Granitic Clasts Research Articles

Sedimentary evidence against large strike-slip translation on the Northern Altyn Tagh fault, NW China

Whether the oblique left-reverse Northern Altyn Tagh fault has large cumulative offset (>120 km) is one of the most controversial issues concerning the deformation mechanics for the northern Tibetan Plateau. In order to obtain material constraints on its total displacement, sedimentologic and bedrock provenance analysis of Oligocene–Pleistocene strata on the northern side of the fault was conducted. Lithofacies analysis and 970 paleocurrent measurements show that these strata were products of fluvial systems originating from south of the fault, making it possible to constrain the fault's slip history by establishing cross-fault sediment/source linkages. Identification of 5217 conglomerate clasts in the field indicates that major clast types are quartzite (30–50%), schist/gneiss (increasing upsection from 10% to 40–50%), and marble (decreasing upsection from 20–30% to <10%). Although granitic clasts can be observed throughout the entire section, they do not constitute a major clast type in any single bed. Amphibolite clasts first occur in the middle part of the Miocene strata, and reach up to 5% in the Pliocene and Pleistocene strata. Accordingly, sandstone heavy mineral analysis indicates: (1) dominance of metamorphic over igneous minerals, (2) absence of contact metamorphic minerals, (3) dominance of greenschist facies minerals lower in the section and coexistence of both greenschist and amphibolite facies minerals in the upper part of the section. These observations are consistent with unroofing of Precambrian basement immediately to the south of the studied Oligocene–Pleistocene section and argue against any left-lateral translation greater than 30 km along the Northern Altyn Tagh fault. Therefore, our results preclude the possibility of large (>30 km) strike-slip translation on the Northern Altyn Tagh fault and support the model that strain induced by Indian indentation has been partitioned into left-slip on the Altyn Tagh fault and reverse dip-slip on the Northern Altyn Tagh fault.

Timing and setting of Guanjiagou conglomerate in South Qinling and their tectonic implications

Located in the southern Qinling Mountains of central China, the Guanjiagou Formation has been a controversial issue with regard to its depositional age and tectonic implications. Being comprised of an approximately 2050 m thick succession of texturally and compositionally immature, presumed marine turbiditic sandstones and conglomerate, the Guanjiagou Formation consists of an overall prograding- and coarsening-upward megasequence. Although bounded by faults on both its northern and southern margins, it is weakly metamorphosed and deformed. To the north is the Devonian Sanhekou Group and to the south is the Neoproterozoic Hengdan Group. The lower portion of the sequence contains granitic and volcanic clasts (Guanjiagou conglomerate). The feldspars from these clasts were dated using the40Ar/39Ar method. Two cooling ages of 219.69±0.49 and 216.46±0.59 Ma, for K-feldspar from a granitic clast and plagioclase from a volcanic clast, respectively, were obtained. These ages are identical to the time of regional igneous activities (ca. 240–220 Ma) and are interpreted as the products of magmatism associated with collision in the Qinling orogenic belt in the Early Mesozoic, suggesting that the Guanjiagou Formation was deposited in the Norian of the Late Triassic, ca. 220 Ma. Therefore,40Ar/39Ar and sedimentary analyses suggest that the Guanjiagou Formation contains sediments that may have filled in a remnant ocean basin, which might be part of the Anyemaqen-Mianlue ocean, or Tethys on the southern side of Central Orogenic Belt in China during the Late Paleozoic to Early Mesozoic.

Provenance of the Murihiku Terrane, New Zealand: evidence from the Jurassic conglomerates and sandstones in Southland

