High-grade Metasedimentary Rocks Research Articles

Organic carbon burial forcing of the carbon cycle from Himalayan erosion

1-4 . Recent attention has focused on increased silicate weathering of tectoni- cally uplifted areas in the India-Asia collision zone as a possible cause for falling atmospheric CO2 levels in the Cenozoic era 5-7 . The chemistry of Neogene sediments from the main locus of sedimentary deposition for Himalayan detritus, the Bengal Fan, can be used to estimate the sinks of CO2 from silicate weathering and from the weathering and burial of organic carbon resulting from Himalayan uplift. Here we show that Neogene CO2 con- sumption from the net burial of organic carbon during Himala- yan sediment deposition was 2-3 times that resulting from the weathering of Himalayan silicates. Thus the dominant effect of Neogene Himalayan erosion on the carbon cycle is an increase in the amount of organic carbon in the sedimentary reservoir, not an increase in silicate weathering fluxes. Silicate weathering is typically incongruent, yielding both a solute and a secondary mineral phase, so direct evidence of chemical weathering can be found in the record of secondary minerals in sedimentary basins. The Bengal Fan and Ganges-Brahmaputra (GB) delta contain a huge volume of sediment derived from erosion of the India-Asia collision zone, with 6 3 10 6 km 3 deposited in the past 20 Myr (ref. 8). Isotopic data for Nd, Sr and O from Bengal Fan sediments show that the source for over 80% of the detritus since 20 Myr ago has been the high-grade metasedimentary rocks of the High Himalayan crystalline (HHC) sequence 9 . Clastic and carbo- nate sediments of the Precambrian Lesser Himalaya (LH) and Palaeozoic-Mesozoic Tethyan Himalaya (TH) are the other impor- tant sources of sediment to the Bengal Fan during the Neogene. Carbon dioxide consumption from silicate weathering can be represented schematically by: CaSiO3 ˛ 2CO2 ˛ 2H2O ! Ca 2˛ ˛ 2HCO 2 3 ˛ H4SiO4

High-grade metasedimentary rocks (Gilgit Formation) in the vicinity of Gilgit, Kohistan, northern Pakistan.

A thick succession of metasedimentary and metavolcanic rocks has been reported for the first time in the Gilgit area. This succession (the Jaglot Group) comprises from bottom to top 1) the Gilgit Formation, 2) the Gashu Confluence Volcanics, and 3) the Thelichi Formation. The Gilgit Formation consists of thick turbiditic sediments interstratified with amphibolites and calc-silicates. The lower contact of the formation is not exposed, but presumably underplated by the Chilas Complex, and the upper contact is conformable with the Gashu-Confluence Volcanics. The Gilgit Formation contains more than 80% paragneisses and schists showing metamorphism up to the sillimanite grade. The paragneisses are strongly deformed and at places migmatized. The paragneisses of the Gilgit Formation are quartz rich, but also contain biotite, garnet, staurolite, kyanite, sillimanite, graphite, together with pyrite and magnetite. Petrographical and geochemical data show that they are metapelites, semi-metapelites and semi-metapsammites. Using geothermometry and geobarometry on garnet-biotite pairs, the estimated maximum temperature and pressure conditions are 640°C and 6.7 kbar, respectively. We assume that igneous intrusions, both the Chilas Complex and the Kohistan Batholith and crustal thickening accompanying arc accretion to the Karakoram plate might have ensued metamorphism in the Gilgit Formation and other lithological units of the Jaglot Group. As depicted by the chemical zonation profiles of garnets in two samples from Jutial, the grade of metamorphism in the metasediments of the Gilgit Formation seems to increase from top to bottom in a normal sequence. The field and laboratory studies show almandine amphibolite subfacies and sillimanite-almandine amphibolite subfacies metamorphic conditions for the Gilgit Formation, and ocean basin of a back-arc affinity for its deposition.

