Pink Gneisses Research Articles

The Logoualé Band: A large Archean crustal block in the Kenema-Man domain (Man-Leo rise, West African Craton) remobilized during Eburnean orogeny (2.05 Ga)

The Archean domain of the Man Rise was strongly remobilized during Eburnean coincidentally with the genesis of the Baoulé-Mossi domain (Birimian). This remobilization has allowed the recycling of Leonian and Liberian formations to generate a large gneissic crust, represented by the Logoualé Band. Zircons dates by laser ablation (LA-ICP-MS) of two samples of biotite-bearing pink gneisses of the Logoualé Band yielded ages of 2709 ± 15 and 2804 ± 11 Ma, confirming the Archean age of the Logoualé Band formations. The Eburnean tectono-metamorphic event in the Logoualé Band has totally reset both U-Th-Pb chronometer with an average age at 2050 ± 16 Ma in recrystallized zones of monazites, and Sm-Nd chronometer of garnets with an age at 2053 ± 15 Ma. Non-recrystallized zones of monazites give an average age at 2712 ± 16 Ma. We propose that the Logoualé Band rocks were originally sediments deposited in some protocratonic rift-type basins. During Eburnean, these sediments were buried, underwent high-grade metamorphism and exhumed in a tectonic context dominated by transcurrent motion. The structural setting of banded iron formations (iron deposits), which are abundant in the Logoualé Band, would date back to the Eburnean.

Did Bushmanland extensionally unroof Namaqualand?

The Namaqua-Natal Orogen evolved between about 2000 and 1000 Ma as part of an orogenic system that assembled the Rodinian supercontinent, and “wraps” the Archaean Kaapvaal Craton to the south and west. The Orogen includes the western ultra-high-temperature sapphirine/hercynite-granulite-facies Namaqualand Terrane, structurally overlain, to the north, by the low-grade, high-level igneous, Richtersveld Terrane (1900–1700 Ma) and, to the east, by the amphibolite-facies granite gneiss/Pb-Zn-rich supracrustal Bushmanland Terrane, which is overthrust, from the north along the Groothoek Thrust, by the high-pressure Hom Terrane. West of the dextral Tantalite Valley/Pofadder Shear Zone, the gently-dipping contacts among these terranes have been considered to be thrusts. With the exception of the Groothoek Thrust, the contacts are low-angle, top-down-to-the-east or northeast, folded extensional detachments with displacements of up to at least 100 km. The Namaqualand part of the Namaqua-Natal Orogen comprises two distinct event sequences, a Kibaran phase of crustal shortening and thickening, and voluminous granitic sheets at about 1200 Ma, then a Namaquan phase of mafic underplating, ultra-high-temperature metamorphism, granitic sheets, dextral transtension, constrictional fabrics, and crustal thinning from about 1060 to 1030 Ma. A dominant constrictional LS fabric was developed in regionally sub-horizontal layered pink granite gneisses and metasediments deformed by ENE-trending folds. Extensional detachments developed late in the Namaquan phase when the ultra-high-temperature rocks were exhumed. In the Bushmanland Terrane, Aggeneys metasediments have mainly constrictional fabrics and are deformed by giant upward-facing, downward-closing sheath folds, developed during transtension, in which lie 1700 Ma Pb/Zn SedEx deposits like those of Broken Hill and Flin Flon. The Namaqualand Terrane may be the largest and oldest extensional core complex on Earth. The Proterozoic Era was characterized by transtensional ultra-high-temperature, horizontally-layered, lineated, pink gneiss terrains with corrugating fold hinges roughly parallel with the finite stretching direction.

Rare earth and other trace element mobility during mylonitization: a comparison of the Brevard and Hope Valley shear zones in the Appalachian Mountains, USA