Sedimentary petrological, geochemical, and geochronological studies were carried out to reveal the provenance of the Murihiku Terrane, Southland, New Zealand. The Murihiku Terrane comprises Late Permian to Early Cretaceous volcaniclastic sediments that were deposited at the fore-arc or back-arc basin of the eastern Gondwana margin. The strata in the Waikawa district, Southland, are Middle or early Late Jurassic and comprise sandstones interbedded with conglomerates and mudstones. The sediments can be petrologically classified into two groups: one dominated by intermediate-mafic volcanic detritus (type-A), and the other more quartz bearing (15–20% in QFL), dominated by felsic volcanic and plutonic detritus (type-B). The chemistry of detrital clinopyroxene in type-A sediments is compatible with their derivation from island arc volcanics. Whole-rock major- and trace-element analyses suggest that granitoid and gabbroid clasts associated with type-B sediments are also of volcanic arc origin. Geochemical characteristics of the plutonic clasts can be compared closely with the Median Batholith (Median Tectonic Zone). Whole-rock K–Ar ages of the clasts (156±8 and 177±9 Ma for gabbroid clasts and 156±8 and 212±11 Ma for alkali feldspar granite clasts) are consistent with the Median Batholith magma cooling. Small quantities of grandite and pyrope-bearing garnet grains in type-B sediments might be derived from metamorphosed intermediate-mafic volcanic rocks to amphibolite facies. Granitic cataclasite and hornfels are also compatible with derivation from the Median Batholith that was accreted to the Brook Street Terrane.

Restoration of Exotic Terranes Along the Median Tectonic Line, Japanese Islands: Overview

This paper reviews recent progress on the geotectonic evolution of exotic Paleozoic terranes in Southwest Japan, namely the Paleo-Ryoke and Kurosegawa terranes. The Paleo-Ryoke Terrane is composed mainly of Permian granitic rocks with hornfels, mid-Cretaceous high-grade metamorphic rocks associated with granitic rocks, and Upper Cretaceous sedimentary cover. They form nappe structures on the Sambagawa metamorphic rocks. The Permian granitic rocks are correlative with granitic clasts in Permian conglomerates in the South Kitakami Terrane, whereas the mid-Cretaceous rocks are correlative with those in the Abukuma Terrane. This correlation suggests that the elements of Northeast Japan to the northeast of the Tanakura Tectonic Line were connected in between the paired metamorphic belt along the Median Tectonic Line, Southwest Japan. The Kurosegawa Terrane is composed of various Paleozoic rocks with serpentinite and occurs as disrupted bodies bounded by faults in the middle part of the Jurassic Chichibu Terrane accretionary complex. It is correlated with the South Kitakami Terrane in Northeast Japan. The constituents of both terranes are considered to have been originally distributed more closely and overlay the Jurassic accretionary terrane as nappes. The current sporadic occurrence of these terranes can possibly be attributed to the difference in erosion level and later stage depression or transtension along strike-slip faults. The constituents of both exotic terranes, especially the Ordovician granite in the Kurosegawa-South Kitakami Terrane and the Permian granite in the Paleo-Ryoke Terrane provide a significant key to reconstructing these exotic terranes by correlating them with Paleozoic granitoids in the eastern Asia continent.

A Back-arc Palaeotectonic Setting for the Augaro Neoproterozoic Magmatic Rocks of Western Eritrea

The Augaro volcano-sedimentary assemblages of western Eritrea are part of the Neoproterozoic, N-S trending belt of low-grade volcano-sedimentary and associated plutonic rocks. In contrast to the volcanic-dominated oceanic-arc assemblages in central Eritrea, the predominant rock types in the west are supracrustal sequences of sedimentary origin with subordinate volcanic rocks. These Augaro supracrustal rocks are overlain, unconformably, by a basin-fill metasedimentary succession known as the Gulgula Group. The Augaro metavolcanic rocks are tholeiitic and range in composition from basalt to basaltic andesite. Comparison of trace element characteristics and N-MORB-normalised spidergrams of these rocks with those of modern volcanic environments and age-comparable metavolcanic rocks of known tectonic association from the Arabian-Nubian Shield suggest that the volcanic assemblages from western Eritrea were generated in a back-arc tectonic setting. Single zircon Pb-Pb evaporation and vapour-transfer U-Pb analyses of magmatic zircons from pre/syn-tectonic granites yield a mean 207Pb/ 206Pb age of 849±20 Ma and an upper concordia intercept age of 849±26 Ma. These ages are interpreted to represent the time of major magmatism in western Eritrea and are comparable to ages of early arc magmatism in central and northern Eritrea and in the southern Nubian Shield. Initial eNd values and initial Sr isotope ratios of whole-rock samples of magmatic rocks calculated for an age of 850 Ma range from +4.0 to +7.1 and 0.7026 to 0.7037, respectively. Single zircon 207Pb/ 206Pb ages, initial eNd value and Sr isotope ratio for a granitic clast in the Gulgula metaconglomerate suggest that the source area for the Gulgula metasedimentary rocks is similar to the surrounding Neoproterozoic rocks of western Eritrea.

