Low-grade Metasedimentary Rocks Research Articles

Provenance constraints from detrital zircon U–Pb ages in the NW Iberian Massif: implications for Palaeozoic plate configuration and Variscan evolution

Detrital zircons from pre-orogenic Upper Ordovician to Devonian low-grade metasedimentary rocks have been dated by laser ablation–inductively coupled plasma mass spectrometry (LA–ICP-MS) to: (1) constrain the variations of zircon age populations within the autochthonous sequences of NW Iberia; (2) compare U–Pb detrital zircon ages with previous data; (3) test the hypothesis of the Armorica microplate as a peri-Gondwanan terrane separated from Gondwana. The similarity in the ages of detrital zircons found in the Palaeozoic samples studied here to those published for the Upper Proterozoic, Lower Cambrian and Lower Ordovician sediments of NW Iberia argues against the separation of Armorica, and points to a common source area for all of the Palaeozoic detrital formations of NW Iberia, the West African craton and the surrounding Pan-African belts. LA–ICP-MS U–Pb dating of zircons from a synorogenic flysch preserved in klippen in the core of a syncline establishes a maximum depositional age for this deposit as Early Namurian. This age is also a maximum age limit for thrusting and refolding in this part of the Iberian Massif. Correlation of the zircon age populations with published ages confirms the link between the emplacement of the allochthonous complexes of NW Iberia and synorogenic sedimentation.

Zircon growth in slate

Clastic sedimentary and low-grade metasedimentary rocks preserve populations of detrital zircons because of the unreactive nature of this mineral. However, evidence of new zircon growth has been found within highly heterogeneous populations of zircon from several greenschist facies slates from the Scottish Highlands. Small (<10 μm), anhedral, unzoned zircons and discrete overgrowths on rounded detrital grains are very common. These new fine-grained zircons have crystallized at temperatures below 350 °C and have been observed only in polished thin sections; they are absent from conventional mineral separates. Typical separation techniques create severe biases in the heavy-mineral populations of metasedimentary rocks, and recognition of the growth of zircon in such conditions may allow isotopic dating of low-temperature events.

Sedimentary and Tectonic Evolution of a Trench-Slope Basin in the Nankai Subduction Zone of Southwest Japan

Leg 190 of the Ocean Drilling Program yielded discoveries about the early stages of tectonic and sedimentary evolution of a trench-slope basin in the Nankai subduction zone of southwest Japan. Lithofacies character, biostratigraphy, and seismic-reflection data show that the basin's architecture was constructed during the early Quaternary by frontal offscraping of coarse-grained trench-wedge deposits. Clast types in muddy gravel beds indicate that one of the trench's polymictic sources was enriched in low-grade metasedimentary rocks. Outcrops of the Shimanto Belt on the island of Shikoku contain comparable lithologic assemblages, so we suggest that some of the turbidity currents and debris flows were funneled from that source through a transverse canyon-channel system. Offscraped trench deposits are mildly deformed and nearly flat lying beneath the slope basin. Bedding within the basin laps onto a hanging-wall anticline that formed above a major out-of-sequence thrust fault. Rapid uplift brought the substrate above the calcite compensation depth soon after the basin was created. The sediment delivery system probably was rerouted during subduction of the Kinan seamounts, thereby isolating the juvenile basin from coarse sediment influx. As a consequence, the upper 200 meters of basin fill consist of nannofossil-rich hemipelagic mud with sparse beds of volcanic ash and thin silty turbidites. Intervals of stratal disruption are also common, and the soft-sediment folding resulted from north- to northeast-directed gravitational failure. The Nankai accretionary prism has grown 40 km in width during the past 1 My, and the slope basin is already filled to its sill point on the seaward side. The stratigraphy displays an upward fining and thinning trend, in contrast to the upward coarsening and thickening mega-sequences depicted by some conceptual models for slope basins.