Orogen-parallel and orogen-perpendicular extension in the central Nepalese Himalayas

Research Article| December 01, 1996 Orogen-parallel and orogen-perpendicular extension in the central Nepalese Himalayas M. E. Coleman M. E. Coleman 1Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02138 Search for other works by this author on: GSW Google Scholar Author and Article Information M. E. Coleman 1Department of Earth Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02138 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1996) 108 (12): 1594–1607. https://doi.org/10.1130/0016-7606(1996)108<1594:OPAOPE>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation M. E. Coleman; Orogen-parallel and orogen-perpendicular extension in the central Nepalese Himalayas. GSA Bulletin 1996;; 108 (12): 1594–1607. doi: https://doi.org/10.1130/0016-7606(1996)108<1594:OPAOPE>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract North-dipping, gravitationally driven extensional structures that unroof the high-grade core of the Himalayas have been recognized in several parts of the orogen. Recent work in west-central Nepal demonstrates that the extensional history at the top of the Greater Himalayan metamorphic sequence in this region is laterally variable. In the upper Marsyandi valley there is evidence for three generations of early to middle Miocene west-directed (orogen-parallel) extensional structures. The study area consists of two tectonostratigraphic sequences. The lower sequence consists of a 12-km-thick section of high-grade metasedimentary rocks of the Greater Himalayan sequence. The higher sequence consists of a 4-km-thick section of medium-grade marble of the Tibetan sedimentary sequence; it overlies the Greater Himalayan sequence in the Marsyandi valley along a northwest-dipping ductile shear zone (the Chame detachment). Mylonitic fabrics in footwall rocks show evidence of west-directed displacement at sillimanite-grade through greenschist-grade conditions. A cataclastic zone indicates progressively more brittle deformation at the eastern end of the outcrop trace of the Chame detachment. In the Dudh Khola valley, 15 km west of the Marsyandi valley, the Tibetan sedimentary sequence is juxtaposed with the Greater Himalayan sequence by an early Miocene north-dipping ductile shear zone (the Dudh Khola detachment) that has well-developed, north-trending stretching lineations formed during sillimanite-grade metamorphism. North-directed (orogen perpendicular) extension in the Dudh Khola region is interpreted as coeval with west-directed (orogen-parallel) extension in the Marsyandi region. Along-strike variations in the orientation of extensional strain at the top of the Greater Himalayan sequence may be explained by lateral crustal thickness gradients. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Thick-skinned deformation of the Archean Wyoming province during Proterozoic arc-continent collision

Two major mylonite zones are exposed in the southeastern Archean Wyoming province: the Cheyenne belt, which marks the boundary between the Wyoming and Colorado provinces, and the Laramie Peak shear zone, across which Archean rocks have been uplifted differentially at least 10 km. Between the two mylonite zones is a 60-100-km-wide belt of Archean migmatitic gneiss, which has been intruded by diabase dikes and peridotites and contains enclaves of high-grade metasedimentary rocks. We interpret this belt of high-grade rocks as a tectonically reactivated block of Archean crust, uplifted during the development of the Cheyenne belt collisional zone at ca. 1.8 Ga. This block does not exhibit a decreasing metamorphic gradient away from the mylonite zone, but is at uniformly high grade. Thus, rather than a thick-skinned thrust-ramp model for uplift, we invoke vertical uplift along a high-angle reverse fault without rotation. This style of uplift may be a tectonic response to transpression, underplating, or tectonic interfingering at depth during collisional orogeny, and it may be a more common feature of basement uplifts within foreland fold-and-thrust belts than has been previously recognized.

Heat-Producing Elements and the Thermal and Baric Patterns of Metamorphic Belts

The character of sedimentary basins, before they are deformed and metamorphosed, may strongly influence the thermal and baric patterns of metamorphic belts. Crustal thickening of anoxic sedimentary basins and subsequent thermal reequilibration may produce large areas of high-grade metamorphic rocks and granites because the concentrations of the heat-producing elements are high in such basins. In New England there is a spatial association among granites and high-grade metasedimentary rocks rich in U and Th that now form the Central Maine terrane. The high content of heat-producing elements in these rocks is attributed to fixing of U and Th in highly reduced sediments that were deposited in an anoxic basin that formed in the Silurian. When the basin was thickened during the Devonian Acadian orogeny, the thermal energy generated by the U- and Th-rich sediments produced the observed broad zone of high-grade rocks and anatectic granites. This hypothesis was tested with thermal calculations that reproduce most of the first-order thermal and baric patterns of the Acadian Appalachians, if pretectonic lateral variations in heat production are assumed.