A new Late Proterozoic stratum in South Korea

The Myeonsan Formation, the lowermost strata of the lower Paleozoic sequence, has basal conglomerate, which consists mostly of orthoquartzite and granitic gneiss clasts. The latter was derived from the basement rock (the Taebaeksan Gneiss Complex) underlying the Myeonsan Formation. However, a Precambrian sedimentary sequence, which is supposed to have supplied orthoquartzite clasts, has not been reported in South Korea. This study reports a newly definable Proterozoic stratum, less than 10 cm thick, formerly known as the lowermost part of the Myeonsan Formation. It is different from the lithology of the Myeonsan Formation, but is same in petrographical and geochemical characteristics to orthoquartzite clasts in basal conglomerate of the Myeonsan Formation. This stratum is interpreted to have been a source rock of orthoquartzite clasts to the lower Paleozoic basin. It is interpreted that the orthoquartzite sequence had been deposited in the Late Proterozoic, which is correlatable with the Guhyeon System in North Korea.

The Eucarro Rhyolite, Gawler Range Volcanics, South Australia: A &gt;675 km3, compositionally zoned lava of Mesoproterozoic age

The Eucarro Rhyolite is remarkable in being far more voluminous (>675 km3) and extensive than most felsic lavas (<1 km3). Its dimensions are comparable to the largest known volcanic units (felsic ignimbrites and flood basalts). It is one of several voluminous felsic units within the Mesoprot erozoic (ca. 1592 Ma), Gawler Range Volcanics, an intracontinental, intraplate volcanic province in South Australia and is typical of the voluminous felsic magmatism associated with Laurentia (cf. Belt Supergroup, Midcontinent United States). The lava is 300 m thick, with groundmass textures and lithofacies characteristics consistent with cooling and degassing as a single unit. There are gradational compositional and textural variations throughout its highly elongate distribution (225 km east-west, 74 wt% SiO2). Pyroxene is the dominant ferromagnesian phase, but at the base in the west, primary amphibole and olivine are present. Granitic and mafic clasts are ubiquitous but most abundant (5 vol%) in western and eastern parts of the unit. The main Eucarro magma was compositionally zoned, and several smaller batches of compositionally distinct magma were tapped during the eruption. Concentrations of granitic clasts coincide with changes in bulk composition and may record the rupture of wall rock between nearby but separate magmas. The distribution, subtle lateral variations in composition and texture, and local presence of compositional flow banding along the northern margins suggest that the Eucarro Rhyolite was derived from several source vents located along an east-trending fissure(?) system. Although very widespread, outflow of the Eucarro lava was probably no more than a few tens of kilometers from source vents.

Significado Tectônico dos Depósitos de Leques Aluviais da Formação Santa Bárbara (Eo-Paleozóico) na Região de Caçapava do Sul (RS, Brasil)

The alluvial fan deposits of Early Paleozoic Santa Bárbara Formation, located in Sul-rio-grandense Shield area of southernmost Brazil, have been analyzed here in terms of facies associations and tectonic significance. The investigated unit comprises conglomerates, sandstones, and siltstones, deposited probably under continental semi-arid climate conditions, in the post-collisional stages of Neoproterozoic Brasiliano/Pan-African Cycle. The location of the alluvial deposits suggests that the depositional locus of this sedimentary unit should probably consist of an individual and restricted fault-bounded basin, named here Santa Bárbara Basin. Its settling was controlled by the extensional and strike-slip reactivation of NE, NW, and N-S structures inherited from Brasiliano/Pan-African Cycle. Alluvial deposits located in the southwestern and northwestern borders show dominance of sheetflood processes in the alluvial fans deposition. On the other hand, the eastern border of Santa Bárbara Basin, named the “Caçapava paleohigh”, comprises the coarser alluvial deposits, with major occurrence of debris-flow features. This fact suggests a possible asymmetry of this basin. These eastern border deposits reveal, in the clast composition of the conglomerates, three main phases of denudation of “Caçapava paleohigh”. The basal conglomerates reflect a stage when this important topographic high was covered by andesites and metasedimentary rocks. Stratigraphically above, metavolcanic fragments compose the coarse deposits, while at the top of the package the conglomerate framework consists mainly of granitic clasts. This third stage reflects the exposure of the deeper parts of “Caçapava paleohigh”, and it can be positioned at the base of the unconformable Sequence III of Santa Bárbara Formation. The total inversion of braided fluvial axial filling system and the occurrence of intrabasinal debris-flow deposits suggest a strong tectonic event and possibly a significant time span at the base of Sequence III.a