Separate lead isotope analyses of leachate and residue rock fractions: implications for metal source tracing in ore deposit studies

Separate lead isotope analyses of leachate and residue fractions are applied to a broad spectrum of rocks commonly investigated in metallogenic studies. Resulting data highlight a systematic behavior of leachate and residue fractions with respect to lead isotope compositions, which essentially depends on the mineralogical composition of the rock. Granitoid and high-grade metamorphic rocks have residue compositions virtually identical to common lead. In contrast, low-grade metasedimentary rocks may have residue compositions swamped by radiogenic lead of leach-resistant zircons. Mafic magmatic rocks have residues that are often more radiogenic than leachates, depending on the ratio of leach-refractory zircons to common lead in the residual fraction of these rocks. Separate leachate and residue analyses of source rocks provide two lead isotope end members whose mixture may represent lead with the appropriate ore fluid composition. Our leaching experiments indicate that hot acid solutions (and by inference hydrothermal fluids) preferentially leach radiogenic lead from medium- to high-grade metamorphic and granitoid rocks, whereas they preferentially leach common lead from low-grade metasedimentary and mafic magmatic rocks. The method presented in this study provides a reliable alternative to other methods (i.e., age-correction of bulk-rock compositions) for the determination of the common lead signature of felsic to intermediate magmatic rocks. This may be preferable to age-corrected bulk-rock analyses, where ages to apply for corrections of bulk-rock data are not known or where moderately to highly altered rocks must be used. Case studies of orogenic gold and MVT districts of Peru (Pataz and San Vicente, respectively) show that separate leachate and residue lead isotope analyses carried out systematically on whole rocks allow a more thorough evaluation of metal source reservoirs than does the standard method of age-corrected or uncorrected bulk-rock analyses.

Neoproterozoic deformation in central Madagascar: a structural section through part of the East African Orogen

Abstract The Itremo region of central Madagascar has an importance in the evolution of the East African Orogen (EAO) that belies its size. Unusually for the southern EAO (Mozambique Belt), it is made up of low-grade metasedimentary rocks and therefore preserves an almost unique window into upper crustal deformation during this key period of the Gondwana supercontinent cycle. In this paper new field mapping of three linked regions in the Itremo Sheet and in the upper part of the underlying mid-crustal Antananarivo Block are presented. From these a complete structural section through the eastern Itremo Sheet is produced and the complex deformation record preserved there is then discussed. An early deformation (D1) consists of 10 km scale recumbent isoclinal folds that predate intrusion of a c. 780–800 ma igneous suite. Metamorphic aureoles around these plutonic bodies overprint D1-related fabrics. Local deformation accompanies intrusion of the c. 780–800 Ma, suite (D2). Extensive E-W contractional deformation occurs between 780 and c. 570 Ma, that is here amalgamated as a composite D3 event, which includes thrusts and at least two phases of upright folds. Post-551 Ma, normal shearing (D4) marks the boundary between the Itremo Sheet and the underlying Antananarivo Block (the Betsileo Shear Zone), and may have also been responsible for formation of the Saronara Shear Zone. Finally, E-W open folding and dextral shear zone development marks a late N-S contractional event that is interpreted as a far-field response to collision between the northern Bemarivo Belt and central Madagascar.

Gabbro Akarem mafic-ultramafic complex, Eastern Desert, Egypt: a Late Precambrian analogue of Alaskan-type complexes

¶Gabbro Akarem is a Late-Precambrian concentrically-zoned mafic-ultramafic intrusion located along a major fracture zone trending NE-SW in the Eastern Desert of Egypt. It intruded low-grade metasedimentary rocks, and has a contact metamorphic aureole a few meters wide. This intrusion comprises a dunite core enveloped by clinopyroxene hornblende-bearing lherzolite, olivine-hornblende clinopyroxenite and plagioclase hornblendite. The contacts between the rock types are gradational. They have cumulate textures and the observed crystallization sequence is: olivine ( + cotectic spinel)-orthopyroxene (Opx)-clinopyroxene (Cpx)-hornblende. Mafic minerals from the core of the intrusion are highly magnesian, a consistent increase in the Mg# of olivine (from 69 to 87), Opx (from 62 to 89), Cpx (from 85 to 96) and hornblends (from 62 to 88) is observed from the mafic to the ultramafic units. Spinel has a wide range of Cr# and Mg# ratios. The various rock units define a fractionation trend. The mafic rocks are slightly LREE-enriched relative to the ultramafic units and chondrites.

