scholarly journals Structure and Petrology of the Red Hill Complex, Nelson

2021 ◽  
Author(s):  
◽  
Richard Irving Walcott
Keyword(s):  
Bulk Composition ◽  
Mineral Chemistry ◽  
Parent Material ◽  
Preferred Orientation ◽  
Ultramafic Rocks ◽  
Rock Types ◽  
Basal Zone ◽  
Upper Paleozoic ◽  
Two Generations ◽  
Thick Lens

<p>The Red Hill Complex is an essentially concordant ultramafic body enclosed in Upper Paleozoic flysch facies sediments which include Pelorus Group (oldest), Lee River Group and Maitai Group. The Pelorus Group contains rare submarine lavas and is largely derived from spilitic volcanics. The Lee River Group consists of spilitic pillow lavas, volcanic breccias and spilitic basalts and dolerites. The Maitai Group consists of limestone, sandstone and argillite; an extensive conglomerate lens in the argillites is largely composed of andesitic pebbles. The Red Hill Complex is a 12,000 ft. thick lens and is part of a sheet of peridotites which may extend 40 miles northward to Dun Mountain. The Complex is divided into a 3000 ft thick Basal Zone of massive harzburgite and a 9000 ft thick Upper Zone of layered harzburgite and dunite with minor variants, feldspathic-peridotite, eucrite, lherzolite, wehrlite and pyroxenite. The bulk composition of both zones is approximately the same but the Upper Zone contains about 0.2 per cent feldspar not present in the Basal Zone. There is no significant regional change in mineral chemistry throughout the Complex and the average composition is about; olivine Fo91, 70 per cent; orthopyroxene, En88, 22 per cent; clinopyroxene, 5 per cent; feldspar An96, less than 0.2 per cent; spinel 2 per cent. Layering and foliation are common in the top of the Upper Zone. Layering is of at least two generations of which at least one is of metamorphic origin. Metamorphic layering was formed by metasomatic replacement probably along subhorizontal shear planes during intrusion of the ultramafic sheet. Pyroxene pegmatites formed after flow ceased. The diversity of rock types in the top of the Upper Zone is considered by the writer to have been caused by metamorphic differentiation of parent material the same composition as the Basal Zone. The preferred orientation of olivine in lineated, foliated, laminated and layered rocks has the same pattern suggesting a close genetic relationship between those structures. Evidence strongly supports a tectonic origin for the preferred orientation. Rocks in the Upper Zone are xenomorphic-granular in texture and those in the Basal Zone are typically protoclastic. Xenomorphic-granular textures are derived in part from protoclastic by post-deformational recrystallization. The ultramafic rocks are cut by a number of dykes composed of hornblende-labradorite, hypersthene-augite-bytownite assemblages or minor variants of these. The dykes were intruded shortly after emplacement of the ultramafic rocks. The Red Hill Complex is considered to have been emplaced as a sheet at shallow depths which intruded superficial deposits on the ocean floor and was later overlain by volcanics</p>

scholarly journals Structure and Petrology of the Red Hill Complex, Nelson

2021 ◽  
Author(s):  
◽  
Richard Irving Walcott
Keyword(s):  
Bulk Composition ◽  
Mineral Chemistry ◽  
Parent Material ◽  
Preferred Orientation ◽  
Ultramafic Rocks ◽  
Rock Types ◽  
Basal Zone ◽  
Upper Paleozoic ◽  
Two Generations ◽  
Thick Lens