Granite: A Planetary Point of View

The telluric planets and the asteroid belt display the same internal structure with a metallic inner core and a silicate outer shell. Experimental data and petrological evidence in silicate systems show that granite can be produced through all types of magmatic processes. On Moon, 4.4-3.9 Ga granite clasts displaying dry mineral assemblages correspond to discrete intrusive events. Large K/Ca enrichment and low REE abundances in granite relative to KREEP are consistent with silicate liquid immiscibility, a process observed in melt inclusions within olivine of Lunar basalts. On Mars, black-and-white rhythmic layers observed on the Tharsis rise along the flanks of Valles Marineris and the peripheral scarps of the Tharsis Montes giant volcanoes suggest the eruption of possibly felsic pyroclastites. Samples analysed in the Mars Pathfinder landing site yield basic to intermediate (45 to 62 wt.% SiO 2) compositions. Though no granites were found so far in the Martian SNC meteorites, a component close to terrestrial continental (granitic) crust is inferred from trace element and isotope systematics. Like the Martian northern hemisphere, Venus has suffered extensive volcanic resurfacing, whereas folded and faulted areas resemble terrestrial continents. The hypothesis of a granitic component is again “tantalizing”. Finally, extra-terrestrial granite is also found as enclaves within iron meteorites and ordinary chondrites. Granite, a major component of the Earth's crust, can be generated in all geodynamic settings. The low density of granite favours continental accretion. Thus, the occurrence or absence of granite and of associated silicic volcanism within the other telluric planets is not a trivial question. Granite is generally thought to be produced through “wet” processes. Lunar evidence shows that dry conditions may apply as well. In Venus, a large planet with high rates of magma production, it is speculated that significant volumes of granite can develop.

Basement chronology of the Antarctic Peninsula: recurrent magmatism and anatexis in the Palaeozoic Gondwana Margin

Abstract: A revised pre-Jurassic chronology is presented for the Antarctic Peninsula on the basis of new U–Pb zircon dating, using both conventional (11 samples) and SHRIMP microprobe data (10 samples). The age range for plutonism and high-grade metamorphism is from 435 ± 8 Ma (Silurian) to c. 206 Ma (approximately the Triassic–Jurassic boundary), with peaks of activity in Devonian, Carboniferous, Permian and mid-Triassic times, although some are represented by rocks with limited outcrops. The new data confirm the importance of Mid- and latest Triassic magmatic events, previously identified using Rb–Sr geochronology. The Adie Inlet gneiss, previously thought to be Neoproterozoic, is recognized as a Permian migmatite derived from paragneiss with a provenance dominated by Cambrian granitoids. Granite gneiss from NW Palmer Land, previously dated as Cambrian, is shown to be Triassic. Detrital zircons in metasedimentary rocks, and inherited zircons in granitoids, are dominated by Mesoproterozoic to Cambrian components, with sparse Palaeoproterozoic to Archaean grains, suggesting sources within Gondwana. At times these sources were nearby, as shown by an Archaean cobble-sized granite clast from a Permo-Triassic conglomerate; another clast in the same conglomerate, previously thought to be Devonian in age, has an Ordovician crystallization age.

