Granitoid Protoliths Research Articles

Geochemistry of granulite‐facies granitic rocks from Battye Glacier, northern Prince Charles Mountains, East Antarctica

Proterozoic basement outcrops in the vicinity of Battye Glacier, northern Prince Charles Mountains, are dominated by granulites and gneisses derived from felsic (granitoid) intrusive igneous rocks, and by pegmatites. Felsic orthopyroxene granulites, garnet leucogneisses and garnet pegmatites have major and trace element compositions of highly felsic, but not strongly fractionated, granites. The garnet leucogneisses and garnet pegmatites have S‐type characteristics, whereas the felsic granulites are probably I‐type, although their high Zr+Nb+Y+Ce abundances suggest possible A‐type affinities. Intermediate orthopyroxene ± clinopyroxene granulites mostly resemble I‐type quartz diorites, except for a small subgroup of samples (characterised by low Na2O and K2O, and high MgO, Ni, Cr and HREE) of uncertain affinities and significance. Element ratios involving LILE (e.g. K/Rb, Rb/Ba, Rb/Sr, K/La, La/Th) closely match those typical of the inferred granitoid protoliths, suggesting that these rocks have experienced relatively little LILE depletion (except possibly for U) during regional metamorphism. It is therefore inferred that metamorphism was probably broadly isochemical. Because the felsic and intermediate granulites and garnet leucogneisses are Sr‐depleted, Y‐undepleted and mostly have negative Eu anomalies they are inferred to be the products of partial melting of felsic crustal sources leaving plagioclase‐bearing residua. Plagioclase fractionation during crystallisation could also account for these characteristics, but K/Rb, Rb/Ba and Rb/Sr ratios in these rocks are not consistent with strong fractionation of feldspar. Garnet pegmatites differ chemically from garnet leucogneisses mainly in their lower Fe, Ti, Nb, Zn, Zr, Th and REE abundances and positive Eu anomalies, related to lower garnet, ilmenite and zircon contents in the garnet pegmatites. A genetic link between these two rock types, probably involving fractionation of these minerals during partial melting or crystallisation, is inferred. Incompatible‐element abundances suggest that generation of the Battye Glacier granitic magmas from felsic crust might have occurred in a mature continental magmatic arc possibly well removed from an active subduction trench or, perhaps, in an intracontinental setting.

Geochemical and Nd-Pb isotopic systematics of late Archean granitoids, southwestern Slave Province, Canada: constraints for granitoid origin and crustal isotopic structure

New geochemical and Nd-Pb isotopic data for ~ 2.62-2.59 Ga granitoids from the southwest Slave Province are used to determine the source(s) of granitoid magmas, to evaluate the role of pre-2.8 Ga basement during this magmatism, and to refine the existing Nd-Pb isotopic structure of the western Slave Province. The Pb isotopic data require crust older than ~3.2 Ga as a granitoid protolith, whereas the Nd isotopic data require input from juvenile crustal material. This discrepancy is explained if the granitoid protoliths are mixtures of ancient basement and ~2.7 Ga juvenile crust in varying proportions. Specifically, granitoids from the southwestern Slave Province require 10-30% basement, whereas granitoids from other parts of the western Slave Province require >50%. Incorporation of basement as a protolith may be achieved indirectly, by assimilation of basement during juvenile ~2.7 Ga magmatism, or directly during ~2.62-2.59 Ga magmatism. The granitoid isotopic data suggest that indirect basement input was important on a regional scale, but direct input may have also taken place in some areas of the western Slave Province, particularly along the ~111°W "isotopic boundary" zone previously recognized. The geochemical characteristics of these granitoids are compatible with an origin by partial melting of dominantly amphibolite and metasedimentary rocks to produce the ~2.61 Ga and ~2.59 Ga magmatism, respectively; partial melting occurred in response to regional crustal thickening at this time.

Geochemical and Nd-Pb isotopic systematics of late Archean granitoids, southwestern Slave Province, Canada: constraints for granitoid origin and crustal isotopic structure

New geochemical and Nd-Pb isotopic data for ~ 2.62-2.59 Ga granitoids from the southwest Slave Province are used to determine the source(s) of granitoid magmas, to evaluate the role of pre-2.8 Ga basement during this magmatism, and to refine the existing Nd-Pb isotopic structure of the western Slave Province. The Pb isotopic data require crust older than ~3.2 Ga as a granitoid protolith, whereas the Nd isotopic data require input from juvenile crustal material. This discrepancy is explained if the granitoid protoliths are mixtures of ancient basement and ~2.7 Ga juvenile crust in varying proportions. Specifically, granitoids from the southwestern Slave Province require 10-30% basement, whereas granitoids from other parts of the western Slave Province require >50%. Incorporation of basement as a protolith may be achieved indirectly, by assimilation of basement during juvenile ~2.7 Ga magmatism, or directly during ~2.62-2.59 Ga magmatism. The granitoid isotopic data suggest that indirect basement input was ...

