. This model is based on WE distribution pattern in enderbites. The komatiitic-tholeiitic series of metavolcanics are invariably present in minor amounts within the enderbitic complexes of shield areas. Their existence is not unusual, and their origin is linked to early sialic crust. The existence of both ultrabasic and basalt volcanics of this series in a uniform section allows us to speculate the formation of melts by varying partial melting of mantle material in a rising diapir under varying P-T conditions (Ohtani, 1984). The high temperatures of melts and presence of reduced gaseous-fluid dominated aureole, along with high geothermal gradient, are the principal factors which drive the
704
RODINIA, GONDWANA AND ASIA
metamorphism of the primary basaltic crust, with subsequent melting and formation of andesitic and dacitic magmas. Available Sm-Nd a n d U-Pb ages of 3067-tl30 a n d 3131+ 74 Ma (Mishkin et al., 1999; Shemyakin et al., 1998) show that the origin of protoliths of the Sutamsky block occurred at the boundary of early and late Archean. In the CentralnoAldansky block the isotope dating of protoliths of enderbitic complex yielded 3335-t 3 Ma (Nutman et al., 1992). These data suggest that the ancient sialic nucleus of Aldan shield (CentralnoAldansky block) be generated at ca. 3.4 Ga ago above the centre of a rising mantle plume. Subsequently, peripheral, younger mantle diapirs caused lateral growth of an early sialic crust towards the end of early Archean leading to the development of the present borders of the Aldan shield.
References Mishkin, M.A., Maslovskaya, M.N. and Lavrik, S.N. (1999) Geochemistry and Rb-Sr isotopy of Archcan melabasitc - enderbitic formation of the Aldan shield south (Sutamsky block). Geochem., v. 9 , pp. 931-940. (in Russian). Nutman, A.P., Chernyshev, I.V. and Baadsgaard, H. (1992) The Aldan shield of Siberia, USSR: the age of its Archacan components and evidence for widespread reworking in the mid Proterozoic. Precamb. Res., v. 54, pp. 195-210. Ohtani, E. (1984) Generation of komatiite magma and gravitational differentiation in the deep upper mantle. Earth Planet. Sci. Lett., V. 67, pp. 261-272. Shemyakin, V.M., Glebovitsky,V.A. and Berezhnaya, N.G. (1998) About age of most ancient formations of the Sutamsky block (Aldansky granulitic areal). DAN. v. 360, pp. 526-529. (in Russian).
Alluvial Fan - Lacustrine Sedimentation and its Tectonic Implications in the Cretaceous Athgarh Gondwana Basin, Orissa, India B. Mishral, K.L. Pandya2and W. Maejima3 l? G. Department of Geology, Khallikote Autonomous College, Berhampur - 760 001, Orissa, India P. G. Department of Geology, Utkal University, Bhubaneswar - 751 004, Orissa, India Department of Geosciences, Osaka City University, Osaka 558-8585, Japan The Athgarh Formation (Lower Cretaceous) forms a n important stratigraphic unit of the thick pile of fluviatile and lacustrine sediments well known in Indian stratigraphy as 'Upper Gondwana'. The formation has been regarded as t h e northernmost extension of the east coast Upper Gondwana sediments of Peninsular India. Little attention has been given on tracing the sedimentation history of the Athgarh Formation. I t is, therefore, felt that the studies of the petrography, palaeocurrents, lithofacies associations and cyclicity, and geochemistry will provide better understanding for tracing the provenance history, palaeoflow and sediment-dispersal pattern, environments of deposition, and local and regional tectonic phenomena. The Athgarh Formation of the present area is essentially a clastic succession, 700 m thick, constituted of matrix-supported conglomerate, clast-supported conglomerate, pebbly sandstone, coarse to fine sandstone, mudstone, interbedded mudstone and shale, and carbonaceous shale. The succession was built against an upland scarp along the north and northwestern boundary of the basin marked by a NE-SW boundary fault. The succession shows a regular change in lithologic characteristics from the basin margin towards the basin centre (northwest to southeast). The marginal part of the basin is marked by dominance of conglomerates, whereas marked decrease in conglomerate and consequent increase in the proportion of sandstone, mudstone and shale is observed in the southeasterly direction. In the extreme southeast, mudstone and shale become dominant lithology. Fifteen lithofacies have been introduced. A regular change in the dominant facies types and association of lithofacies from the basin margin to the basin centre suggest deposition of the
