scholarly journals Petrogenesis of the 91-Mile peridotite in the Grand Canyon: Ophiolite or deep-arc fragment?

Geosphere ◽  
2021 ◽  
Author(s):  
S.J. Seaman ◽  
M.L. Williams ◽  
K.E. Karlstrom ◽  
P.C. Low
Keyword(s):  
High Pressures ◽  
Grand Canyon ◽  
Mafic Magma ◽  
Ocean Basin ◽  
Partial Melt ◽  
Rock Types ◽  
Tectonic Boundaries ◽  
Tectonic Boundary ◽  
Lower Crustal ◽  
Plagioclase Textures

Recognition of fundamental tectonic boundaries has been extremely difficult in the (>1000-km-wide) Proterozoic accretionary orogen of southwestern North America, where the main rock types are similar over large areas, and where the region has experienced multiple postaccretionary deformation events. Discrete ultramafic bodies are present in a number of areas that may mark important boundaries, especially if they can be shown to represent tectonic fragments of ophiolite complexes. However, most ultramafic bodies are small and intensely altered, precluding petrogenetic analysis. The 91-Mile peridotite in the Grand Canyon is the largest and best preserved ultramafic body known in the southwest United States. It presents a special opportunity for tectonic analysis that may illuminate the significance of ultramafic rocks in other parts of the orogen. The 91-Mile peridotite exhibits spectacular cumulate layering. Contacts with the surrounding Vishnu Schist are interpreted to be tectonic, except along one margin, where intrusive relations have been interpreted. Assemblages include olivine, clinopyroxene, orthopyroxene, magnetite, and phlogopite, with very rare plagioclase. Textures suggest that phlogopite is the result of late intercumulus crystallization. Whole-rock compositions and especially mineral modes and compositions support derivation from an arc-related mafic magma. K-enriched subduction-related fluid in the mantle wedge is interpreted to have given rise to a K-rich, hydrous, high-pressure partial melt that produced early magnetite, Al-rich diopside, and primary phlogopite. The modes of silicate minerals, all with high Mg#, the sequence of crystallization, and the lack of early plagioclase are all consistent with crystallization at relatively high pressures. Thus, the 91-Mile peridotite body is not an ophiolite fragment that represents the closure of a former ocean basin. It does, however, mark a significant tectonic boundary where lower-crustal arc cumulates have been juxtaposed against middle-crustal schists and granitoids.

scholarly journals The Atud gabbro–diorite complex: glimpse of the Cryogenian mixing, assimilation, storage and homogenization zone beneath the Eastern Desert of Egypt

2020 ◽  
Vol 177 (5) ◽  
pp. 965-980
Author(s):  
Robert J. Stern ◽  
Kamal Ali ◽  
Paul D. Asimow ◽  
Mokhles K. Azer ◽  
Matthew I. Leybourne ◽  
...  
Keyword(s):  
Fractional Crystallization ◽  
Mafic Magma ◽  
Eastern Desert ◽  
Content Type ◽  
Igneous Complex ◽  
Nubian Shield ◽  
Partial Melts ◽  
Supplementary Material ◽  
Lower Crustal ◽  
Arabian Nubian Shield

We analysed gabbroic and dioritic rocks from the Atud igneous complex in the Eastern Desert of Egypt to understand better the formation of juvenile continental crust of the Arabian–Nubian Shield. Our results show that the rocks are the same age (U–Pb zircon ages of 694.5 ± 2.1 Ma for two diorites and 695.3 ± 3.4 Ma for one gabbronorite). These are partial melts of the mantle and related fractionates (εNd690 = +4.2 to +7.3, 87Sr/86Sri = 0.70246–0.70268, zircon δ18O ∼ +5‰). Trace element patterns indicate that Atud magmas formed above a subduction zone as part of a large and long-lived (c. 60 myr) convergent margin. Atud complex igneous rocks belong to a larger metagabbro–epidiorite–diorite complex that formed as a deep crustal mush into which new pulses of mafic magma were periodically emplaced, incorporated and evolved. The petrological evolution can be explained by fractional crystallization of mafic magma plus variable plagioclase accumulation in a mid- to lower crustal MASH zone. The Atud igneous complex shows that mantle partial melting and fractional crystallization and plagioclase accumulation were important for Cryogenian crust formation in this part of the Arabian–Nubian Shield.Supplementary material: Analytical methods and data, calculated equilibrium mineral temperatures, results of petrogenetic modeling, and cathodluminesence images of zircons can be found at https://doi.org/10.6084/m9.figshare.c.4958822

Metamorphic evolution of the Karimnagar granulite terrane, Eastern Dharwar Craton, south India