Controls on pseudotachylyte formation during tectonic exhumation in the South Mountains metamorphic core complex, Arizona

Abstract Pseudotachylyte formed during the later stages of extension in the South Mountains metamorphic core complex. Microstructural and microchemical evidence of pseudotachylyte formation processes is well preserved in granitoid mylonite host rocks because of rapid cooling during tectonic exhumation. Petrographic studies indicate a cataclasite precursor to pseudotachylyte. Transmission electron microscope analysis shows glass in pseudotachylyte, providing evidence of melting. Electron microprobe analyses demonstrate variations in glass, microlite, and crystallite composition with proximity to quartz clasts, indicating reaction between clasts and melt. Amphibole crystallites exhibit sieve textures, interpreted as recording rapid crystallization and subsequent decompression. Decompression is postulated to result from drops in fluid pressure along jogs in the slip surface or following formation of injection veins. Both pseudotachylyte and cataclasite are typically subparallel to C-surfaces in the host mylonite. Cataclasite not associated with pseudotachylyte is composed of quartz + feldspars ± biotite. Pseudotachylyte everywhere contains biotite. The model proposed for formation of pseudotachylyte and cataclasite in foliated granitoid protoliths includes seismic failure along C-surfaces. Where biotite is absent, slip produces cataclasite. Where biotite is present, initial cataclasis may be followed by frictional melting, facilitated by biotite’s low melting temperature and lowering of the melting temperatures of other phases by release of water during biotite breakdown.

U–Pb zircon ages and structural development of metagranitoids of the Teplá crystalline complex: evidence for pervasive Cambrian plutonism within the Bohemian massif (Czech Republic)

U–Pb zircon dating of three metagranitoids, situated within a tilted crustal section at the northwestern border of the Tepla Barrandian unit (Tepla crystalline complex, TCC), yields similar Cambrian ages. The U–Pb data of zircons of the Tepla orthogneiss define an upper intercept age of 513 +7/–6 Ma. The 207Pb/206Pb ages of 516±10 and 511±10 Ma of nearly concordant zircons of the Hanov orthogneiss and the Lestkov granite are interpreted to be close to the formation age of the granitoid protolith. Similar to the Cambrian granitoids of the southwestern part of the Tepla Barrandian unit (Domažlice crystalline complex, DCC) the Middle Cambrian emplacement of the TCC granitoids postdates Cadomian deformation and metamorphism of the Upper Proterozoic country rocks, but predates Variscan tectonometamorphic imprints. Structural data as well as sedimentological criteria suggest a dextral transtensional setting during the Cambrian plutonism, related to the Early Paleozoic break-up of northern Gondwana. Due to strong Variscan crustal tilting, the degree of Variscan tectonometamorphic overprint is strikingly different in the dated granitoids. It is lowest in the weakly or undeformed Lestkov granite, located in the greenschist-facies domain. The Tepla orthogneiss in the north underwent pervasive top-to-NW mylonitic shearing under amphibolite-facies conditions. There is no indication for a resetting of the U–Pb isotopic system of the Tepla orthogneiss zircons that could be attributed to this imprint. Radiation damages accumulated until recent have probably caused lead loss.

Reflections from a mylonitized zone in central Sweden

Dipping reflections imaged on a deep seismic reflection profile acquired within the Baltic Shield can be indirectly linked to strain‐induced acoustic impedance contrasts within a regional brittle‐ductile shear zone: the Singö shear zone. Measurements made at various scales and frequencies estimate rock velocity within the shear zone and correlate well with the ductile component of strain previously determined by structural analysis. Velocity measurements include some made on cored field samples at confining pressures of ≤ 0.4 GPa in a laboratory (10 MHz sensor frequencies) and some made in situ using ultrasonic (54 kHz) sensors on exposures within the shear zone. Laboratory measurements show a 10% change in seismic anisotropy and a contrast of >0.5 km s−1 in bulk rock velocity between granitoid protolith and highly strained ultramylonite. The in situ velocity measurements demonstrate that the velocity variations persist over considerable volumes, including zones where strain and associated volume loss of silica remain elevated without mylonitization. Synthetic seismograms generated using a one‐dimensional velocity model of the shear zone show that velocity variations observed in the field were large enough in magnitude and of sufficient scale to produce reflections observable on the deep seismic reflection profile collected using 6–25 Hz frequencies. Ray trace, two‐dimensional forward modeling of a high‐density, wide‐angle reflection section demonstrates that deep dipping reflections could result from strain‐induced acoustic impedance contrasts within the Singö or similar, nearby shear zones.