<p>The Red Hill Complex is an essentially concordant ultramafic body enclosed in Upper Paleozoic flysch facies sediments which include Pelorus Group (oldest), Lee River Group and Maitai Group. The Pelorus Group contains rare submarine lavas and is largely derived from spilitic volcanics. The Lee River Group consists of spilitic pillow lavas, volcanic breccias and spilitic basalts and dolerites. The Maitai Group consists of limestone, sandstone and argillite; an extensive conglomerate lens in the argillites is largely composed of andesitic pebbles. The Red Hill Complex is a 12,000 ft. thick lens and is part of a sheet of peridotites which may extend 40 miles northward to Dun Mountain. The Complex is divided into a 3000 ft thick Basal Zone of massive harzburgite and a 9000 ft thick Upper Zone of layered harzburgite and dunite with minor variants, feldspathic-peridotite, eucrite, lherzolite, wehrlite and pyroxenite. The bulk composition of both zones is approximately the same but the Upper Zone contains about 0.2 per cent feldspar not present in the Basal Zone. There is no significant regional change in mineral chemistry throughout the Complex and the average composition is about; olivine Fo91, 70 per cent; orthopyroxene, En88, 22 per cent; clinopyroxene, 5 per cent; feldspar An96, less than 0.2 per cent; spinel 2 per cent. Layering and foliation are common in the top of the Upper Zone. Layering is of at least two generations of which at least one is of metamorphic origin. Metamorphic layering was formed by metasomatic replacement probably along subhorizontal shear planes during intrusion of the ultramafic sheet. Pyroxene pegmatites formed after flow ceased. The diversity of rock types in the top of the Upper Zone is considered by the writer to have been caused by metamorphic differentiation of parent material the same composition as the Basal Zone. The preferred orientation of olivine in lineated, foliated, laminated and layered rocks has the same pattern suggesting a close genetic relationship between those structures. Evidence strongly supports a tectonic origin for the preferred orientation. Rocks in the Upper Zone are xenomorphic-granular in texture and those in the Basal Zone are typically protoclastic. Xenomorphic-granular textures are derived in part from protoclastic by post-deformational recrystallization. The ultramafic rocks are cut by a number of dykes composed of hornblende-labradorite, hypersthene-augite-bytownite assemblages or minor variants of these. The dykes were intruded shortly after emplacement of the ultramafic rocks. The Red Hill Complex is considered to have been emplaced as a sheet at shallow depths which intruded superficial deposits on the ocean floor and was later overlain by volcanics</p>

Geochemistry of Cretaceous subduction initiation related cumulate gabbros in a forearc setting from Chaldoran ophiolite, NW Iran

2020 ◽  
Author(s):  
Mahleqa Rezaei ◽  
Mohssen Moazzen ◽  
Tian-Nan Yang
Keyword(s):  
Source Region ◽  
Mineral Chemistry ◽  
Ultramafic Rocks ◽  
Nw Iran ◽  
Subduction Initiation ◽  
Rock Types ◽  
Original Magma ◽  
Ree Patterns ◽  
Ree Pattern ◽  
Cretaceous Subduction

&lt;p&gt;The Neo-Tethys-related Chaldoran ophiolite in NW Iran and at the Turkish border is a part of the larger Khoy ophiolite. Cumulate and isotropic gabbro along with serpentinized peridotite, pillow basalt, pelagic limestone, rare radiolarites, and volcano-sedimentary units are the main rock types in the area. The gabbros occur as lenses with ultramafic rocks, or as relatively large exposures with fault contact with ultramafic rocks. In this study, we provide new whole-rock geochemistry, mineral chemistry and zircon U/Pb age for the cumulate gabbros from the Chaldoran area. Gabbros have tholeiitic composition and are highly depleted. Chondrite normalized rare earth elements (REE) pattern for gabbros are comparative with REE patterns for N-MORB, but overall with more depleted features. The N-MORB normalized multi-elements pattern shows high depletion in HREE and HFSE and enrichment in some LREE and LILEs. Negative anomaly for some HFSE relative to N-MORB, along with enrichment in LILE for the samples indicates the source region as subduction influenced mantle. The cumulated gabbro whole rock and Clinopyroxenes geochemistry indicate an intra-oceanic forearc setting for the studied samples. They also have many similarities to boninite in mineral and whole rock geochemistry. U-Pb zircon dating of the gabbro samples indicates 95.3-114.1 Ma ages for the generation of the gabbros parent magma. The original magma was related to the later stages of the forearc setting in the subduction initiation (SI) stage. This &amp;#8216;SI&amp;#8217; related Albian-Cenomanian the Chaldoran depleted gabbro likely are the continuation of Taurus SI related late Cretaceous ophiolite complexes in Turkey.&lt;/p&gt;

Sediment- and basalt-hosted regoliths in the Huronian supergroup: role of parent lithology in middle Precambrian weathering profiles