succession in a stream-dominated alluvial fan environment with t h e development of proximal, mid and distal fan subenvironments with the distal part of the fan merging into a lake. Regional palaeocurrent indicates a consistent southeasterly dispersal of detritus from a source terrain located towards the north-west of the basin. The build up of the Athgarh alluvial fan system occurred in a humid climatic setting. Stream flow processes dominated and the fan was drained by braided channels. Debris flow process also operated, but was prominent only in the proximal fan zone. Several fans coalesced along the basin margin, forming a southeasterly sloping, broad and extensive alluvial plain terminating to a lake in the centre of the basin. Aggradation of fans along the subsiding margin of the basin resulted in the Athgarh succession showing remarkable lateral facies change in the down-dip direction. The proximal fan conglomerates pass into the sandstone dominated mid fan deposits which, in turn, grade into the cyclic sequences of sandstone-niudstone of the distal fan origin. Further downsiope, thick sequence of lacustrine shales occur. Progradation of fluvial sediments of distal fan on the lake surface has resulted in coarsening-upward lacustrine sequence. The results of the grain size analysis of the sandstones also matched surprisingly well with the conclusions drawn from the lithofacies association and indicate deposition of sediments in an alluvial fan environment dominated by fluvial processes. The faulted boundary condition of the basin and a thick pile of lacustrine sediments at the centre of the basin suggest that tectonism both in the source area and depositional site has played an important role throughout the deposition of the Athgarh succession of the present area. The vertical succession fines upward with the coarse proximal deposits at the base and fine Gondwana Research, K 4 , No. 4,2001
703
Middle to Late Devonian Paleomagnetic Pole for Gondwanaland: Results from Dulladerry Volcanics, Eastern Australia Masako Mikil and Chris Klootwijk2 Graduate School of Science and Technology, Kobe University, Kobe 6571-8501,Japan, E-mail: [email protected] Department of Geolopj, The Australian National University, Canberra, ACT 0200,Australia, E-mail:[email protected] Paleomagnetic results are reported for the late Middle Devonian Dulladerry Volcanics of the eastern Lachlan Orogen, Tasman Orogenic System. The volcanics had yielded SHRIMP U-Pb zircon age of 376f 4 Ma (Pogson and Watkins, 1998) and Givetian-Frasnian biostratigraphic age (Sherwin, 1996). The characteristic high temperature components were obtained from 11 sites of rhyolitic ignimbrites and rhyolitic lavas. Normal and reverse polarities and a positive fold test indicate a pre-folding (Early-middle Carboniferous Kanimblan Orogeny: terminal deformation event of the Lachlan Orogen), presumably primary, origin. The magnetic carrier of magnetite and hematite were identified from progressive thermal demagnetizations, three component Lowrie-tests and high and low temperature susceptibility cycling. The mean direction for 11sites is D= 177.8 degrees, I = 49.1 degrees with k= 9.7 and the radius of 9 5 % confidence circle of 15.4 degrees. The pole position (86.4 S and 296.5 E) calculated from the mean direction is close to the pole position for the contemporaneous Comerong Volcanics of the eastern Lachlan Orogen (Schmidt et al., 1986).The Dulladerry Volcanics postdate the early Middle Devonian Tabberabberan Orogeny, generally
regarded as a major cratonizing event of the Lachlan Orogen. The preliminary pole for the Dulladerry Volcanics could therefore be indicative of Middle to Late Devonian pole position for the Australian carton and for Gondwanaland. Middle temperature components with unblocking temperature of about 400 degrees are obtained from several sites. The scatter of these directions increases after tilt corrections, although the scatter is still large. The component is characterized by steep inclination of normal polarity. It might be correlated to secondary magnetization at the Carboniferous orogeny.