2010 ◽  
Vol 148 (1) ◽  
pp. 112-132 ◽  
Author(s):  
D. PRAKASH ◽  
I. N. SHARMA
Keyword(s):  
Granulite Facies ◽  
Dharwar Craton ◽  
Ultramafic Rocks ◽  
Chemical Compositions ◽  
Eastern Dharwar Craton ◽  
Geochronological Data ◽  
Rock Types ◽  
Wide Range ◽  
The Subject ◽  
Lower Crustal

AbstractThe Karimnagar granulite terrane is an integral part of the Eastern Dharwar Craton (EDC), India, having been the subject of much interest because of the only reported granulite facies rocks in the EDC. It shows a large variety of rock types with a wide range of mineral parageneses and chemical compositions, namely charnockites (Opx+Pl+perthite+Qtz±Bt±Grt), gneisses (Opx+Crd+Bt+Pl+Qtz+perthite±Sil±Grt±Spl; Bt+Qtz+Pl±Crd±Hbl±Spl), mafic granulites (Cpx+Pl+Qtz±Opx±Hbl), quartz-free granulites (Spr+Spl+Bt+Crd+Kfs+Crn; Bt+Crd+Kfs±Crn±Spl±Krn; And+Bt+Kfs+Chl), granites (Qtz+Pl+Kfs±Bt±Hbl), altered ultramafic rocks (Chl+Trem+Tlc), metadolerites (Cpx+Pl±Bt±Qtz±Chl), banded magnetite quartzites and quartzites. Andalusite- and chlorite-bearing assemblages presumably suggest a retrograde origin. Investigation of quartz-free granulites of the area brings out some interesting and important observations, reflecting the presence of refractory phases. These granulites are devoid of sillimanite and contain corundum instead. Reaction textures in the gneisses include breakdown of garnet to form coronas and symplectites of orthopyroxene+cordierite, formation of cordierite from garnet+sillimanite+quartz and late retrograde biotite and biotite+quartz symplectites. In the mafic granulites, inclusions of quartz and hornblende within orthopyroxene are interpreted as being a part of the prograde assemblage. At a later stage orthopyroxene is also rimmed by hornblende. The quartz-free granulites display a variety of spectacular coronas, for example, successive rims on corundum consisting of spinel+sapphirine+cordierite±orthopyroxene, rare skeletal symplectitic intergrowth of sapphirine+cordierite+potash feldspar, and late retrograde formation of chlorite, corundum, spinel and andalusite from sapphirine±cordierite. Based on chemographic relationships and petrogenetic grids, a sequence of prograde, isothermal decompressive and retrograde reactions have been inferred. Quartz-free sapphirine granulites and mafic granulites record the highest P–T conditions (~7 kbar, 850°C), whereas the gneisses were formed at lower P–T conditions (~5 kbar, 800°C). In addition, the presence of andalusite-bearing rocks suggests a pressure of around 2.5 kbar. This change in pressure from 7 kbar to around 2.5 kbar suggests a decompressive path for the evolution of granulites in the study area, which indicates an uplift for the granulite-facies rocks from lower crustal conditions. The implications for supercontinent history are also addressed in light of available geochronological data.

A Grenville-age layered intrusion in the subsurface of west Texas: petrology, petrography, and possible tectonic setting

1995 ◽  
Vol 32 (12) ◽  
pp. 2159-2166 ◽  
Author(s):  
Hulusi Kargi ◽  
Calvin G. Barnes
Keyword(s):  
Tectonic Setting ◽  
Basaltic Magma ◽  
Layered Intrusion ◽  
Mafic Magma ◽  
Magma Source ◽  
Rock Types ◽  
Hornblende Gabbro ◽  
Tectonic Environment ◽  
Extensional Tectonic ◽  
General Order

The Nellie intrusion is a thick (more than 4420 m) mafic to ultramafic layered intrusion with a radiometric age of ~1163 Ma. Rock types change abruptly with stratigraphic height and include norite, pyroxenite, gabbronorite, hornblende gabbro, gabbro, anorthosite, harzburgite, and lherzolite. Norite is most abundant, but gabbro and hornblende gabbro are locally abundant. Rare olivine-rich layers are also present. The general order of crystallization was olivine, orthopyroxene, plagioclase + clinopyroxene, and hornblende. Mg#'s, expressed as 100 Mg/(Mg + Fe), range from 76.3 to 85.8 for olivine, 56.7 to 84.9 for orthopyroxene, 62.5 to 90.3 for clinopyroxene, and 52.4 to 82.8 for amphibole. Mg#'s vary with height and display abrupt reversals, which indicate open-system addition of new mafic magma. Eleven cyclic units were identified on the basis of evidence for injection of basaltic magma; these can be grouped into three megacyclic units. The abundance of orthopyroxene, and mineral compositional evidence for Fe enrichment within cyclic units, indicates that parental magmas were subalkaline and tholeiitic. Plagioclase in equilibrium with olivine ranges from An65 to An46, which precludes an arc-related magma source. Although the intrusion is approximately coeval with Keweenawan magmatism and with emplacement of diabasic dikes in western North America, it is dissimilar in detail to both suites of rocks. Nevertheless, its composition and geophysical setting are consistent with emplacement in an extensional tectonic environment.