Geochronology of augen gneiss and related rocks, Yukon-Tanana terrane, east-central Alaska

Research Article| May 01, 1986 Geochronology of augen gneiss and related rocks, Yukon-Tanana terrane, east-central Alaska JOHN N. ALEINIKOFF; JOHN N. ALEINIKOFF 1U.S. Geological Survey, Box 25046, M.S. 963, Denver Federal Center, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar CYNTHIA DUSEL-BACON; CYNTHIA DUSEL-BACON 2U.S. Geological Survey, 345 Middlefield Road, M.S. 904, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar HELEN L. FOSTER HELEN L. FOSTER 2U.S. Geological Survey, 345 Middlefield Road, M.S. 904, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar Author and Article Information JOHN N. ALEINIKOFF 1U.S. Geological Survey, Box 25046, M.S. 963, Denver Federal Center, Denver, Colorado 80225 CYNTHIA DUSEL-BACON 2U.S. Geological Survey, 345 Middlefield Road, M.S. 904, Menlo Park, California 94025 HELEN L. FOSTER 2U.S. Geological Survey, 345 Middlefield Road, M.S. 904, Menlo Park, California 94025 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1986) 97 (5): 626–637. https://doi.org/10.1130/0016-7606(1986)97<626:GOAGAR>2.0.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 JOHN N. ALEINIKOFF, CYNTHIA DUSEL-BACON, HELEN L. FOSTER; Geochronology of augen gneiss and related rocks, Yukon-Tanana terrane, east-central Alaska. GSA Bulletin 1986;; 97 (5): 626–637. doi: https://doi.org/10.1130/0016-7606(1986)97<626:GOAGAR>2.0.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 Using several isotopic techniques, we have determined the ages of selected metamorphic rocks in the Yukon-Tanana terrane (YTT) of east-central Alaska. U-Pb zircon data from an augen gneiss body in the Big Delta quadrangle indicate that the granitoid protolith of the gneiss was intruded 341 ± 3 m.y. ago (lower intercept age). An upper intercept age of 2,136 ± 31 m.y. indicates an inherited early Proterozoic component in these zircons. This inheritance age is substantiated by a Sm-Nd whole-rock model age of 2.09 ± 0.08 b.y. from the Big Delta augen gneiss body. Detrital zircons from quartzitic wall rocks to this body were also derived from an early Proterozoic (∼2.1 to 2.3 b.y. old) crustal source(s). Zircons from three other augen gneisses occurring in an east-west belt which extends into the southern Yukon Territory, Canada, have similar Mississippian and early Proterozoic intercept ages. A Rb-Sr whole-rock isochron from widely separated bodies of augen gneiss has an age of 333 ± 26 m.y. and an initial 87Sr/86Sr ratio of 0.728 ± 0.002, confirming the Mississippian intrusive age for the protolith. The high initial 87Sr/86Sr ratio further indicates an old crustal component in these rocks. A Rb-Sr mineral isochron (115 ± 4 m.y.), K-Ar data from hornblende and micas (128 to 107 m.y.), and U-Pb data from sphene (134. m.y.) from augen gneiss and related rocks are similar to many K-Ar ages in this region and confirm the occurrence of an early Cretaceous thermal event. U-Pb ages of zircons from three metavolcanic units in the YTT suggest that extrusion of the protoliths of these rocks occurred 360–380 m.y. ago. Scatter in the data is caused by ubiquitous inheritance and multiple lead-loss events.Significant Devonian-Mississippian igneous activity (380-340 m.y.) took place in the YTT, followed by one, or possibly two, metamorphic episodes (Mississippian? and Cretaceous). As yet, no early Proterozoic source rocks for the Paleozoic magmas have been identified in the YTT, but rocks of similar early Proterozoic ages occur to the east and south in the Northwest Territories and British Columbia, Canada. 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.

Petrology and tectonic significance of augen gneiss from a belt of Mississippian granitoids in the Yukon-Tanana terrane, east-central Alaska

An approximately east-west-trending belt of porphyritic peraluminous granitoids, metamorphosed and deformed to augen gneiss, is exposed for 400 km across the Yukon-Tanana terrane. This belt consists of compositionally, texturally, and isotopically similar, concordant intrusions of augen gneiss in the Big Delta, Eagle, and Tanacross quadrangles of east-central Alaska, in the Fiftymile batholith of west-central Yukon Territory, and in two batholiths in the offset part of the Yukon-Tanana terrane northeast of the Tintina fault in southeastern Yukon Territory. Chemical analyses of augen gneiss from widely separated localities within the Big Delta intrusion suggest that the various samples are related by crystal fractionation. Comparison of major- and trace-element data from concordant layers of augen-poor gneiss and aplitic gneiss with similar data from adjacent augen gneiss suggests that the layers represent co-magmatic sills of more highly differentiated filter-pressed melt. Similarities between elemental abundances, particularly those of rare-earth elements, in augen gneiss from the large bodies in the Big Delta, Eagle, and Tanacross quadrangles, Alaska, and southeastern Yukon Territory suggest a common origin for these bodies. The peraluminous composition of the gneiss, its high initial 87 Sr/ 86 Sr ratio and high Th concentration indicate that the augen gneiss protolith contains a large component of crustal material. Geochronologic studies of augen gneiss in east-central Alaska and southeastern Yukon Territory indicate a Mississippian intrusive age and an early Proterozoic age for the crustal component. The augen gneiss bodies were intruded late in a middle Paleozoic volcanic-plutonic episode, and they may represent a deeply eroded continental magmatic arc. Regional amphibolite-facies metamorphism and mylonitization may have occurred late during the tectonic episode that resulted in intrusion of the porphyritic granitoid protoliths.