1993 ◽  
Vol 30 (1) ◽  
pp. 60-76 ◽  
Author(s):  
S. J. Sutton ◽  
J. B. Maynard
Keyword(s):  
Chemical Evolution ◽  
Bulk Composition ◽  
Mineral Chemistry ◽  
White Mica ◽  
Low Grade ◽  
Low Oxygen ◽  
Mafic Dike ◽  
Rock Types ◽  
Rock Bulk

Weathering profiles developed side-by-side on sandstone and a mafic dike provide an unusual opportunity to examine the role of parent rock bulk composition in the chemical evolution of middle Precambrian regoliths. Because the profiles are adjacent to one another, differences in topography can be eliminated in accounting for differences in the chemical evolution of the two profiles. Both profiles show upward increases in Al, Ti, K, and Rb and decreases in Mg, Ca, and Na. In addition, the mafic regolith increases upward in Zr and Nb and decreases in Zn and Ni. Total Fe decreases upward in both profiles, but the sandstone profile retains significantly more of its initial Fe than does the mafic dike. This difference in Fe loss is consistent with weathering in a low-oxygen atmosphere of rock types with very different initial Fe contents and therefore different atmospheric requirements for complete oxidation of the Fe present. The Fe in the sandstone was mostly oxidized and retained within the profile, whereas much of the Fe in the mafic dike was not oxidized and was removed from the profile in the more soluble ferrous state. Petrographic evidence indicates that both sandstone and mafic dike weathering profiles underwent preweathering diagenesis, postweathering K–Rb metasomatism, and very low-grade metamorphism. Mineral chemistry indicates that, in the absence of chlorite, white mica composition closely reflects variation in bulk composition. Where both white mica and chlorite are present, changes in bulk composition are accommodated by variations in the proportions of these two minerals rather than by variations in white mica composition.

The Metasomatized Mantle beneath the North Atlantic Craton: Insights from Peridotite Xenoliths of the Chidliak Kimberlite Province (NE Canada)

2019 ◽  
Vol 60 (10) ◽  
pp. 1991-2024 ◽  
Author(s):  
M G Kopylova ◽  
E Tso ◽  
F Ma ◽  
J Liu ◽  
D G Pearson
Keyword(s):  
North Atlantic ◽  
Thermal State ◽  
Mineral Chemistry ◽  
Mantle Metasomatism ◽  
Labrador Sea ◽  
The North ◽  
Rock Types ◽  
History Of ◽  
The North Atlantic ◽  
North Atlantic Craton

Abstract We studied the petrography, mineralogy, thermobarometry and whole-rock chemistry of 120 peridotite and pyroxenite xenoliths collected from the 156–138 Ma Chidliak kimberlite province (Southern Baffin Island). Xenoliths from pipes CH-1, -6, -7 and -44 are divided into two garnet-bearing series, dunites–harzburgites–lherzolites and wehrlites–olivine pyroxenites. Both series show widely varying textures, from coarse to sheared, and textures of late formation of garnet and clinopyroxene. Some samples from the lherzolite series may contain spinel, whereas wehrlites may contain ilmenite. In CH-6, rare coarse samples of the lherzolite and wehrlite series were derived from P = 2·8 to 5·6 GPa, whereas predominant sheared and coarse samples of the lherzolite series coexist at P = 5·6–7·5 GPa. Kimberlites CH-1, -7, -44 sample mainly the deeper mantle, at P = 5·0–7·5 GPa, represented by coarse and sheared lherzolite and wehrlite series. The bulk of the pressure–temperature arrays defines a thermal state compatible with 35–39 mW m–2 surface heat flow, but a significant thermal disequilibrium was evident in the large isobaric thermal scatter, especially at depth, and in the low thermal gradients uncharacteristic of conduction. The whole-rock Si and Mg contents of the Chidliak xenoliths and their mineral chemistry reflect initial high levels of melt depletion typical of cratonic mantle and subsequent refertilization in Ca and Al. Unlike the more orthopyroxene-rich mantle of many other cratons, the Chidliak mantle is rich (∼83 vol%) in forsteritic olivine. We assign this to silicate–carbonate metasomatism, which triggered wehrlitization of the mantle. The Chidliak mantle resembles the Greenlandic part of the North Atlantic Craton, suggesting the former contiguous nature of their lithosphere before subsequent rifting into separate continental fragments. Another, more recent type of mantle metasomatism, which affected the Chidliak mantle, is characterized by elevated Ti in pyroxenes and garnet typical of all rock types from CH-1, -7 and -44. These metasomatic samples are largely absent from the CH-6 xenolith suite. The Ti imprint is most intense in xenoliths derived from depths equivalent to 5·5–6·5 GPa where it is associated with higher strain, the presence of sheared samples of the lherzolite series and higher temperatures varying isobarically by up to 200 °C. The horizontal scale of the thermal-metasomatic imprint is more ambiguous and could be as regional as tens of kilometers or as local as &lt;1 km. The time-scale of this metasomatism relates to a conductive length-scale and could be as short as &lt;1 Myr, shortly predating kimberlite formation. A complex protracted metasomatic history of the North Atlantic Craton reconstructed from Chidliak xenoliths matches emplacement patterns of deep CO2-rich and Ti-rich magmatism around the Labrador Sea prior to the craton rifting. The metasomatism may have played a pivotal role in thinning the North Atlantic Craton lithosphere adjacent to the Labrador Sea from ∼240 km in the Jurassic to ∼65 km in the Paleogene.