In progressing from a granitoid mylonite to an ultramylonite in the Brevard shear zone in North Carolina, Ca and LOI (H2O) increase, Si, Mg, K, Na, Ba, Sr, Ta, Cs and Th decrease, while changes in Al, Ti, Fe, P, Sc, Rb, REE, Hf, Cr and U are relatively small. A volume loss of 44% is calculated for the Brevard ultramylonite relative to an Al–Ti–Fe isocon. The increase in Ca and LOI is related to a large increase in retrograde epidote and muscovite in the ultramylonite, the decreases in K, Na, Si, Ba and Sr reflect the destruction of feldspars, and the decrease in Mg is related to the destruction of biotite during mylonitization. In an amphibolite facies fault zone separating grey and pink granitic gneisses in the Hope Valley shear zone in New England, compositional similarity suggests the ultramylonite is composed chiefly of the pink gneisses. Utilizing an Al–Ti–Fe isocon for the pink gneisses, Sc, Cr, Hf, Ta, U, Th and M‐HREE are relatively unchanged, Si, LOI, K, Mg, Rb, Cs and Ba are enriched, and Ca, Na, P, Sr and LREE are lost during deformation. In contrast to the Brevard mylonite, the Hope Valley mylonite appears to have increased in volume by about 70%, chiefly in response to an introduction of quartz.Chondrite‐normalized REE patterns of granitoids from both shear zones are LREE‐enriched and have prominent negative Eu anomalies. Although REE increase in abundance in the Brevard ultramylonites (reflecting the volume loss), the shape of the REE pattern remains unchanged. In contrast, REE and especially LREE decrease in abundance with increasing deformation of the Hope Valley gneisses. Mass balance calculations indicate that ≥95% of the REE in the Brevard rocks reside in titanite. In contrast, in the Hope Valley rocks only 15–40% of the REE can be accounted for collectively by titanite, apatite and zircon. Possible sites for the remaining REE are allanite, fluorite or grain boundaries. Loss of LREE from the pink gneisses during deformation may have resulted from decreases in allanite and perhaps apatite or by leaching ofy REE from grain boundaries by fluids moving through the shear zone.Among the element ratios most resistant to change during mylonitization in the Brevard shear zone are La/Yb, Eu/Eu*, Sm/Nd, La/Sc, Th/Sc, Th/Yb, Cr/Th, Th/U and Hf/Ta, whereas the most stable ratios in the Hope Valley shear zone are K/Rb, Rb/Cs, Th/U, Eu/Eu*, Th/Sc, Th/Yb, Sm/Nd, Th/Ta, Hf/Ta and Hf/Yb. However, until more trace element data are available from other shear zones, these ratios should not be used alone to identify protoliths of deformed rocks.

UPb geochronology and tectonic development of the southern flank of the Kisseynew Domain, Trans-Hudson Orogen, Canada

The southern flank of the Kisseynew Domain, northern Manitoba, underwent polyphase deformation and metamorphism during the Palaeoproterozoic Trans-Hudson Orogen, and represents a lithological, metamorphic and structural transition from greenschist-grade volcano-plutonic rocks of the Flin Flon Domain to amphibolite-grade paragneisses of the central Kisseynew Domain. The timing of geological events has previously been constrained using widely dispersed sample sites, but this study has integrated UPb geochronology with detailed lithological and structural mapping in the Cleunion Lake area. Pink gneisses, interpreted as Missi supracrustal rocks, were deposited after 1855 Ma, but prior to ∼ 1835 Ma, and thus are similar in age to fluvial sedimentary rocks in the lower-grade Flin Flon Domain. All the zircons in the pink gneisses appear to be inherited, or resorbed during later amphibolite-grade metamorphism, and a late Archaean component is also indicated. Corundum-bearing gneisses in the Missi Group and leucosomes in Burntwood River Group rocks are deformed by F 1 and are indicative of an early high- T, low- P metamorphic event (M 1) that may have been synchronous with the intrusion of 1835 to 1827 Ma intrusions, and related to regional crustal extension. The above rocks are folded during a continuous compressional deformation event (D 2) related to terminal collision between the Palaeoproterozoic rocks of the Trans-Hudson Orogen and the Superior Province. Three main fold generations (F 1, F 2, F 3) have been identified, which occurred during D 2. A 1818 Ma granitic vein is approximately synchronous with F 2 and peak metamorphic (M 2) conditions in the Cleunion Lake area. Cooling, uplift, and intrusion of pegmatites during F 3 at ∼ 1800 Ma provides a time limit on the waning stages of compressional deformation. The main phase of compressional deformation (D 2) in the southern flank of the Kisseynew Domain thus lasted for about 30 million years.

Differential response of zircon U?Pb isotopic systematics to metamorphism across a lithologic boundary: an example from the Hope Valley Shear Zone, southeastern Massachusetts, USA