Geochronology of clasts in allochthonous Miocene sedimentary sequences on Mykonos and Paros Islands: implications for back-arc extension in the Aegean Sea

In the Cyclades, low-angle normal faults juxtaposed Miocene sedimentary rock units lying over Alpine blueschist- and greenschist-facies metamorphic rocks and Miocene granites. The sedimentary units in the hanging wall were deposited in fault-bounded basins while their footwalls progressively emerged through the ductile and brittle crust. The sedimentation in the basins evolved from marine turbidites in the Early Miocene to shallow/continental conglomerates during the Late Miocene. The transition from marine to fan delta sedimentation was coeval with widespread magmatism and possibly reflects true crustal uplift. It is inferred that the sequence is no younger than 8 Ma. K–Ar and 40 Ar/ 39 Ar geochronology, petrology and petrography of the clasts deposited on the hanging wall provide proof of progressive exhumation of the footwall and reveal the nature of the overburden that covered the Cycladic blueschist belt during Alpine orogeny in the Tertiary. Abundant metamorphic clasts yielding mica cooling ages between 80 and 100 Ma occur throughout the sedimentary section and probably pertain to a vast Pelagonian-type rock mass that covered the internal Hellenides from the Olympos to the central Cyclades. In addition, volcanic clasts dated at 10 Ma reveal the existence of a hitherto unknown volcanic province of that age in the central Aegean. Miocene crustal extension and exhumation of granitic plutons is recorded in the detrital sequence in a concentration of 10 Ma granitic clasts restricted to the top of the conglomerate sequence. A group of metamorphic clasts that yielded ages of 13–16 Ma possibly represents exhumation of mid crustal levels. Clasts similar to the currently-exposed Cycladic Blueschist Unit, such as 40 Ma old blueschists, eclogites and marble, were not found, thus indicating the very late exposure of these footwall rocks.

Eocambrian granite clasts in southern British Columbia shed light on Cordilleran hinterland crust

The Spa Creek assemblage is a distinctive thin pericratonic succession that crosses the Okanagan Valley in the hinterland of the southern Cordilleran Orogen in Canada. The succession was ductilely deformed and metamorphosed before deposition of overlying Triassic dark metaclastic strata. A metaconglomerate within the succession, locally composed of more than 90% biotite granite clasts, yielded five fractions of euhedral zircon that define a precise U–Pb upper intercept of 555.6 ± 2.5 Ma, inferred to be the age of a nearby pluton. Other clasts in the metaconglomerate are generally more abundant, consisting of quartzite, amphibole schist, chlorite schist, sericite schist, biotite schist, and quartz–feldspar porphyry. They are likely host rocks of the pluton and, if so, are Late Proterozoic or older. The granite is interpreted as a terminal product of the Eocambrian rifting that preceded Paleozoic miogeoclinal sedimentation farther inboard. The continuity of pericratonic rocks west of the miogeocline and the occurrence of Proterozoic cratonic rocks at the surface west of the Okanagan Valley show that the ancient continental margin extends into a region where most of the crustal lithosphere was until now thought to consist of accreted Phanerozoic arc and accretionary complexes.

SIGNIFICANCE OF SANDSTONES INTERBEDDED IN THE JURASSIC BASALTS OF THE BALAGNE OPHIOLITIC NAPPE (CORSICA, FRANCE)