scholarly journals Petrography and zircon morphology of syntectonic granitoids rocks of Hafafit area, South Eastern Desert (Egypt).

2013 ◽  
Vol 47 (1) ◽  
pp. 438 ◽  
Author(s):  
I. Thabet ◽  
A. Kilias ◽  
A. Koroneos ◽  
S. Kamh
Keyword(s):  
First Generation ◽  
Morphological Characteristics ◽  
Crystal Form ◽  
Eastern Desert ◽  
Elongation Ratio ◽  
Crystal Length ◽  
Rock Types ◽  
Two Generations ◽  
Granitoid Rocks ◽  
Zircon Morphology

The study deals with the petrography of the granitoid rocks intruding the five (A to E) domes of Hafafit Culmination. The morphological characteristics of zircon crystals are also examined, in order to elucidate the genesis of zircon and their host Hafafit granitoids. The granitoid rocks are tonalites, granodiorites and quartz diorites, all exhibiting gneissic texture. The zircon crystals from all the rock types intruding each one of the domes are examined in detail for colour, inclusions, crystal length, crystal width, elongation ratio, crystal form and habit, degree of roundness and corrosion, and are distinguished in groups on the basis of the previous characteristics. Two generations of zircons have been recognized; the first generation with magmatic features and the second one with post magmatic features. The first generation of zircon is attributed to the subduction, magmatic and deformation events while the second generation of zircon is attributed to metamorphism.

scholarly journals Building Arc Crust: Plutonic to Volcanic Connections in an Extensional Oceanic Arc, the Southern Alisitos Arc, Baja California

2019 ◽  
Vol 60 (6) ◽  
pp. 1195-1228 ◽  
Author(s):  
Rebecca A Morris ◽  
Susan M DeBari ◽  
Cathy Busby ◽  
Sarah Medynski ◽  
Brian R Jicha
Keyword(s):  
Reference Model ◽  
Volcanic Rocks ◽  
Three Dimensional ◽  
Mineral Chemistry ◽  
Major And Trace Elements ◽  
Low Velocity ◽  
Fractionation Process ◽  
Process Step ◽  
Rock Types ◽  
Plutonic Rocks