References Pogson, D.J. and Watkins J.J. (1998) Bathurst 1250 000 geological sheet SI/55-8: explanatory notes. Geol. Surv. New South Wales, Sydney. Schmidt, P.W., Embleton, B.J.J., Cudahy, T.J. and Powell, C.McA. (1986) Prefolding and premegaking magnetization from the Devonian Comerong volcanics, New South Wales, Australia, and their bearing on the Gondwana pole path. Tectonics, v. 5, pp. 135-150. Sherwin, L. (1996) Narromine 1250 000 geological sheet SI/55-3: explanatory notes. Geol. Surv. New South Wales, Sydney.
A Model for the Origin of Early Crust of East Asia M.A. Mishkin Far East Geological Institute, FEB R A S , Vladivostok, Russia The early crustal formations of granulite - gneissic terrains in ancient shield areas comprise a variety of enderbites that are broadly analogous with grey gneisses of the granite - greenstone areas. Ourcrops of enderbitic complexes are found in the Aldan and Anabar shields of the Siberian craton. During the past decades, petrological studies have demonstrated that the most deepest of them are widespread towards the south of Aldan shield, where outcrops are found in different tectonic blocks - Sutamsky, Zverevsky, Kurultinsky and Djugdjursky. Investigations in these blocks have contributed significantly to our understanding of the origin of early crust. The present work investigates the nature of protoliths and time scale of formation of the Sutamsky block using geochemical and isotope geochronological techniques. The Sutamsky granulitic complex comprises hypersthene plagiogneisses containing interlayers of two-pyroxene rocks, metaultrabasites and, rarely quartzites. We suggest this strata to be metabasite enderbitic formation. Based on geochemical criteria, the Gondwana Research, I? 4, No. 4,2001
protoliths of the metabasite - enderbitic formation are inferred to be volcanics of calc-alkali and komatiitic-tholeiitic series. The calc-alkali series (comprising 90%) consist of basalt-andesitedacite-rhyodacite, with essential prevalence of andesites and dacites. (Mishkin et al., 1999). The calc-alkaline volcanic series are thought to be derived through the partial melting of a metamorphosed mafic primary crust, such as garnet amphibolite. This model is based on WE distribution pattern in enderbites. The komatiitic-tholeiitic series of metavolcanics are invariably present in minor amounts within the enderbitic complexes of shield areas. Their existence is not unusual, and their origin is linked to early sialic crust. The existence of both ultrabasic and basalt volcanics of this series in a uniform section allows us to speculate the formation of melts by varying partial melting of mantle material in a rising diapir under varying P-T conditions (Ohtani, 1984). The high temperatures of melts and presence of reduced gaseous-fluid dominated aureole, along with high geothermal gradient, are the principal factors which drive the
704
RODINIA, GONDWANA AND ASIA
metamorphism of the primary basaltic crust, with subsequent melting and formation of andesitic and dacitic magmas. Available Sm-Nd a n d U-Pb ages of 3067-tl30 a n d 3131+ 74 Ma (Mishkin et al., 1999; Shemyakin et al., 1998) show that the origin of protoliths of the Sutamsky block occurred at the boundary of early and late Archean. In the CentralnoAldansky block the isotope dating of protoliths of enderbitic complex yielded 3335-t 3 Ma (Nutman et al., 1992). These data suggest that the ancient sialic nucleus of Aldan shield (CentralnoAldansky block) be generated at ca. 3.4 Ga ago above the centre of a rising mantle plume. Subsequently, peripheral, younger mantle diapirs caused lateral growth of an early sialic crust towards the end of early Archean leading to the development of the present borders of the Aldan shield.