Crustal architecture and tectonic assembly of the Central Gneiss Belt, southwestern Grenville Province, Canada: a new interpretation

2000 ◽  
Vol 37 (2-3) ◽  
pp. 217-234 ◽  
Author(s):  
J WF Ketchum ◽  
A Davidson
Keyword(s):  
Structural Model ◽  
Hanging Wall ◽  
Mafic Rock ◽  
Grenville Province ◽  
The North ◽  
Rock Types ◽  
Structural Barrier ◽  
New Interpretation ◽  
Central Gneiss Belt ◽  
Tectonic Boundary

The Central Gneiss Belt, southwestern Grenville Province, is characterized by parautochthonous crust in the north and allochthonous lithotectonic domains in the south. Despite nearly two decades of study, the basal décollement to allochthonous domains transported from the southeast, known as the allochthon boundary thrust, has not been precisely located throughout much of the belt. Between Lake Nipissing and Georgian Bay where its surface trace is known, it separates 1.24 Ga Sudbury metadiabase in the footwall from eclogite remnants and 1.17-1.15 Ga coronitic olivine metagabbro confined to its hanging wall. On the premise that this relationship can be used to trace the allochthon boundary thrust elsewhere in the Central Gneiss Belt, we have sought to extend the known distribution of these mafic rock types, making use of field, petrographic, and geochemical criteria to identify them. New occurrences of all three mafic types are identified in a region extending from south of Lake Nipissing to western Quebec, and the mutually exclusive pattern of occurrence is maintained within this region. Structural trends and reconnaissance mapping of high-strain zones that appear to represent a structural barrier to the mafic suites suggest that the allochthon boundary thrust lies well to the north of its previously suggested location. Our preferred surface trace for it passes around the southern end of the Powassan batholith and through the town of North Bay before turning east to join up with the Lac Watson shear zone in western Quebec. This suggests that a large segment of "parautochthonous" crust lying north of, and including, the Algonquin domain is in fact allochthonous. The mutually exclusive distribution of the mafic suites points to significant separation of allochthonous and parautochthonous components prior to the Grenvillian orogeny, in accord with models of pre-Grenvillian continental rifting proposed by others. Despite a relative abundance of geological and geochronological data for the Central Gneiss Belt and a mafic rock distribution that appears to successfully locate a major tectonic boundary, we emphasize the need for additional field and laboratory work aimed at testing our structural model.

scholarly journals Neoproterozoic magmatic evolution of the southern Ouaddaï Massif (Chad)

2020 ◽  
Vol 191 ◽  
pp. 34 ◽  
Author(s):  
Félix Djerossem ◽  
Julien Berger ◽  
Olivier Vanderhaeghe ◽  
Moussa Isseini ◽  
Jérôme Ganne ◽  
...  
Keyword(s):  
Geothermal Gradient ◽  
Isotopic Signature ◽  
Biotite Granite ◽  
Granite Sample ◽  
Metasedimentary Rocks ◽  
Rock Types ◽  
Pan African ◽  
High K ◽  
Back Arc ◽  
Lower Crustal

This paper presents new petrological, geochemical, isotopic (Nd) and geochronological data on magmatic rocks from the poorly known southern Ouaddaï massif, located at the southern edge of the so-called Saharan metacraton. This area is made of greenschist to amphibolite facies metasediments intruded by large pre- to syn-tectonic batholiths of leucogranites and an association of monzonite, granodiorite and biotite granite forming a late tectonic high-K calc-alkaline suite. U-Pb zircon dating yields ages of 635 ± 3 Ma and 613 ± 8 Ma on a peraluminous biotite-leucogranite (containing numerous inherited Archean and Paleoproterozoic zircon cores) and a muscovite-leucogranite, respectively. Geochemical fingerprints are very similar to some evolved Himalayan leucogranites suggesting their parental magmas were formed after muscovite and biotite dehydration melting of metasedimentary rocks. A biotite-granite sample belonging to the late tectonic high-K to shoshonitic suite contains zircon rims that yield an age of 540 ± 5 Ma with concordant inherited cores crystallized around 1050 Ma. Given the high-Mg# (59) andesitic composition of the intermediate pyroxene-monzonite, the very similar trace-element signature between the different rock types and the unradiogenic isotopic signature for Nd, the late-kinematic high-K to shoshonitic rocks formed after melting of the enriched mantle and further differentiation in the crust. These data indicate that the southern Ouaddaï was part of the Pan-African belt. It is proposed that it represents a continental back-arc basin characterized by a high-geothermal gradient during Early Ediacaran leading to anatexis of middle to lower crustal levels. After tectonic inversion during the main Pan-African phase, late kinematic high-K to shoshonitic plutons emplaced during the final post-collisional stage.