Abstract The ∼50 km long Rosario segment of the Cretaceous Alisitos oceanic arc terrane provides undeformed three-dimensional exposures of the upper 7 km of an oceanic extensional arc, where crustal generation processes are recorded in both the volcanic and underlying plutonic rocks. These exceptional exposures allow for the study of the physical and chemical links between the rock units and help constrain the differentiation processes active during the growth and evolution of arc crust. This study focuses on the southern third of the Rosario segment, previously referred to as the southern volcano-bounded basin, and its plutonic underpinnings. Upper crustal rocks in the Rosario segment consist of a 3–5 km thick volcanic–volcaniclastic section with hypabyssal intrusions. Plutons intrude these units at various levels along-strike, but at each intrusive contact the transition is complete over a distance of <150 m, where stoped volcanic blocks are present. There is striking compositional overlap in whole-rock and mineral chemistry between the plutonic and volcanic units, suggesting a comagmatic source. Whole-rock geochemistry shows coherent trends in major and trace elements in mafic to intermediate compositions, but less coherent trends above 63 wt % SiO2. Units are predominantly low-K with flat rare earth element patterns, and show large ion lithophile element enrichment and high field strength element depletion. Initial Nd and Pb isotope ratios overlap for all units and imply no cratonic continental involvement. This agrees with low Sr/Y ratios of all rock types, indicative of thin, immature oceanic arc crust. Modeling results show that closed-system fractional crystallization drove crustal differentiation from mafic to intermediate compositions, but open-system processes likely occurred to produce some of the felsic compositions. Differentiation occurred in a two-step fractionation process. Step 1, from basaltic andesite to andesite, fractionated an anhydrous gabbroic cumulate (∼40% crystallization). Step 2, from andesite to rhyolite, fractionated a hydrous amphibole cumulate (∼65% crystallization, total), which is similar to what fluid dynamical models suggest for production of rhyolite (between 50–70% crystallization). Our results can be used as a reference model for differentiation processes relating to the growth of the middle and upper crust within active extensional arc systems. The Rosario segment plutonic rocks may be analogous to the low-velocity zone (Vp = 6·0–6·5 km s–1) imaged within the extensional Izu–Bonin arc. The chemistry of the plutonic and volcanic rocks is most similar to those of volcanic rocks in the Izu–Bonin active rift.

Forsterite chondrites; the meteorites Kakangari, Mount Morris (Wisconsin), Pontlyfni, and Winona

1977 ◽  
Vol 41 (318) ◽  
pp. 201-210 ◽  
Author(s):  
A. L. Graham ◽  
A. J. Easton ◽  
R. Hutchison
Keyword(s):  
Natural History ◽  
Bulk Composition ◽  
British Museum ◽  
Mineral Chemistry ◽  
Chemical Analyses ◽  
Ordinary Chondrites ◽  
Sulphide Minerals ◽  
Bulk Chemical ◽  
Original Owner ◽  
Cursory Examination

SummaryPontlyfni and Mount Morris (Wisconsin) are briefly described. Chondrule structure is absent from both, the latter is coarsely crystalline and both are sulphide-rich and contain forsterite and enstatite. Kakangari has well-defined chondrules; its silicate and sulphide minerals are unequilibrated, but similar to those of Pont-lyfni and Mount Morris (Wisconsin). New bulk chemical analyses are presented; Pontlyfni has 33·84 % total Fe and 7·04 % S; Mount Morris (Wisconsin) has 19·88 % total Fe and 4·72 % S; and Kakangari has 22·79 % total Fe and 5·30 % S. These three meteorites, together with Winona, have ordinary chondritic Mg/Si ratios, which result in their having abundant forsterite; this distinguishes them from E-chondrites. The four meteorites have Mg/Si ratios lower than those of C-chondrites, and they are more reduced than C or ordinary chondrites. These four unusual stones, therefore, have some chemical similarities, for example all have Mg/Si (atomic) of about 0·95, mean olivine composition ranging from Fa1 to Fa5, and significant Cr in the sulphide. Cumberland Fails chondritic xenoliths and other exotic fragments in polymict meteorites may be related to the four stones, which may ultimately prove to belong to a distinct chemical group of chondrites related to the irons of Group IAB.These meteorites are described and discussed together because they have similarities in mineralogy, bulk composition, and oxidation state. All have more than 10 % forsterite (i.e. an olivine in the range Fa0·10). In our study we investigated the mineral chemistry of, first, Kakangari (Graham and Hutchison, 1974), and then Mount Morris (Wisconsin), with cursory examination of the previously described Winona (Mason and Jarosewich, 1967) and Cumberland Falls chondritic fraction (Binns, 1969; Jarosewich, 1967). By chance, the British Museum (Natural History) acquired the Pontlyfni stone during the course of this work. Pontlyfni fell in Wales in 1931, but is so far undescribed; it proved to be chemically and mineralogically similar to Kakangari. We thank Mr. J. R. Owen, the original owner, for its timely addition to the Museum's Collection.