Structural characteristics of middle and southern Xainza- Dinggye Normal Fault System and its relationship to South-ern Tibetan Detachment System

The Xainza-Dinggye Normal Fault System (XDNFS) is a large-scale nearly-north-south trending extensional structure across central and southern Tibet. Its middle segment developed in Tethys Himalaya with features of earlier magmatic core complex and later normal faults dipping moderately to northwest-west. The magmatic core complex is made up by mylonitic leucogrante with a low-angle detachment fault on the top of it and overlain by lower-grade meta-sedimentary rocks. The structural pattern of the southern segment of XDNFS take the shape of a detachment fault dipping to southeast-east with the High-Himalayan rock series as the lower plate. The Southern Tibetan Detachment System (STDS) is expressed as a ductile shear zone composed of mylonitic leucogranite in the studied area of this note. STDS was cut by the later XDNFS, which presents that nearly-east-west striking STDS is not the controlling or adjusting structure of the nearly-north-south trending extensional structures. The origin of nearly-north-south trending extensional structures in Tibet may be the result of deform ational partition of north-south compression.

O magmatismo alcalino Neoproterozóico na Ilha do Cardoso, Sudeste do Estado de São Paulo

This work focuses on the geology and geochronology of rocks cropping out on Cardoso Island, on the southeastern coast of Sao Paulo State, close to the boundary with Parana State. The island, with an area of about 151 km² is a protected area administered by the Institute of the Secretariat for the Environment of the State of Sao Paulo. It is mountainous, with a peak at 814 m, and is covered by denseAtlantic Forest vegetation. The island is made up mainly of an igneous complex with light grey leucocratic, inequigranular, medium to coarse-grained syenites. The Tres Irmaos Syenite (STI), composed of pyroxene, hornblende, and perthitic to mesoperthitic microcline, predominates has magmatic flow structures, and is cut by the pinkish grey, leucocratic medium-grained Cambriu alkali-feldspar granites (GC). Geochemical analysis of STI and GC demonstrate their metaluminous alkaline nature and late orogenic to anorogenic character. The bodies formed between 620 and 570 Ma according to U-Pb dating of zircons and cooled between 597 and 531 Ma (K-Ar in amphiboles). Whole rock Sm-Nd analyses yield Meso - and Paleoproterozoic TDM ages (1,500 - 2,200 Ma). A belt of low-grade metasedimentary rocks occurs in the northern part of the island. Quartz schist, quartz-mica schist and mica-quartz schist, often-containing andalusite and cordierite, predominate.Geochemical and geochronological data suggest that the sources of the metasediments were continental arc andesites of whose protoliths separated from the mantle between 1,800 and 2,200 Ma during the Paleoproterozoic. These metasediments probably continue on the continent in the Taquari region and extend southwards in narrow strips between the granitoids of the Paranagua Domain.

Geology of new localities on Tabarin Peninsula, northern Antarctic Peninsula

New outcrops of Hope Bay Formation and Larsen Basin rocks have been exposed by recent ice-retreat on Tabarin Peninsula, northern Antarctic Peninsula. Quartzose, very low-grade metasedimentary rocks, cropping out at Balegno Nunatak (63°28′53″S, 57°00′31″W) are assigned to the Permian–Triassic Hope Bay Formation, and dioritic rocks are assigned to the Antarctic Peninsula batholith. Sedimentary rocks exposed at Rubulis Nunatak (63°30′52″S, 57°07′29′W) closely resemble Lower Cretaceous sedimentary rocks cropping out at nearby Troilo Nunatak, western Tabarin Peninsula. These rocks are correlated with the lower Gustav Group (?Barremian–Coniacian) of the Larsen Basin. This new record confirs that the Larsen Basin extends over southern Tabarin Peninsula, where basin sediments are thought to be faulted against sediments of the Antarctic Peninsula magmatic arc.

Composition of Monocrystalline Detrital and Authigenic Minerals, Metamorphic Grade, and Provenance of Torlesse and Waipapa Graywacke, Central North Island, New Zealand