scholarly journals NMR Spectral Characteristics of Ultrahigh Pressure High Temperature Impact Glasses of the Giant Kara Crater (Pay-Khoy, Russia)

Minerals ◽  
2021 ◽  
Vol 11 (12) ◽  
pp. 1418
Author(s):  
Vladimir Lyutoev ◽  
Tatyana Shumilova ◽  
Anton Mazur ◽  
Peter Tolstoy
Keyword(s):  
High Pressures ◽  
Spectral Characteristics ◽  
Glass Network ◽  
Mas Nmr ◽  
Ultrahigh Pressure ◽  
Impact Melt ◽  
Partial Melt ◽  
Natural Glass ◽  
Pressure Impact ◽  
The Impact

In this study, we carried out the analysis of the impact melt vein glasses from the Kara impact crater (Russia) in comparison to low-pressure impact melt glasses (tektites) of the Zhamanshin crater (Kazakhstan). 27Al, 23Na, and 29Si MAS NMR spectra of the samples of these glasses were analyzed. The samples of the natural glass contained inclusions of crystalline phases, paramagnetic elements that greatly complicate and distort the NMR signals from the glass phase itself. Taking into account the Mossbauer distribution of Fe in these glasses, the analysis of the spectra of MAS NMR of glass network-former (Si, Al) and potential network-modifiers (Na) of nuclei leads to the conclusion that the Kara impact melt vein glasses are characterized by complete polymerization of (Si,Al)O4 tetrahedral structural units. The NMR features of the glasses are consistent with the vein hypothesis of their formation under conditions of high pressures and temperatures resulting in their fluidity, relatively slow solidification with partial melt differentiation, polymerization, and precipitation of mineral phases as the impact melt cools. The 70 Ma stability of the Kara impact vein glass can be explained by the stabilization of the glass network with primary fine-dispersed pyroxene and coesite precipitates and by the high polymerization level of the impact glass.

Geochemical evolution of Devonian-Carboniferous igneous rocks of the Magdalen basin, Eastern Canada: Pb- and Nd-isotope evidence for mantle and lower crustal sources

1998 ◽  
Vol 35 (3) ◽  
pp. 201-221 ◽  
Author(s):  
Georgia Pe-Piper ◽  
David JW Piper
Keyword(s):  
Lithospheric Mantle ◽  
Crustal Contamination ◽  
Mafic Magma ◽  
Igneous Rocks ◽  
Geochemical Evolution ◽  
Eastern Canada ◽  
Nd Isotope ◽  
Isotopic Compositions ◽  
Mantle Sources ◽  
Lower Crustal

Magmatism associated with the extensional Magdalen basin includes voluminous tholeiitic gabbro and basalt and local granite and rhyolite. Pb- and (or) Nd-isotope determinations have been made on 70 igneous rocks from throughout the basin, and a further 15 samples of Avalonian basement from the southern margin of the basin, to characterize the contribution of lower crustal blocks and mantle sources to the magmatism and to constrain tectonic models for the basin. Five phases of magmatic evolution are distinguished in the Magdalen basin. (1) Middle to Late Devonian partial melting of lithospheric mantle, producing principally tholeiites and minor alkalic basalt. Tholeiites have Pb isotopic compositions similar to that of younger Triassic tholeiites generated from the same mantle, but experienced less crustal contamination. Regional variations in trace element composition of the mantle can be recognized. (2) The mafic magma triggered anhydrous base-of-crust melting, principally along the transpressive Cobequid and Rockland Brook faults, producing A-type granites in which radiogenic Pb increases northeastward. (3) In the latest Devonian, a large base-of-crust fractionating magma chamber evolved. It contained immiscible mafic and minor felsic magma, with uniform Nd isotopes, and high Ti in the mafic magma. (4) Although late Tournaisian dykes are not strongly fractionated, their evolution involved more crustal assimilation than earlier mafic rocks. (5) Local Viséan-Westphalian alkalic magmas, which ascended along crustal-scale faults, have Pb and Nd isotopic compositions resembling mantle plumes or their mixtures with lithospheric mantle sources. Only these youngest rocks show any isotopic evidence for input from an asthenospheric plume source, suggesting that regional extension was responsible for most of the magmatism.

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