The precursor principle and the possible significance of stratiform ores and related chemical sediments in the elucidation of processes of regional metamorphic mineral formation

The precursor principle and the possible significance of stratiform ores and related chemical sediments in the elucidation of processes of regional metamorphic mineral formation

Rb–Sr geochronology and metamorphic history of Proterozoic to early Archean rocks north of the Cape Smith Fold Belt, Quebec

The Churchill Province north of the Proterozoic Cape Smith volcanic fold belt of Quebec may be divided into two parts. The first is a broad antiform of migmatitic gneisses (Deception gneisses) extending north from the fold belt ~50 km to Sugluk Inlet. The second is a 20 km wide zone of high-grade metasedimentary rocks northwest of Sugluk Inlet. The Deception gneisses yield Rb–Sr isochron ages of 2600–2900 Ma and initial ratios of 0.701–0.703, showing that they are Archean basement to the Cape Smith Belt. The evidence that the basement rocks have been isoclinally refolded in the Proterozoic is clear at the contact with the fold belt. However, the gneisses also contain ubiquitous synclinal keels of metasiltstone with minor metapelite and marble that give isochron ages less than 2150 Ma. These ages, combined with low initial ratios of 0.7036, show that they are not part of the basement, as the average 87Sr/86Sr ratio for the basement rocks was about 0.718 at that time.The rocks west of Sugluk Inlet consist mainly of quartzo-feldspathic sediments, quartzites, para-amphibolites, marbles, and some pelite and iron formation. In contrast to the Proterozoic sediments in the Deception gneisses, these rocks yield dates of 3000–3200 Ma, with high initial ratios of 0.707–0.714. These initial ratios point to an age (or a provenance) much greater than that of the Archean Deception gneisses. The rocks of the Sugluk terrain are intruded by highly deformed sills of granitic rocks with ages of about 1830 Ma, demonstrating again the extent and severity of the Proterozoic overprint. The eastern margin of this possibly early Archean Sugluk block is a discontinuity in age, lithology, and geophysical character that could be a suture between two Archean cratons. It is not known if such a suturing event is of Archean age, or if it is related to the deformation of the Cape Smith Fold Belt.Models of evolution incorporating both the Cape Smith Belt and the Archean rocks to the north need to account for the internal structure of the fold belt, the continental affinity of many of the volcanic rocks, the continuity of basement around the eastern end of the belt, and the increase in metamorphism through the northern part of the belt into a broad area to the north. The Cape Smith volcanic rocks may have been extruded along a continental rift, parallel to a continental margin at Sugluk. Continental collison at Sugluk would have thrust the older and higher grade Sugluk rocks over the Deception gneisses, produced the broad Deception antiform, and displaced the Cape Smith rocks to the south in a series of north-dipping thrust slices.

Garnet, tourmaline and muscovite-bearing leucogranites, gneisses and migmatites of the Higher Himalayas from Zanskar, Kulu, Lahoul and Kashmir

Summary Approximately 30–50% of the map area of the Central Crystalline Complex of the Higher Himalayas in Zanskar, Kulu, Lahoul and Kishtwar-Kashmir (NW India) is composed of Miocene leucogranites intruding metamorphic basement rocks. Ultrametamorphism at deeper crustal levels resulted in in situ anatexis, migmitization and lit-par-lit intrusions or stromatic layering within high-grade metasedimentary rocks. Garnet, tourmaline and muscovite-bearing granites locally contain biotite, apatite or kyanite. Leucogranites at higher crustal levels are associated with a widespread network of anastomosing dykes and sills with high tourmaline content. Rare-earth-element (REE) abundances are relatively low. Garnets are compositionally variable and are usually unzoned. Extremely high 87 Sr/ 86 Sr initial ratios of leucogranites (0.755–0.769) indicate a crustal anatectic origin while the wide range of Sr initial ratios can be explained by melting of a variety of metasedimentary rocks of the subducting Indian Shield. Radiometric ages of Himalayan leucogranites (28–12 Ma) follow closely in time a thermal peak of metamorphism (40–25 Ma). Inversion of metamorphic isograds during large-scale crustal subduction along several major thrust zones in the Higher Himalayas followed immediately after generation of the leucogranites. The leucogranites are interpreted as anatectic melts of high-grade metamorphic crystalline basement, generated at depth by residual (Barrovian) heat with a large amount of volatile and fluid flux. Frictional heating along the N-dipping Main Central Thrust is not considered a major source of heat for generation of the leucogranites.