The Balagne Nappe which includes a thick ophiolitic succession, has an isolated external western position and is underlain by Eocene rocks over the autochthonous Corsican basement. The nappe consists, from base to top, of: 1) basaltic pillow lavas up to 1 km thick, 2) oceanic sediments (radiolarites, limestones showing diverse facies, particularly pelagic, pelites with interbeds of “Palombini”-type limestones, and 3) detrital formations ranging from the Albian- Cenomanian to Middle Eocene. The trace element content of the basalts of the Balagne Nappe s.l. is characteristic of enriched mid-oceanic ridge basalts (EMORB). In contrast, lavas of the more eastern Ligurian Units, such as the Inzecca Unit (Upper ‘Schistes lustres’), have an N-MORB character. We can thus define a marginal “Balagne-Ligurian domain”. In their lower part close to the gabbroic substratum, near Piana di Castifao, the Jurassic basalts have a level of feldspathic and micaceous quartzitic sandstone with calcareous cement (Baud, 1975). This level, 3 m thick, is intercalated within two 8-m layers of basaltic breccia (exploded pillows), sandwiched between flows of pillow lavas in places showing normal polarity. This rules out a tectonic hypothesis for the contacts at the base and top of the sandstones, as both contacts are clearly stratigraphic. The top of the sandstones is coarse grained, with lithic debris of continental origin and basalt fragments. The heavy minerals originate either from the oceanic or continental material. Zircons are particularly abundant and well preserved, which indicates limited transport. The typological study, according to Pupin’s diagram (1980), reveals that zircons plot in the field overlapping that of zircons from calc-alkaline granites (s.l.), highly abundant in the Corsican-Sardinian batholith. The Piana di Castifao sandstone interbed is the only known example in the Corsican Ligurian ophiolites. A similar thin bed has been described from the top of the ophiolitic sequence of the “Lago Nero-Replatte Nappe” to the east of Briancon in the French-Italian Alps. Similarly, in the Queyras region West of Mount Viso were reported sands and gravels with granite clasts, here there are underlying pillow basalts extruded over a serpentinite substratum. In both cases (east of Briancon and Queyras), no geological nor geochemical indications were found to constrain the position of the continent that supplied the detrital material. The detrital material originated from a continental source similar to that of Piana di Castifao. The Balagne-Ligurian domain of the Ligurian Ocean must therefore have been situated, during the Jurassic, relatively close to the Western European continental margin, as indicated by the geochemical facies of the EMORB, the basement fragments within the San Colombano supra-ophiolitic limestones, and the Piana di Castifao quartzitic interbed. The eastern margin of the Ligurian Ocean was also characterized by continental detritus as shown in the Cretaceous External Ligurides in the Northern Apennines.

Eocambrian granite clasts in southern British Columbia shed light on Cordilleran hinterland crust

The Spa Creek assemblage is a distinctive thin pericratonic succession that crosses the Okanagan Valley in the hinterland of the southern Cordilleran Orogen in Canada. The succession was ductilely deformed and metamorphosed before deposition of overlying Triassic dark metaclastic strata. A metaconglomerate within the succession, locally composed of more than 90% biotite granite clasts, yielded five fractions of euhedral zircon that define a precise U–Pb upper intercept of 555.6 ± 2.5 Ma, inferred to be the age of a nearby pluton. Other clasts in the metaconglomerate are generally more abundant, consisting of quartzite, amphibole schist, chlorite schist, sericite schist, biotite schist, and quartz–feldspar porphyry. They are likely host rocks of the pluton and, if so, are Late Proterozoic or older. The granite is interpreted as a terminal product of the Eocambrian rifting that preceded Paleozoic miogeoclinal sedimentation farther inboard. The continuity of pericratonic rocks west of the miogeocline and the...

Distant ejecta from the Lockne marine‐target impact crater, Sweden

Abstract— The Lockne impact event took place in a Middle Ordovician (455 Ma) epicontinental sea. The impact resulted in an at least 13.5 km wide, concentric crater in the sea floor. Lockne is one of very few locations where parts of an ejecta layer have been preserved outside the crater structure. The ejecta from the Lockne impact rests on progressively higher stratigraphic levels with increasing distance from the crater, hence forming a slightly inclined discontinuity surface in the pre‐impact strata. We report on a ∼30 cm thick sandy layer at Hallen, 45 km south of the crater centre. This layer has a fining upward sequence in its lower part, followed by low‐angle cross‐laminations indicating two opposite current directions. It is rich in quartz grains with planar deformation features and contains numerous, up to 15 cm large, granite clasts from the crystalline basement at the Lockne impact site. The layer is within a sequence dated to the Baltoniodus gerdae conodont subzone. The dating is corroborated by chitinozoans indicating the latest Kukruse time below and the late Idavere above the impact layer. According to the chitinozoans biostratigraphy, some erosion may have occurred because of deposition of the impact layer. The Hallen outcrop, today 45 km from the centre of the Lockne crater, is at present the most distant accessible occurrence of ejecta from the Lockne impact. It is also the most distant location so far found where the resurge of water towards the crater has affected the bottom sediments.A greater crater diameter than hitherto assumed, thus representing greater impact energy, might explain the extent of the ejecta blanket. Fluidisation of ejecta, to be expected at a marine‐target impact, might furthermore have facilitated the wide distribution of ejecta.