Sandstones of the juxtaposed and partially coeval quartzofeldspathic Torlesse terrane and volcanogenic Waipapa terrane of North Island, New Zealand, are generally described as having been derived from silicic continental arc and evolved intermediate volcano-plutonic arc sources, respectively. Modal and chemical compositions of the two terranes differ slightly as a result. From textural considerations, their single-grain (unitary) detrital mineral populations are inferred to have been derived largely from the plutonic components in their sources. Intensive microscopic and electron microprobe study of two representative samples shows that the unitary detrital mineral assemblages in the two terranes are virtually identical, comprising quartz, plagioclase, K-feldspar, white mica, epidote, titanite, pumpellyite, ilmenite, rutile, tourmaline, zircon, and apatite. Detrital chlorite, garnet, and graphite also occur in the Torlesse sample, whereas amphibole, clinopyroxene, and prehnite occur in the Waipapa sample. Authigenic mineral assemblages are also similar, consisting of quartz, albite, chlorite, phengitic mica, epidote, titanite, pumpellyite, pyrite, and calcite. Stilpnomelane and pyrrhotite also occur in the Torlesse sample, and prehnite in the Waipapa specimen. These assemblages define upper prehnite-pumpellyite to lower pumpellyite-actinolite facies conditions (Torlesse) and lower prehnite-pumpellyite facies metamorphism (Waipapa). By comparison with published compositional data for minerals from plutonic, metamorphic, and volcanic rocks, electron microprobe analyses of individual minerals confirm that the unitary detrital grains in both terranes were largely derived from calc-alkaline S-type granitoid plutonic rocks. Contrasts in mineral compositions between the two terranes show that the Torlesse unitary mineral detritus was derived almost entirely from granodiorite, whereas the Waipapa grains originated from a mixed diorite, monzonite, and granodiorite plutonic component in their source. In neither terrane was detritus derived from granite in the strict sense. Although the plutonic components in their sources are lithologically similar, the compositional contrasts seen indicate that they were not coeval or spatial components of the same terrane. Detailed electron microprobe analysis of unitary detrital phases in low-grade metasedimentary rocks thus enables identification of specific source terrane lithotypes, and hence is a valuable complement to existing petrographic, modal, and chemical approaches that define more generalized provenances.

Calcite and quartz microstructural geothermometry of low-grade metasedimentary rocks, Northern Range, Trinidad

The Northern Range of Trinidad is a mountainous exposure of deformed metasedimentary rocks with Mesozoic depositional ages and Tertiary metamorphic ages located in the Caribbean–South American plate boundary zone. These rocks lack precise mineralogical indicators of metamorphic grade. Using temperature-sensitive quartz and calcite microstructures, and fission track data, we identify, describe, quantify, and map a nearly continuous spectrum of relatively low (>l50°C) to relatively high (250–400°C) deformation temperatures from east to west across the range. Average estimated rock exhumation rates also increase systematically along this trend. The analysis illuminates a sharp, east–west-trending, major thermal discontinuity (i.e. a fault) along the southern boundary of the range. Kinematic analysis of faults and shear bands adjacent to this boundary and modeling indicate that the fault probably dips southward at 80–85° and accommodated normal dip-slip at rates that decreased from west to east. Earlier experimental studies at high strain rates illustrated a strong strain rate dependency for quartz microstructural transitions. Our work indicates a possible strong temperature control for quartz microstructural transitions at natural strain rates.

SOUTHERN BRASÍLIA BELT (SE BRAZIL): TECTONIC DISCONTINUITIES, K-AR DATA AND EVOLUTION DURING THE NEOPROTEROZOIC BRASILIANO OROGENY

This paper focuses the tectonic evolution of the southern Brasília belt, with emphasis on the Furnas segment, along the 21°S parallel. The uppermost structural unit (Passos Nappe - PN) comprises a highly deformed metasedimentary succession interpreted as a fragment of the Neoproterozoic passive margin of western São Francisco craton. An inverted metamorphic gradient ranging from greenschist to lower granulite facies of medium to high-pressure regime characterizes the PN as relict of a subduction zone. The External Domain displays a complex imbrication of basement rocks (Archean Piumhi greenstones, a turbiditic graywacke succession and a calc-alkaline granitoid suite) with undated siliciclastic low-grade metasedimentary rocks. The São Francisco Craton (SFC) comprises pre-1.8 Ga basement rocks covered by anchimetamorphic Neoproterozoic carbonatic shallow marine platform deposits of the Bambuí group. The Brasiliano thrust stacking generated a coarse clastic influx of molassic character on the foreland zone of São Francisco Craton, coeval with the exhumation of the External Domain thrust sheets. New K-Ar determinations on mineral separates are presented and interpreted among previous data. The SFC basement rocks display Paleo- to Mesoproterozoic cooling ages. The allochthonous units, in contrast, display K-Ar ages within the 560-675 Ma range. Brasiliano thrust stacking is therefore interpreted to have taken place onto a “cold” São Francisco craton foreland, in a thin-skinned style, as basement rocks were not heated enough to have their K-Ar systems reset during the allochthony.