Whole-rock and mineral chemistry of cumulates from the Kızıldağ (Hatay) ophiolite (Turkey): clues for multiple magma generation during crustal accretion in the southern Neotethyan ocean

2005 ◽  
Vol 69 (1) ◽  
pp. 53-76 ◽  
Author(s):  
U. Bağci ◽  
O. Parlak ◽  
V. Höck
Keyword(s):  
Tectonic Setting ◽  
Mineral Chemistry ◽  
Chromian Spinel ◽  
Olivine Gabbro ◽  
Crustal Accretion ◽  
Magma Generation ◽  
The North ◽  
Rock Types ◽  
Suprasubduction Zone ◽  
Ocean Ridge

AbstractThe late Cretaceous Kızıldağ ophiolite forms one of the best exposures of oceanic lithospheric remnants of southern Neotethys to the north of the Arabian promontory in Turkey. The ultramafic to mafic cumulate rocks, displaying variable thickness (ranging from 165 to 700 m), are ductiley deformed, possibly in response to syn-magmatic extension during sea-floor spreading and characterized by wehrlite, olivine gabbro, olivine gabbronorite and gabbro. The gabbroic cumulates have an intrusive contact with the wehrlitic cumulates in some places. The crystallization order of the cumulus and intercumulus phases is olivine (Fo86–77)± chromian spinel, clinopyroxene (Mg#92–76), plagio-clase(An95–83), orthopyroxene(Mg#87–79). The olivine, clinopyroxene, orthopyroxene and plagioclase in ultramafic and mafic cumulate rocks seem to have similar compositional range. This suggests that these rocks cannot represent a simple crystal line of descent. Instead the overlapping ranges in mineral compositions in different rock types suggest multiple magma generation during crustal accretion for the Kızıldağ ophiolite. The presence of high Mg# of olivine, clinopyroxene, orthopyroxene, and the absence of Ca-rich plagioclase as an early fractionating phase co-precipitating with forsteritic olivine, suggest that the Kızıldağ plutonic suite is not likely to have originated in a mid-ocean ridge environment. Instead the whole-rock and mineral chemistry of the cumulates indicates their derivation from an island arc tholeiitic (IAT) magma. All the evidence indicates that the Kızıldağ ophiolite formed along a slow-spreading centre in a fore-arc region of a suprasubduction zone tectonic setting.

The role of magma injection and crystal sorting in the formation of early gabbros at the Coldwell Complex, Ontario, Canada

2019 ◽  
Vol 56 (7) ◽  
pp. 715-737 ◽  
Author(s):  
Yong-hua Cao ◽  
David J. Good ◽  
Robert L. Linnen ◽  
Iain M. Samson
Keyword(s):  
Major Element ◽  
Mineral Chemistry ◽  
Ultramafic Rocks ◽  
Chemical Compositions ◽  
Olivine Gabbro ◽  
Additional Observation ◽  
Midcontinent Rift ◽  
Layered Series ◽  
Incompatible Elements

The Layered Series of the Midcontinent Rift related Coldwell Complex comprises thick sections of gabbro, without any known associated ultramafic rocks. It represents a major early intrusive unit of the Coldwell Complex and consists of thick accumulations of olivine gabbro and oxide augite melatroctolite. This study combines petrography, mineral chemistry, and lithogeochemistry to constrain the magma composition and petrogenesis of the Layered Series. The presence of cumulus orthoclase together with the observation that the Layered Series rocks plot in the alkaline field on a total alkali–silica diagram indicate that the Layered Series magma has an alkaline parentage. The stratigraphy of the Layered Series cannot be fully correlated between different areas using lithogeochemistry and mineral chemistry. This together with observed normal and reverse trends for mineral chemical compositions in different areas suggest that the processes related to magma emplacement and crystallization were different in different locations. The whole-rock concentrations of incompatible elements and the compositions of major minerals of the olivine gabbro and oxide augite melatroctolite units are chemically similar. However, major element lithogeochemistry is variable, dominantly due to differences in the abundances of olivine, clinopyroxene, plagioclase, and magnetite. An additional observation is that olivine and clinopyroxene are not in chemical equilibrium. Together, these observations are interpreted to reflect a combination of multiple injections of magma and crystal sorting in an open system.

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.

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