References Mishkin, M.A., Maslovskaya, M.N. and Lavrik, S.N. (1999) Geochemistry and Rb-Sr isotopy of Archcan melabasitc - enderbitic formation of the Aldan shield south (Sutamsky block). Geochem., v. 9 , pp. 931-940. (in Russian). Nutman, A.P., Chernyshev, I.V. and Baadsgaard, H. (1992) The Aldan shield of Siberia, USSR: the age of its Archacan components and evidence for widespread reworking in the mid Proterozoic. Precamb. Res., v. 54, pp. 195-210. Ohtani, E. (1984) Generation of komatiite magma and gravitational differentiation in the deep upper mantle. Earth Planet. Sci. Lett., V. 67, pp. 261-272. Shemyakin, V.M., Glebovitsky,V.A. and Berezhnaya, N.G. (1998) About age of most ancient formations of the Sutamsky block (Aldansky granulitic areal). DAN. v. 360, pp. 526-529. (in Russian).
Alluvial Fan - Lacustrine Sedimentation and its Tectonic Implications in the Cretaceous Athgarh Gondwana Basin, Orissa, India B. Mishral, K.L. Pandya2and W. Maejima3 l? G. Department of Geology, Khallikote Autonomous College, Berhampur - 760 001, Orissa, India P. G. Department of Geology, Utkal University, Bhubaneswar - 751 004, Orissa, India Department of Geosciences, Osaka City University, Osaka 558-8585, Japan The Athgarh Formation (Lower Cretaceous) forms a n important stratigraphic unit of the thick pile of fluviatile and lacustrine sediments well known in Indian stratigraphy as 'Upper Gondwana'. The formation has been regarded as t h e northernmost extension of the east coast Upper Gondwana sediments of Peninsular India. Little attention has been given on tracing the sedimentation history of the Athgarh Formation. I t is, therefore, felt that the studies of the petrography, palaeocurrents, lithofacies associations and cyclicity, and geochemistry will provide better understanding for tracing the provenance history, palaeoflow and sediment-dispersal pattern, environments of deposition, and local and regional tectonic phenomena. The Athgarh Formation of the present area is essentially a clastic succession, 700 m thick, constituted of matrix-supported conglomerate, clast-supported conglomerate, pebbly sandstone, coarse to fine sandstone, mudstone, interbedded mudstone and shale, and carbonaceous shale. The succession was built against an upland scarp along the north and northwestern boundary of the basin marked by a NE-SW boundary fault. The succession shows a regular change in lithologic characteristics from the basin margin towards the basin centre (northwest to southeast). The marginal part of the basin is marked by dominance of conglomerates, whereas marked decrease in conglomerate and consequent increase in the proportion of sandstone, mudstone and shale is observed in the southeasterly direction. In the extreme southeast, mudstone and shale become dominant lithology. Fifteen lithofacies have been introduced. A regular change in the dominant facies types and association of lithofacies from the basin margin to the basin centre suggest deposition of the
succession in a stream-dominated alluvial fan environment with t h e development of proximal, mid and distal fan subenvironments with the distal part of the fan merging into a lake. Regional palaeocurrent indicates a consistent southeasterly dispersal of detritus from a source terrain located towards the north-west of the basin. The build up of the Athgarh alluvial fan system occurred in a humid climatic setting. Stream flow processes dominated and the fan was drained by braided channels. Debris flow process also operated, but was prominent only in the proximal fan zone. Several fans coalesced along the basin margin, forming a southeasterly sloping, broad and extensive alluvial plain terminating to a lake in the centre of the basin. Aggradation of fans along the subsiding margin of the basin resulted in the Athgarh succession showing remarkable lateral facies change in the down-dip direction. The proximal fan conglomerates pass into the sandstone dominated mid fan deposits which, in turn, grade into the cyclic sequences of sandstone-niudstone of the distal fan origin. Further downsiope, thick sequence of lacustrine shales occur. Progradation of fluvial sediments of distal fan on the lake surface has resulted in coarsening-upward lacustrine sequence. The results of the grain size analysis of the sandstones also matched surprisingly well with the conclusions drawn from the lithofacies association and indicate deposition of sediments in an alluvial fan environment dominated by fluvial processes. The faulted boundary condition of the basin and a thick pile of lacustrine sediments at the centre of the basin suggest that tectonism both in the source area and depositional site has played an important role throughout the deposition of the Athgarh succession of the present area. The vertical succession fines upward with the coarse proximal deposits at the base and fine Gondwana Research, K 4 , No. 4,2001