Southern Brasília Belt (se Brazil): tectonic discontinuities, K-Ar data and evolution during the Neoproterozoic brasíliano orogeny

This paper focuses the tectonic evolution of the southern Brasilia belt, with emphasis on the Furnas segment, along the 21°S parallel. The uppermost structural unit (Passes Nappe - PN) comprises a highly deformed metasedimentary succession interpreted as a fragment of the Neoproterozoic passive margin of western Sao Francisco craton. An inverted metamorphic gradient ranging from greenschist to lower granulite facies of medium to high-pressure regime characterizes the PN as relict of a subduction zone. The External Domain displays a complex imbrication of basement rocks (Archean Piumhi greenstones, a turbiditic graywacke succession and a calc-alkaline granitoid suite) with undated siliciclastic low-grade metasedimentary rocks. The Sap Francisco Craton (SFC) comprises pre-1.8 Ga basement rocks covered by anchimetamorphic Neoproterozoic carbonatic shallow marine platform deposits of the Bambui group. The Brasiliano thrust stacking generated a coarse clastic influx of molassic character on the foreland zone of Sao Francisco Craton, coeval with the exhumation of the External Domainthrust sheets. New K-Ar determinations on mineral separates are presented and interpreted among previous data. The SFC basement rocks display Paleo- to Mesoproterozoic cooling ages. The allochthonous units, in contrast, display K-Ar ages within the 560-675 Ma range. Brasiliano thrust stacking is therefore interpreted to have taken place onto a cold Sao Francisco craton foreland, in a thin-skinned style, as basement rocks were not heated enough to have their K-Ar systems reset during the allochthony.

Structural development of the Mid-Tertiary Doi Suthep Metamorphic Complex and Western Chiang Mai Basin, Northern Thailand

The northern Western Ranges of Thailand contain mylonitic gneisses of the Doi Suthep Metamorphic Complex (DSMC). Near Chiang Mai, mylonitic foliations outline a southeast-trending dome, and stretching lineations trend N80°W. Low-grade metasedimentary rocks and Tertiary fanglomerates flank the dome on the east, and widespread chloritic breccia marks the intervening Doi Suthep detachment fault. We interpret that the metamorphic complex developed between Triassic and early Miocene, with detachment faulting, mylonitization, and uplift during Oligocene to Miocene. Within the DSMC, dikes of mutually cross-cutting, hence coeval, granitic pegmatite and aplite cut mylonitic orthogneiss. Although the contacts of the dikes-are sharp, the dikes contain a mylonitic fabric that parallels the fabric in the surrounding mylonitic orthogneiss; thus, we infer that intrusion occurred during mylonitization. East of Doi Suthep, Tertiary fanglomerates of the Mae Rim Formation overlie the Doi Suthep detachment fault. Variable strikes, with dips of 10°–50°, and several outcrop-scale fold hinges suggest that the Mae Rim Formation is deformed into a set of non-cylindrical folds. Numerous, variably-striking, high-angle normal faults suggest either multiple deformations or deformation in a three-dimensional strain field. The discovery of an intraformational angular unconformity confirms that the Mae Rim Formation accumulated during folding.

Geometrical classification of multilayered folds

A new classification scheme based on the degree of fluctuation in the geometry of different layers of a multilayered fold is suggested. The classification scheme uses the degree of fluctuation in geometry in terms of the standard deviation ( σ n ) of the thickness parameters t α ′ (orthogonal thickness parameter) and T α ′ (axial plane parallel thickness parameter) for n number of layers, and dip angle α. The degree of fluctuation in the geometry of a multilayered fold can be represented by σ n ( t α ′) or σ n ( T α ′) versus α plots on a Cartesian plane. In the proposed classification scheme, multilayered folds have been divided into two broad categories, namely `isodeviatoric' and `anisodeviatoric'. Isodeviatoric folds have a constant fluctuation in the geometry of different layers recorded in terms of σ n ( t α ′) or σ n ( T α ′) for α>10°. A special type of isodeviatoric fold is recognised as `analogous fold' in which each layer exhibits identical geometry [ σ n ( t α ′) or σ n ( T α ′)=0]. Plots of isodeviatoric folds lie parallel to the abscissa ( α) and those of analogous folds lie along the abscissa in the σ n ( t α ′) or σ n ( T α ′) (ordinate) versus α (abscissa) diagram. Analogous folds have been divided into ten varieties (1A1, 1A2, 1A3, 1B, 1C, 2, 3A, 3B, 3C and composite-analogous types). The anisodeviatoric folds do not exhibit constant fluctuation (deviation) in the geometry of different constitutive layers. Such folds have been subdivided into `peri-analogous', `sub-analogous', `sub-non-analogous', `non-analogous' and `strongly non-analogous' types. This classification scheme is applied to folds developed in low-grade metasedimentary rocks of the Mahakoshal Group and low- to medium-grade rocks of the Chhotanagpur Granite Gneiss Complex in central India.