Alteration Rinds on Glacial Diamictite Clasts in the Gowganda Formation: Possible Indicators of Low Atmospheric Oxygen ca. 2.3 Ga

The Coleman Member of the ea. 2.3 Ga Gowganda Formation is beautifully exposed in a glacially polished outcrop near Cobalt, Ontario. This lithologic unit is a glacial diamictite, or tillite, consisting of clasts up to 1.5 m in diameter embedded in a silty to sandy matrix. More than half of the clasts display alteration rinds. On granite clasts these consist of a pale pink outer rim and a dark inner rim, in which orthoclase has been replaced by what is now chlorite. The rinds are inferred to have initially developed as a result of chemical weathering before the clasts were deposited as part of the tillite, and to have been modified by post-depositional metamorphism. The chlorite in the rinds contains Mg and Fe in approximately the same proportion as in the unweathered clasts. The chlorite was probably generated by the low-grade metamorphism of smectites produced by the reaction of feldspars with O2-free groundwater carrying Fe+2 and Mg+2. The absence of O2 in these ground waters is consistent with the deposition of the Gowganda Formation before the rise of O2 in the atmosphere ca. 2.25 Ga.

Rb-Sr isotopic and REE geochemistry on garnet-biotite granodiorite clasts from the Permian Kozaki Formation, Kumamoto, Southwest Japan. A slightly enriched initial Sr isotope ratio and M-type REE tetrad pattern.

Sr isotopic ratios and Rb, Sr and REE abundances are determined for granitic clasts from the middle Permian Kozaki Formation in the Kurosegawa Belt, Kumamoto, Southwest Japan, in order to elucidate their source and formation. Garnet-biotite granodiorite clasts, uniquely found in the Kozaki Formation, show M-type REE tetrad patterns and wide variation of Rb/Sr ratios, which enable their petrogenesis and age determination. The REE patterns indicate shallow formation of the garnet-bearing granodiorite. The Rb-Sr isotopic data yield an age of 251 ± 46 Ma with an initial ratio of 0.7055 ± 0.0006. The Rb-Sr age is close to the sedimentation age of the Kozaki Formation defined by paleontological data and K-Ar ages of the corresponding granitic clasts in the Permian conglomerate (“Usuginu conglomerate”) of the Kitakami area, Northeast Japan. The initial Sr isotopic ratio of 0.7055 is slightly higher than the ratios around 0.7040 reported for the Permian granitic rocks from the Akaishi Range and the eastern Kyushu and the corresponding Permian granitic clasts in the Kitakami area. The initial ratio for the garnet-biotite granodiorite clasts is also slightly higher than the ratios (0.7037 and 0.7032) at 345 and 400 Ma reported for Yatsushiro granite of the Kurosegawa Belt (Nohda, 1973; Yanagi, 1975). These Sr isotopic data suggest that the garnet-biotite granodiorite clasts correspond to rocks differentiated from the Kurosegawa granitic rocks. The Sr initial ratio of 0.7055 at 251 Ma for the garnet-bearing granodiorite clasts is slightly enriched but still primitive, and the Sr initial ratio clearly indicates that the clasts were not derived from typical-continental crust. The Rb-Sr isotopic data and our preliminary Sm-Nd isotopic data for the garnet-biotite granodiorite clasts yield eSr (251 Ma) = +18 and eNd (251 Ma) = +6, suggesting an intra-oceanic arc provenance for the garnet-biotite granodiorite body from which the clasts in the Kozaki Formation had been derived.

Taking the roof off a suture zone: basin setting and provenance of conglomerates in the ORS Dingle Basin of SW Ireland