A detailed morphological, chemical and isotopic study of zircons from a single outcrop of two mineralogically and chemically distinct units of the late Precambrian Ponaganset gneiss was undertaken to investigate the effects of mylonitization and metamorphism on U-Pb isotopic systematics. Late Paleozoic, amphibolite-grade (approx. 600°C) mylonitization of the Ponaganset gneiss at this locality is associated with movement along the Hope Valley Shear Zone. The response of zircon to metamorphism in each gneiss unit is distinct: zircons in gray augen gneiss are uncorroded and not overgrown, whereas zircons from fluorite-bearing pink granitic gneiss are variably corroded and over 50% bear opaque overgrowths. The zircon overgrowths are chemically distinct from the primary cores, and contain high conentrations of Hf, U, HREE, and Th. Mylonite derived from the gray gneiss contains only a small population of Hf-U-rich metamorphic zircon, but zircons in the pink gneiss-derived mylonite are dominated by the Hf-U-rich metamorphic component. In terms of their U-Pb isotopic systematics, overgrowth-free zircons from both units are markedly discordant (gray, 10–20%, pink, 35%), but overgrown zircons from the pink gneiss are up to 70% discordant. Zircons from the mylonites yield younger Pb−Pb and U−Pb ages than those of the protolith gneisses, and isotopic data from each gneiss + mylonite pair define a linear array on concordia plots. Upper intercept ages of the gray gneiss (621+/−27 Ma) and the pink gneiss (635+/−50 Ma) indicate that the crystallization of both units was coeval, and the lower intercept ages (gray, 270+/−92 Ma; pink, 285+/−26 Ma) fall within the range of other published age estimates for Alleghanian metamorphism in southeastern New England (e.g., Zartman et al. 1988). New growth of zircon suggests that Zr was mobile during metamorphism. The presence of fluorite in the pink gneiss, and a discontinuity in log \(\left( {{{f_{{\text{H}}_{\text{2}} {\text{O}}} } \mathord{\left/{\vphantom {{f_{{\text{H}}_{\text{2}} {\text{O}}} } {f_{{\text{HF}}} }}} \right.\kern-\nulldelimiterspace} {f_{{\text{HF}}} }}} \right)\)values obtained from biotite across the pink gneiss-gray gneiss contact indicates that dissolution and reprecipitation of zircon may be related to local variations in HF fugacity. Zircon dissolution/reprecipitation in the pink gneiss, and the lack of similar features in the contiguous gray gneiss, suggests that the degree of isotopic perturbation of zircon during metamorphism is related to bulk chemistry, fluid chemistry and/or the degree of fluid-rock interaction.

A Grenville age for pre-Caledonian rocks in NW Co. Mayo, Ireland

Pink Gneisses at Erris Head in NW Co. Mayo, Ireland have yielded a Rb-Sr whole rock age of 1070±70Ma (λ 87 Rb = 1.42×10 – 11 year – 1 ). Interbanded Grey Gneisses show a somewhat younger but probably related age with a slightly higher 87 Sr/ 86 Sr initial ratio. Isotopic homogenization appears to have been locally controlled by lithology, with the mineralogically more stable suite of tectosilicate-rich gneisses giving the older age. The ages of these 2 main components of the Annagh Gneiss unit at Erris Head may reflect a reworking event rather than the formation of the gneisses as there is evidence locally for a lengthy, tectono-thermal history. At both Erris Head and nearby Cross Point, K-Ar whole rock analyses of the same specimens confirm a metamorphic overprint which is consistent with an early Caledonian metamorphic history seen in the adjacent Moine and Dalradian rocks.

GLACIAL GEOLOGY OF THE MOUNT GRAN AREA, SOUTHERN VICTORIA LAND, ANTARCTICA

Research Article| October 01, 1964 GLACIAL GEOLOGY OF THE MOUNT GRAN AREA, SOUTHERN VICTORIA LAND, ANTARCTICA PARKER E CALKIN PARKER E CALKIN INSTITUTE OF POLAR STUDIES, OHIO STATE UNIVERSITY, COLUMBUS, OHIO (PRESENT ADDRESS: SCIENCE DEPT., STATE UNIVERSITY COLLEGE, BUFFALO, N. Y.) Search for other works by this author on: GSW Google Scholar GSA Bulletin (1964) 75 (10): 1031–1036. https://doi.org/10.1130/0016-7606(1964)75[1031:GGOTMG]2.0.CO;2 Article history received: 04 Mar 1964 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PARKER E CALKIN; GLACIAL GEOLOGY OF THE MOUNT GRAN AREA, SOUTHERN VICTORIA LAND, ANTARCTICA. GSA Bulletin 1964;; 75 (10): 1031–1036. doi: https://doi.org/10.1130/0016-7606(1964)75[1031:GGOTMG]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 The Mount Gran area, encompassing Mount Gran and the ice-free Alatna Valley adjoining to the north, is located 77° S. and 161° E. in the rugged, glaciated mountains of southern Victoria Land, Antarctica.At least two major glaciations are represented in this area. During the first and most extensive glaciation recognized, the A Glaciation, ice came from the inland ice plateau in the west, hollowed out the western end of Alatna Valley, and probably passed through the area to the Ross Sea in the east.During the second or B Glaciation, ice probably invaded the Mount Gran area from the coastal area in the east. This origin is suggested by the orientation of roche moutonnées and end moraines in Alatna Valley and by the occurrence of erratic boulders of pink granite and pink gneiss on the floor of Alatna Valley and lower elevations of Mount Gran. Such an invasion from the coast may have occurred when sea level was low and there was about 1200 m of ice in the McMurdo Sound-Ross Sea area.Ablation phenomena immediately adjacent to the glacier fronts suggest recent recession in the Mount Gran area. 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.