Variscan vs Cadomian tectonothermal activity in northwestern sectors of the Teplá-Barrandian zone, Czech Republic: constraints from 40 Ar/ 39 Ar ages

Five muscovite concentrates from high-grade, pelitic metasedimentary basement rocks exposed in northwestern sectors of the Tepla-Barrandian zone (Czech Republic) record 40Ar/39Ar mineral plateau ages which range between ca. 376 and 362 Ma. Hornblende concentrates from metagabbro (Marianske Lanzě complex) and fine-grained basement amphibolite display plateaux which define 36Ar/40Ar vs 39Ar/40Ar isotope-correlation ages of ca. 370 and ca. 375 Ma. The mineral ages are interpreted to date relatively rapid cooling through appropriate argon retention temperatures following early phases of Variscan (Early Devonian) regional metamorphism. A slate/phyllite basement sample collected within lower-grade metasedimentary rocks in southeastern portions of the Tepla-Barrandian zone is characterized by an internally discordant 40Ar/39Ar whole-rock age spectrum which suggests partial Variscan rejuvenation of intracrystalline argon systems which had cooled through appropriate argon retention temperatures following an initial regional metamorphism at or prior to ca. 500 Ma (Cadomian). Hornblende from undeformed diorite of the Kdynž complex records a well-defined 40Ar/39Ar age plateau which corresponds to an isotope-correlation age of ca. 516 Ma. This is interpreted to date post-magmatic cooling following emplacement.

Neogene foreland basin deposits, erosional unroofing, and the kinematic history of the Himalayan fold-thrust belt, western Nepal