Geology of the Nemo Lakes belt, northern Valhalla Range, southeast British Columbia

High-grade metasedimentary rocks, probably of both early Paleozoic and late Paleozoic – Triassic ages, underlie an area termed the Nemo Lakes belt between Slocan and Arrow Lakes in the northern Valhalla Range, southeastern British Columbia. The rocks have experienced two possibly related periods of major folding. Phase 1, accompanied and outlasted by metamorphism at P–T conditions of 5.0–6.8 kbar (500–680 MPa) and 630–680 °C, involved emplacement of ultramafic rocks, major faulting, and folding. Phase 2 involved large-scale inclined to upright folds which were dominantly south-verging, deforming the phase 1 fabric. Both phases probably occurred in the Middle to Late Jurassic, as part of the Columbian Orogeny.Rocks lithologically and structurally similar to those of the Nemo Lakes belt are found across the Rodd Creek fault near the Columbia River and extend the general continuity of the belt into the Shuswap metamorphic complex.Plutonic rocks, some of which bracket the movement on the Rodd Creek fault, the southern extension of the Columbia River fault zone, range in age from Middle Jurassic to EoceneIn the valley of Slocan Lake, a major normal fault is postulated on structural and metamorphic grounds and may be related to the north–south arching of the Valhalla gneiss complex. It is suggested that this arching and uplift, which followed phase 2 deformation, produced both the fault and a zone of cataclasis on the eastern side of the complex, and gave rise to its domal shape.

Correlation of aeroradioactivity and geology in southeastern Connecticut and adjacent New York and Rhode Island

Radioactivity patterns in southeastern Connecticut, adjacent Rhode Island, and New York closely reflect bedrock units. This relationship is not greatly affected by surficial deposits of Pleistocene age because most deposits are locally derived and bedrock outcrops are relatively abundant. Most bedrock units are medium- to high-grade metasedimentary, metavolcanic, and metaplutonic rocks that range in age from possible Precambrian to middle Paleozoic. The rest are postmetamorphic granite and pegmatite of late Paleozoic age. The area is on the west flank of an anticlinorium of granitic gneiss and subordinate metasedimentary and metavolcanic rocks at a place where the anticlinorium changes trend from north to east. The Honey Hill and Lake Char faults mark a zone of low-angle faulting between the granite-gneiss terrane and stratified rocks of the Merrimack synclinorium to the north and west. The east-trending Honey Hill fault marks the southern end of the Merrimack synclinorium. Four fields denoting different levels and patterns of radioactivity coincide with major geologic provinces. The granite-gneiss terrane, which contains abundant potassic, ledge-forming granitic rocks (the Sterling Plutonic Group), coincides with a field of relatively high radioactivity, in which subordinate lows reflect outcrop areas of quartzitic rocks of the Plainfield Formation. This terrane is flanked to the west and north by layered metavolcanic rocks of mafic to intermediate composition belonging mostly to the Quinebaug Formation. These rocks coincide with a field of relatively low to moderate levels of radioactivity. The lowest level within this field is produced by the Preston Gabbro, which is at the right-angle bend in the structure. A sequence of metasedimentary schist and gneiss of the Tatnic Hill Formation, Hebron Formation, Scotland Schist, and the sill-like Canterbury Gneiss overlies the Quinebaug Formation. These rocks produce a field of moderate levels of radioactivity but have nodes of high radioactivity where pegmatite is abundant. A field of moderate intensity south of the east-trending anticlinorium of granitic gneiss coincides with layered and massive granodioritic and quartz-dioritic gneiss and subordinate amphibolite equivalent stratigraphically to the layered metavolcanic rocks north of the anticlinorium. The late metamorphic to postmetamorphic, low-angle Honey Hill and Lake Char faults, which follow the bend in the structure at the base of and within the metavolcanic rocks on the north and west sides of the anticlinorium, respectively, can be recognized only by the radiometric pattern that reflects the tectonic pinch-out of the Quinebaug Formation westward along the Honey Hill fault. A pluton of Narragansett Pier Granite of late Paleozoic age, which has associated dikes of Westerly Granite, is reflected in a sub field of high radioactivity in southern Rhode Island. Material derived from these rocks is apparently responsible for a relatively high level of radioactivity in the Charlestown moraine immediately south of the out crop area.