Abstract The Late Silurian to Mid-Devonian Dingle Basin occupies a central position within the Iapetus Suture Zone of SW Ireland. The basin is believed to have formed from late Silurian times onwards as a product of sinistral transpression along several major faults within this suture zone. Conglomeratic sediments were deposited by moderately large gravelly fans shed into the basin from the NW (Glashabeg Formation) and the SE (Trabeg Conglomerate Formation). The systems fed into a large apparently through-going sandy, axial river (Slea Head Formation) that flowed towards the NE. The lateral, basin-margin systems were sourced from two disparate source terranes. To the NW lay a basic volcanic hinterland with some intermediate volcanic rocks and limestones, mudstones, sandstones and chert. During the Early Devonian time the SE drainage basin was underlain by sandstones, quartzites, phyllites and limestones probably intruded by a granite. Some of these lithologies can be found in outcrop in the pre-Dingle Group of the peninsula. Others need to be correlated with rocks of the oceanic terranes in the northern part of the suture zone. The southerly derived clasts have corollaries in the rocks of the Avalonian Leinster Terrane south of the suture. Following partial inversion of the Dingle Basin, the southerly hinterland was apparently further unroofed during mid-Devonian time, when the Inch Conglomerate Formation was deposited by alluvial fans shed northwards from a source area formed along the Dingle Bay Lineament. The Inch conglomerates are characterized by distinctive clasts of schist, gneiss, mylonite, tourmalinite and granite. The general picture of the Early Devonian deformation, intrusion by granite and unroofing of terranes currently partially exposed in central and southern Ireland within the Iapetus Suture Zone is largely consistent with clast lithotypes. However, some exploratory isotopic data indicate at least two possible vagaries in the interpretation. First, model ND T DM ages of Trabeg sedimentary clasts yield several results older than typical southern Iapetean or Avalonian crustal material. This suggests a complex history of sedimentary mixing of material across the developing Iapetus Ocean. Second, two of three Rb-Sr muscovite-whole-rock dates of Inch metamorphic clasts indicate Silurian ages. These data similar to Rb-Sr dates derived from the Carnsore and Saltees granites in the Rosslare Terrane, perhaps extending the geographical spread of this Silurian deformation.

High resolution measurement of bedload transport

A bedload movement detector of novel design was installed in a gravel-bed stream as a component of the ongoing research on sediment transport dynamics in the Stuart–Takla Experimental Watersheds in north-central British Columbia, Canada. The device is designed to collect information on the patterns and timing of bedload transport during a flood event. The device is based on a passive magnetic sensor that produces signals of 10−2 to 10−5 V as its magnetic field is disturbed by passing clasts. A series of 82 sensors is housed in an aluminum beam placed across the stream, inserted such that its surface is flush with the gravel bed. The device can be raised or lowered to compensate for bed aggradation and scour. A data acquisition system gathers voltage signals from the sensors at rates of 30 to 100 Hz. This device is sensitive enough to record the movement of most volcanic, metamorphic, granitic and ultramafic clasts larger than a few millimetres. O'Ne-ell Creek watershed is a 68 km2 tributary basin of the Middle River drainage system in the northern headwaters of the Fraser River. Bedload transport generally occurs twice a year in the Stuart–Takla streams: once during spring floods and again during salmon spawning activity. Bedload moved only during two days in 1998, at the peak of the relatively small nival flood in May. Nevertheless, the device detected at least 3×106 passing clasts. A continuous record of bedload transport was obtained, showing: a pulsating pattern of activity seemingly independent of stage, lateral movement of the transport zone, and a sudden onset of bed movement with a tapered cessation. We anticipate that more sophisticated calibration of the sensors and accelerated sampling rates will provide detailed information on the size and/or velocity and magnetic permeability of particles moving over the device, and will contribute to a better understanding of bedload transport. Copyright © 2000 John Wiley & Sons, Ltd.

The Dalradian of Scotland: Missing link between the Vendian of Northern and Southern Scandinavia?

The Dalradian sequence of the Scottish Highlands and the Vendian sequences of Scandinavian accumulated in rifts that evolved into passive margins in late Vendian to early Cambrian time. They closely resemble one another in their evolution. The Dalradian margin faced SE, the Scandinavian margins faced NW (present-day orientation). When plotted on a reconstruction of Laurentia, Baltica and W Gondwana in which Baltica has a more southerly position than the most commonly discussed reconstructions of Pannotia and Rodinia, the Vendian sediments form a coherent pattern. In particular, the enigmatic granitic clasts in the Port Askaig Dalradian tillite appear to have been eroded from the ∼1 Ga Proterozoic basement of southwestern Scandinavia. This reconstruction is supported independently by previous matches of the metamorphic belts of Baltica and Laurentia which have been largely ignored in most reconstructions of Pannotia and Rodinia, and by recent information on the age and distribution of rift-related magmatism.