Sedimentological and provenance data from the lower Miocene–Pliocene Dumri Formation and Siwalik Group in western Nepal provide new information about the timing of thrust faulting and the links between erosional unroofing of the Himalaya and the Cenozoic 87 Sr/ 86 Sr record of the ocean. In western Nepal, the Dumri Formation is an ∼750–1300-m-thick fluvial sandstone and overbank mudstone unit. The Siwalik Group is >4200 m thick and consists of a lower member (>850 m) of 2–12-m-thick fluvial channel sandstones and oxidized calcareous paleosols, a middle member (>2400 m) of very thick (>20 m) channel sandstones and mainly organic-rich Histosols, and an upper member (>1000 m) composed of gravelly braided river deposits. Paleocurrent data indicate that middle Miocene–Pliocene rivers in western Nepal flowed southward, transverse to the thrust belt, throughout deposition of the Siwalik Group. No evidence was found for an axial fluvial trunk system (i.e., the paleo-Ganges River) in Siwalik Group sandstones. A major increase in fluvial channel size is recorded by the transition from lower to middle Siwalik members at ∼10.8 Ma, probably in response to an increase in seasonal discharge. Modal petrographic data from sandstones in the Dumri Formation and the Siwalik Group manifest an upsection enrichment in potassium feldspar, carbonate lithic fragments, and high-grade metamorphic minerals. Modal petrographic analyses of modern river sands provide some control on potential source terranes for the Miocene–Pliocene sandstones. The Dumri Formation was most likely derived from erosion of sedimentary and low-grade metasedimentary rocks in the Tibetan (Tethyan) Himalayan zone during early Miocene emplacement of the Main Central thrust. The presence in Dumri sandstones of plagioclase grains suggests exposure of crystalline rocks of the Greater Himalayan zone, perhaps in response to tectonic unroofing by extensional detachment faults of the South Tibetan detachment system. During deposition of the lower Siwalik Group (∼15–11 Ma), emplacement of the Dadeldhura thrust sheet (one of the synformal crystalline thrust sheets of the southern Himalaya) on top of the Dumri Formation supplied abundant metasedimentary lithic fragments to the foreland basin. A steady supply of plagioclase grains and high-grade minerals was maintained by deeper erosion into the Main Central thrust sheet. From ∼11 Ma to the present, K-feldspar sand increased steadily, suggesting that granitic source rocks became widely exposed during deposition of the upper part of the lower Siwalik Group. This provenance change was caused by erosion of passively uplifted granites and granitic orthogneisses in the Dadeldhura thrust sheet above a large duplex in the Lesser Himalayan rocks. Since the onset of deposition of the conglomeratic upper Siwalik Group (∼4–5 Ma), fault slip in this duplex has been fed updip and southward into the Main Boundary and Main Frontal thrust systems. We obtained 113 U-Pb ages on detrital zircons from modern rivers and Siwalik Group sandstones that cluster at 460–530 Ma, ∼850–1200 Ma, ∼1.8–2.0 Ga, and ∼2.5 Ga. An abundance of Cambrian–Ordovician grains in the Siwalik Group suggests sources of Siwalik detritus in the granites of the Dadeldhura thrust sheet and possibly the Greater Himalayan orthogneisses. The older ages are consistent with sources in the Greater and Lesser Himalayan zones. An overall upsection increase in zircons older than 1.7 Ga suggests increasing aerial exposure of Lesser Himalayan rocks. None of the detrital zircons (even in the modern river samples) yielded a Cenozoic age that might suggest derivation from the Cenozoic Greater Himalayan leucogranites, but this may be attributable to the inheritance problems that characterize the U-Pb geochronology of the leucogranites. When compared with recent studies of the 87 Sr/ 86 Sr composition of paleosol carbonate nodules and detrital carbonate in paleosols from the Siwalik Group, the provenance data suggest that erosion and weathering of metamorphosed carbonate rocks in the Lesser Himalayan zone and Cambrian–Ordovician granitic rocks of the crystalline thrust sheets in central and eastern Nepal may have played a significant role in elevating the 87 Sr/ 86 Sr ratio of middle Miocene synorogenic sediments in the Indo-Gangetic foreland basin and the Bengal fan, as well as global seawater.

On the Antarctic Peninsula batholith

AbstractThe plutonic rocks of the Antarctic Peninsula magmatic arc form one of the major batholiths of the circum-Pacific rim. The Antarctic Peninsula batholith is a 1350 km long by &lt; 210 km wide structure which was emplaced over the period ˜240 to 10 Ma, with a Cretaceous peak of activity that started at 142 Ma and waned during the Late Cretaceous. Early Jurassic and Late Jurassic–Early Cretaceous gaps in intrusive activity probably correspond to episodes of arc compression. In a northern zone of the Antarctic Peninsula, the batholith intrudes Palaeozoic–Mesozoic low-grade meta-sedimentary rocks, and in a central zone it intrudes schists and ortho- and paragneisses which have Late Proterozoic Nd model ages and were deformed during Triassic to Early Jurassic compression. In a southern zone the oldest exposed rocks are Permian sedimentary rocks and deformed Jurassic volcanic and sedimentary rocks. All these pre-batholith rocks formed a belt of relatively immature crust along the Gondwana margin. With few exceptions, Jurassic plutons crop out only within the central zone: many are peraluminous, having ‘S-like’ mineralogies and relatively high 87sr/86sri. They are considered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion. By contrast, Early Cretaceous plutons crop out along the length of the batholith. Few magma compositions appear to have been affected by upper crust, the bulk being compositionally independent of the type of country rock they intrude. They are dominated by metaluminous, calcic, Si-oversaturated, 1-type granitoid rocks with relatively low 87sr/86sri intermediate-silicic compositions (&lt; 5% MgO). We interpret these to represent partial melts of basic to intermediate, igneous, locally garnet-bearing, lower crust. Contemporaneous mafic magmas (e.g. syn-plutonic dykes) form a more alkaline, Si-saturated series having higher 143Nd/144Nd at the same87sr/86sr than the intermediate-silicic series, to which they are not petrogenetically related. The change from limited partial fusion of upper crust in Jurassic times to widespread partial fusion of lower crust in Early Cretaceous times is considered to be a result of an increasing volume of basaltic intrusion into the crust with time.

The Mozambique Belt in northern Mozambique: Neoproterozoic (1100–850 Ma) crustal growth and tectogenesis, and superimposed Pan-African (800–550 Ma) tectonism

Abstract The Precambrian basement of northern Mozambique is composed mainly of high-grade gneiss, granulite, migmatite and orogenic plutonic rocks emplaced and deformed during the Mozambican orogeny between 1100 and 850 Ma. Granulite nappes (Lurio Supergroup) were thrust over allochthonous supracrustal units (Chiure Supergroup), and both sequences were then thrust over an autochthonous migmatitic foreland (Nampula Supergroup). The granulite rocks are mostly of plutonic origin, and a model is proposed of an initial calc-alkalic enderbitic mass extracted from the mantle at ∼1050 Ma and emplaced in the lower crust; this enderbitic parent is considered as the source for successive alkali-rich melts which were emplaced synkinematically between 1050 and 850 Ma. The supracrustal units comprise marine metasedimentary rocks, and calc-alkalic and tholeiitic rocks associated with ultramafic-mafic sequences and synkinematic granite. The autochthonous foreland consists mainly of plutonic migmatite, showing low Sr initial ratios and originating from mantle-derived tonalitic rocks emplaced between 1100 and 950 Ma, associated with crustal-melt granite showing higher Sr initial ratios and dated as late as 850 Ma. Major synmetamorphic shearing and blastomylonitization, and regional thrusting to the east and southeast, are the main manifestations of the Mozambican orogeny. Early structures are preserved in the granulite complex and the migmatite foreland. Low-grade metasedimentary rocks (phyllite, sandstone, conglomerate and marble), with possible glacial affinities, uncomformably overlie the Mozambican basement and are correlated with the Katangan Supergroup. These rocks, together with the basement, were caught up at ∼538 Ma in Pan-African thrusts, transcurrent shear zones and folds in an intracontinental environment. These new data allow a reassessment of the entire Mozambique Belt, until now regarded as a Pan-African mobile belt involving variably remobilized cratons. The authors believe the belt to be a major fan-shaped collision zone between Kalahari craton and an Indo-Antarctic craton.

Deformation across the western Quetico subprovince and adjacent boundarl regions in Minnesota

North- to northwest-directed crustal shortening across the western Quetico subprovince and its boundary regions produced a complex deformation sequence within the Quetico belt and resulted in concentrated zones of dextral ductile shear in the boundary regions within the adjacent greenstone–granite terranes. In this paper, we review and introduce new data on the regional deformation features and their geometries and discuss the history of generation of these features. We attribute the deformation sequence to differential partitioning of shortening and shear strains during dextral transpression associated with oblique convergence and accretion along the southern margin of the Superior Province.The turbiditic wacke in the western Quetico subprovince, now typically amphibolite-facies schist and migmatite, underwent an early deformation stage that included recumbent folding (F1) and the generation of an S1 bedding-parallel foliation. This event is most evident along the northern and southern boundaries of the subprovince, but it is also recognized in the lower grade metasedimentary rocks in the adjacent Wawa and Wabigoon subprovinces. In these subprovinces, F1 folding may have been associated with higher level thrusting and allochthonous emplacement of greenstone units. Despite our F1 designation of this event, it it unlikely that this deformation was synchronous across the subprovinces.Widespread upright folding of the overturned limbs of F1 folds produced moderately to gently plunging F2 folds with east–west-trending axial planes. F, folds, with an associated L, stretching lineation subparallel to fold hinges, are well developed along the southern and northern margins of the Quetico subprovince and in the metasediments of the adjacent Wawa subprovince. During this event, ductile dextral shear was concentrated in steeply dipping east–west-trending shear zones in the Wawa subprovince and in the region of the Rainy Lake – Seine River fault along the Quetico–Wabigoon subprovince boundary. In the northern Wawa subprovince, shear was strongly concentrated in relatively incompetent, steeply dipping metasedimentary and tuffaceous units interlayered with more competent greenstone units. Concentrated zones of ductile shear are not evident within the Quetico subprovince away from its boundary regions. However, emplacement of syntectonic plutons in the central Quetico reoriented F2 folds which were then refolded by large regional F3 folds during continued regional shortening.